Characterization of bimetallic Pd/Co-HMOR used for the CH4-SCR of NOx

Several techniques, XRD, BET, TPR, UV-VIS, FTIR, 27Al-NMR, and 29Si-NMR, were employed to characterize long TOS (time on stream) effects on bimetallic Pd/Co-HMOR samples used in the SCR of NOx by CH4. The irreversible degradation of catalysts in time for NOx and CH4 conversion was fitted to an exponential decay function. Upon aging, the crystalline structure of mordenite is preserved, but the apparent surface area slightly diminished and t-plot analysis of N2 adsorption isotherms showed that the external surface area increased and mesopore volume formed. FTIR, 27Al NMR and 29Si MAS NMR confirmed steam-induced dealumination, mainly in the 12-ring channels. Also, H2-TPR profiles indicated that a higher fraction of Co oxo-ions may be present in the aged samples and changes in the relative Co2+ populations of α, β and γ sites were confirmed by UV-VIS. Furthermore, Pd2+ cations appear to be preferentially located in the side pockets of MOR. The absence of hydride phases in H2-TPR of fresh and aged samples confirms close Pd-Co interactions. Therefore, Pd and Co interactions in the 8-ring channels of MOR may explain the relatively good stability of Pd/Co-HMOR under wet conditions for about 100 h TOS.