Gamma-Al2O3, ZrO2, and TiO2 gold supported model catalysts have been synthesized by laser vaporization. Structural characterization using Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy experiments have shown that the gold clusters deposited on the different supports have similar distribution of size centered around 3 nm and are in the metallic state. However, X-ray photoemission measurements also indicate lower binding energies than the usual Au 4f(7/2) at 84.0 eV for both alumina and titania supported catalysts, indicating a modification of the electronic structure of the metal. One has taken benefit of these features to study the influence of the nature of the support toward CO oxidation activities without being hindered by particle size or gold oxidic species effects. By comparing the activities of the different catalysts, it is concluded that the nature of the support directly affects the activity of gold. The following tendency is observed: titania and zirconia are superior to alumina as supports, titania being slightly better than zirconia. From XPS and activity results we can conclude that the existence of negatively charged clusters is not the key point to explain the high activity observed for Au/ZrO2 and Au/TiO2 catalysts and also that metallic Au is the major catalytically active phase. Hence, due to their very nature, titania and to a less extent zirconia should participate to the catalytic process.

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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.