Voltammetric and rotating ring-disk studies of the influence of anions in the underpotential deposition of zinc on platinum

[摘要] The electrocrystallization of Ni-Zn alloys on different substrates is one of the most interesting subjects in applied electrochemistry. This is mainly due to the intensive utilization of such deposits in several technological applications like protection against corrosion in automotive industry,1 hydrogen production by water electrolysis,2,3 energy storage devices4 and inhibition of hydrogen embrittlement on Ni/Cu Monel K500 alloy5 or AISI 4340 steel.6 As already observed by Brenner,7 the most important characteristic of such system is the anomalous character of Zn deposition. In the electrochemical deposition of alloys, the term anomalous applies for the reduction of the less noble ion component preferentially than the one predicted only from thermodynamic consideration.8 An interesting model proposed by Swathirajan to explain such behavior involves the inhibition of nickel deposition on platinum surfaces by underpotential deposition of zinc.9 The underpotential deposition of metals has been extensively studied due to the possibility of promoting modifications on the catalyst surface in a specially controlled way. This phenomenon is associated to the interactions between ad-atoms and substrate that make it possible to deposit a metal M in a potential range more positive than the Nernst potential for the couple M/Mz+. The main characteristics of these systems have been well described in several papers and reviews.10-12 The underpotential deposition of Zn on different substrates have been recently studied by Aramata et al., mainly on polycrystalline Pt, Pd and Au13,14 and on Pt single crystals.15 These authors have investigated the effect of pH on the anodic shift of the Zn UPD dissolution peak suggesting a two-electron mechanism with repulsive interaction between ad-atoms. Nevertheless, a model for Zn adsorption was not discussed. Dorda et al.4 have analyzed the deposition of Zn on Pt from alkaline solutions by cyclic voltammetry, proposing a poisoning of the surface by zinc ad-atoms, which was revealed by the inhibition of the hydrogen adsorption peaks. El-Shafei16 investigated the sensitivity of the crystallographic orientation of electrochemically oriented Pt surfaces for UPD Zn in acid medium using cyclic voltammetry. The UPD Zn was found to be a very convenient process for surface characterization due to the sensitivity of the ad-atom to surface orientation and to the fact that the deposition occurs in a potential range where no changes in the Pt surface structure are observed.16 Taguchi and Aramata17 have examined the UPD Zn in pH 4.4 solutions with and without halide anions by cyclic voltammetry. The UPD peak potentials were shifted negatively by anion adsorption that also increases the peak width. These authors assumed a three-step mechanism for the UPD process, namely, (i) desorption of initially adsorbed anions from the substrate, (ii) adsorption of the ad-atom and (iii) re-adsorption of the anions on the UPD metal. This model was used to study the adsorption of Zn in phosphate medium by cyclic voltammetry, FTIR and electrochemical quartz crystal microbalance (EQCM).18 The FTIR measurements pointed to the adsorption of HPO42- ions on Zn through a Zn-O bond. The effect of anion adsorption on UPD metal atoms was also discussed by Varga et al.19 for bisulfate on UPD Cu systems. The electrochemistry of Zn2+ in sulfate medium was also analyzed by Zouari and Lapicque.20 The authors had studied the thermodynamics of the Na2SO4-ZnSO4-H 2O system proposing the existence of (Zn2+)(SO42-) ion pairs in the liquid phase. Finally, Horányi and Aramata21, 22 have studied the adsorption of Cl-, HSO4- and H2PO4- on Pt surfaces induced by the co-adsorption of Zn ad-atoms. The authors concluded that the specific adsorption of the species induced by Zn ad-atoms depends on pH and at a given potential on the concentration of Zn2+ ions. From the discussion above it is clear that UPD Zn is not, up to now, sufficiently explored in the literature. Perhaps some difficulties arise in the interpretation of experimental data by the overlap of the Znads oxidation peaks with desorption of UPD H on Pt or by alloy formation with Au. Moreover, there is a lack of studies indicating the structure of the monolayer with regard to the