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

The adsorption and decomposition of methanol (CH 3 OH) and methoxy radical (CH 3 O) on CuO(111) were investigated via density functional theory calculations with a Hubbard U correction. The configurations and electronic structures of CH 3 OH and CH 3 O adsorbed on CuO(111) surfaces were analyzed. CH 3 OH molecules were preferentially adsorbed on Cu top sites with O MeOH atoms and H-O 3C bonds formed simultaneously. 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. On surfaces with oxygen vacancies, adsorption configurations did not change a lot, while there was increased adsorption energy with shorter bond lengths of Cu-O MeOH and H-O CuO and longer bond lengths of H-O MeOH , indicating the formation of oxygen vacancies enhanced the CH 3 OH adsorption and H-O MeOH bond scission, and thus accelerated CH 3 OH decomposition. 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. Compared with CH 3 OH, the adsorption energy of CH 3 O was much higher and reached –1.52 eV in MeO-3C2. The Cu-O MeO and C-O MeO bond distances were 1.80 Å and 1.40 Å, respectively, which were both shorter than CH 3 OH adsorption. The formation of oxygen vacancies significantly enhanced CH 3 O adsorption, as CH 3 O moved to a vacancy and bound with three Cu atoms by O MeO , whose adsorption energy increased to –3.19 eV. Other configurations had O MeO binding with two Cu 3C atoms and formed a bridging bond, with adsorption energies of –2.53 and –2.61 eV.


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 (CH 3 OH) 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, CH 3 OH decomposition is a worthy topic in the research of VOC control. Most CH 3 OH decomposition processes are catalytic reactions on surfaces with active sites. Precious metals (PM) are the most effective catalysts for CH 3 OH 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 CH 3 OH 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 CH 3 OH decomposition. Even in PM/CuO co-catalysts, CuO nanoparticles play an important role in CH 3 OH oxidation (Sharma et al., 2019). Cu-based composite oxides show outstanding performance on CH 3 OH oxidation because of facilitated electron transfer that lowers the reaction activation barrier (Norsic et al., 2016). For instance, on CuO-CuWO 4 , CH 3 OH molecules decompose to surface adsorbed methoxy (CH 3 O) 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 Cu 0 and Cu + , thus improving catalytic activity in CH 3 OH decomposition (Wei et al., 2019). In MnCuO x catalysts, manganese as Mn 3+ and Mn 4+ 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 CH 3 OH 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 Zhang et al., 2021). Density functional theory (DFT) has been employed extensively in exploring CH 3 OH decomposition on metal surfaces. Studies of CH 3 OH adsorption on MO surfaces have suggested that the dehydrogenation pathway of CH 3 OH 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 CH 3 OH on CeO 2 surfaces. Kinetically, the activation barrier of O-H bond cleavage in CH 3 OH is 0.30 eV on CeO 2 (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 CH 3 O formation on MO surfaces are the most possible pathway of CH 3 OH 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 CH 3 OH dehydrogenation and formaldehyde formation, with Cu reduction involving two key steps, from Cu +2 to Cu + with oxygen transposition and from Cu + to Cu 0 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 CH 3 OH decomposition. C-H bond breaking subsequently occurs and methoxide is formed (Vo et al., 2013). On Fe 3 O 4 (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 CH 3 OH adsorption, in this study, the influence of oxygen vacancies on CuO(111) surfaces in CH 3 OH 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 CH 3 OH were identified. Adsorption of CH 3 O that formed by O-H bond scission of CH 3 OH were also examined to extend the understanding of CH 3 OH decomposition.

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 and Wang, 1992) with the projectoraugmented 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.
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).

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 .

CH 3 OH Adsorption on CuO(111) Surfaces
CH 3 OH adsorption on CuO(111) surfaces was studied regarding both Cu and O sites. After relaxation CH 3 OH 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 CH 3 OH adsorption on CuO (111)  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.   Charge transfer between adsorbed CH 3 OH 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 CH 3 OH, 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 CH 3 OH, 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)

CH 3 OH 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 CH 3 OH oxidation activity of CuO catalysts has a direct relation with the concentration of surface oxygen vacancies. Therefore, CH 3 OH 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 CH 3 OH 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 CH 3 OH 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 CH 3 OH 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.

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 CH 3 OH 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 CH 3 OH adsorption and H-O MeOH bond scission.

Methoxy Radical Adsorption on the CuO(111) Surface
After the O-H bond cleavage, CH 3 O was formed on the surface (Fig. 7). Compared with CH 3 OH, 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 CH 3 OH adsorption. It was obvious that as a kind of free radical, CH 3 O has stronger interaction with the surface than CH 3 OH. Again, calculations considering the Hubbard U term produced higher adsorption energies than that without the U term.
For CH 3 O adsorption on a surface with oxygen vacancies, the configurations were different from that of a defect-free surface. In MeO-ov4C1, CH 3 O 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 CH 3 O adsorption was stronger than CH 3 OH adsorption, so the isosurface level was increase to 0.03 e/Å 3 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 .
In the adsorption of CH 3 OH and CH 3 O on a CuO(111) surface, the methyl (CH 3 -) group was remote from the O CuO and adsorption occurred by O MeOH -Cu bond formation. The first step of CH 3 OH 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 CH 3groups (Ren et al., 2018). Through comparison of CDD plots of MeOH-3C2 and MeO-3C1, it can be seen that CH 3 O adsorption leaded stronger charge transfer around H bound with C (H C ). Meanwhile, the C-H bond was longer (1.11 Å) in CH 3 O adsorption than in CH 3 OH adsorption (1.10 Å).  Therefore, C-H bond dissociation tended to occur after CH 3 O adsorption. However, on the surface with oxygen vacancies, the Cu-O MeOH bond was so strong after CH 3 O adsorption that the C-O MeOH bond was stretched longer and C-H bond shorter than defect-free surface, indicating CH 3 and O * might be formed after CH 3 O adsorption on the surface with oxygen vacancies.

CONCLUSIONS
Periodic DFT+U calculations were to study the CH 3 OH and CH 3 O adsorption and decomposition on CuO(111) surfaces. CH 3 OH molecules were preferentially adsorbed on Cu top sites with the O MeOH atoms and H-O 3C bonds simultaneously formed. The CH 3 -group of CH 3 OH was remote from the O CuO and the first step of CH 3 OH 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 CH 3 OH, the adsorption energy of CH 3 O 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 CH 3 OH adsorption.
Oxygen vacancies had the greater influence on the adsorption of CH 3 O than of CH 3 OH. On a surface with oxygen vacancies, the adsorption configurations of CH 3 O were changed and adsorption energy increased to -3.19, -2.53 and -2.61 eV. For CH 3 OH adsorption, the configurations did not change a lot, while there was a slight increase in adsorption energy. Formation of oxygen vacancies enhanced CH 3 OH adsorption and H-O MeOH bond scission and thus accelerated CH 3 OH 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.