A new and different insight into the promotion mechanisms of Ga for the hydrogenation of carbon dioxide to methanol over a Ga-doped Ni(211) bimetallic catalyst

Abstract

The hydrogenation of CO₂ to CH₃OH is one of the most promising technologies for the utilization of captured CO₂ in the future. Nano Ni–Ga bimetallic catalysts have been proven to be excellent catalysts in the hydrogenation of CO₂ to CH₃OH. To investigate the promotion mechanisms of Ga for the hydrogenation of CO₂ to CH₃OH over Ga-doped Ni catalysts and the wide application of these promotion mechanisms in other catalysts and reactions, herein, density functional theory (DFT) was employed. The reaction mechanisms and the properties of Ni(211) and Ga–Ni(211) surfaces were comparatively studied. The results show that the Ni sites on both the Ni(211) and the Ga–Ni(211) surfaces are active sites, and the most stable structures of the intermediates are similar. Moreover, the Ga–Ni(211) surface is more favorable for the hydrogenation of CO₂, whereas Ni(211) is more favorable for the dissociation of CO₂. The activation barrier of the rate-limiting step in the CH₃OH formation pathway on Ni(211) is 0.54 eV higher than that on Ga–Ni(211). According to the analyses of the projected density of states (PDOS) and Hirshfeld charge transfer, the addition of Ga atoms demonstrates the reactivity of the Ga-doped Ni(211) surfaces. Most importantly, the replacement of some secondary active sites of Ni atoms with the non-active Ga atoms may lower the activities of the secondary active sites and strengthen the activities of the active sites at the step edge. These results provide a new perspective for the reaction mechanism of the hydrogenation of CO₂ to CH₃OH over the state-of-the-art Ga-doped catalysts.

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1. Introduction

Converting carbon dioxide (CO₂) to value-added and highly demanded products^1–3 in the future via electrocatalytic reduction,^4,5 photocatalytic reduction,^4,6,7 and thermocatalytic reduction^8–13 is a practical way for dealing with the captured CO₂;^14,15 in the regard, one of the related strategies is the hydrogenation of CO₂ to methanol (CH₃OH), an easily marketable chemical. Catalysis is significant for the success of state-of-the-art conversion technologies. Conventional Cu/ZnO/Al₂O₃ and their modified catalysts^16–18 used for the production of CH₃OH are well known for their advantages. However, their disadvantages, including low stability resulting from CO accumulation, are obvious.

In catalytic reactions, many heterogeneous catalysts have been proven to have an “ensemble effect”;¹⁹ moreover, promoters are very important for catalysts. For example, according to Studt et al.,¹⁰ Ga–Ni bimetallic catalysts are better for the formation of CH₃OH than Cu/ZnO/Al₂O₃ although Ni-based catalysts have been widely used for CO/CO₂ methanation,^20,21 CO₂ decomposition,²² and CO₂ reforming with methane (CH₄).²³ They guessed that the reverse water–gas shift (rWGS) reaction and methanation (at Ni sties) and CH₃OH formation (at Ga sites) occur in parallel or in series at different sites; however, they did not provide any sufficient evidence for this hypothesis. On the other hand, in our previous studies,^24,25 we found that in the process of hydrogenation of CO₂ to CH₃OH on Ga–Ni bimetallic catalysts, the active sites were Ni sites, whereas the Ga sites had weak or no interaction with the intermediates. Obviously, the promotion mechanisms of Ga in the Ga–Ni bimetallic catalysts cannot be fully determined by the increase in the active sites. Moreover, there are a number of studies in which the promotion mechanisms of Ga have been investigated. For example, Collins and coworkers²⁶ experimentally studied the hydrogenation of CO₂ to CH₃OH on a Pd/Ga₂O₃ catalyst and found that the presence of Ga was critical to the generation and evolution of atomic hydrogen. Ladera et al.²⁷ studied the use of Ga as a promoter for Cu–ZnO–ZrO₂ in the hydrogenation of CO₂ to CH₃OH, and their results showed that the presence of Ga increased the surface area of Cu and thus the active sites; this agreed very well with a previous study conducted by Toyir et al.²⁸ Similarly, Li and coworkers²⁹ have claimed that Ga³⁺ can increase the reduction of ZnO to Zn⁰ and further enhance the hydrogenation of CO₂ to CH₃OH. Medina et al.³⁰ suggested that Ga, a promoter for the Cu/SiO₂ catalysts, could create new active sites for the hydrogenation of CO₂ to CH₃OH. Theoretical investigations conducted by Santiago-Rodríguez et al.³¹ and Fiordaliso et al.³² concluded that Ga/Cu(111) and GaPd₂ might be promising catalysts for the hydrogenation of CO₂ to CH₃OH. Thus, the possible promotion mechanisms of Ga vary, and the Ga atom is a promising promoter in the catalytic hydrogenation of CO₂ to CH₃OH.

