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High Coverage H Adsorption on the Reconstructed CuO(111) Surface Xiaohu Yu, Xuemei Zhang, Hongtao Wang, Zhiyin Wang, and Gang Feng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06361 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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The Journal of Physical Chemistry

High Coverage H2 Adsorption on the Reconstructed Cu2O(111) Surface Xiaohu Yua*, Xuemei Zhanga,b, Hongtao Wangb*, Zhiyin Wanga and Gang Fengc a) 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; b) College of Environmental Science and Engineering, Taiyuan University of Technology,Taiyuan 030024, P.R. China; c) College of Chemistry, Nanchang University, Nanchang, Jiangxi 330031, P.R. China. Xiaohu Yu Tel: +86 18739700918; E-mail: [email protected] Hongtao Wang Tel: +86 13834607727; Email: [email protected] RECEIVED DATE (xxxx) Abstract The adsorption of H2 on the Cu2O(111) surface has been studied by spin-polarized density functional theory (DFT+U) calculations and atomic thermodynamics. It has been found that there exists reconstruction on stoichiometric Cu2O(111) surface. Probability distribution of the reconstructed Cu2O(111) surfaces as function of temperature has been analyzed using Boltzman statistics. It has been found that the molecular H2 prefers to adsorption on the CuCUS atom at low coverages (1/4 or 1/2 monolayer), while totally dissociative H2 is preferred on reconstructed Cu2O(111) surface at higher coverages (3/4 or 1 monolayer). For H2 splitting on Cu2O(111) surface, homolytical dissociative adsorption on two surface un-coordinated CuCUS atoms is preferred which is new mechanism for H2 on metal oxides. More interesting is that the surface reconstruction will be recovered for eight hydrogen atoms binding on four un-coordinated CuCUS and four un-coordinated OCUS atoms at saturation coverage. It has been found that the adsorbed H atoms will put out the lattice oxygen to the surface at higher coverage (five and six H2), which agrees well with the experimental findings. The phase diagrams of H2 binding on ideal and reconstructed Cu2O(111) surfaces were plotted and analyzed. In addition, we compared and analyzed the adsorption mechanisms of H2 splitting on different metal oxides.

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1. Introduction The study of the dissociation of the hydrogen molecule at interfaces is an active field that is motivated by the desire to understand many processes, such as heterogeneous catalysis, hydrogen storage, fuels, sensors, doping of semiconducting oxides, etc.1 It is well known that there are two mechanisms of H2 splitting on metal oxide surfaces: heterolytic or homolytic mechanisms, depending on the nature of the metal oxide 2,3. Cuprous oxide (Cu2O)4 is an attractive p-type oxide for photoelectrochemical hydrogen production with a direct bandgap of 2 eV. However, interaction of H2 on Cu2O surfaces is still not clear theoretically. The interaction of H2 with copper oxides attracted many attentions. Althrough the reduction of metal oxide using hydrogen is a frequently used method to prepare active catalysts and electronic devices, the reduction process is generally not well understood at a molecular or atomic level. Using Time-resolved X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), and first-principles density functional calculations, Kim et al.3,5,6 studied the reduction of CuO and Cu2O with H2 and they found that the mechanism for the reduction of CuO is complex, involving an induction period and the embedding of H into the bulk of the oxide. In addition, they found that the reduction of CuO is easier than the reduction of Cu2O and the apparent activation energy for reduction of CuO is 14.5 kcal/mol, while the value is 27.4 kcal/mol for Cu2O. Tabuchi et al. 7 studied control of carrier concentration in thin cuprous oxide Cu2O films by atomic hydrogen, and found that atomic H exposure is a useful tool for improving the properties of Cu2O. Poulston et al.8 studied surface oxidation and reduction of CuO and Cu2O using XPS and XAES, and found that hydrogen-reduced CuO and Cu2O were both reoxidized on vacuum annealing, demonstrating the diffusion of lattice oxygen to the surface. Using ultraviolet photoelectron spectroscopy (UPS) and thermal desorption spectroscopy (TDS) ， Cox and Schulz9 studied water adsorption on the Cu-terminated Cu2O(100) surface and found that adsorption at 110 K is both dissociative and molecular, with only about 10% of a monolayer of water dissociated, while adsorption at 300 K is dissociative only. In addition, they10 studied the interaction of atomic hydrogen with Cu2O(100) and found that surface hydride formation is formed on the metal oxide surface. Nygren et al.11 studied H2O interaction with the polar Cu2O(100) surface and found that dissociative adsorption is favoured than molecular adsorption. Li et al.12 studied H2O adsorption and dissociation on Cu2O(100) and found that activation barrier for HxO(x=1, 2) dehydrogenation are 0.42 eV and 1.86 eV. Using ACS Paragon Plus Environment

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DFT+U, Maimaiti et al.13 studied the reduction mechanisms of the CuO(111) surface through surface oxygen vacancy formation and hydrogen adsorption, and found that the CuO surface is reduced to Cu2O at approximately 360 K and that complete reduction from Cu2O to metallic Cu occurs at 780 K using H2 as the reducing agent. Recently, water adsorption on CuO surfaces at different coverage was studied by us14,15. Using high-resolution photoelectron spectroscopy (PES) and Hubbard U and dispersion corrected density functional theory (PBE-D3+U) calculations, Stenlid et al.16 studied water interactions and reactions on a common Cu2O(100):Cu surface and found that the results are consistent for ambient temperatures under wet/humic and oxygen lean conditions. Cu2O(111) surface attracted extensive researches experimentally and theoretically as main grown surface of Cu2O surfaces17 and having higher catalytic activity in many surface processes18,19. Using scanning tunneling microscopy (STM) and low-energy electron diffraction, Sträter et al.20 studied the growth and surface properties of cuprous oxide films on Au(111) surface and found that the structure of postannealed films is similar to the one of bulk-cut Cu2O(111). Using STM and hybrid density functional theory, Nilius et al.21 studied the stability of Cu2O(111) surface and found that the stoichiometric surface is the most stable at oxygen-lean conditions which is confirmed by an excellent matching between STM spectroscopy data and the calculated surface electronic structure. It is reported that the octahedral Cu2O with exposed (111) facets exhibited much higher photocatalytic activity than cubes

