Theoretical Investigation of the Reaction Mechanism of the

Article pubs.acs.org/JPCC

Theoretical Investigation of the Reaction Mechanism of the Decarboxylation and Decarbonylation of Propanoic Acid on Pd(111) Model Surfaces Jianmin Lu, Sina Behtash, and Andreas Heyden* Department of Chemical Engineering, University of South Carolina, 301 S. Main St., Columbia, South Carolina 29208, United States ABSTRACT: Conversion of biomass into fuels or chemicals often requires a processing step limited by hydrodeoxygenation of organic acids. Various pathways have been proposed for the deoxygenation of these acids into hydrocarbons, with the decarboxylation and decarbonylation requiring less hydrogen than the reductive deoxygenation without C−C bond cleavage. In this paper, we present the reaction mechanism for the decarboxylation and decarbonylation of propanoic acid over Pd(111) model surfaces determined by first-principles electronic structure calculations based on density functional theory. Our calculations suggest that the most significant decarbonylation pathways proceed via a dehydroxylation of the acid to produce propanoyl (CH3CH2CO) followed by either full α-carbon dehydrogenation and CH3C−CO bond scission to produce CH3C and CO, or first α-carbon dehydrogenation followed by β-carbon dehydrogenation and CH2CH−CO bond scission to produce CH2CH and CO. The decarboxylation mechanism starts with O−H bond cleavage followed by direct C− CO2 bond scission or possibly α-carbon dehydrogenation prior to C−CO2 bond cleavage. As a result, in both mechanisms the most favorable pathways likely involve some level of α- and/or β-carbon dehydrogenation steps prior to C−C scission, which distinguishes these deoxygenation pathways from the reductive deoxygenation without C−C bond cleavage that has previously been shown to not involve dehydrogenation steps.

1. INTRODUCTION Diminishing fossil fuel reserves and more concerns about anthropogenic climate change and national energy security have led to an increased interest in liquid fuel production from various renewable raw materials. First generation biofuels such as fatty acid methyl and ethyl esters (FAMEs) are commonly produced by transesterification of triglycerides (the main constituent of vegetable oils and animal fats) with methanol or ethanol. Unfortunately, FAMEs suffer from disadvantages such as high viscosity, high cloud point temperature, poor oxidation stability, and low energy density.1,2 As a result, there is interest in the deoxygenation of triglycerides and their fatty acids into hydrocarbons with similar properties to conventional diesel, a product also known as “green diesel”, a 1.5th generation biofuel. In this study we focused on the hydrodeoxygenation (HDO) of organic acids which has often been found to be rate limiting in the HDO of biomass intermediates.3 Three reaction mechanisms have been identified for the HDO of carboxylic acids: decarbonylation (DCN), decarboxylation (DCX), and reductive deoxygenation without C−C cleavage (RDO). The main difference between these mechanisms is that in the RDO all oxygen atoms in the organic acid are removed in the form of water requiring a substantial amount of hydrogen, while in the DCN and DCX mechanism most of the oxygen is removed in the form of CO2 and CO (which converts to CO2 through the water−gas shift), which requires less hydrogen but also leads to a product one carbon atom shorter than the corresponding acid © 2012 American Chemical Society

molecule. Considering that hydrogen is currently in short supply in refineries and that the demand for hydrogen for, e.g., fuel cell application will likely grow, there is a need for HDO catalysts favoring the DCN and DCX pathways over the RDO pathway.4 In one of the early efforts, Maier and colleagues demonstrated 30 years ago that Pd/SiO2 catalysts are highly selective for the deoxygenation of carboxylic acids.5 Recently, Murzin’s group conducted an extensive investigation of the liquid phase HDO of various long chain fatty acids such as lauric acid,6 palmitic acid,7,8 and stearic acid8,9 on Pd/C catalyst, and they suggested that the DCX pathway is more dominant than the DCN or RDO. In contrast to these observations from Murzin, Boda et al.10 recently concluded from studies of the catalytic HDO of octanoic acid to hydrocarbons over a similar Pd/C catalyst that the DCN is the preferred reaction route. Distinguishing DCN from DCX pathways is often challenging due to the water−gas shift occurring under reaction conditions. In this paper, we report results from an extensive density functional theory (DFT) study of the DCN and DCX mechanism of carboxylic acids on Pd(111) terraces. We aim at determining the preferred mechanism under reaction conditions and identifying possible descriptors relevant for the selectivity of the DCN/DCX versus the RDO. Pd has been selected for this Received: February 27, 2012 Revised: June 5, 2012 Published: June 13, 2012 14328

dx.doi.org/10.1021/jp301926t | J. Phys. Chem. C 2012, 116, 14328−14341

The Journal of Physical Chemistry C

Article

Figure 1. Scheme for possible DCN pathways of propanoic acid to ethane on Pd (111).

The total energy of fcc-Pd bulk reached a minimum when its lattice constant was 3.953 Å, which is in reasonable agreement with the experimental value, 3.891 Å. The Pd(111) surface was constructed as a periodic slab with four Pd layers separated by a vacuum layer of 15 Å in order to eliminate interactions between the slab and its images. Four layers were found to be a reliable trade-off between accuracy and computational cost. Each Pd layer had 12 Pd atoms with a (3 × 2√3) periodicity, allowing for adsorbate coverages as low as 1/12 ML. The bottom two Pd layers were fixed to their bulk configuration during all computations, while the top two layers were fully relaxed. The adsorbates were free to relax in all directions. All atomic coordinates of the adsorbates and the Pd atoms in the relaxed layers were optimized to a force less than 0.03 eV/Å on each atom. All self-consistent field (SCF) calculations were converged to 1 × 10−3 kJ/mol. Brillouin zone integration was performed using a 4 × 4 × 1 Monkhorst−Pack grid and a Methfessel− Paxton smearing of 0.2 eV. In all cases, the convergence of total energy with respect to the k-point mesh and with respect to plane-wave energy cutoff has been confirmed. We note that Pd hydride formation is not favorable at typical process temperatures of 573 K, as long as the hydrogen partial pressure does not exceed 30 bar.26 Adsorption energies of all the surface intermediates reported in this paper were calculated in their most favorable adsorption modes. The adsorption energies, Eads, were calculated by the following equation:

study since Pd/C has been found experimentally to be one of the best metal catalysts (among Pd, Pt, Ni, Rh, Ir, Ru, and Os) to catalyze the organic acid conversion to alkanes.11 To the best of the authors’ knowledge, no theoretical investigation on the DCN/DCX mechanism of carboxylic acid has yet been reported in the literature. The current paper presents the first attempt on this issue. Considering that long-chain carboxylic acids cannot easily be studied computationally, we focus here on the DCN and DCX of propanoic acid. In the Appendix of this paper, we show that reaction energies for the DCN and DCX are converged with respect to the carbon chain length for propanoic acid; thus we believe that as long as the terminal CH3-group is not involved in the reaction mechanism, propanoic acid represents a meaningful model reactant for various longer chain carboxylic acids. We note that the chemisorption behavior of short-chain carboxylic acids has been studied extensively in the last two decades by both experiments12−17 and theoretical modeling,18−20 but no theoretical investigation on the catalytic deoxygenation of propanoic acid has been reported. Finally, we focus here on the flat Pd terraces, although stepped surfaces have often been found to be more active for C−C bond cleavage. A study on the effect of Pd steps is currently underway. Similarly, a detailed microkinetic modeling study will soon be published.

