Isolated Co²⁺ and [Co−O−Co]²⁺ Species in Na-MOR Exchanged with Cobalt to Various Extents:  An FTIR Characterization by CO Adsorption of Oxidized and Prereduced Samples

A commercial Na-MOR was exchanged to various extents with cobalt. The sodium and cobalt contents were determined by atomic absorption. Samples were characterized by UV−vis and by FTIR. The FTIR results with CO show that Co-MOR heated in O 2 at 793 K and evacuated at the same temperature contained isolated Co 2+ (86 to 100% of cobalt detected by CO adsorption at RT) and [Co−O−Co] 2+ . After exposure of samples to CO at RT, FTIR showed that only a minute fraction of [Co−O−Co] 2+ underwent reduction yielding [(CO) n Co + −Co + (CO) n ], with n = 2 or 3, and CO 2 . After exposure to CO at increasing temperature up to 623 K, the subsequent adsorption of CO at RT yielded increasing amounts of [(CO) n Co + −Co + (CO) n ] and CO 2 . Whereas isolated Co 2+ did not undergo reduction, Co 2+ in [Co−O−Co] 2+ reduced to Co + . On evacuation at RT, [(CO) 3 Co + −Co + (CO) 3 ] completely and reversibly transformed into [(CO) 2 Co + −Co + (CO) 2 ]. On evacuation at increasing temperature, [(CO) 2 Co + −Co + (CO) 2 ] progressively disappeared and transformed into a stable bridged species, [Co + (CO)Co + ], and [(CO)Co + −Co + (CO)]. As the total cobalt content in Co-MOR samples increases, the [Co−O−Co] 2+ amount increases exponentially. Hence, we infer that the [Co−O−Co] 2+ species is not the active site for NO abatement with CH 4 in the presence of O 2 .