First-principles Study Of Hydrogen Adsorption Mechanism On Tungsten Oxide Surface

At present,many scholars have studied the technology of preparing ultrafine tungsten powder by hydrogen reduction of tungsten oxide,but its mechanism still remains unclear and uniformed.To understand the mechanism of hydrogen reduction kinetics further,it is necessary to study the mechanism and kinetics of the adsorption of hydrogen on the surface of tungsten oxide.Due to the limitation of experimental conditions,traditional material research equipments can be used to study the case of H atoms and H₂ molecules adsorbing on the surface of tungsten oxide.So it is impraticable to study the kinetic mechanism of H atoms and H₂ molecules with tungsten oxide surface by traditional experimental methods.To solve this problem,this paper employed Materials Studio7.0 software as the platform and conducted first principles calculation to study the crystal structure of tungsten oxide and adsorption mechanism of hydrogen atoms absorbing on tungsten oxide.Also,the results of TG-DSC experiment are also introduced to analyse the kinetics of hydrogen reduction of tungsten oxide and kinetic mechanism,which provides theoretical basis and guidance for industrial production of hydrogen tungsten oxide reduction.The results are shown as below:?1?The WO₃ crystal is a cubic structure,while the W₂₀O₅₈ and W₁₈O₄₉ crystal structure is irregular non-stoichiometric monoclinic structure.All the W atoms in both crystals occupy the central position and O atoms occupy the vertex position,which consist of the octahedron.The band gaps are 0.587,0.8,0.75 eV,respectively.W₂₀O₅₈ and W₁₈O₄₉ crystal show conductive metal characteristic.Both of them show following characteristics: the electrons delocalization is strong,the bond is strong,and the electron density of W and O atoms overlaps,W-O resonance is strong,covalent bond is in majority.?2?WO₃?001?,W₂₀O₅₈?010?and W₁₈O₄₉?010?all contain the WO termination surface and O termination surfaces.By changing the W-O bond length and the W-O-W bond angle,surface relaxation can be realized.?3?Four kinds of hydrogen adsorption configurations of WO₃?001?,the absorption energy of configuration of H-O1c-H is the smallest?-3.684 e V?with the shortest bond length of H-O bond?0.0968nm?,and hydrogen atoms losing the most amount of electrons?0.55e?,indicating that the H-O1c-H absorption configuration is most stable.Two H atoms and O1 c atom form chemical bond,and a H₂ O molecule is generated,leaving an oxygen vacancy on the surface after absorption reaction.?4?Among six hydrogen adsorption configurations of W₂₀O₅₈?010?,the adsorption capacity of O-V-O1 c is the most stable with the adsorption energy of-3.11 eV,the H-O bond length of 0.0983 nm and the atomic number of H atom of 0.55e;H₂ vertically adsorbs on W₂₀O₅₈?010?oxygen termination surface and then dissociates.Two H atoms and O1 c atom form a chemical bond,and a H₂ O molecule is generated,leaving an oxygen vacancy on the surface after absorption reaction.?5?Among four kinds of hydrogen adsorption configurations of W₁₈O₄₉?010?,the adsorption configurations of P-O1 c and V-O1 c are more stable with the adsorption energy of-6.13 e V and-6.807 eV,respectively,and the bond length in the range of 0.0978⁰.0983 nm,the number of electrons that H atoms offering O atoms in the range of 0.56⁰.58e;H₂ vertically adsorbs on W₁₈O₄₉?010?oxygen termination surface and then dissociates.Two H atoms and O1 c atom form a chemical bond,and a H₂ O molecule is generated,leaving an oxygen vacancy on the surface after absorption reaction.?6?The activation energy of hydrogen reduction reaction on tungsten oxide surface was less than 20kJ/mol and the weight loss changes linearly with time.The H₂ diffusion step is the resctrictive link of the reduction reaction,and changes of crystal occur in the reduction process,which clearly demonstrate the kinetic mechanism of tungsten oxide by hydrogen reduction.