Guo Si-Xuan, Feldberg Stephen W, Bond Alan M, Callahan Damien L, Richardt Peter J S, Wedd Anthony G
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia.
J Phys Chem B. 2005 Nov 3;109(43):20641-51. doi: 10.1021/jp0528459.
The one-electron reduction of alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61) at a glassy carbon electrode was investigated using cyclic and rotating-disk-electrode voltammetry in buffered and unbuffered aqueous solutions over the pH range 3.45-7.50 with an ionic strength of approximately 0.6 M maintained. The behavior is well-described by a square-scheme mechanism P + e(-) <--> Q (E(1)(0/) = -0.275 V, k(1)(0/) = 0.008 cm s(-1), and alpha(1) = 1/2), PH(+) + e(-) <--> QH(+) (E(2)(0/) = -0.036 V, k(2)(0/) = 0.014 cm s(-1), and alpha(2) = 1/2), PH(+) <--> P + H(+) (K(P) = 3.02 x 10(-6) M), and QH(+) <--> Q + H(+) (K(Q) = 2.35 x 10(-10) M), where P, Q, PH(+), and QH(+) correspond to alpha(2)-Fe(III)(OH)P(2)W(17)O(61), alpha(2)-Fe(II)(OH)P(2)W(17)O(61), alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61), and alpha(2)-Fe(II)(OH(2))P(2)W(17)O(61), respectively; E(1)(0)' and E(2)(0)' are the formal potentials, k(1)(0)' and k(2)(0)' are the formal (standard) rate constants, and K(P) and K(Q) are the acid dissociation constants for the relevant reactions. The analysis for the buffered media is based on the approach of Laviron who demonstrated that a square scheme with fully reversible protonations, reversible or quasi reversible electron transfers with the assumption that alpha(1) = alpha(2), can be well-described by the behavior of a simple redox couple, ox + e(-) <--> red, whose formal potential, E(app)(0)', and standard rate constant, k(app)(0)', are straightforwardly derived functions of pH, as are the values of E(1)(0)', k(1)(0)', E(2)(0)', k(2)(0)', and K(P) (only three of the four thermodynamic parameters in a square scheme can be specified). It was assumed that alpha(app) = 1/2, and the simulation program DigiSim was used to determine the values of E(app)(0)' and k(app)(0)', which are required to describe the cyclic voltammograms obtained in buffered media in the pH range from 3.45 to 7.52 (buffer-related reactions which effect general acid-base catalysis are included in the simulations). DigiSim simulations of cyclic voltammograms obtained in unbuffered media yielded the values of E(1)(0)' and k(1)(0)'; K(Q) was then directly computed from thermodynamic constraints. These simulations included additional reactions between the redox species and H(2)O. The value of the diffusion coefficient of the alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61), 2.92 x 10(-6) cm(2) s(-1), was determined using DigiSim simulations of voltammograms at a rotating disk electrode in buffered and unbuffered media at pH 3.45. The diffusion coefficients of all redox species were assumed to be identical. When the pH is greater than 6, instability of P (i.e., alpha(2)-Fe(III)(OH)P(2)W(17)O(61)) led to the loss of the reactant and precluded lengthy experimentation.
在约0.6 M离子强度且pH范围为3.45 - 7.50的缓冲和非缓冲水溶液中，使用循环伏安法和旋转圆盘电极伏安法研究了在玻碳电极上α(2)-Fe(III)(OH₂)P₂W₁₇O₆₁的单电子还原。该行为可用方形机理很好地描述：P + e⁻ ⇌ Q （E(1)(0/) = -0.275 V，k(1)(0/) = 0.008 cm s⁻¹，且α(1) = 1/2），PH⁺ + e⁻ ⇌ QH⁺ （E(2)(0/) = -0.036 V，k(2)(0/) = 0.014 cm s⁻¹，且α(2) = 1/2），PH⁺ ⇌ P + H⁺ （K(P) = 3.02 × 10⁻⁶ M），以及QH⁺ ⇌ Q + H⁺ （K(Q) = 2.35 × 10⁻¹⁰ M），其中P、Q、PH⁺和QH⁺分别对应α(2)-Fe(III)(OH)P₂W₁₇O₆₁、α(2)-Fe(II)(OH)P₂W₁₇O₆₁、α(2)-Fe(III)(OH₂)P₂W₁₇O₆₁和α(2)-Fe(II)(OH₂)P₂W₁₇O₆₁；E(1)(0)'和E(2)(0)'是形式电位，k(1)(0)'和k(2)(0)'是形式（标准）速率常数，K(P)和K(Q)是相关反应的酸解离常数。缓冲介质的分析基于Laviron的方法，他证明了具有完全可逆质子化、可逆或准可逆电子转移且假设α(1) = α(2)的方形机理，可用简单氧化还原对ox + e⁻ ⇌ red的行为很好地描述，其形式电位E(app)(0)'和标准速率常数k(app)(0)'是pH的直接推导函数，E(1)(0)'、k(1)(0)'、E(2)(0)'、k(2)(0)'和K(P)的值也是如此（方形机理中的四个热力学参数中只有三个可以指定）。假设α(app) = 1/2，并使用模拟程序DigiSim确定E(app)(0)'和k(app)(0)'的值，这是描述在pH范围为3.45至7.52的缓冲介质中获得的循环伏安图所必需的（模拟中包括影响一般酸碱催化的缓冲相关反应）。在非缓冲介质中获得的循环伏安图的DigiSim模拟得出E(1)(0)'和k(1)(0)'的值；然后根据热力学约束直接计算K(Q)。这些模拟包括氧化还原物种与H₂O之间的其他反应。使用在pH 3.45的缓冲和非缓冲介质中旋转圆盘电极上的伏安图的DigiSim模拟确定了α(2)-Fe(III)(OH₂)P₂W₁₇O₆₁的扩散系数值为2.92 × 10⁻⁶ cm² s⁻¹。假设所有氧化还原物种的扩散系数相同。当pH大于6时，P（即α(2)-Fe(III)(OH)P₂W₁₇O₆₁）的不稳定性导致反应物损失并排除了长时间的实验。
