CO Methanation on La/Ni(111) Surface:Effect of La Electron Delocalization on Activity and Selectivity

ZHI Cuimei, ZHANG Riguang, WANG Baojun

(1. College of Chemistry and bioengineering, Taiyuan University of Science and Technology, Taiyuan 030021，China) (2. Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province,Taiyuan University of Technology, Taiyuan 030024，China)

Abstract：In attempting to promote the activity and selectivity of CO conversion to CH4 and simultaneously suppress CH3OH formation, density functional theory (DFT) calculation has been employed to insight into the reaction mechanism and the effect of the promoter La on CO conversion to CH4 on La/Ni(111). Our results indicate that the promoter La could enrich the outer layer valence electron density of Ni, make the d-band center of La/Ni(111) upward, and thereby lead to a significant increase of the reactivity. Accordingly, the enhanced activity and selectivity to CH4 as well as CH3OH resistance are mainly originated from the electronic effect of the promoter La on La/Ni(111), where the synergistic effect between La and Ni plays an important role. Meanwhile, the microkinetic modeling is used to estimate the production rates of CH4 and CH3OH under the experimental conditions, and the result shows that r(CH4) is larger than r(CH3OH) at the same temperature, and the relative selectivity of CH4 reaches almost as high as 100% in the temperature range of 550 to 750 K, and thereby no CH3OH is formed when La is doped. Further, to clarify the effect of La promoter on CH4 formation at electron level, Bader charges and the projected density of states (PDOS) have been examined for CO,HCO,COH and CH2O, which are the key intermediates of Path1, Path2, Path3, and Path4 for CH4 formation, respectively. The results indicate that it is electron transfer from La to Ni and the strong interaction between La and O that weaken the C—O bond and promote the cleavage of C—O bond, and thereby lead to no CH3OH yield, which controls the selectivity to CH4. Through analyzing the differential charge density of La atom and its surrounding Ni atoms over La/Ni(111), the result of the direction along La→Ni of charge transfer, has been shed light on furtherly. Conclusively, La/Ni(111) shows a significant increase in the activity and selectivity to CH4 compared to Ni(111), which is mainly originated from the synergistic effect between La and Ni.

Key words：DFT; microkinetic modeling; differential charge density; synergistic effect

1 Introduction

Natural gas has been considered as a promising energy source with high calorific value, while the resource reserve has been decreasing in some regions of the world. CO methanation as an available technology to manufacture synthetic natural gas (SNG) from syngas, has attained great attention from academia and industry in recent years[1, 2]. Commonly, Ni catalyst has been employed for methanation due to the high activity and low cost compared with other precious catalysts[3, 4]. However, the strongly exothermic process leads to carbon formation on the surface of nickel-based catalysts[5], besides, the catalytic activity and stability will be significantly affected with the increasing of temperature. Thus, the catalysts should be highly active and be resistant to carbon deposition.

Considerable efforts have contributed to the investigation of carbon deposition resistance[5-7], Ni(111), the greatest exposed among Ni catalyst surface, was less prone for the accumulation of C centers, irrespective of its origin, in comparison to the situation on steps, as less active for C centers formation[5]. That was a real merit the flat Ni(111) as catalytic surface should serve.Unfortunately, CO methanation of Ni(111) could be expressed mainly as a competition of CO+3H2→CH4+H2O and CO+2H2→CH3OH, thereby the catalytic selectivity to CH4will be significantly affected owing to desired product CH3OH[7].

