Effect of manganese oxide catalyst on the dielectric barrier discharge decomposition of toluene

Investigation on removal of multi-component volatile organic compounds in a two-stage plasma catalytic oxidation system - Comparison of X (X=Cu, Fe, Ce and La) doped Mn2O3 catalysts

Mn₂O₃-X catalysts (X = Cu, Fe, Ce and La) were prepared based on γ-Al₂O₃ for the mixture degradation of muti-component volatile organic compounds (VOCs) composed of toluene, acetone, and ethyl acetate. The catalysts were characterized, and the density functional theory (DFT) simulation of ozone adsorption on Mn₂O₃-X were carried out to investigate the influence of adsorption energy on catalytic performance. The results showed that the removal efficiency (RE) of each VOC component was similarly improved by Mn₂O₃-X catalysts, and the greatest increase in VOCs' removal efficiency was obtained (7.8% for toluene, 86.2% for acetone, and 82.5% for ethyl acetate) at a special input energy (SIE) of 700 J L^-1 with Mn₂O₃-La catalyst. Characterization results demonstrated that Mn₂O₃-La catalyst had the highest content of low valence Mn elements and the greatest O_ads/O_latt ratio, as well as the lowest reduction temperature. Mn₂O₃-La catalyst also presented superior catalytic effect in improving carbon balance (CB) and CO₂ selectivity ( [Formula: see text] ). The CB and [Formula: see text] were increased by 47.7% and 12.61% respectively with Mn₂O₃-La at a SIE of 400 J L^-1 compared with that when only γ-Al₂O₃ was applied. The DFT simulation results of ozone adsorption on Mn₂O₃-X catalysts indicated that the adsorption energy of catalyst crystal was related to the catalytic performance of the catalyst. The Mn₂O₃-La/γ-Al₂O₃ catalyst, which had the highest absolute value of adsorption energy, presented the best performance in improving VOCs' RE.

Insights into the Role of Nanorod-Shaped MnO2 and CeO2 in a Plasma Catalysis System for Methanol Oxidation

Published papers highlight the roles of the catalysts in plasma catalysis systems, and it is essential to provide deep insight into the mechanism of the reaction. In this work, a coaxial dielectric barrier discharge (DBD) reactor packed with γ-MnO₂ and CeO₂ with similar nanorod morphologies and particle sizes was used for methanol oxidation at atmospheric pressure and room temperature. The experimental results showed that both γ-MnO₂ and CeO₂ exhibited good performance in methanol conversion (up to 100%), but the CO₂ selectivity of CeO₂ (up to 59.3%) was much higher than that of γ-MnO₂ (up to 28.6%). Catalyst characterization results indicated that CeO₂ contained more surface-active oxygen species, adsorbed more methanol and utilized more plasma-induced active species than γ-MnO₂. In addition, in situ Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR) were applied with a novel in situ cell to reveal the major factors affecting the catalytic performance in methanol oxidation. More reactive oxygen species (O₂^2-, O^2-) from ozone decomposition were produced on CeO₂ compared with γ-MnO₂, and less of the intermediate product formate accumulated on the CeO₂. The combined results showed that CeO₂ was a more effective catalyst than γ-MnO₂ for methanol oxidation in the plasma catalysis system.

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.

Nonthermal plasma in practical-scale honeycomb catalysts for the removal of toluene

Nonthermal plasma combined with a practical-scale honeycomb catalyst of 5.0 cm in height and 9.3 cm in diameter was investigated for the removal of toluene. The creation of plasma in the honeycomb catalyst greatly depended on the humidity of the feed gas and the presence of metals on the honeycomb surface. Compared to the bare ceramic honeycomb, the metal-loaded one gave higher toluene removal efficiency because the decomposition of toluene by the plasma-generated reactive species occurred not only homogeneously in the gas phase but also heterogeneously on the catalyst surface. The present plasma-catalytic reactor was able to successfully remove about 80% of dilute toluene (15 ppm in air) at a large flow rate of 60 L/min with a specific energy input of 58 J/L. The honeycomb-based plasma-catalytic reactor system is promising for practical applications since it can overcome such problems as high-pressure drop and difficulty in scale-up encountered in packed-bed reactors.

