Pure silica SBA-15 supported Cu-Ni catalysts for hydrogen production by ethanol steam reforming

Transcription

1 Pure silica SBA-15 supported Cu-Ni catalysts for hydrogen production by ethanol steam reforming A.J. Vizcaíno*, A.Carrero, J.A. Calles Department of Chemical and Environmental Technology, Rey Juan Carlos University, Escuela Superior de Ciencias Experimentales y Tecnología (ESCET), c/ Tulipán s/n, Móstoles, Spain * Corresponding author: Tel.: ; fax: address: arturo.vizcaino@urjc.es ABSTRACT: Cu-Ni/SBA-15 supported catalysts prepared by the incipient wetness impregnation method were tested in the ethanol steam reforming reaction for hydrogen production. The effect of reaction temperature and metal loading was studied in order to maximize the hydrogen selectivity and the CO 2 /CO x molar ratio. The best catalytic performance was achieved at 600 ºC with a catalyst containing 2 and 7 wt% of copper and nickel, respectively. In addition, two catalysts were prepared by the method of direct insertion of Ni and Cu ions as precursors in the initial stage of the synthesis. XRD, TEM, N 2 adsorption and ICP-AES results evidenced that SBA-15 materials with long range hexagonal ordering could be successfully synthesized in the presence of copper and nickel salts with the (Cu+Ni) contents around 4 6 wt%. However, lower hydrogen selectivity and together with ethanol and water conversions were observed with catalysts prepared by direct synthesis in comparison with those prepared by incipient wetness impregnation method. KEYWORDS: Ethanol steam reforming, SBA-15, Cu-Ni catalysts, hydrogen production. 1. Introduction Hydrogen, the lightest and ubiquitous element in the universe, is a cleaner energy source since its combustion produces only water and energy. A more efficient use of hydrogen is as a fuel for fuel cells. Using fuel cells, the chemical energy of hydrogen can silently be converted to electricity without the excessive thermal energy loss observed in combustion engines and the maximum thermodynamic yield of the Carnot cycle. Fuel cells are becoming a reality as a means of generating clean energy. However, hydrogen rarely exists in its free form in nature. Fuel reforming is a necessary step for the integration of fuel cells into today s society. It is likely that the main source of hydrogen will vary with the geographical location of its demand. Conventional methods for hydrogen production were based on oil fractions, natural gas and methanol steam reforming. However, the catalytic steam reforming of bioethanol offers a highly attractive route for catalytically converting biomass to hydrogen. Bioethanol is easier to reform than gasoline or natural gas based on reaction temperature, it is safer to handle than methanol, and already has the ethanol-to-water ratio required for the reforming reaction, 10% to 25% ethanol. In comparison to fossil-fuel-based systems, the bioethanol-to-hydrogen system has a significant advantage of being nearly CO 2 neutral, since the CO 2 produced from the reforming reaction is consumed for biomass growth. This forms a nearly closed CO 2 loop that is a noteworthy advantage over methanol, which is primarily produced from non-renewable fossil fuels creating fossil carbon pollution. Ethanol has more qualified advantages over methanol for transportation applications since it is much less toxic and offers a high octane number, a high heat of vaporization, and a low photochemical reactivity [1]. Previous studies on ethanol steam reforming have shown that the main reaction is accompanied by side reactions that produce unwanted byproducts such as carbon monoxide and methane [2-4]. This poses a problem for fuel cells because the catalysts used in the fuel cell anodes are very sensitive to CO, which chemisorbs to the active sites of the catalyst. Therefore, it is imperative that the catalyst for the reforming reaction must have a high selectivity towards producing H 2 with minimal side reactions. According with the three main reactions (1), (2) and (3) showed below, carbon monoxide is favoured at high temperatures, and methane is thermodynamically favoured at low temperatures. Other reactions can also occur (4-12): 1/12

