Reaction mechanisms for reduction of CO

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Applied Surface Science

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Full length article

Reaction mechanisms for reduction of CO

₂

to CO on monolayer MoS

₂

Yunlong Xie

^a,b

, Xiang Li

^a

, Yu Wang

^a

, Biwen Li

^a

, Lun Yang

^a

, Nian Zhao

^a

, Meifeng Liu

^a

, Xiuzhang Wang

^a

, Ying Yu

^b,⁎

, J.-M. Liu

^a,c

aInstitute for Advanced Materials, Hubei Normal University, Huangshi 435002, China

bCollege of Physical Science and Technology, Central China Normal University, Wuhan 430079, China

cLaboratory of Solid State Microstructures and Innovative Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

A R T I C L E I N F O

Keywords:

Transition metal dichalcogenide Electrocatalysis

MoS2

DFT CO2reduction

A B S T R A C T

Since the reduction of CO2to fuels by consuming over-generated electricity, which can establish artificial carbon cycle and energy storage at the same time, extensive studies have been devoted to developing suitable catalysts for CO₂conversion in materials science. Recently, MoS₂, a typical member of transition metal dichalcogenides, has been widely investigated for its high activity and low energy cost to catalyze CO2reduction. In this work, we simulate the microscopic dynamic process of the CO2reduction process in the framework of density functional theory (DFT). Our results reveal that Mo exposed edges of MoS₂are inclined to adsorb CO₂molecule and tend to catalytically reduce CO2to CO. CO2molecule is activated by two neighboring Mo atoms and the CeO double bond reconstructs in the adsorption process. Thefirst proton/electron (H⁺+ e⁻) reaction taking place at MoS₂ edges undergoes a different pathway from that on transition metal catalyst, contributing to the product selectivity towards CO. Finally, we demonstrate that desorption of CO from MoS2edges is in virtue of unique diffusion process for adsorbed CO atoms.

1. Introduction

Excessive consumption of fossil fuels disturbs carbon cycle on the surface of our earth, and leads to the worse greenhouse eﬀect. Electro- reduction CO2to industrial raw materials or clean fuels can establish an artiﬁcial carbon cycle, and therefore contribute to the mitigation of global warming [1–3]. Moreover, the conversion of CO2reduction reaction (CO2RR) is also an economical and practical approach to store over-generated nuclear electricity, photovoltaic electricity and so on.

To achieve this appealing blueprint, the key issue is to develop suitable electro-catalyst with outstanding catalytic activity, superior reliability, high product selectivity, environmental benignity, and low cost. In the last several decades, a large number of catalysts have been experi- mentally evaluated for CO2RR. Some metallic electrodes, including Ag [4,5], Cu [6,7], Zn [8] and Pd [9,10], have been veriﬁed to have ac- ceptable performance on electro-catalytic CO2RR. Among them, copper is one of the mostly studied electro-catalysts, which has shown unique performance for CO2RR to hydrocarbon compound at reasonable faradic eﬃciency, but it still needs a relatively large overpotential of

~1.0 V [11]. Ag and Pd are noble metals that are not conducive to large-scale industrial application. Furthermore, the metallic electrocatalysts produce mixed products of H2, C1, C2, and C3 hydrocarbon

species [12]. These disadvantages of metallic catalysts limit their widely practical applications.

Recently, nanostructured molybdenum disulﬁde (MoS2), one of the most common transition metal dichalcogenides (TMDCs), has attracted much attentions for its layered crystal structure and distinctive catalytic features [13–22]. The most stable structure of monolayer MoS2is tri- gonal prismatic polytype, and the Mo layer is sandwiched by two S layers [23] in each S-Mo-S monolayer. As an eﬃcient catalyst, MoS2is widely used for hydrogen evolution reaction (HER) [16,24] and oxygen reduction reaction (ORR) [25]. Owing to the low price and high catalytic activity, MoS2is regarded as a more promising catalyst than noble metal for water splitting.

Unlike those for water splitting, the researches for CO2RR over MoS2

are still in preliminary stage. Asadi et al.ﬁrst reported their study on MoS2 catalyzing CO2RR in 2014 [15]. They found that Mo-exposed edges of MoS2exhibits prominent CO2RR performance in ionic liquid.

