Adsorption of Ammonia and Its Influence on Coadsorbed Carbon Monoxide on Monolayer and Multilayer Palladium Epitaxially Grown on Mo(110)

Abstract

The adsorption of ammonia on a monolayer of Pd on Mo(110), Pd1ML/Mo(110), and multilayer Pd(111)/Mo(110) surfaces has been studied using low-energy electron diffraction (LEED), temperature-programmed desorption (TPD), and high-resolution electron energy loss spectroscopy (HREELS). A diffuse (2 × 2) LEED pattern has been observed for ammonia on the Pd1ML/Mo(110) surface. TPD measurements indicate a shift of the desorption peak maximum of ammonia to a lower temperature for monolayer palladium compared to multilayer palladium. However, both surfaces display virtually identical HREELS spectra with the symmetric deformation (umbrella mode) being the dominating feature, indicating an upright adsorption geometry on both surfaces. The coadsorption of NH3 and CO has been also studied with HREELS over a wide coverage range on both surfaces. A strong interaction between adsorbed NH3 and CO is indicated by the red shift of the CO stretching vibrational mode (as large as 400 cm-1) and a blue shift of the NH3 symmetric deformation (up to 100 cm-1). On Pd1ML/Mo(110), the coadsorption of NH3 induces a CO adsorption site change from the atop site to the three-hollow site. Evidence has been also found for a significantly inclined NH3 molecule in the coadsorbed layer with a relatively high CO coverage.