Manganese oxide octahedral molecular sieve K-OMS-2 as catalyst in post plasma-catalysis for trichloroethylene degradation in humid air

Structured Cerium-Manganese Catalysts Supported on Nickel Foam for Toluene Oxidation by Electric Internal Heating

Structured catalysts are widely used in catalytic oxidation of gaseous pollutants, hot catalysis is usually needed to assist the reaction in the catalytic process. Herein, a Ce-modified manganese oxide octahedral molecular sieves (Ce-OMS-2) structured catalyst supported on foam nickel was prepared through impregnation process. A systematically quantitative testing on the toluene catalytic oxidation effectiveness of this structured catalyst was conducted through catalyst evaluation device, combining a series of characterization methods, such as XRD and SEM, the structure-activity relationship was established. Assisted with electric internal heating and ozone oxidation environments, this structured catalyst exhibits excellent catalytic oxidation performance for oxidative decomposition of toluene even under high humidity conditions. The results showed that the ozone-coupled structured nickel foam catalyst increased the decomposition efficiency of toluene from 25 % (without catalyst and heating) to 55 % (with catalyst and without heating) and the electric internal heating can significantly improve the reactivity and moisture resistance of the structured nickel-foam catalyst, at 90 % RH and 40000 h-1, 50000 ppb O3 and 40 mg/m3 toluene was maintained 100 % catalytic efficiency. The high-efficiency non-precious metal-based electrothermal catalyst prepared herein is expected to have certain enlightenment for the purification of VOCs.

Enabling efficient and economical degradation of PCDD/Fs in MSWIFA via catalysis and dechlorination effect of EMR in synergistic thermal treatment

Catalytic thermal treatment is an efficient and low-energy consumption method for degrading polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in municipal solid waste incineration fly ash (MSWIFA). However, catalysts with high activity are expensive, difficult to separate and reuse from the treated MSWIFA, and they usually pose a risk of heavy metal pollution. Herein, a synergistic thermal treatment method of MSWIFA and electrolytic manganese residue (EMR) at relatively low temperatures was proposed after an in-depth analysis of their mineralogy composition to achieve detoxification of PCDD/Fs in MSWIFA. The mass and WHO-TEQ degradation efficiencies of PCDD/Fs significantly increased from -92.79% and -51.46%-98.57% and 96.10%, respectively, by the addition of electrolytic manganese residue (EMR) with an MSWIFA/EMR ratio of 3:7 in the thermal treatment of MSWIFA at 250 °C for 60 min. The WHO-TEQ concentration of PCDD/Fs in the treated sample decreased to 3.7 ng WHO-TEQ/kg, meeting the European end-of-waste criteria (20 ng WHO-TEQ/kg). The excellent degradation effect of EMR on PCDD/Fs in MSWIFA could be attributed to two aspects: 1) the manganese oxides in EMR has a catalytic effect on the degradation of PCDD/Fs; 2) the NH3 generated by the decomposition of (NH4)2SO4 in EMR is conducive to the degradation and resynthesis inhibition of PCDD/Fs. Besides, the thermodynamic calculations indicated that CaClOH in MSWIFA played a crucial role in the decomposition of (NH4)2SO4 in EMR. In addition, the degradation pathways and mechanisms of PCDD/Fs-homologues under the synergistic effect of manganese oxides, ammonia, and thermal field were investigated through comparative analysis of concentration and fingerprint of PCDD/Fs.

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.

Volatile organic compound removal by post plasma-catalysis over porous TiO2 with enriched oxygen vacancies in a dielectric barrier discharge reactor

Non-thermal plasma (NTP) degradation of volatile organic compounds (VOCs) into CO₂ and H₂O is a promising strategy for addressing ever-growing environment pollution. However, its practical implementation is hindered by low conversion efficiency and emissions of noxious by-products. Herein, an advanced low-oxygen-pressure calcination process is developed to fine-tune the oxygen vacancy concentration of MOF-derived TiO₂ nanocrystals. Vo-poor and Vo-rich TiO₂ catalysts were placed in the back of an NTP reactor to convert harmful ozone molecules into ROS that decompose VOCs via heterogeneous catalytic ozonation processes. The results indicate that Vo-TiO₂-5/NTP with the highest Vo concentration exhibited superior catalytic activity in the degradation of toluene compared to NTP-only and TiO₂/NTP, achieving a maximum 96% elimination efficiency and 76% CO_x selectivity at an SIE of 540 J L^-1. Mechanistic analysis reveals that the ¹O₂, ˙O₂^- and ˙OH species derived from the activation of O₃ molecules on Vo sites contribute to the decomposition of toluene over the Vo-rich TiO₂ surface. With the aid of advanced characterization and density functional theory calculations, the roles of oxygen vacancies in manipulating the synergistic capability of post-NTP systems were explored, and were attributed to increased O₃ adsorption ability and enhanced charge transfer dynamics. This work presents novel insights into the design of high-efficiency NTP catalysts structured with active Vo sites.

