Surface Science 614 (2013) 53–63

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Surface Science

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A first-principles study of CO hydrogenation into methane on molybdenumcarbides catalysts

Ke-Zhen Qi a, Gui-Chang Wang a,b,⁎, Wen-Jun Zheng a,⁎⁎a Department of Chemistry and the Tianjin Key Lab of Metal and Molecule-based Material Chemistry, Nankai University, Tianjin 300071, PR Chinab College of Chemistry and Chemical Engineering, Shanxi Datong University, Datong 037009, Shanxi Province, PR China

⁎ Correspondence to: G.-C. Wang, Department of Cheof Metal and Molecule-based Material Chemistry, NankPR China.⁎⁎ Corresponding author.

E-mail addresses: wangguichang@nankai.edu.cn (G.zhwj@nankai.edu.cn (W.-J. Zheng).

0039-6028/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.susc.2013.04.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 31 January 2013Accepted 1 April 2013Available online 6 April 2013

Keywords:CO hydrogenationMethaneMolybdenum carbideDensity functional calculations

The reaction mechanisms for the CO hydrogenation to produce CH4 on both fcc-Mo2C (100) and hcp-Mo2C(101) surfaces are investigated using density functional theory calculations with the periodic slab model.Through systematic calculations for the mechanisms of the CO hydrogenation on the two surfaces,we found that the reaction mechanisms are the same on both fcc and hcp Mo2C catalysts, that is,CO → HCO → H2CO → H2COH → CH2 → CH3 → CH4. The activation energy of the rate-determining step(CH3 + H → CH4) on fcc-Mo2C (100) (0.84 eV) is lower than that on hcp-Mo2C (101) (1.20 eV), and thatis why catalytic activity of fcc-Mo2C is higher than hcp-Mo2C for CO hydrogenation. Our calculated resultsare consistent with the experimental observations. The activity difference of these two surfaces mainlycomes from the co-adsorption energy difference between initial state (IS) and transition state (TS), thatis, the co-adsorption energy difference between IS and TS is −0.04 eV on fcc Mo2C (100), while it is ashigh as 0.68 eV on hcp Mo2C (101), and thus leading to the lower activation barrier for the reaction ofCH3 + H → CH4 on fcc-Mo2C (100) compared to that of hcp-Mo2C (101).

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Molybdenum carbide (Mo2C) has exhibited excellent catalytic prop-erties similar to those of more expensive noble metals, and has beenwidely used in hydrodesulfurization (HDS) and hydrodenitrogenation(HDN) processes and also exhibits high activity formethanation, alkaneisomerization, and CO hydrogenation [1–5]. In particular, CO hydroge-nation on Mo2C catalysts has been studied extensively because of thetolerance of these catalysts to sulfur poisoning. There are two maincrystalline structures Mo2C, i.e., the orthorhombic phase (fcc-Mo2C)and the hexagonal phase (hcp-Mo2C). Ranhotra and Reimer [6] identi-fied the main product of CO hydrogenation on the two phase (fccand hcp) Mo2C catalysts that is methane, interestingly, the activityof fcc-Mo2C for CO hydrogenation is twofold higher than that ofhcp-Mo2C. Patterson at el. [7] demonstrated that methane and CO2

were found to be the main products of CO hydrogenation overfcc-Mo2C catalyst. Christensen at el. [8] reported that the use of thebulk Mo2C as catalyst is the effective conversion of syngas (CO andH2) into higher alcohols. Furthermore, other workers also studied theCO hydrogenation over unsupported and supported Mo2C catalysts.

mistry and the Tianjin Key Labai University, Tianjin 300071,

-C. Wang),

rights reserved.

Lee at el. [9,10] studied the CO hydrogenation on alkali metal modifiedMo2C, the products are C1–5 alkanes without the assistance of potas-sium, in which methane is the main product. On potassium modifiedsurface, mixture of lower alcohols, especially the selectivity towardC2–7 alcohol is significantly improved. Woo et al. [11] and Xiang etal. [12] reported that the synthesis for mixed alcohol from CO hydro-genation on Mo2C produced hydrocarbons and by adding potassiumas a promoter, the selectivity shifts from hydrocarbons to alcohols,especially methanol and ethanol. Although the interactions of CO andHonMoC2werewell-studied [13–15] and there aremanyexperimentalinvestigations that focus on hydrogenation reactions from syngas onMo2C surfaces that has been done, [16,17] there are very few systematictheoretical studies on CO hydrogenation mechanism over Mo2C sur-faces that have been reported, [18] CO hydrogenation mechanism onMo2C catalysts is still unclear.

Identifying the mechanism of CO hydrogenation is essential for thedesign of better transition metal carbides catalyst that they mightbe used as a cheaper substitute for noble metal catalysts. AlthoughFischer–Tropsch synthesis has been widely studied since 1923, [19]there is still controversy about the CO hydrogenation mechanism,which involves formation and scission of C\H, O\H, and C\Obonds. The hydrogenation mechanism is rather complicated and vari-ous reaction schemes have been proposed. For instance, using densityfunctional theory (DFT) methods, Choi and Liu [20] investigated thereaction of CO with H on Rh (111) and proposed that the reactionpathway is CO hydrogenation to form CH3O, then C\O bond of CH3Obreaks, and finally forms CH4 by CH3 hydrogenation. Huang and Cho

54 K.-Z. Qi et al. / Surface Science 614 (2013) 53–63

[21] calculated CO hydrogenation on MoS2 surface and found that COhydrogenation firstly produces CHxO, followed by C\O bond breakingand CH4 formation. On the non-stoichiometric MoSx surfaces, Shiet al. [22] simulated CO hydrogenation on Mo-termination with 42%S coverage and S-termination with 50% S coverage, and identifiedCH2OH as the prior intermediate for C\O bond breaking and CH4

formation.On the basis of above discussed reaction mechanisms, all interme-

diates and the elementary steps of CH4 formation from CO hydrogena-tion on periodic slab models of hcp and fcc Mo2C has been calculatedby DFT methods. Based on the calculated results, we intend to answerthe following questions: (1) What is the order for CO hydrogenationprocess on the two phase Mo2C surfaces? (2) When is the C\O bondbreaking of CHxOy on the two surfaces? (3) Why does fcc-Mo2C havethe higher catalytic activity for CO hydrogenation than hcp-Mo2C?This paper is organized as follows: Section 2 describes model geome-tries and computational methods while Section 3 presents the calcu-lated results and discussion. Finally, we summarize our conclusionsin Section 4.

