Structure-Activity Relationship And In Situ DRIFTs Study Of Catalysts With Novel Structures For NH₃-SCR Of NO

With the progress of society and development of economic,nitrogen oxide（NO_x）pollution has become increasingly serious and tricky.NO_x not only cause environmental problems such as photochemical smog,acid rain,ozone hole and so on,but also make damage to human health.To date,selective catalytic reduction of NO_x with NH₃（NH₃-SCR）has been regarded as the most effective and widely used technology for control of NO_x.The development of novel and efficient catalysts to meet actual needs and cognition of related reaction mechanism were key core issues.This paper manily focused on the construction of catalysts with novel structures and in situ DRIFTs studies.We design and prepare highly efficient NH₃-SCR catalysts from the aspects of special morphology,additives modification and alkali resistance.Furthermore,we carefully investigated the related reaction mechanism by the boost of in situ diffuse reflectance infrared fourier transform spectrpscopy（in situ DRIFTs）.The main research results were as follows:（1）Crystal facet-dependent mechanism of TiO₂ is significant in the SCR reaction,but the in-depth study of structure-activity relationship is rarely involved.Therefore,in this work,in situ DRIFTS and DFT calculations were applied to address the promotional effect of the（001）plane of TiO₂-NS in the SCR reaction.The behavior of adsorption and desorption of NO_x and NH₃ on the two surfaces was studied.It was found that NH₃ adsorbed on both TiO₂-NS and TiO₂-NP was in the form of NH₃（g）on the Lewis acid sites.NO_x on Ti O₂-NP were mainly trans-（NO）₂ and N₂O₄ which were non-reactive.Differently,NO on TiO₂-NS was mainly in the form of NO₂ which was probably due to the high energy and abundant active oxygen species of the（001）facet.The formed NO₂ could trigger the subsequent‘fast SCR’reaction thereafter promoting the activity.Therefore,TiO₂-NS with more（001）facets had better SCR activity than TiO₂-NP with the exposed（101）facet.（2）Improved NO_x reduction in the presence of alkali metals is still challenging.In this work,we developed novel hollandite Mn-Ti oxides promoted Cu-SAPO-34catalysts（HMT@Cu-S）for the selective catalytic reduction（SCR）of NO_x with NH₃via the isolation of active sites and alkali metal trapping sites.The HMT@Cu-S catalysts exhibited excellent SCR activity and N₂ selectivity.More importantly,the HMT@Cu-S catalysts had stronger resistance against alkali poisoning compared to Cu-SAPO-34 catalysts.It was found that this newly developed catalysts had superior alkali resistance compared to the reported catalysts,making it attractive for environmental application.The hollandite Mn-Ti oxides acted as a protective layer to trap alkali ions according to an ion exchange mechanism.From in situ diffuse reflectance infrared transform spectroscopy（in situ DRIFTs）studies of desorption,it could be concluded that after alkali poisoning,the NH₃ species of the HMT@Cu-S catalysts were more unstable;therefore,could easily participate in the SCR reactions.Additionally,the NO_x species showed no change after introduction of alkali metal ions due to alkali trapping effects.Moreover,the in situ DRIFTs of transient reactions indicated that the NH₃ species on K-HMT@Cu-S catalysts were much easier to adsorb and that the formed NH₃ species that were unaffected by alkali metal ions were highly reactive.The present investigations provide an effective strategy for the design and the application of catalysts with outstanding catalytic activity and alkali resistance.（3）The manganese oxide octahedral molecular sieve（OMS-2）materials had great catalytic performance and unique structure,but low-temperature activity and N₂selectivity needed to be improved.Besides,the NH₃-SCR mechanism on OMS-2materials is still indistinct.In this work,the OMS-2 materials were prepared by using NH₄⁺ionic template as structure-directing agent.The catalytic performance of OMS-2was improved by Fe doping.The 4Fe-OMS-2 catalysts with 4%Fe:Mn molar ratio exhibited excellent NH₃-SCR activity,N₂ selectivity and further improved alkali resistance.Although the crytomelane-type crystal form of OMS-2 had no change after Fe doping,the crystallinity of catalysts decreased with amounts of Fe doping increased.In addition,the Fe-OMS-2 catalysts began to aggregate when the amounts of Fe doping were more than 4%.There were abundant Oα,Mn⁴⁺and Fe³⁺species on the surface of 4Fe-OMS-2 catalysts,making them show outstanding redox properties.The results of in situ DRIFTs indicated that 4Fe-OMS-2 catalysts could adsorb more active NH₃ species and more NO_x species due to promotional effects of Fe doping.The formed intermidates could participate in the SCR reactions more easily.Furthermore,the alkali metal ions poisoned 4Fe-OMS-2 catalysts still have strong capacity of NO_x species adsorption,thus contributed to the remarkable catalytic performance.The above results and discussions prove that the OMS-2 catalysts were modified and improved effectively by the Fe doping.