Facile Synthesis of Pd-Based Core@Shell Catalysts for Improved Stability and Activity

Abstract

Discovery of catalysts with improved durability, activity, and selectivity may provide solutions for environmental and energy issues. One of the recent developments in such materials is to use core@shell architectures, in which two materials are synthesized in two concentric spheres with an oxide shell surrounding a metal core. This morphology provides not only a barrier to sintering but also facilitates strong metal-support interactions by maximizing the number of perimeter sites. However, commonly known synthesis methods for core@shell nanomaterials are often complicated and expensive, which has hindered extensive research using this architecture. In this thesis, some creative uses of core@shell materials as model catalysts are introduced. The outcome of this study provides new and useful information about the thermal stability and metal-support interactions of Pd-based core@shell materials. First, a simple, one-pot and scalable synthesis has been explored, targeting palladium@silica (Pd@SiO2) core@shell catalysts in an aqueous environment at room temperature. The method offers independent tunability of the Pd cores size (D = 1.6±0.4, 2.7±0.4, and 3.5±0.4 nm) and the SiO2 shell thickness (D = 24±3, 31±4, and 43±6 nm). The catalysts also have very high surface areas (BET: 519-1165 m2/g) and high mesopore volumes (1.3-1.8 cm3/g). Based on transmission electron microscopy (TEM) images, a new mechanism for SiO2 shell formation around Pd cores is proposed. CO oxidation and TEM were used to probe the thermal stability of the catalysts. Surprisingly, improved CO light-off was observed after aging above 800 °C, in striking contrast with the catalytic activity of impregnation-prepared Pd/SiO2 supported catalysts with a similar Pd loading. To explain the improved activity after aging at elevated temperature, we used a combination of catalytic activity measurements and TEM studies and obtained direct evidence of Pd redispersion from nanoparticles (~ 4 nm) to smaller atom clusters (≤ 2 nm). The improved catalytic CO oxidation activity of Pd@SiO2 after high temperature aging was consistent with the TEM results. TEM analysis also indicated that the activity enhancement by aging is attributed to the core@shell geometry, which allows the Pd particles to redisperse within the internal mesopores of the silica shells. The synthesis method was also applied to prepare Pd nanoparticles encapsulated by CeO2, aiming to enhance the catalytic activity. A metalloorganic compound, cerium (IV) atrane, was prepared and used as a water-stable CeO2 precursor. The CeO2 shell of the synthesized Pd@CeO2 nanoparticles is composed of 2-5 nm crystals exhibiting both micro- and meso-pores. The catalyntic activity as well as the accessibility of reactants to the Pd core are confirmed by CO oxidation, where excellent low temperature activity was observed. The thermal stability of the as-prepared Pd@CeO2 can be enhanced by two methods: (1) forming CeO2-ZrO2 solid solutions and (2) forming SiO2 secondary shells as barrier to sintering. On successful expansion of the synthesis method to another shell material, application of core@shell materials as three-way auto exhaust catalysts was studied. It was found that the catalysts’ hydrothermal stability could be improved by using different shell materials. However, aging at elevated temperature often destroyed the core@shell architecture due to Pd vaporization and shell recrystallization. Moreover, even this facile synthesis method was found unsuitable for producing catalysts at volumes required by industry. Therefore, the focus shifted towards using the Pd@CeO2 as a model catalyst to study Pd-CeO2 interaction. Using core@shell geometry, the quantification of metal-support interaction was simplified.