Theoretical Investigation of CO2 Adsorption and Dissociation on Low

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Theoretical Investigation of CO Adsorption and Dissociation on Low Index Surfaces of Transition Metals Xuejing Liu, Lei Sun, and Weiqiao Deng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12660 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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The Journal of Physical Chemistry

Theoretical Investigation of CO2 Adsorption and Dissociation on Low Index Surfaces of Transition Metals Xuejing Liu, Lei Sun and Wei-Qiao Deng*

State Key Laboratory of Molecular Reaction Dynamics, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Corresponding Author *

E-mail: [email protected]

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Adsorption and dissociation processes of gas molecules on bulk materials and nano-materials are essential for catalytic conversion of carbon dioxide (CO2). In this work, we systematically investigated the CO2 adsorption and dissociation on low index surfaces of different transition metal by Density Functional Theory (DFT) calculations. A comparison study demonstrates that the open surfaces (Fe(100), Ni(100), Ni(110), Rh(100) and Ir(100)) have stronger interactions with CO2 molecules than the close-packed surfaces. The order of energy barriers for CO2 dissociation is Fe(110), Ir(100) < Ru(0001), Rh(100), Co(0001), Ni(100) < Os(0001), Ni(111) < Ir(111), Rh(111), Ni(110) < Fe(100), Pt(111) < Cu(100), Pd(111) < Cu(111). The interaction order between the dissociative CO*, O* species and the surfaces is Fe(100) ˃ Fe(110) ˃ Ru(0001) ˃ Os(0001) ˃ Ir(100), Rh(100) ˃ Ni(110) ˃ Co(0001) ˃ Rh(111), Ir(111) ˃ Ni(100), Ni(111) ˃ Cu(100) ˃ Pt(111) ˃ Cu(111), Pd(111). In addition, we found that the change trend of adsorption energy is consistent with that of charge transfer amounts from the low index surfaces to CO2. The Brønsted−Evans−Polanyi (BEP) relation showed that the electronic effects of Ni(111) and Ni(110), Cu(111) and Cu(100) and the geometric effects of Fe(110) and Fe(100), Ir(111) and Ir(100) have great influence on the CO2 dissociation, which is closely related to cleavage of C-O in transition states. Our results may provide an insight into the design of highly efficient nano-catalysts for CO2-involved reactions.


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1. Introduction The catalytic conversions of carbon dioxide molecule into valuable fuels and chemicals are attracting growing interest because of their importance in industrial, sustainable energy, and environmental management.1-3 As the global demand for energy grows, fossil fuels will likely continue to dominate world’s energy consumption. Because CO2 emissions from fossil fuels are considered to be an important driving factor of worldwide climate change, 4 , 5 the CO2 reduction will be more and more important in the near future. However, CO2 is such a stable and almost inert compound that numerous reduction reactions often have high activation energy and is still facing a great challenge. The highly effective catalyst is the only alternative to accomplish the reduction of CO2 in mild conditions.6 Transition metal-based catalysts with various advantages have been extensively investigated in heterogeneous catalysis and also commonly used for chemical reactions involving CO2 activation and conversion.7-10 Especially, a wide variety of transition metal nanoparticles with several nanometres are synthesized by shape-controlled methods and mostly bounded by low index facets, such as the {111}, {100}, {110} facets and so on. A fundamental understanding of the activity of small molecules on different low index surfaces of transition metal catalysts has become very important. 11 Different facets of materials possess unique geometric and electronic structure and thermal stability.12 It is well known that the surface energy of different facets of a face-centered cubic (fcc) metal is increased in the order of γ{111} < γ{100} < γ{110} < γ{hkl}.13-15 The (110) facet for a body-centered cubic (bcc) metal and the (0001) facet for a hexagonal close-pack (hcp) metal are the most favorable. However, the low index surfaces are frequently studied because they generally compose the majority of facets of particles due to their low surface energy. As a consequence, transition metal nanoparticles are mostly bounded by low-index facets, such as tetrahedron, octahedron, decahedron, and icosahedron bounded by {111} facets,16-18 cube by {100},19,

