FTIR spectroscopic study of CO adsorption on Co-ZSM-5:: Evidence of formation of Co+(CO)4 species

CO adsorption on Co-ZSM-5 results in formation of two kinds of Co2+-CO species characterized by bands at 2217 and 2207 cm(-1), respectively. The species are highly resistant towards evacuation at ambient temperature. When CO adsorption is performed at low temperature, new kinds of weakly electrophilic Co2+ sites are evidenced by carbonyl bands at 2197 and 2190 cm(-1). The bands detected at ambient temperature are negligibly reduced in intensity, suggesting initial stages of formation of geminal Co2+(CO)(2) dicarbonyls (most probably absorbing at 2201 cm(-1)). Reduction of the sample in a CO atmosphere at temperatures of 423-673 K results in creation of a fraction of Co+ cations. The latter form, with CO, stable Co+ (CO) 2 dicarbonyl species (bands at 2113 and 2042 cm(-1)). Coadsorption of (CO)-C-12 and (CO)-C-13 allows detection of Co+ ((CO)-C-12)((CO)-C-12) (2097 and 2013 cm(-1)) and Co+ ((CO)-C-13)(2) (2065 and 1997 cm(-1)) species. The dicarbonyls are thermally decomposed without formation of a measurable amount of corresponding monocarbonyls. In the presence of CO in the gas phase the Co+ (CO)(2) dicarbonyl species are converted into tricarbonyls (2137, 2089 and 2079 cm(-1)) which are characterized by a disordered C-3v symmetry. As a result, a variety of mixed carbonyls are produced after (CO)-C-12-(CO)-C-13 coadsorption. Lowering of the temperature in the presence of CO results in coordination of a fourth CO molecule to some of the Co+ sites, the tetracarbonyls formed being characterized by a set of bands at 2130, 2105 and 2075 cm(-1). The reasons for the high coordinative unsaturation of the Co+ cations in Co-ZSM-5 and the possible role of Co+ cations in the selective catalytic reduction of nitrogen oxides are discussed.