Performance and Mechanism of Photocatalytic Toluene Degradation and Catalyst Regeneration by Thermal/UV Treatment

Scheelite Catalysts for Thermal Mineralization of Toluene: A Mechanistic Overview

Toluene, a highly stable aromatic hydrocarbon, is utilized as a benchmark molecule for thermal mineralization by the catalytic community. Mostly, the catalysts used for toluene mineralization either use platinum group metals (PGM) as catalysts or are regulated by a plasma incinerator. Though these catalysts/processes promise better efficiency and lower reaction temperature, they are neither cost-effective nor do they produce thermally stable byproducts. However, most of the metal-oxide catalysts used for toluene degradation are less efficient owing to incomplete mineralization and formation of stable intermediates, which results in higher mineralization temperature. The present work showcases tungsten- and molybdenum-based Scheelites [BaXO4 (X = W, Mo, and Mo0.5W0.5)], which have been utilized for toluene mineralization at ∼200 °C. The intermediates formed during adsorption and thermal reaction are deciphered as a function of temperature using in situ FT-IR studies including their kinetic behavior. These surface intermediates formed over the Scheelite catalysts under an oxidative/inert atmosphere elucidate the toluene mineralization mechanism as a function of temperature/time. The surface active sites for these oxide catalysts for both adsorption and formation of reaction intermediates are deciphered using detailed X-ray photoelectron spectroscopy (XPS) studies. It shows the effective role of the oxidation states of constituent oxides M-O (M = Mo/ W) in the reaction mechanism. Mineralization of toluene in a nonoxidative atmosphere shows a Mars and Van Krevelen (MVK) type of mechanism, suggesting participation of lattice oxygen for the catalytic reaction. To the best of our knowledge, this work represents one of the lowest temperatures achieved for toluene mineralization using oxide catalysts. The identification of reaction intermediates can guide further optimization efforts to minimize the mineralization temperature.

Revealing the Principle of Progressively Enhanced Photocatalytic Reactivity in Dual Single-Atoms-Mediated Electronic Interactions Optimization of Cd/Te-TiO2

In this work, a CdTe@TiO2 single atoms (SAs) catalysts is successfully synthesized, realizing unique portion of nonbonding oxygen-coordinated configuration of Cd─O─Te dimers coupling. Astonishingly, the 5th CdTe@TiO2 (0.027 min-1) shows progressively augmenting phenomenon, accompanied with 2.73 times higher than that of fresh CdTe@TiO2 (0.010 min-1) on the photocatalytic rate constant of gaseous toluene conversion. The incrementally enhanced photocatalytic activity is attributed to atomically dispersed Cd/Te SAs sites generation during the photoreduction process, and further leading to the optimized electron interactions between Cd, Te atoms, and TiO2 NTs and causing a positive shift in the d-band center closer to the Fermi level. Density Functional Theory (DFT) calculations reveal that this unique Cd/Te SAs increasing phenomenon can mutually elevate the electronic density around Cd/Te SAs and generate a substantial local electric field at the interface. In essence, the free energy barriers of the benzene intermediates ring-opening as the rate-determining step appeared to significantly diminish tendency from 1.10 to 0.96 eV, in line with the ICOHP calculation of Cd/Te─O bonds in TS promoted from -2.43 to -3.49 eV. This work unearths the mechanism for ascendant electronic states of synergies dual-metal sites, providing a versatile strategy to tailor the SAs catalysts for solar energy conversion.

Synergic action of bamboo-cellulose-supported hydrogen-bonded nano-AgBr for robust photocatalysis

A novel semiconductor photocatalyst was developed using bamboo cellulose fibers (BCFs) embedded with nano-AgBr (AgBr@BCFs) via a simple and rapid method. BCFs prevented the agglomeration of AgBr and provided numerous active reaction sites as a dispersant and structural support. The photocatalytic activity of AgBr@BCFs in removing organic pollutants was investigated and the endogenous factors leading to the high activity were analyzed through a combination of a series of experiments, characterizations and theoretical calculations. We propose that the efficient photocatalytic performance of AgBr@BCFs was attributed to the interface integration facilitated by hydrogen bonds and robust electronic interactions. The interface demonstrated a significantly negative reduction potential (-0.57 eV), enhancing carrier transport efficiency and inhibiting the recombination of photogenerated electron-hole pairs. Compared to the intrinsic activity of AgBr, AgBr@BCFs exhibits 7.2 times higher performance for Rhodamine B and 8.6 times greater intensity for tetracycline (TC). Additionally, the applicability of the photocatalyst across various pH ranges, photocorrosion resistance, and recyclability were evaluated. The mechanism of the photocatalytic process revealed that the synergistic bifunctionality of high adsorption rate and strong degradation activity is the primary reason for the high activity. BCFs-based semiconductor material can be recycled efficiently, which is a promising photocatalyst for the purification of organic sewage.

