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Ind. Eng. Chem. Res. 2007, 46, 8906-8909
Methanol Conversion for the Production of Hydrogen Charles E. Taylor,* Bret H. Howard, and Christina R. Myers U.S. Department of Energy, National Energy Technology Laboratory, P.O. Box 10940, Pittsburgh, PennsylVania 15236-0940
The production of methanol from a variety of biomass sources is gaining favor. Several facilities exist or are under construction throughout the world to convert biogenerated methane from the decomposition of biomass into methanol using conventional steam reforming. Methanol is an excellent liquid-hydrogen-transport medium. When powered by hydrogen, fuel cells have the potential to be the cleanest and most efficient source of electricity for use by the automotive industry. On-board reforming of liquid hydrocarbon fuels is a viable alternative to the storage of compressed hydrogen. A problem in current reforming processes is the quantity of carbon monoxide (CO) produced. Our research is geared toward circumventing the production of carbon monoxide in methanol reforming through the development of novel reforming catalysts. By modifying a copper-based catalyst, we have produced several catalysts that retain their activity and high surface area after extended methanol reforming runs both with and without the addition of steam. Introduction
Experimental Section
Our goal is to develop an efficient and cost-effective process that uses methanol as the hydrogen source for an on-board fuel cell. Methanol has the highest weight percent (12.5%) of hydrogen of any liquid hydrocarbon. This is nearly double the U.S. Department of Energy’s goal for using hydrogen as a fuel for automobiles.1 One major drawback to current reforming catalysts is the production of carbon monoxide. Carbon monoxide acts a poison upon contact with the proton-exchangemembrane (PEM) fuel-cell (Pt-based) catalyst.2,3 The objective of our research is to reduce the quantity of carbon monoxide produced during reforming to near-zero levels. Current investigations favor methanol as the liquid hydrocarbon that acts as the chemical carrier for hydrogen for fuel cells. The reasons are its ready availability, its compatibility with the current liquid-fuel-based infrastructure, and its high energy density. These characteristics allow easier storage and transportation. To eliminate the CO poisoning of the PEM fuel cell, a catalyst must be developed that operates at moderate temperatures (330-400 °C) and that has hydrogen and carbon dioxide as its primary products. Most reforming processes require high temperatures and produce a high CO yield. High operating temperatures are neither cost-effective nor considered safe by the public for on-board fuel cells. We reviewed investigations of various catalysts used in the reforming of liquid hydrocarbons, such as methanol and petroleum-based fuels. On the basis of our literature search, we selected various copper-based catalysts that are active for the steam reforming of methanol.4-8 These catalysts operate at temperatures low enough that carbon monoxide is not a significant product, and they are stable under our selected operating conditions. A copper-based catalyst that has shown much promise is the Cu/Zn/Zr/Al/Y-based catalyst.9,10 We synthesized this catalyst in our laboratory and compared its performance to that reported in the literature. This catalyst’s performance was used as a baseline for our catalyst development effort. Investigative research and testing has begun on several novel copper-based catalyst variations. The variants are Cu/Zn on Zr/Al/Y, Cu/Zr on γ-Al2O3, Cu/Zn/Zr on γ-Al2O3, and Cu/ Zn/Ce on γ-Al2O3.
