Binding energy and C-O stretch vibrational frequency.
\r\n\t
",isbn:"978-1-83968-076-2",printIsbn:"978-1-83968-075-5",pdfIsbn:"978-1-83968-080-9",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"456f82c97eafad5734cd36c48e167781",bookSignature:"Dr. Redmond Ramin Shamshiri",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/10499.jpg",keywords:"Microclimate Control, Prediction Models, Environment Monitoring, Computer Models, Cloud Computing, IoT Monitoring, Simulink, Solar Greenhouses, Exergy, Urban Greenhouses, Virtual Crop Production, Artificial Lighting",numberOfDownloads:67,numberOfWosCitations:0,numberOfCrossrefCitations:0,numberOfDimensionsCitations:0,numberOfTotalCitations:0,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"July 1st 2020",dateEndSecondStepPublish:"July 22nd 2020",dateEndThirdStepPublish:"September 20th 2020",dateEndFourthStepPublish:"December 9th 2020",dateEndFifthStepPublish:"February 7th 2021",remainingDaysToSecondStep:"7 months",secondStepPassed:!0,currentStepOfPublishingProcess:5,editedByType:null,kuFlag:!1,biosketch:"Dr. Shamshiri is the Member of the International Society of Precision Agriculture and Member of the American Society of Agricultural and Biological Engineering. He is also the founder and director of Adaptive AgroTech Consultancy Int, a network of professional experts focused on technology adaptation for food security.",coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"203413",title:"Dr.",name:"Redmond Ramin",middleName:null,surname:"Shamshiri",slug:"redmond-ramin-shamshiri",fullName:"Redmond Ramin Shamshiri",profilePictureURL:"https://mts.intechopen.com/storage/users/203413/images/system/203413.jpg",biography:"Dr. Redmond R. Shamshiri received M.Sc. Dr. Eng, and Ph.D. from the University of Florida and the Universiti Putra in control system and dynamics. He is currently a research scientist at the Leibniz Institute for Agricultural Engineering and Bioeconomy (ATB) in Potsdam, Germany. He is also the founder and director of Adaptive AgroTech Consultancy Int, a network of professional experts focused on technology adaptation for food security. Dr. Shamshiri's research focus is on digital agriculture for food security, involving high-tech control methods, embedded systems, LPWAN sensors, prediction models, and robust data acquisitions for smart farming. He has widely consulted with the industry and academics. 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Venkateswarlu",coverURL:"https://cdn.intechopen.com/books/images_new/371.jpg",editedByType:"Edited by",editors:[{id:"58592",title:"Dr.",name:"Arun",surname:"Shanker",slug:"arun-shanker",fullName:"Arun Shanker"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"878",title:"Phytochemicals",subtitle:"A Global Perspective of Their Role in Nutrition and Health",isOpenForSubmission:!1,hash:"ec77671f63975ef2d16192897deb6835",slug:"phytochemicals-a-global-perspective-of-their-role-in-nutrition-and-health",bookSignature:"Venketeshwer Rao",coverURL:"https://cdn.intechopen.com/books/images_new/878.jpg",editedByType:"Edited by",editors:[{id:"82663",title:"Dr.",name:"Venketeshwer",surname:"Rao",slug:"venketeshwer-rao",fullName:"Venketeshwer Rao"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"4816",title:"Face Recognition",subtitle:null,isOpenForSubmission:!1,hash:"146063b5359146b7718ea86bad47c8eb",slug:"face_recognition",bookSignature:"Kresimir Delac and Mislav Grgic",coverURL:"https://cdn.intechopen.com/books/images_new/4816.jpg",editedByType:"Edited by",editors:[{id:"528",title:"Dr.",name:"Kresimir",surname:"Delac",slug:"kresimir-delac",fullName:"Kresimir Delac"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}}]},chapter:{item:{type:"chapter",id:"51930",title:"Gold-Catalysed Reactions",doi:"10.5772/64103",slug:"gold-catalysed-reactions",body:'\nNanosized gold particles are conveniently synthesised for practical applications in solution or on surfaces. In the latter case the use of supports to derive supported gold catalysts is the most widespread approach. An intermediate method comprises the use of polymerisation to accompany the development of metallic gold from gold salts exemplified by the use of simultaneous polymerisation of polyaniline where very fine control over the mean gold particle size in the range 3–10 nm can be exerted [1].
\nFor solution, an excellent summary was recently published [2]. Examples of the synthesis include the formation of well-defined gold clusters, small nanoparticles of direct interest for catalysis and especially shaped particles such as polyhedra, rods, wires and plates. Very often protecting organic layers are used [3–7] such as thiols, citrate and polyvinylpyrrolidone (PVP) and can act as capping agents. For gold, reduction is relatively easy, so mild reducing agents such as citrate, ascorbic acid or diols can be used. The use of sodium borohydride is also found as well as cases where no additional reductant is used, such as the simultaneous polymerisation of aniline where the hydrogen atoms released by polymerisation effectively do the work [8]. The growing polymer is thereby in intimate contact with the nanogold particles being formed and so electronic effects and nanoparticle stability are considerably enhanced.
\nThe final size distribution often reflects the transport control mechanisms that occur and these have been well discussed [9, 10]. A degree of additional control can be provided by the use of photochemical, electrochemical or sonochemical methods as well as the use of microwave synthesis [2].
\nThe vast majority of work has employed hydrogen tetrachloroaurate as the gold source.
\nThe use of silica-coated gold has been discussed [11–13]. Use is made of tetraethyl orthosilicate and the method is a useful way of preparing unsupported heterogeneous catalysts. The nanoparticles are present in an essentially homogeneous environment by the silica shell, which also assists in providing a physical barrier to particle agglomeration. For solids, much use has been made of the deposition-precipitation (DP) method adopted by the Haruta group. A solution of HAuCl4 is adjusted to the desired pH and gold precipitated onto a slurried support material by controlled addition of a base, usually sodium hydroxide. The point of zero charge of the support is critically important in controlling the interaction with the gold anions, so that the interactions with titania and silica, for example, are very different. Anionic gold entities are easy to adsorb onto titania at pH values above 7–8, for example. Again, a recent review summarises a lot of the essential information [14]. For materials capable of ion exchange, an alternative approach is to use gold in a cationic form and AuHY zeolites have been successfully synthesised in this way [15]. These zeolites have demonstrated activity in CO oxidation and ethylene hydrogenation, depending on the extent of the reduction of the gold, gold in higher oxidation states favouring the olefin hydrogenation reaction [15]. A degree of autoreduction has been observed, and as the ethylene hydrogenation falls, the CO oxidation increases. Once gold forms small particles, these are no longer stabilised as much by electrostatic forces and the gold moves out of the constraints of the ion-exchange sites. The state of gold in the DP samples in the case of titania as support is not simple. Gold in the +3, +1 and 0 oxidation states are found to coexist, revealed by using Mössbauer effect spectroscopy, even when no external reducing conditions have been applied [16]. The +1 state appears to correlate best with CO oxidation activity. The careful retention of all gold in the +3 state has been demonstrated using titania, as evidenced by EXAFS work [17]. The simultaneous reduction of gold and development of activity for CO oxidation has been demonstrated using a combination of reactor studies, EXAFS and TPR, and again Au(+1) seems to be implicated in catalysis, though a role for Au(0) cannot be ruled out entirely.
\nAfter DP the samples are typically hot water washed. It has been shown that extraneous anions and cations can exert an effect on gold catalysis [18]. Anions in particular appear to promote catalysis via some sort of electronic effect and exert an electron-withdrawing effect on the gold centres [18, 19]. The use of urea to give a gradual rise in pH during DP has been recorded [20, 21] and examples of high gold loadings up to about 8 mass% have been given [14]. In studies by the Gates group, the use of organometallic gold precursors under dioxygen-free conditions is used to give a controlled formation of ligand-free small clusters on oxide supports [22, 23]. Further, a simple preparative method was reported by Haruta group who ground supports with solid gold acetylacetonate to yield solid samples [24].
\nCo-precipitation methods have not been entirely neglected and have been applied to the synthesis of transition metal oxide-supported gold [25–28]. Sol-gel methods have also been used, but appear to result in solids with rather large Au particles [14] when tetraethoxysilane or tetrabutoxytitanate is used.
\nVery recently the incorporation of gold into nanorods of rutile, especially prepared in a flower-like structure, has been reported as having very high thermal stability (of the gold), presumably due to substantially reduced gold-gold nanoparticle interactions [29]. Particles as small as 8 nm can be found even after extended exposure to 800°C. This appears to reopen the debate about whether gold-based catalysts could be used for auto emission control catalysts [30]. The addition of relatively small amounts of platinum to gold renders them less liable to sintering [31] and this aspect may also auger well for future high-temperature gold catalysis, particularly in environmental applications. The use of rutile rather than anatase avoids the debilitating effects of the anatase to rutile phase transition which can be expected to take place at about 500°C, though the phase transition temperature is itself raised in the presence of gold, a result, it is assumed, of gold-containing entities being pinned at defect sites which are the seat of the phase transition process.
\nThe effects of increased surface/volume ratio on catalysis are well known. Additionally, the presentation of a relatively high proportion of corner, edge, and misplaced surface atoms is also well known. Rates of CO oxidation, expressed as the turnover frequency, are remarkably increased at low gold particle diameters [32]. It seems that other reactions such as hydrogen oxidation to hydrogen peroxide, organic oxidation and hydrogenation may follow a similar pattern. Relativistic effects have also been discussed for gold [33]. Au-O bonds can be significantly strengthened by the linear O-Au-O structures. The Au-O bond is always stronger when embedded in common surroundings. The result is assigned to the spatial extent of d orbitals of gold, due to the strong relativistic effects. The wider spatial extension of d orbitals of gold leads to two influences. First, gold atoms in clusters or particles with smaller coordination numbers are active due to the ease in forming strong Au-O bonds, especially for the O-Au-O bond. Second, gold atoms in bulk with larger coordination numbers are chemically inert, because the strong suppression by neighbouring gold atoms destabilises the O-Au-O structures. Other discussions based on theoretical treatments have been presented for gold systems [34]. Shell closing aspects can explain the behaviour of clusters of certain sizes and the formation of ligand-protected clusters can provide a basis for the synthesis of nanocatalysts. Ligand engineering may offer the potential to tune the electron states and thereby control catalytic activity. Although tightly bound ligands may act as catalyst modifiers in a negative sense, behaving as catalytic poisons, partial removal by mild thermal treatment may be sufficient in practice to expose active sites. The use of planar model systems to monitor and understand surface structures in gold systems has been well discussed very recently [35]. Electron spectroscopy, low-energy ion scattering, secondary ion mass spectrometry, high-resolution electron loss spectroscopy, infrared spectroscopy, low-energy electron diffraction, small angle X-ray scattering and scanning tunnelling and atomic force microscopies have been particularly useful. These studies help to understand why small clusters of gold behave in a very different manner to bulk gold. Substrate effects can also be monitored with the aid of these methods and underlying film influences can be used to understand how metal-support effects may manifest themselves in practical catalysts. Spectroscopic methods have also been widely used to monitor surface plasmon effects in gold [36], Raman effects, especially enhanced Raman scattering [37] and infrared methods in studying chemisorption and surface reactivity of IR-active molecules such as CO and NO [38, 39]. X-ray photoelectron spectroscopy has often been used to assess gold oxidation states [40, 41], but beam-induced decomposition is well known to result in a gradual drift toward a zero-valent gold states regardless of the nature of the original sample. Low beam energies and short exposure times seem to be the key to help overcome these effects. The example of simultaneous presentation of gold in various oxidation states [16] serves to illustrate that gold may not be homogeneously present in solid catalysts. Careful studies have argued that the activity of gold is proportional to the gold content of gold-titania catalysts for CO oxidation [41] but elsewhere it has been shown that unit gold activity can be made to increase as the gold content falls by removal of less active gold by reaction with cyanide solutions [42]. Cyanide removal of gold from gold-ceria also seems to result in the retention of almost all the original activity for the water-gas shift reaction [43], an observation considered to reveal the importance of Au(+1) entities in catalysis. Cyanide treatment might therefore be useful in the thrifting of gold, but the interaction of cyanide with gold is undoubtedly multifaceted. Cyanide may bind to the gold; rendering active sites useless, it may preferentially dissolve metallic gold (an oxidative process), or it may react preferentially with Au(+1) centres since no oxidation is then required. Indeed the use of cyanide in the absence of dioxygen may be a useful way of enhancing interaction selectively with Au(+1) species [44]. Partial or total removal of cyanide after cyanidation treatments by thermal means seems to be necessary for the resulting solids to display catalytic activity [45]. The complicated action of cyanide makes it difficult to unambiguously decide on the nature of the gold entities being removed or deactivated and hence makes interpretation of the results difficult.
