-
The Electrochemistry of Gold:I The Redox Behaviour of the Metal
in
Aqueous Media
L D Burke and P F NugentChemistry Department, University College
Cork, Ireland
Gold is frequently regarded as the ideal metal for the
investigation of solid electrode behaviour, which inaqueous media
is often considered in very simplistic terms as being that of a
metal which is highly resistantto dissolution. Gold possesses very
weak chemisorbing properties and an extensive double layer region
thatin the presence of most pure electrolytes is often assumed to
be totally free of Faradaic behaviour, andexhibits a monolayer (or
AUZ03) oxide formation/removal reaction at quite positive
potentials. However,recent investigations have revealed that the
electrochemistry of polycrystalline gold in aqueous solution
isconsiderably more complex. Two significantly different types of
oxide deposits, monolayer (or a) andhydrous (or ~), may be produced
on the metal and the behaviour of the ~-deposit is quite unusual.
It issuggested that not only the behaviour of the ~-oxide but, far
more important from a practical viewpoint, thecatalytic and
electrocatalytic behaviour of gold (which will be discussed in more
detail in Part II)' may berationalized in terms of the active state
(or states) of gold. This active state (frequently present only at
verylow coverage) reacts in a manner that is quite different from
that of stable gold. The nature of the active stateof gold deserves
far more attention than it has received to date.
The importance of surface reactions was highlighted ina recent
article by Sachtler (l) who pointed our that ca17% of GNP in the US
and ca 25% in the case ofGermany are derived from materials
produced bycatalytic processes. Most of these processes
involveheterogeneous catalysis of gas phase reactions on metalor
oxide surfaces. The catalysis of electrode reactions isalso
important, egca 13 million tons (valued at ca 2.2billion dollars)
of chlorine gas (2) is produced annuallyin the US, largely with the
aid of electro catalyticallyactive dimensionally stable
(Ru02/Ti02)/Ti) anodes.
The potential in the electro catalysis field is alsoquite
significant; for example, the development ofeffective
electrocatalysts for the direct methanol/air fuelcell would
revolutionise the transport industry.Compared with the internal
combustion engine,widely used at present, fuel cells in general
offer theprospect of more efficient, cleaner and less noisyenergy
conversion. Methanol is a much more easilystored fuel than hydrogen
for use in mobile energyconversion systems and may, with the
development of
CS:9' GoldBulletin 1997, 30(2)
more efficient methanol oxidation electrocatalysts, bethe system
of choice for operation at ambient, orslightly elevated,
temperatures in vehicles.
It is generally accepted that reactions at surfaces,
egheterogeneous catalysis, electrocatalysis and corrosion,occur
predominantly at active sites (3) and in view ofthe importance of
such reactions one could regard suchsites as among the most
important entities in chemistry.Yet they are rarely mentioned in
Physical Chemistrytextbooks, and electrocatalysis is rarely
discussed indepth, even in many of the recently
publishedspecialized textbooks in the field of electrochemistry.The
basic problem in the area seems to be that a clearunderstanding of
the nature, and especially the mode ofoperation, of such sites is
not available (4).
There is an additional problem in theelectrocaralysis area in
that different modes ofoperation are assumed to be involved in
different typesof electrocatalytic processes, viz:(i) Noble metal
electrocatalysis is assumed to involve
activated chemisorption (5), viz:
43
-
HCOOH------+H"""COOH = 2H+ + CO2 + Ze (1)
It is generally assumed that in this case the surfacemetal atoms
(e) do not undergo a redox transition.
(ii) Electrocatalysis by solution species is generallyassumed to
involve a cyclic redox process (6), viz:
(2)
P is the oxidation product of solution species R(a reductant)
and the dissolved oxidation mediator,Mq-n)+, is repeatedly
regenerated at the anodesurface. A similar scheme may be written
for thereduction of a dissolved oxidant (0) in whichcase the
mediator is the reduced form, M:; ,of the redox couple.
(iii) Electrocaralysis by surface-bonded mediators, egredox
groups or enzymes, is assumed to occur in asimilar manner to that
outlined in (ii) except thatin this case the redox couple remains
bonded tothe electrode surface (7). An in teresting variationof
this mode of e1ectrocatalysis is found in thecase of RuOrbased
anodes (8) where themediator is a ruthenate or perruthenate
speciesderived from the surface of the electrode material.
Recent work on noble metal electrochemistryprovides the basis of
a more unified approach toelectrocatalysis. It was demonstrated
(9-11) that inmany instances the cyclic redox or mediator mode
ofclectrocatalysis applies also to noble metal electrodeprocesses.
The importance of adsorption is notdiscounted; it is the major
(indeed sole) factor in someimportant processes, eg H 2 evolution
or ionization,weakening of C-H bonds in methanol, etc. However,the
mediator route for electrocatalysis seems to bemore widespread;
apart from providing a commonbasis for the different types of
electrocatalysis itcorrelates activity for oxidation and reduction
for thesame electrode system, it applies to both metals andoxides,
and rationalizes behaviour in certain importantprocesses, eg
removal of inhibiting COads species,which is a process of
considerable practical importance- especially in the fuel cell
anode area.