The catalysts were prepared using three different methods: sequential precipitation, coprecipitation, and incipient wetness. Table 1 lists the various catalysts prepared for this study, along with the method of preparation and the BET surface areas of the catalyst as prepared and after reaction. An example of the catalyst preparation method for sequential precipitation is as follows: Appropriate amounts of Zr(NO3)2O, Al(NO3)3‚9H2O, and Y(NO3)3‚6H2O were dissolved in 125 mL of distilled water and heated to 70 °C. This solution was added dropwise into a 1500 mL beaker containing 200 mL of water at 70 °C on a hotplate with stirring. The pH was maintained at 7. A pH meter was used to monitor the pH of the solution, and adjustments were made by adding Na2CO3 solution as needed. After a 10min aging period, a solution of Cu(NO3)2 in 250 mL of distilled water at 70 °C was added dropwise to the suspension. After approximately 20 min of aging, a solution of Zn(NO3)2 dissolved in 125 mL of distilled water at 70 °C was added dropwise to the existing suspension. The pH was maintained at 7 during all additions using Na2CO3 solution as above. The product was recovered by vacuum filtration using a Bu¨chner funnel and washed with distilled water. The sample was dried at 120 °C overnight and then calcined in air at 350 °C for 6 h. A typical incipient wetness preparation was used for the synthesis of supported catalysts. A solution containing the metals of interest as nitrate salts was prepared in a volume of water previously determined to just wet the predried support material. The solution was added slowly, with mixing, to the support. The mixture was dried at 120 °C overnight and then calcined in air at 350 °C for 6 h. All reforming reactions were conducted in a 0.5 in. o.d. × 0.375 in. i.d. (1.27 cm o.d. × 0.95 cm i.d.) × 10 in. (25.4 cm) 304 stainless steel reactor. The inside wall of the reactor was coated with Restek Corporation’s Silcosteel coating for surface passivation. The catalyst was supported in the reactor between deactivated quartz wool plugs, and the reactor was operated in an upflow configuration. A gold-plated thermocouple in the center of the catalyst bed was used to control the temperature of reaction to (0.5 °C. Brooks proportional ratio mass-flow controllers were used to control the gas feed to the reactor, and an Isco high-pressure syringe pump provided liquid feed control. The feed streams were preheated to approximately 130 °C prior
10.1021/ie061307v
This article not subject to U.S. Copyright. Published 2007 by the American Chemical Society Published on Web 06/01/2007
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 8907 Table 1. Reforming Catalyst Composition, Preparation Method, and Surface Area
Table 2. Methanol Conversion and Product Yield for the Reactions without Co-fed Water
surface area (m2/g) sample BH5-2 BH5-3 BH5-4 BH5-10 BH5-12
composition (mol %)
preparation method
asprepared
after reaction
Cu/Zn/Zr/Al/Y 69:17:10:2:2 Cu/Zn/Zr/Al/ 8:2:64:13:13 Cu/Zr/γ-Al2O 12:5:83 Cu/Zn/Zr/γ-Al2O 12:2.5:25:83 Cu/Zn/Ce/γ-Al2O 12:2.5:2.5:83
sequential precipitation coprecipitation/ incipient wetness incipient wetness
92
55
21
16
200
188
incipient wetness
198
187
incipient wetness
194
195
to entering the reactor, and the reaction products were held at approximately 150 °C between the exit of the reactor and the cold trap. The cold trap was maintained at -4 °C using a temperature-controlled chiller. Typical reaction conditions were as follows: Exactly 1.0000 g of catalyst was placed in the reactor supported between the quartz wool plugs such that the tip of the gold-plated thermocouple was located in the middle of the catalyst bed. The reactor was purged at ambient temperature with helium at 5 mL/min for at least 2 h prior to reaction. The temperature of the reactor was increased to either 150 or 255 °C under a helium flow. The methanol flow was begun, at a weight hourly space velocity of 1.0 h-1, after the catalyst had reached the selected reaction temperature. Reaction conditions for the methanol/water reforming reactions were identical to those for the pure methanol reforming reactions with the exception that the feed was an equimolar mixture of methanol and distilled-deionized water. Reaction products were directly sampled and analyzed by an online dual-column gas chromatograph (GC). The GC, a Hewlett-Packard 5890 Series II instrument, was equipped with a 6 ft × 0.0125 o.d. 80/100 Porapak Q column and a 20 ft × 0.0125 o.d. 60/80 Molecular Sieve 5 Å column (both obtained from Supelco). The GC was held at a constant temperature of 100 °C during elution. Argon was used as the carrier gas at an average flow rate of 30 mL/min for the Molecular Sieve column and 50 mL/min for the Porapak Q column at 30 °C. The Molecular Sieve column was used to separate the light gaseous products, whereas the Porapak Q column was used to identify liquid hydrocarbons. Both columns were equipped with backflushing capability. Dual thermal conductivity detectors (TCDs) were employed to detect the separated components. Results and Discussion Methanol Reforming Reactions without Water. Five catalysts were prepared and tested for their ability to reform methanol without water in the reactant feed. The majority of methanol reforming catalysts require mixtures of methanol and water to function at high conversions and selectivities. The water is used to convert the byproduct carbon monoxide to hydrogen and carbon dioxide by the water-gas shift reaction. In this study, we chose to initially study the reforming ability of the catalysts without co-feeding water with the methanol. After determining performance under these conditions, performance in a system that included co-fed water was evaluated. The first catalyst tested was our preparation of the catalyst that had been reported in the literature9 to have the best overall methanol conversion and product distribution, Cu/Zn/Zr/Al/Y. The catalyst, prepared according the method described in the
sample BH5-2 literature9 BH5-2 literature9 BH5-2 BH5-3 BH5-3 BH5-4 BH5-10 BH5-12 a
temperature time on CH3OH (°C) stream (h) conversion 150 150 255 255 255 255 255 255 250 250
1.17 b 1.05 b 51.00 1.00 3.00 2.58 1.00 1.08
21.78 5 83.20 80 61.53 100.00 49.09 99.89 79.04 94.25
H2 (%) 1.92 4 90.09 55 92.98 29.41 77.65 48.01 51.73 44.08
CO2 (%)
CO (%)
98.08 a 1 0 9.01 a 12 0 7.02 a 70.59 a 5.78 16.57 21.35 30.64 38.84 9.43 16.19 39.73
Not detected. b Not given.
