Coadsorption-Induced Azimuthal Ordering in Molecular Adsorbate Layers:H2O and NH3

Abstract

Traces of preadsorbed oxygen on a metal surface will induce a high degree of azimuthal order in adsorbed molecules which are disordered azimuthally on the clean surface. This azimuthal-ordering effect has been studied for both NH 3 and H 2 0 on Ni(lll).PACS numbers: 68.20.+t, 68.55.+b, 79.20.Kz The surface chemistry of a number of molecules which are weakly chemisorbed on clean metal surfaces has been shown recently to be influenced profoundly by coadsorption of the molecules with oxygen. The adsorption energies of several hydrogen-containing molecules such as methanol, 1 ethanol, 1 H 2 0, 2 " 5 and NH 3 4 are apparently increased as a result of hydrogen bonding to preadsorbed oxygen atoms on surfaces of Cu, Ag, Pt, and Ir. In most instances, evidence for hydrogen abstraction to form adsorbed OH along with an adsorbed radical fragment 1 " 5 has been seen [e.g., CH 3 OH(ads) + 0(ads)-~ CH 3 0(ads) + OH (ads)].In this paper, we present the first direct evidence that traces of preadsorbed oxygen on a metal surface will induce a high degree of azimuthal order in adsorbed molecules which are disordered azimuthally on the clean surface. We have used electron-simulated-desorption ion angular distributions (ESDIAD), low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD) to study the adsorption of H 2 0 and NH 3 on a Ni(lll) surface, both clean and with preadsorbed oxygen. The ESDIAD method 6 " 8 is well suited for studies of the local geometrical structure of adsorbed molecules, primarily because the directions of ion desorption are directly related to the orientations of the surface bonds broken by electron excitation. Ultraviolet photoemission spectroscopy (UPS) has been used previously to show that NH 3 bonds to clean Ni(lll) via the N atom, 9 with the H atoms oriented away from the surface; molecular H 2 0 bonded to Ni films, 10 Pt(lll), 3 and Ru(OOl) (Ref. 7) is similarly adsorbed via the O atoms, with the H atoms oriented away from the surface. For NH 3 (Ref. 6) and H 2 0 on Ni(lll) there is little or no azimuthal ordering seen in ESDIAD studies for fractional monolayers on the clean surface. For the adsorption of either molecule, however, traces of oxygen preadsorbed on Ni(lll) result in sharp threefold ion-desorption patterns indicative of azimuthal ordering in the adsorbed layer due to hydrogen bonding. The implications of this oxygen-induced surface ordering for the mechanisms of catalytic promoters and poisons are evident.The ultrahigh-vacuum apparatus and methods used for the studies have been described previously in detail. 8 ESDIAD and LEED patterns were viewed directly on a fluorescent screen following image intensification of the desorbingion (or scattered-electron) signal using a double microchannel plate detector in a hemispherical retarding grid analyzer. NH 3 and H 2 0 were deposited onto the front surface of the Ni(lll) sample with use of a calibrated molecular beam doser having a microcapillary array as an effusion source. Mass analyses of ESD ions and of thermal deso...

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