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Chapter 2. Reduction of Platinum under Superdry Conditions

Chapter 2. Reduction of Platinum under Superdry Conditions

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2.4 In Situ EC-AFM Following of the Platinum Charge................................ 124

2.4.1 Charge and Discharge Observed in Real Time............................ 124

2.4.2 Other Electrolytes......................................................................... 128

2.5 Platinum Phases as Reducing Reagents.................................................... 132

2.5.1 Evidence for Reduction Reactions Induced by the Platinum

Phases........................................................................................... 132

2.5.2 The Case of the Reductive Cleavage of Aryldiazonium

Cations: Platinum Surface Modification....................................... 136

2.6 Charge–Discharge Processes of Platinized Platinum Layers................... 140

2.6.1 Charge–Discharge in the Presence of Tetramethylammonium

Tetrafluoroborate.......................................................................... 140

2.6.2 Swelling of Platinized Layers upon Electric Charge.................... 142

2.7 Is Platinum Really Inert toward Electrogenerated Organic Anions

and π-Acceptor-Reduced Forms?............................................................. 148

2.7.1 Reactivity of Acceptors toward Platinum-Reduced Phases.......... 148

2.7.2 Reactivity of Reduced Organic Acceptors toward Platinum........ 151

2.7.3 Indirect Reduction of Platinum Followed by SECM

Experiments.................................................................................. 153

2.8 Cathodic Charge of Palladium in Superdry Electrolytes......................... 157

2.9 Concluding Remarks................................................................................ 164

Acknowledgment............................................................................................... 166

References.......................................................................................................... 166

2.1 Introduction

2.1.1 Does the Perfectly Inert Electrode Really Exist?

The search for the perfect working electrode in terms of stability, ease of use,

cost, and wide potential range remains nowadays one of the goals of most electrochemists in the fields of preparative organic and inorganic electrochemistry,

fuel cells, and electroanalysis [1,2]. In fact, any valuable cathodic and anodic

reaction is generally associated with a specific set of experimental conditions

such as the nature of the solvent, choice of the electrolyte, and, more particularly

for electrochemical processes, reactivity of the electrode material that could be

tuned through intrinsic modifications of the electrode interface [1,2]. Until now,

mercury, platinum, and glassy carbon have been the most used electrode materials [3]. After research by electrochemists determined its environmental impact,

mercury has been banned [4] both in industry and also from the laboratories.

However, are platinum [5] and all forms of carbon [6], generally considered as

stable materials under cathodic and anodic polarizations, the best choices? Many

substrates that are reported as inert materials are not always very stable [7]. As

we will detail in this chapter, this is clearly the case for platinum, and certainly

for other noble metals.

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2.1.2 Reported Reactivity of Common Interfaces

under Electron Transfer Addition of Electrogenerated Species onto Surfaces

Many free radicals, essentially produced after the bond cleavages of some electrogenerated intermediates, lead to activated, alkyl, or aryl radicals, which could

be grafted onto conducting surfaces. This type of reaction is found in essentially

organic, strongly activated substrates and surfaces possessing efficient catalytic

capacities because the condition of immobilization on the surface depends on the

nonreducibility of the radical. One of the most useful substrates belongs to the

family of aryl diazonium cations, and many studies have demonstrated their properties [7,8]. More generally, during many conventional reactions at solid cathodes,

formation of free radicals may react on surfaces and cause inhibiting films (by the

addition of free alkyl radicals onto metallic polarized surfaces or carbons). Insertion of Salts into the Electrode Material

In electrochemistry, the electrolyte plays a crucial role. Generally, the salt is not

expected to affect the interfacial reaction within the range in which it is totally

inactive. In most cases, the supporting electrolyte is composed of nonredox reactive ions that are especially chosen for this purpose to avoid the interference of

their own reactivities (if any) with the considered (or expected) electrochemical

reaction. However, in many studies, strong interactions between the electroactive

organic substrates, cations, and anions have been noted and were therefore considered, respectively, as acceptors and donors [9]. Antecedent and subsequent reactions brought on by salts could influence product distribution. The specific choice

of an electrolyte could be used to direct the global electrode reaction toward a

precise direction (stereochemistry, polymerization, protonation, etc.). For example, the cathodic reduction of acrylonitrile in the presence of alkali metal salts

yields propionitrile, whereas the use of quaternary ammonium salts orients the

reaction to dimer formation (synthesis of adiponitrile) [10]. Different models of

electronic double layers were proposed to explain such determining effects of the

interface [11].

