Rhodium single crystals show a surprisingly wide range of reconstructions induced by light elements such as oxygen and nitrogen, as well a number of chemical reactions of fundamental chemical importance. In this review, we present and critically discuss the current state of knowledge of the interaction of oxygen, nitrogen and mixed O+N layers with such Rh surfaces, emphasising structural aspects and their impact on the surface reactivity. On the basis of the available experimental results we will elucidate some general trends, which shed light on the possible driving forces behind the diffusive and displacive reconstructions induced by adsorbed atomic oxygen and nitrogen. More attention will be paid to the reconstructive interactions on a Rh(1 1 0) surface. The following three topics are reviewed. The first concerns oxygen interaction with low and some high index Rh surfaces, covering oxygen coverages, adsorption sites, bond strength, ordered structures, sub-surface penetration and oxide formation, and observed by various surface sensitive techniques: X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD)(3), high resolution electron energy loss spectroscopy (HREELS), ultraviolet photoemission spectroscopy (UPS), He scattering, scanning tunneling microscopy (STM), low energy electron diffraction (LEED), etc. They are summarised in a manner illustrating the similarities and differences in oxygen interaction with different Rh single crystal planes. The emphasis is on the reaction conditions when oxygen induces restructuring of the substrate surface. The variety of oxygen-induced (1 x n) missing/added row reconstructions on the Rh(1 1 0) surface is reviewed and discussed in some detail. Comparison will be made with (1 1 0) surfaces of other Group VII/IB fee metals Pd, Cu, Ni and Ag where missing/added row reconstructions are also observed. For Pd, the reconstruction seems similar, whereas for the other three metals it is in a direction orthogonal to that on Rh. In addition, the oxygen on Rh is co-ordinated to three substrate atoms, whereas on Cu, Ni and Ag it is co-ordinated to four atoms. The second topic covers the interaction of atomic nitrogen layers with low index Rh single crystal surfaces, which has been studied far less than oxygen. The growth, ordering and stability of the nitrogen layers produced by various deposition procedures is reviewed, whereby particular attention is given to recent studies on the Rh(1 1 0) surface. Factors such as temperature, nitrogen coverage, substrate surface symmetry and presence of oxygen are examined, and their effect on the stability, desorption kinetics and formation of ordered nitrogen layers is described. The techniques most used in these studies are TPD, LEED, HREELS, Angle Resolved UPS (ARUPS) and STM. The kinetic mechanism of the N-induced (n x 1) added/missing row reconstructions on Rh (1 1 0) and the effect of oxygen on the nucleation and growth rate of the ordered phases are discussed. Concerning the effect of oxygen, we address the factors which drive the complex structural changes and phase transitions observed in mixed O+N layers, where competition between (n x 1) N-induced and (1 x 2) O-induced reconstructions takes place. The third part contains a survey of the experimental results obtained by TPD, Electron Energy Loss Spectroscopy (EELS), XPS, STM, and molecular beam techniques on the reactivity of low index Rh sur faces, where N and O are involved as reaction participants. The emphasis is on comparison of the reactivity in systems, where O and N induce restructuring of the substrate surface. The reaction of the clean surface with NO, and the reactivity of various oxygen structures on Rh(1 1 0) and Rh(1 0 0) surfaces with respect to CO, II and NH3 oxidation are considered. The discussion of these data is made in the framework of the impact of the adsorbate-induced structural changes on the surface activity which is a well-recognised but not well-understood phenomenon in practical catalysis. (C) 1998 Elsevier Science B.V. All rights reserved.
