Density Functional Theory plus U Study of Methanol Adsorption and Decomposition on CuO Surfaces with Oxygen Vacancy

Received: November 13, 2021
Revised: December 20, 2021
Accepted: December 20, 2021

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

Cite this article:

Liu, L., Gong, P., Shao, G., Liu, P., Wang, J. (2022). Density Functional Theory plus U Study of Methanol Adsorption and Decomposition on CuO Surfaces with Oxygen Vacancy. Aerosol Air Qual. Res. 22, 210253. https://doi.org/10.4209/aaqr.210253

HIGHLIGHTS

• CH₃OH adsorption occurs on Cu top site by binding with O_MeOH.
• The adsorption on Cu_3C sites is more stable than on Cu_4C sites.
• Dissociative adsorption is observed on CuO with and without oxygen vacancy.
• Oxygen vacancy enhances CH₃OH adsorption and H-O bond scission.
• Binding energy of CH₃O is higher than CH₃OH, especially on defected surface.

Keywords: CuO, Methanol, Adsorption, Decomposition, Density functional theory

1 INTRODUCTION

Volatile organic compounds (VOCs) are one of the main factors leading to atmospheric environment deterioration and are important precursors of PM_2.5, ozone, and photochemical smog (Alvim et al., 2018). Methanol (CH₃OH) is a typical VOC and also an intermediate product or by-product in VOC oxidation (Wang et al., 2019; Sanito et al., 2020), as well as being a threat to human health, urban air and water. Thus, CH₃OH decomposition is a worthy topic in the research of VOC control. Most CH₃OH decomposition processes are catalytic reactions on surfaces with active sites. Precious metals (PM) are the most effective catalysts for CH₃OH oxidation (Liang et al., 2021). However, its scarcity and toxic sensitivity are adverse to large-scale application. Metal oxides (MO) are promising alternatives for decomposing CH₃OH at low temperatures and also economical with high efficiency (Norsic et al., 2016). Active oxygen species have been shown on MO surfaces, which could enhance further decomposition of intermediates and by-products (Cao et al., 2016; Liu et al., 2017).

Among MO catalysts, copper oxide (CuO) has been widely studied and used, as it is a kind of reducible oxide and demonstrates excellent catalytic activity as well as high selectivity for CH₃OH decomposition. Even in PM/CuO co-catalysts, CuO nanoparticles play an important role in CH₃OH oxidation (Sharma et al., 2019). Cu-based composite oxides show outstanding performance on CH₃OH oxidation because of facilitated electron transfer that lowers the reaction activation barrier (Norsic et al., 2016). For instance, on CuO-CuWO₄, CH₃OH molecules decompose to surface adsorbed methoxy (CH₃O) species, which then partly decompose into CH_x and possibly carbon monoxide (CO) (Blatnik et al., 2018). The cobalt (Co) in CuCoOx promotes the formation of Cu⁰ and Cu⁺, thus improving catalytic activity in CH₃OH decomposition (Wei et al., 2019). In MnCuO_x catalysts, manganese as Mn³⁺ and Mn⁴⁺ cations coexist in these oxides, which promote electronic charge transfer and increase catalytic activity (Figueroa et al., 2005). Lee et al. (2014) have found that adding Mn to CuZn oxide catalysts enhances formate decomposition and reduces the accumulation of carbonate species. CuO can also serve as an active support for activate components and then enhances catalytic activity for CH₃OH decomposition (Nazir et al., 2020; Ziadi et al., 2020). A synergistic effect has been found between PMs, MOs, and CuO, which accelerates VOC decomposition and CO oxidation (Yang et al., 2020; Zhang et al., 2021).

