Steam reforming and water gas shift of ethanol on Rh and Rh Ce catalysts in a catalytic wall reactor

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1 Applied Catalysis A: General 276 (2004) Steam reforming and water gas shift of ethanol on Rh and Rh Ce catalysts in a catalytic wall reactor E.C. Wanat, K. Venkataraman, L.D. Schmidt* Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, MN 55455, USA Received 8 April 2004; received in revised form 1 August 2004; accepted 1 August 2004 Available online 11 September 2004 Abstract Ethanol is a renewable, portable, and non-toxic liquid that is a possible source of hydrogen for PEM fuel cells. This work describes an autothermal flat plate catalytic wall reactor for steam reforming of ethanol into H 2 and CO 2. Catalytic methane combustion on Pt is coupled to ethanol steam reforming on Rh and Rh Ce across an auto-thermal wall. At a steam/carbon ratio of 3/1 the reactor gave >99% conversion of ethanol. An H 2 /CO ratio of 3/1 was obtained at a residence time of 100 ms and an upstream temperature of 800 8C. Methane selectivities of less than 1% were obtained with Rh Ce. Water gas shift was also considered in order to increase the H 2 /CO ratio. The reactor was lengthened to include a cooler water gas shift section using a Pt Ce catalyst. The extended reactor produced >99% conversion of ethanol and an H 2 /CO ratio of 30/1 at a steam/carbon ratio of 4/1. The residence time was 400 ms, and the upstream temperature was 900 8C in the extended reactor. The reactor was stable for at least 100 h with no detectable degradation in performance. # 2004 Elsevier B.V. All rights reserved. Keywords: Ethanol steam reforming; Rhodium; Platinum and ceria catalysts; Catalytic wall reactors; Coupled exothermic and endothermic reactions; Short contact time; Water gas shift 1. Introduction In recent years there has been an increased interest in fuel cells to produce clean electricity. Proton-exchange membrane (PEM) fuel cells in particular have generated interest because they operate near room temperature with high power densities. The PEM fuel cell requires hydrogen with 10 ppm of CO as a fuel. Since hydrogen does not occur naturally it must be produced by reforming hydrocarbons, such as methane, or oxygenates, such as methanol or ethanol. Ethanol is an attractive option since it is less toxic than methanol and can be produced renewably from biomass with little net addition of carbon dioxide to the atmosphere. Both steam reforming and partial oxidation are possible routes to produce hydrogen. Studies have shown that it is possible to produce a more pure hydrogen effluent by steam * Corresponding author. Tel.: ; fax: address: schmi001@tc.umn.edu (L.D. Schmidt). reforming [1,2]. However, the partial oxidation of ethanol remains an active area of research [3] Ethanol steam reforming The steam reforming of ethanol is thermodynamically feasible [4 7] and the greatest thermodynamic concern is the formation of carbon leading to deactivation of the catalyst. However, carbon formation occurs only at low H 2 O/EtOH ratios (<2/1) and low temperatures (<600 8C) [5]. At the temperatures ( C) and H 2 O/EtOH ratios (2/1 8/1) used in this research the reactor effluent is predicted to contain only CO, CO 2,CH 4,H 2, and H 2 O [6]. At high temperatures (>800 8C) the equilibrium hydrogen selectivity is nearly 100% [7]. Steam reforming ethanol over noble metal catalysts has been explored [8 11]. On alumina supports, Rh is the most active noble metal showing significantly higher activity and hydrogen selectivities than Pd, Pt, or Ru [8,9]. For Rh at X/$ see front matter # 2004 Elsevier B.V. All rights reserved. doi: /j.apcata

