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DFT studies on H2O adsorption and its effect on CO oxidation over spinel Co3O4 (110) surface
Surface Science 605 (2011) 1962–1967
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Surface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u s c
DFT studies on H2O adsorption and its effect on CO oxidation over spinel Co3O4 (110) surface Xiang Lan Xu a,⁎, Jun Qian Li b a b
Department of Chemistry, School of Science, Nanchang University, Honggutan New District, 999 Xuefu Road, Nanchang 330031, PR China Department of Chemistry, Fuzhou University, Qi Shan Campus, 2 Xue Yuan Road, Fuzhou 350108, PR China
a r t i c l e
i n f o
Article history: Received 6 January 2011 Accepted 18 July 2011 Available online 27 July 2011 Keywords: Spinel oxide Surface active oxygen Hydrogen bond
a b s t r a c t Adsorption of H2O and its effect on CO oxidation over spinel Co3O4 (110) surface were studied by density functional theory calculations. H2O is adsorbed favorably at the octahedral cobalt (Cooct) site through O atom on the surface. Hydrogen bonding interaction between 1s orbitals of H atoms in H2O and the 2p orbitals of surface active oxygen sites plays a key role for H2O adsorption. The inhibition effect of H2O adsorption on the CO oxidation over the surfaces is attributed to the competition between H2O and CO molecules for the surface twofold coordinated oxygen site. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The interaction of H2O with transition-metal oxide surfaces plays an extremely important role in catalysis, surface chemistry, gas sensors, photochemistry, and electrochemistry. The influence of H2O should be considered if a catalyst is used for practical applications, due to the usual presence of H2O under the real conditions. In the group of transition-metal oxides, the spinel-type tricobalt tetraoxide (Co3O4) is of special interest, with many applications in different areas, e.g., sensors [1,2], and batteries [3,4], heterogeneous catalysts. The Co3O4based catalysts have a potential for the hydrocarbon oxidation [5–9] and as alcohol sensor materials [10]. It is well known that Co3O4 and its doped or supported systems are active for CO oxidation at low temperatures [11–20]. Although some of these materials are active even at sub-zero temperatures for the reaction, their high sensitivity to even trace amount of moisture severely limits its practical applications. Petitto et al. explored the surface reactivity toward H2O and reported that Co3O4 is possible to be hydroxylated by H2O [21]. The negative effect of H2O on the oxidation of CO over Co3O4-based catalysts has been investigated in previous studies. Cunningham et al. found that the oxidation of CO over Co3O4 under dry conditions can be observed at temperatures as low as −54 °C, while Co3O4 rapidly deactivates without sufficient drying [22]. Thormählen et al. studied the oxidation of CO over a cobalt-aluminate spinel catalyst with the spinel composition of Co3O4, Co2AlO4 and CoAl2O4 [23]. It is observed that H2O completely inhibits CO oxidation until the temperature reaches 200 °C. The reasons for the inhibition are attributed to the
⁎ Corresponding author. Tel.: + 86 131 7783 8518; fax: + 86 791 396 9064. E-mail address: [email protected] (X.L. Xu). 0039-6028/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2011.07.013
combination of H2O adsorption at lower temperatures and formation of OH− groups at higher temperatures. Grillo et al. investigated also the effect of humidity on CO oxidation over a Co3O4 powder surface and reported that H2O occupy preferably the coordinatively unsaturated cobalt cations, the active sites, than CO and thus inhibit CO oxidation at low temperatures. However, at higher temperatures, the effect disappears and CO2 can form again in steam conditions [24]. Xie et al. reported that Co3O4 nanorods predominantly exposing their (110) planes, which not only catalyze CO oxidation at temperatures as low as −77 °C, but also enhance the stability in the presence of H2O [25]. They attributed these improvements to the exposing of high density catalytically active Co3+ sites on the nanorod surface. Recently, Jia et al. found that adsorption of H2O molecules on the surface of Co3O4–SiO2 catalyst leads to the unusual U-shape activity curve toward CO oxidation for the catalyst [26]. However, the negative effect of H2O on CO oxidation over Co3O4 is still unclear. Does H2O adsorb at the same active sites as CO? If it does, which one is preferred? Up to now, no direct evidences on these issues have been reported. The (110) crystal face was reported to be one of naturally exposed surfaces of Co3O4 [27–29], and plays an essential role for the catalytic oxidation performance of nano Co3O4 catalysts for CO oxidation [25,30], ethylene oxidation [31] and methane combustion [32]. There are two types of surface terminations for the Co3O4 (110) spinel surface, labeled as A and B [33]. In this work, Co3O4 (110)-B surface is chosen as the surface model. As shown in Fig. 1, two fourfold coordinated Co oct cations in the octahedral sites, two twofold coordinated (O 2f) and two threefold coordinated (O 3f) oxygen anions are exposed in the outermost surface. The O 2f anion in the outermost atomic layer is bonded to one Co tet cation and one Co oct cation, while the O 3f anion in the third atomic layer has three Co oct cations as the nearest neighbors.
X.L. Xu, J.Q. Li / Surface Science 605 (2011) 1962–1967
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Co3O4, the spin directions of two Cotet atoms in the same slab layer are set as spin up and down, respectively. These starting values will be subsequently optimized during the calculation. The adsorption and dissociation energies are calculated as follows: Eads = diss = Eadsorbate + Esurface –Eadsorbate = surface
Fig. 1. Side view of the ideal Co3O4 (110)-B surface in 1 × 1 surface unit cell. The substituted surface is created by Al ions replacing two Cooct sites in the surface layer.
