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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Hydrogenation of o-Cresol at the Water/Pt(111) Interface Yaping Li, Zhimin Liu, Yingdi Liu, Steven P. Crossley, Friederike C. Jentoft, and Sanwu Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10546 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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The Journal of Physical Chemistry

Hydrogenation of o-Cresol at the Water/Pt(111) Interface Yaping Lia,b, Zhimin Liuc, Yingdi Liua, Steven P. Crossleyc, Friederike C. Jentoftc,1 and Sanwu Wanga,* a

Department of Physics and Engineering Physics, The University of Tulsa, Tulsa, Oklahoma

74104, USA b

Department of Mechanical Engineering, The University of Tulsa, Tulsa, Oklahoma 74104,

USA c

School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman,

Oklahoma 73019, USA 1

Current address: Department of Chemical Engineering, University of Massachusetts, Amherst,

Massachusetts 01003, USA. *

Author to whom correspondence should be addressed. Tel.: +1 918 631 3022. Electronic mail:

[email protected] (S. Wang).

1 ACS Paragon Plus Environment

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

Abstract Catalytic experiments were performed to investigate the hydrogenation of o-cresol to 2methylcyclohexanol on a platinum catalyst in water. It has been observed experimentally both here and in prior studies that intermediate ketones desorb from the catalyst surface in the vapor phase or in hydrocarbon solvents, but they are not observed when reactions are carried out in the aqueous phase. Density functional theory (DFT) was employed to explore the atomic-scale mechanism of o-cresol hydrogenation at the water/Pt(111) interface. Here we show that water plays a dual role of accelerating the rate of hydrogenation of these ketones by shuttling hydrogen from the surface while also decreasing the rate of desorption of the less polar ketone intermediate from the catalyst surface. These two effects, when combined, explain the lack of observation of these ketone intermediates when this hydrogenation reaction is carried out in the aqueous phase.


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1. Introduction To relieve concerns about increasing energy consumption and environmental pollution, biofuels derived from biomass are attracting more attention recently.1-7 Fast pyrolysis is a process in which biomass can be converted into bio-oil. The bio-oil needs upgrading prior to use.8-10 Phenolics, components of bio-oil, represent the lignin-derived fraction in the evaluation of reaction processes to improve the quality of bio-oils. Hydrogenation and/or deoxygenation of phenolics such as phenols, cresols, and guaiacol have been investigated extensively to understand the catalytic reaction mechanisms.11-17 These mechanisms include direct deoxygenation,11-14 partial hydrogeoxygenation,15 and enol-keto tautomerization.16,17 Especially, when the reactions occur at the solid-liquid interface (water surrounding), the specific roles of water during the processes of hydrogenation and/or deoxygenation of phenolics on catalyst surfaces are still challenging questions. Yoon et al. investigated hydrogenation of phenol on Pt and Ni catalysts in water with firstprinciples. They found that a repulsion between the electrons of the liquid water and the diffuse tail of electron density from the metal surface lowers the metal work function by about 1 eV. Liquid environment changes the relative energetics of intermediates and of proton-transfer reactions.18 Experimental work by Xiang et al. showed that water improves the hydrogenation rate of phenol compared with the rate in an organic phase and the final product cyclohexanol is formed due to the higher uptakes of H2 and the lower desorption rates for cyclohexanone on the Raney Ni catalyst in the aqueous system.19 Yoon et al. illustrated that water enhances the hydrodeoxygenation activity of guaiacol on a bifunctional catalyst with the composition of Rh/SiO2-Al2O3.20 Wan et al. investigated catalytic hydroprocessing of p-cresol (one of the cresol isomers) on Pt/C in water, they declared that water is able to affect the reaction pathways 3 ACS Paragon Plus Environment


