【作者】 高静；

【导师】 郑小明； 侯昭胤；

【作者基本信息】 浙江大学 ， 物理化学， 2009， 博士

【摘要】 甲烷催化转化制合成气是目前重要和高效的甲烷转化和利用途径,其中甲烷临氧二氧化碳重整制合成气是目前的研究热点和主攻方向。该过程通过将吸热反应与放热反应耦合,不仅可以节约能量、保证反应安全进行,而且还可以根据后续工艺的要求调控产物中的H2/CO比；同时02的加入可以完全消除C02重整过程中催化剂表面的积碳。本论文以机械强度高的工业微球硅胶为初载体,首先通过引入Ce02-ZrO2、La203等制备了一系列CeO2-ZrO2/SiO2和La203-Si02复合载体负载的Ni催化剂；另一方面通过选用Ni络合物前驱体制备了不同Ni粒径的Ni/SiO2催化剂；并对镍基催化剂上甲烷临氧二氧化碳重整制合成气反应的催化性能及反应失活机理进行了系统的研究。得到的主要结果如下：CeO2-ZrO2/SiO2复合载体负载的Ni催化剂在甲烷临氧二氧化碳重整制合成气反应中表现出较好的催化活性及稳定性。其中Ni/6CeO2-4ZrO2/SiO2催化剂在70℃、9,000 h-1空速下具有最高的甲烷初始转化率(30min时72.4%)和最好的稳定性；通过改变原料气中C02/02比可以实现产物中的H2/CO比从0.99调节到2.21。表征结果证明：Ce02-Zr02/Si02复合载体有效地结合了Si02和Ce02-ZrO2的优点,具有高的比表面积、强的表面酸碱性、较好的H2吸附和裂解能力,进而促进了Ni的分散。xLa2O3-SiO2(x=10,15,30)复合载体负载的Ni催化剂(Ni/xLa2O3-SiO2)在甲烷临氧CO2重整制合成气反应中表现出很好的活性和稳定性。在700℃、9,000h-1空速下,Ni/30La203-Si02催化剂在线反应100小时,甲烷的转化率只从78.1%缓慢降低到75.5%。表征结果证明：La203的添加增强了Ni金属与载体相互作用力、提高了Ni的分散度(Ni粒径从45.0 nm降低到6.0 nm以下)；同时,La203的添加还有利于CO2的吸附和活化；脉冲实验结果表明：在Ni/30La203-SiO2催化剂上,重整反应中甲烷的比活性高达20.6 s-1。利用简单的浸渍方法以镍的络合物前驱体成功制备了不同镍粒径(4.5-45.0nm)的Ni/Si02催化剂。系列表征实验表明：由于Ni的络合物前驱体的热稳定性高、流动性低、与载体间相互作用强等特性可以保证浸渍在载体上的镍具有很好的锚定作用、可以防止干燥和焙烧过程中镍在载体表面的迁移、凝聚和再分配；因而可以制备出不同粒径的镍基催化剂。考评实验结果表明：Ni基催化剂在流化床中甲烷临氧二氧化碳重整过程中的活性和稳定性强烈依赖于Ni颗粒的大小和反应的空速。大粒径的催化剂(>16 nm)在高空速下(90,000 h-1)下没有活性,在低空速下(18,000 h-1)有活性但失活很快；而小粒径的催化剂(<9.5 nm)在低于54,000 h-1空速下表现出很好的活性和稳定性。在4.5 nm的Ni催化剂上,甲烷二氧化碳重整反应的速率十分接近甲烷的部分氧化速率(分别为20.6 s-1和22.5s-1),缩小了甲烷选择性氧化、甲烷重整之间的速率差,有可能实现部分氧化、重整反应的同步发生,从而有效提高反应的热量利用效率和催化剂的效率。这也是甲烷二氧化碳自热重整反应过程中,重整与部分氧化可以同步进行的第一次直接证明。利用原位红外、脉冲表面反应、甲烷和二氧化碳的程序升温反应以及TG、Raman、HRTEM等技术对镍粒径为4.5 nm和45.0 nm的Ni/Si02催化剂的反应机理和失活机理进行了研究。结果表明：镍粒径小的催化剂(4.5 nm)有利于甲烷的解离,700度时CH4裂解的比活性可达12.6 s-1；而在45.0 nm的镍催化剂上,CH4裂解的比活性仅为9.8 s-1。CO2和O2都可有效地促进CH4的转化。提出流化床甲烷临氧二氧化碳重整反应中Ni基催化剂失活的可能机理是：Ni粒径大的催化剂或在高空速(9,0000 h-1)的条件下,由于甲烷裂解的速率相对较慢,所生成的表面碳物种相对较少,原料气中的02除了将碳物种气化外,还将金属镍逐渐氧化。这可能是甲烷临氧二氧化碳重整反应中催化剂失活的主要原因：CH4+nNi→CHx-Nim+(4-x)H-Ni(x=0-3)(RDS)C-Nim+(1/2)O2→CO+NimC-Nim+CO2→2CO+NimNi+(1/2)O2→NiO.进一步以[Ni(en)3]2+为前躯体、在不同含量的Zr02改性的Si02载体上制备了系列镍粒径较小、但结构不同的催化剂。结果发现：在超高空速(90,000h-1)下,具有较多Ni-Zr02界面的Ni/5Zr02-Si02催化剂表现出最高的活性和最好的稳定性。C02脉冲及原位XRD发现：C02可以在Ni-Zr02边界裂解形成Ni-O物种和CO：含较多Ni-Zr02界面的Ni/5Zr02-Si02催化剂具有更高的甲烷转化速率。根据课题的需要,本人在攻读博士学位的后期,还对甘油催化选择氧化反应进行了初步的探索。发现无碱条件下,由于甘油更易接触到沉积在MWNTs外表面上的Pt使得Pt/MWNTs比Pt/AC具有更好的甘油选择氧化活性和甘油酸收率,Raman结果进一步证实了无碱条件下甘油选择氧化反应中伯位C-H键较伯位CO-H更易活化。进一步研究发现：在Pt/MWNTs催化剂上甘油选择氧化反应对Pt的分散度和粒径大小很敏感。小粒径的Pt/S-MWNTs催化剂具有更高的甘油选择氧化活性。平均粒径为2.4 nm的Pt/S-MWNTs(60-100)催化剂上,60℃反应6h,甘油的转化率高达84.0%,甘油酸(GLYA)的收率达到58.0%。更多还原

【Abstract】 Catalytic transformation of methane and carbon dioxide, the cheapest carbon-containing materials and the most problematic greenhouse gases, into more valuable compounds has attracted attentions of researchers. Combination of CO2 reforming and partial oxidation of methane, also called methane autothermal reforming [MATR] (Eq.