An Efficient Reaction Pathway Search Method Applied to the

ARTICLE pubs.acs.org/JPCC

An Efficient Reaction Pathway Search Method Applied to the Decomposition of Glycerol on Platinum Y. Chen, M. Salciccioli, and D. G. Vlachos* Department of Chemical Engineering, Catalysis Center for Energy Innovation and Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 19716-3110, United States ABSTRACT: First-principles-calculation-based understanding of the reaction mechanisms of processing biomass derivatives is currently lacking due to the cost involved in performing computations of multireaction mechanisms of large molecules. We combine semiempirical methods (group additivity and Brønsted Evans Polanyi (BEP) linear free energy relationships) and density function theory (DFT) calculations to provide an efficient search method of key intermediates and reactions and point out the most likely reaction pathways for conversion of glycerol to synthesis gas on Pt(111). The following pathway, C3H8O3 f CHOHCHOHCH2OH f CHOHCHOHCHOH f CHOHCOHCHOH f COHCOHCHOH f COCOHCHOH f CO + COHCHOH, is identified as the most favorable one and a highly dehydrogenated species (COCOHCHOH) is a key intermediate for C C bond cleavage. The (over-)functionalization of biomass changes the nature of the rate-controlling step to dehydrogenation compared to C C scission in hydrocarbons and monols. Our results indicate that for biomass reforming, catalysts with high initial-step dehydrogenation activity should be developed.

1. INTRODUCTION Replacing petroleum resources with biomass feedstocks for energy and chemical production is important in increasing global energy demand and reduction of greenhouse gas emission. Understanding C C, C H, C O, and O H bond scission is a key to selectively convert biomass to specific chemicals and fuels. Glycerol has recently attracted considerable interest as an important intermediate in building our knowledge on biomass processing.1 4 Reforming of glycerol into mixtures of H2 and CO (synthesis gas) or CO2 over platinum-based catalysts is particularly attractive. Synthesis gas can be used in fuel cells and to produce fuels via Fischer Tropsch synthesis and chemicals, such as methanol and ammonia. In addition, H2 is needed for onsite upgrade of fuels and chemicals derived from biomass. Recently, Skoplyak et al.5 reported the first surface science studies of glycerol on Pt(111) and Ni/Pt(111) bimetallic surfaces using temperature programmed desorption (TPD). They observed several desorption peaks of CO and suggested the existence of multiple pathways in glycerol decomposition. Dumesic and co-workers1,6,7 investigated both aqueous-phase reforming (APR) and gas-phase catalytic conversion of glycerol to synthesis gas over Pt-based catalysts. Periodic density functional theory (DFT) calculations provide fundamental insights into the surface chemistry. DFT calculations have extensively been used to investigate the reforming of methanol, ethanol, and ethylene glycol on Pt(111).8 16 Despite the size discrepancy, small alcohols posses many of the characteristics of more complex polyols. Mavrikakis and co-workers12,13 studied r 2011 American Chemical Society

competitive reaction pathways for methanol decomposition on Pt(111) using a DFT-based microkinetic model. They suggested that the initial decomposition step is C H scission followed by a second C H scission and a quasi-simultaneous O H/C H scission to CO. Similar to methanol reforming, Wang et al.8 showed that the initial C H and O H bond dissociation, forming adsorbed CH2CH2OH and CH2CH2O, rather than initial C C and C O bond breaking, is preferred for ethanol reforming on several metals. Unlike methanol decomposition, C C bond-breaking is a vital step in ethanol reforming. Alcala et al.14 examined selected oxygenated C2 species derived from ethanol on Pt(111) and proposed that ketenyl (CHCO) species may be the key intermediate for C C bond cleavage. Likewise, Liu and co-workers10 suggested that C C bond cleavage of ethanol intermediates (CH2CO or CHCO) is favorable on lowcoordinated surfaces [Pt(211) and Pt(100)]. To date, first-principles-calculation-based understanding of the reaction mechanism of reforming of more complex polyols, such as glycerol, is lacking. This in part is due to the tremendous computational cost needed to perform computations of multireaction mechanisms of large molecules. As shown in Figure 1, computational expense increases very rapidly as the number of carbons within a molecule increases. A better understanding of

Received: June 11, 2011 Revised: August 12, 2011 Published: August 18, 2011 18707

dx.doi.org/10.1021/jp205483m | J. Phys. Chem. C 2011, 115, 18707–18720

The Journal of Physical Chemistry C

Figure 1. Estimated computational expense associated with calculating energetics of all intermediates and transition states via DFT as a function of carbon number.

the active pathways and energetics involved in these mechanisms can enable catalyst design and reactor optimization. In this work, we propose a hierarchical refinement methodology that allows for the accurate calculation of important kinetic parameters for the decomposition of glycerol, while substantially saving on computational cost. This methodology involves the initial application of first-principles-calculation-based semiempirical correlations15 for the prediction of thermochemistry of C3Hx(x=0 8)O3 species on Pt(111), followed by the use of periodic DFT calculations to examine the stability and configurations of selected adsorbed C3Hx(x=0 8)O3 intermediates. Furthermore, Brønsted Evans Polanyi (BEP) relationships are applied to estimate activation barriers of relevant dehydrogenation and C C bond cleavage reactions and identify key reactions in reforming. Finally, we refine selected energy barriers of these key reactions via DFT to point out the most likely reaction pathways for conversion of glycerol to synthesis gas.

2. THEORETICAL METHODOLOGY 2.1. Density Functional Theory (DFT) Calculations. The DFT calculations were performed using the SIESTA program.17 The pseudopotentials were generated using the Troullier Martins scheme.18 For all of the atoms, a double-ζ with polarization quality basis set was employed. The exchangecorrelation energy was determined using the generalized gradient approximation (GGA) functional proposed by Perdew, Burk, and Ernzerhof (PBE).19 The localization radii of the basic functions were determined from an energy shift of 0.01 eV. In this work, the spacing of the grid points corresponds to an equivalent 200 Ry energy cutoff in the associated reciprocal space at which value the geometries and energies have converged. Spin polarization was included whenever necessary. The calculated equilibrium lattice constant was 4.02 Å, which is very similar to the experimental one (3.92 Å)20 and previous theoretical results.21,22 The supercell approach was taken to model the Pt surface; four layers of Pt atoms were modeled, and more than 15 Å vacuum spacing was left between slabs. The bottom two layers of metal atoms were fixed at their bulk truncated position, and the top two layers and the adsorbates were relaxed. A p(3 3) unit cell was used in all the calculations and a Monkhorst Pack mesh of (5 5 1) was employed in the k-point sampling. The transition states (TS) were determined by constraining the distance between the two reacting atoms, while optimizing the remaining degrees of freedom.23 The TSs were thus located when all the forces on the atoms had reached a

ARTICLE

Figure 2. Flowchart of hierarchical mechanism development for glycerol decomposition reaction network.

