【作者】 马令娟；

【导师】 陈诵英； 陆维敏；

【作者基本信息】 浙江大学 ， 化学， 2008， 博士

【摘要】 随着化石燃料的大量使用，人类往大气中排放的CO₂迅速增加，大量的CO₂通过温室效应使大气层吸热能力大大提高。温室效应引起一系列的环境问题，促使人们重视对大气中CO₂的转化和消除这一课题的深入研究。同时随着探索外太空、进军海洋深处活动的开展，人类需要经常处于特殊的密闭环境中，在密闭环境中O₂不断消耗，同时CO₂含量越来越高，空气质量逐渐变坏。研究解决密闭舱内生命保障系统的供氧和CO₂的消除也成为争夺海洋、太空和长期载人飞行的关键之一。因此开展对化学性质极其稳定、活化转化异常困难的CO₂的研究，有重要的理论价值和实际意义。直接分解CO₂为碳和O₂不仅是实现密闭体系中CO₂和O₂循环最有效的方法之一，也是自然界碳和氧循环化学模拟的重要反应。然而，反应CO₂→C+O₂是大量吸热的，ΔG为很大的正值，因此该反应在热力学上是极为不利的，必须借用耦合物质来改善反应热力学，H₂是最易得的耦合物质。CO₂化学转化的基本原理是通过H₂把CO₂的O先转化成水，水再通过电解产生O₂同时回收H₂。1990年Tamaura和Tabata首次报导利用H₂部分还原生成的氧缺位Fe₃O_4-δ在290℃可以完全分解CO₂为C，同时氧缺位Fe₃O_4-δ夺取CO₂中的氧恢复成正常的尖晶石结构。自此直接分解CO₂为C的研究才获得较大的发展。随后大量的文献报导具有尖晶石结构的铁酸盐MFe₂O₄（M=过渡金属）经H₂还原后均可以分解CO₂为C。然而该反应面临着铁酸盐循环使用寿命短、积碳量少及结构稳定性差等缺点，目前尚不能进行工业化应用。而且反应机理还不统一，有人认为起主要作用的是氧缺位尖晶石铁酸盐。但是2002年Shin等人认为起主要作用的是完全还原后形成金属和合金。针对上述问题，本文较系统地研究了铁酸盐制备方法、掺杂等因素对尖晶石铁酸盐的氢还原和CO₂分解反应性能的影响，获得抑制反应过程中晶粒长大，提高循环反应寿命的有效方法；同时还研究了不同铁酸盐MFe₂O₄在还原和氧化反应中的物相变化以及M和掺杂金属的作用，提出了较合理的反应机理。本论文的主要结论如下：1．利用共沉淀法、柠檬酸溶胶凝胶法和水热法均得到单一的尖晶石NiFe₂O₄结构。通过XRD、低温N₂吸脱附测定及H₂-TPR分析发现共沉淀法所得NiFe₂O₄的平均晶粒最小，表面活性最高，具有很高的比表面积及孔容，H₂还原能力最低。而水热法所得样品结晶最好，表面活性低，比表面积及孔容最小。NiFe₂O₄还原机理可以表述为：NiFe₂O₄→NiFe₂O_4-δ→Ni+Fe₃O₄→FeNi alloy。金属镍促进磁铁矿的还原，最终在完全还原时产物为铁镍合金。晶粒愈小愈易于还原。相同还原条件下共沉淀法所得NiFe₂O₄分解CO₂性能最好，水热法最差。活性高生成的碳化铁量愈多，保留的尖晶石结构愈少。还原后的合金、氧缺位铁酸盐甚至金属铁都可能具有分解CO₂的活性。2．考察掺杂Cr³⁺对NiFe₂O₄分解CO₂性能的影响。XRD结果显示Cr³⁺进入NiFe₂O₄晶格的B位，Cr-O-Fe比Fe-O-Fe键牢固，所以随着掺Cr³⁺含量的增加，NiFe₂O₄还原性能下降，导致其分解CO₂的初活性和分解速率都下降。这是由于Cr³⁺的存在既可以使氧缺位NiFe₂O_4-δ的结构破坏或坍塌速率降低约一半，又能防止或减缓铁镍合金的晶粒长大以及碳化铁的生成速率。3．掺Cr³⁺NiFe₂O₄的CO₂分解活性初期主要来自于氧缺位铁酸盐，后期主要来自于纳米级的铁镍合金。合适量的铬掺杂能极大地提高NiFe₂O₄的循环反应次数和使用寿命，纯NiFe₂O₄循环反应20次基本失去活性，而掺4wt％Cr³⁺后循环反应增加到60次后仍保持有初始分解活性的60％，总的分解CO₂的量达到63mmol／g。CO₂分解生成的沉积碳随着循环反应次数的增加逐渐由无定型向有序化发展，在循环反应60次后的样品中发现了碳纳米管。4．利用混合离子共沉淀法可以制得单一的CuFe₂O₄、CoFe₂O₄及ZnFe₂O₄的纳米尖晶石结构，且具有较高的比表面积。H₂-TPR和H₂-TG的结果显示CuFe₂O₄还原最容易，但是完全还原的温度和时间要大于CoFe₂O₄：ZnFe₂O₄还原性能最差，不仅初始还原温度高而且要完全还原需要很高的温度。三种铁酸盐盐的还原产物不一样，CuFe₂O₄完全还原为金属Cu和Fe，CoFe₂O₄还原得到CoFe合金，而ZnFe₂O₄还原产物为Fe、FeO和ZnO的混合物。铁酸钴在310-340℃的等温氢还原遵从成核反应机理，可以使用三维的成核与生长模型来处理，其活化能为72．72kJ／mol。5．比较了CuFe₂O₄、CoFe₂O₄及ZnFe₂O₄在密闭体系中分解CO₂性能的差异。还原和CO₂氧化温度对容易还原的铁酸盐如铁酸铜的CO₂分解量的影响不大，因为在较低290℃它的还原已比较充分；而对较难还原的铁酸盐如铁酸锌，温度的影响是比较大的，在310℃时的CO₂分解量要比290℃时增加17％多。CoFe₂O₄还原的难易程度介于上述两者之间。不同金属铁酸盐MFe₂O₄（M=Cu，Co，Zn）在290℃和310℃经过等温还原-CO₂分解的一个反应循环后，部分尖晶石结构的铁酸盐被破坏，破坏程度随掺杂金属而变，而且随温度升高而增加；生成了大量的无CO₂分解活性的Fe₅C₂，其量也与掺杂金属有关且随温度大幅增加；除了铁酸锌生成金属氧化物外，其余均有金属生成，其量也随温度而变。6．利用TG分析仪比较了敞开体系中MFe₂O₄的分解CO₂性能的差异。在310℃和350℃下等温还原MFe₂O₄时，随着尖晶石结构的破坏生成了金属，CuFe₂O₄完全还原为金属Cu和金属Fe，分解CO₂后，Fe氧化为Fe₃O₄，但是Cu仍以金属形式存在，不能进入到尖晶石结构；CoFe₂O₄完全还原后形成CoFe合金，分解CO₂反应后，基本可以恢复原先的尖晶石结构，只有少量的Co游离出来；ZnFe₂O₄在350℃等温还原为ZnO-FeO及部分金属Fe，分解CO₂后基本可以恢复为原先的尖晶石结构。比较总的增重量，CoFe₂O₄的增重最多分解活性最好。7．利用热重法研究CoFe合金分解CO₂的活性，研究结果发现H₂还原CoFe₂O₄得到的CoFe合金具有非常好的分解CO₂性能。