# Carbon Dioxide Adsorption on Monoclinic Zirconia: DFT Calculations Support the Role of Polycoordinated Carbonates in IR Spectroscopy

Nicola Scafuri, Mathilde Iachella, Yannick Bleu, Dorothée Berthomieu, Hugo Petitjean

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Nicola Scafuri, Mathilde Iachella, Yannick Bleu, Dorothée Berthomieu, Hugo Petitjean. Carbon Dioxide Adsorption on Monoclinic Zirconia: DFT Calculations Support the Role of Polycoordinated Carbonates in IR Spectroscopy. ChemCatChem, 2025, 17 (10), 10.1002/cctc.202401857. hal-05363696

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# Carbon Dioxide Adsorption on Monoclinic Zirconia: DFT Calculations Support the Role of Polycoordinated Carbonates in IR Spectroscopy

Nicola Scafuri,[a] Mathilde Iachella,[a] Yannick Bleu,[a] Dorothée Berthomieu,[a] and Hugo Petitjean*[a]

Developing the conversion of CO₂ in heterogeneous catalysis requires accurate characterization of the species reacting on the surface of metal oxide catalysts or supports. In this field, zirconia is used for its ability to adsorb and activate CO₂. Nevertheless, the first and crucial step of this process, the CO₂ adsorption over ZrO₂ surfaces, is not fully understood experimentally. In particular, the diversity of surface carbonate species makes the IR analysis very complicated. This work presents a computational study on the adsorption of CO₂ on all the sites of the four most stable monoclinic ZrO₂ surfaces, (001), (011), (111), and (−111).

With this exhaustive sampling of real particle surfaces, our main purpose is to characterize the surface carbonates in terms of structures, adsorption energies and nature of surface ions, coupled with a vibrational study and a Bader atomic charge analysis. Our results show that the polycoordinated carbonates are key species in the CO₂ adsorption over monoclinic ZrO₂. The CO₂ adsorption energies largely depend on the coordination numbers of the involved Zr⁴⁺ cations. This study offers a revisited IR assignment of the carbonate adsorbed on zirconia, showing the specificity of metal oxides with highly charged metal cations.

## 1. Introduction

The chemical conversion of CO₂ is nowadays a great scientific challenge.[1,2] The carbon dioxide is a greenhouse gas but also a C₁ substrate, abundant, lowly toxic, and potentially interesting for industrial synthesis of additives and polymers.[3-5] For example, the dimethyl carbonate (DMC) can be obtained via nucleophilic addition of methanol on CO₂,[6,7] instead of using the toxic phosgene as electrophilic reagent.[8,9] In heterogeneous catalysis, this type of reaction requires active sites presenting both acid and basic properties to activate the electrophile and the nucleophile as on cerium or zirconium oxides. In this study, we focus on zirconium oxide (ZrO₂) because it has gained interest both as catalytic support and as a catalyst on its own, mostly due to its amphoteric properties.[10,11] Zirconia exists in monoclinic, tetragonal, and cubic forms. Below 1175 °C, the most stable phase is the monoclinic one (m-ZrO₂).[12] As a catalyst, zirconia is used into nanoparticles in the range of few to hundreds of nanometers, exposing surfaces with several orientations.[11]

Characterizing the CO₂ activation on the catalyst surface is key to establish reaction mechanisms and develop efficient catalysts in CO₂ valorization. Characterization specialists have many

questions to deal with: what is the structure of the adsorbed molecule? How to distinguish the different adsorbed species experimentally? How to quantify them? What surface properties rule the process? One of the most common approaches used to characterize adsorbed species is the infrared (IR) analysis, which provides information about the nature of the interaction between the adsorbate and the surface but also the quantitative distribution of species.[13] Nevertheless, with few adsorbates and CO₂ is one of those, the IR quantification is complex and usually leads to large uncertainties because several species with overlapped bands are present on the surface.[14]

