Formation of Co2+…(CO)n complexes on high surface area CoO–MgO, as investigated by electron spin resonance

Abstract

Upon adsorption of CO (0.05–70 kPa) at 298 K, three different cobalt–carbon-monoxide surface species (A, B and C) are formed on high surface area (50–300 m²g^–1) CoO–MgO samples (Co content 0.1–20 atoms per 100 Mg atoms). The e.s.r. spectrum of species B, recorded at 77 K, shows hyperfine interaction with one Co nucleus. By adsorption from ¹³CO (90% enriched) hyperfine interactions with C nuclei are also observed. A similar analysis is possible for the e.s.r. spectra of species A and C. Relying also on the behaviour of the signals with respect to (i) pressure of CO and (ii) thermal treatments, the three species are assigned to surface cobalt–carbon-monoxide complexes which differ in the cobalt site and/or number of CO molecules in the complex. The variation of the intensity of the signals after redox cycles in O₂ or CO allow the surface complexes to be formulated as Co²⁺…(CO)_n.