The Mechanism of Non-thermal Plasma Catalysis on Volatile Organic Compounds Removal

Reaction mechanism of toluene decomposition in non-thermal plasma: How does it compare with benzene?

Non-thermal plasma (NTP) catalysis is considered one of the most promising technologies to address a wide range of energy and environmental needs, such as carbon dioxide (CO2) conversion, NH3 synthesis, and volatile organic compounds (VOCs) removal. A systematic approach to optimizing NTP systems benefits from understanding VOCs' fundamental NTP destruction behavior and analyzing the correlations between molecular structures and conversion and selectivity. Herein, the mechanical performance of the toluene destruction in NTP is examined and compared with benzene bearing a similar molecular structure. Different experimental and theoretical techniques are applied, including synchrotron vacuum ultraviolet photoionization mass spectrometry(SVUV-PIMS), thermochemistry, and quantum chemistry. Comparatively, toluene is more readily destroyed under the same NTP conditions than benzene. More intriguingly, the distribution of the decomposition species is significantly different. The theoretical calculations reveal that the abundant methyl radicals generated in toluene decomposition mainly lead to the various species distribution. These radicals promote some reactions, such as the decomposition of o-benzoquinone, one of the key intermediates, thus leading to new reaction pathways and products different from benzene. Finally, the critical mechanistic steps of toluene decomposition under the present non-thermal plasma conditions are established, which include the interactions between toluene and electrons or reactive radicals, the cleavage of the aromatic ring, and the various reaction pathways involving of methyl radicals. This study presents an effective approach to elucidate the distinct fundamental reaction mechanisms arising from subtle structural differences, offering new insights into the underlying plasma chemistry crucial for advancing various promising environmental and energy applications of non-thermal plasma systems.

Removal of toluene via non-thermal plasma generated by applying rare-earth tungsten electrode and nanosecond pulsed power supply

The objective of this investigation is to evaluate the characteristics associated with degradation of toluene through the utilization of non-thermal plasma (NTP) generated via application of a low-work-function electrode and nanosecond pulsed power supply. Initially, a comparative analysis is made between toluene removal efficiency utilizing the low-work-function electrode and that achieved with the conventional stainless-steel electrode. The outcomes demonstrate that NTP generated by the low-work-function electrode exhibits markedly superior removal efficiency for toluene in comparison to the stainless-steel electrode operating at the same voltage. Subsequently, the impacts of voltage, pulse frequency, and initial concentration of toluene on the removal efficiency and production of by-products are investigated. It is found that as the voltage and frequency increase, the removal efficiency also increases, and a maximum toluene removal efficiency of 87.2% is achieved at a voltage of 12,000 V and pulse frequency of 2000 Hz. The removal efficiency first increases and then decreases with increasing toluene initial concentration. The investigation also finds that energy yield is negatively correlated with voltage and pulse frequency and positively correlated with the initial concentration. Finally, the reaction products were subjected to quantitative analysis using GC-MS. Based on the analysis results, potential reaction pathways are inferred.

Unravelling the correlation of dielectric barrier discharge power and performance of Pt/CeO2 catalysts for toluene oxidation

Two “volcano” peaks in the relevant activity curve showcased that plasma discharge power had a significant impact on the activity of Pt/CeO2-Px catalysts and modulating discharge power could be regarded as an efficient method to optimize catalyst performance.

Decomposition of gaseous chlorobenzene using a DBD combined CuO/α-Fe2O3 catalysis system

Copper oxide and hematite (CuO/α-Fe₂O₃) composite catalysts were prepared by using goethite as precursor adopted impregnation way and applied to the dielectric barrier discharge (DBD) catalytic decomposition of gaseous chlorobenzene. The CuO/α-Fe₂O₃ composite was characterised by X-ray diffraction, Brunauer-Emmett-Teller method, scanning electron microscopy and X-ray photoelectron spectrometer technique. The decomposition efficiency and energy yield of gaseous chlorobenzene in DBD catalysis system were studied by a function of gas flow rate, initial concentration and input voltage. The results showed that the CuO/α-Fe₂O₃ composite catalyst exhibited remarkable performance on chlorobenzene decomposition when the molar ratio was 0.4 and calcination temperature was 450°C. When the chlorobenzene initial concentration was 230 mg m^-3, the chlorobenzene decomposition efficiency and mineralisation rate on the DBD catalysis system reached 73.33% and 63.37%, respectively, its decomposition and mineralisation efficiency were enhanced about 20.5% and 16.61%, respectively, compared with the bare DBD system, and it also benefited to significantly reduce the ozone and NO₂ by-products. The possible pathway of chlorobenzene decomposition in the DBD catalytic hybrid system was proposed based on the products analysis.

