Catalytic Steam Reforming of Methanol to Produce Hydrogen on Supported Metal Catalysts

1. Introduction

The use of fossil fuels for energy supply in the world has caused various global environmental problems. For this reason it is becoming progressively more important to find ways of providing environmentally friendly energy. One promising alternative to fossil fuels is hydrogen, due to the importance as a clean source of energy, as well as, the increased demand in chemical industry [1; 2]. Also, fuel cells have recently attracted much attention as a potential device for energy transformation. Their performance is based on a clean process, without forming harmful by-products such as sulphur oxides and nitrogen oxides, while having a highly efficient energy transformation compared to conventional power generation processes as in heat engines. Hydrogen is a promising fuel for fuel cells and can be produced by steam reforming of natural gas, methanol and gasoline. At present, most of the world's hydrogen is produced from natural gas (~97 % CH₄) by a process called steam reforming [3-8]. The primary ways in which methane, is converted to hydrogen involve reaction with either steam (steam reforming), oxygen (partial oxidation), or both in sequence (autothermal reforming). In practice, gas mixtures containing carbon monoxide, carbon dioxide and unconverted methane. Reaction of carbon monoxide with steam (water-gas shift) over a catalyst produces additional hydrogen and carbon dioxide, and after purification, high-purity hydrogen is recovered. This reaction is highly endothermic. Although stoichiometry for the SRM suggests that only one mole of water is required for one mole of methane (CH₄ + H₂O→CO + 3H₂), usually excess steam is used to reduce carbon formation. In most cases, carbon dioxide is vented into the atmosphere today, but there are options for capturing it in centralized plants for subsequent sequestration. However, steam reforming of methane does not reduce the use of fossil fuels and it still releases carbon to the environment in the form of CO₂. Thus, to achieve the benefits of the hydrogen economy, it is necessary produce hydrogen from non−fossil resources, such as water, methanol or ethanol using a renewable energy source. Among the different feedstocks available, alcohols are very promising candidates because these are easily decomposed in the presence of water and generate hydrogen-rich mixture at a relatively lower temperature. Steam reforming (SR) of methanol has been extensively studied in recent years [1–7]. Methanol has a low boiling point, a high hydrogen/carbon ratio and no C–C bonds, and can therefore be reformed at a relatively low temperature, reducing the risk of coke formation during the reaction [9]. Moreover, as methanol can be produced from renewable sources, its reforming does not contribute to a net addition of CO₂ to the atmosphere. Methanol can be converted to hydrogen by the following three reactions:

Partial oxidation of methanol

$${\text{CH}}_{\text{3}}\text{OH }+0.{\text{5O}}_{\text{2}}={\text{ 2H}}_{\text{2}}+{\text{ CO}}_{\text{2}} \Delta \text{H}°=-{\text{ 192 kJ mol}}^{-\text{1}}$$E1

Steam reforming of methanol

$${\text{CH}}_{\text{3}}\text{OH }+{\text{ H}}_{\text{2}}\text{O }={\text{ 3H}}_{\text{2}}+{\text{ CO}}_{\text{2}} \Delta \text{H}°=\text{ 5}0{\text{ kJ mol}}^{-\text{1}}$$E2

Oxidative Steam Reforming of Methanol

$${\text{CH}}_{\text{3}}\text{OH }+\text{ 1}/{\text{2H}}_{\text{2}}\text{O }+\text{ 1}/{\text{4O}}_{\text{2}}={\text{ CO}}_{\text{2}}+\text{ 5}/{\text{2H}}_{\text{2}} \Delta \text{H}°=0{\text{ kJ mol}}^{-\text{1}}$$E3

Most studies reported in the literature for the steam reforming reaction were on the application of CuO/ZnO-based and CuO/ZnO/Al₂O₃-based catalysts [10-12]. Alumina is generally added to the catalysts to improve their surface area and mechanical strength, and to prevent catalyst sintering [13]. The in situ characterization of CuO/ZnO reveals that the interaction of Cu and ZnO has a pronounced effect on the catalytic activity [14; 15]. Zinc oxide is known to improve the dispersion of Cu and the reducibility of CuO. The improvement of reducibility has been proposed as a possible cause of the good activity of CuO/ZnO-based catalysts [16]. However, some researchers have proposed that the main reason is the improvement in the adsorption properties, including the adsorption of methanol [17] and the spillover of both hydrogen from Cu to ZnO[18] and oxygen species from ZnO to Cu [19]. ZrO₂ addition to Cu-based alumina-supported catalysts has been shown to increase methanol conversion and reduce CO yields [9; 20]. However, it has been noted that the metal–support interactions in Cu/ZrO₂ are different than in the more conventional Cu/ZnO catalysts [21]. Some authors even describe a ‘‘synergy’’ between the Cu and ZrO₂[13]. The higher activity of Cu-ZrO₂ catalysts has also been attributed to the stabilization of Cu₂O on the surface of the reduced catalysts or during the reaction [22; 23]. It is believed that the formation of Cu₂O leads to both more active and more durable catalysts, since Cu₂O is less susceptible to sintering compared with Cu metal [22; 23]. Cu⁺ species have also been observed in CeO₂-containing Cu catalysts [24; 25] and isolated Cu²⁺ in lattice sites or in surface sites forming a nano-sized two-dimensional structure [26]. Addition of CeO₂ to Cu/Al₂O₃ catalysts has also been shown to increase methanol conversion, decrease CO selectivity and increase catalyst stability [27]. Due to this a strong effort has been directed to

