Characterization of bimetallic Pd/Co-HMOR used for the CH 4-SCR of NO x

Abstract

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 NO x by CH 4. The irreversible degradation of catalysts in time for NO x and CH 4 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 N 2 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, H 2-TPR profiles indicated that a higher fraction of Co oxo-ions may be present in the aged samples and changes in the relative Co 2+ populations of α, β and γ sites were confirmed by UV–VIS. Furthermore, Pd 2+ cations appear to be preferentially located in the side pockets of MOR. The absence of hydride phases in H 2-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.