Evidence for carbocation formation during the coadsorption of methanol and hydrogen on Pt(110)

There are reasons to suspect that carbocations can form during hydrocarbon adsorption on metal surfaces, but so far no one has observed a carbocation on any metal surface spectroscopically. In this paper, we examine the coadsorption of methanol and hydrogen on Pt(110) to see whether a methoxonium species [CH₃OH₂]⁺ can form. We find that a new species does form when we coadsorb hydrogen and methanol on Pt(110) at 100 K and then heat to 200 K. The new species has an EELS spectrum which is consistent with the one expected for a methoxonium ion, and has the isotopic shifts expected for methoxonium. Comparison to the EELS spectrum estimated from ab initio calculations is used to verify the EELS assignment, and to rule out the possibility that a methoxonium radical or other similar species is formed instead. We also observe the chemistry expected when a carbocation forms (i.e. dehydration). These results provide strong evidence that methoxonium forms when hydrogen and methanol coadsorb on platinum. This is the first time that a carbocation has been observed spectroscopically on a metal surface.