20

cuboctahedron by the mixture of {111} and



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{100}, 21 and rhombic dodecahedron by {110}. 22 From the point of view, the relationship of structure-properties of the low index surfaces will play an important role in encouraging the development of new nano-catalysts. On the other hand, some descriptors for certain classes of reactions on transition metal surfaces have been developed in silica design of catalysts. 23 Combining the advantages of both theoretical calculations and experimental studies has been explored as a long-standing goal.24,

25

The study of the dissociation of CO2 molecule catalyzed by different low

index surfaces is therefore useful in understanding the activity of catalysts and designing new catalysts with enhanced activity. In the past decades, there have been many experimental and theoretical studies on the activation of CO2 molecule on transition metal surfaces. Several experimental studies demonstrated that the dissociation of CO2 molecule is affected by the temperature, the presence of promoters, the surface morphology and defects.26-28 Theoretically, quantum chemistry calculations have been trying to establish the relationship between microscopic surface properties and macroscopic catalytic properties. In order to improve the design efficiency of the catalyst, some descriptors have been developed, such as the d center, the Brønsted−Evans−Polanyi (BEP) relation, scaling relation, etc. Wang et al. found that both d-band centers of the transition metal surfaces and the charge transfer should control the chemisorption of CO2.29 The charge transfer of the substrate to the anti-bonding orbital of CO2 induces the production of anionic CO2δ− species and this state is referred to as a precursor of the CO2 dissociation reaction.30-32 A few studies showed that the CO2 dissociation can be possibly enhanced by alkali metals, stepped or kinked surfaces, and so on.33-37 For example, the interaction with CO2 on Co(100) is stronger than on Co(110) and the CO2δ− state as an intermediate is employed to study the CO2 dissociation.37 Li et al. showed that the strong activation CO2 is due to the relatively small work function or electronegativity

of

Ti.

38

Especially,

the

first

theoretical

studies

of

the dynamics of dissociation of CO2 on Ni(100) surface were recently reported.39-41 It was demonstrated that the CO2 molecule from the physisorbed state to chemisorbed state exits an “early” barrier and the “late” dissociation is significantly promoted by ACS Paragon Plus Environment

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vibrational excitations. It was also revealed that CO2 has a large trapping probability in a chemisorption well due to energy transfer and the lattice effect is stronger on the “late” barrier.41 Some studies42,

43

have shown that the CO2δ− state with relatively low

diffusion barriers has high mobility and the reaction pathways pass through the chair-like conformation (hollow 2-fold), regardless of the initial position of CO2 on Ni(110) surface.42 Ko et al investigated the CO2 activation and dissociation on the stable transition metal surfaces with dispersion correction.44 In recent years, more and more single metal systems have been studied together in order to use some developed descriptors to identify the catalytic activity. In this paper, we investigated the catalytic activity of different low index surfaces with high surface CO2 coverage for CO2 dissociation without adding dispersion correction. The low index surfaces include the (110), (100) and (111) facets of bcc Fe, the (111), (100) and (110) facets of fcc Ni, the (111) and (100) facets of fcc metals (Cu, Rh, Ir, Pd, Pt, Ag and Au), and the (0001) facet of hcp metals (Co, Ru and Os). Firstly, we examined the activation of CO2 molecule via the adsorption energy, geometric parameters and net charge transfer amount. Secondly, we compared reaction energies and energy barriers by the Brønsted−Evans−Polanyi relationship, and identified the imaginary vibration frequency and C-O bond lengths of transition states. We finally examined the electronic and geometric effects of low index surfaces on CO2 dissociation, and the interactions between the dissociative CO* and O* species and low index surfaces.

2. Computational Methods All

periodic

DFT

calculations

were

performed

with

the

Vienna Ab-initio simulation package (VASP). 45 , 46 The PBE functional within the generalized gradient approximation (GGA)

