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Active sites of Ni2P/SiO2  catalyst for hydrodeoxygenation of guaiacol:

A joint XAFS and DFT study

 Ji-Sue Moon a, Eung-Gun Kim b, Yong-Kul Lee a,⇑

a Laboratory of Advanced Catalysis for Energy and Environment, Department of Chemical Engineering, Dankook University, 126 Jukjeondong, Yongin 448-701, Republic of Koreab Department of Polymer Science and Engineering, Dankook University, 126 Jukjeondong, Yongin 448-701, Republic of Korea

a r t i c l e i n f o

 Article history:

Received 2 October 2013Revised 19 November 2013Accepted 22 November 2013Available online 22 December 2013

Keywords:

Ni2P catalystHydrodeoxygenationGuaiacolXAFSDFT

a b s t r a c t

A Ni2P/SiO2 catalyst was prepared by temperature-programed reduction (TPR), and applied for the hydro-deoxygenation of guaiacol. The physical properties of the catalyst samples were characterized by N2

adsorption/desorption isotherms and CO uptake chemisorption. X-ray diffraction (XRD) and extendedX-ray absorption fine structure (XAFS) spectroscopy were used to obtain structural properties for the sup-ported Ni2P catalysts. Hydrodeoxygenation (HDO) tests were performed in a continuous flow fixed-bedreactor at 523–573 K, and 1 or 8 atm, and an LHSV of 2.0 h1. The Ni2P/SiO2 gave an HDO conversion over90% with two different reaction pathways being identified; at 1 atm direct deoxygenation was dominantto produce benzene, and at 8 atm prehydrogenation followed by deoxygenation was preferred to producecyclohexane. A combined X-ray absorption fine structure spectroscopy and density functional theoryanalysis revealed that the active site of Ni2P catalysts is composed of threefold hollow Ni and P siteswhich lead to adsorption of H or OH groups. These results suggest that relative populations of H or OHgroups on Ni or P sites of Ni2P surface have an impact on overall reaction pathways of the HDO.

 2013 Elsevier Inc. All rights reserved.

1. Introduction

Bio-oils produced from the pyrolysis of biomass have a poten-tial as an alternative fuel and petrochemical source and have sev-eral advantages over fossil fuels as a clean source of energy. Bio-oils are CO2-neutral and free of SO x  emission upon combustion.However, bio-oils cannot be directly used as fuel because of highoxygen contents (45–50%), which can cause the poor stabilityand low volatility of fuel [1,2]. The oxygen can be catalytically re-moved via two pathways; catalytic cracking and hydrodeoxygen-ation (HDO). The catalytic cracking occurs at atmosphericpressure conditionand high temperature (>450 C) over acidic zeo-lite catalysts, but the cracking gives a lower yield of transport fuel

product due to the high amount of coke formation, reaching 50–60% [3]. Gayubo et al. studied catalytic cracking of bio-oil overZSM-5 zeolite and showed that phenol and guaiacol had low reac-tivity for hydrocarbons formation; deposition of a coke formedthermally by condensation of 2-methoxyphenol was noticeable[3–5]. The hydrodeoxygenation is generally conducted at highpressure and temperature and results in high product yield withhigh H/C ratios. As the reaction condition is similar to the tradi-tional hydrotreating process, it can be grafted onto the existingsystem [6].

Traditionally, the hydrotreatingcatalysts are composed of metalsulfides like NiMoS/Al2O3 or CoMoS/Al2O3 [7–19]. However, thesecatalysts can be poisoned in the lack of sulfur in the feed stream.In other words, the active sites of the catalysts can be oxidizedand deactivated when the bio-oil without sulfur is applied as feed-stock [13]. Therefore, alternative catalysts for HDO of bio-oil arenecessary to replace traditional metal sulfide catalysts  [20–29].Very recently, non-sulfided metal phosphides such as Fe2P, Co2P,Ni2P, MoP and WP have been tested for the HDO of guaiacol, andNi2P catalysts have shown the best activity in the HDO of guaiacolamong the metal phosphides [23,29].

In the present study, our attention is placed on investigating theeffect of reaction conditions on the catalytic activities and the

structural property of Ni2P by using X-ray absorption spectroscopy(XAS). We also used density functional theory (DFT) calculations toexamine the possible structure and energetics of the phosphideoverlayers on the Ni2P (001) surface resulting from H2O dissocia-tive adsorption.