Effects of surface fluoride modification on TiO2 for the photocatalytic oxidation of toluene

In the present study, we investigated the influence of surface fluorine (F) on TiO2 for the photocatalytic oxidation (PCO) of toluene. TiO2 modified with different F content was prepared and tested. It was found that with the increasing of F content, the toluene conversion rate first increased and then decreased. However, CO2 mineralization efficiency showed the opposite trend. Based on the characterizations, we revealed that F substitutes the surface hydroxyl of TiO2 to form the structure of Ti-F. The presence of the appropriate amount of surface Ti-F on TiO2 greatly enhanced the separation of photogenerated carriers, which facilitated the generation of ·OH and promoted the activity for the PCO of toluene. It was further revealed that the increase of only ·OH promoted the conversion of toluene to ring-containing intermediates, causing the accumulation of intermediates and then conversely inhibited the ·OH generation, which led to the decrease of the CO2 mineralization efficiency. The above results could provide guidance for the rational design of photocatalysts for toluene oxidation.

Strong metal-support interaction (SMSI) in environmental catalysis: Mechanisms, application, regulation strategies, and breakthroughs

The strong metal-support interaction (SMSI) in supported catalysts plays a dominant role in catalytic degradation, upgrading, and remanufacturing of environmental pollutants. Previous studies have shown that SMSI is crucial in supported catalysts' activity and stability. However, for redox reactions catalyzed in environmental catalysis, the enhancement mechanism of SMSI-induced oxygen vacancy and electron transfer needs to be clarified. Additionally, the precise control of SMSI interface sites remains to be fully understood. Here we provide a systematic review of SMSI's catalytic mechanisms and control strategies in purifying gaseous pollutants, treating organic wastewater, and valorizing biomass solid waste. We explore the adsorption and activation mechanisms of SMSI in redox reactions by examining interfacial electron transfer, interfacial oxygen vacancy, and interfacial acidic sites. Furthermore, we develop a precise regulation strategy of SMSI from systematical perspectives of interface effect, crystal facet effect, size effect, guest ion doping, and modification effect. Importantly, we point out the drawbacks and breakthrough directions for SMSI regulation in environmental catalysis, including partial encapsulation strategy, size optimization strategy, interface oxygen vacancy strategy, and multi-component strategy. This review article provides the potential applications of SMSI and offers guidance for its controlled regulation in environmental catalysis.

A Review of Metal-Organic Frameworks Derived Hollow-Structured Photocatalysts: Synthesis and Applications

Photocatalysis is a most important approach to addressing global energy shortages and environmental issues due to its environmentally friendly and sustainable properties. The key to realizing efficient photocatalysis relies on developing appropriate catalysts with high efficiency and chemical stability. Among various photocatalysts, Metal-organic frameworks (MOFs)-derived hollow-structured materials have drawn increased attention in photocatalysis based on advantages like more active sites, strong light absorption, efficient transfer of pho-induced charges, excellent stability, high electrical conductivity, and better biocompatibility. Specifically, MOFs-derived hollow-structured materials are widely utilized in photocatalytic CO2 reduction (CO2RR), hydrogen evolution (HER), nitrogen fixation (NRR), degradation, and other reactions. This review starts with the development story of MOFs, the commonly adopted synthesis strategies of MOFs-derived hollow materials, and the latest research progress in various photocatalytic applications are also introduced in detail. Ultimately, the challenges of MOFs-derived hollow-structured materials in practical photocatalytic applications are also prospected. This review holds great potential for developing more applicable and efficient MOFs-derived hollow-structured photocatalysts.

Pt position determining efficiency and stability for photocatalytic toluene degradation over Pt decorated TiO2

Highly durable photocatalytic degradation of gas toluene pollutants is a great challenge due to the easy deactivation of photocatalysts. Herein, we synthesized the Pt embedded at interface of TiO2 nanocomposites (TiO2/Pt/TiO2) and Pt exposed on the surface of TiO2 nanocomposites (Pt/TiO2/TiO2) to investigate the effect of Pt position on the photocatalytic performance of toluene degradation. It was found that the Pt-exposed samples showed inactivation as the reaction progressed because carbonaceous intermediates such as phenol and benzoic acid were observed to be deposited on the exposed Pt to restrain the role of Pt in electron transfer for the production of reactive oxygen species. Whereas, Pt-embedded nanocomposites had excellent activity and stability for toluene degradation and CO2 production more than 60 h. This was attributed to the protective effect of the TiO2 outerlayer. The embedded Pt was not easily poisoned by the degradation intermediates, resulting in a good electron transfer and the continuous production of reactive oxygen species for photocatalytic reaction. Therefore, this work provides an efficient approach for designing of the stability of metal-decorated photocatalyst for the highly durable photocatalytic performance.

