Mechanisms of Formaldehyde and C2

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Article 2

Mechanisms of Formaldehyde and C Formation from Methylene Reacting with CO Adsorbed on Ni(110) 2

Wei Lin, and George C. Schatz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00945 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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The Journal of Physical Chemistry

Mechanisms of Formaldehyde and C2 Formation from Methylene Reacting with CO2 Adsorbed on Ni(110) Wei Lin and George C. Schatz* Department of Chemistry, Northwestern University, Evanston, Illinois, 60208-3113, USA

Supporting Information Placeholder ABSTRACT: Methylene (CH2) is thought to play a significant role as a reaction intermediate in the catalysis of methane dry reforming as well as in converting synthesis gas to light olefins via Fischer-Tropsch synthesis. Here, we report high quality BornOppenheimer molecular dynamics (BOMD) simulations of the reaction mechanisms associated with CH2 impinging on a Ni(110) surface with CO2 adsorbed at 0.33 ML coverage. The results show the formation of formaldehyde, carbon monoxide, C2 species such as H2C–CO2, and others. Furthermore, we provide real-time demonstration of both Eley-Rideal (ER) and hot atom (HA) reaction mechanisms. The ER mechanism mostly happens when CH2 directly collides with an oxygen of CO2, while CH2 attacks the carbon of CO2, dominantly following the HA mechanism. If CH2 reaches the Ni surface, it can easily break one C–H bond to form CH and H on the surface. The mechanistic details of H2CO, H/CO, C2, and H/CH formations are illuminated through the study of bond breaking/formation, charge transfer, and spin density of the reactants and catalytic surface. This illuminates the key contribution of geometry and electronic structure of catalytic surface to the reaction selectivity. Moreover, we find that 3CH2 switches to surfaces of 1CH2 character as soon as the methylene and nickel/CO2 orbitals show significant interaction, and as a result the reactivity is dominated by low barrier mechanisms. Overall, the BOMD simulations provide dynamical information that allows us to monitor details of the reaction mechanisms, confirming and extending current understanding of CH2 radical chemistry in the dry reforming of methane and Fischer-Tropsch synthesis.

1. INTRODUCTION Dry reforming of methane (DRM) is a process for converting two greenhouse gases, CO2 and CH4, into syngas, and from there to produce a wide range of products (e.g. alkanes, oxygenates) by means of Fischer-Tropsch synthesis. Metal-based catalysts such as Ni are generally used to accelerate the dry reforming of both chemically inert and thermodynamically stable gases.1 Metals and metal oxides can dissociate methane directly to form chemically active CHx (x = 1–3) intermediates.2-3 High temperatures are needed for dry reforming, so these species are highly likely to dissociate, leading to coke deposition.1 Alternatively, dry reforming can take place under more modest conditions through a plasma-based process with no catalyst, although then selectivity can be an issue.4 Plasma catalysis, which combines the capabilities of plasma-based dry reforming with the selectivity of catalysis, has therefore been of growing interest for inducing thermodynamically unfavorable reactions at modest temperatures and pressures.5-11 In the plasma-catalysis dry reforming process, CHx radicals, especially CH2, can play a significant role as intermediates, as these are easily produced from the relatively efficient electron impact induced dissociation of methane. Very recently, Wang et al. have reported that CH3/CH2 may be the key intermediates for one-step reforming of CO2 and CH4 into high-value liquid chemicals and fuels by plasma-driven catalysis.7 Even though CH2 and CH3 have higher adsorption energies than CO2 on Ni, since the concentration of CO2 is higher, and it is less easily fragmented than CH4 in a plasma, it is possible that CH2 and CH3 from the plasma subsequently react with CO2 that is adsorbed on the catalytic Ni surfaces. These gas/surface reac-

