Spontaneous deposition of Pt and Ir on Ru: Reduction to intermediate oxidation states

Abstract

X-ray photoelectron spectroscopy (XPS) was used to study the spontaneous deposition of Pt and Ir on Ru(0 0 0 1). The spontaneous deposition experiments were performed in a combined ultra high vacuum/electrochemistry (UHV–EC) system allowing sample transport between UHV and electrochemical environments under controlled conditions. The Ru(0 0 0 1) electrode surface was cleaned in UHV and then brought into contact with either a Pt-containing solution(10 mM K 2PtCl 6 + 0.1 M H 2SO 4 or 10 mM (NH 4) 2PtCl 4 + DI water) or with solutions containing 10 mM (NH 4) 2IrCl 6 in varying concentrations of H 2SO 4 corresponding to pH values of 0.5, 1 and 7 in the electrochemistry chamber for 30 min without an externally applied potential. XPS data acquired after rinsing in DI water indicated that each metal is present in intermediate oxidation states. Pt is found spontaneously deposited as Pt(II) on Ru(0 0 0 1) at pH 1. Similarly, intermediate states of Ir are observed, with higher binding energies, characteristic of higher oxidation states, observed at higher pH values. Annealing the spontaneously deposited adlayer in UHV to >500 K in each case results in the reduction of the Pt or Ir adsorbate to the metallic state. Typical metal surface coverages, determined by XPS, varied with number of immersions and solution conditions, but in all cases was observed to be <0.3 monolayers average thickness.