High-efficiency non-thermal plasma-catalysis of cobalt incorporated mesoporous MCM-41 for toluene removal

Investigation on removal of multi-component volatile organic compounds in a two-stage plasma catalytic oxidation system - Comparison of X (X=Cu, Fe, Ce and La) doped Mn2O3 catalysts

Mn₂O₃-X catalysts (X = Cu, Fe, Ce and La) were prepared based on γ-Al₂O₃ for the mixture degradation of muti-component volatile organic compounds (VOCs) composed of toluene, acetone, and ethyl acetate. The catalysts were characterized, and the density functional theory (DFT) simulation of ozone adsorption on Mn₂O₃-X were carried out to investigate the influence of adsorption energy on catalytic performance. The results showed that the removal efficiency (RE) of each VOC component was similarly improved by Mn₂O₃-X catalysts, and the greatest increase in VOCs' removal efficiency was obtained (7.8% for toluene, 86.2% for acetone, and 82.5% for ethyl acetate) at a special input energy (SIE) of 700 J L^-1 with Mn₂O₃-La catalyst. Characterization results demonstrated that Mn₂O₃-La catalyst had the highest content of low valence Mn elements and the greatest O_ads/O_latt ratio, as well as the lowest reduction temperature. Mn₂O₃-La catalyst also presented superior catalytic effect in improving carbon balance (CB) and CO₂ selectivity ( [Formula: see text] ). The CB and [Formula: see text] were increased by 47.7% and 12.61% respectively with Mn₂O₃-La at a SIE of 400 J L^-1 compared with that when only γ-Al₂O₃ was applied. The DFT simulation results of ozone adsorption on Mn₂O₃-X catalysts indicated that the adsorption energy of catalyst crystal was related to the catalytic performance of the catalyst. The Mn₂O₃-La/γ-Al₂O₃ catalyst, which had the highest absolute value of adsorption energy, presented the best performance in improving VOCs' RE.

Recent Advances in Co3O4-Based Composites: Synthesis and Application in Combustion of Methane

In recent years, it has been found that adjusting the organizational structure of Co₃O₄ through solid solution and other methods can effectively improve its catalytic performance for the oxidation of low concentration methane. Its catalytic activity is close to that of metal Pd, which is expected to replace costly noble metal catalysts. Therefore, the in-depth research on the mechanism and methods of Co₃O₄ microstructure regulation has very important academic value and economic benefits. In this paper, we reviewed the catalytic oxidation mechanism, microstructure regulation mechanism, and methods of nano-Co₃O₄ on methane gas, which provides reference for the development of high-activity Co₃O₄-based methane combustion catalysts. Through literature investigation, it is found that the surface energy state of nano-Co₃O₄ can be adjusted by loading of noble metals, resulting in the reduction of Co-O bond strength, thus accelerating the formation of reactive oxygen species chemical bonds, and improving its catalytic effect. Secondly, the use of metal oxides and non-metallic oxide carriers helps to disperse and stabilize cobalt ions, improve the structural elasticity of Co₃O₄, and ultimately improve its catalytic performance. In addition, the performance of the catalyst can be improved by adjusting the microstructure of the composite catalyst and optimizing the preparation process. In this review, we summarize the catalytic mechanism and microstructure regulation of nano-Co₃O₄ and its composite catalysts (embedded with noble metals or combined with metallic and nonmetallic oxides) for methane combustion. Notably, this review delves into the substance of measures that can be used to improve the catalytic performance of Co₃O₄, highlighting the constructive role of components in composite catalysts that can improve the catalytic capacity of Co₃O₄. Firstly, the research status of Co₃O₄ composite catalyst is reviewed in this paper. It is hoped that relevant researchers can get inspiration from this paper and develop high-activity Co₃O₄-based methane combustion catalyst.