Figure 3. Estimated computational costs of mechanism development using four methods. The “GA, BEP” value represents a methodology only using semiempirical methods for mechanism development (this value is set arbitrarily to 1). The “GA/BEP/DFT” value denotes the methodology used in this work. The “screen conformers via GA/DFT” value represents a methodology using GA for screening conformers and DFT for activation barriers. The “DFT” value estimates the expense of computing all mechanism parameters via DFT. A CPU hour unit is the estimated expense associated with utilizing one processor for one hour.

convergence criterion of 0.15 eV/ Å, and the total energy was a maximum along the reaction coordinate but a minimum with regard to all the other degrees of freedom. 2.2. Mechanism Development for Complex Reaction Networks. It requires significant computational resources to identify the stability of all (84) C3Hx(x=0 8)O3 adsorbates using DFT calculations. In order to reduce computational expense, we developed a group additivity (GA) method to estimate the thermochemistry of 84 C3Hx(x=0 8)O3 adsorbates on Pt(111).15 Within each C3HxO3 isomeric set, the lowest-energy surface species (with respect to gaseous glycerol and the clean slabs) are chosen to identify favorable adsorbed C3HxO3 species. Those C3HxO3 species which are more than 1 eV less stable than the most stable one have been neglected in the reaction network. This leaves 47 stable C3HxO3 species out of 84. We then refine the 47 stable C3HxO3 species accurately using DFT. Consequently, 101 dehydrogenation and 79 C C bond cleavage reactions of these 47 stable C3Hx(x=0 8)O3 species are investigated. Since it is computationally costly to compute transition states (TSs), the two BEP relationships [ETS (eV) = 0.91EFS (eV) + 0.88 and ETS (eV) = 1.00EFS (eV) + 1.90],24 which are developed from dehydrogenation and C C bond cleavage reactions in C2 oxygenates (dehydrogenated species of ethanol), respectively, have been used to predict the activation barriers of these 101 dehydrogenation reactions and 18708

dx.doi.org/10.1021/jp205483m |J. Phys. Chem. C 2011, 115, 18707–18720


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79 C C bond cleavage reactions. Finally, we investigate with DFT calculations those dehydrogenation and C C bond cleavage reactions with low-energy TSs (relative to glycerol in the gas-phase and the clean slabs). In the end, we determine the most probable reaction pathways for glycerol reforming on Pt(111). While not all intermediates and TSs in the mechanism are calculated via DFT, the proposed hierarchical approach allows for an efficient elimination of calculations irrespective of the mechanism of interest. This hierarchical approach is shown in Figure 2. As seen in Figure 3, this approach reduces computation by nearly 2 orders of magnitude without sacrificing accuracy on important mechanism parameters. Savings depend on the initial screening criteria, which in this work were conservative. Larger savings are expected for bigger biomass derivatives, whose number of intermediates and reactions is much larger. The results of this work are presented and discussed in the next sections.

3. RESULTS 3.1. Structure and Adsorption of C3Hx(x=0 8)O3 Intermediates. Using the proposed GA method,15 we estimate the heats of

Figure 4. Comparison of DFT calculated energies to values estimated from GA, for 47 stable C3Hx(x=0 8)O3 species adsorbed on Pt(111).

Table 1. All the Electronic Energies (E) of 84 C3Hx(x=0-8)O3 Intermediates Plus Excess Hydrogen Relative to Gaseous Glycerol and a Clean Pt Slab Using the Group Additivity Method intermediate

E (eV)

C3H8O3a CH2OHCHOHCH2OH

0.68

intermediate

E (eV)

CHOHCHOCHOH

1.00

COCOHCH2O

1.08

CHOHCHOCH2O

0.04

COCHOHCOH

1.78

CH2OCOCH2OH

0.24

COCHOHCHO

1.12

CH2OCHOHCH2OH

0.07

CH2OCOHCH2O

0.03

CHOCHOCHO

0.01

CHOHCHOHCH2OH

1.04

CH2OCHOCH2O

0.93

CH2OHCOHCH2OH

0.90

C3H4O3

CH2OHCHOCH2OH C3H6O3

0.01

COCOHCH2OH COCHOCH2OH

C3H7O3

1.39 0.59

CHOCHOCOH

0.67

CHOCOHCHO

0.82

CHOCOHCOH CHOCOCHOH

1.57 1.20

COHCHOHCH2OH

0.90

COCHOHCHOH

1.86

CHOCOCH2O

0.23

CHOHCOHCH2OH

1.37

COCHOHCH2O

0.67

COHCOCHOH

1.86

CHOCHOHCH2OH

0.52

CHOCOCH2OH

0.21

COHCOCH2O

0.90

CHOHCHOCH2OH

0.35

CHOCOHCHOH

1.55

COHCHOCOH

1.29

CHOHCHOHCHOH

1.41

CHOCOHCH2O

0.46

COHCOHCOH

2.19

CHOHCHOHCH2O

0.65

CHOCHOCHOH

0.34

CH2OCOHCH2OH CH2OCHOCH2OH

0.29 0.61

CHOCHOCH2O CHOCHOHCHO

0.43 0.36

CHOHCOCO CHOCHOCO

1.96 0.77

CH2OCHOHCH2O

0.59

1.67

CH2OHCOCH2OH

0.38

C3H5O3

C3H2O3

CHOCHOHCOH

1.03

CHOCOHCO

COHCOHCHOH

2.16

CHOCOCHO

0.68

COHCHOHCOH

1.69

CHOCOCOH

1.34

CHOCHOHCHOH

1.10

COHCOHCH2O

1.08

CH2OCOCO

0.99

CHOHCOHCH2O

1.06

COHCHOCH2O

0.18

COHCHOCO

1.38

CHOHCOHCHOH

2.14

COHCHOCHOH

1.14

COHCOHCO

2.61

COHCHOHCHOH COHCOHCH2OH

1.76 1.59

COHCOCH2OH CHOHCOCHOH

0.87 1.72

COHCOCHO COHCOCOH

1.34 2.01

COCHOHCH2OH

1.27

CHOHCOCH2O

COHCHOCH2OH

0.50

CH2OCOCH2O

COHCHOHCH2O

0.58

C3H3O3

0.73

CHOCOHCH2OH

0.75 0.21

C3HO3 CHOCOCO

1.44

COCHOCO

1.47

COCOCH2OH

0.96

COHCOCO

2.10

CHOCHOCH2OH

0.17

COCHOCHOH

1.24

COCOHCO

2.37

CHOCHOHCH2O

0.08

COCHOCH2O

0.27

C3O3

COCOHCHOH

2.16

COCOCO

CHOHCOCH2OH a

E (eV)

intermediate

0.73

2.19

Stoichiometric formulas for each set are in bold. 18709



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Figure 5. Top and side view of selected C3HxO3 surface intermediates derived from glycerol dehydrogenation. Pt atoms are represented with larger dark blue spheres, O atoms are represented with red spheres, C atoms are represented with gray spheres, and H atoms are represented with bright blue spheres. This notation is used throughout the paper.

