Mechanisms of Mo2C(101)-Catalyzed Furfural Selective

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Mechanisms of Mo2C(101)-Catalyzed Furfural Selective Hydrodeoxygenation to 2-Methylfuran from Computation Yun Shi, Yong Yang, Yongwang Li, and Haijun Jiao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02000 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 6, 2016

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ACS Catalysis Mechanisms of Mo2C(101)-Catalyzed Furfural Selective Hydrodeoxygenation to 2-Methylfuran from Computation a,b,c

Yun Shi,

a,b

Yong Yang,

a,b

Yong-Wang Li,

a,d

and Haijun Jiao *

a) State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China; b) National Energy Center for Coal to Liquids, Synfuels China Co., Ltd, Huairou District, Beijing, 101400, China; c) University of Chinese Academy of Sciences, No.19A Yuquan Road, Beijing, 100049, PR China; d) Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein Strasse 29a, 18059 Rostock, Germany. E-mail: [email protected]

Abstract: The selective formation of 2-methylfuran (F-CH3) and furan from furfural (F-CHO) hydrogenation and hydrodeoxygenation on the clean and 4H pre-covered Mo2C(101) surfaces has been systematically computed on the basis of periodic density functional theory including dispersion correction (PBE-D3). The clean Mo2C(101) surface has two distinct surface sites; unsaturated C and Mo sites for the adsorption of H and furfural, respectively. The selectivity comes from the different preference of furfural hydrogenation and dissociation (F-CHO+H = F-CH2O vs. F-CHO = F-CO+H) under the variation of H2 partial pressure. On the basis of the computed minimum energy path on the clean surface, micro-kinetics shows that high H2 partial pressure can promote 2-methylfuran formation and suppress furan formation. To verify this proposed selectivity trend of 2-methylfuran at high H2 partial pressure, the 4H pre-covered Mo2C(101) surface (0.25 monolayer hydrogen coverage), which provides neighboring hydrogen for promoting furfural hydrogenation and blocks the active sites for suppressing furfural dissociation, has been used. The computed results are in full agreement with the experimentally observed selective formation of 2-methylfuran and the half H2 reaction order as well as rationalize the need of high H2/furfural ratio (400 to 1). On the basis of these results, a new two-step protocol for experiment is proposed; i.e.; the first step is the pre-treatment of the catalyst with hydrogen and the second step is furfural hydrogenation on H pre-covered catalysts.

Keywords: Furfural, Furfuryl alcohol, 2-Methylfuran, Furan, Mo2C, Micro-kinetics, Selectivity, DFT

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1. Introduction Facing the increasing problems and challenges of climate change, population growth and depletion of fossil fuels, effective usage of renewable biomass as alternative sources of chemical and fuels has attracted considerable attentions in sciences and technologies.

1-4

One of the interesting biomass materials is furfural, which can be converted via hydrodeoxygenation and hydro-

genation into many useful products such as furfuryl alcohol, 2-methylfuran, furan as well as tetrahydrofurfuryl alcohol, 2-methyl5-7

tetrahydrofuran and tetrahydrofuran.

As 2-methylfuran has been considered as fuel additives, selective furfural transforma-

tion into 2-methylfuran becomes the subject of intensive scientific studies. Experimentally, mainly Cu-based

8-22

and Ru-based

22-26

catalysts can convert furfural selectively to 2-methylfuran via hydro-

deoxygenation. Recently, molybdenum carbide (Mo2C), which is noble metal-like in heterogeneous catalysis, in the selective conversion of furfural to 2-methylfuran.

32-35

27-31

has been used

At 423K and atmospheric pressure by using Mo2C catalyst, Xiong et

35

al., studied furfural conversion and found 2-methylfuran as the dominant product with selectivity at around 60%. In addition, 32

Lee et al., studied vapor phase hydrodeoxygenation of furfural over Mo2C catalyst at low temperature (423 K) and ambient pressure and found high 2-methylfuran selectivity (50-60%) and low furan selectivity (< 1%). Very recently, Grazia et al.,

36

re37

ported that furfural can be converted to 2-methylfuran by using MgO and Mg/Fe/O catalysts. In addition, Aldosari et al.,

studied Pd–Ru/TiO2 catalyst for furfural hydrogenation and found that addition of a small amount of Ru (1 wt%) can shift the selectivity towards furfuryl alcohol and 2-methylfuran, however, further increase of the Ru ratio decreased the catalytic activity 38,39

and also gave very poor selectivity of 2-methylfuran. Apart from these catalysts discussed above, there are Ni-based, Pt-based

40-44

and Pd-based

41,45

catalysts used in furfural conversion to 2-methylfuran, but the selectivity is rather low. 21

There are very few systematic theory studies about the mechanisms of furfural conversion to 2-methylfuran. Sitthisa et al.,

only compared two possible mechanisms of furfural hydrogenation based on the order of the first H atom attack to the carbonyl group resulting in two possible surface intermediates adsorbed on the Cu surface. Recently we reported the detailed reaction mechanisms of furfural selective hydrodeoxygenation on the Cu(111) surface,

46

and our results explained perfectly the experim-

entally observed selective formation of furfuryl alcohol and the equilibrium of furfural/furfuryl alcohol under H-rich conditions as well as the role of H2O in suppressing furfural conversion. In this work, we used the dispersion corrected PBE-D3 method to explore the mechanisms of furfural selective conversion to 2-methylfuran on the clean and 4H pre-covered Mo2C(101) surfaces. On the basis of the computed thermodynamic and kinetic results, we identified the minimum reaction paths and set up the rate equation from micro-kinetics. In full agreement with the experimental findings, our results provide insights into Mo2C catalyzed furfural selective conversion at high H2 partial pressure and broaden our fundamental understanding into selective hydrodeoxygenation reactions of biomass-derived oxygenates. 2. Computational method and model 2.1. Method: All calculations were performed by using the plane-wave based periodic DFT method implemented in Vienna ab 47,48

initio simulation package (VASP, version 5.3.5), method.

49,50

where the ionic cores are described by the projector augmented wave (PAW)

The exchange and correlation energies are computed using the Perdew, Burke and Ernzerhof functional along with 51-53

the latest dispersion correction for counting van de Waals interaction (PBE-D3).

To have accurate energies with errors less

than 1 meV per atom, a cutoff energy of 400 eV and the Gaussian electron smearing method with σ = 0.10 eV are used. Geometry optimization is converged until forces acting on atoms are lower than 0.02 eV/Å, whereas the energy threshold-defining –4

self-consistency of the electron density is set to 10 eV. All transition state structures are located using the climbing image nudged elastic band (CI-NEB) method.

