From Core−Shell to Alloys: The Preparation and Characterization of Solution-Synthesized AuPd Nanoparticle Catalysts

This article describes the solution-phase synthesis of 4 nm gold nanoparticles with 0.7 atom-thick, 1.9 atom-thick, and 3.8 atom-thick layers of Pd on their surfaces. These well-defined core−shell nanoparticles were deposited on a silica support, calcined, and reduced at 300 °C to create alloyed nanoparticles containing 10.9, 20.2, and 28.5% Pd (w/w). Monometallic Pd nanoparticles sintered during calcination at 300 °C, but no sintering was observed for AuPd nanoparticles. Diffuse reflectance infrared Fourier transform (DRIFT) spectra of adsorbed CO suggests that Au donates d electron density to Pd in the core−shell and alloy structures and confirms the presence of Au and Pd atoms on the surface of the nanoparticles after calcination and reduction. The properties of the AuPd alloy catalysts were tested in the vapor-phase conversion of α-limonene to p-cymene. AuPd nanoparticles containing 20% or more Pd per particle produced p-cymene yields greater than 80%, equivalent to conventional Pd catalysts prepared by incipient wetness and ion exchange methods. Very low yields of p-cymene were obtained from dehydrogenation of p-menthane under equivalent conditions, suggesting that the production of p-cymene from α-limonene proceeds through terpinene intermediates.