Reaction Selectivity for Plasmon-Driven Carbon Dioxide Reduction on

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Reaction Selectivity for Plasmon-Driven Carbon Dioxide Reduction on Silver Clusters: A Theoretical Prediction Xia-Guang Zhang, Yuxiu Liu, Chao Zhan, Xi Jin, Qijin Chi, De-Yin Wu, Yi Zhao, and ZhongQun Tian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01448 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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The Journal of Physical Chemistry

Reaction Selectivity for Plasmon-Driven Carbon Dioxide Reduction on Silver Clusters: A Theoretical Prediction Xia-Guang Zhang†, Yuxiu Liu†, Chao Zhan†, Xi Jin†, Qijin Chi‡, De-Yin Wu*†, Yi Zhao*†, Zhong-Qun Tian†

†State

Key Laboratory of Physical Chemistry of Solid Surface, Collaborative

Innovation Center of Chemistry for Energy Materials, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China ‡Department

of Chemistry, Technical University of Denmark, DK-2800 Kongens

Lyngby, Denmark

Corresponding Author E-mail: D.Y. W. ([email protected]); Y. Z. ([email protected]).

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Abstract: Density functional theory is employed to investigate the plasmondriven CO2 reduction at the active sites of metallic silver clusters. The results predict that CO2 prefers to adsorb at the bridge site of silver clusters and the C–O bond is difficult to break due to a high activation barrier at electronic ground state. However, as the photo-generated plasmon energy of silver nanoparticles matches with the dissociation energy of the C–O bond, the CO2 easily dissociates into CO and an adsorption oxygen atom. Moreover, our calculated results demonstrate that the generated CO strongly adsorbed on silver clusters is favorable to its succeeding reduction reaction with hydrogen to CH3OH and CH4. The reaction barrier of the generation of CH3OH is lower than that of the formation of CH4, because the proton combines with the carbon more easily than with the oxygen atom at the initial reaction step. It well accounts for the experimental observation that CH3OH can be formed from the CO2 reduction on silver nanoparticles under visible light irradiations.


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Introduction Carbon dioxide reduction to CO and hydrocarbons is becoming a prevalent research field since it is significant to complete carbon cycle for carbon sequestration and generating energy-rich products. In general, CO2 is an inert molecule (ΔGf0 = –394.4 kJ/mol),1 which requires considerable energy for chemical activation. Thus, multiple steps of electron and proton transfers on catalytic surfaces are difficulty of carbon dioxide reduction reaction (CO2RR).2 For electrochemical CO2RR on different electrodes which has been systematically studied by Hori groups,3 the reduction potentials and product distribution strongly depend on experimental conditions, electrolytic solutions and electrode materials. Catalyst morphology4-6, alloying7-8 and doping9 are possible to influence on the reaction activation barrier, reaction efficiency and selectivity. Recently, more and more attentions have been paid to use renewable energy like solar energy to assist photocatalytic or photoelectrochemical process of CO2RR10-11 and tremendous progress has been made. Among the various photo-catalysts, TiO2 as a semiconductor is commonly used due to its high reactivity.12-15 But, the large band gap (~3.2 eV) of TiO2 limits its application in ultraviolet light region,16 which contains only 5% of total solar energy. These drawbacks greatly limit the applications of TiO2 in photocatalytic CO2RR. Coinage metal nanoparticles, such as, gold, silver, and cooper have a good optical property due to localized surface plasmon resonances (LSPR), which 3 ACS Paragon Plus Environment


can enhance the photocatalytic efficiency under visible light irradiations.17-22 The LSPR can significantly induce the formation of hot carriers participating in CO2RR.23-29 Multi-electron transfer can happen in LSPR mediated CO2 reduction, where the C–O bond of chemisorbed CO2 on metal surfaces is directly broken by the LSPR effect.25-26,

30-33

In previous studies, silver

nanoparticles were often used to study LSPR reactions which could increase electrochemical reaction rate and reduce the energy barrier from CO2 to CO.3437

