NO DISSOCIATION ON RHODIUM

The desorption and dissociation of nitric oxide on a polycrystalline R h foil were studied by means of combined temperature-programmed desorp tion (TPD) and static secondary ion mass spectrometry (SSIMS). The lat ter method was employed either under isothermal conditions or in a tem perature-programmed mode (TPSSIMS). After carefully calibrating the se condary ion intensities by means of TPD, the NO dissociation kinetics were quantitatively evaluated for low NO coverages. Fitting the SSIMS data with a Polanyi-Wigner ansatz and assuming first-order kinetics, w e found an activation energy, E = 8 +/- 2 kJ/mol, and a preexponential , v = 10(-0.2+/-0.4) s(-1). The data were confirmed by independent iso thermal SSIMS measurements at several temperatures between 80 and 330 K. The fairly unusual rate parameters are discussed in relation to sim ilar results obtained recently from measurements on Rh single crystal surfaces.