MgO Catalysts

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Dry Reforming of Methane on Single Site Ni/ MgO Catalysts: Importance of Site Confinement Zhijun Zuo, Shizhong Liu, Zichun Wang, Cheng Liu, Wei Huang, Jun Huang, and Ping Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02277 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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Dry Reforming of Methane on Single Site Ni/MgO Catalysts: Importance of Site Confinement

Zhijun Zuo,1† Shizhong Liu,2† Zichun Wang,3 Cheng Liu,4 Wei Huang,1 Jun Huang,3* and Ping Liu5* 1

Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China 2

Chemistry Department, State University of New York (SUNY) at Stony Brook, Stony Brook, NY 11794, USA 3

Laboratory for Catalysis Engineering, School of Chemical and Biomolecular

Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia 4

Mechanical Engineering college, Yangzhou University, 196 Huayang West road, Yangzhou, Jiangsu 225127, P. R. China 5

Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973, USA

* To whom correspondence should be addressed. E-mail: [email protected] and [email protected]. †

Dr. Z. Zuo and S. Liu contributed equally to this work and should be regarded as

co-first authors. 1 ACS Paragon Plus Environment

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Abstract: Single site catalysts have drawn considerable attentions due to their superior behaviors in catalysis. However, the origin of promoting effect of single site is not well understood. Here, we take the single atom Ni1/Mg(100) and single site Ni4/Mg(100) catalysts as a case study to elucidate their behaviors under the complex dry reforming of methane (DRM, CO2 + CH4→ 2CO + 2H2) reaction by combining theoretical modeling (Density Functional Theory and kinetic Monte Carlo simulation) and experimental studies. The synergy between single Ni atom and MgO is found to improve the binding property of MgO; yet it is not enough to dissociate CO2 and CH4. It can be achieved by the single site Ni4/MgO(100) catalyst, enabling the formations of CO, H2 and H2O under the DRM conditions. During this process, coking, as observed for bulk-like Ni particles, is eliminated. By confining the reaction to occur at the isolated Ni sites in the SSC Ni4/MgO(100) catalyst is able to balance the CO2 and CH4 activations, which is identified as the key for tuning the DRM activity and selectivity of Ni/MgO catalysts. The theory-identified promotion introduced by increasing the size of MgO-supported Ni clusters from Ni1 to Ni4 and the MgO-introduced site confinement of single site catalysts is verified by corresponding experimental studies, highlighting the essential roles of confined sites in tuning the performance of single site catalysts during complex catalytic processes.

Keywords: Site Confinement; Dry reforming of Methane; Ni/MgO; Single site catalyst; DFT; KMC; TEM

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1. Introduction Single site catalysts (SSCs) have been found to not only promote the efficiency of expensive catalysts, but also result in the tuning of catalytic activity and selectivity for various important processes, e.g. water-gas shift (WGS) reaction, methane dissociation, CO oxidation, NO reduction and oxygen reduction reaction1-8.

Each

site of a SSC is constructed by one or more atoms, has the same atomic arrangement, and provides the same binding energy to a reactant, with no spectroscopic or other cross-talk among such sites9. Thus, the SSCs are also beneficial for fundamental studies, enabling the quantitative comparison to corresponding homogeneous catalysts involving the same active centers. In addition, it allows the accurate description and mechanistic understanding from state of art theoretical modeling, which is devoted to provide mechanistic understanding and evolves strategic principles for the design of improved catalysts3,

5-6, 10-11

. Several mechanisms have been proposed for the

promoting effect of SSCs, including the generation of low-coordinated active sites, increase of interaction with support, and/or synergy between active sites and support11-14. However, the contribution from each mechanism always interacts with each other3, 10, and the catalytic nature is still not well understood, which hinders the rational development of SSCs. Besides, so far the catalysis for SSCs has been performed for the relatively simple reactions7. By comparison, the performance within complex reaction network, which involves multiple reaction intermediates and products, is paid little attention. In this study, a combined density functional theory (DFT) calculations and 3 ACS Paragon Plus Environment

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kinetic Monte Carlo (KMC) simulations together with the coordinative studies in experiment is employed and the complex dry reforming of methane (DRM, CO2 + CH4 → 2CO + 2H2) on single atom Ni1/MgO(001) and single site Ni4/MgO(001) model catalysts was used as a case study, aiming to understand the behavior of SSCs toward complex catalytic processes. The DRM is one of the effective ways for utilizing two of the greenhouse gases, carbon dioxide (CO2) and methane (CH4) to produce syngas15-17, which can be used as the feedstock for catalytic processes, such as Fischer-Tropsch and methanol synthesis18-20. However, the conventional Ni catalysts for the DRM deactivate quickly due to sintering of the active metal phase and carbon deposition via the Boudouard reaction (2CO → C + CO2) and/or CH4 decomposition (CH4 → C + 4H) 17, 21-24. One of the solutions is the formation of small Ni particles supported on MgO25-27 by reducing NiO-MgO solid solutions, being able to exhibit a high stability against metal sintering and a strong coking resistance28-32. Alternatively, several possibilities including Ni concentration33-34, the oxygen vacancies (Ovac) produced during the reduction30, 35, and the electronic modification introduced by the strong Ni-MgO interaction have been proposed 36. However, due to the complexity of the reaction, the origin of promoting effects remains elusive. The combination of DFT and KMC allows us to gain better understanding of the reaction mechanism on the SSCs: the interplay of different reaction pathways, overall conversion and selectivity, source of carbon deposition (Boudouard reaction or CH4 decomposition)/elimination, the size effect of single Ni site, and the roles that the single Ni site and the MgO support play. In addition, the key descriptors which can 4 ACS Paragon Plus Environment

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control or selectively tune the conversion and selectivity are also provided. More importantly, the independent promoting effect of site confinement on the catalytic activity of the single site Ni/MgO catalysts is identified by the KMC simulations, which is verified by corresponding experimental studies, highlighting the essential roles of confined sites in tuning the performance of single site catalysts during complex catalytic processes. Such fundamental insight can help in guiding the further optimization of Ni-based catalysts for the DRM reaction.

2. Methods 2.1 Theory All the calculations were studied using DFT, which was implemented with the Vienna Ab Initio Simulation Package37-40. The spin polarized generalized gradient approximation (GGA) with the PW91 functional and pseudopotential generated in the projector-augmented wave method were employed 41. Although the GGA functional is not enough to accurately describe adsorption of small molecules on Ni/MgO(100),42-43 it has been shown to works reasonably well to capture the difference in energy from one system to the next36, 44-46, which is the interest in the current study. A 3× 3× 1 k-point mesh was used to sample the Brillouin zone. A cutoff energy of 400 eV was sufficient to obtain a satisfactory convergence of the total energy. The transition state (TS) search was carried out by using the nudged elastic band (NEB) method 47, where the identified transition state (TS) was confirmed by one imaginary frequency. MgO(100) surface was modeled by a five-layer slab with a (3×3) array in 5 ACS Paragon Plus Environment

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each layer. Ni4/MgO(100) was built based on Ni1/MgO(100), which was described by replacing one of Mg ion or filling in a Mg vacancy (Mgvac) in the surface by Ni or (Figure 1a) following the previous study of NiO-MgO solid solutions

46

. Here we

assumed that the substitution of one Mg atom by Ni does not change the lattice of MgO. It is reasonable according to the previous study, showing that NiO is soluble in MgO in the whole composition range without significant changes in lattice parameters due to close ionic radii of Ni2+ and Mg2+ 48. In the case of Ni4/MgO(100), three Ni atoms were located over the Ni ion sat on the Mg position and available for the reaction, to form a Ni tetrahedron cluster with no Ovac (Figure 1b). After relaxation, Ni4 cluster was slightly lifted from the surface and bound over the Mgvac site via Ni-O bonds. Such model was different from the previous study, where Ni4 either anchored on the ideal MgO(100) surface or occupied the Ovac site

36

. In the DFT calculations,

the bottom two layers were fixed, and the other layers and the adsorbates were allowed to relax. In order to ensure no significant interaction between the layers, the vacuum region between adjacent slabs was 15 Å. The average magnetic moment per atom of Ni is 0.50 µB, which is slightly smaller than the experiment value (~0.60 µB)49. We also tested using GGA+U (U = 5.3 eV for Ni) 50-51 method for the binding of CO adsorption on Ni1/defected MgO(100), and the difference of binding energy is only 0.05 eV. Accordingly, the effect of Hubbard U was not included in this work. The adsorption

energy

(Eads)

of

reaction

intermediates

were

calculated

as

E(adsorbate/surface) – E(surface) – E(adsorbate), where E(adsorbate/surface), E(surface) and E(adsorbate) represent the total energy of surface with and without 6 ACS Paragon Plus Environment

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interaction with adsorbate, and the adsorbate in gas phase, respectively. The DFT method currently employed may not be accurate to calculate the absolute values of binding energies and activation barriers, which determine the DRM activity. However, as shown in the following it is capable enough to capture the difference in binding and DRM activity induced by Ni size and site confinement observed experimentally, which is our interest here.

(a) (b) Fig.1. Schematics of structures for single atom Ni1/MgO(100) catalyst (a) and single site Ni4/MgO(100) catalyst (b). Green: Mg; Red: O; Blue: Ni.

