Hydrogen Generation from Dimethyl Ether for Fuel Cell Auxiliary

2164

Energy & Fuels 2006, 20, 2164-2169

Hydrogen Generation from Dimethyl Ether for Fuel Cell Auxiliary Power Units Marita Nilsson,*,† Lars J. Pettersson,† and Bård Lindstro¨m‡ Department of Chemical Engineering and Technology, School of Chemical Science and Engineering, KTH-Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and VolVo PowerCell, SE-412 88 Go¨teborg, Sweden ReceiVed December 15, 2005. ReVised Manuscript ReceiVed June 27, 2006

Copper-based catalysts have been evaluated for the combined partial oxidation and steam reforming of dimethyl ether (DME) in a reformer designed to produce hydrogen for a full-scale heavy-duty truck auxiliary power unit. The experiments were conducted using gas mixtures similar to realistic operating conditions. A Pd-promoted CuZn catalyst was found to be highly selective for hydrogen production from DME at O2/DME ) 0.25 and H2O/DME ) 2.5. The catalysts were characterized using Brunauer-Emmett-Teller surface area measurement, X-ray diffraction, and temperature-programmed reduction. The copper surface area was determined by pulse chemisorption of nitrous oxide. In addition to the reforming catalyst evaluation, a startup sequence was tested, where DME was catalytically ignited and combusted over platinum or iron oxide catalysts.

1. Introduction 1.1. Dimethyl Ether as a Fuel. The debaters over our global oil supplies are divided into optimists and pessimists. The Association for the Study of Peak Oil suggests that the world’s global oil production will peak around the turn of this decade.1 The International Energy Agency’s “World Energy Outlook 2004” predicts that the oil reserves will last over the next three decades, although requiring high investments in enhanced oil recovery as well as an increasing dependence on politically unstable regions.2 Others claim that the oil supply is controlled only by economical and political factors, and therefore, in the foreseeable future, the supply of fuel will be relatively elastic.3 What is known for sure is that fossil energy is physically limited and that the world of today is characterized by a rapid growth in energy utilization. Vehicle manufacturers are rushing ahead with research into alternative fuels such as dimethyl ether (DME), biodiesel, methanol, ethanol, and hydrogen. Matters of environmental problems, as a result of the significant emissions from mobile sources, are additionally a strong driver for decreasing the dependence on crude oil. Fuel cells are focus of extensive research for generating clean electricity in automotive applications, converting chemical energy directly into electrical energy. Polymer electrolyte fuel cells, PEFCs, are generally considered to be the type of fuel cell with the highest potential for use in automotive systems. The low operating temperature of a PEFC could provide quick startups and shutdowns. Hydrogen would be the best-suited fuel for fuel cells; today, however, hydrogen infrastructure costs are unacceptably high compared to those of the existing petroleum * To whom correspondence should be addressed. E-mail: marita@ ket.kth.se. † KTH-Royal Institute of Technology. ‡ Volvo PowerCell. (1) Aleklett, K.; Campbell, C. J. Miner. Energy 2003, 18, 5. (2) World Energy Outlook 2004; International Energy Agency: Paris, France, 2004. (3) Bergman, L.; Radetzki, M. Energy EnViron. 2003, 14, 147.

infrastructure. The hydrogen could alternatively be produced onboard through the processing of a hydrogen-rich fuel. A drawback with the PEFC is the sensitivity of the anode catalyst to carbon monoxide, which needs to be removed prior to entering the fuel cell if not running the fuel cell on pure hydrogen. DME is a possible hydrogen source for PEFCs. DME has several advantages concerning environmental and health aspects. It decomposes in the atmosphere and therefore does not contribute to ozone-layer depletion. Furthermore, DME is biologically nontoxic and degradable. Because it does not have any carbon-carbon bonds, it burns without producing soot, which makes it very interesting as a fuel alternative. In addition to sootless combustion, the cetane number is very high (around 684), which makes DME an option for use in compression ignition engines. A well-to-wheels analysis performed by Concawe, EUCAR, and JRC has proven DME made from renewable feedstock to be a viable future fuel option when considering greenhouse gas emissions and cost.5 The physical properties of DME are similar to those of propane and butane, the main components of liquefied petroleum gas, and can therefore be handled in the same way. DME can be produced from any carbonaceous material, such as natural gas, coal, crude oil, and biomass. Thus, fossil-based DME could bridge the gap until efficient production methods based on renewable fuels have been established. The Swedish company Chemrec has developed a process for producing DME through the gasification of black liquor, a byproduct from the pulp and paper industry.6 The Va¨xjo¨ Va¨rnamo Biomass Gasification Centre is part of the EU-funded project CHRISGAS (Clean Hydrogen-RIch Synthesis GAS) with the aim to (4) Teng, H.; McCandless, J. C.; Schneyer, J. B. SAE Technical Paper 2003-01-0759; SAE International: Warrendale, PA. (5) Well-to-Wheels Analysis of Future AutomotiVe Fuels and Powertrains in the European Context; Well-to-Wheels Report, version 1b; CONCAWEEUCAR-Joint Research Centre: Ispra, Italy, January 2004. (6) Landa¨lv, I.; Lindblom, M. World Patent No 0240768.