IR characterization of CO₂ adsorption relies on an empirical assignment built years after years by spectroscopists, using the knowledge from molecular inorganic chemistry of metal-carbonate complexes[15] combined with insight from in situ FTIR experiments.[13] The adsorption of CO₂ on ZrO₂ surface creates a surface carbonate (CO₃²⁻), induced by a covalent interaction between the adsorbed molecule and a surface oxygen atom.[14,16] In the asymmetric environment of metal oxide surfaces, the carbonate species loses the D₃h symmetry it would have in solution. This symmetry lowering causes the splitting of the doubly degenerate v₃ and v₄ vibrations, which are observed in D₃h symmetry. On the IR spectra of adsorbed CO₂, the spectroscopists identify the v₃ doublet and quantify the Δv₃ values. On the basis of the experimental Δv₃ values and the thermal stability of the species, they assign the bands to different types of carbonate species on ZrO₂ with different coordinations to zirconium atoms as proposed in literature (Figure 1).[13,14,16-18]

Despite the number of both experimental and theoretical studies, the adsorption of CO₂ on monoclinic zirconia, the active sites, the nature of formed species and their attribution in IR spectra are still unclear and debatable in the literature. The main experimental issue in this field comes from the difficulty charac-

<footnote>[a] Dr. N. Scafuri, Dr. M. Iachella, Dr. Y. Bleu, Dr. D. Berthomieu, Dr. H. Petitjean ICGM, Univ Montpellier, CNRS, ENSCM, Montpellier, France
E-mail: hugo.petitjean@enscm.fr</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202401857</footnote>

<footnote>© 2025 The Author(s). ChemCatChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
<figure><img src="image_2.png" /><figcaption>Figure 1. Structures of hydrogen carbonate and carbonate species obtained after adsorption of CO₂ on oxide surfaces. HC = hydrogen carbonate; C1 = monodentate carbonate; C2 = bidentate carbonate; BC = bridged carbonate; PC = polydentate carbonate. Adapted from ref. [16].</figcaption></figure>

terizing the geometry of the adsorbed species at the molecular scale as it would be possible with crystallized species in XRD. On the theoretical side, many studies have been dedicated to the interaction of CO, CO₂, and H₂O on ZrO₂ substrate and their reactivity on the surface either using DFT[11,19–24] or molecular dynamics.[25] In particular, an investigation on the exposed facets in hydrated m-ZrO₂ nanoparticles has revealed that (001), (011), (111), and (-111) surface orientations are more likely to occur and must be taken into account to give a comprehensive view of all the adsorbates observed experimentally.[11] Chen et al. have also identified three adsorption modes on tetragonal zirconia that go further than the empirical IR spectra bands assignment.[21]

Our work aims to clarify the description of CO₂ adsorbed species on m-ZrO₂ and the band assignment in IR spectra by means of density functional calculations. To this end, we performed geometry optimizations providing converged adsorption structures on four different zirconia surface orientations: (001), (011), (111), and (−111). The energetics and vibrational modes of those structures are discussed with respect to experimental and theoretical literature.

## 2. Computational Methods

Periodic DFT calculations were carried out with Vienna ab initio simulation package (VASP)[26–29] code (version 5.3.5) using the projector augmented wave (PAW) method[30,31] with a plane wave energy cutoff of 400 eV and the PBE exchange-correlation functional.[32] The SCF convergence parameter was set at 10⁻⁶ eV. Geometry optimizations and vibrational analysis were performed within a conjugate-gradient algorithm until the convergence criterion on forces (10⁻² eV Å⁻¹) was reached. A dipolar correction perpendicular to the slab was applied in order to remove the effect of electrostatic interaction between the slab and its periodic images along the z-axis. van der Waals interactions were included in the calculations by means of Grimme approximate method with Becke–Johnson damping (DFT-D3).[33,34] This DFT method has already shown its robustness in taking into account dispersion in CO₂ adsorption on zirconia.[35] The integration in the Brillouin zone generated by the Monkhorst-Pack method[36] was performed as follows: 2 × 3 × 1 for the (001) and (011) planes, 3 × 3 × 1 for the (111), and (−111) planes.

