Selective Electrocatalytic Mechanism of the CO2 Reduction Reaction

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

Selective Electrocatalytic Mechanism of the CO Reduction Reaction to CO on Silver Electrodes: A Unique Reaction Intermediate Xia-Guang Zhang, Xi Jin, De-Yin Wu, and Zhong-Qun Tian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08170 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 21, 2018

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The Journal of Physical Chemistry

Selective Electrocatalytic Mechanism of the CO2 Reduction Reaction to CO on Silver Electrodes: A Unique Reaction Intermediate Xia-Guang Zhang, Xi Jin, De-Yin Wu*, Zhong-Qun Tian State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, Fujian, China

*

Email: [email protected]; Tel: +86-592-2189023 1

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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: The mechanism of the CO2 reduction reaction (CO2RR) on silver electrode surfaces has been investigated by using density functional theory based on the geometric and electronic structures of the reactant, transition states, intermediates, and products. The calculated results show that surface adsorbed hydrogen could promote the conversion of CO2 to *COOH and monodentate HCOO* (HCOO*M) intermediates. Electronic structure analysis indicates that bidentate HCOO* (HCOO*B) is more stable than *COOH. In particular, HCOO*M is first formed in the HCOOH path, has a similar thermodynamic energy as *COOH in the CO path on silver surfaces. The transformation of the two intermediates into each other by hydrogen transfer has a larger energy barrier, suggesting that this process is difficult to occur in the present condition. The activation energy barrier from the HCOO*M intermediate back to CO2 is much lower than that from HCOO*M to HCOO*B, leading to kinetic inhibition of the formation of HCOO*B on silver surfaces. H2O could also assist in promoting CO formation. This finding shows that the CO2RR selectivity for CO over HCOOH is a synergetic effect of thermodynamic and kinetic control, which strongly depends on the geometric and electronic structures of silver electrode surfaces.

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INTRODUCTION Carbon dioxide (CO2) reduction to carbon monoxide (CO) or hydrocarbons is a challenging and important research field because it could be used to solve environmental and energy problems.1 CO2 is an extremely thermodynamically stable molecule with a very high reduction potential.2 Different intermediates and various kinds of products have been found under different reaction conditions, including different electrolytes,3-6 pH values,7 catalysts,8-13 and applied potentials.14-16 As a result, the investigation of the CO2 reduction reaction (CO2RR) is very difficult.17 The activity and selectivity of catalysts are the most important factors in controlling the product composition. In the CO2RR field, many metal electrocatalysts have been used to study catalyst activity and selectivity.18-23 Copper, silver and gold as common metal electrodes exhibit good properties: a hydrocarbon mixture could be obtained on copper surfaces, 17, 24-29 and CO could be obtained on gold and silver with high faradaic efficiencies of approximately 87.1% and 81.5%, respectively.25, 30-34 Interest in catalysts based on silver is growing because silver is more earth-abundant and inexpensive than gold, and has a strong cathodic photoeffect35-36 and high reaction activity and selectivity.12, 37-43 Different adsorbed species have been detected by electrochemical, infrared and Raman spectroscopy techniques during the CO2RR on silver surfaces and nanoparticles, such as CO2–, HCOOH, CO, and the intermediates *COOH and HCOO*.37, 44-46 According to these products and intermediates, several CO2RR mechanisms have been proposed in the 3



literature. These mechanisms can be divided into three categories. The first one occurs through conversion of adsorbed CO2– to *COOH or CO.47-49 The second one involves simultaneously obtaining a proton and an electron from CO2 and producing *COOH.25, 50 Additionally, some scientists consider the CO to originate from the interaction between CO2 and HCO3–.51-52 For silver catalysts, CO2 reduction to *COOH or HCOO* is referred to as a one-proton and one-electron reaction, and the first electron transfer step of the CO2RR is the rate-determining step due to its very high equilibrium potential.53 The CO2– anion was observed in Raman spectra recorded on rough silver electrodes54-55 and alkali metal-predosed silver surfaces.8 Furthermore, attenuated total reflection Fourier transform infrared spectroscopy was used to observe the formation of CO2– under a more negative applied potential of -1.6 V versus a Ag/AgCl reference electrode.56 Hori also found that the CO2– anion could be stabilized on many metals at suitable potentials.25, 57 However, according to density functional theory (DFT) calculations, it is difficult to acquire CO2– from low-index single crystal surfaces of silver,58-59 indicating that the CO2– formation on silver surfaces is variable. The intermediate would be driven to form *COOH or HCOO* through the first proton and electron transfer step. From the adsorption configuration, *COOH could be further reduced to CO or HCOOH, while the transformation from *COOH to HCOOH has a larger overpotential than that from *COOH to *CO on metal and alloy surfaces.48 HCOO* could be reduced to HCOOH and H2COO*, and the latter species could be further reduced to hydrocarbons.25,

