19568
J. Phys. Chem. C 2008, 112, 19568–19576
Formation of CO2- Radical Anions from CO2 Adsorption on an Electron-Rich MgO Surface: A Combined ab Initio and Pulse EPR Study Gloria Preda and Gianfranco Pacchioni* Dipartimento di Scienza dei Materiali, UniVersita` di Milano-Bicocca, Via R. Cozzi, 53 - 20125, Milano, Italy
Mario Chiesa and Elio Giamello Dipartimento di Chimica IFM, UniVersita` di Torino and NIS, Nanostructured Interfaces and Surfaces Centre of Excellence, Via P. Giuria 7, I - 10125 Torino, Italy ReceiVed: July 9, 2008; ReVised Manuscript ReceiVed: October 3, 2008
We report the results of DFT cluster model calculations on the formation of carboxylate radical anions, CO2-, by reaction of CO2 with an electron rich MgO surface. The theoretical results are used in conjunction with new pulse electron paramagnetic resonance (EPR) experiments to interpret recently reported experimental data obtained with continuous wave EPR (CW-EPR) (Chiesa, M.; Giamello, E. Chem. Eur. J. 2007, 13, 1261). Three cases have been considered: (1) interaction of CO2 with oxide anions at low-coordinated sites and formation of surface carbonates, CO32-; (2) interaction of CO2 with “free” electrons trapped at lowcoordinated sites of the MgO surface with formation of adsorbed CO2-; and (3) interaction of CO2 with (H+)(e-) centers and formation of CO2- species bound near an adsorbed proton (OH group). CO2 prefers to form surface carbonates and only once most or all the low-coordinated O sites of the surface have reacted it will interact with trapped electrons to form CO2-. Several adsorption sites have been considered, but based on the comparison of measured and computed hyperfine (13C and 17O) and superhyperfine (1H) coupling constants we conclude that CO2- bound at (H+)(e-) centers formed near cationic corners is the most abundant species observed in the experiment. 1. Introduction The chemistry of CO2 adsorbed on solid surfaces is of great importance in a variety of processes, largely related to the conversion of CO2 into fine chemicals. CO2 is in fact a cheap source of C1, and advances in the use of carbon dioxide in organic synthesis could help in reducing the problem of greenhouse effect and global carbon balance.1 CO2 is a very stable molecule, and external sources of energy (thermal, electrical, or radiative) are needed in order to activate it. As an alternative, the molecule can be activated by coordination to transition metal complexes2 or by the interaction with solid surfaces.3-5 A first step in the heterogeneous activation of CO2 can be considered the formation of the corresponding radical anion, CO2-. In the gas-phase this is an unstable species, as the electron affinity (EA) of CO2 is negative; via collisional ionization experiments with alkali metals it has been possible to determine an EA of -0.6 eV.6 The resulting CO2- species is metastable due to the barrier associated to the change in geometry from bent CO2- to linear CO2. A vibrational spectrum has been reported for a CO2- radical anion stabilized in a neon matrix.7 CO2- has also been observed with EPR in alkali halides by γ irradiation of sodium formate; it was concluded that the molecule has an internal angle of 134°.8 From a theoretical point of view the calculation of the CO2species poses some fundamental problems. Due to the negative EA, the ion tends to lose the extra electron and the results of ab initio calculations are strongly dependent on the basis set used. In fact, the addition of more diffuse functions to the basis set causes a lowering of the total energy, indicating the tendency for the system to dissociate into a free electron and neutral CO2. A calibration of the theoretical method used to compute the
CO2- unit can be done by comparing the results with experiments on Na-CO2 9,10 and K-CO2 10,11 complexes. These have been prepared in a rotating cryostat by deposition of Na or K with CO2 at low temperature. Also Li-CO2 complexes have been produced by condensing the metal vapor with CO2.12 These systems have been studied by EPR and show the occurrence of a charge transfer with formation of M+-CO2- complexes. The primary reaction of CO2 with oxide surfaces is the formation of carbonates. However, in the presence of excess electrons one can form stable CO2- species; these have been observed first a long time ago4 on magnesium oxide. Successive work has led to a better characterization of the species.5,13 Formation of CO2- has been reported also on Na/TiO214 and Na/Cr2O3 surfaces;15 examples of chemisorbed CO2- have been reported for doped and undoped metal surfaces (see ref 3 and references therein). The study of the formation of CO2- at the surface of MgO is the scope of this work. The focus is on the interaction of CO2 molecules with (H+)(e-) centers, a recently discovered class of defects on the surface of ionic oxides. Exposure of MgO16-20 or CaO18 to H atoms or to H2 under UV light results in the spontaneous ionization of H• at temperatures as low as 77 K with the subsequent formation of excess electrons and extra protons on the surface, as schematically represented in eq 1: e-
H+
H• + sMgsOs f sMgsO s +
- 16
(1)
These centers are named (H )(e ). They result from the ionization of the H atom with subsequent stabilization of the electron on low-coordinated cation sites and of the proton on surface oxide anions. Reaction 1 leads to surface excess electron color centers thermally stable up to 373 K, well characterized
10.1021/jp806049x CCC: $40.75 2008 American Chemical Society Published on Web 11/12/2008