Following the previous Wulff constructions of m-ZrO₂,[11] we consider four consistent surface orientations in this work: (001), (011), (111), and (−111) depicted Figure 2.

The adsorption energy (ΔE<sup>ads</sup>) is calculated as:

$$ \Delta E^{\text{ads}} = E^{\text{tot}} - E^{\text{slab}} - E^{\text{gas}} $$

where E<sup>tot</sup> is the total energy of the slab with adsorbed CO₂, E<sup>slab</sup> is the energy of the bare slab, and E<sup>gas</sup> the energy of the CO₂ molecule in the gas phase. E<sup>gas</sup> was evaluated by placing a single CO₂ molecule in a 15 × 15 × 15 Å³ cell. With the present definition of the adsorption energy, the more negative (or the “lower”) the adsorption energy, the more stable the adsorption structure.

The complete description of the computational details, surface models and adsorption procedure are provided in Figures S1 and S2 and Tables S1 and S2 of the Supporting Information.

## 3. Results and Discussion

### 3.1. Geometries and Energies of CO₂ Adsorbates

The adsorption of linear CO₂ resulted in the formation of carbonate species at the end of the geometry optimization, for each surface site of the four surface orientations. Among the 16 stable structures calculated for the 4 surface orientations (see Figures S3–S6), it was found that the surface carbonates can adopt only three different geometries (see Figure 3): bridging (BC), polycoordinated either perpendicular to the surface (PC-perp) or lying down on it (PC-flat).

Figure 4 shows three examples of stable carbonate structures, resulting from geometry optimization calculations. They are the most stable of each of the three geometric families. The reader can tell the difference between PC-perp and PC-flat structures by bearing in mind that the zirconia surface is perpendicular to the plane of the page in Figure 4b, whereas it is parallel to the page in Figure 4c.

All these carbonate structures are issued from the primary covalent interaction between the acid carbon center from CO₂ and a basic O²⁻ surface ion. They are differentiated for (i) the adsorption geometry (in structures BC and PC-perp, the carbonate plane is almost perpendicular to the zirconia surface while in structure PC-flat, the carbonate is parallel to the surface) and (ii) the secondary interactions between basic CO₂ oxygen and Zr⁴⁺ surface acidic sites (namely O<sub>CO₂</sub>-Zr⁴⁺ interactions, identified with a distance lower than 2.62 Å). The structure BC is characterized by one O<sub>CO₂</sub>-Zr⁴⁺ interaction, with a free C-O bond, similar to what is observed in organic carbonates. The structure PC-perp shows an interaction between both O<sub>CO₂</sub> and two geminal Zr⁴⁺ ions. The structure PC-flat presents also two O<sub>CO₂</sub>-Zr⁴⁺ interactions at least and a third one can occur, resulting in one O<sub>CO₂</sub> interacting with two Zr⁴⁺ ions (Table S3). Comparing these structures (Figure 3) with the schemes proposed in the experimental literature (Figure 1), we can associate the present structure BC with the bidentate (C2) and bridged (BC) carbonates, while the structures PC-perp and PC-flat with the polycoordinated ones (PC). No stable monodentate species (C1) was observed in the present calculations, either with geometry optimization or complementary ab initio molecular dynamics calculations (see SI).
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Surface structure of m-ZrO₂ (a) (001), (b) (011), (c) (111), and (d) (−111) orientations within the used *p*(2 × 1), *p*(2 × 1), *p*(1 × 1), and *p*(1 × 1) supercells. Zirconium atoms are depicted in green oxygen atoms in red. The supercell has been evidenced by black lines. Surface atoms are represented as balls and sticks, deeper layers as thin sticks.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Proposed structures for CO₂ adsorption on m-ZrO₂ surfaces. In red are depicted the surface atoms, while in black are depicted the CO₂ atoms. On structure PC-flat, the top O atom can either interact with one or two Zr⁴⁺ ions of the m-ZrO₂ surface.</figcaption></figure>

