It is well known that urbanization and industrialization have resulted in the rapidly increasing emissions of volatile organic compounds (VOCs), which are a major contributor to the formation of secondary pollutants (e.g., tropospheric ozone, PAN (peroxyacetyl nitrate), and secondary organic aerosols) and photochemical smog. The emission of these pollutants has led to a large decline in air quality in numerous regions around the world, which has ultimately led to concerns regarding their impact on human health and general well-being. Catalytic oxidation is regarded as one of the most promising strategies for VOC removal from industrial waste streams. This Review systematically documents the progresses and developments made in the understanding and design of heterogeneous catalysts for VOC oxidation over the past two decades. It addresses in detail how catalytic performance is often drastically affected by the pollutant sources and reaction conditions. It also highlights the primary routes for catalyst deactivation and discusses protocols for their subsequent reactivation. Kinetic models and proposed oxidation mechanisms for representative VOCs are also provided. Typical catalytic reactors and oxidizers for industrial VOC destruction are further discussed. We believe that this Review will provide a great foundation and reference point for future design and development in this field.

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Recent Advances in the Catalytic Oxidation of Volatile Organic Compounds: A Review Based on Pollutant Sorts and Sources.

Abatement of various types of VOCs by adsorption/catalytic oxidation: A review

Artificial Z-scheme photocatalyst can not only reduce the recombination of photogeneraged electron–holepairs, but also retain prominent redox ability. In this study, direct solid-state dual Z-scheme WO3/g-C3N4/Bi2O3 photocatalyst was successfully synthesized by one step co-calcination stratage using tungstic acid, melamine and bismuth (III) nitrate pentahydrate as the precursors. Surface, morphological, and structural properties of the resulting materials were comprehensive characterized by XRD, XPS, SEM, TEM, UV–vis diffuse reflection spectroscopy, BET surface areas, photoluminescence and ESR analysis. The WO3/g-C3N4/Bi2O3 composite exhibited superior photocatalytic activities for tetracycline degradation than that of pure g-C3N4, WO3, Bi2O3 and their binary composites under visible light irradiation. The enhanced photocatalytic performance of WO3/g-C3N4/Bi2O3 composite can be ascribed to improved visible light absorption, increased surface area and enhanced separation efficiency of photo-generated electron-hole pairs. In addition, the photocatalyst exhibits high stability and reusability. On the basis of the results, a novel direct solid-state dual Z-scheme photocatalytic mechanism was also proposed.

In-situ synthesis of direct solid-state dual Z-scheme WO3/g-C3N4/Bi2O3 photocatalyst for the degradation of refractory pollutant

Nanoscale Bi-based photocatalysts are promising candidates for visible-light-driven photocatalytic environmental remediation and energy conversion. However, the performance of bulk bismuthal semiconductors is unsatisfactory. Increasing efforts have been focused on enhancing the performance of this photocatalyst family. Many studies have reported on component adjustment, morphology control, heterojunction construction, and surface modification. Herein, recent topics in these fields, including doping, changing stoichiometry, solid solutions, ultrathin nanosheets, hierarchical and hollow architectures, conventional heterojunctions, direct Z-scheme junctions, and surface modification of conductive materials and semiconductors, are reviewed. The progress in the enhancement mechanism involving light absorption, band structure tailoring, and separation and utilization of excited carriers, is also introduced. The challenges and tendencies in the studies of nanoscale Bi-based photocatalysts are discussed and summarized.

Review on nanoscale Bi-based photocatalysts

To elucidate the role of oxygen vacancy defects, various Mn-based oxides with oxygen vacancy defects are employed to the toluene oxidation, which are synthesized by adjusting solvent and double-complexation routes. The MnOx-ET catalyst shows the highest catalytic activity (T90 = 225 °C) for toluene oxidation. Compared with other Mn-based oxides, the as-prepared MnOx-ET catalyst has more surficial oxygen vacancies and good oxygen storage capacity (OSC), which is the reason on its remarkable activity for toluene oxidation. In addition, in situ DRIFTS study reveals that both lattice oxygen and adsorbed oxygen species can participate in the activation-oxidation process of toluene, which results in two reaction routes for the toluene oxidation. The rich oxygen-vacancy concentration of catalysts accelerates the key steps for the activation and generation of oxidized products. Quasi-in situ XPS results further confirm that enrich adsorbed-oxygen species as active oxygen and increasing Mn4+ concentration enhance the superior activity for toluene oxidation.