2. Computational method and models

The DFT calculations are performed by using the Vienna Ab initioSimulation Package (VASP) [23,24]. The exchange-correlation ener-gy and potential are described by generalized gradient approxima-tion (PW91) [25]. The electron–ion interaction is described by theprojector-augmentedwave (PAW)method, [26,27] and the electronicwave functions are expanded by plane waves up to a kinetic energyof 350 eV. The surface Brillouin zone is sampled using a 4 × 4 × 1Monkhorst–Pack mesh [28]. Our DFT-optimized lattice constantsof bulk fcc-Mo2C (a = 6.022 Å, b = 4.725 Å, and c = 5.195 Å) andhcp-Mo2C (a = 3.003 Å, b = 3.003 Å and c = 4.729 Å) agree wellwith the experimental value [29,30]. The selectivity of surface modelis based on the fact that the main dominate planes making up the sur-face of Mo2C (fcc) and Mo2C (hcp) are (100) and (101) respectively(see Fig. 1) [31]. Recent theoretical results verified the configurationwith aMo\C\Mo\C stackingmode to be themost stable [32]. There-fore, our focus is the Mo-terminated surface and the C-terminatedsurface is not considered in this work. The two surface models are

Fig. 1. Top view and side view of (a) fcc-Mo2C (100) and (b) hcp-Mo2C (101) periodicslab models, the blue balls for Mo atoms and the gray ones for C atoms respectively.

built by a four-layer Mo2C slab. The calculated models are chosen asthe unit cells of 2 × 2 with the corresponding coverage of 1/4 mono-layer (ML). During the calculation, the uppermost two layers and theadsorbate are allowed to relax. The slabs are separated by a 15 Åvacuum layer. The molecules in the gas phase have been calculatedusing a 15 × 15 × 15 Å3 cubic unit cell. Spin-polarized calculationswere performed when needed.

Adsorption energy (Eads) and activation energy (Ea) are calculatedby the following formulas: Eads(A) = EA/M − EM − EA and Ea =ETS − EIS, where EA/M, EM, EA, ETS and EIS represent the energies ofthe adsorption system, substrate, adsorbate, transition state (TS)and initial state (IS), respectively. The activation energy is calculatedbased on using the energy of co-adsorptions at IS. The nudged elasticband (NEB) method has been employed to locate the likely TS [33].Secondly, the TS structures are relaxed using a quasi-Newton algo-rithm to make the forces on the atoms less than 0.035 eV/Å. Finally,the frequency analysis is performed for a confirmation of the TS.Then we continued to include the zero point energy (ZPE) into theactivation energy, it can be calculated by ∑

i1=2ð Þhvi, where vi is the

computed real frequencies of the system.

3. Results

3.1. Adsorption properties of CO and possible reaction intermediates

In this section, we present a detailed investigation of stableadsorption of all possible intermediates involved in the processes ofCO hydrogenation on fcc and hcp Mo2C surfaces. The correspondingconfigurations are shown in Fig. 2, and the geometric and energeticinformation is given in Table 1.

3.1.1. Carbon monoxide (CO)CO prefers to adsorb at the near top site through its C atom on

fcc-Mo2C (100), with the C\Mo bond length of 2.24 Å, and theadsorption energy is −2.37 eV. The C\O bond length elongatesfrom 1.14 Å in free CO (experimental value is 1.13 Å [34]) to 1.22 Å,resulting in CO being more reactive and easily dissociated. It wasfound that the C\O bond is tilted by 54.1° to the surface normal. Onhcp-Mo2C (101), CO adsorbs at the bridge site, with the adsorptionenergy of −1.99 eV, which agrees well with the results obtained byLiu et al. [35] as well as Wang et al. [36]. The shortest C\Mo bondlength is 2.03 Å. The bond length of C\O is elongated to 1.27 Å, andthe C\O bond is tilted by 46.8° to the surface normal.

3.1.2. Hydrogen (H)It is found that the most stable adsorption site for the H atom

on fcc-Mo2C (100) is the hollow site, with the adsorption energy of−3.44 eV, with a height of 1.13 Å. On hcp-Mo2C (101), the H atomadsorbs at the step bridge site, with a height of 0.15 Å. The adsorptionenergy is −3.21 eV, which is similar to that −3.32 eV on hcp-Mo2C(001) obtained by Kitchin et al. [37].

3.1.3. Carbon (C) and oxygen (O)C and O are stably adsorbed at the hollow site of fcc-Mo2C (100),

with the adsorption energy of −7.54 and −7.79 eV, respectively.On hcp-Mo2C (101), C and O both prefer to adsorb at bridge sitewith the adsorption energy of −7.64 and −7.49 eV.

3.1.4. Formyl (HCO) and hydroxy methylidyne (COH)HCO binds to fcc-Mo2C (100) through its C atom at the top site,

with the C\Mo bond length of 2.26 Å, and the adsorption energy is−3.63 eV. The C\O bond is tilted by 59.2° to the surface normal.COH adsorbs at the top site of fcc-Mo2C (100), with the C\Mo bondlength of 2.06 Å, and the adsorption energy is −4.76 eV. The C\Obond is nearly vertical to the surface. On hcp-Mo2C (101), HCO pre-fers to adsorb at bridge site through the O and C atoms, forming the

CO HCO COH H2CO HCOH H2COH O OH

H3CO H3COH CH2 CH3 CH4 H C CH

hcp-Mo2C (101)

CO HCO COH H2CO HCOH H2COH O OH

H3CO H3COH CH2 CH3 CH4 H C CH

fcc-Mo2C (100)

Fig. 2. Side view of the most stable adsorption configurations for intermediates involved in the CO hydrogenation reaction on hcp-Mo2C (101) and fcc-Mo2C (100). The blue,gray, red and white balls represent Mo, C, O and H atoms, respectively.

55K.-Z. Qi et al. / Surface Science 614 (2013) 53–63

C\Mo and O\Mo bonds, the bond lengths are 2.11 and 2.06 Å, re-spectively. The calculated adsorption energy is −3.39 eV. The C\Obond tilted by 69.7° to the surface normal and further tilted towardthe surface, resulting in a shorter length of O\Mo bond. This maybe due to the fact that hcp-Mo2C (101) is more open and corrugatedthan fcc-Mo2C (100). COH also prefers vertical adsorption at bridgesite of hcp-Mo2C (101), with the shortest C\Mo bond length of2.08 Å. The adsorption energy is −4.69 eV.

3.1.5. Formaldehyde (H2CO) and hydroxy methylene (HCOH)Similar to HCO, H2CO also adsorbs at a near top site through its

C atom on fcc-Mo2C (100), with adsorption energy of −2.52 eV.The C\O bond in adsorbed H2CO has a tilting angle of 79.4° to the

Table 1The calculated adsorption energy (eV) and the optimized structure parameters (Å and °) offcc-Mo2C (100) surfaces.