Many methods have been investigated for this situation, depending on La as auxiliaries[8, 9]. Doping La brought about modifications on catalytic activity, selectivity, and stability of Ni/La alloys by adjusting the electron properties for CO methanation at comparatively high temperatures with relatively less deactivation[9]. Ru/Ni/La showed much better CO methanation activity, where La was electron donors; adding La to Ru could enhance the electron density of Ru, and facilitate CO dissociation due to the back donation of the electron from Ru to CO[10]. La/Ce/Ni exhibited much higher activity for CO methanation attributing to the confinement of the interacted two promoters La and Ce for Ni nanoparticles[11]. The addition of La in the catalysts could limit the migration of the active Ni and lead to a high metal dispersion of Ni particles[11, 12]as well as good resistance to coke deposition. Additionally, the synergistic effect between La and Ni exhibited high selectivity to CH4[13]. La/Ni alloys had been proved effective to improve the reactivity of syngas conversion to methane, resulted from the nickel lanthanum sosoloid[11-14]. Considering the above excellent performance of La/Ni catalysts, La can be a potential promoter for CO conversion to CH4. Nevertheless, the principal reason for the observed distinctive properties by introducing La promoter was still unclear.

In this paper, using quantitative and qualitative analysis, density functional theory (DFT) calculation and microkinetic modeling were used to understand the effect of La promoter on activity and selectivity in CO methanation on La/Ni(111). To better illustrate the synergy of the promoter La and Ni, the differential charge density of the La atom and Ni atoms on La surrounding surface over La/Ni(111), as well as the d-band average energy of La/Ni(111), were investigated to prove the electron transfer occurrence of La→Ni. Specifically, to further elucidate the role of the promoter La, Bader charge analysis and projected density of states (pDOS) were performed to determine the promoting function of La promoter toward the cleavage of the C—O bond. These identified factors might offer a more comprehensive understanding of CO methanation.

2 Calculation method

2.1 Calculation models

La/Ni alloys were formed by introducing La into Ni[14]. The perovskite LaNiO3solid solution released/stored electron reversibly via La?Ni electron delocalization; La doping was expected to enhance CO conversion and CH4selectivity by ordering and weakening the bond strength of CO and promoting the hydrogenation of CHx(x=0～3), respectively. Herein, La released electrons with the C—O bond-breaking and the C—H and O—H bond-making, which was crucial for achieving high electron transfer rates[15].

A small amount of La could promote the dispersion of Ni nanoparticles, increase the content of the reduced active Ni0, and enhance the thermal stability for CO methanation, but excess La would cover some of the Ni0active sites[16-18]. Besides, the aim of this work was only qualitatively reveal the role of La doping at Ni in CO methanation, and elucidate the synergistic effect between La and Ni, therefore, the structure with one La atom doping at the Ni(111) was merely considered. There is an exposure of Top, Bridge, FCC, and HCP sites on Ni(111). The site preference of La doping at Ni(111) was described based on the formation energy (Ef), as formula (1)[19]:

Ef=ELa/Ni(111)-ENi(111)-ELa

(1)

A negativeEfshows La doping at Ni(111) is exothermic, and the more negativeEfis, the more likely the structure is. The results ofEfwere -6.81 and -6.87 eV corresponding to FCC and HCP sites; and unsurprisingly, La, originally sited at Top and Bridge sites, migrated to the nearest neighbor HCP site. According toEf, it showed that La doping at the HCP site is favorable than other three sites, which was assigned to La/Ni(111) surface, as shown in Fig.1

Fig.1 The structures and preferable sites of side view and top view for La/Ni(111) surface

The La/Ni(111), a three-layer metal slab was modeled using a 3×3 supercell and vacuum regions of 15 ?, in which the bottom layer was fixed, while the two upper layers combined with the adsorbates were relaxed during optimization. The lattice constant of 3.54 ? overestimated the experimental value of 3.52 ?[20]by 0.6%.

2.2 Calculation methods

DFT calculations were carried out using the Vienna Ab Initio Simulation Package. The exchange-correlation energy functions were deduced in terms of spin-polarized Perdew-Wang (PW91) of the generalized gradient approximation (GGA)[21]. The frozen-core interaction was described by the projector-augmented wave (PAW) method[22]. Cutoff energy of 340 eV was set as the convergence of the plane-wave expansion, the Methfessel-Paxton smearing method was described with a width of 0.1 eV[23], the Monkhorst-Pack k-point mesh was set to 5×5×1 for the Brillouin zone integration.

