Plasma-catalytic Oxidation Research Articles

Effect of Catalyst Placement on the Plasma-Catalytic Oxidation of NO

Present study investigated the effect of catalyst placement in the discharge zone on the plasma-catalytic oxidation of NO to NO2. The TiO2 powder catalyst was placed either to the inlet or outlet of the reactor and the results were compared with the results obtained when the catalyst covered the full length of the reactor or was totally absent. The long-term oxidation was improved by the catalyst only when the catalyst was present in the outlet of the reactor or on full length of the reactor. The catalyst placement influenced also adsorption related transient changes of NO and NO2 concentrations in the outlet of the reactor. The observed results can be explained by the increased importance of forward reactions resulting in the oxidation of NO to NO2 in the presence of catalyst.

La0.8M0.2MnO3 (M = Ba, Ca, Ce, Mg and Sr) perovskite catalysts for plasma-catalytic oxidation of ethyl acetate

Plasma-catalytic removal of low concentration ethyl acetate was carried out over La0.8M0.2MnO3 (M=Ba, Ca, Ce, Mg and Sr) catalysts prepared by sol-gel method using citric acid as a mixing complex. The La0.8Ce0.2MnO3 catalyst showed the highest removal of ethyl acetate and CO2 selectivity over the specific energy density (SED) range followed by the La0.8Sr0.2MnO3 catalyst, while Ba, Ca and Mg-doped catalysts inhibited the oxidation of ethyl acetate compared to the pure LaMnO3 catalyst. A wide range of catalyst characterization including N2 adsorption-desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction by H2 (H2-TPR) indicated that the reducibility of the catalysts played an important role in determining the reaction performance of the plasma-catalytic gas cleaning process.

Effect of electric fields on plasma catalytic hydrocarbon oxidation from atomistic simulations

The catalytic oxidative dehydrogenation of hydrocarbons is an industrially important process, in which selectivity is a key issue. We here investigate the conversion of methanol to formaldehyde on a vanadia surface employing long timescale simulations, reaching a time scale of seconds. In particular, we compare the thermal process to the case where an additional external electric field is applied, as would be the case in a direct plasma‐catalysis setup. We find that the electric field influences the retention time of the molecules at the catalyst surface. These simulations provide an atomic scale insight in the thermal catalytic oxidative dehydrogenation process, and in how an external electric field may affect this process.

Study of positive and negative plasma catalytic oxidation of ethylene

ABSTRACTThe effect of introducing a photocatalytically active coating inside a plasma unit is investigated. This technique combines the advantages of high product selectivity from catalysis and the fast start-up from plasma technology. In this study, a preselected TiO2 coating is applied on the collector electrode of a DC corona discharge unit as non-thermal plasma reactor, in order to study the oxidation of ethylene. For both positive and negative polarities an enhanced mineralization is observed while the formation of by-products drastically decreases. The plasma catalytic unit gave the best results when using negative polarity at a voltage of 15 kV. This shows the potential of plasma catalysis as indoor air purification technology.

In situ FT-IR study and evaluation of toluene abatement in different plasma catalytic systems over metal oxides loaded γ-Al2O3

Silver and nickel based catalysts with γ-Al2O3 support were synthesized via impregnation method for the removal of toluene in the presence of continuous discharge or adsorption- discharge plasma. An elaborate FT-IR analysis confirmed the formation of transient species during the plasma-catalytic oxidation of toluene. TOF-MS analysis indicated benzoic acid was not an important pathway of aromatic rings opening. NiO/Al2O3 catalyst displayed a higher enhancement of CO2 selectivity and ozone decomposition ability. Moreover, the active component (O and OH) areas would increase during the discharge process in adsorption-discharge plasma system that were conducive to the removal of the organic byproducts.

Plasma-catalytic oxidation of adsorbed toluene on Ag–Mn/γ-Al2O3: Comparison of gas flow-through and gas circulation treatment

Oxidation of adsorbed toluene was investigated in a dielectric barrier discharge reactor at atmospheric pressure coupled with Ag–Mn/γ-Al2O3 catalysts. Gas flow-through and gas circulation systems were compared in the same reactor with the same operating conditions. After 90min of plasma treatment, the mineralization of adsorbed toluene reached 68% in the O2 flow-through system with some emission of desorbed toluene. For the O2 circulation system, the mineralization of adsorbed toluene increased to 78% without any emission of desorbed toluene. Furthermore, compared to the gas flow-through system, the CO2 selectivity in the gas circulation system was improved in both air and oxygen plasma. Moreover, the circulation system was more efficient than the gas flow-through system for O3 decomposition, resulting in better mineralization of adsorbed toluene and CO2 selectivity.

