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

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.

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.

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.

Atomically dispersed Ti-O clusters anchored on NH2-UiO-66(Zr) as efficient and deactivation-resistant photocatalyst for abatement of gaseous toluene under visible light

Photocatalytic oxidation (PCO) of volatile organic compounds (VOCs) over MOF-based photocatalysts is considerably impeded by the weak activation of reactant molecules on the catalyst surface and low charge carrier mobility. In this study, we demonstrate that atomically dispersed Ti species anchored on NH₂-UiO-66(Zr) (AUiO-66(Zr/Ti)) exhibit high visible-light-responsive photocatalytic activity toward toluene vapor with an 83 % removal efficiency and 89 % CO₂ selectivity. These results are markedly superior to those reported in the literature. More importantly, AUiO-66(Zr/Ti) exhibited excellent catalytic stability during a prolonged reaction, while its pristine AUiO-66(Zr) counterpart underwent rapid catalytic deactivation after a few hours. The optimized sample, AUiO-66(Zr/Ti)-4h, provided extended visible light absorption and enhanced charge carrier mobility due to ligand-to-linker metal charge transfer. Meanwhile, the defect-rich surface of AUiO-66(Zr/Ti)-4h facilitated the activation of H₂O/toluene molecules into the critical intermediates of hydroxyl, benzoic acid, and maleic anhydride, which were effectively converted under visible light illumination. On the basis of the combined results of the PCO of toluene and material characterization, the structure - activity relationship and the related catalytic mechanism are discussed comprehensively.

Recent Developments in Photocatalytic Nanotechnology for Purifying Air Polluted with Volatile Organic Compounds: Effect of Operating Parameters and Catalyst Deactivation

Photocatalytic oxidation (PCO) is a successful method for indoor air purification, especially for removing low-concentration pollutants. Volatile organic compounds (VOCs) form a class of organic pollutants that are released into the atmosphere by consumer goods or via human activities. Once they enter the atmosphere, some might combine with other gases to create new air pollutants, which can have a detrimental effect on the health of living beings. This review focuses on current developments in the degradation of indoor pollutants, with an emphasis on two aspects of PCO: (i) influence of environmental (external) conditions; and (ii) catalyst deactivation and possible solutions. TiO2 is widely used as a photocatalyst in PCO because of its unique properties. Here, the potential effects of the operating parameters, such as the nature of the reactant, catalyst support, light intensity, and relative humidity, are extensively investigated. Then the developments and limitations of the PCO technique are highlighted, especially photocatalyst deactivation. Furthermore, the nature and deactivation mechanisms of photocatalysts are discussed, with possible solutions for reducing catalyst deactivation. Finally, the challenges and future directions of PCO technology for the elimination of indoor pollutants are compared and summarized.

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.

Novel composite of Zn/Ti-layered double hydroxide coupled with MXene for the efficient photocatalytic degradation of pharmaceuticals

In the present study, a hybrid photocatalyst of Zn/Ti layered double hydroxide (LDH) coupled with MXene - Ti₃C₂ was synthesized for the first time and applied in photocatalytic degradation of acetaminophen and ibuprofen, two commonly present in the natural environment and prone to accumulate in the aquatic ecosystem pharmaceuticals. The effect of MXene content (0.5 wt%, 2.5 wt%, and 5 wt%) on the photocatalytic activity of LDH/MXene composite was investigated. The composite of LDH/MXene containing 2.5 wt% of MXene revealed the highest photocatalytic activity in the degradation of acetaminophen (100% within 40 min) and ibuprofen (99.7% within 60 min). Furthermore, an improvement in acetaminophen and ibuprofen mineralization was observed for the composite material. Meanwhile, the introduction of interfering ions (Na⁺, Ca²⁺, Mg²⁺, Cl^-, SO₄^2-) in the model seawater did not affect the removal efficiency of both pharmaceuticals. The photocatalytic experiment performed in the four subsequent cycles, as well as FTIR, TEM, and XPS analyses after the photodegradation process confirmed the excellent stability and reusability of the prepared composite material. In order to evaluate the effect of various reactive oxidizing species (ROS) on the photocatalytic process, the trapping experiment was applied. It was noticed that •O₂^- had the main contribution in photocatalytic degradation of acetaminophen, while •OH and h⁺ mainly affected the degradation of ibuprofen. Finally, based on the results of Mott Schottky analysis, bandgap calculation, and ROS trapping experiment, the possible mechanism for pharmaceuticals degradation was proposed. This research illustrates the feasibility and novelty of the treatment of pharmaceuticals by LDH/MXene composites, implying that MXene plays a significant role in the electron-hole separation and thus high photocatalytic activity.

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.

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.

Enhanced photocatalytic driven hydroxylation of phenylboric acid to phenol over pyrenetetrasulfonic acid intercalated ZnAl-LDHs

Layered double hydroxides (LDHs) own admirable potential due to their controllable composition and exchangeable interlayer anions. Herein, pyrenetetrasulfonic acid (PTS) intercalated ZnAl-LDHs (denoted as ZnAl-xPTS, x represents the amount of NaPTS in the starting material) are synthesized by a co-precipitation method, which display enhanced photocatalytic activity towards the hydroxylation of phenylboric acid to phenol. Various characterizations suggest that PTS plays significant roles in improving the photocatalytic activity: (1) PTS extends the light absorption from ultraviolet to visible light region; (2) the introduction of PTS upshifts the conduction band, which is feasible for the formation of O₂^∙-; (3) ZnAl-xPTS produces more free electrons under light irradiation, which leads to greatly improved activity. This study develops an alternative LDHs based photocatalyst for the production of phenol, which also provides an efficient strategy to improve the photocatalytic activity of LDHs.