DFT Study on the Methane Synthesis from Syngas on a Cerium

Article pubs.acs.org/JPCC

DFT Study on the Methane Synthesis from Syngas on a CeriumDoped Ni(111) Surface Kai Li,† Cong Yin,‡ Yi Zheng,§ Feng He,† Ying Wang,† Menggai Jiao,† Hao Tang,*,‡ and Zhijian Wu*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ Energy Conversion R&D Center, Central Academy of Dongfang Electric Corporation, Chengdu 611731, P. R. China § Network Center, Changchun Normal University, Changchun, 130032, P. R. China S Supporting Information *

ABSTRACT: The methanation of syngas (CO and H2) on the Ce-doped Ni(111) surface (Ce−Ni(111)) has been investigated by using the density functional method. The doped Ce enhances the adsorption energy of the intermediates on the catalytic surface, except for H2, particularly for Ocontaining species. On the Ce−Ni(111) surface, the reaction pathway CO + 3H2 → CHO + 5H → CH + O + 5H → CH4 + H2O is the most favorite, in which the energy barrier is 1.18 eV for the rate-determining step. Compared with the pure Ni(111) surface, the doping of Ce improves the catalytic activity both thermodynamically and kinetically. The microkinetic analysis also supports that the methanation of syngas has high reaction rate on the Ce−Ni(111) compared with the pure Ni(111). The temperature has great influence on the reaction rate, while H2/CO ratio shows only slightly impact. Our study also explains the experimental observation that the doped Ce can reduce the reaction temperature from ∼500 °C on the pure Ni(111) to ∼340 °C on the Ce−Ni(111) surface. The coverage of CHO is the largest on the Ce− Ni(111) surface. We expect that the obtained results could be useful for the future experimental study in searching the high efficient catalysts. the coke deposition.13,14 Thus, improving the catalytic activity and resistance to carbon deposition for the Ni-based catalysts remains a serious challenge. Recently, the addition of promoter has been found to be an effective method to enhance the catalytic activity and stability of the Ni-based catalysts for the methanation of syngas. The related studies revealed that CeO2-decorated Ni/Al2O3 catalyst shows high catalytic activity, coke-resistance, and thermal stability compared with the pure Ni/Al2O3.15−17 Furthermore, the CO conversion and CH4 selectivity on the CeO2 decorated Ni/Al2O3 catalyst are >90%.15−17 On the other hand, for Cedoped Ni catalyst, low-temperature catalytic activity is observed.18−20 The CO conversion and CH4 yield are close to 100%,18−20 and the carbon deposition amount is ∼1/13 compared with the pure Ni.20 Moreover, the doped Ce would promote the CO dissociation to improve the methanation reaction on the Ni(111) surface.20 In addition to the experimental studies, theoretical studies are also available for the mechanism of CO + 3H2 → CH4 + H2O on different catalysts, such as Fe,21,22 Co,23,24 Rh,25 Ru,26,27 and Ni,28−32 etc. For Fe(001), CO dissociates into active carbon atom, then the carbon atom would be successively hydrogenated to form CH, CH2, CH3, and finally

1. INTRODUCTION Natural gas is known to be an efficient and clean energy carrier. A useful technology to synthesize the natural gas is from the syngas (CO and H2). Thus, the methanation of syngas has received much attention as a promising way to utilize the syngas and produce synthetic natural gas.1−3 This reaction process can recycle the poisoning CO on one hand, meanwhile, it is also one of the important processes in Fischer−Tropsch synthesis that converts the syngas into hydrocarbons and oxygenates.4 In addition, the synthesized methane can be used as renewable feedstock for the chemical industry instead of fossil feedstock (oil or coal), which can effectively reduce the environment impacts.5 For the methanation of syngas (CO + 3H2 → CH4 + H2O), the catalysts play a very important role. The previous studies indicated that most of the metals in groups 8−10 show activity for the methanation reaction.1 Among these metals, Ru is the most active,6 while Pt, Pd and Rh have the highest selectivity.7 Nonetheless, due to the high cost of these noble metals, the industrial scale applications are restricted. In this aspect, due to the low cost, high catalytic selectivity and activity, Ni-based catalyst has become one of the most attractive catalysts in the methanation of syngas.8−12 Among Ni, Fe, and Co, Ni shows the highest catalytic activity and highest CH4 selectivity in the methanation of syngas.8 Moreover, for most of the Ni-based catalysts, the CO conversion and CH4 selectivity are above ∼90%.9−12 However, the Ni catalysts deactivate rapidly due to © XXXX American Chemical Society

Received: July 23, 2016 Revised: September 26, 2016

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DOI: 10.1021/acs.jpcc.6b07400 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