CO2- Radical Anions from CO2 Adsorption on MgO by EPR, IR, UV measurements, and theoretical calculations.16-19 Residual OH groups present on partially hydroxylated MgO, CaO, or SrO surfaces can also act as potential electron traps which can be populated by the deposition of tiny amounts of donors like alkali metals, leading to the formation of exactly the same kind of (H+)(e-) centers.21 The exposure of electron-rich MgO surfaces to CO2 results in the formation of paramagnetic species which have been unambiguously identified as adsorbed CO2-.5 However, one important missing experimental detail consists in the superhyperfine interaction of the adsorbed radical and the proton of the original (H+)(e-) center. While this interaction is clearly resolved in the case of anion radicals formed upon interaction of the same centers with adsorbed molecules such as O2 22-24 or N2, 25,26 the (H+)(CO2-) interaction is not resolved in CW EPR spectra.5 Thus, the immediate environment of the adsorbed radical remains undetermined. In order to overcome this limitation we performed pulsed EPR experiments which unambiguously show that the adsorbed radical weakly interacts with a nearby proton. These new experimental data, together with previous results, are interpreted in the light of advanced theoretical calculations in order to identify the sites where CO2species preferentially form and their relative stability. Furthermore, the charge transfer mechanism between the surface and the adsorbed molecule will be discussed in detail. 2. Computational and Experimental Details 2.1. Computational Details. The energetics, structure, and stability of CO2- formed at the surface of MgO have been investigated theoretically by means of embedded cluster models and DFT calculations with hybrid functionals (B3LYP).27,28 In order to properly account for the occurrence of charge transfers, hence, of strong dipoles, we have used a computational method which allows one to take into account the electronic relaxation of the substrate in a wide region using a shell-model approach. The surface of MgO is represented by a nanocluster containing about 5000 atoms. The central part, treated quantum-mechanically (QM), is surrounded by a region of about 300 classical ions whose polarizability is described by a shell-model (SM).29 This part is called Region I. Cations in the SM region at the interface with the QM region are replaced by ions (hereafter indicated as Mg*) on which a semilocal effective pseudopotential (ECP) is centered, in order to reproduce the Pauli repulsion and avoid the nonphysical polarization of QM interface anions. Region I is then surrounded by a large array of point charges (PC) in order to reproduce the long-range electrostatic potential. This scheme is implemented in the GUESS code30 interfaced with the Gaussian03 code,31 and the total energy of the hybrid system is calculated as a sum of classical and QM contributions. Forces acting on all centers in region I, both QM and classical (cores and shells), can be calculated allowing the simultaneous optimization of their position. All centers in the QM region and Mg* interface atoms have been allowed to move during the optimization, while only shells, not cores, have been relaxed in the SM region. Therefore, the electronic polarization has been included in a large portion of the surface, while ionic polarization is restricted to a few tens of atoms. The total energy and the electronic structure of the QM cluster are calculated at the DFT-B3LYP level.27,28 The following QM clusters have been considered to model edge, corner, and reverse corner sites, respectively, on the MgO surface: Mg10O10Mg*14 (edge), Mg10O10Mg*9 (corner), and Mg17O17Mg*24 (reverse corner). The basis sets used are
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19569 6-311+G** on H, and 6-31G on Mg, 6-31G* on O. For the CO2 molecule we used a 6-311+G* basis set; with this basis the EA is -0.37 eV. In order to verify the accuracy of the method used, we have computed the Li-CO2, Na-CO2, and K-CO2 complexes using various basis sets and comparing wave function based methods like UMP2 and CCSD(T) with B3LYP. A full account will be reported elsewhere,32 but some results for LiCO2 are given in Supporting Information. From a comparison with the existing experimental data it can be concluded that the B3LYP method combined with the basis sets described above is sufficiently accurate for the description of the geometry and EPR parameters (both hyperfine coupling constants and g-values) of activated CO2, while non-negligible oscillations are found in the stability of the complex as a function of the method and basis set used. The hyperfine interactions of the electron spin with the nuclear spin of the 1H, 13C, and 17O nuclides have been determined. The hyperfine spin-Hamiltonian, Hhfc ) S · A · I, is given in terms of the hyperfine matrix A which describes the coupling of the electron with the nuclear spin.33 The components of A can be represented as
[
]
[
A1 0 0 T1 0 0 A ) 0 A2 0 ) aisoU + 0 T2 0 0 0 A3 0 0 T3
]
(2)
where U is the unit matrix. The isotropic part, aiso, of each coupling constant is related to the spin density at the nucleus (the Fermi contact term):
aiso ) (2µ0/3)gNβNgeβe〈FS 〉
(3)
where µ0 is the permeability of free space, gN is nuclear g-factor, ge is the electronic g-factor for the site under consideration, βN and βe are the nuclear and Bohr magnetons, and 〈FS〉 is the expectation value at the nucleus of the spin-density operator. In one-electron systems, 〈FS〉 ) |ΨS(0)|2. The anisotropic traceless tensor T results from the dipolar interaction:
∫ (3xixj/r5 - δij/r3)|ψ(r)|2 dV
Tij ) (µ0/4π)gNβNgeβe
(4)
and can be described in its principal axis system by two anisotropic hyperfine interaction constants b ) (1/2)Tzz and b′ ) (1/2)(Txx - Tyy). It follows that in eq 2 we have T1 ) -b + b′; T2 ) -b - b′; T3 ) 2b. The description of g values in terms of electronic structure parameters requires to account for various magnetic contributions34 and to work with sufficiently accurate eigenfunctions. Spin-orbit interaction, which is crucial for quantifying the deviation of g from the free electron value ge,34 can be either accounted for self-consistently or treated as a perturbation. Here we use the spin-orbit perturbation strategy in the scheme proposed by Neese35 and implemented in the code Gaussian0331 adopted for all the calculations. 2.2. Sample Preparation and Experimental Methods. High surface area polycrystalline MgO was prepared by slow decomposition of Mg(OH)2 in vacuum as described elsewhere.36 The MgO powder was activated at 1073 K in order to remove surface adsorbed impurities. After this treatment the surface was rehydrated exposing the solid to vapor pressure of H217O (86% isotopic enrichment supplied by Icon Services New Jersey). This was done in order to enrich the solid surface with oxygen ions containing the 17O isotope. This isotope with nonzero nuclear spin (I ) 5/2) potentially provides information about the magnetic interactions of the surface radical species with its surroundings.36 The solid was then reactivated at 1173 K, and
