Photocatalytic CO ₂ reduction with H ₂O on TiO ₂ nanocrystals: Comparison of anatase, rutile, and brookite polymorphs and exploration of surface chemistry

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.