Preparation And Properties Of Lanthanum Zirconate Catalyst Carrier Orient To Green Monopropellant

Green monopropellant has attracted considerable attention in space propulsion fields because it has advantages of non-toxic,high density specific impulse,easy to store and high safety.The catalytic combustion condition of green monopropellant is a high-temperature,water-vapor-containing,strongly acidic environment with strong mechanical/thermal shock,and the traditional catalyst carrier based on Al2O3 can not suffer such severe condition.To solve this problem,some new materials should be studied in the future.Lanthanum zirconate(La2Zr2O7,LZO)has been considered as a good candicate as catalyst because of its high melting point,excellent thermal stability and high catalytic activity.In this work,preparation of porous LZO was studied.we researched the pore structural evolution of porous LZO in high temperature and made the structure controllable,and we employed different methods to make the pores more stable in high temperature.LZO catalyst carrier was prepared using extrusion process,and the resistance of sintering,water-vapor erosion and thermal shock was tested.Moreover,the catalytic performance of LZO supported rare metal catalyst was investigated.Details are as follows:Porous LZO xerogel was prepared by epoxide-addition(EA)method.Influence of precursors(nitrates and chlorides)on properties of LZO xerogel was investigated.Ths result shows that LZO xerogel prepared by nitrates has small linear shrinkage,low gelation rate,stable in high temperature and without impurity formed,which is beneficial for preparing porous LZO.Adjusting the additive composition and controlling the gelation and drying process,can make the pore structure of LZO xerogel controllable.The as-prepared LZO xerogel has a surface area of 23.9 m2/g,pore volume of 0.071 cm3/g,and average pore size of 3.7 nm.The evolution of porous LZO in high temperature was investigated.With increasing of temperature from ambient to 1200 ?,inorganic transformation occurs at 600 ? and weak crystal t-Zr O2 forms.After 800 ?,La2O3 entrances to lattice of t-Zr O2 to form f-LZO,then it turns into p-LZO at 1200 ?.After heated at 1200 ?,the surface area of porous LZO drops by 82 %.Solvothermal treatment makes the surface area of LZO xerogel increase to more than 300 m2/g,and the sample has a surface area of five times to that without solvothermal treatment.Meanwhile,this process makes the temperature of formation of p-LZO phase reducing from 1200 ? to 1000 ?.-OH on the surface of LZO xerogel is replaced by-OC2H5 in the process of solvothermal treatment,which inhibits the destroy of micro/mesopores in drying and calcination,promoting crystallization of LZO xerogel and making the skeleton stronger.Surface modification using TEOS further rises the surface area up to 400 m2/g and and the sample has a surface area of 2.4 times to that without surface modification.-OH on the surface of LZO xerogel is replaced by-O-Si? in the process of surface modification,which further inhibits the destroy of micro/mesopores in drying and calcination,and inhibits the growth of crystalline grains of LZO.However,surface modification is effective to make the pore stable below 1000 ? due to the formation of Si O2 which blocks the pores.Y and Ce doping inhibits the crystallization and growth of grains.Addition of Ce in porous LZO rises the surface area at 1200 ?.When molar ratio of Ce/Zr/La is 0.3/0.7/1,the surface area of porous LZO is 19.5 m2/g,which increases by 58.5 % compared to porous LZO.LZO catalyst carrier was prepared using extrusion process and sintering resistance,water-vapor-condition endurance and thermal shock resistance were studied.The as-prepared LZO catalyst carrier has a crushing strength of 26.2±2.9 MPa,surface area of 27.7 m2/g,and has a particles-piled structure with a single p-LZO phase.After heated at 1300 ?,the particles-piled structure retains,while particles are obviously sintered when temperature rises to 1500 ?.LZO catalyst carrier has a better water-vapor resistance than ?-Al2O3 catalyst carrier,and has a fine thermal shock resistance.the structure segregation is due to the reaction of La-contained component to water vapor and sintering aids.Ir/LZO and Pt/LZO catalyst were prepared using impregnation process Both Ir and Pt are well dispersed in the surface of LZO catalyst carrier,with crystalline size of 2~5 nm and about 10 nm,respectively.Addition of Ir and Pt can inhibit the crystalline growth of LZO,so the particle sintering is inhibited.After tested in 1300 ?,100% H2O(g)condition,Ir/LZO and Pt/LZO catalyst retains the particles-piled structure.compared to Pt/LZO catalyst,Ir/LZO catalyst has a better adsorption to CO,and Ir particles disperse more uniformly.Moreover,Ir/LZO catalyst can effectively catalyze the decomposition of green monopropellant,with response time of 205 ms,decompositive temperature of 141 ?,which is 49 ? lower than samples without catalyzed.