The Deactivation of Automotive Post-Combustion Catalysts by Lead From Gasoline

Abstract

This paper reviews research that has been going on for several years in an attempt to find the causes of the deactivation of automotive post-combustion catalysts containing precious metals when used with leaded gasoline. Data concerning the amount, nature and location of the lead deposits that form in aged catalysts as well as concerning the interactions that occur with the support and metal crystallites are given to begin with. The ways in which lead deposits (metal or oxide) grow and become desorbed have been examined with unsupported platinum. Up to about 400° C it is shown that relatively thick layers of lead may be formed. At temperatures higher than 500° C, however, lead tends to be desorbed, and in the steady state all that remains on the surface of the platinum is an amount of lead smaller than a monoatomic layer. Lead reduces the adsorption of carbon monoxide on platinum but not that of oxygen. When the amount of lead present on the surface is less than a monoatomic layer, this causes an increase in the CO + O 2 reaction rate compared with what occurs on clean platinum. On clean platinum the high adsorption of CO effectively tends to inhibit its own oxidation. With more than three monoatomic layers of lead, the activity of the platinum becomes very slight no matter what the temperature may be. (change-para-here) With industrial catalysts made of precious metals supported on alumina, it has effectively been found that their impregnation with PbO as well as their use with gasoline containing pure tetraethyl lead do not cause deactivation as long as the amount of lead present is not so high so as to cause the clogging of the porosity. (change-para-here) It is also shown that the deactivation of catalysts that is observed when commercial leaded gasoline is used is mainly the result of the effect of additives containing chlorine and bromine which are habitually added to gasoline at the same time as tetraethyl lead to reduce the fouling of combustion chambers in engines.