COMPLEX PROCESSES RELATED TO THE EARLY STAGES OF MERCURY ELECTRODEPOSITION ON Pt ELECTRODES

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INTRODUCTION
Otherwise, the amount of Ocontaining surface species changes according to t,.

The injuence of surface coverage by Hg adatom on the hydrogen electrode reaction on Pt
From the results described in sections 1 and-2 one can conclude that peak I, involves two main reactions, namely, the electroreduction of the O-containing surfaces species yielding OH-ions and bare Pt sites, and the Hg-adatoms electrodeposition on bare sites.Therefore, from these results it is possible to estimate the charge densities, qox and qHS, coexisting on the substrate at different potentials and anodization times for the eventual electroadsorption of H atoms, and immediately afterwards from 0 V upwards at v = 0.3 Vs-' to follow the anodic stripping of Hadatoms presumably electroadsorbed in the precedent stage.In this case, the difference between the stripping charge density of H-adatom (qH) resulting for plain 0.5 M HClO., and that, qH, resulting in the Hg:+ ion containing solutions, can be related to the degree of surface coverage of Pt by Hg-adatoms attained for each solution composition (Fig. 4).For HgI(NO,), concentrations as low as 10m6 M, and the entire range of ES and t, covered in the present work, the value of qH results are practically constant and equal to that resulting in the absence of Hg: + ions in solution (Fig. 4).This result indicates that for very low Hg: + ton concentrations only negligible amounts of Hg-adatoms are present on the Pt surface probably due to a t, value too low to reach the adsorption equilibrium.Otherwise, when the Hg,(NO,), concentration in the acid solution reaches 10m4 M, then qH changes rather sharply with E,.At E, = 0.70 V, qH is equal to 50 PC cm -2, and as E, increases, qH increases reaching a limiting value of about 190@cm-'.
It should be noticed that at 0.70 V only a fraction of the O-containing monolayer is still present on Pt.These results clearly demonstrate that bare Pt sites created through the electroreduction of the O-containing surface species are immediately occupied by Hgadatoms resulting through the Hg$+ ion discharge.Hence, the Pt surface is always covered by a composite monolayer containing a variable O/Hg atom surface concentration ratio.The latter depends on the applied potential and solution composition, so that the relative O/(0 + Hg) and (Hg)/(O) + (Hg) atom surface concentration ratios change either from 1 to 0 or from 0 to 1 respectively, as the O-species stripping process proceeds.Therefore, one should expect a cooperative effect of Hg and 0-adatoms on the nucleation and 3-D growth of bulk Hg on the electrode surface as seen in the following section.

lnjluence of Hg and 0-adatoms on Pt on the nucleation and 3-D growth of bulk Hg
The influence of Hg and 0-adatoms on Pt on the nucleation and 3-D growth of bulk Hg was mainly followed through potentiostatic current transients and voltammetry in 0.5 M HC104 + lo-' M Hg,(NO,), (E, = 0.73 V).In this case voltammograms were run from 0.65 V to E,,, values positively changed stepwise.Thus, for E,, ,, lower than E,:, the potential of peak 1;.(Fig. 5a), the voltammogram shows up a clear cathodic loop (II,) at potentials close to E, associated with the nucleation and 3-D growth of bulk Hg and a noticeable peak II, related to Hg stripping.The potential cycling under these conditions produces a substantial increase in the heights of both loop II, and peak II,, as it should occur for an enhancement of Hg electrodeposition.
Otherwise, for E,,, > EI: (Fig. Sb) the magnitude of loop II, and stripping peak II, becomes smaller than that observed for E,,, < El;, and, simultaneously peak I;, becomes clearer.Finally, for this between 1.45 and 0.65 V, then potential stepped to E, for t, = 300 s and finally potential stepped to the potential E, -C E,, to record the corresponding current transient (Fig. 6).For E, = 0.90 V, (E, G 0.90 V < E,:), a potential at which the Pt electrode is presumably covered to a large extent by Hg-adatoms, the current transients recorded at E, decrease initially (not shown in the figure), then increase to reach a maximum current (I M) at time t M, and later decays slowly (Fig. 6a).For E, = 1.45 V, a 0 0  3) and parameters assembled in Table 1.
potential at which both the simultaneous stripping of Hg-adatoms and the electroadsorption of 0-adatoms take place, the current transients exhibit the same shape although the overpotential for obtaining a similar bulk Hg electrodeposition charge, turns out to be about 0.06 V higher than that corresponding to E, = 0.90 V (Fig. 6b).These results firmly indicate that nucleation and growth of bulk Hg is progressively impeded as the surface coverage by 0-adatoms increases.

Alloy formation
The possible formation of a Hg-Pt surface alloy was investigated in0.On the other hand, when E, is set at potentials lower than E, both the height of peak 12 and the corresponding bulk Hg stripping charge increase (Fig. 7), with t,.
In this case the charge density resulting from peak 1: exceeds largely that expected for the sum of Hg and Oadatoms monolayer charge densities which is close to 750 PC cm -'.The same type of experiments made up with pfsc Pt microelectrodes show that the charge of the stripping peak I; becomes smaller than that obtained with pc Pt.This means that the structure of the electrode surface plays an important role for the alloying reactions undergoing at E, -C E,.
Current transients were also made by applying to pc Pt electrodes the following potential program: firstly a potential holding at Ed = 0.45 V (E, c E,) during different t,, then potential stepped to Es = 0.80 V, (E, < E, < EI;), to electrooxidate bulk Hg, and finally potential stepped to 1.45 V for recording the current transient related to stripping peak Ib.In this case the charge resulting from the current transients increases according to td yielding a charge density (qo) vs t'12 linear relationship (Fig. 8).The value of q. was
5 M HClO., + 10m4 M Hg,(NO& by electrodepositing Hg at a constant potential E, (0.70 V < E, < 0.45 V) on pc Pt electrodes during the time t, (10 s < td -c 900 s), and subsequently running an anodic stripping at v = 0.1 V s-' between E, and 1.40 V. Runs comprising Ed values more positive than E, show up an increase in the height of peak 1: according to t,, and for td -+ 30 a nearly constant charge density value close to 700 PC cm-* is reached.