【作者】 张彦威；

【导师】 周俊虎； 岑可法； 刘建忠；

【作者基本信息】 浙江大学 ， 工程热物理， 2008， 博士

【摘要】 氢作为能源载体，由于其固有的特性，越来越引起社会各界的关注．大规模低成本制氢是发展氢能经济的基础，热化学循环水分解制氢则被认为是一种较理想的清洁可持续制氢方法，其利用热能通过一系列不同但又相互关联的化学反应，最终将水分解为氢气和氧气．在诸多的热化学循环中，硫碘循环由于其高热效率，低成本且较易实现大规模生产等原因被作为制氢的理想循环。结合中国国情，（1）作为SO₂来源的硫铁矿资源丰富、价格便宜；（2）除氢气以外的产品硫酸有很好的经济和市场价值，因此我们提出了热化学硫碘开路循环联产氢气和硫酸的多联产系统。该系统主要包含以下两个主要反应过程：Bunsen反应过程：xI₂+SO₂+2H₂O=2HIx+H₂SO₄HI分解过程：2HIx=xI₂+H₂在最优条件下，对该多联产系统进行了详细的描述及能量评估，整个系统能量转化率达到63．1％。本文实验研究了溶液成分和温度对Bunsen反应两相分离及副反应的影响，发现溶液越稀，出现分层所需I₂摩尔分数越大，分层时两相分离越不明显，但副反应越不容易发生；I₂摩尔分数的增加有利于两相的分层，有效抑制Bunsen反应副反应的发生；温度对分层现象影响不大，但温度的增加会促进副反应的发生。利用FactSage软件从热力学角度探讨了温度和压力对HI均相分解的影响，以及无水和有水参与时HI分解的热力学特性变化。HI分解对温度十分敏感，但压力对HI分解的热力学平衡状态不产生影响．在温度高于700℃时，水蒸汽的存在可以有效地促进HI的分解。利用CHEMKIN软件从动力学角度研究了温度、压力、停留时间及氧量对HI均相分解的影响，建立了无氧和有氧参与的HI均相分解详细化学动力学模型，无氧模型包括11个基元反应和5个物种，通过敏感性分析发现，反应HI+HI=H₂+I₂和HI+I=I₂+H对H₂的生成有积极意义。有氧模型包括43个基元反应和12个物种，通过敏感性分析可知，反应HI+I=I₂+H和HI+O=OH+I对H₂的生成起到积极的作用，尤其是反应HI+I=I₂+H，O参与HI分解反应时，虽然会消耗HI中的H生成H₂O，但仍在很大程度上促进了H₂的生成。在500℃-700℃的实验温度范围内，无氧模型模拟结果和实验结果可以较好的吻合。通过以上研究发现，HI的均相分解率极低，这意味着在实际的工业应用中，需要通过催化剂来促进HI的分解．接下来的章节中，我们自主开发了多种催化剂用于HI的分解。对不同制备方法及不同温度焙烧CeO₂催化剂的活性进行对比，通过TG-FTIR，BET，XRD，TEM和TPR的催化剂表征，发现低温（300和500℃）下焙烧的溶胶-凝胶CeO₂样品以单独的不规则纳米颗粒存在，表面形态比较疏松；比表面积较大，晶粒较小；表现出更多的晶格缺陷，如氧空位等，具有更好的表面活性及氧迁移能力．催化机理研究表明，表面还原位Ce³⁺、氧空位及0活性基是CeO₂催化分解HI的关键，CeO₂在催化分解HI的过程中扮演了催化剂和氧化剂的双重角色。对不同制备方法、不同温度焙烧及不同气氛处理Pt／CeO₂催化剂的活性进行对比，Pt／CeO₂催化剂具有很好的催化分解HI活性。通过TG-FTIR，BET，XRD，TEM，TPR和XPS的催化剂表征，发现Pt共同参与溶胶一凝胶过程时Pt高子插入CeO₂的晶格取代了Ce⁴⁺，这对提高CeO₂载体的氧迁移能力，提高催化剂的热稳定性以及CeO₂和Pt活性组分之间强相互作用的形成都具有十分重要的意义。溶胶-凝胶法合成、高温下焙烧的Pt／CeO₂催化剂具有更好的催化活性，这与催化剂内复杂的转化反应Ce⁴⁺+pt→Ce³⁺+Pt²⁺及氧空位在Pt-Ce-O体相内的迁移扩散都有直接关系。不同气氛处理的样品中，Reduced和Re-oxidized样品表现出略好的催化活性，本文建立了Ce⁴⁺及氧空位的迁移扩散模型以及Pt颗粒的壳芯结构模型．ZrO₂固溶进入CeO₂晶格生成Ce_xZr_1-xO₂固溶体，和纯的CeO₂相比，大大提高了催化剂的贮氧能力和抗高温烧结性，更利于体相氧的迁移和扩散，使体相反应过程变得活泼。本文对不同Ce-Zr配比的Pt／Ce_xZr_y催化剂也进行了研究，发现Pt／Ce_0.8Zr_0．2的催化活性最好，这与Ce-Zr固溶体的高贮氧能力以及Pt与Ce-Zr固溶体之间的强相互作用有关。对不同制备方法、不同温度焙烧及不同气氛处理Ni／CeO₂催化剂的活性进行对比，Ni／CeO₂催化剂也具有很好的催化分解HI活性，在高的HI分解温度下，可与Pt催化剂相媲美。通过TG-FTIR，BET，XRD，TEM，TPR和XPS的催化剂表征，发现Ni共同参与溶胶-凝胶过程时Ni离子插入CeO₂的品格取代了Ce⁴⁺，这对Ni在CeO₂载体表面形成高度分散且稳定的活性组分，氧空位的形成以及CeO₂和Ni活性组分之间强相互作用的形成都具有十分重要的意义。催化机理研究表明，Ni／CeO₂催化剂在催化分解HI的过程中，纳米CeO₂的催化氧化机理依然是成立的，但由于过渡金属Ni的存在，其重要性肯定会明显降低。我们认为有三种表面活性位对HI的催化分解是十分重要的，一是Ni表面活性位；二是Ni-Ce界面活性位；三是表面氧空位。对不同Ce-Zr配比，不同制备方法及不同温度焙烧Ni／CexZry催化剂的活性进行对比，ZrO₂固溶进入CeO₂晶格生成Ce,Zn_1-xO₂固溶体，和纯的CeO₂相比，大大提高了催化剂的贮氧能力和抗高温烧结性，更利于体相氧的迁移和扩散，使体相反应过程变得活泼。在整个实验温度范围内，700℃焙烧、溶胶凝胶方法制备的Ni／Ce_0.8Zr_0.2催化活性最好，这与Ni在Ce-Zr固溶体表面形成高度分散且稳定的活性组，Ce-Zr固溶体的高贮氧能力和氧迁移扩散能力以及Ni与Ce-Zr固溶体之间的强相互作用息息相关。结合以上对热化学硫碘开路循环的一些基础问题的研究，目前我们实验小组在国家高技术研究发展计划（863计划）探索导向类项目以及能源清洁利用国家重点实验室的资助下正筹备建立实验室规模可连续运行的多联产试验装置。本文对实验室规模可连续运行的多联产试验系统进行了初步的系统设计和质量平衡计算，设计的产氢速率为1 L／h，Bunsen反应温度为60℃，HI分解温度500℃，采用我们自主开发的催化剂。建立实验室规模可连续运行的多联产试验装置是一项复杂的系统过程，还有很多工作要进一步细化。更多还原

【Abstract】 Hydrogen has ideal characteristics as an energy carrier. Therefore, the concept of a hydrogen energy system has attracted worldwide interest. Huge hydrogen demand is expected in order to develop the hydrogen energy system. Thermochemical splitting of water has been proposed as a clean method for hydrogen production. Hydrogen is obtained by decomposition of water by using heat energy through a chemical cycle process that consists of several reactions. Among the large scale, cost effective and environmentally attractive hydrogen production processes, the sulfur-iodine （SI or IS） thermochemical cycle is a quite promising one.The selection of SI cycle to run in an open-loop fashion for China is tied-up with two important facts: （1） sulfur iron ore as SO₂ source is inexpensive and abundantly available; （2） The product sulfuric acid, in addition to hydrogen, is valuable and marketable.The open-loop SI cycle consists of following two reactions:Bunsen