Kinetics and mechanism of propylene hydro-oxidation to acrolein on Au catalysts

Propylene epoxidation by H2 and O2 to propylene oxide (PO) over the Au-Ti bifunctional catalysts, as an ideal reaction for PO production, has attracted great interest. Revealing the mechanism of acrolein formation is of great importance for understanding the mechanism of molecular oxygen activation and the formation of hydroperoxo species on the Au sites. Here, we investigate the reaction mechanism of propylene oxidation to acrolein on the Au/uncalcined TS-1 (Au/TS-1-B) catalyst through a combination of multiple characterization, H2/D2 exchange, kinetics experiment, and modeling. The Ti sites are found to be non-essential to acrolein formation. Moreover, the acrolein formation on the Au/TS-1-B catalyst is confirmed to be promoted by H2 through hydroperoxo species formation, which includes two main steps: propylene dehydrogenation to *C3H5 with the aid of *OOH species, and *C3H5 oxidation by *OOH to acrolein. The latter step is determined to be the rate-determining step because the corresponding kinetics model gives the best description for experimental results. This work not only provides kinetics insights for the propylene hydro-oxidation to acrolein on the Au-Ti bifunctional catalysts, but also facilitates the rational design of Au catalysts with high activity and selectivity in the direct propylene epoxidation with H2 and O2.