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Non-thermal plasma technology has great potential for the removal of the emission of volatile organic compounds (VOCs). However, the plasma-alone process may produce various undesired by-products that cause secondary pollutions. Here, a coaxial dielectric barrier discharge (DBD) reactor has been developed for the removal of xylene over an Al2O3-based catalyst (Mn/Al2O3, Co/Al2O3, Ce/Al2O3) at low temperatures. Compared with the plasma-alone process and other plasma-catalytic combined process, the combination of plasma and the Mn/Al2O3 catalyst obviously enhanced xylene conversion and substantially restrained the formation of by-product organic aerosol (up to 100%), and what is more is that our setup was more efficient than those in other studies. In the meantime, ozone could be effectively converted by the Mn/Al2O3 catalyst; as the main active group with a long period of existence during discharge, ozone adsorbed onto the surface of the catalyst under the action of Mn/Al2O3 and participated in the oxidation of xylene. Mn4+ and the high lattice oxygen content were the reasons for the highest efficiency of 1% Mn.

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.

Carbon oxidation has two stages, the first is the formation of surface oxides and the second is the gasification of the surface oxides to CO2. Calcium carbonate (CaCO3) was used to catalyze the gasification of the surface oxides. The catalytic effect of on graphite oxidation and its catalytic mechanism were studied by using thermogravimetric technique and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It was found that characteristic temperature (T50) of graphite oxidation with CaCO3 was 946 K, 113 K lower than that of graphite only. DRIFTS analysis results show that surface oxides (adsorbed CO2 and carbonate CO32−) were formed on the graphite surface at a temperature above 473 K, carbonate products on graphite surface disappeared when CaCO3 was present; formation of CO32− on CaCO3 surface was confirmed, this CO32− may be more easily gasified into gaseous CO2. The kinetic analysis results showed that CaCO3 promoted graphite oxidation has an activation energy of 74.3 kJ mol−1, far lower than that of graphite (148 kJ mol−1).

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 Oads/Olatt 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.

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.

Plasma catalysis technology has shown a great prospect in the oxidation of volatile organic compounds with a low concentration. However, low energy efficiency and large numbers of nanoparticle by-products are still the bottleneck for its practical application. Herein, a dielectric barrier discharge (DBD) reactor coupled with nano-sized Au (0.1 wt%) supported on γ-Al2O3 (denoted as Au/γ-Al2O3) was used for plasma catalytic oxidation of toluene. It was found that the energy efficiency was 89.5 g*kWh−1 at 25 °C, and improved to 125.0 g*kWh−1 at 250 °C. If a catalyst was not used in the DBD reactor, the by-products including benzaldehyde, phenol and benzoic acid were formed during the toluene oxidation. Those by-products can form nanoparticles which cause another risk to the atmosphere and human beings. With the use of Au/γ-Al2O3 nanocatalyst, the emission of nanoparticle by-products was reduced by 99.99%. Meanwhile, in-situ plasma DRIFTS analysis showed that Au-based catalysts can promote the oxidation of by-products and the gasification of carbonates to CO2.

Non-thermal plasma technology has great potential in reducing polycyclic aromatic hydrocarbons (PAHs) emission. But in plasma-alone process, various undesired by-products are produced, which causes secondary pollutions. Here, a dielectric barrier discharge (DBD) reactor has been developed for the oxidation of naphthalene over a TiO2/diatomite catalyst at low temperature. In comparison to plasma-alone process, the combination of plasma and TiO2/diatomite catalyst significantly enhanced naphthalene conversion (up to 40%) and COx selectivity (up to 92%), and substantially reduced the formation of aerosol (up to 90%) and secondary volatile organic compounds (up to near 100%). The mechanistic study suggested that the presence of the TiO2/diatomite catalyst intensified the electron energy in the DBD. Meantime, the energized electrons generated in the discharge activated TiO2, while the presence of ozone enhanced the activity of the TiO2/diatomite catalyst. This plasma-catalyst interaction led to the synergetic effect resulting from the combination of plasma and TiO2/diatomite catalyst, consequently enhanced the oxidation of naphthalene. Importantly, we have demonstrated the effectiveness of plasma to activate the photocatalyst for the deep oxidation of PAH without external heating, which is potentially valuable in the development of cost-effective gas cleaning process for the removal of PAHs in vehicle applications during cold start conditions.

