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Article

A Theoretical Study of the Oxygen Release Mechanisms of a Cu-Based Oxygen Carrier during Chemical Looping with Oxygen Uncoupling

by
Minjun Wang
1,*,
Shixiong Zhang
1,
Ming Xia
2 and
Mengke Wang
1
1
School of Energy and Power, Jiangsu University of Science and Technology, Zhenjiang 212003, China
2
Chuzhou Cigarette Factory, China Tobacco Anhui Industrial Co., Ltd., Chuzhou 239000, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(3), 332; https://doi.org/10.3390/catal12030332
Submission received: 2 February 2022 / Revised: 13 March 2022 / Accepted: 14 March 2022 / Published: 15 March 2022
(This article belongs to the Special Issue Catalysts in Chemical Looping Technology for Energy Storage and Carbon Emission Reduction)
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Abstract

:
The Cu-based oxygen carrier is a promising material in the chemical looping with oxygen uncoupling (CLOU) process, while its performance in the CLOU is significantly dependent on the oxygen release properties. However, the study of oxygen release mechanisms in CLOU is not comprehensive enough. In this work, the detailed oxygen release mechanisms of CuO(110) and CuO(111) are researched at an atomic level using the density functional theory (DFT) method, including the formation of O2, the desorption of O2 and the diffusion of O anion, as well as the analysis of the density of states. The results show that (1) the most favorable pathway for O2 formation and desorption occurs on the CuO(110) surface of O-terminated with energy barriers of 1.89 eV and 3.22 eV, respectively; (2) the most favorable pathway for O anion diffusion occurs in the CuO(110) slab with the lowest energy barrier of 0.24 eV; and (3) the total density of states for the O atoms in the CuO(110) slab shifts to a lower energy after an O vacancy formation. All of the above results clearly demonstrate that the CuO(110) surface plays a significantly important role in the oxygen release reaction, and the oxygen vacancy defect should be conducive to the reactivity of oxygen release in a Cu-based oxygen carrier.

