The electrochemistry of gold–platinum alloys

Journal ofElectroanalytical

Chemistry

Journal of Electroanalytical Chemistry 570 (2004) 243–255

www.elsevier.com/locate/jelechem

The electrochemistry of gold–platinum alloys

H. M€oller *, P.C. Pistorius

Department of Materials Science and Metallurgical Engineering, University of Pretoria, Mineral Sciences Building, Pretoria 0002, South Africa

Received 22 January 2003; received in revised form 30 October 2003; accepted 4 April 2004

Available online 19 June 2004

Abstract

The electrochemical properties of gold, platinum and gold–platinum alloy electrodes under different heat treatment conditions

have been studied in 0.5 M H2SO4 and 0.5 M NaOH. The electro-oxidation of 0.1 M ethylene glycol in 0.5 M NaOH at these

electrodes has also been studied. It was found that all the gold–platinum electrodes are more active for ethylene glycol electro-

oxidation than both pure gold and platinum, and that the gold–platinum electrodes in the solid solution condition are more active

than the two-phase electrodes. Poisoning of all the electrodes occurs during electrolysis of ethylene glycol at a fixed potential.

Potential pulsing is successful in removing the poisoning species formed at the pure gold and pure platinum electrodes. High ap-

parent current densities are found during the first few cycles at the Au–Pt alloy electrodes. These high current densities are also

associated with more severe poisoning – than at both pure gold and platinum – and longer cleaning cycles are needed to remove the

poisons at these electrodes.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Gold–platinum alloys; Heat treatment; Cyclic voltammetry; Ethylene glycol; Electrode poisoning

1. Introduction

The electro-oxidation of organic compounds at noble

metal electrodes has been studied extensively for possi-

ble applications in electrochemical power sources [1] andelectrochemical wastewater treatment [2]. Gold is the

noblest and most inert of all metals. It also possesses

weak chemisorbing properties due to the absence of

vacancies in its d-bands. Surprisingly, it still displays a

wide range of electro-oxidation activity – especially in

alkaline solutions [3]. It is known that a bimetallic

electrode is usually more active for the electro-oxidation

of organics than the respective pure metals [1]. The goldalloy containing 40% platinum (by mass) has been

identified [4] as being the most active for the oxidation

of various organics in base. However, the effect of the

microstructure (phase composition) of gold–platinum

alloys on their electrochemical properties has largely

been ignored in the past.

* Corresponding author. Tel: +27-12-420-3191; fax: +27-12-362-

5304.

E-mail address: [email protected] (H. M€oller).

0022-0728/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jelechem.2004.04.003

In this paper, the heat treatments of the gold–plati-

num alloys are discussed, as well as the electrochemical

properties of the heat-treated electrodes in acidic and

alkaline solutions without ethylene glycol in the solu-

tion. The electro-oxidation of ethylene glycol in 0.5 MNaOH at gold, platinum and gold–platinum alloy elec-

trodes under different heat treatment conditions is also

reported. The electro-oxidation of ethylene glycol at

gold, platinum and gold–platinum alloy electrodes has

been studied by others [5–7]. However, the influence of

the phase composition of the alloy electrodes on their

electro-oxidation activity has not been investigated

before.

2. Experimental

2.1. Materials

The gold–platinum alloys were manufactured by

melting together gold and platinum (both 99.99%) in therequired ratios in an arc-furnace with a water-cooled

copper hearth, under a protective argon atmosphere.

The material (weighing approximately 10 g) was turned

mail to: [email protected]

244 H. M€oller, P.C. Pistorius / Journal of Electroanalytical Chemistry 570 (2004) 243–255

around after each melt and re-melted three times to

ensure homogeneity. Two samples were prepared, one

containing 60% Au and 40% Pt and the other with 50%

Au and 50% Pt (compositions in mass percentages, as

are all other compositions reported in this paper).

2.2. Heat treatments

Different heat treatments were applied to vary the

amounts and compositions of phases in the two alloys.

Fig. 1 shows the gold–platinum phase diagram; the two

alloys in this study are indicated by vertical broken lines.

As the diagram shows, the equilibrium phases changewith cooling from the fully liquid region: upon initial

solidification, the first solid (aÞ to form is more plati-

num-rich than the remaining liquid. Upon full solidifi-

cation, a single-phase region is entered, before – upon

further cooling – the alloy decomposes into two different

face-centred cubic phases (a1 and a2). The a1 phase (thegold-rich phase) becomes steadily poorer in platinum at

lower temperatures; similarly, the platinum-rich a2phase becomes poorer in gold with cooling. The specific

heat treatments, which were applied to the two alloys,

are detailed below.

Table 1

The heat treatments of the gold–platinum alloy samples

Composition Heat treatment

(code Fig. 17)

Phase composition

60Au–40Pt 1300 �C, 1 h (1300) Dual phase, non-porous

60Au–40Pt 1200 �C, 24 h (1200) Single-phase, porous

60Au–40Pt 800 �C, 50 h (800) Dual-phase, porous

60Au–40Pt 600 �C, 100 h (600) Dual-phase, porous

50Au–50Pt ‘‘Ductile’’-treatment

(50–50-dual)

Dual-phase, non-porous

50Au–50Pt 1250 �C, 24 h (50–50-soln) Single-phase, non-porous

Fig. 1. Gold–platinum phase diagram [8]. The vertical broken lines at

40% Pt and 50% Pt indicate the two alloys studied in this work. The

curved dot-dash line indicates the boundary of the spinodal region.

Reprinted with permission of ASM International.