This study was designed to make progress in the understanding of the function of Ga in Ga-doped Ni bimetallic catalysts towards the hydrogenation of CO₂ to CH₃OH. The research started with the investigation of the stability and surface segregation of Ga–Ni(211), followed by the adsorption characteristics of intermediates and active sites on Ni(211) and Ga–Ni(211) involved in the catalytic process. Then, the activation barriers and reaction energies of the elementary steps were calculated, and main pathways for Ni(211) and Ga–Ni(211) were obtained, followed by the study of micro-kinetic modeling. PDOS of the Ni atoms on Ni(211) and Ga–Ni(211) and the electron transfer abilities of Ni(211) and Ga–Ni(211) were determined for the clarification of the roles of Ga in the catalytic hydrogenation of CO₂ to CH₃OH. The findings obtained in this research can inspire scientists to further prepare new catalysts. Furthermore, it can have broad impacts on the enhancement of the performance of the existing catalysts.

2. Computational details

2.1 Calculation models

In previous studies,^24,25 it has been found that the Ga₃Ni₅(111) step surface is more active than the Ga₃Ni₅(221) flat surface for the promotion of the hydrogenation of CO₂ to CH₃OH, and the structure of the Ni(211) surface is similar to that of the Ga₃Ni₅(111) surface. To investigate the promotion mechanisms of Ga, the first layer of Ni(211) was replaced with Ga atoms, similar to the case of the first layer of the Ga₃Ni₅(111) surface. Therefore, a supercell of the Ni(211) slab with four layers was used, and eight Ni atoms on the first layer of the Ni(211) surface were replaced by Ga atoms; this led to the formation of a Ga–Ni(211) surface. A 15 Å vacuum was used to separate the slab to prevent periodic interactions. The top two layers and the adsorbates were relaxed, whereas the bottom two layers were fixed at their bulk positions. The structures of the Ni(211) and Ga–Ni(211) surfaces are shown in Fig. 1, and the corresponding bond lengths are listed in Table 1, in which the Ga₃Ni₅(111) surface has been used for comparison.

Fig. 1 Side views and top views of the optimized surface structures of the Ni(211), Ga–Ni(211) and Ga₃Ni₅(111) surfaces; the values in the figure are the corresponding bond lengths, which are in Å.

Table 1 The bond lengths of Ni(211) and Ga–Ni(211) of the first layer

No.	Bond length/Å
	Ni(211)	Ga–Ni(211)	Ga3Ni5(111)
SE1	2.480	2.464	2.549
SE2	2.480	2.500	2.517
S1	2.423	2.404	2.447
S2	2.423	2.458	2.536
S3	2.423	2.400	2.395
S4	2.423	2.391	2.423
S5	2.480	2.441	2.452
S6	2.480	2.534	2.613
L1	2.447	2.426	2.394
L2	2.447	2.554	2.618
L3	2.447	2.428	2.480
L4	2.447	2.687	2.394
L5	2.481	2.478	2.558
L6	2.481	2.498	2.537

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2.2 Calculation methods

2.2.1 Surface stability, surface segregation, adsorption energy and activation barrier computation. The version 5.3.5 of the Vienna ab initio simulation package (VASP)^33–35 was used to study the surface stability, surface segregation, adsorption energies of the intermediates and activation barriers of the elementary steps of CO₂ hydrogenation to CH₃OH. The exchange-correction function was described by GGA-PBE (Generalized gradient approximation correction proposed by Perdew, Burke and Ernzerhof).³⁶ The relationship between the electron and core was treated with the projector augmented wave (PAW) method.^37,38

The supercell of the four-layer Ni(211) slabs was used for the Ni(211) and Ga-doped Ni(211) surfaces. The k-points grid of 4 × 2 × 1 was used for the integration of the Brillouin zone. The chosen cutoff energy and Sigma value were tested. Moreoevr, 400 and 500 eV cutoff energies were employed for the Ni(211) and Ga-doped Ni(211) surfaces, respectively. In addition, 0.12 and 0.10 eV sigma values were obtained for the Ni(211) and Ga-doped Ni(211) surfaces, respectively. All calculations were conducted by considering spin polarization due to the magnetic properties of Ni.

Surface stability can represent the degree of ease or difficulty of surface formation. The stability of Ga–Ni(211) was investigated by calculating the formation energy (E_f) of the surface. Surface segregation is a common phenomenon in alloys; thus, the surface segregation of Ga–Ni(211) has been determined. The energies of the configurations with one Ni atom of the first, second and fourth layer of the Ni(211) surfaces replaced by Ga were calculated. Note that the first and the second layers represent the surface and the subsurface, respectively, and the fourth layer represents the bulk Ni catalyst. Thus, three types of replaced sites, i.e. those at the step edge, slope and low edge, were examined.