10.1021/ef050419g CCC: $33.50 © 2006 American Chemical Society Published on Web 07/29/2006

Hydrogen Generation from Dimethyl Ether

demonstrate the production of clean synthesis gas from biomass gasification that can be further processed into renewable fuels such as DME.7 The flexibility of DME, considering both source and application, gives it a strong case in a long-term scenario. 1.2. Hydrogen Production from DME for Fuel Cell Auxiliary Power Units. In 1995, the use of DME as a fuel gained renewed interest, due to the development of a low-cost production process. Most research on DME concerns the complete oxidation process, mainly for propulsion and household purposes.8-12 Since 1995, a number of articles and patents have been published on generating hydrogen from DME.13-25 Three common methods exist to process fuels into hydrogenrich gases: steam reforming, partial oxidation, and autothermal reforming. Steam reforming of DME has been suggested to take place in two steps,13-18 the first one being hydrolysis of DME into methanol:

CH3OCH3 + H2O(g) T 2CH3OH(g) ∆HR0 ) 24 kJ mol-1 (1) followed by steam reforming of methanol:

2CH3OH(g) + 2H2O(g) T 6H2 + 2CO2 ∆HR0 ) 98 kJ mol-1 (2) Solid acid catalysts, typically alumina and zeolites, are active in hydrolyzing DME into methanol. The steam reforming reaction can be promoted by, for example, Pd or Cu catalysts. Steam reforming can yield high hydrogen concentrations; a drawback however is the endothermic nature of the reaction. Partial oxidation (eq 3), on the other hand, is exothermic and could thus form local hot spots in the catalyst bed, leading to deactivation of the catalyst.

1 CH3OCH3 + O2 f 2CO + 3H2 2

∆HR0 ) -38 kJ mol-1 (3)

In the present study, combined steam reforming and partial oxidation, also referred to as autothermal reforming (ATR), is investigated for hydrogen production from DME. Autothermal reforming makes possible a dynamic system with a fairly high (7) CHRISGAS. http://www.chrisgas.com/ (accessed Sept 17, 2005). (8) Sehested, J.; Mogelberg, T.; Wallington, T. J.; Kaiser, E. W.; Nielsen, O. J. J. Phys. Chem. 1996, 100, 17218. (9) Sehested, J.; Sehested, K.; Platz, J.; Egsgaard; H.; Nielsen, O. J. Int. J. Chem. Kinet. 1997, 29, 627. (10) Solymosi, F.; Cserenyi, J.; Ovari, L. J. Catal. 1997, 44, 89. (11) Solymosi, F.; Cserenyi, J.; Ovari, L. Catal. Lett. 1997, 171, 476. (12) Solymosi, F.; Klivenyi, G. Surf. Sci. 1998, 409, 241. (13) Galvita, V.; Semin, G.; Belyaev, V.; Yurieva, T.; Sobyanin, V. Appl. Catal., A 2001, 216, 85. (14) Matsumoto, T.; Nishiguchi, T.; Kanai, H.; Utani, K.; Matsumura, Y.; Imamura, S. Appl. Catal., A 2004, 276, 267. (15) Takeishi, K.; Suzuki, H. Appl. Catal., A 2004, 260, 111. (16) Mathew, T.; Yamada, Y.; Ueda, A.; Shioyama, H.; Kobayashi, T. Catal. Lett. 2005, 100, 247. (17) Tanaka, Y.; Kikuchi, R.; Takeguchi, T.; Eguchi, K. Appl. Catal., B 2005, 57, 211. (18) Semelsberger, T. A.; Ott, K. C.; Borup, R. L.; Greene, H. L. Appl. Catal., B 2005, 61, 281. (19) Wang, S.; Ishihara, T.; Takita, Y. Appl. Catal., A 2002, 228, 167. (20) Semelsberger, T. A.; Brown, L. F.; Borup, R. L.; Inbody, M. A. Int. J. Hydrogen Energy 2004, 29, 1047. (21) Sobyanin, V.; Cavallaro, S.; Freni, S. Energy Fuels 2000, 14, 1139. (22) Bhattacharyya, A.; Basu, A. U.S. Patent 5626794, 1997. (23) Carpenter, I.; Hayes, J. World Patent 9948804, 1999. (24) Topsøe, H.; Dybkjaer, I.; Nielsen, P.; Voss, B. European Patent 0931762, 1999. (25) Takeishi, K.; Yamamoto, K. European Patent 1452230, 2004.

