Durable catalysts for methanol steam reforming are required for on-board hydrogen processors, and Pd-Zn alloy catalysts are the candidates. The addition of a small amount of aluminum to the coprecipitadon of PdO/ZnO results in the formation of fine Pd-Zn particles after reduction with hydrogen at 500 degrees C in comparison to a Pd-Zn/ZnO catalyst prepared by an impregnation technique or by coprecipitation without aluminum. Methanol steam reforming mainly into hydrogen and carbon dioxide proceeds over these catalysts at 300 degrees C. The activity of the impregnated catalyst whose Pd content is 3 wt.% obviously decreases after the durability test at 550 degrees C. On the other hand, that for the coprecipitated catalyst with aluminum is stable during the test and similar to the initial activity of the impregnated catalyst with the same Pd content of 3 wt.%. The surface activity of Pd-Zn on the coprecipitated catalyst is similar to that on the impregnated one after deactivation, but the Pd dispersion in the coprecipitate is significantly high. The activity of a Pd-Zn catalyst (Pd content, 3 wt.%) formed from PdO/ZnO/Al2O3 coprecipitated on a zirconia support is stable and significantly higher than that for the coprecipitated catalyst without the support. In the composite catalyst, the Pd-Zn particles are stabilized and probably ultrafine. It is estimated that a high surface activity is produced on the ultrafine particles by strongly interacting with the ZnO particles stabilized on the support surface. (C) 2013 Elsevier B.V. All rights reserved.