High coverage H2O adsorption on CuAl2O4 surface: A DFT study

Journal Pre-proofs High Coverage H2O Adsorption on CuAl2O4 Surface: A DFT Study Liu Shi, Shuai Meng, Siriporn Jungsuttiwong, Supawadee Namuangruk, Zhang-Hui Lu, Li Li, Rongbin Zhang, Gang Feng, Shaojun Qing, Zhixian Gao, XiaohuYu PII: DOI: Reference:

S0169-4332(19)33979-0 https://doi.org/10.1016/j.apsusc.2019.145162 APSUSC 145162

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

13 September 2019 14 December 2019 21 December 2019

Please cite this article as: L. Shi, S. Meng, S. Jungsuttiwong, S. Namuangruk, Z-H. Lu, L. Li, R. Zhang, G. Feng, S. Qing, Z. Gao, XiaohuYu, High Coverage H2O Adsorption on CuAl2O4 Surface: A DFT Study, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145162

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High Coverage H2O Adsorption on CuAl2O4 Surface: A DFT Study Liu Shi 1, Shuai Meng 2, Siriporn Jungsuttiwong3, Supawadee Namuangruk4, Zhang-Hui Lu5,6, Li Li 1, Rongbin Zhang 1, Gang Feng 1, *, Shaojun Qing 7, Zhixian Gao 7, XiaohuYu 8, 9, * 1 Key

Laboratory of Jiangxi Province for Environment and Energy Catalysis, Institute

of Applied Chemistry, College of Chemistry, Nanchang University, No. 999Xuefu Road, Nanchang 330031, P. R. China 2

Chemistry Examination Department, Patent office, China National Intellectual

Property Administration, No. 6, Xitucheng Lu, Jimenqiao Haidian District, Beijing 100088, P. R. China 3

Department of Chemistry and Center for Innovation in Chemistry, Faculty of

Science, Ubon Ratchathani University, Ubon Ratchathani 34190, Thailand. 4

National Nanotechnology Center, National Science and Technology Development

Agency, Klong Luang, Pathumthani 12120, Thailand 5

Institute of Advanced Materials (IAM), College of Chemistry and Chemical

Engineering, Jiangxi Normal University, Nanchang 330022, China. 6

School of Materials Science and Engineering, Changsha University of Science and

Technology, Changsha 410114, China 7

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese

Academy of Sciences, Taiyuan 030001, P. R. China

1

8

Institute of Theoretical and Computational Chemistry, Shaanxi Key Laboratory of

Catalysis, School of Chemical & Environment Sciences, Shaanxi University of Technology, Hanzhong 723000, P. R. China 9

Department of Chemistry and Key Laboratory of Organic Optoelectronics &

Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China

Corresponding authors: Gang Feng; Email: [email protected] Xiaohu Yu; Email: [email protected]

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ABSTRACT: Investigation into the interaction between water and surface is important for many reactions. Periodic DFT calculations were performed to investigate the adsorption of nH2O (n = 1−8) on the CuAl2O4 (100) and (110) surfaces. The results show that single water molecule is adsorbed on the CuAl2O4 (100) and (110) surfaces via dissociative adsorption with adsorption energies of ‒103 and ‒170 kJ/mol. On O-defective CuAl2O4 surfaces, the O atom of the water molecule prefers to insert into the O vacancy, leaving two isolated H atoms bonds to surface O atoms forming two in-surface hydroxyls. For nH2O (n = 2−8) on CuAl2O4 surfaces, molecular and dissociative water adsorption can coexist. The interaction of H2O with CuAl2O4 (110) surface is much stronger than with the (100) surface. The PDOS analysis revels that the adsorption of water on the surfaces is accompanied by charge transfer. Water adsorption on the CuAl2O4 surfaces leads to the decreases of the work function at low water coverage, while higher water coverage results in the increase of the work function. The phase diagrams of the water adsorption on the CuAl2O4 spinel surfaces show that water will desorb from the CuAl2O4 surfaces at higher temperature and lower H2O pressure. Keywords: CuAl2O4 spinel; adsorption; water; phase diagram; density functional theory; work function

3

1. Introduction CuAl2O4 spinel could be used as catalysts for methanol steam reforming (MSR) due to its high activity and stability [1]. It was reported that the Cu atoms could be gradually released in the reduction atmosphere of the MSR, which results in the longer durability of the CuAl2O4 spinel catalysts than the traditional alumina supported Cu catalysts [2,3]. In addition, the strong bonding of the released Cu atoms with the CuAl2O4 spinel surface also prevents the aggregation of the surface Cu atoms and contributes to the improved stability of the catalysts. Water is one important reactant of the MSR. The adsorption of water molecules on the CuAl2O4 spinel surface is one important elementary step of the MSR reaction. Investigation into the adsorption of water on the CuAl2O4 spinel surface at molecular level is of importance for the understanding of the mechanisms of all reaction systems with water and spinel, e.g. MSR [1,4,5], dehydrogenation reaction [6-8] and catalyst preparation [9,10]. Due to the adsorption of water on various solid surfaces is important in many fields, e.g. heterogeneous catalysis [11-16], electrochemistry [17,18], photocatalytic [19-22], etc. Previous studies have intensively studied the adsorption of water on metal oxides surfaces, such as CuO(111) [23], Cu2O(111) [24], Al2O3 [25,26], CeO2 [27], TiO2 [17]. The water molecular adsorption on metal oxides is mainly combined with surface acidic sites by the lone pair electrons of water O atom [28]. Stronger 4

