ISOTHERMAL OSCILLATIONS OF THE HYDROGEN-OXIDATION REACTION ON PLATINUM - INVESTIGATIONS IN THE FIELD ELECTRON AND FIELD-ION MICROSCOPE

During the reaction of H-2 + 1/2 O2 --> H2O on a platinum field emitter at 300 < T < 400 K, p(H2) = 5 x 10(-7) to 5 X 10(-4) mbar and p(O2) = 3 X 10(-5) to 6 X 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 (O2+ 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 A diameter black areas. The oscillating H-2/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.