The binding of CO to nickel clusters. I. Determination of saturation coverages

The reactions of small nickel clusters Ni-n (n = 3-60) with carbon monoxide are studied in a gas-phase flow-tube reactor. Cluster coverage is determined as a function of CO pressure at temperatures between -160 and 20 degrees C. The reactions are kinetically controlled under these conditions, but the CO uptake is characterized by a transition from a fast kinetics to a slow kinetics process. Sticking probabilities in the fast kinetics region are near unity at low temperature. The coverage at the transition is consistent with random filling of the cluster surface with van der Waals CO molecules in a standing up configuration with the carbon end toward the metal. Higher coverages in most cases are a consequence of changes in nickel cluster structure to more open ones having larger surface areas. (C) 2000 American Institute of Physics. [S0021-9606(00)00806-0].