Photoelectron spectroscopy: a strategy for the study of reactions at solid surfaces

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Abstract
The development of X-ray photoelectron spectroscopy for the study of the nature of chemisorbed species and the mechanisms of surface reactions is described. The methodology of data analysis and the establishment of data banks which enable photoelectron spectra to be assigned to specific surface species is discussed by reference to examples from recent studies. Although in the first instance the primary aim was to establish a qualitative logic in a well defined area of surface chemistry, this was then developed quantitatively and extended to studies of the mechanism of surface catalysed reactions. Emphasis is given here to the activation of molecules at metal surfaces either by surface modification or through chemical specificity which is associated with coadsorbed molecules. As illustration we discuss the activation of O-H and N-H bonds by ‘oxygen’ resulting in reaction pathways being followed which would not have been predicted on the basis of the known reactivities of the individual molecules. The examples chosen also illustrate the severe limitations of mechanistic studies in surface chemistry based (a) on the study of the individual reactants separately; (b) on a post-mortem type of analysis of the surface and (c) on just a gaseous product analysis. An important concept that has emerged is the dual role of surface ‘oxygen’. It may either act as a promoter in activating an otherwise unreactive adsorbate molecule, or it may form an unreactive oxide overlayer. Parallel studies of metal oxides per se provide information on the defect nature of both oxide overlayers at metal surfaces and also bulk oxide surfaces. Close similarities are shown to exist between the ‘oxygen’ activation of molecules at metal surfaces and the catalytic reactivity of defective bulk oxides. A common theme is that the dominant mechanism involves hydrogen abstraction by oxygen adatoms, which are assigned as O−(s) both at metal surfaces (by XPS) and at bulk oxide surfaces (by E.P.R. and XPS).