A method is presented for the measurement of the chemisorptive reactivity of transition metal cluster ions at near room temperature. Similar to a technique introduced previously for neutral clusters lsqb;Rev. Sci. Instrum.56, 2123 (1985)rsqb;, this cluster ion method utilizes a fasthyphen;flow reactor attached to a supersonic, laser vaporization metal cluster source, followed by timehyphen;ofhyphen;flight mass spectral analysis of the cluster ions as a function of reactant concentration. Results are presented for clusters of cobalt and niobium in the 1ndash;22 atom size range for their chemisorptive reactions with CO, CO2, and N2. Both Nb+nand Co+nclusters displayed chemical reactivity that is remarkably similar to that of the corresponding neutral clusters. For both charge states of each metal, CO was found to chemisorb with a rate which varied in a slow, monotonically increasing fashion with cluster size. The chemisorption rate of N2and CO2, on the other hand, was found to be significantly slower than that of CO and sharply dependent upon the cluster size, this dependency being roughly independent of whether the transition metal cluster had a net positive or neutral charge. Photodissociation measurements of the masshyphen;selected positive ion chemisorption products showed that the desorption energy of these products parallels the relative reaction rate.
展开▼
