Catalytic reactions of dioxygen with ethane and methane on platinum clusters: Mechanistic connections, site requirements, and consequences of chemisorbed oxygen

Abstract

C2H6 reactions with O2 only form CO2 and H2O on dispersed Pt clusters at 0.2–28 O2/C2H6 reactant ratios and 723–913K without detectable formation of partial oxidation products. Kinetic and isotopic data, measured under conditions of strict kinetic control, show that CH4 and C2H6 reactions involve similar elementary steps and kinetic regimes. These kinetic regimes exhibit different rate equations, kinetic isotope effects and structure sensitivity, and transitions among regimes are dictated by the prevalent coverages of chemisorbed oxygen (O*). At O2/C2H6 ratios that lead to O*-saturated surfaces, kinetically-relevant CH bond activation steps involve O*O* pairs and transition states with radical-like alkyls. As oxygen vacancies (∗) emerge with decreasing O2/alkane ratios, alkyl groups at transition states are effectively stabilized by vacancy sites and CH bond activation occurs preferentially at O** site pairs. Measured kinetic isotope effects and the catalytic consequences of Pt cluster size are consistent with a monotonic transition in the kinetically-relevant step from CH bond activation on O*O* site pairs, to CH bond activation on O** site pairs, to O2 dissociation on ** site pairs as O* coverage decrease for both C2H6 and CH4 reactants. When CH bond activation limits rates, turnover rates increase with increasing Pt cluster size for both alkanes because coordinatively unsaturated corner and edge atoms prevalent in small clusters lead to more strongly-bound and less-reactive O* species and lower densities of vacancy sites at nearly saturated cluster surfaces. In contrast, the highly exothermic and barrierless nature of O2 activation steps on uncovered clusters leads to similar turnover rates on Pt clusters with 1.8–8.5nm diameter when this step becomes kinetically-relevant at low O2/alkane ratios. Turnover rates and the O2/alkane ratios required for transitions among kinetic regimes differ significantly between CH4 and C2H6 reactants, because of the different CH bond energies, strength of alkylO* interactions, and O2 consumption stoichiometries for these two molecules. Vacancies emerge at higher O2/alkane ratios for C2H6 than for CH4 reactants, because their weaker CH bonds lead to faster scavenging of O* and to lower O* coverages, which are set by the kinetic coupling between CH and OO activation steps. The elementary steps, kinetic regimes, and mechanistic analogies reported here for C2H6 and CH4 reactions with O2 are consistent with all rate and isotopic data, with their differences in CH bond energies and in alkyl binding, and with the catalytic consequences of surface coordination and cluster size. The rigorous mechanistic interpretation of these seemingly complex kinetic data and cluster size effects provides useful kinetic guidance for larger alkanes and other catalytic surfaces based on the thermodynamic properties of these molecules and on the effects of metal identity and surface coordination on oxygen binding and reactivity.