1. Introduction

Carbon emissions from fossil fuel utilization are important sources of greenhouse gases (GHGs) that cause the greenhouse effect [1]. To minimize global CO2 emissions, it is critical to develop effective CO2 capture technologies [2]. Chemical looping combustion (CLC) is a potential approach for capturing 100% carbon from fuel at a cheap cost and with low energy penalties [3]. A two-step reaction can be characterized as the CLC process: (i) the fuel is oxidized in the fuel reactor (FR) by an oxygen carrier, usually a metal oxide, to produce a concentrated stream of H2O and CO2. (ii) The reduced oxygen carrier is then re-oxidized by air in an air reactor (AR) in the next step. The oxidized carrier is then separated and transferred to the fuel reactor in preparation for the next cycle [4]. Below are the simplified reaction equations for the two-step reaction.
Fuel + MeO x   →   MeO x - 1 + CO 2 + H 2 O
O 2 + 2 MeO x - 1   →   2 MeO x
Instead of air, the oxygen carrier is employed in CLC to carry oxygen to the fuel. As a result, the CLC has a CO2 separation advantage over traditional combustion, where CO2 is diluted with primary flue gas components, such as nitrogen [5]. In addition, CLC also reduces NOx generation because the fuel burns in the FR without air, and the oxidation of the oxygen carrier occurs at a lower temperature in the absence of fuel [6]. However, solid fuels are difficult to be applied in the traditional CLC process due to the rate-limiting gasification stage of solid fuels. To address this problem, an innovative CLC technology has been suggested, named chemical looping with oxygen uncoupling (CLOU) [7]. The CLOU technology makes use of the characteristics of particular metal oxides, which, at high temperatures, produce gaseous O2. The CLOU process has three reaction steps: (i) the decomposition of the oxygen carrier generates gaseous O2 in the FR; (ii) the fuel is oxidized by the oxygen carrier in the FR; and (iii) the reduced oxygen carrier is re-oxidized by air in the AR. The simplified reaction equations of CLOU are shown below.
2 MeO x   →   O 2 + 2 MeO x - 1
Fuel + O 2   →   CO 2 + H 2 O
O 2 + 2 MeO x - 1   →   2 MeO x
The gaseous O2 generated by the oxygen carrier in the CLOU process significantly speeds up the conversion rate of solid fuels at the desired combustion temperatures (800–1000 °C) [7]. The reactivity and stability of the oxygen carrier are critical for the CLOU process applicability [7]. Many materials appropriate for CLOU have been studied to date, including perovskite-type oxides [8], natural ores [9], synthetic materials [10], and transitional metal oxides [11]. As a consequence of advanced research, transitional metal oxides in a variety of forms, such as nickel, cobalt, iron, manganese, or copper oxides, were confirmed as potential candidates for oxygen carriers [11,12,13,14,15,16]. Among these metal complexes, copper-based oxygen carriers are appealing due to their high reactivity, high oxygen capacity, and environmental friendliness [17]. On the other hand, CuO has certain drawbacks, including (i) a low mechanical strength, which may lead to defluidization, and (ii) a strong agglomeration trend, which may result in low reactivity due to its low melting point [18]. De Diego et al. [19] found that the reaction rate of CuO dropped rapidly with repeated cycles due to agglomeration and sintering.
Experimental studies revealed that using inert materials as a support for CuO can improve chemical stability and mechanical strength [20,21]. Yet, the addition of inert support inevitably reduces the oxygen transfer capacity and redox ability per unit mass. Therefore, further work has to be conducted to increase the reactivity of the Cu-based oxygen carrier in CLOU. According to the principle described above, the performance of oxygen release for oxygen carriers is a crucial issue that determines the reactivity of CLOU. Numerous experimental and theoretical studies have been conducted on the mechanisms of the reaction in CLOU [22,23,24,25]. The results show that oxygen carriers release oxygen at the solid surface, and lattice oxygen diffusion occurs at the same time; consequently, the total reaction rate is controlled by both surface reactions and bulk oxygen diffusion. When exposed to a gaseous atmosphere (reducing gases), oxygenic species on the surface of oxygen carrier react with reducing gases at specific active sites to produce the desired products. As a result of the surface reactions, surface oxygen is consumed, and an oxygen chemical potential gradient develops; oxygen anions must migrate from the bulk phase to the surface [26,27,28]. To achieve a high reactant conversion and improve the desired product selectivity, the control of the oxygen diffusion and the oxygen release from surface pathways is required [29,30,31].
