Selective Oxidation of Methane to Methanol Using Supported AuPd Catalysts Prepared by Stabilizer-Free Sol-Immobilization

The selective oxidation of methane to methanol, using H2O2, under mild reaction conditions was studied using bimetallic 1 wt % AuPd/TiO2 prepared by stabilizer-free sol-immobilization. The as-prepared catalysts exhibited low, unselective oxidation activity and deleterious H2O2 decomposition, which was ascribed to the small mean particle size of the supported AuPd nanoparticles. Heat treatments were employed to facilitate particle size growth, yielding an improvement in the catalyst turnover frequency and decreasing the H2O2 decomposition rate. The effect of support phase was studied by preparing a range of AuPd catalysts supported on rutile TiO2. The low surface area rutile TiO2 yielded catalysts with effective oxygenate production but poor H2O2 utilization. The influence of the rutile-TiO2 support was investigated further by producing catalysts with a lower metal loading to maintain a consistent metal loading per square meter compared to the 1 wt % AuPd/P25 TiO2 catalyst. When calcined at 800 °C, the 0.13 wt % AuPd catalyst demonstrated significantly improved turnover frequency of 103 h-1. In contrast, the turnover frequency was found to be ca. 2 h-1 for the rutile-supported 1 wt % AuPd catalyst calcined at 800 °C. The catalysts were probed by electron microscopy and X-ray photoelectron spectroscopy to understand the influence of particle size and oxidation state on the utilization of H2O2 and oxygenate productivity. This work shows that the key to highly active catalysts involves the prevention of deleterious H2O2 decomposition, and this can be achieved through carefully controlling the nanoparticle size, metal loading, and metal oxidation state. © 2018 American Chemical Society.