A density functional theory study of the adsorption behaviour of CO 2 on Cu 2 O surfaces

Copper has many applications, particularly in electro-catalysis, where the oxidation state of the copper electrode plays a significant role in the selectivity towards products. Although copper-based materials have clear potential as catalysts in the reduction of CO2 and conversion to products, fundamental understanding of CO2 adsorption and activation on different copper oxide surfaces is still limited. We have used DFT+U methodology to study the surface reconstruction of the three most exposed (111), (110), and (001) surfaces of Cu2O with different possible terminations. Considering several adsorbate geometries, we have investigated CO2 adsorption on five different possible terminations and proposed eight different configurations in which CO2 binds with the surface. Similar to earlier findings, CO2 binds weakly with the most stable Cu2O(111):O surface showing no molecular activation, whereas a number of other surfaces, which can appear in the Cu2O particles morphology, show stronger binding as well as activation of the CO2 molecule. Different CO2 coverages were studied and a detailed structural and electronic charge analysis is presented. The activation of the CO2 molecule is characterized by structural transformations and charge transfer between the surface and the CO2 molecule, which is further confirmed by considerable red shifts in the vibrational frequencies.


I. INTRODUCTION
Copper is a unique metal owing to its ability to selectively produce hydrocarbons through the electro-reduction of CO 2 , 1,2 where the oxidation state of the Cu electrode plays an important role in the product selectivity.The direct reduction of CO 2 to methanol (CH 3 OH) is known to occur on oxidized Cu electrodes, which show an increase in the methanol formation by an order of magnitude compared to metallic copper. 3The surface structure of oxidized copper resembles the Cu 2 O (111) 4,5 surface and it reduces CO 2 to CH 3 OH at rates remarkably higher than either air-oxidized or anodized Cu. 5 In addition, our recent density functional theory (DFT) based calculations of CO 2 hydrogenation on the most stable (111) surface of Cu 2 O show that it is a suitable catalyst for CO 2 conversion to formate and formic acid under mild conditions. 6Surface analysis of these oxides, before and after the reaction, shows mixed oxidation states (Cu 2 O, Cu 4 O 3 , and CuO) depending on the method of preparation. 7Recently, it has been demonstrated that CuO-Cu 2 O nanorod arrays prepared on Cu substrates can drive the efficient solar photoconversion of CO 2 to methanol. 8he catalytic process is affected considerably by the catalyst structure, with different shapes and surface arrangea) Electronic addresses: akmishra@ddn.upes.ac.in, abhishek.mishra@ucl.
ac.uk, and deleeuwn@cardiff.ac.uk ments having a large impact on the catalyst's activity and stability.0][11] By controlling the size and morphology, one can fine tune the strength of surface adsorption and reactivity to meet the stringent selectivity and activity requirements in a catalytic process.For example, our recent investigations of CO 2 activation on a number of Cu() oxide surfaces revealed that surface structures have significant effects on CO 2 activation and binding energies. 121][22][23][24][25] Recently, a study by Sun et al. on the crystal facet-dependent effect of polyhedral Cu 2 O micro-crystals, that exposed different Miller index facets, showed that the catalytic performance can be enhanced by high-index facets, 26 Furthermore, copper() oxide nano clusters have been studied recently to understand the methanol formation through DFT based calculations. 27he adsorption of molecules on a catalyst surface is the first step in their activation and conversion in any catalytic process.CO 2 adsorption on the Cu 2 O(111) surface has been investigated by Wu et al., 18,19 and Bendavid and Carter, 28 using DFT calculations.Wu et al., 19 investigated CO 2 adsorption on the Cu 2 O(111) surface, using the standard generalized gradient approximation (GGA) and identified that CO 2 binds as a linear molecule in a tilted configuration to the surface, with its oxygen atom coordinated to a coordinatively unsaturated surface copper atom, releasing an adsorption energy of 26.8 kJ/mol.However, it is now well known that pure GGA can lead to considerable errors when calculating reactions where 3d-metal oxides are oxidized by means other than by oxygen.Reaction energies for these processes become more accurate when the so-called DFT+U method is applied. 29endavid and Carter 28 recently investigated CO 2 adsorption on the Cu 2 O(111) using the DFT+U method and showed via comparison to adsorption energies derived by standard DFT that the U parameter is necessary.2][33] The selection of their U value was based solely on its accuracy to predict the equilibrium lattice constant for Cu 2 O.However, experimentally it is found that copper oxide surfaces consist of mixed Cu 2 O and CuO surface species, whereas molecule interactions can also alter the oxidation state of the copper oxide surface, i.e., through −OH groups. 30Therefore, we recently determined a single U parameter to describe adequately both Cu 2 O and CuO in terms of experimental properties. 12In the present work, we have employed DFT with this Hubbard U correction to explore CO 2 adsorption on different non-polar stoichiometric terminations of the (111), (110), and (001) surfaces of Cu 2 O.We first describe the reconstruction of the different surfaces and their electronic properties, followed by a detailed discussion of the CO 2 adsorption behaviour.

