Catalytic applications of modified mesoporous silica ordered networks with transitional metal.
\r\n\tIt is a relatively simple process and a standard tool in any industry. Because of the versatility of the titration techniques, nearly all aspects of society depend on various forms of titration to analyze key chemical compounds.
\r\n\tThe aims of this book is to provide the reader with an up-to-date coverage of experimental and theoretical aspects related to titration techniques used in environmental, pharmaceutical, biomedical and food sciences.
The ordered mesoporous silicas (OMSs) containing transition metals are versatile catalytic materials for oxidation of a wide range of organic compounds [1, 2, 3]. Mesoporous materials with various structures such as MCM-41 (2d hexagonal p6mm) [4, 5, 6], MCM-48 (3d cubic Ia3d) [7, 8], SBA-15 (large-pore 2d hexagonal p6mm) [2, 9, 10, 11, 12], SBA-16 (large-pore 3d cubic Im3m) [13], KIT-5 (well-ordered cage-type mesoporous with cubic Fm3m) [2], and KIT-6 (large-pore cubic with interpenetrated cylindrical mesopores Ia3d) [2, 14, 15, 16, 17, 18] have been used as supports for transition metals. These supports with controlled morphology and accessible metallic center for the reactant molecules also offered the opportunity to immobilize the transition metal complexes and their heterogenization [19, 20]. These catalysts have attracted much interest due to the desirable characteristics of the silica supports such as narrow pore size, high surface area and large pore volume, tunable mesoporous channels with well-defined pore-size distribution, controllable wall composition, and modifiable surface properties. Pore diameter of mesoporous silicas (
In order to obtain active catalysts, different active redox metal sites have been introduced into specific locations (mesoporous channels and framework) of the OMSs supports by direct synthesis methods (framework substitution) or postsynthetic methods. In any case, Men+ can be simultaneously present in different coordination geometries and positions (surface, lattice) [16, 23]. In a direct-synthesis preparation, the condensations of silicon and metal species around the organic micelles occur simultaneously, and it is likely that some of the metal species are trapped in the silica walls during the formation of OMSs supports. This may influence the unit cell parameters, the wall thickness, and the long-range ordering of the material. By contrast, metal species introduced by a postsynthesis treatment (template ion exchange, impregnation, grafting, chemical vapor deposition methods) are mostly located at the surface of the mesopores and do not modify the internal composition of the silica walls, mainly when the samples are prepared in alcohol. The synthesis method offers the advantage that the dispersion and location of metal species are easily controlled. This may be a great advantage with respect to conventional synthesis methods to prepare materials with specific applications in catalysis. The activity of the obtained materials was demonstrated in various reactions, mostly oxidation reactions of the organic compounds in the liquid or gaseous phase. All the reported results show that the localization of the metal ion, morphology, particle and pore channel sizes and their interaction with the support, and other metal (bimetallic catalysts) influence the oxidation state of the catalytic sites, respectively their redox properties. Therefore, the redox properties of these materials are the result of the support and metal cation synergistic effect.
Catalytic properties of the incorporated transition metals in OMSs supports were mainly attributed to their location, chemical state, and environment. The location of metals in mesoporous silica network is the result of the synthesis method, the intrinsic properties of the incorporated metal, and silica support. The location, the loading, and the properties of the incorporated metals can also influence the support mesoporous structure. Thus, the variation of (100) peak in X-ray diffractograms obtained at a lower angle has been detected in many Me-MCM-41 patterns, indicating the effect of metal species on the ordered mesoporous structure of the support [3, 24, 25]. The higher concentrations of metal species (Me[(OH)n(H2O)m]) at the interface affect the electrostatic interaction between surfactant and silica precursor and the polymerization processes of the silica system in alkaline media (pH 11) during synthesis. In such conditions, the free energy of the mesostructure formation decreases and a mixture of nonstructurated oxides (SiO2–Co3O4) was exhibited [25]. Changing the Si-O-Si bond angle due to incorporation of metal in the mesoporous silica support increases the number of local defects within the mesoporous structure. IR spectroscopy is one of the first techniques that has been used for the characterization of materials with metal incorporated in mesoporous silica to obtain information on changes of the Si-OH from surface or regarding appearance of the new vibrations as (Si-O-Me). The change in the Si-O-Si bond angle due to incorporation of Me increases the number of local defects within the mesoporous structure. A much disputed is the change in intensity of the band at 960 cm−1 indicating the structural changes in the Si-OH surface due to the presence of metal oxide or to the evolution of new vibrations (Si-O-Me) that appear in the same region [26].
For TiMCM-41 synthesized by surfactant-assisted direct hydrothermal (DHT) method [7, 16, 21], the absence of the characteristic bands for crystalline TiO2 phase in the wide angle XRD patterns reveal that the metal ions were either atomically dispersed in MCM-41 framework or may exist in an amorphous dispersed form on the outside surface of mesoporous support. Diffuse reflectance UV–Vis spectra also revealed distorted tetrahedral environments for titanium inside the MCM-41 matrix or octahedrally coordinated titanium sites, due to the possible hydration effects. The presence of titanium in various mesoporous silica supports in +4 oxidation state was confirmed by ESR and XPS analysis [7, 9, 23]. XPS analysis was used as an additional tool of UV–VIS data since the dispersion of Ti species depends strongly on the synthesis method, properties of the support, and other metals [7, 9, 16]. XRD and spectroscopic results reveal that the titanium was dispersed as titanium ions on SBA-15 silica wall surfaces at low titanium loading, whereas a titanium dioxide anatase film was formed at high titanium loading [16, 23]. Thus, the nature of Ti species on TiMCM-41 s surface, prepared by three different methods, i.e., isomorphous substitution, wet impregnation, and mechanical mixing, was analyzed by means of Raman spectra [27]. The obtained results indicated the presence of anatase for TiO2MCM-41 sample obtained by mechanical mixing. In contrast, no Raman-active bands were observed on TiMCM-41, obtained by direct synthesis, indicating the absence of surface anatase phase. For TiMCM-41, obtained by impregnation method, no surface anatase phase was detected at a low Ti/Si ratio of 0.3, while surface anatase was detected at high Ti/Si ratios such as 0.6 and 0.9. UV–Vis spectra showed for TiMCM-41 a strong absorbance band at 210 nm and a shoulder band at 260 nm. The first was attributed to isolated Ti atoms in tetrahedral coordination, while the band at 260 nm was attributed to isolated Ti atoms in pentahedral or octahedral coordination. Therefore, it is believed most Ti atoms should substitute Si in the framework or surface in TiMCM-41 with the formation of Ti-O-Si-O-Ti bands. For TiO2MCM-41, adsorption bands at 220, 260, and 320 nm were clearly observed in the UV–vis spectra. The band at 320 nm was typical for bulk titania, indicating the existence of bulk TiO2. The band at 220 was attributed to isolated Ti atoms with distorted tetrahedral environment. These Ti species, dispersed on OMSs support, were supported along with other cations (Ce, V, Nb) or was used as support for another active metal as Ce, Pt, Fe [7, 9, 16, 21, 28]. The second or third [16] metal was evidenced by SEM backscattering and TEM microscopy, as extra framework nanoparticles (Figure 1). A good correlation between TEM results (Figure 1C) and H2 chemisorption on Pt nanoparticles’ diameter was observed.
SEM backscattering (A) of PtTi-SBA-15 (unpublished) and TEM images of TiKIT-6 (B) and PtTiKIT-6 (C) samples (with permission from Ref. [16]).
XPS spectroscopy sustained the interaction of the second metal with titanium [29] and together on the third metal [16]. The presence of Pt0 on surface and the effects of titanium loading and of cerium on its percent were explained by metal-support interaction considering TiKIT-6 and CeTiKIT-6 samples as supports for Pt (Figure 2). Due to Ti and Ce redox properties and strong interaction with noble metals, these metal oxides influence Pt/PtO molar ratio on the catalyst surface. The extended X-ray absorption fine structure measurements evidenced for Pt immobilized on SBA-15, in absence of Ti and Ce species, the presence of Pt–oxygen chemical bonds at the surface. The concentration of these Pt species increased for Pt-amino-SBA-15 sample [11].
XPS spectra Pt-modified KIT-6 mesoporous silica and Pt species atomic percent (with permission from Ref. [16]).
Another transition metal that is present as active component in OMSs supports was vanadium. V-MCM-41 has received many applications in oxidation reactions [3, 30, 31, 32]. For ordered mesoporous V-MCM-41 materials synthesized by DHT method [3, 25, 31], vanadium occurs mainly as isolated tetrahedrally coordinated V5+ species incorporated in the pore wall or anchored to the pore wall. UV–Vis spectra reveal that all the samples prepared with low V contents present well-dispersed V species in the silica network as V5+ species. At high surface vanadium coverage, the species are substantially polymerized. The oxidation of V4+ species in the precursor has also been observed. The change in the UV–Vis spectra after calcination was due to the modification in the oxidation state of vanadium (V5+) from the isolated tetrahedral coordination to its distorted octahedral coordination by coming into contact with the water molecules in the atmosphere [31]. Shylesh et al. [32] reported that UV–Vis spectra of VMCM-41-DHT materials showed vanadium incorporated into the framework positions for VMCM-41 samples, while the greater percentage of active species resides on the surface of VMCM-41, enhancing the formation of higher coordinated vanadium species after calcination. Treating MCM-41 with an aqueous or alcoholic solution of vanadyl acetylacetonate can lead either to a grafting of vanadium entities on the silica surface or to an ion exchange between surfactant molecules and vanadium cations in solution. The UV–Vis spectra of the samples prepared in water or alcohol with low V contents (V/Si < 0.1) showed essentially two absorption bands at 275 and 345 nm. The first was assigned to V5+ species inside the silica walls, whereas those corresponding to the band at 345 nm located on the surface of the mesopores. The presence of internal sites is due to the reorganization of the hexagonal tubular structure of MCM-41 upon hydrothermal treatment, during which vanadium species are allowed to penetrate the silica walls. Thus, vanadium species in the samples obtained by impregnation are dispersed on the wall surface while in the samples obtained by direct synthesis they were fixed in the mesoporous framework. Therefore, the surface vanadium species supported on silica are well known to possess an isolated and distorted VO4 structure with a single V〓O terminal bond and three V–O–Si bridging bonds anchored on silica support. The distorted V5+ species with the bridging V–O–Si could be found in different silica environments. The formation of vanadium oxide nanodomains has also been evidenced by ESR spectroscopy. A quantitative measurement of the ESR signal intensity shows that the corresponding V4+ species represent only 0.1% of the total V species. The majority of the species are V5+. The others species are VO2+ which are very well dispersed and isolated inside the pore channels of MCM materials. Raman and UV–Vis spectroscopic characterization of V-MCM-41 materials were used [33] to obtain more definitive information about the possible presence of XRD-amorphous crystalline V2O5 nanoparticles and surface VOx species for samples possessing with higher vanadium content (up to 5.3 wt. %V). DR UV–Vis spectra of Me-MCM-41 (Me = Ti, V, Cr) samples obtained by direct synthesis [34] sustained the framework incorporation of Ti4+ ions in the inorganic silica matrix with tetrahedral or octahedral coordination, vanadium (V5+) isolated species with tetrahedral environments and monochromates, with minor amounts of dichromates as well as polychromate species, respectively. The calcination treatments had changed all the Cr(III) ions to Cr(VI). A large part of these species resided on the surface of silica mesoporous support. These results indicated that the major species formed on the Cr-MCM-41 sample were monochromates, with minor amounts of dichromates as well as polychromate species. The ESR spectrum of as-synthesized chromium-containing mesoporous silica indicated, for a large part of chromium, the presence of trivalent chromium (Cr3+) in octahedral coordination.
The surface vanadium species supported on silica were well known to possess an isolated and distorted VO4 structure with a single V〓O terminal bond and three V–O–Si bridging bonds anchored on silica support. The distorted V5+ species with the bridging V–O–Si can be found in different silica environments. Similar studies on the surface Nb5+ species present in Nb–MCM-41 have revealed that the Nb atoms were predominately isolated NbO4 species under dehydration conditions, and surface polymeric niobium species and/or bulk Nb2O5 are formed at high niobium loading on silica [35]. Raman spectra of Ta-MCM-41 mesoporous materials indicated [6] that the incorporation of Ta atom into the MCM-41 structure forms a distorted and isolated [TaO4] surrounded by the [SiO4] tetrahedrons with the presence of Ta–O–Si bridging bonds, and three types of tantalum oxide species: an isolated TaO4 species in the MCM-41 framework, an isolated surface TaO4 species on the MCM-41 surface, and bulk Ta2O5, can be present individually or coexist on the Ta–MCM-41 catalysts, and it’s relative intensity was dependent on the Ta concentration.
The idea of the MCM-41 impregnation with vanadium and antimony sources was to locate Sb-V-Ox species on the high surface area of mesoporous materials with various compositions [36]. Vanadium species in the prepared samples have been estimated by UV–Vis and ESR spectroscopic study. All mesoporous matrices modified with vanadium and antimony gave rise to well-resolve signals in the hyperfine structure of ESR spectra characteristic for isolated VO2+ species. Such a structure was not registered in the case of V/SiO2 sample suggesting that mesoporous support was important for the isolation of oxovanadium species. Tetrahedrally coordinated vanadium (IV) species were deduced from UV–Vis spectra on all prepared samples. They were the only registered species on SbV/NbMCM-41 and SbV/AlMCM-41, whereas on SbV/MCM-41 and SbV/SiO2, octahedral ones were also present besides them. Octahedral coordination dominated in bulk SbVOx.
There are many studies on cobalt incorporation in mesoporous silica supports. The information about the nature, the co-ordination, and the location of the metal species for cobalt- and cobalt-vanadium-modified MCM-41 materials obtained by direct synthesis were obtained by TPR, DR-UV–Vis, and XPS analysis [25]. These methods indicated different localization of the cations in extra-framework positions or in the framework of MCM-41molecular sieves. DR-UV–Vis spectra from Figure 3A show two different types of V5+ species. The first one was assigned to isolated tetrahedrally coordinated V5+ species and the second originates from polymeric tetrahedral V5+ species grafted on the walls. According to these results, H2-TPR measurements (Figure 3B) suggested that the vanadium interaction with MCM-41 was predominant in VCoMCM-41 samples and pointed to the presence of monomeric or low oligomeric dispersed tetrahedral vanadium species obtained by direct synthesis and the formation of less reducible “polymeric-like” vanadium species by postsynthesis. In the low-loaded cobalt catalysts, Co2+ in tetrahedral position was observed. The increase in metal content led to the appearance of Co3+ in Oh symmetry. In both cases, the cobalt ions were placed outside of the silica framework. In the bimetallic samples, vanadium was incorporated inside the framework of the molecular sieves and on the channel walls. V5+ was in tetrahedral symmetry. In the bimetallic samples, cobalt was presented as Co2+ in Td symmetry. When Co and V were introduced together in the starting gel, a lower quantity of vanadium was incorporated into the mesoporous sieve. At a low vanadium concentration, the essential part of cobalt gives rise to the cobalt silicate phase. The latter was reduced at higher temperature. The rest of cobalt forms CoO particles interacting weakly with the siliceous framework reduced at lower temperature. The peak at 710 K for VCo3 sample was most likely the composite one from the reduction of both cobalt and vanadium species.