Study on the synergy effect of MnOx and support on catalytic ozonation of toluene

MnO_x has received widespread attention in low-temperature catalytic oxidation of VOCs, however, the synergy effect of MnO_x and support on the VOCs catalytic ozonation were rarely studied. In this study, five different MnO_x/X (X: MCM-41, 13X, ZSM-5, HY, USY) were synthesized and found their support greatly affect the catalytic oxidation activity. MnO_x/MCM-41 presents the largest specific surface area, pore volume and unique surface morphology, and thereby provides more sites for MnO_x loading and VOCs adsorption. Moreover, MnO_x/MCM-41 presents a high proportion of Mn³⁺, which helps to enhance the ion exchange capability, and thus promotes the regeneration of oxygen vacancies. Furthermore, a part of Mn was proved to be introduced into the MCM-41 lattice, which can promote the electron transfer between the active components and the support, and thereby effectively improve the surface electronic properties of the catalyst. The toluene catalytic experiments showed that MnO_x/MCM-41 exhibited the best catalytic activity, presenting complete degradation of O₃ and VOCs at room temperature. In addition, 5 wt%MnO_x/MCM-41 exhibited better catalytic activity than other loading, and its higher surface oxygen species endowed it with strong water resistance and stability. In-situ DRIFTs indicated that toluene was initially oxidized into benzyl alcohol during the adsorption process, and then decomposed to intermediate products (benzaldehyde, phenolate, etc.) during the catalytic ozonation process, and finally oxidized to carbon dioxide. In conclusion, the supply of loading sites and the improvement of interfacial electron transfer are the manifestations of the synergy between the support and MnO_x, leading to the promotion of the catalytic ozonation of VOCs.

The Bibliometric Analysis and Review of the Application of Plasma in the Field of VOCs

The application of plasma in the field of volatile organic compounds (VOCs) can be traced back to the 1990s and has gradually developed into an important research field. In this regard, this article primarily sorts and analyzes the literature on the “application of plasma in the field of VOCs” in the Web of Science core collection database from 1992 to 2021 and, subsequently, obtains important data and trends, including the annual number of articles published, country, institution analysis, and journal, as well as discipline analysis, etc. The results show that China is not only in a leading position in the field of research, but also has six top-ten research institutions. This field has more research results in engineering, chemistry, physics, and environmental disciplines. In addition, this article summarizes dielectric barrier discharge (DBD) and titanium-containing catalysts, which represent the discharge characteristics and type of catalyst highlighted through the hot keywords. This review will provide certain guidance for future, related research.

Mn2O3/γ-Al2O3 catalysts synergistic double dielectric barrier discharge (DDBD) degradation of toluene, ethyl-acetate and acetone

Mn₂O₃/γ-Al₂O₃ catalysts was combined with double dielectric barrier discharge (DDBD) for degradation of acetone, toluene and ethyl acetate. Mn₂O₃/γ-Al₂O₃ catalysts with different Mn loading were synthesized by hydrothermal method. XRD, SEM, XPS and H₂-TPR were applied to characterize the catalysts. Among the catalysts prepared, the Mn₂O₃/γ-Al₂O₃ catalysts with 5 wt% Mn loading presented the best performance in multicomponent VOCs degradation, which the highest removal efficiency (58.8% for acetone, 96.3% for toluene and 85.8% for ethyl acetate), the best carbon balance (87.5%) and CO₂ selectivity (51.9%) were obtained at a specific input energy (SIE) of 700 J L^-1. The formation of ozone was obviously inhibited with the introduction of Mn₂O₃/γ-Al₂O₃ catalysts. The higher Mn³⁺/Mn ratio, higher O₂/O^2- ratio and excellent low-temperature reducibility were beneficial for the VOCs degradation. Highly dispersed Mn₂O₃ crystals on the surface of γ-Al₂O₃ also might be an explanation for the improvement of VOCs degradation. According to the result of GC-MS, the variety of organic by-products gradually decreased with the increase of SIE, and the degradation mechanism of the mixed VOCs in plasma and on catalyst surface was discussed.

Efficient photocatalytic toluene degradation over heterojunction of GQDs@BiOCl ultrathin nanosheets with selective benzoic acid activation

Photocatalytic toluene degradation has attracted tremendous attention because of the growing environmental problem. However, conventional photocatalytic materials used for toluene degradation usually suffer from low carrier separation efficiency and poor stability which will degrade the catalytic performance. Herein, we report the synthesis of a novel heterostructure of GQDs@BiOCl ultrathin nanosheets where the GQDs can rapidly capture and transport photogenerated electrons for effective charge separation, promoting the generation of more reactive oxygen species (·O₂^- and ·OH radicals) for toluene degradation. In situ DRIFTS measurement and theoretical calculation are performed to unveil the reaction intermediates and the underlying toluene oxidation mechanism. The GQDs@BiOCl heterojunction could facilitate the adsorption and conversion of toluene and the reaction intermediates. Especially, the heterojunction greatly enhances the activation and conversion of benzoic acid and thus expedites the complete toluene degradation. This work presents a new insight on the design of high-performance photocatalysts for efficient degradation of typical air pollutants.

