| As a result of the less reservation of petroleum and its ascending price, theresearch on the transformation from gas to basic organic chemical products andpreparation of hydrogen fuel by coke oven gas (COG) became the hotspot recently.In the gas preparation industry, the oxygen permeation reaction will enable thepreparation of oxygen and oxidation of methane in one step by the application of airas oxygen resource, which lowered the manipulation cost and increased thecompetiveness. The research of ionic-electric mixed conductor, the key material usedin this industry, were mainly focused on the new system developments and mademany progress. But in the application conditions, the oxygen exchange process in thehigh oxygen partial side of reaction will be the limited point. To solve this problem,surface modification on the air side by nano-coating with catalyst and oxygenpermeation function will be practicable but rarely reported.BaCo0.7Fe0.2Nb0.1O3-(BCFN) is the excellent oxygen permeation material withhigh ionic-electric conductivity and chemical stability. In this paper, a Ce0.8Y0.2O2-(YDC) porous catalytic coated on the air side of BCFN was prepared. We studied theelectric properties of BCFN and YDC and the preparation technology, performanceand mechanism of BCFN-YDC oxygen permeation.The nano-powders of YDC prepared by co-precipitation method was well coatedon the BCFN membrane by the dipping method. BCFN membrane modified by welldistributed porous YDC was prepared after calcination at1000℃. The coatingfastness adhered to the membrane exhibited perfect anti-vibrate property. Under hightemeprature, no reaction occured the BCFN and YDC, which suggest high chemicalstability of both materials.Different crystal size YDC electrolysis were successfully prepared by thesintering temperature adjusting. The rules of crystal growth suggests that the crystalgrowth follow the rules of parabolla in the sintering process, as depicted:D2-D20kt. Compared with micro-size YDC materials, the nano-size YDC materials owns higher total conductivity and lower oxygen ionic migration activationenergy by the EIS results, which lower the working temperature of solid electrolyte.The nano-YDC reached a total conductivity of4.7×10-3S·cm-1at900℃.The mixed conductivity of BCFN was measured by4-probes measuring methodand the ionic conductivity was studied by ion-blocking method. It was found that thetotal conductivity and electric conductivity increased with temperature elevationbelow550℃, no obvious ionic conductivity was observed, which exhibited acharacteristic of P-type semi-conductor. The apparent activation energy of electricconductivity was0.30eV and the electric migration number was nearly100%. Theelectric conductivity slightly decreased as a result of crystal oxygen desorption andconsumption of electric hole carriers at the range of550-650℃. When thetemperature is above650℃, the total conductivity slowly increased as a result ofrapidly increase of ionic conductivity.Accoring to the calculation results of the theory oxygen permeation velocity ofBCFN by Wanger equotation, it suggests that the theory oxygen permeation velocityincreased with the decrease of oxygen partial pressure on the low oxygen partial sideand the esclation of temeprature. When the oxygen partial pressure is around10-10Paat650-825℃, the theory value fitted well with the measured value. The deviationbetween them increased with temperature escalation. On the research of limitingprocess in the oxygen permeation process, there’s “characteristic oxygen partialpressure difference ΔPC” in the surface exchange process and “characteristictemperature TC” in the diffusion process.According to the oxygen permeation experiments, the BCFN oxygen permeationflux remarkable increased when modified by YDC on the air side. When the inlet flowrate increased to the353ml·min-1of Air and162ml·min-1of COG, the oxygenpermeation flux increased to18.78ml·cm-2·min-1with an increase of20.23%. thephase of membrane air side was still single fluorite structure after COG oxidation, nonew phase formed in the BCFN side while BaCO3came out in the cool process. There’s no destruction of porous structure and the thickness and particle size ofcoating was6μm and100-200nm, respectively.In the O2-TPD experiments of YDC and BCFN, it suggests that the oxygendesorption temperature and β oxygen desorption temperature of YDC is lower thanBCFN, which suggest higher oxygen desorption ability. In the H2-TPR comparisonresearch of YDC and BCFN, the YDC owns higher reduction ability. It wassuggested by the XPS experiments that the shift between the Ce4+and Ce3+in YDCcoating increased the oxygen vacancies and facilitated the oxygen surface exchangein the oxygen permeation experiments.ECR results suggest that when the oxygen partial pressure transformed from0.21atm to0.0002atm at500-650℃, the YDC materials has lower oxygenexchange coefficient and higher oxygen migration activation energy than BCFN,which means higher diffusion ability of YDC.The ionization energy of Ce is smaller than Co, Fe, which has larger potential toprovide spontaneous electric reaction and more likely to supply electric under thesame conditions. In the thermal calculation results, the standards Gibbs energy of1/2Ce2O3(s)＋1/2O(g)'CeO2(s) is the lowest, which means the Ce3+in the YDC iseasier to oxidize and release electric and benefit to the catalyst and redox process onthe surface.In the research of BCFN-YDC oxygen permation mechanism, the largerspecifice surface area of porous coating is benefit to the oxygen absorption in thecoating surface and three boundary interface of coating/BCFN/air. The coatingmaterial implied to provide the electric and faciliated the transformation from oxygenmocular to inonics. The huge amount of oxygen vacancies in the coating is benefitedto the oyxgen diffusion in the materials. Consequently, the YDC coating is able toimprove the oxygen permeation effects remarkablely. |
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