Thermally Induced Restructuring of Pd@CeO₂ and Pd@SiO₂ Nanoparticles as a Strategy for Enhancing Low-Temperature Catalytic Activity

Abstract

Retaining high catalytic activity after exposure to elevated temperatures remains a crucial challenge for applications such as automotive emissions control. While catalysts generally sinter and lose activity after aging at high temperature, here we illustrate that palladium in a core@shell morphology responds very differently. After 800 °C aging in oxygen, palladium redisperses into the encapsulating shell. The redispersion is more pronounced, and nearly complete, when palladium is encapsulated by reducible ceria, as opposed to nonreducible silica. This difference is likely due to the availability of lattice oxygen. Through comparisons with polycrystalline ceria nanoparticles, surface decorated with Pd, we show that for favorable restructuring to occur under our simple aging conditions, the process must start from a particular initial configuration, the

Effects of Hydrothermal Aging on CO and NO Oxidation Activity over Monometallic and Bimetallic Pt-Pd Catalysts

“…Carrillo et al [38] reported that PdO nanoparticles can trap mobile Pt species under oxidizing conditions, which is of high relevance for understanding the behavior of bimetallic Pt-Pd catalysts. Lately, a wide variety of advanced materials has been proposed for catalytic emission control under lean conditions, namely encapsulated coreshell catalysts [25,39,40] and nanostructured architectures [41,42] for oxidation reactions and microporous materials for NOx control [43][44][45][46]. Although these materials often show extremely promising catalytic activity or stability, fundamental in-depth understanding, transfer into industrial application and upscaling of preparation procedures remains challenging.…”

Stabilizing Highly Dispersed Halo Sites in Thermally Restructured Palladium Core@shell Nanoparticles for Improved Catalyst Activity and Durability

“…The three model catalyst systems prior to cycled aging (referred to as initial catalysts) are shown in the first column of Figure 1. We note that these catalysts were prepared following previously described methods 15,16,25 and have been exposed to 500°C in air for 2 hours (see Methods Section for more information). The Pd@SiO2 and Pd@CeO2 have comparable Pd core size on the order of 3 -5 nm in diameter and similar oxide shell thickness of approximately 10 -12 nm.…”

“…13,14 We have recently shown that such core@shell architectures can facilitate favorable and counter-intuitive restructuring when exposed to the high-temperature (800°C) conditions common to many catalytic applications. 15,16 Specifically, we discovered that core Pd disintegrated and migrated outward into the encapsulating shell, nearly doubling the number of available sites for reaction. The resulting highly dispersed Pd species formed an arrangement that resembled a "halo" around the original location of the core.…”

“…This was due to the combination of greater redispersion of core Pd, and changes in Pd oxidation state and coordination, which improved the recruitment and activation of lattice oxygen in the reducible CeO2 support as part of highly active Mars van Krevelen (MvK) oxidation mechanisms. [16][17][18] In contrast, Pd/CeO2 catalysts, which were prepared by impregnating Pd on the external surface of CeO2 nanospheres, underwent significant sintering when exposed to identical high-temperature aging conditions. This sintering decreased lattice oxygen recruitment, catalytic performance and ultimately, the utilization of Pd metal.…”

Recent Advances of Ceria‐Based Materials in the Oxidation of Carbon Monoxide

“…Due to its low cost and excellent CO oxidation performance when combined with CeO 2 , Pd has attracted extensive attention. [ 66–69 ] As mentioned earlier, in the low‐temperature catalytic reaction, small metal particle size usually shows good catalytic activity; [ 70,71 ] Pd can be evenly dispersed on the surface of the support and exhibit high catalytic activity. [ 56,72,73 ] The catalytic reaction is usually conducted at a high temperature, which results in the sintering of small metal NPs and the reduction of catalyst active sites.…”