Metal-support interaction and charge distribution in ceria-supported Au particles exposed to CO

Understanding how reaction conditions affect metal-support interactions in catalytic materials is one of the most challenging tasks in heterogeneous catalysis research. Metal nanoparticles and their supports often undergo changes in structure and oxidation state when exposed to reactants, hindering a straightforward understanding of the structure-activity relations using only ex situ or ultrahigh vacuum techniques. Overcoming these limitations, we explored the metal-support interaction between gold nanoparticles and ceria supports in ultrahigh vacuum and after exposure to CO. A combination of in situ methods (on powder and model Au/CeO2 samples) and theoretical calculations was applied to investigate the gold/ceria interface and its reactivity toward CO exposure. X-ray photoelectron spectroscopy measurements rationalized by first-principles calculations reveal a distinctly inhomogeneous charge distribution, with Au+ atoms in contact with the ceria substrate and neutral Au0 atoms at the surface of the Au nanoparticles. Exposure to CO partially reduces the ceria substrate, leading to electron transfer to the supported Au nanoparticles. Transferred electrons can delocalize among the neutral Au atoms of the particle or contribute to forming inert Auδ− atoms near oxygen vacancies at the ceria surface. This charge redistribution is consistent with the evolution of the vibrational frequencies of CO adsorbed on Au particles obtained using diffuse reflectance infrared Fourier transform spectroscopy

Structural and optical properties of a perylene bisimide in aqueous media

The aggregation of a water soluble, dicationic perylene bisimide derivative was studied by means of absorption and emission spectroscopies, X-ray and neutron scattering techniques as well as electron microscopy. The results provide evidence for the existence of higher order molecular aggregates in solution, potentially utilizable in device fabrication as super molecular building blocks

Inter-Backbone Charge Transfer as Prerequisite for Long-Range Conductivity in Perylene Bisimide Hydrogels

Hydrogelation, the self-assembly of molecules into soft, water-loaded networks, is one way to bridge the structural gap between single molecules and functional materials. The potential of hydrogels, such as those based on perylene bisimides, lies in their chemical, physical, optical, and electronic properties, which are governed by the supramolecular structure of the gel. However, the structural motifs and their precise role for long-range conductivity are yet to be explored. Here, we present a comprehensive structural picture of a perylene bisimide hydrogel, suggesting that its long-range conductivity is limited by charge transfer between electronic backbones. We reveal nanocrystalline ribbon-like structures as the electronic and structural backbone units between which charge transfer is mediated by polar solvent bridges. We exemplify this effect with sensing, where exposure to polar vapor enhances conductivity by 5 orders of magnitude, emphasizing the crucial role of the interplay between structural motif and surrounding medium for the rational design of devices based on nanocrystalline hydrogels

Electrochemical activity of the polycrystalline cerium oxide films for hydrogen peroxide detection

Polycrystalline cerium oxide thin films (15 nm) deposited on a glassy carbon substrate were used as an electrode in a mediator-free, non-enzymatic electrochemical sensor for hydrogen peroxide. The electrode surface was characterized by X-ray photoelectron spectroscopy, resonant photoelectron spectroscopy, scanning electron microscopy and atomic force microscopy. The electrode sensitivity, detection limit and pH range of sensor stability were determined by applying electrochemical techniques: cyclic voltammetry and chronoamperometry. It was found that the sensor reactivity to H2O2 is directly related to the presence of electroactive cerium centres of 3+ character on the electrode surface. The Michaelis–Menten mechanism of catalase-like activity of ceria film is suggested as an explanation of the data and discussed. The results confirmed the sensing abilities of technologically well-accessible nanostructured cerium oxide films for hydrogen peroxide detection without using a mediator, i.e. the enzymatic properties of CeO2/GC electrode

Surface Composition of a Highly Active Pt 3

Currently, platinum is the most widely used catalyst for low temperature proton exchange membrane fuel cells (PEMFC). However, the kinetics at the cathode are slow, and the price of platinum is high. To improve oxygen reduction reaction (ORR) kinetics at the cathode, platinum can be alloyed with rare earth elements, such as yttrium. We report that Pt3Y has the potential to be over 2 times more active for the ORR compared with Pt inside a real fuel cell. We present detailed photoemission analysis into the nature of the sputtered catalyst surface, using synchrotron radiation photoelectron spectroscopy (SRPES) to examine if surface adsorbates or impurities are present and can be removed. Pretreatment removes most of the yttrium oxide in the surface leaving behind a Pt overlayer which is only a few monolayers thick. Evidence of a substochiometric oxide peak in the Y 3d core level is presented, this oxide extends into the surface even after Ar+ sputter cleaning in-situ. This information will aid the development of new highly active nanocatalysts for employment in real fuel cell electrodes