NANOSTRUCTURED MIXED OXIDES CATALYSTS: RELATIONSHIPS BETWEEN PHYSICAL-CHEMICAL AND CATALYTIC PROPERTIES

Perovskitic catalysts showed satisfactory activity for the low temperature flameless combustion of methane. This process allows to decrease the emission of CO, NOx and partially unburnt hydrocarbons. Partial substitution in the composition of perovskites may improve catalytic activity and it can modulate their resistance to sulphur poisoning. Samples with nominal composition La1-xA’xBO3, where A’= Sr, Ce, Ag, B=Co, Mn with x = 0; 0.05; 0.10, 0.20 and x%NB/LaBO3, where NB=Pt, Pd, Ag were prepared by flame spray pyrolysis (FP) and by the so-called sol-gel citrate method (SG). Temperature-programmed analysis, X-ray powder diffraction (XRD) and Electron Paramagnetic Resonance (EPR) spectroscopy were used as main characterisation tools. Almost all of the catalysts showed very active for the flameless combustion of methane. The activity of the FP-prepared catalysts was always higher than that of the SG-prepared ones with identical nominal composition. Partial substitution didn’t lead to a significant change of activity, though an evident result was achieved in case of Sr-substituted manganites. Also activity may be increased by the addition of a small amount of Pt. Ag addition led to a complex change of activity and resistance to poisoning for Co-based perovskites. The activity of samples doped with a small amount of Ag (e.g. 5 mol%) was indeed even lower than that of the “pure” LaCoO3. By contrast, a further increase of Ag concentration led to increasing catalytic activity, mainly when extraframework Ag was present. Partial substitution of Ag for La in Mn-based perovskites led to increasing activity both for SG- and FP-prepared catalysts and the catalytic activity increased with increasing Ag substitution. From the point of view of resistance against poisoning Sr and noble metals may be good protectors since they react more promptly with sulphur. Silver is not as effective from the point of view of protection against poisoning. The main advantage is the improved initial activity of some catalysts, which leads to satisfactory results even after more or less severe poisoning. A brief investigation of different poisoning agents (nitrogen-containing compounds) was also made. The transient response of the catalysts during the injection of poisons was also analyzed.