Surface Design,Controllable Synthesis And NH₃-SCR Study Of Cerium-based Catalysts For Rare Earth Concentrates

Selective catalytic reduction of NO_x by NH₃（NH₃-SCR）has been widely used in the catalysis and treatment of nitrogen oxides（NO_x）.During the process,the design of advanced catalysts is the entire of the NH₃-SCR technology.Currently,the traditional V₂O₅-WO₃/Ti O₂catalysts suffer from problems such as narrow temperature window,the presence of vanadium and other toxic substances,so the development of green,efficient,and environmentally friendly catalysts has become a research hotspot in the field.It was found that the Ce catalyst has the advantages of high activity at high temperature and high selectivity of N₂,but it still has some shortcomings,such as poor activity at low temperatures,water resistance and sulfur resistance.Therefore,the development of cerium-based catalyst,it has excellent activity over a wide temperature range,and is resistant to water and sulfur poisoning.It is important for the efficient utilization of rare earth resources and the improvement of NH₃-SCR process.It is well-known that Bayan Obo rare earth concentrate（REC）is rich in Ce element and low content of transition metal oxides.As natural minerals,they have high stability and strong anti-poisoning ability,which have strong potential in catalysis.So,it is of great significance to develop rare earth catalytic materials and study the mechanism of NH₃-SCR catalytic denitrification.Aiming at the demand of flue gas denitrification catalyst with wide temperature window,this thesis has prepared and explored a novel type of Ce based low-temperature NH₃-SCR denitrification catalyst by modifying rare earth concentrate.The control over microstructures and component modification were carried out by roasting,acidizing,and loading active metal Fe.A variety of characterization methods were used to distinguish acid and base active sites,determine adsorption and desorption characteristics and denitration rate,and explore the structure-activity relationship between pore structure,surface properties and catalytic performance.In order to provide theoretical support for the modification of Baiyunbo rare earth concentrate,the change of valence state on active sites and electron transfer path of catalyst during SCR reaction were further investigated in details.The specific contents are as follows:First,the rare earth concentrate was modified by use of H₂SO₄（S）and H₃PO₄（P）,and the influence of acidified concentration on catalyst structure and surface acidity was investigated.The structure-activity relationship was established based on the study of physicochemical properties and catalytic mechanism.It was elucidated that appropriate amount of acid concentration modification could make Ce bring into play on the surface of rare earth concentrate,which increased the strong acid sites and exposed the active sites in concentrate,resulting in higher Ce³⁺concentration and oxygen vacancy of the catalyst.The results showed that the activity of rare earth concentrate catalyst REC-（S/P）modified by sulfuric acid and phosphoric acid increased significantly in the range of 150～450℃.When the concentration of sulfuric acid is 6 mol/L,the denitrification activity of REC-S catalyst reaches 78.2%at 350℃,and the activity increases obviously at high temperature.When the concentration of phosphoric acid was 4 mol/L,the denitrification performance of REC-P catalyst was the best,and the denitrification activity reached 85%at 350℃.The phosphoric acid treatment effect was better than sulfuric acid,and the selectivity of N₂was higher.In addition,the catalyst surface is mainly Br?nsted acidic sites,which mainly come from the P-O bond of phosphoric acid in Ce PO₄.However,NH₃species adsorbed by Lewis acid site have poor thermal stability and are prone to pyrolysis and desorption with the change of temperature.Bridging nitrate species is the main active nitrate species.whereafter,in order to further improve the activity of the catalyst and broaden the temperature window,using acid-modified concentrate as the carrier,Fe₂O₃/REC-（S/P）catalyst was prepared by impregnation method based on acid modification.The effect of Fe/Ce ratio on catalyst activity was investigated.The results show that the temperature window of catalyst can be widened and the selectivity of N₂ can be increased by suitable Fe O_x loading.When Fe/Ce ratio is 0.8,Fe₂O₃/REC-（S/P）catalyst exhibits the best SCR performance.Among them,NO_x conversion of Fe₂O₃/REC-P catalyst is 95%at the reaction temperature of 350℃.The selectivity of N₂ of the catalyst with Fe loading is more than 90%at 100～400℃.It is established that Fe plays a key role in improving the resistance of catalyst to sulfur and water.Fe₂（SO₄）₃ and Fe PO₄ species formed by the interaction of Fe with acid improved the oxidation reducibility of Fe at high temperature.The ratio of Fe²⁺to surface adsorbed oxygen（O_α）increased,which promoted the NH₃-SCR reaction.Then,the decomposition products of Ce CO₃F in concentrate was synthesized by microwave hydrothermal method according to well-designed proportion,and the as-synthesized system was Ce-La-Fe-O.The Ce/La/Fe ratio of the catalyst was consistent with the Ce/La/Fe ratio of the fluocerite.The NO_x conversion rate of Ce La_0.6Fe_0.12Oy reaches 95%and the selectivity of N₂ is greater than 72%in the range of 100～300℃.Superior SCR activity and N₂ selectivity were associated with the presence of Fe species on the surface.NO_x on Ce La_0.6Fe_0.12Oy catalyst mainly exists in the form of monodonts,and then the monodonts oxidize to bicodonts nitrate,which is the active intermediate of NH₃-SCR reaction.The E-R contribution of Ce La_0.6Fe_0.12Oy catalyst was higher at low temperature while both the L-H and E-R mechanism promoted the SCR reaction at high temperature.In the end,the co-precipitation method was used to synthesize M（M=Zr、Co、Mo and Cu）-Ce PO₄ catalyst.The results showed that Mo-Ce PO₄ showed the best catalytic performance,and the NO_x conversion rate reached more than 90%at the reaction temperature of 200～400℃,and the N₂ selectivity of Mo-Ce PO₄was more than 95%.In addition,M-Ce PO₄（Co,Mo,Cu）also showed good activity,stability and resistance to sulfur and water in long-term operation.M-Ce PO₄ can provide more active sites(Ce³⁺)and surface adsorbed oxygen（O’）due to its dopant-decorated microstructure.The main components of Ce PO₄ catalyst are the Lewis acid sites for NH₃ attachment and the disdentate nitrate on the catalyst surface.And the contribution rate of the L-H mechanism of Mo-Ce PO₄ catalyst is greater in the low temperature region while the effect of the E-R mechanism is more obvious in the high temperature catalytic activity.