Despite the fact that many experimental methods can effectively observe and track changes in bulk phase oxygen carriers, experimental technologies for studying the microscopic mechanisms of CLOU are severely limited because experimental characterization is difficult to capture and it is difficult to observe the transformation of reactants at extreme-speed time scales [32,33]. Computational simulation has demonstrated significant benefits in the research of material surface structure and characteristics, as well as chemical reaction mechanisms. In the areas of solid material structure simulation, property prediction, and an in-depth understanding of reaction mechanisms, first-principle calculations can often provide knowledge and understanding that cannot be achieved by experiments [34,35,36]. In recent years, first-principles calculations have been widely used for oxygen carriers, such as NiO, Fe2O3, Fe3O4, and perovskites [31,37,38,39,40,41]. Meanwhile, a number of studies research Cu-based oxygen carriers by density functional theory (DFT) calculations. Zhao et al. [42] investigated the CuO–support interaction in Cu-based oxygen carriers using DFT calculations. The effects of several inert supports on the decomposition reactivity and sintering resistance of Cu-based oxygen carriers were compared. Wu et al. [43] systematically calculated the kinetic mechanisms of CO oxidation on the CuO (111) surface using the DFT calculation to consider the adsorption, reaction, and desorption processes. Zhang et al. [44] studied the decomposition mechanisms of CuO/CuAl2O4 and the effects of inert supports on the Cu-based oxygen carriers. The results show that the O anion migrates from the lattice to the surface and forms an O2 complex. Zheng et al. [45] reported the detailed oxidation steps of the Cu-based oxygen carrier by DFT calculations. According to the findings, the bulk Cu2O phase forms preferentially as a result of copper outward diffusion in the Cu(111) bulk, before being oxidized to produce the bulk CuO phase. Mishra et al. [46] correlated the oxygen vacancy creation energy of the perovskite oxygen carriers with their redox properties by DFT calculations and experiment research. The results indicate that vacancy creation energy is an effective indicator for oxygen release properties of metal oxide oxygen carriers.
Although there has been a lot of research on Cu-based oxygen carriers, the study of oxygen release mechanisms in CLOU is still not comprehensive enough. For instance, the CuO(110) surface has not been examined in CLOU, which is a significant crystal face due to the presence of active sites on it, and X-ray diffraction characterization of CuO also revealed that one of the most observed peaks relates to the (110) facet [46,47,48,49,50,51]. In addition, the current research exclusively considers O anion diffusion from the lattice to the surface. The diffusion of the O anion between the sites on the surface has not been studied. The objective of this work is to investigate the oxygen release mechanisms of the Cu-based oxygen carrier during CLOU more comprehensively. The oxygen release processes of CuO(111) and CuO(110) were simulated through DFT calculations. More O anion diffusion paths are considered. Additionally, the effects of oxygen vacancy formed during O anion migration on CuO were studied, and the oxygen vacancy creation energy in CuO was calculated. This study aims to understand the main oxygen release mechanisms and real reaction active sites of CLOU on the Cu-based oxygen carrier, which is helpful in controlling and improving the CLOU process.