adsorption leads to water dissociation and the dissociated hydrogen atom adsorbs on the surface alkaline O site [29]. Yu et al. studied the adsorption of 1‒4 water molecules on CuO (111) surface, and found that the first water molecule is preferentially form molecular adsorption with the adsorption energy of ‒68 kJ/mol. As the water coverage increases, both molecular and the dissociative adsorptions coexist. Phase diagram of stable H2O adsorption on CuO (111) reveal that water desorb completely at 800 K with the water pressure is 0.5 atm. Digne et al. investigated the detailed mechanisms for the adsorption of water on γ-alumina surface [30]. The adsorption energy for one water molecule on γ-Al2O3 (100) surface is −105 kJ/mol vs. −240 kJ/mol on the γ-Al2O3 (110) surface. The maximum coverage of two surfaces are 17.1 and 17.8 OH/nm2. In addition, on the basis of ab initio thermodynamics analysis, they concluded that the completely desorption temperature for water on γ-Al2O3 (100) and (110) surfaces are 570 and 1100 K, respectively. In addition, it was also reported that the adsorption of water is stronger than that of alcohol [31], though both molecules adsorb on the metal surface via the interaction of surface acidic sites with the lone pair electrons of O atom. In the present work, dispersion corrected density function theory (DFT-D) calculations were performed for the investigation of the interaction of water molecules with CuAl2O4 spinel surfaces. The interaction mechanism between water and surface is explored by density of states, the influence of water adsorption on the

5

work function are calculated and the phase diagram of multiple water molecules on the surface is made on the basis of ab initio thermodynamics analysis.

2. Computation details 2.1 Computational methods All calculations were performed with the DFT-D method with plane wave basis set as implemented in the Vienna Ab initio Simulation Package (VASP) [32,33]. The DFT-D3 method of Grimme was used to take into account the dispersive interactions for the DFT optimized structures to acquire more accurate adsorption energies [34]. The exchange and correlation energies were calculated by the generalized gradient approximation (GGA) formulation with the PBE functional [35]. All calculations are spin polarized. The Kohn-Sham one-electron states were extended in accordance with plane-wave basis sets with a kinetic energy of 400 eV. The projector augmented wave (PAW) method was applied to describe the electron-ion interactions [36,37]. The Brillouin zone was sampled with 5×5×5 and 3×3×1 k-points meshes generated by the Monkhorst-Pack algorithm, respectively, for the unit cell and surface slabs. The convergence criteria were set 1.0×10–5 eV for the SCF energy, 110–4 eV and 0.03 eV/Å for the total energy and the atomic forces, respectively. Vibrational frequencies and normal modes are calculated by diagonalization of the mass-weighted force constant matrix, obtained by numerical differentiation of 6

analytically calculated forces as implemented in VASP. The atoms of the outmost layer and the adsorbate atoms are displaced by 0.02 Å in both directions for each Cartesian coordinate. All reported minimum energy structures have only real frequencies. The adsorption energies (ΔE(H2O)n) for the adsorption of nH2O (n = 1‒8) in the surface were calculated by: ΔE(H2O)n = E(nH2O/slab) – [nE(H2O) + E(slab)]

(1)

Where E(nH2O/slab), E(H2O) and E(slab) are the total energies of the spinel surface slab with adsorbed nH2O (n = 1‒8), isolated the gas phase H2O molecule and the spinel surfaces, respectively. The larger adsorption energy indicates the stronger adsorption for the water molecules on the surface. The stepwise adsorption energy is defined as: ΔΔE(H2O) = ΔE(H2O)n –ΔE(H2O)n-1

(2)

The average adsorption energy is defined as: ΔE(H2O) = ΔE(H2O)n/n

(3)

The vibrational frequencies are used to calculate Gibbs free energies in the standard harmonic oscillator, rigid rotor, ideal gas approximation. For the solid components,

7

only vibrational contributions are considered, whereas for gas phase species also rotational and translational contributions are included. In order to investigate the surface stability of the surfaces, the surface energy (E(surface)) is defined as: E(surface) = [E(slab) – (N(slab)/N(bulk))E(bulk)]/2A

(4)

Where the N(slab) and N(bulk) are the atomic number of CuAl2O4 surface and CuAl2O4 unit cell, respectively. The E(bulk) represents the total energies of the CuAl2O4 unit cell. A represents the surface area of the slab.

2.2 Cu-Al spinel surface model The CuAl2O4 is of Fd-3m space group with Cu2+ cations in tetra-coordination and Al3+ cations in the octa-coordination of the cell [38]. All O2- anions are in tetra-coordination. The unit cell of spinel contains eight CuAl2O4 units, with the optimized lattice parameter of 8.1669 Å, which agrees well with the experiments 8.0778-8.153 Å [39,40]. The Cu‒O and Al‒O bond distances are 198 and 193 pm, respectively. In order to investigated the surface properties of the spinel surfaces, all of the low indexed surfaces of the spinel are cleavaged. It is found that the (010) and (001) have the same structure with the (100) surface, and the (101) and (011) have the same structure with the (110) surface. It should be mentioned the cleavage of the surface broken Cu–O 6.00 and Al–O 11.99 bond/nm2 on (100) surface vs. 4.24 and 16.96 8

bond/nm2 for the (110) surface. The surface energies of the most stable termination of the relaxed (100), (110) and (111) surfaces are 1.738, 2.245 and 2.534 J/m2. It indicates the (100) and (110) surfaces are more stable than the (111) surface. Thus, the present work chooses the (100) and (110) surfaces to present the spinel surface. Fig. 1 shows the side and top views of the p(1×1) slabs of CuAl2O4 spinel (100) and (110) surfaces, which have the surface area of 66.7 and 94.3 Å2. Both the p(1×1) (100) and (110) surface slabs are non-polar and contain 16 CuAl2O4 units (112 atoms), with exposed Cu, Al and O atoms in the surfaces. The bottom half (four layers, below the dashed line, Cu8Al16O32) of the slabs are fixed in their bulk position and the top half (four layers, above the dashed line, Cu8Al16O32) of the slab are fully relaxed for all optimization calculations. The gas phase molecules are calculated using an a = b = c = 20 Å cell.

