Energy & Fuels 2008, 22, 1873–1879
1873
Ethanol Steam Reforming over Pt Catalysts Supported on CexZr1-xO2 Prepared via a Glycine Nitrate Process Yazhong Chen, Zongping Shao,* and Nanping Xu State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing UniVersity of Technology, Nanjing, 210009, P.R. China ReceiVed September 26, 2007. ReVised Manuscript ReceiVed February 15, 2008
Hydrogen production for application to proton exchange membrane fuel cells (PEMFCs) has been a focus of investigation globally. Ethanol as a H2 source benefits from the ready availability of infrastructure and environmental benignity in comparison with those of other hydrocarbon fuels. Thus, H2 production from ethanol has gained much attention worldwide. In this work, H2 production from ethanol steam reforming (ESR) was investigated over Pt catalysts supported on CexZr1-xO2 (x ) 0.2, 0.4, 0.6, or 0.8) developed via a glycine nitrate process. The supports or catalysts were characterized by low temperature N2 physical adsorption, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), H2 temperature-programmed reduction, temperature-programmed desorption of ethanol, and catalytic performance measurements for ESR at 350–550 °C. Initial catalyst stability was also investigated. The results indicated that Pt/CexZr1-xO2 catalysts were highly active for ESR at lower temperatures, only yielding H2, CO, CH4, and CO2 as products. However, selectivity to CH4 was found around 50.0% at 350–400 °C, while selectivity of products at 450–550 °C was found close to thermodynamic control values, indirectly suggesting that ethanol first dehydrogenates on the metallic surface and then aldehyde undergoes decarbonylation, forming CO and CH4. The function of steam was to establish thermodynamic equilibria for methane steam reforming and water gas shift reactions. The high activity and good initial stability made the catalysts suitable for application to portable power generation by using a PEMFC combined with a solid oxide fuel cell or a methane internal combustion engine for capturing the energy in methane.
Introduction Hydrogen production from various hydrocarbon or alcohol fuels has been a focus of investigation recently.1 Regarding the shortage of fossil fuels, development of biomass-derived fuels as a hydrogen source attracted much attention; thus, research on H2 production from renewable fuels also has gained increasing attention in recent years.2–10 Among various resources, bioethanol (a mixture of water and ethanol generated from the fermentation of biomass) has been deemed as a promising candidate for H2 production for application to proton exchange membrane fuel cells (PEMFCs). The research on energy returned on investment also suggested that bioethanol could contribute to energy sustainability and to reduction of CO2 emission.11 During ethanol steam reforming (ESR), the desired products, H2 and CO2, were generated through the reaction described in eq 1: * To whom correspondence should be addressed. Telephone: +86-2583587722. E-mail:
[email protected]. (1) Song, C. S. Catal. Today 2002, 77, 17–49. (2) Deluga, G. A.; Salge, J. R.; Schmidt, L. D.; Verykios, X. E. Science 2004, 303, 993–997. (3) Frusteri, F.; Freni, S.; Chiodo, V.; Spadaro, L.; Blasi, O. D.; Bonura, G.; Cavallaro, S. Appl. Catal., A 2004, 270, 1–7. (4) Haryanto, A.; Fernando, S.; Murali, N.; Adhikari, S. Energy Fuels 2005, 19, 2098–2106. (5) Morgenstern, D. A.; Fornango, J. P. Energy Fuels 2005, 19, 1708– 1716. (6) Kugai, J.; Subramani, V.; Song, C. S.; Engelhard, M. H.; Chin, Y. J. Catal. 2006, 238, 430–440. (7) Adhikari, S.; Fernando, S.; Haryanto, A. Energy Fuels 2007, 21, 2306–2310. (8) Cavallaro, S. Energy Fuels 2000, 14, 1195–1199. (9) Kugai, J.; Velu, S.; Song, C. S. Catal. Lett. 2005, 101, 255–264. (10) Chen, Y. Z.; Xu, H. Y.; Wang, Y. Z.; Xiong, G. X. Catal. Today 2006, 118, 136–143.