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hydrogen content in the product gas (∼40-50% for DME26,27) as well as the avoidance of hot-spot formation in the catalyst bed. The aim is to provide hydrogen for a heavy-duty truck fuel cell auxiliary power unit (APU). Truck drivers idle engines for climate control and power for comfort. Implementation of fuel cell APUs in heavy-duty trucks could eliminate operation in idling mode and thereby drastically reduce emissions of nitrogen oxides and hydrocarbons as well as provide better energy utilization. An APU in combination with NOx aftertreatment, where nitrogen oxides are reduced, either with DME or with hydrogen from the reformer, would provide ultralow emissions from a truck. The present project’s long-term target is the implementation of auxiliary power units in DME trucks. In conjunction with the Synbios conference on biofuels in Stockholm, in 2005, the Volvo Group demonstrated a Volvo FM model equipped with a conventional 9.4 L, sixcylinder diesel engine with a modified fuel system adapted for DME. It is noteworthy that the fuel processing experiments described in this paper have been conducted under realistic conditions: 2.5 kWel and gas mixtures simulating real operating conditions. No inert gas was used for dilution. The focus has been on copper-based catalysts supported on γ-Al2O3. Alumina is used as the catalyst support because of its acidic properties, favorable for DME hydrolysis. Copper metal, active for steam reforming of methanol is deposited onto the support together with a second metal. Copper catalysts have been extensively used in steam reforming of methanol at 250-300 °C, generating hydrogenrich gases.28,29 2. Experimental Section 2.1. Catalyst Preparation. The catalysts were prepared using the wet impregnation method. The active metals were deposited onto spherical γ-Al2O3 pellets from Sasol GmbH, Germany (2.5 mm). Four different ATR catalysts were prepared, all based on copper. A total of 15 wt % of the active metal was deposited onto the pellets, of which 40 wt % is copper and 60 wt % either Fe, Mn, or Zn. One catalyst was additionally doped with 0.5 wt % Pd. The metal precursors were all in the form of nitrates. After impregnation, the catalysts were dried overnight at 120 °C and then calcined in air for 5 h at 350 °C. For the combustion catalysts, a calcination temperature of 600 °C was used. Platinum and iron, deposited on the same γ-Al2O3 pellets as mentioned above, were investigated as catalysts for the combustion of DME. The metal precursors used were platinum(II) 2,4-pentanedionate [Pt(C5H7O2)2] and iron nitrate [Fe(NO3)3‚9H2O]. 2.2. ATR Catalyst Activity Measurements. The testing of the ATR catalyst activity was performed in a full-scale stainless steel reactor with an 80 mm inner diameter at atmospheric pressure, designed to produce an amount of hydrogen that would correspond to 5 kW of electricity in a PEFC. At the reactor inlet, 4 mm inert balls of Duranit were placed to function as a mixing zone. The volume of the catalyst bed was 500 cm3. Preheated air was used to simulate the heat produced in the startup combustors. To simulate realistic operating conditions, no external heat source was used in the reformer. All catalysts were tested at an oxygen-to-DME ratio (O2/DME) of 0.25, a steam-to-DME ratio of 2.5, and a space velocity of 4500 h-1. Prior to reaction, the catalysts were treated in a mixture of 5% H2 in N2 at 300 °C in order to reduce the Cu2+ and Cu+ into Cu0, supposed to be active for H2 and CO2 production. (26) Semelsberger, T. A.; Borup, R. L. J. Power Sources 2005, 152, 87. (27) Semelsberger, T. A.; Borup, R. L. J. Power Sources 2006, 155, 340. (28) Lindstro¨m, B.; Pettersson, L. J. Int. J. Hydrogen Energy 2001, 26, 923. (29) Alejo, L.; Lago, R.; Pen˜a, M. A.; Fierro, J. L. G. Appl. Catal., A 1997, 162, 281.

