| Summary: | 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.
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