Active {001} Facet Exposed TiO2 Nanotubes Photocatalyst Filter for Volatile Organic Compounds Removal: From Material Development to Commercial Indoor Air Cleaner Application

Exceptional Resistance to Chlorine-Induced Photocatalytic Poisoning via Vacuum UV Irradiation

Catalyst deactivation poses a significant challenge in environmental remediation, especially for the photocatalytic oxidation of chlorinated volatile organic compounds (Cl-VOCs). In this study, a functional flower-like TiO2@Mn/rGO (FTMG) catalyst coupled with a vacuum ultraviolet (VUV) lamp was used as a novel photocatalytic oxidation (VUV-PCO) system for chlorobenzene (CB) oxidation. In this system, more than 80% of CB was efficiently oxidized at a high w8 hly space velocity of 600,000 gcat-1 h-1, which was a 6.5-fold increase compared to conventional UV-PCO, and no catalytic deactivation over 1300 min of reaction. Notably, the COx selectivity consistently reached 100%. These outstanding performances were attributed to the synergy of direct VUV photolysis and gas-solid interface photocatalysis. Importantly, the C-Cl bond of CB was efficiently cleaved by VUV photolysis, generating •Cl as the oxidant. Ozone (O3) generated from VUV photolysis was efficiently adsorbed on oxygen vacancies and Mn (Ov + Mn) adjacent sites on FTMG. These adsorbed O3 rapidly captured the photogenerated electrons, thereby effectively preventing Cl reduction and avoiding catalyst deactivation. This study sheds light on the unique dechlorination reaction and Cl-poisoning-resistance mechanism in the VUV-PCO system, offering a novel strategy to boost the catalytic oxidation of Cl-VOCs.

Solar-Active Carbon Nitride Film Integrated by Free Radical Copolymerization for Photocatalytic Indoor Air Purification

The current lack of stable, scalable, and efficient coating technology dramatically limits the exploitation of solar-driven graphitic carbon nitride (CN) photocatalysts. Herein, a unique, efficient, and scalable method is reported to immobilize CN powder on various substrates ranging from Fluorine tin oxide (FTO), glass, Plexiglas, Al foil, Ti foil, and Granite stone, to even wood. The film shows an outstanding thickness of 212 µm, which is the highest value ever reported. The formation process is ascribed to free radical copolymerization between the tri-s-triazine backbone and polyacrylamide, followed by cross-linking. The smooth, non-oxidizable, and well-defined continuous coating exhibits excellent adherence and durability. The distinctive sequence segments preserve the light transition within the film while enhancing the optical absorption in the solar spectrum. Under visible light illumination, the film shows outstanding photodegradation performance toward air pollutants, whether for gaseous acetaldehyde (Act) or toluene (Tol). This method is a great step forward that can open new opportunities for the commercial applications of CN powder.

Structurally and surficially activated TiO2 nanomaterials for photochemical reactions

Renewable fuels and environmental remediation are of paramount importance in today's world due to escalating concerns about climate change, pollution, and the finite nature of fossil fuels. Transitioning to sustainable energy sources and addressing environmental pollution has become an urgent necessity. Photocatalysis, particularly harnessing solar energy to drive chemical reactions for environmental remediation and clean fuel production, holds significant promise among emerging technologies. As a benchmark semiconductor in photocatalysis, TiO2 photocatalyst offers an excellent solution for environmental remediation and serves as a key tool in energy conversion and chemical synthesis. Despite its status as the default photocatalyst, TiO2 suffers from drawbacks such as a high recombination rate of charge carriers, low electrical conductivity, and limited absorption in the visible light spectrum. This review provides an in-depth exploration of the fundamental principles of photocatalytic reactions and presents recent advancements in the development of TiO2 photocatalysts. It specifically focuses on strategic approaches aimed at enhancing the performance of TiO2 photocatalysts, including improving visible light absorption for efficient solar energy harvesting, enhancing charge separation and transportation efficiency, and ensuring stability for robust photocatalysis. Additionally, the review delves into the application of photodegradation and photocatalysis, particularly in critical processes such as water splitting, carbon dioxide reduction, nitrogen fixation, hydrogen peroxide generation, and alcohol oxidation. It also highlights the novel use of TiO2 in plastic polymerization and degradation, showcasing its potential for converting plastic waste into valuable chemicals and fuels, thereby offering sustainable waste management solutions. By addressing these essential areas, the review offers valuable insights into the potential of TiO2 photocatalysis for addressing pressing environmental and energy challenges. Furthermore, the review encompasses the application of TiO2 photochromic systems, expanding its scope to include other innovative research and applications. Finally, it addresses the underlying challenges and provides perspectives on the future development of TiO2 photocatalysts. Through addressing these issues and implementing innovative strategies, TiO2 photocatalysis can continue to evolve and play a pivotal role in sustainable energy and environmental applications.

Modulating crystal facets of photoanodes for photoelectrocatalytic scalable degradation of fluorinated pharmaceuticals in wastewater

Fluorinated pharmaceuticals pollution has become an ever-increasing environmental concern due to its negative impacts. Photoelectrocatalytic (PEC) degradation system is a desirable approach to tackle the pollution problems. However, photogenerated charge separation and interfacial mass transfer are the main bottlenecks for improving the PEC degradation performance. Herein, we report a TiO2 photoanode with tuned (101)/(110) facets in situ grown on a Ti mesh substrate for PEC degradation of fluorinated pharmaceuticals. The exposure of (101) facets facilitates efficient photogenerated charge separation and the desorption of generated •OH radical. Besides, the three-dimensional (3D) architecture of photoanode promotes macroscopic mass transfer. This system performed complete defluorination of 5-fluorouracil and more than 75 % total organic carbon (TOC) removal efficiency. The apparent reaction rate constant of high (101) facet-exposed TiO2 grown on Ti mesh is up to 6.96 h-1, 6‒fold faster than that of photoanode with low (101) facet-exposed TiO2 grown on Ti foil. It is demonstrated that a large-sized PEC system of 1200 cm2 can degrade 100 L of synthetic fluorinated pharmaceutical wastewater with more than 80 % elimination efficiency. This work showcases the facet and substrate modulated strategy of fabricating high-performed photoanode for PEC wastewater purification.

