DFT Study of CO2 Activation on Doped and Ultrathin MgO Films. - The

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DFT Study of CO Activation on Doped and Ultrathin MgO Films. Sergio Tosoni, Davide Spinnato, and Gianfranco Pacchioni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10130 • Publication Date (Web): 18 Nov 2015 Downloaded from http://pubs.acs.org on November 23, 2015

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The Journal of Physical Chemistry

DFT Study of CO2 Activation on Doped and Ultrathin MgO Films.

Sergio Tosoni,* Davide Spinnato, Gianfranco Pacchioni Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 55, 20125 Milano, Italy

Abstract The bonding mode of carbon dioxide with the surface of various forms of MgO has been investigated by means of Density Functional Theory calculations. Four supports have been considered: the bare MgO(100) surface, the surface of Al-doped MgO, and ultrathin MgO/Ag(100) and MgO/Mo(100) films. Three forms of adsorbed CO2 have been investigated: physisorbed CO2, chemisorbed carboxylate, CO2−, and carbonate, CO32−. While on MgO(100) CO2 forms either the physisorbed species or the more stable surface carbonate, on Al-doped MgO carboxylate is the preferred species. The adsorption properties of one- or two-layer MgO films differ completely, as a function of the metal support. On MgO/Ag(100) the properties of adsorbed CO2 are very similar to those of the MgO(100) surface (formation of carbonate, not of carboxylate); on MgO/Mo(100), on the contrary, both carbonate and carboxylate species can form, depending on the film thickness. On a one layer film, both species are formed with a comparable stability, while for thicker films the carboxylate species becomes unstable. The surface of Al-doped MgO(100) exhibits similar features to that of the non defective MgO/Mo(100) one layer film.

Keywords: CO2, MgO, ultrathin films, carbonate, carboxylate, Density Functional Theory

*

Corresponding author: [email protected]

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1. Introduction There is a considerable interest in the study of the interaction of CO2 with solid materials, in particular porous systems.1 This is motivated by the problem of CO2 sequestration and long term storage but also in view of its possible conversion into useful chemicals or fuels. This is a huge field which involves not only chemical but also geochemical and biogeochemical aspects. On the catalytic side, there is a strong effort to transform CO2 into more reactive species. For instance, it has been reported that electrocatalysis by a copper complex helps reduce carbon dioxide to oxalic acid;2 this conversion uses carbon dioxide as a feedstock to generate oxalic acid. One of the most common processes to remove CO2 from combustion residues is to store it in form of stable carbonate minerals. The process involves reacting carbon dioxide with abundantly available metal oxides such as magnesium oxide (MgO) or calcium oxide (CaO) to form stable carbonates. These reactions are exothermic and occur naturally. Of course, one of the main goals remains artificial photosynthesis, i.e. the conversion of water and carbon dioxide into carbohydrates and oxygen with the help of solar energy. Artificial CO2 reduction for fuel production aims mostly at producing reduced carbon compounds from atmospheric CO2. Some transition metal polyphosphine complexes have shown activity in this context.3 Recently, activation and splitting of CO2 on the surface of inorganic electrides has been reported.4 The aim of the present study is to evaluate the possibility to activate CO2 by interaction with various forms of a common oxide, MgO. The species originating from the adsorption of CO2 with MgO is a surface carbonate.5-9,10 It is also possible that CO2 captures an electron from the substrate and forms the much more reactive paramagnetic carboxylate complex, CO2−.4 The formation of carboxylate species on electron-rich polycrystalline MgO samples has been reported long time ago11 and recently has been extensively studied with a combined electron paramagnetic resonance, EPR, and density functional theory approach.12,13 In these studies it was found that CO2 prefers to form surface carbonates and only once most or all the low-coordinated oxygen sites of the MgO surface have been involved in binding the CO2 molecule, formation of carboxylate species occurs by interaction of CO2 with surface trapped electrons.13 Recently, a great interest has been stimulated by the study of oxides in form of ultrathin films. In particular, MgO ultrathin films have been deposited on substrates like Ag(100) or Mo(100) because of the relatively good lattice mismatch, and their properties have been studied in detail.14-22 Adsorption of metal atoms and clusters on MgO ultrathin films has also been considered extensively both theoretically and experimentally, with the aim to study chemical, optical and magnetic properties.23 In some cases, MgO ultrathin films exhibit completely different properties 2 ACS Paragon Plus Environment