54

For each optimized stationary point, we carried out vibrational analysis at the same level of

theory to determine its nature as minimum or saddle point and ensured that each transition state has only one imaginary frequ55

ency along the reaction coordinates. In our previous study,

it is found that zero-point energy (ZPE) correction has negligible

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effects on the computed kinetic and thermodynamic data, and is not included in this work. For the bulk optimization, the lattice parameters of the hexagonal Mo2C phase are determined by minimizing the total energy of the unit cell by using a conjugated gradient algorithm to relax the ions and a 5×5×5 Monkhorst-Pack k-point grid

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is used for sampling the Brillouin zone. 57

2.2. Model: Mo2C mainly has two crystalline structures, orthorhombic Mo2C phase

work, we used the hexagonal Mo2C phase with an eclipsed configuration as the unit cell.

and hexagonal Mo2C phase. 55,60-62

58,59

In this

The calculated lattice parameters

of the unit cell are 2a = 6.079 Å, 2b = 6.073 Å and c = 4.722 Å, in good agreement with the experimental values (a = b = 3.002 Å, c 63

= 4.724 Å).

60,61,64-66

Among all hexagonal Mo2C surfaces, the (101) termination with Mo/C = 1/1 surface ratio is most stable.

To

execute our study, we used the Mo2C(101) surface, which was modeled by periodic slabs with p(2×2) surface size, and the top two layers along with adsorbates were relaxed and the bottom two layers were fixed in the bulk positions.

55

There are two types of C atoms and two types of Mo atoms exposed on the clean Mo2C(101) surface (Figure 1). To describe this surface atoms easily, the 4-coordinated (two surface Mo atoms and two bulky phase Mo atoms) C atom is marked as CA, and the 5-coordinated (four surface Mo atoms and one bulky phase Mo atom) C atom is denoted as CB. The 10-coordinated (three surface Mo atoms, three surface C atoms and four bulky phase Mo atoms) and 11-coordinated (three surface Mo atoms, three surface C atoms and five bulky phase Mo atoms) surface Mo atoms are notated as MoA and MoB. The vacuum layer between the periodically repeated slabs was set as 15 Å to avoid significant interactions between slabs. The total super cell has a Mo64C32 unit within a volume of 15.39 Å × 12.14 Å × 20.94 Å, and the exposed surface has 16 Mo and 16 C atoms. Dipole correction was used to decouple the slabs and to avoid the effect of dipolar interaction on the total energy. The surface structural relaxation and the 55

total energy calculation were performed with the 3×3×1 Monkhorst-Pack k-point sampling. For the relevant gas phase species, we used a cubic box with side length of 15 Å to calculate the structures and total energies. (Figure 1) The adsorption energy (Eads) was calculated according to Eads = EX/slab – Eslab – EX, where EX/slab is the total energy of the slab with adsorbates in its equilibrium geometry, Eslab is the total energy of the slab and EX is the total energy of the free adsorbates in gas phase. Therefore, the more negative the Eads, the stronger the adsorption. The energy barrier (Ea) and reaction energy (Er) were calculated according to Ea = ETS – EIS and Er = EFS – EIS, where EIS, ETS, and EFS are the total energies of the initial state (IS), transition state (TS) and the final state (FS), respectively. 3. Results and Discussion 3.1. Adsorption of furfural, furfuryl alcohol, 2-methylfuran and furan Furfural (F-CHO) has trans and cis configurations, and the trans one is slightly more stable than the cis one by 0.02 eV, in 67

agreement with the previous theoretical result

68

and experimental findings from IR and Raman spectroscopy studies.

We cal-

culated the adsorptions of cis- and trans-furfural as well as furfuryl alcohol (F-CH2OH), 2-methylfuran (F-CH3) and furan (F-H) on the clean Mo2C(101) surface. The adsorption configurations and their structural properties are shown in Figure 2 and Table 1, respectively. (Figure 2 and Table 1) For furfural (F-CHO) adsorption, both furan ring and CHO group interact with the surface MoA atoms through the O7, C6, C2 and C3 atoms; and the furan ring is tilted away from the surface, as indicated by the MoA-O7-C6-C2 dihedral angle of 118.0° and 2

119.7°, respectively, for the cis- and trans-F-CHO. As shown in Figure 2, it is easily to see that F-CHO has a ƞ (C=O) surface bonding mode, since the C=O distances are elongated compared with the gas phase values (1.354 and 1.354 vs. 1.228 and 1.230 Å). The computed adsorption energies of cis- and trans-F-CHO are -2.68 and -2.50 eV, much larger than those (-0.88 and -0.91 eV) 46

1

on the Cu(111) surface, where F-CHO has a ƞ (O) surface bonding mode. In addition, we computed high coverage adsorption of cis-F-CHO. Due to the limited size and the repulsive interaction along the row of the MoA sites, only the adsorption of one ~3~


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furfural molecule in flat configuration is possible, while the adsorption of two furfural molecules in perpendicular configuration is possible (Figure S1); and the stepwise adsorption energy of the first and second furfural molecules is -1.58 and -1.53 eV, respectively, much lower than that of the flat adsorption configuration (-2.68 eV). Since the ratio of H2/F-CHO is up to 400/1 in experiment, only the flat furfural adsorption is used in our study for furfural conversion. Furthermore, we computed the adsorptions of furfuryl alcohol (F-CH2OH), 2-methylfuran (F-CH3) and furan (F-H). For F-CH2OH, the most stable adsorption has the oxygen atom of the OH group at the MoA site with the MoA-O distance of 2.368 Å; as well as the C2, C3 and C4 atoms of the furan ring in contact with the surface. The computed adsorption energy is -2.23 eV. For F-CH3, the molecule interacts with the surface mainly via the C3 and C5 atoms of the furan ring, and the computed adsorption energy is -1.82 eV. Furan has nearly the same adsorption configuration as F-CH3 and the computed adsorption energy is -1.58 eV. As shown in Table 1, the optimized MoA-O and MoA-C distances from PBE and PBE-D3 are very close; but the PBE-D3 adsorption energies are much stronger than those of PBE, indicating the effect of dispersion correction. However, there are no experimental data available for direct comparison and validation. Nevertheless, both PBE and PBE-D3 show that F-CHO has the strongest adsorption, followed by F-CH2OH and F-CH3, while that of furan is the weakest. 3.2. Mechanisms of furfural conversion On the basis of the most stable adsorbed cis-F-CHO in the tilted configuration, we computed the full potential energy surface of furfural selective conversion; where the energy barrier and reaction energy are computed on the basis of the stable species without direct co-adsorption interaction. In our previous work,

