Evidence for a cation intermediate during methanol dehydration on Pt(110)

Cations are thermodynamically stable on certain transition metal surfaces, but so far no one has determined whether cations are catalytic intermediate s or merely spectators. In this paper TPD and HREELS are used to determine whether methoxonium cations ([CH3OH2](+)((ad))) or closely related species are intermediates during the reaction H-2 + CH3OH --> CH4 + H2O on Pt(110). HREELS shows that the methoxonium cations are produced at the same tempera ture where reaction starts, and that the methoxonium cations decay in the s ame temperature range where the reaction stops, suggesting that methoxonium is a likely intermediate during the reaction. Replacement of CH3OH with CD 3OH produces a 1.8-fold change in the yield of water and methane at 180 K d ue to a secondary kinetic isotope effect. The strong secondary kinetic isot ope effect would be expected only if the reaction occurred via an S(N)1 pat hway with a fully ionized transition state with the positive charge on the carbon. Together the HREELS and TPD results provide strong evidence that re action goes via a carbocation pathway. with methoxonium cations or some clo sely related species as likely intermediates. This paper demonstrates for t he first time that carbocations can be reactive intermediates during reacti ons on metal surfaces.