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Size Dependence of Vapor Phase Hydrodeoxygenation of m-Cresol on Ni/SiO2 Catalysts Feifei Yang, Dan Liu, Yuntao Zhao, Hua Wang, Jinyu Han, Qingfeng Ge, and Xinli Zhu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04097 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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ACS Catalysis

Size Dependence of Vapor Phase Hydrodeoxygenation of m-Cresol on Ni/SiO2 Catalysts

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Feifei Yang , Dan Liu , Yuntao Zhao , Hua Wang , Jinyu Han , Qingfeng Ge* , Xinli Zhu*

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Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.

‡

Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901, United States.

Corresponding Author *Email: [email protected] (Q. Ge). *Email: [email protected] (X.L. Zhu).

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ABSTRACT: Understanding the effect of metal particle size on the reactions during hydrodeoxygenation of phenolics is of great importance for rational design of a catalyst for selective control of a desirable reaction. To this end, vapor phase hydrodeoxygenation of m-cresol was studied over 5% Ni/SiO2 catalysts with varying Ni particle sizes (2–22 nm) at 300 °C and 1 atm H2. The Ni particle sizes were confirmed by several characterization techniques and the varying surface concentration of terrace, step and corner sites with Ni particle sizes was verified by H2 temperature programmed desorption. Decreasing the Ni particle size from 22 to 2 nm improves the intrinsic reaction rate by 24 times and the turnover frequency (TOF) by 3 times. The TOFs for toluene and methylcyclohexanone/methylcyclohexanol formation increase by 6 and 4 times, respectively, while the TOF for CH4 formation decreases by 3/4, indicating that smaller particles with more defect sites (step and corner) favor deoxygenation and hydrogenation while larger particles with more terrace sites favor C-C hydrogenolysis. Density functional theory study shows that the barrier for direct dehydroxylation of phenol on Ni(111), Ni(211) and defected Ni(211) decreases from 175.6 to 145.6 and then to 120.5 kJ/mol. The results indicate that highly coordinatively unsaturated surface Ni site is responsible for C–O cleavage through facile adsorption and stabilization of –OH in the transition state, thus facilitating deoxygenation toward toluene. Our results indicate that tuning the metal particle size is an effective approach to control reactions during hydrodeoxygenation.

KEYWORDS: m-cresol, hydrodeoxygenation, Ni, particle size, deoxygenation, C-C hydrogenolysis, hydrogenation

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1. INTRODUCTION Catalytic hydrodeoxygenation (HDO) of phenolics derived from biomass lignin is an important step to convert lignin to valuable chemicals and fuels. During HDO, a number of reactions, such as hydrogenation, deoxygenation, C-C hydrogenolysis, transalkylation and others, may take place at the same time depending on the catalysts and reaction conditions.1-10 These reactions would result in high hydrogen consumption and low liquid products yield if they are not controlled properly. Selective deoxygenation of phenolics to aromatics consumes the lowest amount of hydrogen and is the desired reaction route as aromatics are important chemicals and fuel components.11-13 Understanding the reaction mechanism and selective control of the deoxygenation reaction are therefore of great importance. For supported metal catalysts, the particle size of the metal influences the concentration of different surface sites, i.e., terrace, step, corner and kink. These surface sites with different local environments have different abilities for adsorption and bond breaking/formation, and therefore may lead to different catalytic performances.14,15 Previous studies have demonstrated that many reactions, such as benzene hydrogenation,16,17 hydrocarbon hydrogenolysis18 and furfural decarbonylation/hydrogenation,19,20 are structure sensitive, i.e., the catalytic activity and products distribution are strongly influenced by varying metal particle size and/or surface structure. For HDO of phenolics, a few pioneer works devoted to the effect of metal particle size have been reported, with particular attention paid to the noble metal based catalysts.21-24 Newman et al. performed HDO of phenol at 300 °C and 600 psi H2 and 50 psi N2 over two Ru/TiO2 samples.21 They reported that benzene selectivity on Ru/TiO2 with Ru particle size of 1.5 nm 3


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(86% selectivity at 12% conversion) is dramatically higher than that with Ru particle size of 33 nm (18% selectivity at 4% conversion). They ascribed this difference of selectivity mainly to the increased perimeter of the Ru-TiO2 interface, which is believed to be the active site for deoxygenation.10,12 Souza et al. carried out HDO of phenol and m-cresol at 300 °C and 1 atm H2 over two Pd/ZrO2 catalysts with Pd dispersions of 39.8% and 23.7%, respectively.22,23 They reported little difference on the products distribution over the two catalysts. Since the reducible TiO2 and ZrO2 supports may participate in the deoxygenation, it is difficult to draw a clear conclusion on the effect of metal particle size. Recently, Resasco et al. reported HDO of m-cresol in a pulse reactor over two Rh/SiO2 catalysts with Rh particle sizes of 4 and 2 nm, achieved by varying Rh loadings.24 The turn over number (TON) for toluene formation was moderately improved when Rh size was reduced from 4 to 2 nm, while the TON for the formation of cracking products of C2-C6 was found to be little affected by the change of size. The effect of particle size of the non-noble metal catalyst, such as Ni, on HDO of phenolics has also attracted some attentions since Ni is much cheaper and may be more applicable in practice.25-27 Shin et al. studied the HDO of phenol at 150–300 °C and 1 atm H2 using methanol as the solvent on Ni/SiO2 catalysts with Ni particle size ranging from 1.4 to 3.7 nm, again achieved by varying metal loadings.25 They concluded that selective deoxygenation of phenol to benzene is favored on Ni catalysts with larger sizes. It should be noted that the conclusion was based on results with very different phenol conversions. Since the selectivity to aromatics during HDO is influenced by the conversion,28 this conclusion may be questionable. Recently, Teles et al. studied HDO of phenol at 300 °C and 1 atm H2 over two Ni/SiO2 catalysts with Ni dispersion of 5.4% (~20 nm) and 9.0% (~12 nm), respectively.26 They reported that the particle 4