2 Ethanol steam reforming: CH 3 CH 2 OH + 3H 2 O 2CO 2 + 6H 2 H 0 = 174 kj/mol (1) Ethanol steam reforming to syngas: CH 3 CH 2 OH + H 2 O 2CO + 4H 2 H 0 = 256 kj/mol (2) Ethanol hydrogenolisis: CH 3 CH 2 OH + 2H 2 2CH 4 + H 2 O H 0 = -157 kj/mol (3) Ethanol cracking: CH 3 CH 2 OH 3/2 CH 4 + 1/2CO 2 H 0 = -74 kj/mol (4) CH 3 CH 2 OH CH 4 + CO + H 2 H 0 = 49 kj/mol (5) Ethanol dehydrogenation: CH 3 CH 2 OH CH 3 CHO + H 2 H 0 = 68 kj/mol (6) Acetaldehyde decarbolylation: CH 3 CHO CH 4 + CO H 0 = -19 kj/mol (7) Acetaldehyde steam reforming: CH 3 CHO + 2H 2 O 3H 2 + 2CO H 0 = -56 kj/mol (8) Ethanol dehydration : CH 3 CH 2 OH C 2 H 4 + H 2 O H 0 = 45 kj/mol (9) Ethylene steam reforming : C 2 H H 2 O 2CO + 4H 2 H 0 = 210 kj/mol (10) Water-gas shift: CO + H 2 O CO 2 + H 2 H 0 = -41 kj/mol (11) Methane steam reforming: CH 4 + H 2 O 3H 2 +CO H 0 = 206 kj/mol (12) Reaction (4) is strongly favoured at low temperatures. In fact, calculations showed that CO 2 and CH 4 are the only products thermodynamically favoured at low temperature (T < 200 ºC), with a stoichiometry equal to that of reaction (4) (CH 4 /CO 2 = 3). However, many studies have shown that acetaldehyde and ethylene may form at relatively low temperatures, well before the formation of hydrogen and CO x by reactions (1) and (2). Compared with the steam reforming reaction, the ethanol dehydrogenation and dehydration reactions are much faster, and acetaldehyde and ethylene may be considered important intermediates in the formation of hydrogen. Besides, the formation of coke on the surface of the catalyst is also not uncommon. Coke formation may occur as per the following Boudouard reaction (10) or through ethylene (11). 2CO CO 2 +C H 0 = kj/mol (10) C 2 H 4 polymers + coke (11) Copper-Nickel catalysts have been investigated in detail because of the relatively good activity of Ni in the steam reforming processes. Ni was shown to rapidly deactivate because of coke formation, but copper is known as a strong inhibitor of coke formation, and thus nickel copper bimetallic catalysts were specially designed and studied to avoid such a deactivation [5-13]. Some studies imply that metals alone do not assist hydrogen production significantly. These studies suggest that the performance of metal catalysts could be improved using suitable supporting materials [14]. Given that nickel crystals will sinter quickly above 590 ºC and that the catalyst activity depends on the nickel surface area, the sintering process can be partially prevented by a stable pore system in the support structure [15] In this way, recently discovered mesostructured materials, opened a new area in the application of these materials as supports for various metals and other catalytic active species. Mesoporous molecular sieves may play a key role in accommodating metallic particles due to their controllable pore size, pore volume, and high surface area [16]. SBA-15 material was synthesized by Zhao et al. in 1998 using amphiphillic triblock copolymers to direct the organization of silica species under strong acid conditions. This material is a well ordered hexagonal mesoporous silica structure with uniform pore size, which can be modified from 50 to 300 Å by the use of different triblock copolymers or by adding organic molecules as cosolvent [17]. In fact, it was reported [18] that Cu-Ni/SBA-15 supported catalyst showed better catalytic performance than other amorphous or mesostructured (MCM-41) silica supports, in the ethanol steam reforming reaction. For this reason, we considered interesting to prepare new catalysts introducing nickel and copper together with silicon in the framework of SBA-15. The characteristics and catalytic behaviour of Cu-Ni/SBA-15(DS) materials obtained by direct synthesis are compared with those prepared by the traditional incipient wetness impregnation method reported in reference [18]. 