C2H5OH + 3H2O f 2CO2 + 6H2
∆H298 ) 174 kJ mol-1 (1)
Besides ESR, other reactions such as ethanol decomposition, water gas shift (WGS), ethanol dehydrogenation, ethanol dehydration, and methanation reactions could occur, as described in eqs 2-6: C2H5OH f CO + CH4 + H2
∆H°298 ) 49 kJ mol-1 (2)
CO + H2O T CO2 + H2
°
-1
∆H 298 ) -41 kJ mol
(3) C2H5OH f H2 + CH3CHO
∆H°298 ) 68 kJ mol-1 (4)
C2H5OH T C2H4 + H2O
∆H°298 ) 45 kJ mol-1 (5)
CO + 3H2 T CH4 + H2O
∆H°298 ) -205 kJ mol-1 (6)
In addition, the formation of coke on the surface of catalyst is also common. Coke formation may result from the Boudouard reaction: 2CO T C + CO2
∆H°298 ) -171.5 kJ mol-1
(7)
Another possible route for the formation of carbon is through ethylene, generated from ethanol dehydration. C2H4 f polymers f coke
10.1021/ef700576f CCC: $40.75 2008 American Chemical Society Published on Web 03/26/2008
(8)
1874 Energy & Fuels, Vol. 22, No. 3, 2008
Catalysts for ESR generally could be divided into four kinds: (1) nickel catalysts, (2) cobalt catalysts, (3) copper catalysts, and (4) precious metal based catalysts.12 Nickel catalysts showed relatively low catalytic activity and tended to form carbon, obtaining high selectivity to CH4 at low temperature.13 Cobalt catalysts showed good selectivity for H2 generation. However, byproducts such as aldehyde and ethylene could be generated at lower reaction temperatures.14–17 For copper catalysts, the intrinsic activity was not so good and very sensitive to the redox cycle, tending to deactivate irreversibly when exposed to air. Such a characteristic made copper catalysts unpractical for portable or mobile application. Precious metal catalysts, particularly Rh-based ones,18–21 have been widely investigated, aiming to develop suitable catalysts for application to portable or mobile fuel processors, which were closely related with the clean automobile industry and various auxiliary power units (APUs) using fuel cells. However, Rh-based catalysts were too expensive. Thus, Pt catalysts, especially those supported on CeO2-based materials, have been investigated for partial oxidation of ethanol, ESR at temperatures higher than 550 °C.22–25 As we know, for practical utilization of the high temperature generated H2-rich gas mixture to PEMFCs, a series of reactions and operations should be performed to achieve sufficiently low CO concentration, as the anode catalyst of PEMFCs tended to be poisoned by CO of a certain concentration.26 Generally, if ESR were performed at temperatures higher than 550 °C, the necessary reactions and operations for CO cleanup included high temperature WGS, low temperature WGS, CO preferential oxidation, and heat exchange, which significantly complicated the design of the fuel processor and limited its application to automobile or APU systems. Thermodynamically, ESR generates lower CO concentration at low reaction temperatures and high steam-to-carbon feed ratio. However, an apparent drawback of a low temperature reforming route is the low yield of H2: 2.0 mols of H2 are produced per mole of ethanol after WGS, according to reactions 4 and 3, while conventional high temperature reforming theoretically produces 6.0 mols of H2 per mole of ethanol. For this reason, interest in the low temperature pathway has focused on eliminating coke deposition by breaking the carbon-carbon (11) Hammerschlag, R. EnViron. Sci. Technol. 2006, 40, 1744–1750. (12) Vaidya, P. D.; Rodrigues, A. E. Chem. Eng. J. 2006, 117, 39–49. (13) Fatsikostas, A. N.; Verykios, X. E. J. Catal. 2004, 225, 439–452. (14) Llorca, J.; Homs, N.; Sales, J.; Piscina, P. R. J. Catal. 2002, 209, 306–317. (15) Batista, M. S.; Rudye, K. S. S.; Assaf, E. M.; Ticianelli, E. A. J. Power Sources 2003, 124, 99–103. (16) Batista, M. S.; Rudye, K. S. S.; Assaf, E. M.; Assaf, J. M.; Ticianelli, E. A. J. Power Sources 2004, 134, 27–32. (17) Llorca, J.; Dalmon, J.; Piscina, P. R.; Homs, N. Appl. Catal., A 2003, 243, 261–269. (18) Aupretre, F.; Descorme, C.; Duprez, D.; Casanave, D.; Uzio, D. J. Catal. 2005, 233, 464–477. (19) Diagne, C.; Idriss, H.; Kiennemann, A. Catal. Commun. 2002, 3, 565–571. (20) Wanat, E. C.; Venkataraman, K.; Schmidt, L. D. Appl. Catal., A 2004, 276, 155–162. (21) Frusteri, F.; Freni, S.; Spadaro, L.; Chiodo, V.; Bonura, G.; Donato, S.; Cavallaro, S. Catal. Commun. 2004, 5, 611–615. (22) Salge, J. R.; Deluga, G. A.; Schmidt, L. D. J. Catal. 2005, 235, 69–78. (23) Navarro, R. M.; Alvarez-Galván, M. C.; Scánchez-Scánchez, M. C.; Rosa, F.; Fierro, J. L. G. Appl. Catal., B 2005, 55, 229–241. (24) Dömök, M.; Tóth, M.; Raskó, J.; Erdo˝helyi, A. Appl. Catal., B 2007, 69, 262–272. (25) Raskó, J.; Dömök, M.; Baán, K.; Erdo˝helyi, A. Appl. Catal., A 2006, 299, 202–211. (26) Haug, A. T.; White, R. E.; Weidner, J. W. J. Electrochem. Soc. 2002, 149, A862–867.
