New end-station for operando experiments at BM31

Progress in sustainable technologies relies on the development of innovative materials, which requires an in-depth understanding of the interplay between a material’s structure and its macroscopic properties. The availability of experimental methods that probe a material’s structure at different length and time scales is therefore key for obtaining fundamental insights into technologically relevant materials. To this end, there is a continuous need to improve X-ray-based approaches that allow the study of materials with multiple techniques during their working state (i.e., operando methods).

The new BM31 beamline and end-station combines X-ray diffraction (XRD) and X-ray total scattering (TS) data for pair distribution function (PDF-TS) analysis with multi-edge X-ray absorption spectroscopy (XAS) measurements to allow the acquisition of high-quality, complementary information on a material under relevant working conditions. This covers the length-scale from short- to mid-range atomic arrangements viz. ~ 1 Å to several nm by PDF, the average structure by XRD, as well as the electronic state and geometry around the element of interest by XAS.

The beamline’s potential is illustrated by a study on the formation of single-phase, high-entropy alloy (HEA) nanoparticles as catalysts for the oxygen reduction reaction. The experiments performed at BM31 combined in-situ XRD with XAS on five edges (Pt-Ir-Os-Rh-Ru), complemented with a series of ex-situ PDF-TS (Figure 1) and imaging experiments. This enabled a thorough investigation of the low-temperature formation of single-phase HEA nanoparticles as potential (electro-)catalysts for a series of relevant chemical reactions.

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Fig 1: In-situ XAS-XRD studies on the formation of two different types of HEA nanoparticles (fcc and hcp) from five element mixtures. The linear combination analysis (LCA) on the XANES data and the sequential refinement (lattice parameter a, crystallite domain size and lattice strain) on the fcc HEA are displayed (a-d) and ex-situ Rietveld refinement of the XRD and PDF-TS refinement for the as-synthesied fcc and hcp HEA (e-f).
 

In-situ X-ray absorption near-edge structure (XANES) spectroscopy revealed the kinetics of reductions of all metals involved. It was found that Pt reduced at the lowest temperatures, followed by Os, Ir, Ru, and lastly Rh. Using XRD, the crystallisation of an fcc phase was observed at 170°C, which contracted continuously as the metals with smaller atomic radii incorporated into the alloy.  At the same time, a pronounced increase in lattice strain accompanied the initial lattice contraction at lower temperatures, whereas the crystallite domains grew at higher temperatures above 350°C, where all five elements were reduced. The hcp single-phase HEA displayed a similar formation mechanism.

Rietveld analysis of the in-situ diffraction data indicated a pure, single-phase fcc alloy, while the refinements for the hcp-HEA could not correctly describe the ratios of Bragg peaks, indicating a highly distorted and defect-rich structure for the hcp alloy. The PDF data clarified the nature of the defect-rich structure. The fast damping in the PDF-TS data (pair distances up to ca. 30 Å/fcc phase and ca. 50 Å/hcp phase) is well in line with the disordered lattices and reaffirmed the strained nature of the crystal lattice of the HEA nanoparticles.

Overall, this study demonstrates the great potential of using the combined XAS-XRD and PDF/TS approach available at BM31 (aided by computational simulations and advanced imaging analysis) to thoroughly explore, at the atomic scale, the complex and dynamic structure of industrially relevant materials under working conditions. BM31’s  capabilities can also be exploited for advanced combined studies on energy, materials, batteries and catalytic reactions.

Principal publication and authors
The more the better: on the formation of single-phase high entropy alloy nanoparticles as catalysts for the oxygen reduction reaction, R.K. Pittkowski (a), C.M. Clausen (a), Q. Chen (a), D. Stoian (b), W. van Beek (b), J. Bucher (c), R.L. Welten (c), N. Schlegel (c), J.K. Mathiesen (a,d), T.M. Nielsen (a), J. Du (c), A.W. Rosenkranz (e), E.D. Bøjesen (e), J. Rossmeisl (a), K.M.Ø. Jensen (a), M. Arenz (c), EES. Catal. (2023); https://doi.org/10.1039/D3EY00201B
(a) Center for High Entropy Alloy Catalysis (CHEAC), Department of Chemistry, University of Copenhagen (Denmark)
(b) Swiss Norwegian Beamlines, ESRF
(c) Department of Chemistry, Biochemistry and Pharmaceutical Sciences, University of Bern (Switzerland)
(d) Department of Physics, Technical University of Denmark (Denmark)
(e) Aarhus University, Interdisciplinary Nanoscience Center (Denmark)