Density functional theory (DFT) has been employed extensively in exploring CH₃OH decomposition on metal surfaces. Studies of CH₃OH adsorption on MO surfaces have suggested that the dehydrogenation pathway of CH₃OH depends strongly on surface conditions. Mei et al. (2008) have found that surface coverage affects the thermodynamic stability of C-H, O-H and C-O bonds in CH₃OH on CeO₂ surfaces. Kinetically, the activation barrier of O-H bond cleavage in CH₃OH is 0.30 eV on CeO₂ (110) surfaces and only 0.08 eV on (111) surfaces, while activation barriers for C-O and C-H bond cleavage are 1.08 and 2.05 eV, respectively. In other studies, it has been verified that O-H bond scission and CH₃O formation on MO surfaces are the most possible pathway of CH₃OH decomposition (Vo et al., 2013; Sun et al., 2014). The introduction of oxygen atoms on clean CuO(111) surfaces reduces the activation barrier of O-H bond dissociation (Sun et al., 2014). Cu⁺ is an active species in CH₃OH dehydrogenation and formaldehyde formation, with Cu reduction involving two key steps, from Cu⁺² to Cu⁺ with oxygen transposition and from Cu⁺ to Cu⁰ with proton transfer, which finally produces formaldehyde (Ceron et al., 2011). On zinc oxide (ZnO) surfaces, O-H bond dissociation is also the first step in CH₃OH decomposition. C-H bond breaking subsequently occurs and methoxide is formed (Vo et al., 2013). On Fe₃O₄(111) surfaces, dissociative adsorption is thermodynamically more favorable compared to molecular adsorption (Li and Paier, 2019).

To understand of the impact of surface defects on CH₃OH adsorption, in this study, the influence of oxygen vacancies on CuO(111) surfaces in CH₃OH adsorption was analyzed in detail in this study using DFT calculations with a Hubbard U correction (DFT + U). The topographic signatures and electronic structures of the defective surfaces with adsorbed CH₃OH were identified. Adsorption of CH₃O that formed by O-H bond scission of CH₃OH were also examined to extend the understanding of CH₃OH decomposition.

 
2 METHODS

 
2.1 Computational Details

DFT calculations were performed using the VASP 5.4.4 (Kresse and Furthmuller, 1996a, b) using the Perdew-Burke-Ernzerhof (PBE) form in generalized gradient approximation (GGA) to treat electron exchange-correlation (Perdew et al., 1992; Perdew and Wang, 1992) with the projector-augmented wave (PAW) method (Blochl, 1994; Kresse and Joubert, 1999). CuO(111) was selected as the adsorptive surface because it is the most probable facet to be exposed and the most stable surface with the lowest surface free energy (Hu et al., 2010; Hu et al., 2017). The (111) surface was built using CuO unit cell with a lattice constant of a = 4.70 Å, b = 3.43 Å, c = 5.14 Å and β = 99.54°, which were obtained by calculations and agree with earlier experimental studies (Åsbrink and Norrby, 1970). The slab model was built with three atomic layers thick and a p(4 × 2) supercell was used, as can be seen in Fig. 1. A 15 Å vacuum gap was added in the direction perpendicular to the surface.

Fig. 1. (a) Side view and (b) top view of CuO(111) surface.

The Hubbard U term (Dudarev et al., 1998) was applied to accurately correct the strong on-site Coulomb interactions of Cu 3d states. According to previous study, the U value was chosen as 7 eV for Cu 3d with a local magnetic moment of 0.65 µB per Cu atom (Nolan and Elliott, 2006; Mishra et al., 2016). Relaxation of the model was carried out until the maximum Hellmann-Feynman force was less than 0.02 eV Å^–1. An 2 × 2 × 1 and 7 × 7 × 1 Monkhorst-Pack grid was used for relaxation and density of states calculations, respectively. The adsorption energies (E_ads, kJ mol^–1) and charge density difference (CDD, Δρ, e Å^–3) plots were defined similar to our previous work (Liu et al., 2019a).

 
2.2 Surface properties

In the bulk of CuO, Cu and O atoms have a coordination number of 4. On its surface, half of Cu and O atoms have a coordination number of 3 and form two kinds of Cu and O sites (Fig. 1). After relaxation, Cu_3C atoms were drawn to O atoms on the second lower layer and shiftd to a place 0.14 Å lower, while Cu_4C atoms rose 0.04 Å. O_3C that were at the outmost layer were 0.09 Å lower and O_4C at a lower layer were 0.08 Å higher after relaxation. Hence, Cu_3C-O4C and Cu_3C-O3C bonds changed to 1.95 and 1.85 Å from 1.97 Å, respectively. The Cu_4C-O3C bond was 1.93 and 1.99 Å, which was shorter than the original lengths 1.95 and 1.97 Å, respectively. The Cu_4C-O4C bond was 1.97 and 2.04 Å, much longer than its original length. The relaxed surface structure was in good agreement with previous calculations (Yu et al., 2017).