Species fcc-Mo2C (100)

Sites Eads ∠tilta RC\Mob RO\Mo

b RC\

H Hollow −3.44 (−3.52) – – – –

O Hollow −7.79 (−7.92) – – 1.92 –

OH Hollow −5.43 (−5.52) – – 2.11 –

C Hollow −7.54 (−7.61) – 1.93 – –

CO Top −2.37 (−2.48) 54.1 2.24 – 1.2HCO Top −3.63 (−3.74) 59.2 2.26 2.19 1.2COH Top −4.76 (−4.88) 2.7 2.06 – 1.3H2CO Top −2.52 (−2.63) 79.4 2.41 1.98 1.3HCOH Hollow −5.23 (−5.35) 65.8 2.18 2.34 1.4H2COH Hollow −2.98 (−3.09) 78.7 2.55 2.20 1.4H3CO Top −4.45 (−4.52) 42.2 – 1.95 1.4H3COH Top −0.61 (−0.72) 72.3 2.44 2.56 1.4CH Hollow −7.24 (−7.29) – 2.02 – –

CH2 Top −5.77 (−5.81) – 2.22 – –

CH3 Hollow −3.55 (−3.58) – 1.75 – –

CH4 Bridge −0.15 (−0.17) – 2.66 – –

Note: Adsorption energies with ZPE correction are given in parenthesis.a ∠tilt means the angle of the C\O bond with respect to the normal of surface.b RC–Mo and RO–Mo denote the shortest C\Mo and O\Mo distance, respectively.c RC–O denotes the C\O distance.

surface normal, so that the O atom is at the near top of Mo atom.The O\Mo bond length is 1.98 Å. HCOH prefers to adsorb at thenear hollow site on fcc-Mo2C (100), with the bond length of 2.18 Å.The adsorption energy is calculated to be−5.23 eV. In this configura-tion, the C and Mo atoms can form a σ-type C\Mo bond between thesp2 (C) hybridized and 4dz2 (Mo) orbitals. The C\O bond is tilted by65.8° from the surface normal. On hcp-Mo2C (101), H2CO adsorbs at abridge site, with the adsorption energy of−2.30 eV. It was found thatthe C\O bond tilted by 78.9° to the surface normal. The O\Mo bondlength is 2.02 Å and C\Mo is 2.25 Å. HCOH prefers to adsorb atbridge site of hcp-Mo2C (101), with the shortest C\Mo bond lengthsof 2.12 Å. The C\O bond is inclined by 85.3° to the surface normal.The adsorption energy is −4.62 eV.

adsorption species involved in the CO hydrogenation reaction on hcp-Mo2C (101) and

hcp-Mo2C (101)

Oc Sites Eads ∠tilt RC\Mo RO\Mo RC\O

Top −3.21 (−3.26) – – – –

Bridge −7.49 (−7.61) – – 2.04 –

Bridge −4.93 (−5.02) – – 2.15 –

Bridge −7.64 (−7.71) – 2.00 – –

2 Bridge −1.99 (−2.09) 46.8 2.03 – 1.277 Bridge −3.39 (−3.49) 69.7 2.11 2.06 1.325 Bridge −4.69 (−4.79) 3.0 2.08 – 1.378 Bridge −2.30 (−2.40) 78.9 2.25 2.02 1.373 Bridge −4.62 (−4.72) 85.3 2.12 – 1.468 Bridge −2.90 (−2.99) 89.7 2.22 2.18 1.483 Bridge −4.21 (−4.30) 22.3 2.13 2.14 1.445 Bridge −0.60 (−0.68) 88.7 2.43 2.36 1.45

Bridge −7.07 (−7.11) – 2.06 – –

Bridge −5.46 (−5.49) – 2.04 – –

Bridge −3.27 (−3.29) – 1.74 – –

Top −0.10 (−0.11) – 2.58 – –

56 K.-Z. Qi et al. / Surface Science 614 (2013) 53–63

3.1.6. Hydroxy methylene (H2COH), methoxy (H3CO), and methanol(H3COH)

H2COH adsorbs at the hollow site through it C atom on fcc-Mo2C(100), with the adsorption energy of−2.98 eV. The shortest distanceof C⋯Mo is 2.55 Å. The angle of the C\O bond to the surface normal is78.7°. H3CO adsorbs at the top site of Mo atom, with the O\Mo bondlength of 1.95 Å. The adsorption energy is −4.45 eV. The angle be-tween the C\O bond and the surface normal is 42.2°. H3COH prefersto adsorb at the near top site of fcc-Mo2C (100), with the adsorptionenergy of −0.61 eV. The C\O bond is inclined by 72.3° to the surfacenormal. This configuration is similar to the DFT results of methanoladsorbed on hcp-Mo2C (001) [38]. On hcp-Mo2C (101), H2COHbinds through its C atom and O atom at the bridge site. The bondlengths of C\Mo and O\Mo are 2.22 and 2.18 Å, respectively. TheC\O bond is almost parallel to the surface. The adsorption energy is−2.90 eV. H3CO is stably adsorbed at the bridge site on hcp-Mo2C(101) with the C\Mo bond length of 2.13 Å and O\Mo 2.14 Å. Theangle between the C\O bond and the surface normal is 22.3°. The ad-sorption energy of this configuration is −4.21 eV. H3COH adsorbs atthe bridge site on hcp-Mo2C (101), with the C\Mo distance 2.43 Å,and the C\O bond is nearly parallel to the surface. This relative longerC\Mo distance leads to a relatively weak interaction between themethyl and the surface, and the adsorption energy is −0.60 eV.

3.1.7. Hydroxy (OH)OH is stably adsorbed at the hollow site of fcc-Mo2C (100) with

the adsorption energy of −5.43 eV, and the shortest O\Mo bondlength is 2.11 Å. On hcp-Mo2C (101), OH adsorbs at bridge sitethrough the O atom in an upright configuration with the adsorptionenergy of −4.93 eV. The shortest O\Mo bond length is 2.15 Å.

3.1.8. Methyne (CH)For CH, it is likely to adsorb at the hollow site of fcc-Mo2C (100)

with the shortest C\Mo bond length of 2.02 Å, and the adsorptionenergy is −7.24 eV. On hcp-Mo2C (101), CH adsorbs at bridge sitewith the shortest O\Mo bond length of 2.06 Å. The adsorption energyis −7.07 eV.

3.1.9. Methylene (CH2)The CH2 intermediate adsorbs at the top site of fcc-Mo2C (100),

with the C\Mo bond length of 2.22 Å, and the adsorption energy is−5.77 eV. Differently, CH2 adsorbed at bridge site on hcp-Mo2C(101), with the adsorption energy of −5.46 eV. The bonding capabil-ity of CH2 to surface is very strong, which is consistent with the shortC\Mo bond lengths of 2.04 Å.

Table 2Energetic data (Ea and ΔH in eV), structure parameters (Rx − y in Å) and imaginary frequency(101) surfaces.