2. Experimental

 2.1. Synthesis of Ni 2P catalysts

Supported Ni2P catalysts were prepared by incipient wetnessimpregnation of aqueous metal phosphate precursors. The initialNi/P ratio in precursor was fixed at 1/2. The amount of Ni loading

0021-9517/$ - see front matter    2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jcat.2013.11.023

⇑ Corresponding author.

E-mail address:  yolee@dankook.ac.kr (Y.-K. Lee).

 Journal of Catalysis 311 (2014) 144–152

Contents lists available at  ScienceDirect

 Journal of Catalysis

j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c o m / l o c a t e / j c a t

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was maintained at 1.5 mmol/g of support using a commercial silica(Cabot, Cab-O-Sil, M5, 200 m2 g1). The supported nickel phos-phate precursor was synthesized by incipient wetness impregna-tion with a solution of nickel nitrate, Ni(NO3)26H2O (Alfa Aesar,98%) and ammonium phosphate (NH4)2HPO4 (Samchun, 99%), fol-lowed by drying at 393 K for overnight and calcination at 673 K for4 h. The resulting precursor phosphates were reduced to the corre-sponding phosphides at 873 K for 2 h using a hydrogen flow at100 cm3 min1/g of sample. After reduction, the phosphides werecooled to room temperature and typically were passivated undera 0.2% O2/He flow for 4 h.

 2.2. Catalyst characterization

CO chemisorption uptake measurements were performed onpassivated, air-exposed samples re-reduced in hydrogen at 723 Kfor 2 h to examine the dispersion of Ni2P particles on the support.Pulses (100 ll) of CO at room temperature (300 K) were passedover the sample to measure the total dynamic gas uptake. AMicromeritics ASAP 2010 micropore size analyzer was used tomeasure the N2 adsorption/desorption isotherms at 77 K, and thespecific surface area of the sample was calculated from the linearportion of Brunauer–Emmett–Teller (BET) plots (P /P o = 0.01–0.10). The chemical composition of the samples was determinedby inductively coupled plasma-atomic emission spectroscopy(ICP-AES) (PerkinElmer, Model Optima-4300 DV). X-ray diffraction(XRD) patterns of the samples were analyzed using a diffractome-ter (Rigaku DMAX-2500) operated at 60 kV and 300 mA with Cu Karadiation (k = 0.15418 nm). Infrared spectra of phenol adsorbed onthe Ni2P/SiO2  were collected using a Fourier transform infraredspectrometer (Nicolet 8700, Thermo Scientific) equipped with aheatable in situ transmission cell with water-cooled KBr windows.The samples were pressed into self-supporting wafers with a diam-eter of 20 mm and a weight of 13 mg. IR spectra were collected inabsorbance mode at a resolution of 2 cm1 and 64 scans in the re-gion 4000–400 cm1. In situ FTIR measurements on Ni2P/SiO2 were

made in a gas mixture of 5%H2/He. The Ni2P/SiO2  samples werepretreated in the 5%H2/He flow at 723 K for 2 h. For the measure-ments of phenol adsorption, the samples were exposed to a gas-eous phenol flow vaporized via a bubbler (1% phenol/hexane) at353 K with the carrier at a flow rate of 150 cm3 min1 until satura-tion was achieved. The samples were outgassed under vacuum(106 Torr) for 20 min at 353 K to remove gaseous and weakly ad-sorbed phenol. Measurements were made at 0, 5, 10, 15 and20 min with outgassing (106 Torr) at 353 K.

 2.3. X-ray absorption fine structure (XAFS) studies

XAFS spectra at the Ni K-edge (8.333 keV) of reference and cat-alyst samples were recorded in the energy range 8.233–9.283 keV

using synchrotron radiation at the beamline 8C, Pohang LightSource (PLS). The X-ray ring at the PLS has a flux of 1 1010 pho-tons s1 at 100 mA and 2.5 GeV. The X-ray beamline is equippedwith a Si (111) channel-cut monochromator and has an energyrange capability of 4–33 keV. The samples were prepared in a kap-ton sealed glass cell to avoid air-exposure. The X-ray absorptionspectra were recorded at ambient temperature in transmissionmode using ionization chambers for the detection of primary (I 0,100% N2) and transmitted (I T, 100% N2) beam intensities. The ob-tained XAFS data were analyzed by Winxas 3.1.

