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The activity of the Pd, Pt, Pd-Pt/HZSM-5 and Pd/HZSM-35 catalysts was inves
tigated at temperature ranges of 120-600 degrees C in the conversion of gas
mixtures consisting of CH4, C2H4, CO, naphthalene (model polyaromatic hydr
ocarbon), O-2, CO2, H2O and N-2. The prepared catalysts exhibited high acti
vities in the complete removal of the model pollutants. Increase in the Pd-
content of the catalyst was found mostly to affect the low temperature acti
vity of the catalyst in methane oxidation. The effect of S-poisoning was st
udied by subjecting the catalysts to 5 ppm SO2 in the feed gas mixture. Cat
alyst de-activation due to catalyst ageing in the reactant flow was observe
d. The ageing of the catalyst only affected the methane oxidation. Addition
of Zr and La improved the low-temperature activities of the Pd and Pd-Pt/H
ZSM-5 catalyst and stabilised the catalysts against de-activation. The XPS
and O-2-TPD data indicated that on the freshly prepared catalyst, palladium
is present as Pd+ and Pd2+ species. The ageing of the catalysts decreased
the number of the Pd2+ sites in favour of the PdOx species. (C) 2000 Elsevi
er Science B.V. All rights reserved.