69

H2 dissociative adsorption on the Mo2C(101) surface was com-

puted (Figure S2). Starting from the molecularly adsorbed H2 on the MoA site; the homolytic dissociative adsorption (both H atoms bridging MoA atoms) has barrier of 0.31 eV and is exothermic by 0.39 eV; and the heterolytic dissociative adsorption (one H atom bridging MoA atoms and one H atom at the CA site) has a barrier of 0.25 eV and is exothermic by 0.57 eV. With respect to the molecularly adsorbed H2 on the MoA site, the barrier of H2 homolytic dissociative adsorption over two CA sites is 1.18 eV, much higher than that on the MoA site. Since H2 dissociative adsorption at the CA sites is thermodynamically much more favored than that at the MoA sites, we computed H diffusion from one MoA site to one CA site; which has a barrier of 0.46 eV and is exothermic by 0.23 eV. All these show the kinetic and thermodynamic preference of the CA site over the MoA site for H adsorption. Since F-CHO has a much stronger adsorption energy on the surface MoA (-2.68 eV) than H2 dissociative adsorption on the surface CA sites, we used the co-adsorption of F-CHO on the MoA site and H on the CA site for our reactions. The selected structural parameters of the surface intermediates are given in Supporting Information (Table S2). (a) –CH=O hydrogenation vs. dissociation: For the first reaction step; we computed the selective –CH=O hydrogenation; (i) H addition to the C atom of C=O [R1, F-CHO+H = F-CH2O] and (ii) H addition to the O atom of C=O [R2, F-CHO+H = F-CHOH]. In addition, we computed the parallel and competitive –CH=O dissociation reactions, (iii) C-H dissociation [R3, F-CHO+H = F-CO+2H], (iv) C=O dissociation [R4, F-CHO+H = F-CH+O+H] and (v) -CHO dissociation [R5, F-CHO+H = F+CHO+H]. The optimized structures of the adsorbed intermediates are given in Figure 3. We plotted the potential energy surface on the basis of the consecutive comparison (Figure 4), i.e.; searching all possible paths for a given adsorbed species obtained from the previous step and continuing only the one with the lowest energy barrier while discarding those with obviously higher energy barriers. (Figures 3 and 4) The R1 path begins with H addition to the C atom of the C=O group, leading to furfuryl oxide [R1, F-CHO+H = F-CH2O]. In the co-adsorbed structure (IS1), the H atom is located at the CA site neighboring the C=O group with the CA-H distance of 1.124 Å. In the transition state (TS1), H atom moves to the neighboring bridge site and the forming C6-H distance is 1.669 Å. In F-CH2O (FS1), the adsorption of the C6 atom is away from the surface and the MoA-C6 distance is 3.117 Å. The energy barrier is 1.11 eV, and the reaction is slightly endothermic by 0.12 eV. In the R2 step [F-CHO+H = F-CHOH], the co-adsorbed H atom added to the O ~4~


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atom of the C=O group leads to the F-CHOH intermediate. In the transition state (TS2), the forming O7-H distance is 1.297 Å and the H atom goes to the MoA site with the MoA-H distance of 2.021 Å. The energy barrier is 1.29 eV, and the reaction is endothermic by 0.50 eV. This shows that F-CH2O formation is preferred kinetically (1.11 vs. 1.29) and thermodynamically (0.12 vs. 0.50 eV) than F-CHOH formation. The competitive reactions of –HC=O hydrogenation are C-H, C=O and CHO dissociations. For C-H dissociation [R3, F-CHO+H = F-CO+2H], the energy barrier is 0.97 eV and reaction energy is 0.07 eV. In the transition state (TS3), the breaking C6-H distance is 1.547 Å. For C=O dissociation [R4, F-CHO+H = F-CH+O+H], the energy barrier is 1.60 eV and the reaction energy is -0.16 eV. For CHO dissociation [R5, F-CHO+H = F+CHO+H], the energy barrier is 1.42 eV and the reaction energy is 0.16 eV. This clearly shows that the C-H dissociation has the lowest barrier and is most favored kinetically. Figure 4 shows that the hydrogenation of C6 atom leading to F-CH2O intermediate and the C-H dissociation into surface F-CO are parallel and competitive. They have close barriers (1.11 vs. 0.97 eV) and reaction energies (0.12 vs. 0.07 eV), and the other reaction pathways are much less favored kinetically and not competitive. Therefore, we used surface F-CH2O for the formation of furfuryl alcohol and 2-methylfuran as well as surface F-CO for furan formation. (b) Furfuryl alcohol and 2-methylfuran formation: On the basis of the most stable adsorbed F-CH2O, we computed the co-adsorption of one H atom as initial state (F-CH2O+H) for the parallel and competitive pathways: (i) hydrogenation of O atom leading to furfuryl alcohol [R6, F-CH2O+H = F-CH2OH] and (ii) C-O dissociation into furfuryl [R7, F-CH2O+H = F-CH2+O+H] as well as furfuryl hydrogenation to 2-methylfuran [R8, F-CH2+O+H = F-CH3+O]. For furfuryl alcohol (F-CH2OH) formation from the co-adsorbed F-CH2O+H (IS6) as the initial state, the H atom is at the CA site with the CA-H distance of 1.115 Å. In the transition state (TS6), the forming O7-H distance is 1.303 Å as well as the CA-H distance is 1.374 Å. The energy barrier is 0.91 eV and the reaction is endothermic by 0.46 eV; indicating that the back reaction is much more favorable kinetically (0.45 eV) and thermodynamically (-0.46 eV) under equilibrium condition. In the transition state (TS7) of the C-O dissociation into surface F-CH2+O [R7, F-CH2O+H = F-CH2+O+H], the breaking C-O distance is 1.915 Å. In the final state (FS7), the dissociated O atom shifts to the neighboring MoA site. The energy barrier is 0.63 eV, and the reaction is highly exothermic by 1.19 eV, and this is due to the very strong adsorption of the O atom.