 2.4. Activity test for HDO of guaiacol

Hydrodeoxygenation of guaiacol was measured in a packed bed

reactor at atmospheric pressure and 8 atm for 25 h. The supportedcatalyst (0.326 g, Ni2P/SiO2) was packed in the reactor. Before the

reaction, the passivated catalysts were re-reduced at 723 K usinga hydrogen flow of 100 cm3 min1 for 2 h. The feed was preparedby dissolving 1 wt% of guaiacol (Alfa Aesar, 98+%) in tridecane(TCI, >98%) and was injected using a liquid pump at a liquid hourlyspace velocity (LHSV) of 2.0 h1 along with 100 cm3 min1 of hydrogen flow. The reactions were carried out at different temper-atures of 523, 553 or 573 K, and pressures of 1 or 8 atm. Liquidproduct compositions were analyzed with a HP-6890A gas chro-matograph, equipped with a 60 m dimethylsiloxane column hav-ing 0.32 mm i.d. (Hewlett Packard, HP-1) and flame ionizationdetector (FID), on samples collected at 3–4 h intervals. The prod-ucts were identified with a GC–MS (Agilent Technologies, 6890A,HP-1 capillary column).

 2.5. Computational methods

DFT calculations were carried out to model the Ni2P surface.First, the unit cell of bulk Ni2P was built based on crystallographicdata [30], the geometry of which was then optimized under three-dimensional periodic boundary conditions to further refine thestructure. The model phosphide surface was constructed by using

the standard slab approach, in which a slab of finite thicknesswas cut out of the Ni2P crystal at the (001) plane to expose anatomic layer that has two types of Ni sites, denoted as Ni(I) andNi(II), as shown in Fig. 1. The first type of Ni forms a trigonal prismaround each P atom on the corners of the unit cell, and the secondtype of Ni forms another trigonal prism around the P atom locatedinside the unit cell.  Fig. 2 shows the supercell model used in thepresent work. The slab consists of five layers, each containing bothNi and P atoms, and four surface unit cells, along with a 15 Å-thickvacuum layer in the  z -direction (perpendicular to the surface) tominimize interactions between neighboring image slabs. Two bot-tommost layers were kept fixed during geometry optimization tosimulate bulk constraints [31–33].

All DFT calculations were performed using the DMol3 moduleas part of the Material Studio package from Accelrys (version 5.5)[35]. The double-numerical plus polarization (DNP) basis set witha real space cutoff radius of 4.5 Å and PW91 exchange–correlationfunctional were used. Effective core potentials were used to treatthe core electrons of nickel.

Relative energies of the surface with different oxygen speciesare calculated with similar methods previously used for sulfidecatalysts [33,34]. For the adsorption of H2O, –OH,and–H on a cleanNi2P surface, the energy changes were calculated using the follow-ing equations:

DE H2O  ¼ E ðhydroxylated surfaceÞ E ðclean surfaceÞ

E ðfree H2OÞ ð1Þ

DE OH ¼ E ðhydroxylated surfaceÞ þ 12

E ðFree H2Þ

E ðclean surfaceÞ E ðfree H2OÞ ð2Þ

DE H ¼ E ðhydroxylated surfaceÞ E ðclean surfaceÞ

1

2E ðfree H2Þ ð3Þ

The thermodynamic equilibrium compositions were calculatedby the minimization of the Gibbs free energy, which was madeby RGibbs module in Aspen Plus 2006. The model compounds of guaiacol, phenol, cyclohexane anisole, phenol, cyclohexanol, andmethoxy-cyclohexane were chosen as target compounds consid-ered in the thermodynamic equilibrium calculation for the hydro-

deoxygenation. The reaction temperature ranging from 300 to1200 K, the hydrogen to oxygen ratio (H2/O ratio) ranging from

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0.5 to 30, and the pressure ranging from 1 to 100 atm were appliedfor the thermodynamic equilibrium calculation. The results arepresented in  Supplementary materials.

3. Results and discussion

 3.1. Physical properties of catalysts

The physical properties of the prepared catalyst samples aresummarized in   Table 1. The supported Ni2P catalyst samplesunderwent a loss in the BET surface area due to the pore occupa-tion by the Ni2P loadings. The surface area remained unchangedduring the reaction.