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Nilsson et al. The copper surface area was determined by reaction with nitrous oxide, as described by Osinga et al.31 and Evans et al.,32 on a Micromeritics TPD/TPR 2900 instrument. The samples were reduced in pure hydrogen at 300 °C for 2 h. After the samples were cooled in He to 30 °C, pulse chemisorption of nitrous oxide was conducted, and the copper surface area was calculated from the amount of nitrogen formed, assuming the dissociation of copper takes place on the copper surface according to32 N2O(g) + 2Cus f N2 + Cus - O - Cus

(4)

The amount of nitrogen formed was measured with a TCD. The copper surface planes were assumed to be evenly distributed, and an average atomic density of 1.46 × 1019 atoms m-2 was used.32

3. Results Table 1. Catalyst Performancea

Figure 1. Schematic picture of the DME reforming system.

The product gas was analyzed using a Varian 3800 gas chromatograph equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID) using two packed columns, a HayeSep N and a Molecular Sieve 5A, and with argon as the carrier gas. CO, CO2, CH4, DME, H2, N2, and O2 were quantitatively analyzed, and CH3OH was detectable but not quantified. The temperature was measured before and inside the reactor using thermocouples placed in the center of the reactor. 2.3. Fuel Processor Startup. A compact DME ATR system (Figure 1), with a quick startup procedure, without the need for an external heat source, was evaluated. Stainless steel reactors with 56 mm inner diameter at atmospheric pressure were used. Two combustion reactors were used in order to avoid excessive temperatures in the catalyst bed. DME and air were fed to the combustor and catalytically ignited and combusted at short contact time (SV ) 150 000 h-1). λ values (λ ) actual-to-stoichiometric air/fuel ratio) ranging from 0.25 to 1 and catalyst volumes from 50 to 200 cm3 were investigated. The inlet air was preheated, since the temperature of the inlet air to an automotive APU will be approximately 120 °C (compressor outlet temperature assuming adiabatic conditions30). Platinum is a commonly used and highly active combustion catalyst and was therefore selected for the oxidation of DME (0.5 and 1 wt % Pt on γ-Al2O3). In addition, an iron oxide catalyst (10 wt % on γ-Al2O3) was tested, since earlier studies within the research group have proven iron oxide to be highly active in methanol oxidation reactions, which are assumed to resemble the oxidation of dimethyl ether. 2.4. Catalyst Characterization. The specific surface areas of the catalysts were measured by BET nitrogen adsorption at liquid nitrogen temperature using a Micromeritics ASAP 2010 Instrument. The crystal phases were identified via X-ray diffraction, XRD, with a Siemens Diffractometer 2000 using Cu KR radiation and scanning from 10 to 80°. Temperature-programmed reduction, TPR, experiments were carried out on a Micromeritics Autochem 2910 instrument. Approximately 100 mg of catalyst pellets were crushed, and the powder was pretreated in 5% O2/He at a flow rate of 50 cm3/min, increasing the temperature to 350 °C and dwelling for 30 min. After the powder was cooled in argon to room temperature, TPR experiments were carried out in 5% H2/Ar at 50 cm3/min, increasing the temperature by 10 °C/min up to 400 °C. The hydrogen consumption was measured using a TCD. TPR profiles were collected both before and after the reforming reaction. (30) Heywood, J. B. Internal Combustion Engine Fundamentals; McGrawHill: New York, 1988.

catalyst

DME conversion, %

H2 concentration, %

CO2 selectivity, %

ηrefb %

CuZn CuZn/Pd CuFe CuMn

94 68 99 98

29 37 32 31

82 78 92 77

34 54 32 31

a Reaction conditions: SV ∼ 4500 h-1, O /DME ) 0.25, H O/DME ) 2 2 2.5, pressure ) 1 atm. b On the basis of maximum H2 concentrations.

