Preparation Of Oxygen-rich Manganese-cerium Catalysts And The Mechanism Study On Low-temperature NH₃-SCR

Nitrogen oxides?NO_x?are considered as one of the major pollutants,being precursors to acid rain,haze,photochemical smog,and have acutely adverse effect on the air quality and human health.To date,a low temperature?120-200??selective catalytic reduction?SCR?technology has been recognized as a hot spot for effective NO_x removal.In order to develop an efficient low-temperature catalyst,this study starts from widely concerned low-temperature manganese-cerium catalysts,it breaks the traditional SCR catalyst from the idea of cation modification and creatively explores from the perspective of anion.Through the methods of surface and bulk structure modification,two types of efficient oxygen-enriched?oxygen vacancy defect?manganese-cerium low-temperature SCR catalysts?OVDs-Mn-CeO₂ and Ce-K-OMS-2?were designed.And the novel catalysts will give the advantage of oxygen vacancies and Mn⁴⁺during the catalytic process.Meanwhile,various characterizations and Density Functional Theory?DFT?were utilized to study the SCR mechanism of two oxygen-rich manganese-cerium catalysts.The main results are as follows:?1?Preparation of surface structure modification?OVDs-Mn-CeO₂?oxygen-rich manganese-cerium catalyst and the mechanism study on low-temperature NH₃-SCR.The OVDs-Mn-CeO₂ catalyst was synthesized by one-step co-precipitation method using Mn?NO₃?₂,KMnO₄ and?NH₄?₂Ce?NO₃?₂as precursors.It is found that increasing of oxygen vacancies can improve the SCR activity,obtaining 100% NO conversion at 150?.Meanwhile,various characterizations were used to further study the relationship between catalyst and SCR activity.It was confirmed that the specific surface area and total pore volume of OVDs-Mn-CeO₂ were greatly improved.Furthermore,the OVDs-Mn-CeO₂ catalyst contained more oxygen vacancies and Mn⁴⁺,and the active Mn species were highly dispersed in OVDs-Mn-CeO₂ catalyst,significantly promoting the redox ability of catalyst,thus enhancing the SCR performance at low temperatures.The in situ DRIFTS and DFT calculation were used to further study the SCR mechanism of the OVDs-Mn-CeO₂ catalyst.It showed that the OVDs-Mn-CeO₂ catalyst has a lower value of oxygen vacancy defect formation energy compared to Mn-CeO₂,and following the Langmuir-Hinshelwood?L-H?mechanism.?2?Preparation of bulk structure modification?Ce-K-OMS-2?oxygen-rich manganese-cerium catalyst and the mechanism study on low-temperature NH₃-SCR.Octahedral molecular sieve M-K-OMS-2?M = K,Zn,Fe,Ce,V?catalysts with improved low-temperature SCR performance were prepared by a thermal reflux method.The best active Ce-K-OMS-2 catalyst,achieving 100% NO conversion at 100-230?,was chosen for further study.Characterization results confirmed that Ce doping can significantly increase the specific surface area and total pore volume of the catalyst.Furthermore,compared with K-OMS-2,the much more oxygen vacancies and Mn⁴⁺,stronger redox capacity,more acidic sites in Ce-K-OMS-2 generally promoted the SCR activity.Meanwhile,Ce doping could effectively inhibit catalyst poisoning in the presence of SO₂ and H₂O.This is mainly due to the structural distortion after Ce replaces the Mn in K-OMS-2 octahedral unit,forming more oxygen vacancies,and effectively protecting the active sites of Mn in Ce-K-OMS-2 catalyst.In addition,the DFT study showed that the lower value of oxygen vacancy defect formation energy of Ce-K-OMS-2 than that of K-OMS-2 was conducive to the adsorption of gaseous molecules,thereby improving the catalyst SCR performance.And the L-H mechanism is applicable for Ce-K-OMS-2 catalyst during the reaction process.