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

Abstract

The ageing effect induced by S-compounds over 2%Pd/CeO 2·2ZrO 2, 2%Pd/LaMnO 3·2ZrO 2 and 2%Pd/BaCeO 3·2ZrO 2 catalysts for CH 4 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% CO 2, 18% H 2O, 2% O 2 in N 2, containing also 200 ppmv of SO 2 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/CeO 2·2ZrO 2 increased its CH 4 combustion half-conversion temperature ( T 50, 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/LaMnO 3·2ZrO 2 and 2%Pd/BaCeO 3·2ZrO 2: the T 50 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/LaMnO 3·ZrO 2 and 2%Pd/BaCeO 3·ZrO 2 powders the amount of the perovskite phase strongly decreased during ageing, in favor of the formation of bulk sulfate and of oxides.