A First Principles Study of NO2 on MgO and CaO Surfaces: Structures, Interactions and Vibrational Spectra

The studies on the effects of the properties of metal oxide formed by deposition of nanosized metal clusters on the oxide surfaces on the adsorption and binding of molecules of single and pairwise NO2 on their surface are investigated by density functional theory. In this work, we examined NO2 pair adsorption behaviour over the (MgO)9 and (CaO)9 surface clusters by considering NO2 bind on cations forming nitrite and NO2 binds on anions producing nitrates. In addition, vibrational spectra have been computed by means of the B3LYP hybrid density functional in order to interpret the experiments [J. Phys. Chem. B 2002, 106, 6358] particularly, regarding a transient 1225 cm-1 absorption during accumulation of NO2 in MgO supported BaO. The degree of generality of results was tested by comparing (NO2)x (MO)9 for x=1,2 and M=Mg, Ca. Finger prints are produced for the single bidentate M2+-[ONO]--M2+ surface nitrite ion. A novel single monodentate [Ocluster - NO2]2- ion and the chemisorbed Nitrite/Nitrate ion pair, i.e., M2+-[ONO]--M2+ + [Ocluster -NO2]-. The results showed, the novel monodentate [Ocluster - NO2]2-, to be responsible for the experimentally observed 1225 cm-1 absorption, being a transient towards surface nitrate rather than nitrite formation. This result is consistent with a mechanistic DFT study concerning the initial loading of NO2 in BaO. Furthermore, our results show the computed in stability for a single adsorbed Nitrite is 0.36 eV and (1.66 eV) for Mg9O9 and (Ca9O9), while, pairs adsorption displays a 2.82 eV and (3.64 eV) stability. Hence, the addition of second NO2 causes a pronounced surface relaxation, which changed of bond lengths and bond angles.