【作者】 刘说；

【导师】 李永丹； 林跃生；

【作者基本信息】 天津大学 ， 工业催化， 2008， 博士

【摘要】 目前世界大多数能源来自于化石燃料。但化石燃料的储藏量是有限的,人们认识到石油、天然气和煤能够为人类提供能量的时间是有限的。相对于化石燃料,氢的燃烧不排放污染环境的物质,同时基于燃料电池技术,采用氢为燃料可以构成一个能源系统。所以氢被认为是未来能量的载体。当前大部分的氢气是由化石燃料的水蒸气重整生产的。由化石燃料制氢会排放温室气体并造成大气污染。虽然有很多不同的原料可以用于生产氢气。但是在众多的液体原料中,乙醇有很多的优点,如可再生、容易运输、可生物降解、毒性低等。因此乙醇制氢的相关研究具有重要的意义。采用吉布斯自由能最小原理对乙醇氧化水蒸气重整制氢进行了热力学分析。结果表明氧气的加入有效地减少了反应的吸热。本文给出了系统达到自热的条件。发现自热的反应温度是进料H2O/EtOH和O2/EtOH的摩尔比的函数。在700 K及其以下可以在宽广的进料H2O/EtOH和O2/EtOH摩尔比条件下实现自热,但是如此低的温度下有活性的催化剂是很难找到的。900 K时,在一定的H2O/EtOH和O2/EtOH的摩尔比范围内系统可以实现自热。1100 K,在O2/EtOH < 0.9范围内则不存在H2O/EtOH比可以实现自热。在自热条件下氢气的平衡摩尔数在900 K时达到最大,从而900 K被认为是适宜工作条件。对于非自热条件,重点考察了700,900和1100 K,H2O/EtOH和O2/EtOH分别在1.0-10.0,0.0-0.90等条件下氢、甲烷、一氧化碳和碳的平衡摩尔数。提出了提高氢气产率,降低甲烷产率和避免积炭生成的条件。由Feitknecht化合物母体出发制备出具有不同负载量的镍基催化剂,对乙醇水蒸气重整反应的活性,稳定性和抗积碳性能进行了研究。结果表明,通过改变负载量,镍的分散度能被控制。发现加入镁可以改变载体的酸性。当催化剂组成为NiMgAl-0.5时表现了最好的活性,氢的选择性和抗积碳性能。催化剂反应100小时后没有失活,但该催化剂表面却有碳纤维生成,镍颗粒也轻微烧结。成功地用反相微乳液方法制备出了Feitknecht化合物母体,并将所得到的母体经焙烧后应用于乙醇水蒸气重整反应。与共沉淀和浸渍法制备的具有相同金属负载量的催化剂相比,结果表明反相微乳液法获得的催化剂有较大的比表面积、和较高的镍分散度。该催化剂表现了最好的活性和稳定性,最少量的积碳。由Feitknecht化合物母体出发,制备出镧和铈掺杂的镍镁铝催化剂。将其应用于乙醇水蒸气重整制氢反应。BET结果表明镧和铈掺杂的镍镁铝催化剂比表面积相对于未掺杂的镍镁铝催化剂有明显增加,但是镧和铈掺杂量存在最佳值。氧化物的XRD结果表明一小部分的镧进入到镍镁固溶体中。而铈发生聚集并分布在外表面。TPR结果说明镧和铈的掺杂提高了催化剂的还原能力,其中镧的作用更加明显。XPS数据表明对比于铈掺杂的催化剂,镧掺杂的催化剂被还原成零价镍的量更多。反应后催化剂的XRD结果表明镧掺杂的催化剂中镍晶粒的直径最小,说明镧能更好的抑制镍晶粒的烧结。活性数据表明镧和铈掺杂的催化剂提高了催化活性和稳定性。更多还原

