Decomposition of Toluene and Acetone in Packed Dielectric Barrier Discharge Reactors

The influences of TiO₂ catalytic material and glass pellet packing on the decomposition efficiency of toluene and acetone in air by dielectric barrier discharge (DBD) reactors were experimentally investigated in this study. The effects of both packing materials on the formation of byproducts such as CO and CO₂ were also evaluated. Experimental results indicate that the introduction of glass materials into the plasma zone of a wire-tube reactor would improve the decomposition efficiency of toluene and acetone compared to a nonpacked reactor. The apparent decomposition rate constant of a glass packed-bed reactor was 4.5–4.8 times greater than that of a nonpacked reactor. The results also indicate that the decomposition rate constant of toluene was approximately 2.6 times higher than that of acetone no matter which type reactor was utilized. The application of TiO₂ coated pellets in DBD reactors will enforce the hydrocarbon byproducts to further be oxidized to CO₂, notwithstanding, it will not significantly improve the performance of the reactors in the decomposition of toluene and acetone, and in the formation of CO. The results show that the best selectivity of CO₂ for acetone decomposition in a TiO₂ coated pellets packed-bed reactor was approximately 40% higher than that in a glass packed-bed reactor.     

Effect of porosity of α-alumina on non-thermal plasma decomposition of ethylene in a dielectric-packed bed reactor

The performances of the porous and nonporous α-alumina (α-Al₂O₃) for the decomposition of ethylene in a dielectric-packed bed plasma reactor were comparatively examined with respect to the decomposition efficiency and the formation of byproducts. The decomposition was mainly controlled by discharge power, oxygen content, and properties of the alumina, such as porosity and surface area. The addition of a small quantity of oxygen led to an increase in the generation of oxidative species which eventually increased the ethylene decomposition efficiency. In the presence of 5 % oxygen, ethylene at an initial concentration of 1,898 ppm was completely oxidized into CO or CO₂ when using the porous α-alumina. On the other hand, the nonporous α-alumina resulted in an incomplete oxidation, producing several carbon-containing byproducts other than CO and CO₂. Moreover, with the other conditions kept constant, the decomposition efficiency obtained with the porous α-alumina was higher than that with the nonporous one, suggesting the adsorption capability of the packing material plays an important role in the decomposition process.     

Enhanced effect of plasma on catalytic reduction of CO_2 to CO with hydrogen over Au/CeO_2 at low temperature

In terms of the reaction of CO_2 reduction to CO with hydrogen, CO_2 conversion is very low at low temperature due to the limitation of thermodynamic equilibrium(TE). To overcome this limitation, plasma catalytic reduction of CO_2 to CO in a catalyst-filled dielectric barrier discharge(DBD) reactor is studied. An enhanced effect of plasma on the reaction over Au/CeO_2 catalysts is observed. For both the conventionally catalytic(CC) and plasma catalytic(PC, Pin= 15 W) reactions under conditions of 400 °C, H_2/CO_2= 1,200 SCCM, GHSV = 12,000 mL·g~(-1)cat·h~(-1), CO_2 conversions over Au/CeO_2 reach 15.4% and 25.5% due to the presence of Au, respectively, however, those over CeO_2 are extremely low and negligible. Moreover,CO_2 conversion over Au/CeO_2 in the PC reaction exceeds 22.4% of the TE conversion. Surface intermediate species formed on the catalyst samples during the reactions are determined by in-situ temperatureprogrammed decomposition(TPD) technique. Interestingly, it disclosed that in the PC reaction, the formation of formate intermediate is enhanced by plasma, and the acceleration by plasma in the decomposition of formate species is much greater than that in the formation of formate species on Au/CeO_2. Enhancement factor is introduced to quantify the enhanced effect of plasma. Lower reactor temperature, higher gas hourly space velocity(GHSV), and lower molar ratio of H_2/CO_2 would be associated with larger enhancement factor.     

OH Radicals as Oxidizing Agent for the Abatement of Organic Pollutants in Gas Flows by Dielectric Barrier Discharges

OH radicals play an essential role in various plasma-chemical processes aimed at the abatement of organic and inorganic pollutants from off-air flows. We report about the oxidation of carbon monoxide in nonthermal air and nitrogen plasmas in dependence on CO inlet concentration and flow humidity. Thereby the reaction CO + OH CO₂ + H served as a diagnostic tool for OH radical determination in the dielectric barrier discharge at atmospheric pressure. The results were numerically fitted to the equations of a kinetic model allowing the determination of the average OH production efficiency (G_OH-value) and OH lifetime (T_OH) in dependence on flow humidity. Finally,results on ethyl acetate abatement obtained under similar experimental conditions were modeled by OH radical decomposition.     

