Theoretical Study of the Reverse Water Gas Shift Reaction on Copper

Subscriber access provided by University of Winnipeg Library

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Theoretical Study of the Reverse Water Gas Shift (RWGS) Reaction on Copper Modified #-MoC(001) Surfaces 2

Huijuan Jing, Qiaohong Li, Jian Wang, Diwen Liu, and Kechen Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09884 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 24, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 57 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

The Journal of Physical Chemistry

Theoretical Study of the Reverse Water Gas Shift (RWGS) Reaction on Copper Modified β-Mo2C(001) Surfaces Huijuan Jinga,c, Qiaohong Lia*, Jian Wang a, Diwen Liu a,c, Kechen Wua,b* a

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China. b Center for Advanced Marine Materials and Smart Sensors, Minjiang University, Fuzhou 350116, P. R. China. c University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

ABSTRACT The reverse water-gas shift (RWGS) reaction has attracted great attention in recent years. It is well known that the supported catalysts, especially single-atom catalysts (SACs), exhibit good catalytic activity in many reactions. Thus, we designed the singleatom catalyst (SAC) Cu@Mo2C(001) and the smallest copper cluster catalyst Cu4@Mo2C(001) for the RWGS reaction. In this study, density functional theory (DFT) calculations were used to explore the reaction mechanisms of the RWGS reaction on the surfaces of Cu@Mo2C(001) and Cu4@Mo2C(001). The dissociative adsorption of H2 on these two surfaces is barrier-free and highly exothermic, which is beneficial to the RWGS reaction. Importantly, three possible mechanisms—the COOH mechanism, HCOO mechanism and redox mechanism—have been discussed. The results illustrated that the redox mechanism is most feasible pathway among the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces. By comparing the activation barrier of the rate-limiting step of the redox mechanism on the two surfaces, the results showed that the activation barrier of the rate-limiting step on the Cu4@Mo2C(001) surface is smaller, which is more conducive to the progress of the RWGS reaction. Therefore, the tactic of 1 / 45

ACS Paragon Plus Environment

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

Page 2 of 57

introducing non-noble copper could be a promising way to design highly efficient catalysts.

1. INTRODUCTION It is well known that ocean acidification and greenhouse effects are caused by exorbitant carbon dioxide (CO2), which comes from human activities and the flourishing development of industry.1-3 Thus, the capture and utilization of CO2 as a cheap, renewable and giant carbon C1 source for manufacturing valuable products has become a necessary issue to mitigate this global threat.4-7 Although CO2 is abundant and renewable,8 only a small amount of CO2 is used in the chemical industry or other applications.9 The ability to reasonably and sustainably utilize CO2 as a substantial raw material is still appealing to the scientific community. CO2 catalytic hydrogenation to CO, known as the reverse water-gas shift (RWGS) reaction, has attracted wide interest because this process occupies the critical stage in the transformation of CO2. The product CO is more reactive than CO2. Moreover, the CO product can be converted to multifarious liquid hydrocarbon fuels and high value chemicals via Fischer-Tropsch (FT) synthesis and other syngas processes.10-12 There are three major types of RWGS heterogeneous catalysts: mixed oxide catalysts, supported metal catalysts and transition metal carbides (TMCs).13 The RWGS is an endothermic reaction. Active oxides in mixed oxide catalysts may either be reduced to a metallic state or form coke during the high temperature reaction process. An example of the former is the deactivation of ZnO/Al2O3 catalysts due to the fact that the ZnO in ZnO/Al2O3 was reduced to Zn during the RWGS process;14 an 2 / 45


Page 3 of 57 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


example of the latter is the coke formation on the Fe2O3/Cr2O3 catalyst that results in the drastic deactivation of Fe2O3/Cr2O3.15 Supported metal catalysts frequently exhibit serious drawbacks in sintering under high temperature conditions. For example, supported Au catalysts16-18 are usually used below 400°C because Au particles have lower sintering resistance. Furthermore, noble metal catalysts are relatively expensive and scarce, which limits their ability to be widely used for CO2 hydrogenation. TMCs have attracted considerable interest because they are inexpensive, and their behavior is similar to noble metals. TMCs can be exceedingly useful for the conversion of CO2 into CO,19 CH4,20 CH3OH,21 and other hydrocarbons.22, 23 In addition to their use as catalysts, TMCs can also serve as supports for the dispersion of metals. Among the obtainable TMCs, Mo2C is widely applied in the RWGS because of the double functions for H2 dissociation and C=O bond scission. Recently, studies have experimentally synthesized Mo2C,19,

24

Co/Mo2C25 and Cu/Mo2C26 to improve the conversion of CO2 and the

selectivity of CO. Among these materials, 1 wt% Cu/Mo2C is one of the most active candidates for CO formation. In recent years, since the first practical single-atom catalyst (SAC) Pt1/FeOx was reported by Qiao et al.,27 single-atom catalysts (SACs) have attracted wide attention. Therefore, the single-atom catalyst Cu@Mo2C and the smallest cluster catalyst Cu4@Mo2C were designed for the RWGS reaction by the DFT. The experimental study was not enough to clarify the catalytic process over the catalyst surface. DFT calculations are used as a necessary tool to evaluate the coppermodified Mo2C catalysts for the RWGS reaction. Similar to the water gas shift (WGS) reaction, three main mechanisms have been recommended in a great deal of 3 / 45



Page 4 of 57

experimental and theoretical studies for the RWGS reaction.28-33 The first is the formate (HCOO) mechanism, in which Chen et al.30 proposed that the RWGS reaction on the ALE-Cu/SiO2 catalyst mainly involves the formation of formate species. The second is a redox mechanism. Ginés and co-workers32 have reported that the RWGS reaction on CuO/ZnO/Al2O3 catalysts follows a surface redox mechanism. The third is a carboxyl (COOH)-mediated mechanism. Dietz et al.33 suggested that a COOH-mediated mechanism is favored for the RWGS reaction on Pd, Pt and Ag surfaces, whereas the redox mechanism is the most favorable pathway on Ni, Rh and Cu surfaces. For Cu@Mo2C and Cu4@Mo2C, the most favorable pathway is unknown for CO2 hydrogenation to CO via the RWGS reaction. In addition, the roles of copper are also a puzzle. In this study, we chose the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces to explore the underlying mechanisms of CO2 hydrogenation to CO and the role of supported copper in the RWGS reaction. CO2 hydrogenation to CO was executed on the Cu@Mo2C and Cu4@Mo2C catalysts using DFT calculations and microkinetic modeling. Three reaction routes have been investigated in the RWGS reaction, including the most stable adsorption structures of the intermediates involved in these pathways and the activation barriers and reaction energies of the elementary steps. By comparing the activation barriers of each elementary step and each mechanism, the most viable pathway for CO2 hydrogenation to CO on the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces are obtained.

2. COMPUTATIONAL DETAIL Mo2C mainly has two crystal structures: one is an orthorhombic Mo2C crystal 4 / 45


Page 5 of 57 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


structure denoted as α-Mo2C,34-36 and the other is a hexagonal Mo2C crystal structure called β-Mo2C.26,37,38 According to the literature,39 the orthorhombic structure (α-Mo2C) is less thermodynamically stable than the hexagonal structure (β-Mo2C). The optimized lattice parameter of the β-Mo2C cell is 2a = 2b = 6.006 Å, c =4.729 Å, which is in good agreement with the experimental values (a = b = 3.002 Å, c =4.724 Å).40 In particular, the β-Mo2C(001) surface was proven to be stable and typical in theoretical41,42 and experimental studies.43 For the β-Mo2C(001) system, the surface can either be Moterminated and C-terminated. The C-terminated β-Mo2C(001) surface was proven to be more stable,42 whereas the Mo-terminated β-Mo2C(001) surface was more reactive due to the revealing of Mo atoms with a correspondingly low coordination number.44 Hence, the Mo-terminated β-Mo2C(001) surface was selected as the ideal surface to design the single-atom

catalyst

Cu@Mo2C(001)

and

copper-modified

Mo2C

catalyst

Cu4@Mo2C(001). Afterward, we used Cu@Mo2C(001) and Cu4@Mo2C(001) to investigate CO2 hydrogenation to CO. Several p(2  2) surfaces have been represented by suitable slab models that include four atomic layers;45 the atoms of the top two layers together with the adsorbates were allowed to relax, while the bottom two layers were fixed in the bulk positions (Figure. 1). In all models, the vacuum layer was set to a width range of 15 Å to avoid the interaction between the slabs. All calculations were carried out using the Dmol3 package46,47 based on the DFT. The generalized gradient approximation (GGA) in the form of the Perdew-Burke-Ernzerhof (PBE) functional48 was used to describe the exchange-correlation term. The doublenumerical quality basis set with polarization functions (DNP)46,49 was utilized to expand 5 / 45



Page 6 of 57

the wave functions, which was comparable to 6-31G**.50,51 The basis-set superposition error can be minimized by the numerical basis sets.52 A Fermi smearing of 0.005 hartree was adopted, and a grid of 3  3  2 Monkhorst-Pack k-points was used for the structural relaxation and TS location. The energy, gradient and displacement convergence criteria are 1  10-5 hartree, 2  10-3 hartree/Å, and 5  10-3 Å, respectively. Transition state structures (TS) were located using the complete linear synchronous transit/quadratic synchronous transit (LST/QST) approaches. The adsorption energies with zero-point-corrected ΔEads are defined as follows: ∆𝐸ads = [𝐸slab+adsorbate − (𝐸slab + 𝐸adsorbate )] + ∆ZPEads vibrations

∆ZPEads = (

∑ 𝑖=1

vibrations

ℎ𝑣𝑖 ) 2

− (

adsorbed

∑ 𝑖=1

ℎ𝑣𝑖 ) 2

(E1)

(E2)

gas

where Eslab+adsorbate, Eslab and Eadsorbate represent the total energy of the slab covered with the adsorbate, the energy of the bare slab and the energy of the free adsorbate molecules, respectively. ΔZPEads represents the zero-point vibrational energy (ZPE) correction for the adsorption. Equation (E2) was adopted to calculate the ZPE correction via the vibrational frequencies of the species, including the adsorbed state and the gas phase, where h is the Planck constant and vi represents the vibrational frequency. On the basis of the definition of adsorption energy, the more negative the values of adsorption energy are, the stronger the interactions between the adsorbed species and the slab surface. For a reaction such as R(reactant) → P(product) on the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces, the activation barrier with the zero-point-corrected (ΔEa) was calculated as follows: 6 / 45


Page 7 of 57 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


∆𝐸a = (𝐸TS − 𝐸R ) + 𝛥ZPEbarrier vibrations

∆ZPEbarrier = (

∑ 𝑖=1

(E3) vibrations

ℎ𝑣𝑖 ) 2

− (

∑ 𝑖=1

TS

ℎ𝑣𝑖 ) 2

(E4)

