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

Previously, other researchers have examined the coadsorption of water and hydrogen on Pt(111) with electron energy loss spectroscopy (EELS) and have seen the formation of a new species. This new species was originally designated to be a "hydronium" species, where hydronium referred to H₃O⁺ surrounded by water. However, the formation of H₃O⁺ on platinum has not been widely accepted in the literature because the formation of a bare H₃O⁺ not surrounded by water is not a thermodynamically favored process. In this work, we use ab initio calculations at the MP2(full)/6-31g* level to predict the gas phase frequencies for a host of water and water-cluster species. These frequencies are compared to high resolution EELS spectra to identify which species form on Pt(110). We find that H₅O⁺₂, H₇O⁺₃, and H₉O⁺₄ fit the EELS spectra quite well, while H₃O, H₃O^-, and H₃O⁺ do not show the right vibrational modes. We have also done similar calculations for methanol. Methanol has a higher proton affinity than water, so it seems possible that methanol could also form a positively charged species like methoxonium, CH₃OH⁺₂. Once again, EELS spectra show a new species forms on Pt(110). Ab initio calculations are compared to a high resolution EELS spectrum of hydrogen coadsorbed with methanol on platinum. The most likely species to form here is methoxonium, possibly in a methanol cluster. This conclusion is supported by additional 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 and methanol on Pt(110). The ions are probably hydrated, but the further EELS evidence strongly supports ion formation.