CO2 Electrochemical Reduction Catalyzed by Graphene Supported

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CO2 Electrochemical Reduction Catalyzed by Graphene Supported Palladium Cluster: A Computational Guideline Shiuan-Yau Wu, and Hsin-Tsung Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02174 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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ACS Applied Energy Materials

CO2 Electrochemical Reduction Catalyzed by Graphene Supported Palladium Cluster: A Computational Guideline

Shiuan-Yau Wu, Hsin-Tsung Chen* Department of Chemistry and R&D Center for Membrane Technology, Chung Yuan Christian University, Chung Li District, Taoyuan City, 32023, Taiwan

*Corresponding

author. E-mail: [email protected]; Tel: +886-3-265-3324 1 ACS Paragon Plus Environment

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Abstract By means of periodic density functional theory calculations, we have investigated the heterogeneous catalytic reduction of CO2 to formic acid including electrocatalytic and thermocatalytic reduction on graphene-supported Pd10 and hydride

Pd10 materials. The

hydrogen

proportion

of

palladium

hydride,

nH-Pd10-graphene (n = 1 ~ 10) is considered to mimic the various hydrogen ratio caused by the changed applied potential. We have predicted the limiting potentials (UL) for CO2 reduction on the nH*-Pd10-graphene models and found the UL for the formation of formate intermediate (HCOO*) would be changed with the hydrogen ratio of nH*-Pd10-graphene models. In addition, the HCOO* adsorption strength is found to play an important role for CO2 reduction reaction (CO2RR) on the nH*-Pd10-graphene. The saturated H* metal hydride in our calculation is 10H*Pd10-grahpene, but the CO2RR preferably takes place under the negative potential of –0.17 V on the 8H*-Pd10-graphene. The hydrogen evolution reaction (HER) occurs to compete with the CO2RR when the external negative potential is applied. For n = 9 ~ 10, the HER is more comparable than the CO2RR due to the lower UL of the HER. Over the UL of – 0.41 V, the hydride Pd10-graphene would be refreshed to bare Pd10-graphene and the electrochemical adsorption of CO2 to form HCOO* would become an endergonic process.

KEYWORDS: CO2 reduction reaction, electrocatalysis, formic acid, graphene-supported Pd cluster, DFT calculations 2 ACS Paragon Plus Environment

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1. Introduction The catalytic reaction by using carbon dioxide as a raw material to renewable fuels has the potential to correct the environment imbalance from excess industrial emissions of CO2. Among the possible products of CO2 conversion, formic acid (HCOOH) is suggested as a suitable material for combination of hydrogen storage and CO2 fixation. In addition, the formic acid is also easy to store, transport and release H2 at ambient temperature by employing suitable metal catalysts in direct formic acid fuel cells (DFAFC).1-3 Although the reaction of CO2 and H2 to form HCOOH is an exothermic reaction (∆H298K = –31.2 kJ mol-1), it is unfavorable at high temperature due to the endothermic of reaction free energy at room temperature (∆G298K = 33.0 kJ mol-1, ∆S298K = –0.215 kJ K-1 mol-1).3,

4

Accordingly, the solid-gas interface

thermodynamic CO2 reduction to formic acid is not favorable even at high temperature, and it is also difficult to overcome the reaction barriers at ambient temperature. Therefore, electrochemical promotion to reduction of CO2 is an alternative approach which has been widely investigated.5-12 By applying an electrical current and between the working electrode with the catalyst coated, the reaction free energy, ∆G(U) for the electrochemical CO2 reduction in liquids could be tuned through the proton-electron pair (H+ + e － ) transfer accompanying the change of the 3 ACS Paragon Plus Environment


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external potential (U), and the reaction could become facile.10-12 A good catalyst for the hydrogenation reaction must have the ability to dissociate hydrogen source such as H2O and H2 into atomic H* or promote proton-electron transfer.1-2 Palladium is as an option of metal catalysts for the hydrogenation reaction for its good performance of hydrogen storage in recent experimental and theoretical investigations.13-16 Unfortunately, palladium is considered as an inefficient catalyst for the CO2 electroreduction to produce CO, which is the major intermediate for further hydrogenation processes at high negative potentials (around –1 V).17-20

However,

recent studies also revealed an alternative pathway in which CO2 is reduced to formate intermediate (HCOO or HCOO*) over Pd nanoparticles through the electrochemical process at minimal potentials, and become dominant after formation of surface hydride (PdHx) generated by reaction of Pd nanoparticle with H2.5,

7

While the

hydrogen evolution is the major competing reaction and would affect H* coverage in various applied potential as well as the adsorption behaviors of CO2. To the best of our knowledge, the detailed eletrocatalytic reaction mechanism of CO2 conversion remains lacking.