3.1. ATR Catalyst Activity Measurements. The results from the reforming tests are presented in terms of average DME conversion, H2 concentration (vol %), CO2 selectivity, and reformer efficiency and are summarized in Table 1. The carbon dioxide selectivity, SCO2, and the reformer efficiency, ηref, are defined as follows:

SCO2 )

FCO2 FCO2 + FCO

× 100

(5)

where F corresponds to the molar flows and LHV to the lower

ηref )

FH2produced‚LHVH2 FDME,in‚LHVDME

× 100

(6)

heating values with LHVH2 ) 242 kJ/mol and LHVDME ) 1310 kJ/mol. Because the CO content was too low to give a quantitative analysis when using argon as the carrier gas in the gas chromatograph, the CO concentrations were calculated using atomic balances. The O2 conversion was 100% for all catalysts. Methanol was the main byproduct in the reformer product gas. This result together with a fairly large amount of unreacted steam suggests that the methanol steam reforming reaction is not sufficiently fast in the present catalyst system. A possible explanation for the low conversion of methanol could be the deposition of coke on the copper oxide species with a resulting shortage of catalytically active sites for methanol steam reforming. High DME conversions were obtained on all catalysts examined except for the Pd-doped CuZn catalyst. This catalyst also produced a lower amount of methanol compared to the other catalysts. The highest DME conversion of 99% was obtained for the CuFe catalyst. (31) Osinga, T. J.; Linsen, B. G.; van Beek, W. B. J. Catal. 1967, 7, 277. (32) Evans, J. W.; Wainwright, M. S.; Bridgewater, A. J.; Young, D. J. Appl. Catal. 1983, 7, 75.

Hydrogen Generation from Dimethyl Ether

Figure 2. Hydrogen production vs temperature for the different catalysts. Reaction conditions: SV ∼ 4500 h-1, O2/DME ) 0.25; H2O/ DME ) 2.5; pressure ) 1 atm.

Energy & Fuels, Vol. 20, No. 5, 2006 2167

Figure 4. Catalyst ignition at λ ) 0.5 and temperature stabilization at λ ) 5.

of the exothermic partial oxidation reaction compared to the steam reforming reaction.35 The CO2 selectivity was highest for the CuFe catalyst and lowest for the CuMn catalyst, and the average CO concentrations were 0.9 and 2.8%, respectively. No formation of methane was observed for any of the catalysts. 3.2. Fuel Processor Startup Tests. One of the most important tasks when designing an automotive fuel processing system is to provide the heat required for the reforming reaction to start. During the tests, DME was catalytically oxidized (eq 7), whereupon the hot product gases are directly heat-exchanged, that is, in direct contact with the catalyst bed, enabling a rapid startup of the reformer.

CH3OCH3 + 3O2 f 2CO2 + 3H2O(g) ∆HR0 ) -1330 kJ mol-1 (7) Figure 3. Temperature profile in the ATR catalyst bed for the CuZn/ Pd catalyst. (Lreactor ) 220 mm).

In Figure 2, the hydrogen concentration at different temperatures is shown when using the different catalysts. Observe that the points at increased temperatures are not necessarily collected in chronological order. The Pd-promoted CuZn catalyst was found to be more temperature-dependent regarding hydrogen production. This catalyst exhibited the highest selectivity to hydrogen production at temperatures above 340 °C with a maximum value of 60% at 350 °C. The CuZn and the CuMn catalysts exhibited a lowering of the hydrogen production at the higher temperatures, most probably due to agglomeration of the copper particles with a resulting loss in the catalytically active area.33,34 The system was not fast enough regarding response to changes in load or parameters such as O2/DME. The problems with the dynamics might be overcome by using thin-wall monoliths as the catalyst substrate instead of pellets because they have a lower thermal mass. In addition, monoliths provide a low pressure drop and less attrition compared to pellets. A drawback with the monoliths is that less active material can be deposited per volume of catalyst. The Pd-doped CuZn catalyst was found to give a very stable temperature profile, shown in Figure 3. The temperature increase in the beginning of the experiment is believed to be a result of the faster reaction rate (33) Kung, H. H. Catal. Today 1992, 11, 443.

