Dissociation Mechanism of Water Molecules on the PuO2 (110

Article Cite This: J. Phys. Chem. C 2018, 122, 371−376

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Dissociation Mechanism of Water Molecules on the PuO2(110) Surface: An Ab Initio Molecular Dynamics Study Cui Zhang,† Yu Yang,†,* and Ping Zhang*,†,‡ †

Institute of Applied Physics and Computational Mathematics, P.O. Box 8009, Beijing 100088, China Center for Applied Physics and Technology, Peking University, Beijing 100871, China

J. Phys. Chem. C 2018.122:371-376. Downloaded from pubs.acs.org by NEW MEXICO STATE UNIV on 01/23/19. For personal use only.

‡

ABSTRACT: Reactions between water and plutonium oxide play a key role in determining the oxidation and corrosion rates of plutonium materials. We perform consecutive ab initio molecular dynamics simulations with the DFT + U approach, systematically studying the dissociation dynamics and mechanism of water molecule and small clusters on the PuO2(110) surface. The dissociation of water on the surface is found to be a two-step hydroxylation process for both monomer and clusters, but different dissociation mechanisms are revealed. Water monomer dissociation, as the consequence of the hybridizations between the molecular orbitals of water and the electronic state of the surface, needs to overcome a reaction energy barrier of 0.18 eV. In contrast, the dissociations of hydrogen-bonded small water clusters are exothermic by −0.42 eV with no transition energy barrier. Hydrogen-bonding interactions between water molecules facilitate the dissociation process and result in a more stable dissociation state with respect to that of a single molecule. Such interaction also induces a hydrogen-transfer reaction between formed surface hydroxyl group and the remaining water molecule.

1. INTRODUCTION Plutonium chemistry is attracting much interest for its important role in military weapon security and storage. When exposed to air and moisture, the plutonium metal surface rapidly oxidizes and corrodes, forming a protective layer of dioxide. At room temperature, the corrosion rate of plutonium by water at its equilibrium vapor pressure is more than 100 times faster than that in dry air. Such moisture enhancement is increased to be about 105 times at boiling point.1 To understand the dynamics and mechanism of such moisture-enhanced oxidation process is an essential scientific issue both in fundamental actinide science and nuclear energy application. Experimental studies of water interactions with PuO2 are very challenging.2−9 In early work of Stakebake,2 two distinct water desorption temperature ranges were found, one between 373 and 423 K and the other between 573 and 623 K, which suggested a strong chemisorbed first water layer and a weak physisorbed second layer. The higher temperature desorption was considered to be due to dissociative first-layer water adsorption with an estimated adsorption energy of −2.94 eV, while the lower temperature was attributed to molecular second-layer adsorption with an estimated adsorption energy of −0.88 eV. Paffet et al.7 confirmed the process and revised the estimations, reporting adsorption energy values of firstlayer dissociative adsorption and second-layer molecular adsorption at 371 K to be −1.82 and −1.11 eV, respectively. Reaction of water with plutonium oxide was also proposed to lead to the formation of higher oxide, a PuO2+x phase.3,5,6,10 However, recent photoemission work excluded the stability of © 2017 American Chemical Society

higher plutonium oxide as a bulk system by showing that PuO2 is only chemisorbed by an oxygen layer, which desorbs at elevated temperature.8 Many problems involving PuO2/H2O interactions remain to be further explored. Plutonium oxides are systems involving strong electronic correlations driven by the 5f electrons.11−13 Standard exchange-correlation approximations in density functional theory (DFT), such as the local density approximation (LDA) or the generalized gradient approximation (GGA), fail to predict correct electronic structure and magnetic order of many actinide oxides because of underestimating the strong on-site Coulomb repulsion of 5f electrons. The DFT + U approach is one of the most popular methods that allow correct treatment of systems containing strongly correlated electrons. By addition of an energy term of the effective Hubbard parameter, the antiferromagnetic Mott insulator feature and structural parameters of PuO 2 are well described.13−22 Several previous studies had focused on the properties of plutonium-oxide surfaces.23−26 Jomard and Bottin investigated the influence of electronic correlations on thermodynamic stability of PuO2 surfaces in ref 24 and also pointed out that the electronic structure of the insulating PuO2 surfaces can be well described within the DFT + U method, which is a prerequisite to explore chemical reactions on the PuO2 surfaces. Sun et al. further reported the effects of thickness and oxygen vacancy on the stability and chemical Received: September 6, 2017 Revised: December 20, 2017 Published: December 20, 2017 371