DR-UV–Vis spectra (A) TPR profiles (B) of VCo-MCM-41 and V-MCM-41 samples (with permission from Ref. [25]).
Impregnation of MCM-41 and SBA-15 materials using aqueous solutions of cobalt nitrate has a significantly different impact on their ordered mesoporous structures. Thus, aqueous impregnation of MCM-41 followed in the surface area and pore volume. By comparison, SBA-15 mesoporous structure remained almost intact after the introduction of significant amounts of cobalt (up to 20%). The different behavior of these two mesoporous silicas was principally attributed to the different pore wall thickness in MCM-41 and SBA-15. Cobalt oxide-modified SBA-15, KIT-5, and KIT-6 mesoporous silicas with different pore size/pore entrances have been synthesized by a conventional wet impregnation method using cobalt nitrate as the precursor. UV–Vis spectra indicated the formation of Co3O4 particles with different degree of dispersion, which are in different interaction with the support [2]. XPS spectra showed the variation of surface Co dispersion with aging temperature that facilitated cobalt species migration and agglomeration through the larger pores of the silica matrix. The effect of the pore size was less pronounced for the SBA-15 materials, where the straight cylindrical pores with 2-D arrangement probably leads to homogeneous distribution of the loaded cobalt oxide particles along the pore surface. For the 3D structures with interpenetrated cylindrical mesopores (KIT-6) or cage-like mesopores (KIT-5), the formation of homogeneously dispersed spinel Co3O4 species seems to be facilitated in mesoporous silicas with pores larger than 6 nm. TPR-DTG results evidenced the co-existence of three types of Co3O4 particles for all cobalt-modified SBA-15, KIT-5, and KIT-6 materials. The first type was easily reducible, relatively larger species, loosely interacting with the support; the second type represented hardly reducible and well-dispersed fraction in a moderate interaction with the silica, and the third type was very finely dispersed, strongly interacted with the support, which could not be reduced up to 873 K. The pronounced differences were observed for Co/KIT-6 materials [2]. Therefore, Co/KIT-6 samples presented a significant portion of crystalline species that weakly interacted with the support. For the KIT-6 aging to higher temperature, the presence of more inhomogeneously dispersed cobalt oxide particles, which were not completely reduced to metallic cobalt in the temperature interval 500–750 K, was revealed. For these materials, TPR-DTG analysis in correlation with the FTIR measurements supported formation of spinel-type Co3O4 species in the case of silicas with larger mesopores. The incorporation of Co and/or Fe in HSM and SBA-15 was evidenced by association of XRD, TEM, and TPR techniques [37]. Low-angle XRD patterns indicated that the mesoporous supports remained unchanged after metal impregnation and calcination steps; hence, still retained the ordered structure.
Iron is an interesting metal regarding its properties and applications in catalytic oxidation. In case of its immobilization in mesoporous silica by direct synthesis and hydrothermal treatment [4, 32, 38, 39], most of iron species exist in the tetrahedral coordination located in the support framework. The results obtained from DRUV–Vis, ESR, and XANES showed iron in a mixed environment, indicating that some iron was tetrahedrally coordinated, being sited in the framework, and some iron is present as an extra-framework atom, being octahedrally coordinated [38]. XANES results also suggested that copper was present in the Cu–Al-MCM-41 samples both in the framework and in the extra-framework sites as hydroxide and oxide, respectively. ESR spectra of hierarchical silica structures confirmed the presence of Fe3+ ions with tetrahedral coordination both in framework and extra-framework support. For samples with higher loading, the presence of interstitial oxide phase and iron oxide clusters was displayed [28]. Similar results were obtained for copper. XANES results also suggested that copper is present in the Cu–Al-MCM-41 samples both in framework and extra-framework sites as hydroxide and oxide, respectively. The presence of Al3+-sites on the surface of the support provides considerably better dispersion of copper [26, 40]. When comparing ZnAl-MCM-41 with FeAl-MCM-41 samples, the interaction between the metal and the framework atoms (Zn…Si) was different. 27Al-MASNMR results have indicated that zinc is not substituted for aluminum, which means taking the EXAFS results into account that zinc (II) substitutes for silicon (IV) in the framework. The presence of several, and probably different, silicon-sites in the mesoporous framework explains the higher disorder in Zn–Al-MCM-41 compared to Fe–Al-MCM-41, in which iron substitutes for aluminum.
The incorporation of Ce, another trivalent metal, within the MCM-41 was favored by the greater flexibility of the silica network. However, the size incompatibility between Ce3+ and Si4+ ions led to longer Si\\O\\bonds and caused the strain bond angle in the substituted silica network. Also, the incorporation of Ce induced a drastic reduction in mesopore ordination. These results were probably due to partial substitution of the structural Si4+ for the Ce3+ ion, resulting a substantial change in the textural properties of the hexagonal structure of MCM-41 [41]. The Si/Ce molar ratio is a key factor influencing the textural properties and structural regularity of CeMCM-41 mesoporous molecular sieves. As well, XRD, UV–Vis, and XPS spectra evidenced the presence of cerium species as tetra-coordinated Ce4+/Ce3+ and the formation of CeO2. This was in accord with results obtained on cerium incorporated in SBA-15 [42] and KIT-6 mesoporous silica [16]. The effect of pH on SBA-15 ordered hexagonal structure and incorporation of Ce species was evidenced [42]. For the samples synthesized at pH = 10.0, the position of Ce species was evidenced as deposits only on the surface of SBA-15.
High metal dispersion and incorporation of Ni in MCM-41 framework was evidenced for lower metal loading. Typical XRD diffractograms for ordered hexagonal mesoporous structure, obtained at small angle, evidenced decreasing of structural regularity with metal loading. Considering UV–Vis of NiO as reference, a distorted tetrahedral environment was observed for the most of Ni species in these MCM-41 materials. The effect of Zr4+ on Ni2+ local symmetry and the presence of distorted tetrahedral Ti species were evidenced by UV–Vis for Ni-ZrMCM-41 and Ni-TiMCM-41 bimetallic samples. Thus, a small shoulder at around 293 nm was assigned to penta- or octahedral coordinated Ti species, resulting from the interaction of Ti species with Ni species [27]. For Ni–MnMCM-41 sample, it was assumed that both tetrahedral and octahedral Mn3+ species co-exist. Mn3+ was evidenced both in tetrahedral and octahedral coordination. The results obtained for bimetal samples were compared with them with single metal. Such for Mn-MCM-41 sample, XRD patterns showed the absence of the diffraction peaks of the MnOx species suggesting that a strong interaction between MnOx and silica matrix exists because most of the Mn3+ or Mn2+ cations were either incorporated into the silica framework or highly dispersed on the silica walls. The TPR results on Mn-MCM-41 samples [43] indicated the coexistence of different manganese species. In the samples of different pore dimensions and manganese loadings prepared by impregnation, the nature of the species, identified as well dispersed, strongly interacting with silica surface was similar. In the case of samples prepared by the hydrothermal method, the effect of pore dimensions was more complex. Narrow pores of silica materials caused the formation of small species strongly interacting with silica surface or incorporated into the framework. An increase in Mn loading and pore diameter favored formation of larger particles weakly interacting with silica support. It was observed that the presence of small oxide species of the size partially controlled by pore dimension or preparation method, and simultaneously not strongly interacting with silica support.
The incorporation of larger species into the silica framework was hindered and the formation of extra-framework oxide species was favored. Regarding the incorporation of tungsten species into the MCM-41 framework, there is a critical value for the Si/W ratio of about 30. In the case of smaller Si/W ratio, the formation of extra-framework tungsten oxide species was observed [44]. Variation of the Si/W ratio and the synthesis method has led to various species of W immobilized on HMS silica [45]. Thus, through Raman spectroscopy, isolated [WO4]2− or low condensed oligomeric framework species were displayed. Tin is another metal with redox properties and large size which forms SnO2 clusters distributed on the external pore structure. SnO2 agglomerates were highlighted in the channels or on the external surface, which blocked the pores partially, thereby reducing the surface area. By adjusting the nH2O/nHCl molar ratio, Sn was incorporated into the lattice of SBA-15 at a low Sn concentration [46]. The Sn4+ ions exhibited both tetrahedral and octahedral coordination depending upon the location of these ions either on the walls of the silica or in the corona region of the structure, respectively. The existence of isolated oxide species that have degraded the ordered structure of the silica support and especially the formation of the oxide agglomerations in the pores or on the external surface has been highlighted for other metals with large diameter (Ru, La) but very active in oxidation reactions [24, 47, 48, 49, 50].
The imobilization of active metals in the specific locations of ordered mesoporous silicas by direct synthesis routes with the help of organic groups of surfactants brought a new aspect of creating metal-functionalized OMSs [51]. Although the strong interactions between active metals and support were obtained, the controllable morphology and structure of OMSs synthesized by these direct synthesis routes have not been well developed. The synergistic effect between loccation, its dispersion, and mesoporous ordered silica structure on the metal electronic properties and catalytic needed development of the advanced characterization techniques.
The introduction of the metal cations in the mesoporous silica generated both acid and redox centers depending on their charge and their chemical properties. In oxidation reactions, these properties determine both the activity and the selectivity of the catalyst. To introduce the redox active sites in the OMSs, various transition metals have been chosen. The supports like M41S, SBA-n, and KIT-n families modified by incorporation of one, two, or more transitional metals such as Ti, V, Cr, Fe, Co, Ni, Mn, Cu, La, Ru, Ni–Ru, Cr–Ni, V–Cu, and V–Co created materials with new redox and acidic properties. The introduction of active transition metal into the framework of molecular sieves creates isolated metal sites and these centers are believed for their exceptional catalytic activity. Their catalytic properties were influenced by localization and surroundings of the metal ions. The high dispersion of metals on a support with high surface area, large pore diameter, and uniform pore size distribution determined the formation of new active centers with redox properties different from those of the oxide in the agglomerated form. This explained the increased interest in them and their applications as catalysts.
Table 1 shows a wide range of metals incorporating in mesoporous silica supports with catalytic applications in liquid phase oxidation of organic compounds, with H2O2 or tert-butyl hydroperoxide, and gas phase with O2 from air. Various publications have shown the effects of metals and their associations with silica support and other metal on catalytic activity and selectivity. Thus, a high variety of transition-metals incorporated in mesoporous silica showed interesting catalytic properties in oxidation of organic compounds. Among them, vanadium and titanium were mostly used both single as well as associated with other metals. Vanadium-containing mesoporous materials are found to be active in liquid-phase oxidation reactions as oxidation of cyclohexane to cyclohexanone and cyclohexanol [31], oxidation of aromatic hydrocarbons and alcohols [3] using H2O2 as oxidant. V-MCM-41 catalysts exhibited low activity in the oxidation of alcohols but higher activity and selectivity in oxidation of cyclohexene and aromatic hydrocarbons. This suggested the association of vanadium with another metal more suitable for other oxidation reactions [21, 24]. V-TiMCM-41, V-CoMCM-41 catalysts were used in oxidation of aromatic hydrocarbons and alcohols [21, 24]. In these reactions, FeMCM-41, CoMCM-41, NiMCM-41 [3], NbMCM-41, Nb-TiMCM-41, Co-(Nb, La)MCM-41 [21, 24, 50] and Ru-(Cr, Ni, Cu)MCM-41, La-(Co, Mn)MCM-41 [49], and WHMS were also used as catalysts [45]. It was interesting to note that samples which are active in the oxidation of styrene to benzaldehyde would be less active in the oxidation of benzene to phenol and vice versa, suggesting that active centers for oxidation of styrene might probably be different to those for oxidation of benzene. Reaction data showed that the oxidation activity is higher when H2O2 is used as an oxidant, acetonitrile as solvent, and V-MCM-41 as catalyst. However, the selectivity toward the desired keto derivatives (ethyl benzene to acetophenone and diphenyl methane to benzophenone) follows the order, Ti-MCM-41 > V-MCM-41 > Cr-MCM-41 [34]. The vanadium content in catalysts was evidenced as a key factor in the oxidation of styrene and the conversion increases with metal loading. However, for hydroxylation of benzene, catalysts with an ordered mesostructure presented higher catalytic activity and the V content only serves as secondary role. It can be concluded that high vanadium content favored styrene conversion and higher ordering of mesostructure led to high hydroxylation of benzene. The higher activity of vanadium-incorporated MCM-41 compared to vanadium-grafted MCM-41 may be due to the presence of active isolated tetrahedral-coordinated vanadium ions in the framework positions. The lower activity was a result of the V–O–V bond formed for vanadium-grafted MCM-41. The difference in the selectivity of as-synthesized and calcined vanadium-grafted MCM-41 showed that apart from the active redox sites, the nature of hydrophilic–hydrophobic interactions still play an important role in selective oxidation reactions. Figure 4 depicts the variation in styrene conversion and reaction rate as a function of reaction time. The conversion and reaction rate in oxidation of styrene were modified by the introduction mode of the H2O2 into reaction medium in order to find the optimal reaction conditions.
