Effect of Structure of Pd/WO3-ZrO2 Catalyst on Its Activity for Direct Oxidation of Ethylene to Acetic Acid

A series of 1%Pd/WO3-ZrO2 catalysts with different W/Zr ratios and calcination temperatures of WO3-ZrO2 were prepared by an impregnation method. Their crystal structure, surface state, and acidity were determined using X-ray diffraction, N2 adsorption, NH3 temperature-programmed desorption, pyridine infrared spectroscopy, and temperature-programmed reduction. Special attention was paid to the surface states of tungsten and palladium under different preparation conditions. The results revealed that WOx surface species underwent a transformation from polytungstate species to coexistent polytungstate/crystalline WO3 and further to crystalline WO3 particles with increase of W/Zr ratio and calcination temperature. The W/Zr = 0.2 sample calcined at 1 073 K showed the maximum amount of polytungstates, which were responsible for the excellent activity. Moreover, the state of palladium was only dependent on the calcination temperature. Well-dispersed Pd species were responsible for high selectivity to acetic acid, and large metallic Pd particles were favorable for ethylene combustion.