Grafted iron(iii) ions significantly enhance NO2 oxidation rate and selectivity of TiO2 for photocatalytic NO x abatement

Enhancing Photocatalysis: Understanding the Mechanistic Diversity in Photocatalysts Modified with Single-Atom Catalytic Sites

Surface modification of heterogeneous photocatalysts with single-atom catalysts (SACs) is an attractive approach for achieving enhanced photocatalytic performance. However, there is limited knowledge of the mechanism of photocatalytic enhancement in SAC-modified photocatalysts, which makes the rational design of high-performance SAC-based photocatalysts challenging. Herein, a series of photocatalysts for the aerobic degradation of pollutants based on anatase TiO2 modified with various low-cost, non-noble SACs (vanadate, Cu, and Fe ions) is reported. The most active SAC-modified photocatalysts outperform TiO2 modified with the corresponding metal oxide nanoparticles and state-of-the-art benchmark photocatalysts such as platinized TiO2 and commercial P25 powders. A combination of in situ electron paramagnetic resonance spectroscopy and theoretical calculations reveal that the best-performing photocatalysts modified with Cu(II) and vanadate SACs exhibit significant differences in the mechanism of activity enhancement, particularly with respect to the rate of oxygen reduction. The superior performance of vanadate SAC-modified TiO2 is found to be related to the shallow character of the SAC-induced intragap states, which allows for both the effective extraction of photogenerated electrons and fast catalytic turnover in the reduction of dioxygen, which translates directly into diminished recombination. These results provide essential guidelines for developing efficient SAC-based photocatalysts.

Sustainable nitrate production out of thin air: the photocatalytic oxidation of molecular nitrogen

Dinitrogen can be photocatalytically oxidized by TiO2 to nitrogen oxides and nitrates. This enables the sustainable production of fixed nitrogen essentially from thin air using sunlight.

Visible light-driven novel Bi2Ti2O7/CaTiO3 composite photocatalyst with enhanced photocatalytic activity towards NO removal

Photocatalysis is regarded as a promising technology for removal of nitrogen oxide (NO), however, the low photocatalytic efficiencies under visible light irradiation and the deactivation of the photocatalyst are as yet the significant issues that should be addressed. In this work, visible-light-driven Bi₂Ti₂O₇/CaTiO₃ heterojunction composites were synthesized by a facile in-situ hydrothermal method. The Bi₂Ti₂O₇/CaTiO₃ composites displayed superior visible light photocatalytic activity than pure CaTiO₃ and pure Bi₂Ti₂O₇ in the removal of NO at the 600 ppb level in air. Among all the composites, Bi₂Ti₂O₇/CaTiO₃ containing 20 wt% Bi₂Ti₂O₇ exhibited the best photocatalytic activity, achieving a maximum removal efficiency of 59%. The improved photocatalytic performance is mainly attributed to the strong visible-light-absorbing ability, the presence of an appropriate density of oxygen vacancy defects and the formation of heterojunction between CaTiO₃ and Bi₂Ti₂O₇, resulting in an efficient charge separation at the interface as proven by photoluminescence (PL) and photo-induced current measurements. According to trapping experiments and spin-trapping ESR analysis, the ^•O₂^- and h⁺ are the principal reactive species involved in the photocatalytic NO removal. In addition, the as-obtained Bi₂Ti₂O₇/CaTiO₃ composite showed good chemical stability, which is beneficial for practical applications in air pollution removal.

Intensification of Heterogeneous Photocatalytic Reactions Without Efficiency Losses: The Importance of Surface Catalysis

Advances in LED and photoreactor technology have brought semiconductor photocatalysis to the verge of feasibility of industrial application for the synthesis of value-added chemicals. However, the often observed efficiency losses under intensified illumination conditions still present a great challenge. This perspective discusses the origin of these efficiency losses and what needs to be done to prevent or counteract it and pave the way for efficient, intensified heterogeneous photocatalytic processes. The role of surface catalysis is particularly highlighted as one of the rate-limiting steps.

