CHARACTERIZATION OF OXIDE SURFACES BY INFRARED-SPECTROSCOPY OF ADSORBED CARBON-MONOXIDE - A THEORETICAL INVESTIGATION OF THE FREQUENCY-SHIFT OF CO ON MGO AND NIO

We report the results of ab initio cluster model calculations of the interaction of the CO molecule with a finite cluster representing Mg2+ and Ni2+ chemisorption sites on MgO- and NiO(100) surfaces. We compare the CO adsorption at these two oxide surfaces which have the same NaCl-like crystal structure and very similar bulk moduli but which have different d-orbital occupations, Mg2+ d0 and Ni2+ d8. We found that NiO behaves very similarly as the non-transition metal oxide MgO. In particular, the bonding with CO is almost entirely electrostatic in nature; it does not involve any significant sigma-donation or, in the NiO case, any pi-back-donation. Moreover, we found that the CO vibrational shift toward higher frequencies arises essentially from the repulsion originating when the CO molecule stretches in the presence of the rigid surface (wall effect). Contributions to the frequency shift from chemical bonding mechanisms are negligible.