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Cu (TM = Fe, Co and Ni) surface alloys
Accepted Manuscript Title: First-principles investigation of the activation of CO2 molecule on TM/Cu (TM = Fe, Co and Ni) surface alloys Author: Mei Qiu Zhenxing Fang Yi Li Jia Zhu Xin Huang Kaining Ding Wenkai Chen Yongfan Zhang PII: DOI: Reference:
S0169-4332(15)01513-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.06.165 APSUSC 30686
To appear in:
APSUSC
Received date: Revised date: Accepted date:
21-5-2015 25-6-2015 25-6-2015
Please cite this article as: M. Qiu, Z. Fang, Y. Li, J. Zhu, X. Huang, K. Ding, W. Chen, Y. Zhang, First-principles investigation of the activation of CO2 molecule on TM/Cu (TM = Fe, Co and Ni) surface alloys, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.06.165 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Density of states of TM 3d orbitals with z-component, adsorption structure and -COHP curves for
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the chemisorptions of CO2 molecule on TM/Cu(100) surface alloys.
Highlights
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Among the late 3d TM metals, cobalt is potentially a good dopant to activate CO2 on copper surfaces. The chemical bonding analyses indicated that TM atom primarily provides d orbitals with z-component, namely dz2, dxz, and dyz orbitals to form TM-C and TM-O adsorption bonds. A modified d-band model that only considers the contributions of those d orbitals with z-component was proposed, which could produce a good linear relationship between adsorption energy and the revised d-band center.
First-principles investigation of the activation of CO2 molecule on TM/Cu (TM = Fe, Co and Ni) surface alloys Mei Qiu a, Zhenxing Fang a, Yi Li Yongfan Zhang a,d,∗
a,*
, Jia Zhu b, Xin Huang
a,c
, Kaining Ding a, Wenkai Chen a,
a
College of Chemistry, Fuzhou University, Fuzhou, Fujian, 350116, China College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, Jiangxi, 330022,China c Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Xiamen, Fujian, 361005, China d State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou, Fujian, 350002, China b
∗
Corresponding authors. E-mail addresses: [email protected] (Y. Li), [email protected] (Y. Zhang) 1
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Abstract Periodic density functional theory calculations have been performed to investigate the CO2 activation on a series of TM/Cu surface alloys that are built by dispersing individual later 3d TM atoms (TM = Fe, Co, and Ni) on the Cu(100), Cu(110) and Cu(111) surfaces. The most stable structure with bent CO2δ- configurations on the different TM/Cu surfaces are determined, and the
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results show that among the late 3d-metals, cobalt is potentially a good dopant to activate CO2 on copper surfaces. Using the simple d-band model that treats all five d orbitals of surface TM atom as
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a whole, a poor correlation between adsorption energy and the d-band center is observed. To obtain a deep understanding of the adsorption property, the bonding characteristics of the TM-C and TM-O
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adsorption bonds are carefully examined by the crystal orbital Hamilton population technique, which reveals that the TM atom primarily provides d orbitals with z-component, namely dz2, dxz, and
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dyz orbitals to interact with the CO2. Based on this result, we propose a modified d-band model that only considers the contributions of those d orbitals with z-component, and by employing this
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established.
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approach a good linear relationship between adsorption energy and the revised d-band center can be
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Keywords: Density functional theory; bimetallic alloys; CO2 activation; d-band center
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1. Introduction Bimetallic alloys have attracted considerable interest due to their important applications in heterogeneous catalysis since the concept “bimetal” was proposed in 1960s. The addition of a second metal to the surface of a host can dramatically improve the selectivity, activity, and stability
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in surface-catalyzed reactions[1-7]. Extensive experimental evidence indicates that bimetallic surfaces often exhibit novel properties that are not present on their parent metal surfaces[1,2], and a
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general understanding of the adsorption behavior on bimetallic surfaces can help us discover more efficient catalysts for a given reaction [8].
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It is obvious that the chemisorption on the surface of bimetallic transition metal (TM) alloys involves strong interactions between the adsorbate and the surface TM atom, which leads to the
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formation of adsorption bond between the adsorbate and the TM atom. So the description of such chemical bond is the fundamental basis for exploring surface chemical reactivity of the bimetallic
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TM alloys. Since besides valence s states, the TM atom on the surface primarily uses valence d states to overlap with the orbital of adsorbate, the differences in the adsorption bonds are often described in terms of the interactions between the adsorbate and the TM d-band states. Therefore,
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for a given adsorbate, the strength of the adsorption bond is mainly relied on the characteristics of
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the metal d band. In the well-known d-band model that has been widely used in understanding the bond formation at the transition-metal surfaces[9-11], the center of the d states relative to the Fermi level is chosen as an indicator to quantitatively describe the properties of the metal d band. Basing on the d-band model, the higher the d-band center of a specific TM site is, the stronger the adsorption bond formed at that site will be, and additionally a nearly linear relationship between the binding energies and the surface d-band center have been identified on different TM surfaces, especially on monometallic surfaces [1,2]. However, there are also reports showing that the adsorption behavior on some bimetallic surfaces cannot be well explained from the above simple d-band model [12,13]. So other factors should be considered if we want to obtain a good description of the adsorption properties for bimetallic surfaces. More detailed information about the distribution of the d electrons, such as the width and the shape of d-band states, has been taken into account to improve the d-band model [14,15]. In addition, as pointed out in a recent review by Chen et al. [1], a molecular level understanding of the reaction mechanism on the bimetallic surface would clearly
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require more detailed description of the electronic properties, especially the distribution of individual d-orbitals of the surface metal atom. It is worth noting that in the current d-band model, the d-band center is corresponding to the weighted average energy of total d states projected onto a surface TM atom, or in other words, the five d orbitals of TM atom are treated as a whole during calculating the d-band center. However, in
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general, the distributions of five d orbitals of a surface TM atom are different and may be divided into several groups based on the coordination environment of this TM atom. As an instance, if a
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copper atom on the Cu(111) surface is replaced by another TM atom, the five d orbitals of TM are
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not degenerate and will split into three categories: A1{dz2}, E{dx2-y2, dxy}, and E{dxz, dyz} (where A1 and E are the Mülliken notations for the irreducible representations of point group C3v, and the z-axis is set along the surface normal direction). Furthermore, we must take care that when an
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adsorbate is chemisorbed on the metal substrate, not all d orbitals of a surface TM atom involve in the formations of the adsorption bonds. In most cases, according to the spatial distributions of five d
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orbitals, only those d states vertical to the surface can effectively overlap with the orbitals of the adsorbate. This implies that the d orbitals with z-component, i.e., dz2, dxz, and dyz orbitals can be
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seen as the "active" components of the d states and will play a leading role in the surface reactions
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on the metal surface. For an example, if we assume that a CO2 molecule is chemisorbed on a surface TM atom by the η1(C) linkage, as schematically illustrated in Figure 1a, the σ interactions between TM dz2 and C 2s/2pz orbitals, and the π interactions between TM dxz (dyz) and C 2px (2py) orbitals can be observed. While the interactions associated with TM dx2-y2 and dxy orbitals correspond to the non-bonding overlap and make a small contribution to the formation of TM-C adsorption bond. For a more complex example that the CO2 molecule is bound to surface by the η2(C,O) linkage (Figure 1b), the TM atom still can employ the dz2, dxz and dyz orbitals to form the TM-C and TM-O adsorption bonds although some orbital interactions are different with the η1(C) model. Consequently, we can expect that the specific role of individual d orbital is necessary to take into account during determining the d-band center.
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(a)
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Figure 1 Schematic illustrations for the orbital interactions between CO2 molecule and a surface TM atom when CO2 is chemisorbed by the (a) η1(C) linkage, and (b) η2(C,O) linkage.
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Considering those d orbitals with z-component make significant contributions to the formation of the adsorption bonds on the metal surface, it is interesting to see whether there is an improvement
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in the performance of the d-band model by using above partial d orbitals instead of total d orbitals. To answer this question, in the present work the TM/Cu bimetallic surface alloy is served as a
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prototype, and the carbon dioxide is selected as an adsorbate because with respect to the simple
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atomic adsorbates (such as H, O, etc.), CO2 molecule is able to participate in several distinct types of coordination modes with TM atom [16-19]. This is also motivated by the fact that the interactions between CO2 and metal surfaces have been of great interest in the field of catalytic CO2 activation[20-27]. It has been established that the CO2 activation on the clean metal surfaces usually involves the formation of bent CO2δ- species, which is an important precursor of CO2 dissociation, and therefore, understanding that process is necessary for technical applications such as CO2 utilization. In recent years, some experimental and theoretical investigations have been carried out to study the adsorption and reaction of CO2 molecule on the pure Cu surfaces, as well as on the corresponding bimetallic surfaces, and their results clearly show that the Cu-based surface is an effective catalyst to achieve the activation of inert CO2 molecules [28-34]. However, at present, most attention has been concentrated on the monolayer bimetallic surfaces, few efforts have been devoted to the copper surface that the admetal coverage is very low, especially for the case that the foreign TM atoms are atomically dispersed on the Cu surface [35,36]. In this paper, using density functional theory (DFT), we performed a systematic investigation 5
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of the activation of CO2 molecule on a series of TM/Cu surface alloys that are constructed by dispersing individual and isolated late 3d TM atoms (TM = Fe, Co, and Ni) in Cu(100), Cu(110) and Cu(111) surfaces. We first study the most stable adsorption structure with the bent CO2δconfiguration on different pure and TM-doped Cu surfaces. Next, the bonding characteristics associated with the adsorption bonds are carefully examined by the crystal orbital Hamilton
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population (COHP) technique, and the roles of those 3d orbitals with z-component are discussed. Finally, the d-band center of surface active atom that considers the contributions of the total 3d
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states is calculated by employing the simple d-band model, and the poor correlation between
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adsorption energy and the d-band center is achieved, however a good linear correlation will be observed by using a modified d-band model that only takes into account the contributions of those
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3d orbitals with z-component.
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2. Computational details
All DFT calculations were carried out utilizing the Vienna ab initio simulation package
of
Perdew-Burke-Ernzerhof
(PBE)
exchange-correlation
functionals
was
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approximation
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(VASP)[37-39] and the projected augmented wave (PAW) method[40-42]. The generalized gradient
employed[43]. The kinetic energy cutoff for the plane-wave expansion was set to 400 eV, and the Brillouin-zone integrations were sampled using a (5 × 5 × 1) Monkhorst-Pack mesh[44]. The effects of spin polarization were considered and the dipole correction in the surface normal direction was applied. The convergence thresholds of the energy change and the maximum force for the geometry optimizations were set to 10-6 eV and 0.02 eV/Å, respectively. The Cu(100) and Cu(111) surfaces were modeled by periodic slabs consisting of five atomic layers, whereas a seven-layer slab was used for the Cu(110) surface with a more open structure. During the structural optimization, the atoms at the outermost three layers (four layers for (110) surface) were fully relaxed in all directions, while other Cu atoms at the remaining layers were fixed to their bulk positions. The spacing between the adjacent slabs was set to about 15 Å. To simulate the dispersion of individual TM atoms on the Cu surface and to avoid the obvious interactions between neighboring CO2 adsorbates, a supercell consisting of a (3 × 3) surface unit cell was adopted, in which one Cu atom at the top layer or sublayer was substituted by a TM atom, 6
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corresponding to a dopant coverage of 1/9 ML. Therefore, total 21 different copper surfaces were included in the calculations. Since the physisorption is a nonactivated process of CO2 adsorption and is also insensitive to a given surface, it was not considered in the present work. In such case (for example, CO2 capture in porous materials), the dispersion-corrected DFT method is required to properly describe van der Waals interactions between CO2 molecule and the substrate. On the other
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hand, it is well known that the chemisorption of CO2 on a metal surface usually involves electron transfer from the substrate into the CO2 molecule which in turn leads to the formation of CO2δ-
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anion with a bent geometry[45]. For the chemisorption of CO2 molecule on different TM/Cu surface
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alloys, many possible adsorption configurations were explored, and the thermodynamic stability of different structure was determined by the adsorption energy that was defined as, (1)
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E ads = E slab + ECO2 − ECO2 / slab
where ECO2 / slab , E slab , and ECO2 represent the total energies of the adsorbed system, the clean
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TM/Cu surface alloy, and the CO2 in gas phase, respectively. To understand the chemical-bonding picture of the adsorption bond, the projected COHP method developed by Dronskowski group was
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used[46-48], which can offer a straightforward view onto the orbital-pair interactions. In the
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–COHP diagram, the positive and negative values correspond to the bonding and antibonding states, respectively. Furthermore, the d-band center ( ε dT ) of surface active atom that treats all five d orbitals as a whole was calculated using the following equation[49], EF
ε dT
∫ = ∫
Eρ d ( E )dE
−∞ EF −∞
ρ d ( E )dE
(2)
where E is the energy with respect to the Fermi level (EF), and ρd(E) is the density of states (DOSs) projected onto five d orbitals of the surface active atom at energy E.