Compared to the initial P/Ni molar ratio of 2.0/1.0 in the oxidicprecursor, elemental analysis showed that there was a decrease inphosphorus content in the course of TPR for the fresh sample,which gave the P/Ni molar ratio of 0.91/1.0. During the reaction,a further loss of P was observed to give the P/Ni molar ratio of 0.64 and 0.62/1.0 for the spent Ni

2P/SiO

2 catalysts evaluated at 1

and 8 atm, respectively. The results indicate that the extra P em-ployed in the oxidic precursor might have been removed via theformation of PH3 during the course of TPR and hydrotreating con-dition, which corresponds to the Ni2P stoichiometry, as also sug-gested by previous reports [36,37]. The experimental CO uptakefor both spent samples was smaller than that for the fresh Ni2Psample, and the decrease in CO uptake was found to be 32% and

(A) (B)

Fig. 1.  Unit cell of Ni2P: (A) Ni2P (111), (B) top view.

(B)(A)

Ni

P

Fig. 2.  Supercell models of Ni2P surface: (A) Ni2P (0 01), (B) side view of 5-layered catalyst slab.

 Table 1

Physical properties and elemental analysis of the Ni2P catalyst samples.

Sample CO uptake (lmol g1) BET surface area (m2 g1) Ni (wt%) P (wt%) Molar ratio P/Ni

Fresh 63.9 119.4 5.9 3.0 0.91/1Spent (1 atm) 31.3 77.2 5.6 1.9 0.64/1

Spent (8 atm) 57.8 90.2 5.8 1.9 0.62/1

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9% for the Ni2P samples tested at 1 and 8 atm, respectively. Thisindicates that the surface active site of the catalyst is possiblyblocked by some species that prevent adsorption, such as carbonor oxygen. The loss of active sites is more pronounced for the cat-alyst tested at 1 atm.

In order to confirm the phase of the fresh and spent Ni2P/SiO2

samples after HDO tests at atmospheric and pressured reactioncondition, XRD was measured as shown in Fig. 3. The XRD patternsfor the all Ni2P/SiO2  samples show three main peaks centered at40.5, 44.8  and 47.5  corresponding to the Ni2P phase with littlechanges in the peak positions after reaction. These results demon-strate that the Ni2P phase remains stable during the reaction.

 3.2. XAFS studies

Fig. 4 shows the Ni K-edge XAFS spectra of the fresh and spentNi2P/SiO2  catalyst samples collected after atmospheric or pres-sured reaction conditions. The X-ray absorption near edge struc-ture (XANES) spectra of the fresh samples were measured afterbeing reduced at 673 K. The analysis of XANES region can provideinformation on the symmetry and the oxidation state of the Ni ab-

sorber. The pre-edge intensity is related to the symmetry and theoccupancy of the 3d shell. The Ni K-edge XANES spectra showtwo regions, which exhibit a pre-edge and main edge peak. Thepre-edge peak corresponds to the transition of a photoelectronfrom the 1s to 3d orbital and the main edge peak corresponds tothe transition of 1s to 4p symmetry levels[36,38]. The XANES spec-tra of the spent samples retain a sharp absorption peak from the1s–4p transition, which is generally called a white line. Notably,the spent sample collected after an atmospheric condition gives astronger white line than the case of pressured reaction condition,which implies that the Ni2P/SiO2  underwent the surface oxidiza-tion particularly under atmospheric condition. These results arein line with the lower CO uptake for the spent Ni 2P catalyst testedat 1 atm than the spent sample at 8 atm.

Shown also in Fig. 4 are the Ni K-edge EXAFS spectra and theirFourier transforms for the fresh and spent Ni2P/SiO2 catalysts afteratmospheric and pressured reaction conditions. The EXAFS spec-trum for bulk Ni2P as the reference comprises a little wider oscilla-tion region in 30.0–80.0 nm1 due to the Ni–P contribution and anarrower oscillation region in 80.0–140.0 nm1 due to the Ni–Nicontribution, which give rise to two main peaks in the Fouriertransforms, centered at 0.175 nm and 0.240 nm, corresponding toNi–P and Ni–Ni, respectively. Also for the fresh Ni2P/SiO2  sample,the Fourier transform gives two distinct peaks in the region

0.15–0.30 nm, the shorter distance peak due to the Ni–P bondand the longer distance peak to the Ni–Ni bond [38]. The spentsamples show two distinct peaks located at almost the same posi-tions as the fresh samples, indicating the maintenance of the Ni2Pphase during the reaction. For the sample tested under atmo-spheric reaction condition, the oscillation at 0.4–0.45 nm becomesa little wider than the case of the spent sample tested under thepressured reaction condition, which implies that the spent samplestested at atmospheric pressure condition are slightly oxidized dur-ing the reaction.