More theoretical work [33–40] are devoted to the bulk or/and surface of Co3O4 due to its extensive applications. However, little theoretical work has been published on H2O adsorption and effect on Co3O4 catalytic performance. Zasada et al. [41] investigated H2O adsorption on the Co3O4 (110)-A surface, whereas the Co3O4 (110)-B surface was not considered. In the present study, we investigated the reactivity of surface Co oct sites on the Co3O4 (110)-B surface toward H2O, using DFT calculations, and explored the interaction mechanism by the analysis of density of states and Mulliken population. The effect of Al introduction into Co3O4 (110) surface on the activity for H2O adsorption was investigated in this work by replacing the surface octahedral cobalt (Co oct) atoms. With this way, it is expected to determine further the role of Co oct sites on the reaction with H2O. Effect of H2O adsorption on the CO oxidation performance over the surface was investigated. We expect that our results can contribute to the understanding of the H2O effect on the catalytic activity of Co3O4 for CO oxidation. 2. Calculation details The partially substituted Co3O4 (110)-B surface, which is due to the substitution of two surface Cooct cations by two Al cations (Al-sub-Cooct), is chosen as the substituted surface model. The unsubstituted and partially substituted (110)-B surfaces are modeled using a periodically repeated 1 × 1 slab with five layers, with atomic composition of Co14O20 and Al2Co12O20, respectively. The lattice parameters are a = 8.084 Å, b = 5.716 Å and c = 18.018 Å. The middle layer is fixed and the other four layers are fully relaxed. The vacuum region thickness between the slabs is 12 Å. Surface atomic relaxations and surface energies of the Co3O4 (110) surfaces can be seen in our recent work [33]. For simplification, the (110)-B is abbreviated to (110) in this paper. All calculations are performed with the DMol3 program package [42] in Materials Studio of Accelrys Inc. The DFT-slab approach with GGA-PBE exchange-correlation functional [43] and a double-numerical basis with polarization functions (DNP) are adopted. All electron basis sets are used for H and O atoms. DFT semi-core pseudopots (DSPP) [44] are used for Co atoms, where the outer electrons (3d 74s 2) of Co atoms are treated as valence electrons and the remaining electrons are replaced by a simple potential including some degree of relativistic effects. A fermi smearing of 0.01 hartree and a global cutoff radius of 4.5 Å are used. The self-consistent field convergence criterion is set to be an energy change of 10 −6 hartree. The convergence criterion of optimal geometry based on the energy, force and displacement convergence are 1 × 10− 5 hartree, 2 × 10− 3 hartree/Å and 5 × 10− 3 Å, respectively. In the calculations the spin-polarized approach is used. The Brillouin zone integrations are performed using a 3 × 4 × 1 Monkhorst–Pack grid for (110) surfaces. The initial spin values of Cooct, Cotet and Al atoms are set as 0, 3 and 0, respectively. To keep the antiferromagnetic properties of
ð1Þ
where Eadsorbate, Esurface and Eadsorbate/surface refer to the total energies of isolated adsorbates, the bare surface and the system formed by the adsorbate and the surface, respectively. By this definition, a positive value corresponding to an exothermic process indicates a stable adsorption. The calculated bond length of O\H and bond angle of H\O\H are 0.954 Å and 104.0°, highly consistent with the corresponding experimental value at 0.960 Å and 104.5° [45], respectively. The calculated HO\H bond dissociation energy is 5.227 eV, close to the experimental value 5.169 eV [46]. The results confirm that the H2O molecule is described accurately by the present method. 3. Results and discussion 3.1. Adsorption and dissociation of H2O on the Co3O4 (110) surface H2O molecule has a large dipole moment and often acts as a good electron donor and a Lewis base due to its lone-pair electrons on the oxygen atom. It can be predicted that its oxygen atom will react with the Co oct cation that is a Lewis acid site, and its two hydrogen atoms will interact with the surface oxygen atoms by hydrogen bonding. The assumed states for H2O at the Co oct site with parallel and tilted orientations are optimized and shown in Fig. 2, involving the molecular plane of H2O parallel to the (110) surface or having a certain tilted angle with the surface, respectively. H2O is chemisorbed at the Co oct site with its oxygen atom along the dangling bond direction. The interaction between the oxygen lonepair electrons and the empty orbitals of Co dangling bond results in the formation of a Co\O coordination bond, with lengths of 1.949 Å and 1.972 Å for parallel and tilted states, respectively. For the parallel state, H2O is nearly parallel to the surface, with the angle between them being 10.5°. Furthermore, two hydrogen bonds, namely O 2f⋯H and O 3f⋯H, are formed by the interaction of the two hydrogen atoms of H2O molecule with the adjacent O 2f and O 3f atoms on the surface at lengths of 1.814 Å and 2.015 Å, respectively. For the tilted one, one O\H bond of H2O is rotated by 133° relative to the parallel state and only one hydrogen bond O 2f⋯H is formed with the length of 1.557 Å. As shown in Table 1, the adsorption energies for the parallel and tilted states are 1.612 eV and 1.294 eV, respectively. It indicates that the former is more stable than the latter. To explore the effect of onsite Coulomb interactions of 3d states for Co atoms, the DFT + U calculations were performed with the Hubbard parameter U for Co atoms equal to 2.5 eV, and the corresponding adsorption energies were listed in the parentheses in Table 1. The adsorption energies (1.185 eV and 0.751 eV) from DFT + U calculations for parallel and tilted states are smaller than the DFT results, but the ordering of adsorption energy for two adsorption states remains same for two methods. Though on-site Coulomb interactions should lead to some differences for absolute values of the results in this work, the effect on the results would be systematic. On the other hand, based on our previous work [36], effect of H2O adsorption on the CO oxidation performance over the surface was also investigated. The DFT approach in this work without accounting for on-site Coulomb interactions was performed to keep comparability between H2O and CO adsorption on the Co3O4(110) surface. Ionescu et al. [47] studied H2O adsorption on the γ-Al2O3 (110) surface and reported that the 11% difference occurs for the dissociation energy using the nonstoichiometric surface model compared with the stoichiometric one. In the present work, the 5-
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Fig. 2. H2O adsorption geometries at the Cooct site on the Co3O4 (110) surface (a) parallel one, (b) tilted one. Only the surface and subsurface layers are depicted. The unit of distances is Å.
