A review of the influence of humidity on photocatalytic decomposition of gaseous pollutants on TiO2-based catalysts

Humidity can have an appreciable influence on the photocatalytic degradation of volatile organic compounds (VOCs) in air. It has been proposed that there are multiple layers of water molecules on the surface of the photocatalyst, and a VOC molecule must penetrate this layer and be able to diffuse to the photocatalytic surface for a reaction to occur. An updated analysis of related literature from the past 12 years indicates that this proposed mechanism remains reasonable for explaining observed effects. According to this mechanism the VOC's hydrophilicity should dominate the influence of humidity on the photocatalytic reaction kinetics, and this effect can be seen in a categorization of literature observations based on the octanol-water partition coefficient. The frequently cited Langmuir-Hinshelwood kinetic model does not directly account for humidity effects in this way but can be adapted for this purpose. As an example, the rate of photocatalytic decomposition of chlorobenzene in air under humid conditions can be described by a modified Langmuir-Hinshelwood kinetic model, where the reaction rate coefficient is dependent on the water concentration. Understanding and considering the effect of humidity is useful for optimizing the efficacy of photocatalytic VOC treatment of air.     

纳米光催化剂降解室内微量苯类挥发性有机化合物

社会的迅速发展给人类带来物质文明的同时,也使得环境污染变得日益严重,特别是在近年来由人类活动产生的气态污染物严重威胁到环境和人体健康。习近平总书记曾说过,绿水青山就是金山银山。但是,气态污染物相继排放到大气中,促使人们有必要开发出合适的技术来净化空气。苯类挥发性有机化合物(苯类VOCs)是室内空气中常见的气态污染物之一,应该经过物理吸附并转化成无危害的化合物等。光催化氧化技术(PCO)可以在光照作用下,利用光子的能量使光催化剂表面产生高活性物质来有效降解苯类VOCs,并且不会产生二次污染。目前,已有许多不同结构和物理化学性能的光催化体系被应用于降解苯类VOCs,本文首先对光催化降解苯类VOCs的基本原理、影响光催化反应的因素和提高降解效率的方法进行了系统分析,综述了各类纳米催化剂的设计理念以及其在光催化降解苯类VOCs过程中的性能,并对未来纳米催化剂的设计进行了展望。     

光热协同催化净化挥发性有机物的研究进展及展望

挥发性有机物（VOCs）是一类重要的空气污染物。催化氧化技术可以将VOCs转化为无毒的CO₂和H₂O,是有效的治理方式之一。针对传统的热催化氧化技术的高能耗和光催化净化VOCs技术的低效率问题,光催化耦合强化热催化的光热协同催化净化VOCs技术近些年来受到广泛关注,并表现出比传统热催化或光催化净化技术更优异的净化性能。总结了近年国内外研究者在光热催化净化VOCs领域所取得的主要研究进展,重点讨论了光热协同作用机制的认知和光热协同催化材料的设计理念,包括贵金属型和金属氧化物型光热协同催化材料,并对光热协同催化净化技术的未来发展方向进行了展望。     

Carbonaceous-TiO2 nanomaterials for photocatalytic degradation of pollutants: A review

Semiconductor photocatalysis is one of most appealing and attractive technologies, which has been directly utilized to harvest solar energy for energy and environmental applications. Titanium dioxide (TiO₂) has been proved to be leading semiconductor photocatalyst for the degradation of pollutants. However, it suffers from low photocatalytic activity under visible light activation because of its intrinsic wide band gap. Various strategies have been developed to enhance TiO₂ efficiency in the visible light region. Among them TiO₂ modification with carbonaceous nanomaterials is very effective route for excellent photocatalytic activity. This critical review aims to present recent progress in the design and synthesis of carbonaceous-TiO₂ photocatalysts, covering carbon doping, activated carbon, fullerenes, carbon nanotubes and graphene. Moreover, proposed mechanisms of enhancement, effect of synthesis routes, demonstrations of performance and applications reported in literature are reviewed. Ongoing challenges and possible new directions are outlined.     

