Plasma-catalyst hybrid reactor with CeO2/γ-Al2O3 for benzene decomposition with synergetic effect and nano particle by-product reduction

Enhancement of light-driven adsorption efficacy through the integration of NiCo2O4 onto CeO2 for photo-ozone catalytic degradation of toluene

In this study, a co-catalytic route was explored to enhance the photo-ozone catalytic degradation of volatile organic compounds (VOCs). NiCo2O4 was loaded onto the surface of CeO2 nanoparticles to create a composite catalyst (10%NiCo2O4/CeO2). The integration of NiCo2O4 onto CeO2 enhanced the interaction between the catalyst and toluene, a representative VOC, resulting in significantly increased toluene adsorption without a corresponding increase in specific surface area. This integration also improved the utilization of charge carriers and conversion of ozone to O2-. Under visible light irradiation, H2O accumulated charge carriers at 10%NiCo2O4/CeO2's surface, facilitating both ozone utilization and toluene adsorption. Another benefit of NiCo2O4 loading was its ability to enhance the conversion efficiency of solar energy. Consequently, the toluene removal and mineralization efficiencies of 10%NiCo2O4/CeO2 were enhanced by 182% and 309% compared to CeO2, and by 201% and 357% compared to NiCo2O4, respectively. Overall, this study demonstrated a novel co-catalyst design strategy for enhancing the photo-ozone catalytic degradation of VOCs.

Revealing the intrinsic nature of Ni-, Mn-, and Y-doped CeO2 catalysts with positive, additive, and negative effects on CO oxidation using operando DRIFTS-MS

The catalytic activity of a transition metal (host) oxide can be influenced by doping with a second cation (dopant), but the key factors dominating the activity of the doped catalyst are still controversial. Herein, CeO2 doped with Ni, Mn, and Y catalysts prepared using aerosol pyrolysis were used to demonstrate the positive, negative, and additive effects on CO oxidation as a model reaction. Various characterization results indicated that Ni, Mn, and Y had been successfully doped into the CeO2 lattice. The catalytic activities of each catalyst for CO conversion were in the order of Ni-CeO2 > Mn-CeO2 > CeO2 > Y-CeO2. Operando DRIFTS-MS and various characterization methods were applied to reveal the intrinsic nature of the doping effects. The accumulation rate of the surface bidentate carbonates determined the CO oxidation. A definition to evaluate the doping effect was proposed, which is anticipated to be useful for developing a rational catalyst with a high CO oxidation activity. The CO oxidation reactivities displayed strong correlations with the surface factors obtained from operando DRIFTS-MS analysis and the structure factors from XPS and Raman analyses.

Investigation of photocatalytic-proxone process performance in the degradation of toluene and ethyl benzene from polluted air

In this study, toluene and ethylbenzene were degraded in the photocatalytic-proxone process using BiOI@NH₂-MIL125(Ti)/Zeolite nanocomposite. The simultaneous presence of ozone and hydrogen peroxide is known as the proxone process. Nanocomposite Synthesis was carried out using the solvothermal method. Inlet airflow, ozone concentrations, H₂O₂ concentrations, relative humidity, and initial pollutants concentrations were studied. The nanocomposite was successfully synthesized based on FT-IR, BET, XRD, FESEM, EDS element mapping, UV-Vis spectra and TEM analysis. A flow rate of 0.1 L min^-1, 0.3 mg min^-1 of ozone, 150 ppm of hydrogen peroxide, 45% relative humidity, and 50 ppmv of pollutants were found to be optimal operating conditions. Both pollutants were degraded in excess of 95% under these conditions. For toluene and ethylbenzene, the synergistic of mechanisms effect coefficients were 1.56 and 1.76, respectively. It remained above 95% efficiency 7 times in the hybrid process and had good stability. Photocatalytic-proxone processes were evaluated for stability over 180 min. The remaining ozone levels in the process was insignificant (0.01 mg min^-1). The CO₂ and CO production in the photocatalytic-proxone process were 58.4, 5.7 ppm for toluene and 53.7, and 5.5 ppm for ethylbenzene respectively. Oxygen gas promoted and nitrogen gas had an inhibitory effect on the effective removal of pollutants. During the pollutants oxidation, various organic intermediates were identified.