Reaction fcc-Mo2C (100)

Ea ΔH Rx − y

CO → C + O 1.23 −0.39 2.09CO + H → HCO 0.36 0.13 1.63CO + H → COH 1.10 0.87 2.22HCO → CH + O 0.35 −0.99 2.53HCO + H → H2CO 0.16 −0.56 1.68HCO + H → HCOH 1.08 0.06 1.45H2CO → CH2 + O 0.77 −0.52 2.35H2CO + H → H3CO 0.59 0.05 1.48H3CO → CH3 + O 1.12 −0.52 2.04H2CO + H → H2COH 0.45 −0.62 2.62H2COH → CH2 + OH 0.47 −0.67 2.01H2COH + H → H3COH 0.98 −0.54 1.39CH2 + H → CH3 0.42 −0.46 1.38CH3 + H → CH4 0.84 0.47 1.49CH3 + 2H → CH4 + H 0.89 0.49 1.34CH3 + H + O → CH4 + O 1.20 0.69 1.41

Note: Rx − y is the distance between two reactive atoms in TS.

3.1.10. Methyl (CH3)CH3 adsorbs at the hollow site of fcc-Mo2C (100) with the adsorp-

tion energy of −3.55 eV, and it is 1.75 Å from C to the nearest Moatom. The stable adsorption site agrees well with the result fromthe previous investigations [39,40]. However, on hcp-Mo2C (101),the CH3 is adsorbed at the bridge site, with the C\Mo distance of1.74 Å, and the adsorption energy is −3.27 eV.

3.1.11. Methane (CH4)The most stable adsorption site for CH4 on fcc-Mo2C (100) is at the

bridge site. The adsorption energy is −0.15 eV. This relative weakadsorption energy indicates that CH4 would desorb from the surfaceas soon as it was formed [41]. On hcp-Mo2C (101), CH4 adsorbs pref-erentially at the top site, with the adsorption energy of−0.10 eV. Thedistance between the C atom of CH4 and the nearest surface Mo atomof hcp-Mo2C (101) (2.58 Å) is shorter than that of fcc-Mo2C (100)(2.66 Å).

3.2. Reaction paths of CO hydrogenation at the Mo2C surface

After obtaining the preferred adsorption sites for all possiblespecies involved in the processes of CO hydrogenation, we explorethe details of reaction mechanism by activation energy calculations.The calculated activation barriers (Ea) and the reaction energy (ΔH)for every reaction steps on the two surfaces are listed in Table 2.The structures of IS, TS and final state (FS) for each reaction areshown in Figs. 3 and 4.

3.2.1. Reactions on hcp-Mo2C (101) surface

3.2.1.1. Initial step of CO hydrogenation. The hydrogenation step of COstarts from the formation of the O\H or C\H to initiate the catalyticcycle. To identify which one is the first step on hcp-Mo2C (101), weexamine the two reactions using NEB methods (CO + H → COHand CO + H → HCO). The co-adsorption configuration of CO and Hon hcp-Mo2C (101) is shown in Fig. 3a with CO located near the topsite and H atom located at the near bridge site. The calculated resultsshow that HCO formation is more favorable than COH formationboth kinetically (0.49 vs. 1.02 eV) and thermodynamically (0.01 vs.0.56 eV). Here, direct C\O bond breaking of CO on hcp-Mo2C (101)is also investigated. The activation barrier for the C\O breaking toform C and O is 1.62 eV, which is much higher than that of CO hydro-genation. Thus, these results indicate that HCO will be the first inter-mediate of CO hydrogenation. From the TS structure, we can see thatthe CO and H are adsorbed on the surface in a bridge site and off-top

(ν in cm−1) of transition states of CO hydrogenation on fcc-Mo2C (100) and hcp-Mo2C

hcp-Mo2C (101)

ν Ea ΔH Rx − y ν

453 1.62 −0.86 1.96 401483 0.49 0.01 1.46 583219 1.02 0.56 1.55 256405 0.55 −1.48 1.88 406729 0.19 −0.48 1.60 527

1393 0.35 −0.54 1.28 1681266 1.20 −0.98 2.01 399

1012 1.54 0.14 1.78 937589 0.61 −1.17 2.25 407960 1.05 −0.56 1.28 1287267 1.18 −1.13 2.01 481929 1.43 −0.02 1.36 720828 1.15 −0.37 1.73 934

1030 1.20 0.47 1.63 875952 1.05 0.30 1.38 908928 1.13 0.19 1.56 1183

(a) CO+H → HCO (b) CO+H → COH

(c) HCO → CH+O (d) HCO+H → H2CO

(e) HCO+H → HCOH (f) H2CO → CH2+O

(g) H2CO+H → H3CO (h) H3CO → CH3+O

(i) H2CO+H → H2COH (j) H2COH → CH2+OH

(k) H2COH+H → H3COH (l) CH2+H → CH3

(m) CH3+H → CH4 (n) CO → C+O

(o) CH3+H+O → CH4+O (p) CH3+2H → CH4+H

Fig. 3. The possible steps relevant to CO hydrogenation to methane on hcp-Mo2C (101). The structures to the left denote initial reactants (IS), those on the right refer to final state(FS), and the center structures sketch corresponding transition states (TS). The blue, gray, red and white balls represent Mo, C, O and H atoms, respectively.

57K.-Z. Qi et al. / Surface Science 614 (2013) 53–63

(a) CO+H → HCO (b) CO+H → COH

(c) HCO → CH+O (d) HCO+H → H2CO

(e) HCO+H → HCOH (f) H2CO → CH2+O

(g) H2CO+H → H3CO (h) H3CO → CH3+O

(i) H2CO+H → H2COH (j) H2COH → CH2+OH

(k) H2COH+H → H3COH (l) CH2+H → CH3

(m) CH3+H → CH4 (n) CO → C+O

(o) CH3+H+O → CH4+O (p) CH3+2H → CH4+H

Fig. 4. The possible steps relevant to CO hydrogenation to methane on fcc-Mo2C (100). The structures to the left denote initial reactants (IS), those on the right refer to final state(FS), and the center structures sketch corresponding transition states (TS). The blue, gray, red and white balls represent Mo, C, O and H atoms, respectively.

58 K.-Z. Qi et al. / Surface Science 614 (2013) 53–63

59K.-Z. Qi et al. / Surface Science 614 (2013) 53–63

site, and the C⋯H distance is 1.46 Å. The TS is characterized by animaginary frequency of 583 cm−1.