Catalyst | Reaction |
---|---|
VMCM-41 | Oxidation of cyclohexane to cyclohexanone and cyclohexanol [31], oxidative dehydrogenation of propane (O2 and N2O as oxidant) [33] |
V-MoMCM-41, Cu-FeMCM-41 | Oxidation of o-xylene to phthalic anhydride in air [1], oxidation of adamantane with H2O2 [39] |
VMCM-41, FeMCM-41, CoMCM-41, NiMCM-41 | Oxidation of aromatic hydrocarbons and alcohols with H2O2 [3, 4, 30, 32] |
VMCM-41, NbMCM-41, V-TiMCM-41, Nb-TiMCM-41, Co-(V,Nb,La)MCM-41, V-CoMCM-41 | Oxidation of aromatic hydrocarbons and alcohols [21, 24] |
MeMCM-41 (Me = V, Fe, Co, Ni), WHMS | Oxidation of styrene and benzene with H2O2 [4, 45] |
Ru-(Cr, Ni, or Cu) MCM-41,La-(Co or Mn) MCM-41 | Oxidation of aromatic hydrocarbons with H2O2 [49, 50] |
MMCM-41 (M = Ti, V, Cr) | Oxidation of ethyl benzene and diphenyl methane with H2O2 [34] |
TaMCM-41 | Oxidative dehydrogenation of propane and oxidation of methanol [6] |
CeMCM-41, CeKIT-6 | Hydroxylation of 1-naphthol with H2O2 or tert-butyl hydroperoxide [41], oxidation of cyclohexene [52] |
M (M = Al, Zr, W, B, or P) MCM-41 | Oxidation of ethane [40] |
Pt-SBA-15, Pt-NH2SBA-15, Pt-CoSBA-15, Pt-Co-NH2SBA-15 | 3-Butene-1-ol, cis-2-butene-1, 4-diol, cyclohexene oxidation with H2O2 [11, 30] |
PtSBA-15/PtSiO2 | Oxidation of toluene with O2 [12] |
CoSBA-15, CoKIT-5, CoKIT-6 | Ethyl acetate total oxidation [2] |
LaKIT-6, La-BKIT-6 | Oxidation of styrene with H2O2 [15] |
TiKIT-6, Pt-TiKIT-6, Ce-TiKIT-6, Pt-Ce-TiKIT-6, | Methane in air [16] |
M/KIT-6 (M = Mn, Cu, Fe, Cr, Sn) | Catalytic combustion of chlorobenzene [18] |
CuImph (Imph = bis(4-imidazolyl methyl)benzylamine) on MSNs | Oxidation of toluene in air [19] |
([Cu(acac)(phen) (H2O)](ClO4), [Cu(acac)(Me2bipy)](ClO4)) on HSM or NH2HMS | Oxidation of aromatic compounds (anisole, phenol) with H2O2 [20] |
Catalytic applications of modified mesoporous silica ordered networks with transitional metal.
Variation of the styrene conversion (A) and of the reaction rate (B) as a function of reaction time (h). VTi-1 and NbTi-1: the defined amount of H2O2 was introduced at the beginning of the reaction and VTi-2 and NbTi-2: the defined amount of H2O2 was divided in different portions and introduced step by step ((×) VTi-1, (●) VTi-2, (■) NbTi-1, (▲) NbTi-2) (with permission from Ref. [21]).
The higher conversion and reaction rates were obtained in the first period of reaction when H2O2 was introduced after adsorption the step (samples VTi-2 and NbTi-2). These results sustained the effect of the adsorption step on oxidation reaction confirmed, over time, by photocatalytic reactions, other oxidation processes in which both Ti and V catalysts or other transition metals with redox properties were used immobilized on mesoporous silica [7, 8, 9, 10, 28]. An induction period was needed (VTi-1, NbTi-1catalyst) in oxidation of aromatic hydrocarbons when the oxidant, reagent, catalyst, and solvent were mixed together at the beginning of the reaction and effect of the second metal was significant. The effect of the second metal on the properties of catalysts was evidenced for others materials. Firstly, the introduction of V and La reduced sharply the conversion of styrene.
The increasing of Co/V molar ratio (decreasing V amount) led to increasing of the catalytic activity. The highest conversion was obtained for Co/V molar ratio of 3.0. On the other hand, the effect of the second metal on the reactivity was different for styrene and benzene. The introduction of V or Nb into CoMCM-41 molecular sieves decreased or increased the conversion of styrene. The effect of La on CoMCM-41 was quite surprising since the activity for oxidation of styrene and benzene decreased, implying the inhibitory effect of La for oxidation of aromatics [21, 24, 50]. The main products of reaction were benzaldehyde, for oxidation of styrene, and phenol for oxidation of benzene. After the first utilization, the separated and dried catalysts were reused. The catalytic activity of these reused catalysts increased in the second cycle of reaction and decreased after the third. It was observed that the selectivity decreased gradually with reaction time and in the second cycle of reaction. Some products of polycondensation resulted by polymerization of formaldehyde and/or benzaldehyde were detected after long time of reaction, a possible reason for the decrease in selectivity with reaction time. The characterization of the used and reused materials confirmed, in many cases, the good stability of bimetallic molecular sieves. TEM images of the used catalysts showed a well-ordered structure with a hexagonal arrangement of channels, indicating no effect of the reaction on the support structure after first and second reaction [24]. The IR study of adsorbed phase in discharged catalysts after first cycle reaction of styrene and benzene upon desorption at a series of temperatures (293, 373, 623, and 723 K) was made [50]. The presence of different aromatic species such as benzaldehyde and styrene glycol was observed. These species adsorbed very strongly in the catalyst since a complete desorption of these species can be made only after desorption at 723 K. The strong adsorption of these species was confirmed by thermal analysis.
The association of trivalent cations Ru or La with other transition metals modifies the activity and selectivity of the monometallic molecular sieves [24, 49]. The results evidenced that RuMCM-41 and LaMCM-41 presented good conversion in oxidation of styrene but very low activity for benzene hydroxylation. While all the bimetallic catalysts, except Ru-NiMCM-41 and La-CoMCM-41, have a higher activity and efficiency of the H2O2 (H2O2 quantity used for oxidation/H2O2 quantity transformed) in the benzene hydroxylation. In the styrene oxidation, only RuCr-MCM-41 gives a good conversion. Generally, excepting LaCo-MCM-41, the conversion of catalysts in oxidation of benzene was higher than that in oxidation of styrene. This behavior was specific for these bimetallic samples since all the monometallic modified MCM-41 catalysts by incorporation of the same transition metals have a higher activity in oxidation of styrene and lower conversion in hydroxylation of benzene. LaCo-MCM-41 catalyst has a very low activity and RuCr-MCM-41 catalyst has a high activity both in the styrene and benzene oxidation. Under all investigated experimental conditions for oxidation of styrene, the main reaction products detected by GC analysis were epoxy ethyl benzene (styrene oxide), phenyl ethanediol (styrene glycol), and benzaldehyde.
More transition ions were incorporated into MCM-41, HMS, SBA-15 materials, and tested in liquid-phase oxidation reactions (Table 1). The best activity in oxidation of benzene was obtained for Ti-MCM-41 and in oxidation of styrene for Cr-MCM-41 and CrNi-MCM-41 samples. Their activity decreases with increasing of number of 3d electrons of the metal ions. After immobilization and interaction with support and another metal, it was highlighted that increasing of their oxidation state leads to the growth of their activity. Ti, V, Cr, and Mn ions were more active. After immobilization and interaction with support and another metal, an increase of their oxidation state has been observed. The obtained redox molecular sieves by direct synthesis incorporation of tungsten into hexagonal mesoporous silica (HMS) were tested in oxidation of styrene with hydrogen peroxide. The influence of the synthesis parameters, such as, chemical composition of the gels, surfactant, precursors, and time of the hydrothermal treatment, on the structure, morphology, nature of metal species, and catalytic properties has been examined. The best results were obtained for the catalysts synthesized by oxo-polyoxo mode: ([WO(O2)2(H2O)2]/H2O/H3O+/surfactant/Si(OR)4 in which surfactant was cetylpyridine chloride [45]. The catalysts with more ordered structure, higher surface area, and [WO4]2− species strongly bounded or high dispersed on silica have a higher activity. Conversion and selectivity to benzaldehyde decreased with Si/W molar ratio (Figure 5). The higest conversion of styrene to benzaldehyde has been evidenced the sample with the highest amount of isolated W sites and higher surface area.
Variation of conversion and selectivity with Si/W molar ratio (with permission from Ref. [45]).
In addition to applications in the oxidation reactions [11, 24, 31], cobalt-based catalysts were considered as a suitable alternative to the high cost of catalysts based on noble metals. The catalysts obtained by immobilization of Co on mesoporous silica were used for degradation of toxic compounds from the exhausted automobile and industrial emissions via combustion. Cobalt-modified SBA-15, KIT-5, and KIT-6 mesoporous silicas with different pore sizes/pore entrances have been synthesized and their catalytic activities in total oxidation of ethyl acetate were evaluated [2]. The optimal size of Co3O4 particles and their homogeneous distribution in the porous support are of primary importance for the catalytic activity. Silicas with larger pores facilitate the mass transfer during the sample preparation procedure and lead to more homogeneous distribution of cobalt oxide particles inside the pore system. The Co3O4 particle growth is facilitated by 3D structures with interpenetrating-(KIT-6) or cage-like (KIT-5) pores and much less promoted by 2D arranged straight pores of SBA-15 support.
Furthermore, more open pore structures could facilitate the catalytic process due to enhanced mass transfer, as the catalytic activity of CoKIT-6 materials is generally lower than that of CoSBA-15 and CoKIT-5.
Cerium-containing materials have been used as catalysts for selective oxidation of organic compounds. The liquid-phase oxidation of cyclohexane was carried out over Ce-KIT-6 with various Si/Ce molecular sieves at temperature between 70 and 90°C. Although the catalyst contains both Ce3+ and Ce4+, as evidenced from DR-UV analysis, the main active sites that activate H2O2 were considered to be Ce4+ [52]. Ce4+ was suggested to activate hydrogen peroxide by coordination to oxidize cyclohexane, and cyclohexanol was found to be the major product (74%). Cyclohexyl acetate, as the major secondary product, was considered result of Bronsted acidity generated by Ce3+. Cerium and titanium oxides, immobilized on KIT-6 silica, were used as supports for Pt. The supported Pt nanoparticles on mesoporous silica possess the ability to strongly dissociate toluene to benzene and hydrocarbon fragments (CHx) [12]. The metal–support interaction was evaluated in terms of TiO2 loading and ceria effect on titanium oxide under condition of their dispersion on silica [16]. The highest activity was obtained in oxidation of CH4 for all the catalyst samples containing Pt respectively for PT5K6 sample with lower dispersion of Pt and higher diameter of particles (Figure 6). Two opposite effects determined in this case a very small decrease in the CH4 conversion degree compared to PT5K6 catalyst. The higher concentration of Pt2+on the catalyst surface (CPT4K6 samples) and absence of noble metal (CT5K6, T5K6 samples) decreased the catalytic activity and a slow growth of conversion with temperature was observed. A good correlation was obtained between catalytic activity and reducibility of the catalysts for samples with Ti and Ce. The higher conversion degree of CH4 was obtained for PtTi-KIT-6 samples between 250 and 400°C. KIT-6 was an interesting support for other transition metals such as Mn, Cu, Fe, Cr, Sn, Ln, and the obtained catalysts were used in oxidative degradation of various organic pollutants from air (toluene, chlorobenzene) [18].
CH4 conversion and its variation with temperature for catalysts with different composition: Supported on T5K6 (a), CPTnK6 (B), and TnK6 (C) (with permission from Ref. [16]).
It can be considered that the interest on catalytic performances obtained by incorporating titanium into the zeolite (TS-1 catalyst) and the advantages of silica mesoporous molecular sieves have generated research on the metal-functionalized ordered mesoporous silicas. These catalysts have attracted much attention in the past few years for selective oxidation and oxidative photodegradation of large organic molecules. This explains the particular interest in the synthesis and use of catalysts based on titanium immobilized on various mesoporous silica substrates. The active center of TS-1 catalyst in the selective oxidation reactions was considered the titanium-isolated sites. A systematic study on the function of the different titanium species in TS-1 [53] concluded that the framework Ti species in TS-1 were the active sites for propylene epoxidation, while the nonframework Ti species were responsible for the further conversion of propylene oxide to propylene glycol and methoxy-2-propanol. Similar to TS-1, the remarkably catalytic properties of the metal-functionalized ordered mesoporous silicas result from the isolation of the metal centers in the framework. The high surface area of mesoporous sieves and the presence of ordered arrays of mesopores provide new opportunities for transition metal incorporation. It is possible to obtain metal heteroatoms as framework tetrahedral T atoms, bound in defect sites of the framework, anchored to the surface, extra-framework counter ions or extra-framework oxides.
The isolated heterogeneous single-site catalysts have been attracted great interest in diverse catalytic reactions because of their uniform and distinct geometric and electronic structure [54]. They have great potential to integrate the distinct advantages of homogeneous and heterogeneous catalysts into a single counterpart. The imobilization of active metals in the specific locations of ordered mesoporous silicas by direct synthesis routes with the help of organic groups of surfactants opened a new path in creating of metal-functionalized OMSs. Although the strong interactions between active metals and support were obtained, the controllable morphology and structure of OMSs synthesized by these direct synthesis routes have not been well developed. It is still difficult to understand the relationship between the structural morphology of OMSs and the electronic structure of active species, thus the development of advanced characterization techniques was necesary. Similar to other supported single sites [54], it can be considered that their catalytic activity is due to the following which allows: the ordered arrangement of silica support mesoporous structure; the high dispersion of metal sites, located in framework and extra-framework support, or covalently bounded on the pore surface functionalized with different ligands; the better contact of single site with the reactants thus generating catalysts with high activity and excellent selectivity; the ordered porosity can provide a special environment for the substrate interaction with the catalytic active sites which can further enhance the activity and selectivity.
Considering Ti (IV) species to be the active sites in TiMCM-41 catalyst in the oxidation of cyclohexene to cyclohexene epoxide, a mechanism in three steps was proposed for surface reaction [55]. The main reaction step involved the reaction of Ti single sites with H2O2 to form titanium hydroperoxide, which further reacts with cyclohexene molecules to form cyclohexene epoxide in a concerted manner. It was evidenced that titanium-isolated species from silica framework or surface were the active species. According to titanium location, two Ti-peroxide (η1 and η2) intermediates wherein Ti binds 1, respectively both oxygen atoms of the peroxide (Figure 7), have been identified.
The oxidation mechanism proposed for possible isolated Ti4+ active sites immobilized on mesoporous silica.
The stability and reactivity of Ti-peroxide intermediates was affected by solvent coordination. The best results were obtained in acetonitrile. The extra-framework Ti species were amorphous or crystalline TiO2. These species had a negative effect on the yield of propane oxide. The amorphous Ti species were more acidic and mainly responsible for the further conversion of propane oxide.