3. Results and Discussion 3.1 Chemisorption of CO2 on the TM/Cu(100) Surface Alloys 3.1.1 Adsorption Structures
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(b)
(c)
(d)
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(a)
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Figure 2 The top (upper panel) and side views (bottom panel) of the most stable configuration for the chemisorption of CO2 molecule on (a) pure Cu(100) surface, (b) TM/Cu(100) surface alloys, (c) TM/Cu(110) surface alloys, and (d) TM/Cu(111) surface alloys. The Cu, O, C, and TM atoms are denoted by blue, red, dark gray and green spheres, respectively. We first consider the most stable chemisorption structure of CO2 on the pure Cu(100) surface. As displayed in Figure 2a, CO2 molecule is adsorbed above the fourfold hollow site of Cu(100) surface by one oxygen and carbon atoms, and four adsorption bonds, including two Cu-C and two Cu-O bonds are found at the interface. The lengths of Cu-C and Cu-O bonds are about 2.157, and 2.098 Å (see Table 1), respectively. After adsorption, with respect to the case in the gas phase, two C-O bonds of CO2 are stretched 0.146, and 0.044 Å, respectively, and meanwhile CO2 moiety exhibits a bent configuration with the O-C-O angle of 128.4°. The calculated adsorption energy is about -11.8 kcal/mol, and the negative value indicates that the activation of CO2 is endothermic on the pure Cu(100) surface. Above results are consistent with those reported in a recent theoretical study performed by Liu et al[28]. Furthermore, similar results are also obtained when one Cu atom at the sublayer of Cu(100) surface is substituted by other TM atom, which indicate that the sublayer substitution has small effect on the CO2 activation on the Cu(100) surface. Therefore, we mainly 8
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focus on the results that TM substitution occurs on the top layer of Cu(100) surface.
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Table 1 Some optimized bond lengths (in Å), O-C-O bond angle (in degree), and the calculated adsorption energies (in kcal/mol) for the chemisorption of CO2 molecule on the pure and different TM/Cu surface alloys dC-O2 dTM-C dTM-O1 dCu-O Eads System dC-O1a ∠O-C-O Fe/Cu(100) 1.271 1.257 134.7 1.940 2.025 2.171 5.0 Co/Cu(100) 1.264 1.266 134.6 1.866 2.035 2.140 9.5 Ni/Cu(100) 1.243 1.268 135.0 1.912 2.213 2.121 0.2 b Pure Cu(100) 1.322 1.220 128.4 — -11.8 2.157(×2) 2.098(×2) Fe/Cu(110) 1.301 1.300 120.9 1.972 — -3.2 2.139(×4) Co/Cu(110) 1.304 1.305 120.0 1.895 — 2.5 2.124(×4) Ni/Cu(110) 1.297 1.297 121.0 1.924 — -1.7 2.140(×4) Pure Cu(110) 1.289 1.289 122.5 2.054 — -12.6 2.163(×4) Fe/Cu(111) 1.266 1.226 140.0 1.983 2.006 2.400 -3.1 Co/Cu(111) 1.259 1.236 140.0 1.896 2.042 2.268 3.2 Ni/Cu(111) 1.232 1.239 142.2 1.957 2.286 2.196 -6.7
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a. The symbols O1 and O2 are used to represent two oxygen atoms of CO2 moiety (see Figure 2). For the free CO2 molecule, the optimized length of C-O bond is 1.176 Å. b. The number of bond is shown in parentheses.
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Consider the chemisorption of CO2 molecule on the Fe/Cu(100) surface alloy. The most
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favorable binding conformation of CO2 is shown in Figure 2b, and selected geometrical parameters are listed in Table 1. Compared to the pure Cu(100) surface, CO2 adopts a different configuration on Fe/Cu(100), and now all three atoms of CO2 molecule take part in the interaction with the substrate. It is interesting that CO2 prefers to be adsorbed above the solute Fe atom by the formations of two adsorption bonds (namely, Fe-C, and Fe-O1 bonds), which is corresponding to η2(C,O) linkage. The calculated lengths of Fe-C and Fe-O1 bonds are 1.940 and 2.025Å, respectively. Additionally, the rest oxygen atom (denoted by O2 in Figure 2) of CO2 is attached to one neighboring Cu atom, however the corresponding Cu-O2 distance (2.171 Å) indicates that the interaction between Cu and O2 atoms is weak. In this configuration, the CO2 moiety is also displayed a bent structure with the O-C-O angle of 134.7°. Due to the strong interaction between CO2 and Fe atom, a positive adsorption energy of 5.0 kcal/mol is predicted for Fe/Cu(100) alloy. Thus, by dispersing Fe atoms on Cu(100) surface the chemisorption of CO2 becomes an exothermic process. After CO2 adsorption, an obvious outward movement (0.181 Å) is found for Fe atom. Compared to Fe/Cu(100) surface alloy, similar stable configurations are yielded for the 9
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Co/Cu(100) and Ni/Cu(100) systems (see Figure 2b and Table 1), in which CO2 molecule still binds to the substrate by the formations of TM-C, TM-O1 and Cu-O2 multiple bonds. The calculated adsorption energies are 9.5 (TM = Co) and 0.2 kcal/mol (TM = Ni), respectively. Therefore, the adsorption energies follow a sequence of Co/Cu(100) > Fe/Cu(100) > Ni/Cu(100) > pure Cu(100), indicating that the Co/Cu(100) surface shows the strongest interaction with CO2. Based on above
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results, although CO2 can not be stabilized on the pure Cu(100) surface, the introducing of other TM atoms (especially Co) into the outmost layer of Cu(100) can improve the stability for the
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chemisorption of CO2. On the other hand, besides the Cu(100) surface, it is also necessary to make
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a comparison with another parent metal surface, i.e., the pure TM(100) surface. Liu et al. have studied the adsorption behavior of CO2 on the (100) surfaces of Fe, Co and Ni pure metals with fcc phase using PBE functional[28]. Their results indicated that as observed on the pure Cu(100)
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surface, CO2 tends to sit above the surface hollow site by forming two TM-C and two TM-O bonds (Figure 2a). Moreover, as going from Fe to Cu, the calculated adsorption energies show a
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monotonically decreasing trend of Fe(100) > Co(100) > Ni(100) > Cu(100). Therefore, compared with the case of the parent TM(100) surface, the CO2 molecule exhibits a distinct chemisorption
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behavior on the TM/Cu(100) surface alloys for both the adsorption geometry and the relative
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stability. 3.1.2 Chemical Bonding Analyses
To obtain a deeper insight into the interactions between the CO2 adsorbate and the TM/Cu(100) surface alloy, we calculate the -COHP values of TM-C, TM-O1, and Cu-O2 adsorption bonds, respectively.
As shown in Figure 3a, for the TM-C bond, the Fermi level is located in the -COHP curve between the bonding and antibonding regions. Three sharp bonding peaks are found at -9.1, -8.0, and -7.0 eV, respectively, and an additional broad bonding peak is observed in the region from -3.3 to -0.8 eV. Further analyses indicate that these peaks are mainly corresponding to the σ-bonding interactions between 4s/3dxz/3dyz orbitals of TM atom and 2s/2pz orbitals of C atom. On the other hand, the weak σ-antibonding interactions between TM 4s and C 2s orbitals give rise to the feature near -4.0 eV. Therefore, besides 4s orbital, the TM atoms mainly use 3dxz and 3dyz to form TM-C adsorption bond. As a comparison, for the pure Cu(100) surface, the Cu atom interacts with C mainly by using its 4s orbital. 10
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(b)
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(a)
(c)
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Figure 3 -COHP curves for (a) TM-C, (b) TM-O1, and (c) Cu-O2 adsorption bonds of different TM/Cu(100) surface alloys as well as of the pure Cu(100) surface. The Fermi level (EF) is set at zero. The -COHPs curves shown in figures are the sum of the corresponding spin-up and spin-down states obtained from the spin polarized calculations.
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Figure 3b display the -COHP curves of the TM-O1 adsorption bond of different surfaces. It is noted that the Fermi level falls in a strong TM-O1 antibonding region, implying that removing some
d
electrons from the system may enhance the strength of TM-O1 adsorption bond. For those bonding
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peaks observed below -3.5 eV, TM atom mainly uses 4s and 3dz2 orbital to overlap with O 2s and 2pz orbitals. While in the antibonding region from -3.5 eV to the Fermi level, the contributions of TM 3dxz/3dyz orbitals are also found.
For the Cu-O2 adsorption bond, the Fermi level also falls in a strong Cu-O antibonding region (Figure 3c). In this case the Cu-O σ-bonding peaks are mainly derived from the interactions between Cu 4s and O 2s/2pz orbitals, and the antibonding peaks in the energetic region from -3 eV to 0 eV contain noticeable 3dxz/3dyz contributions of Cu atom. Therefore, according to above chemical bonding analyses, it is clear that among three adsorption bonds, the TM-C bond plays an important role concerning the stability of the system; whereas for the TM-O1 and Cu-O2 adsorption bonds, the Fermi level lied in the clearly antibonding region indicates that the TM-O1 and Cu-O2 adsorption bonds are not strong. 3.1.3 d-band Center Analyses Since the TM atom is the main active site for the activation of CO2 on the TM/Cu(100) surface
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alloys, in here we focus on the d-band center of TM atoms. By using eq. (2), the position of the normal d-band center ( ε dT ) of TM as well as Cu atom that considers the contributions of all five 3d orbitals is determined, and the results are summarized in Table 2. The corresponding d-band centers are -2.25 (Fe), -0.87 (Co), -1.04 (Ni), and -2.69 eV (Cu) relative to the Fermi level, respectively.
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According to the simple d-band model, the higher the d states are in energy, the stronger the adsorption bond is formed between the adsorbate and substrate, which implies that the order of the
cr
adsorption energy is Co/Cu(100) > Ni/Cu(100) > Fe/Cu(100) > pure Cu(100). However, this sequence is different from the calculated result of Co/Cu(100) > Fe/Cu(100) > Ni/Cu(100) > pure
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Cu(100).
