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

Abstract

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.

Hydrodeoxygenation of acetic acid over Ni-promoted Cu-based catalysts: a theoretical mechanism and kinetic study

“…A similar phenomenon happened on the MoPt surface, where the acetic acid adsorbed on the oxophilic Mo site, and the adsorption energies were about 0.58 eV higher than the Pt site. 55,56 The adsorption energies raised by 0.15–0.35 eV towards the reactant and product molecule (CH 3 COOH, C 2 H 5 OH, CH 3 COOC 2 H 5 and H 2 O). For the other intermediates, the increment of 0.49–0.65 eV was observed when only the O atom(s) of the adsorbates participated in bonding with Ni atoms (CH 3 COO, CH 3 CHOO, CH 3 CHOOH, C 2 H 5 O and OH).…”

“…Interestingly, similar situations were reported for MoPt catalysts in the same reaction, where CH 3 COO* transformed from a spectator on the monometallic Pt into the active intermediate, with a relatively low barrier (∼0.38 eV) required for the C–O bond scission after the addition of oxophilic Mo promoter. 56 The formed acetyl would occupy the Ni 3 site and react with hydrogen atom to form the acetaldehyde. Then, the acetaldehyde hydrogenates into the C 2 H 5 O* species on the Cu site.…”

“…The optimal NiCu structure is similar to the MoPt catalyst, where the oxophilic Mo is present in the form of subnanometer-sized clusters on the surface of Pt nanoparticles. 55,56…”

“…54) alloy, while the NiCu, CoCu (ref. 40) and MoPt 55,56 alloys are examples of the latter configurations. After putting the Ni atoms at the surface, several configurations of NiCu (111) catalysts with different Ni dispersions (single, dimer and tetramer shown in Fig.…”

“…The optimal NiCu structure is similar to the MoPt catalyst, where the oxophilic Mo is present in the form of subnanometersized clusters on the surface of Pt nanoparticles. 55,56 The adsorption energy (E ads ), activation energy (E a ), and reaction energies (ΔE) were calculated by the following three formulas:…”