II. COMPUTATIONAL DETAILS
5][36][37] We have employed plane-wave DFT+U 38 with the PBE 39,40 exchange-correlation functional and the formalism of Dudarev et al. 38 The different Cu 2 O surfaces were obtained by the METADISE code, 41 providing different non-polar surface terminations. 42At the base of the surface simulation cell, two layers of atoms were fixed at their optimised bulk positions to simulate the bulk phase of the Cu 2 O. Above these two layers, the surface is represented by three layers of atoms, whose positions are allowed to change freely during optimization.In each case, the vacuum region above the surface was 12 Å, i.e., large enough to avoid interactions between the periodic slabs.We sampled (1 × 1) and (2 × 2) supercells with 5 × 5 × 1 and 3 × 3 × 1 Monkhorst and Pack 43 k-point mesh, respectively.Such dense grids and a truncation kinetic energy of 450 eV for the plane waves ensured an accurate description of properties that are influenced by sharp features in the density of states.A total convergence better than 10 −5 was reached and the interatomic forces were minimized to 0.01 eV/Å for structural relaxation calculations.
The surface energies of the relaxed slabs were obtained using a combination of calculations for the relaxed and unrelaxed surfaces.After surface relaxation, the top and bottom surfaces are not equivalent and therefore we also need to consider the unrelaxed surface energy (γ u ) in order to calculate the final surface energy of the relaxed surface.The unrelaxed surface energy is the surface energy before any surface optimisation and is calculated as where E slab,u is the energy of the unrelaxed slab, nE bulk is the energy of an equal number of bulk atoms, and A is the surface area of one side of the slab.Using this value, it is then possible to calculate the relaxed surface energy (γ r ) from the total energy of the relaxed slab.
The relaxed surface energy, γ r , is given by where E slab,r is the energy of the relaxed slab.
The equilibrium morphology of a Cu 2 O particle (ignoring higher Miller indices) was constructed using Wulff's method, 44 which requires that the distance to a given surface from the center of the particle is proportional to the surface energy.
While modelling the CO 2 molecule, we have also used the implementation of the DFT-D2 approach described by Grimme 45 to account for long-range dispersion forces.The isolated molecule was modelled in the centre of a big cell with broken symmetry and lattice constants of 20 Å, sampling only the gamma-point of the Brillouin zone with the same accuracy parameters described for the surfaces.
The adsorption energy per molecule was calculated from the relation where E surf +mol is the total energy of the adsorbate-substrate system, E surf is the energy of the naked surface slab, and E mol is the energy of the isolated CO 2 molecule.Within this definition, a negative adsorption energy indicates an exothermic process.

III. RESULT AND DISCUSSION
In a recent work, we found that a value of U eff = 7 eV results in the accurate reproduction of the structural parameters of Cu 2 O and a proper description of the Cu() oxide. 12At this U eff value, we found the lattice parameter of Cu 2 O to be 4.270 Å, which is very close to the experimental value of 4.2696 Å. 46 Other structural parameters were also found to be in close agreement with the experimental values. 46We have therefore modelled the different Cu 2 O surfaces using the same U eff value and employing the same bulk structural parameters. 12

