Progress of catalytic oxidation of typical chlorined volatile organic compounds (CVOCs): A review

Insight into the reaction mechanism of NH3-SCR and chlorobenzene oxidation over Mn-based spinel catalysts

To evaluate potential of Mn-based spinel catalysts for multi-pollutant removal applications, a series of Mn-based spinel catalysts were developed and tested for NH3 selective catalytic reduction (NH3-SCR) reaction and chlorobenzene catalytic oxidation. It was found that the CrMn2O4 spinel catalysts showed the best NH3-SCR activity and chlorobenzene catalytic removal activity among these Mn-based spinel catalysts. A NO removal efficiency above 90 % was achieved in the range of 163-283 °C with an apparent activation energy of 32.26 kJ/mol, whereas 90 % of chlorobenzene removal was achieved at nearly 300 °C with an apparent activation energy of 61.41 kJ/mol. CrMn2O4 exhibits the good performance for simultaneous removal of NO and chlorobenzene in the temperature range of 305-315 °C. Stability tests indicates that 6 vol% water inhibits the NH3-SCR reaction, but promoted the chlorobenzene oxidation and CO2 yield. Its porous and fluffy structure provides a large specific surface area of 29.32 m2/g and facilitates the adsorption of reactants. The DFT calculations were used to investigate the valence effect of different A-site metal ions on elemental Mn and the adsorption of reactant molecules on the surface. The results indicate that Mn atoms exhibit a variety of oxidation states and are strongly electrophilic in CrMn2O4 spinel. DFT and in situ DRIFTS were combined to reveal the reaction mechanisms of NH3-SCR and chlorobenzene oxidation. This study lays the foundation for the application of high-performance Mn-based spinel catalysts in multi-pollution abatement.

Synergistic Enhancement of Hydrolysis-Oxidation Drives Efficient Catalytic Elimination of Chlorinated Aromatics over VOx/TiO2 Catalysts at Low Temperature

The efficient catalytic elimination of toxic chlorinated aromatics (i.e., dioxins, chlorobenzenes, etc.) at low temperature is still a great challenge. Based on the VOx/TiO2 catalyst, a hydrolysis-oxidation strategy (CeOx and WOx doping) was built for desirable low-temperature catalytic activity, product selectivity, H2O tolerance, and chlorine desorption. The in situ and ex situ experimental characterizations and density functional theory calculations revealed that hydrolysis sites favored molecular adsorption, C-Cl cleavage, and HCl formation; meanwhile, oxidation sites enhanced the activation of reactive oxygen species and improved oxygen mobility and redox properties. The enhanced oxygen storage/release capacity (33-53 fold) and extended redox cycle (e.g., from V5+↔V4+ to V5+↔V4+↔V3+) favored the deep oxidation. The introduction of H2O triggered the hydrolysis-oxidation process that promoted the catalytic activity and chlorine desorption due to the elevated generation of ·O2- and higher-activity ·OH. Furthermore, the water resistance of the VOx/TiO2-based catalyst was enhanced after the application of the hydrolysis-oxidation strategy. The V-Ce-W/Ti catalyst exhibited remarkable removal efficiency of dioxins (96.7-98.2%), which was reduced from 0.34-0.48 ng I-TEQ Nm-3 to 0.006-0.016 ng I-TEQ Nm-3 during pilot tests at 160-180 °C, achieving ultralow emissions. This work provides practical guidance for industry development for efficiently eliminating chlorinated organics in flue gas.

