Monte Carlo simulation of the oscillatory behavior in partial oxidation of methane on nickel catalyst under nonisothermal conditions

The Monte Carlo method has been used to simulate the kinetic oscillations during partial oxidation of methane under nonisothermal conditions. The oscillatory behavior can be found with the selected parameters by using oxide formation and removal model. From the simulation, the temperature variation during the reaction synchronizes well with the oscillations of product formation rates, and also with the rates of oxide formation and reduction processes. Compared with the isothermal simulation results, the oscillations under the nonisothermal conditions are observed to have a slightly shorter period, lower maximum carbon coverage and higher nickel oxide coverage.     

Catalytic oxidation of carbon nanotubes with noble metal nanoparticles

Catalytic oxidation of multi-walled carbon nanotubes (MWNCTs) with some noble metal nanoparticles was observed by environmental transmission electron microscopy (E-TEM). Amoeba-like movement of the nanoparticles was observed even at a temperature of ∼400 °C, which is much lower than the melting points of any of the metals. In particular, rhodium particles reacted intensely with MWCNTs, and assumed a droplet-like shape. On the other hand, gold particles caused very little erosion of the MWCNTs under the conditions of this study.     

Uncertainty quantification in the catalytic partial oxidation of methane

This work focuses on uncertainty quantification of eight random parameters required as input for 1D modelling of methane catalytic partial oxidation within a highly dense foam reactor. Parameters related to geometrical properties, reactor thermophysics and catalyst loading are taken as uncertain. A widely applied 1D heterogeneous mathematical model that accounts for proper transport and surface chemistry steps is considered for the evaluation of deterministic samples. The non-intrusive spectral projection approach based on polynomial chaos expansion is applied to determine the stochastic temperature and species profiles along the reactor axial direction as well as their ensemble mean and error bars with a confidence interval of 95%. Probability density functions of relevant variables in specific reactor sections are also analysed. A different contribution is noticed from each random input to the total uncertainty range. Porosity, specific surface area and catalyst loading appear as the major sources of uncertainty to bulk gas and surface temperature and species molar profiles. Porosity and the mean pore diameter have an important impact on the pressure drop along the whole reactor as expected. It is also concluded that any trace of uncertainty in the eight input random variables can be almost dissipated near the catalyst outlet section for a long-enough catalyst, mainly due to the approximation to thermodynamic equilibrium.     

The dynamics of partial oxidation of ethane over platinum

Catalysts are increasingly finding application in fuel reformation leading to more reactive blends, micro-combustors and nano-catalysts aimed at improving application performance. In the current work, the interactions of surface and gas phase reactions in C₂H₆/O₂/N₂ mixtures over a supported platinum catalyst are reported. Experiments were combined with computations featuring comprehensive detailed chemistry for both phases in order to explore the dynamics of the conversion processes using high superficial gas velocities, extended Al₂O₃ foam monoliths up to 60 mm in length and with variable Pt loading of 1% and 3% by weight. The residence time was varied between 1 and 3 ms at different oxygen to carbon weight (0.5 < O/C < 0.8) ratios and a constant molar hydrogen to oxygen (H₂/O₂ = 2) ratio. The applied gas phase chemistry features a detailed C₁–C₂ mechanism including low-temperature chemistry. The heterogeneous chemistry is analysed using two independently developed detailed heterogeneous reaction mechanisms [1] and [2]. The study confirms that the heterogeneous chemistry is essential in providing reaction stability and initially contributes to the formation of C₂H₄. It is also shown that increased residence times can result in a consistent heterogeneous consumption of ethylene principally leading to methane, carbon monoxide and carbon deposition. The surface chemistry is analysed in detail and it is shown that residence times can be used to mitigate C₂H₄ loss with the need to balance the stability of the process.     

Propylene deep oxidation on the Pt(111) surface: temperature programmed studies over extended coverage ranges

