- Email: [email protected]
Molecular modelling of the decomposition of NH3 over CoO(100)
Materials Chemistry and Physics xxx (2015) 1e9
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Molecular modelling of the decomposition of NH3 over CoO(100) Kambiz Shojaee, Brian S. Haynes, Alejandro Montoya* School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney NSW 2006, Australia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Minimum reactions pathways of ammonia decomposition were studied using density functional theory. The bonding characteristics of NHx and H on the CoO(100) surface were analysed using Layer-projected density of states. Dehydrogenations of NH3, NH2 and NH are highly activated. The presence of strongly bound lattice oxygen favours the ammonia decomposition towards N2.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 September 2014 Received in revised form 29 January 2015 Accepted 21 February 2015 Available online xxx
Spin-polarised density functional theory using the PBE þ U approach are used to determine reaction pathways of successive NH3 dehydrogenation on the CoO(100) surface. NH3 dehydrogenation promotes noticeable displacements of the surface CoO sites, in particular due to the binding of NH2 and H species. Surface lattice O has low activity towards dehydrogenation, reflected in energy barriers that are in the range of 292 kJ mol1 to 328 kJ mol1. There is a preference of surface NH3 dehydrogenation to N2 rather than towards NO, due to a high-energy penalty of surface O vacancy formation. The presence of CoO in cobalt oxide catalysts not only may decline the ammonia conversion but also alter the selectivity towards N2 rather than NO. © 2015 Elsevier B.V. All rights reserved.
Keywords: Oxides Computer modelling and simulation Electronic structure Surface properties
1. Introduction Cobalt oxides can be synthesised with a variety of morphologies and distinct compositions to obtain specific catalytic and electronic properties for a variety of practical applications. The low-valence CoO and Co3O4 are the most stable forms of cobalt oxides while high valence Co2O3 and CoO2 oxides are thermally less stable [1,2]. CoO crystallises in the rock salt structure with Co2þ sites occupying octahedral positions, while Co3O4 is a normal spinel with cubic crystal structure where Co3þ sites occupy octahedral positions and
* Corresponding author. E-mail address: [email protected] (A. Montoya).
Co2þ sites occupy tetrahedral positions. CoO is a promising oxide for functional materials owing to its magnetic and gas-sensing properties [3], while Co3O4 has diverse applications in heterogeneous catalysts, lithium-ion batteries, solid state gas sensors, supercapacitor devices and solar energy absorbers [4e6]. Cobalt oxides are known to have excellent catalytic activity towards total oxidation of NH3 [7e9] with the potential to reduce catalyst cost by replacing current industrial Pt/Rh catalyst. Among these oxides, Co3O4 has been reported to be the most active oxide catalyst with an NH3 oxidation selectivity toward NO of over 90% [10] and a decrease in the emissions of the undesirable greenhouse gas N2O in NH3 oxidation chemical plants [11,12].
http://dx.doi.org/10.1016/j.matchemphys.2015.02.040 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
2
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
Co3O4 is not suitable for high-temperature operations because it loses oxygen above 750 C under oxygen atmospheric pressure, forming less active CoO [13]. Experiments on the oxidation of NH3 oxidation on unsupported Co3O4 pellets between 450 and 800 C show a decrease of approximately 15% in the NH3 conversion when the reaction temperature is increased from 640 to 800 C, which has been ascribed to the formation of small CoO crystallites [14]. Limited studies are available that demonstrate the reduced activity of CoO for NH3 oxidation under industrial oxidation conditions [14]. Evidence at molecular level of the low activity of CoO comes from FTIR analysis of Co3O4 samples reduced in H2 at 200 Torr and 523 K, which were exposed to NH3 at room temperature. The samples showed bands associated with ammonia coordination on Lewis (Co) sites and the absence of bands associated with NH3 dehydrogenation products like NH2 and NH [15], indicating no decomposition over CoO. The high activity of Co3O4 towards NH3 oxidation has been ascribed to the participation of weak lattice surface oxygen sites in the hydrogen abstraction and NO formation leaving the surface with vacancy sites [16] and possible rapid O2 adsorption to regenerate the O sites and its surface activity [16]. Lattice O in CoO surfaces is more coordinated than in Co3O4 surfaces and therefore, in theory, less active for dehydrogenation than O in Co3O4. However, molecular modelling and substitutional cobalt experimental studies of CO oxidation to CO2 [17,18] and molecular modelling of N2O decomposition to N2 [19] over Co3O4 ascribe some catalytic role during the adsorptionedesorption process to Co in octahedral sites. The Co sites in CoO tend to have lower diffusion energy barriers than O sites [20], and reconstruction of the surface during NH3 dehydrogenation could be expected, leading to the possibility of dynamic participation of Co in NH3 dehydrogenation that needs to be assessed. Despite the technical importance of NH3 oxidation on cobalt oxides, there is no molecular level understanding of the activity of lattice oxygen and cobalt sites in CoO in hydrogen abstraction processes. The site selectivity and possible dehydrogenation pathways have not been considered previously. Knowledge of the NH3 reaction mechanisms on CoO can provide insights into the behaviour of Co3O4 catalyst at high temperature, reducing operating conditions while also enhancing understanding of the activity of CoO in dehydrogenation reactions in general. In this contribution, we study the successive dehydrogenation of ammonia on the CoO(100), which is the predominant exposed surface of CoO [21]. Only limited first-principle studies have been directed towards the atomic description of bulk CoO [22e24], and even fewer studies are devoted to the electronic characteristics of the CoO(100) surface [25,26]. Therefore, we provide additional structural, magnetic and electronic properties of the CoO(100) surface as well as its activity towards ammonia dehydrogenation by determining density of states, binding energies and energy barriers. 2. Computational methodology Spin-polarised periodic DFT calculations were performed with the projector-augmented wave (PAW) method as implemented in the Vienna Ab initio Simulation Package (VASP) [27] using the GGA approximation in the form of the PerdeweBurkeeErnzerhof (PBE) exchange-correlation [28]. On-site Coulomb correction to the 3d electrons of Co atoms was considered using the rotationally invariant formulation proposed by Dudarev et al. [29] with a value of Ueff ¼ (U J) ¼ 3.3 eV, which was obtained by fitting the reaction energy of CoO oxidation to Co3O4 [30]. We have shown in our previous work [16] that the PBE þ U approach with a Ueff ¼ 3.3 eV provides geometry configurations and binding energies of NHx species on Co3O4(100) surfaces in good comparison with the hybrid
functional HSE06 methods. Recently, a value of Ueff ¼ 3.7 eV for bulk CoO was determined using a self-consistent procedure based on the linear response approach [25]. However, we have used a value of 3.3 eV in this study in order to minimise uncertainties when we compare binding energies and minimum energy dehydrogenation pathways of NHx species with our previous PBE þ U study of ammonia oxidation on Co3O4. We will show that a value of Ueff ¼ 3.3 eV is able to predict experimental electronic structure properties of CoO. The convergence criteria for the electronic self-consistent iteration and the forces acting on unconstrained atoms were set to 104 eV and 0.02 eV/Å, respectively. A (2 2 2) rhombohedral unit cell of bulk CoO was modelled with a k-point mesh of (6 6 6) at a cutoff energy of 550 eV, using the tetrahedron € chl corrections [31]. The lattice constant was obmethod with Blo tained using the BircheMurnaghan equation of state [32]. Slab cells, containing five atomic layers exposing the CoO(100) surface, were built from the DFT-relaxed bulk CoO. We found that five layers were adequate for our slab model because the surface free energy was found to converge within 0.02 J m2 for slabs with more than four CoO atomic layers. The adsorption and reactions were studied on one side of the slab, and a vacuum space of 15 Å was used in the direction perpendicular to the surface in order to avoid surfaceesurface interactions. Dipole correction along the axis normal to the surface was applied in order to cancel any spurious effect of dipole moments associated with asymmetric slabs. Atomic relaxations were carried out for the adsorbates and the atoms in the first three top-most layers while atoms in the two bottom-most layers were kept fixed to their bulk DFT-positions. A (2 3 1) Monkhorst-Pack k-point mesh [33] was used for the Brillouin zone integrations at a cutoff energy of 450 eV using the MethfesselePaxton method [34] with a smearing of 0.1 eV. The plane-wave cutoff energy and k-point meshes were found to be adequate for the purpose of this study. Test calculations showed that increasing the cutoff to 550 eV or using a (3 4 1) k-point mesh had a slight (less than 3 kJ mol1) effect on the binding energy of NH3. The Climbing Image Nudged Elastic Band (CI-NEB) method [35] was used to obtain reaction energy barriers, and transition states coordinates. The tolerance for electronic self-consistent iteration was set to 104 eV, and each image was relaxed until the forces were less than 0.05 eV/Å. Vibrational frequencies were calculated using a finite difference method at the G point with an electronic convergence of 107 eV. Each atom of the adsorbate was displaced by 0.02 Å from its equilibrium position while the surface atoms were fixed. Transition states were recognised by having a single imaginary frequency in the direction of the reaction coordinate. Energy barriers of surface reactions that involve hydrogen migration can slightly decrease due to quantum tunnelling effects that increase the reaction rate, especially at low temperatures. Such effects in the NH3, NH2 and NH dehydrogenations are not considered in this study as they are unlikely to have a significant impact at the high temperatures at which cobalt oxide catalysts are used in the industrial combustion of ammonia. 3. Results 3.1. Bulk and CoO(100) surface properties The PBE þ U method with Ueff ¼ 3.3 eV is able to reproduce experimental structural and electronic characteristics of bulk CoO and CoO(100). The optimised lattice constant of bulk CoO is calculated to be 4.28 Å, which is only 0.02 Å larger than the reported experimental value [36] and 0.01 Å larger than a previous DFT prediction using the PW91 þ U (Ueff ¼ 6.1 eV) approach [22]. Fig. 1 shows the magnetic ordering of Co atoms and band structure
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
3
Fig. 1. Magnetic ordering and electronic band structures of bulk CoO and CoO(100). Co atoms (blue atoms) with different spins are distinguished by arrows pointing up and down directions. The {111} planes formed by Co atoms in the bulk CoO structure are highlighted with orange sheets. The high-symmetry points are labelled according to the FCC Brillouin zone. The energy of the highest occupied state (Fermi energy) is set to 0. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
