Unveiling the secondary pollution in the catalytic elimination of chlorinated organics: The formation of dioxins

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.

Efficient Catalytic Elimination of Chlorobenzene Based on the Water Vapor-Promoting Effect within Mn-Based Catalysts: Activity Enhancement and Polychlorinated Byproduct Inhibition

Achieving no or low polychlorinated byproduct selectivity is essential for the chlorinated volatile organic compounds (CVOCs) degradation, and the positive roles of water vapor may contribute to this goal. Herein, the oxidation behaviors of chlorobenzene over typical Mn-based catalysts (MnO2 and acid-modified MnO2) under dry and humid conditions were fully explored. The results showed that the presence of water vapor significantly facilitates the deep mineralization of chlorobenzene and restrains the formation of Cl2 and dichlorobenzene. This remarkable water vapor-promoting effect was conferred by the MnO2 substrate, which could suitably synergize with the postconstructed acidic sites, leading to good activity, stability, and desirable product distribution of acid-modified MnO2 catalysts under humid conditions. A series of experiments including isotope-traced (D2O and H218O) CB-TPO provided complete insights into the direct involvement of water molecules in chlorobenzene oxidation reaction and attributed the root cause of the water vapor-promoting effect to the proton-rich environment and highly reactive water-source oxygen species rather than to the commonly assumed cleaning effect or hydrogen proton transfer processes (generation of active OOH). This work demonstrates the application potential of Mn-based catalysts in CVOCs elimination under practical application conditions (containing water vapor) and provides the guidance for the development of superior industrial catalysts.

Catalytic destruction of chlorobenzene over K-OMS-2: Inhibition of high toxic byproducts via phosphate modification

In the process of catalytic destruction of chlorinated volatile organic compounds (CVOCs), the catalyst is prone to chlorine poisoning and produce polychlorinated byproducts with high toxicity and persistence, bringing great risk to atmospheric environment and human health. To solve these problems, this work applied phosphate to modify K-OMS-2 catalysts. The physicochemical properties of catalysts were determined by using X-ray powder diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), hydrogen temperature programmed reduction (H₂-TPR), pyridine adsorption Fourier-transform infrared (Py-IR) and water temperature programmed desorption (H₂O-TPD), and chlorobenzene was selected as a model pollutant to explore the catalytic performance and byproduct inhibition function of phosphating. Experimental results revealed that 1 wt.% phosphate modification yielded the best catalytic activity for chlorobenzene destruction, with the 90% conversion (T₉₀) at approximately 247°C. The phosphating significantly decreased the types and yields of polychlorinated byproducts in effluent. After phosphating, we observed significant hydroxyl groups on catalyst surface, and the active center was transformed into Mn(IV)-O…H, which promoted the formation of HCl, and enhanced the dechlorination process. Furthermore, the enriched Lewis acid sites by phosphating profoundly enhanced the deep oxidation ability of the catalyst, which promoted a rapid oxidation of reaction intermediates, so as to reduce byproducts generation. This study provided an effective strategy for inhibiting the toxic byproducts for the catalytic destruction of chlorinated organics.

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

Chlorinated volatile organic compounds (CVOCs) are still a part of the current atmospheric environmental problems that cannot be ignored, but unlike conventional VOCs, the presence of Cl causes various catalyst deactivations in the catalytic process. In this paper, we focus on six common CVOCs and discuss various behavioral mechanisms of the whole catalytic process from six aspects: catalyst selection, factors affecting the catalytic effect, changes in catalytic behavior in the presence of different gases, catalyst poisoning deactivation behavior, degradation products and degradation mechanisms to provide guidance for further development of low-temperature and efficient CVOCs catalysts.