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size of Ni has little effect on the selectivity to different products. It should be noted that the dispersion of the two samples are relatively low and similar to each other, which may be the reason of no observable difference. Mortensen et al. investigated the influence of Ni particle size (5–22 nm) of Ni/SiO2 catalysts in liquid phase HDO of phenol at 275 °C and 100 bar H2.27 They found that increasing Ni size promotes phenyl ring hydrogenation to cyclohexanol, while decreasing Ni size favors cyclohexanol deoxygenation to cyclohexane. It is noted that the high H2 pressure causes thermodynamically complete hydrogenation of phenyl ring before deoxygenation to cyclohexane. Analysis of the above literature results clearly indicate that no consensus has been reached on the effect of metal particle size on HDO of phenolics. The reason may be (1) very different reaction conditions (temperature, pressure and solvent), (2) different metal loadings, (3) different active supports, (4) narrow particle size range investigated, and (5) conclusion was drawn based on different conversion levels. The formation of aromatics from phenolics is thermodynamically favorable at relatively high temperature and low pressure. Under these conditions, Ni catalysts were found to be less active than noble metal catalysts. Ni catalysts also favor undesirable C-C hydrogenolysis to methane, which consumes valuable H2 and lowers yield of liquid products.7,13,26,28 In this work, we performed a combined experimental and theoretical study on the effect of Ni particle size on HDO of m-cresol. The 5% Ni/SiO2 catalysts with a wide range of Ni sizes (2–20 nm) were prepared and tested in HDO of m-cresol at 300 °C and 1 atm H2. The results showed that decreasing Ni particle size improves the activity and selectivity to deoxygenation product of toluene, while greatly inhibits undesirable C-C hydrogenolysis to CH4. Theoretical 5


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calculations showed that the highly coordinatively unsaturated surface Ni sites are active for selective deoxygenation through facile adsorption and stabilization of O atom of –OH in the transition state of dehydroxylation.

2. EXPERIMENTAL 2.1. Catalyst preparation Four Ni/SiO2 samples with the same amount of Ni loading (5 wt.%) but varying Ni particle sizes were prepared by different methods. SiO2 (Sigma-Aldrich, SBET = 200 m2/g) and Ni(NO3)2·6H2O (Strem Chemicals) were used as the support and nickel precursor, respectively. The first sample (denoted as 5Ni-1) was prepared by a modified homogenous precipitation-deposition method.29 Briefly, 5 g of SiO2 was added to 200 mL of deionized water in a three-necked flask, followed by sonication for 30 min. After that, a calculated amount of Ni(NO3)2·6H2O and urea (Tianjin Kemiou Chemical) with a urea/Ni molar ratio of 4 were added to the slurry. The resulting slurry was stirred at room temperature for 1 h in the sealed flask, followed by stirring at 90 °C for an additional 4 h. Finally, the flask was opened and the slurry was stirred at 90 °C until water was vaporized. The second (5Ni-2) and the third (5Ni-3) samples were prepared by incipient wetness co-impregnation of SiO2 with an aqueous solution of Ni(NO3)2·6H2O and urea. The molar ratios of urea/Ni were 8 and 4 for 5Ni-2 and 5Ni-3, respectively. After impregnation for 12 h at room temperature, the samples were dried overnight at 90 °C. The fourth sample (5Ni-4) was prepared by incipient wetness impregnation of SiO2 with an aqueous solution of Ni(NO3)2·6H2O for 12 h, which was then dried overnight at 90 °C. All samples were calcined at 400 °C for 4 h. 6


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2.2. Catalyst characterization Nitrogen adsorption was recorded at 77 K in an automatic Micromeritics Digisorb 2600 analyzer. X-ray powder diffraction (XRD) patterns of pre-reduced catalyst samples were measured on a Rigaku D/max 2500 diffractometer in 2θ range of 20 - 80°, with a Cu Kα radiation source (λ=1.54056A) and a scanning rate of 1°/min. Transmission electron microscopy (TEM) observation was performed on a JEM 2010F field emission system, operating at 200 kV. The number-averaged particle size was calculated by d = ∑nidi/∑ni, where ni is the number of particles with size of di. The hydrogen temperature programmed reduction (H2-TPR) was performed on a Chemisorb 2750 (Micrometrics) equipped with a thermal conductive detector. The catalyst sample was pretreated in flowing N2 at 300 °C for 1 h. After the sample was cooled to room temperature, the gas was switched to 10% H2/Ar (25 mL/min). When the signal was stabilized, the sample was heated to 800 °C at a rate of 10 °C/min. CO pulse chemisorption and H2 temperature programmed desorption (H2-TPD) were performed in a microreactor,30 monitored by a Cirrus 200 mass spectrometer (MKS). The catalyst samples were reduced at 450 °C for 1 h, followed by purging with He (30 mL/min) for 30 min. The temperature was then lowered to room temperature. For CO chemisorption, pulses of 5% CO/He (100 µL) were injected to the sample until a constant CO peak (m/z = 28) area was reached. For H2-TPD, the catalyst was exposed to H2 (30 mL/min) for 30 min and then purged with He (30 mL/min) for 30 min. After that, the sample was heated to 800 °C at a rate of 10 °C/min while the signal of m/z = 2 was monitored.

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2.3. Catalytic activity Vapor-phase hydrodeoxygenation of m-cresol were carried out in a fixed-bed quartz tube reactor, as described in previous work.13,28 Briefly, the catalyst sample was placed in the center of the reactor tube between two layers of quartz wool. Prior to reaction, the catalyst was reduced at 450 °C for 1 h in flowing H2 (30 mL/min). After the temperature was decreased to the reaction temperature (250 - 350 °C), m-cresol was injected to the reaction system by a syringe pump (KDS100, KD scientific). All lines were heated at 230 °C to avoid condensation. The products were quantified online with a gas chromatograph (GC7890B, Agilent). The effluent was trapped by methanol in an ice-water bath, which was analyzed by a gas chromatography-mass spectrometer (Shimadzu QP2010SE). The H2/m-cresol molar ratio was kept to be 60 for all runs. The conversion and yield were reported in molcarbon%.