Plasma-catalytic degradation of benzene over Ag–Ce bimetallic oxide catalysts using hybrid surface/packed-bed discharge plasmas

Plasma-assisted catalysis has been employed for the degradation of benzene by hybrid surface/packed-bed discharge (HSPBD) plasmas over a series of Agx Ce1−x/γ-Al2O3 catalysts in in-plasma catalysis (IPC) and post-plasma catalysis (PPC) configurations. In order to study the influence of catalysts placement on discharge characteristics and the consequent synergetic effect in plasma-catalysis process, the catalysts were introduced inside and downstream the surface discharge region (region I) and packed-bed discharge region (region II), respectively, and the benzene degradation performance was investigated in these systems. The effects of the Ag/Ce molar ratio and water vapor have also been investigated in terms of benzene degradation efficiency and CO2 selectivity. Compared with the plasma-only process, the combination of plasma with Agx Ce1−x/γ-Al2O3 catalyst significantly improved the reaction performance, and the combined degradation efficiency is a synergistic effect rather than simply an additive effect. Besides, the emission of discharge products (O3 and NOx) and hazardous intermediates (formic acid and CO) was markedly suppressed with the introduction of catalyst. The highest benzene degradation efficiency of 96.2% and CO2 selectivity of 77.3% can be achieved with Ag0.9Ce0.1/γ-Al2O3 catalyst at the SIE of 400J/L. This result suggests that the interaction between a certain proportion of Ag and Ce species over the catalyst is capable of activating the surface lattice and generating more surface adsorbed oxygen (Oads), which favors the plasma-catalytic oxidation reaction. PPC processes can decompose O3 and destroy benzene more effectively than IPC processes, especially when the catalyst was introduced downstream the region II. Adding a small amount of water vapor into plasma-catalysis system enhanced the catalyst activity, however, further increased the water vapor caused an obvious negative impact on the catalyst activity.

Catalyst screening for acetone removal in a single-stage plasma-catalysis system

Plasma-catalytic removal of acetone over MOx/γ-Al2O3 (M=Ce, Co, Cu, Mn and Ni) catalysts has been investigated in a dielectric barrier discharge (DBD) reactor. The influence of different catalysts on the plasma-catalytic process has been examined in terms of acetone removal efficiency, CO2 selectivity and the formation of by-products. The combination of plasma with the MOx/γ-Al2O3 catalysts significantly enhances the removal efficiency of acetone and CO2 selectivity, whilst substantially reducing the formation of by-products (HCHO and HCOOH). The maximum removal efficiency of 94.2% and CO2 selectivity of 80.1% were achieved when using the CuOx/γ-Al2O3 catalyst. Introducing the CuOx/γ-Al2O3 catalyst into the plasma system also shows the lowest formation of by-products. Different catalyst characterization techniques have been used to understand the effect of catalyst properties on the plasma reactions. It has been found that the reducibility of the MOx/γ-Al2O3 catalysts plays a dominant role in the plasma-catalytic oxidation of acetone, whilst the specific surface area and pore properties of the catalysts have a very weak effect on the performance of the plasma-catalytic reaction. In addition, a simplified kinetic model has been developed, showing the effect of different catalysts on the acceleration of the plasma-catalytic removal of acetone.

Plasma-catalytic removal of formaldehyde over Cu–Ce catalysts in a dielectric barrier discharge reactor

In this study, a coaxial dielectric barrier discharge (DBD) reactor has been used for plasma-catalytic removal of low concentration formaldehyde over a series of Cu–Ce oxide catalysts prepared by the citric acid sol–gel method. The effect of the Cu/Ce molar ratio on the removal of formaldehyde and CO2 selectivity has been investigated as a function of specific energy density (SED). In comparison to the plasma-only process, the combination of plasma with the Cu–Ce binary oxide catalysts significantly enhances the reaction performance, while the presence of CuO or CeO2 in the DBD reactor has a negative effect on the removal of HCHO. This suggests that the interactions between Cu and Ce species change the properties of the catalysts and consequently affect the performance of the plasma-catalytic process. The highest removal efficiency of 94.7% and CO2 selectivity of 97.3% were achieved when the Cu1Ce1 catalyst (Cu/Ce=1:1) was placed in the DBD reactor at the SED of 486JL−1. The interaction between Cu and Ce species results in a larger specific surface area and pore volume, along with a greater formation of surface adsorbed oxygen (Oads), which favors the oxidation of formaldehyde in the plasma process. In addition, the redox cycles between Cu and Ce species facilitate the formation of additional active oxygen atoms and contribute to the plasma-catalytic oxidation reactions. Plausible reaction mechanisms involved in the plasma-catalytic oxidation of HCHO have been proposed.

