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Solvation and Reaction of Ammonia in Molecularly Thin Water Films
Lechner, Barbara A. J.,Kim, Youngsoon,Feibelman, Peter J.,Henkelman, Graeme,Kang, Heon,Salmeron, Miquel American Chemical Society 2015 The Journal of Physical Chemistry Part C Vol.119 No.40
<P>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 NH<SUB>3</SUB> binds preferentially to H<SUB>2</SUB>O molecules that are slightly elevated from the surface and weakly bound to the metal. Density functional theory (DFT) calculations showed that as the NH<SUB>3</SUB> molecule descends onto the water adlayer a high-lying water molecule reorients with zero energy barrier to expose a dangling OH ligand to H-bond NH<SUB>3</SUB>. We also found that NH<SUB>3</SUB> prefers to bind to the metal substrate when water only partially covers the surface, indicating that NH<SUB>3</SUB> is more strongly attracted to the metal than to H<SUB>2</SUB>O. 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 (NH<SUB>4</SUB><SUP>+</SUP>) in addition to molecularly adsorbed NH<SUB>3</SUB>.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2015/jpccck.2015.119.issue-40/acs.jpcc.5b07525/production/images/medium/jp-2015-07525v_0007.gif'></P>