The climbing image nudged elastic band (CI-NEB) method was employed for the minimum energy pathway[24]. The transition state (TS) was deemed converged as follows: the total energy difference was set as less than 10-5eV/atom, and the residual force was set as less than 0.05 eV/?.

The minimum adsorbate structure was obtained by the relaxation of the adsorbate and two surface layers. For the reaction on La/Ni(111), the adsorption energy (Eads), the activation energy(Ea), and reaction energy (ΔE) with the zero-point-energy (ZPE) correction were defined by the equations (2), (3), and (4)[25]:

Eads=(Especies/slab-Eslab-Especies)+ΔZPEads

(2)

Ea=(ETS-EIS)+ΔZPEbarrier

(3)

ΔE=(EFS-EIS)+ΔZPEreaction

(4)

ΔZPEads, ΔZPEbarrier, and ΔZPEreaction, corrected by the ZPE, are corresponding toEads,Ea, and ΔE, respectively; which were determined by the equations (5), (6), and (7)[25].

(5)

(6)

(7)

whereυirepresents the vibrational frequency,his Planck’s constant.

The d-band center (εd) of Ni(111) and La/Ni(111) were evaluated by the equation (8)[26]:

(8)

whereρd(E) refers to the density of d-states at energyE.

3 Results and discussion

In this study, we will explore the role of La in La/Ni(111) toward CO methanation, which is accomplished through the comparison with the DFT results of Ni(111). Furthermore, we will describe the electron effect of the promoter La on La/Ni(111), and explain the synergistic effect between La and Ni on CH4formation.

3.1 Structures and energies of all adsorbed species

The key structural parameters andEadsof the stable structures of all adsorbates on La/Ni(111) were investigated in detail and summarized in Table 1. To facilitate comparison, earlier available research on Ni(111) are also tabulated. The corresponding configurations are displayed in Fig.S2 in the Supplementary material.

Table 1 Adsorption sites, adsorption energies (Eads, eV), and key structural parameters of the stable configurations for the adsorbed species involved in CO methanation on La/Ni(111) surface

H2dissociation can easily occur on La/Ni(111), it is strongly exothermic by 0.96 eV with a moderate activation barrier of 0.50 eV, and the imaginary frequency corresponding to the TS is 568 cm-1, as described in Fig.S1 in the Supplementary material. The dissociated H atom adsorbs at HCP site with anEadsof -2.64 eV, which are the main source of H for CO methanation. That is indeed shown to be the case in some other reports[7, 19].

The structures of C, CH, CH2, CH3, and CH4adsorbed on La/Ni(111) are much closer than those on Ni(111), and increasing order of the chemisorptive abilities are the following: CH4

The geometries of the adsorptions of O, OH, and H2O on La/Ni(111) and Ni(111) are very different, and the adsorption energies of O and OH are increase of 0.59 and 0.79 eV on La/Ni(111) compared to those on Ni(111). Interestingly, when H2O is posited at La/Ni(111), the promoter La exhibits a strong La—O bond, leading to H2O dissociation. OH tends to coordinate with La via O atom.

When La is introduced, CH3O and CH3OH adsorptions become favorable with the larger adsorption energies of -3.53 and -0.93 eV, respectively. The geometries in Fig.S1 in the Supplementary material for the adsorptions of CH3O and CH3OH on La/Ni(111) vary obviously, with O—La rather than O—Ni bonds, compared with the situations on Ni(111).

The adsorbed CO, HCO, CH2O, HCOH, and CH2OH bind to La/Ni(111) via both C and/or O atoms in a similar way to Ni(111). The difference is that O is nearing and pointing toward La, and the adsorption energies on La/Ni(111) are increased by 0.64, 0.89, 0.59, 0.29, and 0.34 eV compared to those on Ni(111), respectively. Coincidentally, the geometry and adsorption intensity of COH on La/Ni(111) resembles that on Ni(111).

Above all, La—O bond can strengthen the adsorption of oxygenates, and the dopant La accepts electrons from the lone pair of which.