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compared to the bare MgO(100) surface. This is a typical feature of nano-oxides and is related to the possibility for electrons to tunnel through the thin dielectric barrier and become trapped at some adsorbed species. This can be used to produce negatively charged metal clusters (e.g. Au)24,25,26 or molecules (e.g. O2 27 or NO2 28 ). Recently, this phenomenon has been used to activate CO2 via formation of carboxylate and then oxalate species by electron transfer stimulated by the presence of small Au particles. 29 The Au adsorbates act as electron scavengers from the metal/oxide support and then transfer the electron to adsorbed CO2 molecules with formation of CO2−. By interaction with a second CO2 molecule a stable (CO2)2− species forms that then evolves into an oxalate with formation of a C-C bond. The reaction thus needs a Au catalyst to occur, while the extra electrons are provided by the MgO/Ag(100) support.29 So far, the direct interaction of CO2 with ultrathin MgO films has not been considered at theoretical level. However, it is interesting to compare the electron transfer ability of these systems as this could provide a way to directly activate the adsorbed CO2 molecule, without the need to use a metal supported catalyst. For this reason we have compared, based on DFT plane wave calculations, the adsorption properties of the bare MgO(100) surface with those of one (1L) or two (2L) layer films grown on Ag(100) and Mo(100) supports, respectively. We also considered an Aldoped MgO(100) surface where the presence of a tri-valent Al atom that substitutes a two-valent Mg in the lattice of MgO results in excess electrons that can be transferred to an adsorbed species.30,31 The effectiveness of doping a simple oxide (e.g. CaO) to induce electron transfer to adsorbed species has been recently demonstrated via combined DFT and STM studies.32,33 The presence of the Al dopant in the supercell results in an extra electron that, once CO2 is adsorbed, can form a surface carboxylate. This will be used for comparison with carboxylate adsorbates formed on the ultrathin MgO films via a different mechanism, i.e. electron tunneling from the metal support. The paper is organized as follows. After a brief description of the computational method, we summarize the properties of CO2 adsorbed on the bare MgO(100) surface. This is followed by the analysis of CO2 adsorption on Al-doped MgO(100). In these two Sections the focus is on the bulk oxide. The effect of producing MgO in form of ultrathin film is discussed in the following sections, with MgO/Ag(100) considered first followed by MgO/Mo(100). In a separate section we report the vibrational frequencies of the observed species, while some general conclusions are summarized in the last Section. 2. Computational details 3 ACS Paragon Plus Environment


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Spin-polarized DFT calculations were performed using the generalized gradient approximation (PBE functional34) and the plane waves code VASP.35,36 The interaction between the ions and the valence electrons is described by the projector augmented wave (PAW) method. 37 The kinetic energy cutoff for the plane-wave expansion was set to 400 eV. Since we are dealing with an insulating material (MgO), one could wonder if the use of a standard generalized gradient approximation (GGA) functional is appropriate. In fact, it is well known that GGA calculations, due to the self-interaction error, underestimate band gaps. This could lead also to an incorrect band alignment in metal/insulator interfaces. However, we have shown recently for similar systems that the use of hybrid functionals results in different absolute values of the adsorption energies but does not change the physical picture emerging from simple GGA calculations.38 The (100) surface of bulk MgO is represented by a MgO 3L film (bottom layer fixed). This approach has been followed in previous studies and provides a robust representation of the surface properties of bulk MgO.39 The experimental lattice constant of Ag (4.09 Å) 40 is about 3% smaller than that of MgO (4.21 Å).41 In the calculations, the optimized Ag and MgO lattice parameters are 4.17 and 4.26 Å, respectively, and their lattice mismatch is reduced to about 2%. Here, the MgO film is adapted to the Ag lattice and is slightly contracted when supported on Ag. The O and Mg atoms are located on top of Ag atoms and on the hollow sites, respectively. The MgO film on the Mo(100) surface is described by depositing the MgO layers on a four-layers thick Mo slab at the optimized lattice parameter (3.15 Å). During the geometry optimization all atoms in the MgO film and in the two outmost metal layers were relaxed, while the remaining two metal layers were frozen at the bulk positions. A surface (3 × 3) supercell was employed, containing 9 MgO units or Ag/Mo atoms per layer, and a (4 × 4 × 1) Monkhorst−Pack grid was used for the k-point sampling. Atomic charges are obtained within the scheme of charge density decomposition proposed by Bader.42 The inclusion of van der Waals (vdW) interactions is particularly important since the adhesion of the MgO film to the metal support is dominated by polarization and dispersion forces. vdW forces can be important also for the adsorption of molecules and metal atoms on an insulating support.43 In this work, dispersion has been included to energies and gradients of all structures by using the pair-wise force field implemented by Grimme (DFT-D2).44 This approach is not free from limitations; for instance, the metallic screening of the dispersive interactions is not considered in the pair-wise evaluation of dispersion forces and leads to an overestimation of the interaction energies.45 In a slightly different approach, PBE-D2’, the C6 parameters and van der Waals Radii R0 of Mg have been replaced by those of Ne since the size of this atom is closer to that of the Mg2+ cation. 4 ACS Paragon Plus Environment