Tuning the Metal-Support Interaction by Modulating CeO2 Oxygen Vacancies to Enhance the Toluene Oxidation Activity of Pt/CeO2 Catalysts

In this research, a range of Pt/CeO2 catalysts featuring varying Pt-O-Ce bond contents were developed by modulating the oxygen vacancies of the CeO2 support for toluene abatement. The Pt/CeO2-HA catalyst generated a maximum quantity of Pt-O-Ce bonds (possessed the strongest metal-support interaction), as evidenced by the visible Raman results, which demonstrated outstanding toluene catalytic performance. Additionally, the UV Raman results revealed that the strong metal-support interaction stimulated a substantial increase in oxygen vacancies, which could facilitate the activation of gaseous oxygen to generate abundant reactive oxygen species accumulated on the Pt/CeO2-HA catalyst surface, a conclusion supported by the H2-TPR, XPS, and toluene-TPSR results. Furthermore, the results from quasi-in situ XPS, in situ DRIFTS, and DFT indicated that the Pt/CeO2-HA catalyst with a strong metal-support interaction led to improved mobility of reactive oxygen species and lower oxygen activation energies, which could transfer a large number of activated reactive oxygen species to the reaction interface to participate in the toluene oxidation, resulting in the relatively superior catalytic performance. The approach of tuning the metal-support interaction of catalysts offers a promising avenue to develop highly active catalysts for toluene degradation.

Boosting Photothermocatalytic Oxidation of Toluene Over Pt/N-TiO2: The Gear Effect of Light and Heat

Photothermal catalysis is extremely promising for the removal of various indoor pollutants owing to its photothermal synergistic effect, while the low light utilization efficiency and unclear catalytic synergistic mechanism hinder its practical applications. Here, nitrogen atoms are introduced, and Pt nanoparticles are loaded on TiO2 to construct Pt/N-TiO2-H2, which exhibits 3.5-fold higher toluene conversion rate than the pure TiO2. Compared to both photocatalytic and thermocatalytic processes, Pt/N-TiO2-H2 exhibited remarkable performance and stability in the photothermocatalytic oxidation of toluene, achieving 98.4% conversion and 98.3% CO2 yield under a light intensity of 260 mW cm-2. Furthermore, Pt/N-TiO2-H2 demonstrated potential practical applicability in the photothermocatalytic elimination of various indoor volatile organic compounds. The synergistic effect occurs as thermocatalysis accelerates the accumulation of carboxylate species and the degradation of aldehyde species, while photocatalysis promotes the generation of aldehyde species and the consumption of carboxylate species. This ultimately enhances the photothermocatalytic process. The photothermal synergistic effect involves the specific conversion of intermediates through the interplay of light and heat, providing novel insights for the design of photothermocatalytic materials and the understanding of photothermal mechanisms.

A theoretical study on toluene oxidization by OH radical

Toluene, a prominent member of volatile organic compounds (VOCs), exerts a substantial adverse influence on both human life and the environment. In the context of advanced oxidation processes, the ·OH radical emerges as a highly efficient oxidant, pivotal in the elimination of VOCs. This study employs computational quantum chemistry methods (G4MP2//B3LYP/6-311++G(d,p)) to systematically investigate the degradation of toluene by ·OH radicals in an implicit solvent model, and validates the rationale of choosing a single-reference method using T1 diagnostics. Our results suggest three possible reaction mechanisms for the oxidation of toluene by ·OH: firstly, the phenyl ring undergoes a hydrogen abstraction reaction followed by direct combination with ·OH to form cresol; secondly, ·OH directly adds to the phenyl ring, leading to ring opening; thirdly, oxidation of sidechain to benzoic acid followed by further addition and ring opening. The last two oxidation pathways involve the ring opening of toluene via the addition of ·OH, significantly facilitating the process. Therefore, both pathways are considered feasible for the degradation of toluene. Subsequently, the UV-H2O2 system was designed to induce the formation of ·OH for toluene degradation and to identify the optimal reaction conditions. It was demonstrated that ·OH and 1O2 are the primary active species for degrading toluene, with their contribution ranking as ·OH > 1O2. The intermediates in the mixture solution after reactions were characterized using GC-MS, demonstrating the validity of theoretical predictions. A comparative study of the toluene consumption rate revealed an experimental comprehensive activation energy of 10.33 kJ/mol, which is consistent with the preliminary activation energies obtained via theoretical analysis of these three mechanisms (0.56 kJ/mol to 13.66 kJ/mol), indicating that this theoretical method can provide a theoretical basis for experimental studies on the oxidation of toluene by ·OH.

