It has long been understood that Au catalysts are highly effective for low temperature CO oxidation, but there remains a debate about the nature of adsorbed oxygen species and the potential role of oxygen vacancies in the catalysis. The ambiguity is largely due to the fact that the quantities of oxygen species adsorbed are very small, making oxygen difficult to characterize. During FTIR analyses of H2 adsorption on Au/TiO2 catalysts, we observed evidence of a strongly adsorbed oxygen species (SAO) that reacted with adsorbed H2. Here we use chemisorption to quantify the amount of SAO, which provides insight into its potential adsorption sites. There are three potential locations: (i) on the Au nanoparticles, (ii) at the metal support interface (MSI), or (iii) directly on the TiO2 support, away from the Au nanoparticles. We prepared two series of Au/TiO2 catalysts. In one series, we varied the Au weight percent loading to measure O2 adsorption as a function of Au sites. In the other, we varied the support material (P25, P90, or rutile) to explore the effects of the TiO2 surface area. Preliminary data indicates that increasing the surface area of TiO2 increases the quantity of SAO adsorbed. Meanwhile, the quantity adsorbed is identical for Au/P25 catalysts with 2.3% and 4.5% Au loadings. The quantity adsorbed stabilizes at ~2 oxygen atoms per Au perimeter site, making it unlikely that the oxygen adsorbs on Au as electron density would be highly localized. All of this suggests that SAO resides on the support rather than directly on the Au atoms or at the Au-TiO2 interface. If this is the case, then Au/TiO2 catalysts have an unusual reduction mechanism, whereby hydrogen is activated at the MSI and transported across the surface to react with oxygen.

Characterization study on a strongly adsorbed oxygen species on Au/TiO2 catalysts

ACS Spring 2022

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