An In Situ ESR Study of Pd/H-ZSM-5 Interaction with Different Adsorbents

In situ ESR at 120–473 K permits to monitor formation of transient paramagnetic ions/complexes (isolated Pd+ sites; Pd+/H2O; Pd+/C6H6) upon interaction of isolated Pd2+ cations stabilized by the H-ZSM-5 matrix with different organic compounds and gas mixtures (NO, O2, H2O, H2, propene, benzene). The in situ study provides insight into the elementary steps of redox processes on isolated Pd species in H-ZSM-5 zeolite under realistic conditions. Adsorbed water stabilizes the transient paramagnetic complex and decreases the rate of Pd2+ to Pd0 reduction by H2. Strong bonding of NOx ligands to Pd2+ species suppresses the reduction of Pd(II) ions. Sorption of benzene on preoxidized Pd2+/HZSM-5 is accompanied by an easy formation of organic cation-radicals and of a Pd+/benzene complex, the paramagnetic Pd+/benzene structure indicating a surprisingly high resistance to further reduction to Pd0. Illumination of the Pd/HZSM-5 by UV-visible light causes no measurable change in the redox properties of the catalyst.