tions have never been characterized experimentally, although it is known that the gas phase reaction of 3CH2 (the triplet ground state) with CO2 to give CH2O + CO has a rate constant of 3.90 × 10-14 cm3/s at room temperature (upper bound estimate), while the corresponding rate constant for CO2 + 1CH2 (the singlet excited state) is 5.0 × 10-12 cm3/s.4, 12 These differences between triplet ground state and singlet excited state reactivities are typical of gas phase chemistry. Regarding the surface chemistry, Michaelides and Hu have pointed out that the structure of chemisorbed CH2 on Ni(111) more closely resembles 1CH2 than the 3CH2,13 but it is an open question as to how these results will change in the presence of CO2 on a Ni surface. Methylene (CH2) itself is the simplest member of the carbene homologous series. It is well known as an intermediate in many heterogeneous catalytic processes.14 Therefore, its hydrogenation or dehydrogenation processes on metal surfaces have been intensively studied to understand elementary steps in catalysis.13, 15-19 Recently, the CH2 radical is also believed to be a primary intermediate in heterogeneous conversion of syngas to light olefins via Fischer-Tropsch synthesis.20-27 Galvis et al. have argued that the carbon chain of the light olefin grows by the addition of methylene monomer units to the adsorbed alkyl species as catalyzed by iron nanoparticles.20 Bao and coworkers have recently reported that CH2 species generated from CO hydrogenation through a ZnCrOx catalyst reacts with CO, forming the relatively less reactive ketene, which then is converted to light olefins in the presence of the confined acidic environment of zeolite pores.22 Furthermore, Zhong et al. have studied the direct production of lower ole-

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fins from syngas showing that cobalt carbide nanoprisms that arise from a CoMn catalyst are responsible for the production of lower olefins (such as CH2CH2) through the dimerization of CH2 radicals.26 However, details of the role and reaction mechanisms of the CH2 radicals in these reactions are unclear. Thus, further theoretical and experimental studies are needed for understanding the role of CHx species at catalytic surfaces. In the present work, Born Oppenheimer molecular dynamics (BOMD) simulations have been carried out to determine the reaction mechanisms for CH2 impinging on a Ni(110) surface under UHV low temperature conditions with CO2 adsorbed at 0.33 ML coverage. The goal of this work is to provide new insights to the role of CH2 radicals in CO2 reduction, both for the specific case of dry reforming, where the Ni(110) surface provides a convenient model for the Ni catalyst, and more generally in Fischer-Tropsch processes. We will also compare the results from the present study with our earlier work on CO/CO2 reduction by atomic hydrogen, which are other reactions of relevance to dry reforming and Fischer-Tropsch chemistry.28-30 As parts of the present study, we report the effect of CH2 adsorption sites, of charge transfer between the reactants and Ni surface, and of the CH2 spin density on the selectivity of the reaction to produce CO, formaldehyde, and C2 species (where we use the symbol C2 to stand for any molecule containing two carbons). Although there have not been experimental studies of this particular reaction, the computational methods being used are at the same level of quality as those used earlier for H + CO2 and H + CO where comparisons with experiment were provided.28-30 We therefore expect that this work should open a new direction of understanding CH2 reactions on catalytic surfaces. 2. METHODOLOGY BOMD simulations of CH2 impinging on a Ni(110) surface with CO2 at 0.33 ML coverage are carried out via the VASP software package.31-32 Spin-polarized DFT with PAW potentials were used for electron-ion interactions, and the GGA-PBE functional,33 which has been widely used to calculate the adsorption energies and transition states for CO2 hydrogenation on Ni(110),28-29, 34-36 was applied to describe the exchangecorrelation interactions. Bader charges and spin densities were calculated from the Bader analysis program.37-40 The translational energy of the CH2 radical is set to 0.1 eV (a typical energy for CH2 produced by electron impact) and the temperature of the rest of the system is set to 90 K to mimic a molecular beam experimental setting similar to that used for CO2 hydrogenation experiments.41 In total, 150 trajectories were run in a constant energy and volume (NVE) ensemble with the initial center of mass of CH2 randomly distributed in a unit cell that is 5 Å above the CO2 covered surface. The initial geometry of CH2 was randomly oriented and with velocities of the atoms determined by combining the translational energy of CH2 normal to the surface with the assumed CH2 rovibrational temperature of 90 K. The Ni(110) surface is treated using a 2 × 3 unit cell with 7 atomic layers, where the top 4 layers are free to relax, and the bottom 3 layers are fixed at their lattice positions. All the trajectories are taken to be 2 picoseconds (ps) in length with 0.25 fs time step. Note that in our BOMD simulations, vibrational mode relaxation by electron-hole pair excitation (which should occur at the >ps timescale that is generally longer than the reaction times we find) is not considered.42