As a classical material used in electrochemistry, mercury is known to react

with electrogenerated alkali metals (sodium, potassium) to form amalgams.

This property impedes the use of alkali metal salts with aprotic organic solvents

(dimethylformamide, acetonitrile, propylene carbonate) at potentials <−1.8 V

versus SCE. Surprisingly, ammonium salts (cation tetraalkylammonium) have

been said to react as well, but at much more negative potentials. The structure

of the so-called tetramethylammonium amalgam has been characterized [12].

The formation of a material analogous to an amalgam was obtained with tetraalkylammonium of longer chains [13]. It was proposed that it could correspond to

the dilution of a hypothetical NR4• radical inside the mercury bulk (n >> 1):

n Hg + R4N+ + e− → R4N(Hg)n

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This hypothesis was supported by Southworth et al. [14] who reported, under

electrolysis of aliphatic ammonium salts on a mercury pool, the formation of a

silvered grey solid. Such electrogenerated materials were later used by Horner

and Neumann [15] as efficient reducing reagents capable of preventing contact of

the reagent with air. More recently, it was established by Garcia and coworkers

[16] that such quaternary ammonium amalgams were in fact Zintl ion salts.

Quite similarly and in the same spirit, Svetlicic and coworkers showed that

mercury is not the lone electrode material that could electrochemically react

in this way with quaternary ammonium salts [17]. Indeed, some posttransition

metals also react with quaternary ammonium salts in a similar way [18]. In particular, lead, tin, antimony, and bismuth were reported to produce thin organic

layers under electrochemical reduction. From coulometric measurements, the

stoichiometries of these electroformed materials were established. Under the

experimental conditions chosen by Kariv-Miller, the cathodic material M was

deeply changed: each ammonium is associated with a homopolyanion derived

from reduction of the metal M according to the following general equation:

5 M+ + R4N+ + e− → R4N+(M5)−

The potentials required for the formation of these organometallic layers and their

stability in aprotic solvents depend on the size of the concerned cations and on

the nature of the metal used as electrode. Thus, in the presence of dimethylpyrrolidinium cations, the stability of the layer obtained with posttransition metals increases as follows: Sb > Sn > Bi > Pb > Hg. All these phases were found

to exhibit a certain reducing power toward weak π-acceptors (aliphatic ketones,

weakly activated phenyl rings) and organic compounds capable of giving an irreversible scission under electron transfer (such as alkyl halides). Quite similarly,

the cathodic insertion of metallic ions (Li+) is well known in rechargeable battery

systems [19,20], and that of tetraalkylammonium cations into graphite (mainly

Me4N+, which occurs without exfoliation of the material) must certainly be cited

as well [21–24]. Several insertion stages were reported with the maximum insertion achieved at somewhat reducing potentials and found to correspond to C6Li

and C12NMe4 as limits. Those species were used as efficient reducing reagents

[24] but were reported to be sensitive to dioxygen. Chemical Formation of Homopolyanions with

Meta-Metals: Zintl Phases, State of the Art

Related to the subject of this chapter are obviously the well-known Zintl phases.

The generation of homopolyanions Mmn− (Figure 2.1) was first reported by Zintl

et al. [25] at the beginning of the 20th century. Their synthesis is based on the

reduction of meta-metal salts (M = lead, tin, antimony, bismuth, and many others)

by alkaline and alkaline-earth metals in liquid ammonia in chemically inert vessels [26–28]. It was found that the stability of complexes with Na+ or Cs+ is fairly

weak. Their stability (case of posttransition metals) was found to be strongly

increased by the use of cryptates [29,30]. Nevertheless, it should be emphasized

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Reduction of Platinum under Superdry Conditions























Figure  2.1  Structures of some Zintl homopolyanions established by means of

x-ray diffraction.

that the electrochemical method (cathodic dissolution of alloys used as cathode

material) permits, at room temperature, the synthesis of a large palette of Zintltype complexes, and allows the insertion of a large variety of cations [31,32].

