Elementary Steps, the Role of Chemisorbed Oxygen, and the Effects of Cluster Size in Catalytic CH4-O2 Reactions on Palladium

Kinetic and isotopic data and effects of cluster size are used to probe elementary steps and their kinetic relevance in CH4-O-2 reactions on Pd clusters that retain a metallic bulk during catalysis. CO2 and H2O were the only products detected, except when O-2 was nearly depleted, during which trace CO amounts were formed. (CH4)-C-13-(CO)-C-12-O-2 reactions showed that CO reacts with chemisorbed oxygen (O*) much faster than CH4 with reactive collision probability ratios for CO and CH4 proportional to O-2/CO ratios via a constant exceeding 500. Thus, even if CO desorbed before forming CO2, it would oxidize via reactions with O* at any reactor residence time required for detectable CH4 conversion, making direct partial oxidation impractical as a molecular route to H-2-CO mixtures on Pd. CH4 turnover rates and effective first-order rate constants initially decreased and then reached constant values as O-2 pressure and O* coverage increased as a result of a transition in the surface species involved in kinetically relevant C-H bond activation steps from O*-* to O*-O* site pairs (*, vacancy site). On O*-O* site pairs, C-H bonds are cleaved via H-abstraction mediated by O* and radical-like CH3 fragments weakly stabilized by the vicinal O* are formed at the transition state. These reactions show larse activation barriers (158 kJ mol(-1)) but involve high entropy transition states that lead to larger pre-exponential factors (1.48 x 10(9) kPa(-1) s(-1)) than for tighter transition states involved in C-H bond activation by *-* site pairs for CH4 reactions with H2O or CO2 (barriers: 82.5 kJ mol and pre-exponential factors: 3.5 x 10(5) kpa(-1) s(-1)) CH3 fragments at the transition state are effectively stabilized by interactions with vacancy sites on O*-* site pairs, which lead to higher turnover rates, as vacancies become available with decreasing O-2 pressure. CH4-O-2 turnover rates and C-H bond activation rate constants on O*-O* site pairs decreased with decreasing Pd cluster size, because coordinatively unsaturated exposed atoms on small clusters bind O* more strongly and decrease its reactivity for H-abstraction. The stronger O* binding on small Pd clusters also causes the kinetic involvement of O*-* sites to become evident at lower O-2 pressures than on large clusters. These effects of metal-oxygen bond strength on O* reactivity also lead to the smaller turnover rates observed on Pd clusters compared with Pt clusters of similar size. These effects of cluster size and metal identity and their O* binding energy are the root cause for reactivity differences and appear to be general for reactions involving vacancies in kinetically relevant steps, as is the case for CH4, C2H6, NO, and CH3OCH3 oxidation on O*-covered surfaces and for hydrogenation of organosulfur compounds on surfaces nearly saturated with chemisorbed sulfur.