\nSeveral studies [46, 47] have suggested that anionic gold species on titania are essentially pinned to defect sites associated with oxygen loss from the oxide lattice. Thus gold inhibits the anatase to rutile phase transition by an appreciable amount. Nevertheless for high-temperature applications, it seems sensible to avoid anatase or anatase-containing samples as supports [29]. A lot of the titania-based gold catalysts examined for low-temperature reaction are predictably unstable at higher temperatures due to the gross structural changes encountered.
\nA compilation of data [48, 49] for the specific activity of gold systems for the CO oxidation reaction reveals that Au/titania is particularly active. Steyn et al. [50] have recently shown that gold-perovskites can exhibit activities essentially equal to those shown by Au/titania for CO oxidation, depending on the nature of the A and B elements in the Au/ABO3. Perovskites are interesting because they themselves tend to exhibit some oxidation activity and their properties can be tuned by choice of A and B elements.
\nOne reaction that has attracted a lot of research interest in the catalysis by gold is the oxidation of carbon monoxide to carbon dioxide at ambient or lower temperatures. Although the pioneering discovery of the catalytic activity of Au for CO oxidation was based on Fe, Ni and Co oxides as support for Au [51], over the years Au supported on TiO2 has been extensively studied because although neither Au nor TiO2 is active independently, their combination generates surprisingly high scatalytic activity for CO oxidation [52–54]. Several factors influence the activity of this class of catalysts which include the type and nature of the support, the gold crystallite size and the method of preparation [53, 55]. Although the Au/TiO2 catalyst is known to be very active for CO oxidation, a major setback is that the catalyst tends to deactivate with storage and/or time on-line at low temperatures [56]. Also controversial is the nature of the active species. Various authors have claimed different states of Au to be responsible for catalytic activity in CO oxidation. Some have claimed ionic gold to be responsible for the active sites [57, 58], whilst some report metallic gold to be the active species [59, 60]. Another group of authors claim that a combination of both ionic and metallic gold is necessary for catalytic activity [61]. We draw attention to these uncertainties concerning the active metal species and deactivation mechanisms resulting from both storage and time on-line as well as to areas involving the support such as use of promoters, mixed metal oxides supports, various types of TiO2 and some unconventional supports (e.g. zeolites and perovskites). We also look at some parameters involved in the preparation of the catalysts and how these influence the activities of the catalysts. Some practical applications of gold catalysts in carbon monoxide oxidations can also be expected.
\nAlthough it is generally agreed that the role of the support of gold-supported catalysts is to stabilise the active gold particles, the nature of the interaction between the support material and the gold particles is very important for catalytic activity [55, 62–64]. For example, for the oxidation of carbon monoxide, exceptionally high activities have been reported for reducible metal oxide supports such as TiO2, Fe2O3 and CeO2 suggesting that the support supplies oxygen to form active oxidic gold sites [64]. It has also been suggested that sites at the gold-support interface are responsible for the activity in CO oxidation [57, 65].
\nTaking these into account, we have been tempted to assume that the contact structure between Au particles and the support is the most important factor for the origin of the activity of gold catalysts and have studied CO oxidation on gold-supported catalysts over a number of supports with different properties.
\nAlthough TiO2 has been one of the most investigated supports for Au for CO oxidation, the catalyst tends to lose its activity with time on stream. This deactivation process has been ascribed to various factors such as the change in the oxidation state of Au [61, 66] and agglomeration/sintering of the gold nanoparticles [67], the presence of moisture [68] as well as other structural changes that may occur on both the support and the metal [69]. Carbonaceous (carbon containing) species such as carbonates, bicarbonates and formates formed as intermediates during the reaction may also accumulate on the catalyst surface causing deactivation [70]. Many studies have been performed not only to improve the activity but also the stability of the catalyst with time on stream. Many transition metal oxides have also been studied as supports for Au for CO oxidation. The use of binary mixed oxide supports has been reported as a possible solution for stabilising the Au nanoparticles and preventing them from sintering, thus preventing catalyst deactivation resulting from sintering. It has also been suggested that increasing the basicity of the support may improve the stability of the catalyst by minimising deactivation resulting from the formation of adsorbed CO, carboxylate and carbonate species on the catalyst surface [71]. Au/TiO2 catalysts modified by Al2O3, CaO, ZnO, NiO, ZrO2 and rare earths were found to be beneficial to the reaction for CO oxidation whereas MoO3 and WO3 had a negative effect on the stability [72]. Moreau and Bond also reported a lowering of the rate of deactivation of Au supported on TiO2, SnO2 and CeO2 when iron was added in the preparation [73].
\nAu/TiO2 and Au/FeOx-TiO2 catalysts were examined in order to get a thorough understanding of the effect of Fe on the Au/TiO2 catalyst and link it to the differences observed in their activities for CO oxidation. The Fe-containing supports were either only dried at 120°C or dried at 120°C followed by calcination at 300°C or 500°C prior to Au addition. Activity depended of the support pretreatment temperature. The Au/FeOx-TiO2 catalysts were more active than the Au/TiO2 catalyst with the catalyst containing FeOx-TiO2 calcined at 300°C showing the highest activity. The FeOx-containing catalysts showed smaller Au particles on average; hence they have higher metal surface area which could possibly lead to the superior activities observed. Our study went further than considering particle size effects alone and examined other effects that the FeOx brought to improve activity. For example, although all the FeOx-containing catalysts had similar Au particle sizes, there were clear differences in the Au-normalised activity shown. From the CO and CO2 desorption profiles of the samples, the activation energy of desorption of CO and CO2 from the samples was quantified using the Redhead method (Eq. (1)) [74]:
\nwhere Ed is the activation energy of desorption, Tp is the temperature at peak maximum, β is the heating rate or ramp rate dT/dt in units of degrees K per unit time and ν is the frequency factor approximated for first order kinetics to be 1013/s. The calculated desorption activation energies showed a significant difference (ca. 24% difference) in the activation energy of desorption for CO between the samples that contain FeOx and those that do not. This difference could manifest itself in the observed reaction rates during CO oxidation. For all intents and purposes, the Ed’s for CO2 of all the tested samples were identical.
\nXPS data indicated that Au/FeOx-TiO2 catalysts all have similar Fe3+ and Fe2+ ratios implying that they contain similar FeOx species. TPR data however shows that the FeOx species distribution is different in the catalysts. The total acid sites in the Brønsted to Lewis site ratio amongst the Au/FeOx-TiO2 catalysts are also distinctly different. Part of the observed activity trend results from the final speciation of the FeOx that seems to be dominated by a mixture of FeO and Fe3O4 with small amounts of Fe2O3 present. The activity and stability increase for the series as the absolute amount of Brønsted acid sites increases. The Au/FeOx-TiO2 (support calcined at 300°C) which shows the highest activity and stability has small Au particles, a high total acid site amount and the highest Brønsted to Lewis site ratio. It is suggested that the increased Brønsted acidity destabilises carbonate species and prevents them from building up on the surface of the catalyst. The activity trend may also be related to the Au particle size on the FeOx-TiO2 supports as it is noted that amongst the three Au-FeOx-TiO2 catalysts the most active are the ones with the smallest Au particles. In particular, the catalyst with FeOx-TiO2 calcined at 500°C has a high activity and stability and the smallest Au particles size but has the lowest amount of acid sites. Thus it is not possible to fully discriminate the effect of the acidity from the Au particle size influence on the activity of this series of catalysts [75].
\nPerovskites (ABO3 structures where A and B represent metals in the 12- and 6-coordinated sites, respectively) are promising catalyst materials due to their low cost, thermal and mechanical stability at relatively high temperature, great diversity and excellent redox properties [76, 77]. They can be manipulated by partial or complete substitution of the cations A and B and are known to be active for CO oxidation, but only at high temperatures for potential use in automobile exhaust catalysts, with no activity being shown at temperatures below 200°C [78–81]. Besides, it is generally known that the addition of a metal to oxides can modify the intrinsic catalytic properties of the oxides themselves, possibly increasing the activity, selectivity or stability of the resulting catalytic systems [82, 83]. Palladium-perovskites have been the subject of studies for potential use in automotive exhaust systems and appear to offer the property of self-generation, associated with palladium’s ability to move in or out of the perovskite structure depending on the oxidising/reducing characteristics of the atmosphere [76, 78]. To the best of our knowledge, only a few reports on the preparation of Au-perovskite catalysts have been seen in the literature. Addition of Au to LaBO3 perovskite catalysts (B = Cr, Mn, Fe and Ni) showed significant enhancement in the rate of CO oxidation with 2 wt%Au-LaNiO3 showing the best performance with CO conversion at temperatures below 150°C [84]. A comparison of the CO oxidation activities of LaCoO3, the mixture of La2O3 and Co3O4 and the Au-supported Au/La-Co-O catalysts showed that the gold catalyst supported on the perovskite had higher catalytic activity and stability than that of the simple oxides or the perovskite [85]. Three-dimensionally ordered macroporous (3DOM) LaFeO3-supported Au also showed superior performance for the oxidation of soot compared to the corresponding perovskite [86]. Despite these, the state of knowledge about Au-perovskite systems is still limited. In particular, very little characterisation of these systems has been done, being limited to XRD, textural studies and some XPS work. More detailed characterisation work as well as the exploration of the powerful ability of perovskites to be modified by substituting other cations in gold systems for CO oxidation is necessary. We have addressed these issues here.
