铜铝尖晶石催化甲醇重整反应过程中结构演变的物理化学基础

The Cu-Al spinel catalysts, which could gradually release the active copper during the methanol reforming reaction for hydrogen production, show excellent performances. Previous investigations reveal that the releasing process is greatly affected by the microstructure of Cu-Al spinel. However the thermodynamic parameters as well as the kinetics mechanisms of the copper releasing process during the reaction remain to be resolved. This project aims at solving the relations of preparation methods, structures, reaction mechanisms and the catalysts performances of the Cu-Al spinel, using combined experimental and theoretical investigation methods. The Cu-Al spinel samples with different micro-structures would be prepared via adjusting the synthetic parameters. The relations between the micro-structure and the stability of the samples, reaction mechanisms over Cu-Al spinel of different micro-structure, as well as the thermodynamic parameters and kinetics mechanisms of the surface structure evolution of the catalysts in the reaction will be investigated on the basis of characterization and theoretical simulation. The mechanisms for the promoter influences to the catalysts structure evolution would also be discussed to acquire the relation of surface structure phase transformation law and gradual released catalytic activity of Cu-Al spinel in the reaction process. Giving consideration of preparation methods, reaction condition, as well as catalytic performances and so on, we will build the dynamic ‘gradual releasing’ model of the Cu-Al spinel catalysts and find out the multiple factors equilibrium points. This project would provide theoretical support and fundamental data for upgrading the performance of the Cu-Al spinel catalysts.

铜铝尖晶石在甲醇重整制氢反应中会逐步缓释活性铜原子并表现出优异的催化性能，研究表明催化剂的微观结构对缓释过程影响较大，目前对反应过程中铜原子缓释的热力学参数及动力学机制尚不清楚。本项目拟将实验研究与理论研究相结合，构筑铜铝尖晶石催化剂制备方法-结构-反应机理-催化效果之间的关系。通过调变催化剂制备参数，制备出具有不同微观结构的铜铝尖晶石催化剂，采用表征及理论模拟方法研究催化剂微观结构与其稳定性的关系、不同微观结构对反应机理的影响、及反应过程中催化剂表面结构演变的热力学参数和动力学机理，并探讨助剂在催化剂结构演变中的作用机制，获得铜铝尖晶石催化剂表面结构在反应过程中的结构相变规律与催化活性缓释的关联性；创建催化剂活性动态缓释模型，兼顾催化剂制备、反应条件、催化效果等因素，找到甲醇制氢尖晶石催化剂的多因素平衡点；为进一步提高尖晶石催化剂的性能提供基础数据和理论支持。

本项目采用理论和实验研究相结合的方法系统的研究了铜铝尖晶石催化剂的制备、催化反应及缓释机理，并评价了催化剂的性能。氧化铝表面Al3+被Cu2+取代在热力学上可行，且形成的Cu掺杂的γ-Al2O3界面增强了Cu与氧化铝表面的结合。高温煅烧下由于原子的热运动导致表面氧分子的形成和脱附；而氢氛围下是氢的吸附导致表面水的形成和脱附。氢氛围可以极大地促进了表面铜的还原‘缓释’。由于铜铝尖晶石表面的酸性较强，与碱性的金属团簇相互作用会发生电荷转移，而导致反应中被还原出的活性铜与表面相互作用较强，而不易团聚导致催化剂失活。Cun团簇的生长和聚集趋势为CuAl2O4<CuAlO2<γ-Al2O3<气相。水分子通过解离吸附方式吸附在尖晶石表面，在含有氧空位的尖晶石表面，水分子的O原子更倾向于填补O空位，解离的H原子与表面O形成两个表面内羟基。甲醇在尖晶石(100)和(110)表面分子吸附的吸附能分别为‒104和‒148 kJ/mol，与水分子在催化剂表面的吸附能相近。在(100)和(110)表面甲醇解离的反应路径相同：CH3OH→ CH3O→ CH2O→ CHO→ CO+ H2， (100)表面的决速步为CH3O→CH2O；而(110)表面的决速步是CHO→CO。过渡金属（Fe、Co和Ni）在CuAl2O4尖晶石的Al、Cu位点掺杂在热力学上是可行的。Ni更倾向于掺杂在尖晶石表面及体相的八面体位点，而Fe和Co更倾向于掺杂在尖晶石表面层的四面体位点，而在尖晶石体相中更倾向于八面体位点。与二元Cu-Al尖晶石相比，Mg掺入尖晶石晶格导致Al3+阳离子在四面体和八面体位点之间的分布发生变化，含镁尖晶石结构的铜释放速率显着下降，有利于更长时间地保持稳定的催化性能。与CuAl二元尖晶石催化剂相比，三元尖晶石氧化物能生成更小的纳米铜金属。Cu0.9Mg0.1Al3具有最高的活性和最好的催化稳定性。镍掺杂进入铜铝尖晶石时，随着焙烧温度的升高，催化剂的尖晶石含量增加，尖晶石晶粒增大，且比表面积下降。Cu NiAl 尖晶石在反应条件下逐渐释放活性铜而使得催化剂稳定性得到提升，Cu/Al =1/3时催化剂显示出最好的催化性能。本项目为甲醇重整铜铝尖晶石催化剂设计奠定理论基础。