 3.3. HDO of guaiacol

It has been suggested that the possible reaction mechanism of guaiacol HDO involves a consecutive reaction pathway from guai-acol to catechol and then to phenol, which then canbe transformedto benzene or cyclohexene over conventional sulfide catalysts [5–19]. More recently, the possibility of direct hydrogenolysis produc-ing phenol from guaiacol by elimination of a methanol moleculewas also suggested in the presence of carbon-supported CoMoScatalysts [17,18]. In a similar manner, Ni2P catalysts mainly pro-duce phenol and benzene without formation of catechol in thecourse of HDO of guaiacol [23].

Fig. 5 presents the catalytic activities in the HDO of guaiacolover Ni2P/SiO2  at 573 K and at 1 or 8 atm. At 1 atm the HDO of guaiacol gave benzene as the major product with a selectivity of 62% and anisole, cyclohexane, and phenol as minor products withproduct selectivities of 30%, 7%, and <1%, respectively, with a guai-acol conversion of 83%. It is noteworthy that the formation of cat-echol, susceptible to coke formation, was not observed during thereaction. These results are consistent with a previous work re-ported by Oyama et al. for the HDO over Ni2P/SiO2 catalyst [23]. In-stead, at 8 atm the HDO of guaiacol gave rise to the formation of cyclohexane (91%) as a major product and benzene (8%), anisole(1%), phenol (<1%), and traces of cyclohexanol and methoxy-cyclo-hexane (<0.1%) as minor products, with a guaiacol conversion of 

100%. Again, the formation of catechol was not observed. Addi-tional activity tests at 4 and 15 atm also followed the pressure-dependent behavior of the HDO as shown in Fig. S1 (Supplemen-tary materials). The increase in pressure from 1 to 4 atm led tothe decrease in the formation of benzene from 62% to 17% withthe increase in the formation of cyclohexane from 7% to 72%. Ateven higher pressure of 15 atm, these trends became more pro-nounced to yield cyclohexane of 99% (Fig. S1).

In order to confirm the pressure-dependent behavior of theguaiacol HDO, thermodynamic equilibrium calculation was carriedout with varying reaction temperature and pressure, as presentedin Fig. S2 (Supplementary materials). Although the formation of reaction products such as anisole, phenol, cyclohexanol, and meth-oxy-cyclohexane are possible, these species are found far less sta-

ble at the reaction condition than benzene or cyclohexane (Fig. S2).At the lowest temperature of 300 K, cyclohexane appears dominantand the equilibriumcomposition is kept constant until the temper-ature of 450 K at 1 atm, at which the formation of benzene is initi-ated and reached a maximum at 650 K at 1 atm. Upon the increasein pressure from 1 to 100 atm, the thermodynamic equilibriumcomposition of cyclohexane is found to shift to higher temperatureand remain dominant until 500 K at 20 atm and 600 K at 100 atm,while that of benzene begins to increase exceeding 500 K at 20 atmand 600K at 100atm (Fig. S2). These results indicate that at a low-er temperature, the formation of cyclohexane is more thermody-namically favorable than benzene at 1 atm and the equilibriumcomposition can shift toward higher temperature region with theincrease in pressure. The thermodynamic equilibrium calculations

are perfectly in line with the HDO tests with varying pressure con-ditions, demonstrating that the Ni2P catalyst is very active in the

20 40 60 80

Spent Ni2P/SiO2 (8 atm)

Spent Ni2P/SiO

2(1 atm)

Fresh Ni2P/SiO

2

2 theta / degree

    S   i  g  n  a   l   I  n   t  e  n  s   i   t  y   /  a .  u .

Ni2P (JCPDS 3-953)

Fig. 3.  XRD patterns for fresh and spent Ni2P/SiO2 catalyst samples.

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HDO with satisfying the thermodynamic equilibrium compositionat the given reaction conditions.

Fig. 6 shows the conversion and product distribution at variousreaction temperatures, 523, 543, and 573 K, at an LHSV of 2.0 h1.