A. Surface reconstructions
In this section (Sec.III A), we have described in detail the reconstruction of the different terminations of three low-index Cu 2 O surfaces: (111), (110), and (001).We   47 we found that the most stable surface is the stoichiometric non-polar oxygen-terminated (111) surface, (111):O, with a surface energy of 1.08 J/m 2 .The work function calculated with DFT+U is 4.98 eV, which is close to the experimental range of 4.62-4.84eV. 48This surface consists of four distinct types of atoms: unsaturated (singly coordinated) surface copper atoms Cu CUS , outermost surface oxygens O SUF , saturated copper atoms with linear O-Cu-O bond symmetry Cu CSA , and sub-surface oxygens that are 4-fold coordinated O SUB (Fig. 2).The unsaturated copper atoms (Cu CUS ) act as Lewis acid sites, where most of the surface reactions are believed to take place. 49fter relaxation, the distance of the Cu CSA atoms to O SUF atoms decreases from 1.85 to 1.82 Å, but increases to the O SUB atom to 1.86 Å.As a result, these Cu CSA atoms become more exposed.The top Cu CUS atoms also move outwards so that the vertical bond length between Cu CUS and the topmost O atoms found in the second trilayer increases from 1.85 to 1.91 Å,  while the vertical bond length from the sub-surface oxygen atoms to the copper atoms in the second layer also increases to 1.89 Å.We investigated the electronic density of states (DOS) (Fig. 3(a)) of this surface and found that the bandgap slightly decreases by 0.78 eV from the calculated value of 0.89 eV for the bulk Cu 2 O material.The calculated values of the bandgap are expected to be under-estimated as DFT+U fails in the accurate prediction of bandgaps for Cu 2 O. 12,32 The calculated projected DOS shows that both valence band maxima (VBM) and conduction band minima (CBM) mainly consist of O (2p) and Cu (3d) orbitals, respectively, while contributions from other orbitals are much less.
b. (111):Cu.We reconstructed another non-polar stoichiometric (111) surface with a Cu termination ((111):Cu), which, however, is found to be less stable by 0.84 J/m 2 than the (111):O surface.The work function is found to increase slightly to 5.10 eV.The presence of two Cu atoms at both top and bottom of the slab makes the (111):Cu surface non-polar, while maintaining the bulk Cu 2 O ratio of Cu and O atoms (an unrelaxed (2 × 2) supercell is shown in Fig. S1 of the supplementary material).After relaxation, we noted significant changes in the positions of the top copper atoms, which moved down below the level of the O atoms.As a result, the O atoms in the relaxed surface are more exposed than the Cu atoms (Fig. 4).The Cu-O bond distance to these two Cu atoms increases slightly by 0.01 Å, while the vertical bond  The Cu-O bond distances in the second tri-layer increase up to 1.89 Å, while there are no structural changes in the third tri-layer.
We calculated the electronic DOS for both terminations and found that the bandgaps for the (110) surfaces are quite low, at 0.30 and 0.15 eV for the (110):Cu and (110):Cu-O terminations, respectively (Fig. 7).energy is calculated at 1.62 J/m 2 , which is 0.46 J/m 2 larger than the surface energy of the most stable Cu 2 O(111):O surface, while the work function is 4.54 eV.This surface consists of Cu atoms in the top layer connected to oxygen atoms below, which in turn are connected to two copper atoms in the layer below (Fig. S4 of the supplementary material).We noted that after relaxation, the top Cu atoms moved down and became less exposed and the Cu-O bond distance increased from 1.85 to 1.88 Å (Fig. 8).Cu atoms in the second layer up to shorten the bond length to oxygen atoms in the top layer from 1.85 to 1.83 Å.We also noted that the Cu-O bond distance in all other relaxed surfaces increases from 1.85 Å and varies from 1.86 to 1.88 Å.With finite states near the Fermi level, this surface is also found to be conducting (Fig. 7).111) surface for CO 2 adsorption and investigated a number of initial configurations with different orientations of the CO 2 molecule.We found that the CO 2 molecule moved away from the (111):O surface for all configurations, except where we placed it near the coordinatively unsaturated surface copper, Cu CUS .In this configuration one of the oxygen atoms, O1, of the CO 2 molecule binds weakly with this Cu CUS copper atom, as shown in Fig. 9.The CO 2 molecule remains almost linear with an angle of 176.9 • .The distance between the oxygen atom O1 of the CO 2 molecule and Cu CUS is found to be 2.05 Å, and the C-O bond between C and this O1 atom is slightly stretched at 1.19 Å, while the C-O2 bond length is found to be around 1.17 Å. Cu-O bond lengths in the slab also change slightly as a result of CO 2 adsorption, where the vertical bond distance between Cu CUS (coordinated to the O1 atom of the CO 2 molecule) and the topmost O atom found in the second trilayer shortens from 1.91 to 1.88 Å.The adsorption energy in this configuration is −51.0 kJ/mol.