increase the overall efficiency of CeO₂ for different applications. CeO₂ has been widely used in purifying vehicle exhausts and became the most important rare oxides for NOx reduction with CO or hydrocarbons [28; 29]. Numerous studies of ZrO₂ or CeO₂ promoted Cu-based methanol steam reforming catalysts are available in the literature [10; 12; 21; 25; 27; 30-34]. However, comparing results between studies is challenging since the reaction evidently is very sensitive to the catalysts used and large differences in Cu loadings and catalyst compositions have been reported. For example, the copper concentrations on these types of catalysts have been varied from a few percent in some publications [35; 36] up to 70% or above in others [13]. Other studies related with nickel based catalysts on the methanol reforming process has been reported [37-39] or ethanol (SRE) as an H₂ source [40-44]. Navarro et al. [45] studied the oxidative reforming of hexadecane over Ni and Pt catalysts supported on Ce/La-doped Al₂O₃. They found for both Ni and Pt catalysts, higher specific activity when active metals were supported on alumina modified with cerium and lanthanum. However, the catalytic activity and H₂ selectivity observed on Ni-based catalysts were higher than on Pt-based catalysts. Recently, Pd-ZnO catalyst systems have been reported to be active and selective for MSR after pre-reducing with H₂[46-48] and Pd supported on ZrO₂-TiO₂[49]. Other catalytic systems containing highly dispersed gold have received great interest from both experimental and theoretical points of view. The role of metal oxide is to stabilize the gold nanoparticles and make the reaction take place on the gold surface reaction. Due to their high catalytic activity, particularly in CO oxidation at low temperature [50] gold-base catalysts are considered promising candidates for hydrogen production, through methanol decomposition and water-gas shift (WGS) reactions [51-54] catalytic combustion of volatile organic compounds (VOCs) [55], selective oxidation of CO in H₂-rich gas [56], adsorption ofCO on Au/CeO₂ catalysts [57]. Methanol steam reforming for H₂ production has been not studied extensively with Au-base catalysts. Nevertheless, some gold-base catalysts has shown high activity for methanol oxidationat 373 K but low H₂ production as a function of time on stream [58]. On methanol decomposition was reported that gold supported on Al₂O₃ was most active than on the CeO₂, however, on the last catalyst the H₂ selectivity was better than on the former catalyst on the range temperature of 300 to 500 °C [58]. But, when water was added in the feed they observed a slight increase in the methanol conversion and, changes in the products distribution. The catalytic activity of the Au-Ag/CeO₂ catalyst and silver supported on ZnO 1D rods catalysts on the steam reforming methanol reaction for hydrogen production was reported [59-61]. The catalytic activity on Ag/ZnO sample with low Ag content showed better performance on the SRM reaction than on high silver loading catalyst. So, the sample with small Ag particle size showed best performance in methanol conversion than catalyst with big Ag particle size. Our group has previously studied the effect of nickel-copper addition to ZrO₂ by impregnation method and compared the catalytic activity of these bimetallic Cu/Ni catalysts on the oxidative steam reforming of methanol to produce H₂[39]. The reactivity of the catalysts showed that the bimetallic samples prepared by successive impregnation had highest catalytic activity among all the catalysts studied.

The goal of this chapter is showed the effect of the metal copper or nickel addition to CeO₂ prepared by co-impregnation and sequential impregnation. Catalytic performance in oxidative steam reforming of methanol for the three Cu–Ni catalysts was compared with corresponding monometallic Cu and Ni catalysts, and Au/CeO₂ catalysts. The comparison is also made with characterization results obtained by BET (N₂ adsorption–desorption), SEM (Scanning Electron Microscopy), EDX (Energy Dispersive X-ray Spectroscopy), XRD (X-ray Diffraction), TEM (Transmission Electron Microscopy) and TPR (Temperature Programmed Reduction). In addition, the relation between the structure of bimetallic particles and catalytic performance in oxidative steam reforming of methanol is discussed.

2. Experimental

2.1. Synthesis of the catalysts

The CeO₂ synthesis was done using the precipitation method of the Ce(NO₃)₃•6H₂O (Aldrich) in NH₄OH (Fluka)at room temperature (r.t.).

$$\text{8Ce}{\left({\text{NO}}_{\text{3}}\right)}_{\text{3}}•{\text{6H}}_{\text{2}}\text{O }+{\text{ 22NH}}_{\text{4}}\text{OH }\leftrightarrow \text{ 8Ce}{\left(\text{OH}\right)}_{\text{4}}\downarrow +{\text{ 23NH}}_{\text{4}}{\text{NO}}_{\text{3}}+{\text{ 41H}}_{\text{2}}\text{O}$$E4

$$\text{Ce}{\left(\text{OH}\right)}_{\text{4}} \stackrel{\Delta }{\to }{\text{CeO}}_{\text{2}}+{\text{ 2H}}_{\text{2}}\text{O}$$E5

The solid obtained was dried at 100 °C and then heated at 650°C for 5 hours in air stream. The prepared supports were impregnated with a solution of NiCl₂•6H₂O, and another with a solution of Cu(CH₃-CO₂)₂•H₂O at an appropriate concentration to yield 3 wt% of copper and nickel respectively. Three bimetallic samples were prepared at 50%Cu and 50%Ni respectively to obtain 3 wt. % of total metallic phase. For the first sample, CeO₂ support was successively impregnated with an aqueous solution of Cu(CH₃COO)₂•H₂O (Merck), after that, the excess of water was removed at 80 °C under constant stirring and the catalyst was dried at 110 ºC and calcined at 500 °C for 2 h followed by cooling down to r.t. Then, an aqueous solution of NiCl₂•6H₂O was added and the resulting solid was calcined at the same temperature and time. The as prepared catalysts will be referred as Ni/Cu/CeO₂. For the second catalyst, the synthesis procedure was changed to the above sample mentioned. The labeling of this catalyst will be referred as Cu/Ni/CeO₂. The third sample (Cu-Ni/CeO₂) was prepared by using a simultaneous impregnation (also called co-impregnation): an aqueous solution of Cu(CH₃COO)₂ and NiCl₂•6H₂Owere added to CeO₂ and calcined at 500 °C for 2 h. All the samples were reduced at 400 °C using a mixture of H₂ (5%)/He (50 mL/min) stream for 1 h before characterization, except for TPR technique in which the sample was calcined.

