(754h) In Situ/Operando Gold Plasmon Sensing of Adsorbed Oxygen (O2-GPS): Activity Trends in CO Oxidation on Supported Gold Catalysts

In this presentation we will describe a novel in situ spectroscopic method for the determination of adsorbed oxygen on gold catalysts. The method termed gold plasmon sensing of adsorbed oxygen (O₂-GPS) in combination with a gold nanoparticle site statistics approach allowed for the first time the detection via in situ UV-visible spectroscopy of oxygen adsorbed at the metal-support perimeter of gold nanoparticles deposited on nonreducible and reducible supports including SiO₂, Al₂O₃, ZrO₂, ZnO, and TiO₂. The O₂-GPS method is based on measurements of gold surface plasmon resonance (Au SPR) peak shifts upon adsorption of oxidizing (e.g., O₂) and reducing (e.g., H₂) species and the quantification of the corresponding relative charge transfer (a surrogate for adsorbed oxygen species) as determined from a simple correlation derived from Drude’s free electron model for gold nanoparticles. A match between the relative charge transfer due to O₂ adsorption at flowing conditions and high temperature (398 K) on gold catalysts with the corresponding statistics (e.g., dispersion) for metal-support perimeter gold atom sites, as derived from a model nanoparticle with a truncated octahedron or truncated cuboctahedron geometry, allowed us to establish the adsorption on O₂ at the gold-support interface. On the studied gold catalysts, we also found that oxygen adsorption depended not only on support reducibility, but also on gold nanoparticle size. Application of in situ/operando GPS during CO oxidation and O₂ adsorption at similar reaction conditions on the supported gold catalysts provided additional experimental evidence for the preferential adsorption of oxygen on all catalysts at the gold-support perimeter, whereas analysis of TOF and Au site statistics suggested that adsorption of CO occurred on all surface gold atoms. The GPS results showed, however, that only a fraction of the oxygen adsorbed at the perimeter sites is active for CO oxidation at the studied conditions. Furthermore, it is shown that CO oxidation catalytic activity increased with the amount of adsorbed oxygen, but it was different for catalysts with reducible and nonreducible supports. All these observations combined were consistent with prior mechanistic proposals where CO adsorbs on the gold nanoparticle and diffuses to the gold-support interface where it reacts with active oxygen species and with possible participation of reducible supports in CO oxidation.