3.2.1.2. CHO species related reactions. After the formation of CHO spe-cies, it can further react with the H atom to form surface specieslike H2CO, HCOH, or dissociate to yield CH and O. The co-adsorptionconfiguration of HCO and H is shown in Fig. 3d with a hydrogenatom located at the top site of subsurface Mo atom and HCO atthe bridge site. The HCO may react with the H atom to form surfacespecies H2CO or HCOH. Here both H2CO and HCOH formation arefavorable thermodynamically (−0.48 vs. −0.54 eV) where the H2COformation is a little more kinetically favorable (0.19 vs. 0.35 eV). Fordissociation of HCO to yield CH and O, our calculations show it is exo-thermic by 1.48 eV with an activation barrier of 0.55 eV. Obviously,these two hydrogenation reactions are both easier than dissociationof adsorbed HCO. Thus the second intermediate for continued COhydrogenation is H2CO. At the TS, the HCO and H locate on the bridgeand near top sites, respectively, with the C⋯H distance of 1.60 Å.Similar to the case of HCO, the frequency calculation obtained animaginary value of 527 cm−1 for the TS configuration.

3.2.1.3. CH2O species related reactions. Fig. 3i shows that the co-adsorption configuration of H2CO and H on hcp-Mo2C (101), whereH2CO adsorbs at the bridge site, and H at the top site of subsurfaceMo. H2CO hydrogenation will lead to H3CO or H2COH as a reactionproduct. The calculated results show that H2COH formation is morefavorable than H3CO both kinetically (1.05 vs. 1.54 eV) and thermo-dynamically (−0.56 vs. 0.14 eV). The dissociation of adsorbed H2COinto CH2 and O is exothermic by 0.98 eV with an activation barrierof 1.20 eV. Obviously, these two hydrogenation reactions are alleasier than dissociation of adsorbed H2CO. Thus the third intermedi-ate for continued CO hydrogenation will be H2COH. This reactionfeatures a TS structure with the H atom being near the bridge site,and the H2CO located at the bridge site. At the TS, the O⋯H distanceis 1.28 Å, and an imaginary frequency of 1287 cm−1 is associatedwith the O\H bond forming.

3.2.1.4. CH2OH species related reactions. The dissociation of H2COH intoCH2 and OH (H2COH → CH2 + OH) is exothermic by 1.13 eV withthe activation barrier of 1.18 eV. At the TS, the dissociated CH2 andOH locate on the neighboring top sites, with the distance of 2.01 Å.There is an imaginary frequency of 481 cm−1 associated with theC\O stretching vibration mode. During another competing step,H2COH may react with another H atom to form H3COH. However,our calculation shows that the hydrogenation of H2COH needs moreenergy than the case of dissociation (i.e., 1.43 vs. 1.18 eV), and soit is difficult to occur due to its higher activation barrier comparedto that of the dissociation process. Thus the following reaction stepis C\O bond cleavage of H2COH.

3.2.1.5. CH2 hydrogenations to produce CH4. As discussed above, COhydrogenation at hcp-Mo2C (101) can yield surface CH2 based onthe step of H2COH → CH2 + OH. This species can react further withadsorbed hydrogen yielding CH4 within two continuously reactionsteps. The co-adsorption configuration shows that H atom locatedclose to top site and CH2 at the bridge site. The reaction is exothermicby 0.37 eV with an activation barrier of 1.15 eV. At the TS, the H atomis located at the top site and the CH2 is at the bridge site, with the C⋯Hdistance of 1.73 Å. An imaginary frequency of 934 cm−1 is involvedto identify this bond formation.

For the subsequent reaction, the adsorbed CH3 can further reactwithanother H atom to produce CH4, where the reaction is endothermicby 0.47 eV with the activation barrier of 1.20 eV. During the reactionprocess, the H atom first approaches to the surface Mo atom and theC⋯Mo distance also starts to decrease. At the TS, the H atom locatedclose to the top site and the CH3 is near the top site, with the C⋯H

distance of 1.63 Å. The frequency calculation obtained an imaginaryvalue of 875 cm−1 for the TS configuration. The formed CH4 speciescan be assumed to bind very weakly with the surface and may easilydesorb.

3.2.2. Reactions on fcc-Mo2C (100) surface

3.2.2.1. Initial step of CO hydrogenation. On fcc-Mo2C (100), thestarting configuration includes CO at the near top site and H at thebridge site (see Fig. 4a). The HCO formation is more favorable thanCOH both kinetically (0.36 vs. 1.10 eV) and thermodynamically(0.13 vs. 0.87 eV). Thus, HCO will be the first intermediate of COreacting with H. At the TS, CO is close to the top site and the hydrogenis nearby the hollow site with the C⋯H distance of 1.63 Å. An imagi-nary frequency of 483 cm−1 is involved to identify this bond forming.Here, we also investigate C\O bond breaking of CO on fcc-Mo2C(100). The activation barrier for the C\O bond breaking to form Cand O is 1.23 eV, which is higher than that of CO hydrogenation.

3.2.2.2. CHO species related reactions. The CHO may react with anotherH atom to form H2CO or CHOH. Fig. 4d shows that the co-adsorptionconfiguration of HCO and H atom on fcc-Mo2C (100), where HCOadsorbs at the top site and H at the bridge site. Here H2CO formationis more favorable than HCOH formation both kinetically (0.16 vs.1.08 eV) and thermodynamically (−0.56 vs. 0.06 eV). The dissocia-tion of adsorbed HCO to yield CH and O is exothermic by 0.99 eVwith an activation barrier of 0.35 eV. Thus the second intermediatefor CO hydrogenation will be H2CO. At the TS, HCO locates at thetop site and H close to the hollow site, with the C⋯H distance of1.68 Å. There is an imaginary frequency of 729 cm−1 correspondingto the C\H stretching vibration mode.

3.2.2.3. CH2O species related reactions. Once the formed H2CO interme-diate reacts further with another H atom, H2COH or H3COwill form. Asbefore the co-adsorption configuration of HCO and H on fcc-Mo2C(100), H atom adsorbs at the bridge site, H2CO at the top site. Thecalculated results show that H2COH formation is more favorablethan H3CO both kinetically (0.45 vs. 0.59 eV) and thermodynamically(−0.62 vs. 0.05 eV). The dissociation of adsorbed H2CO into CH2 andO is exothermic by 0.52 eV with an activation barrier of 0.77 eV.Therefore, these two hydrogenation reactions are both easier than dis-sociation of adsorbed H2CO. Thus the third intermediate for continuedCO hydrogenation will be H2COH. The frequency calculation gave animaginary value of 960 cm−1 for the TS configuration. At the TS, theH2CO still sits at the same top site as in the IS, and the H atom movesto the near top site, with the O⋯H distance of 2.62 Å.

3.2.2.4. CH2OH species related reactions. The activation barrier for thedirect reaction H2COH → CH2 + OH is 0.47 eV, and the process isexothermic by 0.67 eV. Thus, the dissociation of adsorbed H2COH toform CH2 and OH is more likely to occur. H2COH may react with Hatom to form H3COH, and the calculated result shows that the hydro-genation of H2COH needed more energy than the dissociation, whichis less likely to occur due to its higher activation barrier, compared tothat of the dissociation process (0.98 vs. 0.47 eV). Thus the followingreaction step is C\O bond cleavage of H2COH. At the TS, the dissoci-ated OHmoves toward the top site and the CH2 sits at the near hollowsite, with the C⋯O distance of 2.01 Å. The TS is characterized by animaginary frequency of 267 cm−1, which just depicts the C\O bondbreaking movement.

