Mechanistic Insights Into Selective CO2 Conversion via RWGS on

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

Mechanistic Insights Into Selective CO Conversion via RWGS on Transition Metal Phosphides: A DFT Study Utsab Guharoy, Tomas Ramirez Reina, Sai Gu, and Qiong Cai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04122 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 25, 2019

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The Journal of Physical Chemistry

Mechanistic Insights into Selective CO2 Conversion via RWGS on Transition Metal Phosphides: A DFT Study Utsab Guharoy1,2, Tomas Ramirez Reina1, Sai Gu1, Qiong Cai1,*

1Department

of Chemical and Process Engineering, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford GU2 7XH, United Kingdom

2State

Key Laboratory of CatalysisDalian Institute of Chemical Physics, Chinese Academy of Sciences 457 Zhongshan Road, Dalian 116023,China *Corresponding author. Email: [email protected]; Tel:+44(0)1483686561

ABSTRACT

Selective conversion of CO2 to CO via the reverse water gas shift (RWGS) reaction is an attractive CO2 conversion process, which may be integrated with many industrial catalytic processes such as Fischer-Tropsch synthesis to generate added value products. The development of active and cost friendly catalysts is of paramount importance. Among the available catalyst materials, transition metal phosphides (TMPs) such as MoP and Ni2P have remained unexplored in the context of the RWGS reaction. In the present work we have employed density functional theory (DFT) to firstly investigate the stability and geometries of selected RWGS intermediates on the MoP (0001) surface, in comparison to the Ni2P (0001) surface. Higher adsorption energies and Bader charges are observed on MoP (0001), indicating better stability of intermediates on the MoP (0001) surface. Further, mechanistic investigation using potential energy surface (PES) profiles showcased that both MoP and Ni2P to be active towards RWGS reaction with the direct path (CO2*→CO* +O*) to be favourable on MoP (0001), whereas, COOH-mediated path (CO2* + H*→COOH*) favors on the Ni2P (0001) for products (CO and H2O) gas generation. Additionally, PES profiles of initial steps to CO activation revealed that, direct CO decomposition to C* and O* is favoured only on MoP (0001), whilst H-assisted CO activation is more favourable on Ni2P (0001) but could also occur on MoP (0001). Furthermore, our DFT calculations also ascertained the possibility of methane formation on Ni2P (0001) during the RWGS process, while MoP (0001) remaining more selective towards CO generation.

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1.1 INTRODUCTION Carbon dioxide (CO2) is considered to be one of the main greenhouse gases, which poses an adverse effect on the Earth’s atmosphere and climate.1 A selective conversion of CO2 to CO in the presence of H2, commonly known as reverse water gas shift (RWGS, CO2 + H2⇋ CO + H2O) is a promising industrial option of CO2 utilisation.2-6 Especially this is largely due to the versatile nature of CO, which may be transformed into value-added products or used in the downstream Fischer-Trospsch (FT) or MeOH synthesis processes.1-3 Recent research has focused on the development of suitable catalysts for the RWGS process, to overcome the challenges mainly due to the endothermic nature of the process which requires higher temperatures to achieve equilibrium CO2 conversions ranging between 10-50% at (200-500)°C respectively.2 Therefore to improve the kinetics of the RWGS process, efforts have been made to develop more active catalysts towards CO2 dissociation to produce a greater yield of CO.2 In the past decade, both transition metal carbides (TMCs) and phosphides (TMPs) have been investigated for a variety of industrial processes such as the hydrogen evolution reaction (HER)79,

water gas shift (WGS)10-11 and CO hydrogenation.12-14 Among the TMCs a major focus has

been given to molybdenum (Mo) carbides, which have shown to be successful in transforming CO2 to CO, methane, methanol and other hydrocarbons.4, 13, 15-16For instance, DFT investigations indicated spontaneous cleavage of C-O bond on metal (Mo) terminated orthorhombic (β-Mo2C) to transform CO2 to CO.17 Liu et al.16, in their experimental and theoretical investigation on polycrystalline (α-Mo2C) for RWGS, found CO2 conversion to be 16% with CO selectivity (~99%) at 673K, with their theoretical calculations revealing strong binding of CO2, CO, and O species on Mo terminated Mo2C (001) facet.16 The common feature of Mo-carbide catalysts showcased Mo- terminated surfaces to be more active towards CO2 activation13,

15-16, 18

with

Mo/C ratio of one, usually adsorbing CO2 without breaking the C-O bond of the molecule.13, 17 In contrast, TMPs have largely remained unexplored for the RWGS process with limited experimental and theoretical investigations available to understand the interaction between reactant CO2 and catalyst surface. Among TMPs, Mo phosphide has been investigated for CO hydrogenation reactions.14 Zaman and Smith using DFT calculations of elementary reaction steps for the conversion of synthesis gas (CO+H2) to methanol on Mo6P3 cluster model, hypothesised 2 ACS Paragon Plus Environment

