The Oxidation of CO on Small Gold Clusters

Cooperative Coadsorption Effects on Small Gold Clusters. Two examples of cooperative adsorption effects on small gold cluster anions identified in temperature dependent rf-ion trap experiments (see Chemical and Catal3dic Properties of Gas-Phase Clusters for experimental details) will be presented in the following. Au3 does not react with O2 in the ion trap experiment at any reaction temperature [34]. It, however, adsorbs a maximum of two CO molecules at reaction temperatures below 250 K [185]. If the gold trimer is exposed simultaneously to CO and O2 inside the octopole ion trap, still no reaction products are observed at reaction temperatures above 250 K as can be seen 

The following idea for a possible molecular mechanism of this unexpected cooperative action of two adsorbate molecules on the small Au3 cluster is based on recent ab initio simulations of CO adsorption and CO/O2 coadsorption energetics and structures [379] as well as on previous experiments on the 

Catalytic CO Oxidation by Free Au2. The potential catal3dic activity of Au2 in the CO combustion reaction was first predicted by Hakkinen and Land-man [382]. The subsequent experimental investigation employing an rf-ion trap indeed revealed the catalytic reaction of the gold dimer and, in conjunction with theory, a detailed reaction cycle could be formulated [33]. Also for particular larger gold cluster anions evidence for catalytic CO2 formation has 

The solid lines in Fig. 1.64a are obtained by fitting this equilibrium mechanism to the experimental data. However, the extend in which the gold dimer is reformed increases with increasing CO concentration. Thus, the reaction mechanism must involve more intermediate steps representing the influence of CO. 

To reveal the complete reaction mechanism, the reaction was investigated at lower temperatures. The product ion mass spectrum recorded at 100 K with O2 and CO in the ion trap (Fig. 1.63b) shows the appearance of the coadsorption complex Au2(C0)02 discussed above. This complex represents a key intermediate in the reaction mechanism of the catalytic oxidation of CO to CO2 as has been predicted in the earlier theoretical study [382]. The experimental evidence obtained so far demonstrates that O2 adsorption is likely to be the first step in the observed reaction mechanism. Subsequent CO coadsorption yields the observed intermediate (Fig. 1.63b) and finally the bare gold dimer ion must be reformed. The further strategy to reveal the full reaction mechanism consists in varying the available experimental parameters, i.e., reaction temperature and reactant partial pressures. This procedure leads to a series of kinetic traces similar to the one shown in Fig. 1.64b and c [33]. The goal then is to find one reaction mechanism that is able to fit all experimental kinetic data obtained under the various reaction conditions. This kinetic 

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