Oxidation of methane to formaldehyde over Fe/SiO2 and Sn---W mixed oxides

In an attempt to develop new catalysts for the formation of formaldehyde from methane, the promotion effect of Fe on SiO₂ and that of Sn on WO₃ have been studied. The formation of formaldehyde on silica can be appreciably enhanced by the impregnation of Fe, as far as iron loadings are kept below 0.1 atom.% (Fe/Si × 100). In the case of Sn---W---Ox catalysts, both the addition of Sn to WO₃, and that of W on SnO₂ were effective to the selective formaldehyde formation. Absorption spectra (UV-Vis) and ESR measurements revealed that tetrahedrally coordinated Fe³⁺ in the silica network plays an important role in the formation of formaldehyde. A thin surface layer consisting of W and Sn oxides can account for the selective formaldehyde formation on the Sn---W---Ox catalysts.     

The effect of water vapor and inert gases on the carbon-oxygen reaction

The interpretation of the effect of small quantities of water vapor on the kinetics of the gasification of carbon by oxygen is re-examined in the light of recent studies describing conditions under which an explosive reaction may occur in this mixture of reactant and products: oxygen, carbon monoxide, carbon dioxide and water vapor. The generation of water from residual hydrogen in the carbon is a key factor in the development of explosive conditions through a radical chain reaction in the gas phase. Some effects of inert gases on the rate of gasification are also explained by their influence on gas-phase reactions through stabilization of radicals and inhibition of diffusion of atoms to the walls. It is concluded that the possibility of gas-phase radical reactions in mixtures of carbon monoxide and oxygen, which depends critically on the presence of water vapor, should be considered in the interpretation of the kinetics of carbon gasification by oxygen.     

Dynamics of pressure build-up accompanying multicomponent gas transport in porous solids: inert gases

The dynamics of countercurrent transport of binary and ternary gas mixtures through a porous medium accompanied by a spontaneous temporary build-up of pressure inside the porous medium was experimentally studied. From the measurements, it follows that if a lighter gas replaces a heavier gas the pressure increases and vice versa. The larger the difference between molecular weights of the transported gases the larger the change of the pressure. The spontaneous pressure build-up can be satisfactorily described by the Mean Transport Pore Model (MTPM) or the Dusty Gas Model (DGM). Both models contain three parameters (transport parameters), which represent material constants of the porous medium, i.e., are independent of the kind of transported gases and conditions under which the transport takes place (temperature, pressure). Transport parameters have to be determined experimentally, e.g., by measurements similar to those performed in this study. With the use of obtained transport parameters it is possible to predict the transport under different conditions.     

Methane partial oxidation to methanol-solid initiated homogeneous methane oxidation

The initiation temperature of methane partial oxidation was markedly lowered by platinum wire placed upstream of a high pressure reactor. Added hydrogen in the reactant gas promoted the methanol selectivity. The radicals formed on the platinum surface were desorbed from it and initiated the reaction.     

Overall chemical kinetics model for partial oxidation of methane in inert porous media

An overall three-step six-component chemical kinetics model which includes CH₄ + O₂ → CO + H₂O + H₂ and reversible 2CO + O₂ ←→ 2CO₂ and CH₄ + H₂O ←→ CO + 3H₂ reactions is elaborated for the simulation of partial oxidation of methane in inert porous media. Procedure of the model adjusting to the experimental data is described. Kinetic parameters of the model are derived on the basis of temperature–flow rate, H₂ and CO output concentration–flow rate and temperature–pressure experimental correlations. It is found that extremely slow solid body temperature growth with flow rate T_s,max(G) reported in the works on partial oxidation of methane (and other hydrocarbons) in inert porous media may be reproduced by the model. The model is designed for optimization, scale up and design assistance of the reactors of partial oxidation of methane.It is demonstrated that the overall chemical kinetics model can be combined with detailed gas-phase kinetics model for the investigation of detailed composition of syngas and intermediary components.     

