Study on syngas methanation mechanism over Ni 4 /MCM-41 catalyst based on density functional theory

Computational models

We set the parameters of MCM-41 model according to the literature. ¹⁸ The two walls of MCM-41 are connected by bridging the oxygen center (O_b, Figure 1(a)). From the structural optimization of the flat plate model, 535 pm for the edges of a hexagonal unit cell were obtained and α was 60°, and the optimized structural parameters were 165 pm for Si–O_s, 167 pm for Si–O_b, and Si–O_b–Si = 180°, in which, O_s denotes a surface O center, Figure 1(a). MCM-type single-wall structure has recently been employed to model a silicate overlayer on a molybdenum support. ¹⁹ For the adsorption on MCM-41 carrier, a (2 × 2) unit cell of the ideal substrate (Figure 1(a)) was employed and a defect site as O_b vacancy was constructed, in which the Si center of the “bottom” layer is saturated with the silanol group. The unconstrained optimization was performed for the structures of the MCM-41 model. The slabs were separated by setting a vacuum region of 15 Å in the z direction, which were perpendicular to the surface.

OPEN IN VIEWER

Figure 1.

The top and side views of (a) the MCM-41 surface and (b) Ni₄ cluster supported on surface. Blue balls stand for Si atoms, red balls stand for O atoms, and green balls stand for Ni atoms.

In addition, the Ni₄ cluster, which is the smallest three-dimensional structure of nickel atoms, is chosen to study the interaction between nickel nanoparticles and between nickel particles and support. ²⁰

Computational methodology

Dmol³ code on Materials Studio 7.0 Package was employed to carry out DFT calculations. ²¹ The correction and exchange effects were studied with the Perdew–Burke–Ernzerh (PBE) and generalized gradient approximation (GGA) functional. An effective core potential (ECP) was adopted to replace the inner electrons of the nickel atoms based on the double numerical plus polarization (DNP) basis sets, which were kept frozen. ²² In adsorption studies, a grid of (5 × 5 × 1) k-points was employed for the surface model of the mesoporous molecular sieves MCM-41. A 10 × 10 × 10 Å ³ unit cell was adopted to calculate the free adsorbates, and an all-electron basis set was used to calculate other atoms. The convergence criterion for the energy was set at 2.0 × 10⁻⁵ Hartree with the spin unrestricted condition. The convergence criterion for maximum displacement changes and maximum force were set at 5.0 × 10⁻³ Å and 4.0 × 10⁻³ Hartree Å⁻¹, respectively. The thermal smearing value and the k-point was set as 0.005 Ha and 4 × 4 × 1, respectively. The self-consistent field (SCF) convergence was 1 × 10⁻⁵ Hartree. The reaction transition state was identified by the linear synchronous transit (LST/QST) of the Dmol³ code. ²³ Adsorption energy is defined as follows

E ads = E substrate + E adsorbate − E adsorbate / substrate

in which E_{adsorbate/substrate} represents the total energy of the adsorbate–substrate system in equilibrium; E_substrate represents the total energies of the substrate; and E_adsorbate represents the total energies of the substrate, the free adsorbate.

Ni₄ cluster supported on the MCM-41 surface

Figure 1 shows the configurations of MCM-41 and Ni₄ cluster supported on MCM-41 surface. Because the tetrahedral structure is more energy-efficient among the three possible structures of Ni₄ cluster (linear, rhombic, and tetrahedral), we chose the three-dimensional Ni₄ with tetrahedral structure to adsorb on the surface of MCM-41 as the catalyst to study the mechanism of CO methanation. Figure 1(b) shows the most stable adsorption configuration of the Ni₄ cluster on MCM-41 surface and the key structural parameters. The relative bonding energy of Ni₄ cluster on MCM-41 surface is higher than that of Ni₄ cluster on Al₂O₃ surface (67 vs 479 kJ mol⁻¹), which may effectively inhibit the reunion of Ni atoms on the surface of MCM-41 in the process of CO methanation.

Adsorption of species on Ni₄/MCM-41 surface

There is no doubt that the adsorption of species plays an important role in the catalytic process of CO methanation. Therefore, the species adsorbed at different sites on the surface of Ni₄/MCM-41 catalyst involved in CO hydrogenation reaction are studied in this section, and Figure 2 shows the calculated adsorption energy and preferred adsorption sites of adsorption species involved in the CO hydrogenation reaction on the Ni₄/MCM-41 catalyst surface. Because hydrogen molecules could easily dissociate into atomic hydrogen on the surface of nickel metal^12,15,24,25 any possible adsorption sites on the surface of Ni₄/MCM-41 catalyst will be filled with high concentration of H atoms and highly efficient adsorption under the condition of syngas methanation. Detailed configuration descriptions of adsorbed species on the surface of Ni₄/MCM-41 catalysts involved in the CO methanation are provided in the Supplementary Information.

OPEN IN VIEWER

Figure 2.

The calculated adsorption energy and preferred adsorption sites of adsorption species involved in the CO hydrogenation reaction on Ni₄/MCM-41.

