Study On Photocatalytic Properties And Mechanism Of AgCl

The photocatalysis phenomenon was discovered in the 1960's, since then, extensive investigations has been conducted. It is found that photocatalysis can be used to degrade inorganic or organic pollutants in the environment. Researches have shown that silver halides can be used not for only photography, but also for photocatalysis. In particular, AgCl is a new type of photocatalyst which was studied recently by researchers.Some researches had suggested that the semiconductor photocatalyst are able to degrade a wide range of organic pollutants in the environment through photocatalysis effectively and mineralize them into CO₂, water and certain types of small molecule organic materials. When a semiconductor absorbs the photon with enough energy, transion of electron from valence band to conduction band occers , and then a photoelectron (e-) and a hole (h+) generate, which are powerful reductants and oxidants, and are capable to lead to surface catalyst reactions. Titanium dioxide is one of the semiconductor photocatalysts which has been paid extensive attentions. The electron-hole pair on AgCl photocatalyst was found by scientists during the water splitting processes under the UV radiation which was similar to TiO₂. So it was deduced that the photocatalytic mechanism of AgCl used as photocatalyzer to degrade the organic contamination in solution may be the same as TiO₂.In the paper, the AgCl photocatalyst was synthesized by adding of a small excess amount of AgNO₃ to an NaCl aqueous solution with continuous stirring. Characterizations of as-prepared AgCl sample before and after irradiation were carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM) photographs and UV-visible diffuse reflectance spectroscopy (DRUVS) , and the results showed that AgCl was stable and had not been destroyed in the photocatalysis. But after irradiation there are many silver nanoparticles generated on the surface of the AgCl particles, the broad distribution of the silver particle size makes it have strong absorption in the visible-light region, which makes the AgCl photocatalyst use sunlight better. The adsorption and photocatalytic degradation of organic dyes including methyl violet and the direct sky blue 5B by AgCl was investigated. At the same time, the blank tests without the catalyst were done as control. The results showed that the adsorption isotherm for methyl violet and the direct sky blue 5B on AgCl in the dark exhibited a typical IUPAC type II isotherm shape. It was found that the photocatalytic activity of AgCl catalyst in aqueous under the UV and visible light irradiation was very good; the pH had very little impact on AgCl photocatalytic activity; the increase of catalyst dosage was able to improve the photocatalytic effect; the higher the dyestuff concentration was, the smaller the degradation rate was. Via the UV-visible absorption spectrum of dye solutions at different degradation times, the photocatalytic degradation of methyl violet with AgCl photocatalysis was more thorough, but an intermediate product with absorption peak at 400nm wavelength was found and could not be eliminated totally during the photocatalytic degradation of the direct sky blue 5B.Through the study of the effect of ethanol on the photocatalytic activity of AgCl, and the fluorescence spectra of benzene-1, 4-dicarboxylic acid solution with AgCl before and after irradiation, we research the mechanism of using AgCl as the photocatalyst. The results showed that, photocatalytic mechanism of AgCl is not the same as TiO₂, which produced the powerful oxidizing hydroxyl radicals (?OH), but because of producing the chlorine radicals (Cl?).