60

The reduction from HCOO* to

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H2COO* also has a higher Gibbs free energy barrier than that from HCOO* to HCOOH. Therefore, only *COOH and HCOO* were considered as the intermediates of CO and HCOOH, respectively. In a previous study, it was also noted that the reaction path of formic acid decomposition to CO2 is easier than that to CO.61 The former decomposition path has a lower barrier because HCOOH directly dissociates into bidentate HCOO*, then transforms to monodentate HCOO*, and finally forms the product CO2.62 Another high barrier path is the conversion of *COOH to CO2.61 The reverse reaction from CO2 to *COOH was found to have a higher barrier than the reaction from CO2 to monodentate HCOO*. Other previous DFT calculations also suggested that the CO2RR to HCOOH is thermodynamically easier than that to CO on silver surfaces.63 This is a strange phenomenon because the conversion of CO2 to CO has a much higher faradaic efficiency than its conversion to HCOOH on silver electrodes.34, 53 This inconsistency raised a new challenge in understanding the experimental results of the CO2RR on silver electrodes. Thus, understanding the properties of reaction intermediates is an important way to determine the reaction mechanism of the CO2RR. In this work, we focus on looking for the reaction pathway and the reason why CO is the primary product on silver electrodes. Our results demonstrate that the intermediates *COOH and monodentate HCOO* show similar stability on silver surfaces and that the former transforms into CO, whereas the latter contributes to the reversion to CO2. Then we 5



further consider the factors in the surface environment influencing the chemical selectivity of the CO2RR. The presence of H2O would promote *COOH formation and then reduce *COOH to CO, whereas surface H* would promote the formation of intermediates. Our theoretical results of the CO2RR on the silver surface are helpful for clearly understanding the experimental phenomenon regarding the catalytic activity and reaction selectivity of silver compared with copper and gold. COMPUTATIONAL METHODS We mimicked single-crystal surfaces of silver, Ag(111), Ag(100), Ag(110), and Ag(211). A gamma-centered 12×12×12 sampling grid was used to calculate the lattice constant of Ag, which is 4.174 Å, in agreement with the experimental value (4.09 Å).64 A six-layer 2×2 supercell with the bottom two layers fixed was constructed in all calculations, and a Gamma-centered 6×6×1 k-point sampling grid and vacuum spaces of 15 Å were adopted for the single-crystal facets studied in this work. During geometry optimization, energies were converged to 10-5 eV. The Methfessel-Paxton method with a broadening factor of 0.1 eV was used. The electron-ion interaction was described by the projector-augmented wave (PAW) method.65 A plane-wave basis cutoff of 450 eV for a plane-wave basis was used for the wavefunctions. For the density of states (DOS), the k-point sampling grid increased to 18×18×1. The reaction transition states were optimized by using the nudged elastic band (NEB) method and dimer method. Finally, the Bader charge analysis was calculated by using a code from the Henkelman group.66 In addition, the vibrational frequencies of

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adsorbed molecules were calculated with the finite difference method under the condition of all metal atoms frozen. All calculations were performed with the Perdew-BurkeErnzerhof (PBE) functional of generalized gradient approximation (GGA)67 in the Vienna ab initio simulation package (VASP).68 The adsorption energies (Eads) used in this work were evaluated as

Eads  Etot  Emetal  Emole

(1)

where Etot, Emetal, and Emole are the total electronic energies of molecules adsorbed on a slab, a clean metal slab, and free molecules, respectively. Thermodynamic properties were calculated using the Atomic Simulation Environment (ASE) Suite of programs.69 The Gibbs free energies at 298.15 K and 1 atm were calculated with

G  H  TS  EDFT +EZPE  

298.15 K

0

Cv dT  TS

(2)

where EDFT is the total energy obtained from DFT optimization, EZPE is the zero-point vibrational energy, ∫CvdT is the heat capacity, T is the Kelvin temperature, and S is the entropy. The ideal gas approximation was used for CO2, H2, CO, HCOOH, and H2O molecules, and the harmonic approximation was used to describe the adsorbents. The reaction free energy change was also used to calculate electrochemical potentials with the computational hydrogen electrode (CHE) model.70 The CHE model was used in this work to equate the electrochemical potential of proton-electron pairs and hydrogen gas under standard conditions,