22,23

. Zheng et al.24 researched the surface stabilities and photocatalytic properties of Cu2O

microcrystals and found that Cu2O exposing (111) facets can be as a stable photocatalyst. Önsten et al. 25

studied role of defects in suface chemistry on Cu2O(111), and found that there are two different

defects on the surface. Using cluster model, Casarin et al.26 studied H2O and H2S adsorption on Cu2O(111) cluster and found that molecular H2O chemisorption is favoured than the dissociative adsorption on Cu2O cluster. Recently, surface properties of Cu2O surface27, high coverage water adsorption on Cu2O(111) surface28 and O2 adsorption Cu2O surfaces29 were studied by us. CO2 adsorption on Cu2O(111) surface was researched by different groups

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and water interaction with

Cu2O(111) surface was also studied by Riplinger and Carter 34. To shed light on the interaction of H2 on Cu2O(111) surface and unraveling the role of different coverages of H2 adsorbates, systematically studying the different H2 adsorption is very important. To the best of our knowledge, there is only a few reports about the H2 adsorption on the Cu2O surfaces have ACS Paragon Plus Environment

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been reported35,36. Zhang et al.35 studied one single H2 adsorption on Cu2O(111) p(2×2) surface with six-layer model and found that surface un-coordinated CuCUS is the most advantageous position with the side-on type of H2. Li et al.

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researched single H2 adsorption on the different Cu2O(111) p(1×1)

surfaces and found that parallelly H2 adsorbs on surface un-coordinated CuCUS atom. In the present paper, we have performed a systemtically DFT+U study of different coverages of H2 on reconstructed Cu2O(111) surface. The probability distribution of different reconstructed Cu2O(111) surfaces as function of temperature are calculated using Boltzmann statistics. Atomic thermodynamics is used to analyze the adsorption properties of H2 on ideal and reconstructed Cu2O(111) surfaces. The overall trends of the adsorption energy as a function of coverage are discussed and the adsorption properties of H2 on Cu2O(111) surfaces and other metal oxide surfaces are compared. 2. Methodology and Surface Model 2.1. Computational Methodology The exchange and correlation energies were calculated using the Perdew, Burke and Ernzerhof (PBE) functional.37 The projector augmented wave (PAW) method described the electron-ion interaction.38 The well-known GGA+U method, Ueff

= 6 eV for Cu2O which is same to former literature

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, was

introduced for the Cu-3d electrons to describe the on-site Coulomb interaction. The Kohn-Sham oneelectron states were expanded in a plane wave basis set up to 500 eV for bulk, and smaller 350 eV for slab in order to decrease the computational cost. The integrations over the Brillouin zone were performed using Monkhorst-Pack grids39 and the conjugate gradient algorithm was applied to determine the electronic ground state. The results presented in this work were obtained using k-point meshes of 5×5×5 for the bulk and 3×3×1 for the surface calculations. A Gaussian broadening of the Fermi surface of 0.2 eV was applied to improve the convergence of the solutions. In all calculations the positions of all atoms were fully relaxed until the Hellmann-Feynman forces were smaller than 0.02 eV/Å and the energy was smaller than 10-4 eV. The dipole moment induced by adsorbed H molecules was corrected by dipole moment which is perpendicular to the surface.40,41 All calculations were performed using spin polarized plane-wave periodic density functional method as implemented in the Vienna ab initio simulation package (VASP).

40,41

The DFT-D3 method of Grimme42,43, which takes into account the

dispersive interactions, was used to test the adsorption of H2 on the Cu2O(111) surface. It was found that the adsorption energies of the most stable molecular (-0.54 eV), homolytical (-0.34 eV) and heterolytical ACS Paragon Plus Environment

