Design of catalytically active porous gold structures from a bottom-up method: The role of metal traces in CO oxidation and oxidative coupling of methanol

Abstract

By using a bottom-up approach, we prepared 3D self-supported porous structures made of gold nanoparticles and applied them as oxidation catalysts. To do so we used the established cryogelation technique, in which nanoparticle units were frozen in liquid nitrogen and the ice-template was subsequently sublimated by a freeze-drying process, resulting in a 3D self-standing monolith of nanoparticles. The sample was characterized with different techniques to understand the evolution of the microscopic structure before and after exposure to catalytic environments. In addition, the synthesized materials were tested in the CO oxidation and in the oxidative methanol coupling, in the attempt to compare the prepared materials against established nanoporous gold systems synthesized by a top-down approach. The as-prepared gold sample was inactive in the CO oxidation and surprisingly, highly active in the oxidative methanol coupling. A set of analyses revealed the presence of sodium species on the catalyst surface, which correlated to the extremely high activity of the catalyst. When such traces were removed, the pure gold samples was found to be inactive also in the methanol coupling reaction. Finally, our results proved that pure porous gold systems were not able to catalyze the investigated oxidation reactions, but instead traces of alkali metal ions in these systems could enhance the catalytic activity.