COMPETITIVE ELECTROFORMATION OF SILVER AND OXYGEN OVERLAYERS ON POLYCRYSTALLINE RHODIUM IN ACID

The stripping analysis of metal layers at solid electrodes is considerably influenced by strong interactions at the deposited monolayer level [1,2]. For various systems the underpotential deposition (UPD) of metals occurs within a potential range where another surface process such as 0-electrosorption takes place simultaneously. In this case the interactions at the metal UPD level can be modified significantly. Some of these effects can be observed in voltammetric studies of UPD of Ag on Pt performed with Pt ring-disc electrodes [3], accompanied by the inhibition of hydrogen adsorption on Pt [4], the displacement of adsorbed hydrogen by UPD Ag and Cu [5,6], and UPD and overpotential deposition (OPD) of Ag on Pt [7]. In the present work results obtained for Ag deposition on polycrystalline Rh in H,SO, + Ag,SO, solutions are reported. This system offers a relatively wide potential range where both reactions, namely, the UPD of Ag and O-electrodesorption can be studied either simultaneously or independently.


INTRODUCTION
The stripping analysis of metal layers at solid electrodes is considerably influenced by strong interactions at the deposited monolayer level [1,2].For various systems the underpotential deposition (UPD) of metals occurs within a potential range where another surface process such as 0-electrosorption takes place simultaneously.In this case the interactions at the metal UPD level can be modified significantly.Some of these effects can be observed in voltammetric studies of UPD of Ag on Pt performed with Pt ring-disc electrodes [3], accompanied by the inhibition of hydrogen adsorption on Pt [4], the displacement of adsorbed hydrogen by UPD Ag and Cu [5,6], and UPD and overpotential deposition (OPD) of Ag on Pt [7].
In the present work results obtained for Ag deposition on polycrystalline Rh in H,SO, + Ag,SO, solutions are reported.This system offers a relatively wide potential range where both reactions, namely, the UPD of Ag and O-electrodesorption can be studied either simultaneously or independently.

EXPERIMENTAL
The working electrode was a polycrystalline (PC) Rh wire (Johnson Matthey Chemical Co, Spec-pure, 0.1 mm dia., 0.59 cm* apparent area).The electrode pretreatment was the same as described previously [8].A Rh counterelectrode and a Hg/Hg,SO,/l M H2S04 reference electrode were employed, but potentials in the text are referred to the reversible hydrogen electrode in the same solution (RHE).The electrolyte solution was 1 M H,SO, which was prepared from 98% H,SO, (Merck AR) and purified distilled water (Milli Q@).Ag,SO, (Mallinckrodt, p.a.) was added to the electrolyte in the lop5 to 10e3 M range.The experiments were made with nitrogen saturated solutions in the O-65 * C range by using triangular potential scanning combined with potential steps [8,9].

