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

Abstract

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 ¹²CO and ¹³CO allows detection of Co⁺(¹²CO)(¹²CO) (2097 and 2013 cm⁻¹) and Co⁺(¹³CO)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 ¹²CO–¹³CO 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.