H aq   e  

1 H2  g  2

(3) 7



RESULTS AND DISCUSSION Electronic Structures of Adsorbed Species. Both the geometric and electronic structures of reaction intermediates play an important role in the CO2RR on silver surfaces. For the proton and electron transfer reactions of CO2, two intermediates, *COOH and HCOO*, correspond to the products *CO and HCOOH along two parallel paths, respectively. Here the * denotes the atoms binding to surface sites. Figure 1 presents the DOS in the evolution process from CO2 to *COOH and HCOO* through reaction stages. First, from the view of orbital overlap, we assessed the interaction of silver with adsorbed species *COOH. The DOS far away from the surface (10 Å, a) and the physical adsorption state (b) were very similar to each other, which indicates a very weak binding interaction in the physisorption state. In turn, from left to right (Figure 1, a and b), four peaks were attributed successively to the mixture of the lone-pair orbital of the O atoms with the 2s orbital of the C atoms, the mixture of the O lone-pair orbital with the σ bonding orbital of the C=O bond, the 2p orbital of the O atoms, and the π orbital of the C=O bond. However, the chemisorption configuration on silver surfaces could not be predicted from previous DFT calculations due to the abnormal and unstable structural deformation of CO2.66, 71-72 Therefore, we did not consider CO2 chemisorbed on the silver surface in this study. Only a small adsorption energy of approximately -0.02 eV (Table 1) of the physisorption structure could be found in a configuration parallel to silver surfaces,71 and the distances between CO2 and Ag(111), Ag(100), Ag(110) and Ag(211) surfaces were approximately 8


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3.864, 3.605, 3.442 and 3.257 Å, respectively.73 For the *COOH intermediate (Figure 1d), the orbital overlap was found in low energy states (less than -8 eV) for the silver surface interacting with transition states and the adsorption intermediate *COOH. The binding interaction can be closely associated with the σ bonding orbitals of C and Ag atoms. For the *COOH intermediate, the binding interaction between C and Ag is located in the energy range of -8.5 ~ -6.0 eV (Figure S4) and is assigned to the mixture of the s and p orbitals of the C atoms with the d band of the Ag atoms. Furthermore, we also assessed the interaction between Ag and O atoms for *COOH adsorption on the silver surface (Figure S5). For energies less than -6.0 eV, there is a strong orbital overlap between the silver d band and the oxygen p orbital. For energies greater than -6.0 eV, the Ag–O binding interaction plays an important role in stabilizing the *COOH adsorption on silver surfaces. Second, we evaluated the formation of the HCOO* intermediate on the silver surface (Figures 1 e-g and S7), in both lower and higher energy states. There is good orbital overlap between the p orbital of the O atoms and the d band of the Ag atoms (Figure S8). The interaction between silver and monodentate HCOO* (HCOO*M) was weaker than that between silver and bidentate HCOO* (HCOO*B). Thus, HCOO*B has a large adsorption energy. For *COOH and HCOO*B adsorbed on a Ag(100) surface, the d band centers of the surface Ag atoms were estimated to be approximately -3.29 and -3.07 eV, respectively, indicating that the low d band weakens the substrate-adsorbate interaction. Therefore, Ag

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DOS

DOS

DOS

DOS

DOS

DOS

has a stronger binding interaction with HCOO*B than with *COOH.

DOS

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

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a

12 8 4 0 12 8 4 0 12 8 4 0 12 8 4 0 12 8 4 0 12 8 4 0 12 8 4 0

b

c

d

e

f

g

-10

-8

-6

-4

-2

0

2

4

Energy (E-EF,eV) Figure 1. DOS plots at different reaction stages from CO2 to the *COOH and HCOO* intermediates on a (2×2) supercell Ag(100) surface. a) CO2 at approximately 10 Å from the surface, b) physisorbed CO2, c) the transition state of H* transfer to CO2, d) *COOH 10


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adsorption state, e) the transition state of CO2 and H* to HCOO*M, f) HCOO*M adsorption state, g) HCOO*B adsorption state. Blue, CO2 and H atoms; and red, Ag atoms interacting with CO2 and H atoms.

