氧氣還原反應在鉑合金表面的活性和穩定度趨勢
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2015
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本論文針對在一系列Pt合金上氧氣還原反應(Oxygen Reduction Reaction, ORR)的反應機構、Pt合金的效率和穩定度進行理論計算和實驗。理論計算部分，我們比較氧氣還原反應在Pt-cored和 Pt-shelled結構上所有吸附物O*、O2*、OH*、 OOH*、H2O2*、 H2O*和H*的吸附能以及三個O-O斷鍵步驟和四個O-H生成反應步驟的活化能，計算結果發現Pt與小原子Co、Cu、Pd形成Pt-shelled結構以及Pt與大原子Au形成Pt-cored結構能夠提升ORR活性。此外，我們藉由計算乾淨表面以及表面吸附含氧中間產物(ORR的主要中間產物)的segregation energy，確認合金在ORR中的穩定度，根據能量計算的結果發現，小原子和大原子會分別傾向於形成Pt-shelled和Pt-scored構型，而且在ORR都很穩定，其中PtAg合金的穩定度最高。實驗部分，將Pt3M合金(M = Cu, Pd, Ag, Au)吸附在Vulcan XC-72上，探討他們氧氣還原反應的趨勢，樣品的合成是用NaBH4還原法，藉由XRD、SEM、EDS確認晶格常數和化學組成，將樣品滴在玻璃碳電極上，在酸性(0.1 M HClO4 (aq))條件下進行電化學測量，而實驗的結果與計算的一致，根據mass activity(MA)和specific activity (SA)的測量結果，氧氣還原反應的活性為Pt3Au/C > Pt3Cu/C > Pt3Pd/C > Pt3Ag/C > Pt/C。
In this thesis, we computationally and experimentally study oxygen reduction reaction (ORR) on a series of PtM alloys and systematically elucidate the mechanisms for the design catalysts with better efficiency and stability. Computationally, we thoroughly examined compared the energetics for all the possible adspecies, O*, O2*, OH*, OOH*, H2O2*, H2O* and H* and elementary steps, including 3 O-O cleavage and 4 O-H association steps, on both Pt-cored and Pt-shelled structures. The computational result found that ORR activity can be efficiently improved by alloying Pt with smaller dopants, such as Co, Cu, Pd, in Pt-shelled structures, while with larger and less active dopants, such as Au, in a Pt-cored structure. Additionally, the segregation energy for clean and O-containing adspecies, key intermediates in ORR, alloys were computed to investigate the stability of alloys during ORR operation. The energetic result found that smaller and larger dopants preferred Pt-shelled and cored structures, respectively, and both are stable in ORR process, whereas PtAg showed the best stability. Experimentally, Pt3M alloys (M = Cu, Pd, Ag, Au) supported on Vulcan XC-72 carbon were systematically fabricated and investigated to reveal their catalytic trends for ORR. The samples were prepared by NaBH4 reduction method and examined by XRD, SEM, EDS to confirm their lattice constant and chemical composition initially. Additionally, the samples were deposited on glassy carbon electrodes for the electrochemical measurement in acid (0.1 M HClO¬4 (aq)) medium. The experimentally result agreed with the computational prediction and found that ORR activity followed the order of Pt3Au/C > Pt3Cu/C > Pt3Pd/C > Pt3Ag/C > Pt/C, according to the mass activity (MA) and specific activity (SA) measurements.
In this thesis, we computationally and experimentally study oxygen reduction reaction (ORR) on a series of PtM alloys and systematically elucidate the mechanisms for the design catalysts with better efficiency and stability. Computationally, we thoroughly examined compared the energetics for all the possible adspecies, O*, O2*, OH*, OOH*, H2O2*, H2O* and H* and elementary steps, including 3 O-O cleavage and 4 O-H association steps, on both Pt-cored and Pt-shelled structures. The computational result found that ORR activity can be efficiently improved by alloying Pt with smaller dopants, such as Co, Cu, Pd, in Pt-shelled structures, while with larger and less active dopants, such as Au, in a Pt-cored structure. Additionally, the segregation energy for clean and O-containing adspecies, key intermediates in ORR, alloys were computed to investigate the stability of alloys during ORR operation. The energetic result found that smaller and larger dopants preferred Pt-shelled and cored structures, respectively, and both are stable in ORR process, whereas PtAg showed the best stability. Experimentally, Pt3M alloys (M = Cu, Pd, Ag, Au) supported on Vulcan XC-72 carbon were systematically fabricated and investigated to reveal their catalytic trends for ORR. The samples were prepared by NaBH4 reduction method and examined by XRD, SEM, EDS to confirm their lattice constant and chemical composition initially. Additionally, the samples were deposited on glassy carbon electrodes for the electrochemical measurement in acid (0.1 M HClO¬4 (aq)) medium. The experimentally result agreed with the computational prediction and found that ORR activity followed the order of Pt3Au/C > Pt3Cu/C > Pt3Pd/C > Pt3Ag/C > Pt/C, according to the mass activity (MA) and specific activity (SA) measurements.
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氧氣還原反應, 密度泛函理論, 鉑合金, 循環掃描伏安, 線性掃描伏安, Oxygen Reduction Reaction, density functional calculations, Pt-based alloy, Cyclic Voltammetry, Linear Sweep Voltammetry