R

where ETS and ER are the total energies of the transition state (TS) and reactant, respectively. ΔZPEbarrier refers to the ZPE correction for the reaction barrier. In Equation (E4), the front term includes the vibrational frequencies of the species in the TS, and the back term includes the vibrational frequencies of the adsorbed reactants. The reaction energy with the zero-point-corrected (ΔE) was calculated according to Equations (E5) and (E6): ∆𝐸 = (𝐸P − 𝐸R ) + ∆ZPEenergy vibrations

∆ZPEenergy = (

∑ 𝑖=1

(E5) vibrations

ℎ𝑣𝑖 ) − ( 2 P

∑ 𝑖=1

ℎ𝑣𝑖 ) 2

(E6)

R

where EP and ER are the total energies of the product and reactant, respectively. ΔZPEbarrier refers to the ZPE correction for the reaction energy, which is decided by the vibrational frequencies of the products and reactants. Furthermore, copper-supported Mo2C catalysts possess excellent catalytic activity and stability for the RWGS reaction in the temperature range of 300–600°C.26 Therefore, we calculated the rate constants of the RWGS reaction using the harmonic transition state theory (TST)28, 53 at temperatures of 550, 600, 650, 700, 750, 800, 850 and 900 K. Reaction rate constants (k) were calculated using the following functions: 𝑘=

𝑘B 𝑇 𝑞TS 𝐸a exp (− ) ℎ 𝑞R 𝑘B 𝑇

(E7)

7 / 45



𝑞=

1 ∏vibrations 1 − exp (− 𝑖=1

ℎ𝑣𝑖 ) 𝑘B 𝑇

Page 8 of 57

(E8)

where kB is the Boltzmann constant, T is the thermodynamic temperature, Ea is the activation barrier with the zero-point-corrected, and vi represents the vibrational frequency.

3. RESULTS AND DISCUSSION 3.1 Surface Model According to previous studies,41, 43, 54 the Mo-terminated β-Mo2C(001) surface was proven to be stable and representative. Compared to the C-terminated β-Mo2C(001) surface, the metallic β-Mo2C(001) surface was more active, with a surface energy of 2.76 J m-2. Therefore, we built the SAC Cu@Mo2C and Cu4 cluster supported Mo2C catalyst Cu4@Mo2C based on the Mo-terminated β-Mo2C(001) surface. Figure. 1 shows the different views of the Mo-terminated β-Mo2C(001) surface, Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces.

Figure 1. Top and side views of the optimized stable structures of the Mo-terminated β-Mo2C(001) surface, Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces. Here, the Mo, C and Cu atoms are colored in blue, gray and orange, respectively.

8 / 45


Page 9 of 57 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


The formation energy (ΔEform) is used to prove the stability of the Cu@Mo2C. The formation energy (ΔEform) is calculated by using the following equation: ∆𝐸form = 𝐸Cu@Mo2𝑚−1 C𝑚(001) + 𝜇Mo − 𝐸Mo2𝑚C𝑚(001) − 𝜇Cu

(E9)

where m is the number of Mo2C units in the surface unit cell, ECu@Mo2m-1Cm(001) and EMo2mCm(001) are the total energies of the doped and clean Mo2C(001) surface, respectively; Mo and Cu are the assumed energies of bulk Mo and Cu, respectively. The calculated ΔEform of Cu@Mo2C(001) is 0.49 eV, which is a small value for proving the possibility of doping. However, Cu-doped Mo2C catalysts with Cu/Mo molar ratios ranging from 0.02 to 0.05 have already been synthesized,55 and the Cu/Mo molar ratio in the Cu@Mo2C(001) is located in this range. Based on the literature,56 regardless of the Mo-terminated or C-terminated β-Mo2C(001) surfaces, the supported particles tend to become two-dimensional. It was found that the Cu4 rhombus with the Cu atoms above the Mo hollow sites was the most stable configuration. The binding energy (Eb) of the Cu4 nanocluster was calculated according to Equation (E10): 𝐸b = 𝐸Cu4 + 𝐸Mo2 C(001) − 𝐸Cu4 +Mo2 C(001)

(E10)

where ECu4+Mo2C(001) is the total energy of the Cu4 cluster adsorbed on the Mo2C(001) slab; ECu4 is the energy of the Cu4 cluster under vacuum; and EMo2C(001) is the energy of the bare slab of Mo2C(001). The higher the value of Eb is, the stronger the interactions between the Cu4 cluster and the Mo2C(001) surface. The calculated Eb of Cu4@Mo2C is 8.07 eV, which suggests that the Cu4@Mo2C structure is stable (Figure. 1(c)). Next, the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces were used to investigate CO2 hydrogenation to CO. 9 / 45



Page 10 of 57

3.2 Adsorption of Intermediates on the Surfaces It is vital to characterize the adsorption of related species to understand the elementary chemical steps in the RWGS reaction. In this section, the adsorptions of all possible species involved in the RWGS reaction were considered over different adsorption sites on the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces. The most stable structures of these species on the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces are shown in Figure. 2 and Figure. 3, respectively. The optimized structural parameters and adsorption energies are listed in Table 1 and Table 2, respectively. 3.2.1 Adsorbed Intermediates on the Cu@Mo2C(001) Surface CO2*. CO2 prefers to adsorb at the hollow sites of Mo with an adsorption energy of 1.14 eV, which is close to the adsorption energy on the Mo2C(001) surface,19 and with a Mo-C distance of 2.164 Å and a Mo-O distance of 2.140 and 2.152 Å on the Cu@Mo2C(001) surface. Compared with the Cu@Mo2C(001) surface, CO2 has a very weak interaction with the Cu(111) surface,33 which keeps its gas-phase structure approximately 3 Å above the surface. For the optimized geometry of chemisorbed CO2, the bond lengths of C-O are 1.204 and 1.481 Å and the bond angle of OCO (OCO) is ~118, which means that the CO2 molecule is activated on the adsorption sites above the Cu@Mo2C(001) surface. Compared with Cu(111),33 the Cu@Mo2C(001) surface prefers to adsorb and activate the CO2 molecule. CO*. As shown in Figure. 2, CO adsorbs to the surface at the hollow sites, wherein its C atom binds to three Mo atoms. The adsorption energy is -2.18 eV, and the calculated Mo-C distances are 2.007, 2.640 and 2.658 Å, respectively. The adsorption 10 / 45


Page 11 of 57 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


energy of CO is -1.19 eV at the fcc site on Cu(111),33 which is smaller than that on Cu@Mo2C(001). It is worth mentioning that the adsorption energy of CO is -0.62 eV at the top site of Cu on Cu@Mo2C(001), which is beneficial to the subsequent removal of CO. O* and H*. Atomic O prefers to adsorb at the hollow sites of Mo via three Mo-O bonds on the Cu@Mo2C(001) surface. The adsorption energy is -4.27 eV on Cu@Mo2C(001), which is approximately equal to the adsorption on the metallic Mo2C(001) surface.57 This large negative adsorption energy indicates that the O atom strongly adsorbs on the surface. For H atoms, the most stable sites are the hollow sites formed by three Mo atoms, which have an adsorption energy of -0.74 eV; these results are consistent with a previous study.28

Figure 2. The top views (top) and side views (bottom) of the most stable configurations and adsorption energies of the adsorbates on the Cu@Mo2C(001) surface. The adsorption energies are in eV. Here, the C, H, O, Cu and Mo atoms are colored in gray, white, red, orange and blue, respectively, which are the same in Figures. 3–6 and Figures. 8–10.

OH* and H2O*. OH species adsorb with an optimal configuration on the hollow sites through three Mo-O bonds (Figure. 2). The calculated adsorption energy of OH is -4.44 11 / 45



Page 12 of 57

eV, and the bond lengths are 2.207, 2.242 and 2.242 Å. H2O anchors to the top site via a Mo-O bond with an adsorption energy of -0.87 eV, which is 0.15 eV lower than that on the metallic Mo2C(001) surface in previous studies.57 H2O is one of the most important products in the RWGS reaction, and its adsorption energy is neutral to the desorption of H2O. COH* and HCO*. COH prefers to adsorb at the hollow site of three Mo atoms with a corresponding adsorption energy of -4.76 eV, which is much more negative than the calculated result of -3.19 eV on the Cu(111) surface.58 The calculated O-H and C-O lengths are 0.979 and 1.363 Å, respectively. HCO adsorbs at the hollow site of three Mo atoms with an adsorption energy of -3.50 eV. The calculated C-H and C-O distances are 1.100 and 1.374 Å, respectively. COOH*. Two isomers, cis-COOH and trans-COOH, are distinguished by the orientation of the OH group (Figure. 2). Both cis-COOH and trans-COOH adsorb at the hollow sites of three Mo atoms with corresponding adsorption energies of -3.52 and -3.51 eV, respectively, which suggests that the two configurations have a similar stability. HCOO*. Although HCOO has been detected on the catalyst surface, it is generally considered to be a bystander rather than a reactive intermediate because of its high stability.59,

60

HCOO adsorbs at the bridge site through two O atoms with a

corresponding adsorption energy of -4.16 eV. The calculated Mo-O distances are 2.145 and 2.146 Å.