Cu
In order to assess the effect of CO 2 coverage, we repeated our calculation by placing one CO 2 molecule in a (2 × 2) supercell; we found that the adsorption energy increases to −56.1 kJ/mol, but with negligible changes in the CO 2 geometry.Adsorption geometries of the CO 2 molecule on   28 This small change in adsorption energy value is expected as we have not included entropy and enthalpy energy corrections in our calculated adsorption energies.
A Bader charge analysis of the (1 × 1) cell (Table III) shows that the oxygen atom O1 of the CO 2 molecule (bonded to the coordinatively unsaturated surface copper Cu CUS ) gains 0.03e − , resulting from a small charge transfer from the surface copper atom Cu Cus , which becomes more oxidized after CO 2 adsorption.This very small charge transfer between the surface and the CO 2 molecule, as well as small changes in vibrational frequencies (Table III  We noted that due to the orientation of the CO 2 molecule, the lateral distance in the x-direction between the CO 2 molecule and its periodic image is 6.04 Å, while in the y-direction, it is only 3.80 Å.Hence, to minimize the effect of the periodic images on the CO 2 adsorption, we carried out calculations on a (1 × 2) supercell.At this lower coverage, CO 2 adsorbs in a slightly different manner, as the top surface Cu atoms (Cu A and Cu B ) interact with both CO 2 oxygen atoms at distances of 2.03 Å and 2.01 Å, respectively (Fig. 10(b)).As a result, the Cu A and Cu B bond lengths with oxygen atoms in the surface change to 1.91 and 1.90 Å, respectively.Because of the lower coverage of CO 2 molecules on the surface, other surface Cu atoms (further away from the CO 2 molecule) bend inwards to bind to O atoms in the second layer, as shown in Fig. 10(b).As expected, the adsorption energy increases to about −161.5 kJ/mol.Similar to the (1 × 1) cell configuration, the C atom of the CO 2 molecule bends towards a surface oxygen atom O SUF in the second layer (d C-OSUF = 1.42 Å).The angle of the adsorbed CO 2 molecule is 129.0 • and both C-O bond lengths are 1.27 Å.We have given parameters of the CO 2 adsorption geometries in the (1 × 1) and (1 × 2) simulation cells in Table IV.
Bader charge analysis of the (1 × 2) supercell shows charge transfer between the CO 2 molecule and the surface, as both molecular oxygens O1 and O2 gain 0.08e − and 0.07e − charge densities, respectively.This charge transfer originates mainly from the interacting surface copper atoms Cu A and Cu B , which become more positively charged after adsorption.The O SUF atom bound to the molecule also gains 0.11e − charge density (Table V).We also note some charge redistribution on the Cu 2 O surface as a result of CO 2 adsorption.Bader analysis indicates the CO 2 molecule as a chemisorbed anion on the surface, in agreement with the molecular orbital occupation and bending of the molecule.This activation of the CO 2 molecule is also reflected in terms of changes in the vibrational frequencies of the molecule, as asymmetric (υ as ) and symmetric (υ s ) stretching modes change to 1560 and 1200 cm −1 from their values of 2355 and 1316 cm −1 , respectively, in the isolated gas phase molecule (Table V).
In a different configuration (config.2), CO 2 binds to the (111):Cu terminated surface through its C atom to a surface oxygen atom (d C-OSUF = 1.44 Å), while both oxygen atoms of the molecule bind to Cu A and Cu B (d O-Cu = 2.14 Å), as shown in Fig. 10(c).The CO 2 molecule again bends to ∠CO 2 = 133.2• , while the Cu-O-Cu angle in the surface is about 145.4 • .We found the surface Cu-O bonds to be slightly more stretched with bond distances of 1.98 Å.The adsorption energy at this coverage is −97.1 kJ/mol, which is slightly less than the same coverage in config.1.Similar to config. 1, we also investigated a lower coverage of CO 2 at the surface in a (1 × 2) supercell (Fig. 10(d)).At this coverage, after CO 2 adsorption, surface rearrangement takes place where copper atoms Cu A and Cu B break their bonds with the O SUF atom to form new bonds to surface oxygen atoms nearby, as well as bind to both CO 2 oxygen atoms (d O-Cu = 1.85 Å).The carbon atom binds more strongly to surface atom O SUF (d C-OSUF = 1.32 Å) as the CO 2 angle changes to ∠CO 2 = 119.0• , and we noted that ∠O 1 -C-O SUF and ∠O 2 -C-O SUF are ∼120.0• .The adsorption energy increases to −232.6 kJ/mol (Table IV).Despite this large adsorption energy, Bader charge comparison (Table V) of the free CO 2 molecule with that in the adsorbed geometry shows that there is very little charge transfer, although large charge redistribution takes place among the surface atoms bonded to the molecule.O SUF atom gains 0.23e − charge

