Kinetics and Reaction Mechanisms of Acetic Acid Hydrodeoxygenation over Pt and Pt–Mo Catalysts
In this work, 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 acetic acid partial pressure between 7 and 18 kPa at 423-473 K. At 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 surface acetate is not a major reaction intermediate on both Pt and Pt-Mo and, instead, C-OH bond splitting with the formation of acetyl is the first reaction step in hydrodeoxygenation. The activation energy for this step is dramatically lower on Pt-Mo. Furthermore, the activity of acetyl on Pt-Mo is different. Unlike on pure Pt, the reaction of C-O bond splitting becomes exothermic with a lower activation energy on Pt-Mo, and the reaction of C-C bond splitting, in contrast, becomes endothermic with a higher activation energy, explaining the experimentally observed higher activities and selectivities of Pt-Mo. In addition, the calculations demonstrate that a pair of Pt-Mo surface atoms acts as a single active site where Mo serves as a preferential binding anchor for O atoms. The presence of Mo atoms changes the structure and energy of adsorbed and reacting surface species.