【作者】 李辉；

【导师】 张鸿斌；

【作者基本信息】 厦门大学 ， 工业催化， 2008， 硕士

【摘要】 CO₂在大气和水中广泛存在,其含碳量是石油,煤炭,天然气三大能源含碳量的10倍,在自然界中以CO₂形式存在的碳含量约为10¹⁶吨。随着经济的发展,大量CO₂排放带来的温室效应严重威胁着人类的生存环境。因此,如何利用CO₂已经成为世界各国普遍关注的重要课题之一。CO₂加氢合成甲醇被认为是既方便又经济的过程之一。Cu基催化剂被认为是CO₂加氢合成甲醇具有发展潜力的催化剂。然而,传统的甲醇合成Cu-Zn-Al催化剂用于CO₂加氢活性低。开发具有高活性及高的甲醇选择性的催化剂具有重要意义。多壁碳纳米管（MWCNTs,下同）是一类新奇的纳米碳素材料。典型的MWCNTs具有由sp²-C组成的类石墨平面、按一定方式叠合而成的纳米级管状结构。从化学催化的角度考虑,碳纳米管具有高的机械强度、大而可修饰的表面、类石墨化的管壁、纳米级的管腔、优良的导热/导电性能以及对氢强的吸附活化能力并可期产生的促进氢溢流的效应,这些特点使其很有希望作为催化剂的优良载体和/或高效促进剂;选择某些过渡金属对MWCNTs进行表面修饰可望拓宽MWCNTs的应用领域或提高其助催化性能。本文以过渡金属Co修饰碳纳米管复合材料的研制及其在催化中的应用为目标,开展如下工作:以自行制备的MWCNTs作为基质,采用微波助多元醇液相化学还原沉积法,制备Co修饰碳纳米管基复合材料,利用一系列物化观测技术（TEM,SEM,EDS,XRD,H₂-TPD等）对其进行表征;进而用Co修饰MWCNTs作为促进剂,制备一类分步沉淀型Cu-ZrO₂氧化物基催化剂,在固定床加压连续流动反应系统上考察其对CO₂加氢合成甲醇的催化性能,并与单纯MWCNTs促进的对应物以及不添加MWCNTs的基质体系作比较,对催化剂进行一系列物化表征,探讨了Co修饰碳纳米管助剂的促进作用本质。本文结果对于过渡金属修饰MWCNTs基复合材料的制备及其在化学催化领域的应用兼具理论和实践意义。本文得到如下主要结果:1.Co修饰MWCNTs基复合材料的制备和表征所用MWCNTs由本研究组先前报道的方法制备,系为鱼骨型多壁碳纳米管,外径在10-50 nm范围,内径为3-7 nm,比表面积-130 m²/g。用自行制备的MWCNTs作为基质,采用微波助多元醇液相还原沉积法制备金属Co修饰的MWCNTs,记为y%Co/MWCNTs（y%为质量百分数）,所制得材料经TEM、SEM、EDX和XRD技术进行表征,结果显示,金属Co相当均匀地分布在MWCNTs表面,Co_X⁰微晶粒径在10 nm以下。C、O和Co是y%Co/MWCNTs表面仅有的三种元素,对于4.3%Co/MWCNTs而言,其表面元素组成为C:82.08%;O:14.51%:Co:3.41%（原子分率）;表面元素氧可能来自MWCNTs纯化过程中所用浓硝酸对MWCNTs表面的氧化/羧基化。H₂-TPD测试结果表明,少量金属Co对MWCNTs表面的适当修饰使其对氢的吸附能力明显提高。2.Co修饰的MWCNTs促进的Cu-ZrO₂基催化剂上CO₂加氢合成甲醇的反应性能用Co修饰的MWCNTs作为促进剂,制备一类分步沉淀型Cu-ZrO₂基催化剂,记为Co_iZr_j-x%（y%Co/MWCNTs）（x%为质量百分数）。对比实验评价结果显示,Co对MWCNTs的修饰明显地提高单纯MWCNTs促进的Cu-ZrO₂催化剂及非促进原基质催化剂上CO₂加氢合成甲醇的反应活性。在组成经优化的Cu₁Zr₁-10%（4.3%Co/MWCNTs）催化剂上,在经优化的反应条件（即5.0 MPa,513K,V（H₂）:V（CO₂）:V（N₂）=69:23:8,GHSV=8000 mL/（h·g））下,甲醇的时空产率达176 mg/（h·g）,是单纯MWCNTs促进的对应物和非促进原基质在其最佳操作条件下的相应值的1.13和1.16倍,显示Co修饰MWCNTs促进剂的加入有利于提高CO₂加氢合成甲醇的反应活性。3.Co修饰MWCNTs促进的Cu-ZrO₂催化剂的表征反应后准工作态催化剂的XRD测试结果显示,在含与不含MWCNTs-基促进剂的催化剂（即Cu₁Zr₁-10%（4.3%Co/MWCNTs）,Cu₁Zr₁-10%MWCNTs和Cu₁Zr₁）上,可观测到的主要物相皆为:金属Cu（Cu_x⁰）（2θ=43.4°,50.4°和74.1°）,CuO（2θ=35.5°和38.3°）和ZrO₂（2θ:30.5°和31.7°）。比较而言,不含促进剂的原基质Cu₁Zr₁催化剂的CuO相特征衍射峰最强,相应的金属Cu（Cu_x⁰）相特征衍射峰最弱,暗示其在3种工作态催化剂中的还原程度（即CuO的被还原率）最低。预还原催化剂的H₂-TPD测量结果表明,4.3%Co/MWCNTs促进的催化剂对H₂的可逆吸附/脱附容量明显高于单纯MWCNTs促进的催化剂或不含MWCNTs的基质体系:在与CO₂加氢制甲醇密切相关的反应温度区域（≥473K）,三种催化剂H₂-TPD曲线复盖区的相对面积强度比为:S_{Cu1Zr1-10%（4.3%Co/MWCNTs）}/S₍Cu₁Zr₁-10%MWCNTs/S_Cu1Zr1=100/81/61,这个顺序同这3种催化剂上CO₂加氢制甲醇反应活性的高低顺序相一致。XPS测试结果显示,在上述3种催化剂工作态表面,zr物种的价态无明显区别,均为Zr⁴⁺,并以ZrO₂的形态存在;而Cu物种以混合价态存在。参照相关文献利用计算机对多重Cu（2p）-XPS谱峰进行解叠,结果显示,工作态催化剂表面Cu物种主要是Cu⁰和Cu⁺,而Cu²⁺的相对含量在XPS检测极限以下;上述3种催化剂工作态表面Cu⁰物种的相对含量高低顺序为:Cu₁Zr₁-10%（4.3%Co/CNT）>Cu₁Zr₁-10%CNT>Cu₁Zr₁;这个顺序与这些催化剂上CO₂加氢的转化频率（TOF）的高低顺序相一致。这个结果也为“表面Cu⁰是负责CO₂加氢的催化活性Cu物种”的论断提供实验支持.4.MWCNTs基助催剂的促进作用本质表观活化能的对比测量显示,适当添加Co修饰MWCNTs或单纯MWCNTs于Cu-ZrO₂基质催化剂中并不引起CO₂加氢制甲醇反应的表观活化能发生明显变化（在Cu₁Zr₁-10%（4.3%Co/MWCNTs）,Cu₁Zr₁-10%MWCNTs和Cu₁Zr₁三种催化剂上,在2.0 MPa,493-533 K,V（H₂）:V（CO₂）:V（N₂）=69:23:8,GHSV=10000mL/（h·g）的反应条件下所测得表观活化能分别为30.2,29.7和29.9 kJ/mol）,这个结果强烈暗示,MWCNTs基助催剂的加入并不改变CO₂加氢制甲醇的主要反应途径。