through changing p-cresol’s adsorption configurations.21 The effects of water in the catalytic chemical reactions are complicated. Further work needs to be done in clarifying the behavior of water in heterogeneous catalysis.22-24 Our recent work showed that for hydrogenation of o-cresol (one of the cresol isomers) in a hydrocarbon solvent, 2-methyl-cyclohexanone is an intermediate product and the final product is 2-methyl-cyclohexanol.25, 26 The experimental observation was adequately reproduced by DFT calculations that did not include a solvent. However, in a water environment, hydrogenation of ocresol produces 2-methyl-cyclohexanol and no intermediate products were observed in the aqueous phase. With density functional theory and ab initio molecular dynamics simulations, we clarified the catalytic chemical reaction mechanism from an atomic scale for hydrogenation of ocresol in water. In water, the barrier to desorb the ketone is greater than the barrier to continue hydrogenation to the alcohol. On the Pt surface alone in the absence of water, the reverse is true, the lowest barrier pathway involves the desorption. Also 2-methyl-cyclohexanone has a higher desorption energy than 2-methylcyclohexanol does in water. Water plays a dual role through shuttling hydrogen atom and affecting charge distribution of metal surface. These explain the lack of observation of these ketone intermediates when this hydrogenation reaction is carried out in the aqueous phase. Our work will contribute to the research on the roles of water in heterogeneous catalysis.

2. Experimental and Computational Methods 2.1 Experimental method 2.1.1 Materials


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o-Cresol (99%), 2-methyl-cyclohexanone (≥ 99.5%), 2-methyl-cyclohexanol (≥ 99.5%) and n-dodecane ((≥ 99%) were obtained from Sigma Aldrich Company. All chemicals were used without further purification. Platinum on carbon (5 wt. % loading, 205931-Aldrich) was used as catalyst. 2.1.2 Catalytic tests The catalytic reactions were run in a specially designed Parr reactor (300 ml) equipped with an ATR-IR spectroscopic probe in the side wall to avoid settling of catalyst particles onto the crystal. In situ ATR-IR spectra of the liquid phase were recorded using a Mettler-Toledo ReactIR™ iC10 module with a K4 conduit and SentinelTM DiCompTM High Pressure ATR–IR probe. The lid of the reactor was equipped with a pressure transducer. For a typical experiment, 120 mL of solvent (weight percent for all additives is shown) and 60 mg of catalyst were charged into the reactor. The reactor was then purged five times with H2. After that, the reactor was pressurized with 200 psi H2 at R.T. and heated to the reaction temperature (373 to 423 K) and kept at this temperature for 10 min before the IR background was collected. The reaction was started by injection of the reactant (o-cresol). The IR spectra were collected every 30 s in the first 2 hours and every 1 min in the following several hours. Temperature and pressure were computer-monitored every 5 s through ParrCom software (with a pressure resolution of 1 psi). The agitator speed was 700 rpm. After the reaction finished and the reactor was cooled down to R.T., the liquid products were also analyzed using an Agilent GC-MS (5820A-5957) with helium as the carrier gas. A HP-5MS 30 m × 250 μm × 0.25 μm capillary column was used. For the aqueous liquid, only hydrogenation products could be identified.



2.2 Ab initio calculations Ab initio calculations were based on density functional theory, the projector augmented wave (PAW) method, and plane-wave basis sets. All the calculations were performed with the use of the Vienna ab initio simulation package (the VASP code).27-32 The functionals within generalized gradient approximations, developed by Perdew, Burke, and Ernzerhof (PBE), were used to describe the exchange and correlation effects.31 The interface includes one o-cresol molecule on Pt(111) and 80 water molecules. The Pt(111) surface consists of five metal layers with each layer containing 25 platinum atoms in the supercell (a 5×5 surface unit cell). For all the optimization calculations, the Pt atoms of the bottom two layers were fixed while the Pt atoms of the top three layers and all the atoms of the molecule were allowed to move until the forces on all the atoms that were allowed to relax were less than 0.05 eV/Å. Reaction pathways and the associated barriers were determined with DFT using the climbing image nudged elastic band method.33, 34 A plane-wave energy cutoff of 400 eV and two special k points in the irreducible part of the two-dimensional Brillouin zone of the 55 surface cell were used for all the calculations.35-37 The initial water configuration was based on the simple point charge model from Gromacs.38 It consisted of 80 water molecules with the density of water (1g/cm3). For the model of o-cresol adsorbed at the water/Pt(111) interface, ab initio molecular dynamics (AIMD) from VASP were executed. The time step for AIMD was 1 fs and the total steps were 60,000 (over a time span of 60 ps).The configurations at every 2 ps were chosen to relax.39, 40 All the following results were based on relaxed structures.


