Understanding the Room-Temperature Catalytic Activity of Silver-Containing Mixed Oxides: The Role of Ag ²⁺ Surface Species

Mixed oxides AgFeO2 (delafossite) and AgMnO2 (crednerite) were prepared during hydrothermal synthesis. The catalytic properties were investigated in the low-temperature CO oxidation reaction, depending on the pretreatment temperature in CO + O2. Correlations were established between the catalytic activity of mixed oxides, structural features, thermal stability, and silver surface state. Oxide AgMnO2 was characterized by enhanced reactivity toward CO (below 100 °C) and enhanced oxygen mobility in comparison with AgFeO2. In situ XRD and ex situ XPS revealed the relationship between the high catalytic CO oxidation activity of AgMnO2 and the presence of active oxygen forms related to the Ag2+ surface species, which were not found on the AgFeO2 surface. High catalytic activity on the surface of the AgMnO2 particles is considered to involve a redox cycle with the participation of Ag2+, Ag1+, and Ag0 silver states, while the reduced catalytic activity of AgFeO2 is caused by the reaction occurring at the interface between the Ag1+Ox/Ag0 particles and the delafossite surface, excluding the involvement of Ag2+-type species in the redox cycle.