Accelerating Toluene Oxidation over Boron-Titanium-Oxygen Interface: Steric Hindrance of the Methyl Group Induced by the Plane-Adsorption Configuration

Elimination of dilute gaseous toluene is one of the critical concerns within the field of indoor air remediation. The typical degradation route on titanium-based catalysts, "toluene-benzaldehyde-carbon dioxide", necessitates the oxidation of the methyl group as a prerequisite for photocatalytic toluene oxidation. However, the inherent planar adsorption configuration of toluene molecules, dominated by the benzene rings, leads to significant steric hindrance for the methyl group. This steric hindrance prevents the methyl group from contacting the active species on the catalyst surface, thereby limiting the removal of toluene under indoor conditions. To date, no effective strategy to control the steric hindrance of the methyl group has been identified. Herein, we showed a B-Ti-O interface that exhibits significantly enhanced toluene removal efficiency under indoor conditions. In-depth investigations revealed that, compared to typical Ti-based photocatalysts, the steric hindrance between the methyl group and the catalyst surface decreased from 3.42 to 3.03 Å on the designed interface. This reduction originates from the matching of orbital energy levels between Ti 3dz2 and C 2pz of the benzene ring. The decreased steric hindrance improved the efficiency of toluene being attacked by surface active species, allowing for rapid conversion into benzaldehyde and benzoic acid species for subsequent reactions. Our work provides novel insights into the steric hindrance effect in the elimination of aromatic volatile organic compounds.

Facet-controlled growth and soft-chemical exfoliation of two-dimensional titanium dioxide nanosheets

TiO2 remains one of the most popular materials used in catalysts, photovoltaics, coatings, and electronics due to its abundance, chemical stability, and excellent catalytic properties. The tailoring of the TiO2 structure into two-dimensional nanosheets prompted the successful isolation of graphene and MXenes. In this review, facet-controlled TiO2 and monolayer titanate are outlined, covering their synthesis route and formation mechanism. The reactive facet of TiO2 is usually controlled by a capping agent. In contrast, the monolayer titanate is achieved by ion-exchange and delamination of layered titanates. Each route leads to 2D structures with unique physical and chemical properties, which expands its utilisation into several niche applications. We elaborate the detailed outlook for the future use and research studies of facet-controlled TiO2 and monolayer titanates. Advantages and disadvantages of both structures are provided, along with suggested applications for each type of 2D TiO2 nanosheets.

Time-resolved spectroscopic investigation for the practical application of a photocatalytic air purifier

The photocatalytic efficiency for removing volatile organic compounds (VOCs) is significantly influenced by operational parameters like humidity and flow velocity, exhibiting notable and inconsistent fluctuations in both lab-scale and large-scale demonstrations. In this study, operando spectroscopy and isotope analysis were employed to investigate the correlation between humidity levels and degradation of gaseous acetaldehyde using TiO2 photocatalysts, aiming to demonstrate the scaling-up of photocatalytic air purifier. It was observed that rate constants for the mineralization of acetaldehyde rapidly decreased by 30% as relative humidity increased from 25% to 80% in the flow system (with an air velocity, v = 0.78 m/s). However, batch system showed smaller change with only a 10% reduction of the rate constant. Humidity fluctuations were more pronounced under high-speed conditions and were amplified in air purifier (v = 3.8 m/s). Time-resolved operando spectroscopy using an 13C isotope of acetaldehyde revealed that humidity's distinct role in dark adsorption and photocatalytic reactions. Water was found to inhibit the formation of crotonaldehyde during aldol condensation reaction in dark condition. Moreover, water suppressed photocatalytic mineralization by inhibiting acetate oxidation to formate. These findings provide valuable insights for improving realistic air purification processes, underscoring the importance of identifying key intermediates and controlling humidity to enhance the selectivity of gaseous pollutant oxidation reactions.

Quantification of Byproduct Formation from Portable Air Cleaners Using a Proposed Standard Test Method

In response to the COVID-19 pandemic, air cleaning technologies were promoted as useful tools for disinfecting public spaces and combating airborne pathogen transmission. However, no standard method exists to assess the potentially harmful byproduct formation from air cleaners. Through a consensus standard development process, a draft standard test method to assess portable air cleaner performance was developed, and a suite of air cleaners employing seven different technologies was tested. The test method quantifies not only the removal efficiency of a challenge chemical suite and ultrafine particulate matter but also byproduct formation. Clean air delivery rates (CADRs) are used to quantify the chemical and particle removal efficiencies, and an emission rate framework is used to quantify the formation of formaldehyde, ozone, and other volatile organic compounds. We find that the tested photocatalytic oxidation and germicidal ultraviolet light (GUV) technologies produced the highest levels of aldehyde byproducts having emission rates of 202 and 243 μg h-1, respectively. Additionally, GUV using two different wavelengths, 222 and 254 nm, both produced ultrafine particulate matter.

Current trends on dry photocatalytic oxidation technology for BTX removal: Viable light sources and highly efficient photocatalysts

One of the main gaseous pollutants released by chemical production industries are benzene, toluene and xylene (BTX). These dangerous gases require immediate technology to combat them, as they put the health of living organisms at risk. The development of heterogeneous photocatalytic oxidation technology offers several viewpoints, particularly in gaseous-phase decontamination without an additional supply of oxidants in air at atmospheric pressure. However, difficulties such as low quantum efficiency, ability to absorb visible light, affinity towards CO2 and H2O synthesis, and low stability continue to limit its practical use. This review presents recent advances in dry-phase heterogeneous photodegradation as an advanced technology for the practical removal of BTX molecules. This review also examines the impact of low-cost light sources, the roles of the active sites of photocatalysts, and the feasible concentration range of BTX molecules. Numerous studies have demonstrated a significant improvement in the efficiency of the photodegradation of volatile organic compounds by enhancing the photocatalytic reactor system and other factors, such as humidity, temperature, and flow rate. The mechanism for BTX photodegradation based on density functional theory (DFT), electron paramagnetic resonance (EPR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) investigations is also discussed. Finally, the present research complications and anticipated future developments in the field of heterogeneous photocatalytic oxidation technology are discussed.

Enhancing air treatment through controlled fabrication of transition metal-doped titanium dioxide nanocomposites for photocatalytic toluene degradation

Rapid industrial growth and urbanization have resulted in a significant rise in environmental pollution issues, particularly indoor air pollutants. As a result, it is crucial to design and develop technologies and/or catalysts that are not only cost-effective but also promising high performance and practical applicability. However, achieving this goal has been so far remained a challenging task. Herein, a series of transition metal M - TiO2 (M = W, Fe, Mn) nanocrystals was prepared for photocatalytic degradation of volatile organic compounds (VOCs), i.e., toluene. Of the nanocomposites tested, W-TiO2 showed significantly improved photocatalytic activity for VOC degradation under UV irradiation compared to the others. In particular, the optimized W dopant amount of 0.5 wt% resulted in the outstanding degradation performance of toluene (96%) for the obtained W-TiO2(0.5%) nanocomposite. Moreover, W-TiO2(0.5%) nanocomposite exhibited good stability for 32 h working under high toluene concentration (10 ppm) compared to the pristine TiO2. The current work demonstrates the potential usage of M - TiO2 nanocrystals, particularly W-TiO2(0.5%), as a promising photocatalyst for efficient VOCs degradation.

