Construction of the Polyoxometalates Host-guest Composite System Based on Novel Porous Carrier and Investigation of Its Medium-low-temperature NH3-SCR Activity

Author:Ren Zhao Yong

Supervisor:wang zuo


Degree Year:2019





In order to satisfy the increasingly strict NOx emission control standards,it is urgent to develop an efficient,low-pollution and low-cost SCR(selective catalytic reduction)catalyst that suits China’s national conditions for coal-fired thermal power plants.Compared with the traditional vanadium-tungsten-titanium series catalysts,the iron-and copper-based catalysts have the advantages of good selectivity,strong resistance to H2O and SO2 poisoning,low cost and easy availability,and environmental friendliness.However,iron-and copper-based catalysts showed low denitration activity in low temperature range,and it is necessary to modify it to improve the denitration performance of low-temperature SCR reaction.A series of materials(molecular sieve,Popcarbon,iron nanoring)with nanoporous or nanometer size were used as the matrix,and the transition metals was doped via appropriate methods to achieve high dispersion of active components;Polyoxometalates(POMs)with strong Br(?)nsted acid and inherent adsorption capacity of NH3/NO were used as guest material.The assembly of the host and guest materials is achieved by conventional Impregnation(IP),Mechano-Chemical Grinding(MCG)or "Ship in Cage"(SiC).Each component in the composite material has different functional orientations:the host material has micropores and nanometer-sized small mesopores,which can not only attract NH3,NO and other reaction gases to adsorb into the inner surface,but also provide a reaction place for the SCR reaction;highly dispersed copper and iron on the inner surface of the nanopore can avoid the catalyst deactivation caused by the sintering aggregation of the active components;the design of the guest material-POMs is mainly used as the catalyst promoter.Specifically,the promotion of POMs is mainly reflected in the following aspects:on the one hand,its super Br(?)nsted acidity can greatly enhance the surface acidity of the catalyst and enrich the acid species,thereby promoting the adsorption and activation of the reducing agent-NH3;on the other hand,the strong redox properties of POMs can promote the adsorption of NO and its conversion to NO2,thus improving the denitration reaction along the path of the rapid SCR reaction.The main contents and corresponding conclusions of this work include the following aspects:(1)NH3-SCR reaction on the catalysts prepared by impregnation method was carried out in the range of 100-550℃.Denitration activity of CuO and POMs supported by four carriers(NaY,TiO2,MCM-41,SAPO-34)was significantly improved,with NaY catalyst showing the best denitration performance.For two kinds POMs,phosphotungstic acid(HPW)performs better than phosphomolybdic acid(HPMo)with regard to SCR activity.For MCM-41,TiO2 and SAPO-34 supported catalysts,the removal rate of NOx decreased with the increase of CuO loading,with 2%CuO being the optimal loading.For NaY,NOx conversion showed the opposite trend.This may be related to the relative size of copper ion exchange capacity of several carriers.For the 5%CuO-n%HPW/carrier composite system,the denitration efficiency increased with the increase of HPW loading.This may be attributed to super acidic of HPW and it’s "quasi liquid phase" property.Through analysis of the results,it was found that 2%CuO-60%HPW/NaY had excellent low temperature denitration activity(NOx conversion=98%),wide temperature window(220-350℃)and outstanding N2 selectivity.Therefore,the CuO-HPW/NaY composite system can meet the current mainstream trend and the urgent practical application in the current denitration field.(2)Using green and harmless corn as a raw material,Popcorn was rapidly synthesized by microwave irradiation as a precursor of porous carrier Popcarbon.The raw materials are cheap and easy to obtain,with the synthesis method being simple and quick.The influences of preparation conditions including microwave power,carbonization temperature,copper precursor species,copper loading and POMs loading on surface acidity,pore structure characteristics and SCR denitrification activity of catalysts were systematically studied.In addition in this chapter,based on the conversion index of NOx,microwave power,carbonization temperature,CuO loading and HPW loading were taken as impact factors,and the four-factor and three-level orthogonal experiments were designed.HRTEM visually confirmed that Popcarbon obtained via microwave irradiation followed by high-temperature carbonization has large honeycomb macropores,and the pore walls are formed by the disorderly accumulation of graphite-like crystallites.The gap between microcrystals can explain the existence of abundant micropores in carbon materials.XPS,XRD,Raman,FT-IR and other