Syngas reaction over lanthanum-cobalt intermetallic catalysts

Abstract

Some ReM5 (M = Ni, Co) intermetallics are very active and selective catalysts in syngas conversion, especially in methanation. However, one always observes a large increase in activity with time on stream because of partial transformation of the intermetallic. From bulk and surface characterization, other workers concluded that a metal-support system was the active catalyst for the reaction, the rare-earth oxide acting as a support.In this work, we studied the transformation of LaCO5 compound during syngas conversion by XRD, ESCA, EXAFS and electron microscopy. Significant changes in superficial composition during activation steps or (CO, H2) conversion were observed and can be linked to the activation period. Indeed, whatever the pre-treatment of the sample, the activation period was always obtained. However, reduction, oxidation or reaction enrich the surface with cobalt and shorten this activation step. Therefore, contrary to suggestions made by others, such treatment does not lead to the real active sites for (CO, H2) reaction, which are formed during the reaction. In fact, the surface appears to break up into a nodular morphology composed of small carbon filaments formed from metallic particles. Moreover, these particles seem to be hexagonal cobalt rather than LaCO5 alloy and the particles of lanthanum oxide are independent of the preceding filamentous structure.