After the heat treatments, samples of the alloys were

mounted in black phenolic (thermosetting) resin and

polished to a 1-lm finish with diamond paste. The

samples were studied unetched, with a combination of

optical microscopy, elemental mapping using energydispersive X-ray analysis in a scanning electron micro-

scope and X-ray diffraction (with copper Ka radiation).

The different heat treatments and resulting phase

compositions are summarised in Table 1, with more

discussion below.

2.2.1. Heat treatments applied to the 60Au–40Pt alloy2.2.1.1. 1300 �C for 1 h. The sample was kept at 1300 �Cfor 1 h before quenching in water. After quenching, the

sample was cold-rolled to a thickness of approximately

2 mm. Partial melting of the sample is expected to occur

at 1300 �C (yielding a gold-rich liquid phase and a

platinum-rich solid-solution a phase, see Fig. 1). The

resulting dual-phased microstructure is shown in Fig. 2.

It can be seen that the platinum-rich areas formed bythis heat treatment are relatively large, with typical di-

ameters of 30–50 lm.

Approximate equilibrium

composition of Au-rich phase

Approximate equilibrium

composition of Pt-rich phase

wt% Au wt% Pt wt% Au wt% Pt

85 15 40 60

– – – –

75 25 5 95

81 19 2 98

68–75 25–32 5–10 90–95

– – – –

Fig. 2. Dual-phase microstructure produced by a heat-treating sample

containing 60% Au and 40% Pt at 1300 �C (partially molten region).

Optical micrograph (unetched).

Fig. 4. Dual-phased microstructure produced by heat-treating the

60Au–40Pt sample of Fig. 3 at 800 �C for 50 h. Small platinum-rich

areas nucleated heterogeneously. Elemental map based on L lines of

energy-dispersive X-ray analysis.

H. M€oller, P.C. Pistorius / Journal of Electroanalytical Chemistry 570 (2004) 243–255 245

2.2.1.2. 1200 �C for 24 h. Samples of the 60Au–40Pt

alloy in the 1300 �C heat treatment condition were heat-

treated at 1200 �C for 24 h, followed by water quench-

ing. This heat treatment is designed to produce a

homogeneous solid solution of platinum in gold (seeFig. 1). As Fig. 3 shows, a single-phased solid solution

was indeed produced, but porosity formed during this

heat treatment. The porosity can be explained by the

Kirkendall effect: the rates at which two types of atoms

of a binary solution diffuse are not the same – the

element with the lower melting point diffuses faster [9].

The melting points of gold and platinum are 1064 and

1769 �C, respectively (Fig. 1). During the 1200 �C heattreatment, more gold atoms left the gold-rich areas than

platinum atoms which arrived to take their place. Since,

every time an atom makes a jump, a vacancy moves in

the opposite direction, an unequal flow in the two types

of atoms must result in an equivalent flow of vacancies

in the reverse direction. This resulted in the formation of

pores throughout the sample in the areas that had been

Au-rich prior to the solutionising heat treatment. Theporosity greatly increased the surface area of this ma-

terial, as the electrochemical results (later in this paper)

show.

2.2.1.3. 800 �C for 50 h and 600 �C for 100 h. Samples in

the solid solution condition (1200 �C, 24 h) were further

heat-treated at, respectively, 800 �C for 50 h and 600 �Cfor 100 h, followed by water quenching in each case. Anelemental map (using energy-dispersive X-ray analysis

based on the L lines of the two metals) of the 60Au–40Pt

alloy heat-treated at 800 �C for 50 h is shown in Fig. 4.

The distribution of phases indicates that heterogeneous

nucleation of the platinum-rich areas occurred, proba-

bly on grain boundaries. The platinum-rich areas are

small, with diameters of only 1–3 lm. Heterogeneous

nucleation of the platinum-rich areas was observed forthe 60Au–40Pt alloy samples heat-treated at 800 and

Fig. 3. Single-phased porous solid solution produced by heat-treating

the 60Au–40Pt sample of Fig. 2 at 1200 �C for 24 h. Optical micro-

graph (unetched).

600 �C, because this composition lies outside the spin-

odal decomposition region and second-phase formation

occurs by conventional nucleation and growth (unlike

the 50Au–50Pt alloy, as discussed below).

2.2.2. Heat treatments applied to 50Au–50Pt alloy

2.2.2.1. ‘‘Ductile’’ heat treatment. A 50Au–50Pt sample

was given a heat treatment to produce a ductile material[10]. The following procedure was used: The sample was

placed in a furnace at 1000 �C for 2 h. The sample was

left in the furnace and the temperature was reduced to

800 �C (which took approximately 1.5 h). Once the

temperature reached 800 �C, the sample was left in the

furnace for a further 2 h. The sample was then rapidly

cooled by water quenching and subsequently cold-rol-

led. The whole process (heat treatment and cold rolling)was performed three times. The sample therefore spent

16.5 h (5.5� 3) in total in the furnace. The resulting

microstructure is shown in Fig. 5. Evidently nucleation

of the platinum-rich areas occurred throughout the

Fig. 5. Dual-phased microstructure produced by subjecting a 50Au–

50Pt sample to the ‘‘ductile’’ heat treatment (2 h at 1000 �C, cooling to

800 �C over 1.5 h, holding at 800 �C for 2 h). Fine, uniform micro-

structure produced by spinodal decomposition. Optical micrograph

(unetched).


matrix, producing a uniform microstructure. Uniform

microstructures are obtained during spinodal decom-

position, because there is no thermodynamic barrier to

nucleation [11]; this alloy composition does lie in the

spinodal decomposition region in the temperature range800–1000 �C (see Fig. 1).