MoSi2和(Mo,W)Si2涂层的宽温域氧化过程

采用包渗法在Mo及Mo?W基体上分别制备MoSi2及(Mo,W)Si2涂层,研究了W掺杂对MoSi2涂层抗氧化性能的影响规律和作用机理。结果表明,W元素固溶到MoSi2涂层中,形成(Mo,W)Si2固溶体,涂层微观结构更加致密化。在1600℃高温下静态氧化,(Mo,W)Si2涂层抗氧化失效时间长达70 h,1200℃下氧化1000 h仍具有良好的防护性能,抗氧化性能大幅提升。加入W元素阻碍了Si元素与基体间的扩散反应,降低了涂层中Si元素的消耗速率,显著增强了(Mo,W)Si2涂层抗高温氧化性能。在500℃低温下静态氧化50 h,与MoSi2涂层相比,(Mo,W)Si2涂层氧化产生明显的“Pest”现象,涂层严重粉化失效。加入W元素降低了涂层中Si元素的扩散速率,导致低温下涂层表面无法形成致密氧化层,加剧涂层的快速氧化。     

Oxidation of carbon monoxide on poly-oriented and single-crystalline platinum electrodes over a wide range of pH

The oxidative stripping of pre-adsorbed carbon monoxide has been studied on poly-oriented platinum, and on Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0) single-crystal electrodes in phosphate buffer solutions as a function of pH, both stripping voltammetry and chronoamperometry. It was found that the stripping peak potential has a tendency to decrease as a function of pH until a pH of ca. 10–11, which is ascribed to a weaker adsorption of phosphate on platinum with increasing pH. Above a pH of ca. 11, the stripping peak appears to stay constant or increase, depending on the surface structure. We hypothesize that this may be due to the fact that above a pH of ca. 10–11, the main product of carbon monoxide oxidation is carbonate, which may be block active sites on the surface. By comparison with the stripping on the single-crystalline Pt, the stripping voltammetry on the poly-oriented Pt electrode appears as a convolution of the oxidation on the different facets. A similar conclusion is drawn for the chronoamperometry experiments, and it is suggested that this may be the main reason for the asymmetrical transients observed on the poly-oriented Pt electrodes, as well as on other strongly heterogeneous Pt electrodes that have been studied in the literature.     

Promoting effect of water vapor on catalytic oxidation of methane over cobalt/manganese mixed oxides

Methane oxidation was conducted in a fixed bed quartz tubular reactor on a series of mixed oxides of cobalt/manganese prepared by a sol–gel method. A unique promoting effect of water vapor on methane conversion was observed for the first time on these cobalt/manganese mixed oxides calcined at 450 or 600 °C. However, these mixed catalysts lost their catalytic activities after being calcined at 850 °C. The catalytic activity of methane oxidation was significantly improved by supporting the cobalt/manganese mixed species onto the high surface area SiO₂ or Al₂O₃–SiO₂ materials. It was noteworthy that the water enhancement effect was retained on these supported catalysts.     

Oxidation of methane over Pt and Pd supported on alumina in lean-burn natural-gas engine exhaust

Catalytic oxidation of hydrocarbons in lean-burn natural-gas engine exhaust has been studied for Pt and Pd supported on alumina. A Pt–Pd/alumina catalyst exhibited higher and longer-lasting hydrocarbon oxidation activity than Pt–Rh/alumina, Pt/alumina, and Pd/alumina catalysts. Increasing the palladium content in Pt–Pd/alumina catalyst increased the oxidation activity and had more durability. While increasing the platinum content a little bit also improved the activity, adding much more did not. Supporting the platinum on alumina retarded the sintering of Pd and PdO, thus lengthening the oxidation activity of the Pt–Pd/alumina catalyst.     

甲烷部分氧化制合成气反应中床层热点问题研究进展

综述了甲烷部分氧化制合成气反应中催化剂床层热点问题,包括热点产生的原因,热点位置的测定,热点温度的影响因素,以及热点问题的解决方法,对于保护催化剂和反应器,降低反应的危险性起到借鉴作用.     

Improving selectivity during methane partial oxidation by use of a membrane reactor

A reactant-swept catalytic membrane reactor for partial oxidation of methane to formaldehyde has been modeled. Kinetic parameters were taken from the literature for a V₂O₅/Sio₂ methane partial oxidation catalyst, and membrane parameters characteristic of commercially available materials were used. The models show that the selectivity for formaldehyde can be significantly improved by using a membrane reactor.     

