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.