A dielectric barrier discharge (DBD) catalyst hybrid reactor with CeO2/γ-Al2O3 catalyst balls was investigated for benzene decomposition at atmospheric pressure and 30 °C. At an energy density of 37–40 J/L, benzene decomposition was as high as 92.5% when using the hybrid reactor with 5.0wt%CeO2/γ-Al2O3; while it was 10%–20% when using a normal DBD reactor without a catalyst. Benzene decomposition using the hybrid reactor was almost the same as that using an O3 catalyst reactor with the same CeO2/γ-Al2O3 catalyst, indicating that O3 plays a key role in the benzene decomposition. Fourier transform infrared spectroscopy analysis showed that O3 adsorption on CeO2/γ-Al2O3 promotes the production of adsorbed O2− and O22‒, which contribute benzene decomposition over heterogeneous catalysts. Nano particles as by-products (phenol and 1,4-benzoquinone) from benzene decomposition can be significantly reduced using the CeO2/γ-Al2O3 catalyst. H2O inhibits benzene decomposition; however, it improves CO2 selectivity. The deactivated CeO2/γ-Al2O3 catalyst can be regenerated by performing discharges at 100 °C and 192–204 J/L. The decomposition mechanism of benzene over CeO2/γ-Al2O3 catalyst was proposed.

This study proposes a novel four-way plasma-catalytic removal of particulate matter, nitrogen oxides, hydrocarbons, and carbon monoxide without external heating using a pulsed dielectric barrier discharge reactor combined with Au/CaSO4/γ-Al2O3 catalyst balls. With the optimized amount of Au and CaSO4, over 90% of particulate matter, 40% of nitrogen oxides, and nearly 100% of hydrocarbons were removed from the diesel engine exhaust, while the increase of carbon monoxide concentration due to the oxidation of particulate matter and hydrocarbons and the decomposition of carbon dioxide were minimized. Importantly, the performance of the plasma-catalytic reactor was maintained without decline for the duration of the experiment (7 h) with an attractive cost/performance ratio. These findings offer a new approach to the simultaneous treatment of diesel exhausts and provide a novel concept for the design of a practical and compact after-treatment device for the diesel engine exhaust gases.

The activities for nonthermal plasma-induced oxidation of diesel particulate matter (DPM) with various metal sulfates were investigated. It was found that the metal sulfates, MgSO4, K2SO4, and CaSO4 $\cdot$ 2H2O, enhanced the energy yield of DPM oxidation while other sulfates, Na2SO4, Fe2(SO4)3, and Al2(SO4) $\cdot$ 8H2O, did not affect the energy yield. The addition of water vapor caused a promotion of DPM oxidation regardless of a metal sulfate was presented. The effect of temperature and sulfate adding amount was also investigated. The metal sulfates enhanced DPM oxidation is possibly due to the adsorption of O atoms onto the sulfates. The positive effects of H2O might be thought of as due to the formation of reactive OH radicals and the promotion of the hydrolysis of the oxygen-containing compounds on DPM surface.

Naphthalene decomposition in O 2 /N 2 gas mixture with different O 2 concentrations has been studied in a dielectric barrier discharge reactor at atmospheric pressure. O 2 played an important role in the decomposition of naphthalene, especially in the selectivities of CO and CO 2 . There was an optimal naphthalene decomposition rate at an O 2 concentration of about 3%. The CO x selectivity increased up to 83.3% gradually with the O 2 concentration increasing from 1% to 20%. Nanoparticles were found in the gas samples, concentrations of which can be reduced greatly through raising the O 2 concentration. The decomposition byproducts of naphthalene were obviously different under different O 2 concentrations. Some nitrogenous compounds reduced but some oxygenous compounds increased with increasing O 2 concentration. The mechanism of naphthalene decomposition was proposed as that naphthalene was first initiated by dehydrogenation and oxidation, and then followed by deep oxidation to CO and CO 2 .

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.

Decane decomposition in Ar, N2, and O2/N2 gases was investigated using a pulsed dielectric barrier discharge reactor. The mechanism of decomposition was discussed on the basis of the products detected by gas chromatograph (GC) and GC-mass spectrometer techniques. Decane decomposition in Ar was initiated by dehydrogenation to form decane radicals, which were then dehydrogenated to yield decenes, lighter hydrocarbons. When O2 was present in the discharge space, decane radicals were oxidized to decanone, CO, and CO2. The oxidation mechanism of decane radicals to CO and CO2 was suggested due to the radical oxidation by O atoms and OH radicals. Nanoparticles were found in a size range of 5.5–160 nm. The sizes of the nanoparticles were different with the gas components due to the difference in the produced radicals in the discharge space. We proposed that the oxidation and nitridation of the decane led to the formation of the nanoparticles.

Nitrogen fertilizers are important for modern agriculture, among which nitric acid is a basic chemical for ammonium nitrate production. Herein, NOx was produced from N2 and O2 using a spark discharge technology with point-to-point and circle-to-plate types of plasma reactors driven by a nano-second pulsed power supply. NOx could be produced with an energy efficiency of 0.09 mol/kWh under the gases of 50%/50% N2/O2, while NO2 selectivity increased with increasing energy density. The gas temperatures in the spark channels were estimated using the Gibbs free energy equation. Based on kinetics calculations, the reactions of N with O2 and O with NO were the major reactions for NOx production.

