X-Ray absorption spectroscopic study at the cobalt K-edge on the calcination and reduction of the microporous cobalt silicoaluminophosphate catalyst CoSAPO-34

Abstract

The local environments about the cobalt atoms in as-synthesised, calcined and hydrogen-reduced CoSAPO-34 have been studied by X-ray absorption spectroscopy. Calcination of the as-synthesised material and reduction in hydrogen of the product of calcination were carried out at 450 °C and for 6 h, respectively. The results show that cobalt is sited as Co^II in the framework and that calcining CoSAPO-34 with 4.4% Co leads to two-thirds of the Co^II content being oxidised to Co^III thereby lending support to the view that the material is a redox catalyst. Reducing the calcined material with hydrogen leads to a material containing only Co^II. Taken together, the pre-edge, XANES and EXAFS regions indicate that one-third of the original tetrahedrally coordinated divalent cobalt in the as-synthesised product is maintained in the calcined and reduced products with the remaining cobalt being in the form of trivalent hexacoordinated and divalent hexacoordinated cobalt, respectively.