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The adsorption energy of CO2 is defined as Eads = E (CO2/MgO/metal) – E(CO2) – E(MgO/metal)

(1)

with metal = Ag or Mo (or nothing). The same formula applies for pure and doped bulk material. Eads < 0 indicates an exothermic process.

3.

Results and discussion

Three possible adsorption modes will be considered on the various MgO supports, as shown in Figure 1: physisorbed CO2, Figures 1(a) and 1(b), carboxylate CO2− , Figures 1(c) and 1(d), and carbonate CO32−, Figures 1(e) and 1(f). In the following we will discuss the stability and properties of each of these species on the four MgO supports considered, MgO(100), Al-doped MgO(100), MgO/Ag(100) and MgO/Mo(100) films.

Figure 1 - Adsorption modes of CO2 on the MgO surface. (a) and (b) Top and side views of physisorbed CO2; (c) and (d) Top and side views of carboxylate, CO2−; (e) and (f) Top and side views of carbonate, CO32−.



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3.1 MgO(100) surface On the MgO(100) surface CO2 interacts only in two modes. Physisorbed CO2 lies parallel to the surface with the O-ends pointing towards two Mg2+ cations; the C atom is above a four-fold hollow site, Figure 1(a). The adsorption energy, -0.24 eV, Table 1, is typical of weakly adsorbed species and derives entirely from dispersion forces. In this configuration the molecule is practically undistorted, Table 2, consistent with the weak interaction. On the MgO(100) surface the carboxylate species is unstable. Attempts to optimize the structure of CO2− failed and resulted in a physisorbed CO2 molecule. Table 1 – Adsorption energy, Eads, of CO2 on various forms of MgO support Eads(eV)

CO2−

CO2

CO32−

physisorbed carboxylate carbonate MgO(100) Al-MgO(100) MgO/Ag(100) 1L

-0.24 (b)

unstable

-0.23

unstable(a)

-0.61

-1.05

-0.47

unstable

(a)

-0.79

(a)

-0.59

MgO/Ag(100) 2L

-0.26

unstable

MgO/Mo(100) 1L

-0.34

-0.72

-0.76

MgO/Mo(100) 2L

-0.31

unstable(a)

-1.22

MgO/Mo(100) 3L

-

-

-1.05

(a) The system evolves towards physisorbed CO2; (b) The system evolves towards carboxylated CO2−.

A surface complex of lower energy can be obtained when the CO2 molecule interacts with a fivecoordinated O2- ion of the surface. Here, the overlap of the O 2pz orbital with the empty π* MO of CO2 leads to the formation of a typical surface carbonate with an energy gain of -0.61 eV. This energy is similar to that reported recently by Downing et al. on the same system using embedded cluster models (0.68 eV).9 The delocalization of the negative charge of the O2- ion towards the adsorbed CO2 molecule leads to the activation of the molecule. Table 2 – Geometrical properties and Bader charge of CO2 adsorbed on MgO(100) surface d(Mg-OCO2), Å d(C-O), Å α(OCO)° qea

MgO(100)

CO2 CO3

2−

Physisorbed

2.67

1.18

177.2

-0.07

Carbonate

2.16

1.26

133.0

-0.44 6

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a

q is defined as the charge transferred from the substrate to the adsorbed CO2

molecule. The charge delocalization to the π* antibonding MO of CO2 is clear from the Bader charge of the adsorbed molecule (-0.44 e), and as a consequence induces the elongation of the C-O bond (from 1.18 Å in the free molecule to 1.26 Å in the surface carbonate) and a loss of linearity, Table 2. In Figure 2(a) we report the projected density of states (DOS) for the surface carbonate species. The DOS curves indicate a strong mixing of the O 2p states (contributing to the valence band) and the CO2 energy levels, sign of a strong covalent interaction. The empty CO2 states are above the bottom of the conduction band, and therefore no states in the gap appear due to the formation of the carbonate.