3.2.2.5. CH2 hydrogenations to produce CH4. Fig. 4l shows that theco-adsorption configuration of CH2 and H on fcc-Mo2C (100), whereCH2 adsorbs at the top site and the H atom at the bridge site. Thereaction is exothermic by 0.46 eV with an activation barrier of0.42 eV. At the TS, the H atom is near the top site and the CH2 is at

60 K.-Z. Qi et al. / Surface Science 614 (2013) 53–63

the top site, with the C⋯H distance of 1.38 Å. Then the H atom movesto the hollow site and form CH3 with the adsorbed CH2. The frequencycalculation obtained an imaginary value of 828 cm−1 for the TSconfiguration.

For the CH3 hydrogenation, the adsorbed CH3 can combine withanother H atom to produce CH4, where the reaction is endothermicby 0.47 eV with an activation barrier of 0.84 eV. During the reactionprocess, H atom first is apart from the surface and the C⋯H distancestarts to shorten. At the TS, the H atom located close to the bridgesite, while the CH3 is at the hollow site. The C⋯H distance is 1.49 Å.Our frequency calculation gave an imaginary value of 1030 cm−1

for the TS configuration.

3.2.3. The favorable reaction paths for the CO hydrogenation to methaneon Mo2C

After the above investigation for the possible mechanisms of COhydrogenation on fcc-Mo2C (100) and hcp-Mo2C (101), we find theinitial reaction step of CO and H is the formation of HCO intermediate,and the C\O bond breaking is the third step after H2COH formation,which produces co-adsorbed CH2 and OH on the surface. Interestingly,we find that hydrogenation of CO leads to lower dissociation barriersof C\O bond obviously (see Table 2). This indicates that CO dissocia-tion may occur via a hydrogenated intermediate and contribute tothe low selectivity toward alcohols on Mo2C catalysts, which agreeswell with the previous literature reports [18]. For the successive reac-tions steps on both fcc and hcp Mo2C surfaces, the feasible pathwayare the same, that is through the path of CO → HCO → H2CO → H2-

COH → CH2 → CH3 → CH4 (see Fig. 5). The rate-controlling step forthe methane formation is the CH3 hydrogenation. Obviously, the acti-vation energy of CH3 hydrogenation on fcc-Mo2C (100) is smaller thanthat on hcp-Mo2C (101) (0.84 vs. 1.20 eV), means that the formationof methane is more active on fcc Mo2C, which agrees well with theavailable experimental observations that the activity of fcc-Mo2C ishigher than that of hcp-Mo2C in the formation of methane [6]. Inter-estingly, the present reaction mechanism is similar to the previouswork of CO hydrogenation on MoS2, [22] where the reaction productis the CH4 and H2COH as the intermediate prior to C\O bond breaking.

3.2.4. The effect of environment on the CO hydrogenation reactionRecently, Medford et al. have systematically investigated the ele-

mentary steps of syngas reactions on the Mo2C (001) surface, [18]which has drawn attention to the nature of the reaction and the effectof reaction environment. The Mo-terminated Mo2C (001) surface ex-hibits carbon reactivity similar to transitionmetals, but is significantly

Fig. 5. Energetic reaction schemes for sequential CO hydrogenation to methane on the (a) fcctheir adsorbed states at the surface. The most favorable reaction paths are highlighted by thicof the graph.

more reactive toward oxygen. This explains some previous observa-tions for Mo2C catalysts and suggests that Mo2C may exhibit uniqueand potentially useful reactivity or selectivity patterns. Inspired bythis work, we investigate whether the environment also affects theCO hydrogenation reaction on fcc-Mo2C (100) and hcp-Mo2C (101).

Since the CO hydrogenation occurs at the environment of H2

surrounding, and thus the effect of hydrogen on the reaction of COhydrogenation should be taken into account. Considering thatCH3 + H → CH4 is the rate-controlling step, so we explore the effectof pre-adsorbed hydrogen atom on the reaction of CH3 + H on Mo2Csurfaces, that is, CH3 (a) + 2H (a) → CH4 (a) + H (a). On fcc-Mo2C(100), the reaction is endothermic by 0.49 eV with an activationbarrier of 0.89 eV. The activation barrier for the C\H bond formationof CH4 in the presence of hydrogen atom is almost the same as theclean fcc-Mo2C (100) surface. At the TS, the CH3 is located at thebridge site, while the H is at the bop site and the C⋯H distance is1.34 Å. The frequency calculation obtained an imaginary value of952 cm−1 for the TS configuration. For such reaction process, CH3

(a) + 2H (a) → CH4 (a) + H (a) on hcp-Mo2C (101), the C⋯H dis-tance is 1.38 Å at the TS. The CH3 is located at the bridge site, whilethe H atom is at the step-bridge site. The frequency calculation gavean imaginary value of 908 cm−1 for the TS configuration. This reac-tion is endothermic by 0.30 eV and the activation barrier is lowerthan that of the clean hcp-Mo2C (101) (1.05 vs. 1.20 eV). This resultindicates that the pre-adsorbed hydrogen atom will improve the ac-tivity of hcp-Mo2C (101) surface for CH4 formation.

Oxygen is ubiquitous in most environments, the influence of pre-adsorbed oxygen atom on the C-H formation of CH4 on the Mo2C sur-faces was also explored, that is, CH3 (a) + H (a) + O (a) → CH4

(a) + O (a). For such reaction process on fcc-Mo2C (100), the reac-tion is endothermic by 0.69 eV. The activation barrier is higher thanthat of the clean fcc-Mo2C (100) surface (1.20 vs. 0.84 eV), indicatingthe inhibition effect of pre-adsorbed oxygen on the formation in CH4.At the TS, the CH3 is located at the bridge site, while the H atom is atthe near bop site and the C⋯H distance is 1.41 Å. The TS is character-ized by an imaginary frequency of 928 cm−1. On hcp-Mo2C (101), thereaction of CH3 (a) + H (a) + O (a) → CH4 (a) + O (a) was also ex-plored. At the TS, the CH3 is located at the bridge site, while the H isat the step-bridge site and the C⋯H distance is 1.56 Å. The frequencycalculation obtained an imaginary value of 1183 cm−1 for the TSconfiguration. This reaction is endothermic by 0.19 eV. The activationbarrier is lower than that of the clean hcp-Mo2C (101) (1.13 vs.1.20 eV), and it indicates the promotion effect of pre-adsorbed oxy-gen on the formation of CH4.

