A density functional theory study of NH3 and NO adsorption on the β-MnO2 (110) surface

The adsorption of NH3 and NO on the beta-MnO2 (1 1 0) surface has been investigated by density functional theory using periodic models. The energetically favourable sites of adsorption of the gases on the beta-MnO2 surface are 4-fold coordinate Mn (Mn4-top) and 5-fold coordinate Mn (Mn5-top). The relative adsorption energies (E-ads) of these gases on the Mn4-top site and Mn5-top site are in the orders NH3 (E-ads = -1.02 eV) > NO (E-ads = -0.96 eV) and NH3 (E-ads = -0.63 eV) > NO (E-ads = -0.49 eV). The N-H and N-O bond lengths, Mulliken charges and the densities of states of the NH3 and NO molecules are discussed after adsorption. The calculated results indicate that the coordination number of surface Mn ions has a significant influence on the adsorption capacity. Furthermore, the analysis of the results of the density of states determinations shows that when NH3 and NO are adsorbed with the NH3-Mn and NO-Mn configurations, the bonding mechanism is mainly from the interaction between the NH3 or NO molecule and the Mn d orbital, which is the major reason for the strong chemical adsorption of NH3 and NO.