The adsorption energies of the intermediates and the activation barriers and reaction energies of the elementary steps of CO₂ hydrogenation to CH₃OH on the Ni(211) and Ga–Ni(211) surfaces were calculated. A total energy below 1.0 × 10⁻⁵ eV was used to stop the geometric optimization. CI-NEB³⁹ was used to search the transition states, where the maximum force of 0.05 eV Å⁻¹ was used to stop the algorithm. Transition states were verified by frequency calculations.

2.2.2 PDOS and Hirshfeld charge calculations. The PDOS and Hirshfeld charge⁴⁰ of the Ni(211) and Ga–Ni(211) surfaces were calculated using the Cambridge sequential total energy package (CASTEP)^41,42 in materials studio 6.0.^43–45 The exchange-correction function, the tolerance of convergence, and the cutoff energy used for the computations were the same as abovementioned in section 2.2.1. Spin-polarized considerations were included in all the calculations.

3. Results and discussion

3.1 The adsorption behaviors of intermediates on the Ni(211) and Ga–Ni(211) surfaces

3.1.1 The stability of the Ga–Ni(211) surface. To examine the stability of the Ga–Ni(211) surface, its E_f was calculated using the following equation.^46,47

E_f = (E_Ga–Ni(211) + nE_Ni − E_Ni(211) − nE_Ga)/n (1)

where E_f is the formation energy of the Ga–Ni(211) surface, E_Ga–Ni(211) is the energy of the formed Ga–Ni(211) system, E_Ni(211) is the energy of the pure Ni(211) system, E_Ni and E_Ga are the energies of the single Ni and Ga atoms calculated from bulk materials, respectively, and n represents the number of Ni atoms replaced by Ga atoms.

The calculated formation energy for Ga–Ni(211) is −1.34 eV, indicating the ease of surface formation. Based on this definition, the positive E_f value suggests that the formation process is endothermic. The larger the value, the weaker the surface.⁴⁶ Negative E_f value represents both exothermic and easy characteristics of surface formation.

3.1.2 Surface segregation of the Ga–Ni bimetallic alloy. Segregation energy (E_segr) was calculated using the following equation:⁴⁸

E_segr = E_surface1(or E_surface2) − E_surface4 (2)

where E_surface1, E_surface2, and E_surface4 are the surface energies of the Ga atom replacing one of the Ni atoms in the first, second, and fourth layers, respectively. A positive value suggests that there is no surface segregation and vice versa.

The calculated segregation energies are displayed in Table 2. All the energy values are positive except that at the step edge of the first layer; this indicates that surface segregation cannot occur at the slope and low edge on both the first and second layers. Although slight surface segregation can take place at the step edge of the first layer, it does not affect the surface used in this study because only the slope and low edge Ni atoms have been replaced on the Ga–Ni(211) surface.

Table 2 Segregation energies of Ga at different sites on the surface and subsurface

Sites	E segr
	Step edge	Slope	Low edge
First layer	−0.37	0.01	0.39
Second layer	0.30	0.55	0.65

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3.1.3 The adsorption behaviors of intermediates. The adsorption energies of intermediates on the Ni(211) and Ga–Ni(211) surfaces were calculated using eqn (2):

E_ads = E_sys − (E_slab + E_adsorbate) + ΔZPE (3)

where E_ads is the adsorption energy of the intermediate, E_sys is the energy of the adsorption system, E_slab is the energy of the Ni(211) or Ga–Ni(211) slab, E_adsorbate is the energy of the intermediate in vacuum, and ΔZPE is the zero point energy correction.

The intermediates involved in the hydrogenation of CO₂ to CH₃OH on the Ni(211) and Ga–Ni(211) surfaces and their adsorption energies are displayed in Fig. 2. For comparison, their adsorption energies on Ga₃Ni₅(111) are also listed. All the adsorption sites for these intermediates have been examined on the Ni(211) and Ga–Ni(211) surfaces, and the most stable structures are displayed in Fig. S1 and S2 of the ESI.†

Fig. 2 Values of the adsorption energies of the intermediates involved in the hydrogenation of CO₂ to CH₃OH on the Ni(211) and Ga–Ni(211) surfaces. The values obtained for the Ga₃Ni₅(111) surface are used for comparison.²⁵