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3. RESULTS AND DISCUSSION We first present the populations and product distributions for CH2 impact on CO2−adsorbed Ni(110). Subsequently, to explore the details of the reaction mechanisms, we consider bond breaking/formation, charge transfer, and the spin densities of four representative products. Note that surface adsorbed atoms and molecules are denoted with a *, as in CO2*, to distinguish them from gas phase species (CH2). 3.1 Populations and product distributions. Figure 1 presents the products and their corresponding populations, characterized into four groups: (1) CO* plus fragments, (2) the adduct H2C–CO2*, (3) nonreactive scattering, and (4) CH2 adsorption leading to CH2 dissociation (and CO2 desorption). Overall, 45% (with ±6%) of the products form in “CO* plus fragments” group, and all collisions in this group involve a mechanism wherein CH2 collides with one of the O atoms of CO2*. Three categories of products are included in this group, including 12% leading to the formation of H2CO (gas) + CO*, 19% leading to H2CO* thermalization, and 14% involving H2CO* fragmentation (Figure 1). Since the desorption energy of H2CO is 1.25 eV, much smaller than the CH2 adsorption energy (−3.94 eV), it is expected that gaseous H2CO can easily be produced from this reaction. However, thermalization also occurs, which involves making H2CO* with significant internal excitation initially. So, it is not surprising that the H2CO* can also dissociate, leading to 2H* + CO*. No HCO* product was found in any trajectories, which is consistent with our recent study of CO hydrogenation, in which HCO* was found to be a short-lived species on the path to forming H2CO/CH3OH.29 In particular, the barrier to break the C−H bond in H2CO* is 0.5 eV, while it only requires 0.28 eV to form CO* + H* from HCO*. Furthermore, the barrier for the reverse process (CO* + H* to HCO*) is as high as 1.08 eV.29

Figure 1. The products and their populations for CH2 impact on CO2−adsorbed Ni(110). For the three categories in the “CO* plus fragments” group, the reaction mechanism mostly occurs through an Eley-Rideal (ER) mechanism (Table 1), as both O atoms on CO2* face toward vacuum (as the adsorbed CO2 has a bent CO2δ− structure on Ni(110)) facilitating the direct reaction. As labelled by the red symbols in Figure 2c (square for gaseous H2CO, triangle for H2CO*, and circle for 2H* + CO*), the mechanism in which

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CH2 directly abstracts O of the CO2* can occur in most places sampled in the unit cell. However, it barely happens on the areas that are not covered by the van der Waals radii of the two adsorbed CO2*’s (boxes 1-3 in Figure 2b & 2c), as other reaction mechanisms take over.

CH2 is similar to that for the reactions of CH2 with O of the CO2* (red symbols in Figure 2). After carefully examining the trajectories, we found that when the CH2 reaches CO2* with hydrogen (rather than carbon) downward, the CH2 prefers to bounce back to vacuum, whereas when the C of CH2 is downward it is more likely to react with O of CO2*.

Table 1. Number of trajectories (out of 150) following the ER or HA mechanism for different reaction channels.

