Influence Of The Activation Temperature On The Metal Accessibility In Model Three-Way Catalysts

The influence of the reduction temperature on the accessibility of the metallic surface was studied on model ceria-alumina supported platinum or rhodium catalysts. For a 0.5% Pt-Ce/Al solid, the H/R I values, deduced from hydrogen irreversible chemisorption, decrease deeply with T-r, the reduction temperature, from 60% at T-r = 300 degrees C to 19% at T-r = 500 degrees C. This can be attributed to strong interactions between ceria and platinum, since, the initial H-2 chemisorption could be restored after reoxidation. The presence of BaSO4 in the support accelerates the loss of metallic area, because of sulfur poisoning of the platinum surface. For T-r = 300 degrees C, the dispersion values were in agreement with those deduced from FTIR spectroscopy of adsorbed CO. In the case of rhodium, a 37% H/M dispersion was obtained, which did not change when T-r was increased from 300 to 500 degrees C. For two industrial Pt-Rh three-way catalysts, the behaviour was found similar to that of platinum, the amount of chemisorbed hydrogen decreasing for T-r> 350 degrees C. Thus, in the three-way catalysts characterization, the maximum metal accessibility is obtained after a reduction at 300 degrees C.