The effect of zeolitic protons on NO x reduction over Pd/ZSM-5 catalysts

Abstract

Formation of PdO particles is observed in ion-exchanged Pd/HZSM-5 andPd/NaZSM-5 after reduction and reoxidation at 500°C. Formation of Pd 2+ ions is not observed inPd/HZSM-5 in the absence of NO x , but in an atmorphere of NO 2 + O 2 the PdO particles are transformed into Pd 2+ ions. Even inPd/NaZSM-5, roughly 20% of the PdO particles are converted to ions in NO x ; the protons needed for this process are mainly formed during reduction of Pd 2+ to Pd 0. When a reduced and reoxidizedPd/HZSM-5 catalyst is exposed to CH 4 + NO 2 + O 2, the rate of catalytic NO 2 reduction to N 2 slowly increases over 24 h from 20 to 50% while PdO is converted to Pd 2+, but the rate of CH 4 oxidation remains constant with time on-stream. Apparently, Pd 2+ ions are needed for SCR of NO x , but for methane oxidation with O 2, Pd 2+ ions and PdO particles have similar activity.Pd/HZSM-5 is active even when Pd is initially present as PdO particles, butPd/NaZSM-5 remains inactive also when part of the Pd is present as Pd 2+ ions. This shows that protons are not only required to convert PdO into Pd 2+, but also thatPd/HZSM-5 acts as a bifunctional catalyst. Possible routes for CH 4 and C 3H 8 activation are discussed.