2 156 E.C. Wanat et al. / Applied Catalysis A: General 276 (2004) temperatures of 800 8C, complete conversion of ethanol with 90% selectivity to hydrogen has been observed [8]. The formation of side products such as ethylene and acetaldehyde is minimized using Rh [10], and the main side product is methane. Rh also shows good stability in time on stream tests [11]. The GHSV tested ranged from 5000 to 80,000 h 1 [11] Catalytic wall reactors (CWR) Hydrogen is typically produced by steam reforming of natural gas in long tubes packed with Ni catalyst in large furnaces. Heat transfer boundary layers both on the tube and furnace sides cause heat transfer to be rate limiting in these systems and require residence times of 1 s. High flame temperatures in the furnace lead to NO x formation and a reactor system that scales down poorly to produce small or varying amounts of hydrogen. One possible solution is to eliminate the heat transfer boundary layers by coupling the combustion and reforming reactions catalytically on the opposite sides of a thin wall. The catalytic wall reactor (CWR) increases the heat transfer coefficient by 200 times when compared to a traditional steam reforming reactor [12]. This reduces the required residence time for steam reforming by a large factor. Lower catalytic combustion temperatures of C in the CWR eliminate the formation of NO x. Finally, the flat plate CWR allows easy scaling because exothermic and endothermic channels can be alternated to produce a desired amount of hydrogen. Catalytic wall reactors have been investigated for several years, but only recently have working reactors been described. A flat plate CWR where methane combustion is coupled to methane steam reforming followed by water gas shift has been demonstrated [13]. Using Rh catalyst for steam reforming and Pt Ce catalyst for water gas shift an effluent stream with an H 2 /CO ratio of 42/1 was obtained [13]. Several investigators have described or modeled a CWR [14 19]. Those catalytic wall reactors have had problems with material stability that limited maximum temperatures to <800 8C [20,21]. These temperatures are too low to steam reform hydrocarbon fuels in millisecond contact times. Temperatures of 900 8C were obtained with foam monoliths in the exothermic and endothermic channels of a flat plate CWR [22]. However, heat transfer boundary layers were still present in the reactor channels since a foam monolith was used. A flat plate CWR much like the one described here was used to reform higher hydrocarbons although the plate was heated electrically [23]. A tubular reactor with fins extending radially, coated in a combustion catalyst, has been described. Although the goal of that research was to optimize the combustion side of the reactor and no attempt was made to steam reform a fuel inside the tube [24]. Another configuration for a CWR is the concentric tube reactor, which has tested for the homogeneous dehydration of ethane to ethylene [25]. Research into extending this reactor concept for the steam reforming of ethanol is ongoing. This work extends the flat plate CWR described in [13] to steam reforming ethanol, a renewable and portable liquid fuel. Water gas shift is also considered in order to reduce CO selectivity to create a suitable fuel for preferential oxidation of CO, which is the next step in creating a hydrogen fuel for a PEM fuel cell. 2. Reactions The reactions considered were the coupling of methane catalytic combustion on platinum CH 4 þ 2O 2! CO 2 þ 2H 2 O; DH ¼ 802:6kJ=mol to the catalytic steam reforming of ethanol on rhodium or rhodium ceria. C 2 H 5 OH þ H 2 O! 2CO þ 4H 2 DH ¼þ255:9kJ=mol: The combustion of methane provides heat to the endothermic steam reforming reaction. Water gas shift, using platinum ceria, reduced the amount of CO while producing more H 2. CO þ H 2 O! CO 2 þ H 2 ; DH ¼ 41:0kJ=mol: Methanation is an undesired reaction because it consumes H 2. CO þ 3H 2! CH 4 þ H 2 O; DH ¼ 205:6kJ=mol: Platinum ceria was chosen because it showed high activity for water gas shift while minimizing methanation [26]. Reactions producing acetaldehyde, ethylene, and carbon were <1% as long as H 2 O/EtOH was >2/1. From the standard enthalpies of reactions above, combustion of 1 mol of CH 4 can theoretically reform 3 mol of ethanol. However, due to heat losses and sensible heat associated with temperature increases our reactor actually reformed about 1.5 mol of ethanol for every mole of CH 4 combusted. Reduced heat losses and a larger reactor would of course increase energy efficiency beyond the 50% achieved here. 3. Experimental Fig. 1 shows schematic diagrams of the CWR and extended CWR, which have been described previously [13] Reactor structure The thin wall on which the catalyst was coated, was made of fecralloy, a high temperature steel alloy (73% Fe, 20% Cr, 5% Al, and <1% Ni and Si) that can withstand temperatures