影响光催化降解VOCs可操作因素的研究进展

光催化处理可挥发性有机物（VOCs）技术发展迅速,影响该过程的可操作因素较多,本文系统总结了影响光催化降解VOCs的因素和研究方法,围绕光催化反应过程和动力学规律分析,阐明了光传递、目标物的浓度和气速、系统湿度、反应器等因素对光催化降解的作用机理,揭示了光催化反应过程存在的问题和今后的研究方向。     

Photocatalytic degradation of gaseous trichloroethene using immobilized ZnO/SnO2 coupled oxide in a flow‐through photocatalytic reactor

The photocatalytic degradation of gaseous trichloroethene (TCE) was investigated on immobilized ZnO/SnO₂ coupled oxide in a flow‐through photocatalytic reactor. It was found that gaseous photocatalysis is an efficient method for volatile organic compounds' abatement and air purification. Degradation of ～100% was found for TCE at the concentrations examined, up to 400 ppmv, in a flow‐through dry synthetic gas stream. In our tested conditions, the flow rate had little influence on the photocatalytic degradation efficiencies of TCE, while the relative humidity had a significant influence on the photocatalytic degradation of TCE. The photocatalytic degradation efficiencies of TCE increased slowly below 20% relative humidity and then decreased as the relative humidity increased further. The deactivation of used immobilized photocatalyst was not observed within the 200 h testing period in the present experiment, although the surface of the photocatalyst changed greatly during the use of the photocatalyst. Copyright © 2004 Society of Chemical Industry     

Surface modifications,perspectives, and challenges of scheelite metal molybdate photocatalysts for removal of organic pollutants in wastewater

Semiconductor-mediated photocatalysis is a promising green technology which has been widely adopted for solving energy and environmental issues. Among the semiconductor photocatalysts, the scheelite tetragonal structure of metal molybdate materials have attracted tremendous research attention in the application for photocatalytic degradation of organic pollutants due to their diverse band gaps, unique physical and chemical properties, ecofriendly characters, abundant sources of molybdenum (Mo) element, and low cost. However, the fast recombination of photogenerated electron-hole pairs results in decreased efficiency of the photocatalysis. Thus, this review covers the progresses in various unmodified and modified scheelite metal molybdate photocatalysts, which include PbMoO₄, BaMoO₄, CaMoO₄, SrMoO₄, and CdMoO₄. The modification methods are categorized into morphology control, facet modification, doped, plasmonic, and heterojunction, which are devoted to the degradation of organic pollutants (mostly, dyes and drugs) and to overcome the rapid recombination of electron-hole pairs. Furthermore, the challenges and future direction as well as the perspectives toward the designation for more efficient scheelite metal molybdate photocatalysts are discussed through extensive literature review.     

Plant Volatile Organic Compounds (VOCs) in Ozone (O<Subscript>3</Subscript>) Polluted Atmospheres: The Ecological Effects

Tropospheric ozone (O₃) is an important secondary air pollutant formed as a result of photochemical reactions between primary pollutants, such as nitrogen oxides (NO_x), and volatile organic compounds (VOCs). O₃ concentrations in the lower atmosphere (troposphere) are predicted to continue increasing as a result of anthropogenic activity, which will impact strongly on wild and cultivated plants. O₃ affects photosynthesis and induces the development of visible foliar injuries, which are the result of genetically controlled programmed cell death. It also activates many plant defense responses, including the emission of phytogenic VOCs. Plant emitted VOCs play a role in many eco-physiological functions. Besides protecting the plant from abiotic stresses (high temperatures and oxidative stress) and biotic stressors (competing plants, micro- and macroorganisms), they drive multitrophic interactions between plants, herbivores and their natural enemies e.g., predators and parasitoids as well as interactions between plants (plant-to-plant communication). In addition, VOCs have an important role in atmospheric chemistry. They are O₃ precursors, but at the same time are readily oxidized by O₃, thus resulting in a series of new compounds that include secondary organic aerosols (SOAs). Here, we review the effects of O₃ on plants and their VOC emissions. We also review the state of current knowledge on the effects of ozone on ecological interactions based on VOC signaling, and propose further research directions.     

掺杂Fe~(3+)纳米TiO_2膜光催化降解VOCs

利用玻璃弹簧负载TiO_2膜光催化降解VOCs,以室内空气污染物甲醛、丙酮和甲苯为目标污染物,采用自制反应器,分析了初始浓度、空气湿度和气体流量对甲苯降解效果的影响,考察了三种物质混合的降解效果,对比了在相同条件下单一甲苯和VOCs中甲苯的降解效果。结果表明,气体流速5 L·min~(-1)时,甲苯的降解效率为74.6%,空气湿度为30%时,甲苯降解率为81.6%,初始浓度为0.173 mg·L~(-1)时,甲苯降解率为94%,在这三种情况下,甲苯降解效果较好;对VOCs中甲苯和单一甲苯的降解效果进行对比,发现混合气体中甲苯的降解率在反应205 min后高于单一甲苯。     

Indoor air quality and the emissions of VOCs from interior products

Indoor air quality (IAQ) has developed into an important environmental and health concern. IAQ legislation has been introduced in the U.S. Congress. Volatile organic compounds (VOCs) comprise a part of the total concern about IAQ. Types and sources of pollutants contributing to IAQ are identified. Some strategies for reducing VOCs in indoor air are provided, and various sources of VOCs are identified. Various means of reducing VOC emissions from products are presented. Government regulatory agencies have begun to introduce legislation focusing on regulation of IAQ. Discussed is how these regulations, some proposed legislation, and customer concerns will affect both product manufacturers and their suppliers.     