Propylene oxidation was studied on the Pt(111) surface over a wide range of reaction stoichiometries using temperature programmed methods. Reaction of propylene with excess oxygen results in complete oxidation to water and carbon dioxide, with oxydehydrogenation to form water beginning at 290 K. The initiation of skeletal oxidation occurs after water formation begins, except for the highest propylene coverages. A stable dehydrogenated intermediate with a C₃H₅ stoichiometry is formed in the 300 K temperature range during oxidation. Reaction of propylene with substoichiometric amounts of oxygen results in incomplete oxidation with hydrocarbon decomposition dominating after depletion of surface oxygen. Increasing oxygen coverage results in more complete oxidation. Oxidation processes result in water, carbon dioxide, and carbon monoxide, while decomposition results in hydrogen, propylene, and propane desorption with some surface carbon remaining. Propylene-d₆ and selectively labeled propylene-3,3,3-d₃ (CH₂CHCD₃) experiments indicated initial water formation results from oxydehydrogenation of one of the olefinic hydrogens. At the highest propylene and oxygen coverages studied, we observed small amounts of partial oxidation which indicate that the vinyl hydrogen is removed initially, resulting in the formation of an adsorbed H₂CCCH₃ intermediate. The partial oxidation products observed are acetone desorbing at 200 K and acetic acid at 320 K. Removal of the first skeletal carbon begins at 320 K, except for the highest propylene coverages. Preadsorption of molecular oxygen limits adsorption of propylene and preadsorption of propylene limits molecular oxygen adsorption at 110 K. Similar oxidation mechanisms are observed following initial adsorption of both molecular and atomic oxygen, which is expected since molecular oxygen dissociates and/or desorbs well below oxidation temperatures.     

Partial catalytic oxidation of methane to synthesis gas over rhodium: in situ Raman experiments and detailed simulations

The partial catalytic oxidation of methane to synthesis gas over Rh/ZrO₂ was investigated experimentally and numerically at fuel-to-air equivalence ratios of 2.5 and 4.0 and pressures of 4 and 6 bar. Experiments were performed in an optically accessible, laboratory-scale, channel-flow catalytic reactor and involved in situ one-dimensional Raman measurements of major species (CH₄, O₂, H₂O, CO₂, H₂, CO, and N₂) concentrations across the reactor boundary layer. The numerical model included a two-dimensional elliptic code with elementary homogeneous (gaseous) and heterogeneous (catalytic) chemical reaction schemes. Homogeneous ignition experiments and numerical predictions have validated the employed gas-phase reaction mechanism and have further delineated the reactor extent over which the contribution of the homogeneous reaction pathway was negligible. Over the reactor extent where oxygen was still available, the employed heterogeneous reaction scheme provided good agreement with the measured species concentrations, overpredicting only to a small degree the partial over the total oxidation route. In the oxygen-depleted zones of the reactor, however, the heterogeneous scheme overpredicted to a greater degree the impact of steam reforming and water gas shift reactions, resulting in higher computed hydrogen yields at the reactor exit. Additional experiments and predictions were carried out in a sub-scale gas-turbine honeycomb reactor, at operating conditions leading to oxygen breakthrough. The predictions again favored the partial over the total oxidation route. A modified heterogeneous scheme was proposed that provided very good agreement with measurements in the honeycomb reactor and in the oxygen-rich zones of the laboratory-scale reactor. The hydrogen produced during partial oxidation was partly re-adsorbed on the catalyst leading to superadiabatic surface temperatures, thus exemplifying the importance of proper thermal management in commercial reactors.     

Numerical investigation of the partial oxidation process in porous media based reformer

Numerical investigation of the thermal partial oxidation process of Methane in porous media based reformer is performed. A finite volume based CFD code, including radiation modeling, in combination with a detailed chemical kinetics scheme is used to perform the numerical simulation. A heterogeneous approach for the heat transport modeling in porous media (separate coupled energy equations for the gas and solid phases) was used. Validation of the model with experimental data is also performed. The model was able to predict the temperature behavior in the reformer reasonably well. However, the concentrations of H₂ and CO were under predicted while the H₂O concentration was over predicted.     

Promotion of partial oxidation of methanol over thin Pt and Pd film catalysts by resonance oscillation of acoustic waves

The thickness extension mode resonance oscillation (TERO) of bulk acoustic waves generated on z-cut LiNbO₃ by rf electric power was employed to the catalytic oxidation of methanol over thin Pt and Pd film catalysts deposited. Both the catalysts produced formaldehyde, methyl formate, and carbon dioxide as carbon-containing products in the gas phase. The TERO considerably increased the selectivity for formaldehyde production on Pt at a moderate rf power, indicating the ability of promoting the partial oxidation of methanol, whereas a small and monotonous decrease in the selectivity of the reaction on Pd occurred with power. The different TERO effects on the selectivity are discussed.     

Effects of thermal oxidation on the photoluminescence properties of porous silicon

The effects of thermal oxidation on the photoluminescence (PL) properties of powdered porous silicon (PSi) are studied using X-ray photoelectron spectroscopy (XPS). It is found that the PL intensity is steeply quenched after annealing at and recovered at above . The XPS intensity of oxides formed on the PSi surface is also found to strongly depend on the annealing temperature. The comparison between the annealing temperature dependence of PL intensity and that of the oxide XPS intensity suggests that the formation of thin disordered SiO₂ layer accompanies the quenching of the PL intensity, and that the formation of thick high-quality SiO₂ layer results in the PL intensity recovery. These results indicate that the thickness and quality of SiO₂ layer play a crucial role in the PL properties of thermally oxidized PSi.     