Synthesis of BiOI@NH2-MIL125(Ti)/Zeolite as a novel MOF and advanced hybrid oxidation process application in benzene removal from polluted air stream

One of the popular process in volatile organic compounds removal in gas phase is advanced oxidation process. We in this research, synthesized BiOI@NH2-MIL125(Ti)/Zeolite nanocomposite as a novel nanocomposite to degradation of benzene in hybrid advanced oxidation process. The nanocomposite synthesized via solvothermal method. The effect of airflow, ozone gas concentration, hydrogen peroxide concentration, relative humidity and initial benzene concentration are the main parameters in the UV/O3/H2O2/ nanocomposite hybrid process that were studied. The characterization by XRD, FT-IR, FESEM, EDS element mapping, TEM, BET, and UV-vis spectra indicated that nanocomposite were well synthesized. Optimal operating conditions of the process were determined at air flow of 0.1 l/min, ozone concentration of 0.3 mg/min, hydrogen peroxide concentration of 150 ppm, relative humidity of 45 ± 3% and benzene concentration of 50 ppmv. Under these conditions, more than 99% of benzene was degraded. The synergistic effect coefficient of the mechanisms is 1.53. The nanocomposite had good stability in the hybrid process and remained above 99% efficiency up to 5 times. The ozone concentration residual the system was reported to be negligible (0.013 mg/min). The CO and CO2 emissions in the hybrid process was higher than other processes, which indicates better mineralization in the hybrid process. Formaldehyde, octane, noonan, phenol, decanoic acid were reported as the main by-products. The results indicated that UV/O3/H2O2/ nanocomposite hybrid process has fantastic efficiency in the degradation of benzene as one of the indicators of VOCs.

MOF-derived MnO@C with high activity for electric field-assisted catalytic oxidation of aqueous pollutants

The activation of oxygen (O₂) under room condition is important for the utilization of air to perform oxidation. Here, we report a porous carbon-encapsulated MnO (MnO@C) derived from Mn metal-organic framework (MOF)grown in-situ on a graphite felt (GF) support. The MnO@C exhibits superior catalytic activity in an electric field-assisted catalytic oxidation system for the degradation of organic pollutants under room condition. The catalytic oxidation reaction applies a surface reaction pathway in which the surface-bound chemisorbed oxygen species are electro-oxidized and then involved in the oxidation of co-adsorbed organic pollutants. The abundant oxygen vacancies and oxygenated functional groups in MnO@C provide active sites for the chemisorption of O₂, and its conductive mesoporous structure allows facile electrons and mass transfer. As a result, the MnO@C/GF catalyst displays quite high turnover frequency (TOF) value as 0.038 mg-TOC mg-MnO^-1 min^-1, which is 6.66 times higher than that of the MnO/GF catalyst prepared by impregnation method as a comparison. With the aid of + 1.0 V of positive electric field, the catalytic oxidation system exhibits extensive effectiveness in mineralizing a variety of dyes, pharmaceuticals, personal care products, and phenolic compounds under room condition with significantly enhanced biodegradability.

Exploring the roles of oxygen species in H2 oxidation at β-MnO2 surfaces using operando DRIFTS-MS

Understanding of the roles of oxygen species at reducible metal oxide surfaces under real oxidation conditions is important to improve the performance of these catalysts. The present study addresses this issue by applying a combination of operando diffuse reflectance infrared Fourier transform spectroscopy with a temperature-programmed reaction cell and mass spectrometry to explore the behaviors of oxygen species during H2 oxidation in a temperature range of 25-400 °C at β-MnO2 surfaces. It is revealed that O2 is dissociated simultaneously into terminal-type oxygen (M2+-O2-) and bridge-type oxygen (M+-O2--M+) via adsorption at the Mn cation with an oxygen vacancy. O2 adsorption is inhibited if the Mn cation is covered with terminal-adsorbed species (O, OH, or H2O). In a temperature range of 110-150 °C, OH at Mn cation becomes reactive and its reaction product (H2O) can desorb from the Mn cation, resulting in the formation of bare Mn cation for O2 adsorption and dissociation. At a temperature above 150 °C, OH is reactive enough to leave bare Mn cation for O2 adsorption and dissociation. These results suggest that bare metal cations with oxygen vacancies are important to improve the performance of reducible metal oxide catalysts.

Preparation of graphene-based catalysts and combined DBD reactor for VOC degradation

The objective of this study was to compare the transformation of by-products between single dielectric barrier discharge (SDBD) and double dielectric barrier discharge (DDBD), to optimize the preparation of graphene-based catalysts and apply them in combination with DBD for volatile organic compound degradation. We compared the degradation performance of SDBD and DDBD, prepared, and characterized graphene-based catalysts. SEM, BET, XRD, and FTIR analyses showed that the morphologies and internal structures of the three catalysts were the best when 0.25 mL of [BMIM]PF6 was added. When MnO_x/rGO, FeO_x/rGO, and TiO_x/rGO were used in combination with DDBD, the degradation rates of benzene were found to be 83.5%, 77.2%, and 63.8%, respectively, whereas the O₃ transformation rates were 60%, 79%, and 40%, respectively. Moreover, the NO₂ transformation rates were 70%, 55%, and 42.5%, respectively, whereas the NO transformation rates were 69%, 39%, and 33.5%, respectively. The CO₂ selectivity was 62%, 51%, and 49%, respectively. MnO_x/rGO exhibited superior performance in the degradation of benzene series, NO transformation, NO₂ transformation, CO₂ selectivity, and energy efficiency. On the other hand, FeO_x/rGO exhibited superior performance for O₃ transformation. Based upon the XPS analysis, it was found that Mn₃O₄ and Fe₃O₄ played a leading role in promoting the degradation of benzene series and the transformation of by-products.

