疏水性奈米複合光觸媒行光催氫化二氧化碳

Abstract

面臨日趨嚴重的全球暖化與能源議題。利用近乎無限的太陽能，光催化二氧化碳是目前受到科學家大量重視，同時能解決此兩大議題的辦法。經由熱力學計算，使用氫氣作為氫源的光催化二氧化碳氫化反應與傳統的二氧化碳與水的還原反應相比在熱力學上有利、促進碳氫化合物的生成。 有鑑於在二氧化碳光還原反應中，二氧化碳在和水競爭活性位上的不利將導致還原效率的降低。本研究旨在藉由提高觸媒的疏水性，以改善二氧化碳還原光觸媒上二氧化碳與水氣競爭吸附的問題。本研究中之光觸媒以溶膠凝膠法配合陽離子交換法製備鈦柱撐蒙脫土觸媒，並透過吸附不同劑量之界面活性劑CTAB增加觸媒之疏水性。為了釐清觸媒之結構性質，研究裡進行了UV-vis、XRD、TG-DTA、ICP-MS、FE-SEM、EDS、FE-TEM、接觸角等觸媒鑑定。 本研究以8W之UVB筆燈作為燈源，透過單胞批次式反應器測試觸媒在光催化二氧化碳氫化反應照光四小時之光催化活性。實驗結果顯示50PtTiMt的光催化活性最佳、光量子效率也最高，光照四小時後，於45°C產率為:CO:18.59μmol/gcat、CH4:22.74μmol/gcat、光量子效率0.04221%、CH4之選擇性最好。和商用的二氧化鈦Degussa P25相比提升了二倍的還原產物產率。根據實驗結果以及觸媒鑑定所得的資訊，在鈦柱撐蒙脫土後，觸媒之形貌、物理吸附特性發生改變、二氧化鈦之結晶顆粒越小。隨著CTA+吸附的增加，觸媒之疏水性也隨之增加。然而，當CTA+吸附增加，我們推測部分CTA+會吸附於反應活性位上，反而影響到光催化活性。因此，CTA+之吸附於50%陽離子交換能力時達到最適量，疏水的效果最好。研究中詳細探討蒙脫土、界面活性劑等所扮演的角色。

Facing the serious global warming and energy crisis problem, photocatalytic reduction of carbon dioxide is one of the most promising solutions to solve both problems simultaneously. Previous studies show that the hydrogenation of carbon dioxide photoreduction process can make the reaction more thermodynamically favorable, therefore promoting the hydrocarbon yield. In this research we prepared a set of photocatalysts with different hydrophobic properties, on the basis that the inferior ability of carbon dioxide to compete the active sites with water vapor during the carbon dioxide photoreduction process. Montmorillonite with excellent cation exchange ability were combined with titanium dioxide sol gel for the preparation of titanium pillared montmorillonite. With simple cation exchange process, the catalysts were obtained with different hydrophobicity by the control of surfactant addition. To clarify the structural, textural and thermal properties, series of characterizations, including UV-vis, XRD, TGA, TG-DTA, FE-SEM, EDS, FE-TEM, ICP-MS were performed. Using 8W UVB pen ray lamp as the light source, the photocatalytic carbon dioxide reduction activity of the catalysts were studied in a single batch reactor. After 4 hours irradiation under a mixture of carbon dioxide, water vapor, and hydrogen system at 45°C, 50PtTiMt showed the best photoactivity, generating CO:18.59μmol/gcat and CH4:22.74μmol/gcat, quantum efficiency 0.04221% and the highest CH4 selectivity. In comparison with commercial titanium dioxide P25 catalyst, a two-fold enhancement was achieved. Based on the experiment and characterization information, titanium pillared montmorillonite showed significant difference in morphology, adsorption properties compared with original clay, and smaller crystallite size compared with titanium dioxide without montmorillonite. The hydrophobicity of the catalyst increase along with the increase of CTA+ addition. However, we postulate that when the CTA+ addition amount is very high, part of the CTA+ may adsorb on the active sites and affect the photoactivity. In this study, the optimal CTA+ addition amount at 50% cationic exchange ability. Finally, the roles of titanium dioxide, surfactant, and montmorillonite are also well discussed.