Abstract
The mechanism of carbon monoxide oxidation over gold was explored using a model planar catalyst consisting of monodisperse gold nanoparticles periodically arranged on single crystal SiO2/Si(111) substrates using a combination of Grazing Incidence Small Angle X-ray Scattering and Grazing Incidence X-ray Diffraction (GISAXS/GIXD) under reaction conditions. It is shown that nanoparticle composition, size and shape change when the catalyst is exposed to reactive gases. During CO oxidation, the particle's submergence depth with respect to the surface decreases due to the removal of gold oxide at the metal-support edge, meanwhile the particle 'flattens' to maximise the number of the reaction sites along its perimeter. The effect of the CO concentration on the catalyst structure is also discussed. Our results support the dual catalytic sites mechanism whereby CO is activated on the gold surface whereas molecular oxygen is dissociating at the gold-support interface.
Highlights
Oxygen dissociation could be activated by CO–O2 complex formation at the metal–support interface as observed by Green et al.[9] or by Au–OOH species formed near the particle perimeter as shown more recently in the work of Saavedra et al.[31] who stressed a critical role of support OH groups due to the presence of water
We have successfully combined a highly controlled synthesis method that yields uniform gold nanoparticles arranged in regular hexagonal arrays with advanced surface-sensitive X-ray scattering techniques to gain insight into the mechanism of CO oxidation, and the role of the metal–support interface
It was revealed that supported Au nanoparticles undergo size and shape transformations during CO oxidation, primarily due to gold oxide removal at the metal–support interface along the particle perimeter
Summary
In recent decades supported gold nanoparticles (NPs) have received considerable attention in the eld of heterogeneous catalysis due to their extraordinary catalytic performance.[1,2,3,4] When con ned at the nanoscale/atomic level and stabilised on various oxide supports, Au ceases to be inert, and instead becomes a catalyst of choice for many industrially important reactions including acetylene. It is generally agreed that catalytic performance during CO oxidation depends on the nanoparticle size, and the optimal size for the highest turnover frequency was reported to be $3 nm on titania[10] and alumina[11] supports. In the above studies the catalyst’s structure was probed before and/or a er the reaction, which prevents the understanding of which features are pertinent in a catalytic process
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