For CO2 electroreduction on a silver electrode, CO is the main product, and

the Faradaic efficiency is above 81.5%.3 Some mechanisms for the formation of CO on silver surfaces have been proposed. One is CO2 in the chemisorption state directly dissociates into CO,38 another one involves CO2 simultaneously obtains a proton and an electron produces CO via intermediate *COOH.3, 39-40 Recently, researchers also found that CO2 could be reduced to hydrocarbon and alcohol compounds through *CHO and *COH intermediates on nanostructured silver surface under visible light,32 and *CHO is a thermodynamically stable structure on silver nanoparticle.41 While the reaction diagram of LSPR induced CO2RR is perplexing, especially, how to understanding photo-generated hot electron transfers from metal to CO2. In this work, we conduct density functional theory (DFT) to first investigate the interaction between CO2 and silver clusters, simulate active sites and explore the CO2 reduction mechanism on silver electrodes. We find that excited plasmons on silver clusters can promote hot electron transfer to LUMO of CO2, 4 ACS Paragon Plus Environment

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and then relax to the charge transfer state, due to the strong orbital coupling of silver clusters and CO2. Under light excitation, the strong LSPR effect of silver nanoparticles is able to efficiently promote the CO2 adsorption and subsequent dissociation to CO. Then CO is reduced to CH3OH and CH4 due to the high activity of silver clusters. The schematic of reaction path and the characteristic vibrations of Raman spectra are analyzed and drawn for a better understanding of the CO2RR on silver nanoparticles. This will be helpful to identify the reaction intermediates and speculate reaction mechanisms. Computational Methods To gain deep insight on the interaction of CO2 on silver electrodes, metallic cluster models were adopted according to previous work of Pereiro’s group,42 where CO2 chemisorbed and physisorbed at active sites of silver surfaces (Figure S1 and S2). All DFT calculations were performed by Gaussian 09 program.43 The hybrid exchange-correlation functional PBE1PBE44 was employed to optimize all structures of the ground states and transition states. For H, C, and O atoms, the basis set Aug–cc–pVTZ45 was adopted here. For the metal atom Ag, the outer and inner shell valence electrons are described by the effective pseudo potential double–ζ (LANL2DZ) basis set. The solvation model of density (SMD) was adopted for all calculations in solvent water with a dielectric constant of 78.4.46 The tight option of the cutoffs on forces and step size are used to determine convergence. Vibrational frequency calculations show that all optimized structures are minima on the potential energy surfaces 5 ACS Paragon Plus Environment


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of electronic ground states. All the transition states have only an imaginary frequency, and the intrinsic reaction coordinate (IRC) calculations prove the validity of transition state. Nature bond orbital (NBO) analysis47-48 has been used to investigate the bonding mechanism between adsorbent and metal clusters. Absorption spectra and nature transition orbital (NTO) analysis could be obtained from time–dependent DFT (TDDFT) calculation based on optimized structures of electronic ground states. The excited state energies calculated by using TDDFT are known to have errors dependent on all functionals.49 In order to verify that the results of PBE1PBE are consistent with other functionals and basis set, we have repeated the calculations above using the other commonly used functional B3LYP and basis set 6-311+G** for C and O atoms and LANL2DZ basis set for Ag atoms.50 The results are provided in the Supporting Information, showing that the two functionals have similar performance. All activation energies have been corrected by considering zeropoint energies and enthalpies. The reduced two–state model was used to predict the electronic coupling by the overlap of the two diabatic states, which were assumed to be the LUMOs of the two isolated neutral molecules.51-53 The electronic coupling is 1 h12  (h11  h22 )S12 2 VDA = 2 1  S12

(1)

where hij = , Sij = , and the ϕi and ϕj are corresponding orbital wave functions of molecule and the silver clusters respectively. 6 ACS Paragon Plus Environment

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The absolute Raman intensities were calculated by using the differential Raman scattering cross section.54 The calculated harmonic frequencies were scaled by scaling factors of 0.981 for below 2000 cm–1, and 0.967 for above 2000 cm–1.55 In this case, the Raman scatting intensity was calculated using54 IRaman

 2π  

2 4   dα 2  dγ    ω0  ωi  h  45    7   45 8π 2cωi 1  exp  hcωi / kBT    dQi  dQi     

=

 2π 

4

 ω0  ωi  h Si 45 8π cωi 1  exp  hcωi / kBT  4

4

(2)

