Acidity and basicity of clusters
The catalytic behavior of metal oxides is related to the fact that their surface has both Lewis acidic (metal atoms) and basic sites (oxygen atoms).[41] In experiments, the acidity or basicity strength of the surface is typically examined using probe molecules such as CO2 (adsorbed on a basic site), or NH3 (adsorbed on an acidic site).[42] Thus, to understand the cluster’s intrinsic basic or acidic nature, we first investigated the adsorption energy of CO2 or NH3 on the metal oxide clusters where the adsorption energy (E ads) is defined as follows:
E ads = E ads+cE cE adsorbate
E abs+c indicates the energy of the cluster adsorbed by the adsorbate molecule, E c indicates the energy of the bare cluster and E adsourbateindicates the energy of adsorbate molecule.
The results of the optimized structures of the metal oxide clusters adsorbed by the probe molecule are presented in Fig. 2.