NMR evidence of metal-support interaction in syngas conversion catalyst Co-TiO2

Abstract

To examine the relation between catalytic and magnetic properties, the zero-field NMR spectra and hysteresis loops of cobalt supported on silica, alumina, magnesia, titania, and ZSM-5 with and without the promoter thoria were investigated. Cobalt was incorporated on the support by simple physical admixture of precipitated cobalt and support, and by aqueous impregnation technique. Our studies indicate that the particle sizes are consistently lower in the presence of thoria. Of all the catalysts examined, the Co/Th/TiO2 catalyst exhibits a high saturation magnetization value—about 20% higher than pure cobalt. In addition, the NMR spectrum of the aqueous impregnation Co/TiO2 catalyst is distinctly different from the rest. All the NMR lines are shifted to a higher frequency by about 4 MHz. These two features—enhancement of the magnetic moment of cobalt atoms and increases in the hyperfine field at the Co nucleus—clearly indicate that there occurs strong metal-support interaction between cobalt and titania support. The higher hydrocarbon yields observed by the earlier investigators with Co/TiO2 catalysts might be related to this phenomenon.