As-reported catalytic performance of MoS2 has the following three signiﬁcant features: the overpotential of CO2RR is extremely low, about

~54 mV (low overpotential); CO is the only carbonaceous product of CO2RR (high selectivity); the semiconducting nature of MoS2 con- tributes to its photochemical performance (artiﬁcial photosynthesis) [14]. Although the related experimental studies are in rapid progress,

https://doi.org/10.1016/j.apsusc.2019.143964

Received 27 May 2019; Received in revised form 18 August 2019; Accepted 10 September 2019

⁎Corresponding author.

E-mail address:yuying01@mail.ccnu.edu.cn(Y. Yu).

Applied Surface Science 499 (2020) 143964

Available online 11 September 2019

T

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theoretical understanding of the unique features of MoS2 catalyzing CO2RR has been of inadequacy so far. The existing calculation results based on density functional theory (DFT) have shown some explana- tions for the catalytic activity of MoS2[14,17]. The d-band center of exposed Mo atoms at the edges of MoS2is much closer to the Fermi level than that of metallic catalysts, suggesting that Mo exposed edge sites of MoS2have strong interactions with the adsorbed species. In the MoS2catalytic reaction pathway CO2→COOH*→CO*→CO, the free energies of CO* and COOH* are lower than that of CO2and CO in the gas phase, owing to the strong binding interactions between MoS2and adsorbed CO* and COOH*. Furthermore, it is illustrated from the bowl- shaped free energy curve that theﬁrst two reaction steps are exergonic and kinetically favorable.

However, there still exist some unresolved doubts about microscopic catalytic reaction process. For example, how is CO2molecule activated at the edge of MoS2, why does the electro-catalytic reduction of CO2 select the pathway CO2→COOH*→CO, and how does adsorbed CO diﬀuse and desorb from the edges of MoS2. To address these unclear issues, we present our systematically theoretical study about reaction mechanisms for the reduction of CO2 to CO on monolayer MoS2. Our work includes the following three aspects: the activation process of CO2molecule, selectivity-determining steps of CO2reduction to CO and dynamic process of adsorbed CO*.

We employ theﬁrst principles studies in DFT framework [26,27].

Moreover, monolayer MoS2is chosen as the calculated model mainly for the following considerations. First, monolayer MoS2exhibits much better catalytic CO2RR performance than that of bulk MoS2. Second, the morphology of the active edges of MoS2are monolayer-like, as shown in the scanning transmission electron microscopy (STEM) images [14], which indicates that the monolayer model of MoS2can fully cover the nature of bulk MoS2. Finally, monolayer MoS2is a promising photo- catalyst candidate for its semiconducting nature with about 1.9 eV di- rect band gap.

2. Models and computational details

The DFT [28,29] calculations were carried out using the Vienna ab- initio simulation package (VASP, version 5.4.4) [30,31]. The PBE functional [32] was employed for electronic exchange and correlation.

The plane wave pseudopotential with a kinetic cutoﬀenergy of 450 eV and Gaussian smearing method with an electronic temperature of kBT= 0.05 eV were used in all our DFT calculations. During the system relaxing process, the total electronic energies and the residual forces on the atoms were converged to 10⁻⁶eV and 0.02 eV·Å⁻¹respectively.

In our calculations, monolayer MoS2 model was constructed by adding a 15 Å thick vacuum slab to thec-axis direction. Unit cell of monolayer MoS2contained two S atoms and one Mo atom. Brillouin- zone integration was conducted using a 7 × 7 × 1 Monkhorst-Pack grid [33]. The lattice parameters of monolayer MoS2 were obtained by geometry optimization calculations. Our calculations yield a=b= 3.18 Å, in reasonable agreement with the experimental values of 3.16 Å, and the included angle of vectorsaandbis 120°. Incdi- rection, the distance between two nearest S atoms is 3.11 Å, well con- sisting with the as-reported results [34]. The Mo exposed edges were obtained by cut along theb-axis, and another 10 Å thick vacuum slab was added to avoid inter-layer interactions. The constructed Mo exposed nanoribbons included 3 × 4 unit cells with 24 S atoms and 12 Mo atoms. The Brillouin-zone integration on MoS2nanoribbons was per- formed using a 3 × 1 × 1 Monkhorst-Pack gird. The bottom four layers of the nanoribbon wereﬁxed while the top two layers and the asso- ciated adsorbed species were allowed to relax in the geometrical optimization.