2. Experimental 2.1. Catalysts synthesis A pure silica SBA-15 sample was used as support for the Cu-Ni active phase. The synthesis of this material was carried out by a hydrothermal method as described by Zhao et al [17]. The as-synthesized sample was then calcined at 550 ºC in static conditions for 6 h, with a heating rate of 1.8 ºC/min. Active phase was added on the support by incipient wetness impregnation using mixed aqueous solutions of the metal precursors, Ni(NO 3 ) 2 6H 2 O and Cu(NO 3 ) 2 3H 2 O (Scharlab), with the proper concentration to obtain the desired Ni and Cu loadings. For comparison, a Cu-Ni/SiO 2 catalyst was prepared by impregnating 2/12

3 commercial amorphous silica (Ineos Silica), previously calcined at 550 ºC, with an aqueous solution of the copper and nickel nitrates. All the impregnated catalysts were air-dried overnight and calcined at 400 ºC for 15 h with a heating rate of 1.8 ºC/min. Besides, two additional Cu-Ni/SBA-15 catalysts were prepared by direct synthesis, following a technique described elsewhere [19], similar to that previously used for pure silica SBA-15. In this method, the precursors of the metallic phase were incorporated by adding Ni(NO 3 ) 2 6H 2 O and Cu(NO 3 ) 2 3H 2 O just after Pluronic123 dissolution. The solid obtained was recovered by filtration, dried overnight and calcined at 550 ºC in static conditions for 6 h, with a heating rate of 1.8 ºC/min Catalysts characterization Support textural properties were measured by nitrogen adsorption-desorption at 77K using a Micromeritics TRISTAR 2050 sorptometer. Prior to the adsorption, the samples were outgassed under vacuum at 200 ºC for 2h. Surface areas were calculated according to BET method. The final catalysts were characterized by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP- AES), Temperature Programmed Reduction (TPR) and Transmission Electron Microscopy (TEM). ICP-AES technique was used to determine the actual copper and nickel content in the catalysts on a Varian VISTA- PRO AX CCD-Simultaneous ICP-AES spectrophotometer. Previously, solid samples were dissolved by acidic digestion. TPR measurements were performed on a Micromeritics AUTOCHEM 2910, using 300 mg of catalyst, placed in a fixed-bed quartz tube under 10% H 2 in Ar flow (35 ml/min) with a heating rate of 5 C /min from 25 to 800 C. Effluent gas is forced to flow through a cold trap to remove water produced before reaching the detector (TCD). Samples were previously degasified under dry Ar flow (35mL/min) at 400 ºC with a heating rate of 15 ºC/min. TEM micrographs were acquired on a Phillips TECNAI 20 microscope equipped with LaB 6 filament and an accelerating voltage of 200 kv, with a point-to-point resolution of 0.27 nm. Samples were prepared by powder dispersion of the material, finely divided, in acetone and subsequent deposition on a gold grid with carbon support Catalytic test Ethanol steam reforming tests were carried out in a MICROACTIVITY-PRO unit as described in a previous paper [18]. Gaseous products are analyzed on-line using a gas chromatograph Varian CP-3380 equipped with a sampling valve (250 µl, with heating oven), two columns and a thermal conductivity detector (TCD). Hayesep Q 6m column was used to separate some components of the mixture (ethanol, water, acetaldehyde, ethylene, ethane, carbon dioxide and methane). Molecular Sieve 13X 1m column was used to complete the carbon monoxide and hydrogen separation. Helium was used as both the carrier gas and the reference gas. The catalyst (around 315 mg) was placed in the reactor diluted with silicon carbide (catalyst/silicon carbide weight ratio 1:5) to minimize hot