Photocatalytic Activity And Mechanism Of Copper-Based Materials

The global energy supply and the related environmental issues are among the biggest challenges being confronted by chemists and technologists of our time. Semiconductor photocatalytic technology use solar energy to initiate the chemical reaction, and with many advantages, such as mild, cheap, and without causing secondary pollution, will have a great application in environment pollution remediation and energy regeneration. However, the quantum efficiency of semiconductor photocatalysis achieved so far is still very low for practical application. In this case, ways for improving the photocatalytic activity of semiconductor have attracted increasing interest.Base on the above elucidation, this thesis focused on th- enhancement of the semiconductor photocatalytic activity. Catalysts with high activity were synthesized through surface modification of semiconductor with a suitable co-catalyst. And the mechanism of the surface modification can result into highly enhancement in photocatalytic activity had been studied in-depth. In this thesis, results were divided into five sections as follows:(1) Several studies have shown that surface modification of TiO2 with CuO or calcium phosphate can result into enhancement in the photocatalytic activity for organic degradation. In this work, we report on a synergism between cation and anion of copper phosphate on the photocatalytic activity of TiO2, for phenol degradation in aerated aqueous suspension under UV light at wavelengths longer than 320 nm. Samples were prepared by mixing TiO2 and copper phosphate powders in isopropyl alcohol, followed by dryness at 90℃. As copper phosphate loading in the sample increased, the rate of phenol degradation first increased, and then decreased. The optimum loading of copper phosphate onto TiO2 was about 0.1wt%, at which the catalyst activity not only was about 1.9-3.4 times that of bare TiO2 (anatase, rutile and their mixture), but also exceeded that of the modified TiO2 with CuO or calcium phosphate. During five repeated tests, the catalyst activity was stable, without detectable leaching of cupric and phosphate ions into aqueous solution. Solid characterization with X-ray diffraction, N2 adsorption and electron paramagnetic resonance (EPR) spectroscopy revealed that copper phosphate particles at low loading were highly dispersed onto TiO2 as a kind of clusters, whereas TiO2 phase in different samples remained unchanged in terms of the crystal structure, surface area and crystallinity. Upon exposure to UV light, the EPR signal of Cu2+ became weaken in N2, but almost unchanged in air. Moreover, for uptake of 2,4-dichlorophenol in aqueous solution, copper phosphate-modified TiO2 showed a higher capacity than bare TiO2, and CuO or calcium phosphate modified TiO2. It is proposed that Cu(II) acts as an electron scavenger and phosphate as an organic sorbent. Their co-operation would promote electron and hole transfers, consequently enhancing the efficiency of charge separation, and increasing the rate of phenol degradation.(2) Hydrothermal reaction between Cu(NO3)2 and Na2WO4 at 170℃, followed by sintering at 500℃, resulted in the formation of stoichiometric CUWO4 at pH 5.2, while the reactions below and above pH 5.2 gave a mixture of CUWO4 with WO3 and CuO, respectively. For organic degradation in water under visible light, W-rich and Cu-rich samples were less and more active than CuWO4, respectively. A maximum activity was observed with the sample prepared at pH 8.5. Furthermore, this Cu-rich sample shown an activity greatly changing with its sintering temperature, and reaching a maximum at 600℃. A possible mechanism responsible for the observed activity difference among the samples is proposed, involving the interfacial charge transfer between CUWO4 and WO3 or CuO.(3) Several papers have shown that CUWO4 is active under visible light for water oxidation at an applied potential bias, and for organic degradation in an aerated aqueous suspension. In this work, we report that the observed reduction of O2 on the irradiated CUWO4 is a multi-electron transfer process with the formation of H2O2. More importantly, the surface modification of CuWO4 with 1.8wt% of CuO can increase the activity by approximately 9 times under UV light, and by 5 times under visible light, for phenol degradation in aerated aqueous suspension. The catalyst was prepared by a hydrothermal reaction between Cu(NO3)2 and Na2WO4, followed by thermal treatment at 773 K. High resolution transmission electron microscope revealed that triclinic CuWO4(40 nm) were covered by monoclinic CuO (4 nm). Through a combination of photo- and electrochemical measurement, a plausible mechanism responsible for the activity enhancement is proposed, involving an interfacial electron transfer from CuO to CUWO4, and an interfacial hole transfer from CuWO4 to CuO.(4) CuWO4/WO3 composite electrode observed highly faradaic efficiency for oxidation of H2O has been reported. However, the charge transfer between the WO3 and CUWO4 is still unclear. In this work, we have found that simple mixing of WO3 and CuWO4 can result into significant enhancement in activity for phenol degradation in the presence of H2O2 under visible light. The enhanced activity varied with CuWO4 content in the mixed oxide. The maximum activity of the composite catalysts was 1.0wt% CuWO4/WO3, whose activity, in relative to bare WO3 and CuWO4, increased to 2.1 and 4.3-fold, respectively. Through electrochemical measurement, we demonstrated that the excited electron in the CuWO4 conduction band could transfer to WO3, which could promote the charge separation of WO3 and CuWO4, and consequently accelerated phenol degradation, but there was no interfacial hole transfer between WO3 and CuWO4.(5) Surface modification of TiO2 with a certain content of CuO had been prepared through impregnation. However, the photocatalytic activity of TiO2 for CO2 reduction was not significantly enhanced by deposition of CuO under UV light in a CO2 aqueous suspension at pH = 3.8. Through electrochemical measurement of dark cathodic current under N2 or CO2 atmosphere, we found that the current intensity of TiO2 electrode measured under CO2 was larger than that under N2, but as modification of TiO2 electrode with CuO, the cathodic current under CO2 and N2 was overlap. These results indicate that surface modification of TiO2 with CuO could not improve the photocatalytic activity of TiO2 for CO2 reduction indeed. However, it is worth mentioning that CO2 could be selective photocatalytic reduced to produce methanol by modification of TiO2 with CuO.