Plasma-assisted dry reforming of methane has attracted much research attention because this process simultaneously utilizes greenhouse, methane and carbon dioxide, to produce hydrogen-rich syngas at a relative low temperature. Although it is generally recognized that the gas composition of reactant has great effect on the methane conversion and products distribution, systematic studies that clarify the roles that electron-induced chemistry and thermochemistry play are needed for a full understanding of reactant effect. Here, we compared the reforming performance by varying the ratio of CO2/CH4 or O2/CH4 at the similar reduced field intensity (E/N) in a temperature-controlled dielectric barrier discharge reactor to elaborate the role of electron-induced chemistry and thermo-chemistry in the dry reforming process. By conducting optical emission spectrum measurement, the enrichment of O atoms was observed at the increased CO2/CH4 or O2/CH4 ratios. At T = 293 K, methane conversion was only dependent on the electron-induced chemistry regardless of the specific reactant gas composition. At a relative high temperature condition, however, thermochemistry could become pronounce when sufficient O atoms were added into the dry reforming process. In contrast, the chemical pathways to the products were overall controlled by the thermochemistry at the tested background temperatures. Due to the conversion of carbon-based products into the carbon dioxide, the conversion of carbon dioxide was influenced by the thermochemistry when the concentration of O atoms was high. Our findings may improve the understanding of reactant effect and the designs of plasma-reformer.

Nonthermal plasma is a promising technology for the on-board reforming of gasoline or diesel fuels into cleaner fuels. However, the formation of deep cracking products, such as carbon particulates, discourages the development of an environmentally friendly process. The authors report the reformation of n-pentane (C5H12) using methane (CH4) or carbon dioxide (CO2) in a temperature-controlled dielectric barrier discharge reactor to produce the H2 and carbon-based clean products. A mechanistic study suggests that electron-induced chemistry dominates C5H12 and the added gas conversion, whereas the thermochemistry controls the product distribution. On this basis, the product distribution is varied by the enriched CH3 and H, or the enriched O by adding CH4 or CO2. When increasing CH4 percentage from 0 to 75%, the production of short-chain alkanes (C1–C4 alkanes) and H2 were increased by around 29% and 45%, respectively. The increase in CO2 (0–75%) percentage resulted in the increased production of oxygenated fuels (0–9%). Our results emphasize the advantages of using low-temperature plasma for the co-production of H2 and clean carbon-based fuels (chemical intermediates), which may offer a new concept for the design of a practical on-board plasma reformer.

Ozone (O3) generation in a dielectric barrier discharge (DBD) reactor driven by a pulsed power supply was investigated at atmospheric pressure and room temperature. An O3 generation efficiency model is established in which discharge power, O2 concentration, gas flow rate, and volume of the discharge space are included. Constants in the O3 generation efficiency model were obtained by fitting the model with experiment results. O3 concentration can be simply calculated from the energy density and initial O2 concentration. Comparison on O3 concentrations from calculation with references is given.

The effect of gas–liquid phase compositions on NO and NO 2 absorption into ammonium-sulfite and bisulfite solutions is investigated. Preliminary experiment results indicate that the concentrations of (NH 4 ) 2 SO 3 or NH 4 HSO 3 solution and the molar ratio for HSO 3 − to the total solution concentrations all have significant impact on NO 2 and NO absorption rates. While the solution concentration is constant, the absorption of NO x mixture is strongly related to the ratio NO 2 /NO x . The absorption rate of NO is primarily affected by NO 2 inlet concentration, and the NO absorption rate reaches the maximum value in (NH 4 ) 2 SO 3 solution with the increase of NO 2 inlet concentration, which is determined by the reaction of NO and NO 2 with SO 3 2− as well as NO formation. Moreover, when the solution is NH 4 HSO 3 the best ratio of NO 2 /NO for the maximum value of the NO absorption rate becomes less or smaller. Meanwhile, the presence of NO in the gas phase is also favorable to the absorption rate of NO 2 in ammonium-sulfite or bisulfite solutions. The total results suggest that the coexistence of NO and NO 2 in the flue gas could enhance the absorption of each other to some extent.

The naphthalene decomposition in a corona radical shower discharge (CRS) was investigated, with attention paid to the influences of voltage and initial naphthalene density. The OH emission spectra were investigated so as to know the naphthalene decomposing process. The by-products were analyzed and a decomposing theory in discharge was proposed. The results showed that higher voltage and relative humidity were effective on decomposition. The initial concentration affected the decomposing efficiency of naphthalene. When the initial naphthalene density was 17 mg/m3, the decomposition rate was found to be 70% under 14 kV. The main by-products were carbon dioxide and water. However, a small amount of carbonic oxide, 1,2-ethanediol and acetaldehyde were found due to the incomplete oxidization.