Again, at different pressure conditions, the products are quite dif-ferent with a slight change in product selectivity with reactiontemperature. At 1 atm, the major product was benzene, followedby phenol, anisole, and cyclohexane. As the temperature increased,the conversion of guaiacol also rose from 78% to 87% and the prod-uct selectivity of benzene also increased from 54% to 62%. Theselectivity of cyclohexane then decreased from 24% to 8% and theformation of anisole increased from 22% to 30%. The formation of phenol appeared at the beginning of the reaction but disappearedat the steady state. At 8 atm, in contrast, the major product forthe HDO of guaiacol was cyclohexane, followed by anisole and ben-zene. As the temperature increased, the conversion of guaiacol roseto reach 100%. At 573 K, guaiacol was converted almost exclusivelyto cyclohexane (90%), with the amount of anisole down from 11%

to 2%. These results clearly indicate that the Ni2P catalyst can pro-vide parallel reaction pathways of direct hydrodeoxygenation

(DDO) and pre-hydrodeoxygenation (HYD), which give rise to theformation of benzene and cyclohexane, respectively.

The catalytic stability was tested in the HDO with varying tem-perature steps of 573, 543, and 573 K in series at 1 or 8 atm, as pre-

sented in Figs. S3 and S4 (Supplementary materials). At 1 atm, thedecrease in reaction temperature lowered the HDO conversionfrom 80% to 60%, but it was not fully recovered upon the temper-ature ramping, indicating that the catalyst underwent deactivationprobably due to the partial oxidation of the catalyst surface or cokedeposition. At 8 atm, the deactivation behavior was not observedand the HDO activity was recovered upon the increase in temper-ature. These results suggest that the hydrogenation pathway is lesssusceptible toward poisoning or coke deposition than the directdeoxygenation pathway.

 3.4. DFT calculations

Hydrodeoxygenation is generally conducted over traditional

hydrotreating catalysts which consist of metal sulfides like Ni-MoS/Al2O3 or CoMoS/Al2O3  [5–19]. It is commonly suggested that

8.30 8.32 8.34 8.36 8.38 8.40

Ni2P(Bulk)

PhotonEnergy/ keV

FreshNi2P/SiO

2

SpentNi2P/SiO

2

(1atm)

SpentNi2P/SiO

2

(8atm)

4 6 8 10 12 14 0 2 4 6

FreshNi2P/SiO

2

BulkNi2P

  x   (   k   )   *   k   3

   F   T   M   a   g   n   i   t  u   d   e   /   a

 .  u .

Distance / 0.1nmk/10nm-1

SpentNi2P/SiO

2

(1atm)

SpentNi2P/SiO

2

(8atm)

   N  o  r  m  a   l   i  z  e   d  a   b  s  o  r   b  a  n  c  e   /  a .  u .

Fig. 4.  Ni K-edge XANES, EXAFS, Fourier transforms of fresh and spent Ni2P catalysts.

5 10 15 20 25 30

0

20

40

60

80

100

   P  r

  o   d  u  c   t   d   i  s   t  r   i   b  u   t   i  o  n   /   %

Time on stream / h

Benzene

Cyclohexane

 Anisole

Phenol

0

20

40

60

80

100

   X   H   D   O    /

   %

(B) 8 atm

5 10 15 20 25 30

0

20

40

60

80

100

Benzene

Cyclohexane

 Anisole

Phenol

   P  r

  o   d  u  c   t   d   i  s   t  r   i   b  u   t   i  o  n   /   %

Time on stream / h

0

20

40

60

80

100

   X   H   D   O

   /   %

(A) 1 atm

Fig. 5.  Activity test for guaiacol HDO at 1 atm (left) and 8 atm (right) at 573 K.

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the active sits of the catalysts are coordinatively unsaturated sites

at the edges of MoS2 slabs [39,40]. In addition, the surface config-uration of the catalyst is known to strongly affect the reaction [39–42]. Delmon proposed that the active site for hydrogenation con-sists of threefold coordinatively unsaturated Mo atoms, and thatof hydrogenolysis is composed of coordinatively unsaturated Moatoms with neighboring SH or H groups which can be formed byadsorption and dissociation of H2S [41]. Bunch and Ozkan also pro-posed the role of surface OH group in the HDO and revealedhydrogenolysis function similarly to SH groups in sulfide catalysts[42]. In order to better understand the active site configuration of Ni2P catalyst upon the production of H2O during the HDO, weinvestigated three different configurations, namely, OH adsorption,H2 dissociative adsorption, and H2O dissociative adsorption.