2

where h, c, kB and T are the Planck constant, light speed, Boltzmann constant, and Kelvin temperature, respectively. Si is the Raman scattering factor (in Å4/amu) that can be calculated using Gaussian 09 at the equilibrium geometry, and it is only an expression of derivatives is the static isotropic and anisotropic polarizabilities with respect to the given normal coordinate. Here ω0 and ωi denote the frequency (in cm–1) of the incident light and vibrational frequency of the ith mode. The simulated Raman spectra were presented in terms of a Lorentzian expansion with a line width 10 cm–1. The differential Raman scattering cross section values of different vibrational modes were calculated from the Raman scattering factor under the double-harmonic approximation, with a specific excitation wavelength of 532 nm at room temperature. Results and Discussion Adsorption Structure of CO2 at Silver Nanoparticles. For CO2RR on silver electrodes, firstly, an electron transfers from silver to the CO2, forming the CO2–. This process needs a high negative electrode potential about -1.9 V vs. normal 7 ACS Paragon Plus Environment


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hydrogen electrode (NHE).56 In general, the formation of CO2– is very difficult on low-index single crystal surface of silver with a weak surface activity.57

Table 1. Structural parameters, NBO charges (qNBO, a.u.), and adsorption energies (Eads, eV) of CO2 adsorbed on silver clusters.a Physisorptionb Ag2 Ag4 Ag6 Ag8 Ag10 Ag12 Ag14 Ag16 Ag18 Ag20

Chemisorption

NBO

Eads

C=O1c

C=O2d

NBO

Eads

–0.009 0.016 0.019 0.031 0.044 0.030 0.029 0.027 0.048 0.051

–0.14 –0.16 –0.17 –0.22 –0.16 –0.16 –0.17 –0.17 –0.17 –0.18

1.237 1.245 1.250 1.252 1.257 1.258 1.260 1.259 1.251 –

1.237 1.276 1.280 1.293 1.283 1.289 1.289 1.294 1.270 –

–1.058 –1.178 –1.091 –1.201 –1.216 –1.237 –1.275 –1.257 –1.030 –

–2.26 –3.40 –3.00 –3.90 –3.51 –4.03 –4.37 –4.15 –3.25 –

a) The unit of bond length is Å; b) the bond length of C=O for physisorption CO2 is 1.158 Å, and the bond length of C=O for free CO2 is 1.157 Å (the experimental value is 1.163 Å58); c) C=O1, the O atom is far away from silver surfaces; d) C=O2, the O atom is close to silver surfaces.

According to the NBO analysis of adsorbed CO2 molecules, the physisorption CO2 moiety possesses zero charge, whereas the chemisorption CO2 carries one negative charge, (i.e., CO2–) (Figure S1, Table 1). The geometric structure of free CO2– is a bending configuration, in which the OCO is 134.6º and the C=O bond lengths are 1.232 Å. For the CO2– adsorbed on silver clusters, the geometric structure parameters are listed in Table 1. CO2 prefers to bind at the 8 ACS Paragon Plus Environment

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bridge site of the Ag2 cluster, while for other silver clusters, the C and O of CO2 adsorbed at silver top sites and the C–O bond is parallel to the Ag–Ag bond. When CO2 adsorbs on silver clusters, the interaction between CO2 and silver clusters would lead to the change of geometric structures of clusters.59 The calculated Ag–Ag bond length of clusters (~3.0 Å) with adsorbed CO2 is longer than that of Ag(100) and Ag(111) surfaces (2.889 Å).60 It shows that decreasing coordination number and increasing Ag–Ag bond length of silver clusters can increase the surface activity of silver clusters.61 50

Raman Intensity/10-29 cm2/sr

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


1152

700

40

30

20

740

1164

744

1184

1356

Ag14

738

1174

1360

Ag12

736

1188

742

1160

712

10

734 612

0 400

Ag18

1414 1352

600

800

Ag16

1376

Ag10

1380

Ag8

1168

1412 Ag6

1178

1440 Ag4

1162

1585 Ag2

1000

1200

1400

1600

-1

Raman Shift /cm

Figure 1. Calculated Raman spectra of chemisorbed CO2 on different silver clusters. The Raman intensity is expanded on the basis of the Lorentzian line shape with the linewidth of 10 cm–1. For free CO2–, our calculated results show that it has three characteristic vibrational frequencies at 719, 1317, and 1654 cm–1, in accordance with the experimental data at 714,62 1298,63 and 1658 cm–1.62,