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d
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Table 2 The centers and electron numbers of five individual 3d orbitals, as well as the total d-band center (εdT) and partial d-band center (εdP) of TM atom on different TM/Cu surface alloys and Cu on the corresponding pure Cu surfaces a dz2(eV) dxy(eV) dxz(eV) dyz(eV) Systems dx2-y2(eV) εdT(eV) εdP(eV) Fe/Cu(100) -1.04 (0.5)b -1.69 (0.1) -1.66 (0.1) -0.82 (0.4) -0.81 (0.4) -2.25 -0.91 Co/Cu(100) -1.07 (1.5) -0.72 (1.4) -0.97 (1.3) -0.81 (1.5) -0.81 (1.5) -0.87 -0.78 Ni/Cu(100) -1.24 (1.7) -0.90 (1.7) -1.03 (1.7) -1.02 (1.7) -1.02 (1.7) -1.04 -0.98 Pure Cu(100) -2.75 (1.8) -2.63 (1.9) -2.60 (1.9) -2.73 (1.8) -2.73 (1.8) -2.69 -2.70 Fe/Cu(110) -1.48 (0.3) -0.94 (0.4) -1.68 (0.3) -1.19 (0.5) -1.68 (0.2) -2.25 -1.19 Co/Cu(110) -1.05 (0.6) -0.83 (0.7) -1.24 (0.6) -1.13 (0.6) -0.98 (0.5) -1.45 -0.97 Ni/Cu(110) -1.39 (1.7) -1.33 (1.7) -1.47 (1.7) -1.53 (1.6) -1.34 (1.7) -1.41 -1.40 Pure Cu(110) -2.78 (1.8) -2.73 (1.8) -2.85 (1.8) -2.90 (1.8) -2.75 (1.8) -2.80 -2.79 Fe/Cu(111) -1.36 (0.3) -1.55 (0.2) -1.36 (0.3) -0.89 (0.4) -0.89 (0.4) -2.29 -1.02 Co/Cu(111) -0.95 (1.5) -0.94 (1.4) -0.95 (1.5) -0.84 (1.5) -0.83 (1.5) -0.90 -0.87 Ni/Cu(111) -1.29 (1.7) -1.15 (1.7) -1.29 (1.7) -1.11 (1.7) -1.11 (1.7) -1.19 -1.12 a. For three Fe/Cu surface alloys, a large magnetic moment (2.7 ~3.0 μB) is observed around Fe atom, and basing on the results of COHP analyses that Fe atom mainly uses its spin-down states to overlap with the orbitals of CO2 molecule, therefore, the centers of spin-down 3d orbitals are presented in here. A similar picture is also seen in Co/Cu(110) system. b. The electron number is shown in parentheses.
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(a)
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Figure 4 (a) The spin-up and spin-down -COHPs curves for the Fe-C adsorption bond, and (b) the atomic DOSs of the spin-up and spin-down states of Fe 3d orbitals of Fe/Cu(100) surface alloy. The Fermi level (EF) is set at zero.
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It is noted that the above value of d-band center is the weighted average energy level of five 3d orbitals of TM atom, namely, the different role of individual 3d orbital played in the formation of adsorption bond is not taken into account. Actually, above COHP analyses clearly confirm that TM
d
atoms tend to use those 3d orbitals with z-component, i.e. 3dz2 and 3dxz/3dyz (the 3dxz and 3dyz
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orbitals are degenerate in here) to interact with CO2 molecule. Therefore, it is worthy to study the correlation between the adsorption energy and another kind of d-band center that only above specific 3d orbitals are considered. In Table 2, we list the individual d-band center positions of five isolated 3d orbitals for solute TM atom and Cu atom on the pure Cu(100) surface. It is noted that for Fe/Cu(100) system a remarkable magnetic moment (2.97 μB) is observed around Fe atom, and basing on the spin-up and spin-down -COHPs curves (Figure 4a), Fe atom mainly adopts spin-down states to overlap the orbitals of carbon to form the strong bonding peak just below Fermi level. Moreover, the atomic DOSs (see Figure 4b) indicate that the distributions of the spin-up and spin-down states of the Fe 3d orbitals are quite different, and especially the spin-down states are localized in the region near the Fermi level. This is the reason why the spin-down states are the major 3d components to interact with adsorbate. Similar results also can be observed for the cases of Fe/Cu(110) and Fe/Cu(111) surface alloys (data not shown). Hence, for Fe-doped surfaces, we only focus on the spin-down states of 3d orbitals, and the centers of five spin-down 3d orbitals are 13
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reported in Table 2. For Co/Cu(100) and Ni/Cu(100) surface alloys, the magnitudes of magnetic moment around TM atoms are very small (< 0.05 μB). From the data listed in Table 2, it shows that the arrangements of five individual d-band centers are quit different. For instance, the lowest-lying d state is 3dz2 orbital when TM = Fe, while it changes to 3dx2-y2 orbital when TM = Co, Ni, and Cu. Since more than one 3d orbital of TM will involve in the interactions with CO2, we have to
ip t
distinguish their contributions during calculating the new d-band centers. In here, we use a simple and approximate method that takes into account the occupation of each individual d-band, and then
cr
the d-band center can be modified using the following equation, n
ε dP =
i =1 n
i d
ndi
∑ ndi
(3)
an
i =1
us
∑ε
where ndi is the electron number of the associated 3d orbital, and ε di is the center of an individual
M
3d orbital. Using the formula similar to eq. (2), ε di can be calculated when the projected DOS of individual 3d orbital ( ρ di (E ) ) is used. The summation in above equation is only applied to those 3d
Ac ce pt e
d
orbitals that interact with adsorbate. Since ndi is obtained by integrating the projected DOS up to the Fermi level, the eq. (3) can be rearranged as:
⎡ EF Eρ i ( E )dE ⎤ EF i ⎢ ∫−∞ d ⋅ ∫ ρ d ( E )dE ⎥ ∑ EF −∞ ⎢ ⎥ i i =1 ρ ( E )dE ⎣⎢ ∫−∞ d ⎦⎥ n
n
ε dP =
∑ ε di ndi i =1 n
∑ ndi i =1
=
n
∑∫ i =1
EF
−∞
ρ di ( E )dE
n
=
∑∫ i =1 n
EF
−∞
∑∫ i =1
Eρ di ( E ) dE
EF
−∞
ρ di ( E )dE
(4)
So the definition by eq. (3) is equivalent to determine the center of gravity of partial 3d orbitals when only those 3d orbitals with z-component are treated as a whole. To distinguish from the previous ε dT descriptor that is corresponding to the center of gravity of the entire d states, in the following sections we call the value defined by eq. (3) as the partial d-band center ( ε dP ). For the pure and TM-doped Cu(100) surfaces, the values of ε di and ndi of each 3d orbital are listed in Table 2. When the 3dz2, 3dxz and 3dyz orbitals of surface active atoms are concerned, the ε dP value 14
Page 14 of 26
of three surface alloys are -0.91 (Fe), -0.78 (Co), and -0.98 eV (Ni), respectively, and -2.70 eV for the pure Cu(100) surface. Now the order of ε dP is Co/Cu(100) > Fe/Cu(100) > Ni/Cu(100) > pure Cu(100), which is in agreement with the sequence of the adsorption energy. Additionally, in Figure 5a we present the adsorption energies as a function of the ε dP , and a linear relationship between
ip t
adsorption energy and the ε dP can be established. Consequently, it seems that the chemisorption behaviors of CO2 on the different Cu(100) surfaces can be well explained from the partial d-band
(b)
Ac ce pt e
(a)
d
M
an
us
cr
model.
(c)
(d)
Figure 5 The adsorption energies of CO2 on (a) TM/Cu(100), (b) TM/Cu(110), and (c) TM/Cu(111) surface alloys as well as the corresponding pure Cu surfaces as a function of the partial d-band
center ( ε dP ) of surface active atoms. Figure (d) displays the curve if all the same adsorption configurations that CO2 is adsorbed on surface through the formations of TM-C, TM-O1 and Cu-O2 15
Page 15 of 26
bonds (see Figures 2b and 2d) are considered together. The inset in figure (b) is corresponding to the result that the contributions of those 3d orbitals with z-component of the TM atom and the nearest Cu atom are considered together. R: Correlation coefficient, SD: Standard deviation of the fit. 3.2 Chemisorption of CO2 on the TM/Cu(110) Surface Alloys
ip t
3.2.1 Adsorption Structures
The Cu(110) surface exhibits a more open structure, and besides the first layer, the Cu atoms at
cr
the second atomic layer are also exposed to react with the adsorbate (Figure 2c). Correspondingly, there are two TM-doping configurations that come from the replacements of Cu atoms at the first
us
and second layers, respectively. However, by comparing the total energies of two structures, it is indicated that the substitution of TM for Cu atom at the second layer is energetically favorable, and
an
the energy differences between two doping configurations are about 0.33 (Fe), 0.64 (Co), and 0.40 eV (Ni), respectively. Therefore, in the following sections, we focus the chemisorption of CO2 after
M
introducing TM atom at the second layer of Cu(110) surface.
Figure 2c displays the most stable configurations for the chemisorption of CO2 on the different
d
TM/Cu(110) surfaces, and some structural parameters are shown in Table 1. It is clear that the
Ac ce pt e
configurations of CO2 on TM/Cu(110) surface alloys and the pure Cu(110) surface are very similar. Furthermore, for the sublayer substitution of Cu(110), namely the doping of TM atom occurs at the third atomic layer, the corresponding results are nearly identical to that of the pure surface. In the above structure, the carbon atom favors the TM atop site, while two O atoms are adsorbed at the bridge sites of Cu(110) surface. Correspondingly, there are five adsorption bonds, including one TM-C bond and four Cu-O bonds are formed between CO2 moiety and substrate. Compared with the TM/Cu(100) and TM/Cu(111) (see later section) systems, there are more adsorption bonds formed between CO2 moiety and Cu(110) surface. After adsorption, the CO2 molecule on Cu(110) substrate also exhibits a bent structure, and the O-C-O bond angle (about 120°, Table 1) is close to that expected for CO2-. Compared with the configuration on the Cu(100) surface, the small O-C-O bond angle (120° vs. 135°) suggests that CO2 molecule receives more electrons when it is bound to Cu(110) surface. By analyzing the Bader charge distribution, for TM/Cu(110) surfaces about 1.0 e electrons are transferred from the substrate to CO2, while the magnitude of charge transfer is reduced to about 0.7 e for TM/Cu(100) surfaces. This coincides with the fact that, with respect to 16
Page 16 of 26
TM/Cu(100), three more Cu-O adsorption bonds are formed on the TM/Cu(110) surface alloys. The CO2 adsorption energies for the TM/Cu(110) surface alloys and the pure Cu(110) surface are calculated, and as the TM atom varies from Fe to Cu, the adsorption energies of four systems are -3.2, 2.5, -1.7, and -12.6 kcal/mol, respectively. For three TM/Cu(110) surface alloys, the difference in the adsorption energy is not obvious, and the large negative binding energy of the pure
ip t
Cu(110) indicates that the formation of bent CO2δ- anion on this surface is thermodynamically unstable. Therefore, the introducing other TM atoms into Cu(110) surface also can enhance the
cr
attraction interaction between CO2 and substrate.
us
3.2.2 Chemical Bonding Analyses
The -COHP curves of TM-C and Cu-O adsorption bonds of four Cu(110) systems are displayed in Figure 6. For the TM-C bond, two features at -9.6 and -8.1 eV are corresponding to the σ-
an
bonding interactions between TM 4s (also contain some components of 3dz2 states) and C 2s/2pz orbitals. The π-bonding interactions between TM 3dxz/3dyz and C 2px/2py orbitals give rise to the
M
weak peaks near -7.2 eV, and those peaks observed in the region from -3.5 to -1.3 eV are mainly originated from the strong σ-bonding interactions between TM 3dz2 and C 2s/2pz orbitals.
d
Additionally, 3dxz and 3dyz states of TM are also found for the features near the Fermi level. Thus,
Ac ce pt e
among the five d orbitals, the TM atoms primarily use 3dxz and 3dyz, especially 3dz2 to form the TM-C bond. This is different from the previous TM/Cu(100) surfaces (Figure 3a) that the TM 3dz2 orbital has a neglectable contribution to the formation of TM-C bond. From Figure 6a, it is clear that for the pure Cu(110) surface the bonding peaks of Cu-C bond are less pronounced, and meanwhile the Fermi level lies in the antibonding region. Consequently, the weak Cu-C adsorption bond can be expected, which is in line with the fact that the chemisorption of CO2 on the pure Cu(110) is energetically unfavorable.
17
Page 17 of 26
ip t cr us
(a)
(b)
M
an
Figure 6 -COHP curves for (a) TM-C, and (b) Cu-O adsorption bonds of different TM/Cu(110) surface alloys as well as of the pure Cu(110) surface. The Fermi level (EF) is set at zero. The -COHPs curves shown in figures are the sum of the corresponding spin-up and spin-down states obtained from the spin polarized calculations.
