Rates Of Heterogeneous Catalytic Reactions Research Articles

Adlayer structure and lattice size effects on catalytic rates predicted from KMC simulations: NO oxidation on Pt(111).

Repulsive and/or attractive interactions between surface adsorbates have an important effect on the structure of the adsorbate layer and consequently on the rate of heterogeneous catalytic reactions. Thus, developing reaction models that take into account adsorbate-adsorbate interactions is crucial for making accurate predictions of the catalytic rate and surface coverage during reaction. In the present work, we employ kinetic Monte Carlo simulation to model the catalytic NO oxidation on Pt (111), adopting a cluster expansion (CE) Hamiltonian approach for treating the aforementioned interactions. We investigate CEs of increasing complexity, ranging from pairwise 1st nearest neighbor to long-range and many-body terms. We show that energetic models incorporating solely short-range interactions result in ordered adlayer structures, which are disrupted by anti-phase boundaries and defective regions when the size of the periodic lattice is non-commensurate to the structure of the stable adlayer. We find that O2 dissociates on sites located in these defective regions, which are predominantly responsible for the activity, and the predicted catalytic rate is strongly depended on the lattice size. Such effects are absent when employing non-periodic lattices, whereon the catalytic activity appears more intense on edges/corner sites. Finally, inclusion of long-range interactions in the model Hamiltonian induces relative disorder in the adsorbate layer, which is ascribed to the "softening" of the repulsive interactions between adspecies. Under these circumstances, the distribution of activation energies for O2 dissociation is broader as compared to short-range interaction models and on this basis we explain the disparate catalytic rate predictions when using different CEs.

Kinetics of Catalytic Ozonation of Methyl tert‐Butyl Ether in the Presence of Perfluorooctyl Alumina

AbstractIn this work, the kinetics of heterogeneous ozonation of methyl tert‐butyl ether (MTBE) in the presence of perfluorooctyl alumina (PFOAL) catalysts is presented. Activity of the PFOAL catalysts in ozonation reactions of MTBE was studied in a batch reactor over the temperature range of 25 to 40 °C. The results show that PFOAL is an effective catalyst for enhancing the molecular mechanism of ozone reactions. For a catalyst concentration of 5 g/L, the contribution of the rate of heterogeneous catalytic reaction in the total reaction rate is about 50 %, 75 %, and 85 % at the reaction temperatures of 25 °C, 30 °C, and 40 °C, respectively. Kinetic studies showed that the chemical reaction of ozone and MTBE is the controlling step in this catalytic system. The quasi‐homogeneous kinetic model proved to be an appropriate approach in modeling the heterogeneous reaction of ozone with MTBE on PFOAL catalysts.

The nature of permeability anisotropy and catalytic activity

The phenomena of permeability anisotropy and an increase in the rates of catalytic reactions in porous membranes modified with highly dispersed catalytic systems were analyzed. A model of stochastic gas motions was proposed; this model is based on the hypothesis of the specific interaction of molecules with the inner surface of pores resulting in a nonisotropic distribution of molecules over traveling directions. The effects of asymmetric gas transfer in porous and gradient-porous membranes were considered to explain differences in the rates of heterogeneous catalytic reactions in a nanoporous membrane reactor under changes in the direction of supplying a reaction mixture. From the model proposed, it follows that the transversal diffusion of gas molecules is most probable in the porous medium of a ceramic membrane with a pore-size distribution gradient from large to small pores along the flow direction. This diffusion results in an increase in the frequency of molecular collisions with the wall of a microchannel and, correspondingly, in an increase in the contact time. The model proposed explains the intensification of a number of heterogeneous catalytic reactions performed in the porous media of catalytic porous membranes.

The effect of fractal dimensionality and dispersion of catalysts on the rate of heterogeneous catalytic reactions involving carbon oxides

Analytical relationships describing the relation between the morphology of a heterogeneous catalyst and the rate constant of the controlling stage of a reaction taking place on its surface were obtained. It was shown that the structure of the catalyst surface, expressed in terms of fractal dimensionality, affects both the activation energy and the preexponential coefficient of the rate constant. The equations relating the rate constant of the catalytic reaction to the fractal dimensionality were used for both bulk and deposited catalysts. It was shown that the activation energy of the process increases with increase in the nonuniformity of the catalyst surface, characterized by the fractal dimensionality DF.

