Adsorption of NO and H,O/NO coadsorption at Ni(110) surfaces

Abstract

Adsorption of NO and H,O/NO coadsorption at Ni(110) surfaces has been studied using thermal desorption spectroscopy, low-energy electron diffraction, Auger electron spectroscopy, and high-resolution electron energy-loss spectroscopy. At 110 K up to Θ=0.2 NO adsorbs dissociatively. Further uptake occurs at top (1875 cm−1 at Θ=0.5) and bridge (1675 cm−1 at Θ=0.5) sites, initially predominantly at bridge sites. No vibration due to inclined NO is observed, nor a Ni–N vibration loss. At saturation coverage, Θ=0.5, the ratio of the site population is top/bridge=1.7 as deduced from the HREELS intensities. Decomposition of bridge bonded NO starts around 190 K. Simultaneously, bridge bonded NO is shifted to top sites due to a blocking effect of the decomposition product oxygen. Above 340 K the top site NO starts to decompose (predominantly) and desorb, leaving a N and O covered surface. Activation energies for desorption and decomposition are ∼20 kcal/mol. Of the decomposition products oxygen diffuses into the bulk, T>550 K, and nitrogen desorbs recombinatively around 850 K. After preadsorption of hydrogen at the Ni(110) surface NO top site adsorption decreases and is suppressed after 0.5-L H2 exposure. Bridge bonded NO with coadsorbed H present exhibits an enhanced stability with respect to dissociation and starts to decompose only after hydrogen is desorbed. Conversely, hydrogen bonding to the Ni surface is destabilized and the H induced surface reconstruction suppressed. After preadsorption of oxygen NO can be postadsorbed even if the substrate is oxidized, however, in slightly reduced quantity only. Coadsorbed oxygen favors NO adsorption at top sites; at oxygen saturation postadsorbed NO only occupies top sites.