Nanometer-sized titania particles were prepared by thermal decomposition of titanium tetraisopropoxide(TTIP) in a furnace type aerosol reactor. The possibility of controlling particle size and conversion with varying the processing variables such as input concentration, temperature, and residence time was tested. Addition of tert-butanol was proposed as a method of controlling particle size without distributing reaction conditions such as temperature or flow rate. Competition of tert-butanol with precursors made the growth of particles retarded and smaller particles were produced in an aerosol reactor.
Simple reaction coagulation(SRC) model that was valid at high concentration and low conversion. So direct surface reaction(DSR) model was introduced for estimating the size of product particles.
The size of titania particles was increased from 10 to 60 nm with increasing input concentrating of TTIP from $7 \times10^{-10}$ to $2 \times10^{-8}$ mole/ml at 430℃. The particle size was proportional to the 0.46th power of the precursor concentration. The exponent should be 0.171 when calculated by SRC model, which was not consistent with the experimental value. However, the proposed DSR model predicted the experimental result better than SRC model.
The size of product particles was decreased from 55 to 30 nm as the reaction temperature was increased from 250 to 550℃. The input concentration of TTIP was ranged from $6 \times10^{-9}$ to $9 \times10^{-9}$ mole/ml depending on the reactor temperature. The particle size was decreased with a-0.001 power of the temperature. The SRC model simulation predicted that the size should an increasing function of the temperature. However, DSR model predicts the decreasing trend of the particle size with increasing temperature. The conversion and the rate of decomposition of TTIP were increased with increasing reaction temperature.
The conversion of TTIP was increased with increasing residence time and, the conversion was reached to unity when the residence time was exceed 5 sec.
The kinetics of decomposition of TTIP was not affected by the types of inert carrier gases. However, the inert gas with the heavier molecular weight carried in more precursor. Therefore, the light carrier gas such as helium is favorable for controlling the size of particles in small range which were prepared at low concentration of precursor.
The particle size was controlled by the addition of tert-butanol which compared with the TTIP on the surface of titania. The mean size of the produced titania particles ranged between 40 and 80 nm and, the conversion of TTIP was varied from 0.75 to 0.95 depending on the input TTIP mole concentration and mole fraction of tert-butanol. However, the conversion of TTIP was not affected by ethanol with a lower reactivity on titania surface than TTIP.
The photocatalytic degradation of trichloroethylene(TCE) was carried out in order to observe the size effects of the prepared particles. Titania particles between 26 and 90 nm with only anatase phase were prepared. The UV absorbance of titania particle size was decreased from 90 to 26 nm. The TCE initial removal rate was increased linearly from $3.5 \times10^{-6}$ to $3.5 \times10^{-6}$ (mole TCE/㎡ min) as the particle size was increased.