Cu 2 O(110) surface
a. (110):Cu.For this surface, we first considered a (1 × 1) unit cell and tried different initial configurations with several orientations of the CO 2 molecule, but we found only one configuration in which CO 2 binds to the surface.Here, CO 2 binds strongly (E ads = −100 kJ/mol) in a configuration where the molecule bends to bind with an oxygen atom in the second layer (d C-OSUF = 1.45 Å), while its oxygen atoms O1 and O2 bind to surface atoms, Cu A and Cu B , at 1.97 Å (Fig. 11(a)).We noted that the CO 2 molecule is activated with an angle of ∠CO 2 = 128.0• .From Fig. 11(a), we observe that the distance between the CO 2 molecule and its image in the x-direction is 4.3 Å, while in the y-direction it is only 3.78 Å.We therefore repeated the calculations of all the different configurations in (2 × 1) and (1 × 2) supercells.
Keeping the same input orientations, we first assessed the effect of a lower CO 2 coverage by placing one molecule in a (2 × 1) supercell and found that E ads increased to −105.0 kJ/mol, while in the (1 × 2) supercell, E ads increased to −116.7 kJ/mol.This increase in E ads was expected because of the small distance between the CO 2 molecule and its periodic image in the y-direction in the (1 × 1) cell.Because of the significant difference in E ads in the (1 × 2) supercell compared to the (2 × 1) supercell, we have limited our discussion only to the more favourable (1 × 2) supercell system.In the (1 × 2) supercell, the carbon atom of the molecule binds strongly to the surface oxygen atom (d C-OSUF = 1.42 Å), while Cu A -O1 and Cu B -O2 bond lengths reduce to 1.89 Å (Fig. 11(b)).We have given geometrical parameters of the adsorbed geometry of the (1 × 1) and (1 × 2) supercells in Table VI.Bader analysis of the (1 × 2) supercell (Table VII) shows charge transfer between the oxygen atoms of CO 2 and surface copper atoms.Oxygen atoms O1 and O2 gain 0.05 and 0.06e − , respectively, while both surface copper atoms Cu A and Cu B lose 0.12e − .There is a very small charge transfer to the carbon atom of the CO 2 molecule of ∼0.01e − .This amount of charge transfer is consistent with the charge transfer in the ( 111      ) is too small to study adsorption of the isolated molecule, as we discovered earlier that the interaction between neighbouring CO 2 molecules affects the geometry and adsorption energies.Hence, we carried out calculations on the (2 × 2) supercell to minimize the effect of interactions between periodic images.Similar to the other surfaces, we studied different sites at the surface for possible CO 2 adsorption.In the first adsorption configuration (config.1), oxygens O1 and O2 of the molecule bind to the nearest topmost copper atoms   X).Bader charge analysis (Table XI) shows significant charge transfer between the CO 2 molecule and the surface atoms.After CO 2 adsorption, both O1 and O2 oxygens of the CO 2 molecule lose charge density of 0.24e − and 0.22e − , respectively, while surface copper atoms Cu A and Cu B which bind to these two oxygen atoms lose charge density of 0.12 and 0.10e − , respectively.However, the carbon atom gains 0.   first configuration (Table X).The Bader charge analysis shows charge redistribution, with oxygen O1 losing 0.08e − charge density, while the Cu A and Cu B atoms also lose 0.25e − and 0.27e − , respectively.Oxygen atom O2 gains 0.07e − charge density after binding to surface copper atom Cu C , which loses 0.14e − .Charge transfer also occurs between surface oxygen atom, O SUF and the C atom which gains 0.22e − , while O SUF gains 0.04e − (Table XI).Here again, we note considerable changes in the vibrational frequencies of the adsorbed CO 2 molecule, as shown in Table XI.

