耐高温负载型贵金属催化燃烧及甲烷干重整催化剂研究

发布时间：2018-05-23 16:08

  本文选题：耐高温 + 贵金属　； 参考：《山东大学》2017年博士论文

【摘要】：开采页岩气技术不断提升,天然气将逐渐取代煤炭成为第二大化石资源。作为一种储量丰富的清洁资源,天然气除直接作为燃料提供热能和电能外,其作为发动机燃料可替代石油或作为C1化工原料可替代煤制合成气,这对降低氮氧化物及粉尘等污染物及CO2排放具有重要意义。目前,天然气在以上利用过程中存在的核心问题是:1)天然气发动机尾气中残留的低浓度甲烷和一氧化碳需催化燃烧消除;2)甲烷间接转化制合成气尚缺乏更加环保而经济的路线。解决以上两问题依赖于催化一氧化碳及甲烷燃烧和甲烷干重整反应,燃烧产生的高温热点和重整反应所需的长时间高温可导致催化剂不可逆烧结失活,这是这两个过程存在的共性科学问题,开发高活性耐高温催化剂是该领域的最具挑战性课题。基于反应高温条件对催化剂高的活性尤其热稳定性要求,本论文通过合理选择和设计氧化物载体组成与形貌,构建氧化物载体与贵金属纳米物种外延生长界面结构,增强二者之间相互作用及高温稳定性,并进一步通过优化贵金属种类或具有高氧化还原性的氧化物助剂以针对不同反应提高其催化活性,研制了高活性长寿命的甲烷和一氧化碳催化燃烧及甲烷二氧化碳干重整催化剂,并对该类催化剂的稳定机制进行了研究。主要研究内容如下:(1)介孔氧化铈形貌对其直接作为催化剂或催化剂载体对甲烷和一氧化碳催化燃烧性能影响。采用温和的溶剂热方法,通过控制表面活性剂、沉淀剂、氧化剂和溶剂热处理时间获得了空心纳米锥、纳米片和纳米线型介孔氧化铈,发现该具有多级介孔结构的氧化铈纳米催化剂催化CO燃烧活性均明显高于商品氧化铈,其中介孔空心纳米锥型形氧化铈催化CO燃烧活性最高,起燃温度(T50)约为200 ℃;向制备介孔氧化钸空心纳米锥体系中加入过渡金属(Cu、Mn、Co、Ni)离子,所得铜掺杂氧化铈样品催化CO燃烧活性大幅提高,起燃温度可降低到127℃,这与其优异的氧化还原性能相关;对于催化甲烷燃烧反应,不同氧化铈负载贵金属Pd后,起燃温度都在550-580℃之间,反而低于商品氧化铈负载的Pd催化剂,其原因可能在于氧化铈的特殊形貌结构在高温下坍塌后导致Pd更易烧结失活。(2)MgAl204负载的耐高温Pt(Pd)催化剂及Ce02助剂对催化一氧化碳和甲烷燃烧性能影响。基于MgAl204对尖晶石通过外延生长结构对Pt和Pd优异的稳定作用,我们通过传统等体积浸渍过程制备了高温稳定单Pt和Pt-Pd双金属催化剂(质量分数为1wt.%),并进一步通过浸渍法引入氧化铈获得氧化铈修饰的上述催化剂。在800 ℃空气气氛中老化7天后,A1203负载的贵金属Pt发生严重烧结,而Pt/MgAl2O4和Pt-Ce/MgAl204分别实现了贵金属纳米粒子(2-3 nm)的部分和全部稳定,尤其是后者,在氧化铈的辅助下,Pt甚至可稳定在单原子分散状态!相应的,Pt/MgAl204和Pt-Ce/MgAl204在老化处理后仍保持较高CO氧化低温活性,在空速高达500000 mL/g-s时,CO的起燃温度分别低至约190 ℃和160 ℃,表现出耐高温高分散贵金属Pt对CO燃烧的优异催化性能。然而,这些具有高热稳定性的高分散Pt催化剂催化甲烷燃烧反应时(空速50000 mL/g-s),其起燃温度分别为645 ℃和526 ℃,远高于常规大尺寸Pt催化剂,表明小尺寸Pt不利于CH4催化燃烧反应。双金属催化剂Pt-Pd/MgAl204老化后催化甲烷燃烧起燃温度可降低至495 ℃,经氧化铈修饰后催化剂催化甲烷燃烧起燃温度可进一步降低到475 ℃,且低温段活性显著提高。(3)低载量高分散耐高温Ru/MgA1204催化甲烷二氧化碳干重整反应研究。基于MgAl204对Ru纳米粒子的高稳定作用,我们利用氧化钌物种在高温氧化气氛中的高挥发特性,通过物理气相沉积(PVD)过程,在900 ℃流动空气中,沉积氧化钌制备了原子级分散Ru/MgAl204催化剂,经900 ℃H2高温还原后,金属Ru纳米颗粒平均尺寸约为1.1nm,完全避免了通过传统浸渍法制备的Ru催化剂样品中尺寸大于100 nm大颗粒Ru的形成。在重整反应生成氢气和一氧化碳的还原气氛下,金属Ru不再挥发流失。在850 ℃反应温度下,PVD法制备的Ru/MgAl204催化剂(质量分数0.15%)催化甲烷干重整反应速率可达279.50 mol/molRu-s,约是浸渍法Ru/MgAl204催化剂(质量分数1%)反应速率(28.61 mol/molRu-s)的10倍,是目前文献中高活性Ni催化剂NiCoMg/Al203(0.61 mol/molNi-s)的457倍;该催化剂在850℃,空速高达400000 mL/g-h(CH4:C02 =1:1)条件下对催化甲烷干重整反应表现出及其优异的高温稳定性和抗积碳性能,在长达600 h的稳定性实验中,甲烷和二氧化碳转化率(96.7%和98.6)均达到热力学平衡转化率,H2/CO比值接近理论值1;反应后Ru平均尺寸增大为2.7 nm,反应速率反而提高到反应前的1.5倍,催化剂上无可测的积碳量。与此相反的,浸渍法制备的Ru/MgAl204催化剂(质量分数1%)在此反应条件下10小时内迅速失活,同时伴随大量积碳。若降低PVD法制备的Ru/MgAl204催化剂的Ru质量分数到0.07 wt.%,得到的金属Ru粒子尺寸更小,其在同等条件下的反应速率为35.72mol/molRu-s。这些结果表明,至少在金属Ru尺寸低于2.7 nm的范围内,增大Ru纳米粒子尺寸可显著增加其表观活性,且在此尺寸范围内,催化剂上基本不产生积碳;但存在于浸渍法制备催化剂中的大颗粒Ru似乎是导致催化剂严重积碳的重要原因。这种极低金属钌用量的高活性长寿命催化剂有显著的工业应用潜力。综上,该论文针对高温反应对催化剂活性及高温稳定性的要求,利用MgAl204尖晶石作为载体成功制备了在纳米、亚纳米乃至单原子尺度稳定的Pt,Pd和Ru基催化剂,并选择性的利用具有高氧化还原性的氧化铈作为助剂,进一步增强了其催化性能。通过研究耐高温Pt,Pd和Ru催化剂在催化CO和CH4催化燃烧及CH4/C02干重整反应中的行为,发现在被稳定在不同尺寸的贵金属物种中,较大颗粒尺寸的贵金属具有更好的表观活性。因此,适当增大高热稳定贵金属纳米粒子的尺寸有望进一步增强其催化活性,尤其是催化CO和CH4燃烧反应的低温活性。
[Abstract]:With the continuous improvement of shale gas technology, natural gas will gradually replace coal and become the second largest fossil resource. As a rich and abundant clean resource, natural gas is used as fuel to replace petroleum or substitute for coal synthetic gas as C1 chemical raw material. And dust and other pollutants and CO2 emissions are of great significance. At present, the core problems in the utilization of natural gas are as follows: 1) the residual low concentration methane and carbon monoxide in the gas engine exhaust gas need catalytic combustion; 2) the indirect conversion of methane to synthetic gas is still lack of a more environmental and economical route. The above two questions are solved. The problem depends on the catalytic reaction of carbon monoxide and methane combustion and methane dry reforming. The hot hot spots and the long time high temperature required by the combustion can lead to the irreversible inactivation of the catalyst. This is the common scientific problem in these two processes. The development of high activity and high temperature resistant