2.4. Density functional theory calculations of adsorption and deoxygenation of phenol We used the Ni(111), Ni(211) and defected Ni(211) surfaces to simulate the different sites and examined elementary HDO steps using density functional theory (DFT) periodic slab calculations, including spin-polarization, implemented in the Vienna Ab Initio Simulation Package.31,32 The effective cores were described by the projector-augmented wave method.32,33 The Perdew-Burke-Ernzerhof functional was used to evaluate the exchange-correlation energy of interacting electrons.34 The lattice constant of Ni bulk was calculated to be 3.519 Å and was used to build the slab models for Ni(111), Ni(211) and defected Ni(211). Ni(111) surface was modeled with a four layered slab in a p(4 × 4) surface unit cell, as has been detailed in previous work.13 In the present study, we chose a Ni(211) slab consisting of 96 Ni atoms, distributed in 8


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four layers with a vacuum space of 15 Å. The Ni(211) surface, consisting of three-atom-wide (111) terraces and one atom high (100) steps, could be described as [3(111) × (100)] step surface. The unit cell was modeled using a (6 × 4) surface supercell with two terraces. The defected Ni(211) surfaces were constructed by removing one to four atoms from the step (S) or terrace (T) site on the Ni(211) surface, which were referred as Ni(211)-S1, Ni(211)-S2, Ni(211)-T2 and Ni(211)-T4, respectively. The atoms in the bottom two layers were fixed while those in the top two layers were allowed to relax. A cutoff energy of 400 eV and a (2×2×1) k-point grid were found to provide converged results.35 The atomic structures were relaxed using either the quasi-Newton scheme or conjugate gradient algorithm implemented in the VASP code. Phenol instead of cresol was used to reduce the computational cost. The adsorption energy of phenol was defined as: Eads = E(phenol/surface) – E(phenol) – E(bare surface)

(1)

where E(phenol/surface) is the total energy of phenol bound to Ni slab, E(phenol) is the total energy of an isolated phenol molecule, E(bare surface) is the total energy of the bare slab. The transition state for deoxygenation was confirmed to have only one imaginary mode in the normal mode analysis. Although van der Waals interactions are expected to contribute to the adsorption of the molecular species, they were not considered in the present work. This is because PBE functional with wan der waals (vdW) corrections overestimates the adsorption energies of phenolics on transition metals, and different vdW functionals lead to very different adsorption energies.36 In addition, it has been reported that using vdW functionals has little effect on the adsorption structures, reaction energies, and activation energies with respect to without using 9


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vdW functionals.35-38

3. RESULTS AND DISCUSSION 3.1. Catalyst characterizations As expected, different preparation methods have little effect on specific surface areas of the resulting catalysts (Table 1). The specific surface areas are mainly dependent on the surface area of SiO2.

Table 1. The specific surface area, the Ni particle size obtained from XRD and TEM, the Ni dispersion obtained from CO chemisorption, as well as the intrinsic reaction rate and turnover frequency (TOF) of m-cresol conversion on Ni/SiO2 catalysts with different Ni particle sizes.

a

CO/Ni

Intrinsic reaction rate a (µmol•gcat-1•s-1)

TOF (s-1)

1.8 ± 0.6

0.11

13.49

0.14

3.5

4.8 ± 1.3

0.073

6.97

0.11

173

11.0

9.7 ± 3.3

0.041

3.83

0.096

181

22.1

21.8 ± 3.8

0.016

0.57

0.043

Catalyst

SBET (m2/g)

dXRD (nm)

dTEM (nm)

5Ni-1

176

1.8

5Ni-2

163

5Ni-3 5Ni-4

a

measured at conversion < 10%.

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Ni(111)

Ni(200)

Intensity (a.u.)

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Ni(220)

5Ni-4 5Ni-3 5Ni-2 5Ni-1 20

30

40

50

60

70

80

2θ (°)

Figure 1. XRD patterns of the Ni/SiO2 catalysts from different preparation method pre-reduced at 450 C for 1 h, resulting in different Ni particle sizes.

Figure 1 shows XRD patterns of Ni/SiO2 catalysts pre-reduced at 450 °C for 1 h. The 5Ni-4 sample shows sharp diffraction peaks at 2θ of 44.41, 51.85 and 76.37°, which are assigned to Ni(111), Ni(200) and Ni(220), respectively. These peaks are gradually weakened and broadened from sample 5Ni-4 to 5Ni-1, and become barely observable for 5Ni-1. The average particle sizes of Ni, estimated from full width at half maximum of Ni(111) using Scherrer equation, decrease from 22 nm for 5Ni-4 to 1.8 nm for 5Ni-1 (Table 1).

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35 30

Frequency (%)

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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(E) 5Ni-1

(F) 5Ni-2

(G) 5Ni-3

(H) 5Ni-4

1.8±0.6

4.8±1.4

9.7±3.8

21.8±3.8

20 15 10 5 0 0

1 2 3 4 0 2 4 6 8 10 Particle size (nm) Particle size (nm)

5 10 15 20 25 Particle size (nm)

10 20 30 40 50 Particle size (nm)

Figure 2. The TEM images (A to D); and the statistics particle size distributions bar charts with the Gaussian fitted curves (E to H) of Ni/SiO2 catalysts with different Ni particle sizes. All catalysts were pre-reduced at 450 °C for 1 h.

Figure 2A-D displays the representative TEM images of different samples. It appears that Ni particles in spherical shape are deposited on SiO2 support for all samples. However, the particle size varies significantly from the samples with different preparation methods. The particle size distributions (Figure 2E-H), fitted to a Gaussian function, lead to a number-weighted Ni particle size of 1.8 ± 0.6, 4.8 ± 1.3, 9.7 ± 3.3 and 21.8 ± 3.8 nm for 5Ni-1, 5Ni-2, 5Ni-3 and 5Ni-4, respectively. This result is in good agreement with the Ni particle size estimated from the XRD result.

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The dispersion of Ni was measured by CO chemisorption, and the results are summarized in Table 1. It is evident that the Ni dispersion (CO/Ni) drops from 0.11 of 5Ni-1 to 0.016 of 5Ni-4. This trend agrees well with the increase in the Ni particle sizes. The XRD, TEM and CO chemisorption results indicate that catalysts with varying Ni particle sizes were successfully

5Ni-3

100

380

5Ni-2 5Ni-1 200

300

400

480

360

5Ni-4

410

370

330

300

317

prepared.

H2 consumption (a.u.)

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500

600

700

Temperature (°C)

Figure 3. H2-TPR profiles of Ni/SiO2 catalysts with different Ni particle sizes.