Once the desired temperature is reached, steam, air, and DME are fed to the reformer and the combustor can be shut off. The performance of the startup catalysts was evaluated in terms of light-off, that is, the catalysts’ ability to oxidize the DME/air mixture at a desired temperature. The platinum catalyst could ignite the reaction mixture already at room temperature, but reproducible results were hard to achieve. The iron catalyst was able to ignite the reaction at approximately 100 °C. After ignition, a rapid temperature increase was observed. The combustor was operated at a space velocity of 150 000 h-1 and in excess air (λ ) 5) in order to control the temperature at around 600 °C. The ease of ignition increased with a decreasing λ value. The impact of the amount of catalyst and active metal loading was also evaluated. No difference in light-off could be seen down to 50 cm3 of catalyst or between a 1 and 0.5 wt % loading of platinum. Tests with an empty reactor, with only Duranit, and with uncoated γ-Al2O3 pellets were additionally conducted to rule out the possibility of homogeneous ignition. However, because of the rapid temperature increase, the possibility of a catalyst-supported ignition followed by homogeneous combustion could not be ruled out. Figure 4 shows ignition curves for the iron and platinum catalysts using 100 cm3 of the catalyst and preheating the air to 120 °C. The time to full power is of great importance for the fuel processor. It is uncertain if the (34) Twigg, M. V. Catalyst Handbook, 2nd ed.; Wolfe Publishing: London, 1989. (35) Reitz, T. L.; Ahmed, S.; Krumpelt, M.; Kumar, R.; Kung, H. H. J. Mol. Catal. A 2000, 162, 275.

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Table 2. BET Surface Area, Copper Surface Area, and Dispersion for Fresh Samples catalyst

BET surface area, m2/g

Cu0 area, m2/g

dispersion, %

CuZn CuZn/Pd CuFe CuMn

173 176 173 171

4.7 2.7 6.0 3.6

12.1 6.8 15.3 9.3

iron catalyst can fulfill the desired requirements regarding the startup time. A possible approach could be to replace part of the iron catalyst in the bed with platinum catalyst in order to decrease the time to light-off. 3.3. Catalyst Characterization. Table 2 lists the BET surface areas for the different catalyst samples, showing similar values of around 170 m2/g for the catalyst pellets. Differences in catalytic activity could therefore not be correlated to differences in surface area. Figure 5 shows X-ray diffractograms for the CuZn catalyst obtained before and after the reforming reaction. Copper present in the metallic phase could only be observed for the used sample with Cu0 peaks at 43 and 50°. Thus, the active species during reaction is assumed to be metallic copper. Alumina was present in the gamma phase for all catalyst samples. The alumina peaks were overlapping the ZnO peaks. In addition to ZnO, ZnAl2O4 could be observed. Iron was present as Fe2O3, and for the CuMn sample, the manganese was in different oxidation states (MnOx and Mn3O4), overlapping in the diffractogram. The palladium content was below the detection limit of the instrument. The TPR patterns obtained for the fresh catalysts are shown in Figure 6. The TPR study was employed to investigate the reducibility of the copper species (eq 8) in the different catalysts.

CuO + H2 f Cu0 + H2O

(8)

The CuZn profile consists of three overlapping peaks. There is a shoulder at around 125 °C, probably corresponding to surface copper oxide species. The H2 consumption reaches a maximum at 160 °C, and the reduction is complete at around 225 °C. The two-step reduction of copper oxide into metallic copper could indicate an interaction between part of the copper and zinc oxide in the sample. This catalyst exhibited the lowest reduction temperature of the four samples tested. The Pd-promoted CuZn catalyst exhibited the highest temperature for copper oxide reduction of all samples. This could

Figure 5. XRD patterns for the CuZn catalyst before (fresh) and after (used) reaction.