Activation and characterization of environmental catalysts in plasma-catalysis: Status and challenges

Plasma-catalysis has attracted great attentions in environmental/energy-related fields, but the synergetic mechanism still suffers intractable defects. Key issues are that what kind of catalysts are applicable for plasma system, how are they activated in plasma, and how to characterize them in plasma. This review systematically gives a comprehensive summarization of the selection of catalysts and its activation mechanism in plasma, based on the character of plasma, including physical effects containing the enhancement of discharge intensity and adsorption of reactants, and the utilization of plasma-generated active species such as·O, heat, O₃, ultraviolet light and e* . Focus is given to the illumination of the activation mechanisms of catalysts when placed in plasma zone. Subsequently, the novel characterization techniques for catalysts, which may associate properties to performance, are critically overviewed. The challenges and opportunities for the activation and characterizations of catalysts are proposed, and future perspectives are suggested about where the efforts should be made. It is expected that a bridge between catalysts design and character of plasma can be built to shed light on the synergetic mechanism for plasma-catalysis and design of new plasma-catalysis systems.

Recent Manganese Oxide Octahedral Molecular Sieves (OMS–2) with Isomorphically Substituted Cationic Dopants and Their Catalytic Applications

The present report describes the structural and physical–chemical variations of the potassium manganese oxide mineral, α–MnO2, which is a specific manganese octahedral molecular sieve (OMS) named cryptomelane (K–OMS–2), with different transition metal cations. We will describe some frequently used synthesis methods to obtain isomorphic substituted materials [M]–K–OMS–2 by replacing the original manganese cationic species in a controlled way. It is important to note that one of the main effects of doping is related to electronic environmental changes, as well as to an increase of oxygen species mobility, which is ultimately related to the creation of new vacancies. Given the interest and the importance of these materials, here, we collect the most recent advances in [M]–K–OMS–2 oxides (M = Ag, Ce, Mo, V, Nb, W, In, Zr and Ru) that have appeared in the literature during the last ten years, leaving aside other metal–doped [M]–K–OMS–2 oxides that have already been treated in previous reviews. Besides showing the most important structural and physic-chemical features of these oxides, we will highlight their applications in the field of degradation of pollutants, fine chemistry and electrocatalysis, and will suggest potential alternative applications.

Novel cryptomelane nanosheets for the superior catalytic combustion of oxygenated volatile organic compounds

This work offers a novel pathway to prepare cryptomelane manganese oxides nanosheets as an superior catalyst for the catalytic combustion of oxygenated volatile organic compounds. The tunnel cryptomelane manganese oxides nanosheets were prepared from layered birnessite via simultaneously tuning pH and molar ratio (R_OK) of the redox-precipitation between oxalic acid and KMnO₄. Thus, few-layered cryptomelane nanosheets possessing the most predominantly exposed (211) facet are generated at low pH (5.2-5.6), which intensifies the surface area of thin crystal cryptomelane nanosheets up to 177 m²g^-1 and weakens Mn-O bonds. Moreover, high R_OK results in low manganese average oxidation state (AOS), greater oxygen vacancies and better low-temperature reduction and oxygen mobility. Such features significantly maneuver the catalytic activity of the cryptomelane nanosheets catalysts for the complete oxidation of oxygenated volatile organic compound (e.g., 2-propanol, acetone) at low temperature (170-230 °C). Moreover, the catalysts show high stability for 48 h. The presented catalyst discloses an avenue to address current obstacles in the catalytic oxidation of volatile organic compounds.

Simultaneous removal of toluene and styrene by non-thermal plasma-catalysis: Effect of VOCs interaction and system configuration

Toluene and styrene were two typical aromatic VOCs which were commonly used and coexistence in the exhaust gases from industrial manufacturing. Their simultaneous removal performances under non-thermal plasma (NTP) and NTP-catalysis were carried out and compared by a single stage coaxial dielectric barrier discharge (DBD) reactor. The effects of VOCs mixture, humidity, materials filling in the discharge zoon on the removal efficiency, CO_x selectivity, byproducts types and their emission levels were deeply investigated to explore the degradation mechanism and coexistence effect. Experimental results showed that the toluene removal was significantly inhibited when treated together with styrene under plasma treatment. But that of styrene was hardly affected at the same conditions. It was found that benzaldehyde as the primary organic byproducts from styrene consumed the oxidizing particles (O and ^. OH), limiting the conversion of toluene. The introduction of Cu-doped MnO₂ materials significantly improved the VOCs removal performance with nearly 100% conversion to CO_x at a discharge power less than 30 W, as well as O₃ generation from more than 1.2 mg L^-1 by NTP to 1.6 × 10^-3 mg L^-1 by NTP-catalysis. With the help of in situ FT-IR, it was believed that catalysts not only accelerated the adsorption and degradation of pollutants but also utilized ozone to involve this process. At last, a plausible explanation on binary coexistence effect under different conditions had been suggested and discussed.

Removal mechanism and quantitative control of trichloroethylene in a post-plasma-catalytic system over Mn–Ce/HZSM-5 catalysts

The optimization of the TCE degradation process was achieved and the TCE degradation pathway in the PPC system was proposed.

