Effect of alumina and titania on the oxidation of CO over Au nanoparticles evaluated by 13C isotopic transient analysis

Abstract

Titania- and alumina-supported Au nanoparticles were synthesized by a deposition-precipitation method and subsequent thermal treatment in He. X-ray absorption spectroscopy at the Au-L III edge revealed that the as-prepared Au/TiO 2 sample contained cationic Au that was reduced to a predominately metallic state after treatment in He at 623 K. Scanning transmission electron microscopy showed the Au to be highly dispersed over both the metal oxides, with an average particle size of 3.3 nm for Au/TiO 2 and 2.5 nm for Au/Al 2O 3. The global rate, apparent activation energy, and orders of reaction with respect to CO and O 2 of CO oxidation were different for the two metal oxide-supported samples. Steady-state isotopic transient kinetic analysis was used to explore the intrinsic turnover frequency (TOF intr) and coverage of active carbon-containing intermediates ( θ CO x ) that led to carbon dioxide during CO oxidation. After correcting for CO 2 readsorption, the TOF intr was found to be independent of temperature, approximately 3.4 s −1 for Au/TiO 2 (261–303 K) and 2.1 s −1 for Au/Al 2O 3 (272–343 K). At 293 K, the coverage of active carbon-containing intermediates was greater over Au/TiO 2 than over Au/Al 2O 3. The higher coverage of species-forming product on Au/TiO 2 is attributed to the greater availability of active surface oxygen on a titania catalyst compared with an alumina catalyst.