Study On The Catalytic Reactivity Of Titanium Dioxide Through Phenol Oxidation

Titanium dioxide(TiO₂) belongs to the family of transition metal oxides. It has various applications. Besides used as an advanced white pigment, it is a catalytic material due to its chemical stability, high activity and low cost heretofore. It is widely used as catalyst or support in heterogeneous catalysis and heterogeneous photocatalysis. Recently, our research group succeed in preparing TiO₂ whisker with high surface area. The size of the TiO₂ whisker is about micron and it has nanostructures. It exhibits good photocatalytic activities equivalent to P25 which is the best photocatalyst, when applied in methyl orange system, chloroform system and DMF system. Moreover it is favorable for separation. However, the study for TiO₂ whisker still rest on the apparent photocatalytic efficiency. Many problems such as the mechanism of high activity, the performance in heterogeneous catalysis, and so on, haven't been deeply investigated. These restrict TiO₂ whisker in improvement and application. The oxidation of phenol is a complex process with various intermediate products. There are different products for different degree. For example, dihydroxybenzenes are produced at partial oxidation stage(hydroxylation of phenol); phenol wastewater is disposed of at complete oxidation stage(photocatalytic degradation of phenol). Hence, we can explore the activity of catalyst with oxidation of phenol by controlling oxidation degree. The main purpose of this thesis is trying to explore the performance of TiO2 whisker from the point of view of heterogeneous catalyst and heterogeneous photocatalyst. At the same time, we hope to find a novel catalyst of hydroxylation. To explore the activity of TiO₂ whisker using hydroxylation of phenol, the effects of various conditions on reaction and doping with rare earth element were discussed. The results show that at suitable conditions the conversion of phenol was 29.87% , selectivity of catechol was 41.25%, selectivity of hydroquinone was 40.37% with TiO₂ whisker as the catalyst. It is better than that of catalyst used on industry. Rare earth element La had an important influence on the distribution of main products, the mole ratio of catechol to hydroquinone varied from 1:1 to 16:1. TiO₂ whisker may be a hopeful novel hydroxylation catalyst. Hydroxyl free radical (HO?) and hole are both the main reactive oxidants in photocatalysis. There exists dispute all the while that oxidation occur by whether indirect oxidation via hydroxyl free radical or directly via hole. In this thesis, the photocatalytic reactivity of TiO2 whisker had been investigated with photocatalytic degradation of phenol from the point of view of hydroxyl free radical and hole. The results show that TiO2 whisker has good photocatalysis properties equivalent to P25. Because the morphology of the nanoporous particles in the micrometer-size range is favorable for separation, TiO2 fiber is a good candidate for the application of industry. In this thesis, the contribution of HO? in the process of phenol degradation was confirmed by adding HO? scavengers that is a simple method and hole effect was observed by adding hole scavengers. Furthermore, the quantum yield of hole generation during TiO2 whisker photocatalysis estimated by iodide ion oxidation is about 3.3 times higher than that of P25 with the absence of O2. Compared with the result of photocatalysis experiment when dissolved O2 exists, it demonstrates the higher stability of holes of TiO2 whisker which may arise from the elaborating structure of TiO2 whisker. This suggests TiO2 whisker is more suitable for the reaction which main oxidation species is hole. As a result of dynamics research, datum of phenol degradation was more fit with exponential model and the phenol degradation in low concentration can be predicted by the exponential model.