layer Co14O20 slab model for Co3O4 (110) surface does not agree with the Co3O4 stoichiometry. To explore the difference of the adsorption energies between nonstoichiometric and stoichiometric slab models in this work, the 6-layer stoichiometric Co18O24 slab is used to calculate H2O adsorption at the Co oct site with parallel orientations and all atomic positions are optimized. The results show that the calculated adsorption energy is 1.633 eV for H2O on the stoichiometric Co3O4 (110) surface, close to the value of 1.612 eV on the nonstoichiometric surface. The difference of adsorption energy for stoichiometric and nonstoichiometric slab models is proved to be insignificant. The change of Mulliken population in Table 1 shows that H2O donates electrons to the substrate and becomes positively charged. For the parallel state, the H2O molecule transfers 0.282 electrons to the surface, with 71% being accumulated at the O 2f (0.116 e) and O 3f (0.083 e) atoms as the nearest neighbors to H atoms of H2O. For the tilted state, the O 2f atom in the hydrogen bonding interaction retains 68% electrons of the donated electrons from the adsorbed H2O. For both adsorbed states, the Cooct site accepts the same electrons (0.074 e) from the O atom of H2O. It can be referred that the adsorption ability for H2O on the Cooct site relates to the ratio of the accumulated charge on surface hydrogen-bonding oxygen atoms and the donated charge from the adsorbed H2O. The higher this ratio, the more stable the molecular adsorption system. In other words, hydrogen bonding plays an important role on the H2O adsorption. Zasada et al. [41] found that the H2O molecules dissociate on fourfold coordinated Co oct cations upon adsorption on the Co3O4 (110)-A surface and lead to the surface hydroxylation by means of DFT calculations. To explore the possibility for H2O hydroxylation on the Co3O4 (110)-B surface, the complete linear synchronous transit and quadratic synchronous transit (LST/QST) method [48] was used for searching transition states for H2O decomposition reaction on the Co3O4 (110)-B surface. The pathways and energy profiles of the reaction denoted by H2O(ad) → OH(ad) + H(ad) are determined and presented in Fig. 3, which starts from the parallel and tilted adsorbed state for H2O. Two initial states are denoted by IS1 and IS2, respectively. The energy IS1 is lower than IS2 with the energy difference being 0.318 eV. In the final state, the OH − binds to the Co oct site and the proton (H +) binds to a nearby surface O 2f atom. In fact,
the dissociated process of H2O on the Co3O4 (110) perfect surface is a competition for H + between the O 2f atom and O of H2O. Beginning from IS1, the reaction is found to be an endothermic process with the enthalpy change of 0.144 eV and a low activation energy of 0.216 eV. The reaction initiated from IS2 has a barrier of 0.176 eV and releases heat of 0.174 eV, due to the cooperative effect that occurs for the hydrogen bond interaction of O 2f⋯H and the free rotation of OH in H2O. Furthermore, the barrier for the process from FS to IS1 is calculated to be only 0.061 eV. The results indicate that processes from IS2 to FS and then from FS to IS1 is easy. Consequently, H2O dissociation is not preferable, and IS1 is the major species for H2O on the Co3O4 (110)-B surface. It is worth mentioning that our results doesn't mean that the surface hydroxylation is impossible on Co3O4 (110) surfaces. The surface hydroxylation occurs on the Co3O4 (110)A surface due to favorable dissociation process of H2O [41]. 3.2. Adsorption of H2O on Al-sub-Co oct Co3O4 (110) surface The partially substituted surface, namely Al-sub-Co oct surface, was employed to further determine the role of Co oct ions on the interaction with H2O. The corresponding geometries and results for H2O adsorption through its oxygen atom on the Al-sub-Co oct Co3O4 (110) surface were shown in Fig. 4 and Table 1. On the Al-sub-Co oct surface, H2O is adsorbed both molecularly and dissociatively at the Al site with parallel (Fig. 4a) and tilted orientations (Fig. 4b). The adsorption and dissociation energies shown in Table 1 are 1.663 eV and 1.753 eV, respectively. The former is close to the case of the unsubstituted surface (1.612 eV). For the molecular adsorption, the Al\O bond has a length of 1.922 Å, and one O 2f⋯H hydrogen bond forms at a length of 1.679 Å. Two O\H bonds of H2O are stretched to be 1.005 Å and 0.968 Å. For the dissociative adsorption, the geometry in Fig. 4b shows an OH group bonded to the Al site at the length of 1.795 Å and the O 2f atom adjacent to another Al
Table 1 Adsorption energies (Eads), angles and changes of Mulliken charge (Δq) for single H2O at the octahedral site on the Co3O4 (110) and Al-sub-Cooct (110) surfaces. Al-sub-Cooct
Unsubstituted
Eads/eV θHOH/deg θoct M OH/deg Δqadsorbate oct ΔqM 2f ΔqO 3f ΔqO
Co-P
Co-T
Al-P
Al-T
1.612(1.185) 108 104 0.282 − 0.074 − 0.116 − 0.083
1.294(0.751) 105 109 0.246 − 0.074 − 0.167 0.014
1.663 109 105 0.140 0.079 − 0.148 − 0.066
1.753 – – Diss 0.111 − 0.087 − 0.004
“Dissociation” is abbreviated to “Diss”. The letters P and T denote the parallel and tilted modes, respectively. The corresponding adsorption energies calculated from GGA+U method are listed in the parentheses. For Δq, positive and negative values represent losing and getting electrons, respectively.
Fig. 3. Reaction pathways and energy profiles for H2O decomposition on the Co3O4 (110) surface. A Cooct atom, an O2f atom on the surface and a H2O adsorbate are depicted. Ea: activation energy; ΔH: enthalpy change.
X.L. Xu, J.Q. Li / Surface Science 605 (2011) 1962–1967
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Fig. 4. H2O adsorption geometries at the Aloct site on the Al-sub-Cooct (110) surface (a) parallel one (b) tilted one. Only the surface and subsurface layers are depicted. The unit of distances is Å.
site is protonated. Ionescu et al. [47] also found that H2O is dissociated at the octahedral Al site along with the O 2f atom protonated on the γ-Al2O3 (110) surface. This is resulted from that the outermost surface of the γ-Al2O3 (110) slab also exposes the octahedral Al, O 2f and O 3f atoms as the Al-sub-Co oct Co3O4 (110) surface. These results indicate that the replacement of Co by Al has a slight positive effect on the molecular adsorption of H2O and brings about the dissociative adsorption of H2O. The change of Mulliken population in Table 1 shows that for the molecular adsorption of H2O on the Al-sub-Co oct surface, H2O donates 0.140 e to the surface, which is transferred totally to the O 2f site. At the same time, the Al site loses electrons, which is different from the result that the Co oct site gains electrons on the unsubstituted surface. 3.3. Nature of the Co3O4 (110) surface reactivity for H2O In Fig. 5, the total and partial electronic density of states (DOS) were depicted and compared for the unsubstituted and Al-sub-Co oct Co3O4 (110) surfaces before and after H2O parallel adsorption. After H2O adsorption, a new peak at −9.5 eV relative to the Fermi level at energy being zero is clearly observed (a1 and a2 in Fig. 5), which is the main modification of the total DOS. The partial DOS in Fig. 5 a3–a6
illustrates that the peak at − 9.5 eV is derived from three kinds of orbital overlaps, namely between 2p orbitals of O in H2O and 1s orbitals of H, 1s orbitals of H and 2p orbitals of O 2f and O 3f, and 3d orbitals of Co oct and 2p orbitals of O in H2O. Fig. 5 a3 and a6 shows that the OH states of the adsorbed H2O are located at −9.5 eV and −6.7 eV, contributed from 1s orbitals of H atoms interacting with p and sp orbitals of O atom, respectively. The interaction between 3d orbitals of the Co oct atom and the 2p orbitals of H2O is in the range from −9.5 eV to − 1.7 eV, leading to the formation of the Co oct\O bond. In Fig. 5 a3 and a4, it is noted that the formation of O 2f⋯H and O 3f⋯H hydrogen bonds is due to the interaction between the 2p orbitals of O 2f and O 3f atoms and the 1s orbitals of H atoms. In the case of H2O adsorption on the Al-sub-Co oct (110) surface, comparing b1 with b2 in Fig. 5, the new peak of the total DOS is at −9.6 eV after H2O adsorption. The partial DOS in Fig. 5 b3–b6 shows that this new peak is attributed to three kinds of orbital interactions, namely between O 2p orbitals and H 1s orbitals in H2O, H 1s orbitals and O 2f 2p orbitals, Al 3s orbitals and O 2p orbitals in H2O. From Fig. 5 b3 and b6, it can be seen that the OH states of the adsorbed H2O are at −9.6 eV and − 7.3 eV, which is contributed from H 1s orbitals interacting with O 2p orbitals. The interaction between Al 3s orbitals and O 2p orbitals in H2O is located at − 7.3 eV, as indicated by b5 and
Fig. 5. The total and partial electronic density of states (DOS) are depicted for Co3O4 (110) surfaces before and after H2O parallel adsorption. a1 represents the total DOS of clean unsubstituted Co3O4 (110) surface; a2 and a3-a6 represent orderly the total DOS of adsorbed unsubstituted surface and the partial DOS of 2p orbitals of surface O2f and O3f atoms, 1s orbitals of H in H2O, 3d orbitals of the Cooct atom, 2s and 2p orbitals of O in H2O for H2O parallel adsorption on the unsubstituted surface; b1 represents the total DOS of clean Al-subCooct (110) surface; b2 and b3-b6 represent orderly the total DOS of adsorbed Al-sub-Cooct (110) surface and the partial DOS of 2p orbitals of surface O2f atoms, 1s orbitals of H in H2O, 3s orbitals of the Al atom, 2p orbitals of O in H2O for H2O parallel adsorption on the Al-sub-Cooct (110) surface.
