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Solvation and Reaction of Ammonia in Molecularly Thin Water Films Barbara A. J. Lechner, Youngsoon Kim, Peter J. Feibelman, Graeme Henkelman, Heon Kang, and Miquel Salmeron J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07525 • Publication Date (Web): 17 Sep 2015 Downloaded from http://pubs.acs.org on September 19, 2015
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The Journal of Physical Chemistry
Solvation and Reaction of Ammonia in Molecularly Thin Water Films Barbara A. J. Lechner,a† Youngsoon Kim,b† Peter J. Feibelman,c Graeme Henkelman,d Heon Kangb and Miquel Salmerona* a
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA b
Department of Chemistry, Seoul National University, Seoul, 151-747, Korea c
d
1401 Sigma Chi Road NE, Albuquerque, New Mexico 87106, USA
Department of Chemistry and the Institute for Computational Engineering and Sciences, The University of Texas at Austin, Austin, Texas 78712-0165, USA
ABSTRACT. Determining the interaction and solvation structure of molecules with solvents near a surface is of fundamental importance for understanding electro- and photochemical processes. Here we used scanning tunneling microscopy (STM) to investigate the adsorption and solvation structure of ammonia on water monolayers on Pt(111). We found that at low coverage, NH3 binds preferentially to H2O molecules that are slightly elevated from the surface and weakly bound to the metal. Density Functional Theory (DFT) calculations showed that as the NH3 molecule descends onto the water adlayer, a high-lying water molecule reorients with zero
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energy barrier to expose a dangling OH ligand to H-bond NH3. We also found that NH3 prefers to bind to the metal substrate when water only partially covers the surface, indicating that NH3 is more strongly attracted to the metal than to H2O. In addition to this solvation interaction, a proton transfer reaction occurs as revealed by reflection-absorption infrared spectroscopy (RAIRS), leading to the formation of ammonium ions (NH4+) in addition to molecularly adsorbed NH3.
INTRODUCTION Because of the economic importance of corrosion, industrial catalysis, and electrochemistry, the interplay of hydrogen bonding and water–metal interactions has been studied extensively.1–3 As a result of such efforts, the first wetting layers on many metals at cryogenic temperatures were found to have no dangling H bonds,2,4 and thus, potentially, to behave hydrophobically; water monolayers on Pt(111) are a representative example.5 The hydrogen atoms in the first wetting layer either form H-bonds to neighboring molecules or point towards the surface.4,6–8 Growth of metastable multilayer water films that preserve an H-down orientation of water has been proposed to occur on Pt(111), resulting in the formation of “ferroelectric ice”.9 The growth of entropically favorable proton-disordered ice requires rotating some of the molecules in the first layer to expose dangling H atoms.10,11 In addition to energetic effects, kinetic hindrance of such molecular reorientation has been invoked as the reason for the hydrophobic character of water monolayer films.4,12 In previous work, the adsorption of CO and N2 at 20 K was found to flip some H-down water molecules into an H-up configuration on Pt(111),13 implying that the flipping energy barrier is low.
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The Journal of Physical Chemistry
On the more reactive Ru(0001) surface, known to dissociate H2O 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 H2O monolayer is acidic enough to protonate NH3 to form NH4+.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. The present study, which is part of an effort to understand solvation phenomena, provides the first real-space images of molecular adsorption on a water monolayer and the initial adsorption sites, which correspond to slightly lifted molecules. Through DFT calculations we discovered that on Pt(111) the adsorption causes such water molecules to rotate and expose an H atom to form a hydrogen bond with the incoming NH3. Most notably, such a process proceeds without an energy barrier. Further, we have found that ammonia bonds more strongly to Pt than to H2O and thus segregates from water. This preference of adsorption on the metal is in contrast with the intimately mixed structures on Cu(110) predicted by DFT calculations,17 a behavior that we attribute to attraction between NH3 and the unfilled d-bands of Pt.
METHODS STM experiments were performed in a home-built low-temperature microscope described elsewhere.18 A Pt(111) single crystal (Princeton Scientific) was mounted in an ultra-high vacuum (UHV) chamber with a base pressure of 100 nm. Water (Sigma Aldrich, deuterium depleted,