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the formation of methane to be greater compared to methanol on the Mo6P3 cluster model.14 Additionally, the authors revealed the CH2OH to be a common intermediate for the formation of methane or methanol on Mo6P3 cluster.14 Further, Feng et al.19, with their in situ Fourier transform infrared (FT-IR) spectroscopy study of CO adsorption on MoP/γ-Al2O3 obtained a similarity in IR bands corresponding to CO adsorption on noble metals (Pt, Pd, Ru).19 Strong CO adsorption on MoP (001) was also reported by Liu et al.20, in their theoretical study. In another recent theoretical study the CO saturation coverage on Mo terminated MoP (001) was reported to be 1 ML with

CO desorption occurring above 500 K.21 Also experimental investigations

conducted by Yao et al.22, found MoP to be active towards CO2 reforming of methane with better activity and stability compared to Ni/Mo2C.22 Additionally, strong oxygen to phosphorus interaction on nickel phosphide Ni2P (001) catalyst has shown to facilitate direct water gas shift reaction.11 Another interesting feature of TMPs is attributed to its hydrotreating abilities which favour easier H2 dissociation on catalyst surface.9, 23-25 A hydrogen rich environment is typically required during RWGS and CO2 hydrogenation reaction process and has been found to occur efficiently on noble metal (Ru, Rh and Pt) catalyst surfaces.3 Also a recent ab initio thermodynamic study found that, H maintains high saturation coverage (⁓4 ML) during hydrotreating conditions on different facets of MoP surface with low H2 activation barriers.24 Further theoretical evidence of H stability was reported by Liu et al., on Ni2P (0001), with better HER activity compared to noble metals.9 Motivated by the findings of effective usability of Mo-carbide catalysts in RWGS and CO2 hydrogenation reactions, in the present work we have used DFT based mechanistic study to explore the potentiality of TMPs, namely MoP (0001) and Ni2P (0001) surfaces for RWGS reaction. We first develop detailed insights of the energies and geometries for the RWGS intermediates on the MoP (0001) surface and then draw a comparison for the same with the Ni2P (0001) surface. Further, mechanistic insights of RWGS pathways are discussed on both TMPs. Additionally, we have evaluated and compared the initial steps for CO activation, an important reaction in F-T synthesis, on both MoP (0001) and Ni2P (0001) surfaces. Also, the potentiality for methane formation, which is a competing reaction in the RWGS process, is investigated on both the TMP surfaces.



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2. COMPUTATIONAL METHOD AND MODEL In the present work, spin polarized DFT calculations are carried out using Cambridge Sequential Total Energy Package (CASTEP).26The generalized gradient approximation (GGA) using Perdew-Burke-Ernzerhof (PBE) functional27 is used to describe the exchange-correlation energy. Plane wave basis set was used to expand the electron wave functions with a kinetic energy cutoff of 340 eV for all the calculations, chosen after convergence tests. To describe the ionelectron interaction ultra-soft pseudo-potentials, introduced by Vanderbilt28, was used. Gaussian smearing method29 was used to determine the electron occupancies, with a smearing value of 0.1 eV. The self-consistency field (SCF) is considered to be converged when the total energy of the system is below 10-6 eV/atom. All geometries are optimized using the (BFGS)30 algorithm, where the geometries converged until the forces between atoms are less than 0.01 eV/Å. To model the MoP (0001) surface, first, the MoP bulk parameters are calculated with a Monkhorst-Pack31 k-point grid of 9×9×8. Thereafter the optimized bulk was cleaved in the [0001] direction. Five layers were used to construct the surface slab in a (3×3) surface supercell, corresponding to an adsorbate coverage of 1/9 monolayer (ML), with a vacuum gap of 12 Å to avoid interactions between the slabs, is used to model the MoP (0001) surface. Further, the bottom two layers were constrained to their bulk positions, with the top three layers being allowed to relax in all directions. For sampling, the Brillouin zone of MoP (0001) surface a kpoint grid of 2×2×1 was found to be suitable after convergence tests. Additionally, tests were performed with a denser k-point grid of (3×3×1) for CO adsorption on MoP (0001).The results did not show an appreciable increase in the adsorption energies value. Therefore, a lower k-point grid was considered in this work to save computational expense. Additionally the Ni2P (0001) surface model was considered from our previous theoretical work32, where the Ni2P (0001) surface is represented by its Ni3P2-termination. Adsorption energy (Eads) of the surface adsorbed species is defined by the following equation; Eads = Eint+ surface – Esurface – Eint

(1)

where Eint+surfacerepresents the total energy of the adsorbates on the modeled surface, Esurfaceis the total energy of the clean surface slab and Eint corresponds to the energy of the gas phase