spot effects. Previous to reaction, catalyst was in situ reduced under flowing pure hydrogen (30 ml/min) at 550 ºC for 4.5 h with a heating rate of 2 ºC/min. After the catalyst activation, the reaction temperature was fixed and catalytic test was carried out isothermally at atmospheric pressure under nitrogen-diluted conditions. A liquid reaction mixture water/ethanol = 3.7 molar ratio was introduced at a flow rate of ml/min, vaporized at 150 ºC and further eluted by N 2 (30 ml/min). Weight hourly space velocity (WHSV) was defined as the ratio between the inlet feed (water + ethanol) mass flow rate and the mass of catalyst. Carbon deposited during reaction was evaluated from mass difference after coke total combustion process by heating the used catalysts under air static conditions at 550 ºC for 15 h. Reaction parameters were calculated as follows: F H produced 2 SH = 2 [ 3 (F F ) + (F F ) ] ethanol in ethanol out water in water out S i carbon containing product Fi = n (F carbon containing product ethanol in F ethanol out ) 3/12

4 X reac tan t F = reac tan t in F F reac tan t in reac tan t out S i and X reactant represent the products selectivity and the reactants conversion, respectively. F i, in or out is the molar flow rate of the i species at the inlet or at the outlet of the reactor and n is the stoichiometric factor between the carbon-containing products and ethanol. 3. Results and discussion 3.1. Effect of reaction temperature In order to study the influence of reaction temperature on the catalytic behaviour of a Cu-Ni/SBA-15 (10.9 wt% Ni and 1.1 wt% Cu) supported catalyst in the ethanol steam reforming reaction, catalytic tests were carried out at five different temperatures, ranging from 400 to 650 ºC. Figure 1 shows that ethanol conversion is complete in the whole range of temperatures studied, while water conversion continuously increases with reaction temperature. Regarding to selectivities, it can be noticed that hydrogen selectivity increases with temperature, while CH 4 selectivity decreases. However, as reaction temperature increases, it can be observed a minimum in CO selectivity and a maximum in CO 2 selectivity. This behaviour may be explained through the enthalpies of the reactions described in the Introduction section, which indicate that methane is favoured at low temperatures, while carbon monoxide and hydrogen, as well as water conversion, predominate at high temperatures. No presence of intermediate products (acetaldehyde or ethylene) was detected over the range of temperature studied. The main production of CH 4, CO 2 and H 2 at 400 ºC suggests that the cracking (4) reactions, together with the water-gas shift reaction (11), are taking place. Another pathway implies acetaldehyde as intermediate in reactions (6-8), followed by water-gas shift. In the range from 400 ºC to 500 ºC, CO 2 selectivity increases as a consequence of the water-gas shift reaction, which accounts for the decrease of CO selectivity. Besides, CH 4 selectivity slightly decreases, which indicates that methane steam reforming (12) also takes place, but in a lower extent. However, in the range from 500 ºC to 650 ºC, a strong decrease in CH 4 selectivity, together with an increase in CO selectivity, is observed. It may be explained by the methane and ethanol steam reforming which predominate in detriment of the water-gas shift reaction. Also ethanol steam reforming reactions (1) and (2) may take place. As a consequence of the equilibrium between these reactions, CO 2 selectivity draws a maximum. This explanation can also account for the rise in hydrogen selectivity. This behaviour is very similar to that previously reported for silica supported Cu-Ni catalysts [18, 20]. Figure 2 shows the variation in coke deposition on the catalyst with reaction temperature after a time on stream (TOS) of 3h. As it can be seen, coke deposition initially increases with temperature and remains almost constant from 500 ºC. Conversion (mol %) Selectivity (mol %) S H2 SCO2 SCO S CH Temperature (ºC) X H2 O X EtOH Temperature (ºC) Figure 1. Effect of temperature on conversion and selectivity for ethanol steam reforming over a Cu-Ni/SBA-15 catalyst. WHSV: 12.7 h -1, water/ethanol molar ratio: /12

