Determination of cobalt species in niobia supported catalysts

Abstract

Niobia-supported cobalt catalysts were prepared by incipient wetness impregnation and were characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), temperature-programmed reductioon (TPR) and magnetic measurements. At least two different types of cobalt species were present on the calcined catalysts: Co₃O₄ particles and Co²⁺ surface species. At high cobalt content, Co₃O₄ particles are the main species whereas the percentage of Co²⁺ species linked to the support increases as the cobalt loading is decreased. XPS results revealed that the Co²⁺ species could be better represented by a mixture of Co₂Nb₅O₁₄ and CoNb₂O₆. TPR analyses allowed one to quantify the percentage of Co₃O₄ particles on the niobia-supported Co catalyst. However, due to partial reduction of niobia, TPR alone did not permit the quantification of the reduction degree of cobalt in these catalysts. Magnetic measurements linked to a TPR technique is a possible way to measure the reduction degree of cobalt in Co/Nb₂O₅ catalysts. After reduction at high temperature, the mixture of cobalt niobates was reduced and NbO₂ produced led to the strong metal-support interaction (SMSI) effect.