Graphic Abstract

Atomic-Level Charge Separation Strategies in Semiconductor-Based Photocatalysts

Semiconductor-based photocatalysis as a productive technology furnishes a prospective solution to environmental and renewable energy issues, but its efficiency greatly relies on the effective bulk and surface separation of photoexcited charge carriers. Exploitation of atomic-level strategies allows in-depth understanding on the related mechanisms and enables bottom-up precise design of photocatalysts, significantly enhancing photocatalytic activity. Herein, the advances on atomic-level charge separation strategies toward developing robust photocatalysts are highlighted, elucidating the fundamentals of charge separation and transfer processes and advanced probing techniques. The atomic-level bulk charge separation strategies, embodied by regulation of charge movement pathway and migration dynamic, boil down to shortening the charge diffusion distance to the atomic-scale, establishing atomic-level charge transfer channels, and enhancing the charge separation driving force. Meanwhile, regulating the in-plane surface structure and spatial surface structure are summarized as atomic-level surface charge separation strategies. Moreover, collaborative strategies for simultaneous manipulation of bulk and surface photocharges are also introduced. Finally, the existing challenges and future prospects for fabrication of state-of-the-art photocatalysts are discussed on the basis of a thorough comprehension of atomic-level charge separation strategies.

Plasma-liquid synthesis of MoOx and WO3 as potential photocatalysts

Plasmas in contact with liquids represent a green chemistry method for the synthesis of metal oxides. In this work, underwater plasma was used for the synthesis of molybdenum and tungsten oxides. The obtained samples were analyzed by various techniques. Results showed that underwater plasma with Mo electrodes allows obtaining non-stoichiometric molybdenum oxide (MoOx). In the case of tungsten electrodes, monoclinic WO3 was formed. The synthesized oxides have a wide band gap (3.21 eV for MoOx and 3.27 eV for WO3). The photocatalytic and sorption activities of the synthesized oxides towards the decomposition of cationic and anionic dyes (Methylene Blue, Rhodamine B, and Reactive Red 6C) were studied. MoOx shows excellent photocatalytic performance under UV and visible light irradiation. The photocatalytic activity of WO3 under visible light is less than that under UV irradiation.

Mesoporous polycrystalline SnO2 framework synthesized by direct soft templating method for highly selective detection of NO2

SnO2 is one of the most studied oxide materials for gas sensing applications. Investigations have shown that SnO2 is sensitive to a wide range of gaseous compounds. However, its lack of selectivity remains an issue. Here, a mesoporous polycrystalline SnO2 framework was successfully synthesized using a soft templating method at ambient temperature and pressure. The prepared materials were characterized using x-ray diffraction analysis, high-resolution transmission electron microscopy, energy-dispersive x-ray spectroscopy, N2 sorption tests, and x-ray photoelectron spectroscopy. Gas sensing analyses were performed on two batches of the material calcined at 400 °C and 500 °C. The resultant materials were highly conductive at relatively low operating temperatures. The thermal annealing and operating temperatures of the materials had significant effects on their gas sensing response and selectivity. The structure calcined at 400 °C showed a very selective response of 407 to 1 ppm NO2. The superior sensing performance of the obtained mesoporous SnO2 framework is attributed to its small crystal size of 4-5 nm-less than double the thickness of the critical electron depletion layer-as well as its high surface area of 89 m2 g-1 and high pore volume of 0.12 cm3 g-1.

A Holistic Approach to Model the Kinetics of Photocatalytic Reactions

Understanding and modeling kinetics is an essential part of the optimization and implementation of chemical reactions. In the case of photocatalytic reactions this is mostly done one-dimensionally, i.e., only considering the effect of one parameter at the same time. However, as discussed in this study, many of the relevant reaction parameters have mutual interdependencies that call for a holistic multi-dimensional approach to accurately model and understand their influence. Such an approach is described herein, and all the relevant equations given so that researchers can readily implement it to analyze and model their reactions.