【作者】 王璐；

【导师】 王静松； 张国华； 周国治（Kuo-Chih Chou）；

【作者基本信息】 北京科技大学 ， 冶金工程， 2018， 博士

【摘要】 由于良好的优异性能,钼在国民经济和国家军事建设等方面都具有非常广泛的用途,是一种非常重要的战略资源。自工业应用以来,受到了越来越多西方发达国家乃至我国的高度重视。然而,随着现代工业的高速发展,一些普通Mo基材料已远远不能满足尖端领域材料的各项性能指标要求。在超细MoO₃的制备方面,采用较多的仍然为传统的湿法制备,制备出的多为热力学稳定的正交三氧化钼,α-MoO₃;而对于火法制备,尤其针对具有更高优异特性的单斜三氧化钼,β-MoO₃,研究则很少。与此同时,由于具有高硬度、高熔点、高催化活性和耐磨抗毒等优异特性,Mo₂C和Mo₂N被广泛地应用于加氢脱硫加氢脱氮等,对于它们的制备,尤其朝着超细方面,受到了越来越多研究者和企业的关注。本论文采用了 FactSage 7.0热力学软件、HCT-3差热分析仪、XRD衍射、FE-SEM扫描、BET比表面积分析、TEM透射电镜、XPS表面元素分析和氧氮分析仪等技术和手段对超细三氧化钼的制备及其气基还原动力学和反应机理进行了详细的研究。主要内容包括:1)采用高温升华-低温冷凝法制备超细单晶球形β-MoO₃并分析工业氧化钼的升华机理;2)H₂还原超细MoO₃和MoO₂的反应动力学和还原机理;3)超细Mo粉自燃机理的研究;4)CO、CO-CO₂混合气体、NH₃、或者N₂-H₂混合气体对超细MoO₃的还原研究;5)各种还原气氛下各产物的物相转变规律和形貌演变过程等。主要获得了如下研究成果:（1）以工业氧化钼为原料,采用高温升华-低温冷凝法制备出了大小约1μm且粒度均匀的超细MoO₃。并且,此时的MoO为球形的单晶单斜三氧化钼,β-MoO₃,这是一种具有更高优异特性的亚稳态三氧化钼类型。在工业氧化钼的升华过程中,表面MoO₃优先反应而升华出去,而底部的则由于表面MoO₃和杂质的阻碍作用,反应速率稍慢;随着时间的推移,样品中MoO₃含量逐渐下降,杂质浓度逐渐增加,升华速率进一步下降;当反应结束时,残留样品为各种钼酸盐和SiO₂的混合物。（2）在713K至833 K范围内,H₂还原超细MoO₃符合两步还原法:MoO₃首先还原成Mo₄O₁₁,一旦出现Mo₄O₁₁,其会进一步还原成MoO₂,两者同时发生同时进行。当温度比较低时,制备出的超细MoO₂几乎保持着和超细MoO₃原料一致的球形形貌,反应符合假晶转换机理;当温度比较高时,制备出的超细MoO₂呈现出片状或者骨架结构,反应符合化学气相传输机理。并且,本论文采用双界面反应模型对高温下的反应动力学进行了分析,发现一段还原(MoO₃至Mo₄O₁₁)和二段还原(Mo₄O₁₁至MoO₂)分别符合化学反应和扩散模型,提取的活化能分别为122kJ/mol和114kJ/mol。而且,当总反应进度介于0至0.7时,一段还原和二段还原同时存在同时发生,此时MoO₃、Mo₄O₁₁和MoO₂三相共存;当总反应进度介于0.7至1时,一段还原结束,二段还原仍在进行,此时MoO₃完全消失,系统只剩Mo₄O₁₁和MoO₂。（3）H₂还原超细MoO₂符合一步反应法:MoO₂直接还原成金属Mo,没有任何中间产物的形成。当温度较低时,速率控速环节随着反应进度的变化而变化:在0至0.8时,反应符合成核与核心长大模型;在0.8至1时,反应符合扩散模型,此时获得超细Mo粉呈现和MoO₂原料一致的片状形貌,反应符合假晶转化机理。当温度较高时,还原过程由界面化学反应控速,获得的超细Mo粉呈现细小的球形形貌,反应符合化学气相传输机理。（4）873 K采用H₂还原超细MoO₂时,发现H₂露点对超细钼粉的自燃行为和产物组成具有非常重要的影响。低H₂露点导致自燃行为的发生,产物为面心立方的γ-Mo₂N且表面包裹着薄薄的MoO₃层。然而,高H₂露点会抑制自燃行为的发生甚至导致还原不完全。在整个还原和自燃过程中,MoO₂ 原料,中间产物Mo和自燃产物γ-Mo₂N都保持着一致的片状形貌。（5）采用程序升温反应法用CO气体还原超细MoO制备超细Mo₂C的反应过程中,低温下球形MoO₃逐渐还原成表面光滑呈片状结构的MoO₂,期间伴随着中间产物Mo₄O₁₁的生成,即还原过程;然后随着时间的推移,光滑片状结构的MoO₂会进一步碳化成表面粗糙带有众多条纹结构的Mo₂C,即碳化过程。在CO气体中增加一定比例的CO₂（15%CO₂+ 85%CO）有利于抑制CO析碳反应的发生,减少自由碳的形成,获得纯净的Mo₂C;但如果CO₂比例过高则不利于Mo₂C的生成,此时产物为表面光滑呈片状结构的MoO₂。在本论文的研究条件下,得出了制备Mo₂C的最佳反应条件。（6）采用NH₃气体还原超细MoO₃制备出的氮化钼为面心立方的γ-Mo₂N,并且此时的还原产物保持着和原料一致的球形形貌、颗粒大小和单晶晶体结构,具有一定的遗传性。结合大量的相关文献,得出MoO₃前驱体的选择对最终还原产物γ-Mo₂N的特性起着决定性的影响。更多还原

【Abstract】 Due to the good performance,molybdenum（Mo）is widely used in the national economy and national military construction.Many contries including China have payed lots of attentions on the studies about the preparation of ultrafine Mo base materials.Since the beginning of the 21 st century,and with the rapid development of the modern industry,some ordinary Mo base materials have been unable to meet the performance indexes of high technological materials in the corresponding field.In the preparation of high-purity and ultrafine MoO₃ or metal Mo aspect,China has largerly far behind some western developed