5 Coke Deposition (w/w catalyst %) Temperature (ºC) Figure 2. Effect of temperature on coke deposition over a CuNi/SBA-15 supported catalyst during ethanol steam reforming reaction. WHSV: 12.7 h -1, water/ethanol molar ratio: 3.7, TOS: 3 h. As a conclusion, a temperature of 600 ºC seems to be suitable to achieve both high hydrogen selectivity (85.3 mol%) and relatively high CO 2 /CO x ratio (0.66) with a water conversion of 38.6 mol%. Thus, it is an appropriate temperature value to carry out the steam reforming reaction on Cu-Ni catalysts using SBA-15 as support. Similar trends were observed by Klouz and Fierro et al. [11, 12] 3.2. Influence of catalyst preparation procedure In order to determine the effect of the catalyst support on ethanol steam reforming reaction, catalytic tests were performed over catalysts supported on SiO 2 and SBA-15 prepared by incipient wetness impregnation. The influence of the metal incorporation method was studied by comparing Cu-Ni/SBA-15 samples prepared by impregnation (I) and by direct synthesis (DS). The characteristics of the prepared catalysts are summarized in Table 1. Regarding to textural properties, it can be observed that SBA-15 supported catalysts exhibit higher surface area than CuNi/SiO 2 sample, but lower total pore volume and pore size. Comparing the preparation method, direct synthesis leaded to catalysts with slightly higher surface area, as well as pore volume and diameter. This indicates that incorporating the metallic phase during the SBA-15 synthesis, not only may affect the metal dispersion, but also may have influence on textural properties. Figure 3 shows the X-ray difractograms corresponding to samples containing around 2 wt% of both copper and nickel. The XRD pattern at low angle provides information about the support structure (Figure 3a). For samples using SBA-15 as support, low angle peaks related to diffraction on its hexagonal mesostructure can be observed, while catalyst supported over amorphous silica presents no diffraction peaks. CuNi/SBA-15(I) and CuNi/SBA-15(DS) patterns show similar peaks at 2θ = 0.9º, 1.5º and 1.7º, which are characteristics of the planes (1 0 0), (1 1 0) and (2 0 0) of the hexagonal pore mesostructure of SBA-15 material [17]. This verifies that the structure has been maintained in spite of the metallic phase incorporation. Regarding to high angle diffraction, Figure 3b shows the crystal structure of the metallic phase. All samples show peaks of similar intensity at 2θ = 35.5º and 38.7º, corresponding to the planes ( 1 1 1) and (1 1 1) of monoclinic CuO, and 2θ = 37.3º, 43.3º and 62.9º, corresponding to the planes (1 1 1), (2 0 0) and (2 2 0) of cubic NiO, respectively. 5/12

6 Catalyst Table 1. Physicochemical properties of the prepared catalysts. Preparation method Cu a Ni a S BET V b p D c p (wt %) (wt %) (m 2 /g) (cm 3 /g) (nm) CuNi/SiO 2 Impregnation CuNi/SBA-15(I) Impregnation CuNi/SBA-15(DS) Direct Synthesis CuNi/SBA-15(I)2 Impregnation CuNi/SBA-15(DS)2 Direct Synthesis a ICP-AES measurements. b Determined at P/Po = c Calculated through the maximum of the BJH pore size distribution. CuNi/SBA15(DS) CuNi/SiO 2 CuNi/SBA-15(I) 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 2Theta CuNi/SBA15(DS) CuNi/SiO 2 CuNi/SBA-15(I) Theta (a) (b) Figure 3. X-ray difractograms for catalysts containing 2 wt% Cu and 2 wt% Ni: (a) Low angle; (b) High angle. In order to compare the metallic particle distribution over the different supports, TEM micrographs of catalysts containing around 2 wt% Cu and Ni are presented in Figure 4. Images of CuNi/SBA-15 samples