3 E.C. Wanat et al. / Applied Catalysis A: General 276 (2004) Fig. 1. Schematic diagrams of the flat plate configuration CWR (a) and extended CWR (b). The arrows show the direction of flow. The label Endo indicates the channel in which the ethanol water mixture flows. The label Exo indicates the channel in which the combustion mixture flows. All channels were 5 cm wide and 8 cm long except for the extended channel in the extended CWR, which was 13 cm long. The channels were nominally 4 mm in height. of C. No deterioration of the fecralloy plates was ever observed despite exposure to temperatures of >900 8C and several cycles of startup and shutdown. The plates were corrugated which prevented warping and increased the surface area for heat transfer between the exothermic and endothermic channels. The plates were 8 cm long 5 cm wide 0.1 mm thick in the CWR. The plates were extended to 13 cm long in the extended CWR. The distance between the plates was nominally 4 mm. The gases flowed through and 3.2 mm. pipe to get to the reactor. Once in the reactor the gases flowed in the channels formed by the plates. The exothermic channels were left open in order to allow for temperature measurement. The endothermic gases were collected in a 3.2 mm. pipe at the end of the channel in order to allow for analysis of the product gases. The CWR and extended CWR had an extra plate of fecralloy coated with either Rh or Rh Ce was placed in the upstream portion of the endothermic channel. In the extended CWR an extra plate coated in Pt Ce was placed in the extended portion of the endothermic channel. The plates were held in place by stainless steel frames that were bolted tightly together to prevent the escape of gases Catalyst preparation The fecralloy plates were coated with washcoat and catalyst in a procedure described in earlier work [25]. Before the washcoat was applied the Fe Cr alloy plates were heated to 900 8C for 6 h. This caused the aluminum in the fecralloy to form a layer of Al 2 O 3 on the surface of the fecralloy and was verified by EDS. This layer of Al 2 O 3 helped the washcoat adhere to the surface of the fecralloy. The washcoat was a suspension of 10 wt.% g-alumina from 3 mm particles, 1 wt.% Cr from Cr(NO 3 ) 3 9H 2 O, and 0.1 wt.% Y from Y(NO 3 ) 3 6H 2 O [27]. Two coats of washcoat were typically applied, yielding a washcoat that was 10 mm thick. After each application of washcoat, the plate was allowed to dry in air and briefly fired to C, which converted some of the g-alumina to a- alumina. Two steam reforming catalysts were tested, Rh and Rh Ce. The Rh was from a 13.6 wt.% Rh(NO 3 ) 3 solution. The Rh Ce catalyst was a solution containing 5 wt.% Rh and 5 wt.% Ce from Ce(NO 3 ) 3 6H 2 O aqueous solution. The combustion catalyst was Pt from an 8 wt.% H 2 PtCl 6 solution. The water gas shift catalyst was Pt Ce using a 1:1 by volume mixture of 8 wt.% H 2 PtCl 6 solution and 5 wt.% Ce from Ce(NO 3 ) 3 6H 2 O aqueous solution. After applying the catalyst, the plate was dried in air and again briefly fired to C. Two coats of catalyst were applied to each plate. The reforming catalyst was the only catalyst used in the CWR, no water gas shift catalyst was used. The water gas shift catalyst was applied only in the extended region of the extended CWR, downstream of the steam reforming catalyst Gas delivery system and startup Compressed air, methane, and nitrogen were delivered from cylinders and metered by mass flow controllers. The ethanol water mixture was delivered from a pressurized tank via an automotive fuel injector into a quartz tube. Heating tape was used to vaporize the mixture, which was delivered to the endothermic channel at 150 8C. An electronic pulse generator was used to control the mixture flow rate. Nitrogen, flowing in the endothermic channel, was used as the reference gas in the endothermic channel for gas chromatography analysis. The CH 4 -air mixture was delivered to the reactor at 25 8C through similarly sized tubes to ensure an equal amount of mixture was delivered to each of the exothermic channels. A Bunsen burner was used to start the reactor with only the exothermic gases flowing in the reactor. Once the catalytic combustion