How oxides affect the stretching modes of carbon monoxide adsorbed on Ni catalyst?

Abstract

Testing the stretching frequency shift of adsorbed CO has been a popular tool to evaluate the chemical property change of catalysts. We demonstrated first-principles models to reveal the mechanism of oxides effect on the frequency shift of CO on the Ni catalysts. Our calculations show that higher coverage of CO molecules leads to blue frequency shift, and there is negligible frequency shift when co-adsorbed with Al2O3 species and red frequency shift when co-adsorbed with ZrO2 or CeO2 species, which are consistent with the experimental literatures. The frequency shift is conformed to be proportional to the charge transfer amount of the π back-donation from Ni to the π⁎ orbital of CO molecule. The frequency shift direction is not determined by whether the oxide species is an electron acceptor or donor when adsorbed on Ni surface, but by how the oxide species affects the electrons on t2g and eg orbitals of the Ni atom under the CO molecule. The redistribution between t2g and eg orbitals induces the decreasing or increasing back-donation when a CO molecule co-adsorbed with the CO molecules or CeO2 species.