In this work, we investigated the hydrogen adsorption, dissociation from clear Pd10-graphene to hydride 10H*-Pd10-graphene, and calculated the limiting potential (UL) of HER at different H* coverage by using the computational hydrogen electrode 4 ACS Paragon Plus Environment

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(CHE) model developed by Nørskov group.21-23 Then, CO2 adsorption and reduction were investigated under various H* covered Pd10-graphene to calculate the limiting potentials for CO2 reduction at several conditions. By comparing the limiting potential of HER and CO2RR at various H* coverage Pd10-graphene, we could identify the modest value of applied potential for CO2RR on the Pd10-graphene model.

2. Computational Details All calculations were carried out using the Vienna ab initio simulation package (VASP),24-26 based on density-functional theory (DFT) with the projector-augmented wave method (PAW).25,

27

The spin polarization and dipole correction were

considered to obtain more accurate energetics. The Kohn-Sham equations were solved in a self-consistent manner under the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional.28 The Monkhorst-Pack mesh k-points (5 × 5 × 1) was used for surface calculations with energy truncated at 400 eV.29

All

slabs were separated by a vacuum spacing greater than 20 Å, which ensures no interaction between the slabs. In addition, we have accounted for solvation correction in our calculations for describing the CO2RR in aqueous phases. The details of solvation correction and brief discussion about the influence for the HCOO* and HCOOH are described in Supporting Information. 5 ACS Paragon Plus Environment


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In order to explore the effect of electrochemical promotion of catalysis (EPOC), the computational hydrogen electrode (CHE) model suggested by Nørskov et al.21-23 has been used to calculate the Gibbs free energy changes of each elementary reaction under electrochemical condition. According to CHE method, the reaction free energies for the elementary reactions involving an proton-electron pair (H+ + e － ) transfer in the electrode could be tuned by shifting the energy of this state by –eU, where U is the applied electrode potential. The reaction free energy of an electrochemical step can be calculated as ∆Gele (U) = ∆Gele (U = 0) + eU

(1)

where ∆Gele (U) represents the free energy change of the elementary step with the applied electrical potential of U (RHE), and reaction energies for the reductive process would be decrease under the increasingly negative potentials. The corresponding limiting potential UL of each step is at potential causing the free energy difference turn into 0, which can be expressed as ∆Gele (UL) = ∆Gele (U = 0) + eUL = 0



UL = －∆Gele (U = 0)/e.

(2)

The clear Pd10-graphene model was constructed by anchoring a Pd10 cluster onto 5-8-5 defect graphene sheet (with 70 carbon atoms) which had been examined in detail in our previous work and the supported Pd10 cluster was found to have great activities for H2 and suitable adsorption energy for CO2 molecule.30 In this work, we 6 ACS Paragon Plus Environment

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used the Pd10-graphene model as a heterogeneous catalyst to investigate the mechanisms of CO2 reduction reaction (CO2RR) under the thermochemical and electrochemical conditions.

DFT molecular dynamic (MD) simulation was also

employed to predict the H* ratio of nH*-saturated Pd10-graphene by pre-covering 10H* and adsorbing 1-5 H2 onto Pd10-graphene model with the k-points of 3 × 3 × 1 in a canonical (NVT) ensemble condition; all MD simulations were performed using 2fs time step for 3ps at 300 K closed the ambient temperature (The details is shown in Supporting Information).

3. Results and Discussion In experimental environment, H2/CO2-saturated solution (instead of pure CO2) was fed into cathode for the CO2 reduction reaction under an applied negative potential, the thermocatalytic and electrocatalytic reduction of CO2 occur simultaneously7, 31, while the HCOO* was the shared intermediate as follow: (a) Electrocatalytic reduction reaction (i) CO2 (g)+ (H+ + e-) + * → HCOO* (direct electrochemical adsorption) or (ii) CO2(g) + (H+ + e-) + * → CO2* + (H+ + e-) → HCOO* (Chemisorption, then proton-transfer by Eley-Rideal mechanism) (iii) HCOO* + (H+ + e-) → HCOOH* → HCOOH(l) (b) Thermocatalytic reduction reaction 7 ACS Paragon Plus Environment


(i) H2(g) → H2* → 2H* (ii) CO2(g) + H* → HCOO* (Eley-Rideal mechanism) (iii) CO2(g) + * → CO2*, then CO2* + H* → HCOO* (Langmuir–Hinshelwood mechanism) (iv) HCOO* + H* → HCOOH* → HCOOH(l) In this work, we focused on the electrocatalytic reduction condition majorly, with the slightly discussion of possibility for thermocatalytic reduction reaction due to the thermocatalysis over the nH*-Pd10-graphene is energetically unfavorable in most conditions of our calculations.