literature9 (BH5-2), exhibited an as-prepared surface area of 92 m2/g. This value agrees with the value reported in the literature of 92 m2/g. Reforming of methanol was performed at various reaction temperatures. Table 2 lists the data from a series of reforming reactions performed in our laboratory and those for the Cu/Zn/Zr/Al/Y catalyst as reported by Breen and Ross.9 Conversion of methanol with our preparation of the literature catalyst produced results similar to those reported in the literature. As shown in Table 2, methanol conversion increases with reaction temperature. At 150 °C, methanol conversion for BH5-2 was approximately 25%. The majority of product detected by the gas chromatograph was carbon dioxide. When the reaction temperature was increased to 255 °C, conversion of methanol increased initially to 83%. The product distribution also changed, with hydrogen comprising approximately onehalf of the detected product. Of importance is the fact that carbon monoxide was not detected at either reaction temperature in our investigation (without water as a reactant) or in the literature results. Conversion of methanol at 255 °C decreases with time on stream (Figure 1). This decrease in performance is most likely due to coke formation during the reaction. Even though the catalyst exhibits a rapid decrease in conversion over the first 20 h on stream, conversion and product selectivity ultimately stabilize to constant values (Figure 1). As listed in Table 2, conversion decreases to approximately 60% as the product selectivity improves slightly. These experiments were conducted up to 250 h with no change in conversion or selectivity after the initial drop. After reaction, the catalyst was removed from the reactor and examined. The appearance and consistency of the used catalyst suggested that coke had formed during the reaction. The catalyst also exhibited a 40% decrease in surface area during the reaction. This decrease might result from the coke formation and/or sintering of the unsupported metals in the catalyst. According to the literature,9 zirconium/aluminum/yttrium acts as a support for the copper/zinc catalyst. The literature results also suggest a synergy between copper and zirconium that is not evident between copper and aluminum. However, as shown in Table 1, the copper and zinc comprise 86 mol % of the catalyst composition. In an effort to produce a more effective supported copper/zinc catalyst, we synthesized a catalyst (BH53) by first coprecipitating the zirconium/aluminum/yttrium support and then adding the copper and zinc by incipient wetness. This resulted in a catalyst with copper and zinc comprising only 10 mol % of the catalyst. However, this also resulted in a catalyst with a very low surface area of 21 m2/g (Table 1). Initial conversions of methanol on BH5-3 were high (approximately 100%), and product selectivity was poor. As time
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Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007
Figure 1. Results of methanol conversion over the Cu/Zn/Zr/Al/Y (BH52) catalyst as a function of time.
Figure 4. Results of methanol/water conversion over Cu/Zn/Zr on γ-Al2O3 catalyst as a function of time.
Figure 2. Results of methanol conversion over Cu/Zr on γ-Al2O3 (BH54) at 255 °C.