-Mo2C (100) and (b) hcp-Mo2C (101) assuming reaction intermediates and products ink black lines. Alternative intermediates are labeled accordingly and listed at the bottom

61K.-Z. Qi et al. / Surface Science 614 (2013) 53–63

Based on the above calculations, we can see that the activationenergy of CH3 + H → CH4 decreases when hydrogen or oxygenatom adsorbed the hcp-Mo2C (101) surface, while it shows a slightincrease for the fcc-Mo2C (100) surface. This phenomenon may beexplained from the two aspects. First, CH3 + H → CH4 is an endo-thermic reaction. When H/O adsorbed on the hcp-Mo2C (101) sur-face, the reaction heat of CH3 + H → CH4 is smaller than the caseon the clean surface, which will lower its activation energy. However,when H/O adsorbed on the fcc-Mo2C (100) surface, the reaction heatis larger than the case of the clean surface, which will increase the ac-tivation energy. Second, both hcp-Mo2C (101) and fcc-Mo2C (100)surfaces will reconstruct after the adsorption of hydrogen or oxygenatoms, and this reconstruction will also affect the activity of Mo2Ccatalysts. Another interesting phenomenon is that the oxygenmodifiedhcp-Mo2C (101) surface shows a higher catalytic activity for CH4 for-mation than that oxygen modified fcc-Mo2C (100) surface. However,it is worth to mention that the CO + H reaction occurs under thehydrogen atmosphere, [6] the high hydrogen pressure will inhibit theprocess of oxygen adsorption. Thus such a small amount of oxygenwill not effectively affect the whole reaction. All above, althoughthe activation energy of the rate-controlling step can be changed bythe adsorbed hydrogen or oxygen atom, the activity of the fcc-Mo2C(100) surface is still higher that the hcp-Mo2C (101) surface, whichagrees well with the experimental results [6].

4. Discussion

4.1. Thermodynamical trends in CO hydrogenation

Brönsted–Evans–Polanyi (BEP) correlation is useful in the estima-tion of activation energy based on the thermodynamical properties.This relationship is previously used for a given reaction like O2 disso-ciation on different metals, [42–48] and later is applied to the caseof series bonds cleavage reactions on a given metal, such as C\H/O\H/C\O bond scission of ethanol on Pt (111) [49]. Additionally,the BEP-relation is usually applied to the case of bond cleavage typereaction, and it is little used in the case of coupling reaction such ashydrogenation reaction. Hu and co-workers found that the BEP rela-tionship is not efficient for the association reaction due to the lowbarrier difference as compared to the dissociation reaction on variousmetals [46]. Later, they found that the BEP holds for the associationreaction whose reactive center atom with the lower valency butfailed in the case of reactive center atom has the higher valency[50]. For instance, the C(a) + O(a) = CO(a) reaction type obeys the

Fig. 6. Brønsted–Evans–Polanyi (BEP) relationships for (a) C\H/O\H bond formation and (relative to initial state gas phase and clean slab.

BEP relationship, but C(a) + N(a) = CN(a) reaction type cannot beanalyzed by the BEP principle. Recently, Loffreda et al. [51] foundthe activation energy barrier of the hydrogenation reactions cannotcorrelate with the reaction heat for the hydrogenation of acroleinon Pt (111), but the co-adsorption energy of transition state can becorrelated well with the co-adsorption energy of initial state, namelyfollows another type BEP-relation. Although BEP successfully used inmany metal-catalyzed reactions, there is little application in themetal oxide or metal carbide systems. Since Mo2C presents similarelectronic structure to the transition metals and exhibits similar cata-lytic activity, [52] so it is expected that the BEP relation may hold forthe case of Mo2C catalyzed CO hydrogenation reactions as studied inthis work. Here, the energies at the transition states and the energiesof the initial states or the final states are calculated with respect to thecorresponding intermediates in the gas phase and clean slab.

4.1.1. C\H/O\H formationInterestingly, for the C\H/O\H bond formation reactions of CO

hydrogenation on Mo2C investigated in the present work, we cannotfind a linear correlation between reaction energy and activation ener-gy, but a clear linear correlation between the co-adsorption energy oftransition state (EcoadsTS ) and the initial state (EcoadsIS ) (see Fig. 6a) wasobserved, and thus the BEP relation holds for the association typereaction. From Fig. 6a, one can easily conclude that the activationenergy barrier for the C\H and O\H bond formation on fcc-Mo2C(100) is lower than hcp-Mo2C (101) generally because of the moreexothermic C\H/O\H formation reactions on fcc-Mo2C comparedto that of hcp-Mo2C, that is the kinetically properties are correlatedwell with the corresponding thermochemical properties.

4.1.2. C\O bond cleavageBesides the C\H/O\H formation type reaction as discussed

above, the C\O bond cleavages are also analyzed by the BEP principledue to this importance in the whole reaction processes. On hcp-Mo2C(101), C\O bond is elongated with CO hydrogenation, that is, theC\O bond length in H3CO (1.44 Å) is longer than that of H2CO(1.37 Å) and HCO (1.32 Å). This trend of C\O bond elongationseems to be that C\O is more reactive and easily dissociated onhcp-Mo2C (101). However, the activation barriers of C\O bond scis-sion for H3CO (0.61 eV) and HCO (0.55 eV) are lower than that ofH2CO (1.20 eV) (Scheme 1). These three reactions were treated asthe exothermic reaction. The larger reaction energy can account forthe relatively lower energy barriers. The HCO and H3CO cleavage re-actions release more energy than that of H2CO cleavage, which will

b) C\O bond scission reactions on hcp and fcc Mo2C surfaces. IS, FS and TS energies are

Scheme 1. Reaction network and activation barriers (eV) of CO hydrogenation to methane on fcc-Mo2C (100) and hcp-Mo2C (101). Note: The values in parentheses are related tothe hcp-Mo2C (101) surface.

Fig. 7. Variation of the d-projected density of states of the surface of Mo atom due to itsbonding with TS complex in C\O bond cleavage on hcp and fcc Mo2C surfaces.

62 K.-Z. Qi et al. / Surface Science 614 (2013) 53–63

decrease the activation barrier. This striking feature clearly reflectsthat the C\O bond cleavage has the BEP-type relation (see Fig. 6b).

