Mechanisms of Transforming CHx to CO on Ni(111) Surface by Density Functional Theory

To elucidate feasible routes of producing CO from CH3 and unravel the effect of adsorbed O on CHx transformation, the reactivity of CHx (x = 1–3) with and without the assistance of adsorbed atomic O on Ni(111) was explored using density functional theory calculations. The adsorption energies of CHx (x = 0–3) were found to be significantly reduced on an O-preadsorbed Ni(111) surface compared to a pure surface. Furthermore, O-assisted one-step dehydrogenation of CHx (x = 1–3) features energy barriers and thus is difficult to proceed. In terms of energy, the direct dissociation of CH3 is favorable, except for the last CH dehydrogenation, which is energy intensive. Interestingly, in O-assisted two-step CH transformation to CO via CHO intermediate, the barrier is dramatically lowered. The successive dehydrogenations of CHxO (x = 1–3) were also found to be a route for CO formation. Finally, two possible pathways from CH3 to CO are proposed: (a) CH3 → CH2 → CH → CHO → CO; (b) CH3 → CH3O → CH2O → CHO → CO.


Introduction
Methane, the principal component of natural gas, has been a high-quality raw material for the production of various fuels and chemicals in the past several decades [1][2][3][4]. Methane conversion is realized mainly by two strategies, which are the direct and indirect processes. Although great achievements and progress have been attained in the direct activation of methane to chemicals [5][6][7], there is still much to do to realize industrial operation. Currently, only the indirect process via syngas as platform molecules is operated commercially.
As an alternative to steam reforming of methane, partial oxidation of methane (POM) to syngas has attracted much attention [8][9][10][11] because of mild heat release and a H 2 /CO ratio of 2, which is very suitable for methanol synthesis and Fischer-Tropsch synthesis. Among the series of catalysts developed for POM, Ni-based catalyst is the most popular because of its properties, such as good catalytic performance, rich resources, and low cost [8,[12][13][14].
In contrast to the numerous studies on the improvement in catalysts and evaluation of their catalytic performance for POM, less attention has been given to the mechanistic understanding of this process, and the mechanism of POM remains ambiguous [9,15,16]. Since experimentally tracking the details of transient transformation process is challenging, researchers have attempted to employ first-principle density functional theory (DFT) to unravel the reaction pathways. Many studies have reported the adsorption and dissociation of methane on Ni-based and other transitionmetal catalysts [17][18][19][20][21][22][23][24][25][26][27][28]. These studies further clarify the catalytic processes involving methane decomposition but are not extensive enough to demonstrate a full scheme of POM, because they do not consider the effect of adsorbed O, which plays a significant role toward the product CO [29][30][31].
Liu et al. [18,19] investigated the adsorption of CH x (x = 0-4) and the dissociation of CH x (x = 1-4) on NiCo(111) facets of ordered NiCo alloy. They also investigated the dissociation of CH 4 on NiCu(111) surface and later investigated the adsorptions of CH 4 , CH 3 , CH 2 , CH, C, H and dissociations of CH 4 , CH 3 , CH 2 , CH on the (111) catalyst surfaces of NiFe. They found that segregated NiCu is the best among the eight CH 4 /CO 2 reforming catalysts Fe, Co, Ni, Cu, NiFe, NiCo, NiCu, and NiCu(S). Li et al. [21,22] studied the dissociation of CH 4 on NiM(111) (M = Co, Rh or Ir) surface using the DFT. Shen et al. [23] studied primary dehydrogenation of methane on the (100), (110), and (111) active facets of single-crystal metal catalysts of the Co group (Co, Rh, and Ir) and Ni group (Ni, Pd, and Pt). In their study, the main concern was the difference in the catalytic mechanism of alloys and pure metals. Their research was aimed to study the change in the electronic properties of the catalyst surface after alloy addition, and the influence on the reaction process of the element. It was found that when the catalyst was an alloy, it had a positive effect on the dehydrogenation reaction with methane. Moreover, the mechanism of the alloy was proposed to be capable of effectively inhibiting carbon deposition compared to pure metals because a good catalyst should have a moderate d-band center. This can give us insight into the factors controlling carbon deposition and assist in designing the optimal catalyst. In our preliminary work in the laboratory [32], we studied the adsorption and dissociation of methane as well as syngas formation over Ni(111) and NiPt(111) to understand the effect of Ni(111) surface alloying on POM. The results showed that Pt-doped Ni(111) surface is beneficial to the adsorption of CH x and H species and unfavorable to the desorption of H 2 . However, in the above research work, the catalyst was considered only based on a smooth surface. In the actual reaction process, the surface of the catalyst adsorbs a large amount of O atoms; the effect of O atoms on the gradual dehydrogenation of methane was not considered in the above study.
To the best of our knowledge, the role of adsorbed O atom during POM in the literature is controversial. Wise et al. [33,34] found that the activity for methane reaction significantly increased due to the presence of preadsorbed O atom on Ni(100) and Ni(111), while Alstrup et al. [35] pointed that oxygen had no clear enhancement effect on methane dissociation on Ni(100). Valden et al. [36] concluded that in the presence of atomic O, methane dissociation could be poisoned due to a steric hindrance of active adsorption sites. Au et al. [37] reported that oxygen promoted methane dissociation on Pt, Cu, Ag, and Au when atomic O was at hollow sites. Several recent studies [38][39][40] highlight that the reactivity of surface O depends on the nature of the transition-metal surface employed, and the oxygen-promotion effect is beneficial to lower-activity metals as opposed to higher-activity metals.
Inspired by these results and doubts, in this work, we explore adsorbed oxygen-mediated transformations of CH x (x = 1-3) to CO on Ni(111) through DFT calculations. The aim of this study is to identify the effect of adsorbed O on CH x conversion and screen the possible pathways of producing CO from CH 3 . In this work, the reactant begins from CH 3 instead of CH 4 because of the following considerations: CH 4 is physically adsorbed on the surface and is basically unaffected by the adsorbed O; as a result, the initial dehydrogenation of CH 4 proceeds mainly through a direct route. Moreover, CH 3 is a common intermediate with general interest in catalytic partial oxidation of hydrocarbons and many theoretical works have been published relating to CH 3 species [41][42][43][44]. The Ni(111) surface was selected as the catalyst model because it is the most abundant and frequently exposed among all the crystal planes of Ni metal. This paper is organized as follows: In the first section, the details of calculation methods and models are introduced. In the second section, the adsorption of related species and O-assisted two-step and O-assisted one-step dehydrogenations of CH x (x = 1-3) are presented in detail; then combined with our previous study, the results are analyzed and compared. Concluding remarks are given in the final section.

