Effect of Coadsorbed Oxygen on the Chemistry of Ammonia over Ni(110) Single-Crystal Surfaces

Abstract

The surface chemistry of ammonia coadsorbed with atomic oxygen on Ni(110) single-crystal surfaces was studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). At intermediate oxygen coverages, direct ammonia adsorption on nickel sites is suppressed, but a new high-temperature reaction regime is generated. Desorption of ammonia from rehydrogenation of NH x surface fragments (identified by XPS) takes place around 400 K, concurrently with the production of water and molecular hydrogen. Experiments with deuterium labeling indicated extensive isotope scrambling and hydrogen transfer from nitrogen- to oxygen-containing surface intermediates. The optimum yields seen for this 400 K state at intermediate oxygen coverages strongly suggest the direct interaction of the adsorbed ammonia with oxygen atoms at the end of the −Ni−O− rows that form upon reconstruction of the surface. Hydrogen transfer between ammonia and oxygen appears to take place directly via hydrogen bonding, and to be reversible but biased toward water formation. An equilibrium is reached between the produced water and the reacting surface oxygen and hydrogen.

Thermal chemistry of diiodomethane on Ni(110) surfaces II. Effect of coadsorbed oxygen

“…In fact, hydroxide surface groups have been previously shown to facilitate both oxygen insertion and b-hydride elimination steps [9,35,36,38]. Similar chemistry has also been reported for both water [43,44] and ammonia [73,74] coadsorbed with oxygen on nickel surfaces. In those cases the facilitating role of oxygen in dehydrogenation was explained by the potential formation of hydrogen bonds on the surface.…”

Selective Oxidation of Ammonia by Co-adsorbed Oxygen on Iridium Surfaces: Formation of N2O

“…O 2 for different NH 3 /O 2 pressure ratios with 1 9 10 -7 mbar NH 3 [9]. However, N 2 O was not detected during TPD from a co-adsorbed oxygen and ammonia layer under UHV conditions (\1 9 10 -9 Torr) on Ir(110) [9], Ir(111) [9,15], Ir(510) [34], Ru(0001) [33], RuO 2 (110) [10], Au(111) [12], Pt(410) [35], Pt(100) [6,36], Pt(111) [37], Ni(110) [38], and Rh(111) [4]. In particular, Baerns and coworkers have investigated the reaction over a wide range of pressures and temperatures on various Pt catalysts including single crystal Pt(533), knitted Pt gauze and Pt foil [11].…”

Varied Thermal Chemistry of Hydrocarbons on Nickel Single-Crystal Surfaces

“…The experiments reported here were carried out in one of two ultrahigh vacuum (UHV) instruments described in detail previously [7,[53][54][55][56]. They both consist of stainless steel vessels pumped with turbomolecular pumps to a base pressure on the order of 1 9 10 -10 Torr.…”