\nTwo perovskite systems, CaTiO3 and LaCaxFeO3, have been selected to check if any major differences are displayed for different perovskites when used as a support for gold in catalysis. The systems investigated include Au-supported LaFeO3, LaMnO3, LaCuO3 and CaTiO3. However, only catalysts supported on CaTiO3 and LaFeO3 showed activity for CO oxidation for reaction temperatures below 100°C and the other systems were eliminated from the study.
\nTitania is amongst the most effective supports for gold for carbon monoxide oxidation [51–53, 55, 56, 59, 61, 65–68, 71–73]. The biphasic Degussa P25 (75% anatase and 25% rutile TiO2) is commonly used as support material. The phase structure, crystal size, surface and textural properties are important parameters on the catalytic performance of Au/TiO2 for CO oxidation [87]. We have therefore modified TiO2 with the aim of modifying its properties as support for Au for CO oxidation.
\nNitrogen doping of TiO2 results in TiO2-xNx which contain more oxygen vacancies than pure TiO2. According to density functional theory (DFT) calculations, N doping of TiO2 favours the formation of oxygen vacancies [88] and this finding was confirmed by real-time transmission electron microscopy (TEM) [89]. The computed energy cost to create oxygen vacancies is drastically reduced from 4.2 eV in pure TiO2 to 0.6 eV in N-doped TiO2 [89].
\nIn our work [90], we confirm that nitrogen doping of anatase TiO2 creates oxygen vacancies (point defects). These play an essential role as metal cluster nucleation sites. Theoretical studies show that electron transfer from defects to the Au clusters facilitates CO oxidation. Centeno et al. [91] reported that Au/TiO2 catalysts showed higher activity than Au/TiO2-xNx, though our work shows that care must be taken over the pretreatment conditions used, as we find that for catalysts pretreated in an oxidising atmosphere, the Au/TiO2-xNx catalyst clearly shows superior activity over Au/TiO2. Although the Au/TiO2-xNx samples contain smaller Au particles, they were generally less active than the Au/TiO2 catalysts showing no direct positive correlation of activity with Au specific surface area. However, for catalysts pretreated reductively in hydrogen, the Au/TiO2 catalysts showed an increase in activity by about three fold whereas the nitrided catalysts showed only a very slight increase.
\nMoisture plays a major role in promoting the CO oxidation activity of the Au/TiO2 catalyst as well as inhibits its deactivation when introduced from the start of the reaction. This effect is not seen for Au/TiO2-xNx where moisture did not either promote the catalytic activity or prevent deactivation suggesting that even small amounts of nitrogen doping of TiO2 inhibit any positive role that moisture might play in the reaction.
\nTitanate materials have been synthesised recently and used in catalytic applications [92–94]. However, only few reports exist where these have been exploited as support for Au for CO oxidation. We find that gold supported on potassium titanate can, under some circumstances, exhibit superior performance for CO oxidation relative to that obtained with titania as a support [95]. The specific surface area of the titanate material KTiO2(OH) was three times more than that of the commercial TiO2 (Degussa P25), whilst the average Au particle sizes for the Au-supported catalysts were 4.7 and 5.1 nm, respectively. An oxidative pretreatment of both catalysts Au/TiO2 (P25) and Au/KTiO2(OH) generally results in Au/KTiO2(OH) being significantly more active than Au/TiO2 (P25). Au/KTiO2(OH) catalyst pretreated in a reducing atmosphere was also more active than the Au/TiO2 (P25) catalyst treated under the same conditions but it was noted that deactivation of the Au/TiO2 (P25) was more rapid. In general, the treatment conditions that a catalyst is subjected to ultimately affect its composition and in this case the contribution of ionic Au species may have played a significant role. Although we have not fully established whether the enhancement in activity with the titanate support is due to a particle size effect or chemical effects, the fact that both catalysts show similar Au particle size distributions makes it more likely that chemical effects have a major contribution to the differences in activity observed. Generally, KTiO2(OH) is a more basic support than TiO2 and the basicity of the support has to be taken into account as well. Basic oxides such as magnesia have been shown to be particularly active for supported gold for CO oxidation [55, 96, 97], and more acidic supports such as zeolites, for example, are generally less active [16]. Modification of TiO2 by the addition of Group 1 metal ions has been shown to either increase or decrease the activity of the resulting Au-supported catalysts subsequently prepared from the treated support depending on the amount of the Group 1 metal ions added. This effect appears to be related to the electronic environment of the Au in the catalysts [18]. A further factor may well be the detailed structure of the support at the nanoscale. Nanosized ceria, for example, appears to lead to a higher activity when used to support gold [98] than other forms of ceria. Nevertheless, our work adequately demonstrates that titanate materials may well be used in offering a further support system for gold-mediated catalysis.
\nThe stabilisation of gold nanoparticles is of immense importance when nanogold catalysts are considered, as catalytic activity is directly related to gold particle size [51–55, 59, 61, 72]. For example, gold catalysts for potential use in the automotive industry must be able to withstand high temperatures from exhaust gases, where sintering of the gold nanoparticles results in catalyst deactivation. Sintering of gold particles not only occurs at high temperatures but occurs slowly over time at ambient temperatures that can result in the deactivation of the catalysts over extended time [55, 99]. Very few gold catalysts have the durability to withstand temperatures over 400°C for extended periods of time without complete loss of activity [100–103]. Almost all of these catalysts, whilst showing activity after exposure to moderately high temperatures, are not durable enough for long-term catalytic applications. If gold catalysts are to be used in applications above 400°C, such as in automotive catalysts, thermal stability of not only the gold nanoparticles but also of the support is crucial for long-term stability. We carried out a thorough search of the literature to understand the reasons for deactivation of catalysts at high temperatures and used the information gained to develop a support that can combat the deactivation processes.
\nWe have been able to synthesise titanium dioxide catalyst support material comprising rutile nanorods extending radially from a central point, each rod having a free end spaced from adjacent nanorods [104]. The material has a high surface area of ca. 100 g/m2. When gold nanoparticles are deposited on the support, they preferentially locate at or near the free ends of the nanorods. At low gold loadings on the support, the orientation of the gold particles on the support prevents sintering of the particles when heated at high temperatures. The catalysts exposed to temperatures of 550°C for up to 120 hours show very insignificant changes in the catalyst activity for CO oxidation as opposed to a standard Au/TiO2 (P25) catalyst which almost completely loses its activity after exposure to 550°C for only 24 hours as a result of sintering of the gold nanoparticles. When CO oxidation is carried out at 250°C, the heated catalysts show similar performance with the fresh catalysts. Storage of the catalysts at ambient conditions for several months showed no effect on the Au particle sizes demonstrating the long-term stability of the catalyst.
\nThe interface between gold and the support in gold-supported catalysts is crucial for catalytic activity in CO oxidation [105, 106]. Charge transfer between the support, particularly involving negatively charged defect sites, and the Au particles has also been connected with catalytic performance [107]. The addition of anions and cations has been reported to act as promoters for some heterogeneous. The poisoning effect of residual chloride ions on gold-supported catalysts prepared using HAuCl4 solution is well documented [108, 109]. Residual chloride is found to affect activities by facilitation agglomeration during heat treatment and also poisoning the active sites. The addition of low levels of nitrate ions to Au/TiO2 catalyst has been shown to enhance the catalytic activity towards CO oxidation, with high levels leading to a decrease in activity [19]. Our comprehensive study on the effects of the incorporation of varying levels of a number of anions and cations into Au/TiO2 catalysts for CO oxidation [18, 110] reveals activity enhancement in some cases, whilst in others activity is depressed. The effect seen depends on the concentration levels of the ions added and the manner in which they are added. In order to gain an understanding into the nature of the effects operating between the added ions and the support and/or Au, we incorporated these ions into the support before gold introduction and into the catalyst after catalyst preparation. For the sulphate-modified sample, there is a direct evidence that entities containing both Au and S exist on the surface of the final catalyst and it may well be that the promoting effect of sulphate is due, at least in part, to a direct interaction occurring between gold centres and sulphate or sulphate-derived entities. The evidence points strongly to the fact that the enhancement is associated with gold centres having a relatively high electron deficiency. For the anions and cations other than sulphate, it appears that these exert an influence on catalytic activity via the interaction with the support, rather than by direct interaction with the gold centres. Our findings here generally support the idea that the performance of gold catalysts can be extremely sensitive to parameters involved in the catalyst preparation which may include exposure to “foreign” ions [19] and at the same time suggests that the specific activity of gold can be improved through the judicious use of such ions.
\nAn important feature of gold-based catalysts for CO oxidation is the size of the gold particle of which 3 nm is reported as optimum [111]. Various methods have been used for controlling the size of the gold nanoparticles amongst which is to embed the Au nanoparticles within the pores of zeolites. The strong confinement of the nanopores of zeolites can result in a very uniform size distribution of gold nanoparticles. Zeolites have the advantages of high surface area (typically 1000 m2/g) [111], tunable uniform pores (2–10 nm) which can be used to stabilise the small gold particles by inserting them into the small cages and also ion-exchange ability. However, traditionally zeolites are considered to be an “inert” support resulting in poor catalytic activity for gold nanoparticles [64]. This poor activity was generally considered to be due to sintering of the gold nanoparticles as SiO2 is known to have a relatively weak metal-support interaction with Au [112]. We however showed that partial reduction can lead to higher activity of gold-zeolite-Y [16] consistent with the work of Chiang et al. [113]. For gold-HY zeolites, in which gold is initially introduced as Au(III) by ion exchange from [Au(en2)]3+, samples become catalytically active only after a considerable induction period has been exceeded. The induction period is substantially shortened by carrying out a mild reductive pretreatment of the AuHY with reducing agents such as sodium borohydride. This significantly also increases the activity of the catalyst. This behaviour is consistent with the suggestion that gold in a partly reduced state is required for activity. The reduction of Au(3+) in AuHY would result from a sufficiently long exposure of the catalyst to the reacting CO mixture. There is however compelling evidence that the majority species in most active catalysts is in the zero-valent state, the obvious conclusion being that most of the gold might well be considered as a spectator species and do not take part in the catalysis. This has been shown for gold-ceria catalysts where after removing a large fraction of the gold present in the catalysts by oxidative cyanide leaching, no fall in activity for the water-gas shift reaction was observed and that the remaining gold was essentially present as Au(1+) [43].
