Boosting hot electron flux and catalytic activity at metal-oxide interfaces of PtCo bimetallic nanoparticles

Despite numerous studies, the origin of the enhanced catalytic performance of bimetallic nanoparticles (NPs) remains elusive because of the ever-changing surface structures, compositions, and oxidation states of NPs under reaction conditions. An effective strategy for obtaining critical clues for the phenomenon is real-time quantitative detection of hot electrons induced by a chemical reaction on the catalysts. Here, we investigate hot electrons excited on PtCo bimetallic NPs during H-2 oxidation by measuring the chemicurrent on a catalytic nanodiode while changing the Pt composition of the NPs. We reveal that the presence of a CoO/Pt interface enables efficient transport of electrons and higher catalytic activity for PtCo NPs. These results are consistent with theoretical calculations suggesting that lower activation energy and higher exothermicity are required for the reaction at the CoO/Pt interface

Schaltbare Oberfläche. Responsive Polymerschichten

Dünne Polymerschichten auf festen Oberflächen spielen eine wichtige Rolle in vielen technischen Anwendungsfeldern und Produkten des täglichen Gebrauchs. Oft dient der Polymerfilm vor allem als Schutzschicht und soll den direkten Kontakt von Stoffen mit der darunter liegenden Oberfläche verhindern. Zunehmend an Bedeutung gewinnen Polymerschichten mit spezialisierten Eigenschaften, zum Beispiel wasserabweisende Beschichtungen, Beschichtungen gegen Graffiti und zur Verhinderung des Bewuchses von Oberflächen, oder Beschichtungen, die bestimmte Wirkstoffe freisetzen können. Besonders nützlich können solche Polymerschichten werden, wenn es möglich ist, ihre Wirkung von au en gezielt ein und auszuschalte

A SIMPLIFIED THERMODYNAMIC ANALYSIS OF A LiBr-H 2 O VERTICAL TUBE ABSORBER

ABSTRACT One of the most important components of an absorption airconditioning/heat pump system is the absorber, where the refrigerant water vapour is absorbed into the liquid solution. While absorption systems have been in use for several years, the complex transport phenomena occurring in the absorber are not fully elucidated yet. Thus, an attempt is made to model the absorption process of water vapour in aqueous solutions of lithium bromide considering a falling-film, vertical-tube absorber. The proposed analysis is based on the formulation of four differential equations describing the spatial variation (parallel to the tube-axis) of solution mass, temperature, mass fraction and coolant temperature. The system of ordinary differential equations is numerically solved using a non-stiff numerical method. Thermophysical properties and especially, heat and mass transfer coefficients are calculated using widelyaccepted and reliable relationships, which are extracted from the literature using recently published information on wavylaminar flows. In the present study, the questionable assumption of treating the water vapour as an ideal gas is heavily modified utilizing. Consequently, the hypothesis of saturated water vapour at the steam-solution interaction surface is revised by introducing an energy difference between the superheated steam and the liquid water within the binary solution. The last correction encouraged us to compare theoretical results for solution temperature, mass fraction and mass flow rate, which were obtained using both assumptions. It was proved that the initial treatment causes an underestimation of the absorbed steam mass and correspondingly, an underestimation of solution temperature and mass fraction at the mass exchange interface. An attempt is made also to identify the effect of mass transfer coefficient on the effectiveness of the absorption process and on the energy differences between the superheated steam and the liquid water either as pure substance or as component of the binary mixture. It was shown that the increase of mass transfer coefficient leads to an increase of steam mass transfer rate and to a corresponding decrease of solution temperature slope at the entrance of a tube. Correspondingly, the increase of mass transfer coefficient results in an increase of heat of absorption and heat of dilution at the same variation range of the solution mass fraction

Is Steam an Oxidant or a Reductant for Nickel Doped Ceria Cermets?

Nickel/doped‐ceria composites are promising electrocatalysts for solid‐oxide fuel and electrolysis cells. Very often steam is present in the feedstock of the cells, frequently mixed with other gases, such as hydrogen or CO(2). An increase in the steam concentration in the feed mixture is considered accountable for the electrode oxidation and the deactivation of the device. However, direct experimental evidence of the steam interaction with nickel/doped‐ceria composites, with adequate surface specificity, are lacking. Herein we explore in situ the surface state of nickel/gadolinium‐doped ceria (NiGDC) under O(2), H(2), and H(2)O environments by using near‐ambient‐pressure X‐ray photoelectron and absorption spectroscopies. Changes in the surface oxidation state and composition of NiGDC in response to the ambient gas are observed. It is revealed that, in the mbar pressure regime and at intermediate temperature conditions (500–700 °C), steam acts as an oxidant for nickel but has a dual oxidant/reductant function for doped ceria

Synchrotron Radiation X-ray Photoelectron Spectroscopy as a Tool To Resolve the Dimensions of Spherical Core/Shell Nanoparticles

International audienceIn this work we demonstrate the potential of synchrotron X-ray photoelectron spectroscopy (XPS) to provide quantitative information on the intrinsic dimensions of core–shell nanoparticles. The methodology is based on the simulation of depth profiling curves, using simplified quantitative models earlier proposed in the literature. Three model systems consisting of X@Fe2O3 (with X = Au, Pt, and Rh) metal–iron oxide core–shell nanoparticles, formed via oxidation of size-selected 5 nm bimetallic FeX nanoparticles inside the spectrometer, were measured in situ by near ambient pressure XPS. We show that when the shell layer is composed of a unique component, the experimental depth profiling curve can be simulated by the quantitative calculations and reveal the core and the shell thickness of the nanoparticles. On the contrary, a significant offset between the experimental and the theoretical depth profiling curves implies intermixing between the core and the shell layers. In this case the theoretical model has been modified to represent the more complex particle morphology. Transmission electron microscopy results are in good agreement with the XPS findings, confirming the validity of the model to predict the nanoparticle dimensions

Mixing Patterns and Redox Properties of Iron Based Alloy Nanoparticles under Oxidation and Reduction Conditions

The redox behavior of 5 nm Fe-Me alloyed nanoparticles (where Me = Pt, Au, and Rh) was investigated in situ under H2 and O2 atmospheres by near ambient pressure X-ray photoelectron and absorption spectroscopies (NAP-XPS, XAS), together with ex situ transmission electron microscopy (TEM) and XAS spectra simulations. The preparation of well-defined Fe-Me nanoalloys with an initial size of 5 nm was achieved by using the mass-selected low energy cluster beam deposition (LECBD) technique. The spectroscopic methods permit the direct observation of the surface segregation and composition under different gas atmospheres and annealing temperatures. The ambient conditions were found to have a significant influence on the mixing pattern and oxidation state of the nanoparticles. In an oxidative atmosphere, iron oxidizes and segregates to the surface, leading to the formation of core–shell nanoparticles. This structure persists upon mild reduction conditions, while phase separation and formation of heterostructured bimetallic particles is observed upon H2 annealing at a higher temperature (400 °C). Depending on the noble metal core, the iron oxide shell might be partially distorted from its bulk structure, while the reduction in H2 is also significantly influenced. These insights can be of a great importance in understanding the activity and stability of Fe-based bimetallic nanoparticles under reactive environments