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随着现代社会与工业发展的不断进步,人类对能源的需求日益增加。化石能源燃烧引发的环境污染问题,迫切要求人类开发清洁高效的新能源。氢气具有热值高、能量利用率高、来源广泛等优点,且燃烧产物为水,不会造成环境污染,被视为是21世纪最具应用前景的清洁能源[1-2]。目前工业上常用的制氢方法包括甲醇蒸汽重整制氢[3]、电解水制氢[4]、煤气化制氢[5]、天然气或裂解石油气制氢[6]、氨分解制氢[7]等。其中氨分解制氢具有工艺简单、制得氢气纯度高且不会产生COx、投资少、成本低等优点,受到广泛的关注和研究[8]。
氨分解制氢反应主要采用高温催化裂解,反应方程如下:
该反应为可逆的吸热反应,据热力学计算可知,在常压、400 ℃时,理论上可实现NH3几乎完全转化。然而由于该反应活化能非常高,受动力学控制,实际上在常压、700 ℃及不添加催化剂条件下,NH3的转化率不到10%[9]。因此,氨分解制氢反应的关键在于催化剂的选择与构建。
目前,应用于氨分解的催化剂主要包括贵金属催化剂(Ru、Ir、Pt等)、过渡金属催化剂(Fe、Ni、Co、Mo等)以及FeCx、MoCx、FeNx等碳化物催化剂和氮化物催化剂[10-13]。尽管贵金属催化剂(特别是Ru基催化剂)具有很高的氨分解反应活性,但其资源稀少、价格昂贵,从而限制了大规模的应用。而过渡金属催化剂资源丰富、价格低廉,并且具有一定的氨分解活性,因此得到了广泛的关注。其中Fe基催化剂相较于其他过渡金属催化剂具有较高的氨催化活性,催化剂的原料来源广泛,相对于Ni基等催化剂成本更低并且具有更好的稳定性,已经成为现今研究和应用的重要催化剂之一。目前报道的Fe基催化剂主要集中于负载型催化剂、核壳结构催化剂以及助剂修饰的催化剂[14]。Hu等[15]通过固相离子交换法制备了一系列脱铝ZSM-5分子筛负载的Fe基催化剂用于氨分解制氢,表征测试结果显示当负载量为9%时,Fe物种高度分散在载体表面,并且在650 ℃时几乎实现NH3完全转化;当Fe负载量进一步增加到12%时,催化剂活性出现了明显的下降,可能是因为较高的负载量使得载体表面形成结块状的Fe颗粒,降低了活性Fe物种的分散度从而抑制了反应活性。Mathias等[16]研究了具有核壳结构的α-Fe2O3@多孔氧化硅催化剂的氨分解催化性能,实验中首先采用水热法合成α-Fe2O3纳米颗粒,其次通过碱性条件下水解硅源(正硅酸乙酯)进行颗粒的包覆从而合成所需材料。该核壳结构催化剂在空速120000 cm3·(gcat·h)−1、温度750 ℃的条件下能稳定运行33 h,保持转化率约为80%,催化性能相对于物理混合的α-Fe2O3和氧化硅催化剂有了明显提升。为了进一步提升催化剂在氨分解反应中的活性和稳定性,助催化剂的使用是一种有效的方法。碱金属和碱土金属都是比较理想的氨催化分解反应助剂,碱金属能够改善金属活性物种周围的电子环境,促进N原子在催化剂表面的吸附,进而提高催化剂的活性;能够调节催化剂的几何结构和电子结构,提高催化剂的催化活性和热稳定性[17]。Jaclyn等[18]研究了以La2O3和Ce-Zr氧化物为助剂对Al2O3负载的Fe-Mo双金属催化剂在氨分解反应中的性能影响,活性测试和表征技术的结果显示,La2O3修饰的催化剂具有最高的反应活性,在700 ℃时NH3的转化率达到99%以上,其原因在于La2O3使催化剂具有一定的表面碱性,有利于反应产物中氮的脱附。目前的研究中,负载型催化剂面临的主要问题是可负载的活性Fe物种的含量受到限制,提高负载量容易使表面活性金属发生团聚,从而导致催化剂的氨分解活性降低,并且负载型催化剂的热稳定性有待进一步提高。对于核壳结构催化剂,其合成过程通常较为复杂,因此在一定程度上限制此类催化材料在氨分解制氢中的应用。
六铝酸盐是一种具有独特热稳定性的催化材料,其通式为AAl12O19(A位离子为碱金属离子、碱土金属离子或稀土金属离子),在工业上常被用于甲烷催化燃烧反应和重整制合成气等反应中。六铝酸盐其纵向有序、横向无序的立体结构决定了它耐高温、稳定性强的特点,同时晶格的特殊排布可以容纳其它离子在A位和B位(Al)的取代,而过渡金属取代后所得催化剂中活性组分间呈原子级均匀分布、具有较强的协同效应,表现出更高的催化活性。在本研究中,以Fe进行B位Al的部分取代,催化剂的A位选择能够改善活性组分几何结构和电子结构的稀土金属La,合成了一系列不同Fe取代量的LaFexAl12-xO19六铝酸盐催化剂,通过取代量的改变调节催化剂的结构,系统考察了不同Fe取代量对催化剂结构和氨催化分解活性的影响。
LaFexAl12-xO19六铝酸盐催化剂上氨分解制氢性能
Hydrogen production from ammonia decomposition over LaFexAl12-xO19 hexaaluminate catalysts