reaction: xI₂ + SO₂ + 2H₂O = 2HIx + H₂SO₄Hydrogen iodide （HI） decomposition reaction: 2HIx = xI₂ + H₂The mass and heat balance of the open-loop SI cycle were calculated with the optimized conditions. Thermal efficiency for hydrogen production was 63.1% with ideal operating conditions.The Bunsen reaction for the production of hydriodic and sulfuric acids from water, iodine and sulfur dioxide has been studied with the evaluation of the effect of solution temperature and composition of initial solution on separation characteristics of two liquid phase. The effect of solution temperature and composition of initial solution on side-reactions of the Bunsen reaction were also investigated. The separation characteristics were found to improve with the increase in iodine fraction and the decrease of water. The side-reactions can be controlled with the increase in iodine fraction and water. Results show that operative temperature has a minor effect on the phase separation, but the increase of temperature can enhance the side-reactions.Thermodynamics simulation of HI decomposition without and with water is investigated under different temperature by FactSage. Results show that the HI decomposition reaction is sensitive to temperature, but not to pressure. Vapor can improve HI decomposition as temperature increased above 700℃. Detailed kinetic modeling and sensitivity analysis for HI homogeneous decomposition with and without O were investigated. The kinetics model of HI decomposition without O was composed of 11 elemental reactions and 5 typical middle species. The result shows that the reactions HI+HI=H₂+I₂ and HI+I=I₂+H play a major role. The kinetics model of HI decomposition with O was composed of 43 elemental reactions and 12 typical middle species. According to the results, the reactions HI+I=I₂+H and HI+O=OH+I are the most important steps, especially the reaction HI+I=I₂+H. The presence of O can obviously promotes the HI decomposition reaction rate, but simultaneously consume H contained in HI. Kinetic calculations of HI decomposition without O are also compared with the experimental data, and all trends of the experiment can be reproduced by the model. The HI decomposition reaction path diagram was constructed in this dissertation.The conversion of HI homogeneous decomposition is rather low. The use of catalyst allows a substantial reduction in temperature to achieve workable reaction rates. We developed different kinds of catalysts to improve HI decomposition.CeO₂ which acts not only a catalyst but also an oxidant with different preparation methods and calcination temperatures have been tested to evaluate their effect on HI decomposition at various temperatures. According to the results of TG-FTIR, BET, XRD, TEM and TPR, the CeO₂ catalyst synthesized by citric-aided sol-gel method and calcined at low temperature（300 and 500℃） shows more lattice defects, smaller crystallites, larger surface area and better reducibility. Lattice defects, especially the reduced surface sites, i.e., Ce³⁺ and oxygen vacancy, and O species play the dominant role in surface reactions of HI decomposition. An original reaction mechanism for HI catalytic decomposition on CeO₂ catalyst is proposed.The Pt/CeO₂ catalysts with different preparation methods, calcination temperatures and oxidative/reductive treatments have been tested to evaluate their effect on HI decomposition. The Pt/CeO₂ catalysts strongly enhance the decomposition of HI. According to the results of TG-FTIR, BET, XRD, TEM,TPR and XPS, it was found that, through sol-gel method, the Pt ions could be inserted into the ceria lattice. This brought about a different synergistic effect between the Pt and Ce