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Surface-termination dependence of propanoic acid deoxygenation on Mo2C
Catalysis Communications 99 (2017) 61–65
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Surface-termination dependence of propanoic acid deoxygenation on Mo2C a
b,c
MARK
d,⁎
Seok Ki Kim , Jaehoon Kim , Seung-Cheol Lee a
Carbon Resource Institute, Korea Research Institute of Chemical Technology, 141 Gajeongro, Yuseong, Daejeon 34114, Republic of Korea School of Mechanical Engineering, Sungkyunkwan University, 2066 Seobu-Ro, Jangan-Gu, Gyeonggi-Do 440-746, Republic of Korea Sungkyun Advanced Institute of Nano Technology (SAINT), 2066 Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 440-746, Republic of Korea d Electronic Materials Research Center, Korea Institute of Science and Technology, Hwaranno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Molybdenum carbide Structure sensitivity Hydrodeoxygenation Reaction pathway Fatty acid
Although Mo2C is used as an alternative catalyst material for biomass conversion, the dependence of its reaction on the type of surface termination has not been studied. In the present study, we performed density functional theory calculations for the deoxygenation of propanoic acid on the two types of surface terminations possible for orthorhombic Mo2C (100), namely, on Mo- and C-terminated surfaces. The reaction energetics of the three possible deoxygenation pathways, namely, those for hydrodeoxygenation, decarbonylation, and decarboxylation, were compared by calculating the activation energies for their key surface reactions, such as hydrogenation and the scission of CeC, CeO, and OeH bonds. The Mo-terminated surface was advantageous in the case of the hydrodeoxygenation pathway because of its low kinetic energy barrier for the hydrogenation of the oxygen species, while the C-terminated surface preferred the decarboxylation pathway because of its low kinetic energy barrier for OeH bond scission.
1. Introduction
In the present study, propanoic acid was chosen as a model acid compound instead of a typical longer-chain fatty acid. Further, we assumed that the deoxygenation energetics were mainly determined by the reactions between the catalyst surface and the oxygenate part of the reactant and not the alkyl part. The free energies of the deoxygenation of propanoic acid on Mo- and C-terminated Mo2C (100) surfaces were calculated using density functional theory (DFT) calculations, and the reaction mechanisms for the three different deoxygenation pathways were proposed. This study could explain the surface-termination-sensitive deoxygenation properties of Mo2C, and its results can be used to develop a strategy for designing Mo2C-based catalysts for the selective conversion of fatty acids.
The catalytic deoxygenation of triglycerides or fatty acids is one of the most commercially viable processes for producing renewable dieselrange hydrocarbons, which can be used as alternative transportation fuels [1–3]. This reaction primarily takes place via three pathways, hydrodeoxygenation, decarbonylation, and decarboxylation, either individually or in combination [3–6]. Although the amounts of oxocarbons discharged and hydrogen consumed during the process are largely dependent on the pathway, a selective catalyst that can drive the reaction towards a specific pathway has not yet been discovered. Transition metal carbides have received attention as potential catalytic materials that can be used as alternatives to noble metals in various reactions [7,8]. In particular, recent studies have shown that Mo2C is highly active with respect to the deoxygenation of triglycerides or fatty acids but is not selective towards a specific pathway [9–14]. In order to utilize Mo2C as a highly selective catalyst for this reaction, it is imperative to understand the fundamental reaction mechanism correlating the surface structure of the catalyst and its deoxygenation properties [15–17]. Recent theoretical studies focusing on the deoxygenation of butyric acid on the Mo2C (101) surface proposed a deoxygenation mechanism wherein CeO bond scission is favored more than CeC bond scission, with this resulting in selective hydrodeoxygenation [15,16].
⁎
Corresponding author. E-mail address: [email protected] (S.-C. Lee).
http://dx.doi.org/10.1016/j.catcom.2017.05.027 Received 16 February 2017; Received in revised form 10 May 2017; Accepted 28 May 2017 Available online 29 May 2017 1566-7367/ © 2017 Elsevier B.V. All rights reserved.
