Pt single sites are highly attractive due to their high atom economy and can be generated on CeO2 by an oxidative high-temperature treatment. However, their location and activity are strongly debated. Furthermore, reaction-driven structural dynamics have not been addressed so far. Here, we were able to evidence Ptinduced CeO2 surface restructuring, locate Pt single sites on CeO2 and track the variation of the active state under reaction conditions using a complementary approach of density functional theory calculations, in situ infrared spectroscopy, operando high-energy-resolution fluorescence detected X-ray absorption spectroscopy and catalytic CO (as well as C3H6 and CH4) oxidation. We find that the onset of CO oxidation is linked to the migration of Pt single sites from four-fold hollow sites to form small clusters containing few Pt atoms. This demonstrates that operando studies on single sites are essential to assess their fate and the resulting catalytic properties. a promise as they lower the noble metal content significantly as all atoms are potentially active species. [5][6][7][8][9] Exploiting the strong noble metal support interaction between Pt and CeO2, metallic Pt particles can be formed orin contrast to weakly interacting supports like Al2O3redispersed, with tremendous impact on the catalyst activity. [10][11][12][13] The preparation of SAC has been demonstrated for Pt, which can be atomically dispersed when using CeO2 as a support through an oxidizing treatment at 800 °C. 14 However, the exact structure of the single sites, their reactivity and, particularly, their state and dynamics during reaction are still unknown and heavily debated. 4,15,16 The location of Pt single sites is claimed to range from surface adsorbates on {111} ceria steps, 17,18 {111}, 19 {110} 20,21 or {100} 6,22,23 ceria facets to surface 21,24,25 or bulk Ce substitutes [26][27][28] forming Ce1-XPt 2+ XO2-Y-composites. During change of the gas atmosphere and of the temperature, the structure of the single sites may strongly change resulting in a new and more active state. For example, after a high temperature treatment strong Pt-O-Ce bonds are reported to over-stabilize the single sites which are thus less active. 29 During the catalytic oxidation, oxygen is suggested to be provided by the support, while the reactant e.g. CO is adsorbed directly on Pt, 22,25 similar to Pt nanoparticles on CeO2. 30 Bera et al. correlated the intensity of the Pt-O-Ce bond observed by Extended X-ray Absorption Fine Structure (EXAFS) measurements with the catalytic activity for Ce1-XPt 2+ XO2-Y, 31 and Nie et al. 24demonstrated that the catalytic activation of a Pt single atom catalyst can be increased by steam treatment. It is suggested that this treatment leads to the formation of Ce1-XPt 2+ XO2-YH-OH species that are catalytically more active than Ce1-XPt 2+ XO2-Y. 24 In contrast, other studies report an increase in catalytic activity after a reductive treatment at temperatures below 300 °C. 9,32-34 Importantly, such

Pd/Al2O3 and Pd/CeO2 catalysts were investigated for methane oxidation at conditions typical for the exhaust of lean burn gas engines. The results show that catalyst prereduction significantly increases the catalytic activity during the light-off irrespective of the gas composition. Operando X-ray absorption spectroscopy revealed a fully reduced catalyst state after the reductive pretreatment, which is reoxidized with increasing temperature in a lean reaction mixture, resulting in bulk PdO formation at 350 °C. The correlation of catalytic activity with oxidation state during light-off tests led to the conclusion that PdO is a mandatory species for methane oxidation. We attribute the increased conversion after prereduction to the slight sintering of Pd particles and higher reactivity of the formed PdO surface species. Additionally, the H2O inhibition effect was found to be retarded under dry conditions due to the relatively slow palladium reoxidation. The results presented are in particular relevant for the activity of methane oxidation catalysts at low temperature and under dynamic conditions.

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