Study On CuO-Ce_xZr_1-xO₂/TiO₂Catalysts For The Selective Catalytic Reduction Of Noxwith NH₃from Diesel Engines

Currently, selective catalytic reduction (SCR) with ammonia is regarded as one ofthe most efficient technologies for the abatement of NOxemissions from diesel engine，which has been extensively concerned in developed nations. Ce-/Zr-additives are usedas a leveraging solution to prompt the stability of the Cu/TiO2, enlarge temperaturewindow and enhance the SCR activity as well. A series of CuCexZr1-x/TiO2catalystswere prepared by a currently wet impregnation with an excess of deionized water. Thephysic-chemical properties and the reaction mechanism of these catalysts arecharacterized by BET, XRD, TEM, XPS, H2-TPR, NH3-TPD, NO-TPD and In situDRIFT. The research results are crucial in both theoretical significance and practicalapplication to reduce diesel engine NOxemission and adapt the more strict emission.Main conclusions from this study are as follows:(1) CuCe0.25Zr0.75/TiO2sample exhibited nearly100%NOxconversion over a widetemperature range (165-450oC), which is strikingly superior to those ofCu/TiO2(210–389°C).(2) The zirconium and/or cerium introduction promote copper dispersion andinhibit copper crystallization. The amorphous copper clusters formedcontribute to widening the activity window.(3) Zirconium and/or cerium doping is also beneficial to promoting copper speciesenrichment on the surfaces of TiO2grains, promoting the transfer of mass andenergy during the SCR process. Moreover, CuCe0.25Zr0.75/Ti has the highestOα/Oα+Oβratio, which is valuable for the NO oxidation to NO2in the SCRreaction and then facilitate the “fast SCR” reaction.(4) The redox property seems to play a significant part in the process of theNH3-SCR. From H2-TPR results, it is found that the addition of zirconiumand/or cerium, α reduction peaks shift moderately to lower temperatures andthe peak intensities increase. It indicates that the addition of zirconium and/orcerium to Cu/Ti catalyst enhance the surface redox ability at low temperatures,which leads to promote the SCR activity under low temperatures. Moreover,partial copper species have been incorporated into the vacant sites of zirconiumoxides or cerium oxides to come into being coordinated surface structure withthe capping oxygen, which leads to lattice distortion, allowing a highermobility of the lattice oxygen. It may also accelerate the reduction process and consume more hydrogen, which enhance the redox property of catalysts.(5) The in situ diffuse reflectance infrared transform spectroscopy (DRIFT)disclosed two possible reaction mechanism-L-H mechanism at lowtemperature (<200°C) and E-R mechanism at high temperatures (>200°C) forSCR reaction over CuCe0.25Zr0.75/TiO2catalyst. When NH3is bubbled into theCuCe0.25Zr0.75/TiO2catalyst, ammonia bonded to Lewis acid sites is morestable over CuCe0.25Zr0.75/Ti in high temperature, while the Br nsted acid sitesare more important than Lewis acid sites in low temperature. WhenNH3+NO+O2co-adsorption, NH3species have occupied most of activity siteson CuCe0.25Zr0.75/Ti catalyst, NH3species mainly exist in the form of NH+4(atlow temperature) and NH3coordinated(at high temperature), they play acrucial part in NH3-SCR process.