| A commercially viable catalyst for selective catalytic reduction (SCR) of NO{dollar}sb{lcub}rm x{rcub}{dollar} (x=1,2) under lean conditions needs to have high activity and durability. In general, metal/zeolite catalysts have high activity but low durability, whereas metal/oxide catalysts have high durability but low activity. In this study, the deactivation mechanism of Cu/ZSM-5, and the nature of active sites in Co/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} were investigated.; Both fresh and deactivated Cu/ZSM-5 catalysts were characterized by H{dollar}sb2{dollar}-TPR, CO-TPR, EPR, XRD, BET surface area, and steady-state reaction measurements. The results suggest that deactivation of Cu/ZSM-5 is initiated by dealumination of zeolite at the proton exchange sites, followed by the formation of amorphous Al{dollar}sb2{dollar}O{dollar}sb3{dollar} and SiO{dollar}sb2.{dollar} The Cu{dollar}sp{lcub}2+{rcub}{dollar} ions are trapped in two- or three-dimensional compounds such as Cu/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} + SiO{dollar}sb2{dollar} or CuAl{dollar}sb2{dollar}O{dollar}sb4{dollar}, in which they are highly dispersed but virtually inactive for SCR of NO{dollar}sb{lcub}rm x{rcub}.{dollar}; Based on this deactivation mechanism, it should be possible to improve the stability of Cu/ZSM-5 by decreasing the number of zeolite protons without changing the state of the active Cu{dollar}sp{lcub}2+{rcub}{dollar} ions. It was found that increasing the Cu loading and introducing another cation modifier, e.g. La, slowed the rate of deactivation up to a factor of two. Presumably, this is because the excess metal oxide (CuO or La{dollar}sb2{dollar}O{dollar}sb3{dollar}) clusters in the zeolite channels neutralize the zeolite protons.; Co/Al{dollar}sb2{dollar}O{dollar}sb3{dollar}, either of low Co loading or calcined at high temperatures, in which the Al{dollar}sb2{dollar}O{dollar}sb3{dollar} was prepared by the complexing agent-assisted sol-gel method, was found to be very active and stable for lean-NO{dollar}sb{lcub}rm x{rcub}{dollar} reduction by propane or propene. The nature of the active sites in both Co/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} and Co/ZSM-5 was studied by H{dollar}sb2{dollar}-TPR, UV-vis, and steady-state reaction measurements. The results suggest that these catalysts have similar active sites: Co{dollar}sp{lcub}2+{rcub}{dollar} ions on Al{dollar}sb2{dollar}O{dollar}sb3{dollar}, surface or in ZSM-5 channels which are octahedrally coordinated under ambient conditions. The number of active sites in Co/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} is greatly affected by the preparation parameters, such as Co loading, pretreatment conditions, and the source of alumina.; In CH{dollar}sb4{dollar} SCR of NO over Co/Al{dollar}sb2{dollar}O{dollar}sb3{dollar}, a synergistic effect was observed over a mixture of Co/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} and H/zeolite (H/ZSM-5 or H/USY). This is attributed to a bifunctional mechanism, which requires the participation of both Co{dollar}sp{lcub}2+{rcub}{dollar} sites and H{dollar}sp+{dollar} sites. |
