氧氣還原在過渡金屬表面之反應趨勢

Abstract

本論文利用還原劑(NaBH4)還原十種不同金屬錯合物成金屬Co、Ni、Cu(第三周期)、Ru、Rh、Pd、Ag(第四周期)和Ir、Pt、Au(第五周期)，並吸附在Vulcan XC-72上，進一步在樣品上嘗試Ag金屬重量比例不同(80%)和利用實驗方式改變Au、Ag金屬顆粒大小，藉由XRD、SEM、EDS、TEM儀器確認金屬吸附狀況。將RDE玻碳電極滴上不同金屬的Vulcan XC-72樣品在酸性( H2SO4 )及鹼性( KOH )溶液條件下，使用電化學循環伏安法( CV ) 、一氧化碳氧化及線性掃描伏安法( LSV )，測試不同金屬的電化學上的特性，並討論氧氣還原反應( ORR )的差異。 藉由電化學結果可整理出5個結論：(1)Pt相較於其他金屬有利氧氣還原反應(ORR)。 (2)推測Pt、Pd等金屬在鹼性條件下為4個電子反應；Au可能包含2個和4個電子反應；Cu、Ni因活性不佳與碳黑為2個電子反應。(3)Au、Ag顆粒變小也使得氧氣還原活性提升。(4)Ag金屬重量比提升相對也增加氧氣還原活性，但不符合經濟效應。

In the current research, a series of catalysts of 3d( Co、Ni、Cu ), 4d( Ru、Rh、Pd、Ag ), 5d( Ir、Pt、Au ) supported on Vulcan XC-72 carbon have been systematically investigated by electrochemical measurement to investigate their catalytic trends for oxygen reduction reaction ( ORR ). The samples were initially prepared by NaBH4 reduction method and characterized from XRD, SEM, EDS, TEM to examine their chemical compositions and microstructures. The electrochemical active surfaces ( EAS ) of the fabricated carbon-supported metals were then determined by CO oxidation method. The ORR performance and mechanism were investigated by rotating disc electrode ( RDE ) method in both acidic ( H2SO4 ) and basic ( KOH ) conditions-with various rotating speeds. In addition, effects of metal sizes and loading for the electrochemical performance were also examined. The ex-situ characterization of the synthesized catalysts shows that all the metals are uniformly distributed on the carbon supporter with desired loadings, c.a. 20% and similar sizes, c.a. 8 nm, indicating that the electrochemical behavior for those catalysts are mainly controlled by chemical identies of metals. Electrochemical studies of them conclude the following results. First, Pt is most activity metal for ORR and followed by Pd, Au, and etc. Second, Pt, Pd, Rh, Ir, Ru, Ag, Co promoted predominately a four-electron pathway for ORR, while Au includes both two and four-electron pathways. Nevertheless, Cu and Ni are the poorest activity for ORR, behave similar to the carbon supportor shows two-electron pathway for ORR. Finally, metal size have significant effect on electrochemical performance as the small sized Au and Ag, ~3 nm, boost better ORR activity; on the other hand, metal loading shows limit influence in ORR.