Water-Resistance-Based S-Scheme Heterojunction for Deep Mineralization of Toluene

Deep mineralization of low concentration toluene (C7 H8 ) is one of the most significant but challenging reactions in photocatalysis. It is generally assumed that hydroxyl radicals (⋅OH) as the main reactive species contribute to the enhanced photoactivity, however, it remains ambiguous at this stage. Herein, a S-scheme ZnSn(OH)6 -based heterojunction with AlOOH as water resistant surface layer is in situ designed for tuning the free radical species and achieving deep mineralization of C7 H8 . By employing a combination of in situ DRIFTS and materials characterization techniques, we discover that the dominant intermediates such as benzaldehyde and benzoic acid instead of toxic phenols are formed under the action of holes (h+ ) and superoxide radicals (⋅O2 - ). These dominant intermediates turn out to greatly decrease the ring-opening reaction barrier. This study offers new possibilities for rationally tailoring the active species and thus directionally producing dominant intermediates via designing water resistant surface layer.

Surface Hydroxyl and Oxygen Vacancies Engineering in ZnSnAl LDH: Synergistic Promotion of Photocatalytic Oxidation of Aromatic VOCs

Photocatalytic oxidation has gained great interest in environmental remediation, but it is still limited by its low efficiency and catalytic deactivation in the degradation of aromatic VOCs. In this study, we concurrently regulated the surface hydroxyl and oxygen vacancies by introducing Al into ZnSn layered double hydroxide (LDH). The presence of distorted Al species induced local charge redistribution, leading to the remarkable formation of oxygen vacancies. These oxygen vacancies subsequently increased the amount of surface hydroxyl and elongated its bond length. The synergistic effects of surface hydroxyl and oxygen vacancies greatly enhanced reactant adsorption-activation and facilitated charge transfer to generate •OH, •O2-, and 1O2, resulting in highly efficient oxidation and ring-opening of various aromatic VOCs. Compared with commercial TiO2, the optimized ZnSnAl-50 catalyst exhibited about 2-fold activity for the toluene and styrene degradation and 10-fold activity for the chlorobenzene degradation. Moreover, ZnSnAl-50 demonstrated exceptional stability in the photocatalytic oxidation of toluene under a wide humidity range of 0-75%. This work marvelously improves the photocatalytic efficiency, stability, and adaptability through a novel strategy of surface hydroxyl and oxygen vacancies engineering.

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.

Boosting the photocatalytic activity of β-FeOOH catalyst for toluene oxidation by constructing internal electric field at 0D/1D homojunction interfaces

Photocatalytic degradation is considered as the most energy-efficient, environmentally benign, and effective method for treating low fraction organic contaminants. However, the photocatalysts still suffer from low utilization efficiency of visible-light and severe carrier recombination. Heterojunctions can resolve these two main problems in some extent but still be restrained by the low quality of hetero-interface. In this study, homojunction was constructed of β-FeOOH quantum dots and nanorods with the same lattice by a two-step precipitation method, to avoid the heterointerface with too many defects and possess good charge separation as a consequence. The catalysts were characterized by activity test, electron spin resonance, Mott-Schottky plots, photocurrent density tests and open-circuit potential measurements, etc. The results revealed that a strong internal electric fields (IEFs) was created at the interface of catalyst. Beneficently, the electron rearrangement leads to a more rational distribution of oxygen vacancies in the catalyst, resulting in more efficient dissociation of oxygen molecules and formation of active radicals, thus facilitating the efficient degradation of toluene. This study proposes a novel strategy to boosting the photocatalytic activity of low dimensional semiconductors via forming homojunction interfaces to improve their charge transfer.

