Stephen W. T. Price

Exploring the First Steps in Core–Shell Electrocatalyst Preparation: In Situ Characterization of the Underpotential Deposition of Cu on Supported Au Nanoparticles

The underpotential deposition (upd) of a Cu shell on a non-Pt nanoparticle core followed by galvanic displacement of the Cu template shell to form core–shell electrocatalyst materials is one means by which the Pt-based mass activity targets required for commercialization of PEM fuel cells may be reached. In situ EXAFS measurements were conducted at both the Au L3 and the Cu K absorption edges during deposition of Cu onto a carbon-supported Au electrocatalyst to study the initial stages of formation of such a core–shell electrocatalyst. The Au L3 EXAFS data obtained in 0.5 mol dm–3 H2SO4 show that the shape of the Au core is potential dependent, from a flattened to a round spherical shape as the Cu upd potential is approached. Following the addition of 2 mmol dm–3 Cu, the structure was also measured as a function of the applied potential. At +0.2 V vs Hg/Hg2SO4, the Cu2+ species was found to be a hydrated octahedron. As the potential was made more negative, single-crystal studies predict an ordered bilayer of sulfate anions and partially discharged Cu ions, followed by a complete/uniform layer of Cu atoms. In contrast, the model obtained by fitting the Au L3 and Cu K EXAFS data corresponds first to partially discharged Cu ions deposited at the defect sites in the outer shell of the Au nanoparticles at −0.42 V, followed by the growth of clusters of Cu atoms at −0.51 V. The absence of a uniform/complete Cu shell, even at the most negative potentials investigated, has implications for the structure, and the activity and/or stability, of the core–shell catalyst that would be subsequently formed following galvanic displacement of the Cu shell.

In Situ XAS Studies of Core-Shell PEM Fuel Cell Catalysts: The Opportunities and Challenges

Core-shell electrocatalysts are of increased interest in PEM fuel cells as a means of improving activity and reducing costs. In situ x-ray absorption spectroscopy (XAS) provides a unique opportunity to provide characterization of the structure of such materials under operating conditions, enabling the effects of the electrochemical environment on the structures to be explored. However, such measurements present numerous challenges in terms of both data collection and analysis. Here we present two case studies that illustrate both the opportunities and challenges; (i) the modification of a Pt catalyst by Ru and the potential dependent formation of a surface alloy and (ii) a Pt shell - Pd core catalyst with varying shell thickness.

In Situ Multimodal 3D Chemical Imaging of a Hierarchically Structured Core@Shell Catalyst

A Cu/ZnO/Al2O3@ZSM-5 core@shell catalyst active for one-step conversion of synthesis gas to dimethyl ether (DME) was imaged simultaneously and in situ using synchrotron-based micro X-ray fluorescence (μ-XRF), X-ray diffraction (μ-XRD), and scanning transmission X-ray microscopy (STXM) computed tomography (CT) with micrometer spatial resolution. An identical sample volume was imaged stepwise, first under oxidizing and reducing atmospheres (imitating calcination and activation processes), and then under model reaction conditions for DME synthesis (H2:CO:CO2 ratio of 16:8:1, up to 250 °C). The multimodal imaging methods offered insights into the active metal structure and speciation within the catalyst, and allowed imaging of both the catalyst core and zeolite shell in a single acquisition. Dispersion of nanosized Cu species was observed in the catalyst core during reduction, with formation of a metastable Cu+ phase at the core-shell interface. Under DME reaction conditions at 1 bar, the coexistence of Cu0 in the active catalyst core together with partially oxidized Cu species was unraveled. The zeolite shell and core-shell interface remained stable under all conditions, preserving the bifunctional nature of the catalyst. These observations are inaccessible using standard bulk techniques like X-ray absorption spectroscopy (XAS) and XRD, demonstrating the potential of multimodal in situ X-ray CT for characterization of hierarchically designed materials, which stand to benefit tremendously from such 3D spatially resolved measurements.

The application of molecular dynamics to fitting EXAFS data

In recent work, a new method for using the outputs of molecular dynamics simulations to generate enhanced inputs for EXAFS analysis of nanoparticles was reported. Although currently limited to the first coordination shell, the method was demonstrated to improve both the quality of the fit, and the cross-correlation of the EXAFS data with that from TEM and XRD. Here we discuss further the justifications behind this assertion.