The new approach to noble metal electrocatalysis,sometimes
referred to as the IHOAM (IncipientHydrous Oxide/Adatom Mediator)
model, IScontroversial as:(i) It stresses the importance of the
defect state of
surfaces and assumes that, for the same surface,two
significantly different types of electrochemicalresponses may be
observed - one for high
44
coordination and the other for low coordinationsurface metal
atoms.
(ii) Considerable stress is placed in the new approachon two
largely ignored features of noble metalelectrochemistry, namely
hydrous oxides andactive states of the metal.
(iii) In the conventional view of the metal/solutionboundary or
double layer the surface or boundaryis frequently represented as an
inert flat planewhereas in the IHOAM approach the metalatoms at the
surface are regarded as having adegree of mobility and a range of
chemicalreactivities; some of these atoms are assumed toundergo
electro-oxidation reactions in aqueousmedia at potentials that are
significantly morenegative than that normally associated with
theonset of regular monolayer oxide formation.
In this, the first of two articles (the second will
dealspecifically with electrocatalytic processes on gold),
anaccount will be given of the electrochemistry ofpolycrystalline
gold in aqueous media. Since theconventional chemical behaviour of
this electrodesystem has been described earlier by various
authors(12-14), attention will be focused here largely on
morerecent work in which anomalous behaviour wasobserved. In the
present context the behaviour of ametal or oxide is regarded as
anomalous if it deviatessignificantly from that expected from
conventionalthermodynamic data as summarized, for instance,
byPourbaix (15).
THE CONVENTIONAL BEHAVIOUROF GOLD
Gold is the noblest of metals. According to thethermodynamic
data for this element (15) gold shouldbe oxidized to the trivalent
state, according to thereaction:
(3)
at 1.457V if the product is the hydrated oxide (whichmay be
represented as Au(OHh or AU203.3H20), orat 1.511 V if the product
is the anhydrous oxide. Thepotentials quoted here are standard
reversible (EO)values, calculated using standard chemical
potential(u0) data, and the aqueous solution is assumed to befree
of complexing agents. Most potential values arequoted in this
review in terms of the RHE scale, iewith respect to a reversible
hydrogen electrode (PH2 =
@ Gold Bulletin 1997, 30(2)
-
1.0atm) in the same solution. This is a convenient scalefor use
with metal/metal oxide electrodes as, in termsof the conventional
behaviour of such systems, theelectrode potential value for a given
transition shouldbe pH-independent (the pH-dependence of
theworking, or oxide, electrode being the same as that ofthe
reference electrode). The fact that significantlydifferent E/pH
variation is observed, in terms of thisRHE scale, for certain
oxide/metal (and indeedoxide/oxide) transitions was discussed
earlier (10, 16).
A typical cyclic voltammogram* recorded for goldin acid is shown
in Figure 1. The most notable featurein the positive sweep is the
increase in anodic current,
0.30
commencing at ca 1.36V, which is due to monolayer(or o-) oxide
formation at the electrode surface. Thisprocess commences at a
slightly lower potential thanexpected on the basis of the EO values
quoted hereearlier. However, it must be borne in mind that
thelatter were estimated for the stable bulk metal atomsand it is
only for the latter that f-LO(Au) = O. Surfaceatoms are inevitably
more active; the absence of similarmetal neighbours on one side
means that they arelacking some of the lattice stabilization energy
of theirbulk equivalents and, hence, they tend to oxidize at alower
potential.
The early stage of oxidation of noble metal surfacesis usually
discussed in terms of adsorbed hydroxyradical formation (14),
viz:
Figure 1 Tjpical cyclic uoltammogram (0.0 to 1.80 V, 50mV 5'1)
recorded for a polycrystalline gold discelectrode in 1.0 mol dm,3
H2S0 4 at 25C
'Cyclic voltammetry is a widely used electrochemical technique
hasco onthe usc of a potcntiostat to control the potential (by
automatic adjustmentof current flow) of the electrode under
investigation. In this technique thepotential impressed on the
electrode is varied in a triangular manner at apreselected sweep
rate, eg50 mY SI, between preselected upper and lowerlimits, The
resulting response is a cyclic volramrnogram, ic a plot of
current
versus potential for the forward and reverse sweeps. A large
positive
potential corresponds to highly oxidizing conditions at
theelectrode/solution interface and this, depending on the nature
of the metaland solution pH, may result in dissolution of, ot oxide
forrnarion at, theelectrode surface. The technique is normally used
10 study the redoxbehaviour of either solution species or rhe
electrode surface; hut the sameprocedure has been used extensively
ar Cork 10 produce and invcsrigareunusual oxide deposits on
electrode surfaces (see reference 16 for derails)and to modify, or
activate, rhe outer layers of metal electrodes. The study ofrhe
acrive stares of surfaces is a very challenging area of research
but theresults obtained to dare from cyclic volrarnrnerry work are
of considerableinterest in areas such as heterogeneous
catalysis.