catalysts is the most challenging topic in this field. In this paper, the interfacial structure of oxide carrier and noble metal nanoscale is constructed by rational selection and design of the composition and morphology of oxide carrier, and the interaction between the two and the high temperature stability is enhanced by the rational selection and design of the composition and morphology of the oxide carrier. In order to improve its catalytic activity with high oxidation-reducibility, the catalytic combustion of methane and carbon monoxide and carbon dioxide dry reforming catalyst for high active and long life are developed, and the stability mechanism of this kind of catalyst has been studied. The main contents are as follows: (1) the morphology of mesoporous cerium oxide is straight. The effect of catalyst or catalyst carrier on the catalytic combustion performance of methane and carbon monoxide was obtained. The hollow nanoscale, nanoscale and nanoscale mesoporous cerium oxide were obtained by controlling the surface active agent, precipitant, oxidant and solvent heat treatment by mild solvent thermal method. The catalytic activity of CO is obviously higher than that of commercial cerium oxide, in which the mesoporous hollow nano cone shaped cerium oxide catalyzes the highest combustion activity of CO, and the ignition temperature (T50) is about 200 c, and the transition metal (Cu, Mn, Co, Ni) ions are added to the mesoporous hollow plutonium oxide nanocone system, and the copper doped cerium oxide sample catalyzes the combustion of CO. The ignition temperature can be reduced to 127 degrees centigrade, which is related to the excellent oxidation and reduction performance. For the catalytic methane combustion reaction, the ignition temperature is between 550-580 centigrade and less than the commercial cerium oxide supported by different cerium oxide Pd. The reason may be that the special morphology of cerium oxide is high. Pd is more prone to sinter inactivation after the temperature collapse. (2) the effect of high temperature Pt (Pd) catalyst and Ce02 promoter on the catalytic performance of carbon monoxide and methane combustion with MgAl204 load. Based on the excellent stabilizing effect of spinel on Pt and Pd by the epitaxial growth structure of spinel, we prepared high temperature stable single Pt and Pt- through the uniform impregnation process. Pd bimetallic catalyst (mass fraction is 1wt.%), and cerium oxide was further introduced by impregnation to obtain cerium oxide modified catalyst. After 7 days of aging in air atmosphere at 800 C, A1203 loaded precious metal Pt was seriously sintered, and Pt/MgAl2O4 and Pt-Ce/MgAl204 showed part and all of the noble metal nanoparticles (2-3 nm) respectively. Stability, especially the latter, with the aid of cerium oxide, Pt can even be stable in the state of single atom dispersion! Corresponding, Pt/MgAl204 and Pt-Ce/MgAl204 still maintain high CO oxidation activity after aging, and the ignition temperature of CO is lower to about 190 and 160 degrees, respectively, when the air velocity is up to 500000 mL/g-s, showing high temperature and high dispersive noble metal P T has excellent catalytic performance for CO combustion. However, these high thermal stability highly dispersed Pt catalysts catalyze methane combustion (air velocity 50000 mL/g-s), and the ignition temperature is 645 C and 526 C, which is far higher than the conventional large Pt catalyst, indicating that small size Pt is not conducive to the catalytic combustion reaction of CH4. Bimetallic catalyst is old and old. The combustion temperature of methane combustion can be reduced to 495 C after the catalytic combustion of methane, and the catalytic combustion temperature of methane combustion can be further reduced to 475 degrees