Anodic oxidation of porous silicon bilayers

This paper focuses on the study of the effect of anodic oxidation in porous silicon bilayers composed of two porous layers of different porosities. The order of the two types of layers has been alternated, and the thicknesses and refractive indices have been optically characterized by Fourier transform infrared spectroscopy. The results show that the refractive index of anodic oxidized porous silicon is reduced significantly with respect to just formed porous silicon. It is also observed that the quality of the oxidation is related to the porosity of the inner porous layer of the silicon bilayer structure. This effect is interpreted in terms of quantum size effects.     

Catalytic and electrocatalytic oxidation of methane on palladium electrodes in a solid electrolyte cell

The catalytic oxidation of methane on polycrystalline palladium films was studied at 550-750°C and atmosheric total pressure. The reaction was studied under both open and closed-circuit. Under open circuit, and when yttria-stabilized zirconia (YSZ) was used as solid electrolyte, the technique of Solid Electrolyte Potentiometry (SEP) was used to monitor the thermodynamic activity of oxygen adsorbed on the Pd electrode during reaction. The main products were those of complete oxidation, i.e. CO₂ and H₂O. Under closed-circuit, the effect of electrochemical oxygen “pumping” to or from the catalyst was studied. Non-faradaic (NEMCA) phenomena were observed but the reaction rate enhancement factors (A) were not as large as with previously studied catalytic systems. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996.     

Catalytic degradation of organic dyes using biosynthesized silver nanoparticles

The green synthesis of metallic nanoparticles paved the way to improve and protect the environment by decreasing the use of toxic chemicals and eliminating biological risks in biomedical applications. Plant mediated synthesis of metal nanoparticles is gaining more importance owing to its simplicity, rapid rate of synthesis of nanoparticles and eco-friendliness. The present article reports an environmentally benign and unexploited method for the synthesis of silver nanocatalysts using Trigonella foenum-graecum seeds, which is a potential source of phytochemicals. The UV–visible absorption spectra of the silver samples exhibited distinct band centered around 400–440 nm. The major phytochemicals present in the seed extract responsible for the formation of silver nanocatalysts are identified using FTIR spectroscopy. The report emphasizes the effect of the size of silver nanoparticles on the degradation rate of hazardous dyes, methyl orange, methylene blue and eosin Y by NaBH₄. The efficiency of silver nanoparticles as a promising candidate for the catalysis of organic dyes by NaBH₄ through the electron transfer process is established in the present study.     

Preparation and performance of a perovskite-type tubular membrane for the partial oxidation of methane

Thin-walled (∼200 μm) tubular membranes intended for the catalytic partial oxidation of natural gas have been manufactured from La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) powders using viscous plastic processing technology. The perovskite-type powders were synthesised using various conventional and novel techniques. The tubes have been characterised using a custom-built gas analysis rig with on-line mass spectrometry. Porosity levels of the membranes were found to be very low (<0.5%) as calculated using a mass spectrometry leakage test. This was confirmed by microstructural analysis of polished cross-sections using SEM. The spontaneous oxygen flux across the tubular membranes was determined as a function of temperature. Oxygen permeation rates were found to range from 0.1 to 0.3 μmol cm⁻²s⁻¹ at 1273 K. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Catalytic properties of mesoporous materials supported heteropoly acids for Baeyer-Villiger oxidation of cyclic ketones

Three mesoporous molecular sieves loaded silicotungstic acids, named HSiW/SBA-15, HSiW/MCM-41, HSiW/MCM-48, were prepared and characterised by XRD, FT-IR, TEM and SEM. The catalytic performance of the prepared materials for the Baeyer-Villiger oxidation of cyclic ketones was carried out in the presence of 30%H₂O₂ under mild conditions. These loading materials were proved to be efficient and reusable catalysts, they all exhibited excellent catalytic performance for the Baeyer-Villiger oxidation of cyclic ketones with 30% H₂O₂ as oxidant. Many cyclic ketones were efficiently converted to the corresponding lactones with up to 90% conversions and high selectivities under the optimum reaction conditions.

Cyclic ketones were efficiently oxidised by mesoporous materials sopported silicotungstic acid to the corresponding lactones with 30%H₂O₂ as oxidant. All of the catalysts showed promising recyclability in the reactions.     