Considering the most stable configurations of the intermediates on Ni(211) and Ga–Ni(211), as shown in Fig. S1 and S2,† respectively, all the adsorption processes of the intermediates occur at the Ni sites along the step edge. CO₂, OH, mono-HCOO, bi-HCOO, trans-COOH, cis-COOH, t,t-COHOH, t,c-COHOH, c,c-COHOH, HCO, HCOH, CH₂OH and CH₃O prefer to adsorb at the SE-bridge sites. On the other hand, O, H, CO, H₂CO, H₂COOH, and COH likely choose to adsorb at the SE-3-Fold hollow sites. HCOOH has high affinity for adsorption at the P-Bridge sites; H₂COO prefers to adsorb at the SE-4-Fold hollow sites, whereas H₂O and CH₃OH have high selectivity for the SE-Atop sites. Fig. 2 shows that the adsorption energies of most of the intermediates on Ni(211) and Ga–Ni(211) are lower than the corresponding values on the Ga₃Ni₅(111) surface²⁵ and higher than those on the Cu(111)⁴⁹ and ZnO(0001)⁵⁰ surfaces; this means that the Ga₃Ni₅(111) surface more strongly interacts with the intermediates. Note that the adsorption energy of CO₂ on Ga–Ni(211) is −0.49 eV, which is very close to that on the Ga₃Ni₅(111) surface (−0.51 eV), whereas the corresponding value on the Ni(211) surface is −0.38 eV.

In addition, the Hirshfeld charges of CO₂ and CO₂ adsorbed on the Ni(211) and Ga–Ni(211) surfaces were calculated, and the results are displayed in Table 3. Table 3 shows that charge transfer to CO₂ from Ni(211) (0.12 e) is less than that from Ga–Ni(211) (0.14e); this indicates that the interaction between CO₂ and Ga–Ni(211) is stronger.^51,52 Thus, the adsorption energy of CO₂ on Ni(211) is 0.11 eV lower than that on Ga–Ni(211).

Table 3 Hirshfeld charges of CO₂ and adsorbed CO₂ on the Ni(211) and Ga–Ni(211) surfaces

Species	q/e
	CO2	CO2/Ni(211)	CO2/Ga–Ni(211)
C	0.30	0.16	0.15
O1	−0.15	−0.14	−0.15
O2	−0.15	−0.14	−0.14
Sum	0	−0.12	−0.14

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3.2 The main reaction pathways of the hydrogenation of CO₂ to CH₃OH on the Ni(211) and Ga–Ni(211) surfaces

To investigate the main reaction pathways of the hydrogenation of CO₂ to CH₃OH on Ni(211) and Ga–Ni(211), activation barriers and reaction energies of the elementary steps were calculated, which are listed in Table S1 of the ESI.† The structures of the initial states (ISs), transition states (TSs), and final states (FSs) of all the elementary steps on Ni(211) and Ga–Ni(211) are displayed in Fig. S3 and S4 of the ESI.† The activation barrier (E_a) and reaction energy (ΔE) were calculated using the following equations:

E^ZPE_a = E_TS − E_IS + ΔZPE (4)

ΔE = E_FS − E_IS + ΔZPE (5)

where E_IS is the energy of the initial state, E_TS is the energy of the transition state, E_FS is the energy of the final state, and ΔZPE is the zero-point energy correction.

The reaction rate constants were calculated using the equations reported in a previous study.²⁵

3.2.1 Kinetics and thermodynamics of the elementary steps.
3.2.1.1 The dissociation of H₂. Table S1† indicates that the dissociation of H₂ is spontaneous on all the considered surfaces; this suggests that the existing adsorption form of H₂ is dissociative adsorption, and two dissociated H atoms adsorb at two adjacent SE-3-Fold-Hollow sites. The reaction energies on the Ni(211) and Ga–Ni(211) surfaces are −0.72 and −0.76 eV, respectively, which mean that H₂ dissociation is highly exothermic. The calculated results for the dissociation of H₂ on Ni(211) agree very well with those obtained by Zhi et al.⁵³
3.2.1.2 The hydrogenation and dissociation of CO₂. The hydrogenation or dissociation of CO₂ is the first step of the CH₃OH formation process. As previously reported,^24,25 the first step may proceed via three possible ways: dissociation of CO₂ to CO and O, hydrogenation of CO₂ to COOH, and hydrogenation of CO₂ to HCOO. Table S1† shows that the activation barriers of R2, R3, and R4 are 0.85, 0.88 and 0.74 eV on the Ni(211) surface, and they are 0.64, 0.61, and 0.95 eV on the Ga–Ni(211) surface, which are clearly displayed in Fig. 3(a) and (b), respectively. The results show that the easiest CO₂ dissociation occurs on the Ni(211) surface, whereas CO₂ prefers to undergo hydrogenation on the Ga–Ni(211) surface; this is consistent with the results obtained for the previously studied Ga₃Ni₅(111) surface.²⁵ Furthermore, on the Ni(211) surface, the active sites will deactivate rapidly due to CO poisoning. Generally, CO is considered as an inevitable by product in the process of CO₂ hydrogenation to CH₃OH on Cu-based catalysts, resulting in low selectivity of CH₃OH.⁵⁴

Fig. 3 Reaction barriers of CO₂ hydrogenation and dissociation on (a) the Ni(211) and (c) Ga–Ni(211) surfaces, and their elementary steps on (c) the Ni(211) and (d) Ga–Ni(211) surfaces.