55,69

Figure 4 shows that the C-O dissociation of F-CH2O into F-CH2+O is more favored than F-CH2O hydrogenation into furfuryl alcohol both kinetically (0.63 vs. 0.91 eV) and thermodynamically (-1.19 vs. 0.46 eV). Once F-CH2OH is formed, it can easily go back to F-CH2O under equilibrium condition due to the much low back energy barrier (0.45 eV). Therefore, the most favored surface species are the co-adsorbed F-CH2 and O, which are used for the formation of 2-methylfuran and H2O. For F-CH2 hydrogenation into 2-methylfuran [R8, F-CH2+O+H = F-CH3+O], we computed the co-adsorption of F-CH2+O+H, where the H atom is adsorbed at the CA site. The optimized structures of the adsorbed intermediates and the potential energy surface are given in Figures 5 and 6, respectively. In the transition state (TS8), the H atom moves to the neighboring bridge site and the forming C-H distance is 1.674 Å. The energy barrier is 0.95 eV and reaction energy is 0.17 eV. (Figures 5 and 6) The parallel route to R8 is OH formation [R9, F-CH2+O+H = F-CH2+OH]. In the transition state (TS9), the O atom moves to the bridge site, and the forming O-H distance is 1.339 Å as well as the CA-H distance is elongated from 1.129 to 1.376 Å. The energy barrier is 1.06 eV and the reaction energy is 0.18 eV. Compared with F-CH2 hydrogenation into F-CH3 (Figure 6), OH formation is only less favored kinetically by 0.11 eV and thermodynamically by 0.01 eV. These indicate that the formation of F-CH3 and OH should be parallel and competitive under H-rich condition. As shown in Supporting Information (Table S1 and Figure S3), starting from the co-adsorbed F-CH2+OH+H, F-CH3 formation is more favorable than H2O formation kinetically (0.85 vs. 1.14) and thermodynamically (0.23 vs. 0.88 eV). Therefore, H2O formation is not competitive with F-CH3 formation. ~5~


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Finally, we computed the formation of OH and H2O after F-CH3 formation [R10, F-CH3+O+H = F-CH3+OH; R11, F-CH3+OH+H = F-CH3+H2O] because of the very stronger adsorption energy of 2-methylfuran over that of H2O (-1.82 vs. -0.84 eV). On the basis of the most stable co-adsorbed F-CH3+O+H (IS10), we computed OH formation. In the transition state (TS10), the adsorption configurations of the O and H atoms are almost the same as those in TS9, and the forming O-H distance is 1.361 Å. The energy barrier is 0.98 eV and the reaction energy is 0.09 eV, nearly the same as those of R9. For H2O formation from the co-adsorbed F-CH3+OH+H, the energy barrier is 1.26 eV and the reaction energy is 0.99 eV. This indicates that H2O formation from OH+H is not favored kinetically and thermodynamically, and direct surface OH removal should be less likely. Therefore, we computed OH disproportionation (2OH = H2O+O) (Figure S4); the energy barrier is 0.81 eV and the reaction energy is 0.56 eV; much lower than 70,71

those of direct H2O formation. That OH disproportionation has a lower energy barrier than OH hydrogenation is found on Fe and Cu.

46

(c) Furan formation: We computed furan formation from surface F-CO+3H. The optimized structures and energy profile are also given in Figures 5 and 6, respectively. Starting from the co-adsorbed F-CO+3H as the initial state (IS12), we firstly computed the CO dissociation pathway into furanyl [R12, F-CO+3H = F+CO+3H], and the breaking C-C distance is 1.855 Å in the transition state (TS12). In the final state (FS12), the formed CO molecule is nearly vertical to the neighboring MoA site with the MoA-C distance of 2.013 Å. The energy barrier is 0.98 eV and the reaction is nearly thermal-neutral (0.01 eV). Secondly, furan formation from the co-adsorbed F+CO+3H [R13, F+CO+3H = F-H+CO+2H] has been calculated. In the transition state (TS13), the forming C-H distance is 1.494 Å. The energy barrier is 0.83 eV and the reaction is exothermic by 0.40 eV. (d) Potential energy surface: On the basis of the adsorption energies (Table 1), it is clearly to see that furfural prefers surface MoA atoms, and H atoms prefer surface CA atoms; and furfural has a stronger adsorption energy than H2 dissociative adsorption (-2.68 vs. -1.38 eV). In addition, the adsorption energies of furfuryl alcohol, 2-methylfuran and furan are also higher than that of H2; and therefore H2 dissociative adsorption at the CA sites does not affect furfural adsorption. This is consistent with the proposed two distinct sites required for vapor phase furfural hydrodeoxygenation on the basis of the observed H2 dissociative adsorption as well as the zero-reaction order of furfural.

32

On the potential energy surface, there are two competitive and parallel paths starting from F-CHO, one leading to the formation of 2-methylfuran via F-CH2O from H addition and one leading to the formation of furan via F-CO from C-H dissociation. It shows that F-CO formation is slightly more favored than F-CH2O formation kinetically (0.97 vs. 1.11 eV) and thermodynamically (0.07 vs. 0.12 eV); and therefore both steps should be competitive, but in favor of furan formation. To distinguish the selectivity of F-CHO hydrogenation and dissociation, we computed the rate constants (k) on the basis of the transition state theory.

72,73

For

surface reactions, the rate constant of each reaction is calculated according to equation (1), where kB is the Boltzmann constant, T is the reaction temperature, h is the Planck constant, Ea is the activation energy, qTS,vib and qIS,vib are the harmonic vibrational partition functions for the transition state and the initial state, i.e., qvib is calculated on the basis of equation (2), where νi is the vibrational frequency of each vibrational mode of the adsorbed intermediate derived from DFT calculations. The computed activation energies and reaction energies as well as rate constants are listed in Table 2. (Table 2) , 1 = ℎ , 2 =

1

ℎν 1 − exp "− #

On the basis of the computed rate constants, F-CHO dissociation into F-CO+H is 54 times faster than hydrogenation into 1

-1

F-CH2O (2.14×10 vs. 3.95×10 ); and this is not in agreement with the experimental findings under H-rich condition. Compared ~6~


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with F-CH2O dissociation into F-CH2, F-CH2OH formation is neither kinetically (0.63 vs. 0.91 eV) nor thermodynamically (-1.19 vs. 5

0.46 eV) favored. The rate constant of F-CH2O dissociation into F-CH2+O and hydrogenation into F-CH2OH is 2.42×10 and 2

3

1.05×10 , respectively, and F-CH2O dissociation is 2.30×10 times faster than hydrogenation. Despite the disagreement between the experiment and theory, the low activation energies and the near neutral reaction energies reveal their possible competition and equilibrium under reaction condition as well as provide the possibility for selectivity fine-tuning upon change of H2 pressure. For the preferred formation of F-CH2O from hydrogenation, for example, it is necessary to carry out the reaction at high H2 partial pressure. High H2 partial pressure can increase the concentration of surface H and shift the reaction towards F-CH2O formation; as well as block the H adsorption sites and suppress C-H dissociation of F-CHO to F-CO+H. The whole effect of high H2 partial pressure should promote F-CH2O formation and suppress F-CO formation. Indeed, 32

the experimental study used high H2 partial pressure with H2/F-CHO ratio up to 400. On the basis of this analysis, we performed the analysis of micro-kinetics to illustrate the effect of H2 partial pressure. (e) Micro-kinetics of the selective formation of 2-methylfuran and furan: On the basis of the minimum energy paths, we set up the kinetic equations for the selective formation of 2-methylfuran and furan; and the detailed information is given in the 32