relatively large value of −1.92 eV.41 The wave functions at each k-point were expanded with a plane wave basis set and a kinetic cutoff energy was set to 400 eV. The electron occupancies were determined according to the Fermi scheme with an energy smearing of 0.1 eV. Brillouin zone integration was approximated by a sum over special selected k-points using the Monkhorst−Pack method44 and they were set to 3 × 3 × 1. Geometries were optimized until the energy was converged to 1.0 × 10−6 eV/atom and the force to 0.01 eV/Å. Because of the existence of the magnetic atoms, spin polarization was considered in all calculations. The transition state (TS) structures and the reaction pathways were located using the climbing image nudged elastic band (CI-NEB) method.45 The minimum energy path was optimized using the force-based conjugate−gradient method36 until the maximum force was less than 0.05 eV/Å. The harmonic vibrational frequency calculations were performed to characterize the nature of all the stationary points and to obtain the zero point energy (ZPE) correction. To describe the van der Waals (vdW) interaction, the nonlocal van der Waals density functional (vdW-DF) method was employed.46,47 2.2. Model. For Ce-doped Ni, the (111) surface was observed by the XRD pattern and was the dominant surface based on the experimental studies.19,20 The Ni(111) surface was obtained by cutting Ni bulk (fcc) along [111] direction. The thickness of the surface slab was chosen to be a three-layer slab. In the geometry optimizations, the positions of the atoms in the two top surface layers were allowed to relax, while the bottom layer was fixed. A vacuum layer as large as 12 Å was used along the c direction normal to the surface to avoid periodic interactions. A (3 × 3) supercell was used to model the coverage of a 1/9 monolayer and one Ni atom within the outmost surface was replaced by a Ce atom (Figure 1a). The adsorption energy of the adsorbates on the Ce− Ni(111), ΔEads, is defined as follows

CH 4 . 22 For Co(0001), 24 Rh(211), 25 Ru(0001), 26 and Ni(111),29−32 however, CO is hydrogenated first to form CHxO or CHxOH followed by their dissociation into the CHx species, and then hydrogenated to CH4. For Rh(211), CO → CH3O → CH3→ CH4 is the dominant pathway.25 For Co(0001)24 and Ru(0001),26 the pathway CO → CHOH → CH → CH4 is the most favorable. For Ni(111), the most favorable pathway is CO → CHO → CH→ CH4,31,32 in which the rate-determining step is CO + H → CHO with an energy barrier of 1.3831 and 1.44 eV.32 For the potassium-modified Ni(111), the rate-determining step CO + H → CHO has a higher energy barrier of 1.61 eV compared with the pure Ni(111).32 From the previous experimental study, it is know that Cedoped Ni(111) has high catalytic activity for the methanation of syngas,18 but the detailed reaction mechanism is unclear. Therefore, in this work, the reaction mechanism of CO + 3H2 → CH4 + H2O on Ce-doped Ni(111) (denote as the Ce− Ni(111) in the following) surface has been investigated by using the density functional theory (DFT). For comparison, the reaction mechanism for the pure Ni(111) has been also studied. In addition, the microkinetic model based on the DFT results is employed to explore the influence of the reaction temperatures and H2/CO ratios on the reaction rate.

2. COMPUTATIONAL DETAILS 2.1. Method. The calculations were performed using the Vienna ab initio simulation package (VASP).33−36 The interactions between valence electrons and ion cores were treated by Blöchl’s all-electron-like projector augmented wave (PAW) method.37,38 Since the PBE functional overestimates the adsorption energies of the adsorbates,39 the generalized gradient approximation with the revised version of Perdew− Burke−Ernzerhof (revPBE) is employed.40 From Table 1, it is seen that the adsorption energies obtained by revPBE functional is lower than those by PBE.41 The calculated CO adsorption energy of −1.40 eV by revPBE is close to the experimental value of ∼−1.30 eV,42,43 while PBE gives a

ΔEads = Eadsorbate / slab − (Eslab + Eadsorbate)

where Eadsorbate/slab is the total energy of the adsorbate on the Ce−Ni(111) surface, Eslab is the total energy of the clean Ce− Ni(111) surface and Eadsorbate is the total energy of the free adsorbate. The first two terms are calculated with the same parameters. The third term is calculated by setting the isolated adsorbate in a box of 12 Å × 12 Å × 12 Å. The interaction energy ΔEint of the coadsorbed species, i.e., CHx + H, CHxO + H and H + O(OH), is employed to correct the energy barriers and reaction energies in this work, which is defined as follows:

Table 1. Adsorption Energies (in eV) of the Possible Intermediates on Ce−Ni(111) and Ni(111) O C H OH CO H2 H2O CH CH2 CH3 CH4 CHO CH2O CH3O COH CHOH CH2OH CH3OH a

Ce−Ni(111)

Ni(111)-revPBE

Ni(111)-PBEa

−6.16/fccCe −6.33/fccNi −2.81/fccNi −4.01/TCe −1.63/fccNi −0.05/TNi −0.67/TCe −5.63/fccNi −3.60/bri1 −1.59/TNi −0.24/TNi −2.64/bri1 −1.35/bri2 −3.35/TCe −3.82/fccNi −2.82/bri1 −1.96/bri2 −0.84/TCe

−5.24/fcc −6.13/fcc −2.76/fcc −3.08/fcc −1.40/fcc −0.05/T −0.16/T −5.56/fcc −3.54/fcc −1.48/fcc −0.13/T −1.92/fcc −0.29/fcc −1.88/fcc −3.79/fcc −2.39/bri −1.47/bri −0.37/T

−5.67 −6.78 −2.81 −3.42 −1.92 − − −6.43 −4.01 −1.91 −0.02 −2.26 −0.75 −2.63 −4.39 −3.88 −1.54 −0.30

