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HC-SCR is a more attractive approach for removing NOx from lean-burn diesel engines compared with NH3-SCR. The novel SnAg/γ-Al2O3 catalyst exhibit excellent C3H6-SCR activity, due to the optimized redox ability and enriched hydroxyl groups.

Ozone (O3) generated by a dielectric barrier discharge (DBD) is widely used in various industrial processes. In this study, NaCl aqueous solution was used as a novel electric power transmission electrode in a DBD reactor (instead of a traditional metal electrode) for highly efficient ozone generation. The results demonstrated that a high O3 yield of 242 g kWh−1 with a concentration of 14.6 g m−3 O3 was achieved. The power transmission mechanism works because NaCl aqueous solution behaves as a capacitor when an alternating pulse voltage below 8 kHz is used. Compared with the resistance of the discharge barrier and discharge space, the resistance of NaCl aqueous solution can be ignored, which ensures that O3 is generated efficiently. It is expected that O3 generation using NaCl aqueous solution as a novel electrode in a DBD reactor could be an alternative technology with good application prospects.

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.

In this study, La-Ce/γ-Al2O3 materials for the catalytic ozonation of phenol were prepared by impregnation. In order to analyze the physicochemical properties, BET, SEM, XRD, XPS, and FTIR were used to characterize the catalysts. The removal performance of phenol under different reactive conditions (catalyst dosage, initial pH value, initial phenol concentration, and ozone concentration) was investigated, and the possible catalytic oxidation mechanism and conversion pathway of phenol were proposed through the analysis of ROS and intermediates in solution. The results indicated that the conversion rate of 2.5% La-Ce/γ-Al2O3 increased by 20.53% within 20 min compared with ozonation alone. Moreover, the CO2 mineralization rate was more than twice that of ozonation alone within 250 min, reaching 92.53%, and superior CO2 mineralization was maintained after repeated use. The CO2 mineralization rate of phenol after adding 3 g/L 2.5% La-Ce/γ-Al2O3 catalyst reached almost 100%, and could be maintained in a solution with a wide pH range of 3~9.5. From the characterization results, it could be seen that the 2.5% La-Ce/γ-Al2O3 catalyst obtained higher adsorbed oxygen and hydroxyl species content than γ-Al2O3 and Ce/γ-Al2O3. In the process of phenol catalyzed ozonation, the •O2ˉ species in the solution were deduced to be the key ROS.

The effect of N2 discharge products on cyclohexane degradation over a MnO2/γ-Al2O3 catalyst has been evaluated by feeding N2 discharge products to the catalyst using a specially designed dielectric barrier discharge reactor. At a reaction temperature of 100 °C, the cyclohexane conversion increased from 2.46% (without N2 discharge products) to 26.3% (with N2 discharge products). N- and O-containing by-product (3, 4-dehydroproline) was found on the catalyst surface using gas chromatograph-mass spectrometry identification, in which C=N–C and C=N–H bonds were also confirmed from x-ray photoelectron spectroscopy analysis results. Operando analysis results using diffuse reflectance infrared Fourier transform spectroscopy revealed that N atoms can react with surface H2O possibly to NH and OH reactive species that have reactivities to promote CO oxidation to CO2. The mechanism of N-atom-driven cyclohexane degradation to CO and CO2 is proposed.