Third, we focus on our attention on the effect of different crystal surfaces. The calculated adsorption energies of the two intermediates listed in Table 1 indicate that the HCOO*B was more stable in energy than *COOH. The results also show that the adsorption energy significantly increases with the crystal surface index. For *COOH adsorbed on Ag(111), Ag(100), Ag(110) and Ag(211) surfaces (Table 1), the bond angles OCO are 117.4º, 117.2º, 116.1º and 116.3º, respectively. Their Ag–C and Ag–O bond lengths are in order, Ag(111) > Ag(100) > Ag(110) > Ag(211). For example, the bond lengths of Ag–O on Ag(111) > Ag(100) > Ag(110) > Ag(211) surfaces are 2.510, 2.465, 2.388 and 2.388 Å, respectively. All these values are clearly larger than the Ag-O bond lengths of HCOO*B, which are approximately 2.248, 2.232, 2.210 and 2.207 Å on the corresponding silver surfaces. We predicted the asymmetric and symmetric stretching frequencies of the C=O bond in free CO2 to be 2366 and 1388 cm-1, close to experimental frequencies at 2349 and 1388 cm-1, respectively.48 At the physical adsorption states on these surfaces, as shown in Table 2, the two frequencies decrease slightly, in good agreement with previously observed values.58 However, for *COOH and HCOO*B on Ag(111), Ag(100), Ag(110) and Ag(211) surfaces, the vibrational frequencies significantly change to 1534, 1535, 1510 and

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1523 cm-1 for the former and 1527,1534, 1543 and 1540 cm-1 for the latter, respectively. The Bader charge analysis shows that the charge transfer from the silver surface to the HCOO*B intermediate (approximately 0.59 - 0.61 e) was greater than that from the silver surface to *COOH (approximately 0.31 - 0.35 e). The differences between the two intermediates in charge transfer imply that the binding interaction between Ag and HCOO*B was stronger than that between Ag and *COOH, as shown in Table 1. In addition, the free energies of HCOO*B were more negative than those of *COOH on silver surfaces. This finding seemingly indicates that formic acid would form more easily than CO on silver surfaces, similar to previous theoretical studies.74-75 However, this prediction does not conform to experimental results, in which CO is a primary product.25, 57, 76 Nørskov and Studt proposed that there is a kinetic control of the reduction of CO2 to HCOOH on silver surfaces.63 To obtain a clear understanding the selectivity of the CO2RR to CO on silver electrode surfaces, the kinetic control mechanism still needs to be explored.

Table 1. Adsorption energy and structural parameters of physisorbed CO2 and intermediates HCOO* and *COOH on different silver surfaces. HCOO* Eads

dAg-O

*COOH dC-O

OCO

Eads

dAg-C

dAg-O

dC=O

dC-O(H)

OCO

Ag(100)

-2.68 2.231

1.267

129.5

-1.80 2.165

2.465

1.237

1.355

117.2

Ag(111)

-2.65 2.241

1.268

129.5

-1.74 2.190

2.532

1.238

1.353

117.1

12


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Ag(110)

-3.00 2.209

1.267

129.7

-2.02 2.158

2.388

1.244

1.358

116.0

Ag(211)

-2.83 2.207

1.267

129.6

-1.94 2.160

2.389

1.241

1.356

116.3

a) The units of Eabs are eV, those of d are Å, and those of OCO are degrees. All C–H and O–H bond distances are approximately 1.112 and 0.979 Å on these silver surfaces, respectively. The adsorption energy of CO2 adsorption on (100), Ag(111), Ag(110) and Ag(211) surfaces are -0.01, -0.01, -0.02, and -0.02 eV, respectively, and the bond length of C=O is 1.177 Å for them.

Table 2. Vibrational frequencies (cm-1) of physisorbed CO2 and intermediates HCOO* and *COOH on different silver surfaces.a) HCOO*

*COOH

υs(CH) υs(CO) υr(CH)

υs(CO) υb(CH) υb(OCO)

υs(OH) υs(CO) υb(OH) υs(CO) υs(OCO) υr(OH)

Ag(100)

2902 1534 1315

1310 988

728

3655 1570 1191 1088 662

633

Ag(111)

2910 1527 1312

1307 984

729

3658 1559 1189 1093 657

616

Ag(110)

2908 1543 1319

1314 991

731

3665 1543 1189 1088 669

638

Ag(211)

2906 1540 1311

1310 989

730

3655 1556 1189 1091 669

634

a) For CO2 physisorption on Ag (100), Ag(111), Ag(110) and Ag(211) surfaces, the asymmetric stretching frequencies are 2349, 2346, 2345, and 2355 cm-1, and the symmetric stretching frequencies are 1315, 1314, 1314 and 1317 cm-1 respectively. The OCO bending frequencies are 625 and 618, 623 and 620, 622 and 608, 619 and 602 cm-1, respectively. For free CO2, the vibrational frequencies are 2366, 1318, 632, and 631 cm-1, respectively. s denotes symmetrical stretching; as, asymmetric stretching; 13



b, bending, and r, rocking.