(4)Such an approach may be inadequate in the case
of gold as the hydroxy radical is a very high energyspecies; the
EO value for the H 20/OH couple (17) is2.85V(SHE) and gold has an
extremely weakchemisorbing capability, eg the coverage of Hads
ongold at O.OV is only ca0.03% of a monolayer (12). Ifthe
involvement of a hydroxy species is to beretained, it may be more
appropriate to consider theinitial oxidation product as some type
of metal-hydroxy compound, eg Auo+... OHo-, the productbeing a
polar covalently-bonded species. The chargeassociated with further
monolayer oxidation does notgenerally give rise to a single sharp
peak above 1.36Y.Instead, the charge in question, for gold in
acid,tends to be distributed along a plateau (12) (usuallywith some
minor features evident on the latter) withno major change until
oxygen gas evolutioncommences at ca 2.0V (this latter feature is
notshown in Figure 1).
The conditions for complete monolayer oxideformation on gold
surfaces are important from a practicalviewpoint as it forms the
basis of a useful method fordetermining the true surface area (or
roughness value) ofthe metal surface. For this purpose, Woods (12)
hasrecommended that the potential of an initially clean
goldelectrode in 1.0 mol drrr-' H 2S04 be held initially at1.8V for
100 sec to achieve monolayer oxide coverage.The charge for
monolayer oxide reduction, estimatedfollowing the application of a
subsequent negative sweep,may be converted to real surface area
using therecommended conversion factor of 386 f-LC crn? (realarea).
This is a very convenient, though perhaps not veryprecise, method
for the determination of a relativelyimportant electrode
parameter.
1.60.8 1.2FlV (RHE)
004
0)-0.30
-0.60
0
($9' GoldBulletin 1997, 30(2) 45
-
(5)
A very common feature of monolayer oxideIormation/rcduction
behaviour in the case of the noblemetals is hysteresis, ie the
difference in the potentialrange for monolayer oxide formation
(positive sweep)as compared with that for monolayer oxide
reduction(negative sweep) - see Figure 1. It is generally
assumedthat hysteresis in this case is due to post-electrochemical
processes, ie the product of a givenelectrochemical reaction
undergoes a change after theelectron transfer step such that the
species, energiesand potential range involved in going in one
directionare appreciably different to those involved when
theprocess is reversed.
The origin of hysteresis in the monolayer oxideformation/removal
processes is usually attributed (14)to gradual changes in the
nature of the oxide film.During the growth process dipolar
(Auo+.OHO,) speciesare produced which, at appreciable coverage,
repel oneanother. Such repulsion raises the energy required
togenerate additional dipoles; hence, there is an increasein
potential with increasing coverage. This is evidentlythe reason why
the monolayer oxide formationresponse is an extended plateau rather
than a sharppeak. The post electrochemical process involved in
thiscase is outlined schematically in Figure 2. Rotation ofsome of
the surface dipoles, a process also referred to asplace-exchange,
relieves much of the lateral repulsionor stress in the surface
layer, resulting in a more stablesurface deposit. In the negative
sweep the dipolecoverage (and any residual stress) is
progressivelyreduced. No electrostatic repulsion barrier is
developedin this case and a relative sharp cathodic peak
isobserved.
Figure 2 Schematic representation ofthepost-electrochemical,
place-exchange reactioninvolved in a monolayer oxideformation.
Somesurface metal atoms (unshaded) switch positionswith
adsorbedoxygen species (shadedcircles).Obviously ifthese species
are charged, asindicated here, the place-exchange reaction leadsto
a considerable reduction in electrostaticrepulsion energy
46
There is an additional factor, related to changes inthe activity
of the surface metal atoms, that may alsocontribute to the
hysteresis effect. This is the change inactivity of surface metal
atoms during the course ofreaction. Three types (or states of
activity) of goldatoms may be considered, namely:(a) Bulk lattice
atoms (AuO) - these have little
relevance to the reaction at the surface;(b) Regular surface
atoms (Au") which are more
active than those in the bulk.(c) Displaced surface atoms (Au**)
which are in a
state of still higher activity as these are generatedinitially
in a state of unusually low latticecoordination number on reduction
of thepartially place-exchanged surface oxide.
Atoms of type (c) are assumed to undergospontaneous rapid
alteration to the state represen tedhere by (b). The different
states in question hereshould not be viewed in a very rigorous
manner as, inthe first instance, solid surfaces tend to
beheterogeneous (it is more a question of the spread andoccupancy
of different energy states).
