Formation of hydronium and methoxonium on Pt(110): Ab initio determinationof spectroscopically observed species

Previously, other researchers have examined the coadsorption of water and h ydrogen on Pt(111) with electron energy loss spectroscopy (EELS) and have s een the formation, of a new species. This new species was originally design ated to be a ''hydronium'' species, where hydronium referred to H3O+ surrou nded by water. However, the formation of H3O+ on platinum has not been wide ly accepted in the literature because the formation of a bare H3O+ not surr ounded by water is not a thermodynamically favored process. In this work, w e use abinitio calculations at the MP2(full)/6-31g* level to predict the ga s phase frequencies fora host, of water and water-cluster species. These fr equencies are compared to high resolution EELS spectra to identify which sp ecies form on Pt(110). We find that H5O2+, H7O3+, and H9O4+, fit the EELS s pectra quite well, while H3O, H3O-, and H3O+ do not show the right vibratio nal modes. We have alsb done similar calculations for methanol. Methanol ha s a higher proton affinity than water, so it seems possible that methanol c ould also form a positively charged species like methoxonium, CH3OH2+. Once again, EELS spectra show a new species forms on Pt(110). Ab initio calcula tions are;compared to a high resolution EELS spectrum of hydrogen coadsorbe d with methanol on platinum. The most likely species to form here is methox onium, possibly in a methanol cluster. This conclusion is supported by addi tional spectra of deuterated species that show the correct isotopic shifts in frequencies predicted by calculations. The conclusion from our study is that ions can form during coadsorption of hydrogen and water and hydrogen a nd methanol on Pt(110). The ions are probably hydrated, but the further EEL S evidence strongly supports ion formation. (C) 1999 American Vacuum Societ y [S0734-2101(99)19404-1].