Enhanced performance of oxygen vacancies on CO₂ adsorption and activation over different phases of ZrO₂

Abstract

The effect of oxygen vacancies on the adsorption and activation of CO₂ on the surface of different phases of ZrO₂ is investigated by density functional theory (DFT) calculations. The calculations show that the oxygen vacancies contribute greatly to both the adsorption and activation of CO₂. The adsorption energy of CO₂ on the c-ZrO₂, t-ZrO₂ and, m-ZrO₂ surfaces is enhanced to 5, 4, and 3 folds with the help of oxygen vacancies, respectively. Moreover, the energy barrier of CO₂ dissociation on the defective surfaces of c-ZrO₂, t-ZrO₂, and m-ZrO₂ is reduced to 1/2, 1/4, and 1/5 of the perfect surface with the assistance of oxygen vacancies. Furthermore, the activation of CO₂ on the ZrO₂ surface where oxygen vacancies are present, and changes from an endothermic reaction to an exothermic reaction. This finding demonstrates that the presence of oxygen vacancies promotes the activation of CO₂ both kinetically and thermodynamically. These results could provide guidance for the high-efficient utilization of CO₂ at an atomic scale.

Graphical abstract

Keywords

CO₂ activation / oxygen vacancies / ZrO₂ / different phases

1 Introduction

The use of fossil energy has contributed to the development of modern industry. However, the greenhouse gas emission has caused a series of environmental and survival problems, such as global warming, sea level rise, land desertification, and reduction of grain [1–3]. The Paris Agreement, adopted at the 2015 UN Climate Summit, demonstrates the determination of the UN to curb the global warming trend. It is worth noting that CO₂ is a major greenhouse gas. The search for effective ways to collect and utilize CO₂ has become a hot research topic in recent years [4–6]. Currently, the main pathways to achieve chemical conversion of CO₂ include catalytic conversion of CO₂ to carbonate with alcoholic organics such as methanol or ethanol [7], reaction of CO₂ with hydrogen at a high temperature to produce methanol [8–14], and catalytic reforming of CO₂ with alkanes to prepare carbon monoxide and hydrogen [15]. CH₄ is also a type of greenhouse gas, whose greenhouse effect is 21 times more efficient than CO₂ [16].

The CO₂-CH₄ reforming process uses these two major greenhouse gases as feedstock. The syngas produced from reforming process is composed mainly of CO and H₂, which can be further synthesized by Fischer–Tropsch to produce high value-added chemicals or fuels [17–21]. Therefore, CO₂-CH₄ reforming into syngas has good prospects for application. The current technical means of CO₂-CH₄ reforming include steam reforming and dry reforming. Steam reforming was industrialized earlier [22,23], but the involvement of water in the reaction process had high requirements for the selection of the plant location. In contrast, the CO₂-CH₄ dry reforming (DRM) process is much more promising as it does not require the involvement of water. Actually, due to the high energy of C–H bond in CH₄ molecule ((107 ± 4) kcal/mol) [24], CH₄ molecule is uneasy to undergo activation reaction. Therefore, the choice of catalyst is extremely important for DRM reaction.

The catalyst for the DRM reaction mainly consists of two parts: the active metal and the support. When the active metal is a precious metal, the catalyst can obtain an excellent anti-carbon accumulation and thermal stability. However, the high cost makes it difficult to achieve a large-scale industrial application. Nickel has the highest activity among non-precious metals, but it is easily deactivated by sintering and carbon accumulation. The current research hotspot is to find a technical means to improve the comprehensive performance of nickel-based catalysts [25–29]. Many studies have pointed out that doping Ni with other transition metals, such as Cu, Fe, and Co, can improve its overall performance [30,31]. Meanwhile, finding a suitable support can also enhance the comprehensive performance of the catalyst. In fact, the support sometimes interacts synergistically with the active metal or even participates directly in the reaction itself [32–35]. This synergistic effect can greatly improve the comprehensive performance of the catalyst.

