Fe3O4(001)表面合成氨机理研究

Magnetite play an increasingly prominent role in heterogeneous catalysis, drug delivery and spintronics. The surface structure has been well studied by surface analysis techniques and Fe3O4 has attracted continued interest in the past decades. Synthesis of ammonia is one of the most important industry catalytic reactions. Numerous studies on the kinetics and mechanism of this important reaction have been performed on the Fe single crystal surface. They believed this is an atomic reaction and the N2 dissociation is the rate determining step. There is a big gap between Fe and Fe3O4 single crystal surface. We plan to use study the adsorption and reaction of N2 and H2 on the Fe3O4(001) surface with temperature programmed desorption (TPD), scanning tunneling microscopy (STM) and density functional theory calculations. Besides, we want to control the surface defect density by change the surface preparation condition and to get the relationship between N2 dissociation rates, synthesis of ammonia rate and defect density. By this work, we want to get the dynamic information, such as adsorption structure, active sites, elementary reaction process, and the mechanism of synthesis of ammonia.

Fe3O4是一种重要的金属氧化物，被广泛应用到工业催化、生物制药和自旋电子器件等领域。表面分析技术的进步使得Fe3O4(001)表面的原子结构和电子结构逐渐清晰，Fe3O4的成为表面研究的新热点。合成氨工业作为基础化工之一，在国民经济中具有举足轻重的地位。此前，在Fe单晶表面开展的合成氨机理研究认为氮原子和氢原子是反应的活性物种，N2在铁触媒表面的解离是合成氨反应的决速步骤。但Fe单晶与Fe3O4存在较大差异，因此本项目计划使用程序升温脱附谱（TPD）和扫描隧道显微镜（STM）并结合密度泛函理论（DFT）计算研究N2和H2在Fe3O4(001)表面的吸附和反应，特别是吸附构型、反应活性位、表面基元过程等动力学信息，从微观水平理解合成氨机理和金属氧化物表面的催化过程。此外，通过改变样品制备方法控制表面缺陷浓度，研究缺陷和温度对合成氨效率的影响，为构建新型高效催化剂提供指导。

本项目按照原定计划，发展了一套高精度、低成本的精密程序升温脱附（TPD）系统，并利用TPD技术、光电子能谱技术及密度泛函理论计算，研究了Fe3O4（001）表面合成氨机理。通过对NH3在Fe3O4表面的吸附位点及反应过程的详细研究，发现NH3在该表面的吸附为吸热过程，NH3容易吸附在八配位的Fe原子表面并失去一个H原子，形成NH2与OH结构。TPD结果发现有NO生成，且NH3在Fe3O4（001）表面可能以多聚物或环形结构吸附，表现出与H2O在该表面的吸附不同的吸附规律，对Fe3O4表面合成氨机理有了进一步的认识。此外，通过本项目我们对小分子在Fe3O4表面的吸附和催化反应动力学有了进一步的理解和认识，掌握了Fe3O4（001）重构表面的制备和表征方法。