Adsorption and reactions on a surface alloy: CO, NO, O-2 and CO2 on Pd(100)-Mn-c(2x2)

The adsorption properties of the Pd(100)-Mn-c(2 x 2) surface alloy have bee n investigated using photoemission of both core and valence levels. CO adso rbs in a molecular form without affecting the alloy structure. Two CO speci es were found, one bonded to Pd, which desorbs upon heating to 270 K, and o ne bonded to Mn, which desorbs when heating to 400 K. O-2 destroys the allo y, leading to a disordered surface with MnOx complexes. The MnOx aggregates stabilize adsorbed CO, and act as active sites for the following oxidation reactions at 110 K: CO + O-->CO2, CO + 2O-->CO3delta- and CO2 + O-->CO3del ta-. The CO2 species desorbs upon heating to 170 K, whereas the CO3delta- s pecies is stable up to temperatures between 300 and 500 K. When exposed to low amounts of NO at 110 K, the major part of the molecules dissociates in order to form MnOx, thereby destroying the alloy. Larger NO doses yield an increasing amount of molecular NO, which dissociate upon heating to 300 K. CO can react with the dissociated NO to form CO, and another species with a C Is binding energy and thermal stability similar to that of CO3delta-. Th is species was tentatively identified as -NCO or -NCO2. (C) 1999 Elsevier S cience B.V. All rights reserved.