Mechanistic Aspects of the Ethanol Steam Reforming Reaction for

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Mechanistic Aspects of the Ethanol Steam Reforming Reaction for Hydrogen Production on Pt, Ni, and PtNi Catalysts Supported on γ-Al2O3† Maria Cruz Sanchez-Sanchez,*,‡ Rufino M. Navarro Yerga,*,‡ Dimitris I. Kondarides,§ Xenophon E. Verykios,§ and Jose Luis G. Fierro‡ Instituto de Cata´lisis y Petroleoquı´mica (CSIC), C/Marie Curie s/n, Cantoblanco 28049, Madrid, Spain, and Department of Chemical Engineering, UniVersity of Patras, 26504 Patras, Greece ReceiVed: July 10, 2009; ReVised Manuscript ReceiVed: September 21, 2009

Mechanistic aspects of ethanol steam reforming on Pt, Ni, and PtNi catalysts supported on γ-Al2O3 are investigated from the analysis of adsorbed species and gas phase products formed on catalysts during temperature-programmed desorption of ethanol and during ethanol steam reforming reaction. DRIFTS-MS analyses of ethanol decomposition and ethanol steam reforming reactions show that PtNi and Ni catalysts are more stable than the Pt monometallic counterpart. Ethanol TPD results on Ni, Pt, and NiPt catalysts point to ethanol dehydrogenation and acetaldehyde decomposition as the first reaction pathways of ethanol steam reforming over the studied catalysts. The active sites responsible for the acetaldehyde decomposition are easily deactivated in the first minutes on-stream by carbon deposits. For Ni and PtNi catalysts, a second reaction pathway, consisting in the decomposition of acetate intermediates formed over the surface of alumina support, becomes the main reaction pathway operating in steam reforming of ethanol once the acetaldehyde decomposition pathway is deactivated. Taking into account the differences observed in the mechanism of ethanol decomposition, the better stability observed for PtNi catalyst is proposed to be related with a cooperative effect between Pt and Ni activities together with the enhanced ability of Ni to gasify the methyl groups formed by decomposition of acetate species. On the contrary, monometallic catalysts are believed to dehydrogenate these methyl groups forming coke that leads to deactivation of metal particles. ∆H° ) 207.7 kJ/mol

1. Introduction The increase of greenhouse gases in the atmosphere and the scarcity of fossil fuels demand a more rational use of our energy resources. Hydrogen is considered to play a key role in the future energy systems as energy vector when used directly as a fuel in internal combustion engines or, indirectly, to supply electricity through the use of fuel cells. Hydrogen is currently produced from nonrenewable materials (methane, hydrocarbons, etc.) which implies CO2 as a byproduct in the transformation processes. In this context, there is growing interest in the study of hydrogen production from biomass as a potential source of renewable energy. Among the biomolecules derived from biomass, ethanol is of particular interest because (i) it has a low toxicity, (ii) it can be easily produced by fermentation of biomass such as sugar cane, waste materials from agroindustries, or forestry residue, (iii) it is a relatively clean fuel in terms of composition, (iv) its hydrogen content is relatively high, and (v) as a liquid it is easy to handle. Ethanol can be efficiently converted in hydrogen by means of its catalytic reaction with steam according to the following reaction:

CH3CH2OH + 3H2O f 6H2 + 2CO2

(1)

† Part of the special issue “Green Chemistry in Energy Production Symposium”. * Corresponding authors, [email protected] and [email protected]. Phone: +34915854773. Fax: +34915854760. ‡ Instituto de Cata´lisis y Petroleoquı´mica (CSIC). § Department of Chemical Engineering, University of Patras.

The ethanol steam reforming reaction includes several steps that involve the need for catalytic surfaces able to (i) dehydrogenate ethanol, (ii) break the carbon-carbon bond of C2 intermediates to produce CO and CH4, and (iii) water reform the C1 products to generate additional hydrogen. On the basis of the influence of the nature of both the metal and support on the catalytic characteristics of supported metals, choice of these elements is a key factor to develop supported catalysts that fulfills the above requirements. The ethanol steam reforming reaction has been performed on several catalyst systems using Ni,1,2 Co,3,4 Ni/Cu,5,6 and noble metals (Pd, Pt, Rh)7-9 as active phases deposited on different oxide supports (e.g., Al2O3, La2O3, ZnO, MgO, etc.). It should be stressed that the nature of support may strongly influence the catalytic performance of steam reforming catalysts and even participate in the reaction. Alumina-based supports are often used in these catalysts because of their mechanical and chemical resistance under reaction conditions. In this case, it must be taken into consideration that alumina surface acidity leads to an important production of ethylene, which can be easily dehydrogenated to form coke over metal phases. Among transition metals, the high C-C bond-breaking activity and the relatively low cost of Ni make it a suitably active phase for ethanol reforming reactions. However, Ni-based catalysts have a strong tendency to form coke in reforming reactions.10 Several authors11-13 have reported that the addition of small amounts of noble metals to Ni catalysts improved their catalytic performance. Such improvement is mainly related to a lower formation of coke deposits, and, therefore, a higher stability of

