Abstract:
Rates of decomposition of ethyne, ethene and butadiene on niobium wire filaments at high temperatures have been investigated for hydrocarbon pressures between about 100 mN m–2 and 2 µN m–2, using ultra-high vacuum techniques. At high pressures, clean filaments underwent rapid decay of activity to give steady zero-order decomposition rates whose values were independent of the hydrocarbon studied. This phenomenon is discussed in terms of carbon deposition and diffusion of carbon from the surface. At lower pressures, steady first-order rates were observed before very slow decays of activity. Complicating hydrogenation reactions, occurring on the reaction vessel walls, restricted the upper temperatures at which first-order rates could be accurately measured. First-order rates are in reasonable accord with the transition state theory for a mobile transition state.
Experiments were performed with carbon, Nb2C and NbC filaments to indicate the possible effects of carbon contamination on niobium. Kinetics on Nb2C and pure Nb were similar.
Results for ethyne decomposition on niobium using u.h.v. are compared with some results under conventional high vaccum conditions and differences explained by decarburisation by water vapour. The high pre-exponential terms reported are critically examined and discussed in the light of these findings.