2. Methods The DFT calculations were performed using a plane-wave basis set implemented in the Vienna Ab initio Simulation Package (VASP) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional [18–20]. The k-points sampling of a 4 × 4 × 1 Monkhorst-Pack mesh grid was used for the simulations of the surface reactions. The transition states between the initial and final states of the components were calculated using the nudged elastic band (NEB) method [21,22]. The Gibbs free energies of the gas and adsorbed species (including species at the transition state) were calculated based on the ideal gas limit and
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atoms and their adjacent atoms participated. If gaseous hydrogen was involved in a reaction, its dissociative adsorption was calculated first and shown at the initial stage of the pathway to simplify the free-energy diagram. Most exergonic transformations of HDO on Mo- and C-terminated surfaces are shown in Fig. 1(a). As the reaction proceeded, surface species generated on both surfaces were found to be identical, but the energetics between each reaction coordinate appeared to be largely dependent on the surface termination. At the initial stage of the reaction, CeO bond scission of propanoic acid producing surface oxygen species (OH* and O*) and an alkyl group (RC*) (3 → 4 → 5) was exergonic on both surfaces. After formation of the surface oxygen and alkyl species, only endergonic hydrogenation of those species was left to take place (5 →…→ 11). The most energetically demanding step in the hydrogenation of the Mo-terminated surface was the formation of water (9 → 10 or 10 → 11) by 1.24 eV of activation energy, whereas that for the C-terminated surface was the formation of hydroxyl (8 → 9) by 1.52 eV of activation energy. Although the C-terminated surface appeared to weakly bind most of the adsorbates, showing a narrower energy window for HDO as compared to the Mo-terminated surface, the overall reaction kinetics were not necessarily faster on the C-terminated surface because of the higher activation energies of the most endergonic steps between both terminations. Previous experimental studies reported production of small amounts of aldehydes and alcohols as a result of partial HDO of fatty acids [10–14]. To compare the energetics on both terminations, additional transformations that branched out from the acyl group [RCO*, stage 4 in Fig. 1(a)] were calculated and the results are shown in Fig. 1(b). In the process of producing alcohol on the Mo-terminated surface, the formation of RCH2O* {(5) → (6)} and RCH2OH* {(6) → (7)} was calculated to be similarly difficult by 1.26 and 1.35 eV of activation energy, respectively. Because the parallel endergonic reactions producing RCH3 (5 →…→ 11) were more likely to occur, the intermediate path was not expected to be favourable during HDO. In the case of the Cterminated surface, the two most difficult transformations along the intermediate path were the formations of RCHO* {4 → (5)} and RCH2OH* {(6) → (7)} by 1.01 and 0.95 eV, respectively. Unlike the Mo-terminated surface, the intermediate path producing RCH2OH was energetically more favourable than the HDO path that included the difficult OH* formation. Accordingly, the C-terminated surface was considered to be responsible for the production of partially deoxygenated species. The free energies of desorption of RCH2OH {(7) → (8)} also showed easier desorption energetics on the C-terminated surface than on the Mo-terminated surface. The free-energy transformations for another deoxygenation pathway, DCN, are shown in Fig. 2. Analogous to HDO, initial CeO bond scissions producing RCO* and OH* were found to be favored (3′ → 4′) on both terminations. The subsequent CeC bond scission of RCO* (4′ → 5′) significantly stabilized the surface species, making the next steps more difficult. On the Mo-terminated surface, highly endergonic hydrogenation of surface alkyl (5′ → 6′) and hydroxyl (7′ → 8′) constrained the overall reaction by 1.26 and 1.24 eV of activation energy, respectively. On the C-terminated surface, however, the initial CeO bond scission (3′ → 4′) and hydrogenation of alkyl (5′ → 6′) required smaller activation energies by 0.58 and 0.88 eV, respectively, indicating that the C-terminated surface was energetically more beneficial for the DCN pathway. It is noteworthy that in the practical deoxygenation of triglycerides or fatty acids, their constituent alkyl chains are longer and more complex (14–20 carbon atoms with unsaturated bonds) than those of our model compound, propanoic acid. Because those diesel-range hydrocarbons are produced as a liquid phase, their free energies of formation (the last hydrogenation step of alkyl) are expected to be much lower than those of ethane and propane. In the present study as well, the hydrogenation of RCH2* was energetically easier than that of R* on both surfaces. Thus, energy comparisons on the basis of the
