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Samples containing chromium (both 53Cr-enriched and non-enriched) have been prepared by equilibrium adsorption or impregnation methods at low loadings (<0.5%) using ZrO, , y-Al,O, , SnO,, TiO, (anatase) and SiO, as supports. Heating in O,, generally at 773 K, yielded mononuclear CrV species in a square-pyramidal configuration, Crgc (A), on all supports with the exception of SiO, where CrV is in a tetrahedral configuration, Crxc (A). H, O or NH,, both a t room temperature (RT), yielded the Crxv (A) species from CrEc (A), that is, the complex changes its coordination from five to six. After H, O adsorption and evacuation at RT, Crgc (A) is reversibly restored; however, N H, adsorption and evacuation at increasing temperature gives a new CrV species at 473 K. In the EPR signal of this species, designated Cr& (B), the perpendicular component is split into three lines with 14NH3 (A,4N = 4.0 G) and t w o lines with 15NH3 (A,5N = 5.5 G). The species is therefore assigned to a chromyl complex with an equatorial 0,ligand replaced by a nitrogen-containing NH:-3 species, possibly the NH; anion. With H,O or NH, at RT, the Crgc (B) species is transformed into the corresponding hexacoordinated species, Cr& (B).Upon adsorption of small H, O doses on the CrO,/SiO, sample, the Crgc (A) species is transformed into Crgc (A) and Crxc (A). In the presence of excess water, the chromyl species on SiO, becomes unstable, undergoing disproportionation t o Cr"' and Cr"' On adsorption of NH,, Cr& (6) is formed from Cr& (A) at RT. With 5N H,, a small superhyperfine interaction with nitrogen is partially resolved. Computer-calculated spectra enable us t o assign the Cr& (B) species o n SiO, t o a slightly distorted chromyl complex with slightly nonplanar equatorial ligands.

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) n Co+−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) n Co+−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.

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