12 / 45


Page 13 of 57 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


Table 1 Adsorption energies (eV), stable adsorption sites and key geometric parameters (Å) of all possible intermediates involved in the RWGS reaction on the Cu@Mo2C(001) surface. Species

ΔEads

Sites

Key parameters

CO2

-1.14

Hollow of Mo atoms

C-O：1.204，1.481；Mo-C：2.164，2.852，2.922；Mo-O：2.140，2.152，2.970

CO

-2.18

Hollow of Mo atoms

C-O：1.190；Mo-C：2.007，2.640，2.658

O

-4.27

Hollow of Mo atoms

Mo-O：2.070，2.094，2.095

H

-0.74

Hollow of Mo atoms

Mo-H：1.986，1.990，1.991

OH

-4.44

Hollow of Mo atoms

O-H：0.972；Mo-O：2.207，2.242，2.242

H2O

-0.87

Top of Mo atom

O-H：0.980，0.981；Mo-O：2.329

COH

-4.76

Hollow of Mo atoms

O-H：0.979；C-O：1.363；Mo-C：2.113，2.128，2.301

HCO

-3.50

Hollow of Mo atoms

C-H：1.100；C-O：1.374；Mo-C：2.094，2.367，2.938；Mo-O：2.153，2.215，2.988

cis-COOH

-3.52

Hollow of Mo atoms

O-H：0.985；C-O：1.338，1.354；Mo-C：2.117，2.617，2.925；Mo-O：2.236，2.240，2.988

trans-COOH

-3.51

Hollow of Mo atoms

O-H：0.980；C-O：1.340，1.342；Mo-C：2.114，2.615，2.891；Mo-O：2.222，2.232，2.989

HCOO

-4.16

Bridge of Mo atoms

C-H：1.103；C-O：1.274，1.275；Mo-O：2.145，2.146

3.2.2 Adsorbed Intermediates on the Cu4@Mo2C(001) Surface CO2*. The most stable configuration of CO2 is adsorbed at the hollow site of three Mo atoms with a corresponding adsorption energy of -1.30 eV. Compared with the adsorption energy (-1.70 eV) for the absorbed CO2 on the metallic Mo2C(001) surface,57 the adsorption energy on the Cu4@Mo2C(001) surface is more positive. For the optimized structure of chemisorbed CO2, the bond lengths of C-O are 1.272 and 1.367 Å and the bond angle of OCO (OCO) is ~118, which indicates that the CO2 molecule is activated on the adsorption sites above the Cu4@Mo2C(001) surface. Similar to the Cu@Mo2C(001) surface, the Cu4@Mo2C(001) surface can also excellently adsorbs and activates CO2 molecules. CO*. CO prefers to adsorb at the hollow site of three Mo atoms with an adsorption energy of -2.19 eV, which is 0.04 eV higher than that on the Pt@Mo2C(001) surface in previous studies.27 CO is one of the foremost products in the process of CO2 hydrogenation to CO, and compared with CO adsorbed on the hollow site of Mo atoms, CO adsorbed on the top site of copper has a relatively positive adsorption energy (-1.13 13 / 45



Page 14 of 57

eV), which is a great benefit to subsequent CO desorption. O* and H*. For O atoms, the most stable sites are the hollow sites formed by three Mo atoms (Figure. 3), which have a very negative adsorption energy of -4.19 eV (versus -0.67 eV for H); this adsorption energy is 0.08 eV larger than that of O on the Cu@Mo2C(001) surface. Atomic H prefers to adsorb at the hollow sites of Mo via three Mo-H bonds on the Cu4@Mo2C(001) surface. The adsorption energy is -0.67 eV and the calculated Mo-H distances are 1.996, 1.997 and 1.998 Å.

Figure 3. The top views (top) and side views (bottom) of the most stable configurations and adsorption energies of the adsorbates on the Cu4@Mo2C(001) surface. The adsorption energies are given in eV.

OH* and H2O*. The most stable configuration of OH is found to adsorb on the hollow sites via three Mo-O bonds (Figure. 3). The calculated adsorption energy of OH is -4.30 eV, and the Mo-O distances are 2.189, 2.225 and 2.241 Å. H2O prefers to adsorb at the top site through the Mo-O bond. The adsorption energy is -0.83 eV, and the calculated Mo-O distance is 2.340 Å. COH* and HCO*. COH adsorbs at the hollow site of three Mo atoms, with an 14 / 45


Page 15 of 57 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


adsorption energy of -4.72 eV, which is much more positive than the calculated result of -4.76 eV on the Cu@Mo2C(001) surface. The calculated O-H and C-O distances are 0.979 and 1.372 Å, respectively. HCO prefers to adsorb at the hollow sites of three Mo atoms with an adsorption energy of -3.38 eV. The calculated C-H and C-O lengths are 1.101 and 1.375 Å, respectively. COOH*. COOH, which was experimentally detected in a previous study60 on the Ni(110) surface, has two different configurations, cis-COOH and trans-COOH; these configurations are distinguished by the direction of the OH group (Figure. 3). Both cisCOOH and trans-COOH adsorb at the hollow sites of three Mo atoms with corresponding adsorption energies of -3.37 and -3.30 eV, respectively, which indicates that the two isomers have a similar stability. Comparatively, the adsorbed cis-COOH is more stable than the trans-COOH on the Cu4@Mo2C(001) surface. HCOO*. HCOO prefers to adsorb at the bridge site with two O atoms being adsorbed on the top sites of two Mo atoms. The adsorption energy is -3.95 eV and the calculated Mo-O distances are 2.151 and 2.163 Å. Table 2 Adsorption energies (eV), stable adsorption sites and key geometric parameters (Å) of all possible intermediates involved in the RWGS reaction on the Cu@Mo2C(001) surface. Species

ΔEads

Sites

Key parameters

CO2

-1.30

Hollow of Mo atoms

C-O：1.272，1.367；Cu-O：2.098，2.832；Cu-C：2.516，2.787； Mo-C：2.146；Mo-O：2.205，2.234

CO

-2.19

Hollow of Mo atoms

C-O：1.192；Mo-C：2.005，2.651，2.684

O

-4.19

Hollow of Mo atoms

Mo-O：2.061，2.084，2.099

H

-0.67

Hollow of Mo atoms

Mo-H：1.996，1.997，1.998

OH

-4.30

Hollow of Mo atoms

O-H：0.971；Mo-O：2.189，2.225，2.241

H2O

-0.83

Top of Mo atom

O-H：0.979，0.989；Mo-O：2.340

COH

-4.72

Hollow of Mo atoms

O-H：0.979；C-O：1.372；Mo-C：2.096，2.161，2.240

HCO

-3.38

Hollow of Mo atoms

C-H：1.101；C-O：1.375；Mo-C：2.092，2.354，2.970；Mo-O：2.169，2.212，2.974

cis-COOH

-3.37

Hollow of Mo atoms

O-H：0.984；C-O：1.352，1.345；Mo-C：2.113，2.565，2.959；Mo-O：2.244，2.248，2.976

trans-COOH

-3.30

Hollow of Mo atoms

O-H：0.981；C-O：1.354，1.335；Mo-C：2.110，2.543，2.944；Mo-O：2.232，2.253，2.976

HCOO

-3.95

Bridge of Mo atoms

C-H：1.102；C-O：1.272，1.274；Mo-O：2.151，2.163

15 / 45



Page 16 of 57

3.3 Elementary Steps Involved in the RWGS Reaction Table 3 Calculated activation barriers (Ea/eV) and reaction energies (ΔE/eV) for the elementary steps involved in the RWGS reaction on the Cu@Mo2C(001) surface and Cu4@Mo2C(001) surface Activation barrier (Ea)

Reaction energy (ΔE)

Cu@Mo2C(001)

Cu4@Mo2C(001)

Cu@Mo2C(001)

Cu4@Mo2C(001)

Mechanism

Elementary step

Redox mechanism

ER1-1

CO2(g) + * → CO2*

—

—

-1.14

-1.30

ER1-2

H2(g) + 2* → 2H*

0.00

0.00

-1.66

-1.13

ER1-3

CO2* + * → CO* + O*

0.36

0.38

-0.36

-0.49

ER1-4

O* + H* → OH* + *

1.21

0.94

1.00

0.46

ER1-5

OH* + * → O* + H*

1.07

0.48

-0.33

-0.46

ER1-6

OH* + H* → H2O* + *

1.34

1.79

0.81

0.56

ER1-7

OH* + OH* → H2O* + O*

0.69

0.79

0.59

0.71

ER1-8

H2O* + O* → OH* + OH*

0.10

0.08

-0.59

-0.71

ER1-9

CO* → * + CO(g)

—

—

0.62

1.13

ER1-10

H2O* → * +H2O(g)

—

—

0.81

0.77

ER2-1

CO2(g) + * → CO2*

—

—

-1.14

-0.99

ER2-2

CO2* + H* → trans-COOH* + *

1.47

1.30

0.59

0.50

ER2-3

trans-COOH* → cis-COOH*

0.46

0.36

0.05

0.01

ER2-4

cis-COOH* → CO* + OH*

0.95

0.83

0.11

-1.35

ER2-5

cis-COOH* → COH* + O*

1.01

1.63

-1.04

0.80

ER2-6

COH* → CO* + H*

1.35

1.15

-1.32

-1.05

ER2-7

CO* → * + CO(g)

—

—

0.62

2.13

ER3-1

CO2* + H* → HCOO* + *

1.14

1.11

0.07

0.27

ER3-2

HCOO* → HCO* + O*

1.48

0.72

-0.78

-1.43

ER3-3

HCO* → CO* + H*

1.38

0.81

0.98

-0.60

ER3-4

O* + H* → OH* + *

1.33

1.55

0.54

0.94

ER3-5

OH* + * → O* + H*

0.79

0.61

-0.54

-0.94

ER3-6

CO* → * + CO(g)

—

—

0.62

2.09

COOH mechanism

HCOO mechanism

To explore the feasible reaction mechanisms for the RWGS on the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces, the activation barriers and reaction energies for all substantial elementary steps are calculated, as shown in Table 3. 3.3.1 Elementary Steps Involved in the RWGS Reaction on the Cu@Mo2C(001) Surface Table 4 exhibits the rate constant for all elementary reactions on the Cu@Mo2C(001) surface. Figures. 4, 5 and 6 depict the structure of the initial state (IS), transition state (TS) and final state (FS) for each elementary reaction. Figure. 7 displays the calculated potential energy profile for the RWGS reaction on the Cu@Mo2C(001) surface. 16 / 45


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 — — — 8.13  10

— 1.45  10

k2-2 k2-3 k2-4 k2-5 k2-6

k3-1 k3-2 k3-3 k3-4 k3-5

CO* → * + CO(g)

H2O* → * +H2O(g)

CO2(g) + * → CO2*





COH* → CO* + H*

CO* → * + CO(g)

CO2* + H* → HCOO* + *

HCOO* → HCO* + O*

HCO* → CO* + H*

O* + H* → OH* + *

OH* + * → O* + H*

CO* → * + CO(g)

ER1-9

ER1-10

ER2-1

ER2-2

ER2-3

ER2-4

ER2-5

ER2-6

ER2-7


ER3-1

ER3-2

ER3-3

ER3-4

ER3-5

ER3-6

1.35  101

1.26  100

—

—

6.09  10

1.80  10

4.28  10

3.56  102 6

4.74  101

5

4.38  100

—

6

8.79  103

1.13  103

9.90  101

6.25  100

6.94  10-1

5.96  10

1.09  10

3

5.16  10-2

—

1.01  102

5.07  105

6.71  105

— 3

1.08  105

1.74  104

2

1.57  105

2.82  104

0

4.91  108

9.47  10

1.07  10 2.52  108

— 0

—

—

3.20  1011

3.01  107

9.21  102

1.08  105

—

1.14  108

-2

—

—

2.78  1011

2.35  1011

k1-8

H2O* + O* → OH* + OH*

ER1-8

1.09  107

3.24  106

k1-7

OH* + OH* → H2O* + O*

ER1-7

1.21  102

k1-6

OH* + H* → H2O* + *

ER1-6

1.09  101

k1-5

OH* + * → O* + H*

ER1-5

10

7.53  103

2.45  10

1.43  10

2.03  104

k1-4

O* + H* → OH* + *

ER1-4

— 10

—

650K

—

2.84  103

7.49  10

k1-3

CO2* + * → CO* + O*

ER1-3

9

1.18  103

—

H2(g) + 2* → 2H*

ER1-2

—

600K

1.31  102

—

CO2(g) + * → CO2*

550K

ER1-1

Elementary step

Rate constant k (s-1)

