Unraveling the role of Ni13 catalyst supported on ZrO2 for CH4 dehydrogenation:The d-band electron reservoir

ZHI Cui-mei ，YANG Rui-hong ，ZHOU Chang-yu,* ，WANG Gui-ru ，DING Jia-ying ，YANG Wen

（1. College of Chemical and Biological Engineering, Shanxi Key Laboratory of High Value Utilization of Coal Gangue,Taiyuan University of Science and Technology, Taiyuan 030024, China；2. Shanxi Health Vocational College, Taiyuan 030012, China；3. Shanxi Key Laboratory of Metal Forming Theory and Technology, School of Material Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China）

Abstract: The activation of C-H bonds of CH4 is a key step for the conversion of methane to chemical commodities.Loading Ni onto ZrO2 is regarded as a relatively efficient way to harness the beneficial electronic property and the fine dispersion of the Ni catalyst for CH4 dissociation. Herein we demonstrate the crucial role of Ni13 catalyst supported on ZrO2 for the dissociation of CH4. The density functional theory (DFT) results show that the ZrO2 supported Ni13 stabilizes all species better and facilitates CH4 activation. The stepwise dehydrogenations of CH4 on Ni13-ZrO2(111) exhibits longer C-H bond lengths of ISs , lower Ea, and smaller displacements between the detaching H and the remaining CHx fragment in TSs .In addition, they are also thermodynamically more feasible. However, without the ZrO2 support on Ni13, the opposite results are obtained. Consequently, the ZrO2 modified Ni13 is more superior to the original Ni13 in CH4 dehydrogenation. The electronic analysis combining DFT calculations confirmed that the larger overlap between C 2p and Ni 3d, and the electron transfer of Ni→C cause the weaker C 2p-H 1s hybridization. In addition, the reduction of electron transfer of H→C leads to a stronger interaction between Ni and C along with a weak C-H bond. Hence, the ZrO2 support serves as the d-band electron reservoir at Ni13 and it is benefit to the activation of C-H bonds in CH4 dehydrogenation.

Key words: CH4 dehydrogenation；Ni13 catalyst；ZrO2 support；d-band electron；electron transfer

With global warming and the depletion of crude oil reserves, the development of alternative resources attracted more attention over the past decades[1].Methane has been regarded as an important clean energy and it can be converted to various commodity chemicals[2]. The activation of methane to form CHxthrough C-H bond breaking is the key[3-11]. Hence,many efforts have been carried out to develop and design efficient catalyst for the activation of CH4.

Transition metals such as Rh[3], La[4]and Ni[5-11]possess a remarkable catalytic activity for methane dissociation. Typically, supported Ni-based catalysts are of much interest, because of their high activity and low costs. For methane decomposition, the activity and dispersion of Ni are crucial. In other words, the more exposed sites in Ni-based catalyst, the higher reactivity.

Firstly, the activation of C-H bond in methane needs additional electrons. Due to the abundantdorbitals, metal oxide support exhibits better electron donation ability that enhances the reactivity of Ni.Secondly, the support with higher specific surface area is extremely benefit to disperse and anchor the fine Ni particles. This has been confirmed by previous reported literatures[12-16]. Therefore, loading Ni onto a proper support can provide additional electronic property and improve the dispersion of Ni, thereby promoting the dissociation of CH4.

In particular, ZrO2is known to have a superior electronic donor ability to enhance the Ni catalytic activity[12], although ZrO2itself has no activity for CH4dissociation[17,18]. Wang et al.[12]concluded that the Ni/ZrO2catalyst can stabilize the original Ni state due to its small size； the strong electronic donor ability of ZrO2was benefit to the formation of Ni active particles in carbon dioxide reforming of methane. Besides, the introduction of Zr ensured an excellent dispersion of Ni/Zr alloy and promoted the formation of active center, both of which facilitated the activation of the C-H bond in CH4[19], and thus, decreasing theEavalue in dry reforming of methane. Similarly, Han et al.[13]pointed out that in addition to facilitating the Ni dispersion, support can also alter the electronic property of the catalysts that influences the adsorption characteristics, the activation energy and even the reaction pathways.

In fact, the influence of strong interaction between Ni and ZrO2on the activity and dispersion of Ni has been confirmed by the recent reports[19-23]. Thanks to the strong interaction between the support ZrO2and Ni catalyst, the dispersion of Ni active components is considerably enhanced[13,17,19-23]. Besides, the interaction between nickel and support might maintain the initial state of Ni[12]. Meanwhile, the moderate interaction,generated from the partial electron transfer between Ni and Zr, might promote the adsorptions of CHx[12]. Li et al.[24]believed that La-modified Ni/Al2O3is responsible for improving the stability and reactivity of Ni catalyst via modulating the interaction between Ni and Al2O3.

