Adsorption of NO2 on Rh(111) and Pd/Rh(111):: photoemission studies

Synchrotron-based high-resolution photoemission has been used to study the interaction of NO2 with Rh(111) and Pd/Rh(111) surfaces. Clean Rh(111) reacts readily with nitrogen dioxide which decomposes at temperatures less than or equal to 150 K producing adsorbed NO and oxygen adatoms (NO2-->NOa + O-a). The majority of the adsorbed NO further dissociates (NOa-->N-a + O-a) between 300 and 400 K. The bimetallic Pd/Rh(111) system exhibits a significantly lower reactivity towards NO2 in comparison with Pd(111) and Rh(111). On Pd/Rh(111) surfaces (theta(Pd) =0.6-1.2 ML), NO2 dissociates at low temperatures, but essentially no decomposition to atomic nitrogen occurs and all the nitrogen molecular species disappear from the surface by 380 K. The O-Pd bond in a Pd-1.2/Rh(111) system is substantially weaker (adsorption energy lowered by at least 20 kcal mol(-1)) than on the individual metals and most of the oxygen evolves from the surface by 400 K. When the Rh(111) surface is only partially covered with Pd (Pd-0.6/Rh(111)), spillover of adsorbed species from Pd islands to the bare Rh(111) patches is observed when the temperature of the system is increased from 100 to 400 K. (C) 1999 Elsevier Science B.V. All rights reserved.