Table S3 and Figures S3–S6 gathered the most stable adsorption structures calculated on the different ZrO₂ surface orientations. On the ZrO₂ (001) surface, three structures PC-flat and one structure PC-perp were found, while on the ZrO₂ (011) surface three structures PC-flat and two PC-perp. In contrast, on the ZrO₂ (111) surface two structures PC-flat, one PC-perp and one BC; on the ZrO₂ (−111) two structures PC-perp and one BC (see Figures S3–S6). The distance between the CO₂ carbon and the O²⁻ surface ion (C₅O₂⁻O₃ₚ) is a geometric indicator of the Lewis acid-base interaction. In our converged adsorption structures, this value ranges from 1.33 to 1.49 Å. In contrast, the O₅O₂⁻Zr⁴⁺ distances cover a larger range: from 2.10 to 2.61 Å.

Calculated adsorption energies range from −181 to −16 kJ.mol⁻¹ (see Table S3). The (011) and (001) surfaces show the most stabilizing adsorption energies (from −181 to −86 kJ.mol⁻¹),

while the (−111) and (111) surfaces show the lowest adsorption energies (from −132 to −16 kJ.mol⁻¹). Hornebecq et al.[37] carried out microcalorimetric experiments on CO₂ adsorption on cubic ZrO₂ at 303 K. They determined adsorption enthalpies of −35 to −125 kJ.mol⁻¹. Besides, Stelmachowski et al.[14] carried out experiments on CO₂ adsorption on monoclinic ZrO₂ using a gravimetric technique with temperature and pressure ranges of 353–637 K and 10⁻⁴–0.4 bar, respectively. They reported an adsorption enthalpies range between −62 and −128 kJ.mol⁻¹. The range of experimental values is well included in the present set of calculated adsorption energies. The values calculated below −130 kJ.mol⁻¹ are hard to be measured experimentally for two main reasons. First, they concern the sites the most stabilizing and thus the most easily poisoned by atmospheric water or CO₂ during preparation or storage of the sample. Therefore, without high temperature degassing, the sites cannot be probed during the CO₂ adsorption experiment. Secondly, to capture the highly exothermic adsorption phenomena, one needs to add very small amounts of CO₂ and to be able to measure this CO₂ amount, the heat flow (for microcalorimetry) and the pressure (for isotherm analysis) very precisely: this is not always possible or in the scope of the studies. In the theoretical literature, the adsorption of CO₂ has been evaluated on cubic ZrO₂ (23 kJ.mol⁻¹)[20] and on tetragonal ZrO₂ surfaces (88–190 kJ.mol⁻¹),[21] using DFT (Density Functional Theory) approach. Considering the modeling studies on parent materials and the experimental context, our results are thus in fair agreement with both DFT and experimental literature.
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Examples of stable CO₂ adsorption structures on m-ZrO₂ surfaces: (a) bridging carbonate BC on O7 site of (−111), (b) PC-perp on O6 site of (001), and (c) PC-flat on O2 site of (001). Zirconium atoms are depicted in green, oxygen atoms in red, and carbon atoms in brown. The snapshots are side views for (a) and (b) and a top view for (c).</figcaption></figure>