2. Results and Discussion

2.1. Calculations of Bulk CuO and O2

The calculated Cu–O bond length (0.196 nm) of the CuO agrees well with the experimental value (0.195 nm) [52]. The calculated lattice parameters for CuO are a = 4.723 Å, b = 3.418 Å, c = 5.161 Å and β = 99.34°, which is comparable to the experimental results (a = 4.683 Å, b = 3.422 Å, c = 5.129 Å and β = 99.54°) [53]. In addition, the bond length of molecule O2 predicted using our approach is 0.124 nm, which is consistent with experimental result of 0.121 nm [54]. The above results in this investigation support the accuracy of the chosen computation method.

2.2. The Mechanisms of Oxygen Release from the CuO Surface

First, the oxygen release reaction on the CuO(110) surface was studied. In the preliminary work, the oxygen release process on the CuO(110) surface of Cu-terminated was simulated. However, the calculation results show that it is difficult to form stable O2, probably because the oxygen atoms of the Cu-terminated surface are not in the surface layer, which is very stable, and it is difficult to break the bond with the Cu atoms and migrate to the outermost layer. Therefore, the oxygen release process on the CuO(110) surface of O-terminated is simulated and analyzed here.
The IS column in Figure 1 shows the relaxed structure of the CuO(110) surface of O-terminated. The oxygen atoms on the CuO(110) surface of O-terminated are of the same chemical type as the three-fold coordinated atoms. Yet, oxygen atoms may be in different environments and combine in different paths. O1, O2, O3, and O4 were assigned to the four oxygen atoms on the CuO(110) surface of O-terminated (see Figure 1). In order to determine the possible O2 formation process, an O1 atom was removed and reintroduced to bond with three neighboring O atoms, including O2, O3, and O4. Subsequently, these configurations were optimized. The optimization results show that the O1 atom forms bonds with the O2, O3 and O4 atoms with bond lengths of 1.466 Å, 1.276 Å, and 1.370 Å, respectively. These bond lengths are close to those of the free O2 molecule (1.124), implying that an O2 complex is formed in the three structures. Therefore, the O2 formation processes of O1-O2, O1-O3, and O1-O4 were considered and denoted by path 1 (P1), path 2 (P2), and path 3 (P3), respectively. For the calculation of O2 formation processes, the perfect CuO(110) surface of O-terminated was set as the initial state (IS). The final structure was the stable configuration with the O2 complex, which was denoted by the IM (intermediate state). Then, a transition state (TS) search calculation was performed. The TS structure and the corresponding energy profile for the process are shown in Figure 1 and Figure 2. The energy barriers of the path 1, path 2, and path 3 are 1.89 eV, 4.47 eV, and 4.13 eV, respectively. In addition, all three processes are endothermic by 1.72 eV, 2.38 eV, and 1.91 eV, respectively. These results suggest relatively large differences in the three paths of O2 formation processes.
Subsequently, the O2 desorption processes following the O2 formation were simulated. The O2 complex formed in the intermediate state was removed and placed at a distance of 8 Å from the CuO(110) surface of O-terminated. The structures were then optimized and labeled with the FS (final state) (see Figure 1). Thus, a TS search calculation was used to determine the energy barrier between the IM and the FS. The results (see Figure 1) show that all the three O2 desorption processes are largely endothermic by 3.22 eV, 4.13 eV, and 4.21 eV, respectively, which indicates that the oxygen is only released at high temperatures. This is consistent with the thermodynamic calculation result that the temperature of oxygen release on CuO needs to be greater than 800 °C [7]. Furthermore, due to the low energy barriers for O2 formation and desorption, path 1 is regarded as the most favorable pathway for oxygen release on the CuO(110) surface of O-terminated. In the path 1 process, an O2 complex is formed in the IM structure, and the O-O bond length is 1.466 Å. Then, the Cu–O bond is ruptured with a desorption energy of 3.22 eV, and the O2 is desorbed from the CuO(110) surface. The length of the new O–O bond is 1.241 Å, which is similar to the length of a free O2 molecule (1.240 Å).
To provide further insight into the oxygen release reaction, the density of states (DOS) was investigated. Figure 3 shows the DOS for O atoms (O1, O2), O2 complex, and free O2 molecule in the path 1 process, respectively. The DOS of O atoms in the IS delocalizes over a wide range due to the strong Cu–O bonds that exist in the CuO. In the IM, two peaks are observed in s orbitals, which indicates the formation of an O2 complex. However, a large delocalization still appears in the p orbitals, which indicates that the O2 complex still has strong interaction with the CuO(110) surface. After O2 desorption, the DOS of O2 in the FS was nearly the same as that of a free O2 molecule, implying that free O2 molecules are generated.
Next, the oxygen release reaction on the CuO(111) surface was simulated. Of particular note, the oxygen release reaction on the CuO(111) surface was studied by the work of Zhang et al. [44]. However, the calculation methods were slightly different from those in this paper. In order to make the results in this work as comparable as possible, the O2 formation and desorption processes were still calculated here, and the results are shown in Figure 4 and Figure 5. There are two pathways for oxygen release on the CuO(111) surface, which are denoted as path 1 (P1) and path 2 (P2). The details of model construction are not repeated here. The energy barriers of O2 formation in path 1 and path 2 are 3.18 eV and 2.47 eV, respectively. Both O2 desorption processes are largely endothermic by 3.61 eV and 3.31 eV, respectively. The calculated results are quite similar to those that are reported in the literature [44] (see Table 1). Comparing the calculation results of the CuO(110) surface with that of the CuO(111) surface reveals that there is the lowest energy barrier and total energy in path 1 on the CuO(110) surface. In particular, the activation energy values of oxygen release on CuO studied by experiments (3.26 eV and 3.41 eV) [55,56] were close to the energy barrier of path 1 on the CuO(110) surface, while largely smaller than that of other pathways. The analysis above indicates that the CuO(110) surface may also play an important role in the oxygen release reaction, and the active sites in the oxygen release process on the CuO surface exist in pairs.

