Theoretical insights of Ni2P (0001) surface towards its potential applicability in CO2

conversion via dry reforming of methane

Utsab Guharoy, Tomas Ramirez Reina, Emilia Olsson, Sai Gu, Qiong Cai*

Department of Chemical and Process Engineering, Faculty of Engineering and Physical

Sciences, University of Surrey, Guildford GU2 7XH, United Kingdom

*Corresponding author: q.cai@surrey.ac.uk

Abstract

This study reports the potential application of Ni2P as highly effective catalyst for chemical

CO2 recycling via dry reforming of methane (DRM). Our DFT calculations reveal that the

Ni2P (0001) surface is active towards adsorption of the DRM species, with the Ni hollow site

being the most energetically stable site and Ni-P and P contributes as co-adsorption sites in

DRM reaction steps. Free energy analysis at 1000 K found CH-O to be the main pathway for

CO formation. The competition of DRM and reverse water gas shift (RWGS) is also

evidenced with the latter becoming important at relatively low reforming temperatures. Very

interestingly oxygen seems to play a key role in making this surface resistant towards coking.

From microkinetic analysis we have found greater oxygen surface coverage than that of

carbon at high temperatures which may help to oxidize carbon deposits in continuous runs.

The tolerance of Ni2P towards carbon deposition was further corroborated by DFT and micro

kinetic analysis. Along with the higher probability of C oxidation we identify poor capacity

of carbon diffusion on the Ni2P (0001) surface with very limited availability of C nucleation

sites. Overall, this study opens new avenues for research in metal-phosphide catalysis and

expands the application of these materials to CO2 conversion reactions.

Keywords: DRM; nickel phosphide (Ni2P); DFT; reaction mechanism; microkinetics; coking

stability.

1

1. Introduction

The continuous global increase in CO2 emissions spurred from the anthropogenic use of fossil

fuels, has a detrimental effect towards the earth’s atmosphere. This has propagated the

research interest in finding effective industrial solutions for CO2 utilization [1]. Amongst the

various CO2 utilization solutions, production of syngas via dry reforming of methane (DRM)

is a lucrative alternative to reduce the carbon footprint by combining two greenhouse gases to

generate valuable fuels [1-2]. DRM is an endothermic reaction, as shown in Equation (1). It

yields low H2/CO ratio close to 1, which can be potentially used for Fischer-Tropsch

synthesis of liquid fuels and it is particularly interesting for hydrocarboxylation reactions or

for fueling a solid oxide fuel cell [2-3]. The main challenge in the DRM reaction comes from

competing processes such as the reverse water gas shift (RWGS) reaction as shown in

Equation (2), which affects the H2/CO ratio and carbon formation during methane

decomposition (Equation (3) [2-4].

CH 4(g)+CO2( g)

⇌2 CO(g)+2 H 2( g)∆ H=247.4 kJ mol−1(1)

CO2( g)+ H2( g)

⇌CO(g)+H2 O(g )∆ H=74.9 kJ mol−1(2)

CH 4( g)⇌C(s )+2 H 2 (g)∆ H=41.1 kJ mol−1(3)

So far, the main focus in DRM research has been towards the development of suitable

catalysts, by optimizing both cost and catalyst performance. Industrially, Ni-based catalysts

are preferred due to its low cost and good activity. However, these catalysts suffers from

excessive coking under DRM reaction conditions [3-4].

To develop suitable and low-cost stable catalysts for the DRM process, it is worth exploring

alternative materials to replace traditional nickel supported catalysts. In recent years,

interstitial compounds of metal carbide, nitride and phosphides have received attention for

catalyst development [5]. Amongst these interstitial compounds for DRM, predominant

attention has been drawn to Mo-based carbide, nitride and phosphide catalysts [6-9].

Especially, extensive studies on Mo2C has shown that this compound has similar activities

compared to noble metals (Pd, Pt) at slightly elevated pressures (2 bar) in DRM [6-8].

However, for single metal carbides, high reaction pressures are required to achieve good

activity, as at low pressures the catalyst deactivates due to oxidation by CO2 to form

ineffective oxides [6-8]. Recently, Fu et al. [10] showed that bimetallic Ni and Co-doped

molybdenum nitride exhibited high catalytic activity, and are resistant to oxidation at

2

atmospheric pressure [10]. Similarly, a study of DRM with molybdenum phosphide (MoP)

catalyst showcased high coking and oxidation resistance with better stability compared to

Ni/Mo2C catalyst [9]. Apart from MoP, only one other study of metal phosphide catalyst,

tungsten phosphide (WP) has been investigated, which was found to be active for the DRM

reaction [9, 11]. Considering the good activity of Ni-based catalysts towards the reactant

species (CH4 and CO2) [12], it is worthwhile to investigate nickel phosphide (Ni2P) catalyst,

as it has largely remained untouched for exploring its potentiality in DRM.

The versatile nature of the Ni2P family of materials stems from its applicability in important

industrial processes such as hydrodenitrogenation (HDN) and hydrodesulphurization (HDS)

with high catalytic activity [13-14]. Furthermore, both theoretical and experimental studies

have previously demonstrated higher activity and stability of Ni2P compared to Pt electro

catalyst for hydrogen evolution reactions (HER) [14-15]. These enhanced catalytic properties

of Ni2P have been attributed to the ligand (with minor charge transfer observed between Ni

and P), and ensemble (limitation of active metal sites) effects [16-17]. The P atoms in Ni 2P

dilutes the Ni concentration, resulting in the perturbation of the electronic structure of nickel

slightly [16]. X-ray photoelectron spectroscopy (XPS) studies of the Ni2P (0001) surface

under water gas shift reaction (WGSR) reaction conditions indicated the presence of oxygen

mostly occupying the P sites without oxidizing the Ni sites. Additionally, a small amount of