harmonic limit, respectively [23]. In ideal gas limit vibrational, translational, and rotational degree of freedom was taken into account while in harmonic limit only vibrational degree of freedom was taken into account. Since the PBE functional reportedly results in errors when describing the electronic energies of gas-phase molecules, we calibrated the DFT energies based on experimentally determined heat of reaction values. The van der Waals interactions were not included in these calculations. Additional details of the calculation methods are given in Supplementary data. Orthorhombic Mo2C (space group, Pbcn) was used as a model structure in the present study. The calculated lattice parameters were a = 4.74, b = 6.05, and c = 5.22 Å, which are close to the experimental values (a = 4.72, b = 6.01, c = 5.20) [24]. There has been confusion in the nomenclature of the Mo2C phase (α, β,…) used in the literature; [9,25–32] therefore, we have avoided the use of Greek letters to denote the Mo2C modelled in this study. The most closely packed surface of the orthorhombic Mo2C is the (100) surface, which is identical to the (0001) surface of hexagonal Mo2C [33]. The structures of Mo- and C-terminated orthorhombic Mo2C (100) surfaces are shown in Fig. S1 in Supplementary data. 3. Results and discussion The surface energies of the Mo- and C-terminated surfaces were calculated to be 2.52 and 2.67 J/m2, respectively, suggesting that the preferences for the surfaces are similar. Previous theoretical [9,30–32] and experimental [34–39] studies have reported that both terminations can be easily formed and can even be controlled by heat treatment. As shown in Fig. S1b and d, the C-terminated surface is not fully covered with carbon atoms because of the bulk stoichiometry; that is, only 1/2 monolayer (ML) of the surface molybdenum atoms are occupied by carbon atoms. Occupation of 1 ML of molybdenum atoms by carbon atoms increased the surface energy to 4.51 J/m2, indicating that additional carbon deposition on the stoichiometric C-terminated surface is energetically implausible. Oxygen atoms present in fatty acids can be removed via three different pathways: hydrodeoxygenation (HDO), decarbonylation (DCN), and decarboxylation (DCX) [3–6]. The net equations and free energies of each reaction for propanoic acid deoxygenation are summarized in Table 1. In the case of HDO, oxygen atoms are removed by forming water via two successive CeO bond scissions. Using HDO, one can avoid the loss of carbon atoms released as gas-phase molecules, but it requires a relatively large quantity of hydrogen. DCN or DCX involves CeC bond scission, resulting in liberation of a carbon atom by forming CO or CO2. The amount of hydrogen required for DCN or DCX is lower than that for HDO. The free energies of these three pathways on the Mo- and C-terminated surfaces were compared by searching for the most exergonic transformations, starting from propanoic acid adsorption, which was chosen as the most stable state (see Supplementary data) among all possible configurations. The oxygen ends of propanoic acid (eCOOH*) and of the intermediates (eCH2O*, eCHO*, eCOH*, and eCO*) favored adsorption on both surfaces, resulting in an alkyl end (–R) heading towards the vacuum side during the overall reaction process. Calculations were performed for surface reactions in which only oxygen Table 1 Pathways of propanoic acid deoxygenation. Pathway
Net equationa
ΔG/eVb
HDO DCN DCX
RCOOH + 3H2 → RCH3 + 2H2O RCOOH + H2 → RH + CO + H2O RCOOH → RH + CO2
− 1.77 − 1.74 − 1.94
a
R denotes CH3CH2. Free energies of reaction (ΔG) were calculated assuming reaction conditions of T = 623.15 K and P = 0.1 MPa. b
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Fig. 1. Free energies of (a) hydrodeoxygenation (HDO) of propanoic acid and (b) propanol formation on Mo- and C-terminated Mo2C (100). The activation energies at the rate-limiting steps are included. Free energies were calculated assuming reaction conditions of T = 623.15 K and P = 0.1 MPa. Atomic configurations of the rate-limiting reactions for the HDO and propanol formation are shown below. Color guide: Purple = Mo; Red = O; Brown = C; White = H. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the formation energies, were not considered in the present study. We avoided modeling high coverage of coadsorbate because it would have to investigate a very large number of cases to find global minimum atomic configurations for multiple adsorbates at every stage of the reaction. Furthermore, artificially-manipulated coadsorption environment may mislead energetics. However, it is intuitively clear that the energy required for bond scission will generally be higher than that for bond formation if the surface is highly occupied. For instance, hydrogenation of O* to form OH* will become easier if surface that surrounds O* is highly occupied by H*, while CeO bond scission will be facilitated on the surface that has more unoccupied sites. The overall reaction rates of HDO and DCX are mainly constrained by hydrogenation and CeO bond scission, respectively, regardless of the surface termination. Thus, while HDO is expected to be relatively dominant at high surface coverages or at high pressure of H2, while DCX is expected to be promoted in the early stage of the reaction when the surface is relatively cleaner than thereafter. This interpretation is in accordance with a previous experimental study [14], which showed changes in the deoxygenation pathway from DCX or DCN to HDO on Mo2C/C catalysts as oleic acid conversion increased.