Since the support has significant influence on the activity of Ni-based catalyst in CH4activation, it is necessary to investigate the interaction of nickel and support and the electron transfer between them. It is shown that the catalytic activity of small cluster Ni13supported on ZrO2(111) surface differs from that of large Ni(111) in the CO2hydrogenation reaction due to their different surface atomic arrangement and electronic characteristics[25]. Ni13exhibits higher catalytic activity and selectivity, as it has larger number of exposed surface atoms and more active sites with different coordination numbers[26]. Ni13has stable icosahedron configurations that contains 42 Ni-Ni bonds[27,28]. This makes Ni13can well maintain the structure during catalysis process[29]. Ni atom in the center of Ni13shows the highest coordination number,while each Vertex Ni atom of Ni13, possessing the significantly coordinatively unsaturated site, is the predominant active center for Ni catalyst. Yilmazer’s[30]stated that ethylene adsorption energy gradually increases with the decrease of Ni coordination number from 9 to 6 on Ni(111), Ni(100), Ni(110) and Ni13.Therefore, the icosahedral Ni13with Ihsymmetry shows higher the structural stability and reactivity[31,32]among different Nin[33].

Hence, the reaction mechanism for CH4dehydrogenation on pure Ni13and Ni13supported ZrO2(111) catalysts was investigated by using DFT calculations. The effect of support on the adsorption and activation of CHx(x=0-3) was analyzed. The results suggested that the interaction between Ni and ZrO2can improve the electron delocalization of Ni and make the adsorbed CHx(x=0-3) species become more electron-rich in reaction.

1 Computational details

DFT calculations were performed using the Vienna Ab Initio Simulation Package (VASP), where the exchange-correlation energy functional is described with Perdew-Burke-Ernzerhof (PBE) of the generalized gradient approximation (GGA)[34]. The configurations of Ni13and Ni13-ZrO2(111) are shown in Figure 1. Ni13is obtained on the basis of the magic number cluster structure[30], the average Ni-Ni distance in Vertex-Vertex (2.444 ?) and in Center-Vertex (2.325 ?) are in line with the experimental results (2.389 and 2.274 ?,respectively)[35]and theoretical calculation[26]. Thep(3×3) slabs ZrO2(111) has nine layers. A vacuum of 15 ? is added, and a lattice parameter of 5.13 ? is used[36-38]. The top five layers are fully relaxed, while the bottom four layers are fixed at their bulk positions.The energy convergence for electronic relaxation is set to 10-5eV. All atoms are relaxed until the atomic force is less than 0.02 eV/? with a kinetic energy cutoff of 400 eV[39]. The Monkhorst-Pack 2×2×1 k-point mesh is used to Brillouin zones of sample surface[40]. The transition state is searched by using the nudged elastic band method , as described by our previous work[41].

Figure 1 Top view correspond to Ni13 and Ni13-ZrO2(111)morphologies, respectively

For the adsorptions and reactions on Ni13and Ni13-ZrO2(111), the adsorption energies (Eads), the activation energy (Ea) and reaction energy (ΔE) with the zeropoint-energy (ZPE) are defined according to equations(1), (2) and (3)[41,42], whereEcluster,EadsorbateandEadsorbate/clusterare the total energies of the bare cluster,the isolated adsorbate, and the optimized structures of the adsorption configuration, respectively. ΔZPEads,ΔZPEbarrierand ΔZPEreactionrefer toZPEcorrections forEads,Eaand ΔE, respectively, which are determined according to equations (4), (5) and (6)[41,42]；his Planck′s constant,υirefers to the vibrational frequencies. Thedband center (εd) of Ni13and Ni13-ZrO2(111) is calculated by equation (7)[7], where ρdrepresents the density of states projected onto the Ni atoms′d-band；Eis the energy ofd-band.

2 Results and discussion

2.1 Adsorption of all species

The optimization structure in DFT calculation and corresponding energy of all adsorbed species involving in CH4dissociation on Ni13and Ni13-ZrO2(111) are presented in Figure 2 and Table 1. For comparison,previous calculated results on Ni(100) and Ni4are also given in Table 1. The partial density of states (pDOS)and the differential charge density for CH adsorbed on Ni13and Ni13-ZrO2(111) are shown in Figure 3.

Figure 2 Most stable configurations of adsorbed species involved in CH4 dissociation on Ni13 and Ni13-ZrO2(111), respectively The blue, gray, red, white and turquoise balls represent Ni, C, O, H and Zr atoms, respectively Bond length are in ?

