Preparation, Characterization And Performance Of Sulfur-Tolerant Pt-based Catalysts For Reforming Of Gasoline/Diesel-Fuel To Produce Hydrogen

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.