Molecular Aspects of Catalytic Reactivity. Application of EPR Spectroscopy to Stuies of the Mechanism of Heterogeneous Catalytic Reactions

Abstract

The basic objective of mechanistic studies of real catalytic processes is to dissect the course of the reaction into individual steps; ascertain their sequence; and determine the stoichiometry, structure, and electronic states of active sites and intermediates. The electron paramagnetic resonance (EPR) technique is at present widely used to explore many of these principal aspects of heterogeneous catalysis and surface chemistry. The extreme sensitivity compared to the usual spectroscopic methods is perhaps its most acknowledged advantage and makes EPR best suited to investigate and characterize low-abundance active sites and intermediates appearing during catalytic reaction. Additional information can be drawn from the theoretical analysis of the experimental spin Hamiltonian parameters within the ligand field and from angular overlap or Newman's superposition models as well as by more sophisticated quantum chemical calculations. The purpose of this paper is to show how catalysis benefits from EPR spectroscopy and to identify the issues and areas explored by this method. A comprehensive literature review is not attempted in this article; instead, attention is directed toward application of EPR for elucidation of the molecular reaction mechanism that can provide a scientific background for understanding many fundamental aspects of catalytic activity. The major events of mechanistic studies which involve the identification of active sites, activation of reagents, and determination of the reaction pathways are illustrated by selected examples and discussed. An approach that is complementary to mechanistic catalytic test studies is also presented. It consists of spectroscopic investigations of a set of partial reactions, driven by external creation of the supposed active sites and intermediates, with the aim of reproducing and verifying the feasibility of the postulated catalytic cycle. Moreover, to assure some consistency of the subject, basic characteristics of EPR spectroscopy related to surface studies and chemical theories of reactivity are concisely reviewed.