The reducibility of different catalysts was studied by H2-TPR, as shown in Figure 3. The 5Ni-4 sample shows a sharp reduction peak centered at 317 °C. From 5Ni-4 to 5Ni-1, the peak splits into several peaks and shifts to higher temperatures. For 5Ni-1, the main peak is centered at 380 °C with shoulders at 300 and 480 °C, respectively. Three stages of reduction of Ni/SiO2 have been reported: the first stage at 200~300 °C is related to phase transition and partial reduction of NiO, the second stage at 300~400 °C is attributed to complete reduction of the large NiO crystallites interacting weakly with the support, and the third stage at > 400 °C is associated with complete reduction of the small NiO crystallites interacting strongly with 13


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support or the nickel silicate species.39-41 In addition, the reduction profiles of different Ni/SiO2 catalysts are strongly dependent on their preparation methods.40 Our result indicates that sample of 5Ni-4 prepared using the incipient wetness impregnation produces the largest Ni particles that interact the weakest with SiO2. The co-impregnation of urea (sample 5Ni-3 and 5Ni-2) improves the dispersion of Ni and its interaction with SiO2. While the sample prepared by modified precipitation-deposition (5Ni-1) results in the smallest Ni particle size with the strongest interaction with SiO2. The quantitative results from TPR indicate that all catalysts could be completely reduced with a H2/NiO molar ratio > 0.95. To minimize the aggregation of Ni, a reduction temperature of 450 °C was selected. Apparently, this temperature ensures the complete reduction of most samples except for 5Ni-1, which shows a tail extending to 600 °C. Additional H2-TPR was carried out for 5Ni-1 after reduction at 450 °C for 1 h. Figure S1 shows a very weak peak at 570 °C, with a H2/NiO ratio of about 0.074. Therefore, this sample was also treated as completely reduced after reduction at 450 °C.

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91

175 240 305

524

MS signal of m/z = 2 (a.u.)

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5Ni-1

5Ni-2

5Ni-3 480

260

76

0

200

5Ni-4 400

600

800

Temperature (°C)

Figure 4. H2-TPD profiles of Ni/SiO2 catalysts with different Ni particle sizes.

Varying Ni particle size is expected to change the fraction of terrace, step and corner site on the surface of a Ni particle, which may in turn influence the HDO activity. The surface property of Ni was probed by H2-TPD. As shown in Figure 4, the H2 desorption profile comprises two regions for all samples, consistent with literature results performed under similar pretreatment conditions.42-44 The high temperature region at 480-524 °C is well resolved. It can be assigned to desorption of the spillovered H on SiO2 by a reverse process.25,42-44 The peak intensity increased as the Ni particle size decreased from 5Ni-4 to 5Ni-1, implying that smaller Ni particles with more surface sites favor more H spillover. On the other hand, the low temperature 15


ACS Catalysis

region (up to 400 °C) contains several components and was not well resolved. Deconvolution of this region results in four peaks at about 91, 175, 240 and 305 °C. Theoretical calculation and electron energy loss spectroscopy studies show that the adsorption of hydrogen becomes stronger from terrace to step and then to the corner sites of Ni.45,46 We therefore tentatively assign these four peaks to H desorption from decomposition of subsurface NiHx (or weakly adsorbed H2), terrace site, step site and corner site, respectively. The significant increase in intensity of the peak at 76~91 °C with increasing Ni particle size from 5Ni-4 to 5Ni-1 is consistent with the assumption that smaller Ni particles provide more sites for the forming subsurface hydrogen.47 1.0

(A)

Terrace NiHx Step Corner

0.8

(B)

Terrace Step Corner Corner

Step

0.6

Proportion

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0.4 Terrace

0.2

0.0 0

5

10

15

20

Particle size (nm)

25 0

5

10

15

20

25

Particle size (nm)

Figure 5. The proportion of different surface site obtained from H2-TPD (A), and theoretical proportion based on a model of cube-octahedron which is shown in the insert picture (B) as a function of Ni particle size.

The distribution of these sites estimated from H2-TPD with varying Ni particle size was plotted in Figure 5A. For comparison, the distribution of surface sites estimated from a cube-octahedron model48 with varying Ni sizes was shown in Figure 5B. Although absolute 16


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values are different, the measured and predicted fractions of different surface Ni sites show a consistent trend, i.e., with increasing Ni particle sizes, the faction of terrace sites increases while that of corner site decreases. The fraction of step sites increases initially and then decreases. We note that the prediction does not include NHx formation. We also note that the concentration of different sites derived from H2-TPD is only semi-quantitative, since the stoichiometry of H to surface Ni atom of different type of sites was not considered. However, the similar trends of different sites derived from H2-TPD and cube-octahedron model confirm that the H2-TPD results provide a valid relative measure of the variation of different sites as particle size is varied.

3.2. Effect of Ni particle size on catalytic performance The conversion of m-cresol on Ni/SiO2 catalysts at 300 °C as a function of space time (W/F) are compared in Figure 6. To achieve ~95% conversion, the required W/Fs were 0.6, 1, 2, and 7 h, respectively, for samples from 5Ni-1 to 5Ni-4. This result suggests that smaller Ni particles are more active than larger ones for conversion of m-cresol. The intrinsic reaction rates, calculated from conversions < 10%, are summarized in Table 1. It is evident that the rate increased 24 times from 0.57 to 13.49 µmol.gcat-1s-1 as Ni particle size decreased from 22 nm for 5Ni-4 to 2 nm for 5Ni-1. Moreover, the turnover frequency (TOF), based on CO chemisorption measured dispersion, increased more than 3 times from 0.043 to 0.14 s-1 when Ni particle size decreased from 22 nm for 5Ni-4 to 2 nm for 5Ni-1. This comparison clearly indicates that the surface Ni atoms in smaller particles are more active than those in the larger ones. 17


ACS Catalysis

100

Conversion (%)

80 60 40

5Ni-1 5Ni-2 5Ni-3 5Ni-4

20 0 0

1

2

3

4

5

6

7

W/F (h)

Figure 6. m-Cresol conversion as a function of W/F. Reaction condition: T = 300 °C, P = 1 atm, TOS = 30 min. 70

(A) 5Ni-1

(B) 5Ni-2

(C) 5Ni-3

(D) 5Ni-4

60

Tol CH4

50

Yield (%)

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Ph MCHone MCHol

40 30 20 10 0 0

20 40 60 80 100 0

20 40 60 80 100 0

Conversion (%)

Conversion (%)

20 40 60 80 100 0

Conversion (%)

20 40 60 80 100

Conversion (%)

Figure 7. Major primary product distribution as a function of m-cresol conversion over (A) 5Ni-1, (B) 5Ni-2, (C) 5Ni-3, (D) 5Ni-4. Reaction conditions: T = 300 °C, P = 1 atm, TOS = 30 min.

