CO2 Adsorption on Cu2O(111): A DFT+U and DFT-D Study

Article pubs.acs.org/JPCC

CO2 Adsorption on Cu2O(111): A DFT+U and DFT‑D Study Leah Isseroff Bendavid† and Emily A. Carter‡,* Departments of †Chemical and Biological Engineering and ‡Mechanical and Aerospace Engineering, Program in Applied and Computational Mathematics, and Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544-5263, United States S Supporting Information *

ABSTRACT: Adsorption of CO2 on the Cu2O(111) surface is investigated using density functional theory + U with and without dispersion corrections. A number of adsorbate geometries are considered on four different surface terminations that include the bulkterminated surface and surfaces with oxygen and/or copper vacancies. CO2 is found to adsorb most strongly as a tilted linear molecule coordinated to an unsaturated surface cation. Surface vacancies allow for bent adsorbate configurations to be accessed but they are all less stable than the linear adsorbate. Bader analysis confirms that adsorption of bent CO2 is accompanied by charge transfer from the surface to the molecule, whereas minimal charge transfer occurs in linear physisorption. We show that surface oxygen vacancies have a small impact on adsorption free energies, while surface copper vacancies result in a significant reduction of CO2 adsorption. Including dispersion corrections increases the stability of adsorbed CO2, but adsorption is mostly endoergic at 298.15 K and 1 atm. Our findings reveal that adsorption of CO2 on cuprous oxide is contingent on the presence of copper cations at the surface. Thus, this surface’s chemistry is dominated by Lewis acidity, in contrast to other oxide surfaces where oxygen anions may act as Lewis bases to form carbonates. The suppression of carbonate formation is important, as it confirms that this surface may be useful for CO2 reduction to fuels.

1. INTRODUCTION Photocatalytic conversion of CO2 into fuels is attractive as a means of reducing CO2 emissions and as a renewable energy resource. While numerous potential materials are being evaluated for their photocatalytic activity, cuprous oxide (Cu2O) is a particularly attractive photocatalyst for the conversion of CO2 to methanol because of its nontoxicity, abundance, and high absorption coefficient over a substantial portion of the solar spectrum. Cu2O has been employed successfully in the electrochemical reduction of CO2 to methanol1,2 and in hybrid CuO-Cu2O nanoarrays in the photoelectrosynthesis of methanol from CO2.3 However, it is still a nascent photocatalyst for CO2 conversion and its catalytic mechanism is not well understood. To clarify the catalytic mechanism and subsequently optimize the photocatalytic potential of Cu2O in CO 2 conversion, we first require a fundamental understanding of the interactions at the start of this reaction, namely, the interaction between CO2 and the Cu2O surface. Knowledge of the energetics and adsorbate geometries associated with CO2 adsorption may shed light on the catalytic mechanism and also assist in evaluating the availability of CO2 for reaction on the catalytically active oxide surface. Only one experimental study analyzed the interaction between CO2 and Cu2O. Abee used thermal desorption spectroscopy (TDS) and X-ray photoemission spectroscopy under ultrahigh vacuum (UHV) to study CO2 adsorption on Cu2O(111).4 He observed that CO2 uptake was very small and did not detect any adsorbed carbonate, indicating that the primary method of CO2 adsorption is via © 2013 American Chemical Society

physisorption of linear molecules. However, no precise bonding geometry was identified, and Abee did not measure adsorption energies. CO2 adsorption can also be studied with density functional theory (DFT), which enables the computation of adsorption energetics and adsorbate geometries. Using DFT with the B3LYP functional and a point-charge-embedded cluster model, Wu et al. identified two stable adsorbate geometries on Cu2O(111): a linear molecule adsorbed perpendicular to the surface and a slightly bent molecule adsorbed at an angle to the surface.5 They calculated adsorption energies of −21.1 kcal/ mol (−16.7 kcal/mol after correction for basis set superposition error (BSSE)) for the perpendicular geometry and −20.6 kcal/ mol (−11.9 kcal/mol after BSSE correction) for the tilted geometry. These high adsorption energies are far above the typical range for physisorption, which suggests potential problems with their model. In a recent study, Wu et al. used DFT and the generalized gradient approximation to exchangecorrelation parametrized by Perdew-Burke-Ernzerhof (PBE) to study CO2 adsorption at 1/4 ML (monolayer) coverage on a periodic slab model of the Cu2O(111) surface.6 They identified the most stable adsorbate geometry on the nondefective surface as a linear molecule adsorbed tilted to the surface with its oxygen atom coordinated to a coordinatively unsaturated surface copper atom. The adsorption energy for this Received: July 26, 2013 Revised: November 10, 2013 Published: November 19, 2013 26048

dx.doi.org/10.1021/jp407468t | J. Phys. Chem. C 2013, 117, 26048−26059

The Journal of Physical Chemistry C

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configuration was −6.4 kcal/mol, which was reduced to −3.0 kcal/mol when an oxygen vacancy was introduced to the surface. The oxygen vacancy also allows for a bent CO−2 anion to adsorb at the vacancy site, with a low adsorption energy of −0.05 kcal/mol. Their use of the PBE functional is problematic in two ways: (i) PBE poorly describes the electronic structure of Cu2O (the band gap is severely underestimated)7 and (ii) it neglects dispersion interactions, which may significantly influence CO2 adsorption. Additionally, the authors calculated only electronic energies, and did not account for zero point energies in the evaluation of adsorption energies, nor did they consider (important, vide inf ra) free energy corrections. Finally, their study did not consider the effect of other defects at the surface that may also play a role in aiding or inhibiting CO2 adsorption. Due to the limited characterization heretofore, we revisit the analysis of CO2 adsorption on Cu2O. We focus on adsorption on the nonpolar Cu2O(111) surface, and consider a larger number of adsorbate geometries to examine the nature of the adsorbed species and to evaluate its stability. There are a number of manners in which CO2 may adsorb on this surface. Chemisorption of CO2 on the clean bulk-terminated Cu2O(111) surface can be described within Lewis acid-base theory, where the coordinatively unsaturated Cu atoms (CuCUS) can be considered Lewis acid sites and the coordinatively unsaturated O atoms (OCUS) Lewis base sites (shown in Figure 1).8,9 CO2

ideal and reconstructed Cu2O(111) surfaces.15 The first model is the one discussed above, where the ideal surface is bulkterminated and the reconstructed surface only differs by having ordered oxygen vacancies in 1/3 of the terminating ML. However, a second possible model exists, where these two surface structures lack the coordinatively unsaturated copper ions in the surface copper layer. These copper atoms have previously been identified as the most reactive sites of the Cu2O(111) surface,16−21 and their absence might have a significant impact on the ability of the surface to adsorb CO2. We therefore also consider these two models of the ideal and reconstructed surface and examine the role of these surface copper vacancies on adsorption. We apply the DFT+U method22 to optimize adsorbate geometries and calculate adsorption energies, as the correction to the self-interaction error of the highly localized copper d-orbitals is needed to generate more accurate geometric and electronic structures.7 We show that the +U parameter is necessary via comparison to a DFT-derived adsorption energy. We also introduce semiempirical dispersion corrections via the DFT-D2 method of Grimme,23,24 as dispersion interactions may be important when characterizing the CO2−Cu2O(111) interactions. The paper is organized as follows: section 2 describes the computational methods used, section 3 discusses the results of adsorption on each of the four surfaces considered, and we summarize our conclusions in section 4.