2.2 Experiment Catalyst preparation. Ni/MgO catalysts were prepared by a co-precipitation method. Typically, an aqueous solution of magnesium nitrate (1 M) was mixed with desired amount of nickel nitrate aqueous solution (1 M). Then the mixed solution was added dropwise to the Na2CO3 solution (1 M) under vigorous stirring. The pH value was adjusted to 10 by adding the NaOH solution (10 M). The obtained mixture was kept at 75 °C for 1 h under stirring. Subsequently, the resulting gel was filtered and washed, then dried at 120 °C for overnight, followed by calcination at 850 °C for 5 h to obtain the final products. The obtained catalysts were denoted as 2.5% Ni/MgO, 5% Ni/MgO, and 10% Ni/MgO. 7 ACS Paragon Plus Environment

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Catalyst characterization. Transmission electron microscopy (TEM) were employed to characterize the structure and morphology of the Ni/MgO catalysts. High-resolution TEM (HRTEM) was performed on a Philips CM 200 facility operating at 200 kV. The high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images and energy-dispersive X-ray (EDX) spectra were recorded on a JEOL JEM-ARM200F TEM with an EDX detector. Similar as pretreatment for the activity test, the sample was reduced in hydrogen for 1 h, and then used for microscopic imaging. Under the reaction conditions, no obvious activity loss was observed for the catalysts. Therefore, no microscopic imaging was carried out after reaction. Catalytic activity test. The catalytic CO2 reforming of CH4 was carried out under atmospheric pressure in a fixed-bed stainless steel reactor (300 mm long and 6 mm internal diameter) packed with 200 mg of catalyst. The catalyst was preheated under a nitrogen flow (20 mL/min) to 750 °C (heating rate 10 °C/min) and held at this temperature for 1 h. Prior to the reaction, the catalyst was in situ reduced at 750 °C for 1 h with a flow rate of 50% H2/N2 mixture gas (10 mL/min N2 and 10 mL/min H2) and cooled to 600 °C. Then, the reactant gases consisting of CH4 (15 mL/min), CO2 (15 mL/min), and N2 (15 mL/min) with a total flow of 45 mL/min were introduced into the reactor. The reaction temperature was increased from 600 to 800 °C in 50 °C increments with a heating rate of 10 °C/min and held for 0.5 h until the conversion was stabilized at each temperature. A Varian 490 Quad Micro-GC equipped with three thermal conductivity detectors (TCDs) with three columns (Agilent PoroBond Q, 8 ACS Paragon Plus Environment

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CPMolsiever 5A, and HayeSep Q) was used for H2, CO, CO2, and CH4 analysis.

3. Results and Discussion 3.1 DFT calculation DRM on MgO(100).

The DRM on MgO(100) surface was studied as a

reference. CO2 prefers to occupy the Mg-O-Mg site, where two O atoms bind to two Mg sites respectively and C interacts with O atom (Figure S1) to form carbonate-like (*CO3) species. The corresponding Eads is -0.42 eV (Table 1), which is in agreement with the experimental value of -0.41 eV52. The O=C=O bond is bent to 133.5° and the C-O bond is slightly elongated; however the dissociation reaction, *CO2 + * →*CO + *O, is highly endothermic with reaction energy (∆E) of 3.95 eV, as both *CO (Eads =-0.17 eV) and *O (Eads = -2.40 eV) interact with the surface weakly at the Mg and Mg-O-Mg hollow sites respectively (Figure 2 and Table 1), in consistent with the previous studies53-55. Due to the same reason, CH4 decomposition is also unlikely. As observed in other studies54, 56, the DFT calculations show that CH4 is only physisorbed on the surface (Eads= -0.04 eV, Table1). The dehydrogenation is hindered both thermodynamically with ∆E of 2.40 eV and kinetically with barrier (Ea) of 2.59 eV, due to the low stability of methoxy (*CH3) species (Eads= -0.12 eV, Table1), which are barely adsorbed and prefer away from the surface by ~3 Å (Figure 2); yet *H is fairly stable at the O site (Eads= -0.45 eV). Accordingly, the DRM should not occur on MgO(100) due to its low adsorption properties, as observed experimentally57. We note that the step edge of MgO(100) was not considered in the current 9 ACS Paragon Plus Environment

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calculation. According to the calculations by Mazheika and Levchenko, similar enhancement in CO2 and CH4 adsorption energy was observed going from the terrace sites to the step sites on both MgO and Ni1/MgO58. Thus, the difference in DRM activity between MgO and Ni1/MgO likely follows the same trend, which is one of our focuses in the current study. DRM on SAC Ni1/MgO(100). Our calculations show (Table 1) that the formation of SAC, Ni1/MgO(100), does not vary the binding of *CH4 on MgO(100) and the corresponding adsorption energy remains as -0.04 eV. This is consistent with previous DFT calculations using the slab model and the HSE(0.3)+vdW functional, which shows a difference in binding energy of 0.02 eV58. In the case of *CO2 adsorption (Table 1), the adsorption energy on Mg-O-Ni site (Eads= -0.23 eV) is less than that of Mg-O-Mg site (Eads= -0.42 eV). That is, the Ni/MgO is not as favorable as MgO alone for *CO2 adsorption, which is also reported previously58. In our study, Ni1/MgO(100) binds CO more strongly than that on MgO(100) by 0.56 eV (Table 1). Same trend is also observed by Mazheika and Levchenko58. Yet, the corresponding magnitude is different, which may be associated with the variation in Ni concentration on the surface. SACs have been reported to well catalyze various catalytic processes.7 However, for the complex DRM, our calculations show that this is not the case for the SAC Ni1/MgO(100). Compared to MgO(100) (Figure S1), the presence of isolated individual Ni1 cation on MgO(100) in cooperation with MgO facilitates the *CH4 decomposition to *CH3 with ∆E of 1.36 eV and Ea of 1.57 eV (Figure S2); yet it is not 10 ACS Paragon Plus Environment

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as active as Ni(111)59-60 and the sequential activation of *CH3 is still energetically difficult with Ea > 2.3 eV. In addition, the *CO2 dissociation is still very difficult (∆E = 3.62 eV) due to the limited number of neighboring Ni sites and therefore weak bindings to the reaction intermediates as that of MgO(100) (Table 1). That is, the SAC Ni1/MgO(100) is not active enough to help for the C-O bond scission, C-H bond cleavage and overall DRM reaction. We also considered the effect of Ovac on Ni1/MgO(100), which was found to be energetically favorable at low Ni loading in our previous study46. The results show that the Ovac is not likely to survive under the DRM conditions. The presence of Ovac does not help to stabilize CH4 (Eads = -0.14 eV), but substantially increase the binding of CO2 (Eads = -2.45 eV). For *CO2, one of C-O bonds is nearly parallel over Ovac (Figure S3), resulting in the longer C-O bond of 1.33 Å and more bent O=C=O bond of 118.9° than that of Ni1/MgO(100) and MgO(100). Similar promotion in stability over the Ovac site is also observed for *O and *CO (Figure S3). As a result, *CO2 dissociation is greatly facilitated (∆E = -0.28 eV, Ea = 0.54 eV), where the dissociated *O fills the Ovac site. This is also the case for *CH4 dissociation to form CH3* (∆E = 0.15 eV, Ea = 0.70 eV), *CH2 (∆E = 0.60 eV, Ea = 1.35 eV), *CH (∆E = 0.88 eV, Ea = 1.56 eV) and eventually *C (∆E = 2.02 eV, Ea = 2.44 eV) and occupy the Ovac site (Figure S3). According to the DFT calculations, *COx and *CHx species compete for the Ovac sites to gain the increased activity. However, the binding of CH4 is much weaker than that of CO2. That is, under the DRM conditions, the Ovac sites are mostly occupied by *CO2, followed by a facile C-O bond scission to fill the vacancy. Given 11 ACS Paragon Plus Environment

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that, the Ovac cannot survive. The defected Ni1/MgO(100) surface likely transforms back to Ni1/MgO(100) surface and eventually losses the activity toward the DRM. Accordingly, the Ovac was not considered in our Ni4/MgO(100) model as follows. The size increase of MgO-supported Ni cluster from Ni1 to Ni4 allows the formation of a Ni4 ensemble, which consists of three neighboring Ni sites to provide strong binding to the reactants and enable the proceeding of the DRM reaction. Table 1. The adsorption energies (Eads) on MgO(100), Ni1/MgO(100) and Ni4/MgO(100) in eV. Adsorbate *CH4 *CH3 *H *C *CO2 *CO *O

MgO(100) Ni1/MgO(100) Ni4/MgO(100) -0.04 -0.04 -0.17 -0.12 -1.55 -2.60 -0.45 -1.92 -2.99 -2.76 --7.50 -0.42 -0.23 -0.99 -0.17 -0.73 -2.17 -2.40 -3.00 -6.23

DRM on confined SSC Ni4/MgO(100). The binding capability of Ni1/MgO(100) can be enhanced by increasing the size of Ni sites from Ni1 to Ni4 (Figure 1). A three-fold hollow site is formed over a Ni4 cluster of Ni4/MgO(100) (Figure 1b). It is constructed by the three Ni atoms, which are less positively charged than the underneath Ni atom embedded in the MgO(100) surface by ~1 e. Consequently, on Ni4/MgO(100) binds the reaction intermediates more strongly than that on Ni1/MgO(100) (Table 1). Unlike the case of Ni1/MgO(100), all reaction intermediates solely prefer to adsorb at the Ni sites and MgO(100) does not participate in the reaction directly (Figure 2). That is, if the Ni4 clusters are well dispersed and isolated

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from each other by MgO (Figure 1), all reactions are confined to occur within each individual Ni4 cluster. Accordingly, the effect of lateral interaction among the adsorbates on Eads, ∆E and Ea cannot be ignored and plays essential role in promoting the catalytic performance of Ni/MgO toward the DRM reaction as demonstrated in the following.

Fig.2. Adsorption configuration of possible intermediates involving DRM on bare confined SSC Ni4/MgO(100) catalyst. *X represents an intermediate adsorbed on the surface. Green: Mg; Red: O; Blue: Ni; Dark grey: C; White: H. 13 ACS Paragon Plus Environment

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CO2 on Ni4/MgO(100). CO2 favors the Ni bridge site (Eads = -0.99 eV, Table 1), where one of C-O bonds is stretched to 1.27 Å and O-C-O is bent to 135.9o (Figure 2); The increased binding of CO2 on Ni4/MgO(100) facilitates the dissociation to CO chemisorbed on the surface, *CO in our notation, (∆E = -0.70 eV, Ea = 0.35 eV, Table 2), which is more facile than that on Ni(111) surface (Ea = 0.42 eV)59. Here, the *CO2 dissociation is confined to the same Ni4 cluster and the final state corresponds to the co-adsorption of *CO + *O (Figure 3). This is due to the fact that MgO(100) is too inert to directly participate in the binding and allow the diffusion of adsorbed species among the isolated Ni clusters from one to another. Such site confinement by MgO can be lifted by increasing the coverage of Ni4 cluster, which allows the formation of neighboring Ni4 clusters as shown in section 3.2. In this way, the *CO and *O fragments from *CO2 dissociation do not necessarily suffer the lateral repulsion; instead they can diffuse separately to two neighboring Ni4 clusters (Figure 2) under the reaction condition to reach a stabilized final state and the increase in Ea by 0.04 eV for the reverse step. The energy for the forward step remains the same (Table 2). By comparison, such site confinement displays more significant effect on the further decomposition of *CO to *C. In this case, the co-adsorption of *C + *O (Figure 3) destabilizes the final state by 0.88 eV than the case with the separate adsorptions (Figure 2), which facilitates the reverse step, oxidation of *C to *CO (Ea: 0.56 eV vs. 1.44 eV, Table 2) significantly. Again, it does not affect the forward step and the corresponding Ea (1.90 eV) is much higher than that of Ni(111) (0.55 eV)59. Thus, the single site Ni4/MgO(100) likely facilitates the *C oxidation and may help in releasing 14 ACS Paragon Plus Environment

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the long-standing coking problem for Ni-based catalysts via hindering the *C formation from CO2 dissociation.