hydrogenation of alkyl species' are not suitable. If the free energies of hydrogenation of the alkyl species were found to be the most demanding, their energies are denoted in parentheses in the free-energy diagrams. Fig. 3 shows the free-energy diagrams for the DCX pathways, which were much simpler than others because of the lack of hydrogen as a coreactant and CeO bond scission steps. On the C-terminated surface, CeC bond scission producing R* and COOH* (2″ → 3″, 0.38 eV of activation energy) was calculated to be the rate-limiting step right after the adsorption of propanoic acid, whereas OeH bond scission of COOH* producing gaseous CO2 and H* (3″ → 4″, 0.71 eV of activation energy) was the rate-limiting step on the Mo-terminated surface (except R* hydrogenation, 4″ → 5″, as noted earlier). These results indicate that the C-terminated surface was also energetically favourable for DCX relative to the Mo-terminated surface. As shown in the above energy diagrams, the Mo-terminated surface tended to strongly bind adsorbates that easily decomposed to small species by CeC or CeO bond scission. But a great deal of energy was required at the late stage of the reaction, such as during the hydrogenation of OH* to water. On the other hand, the C-terminated surface appeared to weaken the overall adsorption strength, resulting in easy removal of the adsorbates; however, it was difficult to decompose the adsorbates on the C-terminated surface as compared to the Mo-terminated surface. The activation energies of the individual reactions are summarized in Table 2. It should be noted that surface coverages of reactants or intermediates, which have a significant effect on both the bond scission and
4. Conclusions The present DFT study showed that the free energy of propanoic acid deoxygenation was dependent on the surface termination of Mo2C. The two surface terminations preferred different reaction pathways, suggesting that reaction pathways can be controlled by using specific 63
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Fig. 3. Free energies of decarboxylation (DCX) of propanoic acid on Mo- and C-terminated Mo2C (100). The activation energies at the rate-limiting steps are included. Numbers in parentheses denote the activation energies of RH(g) formation. Free energies were calculated assuming reaction conditions of T = 623.15 K and P = 0.1 MPa. Atomic configurations of the rate-limiting reactions are shown below. Color guide: Purple = Mo; Red = O; Brown = C; White = H. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Free energies of decarbonylation (DCN) of propanoic acid on Mo- and C-terminated Mo2C (100). The activation energies at the rate-limiting steps are included. Numbers in parentheses denote the activation energies of RH(g) formation. Free energies were calculated assuming reaction conditions of T = 623.15 K and P = 0.1 MPa. Atomic configurations of the rate-limiting reactions are shown below. Color guide: Purple = Mo; Red = O; Brown = C; White = H. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Acknowledgements We acknowledge support from the core KRICT project (SI1701-06) from Korea Research Institute of Chemical Technology and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under “Energy efficiency & Resources Programs” (Project No. 20152010201910) of the Ministry of Trade, Industry and Energy (MOTIE).
Table 2 Activation energies (eV) of individual reactions on the Mo- and C-terminated Mo2C (100). Reaction
Hydrogenation
CeO bond scission CeC bond scission OeH bond scission
Equation
R* + H* → RH(g) RCH2* + H* → RCH3(g) O* + H* → OH* OH* + H* → H2O(g) RCOOH* → RCO* + OH* RCO* → RC* + O* RCOOH* → R* + COOH* RCO* → R* + CO* COOH* → CO2(g) + H*
Termination Mo
C
1.26 1.05 1.18 1.24 0.49 0.88 0.30 0.26 0.71
0.88 0.67 1.52 0.52 0.58 0.96 0.38 0.36 0.31
Appendix A. Supplementary data Supplementary data including computational details (calculation methods, molecular configurations, and energies). Supplementary data associated with this article can be found in the online version, at doi: http://dx.doi.org/10.1016/j.catcom.2017.05.027. References [1] A. Corma, G.W. Huber, L. Sauvanaud, P. O'Connor, Processing biomass-derived oxygenates in the oil refinery: catalytic cracking (FCC) reaction pathways and role of catalyst, J. Catal. 247 (2007) 307–327. [2] G.W. Huber, A. Corma, Synergies between bio- and oil refineries for the production of fuels from biomass, Angew. Chem. Int. Ed. 46 (2007) 7184–7201. [3] G.W. Huber, P. O'Connor, A. Corma, Processing biomass in conventional oil refineries: production of high quality diesel by hydrotreating vegetable oils in heavy vacuum oil mixtures, Appl. Catal. A 329 (2007) 120–129. [4] M. Snåre, I. Kubičková, P. Mäki-Arvela, K. Eränen, D.Y. Murzin, Heterogeneous catalytic deoxygenation of stearic acid for production of biodiesel, Ind. Eng. Chem.
surface structures. For instance, DCX of fatty acids, consuming no hydrogen, can be facilitated by generating the C-terminated surface of a Mo2C catalyst. The energetics explored in the present study will provide atomic insight into designing Mo2C catalysts for deoxygention reactions. More detailed mechanistic studies that include the effects of alkyl chain length, surface coverage, and surface structure will lead to an indepth understanding of and aid in developing catalysis systems.