For the Cu-O adsorption bond, the -COHP curves of three TM/Cu(110) surface alloys and pure
d
Cu(110) surfaces are very similar (Figure 6b). Four bonding features are found in the region
Ac ce pt e
between -10 to -4 eV, and they are mainly derived from the σ-bonding interactions between Cu 4s and O 2s/2p orbitals. Similar to the previous cases of Cu(100) surfaces, there are several antibonding peaks below the Fermi level, and besides the Cu 4s state, these peaks also contain some contributions of 3dyz and 3dyz orbitals of Cu atom. Since the Fermi level is located in the antibonding region, the Cu-O adsorption bond is not strong. This is the main reason why the binding strength between CO2 and different Cu(110) surface is weak although four Cu-O adsorption bonds are formed.
3.2.3 d-band Center Analyses
Due to the similar adsorption configurations and nearly identical Cu-O interactions, the adsorption property of CO2 on TM/Cu(110) and pure Cu(110) surfaces is mainly relied on the strength of TM-C bond; in other words, it is still related to the d-band center of TM or Cu atom attached to the carbon atom. The ε dT of TM and Cu atoms on Cu(110) surfaces are listed in Table 2, and the corresponding values are -2.25 (Fe), -1.45 (Co), -1.41 (Ni) and -2.80 eV (Cu), respectively.
18
Page 18 of 26
It is obvious that the sequence of ε dT does not follow the order of the adsorption energies: Co/Cu(110) > Ni/Cu(110) > Fe/Cu(110) > pure Cu(110) (Table 1). Therefore, the adsorption behavior of CO2 on the TM/Cu(110) surface cannot be well described from the simple d-band model.
ip t
Based on the preceding discussion, the TM atom is mainly through the 3dz2, 3dxz and 3dyz orbitals to interact with the carbon atom of CO2, so these 3d orbitals play a major role in the
cr
formation of TM-C adsorption bond. According to eq. (3), the ε dP of TM and Cu atoms are obtained if the contributions of 3dz2, 3dxz and 3dyz orbitals are considered (Table 2), and the order of
us
ε dP is Co (-0.97 eV) > Fe (-1.19 eV) > Ni (-1.40 eV) > Cu (-2.79 eV). Compared with the ε dT , the
an
sequence of ε dP is more consistent with the order of adsorption energies, and in Figure 5b a linear relationship between ε dP and the adsorption energies is observed. The linear regression fit is
M
characterized by a correlation coefficient of R = 0.96 and a standard deviation of 2.12 kcal/mol. However, one exception is that the sequence of Fe/Cu(110) and Ni/Cu(110) is reversed if we
d
concern the values of ε dP (Figure 5b). This deviation is due to ignoring the influence of the Cu-O
Ac ce pt e
adsorption bonds. It is noted that, unlike the (100) and (111) surfaces that only one Cu-O adsorption bond is formed, in here there are four Cu-O adsorption bonds (Figure 2c). Hence, to obtain an accurate description of the adsorption behavior of CO2 on Cu(110) surfaces, the small difference for Cu-O bonding must be taken into account. When the contributions of 3dz2, 3dxz and 3dyz orbitals of the TM atom and the nearest Cu atom are examined together, the ε dP values are changed to -2.00, -1.73, and -1.82 eV for Fe/Cu(110), Co/Cu(110), and Ni/Cu(110), respectively. Now the sequence of the ε dP agrees perfectly with the order of adsorption energies. This can be seen more clearly in the inset of Figure 5b, where there is a good linear relationship between ε dP and the adsorption energy. Thus, it seems that the partial d-band model can provide a more reasonable description of the chemisorption behavior of CO2 on Cu(110) surface alloy if the influence of the neighboring Cu atom is also considered. 3.3 Chemisorption of CO2 on the TM/Cu(111) Surface Alloys 3.3.1 Adsorption Structures 19
Page 19 of 26
For the pure and sublayer TM-doped Cu(111) surfaces, our calculations suggest that CO2 molecule weakly binds to the surface, and the CO2 moiety maintains a nearly linear arrangement. Because the physisorption of CO2 is a nonactivated process, it will not be taken into account in here. However, when one Cu atom at the top layer is replaced by other TM atom with a coverage of 1/9 ML, CO2 molecule can be chemisorbed on the Cu(111) surface. Figure 2d presents the most stable
ip t
structures of the chemisorption of CO2 on the TM/Cu(111) surface alloys, in which the CO2 is also attached to a TM center by the formations of TM-C and TM-O1 bonds (namely, a η2(C,O) linkage),
cr
and meanwhile an additional Cu-O2 bond is formed between the rest oxygen and Cu atoms.
us
Therefore, the coordination of CO2 is similar to the case of TM/Cu(100) system (Figure 2b), and three adsorption bonds are formed on TM/Cu(111) surfaces. Additionally, the strong interaction with the substrate also makes CO2 molecule a bent structure that is similar to CO2δ- anion. However,
an
it is noted that the bending degree is reduced with respect to the adsorption configurations on the Cu(100) and Cu(110) substrates, and the largest O-C-O angle (about 140°) is predicted for
M
TM/Cu(111) surface. This difference means that the interactions between CO2 and TM/Cu(111) are relatively weak, and correspondingly the amount of charge transfer from substrate to adsorbate is
d
reduced to less than 0.60 e. The calculated adsorption energies of three TM/Cu(111) surface alloys
Ac ce pt e
are -3.1, 3.2 and -6.7 kcal/mol as TM atom changes from Fe to Ni. The small magnitude of adsorption energies indicate that the CO2 activation on TM/Cu(111) is not energetically favorable although the alloying of TM with Cu(111) helps the chemisorption of CO2. 3.3.2 Chemical Bonding Analyses
Figure 7 displays the -COHP curves of TM-C, TM-O1 and Cu-O2 adsorption bonds on the TM/Cu(111) surface alloys. For the TM-C bond, the sharp peaks at -9.0 eV and in the region from -2.0 to -1.0 eV are related to the σ-bonding interactions between TM 3dyz and C 2s/2pz orbitals. The feature near -8.0 eV is due to the σ-bonding interactions between TM 4s and C 2pz orbitals, and the peak appeared at -7.0 eV is derived from the π-bonding overlap between TM 3dxz and C 2px orbitals.
20
Page 20 of 26
ip t cr us
(a)
(b)
(c)
M
an
Figure 7 -COHP curves for (a) TM-C, (b) TM-O1, and (c) Cu-O2 adsorption bonds of different TM/Cu(111) surface alloys. The Fermi level (EF) is set at zero. The -COHPs curves shown in figures are the sum of the corresponding spin-up and spin-down states obtained from the spin polarized calculations.
As shown in Figure 7b, for the TM-O1 adsorption bond, those peaks below -6.0 eV are
d
dominated by the σ-bonding interactions between TM 4s and O1 2s/2pz orbitals, while the
Ac ce pt e
contributions of TM 3dz2 and 3dyz states are observed for the broad bonding peaks near -4.0 eV. It is noted that the Fermi level falls in a strong TM-O1 antibonding region, and the peaks just below the Fermi level are correspond to the σ-antibonding interactions between TM 4s/3dz2 and O1 2pz orbitals. For the Cu-O2 adsorption bond, the Cu atom mainly uses 4s and 3dz2 to interact with the orbitals of O2 atom, and the Fermi level still lies in the antibonding region. Hence, similar to the case of TM/Cu(100), it can be expected that the strengths of TM-O1 and Cu-O2 bonds in TM/Cu(111) surface are also not strong.
3.3.3 d-band Center Analyses
By employing the eq. (2), the ε dT of TM atom on three Cu(111) surface alloys are calculated to be -2.29 (Fe), -0.90 (Co), and -1.19 eV (Ni), respectively, and the associated sequence of Co > Ni > Fe does not follow the order of adsorption energies of Co/Cu(111) > Fe/Cu(111) > Ni/Cu(111). However, basing on the COHP results of TM-C and TM-O1 adsorption bonds, if we use the ε dP
21
Page 21 of 26
descriptor that takes into account the contributions of 3dz2, 3dxz and 3dyz orbitals of TM atom (Table 2), the sequence of d-band center will change to Co (-0.87 eV) > Fe (-1.02 eV) > Ni (-1.12 eV), which now correlates well to the adsorption energies. Figure 5c presents the variation of adsorption energies on TM/Cu(111) surface alloys as a function of the ε dP , and an excellent linear relationship
ip t
is established between the ε dP and the adsorption energies. Furthermore, to remedy the deficiency that there are few data points in Figure 5c, and by
cr
considering that the coordination features of CO2 molecule are similar on TM-doped Cu(100) and
us
(111) surfaces, we have also verified the relationship between the ε dP and the adsorption energies when both TM/Cu(100) and TM/Cu(111) systems are considered together. As shown in Figure 5d, it
an
is interesting that a good correlation between ε dP and the adsorption energies is still preserved.
M
4. Conclusions
In the present work, the first-principles DFT calculations combined with a slab model have
d
been carried out to study the CO2 activation on various Cu(100), Cu(110) and Cu(111) surfaces, including the pure and late 3d TM-doped (TM = Fe, Co and Ni) surface alloys. Our results indicate
Ac ce pt e
that, from a thermodynamic point of view, the formation of CO2δ- anion on the three pure Cu surfaces are obviously unfavorable, as well as the cases that the Cu atom at the sublayer is substituted by other TM atom. However, the introducing of TM dopant at the top layer can help the chemisorption of carbon dioxide on three copper surfaces, and interestingly for this doping pattern, the most energetically favorable chemisorption structures on three Cu surfaces are all corresponding to the cases of TM = Co. Therefore, it seems that among the late 3d-metals, cobalt is potentially a good dopant to enhance the chemisorption of CO2 on copper surfaces. After binding to the substrate, obvious charge transfer from the surface to the CO2 moiety is observed, and the CO2 exhibits a bent configuration like that of CO2- anion, indicating the activation of carbon dioxide. To further understand the chemisorption behaviors of CO2 on the surface alloys, the chemical bonding analyses are performed to study the characteristics of adsorption bonds by using the COHP technique. Since only those 3d orbitals vertical to the surface can overlap effectively with the orbitals of the adsorbate, as verified by the COHP results, among five individual d orbital the dz2, dxz 22
Page 22 of 26
and dyz orbitals are the "active" components of TM atoms and will interact with CO2 molecule. Although the adsorption configurations are dependent on the surface structures of the substrates, besides 4s states the metal atoms on different Cu surfaces all primarily tend to adopt the above three 3d orbitals to form the adsorption bonds. Therefore, it is clear that the five d orbitals may play different roles in the reactions happened on the metal surfaces. This is the main reason why the
ip t
correlation between surface d-band center and the binding energies of CO2 is poor when all five 3d orbitals are treated as a whole in the d-band center calculation. However, if the partial d-band center
cr
that only considers the contributions of those "active" orbitals of surface TM atoms is employed, it
us
exhibits a good linear relationship with the binding energies of CO2 for three different TM/Cu
an
surface alloys.
Acknowledgements
M
This work was supported by National Natural Science Foundation of China (grant nos. 21373048, 21203027, 21371034, 21403094, and 21171039), Natural Science Foundation of Fujian Province
d
for Distinguished Young Investigator Grant (2013J06004), Fund of Jiangxi Province Office of
Ac ce pt e
Education (GJJ14261), and the Independent Research Project of State Key Laboratory of Photocatalysis on Energy and Environment (2014A02).
23
Page 23 of 26
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cr
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[44] H. J. Monkhorst, J. D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B 13 (1976) 5188-5192. [45] H.-J. Freund, M. W. Roberts, Surface chemistry of carbon dioxide, Surf. Sci. Rep. 25 (1996) 225-273. [46] R. Dronskowski, P.E. Bloechl, Crystal orbital Hamilton populations (COHP): energy-resolved visualization of chemical bonding in solids based on density-functional calculations, J. Phys. Chem. 97 (1993) 8617-8624. [47] V. L. Deringer, A. L. Tchougreeff, R. Dronskowski, Crystal orbital Hamilton population (COHP) analysis as projected from plane-wave basis sets, J. Phys. Chem. A 115 (2011) 5461-5466. [48] S. Maintz, V. L. Deringer, A. L. Tchougreeff, R. Dronskowski, Analytic projection from plane-wave and PAW wavefunctions and application to chemical-bonding analysis in solids, J. Comput. Chem. 34 (2013) 2557-2567. [49] J. R. Kitchin, J. K. Norskov, M. A. Barteau, J. G. Chen, Modification of the surface electronic and chemical properties of Pt(111) by subsurface 3d transition metals, J. Chem. Phys. 120 (2004) 10240-10246.
