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 monitored 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.

Pt/TiO2 photocatalysts were prepared by incipient wetness impregnation followed by oxidative and/or reductive thermal treatments. By varying the TiO 2 form (commercial P25 and P90, and homemade shape-controlled), the Pt loading (0.2-1 wt% Pt) and the treatment temperature (200-600 °C), it has been possible to tune the Pt cluster size. An increase in the ethanol dehydrogenation rate under ultraviolet irradiation as the Pt cluster average diameter decreases from 17 to 9 Å is suggested by our data. Whereas pre-reduction in H 2 leads to Pt clusters, pre-calcination in air leads to atomically dispersed cationic Pt species. The former are more active and stable than the latter. This conclusion is valid both in gas-and liquid-phase reaction conditions for given TiO 2 type and Pt loading. The activity results are consistent with a recent theoretical work showing that 1 nm is an optimal Pt cluster size for favoring both photoelectron transfer from TiO 2 to Pt and hydrogen coupling on Pt. The best catalytic performance is obtained in gas phase for pre-reduced 0.2 wt% Pt/TiO 2-P90, with an H2 production rate of 170 mmol h-1 gcat-1 .