47 , 48

was used to describe the

exchange-correlation interaction. The electron–ion interactions were represented by the projector augmented wave (PAW) method.49,50 The energy cutoff for the kinetic energy of 450 eV was employed. The Fermi-surface effects have been treated by the smearing technique of Methfessel-Paxton, 51 using a smearing parameter of 0.20



eV. The geometries were optimized using a conjugate-gradient method until the forces acting on each atom were less than 0.02 eV·Å–1. All calculations were performed using 4 × 4 × 1 Monkhorst–Pack grids and (2 × 2) four-layer slabs with the top two layers relaxed. Vacuum layers 15 Å in thickness were added above the top layer of slabs in all cases. All energy values presented below were extrapolated to 0 K. We considered spin polarization for Fe, Ni, and Co low index surfaces. Bader charge calculations were performed for the CO2 adsorption systems.52,53 The climbing image nudge-elastic-band (CI-NEB) method 54 , 55 was used to search the transition states. Saddle points and minima were considered converged when the maximum force in every degree of freedom was less than 0.02 eV·Å–1. We calculated the adsorption energies according to the following equation: Eads=Eadsorbate/slab − (Eadsorbate + Eslab) Where Eadsorbate/slab, Eadsorbate, Eslab are the energy of the adsorbed system, the gas-phase molecule, and the bare surface, respectively. A negative value of Ead indicates that the adsorption is exothermic. Similarly, the dissociative adsorption energy for CO* and O* is defined as ∆Echem=Edissociated adsorbate/slab − (Eadsorbate + Eslab) The activation barrier is defined as Ea = ETS – Eadsorbate/slab, where TS and adsorbate/slab correspond to the transition state (TS) and the initial adsorption state (IS), respectively. The reaction energy ∆E is defined as the energy difference between the final state (FS) and the initial state (IS). As displayed in Table S1 of Supporting information, we first calculated lattice constants of twelve transition metals, which are good agreement with the experimental values.56,57 The calculated magnetic moment of 0.63 µB/atom for Ni metal, 1.63 µB/atom for Co metal and 2.19 µB/atom for Fe metal are in agreement with the corresponding experimental values of 0.61 µB/atom, 1.71 µB/atom and 2.22 µB/atom.56, 57

3. Results and Discussion


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Figure 1. Geometric structures of fcc(111), fcc(100), bcc(110) and hcp(0001) surfaces. Each low index surface of transition metal has different adsorption sites for a CO2 molecule. Fcc(111) and hcp(0001) surface have four adsorption sites of top, bridge and three-fold hollow (fcc and hcp) sites. More open fcc(100) surface has three adsorption sites of top, bridge and four-fold hollow sites. Bcc(110) surface has four adsorption sites of top, short/long bridge and three-fold hollow sites (See Figure 1).

3.1 Comparative study of CO2 adsorption on different low index surfaces of transition metals We first investigated the adsorption properties of CO2 on different low index surfaces of transition metals. The adsorption energy and geometry parameters (the distances of dC-M and dC-O, bond angle θoco) are shown in Table 1 and Table S3 of Supporting information. Additionally, we compared the adsorption energies with previous results with and without vdW effects in Table S2. CO2 molecules are preferred to be physisorbed on different adsorption sites of Fe(111), Cu(111), Cu(100), Ag(111), Au(111), Pd(100), Pt(100), Ag(100) and Au(100) surfaces. On top sites of Co(0001), Os(0001), Ni(111), Ni(100), Rh(111), Ir(111),



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Pd(111), Pt(111), Rh(100) and Ir(100) surfaces, CO2 molecules are also preferred to be physisorbed with weak interactions. The physisorption of CO2 is a slight exothermic process, which is considered to be a constant value. For any adsorption sites of the transition metal surfaces that can’t activate CO2 (such as Au, Cu, and Ag), the CO2 is initially placed on the surfaces above 1.9 ~ 2.0 Å and after optimized the CO2 is adsorbed on the surfaces above ~3 Å. For the optimized geometries of physisorbed CO2, the bond lengths of C-O are ~ 1.18 Å, and the bond angles of OCO (θoco) are 179.77°−179.99°, which are agreement with geometric parameters of gas-phase CO2 molecule (dC−O=1.18 Å, θoco = 180°). This implies that the CO2 molecules are not activated on adsorption sites of above transition metal low index surfaces.