A critical review on plasma-catalytic removal of VOCs: Catalyst development, process parameters and synergetic reaction mechanism

It is urgent to control the emission of volatile organic compounds (VOCs) due to their harmful effects on the environment and human health. A hybrid system integrating non-thermal-plasma and catalysis is regarded as one of the most promising technologies for VOCs removal due to their high VOCs removal efficiency, product selectivity and energy efficiency. This review systematically documents the main findings and improvements of VOCs removal using plasma-catalysis technology in recent 10 years. To better understand the fundamental relation between different aspects of this research field, this review mainly addresses the catalyst development, key influential factors, generation of by-products and reaction mechanism of VOCs decomposition in the plasma-catalysis process. Also, a comparison of the performance in various VOCs removal processes is provided. Particular emphasis is given to the importance of the selected catalyst and the synergy of plasma and catalyst in the VOCs removal in the hybrid system, which can be used as a reference point for future studies in this field.

Manganese-doped molybdenum oxide boosts catalytic performance of electrocatalytic wet air oxidation at ambient temperature

Mn-doping strategy was adopted to modify the structure of MoO₂ for enhancing its catalytic activity towards room-temperature electrocatalytic wet air oxidation (ECWAO) reaction. A series of Mn-doped MoO₂ were prepared on carbon support, and their structures were investigated to elucidate the productive effect of Mn doping on the catalytic activity of MoO₂. The incorporation of Mn^III/Mn^II into the MoO₂ lattice induced the transformation from Mo^IV to Mo^V and created more oxygen vacancies. Such structural modifications promoted the electron transfer of MoO₂ through the redox couples between Mo^VI/Mo^V/Mo^IV and Mn^III/Mn^II, and facilitated the transformation from O₂ to adsorbed oxygen species on MoO₂ surface. As a result, the ECWAO catalytic activities of Mn-doped MoO₂/graphite felt (MoO₂/GF) outperformed the activity of MoO₂/GF. Among the synthesized series, Mn_0.066:MoO₂/GF exhibited the highest activity with the maximum turnover frequency (TOF) promoted by 59% than the undoped MoO₂/GF. Under the catalysis of Mn_0.066:MoO₂/GF, the ECWAO process obtains mineralization efficiencies generally above 85% in degrading typical pharmaceutics and person care products (PPCPs). These findings are anticipated to open up a new venue in the design and fabrication of highly active catalysts for air oxidation reactions by using the strategy of selective dopant-induced structure modification.

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.

High ratio of Ce3+/(Ce3++Ce4+) enhanced the plasma catalytic degradation of n-undecane on CeO2/γ-Al2O3

n-Undecane (C11) is the main component of volatile organic compounds (VOCs) emitted from the printing industry, and its emission to the atmosphere should be controlled. In this study, a dielectric barrier discharge reactor coupled with CeO₂/γ-Al₂O₃ catalysts was used to degrade C11. The effect of the chemical state of CeO₂ on C11 degradation was evaluated by varying the CeO₂ loading on γ-Al₂O₃. The C11 conversion and COx selectivity were as high as 92% and 80%, respectively, under mild reaction conditions of energy density 34 J/L and 423 K to degrade 134 mg/m³ C11 in a simulated air using 10 wt%CeO₂ impregnated on γ-Al₂O₃. After analyses using in-situ plasma diffuse reflectance Fourier transform infrared spectroscopy and gas chromatography-mass spectrometry, it was found that most of C11 were degraded to CO₂, and the main by-products on catalyst surfaces were alcohols and ketones. It was concluded from X-ray photoemission spectroscopy that the good performance of the 10 wt%CeO₂/γ-Al₂O₃ catalyst was due to its high Ce³⁺/(Ce³⁺+Ce⁴⁺) ratio as well as the oxygen vacancies. The Ce³⁺/(Ce³⁺+Ce⁴⁺) ratio of CeO₂ on γ-Al₂O₃ is crucial for the degradation of C11, providing a further roadmap for the plasma catalytic oxidation of alkanes.

Degradation of contaminants in plasma technology: An overview

The information of plasma technologies applications for environmental clean-up on treating and degrading metals, metalloids, dyes, biomass, antibiotics, pesticides, volatile organic compounds (VOCs), bacteria, virus and fungi is compiled and organized in the review article. Different reactor configurations of plasma technology have been applied for reactive species generation, responsible for the pollutants removal, hydrogen and methane production and microorganism inactivation. Therefore, in this review article, the reactive species from discharge plasma are presented here to provide the insight into the environmental applications. The combinations of plasma technology with flux agent and photocatalytic are also given in this review paper associated with the setup of the plasma system on the removal process of metals, VOCs, and microorganisms. Furthermore, the potential of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) inactivation via plasma technology is also described in this review paper. Detailed information of plasma parameter configuration is given to support the influence of the critical process in the plasma system to deal with contaminants.

