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2009, 113, 18455–18458 Published on Web 10/05/2009
Stability Enhancement in Ni-Promoted Cu-Fe Spinel Catalysts for Dimethyl Ether Steam Reforming Kajornsak Faungnawakij,† Ryuji Kikuchi,‡ Tetsuya Fukunaga,§ and Koichi Eguchi*,| National Nanotechnology Center (NANOTEC), National Science and Technology DeVelopment Agency, 111 Thailand Science Park, Patumthani 12120, Thailand, Department of Chemical System Engineering, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Central Research Laboratories, Idemitsu Kosan Co., Ltd., Chiba 299-0293, Japan, and Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan ReceiVed: August 30, 2009; ReVised Manuscript ReceiVed: September 28, 2009
Effects of Ni doping on the catalytic behavior of CuxNi1-xFe2O4 (x ) 1, 0.95, 0.90, 0.50) mixed with Al2O3 were investigated in steam reforming of dimethyl ether for hydrogen production. The stability of the catalyst was significantly enhanced by doping Ni species to CuFe2O4. Formation of CuNi alloy was confirmed as the amount of Ni addition increased. The suppression of the sintering rate was clearly evidenced over Ni-doped catalysts as compared with the undoped one, while the effect on carbon deposition rate was unclear. Nevertheless, an increasing amount of Ni dopant to Cu-Fe spinel retarded the conversion of dimethyl ether and led to high selectivity to CH4 and CO. Global warming has a serious impact on civilization and environment. Greenhouse gas emission could lead to global warming and environmental problems.1 A fuel cell is an efficient electrochemical device that converts the chemical energy of hydrogen to electrical energy with higher efficiency and lower pollutant emission compared to conventional processes.2 Hydrogen is a clean fuel that can be produced from hydrocarbon reforming. Dimethyl ether (DME) has been recently considered as a promising H2 source.3 DME is preferable to methanol (MeOH) and to ethanol and methane due to its less toxic nature and relatively lower reforming temperatures, respectively.4 Steam reforming (SR) of DME ((CH3)2O + 3H2O f 6H2 + 2CO2) comprises two reactions in sequence: hydrolysis of DME to MeOH ((CH3)2O + H2O f 2CH3OH) and SR of the resultant MeOH to hydrogen and carbon dioxide (CH3OH + H2O f 3H2 + CO2).5 DME hydrolysis proceeds over solid acids, while MeOH SR proceeds over Cu-based catalysts.5 Consequently, a mixture of acid catalysts and metal-based catalysts is generally needed for DME SR. We have proposed Cu-based spinel catalysts mixed with γ-Al2O3 for DME SR.6,7 The Cu spinels exhibited excellent performance in terms of activity and stability as compared with Cu/ZnO/Al2O3. However, the Cu spinels still experienced a sintering problem after long-term operation at high reaction temperature.6 Improving stability has now become the most important target for the development of catalysts for DME reforming. The addition of metal components is known to affect catalytic performance in various reaction systems;8 it * To whom correspondence should be addressed. Phone: (+81) 75 383 2519. Fax: (+81) 75 383 2520. E-mail:
[email protected]. † National Science and Technology Development Agency. ‡ The University of Tokyo. § Idemitsu Kosan Co., Ltd. | Kyoto University.
10.1021/jp908365a CCC: $40.75
could enhance the interaction between catalyst species, and form new active and stable catalyst phases with increased surface areas. Here, the effects of doping of Ni species to Cu-Fe spinel catalysts on activity, stability, and selectivity have been investigated in DME SR over composites of Ni-doped Cu-Fe spinel and γ-Al2O3. The Ni-doped Cu-Fe spinels were prepared by a citric acid complexation method as reported elsewhere.6,7 A homogeneous aqueous solution of corresponding nitrates of copper, nickel, and iron as well as citric acid was heated to 90 °C to evaporate water, and then to 140-300 °C until fine powders were obtained. Next, calcination of the powders was conducted in air at 900 °C for 10 h. The obtained spinel was mechanically mixed with γ-Al2O3 (ALO8 from Catalysis Society of Japan) at a weight ratio of 2:1. The physical mixture was calcined again at 700 °C for 10 h. The mixture of the spinel and Al2O3 is referred to as CuxNi1-xFe2O4-Al2O3 (x ) 1, 0.95, 0.90, 0.50). The crystalline phase of the catalysts was measured by the powder X-ray diffraction (XRD) technique using a Rigaku Ultima IV with Cu KR radiation source. Temperatureprogrammed oxidation (TPO) was employed to analyze the carbon deposited on catalyst surface. A catalyst sample (50 mg) was oxidized in 5% O2/He mixed gas at a gas flow rate of 30 mL min-1 (25 °C, 1 atm) and a heating rate of 10 °C min-1. The product gases were monitored by an online mass spectrometer. DME SR was performed in a conventional flow reactor. A mixture of DME and steam at a fixed steam-to-carbon ratio (S/C) was supplied to a preheater at a temperature of 150 °C, and then to the catalyst bed set at the reaction temperature. Gaseous compositions were analyzed by online gas chromatographs equipped with a flame ionization detector (Shimadzu, GC-14B) and with a thermal conductivity detector (VARIAN, CP-4900). The steam in feed and reformate was trapped by a condenser at ca. 3 °C before the gas analysis (for full details of 2009 American Chemical Society
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Figure 1. Time-on-stream of DME SR over CuFe2O4-Al2O3 (0,9) and Cu0.95Ni0.05Fe2O4-Al2O3 (O,b). Reaction conditions: GHSV ) 500 h-1; S/C ) 2.5; temperature ) 375 °C.
the catalytic test and catalyst characterization, see the Supporting Information). DME conversion and H2 concentration over CuFe2O4-Al2O3 and Cu0.95Ni0.05Fe2O4-Al2O3 during DME SR at 375 °C are shown in Figure 1. Complete conversion of DME was attained for 300 h over CuFe2O4-Al2O3, followed by a gradual decrease in conversion to ca. 95% at a time-on-stream of 550 h. Cu0.95Ni0.05Fe2O4-Al2O3 exhibited greater stability than CuFe2O4-Al2O3. The complete DME conversion was attained for ca. 700 h over Cu0.95Ni0.05Fe2O4-Al2O3, and then, the conversion gradually decreased to ca. 95% at a time-on-stream of 1000 h. The changes in H2 concentration were in line with those in DME conversion. At the beginning of the tests (
