循環(huán)伏安法：Cyclic Voltammetry（循環(huán)伏安法）

1、1.3.1 Cyclic Voltammetry（循環(huán)伏安法）（循環(huán)伏安法） (R.S.Nicholson and I. Shain, Anal.Chem., 1964, 36, 706) Figure 1.8 Figure 1.9 Figure 1.10 Reversible system: The peak current for a reversible couple (at 250C) is given by Randles-Sevcik equation: Where n is the number of electrons, A is the electrode area (in

2、cm2), C is the concentration (in mol/cm3), D is the diffusion coefficient (in cm2/s), and is the scan rate (in V/s). (1.6) (1.7) (1.8) Figure 1.11 Q. L. Echegoyen et al, J. Am. Chem. Soc. 1992,114,3978 Irreversible and Quasi-reversible Systems: (1.9) (1.10) Figure 1.12 不可逆過(guò)程電流和電勢(shì)隨掃描速度的變化不可逆過(guò)程電流和電勢(shì)隨掃

3、描速度的變化 Study of Adsorption Processes (1.11) Figure 1.13 Figure 1.14 (1.12) Study of Reaction Mechanisms Quantitative Applications Polarography (極譜極譜) and Voltammetry (伏安法伏安法)的區(qū)別？的區(qū)別？ polarography A classical electroanalytical technique discovered in 1922 by J. Heyrovsky, for which he was awarded the

4、 Nobel Prize for Chemistry in 1959. Essentially, it is linear-sweep voltammetry using a dropping-mercury electrode for working electrode and a large mercury pool as counter electrode. voltammetry An electrochemical measuring technique used for electrochemical analysis or for the determination of the

5、 kinetics and mechanism of electrode reactions. Voltammetry is a family of techniques with the common characteristics that the potential of the working electrode is controlled (typically with a potentiostat) and the current flowing through the electrode is measured. In one of the most common applica

6、tions of the technique, the potential is scanned linearly in time; this is called the linear-sweep voltammetry, LSV, or LV. Cyclic voltammetry (CV) is a linear-sweep voltammetry with the scan continued in the reverse direction at the end of the first scan, this cycle can be repeated a number of time

7、s. 6.7多電子轉(zhuǎn)移的電極過(guò)程多電子轉(zhuǎn)移的電極過(guò)程 對(duì)于兩電子還原反應(yīng)，一般的表達(dá)式為：對(duì)于兩電子還原反應(yīng)，一般的表達(dá)式為： A+ e BB+ e C Fig.6.7 兩電子轉(zhuǎn)移反應(yīng)示意圖兩電子轉(zhuǎn)移反應(yīng)示意圖 對(duì)于此過(guò)程，對(duì)于此過(guò)程， 可以分為三個(gè)極限情況進(jìn)可以分為三個(gè)極限情況進(jìn) 行討論行討論： (1) 第二步在更負(fù)的電位下反應(yīng)， 我們可以觀察到兩 個(gè)分開(kāi)的波。即： kf,2 - kb,2 kb,1B + eC fast (3) 第二步是決速步驟，即：kf,2 - kb,2 kb,1 A + e B pre-equilibrium kf,2 kb,1B + eC rate-determini

8、ng step Due to pre-equilibrium, the voltammetric wave is steeper than in case 2. The activated complex is more sensitive than in case 2 to changes in applied potential. Fig.6.8 Voltammograms for the reduction of species A following A+e B +e C according to the relative rates of the two steps. IL = IL

9、 (A B). (a) Second step much more difficult than the first; (b) First step rate-determining; second step fast; ( c)First step pre-equilibrium; second step rate-determining. 3.4 特性吸附特性吸附 即使電場(chǎng)不存在也能發(fā)生的吸附即使電場(chǎng)不存在也能發(fā)生的吸附- 特性吸附特性吸附 特性吸附使零電荷電勢(shì)特性吸附使零電荷電勢(shì)(EPZC)發(fā)生移動(dòng)。發(fā)生移動(dòng)。 負(fù)離子使之負(fù)移，正離子使之正移。為什么？負(fù)離子使之負(fù)移，正離子使之正移。為什