The mechanisms proposed for oxidation of organic compounds with H2O2 on vanadium-modified mesoporous silica supports proposed also the formation of V-peroxide intermediates (η1 and η2). These results were also generalized for the activity of other immobilized metals. Thus, the specific catalytic activity (per one Cu2+-active site accessible to the reactants) depended strongly on the structure of the localized site [40]. Isolated Cu2+-sites grafted to Al-MCM-41 showed relatively high activity for the sample calcined at lower temperature. Thermal treatment at higher temperatures caused a sharp loss of specific Cu2+ catalytic activity of CuAl-MCM-41 as result of copper oxide agglomeration.
If the organic substrate was introduced in the first step of the reaction, it was adsorbed on the surface and reacted more rapidly with the Me-peroxide intermediate formed by the addition of H2O2 in the second step [21]. The presence of the organic compound on the surface prevented H2O2 decomposition more in the presence of extra-framework oxides that favor its decomposition. Depending of their loading and dispersion, the activity of these metal extra-framework species was similar with that of clusters or bulk oxides. For Ru- and La-incorporated MCM-41 molecular silica, the ratio of metal and oxygen ion radii (RM(n+)/RO(2−) > 0.5), an important structural parameter, showed a little possibility to be incorporated metals in the silica framework [49]. For many of these catalysts, the efficiency of the H2O2 (H2O2 quantity used for oxidation/H2O2 quantity transformed) was very low. These catalysts had a good conversion in oxidation of styrene but very low for benzene hydroxylation. Their association with metals with smaller diameter as Co or Mn led to a higher activity and efficiency of the H2O2.
The hydrophobicity of metal-functionalized ordered mesoporous silicas was improved, in many cases, by functionalization of surface with organic groups in direct (co-condensation) or postsynthetic silylation treatment. The coexistence of metal cations with other organic or inorganic species favored the distribution on the pore surface of catalytic active species, thus favoring the access of the reactants during the chemical reaction. However, in this case, the metal species formed oxide agglomerations, which led to the decreasing of the conversion. Thus, for LaKIT-6 catalyst, the coexistence of B hindered the dispersion or incorporation of La into the framework [15]. La3+ species in the framework of LaKIT-6 were favorable to improve the catalytic performance of LaKIT-6 for the oxidation of styrene. When H2O2 was used as an oxidant, LaBKIT-6 showed much lower styrene conversion than LaKIT-6 catalyst with similar product selectivity, which was ascribed to the formation on surface of La2O3 nanocrystals.
The controllable dispersion of metal single sites and a higher hydrophobicity were obtained by immobilization of the metal complexes on mesoporous silica supports. A number of mono- and binuclear metal complexes have been investigated as biomimetic catalysts for organic compound oxidation. New biomimetic catalysts were obtained by immobilization of [Cu(AcAc)(Phen)(H2O)]ClO4, [Cu(AcAc)(Me2bipy)] ClO4 complexes on HMS or NH2-functionalized mesoporous silica [20]. The copper-substituted mesoporous silica was a good catalyst for oxidation of aromatic compounds or a very good support for biomimetic oxidation catalyst due to the possible interactions between the metallic ions of the biomimetic complex and the stabilized cuprous species in the silica framework. The immobilization on different mesoporous silica supports of (Schiff base) copper(II) or Mn(II) complexes was synthesized and applied in oxidation of alcohols in acetonitrile and H2O2. Comparing the effect of silica supports on catalytic activity, the higher performances of the metal complexes supported on MCM-41 were evidenced. A higher catalytic activity was obtained for Mn complexes, especially in oxidation of cyclohexene in conditions of significant lower M/L ratio and metal content. The most probable mechanism for attachment of the aminopropyl groups to the surface of mesoporous silicas was proposed through siloxane linkages (Si–O–Si) between the silicon of the aminopropylsilane group and the surface silicon atoms. It has been assumed the possibility that each aminopropylsilane silicon attaches to the internal surface of the silica via one, as well two and/or three siloxane linkages. Figure 8 proposes the metal complex structure immobilized on SBA-15 mesoporous support.
Copper single site supported by complexation with Schiff-base ligands obtained from 2-hydroxyacetopheneone covalently attached to amino-functionalized SBA-15 (unpublished).
The stability and heterogeneous nature of these catalysts in the oxidation of different substrates have been investigated. The oxidation in the liquid phase with organic hydroperoxides or even H2O2 was often the subject of leaching phenomena and the question about the true nature of the catalytic reaction (homogeneous or heterogeneous) was a serious one.
The photocatalytic activity of supported transition metals (Ti, V, Ce, Fe, Cu, Au, Ag, Ta, Nb) on mesoporous silica supports in mono- or bimetallic photocatalysts was evaluated in degradation of various organic pollutants from water or air. Among the various mesoporous silica materials, MCM-48 and KIT-6 were several advantages in photocatalysis due to their cubic arrangement of the three-dimensional mesopores and their considerable amount of surface silanol groups. The photocatalytic properties of CeTi-MCM-48 photocatalysts were evaluated by phenol photodegradation under UV irradiation at two different wavelengths and the photocatalytic activity was correlated with the active metallic distribution, speciation, and their immobilization method [7, 8]. The incorporation of Ce in the Ti-MCM-48 framework allowed activation by UV light, generating more electrons and holes in the photocatalytic redox reactions and a better photoactivity for phenol. The presence of redox couple Ce4+/Ce3+ exchaged the surface oxygen vacancies that could be transformed in oxygen radicals with formation of superoxide radicals. Many other photocatalysts were obtained by immobilization of the photoactive transition metals on various mesoporous silica supports, and their activity was evaluated in oxidation of organic polutants from water as dyes, functionalized 8-hydroxyquinolinate, antibiotics [9, 10, 28, 56]. In all of these, the active photoactive metallic sites were Ti4+, Co2+, Fe3+, Al3+, Zn2+, Eu3+, Tb3+, Er3+, Nd3+ with various dispersion on MCM-41, SBA-15 or hierarchical silica supports.
In case of the catalysts used in gas-phase reactions, dispersion, specific surface area, and reducibility was observed as the main factors influencing the catalytic activity. In combustion of chlorobenzene (CB), catalytic activity of metal-modified KIT-6 (M = Mn, Cu, Fe, Cr, Sn) mesoporous catalysts was basically in line with their redox properties, with the exception of CrKIT-6, which differs from this trend due to the aggregation of Cr species in the pore structure and surface of the KIT-6 [18]. The proposed mechanism was follows: CB was adsorbed to the catalyst by physical adsorption, then the adsorbed CB molecule dissociated on the active sites (mainly metal oxides) via nucleophilic attacks on C–Cl bond. The results obtained showed decreasing catalytic activity in the following order: MnKIT-6 > CrKIT-6 > CuKIT-6 > FeKIT-6 > Sn/KIT-6, in correlation with redox properties of the catalysts. Therefore, the best catalytic combustion activity of MnKIT-6 catalysts benefited from their large specific surface area, good Mn dispersion, better reducibility, and large amount of chemisorbed oxygen species. However, as the content of Mn species increasing, due to increased agglomeration of Mn, more MnOx clusters gave rise to pore plugging and cover adsorption sites, leading to weaker catalytic activity. Mesoporous silica KIT-6 with 3D interconnected mesopores offered a confined environment, a better dispersion of the oxide phase, and a faster diffusion of the reactants and products. The Mo/KIT-6, Fe/KIT-6, and Mo–Fe/KIT-6 were tested as potential alternatives to noble metals in the conversion of MCP [57]. Only the isolated tetrahedral Fe ions and highly dispersed small FeOx nanoclusters were responsible for the endocyclic C-C bond rupture at substituted C-C. The synergy between Fe and Mo was not observed at low temperatures and for total conversion but only for ring-opening reactions at higher temperature.
In particular, the good performance of the Ti(IV)-doped SBA-15-supported catalysts in CH4 oxidation was due to the combination of Ti(IV) structurally incorporated into the silica lattice and present as surface-dispersed TiO2 particles. The negative effect of the Ti(IV) over the HMS-supported catalysts was related to the high acidity induced by the more homogeneous incorporation of Ti(IV) into the silica structure. The loss of catalyst activity during CH4 oxidation reaction was not necessarily related to the sintering of the active sites but rather to the variation of the oxygen storage capacity and oxygen mobility strictly related to the support properties. For PdTi-SBA-15 catalysts, combustion of methane occurs through a redox or Mars van Krevelen mechanism was accepted [58]. Accordingly, during this reaction, PdO was locally reduced to Pd by methane, producing H2O and CO2, and then Pd was reoxidized by oxygen. Although the active species was considered PdO, it was widely recognized that metallic Pd plays the important role in decomposing and activating the methane molecule. Similar conclusions were obtained also for PtTiKIT-6 catalysts [16]. Therefore, the efficiency of all process was related to the redox properties of the catalyst and to the oxygen mobility. The maintenance of the activity, in spite of the significant palladium oxide sintering, was attributed to the good interaction between surface titania and silica. Such interaction favored the formation of Ti–O–Si linkages with an increase of the oxidizing potential of the Ti(IV) cations. In the presence of a support metal interaction, the increase of the PdO particle size [58], respectively PtO [16] after the long reaction is not detrimental. Another effect of metal–support interaction was observed for Ce-TiKIT-6 samples. In this case, two factors were influencing the Pt activity: dispersion and size of the platinum species and their concentration on the surface. Two opposite effects determined a decreasing in the CH4 conversion. The higher concentration of Pt2+ and lower Pt0 concentration on the catalyst surface. A good correlation was obtained between catalytic activity, and reducibility of the catalysts was also obtained for samples with Ti and Ce supported on KIT-6.
It can be concluded that OMSs containing transition metals showed extremely promising redox properties in the oxidation of organic compounds with larger molecules thanks to the mesoporous support. The applications and performances of the redox couple diversity resulted by association of transition metals on an increasing diversity of mesoporous or hierarchical ordered structures based on silica offer new research directions in this field. However, the decoration of multiple metal active species and their synergistic interactions on the surface of mesoporous silica remain to be explored in the future.
Gas sensing for pollutant monitoring and leaky molecules detection is important when the environmental issues on breath health are revealed. Various gas sensors based on different principles are presented, such as the gas chromatography-mass spectroscopy [1, 2, 3], electrochemical [4], and optical sensors. For the electrochemical sensors [5, 6], the high sensitivity is requested from the high operation temperature, which is risky for explosive gas detection due to the high-power consumption at electrodes. The optical sensing scheme can solve the unsafe problem because of the room-temperature operation without electric contact [7, 8, 9]. THz radiation, which lies between the infrared ray (IR) and microwave regions, can strongly perturb polar gas molecules with the energy level transitions of rotation or vibration. The absorption strength of gas molecules in the THz frequency range is typically on the same order of magnitude as the IR region, and is ∼103–106 stronger than that in the microwave region [10]. The low photon energy of the THz wave is relatively safer than that of IR wave and has the stronger interaction response than that in the microwave region [11, 12, 13].
\nTHz gas sensing methods have been demonstrated based on two main styles. The first style is normally illuminating THz radiation directly on the gaseous analytes and acquiring their spectral response for the sensing purposes. For example, the strong absorption lines at specific frequencies (i.e., fingerprint spectra) or the pulse power decay within one certain THz spectral width have been applied to analyze the gaseous analytes [14]. Such the sensing performance are presented from the photo-mixing [15], heterodyne detection [3, 16], and chirped-pulse THz spectroscopy [12]. Such the spectroscopic systems successfully analyze the gas mixtures of more than 30 chemicals [3] and distinguish gases that possess similar compositions. This recognition scheme provides high selectivity based on the rotational/vibrational transition of gas molecules. Nevertheless, the THz spectroscopic system should be equipped with a long gas cell [3, 12, 14], a cryo/sorbent pre-concentration system, and a heating apparatus [3, 16] to improve the sensing limit from the ppm concentration to the ppb level. The overall configuration is complicated, bulky, expensive, and consumes high power. Although the quantum cascade laser is presented as a compact THz laser source for gas sensing applications to simplify THz wave generation [17]. However, the THz laser source should be operated in the low-temperature condition and is limited for practical applications.
\nThe other method of THz gas sensing is to use the THz resonance field in the photonic or periodic structures [18, 19]. For example, one- and two-dimensional photonic structures respectively based on the silicon slabs [18] and pillar arrays [19] have been validated for the non-specific gas sensing in the THz frequency range. The proposed photonic structures have high-quality factors on THz field resonance and are sensitive to slight changes of refractive index. The demonstrated detection limit for hydrogen gas is about 6% concentration change [18]. The approved minimum detectable amount of oxygen or argon is ∼1 μmol [19]. Although the resonator-type THz gas sensor is relatively compact, portable, and consumes low power, its short interaction length inside the chip essentially leads to the limited sensitivity and poor selectivity.
\nIn this chapter, THz gas sensors with sufficiently long interaction lengths are presented based on THz wave propagation along the dielectric layers. The layered media specifically perform the enhancement of THz resonance and absorption in the dielectric waveguides and porous structures, respectively. The interaction between THz wave and gas analytes thus becomes efficient. The presented waveguide sensor contributes the detection of THz refractive index variation based on the resonator principle of Fabry-Pérot (FP) etalon, and the porous structures detect vapor molecules based on the response of molecular dipole moment for THz wave absorption.
\nOne dielectric layer bent to be a cylindrical structure can be used as one hollow core waveguide to guide THz waves. Those dielectric pipes or tubing used for THz pipe waveguides are available from the hydroelectric materials and demonstrated in the literatures [20]. In the study, a 30 cm-long glass pipe is used to load 0.05 cm3 liquid analytes to evaporate and fill the pipe core. When THz waves input and pass through the glass-pipe core, the vapor molecules interact with the THz wave for the sensing purpose.
\nThe cylindrical layer acts as a FP etalon, and THz waves satisfying the FP resonance condition enables field resonance inside the cylindrical layer, which becomes leaky to form multiple transmission dips. Based on the FP criteria, the resonant wavelength of THz waves in the cylindrical layer is defined as, \n
Transmission spectra of a glass-pipe waveguide: (inset) the cross section of a glass-pipe-waveguide sensor (reprinted from Opt. Express 20, 5858-5866 (2012). © 2012 OSA.
THz resonant fields inside the cylindrical layer must be sufficiently evanescent toward the air core to make the pipe-waveguide resonator sensitive to the vapor molecules. Interaction between the THz resonant field and the vapor molecules is therefore enhanced, consequently resulting in the sensitive detection. Based on the investigation of THz dielectric pipe waveguides, the high order resonance waves have strong evanescent field inside the pipe core [22]. That is, the resonance dips at the high frequency range have stronger core field than those at the low frequency range. The resonant dip at 0.452 THz as shown in \nFigure 1\n is thus suitable to probe vapors within the glass pipe core because of the powerful resonant field to interact vapor molecules.
