Vibrational Disorder Effects on Temperature-Resolved X-Ray Absorption Signatures of Metal Catalysts: From Single-Atoms to Clusters and Nanoparticles.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 42044374.
- Also identified by DOI 10.1021/acsnano.5c20042 and PMC identifier 13173657.
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Abstract
Revealing dynamic local-structure changes of (sub)nanometric metal species under operating conditions is essential. In heterogeneous catalysis, this insight enables the rationalization of operation and optimization of catalyst efficiency and stability. Extended X-ray absorption fine structure (EXAFS) provides element-specific access to metal-metal coordination numbers, interatomic distances, and local disorder, which is pivotal when active motifs lack long-range order. Yet, accurate determination of structural parameters from EXAFS signatures is often complicated by the convolution of static heterogeneity and thermal vibration effects, encoded in the Debye-Waller factor: <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mi>σ</mi></mrow><mrow><mn>2</mn></mrow></msup><mo>=</mo><msubsup><mi>σ</mi><mrow><mi>dynamic</mi></mrow><mn>2</mn></msubsup><mrow><mo>(</mo><mi>T</mi><mo>)</mo></mrow><mo>+</mo><msubsup><mi>σ</mi><mrow><mi>static</mi></mrow><mn>2</mn></msubsup></math>. This coupling, especially at elevated temperatures typical of <i>in situ</i> and <i>operando</i> studies, obscures genuine structural changes. Here we present a temperature-resolved EXAFS study geared toward deconvoluting <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mi>σ</mi><mrow><mi>dynamic</mi></mrow><mn>2</mn></msubsup></math>(<i>T</i>) in three supported Ag catalysts spanning different <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mi>σ</mi><mrow><mi>static</mi></mrow><mn>2</mn></msubsup></math> levels and metal aggregation states: Al<sub>2</sub>O<sub>3</sub>-supported Ag nanocrystals, few-atom Ag clusters confined to a zeotype host, and single-atom Ag dispersed on WO<sub><i>x</i></sub>/Al<sub>2</sub>O<sub>3</sub>. Over 298-723 K, representative of catalyst activation and deployment conditions, we observe a nuclearity-dependent vibrational stiffness: Ag-Ag bonds in nanoparticles show strong thermal disorder, whereas Ag-O bonds in single-atoms and confined clusters remain comparatively rigid, limiting dynamic fluxionality. While a classical formalism, such as the correlated Einstein model, adequately captures nanocrystal dynamics, it fails for few- and single-atom motifs. Therefore, a direct parametrization of σ<sup>2</sup>(<i>T</i>) is proposed, better capturing vibrational disorder in low-nuclearity metal catalysts. The results provide guidance for decoupling thermal and static contributions in temperature-resolved EXAFS studies, enabling a more reliable structural analysis of (sub)nanometric metal species under <i>operando</i> conditions.