The effects of water, substrate, and intermediate adsorption on the photocatalytic decomposition of air pollutants over nano-TiO2 photocatalysts

The photocatalytic performance of nano-TiO2 photocatalysts in air pollutant degradation greatly depends on the adsorption of water, substrates, and intermediates. Especially under excessive humidity, substrate concentration, and intermediate concentration, the competitive adsorption of water, substrates, and intermediates can seriously inhibit the photocatalytic performance. In the past few years, extensive studies have been performed to investigate the influence of humidity, substrate concentration, and intermediates on the photocatalytic performance of TiO2, and significant advances have been made in the area. However, to the best of our knowledge, there is no review focusing on the effects of water, substrate, and intermediate adsorption to date. A comprehensive understanding of their mechanisms is key to overcoming the limited application of nano-TiO2 photocatalysts in the photocatalytic decomposition of air pollutants. In this review, the progress in experimental and theoretical fields, including a recent combination of photocatalytic experiments and adsorption and photocatalytic simulations by density functional theory (DFT), to explore the impact of adsorption of various reaction components on nano-TiO2 photocatalysts is comprehensively summarized. Additionally, the mechanism and broad perspective of the impact of their adsorption on the photocatalytic activity of TiO2 in air treatment are also critically discussed. Finally, several solutions are proposed to resolve the current problems related to environmental factors. In general, this review contributes a comprehensive perspective of water, substrate, and intermediate adsorption toward boosting the photocatalytic application of TiO2 nanomaterials.

Dual function of H2O on interfacial intermediate conversion and surface poisoning regulation in simultaneous photodegradation of NO and toluene

Co-existing air pollutants, especially NOx and VOCs, will generate secondary photochemical pollution under light irradiation. However, simultaneous elimination of multi-pollutants has long been a challenge. Photocatalysis could turn the reaction pathway between pollutants to convert them into harmless products, which is a promising technology for multi-pollutant control. Here we achieved synergistic photocatalytic degradation of NO and C7H8 on InOOH photocatalyst, and the performance can be adjusted by H2O through affecting the interaction between surface species and catalyst. In situ DRIFTS and GC-MS revealed that the improved efficiency originated from the fast conversion of C-N coupling intermediates led by additional H2O. Surface characterizations and DFT simulation determined that accumulated nitrates will compete with the adsorption of NO and C7H8, resulting in a decline in efficiency in the later stage. Although improved efficiency would bring more nitrates, as H2O has comparable adsorption to nitrate at the same site, high humidity can mitigate the deactivation. The photocatalyst can be also simply regenerated by water washing. This work reveals the complex interaction in the multi-pollutant system and provides guidelines for precisely regulating the synergistic removal of NOx and VOCs.

Accelerated Photolysis of H2O2 at the Air-Water Interface of a Microdroplet

Photochemical homolysis of hydrogen peroxide (H2O2) occurs widely in nature and is a key source of hydroxyl radicals (·OH). The kinetics of H2O2 photolysis play a pivotal role in determining the efficiency of ·OH production, which is currently mainly investigated in bulk systems. Here, we report considerably accelerated H2O2 photolysis at the air-water interface of microdroplets, with a rate 1.9 × 103 times faster than that in bulk water. Our simulations show that due to the trans quasiplanar conformational preference of H2O2 at the air-water interface compared to the bulk or gas phase, the absorption peak in the spectrum of H2O2 is significantly redshifted by 45 nm, corresponding to greater absorbance of photons in the sunlight spectrum and faster photolysis of H2O2. This discovery has great potential to solve current problems associated with ·OH-centered heterogeneous photochemical processes in aerosols. For instance, we show that accelerated H2O2 photolysis in microdroplets could lead to markedly enhanced oxidation of SO2 and volatile organic compounds.

Reaction Steps in Heterogeneous Photocatalytic Oxidation of Toluene in Gas Phase-A Review

A review of the current literature shows there is no clear consensus regarding the reaction mechanisms of air-borne aromatic compounds such as toluene by photocatalytic oxidation. Potential oxidation reactions over TiO2 or TiO2-based catalysts under ultraviolet and visible (UV/VIS) illumination are most commonly considered for removal of these pollutants. Along the pathways from a model pollutant, toluene, to final mineralization products (CO2 and H2O), the formation of several intermediates via specific reactions include parallel oxidation reactions and formation of less-reactive intermediates on the TiO2 surface. The latter may occupy active adsorption sites and causes drastic catalyst deactivation in some cases. Major hazardous gas-phase intermediates are benzene and formaldehyde, classified by the International Agency for Research on Cancer (IARC) as Group 1 carcinogenic compounds. Adsorbed intermediates leading to catalyst deactivation are benzaldehyde, benzoic acid, and cresols. The three most typical pathways of toluene photocatalytic oxidation are reviewed: methyl group oxidation, aromatic ring oxidation, and aromatic ring opening.