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65
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Surface-termination dependence of propanoic acid deoxygenation on Mo2C a
b,c
MARK
d,⁎
Seok Ki Kim , Jaehoon Kim , Seung-Cheol Lee a
Carbon Resource Institute, Korea Research Institute of Chemical Technology, 141 Gajeongro, Yuseong, Daejeon 34114, Republic of Korea School of Mechanical Engineering, Sungkyunkwan University, 2066 Seobu-Ro, Jangan-Gu, Gyeonggi-Do 440-746, Republic of Korea Sungkyun Advanced Institute of Nano Technology (SAINT), 2066 Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 440-746, Republic of Korea d Electronic Materials Research Center, Korea Institute of Science and Technology, Hwaranno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Molybdenum carbide Structure sensitivity Hydrodeoxygenation Reaction pathway Fatty acid
Although Mo2C is used as an alternative catalyst material for biomass conversion, the dependence of its reaction on the type of surface termination has not been studied. In the present study, we performed density functional theory calculations for the deoxygenation of propanoic acid on the two types of surface terminations possible for orthorhombic Mo2C (100), namely, on Mo- and C-terminated surfaces. The reaction energetics of the three possible deoxygenation pathways, namely, those for hydrodeoxygenation, decarbonylation, and decarboxylation, were compared by calculating the activation energies for their key surface reactions, such as hydrogenation and the scission of CeC, CeO, and OeH bonds. The Mo-terminated surface was advantageous in the case of the hydrodeoxygenation pathway because of its low kinetic energy barrier for the hydrogenation of the oxygen species, while the C-terminated surface preferred the decarboxylation pathway because of its low kinetic energy barrier for OeH bond scission.
1. Introduction
In the present study, propanoic acid was chosen as a model acid compound instead of a typical longer-chain fatty acid. Further, we assumed that the deoxygenation energetics were mainly determined by the reactions between the catalyst surface and the oxygenate part of the reactant and not the alkyl part. The free energies of the deoxygenation of propanoic acid on Mo- and C-terminated Mo2C (100) surfaces were calculated using density functional theory (DFT) calculations, and the reaction mechanisms for the three different deoxygenation pathways were proposed. This study could explain the surface-termination-sensitive deoxygenation properties of Mo2C, and its results can be used to develop a strategy for designing Mo2C-based catalysts for the selective conversion of fatty acids.
The catalytic deoxygenation of triglycerides or fatty acids is one of the most commercially viable processes for producing renewable dieselrange hydrocarbons, which can be used as alternative transportation fuels [1–3]. This reaction primarily takes place via three pathways, hydrodeoxygenation, decarbonylation, and decarboxylation, either individually or in combination [3–6]. Although the amounts of oxocarbons discharged and hydrogen consumed during the process are largely dependent on the pathway, a selective catalyst that can drive the reaction towards a specific pathway has not yet been discovered. Transition metal carbides have received attention as potential catalytic materials that can be used as alternatives to noble metals in various reactions [7,8]. In particular, recent studies have shown that Mo2C is highly active with respect to the deoxygenation of triglycerides or fatty acids but is not selective towards a specific pathway [9–14]. In order to utilize Mo2C as a highly selective catalyst for this reaction, it is imperative to understand the fundamental reaction mechanism correlating the surface structure of the catalyst and its deoxygenation properties [15–17]. Recent theoretical studies focusing on the deoxygenation of butyric acid on the Mo2C (101) surface proposed a deoxygenation mechanism wherein CeO bond scission is favored more than CeC bond scission, with this resulting in selective hydrodeoxygenation [15,16].
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Corresponding author. E-mail address: [email protected] (S.-C. Lee).
http://dx.doi.org/10.1016/j.catcom.2017.05.027 Received 16 February 2017; Received in revised form 10 May 2017; Accepted 28 May 2017 Available online 29 May 2017 1566-7367/ © 2017 Elsevier B.V. All rights reserved.