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molecules calculated in a simulation box of 15 Å × 15 Å × 15 Å. The charge analysis of the adsorbates and the surface was performed by the Bader charge analysis package.33 Transition state (TS) searches to locate the saddle points are performed using complete linear synchronous transit (LST)/quadratic synchronous transit (QST) method34, as implemented in CASTEP. The convergence criterion for the transition state search is set to be 0.05 eV/Å rootmean-square forces on each atom. The uniqueness of the (TS) structures was confirmed using vibrational frequency analysis using the finite displacement method.35 3 RESULTS AND DISCUSSIONS 3.1 SURFACE PROPERTIES OF MoP (0001)



Figure 1. (a) Top and side views of the Mo-terminated surface layer of MoP (0001) termination with alternating Mo and P layers and HM, HP, top, bri denotes possible adsorption sites.(b) Total density of states (DOS) for MoP (0001) surface. (c) Partial density of states (PDOS) projected onto a Mo atom on the outer layer of the MoP (0001). Cyan spheres represent Mo atoms, and orange P atoms, respectively.

To develop the MoP (0001) surface, the bulk parameters were first optimised. The obtained lattice parameters for the bulk MoP was found to be a=b=3.246 Å, c=3.21Å, which is in good agreement with experimental (a=b=3.223Å c=3.191Å)36 and theoretical (a=b=3.252 Å, c= 3.216 Å)37 findings. The possible adsorption sites on MoP (0001), shown in Figure 1a, may be defined as hollow sites labelled as HM and HP, where HM is a three-fold hollow site with Mo atoms only 6 ACS Paragon Plus Environment

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and HP is also a three-fold hollow site with a P atom attached below. The other two sites are a bridge (bri) and Mo top site (top). The MoP (0001) surface, shown in Figure 1a, is developed with (3 ×3) unit cell consisting of five layers with alternating Mo and P atomic layers (Figure 1a), comprising of 45 atoms (27 Mo and 18P), with 9 atoms per layer. To study the RWGS reaction, a Mo-terminated MoP (0001) surface is developed due to its higher chemical activity towards similar intermediates studied previously37 and lower surface formation energy as compared to its P-terminated counterpart.38 Further, Bader charge analysis of the MoP (0001) surface indicates a clear charge transfer between Mo → P (Mo 0.43 and P -0.77). Similar electronegativity of P on MoP surfaces was reported previously.14, 39 The analysis of the total density of states (DOS) in Figure 1b shows the metallic characteristic of MoP (0001) surface. Also, in the PDOS analysis in Figure 1c, the peak around -10 eV corresponds to the P (s) semi-local states with the region near the Fermi level (EF) mainly dominated by Mo (d) and P (p) bonding and anti-bonding states.

3.2 ADSORPTION RWGS INTERMEDIATES ON TMPs In this section, first the adsorption energies, configurations and preferable sites of possible intermediates involved in the RWGS reaction on the MoP (0001) surface are discussed in details. Thereafter to ascertain the adsorption behaviour of similar intermediates on TMP’s, Ni2P (0001) surface is used as comparison. Table 1: The adsorption energies Eads in (eV), bond lengths (Å) and net Bader charges (e) on the most stable sites of MoP (0001) surface. The * sign denotes an adsorbate. Species

Site

Eads

Bonds

(Å)

(eV)

CO2*

H*

bri

HM

-1.89

-3.24

Bond lengths

dC-O1/ C-O2

1.29/1.29

dC-Mo

2.26, 2.24

dH-Mo

2.02, 2.03


Net Bader charge (e)

-0.97

-0.45


H2*

CO*

H2O*

top

HM

top

-0.73

-2.42

-0.87

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dH-H

0.89

dH-Mo

1.88, 1.89

dC-O

1.25

dC-Mo

2.21, 2.22, 2.22

dO-H

1.02, 1.02

dO-Mo

2.28

-0.12

-0.85

0.002

OH*

HP

-4.43

dO-H dO-Mo

1.0 2.26, 2.27

-0.61

C*

HM

-8.08

dC-Mo

2.03, 2.03, 2.04

-1.08

O*

HP

-7.29

dO-Mo

2.12, 2.13, 2.13

-0.99

HP

-3.77

dC-H / dC-O

1.20 / 1.37

-0.98

dC-Mo

2.18. 2.24, 2.27

dC-O1/dC-O2/ dO-H

1.36/1.37/1.01

dC-Mo/dO-Mo

2.12 / 2.22, 2.23

CHO*

COOH*

HP

-3.79

-0.80

The details about the most preferred adsorption sites, energies, bonds and Bader charges of RWGS intermediates on the MoP (0001) surface are summarised in Table 1. Further details about the interaction of the intermediates on other possible surface sites on MoP (0001) and their respective geometries are shown in the Supporting Information in Table S1 and Figure S1(a-y). In the gas phase CO2 is a thermodynamically stable molecule with a linear geometry (O=C=O) bonds at 180◦. Therefore the reactivity of CO2 on metallic surfaces depends upon charge transfer from the catalyst surface to a great extent.40 Further in the chemisorbed state, CO2 adopts a bent configuration, whereby the OMoP > OHMoP > COOHMoP > CHOMoP > HMoP > COMoP > CO2MoP > the following trends (CMoP ∗ ∗ > H2MoP H2OMoP ) on the MoP (0001) surface and (CNi∗ 2P > ONi∗ 2P > OHNi∗ 2P > HNi∗ 2P > CHONi∗ 2P > ∗ ∗ ∗ ∗ > CONi > H2Ni > H2ONi COOHNi > 𝐶𝑂2 2P 2P 2P 2P