Vitamin C Affinity to TiO2 Nanotubes: A Computational Study by Hybrid Density Functional Theory Calculations

Titanium dioxide nanotubes (TNT) have been extensively studied because of their unique properties, which make such systems ideal candidates for biomedical application, especially for the targeted release of drugs. However, knowledge about the properties of TiO2 nanotubes with typical dimensions of the order of the nanometer is limited, especially concerning the adsorption of molecules that can be potentially loaded in actual devices. In this work, we investigate, by means of simulations based on hybrid density functional theory, the adsorption of Vitamin C molecules on different nanotubes through a comparative analysis of the properties of different structures. We consider two different anatase TiO2 surfaces, the most stable (101) and the more reactive (001)A; we evaluate the role of the curvature, the thickness and of the diameter as well as of the rolling direction of the nanotube. Different orientations of the molecule with respect to the surface are studied in order to identify any trends in the adsorption mechanism. Our results show that there is no preferential functional group of the molecule interacting with the substrate, nor any definite spatial dependency, like a rolling orientation or the concavity of the nanotube. Instead, the adsorption is driven by geometrical factors only, i.e., the favorable matching of the position and the alignment of any functional groups with undercoordinated Ti atoms of the surface, through the interplay between chemical and hydrogen bonds. Differently from flat slabs, thicker nanotubes do not improve the stability of the adsorption, but rather develop weaker interactions, due to the enhanced curvature of the substrate layers.

Dark-Light Tandem Catalytic Oxidation of Formaldehyde over SrBi2Ta2O9 Nanosheets

Formaldehyde (HCHO), as one of the main indoor toxic pollutions, presents a great threat to human health. Hence, it is imperative to efficiently remove HCHO and create a good indoor living environment for people. Herein, a layered perovskite material SrBi2Ta2O9 (SBT), was studied for the first time and exhibited superior photocatalytic efficiency and stability compared to commercial TiO2 (P25). Furthermore, a unique dark-light tandem catalytic mechanism was constructed. In the dark reaction stage, HCHO (Lewis base) site was adsorbed on the terminal (Bi2O2)2+ layer (Lewis acid) site of SBT in the form of Lewis acid-base complexation and was gradually oxidized to CO32- intermediate (HCHO → DOM (dioxymethylene) → HCOO- → CO32-). Then, in the light reaction stage, CO32- was completely converted into CO2 and H2O (CO32- → CO2). Our study contributes to a thorough comprehension of the photocatalytic oxidation of HCHO and points out its potential for day-night continuous work applications in a natural environment.

A curtain purification system based on a rabbit fur-based rotating triboelectric nanogenerator for efficient photocatalytic degradation of volatile organic compounds

Efficient removal of air pollution caused by volatile organic compounds (VOCs) and particulate matter (PM) through distributed energy collected from the environment is an effective strategy to achieve both energy conservation and better air quality. Herein, a curtain purification system based on a rabbit fur-based rotary triboelectric nanogenerator (RR-TENG) and a collaborative photocatalysis technology was designed for indoor air purification. The high electrostatic field from RR-TENG enhances formaldehyde adsorption, while it can also efficiently adsorb PM_2.5 simultaneously. More interestingly, the ultrahigh electric field provided by RR-TENG promotes the separation of photogenerated electron-hole pairs of the g-C₃N₄/TiO₂ composite photocatalyst, generating more superoxide radicals (⋅O₂^-), hydroxyl radicals (⋅OH), and holes (h⁺) and thereby improving the photocatalytic efficiency. In a simulated reaction chamber of 9 L, the formaldehyde removal rate of the system can reach 79.2% within 90 min and RR-TENG rapidly reduces PM_2.5 from 999 μg m^-3 to 50 μg m^-3 within 60 s. This study proposes a curtain purification system integrating the function of energy collection and photocatalytic purification, which can be applied for improving air quality and human health.

Semi-solid electrolyte with layered heterometallic low-valent electron-mediator enabling indirect destruction of gaseous toluene

The electrochemical degradation of air pollutants, particularly volatile organic compounds (VOCs), at their gaseous state is a promising method. However, it remains at an infant stage due to sluggish solid-gas electron transfers at room temperature. We established a triphase reaction condition using a semi-solid electrolyte layer between the electrode and membrane to enhance the electron transfer at room temperature. A polyvinyl alcohol (PVA) gel layer was inserted between a bimetallic layered CuNi(CN)₄ complex coated Cu foam electrode (TCNi-Cu) and Nafion 324 membrane for the degradation of gaseous toluene. The cyclic voltammetry of TCNi-Cu using a sodium hydroxide-coated copper mesh electrode at a triphase showed Cu¹⁺ and Ni¹⁺ stabilization at -0.7 and -0.9 V, respectively, which was similar to the liquid phase electron transfer behavior. The degradation capacity of gaseous toluene without using electrogenerated TCNi-Cu + PVA gel was 0.54 mg cm² min^-1, whereas that of TCNi-Cu + PVA gel layers was 1.17 mg cm^-2min^-1, which revealed the mediation effect at a triphase condition. Toluene was converted into oxygen-containing products, such as butanol, propanol, and acetone (without reduction products), which revealed that indirect oxidation occurred at the cathode using an in-situ generated oxidant, such as OH˙ radical. As an electron-mediator, Cu¹⁺ was used to form oxidants for the degradation of toluene at -0.7 V. The toluene removal rate reached 1.4 μmol h^-1, with an energy efficiency of 0.15 Wh L^-1. This study is the first attempt to describe a liquid-electrolyte-free cathodic half-cell in electrochemical application to VOCs degradation, highlighting the electron transfer at room temperature.

Photocatalytic Oxidation for Volatile Organic Compounds Elimination: From Fundamental Research to Practical Applications

Photocatalysis is regarded as one of the most promising technologies for indoor volatile organic compounds (VOCs) elimination due to its low cost, safe operation, energy efficiency, and high mineralization efficiency under ambient conditions. However, the practical applications of this technology are limited, despite considerable research efforts in recent decades. Until now, most of the works were carried out in the laboratory and focused on exploring new catalytic materials. Only a few works involved the immobilization of catalysts and the design of reactors for practical applications. Therefore, this review systematically summarizes the research and development on photocatalytic oxidation (PCO) of VOCs, with emphasis on recent catalyst's immobilization and reactor designs in detail. First, different types of photocatalytic materials and the mechanisms for PCO of VOCs are briefly discussed. Then, both the catalyst's immobilization techniques and reactor designs are reviewed in detail. Finally, the existing challenges and future perspectives for PCO of VOCs are proposed. This work aims to provide updated information and research inspirations for the commercialization of this technology in the future.