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The simulations neglected the hydrocarbon solvent for two reasons. First, the hydrocarbon solvent does not directly participate in the reactions. Second, explicit inclusion of the hydrocarbon solvent for ab initio calculations are currently too demanding.

3. Results and Discussion 3.1 Experimental observations ATR-IR spectroscopy was used to study the dynamics of the reaction here. The reactant (ocresol) and possible hydrogenation products (2-methylcyclohexanol and 2-methycyclohexanone) in liquid phase can be easily identified and quantified by IR spectroscopy. Our test experiments showed that the IR band intensity was proportional to the concentration of the substances (ocresol, 2-methycyclohexanol, and 2-methylcyclohexanone) in liquid phase. The effect of pressure on the solubility of H2 in water was found to be linear in the pressure range of 0.5-1.5 MPa, consistent with Henry’s law. Considering the solubility of H2 in water and effect of pressure on the solubility of hydrogen, the ratio of amount of H2 in water to that in gas phase was only about 1.3% under the conditions of this study. Therefore, the change of hydrogen pressure in gas phase can also be used to study the kinetics of the hydrogenation reaction in the batch reactor here. Pt/C showed very high activity in the aqueous hydrogenation of o-cresol at 423 K. All the IR bands from o-cresol disappeared within 20 min (see Figure 1 (a)). The appearance of the IR band at 976 cm-1 indicated that 2-methylcyclohexanol was the product. This result was confirmed by GC-MS. The typical IR band of the C=O vibration from 2-methylcyclohexanone (at ca. 1720 cm1)

was not observed. This observation means 2-methylcyclohexanoe was not desorbed in



significant quantities into the aqueous phase. Moreover, the H2 consumption paralleled the concentration change of o-cresol over the entire course of the reaction as shown in Figure 1, consistent with the formation of 2-methylcyclohexanol as the only product. Our previous work showed that 2-methylcyclohexanone was an intermediate product when hydrogenation of ocresol on Pt/C was run in a hydrocarbon solvent at similar conditions.25, 26 This result implies either an inhibited desorption of intermediate ketones in the aqueous phase, or accelerated rates of ketone conversion to form alcohols in the presence of an aqueous phase. In contrast, during hydrogenation of o-cresol in dodecane (see Figure 1 (b)), the intermediate product 2methylcyclohexanone was observed.

Concentration of o-cresol Concentration of methylcyclohexanol

(a)

0.025

Pressure -1 o-Cresol 1102 cm -1 Methylcyclohexanone 1722 cm -1 Methylcyclohexanol 974 cm

(b)

280

0.20

0.020

220 0.10 200

IR band intensity

240

0.15

Pressure (psi)

260

180

0.05

20 30 40 Reaction time (min)

50

260

220 0.010 200 180

160 10

280

240

0.015

0.005

160

0.00 0

300

Pressure (psi)

300

0.25

Concentration (mol/L)

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

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0.000

60

0

60

120

180 240 300 Reaction time (min)

360

420

Figure 1: (a) Hydrogenation of o-cresol on Pt/C in aqueous medium at a temperature of 150 oC. The concentration of o-cresol as extracted from in-situ IR spectra and the total pressure, which reflects the H2 consumption profile, are shown. (b) Hydrogenation of o-cresol in dodecane. The intermediate product 2-methylcyclohexanone is observed. Reactant o-cresol (black stars), intermediate 2-methylcyclohexanone (green hexagons), final product 2-methylcyclohexanol (small wine diamonds), and total pressure (blue dashes).