CH4 on Ni4/MgO(100). Compared to CO2, the promotion for CH4 binding by size increasing is rather small (Eads= -0.17 eV). The dehydrogenation of *CH4 to *CH3 is also accelerated on Ni4/MgO(100) (∆E = -0.36 eV, Ea = 0.53 eV, Table 2) in comparison with Ni1/MgO(100) (Ea = 1.57 eV) and Ni (111) (Ea = 0.91 or 1.17 eV) 59. Interestingly, the corresponding Ea is also lower than those by simply depositing Ni4 cluster on ideal MgO(100) (Ea = 1.15 eV) and Al2O3 (Ea = 0.71 eV) surfaces 36, 59, 61-62. In addition, the decomposition of *CH3 to *CH2 (∆E = 0.21 eV, Ea = 0.55 eV, Table 2) and *CH (∆E = 0.01 eV, Ea = 0.38 eV, Table 2) is facile, while the formation of *C is more difficult from *CH dissociation (∆E = 0.63 eV, Ea = 1.06 eV, Table 2). Like CO2 dissociation, the lift of site confinement for CH4 dissociation allows the diffusion of dehydrogenated states, *CH3 + *H, from coadoption (Figure 3) to separate adsorption (Figure 2), which results in the stabilization and the increased Ea for the reverse hydrogenation steps. Such effect is increasingly significant during the conversion from *CH4 to *C (Table 2) due to the requirement of highly symmetric sites in particular for stabilization of *CH and *C (Figure 2) and therefore the strong lateral repulsion introduced by the coadsorbed *H.

Effect of *H on Ni4/MgO(100). As a result of facile dissociations of CO2 and CH4 on Ni4/MgO(100), *H, *O and *C may be available on the surface during the DRM reaction. These surface species can open new pathways for the reaction of intermediates involved in the DRM reaction. 15 ACS Paragon Plus Environment

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Fig.3. Co-adsorption configurations of possible intermediates involving DRM on confined SSC Ni4/MgO(100) catalyst in presence of *O and *C on the surface. *X represents an intermediate adsorbed on the surface. Green: Mg; Red: O; Blue: Ni; Dark grey: C; White: H. The *H species on the surface can either desorb in the form of H2 (∆E = 1.42 eV, Table 2), or be involved in hydrogenation reactions. The presence of *H may enable the CO2 activation as reported previously for metal/oxide catalysts63-65. However, neither formate (*HCOO; ∆E = 0.07 eV, Ea = 1.39 eV,) nor carboxyl (*HOCO: ∆E = 0.78 eV, Ea = 1.98 eV,) is formed as favorably as the *CO2 dissociation (Ea = 0.22 eV). That is, the CO2 hydrogenation is unlikely, but preferring the direct dissociation. The *CO species can also be hydrogenated to *HCO (∆E = 0.77 eV, Ea = 1.11 eV, Table 2) rather than *COH (∆E = 1.27, Ea = 2.87 eV, Table 2), which is more favorable than the formation of *C via either C-O bond scission (∆E = 1.34, Ea = 1.90 eV, Table 2) or Boudouard reaction (∆E = 0.63, Ea = 3.49 eV, Table 2). According to the KMC simulations below, neither process can compete with *CO desorption due to the 16 ACS Paragon Plus Environment

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entropic contribution under the DRM conditions. In addition, the hydrogenation of *O to *OH on the surface (∆E = 0.19 eV, Ea = 1.13 eV, Table 2) is possible; yet the reverse step is slightly preferred. By comparison, the hydrogenation of *OH to *H2O is more difficult both thermodynamically and kinetically (∆E = 1.34 eV, Ea = 1.80 eV, Table 2) and the reverse step is more preferential. The following KMC simulations show that desorption of *H2O is facile under reaction conditions due to the entropic contribution of H2O gas. Therefore, the *OH species cannot be observed on the surface. For both *CO and *O hydrogenations, the confinement effect introduced by the co-adsorption with *H is small.

Effect of *O on Ni4/MgO(100). The *O species on the surface can directly oxidize *CHx to *CHxO species or assist the dissociation of *CHx via the formation of *OH. The effect may also be indirect via destabilization of the dehydrogenated states due to lateral repulsion. For the first C-H bond breaking of *CH4, the direct participation of *O or the oxidative-dehydrogenation in our notation, stabilizes the dissociated *H by the formation of *OH (∆E = -0.88 eV) compared to the case without *O (∆E = -0.36 eV), or the non-oxidative-dehydrogenation in our notation, while without direct participation, the presence of *O results in a less negative reaction energy (∆E = -0.32 eV). In both cases, the *O species on the surface slightly raises the barrier for C-H bond breaking (Ea: 0.62 eV, 0.60 eV vs. 0.53 eV, Table 2). Similar situation is observed for the second and the third C-H bond cleavage of *CH4, where the direct dissociation on the bare Ni sites (Ea = 0.55 eV for *CH3, 0.38 eV for *CH2) is more favorable than the case in presence of *O with or without formation of 17 ACS Paragon Plus Environment

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*OH (Ea =0.83 eV, 0.64 eV for *CH3, 0.78eV, 0.70 eV for *CH2, Table 2). For both *CH3 and *CH2, the oxidation to form *CHxO is highly activated (Ea =1.44 eV for *CH3, 1.16 eV for *CH2, Table 2) due to their strong interactions with the Ni sites. However, this is not the case for *CH, where the formation of *CHO is favored (∆E = -0.12 eV, Ea = 0.75 eV) over the four kinds of dissociation steps to produce *C (Ea > 1 eV, Table 2). The formed *CHO then either dissociates to *CO (∆E = -0.77, Ea = 0.34 eV, Table 2) or hydrogenates to *CH2O (∆E = 0.27 eV, Ea = 0.57 eV, Table 2), rather than undergoing hydrogenation to *CHOH (∆E = 0.53 eV, Ea = 1.12 eV, Table 2). Eventually, *CO desorbs from the surface, while *CH2O is hydrogenated to either *CH3O (∆E = 0.08 eV, Ea = 0.80 eV, Table 2) or *CH2OH (∆E = 0.90 eV, Ea = 0.95 eV, Table 2) and finally *CH3OH (∆E = 0.87 eV, Ea = 1.72 eV via *CH3O; ∆E = 0.52 eV, Ea = 1.51 eV via *CH2OH, Table 2). Accordingly, the formation of *C seems very difficult, which *CO is more preferential on Ni4/MgO(100).

Effect of *C on Ni4/MgO(100). Under the extreme situation that *C is formed on Ni4/MgO(100), there are three possible pathways for carbon elimination: the first is recombination to form C-C chain (*C + *C →*C2 + *); the second is oxidation (*C+ *O → *CO + *); the third is reverse Boudouard reaction (*C + *CO2 → *CO + *CO), which proceeds via *CCOO intermediate. Among them, the C-C chain formation on the Ni4 cluster is the most preferential with Ea as low as 0.08 eV; by comparison, the other routes are less competitive (Table 2), which correspond to a higher barrier (Ea = 0.56 eV for oxidation; Ea = 0.66 eV for *CCOO formation and Ea = 1.37 eV for *CCOO + *→ 2*CO). Therefore, to avoid coking, the coverage of *C should be 18 ACS Paragon Plus Environment

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minimized to prevent facile carbon accumulation on Ni4/MgO(100). Overall, the DFT calculations clearly addressed the energetics involved the reaction network for the DRM on Ni1,4/MgO(100). The single atom Ni1/MgO(100) catalyst cannot catalyze the DRM reaction, as the Ni sites are too inert and limited to activate CH4 and CO2. The growth of Ni1 to Ni4 provides adequate and active Ni sites, being able to provide binding strong enough to activate CO2 and CH4, but weakly enough to avoid coking. Again, if the site confinement is released, the stability of reactant or products and therefore ∆E and Ea change accordingly for each elementary step involved in the DRM reaction. It is not clear, however, how the energetics calculated using DFT affects the reaction kinetics under the DRM reaction conditions: the dominant pathways, the overall conversion rate and the selectivity among CO, H2, H2O and CH3OH, the resources for carbon deposition and elimination, the key descriptors to promote the catalytic performance, and more importantly the role of site confinement of Ni cluster supported on Mg(100) in tuning the catalytic activity of Ni/MgO. To answer these questions, KMC simulations were conducted to describe the DRM reaction over Ni4/MgO(100) based on the DFT calculations. Table 2. The reaction energies (∆E), forward and reverse activation energies (Eaf, Ear) in eV involving in the DRM on Ni4/MgO(100) with and without site confinement. Reactions CH4 (g) + * → *CH4 CO2 (g) + * → *CO2 *CO → CO(g) + * H* + H* → *H2 + * *H2O → H2O(g) + *

Confined Ni4/MgO Eaf Ear ∆E ---0.17 ---0.99 --2.17 --1.42 --0.04

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Non-confined Ni4/MgO Eaf Ear ∆E ---0.17 ---0.99 --2.17 --1.42 --0.04

