The Reaction
Rates of O<sub>2</sub> with Closed-Shell and Open-Shell Al<sub><i>x</i></sub><sup>–</sup> and Ga<sub><i>x</i></sub><sup>–</sup> Clusters under Single-Collision Conditions:
Experimental and Theoretical Investigations toward a Generally Valid
Model for the Hindered Reactions of O<sub>2</sub> with Metal Atom
Clusters
- Publication date
- Publisher
Abstract
In
order to characterize the oxidation of metallic surfaces, the reactions
of O<sub>2</sub> with a number of Al<sub><i>x</i></sub><sup>–</sup> and, for the first time, Ga<sub><i>x</i></sub><sup>–</sup> 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 O<sub>2</sub> pressures of ∼10<sup>–8</sup> mbar. In this way, the
qualitatively known differences in the reactivities of the even- and
odd-numbered clusters toward O<sub>2</sub> 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<sub><i>x</i></sub>O<sub>2</sub>]<sup>−</sup>, proceeds via the less stable peroxo
species, [M<sub><i>x</i></sub><sup>+</sup>···O<sub>2</sub><sup>2–</sup>]<sup>−</sup>, 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 O<sub>2</sub>. This could be one of the main reasons why O<sub>2</sub> reactions with metal surfaces proceed more slowly than Cl<sub>2</sub> reactions with such surfaces, even though O<sub>2</sub> reactions
with both Al metal and Al clusters are more exothermic than are reactions
of Cl<sub>2</sub> with them. Furthermore, our results indicate that
the spin-forbidden reactions of <sup>3</sup>O<sub>2</sub> with closed-shell
clusters and the spin-allowed reactions with open-shell clusters to
give singlet [M<sub><i>x</i></sub><sup>+</sup>···O<sub>2</sub><sup>2–</sup>]<sup>−</sup> are the root cause
for the observed even/odd differences in reactivity