Tuning the Pt/CeO2 Interface by in Situ Variation of the Pt Particle Size
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Abstract
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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