二氧化碳再利用生成乙烯之研究

Abstract

近百年來由於工業革命促成石化材料的大量使用，人類對於物質生活越來越講究，使得石化產品急劇的產生，並伴隨著污染物迅速的生成，包含大量CO2的排放。日前京都議定書已通過，使得CO2減量議題成為目前全世界所關注的焦點之一。本研究利用金屬光觸媒還原CO2以生成乙烯，提供作為工業上之原料。一方面可以降低溫室效應氣體排放，另一方面可以增加石化資源的產生來源。 本研究利用光觸媒(Cu/TiO2及Ag/TiO2)，經由波長254 nm的UV燈照光產生電子與電洞對，並結合電解程序將CO2還原成石化資源之乙烯。本研究共使用四種不同還原程序，方法與結果分別敘述如下 : 1) Ag/TiO2光催化還原氣相CO2程序：產物主要為甲醇。反應24小時後，最大甲醇產量達到48.3 μg ; 2) Ag/TiO2和Cu/TiO2光催化還原液相CO2程序：反應產物以液相甲醇為主。在溶液初始濃度為5 g L-1 NaOH、飽和CO2、觸媒量為1 g、及反應28小時，結果顯示Ag/TiO2之催化效果較Cu/TiO2為佳。其甲醇產量分別為124.46 及53.93 μg ; 3) 三相（氣(CO2)/液(Na2CO3/CuCl2)/固(Cu/TiO2))光催化還原CO2程序：產物主要為甲烷。當觸媒量為0.5 g，在溶液初始濃度含0.1 M Na2CO3和1 M CuCl2，反應28小時後系統之甲烷產量可達62.5 μg ; 4) 三相（氣(CO2)/液(Na2CO3/CuCl2)/固(Cu/CuCl) )使用紅銅電極電解法還原CO2之程序：此程序反應3小時但無Cu/TiO2觸媒下，最適操作條件為前處理曝CO2一小時下，0.2 M Na2CO3、0.1 M CuCl2濃度、電流為1安培(A)、相對電壓為12伏特(V)時，氣態CO2降解率達88%。乙烯及甲烷產量分別為53,211及3,433 μg，乙烯產量佔氣相有機物達到93.9%。乙烯產量佔氣液相有機物產量總合的73.42%。合成氣 (CO + H2)產量更佔整體氣體產量的49.6%。其他氣體CO、H2、及O2則分別為74,788、80,791、及101,642 μg ; 最後液相甲酸產量為15,832 μg ; 5）三相（氣(CO2)/液(Na2CO3/CuCl2)/固(Cu/TiO2))之光催化結合紅銅電極電解法還原CO2之程序: 在0.2 M Na2CO3，添加0.5 g Cu/TiO2條件下，其甲烷、乙烯、CO、甲酸、H2、及O2之總生成量分別為942、13,160、46,096、4,907、87,975、及100,926 μg。與程序4相比較，乙烯、甲烷、及甲酸產率下降三分之二，CO產率下降二分之一，O2產率不變，而H2產率稍微上昇。由此可知，於較高Na2CO3濃度( 0.2 M )時添加Cu/TiO2光觸媒進行光電解反應會抑制甲烷、乙烯、CO、及甲酸之生成，但會稍微增加氫氣的產生。顯示出電解與光觸媒兩系統結合將CO2還原，其效能有降低之效果。 以上各程序所使用之觸媒量中，程序(1)為Ag/TiO2觸媒0.85 g，程序(2)為Ag/TiO2及Cu/TiO2觸媒1 g，程序(3)及(4)皆為Cu/TiO2觸媒0.5 g，Ag/ TiO2和Cu/TiO2之Ag和Cu為2 wt.%;程序(1)之氣體體積為1060 mL，而程序(2)之液相體積1000 mL，程序(3)及(4)之液相體積皆為500 mL。未來後續研究仍需針對乙烯產率及選擇性做更進一步之提升。

This study investigated the application of the ultra violet (UV) with reduction photocatalyst to convert the green-house air pollutants to the useful chemicals. The method is denoted as UV/photocatalyst process. Carbon dioxide, which has a highest concentration among green-house air pollutants was taken as a model compound. The emission of CO2 to the atmosphere was substantially found in the industrial waste gaseous streams. The operation parameters and influential factors such as the type, amount, and coating of photocatalysts, treatment temperature, initial concentration of CO2, intensity and wave length of UV radiation, type of solutions, and hydrogen sources, etc. were examined in order to find the proper conditions for the effective reduction of CO2 to useful chemicals. The photocatalyts used in this study were Ag/TiO2 and Cu/TiO2 made by the impregnation method. The wavelength of UV irradiation was 254 nm. The photocatalytic reductions of CO2 were performed via three different methods. In method 1, only gases CO2 was presented in the reactor to process the phtocatalytic reaction with catalyst on glass plates. As for method 2, sources of CO2 were from the liquid solution of Na2CO3 and gaseous CO2 above the liquid solution containing suspended particles of catalyst. Method 3 was similar to method 2 with addition of CuCl2 in the liquid solution of Na2CO3. The major product via methods 1 and 2 was methanol. An addition of CuCl2 to the solution containing suspended Cu/TiO2 catalyst can form CuCl/Cu/TiO2 which further catalytically enhanced the reduction of CO2 and CH3OH, producing CH4 and increasing its yield. The determining step for the production of methane is the additions of Na2CO3 and CuCl2 in the aqueous solution. The orders of concentrations of Na2CO3 enhancing the methane yields were 0.1 > 0.2 > 0.3 > 0.05 M Na2CO3 at 0.1 M CuCl2, while those of CuCl2 were 1 > 2 > 0.1 > 0 > 4 M CuCl2 at 0.1 M Na2CO3. The results obtained from this study are useful for better understanding the UV/photocatlyst process for the reuse of CO2. Method 4 was electrolysis using red copper electrode with power supply standards of 12 relative volts and 1 amp. The orders of concentrations of Na2CO3 enhancing the ethylene yields were 0.2 > 0.1 > 0.3 > 0.4 M Na2CO3 at 0.1 M CuCl2. As for the enhancing effects of concentrations of CuCl2 on the methane yields, the orders were 0.1 > 0.05 > 0 > 0.2 M CuCl2 at 0.2 M Na2CO3. The degradation efficiencies of gaseous CO2 was 88% and the products yielded of C2H4, CO, CH4, HCOOH, H2, and O2 were 53211, 74788, 3433, 15832, 80791 and 101642 μg at 0.2 M Na2CO3 and 0.1 M CuCl2. Method 5 was combining photocatalyst and electrolysis processes for the reduction of CO2. The yields of products of method 5 relative to those of method 4 decreased two-thirds for C2H4, CH4 and HCOOH, and half for CO. As for O2, the yields of method 5 and 4 were about the same. However, the yield of H2 of method 5 increased slightly as compared to that of method 4 with the addition of 0.5 g Cu/TiO2 catalyst at 0.2 M Na2CO3 and 0.1 M CuCl2. The result indicated that combining photocatalyst and electrolysis processes would restrain the reduction efficiency of CO2. The technology for the reduction of CO2 to useful chemicals elucidated by this study is environmentally favorable.