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S0169-4332(15)01513-5 http://dx.doi.org/doi:10.1016/j.apsusc.2015.06.165 APSUSC 30686
To appear in:
APSUSC
Received date: Revised date: Accepted date:
21-5-2015 25-6-2015 25-6-2015
Please cite this article as: M. Qiu, Z. Fang, Y. Li, J. Zhu, X. Huang, K. Ding, W. Chen, Y. Zhang, First-principles investigation of the activation of CO2 molecule on TM/Cu (TM = Fe, Co and Ni) surface alloys, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.06.165 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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Density of states of TM 3d orbitals with z-component, adsorption structure and -COHP curves for
M
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the chemisorptions of CO2 molecule on TM/Cu(100) surface alloys.
Highlights
Ac ce pt e
d
Among the late 3d TM metals, cobalt is potentially a good dopant to activate CO2 on copper surfaces. The chemical bonding analyses indicated that TM atom primarily provides d orbitals with z-component, namely dz2, dxz, and dyz orbitals to form TM-C and TM-O adsorption bonds. A modified d-band model that only considers the contributions of those d orbitals with z-component was proposed, which could produce a good linear relationship between adsorption energy and the revised d-band center.
First-principles investigation of the activation of CO2 molecule on TM/Cu (TM = Fe, Co and Ni) surface alloys Mei Qiu a, Zhenxing Fang a, Yi Li Yongfan Zhang a,d,∗
a,*
, Jia Zhu b, Xin Huang
a,c
, Kaining Ding a, Wenkai Chen a,
a
College of Chemistry, Fuzhou University, Fuzhou, Fujian, 350116, China College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, Jiangxi, 330022,China c Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Xiamen, Fujian, 361005, China d State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou, Fujian, 350002, China b
∗
Corresponding authors. E-mail addresses: [email protected] (Y. Li), [email protected] (Y. Zhang) 1
Page 1 of 26
Abstract Periodic density functional theory calculations have been performed to investigate the CO2 activation on a series of TM/Cu surface alloys that are built by dispersing individual later 3d TM atoms (TM = Fe, Co, and Ni) on the Cu(100), Cu(110) and Cu(111) surfaces. The most stable structure with bent CO2δ- configurations on the different TM/Cu surfaces are determined, and the
ip t
results show that among the late 3d-metals, cobalt is potentially a good dopant to activate CO2 on copper surfaces. Using the simple d-band model that treats all five d orbitals of surface TM atom as
cr
a whole, a poor correlation between adsorption energy and the d-band center is observed. To obtain a deep understanding of the adsorption property, the bonding characteristics of the TM-C and TM-O
us
adsorption bonds are carefully examined by the crystal orbital Hamilton population technique, which reveals that the TM atom primarily provides d orbitals with z-component, namely dz2, dxz, and
an
dyz orbitals to interact with the CO2. Based on this result, we propose a modified d-band model that only considers the contributions of those d orbitals with z-component, and by employing this
d
established.
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approach a good linear relationship between adsorption energy and the revised d-band center can be
Ac ce pt e
Keywords: Density functional theory; bimetallic alloys; CO2 activation; d-band center
2
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1. Introduction Bimetallic alloys have attracted considerable interest due to their important applications in heterogeneous catalysis since the concept “bimetal” was proposed in 1960s. The addition of a second metal to the surface of a host can dramatically improve the selectivity, activity, and stability
ip t
in surface-catalyzed reactions[1-7]. Extensive experimental evidence indicates that bimetallic surfaces often exhibit novel properties that are not present on their parent metal surfaces[1,2], and a
cr
general understanding of the adsorption behavior on bimetallic surfaces can help us discover more efficient catalysts for a given reaction [8].
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It is obvious that the chemisorption on the surface of bimetallic transition metal (TM) alloys involves strong interactions between the adsorbate and the surface TM atom, which leads to the
an
formation of adsorption bond between the adsorbate and the TM atom. So the description of such chemical bond is the fundamental basis for exploring surface chemical reactivity of the bimetallic
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TM alloys. Since besides valence s states, the TM atom on the surface primarily uses valence d states to overlap with the orbital of adsorbate, the differences in the adsorption bonds are often described in terms of the interactions between the adsorbate and the TM d-band states. Therefore,
d
for a given adsorbate, the strength of the adsorption bond is mainly relied on the characteristics of
Ac ce pt e
the metal d band. In the well-known d-band model that has been widely used in understanding the bond formation at the transition-metal surfaces[9-11], the center of the d states relative to the Fermi level is chosen as an indicator to quantitatively describe the properties of the metal d band. Basing on the d-band model, the higher the d-band center of a specific TM site is, the stronger the adsorption bond formed at that site will be, and additionally a nearly linear relationship between the binding energies and the surface d-band center have been identified on different TM surfaces, especially on monometallic surfaces [1,2]. However, there are also reports showing that the adsorption behavior on some bimetallic surfaces cannot be well explained from the above simple d-band model [12,13]. So other factors should be considered if we want to obtain a good description of the adsorption properties for bimetallic surfaces. More detailed information about the distribution of the d electrons, such as the width and the shape of d-band states, has been taken into account to improve the d-band model [14,15]. In addition, as pointed out in a recent review by Chen et al. [1], a molecular level understanding of the reaction mechanism on the bimetallic surface would clearly
3
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require more detailed description of the electronic properties, especially the distribution of individual d-orbitals of the surface metal atom. It is worth noting that in the current d-band model, the d-band center is corresponding to the weighted average energy of total d states projected onto a surface TM atom, or in other words, the five d orbitals of TM atom are treated as a whole during calculating the d-band center. However, in
ip t
general, the distributions of five d orbitals of a surface TM atom are different and may be divided into several groups based on the coordination environment of this TM atom. As an instance, if a
cr
copper atom on the Cu(111) surface is replaced by another TM atom, the five d orbitals of TM are
us
not degenerate and will split into three categories: A1{dz2}, E{dx2-y2, dxy}, and E{dxz, dyz} (where A1 and E are the Mülliken notations for the irreducible representations of point group C3v, and the z-axis is set along the surface normal direction). Furthermore, we must take care that when an
an
adsorbate is chemisorbed on the metal substrate, not all d orbitals of a surface TM atom involve in the formations of the adsorption bonds. In most cases, according to the spatial distributions of five d
M
orbitals, only those d states vertical to the surface can effectively overlap with the orbitals of the adsorbate. This implies that the d orbitals with z-component, i.e., dz2, dxz, and dyz orbitals can be
d
seen as the "active" components of the d states and will play a leading role in the surface reactions
Ac ce pt e
on the metal surface. For an example, if we assume that a CO2 molecule is chemisorbed on a surface TM atom by the η1(C) linkage, as schematically illustrated in Figure 1a, the σ interactions between TM dz2 and C 2s/2pz orbitals, and the π interactions between TM dxz (dyz) and C 2px (2py) orbitals can be observed. While the interactions associated with TM dx2-y2 and dxy orbitals correspond to the non-bonding overlap and make a small contribution to the formation of TM-C adsorption bond. For a more complex example that the CO2 molecule is bound to surface by the η2(C,O) linkage (Figure 1b), the TM atom still can employ the dz2, dxz and dyz orbitals to form the TM-C and TM-O adsorption bonds although some orbital interactions are different with the η1(C) model. Consequently, we can expect that the specific role of individual d orbital is necessary to take into account during determining the d-band center.
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(a)
(b)
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Figure 1 Schematic illustrations for the orbital interactions between CO2 molecule and a surface TM atom when CO2 is chemisorbed by the (a) η1(C) linkage, and (b) η2(C,O) linkage.
an
Considering those d orbitals with z-component make significant contributions to the formation of the adsorption bonds on the metal surface, it is interesting to see whether there is an improvement
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in the performance of the d-band model by using above partial d orbitals instead of total d orbitals. To answer this question, in the present work the TM/Cu bimetallic surface alloy is served as a
d
prototype, and the carbon dioxide is selected as an adsorbate because with respect to the simple
Ac ce pt e
atomic adsorbates (such as H, O, etc.), CO2 molecule is able to participate in several distinct types of coordination modes with TM atom [16-19]. This is also motivated by the fact that the interactions between CO2 and metal surfaces have been of great interest in the field of catalytic CO2 activation[20-27]. It has been established that the CO2 activation on the clean metal surfaces usually involves the formation of bent CO2δ- species, which is an important precursor of CO2 dissociation, and therefore, understanding that process is necessary for technical applications such as CO2 utilization. In recent years, some experimental and theoretical investigations have been carried out to study the adsorption and reaction of CO2 molecule on the pure Cu surfaces, as well as on the corresponding bimetallic surfaces, and their results clearly show that the Cu-based surface is an effective catalyst to achieve the activation of inert CO2 molecules [28-34]. However, at present, most attention has been concentrated on the monolayer bimetallic surfaces, few efforts have been devoted to the copper surface that the admetal coverage is very low, especially for the case that the foreign TM atoms are atomically dispersed on the Cu surface [35,36]. In this paper, using density functional theory (DFT), we performed a systematic investigation 5
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of the activation of CO2 molecule on a series of TM/Cu surface alloys that are constructed by dispersing individual and isolated late 3d TM atoms (TM = Fe, Co, and Ni) in Cu(100), Cu(110) and Cu(111) surfaces. We first study the most stable adsorption structure with the bent CO2δconfiguration on different pure and TM-doped Cu surfaces. Next, the bonding characteristics associated with the adsorption bonds are carefully examined by the crystal orbital Hamilton
ip t
population (COHP) technique, and the roles of those 3d orbitals with z-component are discussed. Finally, the d-band center of surface active atom that considers the contributions of the total 3d
cr
states is calculated by employing the simple d-band model, and the poor correlation between
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adsorption energy and the d-band center is achieved, however a good linear correlation will be observed by using a modified d-band model that only takes into account the contributions of those
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3d orbitals with z-component.
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2. Computational details
All DFT calculations were carried out utilizing the Vienna ab initio simulation package
of
Perdew-Burke-Ernzerhof
(PBE)
exchange-correlation
functionals
was
Ac ce pt e
approximation
d
(VASP)[37-39] and the projected augmented wave (PAW) method[40-42]. The generalized gradient
employed[43]. The kinetic energy cutoff for the plane-wave expansion was set to 400 eV, and the Brillouin-zone integrations were sampled using a (5 × 5 × 1) Monkhorst-Pack mesh[44]. The effects of spin polarization were considered and the dipole correction in the surface normal direction was applied. The convergence thresholds of the energy change and the maximum force for the geometry optimizations were set to 10-6 eV and 0.02 eV/Å, respectively. The Cu(100) and Cu(111) surfaces were modeled by periodic slabs consisting of five atomic layers, whereas a seven-layer slab was used for the Cu(110) surface with a more open structure. During the structural optimization, the atoms at the outermost three layers (four layers for (110) surface) were fully relaxed in all directions, while other Cu atoms at the remaining layers were fixed to their bulk positions. The spacing between the adjacent slabs was set to about 15 Å. To simulate the dispersion of individual TM atoms on the Cu surface and to avoid the obvious interactions between neighboring CO2 adsorbates, a supercell consisting of a (3 × 3) surface unit cell was adopted, in which one Cu atom at the top layer or sublayer was substituted by a TM atom, 6
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corresponding to a dopant coverage of 1/9 ML. Therefore, total 21 different copper surfaces were included in the calculations. Since the physisorption is a nonactivated process of CO2 adsorption and is also insensitive to a given surface, it was not considered in the present work. In such case (for example, CO2 capture in porous materials), the dispersion-corrected DFT method is required to properly describe van der Waals interactions between CO2 molecule and the substrate. On the other
ip t
hand, it is well known that the chemisorption of CO2 on a metal surface usually involves electron transfer from the substrate into the CO2 molecule which in turn leads to the formation of CO2δ-
cr
anion with a bent geometry[45]. For the chemisorption of CO2 molecule on different TM/Cu surface
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alloys, many possible adsorption configurations were explored, and the thermodynamic stability of different structure was determined by the adsorption energy that was defined as, (1)
an
E ads = E slab + ECO2 − ECO2 / slab
where ECO2 / slab , E slab , and ECO2 represent the total energies of the adsorbed system, the clean
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TM/Cu surface alloy, and the CO2 in gas phase, respectively. To understand the chemical-bonding picture of the adsorption bond, the projected COHP method developed by Dronskowski group was
d
used[46-48], which can offer a straightforward view onto the orbital-pair interactions. In the
Ac ce pt e
–COHP diagram, the positive and negative values correspond to the bonding and antibonding states, respectively. Furthermore, the d-band center ( ε dT ) of surface active atom that treats all five d orbitals as a whole was calculated using the following equation[49], EF
ε dT
∫ = ∫
Eρ d ( E )dE
−∞ EF −∞
ρ d ( E )dE
(2)
where E is the energy with respect to the Fermi level (EF), and ρd(E) is the density of states (DOSs) projected onto five d orbitals of the surface active atom at energy E.