3.1 Chemisorption and Electrochemical Adsorption of CO2 over Pd10-Grahpene

The chemisorption and electrochemical adsorption onto a heterogeneous catalyst32 can be typically represented as (c) Chemisorption: CO2 + * → CO2* (d) Electrochemical adsorption CO2 + (H+ + e-) + * → HCOO* or COOH* The chemisorption of CO2 mainly depends on the charge transfer ability of the catalyst itself, while electrochemical adsorption can be adjustable by applied electric potential.32, 33 In this section, we focused to discuss both adsorption behaviors and the electrochemical reduction of CO2 to formic acid on clear Pd10-graphene model.

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The Pd10-graphene model is shown in Figure 1a, while the charge difference diagrams of adding or removing an electron onto Pd10-graphene model (Figure 1b and 1c, respectively) are used to predict the preferred site selectivity for the adsorbate (CO2, HCOO, COOH, H2, and H). The main difference regions are located on Pd(1) to Pd(4), indicating it is preferred sites for the adsorbate, and these positions also exhibit strong interactions to the adsorbed species, such as H2, CO2 and so on, in our subsequent study. The most stable structures of adsorbed CO2 and HCOO are shown in Figure 1d and 1e with the calculated adsorption free energy (∆Gads) of chemisorption (CO2*) and the electrochemical adsorption (HCOO*) being –0.31 and –0.22 eV, respectively. The negative free adsorption energies for CO2* and HCOO* indicate both adsorption behaviors would happen spontaneously and competitively, while the chemisorption of CO2 on Pd10-graphene was preferred with no applied potential and electrochemical product of HCOO* became dominant by negatively shifting the applied potential over –0.09 V. In addition, we also calculated the COOH* intermediate (Figure 1f), but the positive free adsorption energy of +0.07 eV suggesting that the carboxyl structure is not preferred electrochemical adsorption over CO2 reduction on Pd10-graphene in our calculation. After the electrochemical adsorption of CO2 to form an adsorbed HCOO*, the subsequent step of electrochemical reaction is through 9 ACS Paragon Plus Environment


another (H+ + e-) pair transfer toward HCOO*, then generates an adsorbed HCOOH* (Figure 1g) or liquid HCOOH.

As the reaction free energy diagram shown in Figure

2(a), the reaction free energies to form HCOOH* is endergonic by 0.25 eV, less than the same counterpart of HCOOH(aq) of 0.46 eV. As a consequence, the negative potential of –0.25 V is the limiting requirement for electrochemical reduction of CO2 to form an adsorbed formic acid on Pd10-graphene, and the HCOOH* could desorb from surface or as an intermediate to the subsequent hydrogenation reaction depending on varied catalytic environments.


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Figure 1. (a)The configuration of Pd10 cluster on defect graphene and the charge difference diagrams of (b) adding or (c) removing an electron onto Pd10-graphene. (d to i) the optimized configurations of various intermediates on Pd10-graphene.


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Figure 2. Free energy diagram of CO2 adsorption and reduction to formic acid on low hydrogen covered condition. The regions (a) to (c) indicate the electrochemical steps of CO2 reduction on clear, 1H*-, and 2H*-Pd10-graphene, respectively. The corresponding reaction free energies in various conditions are summarized in Table 1. 12 ACS Paragon Plus Environment

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Table 1. Gibbs reaction free energies (in eV) of the possible step of CO2 reduction reaction on low hydrogen ratio covered nH*-Pd10-graphene (n = 0 to 2). The Gibbs free energy diagrams for CO2 hydrogenation along thermocatalytic steps is depicted in Supporting Information. Volmer step

chemisorption

Electrocatalytic steps

(H+ + e-) + *

CO2 + *

CO2* + (H+ + e-)

→ H*

→ CO2*

→ HCOO*

→ HCOO*

→ HCOOH*

Pd10-gra.

0.40

0.31

+0.09

0.22

+0.25

1H*-Pd10-gra.

0.40

0.22

+0.06

0.16

+0.22

2H*-Pd10-gra.

0.22

+0.03

+0.01

+0.04

+0.06

CO2 + (H+ + e-) +* HCOO* + (H+ + e-)

Thermocatalytic steps (Langmuir–Hinshelwood mechanism) CO2* + (H+ + e-)

CO2* + H*

HCOO* + (H+ + e-)

HCOO*+ H*

HCOOH*

→CO2* + H*

→ HCOO*

→HCOO* + H*

→ HCOOH*

→ HCOOH(aq)

Pd10-gra.

0.31

1H*-Pd10-gra.