According to this approach hysteresis may beconsidered in terms
of the following scheme, viz:
Au* + H 20 ----- Auo+.OHo, + H+ + e'
1HO'.AuO+
1+ 2H20 - 2H+ - 2e-+3H+ + 3e'Au** + 3H20 .-----Au(OH)"
The two post-electrochemical steps are representedhere by the
vertical arrows. The scheme also highlightsanother complication in
the case of the positive sweep,namely, the conversion of surface
dipoles, Auo+.OHo"to a more regular oxide species, ie formation of
speciessuch as Au(OHh, or AU20".
Examples of cyclic voltammograms recorded forgold in base are
shown in Figure 3. The onset potentialfor the initiation of
monolayer oxide growth in thiscase, ca 1.25Y, is significantly
lower than in acid (18).An unusual feature in the response for gold
in base isthe large anodic peak in the positive sweep (which hasno
cathodic counterpart on the subsequent negativesweep) extending
over the range of ca 1.6 to 2.GY - seeFigure 3(b). The process
giving rise to this response isassumed to be oxygen gas evolution
catalysed in atransient manner by some type of nascent hydrousgold
oxide species formed at the monolayeroxide/aqueous solution
interface (19). The transitory
@ GoldBulletin 1997,30(2)
-
nature of the activity in this region was attributed
toconversion of the active gold species to a less activedeposit
which may well be conventional gold hydrousoxide material. Regular
oxygen gas evolution on goldin base (as in acid) occurs vigorously
only above ca2.0Y. The behaviour shown here on the negative
sweepover the range 2.1 to lo6Y (see Figure 3(b)) may wellbe a
better reflection of the steady state oxygen gasevolution activity
of gold in base than that shown inthe corresponding region of the
positive sweep wherethe influence of transient activity is quite
marked.
(6)
namely, (i) the hydrous oxide electrochemistry of gold,and (ii)
the premonolayer oxidation of gold.
Multilayer hydrous oxide deposits are readilyproduced on gold in
aqueous solution by either dcpolarization (21) at potentials within
the oxygen gasevolution region (E 2>. 2.0Y) or by repetitive
potentialcycling (22) or pulsing (23). Such growth occurs
mostreadily in acid solution and the product obtained is arather
amorphous, hydrated Au(III) oxide. As pointedout here earlier,
thermodynamic data suggest that thismaterial should be produced at
ca 1.46Y; however, atthe latter value most of the gold surface is
alreadycoated with a monolayer oxide () film whichevidently
passivates the surface with regard to hydrous([3) oxide growth. The
a. deposit is assumed to be anadherent layer of compact material
through whichAu3+ ion migration is quite slow. Both oxygen
gasevolution and hydrous oxide growth may involveformation of an
unstable (15) Au(IY) oxide species asoutlined in the following
scheme, viz:
1.60.8EN (RI-IE)
o
0.2(3)
0-0.2
':5 0.6 (b)
~0.2
0
-0.2
Figure 3 7ypical cyclic uoltammograms fir apolycrystalline
goldelectrode in 1.0 mol dm-3NaOH at 25C: (a) 0.0 to 1.60 Vat
50mVs'j; (b) -0.2 to 2.1 Vat 10 mVs'j
An interesting feature of the negative sweep inFigure 3(b) is
the appearance of a second cathodic peakat ca. 0.8SY. The monolayer
oxide reduction peak wasobserved in this case at ca 1.07V and the
subsequentpeak is assumed to be due to reduction of hydrous
goldoxide species formed on the gold surface at the upperend of the
cycle. The growth and subsequent reductionof such hydrous oxide
deposits were discussed in somedepth in earlier reviews (16,
20).
ANO~OUSBEHA~OUROFGOLD
1 Hydrous OxideIn terms of gross departure from behaviour
expectedon the basis of thermodynamic data, two aspects of
theelectrochemistry of gold merit special consideration,
According to this scheme some of the monolayeroxide material is
converted, at E 2>. 2.0V to goldperoxide species which
decomposes to yield oxygengas. This process continues in a cyclic
manner,resulting also (possibly as a side reaction) in
thegeneration of an increasing thickness of the [3-material.There
are other possibilities, eg oxygen evolution onthe o -oxide may
result in the generation of flaws in thelatter through which gold
cations emerge to form the[3-oxide, ie the growth of the latter may
proceedwithout the intervention of any other oxide species.
The use of repetitive potential cycling as atechnique for
producing hydrous oxide deposits hasalready been discussed (16). At
the upper limit of eachcycle some metal atoms are displaced in
accordancewith the scheme shown here in Figure 2. Reduction ofmuch
of the o.-film at the lower limit of the negativesweep results in
the generation of active (low latticecoordination) surface metal
atoms which reactspontaneously with water molecules to yield some
[3-material. Important requirements in this case are that(i) the
[3-material must be of a low density or porous innature to allow
access of water to the surface of the a.-oxide where the hydrous
oxide formation is assumed to
C69' Gold Bulletin 1997, 30(2) 47
-
occur, and (ii) the lower limit of the oxide growth cyclemust be
such that much of the -, but little if any ofthe 13-, oxide is
reduced.