Spatio-kinetic variation of methane oxidizing bacteria in paddy soil at mid-tillering: effect of N-fertilizers

Laboratory experiments were conducted to evaluate the variation ofpopulation size of methanotrophs (MOB) and CH₄ oxidation pattern inflooded rice soils sampled at three spatial points (rhizosphere, bulk and baresoils). Rhizosphere soil had higher MOB population size (301.1 ×10⁵ cells g^–1 dry soil) than bulk(37.2× 10⁵ cells g^–1 dry soil) andbare soil (19.1 × 10⁵ cells g^–1dry soil). The population size of MOB followed a decreasing trend with respectto fertilizer (urea NH₄NO₃ NH₄Cl control). The result indicated that rhizosphere soil presented thestrongestCH₄ oxidation activities, as shown by the highest values of the twokinetic parameters (K _m(app) andV _max). K _m andV _max increased significantly from bare to bulkto rhizosphere soil in control and fertilized soil and ranged from 6.2 to 133.2g g^–1 dry soil and from 0.03 to 0.41g h^–1 g^–1 dry soil,respectively. The differences in K _m andV _max among the three soils (rhizosphere, bulkand bare) in this study could be due to differential species composition ofmethanotrophic community and/or to conditioning of MOB under different soilmicroenvironments. The present study has demonstrated a competitive inhibition effectof NH₄ ⁺-N on CH₄ oxidation.     

Deactivation of palladium catalyst in catalytic combustion of methane

Catalytic combustion of natural gas, for applications such as gas turbines, can reduce NO_x emissions. Palladium-on-stabilised alumina has been found to be the most efficient catalyst for the complete oxidation of methane to carbon dioxide and water. However, its poor durability is considered to be an obstruction for the development of catalytic combustion. This work was aimed at identifying the origin of this deactivation: metal sintering, support sintering, transformation or coking.

Catalytic combustion of methane was studied in a 15 mm i.d. and 50 mm length lab reactor and in a 25 mm i.d. pilot test rig on monolithic honeycomb substrates. Experiments were performed at GHSV of 50 000 h⁻¹ in lab test and 500 000 h⁻¹ in pilot test. The catalysts used were palladium on different supports on cordierite substrate. The catalysts were characterised by XRD, STEM, ATG and XPS.

In steady-state conditions, deactivation has been found to be dependent on the air/methane ratio, the palladium content on the washcoat and the amount of washcoat on the substrate. An oscillating behaviour of the methane conversion was even observed under specific conditions, due to the reducibility of palladium oxide PdO to Pd. The influence of the nature of the support on the catalyst deactivation was also investigated. It has been shown that some supports can surprisingly eliminate this oscillating behaviour. However, in pilot test, deactivation was found to be very rapid, even with stabilised alumina supports. Furthermore, successive tests performed on the same catalyst revealed that the activity (light-off temperature, conversion) falls strongly from one test to another.

Then, the stabilised alumina support was calcined at 1230°C for 16 h prior to its impregnation by palladium, in order to rule out its sintering. Experiments carried out on precalcined catalysts point out that deactivation is mostly correlated to the metal transformation under reaction conditions: activity decreases gradually as PdO sinters, but it dropped much more steeply in relation to appearance of metallic palladium.     

Selective oxidation of methane to CO and H2 over unreduced NiO-rare earth oxide catalysts

NiO-LnO _x (Ln = lanthanide) catalysts (with NiLn=11) without prereduction show high activity/selectivity and very high productivity in the oxidative conversion of methane to CO and H₂. The catalysts are first activated in the initial reaction, which is started at 535–560°C, by the reduction of NiO and creation of active sites. The carbon deposition on the catalysts in the reaction, particularly for the NiO-Gd₂O₃, NiO-Tb₄O₇ and NiO-Dy₂O₃ catalysts, is quite fast but it has caused a little or no influence on the catalytic activity/selectivity. Pulse reaction of pure methane on NiO-Nd₂O₃ (at 600°C) shows involvement of lattice oxygen in the initial reaction and also reveals formation of carbon from CO on the catalyst reduced in the reaction.     

