Ageing of palladium, platinum and manganese-based combustion catalysts forbiogas applications

During recent years, catalytic combustion of low heating value gases has re ceived increased attention. The purpose of the present work was to study th e effect of ageing for 30 days at 1000 degrees C in air saturated with 12% steam on Pd- and Pt-impregnated as well as Mn-substituted lanthanum hexaalu minate materials. Both hexaaluminate powders and 400 cpsi cordierite monoli ths, washcoated with hexaaluminate powder, were aged. Powders were characte rised by BET and XRD, whereas the catalytic activity of the washcoated mono liths was evaluated in a bench-scale rig for conversion of synthetic gasifi ed biomass. The surface areas decreased significantly during the first day of ageing, whereas further ageing had only a minor influence. The pure lant hanum-alumina sample was a mixture of the hexaaluminate LaAl11O18 phase and the less preferable perovskite LaAlO3 phase, which increased after ageing. The Mn-substituted lanthanum-alumina mainly showed pure hexaaluminate phas e both before and after ageing. The catalytic activity tests showed that Pd -impregnated lanthanum hexaaluminate was the most active catalyst for combu stion of carbon monoxide and hydrogen, retaining low light-off temperatures also after 30 days of ageing. However, the ignition temperature for 50% co nversion (T-50) of methane was approximately 300 degrees C higher than for the fresh sample. Pt-impregnated samples were less active than the Pd ones. The Pt-loading decreased after ageing, whereas the Pd-loading remained fai rly constant. However, the amount of Pd oxide decreased after ageing. Furth er, the Mn-substituted samples were less active than the precious metal one s. Here, the activity for the combustion of carbon monoxide was substantial ly affected by ageing. The formation of nitrogen oxides from ammonia was lo wer over the aged samples than over fresh ones; the Mn-substituted sample a ged 30 days showed the lowest yield, only 30% of ammonia was converted to n itrogen oxides. (C) 2000 Elsevier Science B.V. All rights reserved.