2. COMPUTATIONAL METHODS The Vienna Ab-initio Simulation Package25 was used to perform all calculations. Based on previous tests we have performed to ascertain the most accurate means to model bulk Cu2O,7 we employ planewave DFT+U22 with the PBE26,27 exchange-correlation functional (henceforth denoted PBE+U). We use the formalism of Dudarev et al.22 for DFT+U. A U−J value of 6 eV was used for Cu, which was validated in previous calculations on bulk Cu2O.7 Nuclei and core electrons (an [Ar] core for Cu and a [He] core for O) were replaced by potentials according to Blö chl’s all-electron, frozen-core, projector augmented wave (PAW) method.28 All PAW potentials were obtained from the VASP repository, and the standard PAW potential was used for oxygen. Because we expect that long-range dispersion interactions will play a role in CO2 adsorption on the Cu2O(111) surface, we also employ the DFT-D2 method of Grimme23,24 to introduce corrections for dispersion interactions within the PBE+U calculations (denoted henceforth as PBE+U-D2). The DFT-D2 correction term is a semiempirical dispersion potential expressed as a pairwise force field summed over interatomic C6R−6 contributions. The potential is modulated by a damping factor to minimize contributions within typical bonding distances. We used the default values in VASP for the atomic dispersion coefficients, atomic radii, and scaling parameters. We used the two experimentally characterized structures of the (111) surface: the ideal O-terminated surface with (1 × 1) periodicity and the surface with (√3 × √3)R30° periodicity and 2/3 of the terminating oxygen ML.29 We consider both models for these surfaces by including or omitting the surface CuCUS atoms in each structure. CO2 adsorption was first modeled on the Cu2O(111) ideal bulk-terminated (1 × 1) surface, where the surface lattice vectors are fixed to the PBE+U bulk-optimized lengths of u = v = 6.065 Å with an angle of 120° between them. The sensitivity of the adsorption free energy with respect to coverage was tested by studying adsorbate

Figure 1. Ideal bulk-terminated Cu2O(111) surface (unrelaxed), shown here with 2 × 2 periodicity. Only the surface trilayer is shown, which consists of a layer of copper atoms sandwiched by two layers of oxygen atoms. CuCUS and OCUS are the coordinatively unsaturated surface ions. Blue and red spheres represent copper and oxygen atoms, respectively.

chemisorption is therefore a probe of oxide surface basicity,10−13 as the electropositive carbon atom may draw charge from a basic surface oxide ion to form a chemisorbed anion with a bent geometry. This adsorption mechanism can also be modeled as the addition of an oxide ion to CO2 to form an adsorbed carbonate ion, which may adsorb on the Cu2O(111) surface as a monodentate or bidentate complex. Other modes of CO2 adsorption also exist, as CO2 may chemisorb to a metal site on the surface as a bent anion or may physisorb as an unperturbed linear molecule.14 We undertake this study using a fully relaxed, periodic slab model of the surface. We study adsorption on the ideal surface, but also examine the role of surface oxygen vacancies by modeling CO2 adsorption on the reconstructed Cu2O(111) surface. Additionally, there are two proposed models for the 26049



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both sides of the slab; see Supporting Information for more details). The entropy and enthalpic temperature corrections for the free CO2 molecule were determined according to statistical mechanics, where each quantity is defined as a function of the molecular partition function (see Supporting Information for a more detailed explanation of these calculations).30,31 The molecular partition function can be expressed as the product of the electronic, rotational, vibrational, and translational partition functions. If we assume that all excited states are inaccessible and set the ground state energy to zero, then the electronic partition function is given by the degeneracy of the ground state, which for a singlet is 1. The rotational, vibrational, and translational partition functions were derived using the rigidrotor, harmonic oscillator, and ideal gas approximations, respectively.31 The vibrational modes for the harmonic oscillator approximation were obtained by diagonalizing the numerical Hessian, calculated using finite differences of analytic gradients and displacements of 0.01 Å away from equilibrium. These vibrational frequencies were also used to calculate the ZPE correction of the free molecule. The ZPE, enthalpic, and entropic corrections for the free CO2 molecule are reported in Table 1.

coverages of 1 and 1/4 ML on the ideal surface. One ML is defined by saturation of surface oxygen anions (although some adsorbate geometries may be coordinated to surface copper cations). A surface periodicity of 1 × 1 was used to model the 1 ML adsorbate coverage, whereas a periodicity of 2 × 2 was used to model the 1/4 ML coverage. CO2 adsorption was also modeled on the (1 × 1) surface with CuCUS vacancies, here using only a surface periodicity of 1 × 1. Finally, we studied adsorption on the reconstructed (√3 × √3)R30° (111) surface, where u = v = 10.469 Å and 1/3 of the terminating oxygen ML was removed. Surfaces both with and without CuCUS atoms were considered. Adsorption of one CO2 molecule on the reconstructed surface corresponds to a coverage of 1/3 ML on a pristine surface with the same surface area. Five trilayers were used in each slab, which converges the surface energy to within 3 mJ/m2. A trilayer is defined in the ideal 1 × 1 slab as one layer consisting of four copper atoms sandwiched by two layers of one oxygen atom each. A total of 16 Å of vacuum between surfaces was included to prevent interactions between periodic images of the adsorbates and surfaces. Adsorption of one CO2 molecule was modeled on both top and bottom surfaces of the slab to retain inversion symmetry and avoid the creation of a dipole. The adsorption energy of a single CO2 molecule is defined as Eads = ECO2 /Cu 2O − ECO2 − ECu 2O + ΔZPE

Table 1. Contributions to the Molecular (CO2) and Adsorbate (CO2* Structures) Free Energies from the ZPE, Enthalpic Temperature Correction, and Entropy at 298.15 K and 1 atma

(1)

where ECO2/Cu2O is the total energy of one CO2 molecule adsorbed on the Cu2O slab, ECO2 is the total energy of one isolated CO2 molecule, and ECu2O is the total energy of the clean Cu2O slab. The final term is the difference in the zero point energies (ZPE) of the three systems, defined as ΔZPE = ZPECO2 /Cu 2O − ZPECO2 − ZPECu 2O

PBE+U

(2)

Because our model includes a CO2 molecule adsorbed on both top and bottom surfaces to prevent dipole formation, we instead define the adsorption energy for our model as Eads =

[E2CO2 /Cu 2O − 2ECO2 − ECu 2O] 2

+ ΔZPE

(3)

where E2CO2/Cu2O is the total energy of two CO2 molecules adsorbed on the Cu2O slab, while ΔZPE is still defined for one adsorbed CO2 molecule (albeit within a model including an adsorbed CO2 on both sides of the slab; see Supporting Information for a description as to how ΔZPE is calculated on the basis of one molecule). By including the entropic and enthalpic temperature corrections, we can define the adsorption free energy of a single CO2 molecule as Gads =

TO

CpdT ] − T ΔS)

∫ CpdT

−TS

ZPE

∫ CpdT

−TS

CO2 CO*2,∠ (1 ML) CO*2,∠ (1/4 ML) CO*2,VCu‑tri

7.0 8.7 7.5 8.6

2.3 2.0 2.7 1.5

−15.2 −5.4 −7.7 −2.4

7.0 8.7 7.4 8.6

2.3 2.0 2.8 1.5

−15.2 −5.2 −7.7 −2.3

CO*2,∠CuCUS,1

7.4

2.7

−7.1

7.4

2.7

−7.5

CO*2,∠CuCUS,2

7.4

2.7

−8.1

7.4

2.7

−7.4

CO*2,rec‑bi CO*2,rec‑tri CO*2,rec/CuCUS‑bi1

7.6 6.7 6.4

1.9 2.2 2.6

−4.1 −4.5 −6.2

7.6 6.8 6.4

1.9 2.2 2.6

−4.6 −4.4 −6.4

CO*2,rec/CuCUS‑bi2

6.9

2.4

−7.2

6.8

2.4

−7.6

Contributions are calculated with PBE+U or PBE+U-D2, using the structures optimized with the corresponding method. The contribu* (1 ML) were the same as those tions calculated with PBE for CO2,∠ calculated with PBE+U, and so are not listed separately. The CO2 adsorbate geometries are described in section 3. All values are given in kcal/mol.