\nSince the activities of gold-supported catalysts for CO oxidation can be ascribed to a significant metal-support interaction, the degree of interaction and thus the method of preparation of the catalysts will largely affect the properties of the resulting catalysts. Numerous papers have been published describing various methods to incorporate gold nanoparticles on various metal oxide supports including TiO2, Al2O3, CeO2, Fe2O3, Co3O4, ZrO2 and SiO2. Depending on the choice of the metal oxide support, the main synthesis methods include deposition-precipitation, co-precipitation, colloidal dispersion, chemical vapour deposition and photodeposition. Conventional incipient wetness impregnation yields large Au nanoparticles due to weak interaction of the most commonly used Au precursor (HAuCl4) with the metal oxide support. This method also results in a large amount of residual chloride in the catalyst which promotes sintering of the Au nanoparticles and may poison the catalyst active sites [59, 60]. Deposition-precipitation has been most widely used especially for support metal oxides with high IEP, such as TiO2, Fe2O3, CeO2 and Al2O3. Deposition-precipitation is however not suitable for incorporating Au nanoparticles onto supports with low IEP such as SiO2, because under the high pH conditions required to hydrolyse the HAuCl4, which is the most common Au precursor used, there is weak interaction between the negatively charged support surface and the [Au(OH)nCl4-n] species which hinders the gold adsorption and facilitates the mobility of the Au nanoparticles. This can lead to the Au nanoparticles sintering easily during the synthesis process, yielding low gold loadings and inactive catalysts [114]. Particular attention has also been given to TiO2 since Au supported on this oxide has been found to be more active for CO oxidation than many other supports.
\nIt has been shown that the acidity of TiO2 can be strongly increased by treatment with sulphate ions, with the formation of S-O and O-S-O bonds in bulk and surface, creating unbalanced charge on Ti and vacancies and defects in the TiO2 network [115, 116]. Au/TiO2 samples prepared from TiCl4 were inactive up to 100°C, but when sulphated with 2.5 mass%SO42−, the CO oxidation initiation temperature was lowered to 30°C. In the case of the Au/TiO2 when the titania support was prepared from titanium isopropoxide, the CO oxidation activity started at 30°C and gradually increased to 40% at 150°C. But when the sample was impregnated with 2.5 mass%SO42−, the CO conversion increased to 98% at the same temperature. The sample prepared in the presence of sodium dodecyl sulphonate containing sulphate (1.5 mass%) showed 84% conversion without further addition of sulphate. However, the CO conversion is reduced to 42% when loaded with 2.5 mass%SO42−. This showed that low amount of sulphate is responsible for enhancing the activity of the Au/TiO2 catalyst and high amount of sulphate is detrimental for CO oxidation. An examination of a series of low sulphate-loaded 1 wt%Au/TiO2 samples revealed a dramatic effect of sulphate treatment on CO oxidation activity recorded at room temperature where an over 5-fold higher activity was found for relatively low sulphate loadings. The promotional effect of sulphate on CO oxidation was found to be unlikely due to physical or textural changes in the catalyst but more likely that chemical effects are responsible. The source of TiO2 was also found to have a considerable influence on the CO oxidation activity of gold-supported catalysts on the supports [117].
\nOne of the practical routes used to prepare gold catalysts especially for CO oxidation, which achieves high activity and a high gold dispersion, is the deposition-precipitation method [52]. Deposition-precipitation, usually a two-step procedure of deposition of the gold precursor in aqueous phase onto the support, followed by reduction of the gold precursor using a reducing agent is typically carried out at a controlled pH (usually in the range 6–10) and uses HAuCl4 as gold source. The HAuCl4 is often added at a carefully controlled and low rate with vigorous stirring and with pH control, and frequently solutions are heated to 60–70°C to affect the process. A key aspect of the use of high pH appears to be associated with the removal of chloride entities from the coordination sphere of the Au atom which would otherwise tend to deactivate the gold centres and contribute to sintering of the gold nanoparticles during catalytic operation and/or heat treatment. A simple single-step method for preparing Au/TiO2 was reported which proceeds without pH control during the contacting of the support with the gold source HAuCl4 solution, followed by washing with water only, and leads to a highly active and stable CO oxidation catalyst. The method makes use of a suitable reducing agent such as an aqueous solution of sodium borohydride and the number of variables involved in the catalyst preparation is drastically reduced and there is no need to rigorously control the pH. The particle diameters of the gold in the catalyst are in the range 2–5 nm as obtained by the deposition-precipitation method, and no residual sodium- or boron-containing species are present in the vicinity of the gold particles in these catalysts as any residual sodium borohydride is easily washed out during the washing procedure. It is however noted that an excess of the sodium borohydride calculated on the reductive stoichiometry of sodium borohydride undergoing conversion to sodium metaborate and all the gold in HAuCl4 being reduced to the zero-valent state is used. The pH of the system upon the sodium borohydride addition rises to over 8 and so the beneficial effects of an alkaline medium on chloride removal from the coordination sphere of the gold may still be achieved [118].
\nAlthough there is a general agreement on the high activity of gold catalysts for CO oxidation, the need for small Au nanoparticles, and the catalyst preparation methods, the nature of the active Au species in relation to its oxidation state has been quite controversial. Determination of the active oxidation state (Au0, AuI and AuIII) or establishing whether some combination of them is needed turns out to be very difficult because of the extreme sensitivity of supported gold catalysts to their surroundings and the fact that the mere act of examining them may change their composition. Examining the state of the catalyst before and after a catalytic reaction may not necessarily reveal its state during the reaction [119]. This is made even more complicated by the fact that sometimes most often the active sites are present in very low amounts on the catalyst surface as most of the atoms present in the solid sample are located in the bulk. This makes it more difficult to understand the nature of the active sites and has led to the design of model systems using single crystals and well-defined surfaces as these can be interrogated using modern surface spectroscopy. Such surfaces may not be representative of a working catalyst [120]. Another difficulty lies in the fact that gold catalysts contain a multiplicity of Au species in one catalyst and their activation under different conditions can lead to various distributions of the various species. This makes it possible that working with the same catalyst under different conditions will lead to the activation of different sites. It is therefore important to consider which active sites are activated in a given catalyst under applied conditions and not just what the nature of the active sites is, in the catalyst. So, in some cases probably the results of different groups do not contradict but supplement each other [121].
\nA number studies [17, 122–126] have concluded that just the metallic form of gold is active. On the other hand, gold cations have been found to be the catalytically active sites for CO oxidation [127–130]. Other studies reveal that a combination of both metallic and cationic gold is necessary for CO oxidation activity. Using EXAFS and XANES, it was reported that for Au/MgO catalyst under reaction conditions, both Au+ and Au0 were present and that higher concentrations of cationic Au resulted in higher catalytic activity [131]. Similar results were reported for Au/Fe2O3 where both Au+ and Au0 coexisted upon exposure to the reaction gas mixture with the conclusion that the cationic Au species was more active but less stable than the metallic Au [132]. Different electronic states of ionic and metallic Au species were detected in Au/H-mordenite (zeolite)—Au+ and Au3+ ions, charged Aunδ+ and neutral nanoparticles Aum and catalytic tests in CO oxidation revealed the coexistence of several types of active species; gold clusters <1.5 nm were responsible for low-temperature activity whilst gold nanoparticles were responsible for high-temperature activity [121].
\nThe oxidation state of Au can be characterised by several experimental and theoretical techniques, amongst which spectroscopic methods are most commonly used. These include diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure spectroscopy (XANES), extended X-ray absorption fine structure spectroscopy (EXAFS), infrared (IR) spectroscopy and Mössbauer spectroscopy. Temperature-programmed oxidation and reduction (TPO and TPR) can also provide quantitative oxidation state information by determining the uptake of oxidising (e.g. O2) or reducing (e.g. H2, CO) agents.
\nFor Au/TiO2 catalyst system for CO oxidation, DRIFTS was used to show a synergy between positively charged and metallic gold nanoparticles whereby, on the reduced catalyst, CO is weakly adsorbed on Au0 species and, in the presence of oxygen, CO is adsorbed on the Au particle associated with oxygen. Metallic Au particles are believed to be activating an oxygen molecule into two oxygen atoms [133]. In a separate study, using XAS (X-ray absorption spectroscopy) and FTIR (Fourier transform-infrared spectroscopy), it was found that during CO oxidation on Au/TiO2, the activity of the catalyst increased with the degree of reduction up to 70% reduction and then decrease slightly beyond 80% reduction due to an increase in the Au particle size and changes in particle morphology consistent with metallic Au being responsible for catalytic activity [17].
\nFrom the suggestions found in the literature, one might conclude that the choice of the nature of the active Au species for CO oxidation on gold-supported catalysts depends on the technique used for the determination and that different supports may have different active gold species as defect formation is more facile in some supports than others.
\nOne of the major drawbacks of gold catalysts is the fact that they tend to lose their activity during reaction (on-line) [48, 68, 71, 98, 109, 122, 126, 134–138] as well as during storage [139, 140].
\nThe main reason for the loss of activity during reaction is a growth of the gold particles (agglomeration/sintering) [135, 136, 140]. Prior to the reaction, the agglomeration of gold particles may also occur from the conditions of thermal treatment used to reduce AuIII to Au0. The removal of hydroxyl groups from the active sites during thermal treatment may also be responsible for deactivation during thermal treatment [48, 137]. It was found for Au/TiO2 that a lowering of the pretreatment gas flow rate as well as an increase in the amount of sample being pretreated may lead to the gold particle size increasing. Also pretreatment in air instead of hydrogen or argon led to an increase in the gold particle size [140]. Residual chloride in catalysts prepared using HAuCl4 as Au precursor may also promote the sintering of the Au particles during thermal treatment [99].
\nIntermediate carbonate formation during the formation of CO2 also results in loss of catalyst activity by blocking some of the catalyst active sites [135–137].
\nThe on-line deactivation characteristics of Au/TiO2 were studied in an unconventional PROX system using dry, cylinder-stored CO-contaminated hydrogen for fuel cell applications. The results obtained suggest that as opposed to CO removal from air, the accumulation of carbonate species and surface hydration have minor, if any, effect on the on-line deactivation of the catalyst. The deactivation is more likely to be due to an intrinsic transformation in the catalytic properties of the catalyst, by distorting the balance between Au0 and Aux+ through reducing Aux+ to Au0 [50].
\nThe main reasons purported for the loss of activity of gold catalysts during storage are gold particle sintering and change in oxidation state. Indoor light has been reported to cause the slow reduction of ionic gold to metallic gold in Au/TiO2 catalysts during storage. A substantial drop in the gold content on the TiO2 surface was also observed, as light causes the migration of gold on the Au/TiO2 surface into TiO2 solid. However, no growth in the gold nanoparticles was observed in this case. It was recommended that the catalysts be stored in a well-defined dark environment under ambient conditions to preserve catalytic performance even after 5 months [139]. In addition to light, water in ambient air is also reported to lead to deactivation of gold catalysts by causing the reduction of unreduced gold and the sintering of metal gold nanoparticles with a proposal for the catalysts to be stored in a desiccator under vacuum and in the dark [140]. Impurity gases in air or occasionally the accumulation of products on their surface during ambient storage may also lead to deactivation of gold catalysts [141].
\nIn a systematic study of the effect of various storage conditions on Au/TiO2 (refrigeration, vacuum, light, dark and inert gas storage) stored over 12 months, we found that a number of factors contribute to the deactivation of the catalysts. These factors include reduction of ionic gold, agglomeration of Au nanoparticles, loss of hydroxyl groups and moisture as well as formation and accumulation of carbonate and formate species on the catalyst surface. When the catalyst was stored in the refrigerator, the extent of Au reduction and Au particle agglomeration as well as the formation of carbonate species was reduced compared to catalysts stored at ambient temperature in light or dark conditions. Storing the catalyst in vacuum accelerated catalyst deactivation quite drastically caused by Au reduction and agglomeration of the Au nanoparticles, loss of surface hydroxyls and moisture as well as accumulation of carbonates and formates. From the findings of the work, we recommended that gold catalysts be thoroughly purged with inert gas to remove all the atmospheric and adsorbed CO2 from the catalyst and the catalysts stored in a refrigerator or at least a cool and dark place to minimise the effects of temperature and light [142].