Regeneration of Hopcalite used for the adsorption plasma catalytic removal of toluene by non-thermal plasma

A dielectric barrier discharge reactor packed with both Hopcalite & glass beads has been investigated for the total oxidation of toluene adsorbed on Hopcalite. The catalytic activity and selectivity through the possible formation of by-products during the NTP discharge for the abatement of irreversibly adsorbed toluene have been investigated by FT-IR and mass spectrometer. The regeneration of the used Hopcalite by NTP discharge has been established by (i) determining the amount of toluene adsorbed on NTP regenerated Hopcalite, (ii) investigating the catalytic activity of NTP regenerated Hopcalite and (iii) comparing the bulk and surface properties of the fresh calcined and NTP regenerated Hopcalite. The ratio of amount of irreversibly adsorbed toluene to that of the total amount of adsorbed toluene adsorbed is similar for the fresh calcined and NTP (I) regenerated Hopcalite. The catalytic activity of the NTP (I) regenerated Hopcalite is slightly enhanced when compared to that of the fresh calcined Hopcalite. Although the first NTP treatment induces partial transformation of Hopcalite into Mn₃O₄ with no detected related CuO_x and reduces specific surface area by a factor of 2, the toluene adsorption capacity remains less affected. A plausible reaction scheme for toluene decomposition in Hopcalite PBDBD reactor is proposed.

Highly efficient decomposition of toluene using a high-temperature plasma-catalysis reactor

Plasma-catalysis technologies (PCTs) have the potential to control the emissions of volatile organic compounds, although their low-energy efficiency is a bottleneck for their practical applications. A plasma-catalyst reactor filled with a CeO₂/γ-Al₂O₃ catalyst was developed to decompose toluene with a high-energy efficiency enhanced by the elevating reaction temperature. When the reaction temperature was raised from 50 °C to 250 °C, toluene conversion dramatically increased from 45.3% to 95.5% and the energy efficiency increased from 53.5 g/kWh to 113.0 g/kWh. Conversely, the toluene conversion using a thermal catalysis technology (TCT) exhibited a maximum of 16.7%. The activation energy of toluene decomposition using PCTs is 14.0 kJ/mol, which is far lower than those of toluene decomposition using TCTs, which implies that toluene decomposition using PCT differs from that using TCT. The experimental results revealed that the Ce³⁺/Ce⁴⁺ ratio decreased and O_ads/O_latt ratio increased after the 40-h evaluation experiment, suggesting that CeO₂ promoted the formation of the reactive oxygen species that is beneficial for toluene decomposition.

A novel array of double dielectric barrier discharge combined with TiCo catalyst to remove high-flow-rate toluene: Performance evaluation and mechanism analysis

A novel array double dielectric barrier discharge (ADDBD) combined with a TiO₂/Al₂O₃-Co₃O₄/AC (TiCo) catalyst was applied to remove toluene. The effects of catalyst setting distance, catalyst combination mode, and process factors (including specific input energy, initial toluene concentration, and relative humidity) were investigated in terms of the toluene degradation efficiency (η_toluene) and the selectivity of CO₂ (S_CO2). When the specific input energy was 65 J·L^-1, the initial toluene concentration was 100 mg·m^-3, and the relative humidity was 30%, the highest η_toluene of 72% and S_CO2 of 44% could be achieved with TiO₂/Al₂O₃ 10 cm and Co₃O₄/AC 20 cm downstream of the ADDBD. Based on the determination of active substances (e.g., O₃, OH) and the catalyst activation mode, a synergistic effect of active substances and photon between the ADDBD and the TiCo catalyst was proposed for the removal of toluene. Finally, the biodegradability and toxicity of the outlet gas were evaluated, and the results showed that the outlet gas was more convenient for subsequent biopurification and less toxic to the surroundings after the treatment by the ADDBD combined with the TiCo catalyst.

