Applied Catalysis A: General 361 (2009) 18–25

Hydrodesulfurization of 4,6-dimethyldibenzothiophene over high surface areametal phosphides

Rui Wang, Kevin J. Smith *

Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, BC V6T 1Z3, Canada

A R T I C L E I N F O

Article history:

Received 3 February 2009

Received in revised form 5 March 2009

Accepted 6 March 2009

Available online 8 April 2009

Keywords:

Catalyst

Hydrodesulfurization

Metal phosphide

Nickel phosphide

Molybdenum phosphide

4,6-Dimethyldibenzothiophene

Citric acid

A B S T R A C T

Unsupported monometallic phosphides (MoP and Ni2P), as well as a series of bimetallic nickel

molybdenum phosphides (NixMoP), were synthesized with high surface area by adding citric acid (CA) to

precursor metal salt solutions prior to drying, calcination and temperature-programmed reduction

(TPR). The resulting MoP and Ni2P catalysts had surface areas of 139 and 216 m2/g, respectively. The

surface areas of the NixMoP catalysts decreased with increased Ni content and ranged from 121 to 16 m2/

g. In all cases, the catalyst surface area was greater than that obtained when a conventional preparation

procedure was followed that did not use CA. The activity of the catalysts for the hydrodesulfurization

(HDS) of a feed containing 3000 ppm 4,6-dimethyldibenzothiophene (4,6-DMDBT) in toluene was

measured at 583 K and 3.0 MPa. The resulting catalysts were highly active and among the studied

phosphides, Ni2P exhibited the highest HDS activity with a conversion of 87.7% at a WHSV of 26 h�1 and a

H2/liquid feed ratio of 625 (v/v). The HDS TOF decreased in the order Ni2P > NixMoP > MoP and for the

NixMoP catalyst, the observed TOF increased with Ni content.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

Hydrotreating is an important crude oil refining process thatremoves S, N, and O from hydrocarbon molecules by reacting themwith H2 over a catalyst. Transition metal sulfides are commonlyused in commercial hydrotreating reactors. Recently, transitionmetal phosphides [1–5] and bimetal phosphides [6,7] have beeninvestigated as alternatives to metal sulfides, and in some studies,metal phosphides have been shown to be more active than metalsulfides for hydrodesulfurization (HDS) [8–10] and hydrodeni-trogenation (HDN) [2,5,11]. Among the phosphides, MoP and Ni2Phave shown high activity for deep HDS, as evidenced by the resultsfrom HDS studies that have used 4,6-dimethyldibenzothiophene(4,6-DMDBT) as a model reactant.

Both unsupported and supported metal phosphides can beprepared by temperature-programmed reduction (TPR) of thecorresponding metal and phosphate salts [4–7]. Preparation ofunsupported metal phosphides using TPR yields catalysts with lowsurface area (<15 m2/g) and low activity. Consequently, moststudies have used highly porous supports [12] to enhance thedispersion of the metal phosphides. Although this approach hasbeen successful, a strong interaction between the support and the

* Corresponding author. Tel.: +1 604 822 3601; fax: +1 604 822 6003.

E-mail address: kjs@interchange.ubc.ca (K.J. Smith).

0926-860X/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2009.03.037

phosphorous can occur under certain preparation conditions, suchthat a limited amount of the metal phosphide phase is produced.Furthermore, a strong metal–support interaction can complicatethe interpretation of observed activity trends on supportedcatalysts. An alternative approach, therefore, is to prepareunsupported metal phosphides with high surface area. Severalmethods for synthesizing unsupported metal phosphides areknown [6,13–15], including the reaction of phosphine with metalsor metal oxides, electrolysis of molten metal phosphate salts,thermal decomposition of single-source precursors, solvothermalsynthesis, surfactant-assisted synthesis, self-propagation high-temperature synthesis and the use of a non-thermal H2 plasma asthe reduction medium. However, these methods are usually ofrelatively high cost, some are difficult to implement and, moreimportantly, few provide for the synthesis of high surface areametal phosphides. The preparation of high surface area, unsup-ported metal phosphides has been demonstrated in two recentstudies. Unsupported Ni2P with a surface area of 130 m2/g wasprepared by Yang et al. [16] by adding a polymer surfactant (TritonX-114) and ethylene glycol to an aqueous solution of Ni(NO3)2 and(NH4)2HPO4 prior to drying and calcination. In another study,Cheng et al. [17] used citric acid (CA) as a structure template orchelating agent to prepare unsupported MoP, with a maximum BETsurface area of 122 m2/g.