64

Figure 1 shows

calculated Raman spectra of CO2– adsorbed on silver clusters, three vibrational 9 ACS Paragon Plus Environment


frequencies appear in agreement with the experimental values at 750, 1200, and 1410 cm–1.65-66 Furthermore, for the vibrational frequency at 1410 cm–1, it can be attributed to the C=O stretching vibration with the O atom far away the silver clusters. The vibrational frequency at 1160 – 1190 cm–1 with a large Raman activity belongs to the C=O bond stretching vibration on silver surfaces. The vibrational frequency near to 700 cm–1, mainly comes from the O–C–O bending vibration. The above results indicate that the vibrational frequencies and Raman activities of the chemisorption state CO2 can be used to identify the adsorption structures. Therefore, observed Raman spectra provide the information to recognize surface chemical adsorption states and active sites of CO2. Plasmon Driven CO2 Adsorption Dissociation on Silver Clusters. For CO2RR, from the chemisorption CO2 to CO two reaction paths: the direct twoelectron dissociation path38 and two-electron two-proton reaction path3 are proposed. The activation energy barrier of the former is above 2.0 eV (Table S2). The other reaction path is that CO2 is reduced into CO through a *COOH intermediate.59 The reaction energy barrier from adsorbed CO2 to *COOH is above 1.0 eV (Table S3). Both of them need a very negative cathodic polarization in electrochemical interfaces.34 The surface plasmon excitation is the collective excitation of conductance electrons on metal surface, and the photogenerated hot-carriers from LSPR relaxation were proposed to facilitate the C–O bond break of adsorbed CO2 in illuminated on silver and aluminum10 ACS Paragon Plus Environment

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cuprous oxide.30,

38

The conceptual mechanism of LSPR-driven CO2

dissociation is that the hot electron will transfer from metal to the LUMO of adsorbed CO2. In order to understand the detail process of plasmon driven CO2RR, we performed TDDFT calculations to investigate the orbital energy levels of CO2 and silver clusters, and the plasmon-excited charge transfer process. An excellent absorption of visible light on plasmonic nanostructures may be a major reaction channel for the plasmon driven surface catalysis mechanism. 20, 67

Figure S9 shows the calculated absorption spectra of different adsorption

configurations. The simulated results of absorption spectra of silver clusters are in accordance with the results of previous studies.68-69 For all physisorption structures, all the orbitals are localized on the silver clusters in a low energy region (λex > 500 nm). It doesn’t exist any proper orbital for the charge transfer from the silver clusters to CO2. Whereas in the higher photonic energy region (λex < 500 nm), the SPR excitation appears, which originates from excited sp electrons with large oscillator strengths.68-69 In addition, the charge transfer from silver clusters and CO2 can be found in this region. As shown in Figure 2, we analyze the interaction of the Ag18 cluster with CO2. In the energy region of physisorption structures, we notice each of the orbital energies of LUMO+8 ~ LUMO+11 of Ag18 matches well with the orbital energy of the LUMO of CO2 (Table S4), and the orbital energy of CO2 conforms to data in the previous studies.15,

70-71

Meanwhile, the electron excitation state is associated to 11 ACS Paragon Plus Environment


LUMO+8 ~ LUMO+11 orbitals have large oscillator strengths. Thus, we propose LUMO+8 ~ LUMO+11 orbitals are related to the plasmon-driven electron transfer mechanism. To prove our hypothesis, we further investigate the coupling strength of wave functions between CO2 and Ag18 based on the two–state model.53 Herein, the two coupled wave functions belong to the unoccupied orbital of silver clusters and the LUMO of CO2. The strength of orbital coupling is strong, about 106.2 meV (Table S5), leading to the excited hot-electrons on silver clusters can easily transfer to the LUMO of CO2 (Figure S10). 1

0

L+11 L+10 LUMO L+9 coupling: L+8 106.24 meV

L+9

-1

LUMO

HOMO

HOMO

absorption f=1.2136 Eabs=3.23 eV

-3

LUMO


-2


Orbital 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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HOMO

-4 Ag18

CO2

Physical Adsorption

Cluster

Chemical Adsorption

Figure 2. Orbital coupling between the silver cluster and CO2. The red arrow, orange arrow, and violet arrow represents the highest oscillator strength transition of the Ag18 cluster, CO2 physisorption structure, and CO2 chemisorption structure, respectively.

