【作者】 吴国鹏；

【导师】 李灿； 杨学锋；

【作者基本信息】 大连理工大学 ， 应用化学， 2007， 博士

【摘要】 光催化重整生物质及其衍生物制氢，是一种清洁的、可持续发展的制氢技术，将是未来社会解决环境和能源问题的重要途径之一。未来氢能的利用形式主要是利用燃料电池将氢能转化为电能。其中质子交换膜燃料电池（PEMFC）是利用氢作为燃料的一种重要技术，但PEMFC采用Pt作电催化剂，CO极易吸附在Pt表面，导致Pt催化剂中毒，氢气中微量的CO就会使电池性能严重下降。因此，本论文以甲醇、乙醇和葡萄糖作为生物质衍生物的典型代表，研究了光催化重整生物质衍生物制氢过程中气体产物的分布，尤其是CO副产物在H₂中的含量，CO生成的机理，对光催化重整过程中抑制CO的生成进行了初步探索。以M（Pt，Pd，Au，Rh，Ni等）／TiO₂为催化剂，研究了光催化重整甲醇、乙醇、葡萄糖过程中气体产物的分布。结果表明光催化重整生物质衍生物的气体产物中除了H₂和CO₂，还有CO、CH₄等副产物，同时在反应液中可能生成许多中间物种。这些中间物种在催化剂表面发生反复多次的脱附、吸附、反应等过程，最后通过脱羧反应被氧化为CO₂。TiO₂催化剂上产氢活性极低，但CO在氢气中浓度较高；担载金属后，M／TiO₂的光催化活性大幅度提高，同时CO的生成被显著抑制。不同担载金属对光催化重整生物质的产氢活性及CO的生成具有重大影响，而且对不同的反应底物表现有差异。对甲醇和乙醇来讲，Pt／TiO₂的产氢活性最高，Pd／TiO₂的CO／H₂摩尔比值最低；对葡萄糖而言，Rh／TiO₂的产氢活性最高，同时CO／H₂的摩尔比值也最低。Au／TiO₂催化剂光催化重整甲醇可制取较低CO含量的H₂。中间产物甲酸在TiO₂表面的脱水反应是CO副产物的主要来源。Au粒子减小可大幅度提高产氢活性同时抑制CO生成。Au粒子在光催化条件下对CO生成的抑制作用，一方面是由于表面的光生空穴和活性氧化物种有利于甲酸的脱质子降解反应，生成以CO₂和H⁺为主的反应产物；另一方面是由于部分生成的CO在Au／TiO₂催化剂上被进一步氧化的结果。Pt的担载量以及担载方法对Pt／TiO₂催化剂光催化重整甲醇的产氢活性和CO选择性都有很大影响。Pt担载量存在一阈值，担载量低于该值，产氢活性低，同时氢中CO含量高；担载量高于该值，产氢活性迅速升高，同时CO选择性降到最低。浸渍法制备的Pt／TiO₂催化剂比原位光还原法制备的催化剂具有较好的催化活性和较低的CO选择性，同时具有较低的阈值。在反应体系中添加少量的无机阴离子，如SO₄^2-、H₂PO₄^-等，在不降低产氢活性的前提下，可大幅度抑制光催化重整过程中CO的生成。阴离子对CO生成的抑制作用由弱到强的顺序为：Cl^-＜NO₃^-＜HCO₃^-＜SO₄^2-＜H₂PO₄^-。结合光谱表征的结果，我们认为副产物CO主要来源于反应中间物种甲酸在TiO₂表面氧缺陷位的脱水反应；SO₄^2-或H₂PO₄^-等无机阴离子可强烈吸附在TiO₂表面的氧缺陷位，降低了中间物种甲酸在这些氧缺陷位的吸附分解几率，从而显著抑制CO的生成。以Pt／TiO₂、Ni／TiO₂或Pd／TiO₂为催化剂，在葡萄糖反应体系中加入少量的SO₄^2-，或者在催化剂上浸渍少量的SO₄^2-，都可以明显降低光催化重整葡萄糖过程中CO的生成。说明采用少量无机阴离子吸附在TiO₂表面的氧缺陷位，以抑制光催化重整生物质衍生物制氢过程中CO的生成是一种有效方法。在论文工作的早期还进行了生物质的热化学转化研究，结果表明生物质快速热解温度为500℃时，液体产物的产率最高，达到干基原料的55.8％；经酸、碱、或盐预处理后，生物质快速热解的液体产物产率降低，而固体产物的产率提高。更多还原