Reaction Mechanism of the CO2RR to CO through *COOH on Silver Surfaces. Here, we consider the CO2RR to CO on a hydrogen covered silver surface. This reaction can be described by the adsorbed hydrogen transfer mechanism for surface catalytic CO2RRs.30 Figure 2 presents the *CO2–H structure adsorbed on a Ag(100) surface. We predicted a transition state (*CO2–H) in which a hydrogen bond formed between a bent CO2 structure and a surface-adsorbed H*. For *CO2–H adsorbed on Ag(111), Ag(100), Ag(110) and Ag(211) surfaces, the bond angles of OCO become 143.9º, 144.9º, 143.1º and 141.7º, respectively, and the O–H bond lengths are 1.381, 1.378, 1.401 and 1.415 Å, respectively. According to the Bader charge analysis, the charges of all the bent structures of CO2 are approximately -0.3, indicating that significant charge transfer occurs. As shown in Table S3, the energy barriers predicted for Ag(111), Ag(100), Ag(100) and Ag(211) surfaces are 1.74, 1.76, 1.77 eV and 1.68 eV, respectively. Although the energy barriers are lower than that on bare silver surfaces, the CO2RR on a hydrogen-covered electrode surface still needs a high negative potential.

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Figure 2. Optimized structures of transition states for the reaction of CO2 with H* and H2O through *COOH to produce CO on Ag(100) surface. a) and e) Side view and vertical view, respectively, of the transition state for the reaction CO2 with H* to produce *COOH; b) and f) side view and vertical view, respectively, of the transition state for the reaction of *COOH with H* to produce *CO; c) and g) side view and vertical view, respectively, of the transition state for the reaction of CO2 with H2O and H* to produce *COOH; d) and h) side view and vertical view, respectively, of the transition state for the reaction of *COOH with H2O and H* to produce CO.

The solvent water can further decrease the barrier expected in electrochemical reactions. For the CO2RR on silver surfaces, introducing H2O may also reduce the reaction barrier of CO2 reduction to *COOH. Using Ag(100) surface as an example, Figure 2 presents the reaction configuration evolution along the process of the conversion of CO2 to *COOH. 15



The bond angle of OCO becomes 148º. The Gibbs free energy calculation shows that the energy barrier of *COOH formation is approximately 1.0 eV (Table S4), which is lower than the barrier to the direct transfer of H* to CO2. The Bader charge analysis shows that the charge of CO2 is approximately -0.4, indicating that large charge transfer happens. Previous studies on this CO2RR process on copper surfaces considered the solvent effect by introducing interfacial H2O and obtained a lower barrier for the *COOH formation process.30, 77 In addition, the free energy barrier was expected to continuously decrease with increasing H2O.30 Comparison of the two different mechanisms indicated that the solvation effect could reduce the reaction barrier considerably, implying a H2O–assisted H* transfer process.

*CO2H+H++e-

1.5

Gibbs Free Energy (eV)

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

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*CO+H2O

1.0

Ag(100) Ag(111) Ag(110) Ag(211)

*H+*CO2+H++e*+CO+H2O 0.5

0.0

*+CO2+2H++2e-

Reaction Coordinate 16


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Figure 3. Gibbs free energy diagrams for the CO2RR to CO on silver surfaces. * represents the metal surface and adsorption site of surface species. All Gibbs free energies are provided relative to CO2 and H2 in the gas phase and a free metal surface.

As shown in Figure 3, the process from *COOH to CO is thermodynamically spontaneous. The two reaction mechanisms were also used to study the reaction of *COOH to CO. The first mechanism is the adsorbed hydrogen transfer mechanism, where the surface H* interacts with *COOH to form *CO, with a reaction barrier of approximately 1.12 eV on Ag(100) and then *CO desorption occurs. However, our calculated results indicated that *COOH converted to *CO through the H2O-assisted H transfer mechanism with a lower barrier of approximately 0.75 eV (Table S4). This finding shows that the H2O-assisted H transfer reaction for CO2 reduction to CO is favorable. Figure 3 also presents the Gibbs free energy diagrams of the reaction from CO2 to CO on surfaces comprising various silver facets at 298.15 K, 1 atm and 0 V versus the reversible hydrogen electrode (RHE). The energy parameters are listed in Table S1 and Table S2. The Gibbs free energies of *COOH adsorbed on Ag(111), Ag(100), Ag(110) and Ag(211) surfaces are 1.42 eV, 1.38 eV, 1.16 eV and 1.03 eV, respectively, similar to previous studies.53, 78-79 We also noticed that the interaction between *COOH and Ag atoms increased on the high-index surfaces and that the energy barrier decreased from low- to high-index crystal surfaces. This observation is supported by previous studies showing that

