Study On Modified Molecular Sieve Materials For The Selective Catalytic Reduction Of No_X From Diesel Engines

This paper, by means of ZSM-5 and SBA-15 based zeolite catalysts, evaluats the selective catalytic reduction (SCR) of NO_x by NH₃ and soot combustion on a stimulation system, as well as hydrothermal stability and resistance to sulfur. The physic-chemical properties of these catalysts are characterized by N2 adsorption-desorption isotherms, XRD, SEM, TEM, FT-IR, UV-vis, H2-TPR and XPS. Engine tests are also be operated to determine the regular disciplines derived from the stimulation tests. The research results are essential in both theoretical significance and practical aspect to reduce diesel engine NO_x emission and meet the more strict emission. Main conclusions from this study are as follows:(1) Co/ZSM-5 and Co/SBA-5 catalysts were comparatively studied for the selective catalytic reduction of NO_x by NH₃. With respect to Co/ZSM-5 series, partial Co2+ ions have entered the lattice structure of ZSM-5 by exchanging with H+ ions, and these ions could provide the activity at temperature < 300°C, thus corresponding to the low-temperature peak at relatively low temperature. While excess Co ions existed as CoOx, which served as active sites responsible for the high-temperature activity peak at temperature > 300°C. The catalytic activity of Co/SBA-5 series exhibited much lower activity than Co/ZSM-5, since Co2+ ions could exchange only with H+ from Si-OH and excessive Co oxides tended to agglomerate within SBA-15 channels, consequently blocking these channels.(2) A series of M/ZSM-5 catalysts (M=Fe, Ni, Mn, Cu, Sr, Ce, La and Co) were prepared through ion exchange method. Results from activity tests indicated that Cu/ZSM-5, Mn/ZSM-5 and Ni/ZSM-5 exhibited higher activities for the NO_x reduction than others. Results from physic-chemical analysis showed that metallic oxides as active phases enriched and dispersed well on the outer and shallow surface of zeolites. The catalytic activity followed the order: Cu/ZSM-5 > Mn/ZSM-5 > Ni/ZSM-5. At lower metal contents, Cu/ZSM-5, Mn/ZSM-5 and Ni/ZSM-5 all showed double peaks in the NO_x reduction profiles. While Cu, Mn, Ni loadings were up to 0.9%, 1.4% and 14.6%, respectively, the double-peak combined into one unit.(3) The soot catalytic combustion was studied in detail using Cu/ZSM-5 and CuZr/ZSM-5 catalysts. Carbon blended with Cu/ZSM-5 could yield CO and CO2 simultaneously under the O2+N2 atmosphere. The addition of zirconium into Cu/ZSM-5 to form CuZr/ZSM-5 promoted the activity for the soot combustion. Under the O₂+NO+N₂ atmosphere, the CO peak emerged earlier than that of CO₂. All of the Cu/ZSM-5 and CuZr/ZSM-5 were highly active for the soot combustion, since light-off temperature was within the range of diesel engine exhaust temperatures.(4) The CuZr/ZSM-5 catalysts exhibited the higher activity for the reduction of NO_x than Cu/ZSM-5. Doping of zirconium could enhance the low-temperature activity and broadened the active window. More active phase's enrichment and the change of Cu coordination environment could contribute to the increase of catalytic activity. XPS results from the Cu/ZSM-5 sample before and after catalytic experiments indicated that redox propertied between Cu+ and Cu2+ was of importance for the SCR reaction. After 100h ageing tests at 400°C, no significant inactivation was observed, indicating that both Cu/ZSM-5 and CuZr/ZSM-5 catalysts were quite stable. The negative effect derived from water was irreversible because the Cu active sites enriched on the catalyst surface have been transformed to CuO species which are less active after sintering at high temperatures. The existence of SO₂ in the reactive atmosphere would lead to deactivation of catalysts. However, it should be noticed that the deactivation level of CuZr/ZSM-5 was lower than that of Cu/ZSM-5, because the addition of acid ZrO₂ adjusted the cidity on the catalyst surface and prevented the SO₂ adsorption and accumulation.(5) The catalytic properties of Cu/ZSM-5 have been studied in the real SCR system on diesel engine bench tests, and the results showed that SCR converter with 6% Cu/ZSM-5 was more efficient than that with 8% Cu/ZSM-5. After SCR reaction, NO and NO₂ concentrations in diesel engine emission were reduced effectively in the entire working areas. As by-product, N₂O would increase along with the desirable SCR reaction. The elimination rate of NO_x had nothing to do with the initial NO_x amount when temperature was high enough. When temperature was relatively below, the Cu/ZSM-5 monolithic converter was extremely sensible to space velocity: the space velocity increasing led to the decrease of NO_x reduction. At the exhaust temperature > 350°C, the effect of space velocity on the removal of NO_x was weakened. With the exhaust temperature increase, NO_x reduction increased obviously and reached the maximum at about 320°C. The activity window with NO_x reduction > 80% maintained to about 470°C. Compared with powder catalysts, the active window of monolithic moved toward the high temperature by about 25°C.