Table 1. Adsorption energies Eads (in eV) for CO2 and CO2δ− on different adsorption sites in transition metal low index surfaces. Surfaces

Eads

Eads

Eads

(Top)

(Bridge)

(Hollow)

−0.42(lb) Fe(110)

−0.23

Surfaces

Eads

Eads

Eads

(Top)

(Bridge)

(Hollow)

Rh(111)

−0.034

0.13

0.24(h)

Ir(111)

−0.031

0.30

0.30(b)

−0.56

−0.56(h) Fe(100)

−1.01(b)

−1.01

−1.47

Pd(111)

−0.032

0.30

0.30(b)

Co(0001)

−0.044

0.17

0.16(h)

Pt(111)

−0.027

0.39

−0.031(h)

Ni(111)

−0.020

0.40

0.31(h)

Ru(0001)

−0.054(b)

−0.054

−0.16(h)

Ni(100)

−0.052

0.066

−0.25

Os(0001)

−0.12

0.059

−0.13(h)

Ni(110)

−0.33(b)

−0.33

−0.52

Rh(100)

−0.045

−0.13

−0.36

Fe(111)

−0.025

−0.071

−0.078(h)

Ir(100)

−0.038

−0.19

−0.29

Cu(111)

−0.027

−0.034

−0.024

Pd(100)

−0.047

−0.038

−0.057

Cu(100)

−0.048

−0.053

−0.056

Pt(100)

−0.043

−0.043

−0.043


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Ag(111)

−0.031

−0.029

−0.035

Ag(100)

−0.040

−0.032

−0.033

Au(111)

−0.029

−0.031

−0.033

Au(100)

−0.038

−0.030

−0.031

The Eads in bold represents the adsorption energy of CO2 δ− adsorbed on transition metal surface.

However, for other transition metal low index surfaces that can activate CO2 molecules, the CO2 molecule is initially located on the surfaces above 1.90 Å ~ 2.00 Å and after optimized the CO2 molecule is bent and adsorbed on the surfaces above 1.89 Å ~ 2.17 Å. The CO2 on these transition metal surfaces are chemisorption and the chemisorbed CO2 is referred as CO2δ− state. The adsorption energies (Eads) of CO2δ− states adsorbed on transition metal low index surfaces are in bold (See Table 1). Comparing the adsorption energies, we found that the CO2 molecule is preferred to be chemisorbed on Fe(100), Fe(110), Ni(110), Rh(100), Ir(100), Ni(100), Ru(0001) and Os(0001) surfaces in turn. When the CO2 molecules are initially placed on top sites by different orientations and parallel to Fe(110) surface, after optimized the CO2δ− states are adsorbed on the long bridge or hollow sites. The adsorption energies of CO2δ− state on different sites in Fe(110) surface are agreement with those reported, 29,

58

while the adsorption energy value (absolute value) of CO2δ− state on long bridge site is smaller than the reported value with dispersion correction. Besides, the CO2 molecules are initially placed on top sites and parallel to Fe(100), Ni(110) and Ru(0001) surfaces, the optimized CO2δ− states are adsorbed on bridge sites. The most stable adsorption sites for CO2δ− states are hollow sites for Fe(100), Ni(110) and Ru(0001) surfaces. For Co(0001), Ni(111), Rh(111), Ir(111), Pd(111) and Pt(111) surfaces, the adsorption energy values of CO2δ− states adsorbed on bridge and hollow sites are smaller than those for CO2δ− states adsorbed on other transition metal low



index surfaces. The initial CO2 parallel to the hollow sites of Ir(111) and Pd(111) surfaces are optimized to the nearest bridge sites, exhibiting the sensitivity to surface sites. The CO2δ− states adsorbed on hollow sites of Rh(100) and Ir(100) are more favorable than on bridge sites. Compared with the close-packed surfaces, more open Fe(100), Ni(100), Ni(110), Rh(100) and Ir(100) surfaces have stronger interactions with CO2 molecules. The Fe(100), Fe(110), Ni(110), Rh(100), Ir(100), Ni(100), Ru(0001) and Os(0001) surfaces can more easily activate CO2 molecules than the most stable Co(0001), Rh(111), Ir(111), Pd(111) and Pt(111) surfaces. The activation of CO2 chemisorption is significantly important to catalysis of CO2 conversion. The results implied that the chemisorption of CO2 molecules depends not only on the type of transition metal, but also on surface geometry.

