Platinum Metals Rev., 2008, 52, (3), 134
A New Palladium-Based Catalyst for Methanol Steam Reforming in a Miniature Fuel Cell Power Source
- Oleg Ilinich*
- Ye Liu
- Christopher Castellano
- Gerald Koermer
- Ahmad Moini
- Robert Farrauto
- BASF Catalysts LLC,
- 25 Middlesex-Essex Turnpike, Iselin, NJ 08830-0770, U.S.A.
A novel palladium-based catalyst has been developed for use in a miniature fuel cell power source for portable applications, incorporating a polymer electrolyte membrane (PEM) fuel cell. Hydrogen, which is the fuel for the cell, is produced in a ceramic microreactor via the catalytic reaction of methanol steam reforming: CH3OH + H2O → 3H2 + CO2. The need for a new catalyst in this application is driven by the limitations of traditional particulate catalysts based on copper oxide, zinc oxide and alumina (Cu-Zn-Al catalysts), which have low thermal stability and high sensitivity towards air and condensing steam. These features result in a declining activity and mechanical integrity of Cu-Zn-Al catalysts under the frequent start-stop conditions typical of the operational mode of the miniature power source. The new Pd-based catalyst has activity and selectivity similar to those of Cu-Zn-Al catalysts, but is more durable and stable under the duty cycle conditions of a portable power source. In the microreformer, the catalyst is washcoated directly on the walls of the steam reforming section, providing favourable conditions for efficient heat transfer between the heat-generating catalytic combustion section of the microreformer and its heat-consuming steam reforming section.
Current trends in energy demand for portable electronics show that the power consumption in devices such as cell phones, personal digital assistants (PDAs), notebook computers and digital cameras continues to rise. Consumers demand small size, light weight, but long battery life. The batteries most commonly used in these devices are of the rechargeable (secondary) type. However, in certain applications such as those used by exploratory expeditions, first responders and the military, recharging a battery in the field is often difficult, if possible at all. Therefore a heavy load of disposable (primary) batteries must be transported to remote locations. One solution to this problem is to use a small portable battery charger powered by a fuel cell.
Due to the low energy density of compressed hydrogen, using it as a fuel for a portable fuel cell charger is not a viable option. Liquid fuel, in particular methanol, has a much higher energy density and is easier to transport and handle, which makes it practically attractive for this application.
Methanol is used as a fuel in two different types of fuel cells. In the direct methanol fuel cell (DMFC) (Figure 1), methanol is fed directly to the anode where it reacts with water, generating electrons which travel through the external circuit as electric current, Reaction (i):
Protons travel through the proton-conducting polymer electrolyte membrane (for example, Nafion®) to the cathode where they react with oxygen from the atmosphere, to produce water.
The subject catalyst of this article was developed for use in a reformed methanol fuel cell, shown in Figure 2. In essence this is a classic type of fuel cell, invented in 1839 by William Grove. Here the fuel is hydrogen which is fed to the anode, where it splits electrocatalytically into protons and electrons, Reaction (ii):
In contrast to Grove's cell, however, in our application the hydrogen is produced by the catalytic steam reforming of methanol, Reaction (iii):
This reaction has received much attention in the last decades as an attractive route to hydrogen supply. An excellent review on methanol steam reforming (MSR) for hydrogen production has recently been published by Palo et al. (1).
The MSR reaction can be efficiently catalysed by copper-based catalysts (2–7), including the Cu-Zn-Al particulates otherwise used in methanol synthesis (8) and the water-gas shift reaction (9). These catalysts are commercially produced, and have been used successfully in industry for many years. By their nature they are quite sensitive to the process conditions. In particular, they are prone to sintering at temperatures above about 280 to 300°C, which results in a significant decline in activity, and also deteriorate both mechanically and in performance if steam condenses on them. Besides, a Cu-Zn-Al catalyst can develop dangerously strong exotherms if in its oxidised state it is exposed to a reducing environment, or, in its reduced (active) state, to an oxidising environment, such as ambient air (10). Therefore in industrial settings Cu-Zn-Al catalysts are operated under carefully controlled conditions. The operating cycle includes a lengthy start-up with slow reduction in syngas (H2/CO) or hydrogen heavily diluted with nitrogen to minimise the reduction exotherm. The reduced and activated catalysts normally operate under steady-state conditions.
By contrast, the duty cycle anticipated for the MSR catalyst in the miniature fuel cell power source is much more demanding. The miniature power source will be operated with frequent starts and stops, during which liquid (reformate) will condense and may even freeze on the catalyst. Besides, slow reduction in dry gas with low concentrations of a reductant will clearly be unavailable, and the catalyst will have to be activated (reduced) upon direct contact with the methanol/water feed mixture. The properties of Cu-Zn-Al catalysts are incompatible with these requirements, and therefore a new application-specific catalyst had to be developed. This catalyst is a further improvement over the family of palladium-zinc-based catalysts which have been developed for fuel cell applications in recent years (11–16).
