Surface Of Heterogeneous Catalysts Research Articles

Computer-assisted generation of chemical reaction networks. Applications in mechanistic studies of organometallic catalysis

The Chem Net computer program designed for mechanistic studies of organic reactions occurring in solutions of metal complexes and on the surface of heterogeneous catalysts is proposed. The use of the program is illustrated by applications to catalyzed homogeneous hydrocarboxylation and hydroformylation of ethylene and heterogeneous hydrogenolysis of ethane.

Modified Titanium Dioxide Photocatalysts for the Enhanced Photodegradation of Organic Substrates

Two approaches to generate TiO 2 photocatalyst for the enhanced photodegradation of substrates are described. One approach includes the surface modification of TiO 2 , with a layer consisting of a N,N'-bipyridinium π-acceptor layer. The resulting photocatalyst reveals superior photocatalytic activity as compared to the non-modified TiO 2 for the decomposition of π-donor substrates such as 1,4-dimethoxybenzene (1), 1,2-dimethoxybenzene (2) and indole (3). The enhanced photocatalytic activity of the modified TiO 2 , V 2+ - TiO 2 is attributed to the concentration of the pollutant at the heterogeneous catalyst surface via the formation of supramolecular π donor-acceptor complexes. A second approach to improve the photocatalytic activity of TiO 2 , involves its doping with Fe(III)-Phthalocyanine, Fe(III)-Pc. The Fe(III)-Pc doped TiO 2 , Fe(III)-Pc/TiO 2 is formed by the sol-gel method. The series of organic substrates p-nitrobenzoic acid (5), p-aminobenzoic acid (6), p-chlorophenoxyacetic acid (7), salicylic acid (8) and aniline (9), is effectively degraded by the Fe(III)-Pc/TiO 2 catalyst compared to the non-modified catalyst. Photodegradation of the organic pollutants in the presence of Fe(III)-Pc/TiO 2 is substantially enhanced as compared to non-modified TiO 2 and leads to complete mineralization. The enhanced activity of the Fe(III)-Pc doped TiO 2 , is attributed to the synergistic generation of °OH radical, the active species in the degradation of the organic substrates, at the semiconductor surface, via the photochemical cleavage of light-induced generated H 2 O 2 at the semiconductor surface.

A Second Look at Modeling the Multiplicity of Active Site Types of Ziegler-Natta Catalysts with Flory's and Stockmayer's Distributions

ABSTRACTThe presence of multiple site types on the surface of heterogeneous Ziegler-Natta catalysts is generally used to explain the production of polyolefins with broad and sometimes multimodal distributions of molecular weight (MWD) and chemical composition (CCD). These distributions can be modelled as a superposition of several Flory's and Stockmayer's distributions for each site type. However, several individual distributions are generally needed to properly describe MWD and CCD, making it difficult to assign these distributions to chemically distinct catalyst sites. In the model proposed herein, Flory's and Stockmayer's distributions are used to explain the underlying polymerization mechanism on each site type, but catalyst site-surface interactions are considered to broaden these distributions. As a result, fewer Flory's and Stockmayer's distributions are required to describe broad and multimodal MWDs and CCDs.

Reduction of a diruthenium bridging vinylidene species Cp 2Ru 2( μ-C = CH 2)(CO)( μ-CO)(MeCN) by hydrosilane leading to a vinyl complex Cp 2Ru 2( μ-CH = CH 2)(CO) 2(SiR 3) and vinylsilane: a model system for production of an olefinic product from a vinylidene intermediate

Treatment of the labile diruthenium bridging vinylidene species, Cp 2Ru 2( μ-C = CH 2)( μ-CO)(CO)(MeCN) ( 7), with hydrosilane produces the silylated bridging vinyl complex, Cp 2Ru 2( μ- η 1: η 2-CH=CH 2)(SiR 3(CO) 2 ( 8), from which vinylsilane CH 2 = CHSiR 3 ( 19) is eliminated upon further treatment with 2e-donors. The reaction sequence, where hydrosilane works as a pseudohydrogen, can be viewed as a model system for production of an olefinic product from a vinylidene species on a heterogeneous catalyst surface. A similar reaction of 7 with HSnPh 3 gives small amounts of ethylene and Cp 2Ru 2(μ-H) 2(SnPh 3) 2(CO) 2 ( 20). The molecular structure of the SiMe 2Ph derivative of 8 has been determined by X-ray crystallography.