\nDeactivation caused by the adsorption of CO and its accumulation as carbonates and formates may be reversed by heating the catalyst to evolve CO2 from the surface [135, 136]. Activity loss caused by the depletion of hydroxyl groups from the active sites may be restored by treatment with hydrogen or water [48, 137]. Deactivation caused by agglomeration of Au particles is weak but however irreversible. Deactivation of Au/TiO2 catalysts in selective oxidation of CO in the presence of hydrogen caused by a distortion of the balance between Au0 and Aux+ through reducing Aux+ to Au0 was reversed by exposing the catalysts to oxidising atmospheres [50].
\nAn atmospheric pressure non-thermal plasma method using oxygen plasma and O3 injection has been reportedly applied to regenerate Au/TiO2 catalyst deactivated by the adsorption of VOCs. Deactivation of Au/TiO2 exposed to environmental conditions resulting in the blocking of the active sites for CO adsorption was regenerated by irradiation of light (photo-cleaning) without heat treatment [141].
\nGold has come up to take a place alongside the other precious metals (platinum group metals and silver) as a key catalyst in a range of industrial processes and uses. Project AuTEK hosted by Mintek in South Africa is the leading organisation in the commercialisation of important new catalytic applications for gold. Project AuTEK makes kilogram quantities of gold catalysts under the trade name AUROlite™ (1 wt% gold on titania, alumina and zinc oxide supports). The advantages of these Au catalysts over other precious metal catalysts are being demonstrated by achieving high activities and selectivities in both liquid- and gas-phase reactions which have commercial potential. This is likely to result in new industrial applications for gold catalysts in chemical processing and pollution control. Selective oxidation of carbon monoxide in the hydrogen streams used for fuel cells has been achieved using AuTEK catalysts, as is the use of this ambient temperature oxidation process for use in gas masks for protection from CO poisoning and for CO removal from room atmospheres [143, 144].
\nVinyl acetate monomer is used in emulsion-based paints, wallpaper paste and wood glue and has a worldwide annual production of 5 million tonnes. 80% of this is produced by the acetoxylation route employing Pd:Au/SiO2 catalysts. The presence of Au leads to a fourfold increase in space time yield compared with use of Pd alone [144].
\nGluconic acid is used as a food and beverage additive, metals cleaning and applications in pharmaceuticals with a worldwide production of 100,000 t/a. It is produced catalytically from glucose.
\nAu/Al2O3 shows stable activity and selectivity for up to 110 days using a continuous stirred tank reactor. Four tonnes of gluconic acid were produced per gram of Au with no significant deactivation observed [145]. Au has greater activity, selectivity and resistance to deactivation compared to PGM systems. It is also more environmentally friendly and economical compared to the biological, chemical and electrochemical methods [145].
\nThe low-temperature CO oxidation ability of gold catalysts makes them ideal for air-quality applications. Industrial Technology Research Institute (ITRI), Taiwan, has developed a nanogold catalyst for use in CO oxidation fire escape hood [146]. This is available commercially from Taiwan-based Novax Material and Technology.
\nProject AuTEK’s Au/TiO2 catalysts tested under typical EN403 (fire escape mask) test conditions are more active and durable than the established commercial technology, namely, Hopcalite (CuMnOx). The activity of the catalyst is amplified by the presence of moisture, unlike Hopcalite which experiences rapid deactivation.
\nNanostellar has developed an oxidation catalyst for cleaning diesel exhaust gas based on Au-Pt-Pd catalysts. The material increases hydrocarbon oxidation by 40% compared with conventional platinum converters at equivalent precious metal cost.
\nProject AuTEK has developed a new system for hydrogen purification for PEM fuel cells trademarked AuroPureH2 designed to purify cheap hydrogen on board vehicles, drawing the hydrogen feed for fuel cell directly from a cylinder. The system makes use of Au/TiO2 catalyst which is very selective for CO oxidation and can remove high levels from the hydrogen. The low operating temperature gives high selectivity and no additional energy is required to heat the reactor. Fuel efficiency is essentially maintained. The AuroPureH2 system outperforms the PtRu and PtMo CO-tolerant technologies [144].
\nPalladium and platinum catalysts are generally more active for the complete oxidation of hydrocarbons. However, due to the high activity of gold catalysts for CO oxidation at low temperatures and the fact that catalytic performance can be tuned by the choice of the support, a number of reports now exist attempting to develop Au catalysts for the complete oxidation of VOCs. A large number of metal oxides have been reported as support for gold in various VOC oxidation reactions.
\nFormaldehyde is one of the most common and most noxious indoor gaseous pollutants commonly emitted from materials used for building construction as well as decorative materials. Long-term exposure to indoor air containing formaldehyde even at low concentrations is adverse to human health. Supported base metals and precious metals have been applied for catalytic oxidation of formaldehyde. However, base metals require high temperatures [147]. Noble metal catalysts can, however, completely oxidise formaldehyde at ambient temperatures. Fe2O3-, ZrO2- and CeO2-supported Au catalysts have been found to be able to completely oxidise formaldehyde, but at temperatures above 100°C [148, 149]. Room temperature removal of formaldehyde has been reported over Au/TiO2 and Au/CeO2 [147]. For Au/CeO2, it was found that the method of preparation played an important role on the catalyst performance with deposition-precipitation using urea offering a more active system than using NaOH.
\nHydrocarbons are amongst the most prevalent environmental VOC emissions due to their use as transportation fuels as well as being essential feedstocks for chemical production. For hydrocarbons, Co3O4 as support for gold has shown the highest catalytic activity amongst other supported metals [150]. Addition of gold to CoOx, MnOx, CuOx, Fe2O3 and CeO2, which on their own catalyse the oxidation of alkanes, but at high temperature, was found to increase the activity of the catalysts by reducing the temperature at which the reaction occurred. The most effective catalyst for alkane oxidation was Au/CoOx which retained a constant high activity for a 48-h test period; the highest activity catalysts were prepared by coprecipitation rather than impregnation [27]. A detailed study of methane combustion over transition metal oxide-supported gold catalysts prepared by coprecipitation also concluded that the best catalytic performance was obtained with Co3O4 as the support [151].
\nAu/V2O5 supported on titania and zirconia were used for the oxidation of benzene and a strong synergistic effect was observed between Au and V2O5 especially with titania. In this case, activation of oxygen was thought to occur on the gold nanoparticles whilst benzene was activated on vanadium oxide surface [152]. High-surface area ceria prepared by precipitation and calcined at only low temperatures showed surface reducibility and high activity for benzene oxidation at low temperatures. The high-surface area ceria stabilised gold at high dispersion and gold promoted the oxidation of benzene [153].
\nAu supported on ceria or ferric oxide was found to be very active for oxygenated VOCs, e.g. methanol, ethanol, 2-propanol and acetone as well as for aromatic molecules like toluene although for toluene, gold catalysts showed lower activity compared to oxygenated compounds due to the much lower strength of the organic substrate adsorption on the catalyst [154]. The use of various forms of manganese oxide materials in combination with gold for the target total or partial oxidation of 2-propanol, 2-butanol and toluene and the direct comparison made with gold catalysts from the AUROlite™ series (based on the use of alumina, zinc oxide or titania supports) with a further examination of Au-ceria systems has been reported [155]. Amorphous manganese oxides prepared by co-precipitation showed excellent activity due to the mixed oxidation states present. The surface structure of the support was found to play a role in the oxidation reaction. Au//γ-MnO2 was found to be a more superior catalyst than the Au/β-MnO2 catalyst. The gold-based catalysts proved to be superior to Ce/MnO2 catalysts. Ce-based catalysts were less active for the oxidation of 2-propanol than the Au-based samples. Overall, it was determined that the catalytic activity of gold-based catalysts depends on the nature of the support and the nature of the VOC. The order of reactivity observed over the same catalyst was 2-butanol > 2-propanol > toluene. The Au/CeO2 catalyst was found to exhibit superior catalytic activity towards aromatic VOC oxidation. Au/MnOx catalysts showed better CO2 selectivity than Au/Al2O3 and Au/ZnO catalysts.
\nOne of the applications for which low-temperature oxidation is important is automotive pollution abatement. The three major pollutants emitted by internal combustion engines are carbon monoxide, non-methane hydrocarbons and nitrogen oxides (NOx). Environmental legislation governing the emission of these pollutants is becoming increasingly stringent. To comply with these regulation standards, highly efficient catalysts for complete elimination of these compounds are needed. Catalysts that are capable of removing these pollutants simultaneously are generally referred to as three-way catalysts (TWCs) and the design of these catalysts is continually evolving to meet lower emission requirements. For the cleaning of exhaust from diesel engines, particulate matter, especially carbonaceous particulates, also needs to be considered. The platinum group metals (platinum, palladium and rhodium) are the essential constituents of automotive catalysts in the catalytic converter. The support consists of zirconia-stabilised ceria, zirconia and γ-Al2O3 with barium oxide and zinc oxide as promoters [102]. However, low-temperature start-up of catalysts remains an area in which there is need for improvement. PGM-based TWCs are not very efficient during low-temperature start-up and prolonged idling mainly because the catalyst monolith does not operate until light-off temperature (∼300°C) required for oxidation of hydrocarbons is attained. Gold-based catalysts have the ability to be active at relatively low temperatures and be used in conjunction with the TWCs in automotive catalysts systems to overcome the cold engine problem. In addition, Au-based catalysts have shown activity for the lean-burn reduction of NOx, at both high and relatively low temperatures.
\nThe most common industrial-scale process for the production of hydrogen and synthesis gas is steam-methane reforming (SMR) [156]. The main products of SMR are CO, CO2 and H2, which are produced according to Eqs. (2) and (3). The hydrogen and syngas obtained from SMR are used as raw materials in the ammonia, methanol and Fischer-Tropsch syntheses, as well as reducing feed in steel production [156–158].
\nThe reaction represented in Eq. (2) is a composite reaction: first, methane is dissociated on the surface of a catalyst leading to hydrogen formation; second, the remaining carbon is oxidised by water to form additional molecular hydrogen and carbon monoxide [158]. This multistep process is highly endothermic and hence is favoured by high temperatures. It has been reported that with H2O/CH4 feed ratios in the range 3–4, up to 80% of methane conversion can be achieved at 850 °C [159]. However, although these elevated temperatures are desirable for improved reaction rates without being limited by thermodynamics, simultaneous disproportionation of carbon monoxide (Boudouard reaction) and CH4 decomposition can take place [160]. The disproportionation reaction is highly thermodynamically favoured. These two processes are represented by Eqs. (4) and (5), respectively.