C, and the activity of low temperature section increases significantly. (3) study on the dry reforming reaction of Methane Carbon Dioxide Catalyzed by low load and high dispersing high temperature Ru/MgA1204. Based on the height of MgAl204 to Ru nanoparticles We use the high volatilization characteristic of ruthenium oxide in the high temperature oxidation atmosphere. Through the physical vapor deposition (PVD) process, the atomic level dispersed Ru/MgAl204 catalyst is prepared by the deposition of ruthenium oxide at 900 centigrade. The average size of the metal Ru nanoparticles is about 1.1nm after the high temperature reduction at 900 C H2. In the Ru catalyst samples prepared by traditional impregnation, the size of the large particles larger than 100 nm was formed. Under the reduction atmosphere of hydrogen and carbon monoxide in the reforming reaction, the metal Ru no longer volatilized and lost. At the reaction temperature of 850 C, the Ru/MgAl204 catalyst prepared by PVD method (mass fraction 0.15%) catalyzed the methane dry reforming reaction rate of 279.50 mol /molRu-s, about 10 times the reaction rate (28.61 mol/molRu-s) of the impregnation method Ru/MgAl204 catalyst (mass fraction 1%), is 457 times of the present high active Ni catalyst NiCoMg/Al203 (0.61 mol/molNi-s) in the literature. The catalyst shows its excellent catalytic performance on methane dry reforming under the condition of 850 centigrade and high air speed up to 400000 mL/g-h (CH4:C02 =1:1). In the 600 h stability experiments, the conversion rate of methane and carbon dioxide (96.7% and 98.6) reached the thermodynamic equilibrium conversion rate, and the H2/CO ratio was close to the theoretical value of 1. The average size of Ru increased to 2.7 nm after the reaction, and the reaction rate was 1.5 times higher than that before the reaction. There was no measurable carbon accumulation on the catalyst. On the contrary, the Ru/MgAl204 catalyst prepared by the impregnation method (mass fraction 1%) quickly deactivated in 10 hours under the reaction conditions, and accompanied by a large amount of carbon. If the Ru mass fraction of the Ru/MgAl204 catalyst prepared by the PVD method was 0.07 wt.%, the obtained metal Ru particle size was smaller, and the reaction rate under the same condition was 35.72mol/mol Ru-s. results show that, at least in the range of metal Ru size below 2.7 nm, increasing the size of Ru nanoparticles can significantly increase its apparent activity, and in this size range, there is no carbon deposition on the catalyst, but the large particle Ru in the catalyst prepared by impregnation seems to be an important cause of the serious carbon deposition of the catalyst. The highly active and long-life long-life catalysts with extremely low metal ruthenium have significant industrial potential. To sum up, this paper successfully prepared Pt, Pd and Ru based catalysts with MgAl204 spinel as a carrier for the catalytic activity and high temperature stability of the catalyst. The catalytic performance of the high temperature resistant Pt, Pd and Ru catalysts in the catalytic combustion of CO and CH4 and in the CH4/C02 dry reforming reaction was further enhanced by using high oxidizing and reductive cerium oxide as a promoter. It was found that the larger size of noble metals in different sizes of precious metals have a better table in the noble metal species that are stable in different sizes. Therefore, it is expected that the proper increase in the size of high heat and stable noble metal nanoparticles is expected to further enhance its catalytic activity, especially at the low temperature activity of the CO and CH4 combustion reactions.
【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O643.36
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本文编号：1925445