show a hexagonal mesoporous structure, with metallic particles of two different sizes, the largest of which (around nm) exhibit a cubic shape. However, differences between these samples must be noticed. CuNi/SBA-15(I) sample exhibit a lower fraction of large particles, which is in accordance with the broader XRD peaks showed in Figure 3b. In addition, small particles seems to be irregularly dispersed on CuNi/SBA- 15(I), while for CuNi/SBA-15(DS) these are distributed throughout the support channels, which may indicate that these metal particles are arranged inside the pores. These results disagree with those previously reported by Park et al. [19], probably due to the incorporation of Cu into the synthesis gel. CuNi/SiO 2 show an irregular pore system and large metallic particles of similar size (between nm). 6/12

7 500 nm (a) 100 nm 500 nm (b) 200 nm 500 nm (c) Figure 4. TEM micrographs of (a) CuNi/SBA-15(DS), (b) CuNi/SBA-15(I) and (c) CuNi/SiO nm The results of the TPR analysis are shown in Figure 5. It can be noticed that reduction for catalysts prepared by impregnation takes place at a relatively low temperature, from 175 to 275 ºC. However, TPR profile corresponding to the sample prepared by direct synthesis is wider and presents different signals in the range of 175 to 375 ºC. This indicates the existence of different types of Cu-Ni species in this catalyst. The reduction at lower temperature may be due to metal particles of great size located between particles of the support and over its surface. Reduction at higher temperature may correspond to smaller metallic particles, probably located inside the channels of the SBA-15 support and, consequently, with less reducibility due to a stronger metal-support interaction. Even, some metallic particles could be totally or partially located inside the wall of the SBA-15. These particles may not be considered as active sites and therefore they would not participate in the reaction mechanism. 7/12

8 Figure 5. TPR profile for catalysts containing 2 wt% Cu and 2 wt% Ni. Catalytic results on ethanol steam reforming carried out at 600 ºC are shown in Table 2. Comparing CuNi/SiO 2 and CuNi/SBA-15(I) samples, it can be observed that both catalysts achieve complete ethanol conversion, similar water conversion and no significant intermediate products. On the other hand, catalyst supported on amorphous silica presents the highest coke deposition, while CuNi/SBA-15(I) reaches higher selectivity towards main products (H 2 and CO 2 ). This may be related to a better dispersion of the active phase over this material owing to its high surface area (see Table 1), as it can be observed in the TEM micrographs (Figure 4). However, Cu-Ni/SBA-15 (DS) catalysts proceeded with lower conversions and selectivity to hydrogen due to the production of intermediates like acetaldehyde. This may be explained on basis of: (1) a blocking of the pores of SBA-15 structure by some metal particles, which isolates some other particles, reducing the number of active sites available for the reactants, or even (2) a lost of active metal surface due to some metal particles being embedded into the silica wall. This can be verified from TEM images, where these metal particles seem to be arranged inside the pores. TPR analysis (Figure 5) also supports these results, as both pore blockage and strong metal-support interaction lead to the enlargement of the reduction profile. Anyway, this would mean a higher (SBA-15 surface)/(metal surface) ratio, leading to the enhancement of ethanol dehydrogenation to acetaldehyde, which is a main product in the ethanol steam reforming over oxides [21]. As shown in Table 2, increasing the metal content, the catalytic behaviour of impregnated and DS samples present the same trend as those explained above. Table 2. Effect of the catalyst synthesis method and support on ethanol steam reforming reaction. Reaction temperature: 600 ºC, WHSV: 12.7 h -1, water/ethanol molar ratio: 3.7, TOS: 3h. Catalyst X EtOH X H2O Selectivity (mol %) Coke dep (mol %) (mol %) H 2 CO 2 CO CH 4 CH 3 CHO C 2 H 4 (w/w%) CuNi/SiO CuNi/SBA-15(I) CuNi/SBA-15(DS) CuNi/SBA-15(I) CuNi/SBA-15(DS) /12