\nThese reactions are undesirable because they lead to the formation of whisker carbon and other kinds of carbon deposits which are detrimental to the activity of SMR catalysts [161]. A lot of research has been done in the past three decades in trying to solve the problem of coke formation over SMR catalysts [158, 162–167]. Most of these studies focused on Ni-based catalysts since Ni exhibits high activity for SMR and Ni is cheaper (albeit less active) compared to the traditional platinum group (Pt, Ru, Rh and Pd) metals.
\nSeveral ways of alleviating the problem of coking of Ni surfaces have been explored experimentally and theoretically. Based on density functional theory (DFT) calculations, boron was proposed as a viable promoter for improving coking resistance of Ni-based catalysts [168]. The addition of alkali metal salts also improves coking resistance, although at the expense of reforming rate [169–171]. Alloying Ni with slight amounts of Au was proposed to be another viable method of improving resistance towards graphitic carbon formation [171, 172]. Bengaard et al. [171] concluded that there are at least two kinds of active sites with different reactivities for SMR on a Ni-based catalyst: a more active site associated with defect (step) sites and a less active one associated with close-packed facets. The step sites were suggested to be the nucleation sites for graphite formation and these sites could be blocked by additives such as K, S and Au, which preferentially bind to the step sites of Ni.
\nIn order to elucidate how Au can influence the reactivity of Ni catalysts, we have used carbon monoxide as a probe molecule in both Monte Carlo simulations and combined quantum mechanical (QM) and molecular mechanics (MM) force field calculations on a Ni particle with close-packed facets and defect (step) sites. The Ni nanocluster used to mimic a Ni catalyst was made up of 393 atoms (diameter ≅ 2.5 nm) with pyritohedral symmetry (Th). The cluster was alloyed with 24 gold atoms on the step sites without distorting the Th symmetry. Figure 1 shows the Ni393 cluster as well as the alloy Au24Ni369 cluster. In this geometry the clusters have three possible adsorption sites: the step site and two low index close-packed, (1 1 1) and (1 0 0), facets.
\nCPK and wire-frame models of Ni393 (left) and Au24Ni369 (right). All the atoms on the Ni393 cluster are Nithe different colours are just for illustration purpose: brown atoms show (1 0 0); pink atoms show the (1 1 1) facet. The step sites are also shown.
All calculations were done using commercially available software programs from Dassault Systèmes Biovia Corp. and graphical displays generated with Materials Studio. The Forcite module in Materials Studio was used to optimise the geometry of the clusters (and CO) prior to the Monte Carlo simulations and QM/MM calculations. The universal force field was used in all calculations to describe approximately the potential energy hypersurface on which the atomic nuclei move. In order to find low-energy adsorption sites on the clusters, the metaheuristic simulated annealing algorithm, which uses a canonical Monte Carlo sampling of the search space, was applied using the Adsorption Locator module within Materials Studio. The annealing simulation predicted the step site as the preferred adsorption site for CO on both clusters. The QMERA module, at fine setting, was used to further optimise the geometry of the adsorbed CO molecule. QMERA employs DMol3 as the QM server and GULP for the MM calculations. The CO ligand was treated at LDA/PWC level of theory (with DNP basis set) as the QM part of the calculation. All the metal atoms were treated as the MM part of the calculation with the universal force field. Figure 2 shows the final configuration of the adsorbed CO ligand as optimised by the QM/MM settings in QMERA.
\nOptimised configuration of CO on step (bridge) site of Ni393 (left) and Au24Ni369 clusters.
The binding energy and vibrational frequency of CO are summarised in Table 1. Binding energy was described as
\nwhere BE is the binding energy and E_tot is the total energy of the fully relaxed CO adsorbed on the cluster. The binding energy results indicate that CO binds slightly more strongly on the step site of the pure Ni393 cluster than on the same site of the Au24Ni369 alloy. However, the DFT that predicted stretch vibrational frequency of CO (ωCO) adsorbed on the step site of the Au24Ni369 alloy is less than that of its counterpart on the pure Ni393 cluster by 31 cm−1. The implication is that the C–O bond is weaker when it is adsorbed on the step site of the Au24Ni369 alloy than when the molecule is adsorbed on pure Ni393 cluster. This result, where the same ligand exhibits different electronic characteristics as a result of adsorption site modification, indeed indicates that Au can influence the reactivity of Ni if the two metals are alloyed where Au is more of a promoter (i.e. only small amounts of Au used).
\n\n | Adsorption site | \nNi–CO bond Å | \nC–O bond Å | \nωCO (cm−1) | \nBinding energy (kcal/mol) | \n
---|---|---|---|---|---|
Ni393 | \nStep (twofold) | \n1.722 (bond 1) 1.763 (bond 2) | \n1.203 | \n1905 | \n176 | \n
Au24 Ni369 | \nStep (twofold) | \n1.892 (bond 1) 1.892 (bond 2) | \n1.211 | \n1874 | \n174 | \n
Binding energy and C-O stretch vibrational frequency.
Indeed, Nørskov and co-workers [172–174] have carried out surface science, theoretical and microkinetic studies whose findings suggested that alloying Ni with Au was the main reason for the observed suppression of graphite formation in their systems. The recent work of Lazar and co-workers [175–177] has further strengthened the argument that alloying Ni with Au leads to improved CH4 conversion, higher selectivity to CO2 and an improved H2 yield at low temperature (T < 873 K). At a higher temperature, ca 973 K, the Au additive had no significant effect in H2 production and deactivation was accelerated under their testing conditions. This result is in agreement with the earlier findings of Chin et al. [178], who found a lower initial activity and deactivation rate resulting from Au promotion of their Ni catalyst. Our QM/MM findings in this work have shown that alloying Ni with Au on the step sites weakens the CO bond (relative to pure Ni) and this system might accelerate CO disproportionation (Eq. 3) at high temperatures, which would explain the negative effect of high temperature on AuNi systems for SMR. In conclusion, a lot of studies report positive effects of Au promotion of Ni-based SMR catalysts; however, only a few report negative results which can be linked to accelerated graphite formation as a direct consequence of Au presence. Our conclusion is that whether or not Au acts as a promoter for SMR or a catalyst for accelerating the competing disproportionation reaction is partly dependent on the process conditions.
\nThe research efforts in gold catalysis continue at an impressive level. It is true that a more sober view is now being taken concerning the commercial exploitation of gold catalysts as issues such as cost, longevity, stability in storage and others are being more seriously considered. Our increasing understanding of the way in which gold catalysts operate and our ability to manipulate particularly the activity and stability of nanogold could pave the way for realising improved commercialisation. An ever widening array of support materials is now being studied together with very important aspects such as the promotion of gold and also the use of gold in bi- or multi-metallic systems. The debate as to whether high-temperature uses of gold catalysis can be realised continues and it is suggested that recent advances in arriving at formulations that are catalytically active and that exhibit very high thermal stability have thrown this field wide open again for critical examination and further exploration.
\nSpatial competence is essential in everyday life for numerous human activities, as it entails the ability to understand and internalize the representation of the structure, entities, and relations of space with respect to one’s own body [1, 2]. Despite the fact that spatial competence encompasses a diverse set of skills, research in the field has generally focused on identifying the developmental steps that are necessary to acquire from an early age the ability to reason about spatial properties of the environment.
\nThere is a general consensus on the crucial role of visual experience in guiding the maturation of spatial competence [3]. Vision takes advantages respect to other senses in encoding spatial information because it ensures the simultaneous perception of multiple stimuli in the environment despite the apparent motion of the array on the retina during locomotion enabling us to extract more invariant spatial properties from the surrounding layout [4, 5]. Indeed psychophysical data indicate that when sensorial conflict occurs, audition and touch are strongly biased by simultaneously presented visuospatial information, suggesting that sighted people tend to organize spatial information according to a visual frame of reference [6, 7, 8, 9, 10, 11, 12]. Neurophysiological data further confirm the view by suggesting that the visual feedback is fundamental for spatial learning [13, 14, 15, 16, 17, 18], i.e., visual experience allows the alignment and thus the integration of auditory and visuospatial cortical maps [19, 20, 21, 22]. Thus, research on sighted individuals suggests that vision typically provides the most accurate and reliable information about the spatial properties of the external world, therefore it dominates spatial perception. Consequently, if visual experience is necessary to adequately represent spatial information, we would expect blind people to perform worse than sighted people in spatial tasks. This would be especially true if the visual impairment emerges at birth, when multisensory communication is fundamental for the sensorimotor feedback loop that contributes to the development of spatial representations [23, 24].
\nDespite valuable insights into the important guiding role of vision on spatial development, contrasting results indicate that visually impaired people can manifest or enhanced either impaired skills depending on the spatial aspects investigated, leading to the hypothesis that vision could have an essential or facilitating role depending on the nature of the spatial task that individuals carry out [14]. A clearer definition of the underlying processes involved in spatial competence enhancements and deficits caused by visual loss is important not only to quantify to what extent the perceptual consequences of early blindness translate to real-world settings but also to develop effective rehabilitation tools and technologies to improve their spatial skills [25]. Indeed, scientific findings related to spatial competence development in the absence of visual experience have important implications for clinical outcomes and for the design of new rehabilitation activities meant to activate compensatory strategies since an early age.
\nThe first developmental theory of spatial competence was proposed by Jean Piaget and his colleagues [26, 27, 28], who hypothesized that spatial understanding gradually improves with age thanks to a progressively more conscious interaction with the external world that permits to accumulate sensorimotor experiences such as reaching. Nonetheless, the identification of the starting points for spatial development remains one of the most debated topics within the literature of spatial competence.
\nWhile some researchers argue for innate knowledge of spatial understanding in humans [29] by reporting impressive spatial abilities in infants, other researchers advocate for a gradual acquisition of spatial competence during childhood [30] by reporting significant limitation of early spatial skills during infancy. For instance, several studies have demonstrated that already at 3 months infants are able to represent categorical spatial information by distinguishing between above vs. below and left vs. right [31, 32] and that by 5 months of age babies are sensitive to metric properties of space being able to code spatial object dimensions such as height [33, 34, 35], distance location [36], and angles [37]. Conversely, other studies indicate that while sensitivity to spatial properties appears in early infancy, further refinement of spatial accuracy emerges later during development. For instance, coding of categorical and metrical information improves through the primary school years [38, 39, 40] as well as capabilities of estimating and reproducing object size and location [41, 42].
\nThe question of whether spatial capabilities are innate or acquired is of central importance to understand if an early sensory deprivation can negatively impact on the acquisition of adult-like competences. In the case of blindness, a key developmental acquisition is the ability to code auditory and tactile spatial properties of the environment in order to independently orient and navigate in space. Research on auditory spatial perception has shown that sighted infants already possess the ability to differentiate acoustic information and perform adequate actions in different dimensions [43]. Indeed they can turn their heads toward a sound from the moment they are born [44, 45] and at the age of 4–5 months, head-orientation movements become even faster and more precise than in the neonatal period. Further improvements in the ability to code the location of sonorous objects in space manifest at 6 months of age, when infants are sensitive to changes in the location of sounds as small as 13–19 degrees [46, 47]. Nonetheless, this reflexive orientation to sound sources is present at birth but disappears during the first month if large movements of the head are required [48] to appear again at 4–5 months of age: for this reason, it has been hypothesized that the early orientation reflex represents the activity of lower brain stem and provides an initial stage to acquire spatial competence [49] that is later consolidated through concrete experience.
