Dimethyl ether (DME) is expected as a clean fuel for the 21st century [1, 2]. DME has also recently become a hydrogen carrier, storage, and potential fuel for hydrogen production to be used in fuel cells. I have been studying on steam reforming of DME for the hydrogen production. The new type catalysts prepared using a sol-gel method have been developed [2-4]. These catalysts produce hydrogen more effectively at lower reaction temperature than mixed catalysts with DME hydrolysis catalysts and methanol steam reforming catalysts. Cu-Zn(29-1wt%)/Al2O3 is the most effective catalyst for hydrogen production in my recent study. Aiming at improving the catalysis activity for hydrogen production, the preparation method and additive effect have been investigated. In this paper and presentation, I will show my latest development of the sol-gel catalyst for DME steam reforming.
Two Cu(29wt%)-Zn(1wt%)/Al2O3 and two Cu(29wt%)-Zn(1wt%)-CeO2(1wt%)/Al2O3 powder catalysts were prepared using the two different preparation procedures of sol-gel method, respectively. One is prepared in one-step (one-step method) [2-4], and the other is made in two steps (consecutive method), respectively.
BET specific surface area of Cu-Zn/Al2O3 (one-step), Cu-Zn/Al2O3 (consecutive), Cu- Zn-CeO2/Al2O3 (one-step), and Cu-Zn-CeO2/Al2O3 (consecutive) were 209, 190, 208, and 202 m2 g-1, respectively. Specific surface area of catalysts prepared by one-step method is larger than that of catalysts prepared by consecutive method. Amount of adsorbed CO is related to Cu sites of catalyst surface. The values of the four catalysts were 0.18, 0.14, 0.19, and 0.15 mmol g-1, in the same order above-mentioned. Adsorbed CO amount of catalysts prepared by one-step method is larger than that of catalysts prepared by consecutive method like surface area. The physical property data of the catalysts prepared by the consecutive method are lower than those of the catalysts prepared by the one-step method, however, hydrogen production rate of the four catalysts order is Cu-Zn-CeO2/Al2O3 (consecutive) > Cu-Zn/Al2O3 (consecutive) > Cu-Zn/Al2O3 (one-step) > Cu-Zn-CeO2/Al2O3 (one-step). The acidity of the catalysts was analyzed using pyridine adsorption by FT-IR. Lewis acid was detected, but Brønsted acid was not detected. Acidity of the catalysts prepared by the consecutive method is stronger than that of the catalysts prepared by the one-step method. Steam reforming of DME (CH3OCH3 + 3H2O → 6H2 + 2CO2) consists of two reaction steps; the first step is hydrolysis of DME to methanol (CH3OCH3 + H2O → 2CH3OH), and the second step is methanol steam reforming to hydrogen (CH3OH + H2O → 3H2 + CO2). The DME hydrolysis reaction is the rate-determining step. The stronger acidity of Lewis acid sites works for the hydrolysis well and well-produced methanol reforms to hydrogen on the copper sites, therefore copper alumina catalysts prepared by consecutive method produce hydrogen more than copper alumina catalysts prepared by one-step method. CeO2 accelerates methanol steam reforming in case of the catalyst prepared by the consecutive method, but in case of the catalyst prepared by the one-step method CeO2 weakens the acidity of the Lewis acid sites of the catalyst. Therefore, Cu-Zn-CeO2/Al2O3 (one-step) was not work most for hydrogen production among the four catalysts.
I have developed DME steam reforming catalysts. Sol-gel method has been developed, and the consecutive sol-gel method is optimum for preparation of DME steam reforming catalysts. CeO2 improves methanol steam reforming and DME steam reforming.
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