THE EFFECT OF ZEOLITIC PROTONS ON NOX REDUCTION OVER PD ZSM-5 CATALYSTS/

Formation of PdO particles is observed in ion-exchanged Pd/HZSM-5 and Pd/NaZSM-5 after reduction and reoxidation at 500 degrees C. Formation of Pd2+ ions is not observed in Pd/HZSM-5 in the absence of NOx, but in an atmosphere of NO2 + O-2 the PdO particles are transformed into P d2+ ions. Even in Pd/NaZSM-5, roughly 20% of the PdO particles are con verted to ions in NOx; the protons needed for this process are mainly formed during reduction of Pd2+ to Pd-0. When a reduced and reoxidized Pd/HZSM-5 catalyst is exposed to CH4 + NO2 + O-2, the rate of catalyt ic NO2 reduction to N-2 slowly increases over 24 h from 20 to 50% whil e PdO is converted to Pd2+, but the rate of CH4 oxidation remains cons tant with time on-stream. Apparently, Pd2+ ions are needed for SCR of NOx, but for methane oxidation with O-2, Pd2+ ions and PdO particles h ave similar activity. Pd/HZSM-5 is active even when Pd is initially pr esent as PdO particles, but Pd/NaZSM-5 remains inactive also when part of the Pd is present as Pd2+ ions. This shows that protons are not on ly required to convert PdO into Pd2+, but also that Pd/HZSM-5 acts as a bifunctional catalyst. Possible routes for CH4 and C3H8 activation a re discussed. (C) 1997 Elsevier Science B.V.