of bulk CoO (Fig. 1a and b) and CoO(100) slab (Fig. 1c and d). We found that the most stable bulk CoO has a type-II antiferromagnetic (AF-II) state consisting of all Co atoms in a {111} plane with magnetic moments aligned in the same direction, and all adjacent Co atoms in a {111} plane with magnetic moments aligned in the opposite direction, as shown in Fig. 1a. The higher stability of the AF-II state relative to other magnetic orderings is consistent with experimental reports using neutron and synchrotron X-ray diffraction [37] as well as previous PBE þ U calculations (with Ueff varying between 0 and 10 eV) [24]. The magnetic moments of Co atoms are found to 2.64 mB, 0.16 mB lower than the reported experimental values [38]. Electronic band structures of the bulk CoO at various high-symmetry points is depicted in Fig. 1b. As can be seen, the minimum gap between the valence band maximum and the conduction band minimum occurs at the G point and amounts to 2.35 eV, consistent with a value of 2.6 eV obtained via oxygen X-ray absorption spectroscopy measurements [39]. A p(3 2) super-cell, exposing the CoO(100) surface, was constructed from the DFT-relaxed bulk CoO. The top three layers in one side of the slab were allowed to relax while the bottom two layers were frozen. The first and second relaxed interlayer distances increased only by ~0.04 Å (~2%) from the 2.14 Å bulk layer spacing. The minor surface relaxations are consistent with experimental observation using LEED measurements on a UHV cleaved CoO(100) surface in which changes in interlayers distancing as a result of surface relaxations were reported to be within ±3% compared to
the bulk CoO [40]. We observed a break in the surface symmetry in the spin-polarised PBE þ U calculations, resulting in the CoeO bond lengths in the top-most layer varying between 2.12 Å and 2.17 Å. The magnetic ordering of Co atoms in the CoO(100) slab is depicted in Fig. 1c. The surface is antiferromagnetic with the Co atoms retaining the same magnetic ordering as that observed in the bulk. The magnetic moment of exposed Co atoms increases only by ~0.03 mB relative to those in bulk CoO, and in good agreement with a value of 2.64 mB obtained from a LSDA þ U study with U ¼ 6.2 eV and J ¼ 0.95 eV using the rotationally invariant approach introduced by Liechtenstein et al. [26]. The surface free energy (g) of the relaxed five-layer CoO(100) surface is calculated to be 0.84 J m2, consistent with a previously reported PBE þ U (Ueff ¼ 3.7 eV) value of 0.8 J m2 [25]. Relaxation of the slab from the bulk termination reduces the CoO(100) surface energy only by 0.04 J m2, as a result of minor surface relaxations. The low value of g indicates that the cleavage of the bulk CoO to create the (100) surface is not a highly energetic process, which we attribute to a low number of dangling surface bonds (one CoeO bond per surface atom) and non-polarity of the surface due to a charge compensation of Co with O atoms at each layer. The electronic band structures of the CoO(100) surface at various high-symmetry points is depicted in Fig. 1d. The band gap of CoO(100) slab decreases by 0.75 eV compared to that in the bulk, indicating a decrease in the insulating nature of the bulk due to the shift of the surface states closer to the Fermi level.
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
4
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
Fig. 2. Top and side views of relaxed configurations of NHx* and H* species on CoO(100). Only adsorbates and the top-most layer are coloured for clarity and atoms in bottom layers are shown in grey. The blue, red, green and white atoms refer to cobalt, oxygen, nitrogen and hydrogen, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2. Binding characteristics of adsorbed species Fig. 2 shows top and side views of the relaxed geometries of NH3, NH2, NH, N and H species bound to their most stable sites above the top-most layer of CoO(100). The adsorbates and the top-most layer only are shown since only minor atomic relaxations were found in the second layer. The insets in Fig. 2 provide selected geometrical parameters and the corresponding binding energies with no corrections coming from the vibrational modes at 0 K. All the NHx species bind preferentially N-down to the CoO(100) surface sites. NH3 binds atop Co sites with a slightly tilted configuration towards lattice surface oxygen (Fig. 2a). Binding of NH3 barely induces surface relaxation, due to the weak interaction between the NH3 and the surface. The role of lattice oxygen becomes more apparent in binding of the dehydrogenated fragments and H species. Surface hollow sites are the preferred binding positions for NH2, NH and N. NH2 binds in the centre of a four-fold hollow site closer to Co sites, with H atoms pointing towards the lattice oxygen (Fig. 2b), inducing a significant intra and interlayer relaxation of the hollow sites in the top-most layer. The intra-layer relaxation is characterised by 8% contraction of the CoeCo distance and 9% increase in the OeO distance of the hollow site. The interlayer relaxation is characterised by an outward displacement of the Co atoms by 14% and an inward relaxation of the O sites by 8%, inducing a local surface rumpling which is evident in the side view of the adsorbed configuration in Fig. 2b.
NH binds slightly further from the centre of the four-fold hollow site towards the three-fold coordinated CoeOeCo hollow position. No significant rumpling of the surface is noted, although there is a lateral relaxation evidenced by an increase in the CoeCo and OeO distances by 6% and 16%, respectively. Atomic N binds 0.73 Å above a three-fold coordinated hollow site with an NeO distance of 1.38 Å, which is only 0.21 Å longer than the PBE-optimised length of NO in the gas phase (calculated in this study), thus forming a surface NO-like group. The binding of N* causes the CoeCo and OeO bond distances of the hollow site to increase by 4% and 18% respectively with no evidence of interlayer relaxation. Atomic H binds atop O and Co sites with a difference in binding energy of 51 kJ mol1, in favour of atop O sites. Binding of H above O causes the lattice O site to be significantly pulled away from the top layer by 0.74 Å, forming a hydroxyl group (OH) above the surface (Fig. 2e). The surface sites adjacent to the surface OH group, in particular the Co sites, relax only slightly. The distance between O of the OH group to the surface Co sites increases by almost 6% relative to the bare surface. The binding energy of NH3 is 59 kJ mol1, which is low enough that an adsorptionedesorption equilibrium can be expected to establish rapidly under atmospheric conditions. The dehydrogenated NH3 species are strongly bound to the surface, with binding energies of 140, 215 and 265 kJ mol1 for NH2, NH and N, respectively. The increase in binding energy as the NH3 is being dehydrogenated has been reported on various transition metals
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
5
Fig. 3. Layer-projected density of state (LDOS) of the top-most layer of CoO(100) before and after adsorption and partial density of state (PDOS) of the adsorbed NHx, N and H species. Black solid line, red dashed line, and blue thick solid line refer to surface LDOS, N 2p and of H 1s, respectively. Energies are relative to the energy of the highest occupied state (Ef). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[41,42] as well as on metal oxides [43,44]. The binding energy of H atop lattice O is 160 kJ mol1, which is 233 kJ mol1 lower than the binding energy of H atop lattice O in the Co3O4(100) surface [16]. The lower binding energy of H on CoO compared to Co3O4 is attributed to a significant vertical relaxation of lattice O upon binding of H in the CoO(100) surface. In order to analyse the bonding characteristics of the species on the CoO(100) surface, the Densities of States of the bare slab and the slab with the adsorbed species were obtained. With the intention of concentrating on electronic changes at the surface, Fig. 3 shows the Layer-projected density of states (LDOS) composed of the Co 3d and O 2p states of the top-most layer and the partial density of state (PDOS) of the adsorbates consisting of the N 2p and/or H 1s states. The LDOS of the bare slab is presented in Fig. 3a. The valence band consists of Co 3d and O 2p starting from 6.9 eV with two distinguishable peaks at 5.3 and 0.8 eV. The LDOS is barely perturbed upon binding of NH3 (Fig. 3b), consistent with a weak interaction of NH3 with the surface. Binding of NH2, NH and N to the surface considerably changes the LDOS with the corresponding valence bandwidths becoming wider compared to those of bare surface. The N 2p states in the PDOS profiles become more populated near the Fermi level as the stability of NHx* increases. In the case of NH2*, there are overlapping peaks associated with N 2p and Co 3d between 6.7 and 0.8 eV due to strong interaction of N with the Co sites, and an overlap between 6.6 and 6.8 eV involving N 2p and H 1s (Fig. 3c). The PDOS and LDOS profile of NH* (Fig. 3d) show binding peaks between 6.1 and 0.4 eV associated with the interaction of N 2p with both O 2p and Co 3d states of the LDOS, and a binding state at 8 eV originating from the interaction of N 2p with the H 1s of NH* and interaction of N* with the O 2p state of the LDOS (not
clearly visible in Fig. 3d because it coincides with the population of H 1s states), indicating the formation of a nascent surface ONH group. The LDOS and PDOS of N* are shown in Fig. 3e. There are overlapping peaks associated to N 2p states and the LDOS between 7.5 eV and 0.3 eV. The presence of a split band near the Fermi level is due to the partial reduction of Co atoms in the vicinity of N as a result of the NeO formation and weakening of the CoeO bond. The reduction of the Co sites due to the weakening of the CoeO bond can also be observed in the LDOS profile of H adsorption. Fig. 3f shows the interaction between O 2p and H 1s states at 8.3 eV, and the appearance of surface states near the Femi level due to a reduction of the Co sites in the vicinity of the lattice OH. The partial reduction of Co ions is consistent with a decrease of approximately 0.5 mB in the magnetic moments of the Co atoms. 3.3. Dehydrogenation pathways of NHx species on the CoO(100) surface The ammonia dehydrogenation pathways on the CoO(100) surface were studied starting from the most stable binding configurations. The reaction pathway was obtained with a CI-NEB approach with the final states selected to be the most stable coadsorbed configurations. Top and side views of the dehydrogenation pathways including the initial, transition and final states of NH3, NH2, and NH are shown in Fig. 4 and the energy profiles from the initial to the co-adsorbed states and then to the products at infinite separation are displayed in Fig. 5. Dehydrogenation is in all cases assisted by lattice oxygen, with the NeH elongation taking place along the CoeO bridge positions forming OH species in the final states. The configurations of the
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
6
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
Fig. 4. Top and side views of the initial, transition and final states of NHx* dehydrogenation reactions on CoO(100) obtained with the CI-NEB approach.
transitions states show that the dissociating NeH bonds are stretched by 1.75 Å (171%), 1.61 Å (158%) and 1.86 Å (179%) compared to the NeH bonds in the corresponding initial states for dehydrogenation of NH3, NH2 and NH respectively, indicating that the NeH dissociation proceeds via late transition states. The lateral and vertical displacements of surface atoms are more evident for Co sites than O sites, consistent with a lower energy barrier for Co diffusion compared to the O diffusion in bulk CoO [20]. Co sites mostly undergo outwards relaxations whereas O atoms mainly relax inwards upon interaction with NH2 and NH. Consequently, local surface rumpling occurs, which is more evident for the dehydrogenation of NH2 to NH. The dehydrogenation energy barriers are relatively constant, with values varying from 292 to 328 kJ mol1. The dehydrogenation reactions of NH3, NH2 and NH leading to the co-adsorbed product states are all endothermic with reaction energies of 86 kJ mol1, 156 kJ mol1 and 106 kJ mol1, respectively. There are attractive interactions in the co-adsorbed states, and, therefore, the energy of reaction with respect to infinitely separated products increases to 221, 173 and 108 kJ mol1 respectively.