ΔE int = E(A + B)/ slab − (EA / slab + EB / slab) + Eslab

where E(A+B)/slab is the total energy of the coadsorbed A and B on the Ce−Ni(111) surface, EA(B)/slab is the total energy of the isolated adsorbed A(B) on the Ce−Ni(111) surface. Eslab is the total energy of the clean Ce−Ni(111) surface. The positive (negative) ΔEint denotes the repulsive (attractive) interaction between A and B. The results are shown in Table S1, Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Adsorption Sites and Energies on the Ce−Ni(111) Surface. For the Ce−Ni(111) surface, the doped Ce atom is protruded above the Ni(111) surface due to the larger atomic radius of Ce atom (2.70 Å) compared with Ni atom (1.62 Å)

Adsorption energies with PBE from ref 41. B


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Figure 1. Structures and possible adsorption sites of the Ce−Ni(111) surface. The milky white and blue balls denote the Ce and Ni atoms, respectively.

increase of H atoms in CHxO, the Ce−O distance decreases from 2.255 Å in CHO and 2.138 Å in CH2O to 2.091 Å in CH3O. CHxOH (x = 0−3) Adsorption (Figure 2o−r). For COH (Figure 2o), the most stable site is fccNi with the C−O bond almost perpendicular to the Ce−Ni(111) surface. For CHxOH (x = 1−3), the most stable adsorption site is the same as CHxO (x = 1−3), i.e., CHOH at bri1 (Figure 2p), CH2OH at bri2 (Figure 2q), and CH3OH at TCe (Figure 2r) sites. The adsorption energies of CHxOH (x = 0−3) become weaker with the increase of H atoms, i.e., −3.82 eV for COH, −2.82 eV for CHOH, −1.96 eV for CH2OH, and −0.84 eV for CH3OH. 3.2. Comparison of the Adsorption Energies with Pure Ni(111). From Table 1, it is seen that the adsorption energies of all the intermediates (except for H2) on the Ce− Ni(111) surface are enhanced compared with those on the pure Ni(111) surface. This is different from K-modified Ni(111) surface, in which the adsorption of CHx (x = 1−3), OH and H2 are weakened.32 In particular, the intermediates O, OH, H2O, CHxO, and CHxOH (x = 1−3) (the adsorption energies increase by ∼0.43−1.47 eV compared with pure Ni(111)) on the Ce−Ni(111) surface show more strengthened adsorption compared with C, H, CHx, CO, and COH (the adsorption energies increase by ∼0.03−0.23 eV) (Table 1). From the structures shown in Figure 2, we noted that for the former intermediates, the O atom either bonds or points to Ce besides the bonding of C with Ni for CHxO and CHxOH. For the latter intermediates, however, only C atom bonds with Ni. C−O bond is nearly perpendicular to the surface in CO and COH. This means that the doped Ce has great impact on the Ocontaining species due to the interaction between Ce and O (Ce−O distance is in the range of ∼2.0−2.7 Å). This is also confirmed by the calculated differential charge density (Figure S1, Supporting Information). For O-containing species in which O bonds with Ce, the electron transfer between O and Ce is observed, while there is no electron transfer between Ce and O for COH in which only C bonds with Ni. In addition, weak adsorption species such as CH3OH (ΔEads = −0.84 eV) and H2O (ΔEads= −0.67 eV) have less electron transfer than the strong adsorption species such as O, OH, CHxO (x = 1−3), and CHxOH (x = 1−2) (with ΔEads from −6.16 to −1.35 eV), in agreement with the case of pure Ni(111).48 3.3. The Elementary Reactions on the Ce−Ni(111) Surface. According to the previous DFT studies on the methanation of syngas on the Ni(111) surface,29−32 the available pathways on the Ce−Ni(111) surface are designed and summarized in Table 2. The most stable adsorption

(Figure 1b). There are eight possible adsorption sites (Figure 1a). For each adsorbate, the most stable adsorption energies and structures are shown in Table 1 and Figure 2, respectively. C, H and O Adsorption (Figure 2a−c). For C and H, the most stable site is the fccNi (Figure 2, parts a and b) with adsorption energies of −6.33 eV for C and −2.81 eV for H (Table 1), respectively. The adsorption energies on the Ce top site (TCe) are much weaker, i.e., −2.09 eV for C and −1.53 eV for H. This suggests that C and H atoms prefer to connect with the Ni atom. Different from C and H, O prefers to connect with Ce atom with the adsorption energy of −6.16 eV at the most stable fccCe site (Figure 2c). Thus, Ce has more affinity to O atom than C and H atoms in adsorption. OH and H2O Adsorption (Figure 2, parts d and e). For OH, the stable adsorption sites are TCe and bri2, in which the most stable site is TCe with an adsorption energy of −4.01 eV. For H2O, however, only one stable adsorption site is found, i.e., the TCe site, with an adsorption energy of −0.67 eV. For both adsorbates, the O atom bonds to the Ce atom with the Ce−O distance of 2.099 and 2.689 Å, respectively. CO and H2 Adsorption (Figure 2, parts f and g). For CO, either C or O atom could bond to the Ce−Ni(111) surface. However, after geometry optimization, only the configuration with C atom bonding to the Ce−Ni(111) surface is obtained (Figure 2f). Thus, similar to the adsorption of isolated C atom, the most stable adsorption site for CO is the fccNi with the adsorption energy of −1.63 eV. For H2 (Figure 2g), the physisorption on TNi is observed with the adsorption energy of −0.05 eV. CHx (x = 1−4) Adsorption (Figure 2h−k). Similar to CO and C, CH prefers to adsorb at the fccNi site (Figure 2h) with an adsorption energy of −5.63 eV. For CH2 and CH3, the bri1 and TNi are the most favorable sites (Figure 2, parts i and j), with adsorption energies of −3.60 eV and −1.59 eV (Table 1), respectively. This is different from the pure Ni(111), in which fcc site is the most stable for CH2 and CH3 adsorption.29,31 For CH4, the physisorption is observed with an adsorption energy of −0.24 eV (Table 1) at the TNi site (Figure 2k). CHxO Adsorption (x = 1−3) (Figure 2l−n). The bri1 site is the most stable for CHO adsorption with an energy of −2.64 eV (Table 1), in which the C atom bonds to the surface Ni atom and the O atom points to the Ce atom (Figure 2l). For CH2O, bri2 is the most stable adsorption site with an energy of −1.35 eV, in which the C and O atoms also bond to the Ni and Ce atoms, respectively (Figure 2m). For CH3O, it can only be adsorbed at the TCe site through O bonding to the Ce atom (Figure 2n) with an energy of −3.35 eV. Thus, with the C