carbon was found to bind to the Ni2P surface in the same XPS study [17]. Indeed, this oxygen

affinity of the Ni2P surface may be beneficial, as it could potentially oxidise carbon and

prevent carbon nucleation over the catalyst surfaces during DRM reaction conditions [18],

thus improving catalyst stability. Furthermore oxygen binding to P atoms presents another

advantage, as it avoids any possible oxidation of Ni, which must remain on its metallic state

to display high activity. This is in contrast to Ni surface facets which are known to have

severe carbon nucleation problems, due to CH4 sequential dissociation and CO

disproportionate processes during DRM reaction conditions [19-23]. Density functional

theory (DFT) based microkinetic studies have established that at high DRM reaction

temperatures, the close-packed Ni (111) surface has low oxygen concentration, thereby

restricting carbon oxidation and facilitating carbon nucleation over the surface [21-23],

leading to catalyst deactivation. The above knowledge about Ni2P and Ni surfaces indicates

that Ni2P may be a promising DRM catalyst, providing the necessary stability.

In the present work we have used a DFT-based micro kinetic approach to investigate the

suitability of Ni2P (0001) as DRM catalyst. This work reveals for the first time, through DFT

3

studies, the fundamental aspects of DRM reaction steps on the Ni2P (0001) surface. Further

thermodynamic and microkinetic analysis show the reaction paths, behavior of DRM

intermediates at high temperatures and coking stability of the Ni2P (0001) surface.

2. Computational Details

Cambridge Sequential Total Energy Package (CASTEP) [24], which has previously been

shown to be suitable for Ni2P bulk and surface studies [16], is used to perform all the DFT

calculations in the present work. The exchange correlation energy was described by

generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional

[25]. The electron wave functions were expanded in a plane wave basis set with kinetic

energy cut off at 340 eV for all the calculations, chosen after convergence tests. Further ultra-

soft pseudopotentials, introduced by Vanderbilt [26], is considered to describe the ion–

electron interaction. Spin polarization is considered for all the calculations. The electron

occupancies were determined by the Gaussian smearing method [27] 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 were optimized using the

Broyden–Fletcher–Goldfarb–Shanno (BFGS) [28] algorithm, where the geometries

converged until the forces between atoms are less than 0.01 eV/Å.

To model the DRM reaction mechanisms, first the Ni2P bulk parameters are calculated with a

Monkhorst-Pack [29] k-point grid of 4×4×5. Thereafter the optimized bulk was cleaved in the

[0001] direction. To model the Ni2P (0001) surface, four layers were used to construct the

surface slab in a (2×2) surface supercell, corresponding to a surface coverage of ¼

monolayer (ML), with a vacuum gap of 12 Å to avoid interactions between the slabs.

Furthermore, a self-consistent electrostatic dipole correction scheme was applied as

implemented in CASTEP to account for any dipole induced by the adsorbents on the surface.

Further the bottom layer of the surface slab was constrained to its bulk position during all

geometry optimization calculations. To sample the Brillouin zone of Ni2P (0001) surface a

k-point grid of 2×2×1 was found to be suitable after convergence tests. Additionally, tests

were performed with higher k-point grid of (5×5×1) and kinetic energy cut-off (at 520 eV)

for CO adsorption on Ni2P (0001), the results did not show an appreciable increase in the

adsorption energies (as shown in Table S1). Therefore, a lower k-point grid was considered in

this work to save computational expense. Further the density of states (DOS) and partial

4

density of states (PDOS) plots were obtained using OptaDOS program [30]. Also to estimate

the partial charges on the atoms of the developed Ni2P bulk, Mulliken population analysis

[31] was carried out to qualitatively examine trends in charge redistribution.

Adsorption energy (Eads) of the surface adsorbed species is defined by Equation (4):

Eads = ES + surface – Esurface – ES (4)

Where, ES+ surface represents the total energy of the adsorbed species on the Ni2P (0001)

surface, Esurface total energy of the Ni2P (0001) surface without adsorbed species and ES

corresponds to the total energy of the adsorbate species. The total energy of the adsorbates

(O2, H2, CHx, COOH, OH, H2O, CO, CO2, COH, and CHO) were calculated for a single

adsorbate fragment in a neutral cell with dimensions of 12 Å × 13 Å × 15 Å. Es for atomic

O, and H were taken as half the total energy of the molecules, whereas the total energy of

atomic C was calculated as the total energy of a single C in graphite.

To perform the transition state searches, we have used the linear synchronous transit

(LST)/quadratic synchronous transit (QST) method [32] as implemented in CASTEP. The

convergence criterion for the transition state search is set to be 0.05 eV/Å root-mean-square

forces on each atom. The uniqueness of the (TS) structures were confirmed with vibrational

frequency analysis using the finite displacement method [33].

The free energy and microkinetic analysis to examine the activity of the Ni2P (0001) catalyst

surface was performed using the descriptor based micro-kinetic model package CatMAP

[34]. The rates for the elementary reactions of DRM considered in this work are determined

by solving differential equations numerically at steady state approximation. In CatMAP, the

free energies for the adsorbates and the gas phase entropies and enthalpies are calculated

using ideal gas thermochemistry as implemented in atomic simulation environment (ASE)

package [35]. Free energies and rates are calculated from DFT obtained activation energies

using the harmonic transition-state theory. Further details about the microkinetic model are

given in the Supporting Information S2.

