In-situ and operando study of reaction-induced strain on model catalysts using Bragg CDI

Gold nanoparticles are highly active catalysts for CO oxidation, although bulk gold is considered quite inert even to reactive molecules, the nanometer-size particles are efficient catalysts. Several effects contribute to the catalytic properties of supported nanoparticles catalysts such as particle size, morphology, exposed crystal facets, interaction with the support, low-coordinated atoms, strain effects and crystalline defects. During the reaction, nanoparticle surface goes under dynamic restructuration and expose different surface sites and defects with distinct reactivity. Understanding catalysts dynamic behaviours is crucial and requires the study of active sites IN-SITU and operando conditions. A peculiarity of CO oxidation reaction is the temperature hysteresis observed when during a complete cycle the catalytic properties do not match during heating and cooling. Explanations about the origin of this hysteresis loop are still conflicting. In order to investigate the facet-selectivity and dynamics involved in the active sites formation and the hysteresis behaviour we imaged the defects dynamics of gold nanocubes during CO oxidation reaction by Bragg CDI. Bragg CDI uses a coherent X-ray beam and phase-retrieval algorithms to image the 3D morphology of crystals and probe dynamic changes in strain and defects. I will show how the anisotropic strain, occurring during the oxidation reaction, provides new understanding of the formation of the active sites in gold nanocrystals. Significant changes were observed in the strain distribution during the reaction, regions of high tensile and compressed strain are concentrated on preferential facets in shape-controlled nanoparticles. The lattice strain can alter the reactivity of metal surfaces, strained metal surfaces have different chemical properties from those of unstrained surfaces due the shift in the d-band center. The tensile strain generation and release during the reaction directly influences the temperature hysteresis in CO oxidation. IN-SITU/operando strain imaging provide important insights to elucidate catalytic processes and designing catalysts with optimized morphology and defect control. (author)