10.1021/jp906531x  2010 American Chemical Society Published on Web 10/13/2009

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bimetallic catalysts. One explanation of this phenomenon is the activity of noble metal in gasification of coke precursors adsorbed over the surface of the non-noble metal.11 Nevertheless, few studies have been published dealing with the synergy observed in bimetallic catalysts and many questions related to the catalytic performance of bimetallic catalysts are still not answered to date. Depending on the reaction conditions and the catalytic system used, the ethanol steam reforming reaction can proceed through two main pathways: (i) dehydration to produce ethylene and (ii) dehydrogenation to produce acetaldehyde. Both ethylene and acetaldehyde can further react with water producing carbon oxides and methane and/or dehydrogenate to form carbon deposits. In order to maximize hydrogen production, the formation of byproducts such as CO, CH4, C2H4, C2H6, and carbon deposits must be minimized. Although the understanding of ethanol steam reforming reaction has increased significantly in the past few years, only a few attempts to provide information about the reaction steps occurring in the ethanol steam reforming processes are found in the literature.10,14-18 Despite these studies, the detailed surface mechanism for ethanol steam reforming is still unclear, and many questions such as the mechanisms operating in the catalysts deactivation, the role of active phases and supports on the reaction network, the metal interactions in bimetallic catalysts leading to synergetic effects, etc., have not been answered yet. One of the most important issues to investigate is the mechanism of deactivation of the catalysts under reforming conditions. Erdohelyi et al.18 attributed the deactivation of catalysts during the steam reforming of ethanol to the accumulation of acetate-like species on the supports. The high stability of these acetate species might block the migration mechanism of the ethoxide intermediates from the support to the metal particles, where they are decomposed to produce hydrogen. On the other hand, Platon et al.19 suggested that the intermediates ethylene and acetone were the species responsible for the strong deactivation of catalysts observed at low reforming temperatures (523 K). Despite these studies, the catalyst deactivation process is still not entirely understood. In this work, an attempt to unravel the mechanism of the ethanol steam reforming reaction over Pt/Ni/γ-Al2O3 supported catalysts is done. Ethanol interaction with Ni, Pt, and Pt-Ni catalytic systems is studied by the combined analysis of adsorbed intermediates by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and gas products by mass spectrometry, in order to understand the effect of noble metal addition on the reactivity and stability of Ni catalyst. Taking into account that the stability under reaction is one of the most important issues in the design of catalysts for ethanol steam reforming, another objective of this work is to gain insight into the mechanism of deactivation processes operating on Pt and Ni metal phases separately and to compare them with those occurring on the Pt-Ni bimetallic catalyst. Results from this study help us to understand the processes involved in the deactivation of catalysts and also give complementary information on steam reforming reaction pathways and on the role of water in the catalyst activity and stability. 2. Experimental Methods 2.1. Catalyst Preparation. Ni and Pt monometallic catalysts were prepared by impregnation of a commercial γ-Al2O3 (Alfa Aesar, SBET ) 212 m2/g), previously calcined at 923 K, with aqueous solutions of Ni(NO3)2 and Pt(NH3)4(NO3)2, respectively. Ni catalyst (13 wt % Ni) was impregnated for 5 h, subsequently

Sanchez-Sanchez et al. dried at 393 K for 2 h, and finally calcined in air at 773 K for 4 h. Pt sample (2.5 wt % Pt) was impregnated for 2 h and subsequently dried at 383 K for 14 h. The Pt precursor was decomposed by treating the sample at 573 K under vacuum for 3 h. Pt-Ni bimetallic catalyst (2.5 wt % Pt and 13 wt % Ni) was prepared by wet impregnation of the calcined Ni/Al2O3 catalyst with an aqueous solution of Pt(NH3)4(NO3)2 during 2 h. After drying at 383 K for 14 h, the Pt precursor was decomposed following the same procedure under vacuum previously described for the Pt monometallic catalyst. 2.2. Catalyst Characterization. The specific surface areas of the catalysts and the commercial Al2O3 support were calculated by applying the BET method to the N2 adsorption isotherms, measured at liquid nitrogen temperature on a Micromeritics ASAP 2100 apparatus on samples previously outgassed at 473 K for 24 h. Metal dispersion was measured by H2 pulse chemisorption at 298 K using an Ar flow of 50 mL/min and pulses of 0.1 mL (10% H2 in Ar) on a Micromeritics ASAP 2100 unit. Prior to the pulse chemisorption experiment, all samples were reduced under H2/Ar flow (50 mL/min) for 1 h at 623 K (Pt/Al2O3), 823 K (PtNi/Al2O3) or 923 K (Ni/Al2O3) and subsequently flushed under Ar for 15 min at 15 K above the reduction temperature. To calculate metal dispersion, adsorption stoichiometry of H/M ) 1 was assumed. X-ray diffraction (XRD) patterns were recorded on reduced catalysts using a Seifert 3000P vertical diffractometer and nickelfiltered Cu KR radiation (λ ) 0.1538 nm) under constant instrument parameters. For each sample Bragg angles between 5° and 80° were scanned. A rate of 5 s per step (step size: 0.04°2θ) was used during a continuous scan in the abovementioned range. Volume averaged crystallite sizes were determined by applying the Debye-Scherrer equation. X-ray photoelectron spectroscopy (XPS) was used to study the chemical composition of the catalyst surfaces. Photoelectron spectra were recorded with a VG Escalab 200R electron spectrometer equipped with a Mg KR X-ray source (hν ) 1253.6 eV) and a hemispherical electron analyzer operating at constant transmission energy (50 eV). The reduction treatment was carried out ex situ at 623 K (Pt/Al2O3), 823 K (PtNi/Al2O3), or 923 K (Ni/Al2O3) in H2/N2 (1/9 vol) flow for 90 min followed by rereduction in situ at 773K (623 K for Pt/Al2O3) for 30 min. The C 1s, Al 2p, Pt 4d, and Ni 2p core-level spectra were recorded and the corresponding binding energies were referenced to the C 1s line at 284.6 eV (accuracy within (0.1 eV). 2.3. Temperature-Programmed Desorption of Ethanol. Temperature-programmed desorption (TPD) studies were carried out in a U-shape quartz reactor (4 mm internal diameter) connected to a QMS 200 Balzers Prisma quadrupole mass spectrometer. Prior to TPD analysis, the catalysts were activated ex situ at 623 K (Pt/Al2O3), 823 K (PtNi/Al2O3), or 923 K (Ni/ Al2O3) under 10% (vol) of H2 in Ar flow (30 mL/min) for 1 h. Thirty milligrams of pretreated samples were placed in the U-shaped reactor where additional in situ pretreatments were performed. The pretreatments include a surface cleaning under He flow (30 mL/min) at 773 K for 15 min, followed by a reactivation under a 20% (vol) of H2 in He flow (30 mL/min) at 573 K (for Pt/Al2O3), 673 K (for PtNi/Al2O3), or 773 K (for Ni/Al2O3) for 1 h, and finally flushing under He flow (30 mL/ min) at 773 K for 15 min. For TPD experiments, adsorption of ethanol was done under 3% (vol) of ethanol in He flow (30 mL/min) at 300 K for 30 min. After ethanol adsorption, samples were flushed with a 30 mL/min He flow at 300 K for 30 min. Temperature-programmed

Effect of Metal Phase on Ethanol Steam Reforming Mechanism

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TABLE 1: Surface Area, Metal Particle Size (XRD), Metal Dispersion (H2 chemisorption), and Surface Atomic Ratio (XPS) of the Reduced Catalysts particle sizea (nm) sample

SBET (m2/g of Al2O3)