3 Results and discussions

5

3.1 Bulk and surface models

In catalyst synthesis, the formation of nickel phosphide occurs from the reduction of nickel

oxide or salts with a phosphorus compound in the presence of H2 , at temperatures between

400-1000 °C [36]. During this reduction process different bulk Ni-P stoichiometry forms

depending upon the catalyst calcination temperature [36-37]. Where, first Ni3P forms,

thereafter Ni12P5 at 400°C and finally Ni2P at 550 °C for a supported system and at 650 °C for

an unsupported system [36-37]. Therefore considering the high reaction conditions

experienced during DRM (800-1000)°C [12], we have used the Ni2P bulk stoichiometry to

develop the surface model for our DFT calculations. The Ni2P bulk has a hexagonal structure

and belongs to the P6̅2m space group [38]. The DFT calculated lattice parameters (a=b=5.86

Å and c= 3.37 Å) are in good agreement with previous reported theoretical (a=b=5.88 Å and

c= 3.37 Å) and experimental (a=b=5.87 Å and c=3.39 Å) data [38-40]. Further the nearest Ni-

P distances of the Ni3P2 and Ni3P1 planes in the bulk stoichiometry was calculated to be 2.21

Å and 2.28 Å respectively, which is consistent with previous theoretical findings [38].

From the optimized bulk parameters, the Ni2P (0001) surface is constructed, as shown in

Figure 1 (a), with supercell dimensions of a=b=11.78 Å and c=18.79 Å, comprising of 72 (48

Ni and 24 P) atoms. The Ni2P (0001) surface is developed with four atomic layers comprising

of alternating Ni3P2 and Ni3P1 atomic layers representing the bulk stoichiometry along [0001]

termination. This has been observed in previous scanning tunneling microscopy (STM) [41],

and low energy electron diffraction (LEED) [42] studies. Herein, we have considered the

Ni3P2

surface plane shown in Figure 1 (a), for our calculations, as previous DFT calculations found

the Ni3P2 terminated plane is more stable compared to Ni3P1 by 2.75 eV per unit cell [16] with

lower surface free energy [43]. The metallic character of the Ni2P (0001) is evident from the

total surface density of states (DOS) as shown in Figure 1 (b), where the highest peaks in the

DOS plot represents the d states of Ni without any band gap above the Fermi level.

Additionally the partial density of states (PDOS) projected on a Ni atom of the Ni3P2 plane,

as in Figure 1 (c), shows the overlapping of Ni 3d and P 3p states ( 1.34 to -8.2 eV) and (-10

to -14.14 eV) P s states. Additionally the Mulliken charges of the surface ions shows a slight

charge transfer from Ni to P bond (Ni 0.08e and P -0.08e), consistent with the observations

reported elsewhere [16].

6

Figure 1: (a) Top view of the Ni3P2 surface layer of Ni2P (0001) termination. Numbers

denotes possible adsorption sites as discussed in Section 3.1. (b) Total density of states

(DOS) for Ni2P surface. (c) Partial density of states (PDOS) projected onto a Ni and P atom

on the outer layer of the Ni3P2 plane in Ni2P (0001). Green spheres represents Ni atoms, and

orange P atoms, respectively. The Fermi level is located at 0 eV. Negative E-EF represent the

occupied bands and positive the virtual bands.

Further, Hernandez et al. [44] with dynamical LEED observations found the stability of the

Ni3P2 surface plane to increase with the addition of P adatoms onto three fold hollow sites

7

found on the Ni3P2 plane [44]. This increased surface stability of Ni3P2 is mainly attributed to

phosphorus (P) stabilization of the dangling bonds formed due to unbound P (p) and Ni (d)

orbitals of Ni3P2 surface plane [45]. However considering the typically high reaction

prevalent during DRM, the P adatom may diffuse or desorb from the surface plane allowing

the reaction intermediates to stabilize the dangling bonds and occupy the three fold hollow

sites on Ni3P2 plane. Therefore we believe Ni3P2 to be a reliable representative surface plane

to develop an understanding of behavioral characteristics of the DRM reaction intermediates.

Here it may be worth mentioning that a real heterogeneous catalyst will comprise of surface

defects which may influence the adsorption characteristics and kinetics of DRM species to a

great extent. For instance surface vacancies which are typically electronically rich sites are

envisaged as preferential sites for molecules activation [46]. Assuming that such defects may

facilitate the interaction with the catalytic surface they can also facilitate the activation of the

reactant molecules and therefore speed up the reaction. However the present work hasn’t

focused on surface defects on the Ni2P (0001) due to the stability of the Ni3P2 plane.

To study the active sites on the Ni2P (0001) surface for the interaction of the DRM species,

six inequivalent surface sites shown in Figure 1 (a) are considered to study the adsorption of

DRM species. The definition of the sites are given as Ni threefold hollow ‘1’ , Ni bridge ‘2’,

Ni top ‘3’, Ni-P hollow ‘4’, Ni-P bridge ‘5’ and P top ‘6’.

3.2 Adsorption of DRM species on Ni2P (0001) surface

The optimized structures of the DRM species on the Ni2P (0001) surface are shown in Figure

2 (a-o) and their corresponding adsorption energies, preferable adsorption sites and calculated

bond lengths are reported in Table 1. Additionally, the adsorption energies of the species on

all the available adsorption sites on the Ni2P (0001) surface are included in Table S2 for

completeness.

8

Figure 2. Front view of the most stable configurations of the DRM species on the Ni2P (0001). Additional for details of bonds and bond lengths of the adsorbates refer to Table 1. Green spheres represent Ni atoms, orange P atoms, grey carbon, red oxygen, and white hydrogen, respectively.

From Figure 2 (a-o) , Table 1 and S2, it can be seen that the most preferable adsorption sites

for the majority of the DRM species on the Ni2P (0001) surface is Ni ‘1’ site, which is similar

to the hollow sites (hcp, fcc) of Ni (111) surfaces found to be active for adsorption of similar

species [19-20]. Monoatomic carbon prefers to adsorb on both the available hollow sites (Ni

‘1’ and Ni-P ‘4’) with a stronger binding of -6.53 eV (E ads) on the ‘1’ site. Additionally,

compared to Ni (111), C adsorption is weaker on Ni2P. In comparison to Ni (111) the

binding of atomic O is found to be stronger on Ni2P, at -6.00 eV (Eads) on the Ni ‘1’ site.