After Zintl, the chemistry of heteropolyanions was developed. It is remarkable

that species obtained by means of electrochemistry are those of weaker entropy

(Sn94− and Pb94−), while complexes obtained from tertiary alloys such as NaSnPb

in ethylenediamine are species of much larger entropy (heteropolyanions such as

Sn5Pb44− and Sn4Pb54−) [33]. Figure 2.2 exhibits the limit proposed by Zintl with

the extension when the reducing species is an alkali metal.

Zintl limit


























P 11



















Anions complexes of transition

metals and meta-metals



Zintl polyanions of AX compounds

(with A = Li, Na, K, Cs)



Figure 2.2  Zintl classification.

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Electroanalytical Chemistry: A Series of Advances

2.1.3 What Is the Cathodic Reactivity of Transition

Metals Used as Cathode Materials?

Platinum, palladium, copper, and tin are generally considered as relatively inert

materials for cathodic electrochemical processes when experiments are performed under regular experimental conditions such as in an aqueous media or

some organic solutions with a large (or fairly large) amount of moisture. Gold

was also found to react electrochemically with K+ in liquid ammonia to form the

auride ion Au−. The reduction occurs at very negative potential, and the formation

of soluble Au−, which could be deposited as Au at the anode, was observed [34].

In general, the use of Pt, Pd, Cu, and Sn as cathodic material is limited (especially

platinum and palladium) by their weak hydrogen overvoltage. Thus, with technical or commercial weakly acidic solvents such as dimethylformamide (with tetrabutylammonium tetrafluoborate as supporting electrolyte), the cathodic limit is

always larger than –2 V versus SCE. Under such conditions, the cathodic boundary simply corresponds to hydrogen evolution.

2.2 Experimental Conditions Required to

Demonstrate Cathodic Charging of Platinum

2.2.1 Preparation of the Electrolyte, Electrodes

Used , and Voltammetry and Coulometry

For most of the experiments related to this chapter devoted to the reduction of

platinum, the experimental conditions are based on obtaining the so-called superdry conditions described by Hammerich and Parker [35] and Heinze [36]. These

conditions were additionally redefined in previous reports [37–39]. Even though

most of the experiments described in this chapter have been performed in dimethylformamide (DMF), it should be emphasized that acetonitrile (AN) or propylene

carbonate (PC) could be used as well. An important and essential condition is the

use of extremely dry solvents. A simple method for obtaining dry solvents (with

less than 50 ppm of water traces) consists in the direct in situ addition of activated

neutral alumina in the electrochemical cell. For this, the activation of alumina

is achieved by heating at 300°C for 4 h under vacuum. In particular, DMF was

almost constantly checked (by the Karl Fischer method) to ascertain that it contained less than 50 ppm of water. A permanent storage of solvent over alumina

gave best results.

The same precautions versus the presence of water traces have to be taken for

the used electrolyte. In most of the experiments, the electrolyte concentration was

0.1 mol.L−1. Potassium, lithium, sodium, and cesium iodides were used (because

of their large solubilities). Tetraalkylammonium salts (puriss grade) were also

used with purity >99.7%. It is required that all salts be dried by conventional

methods before use [40].

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Reduction of Platinum under Superdry Conditions


Cyclic voltammetry investigations were carried out in a standard threeelectrode cell. For analytical studies, a typical working electrode was a smooth disk

of metal (Pt, Pd, and others) with a surface area of 8 × 10−3 cm2. The counterelectrode

was a glassy carbon rod. It is important to underline the required use of an organic

reference (Ag–AgI 0.1 mol.L−1 NBu4I system in DMF) to avoid any possible water

diffusion in the cell. All potentials are in principle referred to the aqueous SCE. The

potential shift in DMF with this referenced electrode is −0.52 V versus SCE.

Prior to experiments, platinum electrodes were carefully polished with silicon

carbide paper of successively smaller size (18 to 5 µm), then by diamond powder

(6 and 3 µm). Finally, electrodes were rinsed with ethanol and acetone, and then

dried with a hot airflow. Between each scan, the electrode surface was thoroughly

polished according to the procedure given earlier.