18


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ACS Catalysis

Deoxygenation

C-C hydrogenolysis

CH4

Tol C-C hydrogenolysis OH

OH C-C hydrogenolysis

CH4 +

m-Cr Hydrogenation Hydrogenation O

Ph

OH Hydrogenation

MCHone

MCHol

Scheme 1. Apparent major reaction pathways of hydrodeoxygenation of m-cresol on Ni/SiO2 catalysts. Red line indicates the reaction is favored on small Ni particles while blue line indicates the reaction is favored on large Ni particles.

Figure 7 shows the major products distribution as a function of m-cresol conversion on different catalysts. Consistent with previous work,13 toluene (Tol), CH4, phenol (Ph), 3-methylcyclohexanone (MCHone) and 3-methylcyclohexanol (MCHol) are the primary and major products over all catalysts, indicating three parallel reaction pathways in competition: deoxygenation to Tol, C-C hydrogenolysis to CH4 and Ph, and phenyl ring hydrogenation to MCHone and MCHol (Scheme 1). However, the selectivities of these reactions are strongly dependent on the particle size of Ni. The initial selectivity to primary products at a low conversion level of 5.8 ± 0.5% is compared in Figure 8. It is clear that deoxygenation (Tol), hydrogenation (MCHone and MCHol) and C-C hydrogenolysis (CH4 and Ph) products show a similar selectivity on large Ni particle of 22 nm (5Ni-4). Decreasing the Ni particle size to 2 nm (5Ni-1) leads to a significant decrease in the selectivity toward C-C hydrolysis, making it a minor reaction. In contrast, there are remarkable increases in selectivity toward deoxygenation

19


ACS Catalysis

and hydrogenation. The difference in the initial selectivity to different reaction pathways is more evident when the reaction temperature was increased to 350 °C (Figure S2).

40

Tol CH4 Ph MCHone MCHol

30

Selectivity (%)

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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20

10

0 0

5

10

15

20

25

Particle size (nm)

Figure 8. Selectivity toward major products as a function of Ni particle size at 5.8 ± 0.5% m-cresol conversion. Reaction conditions: T = 300 °C, P = 1 atm, TOS = 30 min. W/F was adjusted to maintain a similar m-cresol conversion level.

The yields of hydrogenation products of MCHone and MCHol passed through their respective maximums of ~6% and 2% at intermediate conversion of ~33%, and decreased to zero at higher conversions for all catalysts (Figure 7). Meanwhile, the yield of deoxygenation product of Tol continued increasing. These trends might imply that the formation of Tol stem from MCHone and MCHol by following the typical hydrogenation-deoxygenation (HYD) path, i.e., hydrogenation of m-cresol to MCHone and then to MCHol, deoxygenation (or dehydration) to methylcyclohexene, and finally dehydrogenation to Tol. However, the results from feeding MCHone and MCHol as the reactants on all catalysts (Table S1) indicated that they are not the intermediate products toward Tol or methylcyclohexane by following the HYD path. On the other hand, the formation of these products is due to hydrogenation is faster than the 20


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ACS Catalysis

deoxygenation of m-cresol or its partially hydrogenated surface intermediates.28,38 These products were finally converted to Tol by hydrogenation/dehydrogenation (to partially hydrogenated intermediates) and deoxygenation at high conversion of m-cresol.38 At conversions > 90%, deoxygenation products (Tol and benzene) and C-C products (CH4 and benzene) are the major products for all catalysts (Figure 7 and Table 2). As shown in Table 2, the yields of Tol and CH4 are similar for 5Ni-4 sample with a Ni size of 22 nm. However, the yield of Tol is ~ 8 times higher than that of CH4 on 5Ni-1 with a Ni size of 2 nm. Such differences clearly indicate that varying Ni particle sizes remarkably changes the selectivity toward different reaction pathways. Consequently, decreasing Ni particle size dramatically improves deoxygenation to Tol while greatly inhibits C-C hydrogenolysis to CH4.

Table 2. Product distribution of m-cresol conversion over Ni/SiO2 catalysts with different Ni particle sizes at a similar m-cresol conversion level of ~95%. Catalyst

5Ni-1

5Ni-2

5Ni-3

5Ni-4

W/F (h)

0.6

1

2

7

Conversion (%)

95.6

93.6

98.9

93.9

Tol

67.6

57.6

51.2

30.2

CH4

8.9

13.5

21.6

34.8

Ph

0.8

1.7

0.5

3.5

MCHone

0.3

0.5

0.0

0.2

MCHol

0.1

0.2

0.0

0.1

Benzene

11.6

11.1

16.9

15.6

MCHane a

0.8

0.8

0.6

1.4

Xylenol

1.3

1.5

0.5

0.8

Product yield (%)

21


ACS Catalysis

CHone+CHol b

0.2

0.4

0.0

0.6

C2-C6

4.1

3.0

3.2

4.1

a b

Abbreviation of methylcyclohexane. Abbreviation of cyclohexanone + cyclohexanol.

0.06 Tol CH4

0.05

MCHone+MCHol

0.04 -1

TOF (s )

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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0.03 0.02 0.01 0

5

10

15

20

25

Particle size (nm)

Figure 9. Turnover frequencies of m-cresol conversion to different products. Reaction conditions: T = 300 °C, P = 1 atm, TOS = 30 min. W/F was adjusted to achieve m-cresol conversion < 10%. To further demonstrate how the sizes of Ni particles affect the reactivity of individual reaction pathway, the TOFs for forming typical product of each path are reported in Figure 9 and Figure S3. The TOF for deoxygenation to Tol increased 6 times from 0.008 s-1 for 22 nm Ni of 5Ni-4 to 0.0478 s-1 for 2 nm Ni of 5Ni-1. Meanwhile, the TOF for hydrogenation to MCHone + MCHol increased 4 times from 0.011 s-1 to 0.0447 s-1. In contrast, the TOF for C-C hydrogenolysis to CH4 decreased by 3/4 from 0.0197 s-1 to 0.052 s-1. These results further demonstrate that the surface atoms in smaller Ni particles are more active toward deoxygenation and hydrogenation, while the surface atoms in larger Ni particles are more active for C-C hydrogenolysis.