Ni0

Pt/Al2O3 Ni/Al2O3 PtNi/Al2O3

222 207 225

7 10

a

Obtained from XRD data by Debye-Scherrer equation. ratio from nominal composition.

surface atomic ratioc

Pt0

dispersionb (%)

5

22.1 4.1

20 b

Ni/Al

Pt/Al 0.0037 (0.0069)

0.026 (0.130) 0.050 (0.135)

0.0048 (0.0083)

c

From H2 chemisorption. Calculated from XPS spectra. In parentheses, atomic

desorption experiments were performed heating the catalyst with a rate of 10 K/min up to 900 K. The analysis of gas products in the mass spectrometer was done following the changes in the signal intensity of m/z ) 2, 4, 15, 18, 28, 29, 31, and 44, which corresponds to the main fragments of H2, He, CH4, H2O, CO, CH3COH, C2H5OH, and CO2, respectively. C2H4 and C2H6 signals were followed by their second most intense fragment (m/z ) 26 and m/z ) 30, respectively), because the main fragment was coincident with CO signal (m/z ) 28). The intensity of each product was represented by the selected signal after subtracting the possible contributions of fragments from other compounds. Such contributions were calculated on the basis of the intensity ratios between the fragments characteristic of the individual molecules. 2.4. Isothermal Ethanol Decomposition and Steam Reforming. Isothermal experiments were carried out in the same equipment and using the same pretreatments previously described for TPD measurements. Isothermal reactions were studied by heating the sample up to 673 K under He flow (30 mL/min) and then switching the feed to the reacting mixture. For ethanol decomposition experiments, a 30 mL/min flow of a 1% (vol) of ethanol in He was introduced in the reactor. In the case of ethanol steam reforming experiments, the feed consisted in a 30 mL/min He flow containing 3% (vol) of H2O and 1% (vol) of ethanol. The analysis of gas products in the mass spectrometer was done following the changes in signal intensity corresponding to the main fragments of H2, He, CH4, H2O, CO, CO2, CH3CHO, C2H5OH, C2H4, C2H6, and C3H6O. 2.5. DRIFTS. Temperature-programmed desorption of ethanol and isothermal ethanol decomposition and steam reforming experiments were also studied by DRIFTS. DRIFT spectra were recorded using a Nicolet 740 FTIR spectrometer equipped with a diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) cell, an MCT detector, and a KBr beam splitter. FTIR spectra were collected with a 64-scan data acquisition at a resolution of 4 cm-1 using a DRIFTS cell (Spectra Tech) directly connected to a flow system equipped with mass flow controllers and a set of valves that allowed selection of feed gas composition. In a typical experiment, the catalyst was finely powdered and placed into the sample holder, and its surface was carefully flattened to increase the intensity of the reflected IR beam. Samples were pretreated and activated following the same experimental protocol previously described for TPD and isothermal ethanol decomposition and steam reforming experiments. After pretreatments, the catalyst was cooled to room temperature under He flow (30 mL/min). In the cooling stage, background spectra were collected every 50 K. In the case of isothermal experiments, only background at 673 K was recorded. The gas reaction mixtures for TPD and isothermal experiments were prepared in the same way as for mass spectrometry experiments. For ethanol TPD experiments, adsorption of ethanol was done under 3% (vol) of ethanol in He flow (30

mL/min) at 300 K for 30 min. The first spectrum was recorded at room temperature after flushing under He for 10 min. Then the temperature was increased stepwise up to 773 K and spectra were recorded every 50 K after equilibration for 3 min on stream. Ethanol decomposition experiments at 673 K were done by flowing 30 mL/min of 1% (vol) of ethanol in He through the DRIFT cell while taking spectra as fast as possible (ca. 1 scan per min). In a similar way, ethanol steam reforming reactions were performed flowing 30 mL/min of 1% (vol) of ethanol and 3% (vol) of water in He flow through the DRIFT cell. 3. Results 3.1. Catalyst Characterization. Table 1 summarizes the main results of the characterization of reduced catalysts. Surface areas of catalysts, obtained from N2 adsorption isotherms, show that the addition of Pt and/or Ni does not change the textural properties of the alumina support (SBET ) 212 m2/g). Among the studied catalyst, the Pt monometallic sample shows a relatively high Pt dispersion, as can be inferred from the H2 chemisorption value and its XRD particle size (5 nm). On the contrary, the monometallic Ni catalyst shows a considerably lower metal dispersion as indicated by the H2 chemisorption data and the low Ni/Al atomic ratio at the surface of this catalyst. In the case of the bimetallic sample, Pt and Ni appear with crystalline particle sizes higher than its monometallic counterparts. Nevertheless, the surface atomic ratios calculated from XPS show that the amount of Pt exposed at the surface of the bimetallic catalyst is similar to the amount detected on the monometallic counterpart. In the case of Ni, the Ni/Al ratio observed in the bimetallic sample indicates that the presence of Pt causes an increase in the exposition of Ni metal atoms with respect to that achieved on monometallic Ni/Al2O3 catalyst. 3.2. Temperature-Programmed Desorption of Ethanol. Figure 1 represents temperature-programmed desorption profiles obtained from mass spectrometry after ethanol adsorption on Pt/Al2O3, Ni/Al2O3, and PtNi/Al2O3 catalysts. It is observed that all catalysts showed a first desorption peak at temperatures lower than 400 K, assigned to ethanol physically adsorbed and small amounts of C2 products (acetaldehyde and ethylene). For the Pt-containing catalyst (Pt/Al2O3, Figure 1a, and PtNi/Al2O3, Figure 1c) small amounts of CH4, CO, CO2, or H2 were also detected, indicating that Pt phase has a slight C-C bond rupture activity at this low temperature. The main desorption peaks were observed at 473 K for Pt catalyst (Figure 1a), at 523 K for Ni catalyst (Figure 1b), and at 463 K for PtNi catalyst (Figure 1c). In this temperature range, desorbed species are CH4, CO, H2, and small amounts of acetaldehyde. The formation of these products is probably related to the dehydrogenation of ethanol (reaction 2) and the subsequent decomposition on metal particles of the produced acetaldehyde into CO and CH4 (reaction 3).

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Sanchez-Sanchez et al. the C-C bond at room temperature, in agreement with the desorption of small amounts of H2 and C1 species observed by MS at low temperatures (Figure 1a).

