Single-atom catalysts (SACs) are promising atom-efficient materials, with potentially superior performances with respect to their nanoparticulate counterparts. Owing to its practical importance and relative simplicity, CO oxidation on Pt/-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

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... nitored the Pt dispersion, oxidation state, and CO oxidation activity by operando fast X-ray absorption spectroscopy and diffuse reflectance infrared spectroscopy, both combined with mass spectrometry. It is shown that single Pt m+ atoms (m  2), resulting from the standard impregnation-calcination procedure of SAC preparation, are poorly active. However, they gradually but irreversibly convert into highly active ~1-nm-sized Pt + clusters ( < 2) throughout the heating/cooling reaction cycles, even under highly oxidizing conditions favorable to atomic dispersion. Increase in the Pt loading or the CO/O2 concentration ratio accelerates the clustering-reduction phenomena. This work not only evidences 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.