RE!SULTS AND DISCUSSION
A typical vol~o~am of the pc Rh electrode in 1 M H,SO, + 2.5 X 10T4 M Ag,SO, in the E,, = 0.03 V to Es+ = 1.4 V range at 0.1 V/s is shown in Fig. 1.The main anodic current contributions in the entire potential range are the remaining H-electrodesorption (In,*) at ca. 0.1 V, an asymmetric peak (II,) at 0.75 V, preceded by a shoulder at 0.6 V (II:) and followed by a hump at 0.85 V (II:), and the constant current region (III,) due to Rh oxide formation extending from ca. 1.0 V upwards.The current peaks II,, II: and 11: are related to the stripping of the different Ag layers competing with the Rh oxide formation in the low potential region (see Ag+ ion-free voltammogram, dotted line in Fig. 1) [8,9].
The electroreduction scan exhibits a wide cathodic peak (III,) at ca. 0.45 V and the H-electroadsorption peak (Ii+) at 0.1 V.The sequential formation of the different Ag layers becomes evident as Es,a is decreased stepwise from 1.4 V downwards.Peak III, shifts to more positive potentials and decreases gradually while the shoulder at 0.6 V (II:) becomes an anodic current peak.Further on, two cathodic peaks (II, and II:), which correlate with the corresponding anodic peaks, are progressively distinguished.The H-electrodesorption charge decreases systemati- When the potential hold is set at 0.03 V, close to the hydrogen electrode potential (Fig. 3) the voltammograms show that the H-electrodesorption charge firstly  increases up to a maximum as +T increases and later decreases to the baseline current, whereas the H-electroadsorption peak remains practically unaltered after Ag stripping.Likewise, the charge of peak II,-II: increases to reach a limiting value and immediately afterwards, the height of peak II: begins to increase.As peaks II,-II: and II: can be assigned to UPD and OPD.Ag on an O-free pc Rh surface, respectively, one concludes that at 0.03 V, the H-adatom layer is displaced progressively by electrodeposited Ag atoms.The anodic current contribution found in the 0.55-0.65V range, which depends on both the concentration of Ag+ ion in solution and the characteristics of the potential programme, can be related to two simultaneous reactions, namely, the formation of Rh(OH),, and the stripping of OPD Ag.However, the formation of Rh(OH), is inhibited progressively due to the accumulation of Ag at El, as 7 increases.The stripping voltammograms show definitely that the formation of bulk Ag starts only after electrodeposition of the amount of Ag related to peak II,.Furthermore, as the amount of stripped Ag increases, the negative potential scan shows an increasing negative charge due to Ag electrodeposition overlapping the 0-electroreduction charge.Therefore, from these results one concludes that bulk Ag electrodeposition can apparently occur only after the surface coverage by Ag adatoms (e,,) is slightly lower than two juxtaposed Ag layers (8,, = 1.6).
The electrodeposition of Ag increases the threshold potential for the reaction yielding (RhOH) ad, whereas the presence of the O-containing surface species produces a substantial delay on the formation of Ag electrodeposits at UPD level (Figs.2c and 3).For pc Rh partially covered by O-containing species, the growth of bulk Ag can occur without an appreciable amount of UPD Ag (Figs. 4a,b).The absence of stripping current peaks due to Ag UPD for certain values of Es,c is accompanied by the appearance of a hysteresis loop in the electroreduction scan which has been related to a nucleation and growth process [I, lO,ll].
No dramatic changes in the voltammograms are observed by changing the temperature.The voltammograms run under conditions comparable to those of Figs.2a and 2c but at 0°C either for clean or O-containing Rh surfaces show that peaks II, and II: largely overlap at 0°C on a prereduced pc Rh.Conversely, current peaks II, and II: remain distinguishable and of the same order of magnitude after holding the potential at Es:, = 0.53 V for r = 180 s at 65 o C.
The competitive electroformation of Ag and O-containing surface layers on Rh can be explained in terms of the relative position of the potential window determined by the threshold potential for Rh(OH)., formation, i.e. 0.6 V (vs.RHE) in 1 M H,SO, [8,9], and the value of E,.Thus, when E, is lower than the threshold potential for Rh(OH)., electroformation, Ag electrodeposition t_akes place on a Rh electrode free of O-containing surface species.Then the overall reaction can be interpreted as the initial formation of the Ag submonolayer, followed by the completion of the Ag UPD layer, and finally bulk Ag growth.
Otherwise, when E, is greater than the threshold potential for Rh(OH), formation, Ag electrodeposition occurs on a Rh surface completely covered by an O-containing surface species electroadsorbed monolayer.In this case, Ag elec-  ,b).This charge difference can be related directly to the charge involved in Ag electrodeposition plus the charge required for creating bare Rh sites through 0-electrodesorption.These plots for r < 100 s and El, = 0.53 V (Fig. 5a) approach linear relationships.The slope of these lines as well as the (Q& -Q$)_,, value, that is the charge difference for 7 = 0, both increase with temperature.For r > 100 s, and El, z E,, limiting charge values related to Ag UPD can be observed.Otherwise, for El, = 0.43 V, that is, Es:, < E,, bulk electrodeposited Ag prevails and in this case the linear relationship holds over the whole time window of the experiments.The slope of the line changes with temperature according to an Arrhenius plot, which yields an experimental activation energy equal to 20 f 5 kJ/mol.The linear plots depicted in Fig. 5 and the activation energy value are consistent with a diffusion controlled process for any stage of Ag+ ion electrodeposition.This is consistent with the fact that the value of the exchange current density for that reaction is about 1 A cme2 [12].

Fig. 2 .
Fig. 2. Voltammograms on pc Rh run after including a potential hold at E,' during a time r (0 Q r Q 600 s). 1 M HaSO, + 10T5 M Ag,SO,.(a) Potential hold E,' = E& = E, applied during the electrooxidation sweep which continues after r.The potential scans preceding the potential hold were run at: v = (1) 0.1, (2) 1.0 and (3) 1.0 V/s.(b) E,' = El, = E,.The potential hold is applied during the electroreduction sweep which continues after r.(c) E,' = E& s E,.The potential hold is applied during the electroreduction sweep, and after time T the electrooxidation scan starts from E& upwards.

Fig. 3 .
Fig. 3. Voltammograms on pc Kh run after including a potential hold at E,,= during a time r (0 B r G 600 s) between each sweep. 1 M HaSO, + 10m5 M Ag,SO+