3. Results and Discussion 3.1 Chemisorption of CO2 on the TM/Cu(100) Surface Alloys 3.1.1 Adsorption Structures
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(b)
(c)
(d)
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(a)
Ac ce pt e
d
Figure 2 The top (upper panel) and side views (bottom panel) of the most stable configuration for the chemisorption of CO2 molecule on (a) pure Cu(100) surface, (b) TM/Cu(100) surface alloys, (c) TM/Cu(110) surface alloys, and (d) TM/Cu(111) surface alloys. The Cu, O, C, and TM atoms are denoted by blue, red, dark gray and green spheres, respectively. We first consider the most stable chemisorption structure of CO2 on the pure Cu(100) surface. As displayed in Figure 2a, CO2 molecule is adsorbed above the fourfold hollow site of Cu(100) surface by one oxygen and carbon atoms, and four adsorption bonds, including two Cu-C and two Cu-O bonds are found at the interface. The lengths of Cu-C and Cu-O bonds are about 2.157, and 2.098 Å (see Table 1), respectively. After adsorption, with respect to the case in the gas phase, two C-O bonds of CO2 are stretched 0.146, and 0.044 Å, respectively, and meanwhile CO2 moiety exhibits a bent configuration with the O-C-O angle of 128.4°. The calculated adsorption energy is about -11.8 kcal/mol, and the negative value indicates that the activation of CO2 is endothermic on the pure Cu(100) surface. Above results are consistent with those reported in a recent theoretical study performed by Liu et al[28]. Furthermore, similar results are also obtained when one Cu atom at the sublayer of Cu(100) surface is substituted by other TM atom, which indicate that the sublayer substitution has small effect on the CO2 activation on the Cu(100) surface. Therefore, we mainly 8
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focus on the results that TM substitution occurs on the top layer of Cu(100) surface.
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Table 1 Some optimized bond lengths (in Å), O-C-O bond angle (in degree), and the calculated adsorption energies (in kcal/mol) for the chemisorption of CO2 molecule on the pure and different TM/Cu surface alloys dC-O2 dTM-C dTM-O1 dCu-O Eads System dC-O1a ∠O-C-O Fe/Cu(100) 1.271 1.257 134.7 1.940 2.025 2.171 5.0 Co/Cu(100) 1.264 1.266 134.6 1.866 2.035 2.140 9.5 Ni/Cu(100) 1.243 1.268 135.0 1.912 2.213 2.121 0.2 b Pure Cu(100) 1.322 1.220 128.4 — -11.8 2.157(×2) 2.098(×2) Fe/Cu(110) 1.301 1.300 120.9 1.972 — -3.2 2.139(×4) Co/Cu(110) 1.304 1.305 120.0 1.895 — 2.5 2.124(×4) Ni/Cu(110) 1.297 1.297 121.0 1.924 — -1.7 2.140(×4) Pure Cu(110) 1.289 1.289 122.5 2.054 — -12.6 2.163(×4) Fe/Cu(111) 1.266 1.226 140.0 1.983 2.006 2.400 -3.1 Co/Cu(111) 1.259 1.236 140.0 1.896 2.042 2.268 3.2 Ni/Cu(111) 1.232 1.239 142.2 1.957 2.286 2.196 -6.7
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a. The symbols O1 and O2 are used to represent two oxygen atoms of CO2 moiety (see Figure 2). For the free CO2 molecule, the optimized length of C-O bond is 1.176 Å. b. The number of bond is shown in parentheses.
d
Consider the chemisorption of CO2 molecule on the Fe/Cu(100) surface alloy. The most
Ac ce pt e
favorable binding conformation of CO2 is shown in Figure 2b, and selected geometrical parameters are listed in Table 1. Compared to the pure Cu(100) surface, CO2 adopts a different configuration on Fe/Cu(100), and now all three atoms of CO2 molecule take part in the interaction with the substrate. It is interesting that CO2 prefers to be adsorbed above the solute Fe atom by the formations of two adsorption bonds (namely, Fe-C, and Fe-O1 bonds), which is corresponding to η2(C,O) linkage. The calculated lengths of Fe-C and Fe-O1 bonds are 1.940 and 2.025Å, respectively. Additionally, the rest oxygen atom (denoted by O2 in Figure 2) of CO2 is attached to one neighboring Cu atom, however the corresponding Cu-O2 distance (2.171 Å) indicates that the interaction between Cu and O2 atoms is weak. In this configuration, the CO2 moiety is also displayed a bent structure with the O-C-O angle of 134.7°. Due to the strong interaction between CO2 and Fe atom, a positive adsorption energy of 5.0 kcal/mol is predicted for Fe/Cu(100) alloy. Thus, by dispersing Fe atoms on Cu(100) surface the chemisorption of CO2 becomes an exothermic process. After CO2 adsorption, an obvious outward movement (0.181 Å) is found for Fe atom. Compared to Fe/Cu(100) surface alloy, similar stable configurations are yielded for the 9
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Co/Cu(100) and Ni/Cu(100) systems (see Figure 2b and Table 1), in which CO2 molecule still binds to the substrate by the formations of TM-C, TM-O1 and Cu-O2 multiple bonds. The calculated adsorption energies are 9.5 (TM = Co) and 0.2 kcal/mol (TM = Ni), respectively. Therefore, the adsorption energies follow a sequence of Co/Cu(100) > Fe/Cu(100) > Ni/Cu(100) > pure Cu(100), indicating that the Co/Cu(100) surface shows the strongest interaction with CO2. Based on above
ip t
results, although CO2 can not be stabilized on the pure Cu(100) surface, the introducing of other TM atoms (especially Co) into the outmost layer of Cu(100) can improve the stability for the
cr
chemisorption of CO2. On the other hand, besides the Cu(100) surface, it is also necessary to make
us
a comparison with another parent metal surface, i.e., the pure TM(100) surface. Liu et al. have studied the adsorption behavior of CO2 on the (100) surfaces of Fe, Co and Ni pure metals with fcc phase using PBE functional[28]. Their results indicated that as observed on the pure Cu(100)
an
surface, CO2 tends to sit above the surface hollow site by forming two TM-C and two TM-O bonds (Figure 2a). Moreover, as going from Fe to Cu, the calculated adsorption energies show a
M
monotonically decreasing trend of Fe(100) > Co(100) > Ni(100) > Cu(100). Therefore, compared with the case of the parent TM(100) surface, the CO2 molecule exhibits a distinct chemisorption
d
behavior on the TM/Cu(100) surface alloys for both the adsorption geometry and the relative
Ac ce pt e
stability. 3.1.2 Chemical Bonding Analyses
To obtain a deeper insight into the interactions between the CO2 adsorbate and the TM/Cu(100) surface alloy, we calculate the -COHP values of TM-C, TM-O1, and Cu-O2 adsorption bonds, respectively.
As shown in Figure 3a, for the TM-C bond, the Fermi level is located in the -COHP curve between the bonding and antibonding regions. Three sharp bonding peaks are found at -9.1, -8.0, and -7.0 eV, respectively, and an additional broad bonding peak is observed in the region from -3.3 to -0.8 eV. Further analyses indicate that these peaks are mainly corresponding to the σ-bonding interactions between 4s/3dxz/3dyz orbitals of TM atom and 2s/2pz orbitals of C atom. On the other hand, the weak σ-antibonding interactions between TM 4s and C 2s orbitals give rise to the feature near -4.0 eV. Therefore, besides 4s orbital, the TM atoms mainly use 3dxz and 3dyz to form TM-C adsorption bond. As a comparison, for the pure Cu(100) surface, the Cu atom interacts with C mainly by using its 4s orbital. 10
Page 10 of 26
ip t cr
(b)
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(a)
(c)
an
Figure 3 -COHP curves for (a) TM-C, (b) TM-O1, and (c) Cu-O2 adsorption bonds of different TM/Cu(100) surface alloys as well as of the pure Cu(100) surface. The Fermi level (EF) is set at zero. The -COHPs curves shown in figures are the sum of the corresponding spin-up and spin-down states obtained from the spin polarized calculations.
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Figure 3b display the -COHP curves of the TM-O1 adsorption bond of different surfaces. It is noted that the Fermi level falls in a strong TM-O1 antibonding region, implying that removing some
d
electrons from the system may enhance the strength of TM-O1 adsorption bond. For those bonding
Ac ce pt e
peaks observed below -3.5 eV, TM atom mainly uses 4s and 3dz2 orbital to overlap with O 2s and 2pz orbitals. While in the antibonding region from -3.5 eV to the Fermi level, the contributions of TM 3dxz/3dyz orbitals are also found.
For the Cu-O2 adsorption bond, the Fermi level also falls in a strong Cu-O antibonding region (Figure 3c). In this case the Cu-O σ-bonding peaks are mainly derived from the interactions between Cu 4s and O 2s/2pz orbitals, and the antibonding peaks in the energetic region from -3 eV to 0 eV contain noticeable 3dxz/3dyz contributions of Cu atom. Therefore, according to above chemical bonding analyses, it is clear that among three adsorption bonds, the TM-C bond plays an important role concerning the stability of the system; whereas for the TM-O1 and Cu-O2 adsorption bonds, the Fermi level lied in the clearly antibonding region indicates that the TM-O1 and Cu-O2 adsorption bonds are not strong. 3.1.3 d-band Center Analyses Since the TM atom is the main active site for the activation of CO2 on the TM/Cu(100) surface
11
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alloys, in here we focus on the d-band center of TM atoms. By using eq. (2), the position of the normal d-band center ( ε dT ) of TM as well as Cu atom that considers the contributions of all five 3d orbitals is determined, and the results are summarized in Table 2. The corresponding d-band centers are -2.25 (Fe), -0.87 (Co), -1.04 (Ni), and -2.69 eV (Cu) relative to the Fermi level, respectively.
ip t
According to the simple d-band model, the higher the d states are in energy, the stronger the adsorption bond is formed between the adsorbate and substrate, which implies that the order of the
cr
adsorption energy is Co/Cu(100) > Ni/Cu(100) > Fe/Cu(100) > pure Cu(100). However, this sequence is different from the calculated result of Co/Cu(100) > Fe/Cu(100) > Ni/Cu(100) > pure
us
Cu(100).