CO(a) + OH(a) a HCOO(a)

(4)

The increase in temperature causes a decrease in the intensity of absorption bands corresponding to molecular ethanol and an increase in the intensity of the linear CO band at 2000 cm-1. At 523 K, the highest concentration of CO species on the Pt/ Al2O3 surface was achieved, as can be observed by the shift in the vibration wavenumber toward higher frequency values, typical of dipole coupling between adjacent CO molecules.23 When concentration of CO adsorbed on the surface is high, a second band between 1750 and 1800 cm-1 appeared, which is tentatively assigned to bridged carbonyls on Pt particles.24 The formiate band at 1589 cm-1 was present in all the DRIFT spectra for temperatures higher than 423 K. A second contribution at ca. 1575 cm-1 became progressively visible with the increase in temperature. At 523 K, molecular ethanol bands completely disappear and a new band centered at 1455 cm-1 appears. This latter band, and the one detected at 1575 cm-1 have been ascribed to the symmetric and asymmetric vibration, respectively, of the O-C-O group in acetate species.22 The formation of surface acetates at high temperatures can be related to the oxidation of adsorbed acetaldehyde species by OH groups over the alumina surface,25 following reaction 5. Figure 1. Temperature-programmed profiles after room temperature ethanol adsorption on (a) Pt/Al2O3, (b) Ni/Al2O3, and (c) PtNi/Al2O3.

CH3CH2OH f CH3COH + H2

(2)

CH3COH f CH4 + CO

(3)

TPD profiles registered for Ni/Al2O3 (Figure 1b) and PtNi/ Al2O3 (Figure 1c) show second desorption peaks of CH4 and CO2, of higher intensity in the case of the bimetallic sample, at ca. 550 K. Finally, at temperatures above 650 K desorption of variable amounts of CO, CO2, and H2 is observed. In this temperature range, the Pt catalyst mainly desorbs CO2 and H2, while Ni catalyst produces CO2, H2, and a relatively important amount of CO. For PtNi bimetallic catalyst, only small amounts of CO and H2 are detected in this temperature range, probably because the surface of this catalyst is free of intermediates after the important desorption of CO2 and CH4 previously observed at 550 K. 3.2.1. DRIFTS. The in situ DRIFT spectra obtained during ethanol TPD on Pt/Al2O3, Ni/Al2O3, and PtNi/Al2O3 catalysts are shown in Figure 2. The DRIFT spectra corresponding to Pt/Al2O3 catalyst (Figure 2a) show at room temperature bands at 1450, 1390, and 1278 cm-1 (very weak) which are characteristic of molecularly adsorbed ethanol.20 Furthermore, a band is observed at 972 cm-1, ascribed to the F(CH3) vibration in η2-acetaldehyde,21 that originated from ethanol dehydrogenation over the surface of the catalyst. Besides the preceding vibrations, a band around 2000 cm-1 was detected, which is typical of CO species linearly absorbed on Pt particles, and a band centered at 1589 cm-1 that can be associated with the asymmetric O-C-O vibration of formiate species.22 Formiate intermediate is probably originated from the association of adsorbed CO with the OH surface groups of alumina support (reaction 4). The detection of CO and formiate species adsorbed on the surface of Pt/Al2O3 catalyst points to the capacity of this system to break

CH3COH(a) + OH(a) f CH3COO(a) + H2

(5)

The presence of acetate species is confirmed by the increase in the relative intensity of the band at 2940 cm-1 (not shown), which is characteristic of C-H stretching vibration of the methyl group in acetates.21 The two bands in the 1300-1700 cm-1 region ascribed to formiate and acetate species showed different shoulders (the most important at ca. 1640 cm-1), which indicates the presence of carbonate and/or bicarbonate species adsorbed over the alumina support.26 At 723 K, the main intermediate adsorbed on surface corresponds to acetate species, since the two broad bands in this region are centered at 1575 and 1460 cm-1. However, the typical spectrum for acetate species shows similar intensities for symmetric and asymmetric O-C-O vibration bands while for adsorbed formiate, symmetric band is only about 1/3 as intense as the asymmetric one.22 Therefore, the lower intensity of the band at 1455 cm-1 with respect to the band at 1575 cm-1 observed at 723 K (Figure 2a) points out that an important amount of formiate is still adsorbed on this catalyst. Figure 2b represents DRIFT spectra taken for the Ni/Al2O3 sample during ethanol TPD experiments. At room temperature, bands at 1450, 1390, and 1278 cm-1 corresponding to molecular ethanol and one band at ca. 960 cm-1 corresponding to η2-acetaldehyde are visible. Moreover, bands at 1170, 1120, and 1070 cm-1, which were absent in DRIFT spectra on Pt/Al2O3 (Figure 2a), were detected. Such bands are associated with adsorbed ethoxide species.20 Absorption bands due to formiate or CO adsorbed species are not detected, indicating that this catalyst is not active in C-C bond rupture at room temperature. The absorption bands corresponding to adsorbed ethanol and acetaldehyde species decrease with the increase in temperature, disappearing for temperatures higher than 473 K. Bands corresponding to ethoxide species disappear at 573 K. Similarly to DRIFT spectra for Pt/Al2O3 catalyst, two bands centered at



Figure 2. DRIFT spectra during temperature-programmed desorption of ethanol over (a) Pt/Al2O3, (b) Ni/Al2O3, and (c) Pt-Ni/Al2O3 at different temperatures.