2.2.2.2. Solid solution heat treatment. A 50Au–50Pt

sample in the ‘‘ductile’’ (dual-phased) heat treatment

condition was solutionised at 1250 �C for 24 h (see

Fig. 1) followed by water quenching. X-ray diffraction

confirmed that a single-phase solid solution was pro-

duced with this heat treatment. No significant porositywas formed during solutionising in this case. The reason

why Kirkendall porosity was not formed in this case

during solutionising is not clear; perhaps the platinum-

rich areas were too small prior to solutionising to pro-

duce significant porosity.

2.3. Electrochemical experiments

The experimental work was performed with poly-

crystalline gold (99.99%), platinum (99.99%), 60Au–

40Pt and 50Au–50Pt electrodes. The electrodes were

disc-shaped with 6-mm diameters. All the electrodes

were mounted in a black phenolic thermosetting resin.

Electrical contact was achieved through the rear of the

mounting. The apparent surface area of 0.28 cm2 was

used in all cases to calculate current densities. A water-jacketed perspex electrochemical cell was used for all the

experiments. The temperature was controlled at 25 �Cwith water circulated from a thermostatted water bath,

through the water jacket of the cell. The cover of the

perspex cell had six openings, which allowed the inser-

tion of two platinum wire counter electrodes, the

working electrode, the Luggin capillary, the tube for

nitrogen purging and a thermometer. The surface areaof the counter electrodes was significantly larger than

that of the working electrode. All potentials quoted in

this study are with respect to the silverjsilver chloride

reference electrode (SSC) (with saturated KCl filling

solution).

The experiments in acid were conducted in 0.5 M

H2SO4 solution. The solution was prepared by using

CP grade H2SO4 and twice-distilled water. The exper-iments in base were conducted in 0.5 M NaOH solu-

tion. The solution was prepared by using twice-distilled

water and CP grade NaOH pellets. Prior to each ex-

periment, the solution was purged with nitrogen for

30 min to remove dissolved oxygen. A Solartron 1287

electrochemical interface was used for the cyclic vol-

tammetry experiments. The electrode potential was

cycled between the values for onset of O2 and H2

evolution until the I–E curves were reproducible. It was

found that approximately 15 cycles were needed to

obtain reproducible I–E curves. The 15th cycle is

shown in all cyclic voltammograms reported here. The

scan rate employed was 50 mV/s.

Analytical grade ethylene glycol was used to prepare

solutions containing 0.1 M ethylene glycol in 0.5 M

NaOH. Rotation of the electrodes produced an unac-ceptably high level of noise in the cyclic voltammo-

grams, and hence magnetic stirring was used to agitate

the solution. A rotation speed of 500 rev/min was used.

Cyclic voltammetry was performed as follows:

(a) The 0.5 M NaOH solution (without ethylene glycol)

was purged with nitrogen for 30 min to remove dis-

solved oxygen.

(b) The electrode potential was cycled for 15 cycles be-tween the values for onset of H2 and O2 evolution

()0.95 and 0.65 V, respectively) until the current–

potential curves were reproducible. To check consis-

tency, the cyclic voltammogram (15th cycle) of the

electrode was compared with that determined previ-

ously (in 0.5 M NaOH).

(c) The ethylene glycol was added to the solution.

(d) The electrode was activated at 1.2 V for 10 s (the so-lution was stirred during activation). Poisoning spe-

cies and the electrode surface are oxidised at this

potential. Oxygen gas evolution also occurs at 1.2 V.

(e) The electrode potential was then cycled for 10 cycles

between the values for onset of H2 and O2 evolution.

The solution was stirred in one set of experiments

and not stirred in another.

(f) The electrode was reactivated at 1.2 V for 10 s (thesolution was stirred during reactivation).

(g) The electrode potential was cycled once between the

values for onset of H2 and O2 evolution to see if re-

activation had been successful. The solution was

stirred in one set of experiments and not stirred in

another.

In an additional set of experiments, intermittent po-

tentiostatic oxidation of ethylene glycol was performed,with periodic electrochemical cleaning of the electrode

surface. Solution preparation was identical to step (a)

above. The electrode was prepared by cycling its po-

tential 15 times between the potentials for the onset of

H2 and O2 evolution. The ethylene glycol was then ad-

ded to the solution. The intermittent oxidation test was

subsequently performed as a series of potentiostatic

treatments as follows:(h) The electrode was first cleaned at 1.2 V for 1.5 s.

(i) The surface oxides formed during activation were

then removed at )0.6 V for 1.5 s.

(j) The electrolysis of ethylene glycol was performed for

10 s, controlling the electrode at a potential just be-

low that at which the maximum current density had

been observed during cyclic voltammetry, for the

specific electrode. During this 10-s period the cur-rent density decreased (‘‘poisoning’’ occurred).

Steps (h)–(j) were repeated for 100 cycles. The total

cleaning period – anodic and cathodic legs – was chosen


to be only 3 s per cycle to maximise the fraction of the

total time spent at the electrolysis potential. The solu-

tion was stirred throughout.

Fig. 7. Cyclic voltammograms of 60Au–40Pt electrodes in deaerated

0.5 M H2SO4 at 25 �C. Fifteenth cycle shown. Scan rate 50 mV/s. The

two curves are for two different heat treatment conditions: dual-phase,

non-porous (treated at 1300 �C, thinner line) and single-phase, porous

(treated at 1200 �C, heavier line).

3. Electrochemical testing: results and discussion

3.1. Behaviour in 0.5 M H2SO4

3.1.1. Pure gold and platinum

Cyclic voltammograms for pure gold and platinum in

0.5 M H2SO4 are shown in Fig. 6. Monolayer oxide

formation at gold commences at 1.05 V during the po-sitive sweep followed by oxygen gas evolution at higher

potentials. The surface oxide is reduced at 0.9 V during

the negative sweep.