DOI: 10.1021/acs.jpcc.7b08864 J. Phys. Chem. C 2018, 122, 371−376

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calculated lattice parameter of bulk PuO2 crystal is 5.35 Å, in good agreement with the experimental value of 5.398 Å.6 The atomic positions were allowed to relax until the total energy change was less than 10−4 eV per unit cell and all components of forces were smaller than 0.01 eV/Å. A planewave cutoff of 680 eV was used in all structural optimizations. The Brillouin zone integration was calculated using 5 × 7 × 1 Monkhorst−Pack k-point meshes. The adsorption energy of water molecule is given by

activity of the PuO2 surfaces, where the spin−orbit coupling (SOC) interaction was considered.25 Recently, the stabilities of clean and hydroxylated PuO2 surfaces were compared and discussed in ref 26. The first theoretical study on the interaction between water molecule and the PuO2 surfaces was carried out by Wu and Ray within the DFT framework, where both the strong correlation effect of 5f electrons in plutonium and the van der Waals interactions between the molecule and surfaces were not considered.27 More recently, Jomard et al. applied the DFT + U approach to study the molecular and dissociative adsorptions of water molecules on the PuO2(110) surface.28 Adsorption energies were found to be −0.87 and −1.12 eV for the molecular and dissociative configurations, respectively, with a small energy barrier of 0.05 eV between two states. In ref 26, a dramatic effect of the dissociated water on the relative stability of the PuO2 surfaces was reported. Tegner et al. presented results of water molecular and dissociative adsorptions at monolayer coverage on the PuO2 for all three surfaces.29 They found a mixed molecular and dissociative water adsorption to be most stable on the (111) surface, whereas the fully dissociative configuration is most stable on the (110) and (100) surfaces. Wellington et al. reported a study of water adsorption on the PuO2(110) and (111) surfaces, using hybrid DFT within the periodic electrostatic embedded cluster method.30 Similar energies were found for water molecular and dissociative adsorption on the (111) surface, while a clear preference for dissociative adsorption was shown on the (110) surface. Most previous theoretical studies on the water interactions with the PuO2 surfaces have focused on the adsorption geometries and energies. The dynamics and mechanism of water dissociation on the PuO2 surfaces, which is a key factor in the corrosion and oxidation reactions on the plutonium surfaces, still remain unresolved. In this work, we perform ab initio molecular dynamic (AIMD) simulations, together with DFT-based electronic structure calculations, to investigate the dynamics and reactivity of water molecule and small clusters on the PuO2(110) surface. The rest of this paper is organized as follows. Our methodological approach is presented in section 2, and our results are discussed in section 3. Section 4 contains our main conclusions.

Ead = E(slab + mol) − Eslab − Emol

(1)

where Eslab+mol and Eslab are the energies of the slab with the adsorbed molecule and the clean slab, respectively; Emol is the total energy of the water molecule in the gas phase, calculated in a cubic cell with a dimension of 20 Å. Spin−orbit coupling was considered in the water adsorption energy calculations. All AIMD simulations were carried out in the NVT ensemble within a Born−Oppenheimer framework at 300 K. A plane-wave cutoff of 400 eV and a time step of 1 fs were used. Only Γ point was adopted to sample k space.

3. RESULTS AND DISCUSSIONS We first investigate two types of water adsorption, molecular and dissociative, on the PuO2(110) surface. Optimized structures of two types of adsorption were shown in Figure 1. Molecular adsorption of a single water molecule occurs with

Figure 1. Energy landscape of water molecule dissociation on the PuO2(110) surface: (a) optimized molecular adsorption structure; (b) transition structure of the dissociation extracted from the MD simulation trajectory; and (c) optimized dissociative adsorption structure.

2. COMPUTATIONAL METHODOLOGY All DFT calculations were performed using Vienna ab initio simulation package (VASP),31 a plane-wave DFT code using projector augmented wave (PAW)32,33 potential to describe the electron−ion interactions. The PuO2(110) surface model used in this work consists of six atomic monolayers, and a periodic (2 × 1) supercell with a vacuum spacing of 15 Å along surface normal direction (z) was adopted. The gradientcorrected functional PBE,34,35 with a Hubbard U correction for the 5f electrons of plutonium, was used for the exchange correlation interactions. The Coulomb energy U and exchange energy J were selected with the following values: U = 4.75 eV and J = 0.75 eV, which were tested and used in previous work.25 We adopted optPBE exchange functional for the van der Waals correlation to describe the dispersion forces within the adsorption system.36−39 optPBE is an optimized PBE-style functional, yielding a low mean absolute deviation from the CCSD(T) results on the S22 set (a set of 22 weakly interacting dimers) and precise descriptions for the water hexamers.38,39 Spin polarization was included in all calculations. The

