Sulfate ions adsorbed on Au(hkl) electrodes:: in situ vibrational spectroscopy

The adsorption of sulfate and bisulfate ions has been studied on Au(111), Au(100) and Au(110) single crystal electrodes using in situ reflection absorption IR spectroscopy. It has been found that on Au(111) and Au(100) the adsorbed sulfate presents only two observable bands, coherent with C-3v or C-2v symmetries. Comparing the O-O distances of the sulfate ion with the Au-Au distance at the surface, a single coordination for adsorbed sulfate on Au(111) and Au(100) has been proposed, since both electrodes present the same results for adsorbed sulfate. Only a very small amount of bisulfate ions has been detected for Au(111) electrodes at very low pH (0.23). For Au(100) a larger amount of bisulfate than on Au(111) co-adsorbed with sulfate ions has been observed. The onset of adsorption for both electrodes coincides with the lifting of reconstruction. At Au(111) the adsorption begins to be observable with in situ IR spectroscopy at similar to 0.7 V, while for Au(100) at 0.6 V. The adsorbed sulfate on Au(110) electrode presents three IR bands, indicating that at least part of the adsorbed sulfate is tilted at the surface revealing the surface selection rule forbidden bands. The symmetric band for adsorbed sulfate on Au(110) surface has been observed at 1177-1198 cm(-1) and 960 cm(-1) and the unsymmetric stretching at 1130 and 1100 cm(-1). (C) 1999 Elsevier Science S.A. All rights reserved.