Figure 2. Optimized geometries of CO2δ− adsorbed at Bridge (B), Long-bridge (LB) and Hollow (H) sites on Fe(110), Fe(100), Ni(110), Ni(100), Co(0001) and Ir(111) surfaces in top and side view.


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Figure 2 shows the optimized CO2δ− geometries on Fe(110), Fe(100), Ni(110), Ni(100), Co(0001) and Ir(111) as different surfaces representations in top and side view. From top view, it is clear that that the adsorption sites of CO2δ− include the bridge(B), long-bridge(LB) and hollow(H) sites. The geometric parameters for CO2δ− adsorbed on transition metal low index surfaces are displayed in Table 2. The CO2δ− states on the most stable hollow sites of Fe(110) and Fe(100) are bent with the bond angles θOCO of 127.75° and 120.31°, and the bond lengths (C-O1 and C-O2) of 1.23 Å and 1.36 Å, 1.35 Å and 1.35 Å, respectively. The CO2δ− states on the most stable hollow sites of Ni(100) and Ni(110) are bent with the bond angles θOCO of 125.11° and 121.83°, and the bond lengths (C-O1 and C-O2) of 1.22 Å and 1.37 Å, 1.31 Å and 1.31 Å, respectively. In comparison, the bond angles and bond lengths of CO2δ− states on hollow sites of Fe(110) and Ni(100), Fe(100) and Ni(110) are similar. For (0001) surfaces of hcp Co, Ru and Os metals, the order for the bond angles θOCO for CO2δ− states on hollow sites is θOCO (Co) > θOCO (Ru) > θOCO (Os). CO2δ− states on the Rh(100) and Ir(100) than on the Rh(111) and Ir(111) have smaller bond angles and larger bond lengths, which imply a much higher activation ability for CO2. In short, for the same type of transition metal, the smaller the bond angles θOCO, the larger the values of adsorption energy. More open surfaces can activate CO2 molecule with a larger change in θoco and dc-o.

Table 2. Geometric parameters (the distances of C-M, C-O1 and C-O2 in Å, and OCO angle θ in degrees) of CO2δ− states on transition metal low index surfaces.



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Surfaces

Sites

dC-M

θoco

dC-O1

dC-O2

Surfaces

Sites

dC-M

θoco

dC-O1

dC-O2

Fe(110)

LB

1.98

135.55

1.26

1.26

Rh(111)

B

2.09

136.53

1.22

1.27

H

1.97

127.75

1.23

1.36

H

2.04

129.43

1.27

1.27

B

2.08

136.64

1.26

1.26 Ir(111)

B

2.13

130.97

1.21

1.30

H

2.13

120.31

1.35

1.35

B

2.09

140.42

1.24

1.24

Ru(0001)

B

2.09

138.77

1.24

1.25

H

1.96

136.98

1.24

1.28

H

2.10

124.29

1.30

1.30

B

2.14

143.06

1.21

1.27

B

2.17

129.58

1.21

1.31

H

2.02

134.27

1.27

1.27

H

2.10

118.99

1.32

1.32

B

1.97

137.87

1.21

1.27

B

2.06

135.46

1.22

1.28

H

2.04

125.11

1.22

1.37

H

2.04

123.06

1.27

1.34

B

1.89

137.85

1.24

1.25

B

2.08

129.12

1.21

1.31

H

1.89

121.83

1.31

1.31

H

2.03

117.99

1.28

1.38

B

2.14

143.97

1.21

1.24

B

2.10

133.40

1.21

1.29

Fe(100)

Co(0001)

Ni(111)

Ni(100)

Ni(110)

Pd(111)

Os(0001)

Rh(100)

Ir(100)

Pt(111)

The LB, B and H represent long bridge (for BCC Fe), bridge and three- or four-fold hollow sites, respectively.