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CH3OH* → CH3OH(g) + * *CO2 + * → *O + *CO *CO2 + *C → *C + *CO + *O *CO2 + *O → O* + *CO + *O *CH4 + * → *CH3 + *H *CH4 + *O → *CH3 + *OH *CH4 + *O → *CH3 + *H+ *O *CH4 + *C → *CH3 + *CH *CH4 + *C → *CH3 + *H+ *C *CH3 + * → *CH2 + H* *CH3 + *O → *CH3O + * *CH3 + *O → *CH2 + *OH *CH3 + *O → *CH2 + *H + O* *CH3+*OH→ *CH2+*OH+*H *CH2 + * → CH* + *H *CH2 + *O → *CH2O + * *CH2 + *O → *CH + OH* *CH2 +*O → *CH + *H + O* *CH + * → *C + *H *CH + *O → *CHO + * *CH + *O → *C + *OH *CH+ *O → *C + *H + O* *CH+*OH→*C+*OH+*H *CHO + * → CO* + *H *CHO + *H → *CHOH + * *CHO + *H → *CH2O + * *COH + * → *CO + *H *COH + *H → *CHOH + * *CHOH + *H → *CH2OH + * *CH2O + *H → *CH3O + * *CH2O + *H → *CH2OH + * *CH2OH + *H → *CH3OH + * *CH3O + *H → *CH3OH + * *C + *O → CO* + * *C + *C → *C2 + * *C + *CO2 → *CCOO + * *CCOO + * → *CO + *CO *C + *CO2 → *CO + *CO *H + *O → *OH + * *H + *OH → *H2O + *

-0.35 0.71 1.49 0.53 0.62 0.60 1.31 2.66 0.55 1.44 0.83 0.64 0.65 0.38 1.16 0.78 0.70 1.06 0.75 1.55 1.23 1.52 0.34 1.12 0.57 1.60 0.92 0.89 0.80 0.95 1.51 1.72 0.56 0.08 0.66 1.37 2.86 1.13 1.80

-1.05 0.47 0.91 0.89 1.50 0.92 0.74 1.32 0.34 0.67 1.02 0.15 0.33 0.37 0. 89 0.92 0.16 0.43 0.87 1.89 0.25 0.30 1.11 0.59 0.30 2.87 0.66 0.56 0.72 0. 45 0.99 0.85 1.90 0.96 1.09 2.58 3.49 0.94 0.57

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0.28 -0.70 0.24 0.58 -0.36 -0.88 -0.32 0.57 1.34 0.21 0.77 -0.19 0.49 0.32 0.01 0.27 -0.14 0.54 0.63 -0.12 -0.34 0.98 1.22 -0.77 0.53 0.27 -1.27 -0.26 0.33 0.08 0.50 0.52 0.87 -1.34 -0.88 -0.85 -1.21 -0.63 0.19 1.23

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-0.35 --0.53 0.62 -1.31 2.66 0.55 1.67 1.04 --0.38 1.25 0.90 -1.06 1.17 2.33 --0.34 1.54 0.71 1.60 1.04 0.95 1.20 1.35 1.56 1.87 1.44 1.47 0.81 1.37 3.01 1.25 1.91

-1.09 --1.12 1.91 -1.80 3.25 0.37 0.67 1.52 --0.75 0.89 0.94 -1.17 0.87 2.53 --1.21 0.59 0.30 3.04 0.66 0.56 0.72 0.45 0.99 0.85 1.90 0.96 1.09 2.85 4.21 0.94 0.57

0.28 -0.74 ---0.59 -1.29 --0.49 -0.59 0.18 1.00 -0.48 ---0.37 0.36 -0.04 --0.11 0.30 0.20 ---0.87 0.95 0.41 -1.44 0.38 0.39 0.48 0. 90 0.57 1.02 -0.46 0.51 -0.28 -1.48 -1.20 0.31 1.34

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3.2 KMC simulations The KMC simulations included 45 elementary steps identified in the DFT calculations (Table 2). The rate constant for reaction N was described as kN =

k BT QTS exp( − Ea , n / k BT ) = A exp( − Ea ,n / k BT ) QTS QR h QR , where and are the

partition functions per unit volume for a transition state and a reactant, respectively, while Ea , N is a predicted reaction barrier for reaction N. Predicted prefactors (A) at various temperatures are listed in Table S1. kB, T and h represents for Boltzmann constant, temperature and Planck constant respectively. For the reactions involving molecules in gas-phase, the contribution from the entropy is taken from the NIST database

66

and is included in the KMC simulations. The adsorption coefficient (kads)

was calculated according to PAsiteσ/ඥ2πmk ୆ ܶ

67

, where P, Asite, σ and m represented

the pressure of adsorption gas, the area of a single site, the sticking coefficient and the mass of adsorption gas, respectively. Both forward and backward reactions are included in the KMC model. The turn-over-frequency (TOF) is estimated as N /(Ni sites × time), where N is the number of occurrence for a reaction and is counted according to the KMC simulations. To address the role of site confinement on the catalytic performance of Ni4/Mg(100), two surface matrixes are constructed. One is in the format of SSC with site confinement, where each Ni4 ensemble consists of three neighboring Ni atoms and are well dispersed on the MgO(100) support (Figure 4a). Since MgO is too inert to participate in the reaction directly according to DFT calculations in section 3.1, the overall DRM reaction is confined to occur at Ni triangle sites of a Ni4 cluster and 21 ACS Paragon Plus Environment

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there is no diffusion of adsorbates from one cluster to another. As a result, in many cases the effect of coadsorption involving two or three adsorbed species as initial/final states has to be considered (Table 2). The other is the Ni4/Mg(100) catalyst without site confinement or non-confined Ni4/Mg(100) in our notation. It was constructed by increasing the coverage of Ni4 on MgO(100) to the level high enough to allow connection of three exposed Ni sites of Ni4 (Figure 4b). Compared to the case with site confinement (Figure 4a), the non-confined Ni4/Mg(100) (Figure 4b) catalyst provided the larger area of Ni sites, while the binding and catalytic activity of each Ni site was assumed to be kept as the same. In this way, the site confinement of SSC Ni4/Mg(100) is partially lifted. Each elementary can occur over the large area of constructed by many neighboring Ni4 clusters, rather being limited within an isolated Ni4 cluster of SSC Ni4/Mg(100) (Figure 4). As a result, for a reaction involving two adsorbed surface species, for example, each of them can reside in the most stable site, which can be away from each other to avoid lateral repulsion. This is followed by diffusion, co-adsorption, and finally reaction as the case with site confinement. Similar situation was assumed for the final states, enabling the diffusion of the produced surface species from the neighboring coadsorption sites to the stable location without lateral repulsion. Following this idea, the description of DRM on Ni4/Mg(100) catalyst with and without site confinement uses different sets of energies (Table 2). According to the DFT calculations, the lift of site confinement promotes the stability of initial/final sites including two or three adsorbed species and tunes the 22 ACS Paragon Plus Environment

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corresponding ∆E and Ea as demonstrated in section 3.1. When going from the SSC Ni4/MgO(100) to the non-confined case, only the ensemble of active sites varies featured by the increasing coverage of Ni4 and partial lift of site confinement, while the electronic contribution associated with the lift of site confinement can be excluded. In this way, the comparison in catalytic performance of the two model systems allows us to identify the independent effect of site confinement of single site catalysts.

DRM

(a)

(c)

DRM

(b) (d) Fig. 4. KMC-simulated surface matrix for the Ni4/MgO(100) with (a,c) and without site confinement (b, d) before (a, b) and during the DRM reaction at 700 °C and 1 atm with CO2:CH4 ratio of 1( grey: MgO, blue: Ni, red: *O). We note that the diffusion barriers were not included in the KMC 23 ACS Paragon Plus Environment

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simulations. For the confined SSC Ni4/MgO(100), the diffusion of an adsorbate is not necessary to enable the reaction to occur within the limit of Ni4 cluster. In addition, the diffusion among Ni4 clusters isolated by MgO was limited due to the high preference of the reaction intermediates to the Ni sites of Ni4/MgO over MgO sites (Table 1). In the case of non-confined Ni4/MgO(100), the diffusion of an adsorbate from one Ni site to the neighboring ones was assumed to be facile in consideration of high temperature for the DRM reaction. Activity and selectivity of confined SSC Ni4/MgO(100). According to our KMC simulations, the single site Ni4/MgO(100) catalyst is able to catalyze the DRM well ranging from 600 °C to 900 °C and at 1 atm with CO2:CH4 ratio of 1. Three products are observed, including CO, H2 and H2O, and CH3OH is not observed (Figure 5a). In term of selectivity, CO is the major product (~58 % at 700 °C), while the yield of H2 and H2O (~21 % for each at 700 °C) is less. The increasing temperature from 600 °C to 900 °C is able to enhance the yield of all three products (Figure 5a). Both CH4 and CO2 are the source for CO production. Due to the faster activation of CO2 than CH4, the contribution from CO2 is more significant than that from CH4. CH4 provides the only hydrogen source to produce both H2 and H2O. The selectivity to H2 and H2O is always lower than that to CO (Figure 5b). With the temperature increasing from 600 °C to 900 °C, both the H2/CO ratio and the H2/H2O ratio increase (Figure 5b). That is, the CH4 conversion is more significantly increased than that of CO2; yet the former cannot compete with the latter within the temperature range 24 ACS Paragon Plus Environment

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studied. Our results is in well agreement with the experiment results on the conventional NiO-MgO-Al2O3 catalyst, showing that the conversion and selectivity to H2 increase with the temperature, though both values are lower than the previously reported 34. More importantly, the only stable reaction intermediates observed during the reaction is *O and the carbon deposition is completely eliminated on the single site Ni4/MgO(100) catalyst (Figure 4c).

(a)

(b) Fig. 5. KMC-simulated turn over frequency (TOF) for production of CO, H2 and H2O (a) and H2/CO ratio (b) for the DRM reaction on the confined SSC Ni4/MgO(100) from 600 to 900°C at 1 atm with CO2:CH4 ratio of 1. 25 ACS Paragon Plus Environment

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Reaction pathway of confined SSC Ni4/MgO(100). To simplify the complex reaction network for the DRM reaction on the single site Ni4/MgO(100) catalyst, among the 45 steps studied (Table 2) only those which are observed to significantly contribute the reaction rate are identified by the KMC simulations and considered as part of preferential pathway (Figure 6). Our results show that the reaction starts with the facial CO2 dissociation to *CO and *O and *CO desorption as CO gas. The formed *O is stable and occupies 1/3 of Ni sites on the surface (Figure 4c). It opens the new oxidative mechanism for CH4 dehydrogenation to *CH3 via the *OH intermediate. The further oxidative dehydrogenation to *CH3 produces *CH2 and H2O together with the removal of *O from the surface. Yet, the first and the second C-H bond cleavage of CH4 is preferred via non-oxidative mechanism (Table 2), which competes for the three free Ni sites with the *CO2 dissociation. However, the *CO2 dissociation is more facile than that of CH4 (Tables 1 and 2, Figure 7). It enhances the possibility of *O presence on the surface (1/3 of Ni sites, Figure 4c) and the CH4 dehydrogenation is forced to occur via the oxidative mechanism, which shows the higher TOF than that via the non-oxidative pathway (Figure 7). At the elevated temperatures, the preference for the *CO2 dissociation over the oxidative and dehydrogenation of CH4 is increased; while the promotion for the non-oxidative CH4 dehydrogenation is rather small (Figure 7). The produced *CH2 undergoes two successive non-oxidative dehydrogenation to form *CH and *C, which is accompanied with the production of H2 (Figure 6). Although the *CH non-oxidative dehydrogenation to *C corresponds to a higher 26 ACS Paragon Plus Environment

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barrier than the oxidation to *CHO according to the DFT calculations (Table 2), the presence of *CH2 or *CH hinders the CO2 approaching and dissociation due to the steric repulsion and therefore there is no *O at the neighboring site of *CH. For the same reason, the contribution from the oxidative dehydrogenation of *CH2 is much less, which results in the formation of H2O instead of H2.