a

b

Figure 2 -(a) DOS of a carbonate, CO32−, formed on the MgO(100) surface; (b) DOS of a carboxylate CO2− species formed on the Al-doped MgO(100) surface

3.2 Al-doped MgO(100) surface The presence of an Al dopant substitutional to Mg has been simulated by including a Al impurity in the bottom layer of the MgO(100) 3L slab. The trivalent Al3+ ion replaces a bivalent Mg2+ and one electron is transferred to the lowest available empty state in the system, in this case the bottom of the conduction band. Since we are dealing with a perfect surface, no defects, grain boundaries or other trapping sites are available at lower energy.46 This produces a deep change in the properties of



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the surface, which is immediately apparent from the nature the species resulting from the interaction with CO2. On the Al-doped material, in fact, physisorbed CO2 is unstable, and spontaneously evolves to a carboxylate anion, with a binding energy of more than 1 eV, Eads = -1.05 eV, Table 1. The species is even more stable than the carbonate which is bound by -0.47 eV only. The lowest binding energy of CO2 in carbonate form compared to the bare MgO(100) surface, Table 1, can be attributed to the presence of the excess electron in the conduction band provided by the Al dopant which results in a Coulomb repulsion with the adsorbed species. The results show that on Al-doped MgO the carboxylate anion is the only species that should form in thermodynamic equilibrium. Of course, this is an idealized situation since, as we mentioned above, it is quite difficult in practice to avoid the presence of defects or surface imperfections (steps, corners, etc.) where low coordinated Mg ions can act as better electron traps.47 However, the charge transfer from the surface to CO2 will take place any time that the states occupied by the extra electron will be higher in energy than the accepting levels on the adsorbed CO2 molecule. Apparently, this is the case for O vacancies with a single trapped electron (F+ centers). In fact, recent experiments report on the formation of carboxylated species on MgO/Ag(100) films as due to an electron transfer from surface F+ defects.29

Table 3 – Geometrical properties and Bader charge of CO2 adsorbed on Al-doped MgO(100) surface d(Mg-OCO2), Å d(C-O), Å α(OCO)° qea CO2−

Carboxylate

2.06

1.25

133.5

-0.90

CO32−

Carbonate

2.16

1.26

133.3

-0.45

a


molecule. The formation of an activated species on the surface of Al-doped MgO is apparent also from the geometrical data, Table 3. In the carboxylate form, the C-O bond elongates from the value it has in the gas phase, 1.18 Å, to 1.25 Å and the loss of linearity is pronounced, with the OCO angle that reaches 133°. The molecule is oriented with the two O atoms towards two surface Mg2+ cations, which, thanks to the electrostatic interaction with the negatively charged molecule, exhibit a significant outwards relaxation, Figure 1. The presence of an extra electron on the molecule is clearly shown by the Bader charge, -0.9 e, and by the spin density plot, Figure 3(a).


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a

b

Figure 3 - Spin density plots of a carboxylate CO2− species. (a) Al-doped MgO(100); (b) MgO/Mo(100) 1L film (isodensity plot 0.005 |e|/Å3).

The DOS curves, Figure 2(b), clearly show the spin polarized nature of the surface complex, with one component of the π* MO of CO2 which is singly occupied and below the Fermi level. The carbonate species formed on Al-doped MgO(100) has structural and electronic properties similar to those of the same complex formed on the undoped surface, see Table 2 and 3. Notice than both the C-O distance and the OCO angles are almost identical to those of a carboxylate species, and that only the orientation of the molecule changes, Table 3 and Figure 1.