The fourth group in Figure 1 (23±4% contribution) involves CH2 hitting the Ni surface (areas labelled as 1, 2, and 3 in Figure 2b & 2c). Enough internal excitation occurs to dissociate one C−H bond (forming H* + CH*). In a few cases either two or no C−H bonds are broken, forming 2H* + C* or CH2*, respectively (Figure 1). The barriers to dissociate CH2* to CH* + H* and CH* to C* + H* are both ~ 0.5 eV,46 hence further dissociation of CH* through thermal effects can be expected in this reaction. During the collision of CH2 with Ni atoms, part of the CH2 kinetic energy is transferred to the surface Ni atoms, meanwhile CH2* accepts charge from the Ni atoms (both surface and subsurface layers of Ni atoms as they are both in direct contact with CH2*). Both energy and charge transfer destabilize the adsorbed CO2*. When C−H (of CH2*) is broken, the diffusion of the “hot” hydrogen accelerates the CO2 desorption, since the adsorption energy of CO2 is only −0.45 eV uphill, much lower than the other species on the surface (Table 2). As a result, one of the two CO2 molecules desorbs in half of the trajectories (red columns in Figure 1).

H2CO(gas)

H2CO*

2H*+CO*

H2C-CO2*

ER

17

28

21

3

HA

1

0

0

9

In the second reaction group in Figure 1, CH2 attacks the C of the CO2* to form the adduct H2C–CO2*, with a probability of 8%. The gas phase molecule is a relatively unstable biradical known as dioxatrimethylenemethane43, so it is not surprising that this is a minor product, however this is the only C2 product that we obtain (i.e., no dissociation to ketene + O* is found). Figure 2c shows that H2C–CO2* is mostly obtained when CH2 hits the surface close to the C of CO2* (the blue diamond labels). For this approach direction, CH2 first hits the Ni surface, with an adsorption energy of −3.94 eV. Hence, CH2 gains high translational and internal energy from this initial encounter. This is analogous to what happens when atomic hydrogen hits Ni(110) with an adlayer of CO2*, where it leads to a hot hydrogen atom that can then easily react with CO2*.30 Although CH2 has a much higher mass than atomic hydrogen, it is still relatively “hot” and reactive for a few bounces before thermalization or its C−H bond broken. Within these few bounces, CH2 has a chance to form a bond with the C of CO2* (similar to hot H hitting the C of CO2* to form HCOO*).30 Since the incident CH2 traps and then moves at hyperthermal energy across the surface, reaction with an adsorbed CO2 several angstroms away from the impact center can be considered to be a hot atom (HA) mechanism for reaction.44-45 Overall, the HA mechanism dominates this reaction channel (Table 1).

Table 2. The adsorption energies (Eads, eV) of selected species. Eads

CO2

CH2

CH

C

H

CO

H2CO

-0.45

-3.94

-6.91

7.41

2.71

1.91

-1.25

The product mole ratios (rm) of the CH2 + CO2* reaction are listed in Table 3. Overall, CO has the highest mole ratio as it is produced from all three categories of reaction involving CH2 attacking O of the CO2*. The second-highest mole ratio product is hydrogen, which comes from breaking of the C−H bond of H2CO* or CH2*. Formaldehyde has a 0.31 mole ratio as it is one of the key products of the reaction. A small amount of H2C−CO2* is also produced. As the overall charge of the H2C−CO2* is about −0.85 e, with most of the charge on CO2 (will describe in detail below), it can easily be further reduced to form ketene if a second CH2 impacts nearby (leading to H2C–CO2* + CH2 → H2CCO* + H2CO*). Table 3. Product mole ratios (rm) of CH2 + CO2* reaction.

rm

Figure 2. (a) Side view of the system (H, gray; O, red; C, cyan; Ni, blue; unit cell, green dashed rectangle). (b) Top view of the surface, showing van der Waal radii for CO2. (c) Distribution of outcomes for CH2 impact on CO2−adsorbed Ni(110). About one quarter of the incident CH2 radials are reflected back to vacuum (the third group in Figure 1). As labelled by the green crosses in Figure 2c, the distribution of the reflected