3.2 The mechanism of CO methanation on La/Ni(111) surface

The syngas on La/Ni(111) mainly gain CH4and CH3OH as products. We have investigated the mechanisms consisting of elementary steps, and the main configurations for the ISs, TSs, and FSs. The potential energy diagrams are displayed in Fig.S3 and Fig.S4 in the Supplementary material, respectively. To compareEa, ΔE, andkfor steps at 550 K obtained in this work with the literature, the previous results of Ni(111) are also listed in Table 2.

Table 2 Activation barriers (Ea/eV), the reaction energies (ΔE/eV), and the rate constants k(s-1)(T=550 K) of all possible elementary reactions involved in CO methanation on La/Ni(111) surface

3.2.1 The initial CO activation

CO dissociation on La/Ni(111) only requires anEaof 1.63 eV, which is much lower than the results of 3.74 eV on Ni(111) due to the incorporation of La into Ni. The catalytic activity of CO methanation on Ni catalyst is seen to have close relation with the activity to dissociate CO[27]. Again, the promoter La can promote HCO formation with anEareduced by 0.43 eV in comparison with Ni(111), while not facilitate the formation of COH, the adsorption of which have almost the similar structure characteristic with or without La. Although theEais as high as 2.11 eV for CO interaction with H to form COH formation, the dissociation of COH only needs to overcome as low as 0.36 eV. Given this, COH is also a possible product of CO activation.

3.2.2 All possible pathways for CH4formation

To comprehensively understand the mechanism of CO methanation on La/Ni(111), taking all possible pathways involved in CO methanation is necessary. Based on the reaction network (Fig.2), the potential energy diagrams of ten possible pathways leading to CH4on La/Ni(111) are summarized and presented in Fig.S4 in the Supplementary material.

Fig.2 Reaction network involved in CO methanation to form CH4 and CH3OH on La/Ni(111) surface

It is observed that the corresponding overall activation energies of ten pathways are 2.06, 2.04, 2.11, 2.07, 2.39, 2.41, 2.73, 2.77, 3.37, and 2.65 eV, respectively. For comparison, Path1, Path2, Path3, and Path4, corresponding to the lower overall activation energy of 2.06, 2.04, 2.11, and 2.07 eV, are identified as the energetically favorable pathways for CH4formation; the key intermediates of Path1, Path2, Path3, and Path4 are CO, HCO, COH, and CH2O, correspondingly. And to be clear, there are two main strengths of Path1, Path2, Path3, and Path4, as expected: one is low energy barrier and the other is that no CH3OH forms.

3.2.3 The energetically favorable pathways for CH4formation

As above, on La/Ni(111), Path1, Path2, Path3 and Path4, as presented in Fig.3～6, are mainly responsible for CH4formation.

Fig.3 The potential energy diagram of path1 together with the structures of the ISs, TSs and FSs involved in CO methanation to form CH4 and OH on La/Ni(111) surface(The blue, gray, red, white and ultramarine balls represent Ni, C, O, H and La atoms, respectively; Bond lengths are in ?)

Fig.4 The potential energy diagram of path2 together with the structures of the ISs, TSs and FSs involved in CO methanation to form CH4 and OH on La/Ni(111) surface(Bond lengths are in ?, color coding see Fig.3)

As shown in Path1 (see Fig.3), the initial CO dissociation occurs on La/Ni(111). To produce CH4on La/Ni(111), the dissociated C then undergoes sequential hydrogenations to form CH4. C hydrogenation to CH is exothermic with ΔEof 0.29 eV. The formations of CH2and CH3are thermodynamically uphill with 0.34 eV and 0.10 eV in ΔE, respectively; the formation of CH4is significantly exothermic by 0.33 eV.Eafor CH, CH2, CH3and CH4formations are 0.52, 0.65, 0.89 and 0.45 eV, respectively; and the corresponding rate constants are 7.10×108, 2.57×107, 2.86×105and 9.48×109s-1at 550 K, which are in accordance with earlier research (Table 2).