For the chemisorbed transition metal surfaces, the C atoms are ~2.00 Å above the surfaces and the dC−M for CO2δ− adsorbed on Fe(110) and Ni(110) is the shortest (1.98 Å and 1.89 Å). The symmetric lengths of dC−O1 and dC−O2 are observed except for Rh(111), Ir(111), Rh(100), Ir(100), Ni(100), Pd(111) and Pt(111) (See Figure 2). The dC−M of CO2δ− on bridge site in Fe(100) and Ni(100) is shorter than that of CO2δ− on hollow site while the dC−M of CO2δ− on hollow site of Co(0001), Os(0001), Rh(111) and Ni(111) is shorter than that of CO2δ− on bridge site. Although the dC−O1 , dC−O2 and dC−M varied with different transition metals, the bond angles θOCO for CO2δ− on hollow site is smaller than that for CO2δ− on bridge site. The bond angles θOCO and bond lengths for CO2δ− states show remarkable differences, revealing the capabilities


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of different transition metal to activate CO2 molecule. Table 3. Bader charge of CO2δ− adsorbed on different low index surfaces of transition metals Net charge Surfaces

Net charge Surfaces

Sites

Sites

(CO2δ−, |e|)

(CO2δ−, |e|)

Fe(110)

H

−1.02

Ni(111)

B

−0.42

Fe(100)

H

−1.36

Ni(100)

H

−0.72

Co(0001)

H

−0.69

Ni(110)

H

−0.90

Rh(111)

B

−0.40

Rh(100)

H

−0.70

Ir(111)

B

−0.42

Ir(100)

H

−0.78

Pd(111)

B

−0.28

Ru(0001)

H

−0.76

Pt(111)

B

−0.31

Os(0001)

H

−0.84

The bader charge was calculated for the CO2δ− adsorption systems as summarized in Table 3. It showed that the chemisorbed CO2 possesses a partial negative charge, which is transferred from the low index surfaces. The negative charge on chemisorbed CO2 ranges from −1.36 |e| to −0.28 |e|, depending on the type of transition metal and surface geometry. The apparent net charge for CO2δ− states on Fe(100) and Fe(110) surfaces are respective −1.36 |e| and −1.02 |e|, corresponding the larger C-O bond elongation and the larger adsorption energy. Secondly, the net charge of CO2δ− states on Ni(110), Os(0001), Ir(100), Ru(0001), Ni(100), Rh(100) and Co(0001) surfaces ranges from −0.90 |e| to −0.69 |e|, also corresponding the larger C-O bonds elongation.



Figure 3. Change trends of adsorption energy and Bader charge of CO2 on different transition metals low index surfaces. It was also found that the change trend of adsorption energy is consistent with that of charge transfer amount from the surfaces to CO2, as shown in Figure 3. The charge transfers amount for open surfaces of transition metal is more than the one for the close-packed surfaces. Therefore, the catalyst materials will more easily transfer electrons to improve their ability of activating C=O bond if having the characteristics of Lewis base or smaller surface work function or lesser electronegativity. In addition, we calculated the difference charge density of the systems with CO2 chemisorption, as shown in Figure S1. The yellow and blue regions represent positive and negative charge, respectively. It intuitively shows the charge accumulation and depletion for the CO2 chemisorption on transition metal surfaces. For Ru(0001)- and Os(0001)-CO2 systems, the orbital interactions for C-M-bonding are more obvious due to the nature of transition metal and surface geometry.

3.2 Comparative study of activity of different low index surfaces of


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transition metals on CO2 dissociation It is well known that numerous reduction reactions of CO2 often have high activation energies due to its chemical stability. In many heterogeneous catalytic processes, the dissociative chemisorption of CO2 is a key step. Thus, it is important to study the activity of transition metal surfaces for CO2 dissociation and the interaction between the dissociative species and transition metal surfaces. Comparing the activity and the interaction can provide insight into the geometric and electronic effects of different low index surfaces on the CO2 dissociation and help the design of catalyst materials. Nonetheless, the dissociation of CO2 physisorbed on Cu, Ag, and Au are also studied to extract the general trends over a wide range of transition metal surfaces, although it is expected that these surfaces can’t dissociate the CO2 molecule in mild condition. To model CO2* ⇒ CO* + O* dissociation on transition metal surfaces, we started from the initial state (IS), where CO2 is adsorbed on a stable adsorption site. To find final state (FS) we substantially increase the bond length of C=O. After optimized the CO* species is finally located at the top, bridge or hollow sites for different transition metal surfaces and the O* species is finally located at bridge or hollow sites. The dissociative energies of the CO* and O* species on different adsorption sites are summarized in Table S5. It showed that the O* is stably adsorbed at a hollow site and the CO* is adsorbed at a top or hollow site in Fe(110) and Ni(111) surfaces, where the dissociation energies are comparable. On Co (0001) surface, the CO* and O* species are easier to adsorb at hollow sites, so that the CO* species that