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metal catalysts with more steps and edges have good catalytic activity.63 To verify this phenomenon again, the Gibbs free energy of the *COOH intermediate when adsorbed on the Ag13 cluster is found to be 0.04 eV (Table S2), and the adsorption Gibbs free energy is -0.80 eV, which is close to the energy of CO adsorption on the copper surface.31 This finding shows that the good catalytic activity is strongly dependent on the structure of the crystal surfaces. It is also noted that the adsorption energy of *CO increased with the surface index (Table S5 and S6). From the variation trend, hydrocarbons would be expected to generate on the silver surface, similar to the copper surface.43 Thermodynamic Selectivity of the CO2RR to HCOOH on Silver Surfaces. As previously mentioned, HCOO*B is a more thermodynamically stable intermediate than *COOH in the CO2RR. However, the CO path has a very high faradaic efficiency on silver electrodes.25 To understand the two competitive reactions,24,

80

the reaction kinetic

processes of HCOOH formation were further reinvestigated. The HCOOH formation pathway is the reverse reaction of formic acid decomposing into CO2. Previous theoretical studies showed that formic acid decomposition passes through several intermediates, first forming formate in HCOO*B, then changing to HCOO*M with a single O atom binding to the surface, and finally producing CO2.25, 53, 78 This pathway inspired us to propose here that the mechanism of the CO2RR could first pass through the intermediate HCOO*M adsorbed on a silver surface and then transform HCOO*M to HCOO*B, which is a very stable intermediate, as discussed above. The transition state from CO2 and H* to HCOO*M 18


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on silver surfaces is easier than that from CO2 and H* to HCOO*B.61 The transition state (*H–CO2) of the conversion of CO2 to HCOO*M was also calculated through the adsorbed hydrogen transfer mechanism in this study. Figure 4 presents a single oxygen adsorption bent structure at transition states on the Ag(100) surface. Our calculated results also show that the transformation from CO2 to HCOO*M with a surface H* atom has a small energy barrier of approximately 0.64 eV (Table S4). For transition states on Ag(111), Ag(100), Ag(110) and Ag(211) surfaces (Figure S9), the OCO bond angles are 146.4º, 147.5º, 152.6º and 149.7º, the Ag–O bond lengths are 2.412, 2.491, 2.507 and 2.488 Å, and the C– H bond lengths are 1.486, 1.537, 1.635 and 1.545 Å, respectively. Furthermore, the Bader charge analysis shows that the charge transfer from silver surfaces to CO2 is also approximately -0.3, and that the H atom receives approximately 0.1 charge in these configurations. These results show that the significant surface charge transfer leads to a stable adsorption configuration in which CO2 is close to an adsorbed H atom. Thus, HCOO*M formation can be promoted significantly by a direct reduction of physisorbed CO2.

19



Figure 4. Optimized structures of transition states for the CO2RR to HCOOH on Ag(100) surfaces. a) and c) Side and vertical views, respectively, for the transition state of the reaction of CO2 with H* to produce HCOO*M; b) and d) side and vertical views, respectively, for the transition state of the reaction of HCOO*B with H* to produce HCOOH.

In the CO2RR on transition metals, HCOO*B is usually considered as the reaction intermediate on the pathway to the product formic acid. However, it should be emphasized that the strong binding interaction of HCOO*B inhibits the reduction reaction to formic acid on the silver surface. This is mainly due to a thermodynamically nonspontaneous reductive desorption of HCOO*B to formic acid (Figure 5). We tested the surface H* interaction with HCOO*B by the adsorbed hydrogen transfer mechanism and found that it has a high barrier of approximately 0.6 eV (Table S3) which is close to the HCOO*M formation barrier. The above discussion shows that the surface H* reduction of CO2 to 20


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Page 21 of 44

HCOOH has a low barrier, which may indicate that H* could promote HCOOH production. In addition to the HCOO* formation pathway, the adsorbed H* could also be reduced to H2, which is key to tuning the CO2RR. Thus, H2 was calculated on Ag(111) and Ag(100) surfaces. The results show that the transition state barrier to surface H* combination to form H2 is approximately 0.44 eV (Table S7), which is obviously smaller than the energy barrier of the HCOO* product. We also notice that the experimental faradaic efficiencies of HCOO– and H2 are 0.8% and 12.4% at 1.37 V vs. NHE, respectively.25 The low faradaic efficiencies of the hydrogen evolution reaction may be due to the high overpotential on silver.81-82 This observation could prove the validity of our calculated results.