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b6 in Fig. 5, which results in the formation of Al\O bond. Fig. 5 b3 and b4 shows that the O 2f 2p orbitals interact with the H 1s orbitals at −9.6 eV, which leads to the formation of O 2f⋯H hydrogen bond. Comparing a with b in Fig. 5, the formation of Al\O and Co oct\O bonds are ascribed to the s–p and d–p interactions, respectively. On the other hand, for H2O adsorption on the Al-sub-Co oct and unsubstituted Co3O4 (110) surfaces, the same mechanism of hydrogen bonding occurred, that is 2p orbitals of surface active O atom interacting with the 1s orbitals of H atoms. Furthermore, the change of Mulliken population in Table 1 shows that for H2O adsorption on the Al-sub-Co oct and unsubstituted surfaces, Al lost electrons but Co oct obtain, with the major part of the donated electrons from H2O being accumulated on the surface active O atoms and thus contributing to the formation of hydrogen bonds. Though the bonding natures of the coordination bonds between adsorption sites and H2O are different, the Al-sub-Cooct and unsubstituted surfaces have similar adsorption ability for H2O molecules due to the similar adsorption energies (1.612 eV vs. 1.663 eV). These results demonstrated that the strong oxidative activity of the Co oct site is not the major cause for the reactivity of the Co3O4 (110) surface for H2O. However, the activity of surface oxygen interacting with H atoms contributes predominantly to the reactivity of the Co3O4 (110) surface for H2O.
3.4. Effect of H2O adsorption on CO oxidation over Co3O4 (110) surface We reported previously [36] that CO chemisorbs preferably at Co oct and O 2f sites on the Co3O4 (110) surface by DFT calculations as this work, with adsorption energies of 1.850 eV and 0.917 eV, respectively. The removal of the O 2f atom was proposed to be a key step for CO oxidation, which is also proposed in other works [20,38]. The easy removal of the O 2f atom and simultaneously high oxidative activity of the adjacent Co oct site were proposed to be responsible for the activity of the Co3O4 (110) surface for CO oxidation. Based on the above discussion, H2O adsorption on the surface leads to a slight change of the Co oct valence state because the Co oct site gains small amount of electrons. In other words, the oxidative ability of the Co oct site might remain intact. To explore the effect of H2O adsorption at the Co oct site on CO oxidation, the simultaneous adsorption of H2O at the Co oct site and CO at the O 2f site was studied at the full coverage for Co oct and O 2f sites, namely Co oct–(H2O)O 2f–(CO) configuration. The optimized structure was illustrated in Fig. 6. It is noted that CO forms bonds with both O 2f and Co oct sites. The bond lengths of C\O 2f and C\Co oct are 1.309 Å and 1.862 Å, which is longer and shorter than the corresponding values (1.262 Å and 1.937 Å) for the case of individual CO adsorption at the O 2f site [36], respectively. This indicates that CO interacting with O 2f and Co oct atoms is weakened and strengthened due to H2O adsorption, respectively. As a consequence, the process of CO2 formation by abstracting O 2f atoms is hindered to a certain extent. On the other hand, the adsorption of CO also hindered H in H2O and O 2f atoms from forming hydrogen bond. It is confirmed here that a competition to interact with the O 2f atom occurs between CO and H2O. Moreover, H2O can adsorb at the Co oct site through a O 3f⋯H hydrogen bond with the length of 1.645 Å in the presence of CO at the O 2f atom. Generally, the energy associated with hydrogen bond is close to the value of intermolecular forces, which suggests that low energy barrier is required for breaking hydrogen bonds. It can be assumed that the hydrogen bond O 3f⋯H between H2O and the Co3O4 surface would be weakened and H2O tends to be desorbed as the temperature increases. Consequently, the hindrance effect of H2O on CO oxidation disappears. These results may provide an explanation for the previous experimental observations that H2O inhibits CO oxidation on the Co3O4 surface at low temperatures but not at high temperatures [24]. The above discussion suggests that the inhibition effect can be caused by the competition between H2O and CO molecules for the O 2f site.
Fig. 6. Side view of optimized coadsorption structures of Cooct–(H2O)O2f–(CO) (a) 2 × 2 adsorbed surface, (b) surrounding atoms and distances between them of the Co1 atom. All distances are in Å. Only selected atoms on the surface are depicted.
4. Conclusions In this work, H2O adsorption on the Co3O4 (110) surface was studied using DFT calculations and supercell models. It is found that H2O is adsorbed favorably at Co oct sites on the surface, by O bonding with Co oct sites and hydrogen bond formation between surface active oxygen atoms (O 2f or O 3f) and H. Hydrogen bonding plays a key role for H2O adsorption at the Co oct sites. The introduction of Al to replace the Co oct site on the Co3O4 (110) surface is found to have a slight positive effect on the molecular adsorption of H2O. Moreover, H2O adsorption at the Co oct site by hydrogen bonding to the O 2f site presents an inhibition effect on CO interaction with the O 2f site, and then further hinders the CO2 formation by abstracting O 2f atoms to a certain extent.