3.4. Products formation The NH3 dehydrogenation results in N bound to lattice O sites. The selectivity of NH3 oxidation to NO depends on the relative rate between NO desorption and NeN recombination to N2. The formation of NO in the gas phase leaving an O vacant site is 256 kJ mol1 endothermic. Although the NO formation energy is lower than any of the dehydrogenation processes, the interaction of two adjacent N groups producing N2 is a less energetically demanding pathway compared to NO desorption. Fig. 6a shows the initial, transition and final states of N2 formation obtained with the CI-NEB approach. The initial state consists of two adsorbed N species, each bound to adjacent three-fold coordinated hollow sites. It is noted that binding of two N* causes lateral displacements of surface O sites, and slight vertical displacements of surface Co sites. The surface Co and O sites recover the bare slab positions as the recombination of N* generates a weakly bound N2 species. The interaction of two adjacent adsorbed N species is 42 kJ mol1 attractive, the energy barrier is 157 kJ mol1, and the reaction energy is 500 kJ mol1 exothermic relative to two noninteracting N species at the surface. The energy barrier of N2
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
7
4. Discussion
Fig. 5. The energy profiles of successive ammonia dehydrogenation reactions on CoO(100). Black, red and blue profiles refer to dehydrogenation reactions of NH3*, NH2* and NH*, respectively. Energies are relative with respect to the initial state. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
formation is 99 kJ mol1 lower than the NO formation energy, indicating that formation of N2 is energetically preferred compared to that of NO formation. Fig. 6b and c shows the initial, transition and final states of H2O and H2 formation obtained with the CI-NEB approach. The initial state was obtained by relaxing H species atop adjacent O sites. Surface hydrogenation causes vertical displacements of lattice O sites by 1.09 Å away from the surface, and a lateral displacement of O from the centre of a four-fold Co hollow site to above a CoeCo bridge site. The axes of the OH groups are tilted towards the surface forming surface OH rows maximising the OH/OH interactions. The structural reorganisation is limited to the oxygen sites participating in the hydrogenation since the other Co and O sites remain in their lattice positions. Desorption of H2O from the co-adsorbed OH state is shown in Fig. 6b. A direct hydrogen diffusion from one OH group to the other generates a transition state containing H2O that is 3.35 Å above the surface, and the O site at which the H migration originated returns to the bare slab lattice position. The formation of H2O is 62 kJ mol1 activated and 55 kJ mol1 endothermic, with respect to infinite separation of the initial co-adsorbed state. The reverse process, that is, the hydration of the surface containing an O vacant site is only 7 kJ mol1 activated. It indicates that H2O in the gas phase is unstable with respect to the dehydrogenated surface, consistent with an observation of OH peaks in ultraviolet photoemission spectra after H2O exposure of high energy sputtered CoO [45]. The formation of H2 from a co-adsorbed OH/OH state is presented in Fig. 6c. Formation of H2 involves the rotation of one OH group before the HeH recombination takes place. The rotation of the OH group promotes vertical relaxation of all surface atoms of the co-adsorbed state with the O atoms of the OH groups returning to the centre of a four-fold hollow formed by the Co sites. The distance between the H species in the transition state is 2.40 Å, with H pointing towards adjacent Co sites. The H2 formation is 107 kJ mol1 activated and 81 kJ mol1 exothermic with respect to infinite separation of the lattice OH groups. The formation of H2O is 45 kJ mol1 less activated than that of H2 formation. However, there is a thermodynamic tendency towards H2 formation because the H2O is unstable with respect to the hydration of the surface.
The dehydrogenation energy barriers for NH3, NH2 and NH on CoO(100) are 218 kJ mol1, 219 kJ mol1, and 210 kJ mol1 higher, respectively, than those calculated on the Co3O4(100) surface [16]. The binding energies of NH3, NH2 and NH on CoO(100) are 43 kJ mol1, 5 kJ mol1 and 22 kJ mol1 lower, respectively, than on the Co3O4(100) surface [16]. Clearly, the dominant factor in the loss of catalytic activity of CoO towards NHx dehydrogenation compared to that of the Co3O4(100) surface is associated to the inability of lattice oxygen to assist the hydrogen abstraction process, rather than the stability of the NHx species at the surface. The high energy barrier to hydrogen abstraction by lattice O arises from a low binding energy of H atop O sites of the CoO(100) surface. Comparatively, the binding energy of H atop O lattice sites of the CoO(100) surface is 233 kJ mol1 lower than that on the Co3O4(100) surface. NH3 dehydrogenation on the CoO(100) surface favours the formation of N2 whereas our previous study shows that NH3 decomposition on Co3O4 mainly produces NO. This observation is consistent with a highly reactive O site in the Co3O4(100) surface and a less active O site in the CoO(100). Lattice O at the CoO surface are strongly bound, with vacancy formation energy O2 Evac ¼ 386 kJ mol1 with respect to the energy of 1/2O2 in the gas phase. This is very much greater than the corresponding energy for O2 the Co3O4(100) surface, Evac ¼ 94 kJ mol1 [46]. There is evidence that the presence of CoO crystallites in Co3O4 decreases the NH3 conversion [14]. The information provided in this study indicates that CoO may not only decrease the conversion of NH3 but also affects the NO selectivity since the NO desorption energy -leaving an O vacant site-from the CoO surface is 207 kJ mol1 more endothermic than NO desorption leaving a vacant site at the Co3O4 surface [16]. The presence of lattice oxygen vacancies at the CoO surface is thermodynamically unfavourable compared to that of Co3O4. Point defects could appear at the CoO surface during the NH3 dehydrogenation in the presence of an oxygen atmosphere due to oxidation of CoO to other oxides. The existence of such defects might then affect the binding and dehydrogenation of NHx spices due to the change in coordination of surface sites. A more thorough study especially addressing the fundamentals of CoO oxidation to Co3O4 and the NH3 dehydrogenation reactions on oxidised CoO surfaces is needed in order to make more definitive comparisons with experimental observations and to assess the relative NH3 conversion and selectivity to NO. 5. Conclusion Spin-polarised density functional theory using the PBE þ U (Ueff ¼ 3.3 eV) approach within the periodic boundary conditions predicts electronic properties of bulk CoO and CoO(100) surfaces in good agreement with experimental values. Surface relaxation of the bare CoO(100) slab results in a 2% expansion of the top interlayer distance with no surface reconstruction. As a result, the surface free energy decreases from 0.88 to 0.84 J m2 (only by 0.04 J m2) after relaxation of the three top layers. The response of the surface to adsorption depends on the type of adsorbate and its mode of the adsorption. There is insignificant surface relaxation upon binding of NH3 because the interaction energy is low and the NH3 equilibrates relatively far from the surface. Relaxation of the top-most layer is more evident in the binding of NH2, and H. In the case of NH2, cobalt atoms that participate in the NH2 binding are relaxed vertically away from the second layer inducing local surface rumpling. Lattice oxygen sites bind H, forming lattice OH groups that equilibrate at 1.23 Å away from the
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
8
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
Fig. 6. Reaction pathways of N2, H2O and H2 formation on CoO(100) obtained with the CI-NEB approach. The pale grey site labelled as “V” represents an oxygen vacancy site. Energies are with respect to the non-interacting separate adsorption of initial states.
surface. Analysis of the density of states of the slab and adsorbates indicate that binding of NHx species is attributed mainly to the interaction between N states and surface states. Binding of N and H species results in the partial reduction of vicinal surface Co atoms, giving rise to the appearance of surface states near the Fermi level. Dehydrogenations of NH3, NH2 and NH are highly activated, with energy barriers in the order of 292e328 kJ mol1. The high barriers are associated with the coordination of surface O sites inhibiting the activity of CoO towards dehydrogenation. Formation of N2 is thermodynamically and kinetically preferred if NH3 dehydrogenation occurs. The NO formation via loss of lattice oxygen is hindered by a high-energy barrier as a result of the formation of oxygen vacancy sites. The hydrogenated surface is thermodynamically unstable with respect to the bare surface. The reaction of two vicinal hydroxyl groups result in the formation of H2O and H2 with energy barriers of 62 and 107 kJ mol1, respectively, indicating that H2O is kinetically favoured. The formation of H2 is exothermic by 81 kJ mol1 whereas that of H2O is endothermic by 55 kJ mol1, implying that
H2 is thermodynamically preferred. When temperature is high enough to provide the energy for the formation of both products, the selectivity is dictated by the relative thermodynamic stabilities, and H2 is expected to be the major product under equilibrium conditions. The formation of CoO in Co3O4 catalysts for ammonia oxidation is expected to be accompanied by a decrease in the overall consumption rate, due to the high barriers to NHx (x ¼ 3,2,1) dehydrogenation. At the same time, the high barrier to NO formation over CoO is expected to cause a reduction in selectivity to that product, with increased formation of N2. Acknowledgements This research was undertaken with the assistance of resources provided at the NCI National Facility systems through the National Computational Merit Allocation Scheme supported by the Australian Government. Kambiz Shojaee would like to thank the University of Sydney for providing a postgraduate scholarship to conduct this study.