CoFe合金分解CO₂的过程分为两种方式，一种是在低温区段（低于500℃）较小的纳米CoFe合金可以被CO₂直接氧化为尖晶石结构，没有FeO的生成，反应速率非常快；第二种是在高温区段（500-710℃），随着反应温度的升高，CoFe合金长大，长大的CoFe合金需先氧化为Fe（Co）O，Fe（Co）O再进一步氧化为尖晶石结构，这种的反应速率就相对较低。同时还考察了完全还原的CoCr_xFe_2-xO₄（x=0，0．04，0．08）分解CO₂活性差异。发现CoFe₂O₄掺杂一定量的Cr³⁺后会使还原和氧化反应的温度都有所升高，但是Cr³⁺可以阻止CoFe合金的烧结，降低晶粒生长速度。掺杂Cr³⁺的样品表现出较好的循环反应性能，第一次分解CO₂的性能差别很小，但是在第二次循环分解CO₂时掺杂的样品活性远远高于未掺杂的。8．研究了多金属铁酸盐中过渡金属M对铁酸盐分解CO₂的活性影响，通过考察含多金属金属铁酸盐如Ni_xCu_1-xFe₂O₄（x=0．25,0．50,0．75）系列铁酸盐和Ni-Cu-Zn系列铁酸盐分解CO₂性能的差异，发现在Ni-Cu系列铁酸盐中，Ni_0．50Cu_0．50Fe₂O₄金属Cu的还原反应活化能最小，其次是Ni_0．75Cu_0．25Fe₂O₄，Ni_0.25Cu_0．75Fe₂O₄的活化能最大。完全还原的Ni_0．50Cu_0．50Fe₂O₄铁酸盐分解CO₂初始反应温度最低，且分解活性最高。在Ni-Cu-Zn四金属铁酸盐中，Ni²⁺和Cu²⁺的加入可以提高H₂还原的速度并降低H₂还原的温度，而Zn²⁺的加入可以稳定尖晶石结构，使其还原氧化反应中更不容易发生结构崩塌。310℃等温H₂还原的Ni_0．49Cu_0.24Zn_0．24Fe₂O₄具有非常好的分解CO₂性能，在循环反应中，其高反应活性在反应2次后迅速降低，但是随后反应活性降低的速率减缓。这可能是由于在开始几次反应中其尖晶石结构迅速瓦解，随着尖晶石结构的瓦解，反应活性相变为金属合金Fe_xM_1-x（0<x<1），但是其活性要比氧缺位结构差很多。更多还原

【Abstract】 Amount of CO₂ emitted to atmosphere increases with the increasing of combustion of fossil fuel. The rising of CO₂ concentration in the environment has indeed increased the heat trapping capability of the earth’s atmosphere via the greenhouse effect. Greenhouse effect has brought a series of environmental problems, which make us to study on the utilization and mitigation of CO₂. On the other hand, with the explorer of outer space and deep ocean people have to work and live in a space that is airtight such as space shuttle, spaceport or submarine cabin. A major concern of life-sustaining system is how to clear away CO2 released by the people and supplement O₂ consumed, which is necessary for life-sustaining. As a result, the research on the life-sustaining system makes a key point of dispute ocean, outer space and manned flight. Therefore, the research on CO₂ direct decomposition, that is one reaction of chemical simulation of carbon and oxygen cycle in nature, is of important theoretical value and actual meaning.The decomposition of CO₂ into carbon and O₂ has been one of the most effective methods to achieve the cycle of CO₂ and O₂ in airtight system. The decompositionreaction CO₂→C+O₂, however, in unfavorable in thermodynamics, a auxiliary agentsuch as hydrogen must be used, which make the reaction favorable in thermodynamics to converse oxygen in CO₂ to water that can be electrolyzed to regenerate O₂ and H₂. In 1990, Tamaura and Tabata reported that oxygen-deficient Fe ₃O_4-δ formed by H₂ reduced can decompose CO₂ completely to C at 290℃with 100% efficiency. At the same time, oxygen-deficient Fe₃O_4-δ captured [O] of CO₂ and transformed itself to normal spinel structure. From then on, the research on the decomposition of CO₂ to C directly have got a further development and a lot of papers study