Chen et al.
bond in ethanol, in order to prevent ethylene formation.27 However, the low temperature ethanol reformate possesses additional fuel value in the form of CH4, which could pass through the PEMFC unit without degrading its performance.28 The fuel value of CH4 exhausted from the PEMFC unit could be captured in other forms by using readily available techniques. Surprisingly, this scheme can be expected to attain practical vehicular energy efficiencies equivalent to those achievable by using high temperature reforming. Thus, we believe that low temperature reforming of ethanol may represent a practical and comparatively low cost pathway to high-efficiency vehicles fuelled by biomass sustainable energy resources. Thus, catalysts suitable for ESR at lower temperatures were investigated, aiming to achieve good catalytic activity and stability. In view of the good oxygen storage capacity of CexZr1-xO2 materials, Pt catalysts supported on CexZr1-xO2 were designed and investigated in this work. Experimental Section Catalyst Preparation. All the chemicals were supplied by Shanghai Chem. Ltd., China, with high purity and used as received. For the preparation of CexZr1-xO2 (x ) 0.2, 0.4, 0.6, or 0.8) samples, stoichiometric amounts of Ce(NO3)3 · 6H2O and Zr(NO3)4 · 6H2O were dissolved in deionized water. Then, the solution was concentrated to a certain extent, and glycine was added to the solution at a glycine/total metallic cations molar ratio of 2.0. The solution was further condensed and finally put on a hot plate controlled at a temperature of 240 °C to induce autocombustion. After combustion, products were collected and calcined at 600 °C for 4.0 h by using a muffle furnace in static air. Then, the powder was milled, pressed into disks, and fragmented to a desired size range of 0.42–0.63 mm. The Pt/CexZr1-xO2 catalysts were prepared by an incipientto-wetness impregnation method by using H2PtCl6 · 6H2O as a precursor to obtain 1.5 wt % Pt/CexZr1-xO2 samples. After impregnation, the samples were dried at 100 °C in a vacuum oven and then calcined at 600 °C for 4.0 h in static air. Catalyst Characterizations. The BET specific surface area, pore size distribution, and pore volume of the catalysts were characterized by N2 physical adsorption at liquid nitrogen temperature by using BELSORP II. Prior to adsorption, the sample was degassed at 350 °C for 2.0 h to remove physically adsorbed components. The surface area was determined from the linear portion of the BET equation. Pore volume and average pore diameter were calculated by the BJH method by using the isotherm desorption branch. The X-ray diffraction (XRD) measurements were performed on a Brüker D8 Advance instrument by using nickel filtered Cu KR radiation (λ ) 0.1541 nm); the operating voltage was 40 kV, and the current was 30 mA. The average particle size was calculated by using the Scherrer equation. H2 temperature-programmed reduction was performed on a selfmade microreactor-mass spectroscopy system. About 50.0 mg of sample was first pretreated at 400 °C in an inert argon atmosphere and then cooled to room temperature. Argon was switched to a 5% H2-Ar flux of 30 mL/min. Then, the sample was heated from room temperature to over 600 °C at 10 °C/min; H2 consumption was monitored online by mass spectroscopy (QIC-20, Hiden Analytical). Temperature-programmed desorption (TPD) experiments with adsorbed ethanol were performed with the same equipment as described for the H2 temperature-programmed reduction. Before the TPD experiment, the Pt/Ce0.8Zr0.2O2 sample were pretreated in H2-Ar flux at 400 °C for 2.0 h. After reduction, the system was purged with argon at the reduction temperature and cooled down to room temperature. The adsorption of ethanol was carried out by (27) Fatsikostas, A. N.; Kondarides, D. I.; Verykios, X. E. Catal. Today 2002, 75, 145–155. (28) Amphlett, J. C.; Mann, R. F.; Pepply, B. A. Int. J. Hydrogen Energy 1996, 21, 673–678.