10

1

4

—

1.74  10

7

2.01  103

5.06  104

4.10  101

2.56  10

—

5.61  102

1.91  106

2.33  106

8.69  108

6.11  10

—

—

—

3.63  1011

7.22  107

5.25  103

4.53  105

3.69  104

3.87  10

—

—

700K

10

2

4

—

4.31  10

7

9.00  103

2.29  105

2.09  102

9.08  10

—

2.48  103

5.98  106

6.81  106

1.42  109

3.06  10

—

—

—

4.04  1011

1.54  108

2.37  104

1.58  106

1.46  105

5.73  10

—

—

750K

10

3

5

—

9.60  10

7

3.34  104

8.55  105

8.70  102

2.74  10

—

9.13  103

1.63  107

1.74  107

2.18  109

1.25  10

—

—

—

4.46  1011

2.99  108

8.82  104

4.72  106

4.88  105

8.04  10

—

—

800K

11

3

5

—

1.95  10

8

1.06  105

2.72  106

3.05  103

7.26  10

—

2.88  104

3.92  107

3.97  107

3.16  109

4.32  10

—

—

—

4.86  1011

5.38  108

2.81  105

1.24  107

1.41  106

1.08  10

—

—

850K

—

3.66  108

2.98  105

7.56  106

9.30  103

1.73  106

—

7.98  104

8.55  107

8.26  107

4.40  109

1.30  104

—

—

—

5.27  1011

9.06  108

7.85  105

2.95  107

 106

1.40  1011

—

—

900K

Table 4 The rate constant k (s-1 ) for the elementary reactions in the RWGS reaction at different temperatures on the Cu@Mo2C(001) surface

Page 17 of 57 The Journal of Physical Chemistry

17 / 45


Page 18 of 57

3.3.1.1 Redox Mechanism. CO2 gas prefers to adsorb at the hollow sites of Mo via the C and O atoms bind to the Mo atoms, as shown in ER1-1. The elementary step is exothermic by 1.14 eV. H2 dissociation is an important step in the RWGS reaction process. A spontaneous dissociation of H2 occurs, as displayed in ER1-2. H2 directly dissociates two H atoms adsorbed at the hollow sites of Mo on the Cu@Mo2C(001) surface; this step is exothermic by 1.66 eV. The decomposition of CO2 into CO and O occurs as shown in ER1-3. As shown in Tables 3 and 4, as well as Figure. 4, this decomposition step is feasible on the Cu@Mo2C(001) surface. The step is exothermic (-0.36 eV) with an activation barrier of 0.36 eV, and the reaction rate constant is 2.45  1010 s-1 at 650 K (in the main body of this manuscript, only the rate constants of all the elementary reactions at 650 K are displayed; those at other temperatures are listed in Table 4). In TS1-3, one of the C-O bonds is elongated to 2.467 Å from 1.481 Å in CO2. Nevertheless, this step has a larger activation barrier on the Mo2C(001) surface19,57 than that on the Cu@Mo2C(001) surface. The results of our theoretical calculations implied that C-O bond breaking is likely on the Cu@Mo2C(001) surface. H and O species react as shown in ER1-4, which leads to the formation of OH. Compared to the formation of OH on the Mo2C(001) surface,57 the activation barrier (1.21 eV) is smaller and absorbs less reaction energy. These results indicate that the step is more favorable on the Cu@Mo2C(001) surface. OH is dissociated, and then ER1-5 happens. The direct O-H bond cleavage of the OH 18 / 45


Page 19 of 57 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


adsorbed at the hollow sites of Mo can form O and H via a transition state. This step requires an activation barrier of 1.07 eV with a reaction energy of -0.33 eV, and the reaction rate constant is 1.08  105 s-1. When OH and H species are co-adsorbed at the Cu@Mo2C(001) surface, OH hydrogenation can form H2O via ER1-6. This elementary reaction has an activation barrier of 1.34 eV with a reaction energy of 0.81 eV, and the reaction rate constant is 9.21  102 s-1. It should be mentioned that two OH species can form H2O and O, as shown in ER17. In TS1-7, OH species adsorb at the hollow sites of Mo, and the O-H distance decreases from 3.675 to 1.209 Å. The final state H2O and O are adsorbed at the top sites and hollow sites of Mo, respectively. This elementary reaction has a smaller activation barrier of 0.69 eV with a lower reaction energy of 0.59 eV. Furthermore, the reaction rate constant is 3.01  107 s-1, which is higher than k1-6 (9.21  102 s-1). Thus, we believe that H2O is obtained from ER1-7 on the Cu@Mo2C(001) surface. The H2O and O species can generate two OH, as shown in ER1-8. The O-H bond in H2O is broken, and another O-H bond is formed. This elementary reaction requires an activation barrier of 0.10 eV with a reaction energy of -0.59 eV, and the reaction rate constant is 3.20  1011 s-1. CO separation directly occurs as shown in ER1-9. The desorption of the CO molecule from the Cu@Mo2C(001) surface needs to overcome the barrier of 0.62 eV. This barrier is smaller than the desorption of CO molecules from the Mo2C(001) surface.19,57 H2O gas is released, as presented in ER1-10. The H2O molecule from the 19 / 45



Page 20 of 57

Cu@Mo2C(001) surface requires a reaction heat of 0.81 eV. Generally, the Cu@Mo2C(001) catalyst provides enough H species for the RWGS reaction due to the barrier-free dissociation of H2. Next, O originates from the reaction of the C-O bond cleavage in CO2. Afterwards, O hydrogenation to OH has the highest activation barrier (1.40 eV) in this process. We believe that the rate-limiting step is OH formation in the redox mechanism on the Cu@Mo2C(001) surface. Compared with the direct reaction of OH and H to generate H2O, the reaction of two OH species generates H2O and O has a smaller activation barrier (0.69 eV), which is a great benefit to H2O generation. It is obvious that the redox mechanism of the RWGS reaction occurs first by spontaneous dissociation of H2 to form H species, second by CO and O formation from the C-O cleavage in CO2, third by H and O species reacting to produce OH, and then by the reaction of two OH species reacting to generate H2O, and finally by CO and H2O gas desorption on the Cu@Mo2C(001) catalyst.

Figure 4. The top views of the initial states (IS), transition states (TS), and final states (FS) involved 20 / 45


Page 21 of 57 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


in the redox mechanism on the Cu@Mo2C(001) surface.

3.3.1.2 COOH Mechanism. CO2 reduction through a COOH intermediate on the Cu@Mo2C(001) surface was also considered and is presented in Tables 3 and 4 and Figures. 5 and 7. The COOH intermediate has two different detected configurations: cis-COOH and trans-COOH. The reaction between CO2 and H leads to the formation of trans-COOH, as shown in ER2-2. Our calculation results show that CO2 hydrogenation to trans-COOH has a smaller activation barrier (1.47 eV) than CO2 hydrogenation to cis-COOH, and transCOOH can be converted to cis-COOH. From Table 3, we can see that the elementary reaction CO2* + H* → trans-COOH* + * is endothermic by 0.59 eV with a reaction rate constant of 9.47  100 s-1. The reaction of trans-COOH* → cis-COOH* is shown in ER2-3. This elementary reaction is nearly thermoneutral (0.05 eV) with an activation barrier of 0.46 eV, which is 0.01 eV lower than the value provided by Tang et al.61 The reaction rate constant of the step is 4.91  108 s-1. The cis-COOH can be separated into CO and OH, as shown in ER2-4. This elementary reaction has an activation barrier of 0.95 eV, and the endothermic energy is 0.11 eV with a reaction rate constant of 6.71  105 s-1. The distance between the C atom of CO and the O atom of OH increases from 1.338 Å to 1.691 Å, which indicates the break of the cis-COOH intermediate. For ER2-5, the C-O bond cleavage of cis-COOH at the hollow sites of Mo can also form COH and O via a transition state. This step is exothermic by 1.04 eV with an 21 / 45



Page 22 of 57

activation barrier of 1.01 eV, and the reaction rate constant is 5.07  105 s-1. The COH can be disassociated into CO and H as exhibited in ER2-6. This elementary reaction has an activation barrier of 1.35 eV with the reaction energy (-1.32 eV), and the reaction rate constant is 1.01  102 s-1. In TS2-6, COH adsorbs at the hollow sites of Mo, and the O-H distance increases from 0.979 Å to 1.359 Å. The final state CO and H are adsorbed via the C atom and H atom at the hollow sites of Mo. Next, two OH species react to generate H2O and O, which is the same as its reaction in the redox mechanism. Next, CO and H2O gases are released on the Cu@Mo2C(001) catalyst, which are depicted in detail in the redox mechanism. To sum up, as shown in Figure. 7, our calculation results show that the rate-limiting step is carboxyl formation in the COOH mechanism. In summary, the RWGS reaction should go through five steps (the black line in Figure. 7) in the COOH mechanism: in the first step, the CO2 and H species are co-adsorbed; in the second step, the CO2 hydrogenation leads to adsorbed trans-COOH; in the third step, the trans-COOH can be converted to cis-COOH with a reaction rate constant of 4.91  108 s-1; in the fourth step, the cis-COOH is dissociated into CO and OH with a reaction rate constant of 6.71  105 s-1; and in the final step, the two OH species react to generate H2O and O with a reaction rate constant of 3.01  107 s-1.

22 / 45


Page 23 of 57 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


Figure 5. The top views of the initial states (IS), transition states (TS), and final states (FS) involved in the COOH mechanism on the Cu@Mo2C(001) surface.

