Operando surface spectroscopy-placing catalytic solids at work under the spotlight

Abstract

Heterogeneous catalysis is involved in the vast majority of industrial chemical processes performed nowadays, and an increased understanding of catalytic reactions is of the utmost relevance to develop a sustainable and cleaner technology. In order to make new (or improved) catalytic solids, an increased comprehension of the working principles of heterogeneous catalysis is a prime requirement. Towards that endeavour, many rewarding efforts have been made in the last decades to: (a) determine the nature and distribution of the catalytically active sites present on the surface of the catalyst, (b) unravel the underlying catalytic mechanisms, and (c) reveal the close relationship that exists between the structure (and composition) of the heterogeneous catalysts and their most relevant characteristics, which include activity, selectivity and stability. In this scientific endeavour, characterization techniques play a pivotal role and spectroscopic methods, in particular, have become the key tool to characterize catalytic solids. Spectroscopy (from the Latin spectrum and the Greek skopein meaning ‘to examine’) comprises a large group of techniques that, taken either individually or in several judicious combinations, can profitably be used to determine the nature, quantity, structure and environment of atoms, ions and molecules. Most spectroscopic methods currently used in the field of heterogeneous catalysis are based on the analysis of the interaction of the catalyst with electromagnetic radiation ranging from radio and microwaves through to infrared, visible and ultraviolet light and finally to X-rays. However, characterization of the catalyst (or catalytic precursor) itself constitutes only a small step towards understanding catalytic processes. In the end, one wishes to obtain detailed information on the catalytic solids under technologically relevant working conditions, which can bring about very significant changes to the catalytically active sites. For this purpose, in situ spectroscopic cells were developed that enable the researcher to investigate the physicochemical changes taking place in the catalyst while working at even a high temperature and pressure of the substrate in a gas or liquid phase. In situ conveys the meaning of catalyst characterization at its working place, in contrast to ex-situ measurements. Preferably, the in situ characterization approach should be complemented with simultaneous measurement of catalytic activity and selectivity, which can be accomplished by coupling the in situ spectroscopic cell with (for instance) a mass spectrometer or a chromatography system; the wealth of data thus obtained is most useful for analysing structure– performance relationships of the catalysts at work. And this is how operando (meaning ‘at work’) spectroscopy was born. In other words, operando spectroscopy can be regarded as being a very important class of a much broader group of in situ catalyst characterization techniques. Since it provides detailed information on the surfaces of catalytically active materials at work, operando surface spectroscopy seems to be the proper name for this research field.