The transition state (TS) was located using the Climbing Image Nudged Elastic Band (CINEB) method [35] and was veriﬁed by vibra- tion analyses with only one imaginary frequency. The binding energy (EBE) of the adsorbates is deﬁned as:

= − −

EBE Etot Esub Eads, (1)

whereEtot,Esub, andEadsare the total energy terms of optimized adsorbate + substrate, clean substrate, and gas phase adsorbate, respectively, and they can be directly obtained from the DFT calculations. The free energies (G) of the CO2reduction intermediates are calculated as G=Eele+ ZPE -TS, whereEeledenotes the electronic energy, ZPE the zero point energy,Tthe temperature (20 °C in all our calculations) and S the vibrational entropy. The vibrational entropy is estimated according to the vibrational frequencies determined by using the har- monic normal mode approximation:

∑

= − − − ⁻

S k_B [ /(x e 1) ln(1 e )],

i i xi xi

(2) wherekBis the Boltzmann constant. For each vibrational modexi=hνi/ (kBT),νi denotes the normal mode frequency andhthe Planck's constant. For the transition state, the mode of motion along the reaction coordinate is omitted, i.e., the imaginary part of frequency is not calculated. In our practical calculations, the change of free energy (ΔG) relative to the gas phase CO2and clean substrate are more illustrative.

3. Results and discussion

In the microscopic dynamic process of CO2electro-catalytic reduction reaction, the adsorption of CO2on the catalyst (i.e. CO2→CO2*) is thefirst step, which has a significant impact on the catalytic activity for CO2RR. The hydrogenation steps determine the selectivity of reaction pathways. The diffusion and desorption of reduction products together with the hydrogenation steps determine the efficiency of CO2RR.

3.1. Activation of CO2

The results of geometrical optimization show that the most stable adsorption site is the bridge site of two nearest exposed Mo atoms. In addition, linear CO2molecule transforms into bent CO2after its adsorption at the edges (as shown inFig. 1c and d), and the O-C-O angle of bend CO2is about 134°. To understand the bending eﬀect further, the frontier orbitals of CO2molecule is calculated. The highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) are two common kinds of frontier orbitals. HOMOs and LUMOs of isolated CO2and adsorbed CO2* are shown inFig. 1. Two parallelp orbitals of O form theπbond for isolate CO2(Fig. 1a and b), while the frontier orbitals of bent CO2(Fig. 1c and d) are totally diﬀerent from that of isolate CO2. This phenomenon is attributed to the factors in two aspects. On one hand, theπbond in CO2strongly depends on the linear molecule structure and the recombinant of componentp-orbitals as the bending structure breaks the geometrical symmetry of CO2molecule.

On the other hand, the lone pairpelectrons of O and C together withd electron of Mo form thep-dhybridized orbitals.

The projected density of states (PDOS) of CO2* and exposed Mo atoms interacting with the adsorbed CO2are obtained by decomposing the electron density and wave function into the atomic orbital contributions. The Mo exposed edges of MoS2exhibit the hybridization of d-orbital (Mo atoms) andp-orbitals (C and O atoms of bent CO2*), as shown inFig. 1e. In the energy range from−5.0 to−3.8 eV, there are two robust orbital resonance peaks in PDOS, i.e.,d-orbital of Mo andp- orbital of O, indicating the strong bonding interactions between Mo and O. Comparatively,p-orbital of C resonates more withp-orbital of O than withd-orbital of Mo in the whole energy range, which means that the hybridization of orbitals from C and Mo atoms is relatively weaker. Our PDOS results suggest that the bonding interaction between edged Mo and O breaks the strong CeO bond of CO2and activates the adsorbed bent CO2*.

It was reported by Yu's group that the composite TiO2@MoS2ex- hibited excellent performance on catalyzing CO2RR in a slightly alkaline solution [17]. Herein, we consider the situation that CO2RR takes place in alkaline solution. In such condition, there exist two competing

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reactions, i.e. water splitting and CO2RR, in which the adsorption energies of H2O and CO2are key factors that determine the reaction selectivity towards water splitting or CO2RR. The adsorption energies of H2O* and CO2* are presented inTable 1. The adsorption energy of CO2* is 1.81 eV, almost twice as large as that of H2O* (~0.90 eV). Each CO2* occupies two active Mo sites while H2O* only occupies one site.

Moreover, the unpaired electrons in CO2* have strong bonding interactions withd-electron of Mo. These two aspects attribute to diﬀerence adsorption properties of H2O* and CO2*.

