Decomposition of tetrafluorocarbon in dielectric barrier discharge reactor

The decomposition of CF₄ in dielectric barrier discharge at atmospheric pressure was examined. The effect of O₂ contents, N₂ contents, and total flow rate on CF4 conversion was experimentally investigated. The maximum conversion of CF₄ was about 87% at 5 kV, 15 kHz for the feed gas stream containing 5 sccm CF₄, 7.5 sccm O₂, and 187.5 sccm Ar. CO, CO₂, and COF₂ were the main products when O₂ was used as the additive gas. NO_x was produced when N2 was used as the additive gas. The conversion of CF₄ was increased while the applied voltage and the residence time were increased. When nitrogen was added to argon as the diluent gas, the conversion of CF₄ was decreased with the increase of the nitrogen content.     

THERMODYNAMIC CONSIDERATIONS ON THE PURIFICATION OF H2SO4 AND HIX PHASES IN THE IODINE-SULFUR HYDROGEN PRODUCTION PROCESS

The reaction equilibrium and phase equilibrium in H₂SO₄ and HIx phases produced by the Bunsen reaction of the iodine-sulfur thermochemical hydrogen production process were examined using a chemical process simulator, ESP, with a thermodynamic database based on the mixed solvent electrolyte model. At temperatures lower than ca. 110°C, the reaction of HI and H₂SO₄ produced elemental sulfur in both phases. At higher temperatures, the reverse Bunsen reaction occurred, and SO₂ was produced in the H₂SO₄ phase. In the HIx phase, conversely, SO₂ formation predominated in a narrow temperature range and H₂S was produced with the increase in temperature. The presence of N₂ gas lowered the temperature of the predominant reaction change. A feed of O₂ for purification was proposed to suppress the consumption of objective components in the H₂SO₄ phase purification, and an O₂ feed to the HIx phase for the suppression of H₂S and S impurities was proposed by the simulation.     

About the Thermodynamic and Kinetic Fundamentals of the Self‐Decomposition of Tetrafluoroethylene – Part I: Thermodynamic Computation of the Pressure Increase Ratio

The pressure increase ratio of the self‐decomposition of tetrafluoroethylene to carbon and tetrafluoromethane can be computed with sufficient accuracy via thermodynamic standard procedures by using the ideal gas law. Thermodynamic analysis with regard to real gases showed that the deviations are largely self‐compensating and therefore negligible in the pressure and temperature range of technical interest. The increase ratio is independent of the pressure and significantly dependent on the initial temperature. The flame temperature is about 2650 K like with stoichiometric H₂/O₂ mixtures. The computations were extended to various gaseous TFE mixtures of technical interest.     

Low-temperature catalytic decomposition of hydrogen sulfide on metal catalysts under layer of solvent

When hydrogen sulfide decomposition {2 H₂S ? 2 H₂?+?S₂^(gas)} is carried out in the flow regime at room temperature on metal catalysts placed in a liquid capable of dissolving H₂S and sulfur, the reaction equilibrium can be significantly (up to 100%) shifted to the right yielding the desired product – hydrogen. The process efficiency was demonstrated using aqueous solutions of monoethanolamine (MEA), sodium carbonate, which is widely used in industry for H₂S absorption from tail gases, and aqueous hydrazine as examples. IR and Raman spectroscopy data demonstrated that sulfur obtained in the solutions is in the form of diatomic molecules. DFT calculations showed that diatomic sulfur forms weakly bound coordinative complexes with solvent molecules. Some problems related to sulfur accumulation and recovery from the solvents are discussed.     

Direct Oxidation of H2 to H2O2 and Decomposition of H2O2 Over Oxidized and Reduced Pd-Containing Zeolite Catalysts in Acidic Medium

Both the conversion and H₂O₂ selectivity (or yield) in direct oxidation of H₂-to-H₂O₂ (using 1.7 mol% H₂ in O₂ as a feed) and also the H₂O₂ decomposition over zeolite (viz. H-ZSM-5, H-GaAlMFI and H- ) supported palladium catalysts (at 22 °C and atmospheric pressure) are strongly influenced by the zeolite support and its fluorination, the reaction medium (viz. pure water, 0.016 M or 1.0 M NaCl solution or 0.016 M H₂SO₄, HCl, HNO₃, H₃PO₄ and HClO₄), and also by the form of palladium (Pd⁰ or PdO). The oxidized (PdO-containing) catalysts are active for the H₂-to-H₂O₂ conversion and show very poor activity for the H₂O₂ decomposition. However, the reduced (Pd⁰-containing) catalysts show higher H₂ conversion activity but with no selectivity for H₂O₂, and also show much higher H₂O₂ decomposition activity. No direct correlation is observed between the H₂-to-H₂O₂ conversion activity (or H₂O₂ selectivity) and the Pd dispersion or surface acidity of the catalysts. Higher H₂O₂ yield and lower H₂O₂ decomposition activity are, however, obtained when the non-acidic reaction medium (water with or without NaCl) is replaced by the acidic one.     

