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

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.