【作者】 薛青松；

【导师】 何鸣元； 路勇；

【作者基本信息】 华东师范大学 ， 物理化学， 2009， 博士

【摘要】 H₂—O₂质子交换膜燃料电池（PEMFC）是一项高效环保技术，在燃料电池汽车和散布式电源等方面有巨大的市场前景。由于储氢技术和加氢站距离规模化应用还遥遥无期，基于烃类重整的可移动或站制氢系统的研究开发已成为当前燃料电池领域最具挑战性的课题，其中开发具有优异抗硫性能的催化剂是难点之一。本文研制了一种抗硫中毒汽油／柴油重整制氢Gd₂O₃助剂改性的Pt/CeO₂-Al₂O₃催化剂，在水蒸气重整／自热重整含硫158～1050μg／g汽油／柴油的反应中，表现出优异的重整活性和抗硫中毒稳定性。利用H₂-TPR、XRD、BET、In-situ DRIFTS和脉冲反应等表征手段对Pt／（Gd₂O₃）-CeO₂-Al₂O₃催化剂抗硫中毒的本质进行了较详细的研究，并提出了有机硫可能的转化途径。第一部分，用分步初湿浸渍法制备Pt／CeO₂-Al₂O₃催化剂，以含硫300μg/g的异辛烷为模型汽油，在固定床反应器上考察了Pt／CeO₂-Al₂O₃催化剂水蒸气重整的反应性能，在优化的反应条件(反应温度800℃、水与原料中碳的摩尔比5．3、重时空速1．0 h^-1)下，优化配方的催化剂（γ-Al₂O₃上先负载CeO₂并于450℃焙烧后再负载Pt，然后于600℃焙烧制得，其中CeO₂和Pt的负载量分别为15 wt％和0．8 wt％）上进行了100 h的稳定性考察，发现异辛烷转化率在反应前40 h接近100％，之后略有下降并稳定在95％以上，产物中H₂的摩尔分数在～75％，CH₄的摩尔分数低于1．0％。并用H₂-TPR表征手段证实，在水蒸气重整条件下，Pt／CeO₂-Al₂O₃催化剂中Pt—Ce—Al三者间好的协同作用促进有机硫快速转化生成H₂S，抑制硫物种在催化剂表面的沉积。第二部分，Pt／CeO₂-Al₂O₃催化剂上水蒸气重整甲苯、正庚烷、1-辛烯和环己烷的反应结果表明，300μg/g硫存在时芳香烃和环烷烃较难转化，烯烃易产生积碳，导致催化剂在水蒸气重整零售汽油时活性及稳定性较差。基于Pt／CeO₂-Al₂O₃催化剂中CeO₂的表面晶格氧在O₂中比在水蒸气中容易恢复的事实，在优化了反应条件下，考察了Pt／CeO₂-Al₂O₃催化剂自热重整含硫158～500μg/g模型汽油／零售汽油的反应，达到了明显改善催化剂反应稳定性的预期结果。第三部分，通过引入少量Gd₂O₃，对Pt／CeO₂-Al₂O₃催化剂进行改性，采用分步初湿浸渍法先负载CeO₂后负载Gd₂O₃制得改性的Al₂O₃载体，再在上述改性载体上浸渍Pt，制得Gd₂O₃、CeO₂和Pt含量分别为1．6 wt％，15 wt％和1．2 wt％的催化剂，明显加强了Pt-CeO₂间的相互作用，抑制了Pt及CeO₂的烧结，增强了Pt的缺电子性，在自热重整含158μg/g硫的零售汽油1000 h中，汽油达100％转化，反应300 h后缓慢下降至95％并稳定至反应结束，干气中H₂的摩尔分数一直维持在～67％，甲烷的浓度最终控制在0．6％以下。此外，在水蒸气重整含硫300μg/g的异辛烷250 h和自热重整含硫1050μg／g的零售柴油130 h的反应中，也表现出很好的活性／稳定性。第四部分，采用XRD、H2-TPR、in situ DRIFTS表征手段研究了催化剂结构-性能间的规律关系。结果表明，在水蒸气／自热重整反应条件下，发现Pt／（Gd₂O₃）-CeO₂-Al₂O₃催化剂在反应条件下存在2CeO₂+Al₂O₃（?）2CeAlO₃+[O]的可逆相转移作用，并发现这种作用对抑制Pt的团聚和CeO₂的不可逆烧结、促进水（O₂）的活化和晶格氧的传输具有独特效果，进而导致催化剂具有出色的反应稳定性。此外，Gd₂O₃助剂可明显强化这一过程，从而促进了催化剂表面积碳和沉积硫的快速转化。第五部分，采用四极杆质谱在线检测，以SO₂、噻吩为模型硫化物，通过脉冲反应技术，考察了Pt／Gd₂O₃-CeO₂-Al₂O₃催化剂上有机硫的转化途径。研究发现，在水蒸气重整／自热重整条件下，有机硫在Pt／Gd₂O₃-CeO₂-Al₂O₃催化剂上的转化遵循氧化还原机理：（1）有机硫先沉积在催化剂的表面；（2）在H₂O或／和O₂的作用下深度氧化成吸附态SOx；（3）再在还原气氛中还原生成H₂S。更多还原

【Abstract】 H₂-O₂ fuel cells are most promising alternatives for both stationary power plantsand mobile power systems due to the increasing concern about environmental andpollution problems.Until widespread hydrogen refueling infrastructure exists,however,hydrogen production technology appears to be a practical option in amedium-short perspective starting from a commercial-grade high energy densityliquid hydrocarbon fuels such as gasoline/diesel oil.Gasoline/diesel oil fuelprocessing technologies have been intensively developed for both on board and offboard applications because of the convenient system of service station.Nevertheless,the sulfur tolerance of reforming catalysts remains a particularly challenging area.Sulfur-tolerant catalyst Pt/CeO₂-Al₂O₃ was developed by stepwise incipientwetness impregnation （IWI） method and modified with Gd₂O₃ additive,and