Boosting biodegradation and mineralization efficiencies of chlorinated VOCs: The synergy of H2O2 and biotrickling filtration

This study explores a new biodegradation process of H2O2-stimulated BTFs to remove perchloroethylene (PCE) from the contaminated air stream. BTF stimulated with H2O2 significantly boosted PCE biodegradation compared to conventional methods. Optimal parameters for H2O2-assisted PCE biodegradation were also identified (nominal inlet concentration = 150 ppm, EBCT = 30 s, H2O2/PCE molar ratio = 0.2). Under these optimum conditions, the BTF achieved a maximum PCE removal efficiency of around 95 %, a mineralization rate of 65 %, and an elimination capacity of 117 g/m3.h, demonstrating its effectiveness. The BTF maintained stable performance under various PCE loads, suggesting its applicability for industrial cases. Moreover, the study revealed a positive correlation between increasing H2O2/PCE ratios and the activities of key enzymes responsible for biodegradation including dehydrogenase)increased from 1.3 to 26.1 mg-TF/gbiomass), peroxidase (increased from 0 to 251 U/gbiomass), and catalase (increased from 0 to 7.8 U/gbiomass) when the ratio was increased from 0 to 0.2. The H2O2-stimulated BTF performed efficiently for biodegradation (>90 %) and mineralization (>60 %) of PCE at empty bed contact times between 20 and 60 s. Finally, the absence of PCE and toxic intermediates in the recycled liquid supports the efficient biodegradation of PCE in the developed system. In conclusion, H2O2-stimulated BTF shows promise as a sustainable and cost-effective approach for biodegradation of chlorinated organic compounds in contaminated air streams.

Exceptional performance of chlorobenzene oxidation on antimony-loaded commercial selective catalytic reduction catalyst as a co-benefit of nitrogen oxides reduction: Notable enhancement of chlorobenzene oxidation due to antimony loading

Chlorobenzene (CB) oxidation using commercial selective catalytic reduction catalysts as a co-benefit of nitrogen oxides (NOx) elimination in terms of the synergic temperature window and polychlorinated byproducts formation was unsatisfactory. Herein, antimony (Sb) was loaded onto V2O5-MoO3/TiO2 (VMoTi) to enhance its performance for CB oxidation as a co-benefit of NOx elimination, and the promotion mechanism of CB oxidation by Sb loading was investigated. CB oxidation rates of VMoTi and Sb/VMoTi relied on their oxidizing abilities and their numbers of V5+ ions, adsorbed CB, gaseous CB, lattice oxygen, and adsorbed oxygen. A newly formed SbOVOMo chain was observed on VMoTi after Sb loading, resulting in a modest enhancement of oxidizing ability and a slight increase in the numbers of lattice oxygen and adsorbed oxygen. Moreover, more Brønsted acid sites were formed on VMoTi after Sb loading, which facilitated CB adsorption and Cl species removal as HCl. Hence, Sb loading not only significantly enhanced the CB oxidation activity of VMoTi, thereby expanding the synergistic temperature window for NOx and CB elimination, but also effectively inhibited the formation of polychlorinated byproducts. Therefore, Sb/VMoTi was a promising catalyst for chlorinated volatile organic compounds oxidation as a co-benefit of NOx elimination.

Effect of inserting Cr in promoting the deep oxidation of dichloromethane over Co/WNb catalysts at low temperatures

Enhancing catalytic activity while inhibiting the generation of chlorine byproducts is essential in the catalytic oxidation process of chlorinated volatile organic compounds (CVOCs). In this study, Cr-modified Co/WNb catalysts were synthesized and utilized for the degradation of dichloromethane (DCM). It was found that the moderate introduction of Cr exposed more Cr6+ on the catalyst surface due to the interaction between cobalt and chromium oxides, which promoted the generation of more chemisorbed oxygen on the surface, thus improving the redox properties and enhancing the activity of the catalysts. Additionally, the introduction of Cr increased the B acid sites of the catalysts, promoting the breaking of C-Cl bonds and the removal of dissociated Cl- Meanwhile, the improved redox properties also allowed further oxidation of the dissociated activated intermediate products and inhibited the generation of chlorine byproducts. The catalyst activity was optimal when the Cr to Co molar ratio was 4, which the T90 of DCM was 256 °C and the monochloromethane selectivity was only 1.7 %. Moreover, Co4Cr/WNb showed excellent chlorine and water resistance, making it an ideal candidate for CVOC degradation.

