PtRh alloys are used as versatile multipurpose catalysts for a number of industrial applications, including fertilizer production and ammonia slip catalysts for NO x abatement purposes. For the latter, ammonia is oxidized to nitrogen at intermediate temperatures. To optimize the PtRh alloyed catalysts and explain the role of Pt and Rh for future intermediate-temperature ammonia oxidation operando studies, we prepared a series of distinct RhPt model surfaces. We explore post-annealing and

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... mperature deposition as two routes for preparation of surface alloys, and compare results with literature examples. Scanning tunneling microscopy and X-ray photoelectron spectroscopy provide detailed information on surface morphology and composition, and demonstrate excellent temperature stability of RhPt/Pt(111) in the temperature range targeted for operando catalytic studies. A detailed roadmap summarizes preparation conditions to achieve a broad variety of surface structures. Page 1 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 4 catalytic studies by means of near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and Reactor-scanning tunneling microscopy (STM) (see Figure S1 ). We explore two routes to obtain alloyed surfaces: post-annealing after room temperature deposition of Rh on Pt (111) and direct Rh deposition at higher temperature. We show that the obtained morphologies depend on the preparation method. Additionally, we investigate the temperature stability of the obtained model surfaces, which is of high relevance for systematic studies of catalyst performance. Experimental details The experiments were performed in two setups: an in-house built ReactorSTM at Leiden University 27 and a recently installed commercial ReactorSTM system (Leiden Probe Microscopy B.V., LPM) at the University of Oslo (UiO). The UiO machine is based on the previously reported Leiden ReactorSTM, 27 and contains a preparation and an STM chamber, both with a base pressure of ~1×10 9 mbar. The preparation chamber allows flexible high-quality sample preparation via Ar + -sputtering (IQE 11-35, SPECS), vacuum annealing (up to 1300 K), metal deposition through a four-pocket e-beam evaporator (EBE-4, SPECS), and three leak valves for gas co-feeding. Equipment for Auger electron spectroscopy and low-energy electron diffraction (ErLEED 3000D, SPECS) is integrated to evaluate sample purity and crystallinity. The STM chamber is housing an STM system that is capable of scanning from ultrahigh vacuum (UHV) to high pressure (up to 6 bar). The unique design is described in detail by Herbschleb et al. 27 Sample preparation. A Pt(111) single crystal (99.999 %; Surface Preparation Laboratory (SPL), the Netherlands) was cleaned in repetitive cycles of Ar + -sputtering with an energy of 1 kV for 5 min followed by annealing at 1150 K in both O 2 (10 6 mbar, 5 min) and in UHV (5 min). Crystal quality in terms of cleanness, crystallinity, and flatness was verified by means of AES, LEED, and STM. Rhodium was subsequently deposited onto the Pt(111) single crystal using the