Figure 3 (a) Differential charge density, and (b) projected density of states (pDOS) for CH on Ni13 and Ni13-ZrO2(111)

Table 1 Adsorption sites and adsorption energies (Eads) of the stable configurations for the adsorbed species involved in CH4 dissociation on Ni13 and Ni13-ZrO2(111), respectively

2.1.1 Structures and energies of all adsorbed species

As shown in Figure 2 and Table 1, CH4is weakly adsorbed at Ni-top of Ni13and Ni13-ZrO2(111) with similar adsorption energies of 0.04 and -0.02 eV,respectively. The physisorption nature of CH4arises from the van der Waals interaction between CH4and catalyst. The calculated CH4adsorption energy is in agreement with the results of An et al. (-0.01 eV)[43],despite that Roy and co-workers gave a more negative value (-0.41 eV)[6].

The configurations for CH3, CH2, CH, C and H adsorption on Ni13-ZrO2(111) are quite similar to those on Ni13. CH3adsorbed at the Ni-bridge site of Ni13-ZrO2(111) is more stable than at Ni13； notably, the calculated CH3adsorption energy (-2.46 eV) in this work is similar to previous results (-2.37 eV), proving the credibility of current calculations[6]. CH2, CH, C and H prefer to adsorb at Ni-fold sites (Table 1), and they give higher adsorption energy on Ni13-ZrO2(111) than on Ni13. Moreover, the calculated adsorption energies of CH2and CH (-4.76 and -7.00 eV) on Ni13-ZrO2(111) are also consistent with the reported values(-4.85 and -6.71 eV) on Ni4by Roy et al. (Table 1)[6].But the C is strongly adsorbed in four-fold hollow site with adsorption energy of -8.94 eV[6]than in three-fold hollow site on Ni13-ZrO2(111) (-7.96 eV), thus signifying that C atom tends to satisfy its valence. It should be noticed that the adsorption energies of CH2(-3.76 eV), CH (-6.43 eV), C (-7.27 eV) and H (-2.36 eV) on Ni(100) obtained by Li et al.[7]are different to our results (-4.76, -7.00, -7.96 and -3.02 eV) on Ni13-ZrO2(111). This may be due to the use of different functional.[7]

2.1.2 Stabilizing effect of ZrO2 support on all adsorbed species

Despite the similarity in adsorption configuration and site, the adsorption strength of all these species are stronger on Ni13-ZrO2(111) than on Ni13, possibly due to the existence of ZrO2support that modifies the electronic environment of Ni13. For example, for CH adsorption, as shown in Figure 3(a), the electron accepting behavior of C and charge loss behavior of Ni occur in the yellow areas and in the scattered blue areas, respectively. An accumulation of electron density is observed on the C atom, whereas for Ni atoms bonding to C atom, the electron density is decreased. Thus, the electron transfer between Ni→C turns the chemisorbed state of free CHx[44].

As shown in Figure 3(b), with the addition of ZrO2support, C 1sand C 2porbitals are shifted to higher value, while the Ni 3dorbital gives an opposite result. This leads to a larger overlap between C 2pand Ni 3das well as a stronger interaction between Ni and C atoms. Therefore, a strong adsorption of CH on Ni13-ZrO2(111) is observed. Similar result was also observed by Li et al.[7]: the adsorption strength of C on various Ni surfaces dependents on the mixing between C 2pand Ni 3dorbitals. Furthermore, the more downshift the Ni 3dorbital is, the more stable adsorption of CHxis[34].

In addition, as shown in Figure 2, C-H bond distance for CH4(1.104 ?), CH3(1.143 ?), CH2(1.189 ?) and CH (1.108 ?) are significantly enlarged on Ni13-ZrO2(111), compared to that on Ni13(1.089, 1.107,1.107 and 1.107 ?, respectively). This means that the cleavage of C-H bonds in CHxis easier on Ni13-ZrO2(111).

2.2 CH4 dissociation on Ni13 and Ni13-ZrO2(111)

The corresponding energies of various elementary steps for CH4dissociation are listed in Table 2.Previously reported results on Ni(100) and Ni4are also given in Table 2 for comparison. The energy profile and the main configurations for the initial state (ISs),transition state (TSs) and final states (FSs) on Ni13and Ni13-ZrO2(111) are displayed in Figure 4(a) and 4(b),respectively. The values for C2formation and C elimination are calculated in Figure 5.