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Figure 2. Most stable adsorption structures of the intermediates. The C−Ce, O−Ce and H−Ce denote the distance between C, O, or H atom and Ce atom. Since the adsorption structures of H2, CH4, CH3O, and COH are not clear from the top view; the side views are shown in the insets.

= ΔHDFT + ΔEint. The corresponding results are shown in Table 2. 3.3.1. H2 Dissociation. R1: H2 → 2H (Figure 3a). This dissociation process occurs on the TNi site. In the transition state TS1, the H−H distance is elongated to 1.089 Å from the initial 0.743 Å (Figure 3a). The reaction is exothermic by −0.50 eV with an energy barrier of 0.38 eV (Table 2). Thus, H2 dissociation is relatively easy due to the lower energy barrier.

structures (Figure 2) are set as the initial states (IS) and finial states (FS) during the search of the transition states (TS). The optimized structures are shown in the Figures 3-8, while the barriers (ΔEDFT) and reaction energies (ΔHDFT) are shown in Table S1. For the hydrogenation reaction, the H atom needs to overcome the energy ΔEint to approach the intermediates CHx, CHxO, and O (OH). In this case, the energy barriers are obtained by ΔEa = ΔEDFT + ΔEint and reaction energies by ΔH D


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The Journal of Physical Chemistry C Table 2. Energy Barrier (ΔEa, eV) and Reaction Energy (ΔH, eV) of the Possible Elementary Reactions R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11

elementary reaction

ΔEa

ΔH

H2 → 2H CO → C + O CO + H → COH CO + H → CHO CHO → CH + O CHO + H → CHOH CHO + H → CH2O CH2O → CH2 + O CH2O + H → CH2OH CH2O + H → CH3O CH3O → CH3 + O

0.38 1.66 2.19 0.92 0.81 1.62 0.74 0.89 1.26 0.75 1.79

−0.50 0.97 1.42 0.72 −0.35 1.08 0.18 −0.03 0.77 −0.33 0.29

R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22

elementary reaction

ΔEa

ΔH

CH3O + H → CH3OH COH → C + OH CHOH → CH + OH CH2OH → CH2 + OH CH3OH → CH3 + OH C + H → CH CH + H → CH2 CH2 + H → CH3 CH3 + H → CH4 O + H → OH OH + H → H2O

1.12 0.44 0.02 0.20 1.13 0.78 0.69 0.70 0.84 1.16 1.18

0.81 −0.65 −1.58 −1.04 −0.79 −0.48 0.38 −0.12 −0.45 0.08 0.81

Figure 3. Structures of (a) H2 dissociation and (b−d) CO dissociation and hydrogenation on the Ce−Ni(111) surface. The gray, red, and white balls denote the C, O, and H atoms, respectively.

3.3.2. CO Dissociation and Hydrogenation. R2: CO → C + O (Figure 3b). The direct CO dissociation needs to overcome an energy barrier of 1.66 eV, and the reaction is endothermic by 0.97 eV (Table 2). In TS2, C−O distance is 1.859 Å. Meanwhile, on the pure Ni(111) surface, our calculation indicates that the energy barrier is 2.97 eV for CO dissociation. This value is in agreement with the previous results (∼3.00

eV).32,49 This also means that the CO dissociation is very easy due to the doping of Ce. This is consistent with the experimental speculation that the enhanced Ni−C bond promotes CO dissociation.20 For K-modified Ni(111), however, the CO dissociation energy barrier is nearly the same as pure Ni(111).32 Meanwhile, the adsorption energy (ΔEads = −1.63) of CO is nearly the same as CO dissociation E


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Figure 4. Structures of CHO dissociation and hydrogenation on the Ce−Ni(111) surface. The gray, red, and white balls denote the C, O, and H atom, respectively.