the hydrogen atoms tilting toward the surface and the oxygen atom lying in the position above surface oxygen empty sites, as demonstrated in Figure 1a. The Pu···Ow (Ow denotes the oxygen atom in the adsorbed water molecule) distance for water molecularly adsorbing on the (110) surface is 2.61 Å. For dissociative adsorption, the adsorbed hydroxide (OwH) locates right above a surface Pu atom with its hydrogen pointing against the surface, while the other hydrogen from water is bonded with a surface oxygen atom and angled toward another surface oxygen, shown in Figure 1c. A Pu−Ow bond distance of 2.14 Å is yielded in water dissociative adsorption on the PuO2(110) surface. The adsorption energies calculated based on eq 1 are −0.89 and −1.17 eV for the molecular and dissociative geometries, respectively. It indicates that, on the PuO2(110) surface, water dissociative adsorption is more energetically favorable than the molecular one. Such preference found in our work is consistent with previous studies.26−30 In ref 29 longer plutonium−oxygen distances, 2.72 and 2.44 Å, 372

DOI: 10.1021/acs.jpcc.7b08864 J. Phys. Chem. C 2018, 122, 371−376

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The Journal of Physical Chemistry C and lower adsorption energy, 0.88 and −1.14 eV, were reported for molecular and dissociative adsorption states, respectively. The small geometry and energy difference between our results and those reported by Tegner et al. can be attributed to the inclusion of dispersion corrections in our work.29 To gain insight into the dynamics of water dissociation on the PuO2(110) surface, we performed a series of AIMD simulations of 1−3 water molecules on the PuO2(110) surface at ambient conditions. In the initial configuration of MD simulations, water molecules are randomly settled 4 Å above the surface. For the case of water monomer, as the molecule diffuses toward the surface, the water dissociation occurs by ejecting a hydrogen atom and forming surface hydroxyl group with an oxygen atom in the topmost surface layer. After that, the remaining hydroxyl radical (OwH) forms bond with surface plutonium within tens of femtoseconds. A two-step dissociation of water molecule on the PuO2(110) surface is observed. We extract a transition structure of water monomer dissociation on the PuO2(110) surface from the MD simulation trajectory, where one of the water hydrogen atoms breaks away and bonds to a surface oxygen, producing a hydroxyl group on the surface, as shown in Figure 1b. We compute the normal modes of the dissociative water molecule in Figure 1b, using density functional perturbation theory, and only one imaginary frequency is found. At the transition state, an energy barrier of 0.18 eV is obtained with respect to the molecular adsorption state. The second step of the dissociation, where two surface hydroxyl groups are formed, is an exothermic process with an energy release of 0.46 eV. The projected density of states (PDOS) are then calculated to study the electronic interactions between water molecule and the PuO2(110) surface in the dissociation path. The PDOS of two hydrogen and one oxygen atoms within water molecule are summed together to represent the molecular orbitals. To compare the molecular orbital energy levels of water in the molecular adsorption state and the isolated state, we place another water molecule in the molecular adsorption configuration of water on the PuO2(110) surface (see Figure 1a), 15 Å away from the PuO2 surface, and increase the vacuum spacing to 30 Å along the surface normal direction. As shown in Figure 2a, three highest occupied molecular orbitals of the isolated water molecule, 1b1, 3a1, and 1b2, declare themselves as three localized peaks at −10.09, −6.27, and −4.22 eV, respectively, where the Fermi level is shifted to zero. In the molecular adsorption state, the highest 1b2 orbital of the molecule is greatly broadened, as presented in Figure 2b. The surface O 2p, Pu 5f, and Pu 6d states take part in the hybridizations with the 1b2 molecular orbital. Through Bader topological analysis,40,41 we find 0.03 electron is donated from the surface to the water molecule during the electronic hybridization process. As the dissociation of water molecule proceeds, the electronic states of one surface oxygen atom increasingly interact with those of water molecule, as shown in Figure 2c,d, which indicates the formation of a new OH group between the surface oxygen atom and the dissociating hydrogen from water. In short, the dissociation of water monomer on the PuO2(110) surface is the result of the hybridizations between three frontier orbitals of water molecule and the electronic states of the PuO2 surface. Such dissociation needs to conquer an energy barrier of 0.18 eV. In the case of water dimer, two water molecules are clustered by a hydrogen bond (HB), as shown in Figure 3a. When water

Figure 2. Projected density of states (PDOS) for the (a) isolated water molecule, (b) molecular adsorption state, (c) transition state, and (d) dissociative adsorption state of water molecule on the PuO2(110) surface, respectively. Fermi level is shifted to zero. Black line represents the total PDOS of water molecule. Gray, blue, and red lines represent the PDOS of surface O 2p, Pu 6d, and Pu 5f orbitals, respectively.

Figure 3. Snapshots of the dissociation process of water dimer (a−c) and trimer (d−f) on the PuO2(110) surface. The dashed lines indicate hydrogen bonds (HBs) between water molecules.