Supporting Information. Indeed, Lee et al., built such equation for the formation of 2-methylfuran in very details. We applied the same procedure, but used different surface intermediates from the minimum energy paths. At first, H2 molecule adsorbs dissociatively on the CA (or S1) site (Eq. 1); and furfural molecule adsorbs on the MoA (or S2) site (Eq. 2); and both adsorption steps are considered independently. Next, the adsorbed H atom at the S1 site can attack the -CHO group of the furfural molecule to form surface F-CH2O-S2 intermediate (Eq. 3). Afterward, surface F-CH2O-S2 can dissociate into surface F-CH2-S2 (Eq. 4), which is further hydrogenated to F-CH3-S2 (Eq. 5), followed by 2-methylfuran desorption (Eq. 6). Finally the surface O-S2 is hydrogenated by H-S1 into OH-S2 (Eq. 7) and H2O-S2 (Eq. 8), followed by H2O desorption (Eq. 9) and the regeneration of the free surface sites. H2(g) + 2S1 = 2H-S1

Eq. 1

F-CHO(g) + S2 = F-CHO-S2

Eq. 2

F-CHO-S2 + H-S1 = F-CH2O-S2 + S1

Eq. 3

F-CH2O-S2 + S2 = F-CH2-S2 + O-S2

Eq. 4

F-CH2-S2 + H-S1 = F-CH3-S2 + S1

Eq. 5

F-CH3-S2 = F-CH3(g) + S2

Eq. 6

O-S2 + H-S1 = OH-S2 + S1

Eq. 7

OH-S2 + H-S1 = H2O-S2 + S1

Eq. 8

H2O-S2 = H2O(g) + S2

Eq. 9

Despite different intermediates and sequential surface reactions, we got the same rate equation for F-CH3 formation by considering reaction 3 with the highest energy barrier to be rate-determining and selectivity-controlling; i.e.; r(F-CH3) = 1/2

1/2

0

k3zK1 [L2][H2] [F-CHO] , where k3 is the rate constant of F-CHO-S2 hydrogenation (Eq. 3); K1 is the equilibrium constant of H2 adsorption and desorption; z is the probability of finding adjacent surface [F-CHO-S2] to surface [H-S1], and L2 represents the total surface number of the active S2 sites for F-CHO adsorption; and the reaction rate is half order of H2 and zero order of F-CHO. For furan competitive formation, the first step is F-CHO adsorption (Eq. 2); and the second step is F-CHO-S2 stepwise dissociation into F-CO-S2 (Eq. 10) with the highest energy barrier as the step of selectivity-controlling and rate-determining as well as F-S2 (Eq. 11); and the next step is F-S2 hydrogenation into furan-S2 (Eq. 12), followed by furan desorption (Eq. 13). The formed surface CO-S2 desorbs from the surface (Eq. 14). F-CHO(g) + S2 = F-CHO-S2

Eq. 2 ~7~


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F-CHO-S2 + S1 = F-CO-S2 + H-S1

Eq. 10

F-CO-S2 + S2 = F-S2 + CO-S2

Eq. 11

F-S2 + H-S1 = furan-S2 + S1

Eq. 12

furan-S2 = furan(g) + S2

Eq. 13

CO-S2 = CO(g) + S2

Eq. 14

Page 8 of 25

On the basis of the minimum energy path of these sequential surface reactions, we obtained the rate equation for furan formation (Supporting Information); r(furan) = k10[L2][L1], where k10 is the rate constant of F-CHO-S2 dissociation (Eq. 10); L2 represents the total surface number of the active S2 sites for F-CHO adsorption; and L1 represents the total surface number of the active S1 sites for H2 adsorption. As F-CHO has a stronger adsorption on the MoA sites and fully covers the MoA site, the reaction rate is independent on the F-CHO partial pressure. As H2 can only have dissociative adsorption at the MoA sites and the more strongly adsorbed F-CHO suppresses H2 dissociative adsorption, all active S1 sites are free of hydrogen ([H-S1] = 0) and L1 keeps constant. Therefore, the whole reaction depends only on the adsorbed F-CHO-S2 at the MoA sites and consequently, this will only lead to the formation of furan. Under high H2 pressure, however, all S1 sites become H-S1 and [H-S1] is equal to [L1]; and the rate equation becomes r(furan) = 1/2

k10[L2]K1 [S1][H2]

1/2

(Supporting Information). As [H-S1] = [L1]; the concentration of free S1 sites, [S1], is equal to zero; and the

reaction rate of furan formation also becomes zero. Therefore, high H2 partial pressure can suppress furan formation, while accelerate the formation of 2-methylfuran. This explains the experimentally used high input ratio of H2 to furfural (400 to 1) for the selective formation of F-CH3. Actually this relationship is easily understandable. At very low H2 partial pressure, for example, the more strongly adsorbed furfural will block the MoA site and suppress H2 dissociative adsorption; and the empty CA sites are available for accepting H atoms from furfural dissociation (F-CHO = F-CO+H). At high H2 partial pressure, however, all the CA sites are occupied by H atoms, and there are no free sites available for accepting the H atoms from F-CHO dissociation; and the H atoms adsorbed on the CA sites, neighboring to the adsorbed furfural on the MoA sites, can promote furfural hydrogenation (F-CHO+H = F-CH2O). On the basis of these results, it is easily to propose a two-step procedure for experiment; i.e.; the first step is the pre-treatment of the catalyst with H2, and the second step is to conduct furfural hydrogenation on the hydrogen pre-covered catalysts. (f) Minimum energy path on 4H pre-covered Mo2C(101) surface: In order to verify the micro-kinetics analysis, we computed the hydrogenation and dehydrogenation of F-CHO at high H2 partial pressure on the basis of 4H pre-covered Mo2C(101) surface. This is because that according to the micro-kinetics the pre-covered surface H atoms can accelerate furfural hydrogenation by providing adjacent H atoms and suppress furfural dissociation by blocking the surface sites. This surface model is rationalized on 56

the basis of our previous study of H2 adsorption up to saturation coverage. On the p(2x1) Mo2C(101) surface, the first 4H atoms prefer the CA sites (nH = 1-4; up to 0.25 ML) and have nearly the same stepwise adsorption energies (-0.53 ∼ -0.47 eV); indicating that all these 4H atoms are energetically and structurally equal. With the increase of H coverage, the next 4H atoms prefer the bridge sites of MoA atoms (nH = 5-8; up to 0.50 ML) and also have nearly the same stepwise adsorption energies (-0.37 ∼ -0.28 eV), but lower than those of the first 4H atoms on the CA sites. Furthermore, ab initio atomistic thermodynamics shows that the H surface coverage is between 0.25 and 0.50 ML in the temperature range of 400-600K and at the pressure range of 0.02-50 atmospheres. Since the reaction condition of furfural conversion is at 423K and 1 atmosphere and there is much more than enough H2 available as well as furfural adsorption prefers the MoA site rather than the CA site, the surface model with 0.25 ML corresponds to the 4H pre-covered surface model. Indeed, this surface model has been successively used in studying the conver55

sion of butyric acid into butane to model the conversion mechanism of fatty acids to long-chain alkanes. The optimized structures of the intermediates and potential energy surface are shown in Figures 7 and 8, respectively. The computed energy ~8~