3.3.1.3 HCOO Mechanism. An H atom attacks the C atom in CO2 to form HCOO. The top views of the IS, TS and FS for each step of the HCOO mechanism on the Cu@Mo2C(001) surface are presented in Tables 3 and 4 and Figures. 6 and 7. HCOO is produced by a H atom attacking the C atom in CO2, as shown in ER3-1. The reaction between CO2 and H causes the formation of HCOO with an activation barrier of 1.14 eV and a reaction energy of 0.07 eV. The reaction rate constant is 5.96  103 s-1. In TS3-1, the C-H distance decreases from 3.207 Å to 1.497 Å. The final state HCOO adsorbs at the bridge site of Mo atoms. For ER3-2, the cleavage of C-O from HCOO can form an O atom and HCO. This elementary reaction has a high activation barrier of 1.48 eV, which suggests that HCOO decomposition to HCO and O is unfavorable. The reaction is exothermic by 0.78 eV, and the reaction rate constant is 6.25  100 s-1. 23 / 45



Page 24 of 57

The direct decomposition of HCO, as shown in ER3-3, may lead to the cleavage of C-H from HCO and the formation of a H atom and CO. This elementary reaction has an activation barrier of 1.38 eV with a reaction energy of 0.98 eV, and the reaction rate constant is 8.79  103 s-1. In TS3-3, the C-H distance increases from 1.100 Å to 3.018 Å. The final state CO and H adsorb at the top site of the Cu atom and the hollow site of the Mo atoms, respectively. The reaction between O and H leads to the formation of OH, as presented in ER3-4. The hydrogenation of the adsorbed O atom results in OH with an activation barrier of 1.33 eV and a reaction energy of 0.54 eV; the reaction rate constant is 3.56  102 s-1. Additionally, for ER3-5, the O-H bond cleavage of OH at the hollow site can form O and H via a transition state. This step requires an activation barrier of 0.79 eV with the reaction energy (-0.54 eV), and the reaction rate constant is 6.09  106 s-1. In general, as shown in Figure. 7, our calculation results show that the rate-limiting step is the cleavage of the C-O bond from formate (HCOO) in the HCOO mechanism. Generally, the RWGS reaction should go through six steps (the green line in Figure. 7) in the HCOO mechanism: first, the CO2 and H species are co-adsorbed; second, the CO2 hydrogenation leads to adsorbed HCOO; third, the HCOO is dissociated into HCO and O; four, the HCO is further dissociated into CO and H; five, the H and O species react to produce OH; and finally, the two OH species react to generate H2O and O with a reaction rate constant of 3.01  107 s-1.

24 / 45


Page 25 of 57 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


Figure 6. The top views of the initial states (IS), transition states (TS), and final states (FS) involved in the HCOO mechanism on the Cu@Mo2C(001) surface.

Figure 7. The calculated potential energy profile of the most favorable redox (red line), HCOO (green line) and COOH (black line) mechanisms for the RWGS reaction on the Cu@Mo2C(001) surface. The numbers in the figure are the activation barriers of the elementary steps. The numbers in red, green and black circles are the activation barriers of the rate-limiting steps in the redox, HCOO and COOH pathways, respectively.

In conclusion, our calculation results show that the average error of the activation 25 / 45



Page 26 of 57

barrier of the rate-limiting step of the three reaction mechanisms is less than 0.3 eV, and all three mechanisms on Cu@Mo2C(001) are possible. However, the most favorable pathway of the RWGS reaction on the Cu@Mo2C(001) surface follows the redox mechanism, which is consistent with the experimental results.26 It should be mentioned that the two OH species that produced H2O have a lower activation barrier and less heat absorption than the OH and H that directly formed H2O, which suggests that H2O is more likely to be generated by the two OH species due to both kinetic and thermodynamic feasibility. Compared with CO adsorbed at the Mo site, CO adsorbed at the Cu site has a smaller adsorption energy, which is beneficial to the desorption of CO. In other words, the introduction of Cu contributes to the desorption of CO, thus facilitating the reaction. 3.3.2 Elementary Steps Involved in the RWGS Reaction on the Cu4@Mo2C(001) Surface Table 5 shows the rate constant for all the elementary reactions on the Cu4@Mo2C(001) surface. Figures. 8, 9 and 10 portray the structure of the initial state (IS), transition state (TS) and final state (FS) for each elementary step. Figure. 11 displays the calculated potential energy profile for the RWGS reaction on the Cu4@Mo2C(001) surface. 3.3.2.1 Redox Mechanism. CO2 gas prefers to adsorb on the Cu-Mo2C interface, as shown in ER1-1; this elementary step is exothermic by 1.30 eV. Similar to Cu@Mo2C(001), a spontaneous dissociation of H2 also occurs on the Cu4@Mo2C(001) surface, as presented in ER1-2. H2 directly dissociates two H atoms adsorbed at the hollow sites of the Mo atoms and the bridge sites between the Cu and 26 / 45


Page 27 of 57 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


Mo atoms on the Cu@Mo2C(001) surface; this step is exothermic by 1.13 eV. The decomposition of CO2 into CO and O occurs, as displayed in ER1-3. As shown in Tables 3 and 5 and in Figure. 8, this step is favorable on the Cu4@Mo2C(001) surface. Moreover, this decomposition step is exothermic by 0.49 eV with an activation barrier of 0.38 eV, and the reaction rate constant is 4.13  1010 s-1. In TS1-3, the length of one of the C-O bonds increases from 1.367 Å to 2.640 Å in the CO2 molecule. This step is almost equal to the activation barrier of the Cu@Mo2C(001) surface and Cu4@Mo2C(001) surface, which is lower than that of the Mo2C(001) surface.19, 57 At the same time, our calculations also show that the C-O bond is liable to fracture on the Cu4@Mo2C(001) surface. H and O species react to generate OH, as shown in ER1-4. Compared to the formation of OH on the Mo2C(001)57 and Cu@Mo2C(001) surface, the activation barrier of this process (0.94 eV) is smaller, which indicates that this step is more feasible on the Cu4@Mo2C(001) surface. This elementary reaction is endothermic by 0.46 eV with a reaction rate constant of 1.56  106 s-1. For ER1-5, the direct O-H bond cleavage of OH adsorbed at the hollow sites of Mo can form O and H via a transition state. This elementary reaction requires an activation barrier of 0.48 eV with a reaction energy of -0.46 eV, and the reaction rate constant is 2.75  109 s-1. The co-adsorption of OH and H produces H2O on the Cu4@Mo2C(001) surface, as shown in ER1-6. This elementary reaction has an activation barrier of 1.79 eV with a reaction energy of 0.56 eV, and the reaction rate constant is 2.22  100 s-1. 27 / 45


— — — 2.71  10

— 1.18  10

k2-2 k2-3 k2-4 k2-5

k2-6

k3-1 k3-2 k3-3 k3-4 k3-5

CO* → * + CO(g)

H2O* → * +H2O(g)

CO2(g) + * → CO2*





COH* → CO* + H*

CO* → * + CO(g)

CO2* + H* → HCOO* + *

HCOO* → HCO* + O*

HCO* → CO* + H*

O* + H* → OH* + *

OH* + * → O* + H*

CO* → * + CO(g)

ER1-9

ER1-10

ER2-1

ER2-2

ER2-3

ER2-4

ER2-5

ER2-6

ER2-7


ER3-1

ER3-2

ER3-3

ER3-4

ER3-5

ER3-6

3.44  103

4.21  102

—

—

1.87  10

7.15  10

2.31  10

1.77  101 7

1.65  100

7

1.00  10-1

—

8

1.89  106

5.91  105

1.47  105

1.77  106

6.31  105

4.45  10

8.43  10

4

1.86  105

—

2.05  104

1.84  100

4.04  106

— 3

1.62  10-1

9.09  10-3

3

1.14  106

2.53  105

3

3.84  109

2.09  10

2.85  10 2.28  109

— 2

—

—

4.10  1011

7.09  106

2.22  100

2.75  109

—

1.23  109

1

—

—

3.66  1011

3.22  1011

k1-8

H2O* + O* → OH* + OH*

ER1-8

2.18  106

5.40  105

k1-7

OH* + OH* → H2O* + O*

ER1-7

1.41  10-1

k1-6

OH* + H* → H2O* + *

ER1-6

5.45  10-3

k1-5

OH* + * → O* + H*

ER1-5

10

1.56  106

4.13  10

2.16  10

1.26  109

k1-4

O* + H* → OH* + *

ER1-4

— 10

—

650K

—

5.03  108

1.00  10

k1-3

CO2* + * → CO* + O*

ER1-3

10

3.50  105

—

H2(g) + 2* → 2H*

ER1-2

—

600K

6.00  104

—

CO2(g) + * → CO2*

550K

ER1-1

Elementary step

Rate constant k (s-1)

10

4

5

—

4.27  10

8

1.36  102

5.11  106

4.28  106

1.85  10

—

9.49  104

1.48  101

1.20  107

5.98  109

1.15  10

—

—

—

4.52  1011

1.95  107

2.35  101

5.39  109

5.63  106

7.23  10

—

—

700K

11

4

5

—

8.77  10

8

7.94  102

1.20  107

9.17  106

6.31  10

—

3.60  105

8.95  101

3.05  107

8.74  109

5.05  10

—

—

—

4.93  1011

4.69  107

1.81  102

9.70  109

1.72  107

1.18  10

—

—

750K

11

5

6

—

1.65  10

9

3.72  103

2.51  107

1.78  107

1.85  10

—

1.16  106

4.32  102

6.92  107

1.22  1010

1.85  10

—

—

—

5.33  1011

1.01  108

1.08  103

1.63  1010

4.57  107

1.80  10

—

—

800K

11

5

6

—

2.89  10

9

1.46  104

4.79  107

3.19  107

4.76  10

—

3.27  106

1.73  103

1.42  108

1.62  1010

5.79  10

—

—

—

5.71  1011

1.99  108

5.24  103

2.57  1010

1.08  108

2.62  10

—

—

850K

—

4.77  109

4.91  104

8.48  107

5.35  107

1.10  107

—

8.23  106

5.91  103

2.69  108

1.09  1010

1.60  106

—

—

—

6.09  1011

3.64  108

2.12  104

3.88  1010

2.34  108

3.67  1011

—

—

900K

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

Table 5 The rate constant k (s-1 ) for the elementary reactions in the RWGS reaction at different temperatures on the Cu4@Mo2C(001) surface

The Journal of Physical Chemistry Page 28 of 57

28 / 45

Page 29 of 57 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


Two OH species can form H2O and O, as shown in ER1-7. This elementary reaction has an activation barrier of 0.79 eV, which is slightly higher than that on the Cu@Mo2C(001) surface. The reaction energy of this process is 0.56 eV, and the reaction rate constant is 7.09  106 s-1, which is higher than that of k1-6 (2.22  100 s-1). Similarly, we also believe that the H2O comes from E1-7 on the Cu4@Mo2C(001) surface. When H2O and O species are co-adsorbed at the Cu4@Mo2C(001) surface, two OH species are generated by ER1-8. This elementary reaction requires an activation barrier of 0.08 eV with the reaction energy of -0.71 eV, and the reaction rate constant is 4.10  1011 s-1. CO desorption directly occurs, as shown in ER1-9. The desorption of the CO molecule from the Cu4@Mo2C(001) surface needs to overcome the barrier of 1.13 eV; this barrier is smaller than the desorption of CO molecules from the Mo2C(001) surface.19,56 A H2O gas molecule is released, as presented in ER1-10. The H2O molecule from the Cu4@Mo2C(001) surface requires a reaction heat of 0.77 eV, which is smaller than that from the Cu@Mo2C(001) surface. Overall, the Cu4@Mo2C(001) catalyst also provides sufficient H species for the RWGS reaction due to the barrier-free dissociation of H2. Next, O originates from the reaction of CO2* + * → CO* + O*. Afterward, O hydrogenation to OH has an activation barrier of 0.94 eV, which is smaller than the calculated value of 1.40 eV on the Cu@Mo2C(001) surface. Analogous to Cu@Mo2C(001), the two OH species generate H2O more easily than the direct reaction of OH and H because the former has a smaller 29 / 45



Page 30 of 57

activation barrier. Subsequently, CO and H2O are released, of which the CO desorption requires the most energy of 1.13 eV during the process; this is the rate-limiting step in the redox mechanism on the Cu4@Mo2C(001) surface. It is clear that the redox mechanism of the RWGS reaction on the Cu4@Mo2C(001) catalyst occurs in the following order: first, by the spontaneous dissociation of H2 to form abundant H species; second, by the C-O cleavage in CO2 to form CO and O; third, by the reaction of H and O species to produce OH; and finally, by the reaction of two OH species to generate H2O. In the end, CO and H2O gases are released on the Cu4@Mo2C(001) catalyst.