2. METHODS All adsorption energies as well as the electronic and vibrational properties have been calculated using the Vienna Ab Initio Simulation Package (VASP).21,22 Plane wave basis sets are used to solve the Kohn−Sham equations. The electron-ion interactions are described by the projector-augmented wave method (PAW). The PAW method is a frozen core all-electron method that uses the exact shape of the valence wave functions instead of pseudowave functions.23 The exchange correlation energy has been calculated within the generalized gradient approximation (GGA) using the functional form proposed by Perdew and Wang24,25 usually referred to as Perdew−Wang 91 (PW91). An energy cutoff for plane waves of 400 eV was employed throughout this study. Geometry optimizations were performed using periodic boundary conditions.

Eads = Eslab + adsorbate − Eslab − Eadsorbate(gas)

where Eslab+adsorbate is the total energy of the slab with an adsorbate bound to it, Eslab is the total energy of the clean Pd slab, and Eadsorbate(gas) is the total energy of the adsorbate in the gas phase. Finally, transition states for elementary reaction steps were determined by a combination of the nudged elastic band (NEB) method27 and the dimer method.28−30 In the NEB method, the path between the reactant and product is discretized into a series of structural images. The image that is closest to a likely transition state structure was then employed as an initial guess structure for the dimer method. All adsorption energies and activation barriers 14329



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Figure 2. Scheme for possible DCX pathways of propanoic acid to ethane on Pd (111).

adsorbs about twice as strong as CH3CH2 (Eads = −3.65 eV), and the third dehydrogenation product CH3C adsorbs about 3 times as strong as CH3CH2 (Eads = −5.61 eV), so that to a good approximation each dehydrogenation leads to an increase in adsorption strength of 1.6−2.0 eV. 3.2. Reaction Steps in the DCN Mechanism. In this subsection, we will describe each elementary reaction step investigated for the DCN mechanism. All activation barriers and reaction energies are also summarized in Table 2. Step 1: CH3CH2COOH → CH3CH2CO + OH. Several orientations of the reactant propanoic acid (CH3CH2COOH) adsorbed on the Pd(111) surface have been studied, and the cis mode (Figure 3a) was found to be the most stable configuration, which binds via its carbonyl O at the atop site of one Pd atom with an adsorption energy of −0.28 eV. O binds to the Pd atom through its lone pair electrons and the O−Pd distance is 2.37 Å. A similar value has previously been reported for the adsorption of acetic acid.33 Figure 4 shows the optimized reactant, transition state, and products along the reaction coordinate for this elementary reaction. As the initial state for this reaction, we propose that propanoic acid adsorbs on the Pd(111) surface in a tilted trans mode, in which both O atoms bind to the surface, and the backbone of the molecule forms an angle of around 30° with the Pd(111) plane. This adsorption mode is less favorable than the cis mode shown in Figure 3a (Eads = −0.02 eV), but leads to relatively facile dehydroxylation of propanoic acid to propanoyl via C−OH bond scission. The activation barrier for C−OH bond scission is 0.92 eV. In the transition state (TS), the carbonyl C and the hydroxyl O bind to the same Pd surface atom, and the C−OH bond distance (1.73 Å) is 0.35 Å longer than that in propanoic acid (1.38 Å). The imaginary frequency of the TS is 213i cm−1, and the reaction is endothermic by 0.42 eV. Unless stated otherwise, activation barriers and reaction energies are calculated without considering lateral interactions; meaning all reactants and products are isolated on the metal surface. Step 2: CH3CH2COOH → CH3CHCOOH + H. Alternatively to step 1, the reactant CH3CH2COOH can dehydrogenate its αcarbon to form CH3CHCOOH. This reaction has an activation barrier of 0.62 eV. The TS (Figure 5(2)) is characterized by coadsorption of H and α-carbon to one Pd atom, and elongation

reported in this study have been zero point corrected (ΔZPE). Bader charge analysis (including the core states) has been employed to elucidate the charge state of surface intermediates using the implementation from Henkelman et al.31,32

3. RESULTS Figures 1 and 2 illustrate the reaction pathways investigated for the DCN and DCX of propanoic acid to ethane over Pd(111) surfaces, respectively. Both schemes do not include reactions that lead to C1 or C3 products. C1 products are experimentally not observed, and while C3 products are likely important, we will analyze them in a separate paper. In the following, we will first report the adsorbed intermediates in section 3.1. Then, we will investigate each elementary step shown in Figures 1 and 2 individually in sections 3.2 and 3.3, respectively. Finally, we discuss our results in section 4. 3.1. Adsorbed Intermediates. Adsorption geometries of the reactants, products and possible intermediates involved in both the DCN and DCX are shown in Figure 3 (species a−z). Binding modes, charge of the adsorbate, adsorption energies, and zero point energy corrections of various intermediates are listed in Table 1. We have employed the nomenclature ηiμj to designate that i atoms of the adsorbate are binding to the j atoms of the metal surface. Table 1 illustrates that saturated intermediates, such as propanoic acid, ethane, water, and carbon dioxide only weakly bind to the surface with calculated adsorption energies ranging from −0.03 eV to −0.28 eV. These weak bonds are formed either by interactions between the lone pair electrons on the hydroxyl oxygen with the surface or by weak π-bonded interactions of the CO group with the surface.33 Meanwhile, open-shell intermediates strongly adsorb onto the Pd(111) surface with adsorption energies ranging from −0.89 eV to −5.61 eV. Especially, the CH3C species binds strongly to the surface with an adsorption energy of −5.61 eV. Carbon atoms tend to satisfy their tetrahedral bonding geometry on the surface34 and a deeper dehydrogenation results in stronger adsorption. For example, CH3CH3 adsorbs very weakly on Pd(111) (Eads = −0.11 eV). The first dehydrogenation product CH3CH2 forms a significantly stronger bond with the surface (Eads = −1.68 eV), CH3CH 14330



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Figure 3. Side (upper panel) and top view (lower panel) of the preferred adsorption structure of various intermediates in the reaction networks of the DCN and DCX of propanoic acid on Pd(111): (a) propanoic acid (CH3CH2COOH); (b) ethylidene-1-ol-1-olate (CH3CHCOOH); (c) ethylidyne-1ol-1-olate (CH3CCOOH); (d) propanoyl (CH3CH2CO); (e) carbonylethylidene (CH3CHCO); (f) carbonylethylidyne (CH3CCO); (g) vinyl-1-ol-1olate (CH2CHCOOH); (h) carbonylvinyl (CH2CHCO); (i) ethyne-1-ol-1-olate (CHCHCOOH); (j) carbonylethyne (CHCHCO); (k) propanoate (CH3CH2COO); (l) carboxylethylidene (CH3CHCOO); (m) carbonylethylidyne (CH3CCOO); (n) carboxylic (COOH); (o) ethyne (CHCH); (p) vinyl (CH2CH); (q) ethene (CH2CH2); (r) ethylidyne (CH3C); (s) ethylidene (CH3CH); (t) ethyl (CH3CH2); (u) ethane (CH3CH3); (v) hydrogen (H); (w) hydroxyl (OH); (x) water (H2O); (u) carbon monoxide (CO); (z) carbon dioxide (CO2).

