Carbon monoxide oxidation over platinum powder: A comparison of TAP and step-response experiments

Abstract

In this study carbon monoxide oxidation over platinum-powder was used as a model system to compare the capabilities of TAP (Temporal Analysis of Products) and step-response transient techniques. The catalytic oxidation of carbon monoxide and oxygen over platinum and the adsorption and desorption of carbon monoxide and oxygen on platinum were studied. Both techniques demonstrated that about 6% of the surface platinum atoms can be covered by atomic oxygen below 600 K and that carbon monoxide inhibits oxygen adsorption. A maximum in the carbon dioxide production was found as a function of time when carbon monoxide and oxygen are simultaneously introduced over an empty catalyst, which is caused by the increasing carbon monoxide coverage hampering oxygen adsorption. In the TAP reactor the state of the catalyst continuously changes as a consequence of the pulsing. The pulse frequency can strongly influence the results - therefore kinetic information is not easily acquired via TAP reactor experiments, although quasi steady-state conditions can be reached under special circumstances. In step-response experiments the condition of the catalyst evolves to a steady state. As a result, these experiments are more easily interpreted and yield information on reaction orders and (apparent) activation energies. Modelling of the response curves is less complicated for step-response experiments as is illustrated for the case of a step change from 13CO to CO that yields values for both the adsorption and desorption rate constant. Both the time resolution and the sensitivity, however, are higher in the TAP experiments than in the step-response experiments, although for this particular reaction system it does not provide additional mechanistic information. In contrast, in our study step-response experiments displayed self oscillations of the carbon monoxide oxidation over platinum whereas TAP experiments did not.