Structure and Dynamics of Reactant Coadsorption on Single Crystal Model Catalysts by HP-STM and AP-XPS: A Mini Review

Understanding the reaction mechanism of various heterogeneous catalytic reactions is of fundamental importance in catalysis science. In the past, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) have proved to be powerful surface-sensitive techniques to characterize surface reactions on model catalysts under UHV conditions. The recent development of high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) has largely extended the application of these two excellent surface-sensitive imaging and electron spectroscopy techniques to a variety of catalytic systems under realistic conditions. In this mini review, we will review a series of catalytic systems studied by HP-STM and AP-XPS, including reactant coadsorption systems, coadsorption + reaction systems, and poisoned reaction systems. We will also illustrate one of the main difficulties in the practical execution of experiments where the initial surface cleanliness is easily compromised by the adsorption of adventitious contaminants. All of these examples will demonstrate that the combined use of HP-STM and AP-XPS can provide a deeper understanding of the structure and dynamics of reactant coadsorption on model catalysts, although great care has to been taken to maintain the cleanness of the in situ instrumentation.