Ac ce pt e
d
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an
Table 2 The centers and electron numbers of five individual 3d orbitals, as well as the total d-band center (εdT) and partial d-band center (εdP) of TM atom on different TM/Cu surface alloys and Cu on the corresponding pure Cu surfaces a dz2(eV) dxy(eV) dxz(eV) dyz(eV) Systems dx2-y2(eV) εdT(eV) εdP(eV) Fe/Cu(100) -1.04 (0.5)b -1.69 (0.1) -1.66 (0.1) -0.82 (0.4) -0.81 (0.4) -2.25 -0.91 Co/Cu(100) -1.07 (1.5) -0.72 (1.4) -0.97 (1.3) -0.81 (1.5) -0.81 (1.5) -0.87 -0.78 Ni/Cu(100) -1.24 (1.7) -0.90 (1.7) -1.03 (1.7) -1.02 (1.7) -1.02 (1.7) -1.04 -0.98 Pure Cu(100) -2.75 (1.8) -2.63 (1.9) -2.60 (1.9) -2.73 (1.8) -2.73 (1.8) -2.69 -2.70 Fe/Cu(110) -1.48 (0.3) -0.94 (0.4) -1.68 (0.3) -1.19 (0.5) -1.68 (0.2) -2.25 -1.19 Co/Cu(110) -1.05 (0.6) -0.83 (0.7) -1.24 (0.6) -1.13 (0.6) -0.98 (0.5) -1.45 -0.97 Ni/Cu(110) -1.39 (1.7) -1.33 (1.7) -1.47 (1.7) -1.53 (1.6) -1.34 (1.7) -1.41 -1.40 Pure Cu(110) -2.78 (1.8) -2.73 (1.8) -2.85 (1.8) -2.90 (1.8) -2.75 (1.8) -2.80 -2.79 Fe/Cu(111) -1.36 (0.3) -1.55 (0.2) -1.36 (0.3) -0.89 (0.4) -0.89 (0.4) -2.29 -1.02 Co/Cu(111) -0.95 (1.5) -0.94 (1.4) -0.95 (1.5) -0.84 (1.5) -0.83 (1.5) -0.90 -0.87 Ni/Cu(111) -1.29 (1.7) -1.15 (1.7) -1.29 (1.7) -1.11 (1.7) -1.11 (1.7) -1.19 -1.12 a. For three Fe/Cu surface alloys, a large magnetic moment (2.7 ~3.0 μB) is observed around Fe atom, and basing on the results of COHP analyses that Fe atom mainly uses its spin-down states to overlap with the orbitals of CO2 molecule, therefore, the centers of spin-down 3d orbitals are presented in here. A similar picture is also seen in Co/Cu(110) system. b. The electron number is shown in parentheses.
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(a)
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Figure 4 (a) The spin-up and spin-down -COHPs curves for the Fe-C adsorption bond, and (b) the atomic DOSs of the spin-up and spin-down states of Fe 3d orbitals of Fe/Cu(100) surface alloy. The Fermi level (EF) is set at zero.
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It is noted that the above value of d-band center is the weighted average energy level of five 3d orbitals of TM atom, namely, the different role of individual 3d orbital played in the formation of adsorption bond is not taken into account. Actually, above COHP analyses clearly confirm that TM
d
atoms tend to use those 3d orbitals with z-component, i.e. 3dz2 and 3dxz/3dyz (the 3dxz and 3dyz
Ac ce pt e
orbitals are degenerate in here) to interact with CO2 molecule. Therefore, it is worthy to study the correlation between the adsorption energy and another kind of d-band center that only above specific 3d orbitals are considered. In Table 2, we list the individual d-band center positions of five isolated 3d orbitals for solute TM atom and Cu atom on the pure Cu(100) surface. It is noted that for Fe/Cu(100) system a remarkable magnetic moment (2.97 μB) is observed around Fe atom, and basing on the spin-up and spin-down -COHPs curves (Figure 4a), Fe atom mainly adopts spin-down states to overlap the orbitals of carbon to form the strong bonding peak just below Fermi level. Moreover, the atomic DOSs (see Figure 4b) indicate that the distributions of the spin-up and spin-down states of the Fe 3d orbitals are quite different, and especially the spin-down states are localized in the region near the Fermi level. This is the reason why the spin-down states are the major 3d components to interact with adsorbate. Similar results also can be observed for the cases of Fe/Cu(110) and Fe/Cu(111) surface alloys (data not shown). Hence, for Fe-doped surfaces, we only focus on the spin-down states of 3d orbitals, and the centers of five spin-down 3d orbitals are 13
Page 13 of 26
reported in Table 2. For Co/Cu(100) and Ni/Cu(100) surface alloys, the magnitudes of magnetic moment around TM atoms are very small (< 0.05 μB). From the data listed in Table 2, it shows that the arrangements of five individual d-band centers are quit different. For instance, the lowest-lying d state is 3dz2 orbital when TM = Fe, while it changes to 3dx2-y2 orbital when TM = Co, Ni, and Cu. Since more than one 3d orbital of TM will involve in the interactions with CO2, we have to
ip t
distinguish their contributions during calculating the new d-band centers. In here, we use a simple and approximate method that takes into account the occupation of each individual d-band, and then
cr
the d-band center can be modified using the following equation, n
ε dP =
i =1 n
i d
ndi
∑ ndi
(3)
an
i =1
us
∑ε
where ndi is the electron number of the associated 3d orbital, and ε di is the center of an individual
M
3d orbital. Using the formula similar to eq. (2), ε di can be calculated when the projected DOS of individual 3d orbital ( ρ di (E ) ) is used. The summation in above equation is only applied to those 3d
Ac ce pt e
d
orbitals that interact with adsorbate. Since ndi is obtained by integrating the projected DOS up to the Fermi level, the eq. (3) can be rearranged as:
⎡ EF Eρ i ( E )dE ⎤ EF i ⎢ ∫−∞ d ⋅ ∫ ρ d ( E )dE ⎥ ∑ EF −∞ ⎢ ⎥ i i =1 ρ ( E )dE ⎣⎢ ∫−∞ d ⎦⎥ n
n
ε dP =
∑ ε di ndi i =1 n
∑ ndi i =1
=
n
∑∫ i =1
EF
−∞
ρ di ( E )dE
n
=
∑∫ i =1 n
EF
−∞
∑∫ i =1
Eρ di ( E ) dE
EF
−∞
ρ di ( E )dE
(4)
So the definition by eq. (3) is equivalent to determine the center of gravity of partial 3d orbitals when only those 3d orbitals with z-component are treated as a whole. To distinguish from the previous ε dT descriptor that is corresponding to the center of gravity of the entire d states, in the following sections we call the value defined by eq. (3) as the partial d-band center ( ε dP ). For the pure and TM-doped Cu(100) surfaces, the values of ε di and ndi of each 3d orbital are listed in Table 2. When the 3dz2, 3dxz and 3dyz orbitals of surface active atoms are concerned, the ε dP value 14
Page 14 of 26
of three surface alloys are -0.91 (Fe), -0.78 (Co), and -0.98 eV (Ni), respectively, and -2.70 eV for the pure Cu(100) surface. Now the order of ε dP is Co/Cu(100) > Fe/Cu(100) > Ni/Cu(100) > pure Cu(100), which is in agreement with the sequence of the adsorption energy. Additionally, in Figure 5a we present the adsorption energies as a function of the ε dP , and a linear relationship between
ip t
adsorption energy and the ε dP can be established. Consequently, it seems that the chemisorption behaviors of CO2 on the different Cu(100) surfaces can be well explained from the partial d-band
(b)
Ac ce pt e
(a)
d
M
an
us
cr
model.
(c)
(d)
Figure 5 The adsorption energies of CO2 on (a) TM/Cu(100), (b) TM/Cu(110), and (c) TM/Cu(111) surface alloys as well as the corresponding pure Cu surfaces as a function of the partial d-band
center ( ε dP ) of surface active atoms. Figure (d) displays the curve if all the same adsorption configurations that CO2 is adsorbed on surface through the formations of TM-C, TM-O1 and Cu-O2 15
Page 15 of 26
bonds (see Figures 2b and 2d) are considered together. The inset in figure (b) is corresponding to the result that the contributions of those 3d orbitals with z-component of the TM atom and the nearest Cu atom are considered together. R: Correlation coefficient, SD: Standard deviation of the fit. 3.2 Chemisorption of CO2 on the TM/Cu(110) Surface Alloys
ip t
3.2.1 Adsorption Structures
The Cu(110) surface exhibits a more open structure, and besides the first layer, the Cu atoms at
cr
the second atomic layer are also exposed to react with the adsorbate (Figure 2c). Correspondingly, there are two TM-doping configurations that come from the replacements of Cu atoms at the first
us
and second layers, respectively. However, by comparing the total energies of two structures, it is indicated that the substitution of TM for Cu atom at the second layer is energetically favorable, and
an
the energy differences between two doping configurations are about 0.33 (Fe), 0.64 (Co), and 0.40 eV (Ni), respectively. Therefore, in the following sections, we focus the chemisorption of CO2 after
M
introducing TM atom at the second layer of Cu(110) surface.
Figure 2c displays the most stable configurations for the chemisorption of CO2 on the different
d
TM/Cu(110) surfaces, and some structural parameters are shown in Table 1. It is clear that the
Ac ce pt e
configurations of CO2 on TM/Cu(110) surface alloys and the pure Cu(110) surface are very similar. Furthermore, for the sublayer substitution of Cu(110), namely the doping of TM atom occurs at the third atomic layer, the corresponding results are nearly identical to that of the pure surface. In the above structure, the carbon atom favors the TM atop site, while two O atoms are adsorbed at the bridge sites of Cu(110) surface. Correspondingly, there are five adsorption bonds, including one TM-C bond and four Cu-O bonds are formed between CO2 moiety and substrate. Compared with the TM/Cu(100) and TM/Cu(111) (see later section) systems, there are more adsorption bonds formed between CO2 moiety and Cu(110) surface. After adsorption, the CO2 molecule on Cu(110) substrate also exhibits a bent structure, and the O-C-O bond angle (about 120°, Table 1) is close to that expected for CO2-. Compared with the configuration on the Cu(100) surface, the small O-C-O bond angle (120° vs. 135°) suggests that CO2 molecule receives more electrons when it is bound to Cu(110) surface. By analyzing the Bader charge distribution, for TM/Cu(110) surfaces about 1.0 e electrons are transferred from the substrate to CO2, while the magnitude of charge transfer is reduced to about 0.7 e for TM/Cu(100) surfaces. This coincides with the fact that, with respect to 16
Page 16 of 26
TM/Cu(100), three more Cu-O adsorption bonds are formed on the TM/Cu(110) surface alloys. The CO2 adsorption energies for the TM/Cu(110) surface alloys and the pure Cu(110) surface are calculated, and as the TM atom varies from Fe to Cu, the adsorption energies of four systems are -3.2, 2.5, -1.7, and -12.6 kcal/mol, respectively. For three TM/Cu(110) surface alloys, the difference in the adsorption energy is not obvious, and the large negative binding energy of the pure
ip t
Cu(110) indicates that the formation of bent CO2δ- anion on this surface is thermodynamically unstable. Therefore, the introducing other TM atoms into Cu(110) surface also can enhance the
cr
attraction interaction between CO2 and substrate.
us
3.2.2 Chemical Bonding Analyses
The -COHP curves of TM-C and Cu-O adsorption bonds of four Cu(110) systems are displayed in Figure 6. For the TM-C bond, two features at -9.6 and -8.1 eV are corresponding to the σ-
an
bonding interactions between TM 4s (also contain some components of 3dz2 states) and C 2s/2pz orbitals. The π-bonding interactions between TM 3dxz/3dyz and C 2px/2py orbitals give rise to the
M
weak peaks near -7.2 eV, and those peaks observed in the region from -3.5 to -1.3 eV are mainly originated from the strong σ-bonding interactions between TM 3dz2 and C 2s/2pz orbitals.
d
Additionally, 3dxz and 3dyz states of TM are also found for the features near the Fermi level. Thus,
Ac ce pt e
among the five d orbitals, the TM atoms primarily use 3dxz and 3dyz, especially 3dz2 to form the TM-C bond. This is different from the previous TM/Cu(100) surfaces (Figure 3a) that the TM 3dz2 orbital has a neglectable contribution to the formation of TM-C bond. From Figure 6a, it is clear that for the pure Cu(110) surface the bonding peaks of Cu-C bond are less pronounced, and meanwhile the Fermi level lies in the antibonding region. Consequently, the weak Cu-C adsorption bond can be expected, which is in line with the fact that the chemisorption of CO2 on the pure Cu(110) is energetically unfavorable.
