Ageing mechanisms on PdOx-based catalysts for natural gas combustion in premixed burners

The ageing effect induced by S-compounds over 2%Pd/CeO2·2ZrO2, 2%Pd/LaMnO3·2ZrO2 and 2%Pd/BaCeO3·2ZrO2 catalysts for CH4 combustion was investigated; S-compounds are in fact added as odorants in the natural gas network for safety purposes. Pd-based catalysts were prepared by solution combustion synthesis (SCS), starting from metal nitrates/glycine mixtures. Basic characterization (XRD, BET, FESEM analysis), FT–IR studies and catalytic activity tests were performed on powders and after accelerated ageing carried out up to 2 weeks (hydro-thermal treatment at 900 °C under a flow rate with typical domestic boiler exhaust gas composition, 9% CO2, 18% H2O, 2% O2 in N2, containing also 200 ppmv of SO2 to emphasize any poisoning effect). Over fresh catalysts, IR analysis of CO adsorption evidenced the formation of highly dispersed Pd metal clusters and Pd ions. With 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 421 °C, attained with the same sample aged two weeks. An unexpected improvement was found instead in the overall performance of 2%Pd/LaMnO3·2ZrO2 and 2%Pd/BaCeO3·2ZrO2: the T50 in fact lowered from 570 to 450 °C for the first one, and from 512 to 443 °C for the second one, after two weeks ageing. S-hydro-thermal treatment provoked bulk and surface sulfates formation on all aged samples, with a concentration increasing with the exposure time. Prevailing ageing mechanisms seemed to be Pd metallic clusters coalescence, detected over the Ce–Zr system, and surface-bulk sulfates formation, the latter destroying the initial crystallographic structure. In 2%Pd/LaMnO3·ZrO2 and 2%Pd/BaCeO3·ZrO2 powders the amount of the perovskite phase strongly decreased during ageing, in favor of the formation of bulk sulfate and of oxides.