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.

Bare, low-index periodic surface models are typically used to examine metal-catalyzed reactions in density functional theory (DFT) studies, and these most closely resemble low-pressure surface science reactions and catalyzed reactions that occur on large terraces that prevail on large (>5 nm) supported nanoparticles. Many catalytic reactions, however, occur near conditions at which catalytic surfaces are saturated by one or more adsorbed intermediates, leading to strong coadsorbate interactions and surface reconstruction leading to increased curvature. Alkane hydrogenolysis is such a reaction and has been extensively studied using DFToften on bare metal surfaceswith the assumption that omitted coadsorbed hydrogen atoms (H*) do not significantly alter the relative activation barriers and with ad hoc assumptions about the site requirements for relevant reactions. Here, we use ethane hydrogenolysis on H*-covered Ir catalysts (using a periodic surface model and a nanoparticle model) as a probe reaction to examine coadsorbate interactions and to demonstrate the rigorous determination of site requirements. The kinetically relevant transition state [*CH–CH*]⧧ is larger than the 0–3 coadsorbed H* atoms it replaces, such that the reaction has a positive activation area (a concept analogous to activation volume in homogeneous reactions) and thus repels coadsorbed H* atoms when fewer than four H* vacancies are created. This induced strain cannot be relieved on the periodic surface models, resulting in large effective free energy barriers and predictions that four vacant sites are required (γ = 4). These barriers and site requirements lead to turnover rates that are 4 orders of magnitude lower than measured rates and incorrect H2-pressure dependencies. Furthermore, varying the unit cell size of the Ir(111) surface dramatically alters the calculated reaction energetics, indicating that relevant transition states destabilize one another over long distances through the H* adlayer. Curved H*-covered Ir hemispherical particle models (119 atoms), however, stabilize transition states at a lower number of vacant sites (γ = 2) through lateral relaxation of the adlayer, resulting in correct predictions of H2-pressure dependencies and quantitative agreement between calculated and measured rates.

H 2 chemisorption measurements are used to estimate the size of supported metal particles, often using a hydrogen-tosurface-metal stoichiometry of unity. This technique is most useful for small particles whose sizes are difficult to estimate through electron microscopy or X-ray diffraction. Undercoordinated metal atoms at the edges and corners of particles, however, make up large fractions of small metal clusters, and can accommodate multiple hydrogen atoms leading to coverages which exceed 1 ML (supra-monolayer). Density functional theory was used to calculate hydrogen adsorption energies on Pt and Ir particles (38-586 atoms, 0.8-2.4 nm) at high coverages (3.63 ML). Calculated differential binding energies confirm that Pt and Ir (111) single-crystal surfaces saturate at 1 ML; however, Pt and Ir clusters saturate at supra-monolayer coverages as large as 2.9 ML. Correlations between particle size and saturation coverage are provided that improve particle size estimates from H 2 chemisorption for Pt-group metals.

The effects of metal catalyst identity on the ethane hydrogenolysis rates and mechanism were examined using density functional theory (DFT) for Group 8−11 metals (Ru, Os, Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au). Previously measured turnover rates on Ru, Rh, and Ir clusters show H 2pressure dependence of [H 2 ] −3 , consistent with C−C bond activation in *CHCH* intermediates in reactions that require two H* (chemisorbed H) to desorb from the H*-covered surfaces that prevail at these hydrogenolysis conditions. Previous DFT calculations on Ir catalysts have shown that C−C bonds in alkanes are weakened by forming C−metal bonds through quasi-equilibrated dehydrogenation steps during ethane hydrogenolysis, and these steps form *CHCH* intermediates which undergo a kinetically relevant C−C bond cleavage step. Here, the DFT-calculated free-energy barriers show that *CH−CH* bond activation is also more favorable than all C−C bond activations in other intermediates on Group 8−10 metals by >34 kJ mol −1 with the exception of Pd, where *CHCH* and CH 3 CH* activate with similar activation free energies (242 and 253 kJ mol −1 , respectively, 593 K). The relative free-energy barriers between *CH−CH* bond cleavage and C−C bond cleavage in more saturated intermediates decrease as one moves from left to right in the periodic table until *CH 3 −CH 2 * bond cleavage becomes more favorable on Group 11 coinage metals (Cu, Ag, and Au). Such predicted trends are consistent with the measured turnover rates that decrease as Ru > Rh > Ir > Pt and show H 2 -pressure dependence of ∼[H 2 ] −3 (λ = 3) for Ru, Rh, and Ir clusters and [H 2 ] −2.3 (λ = 2.3) for Pt clusters. The decrease in the measured λ value for Pt, however, is caused by a decrease in the number of desorbed H* atoms from the surface (γ = 0−1) rather than a change in the mechanism as shown here using a H*-covered Pt 119 half-particle model. The lower H*-coverage on Pt compared to other metals and the lateral relaxation of the adlayer in curved nanoparticle models, as reported previously, allow *CH−CH* bond cleavage to occur at a lower number of vacant sites on Pt.

Hydrogenolysis of complex heteroatom-containing organic molecules plays a large role in upgrading fossil- and biomass-based fuel and chemical feedstocks, such as hydrodeoxygenation and desulfurization. Here, we present a fundamental study contrasting the cleavage of C–X bonds in ethane, methylamine, methanol, methanethiol, and chloromethane on group 8–11 transition metals (Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au) using density functional theory (DFT). Previous kinetic and DFT studies have shown that hydrogenolysis of unsubstituted C–C bonds in alkanes occur via unsaturated intermediates (e.g., *CHCH* for ethane) after a series of quasi-equilibrated dehydrogenation steps that weaken the C–C bond by creating C–metal bonds. However, the effects of the substituent group in CH3XH n on the required degree of unsaturation to cleave the C–X have not been systematically studied and are critical to understanding heteroatom removal. DFT-predicted free energy barriers indicate that the carbon atom in C–X generally cleaves after the removal of 2 H atoms (to form CH*) on group 8–10 metals regardless of the identity of the metal or the heteroatom. Group 11 metals (coinage metals: Cu, Ag, and Au) generally cleave the C–X bond in the most H-saturated intermediates with barriers close to thermal activation of C–X in gaseous CH3XH n molecules. The N-leaving group in C–N cleavage depends on the metal identity as it can leave fully dehydrogenated (as N*) on group 8 metals and partially or fully hydrogenated (as NH* or NH2*) on group 9–11 metals. Although O and S are both group 16 elements, C–S bonds always cleave to form S* (losing one H), while C–O bonds generally cleave to form OH* (without preceding H removal). Cl does not have H atoms to be removed before C–Cl cleavage in CH3Cl, and thus the C atom sacrifices an additional H atom to weaken the C–Cl bond on group 8 metals. This study of heteroatom removal from simple organic molecules is the first step to providing fundamental insights into H2-based upgrading of more complex organic molecules.