The responses recorded during the course of anegative sweep for
a hydrous oxide-coated goldelectrode are complicated by the fact
that thebehaviour observed is strongly influenced by factorssuch as
the thickness of the deposit and the pH of thesolution. In most
cases the interface in questioncontains both the a- (monolayer) and
13- (multilayer)materials arranged in the following manner:
gold/a-oxide/Bvoxide/aqueous solution. Generally the firstcathodic
response is that due to reduction of the a-oxide and this usually
results in a peak with a potentialmaximum (Ep) in the region of ca
1.0V (the observedvalue varies (24) with factors such as oxide
formationpotential, oxide coverage, sweep rate, etc). With
thinoxide films in acid solution (21) the reduction of thel3-oxide
deposit gives rise to a single peak whichoverlaps with the
monolayer oxide reduction peak.Similar behaviour was observed for
the reduction ofrelatively thin oxide films on gold in base, except
thatin this case a clear separation, in terms of potential,
isobserved between the two peaks. The a-oxidereduction occurs again
at ca 1.0V (with a minor shiftto lower values of Ep with increasing
pH) but themaximum of the l3-oxide reduction peak occurs (21) atca
0.65Y. Such a drop in potential, ca -0.5 (2.3RT/F)V per pH unit, on
changing from acid to base, is quitecommon for hydrous oxide
systems. Similar behaviour,sometimes referred to as a
super-Nernstian shift, hasbeen observed even in the case of highly
reversibleoxide/oxide transitions (16), eg those involving Ir,
Rh,Fe and Ni ions.
There are complications in the interpretation ofsuper-Nernsrian
E/pH shifts in the case of gold in that(i) the structure,
composition and state of charge of theoxyspecies are unknown, and
(ii) the overall reductionstep occurs in an irreversible manner. It
has beensuggested (24) that the behaviour in question may
beinterpreted in terms of the following scheme (thisdescribes the
reaction in terms of a monomer unit ofthe oxide polymer), viz:
AUz(OH)3-9 + 9H+ + 6e- = 2Au** + 9HzO (fast) (7)
2Au** -- 2Au* (8)
The first step is assumed to be rapid and reversible andthis
process may be treated in a Nernsrian manner.Application of the
Nernst equation to Equation 7yields:
48
o 2.3Rl' 2.3RT (2.3Rl')E=E +~log aor 3F log aAu,,-1.5 -p- pH
(9)(aox represents the activity of the oxyspecies).According to
the latter equation the equilibriumpotential for the process shown
in Equation 7 shoulddecrease by ca -1.5 (2.3RT/F) V/pH unit
(pH-independent scale) or ca -0.5(2.3RT/F) V/pH unit(RHE scale). An
obvious objection to this approach isthat the reduced form of the
couple (Au**) is not atconstant activity. However, the same E/pH
trend isobserved for intermediate (oxide/oxide) transitions inthe
case of other, redox active, hydrous oxide systems(16) where the
decay of metal atom activity is clearlynot involved, ie
supcr-Nernstian E/pH shifts are awidespread feature with
redox-active hydrous oxidedeposits.
The l3-oxide material exists outside the metalsurface, ie it is
clearly not an adsorbed state. Anexample of materials that may be
similar to the l3-oxideare certain soil colloids (25) such as
layered silica claysand hydrous oxides of iron and aluminium. In
most ofthese colloids the oxycation framework bears a
negativecharge with ion-exchangeable cations, eg H 30+, Na",Caz+,
present as counterions in the intercalated water.The assumption
here is that the gold oxide andaqueous components are intermingled
in the 13-layer.The accumulation of negative charge on the
oxidestrands or layers in the l3-material may be viewed interms of
a hydrolysis effect (26), ie as a combination ofexcess hydroxide
ion coordination and loss of protonsfrom coordinated water
molecules, viz:
+OR -H+HO-Au3+-OHz ---Au3+-OHz -- -Au 3+-OH-
(10)at highly charged cation (Au3+) sites. The involvementof
super-Nernsrian E/pH shift effects in theelecrrocatalytic behaviour
of gold has already beendemonstrated (27).
Recent reinvestigation of hydrous oxide growth ongold under
repetitive potential cycling conditions (28)have shown that the
behaviour of these oxides issignificantly more complex than assumed
earlier. Withthicker oxide deposits in acid solution four
oxidereduction peaks were noted, Figure 4(a). The Ep valuesand
oxide assignments are as follows:- Cj , 1.1V(monolayer oxide); C'z
and C"z, 0.97 and 0.94V (theoxide involved, HOI, is assumed to
contain twoslightly different components); C 3, 0.73V (H02);
C4,0.58V (H03). It may be noted that evidence of a
@ GoldBulletin 1997, 30(2)
-
10000
H03
TOT
2500 5000 7500NO. OF CYCLES
1.2r--- - - - - - - - --------.
