【作者】 陈慕华；

【导师】 储伟；

【作者基本信息】 四川大学 ， 化学工艺， 2007， 博士

【摘要】 随着石化资源的不断减少，目前世界上天然气的消费比重增加，寻找清洁能源和高效、低排放的燃烧路线成为各国科研与工程技术人员的重要课题。与普通燃烧相比，催化燃烧具有较高的燃烧效率与能量利用率，因此，关于天然气催化燃烧的研究受到世界各国越来越广泛的关注。本论文以Co-Si体系为研究对象，系统的讨论了催化剂的制备方法及制备参数、等离子体增强处理、助剂修饰等对钴基催化剂在甲烷全氧化中的催化性能影响规律，通过低温液氮吸脱附等温线（BET）、X射线粉末衍射（XRD）、X射线光电子能谱（XPS）、紫外可见及傅立叶红外（UV-vis、FT-IR）、透射电镜（TEM）、热重分析（TGA）、程序升温还原（TPR）以及氢氧滴定（HOT）等多种表征手段探讨各种因素对样品的化学物理特性的影响，同时对构效关系及氧化还原反应活性位进行较深入的研究。对于浸渍法制备的催化剂而言，含硅载体以其适宜的载体-活性组分相互作用有益于甲烷C-H键的初始活化过程和化学吸附氧物种的形成，有助于获得低温高活性，其T_50%较氧化铝载体担载样品约低180℃。大比表面积的氧化硅载体有利于活性钴物种更好的分散和高价钴基氧化物种的形成，对应的钴基催化剂具有良好的低温性能。在钴含量为10 wt%的样品上，随着焙烧温度由300℃提高到700℃，钴物种出现迁移团聚甚至烧结，在XRD中测定的Co₃O₄晶粒尺寸由7.2 nm逐步增加至20.3 nm；另一方面，催化剂的燃烧活化能随样品上Co₃O₄晶粒的平均尺寸增加而呈现线形增加趋势，而单位时间内单位活性物种上的甲烷转换频数随着粒径增加而逐渐降低，这一现象证实了该反应属于典型的结构敏感反应。TPR和XRD结果证实，浸渍法制备的钴基催化剂按Co₃O₄→CoO→Co两步进行还原：催化剂在250℃还原后与载体发生弱相互作用的Co₃O₄被还原为CoO，300℃还原后完全为CoO，再提高还原温度后生成的金属钴可高度分散在载体表面。焙烧温度的增加增强了载体与活性组分的相互作用，不利于氧化还原循环的进行；另外，由于实际反应为临氧气氛，不同的还原温度对于催化剂的反应性能及活化能没有明显影响。研究发现，由于部分金属粒子参与了硅胶网络的形成，溶胶凝胶法制备的钴基催化剂样品中含有两种钴物种：与氧化硅载体发生键合形成Co-O-Si键的钴物种以及可还原的常规尖晶石相Co₃O₄。该钴硅催化剂体系中钴含量的增加使得溢出凝胶而成为Co₃O₄活性组分的钴比例增加到30.8%，催化燃烧反应所需的活性位显著增加，特别是氧化钴物种中OB₃键(对应于八配位的Co³⁺)的比例增加，使得样品催化活性逐渐提升；但过多的钴引入凝胶体系更多地增加了几乎不具有催化活性的Co₂SiO₄而使得反应活性组分Co₃O₄比例下降，另外Co₃O₄绝对量的增加也易造成晶粒团聚长大而导致样品的活性降低。同样，由FT-IR和XRD表征结果可见，适度的提高焙烧温度或选择合适的加水量可减小了Si-O-Co-O-Si键的形成几率，从而提高催化剂中可还原的Co₃O₄活性组分的比例，并使活性组分仍保持良好的分散状态，这样的结果使得样品催化活性逐渐提升至最优值。本实验考察范围内优选钴含量22.5%-26%，焙烧温度600℃，H₂O：TEOS值为15。对于溶胶凝胶催化剂而言，由于高能物种对被处理样品表面的轰击作用，特别是电子和活化气体分子的轰击作用可使被处理材料在短时间内释放大量电子、自由基、分子等，在较低温度下使体系脱水并有部分前驱体的分解，等离子体增强制备的钴基催化剂比表面积由参比样的304.6 m²/g增加到320.1 m²/g。等离子体场内主要活性物种（电子）对Si-O-Co-O-Si及其他钴硅化合物的进攻使得已形成的-Si-O-Co-键断裂，形成新的可还原的Co₃O₄物种，从而使得Co₃O₄的表面含量和程序升温耗氢量增加。等离子体处理催化剂上显示了优良的催化燃烧活性，起活温度(T_10%)下降约50℃，且在活性考察的几个温度中，甲烷转化率到达参比样的2倍以上。优选等离子体处理工艺为：电压80-100 V，时间2 h。等离子体引入与焙烧过程的前后次序对其催化性能几乎没有影响。XPS和HOT表征结果表明，等离子体处理使得钴物种向表面富集，其钴的表面原子比由参比样的2.2提高到8.5，同时，显著提高了钴物种的分散度和活性化学吸附氧量，增加了活性物种的表面浓度和活性表面积，单位比表面积上的可还原的表面钴浓度也由参比样的6.38×10^-3Co/nm²显著增加到20.75×10^-3Co/nm²。为进一步提高催化剂比表面积，采用水热晶化法成功合成了新型中孔分子筛SBA-15，比表面积、孔容和平均孔径分别为：586.6m²/g，0.802 cc/g和4.60nm，微观形貌大多数呈1微米左右的长条绳状，具有较好的六角相构型。以该分子筛作为催化剂载体后发现，大比表面的中孔分子筛可有效的抑制担载的活性组分的聚集，担载钴后的样品耗氢量随钴含量的增加而显著增加，而活性组分Co₃O₄晶粒尺寸则从8.3 nm缓慢增大至15.4 nm。氧化锆助剂由于其结构作用和电子助剂双重效应使得其催化燃烧性能提高到了参比样品的两倍左右，锆含量在2%-20%的范围内均有良好的促进作用，优选锆助剂添加量为5%-10%。而由XPS、HOT和XRD结果可见，贵金属助剂钯的添加可使氧化钯本身具有稳定相态并高度分散在催化剂表面，同时，促进钴物种的表面富集和表面晶格氧向化学吸附氧物种的转化，改善氧化钴的分散性能，其活性物种的表面浓度由2.80 Co/nm²显著提高到3.58 Co/nm²；由于物理表面稀释和覆盖作用减小了钴与硅的相互作用，通过电子传递等相互作用显著促进了氧化钴物种的还原，提高样品的氧化还原能力，从而极大地提高催化剂的低温活性，显著改善样品的催化燃烧性能。适量贵金属钯的加入可使样品在低温下迅速起活，并在350℃之前将甲烷转化率提高到90%，在获得低温高活性的同时具备优良的稳定性。更多还原

【Abstract】 Since world petroleum reserves are on the decline, a great deal of emphasis has been placed on developing alternatives for energy production. Large efforts have been undertaken to find effective processes and technology for the optimum utilization of the abundant natural gas （whose main constituent is methane） for energy production. Compared with the conventional flame combustion, a higher combustion efficiency and a utilization capacity were obtained in the catalytic combustion. Therefore, catalytic combustion has drawn increasing attentions extensively in the past few years. In our work, several series of cobalt-based catalysts were prepared for methane catalytic combustion, the effects of preparation method and conditions, plasma assisted treatment, promoter modification on those properties and catalytic performances were investigated in detail. These samples were characterized using