應用於氧氣還原反應之高效能鉑金陰極催化劑其製備、改質與鑑定; Preparation, Modification and Characterization of Highly Effective PtAu Catalyst for Oxygen Reduction Reaction

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Title:	應用於氧氣還原反應之高效能鉑金陰極催化劑其製備、改質與鑑定;Preparation, Modification and Characterization of Highly Effective PtAu Catalyst for Oxygen Reduction Reaction
Authors:	劉丞偉;Liu,Chen-Wei
Contributors:	材料科學與工程研究所
Keywords:	鉑金催化劑;氧化鈰;二氧化鈦;氧氣還原反應;後熱處理;奈米棒;核殼結構;加速穩定度測試;電化學表面積;PtAu;CeO2;TiO2;Oxygen reduction reaction;Post heat treatment;Nanorods;Core/shell structure;Accelerated durability test;Electrochemical surface area
Date:	2013-07-04
Issue Date:	2013-08-22 11:44:21 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究目標為製備與改質高效能鉑金觸媒以應用於氧氣還原反應(oxygen reduction reaction, ORR)。改質的方法包含添加氧化鈰(CeO2)?或二氧化鈦(TiO2)當做改質劑，在氫氣、氮氣或空氣氣氛中進行熱處理，以及製備具有核殼結構之一維奈米棒，利用上述方法調控鉑金觸媒之結構/表面組成，使其具有較高的催化活性。所製備觸媒之結構、形貌、表面物種、表面組成以及電催化活性分別使用X光繞射儀(X-ray diffraction, XRD)、高解析穿透式電子顯微鏡(high resolution transmission electron microscope, HRTEM)、程式溫控還原儀(temperature programmed reduction, TPR)/光電子能譜儀(X-ray photoelectron spectroscopy, XPS)/循環伏安法(cyclic voltammograms, CV)和旋轉盤電極(rotating disc electrode, RDE)來鑑定。本研究共包含五個部分，第一部分為製備Pt/Au原子比為1且金屬負載量為10 wt %的碳載體鉑金(PtAu/C)觸媒，並添加15 wt % CeO2當作改質劑(PtAuCe)。接著將PtAuCe觸媒放置於520和620 K的氫氣下進行熱處理(H520和H620)。由實驗結果指出PtAuCe樣品具有Au 核/Pt 殼結構且表面物種為富Pt合金的型態，在氫氣熱處理之後，表面Pt和Au的氧化物被還原並伴隨著Au逐漸偏析至觸媒表面，造成表面零價Pt組成的提升。其中以H520的活性表現最佳。第二部分中則將PtAuCe 觸媒在520、570和 620 K的高溫中進行氮氣熱處理(N520、N570和 N620)。。由PtAu系統的熱力學模型可得知，在高溫環境下會發生Au的表面偏析現像產生，但當加入CeO2改質後， CeO2會誘發Pt偏析至表面造成表面重組的現象發生。由實驗結果顯示，表面富含Pt或Au將使得ORR活性提升或劣化。第三部份為製備金屬負載量46 wt % 的Pt90Au10/C 觸媒並以不同劑量的TiOx (x = 1, 3 和 7 wt %)改質。 (PtAuTi-1, PtAuTi-2 和 PtAuTi-3)。實驗結果顯示，在加速穩定度測試(accelerated durability test, ADT)後，PtAuTi-2和PtAuTi-3觸媒的ORR活性分別高於商用Pt/C觸媒2.7 和 1.9 倍，此提升乃由於次氧化物(suboxide)形成和強大的金屬(Pt)與支撐物(TiO2)間的交互作用力(strong metal–support interaction, SMSI) 以及適當的Pt/Au表面組成。在第四部份則製備以TiO2C做支撐物並具有金屬負載量為35 wt %的Pt75Au25觸媒(PtAu/TiO2C)並以空氣或氫氣氣氛熱處理(PtAu/TiO2C(Air)和 PtAu/TiO2C(H2))，以期能提升ORR穩定性，。在ADT後，PtAu/TiO2C(Air) 觸媒，其ORR活性比商用Pt/C觸媒高3.0倍，此活性的提升是因其表面為富Pt的型態。第五部分則成功地以濕式化學還原法製備金屬負載量45 wt % 且具有核殼結構的Pt75Au25/C 奈米棒觸媒，其ORR活性以及穩定度均優於商用材Pt/C，此乃由於一維奈米結構可助於ORR活性的提升，此外，這結果也顯示Au核/Pt殼奈米棒相較於表面富Au的PtAu殼奈米棒有較佳的ORR活性。 In this study, the highly effective PtAu catalysts are prepared and promoted for oxygen reduction reaction (ORR). Several methods, including addition of modifier (ceria (CeO2) and titanium dioxides (TiO2)), post heat treatment (H2, N2 or air) and preparation of 1-dimesional (1-D) nanorods (NRs) with core/shell structure are used to synthesize PtAu/C catalysts with tunable structure/surface compositions and high catalytic activity. The basic comprehension on the correlation between structures, morphologies, surface species, surface compositions, and electrocatalytic activities of prepared catalysts are characterized by X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), temperature