Abellan P.,Daresbury Laboratory |
Parent L.R.,Fundamental and Computational Science Dir. |
Parent L.R.,University of California at San Diego |
Al Hasan N.,Institute for Integrated Catalysis |
And 5 more authors.
Synthesizing nanomaterials of uniform shape and size is of critical importance to access and manipulate the novel structure-property relationships arising at the nanoscale, such as catalytic activity. In this work, we synthesize Pd nanoparticles with well-controlled size in the sub-3 nm range using scanning transmission electron microscopy (STEM) in combination with an in situ liquid stage. We use an aromatic hydrocarbon (toluene) as a solvent that is very resistant to high-energy electron irradiation, which creates a net reducing environment without the need for additives to scavenge oxidizing radicals. The primary reducing species is molecular hydrogen, which is a widely used reductant in the synthesis of supported metal catalysts. We propose a mechanism of particle formation based on the effect of tri-n-octylphosphine (TOP) on size stabilization, relatively low production of radicals, and autocatalytic reduction of Pd(II) compounds. We combine in situ STEM results with insights from in situ small-angle X-ray scattering (SAXS) from alcohol-based synthesis, having similar reduction potential, in a customized microfluidic device as well as ex situ bulk experiments. This has allowed us to develop a fundamental growth model for the synthesis of size-stabilized Pd nanoparticles and demonstrate the utility of correlating different in situ and ex situ characterization techniques to understand, and ultimately control, metal nanostructure synthesis. © 2016 American Chemical Society. Source
Wang Y.-G.,Tsinghua University |
Wang Y.-G.,Institute for Integrated Catalysis |
Mei D.,Institute for Integrated Catalysis |
Li J.,Tsinghua University |
And 2 more authors.
Journal of Physical Chemistry C
We present the results of an extensive density functional theory based electronic structure study of the role of 4f-state localized electron states in the surface chemistry of a partially reduced CeO2(111) surface. These electrons exist in polaronic states, residing at Ce3+ sites, which can be created by either the formation of oxygen vacancies, OV, or other surface defects. Via ab initio molecular dynamics, these localized electrons are found to be able to move freely within the upper surface layer, but penetration into the bulk is inhibited as a result of the higher elastic strain induced by creating a subsurface Ce3+. We found that the water molecule can be easily dissociated into two surface bound hydroxyls at the Ce4+ site associated with OV sites. This dissociation process does not significantly affect the electronic structure of the excess electrons at reduced surface, but does lead to a favorable localization on Ce3+ sites in the vicinity of the resulting OH groups. In the presence of water, a proton-mediated Mars-van Krevelen mechanism for CO oxidation via the formation of bicarbonate species is identified. The localized 4f electrons on the surface facilitate the protonation process of adsorbed O2 species and thus decelerate the further oxidation of CO species. Overall, we find that surface hydroxyls formed via water dissociation at the CeO2 surface lead to inhabitation of the CO oxidation reaction. This is consistent with the experimental observation of requisite elevated temperatures, on the order of 600 K, for this reaction to occur. © 2013 American Chemical Society. Source