Necessary levels of detail in microkinetic models of catalytic reactions on nonuniform surfaces

A quantitative evaluation of approximate models which account for the kinetic effects of nonuniformity of adsorption on heterogeneous catalyst surfaces has been performed. Model adequacy was evaluated by comparing the predictions to a base case model in which the dependence of the reaction rate on the nonuniformity of the surface was rigorously taken into account. None of the approximate models were found to provide predictions sufficiently similar to the kinetics observed on a surface possessing active sites with a known heterogeneous distribution of adsorption energies under a variety of conditions. Surprisingly, a uniform surface model, which treated the catalyst surface as if it were homogeneous in adsorption energy, provided the most accurate representation. The prediction of the uniform site model was always within a factor of 4–5 for all conditions investigated with adsorption energy distribution breadths ranging up to 20 kcal mol −1. The same general conclusions were drawn without requiring the allowed adsorption energy distribution to be centered about the so-called “optimum” reactive site. In fact, the uniform site model prediction was within a factor of 4 whether or not the optimum reactive site was contained within the allowed range of adsorption energies. It was concluded that if the kinetic effects of nonuniform surfaces are of central importance and must be incorporated in a kinetic model, a computationally intensive, site-by-site averaging of the reaction rate at each unique adsorption site must be performed.

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.

Competition effects in surface diffusion controlled reactions: Theory and Brownian dynamics simulations

Surface diffusion controlled reactions on a heterogeneous catalyst surface comprising randomly placed circular reactive sites are considered. The diffusing species adsorbs onto the surface following Langmuir–Hinshelwood kinetics and reacts instantaneously on contact with a reactive site. Approximate theories are formulated to describe the process for high concentrations of the reactive sites, when competition between the sites is significant, following three different approaches: (i) modification of the single sink theory; (ii) using a cell model; and (iii) using an effective medium theory. The predictions of the theories are compared with the results of multiparticle Brownian dynamics simulations for the overall reaction rate, the bulk concentration of the reactive species, and the ensemble averaged concentration profile around a reactive site. The effective medium theory is found to give the best results among the theories considered, and the predictions are in good agreement with the computational results.

On the real nature of aliphatic carbenium ions as active intermediates of homogeneous and heterogeneous acid catalysis

The structure and properties of active intermediates of acid-catalyzed homogeneous and heterogeneous transformations of olefins and alcohols are discussed, taking into account the hydration in homogeneous systems or the interaction with the surface of heterogeneous zeolite catalysts on the basis of nonempirical quantum-chemical calculations. In both cases carbenium ions do not represent the actual existing active intermediates, but rather the excited transition states. They could be formed from the more covalent and more stable surface esters in heterogeneous acid catalysis or from hydrated alkoxonium ions in acid catalysis in aqueous solutions. This results in concerted reaction pathways with low activation energies due to almost complete compensation of the energy required for the rupture of the reacting chemical bonds in the initial compounds by the energy released by the formation of the new bonds in the final products.

Theoretical analysis of vibrational spectra of molecules adsorbed on the surface of heterogeneous catalysts

The principles for choosing adsorbed complex models have been formulated. The factors for determining the differences in vibrational spectra of molecules adsorbed on the surface of various heterogeneous catalysts — the change of the adsorbed molecule itself under the action of a surface field and the contribution of the adsorption bond of adsorbed molecules with the surface site into the potential energy of an adsorbed molecule — compared with their spectra in the free state have been analyzed. The criterion of characteristicity, to determine the ratio of these two factors, has been suggested.

A tri-platinum complex containing a coordinatively naked platinum atom

POLYNUCLEAR clusters of platinum and palladium containing metal–metal bonds may serve as models of the surface of heterogeneous catalysts, and their reactions may mimic those on such surfaces1,2. Ligand-free metal clusters can be created in an ultradispersed condensed phase3, but they are difficult to isolate and characterize because of their high reactivities. Here we report the preparation of a tri-platinum diphosphine complex that con-tains a coordinatively 'naked' platinum atom. The metal cluster in this complex is electron-deficient relative to other tri-platinum complexes. Bonding of ligands to the naked platinum could reflect that of chemisorbed species on catalytic platinum surfaces.

