Characterization of hydrogenation active sites on LaCoO3 perovskite

LaCoO₃ becomes active for hydrogenation of ethene upon reduction in hydrogen at temperatures between 300 and 490 °C. Several aspects of the reacting system were studied in order to ascertain the nature of the active sites generated in this manner. Catalyst deactivation was evaluated by comparing rates between two successive experiments. An upper limit was estimated for the amount of polymeric residues formed after a single run: 1.1 ± 0.5 × 10¹⁴ molecules of C₂ per square centimeter. Reduced LaCoO₃ also catalyzed the self-hydrogenation of ethene. When a mixture of C₂H₄:D₂ = 1:1 was reacted over LaCoO₃ reduced to varying extents multiple-exchanged ethenes and ethanes were formed. The exchange patterns were almost unaffected by the extent of reduction. The effect of pretreatment temperatures was also evaluated. The solid in its reduced form was particularly sensitive to high-temperature treatments. The amounts of CO chemisorbed when plotted vs extent of reduction gave curves that were almost identical to the activity plots. The results reported here, discussed in terms of the current literature, are consistent with a model in which finely dispersed Co⁰, formed in the oxide matrix upon reduction, is the locus of hydrogenation activity.