Fig. 1. Surface structures of spinel CuAl2O4. The perfect and O-defective (100) and (110) surfaces. O, Al and Cu atoms are in red, gray and blue, respectively. The

9

exposed top layer atoms are indexed with numbers and the coordination numbers are shown in subscripts. The top layer of the perfect (100) surface exposes equivalent di-coordinated Cu atoms Cu(2,4)2c slightly above the Al4O8 plane. Tetra-coordinated Cu(1,3)4c are slightly lower than the top layer Al4O8 atoms. O(3,5,6,8)3c are equivalent tri-coordinated and O(1,2,4,7)4c are equivalent tetra-coordinated O atoms. All Al(1,2,3,4)5c atoms in the surfaces are equivalent penta-coordinated. The perfect (110) surface exposes equivalent tri-coordinated Cu(1,2,3,4)3c, equivalent

tri-coordinated

O(1,2,3,4,5,6,7,8)3c,

equivalent

tetra-coordinated

Al(1,2,3,4)4c in the top layer, and the equivalent O(9,10,11,12)4c are in tetra-coordination in the sublayer. The equivalent surfaces atoms, e.g. Cu(2)2c = Cu(4)2c; Cu(1)4c = Cu(3)4c; O(3)3c = O(5)3c = O(6)3c = O(8)3c; O(1)4c = O(2)4c = O(4)4c = O(7)4c; Al(1)5c = Al(2)5c = Al(3)5c = Al(4)5c atoms in (100) surface, as well as the Cu(1)3c = Cu(2)3c = Cu(3)3c = Cu(4)3c; O(1)3c = O(2)3c = O(3)3c = O(4)3c = O(5)3c = O(6)3c = O(7)3c = O(8)3c; Al(1)4c = Al(2)4c = Al(3)4c = Al(4)4c in the (110) surface, should have the same chemical properties for H2O molecules adsorption. Since the surface energy of the perfect (100) and (110) surfaces are 1.738 and 2.245 J/m2, respectively. It indicates that the (100) surface is more stable than the

10

(110) surface. It could be expected that the (100) surface may have weaker ability than (110) surface for the adsorption of water. Besides the water adsorption on the perfect CuAl2O4 spinel surfaces, we also considered the adsorption of water on the O-defective surfaces. The O-defect formation energy (Ef) is defined as: Ef = E(O-defect) + 1/2E(O2) – E(slab)

(5)

Where the E(O-defect), E(O2) and E(slab) are the total energies of the O-defective surface, isolated the gas phase O2 molecule, and the perfect slab surface. The smaller of the formation energy, the easier the defect could be formed. All possible O-defective sites on the surface had been tested, the equation (5) used to calculation the defect formation energy (Ef). It is found that the most stable O-defective surfaces are created by the removal of surface O(3)3c atoms from the perfect (100) and (110) surfaces. The formation of one defect site in the surface and half gas phase O2 molecule from the perfect (100) and (110) surfaces are thermodynamically endothermic by 1.61 and 2.64 eV , respectively, which indicates that the O defect is more easily to be created in the (100) surface than in the (110) surface. Compared with the other oxide surfaces, CuAl2O4 (100) surface is more easily to form oxygen vacancy than the CuO (111), Cu2O (111) and α-Al2O3 e.g. the O vacancy formation energies of CuO (111) [41], Cu2O (111) [24] and α-Al2O3 [42] are 3.02, 2.18 and 5.83 eV, respectively.

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Both (100) and (110) surface are distorted after the removal of the surface O(3)3c atoms. In the O-defective (100) surface, O(6)2c and O(4)3c become di- and tri-coordinated, Al(1)4c and Al(2)4c become tetra-coordinated, Cu(1)3c and Cu(3)3c are tri-coordinated, respectively. The O-defective (110) surface distorted more serious than the (100) surface. In the O-defective (100) surface, Cu(4)2c become di-coordinated, and the surface O(1,2,4,5,6,7,8) and O(9,10,11,12) keeps their tri- and tetra-coordination, respectively. It is also found that the Al(3)4c atom moves to sublayer, and accordingly one of the sublayer Al moves to left.

3. Results and discussion For the adsorption of water molecules on each surface, firstly, we tried to put the first water molecules molecularly/dissociative adsorbed on all available surface sites of the surface, and then get the most stable adsorption configuration (with the largest adsorption energy). On the basis of the most stable adsorption configuration for one water molecular adsorption, the second water molecule were added into the surface, and then the third, fourth, fifth, sixth, seventh, and finally the eighth. Following the review’s suggestion to check the influence of lower coverage to the water adsorption, we calculated the adsorption of one water molecule in the p(2×1) slabs of the (100) and (110) surfaces. It is found that the adsorption energies for one 12

water molecule in the p(2×1) slabs of the (100) and (110) surfaces are 105 and 180 kJ/mol, vs. 103 and 170 kJ/mol in the p(1×1) slabs of the (100) and (110) surfaces. It indicates that the lower coverage have little influence to the water adsorption, and the p(1×1) slabs of the (100) and (110) surfaces has the surface area of 66.7 and 94.3 Å2, which are large enough to reduce the interaction of the single water molecule in the neighbor slabs.

