Semiconductor-catalyzed photochemical and thermal reactions of methanol and ethanol.

摘要

Photolyses of liquid methanol and ethanol have been carried out at 308, 313, 337, 365, 405, 436, and 530 nanometers in the presence of semiconductors as photocatalysts. The following semiconductors have been tested: titanium dioxide, silicon, cadmium sulfide, gallium phosphide, strontium titanate, cadmium selenide, cadmium telluride, samarium oxide, vanadium pentoxide, and tungsten oxide. The coating of platinum (1 weight percent) in titanium dioxide increased the catalytic activity of the titanium dioxide powder by about an order of magnitude.; At 313 nanometers, quantum yields of hydrogen from photolyses of methanol using suspensions of platinized titanium dioxide, cadmium sulfide, silicon, gallium phosphide, titanium sesquioxide, and strontium titanate powders were 0.34, 0.030, 0.027, 0.022, 0.017, and 0.0072, respectively. The quantum yield of hydrogen decreased to 0.061 when titanium dioxide was illuminated at 405 nanometers. Based on the dependence of the quantum yields of hydrogen on the light intensity, the estimated numbers of photoactive sites on platinized titanium dioxide and silicon powders were 5.74 {dollar}times{dollar} 10{dollar}sp{lcub}16{rcub}{dollar} and 1.52 {dollar}times{dollar} 10{dollar}sp{lcub}16{rcub}{dollar} sites per square meter, respectively.; The quantum yield of hydrogen decreased by about a factor of 10 when titanium dioxide powder was sintered into photoelectrodes. The surfaces of photoelectrodes were characterized by ultraviolet and visible reflectance spectroscopy, ultraviolet and x-ray photoelectron spectroscopy, and scanning electron microscopy including energy dispersive spectroscopy. The efficiency of titanium dioxide photoelectrodes decreased upon extended irradiation, possibly due to loss of active sites, such as the +3 oxidation state of titanium, as shown by ultraviolet photoelectron spectroscopy.; In the presence of platinized titanium dioxide powder, the catalytic dark reaction of methanol with oxygen was a diffusion-controlled reaction. The major product was formaldehyde; methyl formate, formic acid, and carbon dioxide were observed as secondary reaction products. Turnover numbers as high as 2200 per titanium atom were measured after 27 hours of reaction.; Based on the distributions of products and on the dependence of quantum yields of products on the wavelength and the light intensity, two reaction mechanisms for photocatalysis were proposed. Proposed mechanisms include photochemical reactions of adsorbed species, such as the methoxy radical, and reactions of photogenerated electrons and holes with the adsorbed alcohol.