Photocatalytic CO2 Reduction with H2O on TiO2 Nanocrystals: Comparison of Anatase, Rutile, and Brookite Polymorphs and Exploration of Surface Chemistry

Abstract

CO 2 photoreduction with water vapor has been studied on three TiO 2 nanocrystal polymorphs (anatase, rutile, and brookite) that were engineered with defect-free and oxygen-deficient surfaces, respectively. It was demonstrated that helium pretreatment of the as-prepared TiO 2 at a moderate temperature resulted in the creation of surface oxygen vacancies (V O ) and Ti 3+ sites on anatase and brookite but not on rutile. The production of CO and CH 4 from CO 2 photoreduction was remarkably enhanced on defective anatase and brookite TiO 2 (up to 10-fold enhancement) as compared to the defect-free surfaces. Defective brookite was photocatalytically more active than anatase and rutile, probably because of a lower formation energy of V O on brookite. The results from in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses suggested that (1) defect-free TiO 2 was not active for CO 2 photoreduction since no CO 2 – is generated, and (2) CO 2 photoreduction to CO possibly underwent different reaction pathways on oxygen-deficient anatase and brookite via different intermediates (e.g., CO 2 – on anatase; CO 2 – and HCOOH on brookite). The combined DRIFTS and photoactivity studies reported in this paper have provided new insights to the role of surface defects in CO 2 photoreduction on TiO 2 nanocrystals, and revealed significant information on the much less studied but promising brookite phase.