Model Discrimination in Chemical Kinetics

ABSTRACT

In studies on chemical kinetics, generally after the rate data have been taken, a mechanism and an associated rate law model are proposed based on the data taken. Frequently, more than one mechanism and rate law may be consistent with data. In order to find the correct rate law, regression techniques (model discrimination) are applied to indentify which model equation best fits the data by choosing the one with the smaller sum of squares. With this non-linear regression technique, rate parameters with 95% confidence limits are calculated along with residues. Of course, model parameters must be realistic. For example, reaction rate constants, activation energies or adsorption equilibrium constants must be positive by comparing the calculated value of parameters with 95% confidence limits, one can judge about the validity of the model.

In this paper, model discrimination will be applied to certain data from the literature along with the suggested heterogeneous catalytic models such as Langmuir-Hinshelwood Kinetic Model or Rideal-Eley Model.

A reaction of A + B → C + D type have been selected (like methanation) with the rate laws given below:

–rA = kPAPB / (1+ KAPA + KBPB + KCPC + KDPD)2 dual-site Langmuir-Hinshelwood Model

–rA = kPAPB / (1+ KAPA +KBPB + KCPC + KDPD ) single-site Langmuir-Hinshelwood Model

–rA= kPAPB / (1+ KAPA + KBPB )2 only reactants are adsorbed

–rA = kPAPB / (1+ KAPA )2 dual-site Rideal-Eley Model

–rA = kPAa . PBb power-law

In this study,

CO + 3H2 → CH4 + H2O

reaction rate data were tested for five different models to find the most suitable rate expression by model discrimination method taking the advantage of powerful POLYMATH package program.