Surface Chemistry on Small Ruthenium Nanoparticles: Evidence for Site Selective Reactions and Influence of Ligands

The reactivity of two classes of ruthenium nanoparticles (Ru NPs) of small size, either sterically stabilized by a polymer (polyvinylpyrrolidone, PVP) or electronically stabilized by a ligand (bisdiphenylphosphinobutane, dppb) was tested towards standard reactions, namely CO oxidation, CO₂ reduction and styrene hydrogenation. The aim of the work was to identify the sites of reactivity on the nanoparticles and to study how the presence of ancillary ligands can influence the course of these catalytic reactions by using NMR and IR spectroscopies. It was found that CO oxidation proceeds at room temperature (RT) on Ru NPs but that the system deactivates rapidly in the absence of ligands because of the formation of RuO₂. In the presence of ligands, the reaction involves exclusively the bridging CO groups and no bulk oxidation is observed at RT under catalytic conditions. The reverse reaction, CO₂ reduction, is achieved at 120 °C in the presence of H₂ and leads to CO, which coordinates exclusively in a bridging mode, hence evidencing the competition between hydrides and CO for coordination on Ru NPs. The effect of ligands localized on the surface is also evidenced in catalytic reactions. Thus, styrene is slowly hydrogenated at RT by the two systems Ru/PVP and Ru/dppb, first into ethylbenzene and then into ethylcyclohexane. Selectively poisoning the nanoparticles with bridging CO groups leads to catalysts that are only able to reduce the vinyl group of styrene whereas a full poisoning with both terminal and bridging CO groups leads to inactive catalysts. These results are interpreted in terms of location of the ligands on the particles surface, and evidence site selectivity for both CO oxidation and arene hydrogenation.     

Control of selectivity in methane conversion reactions in RF plasma: the influence of reaction conditions

RF plasma excitation of methane has been studied in an effort to optimize the reaction conditions for a selective partial oxidation of methane. The reaction products of RF-excited methane are C2 hydrocarbons such as ethane and acetylene when O₂ is not used. The introduction of a few percent of O₂, however, is found to switch the selectivity in favor of CO while CO₂ formation is suppressed down to a level below a few percent. Interestingly, in the low O₂ ratio regime (0–0.6), the selectivity between CO and C2 hydrocarbons is observed to vary systematically in response to the detailed reaction conditions, including flow rate, pressure and applied RF power, which are explained by the competition between coupling and partial oxidation reactions. Variation in the density and the residence time of the active species in the plasma is suggested to determine the overall reaction pathways. The present results suggest a possibility of a selective production of the partial oxidation products of methane such as CO with a high selectivity and a high conversion efficiency using controlled RF plasma from methane and O₂.     

Enhancement of gamma-ray radiolysis of carbon dioxide with the assistance of solid materials

This work is devoted to enhance gamma-ray radiolysis of CO₂ with the assistance of coexisting metal materials. It is found that lower energy electrons which are generated through interactions of γ-photons with the coexisting metal materials and ejected to CO₂ gas actually enhance decomposition of CO₂ to produce CO. The increment of CO production agrees well with the increment of the deposited energy in CO₂, given by the lower energy electrons emitted from the materials, which is calculated by a numerical simulations code MCNP. It is also suggested that the volumetric decomposition of CO₂ dominates the decomposition at the material’s surface.     

Carbon monoxide production in silent discharge plasmas of air and air-methane mixtures

CO production in high-voltage alternating current (HVAC) silent discharge plasmas of air and air-methane mixtures at atmospheric pressure has been investigated by matrix isolation FTIR spectroscopy. In pure air, CO is produced by decomposition of CO₂. A steady-state CO/CO₂ ratio was determined by varying the flow rate. CO production was considerably enhanced when methane was added to the plasmas. CO production was observed even at very low oxygen concentrations, and did not noticeably decrease due to secondary oxidation reactions, even when methane was discharged in a pure oxygen carrier. CO production in air-methane mixtures is shown to depend on input power. CO production from CO₂ and hydrocarbons in air appears to be a significant obstacle for development of a plasma-based air purification device.     

