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Iridium metal xps4/1/2024 Another structure, with expanded chains assembling into honeycombs, was observed after exposure to NO 2 at 450 K in vacuum 5. On Pt(111), one of these structures resembled the spoked-wheel structure and was formed by exposing the surface to an O beam 21. Nevertheless, related surface oxides were found earlier, obtained in an artificial way, by oxidation with stronger oxidants. This spoked-wheel superstructure was never identified before at the low pressures of most other Pt(111)-oxidation studies, nor predicted by theory. As a consequence, the spoked-wheel structure had a unit cell close to (8 × 8) with the Pt atoms in the oxide rows showing an expansion of ~8/7. Given the spoke length of 2.2 ± 0.1 nm, this would lead to a lattice constant within the row of 0.31 ± 0.01 nm, close to, but slightly larger than the directly measured interatomic distance. Most spokes consisted of eight atoms, including both endpoints, with very few being extended to nine or more. Based on this agreement, we consider this structure as a surface oxide comprised of 1D oxide rows. The measured atomic periodicity in the rows was 0.30 ± 0.01 nm, which was significantly larger than the Pt(111)-lattice constant of 0.278 nm and close to that of the UHV-stable surface oxide 5, α-PtO 2 15, 16, 17, 18, 19, and PtO 15, 19, 20. 1c, d, show clear atomic resolution within the rows. These surface oxide are not stable without the high O 2 pressure indicating that the O atoms in these structures are very reactive, making them relevant for catalysis. The second structure consist of a pattern of rows which are lifted from the surface and consisted of nearly half the amount of Pt atoms in the top layer. The lattice constant within the spokes is close to that of α-PtO 2. The first has a structure in which equilateral triangles are arranged into spoked wheels. Interestingly, the formation of α-PtO 2 is not observed, instead two stable surface oxides form. In this work, the oxidation of Pt(111) is probed with O 2 pressures of 1–5 bar and at 300–538 K using in situ scanning tunneling microscopy (STM). What is the structure formed under catalytically relevant conditions? If it is an oxide, is this a surface or bulk oxide? The most important questions remain unanswered. In a recent NAP XPS study, it was found that prolonged exposure to oxidizing conditions was needed to form Pt oxide 14. This surface oxide was found to be an intermediate in the bulk oxidation of Pt, which only started at much higher temperatures. These observations were contradicted by a near-ambient-pressure (NAP) X-ray photoelectron spectroscopy (XPS) study 13, which showed the formation of a surface oxide at similar temperatures as in the SXRD experiments, but at lower pressures. Two independent in situ surface X-ray diffraction (SXRD) studies showed the formation of bulk-like α-PtO 2 11, 12. This structure can only be elucidated when it is probed in situ, i.e., under high-pressure and elevated-temperature conditions. There is no guarantee that the structure of a catalyst observed in UHV is the same as the structure present under reaction conditions. Using these harsh conditions, even PtO 2 could be created 9, 10. This included a surface oxide consisting of one-dimensional (1D) oxidic rows, which were forming honeycomb-like superstructures 5. High-temperature exposure 4 or exposure to stronger oxidants, such as NO 2 5, O 3 6, and atomic oxygen 7, 8, was needed to create higher O coverages. It was found that O 2 binds molecularly below 160 K 2, 3, above which it dissociates readily and forms a p(2 × 2)-O chemisorption overlayer with a saturation coverage of 0.25 ML. The interaction of O 2 with Pt(111) has been extensively studied under traditional surface science conditions, i.e., ultra-high vacuum (UHV). This surface has the lowest surface energy and is expected to form the largest facets in a real catalyst 1. For the Pt-based automotive catalyst, the most essential model is the Pt(111) single-crystal surface. However, this is often too difficult for a technical catalyst under chemical conditions and studies of model catalysts are vital. To improve catalysts by rational design, it is crucial to understand the structure and chemistry of a realistic catalyst under reaction conditions. Research into the automotive catalyst remains highly relevant due to stricter emission regulations. In addition, NO is oxidized when the catalyst is operating in lean burn, i.e., in excess oxygen. Platinum serves as a major component in the automotive catalyst, which oxidizes CO and residual hydrocarbons to CO 2.
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