Abstract
AbstractIn the present work, we report on a comparative study of model catalysts during ethylene epoxidation reaction under industrially relevant conditions. The catalysts consist of Ag nanoparticles <6 nm and a reference sample ∼100 nm. Combining catalytic data with transmission electron microscopy, thermal desorption spectroscopy, and density functional theory allows us to show that catalytic performance is linked to the oxygen concentration in/on the Ag particles. Isotope experiments using 18O2 and C18O2 are conducted to gain insight into the nature and location of oxygen in/on the Ag nanoparticles. The oxygen species responsible for the CO2 formation and inhibition of the overall catalytic activity are identified, and the abundance of those species is shown to depend strongly on the pre‐treatment and reaction conditions, showing both are critical for effective oxygen management. By comparison with a conventional Ag/α‐Al2O3 catalyst, we demonstrate a low concentration of oxygen in/on Ag leads to the highest selectivity regardless of particle size. However, particle size dependent oxophilicity leads to significantly lower TOFs for the Ag nanoparticles. This study provides fundamental understanding of the performance of supported Ag particles in ethylene epoxidation and offers new strategies to improve performance under industrially relevant conditions.
Highlights
Introduction ing to increase the reactivity per unit mass relative to macroscopic crystals of the same substance.[1]
By comparison with a conventional Ag/αAl2O3 catalyst, we demonstrate a low concentration of oxygen in/on Ag leads to the highest selectivity regardless of particle size
After more than 40 years of research[10] it is still debated if a true Ag particle size (PS) effect even exists, much less whether it contributes to EO selectivity and its role in oxygen management of real catalysts
Summary
The investigated catalysts consist of Ag particles on SiO2 and αAl2O3. The low surface area α-Al2O3 support serves as industrial reference with large Ag particles. The continuously decreasing CO2 formation rates and stable EO formation rates lead to an increased S(EO) This serves as (another) experimental evidence that activation means nonreversible consumption of unselective oxygen species towards a selective, oxygen poor state of Ag. Due to the insulating character of supported Ag particles and the oxygen of the supports (SiO2, α-Al2O3), a discrimination in electrophilic or nucleophilic oxygen species was not possible. In light of the oxygen poor Ag surfaces in the selective state, these results might be explained by different strength of the Ag O interaction leading to a decreased TOFs for the Ag nanoparticles independent of the reaction path (EO or CO2 formation) This is generally named as particle size effect
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