For example, preoxidized Pd/H–ZSM-5(A), of which Pd atoms were mostly in the isolated Pd2+ state and distributed almost uniformly in the zeolite particle, showed a high activity for NO reduction from the beginning and rapidly reached steady state. On prereduced Pd/H–ZSM-5(A), Pd was initially in highly dispersed metallic state but oxidized very quickly in the NO–CH4–O2 stream and after that the state of Pd and the catalytic performance exhibited a similar behavior as the preoxidized Pd/H–ZSM-5(A). Pd/H–ZSM-5(B), preoxidized and prereduced, had initially large PdO and Pd0 particles (140–160 Å) on the external surface, respectively, which were dispersed gradually into the micropores as isolated Pd2+. The oxidation of Pd0 was very fast. The catalytic performance changed correspondingly; from low to high activity for NO reduction and from high to low activity for CH4 oxidation. In the steady state at 673 K, all four catalysts showed the same high activity for NO reduction and modest activity for CH4 oxidation, corresponding to the same state of Pd. On preoxidized Pd/Na–ZSM-5, which showed a low activity for NO reduction, Pd atoms were mainly in the form of PdO particles (170 Å) on the external surface, and they changed little during the reaction.
These results clearly demonstrate that isolated Pd2+ ions in the zeolite micropore are active and selective for NO reduction, and PdO particles on the external surface are active mainly for CH4 oxidation. These conclusions are consistent with the results in the literature and our earlier work.