Furthermore, O also showcases adsorption on Ni-P ‘5’ bridge site with Eads of -5.84 eV. The

adsorption energy of monoatomic hydrogen (H) was found to be most stable on the Ni ‘1’

site, at -2.94 eV (Eads), higher compared to that found on Ni (111). Also, H adsorbs with

lower energies onto the P top site ‘6’ at -2.18 eV (Eads) and Ni-P bridge site ‘5’ at -2.32 eV

(Eads)

9

Table 1. Adsorption energies (Eads) in eV and bond lengths (Å) of DRM species on the most

stable sites of Ni2P (0001) surface.

Species Preferred

site

Ni2P(001)

Eads (eV)

Bond lengths of adsorbates on Ni2P (0001)

Bonds Bond length (Å)

C 1 -6.53 C-Ni 1.81, 1.82

H 1 -2.95 H-Ni 1.79, 1.80

O 1 -6.00 O-Ni 1.92

OH 1 -3.63 O-H / O-Ni 1.0 / 2.04

H2 3 -0.71 H-H / H-Ni 0.91 / 1.59, 1.63

H2O 3 -0.50 O-H / O-Ni 1.01 / 2.13

CH4 3 -0.02 C-H / C-Ni 1.11, 1.12 / 3.45

CH3 1 -2.14 C-H / C-Ni 1.12 / 2.18, 2.19

CH2 1 -4.07 C-H

C-Ni

1.11, 1.17

1.95, 1.96, 2.12,

CH 1 -6.13 C-H / C-Ni 1.11 / 1.90

CO 1 -1.97 C-O

C-Ni

1.20

2.02, 2.02, 2.03

CO2 2 -0.05 C-O1 / C-O2

C-Ni

1.31/1.23 2.0

COOH 2 -2.27 C-O / O-H 1.22 / 1.02

COH 1 -4.27 O-H

C-Ni

1.01

1.90, 1.92, 1.94

CHO 1 -2.66 C-O / C-H 1.31 / 1.12

For the diatomic species, the most preferable adsorption site for OH and CO was found to be

Ni ‘1’ hollow site with Eads of -3.63 eV and -1.97 eV respectively. OH also adsorbs on the P

and Ni –P sites (5 and 6) with lower Eads. For CO adsorption only the Ni sites (1-3) are

preferred with physisorption on the P site at -0.25 eV (Eads). H2 preferably adsorbs on the Ni

’3’ site with Eads -0.71 eV and dissociates on the Ni ‘1’ site, which is found similarly

10

occurring on Ni (111) planes. Overall among the diatomic species OH was found to have

moderate binding on Ni-P and P sites whereas CO and H2 prefers the Ni sites.

The adsorption data of the two main reactants in DRM, CO2 and CH4 adsorbs onto the Ni2P

(0001) surface with low adsorption energies of -0.05 eV and -0.02 eV respectively. Where,

CO2 was found to chemisorb with a bent configuration from its gas phase linear form shown

in Figure 2 (l) on the Ni ‘2’ site, while the rest of the sites were unfavorable for CO 2

adsorption (Table S2). In comparison the CH4 molecule physisorbs on the Ni ’3’ site with one

hydrogen atom pointing towards the surface, while the Ni-P and P sites were not favorable

towards CH4 adsorption. The most preferential adsorption site for the CHx (x=0-3) species is

the Ni’1’ site, and it was found that adsorption energy increases with decreasing x:

EadsCH>Eads

CH2>EadsCH3. Additionally, the Ni-P ‘5’ bridge site shows binding with lower adsorption

energies towards the CHx species (Table S2).

Carboxyl (COOH) prefers to only adsorb on the Ni’3’ site with Eads of -2.27 eV while Ni-P

and P sites being ineffective towards adsorption. Again, for CHO and COH the Ni ‘1’site is

found to be the most stable with Eads of -2.66 eV and -4.67 eV respectively. However, COH

adsorbed with low adsorption energy on the Ni-P bridge site in comparison CHO did not

prefer the Ni-P bridge or P sites. Finally, H2O was found to be stable only on the Ni ‘3’ site

with Eads of -0.50 eV. From the adsorption energetics of the DRM species it may be noted that

the Ni sites, especially 1 and 3, showcased stronger binding ability with DRM species when

compared to other available sites on the Ni2P surface. However, the Ni-P bridge site also

showcases moderate stability in regards to binding characteristics to monoatomic DRM

species such as (H, O) and diatomic OH, which contributes as co-adsorption sites in the DRM

reaction steps. This observation also confirms that Ni-P and P sites may actively participate

in the reaction.

3.3 DRM reaction mechanism on Ni2P (0001)

There are a large number of reaction steps involved in a detailed DRM reaction network.

Which makes it difficult for computational screening of complicated catalyst surfaces.

However based on the findings of the main rate determining reaction steps from previous

theoretical studies on plane Ni surfaces [19-22] we have proposed a DRM reaction

mechanism scheme shown in Figure 3, to qualitatively ascertain the catalytic activity of the

Ni2P (0001) surface. The reaction mechanism scheme in Figure 3, firstly takes into account

the CH4 activation via sequential dissociation, CO2 activation through a direct or

11

hydrogenation route. Thereafter the CO formation is assessed through three routes, direct

oxidation of C by O, CH oxidation by O and C oxidation by OH. Additionally, OH

formation, H2 and H2O gas formation routes are also considered.

Figure 3: DRM reaction mechanism scheme considered in this work. The subtraction and

addition of hydrogen in green colour, oxygen and hydroxide in red colour is shown for

required reaction steps.