Table 2. Binding Modes, Geometric and Energetic (E) Parameters for Selected C3Hx(x=0-8)O3 Species on Pt(111) Relative to GasPhase Glycerol and a Clean Pt Slaba intermediates

Figure 1 index

E (eV)

O Pt (Å)

a2b2 (O,O)

0.46

2.40, 2.49

binding modes

C Pt (Å)

C O (Å)

C C (Å)

1.44, 1.46, 1.42

1.54, 1.54

C3H8O3b CH2OHCHOHCH2OH

1

C3H7O3 CHOHCHOHCH2OH

2

a1b1 (C)

1.07

2.10

1.42, 1.45, 1.43

1.54, 1.56

CH2OHCOHCH2OH

3

a1b1 (C)

1.03

2.12

1.44, 1.41, 1.42

1.55, 1.56

CH2OHCHOCH2OH C3H6O3

4

a1b1 (O)

0.15

1.43, 1.43, 1.41

1.55, 1.55

2.03

COHCHOHCH2OH

5

a1b2 (C)

0.81

2.14, 2.12

1.35, 1.42, 1.41

1.58, 1.58

CHOHCOHCH2OH

6

a2b2 (C,C)

1.46

2.12, 2.09

1.38, 1.43, 1.45

1.56, 1.54

CHOCHOHCH2OH

7

a2b2 (C,O)

0.93

2.12

1.37, 1.43, 1.45

1.56, 1.55

CHOHCHOHCHOH

8

a2b2 (C,C)

1.59

2.10, 2.09

1.40, 1.42, 1.40

1.54, 1.56

2.09

CHOHCHOHCH2O

9

a2b2 (C,O)

0.72

2.04

2.09

1.40, 1.41, 1.43

1.55, 1.55

CH2OHCOCH2OH

10

a2b2 (C,O)

0.67

2.09

2.14

1.38, 1.44, 1.44

1.55, 1.55 1.56, 1.57

C3H5O3 CHOCHOHCHOH

11

a3b3 (C,C,O)

1.00

2.13

2.15, 2.10

1.39, 1.36, 1.42

CHOHCOHCH2O

12

a3b3 (C,C,O)

1.04

2.07

2.13, 2.09

1.38, 1.41, 1.38

1.55, 1.57

CHOHCOHCHOH

13

a3b3 (C,C,C)

1.81

2.11, 2.11, 2.08

1.38, 1.43, 1.43

1.54, 1.53

COHCHOHCHOH

14

a2b3 (C,C)

1.88

2.11, 2.10, 2.08

1.37, 1.39, 1.43

1.55, 1.55

COHCOHCH2OH

15

a2b3 (C,C)

1.72

2.11, 2.13, 2.07

1.37, 1.39, 1.42

1.54, 1.56

COCHOHCH2OH

16

a2b2 (C,O)

1.27

2.32

1.98

1.24, 1.43, 1.44

1.56, 1.58

CHOCOHCH2OH

17

a3b3 (C,C,O)

1.18

2.13

2.11, 2.18

1.35, 1.40, 1.44

1.53, 1.55

CHOHCOCH2OH CHOHCHOCHOH

18 19

a3b3 (C,C,O) a3b3 (C,C,O)

0.92 1.33

2.45 2.06

2.11, 2.29 2.09, 2.10

1.28, 1.38, 1.44 1.40, 1.41, 1.40

1.55, 1.55 1.55, 1.55

18710



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Table 2. Continued intermediates

Figure 1 index

binding modes

E (eV)

O Pt (Å)

C Pt (Å)

C O (Å)

C C (Å)

2.11, 2.06, 2.11, 2.09

1.37, 1.40, 1.39

1.55, 1.53

2.15, 2.11, 2.11

1.36, 1.40, 1.40

1.54, 1.53

2.11, 2.11

1.39, 1.40, 1.29

1.51, 1.51 1.56, 1.57

C3H4O3 COHCOHCHOH

20

a3b4 (C,C,C)

2.13

CHOCOHCHOH

21

a4b4 (C,C,C,O)

1.63

CHOHCOCHOH

22

a2b2 (C,C)

1.82

COCHOHCHOH

23

a3b4 (C,C,O)

1.96

2.23

2.12, 2.11, 2.09

1.30, 1.38, 1.43

COHCHOCHOH

24

a3b3 (C,C,O)

1.35

2.06

1.92, 2.08

1.31, 1.40, 1.41

1.54, 1.57

COHCHOHCOH

25

a2b3 (C,C)

1.89

2.06, 1.92, 2.14

1.32, 1.37, 1.43

1.53, 1.54

COHCHOHCHO

26

a3b3 (C,C,O)

1.42

COCOHCH2OH C3H3O3

27

a2b2 (C,C)

1.69

COCHOCHOH

28

a3b3 (C,C,O)

1.79

COCOHCHOH

29

a3b3 (C,C,C)

2.18

2.13

2.08

2.10

2.12, 2.13, 2.13

1.36, 1.37, 1.42

1.56, 1.55

2.09, 2.02

1.39, 1.41, 1.22

1.55, 1.58

2.00, 2.09

1.22, 1.41, 1.41

1.56, 1.56

2.03, 2.11, 2.12

1.23, 1.39, 1.39

1.53, 1.54

COCOHCH2O

30

a3b3 (C,C,O)

1.32

2.05

2.03, 2.07

1.22, 1.40, 1.41

1.58, 1.56

COCHOHCHO

31

a3b3 (C,C,O)

1.63

2.17

1.96, 2.15

1.23, 1.41, 1.37

1.57, 1.58

CHOCOHCOH

32

a4b4 (C,C,C,O)

1.53

2.13

2.14, 2.12, 1.97

1.36, 1.41, 1.35

1.54, 1.48

COHCOCHOH

33

a2b3 (C,C)

1.68

2.16, 2.09, 2.08

1.25, 1.36, 1.40

1.52, 1.53

CHOCOCHOH COCHOHCOH

34 35

a3b3 (C,C,O) a2b3 (C,C)

1.32 2.24

2.14, 2.12 1.99, 2.11, 1.99

1.35, 1.29, 1.38 1.22, 1.41, 1.36

1.54, 1.53 1.56, 1.56

COHCOHCOH

36

a3b3 (C,C,C)