3.1. Single water adsorption on the perfect and O-defective CuAl2O4 surfaces 3.1.1. Structures and adsorption energies The structures and adsorption energies for the adsorption of single H2O molecule on the perfect and O-defective CuAl2O4 (100) (water coverage θ is 1.50 H2O molecule/nm2) and (110) (water coverage θ is 1.06 H2O molecule/nm2) surfaces are shown in Fig. 2. The most stable adsorption structure for one water on the CuAl2O4 spinel (100) surface is the dissociative adsorption configuration in Fig. 2a, with the adsorption energy of ‒103 kJ/mol. The dissociated OwH hydroxyl group (Ow, oxygen atom of water; Os, oxygen atom in the surface) adsorbs on the bridge site of Al(3) and Cu(2) with the Al‒Ow and Cu‒Ow distances of 196 and 197 pm, respectively. Another dissociated H atom bonds to O(8) with the Os‒H distance of 99 pm. In addition, the molecular adsorption of water on the (100) surface is shown in Fig. 2b, with the adsorption energy of ‒101 kJ/mol, which is slightly smaller than water dissociative adsorptions on CuAl2O4 (100) surface Cu and Al atoms. The Ow atom of H2O 13

molecule is bonded to Cu(2) with the Cu‒O distance of 202 pm. The two H atoms form hydrogen bonds with O(5,6), respectively, with the H‒Os(5,6) distance are 160, 184 pm. The H‒Ow distances are 101 and 104 pm, which are longer than the gaseous state H2O of 98 pm. It indicates that the H2O molecule is activated by the CuAl2O4 (100) surface. The adsorption of water on the surface Al atom is also calculated. As shown in Fig. 2c, molecular adsorption of water via Ow atom bonds with Al(3) is exothermic by ‒73 kJ/mol, with the Al‒Ow bond distance of 209 pm and H‒Ow distance of 100 pm, which indicates weaker adsorption on the surface Al atoms than on the surface Cu atoms. For one water molecule adsorption on the perfect CuAl2O4 (110) surface (water coverage θ is 1.06 H2O molecule/nm2), three stable adsorption structures are found and shown in Fig. 2. It is interesting to found that the most stable adsorption for one water on the (110) surface is dissociative adsorption (Fig. 2e), which is similar to the case in (100) surface. In Fig. 2e, the OwH hydroxyl group adsorbs on the bridge site of Al(1,2) and another H atom bonds to O(6), with the Al‒O distances of 193 and 189 pm, and H‒O distance of 99 pm, with the adsorption energy of ‒170 kJ/mol. The structure Fig. 2e is similar to the reported structure of water adsorption on α-alumina(0001) [26,43] and γ-alumina [30], in which the bridge sites hydroxyls are formed. Fig. 2f presents the water molecular adsorption on Al(1), with the two H

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atoms pointing to O(2,6) respectively. The adsorption energy for this structure is ‒102 kJ/mol.

Fig. 2. Single H2O molecule adsorbs on the CuAl2O4 perfect and defective surfaces (water coverage θ of 1.50 H2O molecule/nm2 for (100) surface vs. 1.06 for (110) surface). O, Al and Cu atoms of the surface are in red, gray and blue, respectively. O and H of water are in small red ball and write ball. The bond distances and adsorption energies are in pm and kJ/mol, respectively. It should be noted that water molecules in Fig. 2e and Fig. 2f are adsorbed on the Al adsorption site, however, the dissociative adsorption energy is ‒68 kJ/mol larger than the molecular adsorption. This is different from the CuAl2O4 (100) surface that 15

the dissociative adsorption (Fig. 2a) is slightly stable than the molecular adsorption (Fig. 2b). It indicates that CuAl2O4 (110) surface has stronger ability for water dissociation than the CuAl2O4 (100) surface. Fig. 2g shows that water molecularly adsorbed on Cu(2) with the Cu‒Ow bond distance of 201 pm, one H atom pointing to O(2), and another H point to O(10). The adsorption energy of Fig. 2g is ‒98 kJ/mol, which is smaller than water adsorption on the Al atom in (110) surface. Fig. 2d and 2h shows the results for water adsorption on the O-defective CuAl2O4 (100) and (110) surfaces. The adsorption for one water molecule on the O-defective (100) surface (Fig. 2d) is exothermic by ‒218 kJ/mol, which much larger than on the perfect (100) surface. It is found that the Ow atom goes into the defect site and makes the surface recovered its perfect surface structure. One dissociated H atom bonds to O(8), with the O‒H bond of 99 pm. Water adsorption on the O-defective (110) surface (Fig. 2h) is exothermic by ‒137 kJ/mol which is 33 kJ/mol less than perfect (110) surface. The results show that optimal adsorption sites for one water molecule adsorption on O-defective CuAl2O4 surfaces are the oxygen vacancies, which prefers to form dissociative adsorption. It indicates that the surface oxygen vacancies could promote the dissociation of water, which agree well with the previous works on the Cu2O and Al2O3 [24,44]. The adsorption of water molecule on the O-defective surfaces could also be considered as the adsorption of one H2 molecular on the perfect CuAl2O4 spinel 16

surfaces. It could be calculated that the adsorption energy for H2 adsorption on the perfect CuAl2O4 spinel (100) and (110) surfaces are ‒218 and ‒137 kJ/mol. Table 1 Water adsorption on various surfaces of metals, oxides and sulfides. oxides surfaces

adsorption energies (kJ/mol)

functionals

Ref.

CuAl2O4 (100)

‒103

PBE-D3

This work

CuAl2O4 (110)

‒170

PBE-D3

This work

γ-Al2O3 (110)

‒240

PW91

[45]

Iron promoted γ-Al2O3 (110)

‒177

PW91

[25]

α-Al2O3 (0001)

‒151

PW91

[46]

CuO (111)

‒68

PW91

[47]

Cu2O (111)

‒164

PBE

[24]

Cu (111)

‒38

PW91

[48]

K/Cu (111)

‒76

PW91

[48]

Cu promoted ZnS

‒39

PW91

[49]

ZnGa2O4(100)/(110)

‒96/‒50

PW91

[50]

Co3O4 (110)

‒156

PBE

[51]

Table 1 collects the adsorption energies of single water molecule on various surfaces of metals, oxides and sulfides. Though water adsorption on the perfect CuAl2O4 spinel (110) surface (Fig. 2e) is 67 kJ/mol more stable than on the CuAl2O4 spinel (100) surface (Fig. 2a). It is much smaller than water adsorption on the γ-Al2O3 and iron promoted γ-Al2O3 (110) surfaces. However, it is much stronger than on the CuO (111), Cu (111), and K/Cu (111). Compared with other reported results on water 17

adsorption on other spinel surfaces, water adsorption on the perfect CuAl2O4 spinel (110) surface is more stable than on the ZnGa2O4 (110) and Co3O4 (110) surfaces.