2.3. The O Anion Diffusion Mechanisms in the CuO

As a result of the O2 desorption from the CuO surface, surface oxygen is consumed. Therefore, oxygen anions must diffuse to the surface to maintain the oxygen release reaction. First, the O anion diffusion processes in the CuO(110) slab were studied. According to the research results of the oxygen release from CuO(110) surface, there are active sites on the surface during the oxygen release process. Thus, the O anion will diffuse to the active sites. In view of this, one active site for the O atom was labeled as O1, and the three neighboring atoms on the CuO(110) surface were labeled as O2, O3, and O4, respectively (see Figure 6).
For this calculation, the initial state (IS) was defined as an optimized CuO(110) structure with a surface O1 vacancy. The structure CuO(110) with a surface O2 (O3 or O4) vacancy was optimized and denoted as the final state (FS). Accordingly, three O anion diffusion paths were constructed and denoted as path 1 (P1), path 2 (P2), and path 3 (P3), respectively. In addition, considering that O anion may diffuse from the lattice to the surface, the fourth pathway was constructed and denoted by path 4 (P4), in which IS was defined as an optimized CuO(110) structure with a surface oxygen vacancy (denoted as VOsuf) and the FS was defined as an optimized CuO(110) structure with a subsurface oxygen vacancy (denoted as VOsub). Figure 6 and Figure 7 show the TS search results and the corresponding energy profile for the processes. The energy barriers of path 1, path 2, path 3, and path 4 are 0.51 eV, 1.75 eV, 1.32 eV, and 0.24 eV, respectively, indicating that the energy barriers of O anion diffusion in CuO(110) are extremely low. In particular, the energy barrier of O anion diffusion in path 4 (0.24 eV) is significantly lower than that of the O2 formation (1.89 eV, 4.13 eV, 4.47 eV) and O2 desorption processes (3.22 eV, 4.13 eV, 4.21 eV). In addition, all the four processes are exothermic by −0.43 eV, −0.48 eV, −0.47 eV, and −0.02 eV, respectively. These results imply that O anion diffusion in CuO(110) can react readily and is energetically favorable.
Next, the O anion diffusion processes in the CuO(111) slab were studied. The O4f atom migrating to O3f vacancy, O4f atom migrating to O4f vacancy, O3f atom migrating to O3f vacancy, and subsurface oxygen atom (Osub) migrating to surface oxygen vacancy (VOsuf) were considered as four possible O anion diffusion paths in the CuO(110) slab. Correspondingly, the four paths were denoted by path 5 (P5), path 6 (P6), path 7 (P7), and path 8 (P8), respectively. The optimized structures of IS and FS are shown in Figure 8. The reaction pathway and energy profile for the O anion diffusion in the CuO(110) slab are shown in Figure 8 and Figure 9. The energy barriers of path 5, path 6, path 7, and path 8 are 1.67 eV, 1.58 eV, 1.16 eV, and 0.74 eV, respectively, which are also rather small. The results indicate that the O2 formation and desorption are the rate limiting steps for oxygen release from CuO. Therefore, the key to improving the reactivity of Cu-based oxygen carriers is to decrease the energy barrier of O2 formation and desorption processes. In addition, the average energy barrier of O anion diffusion in the CuO(111) slab is higher than that in the CuO(110) slab. Moreover, the path 5 and path 6 are endothermic by 0.40 eV and 0.28 eV, respectively. On the contrary, path 7 and path 8 are exothermic by −0.13 eV and −0.47 eV, which suggests that path 7 and path 8 are energetically favorable. In addition, the average reaction energy of O anion diffusion in the CuO(111) slab is also higher than that in the CuO(110) slab. By comparing the two slabs, path 4 in the CuO(110) slab is regarded as the most favorable pathway for O anion diffusion due to the lowest energy barrier and exothermic process. In the diffusion process of path 4, the Osub atom migrates from the original location to the intermediate position between the surface and subsurface O vacancy. In addition, the subsurface Cu-Osub bond is gradually rupturing. Before populating the surface oxygen vacancy, the Osub atom bonds to the surface Cu atom. Given the above results, it can be found that the O anion migrates more easily in the CuO(110) slab, which further confirms the possibility that CuO(110) is important during oxygen release from the CuO. If the ratio of CuO(110) can be increased, though it is a difficult challenge, the oxygen release performance of the Cu-based oxygen carrier will be effectively improved. Nevertheless, there is a similarity between CuO(110) and CuO(111) slabs, in that paths 4 and 8 have the lowest energy barriers in their respective slabs, indicating that the O anions diffuse more easily from the lattice to the surface.
To provide further insight into the O anion diffusion process, the density of states in the IS, TS, and FS was investigated. The partial density of states (PDOS) of the O atom during the diffusion process in path 4 is presented in Figure 10. The electrons of the O atom in IS delocalize in a wide range near the Fermi energy level (Ef = 0 eV), implying that the O atom is more chemically active. In comparison to IS, the DOS of the O atom in TS has a higher peak at approximately −4.98 eV, indicating that an energy barrier is required to transition from IS to TS. In addition, the p-orbital of the O atom in FS is similar to that in IS, while the s-orbital of the O atom in FS is slightly lower than that in IS, which is consistent with the finding that the O anion diffusion process in path 4 has an energy barrier of 0.24 eV and is exothermic by −0.02 eV.
Moreover, the energy barriers of the O anion diffusion process are substantially lower than those of the O2 formation and desorption processes, which could be caused by the O vacancy in CuO. In order to illustrate this, the density of states of the perfect CuO(110) slab and that with an O vacancy were studied. The total density of states (TDOS) and the PDOS of the O atoms are presented in Figure 11. It is observed that the TDOS for the CuO(110) slab with an O vacancy is more flat than that for the perfect CuO(110) slab, suggesting a larger delocalization in the former. In addition, for the perfect CuO(110) slab, the Cu d-orbital is hybridized with the O p-orbital as shown by the state overlap at approximately −5.84 eV and 0.10 eV, respectively. While the hybridized peaks occur at approximately −5.70 eV and 0.32 eV in the CuO(110) slab with an O vacancy. As seen from Figure 9, both hybridized peaks shift lower in energy after the formation of an O vacancy, indicating a weaker interaction of Cu with the O atom in the CuO(110) slab with an O vacancy, which facilitates O anion migration. This result may provide an idea for increasing the reactivity of oxygen release, that is, increasing oxygen vacancies. However, the oxygen vacancy creation energy in CuO was calculated to be 7.4 eV. The positive value means that energy is needed to create an oxygen vacancy in CuO.

