Pd/CeO₂ is an active component in emission control catalysts for CO oxidation. Nanostructured CeO₂ powders can be prepared in the form of rods exposing predominantly (111) surfaces and cubes exposing (100) surfaces. While differences in the reactivity of Pd supported on these facets of ceria have been reported, the origins of the reactivity differences are not well understood. Pd supported on (111) surfaces of ceria rods exhibits room-temperature CO oxidation activity, while Pd on (100) surface of ceria cubes shows comparable activity at a temperature that is 60 °C higher. Earlier, we established that Pd/CeO₂-rods are active due to a Langmuir-Hinshelwood mechanism involving isolated Pd atoms in the form of Pd₁O and Pd₁O₂ species. Here, we establish using in situ CO IR spectroscopy and density functional theory (DFT) that, in addition to TEM-visible Pd nanoparticles, Pd/CeO₂-cubes also contain isolated Pd species, predominantly in the form of Pd₁O. DFT calculations show that CO oxidation proceeds via a Mars-van Krevelen pathway, which is possible for the (100) surface because of the lower Ce-O binding energy compared to the (111) surface. Overall, the catalytic cycle for CO oxidation on Pd/CeO₂(100) involves a higher free energy barrier than on Pd/CeO₂(111) in keeping with the experimentally observed activity difference. EXAFS measurements show that the active Pd phase in both Pd/CeO₂-rods and Pd/CeO₂-cubes responds dynamically with respect to reducing and oxidizing conditions. The redispersion of Pd in oxidative conditions is more pronounced for Pd/CeO₂-rods and the catalyst is more active after an oxidative treatment.

Pd / CeO ₂是用于CO氧化的排放控制催化剂中的活性组分。可以以主要暴露（111）表面的棒和暴露（100）表面的立方体的形式制备纳米结构的CeO ₂粉末。虽然已经报道了负载在二氧化铈这些面上的Pd的反应性差异，但是对反应性差异的起因还没有很好的了解。负载在二氧化铈棒（111）表面上的Pd表现出室温的CO氧化活性，而负载在二氧化铈立方体（100）表面上的Pd在60°C更高的温度下表现出可比的活性。早先，我们确定了由于Langmuir-Hinshelwood机理涉及Pd ₁ O和Pd ₁形式的分离的Pd原子而使Pd / CeO _2-棒具有活性。O ₂种。在这里，我们使用原位CO IR光谱学和密度泛函理论（DFT）确定，除了TEM可见的Pd纳米颗粒之外，Pd / CeO ₂立方体还包含孤立的Pd物种，主要为Pd ₁ O形式。DFT计算表明，CO氧化是通过Mars-van Krevelen途径进行的，这对于（100）表面是可能的，因为与（111）表面相比，Ce-O的结合能更低。总体而言，在Pd /铈CO氧化催化循环₂（100）涉及比的Pd /铈更高的自由能屏障₂（111）与实验观察到的活性差异一致。EXAFS测量表明，Pd / CeO ₂棒和Pd / CeO ₂立方体中的活性Pd相在还原和氧化条件方面均具有动态响应。对于Pd / CeO ₂棒，Pd在氧化条件下的重新分散更为明显，并且在氧化处理后，催化剂的活性更高。