The contribution of translational motion to the entropy of the free CO2 molecule is logarithmically dependent on pressure (see Supporting Information for the derivation). For the free CO2 molecule, we find that the entropic contribution to the free energy at 298.15 K in kcal/mol follows TS = −0.591 ln(P) + 15.2, where P is the partial pressure of CO2 in atm. We use a pressure of 1 atm in our calculations to report adsorption free energies at standard states. However, direct comparison to experiment necessitates using a partial pressure of CO2 that corresponds to UHV, which is ∼10−12 atm. The decrease in CO2 pressure from 1 to 10−12 atm at 298.15 K results in increased endoergicity of adsorption (by 16.3 kcal/mol). Entropic and enthalpic temperature corrections for the adsorbed molecule and clean surface arise only from vibrational

2

∫0

ZPE

a

[E2CO2 /Cu 2O − 2ECO2 − ECu 2O] + (ΔZPE + Δ[

PBE+U-D2

structure

(4)

The final two terms in the equation are the contributions to the free energy difference from the enthalpic temperature correction (where the heat capacity is integrated from 0 K to T0 = 298.15 K), and the entropic correction. The corrective terms to the free energy are all defined for one adsorbed CO2 molecule (also within a model including an adsorbed CO2 on 26050



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motion. These corrections and ZPEs were calculated from vibrational frequencies obtained via normal-mode analysis (see Supporting Information for a more detailed description). We calculated the numerical Hessian using a finite difference approximation with a displacement size of 0.01 Å, where the atoms in the adsorbed CO2 molecule and all substrate atoms within 4 Å of the adsorbed molecule were individually displaced in each Cartesian direction. The changes to the vibrational modes of the substrate upon CO2 adsorption are expected to be minimal and primarily localized to changes in the surface atoms near the adsorbate. This justifies our inclusion of only these surface atoms in our derivation of the vibrational frequencies. However, because we include only a subset of vibrational frequencies when evaluating the free energy corrections, the values we obtain for the clean and adsorbed slab no longer have meaning as total energies, and only have meaning as their relative difference. We therefore define their relative difference as the contributions to the entropy, enthalpy, and ZPE of the adsorbed molecule (which we denote CO2*), calculated as the difference between the adsorbed molecule plus substrate and the clean substrate. For example, the ZPE contribution associated with CO*2 is ZPECO*2 = ZPECO2 /Cu 2O − ZPECu 2O

Table 2. Total Free Energy Correction (kcal/mol) from ZPE, Enthalpic Temperature, and Entropy Contributions for CO2 Adsorption for Various Adsorbate Geometries and Surface Structures at 298.15 K and 1 atma T

ΔZPE + Δ[∫ 0 0CpdT] − TΔS PBE+U-D2

11.2 8.4 13.7

11.3 8.4 13.7

CO*2,∠CuCUS,1

9.0

8.5

CO*2,∠CuCUS,2

8.0

8.5

CO2,rec‑bi * CO2,rec‑tri * CO*2,rec/CuCUS‑bi1

11.3 10.5 8.7

10.9 10.5 8.6

CO*2,rec/CuCUS‑bi2

8.0

7.6

a

Contributions are calculated with PBE+U or PBE+U-D2, using the structures optimized with the corresponding method. The contribu* (1 ML) were the same as those tions calculated with PBE for CO2,∠ calculated with PBE+U, and so are not listed separately. The CO2 adsorbate geometries are described in section 3.

(5)

176 bands and a Γ-point-centered k-point mesh of 4 × 4 × 1, whereas calculations on the ideal surface with lower coverage used 640 bands and a Γ-point-centered k-point mesh of 2 × 2 × 1. A total of 480 bands and a Γ-point-centered k-point mesh of 2 × 2 × 1 were used for calculations on the reconstructed surface. The k-point meshes were derived from the converged parameters from bulk Cu2O calculations, where the total energy was converged to within 1 meV/atom.7 Calculations for the isolated CO2 molecule were performed in a cubic cell with sides of 15 Å and used 16 bands.

We can now express ΔZPE as ΔZPE = ZPECO*2 − ZPECO2

PBE+U CO2,∠ * (1 ML) CO*2,∠ (1/4 ML) CO*2,VCu‑tri

(6)

We can similarly define entropic and enthalpic corrections for CO*2 . This formalism allows us to include only a subset of atoms of the slabs in our vibrational frequency calculation without totally neglecting enthalpic, entropic, and ZPE contributions from the slabs. The energy corrections of the substrate are now incorporated in the values for the adsorbed molecule, as the changes in the vibrational modes of the substrate contribute to the evaluation of these corrections. However, we expect that the contributions from the substrate will be minimal in comparison to that of the adsorbed molecule. In Table 1 we report ZPE, enthalpic, and entropic corrections for the adsorbed CO2 molecule, calculated for each stable adsorbate geometry (vide infra) with PBE+U and PBE +U-D2. For CO2 adsorption at 1 ML on the ideal (111) surface (CO2,∠ * ), we had also calculated the adsorption free energy with DFT-PBE to evaluate the effect of the +U parameter. The PBEderived free-energy contributions in this case were the same as those calculated with PBE+U, and so these values are not reported separately in Table 1. From Table 1, it is evident that the enthalpic contribution to the adsorption free energy is quite small, while the largest change results from differences in entropy. The total contribution to the adsorption free energy for each adsorbate geometry is shown in Table 2. The free energy corrections derived with PBE+U and PBE+U-D2 differ by less than 0.6 kcal/mol. Each initial structure was fully relaxed, with all atomic positions optimized with a force threshold of 0.03 eV Å−1. All calculations used a planewave kinetic energy cutoff of 700 eV, which converges the total energy to within 1 meV/atom. For slab structural relaxations, the Brillouin zone was integrated using Gaussian smearing with a width of 0.01 eV, but all final total energies were calculated using the tetrahedron method with Blöchl corrections.32 Because the calculation for molecular CO2 sampled only one k-point at the Γ point, there we used Gaussian smearing with a smearing width of 0.05 eV. Calculations on the (1 × 1) surface with 1 ML coverage used

3. RESULTS AND DISCUSSION Our main objective is to identify the stable adsorbate geometries on the Cu2O(111) surface. As such, we have examined a number of possible configurations for adsorption on the ideal O-terminated surface, the (1 × 1) surface with CuCUS vacancies, the reconstructed surface with 1/3 of the terminating oxygen ML removed, and this reconstructed surface also with CuCUS vacancies. We first discuss the results obtained for the (1 × 1) surface, both with and without CuCUS vacancies, followed by analysis of the results for the reconstructed surface. 3.1. CO2 Adsorption on Ideal Cu2O(111). Eight adsorption geometries were considered for CO2 on the ideal Cu2O(111) surface (Figure 2). The monodentate species, CO2,mono, is formed by the coordination of the carbon atom to a coordinatively unsaturated surface oxygen anion, OCUS. The bidentate species, CO2,bi, differs from CO2,mono by the additional coordination of a CO2 oxygen atom to a coordinatively unsaturated surface copper cation, CuCUS. The third bent adsorbate geometry, CO2,bent, is characterized by chemisorption to a surface metal site. Various adsorbate geometries for the linear molecule were also studied, including geometries perpendicular to the surface above CuCUS (CO2,⊥), parallel to the surface and centered over a surface oxygen (CO2,∥‑O), parallel to the surface and centered over a surface copper (CO2,∥‑Cu), parallel to the surface and extended over two copper atoms (CO2,∥‑Cu/Cu), and tilted to the surface above CuCUS (CO2,∠). The geometries described and shown in Figure 2 are all initial structures constructed prior to structural 26051



