Ethylene oxidation on polycrystalline platinum over eight orders of magnitude in ethylene pressure: A kinetic study in the viscous pressure regime

C2H4 oxidation on polycrystalline Pt films and foils at T = 373-473 K was studied with mass spectrometry in the pressure ranges 10(-6)-10(2) Torr C2H4 and 0.3-1500 Torr O-2 (1 Torr = 133.3 Pa). A new, ''spatially resolved gas sampling'' method enabled true kinetic data to be collected in the viscous pressure regime. In situ measurements of surface hydrogen with a capacitance-voltage technique and ex situ characterization with Auger electron spectroscopy and atomic force microscopy were also performed. No structure sensitivity with respect to sample grain size could be seen. The reaction orders in C2H4 and O-2 were +1 and -1 in oxygen excess, -2 and +3 in weak ethylene excess, and -0.5 and +1 in large ethylene excess, respectively. The apparent activation energy was between 30 and 75 kJ/mol for different reactant mixtures. The rate data could be qualitatively fitted to a simple Langmuir-Hinshelwood reaction scheme in the excess regimes, assuming competitive adsorption of C2H4 and O-2, abundant molecular desorption of the excess reactant, and a strong self-inhibition of C2H4 adsorption. (C) 1996 Academic Press, Inc.