3.2.1.3 Successive reactions. During the reaction of bi-HCOO, the introduction of Ga increases the activation barriers for the dissociation of bi-HCOO into HCO and O and its hydrogenation to H₂COO by 0.15 and 0.13 eV, respectively, whereas it lowers the activation barrier for its hydrogenation to HCOOH by 0.13 eV. Note that the hydrogenation of bi-HCOO is highly endothermic; this indicates that the reverse reaction readily occurs; this is highly consistent with the results reported by Vesselli et al.²² The activation barriers for the formation of H₂COOH from HCOOH on the Ni(211) and Ga–Ni(211) surfaces are almost equal (0.88 and 0.90 eV, respectively), whereas the activation barrier for the formation of H₂COO on Ga–Ni(211) is 0.17 eV lower than that on the Ni(211) surface; moreover, the activation barriers for the dissociation of HCOOH into HCO and OH and the hydrogenation of HCO to H₂CO and H₂CO to CH₂OH and CH₃O on the Ga–Ni(211) surface are lower than those on the Ni(211) surface. For the transition of isomers including trans-COOH and cis-COOH, mono-HCOO and bi-HCOO, and t,t-COHOH, t,c-COHOH and c,c-COHOH, their corresponding activation barriers on both Ni(211) and Ga–Ni(211) are almost equal. The activation barriers of the hydrogenation of trans-COOH and the dissociation of c,c-COHOH are higher on Ga–Ni(211) than those on the Ni(211) surface; however, they are low enough to make the corresponding hydrogenation and dissociation occur. On both the Ni(211) and the Ga–Ni(211) surfaces, the hydrogenation of CO has higher activation barrier. On the Ga–Ni(211) surface, the activation barriers of the hydrogenation of COH to HCOH and HCOH to CH₂OH are 0.08 and 0.10 eV lower than those on the Ni(211) surface, respectively. Note that the activation barrier of H₂O formation on the Ga–Ni(211) surface is 0.54 eV lower than that on the Ni(211) surface; this means that on the Ni(211) surface, OH has reached overcapacity status.

3.2.2 The main reaction pathways. Herein, the HCOO pathway, COOH pathway, and rWGS + CO hydrogenation pathway were considered. The analysis of the main reaction pathways on the Ni(211) and Ga–Ni(211) surfaces was conducted as follows. On the Ni(211) surface, the HCOO generation pathway is bi-HCOO since the dissociation of HCOOH into HCOO and H (0.15 eV) is easier than its hydrogenation (E^ZPE_a = 0.88 eV), dissociation into HCO and OH (E^ZPE_a = 0.87 eV), and desorption (E^ZPE_a = 0.79 eV); this is well consistent with the conclusions reported by Vesselli et al.²² that HCOO is the ‘dead end’ of CO₂ hydrogenation on Ni(100). The COOH pathway starts with the hydrogenation of CO₂ to trans-COOH and then to t,t-COHOH. The transformation of the COHOH isomers is followed by c,c-COHOH dissociation into COH and OH and then hydrogenation of COH to CH₃OH via HCOH and CH₂OH intermediates; moreover, OH is hydrogenated to H₂O. In this pathway, the rate-limiting step⁵⁵ is the formation of H₂O with the activation barrier being 1.44 eV. The rWGS + CO hydrogenation pathway proceeds with the dissociation of CO₂ into CO and O and then hydrogenation of CO to HCO (E^ZPE_a = 1.37 eV) instead of COH due to a low activation barrier of HCO formation as compared to that of COH formation (E^ZPE_a = 1.87 eV). Then, HCO is consecutively hydrogenated to H₂CO, CH₂OH, and CH₃OH. The activation barrier of the formation of OH is 1.19 eV, whereas that of H₂O is 1.44 eV. This pathway has the same rate-limiting step as the COOH pathway. Therefore, CH₃OH is mainly produced via the COOH or rWGS + CO hydrogenation pathway on the Ni(211) surface with the high activation barrier of the rate-limiting step of 1.44 eV, as shown in Fig. 4.

Fig. 4 (a) COOH pathway and (b) rWGS + CO hydrogenation pathway of the CO₂ hydrogenation to methanol on the stepped Ni(211) surface. The values in the figure are the corresponding reaction barriers, which are in eV.