\nTo approve the sensing principle of a pipe waveguide sensor, the vapor molecules of the water, hydrochloric acid (HCl), acetone and ammonia liquids are used as the standard analytes. The sensing results show that the spectral dip of 0.452 THz obviously shifts toward the high frequency range when various vapors fill in the pipe core (\nFigure 2a\n). The spectral dip position is shifted to 0.461, 0.465 and 0.477 THz, respectively, for the HCl, acetone and ammonia vapors. Based on the measured spectral dips and FDTD calculation method (\nFigure 2b\n), the related effective refractive indices of the pipe core (neff.core\n) are obtained as shown in \nFigure 2c\n. The core indices of the glass pipe are 1.016, 1.035 and 1.102 for the vapors of HCl, acetone and ammonia, respectively (\nFigure 2c\n).
\nThe spectral dip shift of (a) sensing results and (b) FDTD calculation for different vapors. (c) Relation between neff.cor\n and ν for different vapors. (d) Relation between spectral dip shift and the estimated vapor densities (reprinted from Opt. Express 20, 5858-5866 (2012). © 2012 OSA).
The dip-frequency-shift only occurs at the resonance dip of 0.452 THz and the other resonance dips in the low-frequency range do not exhibit any spectral shift for sensing the vapors, which is different from the calculated results in \nFigure 2b\n. The zero spectral-shift for the low-frequency-dips comes from the low strength of leaky resonance field at the pipe core. It is too weak to sense the presence of vapor molecules. This performance implies that the resonant fields for detecting vapors in the pipe core require high transmission power to identify slight core-index variation for the low densities of vapors. This low sensitivity phenomenon without any significant spectral shift is straightforward correlated to all the anti-resonant fields, i.e., the spectral peaks at the frequency lower than 0.52 THz.
\n\n\nFigure 2c\n shows the relation between the spectral dip frequencies and the neff.core\n values for different vapors inside the pipe core. The dip frequencies increase with the neff.core\n values. The increment effect of neff.core\n arises from the various vapor pressures of the volatile liquids, which are generated from the 0.05 cm3 liquid volume. High vapor pressure represents large amounts of vapor molecules inside the pipe core. Therefore, the vapor pressures of volatile liquids are approximately proportional to the quantities of vapor molecules in the sealed pipe core. The high density of the vapor molecules in the pipe core results in the large neff.core\n, and the resonant dip of 0.452 THz has an apparent blueshift.
\nThe neff.cor\n value versus the resonance frequency can be fit by curve of neff.cor = 24 − 103ν + 114.9ν2\n (\nFigure 2c\n). The slight index variation could thus be estimated via this fitting cure to identify the molecules in the pipe core. However, the water analyte in the pipe core cannot contribute any spectral shift because the density of water vapor is quite low. The sensing result is reasonable because the vapor pressures of water, HCl, acetone and ammonia at normal atmosphere and room temperature are, respectively, around 17 [23], 38 [24], 202 [24] and 308 [23] mm-Hg.
\nFor qualitative analysis, the vapor molecules discussed in this work are assumed as the ideal gases and their densities in the enclosed pipe would be calculated based on the ideal gas law. \nFigure 2d\n illustrates the relation of molecular density (ρ) and spectral dip shift (Δν), which can be fit as Δν = 26.3 − e−ρ/2.7\n. \nFigure 2d\n illustrates that the blueshift relative to 0.452 THz frequency gradually saturates when the vapor density is over 10 nano-mole/mm3. The saturation response eventually reduces the sensitivity of the pipe sensor. Based on the spectral resolution of 4 GHz and the fitting curve in \nFigure 2d\n, the minimum detectable molecular quantity would be around 7.8 μmole, corresponding to a molecular density of 1.6 nano-mole/mm3.
\nBesides of the single cylindrical layer, multiple layers of MPS can be used as a THz gas sensor. The sensing mechanism of THz wave is monitored from THz absorption of gas molecules, different from the refractive-index detection of the pipe-waveguide resonator. A MPS gas sensor is integrated from multiple layers of polyethylene terephthalate (PET) mesh (SEFAR PET1000, SEFAR AG, Switzerland). To collect and seal the gaseous analytes, MPS is assembled with one microfluidic chamber, which is made of Teflon material. As shown in \nFigure 3a\n and \nb\n, a PET mesh is flexible and consists of periodical square holes. PET mesh layers are stacked and fixed by a rectangular acrylic holder to form a MPS structure. There are large numbers of micropores (i.e., square air holes) in one MPS and the micropores are random inside the composite.
\n(a) Macroscopic and (b) microscopic photographs of a PET mesh. (c) The scheme of a MPS volatile gas sensor. (d) Uniform and periodic configurations of the MPSs in the x-z plane (reprinted from Opt. Express 25, 5651-5661 (2017). © 2017 OSA).
\n\nFigure 3c\n shows the schematic diagram of the THz gas sensor based on a MPS device and a microfluidic chamber. The MPS sensor is compact and low loss for THz waves, different from the bulky gas chambers. A flexible plastic tubing to the fluidic channel is externally connected with the sample chamber for easy control of the liquid analyte and its vapor. The inner volume of the chamber is larger than the MPS dimension and sealed to easily achieve the saturated pressure of vapor analyte. The inner chamber has a width of 21 mm (x-axis), a length of 60 mm (y-axis), and a height of 45 mm (z-axis). One fluidic channel (18 mm long in x-axis, 5 mm wide in y-axis, and 3 mm deep in z-axis) is machined at the bottom of the Teflon chamber for loading the liquid analytes that are injected via the external tubing. The liquid analytes evaporate, becoming vapors, and diffuse into the MPS. In the volatile gas sensing experiment, the sample loading and sensing processes are performed at room temperature and normal atmosphere without enforcing pump or additional heating process.
\nTo study the sensing performance dependent on the MPS dimensions, four kinds of PET meshes with different thicknesses and square micropore sizes are stacked into two MPS configurations, which are the uniform and periodic structures (\nFigure 3d\n). The micropores of each PET meshes in a MPS were not precisely aligned with each other when the PET layers are randomly placed layer by layer for both uniform and periodic MPSs. The periodic MPS is constructed by alternately stacking two kinds of PET meshes that have different micropore sizes. In the other way, the uniform MPS is made by stacking only one kind of PET mesh. Different micropore sizes of PET meshes represent different porosities or different effective refractive indices (e.g., n\n1 and n\n2 in \nFigure 1d\n). To investigate the pore size effect for the vapor sensitivity, large- and small-pore MPSs are prepared in both the uniform and periodic structures using the four kinds of PET meshes.
\nIn the geometric designs, large-pore periodic MPS is composed by the PET meshes with 90 and 249 μm pore widths, denoted as Periodic-90-249 MPS. Periodic-45-133 MPS represents the small-pore periodic MPS, possessing 45 and 133 μm pore widths of the PET meshes. Similarly, the small- and large-pore uniform MPSs are denoted as Uniform-45 and Uniform-90 MPSs, respectively. Their pore widths of the composed PET meshes are 45 and 90 μm, respectively. The specifications of the applied MPS devices are list in \nTable 1\n, including the pore width, mesh thickness, stacking layer number, device thickness, and effective porosity.
\nMPS No. | \nPore width (μm) | \nPET mesh thickness (μm) | \nLayer number | \nMPS thickness (mm) | \nEffective porosity (%) | \n
---|---|---|---|---|---|
Periodic-90-249 | \n249 | \n200 | \n23 | \n3.46 | \n40.5 | \n
90 | \n50 | \n||||
Periodic-45-133 | \n133 | \n100 | \n6 | \n0.45 | \n37.2 | \n
34 | \n2.55 | \n||||
45 | \n50 | \n||||
46 | \n3.45 | \n||||
Uniform-90 | \n90 | \n100 | \n23 | \n2.3 | \n30.1 | \n
Uniform-45 | \n45 | \n50 | \n6 | \n0.3 | \n29.6 | \n
12 | \n0.6 | \n||||
34 | \n1.7 | \n
MPS specification.
Reprinted from Opt. Express 25, 5651-5661 (2017). © 2017 OSA.
The effective absorption coefficients and refractive indices of MPS micropores are observed in the sensing process to distinguish different types and concentrations of volatile gases. The transmitted power spectra of the microporous structure with and without vapor analytes are defined as follows,
\nThe subscripts p and v + p in the Eqs. (1) and (2) represent the sample chamber without and with vapor analytes, respectively. P\n0(ω), α(ω), and L mean the power of input THz wave, effect absorption coefficients of micropores, and thickness of the multilayer-stacked device, respectively. Based on the effective medium concept, the effective absorption coefficient of MPS that is defused with vapor analytes can be written as,
\nFor the blank structure, the effective absorption coefficient is defined as,
\nIn Eqs. (3) and (4), the factors of f, αv\n, αair\n, and αPET\n indicate the air filling ratio of micropore volume, the absorption coefficients of vapor analyte, air, and PET material, respectively. α\n\nv + p\n and αv\n in Eqs. (1) and (2) are substituted by Eqs. (3) and (4). The absorption coefficient variation (αv\n − αair\n) is consequently shown as,
\nThe value of (αv\n − αair\n) approximates to αv\n because the THz wave absorption of air is as small as 10−5 cm−1 [25]. Thus, the absorption coefficient of vapor (αv\n) can be obtained in the approximate condition. The effective medium concept to express the absorption coefficients of the micropore in Eqs. (4) and (5) represents the absorption value within the unit pore space/volume. We thus denote αeff\n as the vapor absorption coefficient instead of αv\n to emphasize the critical assumption of effective medium idea (Eq. (6)).
\nFurthermore, the phase difference for the unit pore volume with and without vapors in the MPS can be defined as (Δϕ/Vpore\n), where Δϕ = ϕv + p\n − ϕp\n and Vpore\n are the phase difference for MPS with-without vapors and the total pore volume, respectively. The phase difference Δϕ and the total volume of MPS, Vpore\n, can be individually estimated from Eqs. (7) and (8),
\n\nΔn, ω, c, and Abeam\n are the refractive index variation, angular frequency, light speed in vacuum and input THz beam size on the MPS structure, respectively. The phase difference for the unit volume of micropore is thus presented as,
\nBased on Eq. (9), we consider the effective refractive index variation of the unit pore volume, Δneff\n, correlating to the macroscopic version of refractive index variation, Δn, as
\nThe macroscopic variation of refractive index (Δn) can be obtained from Eq. (9) as,
\n\nEq. (10) is then substituted by Eq. (11) and re-written as,
\nThe effective refractive index variation within the unit volume of micropore (Δneff\n) can eventually be extracted from the phase information of the measured THz waveforms (Δϕ) and the pore volume that is radiated by the THz-wave beam spot. That is, Vpore = Abeam L f. Based on the sensing principle as shown in Eqs. (1)–(12), the detected THz fields within the unit pore volume of micropore are measured and compared with and without vapors infiltration in the scheme (\nFigure 3c\n). Because of the transparent property of PET material in THz frequency, the scattering loss of surface roughness along the propagation axis can be excluded.
\nThe Periodic-90-249 MPS device was used to study the volatile gas sensing abilities of MPS and observed for the αeff\n response under different amounts of acetone vapor exposure. \nFigure 4a\n illustrates THz power transmission spectra of the Periodic-90-249 MPS before and after exposure to different concentrations of acetone vapor. The THz spectra are apparently identified for acetone aqueous solutions with concentrations from 2.5 to 100%. In the sensing scheme (\nFigure 3a\n), all the acetone aqueous solutions naturally evaporate until the vapor pressure is saturated in the chamber. According to the Raoult’s law [26], the aqueous acetone concentration is approximately proportional to the vapor pressure. That is, the large concentration of acetone aqueous solution generates a large acetone vapor pressure. The increased amount of acetone vapor results in power reduction in the THz frequency range of 0.1–0.45 THz (\nFigure 4a\n).
\n(a) Power transmission spectra and (b) the corresponding effective absorption coefficient spectra within the unit volume of micropore for various aqueous acetone concentrations (reprinted from Opt. Express 23, 2048-2057 (2015). © 2015 OSA).
\n\nFigure 4b\n presents the αeff\n spectra under different concentrations of acetone vapor exposure, which is extracted from the parameters of spectral transmittance, MPS thickness of 3.46 mm, and the micropore filling ratio, 40.5% (Eq. (6)). The absorption coefficient increases with THz frequency and vapor molecular density. Based on the ideal gas formula and vapor pressures of acetone aqueous solutions, the vapor densities of acetone (ρ) can be obtained. The vapor pressures for different concentrations of acetone solutions are estimated based on the experimental database in [27] and not by using Raoult’s law.
\nTHz time-domain spectroscopy (THz-TDS) was used to integrate MPS devices to observe the sensing performance of 0.10–0.45 THz waves. \nFigure 4b\n shows the extracted αeff\n values obviously achieve the largest distinction at 0.4 THz. Above 0.4 THz frequency, the signal-to-noise ratio is too low for the vapor sensing. Therefore, we apply the 0.4 THz wave to probe αeff\n and Δneff\n that are responded from the vapor molecules in the MPS devices. Based on the spectroscopic curves of αeff\n in \nFigures 4b\n and \n5a\n and \nb\n illustrate the sensing results of 0.4 THz wave to detect different concentrations of acetone vapors with and without using the MPS inside the chamber, respectively.
\nDetecting the effective absorption coefficients (blue square dot) and refractive index variation (red circular dot) at 0.4 THz for various acetone vapor densities (a) with and (b) without the Periodic-90-249 MPS. The blue and cyan curves indicate the mathematical fit of the proportional relationship and the Langmuir adsorption isotherm, respectively (reprinted from Opt. Express 23, 2048-2057 (2015). © 2015 OSA).
The αeff\n and Δneff\n values of THz wave apparently rise within an acetone vapor density (ρ) of 6 nmol/mm3 and become saturated while ρ is about 6–14 nmol/mm3. This trace trends between αeff\n and Δneff\n versus ρ are consistent. It implies that the increased acetone vapor molecules not only significantly absorb the THz wave (represented by αeff\n) but also introduce considerable phase retardation in the THz electric field oscillation (represented by Δneff\n). The high ρ value certainly makes the infiltration of vapor molecules into the micropores and increases the molecular adsorption capacity of the hydrophilic surface [28]. The acetone vapor molecules, confined in the micropores and adsorbed on the pore surface of MPSs, consequently cause the increment of αeff\n and Δneff\n.