10、么？ 為什么表面活性劑能夠使微分電容在為什么表面活性劑能夠使微分電容在EPZC處變處變 化很大？化很大？ 答案：見(jiàn)吳浩青，李永舫答案：見(jiàn)吳浩青，李永舫電化學(xué)動(dòng)力學(xué)電化學(xué)動(dòng)力學(xué)p2426. 吸附等溫線(xiàn)吸附等溫線(xiàn)(isotherm)：(1)Langmuir; (2)Temkin; (3)Frumkin. 圖圖3.20 特性吸附對(duì)電毛細(xì)曲線(xiàn)的影響特性吸附對(duì)電毛細(xì)曲線(xiàn)的影響 - + - 陽(yáng)離子的活性順序是：陽(yáng)離子的活性順序是： N(C3 H7)4+ Ti+ K+ 陰離子的活性順序是：陰離子的活性順序是： S2- I- Br- NO3- 圖圖3.21 陰、陽(yáng)離子吸附對(duì)于電毛細(xì)曲線(xiàn)的影響陰、陽(yáng)離子吸附對(duì)于

11、電毛細(xì)曲線(xiàn)的影響 圖圖3.22 吸附等溫線(xiàn)吸附等溫線(xiàn)(isotherm)：(1)Langmuir; (2)Temkin; (3)Frumkin 圖圖3.23 各種吸附等溫線(xiàn)各種吸附等溫線(xiàn) Langmuir isotherm: Langmuir 假設(shè)：假設(shè)：(a)吸附在電極表面的分子彼此沒(méi)有相互作用；吸附在電極表面的分子彼此沒(méi)有相互作用； (b)表面吸附是均勻的；表面吸附是均勻的； (c) 在高濃度時(shí)電極表面達(dá)到飽和濃度在高濃度時(shí)電極表面達(dá)到飽和濃度 (單分子層單分子層)， 用用 s 代表。代表。 = i / s (覆蓋度覆蓋度)。 i i = /(1- ) = i /(s - i ) Esin

12、-Markov coefficient, 吸附等溫線(xiàn)吸附等溫線(xiàn)(isotherm)： Temkin isotherm: 主要考慮到吸附能是主要考慮到吸附能是覆蓋度有關(guān)。覆蓋度有關(guān)。 i = RT/(2g)ln( i i ) g是一個(gè)與吸附物質(zhì)之間相互作用能有關(guān)的參數(shù)。是一個(gè)與吸附物質(zhì)之間相互作用能有關(guān)的參數(shù)。 Frumkin isotherm: i = RT/(2g)ln(i i )ln(s - i )/ i g是正值時(shí)表示吸附物質(zhì)之間是吸引作用，是正值時(shí)表示吸附物質(zhì)之間是吸引作用，g是負(fù)值時(shí)表示是負(fù)值時(shí)表示 吸附物質(zhì)之間是排斥作用。吸附物質(zhì)之間是排斥作用。 常用的研究物質(zhì)在電極表面吸附的電化學(xué)方法有循常用的研究物質(zhì)在電極表面吸附的電化學(xué)方法有循 環(huán)伏安法和電勢(shì)雙階躍法環(huán)伏安法和電勢(shì)雙階躍法 循環(huán)伏安法循環(huán)伏安法 圖圖3.24 吸附物質(zhì)的循環(huán)循環(huán)伏安圖吸附物質(zhì)的循環(huán)循環(huán)伏安圖 圖圖3.25 圖圖3.26 R.H.Wopschall and I.Shain, Anal.Chem.,39, 1514(1967) Why do we view adsorbed neutral Species as being intimately bound to the electrode surface, rather than Bei