Microwave assisted catalytic removal of elemental mercury from flue gas using Mn/zeolite catalyst

The integrated microwave with Mn/zeolite and ozone (MCO) and combined microwave with Mn/zeolite (MC) was employed to oxidize elemental mercury (Hg0) in simulated flue gas. The results show that mercury removal efficiency attained 35.3% in the MC, over 92% of Hg0 removal efficiency could be obtained in the MCO. The optimal microwave power and empty bed residence time (EBRT) in the microwave plasma catalytic oxidation were 264 W and 0.41 s, respectively. The effect of Hg0 oxidation in the MCO was much higher than that in the MC. Microwave accentuated catalytic oxidation of mercury, and increased mercury removal efficiency. The additional use of ozone to the microwave–catalysis over Mn/zeolite led to the enhancement of mercury oxidation. Mn/zeolite catalyst was characterized by X–ray diffraction (XRD), X–ray photoelectron spectroscopy (XPS), Fourier transform infrared spectra (FT–IR), scanning electron microscopy (SEM) and the Brunauer Emmett Teller (BET) method. Microwave catalytic mercury over Mn/zeolite was dominated by a free radical oxidation route. Ozone molecules in air could enhance free radical formation. The coupling role between ozone and radicals on mercury oxidation in the MCO was formed. The MCO appears to be a promising method for emission control of elemental mercury.

금속산화물 촉매상에서 플라즈마를 이용한 IPA 저감

This work investigated the plasma-catalytic decomposition of isopropyl alcohol (IPA) and the behavior of the byproduct compounds over monolith-supported metal oxide catalysts. Iron oxide (Fe2O3) or copper oxide (CuO) was loaded on a monolithic porous α-Al2O3 support, which was placed inside the coaxial electrodes of plasma reactor. The IPA decomposition efficiency itself hardly depended on the presence and type of metal oxides because the rate of plasma-induced decomposition was so fast, but the behavior of byproduct formation was largely affected by them. The concentrations of the unwanted byproducts, including acetone, formaldehyde, acetaldehyde, methane, carbon monoxide, etc., were in order of Fe2O3/α-Al2O3 < CuO/α-Al2O3 < α-Al2O3 from low to high. Under the condition (flow rate: 1 L min -1 ; IPA concentration: 5,000 ppm; O2 content: 10%; discharge power: 47 W), the selectivity towards CO2 was about 40, 80 and 95% for α-Al2O3, CuO/α-Al2O3 and Fe2O3/α-Al2O3, respectively, indicating that Fe2O3/α-Al2O3 is the most effective for plasma-catalytic oxidation of IPA. Unlike plasma-alone processes in which tar-like products formed from volatile organic compounds are deposited, the present plasma-catalyst hybrid system did not exhibit such a phenomenon, thus retaining the original catalytic activity.

Removal of ethylene from air stream by adsorption and plasma-catalytic oxidation using silver-based bimetallic catalysts supported on zeolite

Dynamic adsorption of ethylene on 13X zeolite-supported Ag and Ag–MxOy (M: Co, Cu, Mn, and Fe), and plasma-catalytic oxidation of the adsorbed ethylene were investigated. The experimental results showed that the incorporation of Ag into zeolite afforded a marked enhancement in the adsorptivity for ethylene. The addition of transition metal oxides was found to have a positive influence on the ethylene adsorption, except FexOy. The presence of the additional metal oxides, however, appeared to somewhat interrupt the diffusion of ozone into the zeolite micro-pores, leading to a decrease in the plasma-catalytic oxidation efficiency of the ethylene adsorbed there. Among the second additional metal oxides, FexOy was able to reduce the emission of ozone during the plasma-catalytic oxidation stage while keeping a high effectiveness for the oxidative removal of the adsorbed ethylene. The periodical treatment consisting of adsorption followed by plasma-catalytic oxidation may be a promising energy-efficient ethylene abatement method.

