Study On Multi-metal Modified ZSM-5Catalysts For Selective Catalytic Reduction Of NO_x From Diesel Exhaust Gas

Nitrogen oxide pollution is harmful to the environment and human health. DieselNOxemission control technology has always been a hot topic in the field of internalcombustion engine.The research on performance and reaction of multi-metal modifiedZSM-5SCR catalysts will offer a useful theoretical reference for the development andoptimal design of new type of SCR catalysts. Cu/ZSM-5and a serial ofCuCexZr1xOy/ZSM-5catalysts were prepared by the ion exchange method, andcharacterized by a variety of advanced characterization techniques. By using thesetechniques, physical and chemical properties, catalytic activity, the production andtransformation in NH3-SCR reaction were investigated. Main results obtained are asfollows:(1) The addition of zirconium improves the copper dispersion on the surface ofcatalysts which is helpful to improve the high-temperature window in NH3-SCRreaction. Appropriate amount of cerium improves the redox abilities of metalmodified ZSM-5catalysts, the low-temperature properties are enhanced incerium-riched catalysts. Furthermore, the copper species are enriched on the ZSM-5grain surfaces, then enhancing the SCR catalytic activity.(2) Most of strong acid sites are occupied by copper species, then decrease theammonia adsorption capacity; the NO desorption process on metal modified ZSM-5catalysts includes three process, namely NO molecular physical desorption on thesurface of catalysts (110℃), the decomposition of surface nitrites (200–350℃) andnitrate (350℃) species.(3) After the cerium and/or zirconium addition, the activity of Cu/ZSM-5catalystextends toward both lower and higher temperatures. Among the catalysts tested, theCuCe0.75Zr0.25Oy/ZSM-5catalyst show the highest catalytic activity for the removal ofNO in NH3-SCR reaction, corresponding to the widest active temperature range of175–468°C.(4) At low temperature, two types of NH3species are adsorbed on the metalmodified ZSM-5catalysts. One is asymmetric vibration mode of NH4+which ischemisorbed on Br nsted acidic site, the other one is NH3adsorbed on Lewis acidic site through a covalent bond. The NO adsorbed on the catalysts surface exists aschelating and bridging nitrates species which are active intermediate species. NOxcanform hyponitrites on the surface of catalyst which are not active intermediate species.Furthermore, compared to Cu/ZSM-5, the addition of cerium and zirconium improvesthe contents of chelating and bridging nitrates, then enhancing the catalytic activity ofcatalysts.(5) The NH3-SCR reaction over metal modified ZSM-5catalysts at lowtemperature mainly follows the Langmuir–Hinshelwood mechanism. During thereaction, gas phase NOxadsorbed on the surface of catalyst form transition statespecies, which are important to catalytic activity.