2.2. Characterization

The details of catalysts characterization have been reported in our earlier reports [26; 28; 37; 38; 49; 59; 61-64]. Nitrogen adsorption-desorption of the samples was measured at -196 C on a Belsorp-max Bel Japan equipment. Prior to the measurements the samples were degassed at 150 °C for 1 h. The surface area and pore size distribution were determined using the BET and BJH methods respectively. HRTEM and local chemical analysis of the bimetallic nanoparticles were carried out in a JEM 2200FS microscope with a resolution of 0.19 nm and fitted with an energy dispersive X-ray Spectrometer (NORAN) and a JEM 2010-HT with a point resolution of 0.19 nm fitted with an EDX microprobe Thermo-scientific. JEOL-2010 microscope with a point resolution of 0.19 nm fitted with an NORAN microprobe Thermo-scientific. The samples were dispersed in isopropanol and a drop of such a solution was placed onto copper and gold 300 mesh grids. Surface properties of the catalysts were studied by CO adsorption followed by DRIFT (Fourier Transform Infrared Spectroscopy). Experiments were done in a Nicolect Nexus 470 Spectrometer equipped with environmentally controlled Spectra Tech DRIFT (Diffuse Reflectance Infrared Fourier Transform) cell with KBr windows. For each experiment, 0.025 g of the sample was packed in the sample holder and pretreated in-situ under H₂ flow (30 mL/min) at 300 °C for 1 h. After this treatment the sample was purged with helium flow for half hour and cooled to room temperature in the same gas atmosphere before admittance for 5 min a flow (30 mL/min) of 2.5 %CO diluted in He. Afterwards, pure He was allowed to flow in the system to eliminate the residual CO gas. Spectra were collected from 128 scans with resolution of 4 cm^-1. For all catalysts a FTIR spectrum was obtained by making reference to the freshly reduced solid prior to CO adsorption. The spectrum of dry KBR was taken for IR single-beam background subtraction. Oxidative steam reforming of methanol was carried out at an atmospheric pressure by placing the fixed bed flow reactor (8 mm i.d.) in an electric furnace consisting of two heating zones equipped with omega temperature controllers, using a commercial flow system RIG-100-ISRI. Prior to OSRM reaction, 0.05 g of catalyst diluted in 0.150 g of SiC was reduced in situ, using a stream of H₂ (50 mL/min) increasing temperature from room to 400 °C with a heating rate of 10 °C/min and holding this temperature for 1h. A thermocouple in contact with the catalytic bed was utilized in order to monitor and control the temperature inside the catalyst. For the reaction, O₂ (5%)/He mixture (50 mL/min) and 150 mL/min of He was passed through stainless steel saturator containing methanol and water mixture (we use a hot line in the saturator in order to maintain constant the temperature ~ 25 °C). This gas was added by means of a mass flow controller (RIG-100). The total flow rate was kept at 200 mL/min. Reaction products were analyzed by Gow-Mac 580 Gas chromatograph with thermal conductivity detector equipped with two columns system (molecular sieve 5 Å and Porapack Q columns), double injector controlled by Clarity software V.2.6.04.402 and TCD. The first column was used to separate the gaseous products such as H₂, O₂, CH₄ and CO. The second column was used to separate water, methanol, methyl formate (MF) and CO₂. All the reported data were collected after a run time of 7 h. The following equations were used to determine the methanol conversion and selectivity:

$$X(\%)=\frac{C_{in}-C_{out}}{C_{in}}*100$$

$$S_{CO}{}_{{}_2}(\%)=\frac{nC{O}_{2-out}}{nC{O}_{2-out}+nC{O}_{out}}*100$$E6

and/or

$$S_{H_2}(\%)=\frac{n{H}_{2-out}}{n{H}_{2-out}+nC{H}_{4-out}+nC{O}_2{}_{out}+CO}*100$$E7

The subscripts in and out indicate the inlet and the outlet concentrations of the reactants or products.

3. Results

3.1.Ni/Cu/CeO₂. System

3.1.1. Textural properties of the Cu-Ni/CeO₂ system

Table 1 showed the textural properties of the catalytic materials obtained from N₂physisorption measurement at temperature of liquid nitrogen. It showed that doping the bare CeO₂ support with copper or nickel to obtain the monometallic catalysts, results in a slight decrease on the BET surface area. The same effect was observed on the bimetallic samples, when Cu and Ni were impregnated by successive or co-impregnation method on the CeO₂. Typical SEM image with backscatter analysis of the as-synthesized Cu-Ni/CeO₂ catalyst prepared by co-impregnation method is present in Fig. 1. It showed that the sample is composed by irregular particles. It is important to mention that the bare CeO₂, as well as, the other catalysts under study had the same morphology, as the sample present on Fig. 1. This is expected because we used the CeO₂ previously stabilized at 650 °C to obtain the catalysts.

Sample	m^2/g
CeO2	40.1
Cu/CeO2	34.5
Cu/Ni/CeO2	25.7
Cu-Ni/CeO2	30.4
Ni/Cu/CeO2	27.5
Ni/CeO2	28.6

Table 1.

Specific surface area (BET) of the Ni/Cu-base catalysts.

Figure 1.

Typical SEM image of the fresh Ni-Cu/CeO₂ prepared by co-impregnation.