As shown in Path2 (see Fig.4), the direct dissociation of HCO to CH and O is slightly exothermic by -0.11 eV, the correspondingEais 1.04 eV with the rate constant of 2.17×103s-1, which is much lower than the dissociation of CO (Ea=1.63 eV) with the rate constant of 8.74×10-2s-1. Next, the produced CH is followed by the successive hydrogenations to CH4.

Fig.5 The potential energy diagram of path3 together with the structures of the ISs, TSs and FSs involved in CO methanation to form CH4 and OH on La/Ni(111) surface(Bond lengths are in ?, see Fig.3 for color coding)

Fig.6 The potential energy diagram of path4 together with the structures of the ISs, TSs and FSs involved in CO methanation to form CH4 and OH on La/Ni(111) surface(Bond lengths are in ?, see Fig.3 for color coding)

As shown in Path3 (see Fig.5), COH dissociation is highly exothermic by 0.60 eV with anEaof 0.36 eV, and this step has larger rate constant of 9.78×109s-1. Further, C undergoes four sequential hydrogenations to CH4.

As shown in Path4 (see Fig.6), the formation of CH2O is thermodynamically uphill with 0.69 eV in ΔE, and the correspondingEais 0.95 eV with the rate constant of 2.58×104s-1. The produced CH2O dissociates into CH2and O with anEaof 0.22 eV; then CH2eventually leads to CH4formation.

In addition, the hydrogenation of the dissociated O is endothermic by 0.11 eV with anEaof 0.91 eV, leading to the production of OH.

3.2.4 The effect of the by-product CH3OH formation on the selectivity of CH4formation

During the generation of CH4, the by-product CH3OH can be formed by the hydrogenation of CH2OH and/or CH3O. As seen in Fig.S4 in the Supplementary material, Path7, Path8, Path9 and Path10 for the conversion of CO to CH4and CH3OH on La/Ni(111) share identical steps until the CH2OH or CH3O intermediates are formed. Herein, the selectivity of CH4and CH3OH are controlled by the relative activity of CH2OH or CH3O hydrogenation and dissociation. It can be seen that most clearly, Path9 is kinetically feasible for CH3OH formation with the overall activation energy of 2.71 eV, which is much higher than those of Path1, Path2, Path3 and Path4 (2.06, 2.04, 2.11 and 2.07) for CH4formation. This signifies that CH3OH formation is inferior to CH4formation, and the preferred product from CO methanation on La/Ni(111) should be CH4.

3.2.5 Microkinetic modeling

Although the DFT results obtained show that CH4formation is much more favorable than CH3OH formation, it is incompletly only based on the overall activation energy and the rate constant. More precisely, at the experimental terms ofPCO=25 kPa,PH2=75 kPa andT=550～750 K[28-31], the rates ofrCH4andrCH3OH, calculated using microkinetic modeling[32], can further be used to estimate the selectivity of CO methanation. The detailed calculations ofrCH4andrCH3OHare presented in the Supplementary material.rCH4andrCH3OHare listed in Table 3, and the variation of the relative selectivity ofri/(rCH4+rCH3OH) as a function of temperature is depicted in Fig.7.

As depicted in Table 3, the ratesrincrease with the increasing of temperature on La/Ni(111), andrCH4is larger thanrCH3OHat the same temperatures, suggesting that the productivity for CH4formation from syngas is higher than that for CH3OH formation.

Table 3 The rates r (s-1 site-1) for CH4 and CH3OH formations in CO methanation on La/Ni(111) surface at different temperatures

As shown in Fig.7, inT=550～750 K, the relative selectivity of CH4reaches almost as high as 100%[33], far above that of CH3OH at the same temperatures, signifying that CH3OH is hardly formed on La/Ni(111). The same is true in case that CH4selectivity increased to nearly 100% when La is added to Ni/γ-Al2O3for CO2methanation[8, 9]. Thus, the introducing La into Ni leads to a superior selectivity to CH4rather than CH3OH. This agrees well with earlier researches[28, 31], in which no CH3OH is formed.

