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Reactions of free silver anions Ag[sub n][sup -] (n=1–13) with O[sub 2], CO, and their mixtures are investigated in a temperature controlled radio frequency ion trap setup. Cluster anions Ag[sub n][sup -] (n=1–11) readily react with molecular oxygen to yield Ag[sub n]O[sub m][sup -] (m=2, 4, or 6) oxide products. In contrast, no reaction of the silver cluster anions with carbon monoxide is detected. However, if silver cluster anions are exposed to the mixture of O[sub 2] and CO, new reaction products and a pronounced, discontinuous size dependence in the reaction behavior is observed. In particular, coadsorption complexes Ag[sub n](CO)O[sub 2][sup -] are detected for cluster sizes with n=4 and 6 and, the most striking observation, in the case of the larger odd atom number clusters Ag[sub 7][sup -], Ag[sub 9][sup -], and Ag[sub 11][sup -], the oxide product concentration decreases while a reappearance of the bare metal cluster signal is observed. This leads to the conclusion that carbon monoxide reacts with the activated oxygen on these silver clusters and indicates the prevalence of a catalytic reaction cycle. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mass spectra and kinetic measurements that reveal an unprecedented size dependence in the adsorption of multiple oxygen molecules onto anionic silver clusters are presented. These findings of coadsorption properties on Agn- clusters are conceptually important for the understanding of the role of silver clusters in oxidation processes.

The oxidation of carbon monoxide with nitrous oxide on mass-selected Au3+ and Ag3+ clusters has been investigated under multicollision conditions in an octopole ion trap experiment. The comparative study reveals that for both gold and silver cations carbon dioxide is formed on the clusters. However, whereas in the case of Au3+ the cluster itself acts as reactive species that facilitates the formation of CO2 from N2O and CO, for silver the oxidized clusters Ag3Ox+ (n = 1-3) are identified as active in the CO oxidation reaction. Thus, in the case of the silver cluster cations N2O is dissociated and one oxygen atom is suggested to directly react with CO, whereas a second kind of oxygen strongly bound to silver is acting as a substrate for the reaction. [ABSTRACT FROM AUTHOR]

Abstract: Small cationic gold clusters exhibit a strong affinity toward carbon monoxide. This prevents the coadsorption of water which would be the first step of a catalytic water gas shift chemistry on these clusters. In a gas phase ion trap experiment with mass selected Ag n Au m + it was however possible to demonstrate that the replacement of gold by silver atoms in triatomic cluster ions liberates sites for H2O adsorption. The resulting observed coadsorption effect occurs at a cross-over in the molecular binding energies of carbon monoxide and water to these clusters determined by reaction kinetics measurements and first principles calculations. [Copyright &y& Elsevier]

The binding energies of multiple CO molecules to five-atom silver–gold cluster cations have been obtained employing temperature dependent gas phase ion trap measurements and ab initiocalculations. The CO binding energies to AgnAum+(n+ m= 5) decrease with increasing number of silver atoms. Most strikingly, after the adsorption of the fourth CO to Au5+and of the third CO to Ag5+, respectively, a pronounced decrease in the binding energies of further CO molecules was observed. This is related to a CO-induced structural transformation yielding more compact metal cluster geometries. First principles calculations revealed that the exact structure of the carbonyl complexes with multiple CO and the nature of the CO-induced structural transformation strongly depend on the composition of the metal cluster as well as on the number of adsorbed CO molecules. [ABSTRACT FROM AUTHOR]

Abstract: The reactivity of small silver cluster anions Ag2–5 − toward nitric oxide and mixtures of nitric oxide with carbon monoxide is investigated under multi-collision conditions in a radio frequency octopole ion trap at temperatures of 100 and 300K. A strongly cluster size dependent reaction behavior is observed, where reactive fragmentation dominates for clusters with four or fewer atoms and only Ag5 − is able to form reaction products without dissociation of the metal cluster. The decisive role of charge transfer in the NO bond breakage, NO oxidation, and the formation of free NO2 − and N2O4 − ions, as well as N x O(y>x)-ligands on the silver clusters is discussed. The mass spectrometric data reveal the particular stability of the reaction products AgN2O4 − and Ag3NO−. The reaction product mass spectra obtained for Ag5 − in the presence of NO and CO show the depletion of the N x O(y>x)-ligands on the metal cluster possibly involving the oxidation of CO to CO2. [Copyright &y& Elsevier]