Figure 3. Methanol conversion as a function of time on stream at 255 °C for catalysts with different surface areas (refer to Table 1).
on stream increased, methanol conversion decreased (to approximately 40%), whereas product selectivity improved (Table 2). In an effort to improve the conversion and selectivity of the reforming catalyst, we prepared several new catalysts. Catalysts BH5-4, BH5-10, and BH5-12 were all prepared by addition of the metals to γ-alumina by incipient wetness. The support was changed to γ-alumina in an effort to increase the catalyst’s surface area. These catalysts exhibited the highest surface areas of any catalysts we prepared (Table 1). After reaction, these catalysts retained >94% of their original surface areas. The first catalyst synthesized in this series was BH5-4, copper and zirconium on γ-alumina. This combination was chosen on the basis of a literature9 report that a 2.4:1 molar ratio of copper
to zirconium produced the highest turnover number per copper atom. Figure 2 shows the results of methanol reforming over Cu/Zr on γ-Al2O3. The catalyst exhibited high methanol conversion (>80%); however, hydrogen production was 50 mol %, and carbon monoxide production decreased to 9 mol %. Catalyst BH5-12 was synthesized by replacing zirconium with cerium. Cerium was chosen because it is chemically similar to zirconium and has an additional oxidation state, thus providing the possibility of increasing the turnover number. We observed that methanol conversion decreases to a value less than that of BH5-4 (copper/zirconium only) and the production of carbon dioxide decreases as the production of carbon monoxide increases. The true test of any catalyst is its ability to perform for long periods of time with minimal changes in the product slate. Figure 3 shows the results of methanol reforming on all of the catalysts synthesized in this study over a 150-h period. As shown, all of the catalysts perform well at the 1-h mark. After 50 h of reforming, the BH5-2 and BH5-3 catalysts exhibited marked decreases in methanol conversion. After 150 h of reforming, only BH5-4, BH5-10, and BH5-12 still exhibited catalytic activity. As listed in Table 1, these are the catalyst compositions that exhibited the highest surface area (∼200 m2/g) after synthesis. These catalysts also kept their high surface areas after 150 h on stream. Methanol Reforming Reactions with Water. The main byproduct from a hydrogen-fueled PEM fuel cell is water. This water could be used as a co-feed in the methanol reforming reactor to increase the conversion of methanol to hydrogen and to reduce the amount of carbon monoxide produced by means of a water-gas shift reaction on the catalyst. As reported in the literature,9 methanol reforming reactions were carried out using a sequentially precipitated Cu/ZnO/ZrO2/Al2O3 catalyst and 1.3:1 H2O/CH3OH reactant mixture. In addition, the catalyst was pretreated with a 5% H2/N2 mixture starting at room, increasing the temperature at 5 °C/min to 240 °C and holding at 240 °C for 4 h before reaction. In our experiments, the catalysts prepared for the reforming experiments using only methanol as the feed were tested, as-synthesized without
Ind. Eng. Chem. Res., Vol. 46, No. 26, 2007 8909 Table 3. Methanol Conversion and Product Yield with the Addition of Steam (1:1) sample
composition (mol %)
BH5-2
Cu/Zn/Zr/Al/ Y 69:17:10:2:2 BH5-3 Cu/Zn/Zr/Al/ Y 8:2:64:13:13 BH5-4 Cu/Zr/γ-Al2O3 12:5:83 BH5-10 Cu/Zn/Zr/γ-Al2O3 12:2.5:25:83 BH5-12 Cu/Zn/Ce/γ-Al2O3 12:2.5:2.5:83
H2 temperature CH3OH conversion (%) (°C) 59.0
CO2 CO (%) (%)
250
100.0
250
21.2
250
99.4
43.2
30.4
1.10
250
99.7
60.9
38.0
0.98
250
99.0
57.6
38.5
3.70
7.90
38.5
4.30
4.80 0.00
pretreatment, for their reforming and water-gas shift abilities. The feed consisted of an equimolar methanol/water mixture. Figure 4 displays the methanol reforming results for BH510 (Cu/Zn/Zr on γ-Al2O3), the catalyst that exhibited the highest methanol conversion and the lowest production of carbon monoxide. The BH5-10 catalyst also exhibited a relatively stable conversion and product selectivity over a 200-h reforming experiment. The results of methanol reforming using all of our synthesized catalysts are listed in Table 3. With the exception of catalyst BH5-3 (the low-surface-area Cu/Zn/Zr/Al/Y preparation), all of the catalysts exhibited an improvement in the conversion of methanol over extended periods of time on stream with the addition of water to the reactant feed. A reduction of the byproduct carbon monoxide was also observed for each catalyst when water was added to the feed. Conclusions We have synthesized several catalysts that are capable of reforming methanol at moderate temperatures. These catalysts are stable over several hundred hours of reforming. The main products of reaction are hydrogen and carbon dioxide. Carbon monoxide, a byproduct of reaction and a poison for most fuel cells, was below the detection limit of our gas chromatograph for the catalyst compositions (BH5-2 and BH5-3) that were the same as that reported by Breen and Ross.9 The main difference in experimentation is that, in our experiments, we did not add water to the reactant feed (as was done in the literature). These catalysts, however, lost reforming activity rapidly (in less than 100 h) as a result of coke formation. The catalysts prepared by addition of various metals to γ-alumina by incipient wetness yielded the catalysts with the highest surface areas. These catalysts also retained >94% of their initial surface area after extended (>100 h) time on stream. They also maintained consistent levels of methanol conversion and hydrogen production during the extended reforming reactions.