\nIn the spatial cognition domain, two main distinctions can be made about spatial representations of the environment [50]. The first distinction is between the egocentric and allocentric frame of reference which indicates the strategy to code location of objects, respectively, in a viewer-dependent or a viewer-independent manner. While the egocentric representation is tied to the observer and can be used either when the observer remains stationary or when the observer moves keeping track of the movement (dead reckoning or path integration), the allocentric representation does not depend on the viewer’s current position but on external landmarks that can be adjacent (cue learning) or distal (place learning). Although early spatial representations were originally described as purely egocentric [51], several studies indicated that infants can make use of both intrinsic and external features of the environment to locate objects. There is evidence that infants can update egocentric representations by keeping track of their movement and thus locate objects from novel positions within the first year of life: indeed by 9 months, infants can compensate for simple changes in their position, such as translation along a straight line [52] or rotational movements [53]. Nonetheless, for more complex displacements, infants manifest a general difficulty in keeping track of their changing relation to target location. For example, at 12 months of age, they start to solve complex problems involving both translation and rotation but they perform better when they can make use of adjacent landmarks embedded in the environment [54], and this ability seems to show little improvement between 16 and 36 months [55]. Moreover, previous research has shown that sighted infants reach for sounding objects in the absence of visual clues [47, 56, 57, 58, 59], implying that a sense of auditory space is well consolidated at this stage since sounding objects are localized in relation to one’s body. The allocentric strategy seems to emerge quite early in the development together with the egocentric strategy, but with different maturational rates for cue learning and place learning types of coding. Indeed, studies employing paradigms where the direction of looking from a novel position indicate where infants expect to see an engaging stimulus demonstrate that by 8.5 months of age, infants use an adjacent salient landmark to locate the stimulus, whereas only at 12 months of age, they consistently use relational information of distal landmarks [54]. Several studies confirm the idea that egocentric and allocentric strategies continue to refine during childhood by showing that at 18–24 months of age, toddlers become able to use geometrical cues such as shape to orient themselves [60, 61]. Nonetheless, an important milestone such as the ability to integrate different reference frames within a common system of spatial representation in order to increase accuracy and reduce the variability of spatial judgments emerge only later during the development. Indeed, children aged between 4 and 8 years old are not able to use both self-motion and external landmarks as egocentric and allocentric information, respectively, to reproduce object location because they alternated both strategies instead of combining them as adults usually do [62].
\nThe second distinction in the spatial cognition domain is between categorical and metric spatial representations, which, respectively, represent the coding of spatial information in a relative manner by means of comparisons among entities in space and the coding of spatial information in external coordinates by means of metric cues such as distance or length. It has been shown that at 7 months of age, infants spontaneously show categorical dichotomous discrimination of auditory space by differentiating objects within and beyond reach [57, 58] and by distinguishing spatial categories such as above vs. below and left vs. right [32, 63]. Early sensitivity to metric cues has been observed in 4.5–6.5 months old infants for the dimension of objects [64] and distance [36]. Nonetheless, methodological issues have been raised for the interpretation of such results since experimental paradigms typically used with infants employ observational measures of the infant’s behavior that may reveal more low-level perceptual rather than conceptual representation. Indeed, it has been shown that at the age of 2 years, children are able to match objects by height when these objects are presented in containers of a fixed height, but not when they are presented without containers, indicating that toddlers make use of distance cues only when they can rely on relative cues [65]. A considerable improvement in the ability to code object size and location can be observed between the ages of 4 and 12 [40, 41, 42, 66], for example, in tasks that require to use a configuration of distal landmarks to infer object location [67]. This could be due to the development of a hierarchical coding system, which integrates metrics and categorical information [68]. Given the time course of spatial cognition development and the discrepancy between early and later acquisition of spatial skills, an interactionist approach has been proposed that acknowledges strong potentiality and tries to identify underlying mechanisms implicated in the transformation of early abilities into mature competence [69]. The underlying mechanisms responsible for the refinement of spontaneous spatial orientation skills might be found both in the biological and environmental experiences. Within the biological context, many improvements in spatial functioning have been associated with the maturation of specific brain regions such as the hippocampus. For instance, the maturation of the hippocampus-mediated ability to encode relations among multiple objects may determine an increase in the number of stimuli that children rely on during reorientation and navigation tasks [70]. Within the environmental context, experience involves interactions with objects in the physical world and learning conventional information about symbolic spatial representations, such as maps and models. Spatial competence is strictly dependent on experiential factors such as exploratory activities which are in turn related to the development of locomotor activities. For example, it has been suggested that the emergence of allocentric coding in the form of cue learning might derive from the onset of crawling around 8–9 months, while further locomotor experiences may facilitate place learning by stimulating children to observe and approach object arrays from different directions. Indeed, locomotion is not simply a maturational precursor to psychological changes, but it plays a crucial role in their genesis [71]. For example, crawling provides the infant with concrete experiences that may change his coding strategy, for example, permitting the infant to abandon an egocentric body-oriented localization of objects to one based on the use of environmental landmarks. Recent findings suggest that sighted children acquire spatial capabilities thanks to the reciprocal influence between visual perception and execution of movements [72]: children monitor the success of action through a sensory-motor feedback by matching expected and observed changes of visual information. Indeed, self-generated movements commonly help to perceive the space acoustically because they convey the proprioceptive sensation corresponding to the movement of the ears toward sound sources [73]. In other words, using the dichotomy between the body and its exterior, an individual acquires spatial competence through observation of the body’s actions and the resulting sensory consequences: through self-generated movements, the nervous system learns sensorimotor contingencies [74], which reveal the spatial properties of the auditory space. Moreover, acting successfully entails affordances for action: since affordances change according to action capabilities and bodily characteristics, experiential factors are necessary especially during infancy when new skills are constantly appearing and bodily dimensions are changing rapidly [75].
\nThese results suggest that early interaction between the visual input and other sensory and motor signals provides a powerful background to shape the development of spatial cognition in sighted children. But if vision is so important, how spatial development changes when the visual input is missing?
\nWhile the development of spatial cognition has been extensively studied in sighted individuals [50], less effort has been spent in understanding how the sense of space changes during development in children with visual impairment. Specifically, scientific research on the development of auditory localization skills in visually impaired children has provided contrasting results. For example, it has been shown that children with visual disabilities have an excellent spatial hearing, measured as the ability to discriminate differences in sound localization in the horizontal and vertical plane as well as the ability to reach or walk toward the sound source position [76]. On the contrary, several studies suggested that infants and children with severe congenital blindness have a developmental delay in sound localization abilities [23, 77, 78, 79] and motor responses to sound [80, 81]. For example, blind children do not reach for objects that produced sounds until the end of the first year, while sighted children start around 5 months [82]. Similarly, blind children show worse performances than sighted children in auditory bisection, minimum audible angle tasks [23], and audio depth tasks [78]. Other studies show mixed results, indicating that children with congenital visual disabilities show an initial neuromotor developmental delay but compensate for the lack of vision developing good manipulatory and walking skills thanks to the exploration of sounding objects in the environment [83]. Studies of proprioceptive localization of immediate and memorized targets have been used to compare the proprioceptive performance of sighted and blind individuals. For instance, it has been shown that early visual deprivation does not necessarily prevent the development of spatial representations in both early blind children [84] and adults [85]. Considering that spatial competence emerges gradually thanks to the reciprocal influence between visual perception and execution of movements [72], it is evident that visually impaired children not only lack the visual input necessary to establish the sensorimotor feedback that typically promotes spatial development, but also manifests a general delay in the acquisition of important locomotor and proprioceptive skills, which may cause them to accumulate much less spatial experience compared to their sighted peers [79, 86, 87]. It has long been known that the development of blind infants is delayed in self-initiated postures and locomotion [79, 88, 89]. While sighted children typically start to perform first individual actions and navigation from the first year of age, blind children without cognitive and motor impairments start to walk at about 30–32 months of age [90]. Moreover, from the first month of life, blind infants show delays in the vestibular and proprioceptive functions due to the lack of integration with the visual inputs typically provided during the development [91]. Finally, since visual feedback represents the most important incentive for actions and thus for the development of locomotion and mobility skills, the onset of several motor milestones (e.g., rolling, crawling, standing, and balancing) can be delayed in visually impaired infants [92, 93], suggesting that the visual feedback of the body is fundamental for the development of self-concept.
\nTo perceive space, visually impaired children typically use hearing and touch. Despite the haptic sense provides essential information about the spatial layout of peripersonal space, such as the size, shape, position, and orientation of objects within reach, it typically conveys information only within the scope of the body. The case of hearing is particularly interesting because the auditory sense is not only the main channel for providing distal information but also it might be superior to all other sensory alternatives because it provides spatial information in both active and passive conditions and it does not necessarily involve direct contact with objects [94, 95]. At the same time, the use of hearing to perceive distal information might be particularly difficult for visually impaired children because in this case, they do not have any sensory feedback about sonorous objects in the far space. On the contrary, the haptic-proprioceptive system can provide accurate spatial data only within the scope of the body itself [96], and therefore a blind person must actively move in the environment to sequentially touch all the stimuli embedded in space. Several factors may contribute to increasing the difficulty in interpreting such contrasting results. For example, many studies on spatial hearing have been conducted within the framework of broader research on cognitive and motor skills development [87, 97] and reaching mixing the motor and the perceptual component of the observed behavior [83, 98]. In addition, different methodological approaches and stimuli have been used to assess similar aspects of auditory spatial perception: for instance, studies performed on visually impaired children under 3 years of age do not employ psychophysical procedures but they frequently use the sound of familiar voices or toys to gather information about auditory localization abilities in blind children [97]. In addition, in some cases, sighted and blind groups of children are not perfectly matched for age range and sometimes use also adults as comparison [76]. Finally, the difference between early and later loss of vision has not been often considered: many studies mix data from children with no visual experience with those of children with partial visual experience in the first period of life [76]. Instead, it has been demonstrated that the onset of blindness has a strong impact on spatial performance in adulthood: for example, late blind individuals who lost vision later in life after a normal visual experience during the first year of life perform equally or even better than sighted participants in several auditory spatial tasks (1, 50, 83, and 300). To summarize, although compensatory mechanisms for spatial perception have been demonstrated in blind adults, it is not clear whether an early visual impairment might delay the development of special auditory spatial skills. The development of spatial cognition is strictly related to the development of social cognition: the ability to independently navigate and orient ourselves in space facilitates engagement in social interactions. Indeed, a delay in the acquisition of language, motor or cognitive skills can have a direct impact on a child’s social competence (106, 109, and 246). More recent works highlighted that preschool-age children with visual impairments often have difficulties engaging in positive social interactions, making their assimilation into preschool programs difficult. In fact, many do not display a full range of play behaviors [99, 100, 101, 102, 103] and spend more time engaging in solitary play interacting more with adults than with their sighted peers [81, 87, 89, 102, 103, 104, 105, 106, 107]. Considering that the interaction among peers is essential for the development of cognitive, linguistic, social, and playing skills [108], the aforementioned delay in the acquisition of social competence in visually impaired children gives rise to feelings of frustration, rather than self-efficacy and independence which characterize the social experience of typical children. Indeed, the lack of visual information during early infancy often constitutes a risk for the development of the personality and emotional competence [89]. Nonetheless, when assessing social competence in visually impaired people, some other factors resulting from the loss of vision should be taken into account. For example, it has been shown that parenting style influences the socio-emotional development of sighted children [109, 110, 111, 112, 113] because parents represent the first influential setting that can produce appreciable differences in developmental outcomes in terms of psychological functions [114, 115]. Inconsistent, hostile and nonsensitive parenting behaviors have been associated with adjustment problems and social adversity during childhood [116, 117] and also with anxiety, depression, and other stress-related illnesses during adolescence [118, 119] and adulthood [120]. We speculate that a similar influence of parenting style holds also for blind children, especially because families of children with visual disabilities are more prone to experience various stressors such as concerns about the social acceptance of the child [121] and to face difficulties in initiating and sustaining social interactions [122], thus they might easily develop an overprotective behavior that negatively influences the social development of the visually impaired child. The negative effects of blindness on socio-emotional competence can be observed also in adulthood, with the impoverishment of the ability to perform everyday activities both in private settings like home and in public settings like workplace. Importantly, the decrease of functional abilities has been linked to the emergence of serious psychological problems in the blind population [123]. Indeed adults with visual impairments tend to feel more socially isolated and not properly supported compared to sighted individuals [123, 124, 125, 126] and are at higher risk of developing depressive symptoms [105, 125, 127, 128, 129, 130, 131], principally because social competence depends on the ability to utilize visual cues [132]. Overall, several scientific findings suggest that visual impairments, especially if acquired later in life, can have profound consequences for the physical functioning, psychological well-being, and health service needs of older adults [133]. Consequently, early therapeutic interventions specifically focused on activities fostering the development of perceptual and motor abilities would improve the quality of life of children and adults with visual impairments. In the next section, we will present some tools developed to improve perceptual skills of visually impaired individuals and propose a new solution we recently developed for early intervention in visually impaired children.
