氧化钨表面氢吸附机理的第一性原理研究

发布时间：2018-06-20 00:49

本文选题：氧化钨 + H_2吸附　；参考：《江西理工大学》2017年硕士论文

【摘要】：目前许多学者研究了氧化钨氢还原制取超细钨粉的工艺,但是其机理仍然不统一和明确。关于氧化钨表面氢吸附机理和动力学过程的研究较少,因此有必要对其进行研究,从而进一步了解氧化钨氢还原动力学机理。基于实验条件的限制,传统的材料研究设备都不能被用于研究H原子和H_2分子在氧化钨表面的吸附状况,所以人们无法通过实验的方法来研究H原子和H_2分子与氧化钨表面的相互作用。针对此问题,本文以Materials Studio7.0软件为计算平台,采用第一性原理计算从原子和电子角度探讨了氧化钨表面性质及其氢吸附机理,同时采用TG-DSC实验研究了氧化钨氢还原的动力学过程以及动力学机理,从而为氧化钨氢还原工业生产提供一定的理论基础和指导。研究结果表明:(1)WO_3晶胞为立方结构,而W_(20)O_(58)和W_(18)O_(49)晶胞为单斜不规则非化学计量比结构。此三种氧化钨的晶胞都是由W占据中心位置以及O占据顶点位置的八面体组成,其带隙宽度分别为0.587、0.8、0.75eV,且W_(20)O_(58)晶胞和W_(18)O_(49)晶胞都表现出导电性的金属行为;WO_3、W_(20)O_(58)和W_(18)O_(49)都表现出电子离域性较强,成键强,W和O原子的电子态密度重叠多,W-O共振较强,共价键较多的特性。(2)WO_3(001)、W_(20)O_(58)(010)和W_(18)O_(49)(010)都含有WO终止面和O终止面两种表面原子结构,都是通过改变W-O键的键长和W-O-W的键角来达到表面弛豫的目的。(3)WO_3(001)四种氢吸附构型中,H-O1c-H吸附构型的吸附能最小(-3.684eV),H-O键最短(0.0968nm),H失去电子数最多(0.55e),此吸附构型最稳定。两个H原子分别与O1c原子形成H-O化学键,且吸附反应使得在表面生成了一个H_2O分子结构,同时产生了一个表面氧空位。(4)W_(20)O_(58)(010)六种氢吸附构型中,O-V-O1c吸附构型最稳定,其吸附能为-3.11eV,H-O键长为0.0983nm,H原子为O原子提供的电子数为0.55e;H_2分子垂直吸附在W_(20)O_(58)(010)氧终止表面后解离,且两个H原子与O1c原子形成化学键,最终反应生成H_2O分子和产生一个表面氧空位。(5)W_(18)O_(49)(010)四种氢吸附构型中,P-O1c、V-O1c两种吸附构型都很稳定,吸附能分别为-6.13、-6.807eV,H-O键键长在0.0978~0.0983nm范围内,H原子为O原子提供的电子数在0.56~0.58e范围内;H_2垂直以及水平吸附在W_(20)O_(58)(010)氧终止表面后都会解离,且生成的两个H原子与O1c形成化学键,反应生成H_2O分子和产生一个表面氧空位。(6)氧化钨氢还原反应表观活化能均小于20 kJ·mol-1且还原体系的失重量随着时间呈线性变化,H_2的扩散步骤是还原反应的限制性环节,同时还原过程中可能出现晶型的变化,很好地论证了氧化钨氢还原动力学机理。
[Abstract]:At present, many scholars have studied the process of producing ultrafine tungsten powder by hydrogen reduction by tungsten oxide, but its mechanism is still not uniform and clear. There are few studies on the mechanism and kinetic process of hydrogen adsorption on the surface of tungsten oxide. Therefore, it is necessary to study it so as to further understand the mechanism of the hydrogen reduction kinetics of tungsten oxide. The traditional material research equipment can not be used to study the adsorption of H atoms and H_2 molecules on the surface of tungsten oxide. So it is impossible for people to study the interaction between H atoms and H_2 molecules on the surface of tungsten oxide. For this problem, this paper uses Materials Studio7.0 software as the computing platform and uses the first principle calculation. The surface properties and hydrogen adsorption mechanism of tungsten oxide are discussed from the atomic and electronic angles. At the same time, the kinetics and mechanism of hydrogen reduction of tungsten oxide are studied by TG-DSC experiment. The theoretical basis and guidance are provided for the industrial production of tungsten oxide hydrogen reduction. The results show that: (1) WO_3 cell is a cubic structure, and W_ (20) ) O_ (58) and W_ (18) O_ (49) cells are monoclinic irregular nonstoichiometric structures. The three tungsten oxide crystals are composed of W occupying center position and eight sides of O occupying the vertex position, the band gap width is 0.587,0.8,0.75eV, and W_ (20) O_ (58) and W_ (18) O_ (49) cells all exhibit conductive metal behavior; WO_3, W_ (2) 0) O_ (58) and W_ (18) O_ (49) show strong electron delimitability, strong bond formation, high overlapping of electron state density of W and O atoms, strong W-O resonance and more covalent bonds. (2) WO_3 (001), W_ (20) O_ (58) (010) and W_ (18) O_ (49) (010) all contain WO terminating surface and two surface atomic structure, all by changing the bond length of the bond bond and In the four hydrogen adsorption configurations of (3) WO_3 (001), the adsorption energy of the H-O1c-H adsorption configuration is minimal (-3.684eV), the H-O bond is the shortest (0.0968nm), the H loses the most electron number (0.55e), and the adsorption configuration is the most stable. The two H atoms form the H-O chemical bond with the O1c atom respectively, and the adsorption reaction makes a H_2O on the surface. The molecular structure produces a surface oxygen vacancy at the same time. (4) W_ (20) O_ (58) (010) six hydrogen adsorption configurations, the O-V-O1c adsorption configuration is the most stable, its adsorption energy is -3.11eV, the H-O bond length is 0.0983nm, the H atom provides 0.55e for O atom, and the H_2 molecule dissociates after the W_ (20) O_ (58) (010) oxygen terminated surface, and two atoms are with the O. C atoms form chemical bonds, and the final reaction generates H_2O molecules and produces a surface oxygen vacancy. (5) in the four hydrogen adsorption configurations of W_ (18) O_ (49) (010), the two configurations of P-O1c and V-O1c are all very stable, and the adsorption energy is -6.13, -6.807eV, and H-O bonds in 0.0978~ 0.0983nm, and the number of electrons provided by H atoms is within the range. H_2 is dissociated vertically and horizontally after the W_ (20) O_ (58) (010) oxygen terminated surface, and two H atoms are formed to form a chemical bond with O1c to produce a H_2O molecule and produce a surface oxygen vacancy. (6) the apparent activation energy of the hydrogen reduction reaction of tungsten oxide is less than 20 kJ. Mol-1 and the weight loss of the reduction system is linearly changed with time, H_ The 2 diffusion step is the limiting link of the reduction reaction. At the same time, the change of crystal form may appear during the reduction process. The kinetic mechanism of the hydrogen reduction of tungsten oxide is well demonstrated.
【学位授予单位】：江西理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O647.31;TF841.1

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