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 (g_xx= 2.0021, g_yy= 2.0050 and g_zz= 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 (g_xx= 2.0030, g_yy= 1.9975 and g_zz= 2.0018); the other is a polynuclear CO radical containing four non=equivalent C=nuclei.