Effect of S-compounds on Pd over LaMnO3·2ZrO2 and CeO2·2ZrO2 catalysts for CH4 combustion

The ageing effect induced by S-compounds (added as odorants in the natural gas network for safety purposes) over 2% Pd/LaMnO3·2ZrO2 and 2% Pd/CeO2·2ZrO2 catalysts for CH4 combustion is studied. The catalysts were prepared by solution combustion synthesis (SCS), starting from metal nitrates/glycine mixtures. Basic characterization (XRD, BET, FESEM/EDS, TPD/TPR analysis), FT-IR studies and catalytic activity tests were performed on powder catalysts in fresh status and after accelerated ageing carried out up to 3 weeks (hydro-thermal treatment at 800 °C under a flow of domestic boiler exhaust gases typical composition (9% CO2, 18% H2O, 2% O2 in N2), also including 200 ppmv of SO2 to emphasize any poisoning effect). After ageing, 2% Pd/CeO2·2ZrO2 increased its CH4 combustion half-conversion temperature (T50), regarded as an index of catalytic activity, from 382 °C – recorded for fresh sample – to 490 °C. An unexpected improvement in the overall performance was found instead for 2% Pd/LaMnO3·2ZrO2: T50, in fact, got lowered from 570 °C (fresh sample) to 465 °C (after 3 weeks ageing). S-hydro-thermal treatment induced bulk and surface sulphates formation on all aged samples, with a concentration increasing with the exposure time. The prevailing ageing mechanisms seemed to be the oxidation of Pd metallic clusters, detected over the Ce–Zr system, and the formation of surface-bulk sulphates, the latter destroying the initial crystallographic structure, particularly evident for the La–Mn system. The increased activity of the 2% Pd/LaMnO3·2ZrO2 sample could be explained with the formation of MnOx, detected via XRD and IR, which is active towards CH4 combustion.