Research Of The Selective Catalytic Reduction Of NOx By Propene In Excess Of Oxygen

With the increase of the motor vehicles, the problems caused by exhaust gaspollution are being much more serious, and also the emission control technology hasbeen developed dramatically. However, the traditional three-way catalysts （TWC） arenot effective for the removal of exhausts emitted from lean-burn engines and dieselvehicles. Alternatively，selective catalytic reduction （SCR） of NO_xis one of the mosteffective methods for NO_x removal, and NH₃-SCR technique has already beenapplied into the removal of NO_x emitted stationary source successfully, which isunsuitable for the removal of NO_x emitted from mobile source.Selective catalytic reduction by hydrocarbons （HCs, a natural componentalready present in combustion exhaust） is an effective and convenient techniquewhich can remove both pollutants simultaneously. In this work, the oxygen-rich,low-temperature and low-concentration vehicle exhaust was simulated in lab scale. Aseries of catalysts were prepared for C₃H₆-SCR by sol-gel, hydrothermal andwet-impregnation methods, including Pt/TiO₂, SnO₂/TiO₂and Pt/SBA-15（Pt/Al-SBA-15）. The prepared catalysts were characterized by means of XRD, BETsurface area, TEM, NMR, XPS and evaluated by adsorption （ADS）, temperatureprogrammed desorption （TPD）, temperature programmed oxidation （TPO）, selectivecatalytic reduction （SCR），NH₃-TPD and in situ DRIFTS techniques. The main resultsand conclusions are summarized as follows:1． The NO_xadsorption ability of the support was enhanced significantly afterimpregnation of active components （Pt and SnO₂）, in that case, the adsorptionprocess of NO_xhappened on catalysts not only include physical adsorption, butalso chemical adsorption. In the meantime, both the supports and catalysts hadlimited adsorption capacity of C₃H₆, indicating that main influence factor of propene adsorption depends on the physiochemical properties of supports.2． Impregnation of active components enhanced the NO_xadsorption abilitysignificantly as well as the NO_xdesorption amounts during TPD process. The NOdesorption peaks appeared in the low temperature range could be ascribed tophysical adsorption of NO and the dissociative adsorption of NO, whereas the NO_xdesorption peaks displayed in the high temperature range could be assigned to thedecomposition of nitrate species formed on the catalysts surface. The NO-ADScapacity and NO-TPD amounts depends on the physiochemical properties ofsupports （e.g., BET area, acidic and basic properties）, as well as the activecomponents natural properties.3. It can be concluded that the Pt loading （Pt/TiO₂，Pt/SBA-15） improved thesamples oxidation activities in the TPO processes of NO to NO₂and C₃H₆to CO₂in low temperature range markedly, which correlates well with their SCRactivities. The competitive reaction of reactants （C₃H₆and NO） and intermediateswith the active oxygen species on catalyst surface （e.g., Pt-O, adsorbed oxygen）resulted in mutual inhibitition for the oxidation of NO and C₃H₆in SCR reaction.The SnO₂/TiO₂catalyst reached its maximum NO_xreduction efficiency at relativehigher temperature（>300℃）,which correlates well with its NO/C₃H₆-TPOactivity as well.4. Among all Pt/TiO₂samples, PT500exhibited the best NO_xconversion efficiencyof47.03%and100%C₃H₆conversion simultaneously at180℃under the standardreaction condition，the SCR activity displayed a “volcano shape” variation versustemperature. This enhanced activity of PT500may be due to the strongest NO_xadsorption capacity，the outstanding oxidation activities in TPO process and thehighest Pt dispersion together with its anatase phase. Promotive effect of MOx（M=Mn, Ce, Co） mechanically mixed with PT500on SCR activity was observed,which might be associated with the enhancement formation of intermediatespecies.5. Nevertheless, the SnO₂/TiO₂sample showed maximum40.3％NO_xreduction at 320℃and100％conversion of C₃H₆at280℃, respectively. The difference ofcatalytic performances between Pt/TiO₂and SnO₂/TiO₂correlates well withthe natural properties of active components themselves. The XRD and XPSresults exhibited that SnO₂was loaded onto the TiO₂support and no othercomposite oxide was formed.6. The0.5%Pt/SBA-15sample achieved markedly high80.1%NO_xreduction and87.04%C₃H₆conversion simultaneously at140℃. The XPS results suggested thatPt, PtO and PtO₂were all presented on the catalyst, whereas PtO₂species mightplay a role as main active site. The incorporation of appropriate amount Al intoSBA-15improved catalytic performance, which could be ascribed to theenhancement of catalyst surface acidity caused by tetrahedrally coordinated AlO4.7. Higher concentrations of O₂and C₃H₆led to higher NO_xconversion efficiencies.Higher amount of active components loading also favored NO_xconversion, but anoptimum content was observed. For Pt the optimal amount was0.5%, whereas5%was for SnO₂. In addition, the synthesized catalysts possessed good catalyticstability and physical stability.8. From the in situ DRIFTS spectra obtained in C₃H₆-SCR for PT500and SnO₂/TiO₂,it could be deduced that both samples could be ascribed to nitrates reactionmechanism. However, according to the in situ DRIFTS spectra of coadsorption ofreactants, SCR and stepwise reaction, it was obvious that bidentate nitrates weremore active than other kinds of nitrates and readily participating in C₃H₆-SCR onPT500catalysts, whereas bridged and monodentate nitrates were believed to bemore reactive and readily involved in C₃H₆-SCR on SnO₂/TiO₂catalyst. Theinitial activations of NO and C₃H₆were crucial steps in the SCR reaction.