Computational Details
A Ni(111) surface was modeled by a periodic four-layer slab with a vacuum region of 15 Å in the z direction. The lower two layers were fixed in the bulk positions, while the upper two layers as well as the adsorbates were allowed to relax during geometry optimization and transition state search.
All the DFT calculations were carried out using the program package Dmol 3 in Materials Studio of Accelrys Inc. [45,46]. The generalized gradient approximation with revised Perdew-Burke-Ernzerhof functional was utilized to account for the exchange and correlation effects [47]. The basis set was set as double numerical plus polarization(DNP). The basis set superposition error calculation was not performed in this paper since it is very small and can be neglected [48]. The core electrons of metal atoms were treated using effective core potentials, while other atoms were treated using an all electron basis set. Considering the magnetic properties of Ni, all calculations were spinunrestricted. A 5 × 5 × 2 grid was used to generate k-points according to the Monkhorst-Pack method [49]. The convergence criteria for geometry optimization and energy calculations were set as 1.0 × 10 −5 Ha, 0.002 Ha/Å, and 0.005 Å for the tolerance of energy, maximum force, and maximum displacement, respectively.
The transition state searches were performed at the same theoretical level as those for the reactants and products using the complete linear synchronous transit/quadratic synchronous transit (LST/QST) method [50].
The adsorption energy, E ads , is defined as Eq. (1).
where E adsorbates/slab , E adsorbates , and E slab are the total energy of adsorbates on the surface slabs, total energy of (1) E ads = E adsorbates/slab − E slab − E adsorbates the free adsorbates, and total energy of bare surface slabs, respectively.
The activation barrier E a is defined as Eq. (3).
where E IS , E TS , and E FS refer to the total energy of the initial, transition, and final states, respectively.