exhibitedexcellent high catalytic activity and good sulfur tolerance stability for high efficiencyH₂ production from steam reforming （SR）/autothermal reforming （ATR） of gasoline/ diesel oil containing 158～1 050μg/g sulfur.Characterization technologies such asH₂-TPR,XRD,N₂-BET,in situ DRIFTS and pulse reaction were employed to studythe structure-performance relationship sulfur-tolerant Pt/Gd₂O₃-CeO₂-Al₂O₃ catalyst.In addition,the organo-sulfur transformation chemistry was also discussed.In the first part,the effects of catalyst preparation parameters （e.g.,CeO₂ content,Pt content and calcination temperature of catalyst） and reaction conditions wereinvestigated on the performance of Pt/CeO₂-Al₂O₃ for SR of iso-octane with 300μg/gsulfur of thiophene.Under optimal reaction conditions (reaction temperature of 800℃,molar steam-to-carbon ratio of 5.3,and a iso-octane-equivalent WHSV of 1.0 h^-1),the 0.8wt%Pt/15wt%CeO₂-Al₂O₃ catalyst prepared by stepwise incipient wetnessimpregnation method (CeO₂ first （calcined at 450℃） and Pt then （calcined at 600℃）was the best one and demonstrated excellent catalytic activity and stability during a100 h run.The conversion of iso-octane was almost sustained at 100% within the first 40 h and then decreased slowly to 95% throughout the test while H₂ concentrationremained at 75% with a CH₄ concentration less than 1%.H₂-TPR experiments forPt/CeO₂-Al₂O₃ catalysts were carried out in detail to reveal the strong interactionbetween Pt and CeO₂-Al₂O₃ support.By correlating the reaction results with theinformation provided by the H₂-TPR experiments,it is suggested that good synergisticeffect of Pt-Ce-Al existed in that optimalized catalyst,which might ficilitate theconversion of organo-sulfur into H₂S and therefore the deposition of sulfur poisonspecies on the catalyst surface would be suppressed.In the second part,it was found that the stability and sulfur tolerance of thePt/CeO₂-Al₂O₃ catalyst in the SR of iso-octane with 300μg/g sulfur was better thanthat in the SR of retail gasoline.SR of various typical model fractions of gasoline （e.g.,toluene,n-heptane,1-octene and cyclohexane）,with or without sulfur,was carried outto reveal why the Pt/CeO₂-Al₂O₃ catalyst could work well in the steam reforming ofthe iso-octane fuel but not in the reforming of retail gasoline in the presence of 300ppm sulfur.The real reason seems lied in that the aromatics and cyclohydrocarbonswere converted difficultly and carbon deposition was produced easily from alkenes inthe presence of sulfur in the steam reforming of the retail gasoline.Especially,theH₂-TPR results indicated that the Ce³⁺ in the reduced Pt/CeO₂-Al₂O₃ catalyst could beretrieved to highly active