Co K-edge XAS study on a new cobalt-doped-SiO₂ pillared clay

New Co-doped-SiO₂-sol pillared montmorillonite has been synthesized by interlayer hydrolysis and condensation of tetraethylorthosilicate (TEOS) in the presence of Co²⁺ ion using an organic template. The Co K-edge XANES and EXAFS analyses for the CoO-SiO₂-PILC (before and after calcination) and for the references such as CoO and Co(OH)₂ were performed in order to obtain electronic and local structural information of cobalt species, which may act as a catalytic active site for the selective reduction of NO_x, hydrodesulfurization, and Fischer-Tropsch reaction. According to the XANES spectra, the divalent cobalt ion is stabilized in an octahedral symmetry. The EXAFS result shows a significant change in local symmetry around cobalt ion upon calcination. The EXAFS fitting result before calcination shows that the cobalt species is in the form of hydroxide, with a small number of (Co-Co) pairs compared to the bulk Co(OH)₂. After calcining at 550°C, the first nearest neighbours were fitted to six oxygen atoms with two different distances, and the second and the third neighbours were fitted to two Si and one Co atoms. It is, therefore, reasonable to suggest a structure model, where the cobalt species on the SiO₂ sol exists as a nano cluster of Co(OH)₂ before calcination but transforms to a nanosized cobalt oxide covalently bound to the surface of SiO₂ pillar after calcination.