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

A dynamic structural behavior of Pt nanoparticles on the ceria surface under reducing/oxidizing conditions was found at moderate temperatures (<500 °C) and exploited to enhance the catalytic activity of Pt/CeO -based exhaust gas catalysts. Redispersion of platinum in an oxidizing atmosphere already occurred at 400 °C. A protocol with reducing pulses at 250-400 °C was applied in a subsequent step for controlled Pt-particle formation. Operando X-ray absorption spectroscopy unraveled the different extent of reduction and sintering of Pt particles: The choice of the reductant allowed the tuning of the reduction degree/particle size and thus the catalytic activity (CO>H >C H ). This dynamic nature of Pt on ceria at such low temperatures (250-500 °C) was additionally confirmed by in situ environmental transmission electron microscopy. A general concept is proposed to adjust the noble metal dispersion (size, structure), for example, during operation of an exhaust gas catalyst.

The structure of copper sites in Cu-SSZ-13 during NH3-SCR was unravelled by a combination of novel operando X-ray spectroscopic techniques. Strong adsorption of NH3 on Cu, its reaction with weakly adsorbed NO from the gas phase, and slow re-oxidation of Cu(I) were proven. Thereby the SCR reaction mechanism is significantly different to that observed for Fe-ZSM-5.

Pt-CeO2-Al2O3 catalysts play an important role in diesel oxidation and three-way catalysis. In this study, the fast structural dynamics of both platinum and ceria in a 1 wt %Pt/5 wt %CeO2-Al2O3 catalyst prepared by flame spray pyrolysis have been systematically investigated under reducing and oxidizing conditions to elucidate the role of the Pt–CeO2 interface for CO oxidation and fast oxygen storage/release of ceria. The catalyst showed enhanced catalytic activity, particularly after application of a reducing/oxidizing conditioning step at 250 °C, with a pronounced dependence on the reducing agent (C3H6 < H2 < CO). In situ time-resolved X-ray absorption spectroscopy (XAS) at the Ce L3-edge unraveled a dependence of the reduction extent of ceria during temperature-programmed reduction on the noble metal constituent and the applied reducing agent. Dynamic reducing/oxidizing cycling (2% H2 ↔ 10% O2 or 2% CO ↔ 10% O2) at various temperatures (150, 250, and 350 °C) showed that the reducibility of ceria increased at higher temperature and by using a more strongly reducing reaction mixture. This coincides with the trend in catalytic activity. Time-resolved XAS data recorded at the Pt L3-edge and Ce L3-edge during redox cycling revealed a close relationship between the Pt oxidation state and the ceria redox response. The formation of reduced Pt particles was found to induce variations in ceria reducibility under transient conditions and was identified as a decisive prerequisite for ceria reduction at low temperatures. Variations in the extent of ceria reduction during the reducing/oxidizing cycles indicate an evolution of the Pt–ceria interface from an inactive state toward an optimal activated state due to reduction and slight sintering of the noble metal particles. Further growth of Pt particles leads to a decrease in ceria reduction rate due to the smaller Pt–CeO2 interface perimeter. A schematic model illustrating the role of Pt for ceria reducibility is developed and the optimal Pt particle size derived. The results are relevant for various applications, particularly for catalysts operated at low temperature under highly dynamic reaction conditions such as exhaust gas catalysts.

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