Effect of lead compounds on heterogeneous oxidation catalysts

Abstract

Studies have been made of the structure, physicochemical characteristic and catalytic activity of a number of unsupported and alumina-supported transition metal oxides as well as of alumina-supported platinum. Measurements have also been made of the effects on these properties of varying amounts of lead bromide and lead chloride, which were introduced onto the surface of the catalysts by means of a fluidised-bed reactor and in all cases caused a reduction in activity.X-ray and thermal studies showed that the observed decrease in catalytic activity could not be attributed to the formation of a new catalytically inactive phase. With unsupported nickel (II) oxide, manganese (IV) oxide and possibly copper (II) oxide, the poisoning effect of the lead halides can, however, be satisfactorily accounted for in terms of the formation of a monolayer of the lead compound on the catalyst surface.The results obtained with unsupported cobalt (II, III, III) oxide are less readily explained. At low temperatures, lead (II) chloride in monolayer amounts caused complete deactivation, but at higher temperatures considerably greater quantities of this compound were needed to eliminate catalytic activity. Under all conditions used, a large excess of lead (II) bromide was required to deactivate this oxide completely. With supported catalysts, the lead halides appeared to be preferentially adsorbed on the alumina support. Again, however, Co3O4/Al2O3 catalysts behaved atypically, in that no such preferential adsorption took place and even sub-monolayer amounts of lead (II) chloride caused complete elimination of catalytic activity.