3.1.2. Electronic structure analysis for single H2O on CuAl2O4 surfaces

Fig. 3. PDOS plots for the adsorbed H2O molecule, top layer O, Al, and Cu atoms of the slab for water adsorption on CuAl2O4 (100) and (110) surfaces (correspond to the structure of Fig. 2a and Fig. 2e).

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Fig. 3 presents the PDOS plots for the adsorbed H2O molecule, and top layer O, Al and Cu atoms of the most stable structure for single water adsorption on the perfect (100) (Fig. 2a) and (110) (Fig. 2e) surfaces. For single water molecule adsorbed on the (100) surface, the total PDOS of the adsorbed H2O molecule shifts to lower energy. The 1b1 peak becomes smaller and broader. The PDOS of the top layer Cu atoms resonates with the 1b1 of the adsorbed water, indicating that lone pair electron of the adsorbed water molecule overlap with Cu 3d orbit forms a Ow‒Cu(2) bond. In addition, the 3a1 peak of water and the PDOS of surface layer Al resonates, indicating that the dissociated OwH also bonds with Al atoms. The 1b2 state shifts to a lower energy level of ‒7.34 eV, while a strong surface oxygen atom PDOS peak appears at ‒8.27 eV with a weak H2O molecule peak, indicating that there should be a in-surface hydroxyl, which is consistent with the structure of Fig. 2a. For single water molecule adsorbed on the (110) surface, the 1b2 state of water shifts to lower energy level and changes smaller, which resonates with a strong PDOS peak of surface oxygen atom. This is because the H‒Os bond formed by the dissociated H and O(6). The 3a1 peak of water and the PDOS of surface layer Al resonates, indicating that the dissociated OwH bonds with Al atoms. This is consistent with the structure of Fig. 2e that single water is adsorbed on CuAl2O4 (110) via dissociative adsorption. 19

3.2. 2-8 Water molecules on CuAl2O4 surfaces 3.2.1. CuAl2O4 (100) surface On the basis of the first water molecule adsorption on the surface, more water molecules were added into the surface to investigate the effect of water coverage on the adsorption. Fig. 4 shows the structures and adsorption energies for more H2O molecules adsorb on the CuAl2O4 (100) surface. Two stable adsorption structures were found for two water molecules (θ = 3.00 H2O molecules/nm2) adsorption on the CuAl2O4 (100) surface. One is the dissociative adsorption in Fig. 4a and another is molecule adsorption Fig. 4b. Fig. 4a is the most stable adsorption with the adsorption energy of ‒198 kJ/mol, vs. ‒103 and ‒95 kJ/mol, respectively, of the stepwise adsorption energy for the first and second water molecules. The ‒OwH of the second water molecule bonds to Cu(2) with the bond distance of 192 pm. The dissociated H atom bonds to O(6) with the O‒H distance of 110 pm. In Fig. 4b, the second water molecule form similar molecular adsorption structure to the Fig. 2b, in which the Ow bonds to Cu(4) with the Cu‒O bond distance of 202 pm, and one of the H atom forms hydrogen bond with O(8) with the distance of 147 pm. The total adsorption energy for the two water molecules is ‒176 kJ/mol. It is interesting to find that the third water molecule (θ = 4.50 H2O molecules/nm2) seriously influences the structures of the pre-adsorbed two water molecules. After the adsorption of the third water molecule, the first and the second water became 20

molecular and dissociative adsorption, respectively. As shown in Fig. 4c, The first water molecule bonds to surface Al(3) with the Al‒Ow distance of 198 pm. The Ow of the second water, which bonds to Cu(2), form hydrogen bond with one H atom of the first water molecule. The dissociated H atom bonds to O(3). In addition, the third water molecule bonds to surface Al(1) with the Al‒Ow distance of 206 pm. The total adsorption energy for the three water molecules is ‒297 kJ/mol, vs. the stepwise adsorption energy for the third of ‒99 kJ/mol. As shown in Fig. 4d, the adsorption of the fourth water molecule (θ = 6.00 H2O molecules/nm2) also results the re-distribution of the pre-adsorbed three water molecules. One water is molecular adsorption on Al(4), whereas the other three water molecules are dissociated into H atoms and hydroxyls, with two hydroxyl bonds to Cu(2) and Cu(4) and another on the bridge of Cu(2) and Al(2). The total and stepwise adsorption energies are ‒399 and ‒102 kJ/mol, respectively. For the adsorption of five water molecules (θ = 7.5 H2O molecules/nm2), water molecules adsorbed on the top of Cu(2), Cu(4), Al(1), Al(2) and Al(4), respectively. It is interesting to see that the water molecules on Cu(2) and Al(2) share one H atom with the O‒H‒O bond distances of 116 and 127 pm. The adsorption energy is ‒528 kJ/mol. The stepwise adsorption energy is ‒128 kJ/mol, which is larger than the structure of four water molecules (Fig. 4d, ‒102 kJ/mol) on (100) surface.

21

For the adsorption of six water molecules (θ = 9.00 H2O molecules/nm2), as shown in Fig. 4f, the sixth water molecule adsorbed on the Al(3) site. Three of the six water molecules are dissociated into H atoms (on O(3,5,6)) and hydroxyls (on Cu(2), Cu(4) and Al(2)). The three molecular adsorption water molecules are adsorbed on Al(1,3,4). The total adsorption energy for the six water molecules is ‒665 kJ/mol.