OHadsorption on the Ni2P (001) surface was calculated for fourpossible bonding sites, Ni(I), 3Ni(I), P(II), and Ni(I)/P(II), as shownin Fig. 7. The adsorption of an OH group on the Ni(I) site was sta-bilized with a bond length of 1.345 Å and an adsorption energyof 0.238 eV, as shown in Fig. 7(A). The adsorption of the OH groupon three neighbor Ni(I) sites that form a threefold hollow (TFH), asshown in Fig. 7(B), was calculated to have a Ni–O bond length of 

1.776 Å and an adsorption energy of 0.501 eV, indicating a more

stable configuration than the adsorption on the single Ni(I) site.The adsorption energies of the OH group on the P(II) site and Ni(II)site were  0.163 eV (bond length: 1.676 Å) and 0.303 eV, respec-tively, indicating less stable configurations than the others. In addi-tion, the OH adsorption as a bridge between Ni(I) and P(II) siteswas not stable with bond lengths of 1.776 Å and 2.120 Å, respec-tively. These results demonstrate that the TFH-Ni(I) site acts mostfavorably for adsorption of the OH group.

The calculated results for H2 dissociative adsorption on the Ni2P(001) surface are displayed in Fig. 8. The hydrogen atom, placedinitially on either the Ni or P atom, has migrated to sit on theTFH Ni(I) site with an adsorption energy of   0.637 eV, becomingthe most stable configuration for atomic hydrogen adsorption onthe Ni2P (001) surface. Our results show that both H and OH canadsorb on the TFH-Ni(I) site, with the adsorption of atomic H beingmore favorable than the OH group.

H2O dissociative adsorption on the Ni2P (001) surface was cal-culated for two possible configurations as shown in Fig. 9. It wasassumed that H2O dissociation results in the formation of adsorbedH and OH, as with the H2S dissociation proposed by Nelson et al.

523 543 573

0

20

40

60

80

100

   P  r  o   d  u  c   t   d   i  s   t  r   i   b  u   t   i  o  n   /   %

Temperature /K

Benzene

Cyclohexane

 Anisole

0

20

40

60

80

100

   X   H   D   O

   /   %

(A) 1 atm(B) 8 atm

523 543 5730

20

40

60

80

100

   P  r  o   d  u  c   t   d   i  s   t  r   i   b  u   t   i  o  n   /   %

Temperature / K

Benzene

Cyclohexane

 Anisole

0

20

40

60

80

100

   X   H   D

   O

   /   %

Fig. 6.   Product distribution of guaiacol HDO at 1 atm (left) and 8 atm (right) at 523, 543, and 573 K.

Fig. 7.  Adsorption of OH on the surface of Ni 2P (001) supercell.

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[33]. One of the possible configurations can be of atomic H on theTFH-Ni(I) site and OH on the nearest P(II) site which is surroundedby Ni(II) atoms as shown in Fig. 9(A). The adsorption energy was0.709 eV, and the bond length of P–OH was 1.718 Å and of Ni–

H was 1.790 Å, indicating possible surface configuration of Ni2Pin the course of HDO reaction. In the reversed manner, the adsorp-

tion of the OHgroup on TFH-Ni(I)site and atomic H on the P(II) sitesurrounded by Ni(II) atoms was also calculated. The adsorption en-ergy was 0.638 eV, and the bond lengths of Ni–OH and P–H were2.025 Å and 1.595 Å, respectively, indicating less stable configura-

tion than the case of H on TFH-Ni(I) and OH on P(II) sites. These re-sults are in well accordance with a previous study on in situ FTIR 

(A) (B)

Fig. 8.  Adsorption of H on the surface of Ni 2P (0 01) supercell.

Fig. 9.  The structures and energetics for H2O dissociative adsorption.

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analysis of CO and pyridine adsorption on a hydrotreating Ni2P/SiO2 catalyst [36], in which Ni centers and P–OH groups were alsoidentified for promoting hydrogenation and activation of S or Ncompounds, respectively. In the similar manner, it can be thus sug-gested that the TFH-Ni(I) and neighboring P(II) sites of the Ni 2Psurface could play a crucial role in HDO with dissociative adsorp-tion of H2 or H2O during the HDO reaction.

