Preparation Of Bi₂WO₆ Based Catalysts And Study On The Photocatalytic Performance Of Partial Oxidation Of Methanol To Methyl Formate

Methyl formate?H?CO?OCH₃,MF?is an important chemical intermediate.The synthesis of methyl formate from methanol by photocatalysis is a new green route.Bi₂WO₆ is a new type photocatalytic material.It is innovative,and has important theoretical significance as well as great potential of application for its use in photocatalytic oxidation of methanol to methyl formate.In this study,Bi₂WO₆ catalysts were prepared by hydrothermal and solvent thermal methods,and the influence of preparing parameters on the structures and properties of the catalysts were investigated.Au/Bi₂WO₆ and Ag/Bi₂WO₆ catalysts with different metal contents were prepared by the photodeposition method.The catalysts were characterized by the X-ray diffraction?XRD?,UV-Vis diffuse reflectance spectra?UV-Vis DRS?,Scanning electron microscopy?SEM?,and the specific surface area and pore size distributio?BET?analysis.The catalytic performance of photocatalytic partial oxidation of methanol to methyl formate was investigated in the temperature range of 25-45?with ultraviolet light as the light source.The mechanism of the reaction was preliminarily explored by means of characterization analysis and activity test data.The results are as follows.1.The solvents,concentration of H⁺ions in solution,hydrothermal reaction temperature and hydrothermal reaction time have remarkable effects on the morphology,microstructure,specific surface area of Bi₂WO₆ and the catalytic performance of photocatalytic partial oxidation of methanol to methyl formate.The optimum preparation conditions for Bi₂WO₆ catalyst was:1.8M HNO₃ solution as the solvent,and the hydrothermal reaction carried out at 160?for 18 hours.2.The influence of metal loading on photocatalytic performance was investigated,and it was found that Ag/Bi₂WO₆ catalyst with 5%Ag content had the best catalytic performance in the photocatalytic oxidation of methanol.The methanol conversion rate was about 65%,and methyl formate selectivity was up to 90%.The Au/Bi₂WO₆ catalyst with 3%Au content had the best catalytic performance in the photocatalytic oxidation of methanol.The methanol conversion rate of about 55%and methyl formate selectivity of up to 88%.The Ag/Bi₂WO₆ catalyst was superior to Au/Bi₂WO₆ catalyst under the same condition.3.The highest methyl formate selectivity was reached at 2:1 of the theoretical ratio of methanol to oxygen.The methanol conversion and methyl formate selectivity could be improved to some extent by increasing the UV light intensity.4.For Ag/Bi₂WO₆ and Au/Bi₂WO₆ catalysts,the photoexcited e^-_CBB and h_Vb⁺were produced when the catalysts were irradiated under the ultraviolet light.The photoexcited electrons could transfer to metal sites through the interface between metal and the semiconductor due to the difference of Fermi energy levels between metal and Bi₂WO₆,which enhanced the reductivity of metal and facilitated the chemical adsorption and dissociation of oxygen.Methanol molecules adsorbed on the surface of the catalyst were dehydrogenated into methoxyl,and methoxyl was oxidized into formaldehyde by holes with strong oxidation ability.The formaldehyde reacted with adjacent methoxyl groups to form methyl formate.Oxygen molecules on the surface of the catalyst were dissociated by the negatively charged metal to fill the oxygen vacancy on the surface of the catalyst due to the consumption of hydroxyl group and promoted the reaction of the photocatalytic oxidation of methanol.