Xinbo Zhu

Plasma-assisted conversion of CO2 in a dielectric barrier discharge reactor: understanding the effect of packing materials

A cylindrical dielectric barrier discharge (DBD) reactor has been developed for the conversion of undiluted CO2 into CO and O2 at atmospheric pressure and low temperatures. Both the physical and chemical effects on reaction performance have been investigated for the addition of BaTiO3 and glass beads into the discharge gap. The presence of these packing materials in the DBD reactor changes the physical characteristics of the discharge and leads to a shift of the discharge mode from a typical filamentary discharge with no packing to a combination of filamentary discharge and surface discharge with packing. Highest CO2 conversion and energy efficiency are achieved when the BaTiO3 beads are fully packed into the discharge gap. It is found that adding the BaTiO3 beads into the plasma system enhances the average electric field and mean electron energy of the CO2 discharge by a factor of two, which significantly contributes to the enhancement of CO2 conversion, CO yield, and energy efficiency of the plasma process. In addition, the highly energetic electrons (>3.0 eV) generated by the discharge could activate the BaTiO3 photocatalyst to form electron–hole pairs on its surface, which contributes to the enhanced conversion of CO2.

Plasma-catalytic removal of a low concentration of acetone in humid conditions

A coaxial dielectric barrier discharge (DBD) plasma reactor has been developed for plasma-catalytic removal of low concentration acetone over MOx/γ-Al2O3 (M=Mn, Co, or Cu) catalysts. The effect of relative humidity of air (RH) on the discharge characteristics, acetone removal efficiency, CO2 selectivity, and byproduct formation with and without catalyst has been investigated. The results show that increasing the RH leads to a decrease of the specific energy density (SED) of the DBD, while packing γ-Al2O3 supported metal oxide catalysts into the discharge gap enhances the SED of the discharge. The maximum acetone removal of 75.3% is achieved at an optimum RH of 10% using CoOx/γ-Al2O3 beyond which the removal efficiency of acetone decreases with the increase of the RH. Higher RH inhibits the formation of energetic electrons while water can be adsorbed onto the catalyst surface and block active sites on the catalyst surface. It is found that increasing the air humidity enhances both CO2 selectivity and carbon balance, but decreases the formation of ozone. However, the formation of NOx slightly increases with increasing the gas humidity. In addition, the presence of these catalysts in the discharge significantly decreases the formation of unwanted byproducts (O3 and NOx) and promotes the deep oxidation of acetone towards CO2 with an increased carbon balance.

Controllable synthesis of novel hierarchical V2O5/TiO2 nanofibers with improved acetone oxidation performance

A controllable strategy to fabricate novel hierarchical V2O5/TiO2 nanofiber catalysts was proposed. The catalysts, which comprised primary TiO2 nanofibers and secondary V2O5 nanoparticles, were fabricated by combining electrospinning and hydrothermal growth. The controllable synthesis process and possible formation mechanism were also demonstrated through a series of time-dependent experiments. The hierarchical V2O5/TiO2 nanofiber catalysts were further applied in the oxidation of volatile organic compounds for the first time and were found to present a high oxidation performance for acetone. The morphological, structural, chemical characterization and catalytic performance analyses illustrated the highest catalytic activity was obtained from the synthesized V2O5/TiO2 nanofiber catalyst with 5 wt% V2O5. This finding could be attributed to the combined effect of the specific hierarchical nanofibrous morphology, abundant oxygen vacancies, and appropriate vanadium concentration.

Catalytic oxidation of acetone over CuCeOx nanofibers prepared by an electrospinning method

A series of CuCeOx nanofiber catalysts with different Cu/(Cu + Ce) molar ratios were synthesized by an electrospinning method. The catalysts were evaluated for acetone oxidation at different temperatures (100–280 °C) with a GHSV of 79 000 ml g−1 h−1. The results showed that nanofiber catalysts possessed better catalytic performance than catalysts prepared by urea-nitrate combustion and sol–gel methods. An appropriate Cu/(Cu + Ce) molar ratio could greatly improve the activity of the nanofiber catalysts, and a significant improvement of the activity was obtained on the Cu0.50Ce0.50Ox nanofiber catalyst (∼100% acetone conversion at 270 °C). Characteristic analysis of prepared catalysts suggested (1) a special nanofibrous morphology with large specific surface area; (2) abundant oxygen vacancies and (3) cerium ions with unusual oxidation states were the main factors that would affect catalytic activity of CuCeOx nanofibers catalysts.

Controllable synthesis of hierarchical MnOx/TiO2 composite nanofibers for complete oxidation of low-concentration acetone

A novel hierarchical MnOx/TiO2 composite nanofiber was fabricated by combining the electrospinning technique and hydrothermal growth method. The synthesized nanomaterial, which comprised primary TiO2 nanofibers and secondary MnOx nanoneedles, was further investigated for complete catalytic oxidation of volatile organic compounds for the first time, and this presented high-oxidation performance on low-concentration acetone. The morphological, structural, physicochemical characterization, and catalytic performance analyses demonstrated that the highest catalytic activity was achieved from the obtained MnOx/TiO2 nanofiber catalyst with 30wt.% manganese loading. This finding can be ascribed to the synergistic effect of the specific hierarchical nanofibrous morphology, the abundant surface-adsorbed oxygen, the superior redox property, and the sufficient specific surface.