Figure 8. The top views of the initial states (IS), transition states (TS), and final states (FS) involved in the redox mechanism on Cu4@Mo2C(001).

3.3.2.2 COOH Mechanism. CO2 reduction via a COOH intermediate on the Cu4@Mo2C(001) surface was also considered and is described in Tables 3 and 5 and Figures. 9 and 11. The COOH intermediate has two detected isomers, cis-COOH and trans-COOH. 30 / 45


Page 31 of 57 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


When CO2 and H species are co-adsorbed at the Cu4@Mo2C(001) surface, CO2 hydrogenation to trans-COOH occurs via ER2-2. Our calculation results show that the hydrogenation of CO2 to trans-COOH is easier because the formation of cis-COOH requires a higher activation barrier. Next, trans-COOH can be converted to cis-COOH with a small activation barrier. From Table 3, we can see that the elementary reaction is endothermic by 0.50 eV with an activation barrier of 1.30 eV, and the reaction rate constant is 2.09  103 s-1. For ER2-3, trans-COOH* → cis-COOH*, this elementary reaction is nearly thermoneutral (0.01 eV) with an activation barrier of 0.36 eV, which is 0.10 eV lower than the value calculated on the Cu@Mo2C(001) surface. The reaction rate constant of this step is 3.84  109 s-1. The cis-COOH can be dissociated into CO and OH, as shown in ER2-4. This step has

an activation barrier of 0.83 eV with a reaction energy of -1.35 eV, and the reaction rate constant is 4.04  106 s-1. Additionally, for ER2-5, the C-O bond cleavage of cis-COOH at the hollow sites of Mo can also form O atoms and COH via a transition state. This step is exothermic by 0.80 eV with an activation barrier of 1.63 eV, and the reaction rate constant is 1.84  100 s-1. Then, the COH can be separated into CO and H, as shown in ER2-6. This step has an activation barrier of 1.15 eV with a reaction energy of -1.05 eV, and the reaction rate constant is 2.05  104 s-1. In TS2-6, COH adsorbs at the hollow sites of Mo, and the OH distance increases from 0.979 Å to 1.405 Å. The final state CO and H adsorb at the 31 / 45



Page 32 of 57

top site of the Mo atom and the bridge site between the Mo atom and the C atom. Subsequently, the formation of H2O and O by the two OH species is consistent with the redox mechanism. Next, CO and H2O gases are released on the Cu4@Mo2C(001) surface; this is pictured in detail in the redox mechanism. As shown in Figure. 11, our calculation results show that the desorption of CO is the rate-limiting step in the COOH mechanism. In summary, similar to the Cu@Mo2C(001) catalyst, the RWGS reaction should also undergo five steps (the black line in Figure. 11) in the COOH mechanism on the Cu4@Mo2C(001) surface: in the first step, the CO2 and H species are co-adsorbed; in the second step, the CO2 and H form to produce the adsorbed trans-COOH; in the third step, the trans-COOH can be converted to cisCOOH with a reaction rate constant of 3.84  109 s-1; in the fourth step, the cis-COOH is dissociated into CO and OH; and in the final step, H2O and O are generated by the two OH species with a reaction rate constant of 7.09  106 s-1.

Figure 9. The top views of the initial states (IS), transition states (TS), and final states (FS) involved 32 / 45


Page 33 of 57 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


in the COOH mechanism on Cu4@Mo2C(001).

3.3.2.3 HCOO Mechanism. HCOO is generated by a H atom attacking the C atom in CO2. The top views of the IS, TS and FS for each step of the HCOO mechanism on the Cu4@Mo2C(001) surface are displayed in Tables 3 and 5 and Figures. 10 and 11. The reaction between CO2 and H causes the formation of HCOO, as shown in ER31. This elementary reaction has an activation barrier of 1.11 eV with a reaction energy of 0.27 eV, and the reaction rate constant is 4.45  104 s-1. In TS3-1, the C-H distance decreases from 2.946 Å to 1.243 Å. The final state HCOO adsorbs at the bridge site of Mo atoms via O atoms. For ER3-2, the cleavage of C-O from HCOO can form an O atom and HCO. This elementary reaction has an activation barrier of 0.72 eV, the reaction energy is exothermic by 1.43 eV, and the reaction rate constant is 1.77  106 s-1. The cleavage of C-H from HCO can form an H atom and CO as shown in ER3-3. This step has an activation barrier of 0.81 eV with a reaction energy of -0.60 eV, and the reaction rate constant is 1.89  106 s-1. In TS3-3, the C-H distance increases from 1.101 Å to 1.390 Å. The final state CO and H adsorb at the top site of the Mo atom and the hollow site of the Mo atoms, respectively. For ER3-4, the hydrogenation of the adsorbed O atom results in OH with an activation barrier of 1.55 eV with a reaction energy of 0.94 eV, and the reaction rate constant is 1.77  101 s-1. Also, for ER3-5, OH can be decomposed into O and H via a transition state. This step requires an activation barrier of 0.61 eV with a reaction energy of -0.94 eV, and the 33 / 45



Page 34 of 57

reaction rate constant is 1.87  108 s-1. In general, as shown in Figure. 11, similar to the other two mechanisms, the desorption of CO gas is also the rate-limiting step in the HCOO mechanism. The RWGS reaction should go through six steps (the green line in Figure. 11) in the HCOO mechanism on the Cu4@Mo2C(001) surface: the first step is the co-adsorption of CO2 and H species; the second step is the formation of HCOO; the third step is the decomposition of HCOO to generate HCO and O; the fourth step is to decompose HCO into CO and H; the fifth step is the formation of OH; and the last step is to generate H2O and O by the two OH species with a reaction rate constant of 7.09  106 s-1.

Figure 10. The top views of the initial states (IS), transition states (TS), and final states (FS) involved in the HCOO mechanism on Cu4@Mo2C(001).

34 / 45


Page 35 of 57 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


Figure 11. The calculated potential energy profile of the most favorable redox (red line), HCOO (green line) and COOH (black line) mechanisms for the RWGS reaction on the Cu4@Mo2C(001) surface. The numbers in the figure are the activation barriers of elementary steps. The numbers in red, green and black circles are the activation barriers of the rate-limiting steps in the redox, HCOO and COOH pathways, respectively.

In conclusion, the redox mechanism is the most favorable pathway of the RWGS reaction on the Cu4@Mo2C(001) surface, which is in good agreement with the experimental results.26 Similar to the Cu@Mo2C(001) surface, the two OH species that produced H2O have a lower activation barrier than the OH and H that directly formed H2O, which indicates that H2O is more likely to be generated by the two OH species. Interestingly, CO adsorbed at the Cu site has a smaller adsorption energy than that of the CO adsorbed at the Mo site, which is beneficial to the release of CO. Namely, the introduction of the Cu4 cluster favors the desorption of CO, thereby promoting the reaction.

35 / 45



Page 36 of 57

3.4 Catalytic Characteristics of Cu@Mo2C and Cu4@Mo2C Altogether, the Cu@Mo2C and Cu4@Mo2C surfaces show better catalytic activity than the pure Mo2C catalyst for the RWGS reaction, which indicates the advantage of introducing non-noble metal copper. The RWGS reaction mechanisms have been systematically studied using periodic DFT computations on the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces. The most feasible pathway is the redox mechanism on the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces. Firstly, a spontaneous dissociation of H2 is observed, which results in the HH bond being broken. Secondly, O originates from one of the C-O bond breaks of CO2 molecules with a small activation barrier. Thirdly, H hydrogenation leads to the adsorbed OH species. Fourthly, two OH species can form H2O and O with a smaller activation barrier. Finally, CO and H2O desorb from the top site of the Cu atom and Mo atom, respectively. Comparing the whole process of the redox mechanism on the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces, we found that the activation barrier of the rate-limiting step of the redox mechanism is less on the Cu4@Mo2C(001) surface. Thus, the Cu4@Mo2C catalyst is more beneficial to the RWGS reaction. Next, we will discuss the Cu4@Mo2C catalyst only. The synergic effect of bimetallic (Mo-Cu4) catalysts is conducive to CO production. CO2 adsorbs and activates at the Cu-Mo2C interface, followed by CO2 dissociation via one of the C-O bond cleavages of CO2 to form CO and O at the Cu site and the Mo site, respectively. Then, CO gas is released on the Cu site. Figure. 11 shows the most favorable pathway (the red line) of the RWGS reaction. Initial H2 adsorption 36 / 45


Page 37 of 57 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


and activation occur at the Cu-Mo2C interface, and later H2 spontaneously dissociates at the Cu-Mo2C interface. H has a smaller adsorption energy at the Cu site than at the Mo site, which indicates that H at the Cu site is easier to react. Similar to H, the adsorption energy of CO at the Cu site is smaller than that at the Mo site, which suggests that the desorption of CO at the Cu site is easier. Compared with the Mo2C(001) surface,57 CO release is easier on the Cu4@Mo2C(001) surface because it absorbs less heat. The Cu-Mo2C interface can effectively activate CO2, which is propitious to the dissociation of CO2. In addition, the introduction of the Cu4 cluster in Mo2C facilitates the release of CO. Hence, the Cu4@Mo2C catalyst exhibits excellent catalytic activity in CO production. 3.4.1. The Active Sites of the Cu4@Mo2C(001) Surface. On the basis of the above DFT calculations, it can be confirmed that the adsorption of all species and all elementary steps, except CO release, occur at the Mo sites and CuMo2C interface rather than the Cu site. These results indicate that the Mo sites and CuMo2C interface are active sites for forming water and activating CO2 and H2, whereas the Cu site is the active site for CO release. The synergetic effect between the non-noble metal Cu4 cluster and the support Mo2C for the RWGS reaction on the Cu4@Mo2C(001) surface agrees with the experimental results by zhang and co-workers.26 3.4.2. Role of the Cu4 Cluster. There is a strong interaction between the Cu4 cluster and the Mo2C surface due to its high binding energy. We further discern the role of the Cu4 cluster by comparing the formation and release of CO on the Cu4@Mo2C(001) surface and the Mo2C(001) 37 / 45



