Comparing Rate and Mechanism of Ethane Hydrogenolysis on Transition-Metal Catalysts
journal contributionposted on 01.02.2019 by Abdulrahman Almithn, David Hibbitts
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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 H2-pressure dependence of [H2]–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 CH3CH* 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 *CH3–CH2* 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 H2-pressure dependence of ∼[H2]–3 (λ = 3) for Ru, Rh, and Ir clusters and [H2]–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 Pt119 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.