Adsorption and reactivity during low temperature oxidation of carbon monoxide on Au/TiO₂ catalysts studied by rapid response mass spectrometry

The remarkable activity of Au/TiO₂ for CO oxidation at ambient temperature resides in a very high turnover frequency on sites at the interface between the metal and oxide as shown by a study of adsorption and reaction of CO, CO₂, and O₂ on TiO₂ and Au/TiO₂. Rapid response mass spectrometry found that adsorption of CO on TiO₂ at 300 K is rapidly reversible and less on reduced samples than on oxidized ones, which indicates that the adsorption sites are oxide ions. The amount adsorbed reversibly on reduced Au/TiO₂ is less than that on TiO₂ itself, but additional quantities are taken up more slowly and irreversibly. Adsorption of O₂ is undetectable on a time scale of seconds but that of CO₂ is an order of magnitude greater than that of CO. Under catalytic conditions, the oxidation of CO to CO₂ on Au/TiO₂ starts almost instantly and appears to involve rapid turnover of the irreversibly adsorbed form of CO. There is some concurrent uptake of O₂ beyond that required for catalysis on initial exposure and this capacity is increased if reaction mixtures and individual reactants are alternated, possibly due to the generation of reducible interface sites.