Interactive Oxidation of Photocatalysis and Electrocatalysis for Degradation of Phenol in a Photoreactor

TiO2/C particles as photocatalyst were prepared by dipping TiO2 suspension solution with activated carbon and were applied in the photocatalytic-electrocatalytic degradation of phenol, the Ti/SnO2+Sb2O3/PbO2 electrode and oxygen diffusion electrode were used as anode and cathode respectively, and a 250 W ultraviolet lamp (365 nm) as side light source. The SEM results of TiO2/C and Ti/SnO2+Sb2O3/PbO2 anode indicated that the TiO2 on carbon particles was uniform and PbO2 film on the surface of anode was in cauliflower form, the XRD result of oxygen diffusion electrode showed that only crystalline graphite was found. The influential parameters of degradation process such as applied cell voltage (E), initial concentration of phenol (C0), amount of TiO2 catalyst and air flow rate (v) were discussed. Under the following experimental conditions of C0=50 mg/L, pH=6, E=2 V, TiO2 0.98 mg/mL, v=382.2 mL/min, and light intensity I=10.5 mW/cm2, phenol could be entirely degraded, and about 89% of total organic carbon (TOC) was removed after 3 h degradation.     

Oxidation of methane over platinum in a solid proton-conducting electrolyte cell

Studies of the complete oxidation of methane on a Pt electrode-catalyst in the cell with a solid proton-conducting electrolyte (CH₄ + O₂, Pt ¦ SrCe_0.92Dy_0.08O₃ ¦ Pt, H₂O + N₂) were carried out. The non-Faradaic effect of electrochemical hydrogen pumping on the rate of methane oxidation has been demonstrated. The induced change in the reaction rate at anodic polarization of a Pt electrode-catalyst was over two orders of magnitude higher than the rate of hydrogen pumping from the reaction zone through the electrolyte.     

Investigation of the effect of the total pressure and methane concentration on the growth rate and quality of diamond thin films grown by MPCVD

The influence of total gas pressure (50–125 Torr) and methane concentration (0.75%–10%) on diamond growth by microwave plasma chemical vapor deposition (MPCVD) was investigated. Within the regimes studied, the growth rate was proportional to the methane concentration in the source gas while it exhibited a super-linear dependence on total pressure. For a fixed methane concentration, characterization by Raman spectroscopy, scanning electron microscopy and X-ray diffraction indicated there was a minimum pressure required for the growth of large grain diamond, and conversely, for a fixed pressure, there was a maximum methane concentration that yielded diamond deposition. Higher pressures and higher carbon concentrations yielded diamond growth rates more than 10 times higher than achieved by the conventional low pressure MPCVD process.     

Steady-state and dynamic modeling of indirect partial oxidation of methane in a wall-coated microchannel

Steady and dynamic characteristics of catalytic indirect partial oxidation (combined total oxidation and steam reforming) of methane to hydrogen in a wall-coated microchannel are investigated using computational techniques. Steady-state behavior is initially modeled using a two-dimensional axisymmetrical wall-coated reactor model. Considering the small channel diameter, adiabatic operation and negligible transport resistances, response of the microchannel is also investigated using a one-dimensional pseudohomogeneous tubular reactor model. Simulations of the microchannel are carried out using both models for different feed conditions ranging between 1.89 and 2.24 for CH₄/O₂ and 1.17–2.34 for H₂O/CH₄. Outcomes from both models are found to be close, allowing the use of the low-cost one-dimensional model in dynamic simulations. Analysis of transients during the system start-up indicate that steady state is reached between 100 and 120 s depending on the feed composition. Product temperature and flow rates obtained from steady-state and dynamic simulations are found to be close with some differences arising from the finite difference-based numerical method used to solve partial differential equations of the dynamic model. Dynamic responses of the microchannel to several disturbances in the feed are analyzed. The response to a step increase in the inlet oxygen flow rate (decrease of CH₄/O₂ from 2.24 to 1.89) is the elevation of temperature by ca. 100 K, which in turn leads to ca. 33% in hydrogen yield, and the time to reach the new steady state is around 90 s. If the disturbance involves an increase in inlet steam flow, temperature and hydrogen yield decrease in time to a local minimum within 10 s and then gradually increase to the subsequent steady state within 50 s ending up with net reductions of ca. 1.6% and 9%, respectively.