Photocatalysis is one potential solution to the energy and environmental crisis and greatly relies on the development of the catalysts. Niobium pentoxide (Nb2O5), a typically nontoxic metal oxide, is eco-friendly and exhibits strong oxidation ability, and has attracted considerable attention from researchers. Furthermore, unique Lewis acid sites (LASs) and Bronsted acid sites (BASs) are observed on Nb2O5 prepared by different methods. Herein, the recent advances in the synthesis and application of Nb2O5-based photocatalysts, including the pure Nb2O5, doped Nb2O5, metal species supported on Nb2O5, and other composited Nb2O5 catalysts, are summarized. An overview is provided for the role of size and crystalline phase, unsaturated Nb sites and oxygen vacancies, LASs and BASs, dopants and surface metal species, and heterojunction structure on the Nb2O5-based catalysts in photocatalysis. Finally, the challenges are also presented, which are possibly overcome by integrating the synthetic methodology, developing novel photoelectric characterization techniques, and a profound understanding of the local structure of Nb2O5.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.202003156

Nb2O5-Based Photocatalysts.

Sustainable production of fuels and chemicals from biomass over niobium based catalysts: A review

The wet impregnation method was used to synthesize SnO2/MoO3 heterojunctions containing different amounts of Mo (5, 25 and 50 wt%). The materials obtained were characterized and their photocatalytic properties were evaluated in the photodegradation of methylene blue (MB) dye. The presence of superficial Lewis and Bronsted acid sites enhanced in photocatalytic efficiency of the materials. The order of photocatalytic activity in MB photodegradation was MoO3 > SnO2/MoO3-50 % > SnO2/MoO3-25 % > SnO2/MoO3-5% > SnO2, demonstrating that MoO3 acts as a modulator of acidity and this behavior directly influences the photocatalytic performance. In the photodegradation study, the physical mixture of SnO2+MoO3-25 % provided lower performance values than the heterojunction SnO2/MoO3-25 %, highlighting the importance of the appropriate synthesis of these materials.

Photocatalytic properties of SnO2/MoO3 mixed oxides and their relation to the electronic properties and surface acidity

This study is aimed at investigating the influence of different scavenger species of radicals that might possibly be involved in the TiO2/UVA photocatalytic degradation of Indigo Carmine dye. The effect caused by the presence of hydroxyl radicals (1-butanol and 2-propanol), positive holes (h+) (potassium iodide) and singlet oxygen (azide) was studied. Kinetics and optimal degradation conditions were evaluated using a factorial experiment design. The highest pseudo-first-order kinetics (k = 5.22 × 10−2 ± 0.002 and t1/2 = 13.25 ± 0.49 min) was achieved at pH 4.0, 6 mg L−1 of Indigo Carmine dye and 12 mg L−1 of TiO2. Mineralization was not achieved, and direct photolysis was not observed under the studied conditions. Indigo Carmine degradation occurs mainly due to oxidation in the positive hole (h+) followed by singlet oxygen action and on a smaller scale by hydroxyl radical. The use of the aforementioned radical scavengers made it possible to verify the mechanism and kinetics of Indigo Carmine dye through TiO2 heterogeneous photocatalysis.

Investigation of mechanism and kinetics in the TiO2 photocatalytic degradation of Indigo Carmine dye using radical scavengers

Herein we demonstrate that the deposition of a small loading of Cu nanoparticles (0.51 wt %) by DC-magnetron sputtering over niobium oxide (Nb2O5) increases its photocatalytic activity for the reduction of carbon dioxide (CO2) into methane (CH4), methanol (CH3OH), acetone (CH3COCH3), acetic acid (CH3COOH), isopropanol (CH3CH(OH)CH3) and formate (HCOO−). The improvement in the CO2 photoreduction of the Nb2O5/Cu can be attributed mainly to a reduction in the recombination rate of the electron-holes pairs photogenerated due to the electron trapping by Cu species as observed by the photoluminescence analysis, high dispersion of Cu, and high specific surface area of material even after addition Cu.

Enhancing Nb2O5 activity for CO2 photoreduction through Cu nanoparticles cocatalyst deposited by DC-magnetron sputtering

To better understand the photocatalysis oxidation mechanism of different types of coupled semiconductors, mesoporous niobium(V) oxides (n-type) with relatively high surface areas (140 m2 g−1) were synthesized by Evaporation Induced Self-Assemble method and modified with different amounts of NiO (p-type). The materials are polycrystalline nanostructures, with mesoporous of approximately 12 nm and bandgap values in the UVA range of the spectrum (344–356 nm). The materials showed remarkable photocatalytic activity and the addition of nickel oxide increased the reducibility of the Nb5+ species and improved the charge transfer in the materials. Studies with indigo carmine dye have shown that the niobium-based materials are able to decolorize a 40 ppm solution in 20 and 60 min (1.0% NiO material) in photo-Fenton like reaction, and photocatalysis reactions, respectively, without the strong adsorption of the molecule to the material surface. In addition, in the presence of a radical scavenger the photocatalytic reactions half-life time showed an increase of up to 11 times, which indicates that the indirect oxidation mechanism, with the formation of radicals, is predominant. The formation of hydroxyl radicals during irradiation was confirmed by EPR analysis with the spin trapping method.

Iza F.B. Silva

Papers

Understanding photocatalytic activity and mechanism of nickel-modified niobium mesoporous nanomaterials