COORDINATION-NUMBER AND MOLECULAR-ORIENTATION OF HYDRONIUM CATION BISULFATE ANION ADLAYERS ON PT(111)

The molecular orientation and coordination number of a hydronium catio n (H3O+) coadsorbed with a bisulfate anion (HSO4-) and water on Pt(111 ) were studied by infrared reflection absorption spectroscopy (IRAS) a nd thermal desorption spectroscopy (TDS). H3O+ was adsorbed on Pt(111) at 110 K with its molecular C-3 axis perpendicular to the surface. Ho wever, the H3O+ molecule, when coadsorbed with HSO4-, changed the mole cular orientation on Pt(111) with the C-3 axis parallel to the surface . The hydration number per HSO4- or H3O+ on Pt(111) was examined by TD S measurements as a function of theta(HSO4)-. The lower and higher HSO 4- coverages produced larger and smaller amounts of water molecules in the coordination number, respectively. The results agreed well with t hose of in-situ IRAS.