Reaction Kinetics of Nitric Oxide on Size-Selected Silver Cluster Cations

We report reactions of gas-phase free silver cluster cations, Agn+ (n = 3-18), with nitric oxide molecules, which was studied by kinetics measurements using an ion trap. AgnO(NO2)m-1+ and Agn(NO2)m+ were observed as major products after multiple reactions. The reaction pathway to form these product ions was identified by fitting the data to rate equations for n ≤ 15, except for inert n = 3 and 5. Two different reaction mechanisms were found for the formation of these products depending on cluster size; pseudo-first-order rate constants of each step of elementary reactions were obtained. First, as found for n = 4, 6, and 9, AgnO+ is formed by a reaction with two NO molecules, which is followed by a release of neutral N2O. A further reaction of AgnO+ with another NO molecule produces AgnNO2+. Agn(NO2)m+ (m ≥ 1) is thus successively formed via an intermediate, AgnO(NO2)m-1+. This is analogous to the reaction of NO on silver surfaces to produce NO2. Second, both AgnNO2+ and AgnO+ are formed concurrently, as found for n = 7, 8, 10, 11, 12, and 15; AgnO+ does not act as an intermediate for AgnNO2+. AgnO(NO2)m-1+ and Agn(NO2)m+ (m ≥ 2) are formed by successive addition of NO2 to AgnO+ and AgnNO2+, respectively. It is speculated that the successive addition of NO2 proceeds via disproportionation, i.e., three NO molecules are converted to NO2 and N2O. The reaction pathways of n = 13 and 14 are explained equally well by the two mechanisms. The overall reaction rate coefficients exhibit an odd-even alternation; the higher reactivity for even values of n is due to an odd number of valence electrons.