Acknowledgments The authors gratefully acknowledge Dr. Xiang Wang in revising the manuscript. This work was supported by the Natural Science Foundation of China (20673019, 20303002), the Key Project of the Fujian Province (2005HZ01-2-6).
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Contents lists available at ScienceDirect
Surface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u s c
DFT studies on H2O adsorption and its effect on CO oxidation over spinel Co3O4 (110) surface Xiang Lan Xu a,⁎, Jun Qian Li b a b
Department of Chemistry, School of Science, Nanchang University, Honggutan New District, 999 Xuefu Road, Nanchang 330031, PR China Department of Chemistry, Fuzhou University, Qi Shan Campus, 2 Xue Yuan Road, Fuzhou 350108, PR China
a r t i c l e
i n f o
Article history: Received 6 January 2011 Accepted 18 July 2011 Available online 27 July 2011 Keywords: Spinel oxide Surface active oxygen Hydrogen bond
a b s t r a c t Adsorption of H2O and its effect on CO oxidation over spinel Co3O4 (110) surface were studied by density functional theory calculations. H2O is adsorbed favorably at the octahedral cobalt (Cooct) site through O atom on the surface. Hydrogen bonding interaction between 1s orbitals of H atoms in H2O and the 2p orbitals of surface active oxygen sites plays a key role for H2O adsorption. The inhibition effect of H2O adsorption on the CO oxidation over the surfaces is attributed to the competition between H2O and CO molecules for the surface twofold coordinated oxygen site. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The interaction of H2O with transition-metal oxide surfaces plays an extremely important role in catalysis, surface chemistry, gas sensors, photochemistry, and electrochemistry. The influence of H2O should be considered if a catalyst is used for practical applications, due to the usual presence of H2O under the real conditions. In the group of transition-metal oxides, the spinel-type tricobalt tetraoxide (Co3O4) is of special interest, with many applications in different areas, e.g., sensors [1,2], and batteries [3,4], heterogeneous catalysts. The Co3O4based catalysts have a potential for the hydrocarbon oxidation [5–9] and as alcohol sensor materials [10]. It is well known that Co3O4 and its doped or supported systems are active for CO oxidation at low temperatures [11–20]. Although some of these materials are active even at sub-zero temperatures for the reaction, their high sensitivity to even trace amount of moisture severely limits its practical applications. Petitto et al. explored the surface reactivity toward H2O and reported that Co3O4 is possible to be hydroxylated by H2O [21]. The negative effect of H2O on the oxidation of CO over Co3O4-based catalysts has been investigated in previous studies. Cunningham et al. found that the oxidation of CO over Co3O4 under dry conditions can be observed at temperatures as low as −54 °C, while Co3O4 rapidly deactivates without sufficient drying [22]. Thormählen et al. studied the oxidation of CO over a cobalt-aluminate spinel catalyst with the spinel composition of Co3O4, Co2AlO4 and CoAl2O4 [23]. It is observed that H2O completely inhibits CO oxidation until the temperature reaches 200 °C. The reasons for the inhibition are attributed to the
⁎ Corresponding author. Tel.: + 86 131 7783 8518; fax: + 86 791 396 9064. E-mail address: [email protected] (X.L. Xu). 0039-6028/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2011.07.013
combination of H2O adsorption at lower temperatures and formation of OH− groups at higher temperatures. Grillo et al. investigated also the effect of humidity on CO oxidation over a Co3O4 powder surface and reported that H2O occupy preferably the coordinatively unsaturated cobalt cations, the active sites, than CO and thus inhibit CO oxidation at low temperatures. However, at higher temperatures, the effect disappears and CO2 can form again in steam conditions [24]. Xie et al. reported that Co3O4 nanorods predominantly exposing their (110) planes, which not only catalyze CO oxidation at temperatures as low as −77 °C, but also enhance the stability in the presence of H2O [25]. They attributed these improvements to the exposing of high density catalytically active Co3+ sites on the nanorod surface. Recently, Jia et al. found that adsorption of H2O molecules on the surface of Co3O4–SiO2 catalyst leads to the unusual U-shape activity curve toward CO oxidation for the catalyst [26]. However, the negative effect of H2O on CO oxidation over Co3O4 is still unclear. Does H2O adsorb at the same active sites as CO? If it does, which one is preferred? Up to now, no direct evidences on these issues have been reported. The (110) crystal face was reported to be one of naturally exposed surfaces of Co3O4 [27–29], and plays an essential role for the catalytic oxidation performance of nano Co3O4 catalysts for CO oxidation [25,30], ethylene oxidation [31] and methane combustion [32]. There are two types of surface terminations for the Co3O4 (110) spinel surface, labeled as A and B [33]. In this work, Co3O4 (110)-B surface is chosen as the surface model. As shown in Fig. 1, two fourfold coordinated Co oct cations in the octahedral sites, two twofold coordinated (O 2f) and two threefold coordinated (O 3f) oxygen anions are exposed in the outermost surface. The O 2f anion in the outermost atomic layer is bonded to one Co tet cation and one Co oct cation, while the O 3f anion in the third atomic layer has three Co oct cations as the nearest neighbors.
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Co3O4, the spin directions of two Cotet atoms in the same slab layer are set as spin up and down, respectively. These starting values will be subsequently optimized during the calculation. The adsorption and dissociation energies are calculated as follows: Eads = diss = Eadsorbate + Esurface –Eadsorbate = surface
Fig. 1. Side view of the ideal Co3O4 (110)-B surface in 1 × 1 surface unit cell. The substituted surface is created by Al ions replacing two Cooct sites in the surface layer.
