Calorimetric studies of NO on Ni{211}: criteria for switching from dissociative to molecular adsorption

The cove rage-dependent heats of adsorption and sticking probabilities in the interaction of nitric oxide with clean and oxygen pre-covered Ni{2 1 1} surfaces have been measured at 300 K using single crystal adsorption calorimetry. The results are consistent with a switch from dissociative to molecular chemisorption at 1 ML of O plus N adatoms. Initial dissociative adsorption is attributed to step sites with a heat of 400 U mol(-1). When steps are saturated with adatoms, adsorption proceeds molecularly with a heat of 160 U mol(-1). With 0.24 M L oxygen adatom pre-coverage, the initial heat is only 250 U mol(-1) and with 0.6 ML oxygen adatom, NO adsorption is only molecular with an initial heat of 160 U mol(-1). The NO sticking probability behaviour is consistent with this picture, with successive precursor mediated adsorption at step and terrace sites. The inhibition of dissociation above O, or O plus N, adatom coverages of 1 ML is attributed to the strong lateral repulsive interactions between adatoms, which would drive the dissociative heat of adsorption below that of molecular adsorption at higher coverages. (C) 2003 Published by Elsevier B.V.