The Reaction Rates of O₂ with Closed-Shell and Open-Shell Al_<i>x</i>^– and Ga_<i>x</i>^– Clusters under Single-Collision Conditions: Experimental and Theoretical Investigations toward a Generally Valid Model for the Hindered Reactions of O₂ with Metal Atom Clusters

Abstract

In order to characterize the oxidation of metallic surfaces, the reactions of O₂ with a number of Al_<i>x</i>^– and, for the first time, Ga_<i>x</i>^– clusters as molecular models have been investigated, and the results are presented here for <i>x</i> = 9–14. The rate coefficients were determined with FT-ICR mass spectrometry under single-collision conditions at O₂ pressures of ∼10^–8 mbar. In this way, the qualitatively known differences in the reactivities of the even- and odd-numbered clusters toward O₂ could be quantified experimentally. To obtain information about the elementary steps, we additionally performed density functional theory calculations. The results show that for both even- and odd-numbered clusters the formation of the most stable dioxide species, [M_<i>x</i>O₂]⁻, proceeds via the less stable peroxo species, [M_<i>x</i>⁺···O₂^2–]⁻, which contains M–O–O–M moieties. We conclude that the formation of these peroxo intermediates may be a reason for the decreased reactivity of the metal clusters toward O₂. This could be one of the main reasons why O₂ reactions with metal surfaces proceed more slowly than Cl₂ reactions with such surfaces, even though O₂ reactions with both Al metal and Al clusters are more exothermic than are reactions of Cl₂ with them. Furthermore, our results indicate that the spin-forbidden reactions of ³O₂ with closed-shell clusters and the spin-allowed reactions with open-shell clusters to give singlet [M_<i>x</i>⁺···O₂^2–]⁻ are the root cause for the observed even/odd differences in reactivity