Preparation And Characterization Of CuO@TiO₂ Core-shell Structure And Study On Photocatalytic Oxidation Of Methanol To Methyl Formate

Methyl formate is an important chemical intermediate.The main industrial process for methyl formate production nowadays is under traditional thermal conditions using methanol as reactant.However,the two main drawbacks of the traditional processes are energy-consuming and severe.In this regard,studying and developing more efficient new and green photocatalytic route for methanol conversion to methyl formate has not only scientific significance but also a good prospect of application.In this study,different CuO@TiO₂ catalysts were prepared and characterized by SEM,the X-ray diffraction?XRD?,the UV-vis diffuse reflectance spectra?UV-vis DRS?,Thermal Gravimetric analysis?TGA?,N2 adsorption desorption,the Fourier transform infrared spectroscopy?FTIR?,the H2 temperature programmed reduction?H2-TPR?and the transmission electron microscopy?TEM?.The selective oxidation of methanol to methyl formate by oxygen under UV irradiation in the temperature range of 25-45 ? was investigated and the reaction mechanism was proposed as well.The main research content and results are listed as follows:1.The shell of the as-prepared core-shell catalysts was composed by small anatase TiO₂ nanocrystals with an average diameter around 13 nm with mesopores structure formed by TiO₂ particles.CuO dispersed uniformly inside the shell and formed heterojunction with TiO₂.The amount of Cu,the thickness of the shell,the pore structure and the number of hydroxyl can be adjusted by different method.2.The photocatalytic oxidation of methanol to methyl formate on the catalysts was investigated using the feed gas composed by 1?vol.?%methanol,0.5?vol.?%O₂ and 98.5?vol.?%N2.The results indicate that the amount of Cu has negligible influence on activity as Cu overloading cannot enhance the activity further,only the copper species close contact with the inner side of the shell was beneficial to the reaction.The thickness of the shell could be controlled by modulating the TNBT dosage.The activity was obviously affected by the thickness of shell,the methanol conversion being decreased but the methyl formate selectivity increased with thicker shell.The surface hydroxyls could be modulated by the calcination temperature.The activity had a very close relation the number of oxhydryl on the surface of TiO₂,more hydroxyls resulting in higher methyl formate selectivity.Finally,the time duration of the TNBT hydrolytic process significantly affected the degree of the TiO₂ crystallinity,with longer time,the anatase TiO₂ shown better crystallinity and gave better catalytic performance.3.The oxygen partial pressure,light intensity,activation time and reaction temperature remarkably influence the medthanol conversion and methyl formate selectivity.The methanol conversion increased but the selectivity of methyl formate decreased with increasing the oxygen partial pressure.The best ratio of methanol to oxygen is 2:1 in this model reaction.In addition,reaction temperature and light intensity shown a positive correlation relationship with activity.And the performance of catalysts tended to be stable after a two hour photoactivation process.4.Heterojunction was formed between the shell TiO₂ and CuO at the inner side of the shell.Photoelectrons can be excited from the valence band to the conduction band of TiO₂ under irradiation of ultraviolet light,and then the electrons could psaa through the space-charge region around heterojunction to the conduction band of CuO,resulting in the reduction of CuO to metal Cu.At this stage,an Ohmic contact was formed between metal Cu and TiO₂.The O-H bonds of methanol molecule reacted with the hydroxyl on the surface of TiO₂ shell giving rise to methoxy and H2O,and methoxy was further oxidized by photo-generated holes to form coordinated formaldehyde.Finally,methy formate was produced when coordinated formaldehyde reacted with neighboring methoxy group on the surface of TiO₂.In the mean time,oxygen molecules were dissociated on the Cu surface to fill the oxygen vacancies on the surface of TiO₂ through spillover.And thus a complete redox circle was complished.