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

Kinetic and isotopic data and Pt cluster size effects show that C2H6O2 and CH4O2 form CO2 and H2O via analogous elementary steps; turnover rates are higher for C2H6 in all kinetic regimes where CH bond cleavage limits rates because weaker CH bonds in C2H6 and stronger ethyl interactions with adsorbed oxygens (O*) at transition states lead to lower barriers for C2H6 than for CH4 activation. Reactivity differences cause transitions between kinetic regimes to occur at higher O2/alkane ratios for C2H6 because it scavenges O* species more effectively than CH4 and leads to lower O* coverages. These mechanistic analogies and insights can be rigorously extended to other alkanes and metal clusters. [Display omitted] ► C2H6O2 and CH4O2 form CO2 and H2O via similar elementary steps on Pt clusters. ► Elementary steps and rate equations depend on oxygen (O*) coverages. ► O* coverages are set by kinetic coupling between CH and OO activation steps. ► Rates reflect CH strength in alkanes and alkylO* interactions at transition states. ► Strongly bound O* at low-coordination surfaces are less reactive in CH activation. 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 co