Fig. 6. Schematics of KMC-identified reaction network for the DRM on the confined SSC Ni4/MgO(100). The corresponding Ea for each step is also included in eV. 27 ACS Paragon Plus Environment

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The resource for *C is dominantly from the *CH2 successive dehydrogenation via the non-oxidative mechanism (Figure 6), rather than the Boudouard reaction, where ~80% undergoes the reverse step back to *CH and therefore *CH2, and the rest are eliminated via either oxidation or the reverse Boudouard reaction to form *CO via *CCOO intermediate. This is consistent with the previous studies on Ni(hkl) surfaces, proposing that the carbon deposition is from *CH dissociation and *C elimination is mainly due to oxidation and hydroxylation62, 68. Yet, the main resource for *C was recently suggested from the Boudouard reaction on Ni(111) 69. Different from pure Ni catalysts, though, the carbon deposition, the long-standing problem for Ni-based DRM catalysts, can be completely eliminated by forming the single site Ni4 cluster isolated on MgO(100) according to our KMC simulations, showing that there is no *C presence on the surface during the reaction (Figure 4c). Such SSC Ni/MgO(100) is able to not only limit *C accumulation via *CH dehydrogenation, but also enhances the *C elimination via oxidation or reverse Boudouard reaction. According to the KMC simulations, the DRM on the SSC Ni4/MgO(100) is dominated by *CO2 dissociation to *CO and *O, which is the main contribution for the production of CO (Figure 6). CH4 is the only resource for *H or H2 and *C or carbon deposition; however the CH4 dehydrogenation is not as favorable as that of the CO2 dissociation. As a result, under reaction conditions the active Ni sites are always partially covered by *O at coverage of 1/3 ML (Figure 4c), and *O is the only observable species on Ni4/Mg(100). One of the drawbacks is that 1/3 of Ni sites are blocked by *O, which hinders the CO2 dissociation and in particular for the 28 ACS Paragon Plus Environment

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non-oxidative CH4 dehydrogenation. Nevertheless, the partial coverage of *O does not poison the catalyst completely as that of *C on Ni catalysts. It acts as an oxidizing agent to enable the oxidative dehydrogenation of CH4 to *CH2 and therefore the formation of H2, *H oxidation to *OH and therefore the formation of H2O and the formation of CO from *C (Figures 3 and 6). Yet, the H2/CO ratio for Ni4/Mg(100) is lower than 1. This is due to the hindering of non-oxidative CH4 dissociation on the *O-partially covered Ni4/Mg(100), which allows 100% conversion of the formed *H to H2. Instead, the only way to produce H2 is via the oxidative dehydrogenation of CH4, where the dissociated *H is consumed by the formation of both H2 and H2O. This is very important for the many catalytic processes using H2 and CO mixture as feedstocks, e.g. Fischer–Tropsch reactions and methanol synthesis, which typically require high H2/CO ratio to assure the high conversion and selectivity70-71.

Fig. 7. KMC-simulated turn over frequency (TOF) for CO2 + * → *O + CO, CH4 + 2* → *CH3 + *H and CH4 + *O + * → *CH3 + *OH involved in the DRM reaction on the confined SSC Ni4/MgO(100) catalyst from 600 to 900°C at 1 atm with CO2:CH4 ratio of 1.

Key descriptor. To improve the yield of CO and H2 and the H2 selectivity, the 29 ACS Paragon Plus Environment

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sensitivity analysis is performed to identify the keys which control the activity and selectivity. Each Ea in the KMC model is shifted by a small amount from its original value, ±0.4 eV in this case, and the other parameters are kept constant72-73. It is found that the overall conversion and selectivity of Ni4/MgO(100) toward the DRM reaction are controlled by the Ea of three steps or descriptors. To increase the conversion of CH4, one way is to facilitate the non-oxidative dehydrogenation of *CH4 via lowering the Ea for *CH4 + * →*CH3 + *H (Figure 8a). As a result, the yield for all three products is promoted. More importantly, the increase in yield is less significant for CO than H2, while the least effect is observed for H2O. As shown in Figure 6, the accelerated non-oxidative CH4 dehydrogenation indeed enables the balance with CO2 dissociation, which promotes the pathway to allow the 100% conversion of *H to H2. Therefore, enhancing non-oxidative CH4 dehydrogenation on the SSC Ni4/MgO(100) helps to effectively increase the overall yield and the selectivity to H2 (Figure 8b). Similar increase in production is observed in the presence of *O (Figure 8c), where the increase of overall yield with the lowered Ea for oxidative CH4 dehydrogenation, *CH4 + *O → *CH3 + *OH, is more significantly than that for non-oxidative dehydrogenation (Figure 8a). Again, the balance between CH4 and CO2 activation can be essential to promote the overall conversion and production for the DRM reaction on Ni4/MgO(100). However, the facilitated oxidative CH4 dehydrogenation has little effect on the H2/CO ratio (Figure 8d). Although there is more *H produced, it equally promotes both H2 and H2O production along the oxidative pathway (Figure 6). The increase in Ea, on the other hand, can block the oxidative dehydrogenation and 30 ACS Paragon Plus Environment

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therefore populate the non-oxidative dehydrogenation of CH4, which is able to effectively enhance the H2/CO ratio (Figure 8d); yet the conversion and production remain the same. That is, the suppression of oxidative CH4 dehydrogenation is able to selectively enhance the selectivity toward H2. The increased Ea for the reversion of *CH2 dehydrogenation, *CH + *H → *CH2 + *, can stabilize *CH and promote the formation of H2 from *CH2 successive dehydrogenation (Figure 6). As a result, the H2 yield and the corresponding selectivity is selectively accelerated, while the CO and H2O productions are suppressed (Figure 8e,f). However, for the single site Ni4/MgO(100) catalyst, the tuning in stability of *CH does not make the impact on the yield and selectivity as significantly as that of CH4 dehydrogenation (Figure 8a-d). The sensitivity analysis clearly shows that for the SSC Ni4/MgO(100), the *CO2 dissociation is faster than that of *CH4. To achieve the balance of the two processes and increase the overall yield effectively, the *CH4 dehydrogenation via either non-oxidative or oxidative pathways should be facilitated (Figures 9a, c). We also test the suppress of CO2 scission, which does not show significant effect within the applied variation range in energy, ±0.4 eV. That is, the C-O bond breaking should be deactivated severely. Such big change may also affect other steps as well and cannot be handled by the current sensitivity analysis. Wherein, the acceleration of non-oxidative CH4 dehydrogenation enables the promotion of both yield and H2/CO ratio. In the case of oxidative CH4 dehydrogenation, the facilitation results in the selective enhancement in yield and the suppression enable the selective increase in H2/CO ratio. 31 ACS Paragon Plus Environment

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Fig. 8. Sensitivity in TOF of H2, CO and H2O and the H2/CO ratio during the DRM reaction on the confined SSCNi4/MgO(100) to the activation energies of CH4* + * → *CH3 + *H (a,b), CH4* + *O → *CH3 + *OH (c,d), and *CH + *H → *CH2 (e,f) at 700 °C and 1 atm with CO2:CH4 ratio of 1. The zero-energy point corresponds to the DFT-calculated Ea.

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Role of site confinement. The combined DFT and KMC simulations show that the confined SSC Ni4/MgO(100) is able to catalyze the DRM reaction and completely eliminate the carbon deposition. Now the question is whether the formation of single site catalysts is beneficial. The low-coordinated sites of Ni4 cluster and the strong interaction between Ni4 and MgO(100) modifies the electronic structures of Ni atoms and therefore their binding properties as previously reported for various metal/oxide catalysts3,

5-6, 10-11

. Such electronic effect interplays with the site confinement to

catalyze the DRM on the single site Ni4/MgO(100) catalyst. Using the non-confined Ni4/MgO(100) model (Figure 4b), the confinement of the reaction within an isolated Ni sites is removed as indicated above.