Hydrogen desorption/adsorption at platinum occurs

in the region of )0.2 to 0.1 V. Surface oxidation com-

mences at approximately 0.55 V. The surface oxide is

reduced during the negative sweep at 0.5 V.

3.1.2. 60Au–40Pt alloy

A cyclic voltammogram for the 1300 �C heat-treated

electrode (non-porous, dual-phased) is shown in Fig. 7;

features corresponding to both pure gold and platinum

can be seen. The alloy has a hydrogen adsorption/de-

sorption region ()0.2 to 0.1 V). Two oxide reduction

peaks are found. The one peak corresponds to that of

platinum and the other to gold.A cyclic voltammogram for the 1200 �C heat-treated

electrode (porous, single-phased) is also shown in Fig. 7.

The apparent current densities obtained with this elec-

trode are very high due to the porosity; impedance

testing confirmed the increased exposed area, through a

decrease (by a factor of about 3) in the (capacitive)

Fig. 6. Cyclic voltammograms of pure gold (heavier line) and pure

platinum (thinner line) in deaerated 0.5 M H2SO4 at 25 �C. Fifteenthcycle shown. Scan rate 50 mV/s.

impedance. Another interesting feature of the cyclic

voltammogram is that two oxide reduction peaks are

observed, even though the electrode is a solid solution of

platinum in gold. Woods [12] also found that gold–platinum solid solutions have two oxide reduction

peaks.

Cyclic voltammograms for electrodes of the 60Au–

40Pt alloy after heat treatment in the two-phase region

(at 800 and 600 �C, respectively) are shown in Fig. 8.

Very high apparent current densities due to oxygen gas

Fig. 8. Cyclic voltammograms of porous dual-phase 60Au–40Pt elec-

trodes in deaerated 0.5 M H2SO4 at 25 �C. Fifteenth cycle shown. Scan

rate 50 mV/s. The two curves are for electrodes, which were held at two

different temperatures following the 1200 �C treatment: 600 �C (thinner

line; platinum-rich areas lower in gold) and 800 �C (heavier line;

platinum-rich areas higher in gold). Note that the scale is the same as

in Fig. 7.


evolution are found at the 600 �C heat-treated electrode;

this is thought to be related to the composition of the

platinum-rich phase in this dual-phased alloy: the

equilibrium composition of the platinum-rich areas in

this sample is 98Pt–2Au (Table 1). The fact that theseareas are almost pure platinum may explain the

low oxygen gas evolution potential. The equilibrium

gold content of the platinum-rich phase in the 800 �Cheat-treated sample is higher than that of the 600 �Cheat-treated sample (Table 1). This is likely to cause the

oxygen gas evolution reaction at the 800 �C heat-treated

sample to be inhibited more than at the 600 �C heat-

treated sample (Fig. 8).


Cyclic voltammograms for the 50Au–50Pt electrodes

are shown in Fig. 9. As was the case with the solution-

ised 60Au–40Pt electrode, the solutionised 50Au–50Pt

electrode also has two oxide reduction peaks. Kirkendall

porosity was not found in the sample after the solid

solution heat treatment and it can be seen that the two50Au–50Pt samples have similar apparent current den-

sities, despite their very different microstructures.

However, the oxygen gas evolution reaction is inhibited

at the solid solution sample compared to the ductile

(dual-phase) sample. If one considers the two tempera-

ture limits that were used for the ‘‘ductile’’ heat treat-

ment (Fig. 1 [8]), it is seen that the Pt-rich areas of this

sample have an equilibrium gold content of 5–10%(Table 1). The platinum atoms in the Pt-rich phase of

this sample are, therefore, surrounded by fewer gold

atoms than the platinum atoms in the solid solution

sample. This results in oxygen evolution at a lower

overpotential at the ‘‘ductile’’ sample.

Fig. 9. Cyclic voltammograms of non-porous 50Au–50Pt electrodes in

deaerated 0.5 M H2SO4 at 25 �C. Fifteenth cycle shown. Scan rate

50 mV/s. The two curves are for two different heat treatment condi-

tions: dual-phase (‘‘ductile’’ heat treatment, heavier line) and single-

phase (treated at 1250 �C, thinner line). Note that the scale is the same

as in Fig. 6.

3.2. Behaviour in 0.5 M NaOH

3.2.1. Pure gold and platinum

Cyclic voltammograms for pure gold and platinum in

0.5 M NaOH are shown in Fig. 10. Monolayer oxideformation at gold commences at 0.1 V during the posi-

tive sweep, followed by oxygen gas evolution at higher

potentials. The surface oxide is reduced at 0.06 V during

the negative sweep. A small cathodic peak at )0.18 V is

also observed during the negative sweep. This peak is

thought to be due to the reduction of hydrous gold oxide

species formed on the gold surface at the upper end of

the cycle [13]. Hydrogen desorption/adsorption occursat platinum in the region of )0.9 to )0.55 V. Surface

oxidation commences at approximately )0.4 V. The

surface oxide is reduced during the negative sweep at

)0.3 V. Hydrogen gas evolution occurs at potentials

negative of )0.9 V.


A cyclic voltammogram for the 1300 �C heat-treatedelectrode in 0.5 M NaOH is shown in Fig. 11 and, as was

the case in acid solution, features corresponding to both

pure gold and platinum can be seen. The alloy has

a hydrogen adsorption/desorption region ()0.9 to

)0.55 V). Surface oxide forms at platinum (or platinum-

rich areas) at )0.4 V during the positive sweep. Surface

oxide formation at gold (or gold-rich areas) commences

at 0.1 V. Two oxide reduction peaks are found in thesame potential regions for oxide reduction on pure

platinum and pure gold. Hydrogen gas evolution occurs

at potentials negative of )0.9 V.