CO

H

H2CO

CH

C

H2C-CO2

0.59

0.51

0.31

0.17

0.03

0.08

3.2. Details of the reaction mechanisms. We then use bond distances, charges, and spin densities associated with four representative trajectories (two from group 1, one from group 2 and one from group 4) to study the reaction mechanisms. Important details of the different reaction mechanisms are revealed in Figures 3 − 5. To assess the evolution of atomic charges during the reactions, a Bader charge analysis of the reactants and top two layers of Ni atoms is provided. Before CH2 impacts the surface, both chemisorbed CO2*’s (in the unit cell) are negatively charged on Ni(110) with C being positive and both O’s negative (middle row of Figure 3), while all sur-



face Ni atoms are positively charged (total 1.4 e). This indicates that there is charge transfer from surface Ni atoms to CO2 during its adsorption. More specifically, three groups of Ni charges are shown in Figure S1: two of 0.15 e, 0.27 e, and 0.33 e. Since each CO2* forms three strong bonds with the surface,28 the 0.15 e characterizes charge transfer from Ni to the bonded C. The remainder of the two charges refers to charge transfer from Ni to the two bonded O’s in CO2*. Compared to surface Ni, subsurface Ni atoms have a minor effect (~15%) on CO2 adsorption, as their overall charges are nearly neutral (Figure S1). Reaction 1: CH2 + CO2* → H2CO* + CO*. Selected bond distances for CH2 and CO2 as well as their atomic charges and total charge of the Ni atoms in one representative trajectory producing H2CO* + CO* are shown in Figure 3a. This trajectory follows the ER mechanism: CH2 reacts with O of CO2* directly from the gas phase without adsorption. At t = 200 fs, CH2 slows down as it surmounts the barrier to transferring O from CO2*. Then it vibrates twice in the H2C−OCO* intermediate, during which there is significant charge transfer from C of CH2 to CO2, and even to surface Ni (250–350 fs in Figure 3a). After t = 350 fs, H2CO* and CO* form, carrying −0.07 and −0.30 e of charge, respectively. Since the sum of charges of CH2 and CO2* before reaction is ~0.72 e, after the reaction ~0.35 e of charge has been transferred to surface Ni, mostly to the Ni atom that was initially bonded to O of CO2 (yellow curve in Figure S1a). The time-dependent reaction process for this trajectory is shown in the Supporting Movie denoted H2CO.

Figure 3. Bond distances (top), atomic Bader charges of C, O (middle), H, and total charge for Ni (bottom) of (a) Reaction 1 and (b) Reaction 2. Carbon in CO2 is labelled as C′. Figure 4a shows evolution of the spin density of CH2 during the reaction. The spin density, defined as the difference between the densities of the spin-up and spin-down electrons, measures the localization of unpaired electrons during the reaction. Before the CH2 radical reaches the surface, it is in the triplet (ground) state, as expected.47-49 At about 200 fs, there is a sudden transition from triplet (spin density equals two) to singlet (spin density equals zero) corresponding to a point where CH2 approaches to within ~2.5 Å of the CO2*. Afterward, C−O in the H2C−OCO* intermediate vibrates with transfer of charge between CH2 to CO2, and the spin density of CH2 oscillates. When the products form at ~350 fs, the spin densi-

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ty goes to essentially zero, corresponding to the expected result for H2CO*. Evolution of the spin densities of the reactants and catalytic surface along the reaction are reported in Figure S2, which shows that the Ni atoms have significant spin density initially and then the CH2 spin density is altered (mediated by CO2*) by that of the Ni atoms when orbital overlap with the surface becomes significant.

