Light-off curve of catalytic reaction and kinetics

The light-off curve is the conversion-temperature plot of a catalytic reaction; it is usually used in catalyst development. The possibility to deduce kinetic information such as the apparent reaction order or mass transfer limitation from the shape of the curve is examined here. The light-off plots obtained by numerical simulation with several different rate equations are compared between themselves and with experimental curves. This is illustrated with the oxidation of carbon monoxide and methane on noble metal catalysts with different degree of mass transfer resistance. The comparison is complemented with the Arrhenius plots, i.e. the logarithm of the apparent rate constant versus the reverse of temperature.From the shape of the curves, the three following situations can be discriminated: first-order kinetics, negative first-order kinetics in the absence or in the presence of external mass transfer limitation. The range of diffusional limitation is well defined for negative-order reaction. However, the mass transfer resistance is difficult to evidence for first-order reaction. It is observed that the Langmuir-Hinshelwood reaction kinetic model is not convenient to represent the effect of strong external transport limitation on a reaction in the regime of reactant inhibition (negative apparent reaction order).