CO light-off temperatures on Pt, Pd & Rh supported on gamma alumina

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Category: Emissions Control

Subject: CO light-off temperatures on Pt, Pd & Rh supported on gamma alumina

Question

I would like to locate either fundamental or theoretical data on the approximate light-off temperatures (50% conversion/oxidation) of CO by palladium, platinum & rhodium on a ceramic substrate supported by a “typical” gamma alumina surface.

Answer

A deceptively simple question!

CO light-off not only depends on the nature of the active sites but also on the other components present in the gas stream, and on the stoichiometry of the gas stream (i.e. the ratio of oxidants to reductants).

Some general rules:

(i) Under rich conditions (when reductants exceed oxidants), light-off will be lowest on Pd.

(ii) Under stoichiometric conditions (eg CO + 0.5 O₂), Pd and Pt are comparable.

(iii) Under lean conditions (when oxidants exceed reductants), light-off will be lowest on Pt.

Rh is not very effective at oxidising CO by direct reaction with O₂, but it is the best metal for catalysing the CO + NO reaction (to CO₂ and N₂) under rich conditions.

For more detailed information, I would recommend that the sender does a search of the earlier work in the field of emission-control catalysis. Some of the key authors will be:

Haren Gandhi (H. S. Gandhi, Ford Motor Company)
Kathleen Taylor (K. C. Taylor, General Motors)
Gary Acres (G. J. K. Acres, Johnson Matthey).

The Light-off Temperature

The light-off temperature is a much-used term in automotive catalysis; it is the temperature at which the CO (or HC) conversion efficiency reaches to 50%.

Often, the rate of an exothermic reaction undergoes a sudden transition (driven by the heat being generated), which results in a step change in conversion. Defining exactly where that step occurs is quite difficult.

In emission-control catalysis T₅₀ (the temperature at which the conversion passes through 50%) is used as a comparative measure. It can be used for any oxidisable component of the exhaust gas, and therefore has to be made specific e.g. “the T₅₀ for methane was ......”

Answer posted 16 May 2005

Submitted by: Mr Allen Carroll

Affiliation: Applied Ceramics

Answered by: Dr Stan Golunski

Affiliation: Technology Manager of Gas Phase Catalysis, Johnson Matthey Technology Centre