\nThe αeff\n responsivity in \nFigure 5a\n can be fit by the Langmuir adsorption isotherm with a high R\n2 value (>97%). The Langmuir fitting indicates that the monolayer adsorption of acetone vapor molecules on the hydrophilic microporous surface is mainly caused by physisorption. For the molecular density of <6 nmol/mm3, the proportional response of the 0.4 THz wave absorption versus the acetone vapor density can be linearly fit as α = 0.036 + 0.52ρ and considered the sensitive region of the sensor, which is denoted by the blue line in \nFigure 5a\n. The lowest detected concentration of acetone vapor is 291 pmol/mm3, which is indicated as the first data point of \nFigure 5a\n and corresponding to 17 ppm. From the slope of the linear fitting curve and αeff\n inaccuracy (about 0.01–0.02 cm−1), the minimum detectable concentration change of acetone vapor is <108 pmol/mm3, which is equivalent to 6.29 ppm. Therefore, MPS are particularly advantageous for minute vapor sensing with a detection limit of low ppm level.
\n\n\nFigure 5b\n shows the sensing results for different ρ values inside the blank chamber without MPS. For the same ρ, the αeff\n and Δneff\n values inside the blank chamber are smaller than those measured with the MPS. The air-filling ratio (f) quals 100% when we obtain αeff\n and Δneff\n for the blank chamber and based on Eqs. (6) and (10). The MPS enhances the THz absorption about 20 times higher than that in the blank chamber (\nFigure 5a\n and \nb\n). In addition, the Δneff\n in MPS is two orders of magnitude larger than that in the blank chamber for the same ρ. Both αeff\n and Δneff\n slowly increase with vapor density and without the saturation effect for the blank chamber sensing, which is contrast to the MPS condition.
\nFor an acetone liquid concentration of 100% (∼13 nmol/mm3 vapor molecules), the absorption coefficient at 0.4 THz is ∼0.18 cm−1 in \nFigure 5b\n, which is on the same order and reasonably agreed with the published value of 0.45 cm−1 in [29]. According to the slope of the linear fitting curve in \nFigure 5b\n and the system uncertainty of αeff\n, the minimum detectable molecular density variation of acetone vapor is ∼0.558 nmol/mm3, which equals 32.37 ppm. This result reveals that the sensitivity of volatile gas detection by the MPS is higher than that by a traditional THz-TDS system. Given that the MPS can congregate volatile vapors inside the micropores and adsorb on the hydrophilic surface, the interaction strength between THz radiation and polar gas molecules can be enhanced via the adsorbent medium to significantly increase the absorption and index variations.
\nThe sensing ability of MPS is additionally approved to detect other volatile organic compounds (VOCs), including methanol, ethanol, and ammonia. For the three vapors, both the 0.4 THz αeff\n and Δneff\n increase with vapor densities (\nFigure 6\n). The trends are approximate to the sensing result of acetone vapor (\nFigure 5\n), but the saturation responses of αeff\n and Δneff\n are different. The Periodic-90-249 MPS exhibits obvious differences of αeff\n and Δneff\n among the three VOCs (\nFigure 6\n), which are responded from the enhanced adsorption and infiltration of vapors in the micropores. In this sensing configuration of MPS, capillary condensation of vapors does not occur in the micron-sized pores for all vapor species [30, 31].
\nDetecting (a) effective absorption coefficients and (b) refractive index variations at 0.4 THz for different densities of methanol, ethanol, and ammonia vapors by the Periodic-90-249 MPS (reprinted from Opt. Express 23, 2048-2057 (2015). © 2015 OSA).
The 0.4 THz αeff\n values for different concentrations of acetone, methanol, ethanol, and ammonia vapor exposures are summarized in \nFigure 7a\n. The magnitude order from the largest to smallest αeff\n values is ketone, alcohol and ammonia. The linear fitting curves, denoted by the dashed lines in \nFigure 7a\n, represent the molecular responses of the MPS microporous to the vapor analyte. The sensing ability of MPS is significant to identify the three VOC classes. Each type of vapor has a distinct αeff\n curve that can be used to distinguish from other vapors. THz radiation can strongly perturb the dipole moment of a VOC, and predominantly induce molecular motion to increase the THz absorption coefficients. Based on the investigation in \nFigure 7b\n, the magenta columns present magnitudes of the dipole moments of the four polar vapors in the following order: ketone > alcohol > ammonia [32]. The order of molecular dipole moment is approximate to the magnitude sequence of the THz absorption coefficients (\nFigure 7a\n). The THz interaction strength of acetone vapor is higher than those of the other vapors. Methanol and ethanol vapors belong to alcoholic VOCs and have similar THz absorption coefficients in the MPS. The sensitivities of MPS for detecting the four vapors can be evaluated when considering the linear fitting slopes and the system inaccuracies of αeff\n. The detection limit (i.e., detectable molecular densities) of acetone, methanol, ethanol, and ammonia vapors using the Periodic-90-249 MPS sensor are 0.125, 1.71, 1.62, and 3.35 nmol/mm3, respectively, which are denoted by the blue columns in \nFigure 7b\n. The sensing scheme of MPS device not only recognizes minute concentration changes of VOC with picomolar sensitivity but also exhibits excellent discrimination for different polar VOCs.
\n(a) Detecting the effective absorption coefficients at 0.4 THz under different vapor densities. (b) Investigation of dipole moments and detection limits of the molecular densities (reprinted from Opt. Express 23, 2048-2057 (2015). © 2015 OSA).
To study the micropore size effect of MPS on the detection sensitivity, the pore number of the MPSs to interact a THz beam is fixed and only the pore width is changed. In \nTable 1\n, the micropore number of the 23-layered Periodic-90-249 MPS is consistent with that of the 6-layered Periodic-45-133 MPS, but the pore width of the former device is two times larger than the latter device. These two MPSs are compared to observe the pore-size-dependent sensitivity performance. Similarly, the 23-layered Uniform-90 MPS and 6-layered Uniform-45 MPS devices, having an identical pore number but different pore sizes, are also prepared for this test purpose.
\nHere is the operation detail to observe the sensitivity dependent on the micropore size effect. The micropore amounts within the THz beam spot for the periodic and uniform MPSs are fixed, and the micropore sizes of both structures are changed to compare their sensitivities in terms of αeff\n and Δneff\n. Regarding the periodic MPS configuration, the small-pore structure (Periodic-45-133, average porosity ∼37.2%), composed of 6-layered meshes and corresponding to a 0.45 mm thickness, has ∼1900 micropores covered by the THz beam. To maintain the same pore quantity in Periodic-90-249 MPS (average porosity ∼40.5%) under the same THz beam spot, the large-pore mesh of Periodic-90-249 should be stacked up to 23 layers, corresponding to 3.46 mm thickness. For the MPS uniform configuration, the micropore number covered by the THz beam spot in the 6-layered Uniform-45 MPS (porosity ∼29.6% and thickness ∼0.3 mm) is around 3600. To keep the identical micropore number in the Uniform-90 MPS under the same THz beam spot, the Uniform-90 mesh (i.e., a mesh with the large micropores) with 30.1% porosity should be stacked by 23 layers, corresponding to a 2.3 mm thickness.
\nThe sensitivity of different MPS can be measured by the exposure of the MPS to different VOC amounts, and the THz linear responses in effective absorption and refractive index variation are related to the molecular dipole moment of the VOC (\nFigure 7\n). In this study, acetone vapor molecule is thus applied as the standard VOC to calibrate the sensitivity performance of MPS because its high dipole moment (∼2.88 Debye [33]) is easily perturbed by THz waves. That is, obvious THz electromagnetic attenuation or dispersion can be performed from the acetone vapor molecules. Different amounts of acetone vapor are prepared from different volume concentrations of acetone aqueous solutions, including 2.5, 5, 10, 20, 40, 60, 80, and 100%. All the acetone aqueous solutions are individually loaded inside the microfluidic chamber to naturally evaporate into vapor phase under ambient atmosphere/room temperature until the saturated vapor pressures are approached. The vapor pressure inside the chamber is approximately proportional to the aqueous acetone concentration, following the experiment design in \nFigures 4\n–\n7\n.
\n\n\nFigure 8a\n and \nb\n present the αeff\n and Δneff\n, respectively, detected by the Periodic-45-133 and Periodic-90-249 MPSs. In \nFigure 8a\n, the absorption coefficients in the large (Periodic-90-249)—and small (Periodic-45-133)-pore MPSs both proportionally increase within a molecular density of about 6 nmol/mm3 and become saturated at high vapor density. The trace trends of refractive-index variations (Δneff\n) shown in \nFigure 8b\n are similar to those of the absorption coefficients (αeff\n) (\nFigure 8a\n), except that the saturation phenomenon of Δneff\n for the small-pore structure, Periodic-45-133 MPS, occurs at a much lower vapor density (∼2 nmol/mm3) compared with that of the large-pore structure, Periodic-90-249 MPS (∼6 nmol/mm3). The comparison results in \nFigure 8\n show that the αeff\n and Δneff\n of Periodic-45-133 MPS for all vapor densities are much larger than those of Periodic-90-249 MPS. For example, the absorption coefficients of Periodic-90-249 and Periodic-45-133 MPSs (\nFigure 8a\n) at the vapor molecular density of 13 nmol/mm3 are around 4 and 50 cm−1, respectively. Further shrinking the micropore volume under a certain vapor density exposure is able to increase the molecular occupied densities, which are confined in the micropore and adsorbed on the pore surface to increase THz wave absorption and phase retardation. The enhancement in αeff\n and Δneff\n of Periodic-45-133 MPS is attributed to its small pore size, whereas its pore quantity is identical to that of Periodic-90-249 MPS.
\nDetecting (a) effective absorption coefficients and (b) refractive index variations at 0.4 THz within the unit pore volume by the Periodic-45-133 and Periodic-90-249 MPSs (reprinted from Opt. Express 25, 5651-5661 (2017). © 2017 OSA).
The Langmuir adsorption isotherms in \nFigure 8a\n mean the physisorption of acetone monolayer both occurs on the large and small micropore surfaces of MPS. For the molecular density, <6 nmol/mm3 (∼350 ppm), the THz responsivity of the proportional relation between αeff\n and ρ in \nFigure 8a\n are linearly fitted and can be regarded as the sensitive region of the MPS vapor sensor. The αeff\n of Periodic-45-133 MPS increases more rapidly than that of Periodic-90-249 MPS within the sensitive region (ρ < 350 ppm). The minimum detectable concentration changes of acetone vapor for the Periodic-90-249 and Periodic-45-133 MPS sensors determined from the system deviation and responsivity slope in \nFigure 8\n are estimated as less as 108 and 54 pmol/mm3, corresponding to 6.29 and 3.11 ppm, respectively. The detection limit of Periodic-45-133 MPS is two times higher than that of Periodic-90-249 MPS, thereby indicating that a decrease in micropore volume (such as half of the pore width) obviously raises the detection sensitivity of the MPS gas sensor.
\n\n\nFigure 9a\n and \nb\n show the vapor sensing results of Uniform-90 and Uniform-45 MPSs, respectively. The detected αeff\n and Δneff\n both have linear responsivity versus the vapor density and become saturated at a high vapor density, approximate to the trace trends of the periodic MPSs as shown in \nFigure 8\n. However, the saturation vapor density of the two uniform MPSs is about 4 nmol/mm3 (i.e., 200 ppm), and lower than that of the periodic MPSs, which is about 6 nmol/mm3 or 350 ppm. The THz absorption coefficient curves of Uniform-90 and Uniform-45 MPSs are also well fitted by Langmuir adsorption isotherms with high R\n2 values (\nFigure 9a\n), respectively. According to the slopes of the linear fitting curves in the sensitive region (i.e., ρ < 200 ppm) and the measurement inaccuracy of the THz absorption coefficient, the minimum detectable concentration changes of acetone vapor using Uniform-90 and Uniform-45 MPS gas sensors are <46 and 31 pmol/mm3, corresponding to 2.68 and 1.83 ppm, respectively.
\nDetecting (a) effective absorption coefficients and (b) refractive index variations at 0.4 THz within the unit pore volume by the Uniform-45 and Uniform-90 MPSs (reprinted from Opt. Express 25, 5651-5661 (2017). © 2017 OSA).
The detection sensitivity of Uniform-45 MPS is apparently higher than that of Uniform-90 MPS, consistent to the comparison result between Periodic-90-249 and Periodic-45-133 MPSs. The performance emphasizes again that half of the pore width, whether periodic or uniform configuration, facilitates the infiltration and adsorption of acetone vapor in the micropores and on the pore surface, leading to an enhanced vapor-field interaction to increase the sensitivity. In addition, the two uniform MPSs in \nFigure 9\n have obviously higher αeff\n and Δneff\n than those of the periodic MPSs in \nFigure 8\n under the same vapor density exposure. For example, the largest THz absorption coefficient of Uniform-45 MPS is around 80 cm−1 and evidently larger than 50 cm−1 of Periodic-45-133 MPS based on the same 6-layered MPS thickness. The responsivity of linear fitting slopes in \nFigures 8\n and \n9\n presents that the two sensing parameters, αeff\n and Δneff\n, of uniform MPSs are increased more rapidly within a narrower sensitive region compared with those of periodic MPSs. It means only fewer amounts of vapor molecules that infiltrate the uniform MPSs can drastically increase the THz absorption coefficient and refractive index variation until the desired chamber saturation is achieved. The average pore width of Uniform-90/-45 MPS is smaller than that of Periodic-90-249/-45-133 MPS under the similar MPS thickness (i.e., the same stacked layer number of the PET microporous mesh). Therefore, the simple uniform MPS is particularly advantageous for minute vapor sensing with a detection limit of even lower ppm level compared with the periodic MPS.
\nThe pore number effect on MPS sensitivity is studied by changing the stacking layer numbers of Periodic-45-133 and Uniform-45 MPSs. As shown in \nTable 1\n, three different layer numbers of MPSs with identical pore size are prepared for sensitivity comparison in both the Periodic-45-133 and Uniform-45 MPSs. \nFigure 10\n shows αeff\n and Δneff\n for Periodic-45-133 and Uniform-45 MPSs, which are the small-pore MPSs, respectively, with periodic and uniform configurations. Each type of MPS possesses three kinds of thicknesses formed by stacking different layer numbers of PET mesh. The αeff\n and Δneff\n of Periodic-45-133 MPS for 6-layered thickness are evidently larger than those of 34- and 46-layered thicknesses under all vapor densities (\nFigure 10a\n and \nb\n). For the response of Uniform-45 MPS, both αeff\n and Δneff\n are increased with decreasing device thickness from 34 layers to 12 and 6 layers (\nFigure 10c\n and \nd\n). The highest sensitivities for the periodic and uniform MPSs occur in their thinnest conditions (i.e., 6-layered structure). The experiment in this case indicates that the small pore numbers also increase THz wave responses in αeff\n and Δneff\n of the micropores, thereby enhancing the vapor detection sensitivity.