17
Page 17 of 26
ip t cr us
(a)
(b)
M
an
Figure 6 -COHP curves for (a) TM-C, and (b) Cu-O adsorption bonds of different TM/Cu(110) surface alloys as well as of the pure Cu(110) surface. The Fermi level (EF) is set at zero. The -COHPs curves shown in figures are the sum of the corresponding spin-up and spin-down states obtained from the spin polarized calculations.
For the Cu-O adsorption bond, the -COHP curves of three TM/Cu(110) surface alloys and pure
d
Cu(110) surfaces are very similar (Figure 6b). Four bonding features are found in the region
Ac ce pt e
between -10 to -4 eV, and they are mainly derived from the σ-bonding interactions between Cu 4s and O 2s/2p orbitals. Similar to the previous cases of Cu(100) surfaces, there are several antibonding peaks below the Fermi level, and besides the Cu 4s state, these peaks also contain some contributions of 3dyz and 3dyz orbitals of Cu atom. Since the Fermi level is located in the antibonding region, the Cu-O adsorption bond is not strong. This is the main reason why the binding strength between CO2 and different Cu(110) surface is weak although four Cu-O adsorption bonds are formed.
3.2.3 d-band Center Analyses
Due to the similar adsorption configurations and nearly identical Cu-O interactions, the adsorption property of CO2 on TM/Cu(110) and pure Cu(110) surfaces is mainly relied on the strength of TM-C bond; in other words, it is still related to the d-band center of TM or Cu atom attached to the carbon atom. The ε dT of TM and Cu atoms on Cu(110) surfaces are listed in Table 2, and the corresponding values are -2.25 (Fe), -1.45 (Co), -1.41 (Ni) and -2.80 eV (Cu), respectively.
18
Page 18 of 26
It is obvious that the sequence of ε dT does not follow the order of the adsorption energies: Co/Cu(110) > Ni/Cu(110) > Fe/Cu(110) > pure Cu(110) (Table 1). Therefore, the adsorption behavior of CO2 on the TM/Cu(110) surface cannot be well described from the simple d-band model.
ip t
Based on the preceding discussion, the TM atom is mainly through the 3dz2, 3dxz and 3dyz orbitals to interact with the carbon atom of CO2, so these 3d orbitals play a major role in the
cr
formation of TM-C adsorption bond. According to eq. (3), the ε dP of TM and Cu atoms are obtained if the contributions of 3dz2, 3dxz and 3dyz orbitals are considered (Table 2), and the order of
us
ε dP is Co (-0.97 eV) > Fe (-1.19 eV) > Ni (-1.40 eV) > Cu (-2.79 eV). Compared with the ε dT , the
an
sequence of ε dP is more consistent with the order of adsorption energies, and in Figure 5b a linear relationship between ε dP and the adsorption energies is observed. The linear regression fit is
M
characterized by a correlation coefficient of R = 0.96 and a standard deviation of 2.12 kcal/mol. However, one exception is that the sequence of Fe/Cu(110) and Ni/Cu(110) is reversed if we
d
concern the values of ε dP (Figure 5b). This deviation is due to ignoring the influence of the Cu-O
Ac ce pt e
adsorption bonds. It is noted that, unlike the (100) and (111) surfaces that only one Cu-O adsorption bond is formed, in here there are four Cu-O adsorption bonds (Figure 2c). Hence, to obtain an accurate description of the adsorption behavior of CO2 on Cu(110) surfaces, the small difference for Cu-O bonding must be taken into account. When the contributions of 3dz2, 3dxz and 3dyz orbitals of the TM atom and the nearest Cu atom are examined together, the ε dP values are changed to -2.00, -1.73, and -1.82 eV for Fe/Cu(110), Co/Cu(110), and Ni/Cu(110), respectively. Now the sequence of the ε dP agrees perfectly with the order of adsorption energies. This can be seen more clearly in the inset of Figure 5b, where there is a good linear relationship between ε dP and the adsorption energy. Thus, it seems that the partial d-band model can provide a more reasonable description of the chemisorption behavior of CO2 on Cu(110) surface alloy if the influence of the neighboring Cu atom is also considered. 3.3 Chemisorption of CO2 on the TM/Cu(111) Surface Alloys 3.3.1 Adsorption Structures 19
Page 19 of 26
For the pure and sublayer TM-doped Cu(111) surfaces, our calculations suggest that CO2 molecule weakly binds to the surface, and the CO2 moiety maintains a nearly linear arrangement. Because the physisorption of CO2 is a nonactivated process, it will not be taken into account in here. However, when one Cu atom at the top layer is replaced by other TM atom with a coverage of 1/9 ML, CO2 molecule can be chemisorbed on the Cu(111) surface. Figure 2d presents the most stable
ip t
structures of the chemisorption of CO2 on the TM/Cu(111) surface alloys, in which the CO2 is also attached to a TM center by the formations of TM-C and TM-O1 bonds (namely, a η2(C,O) linkage),
cr
and meanwhile an additional Cu-O2 bond is formed between the rest oxygen and Cu atoms.
us
Therefore, the coordination of CO2 is similar to the case of TM/Cu(100) system (Figure 2b), and three adsorption bonds are formed on TM/Cu(111) surfaces. Additionally, the strong interaction with the substrate also makes CO2 molecule a bent structure that is similar to CO2δ- anion. However,
an
it is noted that the bending degree is reduced with respect to the adsorption configurations on the Cu(100) and Cu(110) substrates, and the largest O-C-O angle (about 140°) is predicted for
M
TM/Cu(111) surface. This difference means that the interactions between CO2 and TM/Cu(111) are relatively weak, and correspondingly the amount of charge transfer from substrate to adsorbate is
d
reduced to less than 0.60 e. The calculated adsorption energies of three TM/Cu(111) surface alloys
Ac ce pt e
are -3.1, 3.2 and -6.7 kcal/mol as TM atom changes from Fe to Ni. The small magnitude of adsorption energies indicate that the CO2 activation on TM/Cu(111) is not energetically favorable although the alloying of TM with Cu(111) helps the chemisorption of CO2. 3.3.2 Chemical Bonding Analyses
Figure 7 displays the -COHP curves of TM-C, TM-O1 and Cu-O2 adsorption bonds on the TM/Cu(111) surface alloys. For the TM-C bond, the sharp peaks at -9.0 eV and in the region from -2.0 to -1.0 eV are related to the σ-bonding interactions between TM 3dyz and C 2s/2pz orbitals. The feature near -8.0 eV is due to the σ-bonding interactions between TM 4s and C 2pz orbitals, and the peak appeared at -7.0 eV is derived from the π-bonding overlap between TM 3dxz and C 2px orbitals.
20
Page 20 of 26
ip t cr us
(a)
(b)
(c)
M
an
Figure 7 -COHP curves for (a) TM-C, (b) TM-O1, and (c) Cu-O2 adsorption bonds of different TM/Cu(111) surface alloys. The Fermi level (EF) is set at zero. The -COHPs curves shown in figures are the sum of the corresponding spin-up and spin-down states obtained from the spin polarized calculations.
As shown in Figure 7b, for the TM-O1 adsorption bond, those peaks below -6.0 eV are
d
dominated by the σ-bonding interactions between TM 4s and O1 2s/2pz orbitals, while the
Ac ce pt e
contributions of TM 3dz2 and 3dyz states are observed for the broad bonding peaks near -4.0 eV. It is noted that the Fermi level falls in a strong TM-O1 antibonding region, and the peaks just below the Fermi level are correspond to the σ-antibonding interactions between TM 4s/3dz2 and O1 2pz orbitals. For the Cu-O2 adsorption bond, the Cu atom mainly uses 4s and 3dz2 to interact with the orbitals of O2 atom, and the Fermi level still lies in the antibonding region. Hence, similar to the case of TM/Cu(100), it can be expected that the strengths of TM-O1 and Cu-O2 bonds in TM/Cu(111) surface are also not strong.
3.3.3 d-band Center Analyses
By employing the eq. (2), the ε dT of TM atom on three Cu(111) surface alloys are calculated to be -2.29 (Fe), -0.90 (Co), and -1.19 eV (Ni), respectively, and the associated sequence of Co > Ni > Fe does not follow the order of adsorption energies of Co/Cu(111) > Fe/Cu(111) > Ni/Cu(111). However, basing on the COHP results of TM-C and TM-O1 adsorption bonds, if we use the ε dP
21
Page 21 of 26
descriptor that takes into account the contributions of 3dz2, 3dxz and 3dyz orbitals of TM atom (Table 2), the sequence of d-band center will change to Co (-0.87 eV) > Fe (-1.02 eV) > Ni (-1.12 eV), which now correlates well to the adsorption energies. Figure 5c presents the variation of adsorption energies on TM/Cu(111) surface alloys as a function of the ε dP , and an excellent linear relationship
ip t
is established between the ε dP and the adsorption energies. Furthermore, to remedy the deficiency that there are few data points in Figure 5c, and by
cr
considering that the coordination features of CO2 molecule are similar on TM-doped Cu(100) and
us
(111) surfaces, we have also verified the relationship between the ε dP and the adsorption energies when both TM/Cu(100) and TM/Cu(111) systems are considered together. As shown in Figure 5d, it
an
is interesting that a good correlation between ε dP and the adsorption energies is still preserved.
M
4. Conclusions
In the present work, the first-principles DFT calculations combined with a slab model have
d
been carried out to study the CO2 activation on various Cu(100), Cu(110) and Cu(111) surfaces, including the pure and late 3d TM-doped (TM = Fe, Co and Ni) surface alloys. Our results indicate
Ac ce pt e
that, from a thermodynamic point of view, the formation of CO2δ- anion on the three pure Cu surfaces are obviously unfavorable, as well as the cases that the Cu atom at the sublayer is substituted by other TM atom. However, the introducing of TM dopant at the top layer can help the chemisorption of carbon dioxide on three copper surfaces, and interestingly for this doping pattern, the most energetically favorable chemisorption structures on three Cu surfaces are all corresponding to the cases of TM = Co. Therefore, it seems that among the late 3d-metals, cobalt is potentially a good dopant to enhance the chemisorption of CO2 on copper surfaces. After binding to the substrate, obvious charge transfer from the surface to the CO2 moiety is observed, and the CO2 exhibits a bent configuration like that of CO2- anion, indicating the activation of carbon dioxide. To further understand the chemisorption behaviors of CO2 on the surface alloys, the chemical bonding analyses are performed to study the characteristics of adsorption bonds by using the COHP technique. Since only those 3d orbitals vertical to the surface can overlap effectively with the orbitals of the adsorbate, as verified by the COHP results, among five individual d orbital the dz2, dxz 22
Page 22 of 26
and dyz orbitals are the "active" components of TM atoms and will interact with CO2 molecule. Although the adsorption configurations are dependent on the surface structures of the substrates, besides 4s states the metal atoms on different Cu surfaces all primarily tend to adopt the above three 3d orbitals to form the adsorption bonds. Therefore, it is clear that the five d orbitals may play different roles in the reactions happened on the metal surfaces. This is the main reason why the
ip t
correlation between surface d-band center and the binding energies of CO2 is poor when all five 3d orbitals are treated as a whole in the d-band center calculation. However, if the partial d-band center
cr
that only considers the contributions of those "active" orbitals of surface TM atoms is employed, it
us
exhibits a good linear relationship with the binding energies of CO2 for three different TM/Cu
an
surface alloys.
Acknowledgements
M
This work was supported by National Natural Science Foundation of China (grant nos. 21373048, 21203027, 21371034, 21403094, and 21171039), Natural Science Foundation of Fujian Province
d
for Distinguished Young Investigator Grant (2013J06004), Fund of Jiangxi Province Office of
Ac ce pt e
Education (GJJ14261), and the Independent Research Project of State Key Laboratory of Photocatalysis on Energy and Environment (2014A02).
23
Page 23 of 26
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Ac ce pt e
d
M
an
us
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