Three-dimensional strain dynamics govern the hysteresis in heterogeneous catalysis

Understanding and controlling the strain dynamics within catalysts is crucial for developing efficient and stable catalytic materials. Lattice strain significantly influences the chemical properties of supported metallic catalysts, altering the reactivity of metal surfaces. The d-band model explains this effect, where lattice distortion changes the surface d-band center, influencing adsorption and dissociation energies. For example, tensile strain in gold (Au), a late transition metal, leads to a narrower d-band and increased population, enhancing oxygen adsorption and lowering CO₂ dissociation barriers in CO oxidation. In nanoparticles (NPs), strain originates from intrinsic factors (size, morphology, facets, defects) and extrinsic factors (lattice mismatch, support interactions, core-shell structures). While techniques like X-ray diffraction and high-resolution transmission electron microscopy provide strain information, in situ and operando methods are needed to observe dynamic changes during reactions. Bragg coherent diffraction imaging (BraggCDI) is particularly useful for obtaining 3D strain maps and dynamic defect information under operating conditions. Previous work using in situ BraggCDI has revealed dynamic faceting of gold nanoparticles during CO oxidation and the formation of defect networks to accommodate strain. This study uses BraggCDI to investigate the relationship between strain, morphology, and catalytic hysteresis in CO oxidation using gold nanocrystals with controlled morphologies (cuboctahedra and nanocubes). CO oxidation is a model reaction of significant environmental and societal importance, often exhibiting inverse and direct hysteresis behavior during heating and cooling cycles. The study aims to elucidate the role of strain and defect evolution in the catalytic activity and hysteresis observed during CO oxidation.

Several studies have highlighted the impact of strain on catalytic activity. Theoretical calculations have demonstrated the ability to optimize adsorption and dissociation energies by controlling lattice strain, as explained by the d-band model. Experimental studies using techniques like X-ray diffraction and high-resolution transmission electron microscopy have shown strain variations in nanoparticles due to size, morphology, and support interactions. However, these methods typically provide average information. In situ and operando imaging techniques, such as environmental TEM, have been employed to study dynamic changes in catalysts under reaction conditions. Recently, BraggCDI has emerged as a powerful tool for obtaining 3D strain maps and dynamic information under operando conditions, providing insights into nanocrystal deformations and active site localization. Previous studies using BraggCDI have shown the dynamic faceting of gold nanoparticles and the formation of defect networks during catalytic reactions. However, a detailed investigation of the relationship between strain dynamics, nanoparticle morphology, and catalytic hysteresis has been lacking.

Morphology-controlled gold nanocrystals (cuboctahedra and nanocubes, ~60 nm) were synthesized using a seed-mediated growth method, controlling the amount of reducing agent (ascorbic acid) to tune the morphology. The nanocrystals were supported on TiO₂ and characterized using scanning electron microscopy (SEM) and small angle X-ray scattering (SAXS). Operando BraggCDI measurements were performed at the 34-ID-C beamline at the Advanced Photon Source. Coherent X-ray diffraction patterns were collected around the Au (111) Bragg peak at 9 keV. 3D diffraction data were acquired as rocking curves around the Bragg peak. Iterative phase retrieval algorithms were used to reconstruct the 3D Bragg electron density and lattice displacement fields. Strain was determined by spatial differentiation of the displacement field. Catalytic activity measurements were performed in a tubular reactor using a CO/O₂/He gas mixture. The CO conversion was monitored as a function of temperature during heating and cooling cycles. Operando BraggCDI measurements were performed simultaneously with mass spectrometry to monitor the CO conversion during the catalytic reaction. The elastic energy landscape was determined using the three-dimensional strain distribution and the bulk modulus of Au. The equation used for this calculation is provided in the original paper. Specifically, the 3D strain maps were obtained from BraggCDI measurements. The amplitude of the reconstructed image represents the electron density and the phase corresponds to the projection of the displacement of the crystal lattice on the scattering vector. The strain sensitivity achieved was around 2 × 10⁻⁴. The iterative phase retrieval algorithms used were Gerchberg-Saxton and hybrid input-output methods. The details regarding the synthesis of TiO2 support, the seed-mediated growth of gold nanoparticles, the catalytic tests, and the BraggCDI data acquisition and analysis were described in the original paper's methods section.