(a) (b) Fig. 9. KMC-simulated turn over frequency (TOF) of CO and H2 production during the DRM on Ni4/Mg(100) with (a) and without (b) site confinement at 700 °C and 1 atm with CO2:CH4 ratio of 1. Our KMC results show that under the DRM conditions the confined SSC Ni4/MgO(100) starts with a big spike for CO production, which decays and reaches to 33 ACS Paragon Plus Environment

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a steady state with time (Figure 9a). The production of H2 during this process is also observed and the corresponding TOF is lower than that of CO. According to the identified reaction pathway (Figure 6), the initial CO spike is due to the rapid CO2 dissociation. It leads to the Ni sites partially covered by *O (Figure 4c), which slows down the CO2 dissociation and reaches to a steady state. Without the site confinement Ni4/MgO(100) behaves differently. After the big CO peak at the initial stage, the reaction stops quickly and the H2 production is not observed (Figure 9b). In this case, the CO2 dissociation is populated more significantly than the case with site confinement. This is due to the fact that the lift of site confinement introduces the decrease in Ea of CO2 dissociation from 1.49 eV to 0.35 eV (Table 2). As a result, the coverage of *O on the Ni sites increases from 1/3 ML (Figure 4c) to 2/3 ML (Figure 4d). Thus, the free sites required for CO2 and CH4 dissociation are not enough and therefore the whole reaction stops. That is, the SSC Ni4/MgO(100) is deactivated by removing the site confinement, though the binding of active Ni sites remain the same. According to the sensitivity analysis, balancing the activation of CO2 and CH4 is essential to promote the DRM reaction on Ni/MgO catalysts. Indeed, by confining the reaction to occur at the isolated three Ni sites the single site Ni4/MgO(100) catalyst slows down the facial CO2 dissociation to a level, which provides enough oxidizing agent, *O, to prevent the carbon deposition, but still allowing the catchup of CH4 dehydrogenation via the oxidative mechanism. This is different from the previously reported promoting effect of site confinement in catalysis for SACs, two-dimensional layered materials, porous materials, interfacial materials and nanotubes7, 74-78, where 34 ACS Paragon Plus Environment

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the direct participation of the environmental sites within the confined area via various forms is necessary to facilitate the defect generation, the bond formation or cleavage. The confined SSC Ni4/MgO(100) forces the adsorption of reaction intermediates to occur at the three neighboring Ni sites provided by each isolated Ni4 cluster, enabling the direct interplay among the adsorbates to tune the binding, the reaction barrier for elementary steps and the overall catalytic performance. Our results highlight the essential roles of confined sites in tuning the catalytic performance in single site catalysis. One has to be very careful when using single site catalysts. Increasing the loading of supported particles may lift the site confinement, which can have significant effect on the catalytic performance. According to our calculations, the DRM catalysis on the confined SSC Ni4/MgO(100) depends on the well interplay between Ni and MgO. The three Ni atoms on the top of Ni4 cluster are the only active sites, which participate in the reaction directly to allow the adsorption of reactant, generation of reaction intermediates and removal of the product. The underneath Ni atom, which is embedded in the Mgvac site, is not exposed for adsorption. Instead, it acts as a contact point with the MgO support. Finally, MgO alone is not active for the DRM reaction and does not involve in the reaction directly even in the combination with Ni4 as the SSC. However, it plays indirect roles. On one hand, the Mgvac on MgO provides a strong anchor for Ni4 to strengthen the metal-support interaction and prevent the diffusion and aggregation of Ni4 cluster, which can result in the carbon deposition during the DRM reaction; on the other hand, the inert nature of MgO to the reaction 35 ACS Paragon Plus Environment

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ensures the isolation of Ni4 clusters and therefore the site confinement of Ni4/MgO(100), which is key for the proceeding of the DRM.

Fig.10 Elemental mappings of TEM images for 2.5% Ni/MgO (a), 5% Ni/MgO (b) and 10% Ni/MgO (c), where green corresponds to Mg and red represents Ni.

3.3 Experimental verification To verify the theory-identified effect of site confinement on the catalytic performance of Ni/MgO catalysts during the DRM reaction, the experimental studies 36 ACS Paragon Plus Environment

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was performed accordingly. The HRTEM (Figure S4) and elemental mapping images (Figure 10) were applied to investigate the size distribution on the synthesized Ni/MgO catalysts. It shows that most of Ni species in Ni/MgO are well mixed throughout the MgO matrix at low Ni loadings (2.5%, 5% and 10%, Figure 10 and Figure S4). Mainly Ni particles with a size is centered at 0.3 nm (1-2 Ni atoms, Figure S5a) can be observed for 2.5% Ni/MgO. With increasing Ni loading to 5%, there is a slight increase of Ni particle size centered within 0.6-0.8 nm (3-4 Ni atoms, Figures S5b and Figure 10b). However, going from 5% to 10%, the size distribution of Ni cluster remains and the Ni clusters with 3-4 Ni atoms are still the majority, while the corresponding coverage increases (Figure 10c and Figure S5c). In addition, the large Ni particles, centered at ca. 1.1 nm, was observed in 5% Ni/MgO and 10% Ni/MgO due to the increased Ni loading (Figure S5b,c). Yet, the corresponding amount is about the same for both cases. Thus, the difference in DRM activity between 5% Ni/MgO and 10% Ni/MgO, which is our interest here, is likely associated with the variation in coverage of Ni clusters with 3-4 Ni atoms. The synthesized Ni/MgO catalysts with different Ni loading can represent well the structural difference in our theoretical models in Section 3.1. The 2.5% Ni/MgO (Figure 10a and Figure S4a) describes the Ni1/MgO, where single Ni atom is isolated from each other by MgO (Figure 1a). The 5% Ni/MgO (Figure 10b and Figure S4b) likely corresponds to the confined SSC Ni4/MgO with each Ni cluster separated by MgO (Figure 4a). 2.5% Ni/MgO and 5% Ni/Mg have the similar atomic dispersion of Ni on MgO surface and the only difference is the cluster size, which is also 37 ACS Paragon Plus Environment

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represented by the structural difference between Ni1/MgO and Ni4/MgO (Figure 1). Upon going from 5% to 10% Ni/MgO, the difference in structure tends to capture the effect of site confinement, which corresponds to an increase in coverage of the Ni clusters (Figure 4b), rather than the particle size (Figure 10c and Figure S5c). Yet, the amount of non-confined Ni clusters in the 10% Ni/MgO is not as extensive as that in the non-confined confined Ni4/MgO (Figure 4b).

Fig.11 The conversion of CO2 and CH4 based on per mg of Ni under Ni loading at 2.5 %, 5 %, and 10 %. Conditions: CH4/CO2/N2=1:1:1, 0.1g catalyst at 800 °C. Before the reaction, the catalyst was reduced in 50% H2 flow at 750 oC for 1 h.

Under the DRM conditions, no carbon deposition was observed for all three Ni/MgO catalysts, which agrees with the theoretical studies. It is shown that 2.5% Ni/MgO displays almost no activity (Figure S6). With the increasing of Ni loading to 5%, not only the conversion of CH4 (Figure S5a) and CO2 (Figure S6b), but also the 38 ACS Paragon Plus Environment

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production of H2 (Figure S6c) and CO (Figure S6d) emerge and increase with the temperature under the DRM conditions, which is also observed by the KMC simulations (Figure 5). Similar behavior is also observed for 10% Ni/MgO (Figure S6) and the corresponding conversion and production are higher than that of 5% Ni/MgO; yet the magnitude of increase is not as significant as that going from 2.5% to 5%. To improve the comparability in activity among the Ni/MgO catalysts with different Ni loadings, the measured activities of Ni/MgO catalysts were expressed based on the reactant conversion over per amount of Ni loaded (Figure 11). Obviously, 5% Ni/MgO exhibits the highest activity, in comparison with both 2.5% Ni/MgO and 10% Ni/MgO. As predicted by the DFT calculations, the Ni/MgO with the low Ni loading (SAC Ni1/MgO in modeling and 2.5% Ni/MgO in experiment) is too inert to activate both CH4 and CO2 during the DRM. Both KMC simulations (Figure 5) and experimental activity test (Figure 11) show that with the same dispersion increasing size of each Ni cluster to confined SSC Ni4/MgO in modeling and 5% Ni/MgO in experiment enables the formation of neighboring Ni sites, being able to work together and significantly boost the activity of Ni/MgO. The further increase in Ni loading can selectively promote the coverage of Ni clusters and enable the formation of connected Ni assembles, rather than the size of Ni cluster (non-confined Ni4/MgO in modeling and 10% Ni/MgO in experiment), which allows the identification of site confinement effect. Experimentally, 10% Ni/MgO is not as active as 5% Ni/MgO (Figure 11). This is due to the formation of connected Ni cluster assembles, which lifts the site confinement for the SSC and results in the poisoning of Ni sites by *O according to 39 ACS Paragon Plus Environment

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the KMC simulations (Figure 4d). However, the amount of non-confined Ni4 ensembles in 10% Ni/MgO (Figure 10c) is not as significant as that in non-confined Ni4/MgO in modeling (Figure 4b). As a result, the DRM activity can still be measured experimentally on 10% Ni/MgO (Figure 11) due to the supported Ni clusters, which still remain as confined SSCs, while no activity is observed for the non-confined Ni4/MgO in modeling (Figure 9b). In fact, the actual amount of confined Ni clusters in 10% Ni/MgO is more than that in 5% Ni/MgO, which is demonstrated by the higher total conversion and production (Figure S5). Overall, formation of the confined SSC Ni/MgO is essential to catalyze the DRM reaction on NiO-MgO solid solution. Moderate amount of Ni, in our case 5%, should be chosen to intermix with MgO as observed experimentally. According to the present theoretical calculations and experiments, if the Ni loading is too low, 2.5% in our case and the SAC Ni1/MgO catalyst is formed under reducing conditions, Ni is embedded in the MgO matrix and stays as ionic. As a result, the catalyst binds the reaction intermediates too weakly and the number of neighboring Ni sites is too limited to catalyze the complex DRM reaction. If the Ni loading is high, even 10% in our case, the site confinement of SSC Ni4/MgO can be partially lifted, which results in the poisoning of active Ni sites by *O. Thus, NiO-MgO solid solution with a high content of Ni shows a low stability. Therefore, a NiO–MgO solid solution with a suitable Ni loading, 5% in our case, allows the formation of SSC Ni4/MgO, which shows a high activity and stability. It provides the bindings more strongly than the SAC Ni1/MgO catalyst and enough number of neighboring Ni sites to work 40 ACS Paragon Plus Environment

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cooperatively for balancing the activation of CO2 and CH4 and therefore H2/CO ratio, while still being able to keep the site confinement for each Ni cluster to eliminate the poisoning by *O.

