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

Yiteng Zheng, Yue Qi, Ziyu Tang, Felix Hanke, Simon G. Podkolzin

Research output: Contribution to journalArticlepeer-review

9 Scopus citations

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.

Original languageEnglish
Pages (from-to)5212-5224
Number of pages13
JournalACS Sustainable Chemistry and Engineering
Volume10
Issue number16
DOIs
StatePublished - 25 Apr 2022

Keywords

  • DFT calculations
  • bimetallic
  • biomass upgrading
  • catalytic active site
  • heterogeneous catalysis
  • metal nanoparticles
  • transition state search

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