over the decomposition on oxygen-deficient MFe₂O_4-δ（M=transition metals）. However, for practical application of ferrite there are several obstacles such as short cycle time, low carbon deposition and low structure stability et al., which must be overcome, more study is necessary. Furthermore, the mechanism is inconsistent. Early researchers thought the active phase was oxygen-deficient spinel structure, while Hyun-Chang Shin thought that the oxygen-deficient mechanism was not appropriate for the redox reaction of Ni（Cu） ferrites. The oxidation with CO₂ at 1073 K could not regenerate the original ferrites and produced the mixture of Fe₃O₄ and metallic Ni or Cu.In order to resolve above problems, we studied the effect of synthesis methods and doping Cr³⁺ on the decomposition of CO₂ to C and found efficient methods to inhibit the increasing of crystallite size and prolong the cycle life. Besides, the function of M in MFe₂O₄ and the mechanism were also studied in this paper.Some conclusions are drawn from this thesis as follows:1.Ultra-fine NiFe₂O₄ particles were prepared by co-precipitation, sol-gel and hydrothermal methods, respectively. The results of XRD, low temperature N₂ adsorption and H2-TPR analysis showed that NiFe₂O₄ prepared by co-precipitation methods with the smallest average crystallite size and the largest BET surface areas has the lowest starting reduced temperature; While, NiFe₂O₄ prepared by hydrothermal method has the biggest average crystallite size and the smallest BET surface areas. It’s starting reduced temperature was the higher than the other samples. The reduction mechanism can be described as follows: NiFe₂O₄→NiFe₂O_4-δ→Ni+Fe₃O₄→FeNi alloy. The smaller crystallite size, the easier the reduction of NiFe₂O₄. At the same reducing conditions, the contents of produced iron carbide increase with the CO₂ decomposition temperature.2.The effect of doping Cr³⁺ in NiFe₂O₄ was investigated by XRD, cycle CO₂ decomposition. The results displayed that Cr³⁺ were located in B site of spinel structure and Cr-O bonds would distribute uniformly in B sites. Furthermore, the strength and stability of Cr-O-Fe covalent bond are stronger than Fe-O-Fe. As a result, with the Cr³⁺ content increasing,the structure stability of NiFe₂O₄ increase and it was more difficult to form NiCr_xFe_2-xO_4-δ. On the other hand, the structure stability of NiCr_xFe_2-xO_4-δ increased and the values of 8 decreased which means the reduction ability decreased. Doping Cr³⁺ in NiFe₂O₄ can decrease the destroy rate of oxygen-deficient structure and iron carbide producing rate, as well as inhibit the increasing of the crystallite size of FeNi alloy.3.At the initial stage