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
References [1] C.R. Brundle, T.J. Chuang, D.W. Rice, X-ray photoemission study of the interaction of oxygen and air with clean cobalt surfaces, Surf. Sci. 60 (1976) 286e300. [2] T. Motohashi, Y. Katsumata, T. Ono, R. Kanno, M. Karppinen, H. Yamauchi, Synthesis and properties of CoO2, the x ¼ 0 end member of the LixCoO2 and NaxCoO2 systems, Chem. Mater. 19 (2007) 5063e5066. [3] N. Koshizaki, K. Yasumoto, T. Sasaki, Mechanism of optical transmittance change by NOx in CoO/SiO2 nanocomposites films, Sens. Actuat. B: Chem. 66 (2000) 122e124. [4] V.R. Shinde, S.B. Mahadik, T.P. Gujar, C.D. Lokhande, Supercapacitive cobalt oxide (Co3O4) thin films by spray pyrolysis, Appl. Surf. Sci. 252 (2006) 7487e7492. [5] D. Zhang, X. Li, X. Guo, C. Lai, Fabrication of cobalt oxide (Co3O4) coating by electrophoretic deposition, Mater. Lett. 126 (2014) 211e213. [6] W. Yang, Z. Gao, J. Ma, X. Zhang, J. Wang, Controlled synthesis of Co3O4 and Co3O4@MnO2 nanoarchitectures and their electrochemical capacitor application, J. Alloys Compd. 611 (2014) 171e178. [7] K. Schmidt-Szałowski, K. Krawczyk, J. Petryk, The properties of cobalt oxide catalyst for ammonia oxidation, Appl. Catal. A: Gen. 175 (1998) 147e157. [8] J. Petryk, E. Kołakowska, Cobalt oxide catalysts for ammonia oxidation activated with cerium and lanthanum, Appl. Catal. B: Environ. 24 (2000) 121e128. [9] S.P.S. Ansrew, in: C. Keleti (Ed.), Nitric Acid and Fertelizer Nitrates, Marcel Dekker, Inc., New York, 1985, pp. 31e40. [10] V.A. Sadykov, L.A. Isupova, I.A. Zolotarskii, L.N. Bobrova, A.S. Noskov, V.N. Parmon, E.A. Brushtein, T.V. Telyatnikova, V.I. Chernyshev, V.V. Lunin, Oxide catalysts for ammonia oxidation in nitric acid production: properties and perspectives, Appl. Catal. A: General 204 (1) (6 November 2000) 59e87. rez-Ramírez, E.V. Kondratenko, Mechanism of ammonia oxidation over [11] J. Pe oxides studied by temporal analysis of products, J. Catal. 250 (2007) 240e246. [12] L. Xue, C. Zhang, H. He, Y. Teraoka, Catalytic decomposition of N2O over CeO2 promoted Co3O4 spinel catalyst, Appl. Catal. B: Environ. 75 (2007) 167e174. [13] C.-W. Tanga, C.-B. Wang, S.-H. Chien, Characterization of cobalt oxides studied by FT-IR, Raman, TPR and TG-MS, Thermochim. Acta 473 (2008) 68e73. [14] W.-K. Fung, L. Ledwaba, N. Modiba, M. Claeys, E.v. Steen, Choosing a suitable support for Co3O4 as an NH3 oxidation catalyst, Catal. Sci. Technol. 3 (2013) 1905e1909. [15] G. Busca, R. Guidetti, V. Lorenzelli, Fourier-transform infrared study of the surface properties of cobalt oxides, J. Chem. Soc. Faraday Trans. 86 (1990) 989e994. [16] K. Shojaee, A. Montoya, B.S. Haynes, The role of oxygen during the catalytic oxidation of ammonia on Co3O4(100), Appl. Surf. Sci. 316 (2014) 355e365. [17] P. Broqvist, I. Panas, H. Persson, A DFT study on CO oxidation over Co3O4, J. Catal. 210 (2002) 198e206. [18] K. Omata, T. Takada, S. Kasahara, M. Yamada, Active site of substituted cobalt spinel oxide for selective oxidation of CO/H2, Part II Appl. Catal. A 146 (1996) 255e267. [19] W. Piskorz, F. Zasada, P. Stelmachowski, A. Kotarba, Z. Sojka, Decomposition of N2O over the surface of cobalt spinel: a DFT account of reactivity experiments, Catal. Today 137 (2008) 418e422. [20] D.-H. Ha, L.M. Moreau, S. Honrao, R.G. Hennig, R.D. Robinson, The oxidation of cobalt nanoparticles into Kirkendall-hollowed CoO and Co3O4: the diffusion mechanisms and atomic structural transformations, J. Phys. Chem. C 117 (2013) 14303e14312. [21] D. Wolf, Reconstruction of NaCl surfaces from a dipolar solution to the Madelung problem, Phys. Rev. Lett. 68 (1992) 3315e3319. [22] U.D. Wdowik, K. Parlinski, Lattice dynamics of CoO from first principles, Phys. Rev. B 75 (2007), 104306-1-104306-5. [23] P.A. Ignatiev, N.N. Negulyaev, D.I. Bazhanov, V.S. Stepanyuk, Doping of cobalt
[24]
[25] [26]
[27] [28] [29]
[30]
[31] [32] [33] [34] [35]
[36]
[37]
[38]
[39]
[40] [41]
[42]
[43] [44]
[45] [46]
9
oxide with transition metal impurities: Ab initio study, Phys. Rev. B 81 (2010), 235123 (1e8). A.-L. Dalverny, J.-S. Filhol, F. Lemoigno, M.-L. Doublet, Interplay between magnetic and orbital ordering in the strongly correlated cobalt oxide: a DFTþU study, J. Phys. Chem. C 114 (2010) 21750e21756. B.S. Youmbi, F. Calvayrac, Structure of CoO(001) surface from DFTþU calculations, Surf. Sci. 621 (2014) 1e6. M.R. Castell, S.L. Dudarev, G.A.D. Briggs, A.P. Sutton, Unexpected differences in the surface electronic structure of NiO and CoO observed by STM and explained by first-principles theory, Phys. Rev. B 59 (1999) 7342e7345. D.J.G. Kresse, From ultrasoft pseudopotentials to the projector augmentedwave method, Phys. Rev. B 59 (1999) 1758e1775. J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865e3868. S.L. Dudarev, G.A. Botton, S.Y. Savrasov, C.J. Humphreys, A.P. Sutton, Electronenergy-loss spectra and the structural stability of nickel oxide: an LSDAþU study, Phys. Rev. B 57 (1998) 1505e1509. L. Wang, T. Maxisch, G. Ceder, Oxidation energies of transition metal oxides within the GGAþU framework, Phys. Rev. B: Condens. Matter Mater. Phys. 73 (2006) 195107. €chl, O. Jepsen, O.K. Andersen, Improved tetrahedron method for P.E. Blo Brillouin-zone integrations, Phys. Rev. B 49 (1994) 16223. F. Birch, Finite elastic strain of cubic crystals, Phys. Rev. 71 (1947) 809e824. H.J. Monkhorst, J.D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B 13 (1976) 5188e5192. A.T.P.M. Methfessel, High-precision sampling for Brillouin-zone integration in metals, Phys. Rev. B 40 (1989) 3616e3621. nsson, A climbing image nudged elastic G. Henkelman, B.P. Uberuaga, H. Jo band method for finding saddle points and minimum energy paths, J. Chem. Phys. 113 (2000) 9901e9904. W. Jauch, M. Reehuis, H.J. Bleif, F. Kubanek, P. Pattison, Crystallographic symmetry and magnetic structure of CoO, Phys. Rev. B 64 (2001), 052102-1052102-3. K. Tomiyasu, T. Inami, N. Ikeda, Magnetic structure of CoO studied by neutron and synchrotron x-ray diffraction, Phys. Rev. B 70 (2004), 184411-1-1844116. S. Imada, T. Jo, Magnetic states of cobalt oxide and ferrites and magnetic dichroism in 2p-3d X-ray absorption spectroscopy, J. Magn. Magn. Mater. 104e107 (1992) 2001e2002. E.Z. Kurmaev, R.G. Wilks, A. Moewes, L.D. Finkelstein, S.N. Shamin, J. Kunes, Oxygen x-ray emission and absorption spectra as a probe of the electronic structure of strongly correlated oxides, Phys. Rev. B 77 (2008), 165127-1165127-5. R.C. Felton, M. Prutton, S.P. Tear, M.R. Welton-Cook, A LEED analysis of the CoO(100) surface, Surf. Sci. 88 (1979) 474e478. X. Duan, J. Ji, G. Qian, C. Fan, Y. Zhu, X. Zhou, D. Chen, W. Yuan, Ammonia decomposition on Fe(1 1 0), Co(1 1 1) and Ni(1 1 1) surfaces: a density functional theory study, J. Mol. Catal. A 357 (2012) 81e86. rez-Ramírez, J.M. Ricart, Ammonia dehyG. Novell-Leruth, A. Valc arcel, J. Pe drogenation over platinum-group metal surfaces. Structure, stability, and reactivity of adsorbed NHx species, J. Phys. Chem. C 111 (2007) 860e868. C.-C. Wang, Y.-J. Yang, J.-C. Jiang, DFT study of NHx (x ¼ 1e3) adsorption on RuO2(110) surfaces, J. Phys. Chem. C 113 (2009) 2816e2821. C.-C. Wang, S.S. Siao, J.-C. Jiang, Density functional theory study of NHx (x ¼ 0e3) and N2 adsorption on IrO2(110) surfaces, J. Phys. Chem. C 114 (2010) 18588e18593. J.L. Mackay, V.E. Henrich, Electronic properties of CoO(100) surfaces: defects and chemisorption, Phys. Rev. B 39 (1989) 6156e6168. K. Shojaee, A. Montoya, B.S. Haynes, Insight into oxygen stability and vacancy formation on Co3O4 model slabs, Comput. Mater. Sci. 72 (2013) 15e25.