3.3 MgO/Ag(100) ultrathin film The adsorption of CO2 has been investigated also on one layer (1L) and two layer (2L) MgO films deposited on the Ag(100) surface. This system has been widely studied in the past, and it has been shown that the oxide layer has the effect to reduce the metal work function from about 4.3 eV to about 3.0 eV, with a ∆Φ = -1.3 eV.48,49 This corresponds to raising the position of the Ag Fermi level, EF, towards the vacuum level of the MgO/Ag(100) interface. If EF falls above the empty CO2 levels, a direct electron transfer from the Ag metal to the adsorbed molecule can occur by tunneling through the ultrathin insulating film.23 This effect has indeed been observed for Au atoms24 and clusters 50 , 51 and for O2 molecules adsorbed on ultrathin MgO films.27 Here what we want to investigate is the possible occurrence of the charge transfer also in the case of the CO2 molecule. The results show that on both 1L and 2L MgO films CO2 can physisorb with similar adsorption energies as on the bare MgO(100) surface, Table 1. Also the geometrical parameters are very similar, Table 4. The only noticeable difference is that on the 1L film the distance from the surface is slightly longer, consistent with a slightly weaker bonding, Table 1. This can be explained with the fact that a single MgO layer is less ionic than a thicker MgO film, due to the reduced 9 ACS Paragon Plus Environment


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Madelung potential. This results in slightly smaller net charges of the constituting ions and hence in a weaker electrostatic interaction.

Table 4 – Geometrical properties and Bader charge of CO2 adsorbed on MgO/Ag(100) thin films film d(Mg-OCO2), Å d(C-O), Å α(OCO)° qea CO2 CO32− a

Physisorbed

Carbonate

1L

2.76

1.17

177.0

-0.08

2L

2.68

1.17

177.2

-0.07

1L

2.14

1.26

132.6

-0.38

2L

2.18

1.26

133.6

-0.44


molecule. However, on the MgO/Ag(100) films there is no tendency to form the carboxylate structure. We tried in several ways to facilitate the formation of this complex, for instance by bending the molecule and adsorbing it in a position similar to that found on Al-doped MgO as the initial structure in the geometrical optimization. However, the system is unstable, the structure is not even a local minimum, and it evolves spontaneously into the physisorbed species described above. Clearly, the position of the Fermi level in MgO/Ag(100) is not sufficiently high to induce the electron transfer. When the CO2 molecule is placed with the C atom above an O2- ion of the surface a carbonate species forms, with slightly different adsorption energies, depending on the thickness of the film. On the 1L MgO film the carbonate is bound by -0.79 eV, i.e. about 0.2 eV higher (in absolute value) than on the bare MgO surface, Table 1. On the 2L MgO film the binding reduces to -0.59 eV, a value almost identical to that found on the bare MgO(100) surface, -0.61 eV, Table 1. This indicates a slightly more favorable interaction with the “flexible” 1L film. This structural flexibility has been observed in several other cases, and is a peculiar property of ultrathin films. It is also possible that some overlap between the tails of the Ag metal wave function and the orbitals of CO2 contributes to reinforce the bond. In any case, already for a 2L film the situation of the bare surface is recovered and the presence of the metal support is almost completely screened. This is also shown by the structural data, Table 4, which show that the CO32− species formed on the 2L MgO/Ag(100) film is almost exactly the same found on the bare MgO(100) surface, while some deviation exists for the 1L case. Similar considerations hold true for the Bader charges of the CO2 fragment, Tables 3 and 4, and by the DOS curves, Fig. 2 and Fig. 4.


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a b 2− Figure 4 - DOS of a carbonate, CO3 , formed on (a) 1L MgO/Ag(100) film; (b) 2L MgO/Ag(100).

3.4 MgO/Mo(100) ultrathin film In the previous section we have shown that a 1L or 2L MgO film grown on Ag(100) is not expected to behave differently from the bare MgO surface. On this support, in fact, CO2 can only interact weakly via a physisorption bond or forming a classical surface carbonate. It is known that the charge transfer from a metal supported oxide film to an adsorbed species is a direct function of the work function of the system, Φ.23 In this respect, MgO thin films grown on Mo(100) are expected to result in a smaller work function. Our present results show in fact that a MgO/Mo(100) 2L film has a work function of 2.04 eV, with a reduction of 1.80 eV compared to the bare Mo(100) surface. A problem related to the theoretical estimate is that this is rather dependent on the distance of the MgO film from the metal surface, a property that in turns depends on the inclusion of van der Waals forces.52 Experimental measurements performed on MgO/Ag(100) and MgO/Mo(100) films have confirmed the theoretical predictions of a reduction of the work function of about 1 eV for MgO/Ag(100),53 while were unable to reproduce the large shift in work function predicted for the MgO/Mo(100) system. 54 The measured ∆Φ for MgO/Mo is in fact about -1.3 eV, i.e. not too different from that found for MgO/Ag. This is probably due to the fact that MgO/Mo(100) films are less regular than the corresponding MgO/Ag(100) ones; in particular, due to the lattice mismatch between MgO and Mo(100), MgO islands form, separated by grain boundaries. These grain boundaries are sites where negative electronic charge can be trapped, with the consequence of changing the surface dipole and raising the value of the work function.55 This does not exclude, however, that in some specific areas of the MgO/Mo(100) films the local work function is really



