Mössbauer spectroscopy of CO shift catalysts promoted with lead

Abstract

Lead promotes the activity of Cr-promoted magnetite CO shift catalysts. An analysis of the Mössbauer parameters of the Pb-promoted catalysts shows that lead enters the spinel structure as Pb 4+ on tetrahedral sites, giving rise to a partly normal spinel structure. Attempts to synthesize the Pb 4+-containing Fe-Cr spinel by firing the oxides in vacuum at 1000 °C failed, indicating Pb 4+ is not stable in the spinel structure at high temperature. In fact it exsolves from the Pb-promoted catalyst upon heating to above 700 °C. The data indicate that the octahedral Fe 3+ ions exhibit an increased covalency as a resutt of promotion by lead. This together with the inherent defect structure of the spinel phase may explain the enhanced activity toward the CO shift reaction at 400 °C.