3. Calculation Method and Model

All DFT calculations were performed using the Cambridge Serial Total Energy Package (CASTEP) program suite [57]. Ultrasoft pseudopotentials were used to represent electron–ion interactions [58]. According to previous work conducted by our research group [47,59], the electron exchange correlation functionals of the Perdew–Wang (PW91) approximation based on the generalized gradient approximation (GGA) [60] offered the most accurate geometrical parameters of bulk CuO. As a result, the GGA-PW91 was employed to perform the calculations. The Hubbard parameter (U) was proposed for the Cu 3d orbitals to correctly reflect the high electron correlations in transition metals. We evaluated the DFT + U with U ranging from 0 to 10 eV and found that for the Cu 3d orbitals, U = 7.5 eV produced appropriate results. Thus, we applied U = 7.5 eV to the Cu 3d orbitals throughout this study. The linear/quadratic synchronous transit (LST/QST) approach [61] was used to study the transition states for each reaction.
It is critical to develop proper models for oxygen release reaction research. The unit cell of CuO that is a monoclinic structure and belongs to the C2/c1 space group is depicted in Figure 12. The surface of CuO(110) was modeled using the supercell method, in which a periodic boundary condition was added to the center supercell, causing it to be replicated periodically throughout space. Taking into consideration that the CuO (110) surface can be terminated by O or Cu atoms, the two models that p(2 × 2) super lattice CuO(110) slab models of the O-terminated and the Cu-terminated four atomic layers were constructed, respectively. Similarly, the p(2 × 2) super lattice CuO(111) slab models of three layers were constructed. There are four chemically distinguishable types of atoms at the CuO(111) surface: three-fold coordinated Cu atoms, four-fold coordinated Cu atoms, three-fold coordinated O atoms, and four-fold coordinated O atoms, which have been labeled as Cu3f, Cu4f, O3f, and O4f, respectively. To avoid interactions between the periodic slabs of atomic layers, a 15 Å vacuum layer was used to separate the slab models. In the calculations, the bottom layer was kept fixed while the remaining layers were allowed to fully relax. The O2 molecules were allowed to relax. All the surface configurations are shown in Figure 13.
The cut-off energy (350 eV) and the density of the Monkhorst Pack scheme k-point of 4 × 4 × 1 were chosen to be sufficiently high. The kinetic energy cut-off employed in all calculations was 350 eV, and was used together with the Monkhorst Pack scheme k-point of 4 × 4 × 1. The criteria for the convergence of all calculations in this work were set to (i) an energy tolerance of 2.0 × 10−5 eV/atom; (ii) a maximum force tolerance of 0.05 eV/Å; (iii) a self-consistent field tolerance of 2.0 × 10−6 eV/atom; and (iv) a maximum displacement tolerance of 0.002 Å. The following equation was used to calculate the energy barriers (Eb) of various reactions:
E b = E TS − E IS
where EIS and ETS are the energies of the initial state (IS) and the transition state (TS), respectively. The oxygen vacancy creation energy (Evac) is calculated by the following equation:
E vac = E def + 0 . 5 E O 2 − E per
where Edef is the energy of a defective CuO supercell containing one oxygen vacancy, and E O 2 is the energy of the ground state of an isolated diatomic oxygen molecule in the gas phase. Eper is the energy of the CuO supercell. A positive value of Evac means that energy is needed to create an oxygen vacancy.

