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Abstract

Oscillations with large amplitudes (>500 K), high frequencies (>8 Hz), and good reproducibility are observed in methylamine decomposition on electrically heated Pt, Rh, and Ir wires at pressures of 0.25–8.0 Torr and temperatures between 900 and 1500 K. The major reaction involves formation of HCN which is endothermic by 37 kcal/mol, so that the reaction strongly cools the wire, which counteracts resistive heating. A photodiode array was used to monitor spatial and temporal oscillations with resolutions of 1 mm and 30 Hz, respectively. On Pt, the frequency increased with increasing pressure and with decreasing wire diameter, while amplitude was a weak function of these parameters. On Ir, oscillations were more complex with regions of the wire often oscillating independently and more variability between experiments. However, the overall behavior resembles that on Pt. On Rh, frequencies were much lower (<0.03 Hz), oscillations occurred over a smaller temperature range and halves of the wire frequently oscillated out of phase with each other. Oscillatory behavior of methylamine decomposition on Pt was modeled by a simple lumped model which gives semiquantitative agreement with observations. The model assumes a simple unimolecular reaction with CH₃NH₂ adsorption blocked at low temperatures by formation of a strongly adsorbed CN which is known to be very stable on Pt (E_d≂50 kcal/mol). Calculated frequencies, pressure dependencies, amplitudes, and temperature range of oscillations agree quite well with experiments using reasonable values for adsorption and reaction rate parameters, and wire geometry and properties.