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 O2 at 793 K and evacuated at the same temperature contained isolated Co2+ (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)nCo+-Co+(CO)n], with n = 2 or 3, and CO2. After exposure to CO at increasing temperature up to 623 K, the subsequent adsorption of CO at RT yielded increasing amounts of [(CO)nCo+-Co+(CO)n] and CO2. Whereas isolated Co2+ did not undergo reduction, Co2+ in [Co-O-Co]2+ reduced to Co+. On evacuation at RT, [(CO)3Co+-Co+(CO)3] completely and reversibly transformed into [(CO)2Co+-Co+(CO)2]. On evacuation at increasing temperature, [(CO)2Co+-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 CH4 in the presence of O2.