Advanced Kinetic Characterization

Abstract

Microkinetic analysis that assumes the steady-state approximation are ubiquitous in the modelling and simulations of redox and non-redox reactions. Operando studies such as photoemission electron microscopic investigations have, however, provided evidence for (a)periodic surface concentration profiles and adspecie mobility. Consequently, the ubiquitous steady-state approximation used in nearly all (micro)-kinetic models may not be universally applicable. Furthermore, on unsupported and unpromoted catalyst surfaces, variations in the nature, quantity, and mobility of active sites are observed leading to various forms of site heterogeneity, induced heterogeneity, or both, as well as (a)periodic heterogeneity in the form of oscillatory waves and self-organised patterns. The relationship between surface-site distribution and the ensuing homogeneity and/or heterogeneity and product distribution and the resulting modelling tools used for analysis have not been explored comprehensively. Consequently, we provide a framework for cataloguing catalysts for advanced kinetic characterisation and modelling. The catalogue has two functions: (1) it relates surface models with reactor models, and (2) for any specific reaction, it shows (visually) the dynamicity of adspecie formation and evolution on the catalyst surface. Indeed, this second advantage has been exemplified by archived studies of carbon monoxide oxidation over Pd(111) catalysts. Using the homotattic patch model that leads to the adsorption integral equation, we catalogue catalysts according to evolving active sites on catalyst surfaces with varied homogeneity. We observe that the conventional microkinetic models are only accurate for a very small subset of chemical reactions where the standard model of catalysis applies. However, for many redox catalytic transformations, these models lose their applicability with respect to the dynamics of the active site, catalyst surface, and reaction. As a substitute, we present microdynamic models which not only account for every elementary reaction step but incorporate a multi-scale approach within kinetic models allowing for changing catalyst states, charge transport, as well as active site, catalyst surface, reaction, and reactor dynamics.