A cyclic voltammogram for the porous 1200 �C heat-

treated electrode is also shown in Fig. 11. The apparent

current densities obtained with this electrode are very

Fig. 10. Cyclic voltammograms of pure gold (heavier line) and pure

platinum (thinner line) in deaerated 0.5 M NaOH at 25 �C. Fifteenthcycle shown. Scan rate 50 mV/s.

Fig. 11. Cyclic voltammograms of 60Au–40Pt electrodes in deaerated

0.5M NaOH at 25 �C. Fifteenth cycle shown. Scan rate 50 mV/s. The

two curves are for two different heat treatment conditions: dual-phase,

non-porous (treated at 1300 �C, heavier line) and single-phase, porous

(treated at 1200 �C, thinner line).


high (due to the porosity) and two oxide reduction peaks

are again observed. Similar results were found in the

sulphuric acid solution (Fig. 7).

Cyclic voltammograms for electrodes of the 60Au–

40Pt alloy in 0.5 M NaOH, after heat treatment in thetwo-phase region (at 800 and 600 �C, respectively), areshown in Fig. 12. The oxygen gas evolution reaction in

alkaline solution is not inhibited on the 800 �C heat-

treated sample compared to the 600 �C heat treated

sample (Fig. 8). This implies that the presence of gold in

the platinum-rich phase is not as effective in inhibiting

Fig. 12. Cyclic voltammograms of porous dual-phased 60Au–40Pt

electrodes in deaerated 0.5 M NaOH at 25 �C. Fifteenth cycle shown.

Scan rate 50 mV/s. The two curves are for electrodes, which were held

at two different temperatures following the 1200 �C treatment: 600 �C(heavier line; platinum-rich areas lower in gold) and 800 �C (thinner

line; platinum-rich areas higher in gold). Note that the scale is the same

as in Fig. 11.

the oxygen evolution reaction in base (compare with

Fig. 8). The oxygen gas evolution reaction at gold in

acid commences at approximately 1.65 V (Fig. 6) and in

base at 0.75 V (Fig. 10). The corresponding values for

oxygen evolution at platinum are 1.25 V in acid (Fig. 6)and 0.60 V in base (Fig. 10). These values compare well

with those found in the literature. Burke and Nugent

[13] found that oxygen evolution starts at gold in acid at

1.7 V (1 M H2SO4, 25 �C) and in base at 0.7 V (1 M

NaOH, 25 �C). Heyd and Harrington [14] found a po-

tential of 1.28 V for the onset of oxygen evolution at

platinum in acid (0.5 M H2SO4, 25 �C). For platinum in

base (0.1 M NaOH, 25 �C), a potential of 0.62 V wasfound by Xia and Birss [15]. The difference in potential

at which oxygen starts to be evolved at gold and plati-

num is generally about 0.40 V in acid and 0.10–0.15 V in

base. Burke and Nugent [13] postulated that oxygen gas

evolution at gold in base over the range 0.7–1.0 VSSC is

catalysed in a transient manner by some type of hydrous

gold oxide species. They pointed out that regular oxygen

gas evolution at gold in base occurs only above 1.0 V.The presence of gold in the platinum-rich areas will

therefore be not as effective in inhibiting the oxygen

evolution reaction in base as it is in acid. This might be

detrimental to the use of these alloys for waste-water

treatment, since oxygen gas evolution lowers the current

efficiency during the electro-oxidation of organic

compounds [16].


The apparent current densities of the two 50Au–50Pt

samples (under the different heat treatment conditions)

are similar (Fig. 13). In both cases, two oxide reduction

peaks are observed. The similarity in current densities

for the dual-phased and single-phased alloys reflects the

absence of porosity in the solution-treated sample. The

oxygen gas evolution reaction at the two electrodes issimilar in base (Fig. 13). This was not the case in acid,

where the oxygen evolution reaction was inhibited more

at the solid solution electrode compared to the dual-

phased ‘‘ductile’’ sample (Fig. 9). This observation

confirms that gold in gold–platinum alloys is less effec-

tive at inhibiting oxygen evolution in base than in acid.

3.3. Electro-oxidation of ethylene glycol in base

3.3.1. Cyclic voltammograms: pure gold and platinum

Cyclic voltammograms (first and 10th cycles) for gold

with 0.1 M ethylene glycol in the solution are shown in

Fig. 14. The solution was not stirred in Fig. 14(a) and

stirred in Fig. 14(b). The first cycles after reactivation of

the gold electrode are also shown in Fig. 14(a) and (b).

It is seen from Fig. 14 that ethylene glycol electro-oxidation commences during the positive sweep at a

potential of )0.3 V. This is in the potential region before

the surface oxide forms on gold. It has been suggested

Fig. 14. Cyclic voltammograms for pure gold in deaerated 0.5 M

NaOH containing 0.1 M ethylene glycol. Scan rate 50 mV/s. The

results for the first cycle are shown with the heaviest line, that for the

10th cycle as a lighter line and the first subsequent cycle after reacti-

vation at 1.2 V for 10 s as the lightest line. Solution stagnant in (a) and

agitated in (b).

Fig. 13. Cyclic voltammograms of non-porous 50Au–50Pt electrodes

in deaerated 0.5 M NaOH at 25 �C. Fifteenth cycle shown. Scan rate

50 mV/s. The two curves are for two different heat treatment condi-

tions: dual-phase (‘‘ductile’’ heat treatment, heavier line) and single-

phase (treated at 1250 �C, thinner line). Note that the scale is the same

as in Fig. 10.


that organic compounds are oxidised in the presence of

AuOH formed in the pre-monolayer region [17].