Fig. 4. H2O molecules (θ = 3.00‒11.99 H2O molecules/nm2) adsorb on the CuAl2O4 (100) surface. O, Al and Cu atoms of the surface are in red, gray and blue, respectively. Ow and H of water in small red ball and write ball. The bond distances and adsorption energies are in pm and kJ/mol, respectively. It should be noted that after the adsorption of six water molecules, all of the Lewis acid sites (surface Al and Cu atoms) of the surface are covered with water/hydroxyls. 22

We further add the seventh and eighth water molecules into the surface. Fig. 4f and 4g shows the optimization results that no chemical bond was formed between the seventh (θ = 10.49 H2O molecules/nm2) and eighth water (θ = 11.99 H2O molecules/nm2) molecules with the surface, and only strong hydrogen bonds were formed between the pre-adsorbed water and the new added water molecules. The adsorption energies for seven and eight water molecules adsorption are ‒761 and ‒864 kJ/mol, vs. the stepwise adsorption energies seventh and eighth water molecules ‒96 and ‒103 kJ/mol, respectively.

3.2.2. 2-8 water molecules on CuAl2O4 (110) surface For two water molecules (θ = 2.12 H2O molecules/nm2) adsorption in the CuAl2O4 (110) surface, the second water molecule has the same dissociative adsorption structure as the first water molecule, with the hydroxyl group adsorb on the bridge site of Al(3,4), and H atom bonds to O(8) (Fig. 5a). The third (θ = 3.18 H2O molecules/nm2) and the fourth (θ = 4.24 H2O molecules/nm2) water molecules have the similar adsorption mode to those of the first and the second water molecules, that all form dissociative adsorption with the hydroxyl groups adsorb on the bridge sites of Al (Fig. 5(b, c)). The adsorption energies for the two to four water molecules are ‒328, ‒542, ‒727 kJ/mol, respectively; and the stepwise adsorption energies are ‒158, ‒214, ‒185 kJ/mol, respectively.

23

Fig. 5. H2O molecules (θ = 2.12‒8.48 H2O molecules/nm2) adsorption on the CuAl2O4 spinel (110) surface. O, Al and Cu atoms of the surface are in red, gray and blue, respectively. Ow and H of water in small red ball and write ball. The bond distances and adsorption energies are in pm and kJ/mol, respectively. Since all Al sites are pre-covered with the initio four water molecules, the fifth water molecule (θ = 5.30 H2O molecules/nm2) could only adsorb on the Cu site of the surface. As shown in Fig. 5d, the dissociated OwH of the fifth water molecule bonds to Cu(1) with the bond distance is 196 pm. The adsorption energy of the adsorption structure is ‒839 kJ/mol, and the stepwise adsorption energy is ‒112 kJ/mol, which is smaller than for the fourth water molecule. It indicates that water adsorbs on two Al sites is more stable than adsorb on one single Cu site of the CuAl2O4 (110) surface. 24

Table 2 Total (ΔE(H2O)n), stepwise (ΔΔE(H2O)) and average (ΔE(H2O)) adsorption energies (kJ/mol) of water adsorption on CuAl2O4 (100) and (110) surfaces. Surf

(100)

(110)

nH2O

ΔE(H2O)n

ΔΔE(H2O)

ΔE(H2O)

ΔE(H2O)n

ΔΔE(H2O)

ΔE(H2O)

1H2O

‒103

‒103

‒103

‒170

‒170

‒170

2H2O

‒198

‒95

‒99

‒328

‒158

‒164

3H2O

‒297

‒99

‒99

‒542

‒214

‒181

4H2O

‒399

‒102

‒100

‒728

‒186

‒182

5H2O

‒528

‒129

‒106

‒839

‒111

‒168

6H2O

‒665

‒137

‒111

‒947

‒108

‒158

7H2O

‒761

‒96

‒109

‒997

‒50

‒142

8H2O

‒864

‒102

‒108

‒1084

‒87

‒136

The sixth (θ = 6.36 H2O molecules/nm2), seventh (θ = 7.42 H2O molecules/nm2) and eight (θ = 8.48 H2O molecules/nm2) water molecules (molecularly and dissociative) adsorb on the Cu(2), Cu(4) and Cu(3), respectively, as shown in Fig. 5e, 5f and 5g. The total adsorption energies for the six to eight molecules are ‒947, ‒997 and ‒1084 kJ/mol, vs. and the stepwise adsorption energies of ‒108, ‒50 kJ/mol and ‒87 kJ/mol. Table 2 summarizes the total of water adsorption on CuAl2O4 (100) and (110) surfaces. As been expected that water on the (110) surface always have larger

25

adsorption energies than on the (100) surface, since the (100) is more stable (inert) than the (110) surface.

3.3 Work function analysis The work function is an important parameter to describe the physical chemistry properties of the surface, which sensitive to surface adsorbates. The work function is defined as: ΔΦ = E(∞) ‒ Efermi. Where the E(∞) and Efermi are vacuum level and Fermi level, then the work function is the difference between the vacuum level and the Fermi level. The influence of water adsorption on the work function of the CuAl2O4 spinel (100) and (110) surfaces are calculated and presented in Fig. 6. It shows that the work functions of CuAl2O4 (100) surface is always larger than that of the (110) surface, since the (100) surface (have smaller surface energy and stronger Lewis acidity) is more stable than the (110) surface (have larger surface energy and weaker Lewis acidity). In addition, the work functions of CuAl2O4 (100) and (110) surfaces change as the water coverage varies. The calculated work functions of the clean CuAl2O4 (100) and (110) surfaces are 5.67 and 5.53 eV, respectively. At low water coverages of lower than 5 and 6 H2O/p(1×1) slab for the (100) and (110) surface, respectively, the work function decrease as the water coverage increases. This agrees well with the previous investigations that the adsorption of water molecules usually results a decrease in surface work function, which had been reported for water adsorption at low coverage on the surfaces of Pt(111) [52], NiO(100) [53] and 26

Cu(110) [54]. However, as the water coverage increases to higher than 7 and 7 H2O/p(1×1) slab for the (100) and (110) surface, the work function decrease.

