| 摘要: | 本研究目標為利用沉澱沉積法(deposition-precipitation, DP)製備不同鉑/鈷組成的鉑鈷合金觸媒以應用於氧化還原反應 (oxygen
reduction reaction, ORR)，並添加錳作改質劑以期能提升ORR活性。所製備觸媒之合金相、表面組成、化學組成、微結構、形貌以及電催化活性分析分別使用X光繞射儀(X-ray diffraction, XRD), 光電子能譜儀(X-ray photoelectron spectroscopy, XPS), 感應耦合電漿原子發射光譜分析儀(inductively coupled plasma-atomic emission spectrometer,ICP-AES), X光吸收光譜(X-ray absorption spectroscopy, XAS), 高解析度穿透式電子顯微鏡(high resolution transmission electron microscopy,HRTEM) 以及旋轉盤電極(rotating disc electrode, RDE)等儀器做鑑定。
本研究分為兩部分，第一部分為製備金屬負載量為38 wt %的鉑鈷
合金觸媒，鉑/鈷組成分別為99/1, 90/10和75/25且命名為PtCo-0,
PtCo-1和PtCo-2。其中，PtCo-2之於其他觸媒有最好的ORR活性，此
乃由於PtCo-2有最高的鈷合金度。但在加速穩定度測試accelerated
durability test, ADT)後，PtCo-1相較其他觸媒呈現最佳ORR活性，此現象說明高鈷含量的成分組成在長時間測試下易使得鈷溶解和鉑產生Ostwald ripening，因此不利於活性。
ii
第二部分用0.4和3.2 wt%的錳作改質劑添加於金屬負載量38 wt %
的鉑鈷觸媒，且分別命名為Mn-1和Mn-2，此外，亦製備不同鉑/錳組
成(97/3和79/21)的鉑錳合金觸媒，並命名為PtMn-1和PtMn-2。錳改質的鉑鈷觸媒為面心立方(face centered cubic, fcc)結構，鉑/鈷原子比為3.5且顆粒大小均為3.0 nm。在ORR活性的表現上，錳改質的鉑鈷觸媒相較於鉑鈷、鉑錳以及商用材鉑觸媒均有較佳的ORR活性，在加速穩定測試後，Mn-2仍表現最佳ORR活性，其效能約為商用材鉑觸媒活性之420 %。錳改質的鉑鈷觸媒係由於存在著鄰近鉑的氧化鈷/錳，其排斥吸附於鉑上的氫氧基使得氫氧基從鉑上脫去增加鉑的活性面積。此外，從XAS分析可以鑑定鉑鈷和錳改質鉑鈷觸媒之鉑的未填滿d軌域以及與鈷/錳混成程度，其結果相似。因此，Mn-2之所以能有效提升ORR效能其主因為富鉑之表面結構、高的鉑利用率以及ORR過程
中最佳的電子轉移數。
In this study, PtCo/C alloy catalysts with varying Pt/Co ratios are prepared by the deposition-precipitation (DP) method for the oxygen reduction reaction (ORR). The effect of Mn addition on the promotion of their ORR performance is investigated. The phases, surface compositions, chemical compositions, fine structures, morphologies and electrochemical properties of prepared catalysts are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-atomic emission spectrometer (ICP-AES), X-ray absorption spectroscopy (XAS), high resolution transmission electron microscopy
(HRTEM) and rotating disc electrode (RDE) technique, espectively.
The study is divided into two parts. In the first part, the 38 wt % of PtCo/C alloy catalysts with the Pt/Co ratios of 99/1, 90/10 and 75/25 (denoted as PtCo-0, PtCo-1 and PtCo-2) are prepared. A significant enhancement of ORR activity for PtCo-2 is noted, which is due to the highly degree of Co alloying. After accelerated durability test (ADT), PtCo-1 presents the best performance among all catalysts, suggesting that high Co contents result in obvious dissolution of the non-noble metal and Ostwald ripening of Pt during long term tests.
In the second part, the 38 wt % of PtCo/C catalysts modified with 0.4 and 3.2 wt% of Mn (denoted as Mn-1 and Mn-2) and PtMn/C alloy catalysts with the Pt/Mn ratios of 97/3 and 79/21 (denoted as PtMn-1 and
PtMn-2) are prepared. For the Mn-modified PtCo/C catalysts, the Pt/Co atomic ratio is 3.5 and the size is about 3.0 nm with the same face centered cubic (fcc) structure. Moreover, the ORR activity of
Mn-modified PtCo/C catalysts is both higher than that of PtCo/C, PtMn/C and commercial Pt/C catalysts. After ADT of 1700 cycles, Mn-2 with 3.2 wt% Mn addition still presents the best ORR mass activity, which is
about 420 % of commercial Pt/C. In the Mn-modified PtCo/C system, due to the existence of the neighboring Co and/or Mn oxide, there is OH repulsion between Pt-OH and these non-noble metal hydroxides or oxides, decreasing the OH coverage on Pt and increasing the number of free Pt
active sites. Besides, based on the XAS analysis, the unfilled d-orbital values and extent of hybridization of Pt and Co/Mn of the PtCo and
Mn-modified samples are very similar. Therefore, the promotional effect on ORR performance of Mn-2 is attributed to the most enhanced Pt-skin structure, highest Pt usage, and comparable electron transfer number
during ORR instead of changes in d-orbital vacancy and alloying degree. |