∗

) on the Ni2P (0001) surface. Further the

Ni2P

adsorption energies showcase a greater stability of the intermediates on the MoP (0001) surface compared to the Ni2P (0001) surface. For instance CO2 interacts with MoP (0001) surface more strongly compared to the Ni2P (0001). This may be attributed to a higher charge transfer from the surface to the CO2 molecule, which shows a higher net Bader charge of CO2 adsorbate on MoP (0001). Notably molecular hydrogen interact similarly on both the TMP surfaces with the adsorption energy being marginally higher on MoP (0001) surface with higher net charge on H2 on the Ni2P (0001) surface. Also, atomic H* is found to be more stable on the MoP (0001) surface with a higher net charge. Interestingly H2O* shows positive net charge on both surface, indicating low charge transfer from the surface with a stronger adsorption of H2O* to the MoP (0001) surface. Further, both C* and O* are found to be the most stable intermediates on the respective TMP surfaces. Overall the stability and net Bader charges on the intermediates are higher on the MoP (0001) surface. 13 ACS Paragon Plus Environment


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3.3 RWGS REACTION MECHANISM ON TMPs

Figure 3.Proposed reaction mechanism scheme in the present work. In the scheme ‘H-H, C-O, O-H, CH’ signifies bond formation or cleavage and ‘+H’ showcases addition of atomic hydrogen.

Previously a number of theoretical investigations have been carried out to elucidate RWGS reaction pathways on transition metals46, interstitial molybdenum carbide13,

47

and sulphide48

catalysts. From these studies the generally accepted RWGS pathways are divided into redox or direct, carboxyl, and the formate mediated routes. Here, the formate (HCOO) intermediate has minor influence on the RWGS reaction due to its greater stability on metal and interstitial catalysts.46,

48

Therefore, to effectively examine the chemical activity of TMPs in RWGS

reaction, we have proposed the mechanism scheme shown in Figure 3. The scheme is divided into three categories, RWGS, CO activation and methane formation. For RWGS the direct and carboxyl (COOH) mediated paths have been considered and thereafter initial steps for CO activation, as an effective RWGS catalyst should perform both C-O bond scission as well as hydrogen activation and subsequent hydrogenation.2 Additionally the CO activation steps lead to 14 ACS Paragon Plus Environment

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the sequential formation of methane on the TMP surfaces. Here-in, firstly the reaction mechanism scheme is used to develop a detailed mechanistic understanding of RWGS on the MoP (0001) surface. Thereafter a comparison is drawn between Ni2P (0001) and MoP (0001) for RWGS reaction, to better understand the chemical activity of these catalysts. Additionally the activation and reaction energies of the RWGS reaction on Ni2P (0001) surface are shown in Table S2 in the Supporting Information. Table 2: Elementary reactions for the considered reaction scheme on MoP (0001) surface with forward activation energy (ΔEa,f) and reaction energy (ΔEr). Additionally, bond lengths for bond forming or breaking reactions in (Å) for the transition state geometries on MoP (0001) are given.

Reaction

MoP (0001) ΔEa,f(eV)

ΔEr (eV)

Bond distances at the transition state Bond

Bond length (Å)

R1

H2(gas) + * → H2*

-

-

-0.73

-

-

R2

CO2(gas) + * → CO2*

-

-

-1.89

-

-

R3

H2*→H* + H *

(TS1)

0.22

-0.60

dH-H

1.45

R4

CO2* →CO* +O*

(TS2)

0.95

-1.51

dC-O

1.61

R5

CO2* + H*→COOH*

(TS3)

1.92

0.96

dO-H

1.31

R6

COOH* →CO* +OH*

(TS4)

1.02

-1.30

dC-O

1.73

R7

O* + H*→OH* + *

(TS5)

1.58

1.03

dO-H

1.18

R8

OH* + H*→H2O* + *

(TS6)

1.64

1.11

dO-H

1.42

R9

CO* + *→ C* + O*

(TS7)

0.94

-0.31

dC-O

1.73

R10

CO* + H*→CHO* + *

(TS8)

0.99

0.77

dC-O/ dC-H

1.31 / 1.54

R11

CO* →CO(gas) +*

-

-

2.42

-

-

R12

H2O* →H2O(gas) + *

-

-

0.50

-

-




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Figure 4. Potential energy surface (PES) and front view of RWGS reactions on MoP (0001) surface showing: (a) H2* → H* + H*, (b) CO2* + *→CO* + O*, and (c) CO2* + H*→COOH*, with initial state (IS), transition state (TS) (1-3) configuration, final state (FS) in the respective reactions. Cyan spheres represents Mo atoms, orange P atoms, grey carbon, red oxygen and white hydrogen, respectively.* denotes surface adsorbate.

