In Situ Studies of the Oxide Structure and Oxide Growth on Single Crystal Platinum Surfaces

Abstract

Understanding the Pt surface oxidation is of key importance for the development of durable oxygen reduction reaction catalysts as used in low temperature fuel cells. The formation of an ultra-thin surface oxide on Platinum electrodes is causing atomic-scale restructuring of the electrode surface and Pt dissolution, which promotes the degradation of the Pt catalyst. The surface restructuring can be attributed to a place-exchange process, where Pt surface atoms are extracted from the electrode and form an oxide layer atop the surface [1]. A detailed atomistic picture of the oxide growth and oxide structure previously only existed for the Pt(111) electrode [2].In this study, we have used High Energy Surface X-ray Diffraction (HESXRD) [3] to analyse the atomic-scale surface structure of Pt(111), Pt(110) and Pt(100) during oxide formation and reduction in 0.1 M HClO4 and 0.1 M H2SO4. These surfaces exhibit distinct differences in stability towards restructuring after potential cycles to the same upper potential limit. On Pt(111), almost no surface roughening can be observed, while Pt(100) and Pt(110) immediately degrade upon a single oxidation. To elucidate this difference we performed a detailed analysis of the crystal truncation rods (CTR) to determine the location of the Pt atoms in the oxide formed on Pt(100). During initial stages of oxide formation at potentials < 1.2 V, the Pt atoms are extracted from the surface layer and moved laterally away from their original lattice sites. The extraction of the first atom initiates the immediate extraction of a second atom, leading to the formation of oxide stripes on the surface. DFT calculations support this scenario and show that, differently from Pt(111), this mechanism produces unstable surface atoms at stripe ends that can dissolve during the oxidation as well as during the subsequent oxide reduction [4]. A saturation coverage of about 0.4 ML extracted Pt atoms in this stripe oxide is reached already at about 1.2 V, representing a surface that is fully covered with oxide stripes. Our recent analysis of the CTRs of the oxide structure at higher potentials (> 1.2 V), shows that oxide growth continues vertically by the formation of a second distinct layer of Pt atoms above the stripe oxide.