Surface oxygen vacancies in gold based catalysts for CO oxidation

Abstract

Experimental catalytic activity measurements, diffuse reflectance infrared Fourier spectroscopy, and density functional theory calculations are used to investigate the role and dynamics of surface oxygen vacancies in CO oxidation with O 2 catalyzed by Au nanoparticles supported on a Y-doped TiO 2 catalyst. Catalytic activity measurements show that the CO conversion is improved in a second cycle of reaction if the reactive flow is composed by CO and O 2 (and inert) while if water is present in the flow, the catalyst shows a similar behaviour in two successive cycles. DRIFTS-MS studies indicate the occurrence of two simultaneous phenomena during the first cycle in dry conditions: the surface is dehydroxylated and a band at 2194 cm À1 increases (proportionally to the number of surface oxygen vacancies). Theoretical calculations were conducted in order to explain these observations. On one hand, the calculations show that there is a competition between gold nanoparticles and OH to occupy the surface oxygen vacancies and that the adsorption energy of gold on these sites increases as the surface is being dehydroxylated. On the other hand, these results evidence that a strong electronic transfer from the surface to the O 2 molecule is produced after its adsorption on the Au/TiO 2 perimeter interface (activation step), leaving the gold particle in a high oxidation state. This explains the appearance of a band at a wavenumber unusually high for the CO adsorbed on oxidized gold particles (2194 cm À1 ) when O 2 is present in the reactive flow.These simultaneous phenomena indicate that a gold redispersion on the surface occurs under reactive flow in dry conditions generating small gold particles which are very active at low temperature. This fact is notably favoured by the presence of surface oxygen vacancies that improve the surface dynamics. The obtained results suggest that the reaction mechanism proceeds through the formation of a peroxo-like complex formed after the electronic transfer from the surface to the gas molecule.

Au/CeO2 Catalysts: Structure and CO Oxidation Activity

“…Moreover, this reversible transformation is strongly affected by the hydroxylation of the reducible oxide surface that, at high temperatures, is involved in the re-oxidation of the cerium sites in a mechanism that implies the competition of hydroxyl groups and gold atoms for oxygen surface vacancies [25]. DFT calculations of the role of water on CO oxidation over Au/TiO2 catalysts support this H2O-Au competition for surface oxygen vacancies [12,18]. Consequently, the catalytic performance of this sort of material during the oxidation reactions is closely related to the interaction between gold and these punctual defects.…”

Synergy between gold and oxygen vacancies in gold supported on Zr-doped ceria catalysts for the CO oxidation

“…For instance, we have recently reported the enhancement of the electronic transfer in gold catalysts prepared on Y-doped TiO 2 in the CO oxidation reaction [68]. When gold nanoparticles were placed over oxygen vacancies of the support, a large electronic transfer took place from gold to the gaseous oxygen molecule, activating it for the CO oxidation.…”

CO Oxidation on Au Nanoparticles Supported on ZrO2: Role of Metal/Oxide Interface and Oxide Reducibility

“…Of course, other experimental conditions may affect the activity of the catalyst, for instance, the level of humidity in the reaction gas and the calcination steps, making it even more difficult to unambiguously elucidate the reaction mechanism. Studies of CO oxidation on Au/TiO 2 catalysts have shown the complex interplay between O 2 adsorption at the Au/TiO 2 perimeter, presence of oxygen vacancies, and electron transfer to the O 2 molecule with formation of gold particles in a high oxidation state …”