22


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ACS Catalysis

Table 3. Simulated TOF of different products over different sites. Simulated TOF (s-1) a Site Tol

CH4

MCHone+MCHol

Terrace

0.0002

0.0464

0.0002

Step

0.0087

0.0001

0.0470

Corner

0.2140

0.0001

0.1811

0.98

0.56

0.97

R2 b a

The simulated TOFs on different active sites were solved by minimizing the residual sum of squares (R = ∑(TOFi,sim- TOFi,real)2) between the simulated TOF (TOFi,sim) and the real TOF (TOFi,real) in Matlab. b

R2 refers to the quality of the fit.

The effect of particle size on different reaction paths can be related to variation of concentrations of different surface sites, i.e., terrace, step and corner. Assuming that the same type of sites has the same TOF for each reaction, the total TOF can be obtained by the following equation: TOFi = TOFi,terrace·xterrace + TOFi,step·xstep + TOFi,corner·xcorner

(2)

where TOFi,terrace, TOFi,step and TOFi,corner are the TOF at terrace, step and corner sites, respectively, for product i; xterrace, xstep and xcorner are the fractional concentration of the sites obtained from H2-TPD. This equation could be solved by fitting the experimental data using the method of least squares. The simulated TOFs for different types of sites are summarized in Table 3. A good fit (R2 > 0.97) was obtained for deoxygenation and hydrogenation, while the fit to C-C hydrogenolysis was not good, probably due to CH4 being both a primary and a secondary product from hydrogenolysis. From Table 3, it is evident that the TOF toward Tol increased by an order of magnitude from the terrace to step site, and increased further by 23


ACS Catalysis

another magnitude on the corner site. These results indicate that the corner and step sites are much more active for deoxygenation than the terrace site. Similar trend was observed for reactions toward MCHone + MCHol. However, the TOF for CH4 is 3 orders of magnitude higher on the terrace site than that on the step and corner sites, suggesting that terrace site is the active site for C-C hydrogenolysis. (A) Tol -3.0 Eact=

-3.5

(B) CH4

-3.0 -3.5

36.8

-4.0

Ln (TOF (s-1))

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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-4.0

52.1

-4.5

-4.5

67.9

-5.0

Eact= 153.3 173.3

-5.5 -5.0 -6.0 76.9

-5.5

1.76 1.80 1.84 1.88 1.92

1000/T (K-1)

-6.5

5Ni-1 5Ni-2 5Ni-3 5Ni-4 1.76

181.5 216.9

1.80

1.84

1.88

1000/T (K-1)

Figure 10. Arrhenius plots for toluene formation (A), and CH4 formation (B) for hydrodeoxygenation of m-cresol on Ni/SiO2 catalysts at 250 – 300 °C. Reaction conditions: P = 1 atm, TOS = 30 min, conversion was < 10% by adjusting the W/F. The insert number shows the apparent activation energy with a unit of kJ/mol.

The Arrhenius plots of Tol and CH4 formation for HDO of m-cresol at 250–300 °C are reported in Figure 10. The apparent activation energy (Eact) for Tol formation decreased by a half from 76.9 to 36.8 kJ/mol when the Ni particle sizes decreased from 22 nm for 5Ni-4 to 2 nm for 5Ni-1. A reversed trend was found for CH4 formation, i.e., an increase of Eact from 153.3 to 216.9 kJ/mol was observed when the Ni particle sizes decreased from 22 to 2 nm.

3.3. DFT study on adsorption and deoxygenation of phenol 24


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ACS Catalysis

The above results clearly demonstrate that varying Ni particle size, i.e., changing concentrations of different types of surface sites (terrace, step, and corner) can greatly change both the activity and selectivity toward different reaction pathways for HDO of m-cresol. To corroborate the argument, we built the Ni(111), Ni(211) and defected Ni(211) surfaces to simulate surface terrace, step and corner sites. Their influence on adsorption and deoxygenation (or dehydroxylation) of phenol was examined. The typical structures with the adsorbates in stable adsorption structures (initial states (IS) or final states (FS)) as well as the transition states (TS) are shown in Figure 11 and Figure S4, with structure parameters being summarized in Table 4. In good agreement with previous work,49,50 vertical adsorption of phenol on different Ni surfaces through O atom was found to be weak. Indeed, phenol prefers flat adsorption on a bridge site of the Ni(111) surface through phenyl ring, with O atom tilting away from the surface.13 The adsorption energy of phenol on Ni(111) is calculated to be -116.7 kJ/mol, which is consistent with the literature results of phenol or benzene adsorption on Ni(111) surfaces.51-57 However, the O atom should adsorb on the Ni surface to facilitate C-O cleavage. This is the case for the TS of dehydroxylation, but resulting in a high barrier of 175.6 kJ/mol and an endothermic process of 54.0 kJ/mol.13

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Figure 11. Side view (Top) and top view (bottom) of structures of initial state (IS), transition state (TS) and final state (FS) for dehydroxylaiton of pehnol on (A) Ni(211); (B) Ni(211)-T4; and (C) Ni(211)-S2 surfaces, respectively. Carbon atoms were labeled with numbers clockwise. The surface Ni, bulk phase Ni, O, H, C atoms are in blue, dark blue, red, white, and grey, respectively. The red cirles represent atoms removed from the surface. The unit of distance is Å.

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ACS Catalysis

Table 4. The adsorption parameters at initial states, and the distances of C-O and O-Ni in transition and final states, as well as the adsorption energy (Ead), activation energy (Eact), and reaction energy (Erxn) for dehydroxylation of phenol on different surfaces. Surface

Ni(111)

Ni(211)

Ni(211)-T2

Ni(211)-T4

Ni(211)-S1

Ni(211)-S2

C1-O (Å)

1.37

1.37

1.37

1.37

1.38

1.39

C1-Ni3 (Å)

2.21

2.10

2.10

2.12

2.10

2.08

C2-Ni4 (Å)

2.07

2.13

2.07

2.08

2.07

2.06

C3-Ni1 (Å)

2.06

2.07

2.04

2.04

2.05

2.06

C4-Ni1 (Å)

2.07

2.05

2.04

2.05

2.03

2.03

C5-Ni2 (Å)

2.05

2.03

2.02

1.99

2.01

2.02

C6-Ni3 (Å)

2.03

2.01

2.14

2.11

2.13

2.13

∠O-C1-plane (°)

