Interface interactions and CO oxidation activity of Ag/CeO₂ catalysts: A new approach using model catalytic systems

In this paper, we studied in details the nature of the Ag-CeO₂ interaction and its influence on the catalytic activity in CO oxidation at low temperature. Ag/CeO₂ catalysts were prepared by pulsed laser ablation in liquids (PLA). This method provided the preparation of highly dispersed particles of both an active component and a support. The initial Ag/CeO₂ composites did not show activity in the CO oxidation at temperatures <100 °C. However, thermal activation in an oxidizing atmosphere above 450 °C led to a significant improvement of the low-temperature catalytic characteristics and decrease of the activation energy of the reaction by 2 times. A detailed study by physicochemical methods clearly showed that the enhancement of the catalytic properties related to the transition of Ag° particles in contact with CeO₂ to ionic Ag⁺ species. The ionic species were stabilized on the surface of CeO₂ without incorporation into the ceria volume. The catalyst activated at 450 °C demonstrated high stability under catalytic conditions due to the effective reversible transition Ag⁺-CeO₂ ↔ Ag_n°/CeO₂, where Ag_n° – small metal clusters on the CeO₂ surface. It is proposed that such reversible transition is facilitated by the defects on the surface of CeO₂ nanoparticles. With an increase of the calcination temperature of Ag/CeO₂ catalysts above 600 °C, the efficiency of the redox transition decreased due to annealing of CeO₂ defects and formation of better-crystallized particles. As a result, catalysts calcined at T>600 °C did not show low-temperature catalytic activity.