화학공학소재연구정보센터
Applied Catalysis B: Environmental, Vol.14, No.1-2, 1-11, 1997
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 Pd2+ ions. Even in Pd/NaZSM-5, roughly 20% of the PdO particles are converted 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 catalytic NO2 reduction to N-2 slowly increases over 24 h from 20 to 50% while PdO is converted to Pd2+, but the rate of CH4 oxidation remains constant 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 have similar activity. Pd/HZSM-5 is active even when Pd is initially present 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 only required to convert PdO into Pd2+, but also that Pd/HZSM-5 acts as a bifunctional catalyst. Possible routes for CH4 and C3H8 activation are discussed.