C. CO 2 adsorption trends in copper oxides
We noted that the activation of the CO 2 molecule is related to the adsorption energy, as shown in Fig. 14(a), where we considered the molecule's angle as a measure of its activation. 50Furthermore, the adsorption energy, E ads , depends almost linearly on the surface stability (Fig. 14

IV. SUMMARY AND CONCLUSIONS
Using DFT+U methodology, we have studied the reconstructions of the (111), (110), and (001) surfaces and proposed different non-polar terminations.We further analysed the structural geometries, energetics, and electronic properties for the process of carbon dioxide adsorption to different stoichiometric Cu 2 O surfaces, at different coverages.While the CO 2 adsorption to stoichiometric Cu 2 O(111):O is weak, causing no significant changes to the geometry or electronic structure of the adsorbate, CO 2 adsorption to all other surfaces is energetically favourable.The (110):Cu surface, which is only less stable by ∼0.16 J/m 2 compared to the most stable (111):O surface, shows adsorption energies up to approximately −117 kJ/mol, while the third most stable (110):Cu-O surface exhibits strong chemisorption of the CO 2 molecule, releasing ∼170 kJ/mol.We found that CO 2 coverage affects the adsorption energy as E ads increases for all surfaces at lower coverage, where CO 2 is found to be chemisorbed as the CO 2 − anion.The Cu 2 O(111) surface with Cu termination is found to be the least stable surface and a detailed structural and Bader charge analysis shows that the CO 2 molecule affects the surface geometry, rearranging itself to resemble a [CO 3 ] − species on the surface.
Activation of carbon dioxide is the most important step in its conversion into valuable chemicals and large structural transformations and significant charge transfer between different surfaces and the CO 2 molecule demonstrate that Cu 2 O is capable of activating CO 2 .For all the bent CO 2 configurations, we note a significant red-shift on the C-O symmetric (υ s ) and asymmetric (υ as ) stretching modes relative to the linear gas phase molecule, indicating that the CO 2 molecule is considerably activated.It is worth noting, however, that our calculations are valid only at 0 K and only stoichiometric surfaces were considered in the present study.Nevertheless, the results presented in this paper provide fundamental mechanistic insights into CO 2 activation on stoichiometric (111), (110), and (001) surfaces, which will still be relevant to our general understanding of CO 2 adsorption by Cu 2 O, as the different surfaces studied here include a wide variety of the kind of surface sites, that can be expected to occur on experimental surfaces.

FIG. 1 .
FIG. 1.The Wulff morphology of Cu 2 O particle determined from calculated surface energies.

FIG. 2 .
FIG. 2. The Cu 2 O(111):O terminated relaxed surface side view (a) and top view (b).We have shown a (2 × 2) cell in side view with periodic images of atoms for clearer visualization of bonding in all surface figures.Blue and red balls indicate Cu and O atoms, respectively, in all figures.The bond length values are in Å.

FIG. 9 .
FIG. 9.The CO 2 molecule adsorbed on the Cu 2 O(111):O terminated surface.Black balls indicate C atom of CO 2 molecule, green balls indicate O atoms of the molecule, while blue and red balls denote the surface Cu and O atoms in all the figures.