Degradation of Benzene Using Dielectric Barrier Discharge Plasma Combined with Transition Metal Oxide Catalyst in Air

In this paper, a uniform and stable dielectric barrier discharge plasma is presented for degradation of benzene combined with a transition metal oxide catalyst. The discharge images, waveforms of discharge current, and the optical emission spectra are measured to investigate the plasma characteristics. The effects of catalyst types, applied voltage, driving frequency, and initial VOCs concentration on the degradation efficiency of benzene are studied. It is found that the addition of the packed dielectric materials can effectively improve the uniformity of discharge and enhance the intensity of discharge, thus promoting the benzene degradation efficiency. At 22 kV, the degradation efficiencies of dielectric barrier discharge plasma packed with CuO, ZnO and Fe3O4 are 93.6%, 93.2% and 76.2%, respectively. When packing with ZnO, the degradation efficiency of the dielectric barrier discharge plasma is improved from 86.8% to 94.9%, as the applied voltage increases from 16 kV to 24 kV. The catalysts were characterized by XPS, XRD and SEM. The synergistic mechanism and the property of the catalyst are responsible for benzene degradation in the plasma–catalysis system. In addition, the main physiochemical processes and possible degradation mechanism of benzene are discussed.

Advanced catalytic ozonation for degradation of pharmaceutical pollutants-A review

Various chemical treatment techniques are involved in removing refractory organic compounds from water and wastewater using the oxidation reaction of hydroxyl radicals (^•OH). The use of catalysts in advanced catalytic ozonation is likely to improve the decomposition of molecular ozone to generate highly active free radicals that facilitate the rapid and efficient mineralization and degradation of numerous organics. For the degradation of toxic organic pollutants in wastewater, the advanced catalytic ozonation process has been widely applied in recent years. Low utilization efficiency of ozone and ineffective mineralization of organic contaminants by ozone can be remedied with advanced catalytic ozonation. Advanced catalytic ozonation has gained popularity because of these merits. However, homogeneous catalytic ozonation has the disadvantage of producing secondary contaminants from the addition of metallic ions. Heterogeneous catalytic ozonation can overcome this drawback by utilizing metals, metallic oxides, and carbon materials as a catalyst of efficacy and stability. This review discusses various aspects of catalytic ozonation in wastewater treatment of pharmaceutical pollutants, application of catalytic ozonation process in typical wastewater, and prospects in advancing the techniques in heterogeneous catalytic ozonation.

Evaluation of Au/γ-Al2O3 nanocatalyst for plasma-catalytic decomposition of toluene

The emission of toluene into the atmosphere can seriously affect the environmental quality and endanger human health. A dielectric barrier discharge reactor filled with a small amount of Au nanocatalysts was used to decompose toluene in He and O₂ gases mixtures at room temperature and atmospheric pressure. Normally, the oxidation of toluene using Au nanocatalysts suffers from low reaction activity and facile catalyst deactivation. Herein, the effects of Au loading, calcination time and calcination temperature were systematically investigated. It was found that 0.1 wt%Au/γ-Al₂O₃ calcined at 300 °C for 5 h can keep an average size around 6 nm with good dispersion on γ-Al₂O₃ surface and display the best catalytic performance. Moreover, the influences of energy density, gas flow rate, toluene concentration and O₂ concentration on toluene degradation using 0.1 wt%Au/γ-Al₂O₃ were evaluated. It showed the best catalytic performance of near 100% conversion for toluene degradation under the reaction conditions of the energy density was 20 J/L, the gas flow rate was 300 mL/min, the concentration of toluene was 376 mg/m³ and the oxygen content was 10%. Combining experimental results and theoretical calculations, the values of reaction constant k were 8.6 × 10^-5, 3.53 × 10^-5 and 3.09 × 10^-5 m⁶/(mol*J), when O₂ concentration, power or flow rate changed, respectively. Therefore, O₂ concentration has the greatest effect on toluene decomposition compared to other factors in the presence of Au/γ-Al₂O₃.

