Methanol synthesis over Cu–ZnO aggregates supported on carbon nanotubes

Abstract

Methanol synthesis was studied with catalysts containing Cu and ZnO in the interior space or on the exterior surface of multi-walled carbon nanotubes (CNTs) and were activated by different procedures – reduction in dilute H2 at 513K with or without subsequent exposure to 10% CO/H2 at 673K for 30min. Characterization of the transition-metal species after these treatments by XRD and XAFS revealed striking differences. After mild reduction, XRD reflections related to Cu were missing or were of weak intensity, which could be assigned to very low primary particle sizes as detected by EXAFS. After treatment in CO/H2, reflections arising from alloy phases were obtained for all samples while non-alloyed Cu, although observed by EXAFS, escaped detection by XRD due to small primary particle sizes. Reduction of Zn2+ to Zn(0) was revealed by ZnK XANES only for some samples, which qualifies Zn(0) as a minority oxidation state for the remaining ones. Based on the XANES evidence, a new feature in ZnK EXAFS developing after CO/H2 treatment was interpreted as arising from either alloying or from an SMSI-type interaction of ZnO1−x entities with Cu nanoparticle surfaces. The catalysts exhibited very different productivities and responses to the CO/H2 treatment. In terms of specific activity (related to m2 Cu), the better samples achieved a multiple of the performance shown by a commercial reference, but suffered from insufficient stability. By contrast, such stability was demonstrated for a catalyst containing Cu/ZnO hosted in SBA-15. A catalyst series based on narrow CNTs previously functionalized by thermal shocks in flowing air stood out due to poor performance for unknown reasons. Due to these uncertainties, the data does not permit clear conclusions on the oxidation state of Zn in the promoting interaction with Cu although the general trends favor Zn2+ over Zn(0).