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(-0.31 eV) H2 adsorptions on the Cu2O(111) surface using (GGA+U)-D3 method are larger than corresponding molecular (-0.47 eV), homolytical (-0.30 eV) and heterolytical (-0.28 eV) adsorptions using GGA+U. However, the vdw force correction does not affect our conclusion. So, the results reported in this work use the GGA+U method. For the location of transition states, the climbing image nudged elastic band (CI-NEB) algorithm was employed44. The nature of the transition states was assessed by performing numerical frequency analysis. The reported Bader charges were calculated using the charge densities obtained with VASP and the program developed by Henkelman et al. 45 2.2. Model and Properties Calculated. It is well known that the Cu2O crystal has a simple cubic unit cell with (Pn3) structure and there are six atoms in Cu2O unit cell: 2O atoms form a bcc lattice and each O atom is at a center of a tetrahedron, whose four vertices are occupied by Cu atoms. Using Ueff = 6 eV, we derived of the unit cell parameters of 4.285 Å which is in reasonable agreement with the experimental value of about 4.270 Å and former theoretical result (4.270 27 with DFT and 4.317 Å46 with DFT+U). For the Cu2O(111) surface, there are three different terminations: O-terminated surface (2.526 J/m2), Cu-terminated surface (2.300 J/m2) and Cu/O terminated Cu2O(111) surface (0.784 J/m2)27,29. Among three terminations, Cu/O terminated Cu2O(111) surface was chosen in this work since it is most stable termination17 and has higher activity in many reactions47,48. In experiment, there are reported that two experimentally characterized structures of the (111) surface: the ideal Cu/O terminated surface with (1× 1) periodicity and the surface with (√3×√3)R30°periodicity and 2/3 of the terminating oxygen ML.49 The most stable perfect Cu/O terminated Cu2O(111) surface is non-polar surface. It is reported that there are six-layered slabs to simulate H235, O250, H2S51, N2O52 adsorption on Cu2O(111) surface, nine-layered slabs to simulate CO oxidation53 and CO2 electrochemical reduction32 on Cu2O(111) surface, twelve-layer slabs to simulate CO adsorption54 on Cu2O(111) surface and fifteen-layer slabs to simulate CO231 and water34 binding on Cu2O(111) surface. Considering computational accuracy and efficient, a nine-layered slabs made of (2×2) supercell was used to represent Cu2O(111) surface in this work (Figure 1). A 15 Å vacuum region was used to separate slabs in order to minimize the spurious interactions between the periodic images. In the whole calculating process, a (12.204 Å×12.204 Å× 21.228 Å) hexagonal supercell was used to simulate the Cu2O(111) surface. It exposed four different types of surface atoms in Cu2O(111) surface17, which were denoted as OCUS (the outmost surface oxygen) ACS Paragon Plus Environment

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in the first layer, OCSA (the subsurface oxygen) in the third layer, CuCUS (coordinative unsaturated copper) in the second layer, and CuCSA (coordinative saturated copper) in the second layer. The most stable molecular H2 adsorption on two nine-layered models was tested: the adsorption energy on nine-layered slab with fixed bottom three-layer is larger than on nine-layered slab with fixed bottom six-layer about 0.03 eV. So in all following reported calculations, the atoms of the top three layers and adsorbed H atoms are fully relaxed, while the atoms of the bottom six layers are fixed to their bulk positions. All possible adsorption sites and structures are considerred for each adsorbate, and only those with the largest adsorption energies are used for discussion and comparison; the less stable adsorption sites and structures are given in the Supporting Information. The total adsorption energies were calculated using equation: ΔE(H2)n = E(nH2 /slab)- E(slab)-nE(H2) where E(nH2/slab) is total energy of the adsorbed system, E(slab) is the energy of slab, and E(H2) is the energy of H2 in the gas phase. Gas-phase H2 molecule was calculated in a box of (10×10×10) Å3 using the same cutoff parameters as for the slab calculation. The stepwise adsorption enery was defined as: ∆∆E(H2)n =∆E(H2)n -∆E(H2)n-1, and the average adsorption energy was defined as: ΔE =ΔE(H2)n/n. The bond length of molecular H2 calculated from our approach is 0.750 Å, the bond energy is about 6.09 eV, which is in well agreement with the experimental values of 0.740 Å 36

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and other theoretical results

. Above results obtained in these tests made us confident in pursuing the next step of our researchs,

namely the interaction of H2 with Cu2O surfaces. (Table 1) and (Figure 1) 3. Results and Discussion 3.1. Reconstructed Cu2O(111) surface: First, we studied the reconstructed Cu2O(111) surface and the most stable reconstructed Cu2O(111) surface is shown in Figure 1a. The optimized structure of perfect Cu2O(111) surface is shown in Figure 1b and there are two different oxygen sites: four un-coordinated oxygen atoms (OCUS) in the first layer, four coordinated oxygen atoms (OCSA) in the third layer. The CuCUS-O bond is 1.848 Å, which is much shorter than the bulk Cu-O bond length of 1.868 Å. In addition, there are four surface un-coordinated CuCUS atoms in the second layer on the Cu2O(111) surface as shown in Figure 1b, A, B, C, D represent four surface un-coordinated CuCUS atoms. In order to shed light on the reconstructed Cu2O(111) surface, ACS Paragon Plus Environment

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we move the surface un-coordinated CuCUS atom in 1% about 0.122 Å in x and y axis, respectively, before fully relaxation. So the AxyBCD means move the A atom in x and y about 0.122 Å, and AxyBxyCD means move the A and B atoms in x and y axis about 0.122 Å, respectively. In total, eight different reconstructed Cu2O(111) surface models were considered in this work. After fully relaxation, the AxyBxCD model was found to be the most stable reconstructed Cu2O(111) surfaces as shown in Figure 1a, which is more stable than ideal Cu2O(111) surface about 0.764 eV. It indicates that the fully relaxed ideal Cu2O(111) surface is metastable surface which is in well agreement with former theoretical results56,57. The four surface un-coordinated CuCUS atoms move to one side of reconstructed Cu2O(111) surface (Figure 1a), the distances of CuCUS-CuCSA change from 3.580 to 2.446, 2.547, 2.649, 2.446, 2.547, 2.649, 2.643, 2.450, 2.555, 2.643, 2.450, and 2.555 Å, respectively. The extra stability of reconstructed Cu2O(111) surface comes from the shortened CuCUS-CuCSA distances. The configurations of least stable reconstructed Cu2O(111) surfaces were shown in Supporting Information Figure S1. In addition, the probability distribution as function of temperature was analyzed according the Boltzmann statistics (Figure 2). Each reconstructed Cu2O(111) surface has a probability of occurrence according Boltzmann statistics, Pm, which is function of the temperature: Pm = 1/Zexp[-∆E/(KBT)], where Z is the canonical partition function, and ∆E is the free energy of reconstructed and ideal Cu2O(111) surfaces. The AxyBxCD model of the reconstructed Cu2O(111) (Figure 2a) is the most preferred at 0 K. Along the much higher temperature, probability distribution of different least stable reconstructed Cu2O(111) surfaces increase. One can clearly see that probability distribution of the different least stable reconstructed Cu2O(111) surfaces (Figure 2c) increase at 1000 K, while the ideal Cu2O(111) surface only have very small probability. It indicates that different reconstructed Cu2O(111) surfaces can exchangeable at higher temperature and the reconstructed Cu2O(111) surface may play an important role in the surface chemistry of Cu2O(111) surface. (Figure 2) 3.2. H2 molecules on reconstructed Cu2O surface: Same to the ideal Cu2O(111) surface, there exposed four surface un-coordinated OCUS atoms in first layer, four surface un-coordinated CuCUS atoms in the second layer, 12 surface coordinated CuCSA atoms in the second layer and four coordinated OCSA in the third layer on the reconstructed Cu2O(111) surface. Here the coverage is defined as the ratio of the number of adsorbed H2 molecules to the number of ACS Paragon Plus Environment

