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.

Efficient oxygen evolution reaction (OER) electrocatalysts are pivotal for sustainable fuel production, where the Ni-Fe oxyhydroxide (OOH) is among the most active catalysts for alkaline OER. Electrolyte alkali metal cations have been shown to modify the activity and reaction intermediates, however, the exact mechanism is at question due to unexplained deviations from the cation size trend. Our X-ray absorption spectroelectrochemical results show that bigger cations shift the Ni2+/(3+δ)+ redox peak and OER activity to lower potentials (however, with typical discrepancies), following the order CsOH > NaOH ≈ KOH > RbOH > LiOH. Here, we find that the OER activity follows the variations in electrolyte pH rather than a specific cation, which accounts for differences both in basicity of the alkali hydroxides and other contributing anomalies. Our density functional theory-derived reactivity descriptors confirm that cations impose negligible effect on the Lewis acidity of Ni, Fe, and O lattice sites, thus strengthening the conclusions of an indirect pH effect.

Novel catalysts consisting of ruthenium nanoparticles encapsulated in cross‐linked matrices based on the poly(propylene imine) dendrimers of the 1st and 3rd generations have been synthesized with a narrow particle size distribution (3.8 and 1.0 nm, respectively). The resulting materials showed high activity for the hydrogenation of phenols in aqueous media (specific catalytic activity reached turnover frequencies of 2975 h−1 with respect to hydrogen uptake). It has been shown that the use of water as a solvent leads to a 1.5 to 50‐fold increase in the reaction rate depending upon the nature of the substrate. It has been established that unlike the traditional heterogeneous catalysts based on ruthenium, during the hydrogenation of dihydroxybenzenes, the hydrogenation rate decreases in the order: resorcinol>hydroquinone≫catechol. The maximum specific activity for resorcinol was a turnover frequency of 243150 h−1 with respect to hydrogen uptake. The catalyst based on the dendrimer of the 3rd generation containing finer particles has significantly inferior activity to the catalyst based on the dendrimer of the 1st generation by virtue of steric factors, as well as the need for prereduction of the ruthenium oxide contained on the surface. These catalysts showed resistance to metal leaching and may be reused several times without loss of activity.

No abstract