Kinetics of NH 3 oxidation on Pt, Rh, and Pd

Abstract

The kinetics of the reactions involved in the catalytic oxidation of NH 3 have been studied as a function of temperature over polycrystalline Pt, Rh, and Pd wires in a steady state flow system using a differentially pumped mass spectrometer to monitor species concentrations. Unique and reproducible steady state rates were observed in all experiments for temperatures up to 1400 °C at all compositions. Rates of individual reactions could be fit by Langmuir-Hinshelwood rate expressions with constant parameters, and comparisons of these parameters on different metals give reasonable qualitative interpretations for observed differences between metals. The competition of NO and NH 3 for adsorption sites appears to be important in determining the rates of various reactions. Differences in selectivity between metals are caused by the different temperature dependences of rates of individual reactions rather than by uniform differences in rates. Kinetics measured at low pressures (1–2 Torr) are used to estimate selectivities of NO and N 2 production at conditions of industrial nitric acid production. The predicted selectivity and optimum temperature and composition on Pt assuming simple models of flow patterns over the catalyst gauze match closely those observed in the high pressure process.