Doping of Nanostructured Co₃O₄ with Cr, Mn, Fe, Ni, and Cu for the Selective Oxidation of 2-Propanol

Abstract

A series of transition-metal-substituted (M = Cr, Mn, Fe, Ni, Cu) ordered mesoporous cobalt oxide catalysts were synthesized via nanocasting method using KIT-6 silica as a hard template. While the pristine Co3O4 formed as a perfect replication of KIT-6, metal substitution resulted in less ordered and smaller domains of the replica oxides. The catalysts were applied in the selective oxidation of 2-propanol in the gas phase to reveal the role of the systematic metal substitution. Cu and Ni substitutions were found to be beneficial for the catalytic activity, while Cr, Mn, and Fe substitutions were detrimental. Cofeeding water vapor shifted the onset temperature of 2-propanol conversion to higher temperatures (ΔT = 10–20 K), while a beneficial effect was observed at high temperatures (>260 °C) decreasing deactivation by slowing the reduction of active Co3+ and/or reducing coke deposition. The activity scaled with the reducibility of the catalysts probed by H2 temperature-programmed reduction with the positive effect of a higher reducibility, indicating the crucial role of oxygen activation during 2-propanol oxidation at the gas–solid interface. 2-Propanol activation probed by adsorption/desorption experiments monitored by diffuse reflectance infrared Fourier transform spectroscopy showed a weakening of the interaction and changing of the adsorption mode from dissociative to molecular adsorption following the periodic table from Cr to Cu, suggesting that the activation of 2-propanol plays a minor role compared with oxygen activation. Fe-substituted Co3O4 was the least active catalyst due to the decrease of the number of active Co3+ sites.