In the present paper, we report on the preparation of highsurface area monometallic phosphides (Ni2P, MoP) and a series of

R. Wang, K.J. Smith / Applied Catalysis A: General 361 (2009) 18–25 19

bimetallic phosphides (NixMoP) using citric acid as a chelatingagent that is mixed with an aqueous solution of the desired metaland phosphate salts prior to drying, calcination and reduction. Theresulting materials are shown to have high surface area and to bemuch more active in the HDS of 4,6-dimethyldibenzothiophenethan the corresponding metal phosphides prepared in the absenceof citric acid.

2. Experimental

2.1. Catalyst preparation

The metal phosphide catalysts were prepared from precursorsalts dispersed in citric acid. For the unsupported MoP and Ni2P,4.37 g of citric acid was added to 30 ml of deionized watercontaining the corresponding metal salt (4 g of (NH4)6Mo7O24�4H2O,4H2O, or 3.36 g of Ni(NO3)2�6H2O) and a corresponding amount ofdiammonium hydrogen phosphate ((NH4)2HPO4), to yield a P/Momolar ratio of 1/1, or a P/Ni molar ratio of 2/1. The precursor solutionwas aged in a covered beaker in a water bath at 363 K for 24 h, driedin an oven at 397 K and calcined at 773 K for 5 h. The calcinedcatalyst precursor was ground to a powder (dP < 0.7 mm) andconverted to the active metal phosphide by temperature-pro-grammed reduction in H2 (Praxair-99.99%) at a flow rate of160 ml(STP)/min. A heating rate of 5 K/min to 573 K, followed bya heating rate of 1 K/min to 923 K was used for the TPR, with the finaltemperature held for 2.5 h. The catalyst was then cooled to roomtemperature in He and passivated in a flow of 1% O2/He for 3 h atroom temperature prior to removal from the reactor. The NixMoPcatalysts were prepared similarly by first dissolving stoichiometricamounts (Mo/P = 1) of (NH4)6Mo7O24�4H2O, (NH4)2HPO4 and citricacid (Mo/CA = 1/2) in 15 ml of water. A second solution, prepared bydissolving an appropriate amount of Ni(NO3)2�6H2O and citric acid(Ni/CA = 1/2) in 15 ml of deionized water to give the desired Nicontent (0.125, 0.25, 0.5, 0.75, 1.0 mol% Ni) in the final NixMoPcatalyst, was added to the first. The prepared mixture was treatedthermally as before.

Unsupported MoP, Ni2P and Ni0.25MoP catalysts were alsoprepared without the use of citric acid, using the conventional TPRmethod to reduce the corresponding phosphate precursors. In thiscase the P/Mo and P/Ni ratios were the same as those used for thecatalysts prepared using CA.

2.2. Catalyst characterization

The chemical compositions of the catalysts were determinedusing inductively coupled plasma-atomic emission spectroscopy(ICP-AES), done by Cantest Laboratories, Vancouver, BC. Powder X-ray diffraction (XRD) analyses were performed on the passivatedcatalysts using a Siemens D500 diffractometer with a Cu Ka X-raysource of wavelength, 1.54 A. The analysis was performed using a40 kV source, a scan range of 10–808 with a step size of 0.048 andstep time of 2 s. The phase identification was carried out aftersubtraction of the background using standard software. Transmis-sion electron microscope (TEM) images were generated using a FeiHitachi H-800 scanning transmission electron microscope operat-ing at 200 kV. Catalyst samples were ground to a fine powder,dispersed in ethanol and sonicated for 2 h. A drop of the catalystsuspension was placed on a 200 mesh copper grid coated withformvar carbon, and left to dry before analysis.

The N2 BET surface areas of the catalysts were measured at 77 Kusing a Micromeritics ASAP 2020. The passivated catalyst wasdegassed at 523 K for 10 h prior to the measurement. TPRexperiments were carried out with a Micromeritics AutoChem II2920 automated catalyst characterization system. Prior to the TPRexperiment, the catalyst precursor was pretreated in Ar at 773 K

for 1 h, and then cooled to room temperature. Subsequently, the Arflow was switched to 10% H2/Ar and the sample was heated fromroom temperature to 1073 K at a rate of 5 K/min. The H2

consumption was measured by a thermal conductivity detector(TCD).

The CO uptake was measured by pulsed chemisorption alsousing a Micromeritics AutoChem II 2920. The passivated catalystwas pretreated to remove the passivation layer by passing50 ml(STP)/min of 10% H2/Ar while heating from 313 to 923 K ata rate of 5 K/min, and maintaining the final temperature for 1 h.The 10% H2/Ar flow was then switched to He (50 ml(STP)/min) at923 K for 1 h in order to remove the adsorbed species. The reactorwas subsequently cooled to 298 K, and 0.5 ml pulses of CO wereinjected into a flow of He (50 ml(STP)/min) and the CO uptake wasmeasured using a TCD. CO pulses were repeatedly injected until nofurther CO uptake was observed after consecutive injections.

