Determining the structure of active species and active sites, and their evolution in reaction conditions is a required step towards understanding catalytic mechanisms. In heterogeneous catalysts, multiple species of the same element: single atoms, larger clusters and nanoparticles may appear, disappear and coexist during the reaction. There is therefore a strong need to monitor the charge and geometry of metal centers, as well as their dynamic changes, in reaction conditions. Even with that capability achieved, the reaction mechanism may not be understood until it is known which species are catalytically active, and which ones – spectators. One possible way to address that is to combine multiple in situ probes that can resolve between the heterogeneously distributed species with the online product analysis. This combination will correlate the changes in the spectra (and thus – in the structure) with the changes in activity and thus help identify the active species during the reaction. To illustrate these ideas, and highlight an exemplary application of this multimodal approach I will show how we treated the complexity of a newly synthesized and highly active selective oxidation catalyst, a model single atom catalyst formed by the dilute substitutional doping of Ceria by isolated Pt atoms, (Pt)CeO2. Among the techniques chosen for its characterization, STEM probes the distribution of single atoms and nanoparticles in the sample, in situ XANES and EXAFS probe the local structure around metal atoms/ions, XPS and RIXS – their charge states, DRIFTS – surface composition, and online MS – product formation. We used this multimodal approach to reveal the correlation of the number of Pt2+ ions in Pt/CeO2 catalysts with the observed change in the water-gas shift reaction activity—data definitively establishing that Pt nanoparticles formed during the reaction, and not the Pt single atoms, are the active species. The Pt atoms on the nanoparticle perimeter with proximate Ceria bonding interactions comprise the catalytically active sites.

Active species and active sites in water gas shift reaction over Pt/CeO2 catalysts

2019 Spring National Meeting

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