Removal of toluene as a toxic VOC from methane gas using a non-thermal plasma dielectric barrier discharge reactor

Methane is the main component of biogas, which could be used as a renewable energy source for electricity, source of heat, and biofuel production after upgrading from biogas. It also contains toxic compounds which cause environmental and human health problems. Therefore, in this work, the removal of a toxic compound (toluene) from methane gas was studied using a dielectric barrier discharge (DBD) reactor. It was observed that the removal of the toxic compound could be achieved from methane carrier gas using a dielectric barrier discharge reactor, and it depends on plasma input power. The maximum removal of the toxic compound was 85.9% at 40 W and 2.86 s. The major gaseous products were H₂ and lower hydrocarbons (LHC) and the yield of these products also increases with input power. In the current study, the yield of gaseous products depends on the decomposition of toxic compounds and methane, because the decomposition of methane also produces H₂ and lower hydrocarbons. The percentage yield of H₂ increases from 0.43-4.74%. Similarly, the yield of LHC increases from 0.56-7.54% under the same reaction conditions. Hence, input power promoted the decomposition of the toxic compound and enhanced the yield of gaseous products.

The post plasma-catalytic decomposition of toluene over K-modified OMS-2 catalysts at ambient temperature: Effect of K+ loading amount and reaction mechanism

The present work is devoted to study the post plasma-catalytic (PPC) degradation of toluene using packed-bed discharge (PBD) plasma over K-modified manganese oxide octahedral molecular sieve (OMS-2) catalysts at ambient temperature. Compared to plasma alone, PPC can significantly improve the toluene degradation and mineralization performance simultaneously, and the generation of discharge byproducts and organic intermediates is suppressed. The catalytic capacity of OMS-2 for toluene degradation is greatly promoted by tuning potassium ions (K⁺) content in OMS-2 tunnel, which might be owing to the formation of more surface active oxygen species derived from weak Mn-O bonds, plenty of oxygen vacancies, as well as more superior low-temperature reducibility. Highest toluene degradation efficiency (89.4%) and CO_x selectivity (88.9%) can be achieved in plasma-catalysis system over K-modified OMS-2 sample with K/Mn molar ratio of 2 at the SIE of 658 J/L. A long-term stability test has also been successfully carried out to evaluate the stability of K-modified OMS-2 with the assistance of plasma. Possible reaction mechanism for plasma-catalytic degradation of toluene on K-modified OMS-2 catalyst has been proposed based on the plasma diagnosis, catalysts characterization, and organic intermediates identification. This work aims to gaina deeperunderstandingof plasma-catalytic degradation mechanism and provides an environmentally friendly and energy-efficient method for practical volatile organic compounds (VOCs) abatement in PPC process.

Electric field assisted benzene oxidation over Pt-Ce-Zr nano-catalysts at low temperature

A novel catalytic system for benzene oxidation at low temperature is constructed by combining electric field with Pt-Ce-Zr nano-catalyst. The 1 wt% Pt/Ce_0.75Zr_0.25O₂ catalyst assisted by electric field shows the best catalytic performance with 90% benzene conversion at 96.5 °C and excellent water resistance. The effect of electric field on catalysts and catalytic process is comprehensively investigated. The results of XRD, TEM, XPS and H₂-TPR reveal that the electric field show negligible influence on the crystal structure and surface morphology of the catalyst, but it can lead to more oxygen vacancies. Therefore, more adsorbed oxygen with higher activity will be produced on the catalyst surface. The redox performance is improved due to the fact that valence distribution of Pt is changed in forms of more active sites composed of high valence oxides (PtO) generated in electric field. In situ DRIFTS is used to investigate the oxidation process of benzene and the results prove that electric field could accelerate the production and consumption of intermediate products, and produce new intermediate products such as carboxylic acid species, indicating that the introduction of electric field may open up a new rapid reaction path and promote the activation of benzene at low temperature.

Effect of supports on plasma catalytic decomposition of toluene using in situ plasma DRIFTS

Plasma catalysis technology has been demonstrated to be effective for the decomposition of volatile organic compounds (VOCs). It is highly desired to explore the effect of supports on VOCs oxidation processes during plasma catalysis. In this work, four supports of SiO₂, ZSM-5-300, ZSM-5-38 and γ-Al₂O₃ loading with transition metal oxides were used to decompose toluene at room temperature. It was found that toluene decomposition with 1 wt%Mn/γ-Al₂O₃ was highest, which was strongly proportional to the ozone decomposition ability of the catalyst. The plasma catalytic decomposition of toluene over 1 wt% MnO₂ on different supports were characterized using in situ plasma diffuse reflectance infrared Fourier transform spectrometer. The results showed that 1 wt%Mn/γ-Al₂O₃ could further catalyze toluene to carbonate and bicarbonate via the breakage of C-C bonds from benzoic acid, while that was difficult for 1 wt% Mn/SiO₂, 1 wt%Mn/ZSM-5-300 and 1 wt%Mn/ZSM-5-38. The reaction mechanism of toluene decomposition on different catalysts were proposed.