Figure 4. Evolution of spin density of CH2. As described above, the direct reaction of CH2 with O of the CO2* also produces 12% of H2CO (gas) + CO*. Most of these reactions follow the ER mechanism, where the impinging CH2 abstracts O from CO2* to form H2CO (gas) directly (Figure S3a). After the reaction, the gaseous H2CO is neutral and CO* carries −0.35 e of charge, hence ~0.4 e of charge has been mostly transferred to the surface Ni (Figure S3). Reaction 2: CH2 + CO2* → 2H* + 2CO*. Fig. 3b shows a variation on the Reaction 1 mechanism in which the H2CO* that is produced acts as an intermediate to form CO* + 2H* through the breaking of both C−H bonds. Here we see the formation of a H2C-OCO* intermediate when CH2 reaches CO2* (~200 fs), and then one of C−H bonds is broken right after H2CO* is formed at 280 fs (red curve in top row of Figure 3b). The second C−H bond of H2CO* is subsequently broken at 330 fs (green curve). In this case, both H atoms carry −0.18 e of charge (bottom row). As with Reaction 1, there is significant charge transfer from the carbon of CH2 to that of CO2*. However, in this channel no significant charge transfer occurs between the reactants and the Ni surface (bottom row of Figure 3b and Figure S4). Figure 4b shows that the evolution of CH2 spin density in this channel is similar to that in Reaction 1, including an oscillation of the spin density when the H2C-OCO* intermediate is formed. Afterwards, the CH2 singlet character is maintained, even during the breaking of both C−H bonds. Reaction 3: CH2 + CO2* → H2C−CO2*. The reactions described above mainly follow the ER mechanism in which CH2 directly strikes CO2*. Hereafter, we analyze details of reactions that occur after the CH2 hits the Ni surface (mostly following the HA mechanism) through two representative trajectories. Figure 5a illustrates the results for Reaction 3 in which CH2 is first trapped on the surface (at ~350 fs), releasing a significant amount of energy. Subsequently it bounces a few times (nonthermal diffusion) on the surface (for ~ 200 fs) and then attacks C of CO2* to form H2C−CO2* at ~600 fs (See Supporting Movie H2CCO2). When CH2 bounces on the surface,


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~0.4 e of charge is transferred from Ni to the C of CH2 (Figure 5a and S5), which raises the reaction probability between the negatively charged CH2 carbon and positively charged CO2* carbon. After H2C−CO2* is formed, its overall charge is −0.85 e, meaning there is not significant charge flow between the product and the Ni surface.

Figure 5. Bond distances (top), atomic Bader charges of C, O (middle), H, and total charge of Ni (bottom) for (a) Reaction 3 and (b) Reaction 4. Carbon in CO2 is labelled as C′. The evolution of the spin density of CH2 during H2C−CO2* formation is shown in Figure 4c. This shows that once the CH2 reaches the Ni surface, transfer to the singlet state is sudden and irreversible. This contrasts with the behavior in Figures 5a and 5b, where the spin-transfer is mediated through CO2*, and oscillates once or twice. Also note that although the gaseous H2C−CO2 radical has been predicted to have a triplet ground state,43, 50 it is stabilized as a singlet state on the Ni(110). Chen et al. reported that the H2C−CO2 has a very lowlying open-shell singlet excited state, so it is perhaps not surprising that the singlet state can be stabilized.43 Reaction 4: CH2 + CO2* → H* + CH* + CO2 (gas). Another fate of CH2 after trapping on the surface is the formation of H* + CH*. As discussed above, 17% of trajectories follow this pathway, with half of them also giving CO2 desorption. One representative trajectory of this reaction type is shown in Figure 5b. Once CH2 is trapped on the surface, one of its C−H bonds elongates, then briefly returns to equilibrium before dissociating completely (green curve in top row of Figure 5b). The newly−formed CH* is then trapped on the bridge formed by two Ni atoms (black curve) due to its high adsorption energy at that site (Table 2). As with Reaction 3, the C of CH2 receives ~0.4 e of charge from Ni after trapping on the surface (Figure 5b and S6). Similar charge transfer occurs for H* (red curve in bottom row of Figure 5b). In the meantime, the diffusing H* transfers energy to the CO2*, weakening CO2 binding and the C of CO2 gradually transfers charge to the Ni atoms (C′ in Figure 5b). This leads to CO2 desorption at t = 1.3 ps (red curve in top row of Figure 5b). The CH2 spin density in Figure 4d shows rapid transfer from triplet to singlet when CH2 reaches the Ni surface, and then a small oscillation as the CH2 briefly bounces away from the surface (at 350 fs) before dissociating. Details of the spin densities of the reactants and catalytic surface along Reaction 4 are presented in Figure S7. Different from