The details about the calculated values of the forward (Ea,f) and backward activation energies

(Ea,b), reaction energy (ΔEr) and bond formation or bond scission at transition state of the

reaction steps is reported in Table S3. The DFT calculated geometries for all the reaction

steps considered in our study comprising of initial (IS), transition (TS) and final (FS) states

are shown in Figures S1-S4 are shown in the Supporting Information.

3.3.1 CH4 activation (R1-R4)

R1: CH4* + *→CH3* + H*: The first sequential step of CH4 activation. The stable structure

of CH4 on site 3 is considered as the IS and co-adsorbed CH3 and H as FS, as shown in Figure

S1 (R1). The TS for the reaction occurs on top of a Ni atom where, the C-H bond stretches

from 1.12 Å in the IS to 1.73 Å during the TS. The calculated E a,f is 1.14 eV with the step

being endothermic 0.56 eV (ΔEr) .

12

R2: CH3* + *→CH2* + H*: In the next sequential step in Figure S1 (R2), CH3

dehydrogenates to CH2 and H with Ea,f of 1.00 eV. The reaction is found to be endothermic by

(ΔEr) 0.86 eV. The C-H bond length at the TS is 1.97 Å, which in the IS geometry is 1.12 Å.

R3: CH2* + *→CH* + H*: The C-H bond length of the IS structure of CH2 is 1.17Å which

stretches to 1.95Å during the TS, as shown in Figure S1 (R3). The endothermicity of the

reaction is considerably lower (ΔEr = 0.53 eV) compared to the previous steps, and Ea,f

required obtained for this step is 0.66 eV.

R4: CH* + *→C* + H*: The final sequential dissociation step is the most energy demanding

step with an Ea,f of 1.77 eV and (ΔEr) being highly endothermic 1.42 eV. The C-H bond

stretches from 1.11Å in the IS to 1.95Å during the TS, Figure S1 (R4). The crucial reaction

mechanism of methane activation is found to be mostly endothermic on the Ni2P (0001)

surface. However the final step showcases the difficulty of atomic carbon formation on the

surface due to high energy barriers, a fundamental feature of a promising reforming catalyst.

3.3.2 CO2 activation (R5-R7)

CO2 activation is typically a limiting step in catalytic CO2 utilisation processes. To study the

CO2 activation on Ni2P (0001) surface two reaction routes namely (i) direct dissociation (R5),

and (ii) CO2 hydrogenation (R6-R7) are considered in the present work. While the former

requires a single step to form CO and O (R6), the latter comprises of two reactions (R7) for

the formation of CO and OH.

R5: CO2* →CO* +O*: The direct dissociation route, shown in Figure S2 (R5) for CO2

activation, is the main source of oxygen formation on the Ni2P (0001) surface, similarly to Ni

monometallic surfaces [19-20]. The reaction occurs via C-O bond scission, where the C-O

bond distance increases from 1.31 Å in the IS to 2.31 Å during the TS. The reaction is

exothermic (ΔEr= -0.61 eV) with an Ea,f of 0.96 eV.

R6: CO2* + H*→COOH*: The CO2 hydrogenation occurs via the combination of carbon

dioxide and hydrogen to form a carboxyl intermediate species on the catalyst surface. On

Ni2P (0001) the formation of this carboxyl occurs with the decrease in the O-H bond distance

from 2.70 Å in the IS to 1.50 Å in the TS, as shown in Figure S2 (R6) indicating the key role

of OH species in the reaction mechanism. The (ΔEr) is less exothermic compared to the direct

dissociation route (R5) with (ΔEr = -0.11 eV) and an activation energy barrier (Ea,f ) of 1.19

eV.

13

R7: COOH* →CO* +OH*, dissociation of the carboxyl intermediate in Figure S2 (R7)

occurs easily with an (Ea,f ) of 0.23 eV and an exothermic reaction energy (Δ Er = - 0.51 eV).

In the dissociation geometry shown in Figure S2 (R7), the C-O bond length of 1.44 Å for the

carboxyl molecule shown in the IS geometry stretches to 2.59 Å in the TS to form CO and

OH on the Ni2P (0001) surface.

3.3.3 C and CH oxidation (R8 – R12)

The main source for the formation of CH and C species on the Ni2P (0001) surface is through

the methane dehydrogenation reactions (R1-R4). In previous theoretical studies on mono-

metallic Ni and other transition metal surfaces, the oxidation reactions of the above

mentioned species influences the formation of CO on the catalysts surfaces both

thermodynamically and kinetically [19-23]. Therefore, in this section we have studied the

mechanism of C oxidation by O forming CO (R8), C oxidation by OH forming COH (R9)

and subsequent reaction of COH dissociation to CO and H (R10). Further the oxidation of

CH species with O is studied via reaction (R11) where CHO is formed, thereafter the

dissociation of CHO to CO and H (R12).

R8: C* + O*→CO* + *: In the IS configuration shown in Figure S3 (R8), the co-adsorbed

C and O is separated with a bond distance of 3.03 Å, which in the TS shortens to form CO

bond of 1.80 Å. The (Δ Er) is highly exothermic, with (Δ Er = -2.26 eV) and an Ea,f of 1.18

eV.

R9: C* + OH*→COH* + *: The IS geometry is similar to (R8) where, the co-adsorbed

species of C and OH (IS) are separated by distance of 3.04 Å, which in the TS, C-OH bond

distance shortens to 2.32 Å Figure S3 (R9). Again (Δ Er) is found to be highly exothermic

with (Δ Er = -1.54 eV) and Ea,f = 0.94 eV.

R10: COH* + *→ CO* + H* dissociation of COH occurs with the stretching of the O-H

bond from 1.01 Å in the IS to 1.36 Å in the TS, as shown in Figure S3 (R10). The reaction is

exothermic, with (Δ Er = -0.50 eV) and an Ea,f of 1.04 eV.