Mitigation of indoor air pollution: A review of recent advances in adsorption materials and catalytic oxidation

Indoor air pollution with toxic volatile organic compounds (VOCs) and fine particulate matter (PM2.5) is a threat to human health, causing cancer, leukemia, fetal malformation, and abortion. Therefore, the development of technologies to mitigate indoor air pollution is important to avoid adverse effects. Adsorption and photocatalytic oxidation are the current approaches for the removal of VOCs and PM2.5 with high efficiency. In this review we focus on the recent development of indoor air pollution mitigation materials based on adsorption and photocatalytic decomposition. First, we review on the primary indoor air pollutants including formaldehyde, benzene compounds, PM2.5, flame retardants, and plasticizer: Next, the recent advances in the use of adsorption materials including traditional biochar and MOF (metal-organic frameworks) as the new emerging porous materials for VOCs absorption is reviewed. We review the mechanism for mitigation of VOCs using biochar (noncarbonized organic matter partition and adsorption) and MOF together with parameters that affect indoor air pollution removal efficiency based on current mitigation approaches including the mitigation of VOCs using photocatalytic oxidation. Finally, we bring forward perspectives and directions for the development of indoor air mitigation technologies.

Prediction and evaluation of plasma arc reforming of naphthalene using a hybrid machine learning model

We have developed a hybrid machine learning (ML) model for the prediction and optimization of a gliding arc plasma tar reforming process using naphthalene as a model tar compound from biomass gasification. A linear combination of three well-known algorithms, including artificial neural network (ANN), support vector regression (SVR) and decision tree (DT) has been established to deal with the multi-scale and complex plasma tar reforming process. The optimization of the hyper-parameters of each algorithm in the hybrid model has been achieved by using the genetic algorithm (GA), which shows a fairly good agreement between the experimental data and the predicted results from the ML model. The steam-to-carbon (S/C) ratio is found to be the most critical parameter for the conversion with a relative importance of 38%, while the discharge power is the most influential parameter in determining the energy efficiency with a relative importance of 58%. The coupling effects of different processing parameters on the key performance of the plasma reforming process have been evaluated. The optimal processing parameters are identified achieving the maximum tar conversion (67.2%), carbon balance (81.7%) and energy efficiency (7.8 g/kWh) simultaneously when the global desirability index I₂ reaches the highest value of 0.65.

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.

High energy efficient degradation of toluene using a novel double dielectric barrier discharge reactor

A double dielectric barrier discharge (DDBD) reactor was established to decompose toluene with high energy efficiency. Differences in discharge characteristics including visual images, voltage-current waveforms, Lissajous figures, and temperature variation, were determined between the DDBD and SDBD reactors. Removal efficiency, mineralization rate, CO₂ selectivity, and energy yield were used to evaluate the toluene abatement performance of the two reactors. Compared to the SDBD reactor, the DDBD reactor exhibited more uniform and stable discharges due to a change in discharge mode. In addition, the DDBD reactor's dissipated power and reactor temperature (including the gas, barrier and ground electrode) were significantly lower than those in the SDBD reactor. At 22-24 kV, the DDBD reactor showed a higher toluene removal efficiency and mineralization rate, while at 14-16 kV, the SDBD reactor exhibited higher respective value. The energy efficiency of the DDBD was 2.5-3 times that of the SDBD reactor, and the overall energy constant k_overall of the DDBD reactor (1.47 mL/J) was significantly higher than that of the SDBD reactor (0.367 mL/J) as revealed by the kinetics study. Lastly, a plausible toluene degradation mechanism in the DDBD and SDBD reactors was proposed based on organic intermediates that formed during toluene decomposition.

Enhanced moisture resistance of Cu/Ce catalysts for CO oxidation via Plasma-Catalyst interactions

Copper/cerium bimetallic catalyst is an efficient material for the removal of carbon monoxide, while the rapid deactivation under moisture-rich conditions in the conventional thermal-catalysis limited its wide application. Here, we investigated the plasma-assisted catalytic oxidation of CO over Cu/Ce oxides supported on γ-Alumina in comparison with the conventional thermal catalytic oxidation. The TOF values of the Cu/Ce catalysts showed that the plasma catalysis was the better catalytic system for CO oxidation (2.96 s^-1 for thermal catalysis, 5.13 s^-1 for plasma catalysis). Importantly, the energy barriers for plasma catalysis were much lower than that for thermal catalysis, especially under moisture-rich conditions (e.g. 130.3 kJ/mol versus 246.1 kJ/mol under 9.8 vol% water vapor). The loss of activity caused by water was reversible for the plasma process, but not for the thermal process. The Cu/Ce catalyst remained good stability within 60 h in the presence of 6.1% water for plasma oxidation, while the thermal catalytic activity declines gradually. Also, water could inhibit the formation of gas byproducts (O₃ and NO_x). The promoting role of plasma could be mainly ascribed to the enhanced strength of oxygen mobility and plasma-assisted decomposition of surface carbonate in the presence of water, as revealed by the in-situ NTP-TPR, XPS, and the ex-situ DRIFTS analyses.