1.86

1.96, 1.96, 2.15

1.34, 1.34, 1.39

1.49, 1.50

2.03, 2.07, 2.06, 2.14

1.22, 1.36, 1.39

1.57, 1.53

1.99, 1.92

1.22, 1.40, 1.32

1.59, 1.54

2.14

C3H2O3 COCOHCOH

37

a3b4 (C,C,C)

2.20

COCHOCOH

38

a3b3 (C,C,O)

1.85

COCHOHCO

39

a2b2 (C,C)

2.33

2.00, 2.00

1.22, 1.22, 1.40

1.58, 1.57

2.10

COHCOCOH

40

a2b3 (C,C)

1.79

1.97, 2.00, 2.22

1.33, 1.37, 1.26

1.51, 1.51

CHOHCOCO CHOCHOCO

41 42

a2b2 (C,C) a4b4 (C,C,O,O)

1.82 1.34

2.10, 2.04

2.10, 2.02 2.12, 2.03

1.23, 1.27, 1.38 1.38, 1.35, 1.22

1.54, 1.57 1.60, 1.59

CHOCOHCO

43

a4b4 (C,C,C,O)

1.90

2.11

2.09, 2.00, 2.14

1.22, 1.35, 1.39

1.55, 1.55

2.06

C3HO3 COCHOCO

44

a3b3 (C,C,O)

2.08

2.01, 2.01

1.22, 1.22, 1.38

1.59, 1.59

COHCOCO

45

a2b3 (C,C)

1.64

1.99, 2.07, 2.13

1.23, 1.22, 1.36

1.58, 1.54

COCOHCO

46

a3b3 (C,C,C)

2.46

2.02, 2.02, 2.09

1.22, 1.22, 1.38

1.56, 1.55

47

a2b2 (C,C)

2.07

1.97, 1.98

1.22, 1.22, 1.21

1.61, 1.60

C3O3 COCOCO a

Reference state: gas phase C3H8O3 and a clean Pt slab. Excess H adsorbed on a separate slab. The nomenclature aibj designates that i atoms of the adsorbate shown in parentheses are bonded to j metal atoms on the surface. b Stoichiometric formulas for each set are in bold.

formation of 84 C3Hx(x=0 8)O3 intermediates using glycerol in the gas phase and the clean slabs as a reference. Table 1 lists all the energies of the C3Hx(x=0 8)O3 intermediates plus excess hydrogen atoms adsorbed on a separate slab. On the basis of these GA results and the criteria of section 2.2, we examined 47 stable C3Hx(x=0 8)O3 species via DFT and compared these DFT values to those from GA estimation (shown in Figure 4). It can be seen that the GA method can predict the stability of C3 oxygenates adequately, as previously suggested.15 Herein, we propose the GA method as a screening tool to determine trends in reaction intermediate stability. The selected adsorption geometries of surface intermediates of glycerol are shown in Figure 5 and geometrical and energetic information is summarized in Table 2. Most of the structures of C3 oxygenates on Pt(111) follow specific GA rules as described in detail previously.15 The structures of C3Hx(x=0 8)O3 species on Pt(111) usually achieve a saturated configuration. There are a few exceptions to the GA rules: (1) only one COH group in COHCHOHCOH species binds to a bridge site which is slightly

more stable than two COH groups on bridge sites (within 0.1 eV) and (2) the first C in COCHOHCHOH binds to a bridge site rather than a top site and the first O binds to a top site, which is around 0.15 eV more stable than the adsorption configuration in which the C O group binds to a top site via C and the O comes off the surface to form a CdO double bond. Finally, according to the criteria proposed in section 2.2, we neglected 37 unstable C3Hx(x=0 8)O3 species and selected 47 C3Hx(x=0 8)O3 species to investigate their corresponding dehydrogenation and C C bond cleavage reactions in the following section. 3.2. Dehydrogenation and C C Bond Cleavage Reactions. As mentioned in section 2.2, the activation barriers of 101 dehydrogenation reactions and 79 C C bond cleavage reactions are estimated, as shown in Table 3. The estimated electronic energies from BEPs were used to guide the search for low-energy TSs (with respect to gaseous glycerol and a clean Pt slab) for dehydrogenation and C C bond-breaking reactions. Accordingly, we investigate 18 dehydrogenation reactions and 6 C C 18711



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Table 3. Activation Barriers (Ea) and Reaction Enthalpy Change (ΔE) of (a) C H Dehydrogenation and O H Bond Cleavage Reactions and (b) C C Bond Cleavage Reactions for C3Hx(x=0-8)O3 Species on Pt(111) Estimated from the Linear Free Energy Relation [ETS (eV) = 0.91EFS (eV) + 0.88 and ETS (eV) = 1.00EFS (eV) + 1.90, respectively] and Electronic Energies (E) for TSs Relative to Gas-Phase Glycerol and Clean Slab(s) no.

Ea (eV)

reactions

ΔE (eV)

E (eV)