R11: CH* + O*→CHO* + *: In the IS geometry the C-O bond distance between the co-

adsorbed CH and O is 2.90 Å, which, during the formation of CHO, shortens to 1.73 Å in the

TS, as shown in Figure S3 (R11). The Ea,f required for the reaction is 1.34 eV and the reaction

is exothermic in nature (ΔEr = -0.51 eV).

14

R12: CHO* + *→CO* + H*: After the formation of CHO, it dissociate with an energy

barrier of 0.55 eV (Ea,f ) and the reaction is exothermic by -0.27 eV (ΔEr). CO and H

formation occurs with the stretching of the C-H bond from 1.12 Å in the initial state to 1.20 Å

TS and the shortening of the C-O bond from 1.31Å in the IS to 1.27Å TS, as shown in

Figure S3 (R12).

3.3.4 OH, H2O and H2 formation (R13 – R15)

R13: O* + H*→OH* + *, OH forms with the combination of the adsorbed atomic oxygen

and hydrogen species which requires a high Ea,f of 1.55 eV with the endothermic reaction

(ΔEr = 0.89 eV). In comparison, the formation of H2O (R14: OH* + H*→H2O* + *) is

exothermic with Δ Er = -0.06 eV and a much lower energy barrier (Ea,f = 0.98 eV). In the TS

configurations shown in Figure S4 (R13-14), for both reactions the O-H bond formation is

similar with a bond length of 1.61 Å and 1.64 Å respectively.

R15: H* + H *→H2*, atomic hydrogen formed during the sequential dissociation steps (R1-

R4) contributes towards the process of forming molecular hydrogen on the Ni2P (0001)

surface, which is easier with an Ea,f of 0.36 eV and the corresponding slightly endothermic

reaction energy (ΔEr = 0.22 eV). The TS of the reaction occurs on the top of a Ni atom where

the H-H bond distance is 1.26 Å, which decreases from 2.04 Å in the IS geometry shown in

Figure S4 (R15).

In summary, the above reported reaction energies and activation energies showcase the active

nature of the Ni2P (0001) surface towards the DRM reaction steps. The catalyst activity and

the reaction pathways are examined using free energy analysis in the next section.

3.4 Free Energy Analysis

In the previous section we have developed a mechanistic understanding of the DRM reaction

steps on the Ni2P (0001) surface using DFT calculations. Building on this knowledge, in this

section we have further assessed the thermodynamic characteristics of the DRM reaction on

Ni2P (0001) with the help of free energy (ΔG) barrier analysis. It is worth noting that the free

energy profiles are calculated assuming the reaction steps are sequential rather than

simultaneous. The temperature and pressure considered for the free energy analysis is 1000 K

and 1 bar respectively, similar to previous theoretical studies of DRM [20, 23]. Additionally

the formation energies and values obtained for (ΔG) of each reaction step are provided in

Table S4-5.

15

Figure 4. Free energy profile diagrams for (a) CH4 sequential dissociation (b) CO2 direct and

H2 assisted activation paths (c) Reverse water gas shift (RWGS) reaction and (d) DRM

reaction paths for syngas formation on Ni2P (0001) surface at 1000 K and 1 bar.

16

The CH4 activation on Ni catalyst has previously been shown to influence the DRM turnover

rates greatly, and simultaneously remaining unaffected by the co-reactant (CO2)

concentration [21, 45]. Herein, Figure 4 (a) shows the CH4 activation on Ni2P (0001) surface,

where the first (CH4 (g) →CH3* + H*) and the final (CH* → C*+ H*) sequential steps are the

most energy demanding with (ΔG) of 2.43 eV and 1.67 eV respectively. Figure 4 (b) shows

the two reaction paths for CO2 activation on the Ni2P (0001) surface, where

thermodynamically and energetically the direct dissociation route is favored over the

hydrogen assisted route. The free energy barrier for the direct route is ΔG = 2.26 eV,

compared to carboxyl (COOH) reaction of ΔG = 3.32 eV. Interestingly, the bond cleavage of

the carboxyl intermediate to form CO and OH occurs at a much lower energy of ΔG = 0.14

eV. Different from our observation with Ni2P, the direct CO2 decomposition path has been

found to be favorable on closed packed Ni (111) surface while CO2 hydrogenation path is

more favored on noble metal surfaces especially on Pd (111) and Pt (111) surfaces [22,47].

We have also considered the RWGS reaction in Figure 4(c), which is a competing reaction in

DRM and occurs due to the hydrogenation of CO2 to produce CO and H2O gas [22]. RWGS

on Ni2P (0001) surface occurs through the direct CO2 dissociation route with the free energy

barrier for H2O formation by combination of OH and H on the surface being less than the CO

formation steps in both routes. This may affect the H2/CO ratio on the Ni2P due to the

hydroxyl (OH) and available surface hydrogen (H) combination during the RWGS reaction.

We have considered three reaction routes for CO formation on Ni2P (0001) surface shown in

the free energy profiles in Figure 4(d): (i) C oxidation by O, (ii) C oxidation by OH, and (iii)

CH oxidation by O. Here in the free energy profile, CO2 gets activated before CH4 owing to

the lower energy barrier. Among the considered routes, the CH oxidation route is found to be

most favorable for CO formation compared to C oxidation route by O and OH. This route has

been found to be favorable on Ni surfaces in previous studies [19-20, 22-23].

3.5. Microkinetic Analysis

To understand the reactivity of the Ni2P (0001) surface, we have used mean field steady state

micro-kinetic analysis to predict the catalytic trends in DRM. For our microkinetic model, a

total of 17 elementary DRM reactions steps (R1-R17) were used, as shown in Supporting

Information (S2), which considers the activation steps for the main gas phase reactants (CH4

and CO2 ), syngas (CO and H2) formation pathways, and H2O and H2 formation steps.