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surface un-coordinated CuCUS atoms. In the following, if we do not particular stress, the total adsorption energy is corresponding to the reconstructed surface. (Figure 3) For a single H2 molecule at a 1/4 monolayer (ML) coverage, molecular, heterolytic and homolytic adsorptions were computed. In the most stable molecular adsorption (Figure 3a), molecular H2 parallelly adsorbs on surface un-coordinated CuCUS atom with H-CuCUS bond lengths of 1.662 and 1.651 Å, and the bond length of H-H changes from 0.750 (free H2 molecule) to 0.825 Å: the total adsorption energy is -0.47 eV. Our adsorption mode agrees well with the former theoretical result 36. The adsorption energy (-0.73 eV)36 of the former result is much larger than our result (-0.47 eV). There should be two reasons: one is that they did not consider the surface reconstruction in former work, the other is that the slab that they used is p(1×1) in former work and we used is slab p(2×2). In the most stable heterolytic adsorption (Figure 3b), one hydrogen atom adsorbs on surface un-coordinated CuCUS atom with H-CuCUS bond length of 1.488 Å, and another hydrogen atom binds on the surface un-coordinated OCUS with HOCUS bond length of 0.988 Å: the total adsorption energy is -0.28 eV. In the most stable homolytic adsorption (Figure 3c), one hydrogen atom adsorbs on one surface un-coordinated CuCUS atom with HCuCUS bond length of 1.490 Å, and another hydrogen goes to another surface un-coordinated CuCUS with H-CuCUS bond length of 1.483 Å; the total adsorption energy is -0.30 eV. Comparing with H2 adsorption on two surface CuCUS atoms via homolytical dissociation, H2 adsorbs on two surface OCUS atoms via homolytical dissociation with total adsorption energy of 0.38 eV (Figure S2f). One can clearly see that there exists an unique mechanism for H2 splitting on Cu2O(111) surface which is different with former mechanisms of H2 splitting on other metal oxides: H2 adsorbs on two surface un-coordinated CuCUS atoms via homolytical dissociation. It should be mentioned that at the initial stage put two hydrogen atoms on surface un-coordinated OCUS atom, the surface oxygen atom will be pulled out and forms a H2O molecule with total adsorption energy of -0.11 eV (Figure S2b). It indicates that the water molecule formation on reconstructed Cu2O(111) surface by two hydrogen atoms binding with surface uncoordinated OCUS atom is favorable thermodynamically. On the basis of the most stable molecular and heterolytic dissociative adsorption (Figure 3a and Figure 3b), we computated dissociation transition state44, in which the breaking H-H distance is elongated from 0.825 to 1.160 Å and formed H-O distance is 1.198 Å: the computated heterolytic dissociation barrier is 1.22 eV (Figure S2j). In the same time, the ACS Paragon Plus Environment