Figure 3d, we propose that the initial structure for this elementary reaction is one in which the O atom does not bind to the surface but the carbonyl C atom binds to a Pd atop site. This structure is only slightly less stable (Eads = −2.34 eV) than the most stable structure. In the transition state the CH3CH2 and

of the C−H bond from 1.12 Å to 1.57 Å. The imaginary frequency of the TS is 908i cm−1, and this elementary step has an exothermicity of −0.10 eV. Step 3: CH3CH2CO → CH3CH2 + CO. Instead of using the most stable adsorption structure of CH3CH2CO as shown in 14331



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Table 1. Binding Modes, Charge of the Adsorbate (A Positive Value Means Losing Electrons), Zero-Point-Energy-Corrected Adsorption Energies (Eads, in eV), and Zero Point Energy Corrections (ΔZPE, in eV) of Reaction Intermediates Calculated from DFT species

stoichiometry

Figure 3 species

binding mode

charge

Eads (eV)

ΔZPE (eV)

propanoic acid ethylidene-1-ol-1-olate ethylidyne-1-ol-1-olate propanoyl carbonylethylidene carbonylethylidyne vinyl-1-ol-1-olate carbonylvinyl ethyne-1-ol-1-olate carbonylethyne propanoate carboxylethylidene carboxylethylidyne carboxylic ethyne vinyl ethene ethylidyne ethylidene ethyl ethane hydrogen hydroxyl water carbon monoxide carbon dioxide

CH3CH2COOH CH3CHCOOH CH3CCOOH CH3CH2CO CH3CHCO CH3CCO CH2CHCOOH CH2CHCO CHCHCOOH CHCHCO CH3CH2COO CH3CHCOO CH3CCOO COOH CHCH CH2CH CH2CH2 CH3C CH3CH CH3CH2 CH3CH3 H OH H2O CO CO2

a b c d e f g h i j k l m n o p q r s t u v w x y z

η1 μ1 (O) η2 μ2 (C,O) η2 μ3 (C,O) η2 μ3 (C,O) η2 μ2 (C,O) η2 μ3 (C,O) η2 μ2 (C,C) η3 μ3 (C,C,O) η3 μ3 (C,C,O) η3 μ4 (C,C,C) η2 μ2 (O,O) η3 μ3 (C,O,O) η2 μ3 (C,O) η2 μ2 (C,O) η2 μ3 (C,C) η2 μ3 (C,C) η2 μ2 (C,C) η1 μ3 (C) η1 μ2 (C) η1 μ1 (C) η1 μ1 (H) η1 μ3 (H) η1 μ3 (O) η1 μ1 (O) η1 μ1 (C) η2 μ2 (O,O)

0.042 −0.106 −0.201 −0.144 0.018 −0.120 −0.130 −0.200 −0.207 −0.145 −0.452 −0.207 −0.470 −0.174 −0.123 −0.028 0.033 −0.042 −0.013 0.043 −0.010 −0.102 −0.437 0.063 −0.233 −0.033

−0.28 −1.42 −3.23 −2.52 −1.22 −3.00 −0.89 −2.38 −2.86 −3.50 −2.58 −1.32 −1.71 −2.10 −1.95 −2.75 −0.93 −5.61 −3.65 −1.68 −0.11 −2.70 −2.59 −0.25 −1.99 −0.03

−0.01 0.05 0.09 0.01 0.02 0.01 −0.01 0.06 0.05 0.04 0.05 0.00 0.05 0.04 0.05 0.10 0.13 0.16 0.12 0.08 −0.03 0.17 0.11 0.04 0.04 0.02

Step 7: CH3CHCOOH → CH3CCOOH + H. Alternatively to steps 5 and 6, CH3CHCOOH can follow a further α-carbon dehydrogenation step to give CH3CCOOH. The transition state (Figure 5(7)) is characterized by H binding to one Pd atom and the α-carbon binding to a neighboring bridge site with a C−H bond length of 1.70 Å. The imaginary frequency of the TS is 798i cm−1, the activation barrier is 1.15 eV, and the reaction has a slight exothermicity by −0.05 eV. Step 8: CH3CHCO → CH3CH + CO. CH3CHCO adsorbs on a bridge site by the carbonyl C binding to one Pd atom and αcarbon binding to another Pd. DCN happens when the C−C bond length is enlarged from 1.49 Å to 2.30 Å in the TS (Figure 5(8)). The imaginary frequency of TS is 410i cm−1, the activation barrier is again quite high (Ea = 1.01 eV), and the elementary step is exothermic by −0.79 eV. Step 9: CH3CHCO → CH3CCO + H. Instead of direct DCN in step 8, CH3CHCO can also dehydrogenate the last H on the αcarbon to form CH3CCO. In the TS (Figure 5(9)), the C−H bond is enlarged from 1.10 Å to 1.66 Å, and both the H and αcarbon bind to the same Pd atom. The imaginary frequency of the TS is 848i cm−1 and the activation barrier is 0.60 eV, which is significantly lower than the barrier of step 8. The reaction has a slight exothermicity of −0.39 eV. Step 10: CH3CHCO → CH2CHCO + H. Alternatively to steps 8 and 9, CH3CHCO can also follow an α-carbon dehydrogenation step to give CH2CHCO. The TS (Figure 5(10)) is characterized by both H and α-carbon binding to one Pd atom with a C−H bond length of 1.47 Å. The imaginary frequency of the TS is 766i cm−1, and the activation barrier is 0.56 eV, which is very close to that of step 9. Overall, the elementary reaction is exothermic by −0.32 eV.

CO species bind two different Pd atoms and the transition state C−C distance is 2.34 Å (Figure 5(3)). The imaginary frequency of this TS is 372i cm−1, and the activation energy barrier is relatively high (1.60 eV) while the overall reaction is mildly exothermic by −0.65 eV. Step 4: CH3CH2CO → CH3CHCO + H. For α-carbon dehydrogenation of CH3CH2CO, we chose as the initial state for the reaction the same adsorption orientation as in step 2. The transition state configuration (Figure 5(4)) is characterized by both the α-carbon and H binding to one Pd atom. The C−Pd bond length is 2.28 Å, the H−Pd bond length is 1.72 Å, and the C−H bond length is 1.57 Å in the transition state. The imaginary frequency of this TS is 901i cm−1. The activation barrier is 0.82 eV, and the reaction is almost thermoneutral by 0.01 eV. Step 5: CH3CHCOOH → CH3CHCO + OH. The dissociation of CH3CHCOOH may either follow a dehydroxylation step to cleave the OH group or by a further dehydrogenation of the αcarbon. In the transition state of the dehydroxylation (Figure 5(5)), the carbonyl C binds to one Pd atom and OH binds to the neighboring atop site by O. The C−O bond length is enlarged from 1.36 Å to 2.21 Å, and the imaginary frequency of the TS is 194i cm−1. The activation barrier is 0.88 eV, and the reaction has a mild endothermicity by 0.53 eV. Step 6: CH3CHCOOH → CH2CHCOOH + H. Alternatively to step 5, CH 3 CHCOOH can follow a further β-carbon dehydrogenation step to give CH2CHCOOH. The transition state (Figure 5(6)) is characterized by both H and β-carbon binding to the same Pd atom with a C−H bond length of 1.52 Å. The imaginary frequency of the TS is 940i cm−1, the activation barrier is 0.55 eV, and the reaction is exothermic by −0.38 eV. 14332



