以第一原理計算探討非均相催化反應：甲醇和乙醇氧化反應

Abstract

本文運用密度泛函理論（DFT）計算來探討醇類的非均相催化反應，包括燃料電池當中的發生在陽極的甲醇氧化反應，以及乙醇氧化反應。 Pt-oxide 催化劑為 Pt 表面吸附氧化物質（oxide），表面上的 oxide 可以幫助乙醇脫氫。由先前的研究得知 Pt-AuO 和 Pt-SnO 分別具有優異的 EOR 催化活性和穩定性。透過 DFT 計算，提出 Pt-AuSnO 三元金屬催化劑，同時具備 Pt-AuO 和 Pt-SnO 的優點。計算結果顯示，Pt-AuSnO 催化劑具有良好的 CO 耐受性，並且確實承繼 Pt-AuO 和 Pt-SnO 優異的 EOR 活性和穩定性。 由上章得知，Pt-AuSnO 表面具有比 Pt-SnO 表面優異的 EOR 催化活性。因此，本章想進一步了解身為與 Au 同族的金屬 Cu 和 Ag 是否都具備比 Pt-SnO 二元金屬催化劑還要優異的醇類催化活性。實驗證實 Pt-AgSnO 和 Pt-AuSnO 在 MOR、EOR 上都具有優於 Pt-SnO 的催化活性，而 Pt-CuSnO 則是只有 EOR 優於 Pt-SnO，MOR 則否。經過 DFT 計算，我們以 Pt-MSnO、Pt-SnO-M 兩種分別代表 M 金屬間接影響和直接影響催化反應的模型，比較兩種模型對MOR及EOR的影響。結果顯示，Pt-MSnO 和 Pt-SnO-M 分別展現不同的催化活性，因此認為不同金屬可能傾向不同的構型，Cu 傾向 Pt-SnO-M 構型，Ag、Au 則傾向 Pt-MSnO 構型。而在 Pt-SnO-Cu 上確實觀察到其 MOR 活性差於 Pt-SnO 的情況。藉由分析表面 CO 吸附能，發現 CO 毒化現象極有可能是造成 Pt-CuSnO 的 MOR 反應活性差於 Pt-SnO 的原因。

Direct alcohol fuel cells can effectively utilize alcohols to electricity are considered as power decives in the future. Our present study focused on examining electrochemical performance and mechanism of Pt-oxide catalysts, in which oxides are decorated on Pt, as anodic materials for the promising direct methanol and ethanol fuel cells (DMFC and DEFC). Initially, we systematically investigated the ethanol oxidation reaciton (EOR) for DEFC application on various bimetallic Pt-oxide (oxide = CoO, RhO, IrO, NiO, PdO, O, CuO, AuO and SnO) and found that Pt-AuO and Pt-SnO demonstrated the best activity and stability, respectively, among those catalysts. The enhanced activity and stability are attributable to that the densely charge AuO assists the key dehydrogenation step in EOR and strongly adsorbed SnO avoids the destruction of Pt surface during the electrochemical reaction. Combined the two excellent catalysts, we designed the ternary Pt-AuSnO and found the catalysts has the best EOR performance. Furthermore, we extented our ternary catalysts with other coinage metals of Pt-CuSnO, Pt-AgSnO and Pt-AuSnO on both methanol oxidation reaction (MOR) and EOR. Our results showed that Pt-AgSnO have the excellent performance as that for Pt-AuSnO, but with lower costs. Pt-CuSnO, on the other hand, only good for EOR. We create two models Pt-MSnO and Pt-SnO-M (M=Cu, Ag, Au) to represent the indirect and direct effect of M metals on the catalytic reactions and compare the effects of the two models on MOR and EOR. The results reveal that Pt-MSnO and Pt-SnO-M exhibit different catalytic activity on MOR and EOR. Therefore, it’s believed that these M metals may tend to different models. The Cu tend to model Pt-SnO-M, while Ag and Au tend to model Pt-MSnO. Analyst adsorption of CO on surface which demonstrate poor performance of Pt-CuSnO for MOR might attributable to the severe CO poisoning.