number of active sites occupied by each ad-atom as well as to the maximum coverage of the surface. The hydrogen adsorption/evolution reaction is an adequate tool to analyze the blockage of active sites on the Pt (and even Au) surfaces. As each hydrogen atom needs one site to adsorb (and evolve H2, in the sequence), any decrease in the hydrogen adsorption charge or inhibition of hydrogen evolution means a blockage of the surface by a different foreign ad-atom. This particular behavior is proposed to be used here in order to postulate adsorption models for UPD Zn. Therefore, the objective of this work is to study the underpotential deposition of Zn on polycrystalline platinum surfaces in 0.5 mol L-1 H2SO4, HClO4 and HF solutions The electrochemical techniques to be used are cyclic voltammetry and rotating ring-disk electrodes and the UPD Zn response will be analyzed in relation to the behavior of the hydrogen adsorption/evolution on platinum and gold, aiming to explore some aspects of this system that still remain under discussion.  Experimental A three-compartment electrochemical cell was constructed in Pyrex glass and provided with a Luggin capillary for the reference electrode. The working electrode for the voltammetric experiments was a platinum disc with 0.199 cm2 electrochemical area, as evaluated by the charge associated with the desorption of one monolayer of Hads.23,24 For the hydrodynamic experiments a Pt-Pt configuration with a 0.001 cm2 Pt ring and a 0.160 cm2 Pt disk was used. The collection coefficient was determined with the Fe2+/Fe3+ couple as 0.195. A 5 cm2 platinum foil was used as the auxiliary electrode while the reversible hydrogen electrode (RHE) was the reference system. The acid solutions used in this work were prepared with H2SO4 (Merck, Suprapur), HClO4 (Merck, P.A.) or HF (Merck, P.A.), ZnO (Merck, P. A.) and water purified in a Milli-Q system (Millipore Inc.) and were deaerated by bubbling N2 (SS White Martins). Cyclic voltammetry was carried out using an EG&G PARC model 273 potentiostat/galvanostat linked to an IBM compatible PC 486 microcomputer controlled by the software M270 (EG&G PARC) while for the rotating ring-disk experiments a model 366A EG&G PARC bipotentiostat, a rotating system model 636 EG&G PARC and a model 7046B Hewlett Packard recorder were used.  Results and Discussion Cyclic voltammetric studiesThe voltammetric behavior of Pt at 0.2 V s-1 in the three different media under investigation is presented in Figure 1. The dotted-line curves represent the steady-state voltammetric response of the surface in the blank solutions. On the sequence, ZnO was added to the blank solutions, the electrode was held for 300 s at the initial potential (0.05 V) and the first voltammetric cycle recorded afterward. To achieve maximum surface coverage with Znads the Zn2+ concentration was varied from 10-5 to 10-3 mol L-1 and further up. It was found that 10-3 mol L-1 Zn2+ in the solution was sufficient for that purpose in three electrolytes under investigation, all other conditions maintained constant. This is shown by the full-line voltammograms in Figure 1.   The two anodic peaks observed in Figure 1A, 1B and 1C in the potential range between 0.05 and 0.4 V could be either associated to the dissolution of Znads deposited for 300 s at 0.05 V or to the oxidation of H-atoms adsorbed on the electrode surface. It is very difficult to distinguish these oxidation processes by cyclic voltammetry. However, the use of a rotating ring-disk electrode system will be useful to clarify this point (see later). Meanwhile, the dissolution responses are clearly affected by the anions of the electrolyte as their shape and peak potentials are quite different in the three cases. Thus, for sulfuric acid solutions (Figure 1A) a well-defined peak is observed at around 0.27 V and this can be associated with the strongly adsorbed Zn while the shoulder appearing at approximately 0.16 V is due to the weakly adsorbed Zn, as previously reported.13 The maximum dissolution charge density obtained here after deposition at 0.05 V during 300 s was 225 mC cm-2. By comparison with the charge density value associated to the desorption of a complete hydrogen monolayer (210 mC cm-2), a well-known one-electron transfer reaction where each H ad-atom is bonded to one active site on the platinum surface23,25 it can be proposed that the present

[学科分类] 化学（综合）