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H atom which binds on surface CuCUS derives 0.35 e by Bader analysis45, while another H atom which binds on surface OCUS loses 0.46 e. On the basis of the most stable heterolytic dissociative adsorption and homolytic dissociative adsorption (Figure 3b and Figure 3c), the H-OCUS distance is 1.216 Å and the H-CuCUS distance is 1.805 Å in the transition state (Figure S2k), and the diffusion barrier of H atom is 0.62 eV. Comparing three adsorption modes, one can easily see that molecular adsorption is the most preferred on reconstructed Cu2O(111) surface at 1/4 ML coverage and H2 adsorption on two CuCUS atoms via homolytical dissociation is new mechamism for H2 splitting. For two H2 molecules at a 2/4 ML coverage, molecular, mixed and dissociative adsorptions were computed. In the most stable molecular H2 adsorption configuration (Figure 3d), two H2 molecules adsorb on two surface un-coordinated CuCUS atoms with four H-CuCUS bond lengths of 1.658, 1.660, 1.659 and 1.650 Å, and two H-H bond lengths change from 0.750 Å (free H2 molecule) to 0.822 and 0.825 Å; the total adsorption energy is -0.87 eV. In the most stable mixed molecular and dissociative adsorption configuration (Figure 3e), one molecular H2 binds on one surface un-coordinated CuCUS atom with two H-CuCUS bond lengths of 1.646 and 1.656 Å, and the bond length of H-H changes from 0.750 to 0.825 Å, and another H2 heterolytically dissociates with one hydrogen atom binding on surface uncoordinated CuCUS atom with H-CuCUS bond length of 1.490 Å, and another H atom goes to surface uncoordinated OCUS atom with H-OCUS bond length of 0.990 Å; the total adsorption energy is -0.66 eV. In the most stable dissociative adsorption configuration (Figure 3f), two hydrogen atoms bind on two surface un-coordinated CuCUS atoms with both H-CuCUS bond lengths of 1.492 Å, other two hydrogen atoms go to two surface un-coordinated OCUS atoms with two H-OCUS bond lengths of 0.989 and 0.991 Å; the total adsorption energy is -0.84 eV. Comparing three adsorption modes, one can clearly see that the molecular adsorption is the most preferred for two H2 molecules at 2/4 ML coverage. For three H2 molecules at a 3/4 ML coverage, molecular, mixed and dissociative adsorptions were computed. In the most stable molecular adsorption configuration (Figure 3g), three molecular H2 parallelly bind on three surface un-coordinated CuCUS atoms with six H-CuCUS distances of 1.655, 1.645, 1.650, 1.644, 1.655 and 1.649 Å, and three H-H bond lengths change from 0.750 to 0.825, 0.827, and 0.826 Å: the total adsorption energy is -1.25 eV. In the most stable mixed molecular and dissociative adsorption configuration (Figure 3h), two molecular H2 parallelly adsorb on the surface un-coordinated CuCUS atoms with four H-CuCUS bond lengths of 1.649, 1.643, 1.654, and 1.646 Å, and the H-H bond ACS Paragon Plus Environment

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lengths change from 0.750 Å to 0.827 and 0.828 Å, and another H2 heterolytically dissociates with one H atom binding surface un-coordinated CuCUS atom with H-CuCUS distance of 1.491 Å and another H atom binding on surface un-coordinated OCUS atom with H-OCUS distance of 0.990 Å: the total adsorption energy is -1.21 eV. In the most stable dissociative adsorption configuration (Figure 3i), three hydrogen atoms adsorb on three surface un-coordinated OCUS atoms and one dissociated hydrogen atom binds on one surface un-coordinated CuCUS atom, and other two dissociated hydrogen atoms go to bridge sites of surface un-coordinated CuCUS and surface coordinated CuCSA atoms; the total adsorption energy is -1.34 eV. Comparing three adsorption modes, it is clearly see that the totally dissociative adsorption is most preferred on reconstructed Cu2O(111) surface at 3/4 ML coverage. For four H2 molecules at a 1 ML coverage, molecular, mixed and dissociative adsorptions were computed. In the most stable molecular adsorption configuration (Figure 3j), four molecular H2 adsorb on four surface un-coordinated CuCUS atoms with total adsorption energy of -1.77 eV. In the most stable mixed molecular and dissociative adsorption configuration (Figure 3k), one molecular H2 adsorbs on one surface un-coordinated CuCUS atom, and other three H2 heterolytically dissociate to six hydrogen atoms: three hydrogen atoms bind on three un-coordinated CuCUS atoms and other three hydrogen atoms go to three surface un-coordinated OCUS atoms; the total adsorption energy is -1.73 eV. In the most stable dissociative adsorption configuration (Figure 3l), four hydrogen atoms adsorb on four surface uncoordinated CuCUS atoms with four H-CuCUS bond lengths of 1.490 Å and other four hydrogen atoms bind on four surface un-coordinated OCUS atoms with four H-Osurf distances of 0.898 Å: the total adsorption energy is -1.98 eV. More interesting is that four H2 adsorption on reconstructed Cu2O(111) surface via heterolytical dissociation can get rid of the surface reconstruction. One can see that totally dissociative adsorption is most preferable for four H2 molecules at 1 ML coverage in which the adsorption of four H2 molecules completely saturates the surface cationic and anionic sites. For five H2 molecules at a 5/4 ML coverage, the molecular, mixed and dissociative adsorptions were computed. No molecular configuration was located for five H2 molecules. In the most stable mixed molecular and dissociative coadsorption configuration (Figure 3m), one can easily see that there are four dissociated H atoms adsorption on surface un-coordinated CuCUS atoms and other four hydrogen atoms binding on four surface un-coordinated OCUS atoms, one molecular H2 physisorbs on surface uncoordinated CuCUS and the distance of H-CuCUS is 3.501 Å: the total adsorption energy is -1.94 eV. Here ACS Paragon Plus Environment