(1)), has been substantial interest in recent years in alternative routes for the conversion of natural gas (methane) to syngas. This process has low-energy requirements due to the opposite contribution of the exothermic partial oxidation of methane (Eq. (2)) and the endothermic CO2 reforming on methane (Eq. (3)). By controlling the feed composition, H2/CO ratio in the product can be modified according to the need of the post processing. Oxygen in feed is helpful to avoid catalyst deactivation by carbon deposition.CH4+xCO2+(1-x)/2 O2→(1+x) CO+2H2,AH298=(285x-38)kJ/mol(0<x<1) (1)CH4+CO2→2 CO+2 H2,ΔH298=247 kJ/mol (2)CH4+1/2 O2→CO+2 H2,ΔH298=-38 kJ/mol (3)Fluidization has a favorable effect on the inhibition of carbon deposition, which is probably because the catalyst particles are circulated between the oxidizing zone and reducing zone, and carbon gasification proceeds readily in the oxidizing zone. Moreover, catalyst can maintain a suitable level of reducibility during fluidization that enhances the conversion of methane. The aim of this work is to investigate the influence of promoters (e.g. CeO2-ZrO2 and La2O3) and the particle size of Ni on MATR in a fluidized bed reactor. Main conclusions are as follows:A series of combined CeO2-ZrO2/SiO2 supported Ni catalysts show higher activity and stability for MATR in a fluidized bed reactor. Ni/6CeO2-4ZrO2/SiO2 catalyst exhibits the best initial activity (72.4% at 30 min) and stability at 700℃, 9,000 h-1. H2/CO ratio in product gas could be controlled successfully in the range of 0.99-2.21 by manipulating the relative concentrations of CO2 and O2 in feed. Characterizations found that the combined supports integrated the advantages of SiO2 and CeO2, ZrO2. That is, they have bigger surface area (about 300 m2/g) than that of pure CeO2 and ZrO2, stronger acidity and alkalescence than that of pure SiO2, and enhanced the mobility of H adatoms. Ni species dispersed highly on these combined CeO2-ZrO2/SiO2 supports, and became more reducible. Ni catalysts on the combined supports possess higher CO2 adsorption ability, higher methane activation ability.Ni/xLa2O3-SiO2 (x=10,15,30) possessed high activity and excellent stability for MATR in a fluidized bed reactor. On Ni/30La2O3-SiO2, the conversion of CH4 reached 78.1%(at 700℃and 9,000 h-1) in initial 18 h, and only slightly decreased to 75.5% in the followed 100 h. Characterizations indicate that La2O3-modified SiO2 has higher surface area, strengthened interaction between Ni and support, and improved dispersion of Ni. La2O3 increased the alkalescence of SiO2 and improved the activation of CO2. Coking reaction (via both temperature-programmed surface reaction of CH4 (CH4-TPSR) and pulse decomposition of CH4) disclosed that La2O3 reduced the dehydrogenation ability of Ni. CO2-TPO, O2-TPO (followed after CH4-TPSR) confirmed that only part amount of carbon species derived from methane decomposition could be removed by CO2, and O2 in feed played a crucial role for the gasification of the inactive surface carbons.Different sized Ni catalysts (4.5-45.0 nm) were prepared by a simple impregnation of aqueous solutions of Ni(NO3)2 and Ni complexes. The combined use of UV-DRS, FT-IR and Raman suggested that all Ni complexes precursors