高温触媒燃焼

This paper describes concept and current status of development of catalytic combustion for NOx (nitrogen oxides) abatement. Catalytic combustion is an alternative technology, currently under development, which has been offering the potential of meeting SCR (Selective Catalytic Reduction of NOx) emission level and cost reduction. A conventional combustion process produces a peak temperature which is far into the thermal NOx formation region. In contrast, a catalytic combustion process maintains a peak temperature well below the NOx formation region without compromising combustion efficiency.Two types of the catalytic combustion concept have been proposed. One employs both homogeneous (gas phase) reaction and heterogeneous (catalyst surface) reaction to accomplish complete combustion within a honeycomb catalyst bed. It suggests that the catalyst temperature reaches the adiabatic temperature of fuelair mixtures and that the catalyst shoud withstand above 1300°C. The other employs only heterogeneous reaction, within a honeycomb catalyst bed, to reduce maximum catalyst temperature. And, complete combustion takes place in a post catalyst bed without a catalyst. The maximum catalyst temperature is maintained around 1000°C.Based on these concepts, several development programs for gas turbine applications have been conducted in USA and Japan. They demonstrated very low NOx emission performance (lower than 10ppmv). Despite the low NOx capability of catalytic combustion, further development efforts are needed in catalyst material, combustor design and control system.

An EXAFS study of oxide and suphided catalysts

This review article summarizes the results of EXAFS studies, mainly carried out in the USSR, on the structure of oxide and sulphided chemical compounds formed on surfaces of various heterogeneous catalysts. It describes supported metallic catalysts in the presence of oxygen, supported oxide catalysts at different preparation and activation conditions, and supports, modified by transition metal oxides and catalysts for hydrodesulphurization processes.

Peculiarities of the vibratioanl spectra of molecules adsorbed on the surface of heterogeneous catalysts: theoretical analysis

The vibrational spectra of adsorption complexes resulting from the adsorption of ammonia on the surface of heterogeneous catalysts are interpreted by computation of frequencies and forms of normal vibrations. The characteristics of normal vibrations are computed from the state of separate fragments of adsorption complexes. It is shown that an interaction of internal coordinates, associated with the adsorption bond, with internal coordinates of the adsorbed molecule leads to a decrease of characteristics of vibrations sensitive to the surface area. A correlation is established between the characteristics of such vibrations and the changes in their frequencies in comparison with the free molecule.

Molybdenum in enzymatic and heterogeneous catalysis

The chemistry common to molybdenum at the active centers of molybdoenzymes and at the surface of heterogeneous catalysts is described Oxomolybdenum(VI) compounds catalyze selective oxidation of unsaturated hydrocarbons, e.g , propene to acrolein Similarly, oxomolybdenum species take part in reactions catalyzed by molybdoenzymes, e.g , xanthine oxidase, sulfite oxidase, nitrate reductase. In these reactions H +, O 2− or HO −, and electrons transfer between substrate molecules and molybdenum atoms and groups at the active centres. The chemistry involved is the acid-base and redox chemistry of molybdenum Molybdenum disulfide catalyzes hydrogenation of unsaturated hydrocarbons, e.g , acetylene. The active site is a coordinately unsaturated molybdenum atom in a sulfur-ligand environment The enzyme nitrogenase, which is a protein-bound iron-molybdenum sulfide, is also an excellent hydrogenation catalyst. Both catalysts exploit the chemistry of lower-valent molybdenum coordinated by sulfur. The extent to which understanding of the catalysis can be transferred between the two types of catalyst is assessed.

Vibrational spectroscopy in the study of the structure of ethylene and propylene complexes with metal cations on the surface of heterogeneous catalysts

The frequency and form of normal vibrations of ethylene, propylene and their π-complexes, as well as π- and μ-allylic complexes of propylene and their deutero derivatives have been analyzed in the framework of normal coordinates analysis. It has been shown for ethylene and propylene complexes that the parameter ƒ′ of normal vibrations with a maximum contribution made by q CC sharply or slowly decreases in the case of H- and D-derivatives, respectively, with delocalization of the double bond (i.e. with the decrease of K CC). The value of ƒ′ and such parameters as Δν CC = ν gas CC  ν bond CC Δν H→D CC = ν bond C n H 2 n CC  ν bond C n D 2 n CC are structural they vary depending on K CC in the same manner for all adsorption types and are not suitable (except for some extreme values) for the identification of the type of the olefin-surface cation interaction. The state of the CC bond can easily be identified from the Δν C=C value when using D derivatives. IR spectra of surface OH-groups and some other information are required to identify the types of C 3H 3 complexes.

Determination of hydroxyl groups and water adsorbed on the surface of heterogeneous catalysts by titration with alkali metal naphthenide solutions

Determination of hydroxyl groups and water adsorbed on the surface of heterogeneous catalysts by titration with alkali metal naphthenide solutions