of CO₂ decomposition, the activity phase can be oxygen-deficient structure. While, at the later stage the activity phase can be transformed to FeNi alloy. Proper contents of Cr³⁺ can significantly improve the cycle decomposition performance of NiFe₂O₄. Pure NiFe₂O₄ almost lost its decomposition activity at the 20^th reaction cycle, while NiFe₂O₄ doped with 4wt% Cr³⁺ there is still about 60% of the initial activity value at the 60^th reaction cycle. The amount of decomposed CO₂ is as high as about 63mmol/g. It is found that deposited carbon is amorphous at first and transformed gradually to crystallite carbon, even carbon nano-tubes which increase with number of cycle reaction.4.Nano crystallite MFe₂O₄ （M= Cu, Co, Zn） with single spinel structure were prepared by mixed ions co-precipitation methods and all samples had high BET surface areas. The results of H₂-TPR and H₂-TG showed that CuFe₂O₄ had the best H₂ reduction ability and ZnFe₂O₄ the worst. While, the completely reduced time of CuFe₂O₄ is longer than CoFe₂O₄. The products of MFe₂O₄ reduced by H₂ are different. ZnFe₂O₄ was reduced to metal Fe, ZnO and FeO, while CoFe₂O₄ was reduced to CoFe alloy and CuFe₂O₄ was reduced to metal Cu and Fe. The iso-thermal H₂-reduction kinetics of CoFe₂O₄ at 310-340℃were studied by classic isothermal analysis theory using TG analysis experiment. The reduction mechanism was proved to be linear growth of product nuclei （m=3） and the activity energy is 72.72kJ/mol.5.The H₂ reduction properties and CO₂ decomposition activity over MFe₂O₄ were studied in airtight system. The experiments results showed at 290℃and 310℃CO₂ decomposition activity of CuFe₂O₄ show little differences. That means at those temperatures CuFe₂O₄ was completely reduced. While for ZnFe₂O₄ which is difficult to be reduced, the CO₂ decomposition activity at 310℃is about 17% higher than at 290℃. The results suggested that reaction temperature for ZnFe₂O₄ is more important than for CuFe₂O₄ at 290-310℃. The reducing activity of CoFe₂O₄ was between CuFe₂O₄ and ZnFe₂O₄. After one cycle reaction （contains a H₂ reduction and a followed CO₂ decomposition）, most of MFe₂O₄（M=Cu, Co, Zn） structures were destroyed to iron carbide and metals （or metal oxide）. The contents of produced iron carbide depended on the reaction temperature and properties of M.6.The decomposition CO₂ activities of reduced MFe₂O₄ in open system were studied by TG analysis. The results showed that CuFe₂O₄ was completely reduced to metals Cu and Fe at 310 and 350℃. After decomposing CO₂ metal Fe were oxidized to Fe₃O₄ and Cu was not oxidized to its oxide form. CoFe alloy was formed after CoFe₂O₄ completely reduced by H₂ and was