ESR oVer Pt Catalysts
Energy & Fuels, Vol. 22, No. 3, 2008 1875
passing the flux of the ethanol-Ar mixture, which was obtained by passing argon gas through a saturator containing ethanol at room temperature, through the catalyst bed for 30.0 min. After adsorption, the catalyst was heated from room temperature to about 500 °C in a flux of argon (50.0 mL/min). The products were monitored with a mass spectrometer. The microstructure of the samples was studied by transmission electron microscopy (TEM) (Hitachi H-7000) by using a 75 kV accelerating voltage. The powders were dispersed in ethanol and deposited on a copper grid covered with carbon for observation. Catalytic Activity Measurements. ESR reactions were carried out on a continuous flow fixed-bed reactor composed of stainless steel with an inner diameter of 8.0 mm. Typically, 100.0 mg of catalyst in the size range of 0.42–0.63 mm, diluted with 500.0 mg of silicon dioxide, was packed into the reactor to achieve a catalyst bed height of 6.0 mm. A K-type thermocouple inserted into the center of the catalyst bed detected the temperatures of reaction. The gases were controlled and delivered through AFC 80 MD digital mass flow controllers (Qualiflow). Ethanol and water were metered and delivered to an evaporator controlled at 200 °C to achieve complete gasification through HPLC pumps. Prior to reaction, the catalyst was heated in a 20% H2-He flux of 50.0 mL/min from room temperature to 400 °C at 10 °C/min and was kept at the temperature for 120.0 min to activate the catalyst. Then, ethanol and steam, including the dilution gas, were delivered to the reactor. The products were monitored online by a Varian CP3800 GC instrument, which was equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). N2 was used as an internal standard for calculation. The possible liquid products were collected after condensation by using a cold trap and were analyzed by the FID after separation by a 30.0 m PEG20 M capillary column at 40 °C. The conversion of ethanol was calculated according to eq 9: conversion ) (Fethanol, in - Fethanol, out)/Fethanol, in
(9)
The selectivity of H2 was defined as the percentage of hydrogen atom in the molecular hydrogen in the products and calculated according to eq 10: SH2 )
2[H2] 2[H2] +
∑ y[C H O ] x
y
× 100%
(10)
z
The selectivity of carbon-containing products was calculated by using eq 11: SCxHyOz )
xFCxHyOz 2FC2H5OH
× 100%
(11)
where Fethanol,in was the flux of ethanol (mL/min) at the inlet of the reactor in the gas phase and Fethanol,out was the flux of ethanol in the gas phase at the outlet of the reactor. Fethanol,in was calculated from the liquid ethanol feed rate, and Fethanol,out was calculated by using the analysis results of GC and the standard gas feed rate. CxHyOz represented the carbon-containing products such as CH4, CO, CO2, and possible other products. The steam-to-carbon ratio was the molar ratio between steam and ethanol at the inlet of the catalyst bed, and the yield of hydrogen was defined as the ratio between the hydrogen produced (moles) and ethanol feed (moles). Thermodynamic analysis for the steam reforming of ethanol was performed by using HSC chemistry for Windows V.3.02 (Outokumpu software). The considered reactions included ESR, methanation, and WGS reactions.
Results and Discussion Characterization Results. The surface areas, average pore size, and pore size distribution of as prepared CexZr1-xO2 (x ) 0.2, 0.4, 0.6, or 0.8) are shown in Table 1. They were typical mesoporous materials with intermediate BET specific surface areas. With the increase in zirconium content, the BET surface
Table 1. Texture Properties of CexZr1-xO2 Prepared via a Glycine Nitrate Process sample
surface area (m2/g)
pore volume (mL/g)
average pore size (nm)
Ce0.2Zr0.8O2 Ce0.4Zr0.6O2 Ce0.6Zr0.4O2 Ce0.8Zr0.2O2
42.1 39.2 32.6 30.9
0.15 0.18 0.21 0.23
3.7 4.1 4.3 4.5
area of the sample increased, which suggested that ZrO2 was more sintering-resistant than CeO2. The average pore size tendency also suggested the same results. Among the available chemical processes for the preparation of CeO2-based materials, self-sustaining solution combustion synthesis was characterized by convenient processing, simple experimental setup, time and energy saving, and homogeneous products.29 It was found that the physical properties of CexZr1-xO2 depended upon not only the preparation method but also the concrete processing parameters. For instance, Aruna and Patil30 synthesized a CexZr1-xO2 solid solution with a specific area in the range of 36–120 m2/g via a solution combustion process by using oxalyldhydrazide and carbon-hydrazine as fuel, which were expensive and carcinogenic. Potdar et al.31 prepared nanosized Ce0.75Zr0.25O2 porous powders with a very wide pore size distribution in the range of 2–250 nm and a surface area about 40.0 m2/g via a glycine nitrate autocombustion process. The CexZr1-xO2 samples developed through a combustion process have a higher surface area than the powders prepared by the conventional solid-state method.29 The specific surface areas of the CexZr1-xO2 samples in this work were comparable with the results of Potdar et al.,31 but the pore structure differed significantly, which may be associated with the different glycine/ nitrate molar ratio and the preheating temperature in the combustion process, which need further investigation to clarify the regulations during synthesis. The XRD patterns of the as prepared CexZr1-xO2 (x ) 0.2, 0.4, 0.6, or 0.8) are presented in Figure 1 (top). For the samples CexZr1-xO2 with x ) 0.6 or 0.8, major peaks are indexed as a cubic fluorite structure, with a ) 0.5340(8) nm for Ce0.8Zr0.2O2 and a ) 0.5316(5) for Ce0.6Zr0.4O2. The XRD lines of the fluorite phases are extremely broad, and consequently, the lattice constant could not be accurately determined. The shrinkage of the lattice cell may be well related with the fact that the radius of Zr4+ (0.84 Å) was smaller than that of Ce4+ (0.97 Å). The results also indicated that when x > 0.5 in the CexZr1-xO2 samples, Zr4+ could be incorporated into the fluorite structure of CeO2. For Ce0.4Zr0.6O2, both the cubic and tetragonal phases probably existed. However, XRD characterization could not distinguish between the two, owing to the very small difference in the cell parameters. For Ce0.2Zr0.8O2, the XRD characterization strongly suggested that the cubic and tetragonal phases existed in the samples, and the 2θ values of reflection peaks shifted to slightly higher ones, due to the overlapping between the reflection peaks for the cubic phase and those for the tetragonal phase. The average crystallite size deduced from X-ray line broadening by using the Scherrer equation was in the range of 14.0–20.0 nm, which agreed with the TEM characterization results, as shown in Figure 2. In the TEM experiment, some of the particles were found aggregated together, but from the well dispersed samples, an average (29) Patil, K. C.; Aruna, S. T.; Mimani, T. Curr. Opin. Solid State Mater. Sci. 2002, 6, 507–512. (30) Aruna, S. T.; Patil, K. C. Nanostruct. Mater. 1998, 10, 955–964. (31) Potdar, H. S.; Deshpande, S. B.; Knollam, Y. B.; Deshpande, A. S.; Date, K. S. Mater. Lett. 2003, 57, 1066–1071.
