Lead-containing solid “oxygen reservoirs” for selective hydrogen combustion

Abstract

Lead-containing catalysts can be applied as solid “oxygen reservoirs” in a novel process for propane oxidative dehydrogenation. The catalyst lattice oxygen selectively burns hydrogen from the dehydrogenation mixture at 550 °C. This shifts the dehydrogenation equilibrium to the desired products side and can generate heat, aiding the endothermic dehydrogenation reaction. We compared the activity, selectivity and stability of three types of lead-containing solid oxygen reservoirs: alumina-supported lead oxide, lead-doped ceria, and lead chromate (PbCrO₄). The first is active and selective, but not stable: part of the lead evaporates during the redox cycling. Stability studies of a biphasic material, consisting of doped ceria with a separate PbO phase, show that the PbO phase is not stabilised by the ceria. Evaporation of lead and segregation of lead from the doped ceria occurs during prolonged redox cycling (125 redox cycles at 550 °C, 73 h on stream). The activity of this catalyst does increase over time, which may be related to the segregation of lead. Segregation of lead into a separate phase also occurs when starting from lead-doped ceria (Ce_0.92Pb_0.08O₂). The activity of this catalyst, however, does not increase with time on stream. Lead chromate (PbCrO₄) shows the highest selectivity (∼100%) and activity (2.8 mol O kg⁻¹) of all solid oxygen reservoirs tested (doped cerias, perovskites, and supported metal oxides). The activity is comparable to the theoretical maximum activity of CeO₂ (2.9 mol O kg⁻¹). This activity does drop, however, during the first 60 redox cycles, to about 25% of the starting value, but this is still higher than the best results reported for doped cerias.