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Table 2. Zero-Point Corrected Activation Barriers and Reaction Energies of All Elementary Steps Investigated for the DCN and DCX Mechanismsa steps

surface reactions

Ea [eV]

1

CH3CH 2COOH* + * → CH3CH 2CO* + OH*

0.92

0.42

2

CH3CH 2COOH* + * → CH3CHCOOH* + H*

0.62

−0.10

3

CH3CH 2CO* + * → CH3CH 2* + CO*

1.60

−0.65

4

CH3CH 2CO* + * → CH3CHCO* + H*

0.82

0.01

5

CH3CHCOOH* + * → CH3CHCO* + OH*

0.88

0.53

6

CH3CHCOOH* + * → CH 2CHCOOH* + H*

0.55

−0.38

7

CH3CHCOOH* + * → CH3CCOOH* + H*

1.15

−0.05

8

CH3CHCO* + * → CH3CH* + CO*

1.01

−0.79

9

CH3CHCO* + * → CH3CCO* + H*

0.60

−0.39

10

CH3CHCO* + * → CH 2CHCO* + H*

0.56

−0.32

11

CH 2CHCOOH* + * → CH 2CHCO* + OH*

1.48

0.59

12

CH 2CHCOOH* + * → CHCHCOOH* + H*

0.90

0.04

13

CH3CCOOH* + * → CH3CCO* + OH*

0.78

0.20

14

CH3CCO* + * → CH3C* + CO*

0.47

−1.35

15

CH 2CHCO* + * → CH 2CH* + CO*

0.82

−0.76

16

CH 2CHCO* + * → CHCHCO* + H*

0.68

0.03

CHCHCOOH* + * → CHCHCO* + OH*

1.09

0.58

18

CHCHCO* + * → CHCH* + CO*

0.37

−1.10

19

CHCH* + H* → CH 2CH* + *

0.95

0.32

20

CH 2CH* + H* → CH 2CH 2* + *

0.88

−0.05

21

CH3C* + H* → CH3CH* + *

1.10

0.94

22

17

CH3CH* + H* → CH3CH 2* + *

0.87

0.01

23

CH3CH 2* + H* → CH3CH3* + *

0.66

0.06

24

CH 2CH 2* + H* → CH3CH 2* + *

0.87

0.33

25

CH3CH 2COOH* + * → CH3CH 2COO* + H*

0.36

−0.41

26

CH3CH 2COO* + * → CH3CH 2* + CO2 *

1.40

0.19

27

CH3CH 2COO* + * → CH3CHCOO* + H*

1.22

0.40

28

CH3CHCOOH* + * → CH3CHCOO* + H*

0.79

0.10

29

CH3CHCOOH* + * → CH3CH* + COOH*

1.39

0.32

30

CH3CHCOO* + * → CH3CH* + CO2 *

0.95

−0.32

31

CH3CHCOO* + * → CH3CCOO* + H*

0.85

−0.07

32

CH3CCOOH* + * → CH3CCOO* + H*

0.93

0.07

33

CH3CCOOH* + * → CH3C* + COOH*

0.91

−0.57

34

CH 2CHCOOH* + * → CH 2CH* + COOH*

2.08

0.72

35

CH3CCOO* + * → CH3C* + CO2 *

0.64

−1.19

36

COOH* + * → CO* + OH*

0.41

−0.60

37

COOH* + * → CO2 * + H*

0.36

−0.54

OH* + H* → H 2O* + *

0.67

−0.20

38 a

ΔErxn [eV]

Figure 4. Reaction path for the dissociation of propanoic acid to propanoyl and hydroxyl intermediates on Pd(111). Step 1: CH3CH2COOH → CH3CH2CO+OH. Upper panels are for side views, and lower ones are for top views. (a) initial state; (b) transition state for C−O bond breaking; (c) final state.

the activation barrier is as high as 1.48 eV, and the reaction has a mild endothermicity of 0.59 eV. Step 12: CH2CHCOOH → CHCHCOOH + H. Alternatively to step 11, CH2CHCOOH can also follow another β-carbon dehydrogenation step to give CHCHCOOH. The TS (Figure 5(12)) is characterized by both the H and β-carbon binding to one Pd atom with a C−H bond length of 1.77 Å and an imaginary frequency of the TS of 780i cm−1. The activation barrier is 0.90 eV and the reaction is almost thermoneutral by 0.04 eV. Step 13: CH 3 CCOOH → CH 3 CCO + OH. For the dehydroxylation of CH3CCOOH, we chose a less stable adsorption configuration than the one shown in Figure 3c as the initial state for this reaction. The carbonyl O does not bind to the surface, and the structure is destabilized by 0.20 eV. In the TS (Figure 5(13)), the carbonyl C binds to one Pd atom and the hydroxyl O binds to another Pd. The C−OH bond is elongated by 0.84 Å to 2.21 Å. The imaginary frequency of the TS is 227i cm−1, the elementary step has an activation barrier of 0.78 eV and an endothermicity of 0.21 eV. Step 14: CH3CCO → CH3C + CO. The C−CO bond cleavage in CH3CCO is rather facile with an activation barrier of merely 0.47 eV. In the TS (Figure 5(14)), the CH3C and CO bind to two neighboring hollow sites and the C−CO bond is elongated by 0.28 Å to 1.73 Å. The imaginary frequency of the TS is 396i cm−1, and the reaction is strongly exothermic by −1.35 eV. Step 15: CH2CHCO → CH2CH + CO. The intermediate CH2CHCO can follow either DCN or further β-carbon dehydrogenation. In the former case, a TS (Figure 5(15)) is found with the CH-carbon and carbonyl carbon binding to two neighboring bridge sites with an activation barrier of 0.82 eV. The C−CO bond length is elongated by 0.53 Å to 1.98 Å, and the imaginary frequency of the TS is 491i cm−1. This reaction has a mild exothermicity of −0.76 eV. Step 16: CH2CHCO → CHCHCO + H. Alternatively, CH2CHCO can follow further β-carbon dehydrogenation. TheTS (Figure 5(16)) is characterized by the β-carbon binding to a Pd atom on top and H binding to a neighboring bridge site with a C−H distance of 1.59 Å. The activation barrier is 0.68 eV, the imaginary frequency of the TS is 631i cm−1, and the reaction is almost thermoneutral (ΔErxn = 0.03 eV). Step 17: CHCHCOOH → CHCHCO + OH. CH2CHCO can also follow further β-carbon dehydrogenation. The TS (Figure 5(17)) is characterized by the β-carbon binding to a Pd atom on

Asterisk (*) represents a surface adsorption site.

Step 11: CH2CHCOOH → CH2CHCO + OH. CH2CHCOOH can follow a dehydroxylation to produce CH2CHCO. The TS (Figure 5(11)) is characterized by carbonyl carbon and hydroxyl oxygen binding to two different Pd atoms with a C−OH bond length of 2.35 Å. The imaginary frequency of the TS is 285i cm−1, 14333



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Figure 5. Snapshots of transition states of 18 elementary steps on the Pd(111) surface. Upper panels are for side views and lower ones for top views.

