Mechanisms and Active Sites for C-O Bond Rupture within 2-Methyltetrahydrofuran over Ni, Ni12P5, and Ni2P Catalysts

Megan E. Witzke, Abdulrahman Almithn, Christian L. Coonrod, David D. Hibbitts, David W. Flaherty

Research output: Contribution to journalArticle

Abstract

Nickel phosphide catalysts (Ni12P5 and Ni2P) preferentially cleave sterically hindered 3C-O bonds over unhindered 2C-O bonds, and Ni2P is up to 50 times more selective toward 3C-O bond cleavage than Ni. Here, we combine kinetic measurements, in situ infrared spectroscopy, and density functional theory (DFT) calculations to describe the mechanism for C-O bond rupture over Ni, Ni12P5, and Ni2P catalysts. Steady-state rate measurements and DFT calculations of C-O bond rupture within 2-methyltetrahydrofuran (MTHF) show that quasi-equilibrated MTHF adsorption and dehydrogenation steps precede kinetically relevant C-O bond rupture at these conditions (1-50 kPa MTHF; 0.1-6 MPa H2; 543 K). Rates for 3C-O and 2C-O bond rupture are inhibited by H2, and the ratio of these rates increases with [H2]1/2, suggesting that the composition of the reactive intermediates for 3C-O and 2C-O rupture differs by one H atom. Site-blocking CO∗, NH3∗, and H∗ inhibit rates without altering the ratio of 3C-O to 2C-O bond rupture, indicating that these C-O bond rupture precursors and transition states bind to identical active sites. DFT-based predictions suggest that these sites are exposed ensembles of 3 Ni atoms on Ni(111) and Ni2P(001) and 4 Ni atoms on Ni12P5(001) and that the incorporation of P disrupts extended Ni ensembles and alters the reactivity of the Ni. Increasing the phosphorus to nickel ratio (P:Ni) decreases measured and DFT-predicted activation enthalpies (ΔH, 473-583 K) for 3C-O bond rupture relative to that of 2C-O bond rupture. Selectivity differences between specific C-O bonds within MTHF reflect differences in the H content of reactive intermediates, activation enthalpy barriers, and P:Ni of Ni, Ni12P5, and Ni2P nanoparticles.

Original languageEnglish (US)
Pages (from-to)7141-7157
Number of pages17
JournalACS Catalysis
Volume8
Issue number8
DOIs
StatePublished - Aug 3 2018

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Keywords

  • 2-methyltetrahydrofuran
  • DFT
  • active site
  • hydrogenolysis
  • kinetics
  • nickel phosphide
  • reaction mechanism

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)

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