components, such as increased the oxygen mobility in ceria support and improved the thermal stability of catalyst. A mechanism is supposed to exist in the Pt/CeO₂ catalysts synthesized by sol-gel at high calcination temperature. It involves the complex conversion such as Ce⁴⁺+Pt→Ce³⁺+Pt²⁺ and oxygen-vacancy diffusion in the Pt-Ce-O system. The oxidative /reductive atmosphere affected the structure and performance of the catalyst by the strong metal-support interaction （SMSI）. The activity of the reduced and re-oxidized samples are better than the as-received and oxidized samples. Models were constructed to describe the diffusion of Ce⁴⁺ and oxygen vacancies as well as the possible shell-core structure of Pt crystallites and the decoration/encapsulation by ceria support. Compared to pure ceria, inserting ZrO₂ into the ceria lattice to form ceria-zirconia solid solutions has improved the thermal stability and oxygen storage capacity. Pt/Ce_0.8Zr_0.2 showed the best catalytic performance as a combined result of high oxygen storage capacity of ceria in Ce-ZrO₂, strong interaction between Ni and Ce-ZrO₂ and basic property of the catalyst. The Ni/CeO₂ catalysts with different preparation methods, calcination temperatures and oxidative/reductive treatments have been tested to evaluate their effect on HI decomposition. The Ni/CeO₂ catalysts also strongly enhance the decomposition of HI. According to the results of TG-FTIR, BET, XRD, HRTEM/TPR and XPS, it was found that the Ni²⁺ ions could be inserted into the ceria lattice. This brought about the strong interaction between Ni and CeO₂ and the generation of oxygen vacancies. An original reaction mechanism of HI catalytic decomposition on Ni/CeO₂ has been constructed. We believe that there are three important reactive sites for HI catalytic decomposition. One is the surface site which exhibits in the Ni-Ce interphase, where interfacial Ni sites are located and the strong interaction between Ni and CeO₂ occurs. The other is oxygen vacancy related to the reduced surface sites of CeO₂ support, i.e., Ce³⁺ and Ni²⁺ ions dissolved into the ceria lattice instead of the Ce⁴⁺ ions. The third reactive sites is reduced Ni surface.The Ni/Ce_xZr_1-x（x=1, 0.8, 0.5, 0.2 and 0） synthesized by citric-aided sol-gel method and calcined at 700℃have been tested to evaluate their effect on HI decomposition. Ni/Ce_0.8Zr_0.2 showed the best catalytic performance. Then Ni/Ce_0.8Zr_0.2 with different preparation methods and calcination temperatures have also been tested to evaluate their effect on HI decomposition. Compared to pure ceria, inserting ZrO₂ into the ceria lattice to form ceria-zirconia solid solutions has improved the thermal stability and oxygen storage capacity. Ni/Ce_0.8Zr_0.2 synthesized by citric-aided sol-gel method and calcined at 700℃showed the best catalytic performance due to its high degrees of metal dispersion, high oxygen storage capacity of ceria in Ce-ZrO₂, surface oxygen mobility and strong interaction between Ni and Ce-ZrO₂.Based on our fundamental research and the support of National High Technology Research and Development Program of China （863 Program） and State Key Laboratory of Clean Energy Utilization. We try to construct a bench-scale open-loop SI cycle for continuous hydrogen production. The system design and mass balance of the open-loop SI cycle have already been investigated with the optimized conditions. Designed H₂ production rate is 1 L/h. The Bunsen reaction and HI decomposition reaction are operated at 60℃and 500℃. The catalyst developed by ourself will be selected for HI decomposition. Continuous operation for hydrogen production in a stable state is one of the challenges.更多还原