Photocatalytic Degradation of Composites with Magnesium Aluminum Hydrotalcite Derived Metal Oxides and g-C3N4

The remove of the organic pollutants in water plays an important role on the environmental protection, thus photocatalysis, as an effective method, has been attained much attention to deal with this problem. In this paper, a composite of g-C₃N₄ and Mg–Al hydrotalcite derived metal oxides was prepared by simple co-precipitation methods followed by calcination. The effective adsorption-photocatalysis synergic effect and highly efficient removal of organic pollutants from water under simulated sunlight irradiation was found in the presence of fabricated metal oxide nanocomposites. The photocatalytic degradation rate of methylene blue reached 97.3% within 1 h under visible light, and the degradation rate constant was 0.0432 min^?1, which is 3.6 times that of g-C₃N₄. Though characterization analysis, more active sites are exposed to absorb more organic pollutants on the mater surface in the presence of the open hierarchical structures of composite materials, and the "face to face" contact structure between 2D materials is formed to promote the separation of electrons and holes. The photocatalytic degradation was stable in the recycling process of the catalyst. In addition, the photocatalytic degradation mechanism of the catalyst was explained by free radical scavenging experiment. The material provides an effective method for removing organic pollutants in water, and has broad application prospects.

     

A review on mechanism,applications and influencing factors of carbon quantum dots based photocatalysis

CQDs (carbon quantum dots) have attracted a lot of attention in the field of photocatalysis due to its absorption of visible light, up-conversion luminescence, rich free groups on the surface and low cost. CQDs doped semiconductor can improve the photocatalytic reaction rate by the following three points: (1) adjust the band structure of photocatalyst; (2) facilitate the absorption of more visible light; (3) facilitate electron transfer and inhibits electron-hole recombination. In this review, the mechanism (photosensitizer, electron acceptor, up-conversion luminescence, etc.) and applications (photocatalytic degradation of organic pollutants, reduction of heavy metals, etc.) of CQDs in the field of photocatalysis are briefly introduced. Finally, the factors affecting the photocatalytic activity were summarized in order to adjust the reaction conditions and show high catalytic activity. It is hoped that this review can provide insights and inspiration for the development of CQDs in the field of photocatalysis.     

Effect of pre‐treatments based on UV photocatalysis and photo‐oxidation on toluene biofiltration performance

BACKGROUND: The integration of UV photocatalysis and biofiltration seems to be a promising combination of technologies for the removal of hydrophobic and poorly biodegradable air pollutants. The influence of pre‐treatments based on UV_{254 nm} photocatalysis and photo‐oxidation on the biofiltration of toluene as a target compound was evaluated in a controlled long‐term experimental study using different system configurations: a standalone biofilter, a combined UV photocatalytic reactor‐biofilter, and a combined UV photo‐oxidation reactor (without catalyst)‐biofilter. RESULTS: Under the operational conditions used (residence time of 2.7 s and toluene concentrations 600–1200 mg C m^?3), relatively low removal efficiencies (6–3%) were reached in the photocatalytic reactor and no degradation of toluene was found when the photo‐oxidation reactor was operated without catalyst. A noticeable improvement in the performance of the biofilter combined with a photocatalytic reactor was observed, and the elimination capacity of the biological process increased by more than 12 g C h^?1 m^?3 at the inlet loads studied of 50–100 g C h^?1 m^?3. No positive effect on toluene removal was observed for the combination of UV photoreactor and biofilter. CONCLUSIONS: Biofilter pre‐treatment based on UV_{254 nm} photocatalysis showed promising results for the removal of hydrophobic and recalcitrant air pollutants, providing synergistic improvement in the removal of toluene. Copyright © 2011 Society of Chemical Industry     

Removal of VOCs from gas streams via plasma and catalysis

Emission of volatile organic compounds (VOCs) has resulted in various environmental issues. Therefore, development of effective VOC removal technology is essential for reducing the adverse effects associated. This work provides a systematic review on VOC removal from gas stream via catalytic oxidation, plasma degradation, and plasma catalysis. For catalytic oxidation of VOCs, possible reaction mechanisms and how physicochemical properties of catalyst influences catalytic performance are presented and discussed, followed by plasma removal of VOCs, VOC degradation, and byproduct formation mechanisms. Next, interactions between plasma and catalyst are interpreted for comprehensive understanding. Last, perspectives are provided for further development of VOC removal technology.     