Highly efficient mesoporous MnO2 catalysts for the total toluene oxidation: Oxygen-Vacancy defect engineering and involved intermediates using in situ DRIFTS

The three-dimensionally ordered macroporous (3DOM) BiVO 4 and its supported iron oxide ( x Fe 2 O 3 /3DOM BiVO 4 , x  = 0.18, 0.97, and 3.40 wt%) photocatalysts were prepared using the ascorbic acid-assisted polymethyl methacrylate-templating and incipient wetness impregnation methods, respectively. Physicochemical properties of the materials were characterized by means of numerous analytical techniques, and their photocatalytic activities were evaluated for the degradation of 4-nitrophenol under visible light illumination. It is found that the BiVO 4 possessed a high-quality 3DOM architecture with a monoclinic crystal phase, and the Fe 2 O 3 was highly dispersed on the surface of 3DOM BiVO 4 . The x Fe 2 O 3 /3DOM BiVO 4 samples much outperformed the 3DOM BiVO 4 sample, and 0.97Fe 2 O 3 /3DOM BiVO 4 showed the best photocatalytic performance (98% 4-nitrophenol was degraded in the presence of 0.6 mL H 2 O 2 within 30 min of visible light illumination) and excellent photocatalytic stability. The introduction of H 2 O 2 to the reaction system could promote the photodegradation of 4-nitrophenol by providing the active OH species generated via the reaction of photoinduced electrons and H 2 O 2 . The pseudo-first-order reaction rate constants (0.0876–0.1295 min −1 ) obtained over x Fe 2 O 3 /3DOM BiVO 4 were much higher than those (0.0033–0.0395 min −1 ) obtained over 3DOM or Bulk BiVO 4 and Fe 2 O 3 /Bulk BiVO 4 , with the 0.97Fe 2 O 3 /3DOM BiVO 4 sample exhibiting the highest rate constant. The enhanced photocatalytic performance of 0.97Fe 2 O 3 /3DOM BiVO 4 was associated with its unique porous architecture, high surface area, Fe 2 O 3  − BiVO 4 heterojunction, good light-harvesting ability, high adsorbed oxygen species concentration, and excellent separation efficiency of photogenerated electrons and holes as well as the photo-Fenton degradation process.

Fe2O3/3DOM BiVO4: High-performance photocatalysts for the visible light-driven degradation of 4-nitrophenol

Three-dimensionally ordered macroporous CeO2-supported Pd@Co nanoparticles: Highly active catalysts for methane oxidation

Palladium-based catalysts are highly active for eliminating volatile organic compounds. Reducing the use of noble metals and enhancing performance of a catalyst are always desirable. The three-dimensionally ordered macroporous (3DOM) Mn2O3-supported transition metal M (M = Mn, Cr, Fe, and Co)-doped Au–Pd nanoparticles (NPs) with an Au–Pd–xM loading of 1.86–1.97 wt% were prepared using the modified polyvinyl alcohol-protected reduction method. It is found that the Au–Pd–xM NPs with a size of 3.6–4.4 nm were highly dispersed on the surface of 3DOM Mn2O3. The 1.94 wt% Au–Pd–0.21Co/3DOM Mn2O3 and 1.94 wt% Au–Pd–0.22Fe/3DOM Mn2O3 samples performed the best for the oxidation of methane and o-xylene, respectively. The methane oxidation rate at 340 °C (339.0 × 10−6 mol/(gPd s)) over 1.94 wt% Au–Pd–0.21Co/3DOM Mn2O3 was three times higher than that (93.8 × 10−6 mol/(gPd s)) over 1.97 wt% Au–Pd/3DOM Mn2O3, and the o-xylene reaction rate at 140 °C (2.59 μmol/(gN s) over 1.94 wt% Au–Pd–0.22Fe/3DOM Mn2O3 was two times higher than that (0.93 μmol/(gN s) over 1.97 wt% Au–Pd/3DOM Mn2O3. It is concluded that doping a certain amount of the transition metal to Au–Pd/3DOM Mn2O3 could modify the microstructure of the alloy NPs, thus improving the oxygen activation and methane adsorption ability. We are sure that the M-doped Au–Pd/3DOM Mn2O3 materials are promising catalysts for the efficient removal of volatile organic compounds.