transition state TS5. R5 is an exothermic reaction (ΔH = −0.35 eV) with an energy barrier of 0.81 eV. Compared with CHO formation reaction (CO + H → CHO, ΔEa = 0.92 eV), the CHO dissociation can be easily occurred due to the lower energy barrier. This suggests that the hydrogen-assisted CO dissociation (CO + H → CHO → CH + O with an energy barrier of 0.92 eV) is more favorable compared with CO direct dissociation (CO → C + O with an energy barrier of 1.66 eV) on the Ce−Ni(111) surface. This confirms the experimental speculation20 that the improved dissociation of CO on the Ce− Ni(111) surface mainly undergo the hydrogen-assisted CO dissociation rather than the CO direct dissociation. R6: CHO + H→ CHOH (Figure 4b). In this reaction, the adsorbed H atom moves from hcpNi in the initial state to TNi in the transition state TS6 with the O−H distance of 1.293 Å. The reaction requires an energy barrier of 1.62 eV and is endothermic by 1.08 eV. R7: CHO + H → CH2O (Figure 4c). In this transition state TS7, the H at TNi site tends to bond with C atom in CHO with C−H distance of 1.535 Å. The product CH2O locates at the bri2 site. The reaction is endothermic by 0.18 eV with an energy barrier of 0.74 eV. Hence, CHO hydrogenation in R7 is more favorable compared with R5 = CHO → CH + O (ΔEa = 0.81 eV) and R6 = CHO + H → CHOH (ΔEa = 1.62 eV). 3.3.4. CH2O Dissociation and Hydrogenation. R8: CH2O → CH2 + O (Figure 5a). In the initial state, O atom bonds with Ce atom (bri2 site). After the dissociation, the isolated O atom sits on the fccCe site. The reaction is in equilibrium thermodynamically (ΔH = −0.03 eV). The Ce will promote the CH2O dissociation due to the low energy barrier of 0.89 eV, compared with the pure Ni(111) (1.41 eV)31 and Ni4 cluster (2.09 eV).28

barrier (ΔEa = 1.66 eV), suggesting that the two processes, i.e., CO desorption and dissociation, are competitive. However, if the temperature effect is considered, the entropy effect would reduce the free energy changes of the CO desorption more than that of CO dissociation.48 In this sense, the CO desorption will be much easier than the CO dissociation under the experimental operating temperature.48 R3: CO + H→ COH (Figure 3c). In this hydrogenation reaction, H bonds to O atom. In the initial state, O−H distance is very large, i.e., 3.172 Å. It shortens to 1.309 Å in the transition state. The reaction has the highest energy barrier of 2.19 eV and the largest endothermic energy of 1.42 eV among the studied elementary reactions shown in Table 2. This suggests that the methanation of syngas process via COH intermediate is hindered both thermodynamically and kinetically. R4: CO + H → CHO (Figure 3d). In this process, H bonds to C atom. In the initial state, H locates at fccNi site. It moves to the TNi site at transition state TS4 with C−H distance of 1.514 Å. Compared with R3 (ΔEa = 2.19 eV and ΔH = 1.42 eV), R4 is more favorable due to the less endothermic energy of 0.72 eV and lower energy barrier of 0.92 eV, (Table 2). This is in agreement with the pure Ni(111) and K-modified Ni(111) surfaces, in which the CO hydrogenation would prefer to undergo CO + H → CHO.31,32,49,50 Thus, Ce, as well as K, shows the same trend for CO hydrogenation. Compared with the CO dissociation on the Ce−Ni(111) surface (ΔEa = 1.66 eV), CO + H → CHO (ΔEa = 0.92 eV) is also favorable. 3.3.3. CHO Dissociation and Hydrogenation. R5: CHO → CH + O (Figure 4a). In this process, C−O distance is elongated from 1.334 Å in initial adsorbed CHO to 1.812 Å in the F


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Figure 5. Structures of (a−c) CH2O and (d and e) CH3O dissociation and hydrogenation on the Ce−Ni(111) surface. The gray, red, and white balls denote the C, O, and H atom, respectively.

R9: CH2O + H → CH2OH (Figure 5b). In this reaction, H bonds with O atom. The reaction has an energy barrier of 1.26 eV and is endothermic by 0.77 eV. Compared with CH2O dissociation (R8, ΔEa = 0.89 eV, ΔH = −0.03 eV), R9 is less favorable both thermodynamically and kinetically. R10: CH2O + H → CH3O (Figure 5c). For the case of H bonding with C atom, the energy barrier is lower, i.e., 0.75 eV and the process is exothermic by −0.33 eV. This makes R10 more favorable than R8 (ΔEa = 0.89 eV, ΔH = −0.03 eV) and R9 (ΔEa = 1.26 eV, ΔH = 0.77 eV). 3.3.5. CH3O Dissociation and Hydrogenation. R11: CH3O → CH3 + O (Figure 5d). In this reaction, CH3 fragment only bonds with O atom in the initial state. After the dissociation, it sits on the TNi site. The C−O distance is 2.309 Å in the

transition state. Compared with CHO (R5) and CH2O (R8) dissociation, CH3O dissociation has high energy barrier of 1.79 eV and large endothermic energy of 0.29 eV, implying that CH3O dissociation is not favorable. R12: CH3O + H → CH3OH (Figure 5e). Since the C atom is saturated in CH3O, the H atom can only be connected with O atom. The O−H distance is 1.368 Å in the transition state. The energy barrier of CH3O hydrogenation (R12, ΔEa = 1.12 eV) is lower than CH3O dissociation (R11, ΔEa = 1.79 eV) (Table 2). Compared with CHxO + H → CHxOH (x = 0−2) (2.19 eV for R3 at x = 0, 1.62 eV for R6 at x = 1 and 1.26 eV for R9 at x = 2), R12 has lower energy barrier. 3.3.6. CHxOH (x = 0−3) Dissociation. R13-R16: CHxOH → CHx + OH (Figure 6a−d). The formation of CHxOH (x = 0−3) G


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Figure 6. Structures of CHxOH (x = 0−3) dissociation on the Ce−Ni(111) surface. The gray, red, and white balls denote the C, O, and H atom, respectively.

