| The synthesis, characterization and application of a new type of catalytic carrier belonging to NZP family with excellent thermal shock resistance performance and same crystal structure but different chemical composition were studied in this thesis. Three different types of carriers belonging to NZP family were synthesized and their pore structure data were obtained. A sol-gel route based on inorganic starting material with adding organic acids was explored successfully to synthesize a stable sol precursor of a compound with NZP structure. Based on this sol-gel method, a unit carrier composed of the coating of NZP family compound and cordierite honeycombed ceramic matrix was prepared. Since a good match of thermal expansion characteristics between NZP family coating and cordierite honeycombed ceramic matrix, the thermal shock resistance performance of the unit carrier is better than the current unit carrier composed ofγ-Al2O3 coating and the cordierite honeycombed ceramic matrix which is widely used in three-way catalytic convert to purify engine exhaust gases. Based on the study of this thesis, the potential applications of the phosphate material belonging to NZP family is extended from the area of structure ceramic to the range of innovative catalytic material.In the first part of this thesis, three different types of phosphate carriers with NZP structure including crystalline compound carrier, porous ceramic carrier and coating carrier were prepared. Their phase and pore structure were characterized by means of X ray diffraction (XRD), and N2 adsorption techniques based on Brunauer-Emmett-Teller (BET) formula and Barrett-Joyner-Halenda (BJH) method.First of all, using (NH4)2HPO4 as phosphorus resource and ZrOCl2·8H2O as zirconium resource, the NZP type crystalline compounds, such as KZr2(PO4)3, CaZr4(PO4)6, SrZr4(PO4)6, BaZr4(PO4)6 and NH4Zr2(PO4)3, were synthesized by coprecipitaion method and sol-gel route followed by calcination as well as by coprecipitaion method followed by hydrothermal treatment respectively. The influence of synthesis methods and conditions on the phase and pore structure of these materials was studied. It was found that the amorphous precursor synthesized by coprecipitation method as well as by sol-gel method was transformed into NZP family crystalline compounds after they were calcined at 700-800℃for 2h. The specific surface area, total pore volume and average pore diameter of these compounds was 17.9-34.3m2/g,0.0479-0.15mL/g and 3.48-22.7nm respectively. The pore size distribution was disorder, and the pore structure was formed from the particles aggregation. On the other hand, the crystalline compound NH4Zr2(PO4)3 were prepared by coprecipitation method followed by hydrothermal treatment at 150℃for 24h with F" as structure-directing agent, but the specific surface area of NH4Zr2(PO4)3 was 3.17m2/g, and total volume was only 0.00493mL/g. These results suggested that NZP crystalline compound NH4Zr2(PO4)3 were still compact materials rather than microspore molecular sieves.The second coating carrier CaZr4(PO4)6 was prepared by coating cordierite honeycombed ceramic with the sol precursor synthesized by sol-gel method and followed calcination under 700℃for 2h. The content of CaZr4(PO4)6 coating was achieved 19.5% (mass percentage) weight of the substrate after being coated twelve times, and the specific surface area of cordierite honeycombed ceramic coated by CaZr4(PO4)6 was increased to 16.4m2/g, which can be used as a unit carrier to prepare three-way catalytic convert to purify engine exhaust gases.Finally, three NZP structure-type porous ceramic carriers with different composition, which was CaZr4(PO4)6, Ca0.85Ba0.15Zr4(PO4)6 and K0.5Sr0.75Zr2(PO4)3 respectively, were prepared. The NZP porous ceramics were obtained by compacting the powders with 3%(mass percentage)ZnO as sintering additive and adding 50%(mass percentage) graphite to create pores as well as adding 1%(mass percentage)SiO2 to suppress grain growth excessively and then sintering under 1100℃for 2h. The sintering temperature to prepare NZP structure-type porous ceramic was lower than that one to prepare cordierite ceramic. Among these three porous ceramics, Ca0.85Ba0.15Zr4(PO4)6 and K0.5Sr0.75Zr2(PO4)3 showed near-zero average thermal expansion coefficients which was 0.6×10-6/℃and 0.8×10-6/℃. Therefore, NZP family porous ceramics are promising to be a new type of candidate material of unit carrier with excellent thermal shock resistance performance.In the second part of this thesis, some representative