Fig. 6. Working function as a function of coverage for water molecules adsorption on CuAl2O4 (100) (black line) and (110) (blue line) surfaces. The values in the line graph represent the magnetic moment of the surface. The adsorption of water could be interpreted as the reaction of Lewis acid and base, in which the metal atoms of the oxide surface functions as Lewis acid and the oxygen atoms of the water molecules work as base. The adsorption of water is accompanied the charger transfer from the water molecules to the oxide surface. As the water coverage increases, more electrons were transferred from water into the surface, leads to the basicity of the surface increases, which results in the smaller work function of the surface. However, as the water coverage increases, the work function of the oxide

27

surfaces will reach its minima. Further increase the water coverage would lead to the increase of the work function since the surface is saturated by water.

Table 3 The charge transfer (e) for the adsorption of water molecule on the CuAl2O4 (100) and (110) surfaces. (100)

1st

2nd

3rd

4th

5th

6th

7th

8th

1H2O

+0.10

2H2O

+0.11

+0.22

3H2O

+0.03

+0.13

+0.02

4H2O

+0.09

+0.14

+0.06

+0.16

5H2O

+0.10

+0.08

+0.06

+0.15

+0.09

6H2O

+0.06

+0.13

+0.07

+0.08

+0.05

+0.03

7H2O

+0.09

+0.13

+0.07

+0.10

+0.05

+0.04

+0.12

8H2O

+0.04

+0.12

+0.06

+0.11

+0.04

+0.06

+0.08

+0.09

+0.60

(110)

1st

2nd

3rd

4th

5th

6th

7th

8th

total

1H2O

+0.01

2H2O

+0.01

+0.02

3H2O

0.00

0.00

‒0.02

4H2O

‒0.01

‒0.01

‒0.01

‒0.01

5H2O

‒0.02

‒0.01

‒0.01

+0.01

+0.16

6H2O

‒0.02

‒0.02

+0.02

+0.02

+0.16

+0.15

7H2O

‒0.01

‒0.03

0.00

0.00

+0.07

+0.20

+0.16

8H2O

‒0.02

‒0.02

‒0.02

0.00

+0.06

+0.17

+0.15

total +0.10 +0.32 +0.18 +0.45 +0.48 +0.42 +0.60

+0.01 +0.03 ‒0.03 ‒0.05 +0.12 +0.31 +0.38 +0.11

+0.43

28

It should be mentioned that the clean CuAl2O4 (100) and (110) surfaces are in ferromagnetic state and the magnetic moments mainly distributed on the surface Cu atoms. We tried different magnetic moments in the inputs and optimization leads to such ferromagnetic state, indicates ferromagnetic state surfaces is stable. This results agree well with the recent work on the Cu2O [55]. Tang [53] reported the work function of water adsorption on NiO(100) surface, and found that the magnetic moment of the surface Ni atoms which covered by water molecules are increased. Fig. 6 also show the magnetic moment of the surface slab with different water coverages (the numbers next to the curve in Fig. 6 present the total magnetic moment of the surface under each water coverage). It shows that the total magnetic moments as well as the work functions of the surface change with the varies of water coverages. Whenever the total magnetic moments changes, the rate for work function decrease will changes correspondingly. It indicates that the work function also relates with the total magnetic moments of the surface. Table 3 shows the charge transfer for 1‒8 water molecules adsorption on the (100) and (110) surfaces. The charges of the water molecules are extracted from the occupied PDOS peak. The charge transfer is calculated by the charge of the adsorbed water molecules minus the charge of the free molecules. The positive number indicates the electron transfer from the molecules to the surface. For one water molecule, the charge transfer to (100) and (110) surfaces are +0.10 and +0.01 e,

29

respectively. It indicates the stronger ability of the (100) surface to get electrons than the (110) surface, and agree well with the larger work function of the (100) surface than the (110) surface. Generally, the total charge transfer from the water molecules into the surface became larger as the coverage increases. However, for the 3rd water molecule in the (100) and the 3rd and 4th of the (110) surface, the charge transfer is abnormally smaller than others structures, and the work function only decrease slightly.

3.4. Water adsorption under given conditions

Fig. 7. The calculated Gibbs free energy changes (ΔG) for the desorption of H2O from the CuAl2O4 (100) and (110) surface with water coverage of θ and water partial pressure of 0.5 Mpa.

On the basis of the optimized structures, the thermodynamics for water molecules adsorption on the CuAl2O4 spinel (100) and (110) surfaces are analyzed with atomic thermodynamics to investigate the effects of temperatures and water pressures [56] on 30

the water adsorption. The Gibbs free energy changes (ΔG) is calculated for the adsorption of water on the spinel surface. As shown in equations (6). The μ presents the chemical potential of each component. For a given pressure, the free energy change is the function of the temperature for a given n. The more negative ΔG indicates the more stable adsorption configuration under given conditions. ΔG(T, PH2O) = μ[nH2O/CuAl2O4] –μ[CuAl2O4] – nμ[H2O] – RT ln[p(H2O)]n

(6)

Fig. 7 is the ΔGH2O(T, PH2O) map for the adsorption of water on the CuAl2O4 (100) and (110) surface. The water pressure is set to PH2O = 0.5 MPa, which is the typical reaction condition experiments [1].