## 3.2. Vibration Properties of CO₂ Adsorbates

With the view of consolidating the IR assignment, we analyzed the vibrational properties of this large panel of adsorbed carbonate species. For each adsorption structure, we calculated the carbonate harmonic frequencies, and we analyzed the $v_3$ modes specifically because experimentalists look at them to track CO₂ adsorbates on the IR spectra. Table S4 gives the calculated $v_{3high}$ and $v_{3low}$ normal modes and their difference ($\Delta v_3 = v_{3high} - v_{3low}$), together with the type of carbonate (BC, PC-perp, or PC-flat), and the surface model. Analyzing these spectroscopic data, the 1850–1000 cm⁻¹ region is dominated by the $v_3$ split bands. The $v_{3high}$ values range from 1532 to 1822 cm⁻¹, while the $v_{3low}$ ranges from 1104 to 1314 cm⁻¹. Finally, the $\Delta v_3$ ranges from 217 to 718 cm⁻¹. The BC structures show the most split frequencies ($\Delta v_3$ = 623 and 718 cm⁻¹ on the (111) and (−111) surfaces, respectively), in contrast with PC-perp and PC-flat structures for which $\Delta v_3$ are in the range 217–512 cm⁻¹. In a DFT study of CO₂ adsorption on MgO surface, Cornu et al. calculated the same range of frequency values (1850–1034 cm⁻¹) for the two highest frequency values.[38] Ramis et al. observed frequency values in the 2000–800 cm⁻¹ range for the CO₂ adsorption on ZrO₂ surface at low temperatures (between 173 and 273 K).[39] The $\Delta v_3$ range is also large from 217 to 717 cm⁻¹ in the ranges [623–717 cm⁻¹], [278–464 cm⁻¹], and [217–512 cm⁻¹] for the structures BC, PC-perp, and PC-flat, respectively.

With a view to identifying the 3 carbonate structures from experimental observables, we analyzed the calculated $\Delta v_3$ values as function of the $\Delta E^{ads}$ (Figure 5) for all 16 structures.

As shown in Figure 5, structures BC can be distinguished from the other structures because of their higher $\Delta v_3$ values and small adsorption energies (−33 and −63 kJ mol⁻¹). At lower $\Delta v_3$ values between 500 and 200 cm⁻¹, the polycoordinated structures appear mixed together with a continuous evolution of the adsorption energies from −182 to −16 kJ·mol⁻¹. This makes it difficult to distinguish PC-perp and PC-flat structures in that region. This overlapping is consistent with the experimental observations of Stelmachowski et al. on monoclinic zirconia: the adsorbed carbonates with low $\Delta v_3$ values were characterized with adsorption enthalpies from −84 ± 14 kJ·mol⁻¹ to larger than −120 kJ·mol⁻¹.[14]

<figure><img src="image_8.png" /><figcaption>**Figure 5.** $v_3$ Modes of adsorbed carbonates: $\Delta v_3$ (cm⁻¹) values as function of adsorption energy $\Delta E^{ads}$ of each converged CO₂ adsorption structure (kJ·mol⁻¹). Black squares depict structures BC, red circles depict structures PC-perp, and blue triangles depict structures PC-flat.</figcaption></figure>

## 3.3. Investigating the Parameters Ruling the Adsorption

The large polycoordination of the carbonate species on ZrO₂ surfaces suggests that the adsorption energies values are strongly correlated with the interactions of CO₂ with the surface ions. The covalent interaction between the surface oxygen and the CO₂ molecule is clearly shown through the bending of the linear CO₂ and the formation of a carbonate with C—O bond length lower than 1.5 Å. Figure S7 shows the behavior of C—O bond length as a function of the adsorption energies. As expected, $C_{CO2}-O_{surf}$ distance tends to decrease when the adsorption energies become lower. (011) and (001) surfaces, which show the strongest $C_{CO2}-O_{surf}$ interactions have the shortest $C_{CO2}-O_{surf}$ distances (< 1.40 Å). However, even if $C_{CO2}-O_{surf}$ is the main interaction, there are secondary interactions between the CO₂ oxygen and surface zirconia atoms ($O_{CO2}-Zr$). Such interactions which are purely electrostatic lead upon CO₂ coordination to the distortion of the O—C—O angle from 180° and therefore to a deviation from the linearity of the correlation shown in Figure S7. These processes involve electron transfer from the surface oxygen to the CO₂ moiety and from the CO₂ oxygen to the Zr⁴⁺ surface ions. To quantify these electron transfers between the surface and CO₂, a Bader analysis[40] was carried out for all the systems under investigation. In Table S7 are reported the variation
<figure><img src="image_9.png" /><figcaption>Figure 6. (a) Relation between Zr<sup>4+</sup><sub>coord</sub> and the adsorption energies ΔE<sup>ads</sup>. (b) Distortion of adsorbed carbonates (δ, degree) as a function of the splitting of C—O elongation bands (Δν<sub>3</sub>, cm<sup>-1</sup>). Black squares depict structures BC, red circles depict structures PC-perp, and blue triangles depict structures PC-flat for surface carbonates.</figcaption></figure>