Selective Ru Adsorption on SnO2/CeO2 Mixed Oxides for Efficient Destruction of Multicomponent Volatile Organic Compounds: From Laboratory to Practical Possibility

Ru-based catalysts have been extensively employed for the catalytic destruction of chlorinated volatile organic compounds (VOCs), but their versatility for other routine VOCs' destruction has been less explored. Herein, we show that Ru-decorated SnO₂/CeO₂ mixed oxides can sustain H₂O and HCl poisonings and are endowed with extraordinary versatility for a wide range of VOCs' destruction. Selective adsorption of Ru on the cassiterite SnO₂ and CeO₂ nanorods through a Coulomb force can rationally tune the oxidation and dechlorination centers on decorated catalysts, where the epitaxial growth of RuO_x on top of SnO₂ is endowed with excellent dechlorination ability and that on CeO₂ is functional as an oxidation center; the latter could also activate H₂O to provide sufficient H protons for HCl formation. Our developed Ru/SnO₂/CeO₂ catalyst can steadily destruct mono-chlorobenzene, ortho-dichlorobenzene, trichloroethylene, dichloromethane, epichlorohydrin, N-hexane, ethyl acetate, toluene, and their mixtures at an optimum temperature of 300 °C, and its monolithic form is also functional at this temperature with few dioxins being detected in the off-gas. Our results imply that the Ru-decorated SnO₂/CeO₂ catalyst can meet the demands of regenerative catalytic oxidation for the treatment of a wide range of VOCs from industrial exhausts.

Oxy-Anionic Doping: A New Strategy for Improving Selectivity of Ru/CeO2 with Synergetic Versatility and Thermal Stability for Catalytic Oxidation of Chlorinated Volatile Organic Compounds

Understanding the formation and inhibition of more toxic polychlorinated byproducts from the catalytic oxidation elimination of chlorinated volatile organic compounds (Cl-VOCs) and unveiling efficient strategies have been essential and challenging. Here, RuO_x supported on CePO₄-doped CeO₂ nanosheets (Ru/Pi-CeO₂) is designed for boosting catalytic oxidation for the removal of dichloromethane (DCM) as a representative Cl-VOC. The promoted acid strength/number and sintering resistance due to the doping of electron-rich and thermally stable CePO₄ are observed along with the undescended redox ability and the exposed multi-active sites, which demonstrates a high activity and durability of DCM oxidation (4000 mg/m³ and 15,000 mL/g·h, stable complete-oxidation at 300 °C), exceptional versatility for different Cl-VOCs, alkanes, aromatics, N-containing VOCs, CO and their multicomponent VOCs, and enhanced thermal stability. The suppression of polychlorinated byproducts is determined over Ru/Pi-CeO₂ and oxy-anionic S, V, Mo, Nb, or W doping CeO₂, thus the oxy-anionic doping strategy is proposed based on the quenching of the electron-rich oxy-anions on chlorine radicals. Moreover, the simple mechanical mixing with these oxy-anionic salts is also workable even for other catalysts such as Co, Sn, Mn, and noble metal-based catalysts. This work offers further insights into the inhibition of polychlorinated byproducts and contributes to the convenient manufacture of monolithic catalysts with superior chlorine-poisoning resistance for the catalytic oxidation of Cl-VOCs.

Atmospheric PCDDs/PCDFs levels and occurrences in Southeast Asia: A review

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are toxic compounds derived from anthropogenic sources that stay in the environment for long periods. Ambient air has become the most important pathway for the transfer of PCDDs/PCDFs from emission sources to the environment. This review intends to summarise the information available on atmospheric PCDDs/PCDFs in the countries of Southeast Asia to provide a detailed description of the trends in PCDDs/PCDFs emissions, key sources, and levels in urban, rural, and industrial air as reported in peer-reviewed literature since 2000 and by the United Nations Environment Programme. As the largest country in Southeast Asia, Indonesia is the major PCDDs/PCDFs emitter, accounting for 72.81% of the total release of PCDDs/PCDFs in the air from all available inventories in this region, while Brunei Darussalam is the lowest emitter, contributing to less than 0.02%. Open burning processes have become the largest source of ambient PCDDs/PCDFs in the region (69.62%), followed by waste incineration (10.69%), and ferrous and non-ferrous metal production (8.78%). PCDDs/PCDFs levels in rural areas ranged between 10 and 38 fg TEQ m^-3; however, where open burning waste has occurred, the levels rose to 12-29 times higher. In urban areas, ambient levels were 15 times greater than in rural areas, varying from 23 to 565 fg TEQ m^-3. Atmospheric concentrations near industrial palm oil and waste incinerator sites were between 64 and 1530 fg TEQ m^-3. The non-cancer risk of ambient exposure to PCDDs/PCDFs through inhalation is low among populations near facilities emitting PCDDs/PCDFs. The lack of local technical capacity, the high economic costs, and the lack of established human resource capacities have been the major challenges in conducting ambient PCDDs/PCDFs studies in most countries in the region.