During the first positive scan, a peak current density

of 16 mA/cm2 is found at 0.27 V when the solution is not

stirred (Fig. 14(a)) and a maximum of 10.5 mA/cm2 at0.25 VSSC when stirred (Fig. 14(b)). Hauffe and Heit-

baum [18] also found that the peak currents were smaller

when the solution was agitated, suggesting that stirring

accelerates transport of partially oxidised intermediates

into the solution. These intermediates cannot be oxi-

dised at the electrode surface, resulting in lower current

densities.

The electro-oxidation of ethylene glycol is apparentlyinhibited by the surface oxide on the gold electrode from

0.25 to 0.65 V during the positive sweep (Fig. 14).

During the negative sweep, oxidation of ethylene glycol

commences only after the surface gold oxide has been

reduced, reaching a current density maximum of

3.5 mA/cm2 (not stirred) and 2.2 mA/cm2 (stirred) at

0.03 V.

The peak current densities of the 10th cycles are lowerthan for the first cycles (see Fig. 14(a) and (b)), indi-

cating electrode poisoning. Kadirgan et al [6] found,

using electromodulated infrared reflectance spectros-

copy (EMIRS), that CO poisons are formed during the

electro-oxidation of ethylene glycol at gold in base.

During the reactivation procedure (10 s at 1.2 V), the

poisons (and the electrode surface) are oxidised. The

cyclic voltammograms (see Fig. 14(a) and (b)) show thatthe original peak current densities (before poisoning)

can be regained by such reactivation.

Similar effects are observed for platinum (Fig. 15), on

which ethylene glycol electro-oxidation commences

during the positive sweep at a potential of )0.6 V (a

lower potential than on gold). This, as with gold, is in

the potential region before the surface oxide is formed.

The electro-oxidation of ethylene glycol is inhibited bythe surface oxide on the platinum electrode. Xia and

Birss [15] postulated that hydroxide is adsorbed in the

positive sweep while hydrogen is desorbed over the po-

tential region from )0.9 to )0.55 V. It is generally ac-

cepted that the electro-oxidation of ethylene glycol at

platinum occurs in the presence of OHads species [19].

Peak current densities are smaller for platinum than

for gold. As with gold, stirring lowers the peak currentdensity on platinum, from 13.5 mA/cm2 at )0.15 V

in the unstirred case (Fig. 15(a)) to 8 mA/cm2 at

)0.18 VSSC with stirring (Fig. 15(b)).

During the negative sweep, oxidation of ethylene

glycol commences only after the surface platinum oxide

has been reduced, reaching a current density maximum

of 1.5 mA/cm2 (not stirred) and 0.4 mA/cm2 (stirred) at

)0.35 V.The peak current densities of the 10th cycles are also

lower than for the first cycles (see Fig. 15(a) and (b)). As

was the case for gold, this is apparently due to electrode

Fig. 16. Cyclic voltammograms for a non-porous dual-phased elec-

trode (60Au–40Pt, 1300 �C heat treatment) in deaerated 0.5 M NaOH

containing 0.1 M ethylene glycol. Scan rate 50 mV/s. The results for

the first cycle are shown with the heaviest line, that for the 10th cycle as

a lighter line and the first subsequent cycle after reactivation at 1.2 V

for 10 s as the lightest line. Solution stagnant in (a) and agitated in (b).

Note that the current densities are significantly higher than for pure

gold and pure platinum (Figs. 14 and 15).

Fig. 15. Cyclic voltammograms for pure platinum in deaerated 0.5 M

NaOH containing 0.1 M ethylene glycol. Scan rate 50 mV/s. The re-

sults for the first cycle are shown with the heaviest line, that for the

10th cycle as a lighter line and the first subsequent cycle after reacti-

vation at 1.2 V for 10 s as the lightest line. Solution stagnant in (a) and

agitated in (b). The sensitivity of the current density scale is identical to

that in Fig. 14.


poisoning. Hahn et al. [20] found that the adsorption of

ethylene glycol at platinum was dissociative. Almost

equal amounts of bridge-bonded and linearly bonded

CO species were found. The linearly bonded CO is

thought to be responsible for the poisoning of the elec-

trode [20].

The cyclic voltammograms (Fig. 15(a) and (b)) show

that the original peak current densities at platinum(before poisoning) can also be regained by reactivation.

3.3.2. Cyclic voltammograms: Au-Pt alloy electrodes

Despite differences in composition and microstruc-

ture, the cyclic voltammograms of all the alloy elec-

trodes for the solution containing 0.1 M ethylene glycol

have several common features. As an example, results

for a non-porous dual-phased electrode (60Au–40Pt,1300 �C heat treatment) are shown in Fig. 16 (solution

not stirred for results in Fig. 16(a) and stirred for

Fig. 16(b)). These show that the electro-oxidation of

ethylene glycol at the alloy electrode occurs in two po-

tential regions: the first region is in the same potential

range as for pure platinum and the second in the same

potential range as pure gold. This was observed whether

the alloy was single-phased (that is, an atomic-scalemixture of platinum and gold) or dual-phased as here

(separated into gold-rich and platinum-rich areas of

several microns in size). As with pure gold and pureplatinum, the current densities on the alloy electrode are

lower when the solution is stirred.