of the charges (Δq) are reported upon the CO₂ adsorption. In the case of O<sub>CO₂</sub> atoms, the atomic charge became about 0.1e more negative upon formation of the surface carbonate while for the other atoms, the charge rearrangements are 10 times lower. Once CO₂ is adsorbed, Zr<sup>4+</sup> ions lose charge density, as well as the surface oxygen (O<sub>surf</sub>). Nevertheless, no correlation has been found between the charge transfer to the CO₂ molecule and the adsorption energies (Figure S9) showing that the molecule is also stabilized by other interactions.

Considering CO₂ as a probe-molecule of surface basicity,[41] one first would link the adsorption energies with the availability of the O²⁻ surface anions to share their electron density with C<sub>CO₂</sub>, and consequently with its formal coordination number. However, in the carbonate structures calculated herein, there is no clear correlation between E<sub>ads</sub> and the variation of O²⁻<sub>coord</sub> (O²⁻<sub>coord</sub> = 2 or 3), as shown Figure S8 in Supporting Information. It appears that the Zr⁴⁺ coordination number has also to be taken into account. Zr⁴⁺ ions can be either 5-fold or 6-fold coordinated and several of them are involved in the surface interaction with CO₂. So, we built a descriptor Zr⁴⁺<sub>coord</sub>, the average coordination number of Zr⁴⁺ ions involved in CO₂ adsorption, calculated as follows:

$$Zr^{4+}_{\text{coord}} = 1/n \sum_{i=Zr^{4+} \text{ linked to } CO_2} c_i (Zr^{4+})$$

Zr⁴⁺<sub>coord</sub> is the sum of the coordination numbers cᵢ(Zr⁴⁺) of each Zr⁴⁺ ions divided by n the number of the Zr⁴⁺ ions linked to adsorbed CO₂ (n = 1 for structures BC, n = 2 for structures PC-perp and n = 2 or 3 for structures PC-flat). All the O<sub>CO₂</sub>—Zr bonds shorter than 2.6 Å were considered for the calculation of n. Zr⁴⁺<sub>coord</sub> values give a qualitative measure of the acidity of lowly coordinated Zr⁴⁺ centers and, therefore, a measure of their tendency to interact with the CO₂ oxygen. The lower the Zr⁴⁺<sub>coord</sub>, the stronger the O<sub>surf</sub>—Zr⁴⁺ interaction. Figure 6a shows the behavior of the adsorption energy with the variation of Zr⁴⁺<sub>coord</sub>. For comparison, Figure S8 shows the behavior of the adsorption energies with the variation of O²⁻<sub>coord</sub> (O²⁻<sub>coord</sub> = 2 or 3). As expected, systems with average 6-fold coordinated Zr⁴⁺ ions

show lower adsorption energies (between −16 to −64 kJ·mol⁻¹). Systems with Zr⁴⁺<sub>coord</sub> = 5.5–5.7 show adsorption energies ranging from −84 to −140 kJ·mol⁻¹, while systems with Zr⁴⁺<sub>coord</sub> = 5 have adsorption energies from −154 to −180 kJ·mol⁻¹.

