Revisiting the Au Particle Size Effect on TiO2-Coated Au/TiO2 Catalysts in CO Oxidation Reaction

It is well-known that the particle size of Au nanoparticles enormously affects the catalytic activity of supported gold catalysts in many reactions. The origin of the Au particle size effect is still widely debated. In this work, we precisely deposited different thicknesses of ultrathin TiO2 overcoats onto three Au/TiO2 catalysts with Au particle sizes of 2.9 +/- 0.6 (Au/TiO2-S), 5.0 +/- 0.8 nm (Au/TiO2-M), and 10.2 +/- 1.6 nm (Au/TiO2-L) using atomic layer deposition (ALD). High-resolution transmission electron microscopy illustrated the Au nanoparticles on these three samples were encapsulated by the TiO2 overcoat. X-ray photoelectron spectroscopy measurements showed that the Au nanoparticles remained at metallic state after applying TiO2 ALD overcoat. Diffuse reflectance infrared Fourier transform spectroscopy measurements of CO chemisorption further revealed that the TiO2 overcoat preferentially decorated at the low-coordination sites of Au nanoparticles and broadly tuned the population of these sites accessible for participating in reactions. In CO oxidation reaction, the TiO2 coated Au/TiO2-S catalysts strikingly demonstrated considerably higher activities than the uncoated Au/TiO2-M and Au/TiO2-L catalysts, even though the former ones contained significantly less amount of CO adsorption sites due to the TiO2 overcoating. Our work shows direct evidence that CO adsorption on the low-coordination Au sites is not the rate-determining step, and the Au particle size effect in CO oxidation is NOT related with either the number of the low-coordination Au sites or the changes in oxidation states. Size-related change in the length of perimeter sites at the Au-TiO2 interface could certainly play a role in the Au particle size effect.