The complete mineralization of acetic acid in a biodegradation process is difficult due to the α-position methyl on the carboxyl group of acetic acid. This study explores the complete oxidation of acetic acid by catalytic ozonation. Metal oxides of MnO2, Co3O4, Fe3O4, and CeO2 loaded on γ-Al2O3 power were used as the catalysts. The experimental results showed that MnO2/γ-Al2O3 catalyst had the best mineralization performance for acetic acid. Typically, the mineralization of acetic acid is as high as 88.4% after 300 min ozonation of 100 mL of 1.0 g L‒1 acetic acid catalysed by 3.0 g 1.0wt.% MnO2/γ-Al2O3 catalyst powder with an energy efficiency of 15 g kWh‒1. However, without a catalyst, the mineralization of acetic acid is only 33.2% with an energy efficiency of 5.1 g kWh−1. The effects of MnO2 loading, catalyst dosage, acetic acid concentration, O3 concentration, ozonation temperature, and initial pH value of the acetic acid solution were systematically investigated. Radical quenchers and in-situ DRIFTS analyses indicated that •OH radical and reactive oxygen species on catalyst surface played an important role in the ozonation of acetic acid to CO2 and H2O. The mechanism of acetic acid oxidation on MnO2/γ-Al2O3 is proposed.

Plasma-assisted catalysis has been demonstrated to be an innovative technology for eliminating diesel particulate matter (DPM) efficiently at low temperature (≤200 °C). Moreover, past studies have demonstrated that CaSO

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 CeO

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).

Electroplating sludge is classified as a hazardous waste due to its extremely high leachability of potentially toxic elements. This study concerns the use of magnesium oxysulfate cement (MOSC) for the stabilisation/solidification (S/S) of Zn-rich electroplating sludge. According to X-ray diffraction and thermogravimetric analyses, Zn was mainly immobilised through both chemical interaction and physical encapsulation in the MOSC hydrates of 5Mg(OH)2·MgSO4.7H2O (5−1−7) phase. The crystal size analysis, elemental mapping, and extended X-ray absorption fine structure (EXAFS) analysis proved that the Zn2+ was also incorporated in the structure of 5–1–7 phase. Unlike Portland cement system, hydration kinetics, setting time, and compressive strength of the MOSC system were only negligibly modified by the presence of Zn, indicating its superior compatibility. Subsequent S/S experiments demonstrated that the MOSC binder exhibited an excellent performance on immobilisation efficiency of Zn (up to 99.9%), as well as satisfying the requirements of setting time and mechanical strength of sludge S/S products. Therefore, MOSC could be an effective and sustainable binder for the treatment of the Zn-rich industrial wastes.

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 O

Cancer stem cells are closely associated with tumor metastasis or recurrence. According to previous literature reports, microRNA (miR)‑26a has an inhibitory effect on head and neck squamous cell carcinoma (HNSCC), and the long non‑coding RNA (lncRNA) non‑coding RNA activated by DNA damage (NORAD) has been found to interact with miR‑26a‑5p. The present study aimed to investigate the regulation and mechanism of NORAD and miR‑26a‑5p in the epithelial‑mesenchymal transition (EMT) of HNSCC stem cells. An ALDEFLUOR stem cell detection kit, a flow cytometer, a self‑renewal ability test and western blotting were used to sort and identify HNSCC stem cells. The ENCORI website and a dual‑luciferase assay were used to assess the relationship between genes. The mRNA and protein expression levels of NORAD, miR‑26a‑5p and EMT‑related genes were detected via reverse transcription‑quantitative PCR and western blotting. Functional experiments (MTT assay, flow cytometry, wound healing assay and Transwell assay) were conducted to analyze the effects of NORAD and miR‑26a‑5p on HNSCC stem cells. The successfully sorted aldehyde dehydrogenase (ALDH)+ cells had a self‑renewal capacity and displayed upregulated expression levels of CD44, Oct‑4 and Nanog. NORAD knockdown, achieved using small interfering (si)RNA, downregulated the expression levels of tumor markers in ALDH+ cells. siNORAD inhibited cell vitality, migration and invasion, as well as promoted apoptosis, increased the expression of epithelial cell markers and decreased the expression of interstitial cell markers in HNSCC stem cells. miR‑26a‑5p was a downstream gene of NORAD, and knockdown of miR‑26a‑5p partially offset the regulatory effect of siNORAD on HNSCC stem cells. Collectively, the present study demonstrated that NORAD knockdown attenuated the migration, invasion and EMT of HNSCC stem cells via miR‑26a‑5p.