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it should be mentioned that in the most stable chemisorption configuration (Figure S5c), four molecular H2 adsorb on four surface un-coordinated CuCUS atoms and two dissociated H atoms adsorb on two surface un-coordinated OCUS atoms with the total adsorption energy of -0.61 eV. In the most stable dissociative adsorption configuration (Figure 3n), four H atoms adsorb on four surface un-coordinated OCUS atoms, and six H atoms bind on the top of surface un-coordinated CuCUS and bridge sites of CuCUS and CuCSA atoms with the total adsorption energy of -0.75 eV. More interesting is that at the beginning, put two H atoms on two surface coordinated OCSA atoms in the third layer (Figure S6a), the two OCSA atoms will move out to surface layer by formed two H-OCSA bonds and induce the surface strong reconstruction with the total adsorption energy of -3.16 eV, which is in well agreement with experimental report8: the diffusion of lattice oxygen atom to surface by H adsorption. Comparing the four H2 and five H2 adsorption on Cu2O(111) surface, one can clearly see that five H2 adsorption on Cu2O(111) is not favorable thermodynamically. For six H2 molecules at a 6/4 ML coverage, the molecular, mixed and dissociative adsorptions were computed. No configuration of six molecular H2 adsorption on the Cu2O(111) surface was located. In the most stable mixed molecular and dissociative coadsorption configuration (Figure 3o), four H atoms favor binding on the four surface un-coordinated CuCUS atoms and other four H atoms favor adsorption on the four surface un-coordinated OCUS atoms, and last two molecular H2 physisorb on the Cu2O(111) surface as indicated the long H2-CuCUS distance of more than 3.5 Å: the total adsorption energy is -1.92 eV. It should be mentioned that four molecular H2 chemisorb on surface un-coordinated CuCUS atoms (Figure S7b), and other four H atoms adsorb on surface un-coordinated OCUS atom with the total adsorption energy of 0.67 eV. In the most stable dissociative adsorption configuration (Figure 3p), four H atoms adsorb on four surface un-coordinated OCUS atoms and other dissociative H atoms bind at bridge sites of CuCUS and CuCSA atoms with the total adsorption energy of -0.85 eV. More interesting is that initially put four hydrogen atoms on the four coordinated OCSA atoms in the third layer (Figure S7c), and four OCSA atoms in the third layer will move out of surface and induce the strong reconstruction with the total adsorption energy of 0.22 eV, which agrees well with experimental report8: the diffusion of lattice oxygen atom to surface by H adsorption. One can easily see that six H2 adsorption on Cu2O(111) surface at 6/4 ML coverage is not favorable thermodynamically. The stepwise adsorption energy for most stable one to six H2 molecules is -0.47, -0.40, -0.37, -0.65, ACS Paragon Plus Environment

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0.04, and 0.02 eV, and corresponding average adsorption energy for one to six H2 molecules is -0.47, 0.44, -0.45, -0.50, -0.35, -0.39, and -0.32 eV, respectively. This indicates that the adsorption of four H2 molecules on the reconstructed Cu2O(111) surface is favorable thermodynamically since the negative value of the stepwise adsorption energy. It is found that there is new mechanism for H2 splitting on Cu2O(111) surface: H2 adsorbs on two surface CuCSA atoms via homolytic dissociation. There is no clear trend about the coverage effect: along higher H2 coverage, both stewpwise and average adsorption energies first increase and then decrease, which is mainly due to the surface reconstruction and Coulomb repulsion between adsorbed H atoms. 3.3. Phase diagram of H2 adsorption at given conditions: To shed light on the stable adsorption structure of H2 on the Cu2O(111) surface under experimental conditions, the effect of different temperature and pressure was considerred using atomic thermodynamics 58,59. It is well known that atomic thermodynamics is very useful tool to study the effect of environment on the bulk60, surface61,62 and cluster63-65. According atomic thermodynamics, we chosed the Gibbs free energy (∆G) of nH2 adsorption on the Cu2O(111) surface as the criterion and defined it as: ads ∆GCu (T , P, nH 2 ) = G[nH 2 /Cu 2 O(111)] - G[Cu 2 O(111)] - nGH 2 (T , pH2 ) 2O

(1)

where G[nH 2 /Cu 2 O(111)] , G[Cu 2 O(111)] and GH 2 (T , pH2 ) are the Gibbs free energies of adsorbed system, support and adsorbates, respectively, n is number of adsorbed H2 molecules, T is the temperature, pH2 is the partial pressure of H2 in the gas atmosphere. Here a more negative ∆G indicates the more stable adsorption structure. We can write GH 2 (T , pH2 ) as:

GH2 (T, pH2 ) = EHtotal + µH2 (T, p0 ) + kΒT ln( p / p0 ) 2

(2)

where EHtotal is the total enery of H2 molecules including zero point vibration energy, µH2(T, p0) term 2 includes vibrational and rotational contributions for H2 gas, and can be taken from tables of thermodynamic data 66. kΒT ln( p / p 0 ) is the contribution of temperature and H2 partial pressure to the H2 chemical potential and kΒ is the Boltzmann constant. Since frequency motions of surface-slab have large entropy contribution to the surface, we included the vibration contribution of Cu2O(111) surface with adsorbed H2 in the DFT calculated total energy to substitute the Gibbs free energies of solid surface. Then, we can rewrite equation (1) as: ads ∆GCu (T , P, nH 2 ) = G[nH 2 /Cu 2 O(111)] - Ε[Cu 2 O(111)] - nGH2 (T , pH2 ) 2O

(3)

One can see that there are three stable surfaces: clean Cu2O(111), one H2 adsorption and four H2 ACS Paragon Plus Environment

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adsorption on ideal Cu2O(111) surface, while there are two stable surfaces: reconstructed Cu2O(111), four H2 adsorption on reconstructed Cu2O(111) surface, as shown in convex hulls of Figure 4a and 4c, One can recast Figure 4a and 4c in another insightful forms, as shown in the phase diagram (Figure 4b and 4d). Inserting equation (2) into (3), one can derive: ads ∆GCu (T , P, nH 2 ) = G[nH 2 /Cu 2 O(111)] - Ε[Cu 2 O(111)] - nEHtotal - nµH2 (T , p 0 ) − nkΒT ln( p / p 0 ) (4) 2O 2