17

The CatMAP [34] code is used to perform all microkinetic calculations in this work. In the

microkinetic model, all the DRM species adsorbed on the Ni2P (0001) surface compete for a

free site (*_s) except for hydrogen, which occupies a special site (*_h), known as the

‘hydrogen reservoir site’. The consideration of a special site for hydrogen arises due to the

smaller size scale of hydrogen compared to other adsorbates, which enables it to adsorb

during high coverages of other adsorbates [48]. From this definition it may be considered that

hydrogen adsorbs onto a separate lattice [48-49]. In our calculations the coverage of all

adsorbates on Ni2P (0001) surface and hydrogen sites are constrained to 1.

The reactant gas feed ratio considered for the kinetic model used in our calculations is

CH4:CO2 =1, which is typical in DRM [12, 22], and the product (H2 and CO) gas

concentrations were taken as 10 % of CH4 conversion. Here, we have not considered the

dependence of products concentration on CO2 conversion, as Wei and Iglesia in their kinetic

and isotopic study on Ni/MgO catalyst found the forward turn-over rates for the DRM

reactants to depend only upon the CH4 partial pressures [50]. Further a low CH4 conversion

of (10% ) was considered to provide kinetic control for the reaction as well as to avoid

reverse methanation reaction on the Ni2P (0001) surface, which occurs due to higher H2

partial pressures on Ni based catalysts [23,51]. Further, formation energies of the adsorbates

and gas phase species were calculated relative to gas phase energies of CH4, H2O and H2.

18

Figure 5. (a) Production rate in TOF (s-1) of CO, H2 and H2O gas production on Ni2P (0001)

surface. (b) Coverage of major species on the Ni2P (0001) surface, and (c) Coverage of minor

species on the Ni2P (0001) surface. All the micro kinetic calculations are performed at a

temperature range of 900- 1300 K with a constant pressure of 1 bar.

For the microkinetic calculations a temperature range of 900 – 1300 K is considered with

pressure kept constant at 1 bar. Figure 5 (a) shows the production rate in (TOF s-1) of CO, H2

and H2O gases on the Ni2P (0001) where the rates for CO and H2O gas production are

observed at lower temperatures of 900 -1050 K. However the rates for H2 gas is not observed

at this temperature range and requires higher temperatures (above 1050 K). The H2/CO ratio

obtained from our calculations from 1100 -1300 K is in the range of 0.68 -0.86, whereas the

H2/CO ratio is low (~0.63) below 1100 K. From these calculated values it may be considered

19

that high temperatures will be required to achieve considerable levels of conversion (and high

quality syngas) on Ni2P (0001) surface. This is in line with what was observed during

catalytic activity tests on molybdenum phosphide (MoP) in DRM where the reactant and

product conversion is greatly dependent on temperature [9]. One reason may be due to the

endothermic nature of the CH4 sequential dissociation mechanism on Ni2P (0001), shown in

the free energy barriers. As this step is the main source for hydrogen and carbon species, a

delayed CH4 activation at low temperature slows down H2 gas production rate, thus giving a

low H2/CO ratio. Another reason may be due to the reverse water gas shift (RWGS) reaction,

which has been seen to affect H2/CO rates even at low CH4 conversions (~15%) on Ni (111)

surface, and was found to reach equilibrium without kinetic limitation [22]. Similar to the Ni

(111) study [22], minor amounts of H2O gas production (<10-2) is observed in our

calculations, confirming the influence of the RWGS.

Coverage of the adsorbed species on the Ni2P (0001) surface in the micro-kinetic model is

divided into two categories – major coverage as shown in Figure 5 (b), and minor coverage as

shown in Figure 5 (c). From Figure 5 (b), the major coverage on the Ni2P (0001) surface is

that of CO and H which gradually decreases with increasing temperature, and participates in

the formation of CO and H2 gas kinetically. In addition, CH maintains a relatively uniform

coverage of (~10-2 ML) in the calculated temperature range. Among minor coverage species,

the carbon concertation is lower compared to the oxygen concentration, indicating the

possibility of less coking on the Ni2P (0001) surface during DRM. Furthermore, the oxygen

concentration increases with increasing temperature. The higher oxygen concentration is

consistent with an experimental XPS study performed by Liu et al. [17], where it was found

that oxy phosphides were formed on the Ni2P (001) surface under WGSR conditions [17]. In

other words, P act as an oxygen sink due to its ability to interact with oxygen, this is very

helpful in DRM since an oxygen-rich surface is less prone to coking. Also the preferential

interaction of O with P (P-O) maintains O far from Ni atoms to avoid their oxidation. This is

again another advantage of the Ni2P catalyst since Ni must remain reduced (metallic) to

achieve high performance in reforming reactions.

3.6 Coking Stability

It has been well established previously that nickel-based catalysts are prone to coking during

DRM reactions [21-22]. The mechanisms mainly responsible for coking on Ni catalysts are

essentially driven by the reaction steps of (i) sequential dissociation of CH4 to atomic C and H

20

(R1-R4) and (ii) CO dissociation to C and O (R8), where carbon atoms are considered to be

precursors to carbon growth on the catalyst surface [22, 52].

Figure 6: Partial density of states (PDOS) (a) C adsorbed on Ni site ‘1’ (b) O adsorbed on

Ni site ‘1’ and (c) O adsorbed on Ni-P site ‘5’.

