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Abstract

During the reaction of H₂ + 12O₂ → H₂O on a platinum field emitter at 300 <T<400 K, pH₂ = 5×10^-7 to 5 × 10^-4 mbar and pO₂ = 3 × 10^-5 to 6 × 10^-4 mbar, three different adsorption phases can be distinguished in-situ via FEM and FIM. In FEM the different local work functions lead to areas of differing brightness, i.e. an O_ad-covered surface with high work function appears black and a H_ad-covered surface with lower work function bright. In FIM, oxygen acts as the imaging gas (O⁺₂ ions) of the O_ad-covered surface areas and even brighter images are formed by the product water, thus identifying active sites of the reaction. At steady-state conditions the adsorption of hydrogen appears in the form of islands with 30 to 50 Å diameter black areas. The oscillating H₂/O₂ reaction follows a Langmuir-Hinshelwood mechanism. The O_ad-covered surface starts at the {111} planes ({331} terraces) and proceeds in an anisotropic reaction sequence of planes until finally the (001) plane is covered by hydrogen. This H_ad layer at the Pt(001) surface is transferred into the O_ad layer by a reaction front traveling in the reverse direction, starting from the central Pt(001) and propagating to the peripheral {111} planes.