Photocatalysts for a sustainable future: Innovations in large-scale environmental and energy applications

The growing environmental and energy crises have prompted researchers to seek new solutions, including large-scale photocatalytic environmental remediation and the production of solar hydrogen using photocatalytic materials. To achieve this goal, scientists have developed numerous photocatalysts with high efficiency and stability. However, the large-scale application of photocatalytic systems under real-world conditions is still limited. These limitations arise at every step, including the large-scale synthesis and deposition of photocatalyst particles on a solid support, and the development of an optimal design with high mass transfer and efficient photon absorption. The purpose of this article is to provide a detailed description of the primary challenges and potential solutions encountered in scaling up photocatalytic systems for use in large-scale water and air purification and solar hydrogen production. Additionally, based on a review of current pilot developments, we draw conclusions and make comparisons regarding the main operating parameters that affect performance, as well as propose strategies for future research.

Amorphous SnO2 decorated ZnSn(OH)6 promotes interfacial hydroxyl polarization for deep photocatalytic toluene mineralization

Surface hydroxyl groups play a decisive role in the generation of hydroxyl radicals with stronger oxidizing ability, which is indispensable in photocatalytic VOCs removal, especially under the condition of low humidity. In this work, non-noble amorphous SnO₂ decorated ZnSn(OH)₆ (ZSH) was synthesized by an in-situ method. The charge transport, reactant activation and hydroxyl polarization are enhanced through decoration of amorphous SnO₂ on ZSH. Combined with the designed experiment, in-situ EPR, DTF calculation and in-situ DRIFTS, the role and mechanism of interfacial hydroxyl polarization are revealed on SnO₂ decorated ZnSn(OH)₆. Compared with pristine ZSH and noble-metal modified ZSH, the toluene degradation rate of amorphous SnO₂ decorated ZSH is increased by 13.0 and 3.8 times, and the toluene mineralization rate is increased by 5.2 and 2.2 times. The ZSH-24 sample maintains a high toluene degradation activity after 6 cyclic utilization without catalyst deactivation. This work emphasizes the role of non-noble metal and the origin of hydroxyl group polarization on ZnSn(OH)₆ for photocatalytic VOCs mineralization.

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.

Catalytic Oxidation of Toluene over Fe-Rich Palygorskite Supported Manganese Oxide: Characterization and Performance

A series of Fe–rich palygorskite supported manganese oxide (X%Mn–Pal) catalysts were prepared by co-precipitation method and used as catalysts for toluene oxidation. The components and structure of the as-prepared catalysts were characterized by XRD, Raman, TEM, XPS, and in situ DRIFTS. The results showed that the 15%Mn–Pal catalyst exhibited the highest catalytic activity (T90 = 227 °C) and excellent cycling stability for the oxidation of toluene compared with other catalysts. The characterization results indicated that remarkable activity of the 15%Mn–Pal catalyst for toluene oxidation should be ascribed to the abundant surface oxygen vacancies. In situ DRIFTS results elucidated that benzoate was the main intermediate, which can be further oxidized into H2O and CO2. The objectives of this study are to (i) investigate the synergistic effect between Fe and Mn for toluene oxidation, (ii) develop an efficient catalyst for toluene abatement with high activity and low–cost, and (iii) promote the application of natural Fe–rich palygorskite in the control of VOCs.

Photothermal Synergistic Effect of Pt1/CuO-CeO2 Single-Atom Catalysts Significantly Improving Toluene Removal

Photothermal synergistic catalytic oxidation of toluene over single-atom Pt catalysts was investigated. Compared with the conventional thermocatalytic oxidation in the dark, toluene conversion and CO₂ yield over 0.39Pt₁/CuO-CeO₂ under simulated solar irradiation (λ = 320-2500 nm, optical power density = 200 mW cm^-2) at 180 °C could be increased about 48%. An amount of CuO was added to CeO₂ to disperse single-atom Pt with a maximal Pt loading of 0.83 wt %. The synergistic effect between photo- and thermocatalysis is very important for the development of new pollutant treatment technology with high efficiency and low energy consumption. Both light and heat played an important role in the present photothermal synergistic catalytic oxidation. 0.39Pt₁/CuO-CeO₂ showed good redox performance and excellent optical properties and utilized the full-spectrum solar energy. Light illumination induced the generation of reactive oxygen species (^•OH and ^•O₂^-), which accelerated the transformation of intermediates, promoted the release of active sites on the catalyst surface, and improved the oxidation reaction.

