CO2 photoreduction with water vapor has been studied on three TiO2 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 TiO2 at a moderate temperature resulted in the creation of surface oxygen vacancies (VO) and Ti3+ sites on anatase and brookite but not on rutile. The production of CO and CH4 from CO2 photoreduction was remarkably enhanced on defective anatase and brookite TiO2 (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 VO on brookite. The results from in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses suggested that (1) defect-free TiO2 was not active for CO2 photoreduction since no CO2 – is generated, and (2) CO2 photoreduction to CO possibly underwent different reaction pathways on oxygen-deficient anatase and brookite via different intermediates (e.g., CO2 – on anatase; CO2 – 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 CO2 photoreduction on TiO2 nanocrystals, and revealed significant information on the much less studied but promising brookite phase.

This work for the first time reports engineered oxygen-deficient, blue TiO2 nanocrystals with coexposed {101}-{001} facets (TiO2–x {001}-{101}) to enhance CO2 photoreduction under visible light. The TiO2–x {001}-{101} material demonstrated a relatively high quantum yield (0.31% under UV–vis light and 0.134% under visible light) for CO2 reduction to CO by water vapor and more than 4 times higher visible light activity in comparison with TiO2 with a single {001} plane or {101} plane and TiO2(P25). Possible reasons are the exposure of more active sites (e.g., undercoordinated Ti atoms and oxygen vacancies), the facilitated electron transfer between {001} and {101} planes, and the formation of a new energy state (Ti3+) within the TiO2 band gap to extend the visible light response. An in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study was applied to understand the roles of coexposed {001}-{101} facets and Ti3+ sites in activating surface intermediates. The in situ DRIFTS analysis suggested that the coexposed {001}-{101} facets increased the capacity of reversible CO2 adsorption and that the combination of {001}-{101} and Ti3+ enhanced the activation and conversion kinetics of adsorbed species. The visible light responsive TiO2–x {001}-{101} material is not oxidized after long-term exposure to an air environment. This work is a significant contribution to the design of efficient and stable solar fuel catalysts.

The activation of CO2 on defective surface of Cu(I)/TiO2–x has been studied using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It was demonstrated that CO2 – species, generated upon an electron attachment to CO2, are spontaneously dissociated into CO even in the dark on a partially oxygen depleted Cu(I)/TiO2–x surface prepared by thermal annealing in an inert environment. The formation of CO bound on Cu+ sites was identified in the DRIFT spectra, and isotopic carbon-labeling experiments confirmed that the produced CO was derived from CO2. The spontaneous dissociation of CO2 – in the dark is to a large extent associated with the surface oxygen vacancies that provide not only the electronic charge (i.e., formation of Ti3+) but also the sites for the adsorption of oxygen atoms from CO2. The surface Cu+ species may facilitate destabilizing adsorbed CO2 – and enhance its subsequent dissociation to CO. The defective surface is much more active than defect-free surface; the healed oxygen vacancies after CO2 reduction can be easily regenerated via inert gas annealing at a moderate temperature (i.e., 300 °C). Compared with in the dark, CO2 activation and dissociation under photoillumination is remarkably improved, possibly because of sustained electron supply and partial regeneration of surface oxygen vacancy induced by irradiation. The results from DRIFTS analysis were verified by the measurement of catalytic activity using gas chromatography. These findings are important to advancing the understanding in the chemistry of CO2 adsorption and photocatalytic reduction on the surface of metal oxide catalysts.

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The present work elucidated the morphology and crystal‐plane effects of nanoscale ceria on the activity of CuO/CeO2 catalysts toward NO reduction. CeO2 Nanopolyhedra were enclosed by (111) and (100) planes; the nanorods predominantly exposed (110) and (100) surfaces, and the nanocubes only showed the polar (100) planes. Moreover, the strongest interaction was between CuO and CeO2 rods, followed by CuO/CeO2 polyhedra, and the CuO/CeO2 cubes showed the least interaction. Importantly, Cu2+ ions could be incorporated into the pore and surface lattices by occupying the vacant sites in the nanostructure CeO2 rods. Partial copper oxide species were segregated on the surface of CeO2 cubes with larger particle sizes. As a result, the site geometry and coordination environment of Cu2+ ions were different on the (111), (110), and (100) surfaces of CeO2. This surface structure effect in turn led to a higher surface reducibility, activity and N2 selectivity of CuO/CeO2 nanorods for NO reduction at low temperatures (below 250 °C); the polyhedra and cubes were less active.