Photocatalytic Oxidation of Carbon Monoxide Using Synergy of Redox-Separated Photocatalyst and Ozone

Separating the redox centers of photocatalysts is the most promising strategy to enhance photocatalytic oxidation efficiency. Herein, I investigate a site-selective loading of Pt on facet-engineered TiO₂ to achieve carbon monoxide (CO) oxidation at room temperature. Spatially loaded Pt on {101} facets of TiO₂ attracts photoinduced electrons efficiently. Thereby, oxygen dissociation is facilitated on the Pt surface, which is confirmed by enhanced oxidation of CO by 2.4 times compared to the benchmark of Pt/TiO₂. The remaining holes on TiO₂ can be utilized for the oxidation of various gaseous pollutants. Specifically, gaseous ozone, which is present in indoor and ambient air, is converted to a hydroxyl radical by reacting with the hole; thus, the poisoned Pt surface is continuously cleaned during the CO oxidation, as confirmed by in situ diffuse reflectance infrared transform spectroscopy. While randomly loaded Pt can act as recombination center, reducing photocatalytic activity, redox-separated photocatalyst enhances charge separation, boosting CO oxidation and catalyst regeneration via simultaneous ozone decomposition.

Volatile organic compounds (VOCs) removal by photocatalysts: A review

Amplified anthropogenic release of volatile organic compounds (VOCs) gets worse air quality and human health. Photocatalytic degradation of VOCs is the practical strategy due to its low cost, simplicity, high efficiency, and environmental sustainability. Different types of photocatalyst activated by UV and visible lights are applied for VOC degradation. This review tries to investigate the state-of-art of recently published papers on this subject with a focus on the high-efficiency photocatalyst. The novel photocatalysts are introduced and enhancing photocatalytic activity strategies such as the hybrid of two/three photocatalyst, impurity doping, and heterojunctions with narrow bandgap semiconductors have been explained. The procedures of visible light activation of the photocatalysts are discussed with attention to current problems and future challenges. In addition, effective operational parameters in the photocatalytic degradation of VOCs have been reviewed with their advantages and drawbacks. A series of strategies are developed for the efficient utilization of visible light photocatalysts and improving new materials or design structures to degrade produced toxic intermediates/by-products during photocatalytic degradation of VOCs. This review shows that there are significant challenges in the applications of photocatalysts in the selective removal of VOCs. Several approaches should be combined to produce synergistic effects, which may lead to much higher photocatalytic performance than individual strategies. Another challenge is to develop efficient photocatalysts to meet real problems on an industrial scale.

Metallic zirconium carbide mediated near-infrared driven photocatalysis and photothermal sterilization for multidirectional water purification

Undoubtedly, taking full advantage of near-infrared light (NIR) for the photocatalytic reaction is a promising way to realize the efficient utilization of solar energy. In this work, zirconium carbide (ZrC) has been exploited as a NIR-driven photoactive substance for the simultaneous photodegradation of organic pollutants and photothermal sterilization of Escherichia coli (E. coli). The metallic nature and NIR-responsive localized surface plasmon resonance (LSPR) behaviors of ZrC are revealed by both experimental evidence and density function theory (DFT) calculations. ZrC exhibits extremely wide spectral absorbance, excellent NIR-triggered photosensitive effect and photothermal conversion efficiency. Activation kinetics was performed with DFT to investigate the activation process of O₂ to •O₂^-. In addition, a possible NIR-mediated photocatalytic mechanism of ZrC was proposed on the basis of above DFT simulation and radical scavenging experiments. Metallic ZrC with NIR-responsive activity provides a new perspective for designing full-spectrum-driven photocatalysts.

In2S3 nanoflakes grounded in Bi2WO6 nanoplates: A novel hierarchical heterojunction catalyst anchored on W mesh for efficient elimination of toluene

Toxic toluene can be completely oxidized in CO₂ and H₂O with novel three-dimensional (3D) In₂S₃@Bi₂WO₆ hierarchical crystals under visible light. Dense and uniform In₂S₃ nanoflakes are rooted in Bi₂WO₆ nanoplates which intercross with each other and are anchored on a pliable tungsten mesh. This leads to the construction of a stable and porous interface for adsorbing and decomposing target gaseous toluene. The firm contact between In₂S₃ and Bi₂WO₆ initiates the formation of a built-in electric field that helps in channeling the photogenerated electrons in Bi₂WO₆ CB to quench the holes in₂S₃ VB. This results in highly capable electrons and holes, as well as notable increase in the yields of •O₂^- and •OH. 99.7% of toluene is removed and 93.4% is converted to CO₂ when it is degraded in simulated air. This validates its remarkable efficacy in detoxifying toluene.

Photocatalytic degradation of gaseous benzene using metal oxide nanocomposites

Rapid industrial growth has been accompanied by the pollution of hazardous volatile organic pollutants (VOCs) in air. Among various options available for the treatment of VOCs, the use of metal oxide composites as photocatalysts has been adopted preferably due to their potential to induce the synergistic interactions between the metal nanoparticles (NPs) and metal oxides (especially titanium dioxide (TiO₂)). In this context, an in-depth review is offered to describe the fundamental mechanism of metal oxide-based photocatalysis for the oxidation of gaseous benzene as a model VOC. The discussion has been extended further to evaluate their performances in terms of key performance metrics (e.g., quantum yield (QY), space-time yield (STY), and figure of merit (FOM)). The TiO₂-based metallic bi-component photocatalysts (e.g., Sr₂CeO₄/TiO₂) generally exhibited better photodegradation efficiency with enhanced light absorption capability than monometallic-TiO₂ (e.g., Pd-TiO₂) composites or other modified photocatalysts (e.g., metal-organic framework (MOF)-based composites). Finally, we address the current challenges and future perspectives in this highly challenging research field.

Photocatalytic destruction of volatile aromatic compounds by platinized titanium dioxide in relation to the relative effect of the number of methyl groups on the benzene ring

The photocatalytic destruction (PCD) of volatile organic compounds (VOC) into environmentally benign compounds is one of the most ideal routes for the management of indoor air quality. It is nevertheless not easy to achieve the mineralization of aromatic VOC through PCD technology because of their recalcitrant structures (i.e., conjugated π benzene ring). In this research, the PCD potential against three model aromatic hydrocarbons (i.e., benzene (B), toluene (T), and m-xylene (X): namely, BTX) has been explored using a titanium dioxide (TiO₂) supported platinum (Pt) catalyst after the high-temperature hydrogen (H₂)-based reduction (R) pre-treatment (i.e., Pt/TiO₂-R). The effects of the key process variables (e.g., relative humidity (RH), oxygen (O₂) content, flow rate, VOC concentration, and the co-presence of VOC) on the PCD efficiency and related mechanisms were also assessed in detail. The PCD efficiency is seen to increase with the rise in the increasing number of methyl groups on the benzene ring (in the order of benzene (46.5%), toluene (68.2%), and m-xylene (95.9%)), as the adsorption and activation of the VOC molecule on the photocatalyst surface are promoted by the increased distribution of electrons on the benzene ring. The BTX were oxidated subsequently by the photogenerated reactive oxygen species (ROS), i.e., the hydroxyl radicals (•OH) and superoxide anion radicals (•O₂^-). The overall results of this study are expected to help expand the applicability of photocatalysis towards air quality management by offering detailed insights into the factors and processes governing the photocatalytic decomposition of aromatic VOCs.