Lastly, for macro electrolysis investigations, Pt sheets (99.99% purity with a

surface area of about 1 cm2 and a thickness 0.05 mm) were used. They were

employed only once for SEM investigations without further treatment.

Coulometric and electrochemical quartz microbalance (EQCM) experiments

were carried out on thin metallic deposits prepared by deposition of metals from

solutions of 10 g L−1 H2PtCl6 in 0.1 mol.L−1 HCl onto polished gold disks (2 ×

10 −3 cm2). The plating was achieved in a galvanostatic mode (current 10 −2 A cm−2).

All the experiments were performed with gold substrates that were experimentally

found to be very weakly reactive toward salts within the potential ranges used

for platinum. The procedure allows the easy preparation of different amounts of

electrodeposited metal. Depending on the type of experiment, the gold substrate

was a gold microelectrode polished before each deposition. When EQCM experiments were carried out, a much larger electrode of gold-coated quartz crystal was

used (see the following text).

2.2.2 EQCM Instrumentation

EQCM was found to be a valuable method to quantify, under given experimental

conditions, the mass cathodic change of platinum in the presence of a large variety of salts [41]. Mass balance experiments were carried out with an oscillator

module quartz crystal analyzer connected to a potentiostat. Mass changes could

be achieved potentiostatically at fixed potentials or in the conditions of voltammetry (linear variation of potential upon time). The EQCM device was computer

controlled. In the experiments described here, 9 MHz AT-cut gold-coated quartz

crystals were electrochemically plated with a thin film of metal (principally platinum, but other metals such as palladium or nickel could also be deposited in thin

layers). Plating was achieved by using the galvanostatic procedure. The deposited

mass was additionally checked by EQCM. EQCM measurements were performed

in a Teflon cell equipped with a glassy carbon counterelectrode, a reference electrode, and the metallized quartz crystal working electrode. The apparent area of

the quartz crystal was about 0.2 cm2.

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Microgravimetric data, reported in terms of mass change, were calculated

using the Sauerbrey equation, which links resonant frequency and mass change.

In most cases, it was found that the mass deposit of platinum (or that of other

metal tested) was not totally reversible and, consequently, a small excess of mass

remained at the start of each experiment. However, the amount of extra charge

gained during the deposition process remained the same.

2.2.3 Electrochemical Atomic Force Microscope

(EC-AFM) and Scanning Electrochemical

Microscope (SECM) in Dimethylformamide

Because of the reactivity of the electrogenerated Pt phase, the electrochemical

“reduction” of platinum was investigated by in situ EC-AFM in dry and deoxygenated DMF. AFM requires the use of a very flat sample for allowing the scanner to

follow surface morphology, and this condition renders difficult experiments with

native samples of platinum. Our samples were prepared by d.c. sputtering of Pt

onto (100)MgO, leading to a distribution of (100)Pt platelets (around 100–200 nm

size and an average thickness around 50 nm; see References 42–44 for details).

The quality of the sample was always checked by x-ray diffraction. An θ–2θ x-ray

diffraction showed the (100) orientation of the Pt films, and the narrow rocking curve recorded on the 200 reflection (full width at half maximum of 0.33°)

showed the high crystalline quality of the film. X-ray diffraction φ-scan of the

(220)Pt reflections evidenced the epitaxial growth of the Pt films (Figure 2.7).

By adjusting the experimental conditions, different general patterns can be

obtained, whether the platinum areas are connected or not. The preparative

conditions were selected in order to get connected or unconnected Pt plates.

Under such conditions, the sample could be either macroscopically conducting

(behaving like a single, flat native electrode) or, on the contrary, macroscopically insulating and composed of independent Pt plates. Samples prepared by

this technique have several advantages: (1) the average roughness of the platelets is low (around 2 nm), and their crystallographic orientations correspond to

a fully defined (100)Pt surface; (2) the sample behaves like a native electrode;

and (3) patterns are easily recognizable, which make the observations easier by

comparison with an unmodified sample. The morphology of the Pt modifications was characterized by contact-mode atomic force. For EC-AFM experiments, the cell was a three-electrode type setup. The working electrode that is

also the AFM sample has a 5 × 5 mm 2 size and is fixed onto the bottom of the

cell by two wires to avoid any movement during AFM scanning. The reference

electrode was an Ag quasi-reference obtained with an Ag wire covered with

AgNO3. The counterelectrode was made by twisting a 50 µm platinum wire

and placing it all around the cell. This geometry produces an almost homogeneous modification of the sample. All experiments were performed under

inert gas (argon) to limit the introduction of water and oxygen because the

electromodifications of platinum are highly sensitive to the presence of water

and oxygen.