Facile Synthesis of Co3O4 Nanoparticle-Functionalized Mesoporous SiO2 for Catalytic Degradation of Methylene Blue from Aqueous Solutions

In this study, a series of Co3O4 nanoparticle-functionalized mesoporous SiO2 (Co–SiO2) were successfully synthesized via a spontaneous infiltration route. Co species were firstly infiltrated into the confined spaces between the surfactant and silica walls, with the assistance of grinding CoCl3·6H2O and the as-prepared mesoporous SiO2. Then, Co3O4 nanoparticles (NPs) were formed and grown in the limited space of the mesopores, after calcination. Structures, morphologies, and compositions of the materials were characterized by X-ray diffraction, transmission electron microscopy, energy dispersion spectrum, N2 adsorption, and Fourier transform infrared spectra. Results showed that the high content of Co (rCo:Si = 0.17) can be efficiently dispersed into the mesoporous SiO2 as forms of Co3O4 NPs, and the structural ordering of the mesoporous SiO2 was well-preserved at the same time. The Co3O4 NP functionalized mesoporous SiO2 materials were used as Fenton-like catalysts for removing methylene blue (MB) from aqueous solutions. The catalyst prepared at rCo:Si = 0.17 could completely remove the high-concentration of MB (120 mg·L−1), and also showed an excellent performance with a removal capacity of 138 mg·g−1 to 180 mg·L−1 of MB. Catalytic mechanisms were further revealed, based on the degradation results.

Niobium doping enhanced catalytic performance of Mn/MCM-41 for toluene degradation in the NTP-catalysis system

The destruction of toluene in a dielectric barrier discharge (DBD) reactor with Nb-Mn/MCM-41 had been investigated and compared with X (X = Cu, Ce, Co)-Mn/MCM-41 catalysts. The XRD and TEM result confirms that the metal species were highly dispersed on the MCM-41. The results of XPS, O₂-TPD and H₂-TPR clearly demonstrate that Nb doping facilitated formation of lattice oxygen (O_latt) and the acid sites, which are all beneficial to catalytic degradation of toluene. Compared to X (Cu, Ce, Co)-Mn/MCM-41, Nb-Mn/MCM-41 had the most contents of O_latt, the most amounts of acid sites and the strongest acidity. Consequently, the catalytic performance tests identify that Nb-Mn/MCM-41 had the best catalytic performance, the highest removal efficiency and CO₂ selectivity as well as carbon balance especially at low SIE. These results indicate that Nb was an important promoter improving the activity and CO₂ selectivity of Mn/MCM-41 for the decomposition of toluene in NTP-catalysis system.

Enhanced plasma-catalytic decomposition of toluene over Co-Ce binary metal oxide catalysts with high energy efficiency

In-plasma catalysis has been considered as a promising technology to degrade volatile organic compounds. Heterogeneous catalysts, especially binary metal oxide catalysts, play an important role in further advancing the catalytic performance of in-plasma catalysis. This work investigates the toluene decomposition performance over Co-Ce binary metal oxide catalysts within the in-plasma catalysis. Co-Ce catalysts with different Co/Ce molar ratios are synthesized by a citric acid method. Results show that the catalytic activity of Co-Ce catalysts is obviously superior to those of monometallic counterparts. Especially, Co0.75Ce0.25O x catalyst simultaneously realizes highly efficient toluene conversion (with a decomposition efficiency of 98.5% and a carbon balance of 97.8%) and a large energy efficiency of 7.12 g kW h-1, among the best performance in the state-of-art literature (0.42 to 6.11 g kW h-1). The superior catalytic performance is further interpreted by the synergistic effect between Co and Ce species and the significant plasma-catalyst interaction. Specifically, the synergistic effect can decrease the catalyst crystallite size, enlarge the specific surface area and improve the amount of oxygen vacancies/mobility, providing more active sites for the adsorption of surface active oxygen species. Meanwhile, the plasma-catalyst interaction is able to generate the surface discharge and reinforce the electric field strength, thereby accelerating the plasma-catalytic reactions. In the end, the plasma-catalytic reaction mechanism and pathways of toluene conversion are demonstrated.

Performance of Toluene Removal in a Nonthermal Plasma Catalysis System over Flake-Like HZSM-5 Zeolite with Tunable Pore Size and Evaluation of Its Byproducts

In this study, a series of HZSM-5 catalysts were prepared by the chemical liquid-phase deposition method, and low concentration toluene degradation was carried out in an atmospheric pressure dielectric barrier discharge (DBD) reactor. The catalysts were characterized by X-ray powder diffraction (XRD), SEM, TEM, and N₂ adsorption analysis techniques. In addition, several organic contaminants were used to evaluate the adsorption performance of the prepared catalysts, and the effect of pore size on the removal efficiency of toluene and byproduct formation was also investigated. The unmodified HZSM-5 zeolite (Z0) exhibited good performance in toluene removal and CO₂ selectivity due to the diffusion resistance of ozone and the amounts of active species (OH• and O•). Meanwhile, the time of flight mass spectrometry (TOF-MS) result showed that there were more byproducts of the benzene ring in the gas phase under the action of small micropore size catalysts. Moreover, the surface byproducts were detected by gas chromatography–mass spectrometry (GC-MS).