(a) C H Dehydrogenation and O H Bond Cleavage Reactions C3H8O3 f C3H7O3 + H 1

C3H8O3 f CHOHCHOHCH2OH + H

0.34

0.61

0.12

2

C3H8O3 f CH2OHCOHCH2OH + H

0.38

0.56

0.08

3

C3H8O3 f CH2OHCHOCH2OH + H

1.04

0.32

0.58

4

CHOHCHOHCH2OH f COHCHOHCH2OH + H

1.13

0.25

0.06

5

CHOHCHOHCH2OH f CHOHCOHCH2OH + H

0.71

0.38

0.36

6 7

CHOHCHOHCH2OH f CHOCHOHCH2OH + H CHOHCHOHCH2OH f CHOHCHOHCHOH + H

0.94 0.59

0.15 0.51

0.13 0.47

0.31

C3H7O3 f C3H6O3 + H

8

CHOHCHOHCH2OH f CHOHCHOHCH2O + H

1.21

9

CH2OHCOHCH2OH f CHOHCOHCH2OH + H

0.70

10

CH2OHCOHCH2OH f CH2OHCOCH2OH + H

0.92

11

CH2OHCHOCH2OH f CH2OHCOCH2OH + H

0.49

12

COHCHOHCH2OH f COHCHOHCHOH + H

0.12

1.08

0.68

13

COHCHOHCH2OH f COHCOHCH2OH + H

0.26

0.93

0.54

13 14

COHCHOHCH2OH f COCHOHCH2OH + H CHOHCOHCH2OH f CHOHCOHCH2O + H

0.71 1.16

0.44 0.43

0.10 0.30

15

CHOHCOHCH2OH f CHOHCOHCHOH + H

0.68

0.36

0.78

16

CHOHCOHCH2OH f COHCOHCH2OH + H

0.73

0.30

0.73

17

CHOHCOHCH2OH f CHOCOHCH2OH + H

1.02

0.24

0.44

18

CHOHCOHCH2OH f CHOHCOCH2OH + H

1.00

0.50

0.45

19

CHOCHOHCH2OH f COCHOHCH2OH + H

0.62

0.33

20

CHOCHOHCH2OH f CHOCHOHCHOH + H

0.84

0.09

0.09

21 22

CHOCHOHCH2OH f CHOCOHCH2OH + H CHOHCHOHCHOH f CHOCHOHCHOH + H

0.67 1.14

0.28 0.57

0.26 0.45

23

CHOHCHOHCHOH f CHOHCOHCHOH + H

1.02

0.23

0.57

24

CHOHCHOHCHOH f COHCHOHCHOH + H

0.94

0.33

0.66

25

CHOHCHOHCHOH f CHOHCHOCHOH + H

1.32

26

CHOHCHOHCH2O f CHOCHOHCHOH + H

0.95

0.26

0.23

27

CHOHCHOHCH2O f CHOHCOHCH2O + H

0.94

0.27

0.22

28 29

CHOCHOHCHOH f CHOCOHCHOH + H CHOCHOHCHOH f COCHOHCHOH + H

0.52 0.22

0.62 0.95

0.48 0.78

30

CHOCHOHCHOH f CHOCHOHCHO + H

0.69

0.44

0.31

31

CHOHCOHCH2O f CHOCOHCHOH + H

0.56

0.61

0.48

32

CHOHCOHCHOH f COHCOHCHOH + H

0.80

0.34

1.01

33

CHOHCOHCHOH f CHOCOHCHOH + H

1.02

0.17

0.79

34

CHOHCOHCHOH f CHOHCOCHOH + H

1.15

0.01

0.73

35

COHCHOHCHOH f COHCOHCHOH + H

1.10

0.25

0.78

36 37

COHCHOHCHOH f COCHOHCHOH + H COHCHOHCHOH f COHCHOCHOH + H

1.26 1.30

0.07 0.54

0.62 0.59

38

COHCHOHCHOH f COHCHOHCOH + H

1.32

0.01

0.57

39

COHCHOHCHOH f COHCHOHCHO + H

1.20

40

COHCOHCH2OH f COHCOHCHOH + H

0.86

0.42 0.30 0.58

0.14 0.33 0.11 0.34

C3H6O3 f C3H5O3 + H

0.23

0.31

0.27

C3H5O3 f C3H4O3 + H

0.45 0.40

41

COHCOHCH2OH f COCOHCH2OH + H

1.18

42

COCHOHCH2OH f COCHOHCHOH + H

0.42

0.71

0.85

43

COCHOHCH2OH f COCOHCH2OH + H

0.67

0.44

0.60

18712

0.05

0.69 0.86 0.54



ARTICLE

Table 3. Continued ΔE (eV)

no.

reactions

Ea (eV)

E (eV)

44 45

CHOCOHCH2OH f CHOCOHCHOH + H CHOCOHCH2OH f COCOHCH2OH + H

0.61 0.55

0.43 0.49

0.57 0.62

46

CHOHCOCH2OH f CHOHCOCHOH + H

0.19

0.85

0.74

47

CHOHCHOCHOH f CHOHCOCHOH + H

0.89

0.45

0.43

48

CHOHCHOCHOH f COHCHOCHOH + H

1.30

0.01

0.02

49

COHCOHCHOH f COCOHCHOH + H

1.19

0.03

0.94

50

COHCOHCHOH f COCOHCH2O + H

1.28

0.82

0.85

51 52

COHCOHCHOH f COHCOCHOH + H COHCOHCHOH f COHCOHCOH + H

1.34 1.41

0.45 0.27

0.78 0.72

53

CHOCOHCHOH f COCOHCHOH + H

0.51

54

CHOCOHCHOH f CHOCOHCOH + H

1.17

0.08

0.45

55

CHOCOHCHOH f CHOCOCHOH + H

1.05

0.31

0.58

56

CHOHCOCHOH f COHCOCHOH + H

1.31

0.11

0.50

57

CHOHCOCHOH f CHOCOCHOH + H

1.06

0.48

0.75

58

COCHOHCHOH f COCHOCHOH + H

1.40

0.19

0.56

59 60

COCHOHCHOH f COCOHCHOH + H COCHOHCHOH f COCHOHCHO + H

1.07 1.13

0.21 0.81

0.89 0.82

61

COCHOHCHOH f COCHOHCOH + H

1.00

0.29

0.96

62

COHCHOCHOH f COCHOCHOH + H

0.87

0.42

0.49

63

COHCHOCHOH f COHCOCHOH + H

0.94

0.34

0.41

64

COHCHOHCOH f COCHOHCOH + H

1.08

0.35

0.81

65

COHCHOHCOH f COHCOHCOH + H

1.39

0.03

66

COHCHOHCHO f COCHOHCHO + H

1.09

0.29

67 68

COHCHOHCHO f CHOCOHCOH + H COHCHOHCHO f COHCHOHCO + H

1.06 0.42

69

COCOHCH2OH f COCOHCHOH + H

0.74

70

COCOHCH2OH f COCOHCH2O + H

1.24

71

COCHOCHOH f COCHOCOH + H

1.32

0.07

0.47

72

COCHOCHOH f COCOCHOH + H

1.34

0.04

0.44

73

COCOHCHOH f COCOHCOH + H

1.07

0.08

1.10

74

COCOHCHOH f COCOCHOH + H

1.52

75 76

COCOHCH2O f COCOHCHO + H COCHOHCHO f COCHOHCO + H

0.67 0.01

0.58 1.16

0.65 1.62

77

COCHOHCHO f COCHOCHO + H

0.90

0.18

0.73

78

COCHOHCHO f COCOHCHO + H

0.38

0.75

1.25

79

CHOCOHCOH f COCOHCOH + H

0.53

0.70

1.00

80

CHOCOHCOH f CHOCOHCO + H

0.85

0.35

0.68

81

COHCOCHOH f COHCOCOH + H

1.19

0.13

0.49

82

COHCOCHOH f COCOCHOH + H

1.19

0.12

0.49

83 84

CHOCOCHOH f COCOCHOH + H COCHOHCOH f COCOHCOH + H

0.57 1.35

0.49 0.01

0.74 0.89

85

COCHOHCOH f COCHOCOH + H

1.50

86

COCHOHCOH f COCHOHCO + H

1.30

0.06

0.94

87

COHCOHCOH f COCOHCOH + H

1.04

0.38

0.82

88

COHCOHCOH f COHCOCOH + H

1.42

0.06

0.43

0.20

0.88

C3H4O3 f C3H3O3 + H

0.54

0.11 0.81 0.48 0.37

1.11

0.50 0.34 0.36 1.00 0.95 0.45

C3H3O3 f C3H2O3 + H

0.36

0.41

0.66

0.74

C3H2O3 f C3HO3 + H 89

COCOHCOH f COCOCOH + H

1.31

90 91

COCOHCOH f COCOHCO + H COCHOCOH f COCHOCO + H

0.98 1.20

92

COCHOCOH f COCOCOH + H

1.26

93

COCHOHCO f COCHOCO + H

1.37 18713

0.21 0.28

1.22 0.65

0.22

0.59

0.19

0.95



ARTICLE


no.