From the above analysis, we find that the hot-electron can transfer from silver 12 ACS Paragon Plus Environment

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clusters to the LUMO of CO2 due to a large orbital coupling. Then, the configuration of CO2 obtained one electron becomes nonlinear through geometric relaxation. Moreover, the transition energy of the excited state with a large oscillator strength of CO2 chemisorption structure is 3.2 eV (Figure 2 and Table S7). Surprisingly, this just matches to the energy of the charge transfer excited state of CO2 physisorption structure (Figure 2 and Table S6). For the CO2 chemisorption state, we analyze the charge transfer between CO2 and silver clusters according to the nature transition orbital. Upon photoexcitation, the photo-generated hot electron can transfer into the C–O bond, resulting in the C–O cleavage (Figure 2). We also notice that the required energy to break the C–O bond of CO2 chemisorbed on silver is about 2.23 ~ 2.45 eV. However, the energy is in agreement with that of silver surface plasmons which comes from hot electrons formed from the decay of plasmon excitation on silver nanostructures. Thus, it implies that an efficient excited electron can be transferred to the C–O antibonding orbital, leading to the C–O bond cleavage.18, 72



Figure 3. Mechanism of the photo-generation hot electron transfer from a silver cluster to CO2. The black line and the blue line represent the density of state of Ag18 cluster and CO2, respectively. The energy level is relative to the vacuum level.

Figure 3 displays the detailed picture of LSPR-driven CO2 adsorbed on silver clusters. It is drawn based on our calculated results and several previous studies of the chemical process induced by hot electrons generated by surface plasmons.22, 73-75 We can see that LSPR-driven hot-electron arisen from a silver nanoparticle is transferred to the LUMO of CO2 in Ag*–CO2 (* denotes the excited state), causing the formation of a charge-transfer state . Then, [Ag+– CO2–]* relaxes to the corresponding charge transfer ground state. LSPR-driven the second hot electron breaks the C–O bond. Finally, the new surface species CO and oxygen atom adsorbed on silver surfaces are formed. It is worth to note that the surface hydrogen atoms from H2O reduction reactions can consume surface adsorbed oxygen atom and then produce the hydroxide anions into aqueous solution.30 Therefore, hot-electron induced by LSPR can promote adsorbed CO2 dissociation into CO on silver clusters. Furthermore, we find that CO can be strongly adsorbed on nanostructures (Table S8) and the adsorption energy is close to that of on copper surfaces.76-77 It is favorable for the further reduction of CO2 on silver nanostructures. 14 ACS Paragon Plus Environment

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Reduction of Adsorbed CO on Silver Clusters. Previous studies indicated that the product of CO could be further transformed into methanol and methane compounds on silver nanoparticles under light irradiations.16, 32, 78 Herein, we calculate thermodynamic energies of the CO2RR to explore the feasible mechanism and the reaction selectivity. We find that strong binding of CO on silver clusters results in the reduction of CO to C1 products. And our calculated results also show that when the surface coverage is large, the binding energy of CO decreases, in agreement with previous studies.79-80 Once a single CO is binds individual Ag atoms and it will be isolated from adjacent Ag atoms. Thus, the C–C coupling is difficult to achieve.81 Figure 4 presents different reaction paths of *CO reduction. We first focus on the reaction from *CO to *CHO in path I. The activation energy of the elementary reaction is 0.53 eV with respect to the reactants *CO and H+. The intermediate *CHO has two parallel branching reaction paths to *CH2O and *CHOH with activation energies about 0.45 and 1.01 eV, respectively. It shows that at the parallel steps the *CHOH formation was a harder step than the *CH2O formation. In such a case, the H+ prefers to bind to C atom. The further reduction reactions of *CH2O also follow a similar rule. The activation energy for *CH3O is only 0.15 eV, much smaller than 1.22 eV for *CH2OH. Thus the final stage is the further reduction of *CH3O to CH3OH with an activation energy about 0.87 eV. Comparing with the activation energy of every elementary reaction, we can see that the reaction from *CO to CH3OH is the rate15 ACS Paragon Plus Environment


determining step for the formation of methanol.