was ignited, the Bunsen burner was removed and the reactor covered with Fiberfrax insulation. Startup of the exothermic channel took less than 1 min. After the exothermic channels were ignited, nitrogen was sent to the endothermic channel. Finally, the fuel injector was turned on which delivered the ethanol water mixture to the endothermic channel. This system also shows potential for faster startup times of 5 s using homogeneous combustion in the exothermic channels followed by catalytic combustion for steady reactor operation Data acquisition and interpretation All flow rates are reported in standard liters per minute (slpm, 25 8C and 1 atm). The starting ethanol water mixture

4 158 E.C. Wanat et al. / Applied Catalysis A: General 276 (2004) is reported as the steam to carbon (S/C) ratio. The S/C ratio is defined as the number of moles of water divided by twice the number of moles of ethanol in the mixture. Residence times were estimated by taking the channel volume divided by the volumetric flow rate. Residence times were corrected for volume change due to reaction and temperature. Temperatures were measured in the exothermic channel with a 0.16 cm Inconel sheathed chromel alumel thermocouple. The upstream temperatures were measured 1 cm from the exothermic gas entrance, and downstream temperatures were measured at the point the endothermic gases exited the reactor. Temperatures are regarded as accurate to 50 8C and positions to 0.5 cm. Gases were analyzed by gas chromatography by injecting samples from a syringe that were taken from the endothermic channel. All selectivities reported are based on carbon atoms except for H 2, which was based on hydrogen atoms. n i n i S i ¼ X n j n j all product species where n is the number of carbon atoms in the species and n the number of moles of the species in the product stream. The same formula is used to calculate H 2 selectivity but n is the number of hydrogen atoms in the species. Hence the hydrogen selectivity is decoupled from all other selectivities. To check the performance of the reactor compared to equilibrium CO selectivity as a function of temperature was calculated using a thermodynamic package, HSC. All species that were observed in the product stream were included in the calculations. The program determined equilibrium species concentrations by minimizing the Gibbs free energy. 4. Results This section presents results of experiments on the CWR and the extended CWR and compares Rh and Rh Ce. Using a CWR it was possible to reform ethanol to H 2 and CO at >99% conversion at millisecond contact times, but with little water gas shift (H 2 /CO 3/1). A millisecond contact time extended CWR with Pt Ce water gas shift catalyst was able to increase the H 2 /CO ratio to 30/1. The methanation activities of the two different ethanol reforming catalysts, Rh and Rh Ce, are compared. Methanation is a major concern in the selection of a water gas shift catalyst since methanation consumes H 2. Finally, the effects of the S/C ratio, H 2 yield, and long-term reactor stability on reactor performance are presented The CWR and extended CWR using Rh Fig. 2 shows the ethanol conversion, CO selectivity (Fig. 2a), H 2 /CO ratio (Fig. 2b), and CH 4 selectivity (Fig. 2c) Fig. 2. Measured results of the CWR and extended CWR on Rh at an S/C ratio of 3/1. (a) Endothermic channel ethanol conversion (X EtOH ) and CO selectivity (S CO ) as a function of endothermic channel flow rate. (b) H 2 /CO ratio as a function of endothermic channel flow rate. (c) CH 4 selectivity (S CH4 ) as a function of endothermic channel flow rate. with Rh catalyst at an S/C of 3/1. Fig. 2a shows that it was possible to obtain complete conversion of ethanol for all flow rates tested in the extended CWR. The CWR started at nearly complete conversion, but dropped to 80% conversion as the flow rate increased. The CO selectivity dropped from 60% in the CWR to 25% in the extended CWR. Concurrently the H 2 /CO ratio increased from 3/1 to 9/1 due to an increase in the water gas shift reaction. This increase in water gas shift was due to the Pt Ce catalyst in the extended portion of the extended CWR. However, while Pt Ce increased the extent of water gas shift, it also increased the extent of methanation.