NZP family materials were selected as carriers to support palladium and transition metal oxide respectively to prepare palladium-only three-way catalyst which is used to clean automatic exhaust gases and vanadium catalyst which is widely used in sulfuric acid industry. The feasibility of using NZP family materials as the carriers in these two kinds of catalyst systems were discussed from technology perspective. The influence of NZP family carriers on the catalytic activities of these two different types of active components was investigated.Firstly, palladium-only catalysts which Pd content was not more than 1% supported on the NZP family compound carriers with different composition, which was KZr2(PO4)3,CaZr4(PO4)6 and BaZr4(PO4)6 respectively, were prepared by impregnation method using Pd(NO3)2·2H2O as precursor of palladium. The interaction between precursor of palladium and NZP family carriers were discussed. The dispersion of palladium on the NZP family carriers as well as the influence of modification of the NZP family carrier by rare-earth oxides CeO2 and La2O3 on the palladium dispersion were also investigated. And then, the catalytic activity of the palladium-only three-way catalysts was tested in a lab-scale plug flow reactor. The experimental results indicated that the proper interaction between KZr2(PO4)3 and the precursor of palladium resulted in the highest three-way catalytic activities of palladium-only three-way catalyst supported on KZr2(PO4)3 carrier. The light-off temperatures, at which a 50% conversion was obtained on the fresh catalyst, were T50(CO)=245℃, T50(CH)=290℃and T50(NOx)= 267℃respectively, and the conversions of monoxide carbon, hydrocarbon and nitrogen oxide was 96%,100% and 100% respectivly. Both the dispersion of Pd supported on CaZr4(PO4)6 carrier modified by 15%CeO2 and its catalytic activity was improved obviously because the excessive interaction between precursor of palladium and the CaZr4(PO4)6 carrier was decreased by modification.Based on the experimental results above, the three-way catalytic activities of palladium-only catalyst supported on the unit carrier composed of CaZr4P6O24 coating and cordierite honeycombed ceramic matrix were further investigated, and meanwhile, the palladium-only catalyst supported on the unit carrier composed ofγ-Al2O3 coating and cordierite honeycombed ceramic matrix was prepared for the purpose of comparing. The test results of the bench demonstrated that when palladium content was no more than 0.6%, the palladium-only catalyst supported on CaZr4P6O24 coating showed very close three-way catalytic activities to that one supported onγ-Al2O3 coating. The light-off temperatures of the former were CO384℃, THC 383℃and NOx397℃respectively, and they were only higher around 30℃than the light-off temperatures of the later. If the optimum active components which are suitable to NZP family carrier are studied further, NZP family materials is promising to be a new type of competitive coating carrier which is not only making active components show high three-way catalytic activities but also providing excellent thermal shock resistance performance for the three-way catalytic convert.Using NH4VO3 as the precursor of active component, the vanadium catalysts supported on KZr2(PO4)3,CaZr4(PO4)6,SrZr4(PO4)6 and diatomite respectively, in which V2O5 content was around 8%, were prepared by wet-blend method according to the active components of domestic S101 vanadium catalyst. Selecting the oxidation of sulfuric dioxide as probe reaction, the activities of vanadium catalysts were tested in lab-scale fixed-bed reactor. The results indicated that, the activity of vanadium catalyst supported on KZr2(PO4)3 was close to the activity of the vanadium catalyst supported on diatomite. However, the activities of vanadium catalyst supported on CaZr4(PO4)6 and SrZr4(PO4)6 carriers were lower. It was suggested that KZr2(PO4)3 was the suitable carrier of vanadium catalyst which contents K2SO4 as promoter. The reason was that the chemical composition of KZr2(PO4)3 carrier and the active component of the vanadium catalyst were stable during the preparation of vanadium catalyst. On the contrary, since the Ca2+and Sr2 occupying in the site of M1 of NZP crystal structure were substituted by K+which was resourced from the promoter K2SO4, the active components of vanadium catalyst was changed. As a result, the adjusting mechanism of K2SO4 to the ratio of V4+and V5+was lost which led to the lower catalytic activity. |
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