The role of zeolite type on the lean NOx reduction by methane over Pd loaded pentasil zeolites

The lean selective catalytic reduction of NOx by methane over protonic pall adium loaded ZSM-5, FER and MOR, as well as, on bimetallic Pd-Pt-HMOR was e xamined. Special emphasis was paid on the combined effects of water and SO2 in the feed stream. Under dry conditions and in the absence of SO2, the de gree of NOx conversion at 450 degreesC decreases as follows: Pd-HZSM-5 > Pd -HMOR > Pd-HFER. Sulfur dioxide alone has no apparent effect on the activit y for NOx reduction, but the coexistence of water and SO2 inhibits both NOx and methane conversions. The extent of inhibition by water and SO2 on NO, reduction is Pd-HFER > Pd-HZSM-5 > :Pd-HMOR. Acid mordenite doped with low levels of Pt and Pd leads to an active catalyst that is more tolerant to th e presence of either water or SO2 than the corresponding monometallic Pt- a nd Pd-HMOR. Nevertheless, NOx reduction is also inhibited at temperatures b elow 450 degreesC when SO2 and water are both present. TPD experiments of w ater over calcined samples of protonic Pd supported pentasil zeolites, Pd/g amma -Al2O3 and Pt-Pd-HMOR with and without pretreatment in SO2 + O-2 indic ate that sulfation of the surface increases water chemisorption by the supp ort. Therefore, the observed decrease of NOx reduction on Pd-loaded zeolite catalysts when SO2 and H2O coexist in the feed stream may be due to enhanc ed water inhibition and presumably active site poisoning. (C) 2000 Elsevier Science B.V. All rights reserved.