A comparative study of the dichloromethane catalytic combustion over ruthenium-based catalysts: Unveiling the roles of acid types in dissociative adsorption and by-products formation

Herein, a comparative investigation of the Ru-based catalysts with different kinds of supports (TiO₂, Al₂O₃, HZSM-5 SiO₂/Al₂O₃ = 27 and 130, respectively) for catalytic combustion of dichloromethane (DCM) has been performed. The characterization results showed that the C-Cl bond of DCM was cleaved on both the Brønsted and Lewis acid sites of the catalysts. However, the Lewis acid sites were more active than the Brønsted acid sites. The relatively strong Lewis acidity of Ru/TiO₂ improved the dissociative adsorption of DCM, accounting for its superior activity. The yield of toxic by-products was strongly associated with the acid types of the catalysts. The Cl species deposited on TiO₂ and Al₂O₃ supports interacted strongly with the Lewis acid sites, thereby promoting the electrophilic chlorination reactions and yielding more polychlorinated by-products, especially highly toxic dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). However, the Cl deposits on Ru/HZSM-5 (SiO₂/Al₂O₃ = 27) with abundant Brønsted acid sites, mainly existed as hydrogen-bonded Cl species, with good mobility and less propensity for chlorinating carbonaceous matter. Moreover, Ru/HZSM-5 (SiO₂/Al₂O₃ = 130) yielded the highest polychlorinated by-products and PCDD/Fs because of its poor redox ability and high surface area. Overall, this study provides valuable insights into the CVOCs catalytic combustion catalysts development.

V2O5-WO3/TiO2 Catalyst for Efficient Synergistic Control of NOx and Chlorinated Organics: Insights into the Arsenic Effect

Municipal solid waste incineration and the iron and steel smelting industry can simultaneously discharge NO_x and chlorinated organics, particularly polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). Synergistic control of these pollutants has been considered among the most cost-effective methods. This work combined experimental and computational methods to investigate the reaction characteristics of a catalytically synergistic approach and gives the first insight into the effect of arsenic (As) on the multipollutant conversion efficiency, synergistic reaction mechanism, and toxic byproduct distribution over a commercial V₂O₅-WO₃/TiO₂ catalyst. The loaded As₂O₃ species were shown to distinctly decrease the formation energy of an oxygen vacancy at the V-O-V site, which likely contributed to the extensive formation of more toxic polychlorinated byproducts in the synergistic reaction. The As₂O₅ species strongly attacked neighboring V═O sites forming the As-O-V bands. Such an interaction deactivated the deNO_x reaction, but led to excessive NO being oxidized into NO₂ that greatly promoted the V⁵⁺-V⁴⁺ redox cycle and in turn facilitated chlorobenzene (CB) oxidation. Subsequent density functional theory (DFT) calculation further reveals that both the As₂O₃ and As₂O₅ loadings can facilitate H₂O adsorption on the V₂O₅-WO₃/TiO₂ catalyst, leading to competitive adsorption between H₂O and CB, and thereby deactivate the CB oxidation with water stream.

HCl-Tolerant HxPO4/RuOx-CeO2 Catalysts for Extremely Efficient Catalytic Elimination of Chlorinated VOCs