H+ 1.4 4e V

OH CH

0.29 eV H+

+

H eV 27 1.

OH CH2

OH CH3

O CH2

H+ 0.45 eV

H+ 0.15 eV

0. H + 87 eV

1. H + 22 eV

V 1e

H+

2.86 eV

V

+

O CH

6e

CH4

eV

O C

H+

H

+

O

0.3

H+

72

C

CH3

0.

O

H+

Path III

H+ 0.93 eV

eV

OH C

Path II

CH2

1.12 eV

1.0

Path I

H+

CH

H+ 1.23 eV

07 1.

H+ 2.30 eV

C

H+ 0.53 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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O CH3

Figure 4. Three reaction paths for CO reduction on Ag18 cluster. The green arrows (Path I) and blue arrows (Path II) present two different reaction path with the same reduction product, CH3OH. And the red arrows are a reaction path from CO to CH4 (Path III). The H2O product is omitted in the scheme for simplicity. The numbers represent reaction activation energies at 298.15 K and 1 atm.

The Path II presents the reduction of CO through intermediate *COH, *CHOH, and *CH2OH and finally converted to CH3OH. The first step from CO to *COH intermediate has a very large activation energy barrier about 2.86 eV. Under the circumstances, we think that the LSPR effect plays an indispensable role in overcoming the high energy barrier and facilitating this reaction. Once *COH is formed, it can be further converted into two intermediates, *CHOH and *C, with different reaction barriers about 0.36 and 2.30 eV, respectively. It suggests that the majority of *COH is reduced to *CHOH rather than *C. As shown in Figure 16 ACS Paragon Plus Environment

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4, the activation energies of several steps from *COH to CH3OH along the Path II are 0.36, 0.29, and 0.72 eV, which are much smaller than the first step. So, along the way with CH3OH as the product, the rate-determining step is the formation of *COH. In all reduction steps on silver clusters of Paths I and II, our calculated results as well as previous studies show that surface hydrogen prefers to binding to surface C atom other than O atom because of lower energy barriers.81-82 For path III, CH4 is the final reduction product, But the condition is a little bit different from Path I and Path II, where all elementary reactions have a large activation energy barrier. The smallest barrier is predicted about 0.93 eV along the reaction path. Therefore, it is very difficult for the CH4 formation via ground state reaction of adsorbed CO on silver surfaces.81 Additionally, an alternative scheme which considers the same excited intermediates of Paths II is also proven to be impractical, as shown in Figure 4.83 Simulated Raman Spectra of Key Adsorption States. In order to clearly assign vibration fundamentals of surface adsorption species, we classify the vibrational frequencies as three regions. Concretely, the vibrational frequencies in the region of 200 – 700, 900 – 1600, and 2500 – 3500 cm–1 are denoted as low, middle and high frequencies, respectively. There is no doubt that the high frequency peaks are ascribed to the C–H (2500 – 3000 cm–1)84 and the O–H ones (larger than 3000 cm–1)85. However, the situation is more complicated in low and middle frequency regions, which are usually termed as fingerprint 17 ACS Paragon Plus Environment


region of surface adsorption species. So we will particularly pay attention to the low and middle frequency regions for further understanding of the reaction mechanism. 424

a

1056

1256

*CH3

528 496 546 464

290 346 422

1

322

2

*CH *CH2

454

3

0

1562

*CH3O

1544

1362

1000 634

1

218

2

*CHO *CH2O

1402

b 3

320 392

1320

0

c

3184

1448

1086

914

1

420

390

2

*COH *CHOH *CH2OH

1218

970

3

1248

Raman Intensity/10-28 cm2/sr

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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0 200 400 600 800 1000 1200 1400 1600

30003300

-1

Raman Shift /cm

Figure 5. Simulated Raman spectra of the intermediates for CO reduction on Ag18 cluster. a) CO reduction to CH4 through *CH, *CH2, and *CH3; b) CO reduction to CH3OH through *CHO, CH2O, and CH3O; c) CO reduction to CH3OH through *COH, *CHOH, and *CH2OH. The excitation wavelength of 532 nm was used here with a Lorentzian line width of 10 cm–1.

