The reactivity of hydroxyl groups toward ammonia on Ni(1 1 0) surfaces

The chemistry of ammonia on Ni(1 1 0) single-crystal surfaces in the presence of hydroxyl (OH) groups was characterized by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy. Submonolayer coverages of OH groups on the Ni(1 1 0) surface were shown to exert a strong influence on the thermal chemistry of ammonia. In particular, NH3 TPD spectra from NH3/OH/Ni(1 1 0) display the following significant changes from those obtained with the NH3/Ni(1 1 0) system: (1) the appearance of high-temperature ammonia, water and hydrogen peaks at ∼400 K; (2) an overall reduction of the ammonia that adsorbs directly on Ni sites (the α-NH3 state); and (3) an upward shift in the desorption temperature of the NH3 hydrogen bonded to other surface species (the β-NH3 state). Although desorption of ammonia at ∼400 K is also seen in the NH3/O/Ni(1 1 0) system, the changes in the α- and β-NH3 states are much less evident there than with the surface hydroxyls. A similar behavior was also observed in the thermal chemistry of ammonia in the presence of surface water, particularly when activated by appropriate annealing prior to ammonia dosing. The influence of OH species on the thermal chemistry of ammonia on Ni(1 1 0) is interpreted in terms of the adsorbing geometry of the OH groups on the surface and of hydrogen bonding between adsorbed OH and NH3 species.