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High-speed X-ray imaging of catalyst nanoparticles during oxidation reactions
08-10-2024
Stroboscopic Bragg coherent diffraction imaging (BCDI) at the ID01 beamline revealed the dynamic strain evolution in platinum nanoparticles during CO oxidation. With sub-second temporal resolution, BCDI offered insights into adsorption dynamics and strain variations at the atomic level, providing a deeper understanding of catalytic reactions under operando conditions.
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Understanding strain dynamics in catalytic nanoparticles (NP) is crucial for developing more efficient, stable, and cost-effective catalysts. Well-faceted NPs can probe the adsorption of reactants on different surface sites, such as {111} and {100} facets, providing valuable insights into catalytically driven reactions [1]. Beyond facet-dependent activity, lattice strain also plays a significant role in influencing metal surface reactivity. As a result, high-resolution, three-dimensional methods are needed to localize minor lattice displacements at the nanometre scale. In-situ BCDI offers such a method, enabling precise strain observations sensitive to displacements on the order of picometres.
Recent advancements in BCDI enable sub-second temporal resolution during operando chemical reactions. One approach involves acquiring 3D rocking curves while continuously sweeping goniometer motors [2]. With a 2D detector operating at maximum speed, an entire rocking curve can be recorded in 5 seconds, allowing real-time imaging of particle strain during gas pressure changes, with a spatial resolution of 15 nm. The second approach, stroboscopic BCDI (SBCDI), uses reversible strain changes induced by pulses of reactant gases. Diffraction data are recorded at each step of a rocking curve, revealing strain evolution with a time resolution of 0.25 seconds and a spatial resolution of 20 nm. These techniques enable the study of adsorption dynamics in catalytic nanomaterials under operando conditions (Figure 1a).
Click image to enlarge
Fig. 1: SBCDI reconstructions. a) Wire-frame plot of particle amplitude. Strain (ε111) through YZ central slice is presented. The initial strain pattern of the particle is shown for the central slices, averaged from scans reconstructed before t0. b) Gas conditions as recorded by the mass spectrometer, averaged across each step of the rocking curve. c) Strain changes within the particle with respect to the initial state in (a) as a function of delay time with respect to the arrival of gas. These are averaged across the shaded regions in (b). The second row shows the changes in strain at the particle surface.
This work focused on platinum (Pt) nanoparticles undergoing gas-phase CO oxidation, with particular attention to strain changes at the {111} facets, known for facilitating CO and O2 adsorption [1]. BCDI was used at beamline ID01 to localize lattice deformations during operando reactions. While previous studies using X-ray diffraction and photoemission spectroscopy examined structural evolution during CO oxidation, the temporal dynamics of strain evolution have remained less understood.
An example of gas conditions applied to a Pt nanoparticle during SBCDI is shown in Figure 1b. By reconstructing the particle strain, changes over time were analyzed (Figure 1c). The 3D strain profile revealed significant changes in surface and subsurface regions, where localized strain was observed along the {111} direction. A rapid increase in tensile strain at the top and bottom of the Pt {111} facets during CO adsorption was evident, consistent with previous observations [1]. By recording exhaust gases with a mass spectrometer, the strain changes were directly correlated with gas flux conditions for the first time.
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Fig. 2: Nanoparticle surface changes. a) Evolution of the mean strain (ε111) computed from the lattice spacing for the two voxel populations; surface (red) and bulk (blue). The strain evolution profile has been smoothed using a Savitsky-Golay filter with window size 11 to highlight the oscillatory behaviour. The surface strain is fit with a piecewise exponential growth model. The rate of strain evolution was determined to be 0.105 s-1. b) The strain oscillation relative to the fit is shown, with the period and amplitude of this oscillation increases over time. c) By applying a fast Fourier transform to this oscillation, the peak frequency was found to be 0.155 Hz, with bandwidth of σ ≈ 0.054 Hz. This corresponds to oscillations with periods ranging from 5.9 to 8.6 s.
The rate of strain evolution in the catalyst NP was estimated with fine spatial resolution, as shown in Figure 2. Reconstructions distinguished strain changes related to adsorption processes, which were limited to the surface and caused heterogeneous strain variations, from thermal changes, which were isotropic and homogeneous across the particle. Figure 2a shows that adsorption processes dominated strain changes, with the rate correlating to gas arrival. Oscillatory strain changes with a 6.4-second period, linked to CO adsorption during oxidation, were detected, with a time resolution of 0.25 seconds. These oscillations were also observed in earlier transmission electron microscopy studies during CO oxidation on Pt nanoparticles [3].
In summary, real-time BCDI investigations during CO oxidation revealed dynamic strain evolution, influenced by the initial gas environment. By pulsing CO and O2 gases while recording diffraction intensity, phase retrieval of the Pt catalyst was achieved with a temporal resolution of 0.25 seconds, showing lattice compression at {111} surfaces under O2 environments, followed by expansion upon CO arrival. The observed strain change rate (τ = 9.6 s) and oscillatory strain evolution (T = 6.4 s) correlated with CO2 production and CO adsorption rates. Time-resolved BCDI, therefore, offers the potential to study a wide range of catalyst-assisted in-situ and operando reactions, providing new insights into catalyst particle strain evolution during these processes.
Principal publication and authors
Capturing Catalyst Strain Dynamics during Operando CO Oxidation, M. Grimes (a), C. Atlan (a), C. Chatelier (a), E. Bellec (a), K. Olson (a), D. Simonne (a), M. Levi (b), T.U. Schülli (c), S.J. Leake (c), E. Rabkin (b), J. Eymery (a), M.-I. Richard (a), ACS Nano 18, 19608-19617 (2024); https://doi.org/10.1021/acsnano.4c04127
(a) CEA Grenoble, Grenoble (France)
(b) Technion-Israel Institute of Technology, Haifa (Israel)
(c) ESRF
References
[1] M. Dupraz et al., Nat. Commun. 13, 1-10 (2022).
[2] N. Li et al., Sci. Rep. 10, 12760 (2020).
[3] S.B. Vendelbo et al., Nat. Mater. 13, 884-890, (2014).
About the beamline: ID01 |
ID01 is a versatile X-ray diffraction and scattering beamline capable of delivering beams as small as 35 nm. It is dedicated to the investigation of a wide range of crystalline materials, from nanostructures to bulk, with the ability to image strain and structure using full-field diffraction imaging, coherent X-ray diffraction methods, and nanodiffraction. Typical samples include microelectronic devices, novel metal-organic solar cells, and battery electrodes. |