Fig.7 The relative selectivities of the products CH4 and CH3OH on La/Ni(111) surface at the different temperatures using the microkinetic modeling

3.3 General discussions

3.3.1 Impact of the promoter La in La/Ni(111) on CO methanation

As schematically shown in Path1, Path2, Path3 and Path4 on La/Ni(111), the overall activation energies of 2.04, 2.07, 2.06 and 2.11 eV are favorable for CH4formation, which are reduced by up to 0.29 eV compared to earlier results of 2.33 eV[7]on Ni(111), indicating that a superior catalytic activity is observed over the modified La/Ni(111). The same is true in case that La increases the activity of Ni/γ-Al2O3for CO2methanation[8, 9].

In the case of the pure Ni(111), the formation of CH4from syngas is competitive with CH3OH formation (2.33vs2.36 eV[7]). This is why the selectivity of Ni(111) for CH4formation needs to be improved by introducing La into Ni, and fortunately, this approach has turned out well. On La/Ni(111), in comparison with 2.71 eV for overall activation energy of CH3OH by-product, CH4formation is much more favorable with a significant decrease of 0.67 eV due to the tuning effect of La, indicating that an excellent selectivity can be exhibited by the incorporation of La into Ni.

As stated above, introducing La into Ni performs the superior CH4activity and selectivity associated with the remarkable resistance to CH3OH formation. This is indeed shown to be the case in some other reports[34]that a CO conversion of almost 100% is obtained on La incorporated Ni-based methanation catalyst. Essentially, the rare earth element La is functioned as the electron modifier, which is helpful for activating CO molecule.

3.3.2 Electronic structure analysis based on the synergistic effect between La and Ni

As indicated by the results of DFT calculation and microkinetic modeling, introducing La into Ni can produce an excellent reactivity of CO methanation, and presumably as a result of the synergistic effect between La and Ni, which have been proven by the differential charge density of the La atom and Ni atoms on La surrounding surface over La/Ni(111) in Fig.8.

Fig.8 The differential charge density of La atom and Ni atoms on La surrounding surface over La/Ni(111): (a) side view, (b) top view (The yellow and blue shaded regions represent charge loss and charge gain, respectively)

As can be seen from Fig.8, charge loss behavior of La is happening in the yellow areas, while electron accepting behavior of Ni is occuring in the scattered blue areas. That is, there is depletion of electron density around the La atom and accumulation of electron density on the Ni atoms nearby the La atom. This “delocalization” allows electron transfer from the La atom to the electron-accepting Ni atoms placed nearby, suggesting that La makes its electrons available to the Ni atom over La/Ni(111). Thus far, the electron transfer occurs,La→Ni.

Fig.9 Projected density of states pDOS for d-band center on Ni(111) and La/Ni(111) surfaces. The vertical black lines donate the Fermi level

As presented in Fig.9, it is the electron transfer from La to Ni that enriches d-band electron density of Ni atoms, leads to a shift ofεdof La/Ni(111), and thereby improves the reactivity of Ni for CO methanation. This is known as the electron “delocalization effect”[35], partly as a result of the synergistic effect, the other result of which is the so-called structure “confinement effect”, which leads to a decline in the crystallite size and a growth in the number of La/Ni reaction sites[8, 9, 11, 36], and ultimately creates a greater increase of the reactivity of CO methanation. Apparently, the addition of the promoter La remarkably modifies the electronic environment of the Ni(111).

3.3.3 Role of promoter La

Throughout the whole reaction network, compared to Ni(111), La/Ni(111) has exhibited a significant increase in reactivity of CO methanation. The main role of the promoter La is to weaken and then promote the cleavage of C—O bonds of CO, HCO, COH and CH2O, which are respectively the key intermediates of Path1, Path2, Path3 and Path4 in CO methanation. This conclusion is further supported by Bader charges analysis[7, 19]and the projected density of states (pDOS)[26], which are depicted in Table 4 and Fig.10, respectively.

