Structure and stability of CN adlayers on Rh(110)

The formation and stability of CN adlayers on Rh(110), formed by dissociative adsorption of C2N2 at 373 K, have been studied as a function of coverage and temperature by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS). Two different CN adsorption states have been distinguished by their different C 1s and N 1s XPS core-level binding energies. The CN-I state is exclusively occupied up to a surface coverage of 0.5 monolayers (ML), where a well-ordered c(2×2) LEED pattern is observed. The CN-II state becomes additionally populated at higher coverages from 0.5 ML to the saturation coverage of 0.87 ML. At CN saturation, a c(4×2) LEED structure is formed. Desorption of CN as molecular C2N2 occurs only for surface coverages >0.5 ML and appears to be mainly derived from the CN-II state. The onset of CN bond rupture is indicated at ∼450–550 K, depending on the CN coverage; the resulting Nad desorbs at ∼580 K from the more crowded surface and in the range of ∼650–950 K, whereas Cad remains at the Rh surface and cannot be desorbed thermally.