The rate of ethylene glycol oxidation in the ‘‘plati-

num-like’’ potential region is much higher than on pure

platinum, but conversely the rate in the ‘‘gold-like’’

potential region is much lower than on pure gold

(compare with Figs. 14 and 15). While the current

densities on the alloy in the platinum-like potential re-gion decrease from the first to the 10th cycles (as with

the pure electrodes), the current densities in the gold-like

potential region increase during cycling (but still remain

substantially lower than on the pure gold electrode).

This difference persists after reactivation at 1.2 V: the

peak current density in the platinum-like potential re-

gion regains its original value and the current density in

the gold-like region decreases again. The cyclic vol-tammograms in Fig. 16 indicate that the alloy electrode

is more susceptible to poisoning than pure gold and pure

platinum: for the alloy electrode, the decrease in peak

current density from the first to the 10th cycles is larger


(relative to the original current density) than for the

pure metals. This effect was confirmed by the potentio-

static results, which are presented later in this paper.

While the same general features were observed in the

cyclic voltammograms of all the alloy electrodes, thepeak current densities did depend on microstructure.

This is shown in summarised form in Fig. 17, which

shows the peak current densities for the first and 10th

scans, for solutions unstirred (Fig. 17(a)) and stirred

(Fig. 17(b)). Table 1 describes the microstructures of the

electrodes mentioned in Fig. 17.

The following observations are made from Fig. 17:

• Au–Pt electrodes in the single-phased solid-solutioncondition (‘‘1200’’ and ‘‘50–50-soln’’ in Fig. 17) are

more active for the electro-oxidation of ethylene gly-

col than the dual-phase electrodes (‘‘1300’’, ‘‘800’’,

‘‘600’’ and ‘‘50–50-dual’’). One suggestion is that this

can be explained by the ‘‘third-body effect’’, as fol-

lows [21]: a platinum atom is presumed to be less

likely to be poisoned by a strongly bound intermedi-

ate when it is surrounded by gold atoms, and sinceplatinum atoms are on average surrounded by fewer

gold atoms in the two-phase electrodes than in the so-

Fig. 17. Peak current densities during the first and 10th cyclic vol-

tammograms, as measured in deaerated 0.5 M NaOH containing 0.1

M ethylene glycol. Scan rate 50 mV/s. Refer to Table 1 for a de-

scription of the microstructures of the different electrodes. Solution

stagnant in (a) and agitated in (b).

lid solution electrodes, the two-phase electrodes are

more likely to be poisoned, giving lower current den-

sities. However, this suggestion is not supported by

the observation that the alloy electrodes do undergo

poisoning (as is evident from the large reduction incurrent density from the first to the 10th cycle, see

Figs. 16 and 17). Hence, while the origin of the en-

hanced activity of the solid-solution electrodes (com-

pared with both the pure metals and the dual-phase

alloys) is currently unknown, the enhanced activity

is clearly not caused by elimination of poisoning.

• While stirring of the solution significantly reduces the

current densities at the non-porous electrodes (de-noted by ‘‘Au’’, ‘‘Pt’’, ‘‘50–50-soln’’, ‘‘1300’’ and

‘‘50–50-dual’’ in Fig. 17), stirring has little effect on

the current densities at the porous electrodes (which

are denoted by ‘‘1200’’, ‘‘800’’ and ‘‘600’’ in

Fig. 17). The solution inside the pores is probably

not affected much by stirring.

• The effect of stirring aside, electrode porosity does

not increase the maximum apparent current densitiessignificantly (compare, for example, electrodes

‘‘1200’’ and ‘‘50–50-soln’’ in Fig. 17). This is in con-

trast with the threefold increase in current density

of gold and platinum oxidation in alkaline and acid

solutions, which do not contain ethylene glycol (Figs.

7 and 11).

3.3.3. Intermittent potentiostatic electrolysis of ethylene

glycol

The experimental procedure for these experiments is

described in Section 2. Briefly, 10-s periods of ethylene

glycol oxidation (at a potential just below that which

gave the maximum current density during cyclic vol-

tammetry) were interspersed with a cleaning treatment

(performed potentiostatically, holding for 1.5 s at po-

tentials of 1.2 and )0.6 V in this order). These experi-ments were designed to be closer to practical electrolysis

treatment than is cyclic voltammetry and to elucidate

differences in poisoning behaviour. Pure gold, pure

platinum and three alloy electrodes were used (the latter

were chosen to cover the two microstructural variables,

namely single-phase or dual-phase structures and the

absence or presence of porosity).

The results are summarised in Fig. 18. In this figure,the current densities measured at the start of the elec-

trolysis period are shown with heavier lines and the

current densities at the end of the electrolysis period

with thinner lines. (The electrolysis period spanned 100

cycles, 1300 s.) The potentials applied during the dif-

ferent sections of the cycle are indicated on the figure.

The potentials during the cleaning period were the same

in all cases (1.2 and )0.6 V), but different potentials wereused for oxidation of ethylene glycol, depending on

electrode composition (0.23 V for gold, 0.20 V for

platinum and )0.05 V for the alloy electrodes).

Fig. 18. Current densities during interrupted potentiostatic oxidation of ethylene glycol on different electrodes (solution stirred). The applied po-

tentials for ethylene glycol oxidation depended on electrode type and are indicated on the curves; holding for 1.5 s each at 1.2 and )0.6 V served to

remove (some) poisoning species. The heavy line is from the start of the 100-cycle test and the lighter line from the end of the test.


In all cases, the current density declined substantially

during oxidation of ethylene glycol (the first period of

10 s in all the graphs). Subsequent polarisation to 1.2 V

resulted in oxygen evolution (note the higher current

density for platinum than for gold). The purpose of thesecond part of the cleaning cycle ()0.6 V for 1.5 s) is re-

duction of the surface oxide that will have formed at 1.2 V

(such surface oxides inhibit oxidation of ethylene glycol).

