Abstract
Ultramicroporous carbons loaded with silver (Ag) nanoparticles were synthesised by submitting tannins to hydrothermal treatment in aqueous solutions of silver nitrate at different concentrations, followed by a pyrolysis step. The resultant materials were oxidised with dioxygen diluted in argon for removing the carbon partly coating the Ag nanoparticles and thus increasing the active metal surface area. Morphological, chemical, structural and textural characterisations were carried out in order to establish relationships between the physicochemical properties of the materials and their performances as catalysts for ethylene epoxidation. The latter reaction was increasingly promoted at higher silver content, and the efficiency of the oxidative post-treatment was evidenced. The relation between selectivity and Ag content was nonlinear, in favour of the improved availability of silver nanoparticles after the removal of the carbon coating, at constant surface area and micropore size distribution of the carbon support. A favourable effect of Ag nanoparticles agglomeration on the selectivity was suggested.