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reduced to values close to 2 eV. In this, case it may be interesting to study the reactivity of such regions towards CO2. The first observation is that CO2 adsorbed on MgO/Mo(100) films can form all three variants of adsorbed CO2: physisorbed, carboxylated, and carbonate, Table 1. This is the only system where all three species could in principle be obtained, although in different conditions. Another interesting observation is that substantial differences exist between 1L and 2L films. We start with the physisorbed molecule, which on MgO/Mo(100) films is slightly more strongly bound, by 0.34 eV (1L) and by 0.31 eV (2L), respectively, compared to the other supports, Table 1. This stronger bond reflects in a shorter Mg-OCO2 distance on the Mo supported films, Table 5. Notice that the OCO angle is slightly bent, α = 175°, sign of a small covalent interaction with the electrons of the support. Also the Bader charges of physisorbed CO2 are slightly larger than on the bare MgO surface, Table 5.

Table 5 – Geometrical properties and Bader charge of CO2 adsorbed on MgO/Mo(100) thin films film d(Mg-OCO2), Å d(C-O), Å α(OCO)° q ea CO2

Physisorbed

1L

2.57

1.18

175.4

-0.09

2L

2.54

1.18

175.5

-0.10

CO2−

carboxylate

1L

2.54

1.25

134.0

-0.88

CO32−

Carbonate

1L

2.14

1.26

132.7

-0.36

2L

2.18

1.27

130.2

-0.42

3L

2.15

1.27

131.2

-0.41

a


molecule. On MgO/Mo(100) 1L film CO2 can form a carboxylate. The species is, indeed, bound by 0.72 eV, Table 1, and results in the typical distortion of CO2−. The occurrence of a net charge transfer is shown by the Bader charge, -0.88 e, Table 5, and by the spin density plot, Fig. 3(b). The DOS plot, Fig. 5(a), shows a single component of the π* MO of CO2 below EF and thus occupied by one electron. The carboxylate is more stable than physisorbed CO2, Table 1. This shows that on the regular MgO/Mo(100) surface the CO2− species can form even in absence of a point defect like a Al impurity. However, the results indicate that this is a metastable species. In fact, a surface carbonate can form on the MgO/Mo(100) 1L film with a slightly larger adsorption energy (in absolute value), Eads = -0.76 eV, Table 1. The energy difference between carbonate, Eads = -0.76 12 ACS Paragon Plus Environment

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eV, and carboxylated, Eads = -0.72 eV, is rather small, and suggests that in principle an equilibrium between the two forms of activated CO2 could be present on the surface. The nature of the surface carbonate is not different from what already described for other forms of MgO support (see the DOS curves shown in Figure 5(b)). When the 2L MgO/Mo(100) film is considered things change completely, and the formation of adsorbed carboxylate species becomes unfavorable. On the 2L MgO/Mo(100) support the most stable species is the carbonate, with an adsorption energy of -1.22 eV, Table 1. This is much larger than the binding of CO2 to the bare MgO(100) surface (Eads = -0.61 eV) or to the 2L MgO/Ag(100) film (Eads = -0.59 eV). The origin of this large stability is not entirely clear. It can arise in part from the fact that the charge delocalization from the surface oxide anion towards CO2 is partly compensated by the metal electrons of the support; the distance of the MgO 2L film from the Mo surface is in fact 2.08 Å, and is considerably shorter than on MgO/Ag(100) where it is of 2.55 Å. In part, the difference can be related to a larger structural flexibility of the MgO/Mo(100) 2L film. The geometrical parameters of the resulting CO32− species are very close to those found for the corresponding 1L film, Table 5, despite a substantial difference in adsorption energy. Also the Bader charges of CO2 fragment and the DOS plots, Fig. 5(c), show a clear resemblance to those found for MgO/Ag(100) films, Figure 4. Given the substantial difference in the carbonate stability on a 2L MgO/Mo(100) film with respect to bulk MgO (-1.22 eV vs -0.61 eV, respectively, Table 1), we have also considered a thicker MgO/Mo(100) film consisting of three MgO layers. The results show a modest reduction of the adsorption energy which becomes -1.05 eV while the other properties do not change significantly. Thus, there is a tendency of the adsorption energy to converge towards that of bulk MgO but clearly this convergence is slow and requires much thicker MgO layers than considered here.