0.15

2H*-Pd10-gra.

desorption

0.34 0.20

+0.40 +0.21


+0.21 +0.59

+0.18

+0.33

+0.14


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3.2 The Influence of Hydrogen Evolution Reaction (HER) and the H2 Dissociative Adsorption over Reduced and Low nH*-Pd10-Graphene (n =1 and 2) According to the environment of CO2 reduction on solid-liquid interface, H2, CO2 and solvated proton from the electrolyte exist under electrochemical conditions, and the following reactions5-7: (i) chemisorption or electrochemical adsorption of CO2, (ii) H2 dissociative adsorption, and (iii) H+ + e + * → H* (Volmer reaction, the initial step of HER)34, 35 would occur simultaneously and affect each other.

Both hydrogen

dissociative adsorption and Volmer reaction would cause the cluster become metal hydride5, and the Volmer reaction is usually consider fast over clear surface and affect the hydrogen coverage on the potential34; in this work, the hydrogen proportion of palladium hydride, nH-Pd10-graphene is considered. By the Kubas-type interaction36, 37, the H2 molecule could anchor on the top Pd atom of Pd10-graphene (Figure 1g) with the calculated adsorption free energy of –0.53 eV. The adsorbed H2 would be dissociated spontaneously to form 2H* over Pd10-graphene (Figure 1h) at ambient temperature due to the small energy barrier of 0.20 eV with exothermic free reaction energy of –0.27 eV and the automatic decomposition of adsorbed H2 to form 2H* is found through the MD calculation under 300K. In other word, H2 dissociated adsorption reaction (H2(g) → 2H*) on Pd10-graphene would occur with ∆Gads of –0.80 eV.

On the other hand, the clear

Pd10-graphene is also attractive for solvated (H+ + e-) pair, and the calculated free adsorption energy for the first and second proton-electron pairs to form 1H*- and 2H*-Pd10-graphene are –0.40 and –0.41 eV, respectively. Accordingly, the formation of nH*-Pd10-graphene through either the H2 dissociative adsorption or Volmer step of HER reaction (H+ + e- + * → H*) would be fast34, 35 and have higher priority than CO2 reduction reaction under the electrochemical condition. Therefore, CO2 reduction 14 ACS Paragon Plus Environment

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reaction over hydride nH*-Pd10-graphene is also need to take into account, while the all the reaction on 1H*- and 2H*-graphene would be discussed to compare with clear Pd10-graphene. All the relative free energies of possible states for CO2 reduction on clear, 1H*-, and 2H*-Pd10-graphene are described in Figure 2, while the marked (a), (b) and (c) regions in Figure 2 presented the electrochemical steps of CO2 reduction on clear, 1H*-, and 2H*-Pd10-graphene, respectively, while the major Gibbs reaction free energies of these possible step of CO2 reduction reaction are tabulated in Table 1 and the corresponding thermocatalytic steps is depicted in supporting information (Figure S6). The adsorption free energies, ∆Gads, of CO2 chemisorption on 0H* and 1H*-Pd10-graphene

are –0.31 and –0.22 eV, respectively, and became positive of +0.03 eV on

2H*-Pd10-graphene, while the calculated electronic adsorption energies, ∆Eads, on 0H*, 1H*- and 2H*-graphene are –0.46, –0.40 and –0.12 eV, respectively.

The

negative ∆Eads but positive ∆Gads (298K) for CO2* indicating the chemisorption of CO2 on 2H*-Pd10-graphene is existent but thermodynamically unstable under room temperature; furthermore, the adsorbed CO2 structure has not been found on 3H*-Pd10-graphene

or higher ratio in our calculation.

Consequently, the CO2 chemisorption

would be inhibited by the gradually hydrogenation process for the catalyst. However, the hydrogenation process also affects the electrochemical steps obviously, in which the adsorption free energies for electrochemical adsorption to form HCOO* on 0H*, 1H*- and 2H*-graphene are –0.22, –0.16 and +0.04 eV, respectively.

The

negative adsorption free energy of HCOO* on 1H*-Pd10-graphene suggesting the electrochemical adsorption steps is also exergonic and would compete with chemisorption step, but become dominant in the initial step of CO2 adsorption by negatively shifting the applied potential over –0.06 V. Both chemisorption and 15 ACS Paragon Plus Environment


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electrochemical adsorption of CO2 become endothermic, but the formation of HCOO* under 2H*-Pd10-graphene could proceed under the applied limiting potential of –0.04 V.