More theoretical work [33–40] are devoted to the bulk or/and surface of Co3O4 due to its extensive applications. However, little theoretical work has been published on H2O adsorption and effect on Co3O4 catalytic performance. Zasada et al. [41] investigated H2O adsorption on the Co3O4 (110)-A surface, whereas the Co3O4 (110)-B surface was not considered. In the present study, we investigated the reactivity of surface Co oct sites on the Co3O4 (110)-B surface toward H2O, using DFT calculations, and explored the interaction mechanism by the analysis of density of states and Mulliken population. The effect of Al introduction into Co3O4 (110) surface on the activity for H2O adsorption was investigated in this work by replacing the surface octahedral cobalt (Co oct) atoms. With this way, it is expected to determine further the role of Co oct sites on the reaction with H2O. Effect of H2O adsorption on the CO oxidation performance over the surface was investigated. We expect that our results can contribute to the understanding of the H2O effect on the catalytic activity of Co3O4 for CO oxidation. 2. Calculation details The partially substituted Co3O4 (110)-B surface, which is due to the substitution of two surface Cooct cations by two Al cations (Al-sub-Cooct), is chosen as the substituted surface model. The unsubstituted and partially substituted (110)-B surfaces are modeled using a periodically repeated 1 × 1 slab with five layers, with atomic composition of Co14O20 and Al2Co12O20, respectively. The lattice parameters are a = 8.084 Å, b = 5.716 Å and c = 18.018 Å. The middle layer is fixed and the other four layers are fully relaxed. The vacuum region thickness between the slabs is 12 Å. Surface atomic relaxations and surface energies of the Co3O4 (110) surfaces can be seen in our recent work [33]. For simplification, the (110)-B is abbreviated to (110) in this paper. All calculations are performed with the DMol3 program package [42] in Materials Studio of Accelrys Inc. The DFT-slab approach with GGA-PBE exchange-correlation functional [43] and a double-numerical basis with polarization functions (DNP) are adopted. All electron basis sets are used for H and O atoms. DFT semi-core pseudopots (DSPP) [44] are used for Co atoms, where the outer electrons (3d 74s 2) of Co atoms are treated as valence electrons and the remaining electrons are replaced by a simple potential including some degree of relativistic effects. A fermi smearing of 0.01 hartree and a global cutoff radius of 4.5 Å are used. The self-consistent field convergence criterion is set to be an energy change of 10 −6 hartree. The convergence criterion of optimal geometry based on the energy, force and displacement convergence are 1 × 10− 5 hartree, 2 × 10− 3 hartree/Å and 5 × 10− 3 Å, respectively. In the calculations the spin-polarized approach is used. The Brillouin zone integrations are performed using a 3 × 4 × 1 Monkhorst–Pack grid for (110) surfaces. The initial spin values of Cooct, Cotet and Al atoms are set as 0, 3 and 0, respectively. To keep the antiferromagnetic properties of
ð1Þ
where Eadsorbate, Esurface and Eadsorbate/surface refer to the total energies of isolated adsorbates, the bare surface and the system formed by the adsorbate and the surface, respectively. By this definition, a positive value corresponding to an exothermic process indicates a stable adsorption. The calculated bond length of O\H and bond angle of H\O\H are 0.954 Å and 104.0°, highly consistent with the corresponding experimental value at 0.960 Å and 104.5° [45], respectively. The calculated HO\H bond dissociation energy is 5.227 eV, close to the experimental value 5.169 eV [46]. The results confirm that the H2O molecule is described accurately by the present method. 3. Results and discussion 3.1. Adsorption and dissociation of H2O on the Co3O4 (110) surface H2O molecule has a large dipole moment and often acts as a good electron donor and a Lewis base due to its lone-pair electrons on the oxygen atom. It can be predicted that its oxygen atom will react with the Co oct cation that is a Lewis acid site, and its two hydrogen atoms will interact with the surface oxygen atoms by hydrogen bonding. The assumed states for H2O at the Co oct site with parallel and tilted orientations are optimized and shown in Fig. 2, involving the molecular plane of H2O parallel to the (110) surface or having a certain tilted angle with the surface, respectively. H2O is chemisorbed at the Co oct site with its oxygen atom along the dangling bond direction. The interaction between the oxygen lonepair electrons and the empty orbitals of Co dangling bond results in the formation of a Co\O coordination bond, with lengths of 1.949 Å and 1.972 Å for parallel and tilted states, respectively. For the parallel state, H2O is nearly parallel to the surface, with the angle between them being 10.5°. Furthermore, two hydrogen bonds, namely O 2f⋯H and O 3f⋯H, are formed by the interaction of the two hydrogen atoms of H2O molecule with the adjacent O 2f and O 3f atoms on the surface at lengths of 1.814 Å and 2.015 Å, respectively. For the tilted one, one O\H bond of H2O is rotated by 133° relative to the parallel state and only one hydrogen bond O 2f⋯H is formed with the length of 1.557 Å. As shown in Table 1, the adsorption energies for the parallel and tilted states are 1.612 eV and 1.294 eV, respectively. It indicates that the former is more stable than the latter. To explore the effect of onsite Coulomb interactions of 3d states for Co atoms, the DFT + U calculations were performed with the Hubbard parameter U for Co atoms equal to 2.5 eV, and the corresponding adsorption energies were listed in the parentheses in Table 1. The adsorption energies (1.185 eV and 0.751 eV) from DFT + U calculations for parallel and tilted states are smaller than the DFT results, but the ordering of adsorption energy for two adsorption states remains same for two methods. Though on-site Coulomb interactions should lead to some differences for absolute values of the results in this work, the effect on the results would be systematic. On the other hand, based on our previous work [36], effect of H2O adsorption on the CO oxidation performance over the surface was also investigated. The DFT approach in this work without accounting for on-site Coulomb interactions was performed to keep comparability between H2O and CO adsorption on the Co3O4(110) surface. Ionescu et al. [47] studied H2O adsorption on the γ-Al2O3 (110) surface and reported that the 11% difference occurs for the dissociation energy using the nonstoichiometric surface model compared with the stoichiometric one. In the present work, the 5-
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Fig. 2. H2O adsorption geometries at the Cooct site on the Co3O4 (110) surface (a) parallel one, (b) tilted one. Only the surface and subsurface layers are depicted. The unit of distances is Å.
