Article pubs.acs.org/JPCA
Comparison of Adsorption Probabilities of O2 and CO on Copper Cluster Cations and Anions Shinichi Hirabayashi East Tokyo Laboratory, Genesis Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan
Masahiko Ichihashi* Cluster Research Laboratory, Toyota Technological Institute: in East Tokyo Laboratory, Genesis Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan
Yoshiyuki Kawazoe Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Tamotsu Kondow† Cluster Research Laboratory, Toyota Technological Institute: in East Tokyo Laboratory, Genesis Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan S Supporting Information *
ABSTRACT: Reactions of size-selected copper cluster cations and anions, Cun±, with O2 and CO have been systematically investigated under single collision conditions by using a tandem-mass spectrometer. In the reactions of Cun± (n = 3−25) with O2, oxidation of the cluster is prominently observed with and without releasing Cu atoms at the collision energy of 0.2 eV. The reactivity of Cun+ is governed to some extent by the electronic shell structure; the relatively small reaction cross sections observed at n = 9 and 21 correspond to the electronic shell closings, and those at odd sizes in n ≤ 16 match with the clusters having no unpaired electron. On the other hand, the reactivity of Cun− exhibits no remarkable decrease by the electronic shell closings and the even-numbered electrons. These behaviors may be due to an influence of the electron detachment of the reaction intermediate, CunO2−. Both the cations and anions show the dominant formation of Cun−1O2± in n ≤ 16 and CunO2± in n ≥ 17 in the experimental time window. By contrast, Cun− (n = 3−11) do not react with CO at the collision energy of 0.2 eV, while Cun+ (n = 3−19) adsorb CO though the cross sections are relatively small. The difference in the reactivity between the charge states can be understood in terms of the frontier orbitals of the Cu cluster and O2 or CO.
1. INTRODUCTION
clusters. Actually, copper clusters have shown characteristic behaviors such as magic numbers and even−odd alternation in reactivity,3,20−25 as well as dissociation energies,26−29 ionization energies,30−34 and electron affinities.35−40 These properties are often explained in terms of the electronic shell model,41,42 where one treats the single s valence electron per atom only, as is the case for gold clusters. On the other hand, relativistic effects reduce the energy difference between 6s and 5d orbitals in a gold atom,43 and for example, because of strong hybridization of the 6s and 5d orbitals due to the relativistic
Since the recent findings that gold nanoparticles exhibit an extremely high activity for carbon monoxide (CO) oxidation,1 a large number of studies have been dedicated to reactions between O2 and CO molecules on Au clusters isolated in the gas phase2−15 and supported on the surface.16,17 These clusters having less than 10−20 atoms can be regarded as an active site for the catalytic reaction of Au nanoparticles. The dispersed Au nanoparticles with the diameter of