reactions

Ea (eV)

E (eV)

94 95

COCHOHCO f COCOHCO + H COHCOCOH f COCOCOH + H

1.15 1.20

0.15 0.24

1.18 0.59

96

CHOHCOCO f COCOCOH + H

0.91

0.24

0.91

97

CHOCHOCO f COCHOCO + H

0.61

0.79

0.73

98

CHOCOHCO f COCOHCO + H

0.58

0.56

1.33

99

COCHOCO f COCOCO + H

1.20

100

COHCOCO f COCOCO + H

1.26

101

COCOHCO f COCOCO + H

1.23

C3HO3 f C3O3 + H 0.02 0.04 0.37

0.88 0.38 1.23

(b) C C Bond Cleavage Reactions C3H8O3 f CHxO + C2H8-xO2 1

C3H8O3 f CH2OH + CHOHCH2OH

1.45

0.44

0.99

2

CHOHCHOHCH2OH f CHOH + CHOHCH2OH

1.66

0.22

0.59

3

CHOHCHOHCH2OH f CHOHCHOH + CH2OH

1.26

0.62

0.20

4

CH2OHCOHCH2OH f CH2OH + COHCH2OH

1.37

0.51

0.34

5

CH2OHCHOCH2OH f CH2OH + CHOCH2OH

0.96

0.92

0.81

6

COHCHOHCH2OH f COH + CHOHCH2OH

0.92

0.96

0.11

7 8

COHCHOHCH2OH f COHCHOH + CH2OH CHOHCOHCH2OH f CHOH + COHCH2OH

0.71 1.42

1.16 0.47

9

CHOHCOHCH2OH f CHOHCOH + CH2OH

1.36

0.53

10

CHOCHOHCH2OH f CHO + CHOHCH2OH

1.46

0.43

0.54

11

CHOCHOHCH2OH f CHOCHOH + CH2OH

1.60

0.29

0.68

12

CHOHCHOHCHOH f CHOH + CHOHCHOH

1.38

0.49

13

CHOHCHOHCH2O f CHOH + CHOHCH2O

1.43

0.45

0.71

14

CHOHCHOHCH2O f CHOHCHOH + CH2O

1.25

0.62

0.53

15

CH2OHCOCH2OH f CH2OH + COCH2OH

0.86

1.03

0.19

16

CHOCHOHCHOH f CHO + CHOHCHOH

0.76

1.12

17

CHOCHOHCHOH f CHOCHOH + CHOH

1.30

0.58

0.30

C3H7O3 f CHxO + C2H7-xO2

C3H6O3 f CHxO + C2H6-xO2 0.09 0.05 0.10

0.21

C3H5O3 f CHxO + C2H5-xO2 0.24

18

CHOHCOHCH2O f CHOH + COHCH2O

1.26

0.62

0.22

19

CHOHCOHCH2O f CHOHCOH + CH2O

1.23

0.64

0.19

20

CHOHCOHCHOH f CHOH + COHCHOH

1.32

0.55

0.49

21

COHCHOHCHOH f COH + CHOHCHOH

1.21

0.65

0.67

22 23

COHCHOHCHOH f COHCHOH + CHOH COHCOHCH2OH f COH + COHCH2OH

1.40 1.21

0.46 0.65

0.48 0.51

24

COHCOHCH2OH f COHCOH + CH2OH

1.54

0.33

0.18

25

COCHOHCH2OH f CO + CHOHCH2OH

0.75

1.13

0.52

26

COCHOHCH2OH f COCHOH + CH2OH

1.05

0.83

27

CHOCOHCH2OH f CHOCOH + CH2OH

1.40

0.49

0.22 0.22

28

CHOCOHCH2OH f CHO + COHCH2OH

1.11

0.78

0.07

29

CHOHCOCH2OH f CHOH + COCH2OH

0.69

1.19

0.23

30 31

CHOHCOCH2OH f CHOHCO + CH2OH CHOHCHOCHOH f CHOH + CHOCHOH

0.75 1.62

1.13 0.24

0.17 0.30

32

COHCOHCHOH f COH + COHCHOH

1.17

0.70

0.96

33

COHCOHCHOH f CHOH + COHCOH

1.55

0.32

0.58

34

CHOCOHCHOH f CHO + COHCHOH

1.10

0.79

0.53

35

CHOCOHCHOH f CHOH + CHOCOH

1.45

0.44

0.18

36

CHOHCOCHOH f COH + COCHOH

1.21

0.67

0.61

37

COCHOHCHOH f CO + CHOHCHOH

0.67

1.19

1.29


18714



ARTICLE


no.

reactions

Ea (eV)

E (eV)