Adsorption of CH x (x = 0-3) With and Without Adsorbed O
For the most stable (111) surface of metal Ni, mainly four adsorption sites exist: top site, bridge site, face center cubic (fcc) site, and hexagonal close packed (hcp) site, as depicted in Fig. 1.
The adsorption energies of the O atom at each site were calculated, and from the results, we found that the adsorption energies of the O atom at the fcc, hcp, and top sites of Ni(111) surface were − 503.7 kJ/mol, − 503.6 kJ/mol, and − 343.0 kJ/mol, respectively. The O atom located at the bridge site can only form metastable adsorption system, which transfers to the threefold hollow site spontaneously.
The results indicate that the O atom preferentially binds to the threefold hollow site on Ni(111) surface, which is in good agreement with the findings of the previous research [51].
To clarify the influence of adsorbed O atom on CH x transformation, we first analyzed the interaction between the O atom and CH x (x = 0-3) species. The stable adsorption sites and corresponding adsorption energies of CH x species in the absence and presence of the O atom are listed in Table 1. The adsorption energies of CH x (x = 0-3) species were found to be significantly reduced on an O-preadsorbed Ni(111) surface compared to a clean Ni(111). This is not surprising since CH x needs partial energy to overcome the repulsion interactions with the preadsorbed O atom on the Ni(111) surface.

Formation and Dissociation of CH x O (x = 1-3)
CH x O species are the main intermediates of the reaction between CH x and O. We systematically studied the formation and dissociation processes of CH x O (x = 1-3) species, and the results are discussed in this section. (111) surface. After optimizing the co-adsorption configuration of CH 3 and O on Ni(111) surface, we chose it as the initial state of the reaction, as shown in Fig. 2a. CH 3 and O atom were adsorbed at the hcp and fcc sites, respectively, and their co-adsorption energy was − 606.5 kJ/mol. Figure 2b shows the transition state of this reaction. The C atom and three H atoms were in the same plane, and the angle between C-O and Ni was 53.72°. The final state of this reaction is seen in Fig. 2c. The O atom of the CH 3 O was adsorbed at the fcc site of Ni(111) surface with C-O bond perpendicular to the Ni(111) surface; the interaction between CH 3 O and Ni surface was mainly achieved by the O atom.

CH 3 and O atom can directly form CH 3 O on the Ni
The reaction energy and the activation barrier of the reaction of CH 3 + O → CH 3 O were 60.2 kJ/mol and 96.0 kJ/mol, respectively. According to the Mulliken analysis, as is seen in Table 2, C atom then became slightly positive charged from the negative charged state. This is because while the Ni surface interacted with the C atom of CH 3 in the initial state (IS), it interacted with the O atom in the final state (FS) of the reaction. CH 3 O may directly dissociate into one H atom and CH 2 O. By optimizing the structures of the reactant and products of the reaction, we obtained the most stable structures, as shown in Fig. 2d, f. Figure 2e shows the transition state (TS) of CH 3 O dehydrogenation. The transferred H atom was adsorbed at the top site of the Ni(111) surface (a little shift to the bridge site), and C atom was adsorbed at the same site as H atom but a little closer to the fcc site, while the O atom was preferably adsorbed at the bridge site. The angles between C-O and Ni and between C-O and H-C-H were 44.04° and 133.50°, respectively. From the FS in Fig. 2f, we can see that the H atom was adsorbed  The distance between C-Ni and H-Ni gradually reduced.
Both O and C atoms interacted with the surface Ni atom, but constrained by the high negativity of the O atom, the C atom connected with the Ni atom was not negatively charged, according to the Mulliken analysis, as shown in Table 3.