3.1.2. Crystalline phases of the Cu-Ni/CeO₂ catalysts

Fig. 2 showed the XRD patterns of the Ni/Cu/CeO₂ catalysts after thermal treatments (calcination and reduction). XRD patterns of the Ni/Cu-base catalysts yield a typical cubic fluorite structure of ceria, in addition, diffraction peak attributed to the metallic Ni was observed at 2Θ= 44.735 on the Ni/CeO₂ sample, indicating that NiO was completely reduced to metallic Ni below 500 °C. On Cu/CeO₂ sample diffraction-peaks of metallic Cu were observed at 2Θ = 43.317 and 50.449 (JCPDS 85-1326) respectively. On the bimetallic samples, diffraction peaks of Cu, Ni or Cu-Ni alloy were not observed, although the samples suffer different thermal treatments; this could be due to its low metal concentration (3.0 wt %) or because the particle size of the active phase is below of the detection limit of the technique. On samples with 3 wt. % of Cu/Ni supported on ZrO₂[39] was observed the same effect. So, no diffraction peaks of metallic phase were observed by XRD technique.

Figure 2.

XRD patterns of the Ni/Cu/CeO2 catalysts. [*] cubic -CeO2

3.1.3. Temperature-programmed reduction of the Cu-Ni-base catalysts supported on CeO₂

Hydrogen consumption curves of the fresh bimetallic Cu/Ni-base catalysts and samples after catalytic reaction are shown in Fig. 3. Although the position of the reduction peaks strongly depends of the particle size or the interaction between metal active phase and the support, the TPR profiles of the catalytic materials are included for comparison. TPR profile of the bare CeO₂ sample showed a broad peak above 500 °C, this is assigned to reduction of surface ceria. CalcinedCuO/CeO₂ catalyst showed three reduction peaks below 300 °C indicating the presence of different kinds of Cu species formed during the preoxidation step [26; 63]. Peaks below 200 °C were attributed to reduction of highly dispersed CuO and the peak above 200 °C was associated with the reduction of the CuO bulk. The NiO/CeO₂ catalyst showed a sharp reduction peak at around 400 °C and may be attributed to reduction of NiO to metallic Ni crystallites. This temperature is higher than that the reported in previous work [38] for a similar catalyst. In that case the Nickel precursor was Ni(NO₃)₂•6H₂O which indicates different interaction between NiO and CeO₂. TPR profile of the bimetallic catalysts showed reduction peaks at lower temperature than Ni/CeO₂ catalyst. It has been reported that NiO supported, could be reduced at low temperatures when Cu or Pt are presented [37; 39; 65]. Because, Cu or Pt causes spillover of hydrogen onto Ni, inducing a simultaneous reduction of both, copper (platinum) oxide and NiO, causing a shift in the reduction of the active phase at low temperatures. In addition, it has been suggested that the first reduction peak observed in the TPR profile of the bimetallic catalyst, corresponded to the reduction of adjacent Cu and Ni atoms, which could be forming a bimetallic phase [37; 39; 65]. This finding indicates that the bimetallic phase had different interaction with the support and promoted the nickel reduction at lower temperatures and slows the copper reduction. In addition, it is clear that the bimetallic samples prepared by successive impregnation showed a broader reduction peak at higher temperatures than that for the Cu/CeO₂ sample, suggesting a broad particle size distribution, and slightly lower than Ni/CeO₂ sample. On the other hand, the TPR profiles of the Ni/CeO₂ and Ni-Cu/CeO₂ samples showed a sharp reduction peak than the rest of the samples. The sharp peak observed on theses samples corresponds to high uniformity in the Ni crystallite size. TPR profiles of the samples after catalytic reaction showed lower H₂ consumption, indicating that under OSRM conditions the active phase is partial oxidized. On Cu/CeO₂ samples was found by EPR technique the presence of the ion Cu²⁺ forming a nano-sized two-dimensional structure after OSRM reaction [26; 66; 67]. Oguchi et al. [22] observed a reduction peak on the CuO/ZrO₂ sample post-reaction. They concluded that the Cu₂O catalyst was stabilized during the SRM reaction. Turco et al. [68] suggested that there was a zone within the catalytic bed where the catalyst is oxidized, and another zone where it was reduced. This phenomena could be occurred on our samples, because, generally in oxidative steam reforming process, evidence suggest that the front of the catalyst bed is partially oxidized and the downstream of the catalyst bed remains in the reduced state.

Figure 3.

Temperature-programmed reduction profiles of the fresh Cu/Ni/CeO₂ catalysts (solid line) and samples after catalytic reaction (clear line).