1876 Energy & Fuels, Vol. 22, No. 3, 2008
Figure 1. XRD patterns of the as prepared CexZr1-xO2 developed via a glycine nitrate process.
particle size of around 20.0 nm could be deduced. The XRD patterns for Ce0.2Zr0.8O2 and Pt/Ce0.2Zr0.8O2 are shown in Figure 1 (bottom). After impregnation of H2PtCl6 and calcination at 600 °C for another 4.0 h, the average size of Ce0.2Zr0.8O2 slightly increased, suggesting that long time calcination benefits the sintering of Ce0.2Zr0.8O2. No characteristic reflection peaks corresponding to PtO2 were found in the XRD pattern of Pt/ Ce0.2Zr0.8O2, suggesting that the Pt species was highly dispersed on the inner surface of the Ce0.2Zr0.8O2 support. Similar results were achieved for other samples, indicating incipient-to-wetness impregnation could efficiently disperse the metal precursor to the mesopore of the supports. The redox properties of the catalysts have been investigated by H2 temperature-programmed reduction, and the results are indicated in Figure 3. All the catalysts exhibited a big reduction peak at around 190 °C, which could be attributed to the reduction of Pt4+ in PtO2 supported on CexZr1-xO2. However, the H2 uptake calculated from integration of the peak area (using NiO as a standard) was significantly higher than the amount actually needed to reduce the 1.5 wt % Pt4+ in PtO2. Several authors have also reported that in CeO2 or CexZr1-xO2 supported catalyst the reduction of surface CeO2 or CexZr1-xO2 occurred below 300 °C, while the reduction of bulk ones took place at temperatures higher than 700 °C.32–34 Thus, the big reduction peak at around 190 °C may also be closely related to the (32) Fornasiero, P.; Kasˇpar, J.; Sergo, V.; Graziani, M. J. Catal. 1999, 182, 56–69.
Chen et al.
reduction of part of the Ce4+ in CexZr1-xO2, which was facilitated by the reduction of PtO2 through hydrogen spillover. With increasing Ce4+ content in the solid solution, the reduction peak temperatures shifted to lower values, suggesting that the content of surface Ce4+ affected the intensity of hydrogen spillover. Vidal et al.35 investigated the redox behavior of CeO2-ZrO2 mixed oxide after redox treatments. It also revealed that the temperature of the main peak in the TPR experiments over fresh samples slightly shifted toward lower temperatures as the ceria content in the mixed oxide increases. However, after the redox treatment, the specific surface areas of the CeO2-ZrO2 mixed oxide all decreased. But, the effect on those with relatively high ZrO2 content was much less pronounced, suggesting the obvious stabilization effect of Zr. It has been reported that the introduction of ZrO2 into the CeO2 strongly affected the reduction feature of ceria and the reduction degree of ceria.33 This occurs through structural modifications of the fluorite-type lattice of ceria as a consequence of the substitution of Ce4+ (ionic radius 0.97 Å) with Zr4+ (ionic radius 0.84 Å). Such substitution decreases cell volume, lowering the activation energy for oxygen-ion diffusion within the lattice and consequently favoring reduction. The introduction of Zr also boosts the formation of structural defects that are considered to play an important role in reduction behavior. The Zr content also significantly affects the reduction degree of Ce4+ (to Ce3+); the reduction degree of Ce4+ in our catalysts showed an obvious difference. The reduction degree calculated by using the integration of the peak area in the TPR profile indicated that they were about 83.0, 40.5, 28.0, and 20.7% for CexZr1-xO2 (x ) 0.2, 0.4, 0.6, and 0.8), respectively. We attributed the big difference in reduction degree to the fact that only surface ceria and/or subsurface ceria could be reduced, while the reductions of bulk ceria are closely related with the oxygen-ion diffusion rate and kinetically limited. As reported by Trovarelli et al.,33 the reduction degrees of Ce4+ in CexZr1-xO2 differed significantly with different