Page 38 of 57

surface.19,57 It is well known that the Mo2C(001) surface can activate CO2 and facilitate the dissociation of CO2.57 Compared to CO adsorption at the Mo site on the Mo2C(001) surface, the adsorption energy of CO at the Cu site on the Cu4@Mo2C(001) surface is smaller. This smaller adsorption energy enables the dissociation of CO2 to have a lower barrier (0.38 eV vs. 0.54 eV), and it is good for the release of CO (1.13 eV vs. 2.67 eV)19 on the Cu4@Mo2C(001) catalyst. Our calculation results show that the activation barrier of CO2 dissociation at the Cu-Mo interface is 0.38 eV and that the release of CO requires a reaction energy of 1.13 eV at the Cu site. Nevertheless, the Cu(111) surface33 has less activity than that of the Cu4@Mo2C(001) surface in breaking the C-O bond of CO2, which is a crucial step for the formation of CO. The Cu-Mo2C interface not only reduces the activation barrier of CO2 dissociation but also decreases the activation barrier of OH formation. The formation of OH is particularly important for the formation of H2O. With the introduction of Cu, the CO2 dissociation step and CO release are promoted. As mentioned above, the Cu-Mo2C interface accelerates CO2 dissociation, which produces the O atom and CO. The Cu4 cluster provides weakened CO adsorption; therefore, the Cu4 cluster facilitates CO release. The Cu-Mo2C interface improves the productivity of the RWGS and the separation of product CO.

4. CONCLUSIONS In this work, the reaction mechanisms of the RWGS reaction on the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces were systematically investigated by periodic DFT and 38 / 45


Page 39 of 57 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


microkinetic modeling calculations. The main points of this article are summarized as follows: (i) On the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces, the desorption of CO at the Cu site needs less heat than at the Mo site. Thus, the introduction of non-noble metal copper is beneficial to CO release. (ii) By comparing the three mechanisms among the two surfaces, it is found that the redox mechanism is not only the most feasible pathway on the Cu@Mo2C(001) surface but also the most favorable pathway on the Cu4@Mo2C(001) surface. Moreover, by comparing the rate-limiting step of the redox mechanism on the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces, the RWGS reaction is more likely to occur on the Cu4@Mo2C(001) surface. (iii) Both the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces have one feature: the activation barrier of the two OH species that form H2O is lower than that of the reaction of OH and H. Compared with the Mo2C catalyst, the Cu4@Mo2C catalyst exhibits better catalytic activity for the RWGS reaction, which illustrates the advantages of introducing a nonnoble Cu4 cluster. The Cu-Mo2C interface promotes CO2 dissociation, and the Cu4 cluster provides weaker CO adsorption that favors CO release. The Cu-Mo2C interface is beneficial to the RWGS reaction. The calculated results revealed profound insights into the mechanisms of the RWGS reaction on the Cu@Mo2C(001) and Cu4@Mo2C(001) surfaces, which provide a better understanding from the microscopic perspective. We hope that this article will help to design efficient transition metal 39 / 45



Page 40 of 57

carbides modified by non-noble metals.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected](Q. Li), [email protected](K. Wu) Fax: +86 591 63173138 ORCID Huijuan Jing: 0000-0003-1627-9803 Qiaohong Li: 0000-0001-9286-3580 Jian Wang: 0000-0002-6991-6530 Diwen Liu: 0000-0001-8119-7666 Kechen Wu: 0000-0002-9531-2239 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Science Foundation of China (No. 21673240) and the Foreign Cooperation Project of Fujian Province (No. 2017I0019). The authors gratefully acknowledge the Supercomputing Center in Fuzhou for providing computational resources.

REFERENCES (1) Karl, T. R.; Trenberth, K. E. Modern Global Climate Change. Science 2003, 302, 1719-1723. (2) Cox, P. M.; Betts, R. A.; Jones, C. D.; Spall, S. A.; Totterdell, I. J. Acceleration of Global Warming Due to Carbon-cycle Feedbacks in a Coupled Climate model. Nature 2000, 408, 184-187. 40 / 45


Page 41 of 57 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


(3) Aresta, M.; Dibenedetto, A.; Angelini, A. Catalysis for the Valorization of Exhaust Carbon: from CO2 to Chemicals, Materials, and Fuels. Technological Use of CO2. Chem. Rev. 2014, 114, 1709-1742. (4) Jiang, G.; Huang, Q.; Kenarsari, S. D.; Hu, X.; Russell, A. G.; Fan, M.; Shen, X. A New Mesoporous Amine-TiO2 Based Pre-combustion CO2 Capture Technology. Appl. Energy 2015, 147, 214-223. (5) Luis, P.; Van Gerven, T.; Van der Bruggen, B. Recent Developments in Membranebased Technologies for CO2 Capture. Prog. Energy Combust. Sci. 2012, 38, 419-448. (6) Wang, W.; Wang, S.; Ma, X.; Gong, J. Recent Advances in Catalytic Hydrogenation of Carbon Dioxide. Chem. Soc. Rev. 2011, 40, 3703-3727. (7) Yu, K. M.; Curcic, I.; Gabriel, J.; Tsang, S. C. Recent Advances in CO2 Capture and Utilization. ChemSusChem 2008, 1, 893-899. (8) Mikkelsen, M.; Jørgensen, M.; Krebs, F. C. The Teraton Challenge. A Review of Fixation and Transformation of Carbon Dioxide. Energy Environ. Sci. 2010, 3, 43-81. (9) Aresta, M.; Dibenedetto, A. Utilisation of CO2 as a Chemical Feedstock: Opportunities and Challenges. Dalton Trans. 2007, 28, 2975-2992. (10) Yang, C.; Zhao, H.; Hou, Y.; Ma, D. Fe5C2 Nanoparticles: A Facile BromideInduced Synthesis and as an Active Phase for Fischer−Tropsch Synthesis. J. Am. Chem. Soc. 2012, 134, 15814-15821. (11) Zaman, S.; Smith, K. J. A Review of Molybdenum Catalysts for Synthesis Gas Conversion to Alcohols: Catalysts, Mechanisms and Kinetics. Catal. Rev.-Sci. Eng. 2012, 54, 41-132. (12) Xiang, M.; Li, D.; Xiao, H.; Zhang, J.; Li, W.; Zhong, B.; Sun, Y. K/Ni/β-Mo2C: A Highly Active and Selective Catalyst for Higher Alcohols Synthesis from CO Hydrogenation. Catal. Today 2008, 131, 489-495. (13) Su, X.; Yang, X.; Zhao, B.; Huang, Y. Designing of Highly Selective and HighTemperature Endurable RWGS Heterogeneous Catalysts: Recent Advances and the Future Directions. J. Energy Chem. 2017, 26, 854-867. (14) Park, S.-W.; Joo, O.-S.; Jung, K.-D.; Kim, H.; Han, S.-H. Development of ZnO/Al2O3 Catalyst for Reverse-Water-Gas-Shift Reaction of CAMERE (Carbon Dioxide Hydrogenation to Form Methanol via a Reverse-Water-Gas-Shift Reaction) Process. Appl. Catal. A-Gen. 2001, 211, 81-90. (15) Park, S.-W.; Joo, O.-S.; Jung, K.-D.; Kim, H.; Han, S.-H. ZnO/Cr2O3 Catalyst for Reverse-Water-Gas-Shift Reaction of CAMERE Process. Korean J. Chem. Eng. 2000, 17, 719-722. (16) Wang, F.; Wei, M.; Evans, D. G.; Duan, X. CeO2-based Heterogeneous Catalysts Toward Catalytic Conversion of CO2. J. Mater. Chem. A 2016, 4, 5773-5783. (17) Wang, L. C.; Tahvildar Khazaneh, M.; Widmann, D.; Behm, R. J. TAP Reactor Studies of the Oxidizing Capability of CO2 on a Au/CeO2 Catalyst – A First Step Toward Identifying a Redox Mechanism in the Reverse Water–Gas Shift Reaction. J. Catal. 2013, 302, 20-30. (18) Gavrila, D.; Georgakab, A.; Loukopoulosb, V.; Karaiskakisb, G.; Nieuwenhuysa, B. E. On the Mechanism of Selective CO Oxidation on Nanosized Au/γ-Al2O3 Catalysts. Gold Bull. 2006, 39, 192-199. 41 / 45