(a) 13 ACS Paragon Plus Environment


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(b)

(c) Figure 5 - DOS of (a) carboxylate CO2− on 1L MgO/Mo(100) film; (b) carbonate CO32− on 1L MgO/Mo(100) film; (c) carbonate CO32− on 2L MgO/Mo(100) film. 4. Vibrational frequencies The experimental identification of various forms of CO2 adsorbed on an oxide surface, and on MgO in particular, can be achieved in principle by using vibrational spectroscopy (IR, Raman, or EELS spectra). In practice, the existence of several adsorption modes and the complex morphology of polycrystalline MgO samples makes the assignment of the observed vibrational features to specific bonding modes rather complex. An accurate attribution has been reported recently by Cornu et al. based also on a comparison with DFT calculations.10 The observed features arising from CO2 interaction with MgO powders are usually interpreted in terms of various forms of surface carbonate.5,6 Given the different adsorption modes of carbonate and caboxylate species, one could hope to be able to detect at least the presence of these two different isomers. In this section we report the computed vibrational frequencies for the systems investigated. We start from physisorbed CO2. Given the weak adsorption energy, this form is expected only in case of adsorption at very low temperature. Under these conditions, in fact the molecules 14 ACS Paragon Plus Environment

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can remain trapped in a physisorbed state without forming carbonate species. Notice that in all cases considered, the physisorbed state is metastable, being other forms much more favorable. The vibrational properties of physisorbed CO2 closely resemble those of the free molecule, Table 6. There is a moderate red shift of the asymmetric stretching, 2344 cm-1 on MgO(100), while the symmetric stretching is almost unchanged with respect to the gas-phase, 1303 cm-1 on MgO(100). The computed bending mode is significantly red-shifted, by about 50 cm-1. Adsorption of CO2 on MgO(100) surfaces prepared by cleaving a MgO(100) single crystal at 82 K in situ under UHV results in sharp infrared absorptions at 2334, 2398, 2306 and 2267 cm-1.56 The appearance of several bands is due to the presence of more than one CO2 molecule in the unit cell and to the collective modes of the CO2 monolayer. The values, however, are close to what we computed for our physisorbed CO2 species, in particular the small red shift of the asymmetric stretching is confirmed. Table 6. Computed vibrational frequencies, cm-1, of various forms of CO2 adsorbed on MgO(100), Al-doped MgO(100), MgO/Ag(100) and MgO/Mo(100) surfaces. ν3

ν1

ν2

ν2

As. Stretch. Sym. Stretch. Bending(⊥) Bending(//) 57 CO2 gas phase (exp. ) 2349 1333 667 667 CO2 gas phase (calc.) 2367 1318 633 633 Physisorbed, CO2 MgO(100) 2344 1303 580 591 MgO/Ag(100) 1L 2350 1305 572 592 MgO/Ag(100) 2L 2345 1302 576 589 MgO/Mo(100) 1L 2329 1283 564 539 MgO/Mo(100) 2L 2333 1290 541 556 Carboxylate, CO2− Al-MgO(100) 1673 1286 698 329 MgO/Mo(100) 1L 1695 1287 686 260 Carbonate, CO32− MgO(100) 1703 1244 832 779 Al-MgO(100) 1636 1191 770 750 MgO/Ag(100) 1L 1690 1247 844 785 MgO/Ag(100) 2L 1665 1250 870 797 MgO/Mo(100) 1L 1647 1216 921 751 MgO/Mo(100) 2L 1629 1257 954 785

Much more interesting is the case of the carboxylate species. Here the asymmetric stretching drops by almost 700 cm-1 compared to the free molecule due to the distortion induced by the capture of an extra electron. The computed values are 1673 cm-1 for CO2− on Al-doped MgO, and 1695 cm-1 for CO2− on MgO/Mo(100) 1L. Both values are very close to the band at 1670-1680 cm-1 measured for 15 ACS Paragon Plus Environment


