Comparing Rate and Mechanism of Ethane Hydrogenolysis on Transition-Metal Catalysts

Abstract

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.