X-ray diffraction, X-photoelectron spectroscopy, UV-vis and FT-IR spectral analysis, thermal gravity analysis, N₂ Adsorption- desorption （BET）, transmission electron microscope, temperature programmed reduction and Hydrogen-oxygen titration technologies. The relation among structure and performance and the active sites for the redox reaction were analyzed and discussed in depth.For the catalysts prepared by the conventional impregnation, the suitable interaction between Si-containing support and active component was beneficial for the activation of C-H band in CH₄ molecule and the existence of active chemical adsorbed oxygen species, the higher activity at lower reaction temperature. T_50%, value on Co-Si catalyst was about 180℃lower than that of Co-Al. SiO₂ support with large surface area promoted the dispersion of the active component and the generation of cobalt oxide with higher valence state. The good catalytic performances were obtained on those catalysts supported by SiO₂. With the increasing calcination temperature from 300℃to 700℃, Co₃O₄ particles augmented and sintered on the samples surface which contained 10 wt. % cobalt, the crystallite size of Co₃O₄ was aggrandized from 7.2 nm to 20.3 nm by inchmeal. On the other hand, the activation energy for the combustion reaction was increased linearly with the augment of average crystallite size of Co₃O₄, the TOF value on prepared catalyst was decreased with it. Thus, the catalytic combustion was a typical structure-sensitive reaction. It could be concluded from the results of XRD and TPR measurements that, reduction process of Co-containing species went along through two steps: Co₃O₄→CoO→Co. After the reduction at 300℃for 30 min, Co₃O₄ was reduced to form CoO which would be further reduced and well dispersed as metallic Co under the higher reduction temperature. Otherwise, the increasing of calcination temperature enhanced the interaction between the active cobalt species and support, This was against the good redox cycle. Because of the existence of oxygen, the reduction temperature had a slight influence on the methane conversion and activation energy.These observations supported the fact that Si atom could band with one or several Co-O band to form different network during the gelation process. As a result, there were two kinds of cobalt species in the Co-Si catalysts prepared by sol-gel method in our work. One was very difficult to be reduced which entered into the framework of SiO₂ gel and formed the -Si-O-Co- band. The other was the conventional reducible cobalt oxide made from the residual part of Co atoms which was spilled over the gel network. With the increasing of cobalt content in the catalysts prepared, proportion of reducible Co₃O₄ within the whole Co-containing species enhanced to 30.8%. Active sites for the catalytic combustion, especially the proportion of OB₃ band in cobalt oxide, were augmented remarkably. As a result, the catalytic activity was promoted gradually. However, too much cobalt added to the gel mixture would increase the amount of Co₂SiO₄ which has no catalytic activity, the augment or sinter of crystal cobalt oxide was obtained. Then, the conversion of methane was decreased. Furthermore, indicated from the results of FT-IR and XRD characterization, the appropriate calcination temperature and H₂O: TEOS ratio could reduce the formation properties of Si-O-Co-O-Si bands and favor the good