Figure 5 Variation of the charge fOr hydrous oxidereduction as a
function ofthe number ofoxidegrowth cycles. The conditions used
(apart fromthe number ofcycles) are as specified in Figure4(a). The
latter diagram also shows the oxidepeak assignments:- C] + C'],
H01; c:~,H02; C4, H03
peak current (ip) values increased linearly, withincreasing
oxide reduction sweep rate. Such behaviourindicates that the oxide
reduction processes (especiallyfor HO 1 and H02 - the peak for H03
was quitebroad at fast, >5mV s', sweep rates) occur rapidly,
tein a quasi-reversible manner (ifEquations 7 and 8).
'1
520 U H020'
.,
~ HOIe 0
:::,
(b)
004 0.8
FJV (RHE)
-5
similar nature was obtained recently for the presence ofthree
distinct components in hydrous oxide filmsgrown on Pt in acid
(29).
Figure 4 Reduction profiles, 1.20 to 0.0 V at 2 m V s:',fOr
multilayer hydrous oxide films grown on amildly abraded gold wire
electrode by repetitivepotential cycling (oxide growth conditions:
-1.0xl ()1 cycles between 0.90 and 2.40 Vat 50VsI) in 1.0 mol dm3
H2S0 4 at (a) 25Cand (b) 35C
One of the interesting features that emerged fromthe recent
investigation (and which explains why thecomplexity of the system
was not appreciated earlier)was that some of the components
involved, especiallyH02, are produced only after an extended number
(ca2500) of oxide growth cycles, Figurc 5. For films ofconstant
thickness the charge for reduction of thevarious oxide components
was virtually independent ofsweep rate (5-100 mV s'). The peak
potential (Ep)values decreased, though not dramatically, while
the
The reduction behaviour of thick hydrous oxidefilms on gold in
base is much more complex than thatobserved for gold in acid.
Conditions for producingsuch films in base, using the repetitive
potential cyclingtechnique, were investigated by Burke and
Hopkins(22) who observed in some instances oxide reductionpeaks
with a maximum (Ep) at ca 0.2\1: More recentwork (30), an example
from which is outlined here inFigure 6, has yielded a behaviour
pattern that is evenmore unusual as a major portion of the charge
requiredfor oxide reduction at a very slow sweep rate wasobserved
in the region between ca -0.1 to -0.3\1: Withan organic base
electrolyte the extent of reduction of agold hydrous oxide film in
the negative sweep atE >O.OV was quite small. The main oxide
reductionresponse occurred in this case (at 10mV s"; Figure 7)only
at E < -0.3\1:
~ GoldBulletin 1997,30(2) 49
-
E/V(RHE)
Figure 6 Reduction profile, 1.2 to -0.4 Vat 2. 0 m V r lfor a
thick oxide film grown on a pre-abradedgold wire electrodein 1.0
mol dm-3 NaOH at2YC (the dashed line shows the subsequentpositive
sweep. The oxide deposit was producedin-situ ~y potential cycling,
0.75 to 2.40 Vat50 V s', 6364 cycles
(11)2M-yr ----C - nF'YlP
Equation 11 is well known from electrochemicalnucleation theory
(rc is the critical radius of thecluster, M is the molar mass of
the metal, P is the
Apart from establishing the basic electrochemistryof these gold
hydrous oxide systems in aqueous media,very little other work has
been devoted to this topic;hence only basic ideas can be outlined
here. Thereactions involved, even in acid solution, obviously donot
occur under ideal thermodynamically reversibleconditions, ie there
is significant cathodic overpotentialinvolved and there are various
possible contributions tothe latter. It seems unlikely that the
monolayer oxidedeposit acts as a barrier to f3-oxide reduction,
especiallyin base where (in the case of thin films) the twocathodic
peaks in the negative sweep are quite farapart. The f3-layer may be
regarded as a semi-rigid,amorphous deposit and since reduction
probablyoccurs at the oxide/metal interface there may well
bedifficulty in maintaining contact between the metalsurface and
much of the nearby oxide deposit when thelatter is undergoing
reduction. Oxides and hydroxidesare generally more reactive (eg
more prone todissolution) in acid solution. It was pointed out
earlier(21,24) that the activity of Au3+ cations, existing in agel
network of OH- and OHz species in the f3-layer, isinfluenced by the
OH-ion activity in the bulk solution.This approach, developed
originally to rationalize thesuper-Nernstian E/pH effect mentioned
earlier,highlights one important contribution to the
markedoverpotential observed in the case of the f3-oxidereduction
in base, as compared to acid.