Adsorption and plasma-catalytic oxidation of acetone over zeolite-supported silver catalyst

The abatement of acetone using a combination of non-thermal plasma, catalysis and adsorption was investigated in a dielectric barrier discharge plasma reactor packed with silver-coated zeolite pellets serving as both adsorbent and catalyst. The removal of acetone in this reactor system was carried out by cyclic operation comprising two repetitive steps, namely, adsorption followed by plasma-catalytic oxidation. The effects of the zeolite-supported silver catalyst on the reduction of unwanted ozone emission and the behavior for the formation of gaseous byproducts were examined. The experimental results showed that the zeolite-supported catalyst had a high acetone adsorption capacity of 1.07 mmol g−1 at 25 °C. Acetone with a concentration of 300 ppm was removed from the gas stream and enriched on the zeolite surface during the adsorption step of the cyclic process (100 min). In the succeeding step, the adsorbed acetone was plasma-catalytically treated under oxygen-flowing atmosphere to recover the adsorption capability of the surface. The plasma-catalytic oxidation of the acetone adsorbed in the previous 100 min adsorption step was completed in 15 min. The abatement of acetone by the cyclic adsorption and plasma-catalytic oxidation process was able to increase the performance of the reactor with respect to the energy efficiency, compared to the case of continuous plasma-catalytic treatment. The use of the zeolite-supported silver catalyst largely decreased the emission of unreacted ozone and increased the amount of gaseous byproducts such as carbon oxides and aldehydes due to the enhanced oxidation of the adsorbed acetone and intermediates.

Plasma-Catalytic Oxidation of Toluene on MnxOy at Atmospheric Pressure and Room Temperature

MnxOy/SBA-15 catalysts were prepared via the impregnation method and utilized for toluene removal in dielectric barrier discharge plasma at atmospheric pressure and room temperature. The catalysts were characterized by X-ray diffraction, N2 adsorption–desorption, Raman spectroscopy, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction, and O2 temperature-programmed desorption methods. The characterization results indicated that manganese loading did not influence the 2D-hexagonal mesoporous structure of SBA-15. The catalyst had various oxidation states of manganese (Mn2+, Mn3+, and Mn4+), with Mn3+ being the dominant oxidation state. Toluene removal was investigated in the environment of pure N2 and 80 % N2 + 20 % O2 plasma, showing that the toluene removal efficiency and CO2 selectivity were noticeably increased by MnxOy/SBA-15, especially in the presence of 5 % Mn/SBA-15. This activity was closely related to the high dispersion of 5 % Mn on SBA-15 and the lowest reduction temperature exhibited by this catalyst. Mn loading increased the yield of CO2 in the N2 plasma and promoted the deep oxidation of toluene. During toluene oxidation, oxygen exchange might follow a pathway, wherein bulk oxygen was released from the MnxOy/SBA-15 surface; gas-phase O2 subsequently filled up the vacancies created on the oxide. Each of the manganese oxidation states played an important role; Mn2O3 was considered as a bridge for oxygen exchange between the gas phase and the catalyst, and Mn3O4 mediated transfer of oxygen between the catalyst and toluene.

Plasma-catalytic oxidation of acetone in annular porous monolithic ceramic-supported catalysts

Oxidative decomposition of acetone over multichannel monolithic ceramic-supported catalysts combined with non-thermal plasma was investigated. Plasma was generated inside the porous ceramic by applying an alternating current (AC) voltage to the coaxial electrodes. The tandem plasma-catalytic reactor consisted of two ceramic-supported catalysts containing zinc oxide (ZnO) and/or manganese oxide (MnO2), in which the first supported catalyst was exposed to the plasma discharge and the second one was placed in the post-plasma region. Several sets of catalyst arrangements such as MnO2-loaded monolith followed by bare monolith, ZnO-loaded monolith followed by bare monolith, ZnO-loaded monolith followed by MnO2-loaded monolith and two consecutive MnO2-loaded monoliths with different Mn contents were examined in relation to the acetone decomposition and the byproducts formation. More than 90% of acetone was decomposed at a specific input energy (SIE) of about 1.0 kJ L−1 with the catalyst arrangement of MnO2 (0.1% Mn) followed by MnO2 (5.0% Mn), while the decomposition efficiency obtained with two consecutive bare monoliths was about 66% at the same SIE. The use of ZnO in the plasma discharge region did not largely improve the acetone decomposition efficiency. Wherever it is placed either in the plasma discharge region or in the post-plasma region, MnO2 catalyst substantially promoted the acetone decomposition, obviously due to the dissociation of ozone into far more reactive oxygen atoms available for oxidizing acetone molecules.