3.2 The structures of cresol at the interface 8 ACS Paragon Plus Environment

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Three typical o-cresol configurations on a hydrated Pt(111) surface are obtained from the total (60ps/2ps = 30) 30 relaxed structures: One of them represents hydrogen bonding (see Figure 2 (a)). The hydrogen atom from -OH of cresol is bonded with an oxygen atom from a water molecule. The distance between the hydrogen and oxygen atoms is 1.65 Å. At the same time, the bond length for the phenolic -OH group increases from 0.99 Å in a vacuum environment to 1.02 Å in water environment because of hydrogen bonding.25 The carbon-oxygen bond length of adsorbed o-cresol is 1.32 Å whereas it is 1.36 Å for cresol in the vapor phase. The second configuration is that the hydrogen from -OH of cresol is bonded with oxygen atoms from both cresol and water (see Figure 2 (b)). The distances between hydrogen and oxygen atoms are 1.12 Å and 1.33 Å, respectively. Carbon-oxygen bond length of cresol is 1.31 Å. Comparing Figures 2 (a) and 2 (b), the hydrogen atom from –OH is covalent with the oxygen from –OH and is bonded with an oxygen atom from a water molecule in Figure 2 (a). However, the hydrogen atom from –OH is almost in the middle between the two oxygen atoms from both –OH and a water molecule in Figure 2 (b).

The third configuration is that the hydrogen from -OH

dissociates into water to form a hydronium. Due to Grotthuss mechanism, a hydronium is far away from –OH in cresol (see Figure 2 (c)). The distance between the hydrogen of hydronium and oxygen of a water molecule is 1.47 Å. The carbon-oxygen bond length in cresol is 1.26 Å, which means a double bond. The oxygen is hydrogen-bonded with two water molecules. The distances between oxygen and hydrogens of water molecules are 1.73 Å and 1.72 Å, respectively. Comparison of the cresol adsorption structure on Pt(111)

25,41,42

with the one at the

water/Pt(111) surface (see Figure 2 (a)), shows that there is not much difference except the longer -OH bond length and shorter C-O bond length due to hydrogen bonding. In Figure 2 (c), 9 ACS Paragon Plus Environment


(b)

1.12Å

2Å

1.33Å

Å

1.0 2Å 1.6 5Å

(c)

1.

1.7

(a)

7 1.4

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

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73 Å

Figure 2: Three typical configurations of cresol at the water/Pt(111) interface: (a) Hydrogen bonding; (b) Hydrogen is bonded with oxygen atoms from both cresol and water; (c) Hydrogen from -OH dissociates into water to form a hydronium. It is far away from –OH in cresol due to Grotthuss mechanism.

the carbon-oxygen double bond length increases from 1.23 Å in the vacuum environment to 1.26 Å in the aqueous one. In vacuum, the H atom in the hydroxyl group migrates to the Pt surface.42 In these three typical configurations, the configuration in Figure 2 (a) has the highest total energy and the configuration (b) has the lowest one. The energy difference is 77 kJ/mol between the configurations (a) and (b). However, the energy difference between (b) and (c) is just 5 kJ/mol. If hydrogenations of the three configurations happen, it is possible for configurations (a) and (b) to form the final product 2-methylcyclohexanol directly due to -OH group. However, questions arise about the configuration (c). For example, since the carbon-oxygen double bond is already there, after hydrogenation of the aromatic ring, it should be possible that the intermediate product 2-methylcyclohexanone is formed. Why is it not observed in the analyses of the liquid reaction medium (Figure 1 (a))? What kind of roles does water play during the hydrogenation process? In order to answer these questions, we concentrate on the configuration (c). Also, Configurations (a) and (b) were collected at 2 ps and 18 ps, respectively, while Configuration (c) was taken at


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~56 ps (the system is almost at equilibrium at the beginning due to a good initial configuration, and the ratio of the energy fluctuation to the total energy is only 0.0002 for the first 4.6 ps. (see Figure 1S in Supporting Information)). Prior to 18 ps, the OH bond of o-cresol was vibrating, and the other configurations were similar to those shown in Figures 2(a) and 2(b). After 18 ps, the hydrogen from the –OH group was bonded to water and never came back to the o-cresol. Therefore, the other configurations were similar to the one shown in Figure 2(c) during the time period of 18-60 ps. Therefore, Configuration (c) represents the most probable structure and it is worth to be further investigated.