The novel MSR catalyst has been developed using a combination of rapid catalytic screening and detailed parametric studies simulating the duty cycle of the miniature power source. The catalyst consists of Pd and Zn on an oxide support with proprietary additives. In the present paper this catalyst is designated as ‘Pd-Zn/oxide support’. It is fabricated in a powder form, then slurried and used as a washcoat on the channels of the micro-reformer (Figure 3) to be incorporated into a miniature fuel cell power source.
The catalytic performance of a MSR catalyst is determined by two characteristics: activity and selectivity. A good MSR catalyst should provide high rates of conversion of methanol and water into hydrogen and carbon dioxide, while side reactions should be minimised. In the MSR process, in addition to the target steam reforming Reaction (iii), several undesired side reactions can occur. In particular, carbon monoxide can be formed via methanol decomposition, Reaction (iv):
and/or reverse water-gas shift, Reaction (v):
Carbon monoxide, a known catalytic poison, can only be tolerated by the fuel cell catalysts if its concentration in the reformate is low. Even for the fuel cells based on polybenzimidazole (PBI) membranes, which operate at elevated temperatures (∼ 180°C) and are more CO-tolerant than the fuel cells with Nafion® membranes which operate at ∼ 80°C, the CO level in the reformate must not exceed 1–2%.
Another possible side product is methane, which could be generated via methanation, Reaction (vi) and Reaction (vii), that consumes considerable amounts of hydrogen, while generating large amounts of heat:
In the context of this article, the selectivity is understood as the ratio of the concentration of a given product at certain conversion of methanol to the sum of concentrations of all carbon-containing gas-phase products (in our catalytic tests no coke formation was observed), for example Equation (viii):
where SCO2 (%) is the selectivity towards CO2, and [CO2], [CO] and [CH4] are the concentrations of the corresponding species.
3.1 Performance Testing of the Cu-Zn-Al and ‘Pd-Zn/Oxide Support’ Catalysts in the Simulated Start-Stop Mode
The anticipated duty cycle of the miniature power source will include frequent starts and stops, with variable periods of steady-state operation. The catalysts to be used in the microreformer must be thoroughly tested under conditions simulating such operation, and a special laboratory test procedure has been developed for this purpose. The test consisted of the following elements as illustrated in Figure 4: (a) initial heat-up of the reactor to the reaction temperature, followed by starting the flow of the methanol/water feed mixture and (b) steady-state operation for approximately sixty hours; (c) multiple start-stop cycles consisting of (d) cooling down to 40°C with the feed flow stopped (simulated shutdown of the power source); (e) heating up to the reaction temperature; (f) restarting the flow of the methanol/water feed mixture with steady-state operation for approximately two hours.
In the catalytic performance test using the above protocol, the commercial Cu-Zn-Al catalyst, initially run under steady-state conditions, showed stable activity for a period of about two weeks. However, soon after the onset of the start-stop temperature cycling the activity began decreasing, which continued for another two weeks of operation, showing no signs of stabilisation. However the CO2 selectivity remained very high (in excess of 99%) throughout the test.
For the catalyst ‘Pd-Zn/oxide support’, tested using the same protocol, the activity under the steady-state conditions was also stable and close to that of Cu-Zn-Al catalyst. At the beginning of the temperature cycling, a slight drop in the methanol conversion was registered but the activity remained stable thereafter. The CO2 selectivity was also stable throughout the test at 98 ± 0.6%.
The temperature dependence of the initial CO2 selectivity (i.e. SCO2 at low conversions of methanol) was further investigated in the range 230 to 320°C, and was found to decrease with the temperature from 98.2 to 94.2% (Figure 5). It was also observed that for the given reaction conditions CO2 selectivity is fairly constant over a broad range of methanol conversions; however it decreases at high conversions (above ∼ 95 to 97%).
3.2 Mechanical Strength of the Cu-Zn-Al and ‘Pd-Zn/Oxide Support’ Catalysts
It is known that exposure to a liquid can cause mechanical deterioration of particulate Cu-Zn-Al catalysts (9). This practically important aspect has not been sufficiently addressed in the open literature. Therefore in addition to catalytic performance we also analysed the Cu-Zn-Al catalyst for its mechanical strength before and after sixty temperature cycles, and found that the catalyst pellets lose about 80% of their initial strength as a result of the temperature cycling. The number of starts and stops of the future miniature power sources will certainly be much greater, and therefore a more significant negative impact on the mechanical strength and finally on the integrity of the catalyst should be anticipated. This is yet another reason why particulate Cu-Zn-Al catalysts cannot be used in the miniature fuel reformer, and why a new and more robust catalyst had to be developed.