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ACS Catalysis

barriers and reaction energy as well as rate constants are also listed in Table 2. (Figures 7 and 8) On the 4H pre-covered Mo2C(101) surface, the energy barrier of F-CHO dissociation into F-CO+H (R16) becomes 0.55 eV higher than that on the clean surface (1.52 vs. 0.97 eV), and the barrier of F-CHO hydrogenation leading to F-CH2O (R14) becomes only 0.10 eV higher than that on the clean surface (1.21 vs. 1.11 eV). This shows that on the clean surface, F-CHO dissociation into F-CO is more favored kinetically than F-CHO hydrogenation into F-CH2O (0.97 vs. 1.11 eV) and furan should be the main product. On the 4H pre-covered surface, however, F-CHO hydrogenation into F-CH2O becomes more favorable than F-CHO dissociation into F-CO both kinetically (1.21 vs. 1.52 eV) and thermodynamically (-0.07 vs. 1.05 eV), and 2-methylfuran selectivity should be improved significantly by considering 2-methylfuran as the only product. Compared with the hydrogenation of F-CHO with the formation of F-CH2O (Figure 8), H addition to the O atom of the C=O group leading to F-CHOH intermediate (R15) as well as the C=O (R17) and CHO (R18) dissociations are neither kinetically (1.21 vs. 1.35, 1.72 and 1.51 eV, respectively) nor thermodynamically (-0.07 vs. 0.34, 0.02 and 0.68 eV, respectively) favorable. This is in agreement with the results on the clean surface. On the 4H pre-covered surface, F-CH2O formation becomes from slightly endothermic (0.12 eV) to somewhat exothermic (-0.07 eV), while F-CO becomes much more endothermic (0.07 vs. 1.05 eV). These results show that high H2 partial pressure can switch the selectivity from F-CO to F-CH2O; and the main product of furfural hydrogenation should be 2-methylfuran rather than 3

furan under H-rich reaction condition. The rate constant of F-CHO hydrogenation becomes 3.37×10 higher than that of F-CHO -2

-6

dissociation (2.39×10 vs. 7.09×10 ), and this results in very high selectivity of 2-methylfuran (> 99%) and very low furan selectivity (< 1%). This agrees with the experimental result, and differs from that on the clean surface. We also computed the formation of furfuryl alcohol at high H2 partial pressure. On the clean surface, F-CH2O dissociation into F-CH2 is more favorable kinetically (0.63 vs. 0.91 eV) and thermodynamically (-1.19 vs. 0.46 eV) than F-CH2O hydrogenation into furfuryl alcohol; and furfuryl alcohol formation is less likely. Starting from the co-adsorbed of F-CH2O+H on the 4H pre-covered surface (Figure 8), it is found that F-CH2O dissociation into F-CH2 (R20) is more favored than F-CH2O hydrogenation into F-CH2OH (R19) both kinetically (0.71 vs. 1.04 eV) and thermodynamically (-0.49 vs. 0.28 eV). It shows that high H2 partial pressure not only increases the barriers but also makes the reaction less exothermic or less endothermic. At high H2 partial pressure, the rate con3

4

0

stant of F-CH2O dissociation is 9.42×10 times higher than that of F-CH2O hydrogenation (3.08×10 vs. 3.27×10 ), and this is in line with the result on the clean surface and 2-methylfuran rather than furfuryl alcohol should be the principal product. Starting from the co-adsorbed F-CH2+O+H as the initial state, we computed the parallel formation of F-CH3 (R21) and OH (R22). The optimized structures of the intermediates and potential energy surface are shown in Figures 9 and 10, respectively. It is found that OH formation is 0.34 eV more favorable than F-CH3 formation kinetically (0.76 vs. 1.10 eV), but 0.28 eV less favorable thermodynamically (-0.47 vs. -0.75 eV). On the basis of the most stable co-adsorption configuration of F-CH2+OH+H, we computed the parallel formation of F-CH3 (R23) and H2O (R24). It is found that F-CH3 formation is more favorable than H2O formation kinetically (0.98 vs. 1.27 eV) and thermodynamically (-0.37 vs. 0.45 eV). In addition, we computed H2O stepwise formation on the basis of the co-adsorbed F-CH3+O+2H. The energy barrier and reaction energy of OH formation (R25) are 1.10 and 0.02 eV, respectively; and H2O formation (R26) has a barrier of 1.10 eV and is endothermic by 0.61 eV. (Figures 9 and 10) Finally we computed OH and H2O formation on the 4H pre-covered surface. The optimized structures of the intermediates are shown in the Supporting Information (Figure S5). The energy barrier of OH and H2O formation is 0.95 and 1.22 eV, respectively, 69

nearly the same as those on the clean surface (0.99 and 1.37 eV). The reaction energy is -0.16 and 0.59 eV, respectively. Due to the high barrier of OH hydrogenation leading to H2O, we computed the OH disproportionation on the 4H pre-covered surface (Figure S6). It is found that both the energy barrier and the reaction energy become lower (0.42 and 0.23 eV, respectively), ~9~


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indicating that at high H2 partial pressure, H2O formation from the OH disproportionation is easier and more preferred than that on the clean surface (0.81 and 0.56 eV). Therefore, it is concluded that OH disproportionation results in surface OH removal. Since OH disproportionation always forms one surface O, surface O content can be kept at a minimum level. Along with the minimum energy path starting from F-CHO on the 4H pre-covered Mo2C(101) surface, the first step is F-CHO hydrogenation into F-CH2O rather than dissociation into F-CO, and the second step is F-CH2O dissociation into F-CH2 instead of hydrogenation into F-CH2OH; and the last step is F-CH3 formation from F-CH2 hydrogenation. The first step has the highest barrier of 1.21 eV, followed by the last step (0.98 eV) and OH formation (0.76 eV). The removal of surface OH via disproportionation has the lowest barrier of 0.42 eV. Therefore, the first barrier should be rate-determining and selectivity controlling. It is interesting to compare the performances of the Cu(111) and Mo2C(101) catalyzed furfural hydrodeoxygenation. On 46