Improvement of the Diluted Propane Efficiency Treatment Using a Non-thermal Plasma

The decomposition of propane diluted in air has been investigated using a pulsed high-voltage dielectric barrier discharges reactor. Effects of the temperature (from 300 to 800 K) and humidity in air on propane conversion and on produced species are studied. CO and CO₂ are the two main carbon species produced but other carbon species can be also obtained as functions of electrical parameters or temperature. Total decomposition of inlet propane to CO₂ is possible when propane is diluted in wet air from 600 K. Thermal energy is an important parameter to limit the energy density injected in the plasma reactor and to reduce the total energetic cost keeping a high propane decomposition yield.     

Geometric stability of PtFe/PdFe embedded in graphene and catalytic activity for CO oxidation

The direct methanol fuel cell (DMFC) is considered as a promising power source, because of its abundant fuel source, high energy density and environmental friendliness. Among DMFC anode materials, Pt and Pt group metals are considered to be the best electrocatalysts. The combination of Pt with some specific transition metal can reduce the cost and improve the tolerance toward CO poisoning of pure Pt catalysts. In this paper, the geometric stabilities of PtFe/PdFe atoms anchored in graphene sheet and catalytic CO oxidation properties were investigated using the density functional theory method. The results show that the Pt (Pd) and Fe atoms can replace C atoms in graphene sheet. The CO oxidation reaction by molecular O₂ on PtFe–graphene and PdFe–graphene was studied. The results show that the Eley–Rideal (ER) mechanism is expected over the Langmuir–Hinshelwood mechanism for CO oxidation on both PtFe–graphene and PdFe–graphene. Further, complete CO oxidation on PtFe–graphene and PdFe–graphene proceeds via a two‐step ER reaction: CO(gas) + O₂(ads) → CO₂(ads) + O(ads) and CO(gas) + O(ads) → CO₂(ads). Our results reveal that PtFe/PdFe commonly embedded in graphene can be used as a catalyst for CO oxidation. The microscopic mechanism of the CO oxidation reaction on the atomic catalysts was explored.     

Decomposition of Benzene Using a Pulse-Modulated DBD Plasma

To improve the energy yield (EY) of plasma volatile organic compound decomposition, a dielectric barrier discharge plasma driven by pulse-modulated AC power was used to experimentally study the abatement of benzene in atmospheric pressure air and at room temperature. The effects of the duty cycle on decomposition efficiency, EY, CO₂ selectivity and the formation of ozone and NO₂ were investigated. The results show that applying pulse modulation improves the EY and the CO₂ selectivity and greatly reduces the wall temperature of the reaction chamber.     

Decomposition and oxidation of methanol on platinum: A study by in situ X-ray photoelectron spectroscopy and mass spectrometry

The reactions of the catalytic oxidation and decomposition of methanol on the atomically smooth and high-defect Pt(111) single-crystal surfaces were studied using in situ temperature-programmed reaction and X-ray photoelectron spectroscopy. It was found that the decomposition of methanol on both of the surfaces occurred via two reaction pathways: complete dehydrogenation to CO and decomposition with the C-O bond cleavage. Although the rate of reaction via the latter pathway was lower than the rate of dehydrogenation by three orders of magnitude, the carbon formed as a result of the C-O bond cleavage can be accumulated on the surface of platinum to prevent the further course of the reaction. It was shown that oxygen exhibits high activity toward the formed carbon deposits. As a result, the rate of methanol conversion in the presence of oxygen in a gas phase increased by one or two orders of magnitude; in this case, CO₂ and water appeared in the composition of the reaction products as a result of the oxidation of CO and hydrogen, respectively. The high-defect surface of platinum was more active in the reactions of methanol decomposition and oxidation than the atomically smooth Pt(111) single-crystal surface. On the former, selectivity for the formation of methanol dehydrogenation products in oxygen deficiency was higher than on the latter. The main reaction pathways of the decomposition and oxidation of methanol on platinum were considered.     

