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

Abstract

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 ac...