 
3 RESULTS AND DISCUSSION

 
3.1 CH₃OH Adsorption on CuO(111) Surfaces

CH₃OH adsorption on CuO(111) surfaces was studied regarding both Cu and O sites. After relaxation CH₃OH molecules preferentially adsorbed to Cu top sites with O_MeOH atoms and weak O_3C-H bonds formed simultaneously (Fig. 2). The adsorption energy and structural parameters for CH₃OH adsorption on CuO(111) surfaces showed that adsorption on Cu_3C sites was more stable than on Cu_4C sites, with higher binding energy and shorter Cu-O_MeOH and H-O_CuO bonds (Table 1). The highest binding energy was –0.69 eV (MeOH-3C1), with a Cu-O_MeOH distance of 2.05 Å and a H-O_3C distance of 1.69 Å. The H-O_MeOH and C-O_MeOH distance were elongated to 1.01 Å and 1.44 Å, respectively. The adsorption of CH₃OH and CH₃O on CuO(111) surfaces was examined by GGA-BLYP without considering the Hubbard U term (Sun et al., 2014). Similar configurations of MeOH-4C1 and MeOH-3C3 were achieved with lower adsorption energy and longer Cu-O_MeOH and H-O_3C bonds. MeOH-4C2, MeOH-3C1 and MeOH-3C2 were newfound configurations in this study.

Stable configurations were also achieved when the O-H bond in CH₃OH was stretched to a length of 1.61 and 1.67 Å (Fig. 2(b) and Table 1). Meanwhile, H-O_CuO bonds formed with lengths of 1.02 and 1.00 Å and the Cu-O_MeOH distance shortened to 1.88 and 1.86 Å, because interactions between Cu and O_MeOH were enhanced. This distance was still longer than the Cu-O_MeOH bond in CH₃O adsorption discussed below, indicating that weak bonds between H and O_MeOH remained. Compared to their corresponding configuration in Fig. 2(a), the adsorption energy did not significantly change. Dissociated adsorption was only found on Cu_3C and O_3C sites.

 Fig. 2. Side view of the configurations of CH₃OH adsorbed on CuO(111) surfaces, (a) common chemiadsorption, (b) dissociated adsorption.

Charge transfer between adsorbed CH₃OH and CuO surfaces was characterized using the CDD plots (Fig. 3). For MeOH-3C2, a large charge accumulation was detected in the region between O_MeOH and CuO, as well as H and O_CuO, and charge depletion observed in the region around H_CH3. For MeOH-3C5, there was charge accumulation around the region of H_CH3. The partial density of states (PDOS) plot of O_MeOH and H atoms that bound to the surface showed that, in free CH₃OH, the orbitals of O_MeOH and H were superimposed, at –0.19, –1.90, –4.20, –4.47, –6.38 and –10.75 eV (Fig. 4). After adsorbing on a CuO surface, the orbitals of O_MeOH and H in MeOH-3C2 shifted to an energy ~2 eV lower than free CH₃OH, with disappearance of the peaks at –0.19 and –1.9 eV near Fermi level, indicating the outer electrons shifted to a more stable state. Dispersion peaks spread from –1.19 eV to –4.30 eV appeared with main peaks at –2.70 and –4.12 eV. The H orbitals overlapped with O_MeOH orbitals at –4.12, –8.38 and –12.69 eV, indicating the H-O_MeOH bond still existed. The orbital of O_MeOH had other peaks at –2.70 and –6.43~–7.09 eV, caused by C-O and Cu-O bonds. In MeOH-3C5, because the H atom preferentially interacted with O_3C, the PDOS of O_3C and H was clearly superimposed, mainly at –7.8 eV (spin down) and –8.32 eV (spin up). These two peaks were the major peaks of O_3C orbitals and H orbitals. On a clean surface, the PDOS of O_3C was mainly near the Fermi level (Fig. S1). After binding with H, states of O_3C near the Fermi level became inconspicuous and the H states near the Fermi level weakened to invisible, indicating that the formation of H-O_3C bond made the H and O_3C atoms became stable. On the contrary, although the O_MeOH orbitals near Fermi level shifted to states a litter lower, the states near Fermi level became stronger, indicating that O_MeOH became more reactive. The orbitals of O_MeOH and H still exhibited superposition at –4.50, –7.8 and –8.32 eV, indicating that weak interactions between H and O_CuO remained.

Fig. 3. CDD plot of CH₃OH adsorbed on CuO(111) surface with an isosurface level of 0.015 e Å^–3; charge accumulation is in yellow and charge depletion is in blue.

