In Situ Electron Microscopy Study of Dynamic Structural Changes in CuZn Bimetallic Nanoparticles

In Situ Electron Microscopy Study of Dynamic Structural Changes in CuZn Bimetallic Nanoparticles

Background Introduction

Bimetallic nanocomposites have gained widespread attention due to their tunable physicochemical properties and significant applications in engineering and catalysis. In nanocomposites, the synergistic effects between metal elements often lead to excellent catalytic performance, even imparting unique properties that are not present in single metal elements. However, bimetallic alloys face issues of structural and chemical instability. It is well known that catalysts exist in a metastable state under reaction conditions. During catalysis, the reaction between the atmosphere and metal alloys may destabilize the alloy structure, leading to dynamic changes in structure and chemical composition, such as phase separation, surface segregation, and the formation of new structures. This transformation becomes more pronounced, especially when the metal elements in the bimetallic alloy have significantly different redox properties. Considering the sensitivity of catalytic performance to structure, changes in the structure of alloy catalysts may lead to alterations in catalyst activity and selectivity. Therefore, studying the redox properties of alloy catalysts is crucial, as it provides important insights into predicting possible structural changes of alloy catalysts under relevant reaction conditions.
This work employs in situ electron microscopy to investigate the structural changes of CuZn alloy particles in different atmospheric environments (including H2, O2, and H2/O2 mixed atmosphere). The choice of CuZn alloy for this study is primarily due to the unique redox properties of its constituent metals (Cu0 and Zn0), which differ significantly; moreover, Cu and Zn are key components of industrial methanol synthesis catalysts (Cu/ZnO/Al2O3). Some studies suggest that the surface CuZn alloy is the active substance/structure for methanol synthesis. However, other studies challenge this view, arguing that the CuZn alloy formed during the activation process may undergo de-alloying under reaction conditions. Therefore, investigating the redox properties of CuZn alloys and their dynamic changes under redox atmospheres is important for better understanding the chemical state and structure of CuZn-based catalysts under relevant reaction conditions.

Results Summary

In this work, we selected CuZn alloy particles as a model system and systematically studied the structural and compositional changes of CuZn alloy particles under high-temperature H2, O2, and H2/O2 mixed (HOR reaction) conditions using in situ TEM combined with ex situ TEM, XRD, and XPS methods. Real-time (S)TEM imaging and in situ compositional analysis revealed the destabilization process of CuZn alloy particles under high-temperature coexistence of H2 and O2: i) Zn gradually segregates and oxidizes, causing de-alloying, which leads to the formation of Cu/ZnO core/shell nanostructures. ii) The Cu core undergoes dynamic structural changes and diffuses outward through a non-dense ZnO shell, resulting in the appearance of Cu nanoparticles on the shell surface and externally (on SiNx support films). The Cu nanoparticles exhibit complex structural dynamics characterized by oscillatory transitions between Cu and Cu2O; the production of H2O and the consumption of O2 (demonstrating the catalytic activity of the catalyst) were verified through online mass spectrometry monitoring. Under the same temperature conditions of pure H2 or O2, the structural and chemical changes of CuZn differ markedly from those in the mixed atmosphere. Under high-temperature H2 conditions, Zn sublimates, and CuZn alloy particles gradually transform into Cu nanoparticles (with trace Zn); during high-temperature O2 treatment, CuZn nanoparticles progressively evolve into particle aggregates composed of CuO and ZnO.
Through in situ (S)TEM observations of structural and compositional transformations of alloy nanoparticles under different redox atmospheres, this work demonstrates that the initial alloy particles merely act as precatalysts, and their structure and composition may undergo significant changes under catalytic reaction conditions. Therefore, conducting in situ studies on alloy catalysts under reaction conditions or performing detailed ex situ characterizations on used catalysts is crucial for understanding the true structure-activity relationships. Furthermore, the distinct transformation pathways and final states of CuZn alloy particles under different redox atmospheres provide guidance for the design and preparation of CuZn-based catalysts for reverse water-gas shift and methanol synthesis reactions.

Illustrative Guide

In Situ Electron Microscopy Study of Dynamic Structural Changes in CuZn Bimetallic Nanoparticles

Figure 1. Morphology, composition, and structural characterization of the initial CuZn alloy particle sample. (a) Low-magnification HAADF-STEM image. (b) Particle size distribution. (c) XRD characterization. (d) High-magnification HAADF-STEM image and (e) HRTEM image, with yellow dashed lines marking phase boundaries. (f) HAADF-STEM image of CuZn NPs and (g) corresponding elemental distribution maps of Cu (red), Zn (green), and O (blue). (h) EDX line profile of Cu, Zn, and O. (i-k) EDX mapping images of Cu, Zn, and O elements.