The supports of catalysts are generally prepared from metal oxides, and the commonly used supports are MgO, Al₂O₃, SiO₂, and ZrO₂. Among them, ZrO₂ has received a lot of attentions due to the presence of both acid-base centers and oxygen vacancies [36,37]. In 1993, Murota et al. [38] first reported that ZrO₂ could promote the ability of CeO₂ to store oxygen and lower the temperature at which the reduction reaction occurs from 1100 to 900 K. In 2014, Chen et al. [39] discovered that the higher the concentration of oxygen vacancies on the surface of ZrO₂ in the water–gas shift (WGS) reaction, the higher the catalytic efficiency of the catalyst. In 2017, Han et al. [40] found that the presence of oxygen vacancies on the ZrO₂ surface improved the selectivity of CH₄ during the hydrogenation of CO to CH₄. In 2021, Petchmark et al. [41] demonstrated that the ability of ZrO₂ to store hydrogen was increased in the presence of oxygen vacancies on its surface. These reports demonstrate that oxygen vacancies on the surface of ZrO₂ can improve its catalytic ability in various reactions. In fact, when ZrO₂ acts as a support, the oxygen vacancies can increase the contact area between the Ni particle and the support surface to keep the Ni particles well dispersed [42]. The high dispersion, on the other hand, allows the Ni particles to remain in a small size, and the particle size can directly affect the performance of the catalyst [43,44]. Overall, ZrO₂ reduces the carbon build-up of the catalysts at high temperatures and increases the activity when it is used as a support.

Currently, studies on the activation of the CH₄-CO₂ reforming process on the Ni/ZrO₂ catalyst surface using DFT and experimental methods have been reported [45–48]. However, studies on the effect of oxygen vacancies on the ZrO₂ surface on CO₂ adsorption and activation processes are still lacking. Elucidating the role of oxygen vacancies in CO₂ adsorption and activation is important for the preparation of high-performance DRM reaction catalysts using ZrO₂. Meanwhile, the metal oxides used as support generally exist in different crystalline phases, which can affect the performance of the catalyst [49,50]. Therefore, clarifying the differences in the adsorption and activation capacity of CO₂ on different phases of ZrO₂ surface is equally important.

In this paper, first, the energy barrier and adsorption energy of CO₂ on the perfect surfaces of different phases of ZrO₂ are obtained by DFT calculation. Next, oxygen vacancy (VO) is constructed on the surface of the different phases of ZrO₂. Afterwards, the adsorption energy and dissociation barrier of CO₂ on the defective ZrO₂ surface are calculated. Finally, the effect of the presence of oxygen vacancies on the ZrO₂ surface on CO₂ adsorption and activation is pointed out by comparison, and the results obtained throughout the study are summarized and their significance is discussed for the study of high-performance catalysts for CO₂ conversion.

2 Computational details

2.1 DFT method and parameter setting

In this paper, the Dmol3 module in Materials Studio 2019 (BIOVIA Ltd.) is used to complete the required density function theory (DFT) calculations [51,52]. The generalized gradient approximation (GGA) and the Perdue–Burke–Enzerhof (PBE) exchange-correlation general functions are used for exchange-correlation electron energies [53]. The valence electron wave function uses double numerical orbital basis set + d-orbit polarization (DND). The spin polarization of the electrons is considered in the calculation since ZrO₂ is a semiconductor. The vacuum layer is set to 20 Å to ensure that the intermolecular interaction forces between the plates are negligible. The calculated energy, force, and displacement convergence criteria are 2 × 10⁻⁵ Ha, 4 × 10⁻³ Ha/Å, and 5 × 10⁻³ Ha/Å, respectively. A smearing value of 0.05 Ha is used to ensure the accuracy of the calculation. Based on the Monkhorst–Pack method, the Brillouin zone integrals are selected with the sum approximation on the special k points. For the c-ZrO₂(111), t-ZrO₂(101), and m-ZrO₂(−111) surface models, the k values are taken as 2 × 2 × 1, 3 × 6 × 2, and 4 × 3 × 2, respectively. The complete linear synchronous transfer and quadratic synchronous transfer (LST/QST) method was chosen to search for transition states and thus obtain the energy barrier of the reaction.

2.2 Model details

Thermodynamically stable surfaces of different phases of ZrO₂, i.e., c-ZrO₂(111) [54], t-ZrO₂(101) [55], and m-ZrO₂(−111) [56] are used in the study. The optimized perfect surface models of different phases of ZrO₂ are shown in Fig. S1 in Electronic Supplementary Material (ESM). For t-ZrO₂(101) and m-ZrO₂(−111), which have a good periodicity, both take two layers of O−Zr−O. For m-ZrO₂(−111), three O−Zr−O layers are taken. All the bottom O−Zr−O layer is fixed, while the rest is relaxed in optimizing the surface model. The cell parameters used in the calculations fit well with those experimentally determined in previous reports, and the specific numerical comparisons are listed in Tab.1.

