Dynamics of Single Pt Atoms on Alumina during CO Oxidation Monitored by Operando X-ray and Infrared Spectroscopies

Single-atom catalysts (SACs) are promising atom-efficient materials, with potentially superior performances with respect to their nanoparticulate counterparts. Because of its practical importance and relative simplicity, CO oxidation on Pt/gamma-Al2O3 is considered as an archetypal catalytic system. The efficiency of the corresponding SAC has recently been the subject of debate. In this work, in addition to systematic high-resolution scanning transmission electron microscopy, we have simultaneously monitored the Pt dispersion, oxidation state, and CO oxidation activity by operdndo fast X-ray absorption spectroscopy and diffuse reflectance infrared spectroscopy, both combined with mass spectrometry. It is shown that single Ptm+ atoms (m >= 2), resulting from the standard impregnation-calcination procedure of SAC preparation, are poorly active. However, they gradually and irreversibly convert into highly active similar to 1 nm sized Pt delta+ clusters (delta < 2) throughout the heating-cooling reaction cycles, even under highly oxidizing conditions favorable to atomic dispersion. An increase in the Pt loading or the CO/O-2 concentration ratio accelerates the clustering-reduction phenomena. This work not only demonstrates a gradual aggregation-activation process for an important catalytic system but also highlights the power of operando spectroscopies to address stability issues in single-atom catalysis.