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Molecular modelling of the decomposition of NH3 over CoO(100) Kambiz Shojaee, Brian S. Haynes, Alejandro Montoya* School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney NSW 2006, Australia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Minimum reactions pathways of ammonia decomposition were studied using density functional theory. The bonding characteristics of NHx and H on the CoO(100) surface were analysed using Layer-projected density of states. Dehydrogenations of NH3, NH2 and NH are highly activated. The presence of strongly bound lattice oxygen favours the ammonia decomposition towards N2.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 September 2014 Received in revised form 29 January 2015 Accepted 21 February 2015 Available online xxx
Spin-polarised density functional theory using the PBE þ U approach are used to determine reaction pathways of successive NH3 dehydrogenation on the CoO(100) surface. NH3 dehydrogenation promotes noticeable displacements of the surface CoO sites, in particular due to the binding of NH2 and H species. Surface lattice O has low activity towards dehydrogenation, reflected in energy barriers that are in the range of 292 kJ mol1 to 328 kJ mol1. There is a preference of surface NH3 dehydrogenation to N2 rather than towards NO, due to a high-energy penalty of surface O vacancy formation. The presence of CoO in cobalt oxide catalysts not only may decline the ammonia conversion but also alter the selectivity towards N2 rather than NO. © 2015 Elsevier B.V. All rights reserved.
Keywords: Oxides Computer modelling and simulation Electronic structure Surface properties
1. Introduction Cobalt oxides can be synthesised with a variety of morphologies and distinct compositions to obtain specific catalytic and electronic properties for a variety of practical applications. The low-valence CoO and Co3O4 are the most stable forms of cobalt oxides while high valence Co2O3 and CoO2 oxides are thermally less stable [1,2]. CoO crystallises in the rock salt structure with Co2þ sites occupying octahedral positions, while Co3O4 is a normal spinel with cubic crystal structure where Co3þ sites occupy octahedral positions and
* Corresponding author. E-mail address: [email protected] (A. Montoya).
Co2þ sites occupy tetrahedral positions. CoO is a promising oxide for functional materials owing to its magnetic and gas-sensing properties [3], while Co3O4 has diverse applications in heterogeneous catalysts, lithium-ion batteries, solid state gas sensors, supercapacitor devices and solar energy absorbers [4e6]. Cobalt oxides are known to have excellent catalytic activity towards total oxidation of NH3 [7e9] with the potential to reduce catalyst cost by replacing current industrial Pt/Rh catalyst. Among these oxides, Co3O4 has been reported to be the most active oxide catalyst with an NH3 oxidation selectivity toward NO of over 90% [10] and a decrease in the emissions of the undesirable greenhouse gas N2O in NH3 oxidation chemical plants [11,12].
http://dx.doi.org/10.1016/j.matchemphys.2015.02.040 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
2
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
Co3O4 is not suitable for high-temperature operations because it loses oxygen above 750 C under oxygen atmospheric pressure, forming less active CoO [13]. Experiments on the oxidation of NH3 oxidation on unsupported Co3O4 pellets between 450 and 800 C show a decrease of approximately 15% in the NH3 conversion when the reaction temperature is increased from 640 to 800 C, which has been ascribed to the formation of small CoO crystallites [14]. Limited studies are available that demonstrate the reduced activity of CoO for NH3 oxidation under industrial oxidation conditions [14]. Evidence at molecular level of the low activity of CoO comes from FTIR analysis of Co3O4 samples reduced in H2 at 200 Torr and 523 K, which were exposed to NH3 at room temperature. The samples showed bands associated with ammonia coordination on Lewis (Co) sites and the absence of bands associated with NH3 dehydrogenation products like NH2 and NH [15], indicating no decomposition over CoO. The high activity of Co3O4 towards NH3 oxidation has been ascribed to the participation of weak lattice surface oxygen sites in the hydrogen abstraction and NO formation leaving the surface with vacancy sites [16] and possible rapid O2 adsorption to regenerate the O sites and its surface activity [16]. Lattice O in CoO surfaces is more coordinated than in Co3O4 surfaces and therefore, in theory, less active for dehydrogenation than O in Co3O4. However, molecular modelling and substitutional cobalt experimental studies of CO oxidation to CO2 [17,18] and molecular modelling of N2O decomposition to N2 [19] over Co3O4 ascribe some catalytic role during the adsorptionedesorption process to Co in octahedral sites. The Co sites in CoO tend to have lower diffusion energy barriers than O sites [20], and reconstruction of the surface during NH3 dehydrogenation could be expected, leading to the possibility of dynamic participation of Co in NH3 dehydrogenation that needs to be assessed. Despite the technical importance of NH3 oxidation on cobalt oxides, there is no molecular level understanding of the activity of lattice oxygen and cobalt sites in CoO in hydrogen abstraction processes. The site selectivity and possible dehydrogenation pathways have not been considered previously. Knowledge of the NH3 reaction mechanisms on CoO can provide insights into the behaviour of Co3O4 catalyst at high temperature, reducing operating conditions while also enhancing understanding of the activity of CoO in dehydrogenation reactions in general. In this contribution, we study the successive dehydrogenation of ammonia on the CoO(100), which is the predominant exposed surface of CoO [21]. Only limited first-principle studies have been directed towards the atomic description of bulk CoO [22e24], and even fewer studies are devoted to the electronic characteristics of the CoO(100) surface [25,26]. Therefore, we provide additional structural, magnetic and electronic properties of the CoO(100) surface as well as its activity towards ammonia dehydrogenation by determining density of states, binding energies and energy barriers. 2. Computational methodology Spin-polarised periodic DFT calculations were performed with the projector-augmented wave (PAW) method as implemented in the Vienna Ab initio Simulation Package (VASP) [27] using the GGA approximation in the form of the PerdeweBurkeeErnzerhof (PBE) exchange-correlation [28]. On-site Coulomb correction to the 3d electrons of Co atoms was considered using the rotationally invariant formulation proposed by Dudarev et al. [29] with a value of Ueff ¼ (U J) ¼ 3.3 eV, which was obtained by fitting the reaction energy of CoO oxidation to Co3O4 [30]. We have shown in our previous work [16] that the PBE þ U approach with a Ueff ¼ 3.3 eV provides geometry configurations and binding energies of NHx species on Co3O4(100) surfaces in good comparison with the hybrid
functional HSE06 methods. Recently, a value of Ueff ¼ 3.7 eV for bulk CoO was determined using a self-consistent procedure based on the linear response approach [25]. However, we have used a value of 3.3 eV in this study in order to minimise uncertainties when we compare binding energies and minimum energy dehydrogenation pathways of NHx species with our previous PBE þ U study of ammonia oxidation on Co3O4. We will show that a value of Ueff ¼ 3.3 eV is able to predict experimental electronic structure properties of CoO. The convergence criteria for the electronic self-consistent iteration and the forces acting on unconstrained atoms were set to 104 eV and 0.02 eV/Å, respectively. A (2 2 2) rhombohedral unit cell of bulk CoO was modelled with a k-point mesh of (6 6 6) at a cutoff energy of 550 eV, using the tetrahedron € chl corrections [31]. The lattice constant was obmethod with Blo tained using the BircheMurnaghan equation of state [32]. Slab cells, containing five atomic layers exposing the CoO(100) surface, were built from the DFT-relaxed bulk CoO. We found that five layers were adequate for our slab model because the surface free energy was found to converge within 0.02 J m2 for slabs with more than four CoO atomic layers. The adsorption and reactions were studied on one side of the slab, and a vacuum space of 15 Å was used in the direction perpendicular to the surface in order to avoid surfaceesurface interactions. Dipole correction along the axis normal to the surface was applied in order to cancel any spurious effect of dipole moments associated with asymmetric slabs. Atomic relaxations were carried out for the adsorbates and the atoms in the first three top-most layers while atoms in the two bottom-most layers were kept fixed to their bulk DFT-positions. A (2 3 1) Monkhorst-Pack k-point mesh [33] was used for the Brillouin zone integrations at a cutoff energy of 450 eV using the MethfesselePaxton method [34] with a smearing of 0.1 eV. The plane-wave cutoff energy and k-point meshes were found to be adequate for the purpose of this study. Test calculations showed that increasing the cutoff to 550 eV or using a (3 4 1) k-point mesh had a slight (less than 3 kJ mol1) effect on the binding energy of NH3. The Climbing Image Nudged Elastic Band (CI-NEB) method [35] was used to obtain reaction energy barriers, and transition states coordinates. The tolerance for electronic self-consistent iteration was set to 104 eV, and each image was relaxed until the forces were less than 0.05 eV/Å. Vibrational frequencies were calculated using a finite difference method at the G point with an electronic convergence of 107 eV. Each atom of the adsorbate was displaced by 0.02 Å from its equilibrium position while the surface atoms were fixed. Transition states were recognised by having a single imaginary frequency in the direction of the reaction coordinate. Energy barriers of surface reactions that involve hydrogen migration can slightly decrease due to quantum tunnelling effects that increase the reaction rate, especially at low temperatures. Such effects in the NH3, NH2 and NH dehydrogenations are not considered in this study as they are unlikely to have a significant impact at the high temperatures at which cobalt oxide catalysts are used in the industrial combustion of ammonia. 3. Results 3.1. Bulk and CoO(100) surface properties The PBE þ U method with Ueff ¼ 3.3 eV is able to reproduce experimental structural and electronic characteristics of bulk CoO and CoO(100). The optimised lattice constant of bulk CoO is calculated to be 4.28 Å, which is only 0.02 Å larger than the reported experimental value [36] and 0.01 Å larger than a previous DFT prediction using the PW91 þ U (Ueff ¼ 6.1 eV) approach [22]. Fig. 1 shows the magnetic ordering of Co atoms and band structure
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
3
Fig. 1. Magnetic ordering and electronic band structures of bulk CoO and CoO(100). Co atoms (blue atoms) with different spins are distinguished by arrows pointing up and down directions. The {111} planes formed by Co atoms in the bulk CoO structure are highlighted with orange sheets. The high-symmetry points are labelled according to the FCC Brillouin zone. The energy of the highest occupied state (Fermi energy) is set to 0. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