oxidized to spinel structure after decomposition with CO₂. Only a little of Co metals was isolated. NiFe₂O₄ can not be isothermal reduced completely by H₂ at 310 or 350℃, and after decomposition with CO₂ the metallic Ni was not oxidized and remained in a metallic state as in the case of the oxidation of the reduced Cu ferrite. ZnFe₂O₄ was isothermally reduced at 350℃to ZnO-FeO solid solution which could be oxidized to normal spinel structure by CO₂. Comparing the weight increase it was found that CoFe₂O₄ showed the best decomposition activity.7.The CO₂ decomposition activity of H₂ reduced CoFe₂O₄ was also investigated by TG analysis. CoFe alloy produced by completely reduced CoFe₂O₄ shows a good CO ₂ decomposition activity and its oxidation performs a two-stage oxidation. In the first step, nano-crystallite FeCo alloy was oxidized to spinel structure directly and no FeO was formed, where the decomposition rate is higher. In the second step, FeCo alloy grew-up with the temperature increasing and was oxidized to FeO. Subsequently, FeO was also oxidized to spinel structure. Doping Cr³⁺ in CoFe₂O₄ results in the increasing of the reduced and oxidized temperature. Most importantly, the doping can inhibit the sintering of CoFe alloy and the increasing of crystallite size. As the main point of improving the cycle decomposition activity of CoFe alloy is the control of the increasing of nano-crystallite size, the method of doping Cr³⁺ in CoFe₂O₄ would have a potential for commercialization application.8.The effects of M in multi-metal ferrite on decomposition CO₂ were researched by comparing the decomposition activity of Ni-Cu series ferrites and Ni-Cu-Zn ferrites. It was found that in Ni-Cu ferrites the activity energy of metal Cu reduction was different. The activity energy of Ni_0.50Cu_0.50Fe₂O₄ is the smallest and is the biggest for Ni_0.25Cu_0.75Fe₂O₄. Completely reduced Ni_0.50Cu_0.50Fe₂O₄ started to react with CO₂ at the lowest temperature and showed the best decomposition activity than others. In Ni-Cu-Zn ferrites, the incorporation of Ni²⁺ and Cu²⁺ in ferrite can improve the H₂ reduction rate and lower H₂ reduction temperature; while incorporation of Zn²⁺ can stabilize the spinel structure of ferrite and inhibit the structure destroy during the redox processes. At 310℃H2-reduced Ni_0.49Cu_0.24Zn_0.24Fe₂O₄ has high initial decomposition CO₂ activity, and the activity decreases quickly after twice cycle decomposition reactions. However, the decreased rate of the activity slows down subsequently. This ascribes to the collapse of the spinel structure at first few cycles, and then the activity phase changes from spinel structure to metal alloys.更多还原