 3.5. In situ FT IR studies

In situ FTIR measurements of phenol adsorption as a function of 

the outgassing time were carried out to characterize the catalystsurface properties. For reduced Ni2P/SiO2, the adsorption of phenolgave rise to IR bands at 3747, 3668, 3399, and 2964 cm1 at 343 Kas shown in Fig. 10. The broad band at 3399 cm1 corresponds toC–H vibrations in phenol, and the peak 2964 cm1 correspondsto C–H vibrations in hexane (solvent). The negative peaks at3747 and 3668 cm1 correspond to hydroxyl vibrations of SiO–Hand PO–H, respectively [36]. Since a background spectrum is sub-tracted from all spectra, the decrease in the negative peaks indi-cates recovery in the O–H concentration with outgassing.Initially, the O–H bond on Si–OH is strongly developed by its inter-action with phenol, but as the phenol desorbs the OH concentra-

tion rises again, indicating that the phenol is only weaklyadsorbed on the SiO2 support. On the other hand, the peak assignedto phenol adsorption on the P–OH groups on Ni2P persists evenafter 20 min of outgassing (106 Torr) at 343 K, indicating thatthe interaction of phenol with the P–OH groups on the Ni2P isstronger than the Si–OH groups on the support. These results thussuggest that the P–OH groups on Ni2P catalyst well adsorb phenoland the vicinity of P–OH and TFH Ni sites facilitates the HDO.

 3.6. Active sites of Ni 2P catalyst in HDO

From these experimental and DFT calculation results, the reac-tion schematic over Ni2P catalyst in the HDO of guaiacol can beproposed as shown in Scheme 1. Overall, it can be noted that directhydrodeoxygenation and pre-hydrodeoxygenation occur simulta-neously over the Ni2P/SiO2 catalyst, in which the HDO of guaiacolcommonly produces anisole or phenol in the early stage of reactionwithout the formation of catechol. At 1 atm, the direct hydrodeox-ygenation pathway appears dominant to form benzene. Instead, ata slightly increased pressure of 8 atm, the prehydrogenation path-way becomes pronounced to give cyclohexane. Based on the DFT

studies together with XAFS and in situ FTIR measurements, the ac-tive sites of Ni2P catalysts for the hydrodeoxygenation of guaiacolcan be suggested in terms of relative populations of H or OHgroupson Ni or P sites of Ni2P surface, influencing overall reaction path-ways. It can be thus proposed that the direct deoxygenation(DDO) pathway is favored by the surface OH groups, while the pre-hydrogenation (HYD) pathway is preferred on the more reducedsurface of the Ni2P catalyst. Moreover, high dispersion of the activesites will be of great importance in facilitating the HDO. Studies onthe correlation between the catalytic activity and the metal sitedensity titrated by the amount of CO uptake for the Ni2P catalystsin hydrotreating [36,37] revealed that for the sample series withdifferent amounts of Ni2P loadings on the same surface area SiO2

support and the other series with same amounts of Ni and P pre-cursors on the different surface area SiO2   supports, the more COuptake the Ni2P gave, the higher activity it showed in both cases.

4. Conclusion

The Ni2P/SiO2 catalysts demonstrated high and stable activity inthe hydrodeoxygenation (HDO) of guaiacol with a guaiacol conver-sion over 90% at 523–573 K, and 1 or 8 atm, and a LHSV of 2.0 h1.It was noteworthy that the conversion of guaiacol over the Ni2Pcatalyst can be tunable to form benzene at 1 atm or cyclohexaneat 8 atm. The characterization of structural properties of freshand spent Ni2P catalysts made by XAFS and XRD analysis revealed

3900 3600 3300 3000 2700

Wavenumber / cm-1

   A   b   s   o   r   b   a   n   c   e

   /   A .   U .

20 min

15 min

10 min

5 min

0 min

29643399

36683747

SiO2

O H- +

Ni2P

O H

Fig. 10.  In situ FTIR spectra of phenol adsorption over Ni2P/SiO2 catalyst.

Scheme 1.  Proposed reaction schematic of guaiacol HDO over Ni2P catalyst.

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that the local structure of Ni2P phase was maintained at 8 atm butslightly oxidized particularly at 1 atmalthough the bulk Ni2P phaseremained stable. DFT calculation for the catalyst surface configura-tion showed that threefold hollow (TFH) Ni site and neighboring Psite are responsible for adsorption of H and OH. In addition, disso-ciation of H2O was also possible on the TFH-Ni and P sites. All theseresults suggest that the direct deoxygenation pathway is favoredover OH surface to form benzene, and the prehydrogenation path-way over the more reduced surface of Ni2P catalyst to generatecyclohexane.

 Acknowledgments

The authors are grateful for the funding supplied by KIST(2E2280-11-212) and NRF (2012R1A1A2008651). The authors arealso thankful to Mr. Y.-T. Kwon at SK innovation for his valuablehelp in FTIR measurement.

 Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at  http://dx.doi.org/10.1016/j.jcat.2013.11.023.

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