Step 21−23: CH3C + 3H → CH3CH3. In these three hydrogenation steps, the first one (CH3C + H → CH3CH) has the highest activation barrier of 1.10 eV (TS in Figure 6(21)) and the largest endothermicity of 0.94 eV mostly due to the extremely strong adsorption of the CH3C group. In the transition state, the C and H atom bind to the same Pd atom with a C−H bond length of 1.13 Å and an imaginary frequency of the TS of 196i cm−1. CH3CH hydrogenation (CH3CH + H → CH3CH2) has a slightly lower activation barrier of 0.87 eV (TS in Figure 6(22)) and is thermal neutral by 0.01 eV. In the transition state the C and H atom bind to the same Pd atom with a C−H bond length of 1.69 Å and an imaginary frequency of the TS of 801i cm−1. The last step, CH3CH2 hydrogenation (CH3CH2 + H → CH3CH3) has the lowest activation barrier of 0.66 eV (TS in Figure 6(23)) and is again almost thermal neutral by 0.06 eV. In the TS, the C and H atom bind to the same Pd atom with a C−H bond length of 1.59 Å and the imaginary frequency of the TS is 941i cm−1. Step 24: CH2CH2 + H → CH3CH2. Finally, the hydrogenation of CH2CH2 to CH3CH2 has a TS (Figure 6(24)) characterized by H and C binding to one Pd atom with a C−H distance of 1.53 Å and a imaginary frequency of the TS of 924i cm−1. The reaction

top and H binding to a neighboring bridge site with a C−H distance of 1.59 Å. The activation barrier of 1.09 eV is rather high, the imaginary frequency of the TS is 217i cm−1, and the reaction is mildly endothermic by 0.58 eV. Step 18: CHCHCO → CHCH + CO. The DCN of CHCHCO has a TS (Figure 5(18)) characterized by the α-carbon and carbonyl carbon binding to two different Pd atoms on top with a C−C distance of 2.05 Å and an imaginary frequency of the TS of 462i cm−1. The reaction has a quite low activation barrier of 0.37 eV and a high exothermicity of −1.10 eV. Step 19−20: CHCH + 2H → CH2CH2. CHCH is hydrogenated into CH2CH2 by two consecutive hydrogenation steps. The first step (CHCH + H → CH2CH) has an activation barrier of 0.95 eV (TS in Figure 5(19)) and an endothermicity of 0.32 eV. In the TS, the C and H atom bind to the same Pd atom with a C−H bond length of 1.57 Å and an imaginary frequency of the TS of 1025i cm−1. The second step (CH2CH + H → CH2CH2) has a slightly lower activation barrier of 0.88 eV (TS in Figure 6(20)) and is almost thermoneutral (ΔErxn = −0.05 eV). In this TS, the C and H atom bind to the same Pd atom with a C−H bond length of 1.75 Å and an imaginary frequency of the TS of 787i cm−1. 14334



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Figure 6. Snapshots of transition states of 18 elementary steps on the Pd(111) surface. Upper panels are for side views and lower ones for top views.

bidentate adsorption mode (Eads=-2.37 eV) shown in Figure 3k as initial state for the elementary reaction. Instead, we chose the less stable monodentate adsorption state (Eads = −1.53 eV) in which only one of the O atoms binds to the surface. The TS (Figure 6(26)) has a C−COO distance of 1.93 Å and the imaginary frequency of the TS is 435i cm−1. Overall, this elementary step has a high activation energy barrier of 1.40 eV and is endothermic by 0.19 eV. Step 27: CH3CH2COO → CH3CHCOO + H. A viable alternative reaction is the dehydrogenation of the α-carbon atom of CH3CH2COO. We chose the same adsorption mode as in step 26 as initial state. In the TS (Figure 6(27)), both the H and αcarbon bind to the same surface Pd atom and the C−H distance is 1.64 Å. The imaginary frequency of the TS is 811i cm−1, and the activation energy barrier is 1.22 eV with respect to the most stable adsorption state of CH3CH2COO. This reaction is endothermic by 0.40 eV. Step 28: CH3CHCOOH → CH3CHCOO + H. There are two more possible reaction pathways for CH3CHCOOH beside those already discussed in steps 5, 6, and 7. One is O−H cleavage,

has an activation barrier of 0.87 eV and an endothermicity of 0.33 eV. 3.3. Reaction Steps in the DCX Mechanism. In this subsection, we will describe each elementary reaction step investigated for the DCX mechanism. All activation barriers and reaction energies are summarized in Table 2. Step 25: CH3CH2COOH → CH3CH2COO + H. The DCX mechanism of propanoic acid starts with the abstraction of the hydroxyl H. We found the trans mode for CH3CH2COOH to have the lowest activation energy barrier for this reaction. In this mode the molecule stands up vertically on the surface and both O atoms bind to Pd atoms. In this case, the transition state (Figure 6(25)) is characterized by the hydroxyl H binding to the surface with the O−H distance enlarged to 1.10 Å. O−H bond scission is rather facile with an activation energy barrier of 0.36 eV in agreement to experimental observations on the Pd(111) surface.35 The imaginary frequency of the TS is 538i cm−1, and the elementary step has a mild exothermicity of −0.41 eV. Step 26: CH3CH2COO → CH3CH2 + CO2. For the abstraction of CO2 from CH3CH2COO, we did not use the most stable 14335



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Figure 7. Potential energy surface of eight possible reaction pathways for DCN of propanoic acid on Pd(111). (a) DCN 1 without dehydrogenation steps and DCN 2−5 with α-carbon dehydrogenation steps; (b) DCN 6−8 with both α- and β-carbon dehydrogenation steps. In addition, DCN 3 is shown for comparison only. All species are surface intermediates except for gas phase species identified by subscript (g).

Step 32: CH3CCOOH → CH3CCOO + H. CH3CCOOH has two more possible pathways besides step 13. One of these is O− H cleavage with an activation barrier of 0.95 eV. This step is slight endothermic by 0.07 eV. In the transition state (Figure 6(32)), the hydroxyl O and H has a bond length of 1.59 Å and the imaginary frequency of the TS is 343i cm−1. Step 33: CH3CCOOH → CH3C + COOH. Another reaction pathway is C−OOH cleavage with an activation energy barrier of 0.91 eV. The TS (Figure 6(33)) is characterized by the carboxylic carbon binding to one Pd atom on top and the α-carbon sitting on a neighboring hollow site at a distance of 2.03 Å. The imaginary frequency of the TS is 397i cm−1, and this step has a mild exothermicity of −0.57 eV. Step 34: CH2CHCOOH → CH2CH + COOH. Direct C−COOH scission of CH2CHCOOH is unlikely due to its extremely high activation barrier of 2.08 eV. The TS (Figure 6(34)) is characterized by the carboxylic carbon sitting almost on a hollow site, while the α-carbon binding to the neighboring Pd atom on top at a distance of 2.20 Å. The imaginary frequency of the TS is 528i cm−1, and the step is mildly endothermic by 0.72 eV. Step 35: CH3CCOO → CH3C + CO2. DCX of CH3CCOO involves C−CO2 bond cleavage, which has a relatively low activation barrier of 0.61 eV. In the TS (Figure 6(35)), the C− COO distance is 2.09 Å, which is elongated by 0.57 Å. The imaginary frequency of the TS is 575i cm−1, and the reaction is strongly exothermic by −1.19 eV.