adsorbs at a top site may migrate. On Ru(0001) and Os(0001) surfaces, the CO* and O* species that are respectively adsorbed at top and hollow sites are more stable than both at hollow sites. On Fe(100) and Ni(100) surfaces, both CO* and O* species are stably adsorbed at hollow sites while on the surface of Ni (110) the CO* and O* species can’t be adsorbed at hollow sites and can only be adsorbed at the bridge sites. The CO* and O* species that respectively are adsorbed at top and hollow sites are more stable in Rh(111), Ir(111) and Ir(100) surfaces while the CO* and O* species that respectively are adsorbed at bridge and the hollow sites are stable in Rh(100) surface. The result implies that the stable adsorption sites of the CO* and O* species are significantly dependent on the type of transition metal and surface geometry. We made use of CO2 chemisorption geometry as the initial state except for Cu, Ag and Au surfaces and the stable adsorption geometry for the CO* and O* along the direction of C=O cleavage as the final state. Transition states were searched the by CINEB method. And the elongated C-O bond lengths and imaginary vibration frequencies of the transition states are shown in Figure 4.


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Figure 4.

Structures of Fe(110)-TS (341i cm-1), Fe(100)-TS (365i cm-1),

Co(0001)-TS (475i cm-1), Ni(111)-TS (485i cm-1), Ni(100)-TS (373i cm-1), Ni(110)-TS (378i cm-1), Cu(111)-TS (426i cm-1), Cu(100)-TS (392i cm-1), Ru(0001)-TS (83i cm-1), Rh(111)-TS (456i cm-1), Rh(100)-TS (427i cm-1), Pd(111)-TS (310i cm-1), Os(0001)-TS (233i cm-1), Ir(111)-TS (385i cm-1), Ir(100)-TS (106i cm-1), Pt(111)-TS (240i cm-1), Ag(111)-TS (235i cm-1) and Au(111)-TS (240i cm-1) searched by CINEB method for dissociation of CO2 on different transition metal surfaces. The bond lengths of C-O of transition states are represented and labeled in angstroms. We identified single imaginary vibration frequency for all transition states in CO2 dissociation,

which

ranges

from 83i cm-1 to

485i cm-1. In Figure

4,

the imaginary vibration frequencies of transition state on Fe, Co, Ni, Cu and Rh surfaces are larger than those of transition states on Ir, Pd, Pt, Ru, Os, Ag and Au surfaces. The imaginary vibration frequencies of transition states on more open TM(100) surfaces are smaller than those of transition states on the TM(111) surfaces



(TM=Rh, Ir, Ni ,and Cu). It was also found that the larger C-O bond lengths of transition states, the larger imaginary vibration frequency of the transition state for Fe, Cu and Ir metal surfaces. However, the elongated C-O bond lengths of transition states are more than 2.00 Å on Pd(111), Pt(111), Ag(111) and Au(111) surfaces, and these surfaces have a very low activity for CO2 dissociation. In addition, the elongated C-O bond lengths of transition states on Os(0001), Ru(0001), Ir(100), Fe(110) and Co(0001) surfaces are smaller from 1.27 Å to 1.64 Å, which are closely related to the adsorption sites of the dissociative CO* and O* species. Obviously, the elongated C-O bond lengths of transition states are also dependent on the type of transition metal and surface geometry. In the development of the qualitative to quantitative research for catalytic reactions, the researchers verified that the reaction energy is linear correlation with energy barrier by DFT calculations, which is called the Brønsted−Evans−Polanyi relation. In the studies of heterogeneous catalysis, the extensive study of BEP relationships can not only help researchers better understand the structure and performance of catalysts but also help design more efficient catalysts. Herein, we investigated the BEP relation between the activation barriers and reaction energy of CO2 dissociation over transition metal low index surfaces. As illustrated in Figure 5 from the data of Table S4, the more thermodynamically favorable the reaction, the lower the activation barrier. The order for energy barriers of CO2 dissociation is Fe(110), Ir(100) < Ru(0001), Rh(100), Co(0001), Ni(100) < Os(0001), Ni(111) < Ir(111), Rh(111), Ni(110) < Fe(100), Pt(111) < Cu(100), Pd(111) < Cu(111). However,