On the basis of H2 adsorption, atomic thermodynamics is applied to study the effects of temperature and pressure on its coverage. The phase diagram of H2 on ideal and reconstructed Cu2O(111) surface at wide pressure and temperature ranges is shown in Figure 5a and 5b. One can see that there are three stable adsorption configurations on ideal Cu2O(111) surface: clean Cu2O(111) surface, one H2 molcules adsorption and four H2 molecules adsorption, while there are two stable adsorption configurations on reconstructed Cu2O(111) surface: reconstructed Cu2O(111) surface and four H2 molecules adsorption. One can see that H2 molecules will desorb on the Cu2O(111) surface at high temperature and low H2 pressure from the diagram which agrees well with physical intuition. Two and three H2 molecules binding on Cu2O(111) surface which is not shown on the two phase diagrams maybe that the process is controlled by kinetically not by thermodynamically. The stable single H2 binding on ideal Cu2O(111) surface shown on phase diagram can be understood from Cu2O(111) surface reconstruction which induced by the H2 adsorption. Under UHV conditions full desorption of the adsorbed H2 molecules takes place in the temperature range of 150-200 K from phase diagram (Figure 5a) of H2 binding on reconstructed Cu2O(111), while full desorption of the adsorbed H2 molecules takes place in the temperature range of 250-300 K from phase diagram (Figure 5b) of H2 binding on ideal Cu2O(111) surface. Overall, the 2H and 8H terminations on ideal Cu2O(111) surface are stabilized in the pressure ranges accessible in UHV experiments, while only 8H termination on reconstructed Cu2O(111) surface is stabilized in the pressure ranges accessible in UHV experiments. (Figure 4) and (Figure 5)

3.4. Discussion: To derive a full picture of the H2 interaction with reconstructed Cu2O(111) surface, we discuss here the energetic trends as a function of H2 coverage. The total adsorption energies for different coverages are plotted in Figure 4 and also displayed in Table 1. Comparing three adsorption modes, parallelly ACS Paragon Plus Environment

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molecular H2 adsorption on surface un-coordinated CuCUS atom is preferred for an isolated H2 on reconstructed Cu2O(111) surface. It indicates that surface un-coordinated CuCUS is the most advantageous position with the parallelly adsorbed H2 which agrees well with former theoretical result35,36. From above data, one can easily see that molecular H2 parallelly adsorption on surface uncoordinated CuCUS atoms with side-on mode is most preferred for one and two H2 molecules, while totally heterolytic dissociative adsorption is most preferred for three and four H2 molecules on reconstructed Cu2O(111) surface. H2 adsorption on reconstructed Cu2O(111) surface is not favorable thermodynamically for five and six H2 molecules. It is interesting to compare the H2 splitting mechanisms on Cu2O(111) surface at different coverage. For one H2 molecule, homolytic (-0.30 eV) and heterolytic (-0.28 eV) dissociation is competitive on reconstructed Cu2O(111) surface; for two H2 molecules, heterolytic dissociation (-0.84 eV) is much more preferred than homolytic dissociation (0.54 eV Figure S3i); for three and four H2 molecules, heterolytic dissociation (-1.34 eV and -1.98 eV) is most preferred on reconstructed Cu2O(111) surface. For much higher H2 coverage (five or six H2 molecules), the coordinatively saturated OCSA atoms in the third layer (lattice oxygen) will diffuse to surface of Cu2O(111), which is in well agreement with experimental report8: the diffusion of lattice oxygen atom to surface by H adsorption. (Table 2) It is also interesting to compare H2 adsorption mechanisms on different metal oxide surfaces (Table 2). For the most stable H2 adsorption, there is no correlation between the surface stability and adsorption energy. For Fe3O4(110), the more stable A layer (0.947 J/m2)67 has homolytic dissociative adsorption energy of -1.73 eV68, and the less stable B layer (1.020 J/m2)67 has homolytic dissociative adsorption energy of -0.78 eV68. For Fe3O4(111), the most stable Fetet1 termination (0.944 J/m2)67 has homolytic dissociative adsorption energy of -1.62 eV69, and the second stable Feoct2 termination (1.132J/m2)67 has heterolytic dissociative adsorption energy of -1.21 eV69. On the different Fe3O4 directions, the H2 adsorption rule is reverse with CO adsorption70: much more stable terminations with large H2 adsorption energy, while much more stable terminations with small CO adsorption energy. For Fe3O4(001), H2 prefers homolytical dissociation with adsorption energy of -0.88 eV71. For CeO2, the more stable (111) surface has homolytic dissociative adsorption of -2.80 eV72, and less stable (110) surface has homolytic dissociative adsorption energy of -3.57 eV72. One can easily see that usually H2 prefers homolytic ACS Paragon Plus Environment

