Effect of Zn addition to supported Pd catalysts in the steam reforming of methanol

Abstract

Supported Pd catalysts exhibit poor selectivity for the steam reforming of methanol, CH 3OH+H 2O→3H 2+CO 2; hydrogen and carbon monoxide are predominantly produced by decomposition of methanol, CH 3OH→2H 2+CO. Upon addition of Zn to the supported Pd catalysts, however, the selectivity for the steam reforming is markedly improved. In particular, over Zn-modified Pd catalysts supported on CeO 2 and active carbon (AC), the selectivity attained is 99%. The rates of hydrogen formation of Zn-modified Pd/CeO 2 and Pd/AC are also enhanced compared with those of Zn-free ones. The selectivity for the steam reforming of Pd/CeO 2 is also improved upon the addition of such metals as Cd, In and Ga as well as Zn. These catalytic features may be ascribed to the alloying of Pd with Zn, Cd, In and Ga by the reduction of these modified Pd/CeO 2 catalysts. The effect of preparation conditions on the Zn-modified Pd/CeO 2 is also investigated. Pd/Zn/CeO 2 catalysts prepared by co-precipitation method are more highly active and selective than those prepared by impregnation or deposition–precipitation method. The rate of hydrogen formation of Zn-modified Pd/CeO 2 catalyst with Pd/Zn ratio of 1/1.5 prepared by co-precipitation method is highest among the Zn-modified Pd/CeO 2 catalysts examined. The Zn-modified Pd/CeO 2 catalyst has good thermal stability as well as high activity and selectivity and no deactivation occurred when the reaction was conducted at 623 K, the temperature at which Cu/ZnO catalyst lost its activity.