1455 and 1575 cm-1 are detected when the temperature reached 473 K (Figure 2b). Therefore, the formation of acetates by oxidation of adsorbed acetaldehyde species by OH groups on alumina support (reaction 5) is also taking place on Ni catalyst. The intensity of acetate bands continuously increased with the increase in temperature. As stated above, the two bands around 1455-1575 cm-1 might also include contributions corresponding to formiate and carbonate species. In the range of temperatures from 523 to 673 K (Figure 2a), the intensity of these two bands around 1455-1575 cm-1 is similar, indicating that acetate is the main intermediate in this temperature range. However, at temperatures higher than 673 K, the intensity of the bands at 1455-1575 cm-1 together with the absence of absorption bands in the C-H stretching region at 2800-3100 cm-1 (not shown) indicates that the main adsorbed intermediates in this temperature range are related to carbonate and bicarbonate species. Figure 2c shows DRIFT spectra for ethanol TPD over bimetallic PtNi/Al2O3 catalyst. At room temperature, absorption bands corresponding to molecular ethanol (1450, 1390, and 1278 cm-1), ethoxide (1170, 1120, and 1070 cm-1), η2-acetaldehyde (960-970 cm-1, weak), and CO species adsorbed on metals (a very weak band centered at ca. 2028 cm-1) were observed. The presence of adsorbed CO species indicates some activity of the catalysts for C-C bond rupture at room temperature but considerably lower than that observed for Pt/Al2O3 catalyst (Figure 2a). At 373 K, the acetaldehyde band at 960-970 cm-1 disappeared, while ethanol and ethoxide bands were visible up to 473-523 K. The band at 2010-2030 cm-1, ascribed to linear CO over reduced metals, increased with temperature and achieved its maximum intensity at 523 K. It is worth noting that the shape of CO band for bimetallic catalyst, with a shoulder at ca. 1980 cm-1, is remarkably different from the CO band recorded for Pt monometallic catalyst (Figure 2b). This fact suggests that different kinds of metallic adsorption sites for CO species are involved in monometallic Pt and bimetallic PtNi catalysts (CO species adsorbed over both Pt and Ni particles or electronic interaction between Pt and Ni particles). In the temperature range from 723 to 773 K, CO is the main intermediate adsorbed on the surface of the PtNi/Al2O3 catalyst. In this temperature range, a broad shoulder at 1824 cm-1 is observed, which was ascribed to bridged CO species. The shape of the two main absorption bands in the 1300-1600 cm-1 region, recorded for PtNi/Al2O3 catalyst at temperatures higher than 523 K, points to the presence of formiate species coexisting with acetate and carbonate species. Evolution with temperature of the absorption bands in the 1300-1600 cm-1 region together with the absence of absorption bands corresponding to the C-H

Figure 3. Product distribution of ethanol decomposition over Pt/Al2O3 catalyst (1% EtOH in a He flow of 30 mL/min, T ) 673 K, P ) 1 atm).

stretching region (not shown) indicates that above 673 K carbonate species were the main adsorbed intermediate together with the linear CO adsorbed over metal particles. The evolution of adsorbed intermediates and gaseous products observed on ethanol TPD on all catalysts are similar to that obtained when ethanol and water are coadsorbed (results not shown here) indicating that the high amount of OH groups existing on the surface of the support react with adsorbed ethanol molecules in a similar way if water is or not coadsorbed with ethanol. 3.3. Ethanol Decomposition Reaction at 673 K. Figure 3 depicts the product distribution obtained from ethanol decomposition reaction at 673 K on Pt/Al2O3 catalyst. As observed in this figure, the main products detected by mass spectrometry in the first 5 min under reaction were C2H6, CH4, CO2, and H2. This product distribution indicates that metallic Pt is highly active in the C-C bond breaking of C2 intermediates (to produce CH4, CO2, and H2) and also in hydrogenation reactions (to produce C2H6 from ethylene originated by ethanol dehydration over alumina acid sites). Nevertheless, it can be observed that after 4-5 min on stream the initial activity of Pt catalyst dramatically drops indicating some kind of deactivation process. The loss in Pt activity is revealed by the fast disappearance of products typically formed on metal particles such as C1 compounds. This loss in activity is accompanied by the simultaneous appearance of C2H4 and H2O in the gas phase, both related to the activity of alumina support in ethanol dehydration (reaction 6).

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C2H5OH f C2H4 + H2O

Sanchez-Sanchez et al.

(6)

Besides C2H4 and H2O, small amounts of CO and H2 were detected after Pt catalyst deactivation. These products are attributed to a residual activity of Pt for secondary reactions such as partial gasification of coke and/or coke precursors by reaction with H2O (reaction 7) produced from the dehydration of ethanol.

CHx + OH(a) f CO + (x + 1)/2 H2

(7)

Figure 4 show the product distribution obtained during ethanol decomposition at 673 K on Ni/Al2O3 catalyst. Initially, Ni catalyst only produces CH4, H2, CO2, and small amounts of CO, indicative of its activity for ethanol dehydrogenation and C-C bond rupture reactions. However, after a few minutes under reaction, there was a decrease in the concentration of C1 compounds and, simultaneously, an increase in the production of H2O. In addition to that, small amounts of acetaldehyde, C2H4 and C2H6 were observed. These changes in gas products were similar to those observed for Pt/Al2O3 catalyst (Figure 3) and, as mentioned above, indicated a loss of activity of the metal particles for the C-C bond rupture. In this first period, H2O signal increases fast and reaches a constant value after ca. 15 min. On the contrary, the MS signal corresponding to C2H4, which is the other product of ethanol dehydration (reaction 6), increases more slowly with a slope remarkably different from the slope of the water signal. This phenomenon could be a consequence of the activity of Ni particles in ethylene dehydrogenation (reaction 8) that consumes the ethylene produced by ethanol dehydration reaction. This is in agreement with the higher H2 production observed on Ni catalyst after partial deactivation (Figure 4) in comparison to the hydrogen production achieved over deactivated Pt catalyst (Figure 3).

C2H4 f 2H2 + C(s)

(8)

Figure 5 shows the product distribution obtained during ethanol decomposition at 673 K on PtNi/Al2O3 catalyst. As observed in the Figure 5, at the start of the reaction ethanol is totally converted into CH4 and CO2. However, the concentration of these products decreased with time on stream with a concomitant increase in the concentration of CO and H2. After 4-5 min under reaction, small amounts of H2O and C2 compounds are detected in the product gases. CO and H2 signals reached a maximum for a reaction time of ca. 18 min. Then, CO, CH4, and H2 slowly decreased while H2O and C2 concentration increased continuously. As previously mentioned, the changes in gas products point to a deactivation of the metal functions in the catalyst. However, it should be stressed that the slope of the change in product concentrations observed on bimetallic PtNi/Al2O3 catalyst (Figure 5) is much smaller than that measured for Pt (Figure 3) and Ni (Figure 4) monometallic counterparts. Another interesting feature observed in bimetallic PtNi catalyst is that C2H6 formation, which is associated with the hydrogenation activity of Pt, remains constant even after the lost in activity for C-C bond rupture of catalyst. Therefore, the observed deactivation seems to be related to a modification of active sites responsible for C-C bond rupture reactions, while a second type of active site, responsible for hydrogenation reactions, remains active.

Figure 4. Product distribution of ethanol decomposition over Ni/Al2O3 catalyst (1% EtOH in a He flow of 30 mL/min, T ) 673 K, P ) 1 atm).