Page 42 of 57

(19) Liu, X.; Kunkel, C.; Ramírez de la Piscina, P.; Homs, N.; Viñes, F.; Illas, F. Effective and Highly Selective CO Generation from CO2 Using a Polycrystalline αMo2C Catalyst. ACS Catal. 2017, 7, 4323-4335. (20) Xu, W.; Ramirez, P. J.; Stacchiola, D.; Brito, J. L.; Rodriguez, J. A. The Carburization of Transition Metal Molybdates (MxMoO4, M = Cu, Ni or Co) and the Generation of Highly Active Metal/Carbide Catalysts for CO2 Hydrogenation. Catal. Lett. 2015, 145, 1365-1373. (21) Posada-Pérez, S.; Ramírez, P. J.; Gutiérrez, R. A.; Stacchiola, D. J.; Viñes, F.; Liu, P.; Illas, F.; Rodriguez, J. A. The Conversion of CO2 to Methanol on Orthorhombic βMo2C and Cu/β-Mo2C Catalysts: Mechanism for Admetal Induced Change in the Selectivity and Activity. Catal. Sci. Technol. 2016, 6, 6766-6777. (22) Li, N.; Chen, X.; Ong, W. J.; MacFarlane, D. R.; Zhao, X.; Cheetham, A. K.; Sun, C. Understanding of Electrochemical Mechanisms for CO2 Capture and Conversion into Hydrocarbon Fuels in Transition-Metal Carbides (MXenes). ACS Nano 2017, 11, 10825-10833. (23) Solymosi, F.; Oszkó, A.; Bánsági, T.; Tolmacsov, P. Adsorption and Reaction of CO2 on Mo2C Catalyst. J. Phys. Chem. B 2002, 106, 9613-9618. (24) Gao, J.; Wu, Y.; Jia, C.; Zhong, Z.; Gao, F.; Yang, Y.; Liu, B. Controllable Synthesis of α-MoC1-x and β-Mo2C Nanowires for Highly Selective CO2 Reduction to CO. Catal. Commun. 2016, 84, 147-150. (25) Porosoff, M. D.; Yang, X.; Boscoboinik, J. A.; Chen, J. G. Molybdenum Carbide as Alternative Catalysts to Precious Metals for Highly Selective Reduction of CO2 to CO. Angew. Chem.-Int. Edit. 2014, 53, 6705-6709. (26) Zhang, X.; Zhu, X.; Lin, L.; Yao, S.; Zhang, M.; Liu, X.; Wang, X.; Li, Y.-W.; Shi, C.; Ma, D. Highly Dispersed Copper over β-Mo2C as an Efficient and Stable Catalyst for the Reverse Water Gas Shift (RWGS) Reaction. ACS Catal. 2017, 7, 912-918. (27) Qiao, B.; Wang, A.; Yang, X.; Allard, L. F.; Jiang, Z.; Cui, Y.; Liu, J.; Li, J.; Zhang, T. Single-Atom Catalysis of CO Oxidation Using Pt1/FeOx. Nat. Chem. 2011, 3, 634641. (28) Li, Q.; Ma, Z.; Sa, R.; Adidharma, H.; Gasem, K. A. M.; Russell, A. G.; Fan, M.; Wu, K. Computation-Predicted, Stable, and Inexpensive Single-Atom Nanocatalyst Pt@Mo2C-An Important Advanced Material for H2 Production. J. Mater. Chem. A 2017, 5, 14658-14672. (29) Cheng, D.; Negreiros, F. R.; Apra, E.; Fortunelli, A. Computational Approaches to the Chemical Conversion of Carbon Dioxide. ChemSusChem 2013, 6, 944-965. (30) Chen, C. S.; Wu, J. H.; Lai, T. W. Carbon Dioxide Hydrogenation on Cu Nanoparticles. J. Phys. Chem. C 2010, 114, 15021-15028. (31) Chen, C.-S.; Cheng, W.-H.; Lin, S.-S. Mechanism of CO Formation in Reverse Water–Gas Shift Reaction over Cu/Al2O3 Catalyst. Catal. Lett. 2000, 68, 45-48. (32) Ginés, M. J. L.; Marchi, A. J.; Apesteguía, C. R. Kinetic Study of the Reverse Water-Gas Shift Reaction over CuO/ZnO/Al2O3 Catalysts. Appl. Catal. A-Gen. 1997, 154, 155-171. (33) Dietz, L.; Piccinin, S.; Maestri, M. Mechanistic Insights into CO2 Activation via Reverse Water–Gas Shift on Metal Surfaces. J. Phys. Chem. C 2015, 119, 4959-4966. 42 / 45


Page 43 of 57 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


(34) Ren, J.; Huo, C.-F.; Wang, J.; Cao, Z.; Li, Y.-W.; Jiao, H. Density Functional Theory Study into the Adsorption of CO2, H and CHx (x = 0-3) as well as C2H4 on αMo2C(0001). Surf. Sci. 2006, 600, 2329-2337. (35) Tominaga, H.; Aoki, Y.; Nagai, M. Hydrogenation of CO on Molybdenum and Cobalt Molybdenum Carbides. Appl. Catal. A-Gen. 2012, 423-424, 192-204. (36) Christensen, A. N. A Neutron Diffraction Investigation on a Crystal of α-Mo2C. Acta Chem. Scand. A 1977, 31, 509-511. (37) E. Rudy; S. Windisch; A.J. Stosick; J.R. Hoffman. The Constitution of Binary Molybdenum-Carbon Alloys. Trans. Metall. Soc. AIME. 1967, 239, 1247-1267. (38) St. Clair, T. P.; Oyama, S. T.; Cox, D. F.; Otani, S.; Ishizawa, Y.; Lo, R.-L.; Fukui, K.-i.; Iwasawa, Y. Surface Characterization of α-Mo2C (0001). Surf. Sci. 1999, 426, 187-198. (39) Nagai, M.; Oshikawa, K.; Kurakami, T.; Miyao, T.; Omi, S. Surface Properties of Carbided Molybdena–Alumina and Its Activity for CO2 Hydrogenation. J. Catal. 1998, 180, 14-23. (40) Parthe, E. The Structure of Dimolybdenum Carbide by Neutron Diffraction Technique. Acta Cryst. 1963, 16, 202-+. (41) Wang, T.; Liu, X.; Wang, S.; Huo, C.; Li, Y.-W.; Wang, J.; Jiao, H. Stability of βMo2C Facets from ab Initio Atomistic Thermodynamics. J. Phys. Chem. C 2011, 115, 22360-22368. (42) Shi, X.-R.; Wang, S.-G.; Wang, H.; Deng, C.-M.; Qin, Z.; Wang, J. Structure and Stability of β-Mo2C Bulk and Surfaces: A Density Functional Theory Study. Surf. Sci. 2009, 603, 852-859. (43) Hanif, A.; Xiao, T.; York, A. P. E.; Sloan, J.; Green, M. L. H. Study on the Structure and Formation Mechanism of Molybdenum Carbides. Chem. Mat. 2002, 14, 1009-1015. (44) Posada-Perez, S.; Vines, F.; Ramirez, P. J.; Vidal, A. B.; Rodriguez, J. A.; Illas, F. The Bending Machine: CO2 Activation and Hydrogenation on δ-MoC (001) and βMo2C (001) Surfaces. Phys. Chem. Chem. Phys. 2014, 16, 14912-14921. (45) Politi, J. R.; Vines, F.; Rodriguez, J. A.; Illas, F. Atomic and Electronic Structure of Molybdenum Carbide Phases: Bulk and Low Miller-Index Surfaces. Phys. Chem. Chem. Phys. 2013, 15, 12617-12625. (46) Delley, B. An All-Electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules. J. Chem. Phys. 1990, 92, 508-517. (47) Delley, B. From Molecules to Solids with the DMol3 Approach. J. Chem. Phys. 2000, 113, 7756-7764. (48) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (49) Bergner, A.; Dolg, M.; Küchle, W.; Stoll, H.; Preuß, H. Ab Initio Energy-Adjusted Pseudopotentials for Elements of Groups 13-17. Mol. Phys. 1993, 80, 1431-1441. (50) Lee, C.-S.; Hwang, T.-S.; Wang, Y.; Peng, S.-M.; Hwang, C.-S. Charge Density and Bonding in Bis(diiminosuccinonitrilo)nickel, Ni(C4N4H2)2: A Combined Experimental and Theoretical Study. J. Phys. Chem 1996, 100, 2934-2941. (51) Lin, T.; Zhang, W.-D.; Huang, J.; He, C. A DFT Study of the Amination of 43 / 45



Page 44 of 57

Fullerenes and Carbon Nanotubes: Reactivity and Curvature. J. Phys. Chem. B 2005, 109, 13755-13760. (52) Matsuzawa, N.; Seto, J. e.; Dixon, D. A. Density Functional Theory Predictions of Second-Order Hyperpolarizabilities of Metallocenes. J. Phys. Chem. A 1997, 101, 9391-9398. (53) Clay, J. P.; Greeley, J. P.; Ribeiro, F. H.; Nicholas Delgass, W.; Schneider, W. F. DFT Comparison of Intrinsic WGS Kinetics over Pd and Pt. J. Catal. 2014, 320, 106117. (54) Miyao, T.; Shishikura, I.; Matsuoka, M.; Nagai, M.; Oyama, S. T. Preparation and Characterization of Alumina-Supported Molybdenum Carbide. Appl. Catal. A-Gen. 1997, 165, 419-428. (55) Liu, Y.; Ding, J.; Bi, J.; Sun, Y.; Zhang, J.; Liu, K.; Kong, F.; Xiao, H.; Chen, J. Effect of Cu-doping on the Structure and Performance of Molybdenum Carbide Catalyst for Low-Temperature Hydrogenation of Dimethyl Oxalate to Ethanol. Appl. Catal. A-Gen. 2017, 529, 143-155. (56) Posada-Perez, S.; Vines, F.; Rodriguez, J. A.; Illas, F. Structure and Electronic Properties of Cu Nanoclusters Supported on Mo2C(001) and MoC(001) Surfaces. J. Chem. Phys. 2015, 143, 114704. (57) Shi, Y.; Yang, Y.; Li, Y.-W.; Jiao, H. Activation Mechanisms of H2, O2, H2O, CO2, CO, CH4 and C2Hx on Metallic Mo2C(001) as well as Mo/C Terminated Mo2C(101) from Density Functional Theory Computations. Appl. Catal. A-Gen. 2016, 524, 223236. (58) Zhao, Y.-F.; Yang, Y.; Mims, C.; Peden, C. H. F.; Li, J.; Mei, D. Insight into Methanol Synthesis from CO2 Hydrogenation on Cu(111): Complex Reaction Network and the Effects of H2O. J. Catal. 2011, 281, 199-211. (59) Pekridis, G.; Kalimeri, K.; Kaklidis, N.; Vakouftsi, E.; Iliopoulou, E. F.; Athanasiou, C.; Marnellos, G. E. Study of the Reverse Water Gas Shift (RWGS) Reaction over Pt in a Solid Oxide Fuel Cell (SOFC) Operating Under Open and ClosedCircuit Conditions. Catal. Today 2007, 127, 337-346. (60) Vesselli, E.; et al. Hydrogen-Assisted Transformation of CO2 on Nickel: The Role of Formate and Carbon Monoxide. J. Phys. Chem. Lett. 2009, 1, 402-406. (61) Tang, Q.; Shen, Z.; Huang, L.; He, T.; Adidharma, H.; Russell, A. G.; Fan, M. Synthesis of Methanol from CO2 Hydrogenation Promoted by Dissociative Adsorption of Hydrogen on a Ga3Ni5(221) Surface. Phys. Chem. Chem. Phys. 2017, 19, 1853918555.

44 / 45


Page 45 of 57 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


TOC Graphic

45 / 45



79x39mm (300 x 300 DPI)


Page 46 of 57

Page 47 of 57 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


177x83mm (300 x 300 DPI)



177x97mm (300 x 300 DPI)


Page 48 of 57

Page 49 of 57 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


177x100mm (300 x 300 DPI)



82x78mm (300 x 300 DPI)


Page 50 of 57

Page 51 of 57 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


82x96mm (300 x 300 DPI)



82x77mm (300 x 300 DPI)


Page 52 of 57

Page 53 of 57 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


177x123mm (300 x 300 DPI)



82x78mm (300 x 300 DPI)


Page 54 of 57

Page 55 of 57 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


82x88mm (300 x 300 DPI)



82x70mm (300 x 300 DPI)


Page 56 of 57

Page 57 of 57 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


177x123mm (300 x 300 DPI)


Theoretical Study of the Reverse Water Gas Shift Reaction on Copper

Recommend Documents