Practical scale evaluation of a photocatalytic air purifier equipped with a Titania-zeolite composite bead filter for VOC removal and viral inactivation

A practical scale photocatalytic air purifier equipped with a TiO₂/H-ZSM-5 composite bead filter was demonstrated to be able to effectively remove indoor volatile organic compounds (VOCs) and viruses with sustainable performances under UVA-LED illumination. TiO₂ hybridized with 5 wt% H-ZSM-5 zeolite significantly enhanced its photocatalytic activity for degrading VOCs including formaldehyde, acetaldehyde, and toluene, than bare TiO₂. H-ZSM-5 provided strong adsorption sites for these compounds, thus accelerating their photocatalytic conversion into CO₂ by adjacent TiO₂ photocatalyst. Moreover, owing to its superior adsorption capacity, the composite bead filter completely prevented the emission of formaldehyde produced by photocatalytic oxidation of toluene. The sustainability of this composite bead filter for VOC removal was confirmed by regeneration and accelerated durability tests. In addition, the photocatalytic air purifier was effective in removing aerosolized viral particles of bacteriophage Phi-X 174. It was confirmed that the viruses on filter surfaces were completely inactivated by photocatalytic oxidation. TiO₂/H-ZSM-5 composite beads also exhibited excellent efficacies for inactivation of pathogenic coronaviruses including SARS-CoV-2. The photocatalytic process degraded viral RNAs of SARS-CoV-2 by more than 99.999% in 1 h, eliminating the viral infectivity. Results of this study suggest that the air purifier equipped with the composite bead filter is ready for practical applications for home and hospital uses.

Recent progress of silver-containing photocatalysts for water disinfection under visible light irradiation: A review

Photocatalysis has emerged as an environmentally friendly approach for microbial disinfection. The development of visible-light-driven (VLD) photocatalysts for water pollution remediation is imperative, considering that visible light constitutes a substantial fraction of the solar spectrum. The modification of photocatalysts by Ag/AgX (X = Cl, Br, I) deposition can be used to improve photocatalytic efficiencies. This is achieved by preventing photogenerated electron-hole pairs recombination through electron trapping mechanisms. With the introduction of silver NPs, visible light absorption can also be increased through its SPR enhancement. Silver also possesses excellent antimicrobial properties. Consequently, a novel class of Ag/AgX-containing hybrid materials has recently emerged as a promising candidate for water disinfection. This review summarizes the latest advances in the synthesis of Ag/AgX-containing photocatalysts using various synthetic methods. The microbial disinfection efficiencies of the as-prepared materials, the main reactive oxygen species and disinfection mechanisms are also reviewed in detail. Finally, some areas that need to be improved are discussed along with new insights as perspectives for future developments in this field.

Catalytic ozonation of VOCs at low temperature: A comprehensive review

VOCs abatement has attracted increasing interest because of the detrimental effects on both atmospheric environment and human beings of VOCs. The assistance of ozone has enabled efficient VOCs removal at low temperature. Thereby, catalytic ozonation is considered as one of the most feasible and effective methods for VOCs elimination. This work systematically reviews the emerging advances of catalytic ozonation of different VOCs (i.e., aromatic hydrocarbons, oxygenated VOCs, chlorinated VOCs, sulfur-containing VOCs, and saturated alkanes) over various functional catalysts. General reaction mechanism of catalytic ozonation including both Langmuir-Hinshelwood and Mars-van-Krevelen mechanisms was proposed depending on the reactive oxygen species involving the reactions. The influence of reaction conditions (water vapor and temperature) is fully discussed. This review also introduces the enhanced VOCs oxidation via catalytic ozonation in the ozone-generating systems including plasma and vacuum ultraviolet. Lastly, the existing challenges of VOCs catalytic ozonation are presented, and the perspective of this technology is envisioned.

Photocatalytic Air Purification Using Functional Polymeric Carbon Nitrides

The techniques for the production of the environment have received attention because of the increasing air pollution, which results in a negative impact on the living environment of mankind. Over the decades, burgeoning interest in polymeric carbon nitride (PCN) based photocatalysts for heterogeneous catalysis of air pollutants has been witnessed, which is improved by harvesting visible light, layered/defective structures, functional groups, suitable/adjustable band positions, and existing Lewis basic sites. PCN-based photocatalytic air purification can reduce the negative impacts of the emission of air pollutants and convert the undesirable and harmful materials into value-added or nontoxic, or low-toxic chemicals. However, based on previous reports, the systematic summary and analysis of PCN-based photocatalysts in the catalytic elimination of air pollutants have not been reported. The research progress of functional PCN-based composite materials as photocatalysts for the removal of air pollutants is reviewed here. The working mechanisms of each enhancement modification are elucidated and discussed on structures (nanostructure, molecular structue, and composite) regarding their effects on light-absorption/utilization, reactant adsorption, intermediate/product desorption, charge kinetics, and reactive oxygen species production. Perspectives related to further challenges and directions as well as design strategies of PCN-based photocatalysts in the heterogeneous catalysis of air pollutants are also provided.

Self-wetting triphase photocatalysis for effective and selective removal of hydrophilic volatile organic compounds in air

Photocatalytic air purification is widely regarded as a promising technology, but it calls for more efficient photocatalytic materials and systems. Here we report a strategy to introduce an in-situ water (self-wetting) layer on WO3 by coating hygroscopic periodic acid (PA) to dramatically enhance the photocatalytic removal of hydrophilic volatile organic compounds (VOCs) in air. In ambient air, water vapor is condensed on WO3 to make a unique tri-phasic (air/water/WO3) system. The in-situ formed water layer selectively concentrates hydrophilic VOCs. PA plays the multiple roles as a water-layer inducer, a surface-complexing ligand enhancing visible light absorption, and a strong electron acceptor. Under visible light, the photogenerated electrons are rapidly scavenged by periodate to produce more •OH. PA/WO3 exhibits excellent photocatalytic activity for acetaldehyde degradation with an apparent quantum efficiency of 64.3% at 460 nm, which is the highest value ever reported. Other hydrophilic VOCs like formaldehyde that are readily dissolved into the in-situ water layer on WO3 are also rapidly degraded, whereas hydrophobic VOCs remain intact during photocatalysis due to the "water barrier effect". PA/WO3 successfully demonstrated an excellent capacity for degrading hydrophilic VOCs selectively in wide-range concentrations (0.5-700 ppmv).