19570 J. Phys. Chem. C, Vol. 112, No. 49, 2008 (H+)(e-) centers were generated on the surface of the activated oxide by contacting the solid with H atoms produced in a 2.45 GHz microwave glove discharge under static conditions. Pulse-EPR spectra were recorded on a Bruker ELEXYS 580 spectrometer operating at a microwave (mw) frequency of 9.77 GHz. The spectra were taken at 298 K with a repetition rate of 1 KHz. Similar spectra were obtained at 10 K. Electron spin echo detected EPR spectra were recorded with the pulse sequence π/2-τ-π-τ-echo with pulse length tπ/2 ) 32 ns and tπ ) 64 ns and a time delay of 200 ns. The intensity of the primary echo was measured as a function of the magnetic field. Hyperfine sublevel correlation (HYSCORE)37 experiments were carried out with the pulse sequence π/2-τ-π/2-t1-π-t2-π/ 2-τ-echo with microwave pulse length tπ/2 ) 16 ns and tπ ) 32 ns. The time intervals t1 and t2 were varied in steps of 8 ns starting from 100 to 2500 ns. Two different τ values were used (τ ) 140 and 196 ns). Matched HYSCORE experiments have been performed with the sequence π/2-τ-(ΗΤΑ)-t1-πt2-(ΗΤΑ)-τ-echo. In the Matched HYSCORE experiments the amplitude of the microwave field of the matching pulses was 15.6 MHz, a value which is sufficiently close to the matching field for proton Zeeman frequencies in the range 12-15 MHz.38 The optimal length of the high turning angle (HTA) pulse was experimentally determined with a 2D threepulse experiment where the pulse length of the second and third pulses were increased in steps of 8 ns starting from 16 ns. A four-step phase cycle was used to eliminate unwanted echoes. The time traces of the HYSCORE spectra were baseline corrected with a third-order polynomial, apodized with a Hamming window, and zero filled. After two-dimensional Fourier transformation, the absolute value spectra were calculated. The spectra were added for the different τ values in order to eliminate blind-spot effects. The HYSCORE spectra were simulated using a program developed at the ETH Zurich and kindly provided by Prof. S. Van Doorslaer. 3. Results and Discussion 3.1. Ab Initio Calculations. We have considered three kinds of processes which can occur when an electron-rich MgO surface is exposed to CO2: (1) interaction of CO2 with oxide anions at low-coordinated sites and formation of surface carbonates, CO32-; (2) interaction of CO2 with “free” electrons trapped at low-coordinated sites of the MgO surface with formation of adsorbed CO2-; and (3) interaction of CO2 with (H+)(e-) centers and formation of CO2- species bound near an adsorbed proton (OH group). In the following we discuss these three situations and, for the paramagnetic species, we compare the results with those derived from EPR experiments. 3.1.1. Surface Carbonates. The surface of MgO is chemically rather inert toward CO2 adsorption. The flat (100) terraces are totally unreactive and do not bind the molecule. It is only at the low-coordinated oxide sites like steps, edges, kinks, corners, and so forth that CO2 reacts and forms a stable carbonate. This behavior has been rationalized theoretically some years ago based on the different basicity of the oxide anions of the MgO surface: high basicity for low-coordinated sites and low basicity for highly coordinated terrace sites.39,40 The reason is that at the low-coordinated sites the Madelung potential is lower, hence the energy levels of the O2- ions are shifted to higher energies and become available for charge transfer into the empty states of the CO2 molecule.39 This prediction and rationalization was later confirmed by specific experiments.41-43 Here we have considered the same process only in order to compare the stability of a carbonate species with the CO2-
Preda et al. TABLE 1: Binding Energy (in eV) of CO2 Adsorbed on MgO Surfaces Forming Carbonate, CO32-, or Carboxylate CO2- Species reaction Os2- + CO2 MgO(e-) + CO2 MgO(H+)(e-) + CO2
species CO32MgO(CO2-)
corner
reverse corner
1.87 not studied 1.41 (µ1) 1.86 (µ1) 1.54 (µ2) MgO(H+)(CO2-) 1.41 (µ1) 1.01 (µ2) 1.55 (µ2)
edge 2.16 0.68a (µ1) 0.54a (µ2) 1.27 (µ1)
a Computed with respect to MgO + CO2 because no convergence has been obtained on MgO(e-).
Figure 1. - Carbonate species formed by interaction of a CO2 molecule with (a) a four-coordinated O2- edge site and (b) a three-coordinated O2- corner site. White spheres: O; gray spheres: Mg; black sphere: C.
carboxylate species formed on the same surface. We have considered only two sites, edge and corner, as representative of four- and three-coordinated oxide anions, respectively; see Table 1 and Figure 1. The CO2 molecule forms a strong bond, of 2.16 eV, with an O4c anion and assumes an orientation “parallel” to the edge which allows a direct electrostatic interaction of the O atoms of the CO2 molecule, which carry a partial negative charge, with the Mg cations of the surface, Figure 1a. If the molecule is rotated by 90° the bonding is only 0.33 eV, showing the importance of the electrostatic interaction with the surface cations. The bonding on a O3c corner ion, Figure 1b, is similar, with an adsorption energy of 1.87 eV, Table 1. On both sites the molecule is considerably bent, due to the charge transfer from the oxide anion to the empty 2πu states of CO2. The species formed in this way are diamagnetic and cannot be detected in EPR. We have considered also the interaction of CO2 with an O2ion near a (H+)(e-) center along the edge. The idea was to see if the presence of the extra electron favors the formation of the surface carbonate. The result, however, is practically identical to that found for the interaction with a “clean” edge site. In fact, CO2 binds strongly with the O4c anion forming a surface carbonate; the interaction of one of the O atoms of CO2 with the Mg cation destabilizes the trapped electron which is displaced to the cluster border. This result clearly shows that CO2 binds to an oxide anion of an edge strongly enough that it is not affected by the presence of a (H+)(e-) center in the vicinity. Actually, if the carbonate forms near the (H+)(e-) center, this paramagnetic defect is destabilized. Also the attempt to add an extra electron to a surface carbonate failed: this means that the CO32- species has no tendency to add extra electrons and to transform into a paramagnetic electron trap. From these results the following conclusions can be drawn: (1) CO2 interacts favorably with the exposed oxide anions to form carbonates, and this process will compete with reactions leading to CO2-; (2) the interaction of CO2 with surface O2-