1.5

Gibbs Free Energy (eV)

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


HCOO*M+H++e-

Ag(100) Ag(111) Ag(110) Ag(211)

1.0

*H+*CO2+H++e-

HCOOH*

0.5

HCOOH+* +

-

HCOO*B+H +e 0.0

*+CO2+2H++2e-

Reaction Coordinate Figure 5. Gibbs free energy diagram for the CO2RR to HCOOH on silver surfaces. * 21



represents the metal surface and adsorption site of surface species. All Gibbs free energies are provided relative to CO2 and H2 in the gas phase and on a free metal surface.

Kinetic Selectivity of the CO2RR to CO on Silver Surfaces. The abovementioned results show that the transition state from CO2 and H* to HCOO*M on silver surfaces is approximately 0.64 eV lower in energy than the transition state for HCOO*B formation. We noticed that HCOO*M and *COOH have similar thermodynamic energies as shown in Figures 3 and 5. This finding indicates that the two surface species have similar thermodynamic properties and similar production potentials. Therefore, the transformation of the two intermediates into each other was considered. However, this reaction involves changes in four bonds: formation of both Ag–C and O–H bonds formed and cleavage of Ag–O and C–H bonds. In this case, our calculated results indicate that there is an energy barrier larger than 1 eV, as seen in Table S4, suggesting that this process is difficult to occur. We also noticed that HCOO*M decomposing along the reverse reaction from HCOO*M to CO2 has a very low energy barrier of approximately 0.02, 0.07, and 0.11 eV on Ag(111), Ag(100) and Ag(110) surfaces, respectively. The energy barriers of the reverse reaction to CO2 from HCOO*M being much smaller than the energy barriers of HCOO*M formation (approximately 0.70, 0.57 and 0.48 eV, respectively) is unexpected. In addition, the transition states of the conversion of HCOO*M to HCOO*B were calculated, and the 22


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reaction barrier on Ag(100), Ag(111) and Ag(110) surfaces were approximately 0.33, 0.44 and 0.14 eV, respectively. This result shows that the reverse reaction of HCOO*M to CO2 is kinetically favorable. According to the Arrhenius law, k  Ae



G k BT

(4)

where k is the reaction rate constant, kB is the Boltzmann constant, T is the temperature, G≠ is the reaction activation energy, and A is the pre-exponential factor. We propose that it is very easy for the reverse reaction to occur. This proposal leads to the conclusion that only a small quantity of HCOO*M could be converted continuously to formic acid through the stable intermediate HCOO*B, in a good agreement with experimental results.25 From the different reaction path calculations mentioned above, we know that water and surface H* paly different roles in influencing the two reaction paths. In acidic solution, HCOOH may become a major product. However, most experiments were performed using CO2RR in alkaline aqueous solution because of the good solubility of CO2 under these conditions. This might explain why CO, not HCOOH, is the primary product of the CO2RR on silver electrodes.61,

63, 83

The oxidation of formic acid to CO2 is known to be fast

kinetically, but the transformation from CO2 to HCOO*M is thought to be the ratedetermining step.84-85 In some cases, it is affected by surface H* concentrations.77 In addition, other metal catalysts were also considered, using Au as an example (Table S8), and HCOO*M has a higher thermodynamic free energy of approximately 0.5 eV than *COOH, which has a similar thermodynamic stability as HCOO*B. As a result, CO is 23