Fig. 8. Phase diagram of stable H2O adsorption on CuAl2O4 (100) and (110) surfaces under different conditions. It shows that the water coverage of CuAl2O4 (100) surface is 11.99 H2O molecules/nm2 when the temperature is lower than 599 K. As the temperature rises, there has two water molecules desorb and become 9.00 H2O molecules/nm2 on the surface (6 water molecules in the slab). The adsorbed six water molecules desorbed 31

and become the clean surface when the temperature exceeds 669 K. While for CuAl2O4 (110) surface, the water coverage of CuAl2O4 (100) surface is 8.48 H2O molecules/nm2 when the temperature is lower than 496 K. In the temperature range of 496−629 K, the coverage is 6.36 H2O molecules/nm2. In the temperature range of 629−780 K, the coverage is 5.30 H2O molecules/nm2. The coverage decreases to 4.24 H2O molecules/nm2 as the temperature in the range of 780−1130 K, and it is totally dehydrated when the temperature is higher than 1130 K. In order to investigation the influences of temperatures and pressures on water adsorption, the Gibbs free energy changes for water adsorption on the CuAl2O4 (100) surface and (110) surface in wider temperature and pressure range (T = 300−1000 K, P = 10-1−104 kPa) are calculated. Fig. 8 is the phase diagram of stable H2O adsorption on CuAl2O4 (100) and (110) surfaces for T = 300−1000 K, P = 10-1−104 kPa. The left side of Fig. 8 shows that case for the CuAl2O4 (100) surface. There are three stable possibilities: clean CuAl2O4 (100) surface, six water and eight water molecules adsorption. For CuAl2O4 (110) surface in the p(1×1) slab, in the right side of Fig. 8, there are five stable adsorption structures: clean CuAl2O4 (110) surface, four, five, six and eight water molecules adsorption in the p(1×1) slab. The diagram shows that water will desorb from the CuAl2O4 (100) and (110) surfaces at higher temperature and lower H2O pressure. Water molecules are adsorbed on CuAl2O4 (110) surface more stable than CuAl2O4 (100) surface. 32

4. Conclusion Periodic density functional theory calculations were performed to investigate the adsorption of nH2O (n = 1−8) on the CuAl2O4 (100) and (110) surfaces. The results show that first water molecule is adsorbed on the CuAl2O4 (100) and (110) surfaces via dissociative adsorption, with the adsorption energies of −103 and −170 kJ/mol. The −OwH groups bond to surface Cu and Al for the (100) surface, while they bond to two surface Al atoms in the (110) surface. For one water molecule adsorbs on oxygen defective CuAl2O4 (100) and (110) surfaces, the O atom of the water molecule prefers to insert into the oxygen vacancy, leaving two isolated H atoms bonds to surface O atoms forming two in-surface hydroxyls. For nH2O (n = 2−8) on CuAl2O4 (100) surface, molecular and dissociative water adsorption coexist, and dissociative water adsorption is more favorable on CuAl2O4 (110) surface. As indicated by the calculated total, stepwise and average adsorption energies, the interaction of H2O with CuAl2O4 (110) surface is much stronger than with the (100) surface. Density of states analysis revels that the adsorption of single water on the spinel surfaces is accompanied by charge transfer. For single water adsorbs on CuAl2O4 (100) surface via lone pair electron of the adsorbed water molecule overlap with Cu 3d orbit forms one Ow−Cu bond. For single water adsorbs on CuAl2O4 (110) surface, the dissociated OwH bonds with Al atoms. The work function analysis shows that the 33

clean (100) surface has larger work function than the (110) surface. Water adsorption on the CuAl2O4 surfaces leads to the decreases of the water function at low water coverage, however, higher water coverages results in the increase of the work function. The phase diagram of water adsorption on the CuAl2O4 spinel surfaces with various water pressure and temperature is analyzed. The results show that there are three stable adsorption structures on the CuAl2O4 (100) surface: clean surface, six and eight water molecules adsorption on the CuAl2O4 (100) surface vs. six possibilities: clean surface, four, five, six, and eight water molecules adsorption on the CuAl2O4 (110) surface. The diagram shows that water will desorb from the CuAl2O4 surfaces at higher temperature and lower H2O pressure.

Acknowledgments This work has been supported by the National Natural Science Foundation of China (Grants Nos. 21763018, 21875096, and 21673270), the special Funding for Transformation of Scientific and Technological Achievements in Qinghai Province (2018-GX-101), the Natural Science Basic Program of Shaanxi Province (2019JM-226), and the Natural Science Foundation of Jiangxi Province, China (Grants No. 20181BAB203016, 20181BCD40004).

34

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Author Contributions Liu Shi does most of the calculations and prepared the first version of the manuscript. Shuai Meng gives guidance to improve the discussions of the manuscript. Siriporn Jungsuttiwong contributes to the model description for the revision of the manuscript. Supawadee Namuangruk contributes to some of the computational methods for the revision of the manuscript. Zhang-Hui Lu contributes to final revision of the manuscript. 42

Li Li analyzed the charge data and made tables and figures together with Liu Shi. Rongbin Zhang contributes to the introduction and the conclusion of the manuscript. Gang Feng is the corresponding author in charge of the whole work. Shaojun Qing from the experiment group to give important opinions to interpret the results data, and does PDOS analysis. Zhixian Gao from the experiment group proposed this project and give important opinions to interpret the results data. XiaohuYu contributes to the work function and magnetization analysis.

Declaration of interests

 The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Graphic Abstract

Highlights 1.

One water molecule is absorbed on the CuAl2O4 (100) and (110) (water coverage θ of 1.50 and 1.06 H2O molecule/nm2) surfaces via dissociative adsorption, with the adsorption energies of −103 and −170 kJ/mol.

2.

The adsorption of water on the spinel surface is accompanied by charge transfer.

3.

Water adsorption on the CuAl2O4 surfaces leads to the decreases of the water function at low water coverage, however, higher water coverages results in the 44

increase of the work function. 4.

There are three stable adsorption structures on the CuAl2O4 (100) surface vs. six possibilities on the CuAl2O4 (110) surface.

45