CO2- Radical Anions from CO2 Adsorption on MgO
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19571
anions does not lead to paramagnetic species, so that the radical anions observed in EPR experiments have a different origin; (3) the strong interaction with the MgO edge destabilizes preexisting (H+)(e-) centers when these are close to the carbonate and induces their migration along the surface. 3.1.2. Interaction of CO2 with Shallow Electron Traps. In this section we consider the interaction of CO2 with electrons trapped at specific sites of the MgO surface like Mg cations at corner or kink sites. These are usually classified as shallow traps, because they correspond to weakly binding sites for electrons, stable only at low temperatures.44 The corresponding energy levels are located a few tens of an eV below the MgO conduction band. For instance, a three-coordinated Mg ion at a corner site has an EA of 0.6 eV.44 Excess electrons can be generated by donating species (hydrogen, alkali metal, etc.) or by irradiating the sample with UV light.45 The possibility to stabilize an excess electron in one of these sites is related to the high electrostatic potential generated by a Mg2+ cation. On the (100) terraces this potential is screened and compensated by the presence of five O2- neighbors, respectively; at lowcoordinated sites the cation is exposed, the attractive potential increases, and electrons can be bound. In the presence of molecular accepting species, like CO2, the electron can be transferred to the adsorbate, with formation of the CO2- radical anion. Once formed, the CO2- species can diffuse on the surface and become stabilized at other sites. The reaction under investigation is thus
MgO(e-) + CO2 f MgO(CO2-)
(5)
Notice that here we are dealing with negatively charged clusters and that the presence of the extra electron is not compensated by a counterion. The three sites considered are a corner, a reverse corner, and an edge, Figure 2. We start the discussion with the corner and reverse corner sites, since they have positive EAs (EA is 0.40 and 0.68 eV, respectively). Reaction 5 is exothermic and associated to an energy gain of 1.41 eV (corner) and 1.86 eV (reverse corner), Table 1. This means that the formation of adsorbed CO2- is thermodynamically less favorable than the formation of a carbonate species and that it will occur only once CO2 has saturated the reactive oxide sites. The dissociation into MgO + CO2- costs 2.18 eV (corner) and 2.91 eV (reverse corner), due to the negative EA of CO2 and the cost to remove one electron from the MgO site. This means that by raising the temperature the system will dissociate CO2 and the extra electron will be transferred back to the surface. On the corner site only one bonding mode of CO2- is found, that is, monodentate, µ1, where CO2 interacts with the Mg3c cation, Figure 2a; on the reverse corner, beside the µ1 species, Figure 2b, also a bidentate µ2-coordination is found, Figure 2c; in this isomer the two O atoms of CO2- are directly pointing toward the two Mg4c ions of the reverse corner but the species is about 0.3 eV less stable than the µ1-coordination, Table 1. In all cases the molecule is bent with a very similar internal angle of 132-133°. The electron is completely transferred from the surface to the adsorbate, as shown by the spin density maps, Figure 3. The spin density, however, is not equally distributed among the atoms of the molecule: the C atom carries about 80% of the spin density or more, the rest being distributed over the O atoms, Table 2. The case of the edge site is slightly different, as this site is not an electron trap (negative EA). The MgO(e-) species is unstable, and obviously no charge transfer can occur from this site. However, it is possible that the CO2- species forms at
Figure 2. Carboxylate species, CO2- formed by interaction of CO2 with trapped electrons, MgO(e-). (a) corner, µ1; (b) reverse corner, µ1; (c) reverse corner, µ2; (d) edge, µ1; (e) edge, µ2. White spheres: O; gray spheres: Mg; black sphere: C.
another site and then diffuses away; the edge can be considered a metastable site in the diffusion process. Indeed, the calculations indicate that the CO2- anion can exist at an edge site; there are two minima for this system, one more stable where the molecule is in a µ1 coordination mode, Figure 2d, similar to that found for the corner and reverse corner sites, and a second one, only 0.14 eV higher in energy, where the molecule is bound with the two O atoms to the Mg4c ion, Figure 2e. The level of activation is similar to previous cases for the µ1 mode, and smaller for the µ2 orientation as reflected by the internal angle of 139°, Figure 2e. The dissociation of CO2- from an edge of MgO costs 1.05 eV. This is a much lower energy compared to the corner and reverse corner sites and indicates that most likely the edge is a transient species in the diffusion of CO2- on the MgO surface; in fact, the two minima found suggest a rotating motion of the molecule along the edge, with barriers which are probably very low. In Table 3 we report the hyperfine coupling constants for CO2- formed at the surface of MgO; they can be compared with the experimental values reported for two species identified on polycrystalline MgO characterized by slightly different spin Hamiltonian parameters, hereafter identified as species (a) and (b).5 All three sites considered above, corner, reverse corner, and edge, exhibit a substantial similarity to the experimentally measured constants and, in particular, a large isotropic value for the 13C atom, due to an important contribution to the spin density of the C 2s orbital. The anisotropic part B is nearly cylindrical around the z axis and originates from spin density in the carbon 2pz orbital, see Figure 3; a fraction of the spin density is located in a 2p orbital perpendicular to the molecular plane. With respect to the experimental 13C values for species
19572 J. Phys. Chem. C, Vol. 112, No. 49, 2008
Preda et al.
Figure 3. - Spin density maps for carboxylate species, CO2-, formed at corner, reverse corner, and edge sites. White spheres: O; gray spheres: Mg; black sphere: C.