十分显然,在MWCNTs基材料促进的催化剂上,高的CO₂加氢制甲醇的反应活性同作为促进剂的NWCNTs基材料（MWCNTs,尤其Co修饰的MWCNTs）的某些特性密切相关。根据本文的H₂-TPD比较测试结果,可以推断,在本文CO₂加氢制甲醇的反应条件下,在MWCNTs基材料（尤其是Co修饰的MWCNTs）促进的催化剂的工作表面存在着大量的H-吸附物种,营造较高稳态浓度可逆吸附氢物种的表面氛围,那些具有反应活性的吸附H-物种通过MWCNTs促进的“氢溢流”容易传输到Cu_x⁰-ZrO_y催化剂活性位,于是有助于提高CO₂加氢制甲醇反应过程中一系列表面加氢反应的速率。在另一方面,较高稳态浓度表面吸附H-物种既有助于正价Cu物种(Cuⁿ⁺)的还原（也即Cu⁰-催化活性位的恢复）。上述这些效应都对CO₂加氢制甲醇反应性的提高有所贡献。更多还原

【Abstract】 CO 2 exists widely in atmosphere and water in which the amount of carbon is 10 times more than that of petroleum, coal and natural gas. Conversion of CO₂ to useful chemicals is widely investigated by many workers from the view of point of finding technology for suppressing the green house effect caused by CO₂ emission.The methanol synthesis from CO2 and H₂ is considered as a promising process. Catalysts containing Cu are known to be efficient for production of methanol from CO₂ and H₂. Because of thermodynamic limitation, the yield of methanol is not high enough for single-pass operation. The utilization of industrial Cu-Zn-Al catalysts which exhibited a high activity for methanol synthesis from syngas, was not successful in CO₂ hydrogenation. It is interesting to develop new catalysts with high activity.Multi-walled carbon-nanotubes （MWCNTs） have been drawing increasing attention over the last decade. This new form of carbon is structurally close to hollow graphite fiber, except that it has a much higher degree of structural perfection. The MWCNTs possesses highly graphitized tube-wall, nanosized channel and sp²-C-constructed surface. They display exceptionally high mechanical strength, high thermal/electrical conductivity, medium to high specific surface areas, and excellent performance for adsorption and spillover of hydrogen, all of which render this kind of nanostructured carbon materials full of promise to be a novel catalyst carrier and/or promoter; while modification of some transition-metals to MWCNTs might be expected to further develop their applicable field and/or improve their some performance.In the present work, a series of Co-decorated MWCNTs, noted as y%Co/ MWCNTs （where y% represented mass percentage）, were prepared by an intermittent microwave irradiation-assisted polyol reduction-deposition method, and characterized through TEM, SEM, EDX, XRD and H₂-TPD （temperature programmed desorption） measurements. Using the y%Co/MWCNTs as promoter, y%Co/MWCNTs-promoted Cu-ZrO₂ oxide-based catalysts were prepared. Their catalytic performance was evaluated for methanol synthesis from CO₂ hydrogenation and compared with the related reference systems. The