Effects of Surface and Bulk Silver on PrMnO_3+δ Perovskite for CO and Soot Oxidation: Experimental Evidence for the Chemical State of Silver

Unambiguous evidence has been obtained to explain the presence and effects of both framework and extra-framework silver on catalytic properties of Pr(Ag)MnO_3+δ perovskite type materials, using various tools such as XPS, HR-TEM, O₂-TPD, and H₂-TPR analysis. Three types of Ag-incorporated PrMnO₃ perovskite samples were synthesized by means of Ag partial substitution in perovskite lattice and Ag dispersion on the surface of the synthesized perovskite phase, using two different calcination temperatures of 200 and 550 C. The amount of silver used was 1 wt % (0.000225 mol), in all three catalysts. On the basis of extensive characterization studies, it was clearly explained that the partially substituted Ag for Pr is present in the lattice along with Pr at the "A" site of the ABO₃ perovskite structure. The Ag surface incorporated PrMnO_3+δ sample calcined at 550 °C shows both surface metallic silver and partially substituted Ag in perovskite lattice, whereas Ag₂O nanoparticles were observed on the surface in the case of another sample calcined at 200 °C. All of the synthesized materials were evaluated for their CO and soot oxidation activity, considering the renewed interest in Ag promoted catalytic materials and ambiguity about its location in the perovskite structure as well as its role in promoting catalytic and other properties of perovskite type materials. Perovskites with surface Ag species show better catalytic activity than the Ag substituted counterpart. This improved and better activity of Ag dispersed samples was because of the presence of the surface active redox couple of Ag/Ag₂O, which facilitates the surface redox reaction pathway of CO and soot oxidation reactions. (Figure Presented).