2.3 Formula

In this paper, the adsorption energy E_ads, energy barrier E_b, and heat of reaction H of the reaction process are calculated by using Eqs. (1)–(3)

(1)E_{\mathrm{a}\mathrm{d}\mathrm{s}}=E_{\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{t}\mathrm{e}/\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}- E_{\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{a}\mathrm{t}\mathrm{e}}-E_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}},

(2)E_{\mathrm{b}}=E_{\mathrm{T}\mathrm{S}}-E_{\mathrm{I}\mathrm{S}},

(3)H=E_{\mathrm{F}\mathrm{S}}-E_{\mathrm{I}\mathrm{S}},

where E_{adsorbate/surf} is the total energy of the metal surface with adsorbate on the surface, E_adsorbate is the energy of the adsorbate alone, E_surf is the energy of the clear metal surface, E_TS is the energy of the transition state, E_IS is the energy of the reactants, and E_FS is the energy of the products.

3 Results and discussion

3.1 Construction of oxygen vacancies

The surface model with oxygen vacancy (V_O) is constructed as demonstrated in Fig. S2, with the oxygen vacancy sites being labeled. There are only 3-coordinated oxygen atoms on the surfaces of c-ZrO₂(111) and t-ZrO₂(101). Therefore, only one type of oxygen vacancy site is constructed, respectively. The defective c-ZrO₂(111) and t-ZrO₂(101) surfaces are denoted as c-ZrO₂^d(111) and t-ZrO₂^d(101), respectively. Three types of coordinated oxygen atoms are observed on the m-ZrO₂(−111) surface: 2-ligand, 3-ligand, and 4-ligand. Only the three oxygen atoms in the outermost layer are selected to construct the oxygen vacancy. The three oxygen vacancy sites are noted as V_O1, V_O2, and V_O3 according to the horizontal depth of the oxygen vacancy on the model surface. Similarly, the latter is noted as m-ZrO₂^d1(−111), m-ZrO₂^d2(−111), and m-ZrO₂^d3(−111) in order for the m-ZrO₂(−111) surface with defects, where the coordination number of oxygen vacancies on the surfaces of m-ZrO₂^d1(−111) and m-ZrO₂^d2(−111) is 2, and the coordination number of oxygen vacancies on the surface of m-ZrO₂^d3(−111) is 3.

In addition, to clarify the ability of oxygen vacancies to accommodate the C and O atoms in CO₂, the adsorption energies of oxygen vacancies on the C and O atoms are calculated separately. The calculation results obtained are tabulated in Tab.2.

The calculations show that the difference between the adsorption energy of the O atom and that of the C atom on the c-ZrO₂^d surface is the smallest, but still as high as 0.4 eV. On other defective surfaces, the adsorption energy of the O atom is about 1 eV higher than that of the C atom. It can, therefore, be assumed that in the presence of both C and O atoms, the oxygen vacancies on the ZrO₂ surface tend to adsorb O atoms. This is in line with previous reports [59]. However, it is worth noting that the adsorption energy of the C atom at the oxygen vacancy is above −2 eV, which is at a high level. Therefore, in the absence of free O atoms on the surface, the oxygen vacancies on the ZrO₂ surface may adsorb a certain amount of C atoms.

3.2 Adsorption of CO₂

The calculated most stable adsorption configurations of CO₂ on perfect and defective surfaces are exhibited in Fig.1, and the adsorption energies are recorded in Tab.3.

As the shown in Fig.2(a), the adsorption energy of CO₂ on the defective surface is always greater than its adsorption energy on the perfect surface. The adsorption energy can be reduced by a maximum of 5-fold, as shown in Fig.2(b). This indicates that the presence of oxygen vacancies can greatly facilitate the adsorption of CO₂ on the ZrO₂ surface. The largest increase in adsorption energy is found on the c-ZrO₂(111) surface, where the adsorption energy of CO₂ on the perfect surface is −0.198 eV, while on the defective surface the adsorption energy increases to −0.996 eV. For the m-ZrO₂(−111) surface, where V_O3 is present, the increase in adsorption energy is minimal, but the adsorption energy of the defective surface can still be about 2.4 times higher than that of the perfect surface. Moreover, for t-ZrO₂(101), the involvement of oxygen vacancies increases the adsorption energy of CO₂ from −0.198 to −0.996 eV. In fact, for the m-ZrO₂(−111) surface, it is observed that the contribution of oxygen vacancies to the adsorption diminishes as they penetrate deeper into the surface.