4. Conclusion The performances of SSCs, Ni/MgO catalysts, during the DRM reaction were systemically studied by combining theoretical modeling (DFT and KMC simulations) and experimental studies. The DFT calculations show that synergy between single Ni atom and MgO in the SAC Ni1/MgO is not active due to the weak bindings to the reaction intermediates as that of MgO and the limited number of neighboring active sites. This is confirmed by the experiment on the 2.5% Ni/MgO catalyst including small Ni cluster of 1-2 Ni atoms. By increasing the Ni size, the single site Ni4/MgO catalyst is able to provide the stronger bindings than Ni1/MgO. It offers enough active Ni sites isolated from each other, being able to work cooperatively for activation of both CH4 and CO2, enable the production of CO, H2 and H2O and completely eliminate carbon deposition, which is also observed experimentally on the 5% Ni/MgO catalyst including Ni cluster of 3-4 Ni atoms. According to the KMC simulations, the site confinement of SSC Ni4/MgO helps to balance the activations of CO2 and CH4, which is essential not only to achieve the high yield and the H2/CO ratio of 1, but also to prevent the poisoning of active sites during the DRM reaction. It is able to slow down the facial CO2 dissociation to a level, which provides enough oxidizing agent, *O, to prevent the carbon deposition, but still allowing the oxidative dehydrogenation of CH4. The complete lift of site confinement, 41 ACS Paragon Plus Environment

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non-confined Ni4/MgO, facilitates the CO2 dissociation and results in the deactivated Ni sites by *O and hinders the DRM reaction. Similar deactivation in DRM activity introduced by removing the site confinement is also verified experimentally on the 10% Ni/MgO catalyst, which enables the partial formation of non-confined Ni4 assembles and therefore removal of site confinement of 5% Ni/MgO. The catalytic performance of confined SSC Ni4/MgO during the DRM reaction depends on the well interplay between Ni and MgO. The Ni cluster provides the only active sites, which participate in the reaction directly. MgO plays indirect roles. It offers the Mgvac as a strong anchor for Ni clusters to prevent the aggregation of Ni cluster and therefore the carbon deposition; on the other hand, the inert nature of MgO to the reaction ensures the isolation and the site confinement of SSC Ni4/MgO, which is key for the proceeding of the DRM. Our study identifies the essential roles of confined sites in tuning the catalytic performance in single site catalysis.

Supporting Information. Details of kinetics for the DRM on the single site Ni4/MgO(100) catalyst and optimized structures for the reaction intermediates involved in the DRM on MgO(100) and the single atom Ni1/MgO(100) catalyst. The activity and selectivity of Ni/MgO catalysts during the DRM reaction measured experimentally. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The research was carried out at Brookhaven National Laboratory under contract DE-SC0012704 with the US Department of Energy, Office of Sciences, Division of Chemical Sciences. The DFT calculations were performed using computational 42 ACS Paragon Plus Environment

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resources at the Center for Functional Nanomaterials, a U.S. DOE Office of Science Facility, and the Scientific Data and Computing Center, a component of the Computational Science Initiative at Brookhaven National Laboratory. Z. Z. and W. H. also gratefully acknowledged the key project of the National Natural Science Foundation of China (21336006), the National Natural Science Foundation of China (21776197 and 21776195), and the Shanxi Province Science Foundation for Youths (201701D211003) for financial support. J. H. and Z.W. acknowledges the Australian Research Council Discovery Projects (DP150103842 and DP180104010), and the SOAR Fellowship from the University of Sydney for the support of this project. References 1. Chen, Y.; Lin, J.; Li, L.; Qiao, B.; Liu, J.; Su, Y.; Wang, X., Identifying Size Effects of Pt as Single Atoms and Nanoparticles Supported on FeOx for the Water-Gas Shift Reaction. ACS Catal. 2018, 8, 859-868. 2. Su, Y.-Q.; Filot, I. A. W.; Liu, J.-X.; Hensen, E. J. M., Stable Pd-Doped Ceria Structures for CH4 Activation and CO Oxidation. ACS Catal. 2018, 8, 75-80. 3. Zhang, S.; Tang, Y.; Nguyen, L.; Zhao, Y.-F.; Wu, Z.; Goh, T.-W.; Liu, J. J.; Li, Y.; Zhu, T.; Huang, W.; Frenkel, A. I.; Li, J.; Tao, F. F., Catalysis on Singly Dispersed Rh Atoms Anchored on an Inert Support. ACS Catal. 2018, 8, 110-121. 4. Zhou, X.; Shen, Q.; Yuan, K.; Yang, W.; Chen, Q.; Geng, Z.; Zhang, J.; Shao, X.; Chen, W.; Xu, G.; Yang, X.; Wu, K., Unraveling Charge State of Supported Au Single-Atoms during CO Oxidation. J. Am. Chem. Soc. 2018, 140 (2), 554-557. 5. Nie, L.; Mei, D.; Xiong, H.; Peng, B.; Ren, Z.; Hernandez, X. I. P.; DeLaRiva, A.; Wang, M.; Engelhard, M. H.; Kovarik, L.; Datye, A. K.; Wang, Y., Activation of surface lattice oxygen in single-atom Pt/CeO2 for low-temperature CO oxidation. Science 2017, 358, 1419-1423. 6. Yang, S.; Tak, Y. J.; Kim, J.; Soon, A.; Lee, H., Support Effects in Single-Atom Platinum Catalysts for Electrochemical Oxygen Reduction. ACS Catal. 2017, 7, 1301-1307. 7. Liu, J., Catalysis by Supported Single Metal Atoms. ACS Catal. 2017, 7, 34-59. 8. Zhang, X.; Sun, Z.; Wang, B.; Tang, Y.; Nguyen, L.; Li, Y.; Tao, F. F., C–C Coupling on Single-Atom-Based Heterogeneous Catalyst. J. Am. Chem. Soc. 2018, 140, 954-962. 9. Thomas, J. M.; Raja, R.; Lewis, D. W., Single-Site Heterogeneous Catalysts. Angew. Chem. Int. Ed. 2005, 44, 6456-6482. 10. Qiao, B.; Wang, A.; Yang, X.; Allard, L. F.; Jiang, Z.; Cui, Y.; Liu, J.; Li, J.; Zhang, T., Single-atom catalysis of CO oxidation using Pt1/FeOx. Nat. Chem. 2011, 3, 634-641. 11. Malta, G.; Kondrat, S. A.; Freakley, S. J.; Davies, C. J.; Lu, L.; Dawson, S.; Thetford, A.; Gibson, E. K.; Morgan, D. J.; Jones, W.; Wells, P. P.; Johnston, P.; 43 ACS Paragon Plus Environment

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74. Li, H.; Xiao, J.; Fu, Q.; Bao, X., Confined catalysis under two-dimensional materials. Pro. Nat. Acad. Sci. 2017, 114, 5930-5934. 75. Dai, W.; Zhang, S.; Yu, Z.; Yan, T.; Wu, G.; Guan, N.; Li, L., Zeolite Structural Confinement Effects Enhance One-Pot Catalytic Conversion of Ethanol to Butadiene. ACS Catal. 2017, 7, 3703-3706. 76. Xiao, J.; Pan, X.; Zhang, F.; Li, H.; Bao, X., Size-dependence of carbon nanotube confinement in catalysis. Chem. Sci. 2017, 8, 278-283. 77. Fu, Q.; Yang, F.; Bao, X., Interface-Confined Oxide Nanostructures for Catalytic Oxidation Reactions. Acc.Chem. Res. 2013, 46, 1692-1701. 78. Göltl, F.; Michel, C.; Andrikopoulos, P. C.; Love, A. M.; Hafner, J.; Hermans, I.; Sautet, P., Computationally Exploring Confinement Effects in the Methane-to-Methanol Conversion Over Iron-Oxo Centers in Zeolites. ACS Catal. 2016, 6, 8404-8409.

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TOC

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Supporting Information

Dry Reforming of Methane on Single Site Ni/MgO Catalysts: Importance of Site Confinement Zhijun Zuo,1† Shizhong Liu,2† Zichun Wang,3 Cheng Liu,4 Wei Huang,1 Jun Huang,3* and Ping Liu5* 1

Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China 2

Chemistry Department, State University of New York (SUNY) at Stony Brook, Stony Brook, NY 11794, USA 3

Laboratory for Catalysis Engineering, School of Chemical and Biomolecular

Engineering, The University of Sydney, Sydney, New South Wales 2006, Australia 4

Mechanical Engineering college, Yangzhou University, 196 Huayang West road, Yangzhou, Jiangsu 225127, P. R. China 5

Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973, USA

* To whom correspondence should be addressed. E-mail: [email protected] and [email protected]. †

Dr. Z. Zuo and S. Liu contributed equally to this work and should be regarded as

co-first authors.

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Results Table S1 The Prefactors A (s−1) calculated at temperatures involving the DRM on confined SSC Ni4/MgO(100) Temperature Reactions 673 K

