Kinetics of NH 3 decomposition on polycrystalline Pt

Abstract

The kinetics of NH 3 decomposition on polycrystalline Pt are examined over a wide range of catalyst temperature (500 < T < 1700 ° K), pressure (0.015 < p nh 3 < 20 Torr), and gas composition in a steady-state flow system with conversions sufficiently low that differential rates are obtained. Calibration of system and mass spectrometer parameters yields an apparent accuracy of better than ± 10% under all conditions. All data in pure NH 3 and in NH 3H 2 and NH 3N 2 mixtures could be fit to a single Langmuir-Hinshelwood (LH) rate expression with experimental data deviating from calculated LH curves by less than 10% in pure NH 3 and 20% in NH 3H 2 mixtures over a range of 10 4 in reaction rates. Orders with respect to NH 3, H 2, and N 2 are determined, and coefficients of pressures are shown to have Arrhenius temperature dependences with activation energies appropriate for heats of adsorption. Several reaction mechanisms which can give this rate expression are described, and the significance of parameters is discussed.