which has an activation barrier of 0.79 eV and is endothermic by 0.10 eV. In the TS (Figure 6(28)), the hydroxyl O and H bind to the same Pd atom with a bond length of 1.59 Å. The imaginary frequency of the TS is 226i cm−1. Step 29: CH3CHCOOH → CH3CH + COOH. Another possible pathway is C−COOH cleavage, which has a relatively high activation barrier of 1.39 eV and is endothermic by 0.32 eV. In the TS (Figure 6(29)), the carboxylic C and α-carbon bind to the same Pd atom with a bond length of 2.03 Å. The imaginary frequency of the TS is 414i cm−1. Step 30: CH3CHCOO → CH3CH + CO2. For direct DCX of CH3CHCOO, we found the less stable adsorption mode (the bidentate mode, Eads = −1.00 eV) to be a preferred initial state to the most stable tridentate mode (Eads = −1.22 eV) shown in Figure 3l. In the TS (Figure 6(30)), the C−COO bond is elongated to 2.10 Å (1.51 Å at the initial state), and the imaginary frequency of the TS is 449i cm−1. The activation barrier of this step is 1.06 eV, and the reaction is exothermic by −0.32 eV. Step 31: CH3CHCOO → CH3CCOO + H. Instead of direct abstraction of CO2, CH3CHCOO may also follow another dehydrogenation step of the α-carbon. The TS (Figure 6(31)) is featured by the coadsorption of α-carbon and H on one Pd atom and a C−H distance of 1.75 Å. The imaginary frequency of the TS is 784i cm−1, and the activation barrier is 0.85 eV. This elementary step has a slight exothermicity of −0.07 eV. 14336



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Figure 8. Potential energy surface of four possible reaction pathways for DCX of propanoic acid on Pd(111). All species are surface intermediates except that gas phase species identified by subscript (g).

Step 36: COOH → CO + OH. COOH is the product of steps 29, 33, and 34 and can either decompose through C−OH scission or O−H scission. The former step has a low activation barrier of 0.41 eV. In the TS (Figure 6(36)), carbon sits on a bridge site and hydroxyl oxygen binds to one Pd atom on top. The C−OH bond distance is 1.88 Å in the TS, and the imaginary frequency of the TS is 297i cm−1. The reaction is exothermic by −0.60 eV. Step 37: COOH → COO + H. COO−H cleavage has an even lower activation barrier of 0.36 eV. In the TS (Figure 6(37)), carbon sits on an atop site and H binds to one bridge site. The C−H bond distance is 1.46 Å in the TS, and the imaginary frequency is 1067i cm−1. The reaction has a mild exothermicity of −0.54 eV. Step 38: OH + H → H2O. Finally, water formation from surface OH and H is a relatively facile process with an activation barrier of 0.67 eV and a heat of reaction of −0.20 eV. In the TS, the O atom binds to one Pd atom on top, while H binds to a neighboring hollow site with an O−H bond length of 1.67 Å. The imaginary frequency of the TS is 564i cm−1.

DCN 5: CH3CH 2COOH → CH3CHCOOH → CH3CCOOH → CH3CCO → CH3C → CH3CH3

DCN 6: CH3CH 2COOH → CH3CH 2CO → CH3CHCO → CH 2CHCO → CH 2CH → CH 2CH 2 DCN 7: CH3CH 2COOH → CH3CH 2CO → CH3CHCO → CH 2CHCO → CHCHCO → CHCH → CH 2CH 2 DCN 8: CH3CH 2COOH → CH3CHCOOH → CH 2CHCOOH → CHCHCOOH → CHCHCO → CHCH → CH 2CH 2

These eight DCN pathways can also be divided into two groups, with pathways 1, 2, 3, 6, and 7 belonging to one group, which starts with initial C−OH cleavage of propanoic acid, and pathways 4, 5, and 8, which start with α-carbon dehydrogenation of the acid molecule prior to C−OH scission. While only detailed microkinetic modeling can unambiguously determine preferred reaction pathways under specific reaction conditions, Figure 7a suggests that pathway DCN 1 is not likely a dominant reaction pathway. The direct DCN step to produce CH3CH2 and CO without H abstraction from the α-carbon has an activation barrier of Ea = 1.60 eV that is too high for the reaction to occur under realistic experimental reaction conditions of 573 K. Instead, if the α-carbon of CH3CH2CO is first dehydrogenated to form CH3CHCO (Ea = 0.88 eV), the DCN step to produce CH3CH and CO becomes more kinetically favorable (Ea = 1.01 eV) as shown in DCN 2. Furthermore, if the α-carbon of CH3CHCO is fully dehydrogenated to CH3CCO (Ea = 0.60 eV), the following DCN step to produce CH3C becomes even more kinetically favorable (Ea = 0.47 eV) as shown in DCN 3. Each dehydrogenation leads to a reduction in the activation barrier of more than 0.5 eV. Parallel to full α-carbon dehydrogenation of CH 3 CHCO, the β-carbon can get dehydrogenated to produce CH2CHCO, which has an even lower activation barrier of 0.56 eV. CH2CHCO can then either decarbonylate to produce CH2CH (DCN 6 in Figure 7b), which requires surmounting a barrier of 0.82 eV or likelier further βcarbon dehydrogenation (Ea = 0.68 eV) to produce CHCHCO followed by DCN to CHCH (Ea = 0.37 eV; DCN 7 in Figure 7b).

4. DISCUSSION 4.1. Reaction Pathways for the DCN of Propanoic Acid on Pd(111). Figure 7 displays the energy diagrams of various DCN pathways of propanoic acid on Pd(111). Eight potentially relevant reaction pathways have been identified. The first one does not involve any dehydrogenation step prior to DCN (DCN 1), the next four involve α-carbon dehydrogenation steps prior to DCN (DCN 2−5), and the remaining three involve both α- and β-carbon dehydrogenation steps prior to DCN (DCN 6−8). DCN 1: CH3CH 2COOH → CH3CH 2CO → CH3CH 2 → CH3CH3 DCN 2: CH3CH 2COOH → CH3CH 2CO → CH3CHCO → CH3CH → CH3CH3 DCN 3: CH3CH 2COOH → CH3CH 2CO → CH3CHCO → CH3CCO → CH3C → CH3CH3 DCN 4: CH3CH 2COOH → CH3CHCOOH → CH3CHCO → CH3CCO → CH3C → CH3CH3 14337