Non-thermal plasma coupled with MOF-74 derived Mn-Co-Ni-O porous composite oxide for toluene efficient degradation

A novel strategy for removal of toluene by non-thermal plasma (NTP) coupled with metal-organic frameworks (MOFs) derived catalyst was proposed in this work. The MOF-derived porous trimetallic oxide catalyst (MnCoNiO_x, MCNO) was prepared by simple pyrolysis of a MOF-74(Mn-Co-Ni) precursor. We found that the MCNO material can well synergy with NTP in total decomposition of toluene owing to its high specific surface area, regular porous structure and excellent reducibility, which endow superior catalytic activity and CO₂ selectivity of NTP-MCNO system compared to that of NTP-MnO_x, NTP-CoO_x and NTP-NiO_x. For instance, the toluene degradation efficiency can reach up to 75.7% in NTP-MCNO system with a low specific input energy of 101 J/L, much higher than that of NTP-MnO_x (59.3%), NTP-CoO_x (70.9%), NTP-NiO_x (65.0%) and NTP alone (42.9%). Moreover, the formed ozone (O₃) can be well-controlled by the NTP-MCNO system due to the spinel-type oxides (MCNO) derived from MOF could generate more open-formwork structure and improve the mobility of oxygen. The results of this work would shed light on rational design and preparation of spinel-type oxides for oxidation applications, which provides guidance for further improvement of plasma-catalysis system.

Steam reforming of toluene and naphthalene as tar surrogate in a gliding arc discharge reactor

Steam reforming of mixed toluene and naphthalene as tar surrogate has been investigated in an AC gliding arc discharge plasma, with particular emphasis on better understanding the effect of steam and CO₂ on the reaction performance. Results show that H₂, C₂H₂ and CO are the major gas products in the plasma steam reforming of tar for energy recovery. The addition of a small amount of steam remarkably enhances the conversions of both toluene and naphthalene, from 60.4% to 76.1% and 57.6% to 67.4%, respectively, as OH radicals formed by water dissociation create more reaction pathways for the conversion of toluene, naphthalene and their fragments. However, introducing CO₂ to this process has a negative effect on the tar reforming. Optical emission spectroscopic diagnostics has shown the formation of a variety of reactive species in the plasma process. Trace amounts of monocyclic and bicyclic aromatic condensable by-products are also detected. The destruction of toluene and naphthalene can be initiated through the collisions of tar surrogates with energetic electrons, N₂ excited species, OH and O radicals etc. Further optimization of the plasma tar destruction is still needed because the complexity of the tar component in a practical gasifier could decrease the tar conversions.

Plasma-catalytic removal of toluene over the supported manganese oxides in DBD reactor: Effect of the structure of zeolites support

The degradation of toluene in dielectric barrier discharge (DBD) reactor packed with zeolites or MnO_x/zeolites was investigated. The supported catalysts were prepared by loading 3 wt% of manganese on different zeolites (MCM-41, ZSM-5 and 13X) and were characterized in detail using N₂ adsorption, XRD, TEM, H₂-TPR and XPS analysis technology. Compared to the non-thermal plasma (NTP) alone system, the toluene degradation was improved significantly in NTP-MnO_x/zeolites system. The highest toluene conversion of 99.4%, the CO₂ selectivity of 73%, the carbon balance of 99.5% can be achieved in DBD reactor packed with MnO_x/MCM-41. Both XRD and TEM results confirm that the manganese oxides were dispersed more uniformly on MnO_x/MCM-41 than on MnO_x/ZSM-5 or MnO_x/13X. H₂-TPR and XPS results suggest that manganese oxides on MnO_x/MCM-41 are MnO₂ and Mn₂O₃, while those on MnO_x/ZSM-5 or MnO_x/13X are MnO₂ and MnO. These results indicate that the structures of zeolites play a significant role in the dispersion and oxidation state of manganese oxides, then affecting the activity of catalyst for toluene removal in plasma-catalysis system.

Direct methanol synthesis from methane in a plasma-catalyst hybrid system at low temperature using metal oxide-coated glass beads

A plasma-catalyst hybrid system was used to synthesize methanol directly from methane. A dielectric barrier discharge (DBD) plasma combined with the catalyst was introduced in order to overcome the difficulties of catalyst-only batch reactions such as high reaction pressure and separation of liquid product. Of the transition metal oxides, Mn₂O₃-coated glass bead showed the highest methanol yield of about 12.3% in the plasma-catalyst hybrid system. The reaction temperature was maintained below 100 °C due to the low plasma input power (from 1.3 kJ/L to 4.5 kJ/L). Furthermore, the reactivity of the catalyst was maintained for 10 hr without changing the selectivity. The results indicated that the plasma-induced OH radical might be produced on the Mn₂O₃ catalyst, which led to methanol synthesis.