programmed reduction (TPR)/X-ray photon spectroscopy (XPS)/cyclic voltammograms (CV), and rotating disc electrode (RDE) techniques, respectively. The study is divided into five parts. In the first part, the 10 wt % of PtAu/C alloy catalysts with the Pt/Au ratio of 1:1 modified by CeO2 (denoted as PtAuCe) are prepared and then further treated at 520 and 620 K under H2 (denoted as H520 and H620). The as-reduced Pt50Au50/Ce15C (PtAuCe) sample has an Au core/Pt shell structure with a Pt-rich alloy surface (APt) species dominating on its surface. After heating in H2, reduction of Pt and Au oxides and surface segregation of Au occur, and thus the surface metallic Pt composition increases. As a result, H520 exhibits the best ORR activity. In the second part, PtAuCe catalysts are treated at 520-620 K under N2 (denoted as N520, N570, and N620). The inherent surface segregation of Au of PtAu systems at high temperatures based on the thermodynamic models has been changed due to the CeO2 modification. CeO2-induced Pt surface segregation and surface reconstruction takes place. Consequently, the enhancement of Pt or Au surface composition causes the promotion or deterioration in ORR activity for heat-treated catalysts, respectively. In the third part, the 46 wt % of Pt90Au10/C catalysts with 1, 3 and 7 wt % of TiOx addition, denoted as PtAuTi-1, PtAuTi-2 and PtAuTi-3, respectively) are prepared. After accelerated durability test (ADT), the ORR activity of PtAu/C catalysts of PtAuTi-2 and (PtAuTi-3 is 2.7 and 1.9 times higher than that of the commercial Pt/C, respectively. The enhancement of ORR activity is attributed to the suboxide formation, strong metal–support interaction (SMSI) between the metal oxides and the adjacent Pt atoms and moderate surface Pt/Au ratio. In the fourth part, the 35 wt % of Pt75Au25 alloy catalysts supported on TiO2C are prepared for ORR (denoted as PtAu/TiO2C) and then treated at air or H2 atmosphere (denoted as PtAu/TiO2C(Air) and PtAu/TiO2C(H2), respectively) in order to enhance their durability. After ADT, the ORR activity of PtAu/TiO2C(Air) catalysts is 3 times higher than that of the commercial Pt/C. The enhancement of ORR activity is attributed to the surface enrichment of Pt. In the fifth part, the 45 wt % Pt75Au25/C NRs with core/shell structure are prepared by a facile wet chemical reduction method successfully. The ORR activities and long-term durability of PtAu NRs are both better than that of commercial Pt/C nanoparticles. The promotional effect of the PtAu NRs may be attributed to the 1-D morphology, favorably enhancing the ORR activity. The results also suggests that the Au in the core part of the Pt shell NRs may show a better promotional effect on the ORR performance than the surface Au in the PtAu shell NRs.
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