聚苯胺修饰超微盘电极上镉(Ⅱ)的表面络合吸附波

Surface Complex Adsorption Wave of Cadmium Ion at Ultra-micro Disc-electrode Modified by Polyaniline

用直径7μm的碳纤维组合成超微圆盘电极,以聚苯胺修饰电级.以阶梯扫描法、循环伏安法、双阶跃计时电量法和交流阻抗法等,研究了Cd2+在该电极上的表面络合吸附特性和电极过程.在循环伏安图上出现两个还原峰,实验和理论都证明,由于电极表面的聚苯胺对Cd2+的特性吸附,形成电活性的表面吸附态络合物.因此,这种表面络合物首先被还原,形成峰电位-0.90V处的表面络合吸附波,还原峰电位比Cd2+直接还原电位(-0.98V)正移,循环反扫时,氧化波无峰形.根据实验数据推测了电极过程的反应机理,证实该还原波具有扩散和表面反应同时控制的表面络合吸附波的特性.理论计算与实验基本一致,并求得了表面吸附态配合物的形成常数、吸附量和表面络合反应的动力学参数.实验还证实,在峰电位-1.06V处的还原波,是Cd2+的表面吸附还原态诱导而产生的催化氢波.

The ultra-micro disc-electrode is combined by adhering with 12-carbon fibers of 7 mum in diameter, after that this electrode is modified by polyaniline in electrolytic polymerization. The absorption characteristics and the electrode process of cadmium ion on this electrode are studied by cyclic voltammetry, step-scan voltammetry, two-step chronocoulometry and AC impedance method. There are two reduction peaks in cyclic voltammetric curve at -0.90 V and -1.06 V, respectively (Fig. 1). Experiments and theory show that the electro-active adsorptive surface complex of Cd2+ is formed from chemical adsorption of Cd2+ by polyaniline at the electrode surface. So, this kind of surface complex rather than Cd2+ is reduced firstly. The potential of the reduction peak is at -0.90 V, which is more positive than the directly reduced potential (-0.98 V) of Cd2+. There is no corresponding oxide peak appearing in cyclic scanning. The reaction mechanism of electrode process is inferred according to experimental data, which proves that the characteristics of reduction peak are simultaneously controlled by diffusing and surface complex reactions. The theoretic curve calculated by this mechanism is agreement with the experimental curve determined by step-scan voltammetry (Fig. 2). The formation constant (K=924) of surface adsorptive complex, the maximum adsorptive capacity (Gamma(o)=2.08x10(-9) mol.cm(-2)), the kinetic parameters (k(f)=4.04x10(-3) mol(-1).cm(3).s(-1), k(b)=4.37x10(-6) s(-1)) of surface complex reaction, the diffusion coefficient( D=8.7x10(-6) cm(2).s(-1)) of Cd2+, the thickness of diffusion layer(delta(d)=2.75x10(-4) cm) and reaction layer(delta(k)=5.14x10(-5) cm) are calculated. Experiment also proves that the reduction peak at -1.06 V is a catalytic reduction peak of hydrogen ion, formed by inductive effect of reduced surface complex of Cd2+.