193Ir Mössbauer spectroscopy of Pt–IrO2 nanoparticle catalysts developed for detection and removal of carbon monoxide from air

Mössbauer spectroscopy of 73.0 keV gamma-ray transition in 193Ir and supplementary analytical techniques were used to study the microstructure and chemical form of polymer-supported hydrophobic bimetallic Pt–Ir catalysts for detection and removal of CO from humid air at ambient conditions. The catalysts, typically with a composition of 9 wt.% Pt and 1 wt.% Ir, were prepared by incipient wetness impregnation of polystyrene–divinylbenzene (SDB) granules with ethanol solutions of hexachloroplatinic and hexachloroiridic acids. This procedure, followed by reduction in H2 or CO at only 200 °C or 250 °C, resulted in formation of highly-dispersed Pt–Ir particles usually smaller than 20 nm and having high catalytic activity and selectivity. Mössbauer spectra of 73.0 keV gamma-ray transition in 193Ir were taken after consecutive steps of preparation and exposure of catalysts to better understand and further improve the fabrication processes. In the as-impregnated state, iridium was found mostly as Ir(III) in [IrCl6]3− ions, with only a small fraction of Ir(IV) in [IrCl6]2− ions. The iridium in bimetallic clusters formed by reduction in hydrogen showed a strong tendency towards oxidation on exposure to air at room temperature, while Pt remained mostly metallic. In the most active and stable catalysts, the Ir and Pt in metallic regions of the clusters did not tend to segregate, unlike in Pt–Ir/silica-supported catalysts studied by us earlier. Further, this study shows that the IrO2-like regions in the clusters exhibit stronger deviations from local symmetry and stoichiometry of crystalline IrO2 than observed previously in Pt–Ir/silica catalysts. Our study also indicates that in the examined Pt–IrO2 nanoparticles iridium largely provides the dissociative O2 adsorption sites, while the CO adsorption occurs primarily at metallic Pt sites.