contries such as Germany,Japan and Austria.Also,the method adopted is mostly traditional hydro metallurgy and the as-prepared product is orthorhombic phase,α-MoO₃.However,as to the pyrometallurgy,especially for the preparation of higher properties monoclinic phase,β-MoO₃,corresponding research is almost blank.What is more,H₂ gas is still the most widely used reductant for the reduction of MoO₃,while as for some other reductants such as CO-CO₂ or NH₃ gas,the corresponding studies are seldom.Therefore,the main aim of the paper is to further solve these problems by using the FactSage 7.0 thermodynamics software,thermo-gravimetric analyzer（TGA-DTA）,X-ray Diffraction technology（XRD）,Field emission scanning electron microscope（FE-SEM）,Brunauer-Emmett-Teller method（BET）,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）and ONH analyzer.The following conclusions can be drawn:（1）Ultrafine monoclinic molybdenum trioxide（β-MoO₃）powders with the average crystal size of less than 1 μm are successfully synthesized by the method of sublimation at higher temperatures.In addition,the particles are perfectly spherical morphology and single-crystalline structure.The sublimation of industrial grade MoO₃ is considerably complex process which incorporated sublimation,melting and vaporization.Impurities such as SiO₂ have serious retarding influences on the sublimation rate.（2）Reduction of ultrafine spherical MoO₃ to MoO₂ by H₂ gas obeys the two-stage reaction mechanism with the formation of intermediate product Mo₄O₁₁ during the temperature ranges of 713 K to 833 K.When the reduction temperature is lower,such as 713 K,the morphologies of as-reduced MoO₂ are nearly kept the same as the raw MoO₃ materials,and then the pseudomorphic transformation mechanism is obeyed;while when the temperature is higher（773 K to 833 K）,the products exhibit platelet-shaped or skeleton structure,and then the chemical vapor transport mechanism（CVT）is obeyed;howerer,when the temperature is in the middle of them,such as 753 K,spherical and platelet-shaped MoO₂ are both existed,and then both pseudomorphic transformation and CVT mechanism are worked.In addition,the reaction kinetics during the temperature ranges of 773 K to 833 K are analyzed.It is found that the rate controlling steps for the reduction reaction of MoO₃ to Mo₄O₁₁ and Mo₄O₁₁ to MoO₂ are interface chemical reaction and diffusion model,respectively;the corresponding activation energies are extracted to be 122 kJ/mol and 114 kJ/mol,respectively.What is more,it is concluded that when the reaction extent is in the range of 0 to 0.7,both the reduction of MoO₃ to Mo₄O₁₁ and Mo₄O₁₁ to MoO₂ are occurred simultaneously,MoO₃,Mo₄O₁₁ and MoO₂ are all existed;while when the reaction extent is in the range of 0.7 to 1,only the reduction of Mo₄O₁₁ to MoO₂ are occurred,and then MoO₃ is disappeared,the system are only included Mo₄O₁₁ and MoO₂.