纳米二氧化钛光催化降解水中有机污染物的研究

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Photocatalytic degradation of gaseous perchloroethylene in continuous flow reactors: Rate enhancement by chlorine radicals

The degradation of perchloroethylene (PCE) by UV/TiO₂ photocatalysis in gas phase was studied. The degradation efficiency has been compared in different continuous flow reactors: a photocatalytic tangential reactor (PTR) where the air flows tangentially over the catalytic medium and two photocatalytic filtering reactors (PFR) where the air flows through the porous catalytic medium. The degradation rate shows a linear dependence with the concentration of pollutants (up to 350 mg PCE/N m³) for the PTR, but the degradation was negligible for the PFR. The degradation rate was enhanced by accelerating the chlorine radicals’ formation (by adding HCl in catalytic quantity in the air flow or by PCE over-heating). In these conditions, the oxidation rate constant of PCE in the PFR was about five times higher than that in the PTR, although the mass of catalyst involved in the PFR was about 10 times lower and the contact time was about a 1000 times shorter than that of the PTR. Thus, the catalyst is globally more efficiently used in the PFR, as the mass transfer is not limiting. As a result, a degradation mechanism of PCE, involving the generation of free chlorine radicals, as the first limiting step, has been confirmed.     

On Vehicle VOC and NOx Emissions from the Standpoint of the Tropospheric Ozone Problem

Ozone is formed in and downwind of urban areas from urban emissions of NO_x and volatile organic compounds (VOCs) in the presence of sunlight. The main sources of VOCs in polluted air are motor vehicles, industrial solvents, processes in the petroleum and chemical industries, and vegetation. The main NO_x sources are stationary-source fuel combustion (mainly electric utilities) and motor vehicles. Recent studies have demonstrated that VOC emissions from motor vehicles have been seriously underestimated, and this may well explain why ambient O₃ has not responded well to control efforts. This review presents an overview of the sources, formation, and potential abatement strategies for O₃ pollution in the troposphere, with particular emphasis on the mobile source contribution to O₃ formation.     

Photocatalytic Degradation of Organic Pollutants Under Visible Light Irradiation

Several TiO₂-based photocatalytic systems that have considerable visible light response have been developed, such as the photodegradation of organic pollutants on sensitized TiO₂ by visible light, construction of visible-light-active novel TiO₂ photocatalysts by matrix or surface modification. In this paper, we review briefly our recent progress in the TiO₂ photocatalytic degradation of organic pollutants by visible light, some related work by other groups is also involved.     

Microbial cellulose as a support for photocatalytic oxidation of toluene using TiO2 nanoparticles

The aim of this study was to investigate the feasibility of toluene degradation using impregnated microbial cellulose (MC) with titanium dioxide (TiO₂) nanoparticles (MC/TiO₂). The effects of the initial toluene concentration and ultraviolet (UV) source on the degradation efficiency of toluene have been evaluated. The experimental results showed that the rate of toluene degradation decreased with an increasing of the inlet toluene concentration. After 40 min reaction time, the decomposition rate (%) of toluene decreased from 72.3% to 36.02% for experiments conducted at 100 and 500 ppm, respectively. The degradation efficiency of toluene decreased with application of UVA source instead of UVC source. The toluene degradation efficiency (%) reached to 87.79% and 76.87% for UVC and UVA irradiation, respectively. At initial toluene concentration of 100 mg/L, toluene degradation efficiency for photocatalysis and photolysis processes were 70.2% and 10.65%, respectively; indicating that the photocatalytic degradation efficiency is significantly higher than that of photolytic degradation efficiency. Furthermore, photocatalytic degradation kinetics of toluene was studied and the rates of degradation were found to conform to pseudo‐second‐order kinetic. As shown in the present study, impregnation of TiO₂ nanoparticles on MC/TiO₂ significantly increases toluene removal for short exposure time. It can be concluded that the MC acted as a local toluene concentrator by adsorbing pollutants from the air stream, and thereby diffusing them to the TiO₂ nanoparticles for photodegradation. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43051.