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Figure 2. Adsorbate geometries considered for CO2 on the ideal Cu2O(111) surface, presented as initial structures prior to relaxation: (a) the monodentate species, (b) the bidentate species, (c) a bent geometry adsorbed to a metal site, (d) adsorbed perpendicularly to the surface, (e, f) adsorbed parallel to the surface and centered over an anion or cation, (g) parallel to the surface and centered between two cations, and (h) tilted above an unsaturated cation. Shown here are the slabs used for 1/4 ML coverage, although the same initial geometries were used for higher coverage as well. Side and top views of each geometry are displayed. Red, blue, and gray spheres represent oxygen, copper, and carbon, respectively. Green spheres are used to differentiate the oxygen atoms of the CO2 molecule.

optimization, and therefore do not necessarily represent stable species. Following the full PBE+U optimization of each of the initial structures shown in Figure 2, all structures either relaxed to one unique adsorbate geometry or resulted in complete desorption of CO2 from the surface. CO2,∠ was identified as the only stable adsorbate geometry for both 1 and 1/4 ML coverages, and these CO2,∠ structures were also subsequently optimized with PBE+U-D2. The 1 ML structure was also optimized with PBE (without U) as a basis of comparison to evaluate the effect of the +U potential. The stability of these adsorbate geometries was verified by confirming that all vibrational frequencies were real. The equilibrium CO2,∠ structure is similar to the tilted geometry predicted in the earlier B3LYP embedded cluster study,5 albeit more linear by ∼6°. It also agrees well with the tilted geometry predicted by the earlier DFT-PBE study.6 We have shown that the other structures identified in the B3LYP study are not stable, highlighting deficiencies in the embedded cluster model’s ability to accurately represent the Cu2O(111) surface. On the other hand, their study did not verify all structures as stable minima via vibrational frequency analysis, and it is possible that the geometries they describe are in fact unstable saddle points. The equilibrium adsorbate geometry and its configurational parameters are shown in Figure 3. Table 3 gives the values of these parameters in the equilibrium configurations at both coverages for PBE+U and PBE+U-D2 structures and at 1 ML for our PBE structure. The adsorbed molecules in all structures are nearly linear, which is characteristic of physisorption. The other distinguishing parameters show that CO2 adsorbs at an angle to the surface with its coordinating oxygen slightly offset above the CuCUS to which it coordinates, likely due to the position of the oxygen lone pair and the nature of the weak donor/acceptor bonding. The coordinating oxygen is least offset above the CuCUS for the structure optimized with PBE. Other parameter values in all structures differ only slightly. Dispersion corrections shorten

Figure 3. Equilibrium adsorbate geometry of CO2 on the ideal Cu2O(111) surface. Important configurational parameters are labeled in the diagram: ∠O−C−O is the molecular bond angle, ∠C−O−s is the angle of the molecule from the surface plane, ∠O−Cu−v is the angle of the Cu−O bond from the vertical, as the molecular oxygen is not positioned directly above the CuCUS to which it coordinates, and dO−Cu is the Cu−O bond length.

dO−Cu by 0.06−0.09 Å, slightly decrease the O−C−O bond angle, and bring the molecule closer to the surface. The adsorption energies and free energies were calculated according to eqs 3 and 4 (Table 4). The difference between adsorption energies at 1 and 1/4 ML within PBE+U and PBE +U-D2 suggests the presence of some repulsive interactions between adsorbate molecules. The relatively low adsorption energies are characteristic of physisorption, in contrast with the spuriously high adsorption energies reported in the B3LYP cluster study.5 (Their high adsorption energies may have been an artifact of using a metastable structure for the clean surface, which would artificially shift the adsorption energy to greater exothermicity. It is crucial to use the true equilibrium model of the clean surface to calculate realistic adsorption energies, and we have shown in other work that the equilibrium surface 26052



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Table 3. Characteristic Parameter Values (Labeled in Figure 3) for the Stable CO2 Adsorbate Geometries at 1 and 1/4 ML on the Ideal Cu2O(111) Surface, Calculated with PBE+U and PBE+U-D2, in Comparison to Earlier PBE Results for 1/4 ML Coverage and Our PBE Results at 1 ML Coverage method

structure

a

CO2,∠ CO2,∠ CO2,∠ CO2,∠ CO2,∠ CO2,∠

PBE PBE PBE+U PBE+U-D2 a

(1/4 ML) (1 ML) (1 ML) (1/4 ML) (1 ML) (1/4 ML)

∠O−C−O (deg)

∠C−O−s (deg)

∠O−Cu−v (deg)

dO−Cu (Å)

176.2 177.6 178.0 178.7 177.1 178.1

35.8 31.8 35.0 25.0 28.5

5.0 12.4 14.8 12.6 13.0

2.07 2.085 2.100 2.181 2.041 2.090

Ref 6.

Table 4. PBE+U and PBE+U-D2 Adsorption and Free Energies (298.15 K, 1 atm) for CO2 Adsorbed at 1 and 1/4 ML on the Ideal Cu2O(111) Surface, in Comparison to Earlier PBE Results for 1/4 ML Coverage and Our PBE Results at 1 ML Coverage method PBEa PBE PBE+U PBE+U-D2 a

structure CO2,∠ CO2,∠ CO2,∠ CO2,∠ CO2,∠ CO2,∠

(1/4 ML) (1 ML) (1 ML) (1/4 ML) (1 ML) (1/4 ML)

Eads (kcal/mol)

Gads (kcal/mol)

−6.4 21.6 −1.9 −6.7 −8.7 −9.2

31.2 7.7 1.2 1.0 −1.1

Table 5. Bader Charges (e) of the Atoms in the CO2 Molecule (C, O1, and O2) and the CuCUS to Which the O1 Coordinates for the CO2,∠ Adsorbate Geometry, at 1 and 1/4 MLa adsorbed CO2 (1 ML) adsorbed CO2 (1/4 ML) CO2 molecule clean Cu2O(111)

C

O1

O2

CuCUS

+2.15 +2.13 +2.08

−1.10 −1.10 −1.04

−1.06 −1.04 −1.04

+0.50 +0.47 +0.37

a

Bader charges of the atoms in the adsorbed configuration are compared to the Bader charges of the atoms in the isolated CO2 molecule and of the CuCUS in the clean ideal Cu2O(111) surface. All charges were derived from the structures optimized with PBE+U.

