Ammonia cluster formation and NH3-decomposition on an Ni(100)-surface

We studied adsorption of NH3 on a clean and nitrogen predosed Ni(100) surface at 80 K. Adsorption experiments on a Ni(100) surface with a well defined atomic nitrogen coverage of ϑ = 0.5 allow us to correlate the observed N(1s) core level shifts of ammonia with absolute coverages. We conclude that at fractional coverages of ϑ ≳ 0.22 NH3 cluster formation occurs. Comparison with published data for NH3 adsorption on a Pt(111) and W(110) surface indicate that clustering of NH3 at small coverages is a general phenomenon on metal surfaces. Angle-resolved N(1s) XPS experiments on Ni(100) suggest that clustering involves hydrogen bridge bonding between the chemisorbed and “second layer” ammonia molecules. A simple calculation based on our model for second layer formation provides a more quantitative understanding of ammonia flash desorption spectra from metal surfaces. We also present data for NH3 decomposition on a clean and oxygen predosed Ni(100) surface, and show that the presence of oxygen exerts a strong influence on NH3 decomposition