In Situ Electron Microscopy Study of Dynamic Structural Changes in CuZn Bimetallic Nanoparticles

Figure 2. In situ observation of structural and compositional changes of CuZn particles. (a) HAADF-STEM images of CuZn nanoparticles recorded at 350 °C, 20% H2, 80% He, (b) 450 °C, and (c) 550 °C, with a total pressure of 135 mbar. (d, e) EDX line profiles of the dashed rectangular regions in images (a, c). (f) Overlay comparison of the contour lines of CuZn nanoparticles in (a-c). (g) Measurement of Zn/Cu atomic ratios in the core and shell regions under 350 °C, 20% H2, 80% He and 550 °C, 2% O2, 20% H2, 78% He conditions. (h-s) In situ (S)TEM real-time tracking of the dynamic changes of CuZn NPs under 550 °C, 2% O2, 20% H2, 78% He, revealing the dynamic reconstruction of Cu particles within the ZnO shell (h-k), the release of Cu material (l-o), and the migration and reconstruction of Cu particles on the surface of core-shell structured particles (p-s).

In Situ Electron Microscopy Study of Dynamic Structural Changes in CuZn Bimetallic Nanoparticles

Figure 3. Correlation between structural dynamics and catalytic activity. In a 10% H2, 2% O2, and 78% He atmosphere, in situ recorded HAADF-STEM images during temperature decrease (a-d) and increase (e-h) (650 °C↔350 °C). (i) In situ mass spectrometry data recorded during the aforementioned temperature cycling.

In Situ Electron Microscopy Study of Dynamic Structural Changes in CuZn Bimetallic Nanoparticles

Figure 4. Structural evolution process under high-temperature pure H2 or O2 atmospheres. HAADF-STEM and SE-STEM images recorded in H2 at 350 °C (a, b), 450 °C (c, d), and 550 °C (e, f) (pH2=5 Pa). HAADF-STEM and SE-STEM images captured in O2 at 350 °C (g, h), 450 °C (i, j), and 550 °C (k, l) (pO2=2 Pa).

In Situ Electron Microscopy Study of Dynamic Structural Changes in CuZn Bimetallic Nanoparticles

Figure 5. Ex situ characterization after heat treatment in H2 or O2. (a) HRTEM image of CuZn nanoparticles after treatment at 550 °C, H2 (pH2=10 Pa), (b) Fourier transform of the overall particle, and HRTEM enlarged image of the region shown in (a), (c) HAADF-STEM image, and (d) EDX mapping image. (e) HRTEM image of CuZn NPs after treatment at 550 °C in O2 (pO2=10 Pa), (f) HRTEM enlarged image of the region shown in (e), (g) HAADF-STEM image, and (h) EDX mapping image. (i) XRD images of samples after O2 treatment and (j) H2 treatment. (k) Zn 2p3/2 and (l) Cu 2p XPS data of original sample, O2 treated, and H2 treated samples. (m) Zn/Cu atomic ratio derived from XPS data.

In Situ Electron Microscopy Study of Dynamic Structural Changes in CuZn Bimetallic Nanoparticles

Figure 6. Schematic diagram of dynamic changes of CuZn nanoparticles in (a) H2/O2 mixed atmosphere and (b) pure H2 or O2 conditions.

Author Introduction

First Author Introduction:
Yue Shengnan, PhD student in Physical Chemistry at Fuzhou University, class of 2022, supervised by Professor Huang Xing. Main research direction: In the fields of materials and catalysis, using multi-field coupled sample rods combined with electron microscopy to study the dynamic evolution of nanomaterials in different environmental media.
Corresponding Author Introduction:
Professor Huang Xing: Graduated with a PhD from the Institute of Physical Chemistry, Chinese Academy of Sciences in 2013, has worked at the Fritz Haber Institute of the Max Planck Society in Germany, the Institute of Chemical Energy Conversion, and ETH Zurich, Switzerland. Joined Fuzhou University in 2020 as an independent PI; mainly engaged in the application research of (in situ) electron microscopy in catalysis and materials (surface/interface structure, structure-activity mechanism studies). Over 100 papers published in renowned academic journals such as Science, Nat. Catal., Nature Commun., Adv. Mater., Angew. Chem. Int. Ed., Adv. Funct. Mater., ACS Nano, Nano Lett., with nearly 8000 citations, an H-index of 45, and 6 Chinese authorized patents. Has received numerous honors or titles such as the Excellent Award from the President of the Chinese Academy of Sciences, JMCA “Emerging Investigator”, Chinese New Talent in Science, National Youth Thousand Talents Program, Class B Talent in Fujian Province, Distinguished Professor of “Minjiang Scholar” in Fujian Province, and Fujian Youth Science and Technology Award.
Research Group Website Link:
https://www.x-mol.com/groups/Huang_X
Research Group Recruitment Information:
Due to the development needs of the research group, we are currently recruiting 2-3 postdoctoral researchers and research assistants based in the School of Chemistry, Fuzhou University, with generous compensation (negotiable). We welcome outstanding talents to join us!
Contact Email: [email protected]

Article Information

Yue S, Li Q, Zeng C, et al. Structural and chemical transformations of CuZn alloy nanoparticles under reactive redox atmospheres: An in situ TEM study. Nano Research, 2024, https://doi.org/10.1007/s12274-024-6538-0.

In Situ Electron Microscopy Study of Dynamic Structural Changes in CuZn Bimetallic Nanoparticles

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In Situ Electron Microscopy Study of Dynamic Structural Changes in CuZn Bimetallic Nanoparticles

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In Situ Electron Microscopy Study of Dynamic Structural Changes in CuZn Bimetallic Nanoparticles
In Situ Electron Microscopy Study of Dynamic Structural Changes in CuZn Bimetallic Nanoparticles

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