According to Figure 2b and 3c, the free reaction energies of HCOO* + (H+ + e-)

→ HCOOH* are +0.22 and +0.06 eV on 1H*- and 2H*-Pd10-graphene, respectively, both less than the counterpart of +0.25 eV over clear Pd10-graphene. Consequently, all the rate determining steps of CO2 electronic reduction to formic acid on clear, 1H*- and 2H*-Pd10-graphene are the proton transfer toward adsorbed formate intermediate, HCOO* + (H+ + e-) → HCOOH*, with the gradually reduced limiting potential of –0.25, –0.22 and –0.06 V accompanying to the increased hydrogen ratio on Pd10-graphene.

3.3 HER/HOR over nH*-Pd10-Graphene (n = 0 to 10) Although we obtained a small limiting potential of –0.04 V for CO2 electro-reduction to formic acid on 2H*-Pd10-graphene and adjustable limiting potential with various adsorbed hydrogen amount, the hydrogen ratio of 2H*-Pd10-graphene is still lower due to the subsequently proceeding of H2 dissociative adsorption and Volmer step to change the hydrogen ratio. Therefore, to clarify the CO2 reduction on various H* ratio of Pd10-graphene, we also investigated the multiple H2 adsorption and subsequently Volmer steps (1H* → 2H* → → nH*) to find the applicable H* ratio of nH*-Pd10-graphene model. By anchoring 1 to 5 H2 onto Pd10-graphene model, the free adsorption energy for nth hydrogen molecules is calculated as ∆Gads, nth = μ(nH2-Pd10-graphene) – μ[(n-1) H2-Pd10-graphene] + μ(H2(g))

(3)

As shown in Figure 3a, the calculated free adsorption energy for 1st to 5th H2 by the Kubas-type interaction are –0.53, –0.47, –0.33, –0.39 and –0.30 eV, respectively 16 ACS Paragon Plus Environment

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(corresponding structures are shown in Figure S4). Due to the facilely dissociation of 1st H2 onto Pd10-graphene from the MD simulation, we also performed the MD simulation to predict the possible results of 2 to 5H2*-Pd10-graphene at 300K for 3 ps, and observed that 2H2* and 3H2* on Pd10-graphene were automatic dissociated to 4H*- and 6H*-Pd10-graphene, respectively, with the exothermic free reaction energies in Figure 3b.

However, for 4H2*-Pd10-graphene, there were just three H2*

dissociated to form 6H* accompanying with an adsorbed H2 molecule. On 5H2*-Pd10-graphene model, the MD result also presented 6H* formation with an adsorbed H2* and a separate H2 molecule. So that the most possible ratio of nH*-Pd10-graphene through H2 dissociated adsorption would be 6H*-Pd10-graphene at room 300K; ignored the Pd atom used to connect the cluster and graphene, the H/Pd ratio of 6H*-Pd10-graphene became 0.66, approached to that of PdH0.706 (JCPDS 18-0951) facilely generated under H2 atmosphere.38 However, the hydrogen ratio of the catalyst in electrochemical environment is also dependent on the Volmer step (H+ + e- + * → H*), and the consecutively H adsorption process on Pd10-graphene forming nH*-Pd10-graphene was also investigated and the relative free energies for 1H*- to 11H*-graphene were shown in Figure 3c. The calculated free energy of each H* is defined as the following equation, ∆Gads, nth H = μ(nH*-Pd10-graphene) – μ[(n-1) H-Pd10-graphene] – 0.5μ(H2(g)) + eU (4) The calculated free adsorption energies were from 1H* of –0.40 eV to 10H* of –0.12 eV, and become positive of 0.18 eV for the 11th H* adsorption. The 6H*-Pd10-graphene is the H*-saturated condition through H2 dissociated adsorption, and further hydrogenation by adsorbing proton would over the coordinately unsaturated site until the formation of 10H*-Pd10-graphene, which as the H*-saturated 17 ACS Paragon Plus Environment


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Pd10-graphene model under the electrolyte. The equilibrium for the H*-saturated condition could be shifted via the reverse Volmer route H* → H+ + e-, the final step of the hydrogen oxidation reaction (HOR), under the positive applied potential.

Another fact to affect the ratio of nH*-Pd10-

-graphene is the Heyrovsky Reaction39 H* + (H+ + e－) → H2 (or nH* + (H+ + e－) → (n –1)H* + H2 in our model), while the reaction will occur under the negative applied potentials, compete with CO2RR and change the limiting potential of CO2RR simultaneously.

Figure 4 shows the various limiting potentials for nH*-Pd10-

-graphene to (n-1)H*-Pd10-graphene via the Heyrovsky Reaction.

From the

saturated 10H*-Pd10-graphene at U = 0, the equilibrium will be shifted to 8H*-Pd10-graphene under the negative applied potential around –0.13 eV.