Fig. 4. PDOS of (a) O_MeOH, O_3C and (b) H atoms in free CH₃OH, MeOH-3C2 and MeOH-3C5. The energy at E = 0 eV represents the Fermi energy.

3.2 CH₃OH Adsorption on the CuO(111) Surface with Oxygen Vacancies

Oxygen vacancies are a kind of surface defect which is ubiquitous on metal oxides surfaces and can be formed on CuO surfaces inevitably (Tang et al., 2016; Liu et al., 2019b). Oxygen vacancies are essential for the oxidative activity of CuO (Liu et al., 2018; Davo-Quinonero et al., 2020). Shi et al. (2020) have found that the CH₃OH oxidation activity of CuO catalysts has a direct relation with the concentration of surface oxygen vacancies. Therefore, CH₃OH adsorption on the CuO surfaces with oxygen vacancies was investigated. After removing an O atom near a Cu adsorption site, there was not great change in CH₃OH adsorption configurations, but the bond lengths of Cu-O_MeOH and H-O_CuO became shorter and the length of H-O_MeOH longer, compared to defect-free surfaces (Table 1). Correspondingly, there was an increase of 0.02–0.07 eV in adsorption energy. This indicated that the formation of oxygen vacancy enhanced CH₃OH adsorption and H-O_MeOH bond scission. The formation of oxygen vacancy leaded the Cu_3C atoms near the oxygen vacancy to bind to the neighboring O and moved further toward the vacancy (Fig. 5), while the positions of Cu_3C adsorption sites did not change in position because coordination was compensated by adsorbed CH₃OH molecules. The dissociative adsorption configuration MeOH-ov3C5 had the highest adsorption energy, at –0.71 eV, with the H-O_CuO bond length at 1.00 Å and H-O_MeOH at 1.70 Å.

Fig. 5. Top view of CH₃OH adsorption on CuO(111) surface with oxygen vacancy.

Comparison of PDOS plots of O_MeOH and H atoms bound to the surface of MeOH-ov3C2 and MeOH-ov3C5 showed that, on a surface with an oxygen vacancy, the DOS of O in MeOH-ov3C2 was lower in intensity at –2.67, –2.86 and –4.12 eV and for H at –4.15 eV. For MeOH-ov3C5, the DOS of O at –1.34 eV and H at –7.75 eV and –8.20 eV was also lower in intensity (Fig. 6). After removing an O atom from the surface, which acts for electron delivery, electron transfer from O_CuO to CH₃OH was restricted, leading to reduced states for H. For MeOH-ov3C2, the states of O and H shifted to states ~0.1 eV lower in energy. Therefore, formation of oxygen vacancy enhanceed CH₃OH adsorption and H-O_MeOH bond scission.

Fig. 6. PDOS of O_MeOH and H atoms in MeOH-ov3C2 and MeOH-ov3C5. The energy at E = 0 eV represents the Fermi energy.

3.3 Methoxy Radical Adsorption on the CuO(111) Surface

After the O-H bond cleavage, CH₃O was formed on the surface (Fig. 7). Compared with CH₃OH, the adsorption energy of this radical was much higher and reached –1.52 eV in 3C-M2, because O_MeO possessed unpaired electrons which preferred to bind with Cu. The Cu-O_MeO bond distance was 1.80 Å and the C-O_MeO bond distance 1.40 Å, which were both shorter than in CH₃OH adsorption. It was obvious that as a kind of free radical, CH₃O has stronger interaction with the surface than CH₃OH. Again, calculations considering the Hubbard U term produced higher adsorption energies than that without the U term.

Fig. 7. Side view of CH₃O adsorption on (a) defect-free CuO(111) surfaces and (b) surfaces with an oxygen vacancy.

For CH₃O adsorption on a surface with oxygen vacancies, the configurations were different from that of a defect-free surface. In MeO-ov4C1, CH₃O moved to a vacancy and bound with three Cu atoms by O_MeO, with adsorption energy increased to –3.19 eV. In MeO-ov3C1 and MeO-ov3C2, O_MeO bound with two Cu_3C atoms and formed bridging bonds, which had adsorption energies of –2.53 and –2.61 eV, respectively. This was because the coordination number of Cu_3C was reduced to 2 and then O_MeO, with unpaired electrons, accepted the electrons from Cu_3C and formed a strong bond with Cu_3C. This observation was demonstrated by the CDD and PDOS analyses below. Adsorption on Cu_4C sites was not detected and there was repulsion between O_MeO and O_3C on defect-free surfaces, which disappeared on surfaces with no oxygen vacancies.

