Preparation,Active Site Regulation And Performance Of NH₃-SCR Catalysts

Selective catalytic reduction of NO_x with NH₃(NH₃-SCR)is one of the most commonly used efficient and commercial technology to eliminate nitrogen oxides(NO_x),and catalyst is the core of this technology.Thus,it is of great social significance to develop high performance SCR catalyst and study its active sites in depth for guiding the design of new SCR catalyst and effectively controlling NO_x emission,as well as realizing the reduction of NO_x.In this paper,SCR active sites with different content and structures were constructed by changing the preparation method and adjusting the preparation conditions for the environment-friendly SCR catalyst system of Fe-based oxides and Cu-based small pore zeolites.The structure and physicochemical properties of the catalysts were obtained by various characterization methods.The structure-activity relationship,reaction mechanism and aging mechanism of the catalysts were investigated by performance test,kinetics study,in situ/operando experiment and density functional theory(DFT)calculations.A highly defective?-Fe₂O₃ was obtained by constructing defective sites in?-Fe₂O₃ and the acidity of?-Fe₂O₃ was improved,thus promoting the low-temperature SCR performance by the synergistic effect of acidity and redox performance.The homonuclear binuclear active sites composed of Fe³⁺and oxygen-vacancy coupled Fe²⁺were confirmed by Rietveld refinement,X-ray near-edge absorption fine structure(XAFS),X-ray photoelectron spectroscopy(XPS),temperature-programmed reaction and in situ infrared(IR)spectroscopy.Such binuclear active sites can significantly improve the redox property and acidity of the catalyst,and the increased acidity promote the low temperature SCR activity as well as water and sulfur resistance.DFT calculations proved that binuclear active sites could enhance the low-temperature activity of the catalyst by promoting the adsorption and activation of NH₃,and revealed the essential nature of the enhancement of low temperature SCR performance of the highly defective?-Fe₂O₃.Cu-SSZ-13 catalysts with different Cu²⁺ions active sites contents were prepared using sol method with adjusted silica-alumina ratio(Si/Al)by changing the amount of Al source.The effect of Cu²⁺ions active sites content on SCR performance of Cu-SSZ-13 catalysts was investigated by X-ray powder diffraction(XRD),inductively coupled plasma(ICP),XAFS,ultraviolet-visible spectroscopy(UV-Vis)and performance measurements.Cu-SSZ-13 with low and medium Si/Al exhibited relatively good SCR performance due to the higher Cu²⁺ions content.However,Cu²⁺ions content is low in Cu-SSZ-13 with high Si/Al,showing a poor SCR performance.As silicon islands are easily formed in Cu-SSZ-13 catalysts prepared by sol method,the performance and quality of products are unstable.Therefore,a heterogeneous seed crystal conversion method was developed to prepare Na-SSZ-13 with different Si/Al,and the effect of organic template content on their structure and properties were studied.The results showed that submicron Na-SSZ-13 with different Si/Al ratio was successfully prepared,and the content of OSDA would affect the crystallinity of Na-SSZ-13.For low Si/Al samples,the higher the content of osda,the lower the relative crystallinity.However,it is opposite for the middle Si/Al samples.Cu-SSZ-13 catalysts were prepared by ion exchange method based on Na-SSZ-13 prepared by heterogeneous seed crystal conversion method,and the effects of template amount on their structure and SCR performance were investigated.The proportion of different Cu²⁺active sites in the catalysts can be adjusted by the amount of template.The more the amount of template,the higher the Cu-Al ratio(Cu/Al),thereby the higher the proportion of[Cu²⁺OH]-Z.The reaction kinetics showed that the apparent activation energy decreased with the increase of[Cu²⁺OH]-Z,which confirmed that[Cu²⁺OH]-Z are more active than Cu²⁺-2Z at low temperature.Life and durability of Cu-SSZ-13 are the important indicators to evaluate its performance.By using XAFS,electron paramagnetic resonance(EPR),in situ IR spectroscopy of NH₃adsorption as well as operando temperature programmed surface reaction(TPSR)of NH₃adsorption monitored by in situ IR combined with mass spectrometry(MS),the evolution of Cu²⁺ions active sites in Cu-SSZ-13 during high temperature aging was studied and the mechanism of aging deactivation was revealed.After hydrothermal aging,a part of Cu2+ions in Cu-SSZ-13 were converted into CuOx,while the interaction between the remaining Cu2+ions and oxygen in zeolite framework is enhanced,which reduces the mobility of Cu²⁺ions and limits the adsorption and activation of NH₃,resulting in the decrease of low temperature SCR performance.