TABLE 2: Spin Population and g Tensor for Free CO2-, MCO2 Complexes (M ) Li, Na, K) and CO2- Adsorbed on Various MgO Sites spin population system CO2LiCO2 NaCO2 MgO(CO2-) corner (µ1) MgO(CO2-) reverse corner (µ1) MgO(CO2-) reverse corner (µ2) MgO(CO2-) edge (µ1) MgO(CO2-) edge (µ2) MgO(H+)(CO2-) corner (µ2) MgO(H+)(CO2-) reverse corner (µ1) MgO(H+)(CO2-) reverse corner (µ2) MgO(H+)(CO2-) edge (µ1) polycrystalline MgO a
B3LYP B3LYP exp. (9,10) B3LYP exp. (10,11) B3LYP B3LYP B3LYP B3LYP B3LYP B3LYP B3LYP B3LYP B3LYP exp. (5)
a
a
C
Oa
Ob
gxx
gyy
gzz
1.06 0.69
-0.03 0.07
-0.03 0.07
0.65
0.07
0.07
0.82 0.75 0.81 0.88 0.78 0.82 0.59 0.79 0.81 0.60
0.14 0.19 0.06 0.11 0.09 0.09 0.22 0.08 0.18 0.20
0.05 0.0 0.06 0.02 0.08 0.08 0.06 0.06 0.0 0.20
2.00292 2.00410 2.0033 2.00302 2.0029 2.00337 2.00358 2.00304 2.00326 2.00256 2.00274 2.00388 2.00319 2.00324 2.0026
1.99731 1.99727 1.9967 1.99799 1.9979 1.99668 1.99679 1.99697 1.99678 1.99623 1.99749 1.99743 1.99704 1.99683 1.9965
2.00159 2.00166 2.0008 2.00165 2.0023 2.00160 2.00163 2.00158 2.00155 2.00162 2.00161 2.00172 2.00161 2.00161 2.0009
Oa is the atom of the CO2 molecule less coordinated, pointing toward the vacuum.
(a) and (b) we notice a close agreement for the dipolar part B, while significant departures are found for the aiso(C) term. A closer inspection, however, shows that the sites considered exhibit a different level of agreement with experiment. In particular, the CO2- radical anion formed at a corner site shows a too low aiso(C) value, 384.8 MHz versus 544.0 MHz in experiment; the reverse corner site (µ1 and µ2 bonding modes) offers the best agreement in terms of aiso(C), 457-470 MHz, and in particular of dipolar part. The edge µ1 bonding mode has a too low aiso(C) value as for the corner site; the µ2 structure has an aiso(C) value intermediate between corner and reverse corner. However, for the energetic reasons illustrated above, the edge can be ruled out as a stable site for CO2-. Thanks to the use of C17O2 it has been possible to evaluate the hyperfine interactions also with the O nucleus.5 As for the case of the 13C hyperfine matrix, two slightly different 17O hyperfine tensors were observed for the two species.5 The differences are however small (Table 3) and do not allow to discriminate between two substantially different configurations of the surface radical. An important result which comes from the experiments is the magnetic equivalence of the two O atoms of CO2-; this is consistent with the adsorption geometry and with the computed hfcc’s of the µ2-species formed at the reverse corner site, Figure 2c, but not with those where CO2- is bound via a single oxygen (µ1), which always exhibit a higher stability, Table 1. The nonequivalence of the O atoms is reflected to some extent in the computed hyperfine constants which are not identical, Table 3. However, considering the strongly asymmetric µ1 bonding mode one would expect a much larger difference in the hyperfine interactions of the two O atoms of CO2-. This is not the case.
Consider for instance µ1-CO2- bound at a corner site, Figure 2a: the hfcc’s of the two O atoms differ at most by a few MHz, Table 3. This can be explained with the fact that the excess electron is entirely transferred to the CO2- unit and is almost equally distributed across the molecule. The small nonequivalence of the two atoms is due to the polarizing effect of the Mg cation to which the molecule is bound, but the charge and spin densities within CO2- are only moderately affected by this interaction. The analysis of the 17O hyperfine interactions indicates µ2-CO2- at a reverse corner, Figure 2c, as the site where the agreement with the experimental data is higher: both aiso(O), -112.8 MHz versus -102.2 MHz, and the dipolar part are well reproduced, at least for species (a). However, we have seen that this species is energetically less favorable, Table 1. From the analysis of the hyperfine constants it has been possible to derive an experimental guess of the spin distribution on the CO2- molecule.5 This is done following a standard procedure which is based on the ratio of the aiso values of a given atom in the molecular compound and a computed aiso for the free atom; this procedure, not free from limitations, provides a spin distribution where the electron is for 60% on the C atom and for the remaining 20% on each O atom.5 Our calculations based on a spin population analysis show that actually the proportion is slightly different, with about 80% of the spin on C, and the rest on O. For the cases where the molecule is in a µ1 orientation, the spin density is larger on the less coordinated O atom, Table 2. Considering that also the spin population analysis is not unambiguous, the results can be considered quite consistent with the experimental analysis. To summarize this section, we have found that CO2- tends to bind as a monodentate ligand on low-coordinated cations of
CO2- Radical Anions from CO2 Adsorption on MgO
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19573
TABLE 3: Hyperfine Coupling Constants (in MHz) aiso
T1
T2
T3
-
13
C 17 Oa 17 Ob 13
C 17 Oa 17 Ob 17 Os 13
C 17 Oa,b 17 Os 13
C 17 Oa 17 Ob 13
C Oa,b 17 Os 17
13
C 17 Oa 17 Ob 1 H 13
C Oa 17 Ob 1 H 17
13
C 17 Oa 17 Ob 1 H 13
C 17 Oa 17 Ob 1 H 13
C, ref 5 17 Oa,b, ref 5 1 H, this work 13
C, ref 5 17 Oa,b, ref 5
MgO(CO2 ) corner (µ1) 384.8 -36.7 -48.8 -72.3 33.4 40.6 -78.2 27.3 32.0
79.5 -74.0 -59.4
MgO(CO2-) reverse corner (µ1) 457.2 -38.7 -45.7 -97.8 21.9 24.7 -61.9 36.0 44.7 -6.4 1.4 1.4
84.5 -46.6 -80.6 -2.8
MgO(CO2-) reverse corner (µ2) 470.5 -40.5 -48.2 -112.8 23.4 29.9 -11.1 2.2 2.2
88.7 -53.3 -4.4
MgO(CO2-) edge (µ1) 355.0 -35.9 -78.2 32.3 -71.6 26.4
-41.5 38.9 31.4
77.3 -71.2 -57.8
MgO(CO2-) edge (µ2) 427.9 -34.6 -114.2 26.6 -29.7 4.2
-41.3 32.6 4.3
75.9 -59.2 -8.5
MgO(H+)(CO2-) corner (µ2) 508.1 -43.3 -51.5 -101.9 22.4 29.3 -106.3 22.1 28.2 -1.4 -2.4 -3.7
94.8 -51.7 -50.4 6.1
MgO(H+)(CO2-) reverse corner (µ1) 319.3 -33.9 -37.6 -81.4 28.0 31.1 -54.6 37.4 43.3 -2.9 -4.0 -5.2
71.5 -59.2 -80.8 9.2
MgO(H+)(CO2-) reverse corner (µ2) 505.4 -41.6 -49.7 -116.4 22.4 27.1 -94.3 25.6 34.2 0.5 -2.4 -3.3
91.3 -49.5 -59.8 5.7
MgO(H+)(CO2-) edge (µ1) 434.7 -36.9 -43.9 -65.6 36.0 43.9 -143.0 20.4 24.8 0.9 -3.1 -4.3
80.8 -79.9 -45.2 7.4
experiment species (a) 544.0 -36.5 -102.2 35.0 -1.33 -2.2
-48.8 30.7 -3.0
85.3 -66.7 5.2
experiment species (b) 570.8 -33.3 -108.2 27.2
-51.9 24.1
85.1 -51.5
