Effect of processes at the reactor surface on the low-pressure hydrogen f lame

The interaction of the low-pressure flame of a 2H₂-O₂ mixture with a quartz reactor surface was studied by the resonance fluorescence technique. The results confirmed the fundamental statement of N. N. Semenov’s theory concerning chain propagation in the gas and termination on the surface in the kinetic region of chain termination (quadratic decay in the heterogeneous negative chain interaction) and in the diffusion region (linear decay). The kinetic curves observed in the kinetic and diffusion chain termination regions on the wall were well matched using N. N. Semenov’s theory, taking into account the heterogeneous catalytic chain initiation and interaction processes occurring on the wall with a variable “rate constant.” The interaction of chains on the wall markedly retards ignition in the gas in the kinetic region and has almost no influence on chain propagation in the gas in the diffusion region of the heterogeneous chain termination. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1301–1308, August, 2006.     

On the surface steps of a heterogeneous catalytic reaction

Based on a study of the properties of intermediate complexes in the reactions of low-temperature ammonia oxidation and nitrogen oxide reduction in the presence and absence of oxygen on various catalysts, it was hypothesized that the sequence of steps in the formation of reaction products on chemically different catalysts can be the same if the initial adsorption forms of reactants are identical. In this case, variations in catalysts or reaction conditions affect the ratio between reaction rates.     

Mathematical modeling of heterogeneous catalytic reactions and processes

The problems of heterogeneous catalysis are considered using mathematical modeling of catalytic reactions beginning from the atomic and molecular level. Some problems of the nonlinear dynamics of heterogeneous catalytic reactions are discussed. Mathematical models of these systems were designed using a combination of laboratory and computer experiments. The results of simulation of the nonlinear phenomena involved in the reactions NO+CO, NO+H₂, and CO+O₂, which are important for environmental catalysis, are discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1895–1904, October, 1998.     

Development of a heterogeneous catalytic cracking reactor utilizing online mass spectrometry analysis

A laboratory system has been designed, constructed, and validated that reduces the complexity, time required, and data variability associated with catalytic microreactors that require post reaction steps prior to product analysis. In this work, a Varian (Walnut Creek, CA, USA) 3600 GC (gas chromatography) system coupled with a Saturn quadrupole ion trap mass spectrometer was used to perform mass spectral analysis in real-time catalytic cracking reactions. As this was an integrated reactor/analyzer, the GC column was exposed to temperatures beyond the degradation point of the column, and so selective ion storage RF waveform was used to remove the siloxane masses from the spectra. This produced lower detection limits and full scan data for identification. Mass/charge segmentation of the mass spectrometer allowed the complete product identification for electron impact spectra. Hexane was reacted over H-ZSM-5 catalyst for instrument validation. This produced a series of alkanes, alkenes, and aromatics with distributions consistent with that reported for the cracking of hexane.     

Simulation of heterogeneous catalysts and catalytic processes using the density functional method

The review is devoted to the use of high-level quantum-chemical calculations by the density functional method for the simulation of heterogeneous catalytic systems based on transition metals. The following problems are considered: (1) the development of methods for simulating metal particles supported on the surfaces of ionic and covalent oxides; (2) the calculation of the properties of individual transition metal atoms and small clusters adsorbed on the surfaces of MgO, α-Al₂O₃, γ-Al₂O₃, and various modifications of SiO₂ and in the pores of zeolites; (3) the mechanisms of hydrogen activation and acrolein hydrogenation on the metallic and partially oxidized surface of silver; and (4) the mechanism of formation of carbon residues upon the decomposition of methanol on nanosized Pd particles.     

Application of parahydrogen for mechanistic investigations of heterogeneous catalytic processes

Parahydrogen-induced polarization technique (PHIP), based on the pairwise addition of molecular hydrogen to a substrate, was successfully applied to obtain novel information on the mechanisms of heterogeneous catalytic hydrogenation, hydrodesulfurization, and oligomerization processes. In particular, the PHIP effects were observed upon hydrogenation with parahydrogen catalyzed by the immobilized neutral complexes of rhodium and iridium, which confirms the similarity in the mechanisms of homogeneous and heterogeneous hydrogenation for such systems. In the study of acetylene oligomerization, a significant NMR signal enhancement was revealed for a number of C₄ oligomers, with the enhancement levels by far exceeding that observed in hydrogenation of carbon-carbon triple bonds. The mechanistic features of heterogeneous hydrogenation of a number of six-membered cyclic hydrocarbons over supported metal catalysts were investigated, and their hydrogenation scheme based on the pairwise addition of molecular hydrogen was proposed. Furthermore, the PHIP technique revealed that heterogeneous hydrodesulfurization of thiophene mainly proceeds via hydrogenation followed by a C—S bond cleavage. A significant enhancement of sensitivity in combination with characteristic line shapes of NMR signals make the PHIP method a unique and highly informative tool for the investigation of heterogeneous catalytic processes.     

