Characterization by EXAFS of Co/MFI Catalysts Prepared by Sublimation

Abstract

Catalysts have been prepared by subliming CoCl 2 or CoBr 2 vapor onto the H form of the zeolite MFI, followed by replacing the halide ions by OH groups and subsequent heat treatments. The Co sites have been characterized by EXAFS and XANES spectra at liquid nitrogen temperature. The data are correlated with the H 2 -TPR profiles of the same samples and their catalytic performance in reducing NO x with hydrocarbons. Materials with fairly large Co 3 O 4 particles after calcination, displaying a Co−Co distance of 3.5 Å and negligible interaction between Co and T sites (T = Si or Al), have a poor selectivity in NO x reduction, because such oxide particles catalyze the combustion of the hydrocarbon. In contrast, highly selective catalysts show no Co 3 O 4 clusters and distinct interaction between Co and T atoms. Heating in H 2 at 400 °C reduces Co−oxo ions, while cobalt oxide clusters are incompletely reduced and isolated Co 2+ ions in exchange positions remain unreduced. The water formed in the reduction of oxo species can interact with Co 2+ ions, increasing their coordination number with oxygen. A preedge XANES peak at 7710 eV indicates an electric-dipole-forbidden, but quadrupole-allowed and vibronically allowed 1s−3d electronic transition.