3.3 Dissociation of hydrogen from -OH into water Calculations were performed for understanding the dissociation of the –OH group and the transfer of hydrogen into water. The initial configuration (see Figure 3 (a)) is the same as the Figure 2 (a), which means the OH bond exists. The transition state (see Figure 3 (b)) shows that the hydrogen from -OH goes into the water, the -OH bond length increases from 1.02 Å to 1.18 Å. At the same time, the distance between the hydrogen of -OH and oxygen of water decreases from 1.65 Å to 1.25 Å. The final state is that hydrogen from -OH of cresol is bonded with oxygen of a water molecule (see Figure 3 (c)), the distance between hydrogen and oxygen of a water molecule is 1.05 Å while it is 1.47 Å for hydrogen and oxygen of cresol. During this reaction, the distance between the water (directly participating in the reaction) and its nearest neighbor water molecule is decreasing from 1.63 Å to 1.57Å and 1.45 Å finally. The reaction energy and barrier for this reaction are 2 kJ/mol and 4 kJ/mol, respectively, which explain why the hydrogen from -OH dissociates into water. A hydronium is formed in water such as Figure 3 (c). The charges for the hydronium are: H = +0.68e, H = +0.68e, H = +0.70e and O = -1.36e. Therefore, the hydronium carries positive 0.70e charge. The Figure 2 (c) is not the final state



because the hydrogen atom that forms the hydronium has experienced Grotthuss mechanism and changed the configuration of the system, which is very different from the initial and transition states. Figure 3 (c) has the similar configuration as Figure 3 (a) and (b).

1.0

2Å

1.6

1.18 Å

1.2 5Å

5Å

1.4

1.4 5Å

(c)

(b)

1.5 7Å

(a)

1.6 3Å

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

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7Å

1.05Å

Figure 3: Schematics of hydrogen dissociating into water: (a) the initial configuration; (b) the transition state; and (c) the final configuration. The reacting hydrogen is highlighted in yellow.

3.4 Hydrogenation of cresol in an aqueous phase After the hydrogen atom from -OH of cresol dissociates into water, hydrogenations of the aromatic ring proceed in a stepwise fashion, which has been proved to be one of the most likely reaction mechanisms 15, 25 and also we can compare reaction energies and barriers for vapor and aqueous phases. Figure 4 shows an example of hydrogenation of the aromatic ring. At the initial state (see Figure 4 (a)), a hydrogen atom is adsorbed on a hollow site of Pt(111), the distance between hydrogen and carbon is 2.86 Å. Then hydrogen moves to the carbon, when the distance is 1.51 Å, it reaches the transition-state, and hydrogen is positioned on top of a Pt atom (see Figure 4 (b)). Finally, the hydrogen is bonded to the carbon and the C-H bond length is 1.11 Å. At the same time, the previous hydrogen bonded with carbon has already rotated up. For this reaction, the reaction energy is 37 kJ/mol and the barrier is 78 kJ/mol. Similar hydrogen additions are performed until the final product 2-methylcyclohexanol is formed. The reaction 12 ACS Paragon Plus Environment

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energies and barriers for each reaction are described in Figure 5 (a). Where step 1 is the hydrogenation of the carbon as shown in Figure 4, the following hydrogenations of carbon atoms are according to the clockwise direction. Therefore, step 5 refers to adding hydrogen to the carbon bonded with the methyl group; step 6 is adding hydrogen to the oxygen and step 7 is adding hydrogen to the carbon bonded to the oxygen. It is not possible to finish step 7 before step 6 because oxygen attracts hydrogen much stronger than carbon. Step 8 is that the product desorbs into water. The H atoms in the system that were added to the cresol were introduced on the surface step by step.

(a)

(b)

(c)

1.51Å

2.86Å

1.11Å

Figure 4: Schematics of hydrogenation of a carbon atom in the ring: (a) the initial configuration; (b) the transition state; and (c) the final configuration. The reacting hydrogen is highlighted in yellow.