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2 µA

Figure  2.3  Voltammetric response of a smooth platinum disk electrode (area:

0.8 mm2) in contact with a superdry solution of 0.1 mol.L−1 NaI in DMF. Scan rate:

0.2 V s−1. Potentials are referred to the SCE reference electrode (Cougnon, C., Ph.D. thesis, Université de Rennes 1, 2002).

2.3 The Platinum Charge under Superdry Conditions

2.3.1 Electroanalytical Evidence of Platinum

Charge in the Presence of Alkali Salts

Initially, the first experiments [45] concerning the charge of platinum were performed in the presence of monovalent cations (Li+, Na+, K+, and Cs+) associated

with iodide anion, mainly for the purpose of solubility of these salts [40]. As will

be explained later, in the study relative to tetraethylammonium salts, bromide,

chloride, tetrafluoroborate, hexafluorophosphate, and several other ions could be

used as well. With these experimental conditions, where no electroactive species

are introduced in the solution, except the alkali metal iodides that was originally

chosen as inert supporting electrolyte, a quasi-reversible peak is observed at the

Pt microcathode (Figures  2.3 and 2.4). This unexpected cathodic current, Ipc,





2 µA

Figure 2.4  Voltammetry for a smooth platinum disk (surface area 0.8 mm2) in a superdry solution of 0.1 mol.L−1 CsI in DMF. Scan rate: 0.2 V s−1. Potentials are referred to the

SCE electrode (Cougnon, C., Ph.D. thesis, Université de Rennes 1, 2002).

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Table 2.1

Voltammetric Data Related to a Stationary Platinum Cathode

of Area 8 × 10−3 cm2 a










Electrolyte (0.1 M)

Epc (V)

Ipc (µA)

Epa (V)

Epc − Epa (V)




































Potentials are referred to the SCE. The sweep rate, v, is 0.2 Vs−1. Potentials (Epc and Epa) and peak

currents (Ipc) obtained from cyclic voltammetry at a platinum cathode relative to 0.1 M salt alkali

halide salts and to tetra-n-butylammonium salt solutions in dry DMF.

was always associated with an anodic step peak of current, Ipa, and was detected

for all the considered alkali cations but at different potentials. It could be shown

that the anodic step corresponds to the reverse reaction of the cathodic step by

simply maintaining the applied potential at its level. Charges observed with

alkali metals ions (Table  2.1) require rather negative potentials (<−2 V versus

SCE). All cathodic steps existing under those experimental conditions vary linearly with the square root of the scan rate, which indicates a diffusion control for

the charge process.

This electrochemical behavior is very similar to what has been observed with

highly oriented pyrolytic graphite (HOPG) electrodes when they are cathodically

charged within comparable experimental conditions (in cases of cation insertion

into the lamellar compound). By analogy with these processes, the very small

currents noticed for the cathodic steps on platinum suggest that the limiting diffusion corresponds to the slow insertion into the metallic bulk. It is also important to stress that the cathodic currents do not diminish when activated alumina

is progressively added to the electrolytic solution, and thus, it cannot be due to

residual water reduction. Additionally, the reduction of alkali metal cations to

produce the corresponding metal at the surface of platinum is an unlikely process

in this case. Indeed, considering the concentrations of alkali cations (0.1 mol.L−1),

the observed currents would have to be much higher to bring a cathodic limit to

the process. However, the intensity of the peak current has been found to depend

both on the concentration of the salt and the nature of the cation. Specifically, in

the presence of alkali iodides, the intensity of the currents follows the order Cs+

< K+ < Na+ << Li+.

To check the specific role played by moisture (traces of water), known amounts

of water were added to the solution (amounts slightly larger than 200–500 ppm).