（3）Reduction of MoO₂ to metal Mo by H₂ obeys one stage reduction process,MoO₂ is reduced to metal Mo directly without any intermediate products occurred.When the reaction temperature is higher,the rate controlling step is the interfacial chemical reaction,the as-reduced Mo exihibits nanosize spherical characteristic and reduction obeys CVT mechanism;while when the reaction temperature is lower,the rate controlling step is changed with the reaction extent,the as-reduced Mo exihibits platelet-shaped morphology as the raw MoO₂ material and the reduction obeys the pseudomorphic transformation mechanism.（4）The pyrophoric behaviour of as-reduced ultrafine Mo powder at 873 K is invertigated.The dew point of the H₂ gas used in the preparation process has a significant effect on the pyrophoric behaviour of the resulting Mo powder as well as the product compositions.A low dew point of H₂ results in pyrophoric behaviour and the formation of γ-Mo₂N with a small amount of MoO₃ forming outside;in contrast,a high dew point of H₂ inhibits the pyrophoric behaviour.The morphology of pyrophoric product is the same as that of the MoO₂ raw material and intermediate product Mo.（5）The reduction of ultrafine spherical β-MoO₃ with CO gases by the temperature-programmed reaction method is composed of two stages:the reduction of MoO₃ to MoO₂ with the formation of Mo₄O₁₁ as the intermediate product at the lower temperatures and the carburization from MoO₂ to M02C at the higher temperatures.Mo₂C are always kept the same platelet-shaped morphologies as the newly formed MoO₂.In addition,it is found that adding a certain proportion of CO₂ into the reducing gases has a large inbibiting effect on the carbon deposition reaction of CO,which is beneficial for the preparation of pure Mo₂C.However,if the proportion of CO₂ is too high,Mo₂C cannot be obtained and the product will turn to be smooth platelet-shaped MoO₂ instead.According to the reseach,the optimum conditions for the preparation of higher performance Mo₂C catalytic are obtained:reaction temperature 1237 K,heating rate 10 K/min and partial pressure of CO P_co=0.85.（6）γ-Mo₂N is successfully synthesized by the reaction between β-MoO₃ and NH₃ gas via the temperature-programmed reaction method during the temperature ranges of 1013 K to 1073 K.It is found that the spherical morphologies,particle size（1 μm）and single-crystalline structure between them are amostly kept the same as each other.The selection of the MoO₃ precursor plays a crucial role on the preparation of different characterization of γ-Mo₂N catalytic.更多还原