Due to

similar limiting potentials of HER reaction of 10H* → 9H* (UL = –0.12 V) and 9H* → 8H* (UL = –0.13 V), it is difficult to control the equilibrium to stay in 9H*-Pd10-graphene.

By negatively shifting the applied potential to –0.21 V, the HER reaction

of 8H* → 7H* will occur accompanying the subsequent HER reaction of 7H* → 6H* to become 6H*-Pd10-graphene. The HER reaction of 7H* → 6H* has a slight limiting potentials of – 0.09 V, indicating that 7H*- Pd10-graphene is unstable and facile to subsequent reaction.

The subsequent equilibrium states of nH*-Pd10-graphene are

4H*-Pd10-graphene around –0.25 V due to the similar limiting potentials of HER reaction of 6H* → 5H* (UL = –0.25 V) and 5H* → 4H* (UL = –0.25 V).

Next, the

HER reaction of 4H* → 3H* (UL = –0.35 V) and 3H* → 2H* (UL = –0.22 V) were started by enhancing the negative potential to –0.35 V.

At U = –0.41 V or more

negative value, we would obtain a fully reduced Pd10-graphene model and all the adsorbed hydrogen through Volmer route would become active to react with another proton-electron pair, forming the H2 molecule (complete HER: 2H+ + 2e- → H2). 18 ACS Paragon Plus Environment

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Figure 3. Relative free energy diagram of (a) 1st to 5th H2 by the Kubas-type interaction, (b) dissociative adsorptions and (c) consecutively H adsorptions on Pd10-graphene to form nH*-Pd10-graphene.



Figure 4. The various limiting potentials for nH*-Pd10-graphene to (n-1)H*-Pd10-graphene via the Heyrovsky Reaction.


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3.4 CO2 Electrochemical Reduction to HCOOH on nH*-Pd10-Graphene In section 3.3, we have examined the hydrogenation reaction for graphene supported Pd10 cluster to form the H*-saturated Pd10-graphene and discussed the HER at various negative potentials. In this section, we investigated the CO2 reduction reaction to formic acid over various nH*-Pd10-graphene and calculated the limiting potential of each elementary step to compare with HER at the same counterparts. The reletive free energies of CO2 reduction reaction on nH*-Pd10-graphene (n = 0 to 10) were illustrated in Figure 5, involving the energetically decreased potential of nH*-Pd10-graphene (n = 0 to 10, blue solid line) as well as the relative free energy potentials

of

HCOO*,

HCOOH*

and

HCOOH(l)

on

corresponding

nH*-Pd10-graphene. The lowest free energy potential in Figure 5 is 10H*-Pd10-graphene indicated the most stable structure without the applied potential, while CO2 reduction reaction to HCOO* through electrocatalysis, 10H* + (H+ + e-) + CO2 →10H* + HCOO* requires a potential of –0.24 V (RHE) to proceed over 10H*-Pd10-graphene due to the endergonic reaction free energies (∆Gr) of 0.24 eV. The further reduction step is hydrogenation of HCOO* to form adsorbed HCOOH* with an exergonic process by 0.04 eV, suggesting that the HCOO* becomes ionized and prefers to catch the proton to form formic acid, spontaneously. As a result, the formation of HCOO is the rate-determining step for CO2 electrochemical reduction over 10H*-Pd10-graphene, occurring around –0.24 V.

However, the limiting potential for HER of 10H* → 9H*

is –0.13 V, much less than the requirement for HCOO* formation, indicating the hydrogen ratio of 10H*-Pd10-graphene would be reduced first at the negative between –0.13 to –0.24 V, and limiting potential of –0.24 V for CO2RR over 10H*-Pd10-graphene would be affect and become inapplicable. Therefore, we further 21 ACS Paragon Plus Environment


Page 22 of 37

examined limiting potentials of two elementary steps of CO2RR over various nH*-Pd10-graphene to compare with the limiting potentials of HER at the same counterparts, and illustrated the comparisons in Figure 6.

Where the red and green

bars represent the limiting potentials for the formation of HCOO* and HCOOH over nH*-Pd10-graphene (n = 0 to 10), respectively. The blue bar reveals the requirement for HER of nH* + (H+ + e－) → (n –1)H* + H2, based on the section 3.3.


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Figure 5. The full reletive free energies of CO2 reduction reaction on nH*-Pd10-graphene (n = 0 to 10).



Figure 6. The limiting potentials of two elementary steps of CO2RR over various nH*-Pd10-graphene and the limiting potentials of HER at the same counterparts.


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As shown in Figure 6, the limiting potentials for the formation of HCOO* (RDS) and HCOOH over 9H*-Pd10-grahpene are –0.21 and –0.02 V, respectively, while the limiting potential of RDS is still larger than that of HER of 9H* → 8H*-Pd10-graphene (UL

=

–0.13 V), indicating the H* ratio of 9H*-Pd10-grahpene was still

changed in advance before the CO2RR over 9H*-Pd10-graphene.