the MgO surface and that it forms stable species at corner and reverse corner sites, while on edge sites it can only be a transient species in a diffusion process. The analysis of the hyperfine constants shows that no site among those considered so far exhibits a close agreement with experiment. In fact, µ1-CO2is characterized in general by a nonequivalence of the hyperfine constants of the two O atoms, at variance with the experiment.5 Where a near-equivalence is present, like for µ1-CO2- at a corner or at an edge, the computed aiso(C) is quite far from that measured experimentally, ruling out these structures as those responsible for the observed signals. So far the analysis has only taken into account CO2- species formed by capture of a “free” electron; in the next section we will consider the same process but starting from a MgO surface rich in (H+)(e-) centers.
Figure 4. Carboxylate species, CO2- formed at (a) corner, (b, c) reverse corner, and (d) edge sites by interaction of CO2 with MgO(H+)(e-) centers. White spheres: O; gray spheres: Mg; black sphere: C; small gray sphere: H.
3.1.3. Interaction of CO2 with (H+)(e-) Centers. The last case considered is that of a CO2 molecule which interacts directly with a (H+)(e-) center leading to the formation of a (H+)(CO2-) surface complex:
MgO(H+)(e-) + CO2 f MgO(H+)(CO2-)
(6)
The main differences with respect to the case discussed above (section 3.1.2) is that here the entire system is neutral (formally corresponds to a MgO surface with an H atom and a CO2 molecule added) and that the CO2- radical anion is stabilized in the proximity of an OH group (formed by the addition of a proton, H+, to an oxide anion, O2-). The additional stabilizing effect of the proton is apparent from the optimal structures of the (H+)(CO2-) species, Figure 4; the CO2- species has always one O atom in a bridge position over a Mg2+ cation and the proton of the OH group. The calculations show that the distance between the proton and the C atom of the carboxylate goes from 2.7 to 3.4 Å. In all cases the molecule is bent and the electron transfer is complete. In general we observe the tendency of CO2- to bind as a monodentate ligand, as found also for the shallow electron traps (section 3.1.2). However, some significant differences are present. Let us consider the corner site, Figure 4a. Due to the particular conformation of the corner, CO2- assumes a conformation with one O atom interacting simultaneously with the Mg3c corner ion and with the OH group, and the other O pointing toward the same Mg3c cation, Figure 4a. In this way the molecule interacts with the surface with both O atoms and can thus be classified as µ2. The need to coordinate both O atoms on the same cation reduces the internal angle to 127°; the stabilization is 1.01 eV, indicating a substantial energy gain associated to the electron transfer, Table 1. Still, this energy gain is smaller than for other sites, probably due to the larger strain induced by bending the OCO bond. Notice that while the two O atoms are formally nonequivalent because of the presence of the hydroxyl group, their distances from Mg3c and C are almost the same (Figure 4a). On a reverse corner site we find both µ1 and µ2 bonding modes, Figure 4b,c, with similar adsorption energies, 1.41 and 1.55 eV, respectively, Table 1. In both cases the molecule interacts simultaneously with a low-coordinated Mg cation and
19574 J. Phys. Chem. C, Vol. 112, No. 49, 2008 with the proton of the OH group; in the µ2 conformation the second O points toward another Mg4c ion and the molecule acts as a µ2-ligand. The two O atoms, however, are no longer equivalent, and this reflects in different C-O (1.26 and 1.22 Å) and Mg-O (2.06 and 2.16 Å) bond lengths. The asymmetry of the two O atoms is found also when CO2 interacts with a (H+)(e-) center along an edge site, Figure 4d. The molecule interacts in µ1 bonding mode with a Mg4c cation and the OH group; the adsorption energy is 1.27 eV. We can consider now the EPR properties of the three sites considered. The monodentate CO2- on the edge or reverse corner sites exhibits two different hyperfine constants for the two O atoms, Table 3; this is in contrast to the experimental evidence which indicates the formation of a more symmetric complex. Also the aiso value for the C atom is a bit too low compared to experiment, Table 3. Therefore, the µ1-CO2complex can be discarded since its EPR properties are clearly inconsistent with experiment. The situation is not very different for the µ2-CO2- complex formed at a reverse corner, Figure 4c. Here the 13C constants, including aiso(C) ) 505.4 MHz, are consistent with the experimental ones. However, the two O atoms are nonequivalent and exhibit aiso(O) values which differ by about 20 MHz. On this basis it is not possible to assign the observed features to this species. Much more appealing in this respect is the CO2- radical formed at a corner site, Figure 4a. Here the 13C isotropic hyperfine constant, 508.1 MHz, is rather close to the measured one; the dipolar part is in almost quantitative agreement with experiment, with deviations of a few MHz at most. Even more important, also the O parameters are fully consistent: the two O atoms are not exactly equivalent but they differ by about 1 MHz; the aiso(O) value, 104 MHz in average, reproduces quantitatively the experimental one for species (a); the dipolar part is only slightly underestimated. In this center a small but non-negligible hyperfine interaction with the proton of the OH group is computed. We will show below that this proton matrix is in excellent agreement with the new experimental findings reported in section 3.2. This is very important to conclude that the CO2- radical anion has formed according to reaction 6. On the basis of all these considerations, the (H+)(CO2-) complex formed at a corner site is an excellent candidate for one of the species observed in experiments on polycrystalline oxides. 3.1.4. Determination of g Tensors. In this section we briefly comment on the calculated g tensors for CO2- adsorbed on various sites, Table 2. The reasons why this is discussed in a separate section and not together with the discussion of other EPR properties like the hyperfine coupling constants is that the g tensor is not particularly diagnostic of the adsorption site being very similar for all considered configurations. We start the discussion by considering the g tensor for free CO2- and for the Li+-CO2- and Na+-CO2- complexes for which experimental data exist.9,12 In the following we use the same orientation of the molecular axes used in ref 5. We notice that for all systems gyy < gzz < gxx. For LiCO2, the computed g tensor is 1.9973, 2.0017, 2.0041 to be compared with the experimental one, 1.9967, 2.0008, 2.0033; for NaCO2, the computed g tensor is 1.9980, 2.0017, 2.0030 versus 1.9979, 2.0023, 2.0029 in the experiment. Thus, the calculated g tensors are similar to the measured ones, although not in quantitative agreement. However, what is important in this context is that the computed values are rather similar for free CO2- and for CO2- adsorbed on various sites of the MgO surface, Table 2. Actually, the values obtained for a variety of situations on MgO
Preda et al.
