THE ELECTIWCATALYTIC properties of platinum-gold and palladium-gold alloys have received attention primarily because of the change in electronic configuration as the d-band vacancies of platinum and palladium are progressively filled by the addition of gold. A major difliculty arises in quantitative interpretation of the data for these alloy systems because the surface composition may differ from the bulk. The bulk structures are well established by X-ray diffraction techniques. These alloys are fee systems and the lattice parameter changes almost linearly with composition for both systems. 1 However the platinum-gold system is heterogeneous, except for alloys quenched from temperatures just below the solidus, and can separate into an a, phase, rich in platinum, and an az phase, rich in go1d.l On the other hand the palladium-gold system is homogeneous.l
In a recent communication,a the electrochemical properties of electrolytic codeposited platinum-gold were discussed in relation to X-ray diffraction studies. The bulk composition of the deposits consisted of a mixture of a platinum phase and a non-equilibrium alloy phase of varying composition but it was concluded that the surface was composed of the equilibrium phases (the cc1 is essentially pure platinum at 25°C). Similar techniques have been applied to examine the bulk and surface properties of electrolytically co-deposited palladium-gold where a homogeneous alloy is anticipated.
The electrodes were prepared by electrodeposition on to tantalum foil at 25 "C at a controlled potential of +0.05 V(she); the electrolyte was made up from 2% solutions of PdCl, and AuCl, in 1 M HCl in varying proportions. The quantities of gold and palladium deposited were determined by dissolution in aqua regia and analysis for both metals by X-ray fluorescence spectroscopy.
X-ray diffraction analysis of the electrodeposits yields a single broad reflexion for each of the various planes. The lattice parameter for the peak corresponds to an alloy composition in agreement with the X-ray fluorescence analysis within a few per cent. The line broadening is proportional to the tangent of the Bragg angle and is, therefore, due to a distribution of lattice parameter.a This is essentially due to a range of composition between the crystallites. The range cannot be determined since the broadening from other factors, such as stress, lattice distortion and crystallite size, is not known. However, if the other factors are neglected, the broadening at half peak height is equivalent to a range of palladium concentrations of 7-27 %f and 51-83 % for the 17 and 67% co-deposits respectively.
Voltammetric curves for hydrogen and oxygen electrosorption on palladium and gold in 1 M sulphuric acid show substantial differences.' Hydrogen is adsorbed to a considerable extent by palladium and adsorbed to only a negligible extent by gold.