results shed some light on the properties of the Co-decorated MWCNTs-based composite material and the prospect of its applications in catalysis. The progress obtained in the present work was briefly described as fellows:1. Preparation and characterization of Co-decorated MWCNTsWith the homemade MWCNTs as substrate material, a type of metallic cobalt-decorated MWCNTs, y%Co/MWCNTs, was prepared using an intermittent microwave irradiation-assisted polyol reduction-deposition method. TEM and SEM measurements of the produced materials showed that Co nanoparticles were quite uniform in shape and size and well dispersed on the MWCNTs surface. Most of the Co-particle diameters were below 10 nm. EDX analysis demonstrated that carbon, oxygen and cobalt were the only three elements at the surface of 4.3%Co/MWCNTs, with atomic percentage of 82.08%, 14.51% and 3.42%, respectively. The surface oxygen most probably originated from the pre-oxidation/carboxylation treatment of the MWCNTs by the concentrated nitric add. The measurement of H₂-TPD showed that the modification of the metallic cobalt to the MWCNTs led to a marked increase in its hydrogen-adsorbing capacity.2. Metallic Co-decorated MWCNTe-promoted Cu-ZrO₂ catalyst for methanol synthesis from CO₂ hydrogenationUsing the y%Co/MWCNTs as promoter, a type of y%Co/MWCNTs-promoted Cu-ZrO₂ catalysts, noted as Co_iZr_j-x%（y%Co/MWCNTs）, for methanol synthesis from CO₂ hydrogenation was prepared by co-precipitation method. It was experimentally found that the Co-decorated MWCNTs-promoted catalyst displayed high activity for CO₂ hydrogenation to methanol. Over the catalyst of Co₁Zr₁-10%（4.3%Co/MWCNTs） under reaction condition of 5.0 MPa, 513 K, V（H₂）:V（CO₂）: V（N₂） = 69:23:8, GHSV = 8000 mL/（h·g）, the observed STY of methanol reached 176 mg/（h·g）. This value was 1.16 and 1.13 times that of the MWCNTs-free substrate Cu₁Zr₁ and the simple MWCNTs-promoted counterpart Cu₁Zr₁-10%MWCNTs, respectively, under the same reaction conditions.3. Characterization of Co-decorated MWCNTs-promoted Cu-ZrO₂ based catalystsThe XRD post-analysis of the tested catalysts showed that in the position and shape of XRD features, there was little marked difference between the XRD patterns taken on the three tested catalysts （i.e., Cu₁Zr₁-10%（4.3%Co/MWCNTs）, Cu-1Zr₁-10% MWCNTs and Cu₁Z₁i）. The Cu-Zr components existed mainly in the forms of Cu （Cu_x⁰） （2θ= 43.4°, 50.4°and 74.1°）, CuO （2θ= 35.5°and 38.3°） and ZrO₂ （2θ= 30.5°and 31.7°）.H₂-TPD measurements showed that the MWCNTs （especially Co-decorated MWCNTs）-containing catalysts could adsorb considerably greater amount of hydrogen than the MWCNTs-free counterpart. The relative area-intensities of the H₂-TPD profiles