Investigation of the catalytic decomposition of methane, ethane, propane, n-butane, n-pentane, neopentane and n-hexane at low pressures on niobium at high temperatures using ultra-high vacuum techniques

Abstract

Rates of decomposition of methane, ethane, propane, n-butane, n-pentane, neopentane and n-hexane on niobium wire filaments at high temperatures were investigated using ultra-high vacuum techniques and mass spectrometric analysis. Reactant hydrocarbon pressures were usually between 100 and 1 µN m^–2. Methane was not perceptibly decomposed at any extremes of pressure and filament temperature. The other hydrocarbons, apart from ethane which produced some ethyl radicals, appeared to decompose completely to carbon and hydrogen. At the lower pressures decomposition rates were first-order with respect to the hydrocarbon pressure. Rates were expressed as reaction probabilities (P) per collision with P=B exp (–E_a/RT). E_a values were highest for ethane and neopentane. The other hydrocarbons, with secondary C—H bonds, had lower E_a values, all within a similar range. This suggests that E_a values are influenced by bond dissociation energies of C—H bonds in the molecules. Values of B tended to increase with hydrocarbon molecular weight, but were all reasonably consistent with the idea of a mobile transition state.

Some of the effects observed in conventional and in ultra-high vacuum systems are compared and discussed in terms of contamination.