Surface Composition and Catalytic Evolution of Au x Pd1−x (x = 0.25, 0.50 and 0.75) Nanoparticles Under CO/O2 Reaction in Torr Pressure Regime and at 200 °C

Abstract

Au x Pd 1-x (x = 0, 0.25, 0.5, 0.75, 1) nanoparticle (NP) catalysts (8-11 nm) were synthesized by a onepot reaction strategy using colloidal chemistry. XPS depth profiles with variable X-ray energies and scanning transmission electron microscopy (STEM) analyses show that the as-synthesized Au x Pd 1-x (x = 0.25 and 0.5) bimetallic NPs have gradient alloy structures with Au-rich cores and Pd-rich shells. The evolution of composition and structure in the surface region corresponding to a mean free path of 0.6-0.8 nm (i.e., 2-3 layers to the bulk from the particle surface) was studied with ambient pressure X-ray photoelectron spectroscopy (

Exploring surface science and restructuring in reactive atmospheres of colloidally prepared bimetallic CuNi and CuCo nanoparticles on SiO2in situ using ambient pressure X-ray photoelectron spectroscopy

“…Accordingly, it appears neither case behaves as expected for the bulk materials. The balance of cohesive energies favours Cu segregation 38 (the higher cohesive energy element being expected to be favourably located in the core, where more bonds with like atoms are possible), but this is not dependant on which gases are 20 present. As outlined in the introduction, for many bimetallic nanoparticles it has been found that they restructure on exposure to reactive gases, such as O 2 , as a result of competition between oxide formation enthalpy and surface free energy.…”

“…41 Although electronic effects have been invoked to explain this involving sub-surface nickel altering the properties of the copper, the present results offer an alternative explanation -nickel rather than copper may be segregated to the surface under the conditions of the reaction. CuNi has also recently been identified by theory and experiment to be an active and highly selective 20 catalyst for methanol synthesis from syngas. 3 Given the strong likelihood of nickel segregation in such catalysts under reducing conditions, their optimisation and development for use as practical catalysts would considerably benefit from an improved understanding of this behaviour.…”

“…9 )  Surface-adsorbate interactions becoming a significant energetic term when balanced by very little 'bulk.' Furthermore, surface oxide or hydride formation a 20 few atomic layers deep may account for a large fraction of the particle (as there are many more surface atoms) and thus be a major energetic term.  Diminished importance of kinetic limitations; in many bulk systems it is valid to assume only the top layer or layers of an alloy are restructured at moderate temperature, 6 whereas for nanoparticles complete restructuring of the material to 25 reach thermodynamic equilibrium has been seen on the 15 minute timescale of an ambient pressure XPS experiment.…”

“…These have then been supported on a planar SiO 2 surface and characterised in oxidising and reducing environments using ambient pressure XPS. With this new data, we aim to inform a discussion of how such bimetallic nanoparticles behave under catalytic conditions and the importance of addressing this issue experimentally and 20 theoretically in the preparation of new heterogeneous bimetallic catalysts.…”

“…Samples were prepared by drop casting the nanoparticles (in toluene) on to an ultra thin carbon coated copper TEM grid. A Zeiss Ultra55 scanning electron microscope was then used for imaging and confirming the integrity of the dipcoated films of nanoparticles on Si wafers before 20 and after the AP-XPS studies.…”