Tuning the local electronic structure of SrTiO3 catalysts to boost plasma-catalytic interfacial synergy

Boosting plasma-catalyst synergy to enhance volatile organic compounds (VOCs) decomposition remains a challenge. Herein, rich oxygen vacancies (V_O) were engineered into the SrTiO₃ catalysts through a facile nitrogen incorporation strategy for the plasma-catalytic decomposition of toluene and ethyl acetate. 100% toluene conversion with 81% CO₂ selectivity at a competitive energy efficiency was achieved under ambient conditions. The characterization results and theoretical calculations evidenced that the partial substitution of oxygen by nitrogen triggered the electronic reconstruction and local disorder, thus modulating the electronic properties and coordination structures contributed to the formation of V_O-Ti³⁺ pairs. Quasi in-situ EPR, operando OES, and operando DRIFTS originally demonstrated that the V_O-Ti³⁺ pairs as active sites promoted the plasma-catalytic synergy instead of isolated V_O. Importantly, the V_O-Ti³⁺ pairs with favorable electron transfer characteristics energetically preferred to capture and utilize vibrationally excited oxygen species. And the lattice oxygen supplied by the V_O-Ti³⁺ pairs were more vigorously activated by the plasma to participate in the surface/interface reaction. This work advances our understanding of the real active sites in plasma-catalytic interfacial synergy and thus paving the way for the rational design of efficiently heterogeneous catalysts.

Atomic-level insight into effect of substrate concentration and relative humidity on photocatalytic degradation mechanism of gaseous styrene

Substrate concentration and relative humidity (RH) impact the photocatalytic efficiency of industrial aromatic hydrocarbons, but how they influence intermediate formation and degradation pathway remains unclear. With the help of oxygen isotope tracing method, the effects of these two environmental parameters on degradation mechanism of styrene were revealed at atomic level. Increasing styrene concentration favored product formation, which was however inhibited by RH elevation. Gaseous products were not directly formed in gaseous phase, but originated from desorption of interfacial intermediates. The volatile aldehydes and furans further exchanged their ¹⁶O with ¹⁸O in H₂¹⁸O. Increase of RH showed higher enhancement on ¹⁸O distribution in all products and pathways than that of substrate concentration. Low RH preferred high generation of ¹⁶O₂^•- and ⁽¹⁶⁾¹O₂, dominating reaction to form 1-phenyl-1,2-ethandiol, 2-hydroxy-1-phenyl-ethanon and phenylglyoxal monohydrate in sequence. Successive production of benzyl alcohol, benzaldehyde and benzoic acid through the reaction of styrene with promoted ^•18OH by increasing RH became predominant. Hydration was firstly observed and confirmed as an important gaseous transformation step of aldehyde and furan products. Our findings provide a deep insight into photocatalytic degradation mechanism of aromatic hydrocarbons regulated by environmental parameters to further improve their industrial purification efficiency, and are helpful predicting environmental geochemistry fate of organics and preventing their negative impact on natural environment.

Promote the activation and ring opening of intermediates for stable photocatalytic toluene degradation over Zn-Ti-LDH

Improving the selectivity of photocatalysis and reducing the generation of toxic by-products are the two key challenges for the development of highly efficient and stable photocatalysts. In this work, it was revealed that Zn-Ti-layered double hydroxide (ZT-LDH) photocatalyst, which generated less intermediates, showed better toluene degradation efficiency (removal ratio, 75.2%) and stability, compared with P25 (removal ratio, 10.9%). During the photocatalytic toluene degradation, benzaldehyde and benzoic acid were the main intermediates existed in the gas phase and on the surface of the catalyst, respectively. By combining experiments with theoretical calculation, it was found that the hydrogen atoms on the hydroxyl groups in the LDH would selectively attract the oxygen atoms in the carbon-oxygen double bond of the two major intermediates, facilitating their adsorption and activation on ZT-LDH. Besides, the surface electronic structure of ZT-LDH was demonstrated to facilitate the ring-opening reaction of the two major intermediates, eventually maintaining high activity and stability. This work could provide new molecular perspectives for understanding the photocatalytic reactions in VOCs degradation and developing efficient and stable photocatalysts.

Investigation into the Phase-Activity Relationship of MnO2 Nanomaterials toward Ozone-Assisted Catalytic Oxidation of Toluene

Manganese dioxide (MnO₂ ), with naturally abundant crystal phases, is one of the most active candidates for toluene degradation. However, it remains ambiguous and controversial of the phase-activity relationship and the origin of the catalytic activity of these multiphase MnO₂ . In this study, six types of MnO₂ with crystal phases corresponding to α-, β-, γ-, ε-, λ-, and δ-MnO₂ are prepared, and their catalytic activity toward ozone-assisted catalytic oxidation of toluene at room temperature are studied, which follow the order of δ-MnO₂  > α-MnO₂  > ε-MnO₂  > γ-MnO₂  > λ-MnO₂  > β-MnO₂ . Further investigation of the specific oxygen species with the toluene oxidation activity indicates that high catalytic activity of MnO₂ is originated from the rich oxygen vacancy and the strong mobility of oxygen species. This work illustrates the important role of crystal phase in determining the oxygen vacancies' density and the mobility of oxygen species, thus influencing the catalytic activity of MnO₂ catalysts, which sheds light on strategies of rational design and synthesis of multiphase MnO₂ catalysts for volatile organic pollutants' (VOCs) degradation.