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The Journal of Physical Chemistry C dimers are adsorbed adjacent to the surface, the dissociation is initiated from the hydrogen atom in the HB acceptor of the dimer. The dissociating hydrogen forms an hydroxyl group with surface oxygen atom and the process is followed by the bonding between the remaining hydroxide from water and a surface plutonium atom. It presents a similar two-step dissociation to that of water monomer on the PuO2(110) surface. Furthermore, we compare the energy of the initial, transition, and final configurations of the dimer dissociation obtained from the MD trajectory, as shown in the upper panel of Figure 3. The reaction is found to be exothermic by −0.42 eV with no energy barrier. It suggests a different type of dissociation mechanism of water dimer from that of water monomer, where an activation energy of 0.18 eV is required. Hydrogen-bonding interaction between molecules facilitates the dimer dissociation on the PuO2(110) surface, yielding a more energetically favorable process. Our simulation of three water molecules on the PuO2(110) surface displays similar dissociation phenomena to that of water dimer. Hydrogen-bonding interaction of water glues molecules in a cluster manner. Typical steps in the dissociation process is exhibited in the lower panel of Figure 3. To further demonstrate the effect of HB in water dissociation on the PuO2(110) surface, we calculate the electron density difference of the initiation configuration of water trimer dissociation on the surface, as displayed in Figure 4. We define the electron

Figure 5. Hydrogen-transfer process between water molecules on the PuO2(110) surface.

dangling hydroxyl group. Because of the hydrogen-bonding interaction, the hydroxyl group first separates from the surface plutonium to produce a free radical, and sequentially a hydrogen transfer arises from water molecule to the hydroxyl radical. The dehydrogenated water molecule turns to be a new hydroxide and bonds with adjacent surface plutonium atom, producing a new surface hydroxyl group. Such hydrogentransfer process consequently induces a hydroxyl group transfer on the PuO2(110) surface.

Figure 4. (a) Configuration of the initiation step of water trimer dissociation on the PuO2(110) surface. (b) Electron density difference contour of the configuration shown in (a), as defined in the text.

4. CONCLUSIONS We have performed a series of AIMD simulations of water molecule and small clusters on the PuO2(110) surface to investigate the dynamics and mechanism of water dissociation. Our results reveal that water dissociation on the PuO2(110) surface is a two-step hydroxylation process both for single molecule and small clusters. The reaction is initiated by the dehydrogenation of water molecule to form hydroxyl group with surface oxygen atom, followed by successive surface hydroxylation on plutonium with the remaining hydroxide ion of the dissociating molecule. Despite the same dissociation steps, the mechanism of the reaction are different for water molecules and clusters. Water monomer dissociation, involving the hybridizations between the molecular orbitals of water and the electronic state of the top surface, requires an activation energy of 0.18 eV. In contrast, the dissociation of water dimer on the surface is found to be an exothermic reaction with no energy barrier. The process starts within the HB acceptor molecule in the cluster and results in a more stable dissociation state with respect to that of a single molecule on the surface. Such facilitation in the dissociation of water clusters can be attributed to the hydrogen-bonding interactions between water molecules. Hydrogen-bonding interaction can also induce a hydrogen-transfer reaction between a surface hydroxyl group and remaining water molecule, resulting in a hydroxyl group transfer along the surface.

HBD density difference as Δρ = ρsurf+3H2O − ρsurf+H2O − ρHBA H2O − ρH2O , where ρsurf+3H2O is the total electron density of water trimer adsorbed on the PuO2(110) surface, ρsurf+H2O is the electron density of the PuO2(110) surface with the water molecule that is not directly hydrogen-bonded to the dissociating molecule, HBD and ρHBA H2O and ρH2O are the electron densities of the HB acceptor and donor molecules, respectively, highlighted in the blue square in Figure 4a. Electron density difference contour in Figure 4b shows a clear electron redistribution around the oxygen atom of the HB acceptor molecule. We also perform Bader atomic charge analysis and find no charge transfer between water molecules or between molecules and the PuO2 surface. It indicates that the electron redistribution occurs within the HB acceptor molecule and it may lead to a weakening of intramolecular OH bond and the initiation of the barrierless dissociation of the water cluster. Interestingly, after the partial dissociation of the HB acceptor in water trimer, the HB donor molecule remains hydrogen-bonded with the dangling hydroxyl group on the plutonium atom. As shown in Figure 5, a hydrogen-transfer reaction takes place between HB donor molecule and surface-

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected]. ORCID

Yu Yang: 0000-0002-1603-3603 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Science Challenge Project under Grant No. TZ2016004, by the Natural Science Foundation of China under Grants No. 21503019 and No. 11625415, by the President Fund of China Academy of Engineering Physics under Grant No. 201402034, and by NSFC-NSAF under Grants No. U1530258 and No. U1630248.

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