On fcc-Mo2C (100), for the adsorption species, the C\O bondlength in H3CO (1.43 Å) is longer than that of H2CO (1.38 Å) andHCO (1.27 Å). This trend of C\O transformation indicates that C\Obond is elongated as the CO hydrogenation and C\O bond alsoseems to be easily dissociated on fcc-Mo2C (100). However, the activa-tion barrier of C\O bond scission was increased as the number of Hatoms increases. Different from the case of hcp-Mo2C (101), the differ-ence of reaction energy is small, and it can be accounted by the stericeffect. As the number of H atoms increases, surface intermediates in-crease in the crowdedness, which could lead to considerable bondingcompetition among Mo atoms. At the TS configuration of C\O bondcleavage of H3CO, for the steric effect, the CH3 is so high above thefcc-Mo2C (100) surface that CH3 cannot bond to any Mo atom. Thus,CH3 cannot be located at a stable site, which brings the TS to ahigher-energy state, and the activation energy is increasing as the COhydrogenation reactions. The situation of C\O bond cleavage of H2COis similar to that of H3CO. In contrast, in the TS of the C\O bond scissionof HCO, the CH can bond to the two surfaceMo atoms to achieve C atombonding saturation and O atom can form the O\Mo bond, and there isno strong repulsive interaction between O atom and CH, which bringsthe TS to a lower-energy state. The BEP plot of C\O bond breakingon fcc-Mo2C (100) is shown in Fig. 6b, clearly, the more stable of theproduct, the more active of the C\O bond cleavage.

In Fig. 6b, it can be seen that the activation barrier increases withincreasing of hydrogen atoms in CHxO species on fcc-Mo2C, whichmeans that the C\O bond cleavage in the initial hydrogenationstage may compete with the hydrogenation process. In fact, a similarphenomenon has been found for the C\C bond scission of ethyleneglycol, [53] and glycerol [54].

4.2. Physical origin of structure-sensitivity in CO hydrogenation

4.2.1. Electronic structure analysisBased above analysis we know that the CH3 hydrogenation is the

rate-controlling step in the whole reaction process, so it is necessaryto explore the possible reaction why CH3 hydrogenation is more activeon fcc-Mo2C than that of hcp-Mo2C. This can be explained by stabiliza-tion energy proposed byWang and Liu [49]. Stabilization energy, Ed, can

be calculated by the formula: Ed ¼ ∫ EF−∞ ε nTS

d −nISd

� �dε, where nd is the

normalized density of states of surface Mo atom in the IS and TS, andε is the energy level. In calculation, the cutoff of 1 Å was used for theMo atom. The Fig. 7 shows the difference of d-projected density ofstate of surface Mo atom between initial state and transition state forthe reaction for the step of CH3 hydrogenation on both fcc and hcp

Mo2C surfaces. Based on the plot of Fig. 7 and the stabilization energies,Ed, were estimated to be −1.41 and −1.17 eV for CH3 hydrogenationon fcc and hcp Mo2C surface, respectively. This means that the TS com-plex of CH3 hydrogenation interacts more strongly with the fcc-Mo2C(100) compared to that of hcp-Mo2C (101), and thus a lower barrierwould be expected on fcc Mo2C one (here we assumed that the initialstate contributes same to the energy barrier for fcc and hcp Mo2C).In fact, our present calculation results give the activation energiesof 0.84 and 1.20 eV for the step of CH3 hydrogenation on fcc and hcpMo2C, respectively.

4.2.2. Barrier decompositionTo further explore the physical nature of the activation barrier for

the rate-determining step of CO hydrogenation (CH3 + H → CH4),we decomposed the calculated activation barrier (Ea) using the fol-lowing formula: Ea = ETSCH3

+ EHTS − EISCH3

− EHIS + E int

CH3⋯H , [55–57]where ETSCH3

(EHTS) is the adsorption energies of CH3 (H) in the geome-try of the TS without H (CH3), E

intCH3⋯H is the interaction between CH3

and H in the TS. EISCH3(EHIS) is the adsorption energies of CH3 (H) in

the geometry of the IS without H (CH3). From the energy decomposi-tion results listed in Table 3, it can be found that the adsorption ener-gies of reactants (CH3 and H) on fcc-Mo2C (100) are much smallerthan that on hcp-Mo2C (101) (−3.65 vs. −6.39 eV), which is helpfulin lowering the activation barrier. But the interaction of CH3 and Hwith the substrate in the TS is weaker than that on hcp-Mo2C (101)

Table 3Energy dissociation for the calculated activation energy (unit: eV).

Surface Ea ETSCH3+ EH

TS ETSCH3EHTS E int

CH3⋯H EISCH3+ EH

IS EISCH3EHIS

fcc-Mo2C(100)

0.84 −3.61 −1.81 −1.80 0.80 −3.65 −2.14 −1.51

hcp-Mo2C(101)

1.20 −5.71 −2.72 −2.99 0.52 −6.39 −3.25 −3.14

63K.-Z. Qi et al. / Surface Science 614 (2013) 53–63

(−3.61 vs. −5.71 eV), which increases the activation barrier. The in-teraction energy between CH3 and H is higher than that on hcp-Mo2C(101) (0.80 vs. 0.52 eV), which increases the activation barrier. Theoverall result is that the activation barrier on fcc-Mo2C (100) islower than that on hcp-Mo2C (101) (0.84 vs. 1.20 eV), that is, theactivity of fcc-Mo2C (100) is higher. Based on the above analysis,the active difference of these two surfaces, it seems that theco-adsorption energy difference between IS and TS could be themost important factor related to the variation of activation energy.For instance, the co-adsorption energy difference between IS and TSis −0.04 eV on fcc Mo2C, while it is as high as 0.68 eV on hcp Mo2C(see Table 3). It may be pointed out that the co-adsorption energy dif-ference between IS state and TS obtained by the barrier decomposi-tion scheme can be approximately related to the stabilizationenergy as we previously studied, that is, the smaller co-adsorptionenergy difference between TS and TS means the TS complex is morestable, corresponding to the more negative stabilization energy, andthus a lower activation energy will be. Indeed, as seen from Table 3,fcc Mo2C has a smaller energy difference than that of hcp Mo2C (i.e.,0.04 vs. 0.68 eV), which is agreement with the trends of stabilizationenergy (−1.41 vs. −1.17 eV).

5. Conclusions

The present theoretical studies give a clear conclusion of theadsorption of reaction intermediates appearing during CO hydroge-nation on fcc-Mo2C (100) and hcp-Mo2C (101) as well as of the ele-mentary reaction steps. The calculated adsorption energies of thespecies on the two surfaces show highly localized strong adsorbate–molybdenum interaction for all adsorbates. Furthermore, DFT studieson possible reaction steps during CO hydrogenation at the twophase Mo2C surfaces are used to identify favorable reaction pathswhich are found to involve C1 type surface species in the sequence:CO → HCO → H2CO → H2COH → CH2 → CH3 → CH4. In particular,the dissociation of adsorbed CH2OH will produce CH2 and OH, whereCH2 can further react with H atoms to form CH4. The catalytic activityof fcc-Mo2C is higher than hcp-Mo2C for CO hydrogenation, becausethe activation energy of the rate-determining step (CH3 hydrogenation)on fcc-Mo2C (100) is lower than that on hcp-Mo2C (101).

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