The limiting

potentials for the formation of HCOO* and HCOOH* over the 8H*-Pd10-graphene are –0.17 and –0.03V, respectively, while the limiting potential of HER of 8H* → 7H*-Pd10-graphene enhanced to –0.21 V.

Upon the negatively shifting from 0 V,

10H*-Pd10-grahpene would be reduced to 8H*-Pd10-grahpene at the potentail around – 0.13 V, the adsorption of 9th and 10th hydrogen become unstable and facile to be reduced through HER; therefore, the CO2RR would occur over the 8H*-Pd10-grahpene when the negative potentail is shifted to –0.17 V, as the first stable limiting potential of CO2RR over nH*-Pd10-graphene.

And the equilibrium would be broken

when the negative potential reached to –0.21 V, the H*-saturated situation would convert to 6H*-Pd10-graphene due to the small limiting potential of –0.09 V for 7H* → 6H*-Pd10-graphene. The next potential to change the steady ratio of nH*-Pd10-graphene

is around –0.25 V, the limiting potential of HER for 6H* → 5H*-Pd10-

-graphene. However, it just required –0.12 to meet the limiting potentials of HCOO* formation. It means that at the negative potentials between –0.21 V and –0.25 V, the 6H*-Pd10-graphene is the most likely model, while CO2RR to form formic acid over 6H*-Pd10-graphene is expected to become facile due to the less limiting potentials for RDS (UL = –0.12 V of HCOO* formation). In addition, the limiting potentials for HER (5H* → 4H*, UL 0-graphene

=

–0.25 V) and CO2RR (UL

=

–0.13 for HCOO*) over 5H*-Pd1-

is similar to the same counterparts over 6H*-Pd10-graphene, implying a

steady process of CO2RR when the conversion of 6H* → 5H*-Pd10-graphene occurs 25 ACS Paragon Plus Environment


around –0.25 V.

Page 26 of 37

At the negative potential between –0.25 V and –0.34 V, the most

likely surface gradually became 4H*- Pd10-graphene, while the limiting potential for HCOO* is reduced to –0.07 V.

The most likely surface would be change to

2H*-Pd10-graphene via the short stay at 3H*-Pd10-graphene by negatively shifting the potential from – 0.34 V to – 0.41 V, while the electrochemical adsorption of CO2 to form HCOO* became more facile due to the reduced limiting potentials of –0.06 V and –0.04 V over 3H*- and 2H*-Pd10-graphene, respectively.

More negatively shift

the potential than – 0.41 V, the hydride Pd10-graphene would be revived to bare Pd10-graphene, and the electrochemical adsorption of CO2 would become an endergonic process like the description in section 3.1. This finding agrees with the experimental study in which they demonstrated that the HER becomes prefer to occur upon shifting the UL from – 0.1 V to – 0.4 V.7 According to abovementioned results, we observed the most likely model under various negative potentials would be shifted from 10H*-Pd10-graphene at 0 V to fully reduced Pd10-graphene at – 0.41 V, while the electrochemical adsorption of CO2 to HCOO* over 10H*-Pd10-graphene is an endothermic process with largest reaction energy of 0.24 eV at 0V, being reduced to 0.04 eV gradually upon negatively shifting potential form 0 V to – 0.34 V, and become an exergonic process over 1H*- and reduced Pd10-graphene, indicating an obvious effect of electrochemical promotion of catalysis to HCOO* formation.

In contrast, the lowest endergonic reaction free

energy of the subsequent HCOO* to formic acid is 0.02 eV over 10H*-Pd10-graphene at 0V, being gradually enhanced to 0.25 eV over reduced-Pd10-graphene at – 0.41 V. The 7H*-Pd10-graphene is the most unstable state during the reduction process from 10H*-Pd10-graphene to Pd10-graphene, which could be ignored the corresponding electrochemical result in Figure 6, and apparent trends for HER and CO2RR over 26 ACS Paragon Plus Environment

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various nH*-Pd10-graphene would be obtained as shown in Figure S7. Except the clear, 1H* and 7H*-Pd10-graphene, the HCOO* formation is an endothermic process and is the rate determining step on the nH*-Pd10-graphene. The HCOO* adsorption strength on various nH*-Pd10-graphene is an important fact of CO2RR, and an obvious influence between the Pd-Pd bond length (Figure 7a), and ∆Gads of HCOO* (Figure 7b) could be obtained, in which the elongated Pd-Pd bond from 2.76 Å of Pd10-graphene to 3.23 Å of 10H*-Pd10-graphene would lead the weakened interaction between HCOO* and graphene supported nH*-Pd10-graphene. In addition, the solvation effect for the HCOO* on nH-Pd10-graphene (n = 0 - 10) become obvious gradually accompanying to the processes of cluster hydrogenation. These suggests that the HCOO* is ionized by degrees accompanying to the reduced interaction between HCOO* and nH-Pd10-graphene models (as shown in Figure S3).