In the reaction mechanism scheme considered in this work, the adsorption of two main RWGS reactants CO2 and H2 on MoP (0001) surface is taken to be the initial steps represented by R1 and R2.Thereafter the RWGS reaction proceeds with activation of H2* (R3). R3: H2*→H* + H *In our calculations, hydrogen was found to either adsorb in the molecular form (H2) onto the top site or dissociate onto the hollow (HM and HP) sites of the MoP (0001) surface. Therefore to calculate the activation energy barrier for H2 dissociation the molecular hydrogen configuration on the top site was considered to be the initial state (IS) for the reaction. Here in the (IS), dH-H bond distance was calculated to be 0.89 Å which in the transition state (TS1) geometry increased to 1.45 Å before occupying the adjacent HP sites in the final state (FS) configuration in Figure 4a. The reaction occurs easily on the MoP (0001) surface with a low forward activation energy barrier ΔEa,f= 0.22 eV, with good exothermicity indicated by a negative reaction energy ΔEr= -0.61 eV. R4:CO2* →CO* +O* From the adsorption energies calculation, we found CO2 to bind strongly on the HM, HP sites with the bri site having the strongest binding on the MoP (0001) surface. Therefore we have considered the CO2 adsorbed on the bri site as the (IS) configuration, as shown in Figure 4b. In the (FS) configuration CO and O are placed initially on the opposite HP 17 ACS Paragon Plus Environment


sites separated by a distance of 3.28 Å. During the (TS) calculation, the dC-O bond of the adsorbed CO2 molecule elongates from 1.29 Å found in the (IS) to 1.77 Å in (TS2) resulting in the C-O bond scission. The reaction is found to be highly exothermic indicated by a negative reaction energy ΔEr= -1.51 eV, with energy barrier obtained for the reaction (i.e. the forward activation energy barrier) found to be ΔEa,f = 0.95 eV. R5: CO2* + H*→COOH* Figure 4c represents the CO2hydrogenation route where, hydrogenation of CO2 first forms the carboxyl (COOH) intermediate and thereafter it subsequently dissociates to form CO and OH species on the MoP (0001) surface. The (IS) of CO2 with co-adsorbed H occupy the bri and the opposite HM sites with the dO-H bond separated by a distance of 2.70Å,which,in the (TS3) geometry, decreases to 1.31 Å to form the dO-H bond of trans- COOH configuration and to the (FS) with an dO-H bond of 1.01 Å. The activation energy barrier for the reaction is high (ΔEa,f= 1.92 eV), with the reaction being endothermic indicated by a positive reaction energy ΔEr= 0.96 eV. R6: COOH* →CO* +OH* The dissociation of COOH intermediate on MoP (0001) is shown in Figure 5a and occurs with the cleavage of the C-O bond of COOH*. In the (IS) geometry, COOH* is considered to be on the HP site with dC-O bond of 1.37 Å, which, in the TS configuration (TS4), stretches to 1.73 Å and eventually forms CO* and OH*, as shown in Figure 5a. The reaction is found to be highly exothermic indicated by a negative reaction energy ΔEr= 1.30 eV, with the forward activation energy barrier ΔEa,f= 1.02 eV.


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Figure 5.Potential energy surface (PES) and front view of RWGS reactions on MoP (0001) surface showing: (a) COOH* → CO* + OH*, (b) O*+H*→OH*, and (c) OH* + H*→H2O*, with initial state (IS), transition state (TS) (4-6) configuration and final state(FS) in the respective reactions. Cyan spheres represents Mo atoms, orange P atoms, grey carbon, red oxygen and white hydrogen, respectively.* denotes surface adsorbate.

R7: O* + H*→OH* + *During RWGS reaction, the atomic oxygen (O) is generated on the MoP (0001) surface from initial C-O bond cleavage (R1), where in the reaction, atomic O forms on the catalyst surface in conjunction with the CO intermediate. The removal of this atomic O from the catalyst surface is important to protect the catalyst surface from being poisoned by O.18 Therefore we have investigated the stepwise hydrogenation of O* by H* to form either OH* or 19 ACS Paragon Plus Environment


H2O*. From Figure 5b, in the (IS) configuration, O* and H* are occupying opposite HP and HM sites respectively with an O-H distance of dO-H= 2.26 Å. During the (TS5) geometry the dO-H decreases to 1.22 Å, whilst in the (FS) dO-H is found to be 1.0 Å representing the OH adsorbate. The energy barrier obtained for the reaction is high at ΔEa,f=1.58 eV; thermodynamically the reaction is endothermic with a positive reaction energy ΔEr= 1.03 eV. R8: OH* + H*→H2O* + * From Figure 5c, the considered route for H2O on MoP (0001) occurs from the combination of OH* and H* in the (IS) geometry occupying the HM and HP sites respectively. OH* and H* are separated by a distance of dO-H= 3.67 Å between the H* and the O* centre within OH*. Both OH* and H* moves to the top of Mo atom during the (TS6) configuration with the dO-H distance decreasing to 1.42 Å, which eventually forms H2O* in the (FS), with an O-H bond distance of 1.02 Å. The reaction is found to be endothermic indicated by a positive reaction energy ΔEr= 1.11 eV, with a high energy barrier ΔEa,f = 1.64 eV.