Ref 6.

physically unstable.6 Our observed changes in atomic charge are certainly not indicative of a complete transfer of an electron to the molecule and confirm that the character of adsorption is predominantly physisorption. 3.2. CO2 Adsorption on Ideal Cu2O(111) with CuCUS Vacancies. The other possible model for the (1 × 1) Cu2O(111) surface is where all CuCUS sites are vacant. Because of the copper vacancies, the slab becomes doped slightly p-type, where the excess positive charge is primarily delocalized among the copper atoms at the surface. This is evident through Bader analysis, where the surface copper atoms have Bader charges of +0.60e in comparison with the bulk charge of +0.51e. The lack of unsaturated surface coppers and increased positive charge on the surface will likely impact the surface chemistry. In the previous section, the CuCUS atom was identified to be crucial in the adsorption of CO2, as there was no stable adsorbate geometry that did not involve coordination to this atom. The lack of CuCUS atoms in this model therefore suggests that CO2 adsorption might be unfavorable on this new Cu2O(111) surface. We have already ruled out many of the adsorbate geometries on the unsaturated surface oxygen atoms for the ideal bulk-terminated surface, and because the Bader charges of the unsaturated oxygen atoms in this surface differ by only 0.01e from the charges in the bulk-terminated surface,33 we can assume that those adsorbate geometries would be similarly unstable on this defective surface. Consequently, here we study only new configurations (Figure 4), including those that involve the copper vacancy. The first configuration is a tridentate species (CO2,VCu‑tri) adsorbed within the vacancy, where the carbon atom is coordinated to the subsurface unsaturated oxygen atom and the molecular oxygen atoms are coordinated to two surface copper atoms. We also consider a linear molecule physisorbed to saturated copper atoms at the surface (CO2,VCu‑Cu), a linear molecule within the vacancy and coordinated only to a subsurface copper atom (CO2,VCu‑Cusub),

structure for Cu2O(111) is easily misidentified.33) Our PBE+U adsorption energy for 1/4 ML agrees well with the calculated adsorption energy of −6.4 kcal/mol for the tilted geometry in the earlier DFT-PBE study. However, our PBE adsorption energy at 1 ML exhibits a significant and physically inappropriate increase in endothermicity, illustrating the importance of the +U potential when evaluating these adsorption energies. While the PBE+U adsorption energies are exothermic, implying that CO2 adsorption should be favorable, the inclusion of the free energy corrections results in adsorption free energies that are endoergic at 298.15 K. This implies that, as should be expected, the entropic losses are significant for gas phase adsorption at this temperature. The adsorption free energy for 1/4 ML coverage is exoergic when dispersion corrections are included, but this value would become endoergic at CO2 pressures corresponding to UHV. The endoergicity at room temperature explains why the CO2 desorption feature in thermal desorption spectroscopy is at a relatively low temperature (∼175 K).4 Adsorption would be more favorable at lower temperatures or higher gas pressures. To better understand the mechanism of adsorption, we used the Bader method34 to analyze the extent of charge transfer between the molecule and the oxide surface. Bader charges were calculated for all atoms in the clean surface, the isolated CO2 molecule, and the molecule adsorbed on the surface at both coverages. We report charges from only the PBE+U structures, as the structures and charges from PBE+U-D2 did not differ significantly. Table 5 presents the charges of the relevant atoms for adsorption, namely, the atoms in the CO2 molecule and the CuCUS upon which it adsorbs. The charges on the atoms do not change significantly upon adsorption, with a slight polarization evident in the CO2 molecule and a small loss of electron charge on CuCUS. This is in contrast with the significant polarization of the molecule reported via Bader charges in the earlier DFT-PBE study, which would seem to be 26053



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Figure 4. Adsorbate geometries considered for CO2 on the (1 × 1) Cu2O(111) surface with CuCUS vacancies, presented as initial structures prior to relaxation: (a) the tridentate species, (b) linearly adsorbed on a surface copper atom, (c) linearly adsorbed on a subsurface copper atom, and (d) linearly adsorbed between subsurface and surface copper atoms. Side and top views of each geometry are displayed. Red, blue, and gray spheres represent oxygen, copper, and carbon, respectively. Green spheres are used to differentiate the oxygen atoms of the CO2 molecule.

and a linear molecule within the vacancy and coordinated to both a surface and subsurface copper atom (CO2,VCu‑Cu/Cusub). Of the four adsorbate geometries considered here, only CO2,VCu‑tri was found to be stable, as the CO2 molecule desorbed from the surface when relaxing from the other initial structures. We verified its stability by confirming that all vibrational frequencies were real. The adsorbed molecule is bent with an O−C−O bond angle of 123.8° (123.9° with PBE +U-D2). The carbon atom is coordinated to a subsurface oxygen atom with a C−O bond length of 1.422 Å (1.424 Å with PBE+U-D2), and the two molecular oxygen atoms are coordinated to surface copper atoms with O−Cu bond lengths of 1.979 Å (1.976 with PBE+U-D2). Because of the cell’s periodicity, the molecular oxygen atoms are both coordinated to the same surface copper atom from opposite sides. It is evident that including dispersion corrections did not significantly modify the equilibrium adsorbate structure. The adsorption energy and free energies calculated with PBE +U and PBE+U-D2 are shown in Table 6. The adsorption free

Table 7. Bader Charges (e) of the Atoms in the CO2 Molecule (C, O1, and O2), the Surface Copper Atom to Which the Molecular Oxygen Atoms Coordinate, and the Subsurface Oxygen (Osub) to Which the Carbon Coordinates for the CO2,VCu‑tri Adsorbate Geometry on the (1 × 1) Cu2O(111) Surface with Copper Vacanciesa

adsorbate geometry

Eads (kcal/mol)

Gads (kcal/mol)

CO2,VCu‑tri

10.5

22.6

PBE+U-D2

CO2,VCu‑tri

−3.4

8.8

method

O1

O2

Cu

Osub

+2.10

−1.14

−1.14

+1.07

−1.05

CO2 molecule clean Cu2O(111)

+2.08

−1.04

−1.04 +0.60

−0.96

a

Bader charges of the atoms in the adsorbed configuration are compared to the Bader charges of the atoms in the isolated CO2 molecule and of the relevant atoms in the (1 × 1) Cu2O(111) surface with copper vacancies. All charges were derived from the structures optimized with PBE+U.

the endothermicity of adsorption. The increase in negative charge on the molecular oxygen atoms is a result of some charge transfer from the surface copper atom, which becomes significantly more positively charged upon adsorption. Because we are studying adsorption at high coverage (1 ML), this single carbon atom is coordinated to two oxygen atoms because of the periodicity of the cell. At lower levels of coverage, each surface copper would only be coordinated to a single molecular oxygen atom, and the difference in its charge will likely be less significant. The ionization of the surface copper atom is a second probable contributor to the increase in the endothermicity of adsorption. We expect that this effect would be reduced at lower coverage. 3.3. CO2 Adsorption on Reconstructed Cu2O(111). The reconstructed Cu2O(111) (√3 × √3)R30° surface is characterized by oxygen vacancies in 1/3 of the surface oxygen layer. The relaxed structure of the reconstructed surface (calculated previously with PBE+U33) is shown in Figure 5, indicating the surface copper cations that were originally unsaturated in the pristine surface (CuCUS) and the three additional coordinatively unsaturated copper cations newly created by the oxygen vacancy (CuNN, for “nearest neighbor” to the vacancy). These unsaturated copper cations are predicted to relax inward toward the vacancy, creating a small cluster of copper cations around the oxygen vacancy. Bader analysis of the clean (√3 × √3)R30° surface identifies an increase in electron density localized on this copper cluster. The removed oxygen donates excess electrons, resulting in a charge of +0.22e

Table 6. PBE+U and PBE+U-D2 Adsorption Energies and Free Energies (298.15 K, 1 atm) for CO2 Adsorbed on the Cu2O(111) (1 × 1) Surface with CuCUS Vacancies PBE+U

C CO2,VCu‑tri

energy here is much more endoergic than that of adsorption on the bulk-terminated surface (Table 4), maintaining endoergicity even when including dispersion corrections, indicating that this mode of adsorption on the surface copper vacancy site is less favorable than adsorption of CO2 on the CuCUS site. This reveals the importance of the CuCUS sites in adsorption and shows that surface copper vacancies will result in decreased CO2 adsorption. The Bader method is used to analyze the extent of charge transfer upon adsorption, using the PBE+U structure (Table 7). Typically, the bent CO2 structure corresponds to an anionic molecule with negative charge localized on the carbon atom. However, in this bent adsorbate geometry, the molecule gains only slight negative charge, which is localized on the terminal oxygen atoms. The bent structure will therefore create a strain on the molecule that is a probable contributor to the increase in 26054



