Reduction of Pd/ZnO Catalyst and Its Catalytic Activity for Steam Reforming of Methanol
Key Words: palldium; zinc oxide; supported catalyst; steam reforming of methanol; PdZn
alloy; active species; reduction
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 partially oxidized to PdZnO1–x again during the reaction. The PdZn alloy and PdZnO1–x species might be the real active species.