Anomalous behaviour of silver catalyst for soot oxidation explained: state of silver when operating and the influence of potassium ions

Abstract

Two independent steps in which CO₂ formed were observed in soot oxidation using silver catalysts supported on cerium-doped zirconia and zirconium-doped ceria. The first was attributed to thermal decomposition of oxidized silver species to metallic silver with simultaneous soot combustion, whereas the second was catalytic soot combustion. The first step was endothermic when the catalyst was heated without soot, whereas it was overall exothermic when the catalyst was heated while in tight contact with soot. Both mechanisms of CO₂ formation are described. The influence of potassium ions on the catalytic activity in soot oxidation was found to be substantial for 1 wt% Ag/Zr_0.93Ce_0.07O₂, though negligible for 10 wt% Ag/Zr_0.93Ce_0.07O₂. In contrast, in the case of the zirconium-doped ceria support, the 1.5 wt% K⁺/Ce_0.85Zr_0.15O₂ sample was equally active in soot combustion with and without silver. Proper interpretation of activity onset of silver catalysts for soot combustion, the state of silver when operating and the influence of potassium ions on the activity was the focus of this study.