Analysis of Plasma Reaction for Decomposition of CHF3 in a Dielectric Barrier Discharge

The decomposition of CHF₃ in a mixture with O₂ and Ar was investigated in a coaxial dielectric barrier discharge at atmospheric pressure. CHF₃ decomposition increased linearly in regard to specific energy input (SEI), whereas energy efficiency decreased. The main product was CO_2, and its selectivity increased with high SEI and the presence of O₂ in the feed, but an increase of O₂ in the feed led to a decrease in decomposition rate. An increase in total flow rate led to an increase of the absolute amount of CHF₃ decomposition and energy efficiency; however, the decomposition of CHF₃ decreased. A complete CHF₃ decomposition occurred under an SEI of 1.54 kJ/L with the selectivity of CO₂ and CO as 89.87% and 7.00%, respectively. Optical emission spectroscopic analysis could explain the available reaction pathways for CHF₃ decomposition in the CHF₃/O₂/Ar atmospheric plasma and show the possibility of F₂ and HF formation.     

Surface energy of the plasma treated Si incorporated diamond-like carbon films

Surface energy and surface chemical bonds of the plasma treated Si incorporated diamond-like carbon films (Si-DLC) were investigated. The Si-DLC films were prepared by r.f. plasma assisted chemical vapor deposition using benzene and diluted silane (SiH₄/H₂ = 10:90) as the precursor gases. The Si-DLC films were subjected to plasma treatment using various gases like N₂, O₂, H₂ and CF₄. The plasma treated Si-DLC films showed a wide range of water contact angles from 13.4° to 92.1°. The surface energies of the plasma treated Si-DLC films revealed a high polar component for O₂ plasma treated Si-DLC films and a low polar component for CF₄ plasma treated Si-DLC films. The CF₄ plasma treated Si-DLC films indicated the minimum surface energy. X-ray photoelectron spectroscopy (XPS) revealed that the polarizability of the bonds present on the surface explains the hydrophilicity and hydrophobicity of the plasma treated Si-DLC films. We also suggest that the O₂ plasma treated surface can provide an excellent hemocompatible surface from the estimated interfacial energy between the plasma treated Si-DLC surface and human blood.     

应用CaSO4载氧体燃烧技术的CaO再生过程

引言近年来CaO作为CO2吸收剂在近零排放煤直接制氢、生物质气化、吸收增强型天然气重整制氢、焦炉煤气重整制氢、燃煤电站CO2捕集等过程的应用受到国内外的持续关注和大量研究。在这些过程中CO2以CaCO3的形式固化下来,为循环利用CaO吸收剂并收集CO2,需要将产物CaCO3煅烧分解,这一过程称为CaO再生过程。CaO再生是强吸热过程,为该过程提供热量的一     

Role of the surface state of Ni/Al2O3 in partial oxidation of CH4

The effect of gas phase O₂ and reversibly adsorbed oxygen on the decomposition of CH₄ and the surface state of a Ni/Al₂O₃ catalyst during partial oxidation of CH₄ were studied using the transient response technique at atmospheric pressure and 700°C. The results show that, when the catalyst surface is completely oxidized under experimental conditions, only a small amount of CO and H₂ can be produced from non‐selective oxidation of CH₄ by reversibly adsorbed oxygen which is more active in oxidizing CH₄ completely than NiO via the Rideal–Eley mechanism and both the conversions of CH₄ and O₂ and the selectivities to CO and H₂ are very low. Therefore, keeping the catalyst surface in the reduced state is the precondition of high conversion of CH₄ and high selectivities to CO and H₂. The surface state of the catalyst decides the reaction mechanism and plays a very important role in the conversions and selectivities of partial oxidation of CH₄. During partial oxidation of CH₄, no oxygen species but a small amount of carbon exists on the catalyst surface, which is favorable for maintaining the catalyst in the reduced state and the selectivity of CO. The results also indicate that direct oxidation is the main route for partial oxidation of CH₄, and the indirect oxidation mechanism is not able to gain dominance in the reaction under the experimental conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sequential production of H2 and CO over supported Ni catalysts

