Influence of Noble Metals on the Direct Oxidation of Ethylene to Acetic Acid over NM/WO3-ZrO2 (NM = Ru, Rh, and Pd) Catalysts

Ru-, Rh-, and Pd/WO3-ZrO2 catalysts were studied for the direct oxidation of ethylene to acetic acid. The catalytic activity and selectivity depended strongly on the noble metals used. Rh/WO3-ZrO2 gave the highest conversion of ethylene, whereas Ru/WO3-ZrO2 was almost inactive. The higher catalytic activity was attributed to a better metal dispersion. Pd/WO3-ZrO2 gave the highest selectivity for acetic acid (75%), with the selectivity of the other two catalysts being rather low (∼10%). Temperature-programmed oxidation and H2 reduction experiments were used to show that the strength of the noble metal–oxygen bond played an important role in product distribution. A weak metal–oxygen bond favored the formation of acetic acid, while a strong metal–oxygen bond favored the combustion of ethylene to COx.