Figure 5a displays the simulated Raman spectra of intermediates for the 18 ACS Paragon Plus Environment

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process of CH4 formation. In the middle frequency region, two characteristic frequencies of 1256 and 1056 cm–1 come from the bending vibrations of *CH2 and *CH3.84, 86-87 And in the low frequency region, two main characteristic peaks of *CH3 are 546 and 528 cm–1, which are considered as a consequence of the C–H rocking vibration. Besides, the characteristic peaks *CH2 at 523 and 464 cm–1 come from the C–H rocking and bending vibrations, respectively. As for the intermediate *CH, its C–H bending vibration reflects on the characteristic peaks of *CH at 454 and 496 cm–1. The simulated Raman spectra presented in Figure5b corresponds to CO reduction to CH3OH through *CHO intermediate (Path I). We note that the characteristic frequencies in the middle frequency region were useful to distinguish the surface species.32 For *CHO intermediate, the peaks at 1544 and 1362 cm–1 come from the C–O stretching vibration and the Ag–C–H bending vibrations, respectively. And the *CH2O intermediate possesses three Raman peaks at 1562, 1402 and 1000 cm–1. The first two peaks can be ascribed to the O–C–H bending vibrations, C=O stretching vibration and Ag– C–H bending vibration, respectively. For *CH3O intermediate (Figure S11), there are three Raman peaks at 1473, 1425, 1398 cm–1, all of them coming from C–H bending, and another peak at 1027 cm–1 coming from C–O stretching.32 In low frequency region, the characteristic frequency 392 cm–1 comes from the C–H rocking of *CH2O. Figure 5c shows the simulated Raman spectra of CO reduction to CH3OH 19 ACS Paragon Plus Environment


through *COH intermediate along the Path II. Similar to the Path I from *CHO to CH3OH, the middle characteristic frequencies play a significant role in identifying the surface species. The 970 cm–1 vibration of *CHO comes from C– O stretching and O–C–H bending vibration. For *CHOH, the 1320 cm–1 peak comes from the C–O stretching vibration, and 1218 cm–1 does from O–H and C-H bending vibration. *CH2OH owns three main signals at 1248, 1086 and 914 cm–1, which can be assigned to C–O–H bending, CH2 bending and C–O stretching vibrations, respectively. For *CH3OH on silver clusters, the 1360 cm–1 peak comes from the mixture of C–O–H bending and O–C–H bending, and the 1070 cm–1 does from C–O stretching, C–O–H bending, and O–C–H bending (Figure S12), which are in agreement with experimental studies.88-89 Finally, it is worth to emphasize that the characteristic stretching frequencies of the C–Ag bond strongly depend on surface species. As shown in our calculated results, there are a fundamental 404 cm–1 for Ag–CH3, 422 and 322 cm–1 for Ag-CH2, 290, 346 and 424 cm–1 for Ag–CH, and 509, 449, and 254 cm–1 for Ag-C. The fundamentals of 424 and 232 cm–1 arise from Ag–CHO, 218 cm–1 from Ag–CH2O. Finally, the fundamentals of 283, 243 and 410 cm–1 can be assigned to Ag–COH, Ag–CHOH, Ag–CH2OH vibrations, respectively.90 The vibrational analysis shows that the characteristic frequencies of the reaction intermediates are located in the low and middle frequency regions. However, surface infrared spectroscopy is difficult to detect the low frequency vibrational peaks of surface species so far. Thus surface-enhanced Raman 20 ACS Paragon Plus Environment