9 3.3 Effect of copper and nickel loading In order to study the effect of copper and nickel loading on the ethanol steam reforming reaction, bimetallic catalysts supported on silica SBA-15 material with different metal contents were tested at 600 ºC, as shown in table 3. Catalysts have been denoted as CuxNiy/SBA-15, where x and y are the nominal copper and nickel content (wt%), respectively. Besides, a catalytic test was carried out over the pure silica SBA-15 support. Table 3. Effect of metal loading on ethanol steam reforming over Cu-Ni/SBA-15 catalysts. Reaction temperature: 600 ºC, WHSV: 12.7 h -1, water/ethanol molar ratio: 3.7, TOS: 3h. Catalyst Cu Ni X EtOH X H2O Selectivity (mol %) Coke dep (wt%) (wt%) (mol %) (mol %) H 2 CO 2 CO CH 4 CH 3 CHO C 2 H 4 (w/w%) SBA Cu2Ni4/SBA Cu2N7/SBA Cu2Ni9/SBA Cu4Ni4/SBA Cu4Ni7/SBA Cu4Ni9/SBA Cu6Ni4/SBA Cu6Ni7/SBA Cu6Ni9/SBA As it can be observed, catalytic performance of silica SBA-15 with no metallic phase is poor, exhibiting low ethanol conversion. Furthermore, water conversion shows a negative value and the main product formed is acetaldehyde, while hydrogen selectivity scarcely reaches a value of 19 %. On the other hand, no presence of ethylene is detected. These results suggest that ethanol dehydrogenation (6) is the main reaction with pure SBA-15. Ethanol hydrogenolysis (3), which accounts for the negative water conversion and low hydrogen selectivity, may also take place. This was the typical behaviour described by Llorca et al. [21] when oxides are used as catalysts. However, catalysts containing copper and nickel lead to ethanol conversion and hydrogen selectivity above 91 mol% and 67 mol%, respectively. Water is converted in the range from 15 to 38 mol% and low acetaldehyde quantities are present in the products, meaning that the products distribution is mainly governed by reactions (1) and (2). Figure 6 shows the ethanol (a) and water (b) conversion, as well as the hydrogen selectivity (c) and the CO 2 /CO x ratio (d), versus the nickel and copper content. Regarding to conversions, ethanol is almost completely converted in the whole metal content range studied, although it reaches the lowest value (about 92 %) for high nickel and copper contents. It is probably due to a poor dispersion of the active phase and a partial pore blockage because of the high metal loading. On the other hand, water conversion presents a maximum for both medium nickel and copper contents. Focus on hydrogen selectivity, a maximum is present at a Ni content of 7 wt% and a copper loading around 2 wt%. At the same time, a maximum in the CO 2 /CO x ratio is observed. Taking it into account, it seems that a catalyst containing about 7 % Ni and 2 % Cu would be the most appropriate to carry out steam reforming reactions when using Cu-Ni/SBA-15 catalysts. Figure 7 shows that coke deposition rises as nickel content decreases from 9 to 4 wt% and copper loading increases from 2 to 6 wt%, reaching the maximum value for the Cu6Ni4/SBA-15 catalyst. Fierro et al. [22] observed the effect on coke deposition during ethanol oxidative steam reforming of Cu addition in Ni- Cu/Al 2 O 3 catalysts and stated that Cu additions over 5 % deactivated the catalyst more rapidly, which agrees with our results. 9/12

10 (a) (b) (c) (d) Figure 6. Ethanol (a) and water (b) conversions, hydrogen selectivity (c) and CO 2 /CO x ratio (d) for ethanol steam reforming over bimetallic catalysts of Cu and Ni supported on silica SBA-15. Figure 7. Effect of copper and nickel loading on coke deposition over Cu-Ni/SBA-15 catalysts. 10/12

