新型Ni催化剂上的乙二醇液相重整制氢反应研究

【作者】 朱玲君；

【导师】 闫世润； 乔明华；

【作者基本信息】 复旦大学 ， 物理化学， 2008， 硕士

【摘要】 乙二醇液相重整反应（Aqueous Phase Reforming,APR）是一条极具发展潜力的制取H₂和低碳烷烃等替代能源的新路线。Ni催化剂不但价格低廉,而且具有较强的C-C键断裂能力和较高的水煤气变换反应活性,所以Ni催化剂在APR反应中的应用受到了催化工作者的广泛关注。本文系统研究了一种新的催化剂制备方法对Ni催化剂催化性能的影响。采用制备传统Raney Ni催化剂用量的1/10的碱抽提Raney Ni-Al合金,得到了一种新型的不易着火的NP Ni催化剂（Non-pyrophoric Ni）。表征结果显示,NP Ni催化剂主要由金属态的Ni以及水铝矿和三羟铝石等Al（OH）₃物相组成,该催化剂可以看成是Ni-Al（OH）₃复合催化剂。在乙二醇液相重整反应100h后,催化剂中的水铝矿和三羟铝石转变成片状的薄水铝石,同时Ni晶粒增大并且金属态的Ni部分发生氧化生成Ni（OH）₂。在相同的反应条件下,由于水合氧化铝对Ni晶粒的稳定作用,在NP Ni催化剂上乙二醇转变成气体产物的转化率比之Raney Ni催化剂高40-52%。在NP Ni催化剂上具有较高的H₂选择性,并且气体产物中的CO浓度较低,是因为水合氧化铝的存在有利于水的活化解离从而促进了水煤气变换反应。采用与NP Ni催化剂相同的制备方法抽提急冷Ni-Al合金,得到了NPRQ Ni（Non-pyrophoric Rapidly Quenched Ni）催化剂。在乙二醇液相重整反应中,由于NPRQ Ni催化剂具有较高的活性比表面,在NPRQ Ni催化剂上的催化活性比之RQNi催化剂提高了一倍;由于水合氧化铝对NPNi催化剂的稳定作用等抑制了Ni催化剂的氧化和流失,使得NP Ni催化剂在反应100h后仍能保持82%的初活性。采用浸渍法对NP Ni催化剂进行修饰,得到了NP NiSn催化剂。Sn修饰有利于H₂的生成,在NP Ni₇Sn催化剂上,当乙二醇液相重整反应达到稳态时,H₂选择性可达85%,比之Raney Ni催化剂上升了80%左右。H₂-TPD表征结果表明,添加Sn修饰后,NP NiSn催化剂中出现了新的活性位,而这些活性位可能是导致H₂选择性上升的主要原因。在乙二醇液相重整反应中,以KOH作为CO₂的吸收剂,采用一步法制取得到不含CO和CO₂的高纯H₂。在乙二醇水溶液中添加KOH可以明显提高H₂的TOF值而降低烷烃的TOF值。当KOH的浓度为2 mol L^-1时,H₂的得率上升至151%,接近理论得率（167%）,气体产物中H₂的含量可达到98%以上,其余气体为甲烷。这是因为K元素的存在有利于水煤气变换反应以及KOH吸收了CO₂,从而促进了水煤气变换反应。KOH的添加还可以明显提高NP Ni催化剂的稳定性,在反应36h后,乙二醇的转化率仍为100%,比之反应刚开始时的转化率几乎没有变化;乙二醇转变成气体产物的转化率为90%,比之反应刚开始时只降低了5%。催化剂稳定性的提高可归因于K元素对Ni催化剂的稳定作用抑制了Ni晶粒的长大。更多还原

【Abstract】 Aqueous phase reforming（APR） of ethylene glycol is a promising new route for the catalytic production of high-purity hydrogen for fuel cells and light alkanes.Since Ni shows high activities for C-C bond scission and methanation,moderate WGS activity,and is much less expensive than the noble Pt,Ni-based catalysts have been extensively studied in APR of ethylene glycol.In the present work,Ni catalysts prepared from a new method for the aqueous phase reforming of ethylene glycol were investigated.A non-pyrophoric Ni catalyst（NP Ni） was prepared by alkali leaching of a Ni₅₀Al₅₀ alloy using only～1/10 of the amount of NaOH required for the preparation of the conventional Raney Ni catalyst.Characterizations reveal that the as-prepared NP Ni catalyst can be looked as a Ni-Al（OH）₃ composite catalyst with Ni in the metallic state and Al（OH）₃ in forms of gibbsite and bayerite.After 100 h on stream in APR of ethylene glycol,phase transformation of gibbsite and bayerite to flake-like boehmite occurred,along with the growth of Ni crystallites and partial oxidation of metallic Ni to Ni（OH）₂.Under identical reaction conditions for APR of ethylene glycol,the NP Ni catalyst is about 40-52%more active than Raney Ni in terms of the conversion of ethylene glycol to gas products,which is attributed to the stabilizing effect of hydrated alumina on Ni crystallites.The higher selectivity toward H2 and the lower concentration of CO in the product gas on the NP Ni catalyst are attributed to the activation of water by hydrated alumina which is beneficial to the WGS reaction.The non-pyrophoric rapidly quenched Ni catalyst（NPRQ Ni） was prepared by alkali leaching of a rapidly quenched Ni₅₀Al₅₀(RQ Ni₅₀Al₅₀) alloy using the same method as the preparation of the NP Ni catalyst.In APR of ethylene glycol,the NPRQ Ni catalyst exhibited an catalytic activity doubled that of the RQ Ni catalyst due to its larger active surface area,and maintained～82%of its initial activity even after 100 h on stream due to its tolerance to oxidation and metal leaching.Sn-modified NP Ni catalysts（NP NiSn） were obtained by impregnating of NP Ni catalyst with SnCl₄.In APR of ethylene glycol,the addition of Sn facilitated the H₂ production.On the NP Ni₇Sn catalyst,H₂ selectivity of 85%was achieved at the steady state,which is about 80%higher than that on Raney Ni catalyst. Characterization by H₂-TPD has shown that new active sites were formed on the NP NiSn catalysts,and the increased H₂ selectivity may be ascribed to such active sites. H₂ without CO and CO₂ was produced through one-pot H₂ production from APR of ethylene glycol with KOH as the CO₂ absorbent.The formation of H₂ was enhanced by the addition of KOH to the ethylene glycol solution.Gas product composed of 98 mol%of H₂ and 2 mol%of methane obtained at high conversion of ethylene glycol when the concentration of KOH was up to 2 mol L^-1,and the H₂ yield was dramatically improved to 151%,which is closed to the theoretical yield（167%）. The WGS reaction was promoted by potassium and the absorption of CO₂ with KOH, which leads to the higher H₂ selectivity.The presence of KOH also enhanced the stability of NP Ni catalyst.The conversion of ethylene glycol and the conversion of ethylene glycol to gas products on NP Ni catalyst were about 100%and 90%, respectively,even after 36 h on stream due to the suppressing of Ni sintering by potassium.更多还原