Photocatalytic Oxidation of Gaseous Oxygenates using Thin Film TiO2
Dr. Jong-Nan Cheng
Dr. Ming-Chun Lu
光催化氧化過程，處理對象為異丙醇或丙酮，並未發現二氧化鈦活性衰減；若處理對象為正丙醇、丙醛或二甲基甲醯胺，則有活性衰減現象發生。進流濃度愈高、流量越大、氧含量越低、溼度越低，觸媒活性衰減越嚴重；處理對象不同，反應溫度的影響也不同，對氧化丙醛而言，溫度越高，觸媒活性衰退越嚴重；若對象是二甲基甲醯胺，則溫度越低，觸媒活性衰退越嚴重。處理二甲基甲醯胺後的二氧化鈦表面，含有酸類、醛類、胺類物質，也有NH4+及NO3-離子的存在，這些物種可能是造成觸媒活性衰退的原因。以四種氣相再生處理方式(Dry Air、Dry Air/UV、Wet Air/UV、O2/UV)與三種液相再生處理方式(H2O/UV、H2O2、H2O2/UV)進行觸媒再生，再生效果以後者為佳，其中以H2O2/UV效果最好，觸媒活性接近完全恢復。
The interest in heterogeneous photocatalysis to remove trace organic compounds present in air exhaust streams and in indoor environments is intense and increasing. The attractive advantages of this technology are: (i) photocatalytic oxidation can proceed at ambient temperature and pressure; (ii) the excitation source can be sunlight or low-cost fluorescent light sources; (iii) photocatalysts are generally nontoxic, inexpensive, and chemically and physically stable; and (iv) final oxidation products are usually innocuous. However, it must be consider the conversion of pollutants, the toxicity of intermediates when the pollutant is not completely mineralized, as well as the reduction of reaction rate when the catalyst is deactivated. In the study, we chose five oxygenates (1-propanol, 2-propanol, propionaldehyde, acetone and N, N-dimethylformamide) which are commonly used in industries, laboratories and household. The effect of operating factor, the production of intermediates and the deactivation of catalyst on the photooxidation of oxygenates on TiO2 surface was investigated. The photocatalytic decomposition reaction of oxygenates obeyed the first-order equation. The higher initial oxygenates concentration, the faster the reaction rate. The initial rate of oxygenates degradation can be well described by the Langmuir-Hinshelwood rate form. The specific reaction rate constant and the equilibrium adsorption can be found from Langmuir-Hinshelwood rate form. The decomposition rate increased with increasing the oxygen content. The rate of oxygenates oxidation increased with increasing the concentration of water vapor, but decreased at high water vapor concentrations. The rate of oxygenates decomposition increased with increasing the temperature, but reduced at temperatures higher than 100oC. There were gaseous intermediates during photocatalytic oxidation of 2-propanol, 1-propanol and propionaldehyde. Acetone was the reaction intermediate of 2-propanol. Both of propionaldehyde and acetaldehyde were the reaction intermediates of 1-propanol. Acetaldehyde was the propionaldehyde intermediate. The kinetic model of 2-propanol photooxidation was successfully developed by the competitive Langmuir-Hinshelwood rate form, incorporating the inhibition effect coming from the formation of acetone. The difference between observed and estimated half-lives became larger when the initial concentration of acetone was increased. It is assumed that the intermediates competed with parent compound so that delayed the half-life. The detection of CO2 production can support this assumption. Catalyst deactivation during photocatalytic oxidation of 2–propanol and acetone was not found but photocatalytic deactivation was observed in oxidation of 1–propanol, propionaldehyde and DMF. The Levenspiel deactivation kinetic model and exponentially decaying model were used to describe the decay of catalyst activity. Fourier transform infrared (FTIR) was used to characterize the surface and the deactivation mechanism of the photocatalyst. Results revealed that carbonylic acids, aldehydes, amines, carbonate and nitrate were adsorbed on the TiO2 surface during the photocatalytic reaction of DMF. The ions, NH4+ and NO3-, causing the deactivation of catalysts were detected on the TiO2 surface. Several treatment processes were applied to find a suitable procedure for the regeneration of catalytic activity. Among these procedures, the best one was found to be the H2O2/UV process. Summary, it is feasible to remove the gaseous oxygenates by photocatalysis. The best operating condition was low inlet concentration of reactant, low flow rate, moderate concentration of water vapor, temperature at 100oC and oxygen content more than 20%. The suitable process of regeneration was H2O2/UV process; the activity of catalyst was almost recovered.
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