Plasma-catalytic oxidation of particulate matter (PM) has potential applications for diesel exhaust cleaning. There is a grand requirement to explore the mechanism of carbonaceous PM oxidation for the development of plasma catalysts. Herein, Au/γ-Al

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.

Ultrafine particles (UFPs) are harmful to human beings, and their effective removal from the environment is an urgent necessity. In this study, a dielectric barrier discharge (DBD) reactor packed with porous alumina (PA) balls driven by a pulse power supply was developed to remove the UFPs (ranging from 20 to 100 nm) from the exhaust gases of kerosene combustion. Five types of DBD reactors were established to evaluate the effect of plasma catalysis on the removal efficiency of UFPs. The influences of gas flow rate, peak voltage and pulse frequency of different reactors on UFPs removal were investigated. It was found that a high total UFP removal of 91.4% can be achieved in the DBD reactor entirely packed with PA balls. The results can be attributed to the enhanced charge effect of the UFPs with PA balls in the discharge space. The UFP removals by diffusion deposition and electrostatic attraction were further calculated, indicating that particle charging is vital to achieve high removal efficiency for UFPs.

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.

To reveal the seasonal variations and sources of PM2.5-bound polycyclic aromatic hydrocarbons (PAHs) during haze and non-haze episodes, daily PM2.5 samples were collected from March 2015 to February 2016 in a mixed multi-function area in Hangzhou, China. Ambient concentrations of 16 priority-controlled PAHs were determined. The sums of PM2.5-bound PAH concentrations during the haze episodes were 4.52 ± 3.32 and 13.6 ± 6.29 ng m−3 in warm and cold seasons, respectively, which were 1.99 and 1.49 times those during the non-haze episodes. Four PAH sources were identified using the positive matrix factorization model and conditional probability function, which were vehicular emissions (45%), heavy oil combustion (23%), coal and natural gas combustion (22%), and biomass combustion (10%). The four source concentrations of PAHs consistently showed higher levels in the cold season, compared with those in the warm season. Vehicular emissions were the most considerable sources that result in the increase of PM2.5-bound PAH levels during the haze episodes, and heavy oil combustion played an important role in the aggravation of haze pollution. The analysis of air mass back trajectories indicated that air mass transport had an influence on the PM2.5-bound PAH pollution, especially on the increased contributions from coal combustion and vehicular emissions in the cold season.

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 .

Here, we compared the conversion of gasoline-ranged n-alkanes (C6–C9) using dielectric barrier discharge. For an energy density of ~68 J/L and an initial n-alkane concentration of ~230 ppm, when carbon number increased from 6 to 9, the energy efficiency of n-alkane conversion increased from 117 to 240 mmol/kWh, CO x selectivity decreased from 46 to 20%, and ozone concentration increased from 216 to 240 ppm. The effect of energy density and initial n-alkane concentration were also investigated. The understanding of initial step of conversion was greatly aided by a proposed kinetic model. The pathways of consecutive reactions from the initiation to products were also discussed.

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.

We studied characteristics of OH production by pulsed dielectric barrier discharge (DBD) with O2/H2O gas mixtures. We found that the light emission from the O2/H2O DBD was due to OH (A–X) transition. Therefore, it is allowed to measure the spatial distribution and the intensity of OH (A–X) by simply direct photography of discharge image. The OH (A–X) intensity became weaker approaching the dielectric barriers and away from the electrodes along the direction of the axis of and the surface of the electrodes, respectively. Regardless of the specific operating parameters, the intensity of OH (A–X) linearly increased with discharge power. At a similar level of the discharge power, increasing the applied voltage was most effective for the OH production. In addition, we found that the production of OH radical could be enhanced by increasing O(1D) radical. Our study not only introduced a simple method for the semi-quantification of OH (A–X) in the O2/H2O DBD, but also suggested a way for the optimization of OH production in a plasma process.