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摘要: 采用并流共沉淀法制备了系列Fe取代的LaFexAl12-xO19(LaFex,其中x=0,2,4,6,8)六铝酸盐催化剂,分别考察了Fe取代量、反应温度、空速对NH3分解反应活性的影响。研究结果表明,升高温度有利于提高NH3的转化率;NH3分解反应活性随着Fe取代量的增加先增加后降低,其中LaFe4具有最佳的催化性能,在550 ℃即可实现NH3的完全转化,并能稳定运行50 h以上。采用X射线衍射仪(XRD)、氮气吸脱附(BET)、程序升温还原(TPR)和X射线光电子能谱(XPS)等技术手段对催化剂进行了表征,结果表明,氨分解活性应该与反应温度、催化剂的氧化还原性能以及催化剂中八面体配位的Fe3+的数量密切相关。Abstract: A series of Fe-substituted LaFexAl12-xO19 (where x=0, 2, 4, 6, 8) catalysts were prepared by the co-precipitation method, and the impacts of Fe substitution, reaction temperature and space velocity on reaction activity were investigated. The research results showed that raising temperature was beneficial to improve the conversion of NH3. Meanwhile, the activity of NH3 decomposition increased firstly and then decreased accompanied with the augment of Fe substitution. LaFe4 catalyst exhibited the best catalytic performance among the catalysts, which could achieve complete conversion of NH3 at 550 °C and run stably for more than 50 h. X-ray diffraction (XRD), N2 adsorption-desorption (BET), Temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) were used to characterize the catalysts. The research results showed that the activity of NH3 decomposition is closely related to the reaction temperature, redox ability of the catalysts and the amounts of Fe3+ in octahedral coordination in the catalysts.
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表 1 LaFex催化剂的织构参数
Table 1. Textural properties of LaFex catalysts
催化剂名称
Catalysts比表面积/(m2·g−1)
Surface area孔体积/(cm3·g−1)
Pore volume孔尺寸/nm
Average pore size晶粒尺寸/nm
Crystallite sizeLaAl 27.9 0.15 19.4 18.1 LaFe2 14.9 0.05 11.4 37.1 LaFe4 11.1 0.03 10.8 40.7 LaFe6 8.0 0.01 7.3 42.7 LaFe8 5.6 0.01 8.4 54.5 表 2 LaFex催化剂中不同配位Fe3+比例对比
Table 2. Comparison of different coordination Fe3+ ratios in LaFex catalysts
催化剂
CatalystsFe3+/(Fe2++Fe3+) Fe3+oct/ Fe3+ Fe3+oct/(Fe2++Fe3+) LaFe2 0.70 0.41 0.29 LaFe4 0.80 0.74 0.59 LaFe6 0.89 0.65 0.58 LaFe8 0.91 0.49 0.44 LaFe4稳定性后 0.77 0.73 0.56 Fe3+oct,八面体配位Fe3+。 Fe3+oct,Octahedral coordination of Fe3+ -
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