5 E.C. Wanat et al. / Applied Catalysis A: General 276 (2004) Ethanol reforming catalysts Results from Rh and Rh Ce catalysts at an S/C ratio of 3/1 are shown in Fig. 3. Fig. 3a shows that ethanol conversion was maintained at >99% for Rh Ce catalyst for all flow rates while the conversion decreased to 80% for Rh catalyst at high flow rates. The CO selectivity was 15% lower for Rh Ce as compared to Rh. Fig. 3b shows that Rh Ce produced an effluent that had an H 2 /CO ratio of 3/1 for Rh and 5/1 for Rh Ce. This increase in H 2 /CO ratio was due in part to the dramatic decrease in CH 4 selectivity observed with Rh Ce. The CH 4 selectivity decreased from about 10% with Rh catalyst to less than 1% for most flow rates with Rh Ce. This decrease in CH 4 selectivity was responsible for the 99% H 2 selectivity observed for Rh Ce catalyst compared to 85% for Rh shown in Fig. 3c. The measured upstream temperatures are also shown in Fig. 3c and they were nearly equal for both Rh and Rh Ce. Thus the differences in CO and CH 4 selectivity between Rh and Rh Ce were not simply due to a difference in temperature in the reactor Effect of S/C ratio Fig. 4 shows the ethanol conversion (Fig. 4a) and H 2 /CO ratio (Fig. 4b) for several S/C ratios in the CWR with Rh catalyst. The results show that both the ethanol conversion and H 2 /CO ratio increase with increasing S/C ratio. These results were not unexpected, from reaction stoichiometry for steam reforming and water gas shift, one would expect more steam to assist both reactions. Stable operation of the reactor at a higher GHSV was obtained with more dilute mixtures of ethanol and water (higher S/C ratios). This was because the flow rate of ethanol was lower at higher S/C ratios for a constant GHSV. Hence, less heat had to be provided by the exothermic side of the reactor in order to steam reform the ethanol. As the flow rate increased the amount of heat needed for reaction eventually overwhelmed Fig. 3. Comparison of steam reforming on Rh and Rh Ce catalysts in the CWR. The S/C ratio was 3/1 and flow rates of the exothermic gases were 0.32 slpm CH 4 and 3.0 slpm air. (a) Endothermic channel ethanol conversion (X EtOH ) and CO selectivity (S CO ) as a function of GHSV. (b) H 2 /CO ratio and CH 4 selectivity (S CH4 ) as a function of GHSV. (c) H 2 selectivity (S H2 ) and measured upstream temperature (T u ) as a function of GHSV. Fig. 4. Measured results of the CWR for various S/C ratios and Rh catalyst. The flow rates of the exothermic gases were 0.32 slpm CH 4 and 3.0 slpm air. (a) Endothermic channel ethanol conversion (X EtOH ) as a function of GHSV. (b) Endothermic channel H 2 /CO ratio as a function of GHSV.

6 160 E.C. Wanat et al. / Applied Catalysis A: General 276 (2004) the ability of the exothermic side to provide energy, and the Pt combustion catalyst began to extinguish H 2 yield and CO selectivity Fig. 5 shows the H 2 yield and CO selectivity for the different combinations of reactors and catalysts. Hydrogen yield is expressed as flow rate of H 2 in the reactor effluent divided by the flow rate of ethanol to the reactor. From Fig. 6 if all of the CO formed by steam reforming were converted to CO 2 by water gas shift, 6 mol of H 2 would be formed for every mole of ethanol fed to the reactor. Fig. 5a shows that Rh Ce in the extended CWR obtained values of over 5 mol of H 2 per mole of ethanol fed to the reactor. Fig. 5b shows the CO selectivity as a function of downstream temperature along with the equilibrium CO selectivity for the ethanol steam reforming system. For an S/ C ratio of 3/1 a maximum H 2 /CO ratio of 18/1 was obtained for Rh Ce catalyst in the extended CWR. The residence time of the process gases was 400 ms. By increasing the S/C ratio to 4/1 the H 2 /CO ratio was observed to increase to 30/1. The hydrogen produced by this reactor could be expected to power a 90 W fuel cell Time on stream Fig. 7 shows the results of a time on stream test. The CWR was used with Rh Ce at an S/C of 3/1. The CWR operated for 100 h with little degradation in activity or selectivity of the catalyst. The ethanol conversion remained at 96 99% throughout the entire time and H 2 selectivity was 95%. The only other product produced that contained hydrogen was methane, which remained below 5%. The discontinuity in the selectivities observed at 35 h was due to shutdown of the reactor for maintenance. However, the catalyst returned to its previous activity except for methane selectivity, which remained slightly higher than its previous value. The CO and CO 2 selectivities both remained at 50% throughout the run. An H 2 yield of 4.7 mol H 2 per mole of ethanol and a H 2 /CO ratio of 5/1 were obtained. 