The addition of cerium to the catalyst decreases methanol conversion and increases the production of carbon monoxide. Increased methanol conversion and hydrogen production was observed in experiments when we added water the reactant stream to facilitate the water-gas shift reaction. The addition of water to the feed also reduced the amount of carbon monoxide produced and enhanced the long-term stability of the catalysts. Acknowledgment The authors acknowledge the technical assistance of Richard R. Anderson, Heather Elsen, Kris Howard, Edward P. Ladner, Dirk D. Link, and Parsons Project Services, Inc. Reference in this report to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply its endorsement or favoring by the United States Department of Energy. Literature Cited (1) Mission Statement. Hydrogen, Fuel Cells and Infrastructure Technologies Program, Energy Efficiency and Renewable Energy, U.S. Department of Energy: Washington, D.C. See http://www1.eere.energy.gov/ hydrogenandfuelcells/mission.html#storage. (2) Borroni-Bird, C. E. Fuel cell commercialization issues for lightduty vehicle applications. J. Power Sources 1996, 61, 33-48. (3) Hydrogen Program Overview; Report DOE/GO-10095-088, DE94011827; National Renewable Energy Laboratory: Golden, CO, Feb 1995. (4) Zhang, X. R.; Shi, P; Zhao, J.; Zhao, M.; Liu, C. Production of hydrogen for fuel cells by steam reforming of methanol on Cu/ZrO2/Al2O3 catalysts. Fuel Process. Technol. 2003, 83, 183-192. (5) Agrell, J.; Birgersson, H.; Boutonnet, M.; Melia´n-Cabrera, I.; Navarro, R. M.; Fierro, J. L. G. Production of hydrogen from methanol over Cu/ZnO catalysts promoted by ZrO2 and Al2O3. J. Catal. 2003, 219, 389-403. (6) Lindstro¨m, B.; Pettersson, L. J.; Govind Menon, P. Activity and characterization of Cu/Zn, Cu/Cr and Cu/Zr on γ-alumina for methanol reforming for fuel cell vehicles. Appl. Catal. A: Gen. 2002, 234, 111125. (7) Cheng, W.-H.; Chen, I.; Liou, J.-S.; Lin, S.-S. Supported Cu catalysts with yttria-doped ceria for steam reforming of methanol. Top. Catal. 2003, 22 (3-4), 225-233. (8) Liu, Y.; Hayakawa, T.; Suzuki, K.; Hamakawa, S.; Tsunoda, T.; Ishii, T.; Kumagai, M. Highly active copper/ceria catalysts for steam reforming of methanol. Appl. Catal. A: Gen. 2002, 223, 137-145. (9) Breen, J. P.; Ross, J. R. H. Methanol reforming for fuel-cell applications: development of zirconia-containing Cu-Zn-Al catalysts. Catal. Today 1999, 51, 521-533. (10) Velu, S.; Suziki, K.; Kapoor, M. P.; Ohashi, F.; Osaki, T. Selective production of hydrogen for fuel cells via oxidative steam reforming of methanol over CuZnAl(Zr)-oxide catalysts. Appl. Catal. A: Gen. 2001, 213 (1), 47-63.
ReceiVed for reView October 12, 2006 ReVised manuscript receiVed April 13, 2007 Accepted April 13, 2007 IE061307V