Removal of Isopropanol by synergistic non-thermal plasma and photocatalyst

The dielectric barrier discharge (DBD) of non-thermal plasmas was combined with a self-made photocatalyst to remove isopropanol (IPA). Synthesis conditions for the novel photocatalyst, including calcination temperature and copper loading, were varied before photocatalysis to obtain at the optimal reaction efficiency. The effects of initial IPA concentration, oxygen content, and catalyst dosage were also observed. Finally, catalyst reusability was analyzed. X-ray photoelectron spectroscopy fitting revealed Ti, Cu, C, and O peaks in the synthesized catalyst. After a 60-min reaction with 100% oxygen as the carrying gas, nearly 100% of the IPA was converted. Overall, the optimal IPA conversion efficiency and acetone and carbon dioxide selectivity were achieved when the photocatalyst was synthesized at a calcination temperature of 550 °C and copper loading of 1.8%, along with a 100% oxygen carrying gas and a 3-mm discharge gap.

C-dot doping for enhanced catalytic performance of TiO2/5A for toluene degradation in non-thermal plasma-catalyst system

Non-thermal plasma (NTP) is gaining attention as a powerful tool to induce various reactions. The combination of NTP with catalysts has been successfully used to degrade volatile organic compounds (VOCs) for pollution control. In this study, a series of TiO₂-C/5A catalysts, synthesized by carbon dots (C-dots) that decorate TiO₂ by sol-gel and wetness impregnation methods, were incorporated with a dielectric barrier discharge (DBD) reactor in a single-stage structure to degrade toluene at atmospheric pressure and room temperature. A proton-transfer reaction mass spectrometer and a CO₂ analyzer were used to monitor the concentration variations of organic by-products and CO₂ online. The effects of input power, mass ratio of C-dots/TiO₂ (TiO₂/5A (0 wt%), TiO₂-C1/5A (2.5 wt%), TiO₂-C2/5A (5 wt%), TiO₂-C3/5A (10 wt%)), gas flow rate, initial concentration of toluene on the toluene degradation efficiency, and CO₂ selectivity were studied. The plasma-catalyst hybrid system could effectively improve the energy efficiency and reaction selectivity, attaining a maximum toluene degradation efficiency of 99.6% and CO₂ selectivity of 83.0% compared to 79.5% and 37.5%, respectively, using the conventional plasma alone. Moreover, the generation of organic by-products also declined dramatically, averaging only half as much in plasma alone. The results also indicated that the appropriate amount of C-dot doping could greatly improve the catalyst efficiency in the hybrid plasma system. This is because the interaction between C-dots and TiO₂ favors the formation of photoelectron holes and reduces the energy band gap and the recombination rate of photogenerated electron holes, which facilitates the generation of more active species on the catalyst surface, thereby leading to a more effective degradation reaction. These observations will provide guidance for the interaction studies between NTP and catalysts, not only for the exploration of new chemical mechanisms of aromatic compounds, but also for the screening of favorable materials for the desired reactions.

