Electrochemical behavior of palladium–gold alloys
Introduction
Electrochemical behavior of binary alloys of noble metals has been widely investigated [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. Among various systems studied there are palladium alloys, which are of interest because of their ability to absorb hydrogen [26], [27], [28], [29], [30] as well as valuable electrocatalytic properties [10], [13], [14], [16], [17], [19], [20], [22], [23].
Palladium and gold form a continuous series of f.c.c. solid solutions, homogeneous at normal conditions, with the lattice parameter varying almost linearly with alloy composition [31], [32]. According to AES data, the surface composition of Pd–Au alloys under vacuum conditions is very close to their bulk composition [33], [34], [35].
Cyclic voltammetry experiments [4], [7], [9], [11], [14], [15], [21] have revealed that in acidic solutions the surface of Pd–Au electrodes, polarized to high potentials, changes significantly. Since Pd is much less resistive to oxidation than Au [36], selective electrochemical dissolution of this metal takes place during repetitive potential cycling which is reflected in a dramatic transformation of voltammograms recorded.
Rand and Woods [7] have pointed out a useful in situ technique of surface analysis of homogeneous noble metal alloys in a cyclic voltammetry experiment. The method is based on a linear dependence of the potential of surface oxides reduction (oxygen desorption) peak on alloy surface composition. Some limitations of this method towards Pd–Au electrodes were found by Gossner and Mizera [15] as well as Lamy et al. [14], according to whom the linear relationship could be used only for alloys containing not less than 40% at. of Pd.
The investigations of thin Pd layers deposited on Au substrate [37], [38], [39] have demonstrated the process of alloy formation at the metals interface occurring at room temperature due to the relatively high value of diffusion coefficient of Pd in Au [39].
The studies of the Pd–Au system in the aspect of electrocatalysis showed synergistic effects, as reported for oxidation of such compounds as carbon monoxide [13], [16], sodium formate [14], [22] and methanol [40].
Recently, we have studied voltammetrically the process of hydrogen electrosorption into Pd–Au alloys [27]. In this paper we report on the electrochemical behavior of this system in the potential range where surface oxides are formed (oxygen region). In particular, the influence of hydrogen absorption on electrochemical properties of surface oxides is shown. The discussion on the nature of oxygen electrochemisorbed on Pd–Au alloys is also presented. Although the Pd–Au system was the subject of many investigations, our experiments carried out in a wide composition range have revealed some facts not reported yet in the literature.
Section snippets
Experimental
All the experiments were performed at room temperature in 1 M H2SO4 solutions prepared from triply distilled water and analytical grade reagents. The solutions were deoxygenated with an argon stream for 25 min. A platinum gauze and Hg ∣ Hg2SO4 ∣ 1 M H2SO4 were used as the auxiliary and the reference electrodes, respectively. All potentials in the text and on figures are referred to RHE.
Pd–Au alloys were deposited potentiostatically on a gold wire (0.5 mm diameter) from a bath containing PdCl2 and
Cyclic voltammetry behavior of Pd–Au alloys with various compositions
The shape of initial cycles of i–E curves recorded in the oxygen potential region (0.365/1.565 V; scan rate 0.1 or 0.5 V s−1), reflecting the state of the electrode surface not subjected to any forms of pretreatment, depends on the alloy composition. For almost all samples containing more than approximately 70% of Pd in the bulk we obtained voltammograms with a single cathodic signal (surface oxides reduction peak) present. Its potential was intermediate between the potentials of respective
Conclusions
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Besides the behavior reported earlier in the literature, some other kinds of voltammogram changes have been observed during continuous potential cycling of Pd–Au electrodes in the oxygen region. In the case of Pd-rich alloys (mainly 65–80% of Pd in the bulk) a third cathodic peak appears at potential very close or almost equal with the potential of oxygen desorption peak on pure Pd electrode. For alloys initially very rich in Pd (above 90%) the ‘AuO’ reduction signal is not observed until the
Acknowledgements
This work was financially supported by the Polish State Committee for Scientific Research (KBN) Grant 3T09A 003 19.
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