taken on the three H2-prereduced catalysts in the temperature region of 473-773 K was: S₍Cu1Zr1-10%（4.3%Co/MWCNTs） / S₍Cu1Zr1-10%MWCNTs/ S_Cu1Zr1 = 100/81/61. This sequence was in line with the sequence of reactivity of CO₂ hydrogenation to methanol over these catalysts.XPS measurements of the tested catalysts demonstrated that little marked difference in the Zr（3d）-XPS spectra but a certain difference in the Cu（2p）-XPS spectra existed among the three tested catalysts in the position and shape as well as relative intensity of the XPS peaks associated with the Zr or Cu species. The analysis and fitting of those XPS spectra have been done. The results showed that, at the surface of functioning catalysts, the observed Zr-species all were +4 valence-state, with the corresponding binding energy （B. E.） of Zr⁴⁺(3d_5/2,3/2) at 182.2/184.5 eV; while the dominant Cu-species were Cu⁰ and Cu⁺, with the corresponding B. E. of Cu⁰(2p_3/2,1/2 and Cu⁺(2p_3/2,1/2 at 932.7/952.7 and 931.6/951.6 eV. The sequence of relative content of Cu⁰-species at the surface of the three tested catalysts was Gu₁Zr₁-10%（4.3%Co/CNT） > Cu₁Zr₁-10%CNT > Cu₁Zr₁, in line with the sequence of turn-over frequency （TOF） of CO₂ hydrogenation over the three catalysts. This result provided a strong support to the suggestion that the surface Cu⁰ was the catalytically active Cu-species responsible for CO₂ hydrogenation.4. Nature of the Promoter action by MWCNTs-based materialsIt was experimentally found that proper incorporation of a minor amount of the MWCNTs or Co-decoreted MWCNTs into the Cu-ZrO₂ host catalyst did not cause a marked change in the apparent activation energy （E_α） for the methanol synthesis from CO₂ hydrogenation reaction. On the above three catalysts, the observed E_αof CO₂ hydrogenation was 30.2, 29.7 and 29.9 kJ/mol, respectively. This most likely implied that the addition of a minor amount of the MWCNTs or the Co-decorated MWCNTs to the Cu-ZrO₂ did not alter the major reaction pathway of methanol synthesis from CO₂ hydrogenation.It is quite evident that the considerably better performance of the MWCNT-containing catalysts for CO₂ hydrogenation to methanol is closely associated with the unique structure and properties of the MWCNTs or Co-decorated MWCNTs as promoter. The TPD tests revealed that the MWCNTs, especially Co-decorated MWCNTs, promoted Cu-ZrO₂ catalyst could reversibly adsorb a greater amount of hydrogen at temperatures ranging from room temperature to 623 K. This unique feature would help to generate a micro-environment with higher concentration of active H-adspecies at the surface of the functioning catalyst, thus increasing the rate of surface hydrogenation reactions.更多还原