hydrogenation reactions, respectively (Table 2). The reactions are exothermic by −0.48, −0.12, and −0.45 eV for R17, R19, and R20, respectively, while it is endothermic by 0.38 eV for R18 (Table 2). 3.3.8. O and OH Hydrogenation. R21, O + H → OH (Figure 8a), and R22, OH + H → H2O (Figure 8b). O atom hydrogenation into OH (R21) is in equilibrium thermodynamically (ΔH = 0.08 eV), while OH hydrogenation into H2O (R22) is endothermic by 0.81 eV. The energy barrier is similar for both reactions, i.e., 1.16 eV for R21, 1.18 eV for R22. These values are close to those on the pure Ni(111) surface, i.e., 1.16 eV for R21 and 1.13 eV for R22. This implies that the Ce has little influence on the O + H → OH and OH + H → H2O reactions. 3.3.9. The Possible Pathways for CO + 3H2 → CH4 + H2O. Since the reaction CO → C + O (R2) and CO + H → COH (R3) has high energy barriers (1.66 eV for the former, 2.19 eV for the latter), the pathways of CO → C → CH4 and CO → COH → C → CH4 are excluded. In addition, the isomerization reactions of CHxO ↔ CHx−1OH (x = 1−3) are also excluded due to the high energy barriers (Figure S2, Supporting

has a relative high energy barrier of 2.19, 1.62, 1.26, and 1.12 eV, respectively (Table 2). Here, we wish to know what happens with their dissociation. For R13: COH → C + OH (Figure 6a), the dissociation process is exothermic by −0.65 eV) with an energy barrier of 0.44 eV. Similarly, CHxOH (x = 1−3) dissociation are all exothermic with the energy barriers of 0.02, 0.20, and 1.13 eV for CHOH (R14, Figure 6b), CH2OH (R15, Figure 6c) and CH3OH (R16, Figure 6d), respectively. Thus, from Table 2, we can find that CHxOH (x = 0−2) dissociation is relatively easy compared with the formation. For CH3OH, its dissociation (R16 with ΔEa = 1.13 eV and ΔH = −0.79 eV) and formation (R12 with ΔEa = 1.12 eV and ΔH = 0.81 eV) are competitive kinetically, in which the former one is more favorable thermodynamically. This reveals that CHxOH can be dissociated easily into CHx and OH on the Ce−Ni(111) surface. 3.3.7. CHx Hydrogenation. R17-R20: CHx + H → CHx+1 (x = 0−3) (Figure 7a−d). The CHx hydrogenation reactions occur only at the TCe and fccCe sites. The energy barriers are 0.78, 0.69, 0.70, and 0.84 eV for C (R17, Figure 7a), CH (R18, Figure 7b), CH2 (R19, Figure 7c), and CH3 (R20, Figure 7d) H


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Figure 7. Structures of CHx hydrogenation on the Ce−Ni(111) surface. The gray and white balls denote the C and H atom, respectively.

Figure 8. Structures of O and OH hydrogenation on the Ce−Ni(111) surface. The red and white balls denote the O and H atoms, respectively.

I


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Figure 9. Potential energy profiles of CO + 3H2 → CH4 + H2O for the possible pathways on the Ce−Ni(111) surface. CO(g) denotes the gas phase CO. CHx−O and CHx−OH denote the separated CHx and O (or OH) species. The CHx+ O means the coadsorbed of CHx species and O atom. The separate energy of CHx and O (or OH) is estimated by ΔEint in section 2.2. The black and red lines denote the process of CO → CHxO (CHxOH) → CHx → CH4 and O →OH → H2O on the Ce−Ni(111) surface, respectively. For comparison, the potential energy profile on the pure Ni(111) is shown in part a, in which the blue and green lines denote the process of CO → CHxO (CHxOH) → CHx → CH4 and O → OH → H2O on the pure Ni(111) surface, respectively.

Information). The summarized possible pathways are as follows:

P5: CO + 6H → CH 2O + 4H → CH 2OH + 3H → CH 2 + OH + 3H → CH4 + H 2O

P1: CO + 6H → CHO + 5H → CH + O + 5H P6: CO + 6H → CH3O + 3H → CH3OH + 2H

→ CH4 + H 2O

→ CH3 + OH + 2H → CH4 + H 2O

P2: CO + 6H → CH 2O + 4H → CH 2 + O + 4H

The corresponding potential energy curves are shown in Figure 9, in which the energy of the gas CO and 6 adsorbed H atoms (no interaction) is set as zero energy. It is seen that all the energy barriers for P1 are negative compared with the zero energy (Figure 9a). The highest energy barrier is −0.10 eV. For the remaining pathways, the highest barriers are positive, i.e., 0.24 eV for P2 (Figure 9b), 0.73 eV for P3 (Figure 9c), 0.71 eV for P4 (Figure 9d), 0.53 eV for P5 (Figure 9e), and 0.88 eV for P6 (Figure 9f). This indicates that P1 is the main channel for CO + 3H2 → CH4 + H2O on the Ce−Ni(111) surface, same as the pure Ni(111).31,32 For the most favorite pathway P1, for