The graphs in Fig. 18 can be used to assess the extent

of poisoning during a single oxidation period and also

the success of the cleaning treatments. Evidently, for the

pure gold and platinum electrodes, the cleaning cycle is

sufficient to maintain the same level of activity for theentire 100-cycle test (and a higher average current den-

sity is maintained on gold than on platinum, in line with

the results of cyclic voltammetry as shown in Figs. 14

and 15).

For the porous, single-phase 60Au–40Pt electrode, the

initial apparent current density for ethylene glycol oxi-

dation during the first cycle is very high (approximately

55 mA/cm2). However, the current density of each sub-

sequent cycle is lower than that of the previous cycle andthe current densities at the end of the 100-cycle test

(Fig. 18(c), thinner line) are much lower than at the start.

This demonstrates that the cleaning procedure used here

was insufficient to remove the poisoning species from the

alloy electrodes. Similar observations were made for the

other two alloy electrodes, both single-phased non-po-

rous (Fig. 18(d)) and dual-phase non-porous (Fig. 18(e)).

This confirms the conclusion (stated earlier in this paper)that the reason for the higher current density on the alloy

electrodes cannot be that these electrodes are not poi-

soned – on the contrary, these electrodes are poisoned

more severely than the pure metal electrodes.


Fig. 18 confirms the ranking of the alloy electrodes as

presented in Fig. 17: under these conditions, where the

electrolyte is stirred, the single-phase electrodes

(Fig. 18(c) and (d)) maintain higher current densities

than the dual-phase electrode (Fig. 18(e)) and the po-rous single-phase electrode (Fig. 18(c)) maintains a sig-

nificantly higher current density than the non-porous

single-phase electrode (Fig. 18(d)).

It should be noted that, because of the severe poi-

soning of even the best-performing alloy electrode, its

average current density dropped below that of the gold

electrode at the end of the 100-cycle test. To confirm

that this loss of performance of this alloy electrode wasindeed caused by poisoning rather than, for example,

changes in alloy surface composition by selective dis-

solution, an extended cleaning cycle was applied at the

end of the 100-cycle test. Using 30 s at 1.2 V and 10 s at

)0.6 V, together with the previous ethylene glycol oxi-

dation period of 10 s at )0.05 V, did indeed cause the

current density to recover. The maximum current den-

sity for ethylene glycol electro-oxidation at the end ofthe 100-cycle test was only 15 mA/cm2 (Fig. 18 (d)).

After the extended cleaning cycle a maximum current

density of 45 mA/cm2 could be achieved.

These results are summarised in a different form in

Fig. 19, which shows the charge passed during the

electro-oxidation of ethylene glycol over the time period

of the 100 cycles, for the different electrode composi-

tions. The charge was calculated by numerical integra-tion, taking into account only the periods of ethylene

glycol oxidation (that is, the charge passed during the

cleaning period of each cycle was not included in the

total). In Fig. 19, the curves for pure gold and pure

platinum are linear, because the average rate (per cycle)

at which ethylene glycol is oxidised at these electrodes

remained constant over the period of 100 cycles (see

Fig. 19. Total charge passed during ethylene glycol oxidation for the

tests reported in Fig. 18. Refer to Table 1 for a description of the

microstructures of the different electrodes.

Fig. 18(a) and (b)). However, for the alloy electrodes,

the slope of the charge curve rapidly decreases (indi-

cating poisoning which is not eliminated by the cleaning

cycle). The net result is that only the porous (single-

phase) alloy electrode gave a higher total charge thanthe pure gold electrode, for the full 100-cycle test.

It is worth noting that the integrated charge does

indicate that the decrease in current density with time is

not a result of depletion of the ethylene glycol concen-

tration: in all cases, the charge passed corresponded to

less than 1% of the original ethylene glycol content.

4. Conclusions

It appears that the metal oxidation, oxide reduction,

oxygen evolution and hydrogen evolution behaviour of

gold–platinum alloys (in acid and base without ethylene

glycol) are roughly linear combinations of the behaviour

of pure gold and pure platinum, with secondary effects

of phase composition, and a strong effect of the areaincrease imparted by porosity.

The following conclusions on the electro-oxidation of

ethylene glycol at gold, platinum and alloy electrodes can

be made based on the results found during this work:

• The solid solution Au–Pt electrodes are more active

for the electro-oxidation of ethylene glycol than the

two-phase electrodes. However, poisoning of all elec-

trodes occurs. In general, agitation of the solution re-sults in a decrease of the current density from

oxidation of ethylene glycol.

• Positive and negative potential pulsing (for periods of

1.5 s each) is successful in removing the poisoning

species formed at the pure gold and pure platinum

electrodes during potentiostatic oxidation of ethylene

glycol.

• Initially, high apparent current densities are found atAu–Pt alloy electrodes. However, poisoning is more

pronounced for these alloy electrodes and the clean-

ing cycles (as mentioned above) are insufficient to

eliminate poisoning. With this cleaning cycle, the ac-

tivity of the Au–Pt electrodes declines to less than

that of pure gold. An extended cleaning cycle does in-

crease the current densities.

• Microstructure does affect electrode activity, with sin-gle-phase electrodes giving higher current densities

and a porous electrode showing little reduction in

current density with stirring. However, the severe poi-

soning of alloy electrodes indicates that the initially

higher activity of these electrodes cannot be the result

of decreased poisoning through a ‘‘third-body’’ effect.

Acknowledgements

Financial support from Mintek and the AuTEK

project is gratefully acknowledged.


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The electrochemistry of gold–platinum alloys

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