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.

The mechanisms of atmospheric photooxidation of aromatic compounds are of seminal importance in the chemistry of the urban and regional atmosphere. It has been difficult to experimentally account for the full spectrum of oxidation products in laboratory studies. In an effort to fully elucidate the atmospheric reaction pathways for the aromatic−OH reaction, we have conducted theoretical calculations on aromatic intermediates. Energies have been determined for these intermediates by using semiempirical UHF/PM3 geometry optimizations combined with ab initio calculations using density functional theory (DFT). A hybrid DFT model, the Becke3 parameter function with the nonlocal correlation function of Lee, Yang, and Parr, was used in conjunction with the 6-31G(d,p) basis set to study the intermediate structures. Full mechanisms for the OH-initiated photooxidation of toluene, m-xylene, p-xylene, 1,2,4-trimethylbenzene, and m-ethyltoluene are developed. The lowest energy intermediates have been determined, and predicted products from these structures are compared to available experimental product data. These studies serve to refine proposed mechanisms currently available for toluene, m-xylene, and p-xylene, while providing new information on the 1,2,4-trimethylbenzene and m-ethyltoluene reaction pathways.

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Among the three naturally existing phases of TiO 2 , brookite is the least studied as a photocatalyst. In this study, single-phase anatase and brookite, and mixed-phase anatase-brookite TiO 2 nanomaterials were synthesized through a hydrothermal method. The anatase-brookite phase content was controlled by adjusting the concentration of urea in the precursor solution. XRD, Raman spectroscopy, and highresolution TEM were used to confirm the crystal structures. SEM and TEM analyses demonstrated that anatase TiO 2 were nearly spherical nanoparticles while brookite TiO 2 were rod-shaped nanoparticles.UV-vis diffuse reflectance spectroscopy showed a blue shift in absorption spectra with increasing brookite content. The photocatalytic activities of the prepared bicrystalline TiO 2 were evaluated for CO 2 photoreduction in the presence of water vapor for production of solar fuels (CO and CH 4 ). The activities were compared with those of pure anatase, pure brookite, and a commercial anatase-rutile TiO 2 (P25).The results showed that bicrystalline anatase-brookite was generally more active than single-phase anatase, brookite, and P25. The bicrystalline mixture with a composition of 75% anatase and 25% brookite showed the highest photocatalytic activity, likely due to the enhanced interfacial charge transfer between anatase and brookite nanocrystals. In situ DRIFTS analysis showed that CO 2 À and HCO 3 À species were active reaction intermediates for CO 2 photoreduction while the accumulation of non-reactive CO 3 2À species on the TiO 2 surface may be detrimental.

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