044709- 9 Mishra,
FIG. 11.The CO 2 molecule adsorbed on the Cu 2 O(110):Cu terminated surface in the (a) (1 × 1) cell and in the (b) (1 × 2) supercell.density, while Cu A and Cu B both lose 0.16e − and 0.17e − in charge densities, respectively.This charge redistribution together with the change in the surface results in a CO 3 − like-species on the (111):Cu surface (Fig. 10(d)).Unstable surfaces are often highly reactive, which is exemplified by this behaviour of the (111):Cu surface.This strong activation of the CO 2 molecule is further confirmed by considerable changes in the vibrational modes of the adsorbed CO 2 molecule, where asymmetric stretch (υ as ), symmetric stretch (υ s ), and bending (υ b ) frequencies change to 1395, 1257, and 858 cm −1 , respectively, from their original values of 2355, 1316, and 632 cm −1 in the isolated gas phase molecule.
):Cu surface, where the molecule's oxygen gains ∼0.08e − and surface copper atoms lose charge of ∼0.15e − .Here also, frequencies for asymmetric stretch (υ as ), symmetric stretch (υ s ), and bending (υ b ) vibrations change to 1639, 1247, and 808 cm −1 , indicating activation of the CO 2 molecule on the (110):Cu surface.In all other configurations considered, the CO 2 molecule does not bind to the copper oxide surface.b. (110):Cu-O.Here again, we carried out calculations on a (2 × 1) supercell, exploring different configurations for CO 2 to bind with the surface.In the first configuration (config.1), after placing the CO 2 molecule parallel to the Cu-O-Cu linear bond in the top layer, we found that this bond breaks when Cu atoms move up to bind to oxygen atoms of the CO 2 molecule, while the carbon atom bends down to bind to the oxygen atom of the top surface layer (d C-OSUF = 1.36 Å), as shown in Fig. 12(a).One of the CO 2 oxygen atoms (O1) binds to one of the nearest Cu atoms (Cu A ) in the top layer with a bond distance d O1-CuA = 1.84 Å, while the second oxygen (O2) binds to another surface copper atom with a bond distance d O2-CuB = 1.86 Å, causing the Cu-O distances of Cu A and Cu B to their neighbouring surface oxygen atoms to change from 2.10 and 2.18 Å to 1.83 and 1.84 Å, respectively.The CO 2 molecule bends to an angle of ∠CO 2 = 123.6 • and adsorbs strongly with an adsorption

FIG. 12 .
FIG. 12.The CO 2 molecule adsorbed on the Cu 2 O(110):Cu-O terminated surface in the (2 × 1) cell in (a) config. 1 and (b) config.2.energy E ads = −138.1 kJ/mol.In this configuration, the C-O bond length is 1.27 Å, which is slightly longer than the normal bond length in a CO 2 molecule.The adsorption energy and geometrical parameters of the system are given in TableVIII.Bader charge analysis (TableIX) reveals that the O1 and O2 oxygen atoms of the CO 2 molecule gain 0.13 and 0.10e − in charge density, respectively.Meanwhile, the surface copper atoms, Cu A and Cu B , which are bound to the O1 and O2 lose a charge density of 0.49 and 0.39e − , respectively, while the surface oxygen atom (O SUF ) bound to the carbon atom of the CO 2 molecule gains 0.14e − .Hence, upon CO 2 adsorption, a large charge redistribution occurs at the surface.This strong adsorption and activation of the CO 2 molecule are reflected in changes in the vibrational frequencies, as listed in TableIX.In the second configuration (config.2) (Fig. 12(b)), the carbon atom binds to the O SUF atom (d C-OSUF = 1.35 Å) and O1 binds to Cu A (d O1-CuA = 1.85 Å) in the top layer.The CO 2 molecule bends so that O2 binds with Cu B of the second

3 .
layer (d O2-CuB = 1.86 Å), as shown in Fig.12(b).In this configuration, we found that CO 2 binds more strongly with an adsorption energy of about −170.0 kJ/mol.CO 2 bends to an angle of ∠CO 2 = 122.7 • which indicates stronger activation (TableVIII).Oxygen-carbon bond lengths, i.e., d C-O1 and d C-O2 , are found to be 1.27 Å and 1.30 Å, respectively.After CO 2 adsorption, the Cu-O bonds in the top of the surface change from 2.10 Å and 2.18 Å to 1.86 Å and 1.82 Å for Cu A and Cu B , respectively.Bader charge analysis (TableIX) shows that the oxygen atoms O1 and O2 of the CO 2 molecule gain −0.13 and −0.08e − , respectively, while the change in the carbon atom charge is negligible.The surface copper atom Cu A bonded to one of the oxygen atoms of the CO 2 molecule becomes more positively charged with a loss of 0.26e − , while the surface oxygen atom O SUF gains 0.23e − .Charge transfer to Cu B is negligible.This charge transfer, the adsorption energy, geometry changes together with large changes in vibrational frequencies (TableIX) indicates activation of the CO 2 molecule.Cu 2 O(001) surface a. (001):Cu.The (1 × 1) cell (a = 4.27 Å, b = 4.27 Å
56e − after binding to surface oxygen atom O SUF , which loses 0.12e − .Hence, charge transfer has occurred to the CO 2 molecule from nearby surface atoms.As shown in Table XI, we note considerable changes in the frequencies of the different vibrational modes (υ as = 1509, υ s = 1281, and υ b = 869 cm −1 ) of the activated CO 2 molecule on this surface.In another configuration (config.2) the CO 2 molecule remains almost parallel to the surface, as shown in Fig. 13(b).The carbon atom binds to a top oxygen atom (O SUF ) (d C-OSUF = 1.32 Å), while one oxygen (O1) of the CO 2 molecule binds to surface copper atoms Cu A and Cu B , with bond lengths 1.95 and 1.91 Å, respectively.The second oxygen atom (O2) of the molecule binds to another surface copper atom, Cu C with a bond length of 1.91 Å.In the adsorbed CO 2 molecule we find O-C bond lengths of 1.32 and 1.27 Å for oxygen atoms O1 and O2, respectively, and the CO 2 angle is ∠CO 2 = 121.7 • .The adsorption energy in this configuration is −98.7 kJ/mol, which is almost 40 kJ/mol less than in the (b)): The most stable (111):O surface shows weak adsorption (approximately −56 kJ/mol) and the CO 2 molecule remains linear, while the least stable (111):Cu surface shows the strongest binding (E ads = −233 kJ/mol) with the CO 2 molecule bent to 119.2 • .The second most stable surface, (110):Cu, shows an adsorption energy of approximately −117 kJ/mol, while the (001):Cu surface, binds the CO 2 molecule with the release of ∼138 kJ/mol, with the CO 2 molecule bending to 126.2 and 121.5• , respectively.This trend is similar to that found in CO 2 adsorption on CuO surfaces, where the most stable surface, (111), shows weak binding of the CO 2 molecule compared with other low index surfaces, which cause significant CO 2 activation.11 Reuse of AIP Publishing content is subject to the terms: https://publishing.aip.org/authors/rights-and-permissions.Downloaded to IP: 131.251.254.109On: Mon, 12