Another major factor in the oxide film reductionprocess is a
combination of lattice stabilization energyand particle size effect
at the gold surface. There isevidence from Scanning Tunnelling
Microscopy(STM) (31) that discharge of metal ions from
solutionoccurs most readily at ledge sites on the surface.
Metalions in the f3-layer (especially in base where the layer
ismore stable) are less free to move to such sites; theremay, for
instance, be some preliminary discharge atsuch sites, followed by
localized loss of contact. Then,over much of the surface, oxide
reduction may have tooccur under conditions where the metal atoms
areproduced in a poorly lattice stabilized, active form.Small
clusters of atoms formed at the interface alsoconstitute a high
energy state of the metal - the inverserelationship between cluster
radius and overpotential(32).
2.0
1.20.80.4E/V(RHE)
o-0.4
(b)0
0.2 (a)Ei -10 0
-0.2
Figure 7 Reduction profile, 1.2 to -0.5 V at 10mV s:!fir a thick
oxide (produced in-situ by dcpolarization at 2.3 Vfor 90 min) in
1.0 moldm-3 {C2H5)qNOH at 25C. The inset showsa
cyclicuoltarnmogram, 0.0 to 1.6 Vat 50m V 5- 1, for the same
electrodein the absence ofthe hydrous oxide deposit
50 C69' GoldBulletin 1997, 30(2)
-
density of the cluster which has a surface tension orsurface
energy 'Y). Since the cluster is unstable (orcannot form) until r
> rc the driving force oroverpotential ('Y]) must be increased
(this also has theeffect of reducing the nucleation barrier energy
and rcvalue) until oxide reduction occurs at a rapid rate.
Theunusually high overpotential observed in someinstances in
alkaline solution, Figures 6 and 7, mayreflect a combination of
very low Au 3+ activity (theoxide matrix being more stable in base)
and high goldatom activity. The unusually low reduction
potentialsobserved in the presence of the
terralkylamrnoniurnhydroxide electrolyte, Figure 7, may reflect the
effect ofadsorption of Nit;+ ions on either the gold surface orthe
initially produced adarorns (32) - this may result ininhibition of
cluster growth, an unusually high metalactivity and a very low
reduction potential. Anotherpossibility is that accumulation of
hydrophobic cationsat the interface affects either the oxide
polymer or somepre-electrochemical step (perhaps
depolyrnerization).
The presence of several peaks in the oxidereduction process, sec
Figure 4, may be due to theexistence of more than one type of
hydrous oxidespecies or more than one state of activity of the
freshlyproduced gold atoms - the influence of activity, inNernstian
terms, is evident from Equation 9. Whilethe precise origin of such
behaviour is unknown, thephenomenon is of significant importance as
theelectrocatalytic behaviour of gold (33) (and indeedplatinum (1
1)) is also assumed to involve the reactionof two distinct surface
oxide mediator species.
2 Pre-Monolayer OxidationGold is frequently regarded as the
ideal solid electrodesystem for fundamental investigations
inelectrochemistry as, in the absence of redox activespecies in the
aqueous phase, the system apparentlyexhibits only non-Faradaic (or
double layer) behaviourover the range of about 0 to 1.3 V in acid
or 0 to 1.2 Vin base. However, as pointed out by Woods (12),
therehave been repeated assertions that Faradaic behaviourdue to
the formation of oxyspecies at the gold surfaceoccur well within
the double layer region. Such reportswere usually dismissed, the
effects in question beingattributed to the presence of impurities
either on themetal surface or in the bulk solution.
More convincing evidence for the formation ofoxide species in
the double layer region of gold, iepremonolayer oxidation of the
latter, has beenobtained more recently with the aid
ofoptical/spectroscopic techniques. Nguyen Van Huongand co-workers
(34), on the basis of electrochemical
($i9' Gold Bulletin 1997, 30(2)
and spectroscopic data, postulated in 1980 that sometype of
oxyspecies was formed in the double layerregion (to a coverage of
10-20% of the surface) on goldin base. In the following year
Watanabe and Gerischer(35) postulated, on the basis of
photochemical data,that gold in acid exhibited premonolayer
oxidationextending over the potential range 0.85 to 1.35
V.Desilvestro and Weaver (36), using Surface EnhancedRaman
Spectroscopy (SERS), established that for goldin base the product
of prernonolayer oxidation was ahydroxy species, and, quite
importantly, one that wasof different character to the species
involved in theregular monolayer oxidation reaction. Hutton
andWilliams (37), on the basis of scanning lasermicroscopy data,
also claimed that some oxidation ofthe gold surface occurred prior
to regular monolayeroxide formation; furthermore, they found that
theincipient oxide involved was exceptionally stable - onlybeing
removed by prolonged evolution of hydrogengas.
