Environmental plasma-catalysis for the energy-efficient treatment of volatile organic compounds

Sequential Dosing Strategies for Controlling Selectivity and Plasma-Phase Contributions in Plasma Catalysis

Plasma-assisted catalysis has advanced in recent years, particularly for transforming stable reactants at atmospheric pressure and ambient temperature. However, achieving a deeper understanding of the many plasma and catalytic contributions remains a significant goal, as improving product yield and selectivity in plasma catalysis depends on proper catalyst selection, which is often challenging due to the complex interplay between plasma-phase and plasma-surface reactions. A sequential methodology has emerged as a means to decouple the catalyst activity from plasma-phase reactions. In this approach, nonthermal plasma is used in one step to activate and/or convert a gas phase or surface bound reactant, while in a second step, the catalyst directs product formation under steady-state or temperature-programmed conditions. This review examines studies using this technique for reactions involving N2, CO2, and SO2, offering insights into reaction mechanisms and catalyst behavior/selection for these transformations. These systematic studies provide a framework that can be applied to other plasma-assisted reactions. We also highlight remaining questions, propose directions for future studies, and discuss the potential of applying this methodology to other reaction systems.

Removal and Oxidation of Low Concentration tert-Butanol from Potable Water using Nonthermal Plasma Coupled with Metal Oxide Adsorption

Taste and odor are crucial factors in evaluating the quality of drinking water for consumers. Geosmin is an example of a pollutant commonly found in potable water responsible for earthy and musty taste, and odor even at low concentrations. We have investigated the use of a hybrid two-step adsorption-mineralization process for low-level volatile organic compounds removal from potable water using dielectric barrier discharge over common metal oxides (MO). The system proposed is a proof of principle with tert-butanol (TBA) used as a model compound for geosmin removal/degradation during wastewater treatment when combined with an appropriate metal oxide adsorbent. Initial assessments of the adsorption properties of titania by density functional theory (DFT) calculations and experimental tests indicated that the adsorption of geosmin and TBA with water present results in only weak interactions between the sorbate and the metal oxide. In contrast, the DFT results show that alumina could be a suitable adsorbent for these tertiary alcohols and were reinforced by experimental studies. We find that while there is a competitive effect between the water and TBA adsorption from gaseous/liquid feed, the VOC can be removed, and the alumina will be regenerated by the reactive oxygen species (ROS) produced by a dielectric barrier discharge (DBD). The use of alumina in conjunction with NTP leads to efficient degradation of the adsorbate and the formation of oxygenated intermediates (formates, carbonates, and carboxylate-type species), which could then be mineralized for the regeneration of the adsorbent. A reaction mechanism has been proposed based on the in-situ infrared measurements and DFT calculations, while the removal of TBA with conventional heating is indicative of a gradual desorption process as a function of temperature rather than the destruction of the adsorbate. Furthermore, steady performance was observed after several adsorption-regeneration cycles, indicating no alteration of the adsorption properties of alumina during the NTP treatment and demonstrating the potential of the approach to be applied in the treatment of high throughput of water, without the challenges faced by the biocatalysts or formation of toxic byproducts.

Efficient degradation of p-nitrophenol from water by enhancing dielectric barrier discharge (DBD) plasma through ozone circulation: Optimization, kinetics and mechanism

Non-thermal dielectric barrier discharge (DBD) plasma has received great attention for degradation of persistent organic pollutants such as p-nitrophenol (PNP). However, the feasibility of the DBD implementation is not clear due to its high energy consumption and relatively low degradation efficiency. In this research, a novel strategy was suggested based on re-circulation of the generated O3 in the DBD system to enhance the PNP degradation efficiency and energy yield. The potential mechanism and possible pathway of PNP degradation were studied by EPR, ESR, DFT and GS-MS analytical tests. According to the results, the PNP degradation efficiency and energy yield increased from 57.4% to 94.4% and from 0.52 to 1.18 g kW-1h-1, respectively through ozone circulation into the DBD reactor. This was due to the more release of long-lived and short-lived reactive species (ROS) in the DBD-O3 system by the O3 circulation. The variations in pH (4-10), initial concentration (50-90 mg L-1), and the presence of co-existing substances in the water matrix had minimal impact on the DBD-O3 system, in comparison to the conventional system. The biological toxicity evaluation revealed that the hybrid DBD-O3 system transform PNP to less toxic intermediates. This study proposes a promising strategy to improve the utilization of DBD for the degradation of PNP.