Evaluation of the probability of reaction of fast oxygen atoms with polymers and graphite

On the basis of analysis of published data on the reaction efficiency of various polymer materials and graphite in their interaction with fast oxygen atoms (energy of about 4.5 eV) as obtained in flight tests of materials in low-Earth orbits of the International Space Station and ground tests, probability P _r of chemical oxidation reactions accompanied by ablation has been evaluated. Estimates have been made for 33 polymers consisting of carbon, hydrogen, oxygen, and nitrogen and graphite for two extreme cases when the carboncontaining oxidation products are either CO or CO₂ alone. The average probability values found are P _r(CO)(av) = 0.184 and P _r(CO₂)(av) = 0.317. The probability values range from P _r(CO) = 0.604 and P _r(CO₂) = 0.963 for allyl diglycol carbonate to P _r(CO) = 0.038 and P _r(CO₂) = 0.075 for pyrolytic graphite.     

Properties of surface compounds in methanol conversion on copper-containing catalysts based on CeO2 according to in situ IR-spectroscopic data

Formate and carbonate complexes and bridging and linear methoxy groups were detected on the surfaces of CeO₂ and 5.0% Cu/CeO₂ under the reaction conditions of methanol conversion using IR spectroscopy. The reaction products were H₂, methyl formate, CO, CO₂, and H₂O. The bridging and linear methoxy groups were the sources of formation of bi- and monodentate formate complexes, respectively. Methyl formate was formed as a result of the interaction of the linear methoxy group and the formate complex. The study demonstrated that the recombination of hydrogen atoms on copper clusters and the decomposition of methyl formate were the main reactions of hydrogen formation. Formate and carbonate complexes were the source of CO₂ formation in the gas phase, and the decomposition of methyl formate was the source of CO. It was found that the addition of water vapor to the reaction flow considerably decreased the rate of CO formation at a constant yield of hydrogen. The effects of water vapor and oxygen on the course of surface reactions and the formation of products are discussed. To explain the mechanism of methanol conversion, a scheme of surface reactions is proposed.     

Formation Mechanism,Geometric Stability and Catalytic Activity of a Single Iron Atom Supported on N-Doped Graphene

Based on density functional theory (DFT) calculations, the formation geometries, stability and catalytic properties of single-atom iron anchored on xN-doped graphene (xN-graphene-Fe, x=1, 2, 3) sheet are systemically investigated. It is found that the different kinds and numbers of gas reactants can effectively regulate the electronic structure and magnetic properties of the 3 N-graphene-Fe system. For NO and CO oxidation reactions, the coadsorption configurations of NO/O₂ and CO/O₂ molecules on a reactive substrate as the initial state are comparably analyzed. The NO oxidation reactions through the Langmuir–Hinshelwood (LH) and Eley-Rideal (ER) mechanisms have relatively smaller energy barriers than those of the CO oxidation processes. In comparison, the preadsorbed 2NO reacting with 2CO molecules (2NO+2CO→2CO₂+N₂) through ER reactions (<0.4 eV) are energetically more favorable processes. These results can provide beneficial references for theoretical studies on NO and CO oxidation and designing graphene-based catalyst for toxic gas removal.     

Enhancement of CO2 Conversion Rate and Conversion Efficiency by Homogeneous Discharges

The CO₂ conversion rate and conversion efficiency were greatly enhanced by homogeneous dielectric barrier discharges generated in our experiment. Influence of CaO?CB₂O₃?CSiO₂ glass addition on dielectric properties and microstructures of Ca_0.8Sr_0.2TiO₃ were investigated for the purpose of discerning the effect of dielectric barrier material on the dielectric barrier discharge performance so as to improve the CO₂ conversion rate and conversion efficiency. It was found that considerable grain boundaries on the dielectric barrier surface serving as charge trapping sites contribute a great many charges during plasma generation. And low resistance of the dielectric barrier surface distributes the charges effectively. More importantly, when the gap of the discharge is narrowed down, the surface charges on the dielectric barrier will play a dominant role during the discharge. As a result, for the 5.0 wt% glass addition, the CO₂ conversion rate and conversion efficiency reached the maximum values of 48.71?% and 1.14?W/%, respectively.     

