Sr1-xAgxTiO3 +/-delta (x=0, 0.1) perovskite-structured catalysts for the flameless combustion of methane

Several samples of Sr1-xAgxTiO3 +/- delta (x = 0, 0.1) perovskite-stuctured catalysts were prepared and tested for the title reaction under lean-burn conditions. For the same overall nominal composition, catalyst activity and durability depend strongly on the method of preparation and, for the samples prepared by the same method, mainly on temperature and on the residence time at Such a temperature during preparation. In Sr0.9Ag0.1TiO3 +/- delta catalysts Ag is present either as intra-crystalline Ag2+, likely substituting for Sr2+ in the perovskite lattice, or as inter-crystalline metallic Ag. The catalytic activity of Sr0.9Ag0.1TiO3 +/- delta is connected with the presence of O-2(-)/Ti4+, O-x(-)/Ag, and O-x(-)/Au2+; of O-3(-)/Ti4+, O-/Sr2+; or of O-/Ti4+ species, depending on preparation method and conditions. Furthermore, inter-crystalline metallic Ag plays an important role in improving catalyst resistance to sintering; the most durable samples are those calcined for a sufficient residence time at the highest temperature during preparation, that is, under conditions more favourable to the segregation of this metal from the perovskite lattice.