Zr4+ content even when they were reduced at temperatures above 1000 °C; for pure CeO2, the reduction degree was lower than 50%, while, for Ce0.2Zr0.8O2, it reached about 90%. The TPR results in this work also suggested that during the activation process the bulk CeO2 probably remained in the Ce4+ state, while surface Ce4+ was reduced into Ce3+, forming a Pt-Ce3+ couple on the surface of the catalysts. The TPD of ethanol over Pt/Ce0.8Zr0.2O2 characterization results are indicated in Figure 4. Ethanol desorbed at only one peak at around 150 °C; this could be closely related with some physically adsorbed ethanol. At around 200 °C, the occurrence of strong H2 and CH4 signals indicated the decomposition of ethanol into H2, CH4, and CO. However, due to the fact that most CO were strongly adsorbed on the surface of Pt/ Ce0.8Zr0.2O2, another desorption peak around 372 °C appeared. The formation of CO2 during the experiment was not obvious. No acetaldehyde, acetone, or benzene was observed in the TPD experiment. Catalytic Performance. Influence of Reaction Temperature. The influence of reaction temperature on catalytic performances was investigated at a gas hourly space velocity (GHSV) of 30 000 mL g-1 h-1, a steam-to-carbon feed ratio (mol/mol) of 5.0, and an atmospheric pressure and temperature in the range of 350–550 °C; the results are shown in Table 2. The catalysts all showed good catalytic activity for the ESR reaction, completely (33) Kiss, J. (34) 29–39. (35)
Trovarelli, A.; Zamar, F.; Llorca, J.; Leitenburg, C.; Dolcetti, G.; T. J. Catal. 1997, 169, 490–502. Laosiripojana, N.; Assaburnrungrat, S. Appl. Catal., B 2006, 66, Vidal, H.; Kasˇpar, J.; Pijolat, M. Appl. Catal., B 2000, 27, 49–63.
ESR oVer Pt Catalysts
Energy & Fuels, Vol. 22, No. 3, 2008 1877
Figure 2. TEM images of the as prepared CexZr1-xO2 (x ) 0.2 (A), 0.4 (B), 0.6 (C), and 0.8 (D)) using a glycine nitrate process. Table 2. Influence of Reaction Temperature on the Performance of Catalysts in Terms of Ethanol Conversion, Selectivity of Products, and Yield of Hydrogen catalyst
T (°C)
H2 yield
sel. H2
sel. CH4
CO2/CO
Pt/Ce0.2Zr0.8O2
350 400 450 500 550 350 400 450 500 550 350 400 450 500 550 350 400 450 500 550
1.87 2.34 3.04 3.97 4.60 1.89 2.24 2.96 3.96 4.57 1.74 2.05 2.74 3.79 4.49 1.70 1.99 2.71 3.88 4.52
48.2 56.4 67.8 80.7 88.6 48.4 54.6 66.5 80.4 88.2 45.5 51.1 63.2 78.3 87.2 44.7 50.1 62.2 79.2 87.8
50.3 45.2 36.1 23.8 14.8 50.2 46.5 37.2 24.1 15.2 52.1 49.0 40.0 26.3 16.4 52.6 49.7 40.5 25.2 16.1
8.3 22.0 17.5 11.2 6.4 9.1 24.2 18.6 12.1 7.0 9.6 28.6 20.4 12.9 7.4 9.8 32.5 21.9 13.9 7.9
Pt/Ce0.4Zr0.6O2
Pt/Ce0.6Zr0.4O2
Figure 3. H2 temperature-programmed profiles of Pt/CexZr1-xO2 catalysts. Pt/Ce0.8Zr0.2O2
Figure 4. TPD profile of ethanol adsorbed on Pt/Ce0.8Zr0.2O2.
converting ethanol at a temperature as low as 350 °C. Furthermore, the reaction products were very simple, including only H2, CH4, CO, and CO2, which benefited the practical application. However, at lower reaction temperatures, the yield of hydrogen was low, approaching 2.0 when the reactions were
performed at 350 or 400 °C. At higher reaction temperatures, the selectivity to products was found to be close to the thermodynamic control ones, suggesting that in the complex ESR reactions thermodynamic equilibria for methane steam reforming and WGS could be established as long as ethanol was completely converted. Previous investigations36–38 on the reaction mechanism of ethanol partial oxidation or steam reforming over M/CeO2 catalysts suggested ethoxy species probably acted as the reaction intermediate. In addition, the rate for the decomposition of the intermediate may be faster than that for ethanol decomposition into acetaldehyde, as reported (36) Mattos, L. V.; Moronaha, F. B. J. Catal. 2005, 233, 453–463. (37) Chen, H.; Liu, S.; Ho, J. J. Phys. Chem. B 2006, 110, 14816– 14823. (38) Jacbos, G.; Keogh, R. A.; Davis, B. H. J. Catal. 2007, 245, 326– 337.
