Lu Liu This email address is being protected from spambots. You need JavaScript enabled to view it.1, Peng Gong1, Guangcai Shao1, Pengfei Liu2,3, Junfeng Wang1 

1 School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China
2 Institute of High Energy Physics, Chinese Academy of Sciences (CAS), Beijing 100049, China
3 Spallation Neutron Source Science Center (SNSSC), Dongguan 523803, China


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.


Download Citation: ||https://doi.org/10.4209/aaqr.210253  


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 OMeOH.
  • The adsorption on Cu3C sites is more stable than on Cu4C 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.
 

ABSTRACT


The adsorption and decomposition of methanol (CH3OH) and methoxy radical (CH3O) on CuO(111) were investigated via density functional theory calculations with a Hubbard U correction. The configurations and electronic structures of CH3OH and CH3O adsorbed on CuO(111) surfaces were analyzed. CH3OH molecules were preferentially adsorbed on Cu top sites with OMeOH atoms and H-O3C bonds formed simultaneously. Adsorption on Cu3C sites was more stable than on Cu4C sites, with higher binding energy and shorter Cu-OMeOH and H-OCuO bonds. Stable configurations were also achieved with OMeOH-H bond scission, which were only found on Cu3C and O3C sites. On surfaces with oxygen vacancies, adsorption configurations did not change a lot, while there was increased adsorption energy with shorter bond lengths of Cu-OMeOH and H-OCuO and longer bond lengths of H-OMeOH, indicating the formation of oxygen vacancies enhanced the CH3OH adsorption and H-OMeOH bond scission, and thus accelerated CH3OH decomposition. The dissociative adsorption configuration MeOH-ov3C5 had the highest adsorption energy, at –0.71 eV, with the H-OCuO bond length at 1.00 Å and H-OMeOH at 1.70 Å. Compared with CH3OH, the adsorption energy of CH3O was much higher and reached –1.52 eV in MeO-3C2. The Cu-OMeO and C-OMeO bond distances were 1.80 Å and 1.40 Å, respectively, which were both shorter than CH3OH adsorption. The formation of oxygen vacancies significantly enhanced CH3O adsorption, as CH3O moved to a vacancy and bound with three Cu atoms by OMeO, whose adsorption energy increased to –3.19 eV. Other configurations had OMeO binding with two Cu3C atoms and formed a bridging bond, with adsorption energies of –2.53 and –2.61 eV.


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 PM2.5, 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 CHx 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 MnCuOx 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.
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 (Eads, kJ mol1) and charge density difference (CDD, Δρ, e Å3) 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, Cu3C atoms were drawn to O atoms on the second lower layer and shiftd to a place 0.14 Å lower, while Cu4C atoms rose 0.04 Å. O3C that were at the outmost layer were 0.09 Å lower and O4C at a lower layer were 0.08 Å higher after relaxation. Hence, Cu3C-O4C and Cu3C-O3C bonds changed to 1.95 and 1.85 Å from 1.97 Å, respectively. The Cu4C-O3C bond was 1.93 and 1.99 Å, which was shorter than the original lengths 1.95 and 1.97 Å, respectively. The Cu4C-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 OMeOH atoms and weak O3C-H bonds formed simultaneously (Fig. 2). The adsorption energy and structural parameters for CH3OH adsorption on CuO(111) surfaces showed that adsorption on Cu3C sites was more stable than on Cu4C sites, with higher binding energy and shorter Cu-OMeOH and H-OCuO bonds (Table 1). The highest binding energy was –0.69 eV (MeOH-3C1), with a Cu-OMeOH distance of 2.05 Å and a H-O3C distance of 1.69 Å. The H-OMeOH and C-OMeOH 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-OMeOH and H-O3C bonds. MeOH-4C2, MeOH-3C1 and MeOH-3C2 were newfound configurations in this study.

Table 1. Summary of the adsorption energies and structural parameters for CH3OH and CH3O adsorption on CuO(111) surface with and without oxygen vacancy.

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-OCuO bonds formed with lengths of 1.02 and 1.00 Å and the Cu-OMeOH distance shortened to 1.88 and 1.86 Å, because interactions between Cu and OMeOH were enhanced. This distance was still longer than the Cu-OMeOH bond in CH3O adsorption discussed below, indicating that weak bonds between H and OMeOH remained. Compared to their corresponding configuration in Fig. 2(a), the adsorption energy did not significantly change. Dissociated adsorption was only found on Cu3C and O3C sites.