On the Ga–Ni(211) surface, the HCOO pathway starts with the hydrogenation of CO₂ to bi-HCOO (E^ZPE_a = 0.64 eV) followed by HCOOH (E^ZPE_a = 0.95 eV). Compared with the hydrogenation (E^ZPE_a = 0.90 eV) and desorption of HCOOH (E^ZPE_a = 0.70 eV), the dissociation of HCOOH (E^ZPE_a = 0.65 eV) is much easier. Then, the generated HCO is hydrogenated to CH₃OH via H₂CO (E^ZPE_a = 0.84 eV) and CH₂OH (E^ZPE_a = 0.60 eV) intermediates. OH can be hydrogenated to H₂O with the activation barrier of 0.90 eV. Thus, HCOOH formation is the rate-limiting step in this pathway with the reaction barrier of 0.95 eV. In the COOH pathway, CO₂ is first hydrogenated to trans-COOH (E^ZPE_a = 0.61 eV) and then to t,t-COHOH (E^ZPE_a = 0.60 eV). t,t-COHOH is subsequently transformed into t,c-COHOH (E^ZPE_a = 0.38 eV) and c,c-COHOH (E^ZPE_a = 0.47 eV) isomers, followed by its dissociation into COH and OH (E^ZPE_a = 0.75 eV). COH is further hydrogenated to CH₃OH (E^ZPE_a = 0.60 eV) via HCOH (E^ZPE_a = 0.56 eV) and CH₂OH (E^ZPE_a = 0.50 eV). The rate-limiting step is the formation of H₂O (E^ZPE_a = 0.90 eV). The pathway of rWGS + CO hydrogenation has the same intermediates as those in the case of the Ni(211) surface, in which CO₂ is first dissociated into CO and O (E^ZPE_a = 0.95 eV), and then, hydrogenation of CO to HCO (E^ZPE_a = 1.28 eV) occurs. HCO is consecutively hydrogenated to H₂CO (E^ZPE_a = 0.39 eV), CH₂OH (E^ZPE_a = 0.84 eV), and CH₃OH (E^ZPE_a = 0.60 eV). The activation barriers of the OH and H₂O formations are 1.20 and 0.90 eV, respectively. The rate-limiting step of the rWGS + CO hydrogenation pathway is the hydrogenation of CO to HCO with the activation barrier of 1.28 eV, a 0.38 eV increase as compared to that in the COOH pathway.

In brief, on the Ga–Ni(211) surface, initial generation of COOH is the main pathway with the activation barrier of the rate-limiting step being 0.90 eV, as shown in Fig. 5. The value is 0.42 eV lower than that calculated by Behrens et al.⁵⁶ on the Cu (211) surface. To the best of our knowledge, the COOH pathway as the main pathway has only been reported on the Cu(111) surfaces with the presence of H₂O ⁴⁹ and the Ga₃Ni₅(221)²⁴ and Ga₃Ni₅(111)²⁵ surfaces. In addition, the dissociation of CO₂ is more difficult than its hydrogenation because the activation energy of the former is 0.30 eV higher than that of the latter, and thus, the accumulation of CO is less likely to occur. Therefore, the doping of Ga into Ni can lower the activation barrier of the rate-limiting step in CH₃OH synthesis.

Fig. 5 Most possible reaction pathway of CO₂ hydrogenation to methanol on the Ga–Ni(211) surface. The values in the figure are the corresponding reaction barriers, which are in eV.

Interestingly, many previous studies have indicated that the use of a second metal in addition to the abovementioned Ga can change the activities of the catalysts. For example, Ling et al.⁵⁷ studied the role of Fe in Cu-based catalysts for the conversion of syngas to C₂ oxygenates and ethanol. Their results show that both the Fe and the Fe–Cu sites are active, indicating that Fe can create new active sites. In addition, Zhang et al.⁵⁸ studied C₂ oxygenates and ethanol syntheses using Co-doped Cu bimetallic catalysts, and the results show that Co can promote C–C formation due to the synergetic effect between Cu and Co. Therefore, bimetallic catalysts are a promising perspective for the preparation of new catalysts.

3.2.3 Micro-kinetic modeling. Based on the abovementioned discussion, micro-kinetic modeling^59,60 was used for the estimation of the generation rate of CH₃OH and CO under experimental conditions (101.325 kPa, T = 500–600 K, H₂ : CO₂ = 3 : 1) on the Ga–Ni(211) surface. The calculated reaction rate constants that will be used in the modeling are displayed in Table S2 of the ESI.† Under the estimation conditions, only the reactions related with the main pathway were considered, as presented in Table 4. The reaction rate for the hydrogenation of CO₂ to trans-COOH is 5 orders of magnitude higher than that of CO₂ dissociation into CO and O, indicating that Ga–Ni(211) prefers CO₂ hydrogenation instead of dissociation.

