Formation of polymeric radical anions by adsorption of CO on high surface area CoO—MgO

Abstract

Adsorption of CO at 298 K has been studied by e.s.r. spectroscopy on high surface area CoO—MgO samples (cobalt content from 0.1 to 5 atoms per 100 Mg atoms) and pure MgO. After activation under vacuum at 1173 K and CO adsorption at 298 K a weak three g-value signal (gxx= 2.0021, gyy= 2.0050 and gzz= 2.0060) is observed on pure MgO. The same species is also formed on CoO–MgO samples, although at a much higher concentration (≈ 25 times). The use of 13CO +12CO mixtures, with various 13C contents (from non-enriched to 90 %13C-enriched mixtures) allowed a satisfactory analysis of the e.s.r. spectra. The species contains two equivalent carbon atoms. Spectra taken at K-band frequencies are also consistent with this analysis. The species is tentatively identified as C2O–2.Two other species, in addition to the previous one, are formed on CoO–MgO specimens. One is the CO–2 radical (gxx= 2.0030, gyy= 1.9975 and gzz= 2.0018); the other is a polynuclear CO radical containing four non=equivalent C=nuclei.