Kinetics of Oxygen Reduction at Modified Platinium Group Electrodes in Alkaline Solution

• Main Authors: Chang, Chia-Chin, 張家欽
• Other Authors: Wen, Ten-Chin
• Format: Others
• Language: zh-TW
• Published: 1996
• Online Access: http://ndltd.ncl.edu.tw/handle/11986707941381672798

博士 === 國立成功大學 === 化學工程學系 === 84 === The electrocatalytic activity of modified platinum group electrodes and kinetics for oxygen reduction in alkaline solutions are studied in this dissertation.The electrocatalytic activity and stability of various thermally prepared titanium electrodes (of metal species Pd, Pt, Ir, Co, Ru, and Ni) for oxygen reduction in 1 moldm-3 KOH are investigated. The reaction pathway of oxygen reduction at Pt, Pd, SPR(coating solution containing 33 mol % Sn, 33 mol % Pd, and 33% mol Ru) electrodes are mainly via a four-electron pathway. At IrO2, Co3O4, RuO2, and NiO electrodes,those are a four-electron via two-electron pathway or a two electron pathway (e.g. NiO and RuO2, the current efficiency of H2O2 production on those electrodes is about 70%). SEM, XRD, and CV results show that most thermally prepared electrodes remainstable during cathodic polarization with purging oxygen, with the exception of the Co3O4(HCl) electrode.Voltammetric results revealed that oxygen reduction on Pt during various positive potentials is significantly influenced by the electrode surface oxide/hydrous oxide states. The amount of Pt oxide increase with increasing positive potentials, while oxygen reduction is inhibited by Pt oxides. The Tafel slopes of oxygen reduction on Pt electrodes after cathodic and anodic polarization are respectively about -60 mV dec-1 and about -120 mV dec-1. The rotating ring disc electrode( RRDE) results show thatthe mechanism of oxygen reduction on Pt and PtO electrodes are proposed to obtainthe corresponding rate constants. The rate constant of H2O2 production on PtO electrode is larger than that on Pt electrode. Meanwhile, the rate constant of H2O2 decomposition on PtO electrode is larger than that on Pt electrode. As for the thermally prepared Pt/Ti electrode, the oxygen reduction behavior at low and high polarizations are respectively similar to those on Pt and on PtO electrodes. The RRDE data analysis reveals that oxygen reduction on Pt and PtO electrodes and thermally prepared Pt/Ti electrode are mainly via a four-electron pathway.As for oxide-derived Pd electrodes, oxygen reduction is investigated by using thetechniques of rotating-disc electrode. On the basis of rotating disc electrode experiments along with further data analysis (i.e. reaction orders, Tafel slopes, etc.), a mechanism of oxygen reduction on a oxide-derived Pd electrode is proposed to obtain the corresponding rate equation. Oxygen reduction on the oxide-derived Pd electrode in alkaline solution at pH from 10.8 to 13.8 is catalyzed by Pd(I) species; has a Tafelslope of -60 mV/decade; depends on the solution pH with a reaction order of -1 in OH- concentration; and owns first order for oxygen concentration. Oxygen reduction at IrO2-coated titanium electrodes was investigated by employing cyclic voltammetry, rotating-disc electrode techniques, and galvanostate electrolysis. Cyclic voltammetric results indicated that oxygen reduction begins during the Ir(III)/Ir(IV) transition on an IrO2 electrode. On the basis of measurements using rotating disc electrode and rotating ring disk electrode together with polarization curves, Tafel slopes, and stoichiometric number determinations, a mechanism foroxygen reduction on an IrO2- coated titanium electrode was proposed. In the lowpolarization region, oxygen reduction with a Tafel slope of -40 mV dec-1 depends on the solution pH and has a reaction order of -2 in OH-. In the high polarization region,the process is practically pH independent. Both cases are first order in the dissolved O2 concentration. The mechanism and rate constants were investigated by using RRDE technique. The RRDE results show that the rate constants for the O2 to H2O2 and to OH- reduction reaction increases with increasing cathodic potential. The rate constant for O2 to H2O2 in high OH- concentration is larger than that in low OH- concentration. The effect of calcination temperature of IrO2 electrodes on oxygen reduction pathway were examined by galvanostatic electrolysis. The galvanostatic electrolysis results show that the reaction pathway on IrO2 electrode at high calcination temperature is mainly two electron pathway to form H2O2. As for RuO2-coated titanium electrodes, oxygen reduction was investigated by employing cyclic voltammetry, rotating-disc electrode techniques,and potentiostate electrolysis. Cyclic voltammetric results indicated that oxygen reduction wascatalyzed by at least some hydrous oxyruthenium species, i.e. Ru(III), at low polarization (E > -0.45V) and by at least some low oxide states of ruthenium species at high polarization (E< -0.45V). On the basis of measurements using a rotating disc electrode together with polarization curves, Tafel slopes, and stoichiometric number determinations, two mechanisms for oxygen reduction on an RuO2- coated titanium electrode are proposed. In the low polarization region, oxygen reduction with a Tafel slope of -270 mV dec-1 depends on the solution pH and has an OH- reaction order of 0.25 when in the pH range of 10.8 to 13.8. In the high polarization region,oxygen reduction proceeds with a Tafel slope of -150 mV dec-1 and the process is practically pH independent. Both cases are first order in dissolved O2.. The potentiostat electrolysis results show that at low polarization, the major product ofoxygen reduction is hydrogen peroxide and at high polarization (E<-0.45 V), oxygen reduction is mainly via a four electron pathway.The electrocatalytic activity of various thermally prepared graphite electrodes (of metal species Pd, Pt, Ir, Co, Ru, and Ni) for oxygen reduction in 1 mol dm-3 KOH is investigated by galvanostatic electrolysis. From the galvanostatic electrolysis results,the oxygen reduction behavior on these electrodes are almost similar to those on various thermally prepared titanium electrodes.In summary, oxygen reduction on thermally prepared electrodes is a complex process, with mechanisms determined by the electrode materials as well as their physical,chemical and adsorption properties. In general, the surface of oxide active species iscovered by OH groups and H2O molecules in aqueous solution. The adsorption of O2 molecules on oxide active species is then hindered by this steric hindrance from OH/H2O covering, which possibly causes a two-electron pathway for oxygen reduction such as IrO2, Co3O4, RuO2, NiO electrodes. However, oxygen adsorptionon metal active species is possibly the metal-O2 interaction in a "bridge" configuration,which leads to O－bond scission and consequently causes a four-electron pathway foroxygen reduction such as Pt, Pd, SPR electrodes.