3.2. Selectivity determining steps of CO2reduction to CO

The most distinct feature of electrocatalystic CO2RR over MoS2is the product selectivity towards CO. Unlike the product over the metal catalysts, CO is the only carbonaceous product of CO2RR over this material according to the previous experimental works [15,17,36]. To

understand the reaction mechanisms for the reduction of CO2RR over MoS2, it is necessary to concern two fundamental questions: Why the selective product is CO alone and how CO2is reduced to CO.Fig. 2 shows the possible reaction pathways of CO2RR based on previous DFT calculations [14,36–39]. Reaction step I is the adsorption of CO2, during which the CO2molecule is activated. Step II refers to the hydrogenation process of CO2. For the CO2RR on Cu(111) [37,39], the product of this step is COOH*, while for CO2RR over MoS2it is com- plicated due to the reason that the reaction takes place in two neighboring sites. So, there are two possible reactions for step II (II¹and II²).

Reaction II¹is CO2*→COOH*, which is the same as that on Cu surfaces. In reaction II², CO2*first reacts with (H⁺+e⁻), forming an un- stable intermediate HO-CO*, and then the intermediate product de- composes into OH* and CO* spontaneously. The proposed reaction step III is the hydrogenation of COOH*, which will happen in two different directions (i.e., reaction III¹ and III²). Reaction III¹ gives the same product of CO* as reaction II², and reaction step III²produces hydrocarbon species HxCyOz. It can be seen that both the reaction paths of I→II¹→III¹→IV¹and I→II²→IV¹can result in the reaction CO2→ CO. We need tofigure out which reaction path is more possible in ex- periment.

Herein, the reaction dynamics of these two paths are obtained using the CI-NEB method coded in VASP [35] program. The reactions II¹: CO2* + (H⁺ + e⁻)→COOH* and II²: CO2* + (H⁺ + e⁻)→ CO* + OH* generate two diﬀerent intermediates. The simulated reaction progresses are shown inFig. 3, where the transition states of reactions II¹, II², and III¹are represented as TS-II¹, TS-II², and TS-III¹, respectively. The activation energy of reaction II¹is 1.92 eV as shown in Fig. 3a, larger than that of reaction II²(~1.01 eV). Moreover, the free energy of COOH* (reaction II¹) is higher than that of CO* + OH* (reaction II²). Therefore, the ﬁrst hydrogenation of CO2* preferentially selects reaction II²owing to the lower activation barrier and less energy cost. The reaction CO2*→CO* needs only one step through path II²but at least two steps through path II¹→III¹. The reaction III¹will bring an additional reaction barrier of about 0.40 eV (Fig. 3a).

Given the selectivity that CO is the only carbonaceous product in CO2RR over MoS2from previous experimental results, we further studied the hydrogenation of intermediate COOH* and CO* to clarify the underlying mechanisms. Although formation of COOH* is proved to be unfavorable in CO2RR over MoS2according to the results and analysis above, it is still necessary to carefully study the hydrogenation process of COOH* because it's the key reaction step to understand the formation of CO and HxCyOzon metallic catalysts, e.g. Cu. The hydrogenation of COOH* can go along two diﬀerent reaction paths, III¹: COOH* + (H⁺ + e⁻)→CO* + H2O and III²: COOH* + (H⁺+ e⁻)→COH* + OH*.

The activation energy of reaction III²is about 1.47 eV, much larger than that of reaction III¹ (~0.40 eV). Moreover, the free energy of COH* + OH* is about 1.0 eV, higher than that of CO* + H2O* (as shown inFig. 4a). Our results indicate that hydrocarbon (HxCyOz) is produced by hydrogenation of CO* instead of COOH*. The reaction Fig. 1.Calculated frontier orbitals: top and side views of (a) the highest oc-

cupied molecular orbitals (HOMO) and (b) the lowest unoccupied molecular orbitals (LUMO) of isolated CO2molecule; (c) HOMO and (d) LUMO of adsorbed bent CO₂*. All the cutoﬀvalue of isosurface is 0.01 a.u. (e) Projected density of states (PDOS) of bent CO₂*, only the contributions fromd-orbital of edged Mo,p-orbital of O andp-orbital of C are presented here.

Table 1

Adsorption energy of H*, H2O* and CO2*.

Adsorbed species H* H2O* CO2*

Optimized structure

Adsorption energy 1.106 eV 0.90 eV 1.81 eV

Fig. 2.Proposed reaction pathways for CO2electroreduction at the edge of MoS₂. The blue species are isolated products and the red ones are intermediate species. The grey bases represent the edges of MoS2and the semi-cylindrical hills are active sites. The reactions of OH* and H2O* are omitted.