Influence of Plasma Regimes and Catalysts on Ethanethiol Oxidation

Plasma oxidation of ethanethiol in air was investigated using three plasma regimes: surface dielectric pulsed corona discharge, surface dielectric barrier discharge and pulsed corona discharge (PCD) in the plasma reactor. Catalytic plasma degradation of ethanethiol was also performed on the singular or binary metals doped ?èCAl₂O₃. The ethanethiol removal rate increased with increasing energy density but energy efficiency was first increased and then decreased with increasing energy density under three various types of discharges. PCD plasma required the lowest energy density at the similar ethanethiol removal performance compared with the other two plasma discharges. The main intermediate by-products of ethanethiol oxidation by plasma are CH₃CHO, HCHO, CO and CO₂. The sum of these intermediate products selectivities is 19?C43?%, implying that some other intermediates containing carbon were undetermined. When using PCD plasma combined with catalysts, ethanethiol removal rate and energy efficiency were all evidently improved. The maximum energy efficiency was achieved about 200?g kWh^?1 using Fe?CMn/?èCAl₂O₃ assisted PCD plasma, which was about 4.4 times when using PCD plasma alone. The mechanism of ethanethiol oxidation is also discussed.     

Catalytic performance of Ni-Al layered double hydroxides in CO purification processes

Ni-Al layered double hydroxides with Ni²⁺/Al³⁺ molar ratios of 1.5 and 3.0 have been synthesized by co-precipitation and studied as catalyst precursors for purification of CO-containing gas-mixtures by means of CO oxidation to CO₂ and conversion of CO by water vapor (water-gas shift reaction). The influence of the alkali additives (K⁺ ions) on the water-gas shift activity has been also examined. It was established that the catalytic activity of both reactions increases with the temperature and the nickel content. Hypothetic schemes are proposed about activation of the catalysts in the WGSR and CO oxidation including redox Ni²⁺ ? Ni³⁺ transition on the catalyst surface. The activity in WGSR is positively affected by the presence of potassium promoter, depending on its amount. The sample with higher nickel loading is the most effective catalyst as for CO oxidation as well as for WGSR at intermediate temperatures after potassium promotion.     

The mercury-sensitized oxidation of carbon monoxide

The mercury-photosensitized oxidation of CO was studied at 275°C over a wide range of [O₂]/[CO] ratios in the absence and presence of the oxygen atom scavenger 2-trifluoromethylpropene (TMP) and at 25°C at low [O₂]/[CO] ratios in the presence of TMP. By following the quantum yield of CO₂ production, Φ {CO₂}, as a function of the [O₂]/[CO] ratio, the reactions of vibrationally excited CO (v υ 9) and electronically excited O₂, probably in the c¹Σ^?_u state, were studied. At low [O₂]/[CO] ratios the predominant reactions are of vibrationally excited CO (v υ 9). Relative rate constants for chemical reaction versus deactivation of CO (v υ 9) were obtained. At higher [O₂]/[CO] ratios, the principal reactions are of electronically excited O₂. Relative rate constants for chemical reactions and deactivation of this electronically excited O₂ with CO, O₂, and TMP were obtained. From the effect of total pressure on Φ {CO₂}, it is proposed that an intermediate CO₃ is formed in the reaction of electronically excited O₂ with CO.     

Influence of structural defects on the electrocatalytic activity of platinum

Structural defects play major role in catalysis and electrocatalysis. Nanocrystalline (or nanostructured) materials composed of nanometer-sized crystallites joined via grain boundaries have been recognized for their specific structure and properties, differentiating them from single crystals, coarsely grained materials or nanometer-sized supported single-grained particles (Gleiter, Nanostruct Mater 1:1–19, 1992). In this paper, we use Pt electrodes, prepared by electrodeposition on glassy carbon and gold supports, as model nanocrystalline materials to explore the influence of grain boundaries and other structural defects on electrocatalysis of CO and methanol oxidation. We build on the recently established correlations between the nanostructure (lattice parameter, grain size, and microstrains) of electrodeposited Pt and the deposition potential (Plyasova et al., Electrochim. Acta 51:4447–4488, 2006) and use the latter to obtain materials with variable density of grain boundary regions. The activity of electrodeposited Pt in the oxidation of methanol and adsorbed CO exceeds greatly that for Pt(111), polycrystalline Pt, or single-grained Pt particles. It is proposed that active sites in nanostructured Pt are located at the emergence of grain boundaries at the surface. For methanol electrooxidation, the electrodes with optimal nanostructure exhibit relatively high rates of the “direct” oxidation pathway and of the oxidation of strongly adsorbed poisoning intermediate (CO_ads), but not-too-high methanol dehydrogenation rate constant. These electrodes exhibit an initial current increase during potentiostatic methanol oxidation explained by the CO_ads oxidation rate constant exceeding the methanol decomposition rate constant.