Figure. 2 . Optimized structures of probe molecules ((a) CO2, (b)NH3) adsorbed on MgO or ZnO clusters. (Color description Mg: green; O: red; Zn: cyan; C: grey, H: white, N: blue)
For MgO, the distance between the C atom of CO2 and one of the O atoms of the MgO cluster was computed to be 2.51 Å andE ads was computed to be -7.8 kcal/mol where the (-) sign indicates net stabilization upon the complex formation. For ZnO, the figure indicates that CO2 interacts more strongly with the metal oxide, which is manifested by the short distance (1.37 Å) between the C atom of CO2 and one of the O atoms of the ZnO cluster. The resulting E ads was computed to be -36.8 kcal/mol, which indicates that CO2is chemically adsorbed. Moreover, it is seen in the figure that the distances between the Zn atom and the two O atoms of CO2are around 2.1 Å, implying that, in addition to the C atom, the two O atoms interact with the cluster. Such a structure is similar to the tridentate configuration of chemically absorbed CO2 on a Zn (0001) surface.[43] Also, the above trend is in accordance with a result of a microcalorimetric study by Auroux et al. , where they reported that the order of basicity of pure metal oxide surfaces is ZnO > MgO.[44] In Fig. 2 (b), the optimized structure of metal oxides with NH3 is presented. As shown, the distance between the Lewis acidic site (the metal atom) of the cluster with the N atom of NH3 was computed to be 2.21 and 2.00 Å for MgO and ZnO clusters, respectively. A corresponding E ads value for ZnO was computed to be -27.3 kcal/mol, which is, again, larger than that of MgO (-21.5 kcal/mol). Thus, DFT result indicates that the Lewis acidity of ZnO is stronger than that of MgO.