Table 4 Reaction rate constants of the elementary steps related to the main pathway in the process of CO₂ hydrogenation to CH₃OH in the 500–600 K temperature range

			k/s^−1
Reactions			500 K	525 K	550 K	575 K	600 K
1	H2 → H + H	k 1	1.59 × 10^13	1.57 × 10^13	1.56 × 10^13	1.54 × 10^13	1.53 × 10^13
3	CO2 + H → trans-COOH	k 3	2.53 × 10^8	5.23 × 10^8	1.02 × 10^9	1.86 × 10^9	3.26 × 10^9
4	CO2 → CO + O	k 4	3.00 × 10^3	9.05 × 10^3	2.47 × 10^4	6.20 × 10^4	1.44 × 10^5
16	trans-COOH → cis-COOH	k 16	2.08 × 10^8	3.60 × 10^8	5.95 × 10^8	9.43 × 10^8	1.44 × 10^9
17	cis-COOH → CO + OH	k 17	6.23 × 10^3	1.81 × 10^4	4.78 × 10^4	1.16 × 10^5	2.63 × 10^5
20	trans-COOH + H → t,t-COHOH	k 20	2.96 × 10^6	6.05 × 10^6	1.16 × 10^7	2.11 × 10^7	3.65 × 10^7
21	t,t-COHOH → t,c-COHOH	k 21	3.27 × 10^9	5.21 × 10^9	7.98 × 10^9	1.18 × 10^10	1.69 × 10^10
22	t,c-COHOH → c,c-COHOH	k 22	4.43 × 10^8	7.72 × 10^8	1.28 × 10^9	2.04 × 10^9	3.12 × 10^9
23	t,t-COHOH → COH + OH	k 23	5.19 × 10^1	1.93 × 10^2	6.37 × 10^2	1.90 × 10^3	5.18 × 10^3
24	t,c-COHOH → COH + OH	k 24	1.39 × 10^3	4.47 × 10^3	1.30 × 10^4	3.44 × 10^4	8.42 × 10^4
25	c,c-COHOH → COH + OH	k 25	5.78 × 10^5	1.39 × 10^6	3.09 × 10^6	6.43 × 10^6	1.26 × 10^7
26	COH + H → HCOH	k 26	2.41 × 10^7	4.70 × 10^7	8.65 × 10^7	1.51 × 10^8	2.52 × 10^8
27	HCOH + H → CH2OH	k 27	3.07 × 10^7	5.62 × 10^7	9.75 × 10^7	1.62 × 10^8	2.57 × 10^8
28	CH2OH + H → CH3OH	k 28	4.16 × 10^6	8.50 × 10^6	1.63 × 10^7	2.96 × 10^7	5.13 × 10^7
31	OH + H → H2O	k 31	8.65 × 10^3	2.47 × 10^4	6.40 × 10^4	1.53 × 10^5	3.42 × 10^5

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The detailed micro-kinetic modeling calculation process is presented in the ESI.† The formation rates of CH₃OH and CO were calculated using the following equations:

k_CH3OH = k₂₈ × θ_CH2OH × θ_H (6)

k_CO = k₄ × θ_CO2 (7)

where k_CH3OH and k_CO are the formation rates of CH₃OH and CO, respectively; k₂₈ and k₄ are the reaction rates of the reaction 4 and 28, respectively; θ_CH2OH, θ_H, and θ_CO2 are the coverage rates of CH₂OH, H and CO₂, respectively.

The productivities of CH₃OH and CO are 0.097 and 0.20 per site at 500 K, and therefore, the relative selectivity values of CH₃OH and CO are 32.36% and 67.64%, respectively, which are very close to the results obtained for Ga₃Ni₅ studied by Studt et al.¹⁰ Similarly, the productivities of CH₃OH and CO at 525, 550, 575, and 600 K are obtained, and the relative selectivity of CH₃OH increases with an increase in temperature.

3.3 General discussion

To analyze the promotion mechanism of Ga atom in Ga-doped Ni catalysts, PDOS of Ni in Ni(211) and Ga–Ni(211) was calculated. The calculated results for the d-band centers are displayed in Fig. 6(a). Both the d-band center and the peak of the Ni atom in the Ga–Ni(211) surface shift to Fermi level when compared with those in the Ni(211) surface, and the proximity to Fermi level is beneficial to the improvement of the activity of the catalyst according to previous studies.^52,61,62 Hence, the Ga–Ni(211) surface is more active than Ni(211).