Experimental study of NO2 reduction in N2/Ar and O2/Ar mixtures by pulsed corona discharge

Non-thermal plasma technology has been regarded as a promising alternative technology for NOx removal. The understanding of NO2 reduction characteristics is extremely important since NO2 reduction could lower the total NO oxidation rate in the plasma atmosphere. In this study, NO2 reduction was experimentally investigated using a non-thermal plasma reactor driven by a pulsed power supply for different simulated gas compositions and operating parameters. The NO2 reduction was promoted by increasing the specific energy density (SED), and the highest conversion rates were 33.7%, 42.1% and 25.7% for Ar, N2/Ar and O2/Ar, respectively. For a given SED, the NO2 conversion rate had the order N2/Ar>Ar>O2/Ar. The highest energy yield of 3.31g/kWh was obtained in N2/Ar plasma and decreased with increasing SED; the same trends were also found in the other two gas compositions. The conversion rate decreased with increasing initial NO2 concentration. Furthermore, the presence of N2 or O2 led to different reaction pathways for NO2 conversion due to the formation of different dominating reactive radicals.

Planar Laser-Induced Fluorescence Diagnostics for Spatiotemporal OH Evolution in Pulsed Corona Discharge

OH radicals play an important role in pollutant removal in nonthermal plasmas. It is crucial to clarify the behavior of OH radicals in this process. A time-resolved 2-D OH radial distribution was investigated in a pulsed corona discharge by planar laser-induced fluorescence at atmospheric pressure and room temperature. The OH evolutions under different gas components were studied, and the evolution process was simulated. The OH decay processes were found to be divided into two periods: a fast decay period and a slow decay period. The O, N, and HO2 are dominant radicals for OH generation and decay. The OH radicals are mainly generated near a nozzle electrode. The concentration variations of O2, NO, and H2O in the background gas led to different OH density evolutions. The OH distribution zones were different as gas components varied. The maximum area of OH radical distribution after discharge decreased by 20% as O2 increased from 5% to 8 %, and it decreased by 69% as NO (150 ppm) was added into the background gas.

Catalytic Oxidation of Dimethyl Sulfide Over Commercial V-W/Ti Catalysts: Plasma Activation at Low Temperatures

This paper investigated the enhancement of plasma on dimethyl sulfide (DMS) removal over commercial V-W/Ti catalysts. The effects of catalyst composition, discharge power, and gas temperature on DMS removal in the plasma-based system were analyzed. The results showed that the V0.9-W/Ti catalyst exhibited the best performance toward DMS removal due to its highest reducibility among the employed samples. The introduction of plasma exhibited a synergistic effect on catalysts and significantly enhanced the removal of DMS over the studied temperature range, especially in high discharge power cases. The activation energy Ea of DMS removal in the presence of plasma was dramatically reduced to less than 10% of the catalysis only case, indicating different DMS removal pathways in plasmaactivated catalysis system. The reaction mechanisms of DMS were also discussed based on the reaction products.

New insights into catalytic pyrolysis mechanisms and reaction pathways of urea pyrolysis on V–Ti catalyst surfaces

A series of V–Ti catalysts with different vanadium loadings for urea pyrolysis were synthesized using the impregnation method and showed desirable catalytic activity. The results showed that the catalysts could accelerate the pyrolysis of urea and inhibit the formation of byproducts. The urea conversion over the catalysts was in the order of 1% V–Ti > 0.5% V–Ti ≈ TiO2 > 5% V–Ti > 10% V–Ti > pure urea, which was associated with the total acidity of the catalysts. The identification and quantification of major byproducts, e.g. biuret, cyanuric acid and melamine, were conducted by HPLC and FT-IR between 100 °C and 450 °C. The reaction pathways of urea pyrolysis in the presence of 1% V–Ti catalysts were proposed based on the byproduct distributions. In the presence of the catalysts, urea pyrolysis was accelerated with a lower initial decomposition temperature. Moreover, the catalysts could promote the further conversion of major byproducts, biuret and cyanuric acid, to the final products NH3 and HNCO at lower temperatures compared to the cases without catalysts.

Plasma-catalytic oxidation of diluted formaldehyde over Cu-Ce oxide catalysts

Summary form only given. With the rapid development of industry and economy, the emission of volatile organic compounds (VOCs) is becoming a global focus because of its negative impact on both human health and environment 1. The combination of cold plasma and catalysis (plasma-catalysis) provides an attractive alternative to the conventional thermal catalytic route for the removal of low concentration of gas pollutants in high volume waste gas streams 2. Catalyst is a key factor affecting the performance of a plasma-catalytic process 3. Cu-based catalysts have been investigated in thermal catalytic oxidation of VOCs due to its high catalytic activity and relative low cost. The addition of promoter CeO2 could further improve the performance for VOC oxidation. However, the use of Cu-Ce catalysts in plasma-catalytic gas cleaning process for the oxidation of VOCs has not been studied before. The synergy resulting from the combination of cold plasma and Cu-Ce catalysts is still not clear. In this study, a coaxial dielectric barrier discharge (DBD) reactor has been developed for the plasma-catalytic oxidation of formaldehyde over Cu-Ce oxide catalysts. It is found that the process performance has been significantly enhanced when using binary oxide catalysts compared to pure CuO and CeO2 catalysts. A maximum removal efficiency (94.7%) and CO2 selectivity (97.3%) has been achieved when the Cu-Ce catalyst (CuO/CeO2=1:1) is directly placed in the DBD reactor. The combination of Cu and Ce oxides results in larger specific surface area and pores, and smaller crystalline size, which favors the oxidation of formaldehyde. In addition, two redox cycles of Cu and Ce could facilitate the formation of active oxygen atoms and contribute to the plasma-catalytic oxidation reactions.