3.1.4. DRIFTS of CO adsorption

In situ DRIFT spectra of the monometallic Ni/CeO₂ and Cu/CeO₂ samples and the three bimetallic Cu/Ni-base samples exposed to a 2.5%CO/He gas mixture recorded at room temperature with the aim to evaluate the influence of the metal addition to CeO₂ on the type and amount of different surface species. Fig. 4 shows an infrared spectrum in the 2200–2000 cm⁻¹ regions of the Cu/Ni-base catalysts.The CO absorption band was observed at 2130 and 2100 cm^-1 on the Ni/CeO₂ and Cu/CeO₂ samples respectively.It is generally acknowledged that carbonyl bands at wavenumbers lower than ca. 2115 cm^-1 are due to carbonyl species adsorbed on metallic copper particles [69] while those at higher wavenumber correspond to carbonyls adsorbed on oxidized copper sites, so, the wavenumber increasing with the copper oxidation state. Variations in the frequency of these carbonyls have been related to changes in the nature of the exposed faces (i.e., in the degree of coordination of the copper centers). The main component at 2100 cm^-1 was associated to CO adsorption on Cu sites of stepped particles (i.e., ‹110› plane) [70]. The band at 2135 cm^-1 observed on the bimetallic samples was close to Ni/CeO2 catalyst, although it is slightly shifted to higherwavenumbers but it is virtually the same independently of the Cu and Ni addition to CO₂. In these bimetallic samples the absorption band corresponding to CO adsorption on Cu is totally suppressed (Fig. 4b). This finding suggests that the bimetallic samples are richer with Ni atoms in the surface of the catalysts. On the other hand, the high intensity in the CO-band observed on the bimetallic samples than monometallic catalysts could be associated to major dispersion of the metal active phase on the CeO₂. Differences in the CO-chemisorption were observed on the region of 1800–1000 cm⁻¹ (Fig. 4c). CO chemisorption on the reduced surface CeO₂ and Mo/CeO₂ samples showed bidentate carbonate (as-1340, s-1680 and as-1320, s-1690 cm^–1), bicarbonate (1220, s-1460, s-1490 and as-1630 cm^–1), and bridged carbonate (as-1285 and s-1750 cm^–1) [71; 72]. No bands for the carbonate-like species was detected in the region of 1800–1000 cm⁻¹ on the bimetallic samples prepared by successive impregnation as well for Cu/CeO₂ sample which indicates that CO does not adsorb on these materials.

Figure 4.

Diffuse reflectance FTIR spectra of CO adsorbed on monometallic-a and bimetallic-b catalysts. CO-absorption region of 1800–1000 cm⁻¹ on all samples-c.

3.1.5. Catalytic activity of the Cu-Ni/CeO₂ catalysts on the OSRM reaction

The effect of the Cu and Ni addition to CeO₂ was evaluated on the oxidative steam reforming of methanol (OSRM) reaction from 200 to 400 °C. Fig. 5a-b summarizes the results of the CH₃OH conversion and H₂ selectivity over various catalysts as a function of the reaction temperature. It is clear that the bare-CeO₂ showed poor catalytic activity at the maximum reaction temperature. In general in all the samples the methanol conversion increased with an increase in the reaction temperature but, it is different when Cu and/or Ni were impregnated to CeO₂. At the beginning of the reaction the Cu/CeO₂ catalyst showed better methanol conversion than the other samples. When the temperature was raising at 300 °C the Cu/CeO₂ and the bimetallic Cu-Ni/CeO₂ (prepared by co-impregnation) catalysts had the same methanol conversion (40 %). Following by the bimetallic samples prepared by successive impregnation and the worst catalyst for methanol conversion was the Ni/CeO₂ sample. At the maximum reaction temperature the methanol conversion showed the following order: Ni/CeO₂> Cu-Ni/CeO₂> Ni/Cu/CeO₂> Cu/Ni/CeO₂> CeO₂. In previous study was observed that when Ni was supported on CeO₂ it showed better methanol conversion than Ni/ZrO₂ sample [38]. López et al. [39] reported that the Ni/Cu/ZrO₂ and Cu/Ni/ZrO₂ catalysts prepared by successive impregnation, showed high catalytic activity and H₂ selectivity than bimetallic sample prepared by simultaneous impregnation and the monometallic catalysts on the OSRM reaction. They calculated the reactivity of the model catalysts prepared by successive impregnation and observed that the band gap of the bimetallic models decreases, then, an electron transfer mechanism is favored at the interface between the bimetallic structures and the support, facilitating the redox properties of the catalysts, giving a higher OSRM activity [39]. In our case, we observed that the Ni/CeO₂ and Cu-Ni/CeO₂ (prepared by co-impregnation) samples had the best catalytic activity at the maximum reaction temperature. On these samples was observed by DRIFT technique the CO-band at 1625 cm⁻¹ which was not present on the other samples. This finding can be attributed that the CO adsorption, in the carbonate species range were not favored on the other catalysts, indicating that CO does not adsorb on these materials, and so there are some blockade sites for catalytic reaction. The selectivity towards H₂ carried out at 200–400 °C on Cu-Ni-base catalysts supported on CeO₂ catalysts increased progressively by increasing the reaction temperature. It is clear that the Ni/CeO₂ and Cu-Ni/CeO₂ catalysts showed higher selectivity toward H₂ than the others samples.

Figure 5.

Methanol conversion of the Ni/Cu supported on CeO₂-(a) and H₂ selectivity-(b)

3.2. Au/CeO₂ system

3.2.1. Experimental section

CeO₂ catalyst was prepared in advance by precipitation method. NH₄OH (Baker) was added drop wise to an aqueous solution of (Ce(NO₃)₃•6H₂O). The precipitated solid was aged for 24 h and the residual liquid was removed by decanting, then the solid was dried at 100 °C for 24 h. The solid material was calcined at 100 °C 1h under an air stream and then at 500 °C for 5 h. The prepared support was impregnated with an aqueous solution of HAuCl₄ at an appropriate concentration to yield 1 and 3 wt % of total Au in the catalysts. The samples were dried at 100°C for 1h and then calcined at 400 °C for 2h in static air and finally reduced with a H₂(5%)/He stream at 350 °C 1h before the characterization and activity test. The labeling of different catalysts will be referred as follows: nAu/CeO₂ where n = 1 and 3 wt. % of Au in the catalyst respectively. The steady-state activity in the SRM reaction was performed in a conventional fixed-bed flow reactor (8 mm i.d.) using 0.1 g of the catalyst in a temperature range from 300 to 475 °C with steps of 25 °C with 6 h of stabilization time at each temperature and atmospheric pressure on an automatic multitask unit RIG-100 from ISR INC. The catalyst was first activated in a stream of H₂ (60 mL/min) from room temperature to 350 °C with a heating rate of 10°C/min and held at this temperature for 1h. A thermocouple in contact with the catalytic bed allowed the control of the temperature inside the catalyst was used. The sample was brought up to the reaction temperature in He and the reaction mixture was introduced. For the SRM reaction, He (60 mL/min, GHSV= 30,000 h^-1 based on the total flow) was added by means of a mass flow controller (RIG-100) and bubbled through a tank containing mixture of water and methanol, the partial pressure of CH₃OH and H₂O was 9999.18 and 1699.86 Pa respectively. The molar ratio in the steam was CH₃OH (1.95µmol)/H₂O (1.97 µmol) = R ≈ 1.0 and the other concentration tested was CH₃OH (4.7 µmol)/H₂O (1.97 µmol) = R ≈ 2.4. The effluent gas of the reactor was analyzed by gas-chromatography (Gow-Mac 580 instrument) equipped with a two columns system (molecular sieve 5 Å and Porapack Q columns), double injector controlled by Clarity software V.2.6.04.402 and TCD. The first column was used to separate the gaseous products such as H₂, O₂, CH₄ and CO. The second column was used to separate water, methanol, methyl formate (MF) and CO₂. The GC analysis was performed in isothermal conditions (oven temperature = 100 ◦C). The equations used to determine the methanol conversion and selectivity was showed above.

