摘要:
采用基于密度泛函理论的第一性原理平面波超软赝势方法,在广义梯度近似下,研究了W20O58晶胞、W20O58(010)表面结构及其氢吸附机理.计算结果表明:W20O58晶体理论带隙宽度为0.8 eV,为间接带隙,具有金属性.W20O58晶体中W—O共振较强,以共价键居多.W20O58(010)表面有WO终止(010)表面和O终止(010)表面,表面结构优化后使得W—O键长和W—O—W键角改变,从而实现表面弛豫.分别计算了H2分子吸附在WO终止(001)表面和O终止(001)表面的WO-L-O1c,WO-V-O1c,WO-L-O2c,WO-V-O2c,O-L-O1c和O-V-O1c六种吸附构型,其中WO-L-O1c,WO-V-O1c和WO-L-O2c这三种吸附构型不稳定;而WO-V-O2c,O-L-O1c和O-V-O1c这三种吸附构型都很稳定,H2分子都解离成两个H原子,吸附能均为负值,分别为-1.164,-1.021和-3.11 eV.WO-V-O2c吸附构型的两个H原子分别吸附在O和W原子上;O-L-O1c吸附构型的两个H原子,一个与O原子成键,另一个远离了表面.其中O-V-O1c吸附构型最稳定,两个H原子失去电子,为O原子提供电子.分析其吸附前后的态密度,H的1s轨道电子与O的2p,2s轨道电子相互作用,均形成了一些较强的成键电子峰,两个H原子分别与O1c形成化学键,最终吸附反应生成了一个H2 O分子,同时产生了一个表面氧空位.
Abstract:
With the development of modern industrial technology, tungsten products prepared from traditional tungsten powder cannot meet the demands of industry. However, the properties of tungsten products produced from ultra-fine tungsten powder have been greatly improved: they have high strength, high toughness, and low metal plasticity-brittle transition temperature. Hence, it is necessary to carry out theoretical research of the micro-adsorption dynamics during hydrogen reduction of W20O58, which is beneficial to synthetizing ultra-fine tungsten powder. In this article, to comprehend the crystal characteristics of W20O58 (010) surface and provide the theoretical reaction law for hydrogen reduction on W20O58 (010) surface, the absorption mechanism of H2 molecule on W20O58 (010) surface is studied by the first-principles calculation based on density functional theory in a plane wave pseudo-potential framework. The results show that the indirect band gap of W20O58 is 0.8 eV, indicating that it has metallic characteristic. The W20O58 (010) surface has different terminations, i.e., WO-terminated (010) surface and O-terminated (010) surface. After the geometrical optimization of the two surfaces, the W—O bond length and bond angle of W—O—W are both changed. In addition, six absorption configurations of H2 on W20O58 (010) surface, including WO-L-O1c, WO-V-O1c, WO-L-O2c, WO-V-O2c, O-L-O1c and O-V-O1c, are chosen to be investigated. The calculation results show that the WO-L-O1c, WO-V-O1c and WO-L-O2c absorption system are unstable, while the WO-V-O2c, O-L-O1c and O-V-O1c absorption configuration are stable. When H2 molecule is dissociated into two H atoms, the absorption energies of the three stable configurations are -1.164 eV, -1.021 eV and -3.11 eV, respectively. It is obvious that the O-V-O1c absorption configuration is the most stable one. The analysis of density of states reveals that the 1s state of H atom interacts with the 2p and 2s states of O atom. The outermost O1c atom of O-terminated (010) surface contains an unsaturated bond, which results in the formation of bonding between two H atoms and O1c atom. As a result, an H2O molecule is formed and an oxygen vacancy on the surface is generated after absorption reaction. By combining experimental observations with simulation calculations, the mechanism of hydrogen reduction of W20O58 can be revealed from a microscopic view.
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