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dissociative adsorption on reducible transition metal oxide surface, which agrees well with experimental results. In addition, H2 adsorption mechanism on reducible metal oxide surfaces is in fact often controlled by specific properties of metal oxide surfaces, including electronic and structural surface defects. For example, H2 prefers to heterolytical dissociation on Feoct2 termination of Fe3O4(111) 69. On the contrary, H2 prefers to heterolytical dissociation on nonreducible oxides. For MgO(001), H2 prefers to heterolytical dissociation on terrace with adsorption energy of 1.80 eV73, and H2 prefers to heterolytical dissociation on step with adsorption energy of -0.43 eV73. H2 prefers to hereolytical dissociation on γ-Al2O3(110) surface with adsorption energy of -0.97 eV74. ZrO2 is a poor reducible oxide, H2 prefers to heterolytical dissociation on ZrO2(111) with adsorption energy of 0.24 eV75, and H2 prefers to heterolytical dissociation on ZrO2(101) with adsorption energy of -0.08 eV75. On Cu2O(111) surface, molecular H2 adsorption on surface un-coordinated Cuus atom from side-on mode is most preferred for one or two H2 molecules, while totally heterolytically dissociative adsorption on Cu2O(111) surface is most preferred for three and four H2 molecules. Comparing H2 dissociative adsorption on Cu2O(111) surface for one H2 molecule, H2 adsorption on two un-coordinated CuCUS atoms via homolytical dissociation is much more favorable. It indicates that there is unique mechanism for H2 dissociative adsorption on Cu2O(111) surface which is different with former H2 splitting mechanisms. Table 2 also shows the adsorption energies of H2 dissociative adsorption on different metal oxides, but these results are not comparable. 4. Conclusion Using spin-polarized density functional theory calculations (DFT+U) and atomic thermodynamics we systematically study H2 adsorption on the Cu2O(111) surface at different coverage. It has been found that there exists surface reconstruction of Cu/O terminated Cu2O(111) surface. Boltzmann statistics was used to analyze the probability distribution of different reconstructed Cu2O(111) surfaces as function of temperature. It has been found that molecular H2 adsorption on reconstructed Cu2O(111) surface is favorable at low coverage (1/4 and 2/4 ML), while totally dissociative adsorption is preferred at high coverage (3/4 and 1 ML). At higher H2 coverage (5/4 and 6/4 ML), the lattice oxygen atoms will diffuse to surface induced by H adsorption, which is in well agreement with experimental results. Four H2 molecules adsorption on reconstructed Cu2O(111) surface is favorable thermodynamically according to the stepwise adsorption energy. In addition, four totally dissociative adsorbed H2 molecules will get rid ACS Paragon Plus Environment

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of the surface reconstruction. The phase diagram of H2 molecules on ideal and reconstructed Cu2O(111) surfaces were plotted to analyze the different H2 adsorption mechanisms. A deeper understanding of the reaction mechanism of H2 adsorption on Cu2O(111) surfaces has been obtained. More interesting is that four H2 molecules adsorption on four surface un-coordinated CuCUS atoms and four surface un-coordinated OCUS atoms via heterolytical dissociation is saturation adsorption. The adsorption mechanisms of H2 on different metal oxide surfaces are compared. It has been found that there exists an unique mechanism for H2 splitting on Cu2O(111) surface which is different with former mechanisms. The different H2 adsorption mechanisms on different metal oxides may benefit for new H2splitting metal oxides catalyst designing. The carefully designed experiments are also suggested to be conducted to verify the phenomenon.

Supporting Information: The computed less stable reconstructed Cu2O(111) surfaces, less stable configurations of one, two, three, four, five and six H2 adsorption on Cu2O(111) surface. This material is available free of charge via the Internet at http://xxx/xxx

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 21673270) and the National Natural Science Foundation of Henan (No. 162300410001).


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Figure 1. Top and side views of (a) the reconstructed and (b) the ideal Cu2O(111) surface. The energy of the ground state is set as the reference: A more positive value denotes a less stable ideal surface. (first layer O atoms in bigger red ball; second layer coordinatively un-saturated Cu atoms in green ball; other Cu and O atoms in blue and red ball)


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Figure 2. Probability distribution of different reconstructed Cu2O(111) surfaces as function of temperature.


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Figure 3. Configurations of the most preferred (a)-(c) 1H2; (d)-(f) 2H2; (g)-(i) 3H2; (j)-(l) 4H2; (m)-(n) 5H2; (o)-(p) 6H2 adsorption on the Cu2O(111) surface at different coverage (first layer O atoms in bigger red ball; second layer coordinatively un-saturated Cu atoms in green ball; other Cu and O atoms in blue and red ball , adsorbed H atoms in smaller white ball)


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Figure 4. Phase diagram of stable H2 adsorption as a function of (a) number of H2 molecules and (b) H2 chemical potential on reconstructed Cu2O(111) surface; (c) number of H2 molecules and (d) H2 chemical potential on ideal Cu2O(111) surface. (Solid lines show the most stable adsorption configurations at different coverage)


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Figure 5. Phase diagram of stable H2 adsorption on (a) reconstructed and (b) ideal Cu2O(111) surfaces under different conditions.


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Table 1. Total adsorption energy (eV) of H2 on reconstructed Cu2O(111) surface at different coverage. 1 H2

2H2

3H2

4H2

molecular

-0.47

-0.87

-1.25

-1.77

mixed

-0.26

-0.66

-1.21

dissociative

-0.30

-0.84

-1.34

Adsorption

5H2

6H2

-1.73

-0.75

-0.85

-1.98

-0.61

0.67

states Cu2O(111)


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Table 2. Comparison of the largest adsorption energies (eV) for H2 on different surfaces of different metal oxides at low coverage

a

adsorption states

site

PBE

PW91

GGA+U

Fe3O4(110) A-layer

homolytic

O-top

-1.7368

Fe3O4(110) B layer

homolytic

O-top

-0.7868

Fe3O4(001)

homolytic

O-top

-0.8871

Fe3O4(111) Fetet1

homolytic

O-top

-1.6269

Fe3O4(111) Feoct2

heterolytic

Fe, O

-1.2169

CeO2(111)

homolytic

O-top

-0.1872

-2.4076, -2.8072

CeO2(110)

homolytic

O-top

-1.1672

-3.5772

ZrO2(111)

heterolytic

Zr, O

0.2475

ZrO2(101)

heterolytic

Zr, O

-0.0875

MgO(001) terrace

heterolytic

Mg, O

1.8073

MgO(001) step

heterolytic

Mg, O

-0.4373

γ-Al2O3(110)

heterolytic

Al,O

Cu2O(111)

molecular

CuCUS

-0.1977

-0.9774 -0.7336

-0.47a

this work


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