\nThe acquisition of spatial competence is typically a good indicator of the future ability to independently navigate in the environment and engage in positive social interaction with peers. While for sighted individuals, the visual feedback represents the most important incentive for actions and thus for the development of mobility and social skills, visually impaired individuals strongly rely on auditory and tactile landmarks to encode spatial and social information. Thus, the creation of technological devices to support visually impaired children in their spatial and social development would be a need. Nonetheless, despite the huge recent advancements in technological industry, most of the devices developed so far to address visually impaired population’s needs are not widely accepted by adults and not easily adaptable to children [134].
\nAs reported in the previous sections, visual impairments can determine spatial and social impairments during development. Technological support for the blind should fulfill two different but complementary tasks: the first is to substitute the absent sensory information (vision) with other sensory signals (audition and touch) for daily activities, and the second is to support the rehabilitation of impaired functions following sensory loss. This latter aspect is particularly important when the visual impairment occurs during the first year of life, because technological devices might represent an opportunity for children to develop perceptual and cognitive abilities by compensating for the sensory deprivation. Most of the technological supports developed to date have fulfilled mainly the first task, namely the substitution of vision with other modalities for everyday tasks such as object recognition.
\nSensory substitution devices (SSDs) convert the stimuli, normally accessed through one sensory modality, into stimuli accessible to another sensory modality. Specifically, sensory substitution devices for visually impaired individuals aim at supplying the missing visual information with visual-to-tactile or visual-to-auditory conversion systems [135]. Typically, substitution systems based on visual-to-tactile conversion transforms images captured by a camera into tactile stimulations directed to users. From the first device developed in the mid-1960s by Bach-y-Rita (Tactile-Visual Sensory Substitution device or TVSS), that converts signals from a video camera into tactile stimulation applied to the back of the subject allowing for the recognition of lines and shapes [136], recent technological progress allowed the development of much smaller, portable, and wearable devices. For instance, wristbands, vests, belts, and shoes which allow hands-free interactions [137] and devices that can be placed on various body surfaces (e.g., fingers, wrist, head, abdomen, and feet) [138, 139]. Conversely, systems based on visual-to-auditory conversion transform the images captured by a camera into sounds transmitted to users via headphones. One of the most famous visual-to-auditory devices is the vOICe developed by Meijer [140] that associates height with pitch and brightness with loudness in a left-to-right scan of the visual image.
\nIn our recent review, we listed the SSDs designed for visually impaired individuals by highlighting their main features and limitations for daily use [134]. In particular, we identified six main limitations that might determine low acceptance rate in adults and low adaptability in children:
Invasiveness: SSDs can be physically invasive in the sense that in order to be used, they must be positioned on crucial body parts (e.g., ears or mouth), thus limiting perceptual functions in users or they must be transported (e.g., in backpacks), thus limiting users’ navigation for weight and size;
Extensive training: SSDs typically require long periods of training in order to be used because users need to learn how to interpret the output of the device, which is typically not immediate (e.g., sound loudness corresponds to pixel brightness in the vOICe [141]);
High cognitive load: SSDs usually require high attentional resources, which makes it difficult for the user to focus on the main task they are performing when using the device;
No clinical validation: SSDs frequently remain prototypes and do not reach the blind users market, principally because they are not validated on large sample patients through standardized clinical trials;
Artificiality: SSDs are generally based on the idea that users can understand the properties of visual stimulus by listening (in the case of visual-to-auditory SSDs) or feeling (in the case of visual-to-tactile SSDs) a stimulus resulting from an artificial transformation code, missing an important aspect of the learning process, which is the association of action and perception.
Therefore, while sensory substitution devices have been shown to provide support for specific perceptual tasks in adults [142], they have never been tested in children principally because their use might too overwhelming for children. Nonetheless, technological development should be addressed especially to visually impaired children needs because cortical plasticity is maximal during the first year of life, therefore the benefit deriving from early interventions should be higher. Moreover, technological development should lead to multimodal stimulation whose benefits have been repeatedly reported compared to unimodal stimulation [143, 144, 145], while most of the SSDs developed so far substitute the visual function with either the auditory or the tactile modality alone.
\nWith this in mind, we developed a new device for visually impaired children (Audio Bracelet for Blind Interaction, ABBI, [146]), which is an audio bracelet that produces an auditory feedback of body movements when positioned on a main effector such as the wrist in order to provide a sensorimotor signal similar to that used by sighted children to construct a sense of space. Indeed, several reports indicate that sighted children typically acquire spatial competence by experiencing visuomotor correspondences [72]. In this sense, our device could be used to align the spatial understanding between one’s own body and the external space through coupling auditory feedback with intentional motor actions. The audio movement created by the bracelet conveys spatial information and allows the blind user to build a representation of the movement in space in an intuitive and direct manner.
\nWe validated the ABBI device with a clinical trial on an Italian sample of 44 visually impaired children aged 6–17 years old assigned to an experimental (ABBI training) or a control (classical training) rehabilitation condition. The experimental training group followed an intensive but entertaining rehabilitation for 12 weeks during which children performed ad-hoc developed audio-spatial exercises with the Audio Bracelet for Blind Interaction (ABBI). The clinical trial consisted of three sessions: pre-evaluation, training, and post-evaluation. Pre- and post-evaluation sessions lasted 60 min during which a battery of spatial and motor tests were performed [147]. The BSP (Blind Spatial Perception) battery comprised six tests: (1) auditory localization: the child listens to the sound produced by a set of loudspeakers positioned horizontally in front of him/her and localizes the sound source by pointing to it with a white cane; (2) auditory bisection: the child listens to a sequence of three sounds presented successively by a set of loudspeakers positioned horizontally in front of him/her and verbally reports whether the second sound is closer in space to the first or to the third one presented; (3) auditory distance: the child listens to two consecutive sounds produced by a set of loudspeakers positioned vertically in front of him/her in depth and verbally reports which of the two stimuli presented is closer in space to his/her own body; (4) auditory reaching: the child listens to a static sound positioned in far space and reaches the position of the sound by walking toward it; (5) proprioceptive reaching: the child repeats a movement trajectory after being presented with it by an external operator; (6) general mobility: the child walks straight on for three meters and then back to the starting position at his/her own pace. The training session lasted 12 weeks and children were assigned to the experimental training condition based on activities with the use of ABBI or to the classical training condition based on psychomotor lessons not necessarily involving sound localization activities. All children enrolled in the ABBI training group performed weekly training exercises with a trained rehabilitator for 45 min (9 h over 12 weeks) and weekly training sessions with a relative at home for 5 h (60 h over 12 weeks) for a total training period of 69 h. All training exercises were developed to train children’ ability to recognize and localize sounds in space according to different levels of difficulty: (a) recognize and localize simple sound movements, such as a straight motion flow performed along the horizontal or sagittal planes in the front peri-personal space (first level); (b) recognize and localize complex sound movements, such as a motion flow performed randomly in space in the front peri-personal space, e.g., composite geometrical and nongeometrical figures (second level); (c) recognize and localize simple and complex sound movements in the back peri-personal space (third level); (d) recognize and localize simple and complex sound movements in the front and back in the extra-personal space (fourth level). The comparison of overall spatial performance before and after the training with a dedicated assessment battery indicated that the ABBI device is effective in improving spatial skills in an intuitive manner (see Table 1 for a summary of results), confirming that in the case of blindness perceptual development can be enhanced with naturally associated auditory feedbacks to body movements [148]. Moreover, the validation of the ABBI device demonstrated that the early introduction of a tailored audio-motor training could potentially prevent spatial developmental delays in visually impaired children [149].
\nScore difference (Δ) after 12 weeks training (T1-T0).
One year follow-up of the ABBI group (T2-T0). In order to evaluate the effects within groups, two-tailed t-tests assuming equal variances were performed between groups at baseline (T0) and post-training period (T1). Changes in the outcome measures were then calculated between baseline (T0) and post-training period (T1) in the ABBI training and classical training group (ΔΑ and ΔC), and between baseline (T0) and follow-up period (T2) in the ABBI training group (ΔΑ2). Data are presented as mean and standard deviation. The stars indicate the statistical significance of the corresponding t-test of the score difference (*p < 0.05; **p < 0.01; ***p < 0.001). Table readapted from [148].
Visual experience is deemed to be fundamental for the acquisition of spatial competence; indeed, visually impaired children tend to manifest impairments in spatial and locomotor skills, causing a general developmental delay. The hearing sense can be boosted since an early age to foster compensatory mechanisms for the development of spatial perception, principally because compared to touch it can provide distal information [150]. There is evidence that multisensory training based on the action-perception link can improve spatial abilities in visually impaired children and prevent the risk of developmental delays and social exclusion [148, 149, 151].
\nWe would like to thank all the children and parents for their willing participation in our studies and the Unit for Visually Impaired People (UVIP) members for their passionate work on visually impaired individuals.
\nThe authors declare no conflict of interest.
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