Ambient Air Purification by Nanotechnologies: From Theory to Application

Air pollution has been a recurring problem in northern Chinese cities, and high concentrations of PM2.5 in winter have been a particular cause for concern. Secondary aerosols converted from precursor gases (i.e., nitrogen oxides and volatile organic compounds) evidently account for a large fraction of the PM2.5. Conventional control methods, such as dust removal, desulfurization, and denitrification, help reduce emissions from stationary combustion sources, but these measures have not led to decreases in haze events. Recent advances in nanomaterials and nanotechnology provide new opportunities for removing fine particles and gaseous pollutants from ambient air and reducing the impacts on human health. This review begins with overviews of air pollution and traditional abatement technologies, and then advances in ambient air purification by nanotechnologies, including filtration, adsorption, photocatalysis, and ambient-temperature catalysis are presented—from fundamental principles to applications. Current state-of-the-art developments in the use of nanomaterials for particle removal, gas adsorption, and catalysis are summarized, and practical applications of catalysis-based techniques for air purification by nanomaterials in indoor, semi-enclosed, and open spaces are highlighted. Finally, we propose future directions for the development of novel disinfectant nanomaterials and the construction of advanced air purification devices.

Photocatalysis and Li-Ion Battery Applications of {001} Faceted Anatase TiO2-Based Composites

Anatase TiO2 are the most widely used photocatalysts because of their unique electronic, optical and catalytic properties. Surface chemistry plays a very important role in the various applications of anatase TiO2 especially in the catalysis, photocatalysis, energy conversion and energy storage. Control of the surface structure by crystal facet engineering has become an important strategy for tuning and optimizing the physicochemical properties of TiO2. For anatase TiO2, the {001} crystal facets are the most reactive because they exhibit unique surface characteristics such as visible light responsiveness, dissociative adsorption, efficient charge separation capabilities and photocatalytic selectivity. In this review, a concise survey of the literature in the field of {001} dominated anatase TiO2 crystals and their composites is presented. To begin, the existing strategies for the synthesis of {001} dominated anatase TiO2 and their composites are discussed. These synthesis strategies include both fluorine-mediated and fluorine-free synthesis routes. Then, a detailed account of the effect of {001} facets on the physicochemical properties of TiO2 and their composites are reviewed, with a particular focus on photocatalysis and Li-ion batteries applications. Finally, an outlook is given on future strategies discussing the remaining challenges for the development of {001} dominated TiO2 nanomaterials and their potential applications.

Surface Lattice Oxygen Activation on Sr2Sb2O7 Enhances the Photocatalytic Mineralization of Toluene: from Reactant Activation, Intermediate Conversion to Product Desorption

Transition-metal oxide photocatalysis has attracted increasing attention in environmental remediation and solar energy conversion. Surface lattice oxygen is the key active site on the metal oxide, but its role and activation mechanism in the photocatalytic VOC mineralization are still unclear. In this work, we have demonstrated that Sr₂Sb₂O₇ exhibits an excellent photocatalytic activity and stability compared to TiO₂ (P25) in gaseous toluene mineralization because the lattice oxygen on Sr₂Sb₂O₇ can be activated efficiently. The lattice oxygen of Sr₂Sb₂O₇ promotes the adsorption and activation of O₂ and H₂O molecules and enhances the production of ^•O₂^- and ^•OH radicals, as confirmed by the electron spin resonance and DFT calculations. The in situ diffuse reflectance infrared Fourier transform spectroscopy spectra are applied to dynamically monitor the intermediate activation and selective conversion. Combined with DFT calculation, the role and the mechanism of lattice oxygen in photocatalysis have been revealed. Owing to the promoted surface lattice oxygen, the selectivity for benzoic acid formation is enhanced and final product desorption is promoted, which could largely advance the ring opening and mineralization of toluene. This work reveals the origin of lattice oxygen activation and the role for efficient VOC degradation at the atomic scale.

Anatase versus Triphasic TiO2: Near-identical synthesis and comparative structure-sensitive photocatalytic degradation of methylene blue and 4-chlorophenol

Studies on photocatalytic activity of monophasic and biphasic TiO₂ have been well explored. However, detailed studies on the photocatalytic activity of triphasic titania, as opposed to monophasic or biphasic TiO₂ are scarce. Here we report a comparative structure-sensitive photocatalytic study of triphasic versus anatase TiO₂, both have been synthesized under near-identical conditions through a customized sol-gel approach. The composition of the phases is tuned just by varying the thermal pre-treatment conditions of TiO₂ gel that has been subsequently subjected to calcination at 300 °C. Interestingly, when the pre-treatment temperature of the gel is systematically increased from 50 to 250 °C, a transition from anatase to triphasic (anatase, rutile, and brookite) and then again to anatase has been observed. The synthesized TiO₂ phase compositions have been thoroughly characterized for their structural, optical, electrical, surface and morphological properties. Among the different phase compositions, triphasic titania having a significant proportion of rutile has been found to exhibit the highest photocatalytic activity, as probed using model organic pollutants, Methylene Blue (MB) and 4-Chlorophenol (4-CP). In addition to the earlier known factors such as effective heterojunction, and favorable position of the valence band (VB), an important contribution to the high photocatalytic activity of triphasic TiO₂ has been experimentally found to stem from the additional electron density in VB that is attributed to the lattice contraction of anatase phase owing to the coexistence of other two phases. The study provides fundamental insights into the energetics that impact the photocatalytic activity of triphasic versus anatase TiO₂.

Dual-function surface hydrogen bonds enable robust O2 activation for deep photocatalytic toluene oxidation

Extensive hydrogen-bonds between the hydroxyl-rich BiOCl surface and phosphoric acid significantly facilitate oxygen vacancy formation, O₂ activation, and deep toluene oxidation.

Nature-inspired CaCO3 loading TiO2 composites for efficient and durable photocatalytic mineralization of gaseous toluene

The accumulation of intermediates or final products on TiO₂ during photocatalytic volatile organic compounds (VOCs) degradation is typically neglected, despite the fact that it could result in the block of active sites and the deactivation of photocatalysts. Inspired from the natural formation of stalactite (CaCO₃ + H₂O + CO₂ ↔ Ca(HCO₃)₂), we fabricated CaCO₃ loading TiO₂ composites (CCT21) to realize the spontaneously transfer of accumulated final products (CO₂ and H₂O). Efficient and durable performance for gaseous toluene removal has been demonstrated and the cost of photocatalyst is greatly reduced by the comparison of specific activity. The introduction of CaCO₃ induces the interaction between TiO₂ and CaCO₃ to stimulate abundant activated electrons for the improvement on the adsorption and activation of reactants and the transformation of photogenerated carriers, and most importantly, facilitates the transfer of final products to release active sites and thus suppress the deactivation of TiO₂. Furthermore, we develop a facile method to immobilize CCT21 powder on flexible support, which greatly reduces the loss of photocatalysts and correspondingly enables the practical application of TiO₂-based products. Therefore, this work presents a novel nature-inspired strategy to address the challenge of deactivation, and advances the development of photocatalytic technology for environmental remediation.

