Rational Design of Supported PdAu Nanoparticle Catalysts from Structured Nanoparticle Precursors

摘要

A series of poly(vinylpyrrolidone) (PVP)-stabilized metallic and bimetallic PdAu nanoparticles (coreduced and core-shell) with narrow size distributions were encapsulated into alumina matrixes by sol-gel chemistry, and their chemical, structural, electronic, and catalytic behaviors were investigated. Monodisperse nanoparticles were uniformly distributed in the alumina frameworks as observed by TEM images, and single-particle energy-dispersive spectroscopy (EDS) analyses confirmed the high compositional uniformity of the bimetallic nanoparticles. A combination of TEM, EDS mapping, TGA, XANES and EXAFS studies were used to fully characterize the alumina-supported nanoparticles before and after thermal treatments. It was observed that the size distribution of the final PdAu nanoparticles was highly dependent on calcination conditions, and careful high-temperature calcinations at 300 °C could be used to remove organic PVP stabilizers with minimal particle aggregation and/or structural transformations. The resulting supported nanoparticle catalysts were found to be active as hydrogenation catalysts. EXAFS analysis of coreduced PdAu nanoparticles indicated they had near-alloy structures with slightly Au-rich cores and Pd-rich shells before and after calcination, while intentionally designed Pd-core Au-shell nanoparticles retained their structures after calcination. XANES spectra of both coreduced and core-shell PdAu nanoparticles were also examined and showed that the PdAu coreduced nanoparticles had fewer Au valence d-band vacancies in comparison to monometallic nanoparticles while the PdAu core-shell nanoparticles had relatively higher Au valence d-band vacancies than the coreduced PdAu nanoparticles.