【作者】 王瑞；

【导师】 袁晓俭； 邓辉球；

【作者基本信息】 湖南大学 ， 凝聚态物理， 2007， 硕士

【摘要】 在异相催化过程中气体在过渡族金属表面的吸附是一个重要步骤,所以一直是表面科学领域的热点,无论是实验还是理论方面人们都作了大量的工作。而理论也越来越多的被证实是一个有效的研究手段。在解释吸附的微观机理,说明实验现象,以及去除实验结论的不确定性方面发挥着越来越重要的作用。吸附动力学是研究化学吸附和脱附速率以及其他各种因素对吸附的影响。对于吸附动力学的研究,有助于我们了解哪些因素影响吸附或脱附速度,以便得出机理性的反应速率方程;可以从吸附和脱附速率的基本方程出发,使我们对等温式有进一步的了解;将过渡态理论用于研究化学吸附和脱附速率,可以从分子的性质和一些基本常数来估计速率的大小,从而使我们对动力学吸附的机理有较为本质的了解。本文根据实验和理论关于氢在金属表面吸附的热力学性质的结果为基础,应用蒙特卡罗(Monte Carlo)方法,分别模拟巨正则系综中氢在金属镍、钯(111)和(100)表面的动力学吸附过程。根据氢在这两种金属的不同表面的吸附特性的不同,我们建立了不同的吸附动力学模型。模型中关于氢在金属表面的动力学吸附主要包括以下三个过程:氢分子的解离吸附,氢原子在金属表面的扩散以及氢分子的在结合脱附。并对模型建立过程中的一些细节对于模型计算结果的影响进行了比较和分析。应用我们提出的模型,我们分别对吸附氢原子在表面的吸附形貌、吸附等温线、附着系数、吸附速度等动力学吸附特性进行了计算和分析。模型对于氢原子表面吸附形貌的计算结果表明,在Ni, Pd(111)表面,氢原子主要吸附在Fcc洞位上,而在(100)表面氢原子主要吸附在四重洞位上。氢原子在Pd(111)表面吸附时具有一定的规则结构,而在其他三个表面上吸附时,均表现出无规则结构。按照理论的分析,氢在(111)和(100)表面的饱和覆盖率分别为2ML和3ML,我们的计算结果:氢在Ni(111)和(100)表面的饱和覆盖率约为0.93ML,在Pd(111)和(100)表面的饱和覆盖率分别约为0.92ML和1.40ML,这都与实验符合的较好,同时也验证的模型的正确性。对于吸附等温线的计算结果表明,压强的增大导致覆盖率的升高,而温度的升高使覆盖率降低。附着系数会随覆盖率的升高而降低,同时也引起吸附速度的降低。统计结果表明当体系达到平衡时,氢-金属系统处在动态平衡,吸附速度和脱附速度相等。更多还原

【Abstract】 The adsorption of gas on transitional metal surfaces is an elementary step in the process of heterogeneous catalysis. Nowadays it has been one of the most important subjects in the field of surface science and numerous studies have been carried out in experiments as well as in theories. Theoretical calculation has been playing an essential role in the aspects of helping us to understand the adsorption mechanisms and explain the experimental phenomenon. Adsorption kinetics can be used to study the effect on adsorption caused by chemisorption, desorption rate and other factors.The research of adsorption kinetics facilitates us to find out the factors which can affect the rate of adsorption and desorption, educe the equation of reaction rate. Also it is helpful for us to have a further understanding of adsorption isotherm formula, based on the elementary function of adsorption rate and desorption rate. Furthermore, with the characters of the molecules and some fundamental constants, it can help us to well understand the essence of the mechanism of adsorption kinetics by applying the transition state theory to the research of the adsorption rate and desorption rate. According to the thermodynamic properties which given by experiments and theories, the Grand canonical Monte Carlo method is applied to simulate the adsorption kinetics of hydrogen molecule on the surface of nickel and palladium in this thesis. For the different properties of adsorption when hydrogen molecules are adsorbed on the surfaces of Pd, Ni (111) and (100), different models are presented to simulate the process of adsorption. Three processes are considered when hydrogen molecules adsorb on the surfaces of metal: dissociative adsorption of hydrogen molecules, the diffusion of hydrogen atoms, the desorption of hydrogen molecules. Surface structure of hydrogen/metal systems, equilibrium adsorption isotherms, sticking coefficient and the rates of adsorption and desorption are calculated with our models.The present calculations suggest that hydrogen atoms are mainly adsorbed at Fcc hollow sites on the surfaces of Ni (111) and Pd (111). However, on the surfaces of (100), hydrogen atoms are mainly adsorbed at 4-fold hollow site. Hydrogen atoms have an ordered structure when they are adsorbed on surface of Pd (111); while on other three kinds of surface, hydrogen atoms form a disordered structure. The maximum coverage of hydrogen given by our model are H/Ni (111) and H/Ni (111) 0.93ML, H/Pd (111) 0.92ML, H/Pd (111) 1.4ML, respectively. All these results agree well with experimental results, which prove that the models we propose are reasonable. The results of equilibrium adsorption isotherms suggest that the higher the temperature, the lower the coverage will be; and the higher the pressure, the higher the coverage will be. The sticking coefficient will reduce when coverage increases, reducing the adsorption rate. Statistics show that H-Metal system reaches equilibrium when the adsorption rate equals to the desorption rate.更多还原