→ CH4 + H 2O P3: CO + 6H → CH3O + 4H → CH3 + O + 3H → CH4 + H 2O

P4: CO + 6H → CHO + 5H → CHOH + 4H → CH + OH + 4H → CH4 + H 2O J


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The Journal of Physical Chemistry C Table 3. Calculated Rate Constants (s−1) Involved in the Pathway P1 at 300 °C on the Ce−Ni(111) Surfacea reaction

rate equation

k(forward)

k−1(reverse)

1.83 × 105

1.33 × 1011

1.27 × 106

1.38 × 102

2.48 × 107

3.35 × 1010

R1 R2′

H2+* ↔ 2H* CO+* ↔ CO*

R4

CO*+H* ↔ CHO* + *

R5

CHO*+* ↔ CH* + O*

R18

CH*+H* ↔ CH2* + *

k 2θCOθH − k 2−1θCHOθ * k5θCHOθ − k5−1θCHθO * k18θCHθH − k18−1θCH2θ *

R19

CH2*+H* ↔ CH3*+*

k19θCH2θH − k19−1θCH3θ *

1.72 × 107

4.48 × 105

R20

CH3*+H* ↔ CH4* + *

k 20θCH3θH − k 20−1θCH4θ *

1.35 × 106

0.12

k 21−1θOHθ *

5.02 × 10

4

2.87

−1

4.46 × 10

2

9.53 × 109

R21 R22

O*+H* ↔ OH* + * OH*+H* ↔ H2O* + *

k 21θOθH −

k 22θHθOH − k 22 θH2Oθ *

The k and k−1 denote the forward and reverse rate constants. The θ denotes the coverage of the adsorbed species on the Ni(111) surface. The asterisk denotes the free surface sites.

a

comparison, we also calculated the corresponding energy (ΔEa and ΔH in Table S2, Supporting Information) for the pure Ni(111) and the potential energy curves are shown in Figure 9a. It can be found that CO + 3H2 → CH4 + H2O via P1 are more favorable on the Ce−Ni(111) surface (the highest energy barrier is −0.10 eV) compared with the pure Ni(111) surface (the highest energy barrier of 0.82 eV) both thermodynamically and kinetically. In addition, from the elementary reaction barriers involved in P1 on the Ce−Ni(111)(Table 2), the ratedetermining step barrier (OH + H → H2O, ΔEa = 1.18 eV) is also lower than that on the pure Ni(111) surface (CO + H → CHO, ΔEa = 1.36 eV) (Table S2). This is in good agreement with the experimental observation that the introduced Ce will improve the catalytic activity for methanation of syngas on the Ni(111).18,20 3.4. Microkinetic Model. To further explore the influence of the reaction temperatures and H2/CO ratios on the reaction rate on the Ce−Ni(111) surface, a microkinetic analysis based on the DFT studies51,52 is employed on the most favorable pathway P1. The details about the microkinetic model are shown in Supporting Information (under the section “Brief Description of the Model”). In the microkinetic model, we have adopted the temperatures from 300 to 550 °C,11,12,15−20 H2/ CO = 720 and the syngas pressure of 0.1 MPa20 to simulate the experimental conditions. To accurately describe the reaction at high temperatures (from 300 to 550 °C), the entropy and enthalpy corrections for the energy barrier ΔEa and reaction energy ΔH should be considered. The entropy effect is known to be included in the pre-exponential factor.53−55 The obtained results are shown in Tables S3−S8, Supporting Information. For entropy at the considered temperatures, the correction is usually less than 0.07 eV, except for R20 (CH3 + H → CH4) (ranging from 0.255 to 0.345 eV in ΔH depending on temperatures), R22 (OH + H → H2O) (−0.088 to −0.148 eV in ΔEa and 0.191 to 0.270 eV in ΔH), R5 (CHO → CH + O) (−0.084 to −0.112 eV in ΔEa), R21 (O + H → OH) (0.095 to 0.18 eV in ΔEa). For enthalpy, the correction is below 0.05 eV at the considered temperatures. The rate constants of the elementary reaction steps involved in the P1 at 300 °C are shown in Table 3, while the values for the other temperatures are shown in Table S9, Supporting Information. In addition, the calculated reaction rates for the P1 on the Ce−Ni(111) and the pure Ni(111) surfaces at temperatures of 300−550 °C are shown in Figure 10a. It is

Figure 10. (a) Reaction rates of P1 (CO + 3H2 → CHO + 5H → CH + O + 5H → CH4 + H2O) as a function of the temperatures on the Ce−Ni(111) and pure Ni(111) surfaces. The cross point between the green dotted line and black line denotes the experimental temperature 340 °C on the Ce−Ni(111).20 (b) Reaction rate of P1 as a function of H2/CO ratios on the Ce−Ni(111) at 300 and 550 °C, respectively.

seen clearly that the reaction rate for the P1 increases with the increase of the temperatures on both Ce−Ni(111) and pure Ni(111) surfaces. Compared with the reaction rate for the P1 on the pure Ni(111), the reaction rate for the P1 is much higher on the Ce−Ni(111) surface at the same temperature, indicating that the doped Ce improves the reaction rate K


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Table 4. Coverage (molecule/cm2) of CHO and CHx in the Pathway P1 on the Ce−Ni(111) Surface at 300 and 550 °C 300 °C H2/CO 3 4 5 6 7 8

