Reduction of Pd/ZnO catalyst and its catalytic activity for steam reforming of methanol

The effect of reduction temperature of the coprecipitated 15.9 % Pd/ZnO catalyst on the catalytic activity for steam reforming of methanol was investigated. The results showed that methanol conversion at 523 K reached a maximum of 41.6 % with a CO2 selectivity of 94.6 % and an outlet CO concentration of 1.26 % over the catalyst reduced at 523-573 K. X-ray diffraction analysis revealed that a PdZn alloy began to form at a reduction temperature of 523 K. The improvement in activity at reduction temperature ranging from 523 to 573 K was attributed to the formation of the PdZn alloy with crystal size of 5-14 nm. The interaction between Pd and ZnO upon reduction was also explored by means of temperature-programmed reduction and X-ray diffraction. The results demonstrated that the reduction over Pd/ZnO might undergo a process PdO/ZnO -> Pd/ZnO -> PdZnO1-x/ZnO -> PdZn alloy/ZnO. The PdZn alloy was found to be partially oxidized to PdZnO1-x again during the reaction. The PdZn alloy and PdZnO1-x species might be the real active species