Vibrational spectra of ammonia chemisorbed on platinum (111): I. Identification of chemisorbed states

Ammonia chemisorption on Pt(111) has been studied with high resolution electron energy loss spectroscopy (EELS), combined with thermal desorption spectroscopy (TDS). We detect two distinct molecular states of ammonia at different coverages. Near saturation monolayer coverage, ammonia is weakly chemisorbed (ΔH_ads ～ 9 kcal mol^?1) and coordinated to the metal surface via the nitrogen atom. The vibrational frequencies are shifted from the gas phase values, but not as strongly perturbed as in stable platinum-ammine complexes. Below 40% saturation coverage, a new molecular ammonia state is detected, which has a distinct vibrational and thermal desorption spectrum. This state has a considerably reduced ν(PtN) intensity, and the other frequencies are closer to those in solid ammonia, indicating a weaker interaction with the Pt surface. The thermal desorption spectrum of this lower coverage state is broad, from 160 to 450 K, and coverage dependent. Conversion between the two molecular states appears to be only a function of coverage. We propose that the two molecular states have different adsorption sites, and convert from one form to the other as the coverage is changed. No evidence is found for significant dissociation of ammonia on the Pt surface. At very low temperatures (100 K), solid ammonia multilayers may be grown.