For absorbed CO, HCO, COH and CH2O, it can be clearly seen from Table 4 that the accumulation of charge is around the C atom when La is doped. The more charge number the C atoms (2.88, 3.03, 3.76 and 3.61 e) on La/Ni(111) carries, the less C positive electricity is, compared to those (2.52, 2.96, 3.29 and 3.12 e) on Ni(111), then it offers the weaker polar C—O linkage, and ultimately introduces the lower activation energy for C—O bonds breaking.

Besides, takingdC—Oof CO, HCO, COH and CH2O on Ni(111) as the reference, the C—O bonds of CO, HCO, COH and CH2O on La/Ni(111) are elongated, and the O atoms of CO, HCO, COH and CH2O move towards the La due to the weakened C—O bonds, as seen from Table 4. This agrees with the calculated activation energy for breaking C—O bonds of CO, HCO, COH and CH2O, as summarized in Table 2. Clearly, asdC—Ostretched, the activation energies of 1.63, 1.04, 0.36 and 0.22 eV corresponding to the C—O bonds cleavage of CO, HCO, COH and CH2O gradually decrease, then until CH2O has a minimum at a much longer C—O distancedC—O=1.432?, that means the C—O bonds cleavage of CO, HCO, COH and CH2O are much favorable when La is doped.

Table 4 Charges q of C and O atoms of CO, HCO, COH, and CH2O as well as the promoter La and its nearest Ni atoms of La/Ni(111) and Ni(111) surfaces

Fig.10 Projected density of states (pDOS) for CO (a)，HCO (b)，COH (c) and CH2O (d) chemisorptions on La/Ni(111) and Ni(111) surfaces. The vertical line indicates the Fermi level

As plotted in Fig.10, on La/Ni(111), there are the hybridizations between C2p, O2pand La5dorbitals, and note that the bonding states of C—O bonds of CO, HCO, COH and CH2O, located below theEf, move away theEfrelative to those on Ni(111). This is caused by electron donation from La to these molecules, similarly to which found for Cu cluster[37]. Namely, the C2p—O2pstates is pulled below theEfby the initial empty antibonding La5d, which is partially filled due to La5dparticipating in the charge transfer. Thus, the downshift of C2p—O2pstates accelerates the C—O bonds cleavage of CO, HCO, COH and CH2O.

As a consequence, the promoter La can weaken C—O bond strength, accelerate the cleavage of C—O bonds of CO, HCO, COH and CH2O, and promote CO methanation; and the increase of the reactivity of CO methanation on La/Ni(111) can be ascribed to the introduction of the promoter La.

4 Conclusions

DFT results indicate that the enhanced catalytic activity of La/Ni(111) observed theoretically is the result of a decrease in the reaction barrier for C—O bond cleavage of CO、HCO、CH2O and COH, which are the key intermediates of Path1, Path2, Path3 and Path4 for CH4formation, respectively. On the other hand, the enhanced selectivity of La/Ni(111) compared to undoped Ni(111) in CO methanation is due to the lowering of the overall activation energy toward CH4formation and the significant increasing of that toward CH3OH formation, and thereby leads to no CH3OH yield.

Meanwhile, the results of the microkinetic modeling show that CH4selectivity is far above CH3OH at any temperature between 550 K and 750 K, and CH3OH is hardly formed on La/Ni(111). Moreover, the increase of CH4selectivity can be ascribed to introducting La into Ni, which have been proven by the differential charge density of the La atom and Ni atoms on La surrounding surface over La/Ni(111). The synergistic effect between La and Ni in La/Ni(111) displays significant effectiveness in maximizing CH4and minimizing CH3OH.

Conclusively, the promoter La can weaken C—O bond strength, introduces the lower activation energy for C—O bonds breaking, and promote CO methanation. Namely, the superior activity and selectivity of La/Ni(111) is mainly originated from the synergistic effect between La and Ni.

Supplementary material：download link：http://www.mat-china.com/oa/DArticle.aspx?type=view&id=202009183