The conversion of CO2 to low-carbon fuels using solar energy is considered an economically attractive and environmentally friendly route. The development of novel catalysts and the use of solar energy via photocatalysis are key to achieving the goal of chemically reducing CO2 under mild conditions. TiO2 is not very effective for the photocatalytic reduction of CO2 to low-carbon chemicals such as methanol (CH3OH). Thus, in this work, novel Cu2O/TiO2 heterojunctions that can effectively separate photogenerated electrons and holes were prepared for photocatalytic CO2-to-CH3OH. More visible light-active Cu2O in the Cu2O/TiO2 heterojunctions favors the formation of methanol under visible light irradiation. On the other hand, under UV-Vis irradiation for 6 h, the CH3OH yielded from the photocatalytic CO2-to-CH3OH by the Cu2O/TiO2 heterojunctions is 21.0–70.6 µmol/g-catalyst. In contrast, the yield of CH3OH decreases with an increase in the Cu2O fraction in the Cu2O/TiO2 heterojunctions. It seems that excess Cu2O in Cu2O/TiO2 heterojunctions may lead to less UV light exposure for the photocatalysts, and may decrease the conversion efficiency of CO2 to CH3OH.

Arsenic in groundwater caused the black-foot disease (BFD) in many countries in the 1950–1960s. It is of great importance to develop a feasible method for removal of arsenic from contaminated groundwater in BFD endemic areas. Photocatalytic oxidation of As(III) to less toxic As(V) is, therefore, of significance for preventing any arsenic-related disease that may occur. By in situ synchrotron X-ray absorption spectroscopy, the formation of As(V) is related to the expense of As(III) disappearance during photocatalysis by TiO2 nanotubes (TNTs). Under UV/Vis light irradiation, the apparent first-order rate constant for the photocatalytic oxidation of As(III) to As(V) is 0.0148 min−1. It seems that As(III) can be oxidized with photo-excited holes while the not-recombined electrons may be scavenged with O2 in the channels of the well defined TNTs (an opening of 7 nm in diameter). In the absence of O2, on the contrary, As(III) can be reduced to As(0), to some extent. Cu(II) (CuO), as an electron acceptor, was impregnated on the TNTs surfaces in order to gain a better understanding of electron transfer during photocatalysis. It appears that As(III) can be oxidized to As(V) while Cu(II) is reduced to Cu(I) and Cu(0). The molecular-scale data are very useful in revealing the oxidation states and interconversions of arsenic during the photocatalytic reactions. This work has implications in that the toxicity of arsenic in contaminated groundwater or wastewater can be effectively decreased via solar-driven photocatalysis, which may facilitate further treatments by coagulation.

Toxic disinfection byproducts such as trihalomethanes (e.g. CHCl3) are often found after chlorination of drinking water. It has been found that photocatalytic degradation of trace CHCl3 in drinking water generally lacks an expected relationship with the crystalline phase, band-gap energy or the particle sizes of the TiO2-based photocatalysts used such as nano TiO2 on SBA-15 (Santa Barbara amorphous-15), TiO2 clusters (TiO2–SiO2) and atomic dispersed Ti [Ti-MCM-41 (Mobil Composition of Matter)]. To engineer capable TiO2 photocatalysts, a better understanding of their photoactive sites is of great importance and interest. Using in situ X-ray absorption near-edge structure (XANES) spectroscopy, the A1 (4969 eV), A2 (4971 eV) and A3 (4972 eV) sites in TiO2 can be distinguished as four-, five- and six- coordinated Ti species, respectively. Notably, the A2 Ti sites that are the main photocatalytic species of TiO2 are shown to be accountable for about 95% of the photocatalytic degradation of trace CHCl3 in drinking water (7.2 p.p.m. CHCl3 gTiO2 −1 h−1). This work reveals that the A2 Ti species of a TiO2-based photocatalyst are mainly responsible for the photocatalytic reactivity, especially in photocatalytic degradation of CHCl3 in drinking water.

Summary The 21st Conference of Parties suggested that a significant increase in solar‐driven electricity could assist in the practicable reduction of the continuously increasing global CO2 concentrations. Since the conversion efficiencies of TiO2 for photocatalytic H2O‐to‐H2O2 were relatively low (ie, 0.14 mmole H2O2/gTiO2/min), the novel Pd‐TiO2‐δ/TiO2 photocatalysts for H2O‐to‐H2O2 and ‐H2 were, thus, prepared by dispersion of palladium (36‐86 ppm) on the Ti3+ self‐doped TiO2 (Pd‐TiO2‐δ/TiO2). Specifically, a feasibility study for photocatalytic H2O(l)‐to‐H2O2(aq) and ‐H2(g) affected by the Pd‐TiO2‐δ/TiO2 photocatalysts for green energy was carried out in the present work. Note that H2O2 and H2 can be yielded through the photocatalytic two‐electron transfer reactions (O2 + 2H+ + 2e− → 2H2O2 and 2H+ +2e− → H2) that are thermodynamically feasible. Palladium on TiO2 surfaces could effectively retard the recombination of photoexcited charges and facilitated the simultaneous formation of H2O2 and H2 from H2O. The photoactive Ti4+ on the TiO2 surface could form peroxo‐species (Ti4+‐O2H−) that further induced the formation of H2O2. In the presence of the Pd‐TiO2‐δ/TiO2 photocatalyst, under UV light irradiation for 90 min, very high yields of naturally separated H2O2(aq) (0.45 mmole/gTiO2/min) and H2(g) (0.16 mmole/gTiO2/min) could be obtained simultaneously. A relatively high photo‐conversion efficiency (14%) for the Pd‐TiO2‐δ/TiO2 photocatalysts was achieved. This work demonstrates that using the unlimited solar energy, the effective Pd‐TiO2‐δ/TiO2 photocatalysts can enhance H2O(l)‐to‐H2O2(aq) and ‐H2(g) for green energy.