Reaction pathways of syngas methanation

The calculation of activation energy (E_a) is employed to investigate the reaction mechanism of syngas methanation after identifying the preferred adsorption sites of the important possible species involved in the reaction. There are a total of 12 elemental steps involved in methanation of syngas to methane, as shown in Figure 3. The calculated activation energies (E_a) and reaction enthalpies (ΔH) for each elemental steps over Ni₄/MCM-41 surface are also shown in Figure 3. DFT calculation is adopted to evaluate multiple reaction pathways to obtain the optimal pathways on the Ni₄/MCM-41 catalyst.

OPEN IN VIEWER

Figure 3.

Elementary steps and the corresponding calculated E_a and ΔH values involved in CO hydrogenation over Ni₄/MCM-41 surface. TS denotes the transition state, the first number in parentheses denotes the activation energy (E_a), and the second number denotes the reaction enthalpy (ΔH) with the unit of kJ mol⁻¹.

Formation of formyl (CHO)

As shown in Figure 3, two kinds of reactions may occur on the CO species adsorbed on the Ni₄/MCM-41 surface, including the adsorbed CO species directly breaking into C and O atoms and reacting with H to form formyl species (COH or CHO) by hydrogenation. The relative energy files of the three possible reactions (R1–R3) and the structures of the initial states (ISs), transition states (TSs), and final states (FSs) are shown in Figure 4. It can be seen that the activation energies for direct dissociation of CO, CHO, and COH species formation are 312.8, 168.3, and 287.4 kJ mol⁻¹. Compared with the direct dissociation of CO, the activation energy required for the formation of CHO and COH is lower, with the CHO species as the lowest. These results indicate that the first intermediate of CO hydrogenation reaction should be CHO species based on activation energy.

OPEN IN VIEWER

Figure 4.

Potential energy profile for R1–R3 over Ni₄/MCM-41 catalyst with the configurations of initial states, transition states, and final states.

During the formation of CHO species, CO and H coadsorbed on the surface of Ni₄/MCM-41 catalyst, where CO is strongly present at the bridge site through C atom with the C–Ni–1 and C–Ni–2, and the H is located at the top of the Ni₄ atom with an H–Ni–1 bond. The formation of TS2 is through the bonding of H atom with C atom of the absorbed CO, which is an endothermic step with a reaction enthalpy of 42.6 kJ mol⁻¹. In TS2, initially formed CHO species are bound to the substrate by the C–Ni–1, C–Ni–2, and H–Ni–1 bonds. The lengths of H–Ni–1, C–O bonds, and newly formed C–H bonds are 1.725, 1.328, and 1.346 Å, respectively. Subsequently, TS2 transforms into a more stable form, the final CHO species.

Formation of formaldehyde (CH₂O)

The CHO intermediate formed in R2 may dissociate directly into CH and O atom or hydrogenates with other H atom to form hydroxymethylene (CHOH) or formaldehyde (CH₂O) intermediates. The three element steps (R4–R6) corresponding to the above two possibilities are shown in Figure 3 and the corresponding configurations of ISs, TSs, and FSs and potential energy profile are presented in Figure 5. It can be seen that the activation energies for direct dissociation of CHO, CH₂O, and CHOH species formation are 323.4, 142.7, and 261.8 kJ mol⁻¹. The activation energy of CH₂O formation is 180.7 and 119.1 kJ mol⁻¹ lower than those of direct dissociation of CHO and CHOH species formation, respectively. Therefore, CHO species are more likely to form CH₂O intermediate species by hydrogenation with other H atoms and continue the following reactions.

OPEN IN VIEWER

Figure 5.

Potential energy profile for R4–R6 over Ni₄/MCM-41 catalyst with the configurations of initial states, transition states, and final states.

In the pathway of CH₂O species formation, CHO species and H atom are coadsorbed on the Ni₄ clusters, in which CHO species occupy the hollow site of Ni₄ clusters through the C and O atoms, while the H atom is located at the Ni₄ clusters’ top site via the newly formed H–Ni–1 bond. Subsequently, with the breaking of C–Ni–3 bond and O–Ni–4 bond, CHO species migrate to the top site of the Ni₄ cluster, forming the transition state TS5. Ultimately, the transition state TS5 is transformed into a more stable form, the final CH₂O species.

Formation of methoxy (CH₃O)

As shown in Figure 3, methoxy (CH₃O, R8) or hydroxymethyl (CH₂OH, R9) species can be formed by hydrogenation of the CH₂O intermediates with an H atom. Moreover, the CH₂O intermediates may also dissociate directly to form CH₂ and O atoms (R7). The configurations of ISs, TSs, and FSs corresponding to the three element steps (R7–R9) and potential energy profile are presented in Figure 6. The activation energies of CH₂O direct dissociation, formation of CH₃O, and formation of CH₂OH are 312.6, 148.2, and 202.4 kJ mol⁻¹, respectively. Obviously, in terms of energy barrier, CH₂O is more likely to generate CH₃O species by hydrogenation. Therefore, CH₃O species should be the third intermediate to continue the reaction.

OPEN IN VIEWER

Figure 6.