5. Discussion 5.1. Renewable energy cycle One important motivation for this research is the need to develop renewable energy resources that add little or no net carbon dioxide to the atmosphere. Examining the carbon cycle for H 2 produced from ethanol (Fig. 6), shows that ethanol is an efficient energy source. Starting with 6 mol of CO 2, 12 mol of H 2 O, and sunlight, plants are able to produce 1 mol of glucose while liberating 6 mol of O 2. About 6 mol of H 2 O remain to be used later in the energy cycle along with the liberated O 2 (Fig. 6a). Ideally, a fuel cell could use the glucose directly to produce electricity. However, the reaction to liberate hydrogen from glucose is very slow [28]. Yeast converts the glucose to 2 mol of ethanol and 2 mol of CO 2 (Fig. 6b). This step is nearly thermally neutral, requiring little energy to complete. Now the 6 mol of H 2 O that were left from the production of glucose can be used for the steam reforming and water gas shift of ethanol to produce 12 mol of H 2 and 4 mol of CO 2 (Fig. 6c). The 6 mol of O 2, liberated from the production of glucose, and the 12 mol of H 2 can now be reacted in a PEM fuel cell to produce electricity and 12 mol of H 2 O(Fig. 6d) Minor products Fig. 5. H 2 yield and CO selectivity. (a) H 2 yield from the reactor expressed as the flow rate of H 2 coming out of the reactor divided by the flow rate of ethanol fed to the reactor as a function of measured downstream temperature (T d ). (b) Endothermic channel CO selectivity (S CO ) as a function of measured downstream temperature. The solid curve represents the equilibrium CO selectivity for an ethanol-water mixture with an S/C of 3/1. For all S/C ratios except 1/1 the formation of all minor products (ethylene, ethane, and acetaldehyde) was less than 1% for all conditions tested. For an S/C ratio of 1/1 the formation of ethylene became significant rising to 15% in the CWR and 7% in the extended CWR for Rh catalyst. Ethylene formation dropped to 1% in both reactors for Rh Ce at an S/C ratio of 1/1. Ethane formation also increased

7 E.C. Wanat et al. / Applied Catalysis A: General 276 (2004) Fig. 6. The renewable energy diagram for the production of H 2 from ethanol. Glucose is produced using water, carbon dioxide, and sunlight, which is fermented to ethanol by yeast. This research describes the steam reforming and water gas shift of ethanol to H 2. The H 2 can then be used by a PEM fuel cell to produce electricity and water. The cycle can be repeated indefinitely with no net addition of CO 2 to the atmosphere. from <1% at higher S/C ratios to 2% for Rh catalyst at an S/C ratio of 1/1. However, little ethane was observed for Rh Ce catalyst, even at an S/C ratio of 1/1. Acetaldehyde formation was <1% for most conditions except for Rh catalyst at an S/C of 1/1 when 3 5% was observed. Except at the S/C ratio of 1/1, methane was the only product observed other than CO, CO 2, and H 2. In addition, the methane selectivity was higher in the extended CWR as shown in Fig. 2. Fig. 3 shows that in the CWR virtually no methane was formed with Rh Ce catalyst. However, the methane selectivity increased to 7% in the extended CWR (not shown in Fig. 3b). Thus, the methane was formed by methanation of CO and H 2 on Pt Ce as opposed to being formed directly from ethanol since 100% conversion of ethanol was obtained in both the CWR and extended CWR Carbon formation For S/C ratios 2/1, 3/1, and 4/1 no carbon formation was ever observed, including the 100 h time on stream test. At an S/C ratio of 1/1, carbon formation was always observed quickly after starting the reactor and only 3 4 h of operation could be sustained before the catalyst deactivated. This was not unexpected since carbon is an equilibrium product at an S/C ratio of 1/1 [5]. In the extended CWR carbon formation was observed only on the reforming portion of the fecralloy plate. No carbon was ever observed on the water gas shift catalyst Catalyst regeneration Fig. 7. Measured results vs. time on stream. The CWR with Rh catalyst was used and the figure depicts ethanol conversion (X EtOH ) and selectivities (S) for CO and CH 4 as functions of time (a). H 2 and CO 2 selectivities as functions of time are shown in (b). The flow rates of the exothermic gases were 0.32 slpm CH 4 and 3.0 slpm air. The S/C ratio was 3/1 at a GHSV of 5440 h 1. After the catalyst was deactivated by carbon formation, it could be regenerated by heating in air at 800 8C for 15 min. No carbon was visible on the surface of the fecralloy plate after this heat treatment. The catalyst was then tested at an S/C ratio of 2/1 and the results were compared to the fresh catalyst at an S/C ratio of 2/1. The performance of the regenerated catalyst was somewhat worse than that of the fresh catalyst for all conditions tested. For example, in the CWR with Rh reforming catalyst, the selectivities to methane and ethylene significantly increased to 20 and 15%, respectively compared to 10%