Enhanced removal of toluene by pulse discharge plasma coupled with MgO cathode and graphene Mn-Ce bimetallic oxide

Herein, MgO cathode and graphene Mn-Ce bimetallic oxide were utilized to jointly enhance the removal of toluene in pulsed discharge plasma (PDP). Compared to the common cathode, the MgO cathode enhanced the density of high energy electrons, and then induced to higher removal of toluene. However, the removal of toluene by PDP/MgO system was still insufficient, and there was a large amount of underutilized O₃ in the products. Based on this, Mn-Ce/graphene catalysts were introduced into PDP/MgO system. The Mn-Ce (8:1)/graphene catalyst had the highest catalytic activity. Under the discharge power of 2.1 W, toluene degradation rate and CO₂ selectivity increased by 27.5% and 22.0%, respectively. This was ascribed to the synergistic effect of the solid solution formed between MnO_x and CeO_x, increasing the proportion of O_ads on the surface of the catalyst. The higher O_ads/O_latt ratio lead to the better catalytic activity, which was conducive to the complete transformation of the intermediate products to CO₂ and H₂O. According to the detected products, the degradation pathway and the mechanism of toluene degradation were proposed finally. The PDP itself, field emission effect of MgO cathode and catalytic effect of Mn-Ce/graphene for jointly improve the toluene removal and CO₂ selectivity.

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.

Catalytic ozonation for water and wastewater treatment: Recent advances and perspective

Ozonation process has been widely applied in water and wastewater treatment, such as for disinfection, for degradation of toxic organic pollutants. However, the utilization efficiency of ozone is low and the mineralization of organic pollutants by ozone oxidation is ineffective, and some toxic disinfection byproducts (DBPs) may be formed during ozonation process. Catalytic ozonation process can overcome these problems to some extent, which has received increasing attention in recent years. During catalytic ozonation, catalysts can promote O₃ decomposition and generate active free radicals, which can enhance the degradation and mineralization of organic pollutants. In this paper, the history of ozonation application in water treatment was briefly reviewed. The properties of the ozone molecule, the ozonation types and several ozone-based water treatment processes were briefly introduced. Various catalysts for catalytic ozonation, including homogeneous and heterogeneous catalysts, such as metal ions, metal oxidizes, carbon-based materials and their possible catalytic mechanisms were analyzed and summarized in detail. Furthermore, some inconsistent results of previous research on catalytic ozonation were analyzed and discussed. The application of catalytic oxidation for the degradation of toxic organic pollutants, including phenols, pesticides, dyes, pharmaceuticals and others, was summarized. Finally, several key aspects of catalytic ozonation, such as pH effect, the catalyst performance, the catalytic mechanism were proposed, to which more attention should be paid in future study.

Silver-Copper Oxide Heteronanostructures for the Plasmonic-Enhanced Photocatalytic Oxidation of N-Hexane in the Visible-NIR Range

Volatile organic compounds (VOCs) are recognized as hazardous contributors to air pollution, precursors of multiple secondary byproducts, troposphere aerosols, and recognized contributors to respiratory and cancer-related issues in highly populated areas. Moreover, VOCs present in indoor environments represent a challenging issue that need to be addressed due to its increasing presence in nowadays society. Catalytic oxidation by noble metals represents the most effective but costly solution. The use of photocatalytic oxidation has become one of the most explored alternatives given the green and sustainable advantages of using solar light or low-consumption light emitting devices. Herein, we have tried to address the shortcomings of the most studied photocatalytic systems based on titania (TiO₂) with limited response in the UV-range or alternatively the high recombination rates detected in other transition metal-based oxide systems. We have developed a silver-copper oxide heteronanostructure able to combine the plasmonic-enhanced properties of Ag nanostructures with the visible-light driven photoresponse of CuO nanoarchitectures. The entangled Ag-CuO heteronanostructure exhibits a broad absorption towards the visible-near infrared (NIR) range and achieves total photo-oxidation of n-hexane under irradiation with different light-emitting diodes (LEDs) specific wavelengths at temperatures below 180 °C and outperforming its thermal catalytic response or its silver-free CuO illuminated counterpart.

Electro-activation of O2 on MnO2/graphite felt for efficient oxidation of water contaminants under room condition

The activation of oxygen (O₂) under room condition is highly desirable for oxidative removal of organic pollutants in water. Herein, we report a graphite felt (GF)-supported α-MnO₂ catalyst which is active for activating O₂ with assistance of an anodic electric field. The electro-assisted catalytic wet air oxidation (ECWAO) process on MnO₂/GF is able to rapidly degrade a variety of dyes, pharmaceutics and personal care products (PPCPs) under room condition. The congo red, basic fuchsin, neutral red and methylene blue are completely mineralized in 160 min, and the bisphenol A, triclosan and ciprofloxacin are mineralized by 89.9%, 81.5% and 65.4%, respectively, in 300 min. Mechanistic study indicates a surface-catalyzed non-free radical pathway for the oxidation of organic pollutants by O₂ in the ECWAO process. The oxygen vacancies on MnO₂ are identified as the catalytically active sites, at which oxygen atom is transferred from O₂ to organic molecule through chemisorbed oxygen species. The anodic electric field assists such an oxygen transfer pathway by activating the complex of chemisorbed oxygen species and organic molecule through electro-oxidation reaction. The ECWAO process on MnO₂/GF electrode exhibits a great potential for practical wastewater treatment under room condition.

