Activation and Chemisorption of Hydrogen on Aluminum Clusters
We present results from theoretical and experimental investigations of the chemical (H2 activation) and electronic properties (ionization potentials) of aluminum clusters. The chemisorption of H2 on aluminum clusters exhibits a remarkable sensitivity to the number of metal atoms in the cluster. Al6 is the smallest cluster for which chemisorption of H2 is observed experimentally and for which a stable dissociately chemisorbed state for H2 is predicted. For clusters containing more than 6 atoms, the reactivity decreases rapidly with increasing cluster size. For the bare aluminum clusters, theoretical predictions and experimental measurements of ionization thresholds are in good agreement. Using the reactive Al6 cluster as model, we discuss how electronic factors influence H2 dissociative chemisorption on metals. We find that while charge transfer from the cluster to the H2 antibonding orbital is important, the activation barrier is dominated by repulsive interactions between the H2 and the cluster. The charge state of the cluster (anion, neutral or cation) has only a small effect on the activation barrier, which suggests that similar size selectivity might be expected for charged and neutral clusters.
KeywordsActivation Barrier Dissociative Adsorption Ionization Threshold Hydrogen Chemisorption Increase Cluster Size
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