CHO 8.18 8.45 8.62 8.74 8.83 8.90

× × × × × ×

1010 1010 1010 1010 1010 1010

CH 1.99 2.01 2.02 2.03 2.04 2.05

× × × × × ×

1010 1010 1010 1010 1010 1010

550 °C CH2 2.70 2.81 2.89 2.95 2.99 3.03

× × × × × ×

108 108 108 108 108 108

CH3 3.39 3.53 3.63 3.71 3.76 3.81

× × × × × ×

CHO

109 109 109 109 109 109

significantly. Moreover, the reaction rate for the P1 at ∼340 °C on the Ce−Ni(111) surface is similar to that of the pure Ni(111) surface at ∼450 °C (Figure 10a, green dotted line for the eye). This is in agreement with the experimental observation that the doped Ce can reduce the reaction temperature from ∼500 to ∼340 °C on the Ni(111) surface.11,12,18−20 To explore the influence of H2 partial pressure, different H2/ CO ratios from 3 to 8 have been investigated at T = 300 and 550 °C, respectively. From Figure 10b, it is found that with the increase of H2/CO ratios, the reaction rate for the P1 increases slightly, while large influence is found for the different temperatures. Thus, the methanation of syngas on the Ce− Ni(111) surface is more sensitive to the reaction temperatures than H2/CO ratios. Furthermore, the coverage of CHO and CHx (x = 1−3) species in P1 with different H2/CO ratios and temperatures on the Ce−Ni(111) surface is also calculated. The corresponding values at 300 and 550 °C are shown in Table 4, while the values for the other temperatures (350, 400, 450, 500 °C) are shown in Tables S10−S13, Supporting Information. From Table 4, it can be found that the coverage of CHO (CH) increases (decreases) as the H2/CO ratios increase from 3 to 8, while the coverage of CH2 and CH3 is almost unchanged. Because of the highest coverage at various H2/CO ratios (from 3 to 8), CHO species is the major intermediate on the catalytic surface, followed by CH species. In addition, the rate constants of CH → C + H at the reaction temperatures of 300−550 °C indicate that CH → C + H has higher rate constant on the pure Ni(111) than on the Ce−Ni(111) (Figure S3, Supporting Information). This implies that less C atom will be formed on the Ce−Ni(111) surface. Thus, the doped Ce can improve the resistance to deposition of carbon on the Ni(111), which is in agreement with the experimental observation.19,20

8.90 9.00 9.19 9.32 9.41 9.49

× × × × × ×

1011 1011 1011 1011 1011 1011

CH 8.72 8.69 8.52 8.40 8.31 8.24

× × × × × ×

1011 1011 1011 1011 1011 1011

CH2 2.91 2.90 2.90 2.90 2.90 2.90

× × × × × ×

1010 1010 1010 1010 1010 1010

CH3 1.36 1.36 1.36 1.36 1.36 1.36

× × × × × ×

1011 1011 1011 1011 1011 1011

increase of the temperatures, but only slightly with the increase of H2/CO ratios on the Ce−Ni(111) surface. CHO is the major intermediate on the catalytic surface.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07400. Brief description of the microkinetic model; the energy barriers, reaction energies and interaction in initial state for the possible elementary reactions; the energy barriers (ΔEa, eV) and reaction energies (ΔH,eV) involved in P1 for the pure Ni(111); the entropy and enthalpy corrections for the energy barriers and reaction energies at temperatures of 300, 350, 400, 450, 500, and 550 °C; the calculated rate constants (s−1) involved in P1 at temperatures from 350 to 550 °C on the Ce−Ni(111) surface; the coverage (molecule/cm2) of CHO and CHx in P1 on the Ce−Ni(111) surface at 350, 400, 450, and 500 °C; the differential charge densities of O, OH, H2O, CHxO, and CHxOH (x = 1−3); the barriers and structures of CHxO ↔ CHx−1OH (x = 1−3) reactions; and the rate constant of CH → C + H as a function of the temperatures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.T.). *E-mail: [email protected] (Z.W.). Notes

The authors declare no competing financial interest.

4. CONCLUSIONS The possible elementary reactions for the methanation of syngas on the Ce−Ni(111) surface has been studied theoretically. The results indicate that compared with pure Ni(111), the adsorption of the intermediates is enhanced on the Ce−Ni(111) surface, except for H2, in particular for the Ocontaining species. The reaction pathway CO + 3H2 → CHO + 5H → CH + O + 5H → CH4 + H2O is the most favorable on the Ce−Ni(111) surface. The rate-determining step is OH + H → H2O with an energy barrier of 1.18 eV on the Ce−Ni(111) surface, lower than 1.36 eV on the pure Ni(111) surface (CO + H → CHO). Thus, the methanation of syngas on the Ce− Ni(111) is more favorable both thermodynamically and kinetically compared with the pure Ni(111). The microkinetic analysis indicates that the methanation of syngas has high reaction rate on the Ce−Ni(111) compared with the pure Ni(111). The reaction rate increases dramatically with the

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ACKNOWLEDGMENTS

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REFERENCES

This work is supported by the National Natural Science Foundation of China (21503210, 21521092), Jilin Province Youth Fund (20130522141JH), Jilin Province Natural Science Foundation (20150101012JC) and Special Program for Applied Research on Super Computation of the NSFCGuangdong Joint Fund (the second phase). The authors also express thanks for the financial support from the Department of Science and Technology of Sichuan Province (2011GZX0077, 2012JZ0007, and 2014HH0049). Part of the computational time is supported by the Performance Computing Center of Jilin University and Changchun Normal University.

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DFT Study on the Methane Synthesis from Syngas on a Cerium

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