A facile method for preparing the CeMnO3 catalyst with high activity and stability of toluene oxidation: The critical role of small crystal size and Mn3+-Ov-Ce4+ sites

Volatile organic compounds (VOCs) cause severe environmental pollution and are potentially toxic to humans who have no defense against exposure. Catalytic oxidation of these compounds has thus become an interesting research topic. In this study, microcrystalline CeMnO3 catalysts were prepared by a precipitant-concentration-induced strategy and evaluated for the catalytic oxidation of toluene/benzene. The effect of crystal size on catalytic performance was confirmed by XRD, TEM, N2 adsorption-desorption, XPS, Raman, H2-TPR, and TPSR. The CeMnO3 catalyst with more Mn3+-Ov-Ce4+ active sites exhibited enhanced VOCs catalytic oxidation performance, lowest active energy, and highest turnover frequency, which was attributed to its larger surface area, lower crystal size, higher low-temperature reducibility, and presence of more oxygen defects. In-situ FTIR results suggested more oxygen vacancies can profoundly promote the conversion of benzoate to maleate species, the rate-determining step of toluene oxidation. The work provides a convenient and efficient strategy to prepare single-metal or multi-metal oxide catalysts with smaller crystal sizes for VOC oxidation or other oxidation reactions.

Non-Thermal Plasma Incorporated with Cu-Mn/γ-Al2O3 for Mixed Benzene Series VOCs’ Degradation

In this work, a coaxial dielectric barrier discharge (DBD) reactor was constructed to degrade the mixture of toluene and o-xylene, two typical benzene series. The Cu-MnO2/γ-Al2O3 series catalysts prepared by redox and impregnation methods were filled into the plasma device to degrade VOCs synergistically and explore the degradation effect. The experimental results showed that the introduction of a Cu-doped MnO2 catalyst significantly improved the pollutants’ removal efficiency and CO2 selectivity, and greatly inhibited the formation of by-products. Among them, Cu0.15Mn/γ-Al2O3 showed the highest removal efficiency (toluene was 100% and o-xylene was 100%), and the best CO2 selectivity (92.73%). The XRD, BET, XPS and SEM results confirmed that the synergistic effect between Cu and Mn in the Cu-Mn solid solution could promote the amount and reducibility of the surface active oxygen species, which improved the catalytic performance. Finally, the toluene and o-xylene decomposition pathways in the NTP catalytic system were speculated according to the detected organic matter. This work provides a theoretical and experimental basis for the application of DBD-catalyzed hybrid benzene series VOCs.

Synthesis and characterization of TiO2/ZnO heterostructural composite for ultraviolet photocatalytic degrading DOM in landfill leachate

In order to investigate the photocatalytic degradation of dissolved organic matter (DOM) in landfill leachate, TiO₂/ZnO heterostructural composite powders were fabricated combining with hydrothermal synthesis and solid-state reaction method. The prepared TiO₂/ZnO composite powders consist of anatase TiO₂ nanoparticles distributing on the surface of wurtzite ZnO particles. The optical band gap of TiO₂/ZnO powder is less than that of pure ZnO or TiO₂ powder. TiO₂/ZnO catalyzers show high ultraviolet-degradation efficiency for methylene blue and dissolved organic matter. The degradation rate of TiO₂/ZnO powder for fulvic acid-like substances in landfill leachate is 2.99 times that of pure ZnO powder, and is 1.30 times that of pure TiO₂ powder. The degradation of fulvic acid-like substances by TiO₂/ZnO photocatalyst reduced some molecular weight of benzene ring structure substances in leachate. The effective separation of electron and hole in heterostructural TiO₂/ZnO photocatalyst is the main reason for its high photocatalytic degradation efficiency of DOM in landfill leachate.

Elimination or Removal of Ethylene for Fruit and Vegetable Storage via Low-Temperature Catalytic Oxidation

Ethylene acts as an important hormone to trigger the ripening and senescence of fruits and vegetables (F&V). Thus, it is essential to eliminate trace ethylene and prevent F&V losses effectively. There are several technologies currently applying to control the ethylene concentration in the storage and transportation environment, including adsorption, gene modification, oxidation, etc. These protocols will be compared, and special attention will be paid to the low-temperature catalytic oxidation that has already been applied to practical production in this review. The active sites, supports, and reaction and deactivation mechanism of the catalysts for the low-temperature ethylene oxidation will be discussed and evaluated systematically to provide new insights for the development of effective catalysts, along with the suggestion of some perspectives for future research on this important catalytic system for F&V preservation.