Figure 5. Product distribution of ethanol decomposition over PtNi/ Al2O3 catalyst (1% EtOH in a He flow of 30 mL/min, T ) 673 K, P ) 1 atm).

3.3.1. DRIFTS. Figure 6 shows DRIFT spectra of intermediate species adsorbed over Pt, Ni, and PtNi catalysts during ethanol decomposition reaction. On Pt/Al2O3 catalyst (Figure 6a) during the first 4 min under reaction, a vibration band is observed at 2050 cm-1 corresponding to CO linearly adsorbed on Pt particles. The intensity (and wavenumber) of this band decreased with time, appearing, simultaneously, as weak bands at 1592 and 1378 cm-1 associated to formiate and a band at 1454 cm-1 corresponding to acetate species. After 10 min onstream, the acetate absorption bands became more intense than those corresponding to formiate species. The DRIFT spectrum at the beginning of the ethanol decomposition reaction over Ni/Al2O3 catalyst (Figure 6b) only shows a band at 2024 cm-1, with two shoulders at 1920 and 1865 cm-1 assigned to CO species adsorbed on Ni particles by one (linear), two (bridged), or three bonds (2-fold bridged, Ni3-CO), respectively.11,27 After a few minutes under reaction, the above CO bands completely disappear becoming progressively visible new absorption bands ascribed to acetate species (at 1575 and 1455 cm-1). However taking into account the width of these bands and the presence of shoulders at ca. 1340, 1420, and 1650 cm-1, the existence of formiate and carbonates species accompanying acetate species cannot be discarded. DRIFT spectra corresponding to ethanol decomposition at 673 K on PtNi/Al2O3 catalyst are depicted in Figure 6c. At the initial stage of the reaction only CO bands, with the main contribution at 2026 cm-1 and two shoulders at 1920 and 1850



Figure 6. DRIFT spectra obtained during ethanol decomposition reaction over (a) Pt/Al2O3, (b) Ni/Al2O3, and (c) Pt-Ni/Al2O3 (1% EtOH in a He flow of 30 mL/min, T ) 673 K, P ) 1 atm).

Figure 7. Product distribution of ethanol steam reforming over Pt/ Al2O3 catalyst (1% EtOH/3% H2O in a He flow of 30 mL/min, T ) 673 K, P ) 1 atm).

cm-1, are detected on this catalyst. Although it is not possible to associate the different contributions of CO band to Pt or Ni phases, the comparison of this spectrum with the spectra of monometallic catalysts (Figure 6, parts a and b) indicates that the band at 1850 cm-1 might be due to 2-fold bridged CO adsorbed over Ni particles. The absorption profile of the band corresponding to CO on PtNi/Al2O3 catalyst is very different from those observed over Pt/Al2O3 or Ni/Al2O3 catalysts. This fact points to a coexistence in the bimetallic catalysts of CO species adsorbed over Ni and Pt sites and/or an electronic Pt-Ni interaction that modifies the CO absorption. As observed for monometallic samples, intermediates adsorbed over PtNi catalyst change with time on stream. In this way, after the first 2-3 min, the CO band decreased and formiate and acetate bands appeared at 1590, 1455, and 1378 cm-1. After 25 min, the more intense band in the 1300-1700 cm-1 region appears at 1589 cm-1, which indicates that formiate species are the main adsorbed intermediates on the PtNi/Al2O3 surface at long reaction times. 3.4. Ethanol Steam Reforming Reaction at 673 K. Figure 7 shows the product distribution obtained in the ethanol reforming reaction on Pt/Al2O3 catalyst at 673 K. Initially, the main products of reaction on Pt/Al2O3 catalyst are CH4, CO2, and H2, and small amounts of CO, C2H4, and C2H6. After 20 min on stream, CH4, CO2, and H2 concentrations reach a maximum and decrease at longer times of reaction. In the case of C2H6, the maximum concentration was achieved after 48 min under reaction, and its decrease follows the same trend

Figure 8. Product distribution of ethanol steam reforming over Ni/ Al2O3 catalyst (1% EtOH/3% H2O in a He flow of 30 mL/min, T ) 673 K, P ) 1 atm).

as that observed for H2 production. On the contrary, C2H4 concentration continuously increased, and, after 2 h on stream, it became the main product. As previously mentioned (section 3.2) both ethylene formation and the decrease in C1 products are indicative of the deactivation of Pt active sites responsible for the rupture of C-C bonds. In contrast, the C2H6 profile indicates that Pt/Al2O3 catalyst maintains its activity in ethylene hydrogenation, as far as H2 is available on the surface. Steam reforming of ethanol over Ni/Al2O3 (Figure 8) and PtNi/Al2O3 (Figure 9) samples produces H2, CO2, CH4, and CO as main reaction products. This means that both catalysts are highly active in reactions that imply C-C bond rupture of ethanol or C2 intermediate molecules. In addition, the product distribution obtained for these systems was found to be constant for more than 2 h on stream. This result reveals that Ni and PtNi catalysts have better stability in ethanol steam reforming than Pt/Al2O3 catalyst. 3.4.1. DRIFTS. Figure 10a shows DRIFT spectra of intermediate species adsorbed over Pt/Al2O3 catalyst during the ethanol steam reforming reaction. At the start of the reaction, the main adsorbed intermediate on Pt/Al2O3 catalyst was CO linearly adsorbed over metallic Pt sites (absorption band at 2050 cm-1). Weak bands at 1590 and 1380 cm-1 corresponding to formiate species are also detected. However, after 15 min on stream, formiate bands tend to disappear while bands at 1455 and 1575 cm-1 simultaneously appear associated with adsorbed acetates. The intensity of CO band starts to decrease after 25 min on-stream. Figure 10b shows DRIFT spectra obtained for Ni/Al2O3 during ethanol steam reforming at 673 K. From the Figure, it

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Sanchez-Sanchez et al. generated as intermediates in the ethanol dehydrogenation reaction (reaction 9).

