Solvation and Reaction of Ammonia in Molecularly Thin Water Films

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 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.