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Clearly, α- and β-carbon dehydrogenation facilitate DCN such that not C−C bond scission but the initial dehydroxylation becomes the rate-limiting step in these pathways (Ea = 0.92 eV). Furthermore, CH3C is a strongly adsorbed intermediate that can possibly be identified spectroscopically (wave numbers (cm−1): 110, 160, 181, 390, 455, 476, 934, 938, 1083, 1322, 1392, 1396, 2971, 3027, 3040). For reaction pathways that involve dehydrogenation of the αcarbon prior to C−OH cleavage, the activation barrier for dehydrogenation (Ea = 0.62 eV) is lower than for dehydroxylation (E a = 0.92 eV). The dehydrogenated species CH3CHCOOH can subsequently be dehydroxylated (Ea = 0.88 eV), which has a slightly lower barrier but is also slightly more endothermic (ΔErxn = 0.53 eV versus 0.42 eV) than the direct dehydroxylation (DCN 4 in Figure 7a). Interestingly, further dehydrogenation of the α-carbon to produce CH3CCOOH as in DCN 5 is not likely due to the high energy barrier for this process (Ea = 1.15 eV). Also, while β-carbon dehydrogenation of CH3CHCOOH to produce CH2CHCOOH is facile (Ea = 0.55 eV), subsequent barriers for dehydroxylation are independent of further α- and/or β-carbon dehydrogenation (DCN 8 in Figure 7b) and are larger than the dehydroxylation barriers without any prior dehydrogenation steps so that we do not believe these pathways to be important. To summarize, DCN 3, DCN 4, and DCN 7 are likely kinetically competitive pathways for the DCN of propanoic acid, and all three pathways involve αand β-carbon dehydrogenation steps. 4.2. Reaction Pathways for the DCX of Propanoic Acid on Pd(111). Figure 8 displays the energy diagrams of various DCX pathways of propanoic acid on Pd(111). We identified four potentially relevant reaction pathways: DCX 1 involves no dehydrogenation step prior to DCX, DCX 2 involves one dehydrogenation step of the α-carbon prior to DCX, DCX 3 involves full α-carbon dehydrogenation prior to DCX, and DCX 4 involves dehydrogenation steps of the α-carbon prior to O−H cleavage and DCX.

activation barrier of Ea = 1.40 eV. α-Carbon dehydrogenation of CH3CH2COO to give CH3CHCOO lowers the DCX barrier to Ea = 0.95 eV, but involves an endothermic dehydrogenation step (ΔErxn = 0.40 eV) with a substantial energy barrier of Ea = 1.22 eV (DCX 2). Interestingly, this dehydrogenation barrier is significantly larger than the dehydrogenation barriers encountered in the DCN pathways of CH3CH2CO → CH3CHCO + H with Ea = 0.82 eV. Further α-carbon dehydrogenation of CH3CHCOO to CH3CCOO reduces the DCX barrier further to Ea = 0.64 eV (DCX 3), but the dehydrogenation step (CH3CHCOO → CH3CCOO + H) involves a relatively high activation barrier of Ea = 0.85 eV. DCX 4 starts with two α-carbon dehydrogenation steps prior to O−H bond scission producing CH3CCOOH. The activation barriers for these steps are as high as 0.62 and 1.15 eV. α-Carbon dehydrogenation furthermore increases the following O−H bond scission barrier to 0.93 eV so that we do not believe dehydrogenation steps occur before O−H bond scission in the DCX mechanism. To summarize, the direct DCX pathway (DCX 1) and various DCX pathways with prior α-carbon dehydrogenation (DCX2/3) are in rather close competition. α-Carbon dehydrogenation might facilitate DCX pathways but likely not to the same degree as in the DCN pathways. C−C and C−H bond scissions are likely rate-limiting in the DCX pathways rather than the C−O scission in the DCN mechanism. Overall, the activation barriers for the DCX pathways seem to be larger than the barriers in the DCN. 4.3. Importance of α- and β-Carbon Dehydrogenation. The previous two subsections illustrate the importance of carbon dehydrogenation steps prior to C−CO/CO2 bond scissions for both DCN and DCX. In the DCN mechanism, CH3CH2−CO bond scission is prohibitively difficult (Ea = 1.60 eV), while CH3CH−CO scission has a much lower activation barrier (Ea = 1.01 eV), and CH3C−CO bond cleavage is spontaneous (Ea = 0.47 eV, ΔErxn = −1.35 eV). Similarly, in the DCX mechanism, CH3CH2−COO bond scission is significantly more difficult (Ea = 1.40 eV) than CH3CH−COO scission (Ea = 0.95 eV) and CH3C−COO bond cleavage, which is spontaneous due to the stability of CH3C (Ea = 0.64 eV, ΔErxn = −1.19 eV). Also, βcarbon dehydrogenation is able to facilitate C−C bond cleavage as illustrated by the CHCH−CO scission (Ea = 0.37 eV), which is by about 0.45 eV more facile than the CH2CH−CO bond scission (Ea = 0.82 eV) that is again by about 0.2 eV more facile than the CH3CH−CO bond scission (Ea = 1.01 eV). This phenomenon of facilitated C−C bond cleavage due to carbon dehydrogenation is in agreement with several experimental observations. For example, Davis and co-workers36 performed isotope experiments for CO elimination from CH3CO and CD3CO (D is deuterium, the isotope of hydrogen) and demonstrated that C−H bond cleavage is the rate-determining step in DCN of CH3CO and that this step must precede C−C bond cleavage. Further, Ponec and co-workers37 observed significant yield of acetaldehyde (CH3CHO) from CO and H2 on a Pd/V2O3 catalyst when CH2Cl2 was added to the feed gas, while no acetaldehyde was observed when CH3Cl was added to the feed. This reaction is the reverse reaction of the C−C bond cleavage in CH2CO and the observations indicate that CO binds to CH2 rather than CH3 to produce C2 oxygenates. Dehydrogenation steps play an important role in the activation of esters too. Recently, Xu and co-workers34 investigated the dissociation of methyl acetate (CH3COOCH3) on Pd(111) by DFT and found that while CH3COOCH3 dissociates by direct C−O bond cleavage to produce CH3CO and OCH3 with an

DCX 1: CH3CH 2COOH → CH3CH 2COO → CH3CH 2 → CH3CH3 DCX 2: CH3CH 2COOH → CH3CH 2COO → CH3C HCOO → CH3CH → CH3CH3 DCX 3: CH3CH 2COOH → CH3CH 2COO → CH3CHCOO → CH3CCOO → CH3C → CH3CH3 DCX 4: CH3CH 2COOH → CH3CHCOOH → CH3CCOOH → CH3CCOO → CH3C → CH3CH3

While, in principle, pathways involving C−COOH scissions could also be considered, we found C−COOH scissions to be extremely difficult and will therefore not discuss these pathways in detail. For example, CH3CH−COOH scission has an activation barrier of 1.39 eV, and only CH3C−COOH scission has a competitive activation barrier of 0.91 eV, but the formation of this species requires overcoming dehydrogenation barriers as large as 1.15 eV (see above). Pathways DCX 1, DCX 2, and DCX 3 share the same initial surface intermediate CH3CH2COO produced by facile O−H cleavage of the acid group (Ea = 0.36 eV). Direct DCX to produce CH3CH2 and CO2 (DCX 1) requires surmounting a high 14338



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these correlations is similar, while the intercepts are expectedly different due to the much stronger adsorption on stepped (211) surfaces than on flat (111) terraces. Interestingly, Alcala et al.41 and Ferrin et al.44 obtained BEP relations for C−O and C−C bond scissions on Pt(111) (ETS = 1.00EFS + 1.45 eV) and Ru(0001) (ETS = 0.88EFS + 1.07 eV), respectively, that yield transition state energies to within 0.2 eV of our BEP relationship for final state energies ranging from −2.3 to −4 eV (Figure 9). More work still needs to be done to determine how dependent BEP relations are on the transition metal.

