Role of the Surface Lewis Acid and Base Sites in the Adsorption of CO₂ on Titania Nanotubes and Platinized Titania Nanotubes: An in Situ FT-IR Study

An understanding of the adsorption of CO₂, the first step in its photoreduction, is necessary for a full understanding of the photoreduction process. As such, the reactive adsorption of CO₂ on oxidized, reduced, and platinized TiO₂ nanotubes (Ti-NTs) was studied using infrared spectroscopy. The Ti-NTs were characterized with TEM and XRD, and XPS was used to determine the oxidation state as a function of oxidation, reduction, and platinization. The XPS data demonstrate that upon oxidation, surface O atoms become more electronegative, producing sites that can be characterized as strong Lewis bases, and the corresponding Ti becomes more electropositive producing sites that can be characterized as strong Lewis acids. Reduction of the Ti-NTs produces Ti³⁺ species, a very weak Lewis acid, along with a splitting of the Ti⁴⁺ peak, representing two sites, which correlate with O sites with a corresponding change in oxidation state. Ti³⁺ is not observed on reduction of the platinized Ti-NTs, presumably because Pt acts as an electron sink. Exposure of the treated Ti-NTs to CO₂ leads to the formation of differing amounts of bidentate and monodentate carbonates, as well as bicarbonates, where the preference for formation of a given species is rationalized in terms of surface Lewis acidity and or Lewis basicity and the availability of hydrogen. Our data suggest that one source of hydrogen is water that remains adsorbed to the Ti-NTs even after heating to 350 °C and that reduced platinized NTs can activate H₂. Carboxylates, which involve CO₂^– moieties and are similar to what would be expected for adsorbed CO₂^–, a postulated intermediate in CO₂ photoreduction, are also observed but only on the reduced Ti-NTs, which is the only surface on which Ti³⁺/O vacancy formation is observed