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Figure 6.Potential energy surface (PES) and front view of RWGS reactions on the MoP (0001) surface showing: (a) CO* → C* + O* and (b) CO*+H*→CHO*, with initial state (IS), transition state (TS) (7-8) configuration and final state (FS) in the respective reactions. Cyan spheres represents Mo atoms, orange P atoms, grey carbon, red oxygen and white hydrogen, respectively.* denotes surface adsorbate.

From the above analysis of the RWGS route, CO formation on the MoP (0001) surface occurs through the direct CO2 activation route shown in (R3). Furthermore, the activation of the C-O bond, either directly via CO dissociation or H assisted hydrogenation, is a point of great interest as it is a key step in the initiation of the Fischer-Tropsch (FT) synthesis.2 Therefore we have investigated the CO activation pathways on MoP (0001), which are presented in R9 and R10. R9: CO* + *→ C* + O* Figure 6a shows the direct C-O bond cleavage reaction for CO dissociation on MoP (0001). The stable configuration of CO adsorbed on the HM site is considered to be the (IS), where the dC-O bond is found to be 1.25 Å. During the (TS7) the oxygen atom of CO moves to the opposite HP site, with the dC-O bond stretching to 1.73 Å. The reaction is found to be exothermic with ΔEr=-0.31 eV and the activation energy barrier is found to be ΔEa,f= 0.94 eV. This is in line with other theoretical observation of low activation energy barriers of CO dissociation on plane Mo (100) caclulated to be 0.58 eV at 1/9 ML adsorbate coverage.49 Further, similar activation energy barriers of 0.89 eV for CO dissociation on β-Mo2C (0001) was also reported previously.50



R10: CO* + H*→CHO* + * Figure 6b showcases CO activation with hydrogen, forming the CHO* intermediate. In the (IS) geometry the co-adsorbed species of CO* and H* are occupying HM sites with dC-H distance of 2.97 Å, which, during the (TS8), decreases as the H atom moves to the top site, forming dC-H bond distance of 1.51 Å. The reaction is found to be endothermic with a positive reaction energy ΔEr=0.77 eV. A slightly higher energy barrier for the reaction (ΔEa,f= 0.99 eV)is compared to a direct CO dissociation reaction step. The energy barrier obtained in this work is found to be considerably lower compared to the value of 41.37 kcal/mol (⁓1.79 eV) reported by Zaman and Smith.14 This activation energy difference for CHO* formation may be due to the cluster model (Mo6P3) used by Zaman and Smith for their DFT calculations, which was a representative of a MoP (100) facet compared to the calculations performed on MoP (0001) facet in present work. Finally, R11 and R12 in Table 2 represent the RWGS product formation reactions whereby, the products CO* and H2O* formed on the MoP (0001) surface desorbs into its respective gas phases. 3.3.1 ACTIVITY OF RWGS REACTION ON TMPs


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Figure 7. Potential energy surface (PES) for RWGS reaction on (a) MoP (0001) and (b) Ni2P (0001) surfaces. In the PES profiles the transition states are denoted as TS (1-6) while R (1-2) indicates H2* & CO2* adsorption steps and R (11-12) CO* and H2O* desorption to gas phase products on the TMP surfaces.

Figure 7 (a-b) represent the potential energy surface (PES) profiles of RWGS reaction on MoP (0001) and Ni2P (0001) respectively. The PES profiles are drawn considering two RWGS reaction pathways, i.e. direct path and COOH-mediated path. It may be noted here that the PES profile is drawn assuming the RWGS reaction steps to occur sequentially. Among the gas phase reactants of the RWGS process, CO2 adsorbs with a higher energy compared to H2 on MoP (0001), therefore this step is considered to be the first reaction step (R2), as shown in Figure 7a, followed by H2 dissociation. Among the two examined pathways on MoP (0001), the direct reaction pathway is seen to be favourable towards CO* and H2O* formation compared to the COOH-mediated path. This is due to the high energy demand for COOH* (TS3) intermediate formation and thereafter its subsequent decomposition (TS4) on MoP (0001). Here the direct-path is found to be the most favourable pathway where CO* formation occurs from direct CO2 dissociation (TS2). H2O* formation on MoP (0001) is found to be kinetically and 23 ACS Paragon Plus Environment


