Reductive Activation of the Nitrogen Molecule at the Surface of “Electron-Rich” MgO and CaO. The N₂^- Surface Adsorbed Radical Ion

Abstract

Upon nitrogen adsorption at low temperature onto the surface of magnesium oxide and calcium oxide containing FS + centers (single electron trapped in a suitable surface vacancy), electron transfer occurs from the solid to the adsorbed molecule. About 90% of the total electron density is localized on the adsorbed molecule. The 11-electron N2 - radical anion so formed has been detected by electron paramagnetic resonance for both 14N2 and 15N2. The electron transfer is reversible and, when the pressure is lowered or the temperature increased, the N2 molecules desorb, regenerating the original FS + center. The diatomic radical ion lies parallel to the surface, and the electron density is mainly confined in the π y * orbital. Theoretical calculations at the DFT level indicate that a small energy barrier separates the unbound FS +/N2 state from the bound FS 2+/N2 - state. This explains the facile reversibility of the electron-transfer process. The calculated spin densities are in excellent agreement with those derived from the experiments. The results reported in this paper represent a new method for N2 activation over the basic alkaline earth oxides, which are also known to activate H2 by dissociative adsorption.

Oxygen Vacancy: The Invisible Agent on Oxide Surfaces

“…Another example of the unusually high chemical activity of oxygen vacancies is that of the adsorption of molecular N 2 on a MgO surface activated by the presence of paramagnetic color centers. [20] In this case, electrons trapped at oxygen vacancies cause a special reactivity of the surface. N 2 is a notoriously unreactive molecule, and it is quite surprising that the activated MgO surface reacts by transferring one electron from the vacancy to the N 2 molecule, with formation of the radical anion N 2 À , as shown in Figure 8.…”

Carbon Dioxide Activation by Surface Excess Electrons: An EPR Study of the CO2− Radical Ion Adsorbed on the Surface of MgO

“…[14][15][16][17] Most importantly the magnetic equivalence of the two oxygen nuclei also requires structural equivalence, as already observed in the case of other radicals such as N 2 À and O 2 À stabilized on the same surface. [29,31] This poses, together with the known morphology of MgO, and the topographical details of the (H + )(e À ) centers, which have been recently clarified, [27] some important constraints to the possible adsorption sites for the CO 2 À radical. We recently proposed a general assignment of the (H + )(e À ) paramagnetic centers formed at the MgO surface, which consists of electron-proton pairs stabilized at low coordinate Mg 2 + -O 2À ions at edges, corners, and reverse corners.…”

“…A relevant example of this reactivity is the reduction of the nitrogen molecule to form the surface-stabilized N 2 À radical, which was reported by our group. [29] The electron-rich MgO surface thus represents an ideal model system to investigate the nature of the surface-adsorbed CO 2 À radical formed by surface-to-adsorbate direct electron transfer. In an attempt to learn more about the details of formation of the CO In this way, a complete mapping of the unpaired electron spin density over the entire radical is obtained for the first time, as well as relevant structural information on the geometry of adsorption.…”

N2.3−: Filling a Gap in the N2n− Series

“…Thus, the one-electron addition to N 2 C À [3] and the stabilization of two-electron reduced diazenido(2À) ions ("pernitride", N 2…”