Correspond with the increased adsorption energy, the charge transfer in CH₃O adsorption was stronger than CH₃OH adsorption, so the isosurface level was increase to 0.03 e/ų in Fig. 8(a). CDD analysis revealed that the p orbitals of O_MeO binding to the surface accumulated, while p orbitals of O_3C were depleted. There was also charge depletion near Cu, indicating a charge transfer from Cu to O_MeO. Because of strong interactions between Cu and O_MeO, Cu orbitals greatly changed. Charge depletion also occurred in the area between C and O_MeO and there was a corresponding longer C-O_MeO bond length. The PDOS plot of O_MeO of MeO-3C1 and MeO-ov3C1 showed, on a surface with oxygen vacancies, the DOS of O_MeO had states ~0.4 eV lower in energy, compared with a defect-free surface (Fig. 8(b)). The PDOS of O_MeO in MeO-ov3C1 was similar to that of O_3C on a clean CuO(111) surface, indicating strong interactions between Cu and O_MeO.

Fig. 8. (a) CDD plot of MeO-3C1 and MeO-ov3C1 with an isosurface level of 0.03 e Å^–3; charge accumulation is in yellow and charge depletion is in blue; (b) PDOS of O_MeO in MeO-3C1 and MeO-ov3C1. The energy at E = 0 eV represents the Fermi energy.

In the adsorption of CH₃OH and CH₃O on a CuO(111) surface, the methyl (CH₃-) group was remote from the O_CuO and adsorption occurred by O_MeOH-Cu bond formation. The first step of CH₃OH decomposition on a CuO(111) surface should thus be O-H bond scission. This result was consistent with the GGA+U study of the activation of organics containing hydroxyl (OH-) and CH₃- groups (Ren et al., 2018). Through comparison of CDD plots of MeOH-3C2 and MeO-3C1, it can be seen that CH₃O adsorption leaded stronger charge transfer around H bound with C (H_C). Meanwhile, the C-H bond was longer (1.11 Å) in CH₃O adsorption than in CH₃OH adsorption (1.10 Å).

Therefore, C-H bond dissociation tended to occur after CH₃O adsorption. However, on the surface with oxygen vacancies, the Cu-O_MeOH bond was so strong after CH₃O adsorption that the C-O_MeOH bond was stretched longer and C-H bond shorter than defect-free surface, indicating CH₃ and O^* might be formed after CH₃O adsorption on the surface with oxygen vacancies.

 
4 CONCLUSIONS

Periodic DFT+U calculations were to study the CH₃OH and CH₃O adsorption and decomposition on CuO(111) surfaces. CH₃OH molecules were preferentially adsorbed on Cu top sites with the O_MeOH atoms and H-O_3C bonds simultaneously formed. The CH₃- group of CH₃OH was remote from the O_CuO and the first step of CH₃OH oxidation on the CuO(111) surface should be O-H bond scission. The adsorption on Cu_3C sites was more stable than on Cu_4C sites, with higher binding energy and shorter Cu-O_MeOH and H-O_CuO bonds. Stable configurations were also achieved with O_MeOH-H bond scission, which were only found on Cu_3C and O_3C sites. Compared with CH₃OH, the adsorption energy of CH₃O was much higher, reaching –1.52 eV in MeO-3C2. The Cu-O_MeO bond distance was 1.80 Å and C-O_MeO 1.40 Å, which were both shorter than with CH₃OH adsorption.

Oxygen vacancies had the greater influence on the adsorption of CH₃O than of CH₃OH. On a surface with oxygen vacancies, the adsorption configurations of CH₃O were changed and adsorption energy increased to –3.19, –2.53 and –2.61 eV. For CH₃OH adsorption, the configurations did not change a lot, while there was a slight increase in adsorption energy. Formation of oxygen vacancies enhanced CH₃OH adsorption and H-O_MeOH bond scission and thus accelerated CH₃OH decomposition. Compared with a previous study, if the Hubbard U term was considered, higher adsorption energy and shorter lengths of Cu-O_MeOH and H-O_3C were detected.

DISCLAIMER 

The authors declare no conflict of interest.

 
ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (51906090) and Jiangsu Government Scholarship for Overseas Studies (JS-2019-221).