Figure 6. TPR profiles (TCD signal) of the fresh catalyst samples. (a) CuZn, (b) CuZn/Pd, (c) CuFe, (d) CuMn.

be correlated to this catalyst exhibiting the lowest DME conversion. The shoulder at around 180 °C was unlike the CuZn catalyst observed both before and after reaction, indicating a possible interaction between the surface copper oxide species and palladium. The single copper oxide reduction peak for the CuFe catalyst was located at around 200 °C. A small and broad peak at 320 °C could be attributed to the reduction of some of the Fe2O3 into Fe3O4. The reduction profile of CuMn exhibits two maxima, at 210 and 275 °C. The lower peak starts at 150 °C. This peak is proposed to contain highly dispersed CuO species followed by bulk CuO species and highly dispersed CuO species interacting with Mn, similar to the results of Qi et al.36 The large peak at around 275 °C is believed to correspond to the reduction of MnO2 to Mn3O4. This peak was not observed at the TPR collected after the reforming reaction, showing that the manganese was in an oxidation state more difficult to reduce. A broadening of the copper oxide reduction peak also indicates sintering of the copper particles, which could be due to the lack of interaction with the manganese during reaction. The results from the N2O chemisorption measurements are given in Table 2. The copper surface areas are presented as m2 surface copper/g catalyst. The dispersion is defined as the ratio between the amount of surface copper and the total amount of copper in the sample. A reasonable explanation for the improvement of the CuZn catalyst with the addition of Pd would be an interaction of the Pd particles with highly dispersed copper oxide species. Given that this catalyst was found to have the lowest dispersion of copper, this does not seem to be the case. For this reason, it is concluded that it is the Pd particles, possibly in interaction with the ZnO species, that contribute to the highly selective catalytic sites for generating hydrogen from dimethyl ether in the present catalyst system. An interaction of the Pd particles with ZnO could limit the interaction of Cu with ZnO, leading to a decrease in the stability of the copper species and a resulting loss in copper dispersion. Pd catalysts typically exhibit high methanol decomposition activity, producing CO and H2.37,38 The relatively (36) Qi, G. X.; Zheng, X. M.; Fei, J. H.; Hou, Z. Y. J. Mol. Catal. A 2001, 176, 195. (37) Usami, Y.; Kagawa, K.; Kawazoe, M.; Matsumura, Y.; Sakurai, H.; Haruta, M. Appl. Catal., A 1998, 171, 123. (38) Matsumura, Y.; Okumura, M.; Usami, Y.; Kagawa, K.; Yamashita, H.; Anpo, M.; Haruta, M. Catal. Lett. 1997, 44, 189.

Hydrogen Generation from Dimethyl Ether

low amounts of carbon monoxide formed during the ATR reaction therefore indicate an interaction of the ZnO with Pd, shown to be selective for methanol steam reforming, producing mainly H2 and CO2.39,40 4. Conclusions A rapid startup of a DME fuel processing system could be achieved by catalytic oxidation of the fuel, to supply heat necessary for the autothermal reforming reaction. The ATR system will thus be more compact since the need for electrical power will decrease. Iron could be used as a DME oxidation catalyst; a portion of the catalyst bed could be replaced with platinum to increase the ignitability. (39) Chin, Y.; Dagle, R.; Hu, J.; Dohnalkova, A. C.; Wang, Y. Catal. Today 2002, 77, 79. (40) Iwasa, N.; Masuda, S.; Ogawa, N.; Takezawa, N. Appl. Catal., A 1995, 125, 145.

Energy & Fuels, Vol. 20, No. 5, 2006 2169

Autothermal reforming tests using copper-based catalysts exhibited high conversions of dimethyl ether with the exception of a Pd-doped CuZn catalyst, which on the other hand exhibited high selectivity toward hydrogen production. The CO concentrations were fairly low, ranging from 0.9 to 2.8%, which is of importance when considering PEFC applications. The overall conclusion from this study is that the evaluated reforming system could be a viable solution for hydrogen generation from dimethyl ether for use in auxiliary power units. Acknowledgment. The authors wish to thank the Swedish Agency for Innovation Systems, the Swedish Road Administration, and the Swedish Environmental Protection Agency for financial support. Further, thanks to Sasol GmbH, Germany, for supplying the catalyst substrate. Thanks also to Jonas Nystro¨m for his help in the lab and to Henrik Birgersson for his input on the TPR results. EF050419G