\nDetecting (a, c) effective absorption coefficients and (b, d) refractive index variations within unit pore volume by Periodic-45-133 and Uniform-45 MPSs with different thicknesses (reprinted from Opt. Express 25, 5651-5661 (2017). © 2017 OSA).
Under a constant amount of vapor exposure, increasing pore quantity or size is equivalent to expanding the pore volume of the microporous structure. The vapor density, congregated within the micropore and adsorbed on the hydrophilic surface, is thus diluted and eventually decreases the measured values in αeff\n and Δneff\n. \nFigure 10\n also shows that 6-layered Uniform-45 MPS requires less acetone vapor amounts to saturate the responsivities of αeff\n and Δneff\n because of its less micropore volume/number, comparing to those of 12- or 34-layered Uniform-45 MPS. The vapor saturation density of 6-layered Uniform-45 MPS occurs at around 4 nmol/mm3 (i.e., 200 ppm), indicating the dynamic range of linear responsivity is sufficiently wide for minute vapor detection.
\nThe micropore size dependent sensitivity of the four types of MPSs is summarized in \nTable 2\n, where the sensitivity corresponds to the slope of linear fit. The blank chamber represents the vapor sensing performance of the microfluidic chamber without the MPS. It is THz vapor sensing in the free space measured by traditional THz-TDS. The Uniform-45 MPS with a 6-layered thickness has the highest sensitivity, down to 1 ppm-level acetone vapor molecule. Such the sensitivity is more excellent than that of the 23-layered Periodic-90-249 MPS and much higher than that of blank chamber.
\nMPS No. | \nLayer number | \nError bar at 0.4 THz (cm−1) | \nSensitivity (cm−1/nmol mm−3) | \nDetection limit (ppm) | \n
---|---|---|---|---|
Periodic-90-249 | \n23 | \n0.065 | \n0.60 | \n6.288 | \n
Periodic-45-133 | \n6 | \n0.303 | \n5.66 | \n3.105 | \n
Uniform-90 | \n23 | \n0.062 | \n1.34 | \n2.683 | \n
Uniform-45 | \n6 | \n0.320 | \n10.17 | \n1.825 | \n
Blank chamber | \n0 | \n0.007 | \n0.013 | \n32.37 | \n
Sensing performance of MPS for acetone vapor detection.
Reprinted from Opt. Express 25, 5651-5661 (2017). © 2017 OSA.
The 6-layered Uniform-45 MPS can thus be used for identifying toxic methanol adulterated in alcoholic solutions. Adulterated alcoholic solutions are prepared by mixing different volume ratios of methanol with ethanol, including 1:0, 7:3, 5:5, 3:7, and 0:1 (ethanol/methanol). The adulterated alcoholic solution is injected into the microfluidic channel of the sealed microfluidic chamber, as shown in \nFigure 3c\n, and becomes the vapor molecules via natural evaporation to be detected by THz waves. \nFigure 11a\n illustrates the THz transmission spectrum of Uniform-45 MPS exposed to the vaporized mixtures, which are generated from various concentrations of adulterated alcoholic solutions. THz transmission power apparently decreases in the frequency range of 0.25–0.45 THz as the volume ratio of methanol increases. The THz absorption coefficient spectra for the different concentrations of adulterated alcoholic vapors can be estimated and shown in the inset of \nFigure 11a\n. The measured THz absorption coefficients for each concentration of alcoholic vapor are almost constant in the frequency range of 0.25–0.45 THz. The relatively high absorption coefficients are resulted from the increment of the adulterated methanol concentration. The refractive index variation before and after exposure to different concentrations of alcoholic vapors can also be calculated. \nFigure 11b\n plots the relations of the αeff\n and Δneff\n at 0.4 THz against different concentrations of alcoholic aqueous solutions. The αeff\n and Δneff\n increase with the methanol concentration adulterated in the alcoholic solution. The proportional relation of αeff\n and ρ is linearly fitted as αeff = 1.2 + 0.67ρ. The sensing result of \nFigure 11\n reveals that the colorless and high THz-absorbed alcoholic aqueous solutions with different concentrations of toxic methanol adulteration can be easily distinguished using the MPS gas sensor composed of 6-layered Uniform-45 MPS.
\n(a) THz-wave transmission spectrum for sensing toxic methanol adulterated in alcoholic solutions: (inset) detecting THz absorption coefficient spectra by the 6-layered Unifrom-45 MPS. (b) Detecting absorption coefficient and refractive index variation versus different concentrations of adulterated alcoholic aqueous solutions at 0.4 THz (reprinted from Opt. Express 25, 5651-5661 (2017). © 2017 OSA).
Optical gas sensors are experimentally demonstrated using the THz refractive indices and THz absorption coefficients when THz waves propagating through the dielectric-layer media are monitored in a spectroscopic system (THz-TDS). The cylindrical layer is applied from a glass dielectric pipe to be the waveguide resonator. Based on the FP criteria and FDTD simulation, the THz frequency of pipe-waveguide resonance field is approximately proportional to the refractive index of the pipe core. The experimental results present that only the high-order resonant modes are sensitive to the refractive-index variation due to the high evanescent power toward the pipe core. Different analytes with different vapor pressures, such as water, HCl, acetone and ammonia, are thus identified by a pipe-waveguide resonator. To further improve the detection sensitivity and selectivity, the MPS structures are applied as 1 THz artificial material to adsorb vapor molecules. THz absorption coefficients of the unit volume are defined based on the effective medium concept and demonstrated to identify various vapor molecules in the investigation. The molecular dipole moment dominates THz absorption in the unit volume of micropore when several analytes, such as the acetone, methanol, ethanol and ammonia, are test in one MPS sensor. The sensing performance based on the MPS geometry is studied for the sensitivity and the possible detection limit. For the acetone molecule, the 6-layered Uniform-45 MPS sensor has the high sensitivity and the detection limit is potentially down to 1.8 ppm. The 6-layered Uniform-45 MPS sensor is eventually applied for one sensing application to distinct methanol and ethanol vapor molecules from various liquid mixtures. The MPS sensing scheme is therefore applicable to realize one optical gas sensor.
\nThis work was supported by grants-in-aid for scientific research from the Ministry of Science and Technology of Taiwan (MOST 107-2221-E-006-183-MY3) and Japan Society for the Promotion of Science (JSPS, KAKENHI, JP16K17525).
\nIntechOpen aims to ensure that original material is published while at the same time giving significant freedom to our Authors. To that end we maintain a flexible Copyright Policy guaranteeing that there is no transfer of copyright to the publisher and Authors retain exclusive copyright to their Work.
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\\n\\nSubject to the license granted above, copyright in the Chapter and all versions of it created during IntechOpen's editing process (including the published version) is retained by the Corresponding Author and any Co-Author.
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\\n\\n3. CORRESPONDING AUTHOR'S DUTIES
\\n\\n3.1 When distributing or re-publishing the Chapter, the Corresponding Author agrees to credit the Book in which the Chapter has been published as the source of first publication, as well as IntechOpen. The Corresponding Author warrants that each Co-Author will also credit the Book in which the Chapter has been published as the source of first publication, as well as IntechOpen, when they are distributing or re-publishing the Chapter.
\\n\\n3.2 When submitting the Chapter, the Corresponding Author agrees to:
\\n\\nThe Corresponding Author will be held responsible for the payment of the Open Access Publishing Fees.
\\n\\nAll payments shall be due 30 days from the date of the issued invoice. The Corresponding Author or the payer on the Corresponding Author's and Co-Authors' behalf will bear all banking and similar charges incurred.
\\n\\n3.3 The Corresponding Author shall obtain in writing all consents necessary for the reproduction of any material in which a third-party right exists, including quotations, photographs and illustrations, in all editions of the Chapter worldwide for the full term of the above licenses, and shall provide to IntechOpen upon request the original copies of such consents for inspection (at IntechOpen's option) or photocopies of such consents.
\\n\\nThe Corresponding Author shall obtain written informed consent for publication from people who might recognize themselves or be identified by others (e.g. from case reports or photographs).
\\n\\n3.4 The Corresponding Author and any Co-Author shall respect confidentiality rights during and after the termination of this Agreement. The information contained in all correspondence and documents as part of the publishing activity between IntechOpen and the Corresponding Author and any Co-Author are confidential and are intended only for the recipient. The contents may not be disclosed publicly and are not intended for unauthorized use or distribution. Any use, disclosure, copying, or distribution is prohibited and may be unlawful.
\\n\\n4. CORRESPONDING AUTHOR'S WARRANTY
\\n\\n4.1 The Corresponding Author represents and warrants that the Chapter does not and will not breach any applicable law or the rights of any third party and, specifically, that the Chapter contains no matter that is defamatory or that infringes any literary or proprietary rights, intellectual property rights, or any rights of privacy. The Corresponding Author warrants and represents that: (i) the Chapter is the original work of themselves and any Co-Author and is not copied wholly or substantially from any other work or material or any other source; (ii) the Chapter has not been formally published in any other peer-reviewed journal or in a book or edited collection, and is not under consideration for any such publication; (iii) they themselves and any Co-Author are qualifying persons under section 154 of the Copyright, Designs and Patents Act 1988; (iv) they themselves and any Co-Author have not assigned and will not during the term of this Publication Agreement purport to assign any of the rights granted to IntechOpen under this Publication Agreement; and (v) the rights granted by this Publication Agreement are free from any security interest, option, mortgage, charge or lien.
\\n\\nThe Corresponding Author also warrants and represents that: (i) they have the full power to enter into this Publication Agreement on their own behalf and on behalf of each Co-Author; and (ii) they have the necessary rights and/or title in and to the Chapter to grant IntechOpen, on behalf of themselves and any Co-Author, the rights and licenses expressed to be granted in this Publication Agreement. If the Chapter was prepared jointly by the Corresponding Author and any Co-Author, the Corresponding Author warrants and represents that: (i) each Co-Author agrees to the submission, license and publication of the Chapter on the terms of this Publication Agreement; and (ii) they have the authority to enter into this Publication Agreement on behalf of and bind each Co-Author. The Corresponding Author shall: (i) ensure each Co-Author complies with all relevant provisions of this Publication Agreement, including those relating to confidentiality, performance and standards, as if a party to this Publication Agreement; and (ii) remain primarily liable for all acts and/or omissions of each such Co-Author.
\\n\\nThe Corresponding Author agrees to indemnify and hold IntechOpen harmless against all liabilities, costs, expenses, damages and losses and all reasonable legal costs and expenses suffered or incurred by IntechOpen arising out of or in connection with any breach of the aforementioned representations and warranties. This indemnity shall not cover IntechOpen to the extent that a claim under it results from IntechOpen's negligence or willful misconduct.
\\n\\n4.2 Nothing in this Publication Agreement shall have the effect of excluding or limiting any liability for death or personal injury caused by negligence or any other liability that cannot be excluded or limited by applicable law.
\\n\\n5. TERMINATION
\\n\\n5.1 IntechOpen has a right to terminate this Publication Agreement for quality, program, technical or other reasons with immediate effect, including without limitation (i) if the Corresponding Author or any Co-Author commits a material breach of this Publication Agreement; (ii) if the Corresponding Author or any Co-Author (being an individual) is the subject of a bankruptcy petition, application or order; or (iii) if the Corresponding Author or any Co-Author (being a company) commences negotiations with all or any class of its creditors with a view to rescheduling any of its debts, or makes a proposal for or enters into any compromise or arrangement with any of its creditors.
\\n\\nIn case of termination, IntechOpen will notify the Corresponding Author, in writing, of the decision.
\\n\\n6. INTECHOPEN’S DUTIES AND RIGHTS
\\n\\n6.1 Unless prevented from doing so by events outside its reasonable control, IntechOpen, in its discretion, agrees to publish the Chapter attributing it to the Corresponding Author and any Co-Author.
\\n\\n6.2 IntechOpen has the right to use the Corresponding Author’s and any Co-Author’s names and likeness in connection with scientific dissemination, retrieval, archiving, web hosting and promotion and marketing of the Chapter and has the right to contact the Corresponding Author and any Co-Author until the Chapter is publicly available on any platform owned and/or operated by IntechOpen.
\\n\\n6.3 IntechOpen is granted the authority to enforce the rights from this Publication Agreement, on behalf of the Corresponding Author and any Co-Author, against third parties (for example in cases of plagiarism or copyright infringements). In respect of any such infringement or suspected infringement of the copyright in the Chapter, IntechOpen shall have absolute discretion in addressing any such infringement which is likely to affect IntechOpen's rights under this Publication Agreement, including issuing and conducting proceedings against the suspected infringer.
\\n\\n7. MISCELLANEOUS
\\n\\n7.1 Further Assurance: The Corresponding Author shall and will ensure that any relevant third party (including any Co-Author) shall, execute and deliver whatever further documents or deeds and perform such acts as IntechOpen reasonably requires from time to time for the purpose of giving IntechOpen the full benefit of the provisions of this Publication Agreement.
\\n\\n7.2 Third Party Rights: A person who is not a party to this Publication Agreement may not enforce any of its provisions under the Contracts (Rights of Third Parties) Act 1999.
\\n\\n7.3 Entire Agreement: This Publication Agreement constitutes the entire agreement between the parties in relation to its subject matter. It replaces and extinguishes all prior agreements, draft agreements, arrangements, collateral warranties, collateral contracts, statements, assurances, representations and undertakings of any nature made by or on behalf of the parties, whether oral or written, in relation to that subject matter. Each party acknowledges that in entering into this Publication Agreement it has not relied upon any oral or written statements, collateral or other warranties, assurances, representations or undertakings which were made by or on behalf of the other party in relation to the subject matter of this Publication Agreement at any time before its signature (together "Pre-Contractual Statements"), other than those which are set out in this Publication Agreement. Each party hereby waives all rights and remedies which might otherwise be available to it in relation to such Pre-Contractual Statements. Nothing in this clause shall exclude or restrict the liability of either party arising out of its pre-contract fraudulent misrepresentation or fraudulent concealment.
