生物油催化重整制氢中积碳形成机制和催化剂关系的研究

Steam reforming of organics is still the main route to produce hydrogen in large scale in industry both currently and in the near future. How to selectively produce hydrogen and how to avoid the coke formation are the two key challenges in steam reforming. This project intend to cope with these two key problems via the understanding of the relationship between the molecular structures of organics and their catalytic behaviours in steam reforming and the design of coke-resistant catalysts. Various organics can be used or have been used as the feedstock to produce hydrogen via steam reforming. However, what is the relationship between their catalytic behaviours and their molecular structures? Understanding of this can help us to predict the catalytic behaviours of an organics in steam reforming to choose the appropriate conditions to avoid the side reactions to occur. More importantly, it provides very useful information for steam reforming of bio-oil as bio-oil is a very complex mixture of organics with distinct molecular structures. This project tries to relate the molecular structure of the organics with their catalytic behaviours in steam reforming, extracting the fundamental relationships between them.Design of the coke-resistant catalyst is another target in this project. Due to the incomplete steam reforming, coke is always formed, leading to the rapid deactivation of reforming catalysts. However, the water and the CO2 formed in steam reforming are the favourable factors to eliminate coke formation via gasification. Thus, the coke-resistant catalysts have to be able to adsorb and activate the water and CO2 efficiently. The La2O3 will be used as the support to enhance the adsorption of water and CO2 in this project. The catalytic behaviours of Ni and Co will be compared in terms of the activity, selectivity, stability and their ability to gasify coke or coke precursors. In addition, noble metals modified transition metal catalysts also will be prepared as the noble metals have much better activity to active hydrogen. Hydrogen is also a favourable factor to gasify the coke precursors. Moreover, the micro-kinetics for the formation of coke on catalysts will also be investigated. The effects of molecular structures on the formation of the coke precursors, the tendencies of coke formation, the position of coke and the morphology of coke will be investigated as well.

积碳是水蒸气重整生物质裂解油（生物油）制氢中的一个瓶颈问题。本课题重点研究生物油中主要组分的分子结构和重整催化剂的性质与水蒸气重整反应中积碳形成的关系。建立这些有机分子分子结构与它们在水蒸气重整反应中催化行为（反应活性、产物分布和积碳）联系的普遍规律，针对不同有机分子的水蒸气重整反应的特点提出相应的适宜条件来避免积碳等副反应的发生。选择氧化镧等载体来提高催化剂对水和二氧化碳等的吸附活化来提高催化剂抗积碳的能力。研究重整反应中活性金属（过渡金属和贵金属）催化行为差别来对活性金属进行优化选择和改性以提高其对积碳的气化能力。此外，课题将对积碳形成的微观动力学进行深入研究。对水蒸气重整不同有机分子过程中积碳形成的前驱物、在催化剂表面分布特点和积碳的性质进行研究。了解催化剂失活的机理并探索催化剂再生的有效方法。

有机生物质水蒸气重整目前仍然是工业领域大规模制氢比较成熟的技术路线，高的转化率、高的氢选择性、强的抗积碳性能是该技术面临的三个核心问题。本项目针对这三个问题，系统构建和制备了单组分、双组分和三组分催化剂，考察了活性中心物种行为差异与协同作用对催化性能改善，并取得了一些较好的研究结果：(i) 对于生物质重整反应来说，单组分Ni金属是非常普遍应用的活性组分，然而Ni基催化剂经常面临着积碳引起的催化剂失活问题的困扰。本项目采用一种催化剂再生新策略，即在失活的Ni/SiO2催化剂表面再沉积少量Ni代替原有的有氧燃烧策略，再生后催化剂活性和稳定高于原催化剂。后负载的Ni物种的粒子尺寸更小（原尺寸17.5nm, 再生后尺寸9.5nm），分散度更高（原分散度9.4%, 再生后分散度19.3%）。(ii) 本项目通过优化双组分过渡金属Ni-Co催化剂制备方法实现对活性组分结构与性能的调控，研究结果显示，共浸渍催化剂Ni-Co和分步浸渍催化剂Co/Ni、Ni/Co在乙醇水蒸气重整反应中选择性、稳定性、抗积碳性能都与表面活性物种的粒子尺寸和分散度有着很好的对应关系：Ni-Co, Co/Ni和Ni/Co催化剂在乙醇重整反应中转化率依次为68.7%, 50.9%和36.6%；载体表面活性金属物种分散度依次为31.5%, 29.1% 和 28.0%；载体表面活性金属物种粒子尺寸依次为4-7nm，5-8nm和7-10nm，实验事实证明共浸渍工艺更有助于获得高分散度、高活性负载型催化剂。(iii) 本项目设计并制备了Cu-Co-Zn三组分过渡金属催化剂，结果显示，在醋酸水蒸气重整制氢反应中，三组分催化剂活性和抗积碳性能均高于双组分(Cu-Zn, Cu-Co, Co-Zn)和单组分催化剂(Cu, Co, Zn)，这可归结于三种活性组分之间的协同作用。Cu, Co, Zn三种活性中心物种在醋酸水蒸气重整制氢反应中扮演着不同的作用，Cu是水气变换反应过程中抑制CO形成的重要活性中心物种；Co是有机物及其中间体重整的重要活性中心物种，支配着整个催化反应的活性；Zn扮演着催化反应“开启开关”的角色，特别是低温下的重整催化反应。这三种金属活性中心的合理组合和协同作用，表现出了高效稳定的醋酸水蒸气重整制氢反应性能。