Removal of gas phase low-concentration toluene over Mn, Ag and Ce modified HZSM-5 catalysts by periodical operation of adsorption and non-thermal plasma regeneration

Ag/HZSM-5, Mn/HZSM-5, Ce/HZSM-5, Ag-Mn/HZSM-5 and Ce-Mn/HZSM-5 were prepared by impregnation method. Both their adsorption capacity and catalytic activity were investigated for the removal of gas phase low-concentration toluene by periodical operation of adsorption and non-thermal plasma regeneration. Results show that catalysts loaded with Ag (Ag/HZSM-5 and Ag-Mn/HZSM-5) had larger adsorption capacity for toluene than the other catalysts. And Ag-Mn/HZSM-5 displayed the best catalytic performance for both toluene oxidation by non-thermal plasma and byproducts suppression. On the other hand, the deactivated catalyst can be fully regenerated by calcining in air stream when its adsorption capacity and catalytic activity of the Ag-Mn/HZSM-5 catalyst was found to be decreased after 10 cycles of periodical adsorption and non-thermal regeneration.

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-M(x)O(y) (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 Fe(x)O(y). 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, Fe(x)O(y) 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.

Toluene decomposition performance and NOx by-product formation during a DBD-catalyst process

Characteristics of toluene decomposition and formation of nitrogen oxide (NOx) by-products were investigated in a dielectric barrier discharge (DBD) reactor with/without catalyst at room temperature and atmospheric pressure. Four kinds of metal oxides, i.e., manganese oxide (MnOx), iron oxide (FeOx), cobalt oxide (CoOx) and copper oxide (CuO), supported on Al2O3/nickel foam, were used as catalysts. It was found that introducing catalysts could improve toluene removal efficiency, promote decomposition of by-product ozone and enhance CO2 selectivity. In addition, NOx was suppressed with the decrease of specific energy density (SED) and the increase of humidity, gas flow rate and toluene concentration, or catalyst introduction. Among the four kinds of catalysts, the CuO catalyst showed the best performance in NOx suppression. The MnOx catalyst exhibited the lowest concentration of O3 and highest CO2 selectivity but the highest concentration of NOx. A possible pathway for NOx production in DBD was discussed. The contributions of oxygen active species and hydroxyl radicals are dominant in NOx suppression.

Enhancement of the non-thermal plasma-catalytic system with different zeolites for toluene removal

Based on the important effect of catalyst on the plasma-catalytic system, various types of zeolites (5A, HZSM-5, Hβ, HY and Ag/HY) were chosen as catalysts to remove toluene under non-thermal plasma conditions in this work.

Synergistic effects of non-thermal plasma-assisted catalyst and ultrasound on toluene removal

A wire-mesh catalyst coated by La0.8Sr0.2MnO3 was combined with a dielectric barrier discharge (DBD) reactor for toluene removal at atmospheric pressure. It was found that toluene removal efficiency and carbon dioxide selectivity were enhanced in the catalytic packed-bed reactor. In addition, ozone and nitrogen monoxide from the gas effluent byproducts decreased. This is the first time that ultrasound combined with plasma has been used for toluene removal. A synergistic effect on toluene removal was observed in the plasma-assisted ultrasound system. At the same time, the system increased toluene conversion and reduced ozone emission.

The roles of various plasma species in the plasma and plasma-catalytic removal of low-concentration formaldehyde in air

The contributions of various plasma species to the removal of low-concentration formaldehyde (HCHO) in air by DC corona discharge plasma in the presence and absence of downstream MnO(x)/Al(2)O(3) catalyst were systematically investigated in this study. Experimental results show that HCHO can be removed not only by short-living active species in the discharge zone, but also by long-living species except O(3) downstream the plasma reactor. O(3) on its own is incapable of removing HCHO in the gas phase but when combined with the MnO(x)/Al(2)O(3) catalyst, considerable HCHO conversion is seen, well explaining the greatly enhanced HCHO removal by combining plasma with catalysis. The plasma-catalysis hybrid process where HCHO is introduced through the discharge zone and then the catalyst bed exhibits the highest energy efficiency concerning HCHO conversion, due to the best use of plasma-generated active species in a two-stage HCHO destruction process. Moreover, the presence of downstream MnO(x)/Al(2)O(3) catalyst significantly reduced the emission of discharge byproducts (O(3)) and organic intermediates (HCOOH).

Non-thermal plasmas for non-catalytic and catalytic VOC abatement

This paper reviews recent achievements and the current status of non-thermal plasma (NTP) technology for the abatement of volatile organic compounds (VOCs). Many reactor configurations have been developed to generate a NTP at atmospheric pressure. Therefore in this review article, the principles of generating NTPs are outlined. Further on, this paper is divided in two equally important parts: plasma-alone and plasma-catalytic systems. Combination of NTP with heterogeneous catalysis has attracted increased attention in order to overcome the weaknesses of plasma-alone systems. An overview is given of the present understanding of the mechanisms involved in plasma-catalytic processes. In both parts (plasma-alone systems and plasma-catalysis), literature on the abatement of VOCs is reviewed in close detail. Special attention is given to the influence of critical process parameters on the removal process.