Thermodynamics of faceted palladium(-gold) nanoparticles supported on rutile titania nanorods studied using transmission electron microscopy.

Many physical properties of nanoparticles (NPs) are driven by their equilibrium shape (ES). Thus, knowing the kinetic and thermodynamic parameters that affect the particle morphology is key for the rational design of NPs with targeted properties. Here, we report on the thermodynamic ES of supported monometallic palladium and bimetallic palladium-gold (Pd-Au) single-crystalline truncated nano-octahedra (TOs) studied using aberration-corrected transmission electron microscopy (TEM). Monometallic palladium and bimetallic Pd62Au38 and Pd43Au57 TOs were grown by pulsed laser deposition on rutile titania (r-TiO2) nanorods exposing mainly (110) facets. Particle structure and dimension were first obtained from aberration-corrected high resolution TEM (HRTEM) images acquired parallel to the metal-oxide interface. By fitting an extended Wulff-Kaishev rule to the HRTEM data of the truncated octahedral thermodynamic ES in the size range of 2 to 5 nm, we secondly determined the interface and excess line energies associated with the particle-oxide-vacuum triple phase junction in Pd and Pd43Au57 TOs in the epitaxial relationship Pd(-Au)(111)101‖r-TiO2(110)[1-1-1] and in Pd62Au38 TOs in the epitaxial relationship Pd62Au38(100)101‖r-TiO2(110)[1-10]. Our results show a decrease in particle adhesion to the oxide support upon alloying Pd with Au. The loss in adhesion is tentatively attributed to an increase of the lattice strain induced at the metal-oxide interface as gold atoms are added to the palladium lattice.

Ostwald-Driven Phase Separation in Bimetallic Nanoparticle Assemblies.

The compositional stability of bimetallic nanoparticles (NPs) is crucial for many applications. We have studied the coarsening of amorphous carbon-supported Au-Pd NPs during annealing at 873 K. Using scanning transmission electron microscopy and energy-dispersive spectroscopy measurements, we show that, despite a complete miscibility of the two metals, the particle assembly undergoes a phase separation during annealing, which leads to two distinct populations: Au-rich NPs with a mean radius of 3.5 nm and large Pd-rich NPs with a mean radius of 25 nm. Thermodynamic calculations and kinetic Monte Carlo simulations explain this behavior that is driven by the competition between surface and mixing energy and by the different mobilities of the two atomic species.

Au-Rh and Au-Pd nanocatalysts supported on rutile titania nanorods: structure and chemical stability.

Au, Rh, Pd, Au-Rh and Au-Pd nanoparticles (NPs) were synthesized by colloidal chemical reduction and immobilized on hydrothermally-prepared rutile titania nanorods. The catalysts were characterized by aberration-corrected TEM/STEM, XPS, and FTIR, and were evaluated in the hydrogenation of tetralin in the presence of H2S. Oxidizing and reducing thermal treatments were employed to remove the polyvinyl alcohol (PVA) surfactant. Reduction in H2 at 350 °C was found efficient for removing the PVA while preserving the size (ca. 3 nm), shape and bimetallic nature of the NPs. While Au-Pd NPs are alloyed at the atomic scale, Au-Rh NPs contain randomly distributed single-phase domains. Calcination-reduction of Au-Rh NPs mostly leads to separated Au and Rh NPs, while pre-reduction generates a well-defined segregated structure with Rh located at the interface between Au and TiO2 and possibly present around the NPs as a thin overlayer. Both the titania support and gold increase the resistance of Rh and Pd to oxidation. Furthermore, although detrimental to tetralin hydrogenation initial activity, gold stabilizes the NPs against surface sulfidation in the presence of 50 ppm H2S, leading to increased catalytic performances of the Au-Rh and Au-Pd systems as compared to their Rh and Pd counterparts.

Long-range chemical orders in Au-Pd nanoparticles revealed by aberration-corrected electron microscopy.

Despite the importance of gold-palladium nanoalloys in heterogeneous catalysis, the phase stability of Au-Pd alloys still remains unclear. We report here on the alloying and chemical ordering in epitaxially-grown and post-annealed gold-palladium nanoparticles (NPs) using aberration-corrected transmission electron microscopy. Au-Pd NPs with a controlled size, composition and structure were grown by pulsed laser deposition on freshly-cleaved NaCl(001) single crystals heated at 300 °C. After transfer to an amorphous carbon support, the NPs were annealed in vacuum at elevated temperatures above 400 °C for a few hours (6-10 hours) to promote chemical ordering. The as-grown NPs were mostly monocrystalline with a chemically-disordered face-centered cubic structure. Upon high-temperature annealing, a high degree of chemical ordering was observed in nanometer-sized NPs. Electron microscopy measurements showed that both L10 and L12 orders are stabilized in the Au-rich region of the Au-Pd phase diagram. These ordered phases exist at temperatures as high as 600 °C. Moreover, compositional analysis of single annealed particles revealed that the observed chemical ordering occurs in parallel to a two-tiered Ostwald ripening process. Due to this ripening process, a clear dependence between chemical composition and particle size is established during annealing with an enrichment in Pd as the NPs grow in size. Our results, besides clarifying some controversial aspects about long-range order in Au-Pd alloys, shed light on the structural stability of Au-Pd nanoalloys at elevated temperatures.