1

Cu(111), F-CHO has a weak ƞ (O) surface bonding mode, and only the O7 atom is in contact with the Cu(111) surface. On the Mo2C(101) surface, not only the –CH=O group but also the furan ring interact with the surface MoA atoms. The adsorption energies of cis- and trans-F-CHO on the Cu(111) surface (-0.88 and -0.91 eV) are much lower than those on the Mo2C(101) surface (-2.68 and -2.50 eV). On Cu(111), furfuryl oxide (F-CH2O) formation is preferred, in line with that on Mo2C(101). Differences have been found for the subsequent reactions. On Cu(111), F-CHO and F-CH2OH can form equilibrium; and F-CH2O hydrogenation into F-CH2OH and F-CH2O dissociation into F-CH2 are competitive kinetically, while F-CH2O dissociation on Mo2C(101) is more favored kinetically and thermodynamically than F-CH2OH formation. The largest difference is found between F-CHO hydrogenation and dissociation. On Cu(111), F-CHO hydrogenation into F-CH2O is much more favored kinetically and thermodynamically than F-CHO dissociation into F-CO, while on the clean Mo2C(101) surface, F-CHO dehydrogenation is slightly more favored kinetically and thermodynamically. Under high H2 partial pressure on Mo2C(101), F-CHO hydrogenation into F-CH2O becomes more favored kinetically and thermodynamically than F-CHO dissociation into F-CO. On both catalysts, high H2 partial pressure plays the decisive role, but H2 partial pressure is different as indicated by the experimental H2/furfural ratio, about 25 to 1 on Cu(111), while about 400 to 1 on Mo2C(101). This difference is indeed associated with their different reaction mechanisms, particularly in the first step of the reaction, i.e.; furfural dissociation vs. furfural hydrogenation. 4. Conclusion In this work, we investigated the potential energy surfaces of furfural conversion into 2-methylfuran and furan on the clean and 4H pre-covered Mo2C(101) surfaces on the basis of periodic density functional theory computation with the latest correction of long-range dispersion interaction (PBE-D3). Our goal is the reaction mechanism in explaining the experimentally observed selectivity of 2-methylfuran over furan as well as the decisive role of H2 partial pressure in furfural selective conversion. Our results provide the insights into the selective 2-methylfuran formation as well as the possibility of fine-tuning of selectivity. All these will broaden our fundamental understanding into the deoxygenation reactions of oxygenates from biomass materials. Comparison between Cu(111) and Mo2C(101) in furfural conversion shows their intrinsic differences in selectivity, and in both cases, H2 partial pressure plays the decisive role in 2-methylfuran selectivity. The adsorptions of furfural, furfuryl alcohol, 2-methylfuran and furan prefer the surface unsaturated MoA sites, while H2 dissociative adsorption prefers the surface unsaturated CA sites. This specific surface preference is supported by the proposed two distinct sites required for the vapor phase furfural hydrodeoxygenation on the basis of the observed H2 dissociative adsorption as well as the zero-reaction order of furfural. For furfural selective conversion, there are two parallel and competitive reaction paths, one starting from furfural (F-CHO) hydrogenation into furfuryl oxide (F-CHO+H = F-CH2O), and furfuryl oxide dissociation into furfuryl (F-CH2O = F-CH2+O) is more favorable kinetically and thermodynamically than furfuryl alcohol formation via hydrogenation (F-CH2O+H = F-CH2OH) and the ~ 10 ~


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final product is 2-methylfuran (F-CH2+H = F-CH3). Alternatively, furfural can dissociate into F-CO leading to furan formation. On the clean surface, furfural dissociation into F-CO is fairly more favored kinetically and thermodynamically than furfural hydrogenation into F-CH2O. On the basis of the estimated rate constants, furan rather than 2-methylfuran should be the product. However, the comparable barriers and reaction energies of furfural dissociation and hydrogenation provide the basis for their competition as well as selectivity fine-tuning. On the basis of the minimum energy paths, micro-kinetics for the selective and competitive formation of 2-methylfuran and 1/2

1/2

0

furan was established. For 2-methylfuran formation, the rate equation, r(F-CH3) = k3zK1 [L2][H2] [F-CHO] , depends only on H2 partial pressure, i.e.; half order of H2 and zero order of furfural. For furan formation, the rate equation, r(furan) = k10[L2][L1], only depends on the total number of S1 site; and the concentration of surface hydrogen, [H-S1] = 0. Under high H2 pressure, all S1 sites 1/2

1/2

can become H-S1 and [H-S1] = [L1]; and the rate equation becomes r(furan) = k10[L2]K1 [S1][H2] . As [H-S1] = [L1] and [S1] = 0, the reaction rate of furan formation also becomes zero. All these are easily understandable on the basis of the availability of surface H atoms; i.e.; high H2 partial pressure associates with the high concentration of surface H atoms, which can promote 2-methylfuran formation and suppress furan formation, while low H2 partial pressure corresponds to high concentration of surface free sites, which are available for accepting H atoms from furfural dissociation and accelerate furan formation. On the basis of the distinct sites of H2 dissociative adsorption on the surface CA site and furfural on the surface MoA site, we used the 4H pre-covered Mo2C(101) surface to model the effect of high H2 partial pressure. On the 4H pre-covered Mo2C(101) surface, furfural hydrogenation into F-CH2O becomes more favored kinetically and thermodynamically than furfural dissociation into F-CO+H; and this preference results in the formation of 2-methylfuran instead of furan. It shows clearly that high H2 partial pressure can switch the selectivity of furfural conversion from furan on the clean surface into 2-methylfuran on the 4H pre-covered surface. It is found that H2O formation from OH hydrogenation has a higher barrier than OH formation from surface O, formed from F-CH2O dissociation. Instead of the direct H2O formation from surface OH hydrogenation, surface OH disproportionation has a much lower barrier. Therefore, surface O removal should take place via OH disproportionation and the coverage of surface O can be kept at a minimum level. Most importantly, surface O and H2O formation do not affect the reaction mechanisms and the product selectivity. Furthermore, high H2 partial pressure can lower the barrier of H2O formation. Our results are in full agreement with the experimental observation: (a) The Mo2C(101) surface has two distinct surface sites, unsaturated CA atoms for H adsorption and unsaturated MoA atoms for furfural adsorption; (b) the much higher adsorption energy of F-CHO at the MoA site than those of H at the MoA and CA sites reveals that the surface will be mainly covered by F-CHO ([L2] = [F-CHO-S2]), the reaction rate is independent on F-CHO partial pressure; (c) H2 dissociative adsorption can only take place at the MoA sites; the more strongly adsorbed F-CHO will suppress H2 dissociative adsorption and high H2 partial pressure is needed to promote F-CHO-S2 hydrogenation; (d) the micro-kinetics explains the half reaction order of H2 and the need of high input ratio of H2 to furfural. Finally, a two-step protocol for experiment is proposed; i.e.; the first step is the pre-treatment of the catalyst with H2 and the second step is to conduct furfural hydrogenation on the hydrogen pre-covered catalysts.

