Study On Low-temperature Selective Catalytic Reduction Of NO_x With Propylene In Excess Of Oxygen

With the number of motor vehicles increasing instantly, exhaust gas pollution is being more and more serious. The traditional three-way catalysts (TWC) cannot be effective for lean burn engines and diesel vehicles. Moreover, low-concentration, oxygen-rich and low-temperature exhausts emitted from urban road tunnels and underground parking lots, are easily ignored sources of air pollution and can evoke serious environmental problems. Low-temperature removal of NOx by SCR can be achieved with the application of the toxic reducing agent NH3, but it is not suitable for the mobile engines and stationary constructions. Alternatively, low-temperature and simultaneous removal of pollutants can be accomplished by noble metal supported oxide and zeolite catalysts, using hydrocarbons (HC) as reducing agent.In this work, the oxygen-rich and low-concentration vehicle exhaust was simulated in lab scale. A series of catalysts were prepared by wet-impregnation method, including Pt supported MOx (M= Al, Ce, Ti, W, Zr), noble metals (Pt, Pd, Rh, Ru) supported TiO₂ and Pt supported zeolites (MCM-41, ZSM-5). The catalysts were characterized by BET, TEM,XRD and in situ DRIFTS techniques and low-temperature adsorption (ADS), temperature programmed desorption (TPD), temperature programmed oxidation (TPO), selective catalytic reduction (SCR). The main contents and results of this paper are as follows:1. From the results of the adsorption and desorption of C₃H₆ over Pt/MOx, it can be observed that only simple physical adsorption process happened on the catalyst, as well as the supports. The corresponding desorption process was also simple, suggesting that the support played important role in C₃H₆ adsorption, not the promotion of support (precious metal loading). Otherwise, Pt/ZSM-5 showed stronger ability of C₃H₆ adsorption than ZSM-5. This is possibly due to the slight change of microspores structure of ZSM-5 during the impregnation of Pt.2. Results of the NO-ADS and NO-TPD over the catalysts and supports indicated that Pt loading enhanced the NOx adsorption ability of MOx and MCM-41, ZSM-5 dramaticly as well as the NO-TPD amounts. The NO desorption peak displayed in low temperature range, attributing to the physical adsorption of NO and the dissociative adsorption of NO on Pt sites; while NO and NO₂ desorbed at higher temperature can be assigned to the decomposition of surface nitrates. The NO-ADS capacity and NO-TPD amounts depends on the physiochemical properties of supports (e.g., BET area, acidic and basic properties), as well as the modification of supports (Pt loading).3. It can be observed from the C₃H₆/NO-TPO experiments that the low-temperature oxidation activity of C₃H₆/NO were increased distinctly over Pt/MOx, Pt/MCM-41 and Pt/ZSM-5. The SCR activity of catalyst correlated well with its catalytic performance for TPO of reactants (C₃H₆, NO) and displayed a volcanic variation versus temperature. Owing to high selectivity for oxdization of C₃H₆, no CO was detected during the reactions of TPO and SCR. The competitive reaction of reactants (C₃H₆ and NO) and intermediates with the active oxygen species on catalyst surface (e.g., Pt-O, adsorbed oxygen) resulted in mutual inhibitition for the oxidation of NO and C₃H₆ in SCR reaction.4. It was found that Pt/TiO₂ catalyst calcined at 450 oC showed the best low-temperature C₃H₆-SCR catalytic activity,which can be explained by better dispersion of Pt on the catalyst surface, as well as unchanged phase compositions of the support. However, high pretreatment temperature of support ( > 450 oC) resulted in drop of the BET surface area and derease of the active rutile TiO₂, then caused the decrease of catalytic activity. With Pt content increasing, the low-temperature C₃H₆-SCR activity increased obviously. 0.5 wt% Pt was found to be proper for the enhancement of catalytic activity, while more Pt loading did not have profound effect on the catalytic performace any more.5. Among noble metal supported oxide catalysts, 0.5 wt% Pt/TiO₂ displays outstanding catalytic activity for C₃H₆-SCR and selective reduction efficiency of C₃H₆ (C3 H6 ), in which 63.4% NOx was reduced to N2+N2O and 83.7% C₃H₆ was oxidized with 75.7% C3 H6? at 140 oC. At the same temperature, Pt/ZSM-5 showed higher catalytic activity and C3 H6? than oxide catalysts, with 77.1% NOx reduction, 79.7% C₃H₆ conversiton and 96.7%C3 H6? . Under the oxygen-rich condition, Pt/TiO₂ and Pt/ZSM-5 showed almost the same C₃H₆-SCR catalytic properties. With the increase of O₂ concentration, the low-temperature C₃H₆-SCR activity and C3 H6? was enhanced. With the concentration of C₃H₆ increasing, the maximum conversion of NOx was elevated, but the SCR reaction temperature range shifted to higher temperatures, bringing about the decrease ofC3 H6 .6. The low-termperature SCR of NO with C₃H₆ in the excess of O₂ was investigated by in situ DRIFTS. NO dissociation mechanism and nitrates reaction mechanism had been proposed for the SCR over Pt/TiO₂ and Pt/ZSM-5 respectively; and the C₃H₆-SCR mechanism varies from the physicochemical property (e.g., acid-base pair, charge transfer) of support. During the reaction of C₃H₆-SCR over Pt/TiO₂, the formation of N2 and N2O was from the combinations of NO* and N* radicals. The interaction of NO*, CO* and other CxHyOz species was thought to be the important step during the SCR reaction. On the other hand, oxidizing nitrates induced by the oxidation of NO to NO₂, could react with the reducing CxHyOz species over Pt/ZSM-5 catalyst. The activations of NO and C₃H₆ were crucial steps in the SCR reaction.In conclution, the Pt supported metal oxide ( Pt/TiO₂ ) and zeolite ( ZSM-5 ) catalysts for the low-temperature C₃H₆-SCR catalysts showed high catalytic activity in excess O₂ at temperature as low as that of NH3-SCR. Moreover, HC and NOx were removed simultantly in C₃H₆-SCR. The low-temperature C₃H₆-SCR is a promising technology for NOx emission control in the fields of motor vehicles, even urban road tunnels and underground parking lots.