An Investigation On Selective Catalytic Reduction Of NOjc By Hydrogen Over Supported Pt Catalysts

NO*is one of the main pollutants in atmosphere. Selective catalytic reduction (SCR) is an extensively investigated and effective way to remove NOx in the presence of excess oxygen. Recently, hydrogen has attracted the investigators’attention in the NOx removal field due to its advantage of being with low cost, as a clean reductant and with high activity at low temperature. Up to now, Pt and Pd are the main active components in the catalyst that was used for the investigation of selective catalytic reduction of NOx by H2(H2-SCR). The supported Pt catalyst exhibited higher catalytic performance. Previous investigations reveal that the catalytic performance of supported Pt catalysts was significantly influenced by Pt loading amount, pretreating conditions of the catalyst, feed gas composition to be treated as well as doping material. However, there are large divergences for the investigators on understanding the reaction mechanism as well as the influence mentioned above due to the complexity of heterogeneous catalysis. Furthermore, for practical application, there are some problems should be urgently resolved. For instance, Pt loading of the catalysts is too high to be accepted in economic, and resistance of the catalysts to SO2is much poor.Herein, H2-SCR reaction mechanism was proposed by investigating the reaction over Pt/HY catalyst. On the basis of the understanding of reaction mechanism, W was used as additive to Pt/HZSM-5with low Pt loading (0.1wt.%). Catalytic performance of the catalyst was significantly increased, and the promotional role of W was proposed. The influence of SO2on the H2-SCR reaction activity over HY and HZSM-5supported Pt catalysts as well as Pt/ZrO2was investigated. The influence mechanism of SO2on the catalytic performance of Pt/ZrO2was proposed. The detailed research content is as followings:(1) The influence of reaction conditions on oscillatory behavior of H2-SCR reaction over Pt/HY catalyst was investigated, and the oscillatory mechanism was proposed. It was found that H2-SCR oscillation only occurred over reduced Pt/HY catalyst and in the temperature range of60～120℃. It was also found that oscillation frequency was increased when reaction temperature over Pt/HY catalyst bed, catalyst weight and H2concentration in the feed gas was increased or NOx concentration was decreased. However, the oscillation frequency was decreased when NO2concentration in the feed gas was increased. On the basis of the phenomenon mentioned above, oscillatory mechanism of H2-SCR is proposed as follows:by the harmonization of reaction heat and NH3(act as intermediate in the reaction) diffusion, the oscillation of the adsorption-desorption of NOx on Pt particles in part of or in the whole catalyst bed synchronously occurs. The model of two typical states between Pt-NOx and Pt-H on Pt particle surface during oscillation process was firstly proposed. The proposition that NH3is involved in H2-SCR oscillation and the catalyst surface is alternated between the two states was confirmed by FTIR(2) The influence of pretreating conditions of the catalyst, Pt loading as well as O2and NO2concentration in the feed gas on H2-SCR reaction activity and N2selectivity over Pt/HY was investigated. It was found that the N2selectivity has a relationship with O2concentration in the feed gas, and the maximum value was obtained at10%O2. In the feed gas with2%O2, the maximum value was obtained at250ppm NO2, whereas in the feed gas with10%O2, the N2selectivity was monotonously decreased with the increase of NO2concentration. Based on the above study, two main N2O formation route over the catalyst in the reaction was proposed as follows:NOx adsorbed on Pt surface is reduced by Had to N2and N2O, the higher the ratio of Had/Nox there, the higher the relative amount of N2to N2O; NOx adsorbed at Pt-support interafce region is reduced by spillover Had mainly to N2O.(3) In Pt/HZSM-5, a part of Pt was present in high and inactive state. It was found that the interaction between W and Pt in the W modified catalyst is favorable for Pt maintaining in active metallic state. Just due to the role of W mentioned above, the H2-SCR reaction activity of the catalyst with lower Pt loading amount was effectively increased. In addition, the formation of unfavorable nitrate species on Pt surface was also suppressed; the dissociation of NO and the activation of H2on Pt was significantly accelerated.(4) It was found that the H2-SCR activity of Pt/ZrO2was rapidly increased due to the presence of SO2within a short time (ca.46min). However, the catalyst began to deactivate when the reaction time was prolonged. The promotional role of SO2is proposed as follows: the formation of nitrite/nitrate species on the support was inhibited by a small amount of sulfate species; the formation of NH4+species was promoted, which makes the formation of N2on the support accerated. The poison effect of SO2lies in that the catalyst surface was covered by a large amount of sulfate species, and the formation of nitrite/nitrate species was excessively suppressed.