The correlation on Figure 5a suggests that on monoclinic zirconia, CO₂ molecule behaves as a good probe of the Zr⁴⁺ coordination number when we consider its adsorption energy. This idea of CO₂ as probe of acidic sites could seem quite paradoxical with the status of CO₂ as basic probe molecule but it should be contextualized in the framework of an amphoteric material where the Lewis basicity is not so high. This new point of view is consistent with the main finding of the present study: among all the sites exposed on the four most stable facets of m-ZrO₂, the vast majority of them lead to the adsorption of CO₂ in the form of carbonates polycoordinated with Zr⁴⁺ ions.

To go further the difficulty of distinguishing the two types of polycoordinated structures with IR spectroscopy, we considered that the Δν₃ splitting is not influenced only by the adsorption energy, but also by the geometry of the adsorbed carbonate. Indeed, the free carbonate has a trigonal planar geometry but the carbonates BC, PC-perp, and PC-flat are distorted, with some O—C—O angles smaller than 120 °C and others larger. In order to have a measure of this distortion, we considered the calculated δ magnitude, which is the difference between the largest and the smallest computed O—C—O angles in the adsorbed carbonates. The δ value appears to be a quantitative marker of the deviation from the D₃h symmetry. Table S4 reports for each adsorption structure the corresponding δ values. Figure 6b depicts the correlations between δ and Δν₃.

The correlations in Figure 6b show that Δν₃ values are influenced by the CO₃²⁻ geometry deviation from the trigonal planar geometry. Structures BC have the most distorted geometry (δ = 26 and 25 degrees) whereby they show the highest Δν₃ values (623 and 718 cm⁻¹). Structures PC-perp and PC-flat show similar δ values (going from δ = 11 degrees to δ = 23 degrees). Once again, it is not possible to separate the structures PC-perp and PC-flat based on the δ magnitude.
<figure><img src="image_10.png" /></figure>

### 3.4. Revisited Assignment for the IR Spectra of Surface Carbonates on M-ZrO₂

At the light of our computational study, it is possible to revisit the description of the CO₂ adsorption on monoclinic ZrO₂ surfaces. In our calculations, we did not take into account the hydrogen carbonates possibility (see Figure 1, structure HC) but infrared spectra are precise enough to unveil this kind of structures, with the complementary information of the hydroxyl vibrations. The empirical assignment of hydrogen carbonates IR bands appears consensual. On the contrary, the IR assignment of surface carbonate species was done upon an analogy between the molecular metal carbonate complexes and the surface carbonates,[13] without the possibility to characterize the adsorption geometry directly. This blind spot could be clarified with the present modeling of the CO₂ adsorption structures, on all sites available on m-ZrO₂ (excluding defects and junctions between facets).

In the present work, three new adsorbed carbonate species (BC, PC-perp, and PC-flat, as depicted in Figure 3) were proposed. These structures differ from the number and the nature of the CO₂-Zr⁴⁺ interactions. Monodentate species (Figure 1, structure C1) were not found under geometry optimizations and should be seen as very unlikely on m-ZrO₂: our calculations show that a hypothetical monodentate carbonate would be instantly coordinated to any further Zr⁴⁺ ion that could be in its vicinity. The carbonate structures from the present study are linkable with the assignment actually used by spectroscopists; indeed, if the reader compares the sketches from Figures 1 and 2: (i) the present structure BC can be seen as a combination of the bidentate (Figure 1, structure C2) and bridged (Figure 1, structure BC) carbonates structures while (ii) structures PC-perp and PC-flat could be seen as members of the polycoordinated family (Figure 1, structure PC).

