Reactions of Ti- and V-Doped Cu Cluster Cations with Nitric Oxide and Oxygen: Size Dependence and Preferential NO Adsorption.

Reactions of copper cluster cations doped with an early transition metal atom, CunTi(+) (n = 4-15) and CunV(+) (n = 5-14, 16), with NO and O2 were investigated at a near-thermal collision energy using a guided ion beam tandem mass spectrometer. Most of the clusters adsorb NO and O2 under single collision conditions, and this reaction is often followed by the release of Cu atoms. For both Ti- and V-doped Cu clusters, the total cross sections for the reaction with NO increase gradually with the cluster size up to n ≈ 11 and then decrease rapidly, whereas those with O2 are almost constant in n ≤ 12 and then decrease. The size dependence of the reactivity toward NO is found to correlate with that of the adsorption energy calculated by the density functional theory method; CunTi(+) clusters exhibit the larger reaction cross sections when they have the larger adsorption energies. The calculations of CunTi(+) also show that a structural transition from a Ti-exposed structure to Ti-encapsulated one occurs around n = 12. This indicates that a geometric property of the clusters, i.e., the position of the dopant atom, is a determining factor of reactivity. In addition, the Ti- and V-doping dramatically improves the reactivity of Cu cluster cations toward NO but it does not affect that toward O2 significantly. As a result, most of the Ti- and V-doped Cu clusters are more reactive toward NO than toward O2. We also studied the multiple-collision reaction of Cu7Ti(+) with NO and obtained the cluster dioxide, Cu3TiO2(+), as a product ion, which suggests that the dissociation of NO and the subsequent formation/release of N2 take place.