Formation and Dissociation of CH 2 O
There are two possible pathways to produce CH 2 O on the Ni(111) surface: the direct dehydrogenation of CH 3 O, which is discussed in the above section, and CH 2 interaction with O, which is presented in this section. After optimizing the co-adsorption configuration of CH 2 and O on the Ni(111) surface, we chose CH 2 + O as the IS of the reaction, as shown in Fig. 3a. CH 2 was adsorbed at the hcp site, and O atom at the fcc site. The co-adsorption energy of CH 2 and O was 814.2 kJ/mol. Figure 3b shows the TS of this reaction. The FS of the reaction is presented in Fig. 3c. The C atom of CH 2 O was adsorbed at the hcp site of the Ni(111) surface, and the O atom was adsorbed at the top site of the Ni atom,   Table 4, the negativity of the C atom gradually reduced. In the initial state, CH 2 was electronegative overall, while in the FS it had a charge of 0.219e. In the FS, the O atom, in addition to bonding with C, also bonded with Ni atom at the bottom. CH 2 O may directly dissociate one H atom and form CHO. By optimization, we obtained the initial and final states of the reaction system, as shown in Fig. 3d, f, respectively.  Table 5, and the dissociation process of CH 2 O was similar to that of CH 3 O.

Formation and Dissociation of CHO
Similar to CH 2 O formation, there are also two possible pathways for CHO formation: direct dehydrogenation of CH 2 O to CHO, which is discussed in the above section and

O-assisted CH x (x = 1-3) Dehydrogenation to CH x−1
In addition to the transformation of CH x via CH x O intermediate presented in the above section, there is another potential O-assisted CH x (x = 1-3) dehydrogenation route: CH x + O → CH x−1 + OH, which is discussed in this section.

CH 3 + O → CH 2 + OH
The reaction CH 3 + O → CH 2 + OH competes with the formation of CH 3 O, i.e., the reaction CH 3 + O → CH 3 O. After we optimized the co-adsorption configurations of "CH 3 and O" and "CH 2 and OH" on Ni(111) surface, we chose them as the IS and the FS of the reaction, respectively, as shown in Fig. 4a, c. Figure 4b presents the TS. In the TS, the angles between two C-H bonds from the CH 2 and Ni surface were 26.88° and 33.18°, respectively. The angle between O-H bond and Ni surface was 20.24°. In the FS, as is seen from Fig. 4c, CH 2 was adsorbed at the hcp site, whereby the C atom shifted to the bridge site a little, and OH was adsorbed at the fcc site on the surface of Ni atom, which was separated by a Ni atom with the adsorption site of the CH 2 . As opposed to separate adsorption, in the co-adsorption, OH was not perpendicular to the Ni(111) surface, and the angle between them was about 45°. The co-adsorption energy of CH 2 and OH was − 575.8 kJ/mol. The heat and activation energy of the reaction CH 3 + O → CH 2 + OH were 56.0 kJ/ mol and 119.8 kJ/mol, respectively. In the TS, the bond lengths of H-C and H-O were 1.502 Å and 1.197 Å, respectively. Both C and O atoms were attracted to H atom to some extent, according to the analysis of Mulliken charge from Table 6. The charge of C was then − 0.641e, which was more electronegative than the IS, while O had a charge of − 0.634e, which was more electronegative than the FS. The transferred H atom had a charge of 0.317e, which was more electropositive than the IS and FS. In the reaction process CH 3 + O → CH 2 + OH, C and O simultaneously compete for H atom. When the O atom interacts with the H atom of the CH 3 in the beginning, the charge in the H atom begins to transfer to the O atom. With the gradual increase in the interaction force between O and H, the H atom becomes positively charged. The interaction between

CH 2 + O → CH + OH
The reaction CH 2 + O → CH + OH competes with CH 2 O formation, i.e., the reaction CH 2 + O → CH 2 O. After the optimization of the co-adsorption configurations of "CH 2 and O" and "CH and OH" on Ni(111) surface, we chose them as the IS and the FS of the reaction, respectively, as shown in Fig. 5a, c. Figure 5b shows the TS of the reaction. The angles between C-H and Ni and O-H and Ni were 73.95° and 80.22°, respectively. In the FS, CH was adsorbed at the hcp site, OH was adsorbed at the fcc site on the surface of Ni atom, which was separated by a Ni atom with the adsorption site of CH. Both CH and OH were perpendicular to the Ni surface, and the co-adsorption energy of the system was − 811.4 kJ/mol. The heat and activation energy of CH 2 + O → CH + OH on the Ni(111) surface were − 2.0 kJ/mol and 105.1 kJ/mol, respectively. According to the analysis of Mulliken charge in the TS, as shown in Table 7, the C atom had a charge of − 0.477e, which was more electronegative than the IS, and the O atom had a charge of − 0.598e, which was more electronegative than the FS. The transferred H atom had a charge of 0.290e, which was more electropositive than the IS and FS. The trend is similar to the process illustrated in the above section. The distances between the transferred H and C and O were 1.480 Å and 1.355 Å, respectively.

