Ove Hjortsberg

Sulfur adsorption and desorption on fresh and aged Ce containing catalysts

Abstract The effect of field ageing on the transient H 2 S emissions from Ce-containing three-way catalysts was studied with TPR experiments and XPS analysis. A set of differently aged catalyst samples was analyzed by SO 2 exposure followed by TPR treatment with H 2 , in combination with successive XPS analysis of the oxidation state of Ce. TPR studies were also made using H 2 , CH 4 , C 2 H 2 , C 3 H 6 and C 3 H 8 as reducing agents. It was found that the H 2 S desorption declines rapidly as a function of field ageing. Simultaneously, the TPR (with H 2 ) desorption peak temperature of H 2 S increases and the SO 2 peak temperature decreases. For the catalyst with high activity the Ce3d XPS spectra indicated a shift in the oxidation state from (III) to (IV) during sulfur adsorption. The severely aged catalyst stayed in the trivalent state, during sulfur storage and TPR. The temperature for maximum H 2 S formation strongly depend on the type of reducing agent. A fullscale engine bench test was performed, by use of chemical ionization mass spectrometry. The results are dicussed in the light of the flow reactor results. Possible explanations are discussed, as well as a possible mechanism for changes in the H 2 S formation, due to deactivation.