remained their structure on the surface of SiO2 in as-synthesized samples and contacted strongly with the support, drying of the as-synthesized samples resulted in further stabilization of the isolated Ni centers by an additional link to a silanol group (Si-O(H)-Ni). This strong interaction inhibits the redistribution of impregnated metal ions during drying and calcination, and results in highly dispersed Ni catalysts. It was found that the activity and stability of Ni catalysts for MATR in fluidized reactor depend strongly on the particle size and the operating space velocity. Larger-particle Ni catalysts (>16 nm) had no activities at higher space velocity (90,000 h-1) and deactivated rapidly at lowe space velocity (18,000 h-1). While small sized Ni (<9.5 nm) is more active and stable at space velocity (<54,000 h-1).Characterizations disclosed that methane decomposition rate decreases with the enlarging Ni particle size and the temperature of methane decomposition was lower on small sized Ni. CO2 can not dissociate on the Ni/SiO2. The results of pulse-MS illustrates that activity of CH4 decomposition was 9.8 s-1 on the 45.0 nm Ni catalyst, comparable with previously reported values. On the 4.5 nm Ni catalyst, the calculated activity of CH4 increased to 12.6 s-1. CO2 and O2 accelerated the conversion of methane, but the small particle Ni catalyst was more effective and stable. As the methane decomposition rate slows on larger Ni particles and at higher space velocity to ensure complete conversion of the oxygen, surface Ni will be gradually oxidized by remaining O2, leading to Ni deactivation:CH4+nNi→CHx-Nim+(4-x)H-Ni (x=0-3) (RDS) (4)C-Nim+(1/2)O2→CO+Nim (5)C-Nim+CO2→2CO+Nim (6)Ni+(1/2)O2→NiO (7)On the basis of above reaction mechanism, a series of different amount ZrO2-promoted SiO2 supported Ni catalysts prepared from [Ni(en)3]2+ were used for MATR to synthesis gas in a fluidized bed reactor in order to enhance the methane decomposition rate (RDS). It was found that Ni/5ZrO2-SiO2 with larger Ni-ZrO2 boundary exhibited the best activity and stability for MATR even at an extremely space velocity 90,000 h-1. Pulse-injected surface reactions and in situ XRD characterizations disclosed that CO2 dissociated exclusively at the boundary between Ni and ZrO2. The decomposition rate of CH4 was enhanced at the boundary between Ni and ZrO2.Catalytic oxidation of glycerol with molecular oxygen to glyceric acid was primarily studied in a base-free aqueous solution over Pt/MWNTs and Pt/AC catalysts at atmosphere. Pt/MWNTs was more active than Pt/AC for the easier accessibility of Pt on the external wall of MWNTs. Raman analysis confirmed the primary C-H was more active than the primary CO-H. It was found that the conversion of glycerol was effected strongly on the particle size of Pt on MWNTs. Smaller Pt particles are more active. On Pt/S-MWNTs(60-100) with 2.4 nm Pt particles, the conversion of glycerol and the yield of GLYA reached 84.0% and 58.0%, respectively.更多还原