ISOTHERMAL HYDROGENATION OF CARBONACEOUS ADSORBED SPECIES ON AN IRON CATALYST .3. KINETIC-MODEL AND EXPERIMENTAL-VERIFICATION OF THE VARIATION OF THE SURFACE HYDROGEN CONCENTRATION

A kinetic model is studied as an explanation of the results observed during the isothermal hydrogenation (into CH4) of adsorbed carbonaceous species on Fe/Al2O3. On various metal supported catalysts this experiment leads to the observation of peaks in the rate of production of CH4 as a function of time. In previous studies, we have shown that some results can be explained by the presence of at least two steps with comparable rate constants in the process of CH4 formation. An alternative qualitative explanation is that the peaks are due to a simultaneous increase of the hydrogen surface concentration. Using a specific situation where this model is fully applicable, a quantitative formulation of these assumptions is presented, supported by various experiments. It is shown that the rate of hydrogen increase is linked to the rate constant of hydrogenation and that a peak can be obtained only if the experimental conditions agree with a specific criterion. Associated with previous studies, this analysis contributes to the interpretation of results obtained from this type of transient experiment. © 1992.