Kinetics and Reaction Mechanisms of Acetic Acid Hydrodeoxygenation over Pt and Pt–Mo Catalysts

Kinetic measurements for silica-supported Pt and Pt–Mo catalysts were collected in vapor-phase acetic acid hydrodeoxygenation by varying the hydrogen partial pressure between 18 and 72 kPa and the acetic acid partial pressure between 7 and 18 kPa at 423–473 K. Under all testing conditions, the Pt–Mo catalyst was more active and selective. In addition, the apparent activation energy for Pt–Mo of 76 ± 3 kJ/mol was lower than that of 84 ± 4 kJ/mol for Pt. The apparent reaction orders were also different. The order in hydrogen of 0.8 ± 0.1 for Pt–Mo changed to zero at higher hydrogen partial pressures while that for Pt remained constant at 0.6 ± 0.1. A near-zero order in acetic acid for Pt–Mo changed to −2.1 at higher acetic acid pressures while that for Pt remained constant at −2.9 ± 0.3. These differences in reaction kinetics as well as in selectivity trends with changes in temperature and feed composition indicated a change in the reaction mechanism for Pt–Mo. The catalysts were characterized with hydrogen temperature programmed desorption, oxygen temperature programmed oxidation and transmission electron microscopy with energy-dispersed X-ray spectroscopy elemental mapping. Mo was present in the form of subnanometer-size clusters on the surface of Pt nanoparticles. Both Pt and Pt–Mo catalysts were stable under the reaction conditions for 10 h, and the size and structure of Pt and Pt–Mo particles remained mostly unchanged, without coke accumulation. Density functional theory calculations show that neighboring Pt–Mo surface atoms act as a single active site where Mo serves as a preferential binding anchor for O atoms. The presence of Mo changes the structure and reactivity of hydrocarbon surface species, leading to a change in the reaction mechanism. In contrast with Pt, C–O bond breaking reactions become more favorable on Pt–Mo and, conversely, C–C bond breaking reactions become less favorable on Pt–Mo, explaining the experimentally observed higher activities and selectivities of Pt–Mo.