8 162 E.C. Wanat et al. / Applied Catalysis A: General 276 (2004) methane selectivity and <1% ethylene selectivity with fresh catalyst. The H 2 /CO ratio decreased from 3/1 to 2/1 and the H 2 selectivity dropped from 90 to 60%. Finally, carbon formation was observed on the regenerated catalyst, which was not observed on fresh catalyst at an S/C ratio of 2/1 and limited reactor operation to 3 4 h Water gas shift High H 2 /CO ratios of 30/1 at an S/C ratio of 4/1 were observed in the extended CWR. However, H 2 /CO ratios of greater than 50/1 are needed for preferential oxidation of CO to produce a fuel suitable for a PEM fuel cell with a CO concentration of <10 ppm. Hence, the reactor must be modified to obtain a more optimal temperature profile, which will lead to high H 2 /CO ratios. The water gas shift is an equilibrium-limited reaction requiring low temperatures for a favorable equilibrium conversion as seen in Fig. 5b. However, at low temperatures the reaction becomes kinetically limited [29]. This requires more catalytic surface area to increase the H 2 /CO ratio. One possible solution would be to extend the length of the extended CWR. High H 2 /CO ratios are not the only concern in selecting the water gas shift catalyst. Suppressing methanation is of equal importance to water gas shift activity in catalyst selection because it consumes 3 mol of H 2 for every mole of methane formed. Pt Ce was chosen because it exhibited the lowest methanation activity when tested among several noble metal catalysts [26]. Fig. 2b shows clearly that Pt Ce increases the H 2 /CO ratio, but the methane selectivity was also higher. Thus, the effluent is more suitable for use in a PEM fuel cell after the water gas shift portion of the reactor, but some of the H 2 was consumed by methanation. Comparing the H 2 yield for the CWR and extended CWR, the H 2 yield increases only slightly for Rh and not at all for Rh Ce. However, the CO selectivity was much lower for the extended CWR, which indicates a significant amount of water gas shift had occurred (Fig. 5b). The extra H 2 produced by water gas shift was consumed by methanation because the H 2 yield would have increased if no methanation had occurred. 6. Summary Ethanol shows promise as a renewable source of hydrogen to power PEM fuel cells, eliminating net CO 2 emissions into the atmosphere. We have examined the steam reforming and water gas shift of ethanol in a catalytic wall reactor. The catalytic wall reactor eliminated heat transfer boundary layers by coupling catalytic combustion (Pt) with catalytic steam reforming (Rh or Rh Ce). The first reactor built, while successfully reforming ethanol, little water gas shift occurred as H 2 /CO ratios were 3/1. Extending the length of the endothermic channel enhanced water gas shift. The extended portion of the reactor was coated with water gas shift catalyst (Pt Ce) but no combustion catalyst. The extended CWR produced an effluent with an H 2 /CO ratio of 30/1. A time on stream test showed that the catalyst and Fe Cr alloy plate were both stable for at least 100 h. Acknowledgements The authors would like to acknowledge Abhishek Jhalani for his assistance with the time on stream experiment. This research supported by NSF under Grant CTS References [1] L.F. Brown, Int. J. Hydrogen Energy 26 (2001) 381. [2] P. Tsiakaras, A. Demin, J. Power Sour. 102 (2001) 210. [3] G.A. Deluga, J.R. Salge, L.D. Schmidt, X.E. Verykios, Science 303 (2004) 993. [4] E.Y. Garcia, M.A. Laborde, Int. J. Hydrogen Energy 16 (1991) 307. [5] K. Vasudeva, N. Mitra, P. Umasankar, S.C. Dhingra, Int. J. Hydrogen Energy 21 (1996) 13. [6] I. Fishtik, A. Alexander, R. Datta, D. Geana, Int. J. Hydrogen Energy 25 (2000) 31. [7] T. Ioannides, J. Power Sour. 92 (2001) 17. [8] D.K. Liguras, D.I. Kondarides, X.E. Verykios, Appl. Catal. B 43 (2003) 345. [9] J.P. Breen, R. Burch, H.M. Coleman, Appl. Catal. 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