【Abstract】 Most of the present world energy supplys comes from fossil fuels. But reserves of fossil fuels are limited. People recognize that reserves of petroleum, natural gas and coal are finite for providing energy. Unlike fossil fuels, hydrogen can burn cleanly, without emitting any environmental pollutants. FCs use hydrogen as a fuel which results in the formation ofwater vapor only and thus they provide clean energy. H2 is considered to be the energy carrier of the future and could have an important role in reducing environmental emissions. Currently, most of the H2 is produced via the steam reforming of fossil fuels. However, hydrogen production from fossil fuels is always associated with the emission of greenhouse gases and local pollutants. In nature, H2 can be produced from different resources. Among the liquid H2 resources, ethanol is a good candidate for several reasons. Ethanol is renewable and is becoming available easily. It is easy to transport, biodegradable, and low in toxicity.A thermodynamic analysis of ethanol oxidative steam reforming was carried out with a Gibbs free energy minimization method. The addition of oxygen lowers the enthalpy of the system and favors the heat recycle. Thermal-neutral conditions are obtained, at which the heat released from exothermic reactions makes up exactly for the requirement of the endothermic reactions. Thermal-neutral temperature is a function of the feed composition. For 700 K thermal-neutral conditions can be reached in a wide range of H2O/EtOH and O2/EtOH ratios. However, this condition is not practical because that a catalyst active in the range is difficult to find. At 900 K, a given condition range exists for thermal-neutral operation. At 1100 K, no thermal-neutral condition exists in the condition range examined. Under thermal-neutral conditions, the equilibrium moles of hydrogen, methane, carbon monoxide and carbon are examined. 900 K is favorable for hydrogen production where the maximum equilibrium mole of hydrogen appears. For the non- thermal-neutral operations, a detailed calculation is presented on a range of reaction conditions, i.e. temperature 700– 1100 K, H2O/EtOH and O2/EtOH feed ratios in 1.0– 10.0 and 0.0– 0.9, respectively. The equilibrium moles of H2, CH4, CO and C are examined. Hydrogen is favored at low O2/EtOH ratio, high H2O/EtOH ratio and 900 K. Methane is not favored at high temperatures and high O2/EtOH and H2O/EtOH ratios. Carbon formation can be avoided by adjusting the reaction condition in a reasonable range.The ethanol steam reforming over nickel supported catalysts with different Ni loadings prepared from Feitknecht compound precursors was studied. By varying the Ni loading, Ni dispersion and nickel phases could be controlled. It was found that the amount of Mg in the catalyst affect the acidity of support. It was shown that NiMgAl-0.5 catalyst obtained from Feitknecht compound precursor showed the best activity, selectivity and resistance to carbon deposition. The deposited carbon has a filamentous structure after 100 hour, and the size of Ni particles only increased slightly.Feitknecht compound precursor for preparing mixed oxide catalyst has been successfully synthesized by a novel method. And the mixed oxide obtained from the above method was applied in ethanol steam reforming. Furthermore, for comparison, catalysts prepared from conventional coprecipitation and impregnation methods had the same composition with the catalyst prepared from the new method. The high BET surface area of the catalyst obtained from reverse microemulsion method enhanced the nickel dispersion and the nickel surface area. The catalyst obtained from reverse microemulsion exhibited the best activity, stability, and least carbon deposition. Feitknecht compound precursors for preparing lanthanide promoted catalysts have been applied to ethanol steam reforming. The results showed that the surface area increased greatly for lanthanide promoted catalysts. And there was an optimal value for lanthanide promoted contents. XRD of the mixed oxides indicated that a small amount of La3+ was doped into the Ni-Mg-O solid solution and CeO2 was conglomerated on the surface of the support. TPR results revealed that the presence of lanthanide elements enhanced the catalyst reducibility, which was most evident with lanthanum promoted catalysts. XPS data indicated that lanthanum promoted catalysts exhibited higher Ni0 concentration on surface area compared to cerium promoted catalysts. XRD of the used catalysts indicated that lanthanum prevented the growth of crystallite sizes. Activity experiments showed that adding small amounts of lanthanide elements could improve the catalytic activity and stability significantly.更多还原