Possible Mechanism Of TiO₂ Modification By Anions And Evaluation Of Photocatalytic Activity

Semiconductor photocatalysis has been studied more than 40 years,by using the clean and renewable solar energy as input energy,photocatalysis can be applied in organic contaminants degradation and H2 production without causing secondary pollution.So,semiconductor photocatalysis is expected to be a new technology for environmental pollutant treatment and clean energy regeneration.Among the many semiconductor photocatalysts,TiO2 is the most studied for its high efficiency,stability,non-toxic and so on.To further improve the photocatalytic performance of TiO2,people adopt some ways such as modification with noble metal or inorganic anions.But the effect of inorganic anios has always been controversial.Besides,the evaluation of photocatalytic activity is very important to semiconductor photocatalysis research.For this,we studied the effect of fluoride and phosphate on the degradation of phenol over anatase TiO2 quantificationally from the point of surface adsorption.And the effect of adsorption and solution absorbance on the photocatalytic reduction of Cr(VI)ion was also investigated.The thesis consists mainly of two parts,as below:In the first part,we synthesized anion-free anatase TiO2 by using a sol-gel method,and through a photochemical deposition method,Pt was loaded onto TiO2.All the samples were characterized by XRD,XPS and Raman spectroscopy etc.The effect of fluoride and phosphate on the photocatalytic degradation of phenol was examined.In a suspension at initial pH 5.2,on the addition of 0.1-30 mM anions,the rate constant of phenol degradation were all increased,and the positive effect of fluoride was larger than that of phosphate.The amount of anion adsorption over the solid was measured with Langmuir monolayer adsorption model,the results showed the amount of phosphate adsorption was larger than that of fluoride.Interestingly,there was a linear relationship between the increase of rate constant and the amount of anion adsorption,and the slope of fluoride was 2.7 fold than that of phosphate.These observations indicated that the positive effect of anions originated from the adsorbed anions on solid,and that fluoride was more active than phosphate.Photoelectrochemical experiments indicated that fluoride and phosphate were all beneficial for phenol oxidation,but had a negative and positive effect on water oxidation,respectively.Furthermore,Mott-Schottky plot showed that the flat band potential of TiO2 negatively shifted 159 mV and positively shifted 89 mV in the presence of fluoride and phosphate,respectively.The former favored orbital overlapping of phenol with TiO2 valence band,and the latter favored orbital overlapping of O2 with TiO2 conduction band,all of which promoted the interfacial charge transfers.The value of negative shift was larger than that of positive shift,then,fluoride had a larger positive effect than phosphate.In the second part,we had a preliminary research on the photocatalytic reduction of Cr(VI)ion used for evaluation of photocatalytic activity.The concentration of Cr(VI)ion was measured by a colorimetric method.Under the condition of pH 3.5,the adsorption isotherms of Cr(VI)ion over anatase TiO2 was measured.The results indicated that it's a multilayer adsorption,but in the range of monolayer adsorption,the Langmuir adsorption equation was still applicative.The effect of adsorption amount on the initial rate of Cr(VI)ion reduction was studied under the condition of ultraviolet radiation(?>320 nm)and pH 3.5.The results showed that the initial rate decreased with the increase of adsorption amount.However,after taking the solution absorbance into consideration,the revised initial rate increased and then decreased with the increase of adsorption amount.On the one hand,adsorption had a positive effect,and on the other hand,solution absorbance had a negative effect.When the positive effect was larger than the negative effect,the initial rate would increase.After the adsorption came to a plateau,the positive effect of adsorption was no longer increased and the negative effect of solution absorbance was still increased,and consequently,the initial rate started to decrease.