11 The catalyst reaching the best equilibrium between hydrogen production and CO 2 /CO x ratio, at medium water conversion level, seems to be is the one containing low Cu and medium Ni content, i.e. Cu2Ni7/SBA- 15, as previously discussed. 4. Conclusions Cu-Ni/SBA-15 supported catalysts prepared by the incipient wetness impregnation method were tested in the ethanol steam reforming reaction for hydrogen production. Ethanol conversion is complete in the whole range of temperatures studied ( ºC), while water conversion continuously increases with reaction temperature. Methane and carbon dioxide are the main products at reaction temperatures lower than 500 ºC, while higher hydrogen selectivity and (CO 2 /CO x ) molar ratio were achieved at 600 ºC. The effect of amount and type of metallic phase was studied through a series of catalysts prepared with different nickel (4 9 wt %) and copper (2 6 wt %) loadings. Better catalytic results in terms of reactants conversion and hydrogen selectivity were observed for metallic contents ranging from 6 to 13 (Cu+Ni) wt %. Particularly, Cu-Ni/SBA-15 sample with 2 and 7 wt% of copper and nickel, respectively, exhibited a 77.2 % of hydrogen selectivity with a CO 2 /CO x molar ratio of Two additional catalysts, denoted as Cu-Ni/SBA-15 (DS), were prepared by the method of direct insertion of Ni and Cu ions as precursors in the initial stage of the synthesis. TEM, N 2 adsorption and ICP-AES results evidenced that SBA-15 materials with long range hexagonal ordering could be successfully synthesized in the presence of copper and nickel salts with the (Cu+Ni) contents around 4 6 wt%. Both types of Cu- Ni/SBA-15 samples (impregnated and synthesized directly) presented XRD peaks corresponding to monoclinic CuO and cubic NiO. TEM micrographs showed metallic particles irregularly dispersed on impregnated samples while they are distributed throughout the support channels of Cu-Ni/SBA-15 (DS), indicating that metal particles may be arranged inside the pores. Cu-Ni species observed in the impregnated catalysts showed lower reduction temperatures ( ºC) than those observed in the direct synthesis samples ( ºC). This increase in the reduction temperature may be related with a stronger metal-support interaction from metallic particles probably located inside the channels of the SBA-15 support. In contrast with impregnated samples, the ethanol steam reforming over Cu-Ni/SBA-15 (DS) catalysts proceeded with lower conversions and selectivity to hydrogen due to the production of intermediates like acetaldehyde. References: [1] C. Wyman. Handbook on bioethanol production and utilization. Taylor & Francis, Washington D.C, [2] F. Aupretre, C. Descorme, D. Duprez. Bio-ethanol catalytic steam reforming over supported metal catalysts. Catal. Comm. 3, , [3] F. Aupretre, C. Descorme, D. Duprez. Hydrogen Production for Fuel Cell from the Catalytic Ethanol Steam Reforming. Top. Catal. 30, , [4] F. Aupretre, C. Descorme, D. Duprez, D. Casanave, D. Uzio. Ethanol steam reforming over Mg x Ni 1-x Al 2 O 3 spinel oxide-supported Rh catalysts. J. Catal. 233, , [5] F. Mariño, M. Bover, G. Baronetti, M. Laborde. Hydrogen production from steam reforming of bioethanol using Cu/Ni/K/γ-Al 2 O 3 catalysts: Effect of Ni. Int. J. Hydrogen Energy, 26, , [6] F. Mariño, E.G. Cerrella, S. Duhalde, M. Jobbagy, M. Laborde. Hydrogen from steam reforming of ethanol: Characterization and performance of copper-nickel supported catalysts. Int. J. Hydrogen Energy 23, , [7] F. Mariño, G. Baronetti, M. Jobbagy, M. Laborde. Cu-Ni-K/γ-Al 2 O 3 supported catalysts for ethanol steam reforming: Formation of hydrotalcite-type compounds as a result of metal-support interactions. Appl. Catal. A: Gen. 238, 41-54, [8] F. Mariño, M. Boveri, G. Baronetti, M. Laborde. Hydrogen production via catalytic gasification of ethanol. A mechanism proposal over copper-nickel catalysts. Int. J. Hydrogen Energy 29, 67-71, /12

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