activation barrier of 1.00 eV, dehydrogenation to intermediate CH3COOCH2 is more facile (Ea = 0.82 eV) and leads to CH3CO−OCH2 bond cleavage with an activation barrier of only 0.61 eV. Finally, we note that dehydrogenation leads both in the DCN (e.g., DCN 3) and DCX (e.g., DCX 4) mechanisms to the formation of the stable surface intermediate CH3C. CH3C is the strongest adsorbing intermediate on the Pd(111) surface (Ea = −5.61 eV) studied, and hydrogenation of CH3C to CH3CH involves a relatively difficult endothermic step (Ea = 1.10 eV, ΔErxn = 0.94). As a result, we expect CH3C species to be next to H and possibly CO to be the main surface species on Pd(111) under reaction conditions. 4.4. BEP Relationships for C−H, C−O, and C−C Bond Cleavages on Pd(111). Brønsted−Evans−Polanyi (BEP) linear energy relationships38,39 are often used to describe the relation between the energy of the transition state (TS) and the final state (FS) for an elementary step. That is, the transition state energy of an elementary step is often found to be linearly dependent on the reaction energy. These relations have been reported for a large variety of molecules on different metal surfaces.40−44 Our calculations suggest a BEP relationship for C−H bond scissions that is ETS = 0.93EFS + 0.77 eV (R2=0.95), which is in reasonable agreement with what Xu et al.34 reported (ETS = 0.98EFS + 0.82 eV, R2 = 0.99) for the dissociation of methyl acetate on Pd(111). The largest error among the data points comes from the CH3CCOOH−H scission, which has an error of 0.18 eV. The BEP relationship for C−O bond scission is ETS = 0.99EFS + 0.48 eV (R2 = 0.99), which is again almost the same as what Xu et al. found for similar compounds (ETS = 0.99EFS + 0.47 eV, R2 = 1.00). The largest error among the data points comes from the CH3CHCO−OH scission with an error of 0.23 eV. Finally, the BEP relation for C−C bond scission is ETS = 0.88EFS + 1.25 eV (R2 = 0.94). The largest error among the data points comes from the CH3CH2−CO scission with an error of 0.56 eV, which highlights the risk of applying BEP relationships to C−C bond scissions. All three relationships have slopes close to unity, which indicates that the structures of the transition states of these elementary steps are similar to their corresponding final states. The large intercept in the C−H BEP relation (0.77 eV) might reflect that the C and H atoms are bonded to the same Pd atom in the transition states as has previously been argued by Xu et al.,34 and which can also be seen in Figures 5(2),(4),(7),(9),(10),(12), (16) and 6(27),(31). By contrast, the small intercept in the C−O BEP relationship (0.47 eV) might originate from the C and O atoms to not share any Pd atom in the transition state as seen in Figures 4b, and 5(5),(11),(13),(17). While the BEP relationship for C−C bond cleavage is large (1.25 eV), transition states with (Figures 5(14),(15) and 6(29),(30),(33),(34)) and without (Figures 5(3),(8),(18) and 6(26),(36)) both C atoms sharing the same Pd atom have been found, possibly explaining the poorer quality of the BEP relationship and suggesting that C−C cleavage is often difficult although the reaction products adsorb strongly. Finally, Wang et al.45 recently proposed a universal BEP relation, ETS = 0.84EFS + 1.92 eV, for C−C, C−O, C−N, N−O, N−N, and O−O bond dissociation reactions on (211) steps of various transition metals such as Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au. Mixing our data for C−O and C−C bond scissions leads to a BEP relation of ETS = 0.77EFS + 0.72 eV with relatively low fitting accuracy of R2 = 0.84. Interestingly, the slope between

Figure 9. BEP relations for C−OH cleavage (green squares and line), C−H cleavage (red dots and line), and C−C cleavage (blue triangles and line) for the DCN and DCX of propanoic acid on Pd(111). The energies of the final state (EFS) and transition state (ETS) are calculated relative to the energy of the corresponding reactant state in the gas phase. EFS is calculated from dissociated intermediates infinitely separated.

5. CONCLUSIONS We systematically investigated from first-principles the DCN and DCX mechanisms for the HDO of propanoic acid to ethane on Pd(111) surfaces. The most kinetically favorable DCN pathways proceed via dehydroxylation of the acid (CH3CH2COOH) to produce propanoyl (CH3CH2CO) and then either full α-carbon dehydrogenation of CH3CH2CO to produce CH3C or first αcarbon dehydrogenation of CH3CH2CO followed by β-carbon dehydrogenation to produce CH2CH. CH3C and CH2CH are then (depending on the amount of hydrogen in the system) hydrogenated to CH3CH3 or CH2CH2. α- and β-carbon dehydrogenation facilitate DCN such that the initial dehydroxylation (and not C−C bond scission) becomes rate-limiting. The most favorable pathway in the DCX mechanism involves first O−H scission of CH3CH2COOH to CH3CH2COO, followed by direct C−CO2 scission or possibly full dehydrogenation of the α-carbon to produce CH3CCOO prior to C−CO2 scission to CH3C and CO2. Dehydrogenation of the α-carbon might again facilitate DCX, but likely not to the same degree as in the DCN pathways. We expect C−C and possibly C−H bond scissions to be rate-limiting in the DCX; although on Pd (111) the DCX is likely slower than the DCN. Finally, considering that the RDO does not involve dehydrogenation steps, the ability of a catalyst surface to dehydrogenate the α-carbon atom is possibly a selectivity 14339



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descriptor for the rational design of transition metal catalysts that preferentially deoxygenate organic acids by removing the oxygen primarily in the form of CO and CO2 versus H2O.

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APPENDIX We have calculated reaction energies of the gas phase DCN and DCX reactions of carboxylic acids to alkanes as a function of carbon chain length (CxH2xO2 + H2 → Cx−1H2x + CO + H2O, x = 2, 3, 4, 5, 6 and CxH2xO2 → Cx−1H2x + CO2, x = 2, 3, 4, 5, 6, respectively). Figure A1 illustrates that the reaction energy is

Figure A1. Reaction energies of the gas phase DCN (CxH2xO2 + H2 → C−‑1H2x + CO + H2O, x = 2, 3, 4, 5, 6) and DCX (CxH2xO2 → Cx−1H2x + CO2, x = 2, 3, 4, 5, 6) of carboxylic acids to alkanes as functions of carbon chain length.

approximately converged with respect to chain length for propanoic acid, suggesting that propanoic acid might be the smallest carboxylic acid model molecule with similar reaction properties to longer carbon chain carboxylic acids.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the USC future fuels program, USC NanoCenter, and the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division under Contract DE-FG02-11ER16268 (DE-SC0007167). Computational resources have been provided by the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the U.S. Department of Energy and in part by TeraGrid under Grant Number TGCTS090100. Finally, computing resources from the USC NanoCenter and USC’s High Performance Computing Group are gratefully acknowledged.

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Theoretical Investigation of the Reaction Mechanism of the

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