Mechanism of CO2-formation promotion by Au in plasma-catalytic oxidation of CH4 over Au/γ-Al2O3 at room temperature

The plasma-catalytic oxidation of methane (CH₄) is a potential reaction for controlling CH₄ emissions at low temperatures. However, the mechanism of the CH₄ plasma-catalytic oxidation is still unknown, which inhibits the further optimization of the oxidation process. Herein, a CH₄ oxidation mechanism over an Au/γ-Al₂O₃ catalyst was proposed based on our experimental findings. CH₄ is first decomposed to CH₃ and H by the discharge, and a fraction of the CH₃ is adsorbed on γ-Al₂O₃ surface for deep oxidation. The oxygen atoms produced by the discharge react with H₂O to yield surface reactive OH groups that contribute to the CH₃ oxidation. Oxygen atoms also promote the release of H₂O from the surfaces of the γ-Al₂O₃ and Au/γ-Al₂O₃ and especially promote CO₂ desorption from the surface of the Au/γ-Al₂O₃. When γ-Al₂O₃ was used as the catalyst, the CO₂ selectivity was only 15 vol.%, and the CH₄ conversion decreased after 7 h of plasma-catalytic oxidation. In contrast, when Au/γ-Al₂O₃ was used, the CO₂ selectivity was 80 vol.%, long-term CH₄ conversion was obtained. Experimental results revealed that Au was beneficial for the decomposition of surface carbonate species into gaseous CO₂, whereas the carbonate species accumulated on γ-Al₂O₃ when Au was absent.

Kinetics study on non-thermal plasma mineralization of adsorbed toluene over γ-Al2O3 hybrid with zeolite

Non-thermal plasma mineralization of the adsorbed toluene over γ-Al₂O₃ hybrid with 13X, ZSM-5, and HY was investigated in a sequential adsorption and plasma oxidation system. The γ-Al₂O₃-13X was shown to have a better plasma oxidation performance with fewer by-products as compared to γ-Al₂O₃-ZSM-5 and γ-Al₂O₃-HY, which was due to its better discharge performance and O₃ decomposition ability. For all of the tested materials, the plasma mineralization of the adsorbed toluene process had a good match with the pseudo-second-order kinetic model: kt = 1/n - 1/n_o, where n₀ and n are the amount of adsorbed toluene (mmol) at discharge time = 0 and t, respectively. The overall reaction constant (k) was shown to be affected by the packing materials. The reason for the kinetic model following the pseudo-second-order in the sequential process was analyzed based on the chemical reaction and mineralization mechanism.

Enhancement mechanism of Sn on the catalytic performance of Cu/KIT-6 during the catalytic combustion of chlorobenzene

Sn promoted the redox capacity of Cu²⁺ and Sn⁴⁺, thus greatly reducing the surface chloride-deposition during the chlorobenzene catalytic combustion process.

Plasma-Catalytic Mineralization of Toluene Adsorbed on CeO2

In the context of coupling nonthermal plasmas with catalytic materials, CeO2 is used as adsorbent for toluene and combined with plasma for toluene oxidation. Two configurations are addressed for the regeneration of toluene saturated CeO2: (i) in plasma-catalysis (IPC); and (ii) post plasma-catalysis (PPC). As an advanced oxidation technique, the performances of toluene mineralization by the plasma-catalytic systems are evaluated and compared through the formation of CO2. First, the adsorption of 100 ppm of toluene onto CeO2 is characterized in detail. Total, reversible and irreversible adsorbed fractions are quantified. Specific attention is paid to the influence of relative humidity (RH): (i) on the adsorption of toluene on CeO2; and (ii) on the formation of ozone in IPC and PPC reactors. Then, the mineralization yield and the mineralization efficiency of adsorbed toluene are defined and investigated as a function of the specific input energy (SIE). Under these conditions, IPC and PPC reactors are compared. Interestingly, the highest mineralization yield and efficiency are achieved using the in-situ configuration operated with the lowest SIE, that is, lean conditions of ozone. Based on these results, the specific impact of RH on the IPC treatment of toluene adsorbed on CeO2 is addressed. Taking into account the impact of RH on toluene adsorption and ozone production, it is evidenced that the mineralization of toluene adsorbed on CeO2 is directly controlled by the amount of ozone produced by the discharge and decomposed on the surface of the coupling material. Results highlight the key role of ozone in the mineralization process and the possible detrimental effect of moisture.