Figure 4 Structures of the ISs, TSs and FSs relevant to CH4 dissociation on (a) Ni13 and (b) Ni13-ZrO2(111)Bond lengths are in ? see Figure 2 for color coding

Figure 5 Potential energy profile of C2 formations and C eliminations on (a) Ni13 and (b) Ni13-ZrO2(111) together with the corresponding structures Bond lengths are in ? see Figure 2 for color coding

Table 2 Activation energies the reaction energies the C-H bonds length of ISs and TSs (dC-H/?) and H displacements (?) involved in CH4 dissociation on Ni13 and Ni13-ZrO2(111)

2.2.1 Effect of ZrO2 support on activating the C-H bonds

We mainly focus on theEa,ΔEand the C-H bonds length of ISs and TSs. The activation energies for the dehydrogenation of CHx(x=4-1) on Ni13-ZrO2(111) are lower than on Ni13, indicating that the dehydrogenation of CHxis kinetically more favorable on the former than on the latter.

As shown in Table 2, the calculated activation energy (Ea, 0.61 eV) on Ni13is lower than the reported result (1.23 eV) by Li et al.[7]on Ni(100), whereas that for CH3(0.46 eV) and CH2(0.48 eV) dehydrogenations are similar to the obtained values by Li et al. (0.62 and 0.22 eV)[7]and Roy et al. (0.84 and 0.33 eV)[6]. For CH dehydrogenation, the activation energy on Ni13(0.70 eV) and on Ni(100) (0.64 eV) are similar, but both of them are much lower than on Ni4cluster (1.37 eV). It should be noticed that the dissociations of CHx(x=4-1)on Ni13-ZrO2(111) are all exothermic, with more negativeΔEthan Ni13.

For CH4→CH3+H on Ni13-ZrO2(111), it shows a lowEaof 0.14 eV, with exothermic by -0.66 eV； the C-H bond is increased from 1.104 ? in CH4to 1.122 ? in TS2-1 . After a rearrangement between H and CH3fragments, the structure of TS2-1 is obtained. Similar procedure is used for TS1-1 , despite that theEais increased to 0.61 eV, with a weak exothermic by -0.50 eV on Ni13. The C-H distance of TS1-1 is enlarged from 1.089 ? to 1.561 ?. Since the dissociation of C-H bond is more exothermic on Ni13-ZrO2(111) (TS2-1), it leads to the reduction ofEa[45]. Accordingly, the ZrO2support facilitates CH4→CH3+H, due to the strong interaction between Ni and ZrO2, being line with previous investigation[17].

Similar to CH4→CH3+H, stepwise dehydrogenations of CH3to CH2, CH and C show lowEavalues of 0.35, 0.14 and 0.40 eV for TS2-2 , TS2-3 and TS2-4 , with exothermic by -0.47, -0.53 and-0.30 eV, respectively, on Ni13-ZrO2(111). The displacements between the detaching H and the remaining fragment are 0.519, 0.593 and 0.281 ?,respectively, as shown in Table 2. However, without ZrO2, the activation energy of TS1-2 , TS1-3 and TS1-4 is elevated to 0.46, 0.48 and 0.70 eV, along with the reduction of exothermic energy to -0.15, -0.22 and-0.07 eV, respectively. Meanwhile, the displacements between H and remaining fragments are also increased to 1.230, 0.688 and 0.372 ?, respectively (Table 2).Thus, the ZrO2support has a significant influence on CH4dissociation. This observation is consistent with the previous study[12]where the ZrO2support can modulate the electronic environment of Ni and promote the dissociation of C-H bond.

2.2.2 Effect of ZrO2 support on the activity of CH4 dissociation

Based onEaandΔEof CH4stepwise dehydrogenations to C, the overall activation energies of CH4dissociation on Ni13and Ni13-ZrO2(111) are calculated to be 0.61 and 0.14 eV, respectively, as plotted in Figure 4(a) and 4(b). This gives additional evidence that ZrO2support effectively promotes CH4dissociation. Similar situation is also observed in previous reports[12,19]. As stated by Han et al.[13], this is because the support can tune the adsorption behaviors,decrease the activation barrier and even adjust pathways of dry reforming of methane by modifying the Ni catalyst.

2.2.3 C nucleation and C elimination

Once C is formed, C nucleation may lead to coke deposition. Interestingly, as shown in Figure 5,C+H→CH, as the reverse steps of CH scission, is preferred to C+C→C2on Ni13and Ni13-ZrO2(111),suggesting that carbon deposition is unlikely to occur on both catalyst surfaces. Furthermore, the activation energy (Ea) of 1.90 eV for C+C→C2on Ni13-ZrO2(111) is similar to that of C2dimmer formation(1.89 eV)[46]. Therefore, the undesired coke deposition may be observed only when the residual C can not be timely removed from the catalyst surface[47]. Thus, both the highly dispersed Ni13and the superior electronic donor ability of ZrO2support suppress the coke deposition. This is because C nucleation is inferior to C elimination.

