The adsorption of NH 3 on Rh(111)

Abstract

The adsorption of NH 3 on Rh(111) has been investigated by temperature programmed desorption, work function measurements, low energy electron diffraction and secondary ion mass spectrometry. TPD indicates the existence of three distinct desorption states; α 1-, α 2- and β-NH 3 with peak maxima at 320, 155 and 130 K, respectively. For the most strongly chemisorbed state, α 1-NH 3, the desorption energy is 81.5 kJ mol −1. A (2 × 2) LEED pattern is observed after complete filling of the α 1- and α 2-NH 3 states, which is attributed to saturation of the first adsorption layer, corresponding to an NH 3 coverage of 0.25 ML. The sticking coefficient for NH 3 adsorption at 120 K, is on the order of unity and independent of coverage during filling of the first adsorption layer. NH 3 adsorption causes a significant work function decrease of 2.4 eV below the value of clean Rh(111). The initial work function decrease corresponds to an average dipole per NH 3 molecule of 1.9 D, i.e. higher than that of the NH 3 molecule in the gas phase. SIMS spectra of NH 3 on Rh(111) contain Rh(NH 3) + as the predominant ammonia-derived cluster ion. The intensity of the Rh(NH 3) + cluster ion behaves strongly non-linear with the NH 3 coverage and indicates some changes in the NH 3 adlayer at a coverage of ≈0.12 ML.