Tracking and Understanding Dynamics of Atoms and Clusters of Late Transition Metals with In-Situ DRIFT and XAS Spectroscopy Assisted by DFT

Dynamic structural changes of single-atom catalysts (SACs) are key to many reactions that were reported to be catalyzed by supported single atoms. To understand these changes, systematic in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS) experiments were performed under reducing (1% CO) and oxidizing (1% CO + 1% O2) reaction atmospheres at room temperature over CeO2-supported late transition metals (Ru, Rh, Pd, Ir, and Pt) synthesized via two different methods (wet impregnation and precipitation) to account for the influence of surface properties. As a general trend, the CO vibrational frequencies downshifted under the CO atmosphere, which we assigned to the formation of clusters. Upon changing the gas mixture to more oxidizing (1% CO + 1% O2), single sites are retained as evidenced by the CO vibrational frequencies at higher wavenumbers. Among the investigated metals, Pt2+ and Pd2+ are more prone to cluster formation, and Rh3+ and Ru4+ are found to be stable as single sites following the order Rh > Ru > Ir > Pt > Pd. In combination with the density functional theory (DFT) calculations of CO vibrational frequencies, we were able to assign shifts to changes in the oxidation state of the metals. These findings thus serve as a benchmark for ceria-supported Pd, Pt, Ru, Ir, and Rh SACs.