layer Co14O20 slab model for Co3O4 (110) surface does not agree with the Co3O4 stoichiometry. To explore the difference of the adsorption energies between nonstoichiometric and stoichiometric slab models in this work, the 6-layer stoichiometric Co18O24 slab is used to calculate H2O adsorption at the Co oct site with parallel orientations and all atomic positions are optimized. The results show that the calculated adsorption energy is 1.633 eV for H2O on the stoichiometric Co3O4 (110) surface, close to the value of 1.612 eV on the nonstoichiometric surface. The difference of adsorption energy for stoichiometric and nonstoichiometric slab models is proved to be insignificant. The change of Mulliken population in Table 1 shows that H2O donates electrons to the substrate and becomes positively charged. For the parallel state, the H2O molecule transfers 0.282 electrons to the surface, with 71% being accumulated at the O 2f (0.116 e) and O 3f (0.083 e) atoms as the nearest neighbors to H atoms of H2O. For the tilted state, the O 2f atom in the hydrogen bonding interaction retains 68% electrons of the donated electrons from the adsorbed H2O. For both adsorbed states, the Cooct site accepts the same electrons (0.074 e) from the O atom of H2O. It can be referred that the adsorption ability for H2O on the Cooct site relates to the ratio of the accumulated charge on surface hydrogen-bonding oxygen atoms and the donated charge from the adsorbed H2O. The higher this ratio, the more stable the molecular adsorption system. In other words, hydrogen bonding plays an important role on the H2O adsorption. Zasada et al. [41] found that the H2O molecules dissociate on fourfold coordinated Co oct cations upon adsorption on the Co3O4 (110)-A surface and lead to the surface hydroxylation by means of DFT calculations. To explore the possibility for H2O hydroxylation on the Co3O4 (110)-B surface, the complete linear synchronous transit and quadratic synchronous transit (LST/QST) method [48] was used for searching transition states for H2O decomposition reaction on the Co3O4 (110)-B surface. The pathways and energy profiles of the reaction denoted by H2O(ad) → OH(ad) + H(ad) are determined and presented in Fig. 3, which starts from the parallel and tilted adsorbed state for H2O. Two initial states are denoted by IS1 and IS2, respectively. The energy IS1 is lower than IS2 with the energy difference being 0.318 eV. In the final state, the OH − binds to the Co oct site and the proton (H +) binds to a nearby surface O 2f atom. In fact,
the dissociated process of H2O on the Co3O4 (110) perfect surface is a competition for H + between the O 2f atom and O of H2O. Beginning from IS1, the reaction is found to be an endothermic process with the enthalpy change of 0.144 eV and a low activation energy of 0.216 eV. The reaction initiated from IS2 has a barrier of 0.176 eV and releases heat of 0.174 eV, due to the cooperative effect that occurs for the hydrogen bond interaction of O 2f⋯H and the free rotation of OH in H2O. Furthermore, the barrier for the process from FS to IS1 is calculated to be only 0.061 eV. The results indicate that processes from IS2 to FS and then from FS to IS1 is easy. Consequently, H2O dissociation is not preferable, and IS1 is the major species for H2O on the Co3O4 (110)-B surface. It is worth mentioning that our results doesn't mean that the surface hydroxylation is impossible on Co3O4 (110) surfaces. The surface hydroxylation occurs on the Co3O4 (110)A surface due to favorable dissociation process of H2O [41]. 3.2. Adsorption of H2O on Al-sub-Co oct Co3O4 (110) surface The partially substituted surface, namely Al-sub-Co oct surface, was employed to further determine the role of Co oct ions on the interaction with H2O. The corresponding geometries and results for H2O adsorption through its oxygen atom on the Al-sub-Co oct Co3O4 (110) surface were shown in Fig. 4 and Table 1. On the Al-sub-Co oct surface, H2O is adsorbed both molecularly and dissociatively at the Al site with parallel (Fig. 4a) and tilted orientations (Fig. 4b). The adsorption and dissociation energies shown in Table 1 are 1.663 eV and 1.753 eV, respectively. The former is close to the case of the unsubstituted surface (1.612 eV). For the molecular adsorption, the Al\O bond has a length of 1.922 Å, and one O 2f⋯H hydrogen bond forms at a length of 1.679 Å. Two O\H bonds of H2O are stretched to be 1.005 Å and 0.968 Å. For the dissociative adsorption, the geometry in Fig. 4b shows an OH group bonded to the Al site at the length of 1.795 Å and the O 2f atom adjacent to another Al
Table 1 Adsorption energies (Eads), angles and changes of Mulliken charge (Δq) for single H2O at the octahedral site on the Co3O4 (110) and Al-sub-Cooct (110) surfaces. Al-sub-Cooct
Unsubstituted
Eads/eV θHOH/deg θoct M OH/deg Δqadsorbate oct ΔqM 2f ΔqO 3f ΔqO
Co-P
Co-T
Al-P
Al-T
1.612(1.185) 108 104 0.282 − 0.074 − 0.116 − 0.083
1.294(0.751) 105 109 0.246 − 0.074 − 0.167 0.014
1.663 109 105 0.140 0.079 − 0.148 − 0.066
1.753 – – Diss 0.111 − 0.087 − 0.004
“Dissociation” is abbreviated to “Diss”. The letters P and T denote the parallel and tilted modes, respectively. The corresponding adsorption energies calculated from GGA+U method are listed in the parentheses. For Δq, positive and negative values represent losing and getting electrons, respectively.
Fig. 3. Reaction pathways and energy profiles for H2O decomposition on the Co3O4 (110) surface. A Cooct atom, an O2f atom on the surface and a H2O adsorbate are depicted. Ea: activation energy; ΔH: enthalpy change.
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Fig. 4. H2O adsorption geometries at the Aloct site on the Al-sub-Cooct (110) surface (a) parallel one (b) tilted one. Only the surface and subsurface layers are depicted. The unit of distances is Å.
site is protonated. Ionescu et al. [47] also found that H2O is dissociated at the octahedral Al site along with the O 2f atom protonated on the γ-Al2O3 (110) surface. This is resulted from that the outermost surface of the γ-Al2O3 (110) slab also exposes the octahedral Al, O 2f and O 3f atoms as the Al-sub-Co oct Co3O4 (110) surface. These results indicate that the replacement of Co by Al has a slight positive effect on the molecular adsorption of H2O and brings about the dissociative adsorption of H2O. The change of Mulliken population in Table 1 shows that for the molecular adsorption of H2O on the Al-sub-Co oct surface, H2O donates 0.140 e to the surface, which is transferred totally to the O 2f site. At the same time, the Al site loses electrons, which is different from the result that the Co oct site gains electrons on the unsubstituted surface. 3.3. Nature of the Co3O4 (110) surface reactivity for H2O In Fig. 5, the total and partial electronic density of states (DOS) were depicted and compared for the unsubstituted and Al-sub-Co oct Co3O4 (110) surfaces before and after H2O parallel adsorption. After H2O adsorption, a new peak at −9.5 eV relative to the Fermi level at energy being zero is clearly observed (a1 and a2 in Fig. 5), which is the main modification of the total DOS. The partial DOS in Fig. 5 a3–a6
illustrates that the peak at − 9.5 eV is derived from three kinds of orbital overlaps, namely between 2p orbitals of O in H2O and 1s orbitals of H, 1s orbitals of H and 2p orbitals of O 2f and O 3f, and 3d orbitals of Co oct and 2p orbitals of O in H2O. Fig. 5 a3 and a6 shows that the OH states of the adsorbed H2O are located at −9.5 eV and −6.7 eV, contributed from 1s orbitals of H atoms interacting with p and sp orbitals of O atom, respectively. The interaction between 3d orbitals of the Co oct atom and the 2p orbitals of H2O is in the range from −9.5 eV to − 1.7 eV, leading to the formation of the Co oct\O bond. In Fig. 5 a3 and a4, it is noted that the formation of O 2f⋯H and O 3f⋯H hydrogen bonds is due to the interaction between the 2p orbitals of O 2f and O 3f atoms and the 1s orbitals of H atoms. In the case of H2O adsorption on the Al-sub-Co oct (110) surface, comparing b1 with b2 in Fig. 5, the new peak of the total DOS is at −9.6 eV after H2O adsorption. The partial DOS in Fig. 5 b3–b6 shows that this new peak is attributed to three kinds of orbital interactions, namely between O 2p orbitals and H 1s orbitals in H2O, H 1s orbitals and O 2f 2p orbitals, Al 3s orbitals and O 2p orbitals in H2O. From Fig. 5 b3 and b6, it can be seen that the OH states of the adsorbed H2O are at −9.6 eV and − 7.3 eV, which is contributed from H 1s orbitals interacting with O 2p orbitals. The interaction between Al 3s orbitals and O 2p orbitals in H2O is located at − 7.3 eV, as indicated by b5 and
Fig. 5. The total and partial electronic density of states (DOS) are depicted for Co3O4 (110) surfaces before and after H2O parallel adsorption. a1 represents the total DOS of clean unsubstituted Co3O4 (110) surface; a2 and a3-a6 represent orderly the total DOS of adsorbed unsubstituted surface and the partial DOS of 2p orbitals of surface O2f and O3f atoms, 1s orbitals of H in H2O, 3d orbitals of the Cooct atom, 2s and 2p orbitals of O in H2O for H2O parallel adsorption on the unsubstituted surface; b1 represents the total DOS of clean Al-subCooct (110) surface; b2 and b3-b6 represent orderly the total DOS of adsorbed Al-sub-Cooct (110) surface and the partial DOS of 2p orbitals of surface O2f atoms, 1s orbitals of H in H2O, 3s orbitals of the Al atom, 2p orbitals of O in H2O for H2O parallel adsorption on the Al-sub-Cooct (110) surface.