2. Methods The DFT calculations were performed using a plane-wave basis set implemented in the Vienna Ab initio Simulation Package (VASP) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional [18–20]. The k-points sampling of a 4 × 4 × 1 Monkhorst-Pack mesh grid was used for the simulations of the surface reactions. The transition states between the initial and final states of the components were calculated using the nudged elastic band (NEB) method [21,22]. The Gibbs free energies of the gas and adsorbed species (including species at the transition state) were calculated based on the ideal gas limit and
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atoms and their adjacent atoms participated. If gaseous hydrogen was involved in a reaction, its dissociative adsorption was calculated first and shown at the initial stage of the pathway to simplify the free-energy diagram. Most exergonic transformations of HDO on Mo- and C-terminated surfaces are shown in Fig. 1(a). As the reaction proceeded, surface species generated on both surfaces were found to be identical, but the energetics between each reaction coordinate appeared to be largely dependent on the surface termination. At the initial stage of the reaction, CeO bond scission of propanoic acid producing surface oxygen species (OH* and O*) and an alkyl group (RC*) (3 → 4 → 5) was exergonic on both surfaces. After formation of the surface oxygen and alkyl species, only endergonic hydrogenation of those species was left to take place (5 →…→ 11). The most energetically demanding step in the hydrogenation of the Mo-terminated surface was the formation of water (9 → 10 or 10 → 11) by 1.24 eV of activation energy, whereas that for the C-terminated surface was the formation of hydroxyl (8 → 9) by 1.52 eV of activation energy. Although the C-terminated surface appeared to weakly bind most of the adsorbates, showing a narrower energy window for HDO as compared to the Mo-terminated surface, the overall reaction kinetics were not necessarily faster on the C-terminated surface because of the higher activation energies of the most endergonic steps between both terminations. Previous experimental studies reported production of small amounts of aldehydes and alcohols as a result of partial HDO of fatty acids [10–14]. To compare the energetics on both terminations, additional transformations that branched out from the acyl group [RCO*, stage 4 in Fig. 1(a)] were calculated and the results are shown in Fig. 1(b). In the process of producing alcohol on the Mo-terminated surface, the formation of RCH2O* {(5) → (6)} and RCH2OH* {(6) → (7)} was calculated to be similarly difficult by 1.26 and 1.35 eV of activation energy, respectively. Because the parallel endergonic reactions producing RCH3 (5 →…→ 11) were more likely to occur, the intermediate path was not expected to be favourable during HDO. In the case of the Cterminated surface, the two most difficult transformations along the intermediate path were the formations of RCHO* {4 → (5)} and RCH2OH* {(6) → (7)} by 1.01 and 0.95 eV, respectively. Unlike the Mo-terminated surface, the intermediate path producing RCH2OH was energetically more favourable than the HDO path that included the difficult OH* formation. Accordingly, the C-terminated surface was considered to be responsible for the production of partially deoxygenated species. The free energies of desorption of RCH2OH {(7) → (8)} also showed easier desorption energetics on the C-terminated surface than on the Mo-terminated surface. The free-energy transformations for another deoxygenation pathway, DCN, are shown in Fig. 2. Analogous to HDO, initial CeO bond scissions producing RCO* and OH* were found to be favored (3′ → 4′) on both terminations. The subsequent CeC bond scission of RCO* (4′ → 5′) significantly stabilized the surface species, making the next steps more difficult. On the Mo-terminated surface, highly endergonic hydrogenation of surface alkyl (5′ → 6′) and hydroxyl (7′ → 8′) constrained the overall reaction by 1.26 and 1.24 eV of activation energy, respectively. On the C-terminated surface, however, the initial CeO bond scission (3′ → 4′) and hydrogenation of alkyl (5′ → 6′) required smaller activation energies by 0.58 and 0.88 eV, respectively, indicating that the C-terminated surface was energetically more beneficial for the DCN pathway. It is noteworthy that in the practical deoxygenation of triglycerides or fatty acids, their constituent alkyl chains are longer and more complex (14–20 carbon atoms with unsaturated bonds) than those of our model compound, propanoic acid. Because those diesel-range hydrocarbons are produced as a liquid phase, their free energies of formation (the last hydrogenation step of alkyl) are expected to be much lower than those of ethane and propane. In the present study as well, the hydrogenation of RCH2* was energetically easier than that of R* on both surfaces. Thus, energy comparisons on the basis of the