3.3 Activation of CO₂

The transition state models for the activation process of CO₂ on perfect and defective surfaces are obtained, as manifested in Fig.3. The calculated energy barrier, reaction heat, and Mulliken atomic charge are presented in Tab.4. The electron density of CO₂ binding on the surface of different phases of ZrO₂ are displayed in Fig.4.

As shown in Fig.5(a), the presence of oxygen vacancies significantly reduces the energy barrier of CO₂ dissociation. The energy barrier of CO₂ dissociation can be reduced to a maximum of 1/5 of a perfect surface, as shown in Fig.5(b). For m-ZrO₂^d2(−111), oxygen vacancies can reduce the energy barrier of CO₂ dissociation from 1.593 to 0.312 eV. For c-ZrO₂^d(111), the oxygen vacancy reduces the energy barrier from 1.671 to 0.463 eV, a reduction of roughly 3.6-fold. For t-ZrO₂^d(101), the energy barrier is reduced from 1.713 to 0.352 eV with the help of oxygen vacancies, a reduction of 4.9 times.

In addition, it can be observed from Fig.6 that of the perfect surfaces, the m-ZrO₂(−111) and t-ZrO₂(101) surfaces are the most suitable for CO₂ activation, whereas, of the defective surfaces, the t-ZrO₂^d(101) surface is the most favorable for CO₂ activation, followed by m-ZrO₂^d2(−111). At the same time, the activation of CO₂ is transformed from an endothermic to an exothermic reaction on all defective surfaces. This suggests that oxygen vacancies can contribute kinetically and thermodynamically to the activation process of CO₂.

The nature of the oxygen vacancies that promotes CO₂ activation can be revealed clearly from the electron perspective. The Mulliken atomic charge listed in Tab.4 indicates that oxygen vacancies could reduce the energy barrier of CO₂ dissociation by increasing the charge transfer from the ZrO₂ surface to the CO₂ molecule. For the c-ZrO₂^d(111) and t-ZrO₂^d(101) surface, the charge transfer can be increased about 2.8 times. For the m-ZrO₂^d2(−111) surface, the enhancement is up to 3.7 times. The electron density of the CO₂ molecule depicted in Fig.4 then visually illustrates the intensity of the electron transfer between the CO₂ molecule and the ZrO₂ surface. It can be noticed that oxygen vacancies greatly facilitate the charge exchange between the CO₂ molecule and the ZrO₂ surface.

4 Conclusions

The processes of CO₂ adsorption and activation on perfect and defective ZrO₂ surfaces are calculated separately in this paper. By comparing the results of the calculations, the following main conclusions can be drawn.

1) The oxygen vacancies on the ZrO₂ surface are more inclined to accommodate O atoms. Meanwhile, when CO is adsorbed on the ZrO₂ surface, the Zr atoms prefer to adsorb the C atom in CO.

2) The adsorption energy of CO₂ is greatest on m-ZrO₂(−111) and t-ZrO₂(101) surfaces in perfect surfaces, and it is greatest on t-ZrO₂^d(101) and m-ZrO₂^d2(−111) surfaces among the defective surfaces. In fact, the presence of oxygen vacancies greatly enhances the adsorption efficiency of CO₂ on the ZrO₂ surface. For the m-ZrO₂^d3(−111) surface, the oxygen vacancies increase the adsorption energy of CO₂ by a factor of more than two, and for c-ZrO₂^d(111) surfaces, the adsorption energy increases by a factor of fully five.

3) The energy barrier for CO₂ dissociation are lowest on t-ZrO₂(101) and m-ZrO₂(−111) surfaces in perfect surfaces while they are lowest on t-ZrO₂^d(101) and m-ZrO₂^d2(−111) surfaces in defective surfaces. The activation process of CO₂ is greatly facilitated by oxygen vacancies, both thermodynamically and kinetically. From a kinetic point of view, the energy barrier of CO₂ dissociation can be reduced to at most 1/5 of its initial value on the t-ZrO₂^d(101) and m-ZrO₂^d2(−111) surface. From the thermodynamic point of view, the activation process of CO₂ on the ZrO₂ surface of different phases changes from an absorbing to an exothermic reaction.

These results could indicate the outstanding contribution of oxygen vacancies to the adsorption and activation of CO₂, and provide guidance for the design of efficient DRM catalysts at an atomic scale.

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