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CO2− formed in NaBr, KBr or KCl matrices.58 Also the symmetric stretching is affected but to a much smaller extent, Table 6, and is found at the same frequency, 1286-1287 cm-1, for the two systems. These latter values are very close to the band observed by Calaza et al.29 after exposure of MgO/Ag(100) thin films to CO2 at 220 K (1295 cm-1). In this work, the formation of a carboxylate is demonstrated using isotope labeling experiments. The formation of the CO2− paramagnetic species is attributed, however, to the interaction with trapped electrons at point defects, in particular F+ centers formed at steps. This conclusion is consistent with our findings that show that CO2− does not form on the regular terrace sites of MgO ultrathin films grown on Ag(100); it can form in the presence of excess electrons (as in Al-doped MgO). Finally, we consider the frequencies of the surface carbonate. The first comment is that they are surprisingly close to those of the carboxylate species described above, making a distinction based only on vibrational spectroscopy rather difficult. In fact, the asymmetric stretching of the CO32- species is found in our systems in a range between 1629 and 1703 cm-1, while the symmetric stretching is around 1250 cm-1 (excluding the less stable forms of surface carbonates). These values are close to those found for the carboxylate species. They are also in the same range of those reported in the literature and assigned to the carbonate species shown in Figure 1(f). In IR spectra clear bands have been observed at 1626-1659 cm-1, and 1273-1329 cm-1.59 Other reports have found values in the same range (see ref. 10 and references therein). The values computed in the present study are thus consistent with the bands assigned to surface carbonates and also to those reported based on theoretical calculations. 5. Conclusions We have investigated the adsorption modes of CO2 with four variants of the MgO(100) surface. The bare MgO surface has been taken as a reference; on this surface CO2 can physisorb, by effect of dispersion forces, or form a surface carbonate by reaction with a five-coordinated O2- ion. Stronger species are expected to form in correspondence of low-coordinated ions (steps, edges, kinks) as it has been extensively shown both theoretically and experimentally. Thus, no carboxylate species can form on MgO(100) terraces, in absence of point defects. The situation is different on an Al-doped MgO(100) surface. Here, the presence of excess electrons provided by the Al impurity favors the formation of the paramagnetic carboxylate species, CO2−. The physisorbed state is unstable and spontaneously evolves into the carboxylate complex. This latter is much more stable than the surface carbonate species, indicating a completely different reactivity of the surface by effect of the


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Al-doping. Of course, here Al is just an example of an electron donor in the lattice, and its role can be taken by any chemical species or defect able to provide excess electrons. The main motivation of the study was to compare the properties of the bare MgO surface, doped and undoped, with that of ultrathin MgO films. These have been considered on two supports, Ag(100) and Mo(100). The results show the key role of the metal support in determining the properties of the thin film. In fact, on MgO/Ag(100) the results are practically the same as on the bare MgO(100) surface: only physisorbed CO2 and a surface carbonate are stable species, and the carboxylate is unstable. Also the adsorption properties and vibrational frequencies are similar. Small differences exist between one layer and two layer films, but in general we can say that a MgO/Ag(100) film behaves towards CO2 as a MgO(100) single crystal surface. Things are completely different on MgO/Mo(100). Here a spontaneous charge transfer occurs from the metal to adsorbed CO2 and the carboxylate species becomes stable (or metastable). There is a clear dependence of the results on film thickness: for the thinner one layer film, the stability of the carboxylate competes with that of a carbonate. The reason for the different behavior is the smaller work function of MgO/Mo compared to MgO/Ag, an effect that facilitates the charge transfer to an adsorbate. However, it is interesting to note that the undoped MgO/Mo(100) support behaves qualitatively as the doped Al-MgO(100) bulk material. Recently, it has been shown that indeed, charge transfer coming via electron tunneling through ultrathin films or via doping leads to exactly the same kind of supported species, thus reinforcing the idea that the ultrathin films can play a role as catalytic materials.60 Acknowledgments The work has been supported by the Italian MIUR through the FIRB Project RBAP115AYN “Oxides at the nanoscale: multifunctionality and applications”. The support of the COST Action CM1104 “Reducible oxide chemistry, structure and functions” is also gratefully acknowledged.



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DFT Study of CO2 Activation on Doped and Ultrathin MgO Films. - The

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