Probing boundary sites on a Pt/Al2O3 model catalyst by CO2 hydrogenation and in situ ATR-IR spectroscopy of catalytic solid-liquid interfaces

In situ ATR infrared spectroscopy was used to follow the reaction of CO2 with hydrogen at the solid liquid interface of a model Pt/Al2O3 catalyst in cyclohexane at 313 K. CO2 adsorbs on Al2O3 to form carbonate-like species, which react with hydrogen to form CO. The observed spectrum exhibits unusually low vibrational frequencies and ratio of linearly to bridged bonded CO. In contrast, CO adsorbed from the solution onto the Pt/Al2O3 model catalyst results in a normal spectrum for CO on Pt. The results suggest that the reaction of CO2 and hydrogen takes place at the boundary sites of the interface between the Pt and the Al2O3 and that the resulting CO selectively probes these sites. The unusual CO spectrum indicates that these sites are characterised by a low ionisation potential of the d-valence orbitals of Pt. It is likely that the unusual property of these Pt sites are caused by a nearby oxygen vacancy of the Al2O3, onto which the CO2 initially adsorbs.