4. Results and discussion

BET surface area calculated by the N₂ adsorption-desorption through the single point method of the 1Au/CeO₂ and 3Au/CeO₂ catalysts after thermal pretreatments were 44 and 34 m²/g respectively. Figure 6 (a, b) showed a representative area of the 1Au/CeO₂ and 3Au/CeO₂ catalysts respectively. It showed that both samples are constituted by white spots identified as Au on large CeO₂ particles. Among these catalysts the sample with high Au loading showed big Au nanoparticles (large white spots) than on the 1Au/CeO₂ catalyst. Inset image in Figure 6a, showed an amplification of CeO₂ support using FE-SEM technique. Under this analysis we found that the CeO₂ is constituted by nanoparticles with diameters ~ 20 nm.

Figure 6.

SEM image of: (a) 1Au/CeO₂ catalyst. Inset image corresponds to CeO₂ support obtained by FE-SEM. (b) 3Au/CeO₂ catalyst.

Figure 7 showed the XRD patterns of the 1Au/CeO₂ and 3Au/CeO₂ catalysts. It is possible to observe on the XRD pattern; characteristic peaks of the metallic gold and the others corresponding to the fluorite structure of ceria (CeO₂-cerianite).In addition, it is clear that the intensity of the diffraction peaks of the Au⁰increases proportionally as the Au was loading on CeO₂ suggesting on this sample a big Au crystallite size. The diffraction patterns of the Au/CeO₂ samples showed considerable line widths and no overlapping Au and CeO₂ diffraction peaks. We use the peaks display in the 35-45 = 2θ range to estimate the average Au crystallite size. The average value of the CeO₂ and the Au metal crystallite sizes on 1Au/CeO₂ and 3Au/CeO₂ samples were determined by Scherrer equation and it corresponds to 19, 23 and 33 nm respectively.

Figure 7.

X ray diffraction patterns of the 1Au/CeO₂ and 3Au/CeO₂ catalysts. Cubic structure CeO₂ (fluorite structure) and characteristic peaks of the metallic gold.

TEM analysis of the 1Au/CeO₂ catalyst is showed on Figure 8a; it is possible to observe the homogeneous distribution of the CeO₂ particles about 20 nm of diameter. This value is close to the FE-SEM observation and the results calculated by Scherrer equation. High resolution image of Au nanoparticle (Figure 8b) revealed that after thermal treatments, the Au nanoparticle generally had a hemi-spherical shape. The Au particle size measure was ~ 17 nm. Analysis of the electron diffraction patterns of the Au nanostructures, inset on Figure 8b, show that the crystalline structure grows as a gold fcc single crystal. These Au nanoparticles was recorded along [110] orientation. Figure 8c showed the TEM image of the 3Au/CeO₂ catalyst, inset image on Figure 8c, showed the EDS spectra of the CeO₂ support and a gold nanoparticle the last one about 30 nm of diameter.

Figure 8.

a) Low magnification TEM images of the 1Au/CeO₂ catalyst. b) HRTEM image of the Au nanoparticle from 1Au/CeO₂ catalyst which was recorded along [110] orientation. The d-spacing measured is showed inset image. The particle size was about 17 nm. c) Low magnification TEM images of the 3Au/CeO₂ image. Inset images showed the EDS spectra of the Au nanoparticle about 30 nm of diameter and CeO₂ support.

The TPR profiles of the Au-base catalysts deposited on CeO₂ are depicted in Figure 9. This technique was also employed to found the optimal reduction temperature in the catalysts.TPR profiles showed differences in the hydrogen consumption depending of the gold content. Reduction of Au/CeO₂ samples were observed from 150 to 330 °C. For this reason, the catalysts were first activated in H₂ stream at 350 °C before all the characterization. As reported in literature, pure CeO₂ showed two reduction peaks at about 500 and 800 ° C, and were interpreted as the reduction of surface capping oxygen and bulk phase lattice oxygen, respectively [26; 73]. For Au/CeO₂ catalysts, the peak assigned to ceria surface layer reduction was reported from 120 to 178 °C [54; 74]. In our calcined 1Au/CeO₂ catalyst, the hydrogen consumption peaks were observed within temperature range 175-325 °C, which could be deconvoluted into three components at reduction temperatures of 215, 257 and 294 °C respectively. Whereas, the calcined 3Au/CeO₂ catalyst exhibited a broad reduction peak, which could be decompounds into three components at reduction temperatures of 184, 267 and 297 °C. It is clear that the intensity of the peak at 257 °C observed on the 1Au/CeO₂ sample diminish significantly, and was shifted to the high-temperature region when the amount of Au was increased, because, the large crystallites tend to be reduced slower than the small ones due to their relatively lower surface area exposed to H₂. Rodriguez et al. Showed by XAFS that Au facilitates the oxide reduction of the matrix respect to pure ceria [75].Andreeva et al. [52]observed by TPR technique two reducible species on Au-CeO₂ samples. The low-temperature peak on the TPR profile was connected with the reduction of the oxygen species on the fine gold particles, and the high-temperature peak was due to the reduction of the surface ceria. Taking into account these results and the differences in the gold loading in our samples, we assume that the former peak corresponds to the reduction of Au oxide nanoparticles, then it causes spill-over of hydrogen onto the support inducing a concurrent reduction of both the Au oxide and the surface of CeO₂ as was reported on other Ce-base catalysts [26; 28; 52].

