Preparation And Characterization Of Titania Supported Copper Catalysts And Study On Photocatalytic Oxidation Of Methanol To Methyl Formate

Methyl formate, known as the universal intermediate, is a kind of important industrial chemicals and is usually produced from methanol. In recent years, it is of practical significance to convert methanol into methyl formate due to the excess production capacity of methanol in China, which has become a hot research issue in the field of chemistry.In this study, three series of catalysts Ag-Ni/TiO2, Ag-CuO/TiO2 and Cu/TiO2 have been prepared by the metal sol method. The characterizations and activity tests of the catalysts have been carried out and the catalyst with good catalytic performance has been screened eventually,which is Cu/TiO2.Then, a series of Cu/TiO2 catalysts with different molar ratios of Cu:TiO2 have been prepared and characterized by X-ray diffraction(XRD), temperature-programmed reduction(TPR), the UV-Vis diffuse reflectance spectra(UV-Vis DRS), the transmission electron microscopy(TEM), X-ray photoelectron spectroscopy(XPS), and energy dispersive spectrometer(EDS). At the low temperature from 25 to 45℃ and under UV irradiation, the photocatalytic performance of the catalyst for the selective oxidation of methanol to methyl formate has been investigated. Besides, the reaction mechanisms has been briefly studied and discussed. The main research contents and results are listed below:1、The results of characterizations of the three kinds of catalysts show that copper species in the catalyst Cu/TiO2 is completely oxidized into CuO during the preparation process. The CuO nanoparticles are uniform in size and finely dispersed without the agglomeration. The average diameter of the nanoparticles is approximately 4.5 nm. The photocatalytic performances of three catalysts Ag-Ni/TiO2, Ag-CuO/TiO2 and Cu/TiO2 for selective oxidation of methanol to methyl formate have been investigated at the low temperature from 15 to 45℃ under UV irradiation with the feed gas composed by 1(vol.)% methanol, 0.5(vol.)%02 and 98.5(vol.)% N2. The results indicate that Cu/TiO2 exhibits the best photocatalytic performance. A series of Cu/TiO2 catalysts with different Cu loadings have been prepared and characterized to investigate the morphology, structure, metal content and valence, etc.The photocatalytic activities of Cu/TiO2 catalysts for selective oxidation of methanol into methyl formate have been investigated with the feed gas of 3 vol.% methanol and 1.5 vol.% oxygen. It has been found that when the catalyst with 7wt.% copper loading exibits the best catalytic performance, with 56.4mmol/g/h of the methyl formate rate at 45℃.2、The morphology, valence, absorption wavelength and peak position of hydrogen consumption of Cu species in the 7% Cu/TiO2 sample before, after reaction and during the reaction are characterized by HRTEM, XPS, UV-visible and H2-TPR. The results show that in the as-prepared catalyst, supported copper exists in the form of CuO. Exposed to ultraviolet irradiation during the reaction, CuO is reduced to Cu. After exposed in air for a week after reaction, Cu in the used 7% Cu/TiO2 sample is reoxidized into CuO and Cu2O.3、The influences of oxygen partial pressure, methanol partial pressure, light intensity and reaction temperature on the methanol conversion and the selectivity of methyl formate have been investigated. It is found that the methanol partial pressure should not be too high, less than 3% being suitable. The oxygen partial pressure has remarkable influence on the methanol conversion and the decrease of methyl formate selectivity. The highest methanol conversion and methyl formate selectivity can be achieved at 2/1 of the ratio of methanol/02. The catalytic activity is propotional to the light intensity, and increases with increasing the reaction temperature at the temperature range from 15 to 45℃.4、The heterojuncture can be formed at the interface between CuO and titania. At the initial stage of UV irradiation, the photo-excited electrons in the conduction band from the valence band of titania move through the space charge region of the heterojuncture to the conduction band of CuO. They are inclined to reduce CuO to Cu. When Cu and titania are electronically connected, an Ohmic contact between Cu and titania can be formed. The photo-excited electrons in the conduction band from the valence band of titania are more readily to transfer to the Cu surface, which are responsible for the dissociation of oxygen molecules. Methanol molecules can be chemisorbed on the surface of titania and partly dissociated to methoxy groups at room temperature. The holes in titania can oxidize the coordinated methoxy into coordinated formaldehyde. Oxygen molecules could be dissociated to oxygen atoms on the metal surface, spill out, and eventually fill out the vacancy of the appearances of the catalysts.5、The power-law reaction dynamics equation is determined as following after parameter estimation: where CmF,CM,Co and I are the concentration of methyl formate, the concentration of methanol, the concentration of oxygen and the light intensity, respectively.