Figure 7. The comparison of (a) the Pd-Pd bond length and (b) ∆Gads of HCOO* for the various graphene supported nH*-Pd10-graphene.


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3.5 Thermocatalysis of CO2 reduction over nH*-Pd10-graphene models According to experimental report about the CO2 reduction to produce HCOO* over Pd nanoparticles under the electrochemical condition7, the thermocatalysis and electrocatalysis would occur simultaneously and compete to each other at the negative potential of – 0.1 and – 0.2 V, while the former would sharply decreased at – 0.3 V and disappeared at – 0.4 V.

In our calculation, the most likely model between – 0.1

and – 0.2 V is 8H*-Pd10-graphene, and the calculated reaction free energies in Figure 5 show that the electrochemical CO2 reduction, CO2 + (H+ + e － ) → HCOO* over 8H*-Pd10-graphene is 0.19 eV, higher than that (0.12 eV) for thermocatalytic step (CO2 + 8H* → HCOO* + 7H*), indicating the CO2 reduction through the thermocatalysis could occur under low negative potentials.

At the negative potential

around – 0.4 V, the nH*-Pd10-graphene is refreshed to clear Pd10-graphene, resulting the disappear of thermocatalysis pathway (CO2 + H* → HCOO*). However, over the gas-phase environment, the H2 would automatic dissociated to hydrogenate the Pd10-graphene to from 6H*-Pd10-graphene, and the reaction free energy (Gr) of thermocatalytic step (CO2 + 6H* → HCOO* + 5H*) is +0.38 eV, being larger than that of all the electrocatalysis steps, indicating the CO2 reduction reaction could promote in aqueous environment.



4. Conclusion In this work, we have carried out a comprehensive study of the CO2RR and HER on nH*-Pd10-graphene models (n = 0 to 10). The calculation results demonstrate that the adsorption strength of formate intermediate (HCOO*) plays an important role for CO2RR and the UL for the formation of HCOO* intermediate changes with the hydrogen ratio on the nH*-Pd10-graphene. The UL of Heyrovsky reaction for each nH*-Pd10-graphene is calculated to identify the competing relationship between the CO2RR and HER to find out the suitable range of the negative potentials. The CO2RR is likely to startup under the negative potential of –0.17 V on the 8H*-Pd10-graphene. The HER via nH* + (H+ + e － ) → (n –1)H* + H2 occurs and competes with the CO2RR when the external negative potential is applied. For n = 9 ~ 10, the HER is more comparable than the CO2RR. Over the UL of –0.41 V, the hydride Pd10-graphene would be reanimated to bare Pd10-graphene and the CO2 electrochemical adsorption become an endergonic process, and HCOO* + (H+ + e － ) become the rate-determined step for the HCOOH formation. Our computational results are consistence with the experimental studies5, 7 which show CO2 preferably reduces to formic acid or formate on the Pd nanoparticle at low overpotential and demonstrate that the applied potential would affect the hydration degree of cluster and the trend of CO2RR. This investigation could shed light on the exploration of the graphene-supported metal cluster as potential CO2RR electrocatalysts.


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Supporting Information. The detail of free energy correction, solvation correction including the major influence for HCOOH adsorption and HCOO* intermediates, computational hydrogen electrode (CHE) model. The Kubas adsorption and dissociated adsorption of 1-5H2 over

Pd10-graphene.

MD-calculation

for

1

to

5H2*-Pd10-graphene

and

10H*-Pd10-graphene at 300K. Thermcatalytic mechanism of CO2 reduction to HCOOH on Pd10-grahpene. The limiting potentials of two elementary steps of CO2RR over various nH*-Pd10-graphene (without 7H*-Pd10-graphene) and the limiting potentials of HER at the same counterparts with the apparent trends.

Acknowledgement. This study was supported by the Ministry of Science and Technology

(MOST)

under

Grant

Numbers

MOST

107-2113-M-033-004,

107-2811-M-033-005, 106-2113-M-033-003, 105-2113-M-033-008, Chung Yuan Christian University (CYCU), and National Center for Theoretical Sciences (NCTS), Taiwan, and the use of facilities at the National Center for High-Performance Computing, Taiwan.



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TOC Graphic


CO2 Electrochemical Reduction Catalyzed by Graphene Supported

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