Methane decomposition into H₂ and carbon nanofibers at 823 K and subsequent gasification of the carbon nanofibers with CO₂ into CO at 923 K were performed over supported Ni catalysts (Ni/SiO₂, Ni/TiO₂ and Ni/Al₂O₃). Supported Ni catalysts were deactivated for CH₄ decomposition with time on stream due to deposition of a large amount of carbon nanofibers. Subsequent contact of CO₂ with carbon nanofibers on the deactivated catalysts resulted in the formation of CO with a conversion of the carbons higher than 95%. In addition, gasification with CO₂ regenerated the activity of supported Ni catalysts for CH₄ decomposition, indicating that H₂ formation through CH₄ decomposition and CO formation through gasification with CO₂ could be carried out repeatedly. Conversions of carbon nanofibers into CO were kept higher than 95% in the repeated gasification over all the catalysts, while change in the catalytic activity for CH₄ decomposition with the repeated cycles depended on the kind of catalytic supports. Catalytic activity of Ni/SiO₂ for CH₄ decomposition was high at early cycles, however, the activity decreased gradually with the repeated cycles. On the other hand, Ni/TiO₂ and Ni/Al₂O₃ showed high activity for CH₄ decomposition and the activity was kept high during the repeated cycles. These changes of catalytic activities for CH₄ decomposition could be explained by changes in particle sizes of Ni metal, i.e. Ni metal particles in Ni/SiO₂ aggregated into ones larger than 150 nm with the repeated cycles, while the particle sizes of Ni metal in Ni/TiO₂ and Ni/Al₂O₃ remained at an effective range for CH₄ decomposition (60-100 nm).     

A Novel KCaNi/α-Al2O3 Catalyst for CH4 Reforming with CO2

A potassium and calcium co-promoted nickel catalyst (KCaNi/-Al₂O₃) prepared by a direct impregnation method possessed a high activity, high stability and excellent coke resistance properties in CH₄ reforming with CO₂. XRD, XPS and H₂-TPR characterizations indicated that (i) Ca and K strengthened the interaction between Ni and -Al₂O₃ and promoted the formation of a unique NiAl₂O₄ phase on the surface of the catalyst and (ii) Ca and K increased the dispersion of Ni and retarded its sintering. Coking reactions (CH₄ temperature-programmed decomposition and O₂-TPO) disclosed that K reduced carbon formation via CH₄ decomposition.     

State Parameters and Phase Transformations in Carbonized Periclase Refractories

The effect of temperature and formulation (relative volumes of the gas phase) on phase and chemical transformations in the MgO – C – H₂O – air system at 298 – 2400 K is considered using methods of thermodynamic and physicochemical simulation with a view to using carbonized periclase refractories under industrial conditions. Temperature ranges for stable existence of refractory phases and formulation ranges (for the gas phase) with allowance for different chemical homo- and heterophase interactions are determined. Effects of temperature, concentration, and pressure of pore gases (CO₂, CO, CH₄, H₂O, H₂, etc.) on the bulk material and effects of the carbon component, porosity, intrinsic and atmospheric moisture on gasification processes are considered in terms of a theoretical simulation model.     

Activation of Supported Pd Metal Catalysts for Selective Oxidation of Hydrogen to Hydrogen Peroxide

Catalytic activity of supported Pd metal catalysts (Pd metal deposited on carbon, alumina, gallia, ceria or thoria) showing almost no activity in the liquid-phase direct oxidation of H₂ to H₂O₂ (at 295 K) in acidic medium (0.02 M H₂SO₄) can be increased drastically by oxidizing them using different oxidizing agents, such as perchloric acid, H₂O₂, N₂O and air. In the case of the Pd/carbon (or alumina) catalyst, perchloric acid was found to be the most effective oxidizing agent. The order of the H₂-to-H₂O₂ conversion activity for the perchloric-acid-oxidized Pd/carbon (or alumina) and air-oxidized other metal oxide supported Pd catalysts is as follows: Pd/alumina < Pd/carbon < Pd/CeO₂ < Pd/ThO₂ < Pd/Ga₂O₃. The H₂ oxidation involves lattice oxygen from the oxidized catalysts. The catalyst activation results mostly from the oxidation of Pd metal from the catalyst producing bulk or sub-surface PdO. It also caused a drastic reduction in the H₂O₂ decomposition activity of the catalysts. There exists a close relationship between the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity in the oxidation process and the H₂O₂ decomposition activity of the catalysts; the higher the H₂O₂ decomposition activity, the lower the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity.     

