Microscopic Insights into the Initial Oxidation Process of Single Crystalline Platinum Group Metal Surfaces: from Subsurface Oxygen, a Ghost Species, Towards Surface Oxide

In this review, the initial oxidation process of low-index surfaces of single-crystalline platinum group metals (PGMs: Ru, Rh, Pd, Ir, and Pt) is discussed in detail at the atomic level, involving several types of oxygen species: chemisorbed O, subsurface O, dissolved O, oxidic O. The oxidation of PGMs begins only when the surface of the PGM is saturated with chemisorbed O. Oxygen penetration into the metal is a critical next step in surface oxide formation, which can occur either through the step edge or directly through the terrace, depending on the oxidants chosen (O2, NO2, atomic O, and ozone O3). However, subsurface oxygen (oxygen directly below the top metal layer) does not form a separate phase in PGM. Instead, a surface oxide consisting of a single O-Me-O trilayer nucleates and grows (heterogeneous growth mode). The oxidation process is a nonlinear process with self-acceleration and passivation behavior, where many processes occur in parallel and in sequence, so that patterning can occur on different length scales. For this reason, oxidation studies must be performed at both the atomic and mesoscale using powerful combinations of surface science techniques such as scanning tunneling microscopy (STM) and low-energy electron microscopy (LEEM).