Benzene decomposition by non-thermal plasma: A detailed mechanism study by synchrotron radiation photoionization mass spectrometry and theoretical calculations

Non-thermal Plasma (NTP) catalysis is considered as one of the most promising technologies to address a wide range of environmental needs, such as volatile organic compounds (VOCs) and NOx removal. To meet the updated environmental emission standard, the NTP catalysis reaction system needs to be better understood and further optimized. In this work, the degradation process of benzene in NTP, which is still regarded as a "black box" process, was explored by synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). For the first time, we observed over 20 representative species by PIMS and identified their structures accurately by photoionization efficiency (PIE) spectra. Phenol, acetylene and acrolein were recognized as the three main products. More intriguingly, concentration profiles demonstrated that a large amount of acrolein and also several higher-order products, which were usually neglected in previous research, were produced during the NTP destruction process. The details of the benzene degradation reaction mechanism, were finally established by the combination of SVUV-PIMS results, thermochemistry and theoretical calculations. This work helps to complete the mechanistic picture of plasma chemistry, which may be helpful on raveling the more complicated NTP catalysis mechanism in the future therefore contributing to design of improved NTP system for environmental applications.

Gas-phase isopropanol degradation by nonthermal plasma combined with Mn-Cu/-Al2O3

In this study, the dielectric barrier discharge (DBD) induced by nonthermal plasma (NTP) technology was used for isopropanol (IPA) degradation. IPA, intermediate, final product, and ozone concentrations were analyzed using GC-MS, carbon dioxide detector, and ozone detector. The experimental flow rate and concentration were fixed to 1 L/min and 1200 ppm ± 10%, respectively. Different reaction procedures were proposed for self-made metal catalyst combined with a plasma system (plasma alone and γ-Al₂O₃ combined with plasma, Cu (5 wt%)/γ-Al₂O₃ combined with plasma, Mn (3 wt%)-Cu (5 wt%)/γ-Al₂O₃ combined with plasma). In addition, the effect of the carrier gas oxygen content (0%, 20%, and 100%) on IPA conversion and intermediate and carbon dioxide selectivity was also investigated. The results revealed that the Mn (F)-Cu/γ-Al₂O₃ combined with plasma exhibited more efficient IPA conversion. In the 100% oxygen environment, the IPA conversion rate increased from 79.32 to 99.99%, and carbon dioxide selectivity increased from 3.82 to 50.23%. IPA was completely converted after 60 min of plasma treatment with the acetone selectivity, carbon dioxide selectivity, and tail ozone concentration of 26.71% ± 1.27%, 50.23% ± 0.56%, and 1761 ± 11 ppm, respectively. This study proved that the current single planar DBD configuration is an effective advanced treatment technology for the decomposition of VOCs.

Removal of VOCs by Ozone: n-Alkane Oxidation under Mild Conditions

Volatile organic compounds (VOCs) have a negative effect on both humans and the environment; therefore, it is crucial to minimize their emission. The conventional solution is the catalytic oxidation of VOCs by air; however, in some cases this method requires relatively high temperatures. Thus, the oxidation of short-chain alkanes, which demonstrate the lowest reactivity among VOCs, starts at 250–350 °C. This research deals with the ozone catalytic oxidation (OZCO) of alkanes at temperatures as low as 25–200 °C using an alumina-supported manganese oxide catalyst. Our data demonstrate that oxidation can be significantly accelerated in the presence of a small amount of O3. In particular, it was found that n-C4H10 can be readily oxidized by an air/O3 mixture over the Mn/Al2O3 catalyst at temperatures as low as 25 °C. According to the characterization data (SEM-EDX, XRD, H2-TPR, and XPS) the superior catalytic performance of the Mn/Al2O3 catalyst in OZCO stems from a high concentration of Mn2O3 species and oxygen vacancies.

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.

Dielectric Barrier Discharge Coupling Catalytic Oxidation for Highly Efficient Hg0 Conversion

In this work, we prepared CuCe/Ti catalysts in a dielectric barrier discharge (DBD) reactor and proposed a new method for flue gas mercury oxidation using DBD coupling CuCe/Ti catalyst. Our experiments verified the oxidation efficiency of flue gas Hg⁰ (η_Hg) and clarified the influence of O₂ content, NO concentration, SO₂ concentration, water vapor content, and discharge voltage on η_Hg. The oxidation mechanism of Hg⁰ in the DBD-CuCe/Ti reactor was also illustrated. The Hg⁰ oxidation experiment on the simulated flue gas (70 μg/m³ Hg⁰ + 300 mg/m³ NO + 1000 mg/m³ SO₂ + 6%O₂) with a flow rate of 1 L/min showed that when the amount of catalyst was 1.25 g and the discharge voltage was 9.5 kV, a η_Hg of 93% can be achieved, which indicates that the DBD coupling CuCe/Ti technology is suitable for Hg⁰ conversion and flue gas mercury removal.