harmonic limit, respectively [23]. In ideal gas limit vibrational, translational, and rotational degree of freedom was taken into account while in harmonic limit only vibrational degree of freedom was taken into account. Since the PBE functional reportedly results in errors when describing the electronic energies of gas-phase molecules, we calibrated the DFT energies based on experimentally determined heat of reaction values. The van der Waals interactions were not included in these calculations. Additional details of the calculation methods are given in Supplementary data. Orthorhombic Mo2C (space group, Pbcn) was used as a model structure in the present study. The calculated lattice parameters were a = 4.74, b = 6.05, and c = 5.22 Å, which are close to the experimental values (a = 4.72, b = 6.01, c = 5.20) [24]. There has been confusion in the nomenclature of the Mo2C phase (α, β,…) used in the literature; [9,25–32] therefore, we have avoided the use of Greek letters to denote the Mo2C modelled in this study. The most closely packed surface of the orthorhombic Mo2C is the (100) surface, which is identical to the (0001) surface of hexagonal Mo2C [33]. The structures of Mo- and C-terminated orthorhombic Mo2C (100) surfaces are shown in Fig. S1 in Supplementary data. 3. Results and discussion The surface energies of the Mo- and C-terminated surfaces were calculated to be 2.52 and 2.67 J/m2, respectively, suggesting that the preferences for the surfaces are similar. Previous theoretical [9,30–32] and experimental [34–39] studies have reported that both terminations can be easily formed and can even be controlled by heat treatment. As shown in Fig. S1b and d, the C-terminated surface is not fully covered with carbon atoms because of the bulk stoichiometry; that is, only 1/2 monolayer (ML) of the surface molybdenum atoms are occupied by carbon atoms. Occupation of 1 ML of molybdenum atoms by carbon atoms increased the surface energy to 4.51 J/m2, indicating that additional carbon deposition on the stoichiometric C-terminated surface is energetically implausible. Oxygen atoms present in fatty acids can be removed via three different pathways: hydrodeoxygenation (HDO), decarbonylation (DCN), and decarboxylation (DCX) [3–6]. The net equations and free energies of each reaction for propanoic acid deoxygenation are summarized in Table 1. In the case of HDO, oxygen atoms are removed by forming water via two successive CeO bond scissions. Using HDO, one can avoid the loss of carbon atoms released as gas-phase molecules, but it requires a relatively large quantity of hydrogen. DCN or DCX involves CeC bond scission, resulting in liberation of a carbon atom by forming CO or CO2. The amount of hydrogen required for DCN or DCX is lower than that for HDO. The free energies of these three pathways on the Mo- and C-terminated surfaces were compared by searching for the most exergonic transformations, starting from propanoic acid adsorption, which was chosen as the most stable state (see Supplementary data) among all possible configurations. The oxygen ends of propanoic acid (eCOOH*) and of the intermediates (eCH2O*, eCHO*, eCOH*, and eCO*) favored adsorption on both surfaces, resulting in an alkyl end (–R) heading towards the vacuum side during the overall reaction process. Calculations were performed for surface reactions in which only oxygen Table 1 Pathways of propanoic acid deoxygenation. Pathway
Net equationa
ΔG/eVb
HDO DCN DCX
RCOOH + 3H2 → RCH3 + 2H2O RCOOH + H2 → RH + CO + H2O RCOOH → RH + CO2
− 1.77 − 1.74 − 1.94
a
R denotes CH3CH2. Free energies of reaction (ΔG) were calculated assuming reaction conditions of T = 623.15 K and P = 0.1 MPa. b
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Fig. 1. Free energies of (a) hydrodeoxygenation (HDO) of propanoic acid and (b) propanol formation on Mo- and C-terminated Mo2C (100). The activation energies at the rate-limiting steps are included. Free energies were calculated assuming reaction conditions of T = 623.15 K and P = 0.1 MPa. Atomic configurations of the rate-limiting reactions for the HDO and propanol formation are shown below. Color guide: Purple = Mo; Red = O; Brown = C; White = H. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the formation energies, were not considered in the present study. We avoided modeling high coverage of coadsorbate because it would have to investigate a very large number of cases to find global minimum atomic configurations for multiple adsorbates at every stage of the reaction. Furthermore, artificially-manipulated coadsorption environment may mislead energetics. However, it is intuitively clear that the energy required for bond scission will generally be higher than that for bond formation if the surface is highly occupied. For instance, hydrogenation of O* to form OH* will become easier if surface that surrounds O* is highly occupied by H*, while CeO bond scission will be facilitated on the surface that has more unoccupied sites. The overall reaction rates of HDO and DCX are mainly constrained by hydrogenation and CeO bond scission, respectively, regardless of the surface termination. Thus, while HDO is expected to be relatively dominant at high surface coverages or at high pressure of H2, while DCX is expected to be promoted in the early stage of the reaction when the surface is relatively cleaner than thereafter. This interpretation is in accordance with a previous experimental study [14], which showed changes in the deoxygenation pathway from DCX or DCN to HDO on Mo2C/C catalysts as oleic acid conversion increased.