1878 Energy & Fuels, Vol. 22, No. 3, 2008
Chen et al. Table 3. Influence of the Steam-to-Carbon Ratio on Catalytic Performance for Ethanol Steam Reforming of Pt/Ce0.8Zr0.2O2
Figure 5. Proposed reaction mechanisms for ethanol steam reforming over M/CeO2 catalysts. [O] denotes surface oxygen, and M denotes a metal or a Ce ion site.
by Morgenstern and Fornango,5 who studied the low temperature reforming of ethanol over copper-plated Raney nickel. Chen et al.37 applied density-functional theory to investigate the dehydrogenation and steam reforming of ethanol on a Rh/CeO2 (111) surface. Ethanol was calculated to have the greatest energy of adsorption when the oxygen atom of the molecule was adsorbed onto a Ce atom in the surface, relative to other surface atoms. Before forming a six-membered ring of an oxametallacyclic compound, two hydrogen atoms from ethanol were first eliminated, as described in Figure 5. The barriers for dissociation of O–H and the β-carbon (CH2–H) were calculated to be 12.0 and 28.6 kcal/mol, respectively. In situ IR39 and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)38 investigation on Pt/CeO2 also confirmed that ethanol adsorbed dissociatively on the surface of the reduced ceria or ceriasupported catalysts. Furthermore, from our TPD characterization results, as indicated in Figure 4, the lack of acetaldehyde formation also suggested that the six-membered ring of an oxametallacyclic compound was selectively transformed into CO and CH4 instead of acetaldehyde. CO instead of CO2 was the main product in the TPD experiment, suggesting that the introduction of steam during the ESR reaction was to promote the WGS reaction; thus, CO2 and CH4 were the main carboncontaining products, while acetaldehyde was not observed. It was believed that, after pretreatment of the catalysts, the partial reduction of the support took place on the periphery of the metal particle, where the active sites were supposed to be for the ethanol steam reforming reaction.36 Thus, we proposed that, on our Pt/CexZr1-xO2 catalysts, pretreatment in H2-He resulted in the Pt-Ce3+, which was believed to be the active site for ethanol adsorption. Zr4+ effectively separated the Pt-Ce3+ sites and affected the density of the Pt-Ce3+ because Zr4+ content was found to influence the reduction degree of Ce4+ in the sample.33 Thus, although ethanol first adsorbed on ceria, producing an ethoxy species,36 the ethoxy species could migrate to the metal particles under the reaction conditions; the following reactions were proposed to form a catalytic cycle: CH3CH2OH + 2Pt/ T Pt-H + Pt-OCH2CH3
(12)
Pt-OCH2CH3 + 2Pt/ T Pt-H + Pt-O-CH2-CH2-Pt (13) Pt-O-CH2-CH2-Pt T 2Pt/ + CO + CH4
(14)
Pt-H + Pt-H T 2Pt/ + H2
(15)
where Pt* was the free active site. At lower reaction temperatures such as 350 and 400 °C, due to the intrinsic low reaction kinetics for the methane steam reforming reaction and the selective catalytic activity of Pt/CexZr1-xO2 for the WGS reaction,40,41 the theoretical maximum hydrogen yield was 2.0. (39) Idriss, H.; Diagne, C.; Hindermann, J. P. J. Catal. 1995, 155, 219– 237. (40) Ruettinger, W.; Liu, X. S.; Farrauto, R. J. Appl. Catal., B 2006, 65, 135–141. (41) Ricote, S.; Jacobs, G.; Milling, M.; Ji, Y. Y.; Patterson, P. M.; Davis, B. H. Appl. Catal., A 2006, 303, 35–47.
T (°C)
S/C
yield of H2
sel. H2
sel. CH4
CO2/CO
350
3.0 4.0 5.0 3.0 4.0 5.0 3.0 4.0 5.0 3.0 4.0 5.0 3.0 4.0 5.0
1.60 1.67 1.70 1.86 1.94 1.99 1.94 2.37 2.71 2.81 3.41 3.88 3.71 4.23 4.52
42.8 44.2 44.7 47.6 49.1 50.1 49.1 57.1 62.2 64.9 73.3 79.2 79.9 84.6 87.8
53.4 52.8 52.6 51.2 50.3 49.7 50.1 44.6 40.5 38.1 31.0 25.2 24.9 19.2 16.1
6.9 8.2 9.8 21.3 28.6 32.5 16.5 18.6 21.9 7.8 11.6 13.9 4.1 5.8 7.9
400 450 500 550
Our experimental results also confirmed the proposal. Increasing reaction temperature resulted in a higher yield of H2 and selectivity to hydrogen but a lower selectivity to methane. However, the CO2/CO ratio decreased due to the intrinsic unfavorable effect of temperature on the WGS reaction. For instance, increasing the reaction temperature from 400 to 550 °C enhanced the yield of hydrogen from 2.34 to 4.60 over Pt/ Ce0.2Zr0.8O2 and the hydrogen selectivity from 56.4 to 88.6%, respectively. Simultaneously, the selectivity to methane decreased from about 50.0 to 14.8%, and the CO2/CO ratio decreased from 22.0 to 6.4. Noticeably, the CO2/CO ratio achieved at 400 °C was higher than those at 350 °C, which indicated that Pt/Ce0.2Zr0.8O2 was more active for the WGS reaction at 400 °C, and CO formation may directly result from the decomposition of the six-membered ring of an oxametallacyclic compound in Figure 5. The continuous decrease of selectivity to methane suggested that Pt/CexZr1-xO2 also showed good activity for methane steam reforming when the reactions were performed at temperatures higher than 400 °C. The decreasing CO2/CO ratio indicated that, due to