Collision of Metal Clusters with Simple Molecules: Adsorption and Reaction

Collisional reactions of isolated metal cluster ions (Cu and Cr) were studied at collision energies less than 2 eV by use of a tandem-type mass spectrometer. In the reaction of Cu_n⁺ with a methanol molecule, dominant reactions were methanol chemisorption, demethanation, and H(OH) formation on the cluster ions. The absolute cross sections of each reaction were measured, and found to change drastically with the cluster size; the demethanation proceeds efficiently on Cu_6-8⁺, H(OH) formation on Cu_4,5⁺, and the chemisorption on Cu_n⁺ (n ≥ 9). Structures of the reaction intermediates were calculated by use of the density functional method, and the origin of the size-specific reactivity was examined. In the reaction of Cr_n⁺ and Cr_nO⁺ (n ≥ 2) with an ethylene molecule, the abstraction of one Cr atom from the cluster by the ethylene molecule was found to proceed dominantly through ethylene chemisorption on Cr_n⁺. On the other hand, it was found that one ethylene molecule chemisorbs on CrO⁺ and CrOH⁺ under a single collision condition, and one more ethylene molecule chemisorbs further under a multiple collision condition. The density functional calculation and the measured threshold energy for the elimination of the chemisorbed ethylene molecules indicate that the chemisorbed ethylene molecules polymerize on CrOH⁺ and CrO⁺.