Effect of transition metal doping on the catalytic performance of Au–Pd/3DOM Mn2O3 for the oxidation of methane and o-xylene

Leaf-like monoclinic BiVO4 and yCoxPd/BiVO4 (CoxPd loading (y) = 0.060 − 0.092 wt%; Co/Pd molar ratio (x) = 0.26 − 1.70) photocatalysts were prepared using the hydrothermal and polyvinyl alcohol-protected reduction methods, respectively. The Co−Pd nanoparticles (NPs) with a size of 4–6 nm were well dispersed on the surface of leaf-like BiVO4. The bimetallic Co−Pd-loaded BiVO4 samples performed much better than the monometallic Co- or Pd-loaded counterpart, with the 0.062Co1.70Pd/BiVO4 sample showing the best photocatalytic performance (the time for 90% phenol removal was 3 h under visible light irradiation) and good photocatalytic stability. The pseudo-first-order reaction rate constants (0.4753 − 0.8367 h−1) obtained over yCoxPd/BiVO4 were much higher than those (0.0619 − 0.3788 h−1) obtained over BiVO4, 0.058Co/BiVO4, and 0.083Pd/BiVO4, with the 0.062Co1.70Pd/BiVO4 sample possessing the highest rate constant. In-depth investigations of X-ray photoelectron spectroscopy, photoluminescence spectroscopy, and photoelectrochemical measurements reveal that high dispersion of bimetallic Co–Pd NPs increased the surface Pd0 and superoxide anion radical concentrations and suppressed the recombination of photoinduced electrons and holes (hence enhancing the photocatalytic activity of yCoxPd/BiVO4). The partial deactivation of the 0.062Co1.70Pd/BiVO4 sample after 15 h of three recycle tests was mainly due to the decrease in adsorbed oxygen species concentration. In addition, the possible photocatalytic phenol degradation mechanism over the 0.062Co1.70Pd/BiVO4 sample was also proposed. We believe that the BiVO4-supported Co–Pd NPs have promising applications for the photocatalytic elimination of organic pollutants in wastewater.

Co–Pd/BiVO4: High-performance photocatalysts for the degradation of phenol under visible light irradiation

Polymethyl methacrylate-templating and ethylene glycol reduction methods were adopted to prepare the nanosized Pt catalysts (x wt% Pt/3DOM Mn2O3; x = 0.2–2.3) that were partially embedded in the skeleton of three-dimensionally ordered macroporous (3DOM) Mn2O3. These materials possessed a surface area of 33–36 m2/g, with the Pt NPs (3.6–4.4 nm in size) being well embedded in the skeleton of 3DOM Mn2O3. The 2.3 wt% Pt/3DOM Mn2O3 sample showed the best activity and the lowest apparent activation energy (41 kJ/mol) for toluene combustion, which was related to its high adsorbed oxygen species concentration and good low-temperature reducibility. Compared with 2.0 wt% Pt/3DOM Mn2O3-imp derived from the colloid adsorption method, 2.3 wt% Pt/3DOM Mn2O3 exhibited a better catalytic stability within 60 h of toluene combustion. After calcination at 650 °C for 3 h, the average particle size of Pt nanoparticles (NPs) in 2.3 wt% Pt/3DOM Mn2O3 grew up slightly from 4.3 to 4.9 nm and toluene conversions decreased slightly, while that of Pt NPs in 2.0 wt% Pt/3DOM Mn2O3-imp increased greatly from 4.4 to 13.7 nm and toluene conversions dropped significantly. Effects of H2O, CO2, and SO2 on activity of 2.3 wt% Pt/3DOM Mn2O3 and 2.0 wt% Pt/3DOM Mn2O3-imp were also examined. Partial deactivation induced by H2O or CO2 addition was reversible, whereas that due to SO2 introduction was irreversible. It is concluded that the strong interaction between Pt NPs and 3DOM Mn2O3 was responsible for excellent stability of the partially Pt-embedded 3DOM Mn2O3 sample in toluene combustion.

Kunfeng Zhang

Papers

Fe2O3/3DOM BiVO4: High-performance photocatalysts for the visible light-driven degradation of 4-nitrophenol

Three-dimensionally ordered macroporous CeO2-supported Pd@Co nanoparticles: Highly active catalysts for methane oxidation

Effect of transition metal doping on the catalytic performance of Au–Pd/3DOM Mn2O3 for the oxidation of methane and o-xylene

Co–Pd/BiVO4: High-performance photocatalysts for the degradation of phenol under visible light irradiation

Partially embedding Pt nanoparticles in the skeleton of 3DOM Mn2O3: An effective strategy for enhancing catalytic stability in toluene combustion