The study revealed several key findings: 1. **Anisotropic Strain Dynamics:** 3D strain mapping showed anisotropic strain distribution in both cuboctahedral and cubic gold nanocrystals. Strain was most pronounced at edges and corners. Upon calcination, both showed compressive surface strain. 2. **Morphology-Dependent Hysteresis:** The gold nanocrystals exhibited distinct hysteresis loops during CO oxidation: cuboctahedra showed inverse hysteresis (higher conversion during heating), while cubes showed normal hysteresis (higher conversion during cooling). 3. **Strain-Activity Correlation:** The hysteresis loops correlated with the nanocrystals' surface strain dynamics. A switch from compressive to tensile strain occurred upon flowing the CO/O₂ gas mixture. Tensile strain, particularly on {111} facets for cuboctahedra and {100} facets for cubes, was linked to active sites. The tensile strain propagated into the nanocrystal interior during the reaction and released during cooling. 4. **Facet-Dependent Reactivity:** Interestingly, even identical facets in a single nanocube exhibited unequal reactivity, highlighting that strain significantly influences catalytic activity at the nanoscale. 5. **Single-Particle Hysteresis:** The elastic energy landscape, mapped with attojoule resolution, revealed that hysteresis is present at the single-particle level. Cuboctahedra exhibited energy loss during the catalytic cycle, explaining the inverse hysteresis, while cubes showed no significant energy change, aligning with normal hysteresis. The energy landscape mapping provided unprecedented detail on the energy changes at the single nanoparticle level and indicated irreversible elastic energy release via lattice deformation and heat dissipation. The cuboctahedron did not recover its initial strain or energy after a CO oxidation cycle, while the cube did. This linked hysteresis to the structural and strain properties of the catalysts.

The findings directly address the research question by demonstrating a strong correlation between 3D strain dynamics, nanoparticle morphology, and catalytic hysteresis in CO oxidation. The observation of anisotropic strain formation and propagation, with preferential strain effects on specific facets, provides a detailed explanation for the observed facet-dependent reactivity. The demonstration of hysteresis at the single-particle level highlights the importance of considering nanoscale structural effects when understanding catalytic behavior. The energy landscape analysis revealed that the irreversible energy loss in cuboctahedra, but not cubes, during the catalytic cycle is directly linked to the inverse hysteresis, which contrasts with the long-held belief that hysteresis is solely due to reaction condition variations. These results are significant because they provide a fundamental understanding of how strain dynamics at the nanoscale govern the catalytic performance and hysteresis observed in CO oxidation, paving the way for more controlled design and tuning of catalysts.

This study provides unprecedented insights into the relationship between three-dimensional strain dynamics, nanoparticle morphology, and catalytic hysteresis using operando BraggCDI. The anisotropic strain patterns and facet-dependent reactivity observed demonstrate the crucial role of nanoscale strain in determining catalytic activity. The attojoule-level energy landscape mapping revealed that hysteresis is a single-particle phenomenon, linked to structural properties and strain dynamics. Future research could explore other catalytic reactions and materials, investigate the effects of different supports and promoters on strain dynamics, and develop strategies for manipulating the elastic energy of nanomaterials to enhance catalytic performance.

The study focused on a specific model system (gold nanocrystals on TiO₂ support) and a specific reaction (CO oxidation). The analysis focused on the (111) reflection; obtaining the full strain tensor would require multiple reflections, although this was not feasible due to the experimental limitations of the operando cell. The spatial resolution of the Bragg CDI measurements limited the ability to investigate very fine details of the strain distribution. The model system used, while well-suited for detailed investigation, may not be fully representative of all catalytic systems. These limitations suggest the need for further research to extend these findings to a wider range of catalytic systems and reaction conditions.