DFT calculations of the CO adsorption on Mn, Fe, Co, and Au deposited at MgO (1 0 0) and CdO (1 0 0)
Highlights
► Adsorption properties of Mn, Fe, Co, and Au atoms adsorbed on various sites of MgO (1 0 0) and CdO (1 0 0) surfaces. ► The characteristics of the CO adsorption on these supported metal atoms. ► This paper provides a detailed account of both geometrical and energetic data as well of the CO/TM/MO surface complexes.
Introduction
Transition metals supported on oxide surfaces constitute a class of materials with broad applications in heterogeneous catalysis, microelectronics, photovoltaic cells, corrosion protection, coating for thermal applications, and other technological important fields [1], [2], [3], [4], [5], [6], [7], [8].
It is well known that transition metal (TM) atoms interact rather weakly with the acidic sites and moderately stronger above the basic sites of MgO (1 0 0) surface [9], [10]. The position on top of a regular O2− site has been identified by a number of theoretical studies [11], [12], [13], [14], [15] as the most preferred location for metal atom adsorption on the MgO (0 0 1) surface.
Chiesa et al. [16] reported that: for a given metal, the metal-support interactions are expected to depend on three main factors: (i) the physical and chemical properties of the oxide (band structure and conductivity and acid-base character, nature of the cation), (ii) its morphological habit (type of exposed faces, presence of steps, kinks and other similar features), (iii) the defective state of the surface (presence of point defects such as ion vacancies and trapped electrons).
It is well known that metal atoms and clusters favor binding to defective surfaces. These defects play a twofold role; they act as trapping sites as well as centers of nucleation for the adatom(s) as refereed by experimental studies [17], [18], [19]. On the contrary, surface defects can modify the oxide–metal interaction and consequently the catalytic activity of the metal unit [20], [21], [22], [23], [24], [25].
Defective sites on oxide supports are assumed to be more reactive than regular sites and to elucidate the formation mechanism of metal–oxide interfaces, we should examine the interaction of metal species with different kinds of defects that can be naturally occurred at the surfaces of supports [7], [26], [27].
Oxygen vacancies of oxide supports are designated as F centers. A neutral F vacancy is formed when an oxygen atom is removed from the surface. The two electrons associated with the formal O2− dianion remain on the surface and are trapped in the cavity that is left behind by the missing oxygen atom, due to electrostatic stabilization by Madelung potential of the crystal [28]. On the contrary, a singly charged defect F+ contains one electron less than an F center. Localization of the unpaired electron mainly in the vacancy region is corroborated by both quantum chemical calculations [28] and ESR experiments [29].
As far as the direct identification of the nature of point defects at the surface of oxide thin films is few reported [30]. The nature of surface defect can be understood through indirect information. The use of probe molecules is a powerful tool to achieve this task regarding the proper site where a metal atom is bound. One of the most popular probe molecule is CO owing to its low reactivity and high sensitivity of the CO bonding to changes in the electronic structure of metal atom to which CO is bound [31], [32], [33], [34], [35].
The adsorption properties of CO molecules on Rh, Pd, and Ag atoms deposited on various sites of the MgO surfaces (O2−, F, and F+) have been studied by means of DFT. The authors [31] provided a complete account of the geometrical, energetic, and vibrational data as well as a deeper analysis of the electronic structure of the CO/M/MgO surface complexes (M: Rh, Pd, Ag). As concluded in that study, compared to the free MCO complexes, the deposition of MCO on the oxide sites resulted in enhancing back donation of charge. Such effect is more pronounced on F centers where the electrons trapped in the cavity are more easily redistributed over the CO empty levels. On charged F+ centers the presence of an electric field counteracts the effect of the back donation.
The adsorption properties of CO molecules adsorbed on Ni, Pd, Cu, and Ag atoms deposited on O2−, F, and F+ sites of MgO, CaO, SrO, and BaO terrace surfaces have been studied by means of density functional DF calculations and embedded cluster model [36]. The adsorption properties correlate linearly with the basicity and energy gaps of the oxide support where the electrostatic potential generated by the oxide modifies the physical and chemical properties of the adsorbed metal and therefore its reactivity versus the CO adsorbate.
In the present work, the authors present results on the adsorption properties of Mn, Fe, Co, and Au atoms adsorbed on various sites (regular O2−, neutral F center, and charged F+ centers) of the basic, low coordinated anionic sites of MgO (1 0 0) and CdO (1 0 0) surfaces, and the characteristics of the CO adsorption on these supported metal atoms. This paper provides a detailed account of both geometrical and energetic data as well of the CO/TM/MO surface complexes (TM = Mn, Fe, Co, Au; and MO = MgO, CdO).
Section snippets
Computational details
Density functional theory (DFT) calculations have been performed using Becke's three parameters hybrid exchange functional B3 [37] in combination with the correlation functional of Lee, Yang, and Parr (B3LYP) [38]. This B3LYP hybrid functional is based on the exact form of the Vosko–Wilk–Nusair correlation potential that is used to extract the local part of the LYP correlation potential [37], [38]. Originally the functional B included the Slater exchange along with corrections involving the
Adsorption of single transition metal (TM) atom
It is well known that TM atoms interact rather weakly with the acidic sites and moderately stronger above the basic sites of MgO (1 0 0) surface [9]. The interaction of metal with the low-coordinated Mg site has been found to be almost negligible with respect to theoretical and experimental viewpoints [10].
Our calculations began with studying the adsorption properties of a single transition metal atom; Mn, Fe, Co, and Au at the un-relaxed terrace surfaces of MgO and CdO crystals. The ad-atoms are
Conclusions
The adsorptivity of CO molecule on the transition metal atoms Mn, Fe, Co, and Au on the non-defective O2− site as well as neutral and single charged (F and F+) sites at the terraces of the insulating MgO (1 0 0) and semiconducting CdO (1 0 0) surfaces using B3LPY level of theory and the embedded cluster approach. The results show that not only the presence or absence of surface-defects but also the nature of substrate governs the adsorption. Moreover, calculations of total interaction energies, the
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