All the reactions are endothermic except the reaction in step 7, which forms the final product2-methyl-cyclohexanol. This exothermic reaction means the final product is stable, compared with other intermediate products during the hydrogenation process. When the reaction in step 5 is finished, the intermediate product-2-methyl-cyclohexanone is formed. However, in the



following reaction of step 6, the reaction barrier is just 31 kJ/mol, the intermediate product will continue hydrogenation until the final product is generated. In order to better understand why the intermediate product is observed on Pt(111) and not at the Pt(111)/water interface from experimental work, the reaction energies and barriers for the two cases are plot in Figure 5 (b). From the reaction energies of the two cases, one interesting aspect is about the fifth step. On the Pt(111) surface, the reaction is exothermic. The product in the fifth step exactly is the intermediate product, 2-methyl-cyclohexanone. It can be observed experimentally in a

(b)

(a)

Figure 5: (a) Diagram of reaction energies (in blue) and activation energies (in red) for reactions resulting in the formations of 2-methyl-cyclohexanol at the water/Pt(111) interface. The species at the interface are indicated by ‘‘*”; (b) Reaction energies and activation energies in the presence of water (circles) and without the presence of water (squares).

hydrocarbon solvent (Figure 1 (b)).25 However, in an aqueous phase, the reaction in the fifth step is very similar to the previous reactions (step 1 to step 4), and all of them are endothermic.


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Especially, in the two cases, the intermediate products have different adsorption configurations. For example, on Pt(111), the intermediate prefers tilted configuration(see Figure 6 (a)). At the Pt(111)/water interface, it prefers parallel orientation to the surface, because the oxygen is bonded to two water molecules(see Figure 6 (b)). In other words, on Pt(111), the intermediate product prefers keeping its structure. However, at Pt(111)/water, the intermediate product favors continuing hydrogenation to form the final product due to smaller reaction energy and barrier for the following hydrogenation of oxygen, which agrees well with the experimental data.

(b)

1.

77

Å

(a)

8 1.

2.17Å

0Å

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


Figure 6: Configurations of 2-methyl-cyclohexanone on Pt(111) and at water/Pt(111): (a) tilted on Pt(111); (b) parallel at water/Pt(111).

When comparing the barriers for the two cases for the first 5 steps (see Figure 5 (b)), it is noticed that the barriers in steps 1, 2, 5 in water decrease some. However, barriers in steps 3 and 4 increase a little bit. In general, it seems that there is not much difference between the two cases. However, in the last two steps 6 and 7, the barriers in water decrease 39% and 58% compared with the ones on the bare surface, which is the reason that the intermediate product was not observed because it prefers continuing forming the final product 2-methyl-cyclohexanol.



One important point needs mention here: hydrogenation of the oxygen atom of 2-methylcyclohexanone in water involves a water molecule, which is a water-shuttling (see Figure 7) hydrogenation. The water-shuttling process includes a water molecule between the oxygen atom of 2-methyl-cyclohexanone and the hydrogen atom on the surface (see Figure 7 (a)). In the initial state, the distance between the hydrogen atom and oxygen of the water molecule is 2.49 Å while the distance between the hydrogen of water and oxygen of 2-methyl-cyclohexanone is 1.81 Å. In the transition state, the hydrogen on the surface moves to the water, the distances decrease to 1.82 Å and 1.73 Å, respectively. At the same time, although the distance between hydrogen and oxygen of the water molecule keeps the same 1.00 Å, the water molecule has some rotation. In the final state, the hydrogen from the surface is bonded with oxygen of water and the hydrogen from water is bonded with oxygen of 2-methyl-cyclohexanone. The distances are 1.01 Å and 1.04 Å, respectively. The distance between the oxygen of water and its former hydrogen increases from 1.00 Å to 1.56 Å. During this process, the water molecule plays a shuttling role. The barrier for this reaction is 31 kJ/mol, which is 20 kJ/mol smaller than the one without water.

(b)

(c)

81 Å

1.0

1.

6Å

1.82Å

4Å

1.00Å

1.5

3Å

2.49Å 1.00Å

1.01Å

(a)

1.7

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

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Figure 7: Schematics of water shuttling role: (a) the initial configuration; (b) the transition state; and (c) the final configuration. The reacting hydrogen is highlighted in yellow.