Figure 5. (a) ESE-detected EPR spectrum of the CO2- radical on MgO. The experimental conditions were tπ/2 ) 32 ns, tπ ) 64 ns, τ ) 200 ns, T ) 298 K; (b) computer simulation. The arrows indicate the observer positions taken in the ESEEM experiments. The asterisk indicates an impurity due to a matrix radical.
are often indistinguishable, indicating that the g tensor only reflects the electronic structure of the CO2- fragment and that it is almost insensitive to the local environment or to the change in coordination (for instance no significant differences are found for µ1 or µ2 complexes or for CO2- species formed in the presence or in the absence of a vicinal OH group). This fact is in full agreement with what expected for a 17 electrons radical which has a non-degenerate ground state. This is further corroborated by the fact that the computed g tensor is very close in LiCO2 or NaCO2 and in the MgO surface species, Table 2. The similarity of the g tensor in molecular complexes and for CO2- formed on oxide surfaces has been used also in experiments as a fingerprint of the formation of the CO2- radical anion. 3.2. Pulse EPR Experiments. The adsorbed CO2- radical was generated on the MgO surface by reacting the (H+)(e-) centers with a small amount of CO2 (∼0.5 mbar) at room temperature, following the procedure adopted in our previous study.5 The two pulse echo detected EPR spectrum is shown in Figure 5 together with its computer simulation. The g factors employed in the simulation are the same previously reported by some of us (gxx ) 2.0026, gyy ) 1.9965, gzz ) 2.0009; for the adopted reference axis system see ref 5). A sharp feature at g ) 2.003, marked with an asterisk in Figure 5, is due to a radical impurity of the matrix and is of no interest for this study. In Figure 5 the arrows indicate the observer positions taken in the electron spin echo envelope modulation (ESEEM) experiments. As mentioned in the introduction, no sign of the superhyperfine interaction with the parent proton of the original (H+)(e-) center is detectable in the spectrum. In order to investigate more deeply the local environment of the CO2- ions, pulsed EPR measurements were carried out. Standard two and three pulse ESEEM experiments, however, did not show any trace of proton couplings. Considerable enhancement of echo modulations can be obtained by the use of matched microwave pulses.38,46 Considering that the hyperfine coupling is expected to be weak, the field strength of the microwave field was adjusted to the nuclear Zeeman frequency of the proton. The optimal length of the HTA pulse was then determined performing a 2D three pulse ESEEM experiment and was found to be 128 ns. Indeed, as shown in Figure 6, a drastic increase in the sensitivity of the three pulse ESEEM experiment is obtained, which provides a nice example of the effect of microwave pulse matching. Figure 6a shows the time trace of a normal three pulse
CO2- Radical Anions from CO2 Adsorption on MgO
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19575
Figure 6. (a) Three pulse ESEEM spectral pattern for the CO2- radical. (a) Standard three pulse ESEEM, 16-τ-16-Τ-16-τ-echo, τ ) 196 ns. (b) Matched three pulse ESEEM, 16-τ-128(ΗΤΑ)-Τ-128(ΗΤΑ)-τ-echo, νmatch ) 15.2 MHz, τ ) 196 ns. In the inset the corresponding frequency domain spectra are shown. The spectra were taken at 343.5 mT corresponding to observer position 1 in Figure 5.
