Association Complex Formation in Gas-Phase Ta Cluster Reactions with Simple Alkanes:  Probing the Role of Entropy in Rate Determination for Barrierless Adsorption Processes

Rate coefficients for the reactions of ethane, propane, n-butane, isobutane, and neopentane with Ta atoms and Ta clusters consisting of up to 30 Ta atoms have been measured in a fast-flow reactor. A near-monotonic dependency of the rate coefficients on cluster size is observed for each of these hydrocarbons, with the exception of Ta₁₁, which exhibits rate coefficients that are several times that observed for the analogous reaction with Ta₁₀ and Ta₁₂, for propane, n-butane, and isobutane. Modeling the general trend in the rate coefficients using transition state theory is found to require a relatively loose transition state with free rotation of the hydrocarbon about the bonding point on the cluster, consistent with the rate-determining step being cluster removal by an association reaction process. It is demonstrated that, in general, metal cluster reactions with reagents that can form strongly bound association complexes may exhibit little evidence of cluster-size-based selectivity in the rate of cluster removal, despite exhibiting significant selectivity in terms of the extent of the dehydrogenation reaction involved.