thermodynamically difficult may be due to the high adsorption energy of oxygen with the MoP (0001). Therefore, H2O* formation (TS6) may be considered to be the rate determining step. This has also been found to occur on transition metal carbide surfaces TiC (111) and Mo2C (0001) in a previous study and it is consistent with the fact that water activation is the rate determinant step in the forward WGS reaction which agrees with the microscopic reversibility principle .18 In Figure 7b, H2 is considered to get adsorbed first among the RWGS reactants on Ni2P (0001) surface owing to lower adsorption energy of CO2 (Ea= – 0.05 eV). Therefore, H2 adsorption (R1) is considered to be the initial step with H2 dissociation (TS1) taken to be the next sequential step in the PES profile. Interestingly the most favourable RWGS reaction pathway on Ni2P (0001) is calculated to be the COOH-mediated path. This may be pertaining to the low energy barriers encountered in the COOH* decomposition (TS4) to CO* and OH* intermediates. This COOHmediated path has been found to be preferred on noble metals (Pd and Pt)51 in CO2 activation reactions. However, the reaction step for the formation of COOH* intermediate may be considered to the main rate-determining step on Ni2P (0001) in RWGS reaction. Although the energy barrier for the CO2 dissociation step (TS2) on Ni2P (0001) is lower compared to that required for the CO2 hydrogenation step, the corresponding OH* (TS5) formation step is energy demanding, as shown in the direct-path. Overall, from the PES profiles in Figure 7, both MoP (0001) and Ni2P (0001) surfaces are found to be active towards CO and H2O production via two different reaction paths. Additionally, H2 activation is evident to be favourable from similarly low energy barriers for H2 dissociation experienced on both the MoP (0001) and Ni2P (0001) surfaces, which makes the MoP and Ni2P surfaces good candidates for RWGS reaction. 3.3.2 CO ACTIVATION ON MoP (0001) AND Ni2P (0001)


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Figure 8. Potential energy surface (PES) of (a) comparison of direct-CO activation on MoP (0001) and Ni2P (0001), (b) comparison of H-assisted CO activation on MoP (0001) and Ni2P (0001).

Considering that CO formation indicates favourable activity on the RWGS reaction, here we have briefly explored CO activation reactions on the MoP (0001) and Ni2P (0001) surfaces. Especially the focus has been given to CO direct and H-assisted activation reactions, which have been previously considered to be the main activation reactions on FT catalysts such as Fe and Co.52-54 In FT- synthesis the CO activation reaction mechanisms plays a key role in ascertaining catalytic performance and favourable reaction mechanisms.54 Here the initial CO activations steps on MoP (0001) and Ni2P (0001) has been analysed. Figure 8 (a-b) shows the PES profiles for CO activation reactions (R7-R8) on MoP (0001) and Ni2P (0001) surfaces. Interestingly in Figure 8 (a) it may be observed, that direct CO dissociation is not possible on Ni2P (0001), as the energy barrier (3.44 eV) is much higher than the adsorption energy of CO (-1.97 eV) which 25 ACS Paragon Plus Environment


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showcases that CO will prefer to desorbs from Ni2P (0001) surface rather than dissociate. Similarly on FT catalysts such as Co (0001), direct CO dissociation is unfavourable due to high activation energy barrier (2.82 eV).53 Comparatively, direct CO dissociation is preferred on the MoP (0001) surface exothermically. This may showcase the potentiality of MoP (0001) surface to form C*which may be further hydrogenated to form carbene (CH2*), leading to further polymerization for higher chained hydrocarbons.52 However, CO dissociation on MoP may be structure sensitive, as previous theoretical calculations on MoP surface (101), (110), (100) and (112) terminations have demonstrated non-dissociative CO adsorption properties.21 Further Figure 8 (b) shows CO hydrogenation to formyl (CHO*) intermediate to be favourable on Ni2P (0001) surface compared to MoP (0001). It is worth noting that the reaction is endothermic and the activation energy is comparably low on both TMP surfaces, indicating that this H-assisted CO activation could potentially occur on both TMP surfaces. Similarly endothermic reaction for CHO* formation was reported for Fe (100) with activation energy barrier of 0.90 eV52 and 1.31 eV53 on Co (0001) surfaces. It may be observed from the above analysis that, overall CO activation is favourable on both MoP (0001) and Ni2P (0001) surfaces with different activation routes (direct or H-assisted), which means both surfaces may be considered to be good candidates for CO activation process. Although it may be premature for the consideration of MoP (0001) and Ni2P (0001) in FT-synthesis, as a more detailed theoretical study, considering the formation of important intermediates such as CHx and CHxO will provide valuable insights towards hydrocarbon chain growth on these catalysts. Nevertheless we believe the above analysis provides a good indicator towards the potential use of TMPs in FT-synthesis. 3.3.3 METHANE FORMATION ON THE TMP’s Due to the favourability of CO activation reactions on both the TMP surfaces, evident from our DFT calculations in the previous section, it is worthwhile to investigate the potentiality of methane formation during RWGS process on the considered TMP surfaces. This is because methanation is a competing reaction during the RWGS process and may contribute towards defining product formation (CH4 or CO), on individual catalyst surfaces.6,

55-56

Here we have

used the most preferable CO activation routes as initial steps accompanied by sequential CH4 formation steps on MoP (0001) and Ni2P (0001) surfaces to ascertain the product selectivity. Additionally the activation and reaction energies for methane formation reactions (R13-R18& 26 ACS Paragon Plus Environment

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TS7-TS13) on MoP (0001) and Ni2P (0001) are reported in Table S4 & S5 respectively and transitions state geometries of methane formation on MoP (0001) surface are shown in Figure S3(R13-R17) in the Supporting Information.