38 39

COCHOHCHOH f COCHOH + CHOH COHCHOCHOH f COH + CHOCHOH

1.37 1.15

0.50 0.72

0.59 0.21

40

COHCHOCHOH f CHOH + COHCHO

1.15

0.72

0.21

41

COHCHOHCOH f COH + CHOHCOH

0.92

0.94

0.98

42

COHCHOHCHO f COH + CHOHCHO

1.25

0.62

0.17

43

COHCHOHCHO f CHO + COHCHOH

0.90

0.97

0.52

44

COCOHCH2OH f CO + COHCH2OH

0.55

1.32

1.14

45

COCOHCH2OH f CH2OH + COCOH

1.10

0.77

0.59

46

COCHOCHOH f CO + CHOCHOH

0.94

0.91

0.84

47

COCHOCHOH f CHOH + COCHO

1.45

0.41

0.34

48

COCOHCHOH f CO + COHCHOH

0.60

1.27

1.58

49

COCOHCHOH f COCOH + CHOH

1.20

0.67

0.97

50

COCOHCH2O f CO + COHCH2O

0.45

1.41

0.87

51

COCOHCH2O f COCOH + CH2O

1.03

0.83

0.29

52

COCHOHCHO f CO + CHOHCHO

0.35

1.53

1.28

53 54

COCHOHCHO f COCHOH + CHO CHOCOHCOH f CHO + COHCOH

0.50 0.90

1.37 0.97

1.12 0.63

55

CHOCOHCOH f COH + CHOCOH

0.87

1.00

0.66

56

COHCOCHOH f COH + COCHOH

0.60

1.25

1.08

57

COHCOCHOH f COHCO + CHOH

0.71

1.15

0.98

58

CHOCOCHOH f CHO + COCHOH

0.67

1.21

0.64

59

CHOCOCHOH f CHOCO + CHOH

1.02

0.86

0.29

60

COCHOHCOH f CO + CHOHCOH

0.66

1.20

1.58

61 62

COCHOHCOH f COCHOH + COH COHCOHCOH f COH + COHCOH

1.16 0.79

0.69 1.07

1.08 1.07

63

COCOHCOH f CO + COHCOH

0.57

1.30

1.63

64

COCOHCOH f COH + COCOH

0.80

1.07

1.40

65

COCHOCOH f CO + CHOCOH

0.52

1.33

1.33

66

COCHOCOH f COCHO + COH

1.02

0.83

0.83

67

COCHOHCO f CO + CHOHCO

0.61

1.24

1.71

68

COHCOCOH f COH + COCOH

0.34

1.51

1.45

69 70

CHOHCOCO f CO + CHOHCO CHOHCOCO f CHOH + COCO

0.12 0.89

1.74 0.98

1.69 0.93

71

CHOCHOCO f CHO + CHOCO

0.95

0.91

0.39

72

CHOCHOCO f CO + CHOCHO

0.89

0.97

0.45

73

CHOCOHCO f CO + CHOCOH

0.61

1.26

1.29

74

CHOCOHCO f CHO + COHCO

0.87

1.01

1.03

75

COCHOCO f CO + COCHO

0.70

1.15

1.38

76 77

COHCOCO f CO + COHCO COHCOCO f COH + COCO

0.01 0.64

1.88 1.22

1.67 1.00

78

COCOHCO f CO + COHCO

0.39

1.47

2.07

79

COCOCO f CO + COCO

0.07

1.79

2.00



C3HO3 f CHxO + C2H1-xO2

C3O3 f CO + C2O2

bond cleavage reactions via DFT. The geometries of TSs for these 18 reactions are shown in Figure 6a. Activation barriers and geometric parameters corresponding to these TSs are listed in Table 4a. We identified four different C H or O H bond scission pathways for the initial dehydrogenation of glycerol. C C bond breaking is unlikely to take place in C3H8O3 due to

the high estimated activation barrier (1.45 eV). At the TSs of dehydrogenation reactions, breaking H atoms are always at a bridge-atop site, as reported in our previous works.25,26 The TSs of dehydrogenation reactions are structurally close to the final product states along the reaction coordinate. We found that (as seen in Table 4a) all of the reaction barriers are below 1 eV except 18715



ARTICLE

listed in Table 4b. At the TSs, CO fragments always sit at atop sites and C C bond lengths are above 2.2 Å. It is very surprising to find that most reaction barriers for C C bond breaking are below 0.70 eV on Pt(111), which means that it is energetically easy for highly dehydrogenated C3 oxygenated intermediates to break a C C bond. Figure 7 compares activation barriers predicted from BEP relationships to DFT calculated values. Reasonable agreement is found.

Figure 6. (a) Top and side view of selected TSs for dehydrogenation and O H bond breaking reactions of C3 oxygenates: (1) H CHOHCHOHCH2OH, (2) CH2OH(H) COHCH2OH, (3) CH2OHCHO (H)CH2OH, (4) CH2OHCHOHCH2O H, (5) H CHOHCOHCH2OH, (6) H CHOHCHOHCHOH, (7) CHOH(H) COHCH2OH, (8) H COHCHOHCHOH, (9) CHOH(H) COHCHOH, (10) H CHOHCOHCHOH, (11) H COHCOHCH2OH, (12) COH(H) COHCHOH, (13) H OCHCOHCHOH, (14) H COHCOHCHOH, (15) H CHOHCOHCOH, (16) H COCOHCHOH, (17) H OCCOHCHOH, and (18) H COHCOHCO. (b) Top and side view of selected TSs for C C bond-breaking reactions of C3 oxygenates: (1) OC CHOHCHOH, (2) OC COHCHOH, (3) OC CHOHCOH (4) OC COHCOH, (5) OC CHOCO, and (6) OC COHCO.

reactions 12 and 18; i.e., dehydrogenation reactions can occur easily under typical conditions. Previous theoretical works10,14,16 indicate that the TSs with the lowest energy for cleavage of the C C bond on Pt correspond to reactions of the adsorbed CHx(x=1 2)CO species. Indeed, our estimated results, listed in Table 3b, show that the activation barriers of C C bond breaking in C3 oxygenated species terminated with a CO group are generally low. Thus, we examine several C C bond-breaking reactions in such C3 species on Pt(111) based on this thermodynamic rule. Their located TSs are shown in Figure 6b and the structural parameters at the TSs and the reaction barriers are

4. DISCUSSION The adsorption of C1 and C2 oxygenates on Pt(111) has been well-studied theoretically.12,14,15,27 In general, the polyol molecules bind to the top site of the metallic surface through oxygen atoms. Accordingly, we consider all the possible configurations for glycerol adsorption on Pt(111), as shown in Figure 8. The adsorption state (3), shown in Figure 8, is the most favorable one, and its binding energy is 0.46 eV. Structurally, this adsorption state and binding energy are similar to the adsorbed ethylene glycol,15 the only difference being the simple substitution of hydrogen in glycerol with a CH2OH group. TPD results reported by Skoplyak et al.5,28 indicate that desorption of ethylene glycol and glycerol from Pt(111) occurs at 266 and 274 K, respectively, which points to the binding energies of ethylene glycol and glycerol on Pt(111) being similar. Our glycerol thermochemistry results are consistent with experimental data. In addition, configuration (2) is only 0.02 eV less stable than the most stable adsorption state, which reveals that the major contribution to binding energy is from the CHOH fragment in glycerol rather than the CH2OH fragment. According to Table 2, the thermodynamically favored intermediates within each C3HxO3 isomeric set are CHOHCHOHCH2OH, CHOHCHOHCHOH, COHCHOHCHOH, COHCOHCHOH, COCHOHCOH, COCOHCOH, COCOHCO, and COCOCO species. All of the favorable intermediates bind to the surface through C atoms. Similarly, the most stable adsorbates derived from ethanol14 and ethylene glycol15 are those which are bound to the surface via C atoms. The initial decomposition of alcohols is still under debate. Most theoretical works concluded that the activation barriers for C H bond scission in small alcohols (e.g., methanol and ethanol) are slightly lower than those for O H bond scission,8,10,13 which means that C H bond cleavage is kinetically favorable over O H bond cleavage. However, using TPD in combination with HREELS, Chen and co-workers28 30 proposed that ethanol and ethylene glycol undergo initial decomposition through O H bond cleavage. This was later confirmed through the use of DFT and TPD of isotopically labeled ethylene glycols.16 Hence, in the initial decomposition of glycerol, we compute all the possible elementary steps of C H and O H bond-breaking processes using DFT, as shown in Figure 9. Likely, C H bond breaking is slightly favorable over O H bond breaking, as mentioned previously. Also, upon loss of one hydrogen atom, CHOHCHOHCH2OH and CH2OHCOHCH2OH are around 1 eV more stable than CH2OCHOHCH2OH and CH2OHCHOCH2OH, which implies that the initial C H bond scission is thermodynamically favored over initial O H bond scission. Although Chen and co-workers28 31 claimed that the initial O H bond scission of ethylene glycol and ethanol is kinetically important, our DFT calculations show that initial cleavage of the C H bond in glycerol is energetically favored from both thermodynamic and kinetic considerations. 18716