Soot oxidation in a corona plasma-catalytic reactor

Oxidation of soot by corona plasma was investigated at conditions of exhaust gases from diesel engines, both in the absence and presence of CoO x as a catalyst. The CoO x catalyst nanoparticles were synthesized by a precipitation method. The BET surface area of the catalyst was 50 m2/g, corresponding to 23 nm particles. An aluminum grid was sequentially dip-coated for several times by suspensions of the soot in toluene and/or fine catalyst powder in DI water. The grid was used as the plate of a pin-to-plate corona reactor. Air at 180 °C was passed through the corona reactor to oxidize the soot, oxidation products of which were analyzed by both gas chromatograph and FTIR with a gas cell. Soot oxidation rate linearly increased with an increase of input energy. When the soot was deposited on a layer of the CoO x catalyst, the soot oxidation rate increased up to 2 times. The only product of the plasma (catalytic) oxidation of soot was CO 2 determined by FTIR. O produced in the plasma discharge oxidized the soot and the active surface oxygen enhanced its rate.

Ozone catalytic oxidation of adsorbed benzene over AgMn/HZSM-5 catalysts at room temperature

Investigation of ozone catalytic oxidation of adsorbed benzene over AgMn/HZSM-5 to provide insight into plasma catalytic oxidation of adsorbed benzene in the cycled storage–discharge process.

Plasma-catalytic oxidation of adsorbed toluene with gas circulation

Abstract The plasma-catalytic oxidation of adsorbed toluene was investigated using gas circulation with MnO x and AgO x catalyst. The experimental results indicated that CO x yield, CO 2 selectivity and ozone decomposition ability enhanced significantly in presence of catalysts. It was observed that adsorbed toluene could be effectively oxidized to CO 2 in presence of catalysts with gas circulation, probably due to the enhanced utilization of reactive species in the system. Typically, when background gas was oxygen, the CO 2 selectivity of MnO x /γ-Al 2 O 3 and AgO x /γ-Al 2 O 3 catalysts were both close to 100% in 60 min. The results showed that N 2 O was generated as a by-product with air background gas.

Plasma Catalytic Oxidation of Stored Benzene in a Cycled Storage-Discharge (CSD) Process: Catalysts, Reactors and Operation Conditions

High energy cost and secondary pollutants formation makes the plasma-based technique impractical. To solve these problems, the effects of different catalysts, reactor configurations and operating conditions on plasma catalytic oxidation of stored benzene in a cycled storage-discharge process were investigated in detail. It is shown that the catalysts and reactor configurations were the main factors affecting the plasma catalytic oxidation of stored benzene. When 0.8 wt% Ag/HZSM-5 catalysts and in-plasma catalytic reactor are used, the stored benzene could be oxidized completely to CO2 in a very short discharge period and almost no secondary pollutant formation is observed. In addition, the relative humidity of air streams at storage stage showed little influence on the plasma catalytic oxidation of stored benzene. When the storage period increased from 1 to 14 h, a small increase of discharge period from 9 to 24 min was required to achieve ~100% conversion of stored benzene to CO2.

Low-concentration formaldehyde removal from air using a cycled storage–discharge (CSD) plasma catalytic process

Cycled storage–discharge (CSD) plasma catalytic process was applied to low-concentration formaldehyde removal from air for the first time. HZSM-5, Ag/HZSM-5 (Ag/HZ), Cu/HZSM-5 (Cu/HZ) and AgCu/HZSM-5 (AgCu/HZ) catalysts were used for the process and their performance for formaldehyde storage and plasma catalytic oxidation of stored formaldehyde towards carbon dioxide was investigated in details. AgCu/HZ exhibited the highest formaldehyde adsorption capacity among these catalysts due to the synergistic effect of Ag and Cu, and it was very selective for oxidizing stored formaldehyde into CO 2 by either oxygen or air plasmas at the discharge stage. The CSD process for low-concentration formaldehyde removal had extremely low energy cost, excellent humidity tolerance and almost no secondary pollution. At the discharge stage, in addition to pure oxygen, air can also be used as the discharge gas for complete oxidation of formaldehyde, and the nitrogen oxides produced can be negligible because the discharge period was much shorter than the storage period. The stability of the AgCu/HZ catalysts during the CSD process was also investigated.