Fig. 6 (a) Difference in the d-band center of the Ni atom in Ni(211) and Ga–Ni(211) and (b) Hirshfeld charge of the atoms on the first layer of the Ni(211) and Ga–Ni(211) surfaces.

To further study the promotion mechanism of Ga, the Hirshfeld charges of the atoms on the first layer of the Ni(211) and Ga–Ni(211) surfaces were studied, and the results are shown in Fig. 6(b). As abovementioned, the Ni(211) surface has a step edge effect on CO₂ hydrogenation; this means that the atoms at the step edge are more active than those at the slope and low edge. Replacements with Ga atoms were conducted to substitute some of the slope and low edge Ni atoms, that is, Ga atoms partially replace the second active sites. Fig. 6(b) shows that the Hirshfeld charge of each Ni atom at the step edge is 0.01 on Ni(211). After the introduction of Ga, two charges (−0.03 and −0.06) occurred. The charge of the slope Ni atoms changed from −0.01 to −0.10. The charge of the lower edge Ni atoms changed from −0.03 to −0.10. This demonstrates that the addition of Ga to the Ni(211) atoms leads to a decrease in the Hirshfeld charge of the Ni atoms; this indicates that the Ni atoms obtain electrons from the Ga atoms. It is generally known that chemical synthesis reactions highly depend on electron transfer processes, and many metal catalysts are electron donors, especially for reduction reactions. Therefore, the introduction of Ga atoms increases the electron losing or reduction abilities of Ni atoms that are the active sites for CH₃OH formation on Ga–Ni(211), especially the atoms along the step edge. This is also demonstrated by the adsorption of CO₂ and the Hirshfeld charge transfers between CO₂ and Ni(211) and Ga–Ni(211), as shown in Table 3. Therefore, the promotion mechanism of Ga in Ga-doped Ni bimetallic catalysts involves the replacement of a part of the secondary active centers or sites and enhancement of the activity of the active centers or sites, which is different from the previously reported mechanisms.¹⁰ In summary, the conditions for the catalyst to have high-performance are as follows: the replaced atoms should not be the active sites and should transfer electrons to the active sites.

4. Conclusions

In this study, DFT was employed to study the promotion mechanisms of Ga over Ga-doped Ni bimetallic catalysts for the hydrogenation of CO₂ to CH₃OH. The adsorption behaviors of the intermediates on Ni(211) and Ga–Ni(211) were studied. The results show that the active sites of the intermediates on both surfaces are Ni sites along the step edge, and the differences in adsorption energies of most intermediates on both surfaces are lower than 0.10 eV except for CO₂, COH and CH₃O. From the aspect of electron transfers, the electrons transfer to CO₂ from Ga–Ni(211) (0.14 e) is more than that (0.12 e) from the Ni(211) surface; this means that the interaction between CO₂ and Ga–Ni(211) is stronger; this consistent with the calculated adsorption energies.

In addition, the activation barriers and reaction energies of the elementary steps during the process on both Ni(211) and Ga–Ni(211) were calculated. The results show that on both surfaces, H₂ dissociation is spontaneous. Moreover, the Ga–Ni(211) surface is favorable for CO₂ hydrogenation, whereas the Ni(211) surface is advantageous for CO₂ dissociation. The activation barriers of CO₂ hydrogenation to COOH (0.61 eV) and HCOO (0.64 eV) on the Ga–Ni(211) surface are 0.34 and 0.31 eV lower than those for CO₂ dissociation on the surface, respectively. On the Ni(211) surface, the activation barrier of CO₂ dissociation (0.74 eV) is 0.14 eV and 0.11 eV higher than that of CO₂ hydrogenation to COOH and HCOO, respectively. The introduction of Ga into the Ni(211) surface does not lower all the activation barriers of the elementary steps in the process of CO₂ hydrogenation to CH₃OH; however, it can change the reaction pathway of CO₂ hydrogenation. Moreover, the PDOS and Hirshfeld charge of Ni in both the Ni(211) and the Ga–Ni(211) surfaces were calculated. The results show that the PDOS of Ga–Ni(211) is closer to the Fermi level; this means that the Ga–Ni(211) surface is more active. Moreover, the introduction of Ga into Ni increases the reducing ability of the active sites (along the step edge) of Ni via transfer of electrons; this further enhances the catalytic activity of the catalyst. This new finding may provide an insight into the theoretical guidance for the development of high-performance catalysts for the hydrogenation of CO₂ to CH₃OH.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The computational calculation in this study was performed using the resources of the Advanced Research Computing Center (2012. Mount Moran: IBM System X cluster. Laramie, WY: University of Wyoming) and the USNSF-sponsored NCAR-Wyoming Supercomputing Center (NWSC). This work was financially supported by the China Scholarship Council (CSC).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nr01245a

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