30.1

24.4

23.7

23.6

23.5

23.3

C-O (Å)

2.09

2.27

2.30

2.35

2.35

2.28

O-Ni (Å)

2.22

1.88

1.87

1.86

1.85

1.86

C-O (Å)

3.14

3.83

3.99

3.97

3.86

5.95

O-Ni (Å)

2.22

1.90

1.90

1.90

1.90

1.89

Ead (kJ/mol)

-116.7

-159.1

-171.7

-194.8

-161.1

-158.2

Eact (kJ/mol)

175.6

145.6

145.6

139.8

142.7

120.5

∆Erxn (kJ/mol)

54.0

13.5

5.8

6.8

17.4

33.8

Initial state

Transition state

Final state

As shown in Figure 11A, on the stepped Ni(211) surface phenol also favors a flat adsorption on a bridge site on the terrace with O atom pointing to the step and away from the terrace. Compared to adsorption on flat Ni(111), phenol on the bridge site is slightly shifted toward the 27


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Page 28 of 38

edge of terrace. This shift results in the nearest distance between O and terrace surface Ni to increase to 3.16 Å, which is significantly longer than the nearest distance between O and Ni in the step (2.84 Å). In addition, the C1-Ni bond length is reduced by 0.11 Å to 2.10 Å, and the angle of O-C1-plane is decreased by ~6° to 24.4° (Table 4). These structural changes suggest that the interaction between phenyl ring and surface is strengthened while the repulsion between O and surface is reduced, possibly due to the increased d state energy levels of Ni atoms in the Ni(211) terrace site.58 Indeed, the calculated adsorption energy is -159.1 kJ/mol on Ni(211), which is much stronger than that of -116.7 kJ/mol on Ni(111). The adsorption configuration on Ni(211) is similar to those on Pt(211), Ru(211) and Rh(533), but is different from that on Rh(211).59-61 We also tested the adsorption configuration of phenyl ring on terrace with O pointing 180° away from the step, similar to that on Rh(211). However, the result (Figure S5) showed that this configuration is less stable (Ead = -145.6 kJ/mol). From IS of the most stable configuration on Ni(211), the O atom of –OH moves onto the atop of the nearest Ni atom in the step and is stabilized in TS with an O-Ni5 bond length of 1.88 Å, while C-O bond length increases to 2.27 Å in TS. In FS, the –OH migrates further to adjacent bridge site in the step, while the phenyl ring moves toward the edge of terrace slightly with the C-O distance increased further to 3.83 Å. It should be noted that in TS the –OH is on the atop of step on Ni(211), which is different from that on the terrace on pure Ni(111). Furthermore, O-Ni bond length of 1.88 Å in TS on Ni(211) is shorter than that (2.22 Å) on pure Ni(111) surface. Such differences suggest that the coordinatively unsaturated step Ni atom is more oxophilic than that in pure Ni(111) and thus more effectively stabilizes the TS. Consequently, the barrier

28


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ACS Catalysis

for dehydroxylation of phenol on Ni(211) is reduced to 145.6 kJ/mol with respect to Ni(111), and the reaction becomes less endothermic (Erxn = 13.5 kJ/mol). To further demonstrate how the location of defect sites affect the adsorption and deoxygenation of phenol, two conditions were tested: defect sites in the terrace and in the step of Ni(211). When two atoms or four atoms in the terrace of Ni(211) are removed (denoted as Ni(211)-T2 and Ni(211)-T4, respectively), phenol still prefers flat adsorption on the bridge site with a similar configuration as that on perfect Ni(211) (Figure S4A and Figure 11B). However, the adsorption energy is remarkably increased to -171.7 and -194.8 kJ/mol on Ni(211)-T2 and Ni(211)-T4, respectively (Table 4). The results imply that creating defect sites adjacent to the bridge site where phenol is adsorbed, leads to an increase in energy level in d state of the bridge site58 and therefore results in enhanced interaction between d electrons in Ni with π electrons in the phenyl ring.50 Along the reaction coordinate of dehydroxylation on Ni(211)-T2 and Ni(211)-T4, the structural evolutions are quite similar to that on perfect Ni(211). Compared to perfect Ni(211), the barrier for dehydroxylation is almost the same on Ni(211)-T2 and only slightly reduced on Ni(211)-T4. The results clearly indicate that defect sites in terrace improve the adsorption strength of phenol but have little effect on C-O cleavage. On the other hand, when phenol is adsorbed on defected Ni(211) with one or two atoms removed from the step, denoted as Ni(211)-S1 and Ni(211)-S2 (see Figure S4B and Figure 11C), the adsorption energy is found be similar to that on perfect Ni(211) (see Table 4). This suggests that defect site in the step has little effect on the electronic structure of Ni atoms away in the terrace, and thus has little effect on their interaction with phenyl ring. A close examination of the adsorption structure revealed that the distances between the O atom and the 29


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Page 30 of 38

nearest Ni in the step and in the terrace on Ni(211)-S2 are reduced notably to 2.72 and 3.12 Å, respectively, relative to Ni(211) and Ni(211)-T4 (Figure 11C). This difference may indicate that repulsion between O and Ni atoms in both step and terrace is reduced. The highly coordinately unsaturated Ni5 atom in the step due to removal of two adjacent Ni atoms (see Figure 11C) becomes highly oxophilic, causing the C-O bond of phenol lengthened slightly to 1.39 Å on Ni(211)-S2. Along the reaction coordinate, the O atom moves to atop of Ni5 in the step in TS and further to vicinal bridge site in FS while the phenyl ring moves toward edge of terrace slightly. The reaction barrier of this dehydroxylation step decreases significantly to 120.5 kJ/mol on Ni(211)-S2, probably due to the IS resembles TS more closely than those on Ni(111) or perfect Ni(211). On the other hand, the reaction energy on Ni(211)-S2 slightly increased (Table 4), which may be a result of longer diffusion path of surface –OH between TS and FS due to the absence of bridge site next to Ni5 along the step edge (Figure 11C).