\\n\\n7.4 Waiver: No failure or delay by a party to exercise any right or remedy provided under this Publication Agreement or by law shall constitute a waiver of that or any other right or remedy, nor shall it preclude or restrict the further exercise of that or any other right or remedy. No single or partial exercise of such right or remedy shall preclude or restrict the further exercise of that or any other right or remedy.
\\n\\n7.5 Variation: No variation of this Publication Agreement shall be effective unless it is in writing and signed by the parties (or their duly authorized representatives).
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\\n\\nAny modification to or deletion of a provision or part-provision under this clause shall not affect the validity and enforceability of the rest of this Publication Agreement.
\\n\\n7.7 No partnership: Nothing in this Publication Agreement is intended to, or shall be deemed to, establish or create any partnership or joint venture or the relationship of principal and agent or employer and employee between IntechOpen and the Corresponding Author or any Co-Author, nor authorize any party to make or enter into any commitments for or on behalf of any other party.
\\n\\n7.8 Governing law: This Publication Agreement and any dispute or claim (including non-contractual disputes or claims) arising out of or in connection with it or its subject matter or formation shall be governed by and construed in accordance with the law of England and Wales. The parties submit to the exclusive jurisdiction of the English courts to settle any dispute or claim arising out of or in connection with this Publication Agreement (including any non-contractual disputes or claims).
\\n\\nLast updated: 2020-11-27
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The Corresponding Author (acting on behalf of all Authors) and INTECHOPEN LIMITED, incorporated and registered in England and Wales with company number 11086078 and a registered office at 5 Princes Gate Court, London, United Kingdom, SW7 2QJ conclude the following Agreement regarding the publication of a Book Chapter:
\n\n1. DEFINITIONS
\n\nCorresponding Author: The Author of the Chapter who serves as a Signatory to this Agreement. The Corresponding Author acts on behalf of any other Co-Author.
\n\nCo-Author: All other Authors of the Chapter besides the Corresponding Author.
\n\nIntechOpen: IntechOpen Ltd., the Publisher of the Book.
\n\nBook: The publication as a collection of chapters compiled by IntechOpen including the Chapter. Chapter: The original literary work created by Corresponding Author and any Co-Author that is the subject of this Agreement.
\n\n2. CORRESPONDING AUTHOR'S GRANT OF RIGHTS
\n\n2.1 Subject to the following Article, the Corresponding Author grants and shall ensure that each Co-Author grants, to IntechOpen, during the full term of copyright and any extensions or renewals of that term the following:
\n\nThe aforementioned licenses shall survive the expiry or termination of this Agreement for any reason.
\n\n2.2 The Corresponding Author (on their own behalf and on behalf of any Co-Author) reserves the following rights to the Chapter but agrees not to exercise them in such a way as to adversely affect IntechOpen's ability to utilize the full benefit of this Publication Agreement: (i) reprographic rights worldwide, other than those which subsist in the typographical arrangement of the Chapter as published by IntechOpen; and (ii) public lending rights arising under the Public Lending Right Act 1979, as amended from time to time, and any similar rights arising in any part of the world.
\n\nThe Corresponding Author confirms that they (and any Co-Author) are and will remain a member of any applicable licensing and collecting society and any successor to that body responsible for administering royalties for the reprographic reproduction of copyright works.
\n\nSubject to the license granted above, copyright in the Chapter and all versions of it created during IntechOpen's editing process (including the published version) is retained by the Corresponding Author and any Co-Author.
\n\nSubject to the license granted above, the Corresponding Author and any Co-Author retains patent, trademark and other intellectual property rights to the Chapter.
\n\n2.3 All rights granted to IntechOpen in this Article are assignable, sublicensable or otherwise transferrable to third parties without the Corresponding Author's or any Co-Author’s specific approval.
\n\n2.4 The Corresponding Author (on their own behalf and on behalf of each Co-Author) will not assert any rights under the Copyright, Designs and Patents Act 1988 to object to derogatory treatment of the Chapter as a consequence of IntechOpen's changes to the Chapter arising from translation of it, corrections and edits for house style, removal of problematic material and other reasonable edits.
\n\n3. CORRESPONDING AUTHOR'S DUTIES
\n\n3.1 When distributing or re-publishing the Chapter, the Corresponding Author agrees to credit the Book in which the Chapter has been published as the source of first publication, as well as IntechOpen. The Corresponding Author warrants that each Co-Author will also credit the Book in which the Chapter has been published as the source of first publication, as well as IntechOpen, when they are distributing or re-publishing the Chapter.
\n\n3.2 When submitting the Chapter, the Corresponding Author agrees to:
\n\nThe Corresponding Author will be held responsible for the payment of the Open Access Publishing Fees.
\n\nAll payments shall be due 30 days from the date of the issued invoice. The Corresponding Author or the payer on the Corresponding Author's and Co-Authors' behalf will bear all banking and similar charges incurred.
\n\n3.3 The Corresponding Author shall obtain in writing all consents necessary for the reproduction of any material in which a third-party right exists, including quotations, photographs and illustrations, in all editions of the Chapter worldwide for the full term of the above licenses, and shall provide to IntechOpen upon request the original copies of such consents for inspection (at IntechOpen's option) or photocopies of such consents.
\n\nThe Corresponding Author shall obtain written informed consent for publication from people who might recognize themselves or be identified by others (e.g. from case reports or photographs).
\n\n3.4 The Corresponding Author and any Co-Author shall respect confidentiality rights during and after the termination of this Agreement. The information contained in all correspondence and documents as part of the publishing activity between IntechOpen and the Corresponding Author and any Co-Author are confidential and are intended only for the recipient. The contents may not be disclosed publicly and are not intended for unauthorized use or distribution. Any use, disclosure, copying, or distribution is prohibited and may be unlawful.
\n\n4. CORRESPONDING AUTHOR'S WARRANTY
\n\n4.1 The Corresponding Author represents and warrants that the Chapter does not and will not breach any applicable law or the rights of any third party and, specifically, that the Chapter contains no matter that is defamatory or that infringes any literary or proprietary rights, intellectual property rights, or any rights of privacy. The Corresponding Author warrants and represents that: (i) the Chapter is the original work of themselves and any Co-Author and is not copied wholly or substantially from any other work or material or any other source; (ii) the Chapter has not been formally published in any other peer-reviewed journal or in a book or edited collection, and is not under consideration for any such publication; (iii) they themselves and any Co-Author are qualifying persons under section 154 of the Copyright, Designs and Patents Act 1988; (iv) they themselves and any Co-Author have not assigned and will not during the term of this Publication Agreement purport to assign any of the rights granted to IntechOpen under this Publication Agreement; and (v) the rights granted by this Publication Agreement are free from any security interest, option, mortgage, charge or lien.
\n\nThe Corresponding Author also warrants and represents that: (i) they have the full power to enter into this Publication Agreement on their own behalf and on behalf of each Co-Author; and (ii) they have the necessary rights and/or title in and to the Chapter to grant IntechOpen, on behalf of themselves and any Co-Author, the rights and licenses expressed to be granted in this Publication Agreement. If the Chapter was prepared jointly by the Corresponding Author and any Co-Author, the Corresponding Author warrants and represents that: (i) each Co-Author agrees to the submission, license and publication of the Chapter on the terms of this Publication Agreement; and (ii) they have the authority to enter into this Publication Agreement on behalf of and bind each Co-Author. The Corresponding Author shall: (i) ensure each Co-Author complies with all relevant provisions of this Publication Agreement, including those relating to confidentiality, performance and standards, as if a party to this Publication Agreement; and (ii) remain primarily liable for all acts and/or omissions of each such Co-Author.
\n\nThe Corresponding Author agrees to indemnify and hold IntechOpen harmless against all liabilities, costs, expenses, damages and losses and all reasonable legal costs and expenses suffered or incurred by IntechOpen arising out of or in connection with any breach of the aforementioned representations and warranties. This indemnity shall not cover IntechOpen to the extent that a claim under it results from IntechOpen's negligence or willful misconduct.
\n\n4.2 Nothing in this Publication Agreement shall have the effect of excluding or limiting any liability for death or personal injury caused by negligence or any other liability that cannot be excluded or limited by applicable law.
\n\n5. TERMINATION
\n\n5.1 IntechOpen has a right to terminate this Publication Agreement for quality, program, technical or other reasons with immediate effect, including without limitation (i) if the Corresponding Author or any Co-Author commits a material breach of this Publication Agreement; (ii) if the Corresponding Author or any Co-Author (being an individual) is the subject of a bankruptcy petition, application or order; or (iii) if the Corresponding Author or any Co-Author (being a company) commences negotiations with all or any class of its creditors with a view to rescheduling any of its debts, or makes a proposal for or enters into any compromise or arrangement with any of its creditors.
\n\nIn case of termination, IntechOpen will notify the Corresponding Author, in writing, of the decision.
\n\n6. INTECHOPEN’S DUTIES AND RIGHTS
\n\n6.1 Unless prevented from doing so by events outside its reasonable control, IntechOpen, in its discretion, agrees to publish the Chapter attributing it to the Corresponding Author and any Co-Author.
\n\n6.2 IntechOpen has the right to use the Corresponding Author’s and any Co-Author’s names and likeness in connection with scientific dissemination, retrieval, archiving, web hosting and promotion and marketing of the Chapter and has the right to contact the Corresponding Author and any Co-Author until the Chapter is publicly available on any platform owned and/or operated by IntechOpen.
\n\n6.3 IntechOpen is granted the authority to enforce the rights from this Publication Agreement, on behalf of the Corresponding Author and any Co-Author, against third parties (for example in cases of plagiarism or copyright infringements). In respect of any such infringement or suspected infringement of the copyright in the Chapter, IntechOpen shall have absolute discretion in addressing any such infringement which is likely to affect IntechOpen's rights under this Publication Agreement, including issuing and conducting proceedings against the suspected infringer.
\n\n7. MISCELLANEOUS
\n\n7.1 Further Assurance: The Corresponding Author shall and will ensure that any relevant third party (including any Co-Author) shall, execute and deliver whatever further documents or deeds and perform such acts as IntechOpen reasonably requires from time to time for the purpose of giving IntechOpen the full benefit of the provisions of this Publication Agreement.
\n\n7.2 Third Party Rights: A person who is not a party to this Publication Agreement may not enforce any of its provisions under the Contracts (Rights of Third Parties) Act 1999.
\n\n7.3 Entire Agreement: This Publication Agreement constitutes the entire agreement between the parties in relation to its subject matter. It replaces and extinguishes all prior agreements, draft agreements, arrangements, collateral warranties, collateral contracts, statements, assurances, representations and undertakings of any nature made by or on behalf of the parties, whether oral or written, in relation to that subject matter. Each party acknowledges that in entering into this Publication Agreement it has not relied upon any oral or written statements, collateral or other warranties, assurances, representations or undertakings which were made by or on behalf of the other party in relation to the subject matter of this Publication Agreement at any time before its signature (together "Pre-Contractual Statements"), other than those which are set out in this Publication Agreement. Each party hereby waives all rights and remedies which might otherwise be available to it in relation to such Pre-Contractual Statements. Nothing in this clause shall exclude or restrict the liability of either party arising out of its pre-contract fraudulent misrepresentation or fraudulent concealment.
\n\n7.4 Waiver: No failure or delay by a party to exercise any right or remedy provided under this Publication Agreement or by law shall constitute a waiver of that or any other right or remedy, nor shall it preclude or restrict the further exercise of that or any other right or remedy. No single or partial exercise of such right or remedy shall preclude or restrict the further exercise of that or any other right or remedy.
\n\n7.5 Variation: No variation of this Publication Agreement shall be effective unless it is in writing and signed by the parties (or their duly authorized representatives).
\n\n7.6 Severance: If any provision or part-provision of this Publication Agreement is or becomes invalid, illegal or unenforceable, it shall be deemed modified to the minimum extent necessary to make it valid, legal and enforceable. If such modification is not possible, the relevant provision or part-provision shall be deemed deleted.
\n\nAny modification to or deletion of a provision or part-provision under this clause shall not affect the validity and enforceability of the rest of this Publication Agreement.
\n\n7.7 No partnership: Nothing in this Publication Agreement is intended to, or shall be deemed to, establish or create any partnership or joint venture or the relationship of principal and agent or employer and employee between IntechOpen and the Corresponding Author or any Co-Author, nor authorize any party to make or enter into any commitments for or on behalf of any other party.
\n\n7.8 Governing law: This Publication Agreement and any dispute or claim (including non-contractual disputes or claims) arising out of or in connection with it or its subject matter or formation shall be governed by and construed in accordance with the law of England and Wales. The parties submit to the exclusive jurisdiction of the English courts to settle any dispute or claim arising out of or in connection with this Publication Agreement (including any non-contractual disputes or claims).
\n\nLast updated: 2020-11-27
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I am also a member of the team in charge for the supervision of Ph.D. students in the fields of development of silicon based planar waveguide sensor devices, study of inelastic electron tunnelling in planar tunnelling nanostructures for sensing applications and development of organotellurium(IV) compounds for semiconductor applications. I am a specialist in data analysis techniques and nanosurface structure. 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After obtaining a Master's degree in Mechanical Engineering, he continued his PhD studies in Robotics at the Vienna University of Technology. Here he worked as a robotic researcher with the university's Intelligent Manufacturing Systems Group as well as a guest researcher at various European universities, including the Swiss Federal Institute of Technology Lausanne (EPFL). During this time he published more than 20 scientific papers, gave presentations, served as a reviewer for major robotic journals and conferences and most importantly he co-founded and built the International Journal of Advanced Robotic Systems- world's first Open Access journal in the field of robotics. Starting this journal was a pivotal point in his career, since it was a pathway to founding IntechOpen - Open Access publisher focused on addressing academic researchers needs. Alex is a personification of IntechOpen key values being trusted, open and entrepreneurial. Today his focus is on defining the growth and development strategy for the company.",institutionString:null,institution:{name:"TU Wien",country:{name:"Austria"}}},{id:"19816",title:"Prof.",name:"Alexander",middleName:null,surname:"Kokorin",slug:"alexander-kokorin",fullName:"Alexander Kokorin",position:null,profilePictureURL:"https://mts.intechopen.com/storage/users/19816/images/1607_n.jpg",biography:"Alexander I. Kokorin: born: 1947, Moscow; DSc., PhD; Principal Research Fellow (Research Professor) of Department of Kinetics and Catalysis, N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow.\r\nArea of research interests: physical chemistry of complex-organized molecular and nanosized systems, including polymer-metal complexes; the surface of doped oxide semiconductors. 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