of bulk CoO (Fig. 1a and b) and CoO(100) slab (Fig. 1c and d). We found that the most stable bulk CoO has a type-II antiferromagnetic (AF-II) state consisting of all Co atoms in a {111} plane with magnetic moments aligned in the same direction, and all adjacent Co atoms in a {111} plane with magnetic moments aligned in the opposite direction, as shown in Fig. 1a. The higher stability of the AF-II state relative to other magnetic orderings is consistent with experimental reports using neutron and synchrotron X-ray diffraction [37] as well as previous PBE þ U calculations (with Ueff varying between 0 and 10 eV) [24]. The magnetic moments of Co atoms are found to 2.64 mB, 0.16 mB lower than the reported experimental values [38]. Electronic band structures of the bulk CoO at various high-symmetry points is depicted in Fig. 1b. As can be seen, the minimum gap between the valence band maximum and the conduction band minimum occurs at the G point and amounts to 2.35 eV, consistent with a value of 2.6 eV obtained via oxygen X-ray absorption spectroscopy measurements [39]. A p(3 2) super-cell, exposing the CoO(100) surface, was constructed from the DFT-relaxed bulk CoO. The top three layers in one side of the slab were allowed to relax while the bottom two layers were frozen. The first and second relaxed interlayer distances increased only by ~0.04 Å (~2%) from the 2.14 Å bulk layer spacing. The minor surface relaxations are consistent with experimental observation using LEED measurements on a UHV cleaved CoO(100) surface in which changes in interlayers distancing as a result of surface relaxations were reported to be within ±3% compared to
the bulk CoO [40]. We observed a break in the surface symmetry in the spin-polarised PBE þ U calculations, resulting in the CoeO bond lengths in the top-most layer varying between 2.12 Å and 2.17 Å. The magnetic ordering of Co atoms in the CoO(100) slab is depicted in Fig. 1c. The surface is antiferromagnetic with the Co atoms retaining the same magnetic ordering as that observed in the bulk. The magnetic moment of exposed Co atoms increases only by ~0.03 mB relative to those in bulk CoO, and in good agreement with a value of 2.64 mB obtained from a LSDA þ U study with U ¼ 6.2 eV and J ¼ 0.95 eV using the rotationally invariant approach introduced by Liechtenstein et al. [26]. The surface free energy (g) of the relaxed five-layer CoO(100) surface is calculated to be 0.84 J m2, consistent with a previously reported PBE þ U (Ueff ¼ 3.7 eV) value of 0.8 J m2 [25]. Relaxation of the slab from the bulk termination reduces the CoO(100) surface energy only by 0.04 J m2, as a result of minor surface relaxations. The low value of g indicates that the cleavage of the bulk CoO to create the (100) surface is not a highly energetic process, which we attribute to a low number of dangling surface bonds (one CoeO bond per surface atom) and non-polarity of the surface due to a charge compensation of Co with O atoms at each layer. The electronic band structures of the CoO(100) surface at various high-symmetry points is depicted in Fig. 1d. The band gap of CoO(100) slab decreases by 0.75 eV compared to that in the bulk, indicating a decrease in the insulating nature of the bulk due to the shift of the surface states closer to the Fermi level.
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
4
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
Fig. 2. Top and side views of relaxed configurations of NHx* and H* species on CoO(100). Only adsorbates and the top-most layer are coloured for clarity and atoms in bottom layers are shown in grey. The blue, red, green and white atoms refer to cobalt, oxygen, nitrogen and hydrogen, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2. Binding characteristics of adsorbed species Fig. 2 shows top and side views of the relaxed geometries of NH3, NH2, NH, N and H species bound to their most stable sites above the top-most layer of CoO(100). The adsorbates and the top-most layer only are shown since only minor atomic relaxations were found in the second layer. The insets in Fig. 2 provide selected geometrical parameters and the corresponding binding energies with no corrections coming from the vibrational modes at 0 K. All the NHx species bind preferentially N-down to the CoO(100) surface sites. NH3 binds atop Co sites with a slightly tilted configuration towards lattice surface oxygen (Fig. 2a). Binding of NH3 barely induces surface relaxation, due to the weak interaction between the NH3 and the surface. The role of lattice oxygen becomes more apparent in binding of the dehydrogenated fragments and H species. Surface hollow sites are the preferred binding positions for NH2, NH and N. NH2 binds in the centre of a four-fold hollow site closer to Co sites, with H atoms pointing towards the lattice oxygen (Fig. 2b), inducing a significant intra and interlayer relaxation of the hollow sites in the top-most layer. The intra-layer relaxation is characterised by 8% contraction of the CoeCo distance and 9% increase in the OeO distance of the hollow site. The interlayer relaxation is characterised by an outward displacement of the Co atoms by 14% and an inward relaxation of the O sites by 8%, inducing a local surface rumpling which is evident in the side view of the adsorbed configuration in Fig. 2b.
NH binds slightly further from the centre of the four-fold hollow site towards the three-fold coordinated CoeOeCo hollow position. No significant rumpling of the surface is noted, although there is a lateral relaxation evidenced by an increase in the CoeCo and OeO distances by 6% and 16%, respectively. Atomic N binds 0.73 Å above a three-fold coordinated hollow site with an NeO distance of 1.38 Å, which is only 0.21 Å longer than the PBE-optimised length of NO in the gas phase (calculated in this study), thus forming a surface NO-like group. The binding of N* causes the CoeCo and OeO bond distances of the hollow site to increase by 4% and 18% respectively with no evidence of interlayer relaxation. Atomic H binds atop O and Co sites with a difference in binding energy of 51 kJ mol1, in favour of atop O sites. Binding of H above O causes the lattice O site to be significantly pulled away from the top layer by 0.74 Å, forming a hydroxyl group (OH) above the surface (Fig. 2e). The surface sites adjacent to the surface OH group, in particular the Co sites, relax only slightly. The distance between O of the OH group to the surface Co sites increases by almost 6% relative to the bare surface. The binding energy of NH3 is 59 kJ mol1, which is low enough that an adsorptionedesorption equilibrium can be expected to establish rapidly under atmospheric conditions. The dehydrogenated NH3 species are strongly bound to the surface, with binding energies of 140, 215 and 265 kJ mol1 for NH2, NH and N, respectively. The increase in binding energy as the NH3 is being dehydrogenated has been reported on various transition metals
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
5
Fig. 3. Layer-projected density of state (LDOS) of the top-most layer of CoO(100) before and after adsorption and partial density of state (PDOS) of the adsorbed NHx, N and H species. Black solid line, red dashed line, and blue thick solid line refer to surface LDOS, N 2p and of H 1s, respectively. Energies are relative to the energy of the highest occupied state (Ef). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[41,42] as well as on metal oxides [43,44]. The binding energy of H atop lattice O is 160 kJ mol1, which is 233 kJ mol1 lower than the binding energy of H atop lattice O in the Co3O4(100) surface [16]. The lower binding energy of H on CoO compared to Co3O4 is attributed to a significant vertical relaxation of lattice O upon binding of H in the CoO(100) surface. In order to analyse the bonding characteristics of the species on the CoO(100) surface, the Densities of States of the bare slab and the slab with the adsorbed species were obtained. With the intention of concentrating on electronic changes at the surface, Fig. 3 shows the Layer-projected density of states (LDOS) composed of the Co 3d and O 2p states of the top-most layer and the partial density of state (PDOS) of the adsorbates consisting of the N 2p and/or H 1s states. The LDOS of the bare slab is presented in Fig. 3a. The valence band consists of Co 3d and O 2p starting from 6.9 eV with two distinguishable peaks at 5.3 and 0.8 eV. The LDOS is barely perturbed upon binding of NH3 (Fig. 3b), consistent with a weak interaction of NH3 with the surface. Binding of NH2, NH and N to the surface considerably changes the LDOS with the corresponding valence bandwidths becoming wider compared to those of bare surface. The N 2p states in the PDOS profiles become more populated near the Fermi level as the stability of NHx* increases. In the case of NH2*, there are overlapping peaks associated with N 2p and Co 3d between 6.7 and 0.8 eV due to strong interaction of N with the Co sites, and an overlap between 6.6 and 6.8 eV involving N 2p and H 1s (Fig. 3c). The PDOS and LDOS profile of NH* (Fig. 3d) show binding peaks between 6.1 and 0.4 eV associated with the interaction of N 2p with both O 2p and Co 3d states of the LDOS, and a binding state at 8 eV originating from the interaction of N 2p with the H 1s of NH* and interaction of N* with the O 2p state of the LDOS (not
clearly visible in Fig. 3d because it coincides with the population of H 1s states), indicating the formation of a nascent surface ONH group. The LDOS and PDOS of N* are shown in Fig. 3e. There are overlapping peaks associated to N 2p states and the LDOS between 7.5 eV and 0.3 eV. The presence of a split band near the Fermi level is due to the partial reduction of Co atoms in the vicinity of N as a result of the NeO formation and weakening of the CoeO bond. The reduction of the Co sites due to the weakening of the CoeO bond can also be observed in the LDOS profile of H adsorption. Fig. 3f shows the interaction between O 2p and H 1s states at 8.3 eV, and the appearance of surface states near the Femi level due to a reduction of the Co sites in the vicinity of the lattice OH. The partial reduction of Co ions is consistent with a decrease of approximately 0.5 mB in the magnetic moments of the Co atoms. 3.3. Dehydrogenation pathways of NHx species on the CoO(100) surface The ammonia dehydrogenation pathways on the CoO(100) surface were studied starting from the most stable binding configurations. The reaction pathway was obtained with a CI-NEB approach with the final states selected to be the most stable coadsorbed configurations. Top and side views of the dehydrogenation pathways including the initial, transition and final states of NH3, NH2, and NH are shown in Fig. 4 and the energy profiles from the initial to the co-adsorbed states and then to the products at infinite separation are displayed in Fig. 5. Dehydrogenation is in all cases assisted by lattice oxygen, with the NeH elongation taking place along the CoeO bridge positions forming OH species in the final states. The configurations of the
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
6
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
Fig. 4. Top and side views of the initial, transition and final states of NHx* dehydrogenation reactions on CoO(100) obtained with the CI-NEB approach.
transitions states show that the dissociating NeH bonds are stretched by 1.75 Å (171%), 1.61 Å (158%) and 1.86 Å (179%) compared to the NeH bonds in the corresponding initial states for dehydrogenation of NH3, NH2 and NH respectively, indicating that the NeH dissociation proceeds via late transition states. The lateral and vertical displacements of surface atoms are more evident for Co sites than O sites, consistent with a lower energy barrier for Co diffusion compared to the O diffusion in bulk CoO [20]. Co sites mostly undergo outwards relaxations whereas O atoms mainly relax inwards upon interaction with NH2 and NH. Consequently, local surface rumpling occurs, which is more evident for the dehydrogenation of NH2 to NH. The dehydrogenation energy barriers are relatively constant, with values varying from 292 to 328 kJ mol1. The dehydrogenation reactions of NH3, NH2 and NH leading to the co-adsorbed product states are all endothermic with reaction energies of 86 kJ mol1, 156 kJ mol1 and 106 kJ mol1, respectively. There are attractive interactions in the co-adsorbed states, and, therefore, the energy of reaction with respect to infinitely separated products increases to 221, 173 and 108 kJ mol1 respectively.
