Density functional study of propylene oxidation on Ag and Au surfaces. Comparison to ethylene oxidation

Density functional theory (DFT) calculations were carried out to investigate the reaction mechanisms for propylene oxide formation on Ag and Au catalysts. The energy profile and structural change along reactions were mutually compared between ethylene and propylene, and between Ag and Au catalysts using 5 and 7 atom cluster models. Ag is known as unique catalyst for ethylene epoxidation, but Au is not a good catalyst. For propylene oxidation, the influences of doping co-catalysts and supporting metal-oxides were investigated by varying the charge of clusters. The reaction profiles were calculated repeatedly using neutral, cationic and anionic clusters. In almost all cases considered on the Ag and Au clusters, propylene oxide formation is less favorable than hydrogen elimination which leads to π-allyl intermediates. The reaction with O atom as oxidizing agent results in both lower reaction barrier and lower selectivity in the products. For the interaction with O2 molecule, cationic Ag cluster favors propylene oxide formation and disadvantages π-allyl formation. This unique result is also obtained for cationic Au cluster. As for propylene oxidation, the difference between Ag and Au metals is not so distinct compared to that for ethylene oxidation.