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resulting in the additional coordination of one of the molecular oxygen atoms to the two other available CuNN ions. The tridentate structure (CO2,rec‑tri) has a bent CO2 molecule centered above the vacancy such that the carbon atom fills the vacancy, allowing all three atoms of CO2 to be coordinated to the surrounding unsaturated copper cations. We also consider new geometries for physisorption of a linear molecule: the CO2 molecule adsorbed perpendicular to the surface in the vacancy site (CO2,⊥vac), CO2 parallel to the surface with its two oxygen molecules each atop a CuNN (CO2,∥CuNN), and CO2 tilted in two directions atop CuNN (CO2,∠CuNN,1) and (CO2,∠CuNN,2). As a comparison to adsorption energies on the ideal surface, we model the stable geometry identified previously, with CO2 tilted in two directions atop CuCUS (CO2,∠CuCUS,1) and (CO2,∠CuCUS,2). Of the nine adsorbate geometries considered here, only CO2,rec‑bi, CO2,rec‑tri, CO2,∠CuCUS,1, and CO2,∠CuCUS,2 were found to be stable. The introduction of a stable bent adsorbate geometry due to the oxygen vacancy agrees with earlier observations in the DFT-PBE study.6 All other configurations relaxed to desorbed CO2 molecules, except for CO2,⊥vac, where CO2 dissociated to form an adsorbed oxygen atom and a desorbed CO molecule (the adsorption energy corresponding to this dissociation was highly endothermic and, therefore, this structure was not pursued). The stable geometries were also subsequently relaxed using PBE+U-D2. We confirmed that each adsorbate geometry was a local minimum by verifying that all vibrational frequencies are real. The tilted configurations can be described with the same parameters defined in Figure 3 for the stable adsorbate geometry on the ideal surface. These configurational parameter values for CO 2,∠Cu CUS ,1 and CO2,∠CuCUS,2 are shown in Table 8 for both PBE+U and PBE +U-D2 structures. In all cases, CO2 adsorbs again as a nearly linear molecule. These tilted adsorbate geometries do not differ

Figure 5. Relaxed structure of the reconstructed Cu2O(111) (√3 × √3)R30° surface, indicating the two types of coordinatively unsaturated surface copper cations. A surface oxygen atom has been removed from the corner of the surface unit cell. Red and blue spheres represent oxygen and copper, respectively.

on each clustered CuNN, in comparison with charges of +0.51e on the bulk copper atoms. (The charge on CuCUS does not change significantly from the ideal surface, with a charge of +0.37 e in both surfaces.) Consequently, as a result of these oxygen vacancies, the electron distribution on the (√3 × √3)R30° surface differs significantly from the ideal surface, which may lead to different surface chemistry. For example, because the CuNN atoms have excess electron density, they may behave similarly to a Lewis base site. The oxygen vacancy and localized excess negative charge therefore create a new adsorption site that may facilitate chemisorption or physisorption of CO2 with different stable adsorbate geometries. We sample nine new unique adsorbate geometries for CO2 adsorption on the reconstructed Cu2O(111) surface (Figure 6). Here, a new monodentate species (CO2,rec‑mono) can be formed via the coordination of the carbon atom in a bent CO2 molecule to CuNN, where the carbon atom is centered directly over the unsaturated copper cation. The bidentate species (CO2,rec‑bi) differs from the monodentate structure by placing the oxygen directly in the vacancy site,

Figure 6. Adsorbate geometries considered for CO2 on the reconstructed Cu2O(111) surface, presented as initial structures prior to relaxation: (a) the monodentate species, (b) the bidentate species, (c) the tridentate species, (d) adsorbed perpendicular to the surface in the vacancy site, (e) adsorbed parallel to the surface above two CuNN cations, and (f−i) adsorbed tilted on the surface above CuNN or CuCUS. Red, blue, and gray spheres represent oxygen, copper, and carbon, respectively. Green spheres are used to differentiate the oxygen atoms of the CO2 molecule. 26055



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adsorption energies fall between the adsorption energies on the stoichiometric surface at lower and higher coverage (Table 9), which shows that according to this model, the oxygen vacancies of the reconstructed surface will have little effect on CO2 adsorption on the Cu2O(111) surface. However, when dispersion corrections are included, linear adsorption on this surface is more exothermic than adsorption on the stoichiometric surface, signifying that oxygen vacancies may in fact slightly aid CO2 adsorption. These observations conflict with the earlier prediction from DFT-PBE that oxygen vacancies negatively impact CO2 adsorption.6 CO2,rec‑tri has a slightly less negative adsorption energy, indicating that carbonate formation is somewhat less favored than linear physisorption. CO2,rec‑bi has a significantly more positive adsorption energy, demonstrating that while this adsorbate geometry may be a local minimum, it is not a very stable means of adsorption. All adsorption free energies calculated with PBE+U are endoergic at 298.15 K, once again illustrating the significant impact of entropic effects, in agreement with the observed desorption at 175 K.4 The inclusion of dispersion corrections shifts adsorption free energies to be slightly exoergic; however, at a CO2 pressure corresponding to UHV, these would become endoergic as well, in agreement with experiment. We used Bader analysis to evaluate the extent of charge transfer upon adsorption, using the PBE+U structures. Table 10

Table 8. Characteristic Parameter Values (Labeled in Figure 3) for the Two Stable Tilted CO2 Adsorbate Geometries on the Reconstructed Cu2O(111) (√3×√3)R30° Surface, Calculated with PBE+U and PBE+U-D2 method PBE+U PBE+U-D2

geometry

∠O−C−O (°)

∠C−O−s (°)

∠O−Cu−v (°)

dO−Cu (Å)

CO2,∠CuCUS,1

178.4

38.3

8.6

2.062

CO2,∠CuCUS,2

178.2

31.0

11.6

2.067

CO2,∠CuCUS,1

177.0

27.6

12.3

2.015

CO2,∠CuCUS,2

177.4

25.4

11.7

2.004

significantly from the stable adsorbate geometry on the pristine surface. Dispersion corrections shorten dO−Cu by 0.05 Å, slightly decrease the O−C−O bond angle, and bring the molecule closer to the surface. The bidentate and tridentate species each have their own unique configurations, and cannot be described by the same structural parameters as the tilted configuration. Adsorption in both configurations is characterized by a bent molecule, with an O−C−O bond angle of 121.6° (121.2° with PBE+U-D2) for CO2,rec‑bi and 132.1° (132.8° with PBE+U-D2) for CO2,rec‑tri. CO2,rec‑bi coordinates its carbon atom to one CuNN and one of its oxygen atoms to two CuNN atoms, with a C−Cu bond length of 1.876 Å (1.866 Å with PBE+U-D2) and two O−Cu bond lengths of 1.844 and 1.856 Å (1.834 and 1.846 Å with PBE+U-D2). The initial structure for CO2,rec‑tri had the CO2 molecule coordinated to the surface via all three of its atoms, which is why it was originally labeled as a tridentate structure. However, the structure relaxes so that the two Cu−O bonds lengths (between the two molecular oxygen atoms and surface copper cations) are 1.878 and 3.050 Å (1.871 and 3.204 Å with PBE+U-D2), indicating that the second oxygen is no longer truly coordinated to the surface. Therefore, this configuration may be better described as a second bidentate structure. The carbon atom provides the second point of coordination to two CuNN atoms, with both bond lengths equal to 2.132 Å (2.141 Å with PBE+U-D2). Overall, including dispersion corrections did not significantly change the adsorbate structures. The adsorption free energies for the stable adsorbate geometries are presented in Table 9. The most stable adsorbate geometries on the reconstructed surface are the same as those on the ideal surface, with CO2 adsorbed as a linear molecule tilted on the Cu2O surface. CO2,∠CuCUS,2 is slightly more stable than CO2,∠CuCUS,1, indicating that it is more favorable to have CO2 directed away from the oxygen vacancy. Their PBE+U