Probing the nature of active chromium species and promotional effects of potassium in Cr/MCM-41 catalysts for methyl mercaptan abatement

The introduction of K enables a large number of CrO₄²⁻ active species to be anchored and dispersed on the surface of Cr-based catalysts.

Plasma-catalytic removal of toluene over the supported manganese oxides in DBD reactor: Effect of the structure of zeolites support

The degradation of toluene in dielectric barrier discharge (DBD) reactor packed with zeolites or MnO_x/zeolites was investigated. The supported catalysts were prepared by loading 3 wt% of manganese on different zeolites (MCM-41, ZSM-5 and 13X) and were characterized in detail using N₂ adsorption, XRD, TEM, H₂-TPR and XPS analysis technology. Compared to the non-thermal plasma (NTP) alone system, the toluene degradation was improved significantly in NTP-MnO_x/zeolites system. The highest toluene conversion of 99.4%, the CO₂ selectivity of 73%, the carbon balance of 99.5% can be achieved in DBD reactor packed with MnO_x/MCM-41. Both XRD and TEM results confirm that the manganese oxides were dispersed more uniformly on MnO_x/MCM-41 than on MnO_x/ZSM-5 or MnO_x/13X. H₂-TPR and XPS results suggest that manganese oxides on MnO_x/MCM-41 are MnO₂ and Mn₂O₃, while those on MnO_x/ZSM-5 or MnO_x/13X are MnO₂ and MnO. These results indicate that the structures of zeolites play a significant role in the dispersion and oxidation state of manganese oxides, then affecting the activity of catalyst for toluene removal in plasma-catalysis system.

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.

Co-modified MCM-41 as an effective adsorbent for levofloxacin removal from aqueous solution: optimization of process parameters, isotherm, and thermodynamic studies

Antibiotics are emerging contaminants due to their potential risks to human health and ecosystems. Poor biodegradability makes it necessary to develop effective physical-chemical methods to eliminate these contaminants from water. The cobalt-modified MCM-41 was prepared by a one-pot hydrothermal method and characterized by SAXRD, N₂ adsorption-desorption, SEM, UV-Vis DR, and FTIR spectroscopy. The results revealed that the prepared 3% Co-MCM-41 possessed mesoporous structure with BET surface areas at around 898.5 m²g^-1. The adsorption performance of 3% Co-MCM-41 toward levofloxacin (LVF) was investigated by batch experiments. The adsorption of LVF on 3% Co-MCM-41 was very fast and reached equilibrium within 2 h. The adsorption kinetics followed the pseudo-second-order kinetic model with the second-order rate constants in the range of 0.00198-0.00391 g mg^-1 min^-1. The adsorption isotherms could be well represented by the Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm equations. Nevertheless, D-R isotherm provided the best fit based on the coefficient of determination and average relative error values. The mean free energy of adsorption (E) calculated from D-R model was about 11 kJ mol^-1, indicating that the adsorption was mainly governed by a chemisorption process. Moreover, the adsorption capacity was investigated as a function of pH, adsorbent dosage, LVF concentration, and temperature with help of respond surface methodology (RSM). A quadratic model was established, and an optimal condition was obtained as follows: pH 8.5, adsorbent dosage of 1 g L^-1, initial LVF concentration of 119.8 mg L^-1, and temperature of 31.6 °C. Under the optimal condition, the adsorption capacity of 3% Co-MCM-41 to LVF could reach about 108.1 mg g^-1. The solution pH, adsorbent dosage, LVF concentration, and a combination of adsorbent dose and LVF concentration were significant factors affecting the adsorption process. The adsorption thermodynamic functions were also determined. The negative ΔH ⁰ (-33.50 kJ mol^-1) and ΔS ⁰ (-43.57 J mol^-1 K^-1) suggested that the adsorption was an exothermic process accompanied by decreasing disorder. This study may indicate that 3% Co-MCM-41 is a promising adsorbent for removing emerging pollutants of LVF from water.