 Fig. 2. Side view of the configurations of CH3OH adsorbed on CuO(111) surfaces, (a) common chemiadsorption, (b) dissociated adsorption.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 OMeOH and CuO, as well as H and OCuO, and charge depletion observed in the region around HCH3. For MeOH-3C5, there was charge accumulation around the region of HCH3. The partial density of states (PDOS) plot of OMeOH and H atoms that bound to the surface showed that, in free CH3OH, the orbitals of OMeOH 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 OMeOH 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 OMeOH orbitals at –4.12, –8.38 and –12.69 eV, indicating the H-OMeOH bond still existed. The orbital of OMeOH 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 O3C, the PDOS of O3C 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 O3C orbitals and H orbitals. On a clean surface, the PDOS of O3C was mainly near the Fermi level (Fig. S1). After binding with H, states of O3C near the Fermi level became inconspicuous and the H states near the Fermi level weakened to invisible, indicating that the formation of H-O3C bond made the H and O3C atoms became stable. On the contrary, although the OMeOH orbitals near Fermi level shifted to states a litter lower, the states near Fermi level became stronger, indicating that OMeOH became more reactive. The orbitals of OMeOH and H still exhibited superposition at –4.50, –7.8 and –8.32 eV, indicating that weak interactions between H and OCuO 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. 3. CDD plot of CH3OH adsorbed on CuO(111) surface with an isosurface level of 0.015 e Å3; charge accumulation is in yellow and charge depletion is in blue.

Fig. 4. PDOS of (a) OMeOH, O3C and (b) H atoms in free CH3OH, MeOH-3C2 and MeOH-3C5. The energy at E = 0 eV represents the Fermi energy.Fig. 4. PDOS of (a) OMeOH, O3C and (b) H atoms in free CH3OH, MeOH-3C2 and MeOH-3C5. The energy at = 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-OMeOH and H-OCuO became shorter and the length of H-OMeOH 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-OMeOH bond scission. The formation of oxygen vacancy leaded the Cu3C atoms near the oxygen vacancy to bind to the neighboring O and moved further toward the vacancy (Fig. 5), while the positions of Cu3C 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-OCuO bond length at 1.00 Å and H-OMeOH at 1.70 Å.

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

Comparison of PDOS plots of OMeOH 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 OCuO 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-OMeOH bond scission.

Fig. 6. PDOS of OMeOH and H atoms in MeOH-ov3C2 and MeOH-ov3C5. The energy at E = 0 eV represents the Fermi energy.Fig. 6. PDOS of OMeOH and H atoms in MeOH-ov3C2 and MeOH-ov3C5. The energy at = 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 OMeO possessed unpaired electrons which preferred to bind with Cu. The Cu-OMeO bond distance was 1.80 Å and the C-OMeO 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.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 OMeO, with adsorption energy increased to –3.19 eV. In MeO-ov3C1 and MeO-ov3C2, OMeO bound with two Cu3C atoms and formed bridging bonds, which had adsorption energies of –2.53 and –2.61 eV, respectively. This was because the coordination number of Cu3C was reduced to 2 and then OMeO, with unpaired electrons, accepted the electrons from Cu3C and formed a strong bond with Cu3C. This observation was demonstrated by the CDD and PDOS analyses below. Adsorption on Cu4C sites was not detected and there was repulsion between OMeO and O3C 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/Å3 in Fig. 8(a). CDD analysis revealed that the p orbitals of OMeO binding to the surface accumulated, while p orbitals of O3C were depleted. There was also charge depletion near Cu, indicating a charge transfer from Cu to OMeO. Because of strong interactions between Cu and OMeO, Cu orbitals greatly changed. Charge depletion also occurred in the area between C and OMeO and there was a corresponding longer C-OMeO bond length. The PDOS plot of OMeO of MeO-3C1 and MeO-ov3C1 showed, on a surface with oxygen vacancies, the DOS of OMeO had states ~0.4 eV lower in energy, compared with a defect-free surface (Fig. 8(b)). The PDOS of OMeO in MeO-ov3C1 was similar to that of O3C on a clean CuO(111) surface, indicating strong interactions between Cu and OMeO.

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 OMeO in MeO-3C1 and MeO-ov3C1. The energy at E = 0 eV represents the Fermi energy.Fig. 8. (a) CDD plot of MeO-3C1 and MeO-ov3C1 with an isosurface level of 0.03 e Å3; charge accumulation is in yellow and charge depletion is in blue; (b) PDOS of OMeO in MeO-3C1 and MeO-ov3C1. The energy at = 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 OCuO and adsorption occurred by OMeOH-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 (HC). 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-OMeOH bond was so strong after CH3O adsorption that the C-OMeOH 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 OMeOH atoms and H-O3C bonds simultaneously formed. The CH3- group of CH3OH was remote from the OCuO and the first step of CH3OH oxidation on the CuO(111) surface should be O-H bond scission. The adsorption on Cu3C sites was more stable than on Cu4C sites, with higher binding energy and shorter Cu-OMeOH and H-OCuO bonds. Stable configurations were also achieved with OMeOH-H bond scission, which were only found on Cu3C and O3C sites. Compared with CH3OH, the adsorption energy of CH3O was much higher, reaching –1.52 eV in MeO-3C2. The Cu-OMeO bond distance was 1.80 Å and C-OMeO 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-OMeOH 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-OMeOH and H-O3C 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).


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