generated more readily on gold surfaces than on silver surfaces25. Our calculations revealed that both monodentate and bidentate adsorbed HCOO* should be considered in electrocatalytic reactions and that their relative stability plays a crucial role in the kinetic control of the CO2RR on metal surfaces. CONCLUSION In summary, we have investigated the geometric and electronic structures and energy barriers of the reaction paths for the CO2RR on silver surfaces. The electronic structure of the reaction intermediate *COOH shows that the interaction of σ bonding and π back donation for C and Ag takes place at energies lower than -6.0 eV with respect to the Fermi level. In contrast, the O and Ag interaction is very effective for HCOO*B, especially near the Fermi level. Consequently, HCOO*B is more stable than *COOH. The HCOOH formation through the HCOO*M intermediate was promoted in the presence of surface absorbed H* species. Notably, a high surface H* concentration will lead to the hydrogen evolution reaction, which is a side reaction in competition with the CO2RR. In addition, HCOOH formation is unfavorable kinetically due to the low barrier of the reverse reaction involving HCOO*M and the high barrier of the forward reaction to the HCOO*B intermediate. In particular, our calculated results show that the HCOO*M intermediate, which plays an important role in the CO selectivity of the CO2RR on silver electrodes. And the transformation of two intermediates HCOO*M and *COOH each other through H transfer has a large reaction energy barrier, suggesting that this process is

24


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difficult to occur. For the CO2RR to CO, the H2O-assisted H* transfer mechanism has a lower reaction barrier when water takes part in the CO2RR reaction. This could help elucidate the reaction mechanism and the large CO faradic efficiency on silver electrodes from thermodynamic and kinetic perspectives. Finally, based on quantum chemical calculations, the reaction free energy diagram was used to predict the reaction barrier in certain catalyst morphologies, especially for different single-crystal surfaces. For the CO2RR on silver electrodes, the CO path should be the dominant path depending on the surface electronic structures. Through changing the reaction conditions and tailored design, ones have an expectation to selectively obtain rich products as observed on copper-like catalysts. ACKNOWLEDGEMENTS The authors acknowledge support from the National Natural Science Foundation of China (No. 21533006, 21621091), the 973 Program of the Chinese Ministry of Science and Technology (NO. 2015CB932303) and Funds of State Key Laboratory of Physical Chemistry of Solid Surface and Fujian Science and Technology Office.

Supporting Information There are data for DOS and project DOS plots of all intermediates adsorbed on four single crystal surfaces of silver, calculated geometrical configurations of adsorption intermediates

25



and transition states, calculated molecular orbitals of CO2, simulation parameters calculated from DFT, and adsorption energies of *CO listed in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. DOS plots at different reaction stages from CO2 to the *COOH and HCOO* intermediates on a (2×2) supercell Ag(100) surface. a) CO2 at approximately 10 Å from the surface, b) physisorbed CO2, c) H* to *COOH at the transition state of CO2, d) *COOH adsorption state, e) transition states of CO2 and H* to monodentate HCOO*, f) monodentate HCOO* adsorption state, g) bidentate HCOO* adsorption state. Blue, CO2 and H atoms; and red, Ag atoms interacting with CO2 and H atoms. 175x253mm (300 x 300 DPI)



Figure 2. Optimized structures of transition states for the reaction of CO2 with H* and H2O through *COOH to produce CO on Ag(100) surface. a) and e) Side view and vertical view, respectively, of the transition state for the reaction CO2 with H* to produce *COOH; b) and f) side view and vertical view, respectively, of the transition state for the reaction of *COOH with H* to produce *CO; c) and g) side view and vertical view, respectively, of the transition state for the reaction of CO2 with H2O and H* to produce *COOH; d) and h) side view and vertical view, respectively, of the transition state for the reaction of *COOH with H2O and H* to produce CO. 175x96mm (300 x 300 DPI)


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Figure 3. Gibbs free energy diagrams for the CO2RR to CO on silver surfaces. * represents the metal surface and adsorption site of surface species. All Gibbs free energies are provided relative to CO2 and H2 in the gas phase and a free metal surface. 216x164mm (300 x 300 DPI)



Figure 4. Optimized structures of transition states for the CO2RR to HCOOH on Ag(100) surfaces. a) and c) Side and vertical view, respectively, for the transition state of the reaction of CO2 with H* to produce HCOO*M; b) and d) side and vertical view, respectively, for the transition state of the reaction of HCOO*B with H* to produce HCOOH. 175x168mm (300 x 300 DPI)


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Figure 5. Gibbs free energy diagram for the CO2RR to HCOOH on silver surfaces. * represents the metal surface and adsorption site of surface species. All Gibbs free energies are provided relative to CO2 and H2 in the gas phase and on a free metal surface. 216x164mm (300 x 300 DPI)



Table of Contents 84x41mm (300 x 300 DPI)


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Selective Electrocatalytic Mechanism of the CO2 Reduction Reaction

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