4. Conclusions

In this study, the detailed oxygen release mechanisms of CuO(110) and CuO(111) were researched at an atomic level using the DFT method. A total of five oxygen release paths on CuO(110) and CuO(111) surfaces were considered. The most favorable pathway for oxygen formation and desorption occurs on the CuO(110) surface of O-terminated. The energy barriers of oxygen formation and desorption processes are 1.89 eV and 3.22 eV, respectively, which are largely lower than other paths. The results indicate that there are active sites on the CuO surface during the oxygen release process, and they exist in pairs. The O anion diffusion processes in the CuO were further investigated. A total of eight O anion diffusion paths in CuO(110) and CuO(111) slabs were considered. The most favorable pathway for O anion diffusion is also found in the CuO(110) slab with the lowest energy barrier of 0.24 eV. By comparing both surfaces, it can be concluded that the CuO(110) surface may play a significantly important role in the oxygen release reaction due to the lowest energy barriers. On the other hand, the energy required for the subsurface oxygen to migrate to the surface oxygen vacancy is the lowest among all the O anion diffusion paths, suggesting that the O anions diffuse more easily from the lattice to the surface. In addition, the energy barriers of the O anion diffusion process are found to be substantially lower than those of the O2 formation and desorption processes, indicating the O2 formation and desorption are the rate-limiting steps for oxygen release from CuO, and decreasing the energy barriers of O2 formation and desorption is the key to improving the reactivity of Cu-based oxygen carriers in CLOU. The density of states of the perfect CuO(110) slab and that with an O vacancy were studied. The results show that the TDOS for the O atoms in the CuO(110) slab shifts lower in energy after an O vacancy formation, which implies that the oxygen vacancy defect is conducive to O anion migration, which may provide an idea for increasing the reactivity of oxygen release. However, the oxygen vacancy creation energy in CuO is considerably high (7.4 eV), which means that energy is needed. Therefore, how to reduce oxygen vacancy creation energy is a topic worth studying in the future. This study would be helpful in rationalizing the development of a high-performance oxygen carrier for CLOU.