Beyond these similarities from a "drawing" point of view, our proposition of assignment differs from the old one in several important respects for analyzing the IR spectra of m-ZrO₂ carbonates in the 1800–1200 cm⁻¹ range: (i) no contribution of the monodentate species, (ii) contributions with large Δν₃ and high ν₃high are assigned to bridging carbonates BC (as in the empirical model), and (iii) contributions with low to moderate Δν₃ are assigned to polycoordinated carbonates PC-perp and PC-flat. The C2 molecular structure does not appear as being relevant in the case of m-ZrO₂: from our calculations, all the species with moderate Δν₃ are polycoordinated and all the BC species with a free C=O bond gives signals with high Δν₃ and a especially high ν₃high. From a historical perspective, it is not so surprising because surface IR spectroscopists have built the empirical assignment with the few structure propositions coming from molecular spectroscopy without possibility of checking the actual geometries on surfaces. More impressive is the idea of Busca et al.[13] who proposed in the 1980's to complement the C1-C2-BC assignment with "the hypothetical structures" of polycoordinated carbonates which are now proving to be part of the main species contributing.

The revisited assignment allows the reader to see if old-but-good papers from in situ IR spectroscopy of CO₂ could bring

new mechanistic insights at the molecular scale (what is the role of the carbonate geometry at the molecular scale?). Other carbonate species could be present experimentally in addition to the three BC, PC-perp, and PC-flat and the well-known hydrogen carbonates because the present study has not considered the defects of m-ZrO₂ nor the effect of rising the CO₂ coverage beyond 25%. Nevertheless, having taken into account all the sites of the stable four facets of m-ZrO₂ shows that the polycoordination of surface carbonates by Zr⁴⁺ cations should be seen as an important process to understand the reactivity of CO₂ on zirconia.

### 4. Conclusion

In our work, we investigated the formation of surface carbonates on four m-ZrO₂ surface models: (001), (011), (111), and (−111) considering all the possible adsorption sites of these facets (excluding the defects). In these conditions, with a coverage of 25%, we identified three carbonate structures that are in agreement with both theoretical and experimental literatures: bridging BC species and two different polycoordinated species; PC-perp and PC-flat. We propose a revisited IR assignment for the spectra of carbonates on m-ZrO₂ (1800–1200 cm⁻¹): band pairs with high Δν₃ values are helpful to identify BC bridging carbonates whereas band pairs with low to moderate Δν₃ values correspond to PC-perp or PC-flat polycoordinated carbonates. Consistently with this importance of the multiple coordination of carbonates with Zr⁴⁺ surface cations, the adsorption energies of CO₂ on m-ZrO₂ are correlated with the average coordination number of the zirconium ions involved in the adsorption process. The Δν₃ splitting does not arise only from the chemical properties of the surface (interaction with the surface Zr ions, charge transfer) of the adsorbed carbonate, but also from the geometry distortion of the carbonate with respect to the ideal D₃<sub>h</sub> symmetry. This theoretical work paves the way towards better comprehension of surface carbonates properties and reactivity. In particular, the change in the IR assignment of surface carbonates allows the community to look at CO₂ reaction mechanisms from in situ and operando studies with fresh eyes, whether in recent or older literature.

### Supporting Information

Computational Details, Dispersion Correction, m-ZrO₂ Surfaces, Energetic, Structural and Vibrational Analysis, Bader Analysis.[42–53]

### Acknowledgements

Hugo Petitjean thanks CNRS Chimie for NS's postdoc fellowship. This work was granted access to the HPC resources of CINES under the allocation A0030807394 attributed by GENCI (Grand Equipement National de Calcul Intensif). This project has
<figure><img src="image_11.png" /></figure>

received funding from the Agence Nationale de la Recherche
(CO2KARMA project, grant ANR-17-CE07-0024-01).

Conflict of Interests

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available
from the corresponding author upon reasonable request.

Keywords: Bridging carbonate · Density functional theory · Polydentate carbonate · Vibrational spectroscopy · Zirconium

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Manuscript received: November 9, 2024

Revised manuscript received: February 10, 2025

Accepted manuscript online: February 26, 2025

Version of record online: April 9, 2025