Showing papers by "Emiliano Fonda published in 2018"

Smart Pd Catalyst with Improved Thermal Stability Supported on High-Surface-Area LaFeO3 Prepared by Atomic Layer Deposition.

Abstract: The concept of self-regenerating or “smart” catalysts, developed to mitigate the problem of supported metal particle coarsening in high-temperature applications, involves redispersing large metal particles by incorporating them into a perovskite-structured support under oxidizing conditions and then exsolving them as small metal particles under reducing conditions. Unfortunately, the redispersion process does not appear to work in practice because the surface areas of the perovskite supports are too low and the diffusion lengths for the metal ions within the bulk perovskite too short. Here, we demonstrate reversible activation upon redox cycling for CH4 oxidation and CO oxidation on Pd supported on high-surface-area LaFeO3, prepared as a thin conformal coating on a porous MgAl2O4 support using atomic layer deposition. The LaFeO3 film, less than 1.5 nm thick, was shown to be initially stable to at least 900 °C. The activated catalysts exhibit stable catalytic performance for methane oxidation after high-te...

Nanostructured PdPt nanoparticles: evidences of structure/performance relations in catalytic H2 production reactions

Abstract: A widespread approach to modulate the performances of heterogeneous catalysts is the use of bimetallic nanoparticles (NPs) as the active phase. However, studying the relationship between the NPs structure and catalytic properties requires well-defined systems, having uniform composition, size and nanostructure, which cannot be achieved by traditional methods (e.g. impregnation). Here, we developed wet-chemistry synthetic routes to prepare Pd Pt NPs or Pt-core@Pd-shell NPs of small size and well-controlled composition and structure, protected by mercaptoundecanoic acid (MUA) moieties. The pristine NPs were tested for H2 production by NH3BH3 hydrolysis, in order to systematically investigate the effect of composition and of synthetic route on the activity of the systems. Depending on the preparation method, two distinct trends of activity were observed, rationalized in terms of the extent of surface functionalization by MUA. The MUA protective layer was found to effectively stabilize the NPs dispersion while maintaining high activity in certain cases (Pt-rich NPs), and was demonstrated to be essential for catalyst recycling. In order to further study structure-activity relationships of Pd Pt NPs after ligand removal, nanostructured Pd Pt@CeO2-based catalysts were prepared by self-assembly route. Regardless of the starting NPs structure (alloy or core-shell), similar water gas shift reaction performances were observed, due to the structural rearrangements occurring upon oxidation and reduction thermal treatments, which led to the formation of Pt-rich core@Pd Pt-shell under reducing conditions.

Ultrathin Ni nanowires embedded in SrTiO 3 : Vertical epitaxy, strain relaxation mechanisms, and solid-state amorphization

Abstract: Strain is a key parameter affecting the physical properties of heterostructured thin films and nanosized objects. Generally, the design of application-optimized hybrid structures requires good structural compatibility between the involved phases. However, when controlled appropriately, lattice mismatch can turn from a detrimental to a beneficial property, enabling further functionality tuning. Due to their large heterointerface, nanocolumnar composites are an ideal test bed for such strain engineering approaches, but coupling mechanisms at vertical interfaces are still poorly understood. In the present paper, we therefore present a detailed analysis of ultrathin Ni nanowires, with diameters between 1.7 nm and 5.3 nm, vertically epitaxied in a ${\mathrm{SrTiO}}_{3}$/${\mathrm{SrTiO}}_{3}$(001) matrix. Using a combination of x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and x-ray absorption spectroscopy (XAS) measurements, we unveil peculiar structural features of this hybrid system. We show that the axial deformation of the nanowires depends on their diameter and that their radial strain differs sensitively from the value expected when considering the Poisson ratio. We also provide evidence for the existence of a relaxation mechanism consisting in a slight tilting of crystallographic nanowire domains which reduces the misfit at the Ni-${\mathrm{SrTiO}}_{3}$ heterointerface. This, in turn, induces significant structural disorder and results in a successive amorphization of the metallic phase upon diameter reduction of the nanowires.