3.5 Desorption energies of products 16 ACS Paragon Plus Environment

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In order to calculate the desorption energies of the 2-methylcyclohexanone and 2methylcyclohexanol, the products were inserted in the water network, and ab initio MD simulations were then executed. After the systems reached equilibrium, another 8 ps MD simulations were run for the production. The configuration that had the lowest total energy was relaxed. The desorption energy is defined as the total energy difference between the structure of 2-methylcyclohexanone (or 2–methylcyclohexanol) adsorbed at the Pt(111)/water interface and the structure of 2-methylcyclohexanone (or 2–methylcyclohexanol) completely immersed in water. The desorption energy for 2-methylcyclohexanone is 49 kJ/mol and 37 kJ/mol for 2methylcyclohexanol, which means that it is easier for 2- methylcyclohexanol to desorb from the Pt(111)/water interface into water than it is for 2-methylcyclohexanone. This is another reason why the intermediate product is not observed in the aqueous reaction medium. Comparison of the desorption energy for 2-methylcyclohexanone 49 kJ/mol with the activation energy 31 kJ/mol for continuing reaction on the surface explains that 2-methylcyclohexanone prefers pursuing hydrogenation to form the final product to desorbing from the interface. However, on the bare Pt(111) surface, the desorption energy is 40 kJ/mol and the continuing hydrogenation barrier is 51 kJ/mol. Therefore, the 2-methylcyclohexanone desorbs from the surface and is observed. When 2-methylcyclohexanone is adsorbed on Pt(111) or at the Pt(111)/water interface, the bindings mainly result from the top Pt surface atoms and the hydrogens of

2-

methylcyclohexanone facing the surface.43,44 Through calculating the charges of 2methylcyclohexanone on Pt(111) and at the Pt(111)/water interface, we find that the top Pt surface atoms obtain more negative charge and the hydrogen atoms lose more negative charge at the Pt(111)/water interface than on Pt(111). This charge distribution explains that for 2-



methylcyclohexanone, the desorption energy (49 kJ/mol) at the Pt(111)/water interface is larger than binding energy (16 kJ/mol) on Pt(111) (see Figures 8 (a) and (b)). In Figure 8 (b), the charges for the last 3 Pt atoms (numbers 23 through 25) decrease because water molecules are adsorbed on them.

(a)

(b)

Figure 8: Charges of hydrogen and platinum atoms for 2-methylcyclohexanone adsorbed in a flat configuration on Pt(111) in the absence and the presence of H2O: (a) Positive charge of hydrogen atoms bonded with Pt atoms; (b) Negative charge of the top layer of Pt atoms. Hydrogen atoms 1 through 5 are in the proximity of platinum atoms 19, 14, close to a hollow position (among 3, 8, 9), 8 and 13.

4. Conclusions In conclusion, catalytic experiments show that hydrogenation of o-cresol at the Pt(111)/water interface results in the formation of the final product 2-methylcyclohexanol, and 2methylcyclohexanone as an intermediate product is not observed. Ab initio calculations explain


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the reaction mechanism on the atomic scale. The hydrogen from the hydroxyl group dissociates into water and a carbon-oxygen double bond is generated. 2-Methylcyclohexanone continues hydrogenation to 2-methylcyclohexanol rather than desorbing from the surface, because water shuttles hydrogen for facile hydrogenation and affects the charge distribution of the metal surface. Moreover, the calculation results for desorption energies demonstrate that the ketone is held more strongly on the surface than the alcohol, which also promotes complete hydrogenation.

Acknowledgements This work was supported by the United States Department of Energy under Grant No. DESC0004600. The research used supercomputer resources of the Extreme Science and Engineering Discovery Environment (XSEDE) and the Texas Advanced Computing Center (TACC) at The University of Texas at Austin, of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, of the OU Supercomputing Center for Education & Research (OSCER) at the University of Oklahoma, and of the Tandy Supercomputing Center (TSC) at Tulsa, Oklahoma.

Supporting Information The Supporting Information Available: The energetic evolution with time over the first 4600 time steps (4.6 ps) is shown. This material is available free of charge via the Internet at https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fpubs.acs.org&data=02 19 ACS Paragon Plus Environment


%7C01%7Csanwuwang%40utulsa.edu%7Cfa5f0f45732f4c287a8908d68dc051ce%7Cd4ff013c6 2b74167924f5bd93e8202d3%7C0%7C0%7C636852254493979626&sdata=pn9d%2Fybs4k LandXXVf78RCdcy9F2kZopMiCusddbqcs%3D&reserved=0.

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