A novel UV-assisted PEC-MFC system with CeO2/TiO2/ACF catalytic cathode for gas phase VOCs treatment

Emissions of volatile organic compounds (VOCs) air pollutants could worsen air quality and adversely affect human health, thus developing more efficient low-temperature VOCs removal techniques is desired. A novel continuous system integrating UV-assisted photo-electrochemical catalysis with microbial fuel cell (UV-assisted PEC-MFC) has been established for promoting removal of gaseous ethyl acetate or toluene and generating electricity simultaneously. In this system, CeO₂/TiO₂/ACF catalytic cathode is prepared and used for combination with bio-anode for accelerating cathodic reaction. This UV-assisted PEC-MFC system exhibits an excellent elimination capacity (EC) of ethyl acetate (∼0.39 g/m³, EC: ∼2.52 g/m³/h) or toluene (∼0.29 g/m³, EC: 1.89 g/m³/h) under close-circuit condition. Furthermore, an outstanding elimination capacity (EC: 28.04 g/m³/h) for high concentration toluene (∼4.10 g/m³) removal is obtained after toluene gas passes sequentially through the catalytic cathode then the bio-anode. This way of PEC degradation and biodegradation, avoids inhibition of exoelectrogens activity from toxicity of high concentration toluene. Simultaneously, the cell voltage of UV-assisted PEC-MFC system is stable at 0.11 V (vs. SCE) and 1.452×10^-4 kWh is generated from treatment of toluene gas stream in 6 h duration time. The possible mechanism of VOCs removal in this novel system has been proposed and discussed. This study provides new technical basis for treating gaseous pollutants via integrating photo-electrochemical catalysis with electricity generating microbial fuel cell for energy conversion.

Delocalization Effect Promoted the Indoor Air Purification via Directly Unlocking the Ring-Opening Pathway of Toluene

The ring-opening process was generally considered as the rate-determining step for aromatic volatile organic compound photocatalytic degradation. A sophisticated and intensive degradation pathway is critical to the poor removal efficiency and low mineralization. In the present contribution, we successfully tailored and identified the ring-opening pathway of toluene elimination by electron delocalization in a borocarbonitride photocatalyst. By means of modulation of the dopant coordination configuration and electron geometry in the catalyst, the lone electrons of carbon transform into delocalized counterparts, sequentially elevating the interaction between the toluene molecules and photocatalyst. The aromatic ring of toluene can be attacked directly in the effect of electron delocalization without engendering additional intermediate species, significantly facilitating the removal and mineralization of toluene. This unprecedented route-control strategy alters the aromatic-ring-based reaction behavior from toluene to CO₂ and paves a way to purify the refractory pollutants from the top design.

Probing toluene catalytic removal mechanism over supported Pt nano- and single-atom-catalyst

Commercial TiO₂ supported 0.20 wt% Pt catalyst is obtained via the molten salt method, and both Pt nanoparticles and single atom Pt sites are observed. It exhibits high catalytic performance for toluene oxidation, with T₅₀ and T₉₀ being 173 and 183 °C, respectively. Reaction intermediates including benzene, p-xylene, o-xylene, benzaldehyde, phthalic acid, maleic anhydride, itaconic anhydride, acetone, and acetic acid, are detected during toluene oxidation. On this basis, likely toluene combustion reaction pathway is provided. Benzaldehyde is the most stable surface intermediate, and its oxidation can be rate-limiting for the entire toluene oxidation reaction. 2-10.0 vol% H₂O slightly inhibits the reaction by competing surface sites with the reactant, while it does not poison the catalyst. 2.5-10.0 vol% CO₂ slightly poisons the catalyst by surface carbonate formation, whereas 50 ppm SO₂ severely poisons the catalyst by sulfite/sulfate formation.

Photochemical pollutant degradation on facet junction-engineered TiO2 promoted by organic arsenical: Governing roles of arsenic-terminated surface chemistry and bulk-free radical speciation

Photochemical oxidation based on semiconducting metal oxides is an efficient strategy to remove environmental pollutants in water, air and soil. The fine manipulation of photo-carriers separation, surface chemistry and radical speciation is of considerable interest for environmental remediation. In this work, the morphology- and structure-tailored TiO₂ single crystals with epitaxial {101}/{001} facet junction were designed, prepared and tested for photochemical pollutant oxidation in the presence of organic arsenicals, the main component in swine wastewater from livestock industry, although they have been forbidden for several years. The facet junction-tailored TiO₂ deserved an efficient photo-carriers separation with high quantum efficiency. The photochemical oxidation of 4-chlorophenol (4-CP), phenol and bisphenol A (BPA) was substantially improved by roxarsone (ROX). ROX-enhanced photochemical activity of TiO₂ was mainly attributed to the in-situ arsenic-terminated surface chemistry by Ti-OAs^VO₃/-OAs^IIIO₂. This surface played governing roles in water/TiO₂ interactions, and changed water adsorption from dissociative to molecular configuration. Furthermore, ·OH was finely regulated from low-activity surface-bound to high-activity bulk-free speciation between as-generated photo-holes with free water molecules. Our findings provided a new chance to refine the TiO₂-based photochemical oxidation, and a modifying technology to treat swine wastewater from livestock industry with much reduced secondary pollution.

Catalytic degradation of ciprofloxacin by a visible-light-assisted peroxymonosulfate activation system: Performance and mechanism

Peroxymonosulfate (PMS) is extensively used as an oxidant to develop the sulfate radical-based advanced oxidation processes in the decontamination of organic pollutants and various PMS activation methods have been explored. Visible-light-assisted PMS activation to construct a Fenton-like process has shown a great potential for pollution control. In our work, BiVO₄ nanosheets were prepared using a hydrothermal process and used to activate PMS under visible light. A rapid degradation of ciprofloxacin (CIP) was achieved by dosing PMS (0.96 g/L), BiVO₄ (0.32 g/L) under visible light with a reaction rate constant of 77.72-fold higher than that in the BiVO₄/visible light process. The electron spin resonance and free radical quenching experiments indicate that reactive species of ^•O₂^-, h⁺, •OH and SO₄^•- all worked, where h⁺, •OH and SO₄^•- were found as the dominant contributors to the CIP degradation. The spectroscopic analyses further demonstrate that the photoinduced electrons were directly involved in the PMS activation process. The generated ^•O₂^- was partially utilized to activate PMS and more •OH was produced because of the chain reactions between SO₄^•- and H₂O/OH^-. In this process, PMS acted as an electron acceptor to transfer the photo-induced charges from the conduction band of BiVO₄ and PMS was successfully activated to yield the high-powered oxidative species. From the degradation intermediates of CIP detected by a liquid-chromatography-mass spectrometer, the possible degradation pathways were proposed. The substantially decreased toxicity of CIP after the reaction was also observed. This work might provide new insights into the visible-light-assisted PMS activation mechanisms and is useful to construct environmentally-friendly catalytic processes for the efficient degradation of organic pollutants.