In our DFT adsorption energy analysis of the DRM species on Ni2P (0001), we found atomic

carbon to have the strongest adsorption energy (Eads) with the surface, followed closely by

atomic oxygen (see Table 1). This is also shown in Figure 6 (a-b) where the p orbitals of C

has stronger overlap with the d orbitals of a Ni atom below the fermi level (E F) on Ni2P

(0001) at the three fold hollow Ni ‘1’ site. In comparison, the p orbitals of an O atom

21

adsorbed on the same surface site also overlaps with the d orbital of the Ni atom, as shown in

Figure 6 (b), but to a lesser extent. Hence C is more stable than O on Ni ‘1’ site. However

atomic O is also found to be stable on the Ni-P bridge site ‘5’ where the p orbitals of O

overlaps with the p orbitals of P and Ni, as shown in Figure 6 (c). Additionally the

overlapping of the p orbital of O with that of P shown in Figure 6 (c), is much stronger and is

in good agreement with previous observation by Liu et al. [17]. In our DFT calculation we

have found C to be unstable on the Ni-P ‘5’ and P ‘6’ sites. Essentially, on Ni2P (0001)

surface, the concentration of P atoms decreases the availability of stable three fold Ni site ‘1’

for C species to adsorb, which may restrict carbon growth. This was also found in our

previous work on bimetallic Ni-Sn, where the inclusion of Sn (tin) to closed packed Ni (111)

outermost plane made C adsorption unstable on three fold (fcc and hcp) sites [53]. Therefore

from this analysis it may be considered the available stable adsorption sites for C formation

are much lower than those for O adsorption on Ni2P (0001) surface. The adsorbed oxygen on

the Ni-P site ’5’ will propagate carbon oxidation during the DRM reaction, hence hindering

carbon growth to a larger extent on Ni2P (0001).

Furthermore, to analyze the mobility of the C atoms on Ni2P (0001), we performed DFT

calculations for carbon atom surface diffusion on Ni2P (0001) and have used Ni (111) surface

for comparison. Figure S5 shows carbon diffusion from one stable adsorption site ‘1’ to a less

stable site ‘4’ on Ni2P (0001). Comparing the C atom diffusion on both Ni2P (0001) and Ni

(111) surfaces, as shown in Table S6 and Figure S5, the hollow adsorption site is preferred

for C adsorption on hcp for Ni (111) and Ni ‘1’ for Ni2P (0001). Carbon diffusion on Ni2P

(0001) is found to be more energy demanding with (Ea,f ) being 1.34 eV compared to 0.31 eV

on Ni (111). This analysis shows the mobility of C atoms will be difficult and C will be

constrained to the hollow site ‘1’ on Ni2P (0001).

In regards to carbon stability on Ni2P (0001), our microkinetic calculations showcased

reduced carbon coverage but increased oxygen coverage with the increase of temperature, as

shown in Figure 5 (c). The increased O coverage at higher reaction temperature. This shows

that the probability of carbon oxidation (C-O) to be more compared to the C-C bond

formation on Ni2P (0001). Additionally, CO is found to be the most abundant species at DRM

reaction conditions on Ni2P (0001), as shown in Figure 5 (b). The probability of carbon

formation from the dissociation of CO is less, due to its high backward activation energy

barrier (Ea,b) of 3.44 eV (Table S3), found in our DFT calculations. Overall it may be

considered that Ni2P (0001) catalyst surface with Ni3P2 plane to be active and resistant to

22

carbon formation, with longer catalyst life span in DRM when compared to traditional Ni

plane monometallic surfaces.

4. Conclusions

The theoretical analysis performed in this work to examine the potentiality of Ni2P (0001) for

DRM provided some novel and interesting insights. The positive feature of the Ni3P2

terminated Ni2P (0001) surface was found to be active towards most of the DRM species.

Where the main reactants CH4 is found to physisorb while CO2 chemisorbs on the surface

with C and O atoms adsorbing strongly to the Ni2P (0001) surface. Furthermore, Ni-P bridge

sites contributed as co–adsorption sites for DRM reaction mechanisms. The proposed DRM

reaction mechanisms on Ni2P (0001) was studied using free energy analysis at 1000 K and 1

bar, where the CH4 activation step (CH4(g)→CH3* + H*) and CH cracking step (CH* →C* +

H*) was found to require higher energy barriers. Furthermore, free energy analysis also

showed that CO2 activation prefer the direct decomposition route instead of a sequential

hydrogenation route. Also, the preferred route for CO formation on Ni2P (0001) is via the

CH-O route.

The steady state micro-kinetic analysis was performed in the temperature range of 900-1300

K at 1 bar with 10% CH4 conversion, where TOF for CO gas production occurred at

temperatures below 1000 K and H2 gas production was visible only at temperatures over

1100 K. The kinetic studies also indicated RWGS may occur on Ni2P (0001) with evidence of

H2O (gas) production at lower DRM temperature range. The micro-kinetic calculations also

showed that the O coverage was higher compared to C on Ni2P (0001) and increased with

temperature. The higher O coverage will inhibit C nucleation and may propagate C oxidation

on the surface. Furthermore, the diffusion of C atoms, which are precursors to carbon

nucleation, is greatly deterred on Ni2P (0001) compared to Ni (111) plane surface. This was

obtained from our DFT study, where much higher activation energy barrier for C diffusion

was required on Ni2P (0001) compared to Ni (111). In summary, a higher probability of C

oxidation, lower C diffusion and limited availability of C preferred three fold Ni sites on Ni2P

(0001) surface, indicate long term stability of Ni2P (0001) surface under DRM reaction

conditions.

Overall, from the theoretical analysis performed, we can conclude that the Ni2P (0001)

surface conserves the active characteristics of Ni surfaces towards the DRM reaction, but

with increased resistance towards carbon formation in the relevant DRM temperature

23

window. Hence, we believe that the Ni2P (0001) surface is a promising catalyst that should be

further studied with experimental investigations to validate its catalytic performance for

realistic applications for DRM and CO2 conversion units.

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

Details of the convergence test for k point and adsorption energies of the DRM species on all

possible sites on the Ni2P (0001) surface. Theoretical background on microkinetic modelling

using CatMAP module. Elementary reaction steps and CatMAP input file used for

microkinetic study. Values obtained for the free energy barriers (ΔG) for the considered

reaction steps. Finally, the transition state figures for the DFT study of carbon diffusion on

Ni2P (0001) and Ni (111) surfaces.

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