The ‘Pd-Zn/oxide support’ catalyst was deposited on the walls of the prototype microchannel reformer, with the hydraulic diameter of a channel measuring a few hundred microns (Figure 3). The catalyst was successfully tested in the microreformer, producing hydrogen-rich reformate via MSR. The same catalyst on different support structures is now used commercially in other applications.
3.3 ‘Pd-ZnO/Oxide Support’ Compared to ‘Pd-ZnO/Al2O3’, Pd/Al2O3 and Cu/CeO2 Catalysts
High MSR activity with high CO2 selectivity is well documented for Pd-Zn catalysts, which were first discovered by Iwasa et al. (17), and are now being extensively investigated for hydrogen generation in portable fuel cell power systems. The literature information pertaining to the MSR performance of the Pd-Zn catalytic system, and available to the authors, deals with catalysts composed of Pd supported on zinc oxide and also Pd-ZnO compositions supported on alumina (12–17).
Driven by a continuing interest in inexpensive non-precious metal catalysts for MSR applications, new copper-based catalytic compositions are also being developed (18, 19). To better understand the strengths and possible limitations of our Pd-based catalyst it is important to compare performance of this catalyst with that of other Pd- and Cu-based catalysts. To that end, following a procedure similar to those described in References (14) and (16), we prepared two Pd-ZnO catalysts on different supports: alumina and an oxide support material used in preparation of the new Pd-based catalyst, both catalysts having equivalent contents of Pd and ZnO. These catalysts are hereinafter referred to as ‘Pd-ZnO/Al2O3’ and ‘Pd-ZnO/oxide support’, respectively. For comparison, alumina-supported palladium catalyst (Pd/Al2O3) with the same amount of palladium as in Pd-ZnO samples was also prepared via the same procedure. In addition, a copper/ceria catalyst with 20 wt.% CuO (Cu/CeO2), similar to that used in (18), was prepared by incipient wetness impregnation of ceria with aqueous copper nitrate solution followed by drying and calcining. All catalysts were compared in terms of their activities and selectivities, now at elevated temperatures typical for certain advanced applications. The samples (3 g of each) were tested in a laboratory flow reactor as granules 250 to 710 µm in size. This particle size ensures test conditions free of pore diffusion. The feed mixture consisted of a reagent grade methanol (Aldrich) and deionised water in volume ratio MeOH:H2O = 1:1 (molar ratio 0.88:1). The feed flow rates (from 0.3 cm3 min−1 to 2.0 cm3 min−1) and the catalyst temperatures (375°C to 475°C) were programmed and controlled throughout the test as shown in Figure 6, Figure 7, Figure 8 and Figure 9. The automated gas chromatographic analysis was conducted at twenty minute intervals by sampling the gas mixtures exiting the reactor. Performance of all of the catalysts described above is analysed below.
Methanol conversion and CO2 selectivity for the catalyst ‘Pd-ZnO/oxide support’ are plotted in Figure 6 for temperatures 375°C, 425°C and 475°C and a range of flow rates. It can be seen that the catalyst has stable activity (100% methanol conversion in the first and the last segments of the run, these segments having identical experimental conditions) with CO2 selectivity ranging from around 70% at full conversion of methanol to around 82% at lower conversions.
Under the same experimental conditions the sample ‘Pd-ZnO/Al2O3’ is less active, and has much lower CO2 selectivity (Figure 7). This catalyst ages, partially losing activity in the course of the run. This is illustrated by lower methanol conversion in the last segment of the run as compared with the first. The main product with this sample is CO; under the experimental conditions of the test, methane is also produced with selectivity ranging from 1 to 6%. This implies significantly less efficient production of hydrogen with ‘Pd-ZnO/Al2O3’, and also different reaction pathways for the two similar catalysts – ‘Pd-ZnO/oxide support’ and ‘Pd-ZnO/Al2O3’.
The Pd/Al2O3 sample is even less active than ‘Pd-ZnO/Al2O3’, is more prone to ageing and has very poor CO2 selectivity (Figure 8). CO and methane are the dominant carbon-containing products with the Pd/Al2O3 catalyst under the experimental conditions employed, rendering this catalyst composition unsuitable for hydrogen generation.
The Pd-free sample with Cu impregnated on ceria has high initial activity and relatively high CO2 selectivity (up to about 75%); however it ages rapidly with a significant loss in activity (Figure 9). The ageing in this case is probably due to the thermal sintering of Cu (20). The fresh catalyst at high temperatures also produces small amounts of methane, but methanation stops as the catalyst ages.