3.4. Products formation The NH3 dehydrogenation results in N bound to lattice O sites. The selectivity of NH3 oxidation to NO depends on the relative rate between NO desorption and NeN recombination to N2. The formation of NO in the gas phase leaving an O vacant site is 256 kJ mol1 endothermic. Although the NO formation energy is lower than any of the dehydrogenation processes, the interaction of two adjacent N groups producing N2 is a less energetically demanding pathway compared to NO desorption. Fig. 6a shows the initial, transition and final states of N2 formation obtained with the CI-NEB approach. The initial state consists of two adsorbed N species, each bound to adjacent three-fold coordinated hollow sites. It is noted that binding of two N* causes lateral displacements of surface O sites, and slight vertical displacements of surface Co sites. The surface Co and O sites recover the bare slab positions as the recombination of N* generates a weakly bound N2 species. The interaction of two adjacent adsorbed N species is 42 kJ mol1 attractive, the energy barrier is 157 kJ mol1, and the reaction energy is 500 kJ mol1 exothermic relative to two noninteracting N species at the surface. The energy barrier of N2
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
7
4. Discussion
Fig. 5. The energy profiles of successive ammonia dehydrogenation reactions on CoO(100). Black, red and blue profiles refer to dehydrogenation reactions of NH3*, NH2* and NH*, respectively. Energies are relative with respect to the initial state. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
formation is 99 kJ mol1 lower than the NO formation energy, indicating that formation of N2 is energetically preferred compared to that of NO formation. Fig. 6b and c shows the initial, transition and final states of H2O and H2 formation obtained with the CI-NEB approach. The initial state was obtained by relaxing H species atop adjacent O sites. Surface hydrogenation causes vertical displacements of lattice O sites by 1.09 Å away from the surface, and a lateral displacement of O from the centre of a four-fold Co hollow site to above a CoeCo bridge site. The axes of the OH groups are tilted towards the surface forming surface OH rows maximising the OH/OH interactions. The structural reorganisation is limited to the oxygen sites participating in the hydrogenation since the other Co and O sites remain in their lattice positions. Desorption of H2O from the co-adsorbed OH state is shown in Fig. 6b. A direct hydrogen diffusion from one OH group to the other generates a transition state containing H2O that is 3.35 Å above the surface, and the O site at which the H migration originated returns to the bare slab lattice position. The formation of H2O is 62 kJ mol1 activated and 55 kJ mol1 endothermic, with respect to infinite separation of the initial co-adsorbed state. The reverse process, that is, the hydration of the surface containing an O vacant site is only 7 kJ mol1 activated. It indicates that H2O in the gas phase is unstable with respect to the dehydrogenated surface, consistent with an observation of OH peaks in ultraviolet photoemission spectra after H2O exposure of high energy sputtered CoO [45]. The formation of H2 from a co-adsorbed OH/OH state is presented in Fig. 6c. Formation of H2 involves the rotation of one OH group before the HeH recombination takes place. The rotation of the OH group promotes vertical relaxation of all surface atoms of the co-adsorbed state with the O atoms of the OH groups returning to the centre of a four-fold hollow formed by the Co sites. The distance between the H species in the transition state is 2.40 Å, with H pointing towards adjacent Co sites. The H2 formation is 107 kJ mol1 activated and 81 kJ mol1 exothermic with respect to infinite separation of the lattice OH groups. The formation of H2O is 45 kJ mol1 less activated than that of H2 formation. However, there is a thermodynamic tendency towards H2 formation because the H2O is unstable with respect to the hydration of the surface.
The dehydrogenation energy barriers for NH3, NH2 and NH on CoO(100) are 218 kJ mol1, 219 kJ mol1, and 210 kJ mol1 higher, respectively, than those calculated on the Co3O4(100) surface [16]. The binding energies of NH3, NH2 and NH on CoO(100) are 43 kJ mol1, 5 kJ mol1 and 22 kJ mol1 lower, respectively, than on the Co3O4(100) surface [16]. Clearly, the dominant factor in the loss of catalytic activity of CoO towards NHx dehydrogenation compared to that of the Co3O4(100) surface is associated to the inability of lattice oxygen to assist the hydrogen abstraction process, rather than the stability of the NHx species at the surface. The high energy barrier to hydrogen abstraction by lattice O arises from a low binding energy of H atop O sites of the CoO(100) surface. Comparatively, the binding energy of H atop O lattice sites of the CoO(100) surface is 233 kJ mol1 lower than that on the Co3O4(100) surface. NH3 dehydrogenation on the CoO(100) surface favours the formation of N2 whereas our previous study shows that NH3 decomposition on Co3O4 mainly produces NO. This observation is consistent with a highly reactive O site in the Co3O4(100) surface and a less active O site in the CoO(100). Lattice O at the CoO surface are strongly bound, with vacancy formation energy O2 Evac ¼ 386 kJ mol1 with respect to the energy of 1/2O2 in the gas phase. This is very much greater than the corresponding energy for O2 the Co3O4(100) surface, Evac ¼ 94 kJ mol1 [46]. There is evidence that the presence of CoO crystallites in Co3O4 decreases the NH3 conversion [14]. The information provided in this study indicates that CoO may not only decrease the conversion of NH3 but also affects the NO selectivity since the NO desorption energy -leaving an O vacant site-from the CoO surface is 207 kJ mol1 more endothermic than NO desorption leaving a vacant site at the Co3O4 surface [16]. The presence of lattice oxygen vacancies at the CoO surface is thermodynamically unfavourable compared to that of Co3O4. Point defects could appear at the CoO surface during the NH3 dehydrogenation in the presence of an oxygen atmosphere due to oxidation of CoO to other oxides. The existence of such defects might then affect the binding and dehydrogenation of NHx spices due to the change in coordination of surface sites. A more thorough study especially addressing the fundamentals of CoO oxidation to Co3O4 and the NH3 dehydrogenation reactions on oxidised CoO surfaces is needed in order to make more definitive comparisons with experimental observations and to assess the relative NH3 conversion and selectivity to NO. 5. Conclusion Spin-polarised density functional theory using the PBE þ U (Ueff ¼ 3.3 eV) approach within the periodic boundary conditions predicts electronic properties of bulk CoO and CoO(100) surfaces in good agreement with experimental values. Surface relaxation of the bare CoO(100) slab results in a 2% expansion of the top interlayer distance with no surface reconstruction. As a result, the surface free energy decreases from 0.88 to 0.84 J m2 (only by 0.04 J m2) after relaxation of the three top layers. The response of the surface to adsorption depends on the type of adsorbate and its mode of the adsorption. There is insignificant surface relaxation upon binding of NH3 because the interaction energy is low and the NH3 equilibrates relatively far from the surface. Relaxation of the top-most layer is more evident in the binding of NH2, and H. In the case of NH2, cobalt atoms that participate in the NH2 binding are relaxed vertically away from the second layer inducing local surface rumpling. Lattice oxygen sites bind H, forming lattice OH groups that equilibrate at 1.23 Å away from the
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
8
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
Fig. 6. Reaction pathways of N2, H2O and H2 formation on CoO(100) obtained with the CI-NEB approach. The pale grey site labelled as “V” represents an oxygen vacancy site. Energies are with respect to the non-interacting separate adsorption of initial states.
surface. Analysis of the density of states of the slab and adsorbates indicate that binding of NHx species is attributed mainly to the interaction between N states and surface states. Binding of N and H species results in the partial reduction of vicinal surface Co atoms, giving rise to the appearance of surface states near the Fermi level. Dehydrogenations of NH3, NH2 and NH are highly activated, with energy barriers in the order of 292e328 kJ mol1. The high barriers are associated with the coordination of surface O sites inhibiting the activity of CoO towards dehydrogenation. Formation of N2 is thermodynamically and kinetically preferred if NH3 dehydrogenation occurs. The NO formation via loss of lattice oxygen is hindered by a high-energy barrier as a result of the formation of oxygen vacancy sites. The hydrogenated surface is thermodynamically unstable with respect to the bare surface. The reaction of two vicinal hydroxyl groups result in the formation of H2O and H2 with energy barriers of 62 and 107 kJ mol1, respectively, indicating that H2O is kinetically favoured. The formation of H2 is exothermic by 81 kJ mol1 whereas that of H2O is endothermic by 55 kJ mol1, implying that
H2 is thermodynamically preferred. When temperature is high enough to provide the energy for the formation of both products, the selectivity is dictated by the relative thermodynamic stabilities, and H2 is expected to be the major product under equilibrium conditions. The formation of CoO in Co3O4 catalysts for ammonia oxidation is expected to be accompanied by a decrease in the overall consumption rate, due to the high barriers to NHx (x ¼ 3,2,1) dehydrogenation. At the same time, the high barrier to NO formation over CoO is expected to cause a reduction in selectivity to that product, with increased formation of N2. Acknowledgements This research was undertaken with the assistance of resources provided at the NCI National Facility systems through the National Computational Merit Allocation Scheme supported by the Australian Government. Kambiz Shojaee would like to thank the University of Sydney for providing a postgraduate scholarship to conduct this study.
