Effective Ag doping and resistance to sulfur poisoning of La–Mn perovskites for the catalytic flameless combustion of methane

Abstract

Perovskite-like structured catalysts showed satisfactory activity for the low temperature flameless combustion of methane. This process reduces the emission of CO, NO_x and unburnt hydrocarbons. Partial metal ion substitution in the composition of perovskites may improve catalytic activity and it can modulate their resistance to sulfur poisoning. Silver is an interesting dopant due to its limited solubility in the perovskite structure and suitable activity both in extra- and intra-framework positions. The amount of lattice silver tightly depends on the preparation procedure. Samples with nominal composition La_1 − xAg_xMnO_3 ± δ with x = 0; 0.05; 0.10 were prepared by flame spray pyrolysis (FP) and by the so-called sol–gel citrate method (SG). Temperature-programmed analysis, X-ray powder diffraction (XRPD) and Electron Paramagnetic Resonance (EPR) spectroscopy were used as the main characterisation tools. Almost all of the catalysts were 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. Furthermore, partial substitution of Ag for La led to substantially higher activity both for SG- and FP-prepared catalysts and the catalytic activity increased with increasing Ag substitution. The residual activity after poisoning with tetrahydrothiophene, a common odoriser used in the natural gas grid, is also presented, together with the transient response of the samples upon poisoning.