Volume 138, 2008

Binding energy and preferred adsorption sites of CO on gold and silver–gold cluster cations: Adsorption kinetics and quantum chemical calculations

Abstract

We revisit the reactivity of trapped pure gold (Aun+, n < 26) and silver–gold alloy cluster cations (AgmAun+, m + n < 7) with carbon monoxide as studied in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The experimental results are discussed in terms of ab initio computations which provide a comprehensive picture of the chemical binding behaviour (like binding energy, adsorption sites, associated vibrational frequencies) of CO to the noble metal as a function of cluster size and composition. Starting from results for pure gold cluster cations for which an overall decrease of CO binding energy with increasing cluster size was experimentally observed—from about 1.09 ± 0.1 eV (for n = 6) to below 0.65 ± 0.1 eV (for n > 26)—we demonstrate that metal–CO bond energies correlate with the total electron density and with the energy of the lowest unoccupied molecular orbital (LUMO) on the bare metal cluster cation as obtained by density functional theory (DFT) computations. This is a consequence of the predominantly σ-donating character of the CO–M bond. Further support for this concept is found by contrasting the predictions of binding energies to the experimental results for small alloy cluster cations (AgmAun+, 4 < m + n < 7) as a function of composition. Here, binding energy drops with increasing silver content, while CO still binds always in a head-on fashion to a gold atom. Finally we show how the CO stretch frequency of AgmAunCO+ may be used to identify possible adsorption sites and pre-screen favorable isomers.

  • This article is part of the themed collection: Nanoalloys

Article information

Article type
Paper
Submitted
03 Apr 2007
Accepted
27 Apr 2007
First published
12 Sep 2007

Faraday Discuss., 2008,138, 393-406

Binding energy and preferred adsorption sites of CO on gold and silver–gold cluster cations: Adsorption kinetics and quantum chemical calculations

M. Neumaier, F. Weigend, O. Hampe and M. M. Kappes, Faraday Discuss., 2008, 138, 393 DOI: 10.1039/B705043G

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