the exothermic nature of the WGS, high reaction temperature necessarily generated reformate with a high CO concentration. Influence of the Steam-to-Carbon Ratio. The influences of the steam-to-carbon ratio on the performance of Pt/Ce0.8Zr0.2O2 for the ESR reaction were investigated in the range of 3.0–5.0, which may be the practical steam-to-carbon feed ratio for bioethanol steam reforming; the results are shown in Table 3. It could be clearly seen from the table that when the reaction was performed at a lower temperature, such as 350 and 400 °C, the effect of the steam-to-carbon ratio on the catalytic performance in terms of yield of hydrogen, selectivity to methane, and the CO2/CO ratio was not as pronounced as those at higher temperatures, particularly on the selectivity to methane. We attributed the observation to the fact that, at 350 and 400 °C, the kinetics of methane steam reforming over Pt/Ce0.8Zr0.2O2 were low enough to keep around 50.0% selectivity to methane, which was generated from the decomposition of the sixmembered ring of an oxametallacyclic compound in Figure 5, while, in the temperature range of 450–550 °C, the methane steam reforming reaction occurred over the Pt/Ce0.8Zr0.2O2 catalyst obviously, which resulted in a lower selectivity to methane and a higher yield of H2 and CO2/CO ratio. Initial Stability of Catalyst. The initial stability of the Pt/ Ce0.8Zr0.2O2 catalyst was investigated at 400 °C, a GHSV of 30 000 mL g-1 h-1, and a steam-to-carbon feed ratio of 5.0; the results are shown in Figure 6. The catalyst showed good initial stability. During the test of about 900 min, ethanol conversion was maintained at around 100%, and the selectivity
ESR oVer Pt Catalysts
Energy & Fuels, Vol. 22, No. 3, 2008 1879
the carbon species and oxygen species. Zr4+ incorporation into the lattice of CeO2 significantly promoted the oxygen-ion diffusion, which enhanced catalytic activity for the WGS and ESR as well as stability. The particular reaction route over M/CeO2 also benefited the stability of the catalyst; for example, ethylene, which was believed to be a precursor of carbon deposition, would not form in such a reaction mechanism. Conclusions
Figure 6. Initial stability test of Pt/Ce0.8Zr0.2O2 catalyst at 400 °C.
Figure 7. TEM images of the utilized Pt/Ce0.8Zr0.2O2 catalyst.
to products showed substantially no change. After the stability investigation, the catalyst was cooled down in helium flux, and the utilized catalyst was characterized by TEM; the results are shown in Figure 7. No carbon or serious sintering of the catalyst was observed. The platinum particles were found in the range of 2.0-4.0 nm, suggesting that, even after continuous reaction for 900 min under steam-rich conditions, no serious aggregation of the Pt particle occurred. We attributed the good stability of the catalyst to the good oxygen storage capacity of the CexZ1-xO2 support, which benefited the surface reaction between
Ethanol steam reforming at low temperature represented another route to H2 production for application to a fuel cell vehicle, although traditional high temperature reforming was widely investigated. In this work, a glycine nitrate process was successfully applied to develop fine, nanosized CexZr1-xO2 (x ) 0.2, 0.4, 0.6, or 0.8) quickly and conveniently. The Pt/ CexZr1-xO2 catalysts prepared via incipient-to-wetness impregnation showed high activity for ethanol steam reforming, achieving almost complete ethanol conversion at low temperatures and yielding only H2, CO, CH4, and CO2. The low CO concentration, about 50.0% selectivity to CH4, and about 50.0% selectivity to CO2 at low temperatures suggested that ethanol steam reforming over Pt/CexZr1-xO2 underwent a series of steps: (1) ethanol dehydrogenated to aldehyde, (2) aldehyde decomposed into CH4, CO, and H2, and (3) reactions such as WGS and methane steam reforming occurred. The final reactions determined the selectivity of the products. The agreement between the experimental distribution of the products and the thermodynamic control ones suggested that the Pt/CexZr1-xO2 catalyst also had good activity for steam reforming of methane between 450 and 550 °C. However, the reformate from low temperature ethanol steam reforming contained considerable methane content, and the energy in methane should be captured by using high temperature fuel cells or a natural gas internal combustion engine; therefore, such a low temperature reforming process is highly attractive and applicable in a hybrid power generation system using both low temperature and high temperature fuel cells or a combination of a low temperature fuel cell and a natural gas internal combustion engine. Acknowledgment. One of the authors, Y.C., greatly acknowledges the financial support from the postdoctorial fund of China (No. 20060400928) and the postdoctorial fund of Jiangsu Province (No. 0602001B). EF700576F
