Selective oxidation of propane to acetone (AC) with H₂ and O₂ provides a direct route to convert low-cost propane into value-added products. Unfortunately, the catalytic activity of conventional Au/Ti-based catalysts is constrained by the high energy barrier for H₂ dissociation. Herein, uncalcined TS-1 supported Au-Pd bimetallic catalysts were prepared, and the relationship between the active-site structure and corresponding performance in the selective oxidation of propane with H₂ and O₂ in the gas phase was systematically investigated. In contrast to the liquid-phase reaction, trace Pd alloyed with Au triggered an increase in both catalytic activity and selectivity, in which Au₂₀-Pd₁/TS-1-B catalyst exhibited excellent activity (170 g_AC·h⁻¹·kg_cat⁻¹) and AC selectivity (90.6%), much higher than those of the Au/TS-1-B catalyst (AC formation rate of 100 g_AC·h⁻¹·kg_cat⁻¹ and AC selectivity of 86.3%). It was found that Pd was gradually isolated into monomers with the increase of Au/Pd molar ratio, and the synergy between Pd single atoms and Au improved the catalytic performance via enhancing hydrogen dissociation and modulating the electronic structure of Au. Furthermore, the reaction conditions were optimized based on the kinetics studies and the Au₂₀-Pd₁/TS-1-B catalyst exhibited enhanced H₂ selectivity (45%) and long-term stability (over 130 h). The insights gained here can offer valuable guidance for the design of Au-Pd catalysts applicable to other gas-phase oxidation reactions.

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.

The catalytic performances over propylene epoxidation with H₂ and O₂ (HOPO process) are significantly affected by the properties (e.g., surface properties, Ti coordination, and morphology) of titanosilicate zeolite. Introducing urea into zeolite synthesis is a simple and convenient method to modify these properties of titanosilicate zeolite. Uncalcined pore-blocked titanium silicalite-1 (TS-1, i.e., TS-1-B) with the lower urea dosage possesses more defective structure and unsaturated coordinated Ti sites verified by ²⁹Si nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) analysis, which results in a high initial activity and hydrogen efficiency; while the high surface acidity generated by these Ti species leads to a continuous decrease in the activity and the propylene oxide (PO) selectivity during the reaction. As the amount of urea gradually increases, the TS-1-B samples present the reduced surface defects and defective and unsaturated Ti species. Specially, TS-1-B-0.30U presents the weaker PO adsorption on PO-diffusion reflectance infrared Fourier transform spectra (DRIFTS), and thus results in the high stable PO formation rate and selectivity over its Au catalyst. Furthermore, a flat-plate-like shape with a shorter thickness of 100 nm along the b-axis direction is observed on the urea-modified TS-1. Compared with the conventional ellipsoidal TS-1 with crystal sizes of 200 and 500 nm, the flat-plate-like TS-1-0.30U displays the less surface defects, unsaturated Ti species, and the weaker Lewis acid, which is favorable for the desorption and intracrystalline diffusion of PO, thus reduces the occurrence of side reactions for the improved selectivity and stability. This work may provide a reference for developing titanium-containing materials with high activity and stability over HOPO reaction.