hydrogenation of alkyl species' are not suitable. If the free energies of hydrogenation of the alkyl species were found to be the most demanding, their energies are denoted in parentheses in the free-energy diagrams. Fig. 3 shows the free-energy diagrams for the DCX pathways, which were much simpler than others because of the lack of hydrogen as a coreactant and CeO bond scission steps. On the C-terminated surface, CeC bond scission producing R* and COOH* (2″ → 3″, 0.38 eV of activation energy) was calculated to be the rate-limiting step right after the adsorption of propanoic acid, whereas OeH bond scission of COOH* producing gaseous CO2 and H* (3″ → 4″, 0.71 eV of activation energy) was the rate-limiting step on the Mo-terminated surface (except R* hydrogenation, 4″ → 5″, as noted earlier). These results indicate that the C-terminated surface was also energetically favourable for DCX relative to the Mo-terminated surface. As shown in the above energy diagrams, the Mo-terminated surface tended to strongly bind adsorbates that easily decomposed to small species by CeC or CeO bond scission. But a great deal of energy was required at the late stage of the reaction, such as during the hydrogenation of OH* to water. On the other hand, the C-terminated surface appeared to weaken the overall adsorption strength, resulting in easy removal of the adsorbates; however, it was difficult to decompose the adsorbates on the C-terminated surface as compared to the Mo-terminated surface. The activation energies of the individual reactions are summarized in Table 2. It should be noted that surface coverages of reactants or intermediates, which have a significant effect on both the bond scission and
4. Conclusions The present DFT study showed that the free energy of propanoic acid deoxygenation was dependent on the surface termination of Mo2C. The two surface terminations preferred different reaction pathways, suggesting that reaction pathways can be controlled by using specific 63
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Fig. 3. Free energies of decarboxylation (DCX) of propanoic acid on Mo- and C-terminated Mo2C (100). The activation energies at the rate-limiting steps are included. Numbers in parentheses denote the activation energies of RH(g) formation. Free energies were calculated assuming reaction conditions of T = 623.15 K and P = 0.1 MPa. Atomic configurations of the rate-limiting reactions are shown below. Color guide: Purple = Mo; Red = O; Brown = C; White = H. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Free energies of decarbonylation (DCN) of propanoic acid on Mo- and C-terminated Mo2C (100). The activation energies at the rate-limiting steps are included. Numbers in parentheses denote the activation energies of RH(g) formation. Free energies were calculated assuming reaction conditions of T = 623.15 K and P = 0.1 MPa. Atomic configurations of the rate-limiting reactions are shown below. Color guide: Purple = Mo; Red = O; Brown = C; White = H. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Acknowledgements We acknowledge support from the core KRICT project (SI1701-06) from Korea Research Institute of Chemical Technology and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under “Energy efficiency & Resources Programs” (Project No. 20152010201910) of the Ministry of Trade, Industry and Energy (MOTIE).
Table 2 Activation energies (eV) of individual reactions on the Mo- and C-terminated Mo2C (100). Reaction
Hydrogenation
CeO bond scission CeC bond scission OeH bond scission
Equation
R* + H* → RH(g) RCH2* + H* → RCH3(g) O* + H* → OH* OH* + H* → H2O(g) RCOOH* → RCO* + OH* RCO* → RC* + O* RCOOH* → R* + COOH* RCO* → R* + CO* COOH* → CO2(g) + H*
Termination Mo
C
1.26 1.05 1.18 1.24 0.49 0.88 0.30 0.26 0.71
0.88 0.67 1.52 0.52 0.58 0.96 0.38 0.36 0.31
Appendix A. Supplementary data Supplementary data including computational details (calculation methods, molecular configurations, and energies). Supplementary data associated with this article can be found in the online version, at doi: http://dx.doi.org/10.1016/j.catcom.2017.05.027. References [1] A. Corma, G.W. Huber, L. Sauvanaud, P. O'Connor, Processing biomass-derived oxygenates in the oil refinery: catalytic cracking (FCC) reaction pathways and role of catalyst, J. Catal. 247 (2007) 307–327. [2] G.W. Huber, A. Corma, Synergies between bio- and oil refineries for the production of fuels from biomass, Angew. Chem. Int. Ed. 46 (2007) 7184–7201. [3] G.W. Huber, P. O'Connor, A. Corma, Processing biomass in conventional oil refineries: production of high quality diesel by hydrotreating vegetable oils in heavy vacuum oil mixtures, Appl. Catal. A 329 (2007) 120–129. [4] M. Snåre, I. Kubičková, P. Mäki-Arvela, K. Eränen, D.Y. Murzin, Heterogeneous catalytic deoxygenation of stearic acid for production of biodiesel, Ind. Eng. Chem.
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