以二氧化鈦催化光氧化氣相含氧有機物

Abstract

光催化氧化技術具有(1)可在常溫常壓下操作，(2)可利用可見光或紫外光，(3)催化劑無毒、便宜且性質穩定，(4)最終產物通成無害等優點，因此以紫外光/二氧化鈦程序處理氣相揮發性有機物，近年來開始受到重視。但是，光催化技術若要實際應用，需要注意的不僅是污染物的去除率要高，也要考慮礦化不完全時，中間產物產生的破壞力、以及催化劑活性衰減造成操作效率降低等問題。因此本研究選擇五種常見的含氧有機物(正丙醇、異丙醇、丙酮、丙醛、二甲基甲醯胺)，針對各項操作因子的影響進行研究，並對中間產物的生成與二氧化鈦減活性的現象加以探討。 光催化氧化含氧有機物的速率受到初始濃度、反應溫度、溼度、氧含量的影響。初始濃度越高反應速率越快，以Langmuir-Hinshelwood方程式模擬反應速率與濃度的關係，可以得到反應速率常數與吸附常數。反應溫度越高反應速率越快，當溫度高於100℃，隨著溫度越高反應速率越慢；不過，二甲基甲醯胺的最大反應速率出現於150℃。溼度低時，溼度越高反應速率越快，但是在高溼度，溼度越高反而會抑制含氧有機物分解。氧含量越高，可以促進含氧化合物分解速率，當氧含量高於20%，反應速率趨於平穩。光催化氧化過程，正丙醇、異丙醇、丙醛皆有氣相中間產物；正丙醇的中間產物為丙醛與乙醛，異丙醇的中間產物為丙酮，丙醛的中間產物為乙醛。中間產物在光催化過程會與反應物競爭氧化，而以Langmuir-Hinshelwood競爭氧化模式模擬可得良好結果。光催化分解丙酮的過程雖未發現氣相中間產物，不過比較丙酮實測與理論的半生期，則初始濃度越高，兩者的差距越大，礦化率也越低，推測有非氣相中間產物在氧化丙酮的過程中產生，而此中間產物與丙酮也有競爭氧化的情形。 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。 綜合上述，以光催化分解含氧有機物是可行的，操作條件在低進流濃度、低進流量、中溼度、溫度在100℃、氧含量大於20%，有最佳的去除效率、少量的中間產物與緩慢的觸媒活性衰退。而觸媒活性再生的方式可以過氧化氫淋洗後再照光。

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.