surface science ELSEVIER Surface Science 356 (1996) 189-194 HREELS, LEED and angle-scanned XPD investigation of the coadsorption of CO and NO on Ni(111) L.S. Caputi *, G. Chiarello, A. Amoddeo, R.G. Agostino, L. Papagno, E. Colavita Istituto Nazionale di Fisica della Materia-Unith di Cosenza and Dipartimento di Fisica, Universith della Calabria, 87036 Arcavacata di Rende, Cosenza, Italy Received 16 February 1995; accepted for publication 13 December 1995 Abstract The coadsorption of CO and NO on Ni(lll) at room temperature has been studied by HREELS, LEED and angle-scanned XPD techniques. The experiments have been carried out by first exposing the clean surface to one gas up to the observation of a c(4 × 2) LEED pattern, and then by monitoring the step-wise addition of the other. CO does not modify or adsorb on a c(4 × 2)-NO layer, while 1 L NO causes the complete desorption of a c(4 × 2)-CO layer. The LEED pattern is always c(4 × 2), even if both CO and NO species are present on the surface. For NO exposures lower than 1 L of the CO-covered Ni(111) surface, XPD measurements show that NO molecules are in a bent geometry, while CO molecules are perpendicular to the surface. Our results indicate that the NO-Ni interaction is very weakly affected by the presence of coadsorbed CO. Keywords: Carbon monoxide; Chemisorption; Electron energy loss spectroscopy; Low energy electron diffraction (LEED); Low index single crystal surfaces; Nickel; Nitrogen oxides; Photoelectrondiffraction 1. Introduction The interaction between NO and CO on transition metal surfaces is of great technological interest in view of the development of novel catalysts for the catalytic reduction of NO in automobile exhaust emissions. Spectroscopic investigations of the coad- sorption of NO and CO have been reported for Pt(100) [1-3], Pt(lll) [4], Pt(410) [5], Ru(001) [6], Rh(111) [7] and Ni(lll) [8] surfaces. Gorte and Schmidt [4] found a repulsive interaction between CO and NO coadsorbed on Pt(111). On Pt(100), the chemical reaction between CO and NO, coadsorbed * Corresponding author. Fax: +39 984 839 389; e-mail: caputi@fis.unical.it. at low temperature, takes place around 410 K, with the formation of CO 2 and N 2 [1,3]. Very different results have been obtained for coadsorption of CO and NO at room temperature on the same surface. Gardner et al. [2] found that at room temperature a saturated NO layer is easily removed by subsequent exposures to CO, while a saturated CO layer is unperturbed by NO exposures. Banholzer et al. [5] found a partial removal of the preadsorbed layer regardless of the order of exposure, although CO is removed by NO more easily than vice-versa. They also hypothize a CO/NO reaction at room tempera- ture. The adsorption of NO on CO-predosed Ni(11 l) at 85 K seems to result in a strong local interaction which gives rise to the formation of CO islands with 0039-6028/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S 0039-6028(96)00009-X