ARTICLE

Table 4. Activation Barriers (Ea) and Distances at the TSs of (a) C H Dehydrogenation and O H Bond Breaking Reactions and (b) C C Bond Breaking Reactions of C3HxO3 Species on Pt(111) Using DFT Calculations reactions

Ea (eV)

distance (Å)

(a) C H Dehydrogenation and O H Bond Breaking Reactionsa 1

C3H8O3 f CHOHCHOHCH2OH + H

0.46

1.46

2 3

C3H8O3 f CH2OHCOHCH2OH + H C3H8O3 f CH2OHCHOCH2OH + H

0.50 0.66

1.67 1.52

4

C3H8O3 f CH2OHCHOHCH2O + H

0.78

1.49

5

CH2OHCOHCH2OH f CHOHCOHCH2OH + H

0.83

1.46

6

CHOHCHOHCH2OH f CHOHCHOHCHOH + H

0.61

1.54

7

CHOHCHOHCH2OH f CHOHCOHCH2OH + H

0.82

1.57

8

CHOHCHOHCHOH f COHCHOHCHOH + H

0.61

1.55

9

CHOHCHOHCHOH f CHOHCOHCHOH + H

0.52

1.65

10 11

CHOHCOHCH2OH f CHOHCOHCHOH + H CHOHCOHCH2OH f COHCOHCH2OH + H

0.87 0.73

1.50 1.48

12

COHCHOHCHOH f COHCOHCHOH + H

1.02

1.65

13

CHOHCOHCHOH f CHOCOHCHOH + H

0.80

1.77

14

CHOHCOHCHOH f COHCOHCHOH + H

0.72

1.45

15

COHCOHCH2OH f COHCOHCHOH + H

0.68

1.59

16

CHOCOHCHOH f COCOHCHOH + H

0.43

1.48

17

COHCOHCHOH f COCOHCHOH + H

0.63

1.34

18

COCOHCHOH f COCOHCOH + H

1.05

1.55

(b) C C Bond Breaking Reactionsb

a

1

COCHOHCHOH f CO + CHOHCHOH

1.00

2.49

2

COCOHCHOH f CO + COHCHOH

0.58

2.15

3

COCHOHCOH f CO + CHOHCOH

0.67

2.33

4

COCOHCOH f CO + COHCOH

0.60

2.26

5

COCHOCO f CO + CHOCO

0.40

2.22

6

COCOHCO f CO + COHCO

0.55

2.19

The distances are those between C or O and H at the TSs. b The distances are those between C and C at the TSs.

Figure 8. Top and side view of all the configurations for glycerol adsorption on Pt(111) surface. Values are glycerol adsorption energies on Pt(111).

Figure 7. Comparison of DFT-calculated energies to values estimated from BEP correlations for dehydrogenation and C C bond cleavage reactions for C3Hx(x=0 8)O3 on Pt(111).

Perhaps the discrepancy between theoretical and experimental observations can be explained as follows: (1) adsorbed glycerol needs to be deformed to form one metastable state (MS) before C H bond cleavage occurs (see Figure 9, states 1 and 3), while

O H bond scission can take place directly from adsorbed glycerol, and (2) due to limitations of HREEL experiments, the reduction in intensity of the v(CH) mode cannot be observed clearly. It is interesting to find that the activation barriers of C C bond breaking in highly dehydrogenated C3 oxygenated species and especially those terminated with a CO group are generally low, as shown in Table 3b and Table 4b. Previous work has shown that ethylene glycol dehydrogenated intermediates, which are CO terminated, also have very low activation barriers for 18717



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Figure 9. Energy profiles of all the elementary steps of C H and O H bond breaking in adsorbed glycerol: (1) C3H8O3(g) f C3H8O3ad f CHOHCHOHCH2OHad + Had ; (2) C3H8O3(g) f C3H8O3ad f CH2OCHOHCH2OHad + Had; (3) C3H8O3(g) f C3H8O3ad f CH2OHCOHCH2OHad + Had; (4) C3H8O3(g) f C3H8O3ad f CH2OHCHOCH2OHad + Had.

C C bond scission on Pt.16 In the ethanol decomposition mechanism,14 the TS with the lowest energy for C C bond cleavage is in ketenyl (CHCO) and is very low, only 4 kJ/mol (with respect to gaseous ethanol and a clean Pt slab). Taken together, it appears that C C scission occurring from COterminated species is a generic feature of alcohols and polyols. Comparing dehydrogenation and C C scission barriers, the barriers for early C H and O H bond scission exceed the lowest C C bond scission barriers (found in CO terminated intermediates). This is in stark contrast to ethane, in which C C bond scission appears to be rate-controlling.32 The presence of adjacent oxygen functionalization in biomass withdraws enough electron density to destabilize the corresponding C C bond and lower significantly the barrier, potentially changing the ratecontrolling reaction compared to those of hydrocarbons and monols. Indeed, our previous work also showed that C C bond scission is not kinetically important in ethylene glycol thermal decomposition on Pt(111).16 In this work, we did not investigate any C O bond cleavage reactions, which are predicted to be much more difficult than C C bond cleavage in ethanol decomposition14 on Pt(111). C O bond cleaving reactions should be a minor pathway in decomposition of glycerol to synthesis gas.5 The calculated reaction pathway is schematically summarized in Figure 10. It can be seen that there are multiple pathways for the dissociation of glycerol, a finding which is consistent with experiments.5 Moreover, all the key intermediates during these decomposition pathways are very stable. Proceeding to high levels of dehydrogenation (C3Hx(x=5 8)O3 f C3Hx(x=4 7)O3 + H), C H bond cleaving reactions possess a lower barrier than O H bond cleaving reactions. Most of the C3 oxygenates can easily be dehydrogenated with low barriers (

An Efficient Reaction Pathway Search Method Applied to the

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