Supporting information available: Micro-kinetics; Energy barrier Ea (eV) and reaction energy Er (eV) of furfural hydrodeoxygenation on the clean as well as 4H pre-covered Mo2C(101) surfaces (Table S1); Bond distances (d, Å) of the IS, TS and FS for furfural hydrodeoxygenation on the clean as well as 4H pre-covered Mo2C(101) surfaces (Table S2); Top and side views of perpendicular adsorption configurations of cis-F-CHO as well as the adsorption energy (eV) on the clean Mo2C(101) surface (Figure S1); Potential energy surface of H2 dissociative adsorption on the clean Mo2C(101) surface (Figure S2); The optimized geometries for the reaction routes R27-R28 on the clean Mo2C(101) surface (Figure S3); Top and side views of the optimized geometries for the ~ 11 ~


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OH disproportionation reaction on the clean Mo2C(101) surface (Figure S4); Side views of the optimized geometries for OH and H2O formation on 4H pre-covered Mo2C(101) surface (Figure S5); Side views of the optimized geometries for OH disproportionation reaction on 4H pre-covered Mo2C(101) surface (Figure S6). This material is available free of charge via the internet at http://pubs.acs.org.

Acknowledgments This work was supported by the Chinese Academy of Science and Synfuels CHINA. Co., Ltd. We also acknowledge the general support from the Federal Ministry of Education and Research Bundesministerium für Bildung und Forschung (BMBF) and the state of Mecklenburg-Vorpommern, Germany.

~ 12 ~


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Figure 1: Side views of the clean Mo2C(101) surface structure (Mo/blue, C/grey)

Figure 2：：Top and side views of the adsorption configurations of surface intermediates as well as the adsorption energy (eV) on the clean Mo2C(101) surface (Mo/blue, bulk-C/grey, adsorbed-C/black, H/yellow, O/red; F = 2-furanyl)

~ 13 ~


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Figure 3: Top and side views of the optimized geometries for the reaction routes R1-R7 on the clean Mo2C(101) surface (Mo/blue, bulk-C/grey, adsorbed-C/black, H/yellow, O/red; F = 2-furanyl)

~ 14 ~


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Figure 4: Potential energy surfaces (in eV) for the reaction routes R1-R7 on the clean Mo2C(101) surface (F = 2-furanyl)

~ 15 ~


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Figure 5: Top and side views of the optimized geometries for the reaction routes R8-R13 on the clean Mo2C(101) surface (Mo/blue, bulk-C/grey, adsorbed-C/black, H/yellow, O/red; F = 2-furanyl)

~ 16 ~


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Figure 6: Potential energy surfaces (in eV) for the reaction routes R8-R13 on the clean Mo2C(101) surface (F = 2-furanyl)

~ 17 ~


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Figure 7: Side views of the optimized geometries for the reaction routes R14-R20 on the 4H pre-covered Mo2C(101) surface (Mo/blue, bulk-C/grey, adsorbed-C/black, H/yellow, O/red; F = 2-furanyl)

~ 18 ~


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Figure 8: Potential energy surfaces (in eV) for the reaction routes R14-R20 on the 4H pre-covered Mo2C(101) surface (F = 2-furanyl)

~ 19 ~


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Figure 9: Side views of the optimized geometries for the reaction routes R21-R26 on the 4H pre-covered Mo2C(101) surface (Mo/blue, bulk-C/grey, adsorbed-C/black, H/yellow, O/red; F = 2-furanyl)

~ 20 ~


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Figure 10: Potential energy surfaces (in eV) for the reaction routes R21-R26 on the 4H pre-covered Mo2C(101) surface (F = 2-furanyl)

~ 21 ~


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Table 1. Adsorption energies Eads (eV) and bond distances (Å) of surface intermediates on the clean Mo2C(101) surface (Values of

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PBE in parentheses) configuration

Eads

cis-F-CHO

dMoA-H

dCA-H

dMoA-O

dMoA-C

-2.68 (-1.78)

2.044 (2.023)

2.462,2.467,2.277,2.467 (2.406,2.535,2.371,2.555)

trans-F-CHO

-2.50 (-1.57)

2.050 (2.020)

2.445,2.344,2.453 (2.354,2.428,2.639)

F-CH2OH

-2.23 (-1.27)

2.368 (2.395)

2.211,2.450,2.425 (2.235,2.465,2.464)

F-CH3

-1.82 (-0.97)

2.347,2.241 (2.382,2.255)

F-H

-1.58 (-0.88)

2.265,2.352 (2.523,2.525)

H2O

-0.84 (-0.66)

2.320 (2.344)

HO+H

-1.84 (-1.51)

1.121 (1.122)

2.153,2.151 (2.155,2.153)

O+2H

-1.94 (-1.62)

1.118 (1.119)

1.721 (1.721)

H2

-0.53 (-0.39)

1.968,1.967 (1.978,1.980)

H+H(MoA)

-0.92 (-0.75)

1.925,1.903 (1.936,1.909)

H+H(CA)

-1.38 (-1.20)

1.117 (1.118)

Table 2. Energy barrier Ea (eV) and reaction energy Er (eV) as well as the rate constant k (423K) of furfural hydrodeoxygenation on the clean as well as 4H pre-covered Mo2C(101) surfaces Clean Mo2C(101) surface Reaction F-CHO+H = F-CH2O

Ea 1.11

Er 0.12

4H pre-covered Mo2C(101) surface

K

Ea

Er

k

-1

1.21

-0.07

2.39×10-2

-3

1.35

0.34

4.84×10

3.95×10

-4

F-CHO+H = F-CHOH

1.29

0.50

2.72×10

F-CHO+H = F-CO+2H

0.97

0.07

2.14×10

1

1.52

1.05

7.09×10

F-CHO+H = F-CH+O+H

1.60

-0.16

6.60×10-7

1.72

0.02

2.75×10-8

-4

-6

F-CHO+H = F+CHO+H

1.42

0.16

1.12×10

1.51

0.68

1.14×10-5

F-CH2O+H = F-CH2OH

0.91

0.46

1.05×102

1.04

0.28

3.27×100

F-CH2O+H = F-CH2+O+H

0.63

-1.19

2.42×10

5

0.71

-0.49

3.08×10

1

1.10

-0.75

5.00×10

0.76

-0.47

4.27×103

F-CH2+O+H = F-CH3+O

0.95

0.17

3.34×10

F-CH2+O+H = F-CH2+OH

1.06

0.18

1.02×100

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TOC

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Mechanisms of Mo2C(101)-Catalyzed Furfural Selective

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