Factors Governing the Activity of α-MnO2 Catalysts in the Oxygen Evolution Reaction: Conductivity versus Exposed Surface Area of Cryptomelane

Cryptomelane (α-(K)MnO2 ) powders were synthesized by different methods leading to only slight differences in their bulk crystal structure and chemical composition, while the BET surface area and the crystallite size differed significantly. Their performance in the oxygen evolution reaction (OER) covered a wide range and their sequence of increasing activity differed when electrocatalysis in alkaline electrolyte and chemical water oxidation using Ce4+ were compared. The decisive factors that explain this difference were identified in the catalysts' microstructure. Chemical water oxidation activity is substantially governed by the exposed surface area, while the electrocatalytic activity is determined largely by the electric conductivity, which was found to correlate with the particle morphology in terms of needle length and aspect ratio in this sample series. This correlation is rather explained by an improved conductivity due to longer needles than by structure sensitivity as was supported by reference experiments using H2 O2 decomposition and carbon black as additive. The most active catalyst R-cryptomelane reached a current density of 10 mA cm-2 at a potential 1.73 V without, and at 1.71 V in the presence of carbon black. The improvement was significantly higher for the catalyst with lower initial activity. However, the materials showed a disappointing catalytic stability during alkaline electrochemical OER, whereas the crystal structure was found to be stable at working conditions.

Solar-Enhanced Plasma-Catalytic Oxidation of Toluene over a Bifunctional Graphene Fin Foam Decorated with Nanofin-like MnO2

In this work, we propose a hybrid and unique process combining solar irradiation and post-plasma catalysis (PPC) for the effective oxidation of toluene over a highly active and stable MnO₂/GFF (bifunctional graphene fin foam) catalyst. The bifunctional GFF, serving as both the catalyst support and light absorber, is decorated with MnO₂ nanofins, forming a hierarchical fin-on-fin structure. The results show that the MnO₂/GFF catalyst can effectively capture and convert renewable solar energy into heat (absorption of >95%), leading to a temperature rise (55.6 °C) of the catalyst bed under solar irradiation (1 sun, light intensity 1000 W m^-2). The catalyst weight (9.8 mg) used in this work was significantly lower (10-100 times lower) than that used in previous studies (usually 100-1000 mg). Introducing solar energy into the typical PPC process via solar thermal conversion significantly enhances the conversion of toluene and CO₂ selectivity by 36-63%, reaching ∼93% for toluene conversion and ∼83% for CO₂ selectivity at a specific input energy of ∼350 J L^-1, thus remarkably reducing the energy consumption of the plasma-catalytic gas cleaning process. The energy efficiency for toluene conversion in the solar-enhanced post-plasma catalytic (SEPPC) process reaches up to 12.7 g kWh^-1, ∼57% higher than that using the PPC process without solar irradiation (8.1 g kWh^-1), whereas the energy consumption of the SEPPC process is reduced by 35-52%. Moreover, the MnO₂/GFF catalyst exhibits an excellent self-cleaning capability induced by solar irradiation, demonstrating a superior long-term catalytic stability of 72 h at 1 sun, significantly better than that reported in previous works. The prominent synergistic effect of solar irradiation and PPC with a synergistic capacity of ∼42% can be mainly attributed to the solar-induced thermal effect on the catalyst bed, boosting ozone decomposition (an almost triple enhancement from ∼0.18 g_O3 g^-1 h^-1 for PPC to ∼0.52 g_O3 g^-1 h^-1 for SEPPC) to generate more oxidative species (e.g., O radicals) and enhancing the catalytic oxidation on the catalyst surfaces, as well as the self-cleaning capacity of the catalyst at elevated temperatures driven by solar irradiation. This work opens a rational route to use abundant, renewable solar power to achieve high-performance and energy-efficient removal of volatile organic compounds.

Plasma assisted Cu-Mn mixed oxide catalysts for trichloroethylene abatement in moist air

The removal of dilute trichloroethylene (TCE) in moist air by post-plasma catalysis (PPC) using Cu-Mn mixed oxides heated at 150 °C was investigated. Cu-Mn mixed oxides were prepared by redox- and co-precipitation method. In comparison to the catalytic oxidation and non-thermal plasma (NTP) process, PPC was found to be the best process to convert TCE into CO₂, in particular when Cu-Mn oxide was synthetized by redox precipitation method. The highest TCE conversion efficiency of more than 80% was obtained at the energy density of 60 J.L^-1 using the catalyst prepared by redox-precipitation process in PPC configuration. The performance of Cu-Mn oxide prepared by redox-precipitation method did not show increase in TCE conversion with energy density which is attributed to the changes on the catalyst surface (such as reduction in S_BET, chlorine poisoning and Mn enrichment). Although, Cu-Mn oxide prepared by co-precipitation method showed a lower TCE conversion, it exhibited a better stability in the PPC process for TCE abatement.

Synergistic effects and mechanism of a non-thermal plasma catalysis system in volatile organic compound removal: a review

Non-thermal plasma catalysis with high efficiency, high by-product selectivity and superior carbon balance is one of the most promising technologies in the control of volatile organic compounds (VOCs).