A Comparison of Rate Laws for CO Oxidation Over Pt on Alumina

This study compares global rate expressions for CO oxidation over Pt/Al₂O₃ catalysts from the literature to a recent rate law based on a novel experimental approach. Our new method infers local values of reaction rates from measured concentration profiles along an enlarged isothermal catalytic passage at typical automotive exhaust conditions. Experimental uncertainties are relatively small, so parameters in the rate law can be assigned within close tolerances. Rate laws based on dual-site Langmuir Hinshelwood (LH) mechanisms correlate several of the available databases, including the new one. And activation energies for the CO-O₂ reaction in several global rate laws agree with values reported in the surface science literature. However, assigned energies in the CO adsorption equilibrium constant are 25 to 35 kcal/mole lower than they should be. All of the most accurate global rate laws have coefficients for CO inhibition that are virtually independent of temperature, whereas energy parameters in the actual equilbrium constants are between 30 and 40 kcal/mole.

Simulated light off curves for a typical passage in a monolithic converter are shown to be very sensitive to the activation energy for the CO-O₂ reaction, so it is reassuring that reported values based on different databases are consistent. They are also affected by different forms of the rate expressions, even among those that have the same regression coefficients. But this factor is secondary compared to the influence of activation energy.