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Reduction of Platinum under Superdry Conditions

Upon water addition, the specific redox signal corresponding to the platinum

charging disappears, and the classical cathodic limit assigned to water reduction

is then observed.

In order to get more information about the existence of a charge–discharge

process, employing the coulometric technique appeared very helpful. The idea is

that the charge amount stored in the material (or at its surface) during the cathodic

process could be anodically restored. However, it must be underlined that fully

reversible charge systems are difficult to obtain owing to the presence of residual

water or acidic impurities, even in very small amounts. Thus, the value of the

charge measured during cathodic reduction, Qf, in the course of rather long fixedpotential electrolyses, is expected to be too large because it contains a contribution due to unavoidable hydrogen evolution. On average, the charge proportion

due to hydrogen evolution is around 65% to 75%, depending on the salt used and

the quality of preparation of the DMF solution. Generally, it is observed that the

electric charge stored in material that is specific to the platinum redox process

(named Qa; see Figure  2.5), could not be easily correlated to the total charge

amount measured during the charge process. Thus, Qa values that become invariant with long-enough charge processes could be assigned to the reduction of platinum and the experiments described here, taking into account those maximum

charge values.

In order to determine the exact proportion of metal involved in the insertion

process, experiments were performed with samples containing a known amount

of platinum that was electrochemically deposited onto inert substrates such as

gold or glassy carbon. When gold was used as the substrate, the accuracy of the

mass of Pt was also checked by the EQCM technique. Experimental conditions


























Figure  2.5  Successive charges and discharges of a platinized smooth gold disk

(area: 0.8  mm2) at potentials of −2.15 V and –0.5 V (versus SCE), respectively.

Electrolyte: 0.1 mol.L−1 NaI in superdry DMF. Amount of deposited platinum: 60 µg. (a)

Chronocoulometric curves for charges and discharges. (b) Variation of quantity of charge

Qa (recovered charge during the reoxidation process) upon charge times. The limit of Qa

corresponds to the saturation of the platinized layer (Cougnon, C., Ph.D. thesis, Université

de Rennes 1, 2002).

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Figure 2.6  EQCM experiments related to the charge of platinum upon time in the presence of 0.1 mol.L−1 CsI in DMF. Applied potential: −2.3 V versus SCE. (a) Gold-filmed

quartz. (b, c, d, and e) the same substrate covered with platinum. Deposited masses of Pt

are 0.9 µg, 1.5 µg, 3.5 µg, and 6 µg, respectively (Cougnon, C., Ph.D. thesis, Université de

Rennes 1, 2002).

were always those given in Figure 2.5: the length of the electrolysis time (cathodic)

was checked to be sure it was long enough to ensure the total insertion of the

salt inside the platinum. Thus, as displayed in the example given in Figure 2.6,

the reduction of Pt in CsI–DMF was achieved at –2.3 V versus SCE, while the

reoxidation of the modified platinum was obtained at 0 V. It is remarkable that

the limit, Qa, was reached almost immediately. Additionally, the value of Qa was

found to be proportional to the electrodeposited platinum amounts. The slope of

the lines Qa = f (Pt thickness) was found to be 2, with an accuracy of 2% with the

tested alkali metals. The assumption relative to a reaction in mass was therefore

verified, and confirms the hypothesis of an insertion of the alkali metal cations

M+ inside the platinum. Thus, an ionometallic structure of the general form [Pt2−,

M+, X] could be proposed at this stage [46].

For an exhaustive determination of the mass, it is also necessary to characterize the possible insertion of other species (X-neutral) like the solvent or the salt

that was not concomitantly involved in the process. In this purpose, the variation

of the mass of the platinum sample was followed during the whole charge process.

Such experiments led to the data exemplified in Figures 2.6 and 2.7.

At the level of the redox step, a mass increase was found for the platinum sample in the presence of different salts. This increase was found to be proportional to

the initial mass of deposited metal, and does not depend on the salt concentration.

Mass increase was larger with cesium iodide than with the other alkali cations.

The linear increase of mass of the layer after saturation (for experimental time

>200–300 s under the conditions of Figure  2.6) on the mass of Pt allowed the

proposition of a general global formula for the platinum phase. These formulas

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