TABLE II .
The adsorption energies and the characteristic parameter values of the CO 2 adsorbed geometry in the (1 × 1) and the (2 × 2) supercell of the Cu 2 O(111):O surface.

TABLE III .
Vibrational frequencies (cm −1 ) and Bader charges (e − ) comparison of the atoms in the adsorbed CO 2 molecule and the Cu 2 O(111):O surface atoms bonded with the molecule to that of the atoms in the isolated CO 2 molecule and the bare surface in the (1 × 1) cell.Our calculated geometrical parameters of the adsorbed CO 2 molecule and the binding energies are in reasonable agreement with the recent work of Bendavid et al., where they used similar DFT(D)+U (6 eV) methodology and found ∠CO 2 to be 177.1 • and an adsorption energy of −36.4 kJ/mol.

TABLE IV .
The adsorption energies and the characteristic parameter values of the CO 2 adsorbed geometry in the (1 × 1) and the (1 × 2) supercell of the Cu 2 O(111):Cu surface in config. 1 and config.2.

TABLE V .
Vibrational frequencies (cm −1 ) and Bader charges (e − ) comparison of the atoms in the adsorbed CO 2 molecule and the Cu 2 O(111):Cu surface atoms bonded with the molecule in the (1 × 2) supercell to that of the atoms in the isolated CO 2 molecule and the bare surface in config. 1 and config.2.

TABLE VI .
The adsorption energies and the characteristic parameter values of the CO 2 adsorbed geometry in the (1 × 1) and the (1 × 2) supercell of the Cu 2 O(110):Cu surface.

TABLE VII .
Vibrational frequencies (cm −1 ) and Bader charges (e − ) comparison of the atoms in the adsorbed CO 2 molecule and the Cu 2 O(110):Cu surface atoms bonded with the molecule to that of in the isolated CO 2 molecule and the bare surface in the (1 × 2) supercell.

TABLE VIII .
The adsorption energies and the characteristic parameter values of the CO 2 adsorbed geometry in the (2 × 1) supercell of the Cu 2 O(110):Cu-O surface in config. 1 and config.2.

TABLE IX .
Vibrational frequencies (cm −1 ) and Bader charges (e − ) comparison of the atoms in the adsorbed CO 2 molecule and the Cu 2 O(110):Cu-O surface atoms bonded with the molecule to that of in the isolated CO 2 molecule and the bare surface in config. 1 and config. 2 of (2 × 1) supercell.

TABLE X .
The adsorption energies and the characteristic parameter values of the CO 2 adsorbed geometry in the (2 × 2) supercell of Cu 2 O(001):Cu surface in config. 1 and config.2.