Correlation between the size and the magnetic properties of Ag2+ clusters loaded on ceria surface and their catalytic performance in the total oxidation of propylene. EPR study

Abstract Three different types of Ag 2+ ions (“a”, “b”, and “c”) have been identified and examined by electron paramagnetic resonance (EPR) on 10% wt Ag/CeO 2 prepared by impregnation method. One of them, Ag 2+ (b) , behaves differently than the two others, Ag 2+ (a) and Ag 2+ (c) , under redox atmospheres. The fact that, in reducing conditions (vacuum, propylene, hydrogen, and carbon black), Ag 2+ (a) and Ag 2+ (c) species were more easily reduced than Ag 2+ (b) ones, could not explain the catalytic performance and stability of this latter species compared to the first ones in the reaction of total oxidation of propylene. The EPR technique evidenced that Ag 2+ (b) species form, upon propene oxidation, a cluster. This cluster is composed of two parallel electron spins (dimer) and three nuclear spins (trimer). It seems that before propylene oxidation, Ag 2+ (b) clusters were ferromagnetic. This ferromagnetic character of Ag 2+ (b) species may explain their better catalytic performance, in propylene oxidation, than those of Ag 2+ (a) and Ag 2+ (c) ones.