CO adsorption on MgO and CaO. Spectroscopic investigations of stages prior to cyclic anion cluster formation

Abstract

The adsorption of CO on microcrystalline MgO and CaO has been studied by u.v.–visible and i.r. spectroscopies. U.v.–visible absorption is generated by CO-derived species in two distinct bands, namely a ‘high-v’ absorption at 28 000–35 000 cm^–1, and a ‘low-v’ absorption at 18 000–25 000 cm^–1. The species contributing to these respective absorptions have been discriminated by observing selective reaction with O₂, N₂O and propene. The low-v absorption is due to reduced polymeric species, viz. cyclic anion clusters (CO)^2–_n, and the high-v absorption to lower order non-reduced species incorporating surface oxide ions, with (CO)_nO^2– stoichiometry. There are marked differences in behaviour between CaO and MgO. I.r. studies using ¹³C- and ¹⁸O-labelled CO show that, under conditions where high-v absorption develops in the u.v. on CaO, vibrational spectra are generated which are characteristic of a C₁ species akin to a metallated formate [Ca²⁺…(CO₂)^2–], with oxygen atoms which, although non-equivalent, undergo rapid exchange. The high-v u.v. absorption on CaO is therefore ascribed to this species. With MgO, on the other hand, experiments using propene as a probe show the high-v u.v. absorption in that case is mainly due to higher-order species, notably the (CO)₃O^2– ketenic entity (OCCRR′)^2–, with R = O^– and R′= O^–—CO. Although these non-reduced species are stable as such at many surface locations, certain sites are sufficiently reactive to allow them to undergo reaction with CO to yield the cyclized clusters (CO)^2–_n(especially n= 5 or 6) and concomitant formation of CO^2–₃. These reduced clusters are reactive towards oxygen and N₂O, but not towards propene. By contrast, the simpler non-reduced (CO)_nO^2– species are shown by i.r. to couple with propene, yielding ultimately alkoxide and carboxylate.