Microkinetic Modeling: A Tool for Rational Catalyst Design

Abstract

The design of heterogeneous catalysts relies on understanding the fundamental surface kinetics that controls catalyst performance, and microkinetic modeling is a tool that can help the researcher in streamlining the process of catalyst design. Microkinetic modeling is used to identify critical reaction intermediates and rate-determining elementary reactions, thereby providing vital information for designing an improved catalyst. In this review, we summarize general procedures for developing microkinetic models using reaction kinetics parameters obtained from experimental data, theoretical correlations, and quantum chemical calculations. We examine the methods required to ensure the thermodynamic consistency of the microkinetic model. We describe procedures required for parameter adjustments to account for the heterogeneity of the catalyst and the inherent errors in parameter estimation. We discuss the analysis of microkinetic models to determine the rate-determining reactions using the degree of rate control and reversibility of each elementary reaction. We introduce incorporation of Brønsted–Evans–Polanyi relations and scaling relations in microkinetic models and the effects of these relations on catalytic performance and formation of volcano curves are discussed. We review the analysis of reaction schemes in terms of the maximum rate of elementary reactions, and we outline a procedure to identify kinetically significant transition states and adsorbed intermediates. We explore the application of generalized rate expressions for the prediction of optimal binding energies of important surface intermediates and to estimate the extent of potential rate improvement. We also explore the application of microkinetic modeling in homogeneous catalysis, electro-catalysis, and transient reaction kinetics. We conclude by highlighting the challenges and opportunities in the application of microkinetic modeling for catalyst design.

Mechanistic insights into the conversion of dimethyl ether over ZSM-5 catalysts: A combined temperature-programmed surface reaction and microkinetic modelling study

“…Consistency of reaction steps across the different sites would lead to an explosion in the number of parameters to be fitted and cross-correlation between the parameters. Thus, according to the microkinetic method proposed by Dumesic et al 77,78 , the desire to include all reaction intermediates on the catalyst surface is balanced with the need to express the mechanism in terms of kinetic parameters that are accessible to experimental measurements or theoretical prediction. This compromise between the mechanistic detail and kinetic parameter estimation plays an important role in the use of microkinetic analysis for catalytic reaction synthesis.…”

Concepts, models, and methods in computational heterogeneous catalysis illustrated throughCO2conversion

“…Next, the information about the reaction rates is used to formulate coupled differential equations for each elementary step which are then solved numerically for a given set of initial conditions, with further details reviewed recently by Motagamwala and Dumesic. 164 In this way, MKM connects quantitatively molecular-level modeling with experimental data by using as inputs the outputs obtained from first-principles calculations. MKM is particularly relevant in heterogeneous catalysis because it provides a description of complex reactions by identifying key reaction intermediates and the rate-limiting step, which ultimately controls the catalytic performance.…”

Parameter estimation in kinetic models of complex heterogeneous catalytic reactions using Bayesian statistics

“…Reaction kinetic data are used extensively in reactor design [1,2]. While even simple power law expressions can sometimes be used in simulation of industrial reactors [3], such an approach has a limited insight into the reaction mechanism and can be applied only in a limited range of reaction conditions [4]. More complex reaction mechanisms, such as those usually referred to as Langmuir-Hinshelwood or Eley-Rideal [1] mechanisms assume quasi-equilibria of adsorption/desorption steps and certain rate determining steps.…”

“…[10,11] it was suggested that theoretically sound values of pre-exponential factors, activation energies, adsorption enthalpy and entropy, distribution functions for the number of actives sites with different heats of adsorption, Polanyi parameters for elementary steps, etc., should be introduced as an input to computer subroutines. A more refined methodology coined microkinetic modelling [4,[13][14][15][16], has become a widespread approach to incorporate the essential features of surface chemistry. For relatively simple cases, such as ammonia synthesis, based on the surface science data it is possible to describe the experimental observations without any fitting of the parameters.…”

“…Unfortunately, not all reaction intermediates as well as for example lateral interactions between them are accessible to experimental measurements or theoretical predictions [4]. Besides the often unknown effects of the coadsorbed species, application of model instead of real surfaces in density functional theory (DFT) calculations or an assumption of rapid diffusion in the adsorbed layer, also dynamic changes in the catalytically active phase during the reaction can lead to limitations in not only design of catalytic materials [4], but in reliable description of kinetic data as well. While microkinetic modelling per se aids in understanding the reaction mechanisms, it does not eliminate the need for numerical data fitting and statistical analysis of kinetic parameters.…”