Praseodymium-Doped Cr2O3 Prepared by In Situ Pyrolysis of MIL-101(Cr) for Highly Efficient Catalytic Oxidation of 1,2-Dichloroethane

The development of economical catalysts that exhibit both high activity and durability for chlorinated volatile organic compounds (CVOCs) elimination remains a challenge. The oxidizing and acidic sites play a crucial role in the oxidation process of CVOCs; herein, praseodymium (Pr) was introduced into CrOx catalysts via in situ pyrolysis of MIL-101(Cr). With the decomposition of the ligand, a mixed micro-mesoporous structure was formed within the M-Cr catalyst, thereby reducing the contact resistance between catalyst active sites and the 1,2-dichloroethane molecule. Moreover, the synergistic interaction between chromium and praseodymium facilitates Oβ species and acidic sites, significantly enhancing the low-temperature catalytic performance and durability of the M-PrCr catalyst for 1,2-dichloroethane (1,2-DCE) oxidation. The M-30PrCr catalyst possess enhanced active oxygen sites and acid sites, thereby exhibiting the highest catalytic activity and stability. This study may provide a novel and promising strategy for practical applications in the elimination of 1,2-DCE.

Fundamental insights and recent advances in catalytic oxidation processes using ozone for the control of volatile organic compounds

The development of technologies for highly efficient treatment of emissions containing low concentrations of volatile organic compounds (VOCs) remains an important challenge. Catalytic oxidation with ozone (catalytic ozonation) is useful for the oxidative decomposition of VOCs, particularly aromatic compounds, under ambient temperature conditions. Only inexpensive transition metal oxides are required as catalysts, and Mn-based catalysts are widely used for catalytic ozonation. This review describes the oxidation reaction mechanisms, reaction pathways of aromatic hydrocarbons, and dependence of the catalytic ozonation activity on the reaction conditions. The reasons why Mn oxides are effective in catalytic ozonation are also explained. The structure of the catalytic active sites and the types of supporting materials contributing to the reaction are also discussed in detail, with the aim of establishing a VOC control technology. In addition, recent progress in catalytic oxidation processes using ozone as an oxidant has been outlined, focusing on catalyst materials and reaction conditions.

Unveiling the Function of Oxygen Vacancy on Facet-Dependent CeO2 for the Catalytic Destruction of Monochloromethane: Guidance for Industrial Catalyst Design

Secondary pollution remains a critical challenge for the catalytic destruction of chlorinated volatile organic compounds (CVOCs). By employing experimental studies and theoretical calculations, we provide valuable insights into the catalytic behaviors exhibited by ceria rods, cubes, and octahedra for monochloromethane (MCM) destruction, shedding light on the elementary reactions over facet-dependent CeO2. Our findings demonstrate that CeO2 nanorods with the (110) facet exhibit the best performance in MCM destruction, and the role of vacancies is mainly to form a longer distance (4.63 Å) of frustrated Lewis pairs (FLPs) compared to the stoichiometric surface, thereby enhancing the activation of MCM molecules. Subsequent molecular orbital analysis showed that the adsorption of MCM mainly transferred electrons from the 3σ and 4π* orbitals to the Ce 4f orbitals, and the activation was mainly caused by weakening of the 3σ bonding orbitals. Furthermore, isotopic experiments and theoretical calculations demonstrated that the hydrogen chloride generated is mainly derived from methyl in MCM rather than from water, and the primary function of water is to form excess saturated H on the surface, facilitating the desorption of generated hydrogen chloride.