Figure 9.Potential energy surface (PES) of (a) CH4 formation on MoP (0001) surface following the direct-CO activation route (b) CH4 formation on Ni2P (0001) surface following the Hassisted CO activation route The PES profile for methane formation on the MoP (0001) surface is shown in Figure 9a. Here the direct-CO activation route is considered to be the first step of the reaction, thereafter the 27 ACS Paragon Plus Environment


atomic C* formed gets hydrogenated sequentially to produce CH4* on the surface. Further, from the PES profile, it may be observed that thermodynamically CH4* formation relative to CO* on MoP (0001) surface requires higher energy. However the favorability of CH4 formation relative to CO is found to be easier on the Ni2P (0001) surface (Figure 9b). In the initial step CO hydrogenates to form CHO* intermediate, which in the subsequent step dissociates to form CH* and O* species on the surface. Here the CH* species gets sequentially hydrogenated which leads to formation of CH4 on the Ni2P (0001) surface. Interestingly the reaction for sequential CHx(x=14)

steps are evidently found to be exothermic, thereby making CH4 formation thermodynamically

feasible on the Ni2P (0001) surface. From the DFT calculations it may be considered that during RWGS reaction, Ni2P (0001) surface may be more selective towards formation of methane compared to MoP (0001) surface being more selective towards CO formation. 4. CONCLUSIONS A systematic DFT study was performed on two TMPs, MoP (0001) and Ni2P (0001), to explore its potentiality for applications in chemical CO2 recycling via RWGS reaction. Adsorption study of the RWGS intermediates on Mo-terminated MoP (0001) showcased that most of the intermediates interact strongly with the surface, where the hollow sites (HM and HP) are the most preferred adsorption sites with the bridge and top sites only preferred by CO2, H2 and H2O respectively. Further, the stability of the intermediates with greater net Bader charges was found on MoP (0001) compared to Ni2P (0001) surfaces. The PES profiles were used to investigate and elucidate the RWGS pathways, which showed RWGS favours two different pathways for products (CO and H2O) generation on MoP (0001) and Ni2P (0001). MoP (0001) favours a direct-path of CO2 dissociation, whilst Ni2P favours a COOH-mediated path. Interestingly H2 dissociation, which is a crucial step during RWGS reaction, was found to very favourable on both the TMPs. In conclusion, considering the favourable interaction of the intermediates and mechanistic analysis, both MoP (0001) and Ni2P (0001) surfaces are found to be good candidates for the RWGS reaction. Additionally, CO activation reactions were studied to explore the potentiality of the TMPs as Fischer-Tropsch catalysts. CO activation steps on the TMPs occur with two different paths. The direct decomposition of CO to C* and O* favours only the MoP (0001) surface. The H-assisted path to form CHO* intermediate may occur on both MoP (0001) and Ni2P (0001) surfaces, although it is more favourable on Ni2P (0001). The study indicates that MoP and Ni2P might be promising catalysts for the FT process. However, more detailed 28 ACS Paragon Plus Environment

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theoretical study considering important F-T reaction intermediates will be required to confirm the potentiality of MoP (0001) and Ni2P (0001) in FT-synthesis. Furthermore, the possibility of methanation on the TMP surfaces were investigated using PES profiles. Ni2P (0001) surface was found to be susceptible towards methane formation during RWGS process, whereas MoP (0001) surface being more selective towards CO generation. In the best case scenario these catalytic surfaces could be applied in both the RWGS and FT synthesis opening the possibility of the direct conversion of CO2 to fuels – a worth exploring route that opens some scope for further research in catalysis science using both experimental and computational tools.

ACKNOWLEDGEMENTS Financial support for this work was provided by the Department of Chemical and Process Engineering at the University of Surrey and the EPSRC grants EP/ M027066/1, EP/R512904, EP/J020184/2 and EP/P003354/1. The authors would like to acknowledge the access to HPC clusters including Eureka and KARA (University of Surrey), and ARCHER (UK National HPC resource) for performing the DFT simulations.

SUPPORTING INFORMATION Figures and details for RWGS intermediates on all possible adsorption sites on MoP (0001) surface. Additionally, adsorption and transition state details of RWGS reaction on Ni2P (0001) surface for comparison. Also the transition state details for methane formation reactions on MoP (0001) and Ni2P (0001) are provided. Finally, DFT calculated data for vibrational frequencies of intermediates and transition states on MoP (0001) is reported.

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