Claims based on electrochemical data forpremonolayer oxidation
in the double layer regioncontinue to appear. Of particular
interest is theprocedure used by Kirk and co-workers (38) who useda
relatively low upper sweep limit. By avoiding thelarge response for
the monolayer oxideformation/ removal process, and using fast sweep
ratesand high recorder sensitivities, they observed markedFaradaic
responses in the double-layer region for goldin base. Other authors
who have reported unusualresponses attributable to the formation of
oxyspecieson gold in aqueous media include Conway and co-workers
(for both acid (39-4 I) and base (42)), and oneof the present
authors (19, 43). Further data obtainedusing non-electrochemical
techniques that support thepremonolayer oxidation approach include
theradioactive tracer work of Horanyi and Rizmayer (44),and the
Quartz Crystal Microbalance (QCM)investigations of Gordon and
Johnson (45) (these twogroups worked with gold in base and
acid,respectively). More recent QCM data for gold in baseby Kaurek
and co-workers (46) were also interpreted interms of prernonolayer
oxidation behaviour.
The essential point seems to be that nearly allmetal surfaces
contain defects which from a catalytic orelectrocatalytic viewpoint
may be considered as thesites at which catalytic and
electrocatalytic reactionsoccur (11). The metal atoms at such sites
are lowcoordination species - this was established recently byErtl
and co-workers (47), using STM, in the case ofnitric oxide
decomposition on ruthenium single crystalplane surfaces. The low
lattice stabilization energy of
51
-
the active site metal atoms means that these atoms
aresignificantly more electronegative than their bulklattice
equivalents, ie these protruding metal atomsoxidize at unusually
low potentials.
In general, very little is known about active sitebehaviour (4).
Apart from the fact that the surfacecoverage of such sites is low,
the material or atomspresent at such sites constitute an active
state of themetal and the atoms involved are (i) far from being
inthermodynamic equilibrium with the bulk phase, and(ii) apparently
exhibit an unusual mode ofelectrochemical behaviour (11). It is
assumed thatadarorns and small clusters of same at the surface
areoxidized in aqueous media to form incipient hydrousoxide (rather
than OHads-type) species. It was pointedout earlier (48) that
whereas the electrocatalyticactivity of metals is usually quite
marked in the doublelayer region (due to the involvement of low
coverageincipient oxides), the onset of monolayer oxideformation
frequently results in inhibition.
It is known, eg from the work of Henglein (49),that with
extremely small metal particles the redoxpotential of the metal
shifts in the negative directionwith decreasing particle size; this
effect is quite dramaticwhen the number of metal atoms involved is
extremelysmall. The same concept is employed in the IHOAJAapproach
(11) except that the active metal atoms,instead of being in a free
particle, are attached to thesurface of the electrode. Evidence
supporting the idea ofactive states of metals is provided by the
work ofParmigiani and co-workers (50) who found thatsupported
platinum microclusters also undergoanomalous oxidation, ie they
react with oxygen underfar milder conditions than the bulk metal.
Suggs andBard (51) have emphasized that microscopic potentialsof
metal atoms at different surface sites are not thesame; however,
they also indicated that there is nomethod of determining the
surface energetics ofdifferent sites (the standard P value for a
metalrepresents an averaging of values over atoms at
differentsites). Pre-monolayer responses, as observed in
cyclicvoltammetry experiments (43), may be regarded as apreliminary
attempt to determine the redox behaviourof active surface atoms
which playa vital role in thecatalysis of many surface and
interfacial reactions.
CONCLUSIONSFrom the work described in this, the first of
twoarticles reviewing the electrochemistry of gold, we
havedescribed recent work on the behaviour of
52
polycrystalline gold in acidic and basic aqueous media.Two
significantly different types of oxide deposits maybe produced on
the metal and the conditions underwhich these are formed and their
properties have beenconsidered in Part 1. In Part II the catalytic,
andparticularly the electrocatalytic, behaviour of gold willbe
considered in relation to the results described above.The special
properties of gold, as revealed in thesearticles, will undoubtedly
lead to further investigationsand could lead to new
applications.
ABOUT THE AUTHORSLawrence Declan Burke received his BSc and
MScfrom University College Cork in 1959 and 1961,respectively, and
a PhD in 1964 from Queen'sUniversity Belfast where he worked with
FA. Lewis onthe palladium hydride system. He spent a year as
anAlexander von Humbolt Fellow in KarlsruheUniversity, Germany,
where he worked on solid stateelectrochemistry with Professor Hans
Rickert. Sincereturning to Cork he has been involved in an
extensiveinvestigation of the electrochemistry of metals, oxidesand
especially hydrous oxides. This work, which isconcerned largely
with the unusual or non-idealbehaviour of these systems, is
relevant to such areas aselectrocatalysis, electrochromic systems,
electrolessdeposition of metals, etc. Professor Burke was
awardedFellowshi p of the Electrochemical Society in 1995.
Patrick Francis Nugent received his BSc degree inchemistry from
University College Cork in 1992. He iscurrently completing his PhD
degree on theelectrochemistry of gold in aqueous media.
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53