Figure 9.

TPR profiles of the fresh Au/CeO₂ catalysts calcined at 400 °C.

The catalytic activity of the Au/CeO₂ catalysts as a function of the reaction temperature on the steam reforming of methanol reaction is presented on Figure 10. The light-off temperature of both samples started at ~ 350 °C at the molar CH₃OH/H₂O = 2.4 ratio, and the activity increased as temperature was rising. At the maximum reaction temperature, the methanol conversion observed on the 1Au/CeO₂ and 3Au/CeO₂ catalysts was 95 and 90 % respectively. The better catalytic activity observed on the 1Au/CeO₂ sample than on the 3Au/CeO₂ catalyst, could be attributed to differences in the Au particle size how was observed by XRD, SEM and TEM analysis. So, on the 3Au/CeO₂ catalyst the Au particle size was bigger than on the 1Au/CeO₂ catalyst. Croy et al. [76] studied the H₂ production through methanol decomposition on Pt/TiO₂ catalysts. They observed high catalytic activity on the catalysts with small particle size and diminish as the particle size increase. On methane combustion was observed better catalytic activity on the catalyst with low gold loading than the one with high gold loading, this result was associated with the dispersion of Au and the atomic ratio of Au³⁺/Au⁰[77; 78]. However, Guzman and Gates [79] not found evidence between the Au cluster size and the catalytic activity on Au/MgO catalyst during the CO oxidation reaction with EXAFS technique. Wang et al. [80] suggested that the active sites in Au-ceria catalysts for the WGS probably contain Au nanoparticles and partially reduced ceria. It has been suggested that the presence of gold clusters weakens the bonding of the oxygen species on CeO₂ and facilitates the formation of more reactive species [53; 80; 81].

Figure 10.

Temperature dependence of SRM activity of Au/CeO₂ catalysts. Partial pressure of CH₃OH and H₂O was 9999.18 and 1699.86 Pa respectively. GHVS=30,000 h^-1. The molar ratio in the steam was CH₃OH (1.95µmol)/H₂O (1.97 µmol) = R ≈ 1.0 and CH₃OH (4.7 µmol)/H₂O (1.97 µmol) = R ≈ 2.4.

The effect of the molar CH₃OH/H₂O ratio was evaluated on the 1Au/CeO₂ catalyst and presented on Figure 10. It showed clearly that this sample had better catalytic activity when the molar CH₃OH/H₂O ratio is close to R = 1 than on the R = 2.4. At the beginning of the reaction (300 °C) the conversion of methanol was 11 % and, increase when the temperature was rising. At 375 °C the catalyst reached near 61 % conversion whereas on the R = 2.4 ratio the methanol conversion only reached ~ 22 %. This effect is caused by the modification on the feed concentrations of the fuel as was reported on [82]. The methanol conversion observed at the maximum reaction temperature, reached almost 100 % on the molar CH₃OH/H₂O ratio R = 1, while on the R = 2.4 ratio only 95 % conversion was observed. This finding suggests that the methanol is adsorbed preferable on the surface of the catalysts than water. Table 2 showed the catalytic performance on steam reforming of methanol reaction from 300 to 475 °C range of the bare CeO₂ and 1Au/CeO₂ catalyst using the molar CH₃OH/H₂O ratio R = 1. As it can be seen, CeO₂ support showed low catalytic activity in almost all temperature range. However, at the end of the reaction the methanol conversion is almost 100 % for these samples. This finding showed the effect of the gold nanoparticles on CeO₂ during the catalytic activity. Gazsi et al. [83] suggested a cooperative effect between Au nanoparticles and CeO₂ support on the decomposition and reforming of methanol as was reported for other catalysts [9; 26; 64; 84].

Temperature (°C)	CeO2; Methanol conversion and selectivity (%)					1Au/CeO2; Methanol conversion and selectivity (%)
Conv.	H2	CH4	CO	CO2	Conv.	H2	CH4	CO	CO2
300	9.2	0	0	41.5	58.5	10.8	51.6	0	O	48.4
350	8.7	0	0	71.4	28.6	39.0	52.4	0.8	0.1	46.7
400	18.2	74.8	7.4	14.2	3.5	79.7	81.5	1.7	3.0	13.8
425	43.5	60.7	13.1	21.8	4.4	86.7	86.5	2.3	7.1	4.1
450	83.6	54.7	13.0	27.4	4.9	96.2	84.4	3.0	10.5	2.1
475	100	58.2	7.5	28.6	5.7	100	84.4	3.1	10.6	1.9

Table 2.