the Reaction 1, in which the spin density transition is mediated by CO2*, the spin density of Ni atoms alters that of CH2 directly since CH2 is directly trapped on the Ni surface. Overall, in the four representative trajectories, we have illuminated several distinct reaction mechanisms. If the CH2 radical encounters CO2* first, it prefers to attack O of CO2* to form H2CO + CO* or 2H* + 2CO* through an ER mechanism. However, if CH2 does reach the Ni surface, it can either attack CO2 to form a stable C2 species via a HA mechanism or dissociate the C−H bond after collision with the Ni atoms. In general, the reactions happen within the first 100 fs after the encounter, with significant of charge transfer between reactants and catalytic surface. Here the Ni catalyst can act as either a charge acceptor or donor depending on the reaction path. Furthermore, once CH2 is within the van der Waal radius of the Ni surface, the triplet state of CH2 rapidly transitions to a singlet state (possibly mediated by CO2) due to mixing of its unpaired spin density with the Ni spins. Similar quenching of the spin states (triplet to singlet) on O2 + Al(111) have been reported with the argument that conventional density functional theory fails predicting the O2 adsorption barrier due to the unaccounted spin selection rules.51-53 One may therefore wonder if this is the case in CH2 + CO2adsorbed Ni(110). Our results show that the sticking probability for CH2 is not unity; we found one quarter of CH2 are reflected to vacuum, indicating an adsorption barrier of CH2. Furthermore, we have also noted the crossing of triplet and singlet states during the impacting of CH2 (at around 2.5 Å above the surface), which is consistent with the previous calculation of 1CH2 adsorbed on Ni(111).13 4. CONCLUSIONS In this work, we have performed BOMD simulations to study the reaction of CH2 on a CO2−coated Ni(110) surface and the results have been used to characterize the reaction mechanisms. Overall, our results demonstrate that the ER mechanism dominates when CH2 directly hits CO2*, while a HA mechanism arises when the CH2 hits exposed Ni atoms. Almost half of the total reactions involve the ER mechanism to form H2CO*, and then it can thermalize on surface, fly to vacuum, or dissociate to give 2H* + CO*. If the CH2 radical collides with the Ni surface, a C−H bond breaks to form H* + CH* with about 50% probability, or to form the addition product (a C2 species) H2C−CO2* with 25% probability. Although the overall C2 yield is low (8%), we have demonstrated that C−C bond formation occurs through an addition reaction, which illuminates a key reaction mechanism relevant to olefin production. Also, the charge transfer between the methylene carbon and CO2 carbon that occurs during this reaction suggests that there is an optimal CO2 coverage for C-C bond formation such that charge transfer neutralizes the carbon charges. From the perspective of dry reforming, the intermediate H2C−CO2* that results from CH2 addition to CO2 can be further reduced to form ketene (H2CCO*) if a second CH2 attacks the O in H2C−CO2* (forming H2CCO* + H2CO*, similar to the ER mechanism of group 1 in this study). Ketene can also be produced by the reaction of H2C−CO2* with atomic hydrogen, where it would lead to H-assisted C-O bond breaking to give H2CCO* + OH*, similar to H-assisted CO2* reduction on Ni(110).30 Furthermore, reaction of a second CH2 with CO*, formed from the first CH2 reacting with CO2*, can also produce ketene.



This mechanistic insight for C−C bond formation is also relevant to CO hydrogenation through the ZnCrOx catalyst, where CH2* + CO* forms ketene, and then this is converted to light olefins (rather than paraffins that arise from CH2* polymerization) following a mechanism proposed by Jiao et al.22 Overall, we have revealed a unique mechanism for understanding the role of CH2 reactions in catalysis.

ASSOCIATED CONTENT Supporting Information Videos of two trajectories shown in Figures 3a and 5a. Bond distances, charges, and spin density for representative trajectories. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work is supported by the Air Force Office of Scientific Research through Basic Research Initiative award no. FA9550-14-1-0053 and by the Northwestern University Institute for Catalysis in Energy Processes (ICEP), as funded through the US Department of Energy, Office of Basic Energy Sciences (Award Number DE-FG02-03-ER15457).

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