Surface segregation and reconstructive behaviour of the (1 0 0) and (1 1 0) surfaces of platinum–palladium bulk alloy single crystals: a voltammetric and LEED/AES study

Using a modification of the Clavilier crystal preparation method, a series of palladium–platinum (1 0 0) and (1 1 0) single crystal alloy electrodes have been prepared covering the bulk composition range 3%<Pd<25%. The clean, thermally annealed surface alloys were each characterised using LEED, AES and cyclic voltammetry in order to make structural comparisons with previously published data from the corresponding (1 1 1) palladium–platinum surfaces [1]. Voltammetric data displayed systematic changes as a function of bulk palladium composition. These changes could be related to a gradual increase in the population of surface terrace sites by randomly distributed palladium atoms leading to attenuation of voltammetric peaks associated with platinum terraces. It was found also that at certain alloy compositions, the addition of palladium was sufficient to suppress the reconstruction characteristics of the pure platinum (1 0 0) and (1 1 0) surfaces. It was also established that palladium strongly surface segregates. Furthermore, the segregation was surface structure dependent with the more open (1 1 0) surfaces exhibiting greater segregation than the closer-packed (1 0 0) and (1 1 1) surfaces for a fixed bulk single crystal alloy composition. On the basis of these results, it is proposed that the surface segregation characteristics of platinum–palladium alloys are controlled by thermodynamics and that the composition of the selvedge region plays a crucial role in disfavouring surface reconstruction.