Molecular Oxygen Network as a Template for Adsorption of Ammonia on Pt(111).

Low-temperature scanning tunneling microscopy (STM) was used to observe a mixed NH3-O2 overlayer on Pt(111). At adsorption temperatures below 50 K, the chemisorbed O2 molecules form an ordered network at high coverages. The STM images reveal that this network features a distributed set of holes corresponding to on-top sites of the Pt lattice that are surrounded by two or three O2 molecules. Different hole-hole distances are observed with 0.73 nm most common. These holes in the O2 network act as preferential adsorption sites for the ammonia molecules leading to the formation of an NH3-O2 complex that serves as a precursor to ammonia oxydehydrogenation.