Propene epoxidation with O2 or H2–O2 mixtures over silver catalysts: theoretical insights into the role of the particle size

Abstract

The adsorption and dissociation of molecular O₂ over silver extended surfaces, nanoparticles and clusters of different size, as well as the reaction between adsorbed O₂ and H₂ to form less nucleophilic hydroperoxide groups have been investigated by means of periodic DFT calculations. Hydroperoxide formation from O₂ and H₂ is in all systems kinetically favoured over O₂ dissociation, which becomes energetically forbidden on small planar clusters. The nature and reactivity towards propene of all oxygen species present on silver, including adsorbed molecular O₂, atomic O, and hydroperoxide groups, have been theoretically explored. Formation of the oxametallacycle intermediate involved in propene epoxidation is energetically accessible on subnanometric three-dimensional silver nanoparticles, but competitive pathways leading to hydrogen abstraction and allyl formation always involve lower activation barriers. Theoretical findings have been experimentally confirmed by Raman spectroscopy of O₂ adsorption and catalytic testing of planar and three dimensional silver clusters.