ESEEM experiment with all π/2 pulse length equal to 16 ns, taken at observer position 1 (see Figure 5, similar results were obtained for spectra taken at position 2). Only slow modulations due to the 17O isotopes are observed and no proton frequencies are detected (the frequency domain spectrum is shown in the inset). Figure 6b shows the spectrum observed when optimal matching pulses (HTA ) 128 ns) are used. A drastic change in the modulation pattern is observed, and proton modulations are now clearly visible, superimposed to the 17O modulation pattern. Correspondingly, a clear signal centered at νH is now detected in the frequency domain spectrum. This result proves unambiguously that the surface adsorbed CO2- radical interacts with at least one proton. In order to better characterize this interaction, matched HYSCORE experiments were performed. HYSCORE is a two-dimensional experiment where correlation of nuclear frequencies in one electron spin (mS) manifold to nuclear frequencies in the other manifold is created by means of a mixing π pulse. In the HYSCORE spectrum recorded at observer position B0 ) 343.5 mT (position 1 in Figure 5) only a signal at (νO, νO) was observed in the (+, +) quadrant, which is due to the interaction of the radical with distant (matrix) 17O nuclei (spectrum not shown). In the matched HYSCORE spectrum (Figure 7a) beside the 17O signal, a clear ridge in the (+, +) quadrant, centered at about 15 MHz (the 1H Larmor frequency) with a width of about 4.0 MHz, is present. Assuming that the width of the ridge is due to the anisotropy of the hyperfine tensor, then the following conditions hold:
|aiso + 2T| ≈ 4.0 MHz
and
|aiso - T| e 4.0 MHz
|aiso + 2T| e 4.0 MHz
and
|aiso - T| ≈ 4.0 MHz
or
Under these circumstances, the highest possible value for |T| can be deduced from the condition (aiso + 2T) ) -(aiso - T); thus, -T ) 2aiso. Since T > 0 (due to the positive gn), it follows that T ) 2.6 MHz and aiso ) 1.3 MHz. Any higher value of |T| gives a broader ridge. The magnitude and the sign of both aiso and T are in good agreement with those obtained by means of computer modeling and have been taken as starting parameters for the simulation of the experimental spectrum. Due to the asymmetry of the experimental ridge, which is frequently encountered in matched HYSCORE spectra, the simulations have been compared with the symmetrized spectrum, in order to make sure that the full width of the ridge was reproduced
Figure 7. (a) Matched HYSCORE spectrum of the CO2- radical on MgO taken at observer position B0 ) 343.5 mT (see 1 in Figure 5); HTA pulse was 128 ns. Two τ values (140 and 196 ns) were added together in order to eliminate blind spot effects. (b) Computer simulation of the spectrum.
(see Supporting Information). The computer simulation of the spectrum (Figure 7b) obtained starting from the above values shows that the relevant features of the small proton ridge can be reproduced assuming the interaction of the unpaired electron with a single type of proton nuclei with a coupling tensor A(H) ) [-3.5 -4.3 +3.9] MHz. Due to the small g anisotropy of the spectrum it was not possible to determine the reciprocal orientation of the g and A(H) matrices. It should be noted that the intensity of the experimental ridge at the proton Larmor frequency is actually higher than that obtained from the simulation reported in Figure 7b. This fact can be accounted for by the presence of remote protons which can contribute to spectrum. Considering that all the edges are decorated with protons arising from the precursor (H+)(e-) centers this seems reasonable (the effect of remote protons on the simulated spectrum is reported in the Supporting Information). The above analysis shows that the ridge pattern can be explained in terms of the interaction of the unpaired electron with a proximal proton. The T value deduced from the above analysis sets the under-limit for the H+ · · · CO2- distance, which can be estimated to be of the order of 3 Å from the following equation:
B)
µ0 1 ggββ 4π e n e n r3
(7)
with r being the distance between the unpaired electron localized in the CO2- orbitals and the 1H nucleus. This distance is in good agreement to that estimated theoretically assuming that the center of charge is around the carbon atom (see section 3.1.3), showing that the calculated values provide an acceptable solution. This distance is also in agreement with that observed by CW-EPR in the case of O2 adsorbed on (H+)(e-) centers at the surface of MgO forming superoxide radical anions (O2-).22 In that case the distance of the surface H+ from the center of mass of the oxygen moiety was found to be 2.31 Å.
19576 J. Phys. Chem. C, Vol. 112, No. 49, 2008 The pulse experiment described above adds important information to the whole picture of the CO2-MgO interaction indicating that, after the electron transfer, the adsorbed CO2remains adsorbed on the same site of the surface electron center having as first neighbor an OH group at a distance compatible with the Mg-O distance in the oxide (2.05 Å). 4. Conclusions We have studied the formation of CO2- radicals on a MgO surface where (H+)(e-) defect centers have been generated using ab initio cluster model calculations. The results have been compared with EPR data taken from the literature and new EPR measurements on the superhyperfine interaction of CO2- with the proton of an OH group. The following main conclusions can be drawn: (1) CO2 forms preferentially diamagnetic carbonates by reaction with the low-coordinated oxide anions. If a (H+)(e-) center is present in the vicinity of the carbonate this is destabilized. We suggest that the (H+)(e-) pair diffuses away migrating along the edges of the nanocrystals and becomes stabilized at Mg3c ions at corner or kink sites. (2) CO2 interacts with these sites and leads to the formation of CO2- species stabilized near an OH group. This is shown unambiguously by the small hyperfine interaction with the proton, detected via matched three pulse and HYSCORE experiments. The calculations show that a close agreement with experiment is obtained only when CO2- forms by reaction of CO2 with a (H+)(e-) center at a cationic corner site. On most other sites in fact the calculations show the formation of monodentate CO2- which is more stable than bidentate forms. The fact that this monodentate CO2- is not seen in experiment can be explained with the fact that all the edge sites are saturated with carbonate species and there is no room left to form CO2complexes. (3) The dominant species (a) observed in the experiments should correspond to CO2- formed at Mg3c corner sites. The heterogeneity of species observed experimentally may be ascribed to carboxylate anion radicals stabilized at three coordinated Mg2+ ions with slightly different local environment (for instance on a kink), a common situation in highly dispersed polycrystalline systems. Acknowledgment. M.C. thanks prof. S. Van Doorslaer for useful comments and advice on the pulsed experiments. This work has been supported by the Italian MIUR through a PRIN 2005 project and the COST Action D41 “Inorganic oxide surfaces and interfaces”. Supporting Information Available: Table 1, Computed properties of the LiCO2 complex at various levels of theory; Figure 1, simulation of the HYSCORE spectrum including the contribution of one remote nucleus; Figure 2, comparison of the symmetrized experimental spectrum with the simulation; Figure 3, experimental matched HYSCORE spectra taken at different observer positions (a) B0 ) 344.0 mT and (b) B0 ) 343.5 mT; complete ref 31. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E. J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.; Domen, K.; DuBois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard, W. A.; Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons, J. E.; Manzer,
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