On-line gas chromatographic monitoring of catalyst processes in a microfabricated chemical reactor

A microfabricated catalyst reactor, prepared from glass and polydimethylsiloxane, has been directly interfaced to a gas chromatograph permitting real time reaction monitoring allowing rapid catalyst characterisation.     

固定床天然气与煤共气化火焰区温度影响因素的研究

用实验室固定床反应器模拟合成气制备炉，考察了该工艺中不同因素对火焰区温度的影响。实验中首先确定了使火焰区温度最低时的甲烷和氧气相对入口位置，然后在此条件下分别考察了进料中H2O/O2和CH4/O2摩尔比变化对火焰区温度的影响。结果表明，甲烷和氧气相对入口位置平齐时火焰区温度最低，火焰区温度均随进料中CH4/O2和H2O/O2摩尔比的增大而降低。     

Atomic-scale assembly of a heterogeneous catalytic site

The distance between surface Pd atoms has been shown to control the catalytic formation of vinyl acetate from ethylene and acetic acid by AuPd catalysts. Here, we use the bulk alloy's thermodynamic properties, as well as the surface lattice spacing of a AuPd(100) alloy single-crystal model catalyst to control and optimize the concentration of the active site (Pd atom pairs at a specific Pd-Pd distance with Au nearest-neighbors). Scanning tunneling microscopy reveals that sample annealing has a direct effect on the surface Pd arrangements: short-range order preferentially forms Pd pairs located in the c(2 x 2) sites, which are known to be optimal for vinyl acetate synthesis. This effect could be harnessed for future industrial catalyst design.     

Reactions of peroxide radicals with the surface of heterogeneous catalysts in homolytic and heterolytic processes

An interrelation between homolytic and heterolytic stages is found for olefin epoxidation by hydroperoxides and cumene hydroperoxide decomposition in the presence of a heterogeneous catalyst (molybdenum selenide). Peroxide radicals that are formed in homolytic decomposition of the hydroperoxide interact with the catalyst surface. As a result, molybdenum atoms are oxidized to the highest valence state, and new reaction sites are created, on which heterolytic reactions occur. It is shown that olefin, as an electron donor, decreases the catalyst activity in the heterolytic reactions. The limited value for the olefin epoxidation rate, which is independent of the amount of the catalyst used, is explained by the disappearance of active sites on the catalyst surface due to their interaction with the neighboring sites containing adsorbed olefin molecules.     

Spatial distributions of desorbing products in basic catalytic processes and surface nano-structures

Reviews of recent progress in angle-resolved measurements of desorbing surface reaction products are discussed. The angular and velocity distributions of desorbing products deliver information about the reaction site as well as the reaction mechanism when the products are repulsively desorbed. These distribution measurements can yield symmetry and orientation information of the reaction site for associative processes whereas, in dissociative desorption, the collimation of fragment desorption is related to the orientation of the intermediate species immediately before dissociation. These different collimations provide information on desorption steps whenever any step becomes rate determining.     

Use of Si-N-Fe and B-N-Fe compositions for formaldehyde degradation in combined heterogeneous and homogeneous catalytic processes

The catalytic activity of B-N-Fe and Si-N-Fe compositions toward formaldehyde degradation in aqueous solutions by ozonation and UV irradiation was examined in relation to the time of exposure, phase composition and weight of the catalyst, and amount of oxalic acid added. The conditions for complete degradation of formaldehyde were determined, and high workability of the composites was demonstrated. The application of the catalysts in drinking water treatment to remove formaldehyde was tested.     

Solvent effects on reaction rates in a catalytic slurry reactor

By carrying out static adsorption measurements, it is shown that absolute values of adsorption strength parameters required in an earlier Langmuir type model for liquid phase catalytic hydrogenation in the presence of an adsorbing solvent can be obtained. These can be used to predict the relationship between reaction rate and reactant concentration in the solvent.

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In situ 13C DEPT-MRI as a tool to spatially resolve chemical conversion and selectivity of a heterogeneous catalytic reaction occurring in a fixed-bed reactor

The distortionless enhancement by polarisation transfer (DEPT) nuclear magnetic resonance (NMR) technique, combined with magnetic resonance imaging (MRI), has been used to provide the first in situ spatially-resolved and quantitative measurement of chemical conversion and selectivity within a fixed-bed reactor using natural abundance 13C NMR.     

A simple model of a kinetic heterogeneous catalytic oscillator

A model of a heterogeneous catalytic oscillator with minimum number of kinetic parameters is proposed. Feedback in the oscillator is the dependence of the rate constant of one of the steps on the filling of the surface of the catalyst by adsorbed particles.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 2, pp. 152–154, March–April, 1992.     

Studies of the dynamic properties of heterogeneous catalytic reactions in a closed adiabatic gradientless system over biographically inhomogeneous catalyst surface

The dynamic of a closed adiabatic system of constant volume consisting of an ideal gas mixture and particles adsorbed over the biographically inhomogeneous catalyst surface, has been studied. The asymptotic stability of a point of detailed equilibrium in each reaction simplex has been proved.

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