Photocatalytic oxidation technology for indoor air pollutants elimination: A review

As more people are spending the majority of their daily lives indoors, indoor air quality has been acknowledged as an important factor influencing human health, with increasing research attention in recent decades. Indoor air pollutants (IAPs), such as volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs), can cause acute irritation and chronic diseases. Photocatalytic oxidation (PCO) technology is an efficient approach for eliminating IAPs. In this review, the development of PCO technology was explained and discussed to promote future development of PCO technology for IAP elimination. First, the health effects and the measured concentrations of typical VOCs and SVOCs in indoor environments worldwide were briefly introduced. Subsequently, the development and limitations of some typical photocatalytic reactors (including packed-bed reactors, monolithic reactors, optical fiber reactors, and microreactors) were summarized and compared. Then, the influences of operating parameters (including initial concentration of contaminants, relative humidity, space velocity, light source and intensity, catalyst support materials, and immobilization method) and the degradation pathways as well as intermediates of PCO technology were elucidated. Finally, the possible challenges and future development directions regarding PCO technology for IAP elimination were critically proposed and addressed.

Core-shell ZIF-8@MIL-68(In) derived ZnO nanoparticles-embedded In2O3 hollow tubular with oxygen vacancy for photocatalytic degradation of antibiotic pollutant

Developing a novel core-multishelled metal oxide hollow tube with rich oxygen vacancy is highly attractive in photocatalytic degradation of antibiotic pollutant. Herein, ZnO@In₂O₃ core-shell hollow microtubes were synthesized via one-step calcination of ZIF-8@MIL-68(In) formed by an in-situ self-assembly. TEM images demonstrate that 0D ZnO quantum dots (QDs) shell with 0.2 µm were well coated on the surface of 1D In₂O₃ hollow tube as the core with 1.2 µm. The synthesized heterostructure indicates the enhanced photocatalytic performance in tetracycline (TC) degradation compared with single ZIF-derived ZnO and MIL-68(In)-derived In₂O₃ under simulated solar irradiation. Besides, organic pollutants including malachite green (MG), methylene blue (MB) and rhodamine B (RhB) are further used to evaluate the photocatalytic activity of ZnO@In₂O₃, and the effect of weight ratios between ZnO and In₂O₃ on degradation efficiency is also studies. The ZnO@In₂O₃ heterojunction can provide higher specific surface area, expose more active sites, possess appropriate number of oxygen vacancies, enhance light absorption and further effectively boost the transfer and separation of photoinduced charge carriers. In addition, the proposed photocatalytic mechanism and degradation pathway are discussed in detail based on active species trapping test, electron spin resonance (ESR) and LCMS.

New Insights on Competitive Adsorption of NO/SO2 on TiO2 Anatase for Photocatalytic NO Oxidation

Here, we investigate competitive adsorption and photocatalytic reaction over TiO₂@SiO₂: NO conversion efficiency decreases by 29.1%, and the adsorption capacity decreases from 0.125 to 0.095 mmol/g due to the influence of SO₂. According to identification and comparative analysis of the IR signal, SO₂ has little effect on the NO conversion route and intermediates (adsorbed NO → nitrite → nitrate), but accelerates the deactivation of catalysts. The electronic interaction scheme from density functional theory (DFT) confirms that surface hydroxyls create an unsaturated coordination of neighboring Ti or O atoms, which is favorable for NO/SO₂ adsorption on anatase (101). In addition, the lone pair electrons of N or S atoms prefer to be delocalized and form covalent bonds with active surface-O on the (101) facet with terminal hydroxyls. However, preadsorbed SO₂ could offset the increase of hydroxyls and strongly inhibit NO adsorption, which is consistent with the result performance evaluation. A possible reaction mechanism characterized by oxygen vacancies and·O₂^- is proposed, while the essential reason of catalyst deactivation and regeneration is theoretically analyzed based on the experimental and DFT calculation.

Photocatalytic reaction mechanisms at the gas–solid interface for environmental and energy applications

Five gas–solid photocatalytic reactions including the oxidation of NOx, VOCs and NH3, and reduction of CO2 and N2 are summarized. Besides, basic properties of gas molecules, their adsorption and activation, and various reaction pathways are analyzed.