Reactions of cobalt clusters with deuterium: Temperature dependence

The kinetics of chemical reactions of cobalt clusters (Con, n=9-21) with deuterium are studied in a continuous flow-tube-reactor over a temperature range 133-373 K. Rate constants for the dissociative addition of the first D2 molecule to Con have been measured. The rate constants, as well as their temperature dependences, are found to be strongly cluster-size dependent. Co10-13 and Co15-17 are relatively reactive, and their reaction probabilities are essentially independent of temperature, suggesting no significant reaction barrier for hydrogen adsorption. For Co20, the reaction probability remains nearly constant above 213 K, but increases at lower temperature. For Co9 and Co14, the reaction probabilities increase both at high and low temperatures. The increase at higher temperature suggests that presence of an activation barrier, while the increase at low temperature is interpreted as due to an increase in the initial trapping of D2onto the cluster's surface. For Co18-19 and Co21, evidence for more than one isomer is observed at low temperature. The results provide insight into the mechanism of the D2 dissociative chemisorption on metal surfaces