Ag represents an interesting dopant for the highly active LaCoO3 perovskites used for the catalytic flameless combustion (CFC) of methane, due to its ability to adsorb and activate oxygen and to the possibility of incorporation into the framework Higher activity was observed, in general, with fresh catalysts synthesised by FP. The SG samples demonstrated a slightly better resistance to sulphur poisoning when considering the conversion decrease between the fresh and the poisoned samples, due to lower surface exposure. However, interesting data have been
Several samples of Sr1-xAgxTiO3 (x = 0, 0.1) perovskite-structured catalysts have been 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 residence time at such a temperature during preparation. In Sr0.9Ag0.1TiO3 catalysts Ag is present either as intra-crystalline Ag 2+ , likely substituting for Sr 2+ in the perovskite lattice, or as inter-crystalline metallic Ag. The catalytic activity of Sr0.9Ag0.1TiO3 is connected with the presence of O2 -/Ti 4+ , Ox -/Ag and Ox -/Ag 2+ or of O3 -/Ti 4+ , O -/Sr 2+ and of O -/Ti 4+ species, depending on preparation method and conditions. Furthermore, intercrystalline metallic Ag plays an important role in improving catalyst resistance to sintering, the most durable samples being those calcined for a sufficient residence time at the highest temperature during preparation, i.e. under conditions better favouring the segregation of this metal from the perovskite lattice.
Sr 1−x M x TiO 3±δ (x = 0 or 0.1; M = K or Gd) have been compared as catalysts for the flameless combustion of methane. The samples were prepared both by traditional sol-gel (SG) and by flame-hydrolysis (FH) procedure. All the catalysts possessed the perovskite-like structure, with some SrCO 3 as impurity with Sr 0.9 K 0.1 TiO 3±δ prepared by SG procedure. All the samples prepared by FH were better catalysts than those prepared by SG. A further improvement in the catalytic activity was obtained with M = K, but only at temperatures lower than 550 • C. X-band and Q-band EPR spectroscopies provided an explanation of this observation, based on the high mobility of surface O 3 − species on the surface of the (FH) Sr 0.9 K 0.1 TiO 3±δ sample.
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