Catalytic activity of the bare CeO₂ and 1Au/CeO₂ catalyst. The molar ratio in the steam was CH₃OH (1.95µmol)/H₂O (1.97 µmol) = R ≈ 1.0

The reaction products observed on the steam reforming of methanol reaction of the Au/CeO₂ catalysts were H₂, CO, CO₂, CH₄ and H₂O. Small production of methyl formate as by-product of the reaction was observed in both samples. Figure 11a showed the distribution of hydrogen on the Au-base catalysts. Higher H₂ production was observed from 300 to 400 °C range on the catalysts tested with high molar CH₃OH/H₂O ratio R = 2.4 than R = 1. The drop of the hydrogen selectivity at higher temperatures could be attributed at the formation of CH₄. At the beginning of the reaction all samples showed low CO selectivity, Figure 11b. However, as the reaction temperature raise from 375 to 470 °C, the CO production increase and the selectivity toward CO₂ decrease. It is clear from Figure 11b that the 3Au/CeO₂ sample showed high selectivity toward CO than on the 1AuCeO₂ sample. In addition, we observed that the CO production is practically the same on the 1AuCeO₂ catalyst (~ 10 %) independently of the molar CH₃OH/H₂O ratio. Table 2 showed the selectivity of the bare CeO₂ compared with 1Au/CeO₂ catalyst. It showed the beneficial effect of the gold nanoparticles on the CeO₂. Because, the selectivity toward undesirable by-products such as CO and CH₄ observed on the bare CeO₂, were drop on the 1Au/CeO₂ catalyst and the selectivity toward H₂ was improved on this catalyst.

Time on-stream studies (Fig. 12) at 350 °C in the 1Au/CeO₂ sample with CH₃OH/H₂O ratio = 1.0, reveal high stability on the activity during the steam reforming of methanol reaction during a 65 h of reaction period, as well as high stability in the reaction products. The stability of catalysts under operating conditions is desirable for commercial applications. Fu et al. [73] found no significant change in activity after 120 h on stream on WGS. This behavior could be suggested that the catalyst maintains the same catalytic species during the SRM reaction and it does not lose by effect of the reaction conditions. So, these results showed that the CeO₂ matrix could be use to prevent the vanished of active phase during the reaction. Thus, a cooperative redox mechanism for the SRM reaction on Au–ceria is possible, similar to Cu–ceria or another kind of catalysts [26; 37; 63; 64; 73; 85] and for the WGS reaction [73].

Figure 11.

a) H₂ and CH₄ selectivity as a function of reaction temperature. b) CO and CO₂ selectivity as a function of reaction temperature.Partial pressure of CH₃OH and H₂O was 9999.18 and 1699.86 Pa respectively. GHVS=30,000 h^-1.The molar ratio in the steam was CH₃OH (1.95µmol)/H₂O (1.97 µmol) = R ≈ 1.0 and CH₃OH (4.7 µmol)/H2O (1.97 µmol) = R ≈ 2.4.

Figure 12.

Stability of 1Au/CeO₂ catalyst at 350 °C. Partial pressure of CH₃OH and H₂O was 9999.18 and 1699.86 Pa respectively. GHVS=30,000 h^-1. The molar ratio in the steam was CH₃OH (1.95µmol)/H₂O (1.97 µmol) = R ≈ 1.0

5. Conclusion

Cu/CeO₂, Ni/CeO₂ and three bimetallic copper-nickel catalysts supported on CeO₂ were prepared by the impregnation method and tested in the OSRM reaction. The monometallic Ni/CeO₂ and the bimetallic Cu-Ni/CeO₂ (synthesized by co-impregnation) catalysts demonstrate both a higher catalytic activity in the OSRM reaction than the other catalysts. CO-chemisorption followed by DRIFT technique showed differences in the former samples. So, in the Ni/CeO₂ and Cu-Ni/CeO₂ catalysts was observed a band at 1625 cm⁻¹ that was not present on the bimetallic samples prepared by successive impregnation as well for the Cu/CeO₂samples which indicates that CO does not adsorb on these materials. This suggests that the metal active phase blockage some sites on the support or the active phase, modifies the surface of the catalyst for the adsorption of methanol and water inhibited the catalytic reaction on this system. In the case of the Au/CeO₂ system, nanosized ceria was prepared in advance with particle size of 19 nm and used as a support for gold nanoparticles. The average Au crystallite size in the 1Au/CeO₂ and 3Au/CeO₂ catalysts was 21 and 31 nm respectively. Differences in the reducibility of the Au/CeO₂ catalysts were observed depending of the Au loading. H₂-TPR results showed a shift of the reduction peaks toward high-temperature when the amount of Au was increased associated with the Au particle size present on the catalysts. The gold-base catalysts supported on CeO₂ showed catalytic activity on the SRM reaction and high selectivity toward H₂. Among these catalysts the sample with small Au particle size showed best performance in methanol conversion than on the catalyst with big Au particle size. These finding show the relationship between the Au particle size and the catalytic activity. In addition, it was demonstrate the beneficial effect of gold nanoparticles on the catalytic activity, as well as on the selectivity toward undesirable by-products such as CO and CH₄ and the selectivity toward H₂. The stability of the 1Au/CeO₂ catalyst at 350 °C was followed as a function of time on stream. This result showed high stability during the reaction. It is assumed that the interface between Au and partially reduced ceria is responsible for the high activity of Au/CeO₂ catalyst. In general we observed different behavior in the catalytic activity in our catalysts in the OSRM reaction to those reported in the literature. This difference could be attributed to the nature of the metal active phase and metal addition to the support.In this way, we suggest that the OSRM reaction could be a structure sensitive reaction according with the literature. However, further work is needed to refine and optimize the catalysts to improve the methanol conversion to produce CO-free hydrogen from the reaction under study.

Acknowledgement

Thanks to C. Salinas for technical support and to the projects ININ-CA-009 and CONACYT CB-2008-01-104540 for financial support. Authors would like to acknowledge Dr. Carlos Ángeles for its valuable comments and suggestions on the manuscript.