Interaction between Pd and ZnO during reduction of Pd/ZnO catalyst for steam reforming of methanol to hydrogen

The changes in catalyst structure and species and charge transfer during the reduction of a co-precipitated Pd/ZnO catalyst with 15.9% Pd were investigated by temperature-programmed reduction, temperature-programmed desorption, temperature-programmed electronic conductivity, and X-ray diffraction. The effect of reduction temperature of the catalyst on the catalytic performance in steam reforming of methanol was also tested. The results showed that metallic Pd highly dispersed on ZnO over Pd/ZnO. The strong interaction between Pd and ZnO during the catalyst reduction with hydrogen led to hydrogen spillover from Pd to ZnO, which caused the reduction of ZnO near the metallic Pd and the formation of PdZn alloy. The reduction process proceeded as follows: PdO/ZnO -> Pd/ZnO -> PdZnO1-x/ZnO -> amorphous PdZn alloy/ZnO -> crystalline PdZn alloy/ZnO. Metallic Pd was present in a finely dispersed and amorphous-like state in Pd/ZnO prepared by co-precipitation, and because of that PdZn alloy could be formed at lower temperature than reported in the literature. When catalysts were reduced in the temperature range from 523 K to 573 K, PdZn alloy with size of 5 similar to 14 nm was obtained. At this moment, methanol conversion and CO2 selectivity reached maxima.