【Abstract】 Concerns about the depletion of fossil fuel reserves and the pollution caused bycontinuously increasing energy demands make H₂ an attractive alternative energy source. H₂production via photocatalytic reforming of biomass is a sustainable-energy technology due tothe potential application for the conversion of solar energy. Proton exchange membrane fuelcells （PEMFCs）, using H2 as the fuel source, have received much attention as a potentialelectric power source. However, the Pt-anode catalyst of PEMFC is extremely sensitive to COcontaminant in H₂ feed gas. Small amount of CO can poison the catalyst and decrease thecatalytic performance. It is an urgent and challenging objective to reduce the COconcentration in H₂ for the applications in PEMFCs. Therefore, the effects of metals andmetal particle size on the CO formation, as well as the mechanism of CO formation areinvestigated using methanol, ethanol and glucose as the typical representatives ofbiomass-derived compounds.Among the gaseous products, the H₂, CO, CO₂ and CH₄ were detected as the majorproducts in the photocatalytic reforming of methanol, ethanol and glucose on M（Pt, Pd, Au,Rh, Ni etc.）/TiO₂ catalysts. It is shown that the loaded metals generally enhances the rate ofH₂ production while depresses the CO formation. Both H₂ production and CO formation arestrongly dependent on the kind of deposited metals. In the reforming of methanol and ethanol,the highest photoactivity was achieved on Pt/TiO₂ catalyst, while the CO formation was thelowest on Pd/TiO₂ catalyst. Rh/TiO₂ catalyst was found to be most active for H₂ productionwhile with the lowest CO concentration in the photocatalytic reforming of glucose. The factthat the molar ratio of CO₂/H₂ is much lower than the stiochiometric value indicates that a lotof oxidized fragments of biomass are produced and diffuse into the bulk solution. Theseintermediates are successively adsorbed and oxidized on the M/TiO₂ catalyst for several timesfinally forming CO₂ through the photo-Kolbe reaction.H₂ with low CO concentration was produced via photocatalytic reforming of methanol onAu/TiO₂ catalyst. The rate of H₂ production is greatly increased when the gold particle size isreduced from 10 nm to smaller than 3 nm. The concentration of CO in H₂ decreases withreducing the gold particle size of the catalyst. It is suggested that the by-product CO is mostlyproduced via decomposition of the intermediate formic acid species derived from methanol.The smaller gold particles possibly switch the HCOOH decomposition reaction mainly to H₂ and CO₂ products while suppress the CO and H₂O formation. In addition, some CO may beoxidized to CO₂ by photo-generated oxidizing species at the perimeter interface between thesmall gold particles and TiO₂ under photocatalytic condition.The loading of platinum on TiO₂ influences both the H₂ production and the CO formation.When the loading of Pt is higher than a threshold value, the rate of H₂ production is greatlyenhanced while the CO formation is largely depressed. The CO formation was significantlydepressed with a slight increase in H₂ production in the photocatalytic reforming of methanolon Pt/TiO₂ catalyst with addition of a small amount of inorganic anions, such as SO₄^2- andH₂PO₄^- in the reaction system. The ability of the anions for depressing CO formation is in theorder: Cl^-＜NO₃^-＜HCO₃^-＜SO₄^2-＜H₂PO₄^-. It is suggested that the byproduct CO is formedat oxygen vacancy sites on TiO₂ where formic acid species derived from methanol decomposeto CO and water. The deexcitation of the trapped photoexcited electrons at oxygen vacancysites may accelerate the dehydration reaction of formic acid. Sulfate and phosphate ions caneffectively suppress the CO formation via competing adsorption on oxygen vacancies with theformic acid species derived from methanol.The inhibition effect of the SO₄^2- anions on CO formation was also observed in thephotocatalytic reforming of glucose on Pt/TiO₂, Ni/TiO₂, and Pd/TiO₂ catalysts, indicatingthat the addition of some inorganic anions to the reaction solution is an efficient method todeppress CO formation in the photocatalytic reforming of biomass-derived compounds for H₂production.更多还原