Plasma-Catalysis for Volatile Organic Compounds Decomposition: Complexity of the Reaction Pathways during Acetaldehyde Removal

Acetaldehyde removal was carried out using non-thermal plasma (NTP) at 150 J·L−1, and plasma-driven catalysis (PDC) using Ag/TiO2/SiO2, at three different input energies—70, 350 and 1150 J·L−1. For the experimental configuration used, the PDC process showed better results in acetaldehyde (CH3CHO) degradation. At the exit of the reactor, for both processes and for all the used energies, the same intermediates in CH3CHO decomposition were identified, except for acetone which was only produced in the PDC process. In order to contribute to a better understanding of the synergistic effect between the plasma and the catalyst, acetaldehyde/catalyst surface interactions were studied by diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS). These measurements showed that different species such as acetate, formate, methoxy, ethoxy and formaldehyde are present on the surface, once it has been in contact with the plasma. A reaction pathway for CH3CHO degradation is proposed taking into account all the identified compounds in both the gas phase and the catalyst surface. It is very likely that in CH3CHO degradation the presence of methanol, one of the intermediates, combined with oxygen activation by silver atoms on the surface, are key elements in the performance of the PDC process.

Abatement of Toluene by Reverse-Flow Nonthermal Plasma Reactor Coupled with Catalyst

In order to make full use of the heat in nonthermal plasma systems and decrease the generation of by-products, a reverse-flow nonthermal plasma reactor coupled with catalyst was used for the abatement of toluene. In this study, the toluene degradation performance of different reactors was compared under the same conditions. The mechanism of toluene abatement by nonthermal plasma coupled with catalyst was explored, combined with the generation of ozone (O3), NO2, and organic by-products during the reaction process. It was found that a long reverse cycle time of the reactor and a short residence time of toluene decreased the internal reactor temperature, which was not beneficial for the degradation of toluene. Compared with the dielectric barrier discharge (DBD) reactor, toluene degradation efficiency in the double dielectric barrier discharge (DDBD) reactor was improved at the same discharge energy level, but the concentrations of NO2 and O3 in the effluent were relatively high; this was improved after the introduction of a catalyst. In the reverse-flow nonthermal plasma reactor coupled with catalyst, the CO2 selectivity was the highest, while the selectivity and amount of NO2 was the lowest and aromatics, acids, and ketones were the main gaseous organic by-products in the effluent. The reverse-flow DBD-catalyst reactor was successful in decreasing organic by-products, while the types of organic by-products in the DDBD reactor were much more than those in the DBD reactor.

Plasma-Assisted Surface Interactions of Pt/CeO2 Catalyst for Enhanced Toluene Catalytic Oxidation

The performance of plasma-modified Pt/CeO2 for toluene catalytic oxidation was investigated. Pt/CeO2 nanorods were prepared by wet impregnation and were modified by thermal (PC-T), plasma (PC-P), and combined (PC-TP and PC-PT) treatments. The modified catalysts were characterized by TEM (transmission electron microscope), BET (Brunauer-Emmett-Teller), H2-TPR, O2-TPD, XPS, UV-Raman, and OSC tests. The significant variation of the surface morphologies and surface oxygen defects could have contributed to the modification of the Pt/CeO2 catalysts via the plasma treatment. It was found that plasma could promote the surface interaction between Pt and CeO2, resulting in the thermal stability of the catalyst. The Pt-Ce interaction was also conducive to an increase in the number of oxygen vacancies. Furthermore, PC-PT and PC-TP showed a significant difference in oxygen vacancy concentrations and catalytic activities, which illustrated that the treatment sequence (plasma and thermal treatment) affected the performance of Pt/CeO2. The PC-PT sample showed the highest catalytic activity with T100 at 205 °C. This work thus demonstrates that plasma in combined treatment sequences could assist surface interactions of catalysts for enhanced toluene catalytic oxidation.