Table 10. Bader Charges (e) of the Atoms in the CO2 Molecule (C, O1, and O2) and the CuCUS to Which It Coordinates for the Two Tilted Adsorbate Geometries on the Reconstructed Surfacea CO2,rec‑∠1 CO2,rec‑∠2 CO2 molecule clean Cu2O(111) (√3 × √3)R30°

PBE+U

PBE+U-D2

adsorbate geometry

Eads (kcal/mol)

CO2,∠CuCUS,1

−4.5

4.1

−5.1

2.6

CO2,rec‑bi CO2,rec‑tri CO2,∠CuCUS,1

2.8 −3.9 −10.9

13.6 6.8 −2.8

CO2,∠CuCUS,2

−11.7

−3.5

CO2,rec‑bi CO2,rec‑tri

−1.7 −12.5

8.6 −1.7

O2

CuCUS

−1.03 −1.03 −1.04

+0.52 +0.50 +0.37

Bader charges of the atoms in the adsorbed configuration are compared to the Bader charges of the atoms in the isolated CO2 molecule and of the CuCUS in the clean ideal Cu2O(111) surface. All charges were derived from the structures optimized with PBE+U.

shows the Bader charges associated with adsorption in the tilted geometry. The charges here are similar to those observed in adsorption on the ideal bulk-terminated surface (Table 5); here too there is a minor change to the charges upon adsorption, indicating that adsorption occurs without a significant amount of charge transfer, as anticipated in physisorption. However, Bader analysis of the two bent configurations shows that there is much more significant charge transfer from the surface to the CO2 molecule (Tables 11 and 12). Because there is a greater number of unsaturated surface copper cations coordinating the adsorbed molecules (three in each case), the change in the charges of these atoms, while not so different from the changes observed in physisorption (Tables 5 and 10), sum to a significant amount of charge being transferred to CO2. This is evident by the significant reduction in the Bader charge of carbon, where this transferred charge is subsequently localized. The excess charge causes the molecule to be nearly isoelectronic with NO2, which is why it assumes its bent molecular geometry. CO2,rec‑bi has slightly more negative charge than CO2,rec‑tri, which results in a structure that is more bent by ∼11°. The Bader charges indicate that adsorption in these two configurations can be appropriately described as a chemisorbed

Gads (kcal/mol)

CO2,∠CuCUS,2

O1 −1.10 −1.10 −1.04

a

Table 9. PBE+U Adsorption Energies and Free Energies (298.15 K, 1 atm) of CO2 on the Reconstructed Cu2O(111) (√3×√3)R30° Surface for a Number of Stable Adsorbate Geometries, Calculated with PBE+U and PBE+U-D2 method

C +2.13 +2.14 +2.08

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Table 11. Bader Charges (e) of the Atoms in the CO2 Molecule (C, O1, and O2), the Copper Cation Neighboring the Oxygen Vacancy that Coordinates to the Carbon Atom (CuNN‑C), and the Copper Cation Neighboring the Oxygen Vacancy that Coordinates to O1 (CuNN‑O; Averaged for the Two) for the Bidentate Adsorption Geometry on the Reconstructed Surfacea CO2,rec‑bi CO2 molecule clean Cu2O(111) (√3 × √3)R30°

C

O1

O2

CuNN‑C

CuNN‑O

+1.25 +2.08

−1.05 −1.04

−1.08 −1.04

+0.36

+0.55

+0.22

+0.22

Figure 7. Adsorbate geometries considered for CO2 on the reconstructed Cu2O(111) surface with CuCUS vacancies, presented as initial structures prior to relaxation: (a) the first bidentate geometry, where carbon is doubly coordinated and more central to the oxygen vacancy site, and (b) the second bidentate geometry, where the molecular oxygen is doubly coordinated and more central to the oxygen vacancy site. Side and top views of each structure are shown. Red, blue, and gray spheres represent oxygen, copper, and carbon, respectively. Green spheres are used to differentiate the oxygen atoms of the CO2 molecule.

a

Bader charges in the adsorbed configuration are compared to the Bader charges of the isolated CO2 molecule and the relevant ions in the clean Cu2O(111) reconstructed surface. All charges were derived from the structures optimized with PBE+U.

Table 12. Bader Charges (e) of the CO2 Molecule (C, O1, and O2), the CuCUS that Coordinates to O1 (an Average of the Two Such Atoms), and the Copper Cation Neighboring the Oxygen Vacancy that Coordinates to the Carbon Atom (CuNN) for the Tridentate Adsorption Geometry on the Reconstructed Surfacesa CO2,rec‑tri CO2 molecule clean Cu2O(111) (√3 × √3)R30°

C

O1

O2

CuCUS

CuNN

+1.51 +2.08

−1.03 −1.04

−1.07 −1.04

+0.56

+0.34

+0.37

+0.22

CO2,rec/VCu‑bi2. CO2,rec/VCu‑bi1 has C−Cu bond lengths of 2.140 Å (2.151 Å with PBE+U-D2) and an O−Cu (molecular oxygen to substrate copper) bond length of 2.047 Å (2.055 Å with PBE +U-D2). CO2,rec/VCu‑bi2 has O−Cu (molecular oxygen to substrate copper) bond lengths of 1.872 Å (1.873 Å with PBE+U-D2) and a C−Cu bond length of 1.892 Å (1.903 Å with PBE+U-D2). The adsorption energies and free energies were calculated with PBE+U and PBE+U-D2 for these two adsorbate geometries (Table 13). These energies are

a

Bader charges in the adsorbed configuration are compared to the Bader charges of the isolated CO2 molecule and the relevant ions in the clean Cu2O(111) reconstructed surface. All charges were derived from the structures optimized with PBE+U.

Table 13. PBE+U Adsorption Energies and Free Energies (298.15 K, 1 atm) of CO2 on the Reconstructed Cu2O(111) (√3×√3)R30° Surface with CuCUS Vacancies

CO−2 anion. However, the adsorption energies for these configurations are not within the conventional range expected for chemisorptive processes. 3.4. CO2 Adsorption on Reconstructed Cu2O(111) with CuCUS Vacancies. The second model of the reconstructed (√3 × √3)R30° surface is largely the same as the previous structure, but is also characterized by CuCUS vacancies. There is still a small cluster of CuNN atoms around the oxygen vacancy in the PBE+U-relaxed surface, each with a charge of +0.28e.33 This local defect structure is therefore similar to the structure around the oxygen vacancy in the reconstructed surface from section 3.3 and will likely result in similar adsorbate geometries. In this section, we study the effect of the additional copper vacancies on the CO2 adsorbate geometries that are unique to the reconstructed surface, namely, the two final bidentate structures that adsorb in the oxygen vacancy (see section 3.3). Figure 7 shows the initial adsorbate geometries of these two structures, where in the first bidentate structure (CO2,rec/VCu‑bi1), the carbon atom is located more centrally within the oxygen vacancy site and is coordinated to two CuNN atoms, while one molecular oxygen atom is coordinated to the third CuNN. In the second bidentate structure (CO2,rec/VCu‑bi2), a molecular oxygen atom is located more centrally within the oxygen vacancy site and is coordinated to two CuNN atoms, while the carbon atom is coordinated to the third CuNN. Both geometries retain bidentate structures upon relaxation, with O−C−O bond angles of 139.9° (140.2° with PBE+U-D2) in CO2,rec/VCu‑bi1 and 126.1° (126.4° with PBE+U-D2) in

method PBE+U PBE+U-D2

adsorbate geometry

Eads (kcal/mol)