Author Contributions

Data curation, M.W. (Minjun Wang) and S.Z.; Investigation, M.W. (Minjun Wang), S.Z., M.X. and M.W. (Mengke Wang); Writing—original draft, M.W. (Minjun Wang) and M.X.; Writing—review and editing, M.W. (Minjun Wang), S.Z., M.X. and M.W. (Mengke Wang). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 51806089), the General Project of Natural Science Research of Jiangsu Universities (No. 18KJD470001).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The initial state (IS), transition state (TS), intermediate state (IM), and final state (FS) structures for the O2 formation and O2 desorption processes in different pathways on the CuO(110) surface of O-terminated. The bond lengths are presented in Å. The salmon pink and red spheres denote Cu and O atoms, respectively.
Figure 1. The initial state (IS), transition state (TS), intermediate state (IM), and final state (FS) structures for the O2 formation and O2 desorption processes in different pathways on the CuO(110) surface of O-terminated. The bond lengths are presented in Å. The salmon pink and red spheres denote Cu and O atoms, respectively.
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Figure 2. Potential energy diagram for the O2 formation and desorption processes following different pathways on the CuO(110) surface of O-terminated.
Figure 2. Potential energy diagram for the O2 formation and desorption processes following different pathways on the CuO(110) surface of O-terminated.
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Figure 3. The DOS for O atoms (O1, O2), O2 complex, and free O2 molecule in the path 1 process.
Figure 3. The DOS for O atoms (O1, O2), O2 complex, and free O2 molecule in the path 1 process.
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Figure 4. The initial state (IS), transition state (TS), intermediate state (IM), and final state (FS) structures for the O2 formation and O2 desorption processes in different pathways on the CuO(111) surface. The bond lengths are presented in Å. The salmon pink and red spheres denote Cu and O atoms, respectively.
Figure 4. The initial state (IS), transition state (TS), intermediate state (IM), and final state (FS) structures for the O2 formation and O2 desorption processes in different pathways on the CuO(111) surface. The bond lengths are presented in Å. The salmon pink and red spheres denote Cu and O atoms, respectively.
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Figure 5. Potential energy diagram for the O2 formation and desorption processes following different pathways on the CuO(111) surface.
Figure 5. Potential energy diagram for the O2 formation and desorption processes following different pathways on the CuO(111) surface.
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Figure 6. The initial state (IS), transition state (TS), and final state (FS) structures for the O anion diffusion processes in different pathways on the CuO(110) surface of O-terminated. The salmon pink and red spheres denote Cu and O atoms, respectively.
Figure 6. The initial state (IS), transition state (TS), and final state (FS) structures for the O anion diffusion processes in different pathways on the CuO(110) surface of O-terminated. The salmon pink and red spheres denote Cu and O atoms, respectively.
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Figure 7. Potential energy diagram for the O anion diffusion processes in different pathways on the CuO(110) surface of O-terminated.
Figure 7. Potential energy diagram for the O anion diffusion processes in different pathways on the CuO(110) surface of O-terminated.
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Figure 8. The initial state (IS), transition state (TS), and final state (FS) structures for the O anion diffusion processes in different pathways on the CuO(111) surface. The salmon pink and red spheres denote Cu and O atoms, respectively.
Figure 8. The initial state (IS), transition state (TS), and final state (FS) structures for the O anion diffusion processes in different pathways on the CuO(111) surface. The salmon pink and red spheres denote Cu and O atoms, respectively.
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Figure 9. Potential energy diagram for the O anion diffusion processes in different pathways on the CuO(111) surface.
Figure 9. Potential energy diagram for the O anion diffusion processes in different pathways on the CuO(111) surface.
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Figure 10. The partial density of states (PDOS) of the O atom during diffusion process in the path 4.
Figure 10. The partial density of states (PDOS) of the O atom during diffusion process in the path 4.
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Figure 11. The TDOS of Cu(110) slab and the PDOS of the O atoms.
Figure 11. The TDOS of Cu(110) slab and the PDOS of the O atoms.
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Figure 12. Schematic representation of a CuO unit cell. The salmon pink spheres denote Cu atoms, and the red spheres denote O atoms, respectively.
Figure 12. Schematic representation of a CuO unit cell. The salmon pink spheres denote Cu atoms, and the red spheres denote O atoms, respectively.
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Figure 13. (a) O-terminated CuO(110) surface, (b) Cu-terminated CuO(110) surface, (c) the top view of a CuO(111) slab with different types of Cu and O atoms, and (d) the side view of a CuO(111) slab with different types of Cu and O atoms. The salmon pink spheres denote Cu atoms, and the red spheres denote O atoms, respectively.
Figure 13. (a) O-terminated CuO(110) surface, (b) Cu-terminated CuO(110) surface, (c) the top view of a CuO(111) slab with different types of Cu and O atoms, and (d) the side view of a CuO(111) slab with different types of Cu and O atoms. The salmon pink spheres denote Cu atoms, and the red spheres denote O atoms, respectively.
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Table
Table 1. The energy barriers of O2 formation (Ef), O2 desorption (Ed), total energy (Et), and activation energy (Ea).
Table 1. The energy barriers of O2 formation (Ef), O2 desorption (Ed), total energy (Et), and activation energy (Ea).
PathwayEf (eV)Ed (eV)Et (eV)Ea (eV)
CuO(110) surfaceP11.893.224.94–
P24.474.136.51–
P34.134.216.12–
CuO(111) surfaceP12.473.315.45–
P23.183.616.24–
CuO(111) surface in literatureP12.33.55.2
P23.03.16.0
Experiment values––––3.26
––––3.41
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Wang, M.; Zhang, S.; Xia, M.; Wang, M. A Theoretical Study of the Oxygen Release Mechanisms of a Cu-Based Oxygen Carrier during Chemical Looping with Oxygen Uncoupling. Catalysts 2022, 12, 332. https://doi.org/10.3390/catal12030332

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Wang M, Zhang S, Xia M, Wang M. A Theoretical Study of the Oxygen Release Mechanisms of a Cu-Based Oxygen Carrier during Chemical Looping with Oxygen Uncoupling. Catalysts. 2022; 12(3):332. https://doi.org/10.3390/catal12030332

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Wang, Minjun, Shixiong Zhang, Ming Xia, and Mengke Wang. 2022. "A Theoretical Study of the Oxygen Release Mechanisms of a Cu-Based Oxygen Carrier during Chemical Looping with Oxygen Uncoupling" Catalysts 12, no. 3: 332. https://doi.org/10.3390/catal12030332

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