A ternary compound additive for vacuum densification of β-silicon carbide at low temperature

The thermal decomposition behavior of a ternary carbide compound (Al₄SiC₄) was investigated under vacuum conditions. Decomposition of Al₄SiC₄ occurred above 1450 °C, resulting in the formation of SiC and carbon phases in the matrix, with some losses of Al. To simultaneously obtain the densification and refinement of SiC, the potential of the compound as a sintering additive for low-temperature sintering of SiC was evaluated and compared to cases of SiC with Al₄C₃ and Al₂O₃ additives. SiC that was almost entirely densified with fine and elongated grains was successfully formed using a 10 wt% Al₄SiC₄ additive by hot pressing at 1700 °C for 2 h in a vacuum. During the densification, the decomposition behavior of the Al₄SiC₄ was strongly related to the densification behavior of the SiC.     

Nanocrystalline Nd2O3: Preparation,phase evolution,and kinetics of thermal decomposition of precursor

Nd₂O₃ was synthesized by calcining Nd₂(C₂O₄)₃·10H₂O in air. The precursor and its calcined products were characterized by thermogravimetry and differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray powder diffraction, and scanning electron microscopy. The results showed that high-crystallized Nd₂O₃ with hexagonal structure was obtained when the precursor was calcined at 1223 K in air for 2 h. The crystallite size of Nd₂O₃ synthesized at 1223 K for 2 h was about 48 nm. The thermal decomposition of the precursor in air experienced three steps, which are first, the dehydration of 10 crystal water molecules; then, the decomposition of Nd₂(C₂O₄)₃ into Nd₂O₂CO₃; and last, the decomposition of Nd₂O₂CO₃ into hexagonal Nd₂O₃. Based on the KAS equation, the values of the activation energies associated with the thermal decomposition of Nd₂(C₂O₄)₃?10H₂O were determined.     

Theoretical and experimental study on the emission characteristics of waste plastics incineration by modified O2/RFG combustion technology

For mitigating the emission of greenhouse gas CO₂ from general air combustion systems, a clean combustion technology O₂/RFG is in development. The O₂/RFG combustion technology can significantly enhance the CO₂ concentration in the flue gas; however, using almost pure oxygen or pure CO₂ as feed gas is uneconomic and impractical. As a result, this study proposes a modified O₂/RFG combustion technology in which the minimum pure oxygen is mixed with the recycled flue gas and air to serve as the feed gas. The effects of different feed gas compositions and ratios of recycled flue gas on the emission characteristics of CO₂, CO and NO_x during the plastics incineration are investigated by theoretical and experimental approaches.Theoretical calculations were carried out by a thermodynamic equilibrium program and the results indicated that the emissions of CO₂ were increased with the O₂ concentrations in the feed gas and the ratios of recycled flue gas increased. Experimental results did not have the same trends with theoretical calculations. The best feed gas composition of the modified O₂/RFG combustion was 40% O₂ + 60% N₂ and the best ratio of recycled flue gas was 15%. As the O₂ concentration in feed gas and the ratio of recycled flue gas increased, the total flow rates and pressures of feed gas reduced. The mixing of solid waste and feed gas was incomplete and the formation of CO₂ decreased. Moreover, the emission of CO was decreased as the O₂ concentration in feed gas and the ratio of recycled flue gas increased. The emission of NO_x gradually increased with rising the ratio of recycled flue gas at lower O₂ concentration (<40%) but decreased at higher O₂ concentration (>60%).     

A molecular dynamics and grand canonical Monte Carlo study of silicalite-1 as a membrane material for energy-related gas separations

Hydrogen separation and combustion subsequent to coal gasification is highly attractive as an environmentally benign method of energy generation. Siliceous zeolites are thermally and chemically stable microporous materials that can satisfy the function of a gas separation membrane for such high temperature (>473 K) processes. Ensuing steam generation via hydrogen combustion can consequently occur without significant energy loss. Silicalite-1 is attractive for the separation of smaller H₂ (2.89 Å) from larger CO₂, CH₄, N₂ and O₂ molecules with kinetic diameters of 3.30, 3.80, 3.64 and 3.46 Å, respectively. The current study employs molecular dynamics and grand canonical Monte Carlo approaches to predict single-component gas diffusivities and adsorption isotherms for H₂, CO₂, CH₄, N₂ and O₂ in silicalite-1 at 273–1,073 K. The respective gas diffusivities and adsorption loadings determined in this study enable prediction of separation characteristics of silicalite-1 at relevant process conditions. Adsorption of all gases, excluding H₂, is relatively high at ambient temperature and significantly affects overall mass transport and separation selectivity. Hydrogen adsorption is relatively low even at ambient temperature, and at elevated temperatures (>473 K), adsorption of all gases is low, resulting in mass transport and separation selectivity that is dependent upon molecular diffusivity.     

