Elementary Steps, the Role of Chemisorbed Oxygen, and the Effects of Cluster Size in Catalytic CH4鈥揙2 Reactions on Palladium
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- 作者：Ya-Huei (Cathy) Chin ; Enrique Iglesia
- 刊名：Journal of Physical Chemistry C
- 出版年：2011
- 出版时间：September 15, 2011
- 年：2011
- 卷：115
- 期：36
- 页码：17845-17855
- 全文大小：1024K
- 年卷期：v.115,no.36(September 15, 2011)
- ISSN：1932-7455
文摘
Kinetic and isotopic data and effects of cluster size are used to probe elementary steps and their kinetic relevance in CH4鈥揙2 reactions on Pd clusters that retain a metallic bulk during catalysis. CO2 and H2O were the only products detected, except when O2 was nearly depleted, during which trace CO amounts were formed. 13CH4鈥?sup>12CO鈥揙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 O2/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 H2鈥揅O mixtures on Pd. CH4 turnover rates and effective first-order rate constants initially decreased and then reached constant values as O2 pressure and O* coverage increased as a result of a transition in the surface species involved in kinetically relevant C鈥揌 bond activation steps from O*鈥? to O*鈥揙* site pairs (*, vacancy site). On O*鈥揙* site pairs, C鈥揌 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 large activation barriers (158 kJ mol鈥?) but involve high entropy transition states that lead to larger pre-exponential factors (1.48 脳 109 kPa鈥? s鈥?) than for tighter transition states involved in C鈥揌 bond activation by *鈥? site pairs for CH4 reactions with H2O or CO2 (barriers: 82.5 kJ mol鈥? and pre-exponential factors: 3.5 脳 105 kPa鈥? s鈥?). CH3 fragments at the transition state are effectively stabilized by interactions with vacancy sites on O*鈥? site pairs, which lead to higher turnover rates, as vacancies become available with decreasing O2 pressure. CH4鈥揙2 turnover rates and C鈥揌 bond activation rate constants on O*鈥揙* 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*鈥? sites to become evident at lower O2 pressures than on large clusters. These effects of metal鈥搊xygen 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.