CH3CH2OH f CH3CH2O- + H f CH3COH + H2

(9)

Figure 9. Product distribution of ethanol steam reforming over PtNi/ Al2O3 catalyst (1% EtOH/3% H2O in a He flow of 30 mL/min, T ) 673 K, P ) 1 atm).

can be observed that adsorbed intermediates evolve in the course of reaction. The band at 2015 cm-1 and its shoulders at 1905 and 1845 cm-1, associated with linear and bridged CO species adsorbed on metallic Ni, are intense in the first 10 min on stream, but they decrease with time disappearing after 35 min under reaction. Acetate bands can be also observed on this sample after only 2 or 3 min under reaction (Figure 10b). Acetate intermediate accumulates on the surface with time and, after 15 min, it becomes the main adsorbed species over the Ni/Al2O3 catalyst. DRIFT spectra of PtNi catalyst surface during ethanol steam reforming reaction at 673 K are displayed in Figure 10c. Initially, the only intermediate detected at the surface of this catalyst is CO adsorbed over metal particles. The main band, at 2015 cm-1, is due to linear CO, and the shoulder at 1850 cm-1 is ascribed to CO bridged bond species, probably related to Ni particles. It must be pointed out that the CO band profile in Figure 10c substantially differs from those detected in monometallic catalysts (Figure 10, parts a and b). A similar observation was reported for DRIFT spectra recorded during ethanol decomposition reaction (section 3.2.1). Evolution of intermediates with time on stream is observed by DRIFT spectra (Figure 10c). After 15 min on stream, the intensity of the two low-frequency shoulders of CO band starts to decrease and finally disappears after 35 min on reaction. In the 1300-1700 cm-1 region, weak bands corresponding to formiate and acetate species were detected after 15-20 min on stream. These species accumulate on the surface, as inferred from the dramatic increase in its intensity between 15 and 50 min (Figure 10c). 4. Discussion The interaction of ethanol with Pt, Ni, and PtNi catalysts supported over alumina was investigated by ethanol temperatureprogrammed desorption experiments followed by mass spectrometry (Figure 1) and in situ DRIFT spectroscopy (Figure 2). At room temperature, it was observed that molecular ethanol and η2-acetaldehyde, from ethanol dehydrogenation (reaction 2), are present over the catalysts under study (Figure 2). In the case of Pt monometallic catalyst, the DRIFT spectrum at room temperature (Figure 2a) also showed vibration bands attributed to CO and formiate species. This fact points to a slight activity of Pt/Al2O3 in C-C bond breaking at low temperatures, which is confirmed by desorption at this temperature of small amounts of C1 compounds (Figure 1a). For Ni catalyst, adsorbed ethoxide species are detected at room temperature (Figure 2b), probably

The fast decomposition of acetaldehyde on Pt particles to form C1 compounds (COx and CH4) may explain the absence of ethoxide bands over this catalyst (Figure 1a). On the contrary, the Ni metallic phase is not active in C-C bond rupture at room temperature and therefore acetaldehyde and ethoxide species from dehydrogenation of ethanol accumulate on the catalysts surface. In the bimetallic PtNi sample, both linear CO and ethoxide intermediates were observed at room temperature (Figure 2c), which suggests that at this temperature the activity of Ni and Pt metal phases is apparently independent of the presence of the second metal phase. TPD of ethanol on Pt, Ni, and PtNi catalysts (Figure 1) shows the most intense desorption peaks, mainly consisting in CO, H2, and CH4, in the temperature range between 380 and 560 K. This indicates that ethanol dehydrogenation and subsequent acetaldehyde decomposition (reactions 2 and 3) are the main ethanol reaction pathway operating at this temperature on the three catalysts studied. Nevertheless, the temperature at which the main desorption peak appears is different depending on the nature of the active phase. The PtNi bimetallic catalyst showed the lowest desorption temperature (463 K, Figure 1), followed by Pt monometallic catalyst (473 K, Figure 1) and finally, Ni catalyst (523 K, Figure 1). The lower desorption temperature observed for the PtNi sample suggests that the activity of this catalyst in the C-C bond rupture is higher than that of monometallic counterparts. This indicates some type of cooperation between Ni and Pt active phases in the bimetallic formulation. Adsorbed acetate species, from the reaction of adsorbed acetaldehyde with OH groups of the alumina support (reaction 5), were detected on the surface of all the catalysts during TPD of ethanol when temperature reached 573 K (Figure 2). At high temperatures, these acetate species react on metallic phases forming different products depending on the nature of the metal. In the case of Ni/Al2O3 catalyst, the desorption peaks detected in the temperature range from 523 to 823 K (see Figure 1b) may be assigned to the direct decomposition of acetate species as indicated by reaction 10.

CH3COO(a) f CO2 + CH3(a)

(10)

Part of the methyl groups that originated from acetate decomposition (reaction 10) are desorbed at 573 K as methane, following the mechanism proposed by Jacobs et al.28 Nevertheless, the small intensity of CH4 desorption peak recorded at 573 K in the TPD profile of Ni catalyst (Figure 1b), together with the relatively high amount of H2 desorbed at higher temperatures, suggests that most of the methyl groups generated from acetate decomposition undergo dehydrogenation forming coke deposits on metallic Ni particles. This assertion is consistent with the dramatic decrease observed in the DRIFT intensity of acetate bands at 1575 and 1455 cm-1 when temperature becomes higher than 673 K (Figure 2a). In the case of the Pt/Al2O3 sample, its TPD profile (Figure 1c) does not show any desorption peaks associated to CO2 and/or CH4 in the temperature range between 550 and 600 K, which is indicative of a low activity of this monometallic Pt sample for the acetate decomposition reaction, in agreement with literature.28 On the contrary, the



Figure 10. DRIFT spectra obtained during ethanol steam reforming reaction over (a) Pt/Al2O3, (b) Ni/Al2O3, and (c) PtNi/Al2O3 (1% EtOH/3% H2O in a He flow of 30 mL/min, T ) 673 K, P ) 1 atm).

