Formation of hydronium and water-hydronium complexes during coadsorption of hydrogen and water on (2x1)Pt(110)

In the previous literature, people have found that a new species forms when hydrogen and water coadsorb on various faces of platinum. The new species was identified as "hydronium" based on a preliminary analysis of electron energy loss spectroscopy (EELS) spectra. At present, this assignment is not well accepted in the literature since thermodynamically a bare hydronium ion would not be stable on platinum. In this paper we do the measurements and the calculations to tie down the identity of the species. Ab initio methods are used to estimate the vibrational spectrum of H3O+, and [(H2O)(x)H+] with x = 2, 3 or 4, H3O radicals, H-2-H2O complexes and H3O-. We then measured better quality EELS spectra than had appeared in the previous literature for coadsorption of hydrogen and water on Pt(110). By comparing our experimental spectra and calculated spectra, we found the calculated spectrum of H5O2+ and H7O3+ fit our experimental data the best. These results suggest that the species which forms when hydrogen and water coadsorb on platinum is a cationic species similar to hydronium. It is probably not H3O+. However, it is likely that the surface includes a mixture of hydrated intermediates such as H3O2+, H7O3+ and H9O4+. (C) 1999 Elsevier Science B.V. All rights reserved.