Gads (kcal/mol)

CO2,rec/VCu‑bi1

7.4

16.8

CO2,rec/VCu‑bi2

14.6

22.8

CO2,rec/VCu‑bi1

2.4

11.7

CO2,rec/VCu‑bi2

9.0

16.8

significantly more positive than the adsorption energies for the similar adsorbate geometries on the reconstructed surface without copper vacancies (Table 9), once again indicating that CuCUS vacancies result in decreased CO2 adsorption. Finally, we analyze the extent of charge transfer between the substrate and CO2 upon adsorption for the PBE+U structure (Table 14). The Bader charges of the structures are similar to those of the bidentate structures on the previous reconstructed surface. A significant amount of negative charge has been transferred from the surface to the molecule and is localized on the carbon atom. Once again, this results in an electronic structure similar to NO2, which explains the bent geometries. CO2,rec/VCu‑bi2 has a less positive charge on its carbon atom, and is therefore more bent than CO2,rec/VCu‑bi1. 3.5. Discussion. Based on the collected adsorption energies, physisorption of a linear molecule is favored over adsorption of a bent anionic molecule. This conclusion raises questions regarding the previously proposed catalytic mechanism. In the reduction mechanism proposed by Frese et al.,1 CO2 injects a hole and binds as CO−2 before splitting into 26057



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Lewis acid-base description of the oxide surface, where the surface oxygen anions would act as electron donors for the adsorbate. Rather, many adsorbate geometries involve coordination to surface copper atoms. While the most stable adsorbate geometry is a linearly physisorbed molecule with little evidence of charge transfer, electron transfer from the surface copper atoms to the adsorbate does occur where the adsorbate is bent. Negative charge is readily available on the adsorption site as coordinatively unsaturated copper atoms contain excess negative charge in comparison with the usual linearly coordinated copper ions. The dominant adsorbate geometry for CO2 on the Cu2O(111) surface is a nearly linear molecule adsorbed tilted to the surface, where CO2 binds to a coordinatively unsaturated surface Cu atom. This tilted adsorbate geometry is the only stable configuration on the ideal bulk-terminated surface. Bent adsorbate geometries are made possible by surface defects and are less stable than the linear adsorbate, in spite of the electron transfer that occurs upon their adsorption. While adsorption energies are often exothermic, PBE+U adsorption free energies for all structures are endoergic at 298.15 K, highlighting the significance of entropic effects. Including dispersion corrections increases the stability of adsorbed CO2, but most adsorption free energies are still endoergic or would be endoergic at CO2 pressures corresponding to UHV conditions. Adsorption will therefore be more favorable at lower temperatures or higher CO2 gas pressures. Additionally, adsorption may be aided in non-UHV experimental conditions by coadsorption with other adsorbates or by local structural defects. Solvation by water will be especially helpful in stabilizing the adsorbed CO−2 anion via hydrogen bonding. In studying the differing surface terminations for the Cu2O(111) surface, we found that surface oxygen vacancies do not significantly change adsorption energies, indicating that they have a minimal impact on CO2 adsorption. However, surface copper vacancies, specifically vacancies of the coordinatively unsaturated copper atoms, remove the most active adsorption site and result in significantly more positive adsorption energies. This implies that surface copper vacancies will result in reduced CO2 adsorption. This has important implications for the structure of the active catalyst surface, as only the surface that contains unsaturated copper atoms is expected to be promote CO2 adsorption. Greater knowledge of the true surface structure will thus aid in further evaluating the catalytic potential of Cu2O.

Table 14. Bader Charges (e) of the Atoms in the CO2 Molecule (C, O1, and O2), the Copper Cation Neighboring the Oxygen Vacancy that Coordinates to the Carbon Atom (CuNN‑C; Averaged over the Two Copper Atoms for CO2,rec/VCu‑bi1), and the Copper Cation Neighboring the Oxygen Vacancy that Coordinates to the CO2 Oxygen (CuNN‑O; Averaged over the Two Copper Atoms for CO2,rec/VCu‑bi2) for the CO2,rec/VCu‑bi1 and CO2,rec/VCu‑bi2 Adsorbate Geometries on the Reconstructed Surface with Copper Vacanciesa CO2,rec/VCu‑bi1

C

O1

O2

CuNN‑C

CuNN‑O

+1.56

−1.03

−1.03

+0.41

+0.53

+0.48

+0.60

+0.28

+0.28

CO2,rec/VCu‑bi2

+1.36

−1.02

−1.03

CO2 molecule clean Cu2O(111) (√3 × √3) R30°−CuCUS

+2.08

−1.04

−1.04

a Bader charges in the adsorbed configurations are compared to the Bader charges of the isolated CO2 molecule and the relevant ions in the clean Cu2O(111) reconstructed surface with copper vacancies. All charges were derived from the structures optimized with PBE+U.

adsorbed CO and a subsequently neutralized O−. Our evidence challenges the likelihood that CO2 will accept an electron and chemisorb in this first step. However, our theoretical model represents conditions under UHV, where only CO2 is present. In experimental conditions, it is possible that adsorption is aided by other factors such as surface impurities or coadsorption with dissociated or molecular water. Solvation by water can also facilitate adsorption, as an adsorbed anion will be greatly stabilized by hydrogen bonding. Consequently, this first mechanistic step may need to be further studied under water coadsorption conditions, with a theory accounting for coordination to and solvation by water, for a better understanding of the reaction mechanism. While we have identified some stable adsorbate geometries with bent anionic molecular structures, it is important to note that in most of these cases, the unsaturated surface copper atoms acted as the electron donors. The carbon atoms were typically not coordinated to oxygen atoms on the surface as in the typical Lewis acid−base model, and as such, these adsorbates cannot technically be characterized as carbonates. This study uses PBE+U with and without dispersion corrections to study CO2 adsorption. One means with which to compare the accuracy of PBE+U and PBE+U-D2 is to calculate the free energy of adsorption at 175 K and 10−12 atm. As CO2 desorption was experimentally observed under these conditions,4 the computed adsorption free energy should be equal to zero. We therefore calculate the PBE+U and PBE+UD2 adsorption free energies at 175 K and 10−12 atm for CO2 adsorption at 1/4 ML on the bulk-terminated surface, which was predicted with PBE+U to be the most stable mechanism of adsorption. PBE+U predicts an adsorption free energy of 8.1 kcal/mol, whereas PBE+U-D2 predicts an adsorption free energy of 5.3 kcal/mol. The PBE+U-D2 result is closer to zero and is therefore more accurate, showing that dispersion corrections are needed to better describe the energetics of CO2 adsorption on the Cu2O(111) surface.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed derivations for calculations of thermochemical quantities for gaseous CO2 and the Cu2O slab that contribute to the adsorption free energy. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 609-258-5391. Fax: 609-258-5877. E-mail: eac@ princeton.edu.

4. CONCLUSIONS In summary, we showed that CO2 adsorption on the Cu2O(111) surface does not occur according to the typical

Notes

The authors declare no competing financial interest. 26058



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Article

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ACKNOWLEDGMENTS E.A.C. acknowledges the support of the Air Force Office of Scientific Research for funding and the DoD High Performance Computing Modernization Program at the NAVY, AFRL, and ERDC DSRC for supercomputer resources.

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CO2 Adsorption on Cu2O(111): A DFT+U and DFT-D Study

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