Catalytic soot oxidation in microscale experiments: Simulation of interactions between co-deposited graphitic nanoparticle agglomerates and platinum nanoparticles

Continuously regenerating catalytic soot traps are under development to reduce particulate emissions from diesel exhaust. A good understanding of the processes that take place during soot oxidation is needed to optimize diesel soot trap performance. To gain insight into these processes from the perspective of nanoparticle technology, the effects of catalyst particle size and the interparticle distance between soot and catalyst particles were measured. A model catalyst was prepared by depositing Pt nanoparticles on a SiO/SiO₂-coated transmission electron microscope (TEM) grid. A soot surrogate composed of graphitic nanoparticle agglomerates generated by laser ablation was deposited on the same surface. This system simulates, morphologically, catalytic soot traps used in practice. The reaction was carried out in a tubular flow reactor in which the gas phase simulated diesel exhaust gas, composed of a mixture of 10% O₂ and 1000 ppm NO with the remainder N₂. The progress of the carbon nanoparticle oxidation was monitored off-line by analysis of electron microscopy images of the agglomerates before and after reaction. This experimental method permitted the correlation of reaction rate with particle sizes and separation distances as well as catalyst surface area in the direct environs of the soot particles. The experimental results revealed no effect of Pt catalyst particle size in the range 7–31 nm on the rate of reaction. Also observed were a decrease in the rate of reaction with increasing distance between carbon agglomerates and catalyst particles and a linear dependence of the reaction rate on the fractional catalyst surface area coverage.     

Studies of catalytic process of complete oxidation of methane by SSITKA method

This paper presents the results obtained by means of the steady state isotopic transient kinetic analysis for complete methane oxidation over the Pd(PdO)Al₂O₃ catalyst. The average surface life-time and surface concentration of methane and carbon dioxide were determined. It was found out that on the palladium catalyst there are adsorbed small amounts of methane (which does not take part in the process of oxidation) only at the temperature corresponding to the starting point of methane oxidation. Additionally, in the steady state of methane oxidation on the palladium catalyst there are present two different kinds of carbon dioxide: short- and long-resided on the catalyst surface. The average surface life-time of both kinds of carbon dioxide decreases with temperature. The surface concentration of long-resided carbon dioxide increases with temperature whereas the small maximum at about 380 °C is noticed for the surface concentration of short-resided carbon dioxide.     

Observation of molecular migration in porous media using 2D exchange spectroscopy in the inhomogeneous magnetic field

We present a new method for observing fluid diffusion in a porous medium. The method employs 2D exchange spectroscopy for molecules diffusing in the presence of local magnetic field inhomogeneities, in our case distilled water in various sized glass bead packs. Our experiment involves an acquisition and evolution time domain with the two Fourier domains corresponding to the spectral distribution of local fields. We show that exchange in the internal magnetic field can be seen in a 2D spectrum with a characteristic time on the order of that required to diffuse 0.15 sphere diameters with similar behavior found for computer simulations. The method is potentially useful for studying the internal migrations in more complicated systems such as sandstones or other porous media.     

Near-infrared spectroscopy as a rapid tool for water content analysis in the partial oxidation of ethanol

Abstract

The oxidation of ethanol to acetaldehyde was previously investigated in a continuous flow bench scale reactor using a zirconium-oxide-supported vanadium catalyst. Products were analyzed by near-infrared spectroscopy, a fast and accurate tool for the determination of water, ethanol, and acetaldehyde content. Water content was calibrated by Karl Fischer titration while gas chromatography was used to calibrate ethanol and acetaldehyde contents. Near-infrared spectroscopy is a fast, cost-effective method for the determination of water, ethanol, and acetaldehyde products from partial ethanol oxidation.     

C24N24负载Sc单原子催化剂催化甲烷氧化为甲醇的理论计算

甲醇是一种很有前途的清洁能源,有望替代不可再生的石油能源.因此,将储量巨大,但不易运输的甲烷氧化为甲醇具有十分重要的意义.首先通过密度泛函理论(density functional theory, DFT)计算研究了Sc原子与C₂₄N₂₄之间的稳定性.结果表明,Sc原子与C₂₄N₂₄的结合能(-9.064 eV)小于Sc原子的内聚能(-4.518 eV),即Sc@C₂₄N₂₄具有良好的稳定性.在此基础上,进一步研究了甲烷在Sc@C₂₄N₂₄表面催化氧化制备甲醇的工艺过程,以N₂O为氧化剂在Sc@C₂₄N₂₄单原子上进行甲醇的催化氧化反应.结果表明：N₂O首先吸附在Sc@C₂₄N₂₄上,然后直接分解为N₂和O_ads.N₂...