Bulk metal doping and surface phosphate modification were synergically adopted in a rational design to upgrade the CeO₂ catalyst, which is highly active but easily deactivated for the catalytic oxidation of chlorinated volatile organic compounds (Cl-VOCs). The metal doping increased the redox ability and defect sites of CeO₂, which mostly promoted catalytic activity and inhibited the formation of dechlorinated byproducts but generated polychlorinated byproducts. The subsequent surface modification of the metal-doped CeO₂ catalysts with nonmetallic phosphate completely suppressed the formation of polychlorinated byproducts and, more importantly, enhanced the stability of the surface structure by forming a chainmail layer. A highly active, durable, and selective catalyst of phosphate-functionalized RuO_x-CeO₂ was the most promising among all the metal-doped (Ru, Pd, Pt, Cr, Mn, Fe, Co, and Cu) CeO₂ catalysts investigated owing to the prominent chemical stability of RuO_x and its superior versatility in the catalytic oxidation of different kinds of Cl-VOCs and other typical pollutants, including dimethyl sulfide, CO, and C₃H₈. Moreover, the chemical stability of the catalyst, including its bulk and surface structural stability, was investigated by combining intensive treatment with HCl/H₂O or HCl with subsequent ex situ ultraviolet-visible light Raman spectroscopy and confirmed the superior resistance to Cl poisoning of the phosphate-functionalized RuO_x-CeO₂. This work exemplifies a promising strategy for developing ideal catalysts for the removal of Cl-VOCs and provides a catalyst with the superior catalytic performance in Cl-VOC oxidation to date.

Bimetal oxide CuO/Co3O4 derived from Cu ions partly-substituted framework of ZIF-67 for toluene catalytic oxidation

A MOF-templated method is developed to prepare bimetal oxide CuO/Co₃O₄ by in situ pyrolysis of Cu²⁺ partly-substituted ZIF-67 precursor. The physicochemical properties of CuO/Co₃O₄ are studied by various characterizations such as X-ray diffraction, Raman analysis, transmission electron microscope, scanning electron microscope, N₂ adsorption-desorption measurement, X-ray photoelectron spectroscope, O₂ temperature-programmed desorption, H₂ temperature-programmed reduction, etc. Comparison with CuO, Co₃O₄ and Mix-CuO/Co₃O₄, 90 % of both toluene conversion and mineralization over CuO/Co₃O₄ are fulfilled at around 229 °C under the condition of 1000 ppm toluene and weight hour space velocity =20,000 mL/(g h), which is promoted more than 40 °C. The better catalytic performance of CuO/Co₃O₄ attributes to high mutual dispersion of two oxides, porous structure, lower temperature reducibility, abundant lattice defects, more active oxygen species, higher Co³⁺/Co²⁺ and O_latt/O_ads molar ratios. Meanwhile, CuO/Co₃O₄ exhibits a better catalytic stability at different conversions and a good tolerance to 10 vol.% of water vapour. The investigation of temperature-dependent active oxygen species and in-situ DRIFTS results reveal that toluene oxidation on CuO/Co₃O₄ obeys Mars van Krevelen mechanism.

Influence of oxygen and water content on the formation of polychlorinated organic by-products from catalytic degradation of 1,2-dichlorobenzene over a Pd/ZSM-5 catalyst

Understanding the generation and influence mechanism of polychlorinated organic by-products during the catalytic degradation of chlorinated volatile organic compounds (CVOCs) is essential to the safe and environmentally friendly treatment of those pollutants. In this study, a systematic investigation of the catalytic oxidation of 1,2-dichlorobenzene (1,2-DCB) was conducted using various oxygen and water contents over a Pd/ZSM-5(25) catalyst. It was found that decreasing the oxygen content and increasing the water content resulted in the improvement of the 1,2-DCB catalytic activity, while the amount and variety of polychlorinated organic by-products decreased. More importantly, when water was the sole oxidant, the Pd/ZSM-5(25) catalyst also demonstrated high activity towards 1,2-DCB catalytic degradation. Only chlorobenzene and 1,3-dichlorobenzene were detected as by-products. X-ray photoelectron spectra (XPS) and UV-vis DRS spectra results indicated that the polychlorinated organic by-products were suppressed mainly due to inhibition of the chlorination of the palladium species by regulating the oxygen and water content in the reaction atmosphere. Similar surface species were formed under aerobic and anaerobic atmospheres via the study of the in situ FTIR spectra. We therefore proposed that 1,2-DCB undergoes similar catalytic degradation reaction mechanisms under both aerobic and anaerobic conditions.

Tuning the degradation activity and pathways of chlorinated organic pollutants over CeO2 catalyst with acid sites: synergistic effect of Lewis and Brønsted acid sites

The synergistic effect of Lewis and Brønsted acid sites can promote the effective degradation of chlorobenzene following the hydrolysis pathway of producing less toxic by-products.