773 K

873 K

973 K

1073 K

1173 K

CO2* + * → O* + CO*

2.11E+13 2.42E+13 2.73E+13 3.04E+13 3.35E+13 3.67E+13

O* + CO* → CO2* + *

4.71E+13 5.40E+13 6.10E+13 6.78E+13 7.46E+13 8.14E+13

CH4 * + * → CH3* + H*

1.80E+14 2.05E+14 2.30E+14 2.56E+14 2.81E+14 3.06E+14

CH3* + H* → CH4 * + *

3.38E+13 3.87E+13 4.36E+13 4.86E+13 5.34E+13 5.83E+13

CH3 * + * → CH2* + H*

8.82E+13 1.01E+14 1.14E+14 1.27E+14 1.39E+14 1.52E+14

CH2* + H* →CH3 * + *

4.19E+14 4.79E+14 5.37E+14 5.96E+14 6.55E+14 7.15E+14

CH3 * + O* → CH3O* +

5.24E+12 6.03E+12 6.83E+12 7.63E+12 8.40E+12 9.21E+12

CH3O* + *→CH3 * + O*

1.85E+14 2.12E+14 2.39E+14 2.66E+14 2.92E+14 3.19E+14

CH2 * + * → CH* + H*

8.05E+12 9.23E+12 1.04E+13 1.16E+13 1.28E+13 1.40E+13

CH* + H* →CH2 * + *

4.17E+13 4.79E+13 5.41E+13 6.02E+13 6.64E+13 7.26E+13

CH2 * +O* →CH2O* + *

6.74E+11 7.80E+11 8.86E+11 9.92E+11 1.10E+12 1.20E+12

CH2O* + *→CH2 * + O*

1.28E+13 1.47E+13 1.67E+13 1.86E+13 2.05E+13 2.25E+13

CH* + * → C* + H*

4.75E+13 5.45E+13 6.14E+13 6.83E+13 7.54E+13 8.22E+13

C* + H* → CH* + *

2.18E+13 2.50E+13 2.82E+13 3.15E+13 3.47E+13 3.79E+13

CH* + O* → CHO* + *

3.42E+13 3.92E+13 4.42E+13 4.92E+13 5.43E+13 5.92E+13

CHO* + * → CH* + O*

3.25E+13 3.72E+13 4.20E+13 4.68E+13 5.16E+13 5.64E+13

CHO* + * → CO* + H*

8.56E+13 9.81E+13 1.11E+14 1.23E+14 1.36E+14 1.48E+14

CO* + H* → CHO* + *

2.82E+13 3.24E+13 3.66E+13 4.07E+13 4.50E+13 4.91E+13

C* + O* → CO* + *

5.23E+13 6.01E+13 6.78E+13 7.57E+13 8.34E+13 9.13E+13

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CO* + * → C* + O*

2.38E+13 2.73E+13 3.08E+13 3.43E+13 3.78E+13 4.13E+13

C* + C* → C2* + *

8.36E+12 9.62E+12 1.09E+13 1.22E+13 1.34E+13 1.47E+13

C2* + * → C* + C*

2.85E+12 3.28E+12 3.71E+12 4.15E+12 4.58E+12 5.01E+12

C* +CO2* →CCOO*+ *

5.49E+12 6.30E+12 7.12E+12 7.94E+12 8.76E+12 9.58E+12

CCOO*+ *→C* + CO2*

6.29E+13 7.21E+13 8.14E+13 9.07E+13 9.99E+13 1.09E+14

CCOO*+*→CO* + CO*

8.70E+13 9.99E+13 1.13E+14 1.25E+14 1.38E+14 1.51E+14

CO*+CO*→CCOO* + *

4.88E+12 5.60E+12 6.33E+12 7.03E+12 7.75E+12 8.49E+12

C* +CO2*→CO* + CO*

1.15E+14 1.33E+14 1.50E+14 1.67E+14 1.84E+14 2.00E+14

CO* + CO*→C*+ CO2*

7.41E+13 8.50E+13 9.59E+13 1.07E+14 1.18E+14 1.28E+14

COH* + * → CO* + H*

2.40E+13 2.76E+13 3.12E+13 3.48E+13 3.84E+13 4.19E+13

CO* + H* → COH* + *

1.47E+14 1.69E+14 1.90E+14 2.12E+14 2.34E+14 2.56E+14

COH*+H*→CHOH*+ *

6.75E+13 7.74E+13 8.75E+13 9.75E+13 1.08E+14 1.17E+14

CHOH*+*→COH*+ H*

7.54E+13 8.63E+13 9.75E+13 1.08E+14 1.19E+14 1.30E+14

CHOH*+H*→CH2OH*+*

1.98E+14 2.27E+14 2.56E+14 2.86E+14 3.15E+14 3.44E+14

CH2OH*+*→CHOH*+ *

5.19E+13 5.95E+13 6.71E+13 7.45E+13 8.22E+13 8.97E+13

CHO* + H*→CHOH* + *

1.69E+14 1.94E+14 2.18E+14 2.43E+14 2.68E+14 2.92E+14

CHOH*+ * →CHO* + H*

1.58E+14 1.81E+14 2.04E+14 2.26E+14 2.49E+14 2.73E+14

CHO* + H*→ CH2O* + *

6.72E+12 7.72E+12 8.72E+12 9.71E+12 1.07E+13 1.17E+13

CH2O*+ * → CHO* + H*

8.96E+12 1.03E+13 1.16E+13 1.29E+13 1.43E+13 1.56E+13

H* + O* → OH* + *

6.87E+13 8.08E+13 9.08E+13 9.89E+13 1.09E+14 1.19E+14

OH* + * → H* + O*

1.44E+14 1.64E+14 1.85E+14 2.06E+14 2.26E+14 2.47E+14

H* + OH* → H2O* + *

2.58E+13 2.96E+13 3.33E+13 3.71E+13 4.09E+13 4.46E+13

H2O* + * → H* + OH*

8.26E+12 9.53E+12 1.08E+13 1.20E+13 1.33E+13 1.46E+13

CH2O*+H* → CH3O* + *

7.59E+13 8.72E+13 9.86E+13 1.10E+14 1.21E+14 1.33E+14

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CH3O* + *→CH2O* + H*

1.88E+14 2.15E+14 2.41E+14 2.68E+14 2.94E+14 3.21E+14

CH2O*+H*→CH2OH*+ *

8.16E+13 9.36E+13 1.06E+14 1.18E+14 1.29E+14 1.41E+14

CH2OH*+*→CH2O*+ H*

1.11E+14 1.27E+14 1.43E+14 1.59E+14 1.75E+14 1.91E+14

CH3O*+H*→CH3OH*+ *

1.30E+12 1.51E+12 1.73E+12 1.94E+12 2.16E+12 2.37E+12

CH3OH*+*→CH3O*+ H*

7.32E+12 8.49E+12 9.67E+12 1.08E+13 1.20E+13 1.32E+13

CH2OH*+H*→CH3OH*+* 2.63E+12 3.06E+12 3.49E+12 3.93E+12 4.36E+12 4.79E+12 CH3OH*+*→CH2OH*+H* 2.35E+12 2.73E+12 3.11E+12 3.48E+12 3.87E+12 4.25E+12 CH4 *+ O* → CH3* + OH*

7.13E+13 8.15E+13 9.14E+13 1.02E+14 1.12E+14 1.21E+14

CH3* + OH*→CH4 * + O*

8.03E+13 9.23E+13 1.04E+14 1.16E+14 1.28E+14 1.40E+14

CH4*+O*→CH3*+H*+ O*

1.70E+13 1.93E+13 2.18E+13 2.41E+13 2.65E+13 2.88E+13

CH3*+H*+O*→CH4 *+O*

7.06E+13 8.10E+13 9.08E+14 1.02E+14 1.12E+14 1.23E+14

CH3 *+ O* → CH2* + OH*

1.06E+14 1.22E+14 1.36E+14 1.51E+14 1.68E+14 1.83E+14

CH2* + OH*→CH3 * + O*

2.19E+15 2.49E+15 2.79E+15 3.08E+15 3.40E+15 3.70E+15

CH3*+O*→CH2*+H*+ O*

8.29E+13 9.49E+13 1.07E+14 1.19E+14 1.31E+14 1.43E+14

CH2*+H*+O*→CH3 *+O*

1.20E+14 1.37E+14 1.55E+14 1.72E+14 1.90E+14 2.08E+14

CH2 * + O* → CH* + OH*

1.58E+14 1.82E+14 2.05E+14 2.28E+14 2.51E+14 2.74E+14

CH* + OH*→CH2 * + O*

3.97E+13 4.54E+13 5.12E+13 5.70E+13 6.27E+13 6.85E+13

CH2 *+O* →CH*+H*+ O*

5.10E+13 5.85E+13 6.59E+13 7.33E+13 8.08E+13 8.82E+13

CH*+ H*+ O*→CH2 *+O*

2.61E+13 2.99E+13 3.38E+13 3.77E+13 4.15E+13 4.55E+13

CH * + O* → C* + OH*

9.17E+13 1.05E+14 1.18E+14 1.31E+14 1.44E+14 1.57E+14

C* + OH* → CH * + O*

1.46E+13 1.67E+13 1.88E+13 2.09E+13 2.30E+13 2.52E+13

CH*+O* → C* + H*+ O*

3.45E+14 3.93E+14 4.41E+14 4.90E+14 5.38E+14 5.85E+14

C* + H*+ O* → CH*+O*

4.68E+13 5.37E+13 6.04E+13 6.74E+13 7.42E+13 8.10E+13

C*+CO2*→ C* +CO* +O*

1.17E+14 1.34E+14 1.51E+14 1.68E+14 1.86E+14 2.03E+14

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C*+CO* +O*→C* + CO2*

7.27E+13 8.34E+13 9.41E+13 1.05E+14 1.15E+14 1.26E+14

O*+CO2*→O* +CO* +O*

4.94E+13 5.68E+13 6.40E+13 7.12E+13 7.85E+13 8.55E+13

O*+CO* +O*→O*+ CO2*

1.31E+14 1.50E+14 1.68E+14 1.87E+14 2.06E+14 2.24E+14

CH4 *+ C* → CH3* + CH*

7.39E+12 8.46E+12 9.54E+12 1.06E+13 1.17E+13 1.28E+13

CH3* + CH* →CH4 * + C*

3.98E+12 4.57E+12 5.16E+12 5.75E+12 6.33E+12 6.92E+12

CH4 *+C*→CH3*+H*+ C*

5.15E+12 5.89E+12 6.63E+12 7.37E+12 8.11E+12 8.85E+12

CH3*+H*+C* →CH4 *+C*

2.77E+12 3.18E+12 3.58E+12 3.99E+12 4.39E+12 4.80E+12

Fig.S1. Adsorption configurations of possible intermediates involved in the DRM reaction on the MgO(100) surface. Green: Mg; Red: O; Blue: Ni; Dark grey: C; White: H.

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ACS Catalysis

Fig.S2. Adsorption configurations of possible intermediates involved in the DRM reaction on the SAC Ni1/MgO(100). Green: Mg; Red: O; Blue: Ni; Dark grey: C; White: H.

Fig.S3. Adsorption configurations of possible intermediates involved in the DRM reaction on the defected SAC Ni1/MgO(100). Green: Mg; Red: O; Blue: Ni; Dark grey: C; White: H.

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(a)

(b)

(c)

Fig.S4. HRTEM images of 2.5% Ni/MgO (a), 5% Ni/MgO (b), and 10% Ni/MgO (c).

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40

(a)

35 30

Counts

25 20 15 10 5 0 0.0

0.5

1.0

1.5

2.0

2.5

Size (nm) 40

(b)

35 30

Counts

25 20 15 10 5 0 0.0

0.5

1.0

1.5

2.0

2.5

Size (nm)

40 35

(c)

30 25

Counts

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ACS Catalysis

20 15 10 5 0 0.0

0.5

1.0

1.5

2.0

2.5

Size (nm)

Fig. S5. Particle size distribution of Ni particles supported on MgO: a) 2.5% Ni/MgO, b) 5% Ni/MgO, and c) 10% Ni/MgO according to the TEM images.

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Fig.S6. Conversion of CH4 (a) and CO2 (b), 5he production of H2 (c) and CO (d) as a function of temperature in the reforming of CO2 with CH4 over Ni/MgO catalysts under Ni loading at 2.5 %, 5 %, and 10 % at 800 oC. Conditions: CH4/CO2/N2=1:1:1, 0.1 g catalyst. Before the reaction, the catalyst was reduced in 50% H2 flow at 750 oC for 1 h.

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