Effect of metal nanoparticle size and titania crystallinity on the performance of Au/TiO2 catalysts for the promotion of carbon monoxide oxidation at cryogenic temperatures

The ability of Au/TiO2 catalysts to promote the oxidation of carbon monoxide at cryogenic temperatures was probed by using diffuse-reflectance infrared absorption spectroscopy (DRIFTS). The focus of this work was to identify correlations between the catalytic activity and both the nature of the titania support and the size of the Au nanoparticles (NPs). Two key intermediates were identified: an initial CO adsorbed on defect sites within the titania support with a 2175 cm(-1) C-O stretching frequency signature, and a second species with a DRIFTS peak at 2163 cm(-1) associated with the intermediate at Au-TiO2 interfacial sites that incorporates atomic oxygen to produce CO2. CO adsorption at the low temperatures associated with this process (120 K) appears to require defective sites, possibly titanate-type surface species: an amorphous (and NaOH-treated) support showed higher activity than crystalline titania. In addition, the catalytic activity peaks at an Au NP average size of about 3 nm, which, in turn, correlates with the population of the second CO intermediate mentioned above. The mechanism that explains this cryogenic oxidation reaction is different than that extensively reported for the room-temperature processes, but both appear to display similar dependence on Au NP diameter.