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 CH 4 –O 2 reactions on Pd clusters that retain a metallic bulk during catalysis. CO 2 and H 2 O were the only products detected, except when O 2 was nearly depleted, during which trace CO amounts were formed. 13 CH 4 – 12 CO–O 2 reactions showed that CO reacts with chemisorbed oxygen (O*) much faster than CH 4 with reactive collision probability ratios for CO and CH 4 proportional to O 2 /CO ratios via a constant exceeding 500. Thus, even if CO desorbed before forming CO 2, it would oxidize via reactions with O* at any reactor residence time required for detectable CH 4 conversion, making direct partial oxidation impractical as a molecular route to H 2 –CO mixtures on Pd. CH 4 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 CH 3 fragments weakly stabilized by the vicinal O* are formed at the transition state. These reactions show large activation barriers (158 kJ mol –1 ) but involve high entropy transition states that lead to larger pre-exponential factors (1.48 × 10 9 kPa –1 s –1 ) than for tighter transition states involved in C–H bond activation by *–* site pairs for CH 4 reactions with H 2 O or CO 2 (barriers: 82.5 kJ mol –1 and pre-exponential factors: 3.5 × 10 5 kPa –1 s –1 ). CH 3 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. CH 4 –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 CH 4, C 2 H 6, NO, and CH 3 OCH 3 oxidation on O*-covered surfaces and for hydrogenation of organosulfur compounds on surfaces nearly saturated with chemisorbed sulfur.