Changes of Nitrate Activity and Byproduct Distribution Characteristics for Synergistic NOx and Dioxin Abatement over V2O5/AC Catalyst

The simultaneous removal of NOx and dioxins has been considered an economical and effective technology of controlling multipollutant flue gas in the context of "carbon peaking and carbon neutrality". However, this technology has not yet been implemented in practical situations, because the interactive relationship between the selective catalytic reduction (SCR) reaction and dioxin catalytic oxidation lacks a deep understanding, especially on a carbon-based catalyst. In this research, the influence of NO and NH3 on the oxidation characteristics and byproducts distribution of dibenzofuran (DBF) was studied on V2O5/AC catalyst. Results indicated that NH3 has a stronger inhibition effect for DBF catalytic oxidation than NO due to obvious competitive adsorption between NH3 and DBF on the V2O5/AC catalyst. In addition, although both NO and NH3 inhibit the complete degradation of DBF, their effects on the byproduct distribution are not consistent. NO primarily affects the level of oxygen-containing byproducts, while NH3 primarily affects the level of alkane byproducts. Furthermore, the SCR reaction activity demonstrated a reduction when DBF was present. The occupation of V2O5 sites by DBF and its oxidizing intermediates has hindered the production of monodentate nitrate and the reactivity of bridged nitrate, resulting in a decrease in SCR activity via the L-H mechanism. This work aims to provide theoretical guidance for simultaneous removal of NOx and dioxins in industrial fumes.

New mechanistic insight into catalytic decomposition of dioxins over MnOx-CeO2/TiO2 catalysts: A combined experimental and density functional theory study

Elucidation of the catalytic decomposition mechanism of dioxins is pivotal in developing highly efficient dioxin degradation catalysts. In order to accurately simulate the whole molecular structure of dioxins, two model compounds, o-dichlorobenzene (o-DCB) and furan, were employed to represent the chlorinated benzene ring and oxygenated central ring within a dioxin molecule, respectively. Experiments and Density Functional Theory (DFT) calculations were combined to investigate the adsorption as well as oxidation of o-DCB and furan over MnOx-CeO2/TiO2 catalyst (denoted as MnCe/Ti). The results indicate that competitive adsorption exists between furan and o-DCB. The former exhibits superior adsorption capacity on MnCe/Ti catalyst at 100 °C - 150 °C, for it can adsorb on both surface metal atom and surface oxygen vacancies (Ov) via its O-terminal; while the latter adsorbs primarily by anchoring its Cl atom to surface Ov. Regarding oxidation, furan can be completely oxidized at 150 °C - 300 °C with a high CO2 selectivity (above 80 %). However, o-DCB cannot be totally oxidized and the resulting intermediates cause the deactivation of catalyst. Interestingly, the pre-adsorption of furan on catalyst surface can facilitate the catalytic oxidation of o-DCB below 200 °C, possibly because the dissociated adsorption of furan may form additional reactive oxygen species on catalyst surface. Therefore, this work provides new insights into the catalytic decomposition mechanism of dioxins as well as the optimization strategies for developing dioxin-degradation catalysts with high efficiency at low temperature.

Chitosan -NH2 derived efficient Co3O4 catalyst for styrene catalytic oxidation: Simultaneously regulating particle size and Co valence

In this study, a Co3O4 catalyst is synthesised using the chitosan-assisted sol-gel method, which simultaneously regulates the grain size, Co valence and surface acidity of the catalyst through a chitosan functional group. The complexation of the free -NH2 complex inhibits particle agglomeration; thus, the average particle size of the catalyst decreases from 82 to 31 nm. Concurrently, Raman spectroscopy, hydrogen temperature-programmed reduction, electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy experiments demonstrate that doping with chitosan N sources effectively modulates Co2+ to promote the formation of oxygen vacancies. In addition, water washing after catalyst preparation can considerably improve the low-temperature (below 250 °C) activity of the catalyst and eliminate the side effects of alkali metal on catalyst activity. Moreover, the presence of Brønsted and Lewis acid sites promotes the adsorption of C8H8. Consequently, CS/Co3O4-W presents the highest catalytic oxidation activity for C8H8 at low temperatures (R250 °C = 8.33 μmol g-1 s-1, WHSV = 120,000 mL hr-1∙g-1). In situ DRIFTS and 18O2 isotope experiments demonstrate that the oxidation of the C8H8 reaction is primarily dominated by the Mars-van Krevelen mechanism. Furthermore, CS/Co3O4-W exhibits superior water resistance (1- and 2- vol% H2O), which has the potential to be implemented in industrial applications.