High temperature SrCe0.9Eu0.1O3−δ proton conducting membrane reactor for H2 production using the water–gas shift reaction

The water–gas shift (WGS) reaction is used to shift the CO/H₂ ratio prior to Fischer–Tropsch synthesis and/or to increase H₂ yield. A WGS membrane reactor was developed using a mixed protonic–electronic conducting SrCe_0.9Eu_0.1O_3−δ membrane coated on a Ni–SrCeO_3−δ support. The membrane reactor overcomes the thermodynamic equilibrium limitations. A 46% increase in CO conversion and total H₂ yield was achieved at 900 °C under 3% CO and 6% H₂O, resulting in a 92% single pass H₂ production yield and 32% single pass yield of pure permeated H₂.     

Growth of multi-walled C nanotubes from pre-heated CH4 using Fe3O4 as a catalyst precursor

The present study aims to investigate influence of pre-heating of CH₄ on the growth of multi-walled C nanotubes (MWCNTs) on a Si (100) substrate by chemical vapor deposition technique using Fe₃O₄ powder as catalyst precursor. Reduction behavior of Fe₃O₄ was also studied in a flowing undiluted CH₄ atmosphere in order to gain better insight into MWCNT synthesis. Mass measurements, XRD and thermodynamic analyses were carried out to determine the extent of reduction of Fe₃O₄ by CH₄. It was found that Fe₃O₄ initially transformed to Fe via FeO within 30 min at 1200 K. Fe₃C and C then formed as reaction time increased to 60 min. It was postulated that reduction of Fe₃O₄ took place by H₂, a product of CH₄ decomposition. The overall reactions leading to the formations of Fe and Fe₃C phases were proposed using equilibrium thermodynamic analysis and the experimental results. Undiluted CH₄ was used to synthesize MWCNTs at temperatures in the range of 1050–1300 K. It was observed that a dense carbon coating was formed at 1300 K owing to self pyrolysis of CH₄, while at 1200 K individual MWCNTs were observed on the Si substrate. Growth of MWCNTs did not take place at the temperature range of 1050–1150 K. The use of CH₄ pre-heated at 1200 K, however, yielded MWCNTs at this temperature range. Experimental results and thermodynamic analysis of the C–H system (excluding graphite) indicated that pre-heating treatment of CH₄ promoted Fe₃O₄ reduction by H₂ and C formations from active intermediate hydrocarbon species of high molecular weights (especially C₆H₆).     

Electrochemical decomposition of CO2 and CO gases using porous yttria-stabilized zirconia cell

Electrochemical decomposition of CO₂ and CO gases using a porous cell of Ru-8 mol% yttria-stabilized zirconia (YSZ) anode/porous YSZ electrolyte/Ni–YSZ cathode system at 400–800 °C was studied by analyzing the flow rate and composition of outlet gas, current density, and phases and elementary distribution of the electrodes and electrolyte. A part of CO₂ gas supplied at 50 ml/min was decomposed to solid carbon and O₂ gas through the cell at the electric field strengths of 0.9–1.0 V/cm. The outlet gas at a flow rate of 3 ml/min included 61–63% CO₂ and 37–39% O₂ at 700–800 °C and the outlet gas at a flow rate of 50 ml/min included 73–96% (average 85%) CO₂ and 4–27% (average 15%) O₂ at 800 °C. On the other hand, the supplied CO gas was also decomposed to solid carbon, O₂ and CO₂ gases at 800 °C. The fraction of outlet gas at a flow rate of 50 ml/min during the CO decomposition at 800 °C for 5 h was 11–36% CO, 59–81% O₂ and 2–9% CO₂. The detailed decomposition mechanisms of CO₂ and CO gases are discussed. Both Ni metal in the cathode and porous YSZ grains under the DC electric field have the ability to decompose CO gas into solid carbon and O²⁻ ions or O₂ gas.