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features of COOH* hydrogenation over MoS2is consistent with that over Cu-like catalysts [6,37–40].

Next, we calculate reaction dynamics of CO* hydrogenation. The possible products of reaction CO* + (H⁺+ e⁻) are COH* and CHO*.

Here we only consider the reaction CO* + (H⁺+ e⁻)→COH* for its lower energy barrier [38]. The reactants CO* and H* occupy two neighbor active sites, and H* needs to traverse over a long distance to form COH*. Thus CI-NEB method is used in a strategy of H shuttling through H2O proposed in Ref. [38]. The initial state, transition state andfinal state are illustrated inFig. 4c. Our calculated activation energy of reaction IV²is 2.1 eV, which is about treble as that of the reaction on Cu (111) surface. The differences of reaction dynamics on MoS2 edges and Cu surfaces come from their different adsorption properties. The adsorption energy of H on MoS2(~1.06 eV) is larger than that on Cu (111) (~0.52 eV) [41], thus it needs much more energy to break MoeH bond than CueH bond. Therefore, the formation of hydrocarbon species (HxCyOz) is blocked by the high activation energy in reaction IV².

3.3. Dynamic processes of adsorbed CO

For TMDCs, the strong binding eﬀects for TM-CO (bond energy range from 0.8 to 1.1 eV) signiﬁcantly prevent the CO desorption.

Therefore, Asadi et al. proposed that the over-saturated binding of CO on active sites could decrease the bond energy of TM-C to 0.3–0.5 eV [14] and maintain a high turnover rate. To understand the over-saturated binding process, we conduct our calculations on the dynamics of adsorbed CO on MoS2here.

The dynamics processes of CO* include the following: (1) CO* + *→* + CO*; (2) CO* + CO*→COCO* + *. Process (1) means that adsorbed CO* diﬀuses from one active site to the neighbor active site, and process (2) means that two nearest adsorbed CO* diﬀuse to the same active site, which can lead to the desorption of CO. The active

energies of diffusion processes (1) and (2) are 0.61 eV and 0.40 eV respectively as shown inFig. 5, both of which are smaller than bonding energy of CO*. The diffusion processes (1) and (2) are more possible to take place than the desorption process of CO*→CO + *, while less possible than the desorption process of COCO*→CO* + CO. The free energy of COCO* is larger than that of CO* + CO*, indicating that the process (2) is endothermic. Our results of the diffusion processes further prove that the diffusion processes of adsorbed CO* play important role in the desorption of CO*.

4. Conclusion

In summary, we have simulated the microscopic dynamic process of the CO2 reduction process in the framework of density functional theory. The results reveal that Mo exposed edges of MoS2are inclined to Fig. 3.(a) Free energy diagrams for two paths of reaction CO2+ (H⁺+ e⁻).

Top and side views of adsorbed species and transition states involved in reactions II¹(b), and II²(c). O: red, S: yellow, H: white, C: grey, Mo: light blue.

Fig. 4.Free energy diagrams for (a) the reaction paths II¹→III¹and II¹→III², and (b) for the reaction CO*→COH* over MoS₂(blue coloured) and Cu(111) (brown coloured, data taken from ref. [38]). (c) Detailed H shuttling strategy steps of reaction IV²on MoS2. From left to right, they are initial state, transition state andﬁnal state respectively. O: red, S: yellow, H: white, C: grey, Mo: light blue.

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adsorb CO2atoms for preferentially catalytic reduce CO2to CO. CO2

molecule is activated by two neighboring Mo atoms and the CeO bonds reconstruct in the adsorbing process. The first proton/electron (H⁺+e⁻) reaction taking place at MoS2edges undergoes a different pathway from that on transition metal catalyst, contributing to the product selectivity towards CO. Finally, we demonstrate that the desorption of CO from MoS2edges is in virtue of unique diffusion processes of adsorbed CO atoms. Our study gives reasonable explanation for electro-catalytic CO2RR over MoS2, and the understanding of high ef- ficient and unique selectivity will provide significant insights for the design of new CO2RR catalysts.

Acknowledgements

The work was supported by the National Natural Science Foundation of China, China (11704139, 21573085, and 51872108), the China Postdoctoral Science Foundation, China (2017M612486). The frontier orbitals of CO2are obtained with the help of vaspmo program developed by Yang Wang.

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