dispersion of Co₃O₄. The optimum cobalt content, calcination temperature and H₂O: TEOS value in our work were 22.5 %-26%, 600℃and 15.As to the catalyst prepared by sol-gel method, the results of thermal gravity analysis proved that decomposition of cobalt nitrate and the remaining organic compound removal was partially in progress during the plasma treatment, though the temperature was much lower than that of calcination. Owing to the high active species bombardment on the treated surface during the plasma treatment, the breakage of -Si-O-Co- bonds and the formation of Co₃O₄ was observed obviously. It could be concluded that the discharge treatment of samples prepared by sol-gel method induced a better generation of active cobalt species. The XPS characterizations suggested that plasma treatment was favorable for the enrichment of surface cobalt and active chemical adsorbed oxygen species, a value of surface cobalt was increased from 2.2% to 8.5% in mole. Meanwhile, a smaller diameter of crystallite particles （11.2 nm） and a better dispersion of Co₃O₄ on catalyst surface were obtained on Co-Plas-Solgel-2 compared with those of the conventional sample. The specific surface area of the glow plasma assised sample was increased from 305 m²/g to 320 m²/g. The ignition temperature (T_10% of plasma assisted catalyst was about 50℃lower than that of Co-Sol-gel-1, and its CH₄ conversion was two times higher than that of the conventional one during the whole range of catalytic combustion activity test（340-520℃）. The plasma assisted sample exhibited significant enhancement in catalytic performances because of a better dispersion and more active sites, in addition, the sequence of plasma treatment and conventional calcinations had very little effect on its catalytic performance.The mesoporous SBA-15 was synthesized with hydrothermal crystallization method successfully, specific surface area, total pore volume and average pore diameter was 586.6 m²/g, 0.802 cc/g and 4.60 nm, respectively. 1μm rope-like block was observed in the investigation of the morphology by TEM, and a high ordered hexagonal structure was proved by low angle X-ray diffraction. As a catalyst support, mesoporous material with large specific surface area was effective on the restrain of the aggrandizement of supported active component. Hydrogen consumed in TPR experiment was enhanced obviously with the increase of cobalt content, and the crystallite size of Co₃O₄ was augmented from 8.3 nm to 15.4 nm step by step. Due to the structural and electronic effect of zirconia promoter, the reaction activity was neatly twice higher than that of conventional sample. The optimum zirconia content was 5%-10%. The characterization results indicated that the addition of palladium promoted the reduction and surface enrichment of cobalt oxide species obviously. Furthermore, higher dispersion and active surface area and more active sites were obtained on palladium promoted samples. Surface cobalt density was increased from 2.80 Co/nm² to 3.58 Co/nm² after the modification with Pd promoter. The existence of PdO promoted the translation from the surface lattice oxygen of Co₃O₄ into an active chemical adsorbed oxygen species. As a result, T_90% values of the catalysts containing more than 0.7 wt% Pd were lower than 350℃; their activities were 75% higher than that of the conventional sample. Among them, 2Pd-40Co/SBA-15 showed a good stability.更多还原