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spectroscopy may be a powerful technique to detect and identify surface species and the reaction intermediates of CO reduction on silver nanoparticles. Conclusions In summary, we have explored the mechanism of plasmon-driven CO2RR at silver clusters by DFT calculations. From the electron excitation of silver clusters, we find that the LUMO energy of CO2 well matches with sp electrons’ excitation energies of silver clusters, and the excited hot electrons on silver clusters can easily transfer to the LUMO orbital of CO2 because of the strong orbital coupling. The performed charge transfer state, having a chemisorption structure, greatly lowers the energy barriers of the C–O bond cleavage to form CO. In the succeeding reduction reactions of adsorbed CO with hydrogen, we have found that the favorable reduction path to the formation of CH3OH is through the *CHO intermediate, which is consistent with a recent experimental work of CO2RR on silver nanoparticles. Furthermore, we have calculated the Raman spectra for different intermediates to identify the surface species and deduce possible reaction paths. The present work may provide an encouraging motivation for experimental efforts to photo-electrochemical reduction of CO2. Supporting Information Computational details, calculated data of the geometric structures of chemisorbed and physisorbed states for CO2 on silver clusters, and the reaction barriers of C–O dissociation and CO2 with H interaction. Simulated absorption 21 ACS Paragon Plus Environment


spectra and electronic coupling, and assignment of the electronic transitions. All calculated results of B3LYP functional calculations. Acknowledgements X.G.Z, C.Z, X.J, D.Y.W and Z.Q.T acknowledge support from the National Natural Science Foundation of China (NSFC) (21533006, 21621091, 21373172), the 973 Program of the Chinese Ministry of Science and Technology (2015CB932303 and Y2018YFC1602802) and Funds of State Key Laboratory of Physical Chemistry of Solid surface and Fujian Science and Technology Office. Y.X.L, Y.Z. thanks the support from NSFC (No. 21833006). D.Y.W thanks the Otto Mønsted fund. The authors thank Sheng-Hisen Lin and Martin Moskovits for helpful discussions. References 1. White, J. L.; Baruch, M. F.; Pander, J. E.; Hu, Y.; Fortmeyer, I. C.; Park, J. E.; Zhang, T.; Liao, K.; Gu, J.; Yan, Y., et al. Light-Driven Heterogeneous Reduction of Carbon Dioxide: Photocatalysts and Photoelectrodes. Chem. Rev. 2015, 115, 12888-12935. 2. Ager, J. W.; Lapkin, A. A. Chemical Storage of Renewable Energy. Science 2018, 360, 707. 3. Hori, Y.; Wakebe, H.; Tsukamoto, T.; Koga, O. Electrocatalytic Process of CO Selectivity in Electrochemical Reduction of CO2 at Metal Electrodes in Aqueous Media. Electrochim. Acta 1994, 39, 1833-1839. 4. De Luna, P.; Quintero-Bermudez, R.; Dinh, C.-T.; Ross, M. B.; Bushuyev, 22 ACS Paragon Plus Environment

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TOC Graphic



Figure 1. Calculated Raman spectra of chemisorbed CO2 on different silver clusters. The Raman intensity is expanded on the basis of the Lorentzian line shape with the linewidth of 10 cm–1. 222x175mm (300 x 300 DPI)

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Figure 2. Orbital coupling between the silver cluster and CO2. The red arrow, orange arrow, and violet arrow represents the highest oscillator strength transition of the Ag18 cluster, CO2 physisorption structure, and CO2 chemisorption structure, respectively. 231x191mm (300 x 300 DPI)



Figure 3. Mechanism of the photo-generation hot electron transfer from a silver cluster to CO2. The black line and the blue line represent the density of state of Ag18 cluster and CO2, respectively. The energy level is relative to the vacuum level. 252x191mm (300 x 300 DPI)


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Figure 4. Three reaction paths for CO reduction on Ag18 cluster. The green arrows (Path I) and blue arrows (Path II) present two different reaction path with the same reduction product, CH3OH. And the red arrows are a reaction path from CO to CH4 (Path III). The H2O product is omitted in the scheme for simplicity. The numbers represent reaction activation energies at 298.15 K and 1 atm. 190x113mm (300 x 300 DPI)



Figure 5. Simulated Raman spectra of the intermediates for CO reduction on Ag18 cluster. a) CO reduction to CH4 through *CH, *CH2, and *CH3; b) CO reduction to CH3OH through *CHO, CH2O, and CH3O; c) CO reduction to CH3OH through *COH, *CHOH, and *CH2OH. The excitation wavelength of 532 nm was used here with a Lorentzian line width of 10 cm–1. 172x255mm (300 x 300 DPI)


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TOC Graphic


Reaction Selectivity for Plasmon-Driven Carbon Dioxide Reduction on

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