3.4. Discussion Although aromatics are observed as a major product,7-10,12,13,21 it is generally believed that direct deoxygenation of phenolics to aromatics is not possible due to the strengthened Caroamtic-O bond resulting from delocalization effect.9,62 Indeed, the high barrier of 175.6 kJ/mol for direct dehydroxylation of phenol on Ni(111) also implies that it is unlikely under current mild conditions. Our DFT results suggest that C-O cleavage requires both C and O adsorption on the surface in TS. Since O atom of phenol is repelled away from the Ni(111) in IS, adsorption of O onto the surface requires it to overcome a high barrier, thus leading to high barrier for direct dehydroxylation. On stepped Ni(211), O atom is stabilized on the step site in 30


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ACS Catalysis

TS instead of the terrace site on Ni(111), leading to lowered barrier for direct dehydroxylation of 145.6 kJ/mol. The result is in good agreement with theoretical results of phenol or cresol deoxygenation on Ru(211), Rh(211) and Rh(533) surfaces.24,59,61 When defect site is present in step of Ni(211), the structure of IS resembles more closely with that of TS, resulting in a further lowered dehydroxylation barrier of 120.5 kJ/mol. The lowered activation barrier of the defected Ni(211) suggests that the highly coordinately unsaturated Ni sites may be accessible by direct dehydroxylation reaction under mild conditions. The decrease in activation barriers for dehydroxylation from terrace to step and then to defected step (Table 4) is in good accordance with the orders of magnitude of increase in the experimentally fitted TOFs for toluene formation on corresponding surface sites (Table 3). In addition, the experimental measured apparent activation energies with decreasing particle sizes show a similar trend to the DFT calculated activation barriers with increasing defects in surface. The structural details revealed in the computational study show that the highly coordinately unsaturated surface Ni site is the active site for facile adsorption and for stabilization of –OH in TS, resulting in lowered barrier for C-O cleavage. However, we note that the experimental measured activation energy as a function of Ni size (Figure 10) is significantly lower than the DFT calculated activation barriers for dehydroxylation on different sites (Table 4). Although different concentration of surface sites in different sized Ni particles and further hydrogenolysis of Tol may account for this difference to a small extent, one major reason may be that the direct deoxygenation is not the major path for the formation of Tol. Indeed, previous calculation results indicate that partial hydrogenation of phenyl ring significantly lowers the barrier for dehydroxylation,38,63 and these dehydroxylated surface species form aromatics easily as the 31


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Page 32 of 38

major final products under mild conditions. Since phenol prefers flat adsorption on a bridge site over different surfaces, one may expect that partial hydrogenation happens readily, and lowers the C-O breaking barrier toward the formation of aromatics on all surfaces. The hydrogenation activity toward the formation of MCHone and MCHol is also improved at low conversion of m-cresol with decreasing Ni particle size. This result is in good agreement with benzene or Tol hydrogenation on Ni/Al2O3 or Pt/SiO2 catalysts with varying Ni or Pt particle sizes.16,17 The DFT results indicate that defect sites in terrace of Ni(211) increase the adsorption energy of phenol on the terrace and may facilitate its ring hydrogenation with activated hydrogenation from defect site. This improved hydrogenation activity may improve the deoxygenation through partial hydrogenation of phenyl ring. It is important to note that at high conversion the hydrogenation products eventually dehydrogenated and deoxygenated to aromatics under current experimental conditions. Consistent with previous work that activity toward benzene hydrogenolysis is reduced with decreasing Ni particle sizes,18 the CH4 formation through C-C hydrogenolysis is greatly inhibited with decreasing Ni particle sizes for HDO of m-cresol. Hydrogenolysis of C-C bond requires an additional pair of surface Ni sites to form three C-Ni bonds for each of the two carbon atoms to facilitate C-C breakage in TS.64,65 Decreasing particle size reduces the availability of terrace sites, which favor the formation of TS for C-C hydrogenolysis, leading to lowered selectivity to CH4. The best catalyst in this work, 5Ni-1, shows a similar activity to 1% Pt/SiO228 and Ni-Re bimetallic catalyst,13 although the selectivity toward toluene is slightly lower than those catalysts. This comparison suggests that tuning metal particle size may be an effective approach 32


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ACS Catalysis

for designing highly active and selective deoxygenation catalyst for conversion of phenolics.

4. CONCLUSION 5% Ni/SiO2 catalysts were successfully prepared with varying Ni particle sizes ranging from 2 to 22 nm. For hydrodeoxygenation of m-cresol at 300 °C and atmospheric pressure of H2, the intrinsic reaction rate increases by a factor of 24 and TOF by a factor of 3 when the size of Ni particles decreases from 22 to 2 nm. The result demonstrates that surface Ni atoms on smaller particles are more active than those in larger ones. The selectivity to different reaction pathway is greatly changed with varying Ni particle sizes. Decreasing size of the Ni particles from 22 to 2 nm, the TOF for selective deoxygenation to toluene increases 6 times and that for hydrogenation increases 4 times whereas the TOF for C-C hydrogenolysis to CH4 decreases by 3/4. The results from fitted TOFs for different reactions on different surface sites and the apparent activation energy for different reactions indicate that defect sites (step, corner) are more active for deoxygenation while terrace site are more active for C-C hydrogenolysis. Decreasing Ni particle size increases defect sites and therefore improves selective deoxygenation to toluene, while inhibits C-C hydrogenolysis to undesired CH4. DFT results show that the barrier for direct dehydroxylation of phenol on Ni(111), Ni(211) and defected Ni(211) decreases from 175.6 to 145.6 and to 120.5 kJ/mol, respectively. The results indicate that the highly coordinately unsaturated surface Ni sites is active for deoxygenation through facile adsorption and stabilization of the –OH in transition state, facilitating C-O cleavage toward toluene.

33


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ASSOCIATED CONTENT Supporting Information Figure of reduction of 5Ni-1 and further H2-TPR, Figure of product distributions for m-cresol conversion at 350 °C, Bar graph of Figure 9, Table of MCHone and MCHol conversion, DFT calculations for phenol adsorption and deoxygenation on Ni(211)-T2 and Ni(211)-S1 surfaces, Structure of phenol adsorption on Ni(211) surface with O pointing away from the step site.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Q. Ge). *Email: [email protected] (X.L. Zhu). ORCID Qingfeng Ge: 0000-0001-6026-6693 Xinli Zhu: 0000-0002-8681-9994 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank the support from National Natural Science Foundation of China (21676194 and 21576204) and Ministry of Education of China for Program of New Century Excellent Talents in University (NCET-12-0407).

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Table of Contents Graphic

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Cat - ACS Publications - American Chemical Society

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