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the CO2 conversion reaction depends not only on the low energy barrier of CO2 dissociation but also on the moderate interaction between the dissociative CO* and O* species and the surfaces. The study of dissociative adsorption showed that the interaction between the species of CO* and O* and the surfaces also depends on both the type of transition metal and surface geometry. The BEP relation showed that the energy barriers are high for CO2 dissociation on Cu(111), Cu(100), Ag(111), Au(100), Pd(111) and Pt(111) surfaces. Because there is a weak physisorption and the CO2 molecule cannot be effectively activated. Meanwhile, it was observed that the calculated reaction energy for the dissociative CO* and O* species with these surfaces was almost equal to the activation energy so that the CO2 would be easily generated through the reverse reaction. The dissociative CO* and O* species on Pd(100), Pt(100), Ag(100) and Au(100) surfaces can’t be stably adsorbed, which leads to no transition state searched by CINEB method. This suggested that the CO2 dissociation can’t be driven by the interactive energy of the dissociative species of CO* and O*.



Figure 5. The Brønsted−Evans−Polanyi relationship between reaction energy and activation barrier for CO2 dissociation on the transition metal surfaces. Conversely, if the dissociation has a lower barrier for chemisorption CO2 states, the CO* and O* desorption will need a higher energy depending on the type of transition metal and surface geometry, which will limit the following oxidation reactions. Therefore, the optimum of interaction strength is the volcanic peak, which is known as the Sabatier principle. The comparison of dissociation adsorption energy shows that the order of interaction between the dissociative CO* and O* species and the surfaces is Fe(100) ˃ Fe(110) ˃ Ru(0001) ˃ Os(0001) ˃ Ir(100), Rh(100) ˃ Ni(110) ˃ Co(0001) ˃ Rh(111), Ir(111) ˃ Ni(100), Ni(111) ˃ Cu(100) ˃ Pt(111) ˃ Cu(111), Pd(111). For the same transition metal, the geometric and electronic effects of different low index surfaces on CO2 dissociation can be distinguished from the BEP relation. On Ni(111), Ni(110), Cu(111), Cu(100) surfaces, the change of reaction


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energies and energy barriers is along the direction of BEP line, revealing that the electronic effect on the CO2 dissociation is prominent. It is closely related to the similar C-O distances in their transition states (1.65 Å and 1.75 Å, 1.77 Å and 1.67 Å, in Figure 4). However, on Fe(110), Fe(100), Ir(111) and Ir(100) surfaces, the reaction energies are almost equal while the energy barrier greatly changes, indicating that the geometric effect has great influence on the CO2 dissociation. The distances of C-O in their transition states are different for 1.59 Å and 1.90 Å, 1.79 Å and 1.43 Å.

4. Conclusions We systematically studied the adsorption and dissociation of CO2 on different low index surfaces by DFT calculations. The results showed that the CO2 is physisorbed on transition metals Cu, Ag and Au low index surfaces, Pd(100) and Pt(100) surfaces. For other low index surfaces of transition metals, the CO2 is chemisorbed with the bent of the structure. The analyses of adsorption energy bond anglees θOCO, C-O bond lengths and bader charge transfers demonstrated that the activation of CO2 depends on the type of transition metal and surface geometry. The most stable Fe(110), Ru(0001) and Os(0001) surfaces can better activate CO2 molecules than the most stable Co(0001), Rh(111), Ir(111), Pd(111) and Pt(111) surfaces. More open Fe(100), Ni(100), Ni(110), Rh(100) and Ir(100) surfaces can more activate CO2 molecule. The change of charge transfer is consistent with that of adsorption energy. The BEP relation showed that the order of energy barriers for CO2 dissociation is Fe(110), Ir(100) < Ru(0001), Rh(100), Co(0001), Ni(100) < Os(0001), Ni(111)

Theoretical Investigation of CO2 Adsorption and Dissociation on Low

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