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040
K. Shojaee et al. / Materials Chemistry and Physics xxx (2015) 1e9
References [1] C.R. Brundle, T.J. Chuang, D.W. Rice, X-ray photoemission study of the interaction of oxygen and air with clean cobalt surfaces, Surf. Sci. 60 (1976) 286e300. [2] T. Motohashi, Y. Katsumata, T. Ono, R. Kanno, M. Karppinen, H. Yamauchi, Synthesis and properties of CoO2, the x ¼ 0 end member of the LixCoO2 and NaxCoO2 systems, Chem. Mater. 19 (2007) 5063e5066. [3] N. Koshizaki, K. Yasumoto, T. Sasaki, Mechanism of optical transmittance change by NOx in CoO/SiO2 nanocomposites films, Sens. Actuat. B: Chem. 66 (2000) 122e124. [4] V.R. Shinde, S.B. Mahadik, T.P. Gujar, C.D. Lokhande, Supercapacitive cobalt oxide (Co3O4) thin films by spray pyrolysis, Appl. Surf. Sci. 252 (2006) 7487e7492. [5] D. Zhang, X. Li, X. Guo, C. Lai, Fabrication of cobalt oxide (Co3O4) coating by electrophoretic deposition, Mater. Lett. 126 (2014) 211e213. [6] W. Yang, Z. Gao, J. Ma, X. Zhang, J. Wang, Controlled synthesis of Co3O4 and Co3O4@MnO2 nanoarchitectures and their electrochemical capacitor application, J. Alloys Compd. 611 (2014) 171e178. [7] K. Schmidt-Szałowski, K. Krawczyk, J. Petryk, The properties of cobalt oxide catalyst for ammonia oxidation, Appl. Catal. A: Gen. 175 (1998) 147e157. [8] J. Petryk, E. Kołakowska, Cobalt oxide catalysts for ammonia oxidation activated with cerium and lanthanum, Appl. Catal. B: Environ. 24 (2000) 121e128. [9] S.P.S. Ansrew, in: C. Keleti (Ed.), Nitric Acid and Fertelizer Nitrates, Marcel Dekker, Inc., New York, 1985, pp. 31e40. [10] V.A. Sadykov, L.A. Isupova, I.A. Zolotarskii, L.N. Bobrova, A.S. Noskov, V.N. Parmon, E.A. Brushtein, T.V. Telyatnikova, V.I. Chernyshev, V.V. Lunin, Oxide catalysts for ammonia oxidation in nitric acid production: properties and perspectives, Appl. Catal. A: General 204 (1) (6 November 2000) 59e87. rez-Ramírez, E.V. Kondratenko, Mechanism of ammonia oxidation over [11] J. Pe oxides studied by temporal analysis of products, J. Catal. 250 (2007) 240e246. [12] L. Xue, C. Zhang, H. He, Y. Teraoka, Catalytic decomposition of N2O over CeO2 promoted Co3O4 spinel catalyst, Appl. Catal. B: Environ. 75 (2007) 167e174. [13] C.-W. Tanga, C.-B. Wang, S.-H. Chien, Characterization of cobalt oxides studied by FT-IR, Raman, TPR and TG-MS, Thermochim. Acta 473 (2008) 68e73. [14] W.-K. Fung, L. Ledwaba, N. Modiba, M. Claeys, E.v. Steen, Choosing a suitable support for Co3O4 as an NH3 oxidation catalyst, Catal. Sci. Technol. 3 (2013) 1905e1909. [15] G. Busca, R. Guidetti, V. Lorenzelli, Fourier-transform infrared study of the surface properties of cobalt oxides, J. Chem. Soc. Faraday Trans. 86 (1990) 989e994. [16] K. Shojaee, A. Montoya, B.S. Haynes, The role of oxygen during the catalytic oxidation of ammonia on Co3O4(100), Appl. Surf. Sci. 316 (2014) 355e365. [17] P. Broqvist, I. Panas, H. Persson, A DFT study on CO oxidation over Co3O4, J. Catal. 210 (2002) 198e206. [18] K. Omata, T. Takada, S. Kasahara, M. Yamada, Active site of substituted cobalt spinel oxide for selective oxidation of CO/H2, Part II Appl. Catal. A 146 (1996) 255e267. [19] W. Piskorz, F. Zasada, P. Stelmachowski, A. Kotarba, Z. Sojka, Decomposition of N2O over the surface of cobalt spinel: a DFT account of reactivity experiments, Catal. Today 137 (2008) 418e422. [20] D.-H. Ha, L.M. Moreau, S. Honrao, R.G. Hennig, R.D. Robinson, The oxidation of cobalt nanoparticles into Kirkendall-hollowed CoO and Co3O4: the diffusion mechanisms and atomic structural transformations, J. Phys. Chem. C 117 (2013) 14303e14312. [21] D. Wolf, Reconstruction of NaCl surfaces from a dipolar solution to the Madelung problem, Phys. Rev. Lett. 68 (1992) 3315e3319. [22] U.D. Wdowik, K. Parlinski, Lattice dynamics of CoO from first principles, Phys. Rev. B 75 (2007), 104306-1-104306-5. [23] P.A. Ignatiev, N.N. Negulyaev, D.I. Bazhanov, V.S. Stepanyuk, Doping of cobalt
[24]
[25] [26]
[27] [28] [29]
[30]
[31] [32] [33] [34] [35]
[36]
[37]
[38]
[39]
[40] [41]
[42]
[43] [44]
[45] [46]
9
oxide with transition metal impurities: Ab initio study, Phys. Rev. B 81 (2010), 235123 (1e8). A.-L. Dalverny, J.-S. Filhol, F. Lemoigno, M.-L. Doublet, Interplay between magnetic and orbital ordering in the strongly correlated cobalt oxide: a DFTþU study, J. Phys. Chem. C 114 (2010) 21750e21756. B.S. Youmbi, F. Calvayrac, Structure of CoO(001) surface from DFTþU calculations, Surf. Sci. 621 (2014) 1e6. M.R. Castell, S.L. Dudarev, G.A.D. Briggs, A.P. Sutton, Unexpected differences in the surface electronic structure of NiO and CoO observed by STM and explained by first-principles theory, Phys. Rev. B 59 (1999) 7342e7345. D.J.G. Kresse, From ultrasoft pseudopotentials to the projector augmentedwave method, Phys. Rev. B 59 (1999) 1758e1775. J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865e3868. S.L. Dudarev, G.A. Botton, S.Y. Savrasov, C.J. Humphreys, A.P. Sutton, Electronenergy-loss spectra and the structural stability of nickel oxide: an LSDAþU study, Phys. Rev. B 57 (1998) 1505e1509. L. Wang, T. Maxisch, G. Ceder, Oxidation energies of transition metal oxides within the GGAþU framework, Phys. Rev. B: Condens. Matter Mater. Phys. 73 (2006) 195107. €chl, O. Jepsen, O.K. Andersen, Improved tetrahedron method for P.E. Blo Brillouin-zone integrations, Phys. Rev. B 49 (1994) 16223. F. Birch, Finite elastic strain of cubic crystals, Phys. Rev. 71 (1947) 809e824. H.J. Monkhorst, J.D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B 13 (1976) 5188e5192. A.T.P.M. Methfessel, High-precision sampling for Brillouin-zone integration in metals, Phys. Rev. B 40 (1989) 3616e3621. nsson, A climbing image nudged elastic G. Henkelman, B.P. Uberuaga, H. Jo band method for finding saddle points and minimum energy paths, J. Chem. Phys. 113 (2000) 9901e9904. W. Jauch, M. Reehuis, H.J. Bleif, F. Kubanek, P. Pattison, Crystallographic symmetry and magnetic structure of CoO, Phys. Rev. B 64 (2001), 052102-1052102-3. K. Tomiyasu, T. Inami, N. Ikeda, Magnetic structure of CoO studied by neutron and synchrotron x-ray diffraction, Phys. Rev. B 70 (2004), 184411-1-1844116. S. Imada, T. Jo, Magnetic states of cobalt oxide and ferrites and magnetic dichroism in 2p-3d X-ray absorption spectroscopy, J. Magn. Magn. Mater. 104e107 (1992) 2001e2002. E.Z. Kurmaev, R.G. Wilks, A. Moewes, L.D. Finkelstein, S.N. Shamin, J. Kunes, Oxygen x-ray emission and absorption spectra as a probe of the electronic structure of strongly correlated oxides, Phys. Rev. B 77 (2008), 165127-1165127-5. R.C. Felton, M. Prutton, S.P. Tear, M.R. Welton-Cook, A LEED analysis of the CoO(100) surface, Surf. Sci. 88 (1979) 474e478. X. Duan, J. Ji, G. Qian, C. Fan, Y. Zhu, X. Zhou, D. Chen, W. Yuan, Ammonia decomposition on Fe(1 1 0), Co(1 1 1) and Ni(1 1 1) surfaces: a density functional theory study, J. Mol. Catal. A 357 (2012) 81e86. rez-Ramírez, J.M. Ricart, Ammonia dehyG. Novell-Leruth, A. Valc arcel, J. Pe drogenation over platinum-group metal surfaces. Structure, stability, and reactivity of adsorbed NHx species, J. Phys. Chem. C 111 (2007) 860e868. C.-C. Wang, Y.-J. Yang, J.-C. Jiang, DFT study of NHx (x ¼ 1e3) adsorption on RuO2(110) surfaces, J. Phys. Chem. C 113 (2009) 2816e2821. C.-C. Wang, S.S. Siao, J.-C. Jiang, Density functional theory study of NHx (x ¼ 0e3) and N2 adsorption on IrO2(110) surfaces, J. Phys. Chem. C 114 (2010) 18588e18593. J.L. Mackay, V.E. Henrich, Electronic properties of CoO(100) surfaces: defects and chemisorption, Phys. Rev. B 39 (1989) 6156e6168. K. Shojaee, A. Montoya, B.S. Haynes, Insight into oxygen stability and vacancy formation on Co3O4 model slabs, Comput. Mater. Sci. 72 (2013) 15e25.
Please cite this article in press as: K. Shojaee, et al., Molecular modelling of the decomposition of NH3 over CoO(100), Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.02.040