characterizations revealed the effects of HPW and CuO on the internal microstructure of Popcarbon(the size of graphite-like crystallites and the size of the graphite network plane)and the type,as well as the content of surface functional groups.The rich functional groups on the surface of Popcarbon not only provide anchor sites for HPW and active component-Cu species,but also facilitate high dispersion of active components,thereby effectively avoiding sintering and agglomeration problems of the active center.The interaction between HPW,Cu species and carbon support accelerates electron transport efficiency and further promotes the adsorption and activation of NH3.In addition,experiments have confirmed that the introduction of HPW can also enhance the sulfur toxicity and hydrothermal stability of the catalyst.The orthogonal results showed that the composites have the best catalytic activity when the microwave power was 800 W,the carbonization temperature was 800℃,the CuO loading was 5%,and the HPW loading was 40%.(3)Selecting NaY and USY molecular sieves as the carrier,whose pore size was strictly matched with specific size of the POMs,copper nitrate was used as the precursor of CuO.The effects of traditional wet impregnation(IP)method,solid ion exchange(SSIE)method for loading copper,as well as the conventional impregnation and "cage in ship"(SiC)for loading HPW on the denitration performance of composites were systematically studied.It was found that changes in the loading method of copper and HPW could affect both the NOx adsorption capacity and removal rate of the as-prepared catalyst.The denitration performance of the composite existed in the following order:Cu(SSIE)-HPW(SiC)/NaY>Cu(IP)-HPW(SiC)/NaY>Cu(SSIE)-HPW(IP)/NaY>Cu(IP)-HPW(IP)/NaY.It is also found that the HPW-loading method had a significant effect on the denitration performance of the catalyst.The Cu-loading method has a significant effect on the active temperature window,with HPW-loading method tending to affect the overall denitration efficiency of the catalyst.When the Cu-loading method is fixed,the SiC-loaded HPW can increase the NOx conversion rate by 7-10%as compared to the IP method.The reason is that the SSIE-loaded CuO can achieve high dispersion of the active component in the carrier.Simultaneously,the lattice defects caused during the loading process are beneficial to enhance the adsorption ability for oxygen of the catalyst.With the help of the SiC method,POMs can be dispersed at molecular level and the relative anastomotic size between the POMs and the pore of carrier can also prevent the loss of the POMs during the reaction process.(4)Microwave-assisted hydrothermal method was used to rapidly synthesize Fe2O3 nanorings(NRs)with uniform size.The immobilization of HPW on the surface of Fe2O3 NRs was achieved by mechanical chemical polishing and then the composite catalysts of HPW/Fe2O3 NRs were obtained.The catalyst exhibited excellent SCR denitration activity,good resistance to SO2 and water in a wide temperature range.HRTEM characterization indicated that a layer of HPW film with a thickness of about 2.5-3 nm was formed on the surface of Fe2O3 NRs.The synergistic effect of surface iron species and HPW anions improved the thermal stability of HPW,which in turn enhanced the ability of the catalyst to selectively remove NOx at high temperatures.Since the HPW cluster contains a plurality of crystal waters and has a natural affinity for water,and the number of crystal water affects the proton conductivity and acidity of the POMs,while SO2 cannot enter its bulk phase.Based on this,we can regard the thin layer of HPW supported on the surface of the nanoring as a super skin with special functions.This protective effect of nano skin makes the HPW/Fe2O3 NRs exhibited resistance to sulfur poisoning in SCR reaction.(5)Taking CuO-HPW/Popcarbon as the research object,the adsorption kinetics,reaction thermodynamics and reaction kinetics involved with SCR were investigated.Studies on adsorption kinetics showed that the adsorption of NO2 on CuO-HPW/Popcarbon surface was controlled by gas film diffusion and intra-particle diffusion based on the boundary layer effect induced by HPW and the adsorption microporous-filling nature of NO2 adsorption over the surface of carbon material.The reaction thermodynamic calculations showed that at 200℃,the standard SCR reaction,the fast SCR reaction and the slow SCR reaction can all proceed in the forward direction,and the reaction limit is very deep.The intrinsic rate equation of the SCR reaction was constructed through the study of the reaction kinetics,and the activation energy of the reaction was 24.79 kJ·mol-1.According to the reaction order of NO determined by experiment,it could be preliminarily concluded that the E-R and L-H mechanism existed simultaneously in the denitration reaction catalyzed by the CuO-HPW/Popcarbon system.