How g-C3N4 Works and Is Different from TiO2 as an Environmental Photocatalyst: Mechanistic View

Graphitic carbon nitride (CN) as a popular visible light photocatalyst needs to be better understood for environmental applications. The behaviors of CN as an environmental photocatalyst were systematically studied in comparison with a well-known TiO₂ photocatalyst. The two photocatalysts exhibit different photocatalytic oxidation (PCO) behaviors and dependences on the experimental conditions (e.g., pH, Pt loading, and the kind of organic substrate and scavenger). The PCO of organic substrates was significantly enhanced by loading Pt on TiO₂ under UV light (λ > 320 nm), whereas Pt-CN exhibited a lower PCO activity than bare CN under visible light (λ > 420 nm). While the presence of Pt enhances the charge separation in both TiO₂/UV and CN/visible light systems (confirmed by transient IR absorption spectroscopic analysis), the opposite effects of Pt are ascribed to the different mechanisms of ^•OH generation in the two photocatalytic systems. The negative effect of Pt on CN is ascribed to the fact that Pt catalytically decomposes in situ-generated H₂O₂ (a main precursor of OH radical), which hinders ^•OH production. The production of OH radicals on CN is favored only at acidic pH but ¹O₂ generation is dominant in alkaline pH. The pH-dependent behaviors of reactive oxygen species generation on CN were confirmed by electron paramagnetic resonance spin trap measurements.

Visible light driven CuBi2O4/Bi2MoO6 p-n heterojunction with enhanced photocatalytic inactivation of E. coli and mechanism insight

Here, a novel CuBi₂O₄/Bi₂MoO₆ (CBO/BMO) p-n heterojunction was fabricated and exhibited markedly improved photocatalytic inactivation capacity of E. coli cells under visible light excitation (λ > 420 nm) compared with pure CuBi₂O₄ and Bi₂MoO₆. The CBO/BMO-0.5 hybrid displayed the highest photoinactivation ability which could completely inactivate the E. coli cellswithin 4 h. The mechanism of photocatalytic disinfection towards E. coli of CBO/BMO heterojunctions was attributed to the disruption of cell-membrane, leakage and damage of cellular content including total protein and DNA as verified with SEM, fluorescence-base dead/live stain, sodium dodecyl sulfate polyacrylamide gel electropheresis (SDS-PAGE) and agarose gel electrophoresis (AGE). Additionally, the scavenge experiments showed that the reactive species h⁺, e^- and •O_2^-play the predominant role in the photocatalytic system of CBO/BMO hybrids. The improved photocatalytic activity of CBO/BMO composites was mainly attributed to the promotion of spatial separation and migration rate of photoproduced electron-hole pairs, enhancement of visible light absorption and more generation of reactive species (•O₂^-) on the interface of catalyst and water which was demonstrated by nitroblue tetrazolium (NBT) and EPR. Our work indicated that construction of CuBi₂O₄/Bi₂MoO₆ p-n heterostructure photocatalyst is a promising environmental friendly alternative method to deal with the biohazards of pathogenic microorganisms.

A Critical Review on Energy Conversion and Environmental Remediation of Photocatalysts with Remodeling Crystal Lattice, Surface, and Interface

Solar energy is a renewable resource that can supply our energy needs in the long term. A semiconductor photocatalysis that is capable of utilizing solar energy has appealed to considerable interests for recent decades, owing to the ability to aim at environmental problems and produce renewal energy. Much effort has been put into the synthesis of a highly efficient semiconductor photocatalyst to promote its real application potential. Hence, we reviewed the most advanced methods and strategies in terms of (i) broadening the light absorption wavelengths, (ii) design of active reaction sites, and (iii) control of the electron-hole (e^--h⁺) recombination, while these three processes could be influenced by remodeling the crystal lattice, surface, and interface. Additionally, we individually examined their current applications in energy conversion (i.e., hydrogen evolution, CO₂ reduction, nitrogen fixation, and oriented synthesis) and environmental remediation (i.e., air purification and wastewater treatment). Overall, in this review, we particularly focused on advanced photocatalytic activity with simultaneous wastewater decontamination and energy conversion and further enriched the mechanism by proposing the electron flow and substance conversion. Finally, this review offers the prospects of semiconductor photocatalysts in the following three vital (distinct) aspects: (i) the large-scale preparation of highly efficient photocatalysts, (ii) the development of sustainable photocatalysis systems, and (iii) the optimization of the photocatalytic process for practical application.

Active Complexes on Engineered Crystal Facets of MnOx-CeO2 and Scale-Up Demonstration on an Air Cleaner for Indoor Formaldehyde Removal

Crystal facet-dominated surfaces determine the formation of surface-active complexes, and engineering specific facets is desirable for improving the catalytic activity of routine transition-metal oxides that often deactivate at low temperatures. Herein, MnO_x-CeO₂ was synthetically administered to tailor the exposure of three major facets, and their distinct surface-active complexes concerning the formation and quantitative effects of oxygen vacancies, catalytically active zones, and active-site behaviors were unraveled. Compared with two other low-index facets {110} and {001}, MnO_x-CeO₂ with exposed {111} facet showed higher activity for formaldehyde oxidation and CO₂ selectivity. However, the {110} facet did not increase activity despite generating additional oxygen vacancies. Oxygen vacancies were highly stable on the {111} facet, and its bulk lattice oxygen at high migration rates could replenish the consumption of surface lattice oxygen, which was associated with activity and stability. High catalytically active regions were exposed at the {111}-dominated surfaces, wherein the predominated Lewis acid-base properties facilitated oxygen mobility and activation. The mineralization pathways of formaldehyde were examined by a combination of in situ X-ray photoemission spectroscopy and diffuse reflectance infrared Fourier transform spectrometry. The MnO_x-CeO₂-111 catalysts were subsequently scaled up to work as filter substrates in a household air cleaner. In in-field pilot tests, 8 h of exposure to an average concentration of formaldehyde after start-up of the air cleaner attained the Excellent Class of Indoor Air Quality Objectives in Hong Kong.