Analysis of CH x (x = 1-3) Transformation to CO
Based on the above results and our previous work [32], we explored the reactivity of CH x (x = 1-3) species on Ni(111) in order to screen feasible routes toward CO and identify the effect of adsorbed O on CH x transformation. Figure 6a presents the barrier heights of direct, O-assisted one-step, and O-assisted two-step dehydrogenation routes from CH 3 . The orange bar, which indicates the interaction between CH 3 and O to form CH 2 and OH, features the highest barrier of 119.8 kJ/mol, making this route the least likely for CH 3 transformation. The direct dissociation of CH 3 into CH 2 having the lowest barrier (79.1 kJ/mol) is favorable. However, on O-preadsorbed Ni(111) surface, CH 3 also has a chance to interact with O atom to form the CH 3 O intermediate, followed by CH 3 O dehydrogenation to CH 2 O since the barrier difference between this route and CH 3 direct dissociation is not large.
The relative barriers of three transformation routes for CH 2 are displayed in Fig. 6b. Unlike CH 3 , both O-assisted one-step and O-assisted two-step dehydrogenation processes exhibit much higher overall barriers than CH 2 direct dehydrogenation into CH. Thus, direct dissociation is preferable for the CH 2 species. It should also be noted that the energy required for the dehydrogenation of CH 2 O to CHO is moderate, because only a barrier of 56.0 kJ/mol needs to be overcome.   Figure 6c gives the energy bars for related reactions beginning from CH. The activation energies of CH + O → C + OH and CH → C + H are 148.9 kJ/mol and 140.5 kJ/mol, respectively, which block these reactions from proceeding. On the contrary, combining CH and O to CHO is much easier as the barrier is 88.9 kJ/mol. Moreover, the subsequent dehydrogenation of CHO to CO is quite easy since only a height of 28.7 kJ/mol needs to be overcome.
Having obtained the above results and the insight from our comparative analysis, we propose two possible reaction pathways for CH 3 transformation to CO, which are shown in Fig. 7. In the first pathway, by dissociating H atoms, CH 3 is converted to CH 2 and CH successively; then, CH interacts with adsorbed O to generate CHO, which further dehydrogenates to CO. In the second pathway, CH 3 first combines with adsorbed O to form CH 3 O, which subsequently undergoes three dehydrogenation steps to produce CO.

Conclusions
In this study, systematic DFT calculations were carried out to explore the mechanisms of CH x (x = 1-3) species with and without the assistance of adsorbed O atom on Ni(111). The aim of the study is to elucidate possible reaction routes of producing CO from CH 3 . We also unravel the effect of adsorbed O on CH x transformation. The following conclusions were drawn: (1) The adsorption energies of CH x (x = 0-3) were found to be significantly reduced on an O-preadsorbed Ni(111) surface compared to a pure surface. (2) O-assisted one-step dehydrogenation of CH x (x = 1-3), i.e., CH x + O → CH x−1 + OH, contributes little to the overall transformation of CH x because of the high energy required to overcome the barrier. (3) Direct dissociations of CH 3 to CH 2 and then to CH feature relatively lower barriers and proceed preferentially. However, the energy for scission of CH to C and H is as high as 140.5 kJ/mol. Interestingly, O-assisted two-step CH transformation to CO via CHO intermediate only needs to overcome a barrier of 88.9 kJ/mol, opening a quite feasible route. It should be noted that the successive dehydrogenations of CH x O (x = 1-3) are also favorable from the viewpoint of energy. (4) Through comparison and screening, two possible pathways from CH 3 to CO were found: Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.