Location of isolated Co2+ and [Co-O-Co]2+ in Co-MOR as investigated by means of FTIR with acetonitrile and 2,4,5- trimethylbenzonitrile as probe molecules

Co-Na-MOR and Co-H-MOR were prepared by ion-exchange of Na-MOR or H-MOR with Co(acetate)2 solutions at 350 K. In these Co-Na-MOR and Co-H-MOR samples exchanged to various extents with cobalt (Co/Al = 0.1 to 0.5), isolated Co2+ is the most abundant species, the residual cobalt being present as [Co-O-Co]2+. To find out whether isolated Co2+ and dimers [Co-O-Co]2+ were located inside or outside the zeolite channels we used as probe molecules acetonitrile (AN) and 2,4,5- trimethylbenzonitrile (TMBN), hypothesizing that AN enters the MOR main channels, whereas TMBN, having a higher steric hindrance than AN, does not enter the MOR channels, and only detects external cobalt. The comparison discriminates between internal and external cobalt. This feature was further investigated by presaturating Co-MOR samples with AN or TMBN and subsequently adsorbing CO or NO. FTIR spectra were run at room temperature on samples heated in O2 at 793 K and evacuated at the same temperature for 1 h before exposure to CO, NO, AN, or TMBN. The FTIR results demonstrate that in all Co-MOR samples, irrespective of the cobalt content, cobalt is almost exclusively located inside the mordenite channels. © 2010 American Chemical Society.