One of the most common methods for preparation of heterogeneous catalysts involves impregnation of oxide supports with active metal precursors that upon reduction form highly dispersed metal particles. The objective is to maximize the exposure of the active metal surface. However, these types of catalysts tend to suffer from deactivation because of thermal sintering of active metals via particle migration as well as atomic diffusion. As one of the strategies to prevent such deactivation, core@shell geometries have been explored [1, 2]. So far, the results of various studies have shown that core@shell geometries may not only improve the thermal stability but also promote catalytic selectivity or activity. Therefore, more in-depth investigation of core@shell catalysts is highly desirable.
One limiting factor for the preparation of core@shell nanoparticles is that the preparation itself requires complex chemistry. Common methods for preparation of core@shell particles require carefully controlled chemical environments such as pH, ion concentration, and polarity of the solvent. In addition, the reactivity of the shell material must be carefully controlled to be just high enough to induce sol-gel condensation. In order to avoid these complexities, we have developed a new synthesis method for the formation of core@shell nanoparticles via flame spray pyrolysis. Studies by other groups have shown that when mixed liquid precursors are co-pyrolyzed, noble metals are deposited on the outer surface of oxide nanopowders because of vapor pressure differences between precursors [3, 4]. In contrast, the material prepared by us forms a core@shell structure because we provide Pd metal nanoparticle seeds for the zirconium oxide shell precursor to adsorb and condense. This leads to a structure where a Pd metal nanoparticle forms a core that is surrounded by a porous zirconia shell. The synthesized Pd@ZrO2 catalysts were tested in probe reactions that are relevant for three-way catalysts for automotive exhaust aftertreatment.
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