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.

Decomposition of volatile organic compounds using corona discharge plasma technology

This paper explores the application of corona plasma technology as a tool in treatment of volatile organic compounds (VOCs). The review introduces the principle of corona discharge and describes the characteristics of plasma, especially of various corona plasma reactors. By summarizing the main features of such reactors, this paper provides a brief background to different power sources and reactor configurations and their application to VOC treatment design. Considering chlorinated compounds, benzene series and sulfur compounds, this paper reveals the probable mechanism of corona plasma in VOC degradation. Additionally, the effects of numerous technical parameters - such as reactor structure, shape and materials of electrodes, and humidity - are analyzed comprehensively. Product distribution, energy efficiency and economic benefits are invoked as factors to evaluate the performance of VOC degradation. Finally, the practical application of corona plasma and its advantages are briefly introduced. The review aims to illustrate the enormous potential of corona plasma technology in the treatment of VOCs, and identifies future directions. Implications: This paper comprehensively describes the principle, characteristics, research progress and engineering application examples of the degradation of volatile organics by corona discharge plasma, to provide a theoretical basis for the industrial application of this process.

A Hybrid Reactor System Comprised of Non-Thermal Plasma and Mn/Natural Zeolite for the Removal of Acetaldehyde from Food Waste

The degradation of low concentrations of acetaldehyde while using a non-thermal plasma (NTP)/catalyst hybrid reactor system was investigated while using humidified air at ambient temperature. A series of highly active manganese-impregnated natural zeolite (Mn/NZ) catalysts were synthesized by the incipient wetness method using sonication. The Mn/NZ catalysts were analyzed by Brunauer-Emmett-Teller surface area measurements and X-ray photoelectron spectroscopy. The Mn/NZ catalyst located at the downstream of a dc corona was used for the decomposition of ozone and acetaldehyde. The decomposition efficiency of ozone and acetaldehyde was increased significantly using the Mn/NZ catalyst with NTP. Among the various types of Mn/NZ catalysts with different Mn contents, the 10 wt.% Mn/NZ catalyst under the NTP resulted the highest ozone and acetaldehyde removal efficiency, almost 100% within 5 min. Moreover, this high efficiency was maintained for 15 h. The main reason for the high catalytic activity and stability was attributed to the high dispersion of Mn on the NZ made by the appropriate impregnation method using sonication. This system is expected to be efficient to decompose a wide range of volatile organic compounds with low concentrations.

Direct methanol synthesis from methane in a plasma-catalyst hybrid system at low temperature using metal oxide-coated glass beads

A plasma-catalyst hybrid system was used to synthesize methanol directly from methane. A dielectric barrier discharge (DBD) plasma combined with the catalyst was introduced in order to overcome the difficulties of catalyst-only batch reactions such as high reaction pressure and separation of liquid product. Of the transition metal oxides, Mn₂O₃-coated glass bead showed the highest methanol yield of about 12.3% in the plasma-catalyst hybrid system. The reaction temperature was maintained below 100 °C due to the low plasma input power (from 1.3 kJ/L to 4.5 kJ/L). Furthermore, the reactivity of the catalyst was maintained for 10 hr without changing the selectivity. The results indicated that the plasma-induced OH radical might be produced on the Mn₂O₃ catalyst, which led to methanol synthesis.

Tailoring the wettability of glass using a double-dielectric barrier discharge reactor

A double dielectric barrier discharge reactor operated at a low power frequency of 400 Hz and atmospheric pressure was utilized for regulating the wettability of glass surface. The hydrophobic treatment was performed by plasma polymerization of tetramethylsilane (TMS, in argon gas). The obtained results showed that the TMS coatings formed on the glass substrates without oxygen addition were smooth, uniform films with the maximum water contact angle (WCA) of about 106°, which were similar to those obtained by low pressure, high power frequency plasmas reported in the literature. The addition of oxygen into TMS/Ar plasma gas decreased the WCA and induced the formation of SiOSi and/or SiOC linkages, which dominated the existence of Si(CH₂)_nSi network formed in TMS/Ar (without oxygen) plasma.

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).