Surface Modification of TiO2 by Hyper-Cross-Linked Polymers for Efficient Visible-Light-Driven Photocatalytic NO Oxidation.

Solar-driven photocatalysis offers an environmentally friendly and sustainable approach for the removal of air pollutants such as nitric oxides without chemical addition. However, the low specific surface area and adsorption capacity of common photocatalysts restrict the surface reactions with NO at the ppb-level. In this study, imidazolium-based hyper-cross-linked polymer (IHP) was introduced to modify the surface of TiO2 to construct a porous TiO2/IHP composite photocatalyst. The as-prepared composite with hierarchical porous structure achieves a larger specific surface area as 309 m2/g than that of TiO2 (119 m2/g). Meanwhile, the wide light absorption range of the polymer has brought about the strong visible-light absorption of the TiO2/IHP composite. In consequence, the composite photocatalyst exhibits excellent performance toward NO oxidation at a low concentration of 600 ppb under visible-light irradiation, reaching a removal efficiency of 51.7%, while the generation of the toxic NO2 intermediate was suppressed to less than 1 ppb. The enhanced NO adsorption and the suppressed NO2 generation on the TiO2/IHP surface were confirmed by in situ monitoring technology. This work demonstrates that the construction of a porous structure is an effective approach for efficient NO adsorption and photocatalytic oxidation.

Synergistic effects of holey nanosheet and sulfur-doping on the photocatalytic activity of carbon nitride towards NO removal.

Photocatalysis provides a sustainable way for NOx elimination. However, efficient and safe photocatalytic removal of NOx remain a great challenge due to the limited light-harvesting ability and quick recombination of charge carriers. Herein, holey sulfur-doped g-C3N4 nanosheets (CNN-S) was reported by directly calcining a mixture of hydrolyzed dicyandiamide and thioacetamide. The specific surface area of the pristine g-C3N4 nanosheets (CNN-S0) is 3-4 times higher than bulk g-C3N4 (BCN), and the photocatalytic NO removal rate also increased from 17% (BCN) to 35% (CNN-S0). The effect of sulfur content on the photocatalytic performance was systematic studied, and CNN-S0.5 sample exhibits the highest NO removal rate (53%). The high photoreactivity of S-doped g-C3N4 nanosheets can be attributed to enhanced visible light absorption, increased specific surface area, and effective separation and transfer of photo-generated charges owing to the synergistic effect of the nanosheet structure and sulfur doping. In addition, density functional theory calculations show that the doping of S is also beneficial to the adsorption and activation of the reactants on CN.

Synergistic effect of cyano defects and CaCO3 in graphitic carbon nitride nanosheets for efficient visible-light-driven photocatalytic NO removal.

Photo-oxidation with semiconductor photocatalysts provides a sustainable and green solution for NOx elimination. Nevertheless, the utilization of traditional photocatalysts in efficient and safe photocatalytic NOx removal is still a challenge due to the slow charge kinetic process and insufficient optical absorption. In this paper, we report a novel porous g-C3N4 nanosheet photocatalyst modified with cyano defects and CaCO3 (xCa-CN). The best performing sample (0.5Ca-CN) exhibits an enhanced photo-oxidation NO removal rate (51.18 %) under visible light irradiation, largely surpassing the value of pristine g-C3N4 nanosheets (34.05 %). Such an enhancement is mainly derived from an extended visible-light response, improved electron excitation and transfer, which are associated with the synergy of cyano defects and CaCO3, as evidenced by a series of spectroscopic analyses. More importantly, in-situ DRIFTS and density functional theory (DFT) results suggest that the introduction of cyano defects and CaCO3 enables control over NO adsorption and activation processes, making it possible to implement a preference pathway (NO → NO+ → NO3¯) and reduce the emission of toxic intermediate NO2. This work demonstrates the potential of integrating defect engineering and insulator modification to design highly efficient g-C3N4-based photocatalysts for air purification.

Research progress in metal sulfides for photocatalysis: From activity to stability.

Metal sulfides are a type of reduction semiconductor photocatalysts with narrow bandgap and negative conduction band potential, which make them have unique photocatalytic performance in solar-to-fuel conversion and environmental purification. However, metal sulfides also suffer from low quantum efficiency and photocorrosion. In this review, the strategies to improve the photocatalytic activity of metal sulfide photocatalysts by stimulating the charge separation and improving light-harvesting ability are introduced, including morphology control, semiconductor coupling and surface modification. In addition, the recent research progress aiming at improving their photostability is also illustrated, such as, construction of hole transfer heterojunctions and deposition of hole transfer cocatalysts. Based on the electronic band structures, the applications of metal sulfides in photocatalysis, namely, hydrogen production, degradation of organic pollutants and reduction of CO2, are summarized. Finally, the perspectives of the promising future of metal-sulfide based photocatalysts and the challenges remaining to overcome are also presented.

Plasmonic semiconductor photocatalyst: Non-stoichiometric tungsten oxide.

Semiconductor photocatalysis has attracted increasing attention due to its potential application in solving the problems related to energy crisis and environmental pollution. As a typical plasmonic semiconductor, non-stoichiometric tungsten oxide (WO3-X) has invoked significant interest for its unique property and excellent photocatalytic performance. In this review, we briefly introduce the fundamental properties of the WO3-x, and then summarize the synthesis methods such as solvothermal reaction, solid phase reduction and exfoliation treatment, together with the modification strategies such as doping and constructing homo-/hetero-junctions. Additionally, we emphasize the practical applications of WO3-x in hydrogen evolution, nitrogen fixation, carbon dioxide reduction, and pollutant degradation. Finally, comprehensive conclusions and perspectives on the fabrication of WO3-x photocatalyst leading to satisfactory performance are given as well.

Fe1 /TiO2 Hollow Microspheres: Fe and Ti Dual Active Sites Boosting the Photocatalytic Oxidation of NO.

Recently, single-atom catalysts have aroused extensive attention in fields of clean energy and environmental protection due to their unique activity and efficient utilization of the active atoms. It is of great importance but still remains a great challenge to unveil the effect of single atoms on precise catalysis. Herein, it is reported that doping TiO2 hollow microspheres (TiO2 -HMSs) with single atomic Fe can boost the photoreactivity of TiO2 -HMSs towards NO oxidation due to the synergistic effects of atomically dispersed Fe and bonded Ti atom which act as dual active sites. The atomically dispersed Fe atoms occupy the subsurface Ti vacancies, and the interaction between Ti 3d and Fe 3d orbitals result in the formation of FeTi bond. Single atomic Fe modulates the electronic structure of the bonded Ti atoms by electron transfer, which facilitates the adsorption and activation of NO and O2 at Fe and bonded Ti sites, respectively. In addition, the introduction of single atomic Fe sharply suppresses the production of toxic NO2 byproduct. The synergistic effects of the dual active sites then cause a drastic promotion in photocatalytic oxidation of NO.