2.3 Role of ZrO2 support in stabilizing Ni-C bond and activating C-H bond

The most important steps in CH4dissociation are adsorption and dissociation, which leads to the formation of Ni-C bond and the dissociation of C-H bond, respectively. To further explore the effect of ZrO2support on CH4decomposition, the pDOS of CH3on Ni13and Ni13-ZrO2(111), as an example, is calculated in Figure 6. Accordingly, the role of ZrO2support in CH4dissociation is obtained.

The result of electronic analysis indicates that the presence of ZrO2support broadens thed-band electrons at C of CH3bonding on Ni (Figure 6(a)). This causes a larger overlap between C 2pand Ni 3d, i.e. and a stronger interaction between Ni and C (Figure 6(b)),thereby leading to more stable adsorptions of CHx(x=3-1) and higher exothermic energy for CH4dehydrogenation. As stated by Boukelkoul et al.[48], CH4dehydrogenation on W-Cu(100) is exothermic because W-doping stabilizes better for all species. Ou et al.[44]also proved that the adsorption state becomes highly stable, when more electrons are transferred from Ni to C of CHx.

Besides, the charge distribution on C and H is influenced due to the enhancement of electron transfer Ni→C on Ni13-ZrO2(111), whereas that of H→C is reduced. This causes a weaker C 2p-H 1shybridization than the situation on Ni13, as seen in Figure 6(c),thereby decreasing theEafor C-H bond breaking.Similar results[44]are observed on Co/Ni, where the low activation energy for C-H bond dissociation in CH4dehydrogenation resulted from the abundantd-band electrons on Co/Ni catalyst.

Figure 6 Partial density of states for CH3 adsorbed on Ni13 and Ni13-ZrO2(111) for the role of the support ZrO2: (a) broadening d-band, (b) stabilizing Ni-C bond and(c) activating C-H bond

As a result, the overall activation energy for CH4dissociation drastically decreases with the addition of ZrO2support. This is because ZrO2support can act as thed-band electron reservoir at Ni13that promotes the activation of C-H bond in CH4dehydrogenation reactions. As stated by Ou et al.[44], introduction of Co into Ni leaded to the downshift ofd-band center of Co/Ni that increased theEadsof the CHx, but decreased theEaof C-H bond breakage. These results are further supported by the work Li et al.[8]； the strong C-Ni chemical bonding can electronically adjust the ability of Ni atoms in adsorbing and dissociating CHx(x=4-1)species.

3 Conclusions

DFT calculations are used to investigate the dehydrogenation mechanism of CH4on Ni13and Ni13-ZrO2(111). The results show that ZrO2support has a significant influence on promoting CH4dissociation.

TheEads,Ea,ΔEand the displacements of the detaching H are obtained. Although the configuration and adsorption site are similar, the adsorptions of all species involving in CH4dissociation are stronger on Ni13-ZrO2(111) than on Ni13. This is because the ZrO2support remarkably modifies the electronic environment of Ni13. Besides, introduction of the ZrO2support can obviously decreaseEaand simultaneously increaseΔEfor all the elementary steps in CH4dissociation. In particular, theEafor conversion of CH4to CH3is reduced from 0.61 eV on Ni13to 0.14 eV on Ni13-ZrO2(111). This results in the decrease of overall activation energy for CH4dissociation on Ni13-ZrO2(111) than on Ni13. Thus, the ZrO2modified Ni13can improve the activity towards CH4dissociation,compared with the pure Ni13.

Moreover, the C-H bonds distance in the ISs of CHx(x=4-1) dissociation is significantly enlarged when the ZrO2support is added. This implies that the cleavage of C-H bonds of CHx(x=4-1) becomes easier. Based on TSs , the stepwise detachment of H from CH4to CH3, CH2, CH and C needs to shift smaller distances to reach the final state. This also benefits to decrease the activation energy (Ea) of C-H bond dissociation.

In viewpoint of the electronic environment, the ZrO2support can broadend-band electrons at C atom bonding to Ni that enhances the electron transfer Ni→C, but reduces that of H→C. This causes a stronger interaction between Ni and C, along with the weakness of C-H polar bonds, thereby leading to a strong adsorption of CHx(x=3-1) and a lowEafor C-H bond dissociation. Hence, the presence of ZrO2support can stabilize the Ni-C bond and activate the C-H bond.The mechanistic investigation of CH4dehydrogenation over Ni13and Ni13-ZrO2(111) may provide an approach to reveal the role of ZrO2support and design new Nibased catalysts for CH4dissociation.