The results of our comparative study show that the catalyst ‘Pd-Zn/oxide support’ has an optimum composition for the MSR reaction. It possesses high and stable activity, as well as the highest CO2 selectivity over a broad range of MSR process conditions. The catalyst ‘Pd-Zn/oxide support’ is hence the most efficient among the catalysts tested in this study for hydrogen generation.
A new active Pd-based MSR catalyst has been developed for use as a washcoat in a microchannel reformer integrated to a miniature fuel cell power source. The catalyst ‘Pd-Zn/oxide support’ with proprietary additives shows stable performance under the frequent start-stop operating conditions typical of a portable fuel cell power source. The new catalyst is also thermally stable, which enables its extended operation over a broad range of temperatures and simplifies fabrication of the miniature power source.
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This article contains information produced in partnership with the Motorola Energy Technologies Lab within the National Institute of Standards and Technology (NIST) Advanced Technology Program: “Hydrogen Generator for a Miniature Fuel-Cell Power Source”. Part of this information was presented at the 31st Annual Conference of the International Precious Metals Institute (IPMI) (21). The authors gratefully acknowledge the kind permission of the IPMI to publish this article.
Dr Oleg Ilinich is a Senior Chemist at BASF Catalysts LLC in Iselin, New Jersey, U.S.A. He received his M.S. in Chemical Engineering from the St. Petersburg Institute of Technology in Russia, which was later followed by Ph.D. and D.Sci. degrees in Catalysis from the Boreskov Institute of Catalysis (Novosibirsk, Russia). His recent work includes catalyst development as well as kinetic and mechanistic studies for the water-gas shift reaction, methanol steam reforming and direct methanol fuel cells. Prior to joining BASF Catalysts, Dr Ilinich was involved in fundamental and applied research in selective heterogeneous catalysis, and catalytic and gas separation membranes.
Dr Ye Liu is a Senior Research Engineer at BASF Catalysts LLC. He received his B.S. and M.S. in Chemical Engineering from the Dalian University of Technology, China, and his Ph.D. in Material Science and Engineering from the Pennsylvania State University, U.S.A. His current work involves catalyst development for three-way catalysis, partial oxidation, steam reforming and the autothermal reforming of hydrocarbons. His past research involvement has included catalytic gasification, combustion, sulfur removal in flue gases, and product and process development for carbonaceous materials – activated carbons and carbon blacks.
Christopher Castellano is a Chemist at BASF Catalysts LLC. He received his B.S. in Ceramic Engineering from Rutgers University, U.S.A.; and is currently working on a M.S. in Material Science and Engineering from Columbia University, U.S.A. His current work involves combinatorial catalyst development for environmental applications, as well as flexible fuel catalysts. His previous research involvement has included methanol steam reforming catalysis, NH3-selective catalytic reduction technology, light-duty diesel catalysts, ozone conversion and high-throughput zeolite synthesis. He is also currently serving as the President of the Rutgers Engineering Society.
Dr Gerald Koermer received a Ph.D. from the University of Wisconsin, U.S.A. in Physical Organic Chemistry and a B.A. in Chemistry from Rutgers College. He has over twenty-four years of experience in catalysis and currently leads the Materials Science and Enabling Technology group at BASF Catalysts LLC's Research and Development.
Dr Ahmad Moini is a Senior Research Associate at BASF Catalysts LLC. He obtained his Ph.D. in Chemistry from Texas A&M University, U.S.A., followed by a postdoctoral appointment at Michigan State University, U.S.A. Dr Moini started his career at Mobil Research & Development Corporation, where he conducted research in microporous materials. He joined Engelhard Corporation (now BASF) in 1996. His research focus is on the synthesis and development of novel catalysts and materials.
Dr Robert J. Farrauto is a Research Fellow at BASF Catalysts LLC. He obtained a B.S. in Chemistry from Manhattan College, New York City and a Ph.D. in Electrochemistry from Rensselaer Polytechnic Institute, Troy, New York, U.S.A. His major responsibilities have included the development of advanced automobile emission control catalysts and process catalysts for the chemical industry. He managed an Engelhard research team that developed and commercialised diesel oxidation catalysts for the European, North American and Asian markets for passenger cars and heavy-duty trucks. Currently he manages a research team developing new catalyst technology for the hydrogen economy, including hydrogen refueling stations and fuel cells for stationary, portable power and vehicular applications. He is also Adjunct Professor in the Earth and Environmental Engineering Department of Columbia University, in the City of New York, where he teaches a course in catalysis.