Partial Ionization of Cesium Atoms at Point Defects over Polycrystalline Magnesium Oxide

Abstract

Evaporation of Cs atoms onto dehydrated polycrystalline MgO leads to the formation of surface color centers in correspondence of surface point defects. EPR spectroscopy has revealed that the adsorbed Cs atoms are partially ionized, and a fraction of the electron spin density is delocalized onto a surface oxygen vacancy or trap. The observed defect can thus be written as Cs δ+ (trap) δ-. These results give evidence of the preferential interaction of the metal atoms with specific surface defect sites in the early stages of the metal-support interaction. The reaction of these centers with molecular oxygen leads to bleaching of the surface with formation of the O 2 -superoxide radical anion. A fraction of the adsorbed superoxide ions are adsorbed on "regular" Mg 2+ sites while the remaining ones are adsorbed on top of Cs + ions.The nature of the metal-support interaction in heterogeneous catalysis has both intrigued and stimulated researchers for many years. 1 To unravel the complexities of these interactions, elaborate experimental and theoretical methodologies have developed, in particular by studying the deposition of single metal atoms or small metal clusters on metal oxide thin films. [2][3][4][5] These investigations have revealed that, far from being an inert host, the support can influence the electronic properties of the deposited atoms through specific interactions with surface sites, and in some cases enhanced catalytic reactivity has been observed. 3,4 These studies, carried out on well-defined oxide surfaces, are extremely important because they provide clear evidence for the size-dependent catalytic activity of small clusters and the relationship with the intrinsic electronic and geometric structure of the cluster. [3][4][5][6] The changes in catalytic activity have been interpreted by assuming a size-dependent interaction of the small clusters with the substrate. 3,7 In other words, the morphological defects such as low coordination ions and point defects clearly play a crucial role in stabilizing metal atoms at the metaloxide interface and ultimately influencing the catalysis of the supported metal particles.Investigations into the metal-oxide interface, and the changes to the electronic structure of the supported clusters, has been explored for a number of years also over polycrystalline metal oxide surfaces. Important concepts related to this area, for instance, have been advanced by studying the interaction of low ionization energy metals (essentially alkali metals) with highly ionic polycrystalline oxides such as zeolites. 8-10 Also in the case of nonporous ionic oxide like MgO the study of the early stages of the interaction between low ionization energy metals and the surface provides interesting evidence about the defects present at the surface and their reactivity. Me-MgO systems (Me ) Li, Na, K, Rb, Mg) have been investigated by our group over the years using EPR spectroscopy, and a number of different paramagnetic centers were identified on the oxide surface depending on the amount and type ...

Formation of Superoxo Species by Interaction of O2 with Na Atoms Deposited on MgO Powders: A Combined Continuous‐Wave EPR (CW‐EPR), Hyperfine Sublevel Correlation (HYSCORE) and DFT Study

“…Such a g value is typical of superoxide anions stabilised on monovalent cations and is in line with g values observed for matrix-isolated alkali metal superoxides [26] and O 2 À adsorbed on Cs + ions stabilised at the surface of MgO. [10] The broad linewidth that characterises the spectrum is indicative of a relatively large distribution of values (strains) due to slightly different local environments probed by the adsorbed superoxide on different surface sites. This effect, also referred to as speciation, is often encountered in the case of paramagnetic species stabilised on polycrystalline surfaces.…”

“…This same effect was also observed in the case of O 2 À species bound to Cs + cations on the surface of MgO. [10] This aspect will be further explored by means of quantum chemical calculations.…”

“…[8,9] The surface-adsorbed superoxide anion is usually identified on the basis of its g tensor, which is a very sensitive parameter of the electrostatic field of the adsorption site; however, only few examples are present in the literature in which the superhyperfine interaction with the adsorption site is detected. [9,10] Herein, we will show by means of hyperfine sublevel correlation (HYSCORE) spectroscopy that Na + O 2 À complexes are formed at the surface of MgO by interaction of gas-phase O 2 with deposited Na atoms. HYS-CORE experiments allow the direct coordination of the O 2 À adsorbate to surface Na + cations to be proven, and in conjunction with DFT calculations, enable us to determine the structural details of the formed species.…”

An EPR characterisation of stable and transient reactive oxygen species formed under radiative and non-radiative conditions

“…For paramagnetic oxygen species, electron paramagnetic resonance (EPR) spectroscopy is a vital tool in the characterisation of these oxygen-centred radicals and their subsequent reactivity over polycrystalline oxide surfaces. Several research groups have demonstrated how the superoxide radical anion can act as a probe of surface morphology, not only on TiO 2 but also on other polycrystalline metal oxides [9][10][11][12][13][14][15][16][17][18]. Although the g-values are invaluable in providing important information about the radicals, particularly the surface speciation, the 17 O hyperfine tensor is far more informative for determining the electronic structure.…”

EPR properties of Au atoms adsorbed on various sites of the MgO(100) surface from relativistic DFT calculations

“…This technique is usually employed on polycrystalline materials with high-surface area, like for instance MgO powders. Using this technique, the group of Giamello has been able to identify the properties of isolated alkali metal atoms adsorbed on MgO powders [11][12][13][14][15]. Recently, it has been found that K atoms are stabilized at specific morphological sites of MgO, the reverse corners, where they are sufficiently strongly bound to survive up to 380 K [16].…”