ABSTRACT
Linear scaling relationships (SRs), which relate binding energies of adsorbates across a space of catalyst surfaces, have been extensively explored for metal and oxide surfaces, but little is known about their properties at interfaces between metal nanoparticles and oxide supports, which are ubiquitous in heterogeneous catalysis. Using periodic DFT calculations, scaling principles are extended to bifunctional Au/oxide interfaces. Adopting a Au nanorod on doped MgO (100) as a model, SRs for species participating in water gas shift, methanol synthesis, and oxidation reactions are reported. SR slopes are not constrained by the bond order conservation rule postulated for metals, oxides, and zeolites, potentially permitting greater flexibility in catalyst design strategies. The deviation from bond counting, along with the physical origin of scaling behavior at interfaces, are explored using a conceptual framework involving electrostatic interactions at the Au/oxide interface.
ABSTRACT
The strength of metal-support bonding in heterogeneous catalysts determines their thermal stability, therefore, a tremendous amount of effort has been expended to understand metal-support interactions. Herein, we report the discovery of an anomalous "strong metal-support bonding" between gold nanoparticles and "nano-engineered" Fe3O4 substrates by in situ microscopy. During in situ vacuum annealing of Au-Fe3O4 dumbbell-like nanoparticles, synthesized by the epitaxial growth of nano-Fe3O4 on Au nanoparticles, the gold nanoparticles transform into the gold thin films and wet the surface of nano-Fe3O4, as the surface reduction of nano-Fe3O4 proceeds. This phenomenon results from a unique coupling of the size-and shape-dependent high surface reducibility of nano-Fe3O4 and the extremely strong adhesion between Au and the reduced Fe3O4. This strong metal-support bonding reveals the significance of controlling the metal oxide support size and morphology for optimizing metal-support bonding and ultimately for the development of improved catalysts and functional nanostructures.
ABSTRACT
It has been a long-lived research topic in the field of heterogeneous catalysts to find a way of stabilizing supported gold catalyst against sintering. Herein, we report highly stable AuIr bimetallic nanoparticles on TiO2 synthesized by sequential deposition-precipitation. To reveal the physical origin of the high stability of AuIr/TiO2, we used aberration-corrected scanning transmission electron microscopy (STEM), STEM-tomography, and density functional theory (DFT) calculations. Three-dimensional structures of AuIr/TiO2 obtained by STEM-tomography indicate that AuIr nanoparticles on TiO2 have intrinsically lower free energy and less driving force for sintering than Au nanoparticles. DFT calculations on segregation behavior of AuIr slabs on TiO2 showed that the presence of Ir near the TiO2 surface increases the adhesion energy of the bimetallic slabs to the TiO2 and the attractive interactions between Ir and TiO2 lead to higher stability of AuIr nanoparticles as compared to Au nanoparticles.