Non-thermal plasma has been proposed as a promising technique for the reformation of fossil fuel into cleaner fuel. Despite recent achievements in the economic and technical development of plasma-assisted reforming techniques, a better understanding of thermochemistry and electron-induced chemistry is required to optimize the performance of the relevant systems. We study the relative importance of electron-induced chemistry and thermochemistry for C5H12 (model gasoline) activation and products formation using a temperature-controlled dielectric barrier discharge (DBD) reactor. Important mechanical insight is obtained from the comparison between high-temperature and low-pressure conditions under similar reduced field intensities. In a tested range of background temperatures (303 l T l 623 K), we found that the conversion of C5H12 in DBD depended only on electron-induced chemistry for the Ar/C5H12 and He/C5H12 mixtures, while it was affected by both electron-induced chemistry and thermochemistry for the N2/C5H12 mixture. However, the consecutive reactions from the initiation to the product always depended upon the corresponding thermochemistry for a given temperature. Due to an enhanced electron density and electron energy in the Ar/C5H12 and He/C5H12 DBD, increased C5H12 conversion was observed as compared to N2/C5H12. More importantly, cleaner products were produced from Ar/C5H12 and He/C5H12 DBD because He and Ar were not involved in the product formation reaction pathways. We also found that extensive thermochemical involvement is favorable for enhancing the performance of plasma-assisted C5H12 reforming. Our findings may not only be useful for a greater understanding of the plasma-assisted reforming process but also helpful in designing a cost-effective plasma reformer.

The mass transfer and kinetics of NOx absorption into (NH4)2SO3 solution, the main compound of an ammonia-based wet flue gas desulfurization process, have been investigated in a double-stirred reactor. Under the experimental conditions, the gas−liquid reaction between NOx and the (NH4)2SO3 solution without O2 coexisting is controlled mainly by the gas film because the (NH4)2SO3 concentration is higher than 0.05 mol/L. In this case, the absorption rate of NOx is found to be zero-order with respect to the (NH4)2SO3 concentration. The inlet partial pressure and the oxidation degree (Φ = NO2/NOx) have an apparent effect on the absorption rate of NOx. In this research, a simplified mathematical calculated model is applied to the simulation of the absorption process. The experimental results demonstrate that the orders of the reaction with respect to the concentration of NOx (NO2* or NO*) in the gas phase and the reaction rate constants of NOx (NO2* or NO*) with (NH4)2SO3 are all a function of the oxidation deg...

In order to investigate the characteristics of the reaction between ammonium sulfite, the main desulfurizing solution, and the flue-gas-contained sulfur dioxide during the process of ammonia-based WFGD (wet flue gas desulfurization) in a power plant, the gas–liquid absorption reaction between sulfur dioxide and an ammonium sulfite solution was studied in a stirred tank reactor. The experimental results indicate that the absorption of sulfur dioxide is controlled by both the gas- and liquid-films when the ammonium sulfite concentration is lower than 0.05 mol/L, and mainly by the gas-film at higher concentrations. In the latter case, the reaction rates are found to be zero-order with respect to the concentration of ammonium sulfite. The absorption rates of sulfur dioxide increase as the concentration of sulfur dioxide in inlet gas and the temperature increase. The reaction rate is of 0.6th-order with respect to the concentration of sulfur dioxide.

Iron leaching from fly ash has been investigated under simulated ammonia-based wet flue gas desulfurization conditions to determine the effects of the reaction temperature, initial pH value, liquid/solid ratio, and vibration frequency. The experimental results indicated that the ferric ion concentration in solution increased with the increase of the temperature and vibration frequency and the decrease of the pH value and liquid/solid ratio. The kinetics of iron leaching can be expressed as diffusion combined with a surface chemical reaction model. The reaction path can be described by the equation: 1 − (1 − α)1/3 = kt. The apparent activation energy was estimated to be about 20.01 kJ/mol. Such a value of the activation energy indicates that the leaching of iron under experimental conditions is controlled by both chemical reaction and diffusion.

In this study, the influences of different geometric nozzle electrodes on corona discharge characteristics, ozone and NOx formation characteristics, and NO conversion characteristics were investigated. In addition, an experiment that involved the NO conversion characteristics, relative to multinozzle electrodes, was performed. The results show that the different nozzle radii may transform the discharge modes. The increasing nozzle quantity can enlarge the corona region and increase the discharge current, while it almost has no effect on the onset voltage and sparking voltage. The ozone formation during corona discharge is affected by different geometric nozzle electrodes. More ozone is generated with the increasing nozzle radius and nozzle quantity. However, NOx concentrations are not affected much and the NOx concentrations are basically