Potential energy profile for R7–R9 over Ni₄/MCM-41 catalyst with the configurations of initial states, transition states, and final states.

The pathway of CH₃O species formation starts from the coadsorption of the CH₂O species and H atoms on the Ni₄/MCM-41 surface. Initially, CH₂O species occupy the top site of the Ni₄ cluster via the C–Ni–1 bond and O–Ni–1 bond and the H atom is located at the bridge site of Ni–1–Ni–4. Subsequently, with the breaking of C–Ni–1 bond and H–Ni–4 bond, the coadsorption state of the CH₂O species and H atom is converted into the transition state TS8. For TS8, the bond lengths of O–Ni–1 and H–Ni–1 are 1.628 and 1.014 Å, respectively, with the C–O–Ni angle of 106°. Subsequently, the transition state TS8 is transformed into a more stable form, the final CH₃O intermediate species, which binds to the Ni₄/MCM-41 via the O atom.

Formation of CH₄

As shown in Figure 3, the preceding intermediate species of CH₃O can react with an H atom to form CH₃OH or dissociate directly into the CH₃ species and O atoms, which could further react with one more adsorbed H atom to form the final product CH₄ via TS12. The configurations of ISs, TSs, and FSs corresponding to the involved element steps (R10–R12) and potential energy profile are presented in Figure 7. It can be found that the direct dissociation of CH₃O is more likely to occur than the formation of CH₃OH in terms of activation energy (198.6 vs 264.8 kJ mol⁻¹), which is consistent with the findings of Wang et al. ¹⁵ and Han et al. ²⁵ However, unlike the results of Wang et al. ¹⁵ and Han et al., ²⁵ the activation energy required for the CH₃OH formation is 33% higher than that for the direct dissociation of CH₃O. In case the CH₃ intermediate species appeared on the surface of the Ni₄/MCM-41 catalyst, it can react rapidly with the adsorbed H atom to form CH₄, which is an exothermic reaction with 59.8 kJ mol⁻¹.

OPEN IN VIEWER

Figure 7.

Potential energy profile for R10–R12 over Ni₄/MCM-41 catalyst with the configurations of initial states, transition states, and final states.

Since the activation energy required for the CH₃OH formation is much larger than that for the CH₃O direct dissociation, it is almost impossible to form CH₃OH by CO hydrogenation over Ni₄/MCM-41 catalyst, which is consistent with our previous experimental results. ²⁶ Therefore, the formation of CH₃OH is not discussed too much. The adsorbed CH₃O species are transformed into the transitional state TS10 via breaking C–O bond and the newly formed C–Ni–1 bond. Subsequently, TS10 splits into the adsorbed CH₃ species and O atoms. For TS10, the CH₃ is located at the top site of Ni₄ cluster via the newly formed C–Ni–1 bond with a bond length of 2.024 Å. The pathway of CH₄ species formation starts from the coadsorption of the CH₃ species and H atoms on the Ni₄/MCM-41 surface. Initially, CH₃ species occupy the top site of the Ni₄ cluster via the C–Ni–1 bond and the H atom is located at the bridge site of Ni–1–Ni–4. Subsequently, the H atom migrates to the C atom to form the CH₄ species with the breaking of the H–Ni–4 bond via the transition state TS12.

General discussion

Based on the above-mentioned results, the optimal pathway of CH₄ formation over Ni₄/MCM-41 catalyst surface is CO + H → CHO + H → CH₂O + H → CH₃O → CH₃ + H → CH₄ and the rate-determining step is the direct dissociation of CH₃O species, which is consistent with those on the Ni₄/γ–Al₂O₃ ¹⁵ and Ni₄/SiC catalyst surfaces. ²⁵ Figure 8 shows the potential energy profile for syngas methanation on the Ni₄/MCM-41 catalyst surface. For Ni₄/γ–Al₂O₃, ¹⁵ and Ni₄/t–ZrO₂ surfaces, ²⁷ due to the approximate activation energy of CH₃OH formation and direct dissociation of CH₃O species, the formation of CH₃OH can significantly reduce the yield and selectivity of CH₄. However, according to our calculations, the activation energy required for the CH₃OH formation is 33% higher than that for the direct dissociation of CH₃O (198.6 vs 264.8 kJ mol⁻¹), which may predicate that the formation of CH₃OH may not occur.

OPEN IN VIEWER

Figure 8.

Potential energy profile for syngas methanation on the Ni₄/MCM-41 catalyst surface.

As the by-product CH₃OH can obviously affect the selectivity and yield of target product CH₄, the calculation results in this article indicate that compared with other conventional nickel-based catalysts, Ni₄/MCM-41 catalyst can significantly improve the selectivity and yield of CH₄ in methanation of syngas. The calculated results in this article are consistent with our previous experimental results. ²⁶ No CH₃OH by-products were found in the products of syngas methanation based on Ni₄/MCM-41 catalyst. The stability tests in our previous work show that the selectivity and yield of CH₄ over Ni₄/MCM-41 are much higher than those over Ni/γ–Al₂O₃ and Ni/SiO₂ in the syngas methanation reaction, which verifies the validity of our calculation results.