Spectroscopic Monitoring of the Acidity of Water Films on Ru(0001): Orientation‐Specific Acidity of Adsorbed Water

Abstract

Abstract:We examined the acid-base properties of water films adsorbed onto a Ru(0001) substrate by using surface spectroscopic methods in vacuum environments. Ammonia adsorption experiments combined with low-energy sputtering (LES), reactive ion scattering (RIS), reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) measurements showed that the adsorbed water is acidic enough to transfer protons to ammonia. Only the water molecules in an intact water monolayer and water clusters larger than the hexamer exhibit such acidity, whereas small clusters, a thick ice film or a partially dissociated water monolayer that contains OH, H 2 O and H species are not acidic. The observations indicate the orientation-specific acidity of adsorbed water. The acidity stems from water molecules with H-down adsorption geometry present in the monolayer. However, the dissociation of water into H and OH on the surface does not promote but rather suppresses the proton transfer to ammonia.

Solvation and Reaction of Ammonia in Molecularly Thin Water Films

“…Figure b) the intensity of the dangling OH band decreases, also in line with the STM observation that ammonia bonds to the dangling H of the second layer water molecules to form an O–H···N bond. At the same time, two strong bands at ∼1110 and ∼1470 cm –1 and a weaker one at ∼1250 cm –1 appear, which can be assigned to the ν 2 (umbrella) mode of NH 3 adsorbed on water, the ν 4 (asymmetric deformation) mode of ammonium (NH 4 + ), ,,− and the ν 2 mode of NH 3 bonded to both Pt and H 2 O, e.g., at the edges of holes in the ML, , respectively. RAIRS experiments thus indicate that ammonia can adsorb onto the water ML, bonding to the dangling H atom of second layer H 2 O and also to the Pt substrate, where available.…”

“…The conspicuous absence of a free O−H stretch band at ∼3710 cm −1 , characteristic of dangling OH groups, indicates that the clean R37 water layer exposes no unsaturated H atoms. 4 Adsorption of ammonia onto this surface results in vibrational features for NH 3 16 Atomic-Scale Details of the NH 3 Adsorption Process. Our STM measurements show that NH 3 only bonds to regions in an R3 and R37 water film where H 2 O molecules are lifted off the surface and thus weakly bonded to the metal, suggesting that an H 2 O reorients to accommodate the NH 3 adsorbate.…”

“…+ is likely the more mobile species on H 2 O than NH 3 and can thus move off the H 2 O islands onto bare Pt under the STM tip, where it can decompose into NH 3 and H. 16 Atomic-Scale Details of the NH 3 Adsorption Process. Our STM measurements show that NH 3 only bonds to regions in an R3 and R37 water film where H 2 O molecules are lifted off the surface and thus weakly bonded to the metal, suggesting that an H 2 O reorients to accommodate the NH 3 adsorbate.…”

“…RAIRS thus suggests that, in addition to H-bonding NH 3 , the water provides protons to form ammonium ions. As the N–H···O bond is weaker than the O–H···N bond, NH 4 + is likely the more mobile species on H 2 O than NH 3 and can thus move off the H 2 O islands onto bare Pt under the STM tip, where it can decompose into NH 3 and H …”

“…On the more reactive Ru(0001) surface, known to dissociate H 2 O at temperatures as low as 138 K, 14 low-energy sputtering (LES), reactive ion scattering (RIS), and reflection−absorption infrared spectroscopy (RAIRS) showed that the H 2 O monolayer is acidic enough to protonate NH 3 to form NH 4 + . 15,16 However, owing to the spatially averaging nature of the techniques used in these experiments, no information could be obtained on the structure and spatial distribution of the products formed.…”