Discriminating spectator and reaction intermediate Pt-carbonyls over room-temperature CO oxidation catalysts by in situ and quantitative operando IR.

Common pitfalls and evolution of optical pathlengths during diffuse reflectance FT-IR spectroscopy (DRIFTS) studies of heterogeneous catalytic reactions will be first discussed. Pitfalls include temperature control, Ni(CO)₄ contamination, gas-phase signals (problems also common with transmission IR) and unit selection (i.e. Kubelka-Munk versus Absorbance). In particular, it will be shown how Kubelka-Munk units can conceal bands with weak intensities and lead to falsified data interpretation.

The nature and reactivity of Pt-bound carbonyls over oxide-supported platinum catalyst will be presented, stressing the higher reactivity of metallic Pt-bound species and the involvement of support lattice oxygen over redox oxides such as CeO₂ and TiO₂ during CO oxidation. In contrast, CO adsorbed on positively charged Pt, possibly single atoms, are typically spectator species at room temperature. The use of quantitative DRIFTS analyses allows proving the relevance of carbonyls bound to metallic Pt during room temperature oxidation of CO by comparing the rate of adsorbate decomposition and that of CO₂ formation.

The presence of non-reactive storage sites will also be discussed, where surface species accumulate before surface-diffusing to reaction sites, leading to an overestimation of the concentration of active sites. The presence of storage sites also complicates the determination of the nature of the true reaction intermediates. For instance, linearly adsorbed CO appeared to reacts as fast as the true intermediate multi-bonded CO, only because all carbonyls could readily surface diffuse to reaction site. To conclude, the relevance of transient and quantitative DRIFTS analysis will be extended to other reactions such as CO₂ hydrogenation to hydrocarbons and methanol.