Dynamics of a system of nonlinear differential equations

This is a qualitative analysis of a system of two nonlinear ordinary differential equations which arises in modeling the self-oscillations of the rate of heterogeneous catalytic reaction. The kinetic model under study accounts for the influence of the reaction environment on the catalyst; namely, we consider the reaction rate constant to be an exponential function of the surface concentration of oxygen with an exponent μ. We study the necessary and sufficient conditions for the existence of periodic solutions of differential equations as depending on μ. We formulate some sufficient conditions for all trajectories to converge to a steady state and study global behavior of the stable manifolds of singular saddle points.

On the rate of heterogeneous catalytic reactions with ionic intermediates

A facile way to compute the rate law of a catalytic reaction with ionic intermediates is proposed, based on the theory of complex reactions. When the ionic nature of intermediates is taken into account it is necessary to add balance equations which correspond to the electroneutrality of the catalyst. Several mechanisms of the olefin hydrogenation were considered as examples.

Rate of heterogeneous catalytic reactions involving ionic intermediates

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTRate of heterogeneous catalytic reactions involving ionic intermediatesSlavik KasztelanCite this: Ind. Eng. Chem. Res. 1992, 31, 11, 2497–2502Publication Date (Print):November 1, 1992Publication History Published online1 May 2002Published inissue 1 November 1992https://doi.org/10.1021/ie00011a014RIGHTS & PERMISSIONSArticle Views105Altmetric-Citations7LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (703 KB) Get e-Alerts Get e-Alerts

Experimental criterion for the absence of artifacts in the measurement of rates of heterogeneous catalytic reactions

Experimental criterion for the absence of artifacts in the measurement of rates of heterogeneous catalytic reactions

The structural factor in chemisorption and heterogeneous catalysis—a review

We have now entered an era where the influence of surface structure on the kinetics of catalytic processes may be directly investigated using single crystal substrates. Catalytic processes whose rates are sensitive to substrate structure, and those which seem to be structure insensitive have now been studied, and several examples are described here. It is shown that there may be large differences in both reactivity and reaction mechanism for ordered and disordered chemisorbed species. It is also shown that structure insensitive heterogeneous reactions may occur in cases where the coverage of a reactant is below one monolayer—suggesting that the leveling of crystallographic differences between different planes by an adsorbate is not a useful general model for explaining rate insensitivity to substrate structure. Conversely, structural sensitivity may occur in cases where a high coverage of adsorbate is present, again in violation of the general model.Because of the many complex factors known to be involved in governing the rate of heterogeneous catalytic reaction, it is concluded that there may be a number of system-dependent explanations for the common occurrence of structural insensitivity, and that a general model for this surprising behavior may not exist.

Measurement of the rates of heterogeneous catalytic reactions using a calorimetric bead system

Investigations have been made of the reliability of the calorimetric bead system for studying the kinetics of catalytic reactions. Measurements of the rate of dissipation of energy from a catalytic reactor in both unreactive and reactive gaseous atmospheres show that the change in temperature of the reactor during an individual experiment accurately reflects the overall rate of the chemical changes taking place at the surface of the reactor. The applicability of the bead system has also been tested directly with reference to the oxidation of methane over palladium. The rate measurements are consistent with independent analyses of the reaction products.

A simple method of calculating mass transfer factors for heterogeneous catalytic gas reactions

The effect of external mass transfer on the overall rate of heterogeneous catalytic reactions is briefly discussed and general equations are developed which make it possible to calculate mass transfer factors directly from kinetic data, for any system in which diffusion within the catalyst pores occurs by a molecular mechanism. The application of these equations is illustrated by considering published kinetic data for the water gas shift reaction. It is shown that if the ratio of superficial gas velocity to catalyst pellet diameter is greater than about 24 sec −1 (at 400°C) resistance to mass transfer is small. This condition is generally met in industrial shift converters and mass transfer restriction therefore becomes significant only at high temperatures. In laboratory reactors lower gas velocities are often used and mass transfer resistance may be quite large. In order to avoid this type of error when measuring design rate constants it is essential to use the same size of catalyst pellet and the same gas velosity in the laboratory as are to be used in the full scale converter.