Methane hydrogenation and confirmation of CHx intermediate species on NaY encapsulated cobalt clusters and Co/SiO2 catalysts: EXAFS, FTIR, UV characterization and catalytic performances

Abstract

The adsorption of Co ₂ (CO) ₈ onto dehydrated NaY powder under an N ₂ atmosphere predominantly yielded supported Co ₄ (CO) ₁₂ . The molecular cobalt carbonyl clusters and their decarbonylated products have been structurally characterized by in situ IR, extended X-ray absorption fine structure (EXAFS) and diffuse reflectance spectroscopies. The IR spectrum assigned to the species Co ₄ (CO) ₁₂ /NaY is shifted significantly from that observed for externally supported analogues on NaY and for this cluster in solution, which indicates that the cobalt carbonyl clusters occurring on the NaY are similar to those occurring in weakly basic solution. EXAFS coordination numbers (N) show that the successively decarbonylated samples maintain small cluster sizes, which depend on the temperature-programmed oxidation of the precursor. Methane hydrogenation was carried out on intrazeolitic cobalt clusters using a two-step process. It showed a relatively higher activity and selectivity to C ₂₊ hydrocarbons in comparison with Co/SiO ₂ catalysts. On the basis of IR (ν _C-H 2960, 2880 cm ^-1 and δ _C-H 1520, 1393 cm ^-1 ) spectroscopy, mass spectrometry and reaction studies, one can conclude that CH _x (x=0, 1 or 2) surface carbonaceous species were generated by CH ₄ dissociation on the activated cobalt catalyst; the CH ₂ species was quite reactive, and propagated higher hydrocarbons.