Probing active sites for carbon oxides hydrogenation on Cu/TiO₂ using infrared spectroscopy

The valorization of carbon oxides on metal/metal oxide catalysts has been extensively investigated because of its ecological and economical relevance. However, the ambiguity surrounding the active sites in such catalysts hampers their rational development. Here, in situ infrared spectroscopy in combination with isotope labeling revealed that CO molecules adsorbed on Ti³⁺ and Cu⁺ interfacial sites in Cu/TiO₂ gave two disparate carbonyl peaks. Monitoring each of these peaks under various conditions enabled tracking the adsorption of CO, CO₂, H_2, and H₂O molecules on the surface. At room temperature, CO was initially adsorbed on the oxygen vacancies to produce a high frequency CO peak, Ti³⁺-CO. Competitive adsorption of water molecules on the oxygen vacancies eventually promoted CO migration to copper sites to produce a low-frequency CO peak. In comparison, the presence of gaseous CO₂ inhibits such migration by competitive adsorption on the copper sites. At temperatures necessary to drive CO₂ and CO hydrogenation reactions, oxygen vacancies can still bind CO molecules, and H₂ spilled-over from copper also competed for adsorption on such sites. Our spectroscopic observations demonstrate the existence of bifunctional active sites in which the metal sites catalyze CO₂ dissociation whereas oxygen vacancies bind and activate CO molecules.