state of CeO₂ by reacting with O₂ rather than H₂O.Accordingly,catalyst stability for the ATR of gasoline was tested in the presence ofsulfur under the optimal reaction conditions.As expected,the optimizedPt/CeO₂-Al₂O₃ catalyst showed dramatic improvement of the reaction stability in theATR process.In the third part,Pt/CeO₂-Al₂O₃ catalysts modified with Gd₂O₃ additive wereprepared.The Al₂O₃ supports was pre-impregnated with Ce and Gd nitrates to 15 wt%CeO₂ and 1.6 wt% Gd₂O₃ and calcined at 450℃.Pt was then placed onto the aboveresulting composite supports by incipient wetness impregnation method and calcinedat 600℃.It was found that Gd₂O₃ additive promoted the interaction between Pt andCeO₂ at their interface in the Pt/Gd₂O₃-CeO₂-Al₂O₃ （for support preparation:CeO₂first and Gd₂O₃ then）,suppressed the sintering of both Pt and CeO₂,and enhanced electron-deficiency of Pt sites.The resulting catalyst showed high reactivity andexcellent stability in ATR of retail gasoline containing 158～500μg/g sulfur.A 1000-htest showed that gasoline conversion was slowly decreased to 95% after 300-h runand then remained at～95% throughout the test,while H₂ fraction in reformateremained at～67% with a CH₄ fraction less than 0.6%.Furthermore,this optimalizedcatalyst also exhibited excellent activity and sulfur-tolerance stability for both SR ofiso-octane with 300μg/g sulfur during 250 h test and ATR of retail diesel with 1 050μg/g sulfur during 130 h test.In the fourth part,XRD,H₂-TPR,in situ DRFITS characterization techniqueswere employed to study the structure-performance relationship for thePt/（Gd₂O₃）-CeO₂-Al₂O₃ catalyst.During the reaction,in the Pt/（Gd₂O₃）-CeO₂-Al₂O₃catalysts a reversible phase transfer process was presented:2CeO₂+Al₂O₃←→2CeAlO₃+[O].This resulted in unique benefitial effects including significantlysuppressing the Pt sintering and CeO₂ aggregation,promoting the activation ofH₂O/O₂ as well as the transformation of active O species.It should be noted that theGd₂O₃ additives could significantly promote this reversible phase transfer processs inthe catalyst thereby significantly facilitating the conversion of carbon and sulfurdeposits on the catalyst surface.In the last part,the organo-sulfur transformation chemistry on the sulfur-tolerantPt catalysts in the reforming reaction was tentatively studied by the pulse reactiontechnique using thiophene and SO₂ as probe molecules.During the reaction,it wasfound that organo-sulfur compounds were completely converted into H₂S whilecomplying with a redox mechanism,including three steps:（1）deposition on thecatalyst surface as coke format,（2）oxidation of coke to form COx and SOx（ad）,and（3）hydrogenation of SOx（ad） to form H₂S.更多还原