TPD profile for PtNi/Al2O3 catalyst (Figure 1c) shows important desorption peaks of CO2 and CH4 at 550 K. Therefore, when both Pt and Ni are present on the catalyst, most of the methyl groups originating from acetate decomposition (reaction 10) are desorbed as methane and only a small amount suffers dehydrogenation producing C and H2. This fact can be related to the higher stability and the superior catalytic performance of bimetallic PtNi catalyst when compared to monometallic counterparts, which will be further discussed. Decomposition of ethanol at 673 K over Ni, Pt, and PtNi catalysts (Figures 3-5) showed that all samples initially display high activity for ethanol dehydrogenation and for the rupture of the C-C bond of the C2 intermediates. In addition, Pt catalyst shows activity in the hydrogenation of ethylene intermediate, as can be inferred from the formation of important amounts of ethane (Figure 3). After a few minutes of reaction, the product distribution (Figures 3-5) and surface species detected by DRIFTS on all catalysts gradually change (Figure 6). One can notice that the intensity of DRIFT bands of adsorbed CO and formiate species on all catalysts decreases with time on stream, which is in line with the decrease in the concentration of gaseous C1 products (COx and CH4) detected by mass spectrometry (Figures 3-5). Simultaneously, the C2 gas products (ethylene and ethane) increase and acetate intermediate accumulates on the catalyst surfaces (Figure 6). The observed changes with time on-stream are interpreted in terms of deactivation processes on metal phase occurring when the catalysts are exposed to ethanol at 673 K in the absence of water. The highest deactivation rate was observed for Pt catalyst, followed by Ni monometallic catalyst. A higher stability was achieved when both Pt and Ni are present in the catalyst, as can be inferred from the smaller slope of the decrease in C1 products observed by MS. Ethanol steam reforming tests on catalysts at 673 K (Figures 7-9) showed that Pt, Ni, and PtNi catalysts are highly active in the conversion of ethanol into H2, CO2, CH4, and small amounts of CO. In the case of Pt/Al2O3 catalyst (Figure 7) an important amount of C2H6 is also detected, probably due to the Pt activity for ethylene hydrogenation. Evolution of product distribution with time on-stream shows that monometallic Pt catalyst deactivates after 30 min (Figure 7) while Ni/Al2O3 (Figure 8) and PtNi/Al2O3 (Figure 9) show stable performance. Despite the stable product distribution observed on Ni and PtNi catalysts, some variations in the number and type of intermediates adsorbed on its surfaces are detected by DRIFT spectroscopy (Figure 10, parts b and c). The surface changes with time on-stream are characterized by a decrease in the intensity of CO bands and by an increase in the accumulation of adsorbed

acetates. This evolution is similar to those observed in ethanol decomposition (Figure 7, parts b and c) but it appears at longer reaction times. The evolution of surface intermediates during reaction is hypothesized to be a consequence of deactivation processes on the active sites responsible for the decomposition of acetaldehyde intermediate into CO and CH4 (reaction 3). The accumulation of acetates on surface with time on stream suggests that once the active sites responsible for the decomposition of acetaldehyde are deactivated, the ethanol steam reforming proceeds through the decomposition of acetate intermediates (reaction 10). The accumulation of acetate intermediates on the catalyst surface suggests that the rate-determining step, when this pathway operates, is the access of acetate species, formed over alumina, to active sites located on metal particles. These observations are in agreement with the fast deactivation observed in ethanol steam reforming on Pt/Al2O3 catalysts (Figure 7), since, as indicated above, Pt is found to be unable to decompose the acetate intermediates. Deactivation processes in steam reforming catalysts are usually related to the formation of coke deposits by dehydrogenation of CxHy intermediates.29 In the case of ethanol reforming, the species related with the formation of coke on catalysts are probably methyl groups from C-C breaking reactions and ethylene species formed by dehydration of ethanol over alumina. The results obtained in this work suggest that the mechanism of the catalyst deactivation depends on the nature of the active phase. Ethylene dehydrogenation on Ni/Al2O3 catalyst during ethanol decomposition is inferred from the difference between the formation rate of C2H4 and H2O (Figure 5), while the methyl dehydrogenation on Ni/Al2O3 and Pt/Al2O3 catalyst is inferred from the low amount of CH4 desorbed after acetate decomposition in TPD experiments (Figure 1). PtNi catalyst shows higher ability to desorb methyl groups, as inferred from the CO2 and CH4 desorption peaks at 550 K in ethanol TPD profile (Figure 1c), which could explain the higher stability of this catalyst. The improvement in activity and stability observed when both Pt and Ni are present in the catalyst can be also explained in terms of a cooperative effect of the activity of both metals. Adsorbed intermediates detected at low temperature by DRIFT spectroscopy during TPD of ethanol (Figure 2) indicated that ethanol reacts over Ni and Pt particles in bimetallic catalyst in a similar way as it reacts over monometallic catalysts. However, the products formed over each metal could favor the activity of the other metal. In this way, ethylene hydrogenation over Pt competes with ethylene dehydrogenation over Ni particles minimizing coke formation. At the same time, the high activity

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of Ni in the rupture of C-C bonds releases hydrogen that allows the Pt particles to hydrogenate the ethylene formed on support. 5. Conclusions Mechanistic aspects of the ethanol steam reforming on Pt, Ni, and PtNi catalysts supported on γ-Al2O3 are investigated by ethanol temperature-programmed desorption/reaction experiments followed by mass spectrometry and in situ DRIFT spectroscopy. The main reaction pathway for ethanol reforming over the three catalysts studied was found to be the ethanol dehydrogenation and subsequent acetaldehyde decomposition. The active sites responsible for acetaldehyde decomposition are easily deactivated in the first minutes on-stream by carbon deposits formed from dehydrogenation of CxHy intermediates. For Ni and PtNi catalysts, a second reaction pathway, consisting in the decomposition of acetate intermediates formed over the surface of alumina support, became the main reaction pathway operating in the steam reforming of ethanol once the acetaldehyde decomposition pathway is deactivated. It is found that most of the methyl groups formed from acetate decomposition remained on Ni/Al2O3 catalyst leading to further dehydrogenation that produces deactivation by coke. A DRIFTS-MS study of ethanol reactions revealed that bimetallic PtNi/Al2O3 catalyst possesses a higher activity and stability in converting ethanol into hydrogen and C1 products than monometallic counterparts. Such improvement in catalytic performance was associated with the higher activity in the gasification of methyl groups formed in the decomposition of acetate species together with a cooperative effect between Pt and Ni activities. Acknowledgment. The authors are grateful for financial support from the Ministerio de Ciencia e Innovacio´n of Spain (Project ENE-67533-C02-01). References and Notes (1) Fatsikostas, A. N.; Kondarides, D. I.; Verykios, X. E. Catal. Today 2002, 75 (1-4), 145–155. (2) Freni, S.; Cavallaro, S.; Mondello, N.; Spadaro, L.; Frusteri, F. J. Power Sources 2002, 108 (1-2), 53–57. (3) Haga, F.; Nakajima, T.; Miya, H.; Mishima, S. Catal. Lett. 1997, 48 (3-4), 223–227.

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Mechanistic Aspects of the Ethanol Steam Reforming Reaction for

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