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b6 in Fig. 5, which results in the formation of Al\O bond. Fig. 5 b3 and b4 shows that the O 2f 2p orbitals interact with the H 1s orbitals at −9.6 eV, which leads to the formation of O 2f⋯H hydrogen bond. Comparing a with b in Fig. 5, the formation of Al\O and Co oct\O bonds are ascribed to the s–p and d–p interactions, respectively. On the other hand, for H2O adsorption on the Al-sub-Co oct and unsubstituted Co3O4 (110) surfaces, the same mechanism of hydrogen bonding occurred, that is 2p orbitals of surface active O atom interacting with the 1s orbitals of H atoms. Furthermore, the change of Mulliken population in Table 1 shows that for H2O adsorption on the Al-sub-Co oct and unsubstituted surfaces, Al lost electrons but Co oct obtain, with the major part of the donated electrons from H2O being accumulated on the surface active O atoms and thus contributing to the formation of hydrogen bonds. Though the bonding natures of the coordination bonds between adsorption sites and H2O are different, the Al-sub-Cooct and unsubstituted surfaces have similar adsorption ability for H2O molecules due to the similar adsorption energies (1.612 eV vs. 1.663 eV). These results demonstrated that the strong oxidative activity of the Co oct site is not the major cause for the reactivity of the Co3O4 (110) surface for H2O. However, the activity of surface oxygen interacting with H atoms contributes predominantly to the reactivity of the Co3O4 (110) surface for H2O.
3.4. Effect of H2O adsorption on CO oxidation over Co3O4 (110) surface We reported previously [36] that CO chemisorbs preferably at Co oct and O 2f sites on the Co3O4 (110) surface by DFT calculations as this work, with adsorption energies of 1.850 eV and 0.917 eV, respectively. The removal of the O 2f atom was proposed to be a key step for CO oxidation, which is also proposed in other works [20,38]. The easy removal of the O 2f atom and simultaneously high oxidative activity of the adjacent Co oct site were proposed to be responsible for the activity of the Co3O4 (110) surface for CO oxidation. Based on the above discussion, H2O adsorption on the surface leads to a slight change of the Co oct valence state because the Co oct site gains small amount of electrons. In other words, the oxidative ability of the Co oct site might remain intact. To explore the effect of H2O adsorption at the Co oct site on CO oxidation, the simultaneous adsorption of H2O at the Co oct site and CO at the O 2f site was studied at the full coverage for Co oct and O 2f sites, namely Co oct–(H2O)O 2f–(CO) configuration. The optimized structure was illustrated in Fig. 6. It is noted that CO forms bonds with both O 2f and Co oct sites. The bond lengths of C\O 2f and C\Co oct are 1.309 Å and 1.862 Å, which is longer and shorter than the corresponding values (1.262 Å and 1.937 Å) for the case of individual CO adsorption at the O 2f site [36], respectively. This indicates that CO interacting with O 2f and Co oct atoms is weakened and strengthened due to H2O adsorption, respectively. As a consequence, the process of CO2 formation by abstracting O 2f atoms is hindered to a certain extent. On the other hand, the adsorption of CO also hindered H in H2O and O 2f atoms from forming hydrogen bond. It is confirmed here that a competition to interact with the O 2f atom occurs between CO and H2O. Moreover, H2O can adsorb at the Co oct site through a O 3f⋯H hydrogen bond with the length of 1.645 Å in the presence of CO at the O 2f atom. Generally, the energy associated with hydrogen bond is close to the value of intermolecular forces, which suggests that low energy barrier is required for breaking hydrogen bonds. It can be assumed that the hydrogen bond O 3f⋯H between H2O and the Co3O4 surface would be weakened and H2O tends to be desorbed as the temperature increases. Consequently, the hindrance effect of H2O on CO oxidation disappears. These results may provide an explanation for the previous experimental observations that H2O inhibits CO oxidation on the Co3O4 surface at low temperatures but not at high temperatures [24]. The above discussion suggests that the inhibition effect can be caused by the competition between H2O and CO molecules for the O 2f site.
Fig. 6. Side view of optimized coadsorption structures of Cooct–(H2O)O2f–(CO) (a) 2 × 2 adsorbed surface, (b) surrounding atoms and distances between them of the Co1 atom. All distances are in Å. Only selected atoms on the surface are depicted.
4. Conclusions In this work, H2O adsorption on the Co3O4 (110) surface was studied using DFT calculations and supercell models. It is found that H2O is adsorbed favorably at Co oct sites on the surface, by O bonding with Co oct sites and hydrogen bond formation between surface active oxygen atoms (O 2f or O 3f) and H. Hydrogen bonding plays a key role for H2O adsorption at the Co oct sites. The introduction of Al to replace the Co oct site on the Co3O4 (110) surface is found to have a slight positive effect on the molecular adsorption of H2O. Moreover, H2O adsorption at the Co oct site by hydrogen bonding to the O 2f site presents an inhibition effect on CO interaction with the O 2f site, and then further hinders the CO2 formation by abstracting O 2f atoms to a certain extent.
Acknowledgments The authors gratefully acknowledge Dr. Xiang Wang in revising the manuscript. This work was supported by the Natural Science Foundation of China (20673019, 20303002), the Key Project of the Fujian Province (2005HZ01-2-6).
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