A Leybold Max200 X-ray photoelectron spectrometer was usedfor XPS studies. Al Ka was used as the photon source generated at15 kV and 20 mA. The pass energy was set at 192 eV for the surveyscan and 48 eV for the narrow scan. All catalyst samples wereanalyzed after passivation at room temperature. Exposure of thesamples to ambient atmosphere was minimized by transferringthe samples either in vacuum or under N2. All XPS spectra werecorrected to the C 1s peak at 285.0 eV.

2.3. Catalyst activity

The HDS of 4,6-DMDBT was measured in a fixed bed reactor(i.d. = 9 mm) at 583 K and 3.0 MPa H2. The passivated catalysts(dP < 0.7 mm) were activated at 873 K for 1 h in H2 at a flow rate of160 ml(STP)/min prior to the reaction. The temperature was thencooled to the reaction temperature of 583 K and the reactioninitiated using the appropriate feed flow conditions. The con-centration of 4,6-DMDBT was 3000 ppm and the 4,6-DMDBT wasdissolved in toluene. The feed was introduced to the reactor using aGilson Model 0154E metering pump. Prior to entering the reactor,the liquid was evaporated into a stream of flowing H2. Gas andliquid flows and catalyst charged to the reactor were chosen to givea WHSV = 26 h�1 and a H2/liquid feed ratio of 625 (v/v). Theproduct was collected periodically and analyzed using a VarianStar 3400 Gas Chromatograph equipped with a flame ionizationdetector (FID). Component separation was achieved using acapillary column (CP-Sil 19 CB, 25 m length and 0.53 mm i.d.).Component identification was confirmed using the same columnand a GC–MS (Agilent 6890/5973N).

For all the activity data reported herein, the carbon balanceacross the reactor was >95% and a number of the experimentswere repeated to ensure repeatability of the data.

3. Results and discussion

3.1. Catalyst characterization

The X-ray diffractograms of the bulk monometallic (MoP, Ni2P)and the bimetallic (NixMoP, x = 0.125–1.0) phosphides preparedusing citric acid, presented in Figs. 1–4, confirmed the formation ofthe desired metal phosphides. Furthermore, the diffractograms arein excellent agreement with those obtained for samples preparedby conventional TPR in the absence of CA (Figs. 1–3). As shown inFig. 1, phase pure MoP was obtained for the MoP catalyst preparedin the presence of CA under the optimized conditions described byCheng et al. [17]. When Ni was added to the MoP, a NiMoP phaseappeared (Fig. 4) and as more Ni was added to the NixMoP, theformation of the NiMoP phase became more evident. For theNiMoP formulation (x = 1), the NiMoP phase was dominant. In ourprevious studies [18], a series of NixMoP catalysts prepared by the

Fig. 1. X-ray diffractogram of MoP catalysts prepared with and without CA.

Fig. 2. X-ray diffractogram of Ni2P catalysts prepared with and without CA.

Fig. 4. X-ray diffractogram of NixMoP catalysts prepared with CA.

R. Wang, K.J. Smith / Applied Catalysis A: General 361 (2009) 18–2520

conventional method showed diffraction peaks attributable to theNiMoP phase at a Ni/Mo ratio as low as 0.07, whereas in the presentstudy, the peaks of the NiMoP phase were still very weak at a muchhigher Ni/Mo ratio (0.25), and no diffraction lines between 2Q of

Fig. 3. X-ray diffractogram of NixMoP catalysts prepared with and without CA.

468 and 518 were observed. Based on these comparisons, it isconcluded that the citric acid facilitated a highly homogenousdispersion of Ni in the NixMoP catalyst.

For the catalysts prepared without CA, MoP was the dominantphase in the MoP sample, Ni2P was the dominant phase in the Ni2Psample, whereas in the Ni0.25MoP preparation, both MoP andNiMoP were identified (Fig. 3). Note that the diffractograms showless peak broadening over all the samples prepared without CAcompared to the corresponding CA prepared samples.

Table 1 summarizes the crystallite size calculated by Scherrer’sequation applied to the XRD data. For all of the phosphides, thecrystallite size of the samples prepared using citric acid wassmaller than that of samples prepared in the conventional way(samples annotated as ‘‘no CA’’ in Table 1). For the NixMoPphosphides, the crystallite size ranged from 14 to 30 nm for0 � x � 1 and as the Ni content of the NixMoP catalyst increased,the MoP crystallite size also increased. A similar trend wasobtained over NixMoP catalysts prepared by the conventionalmethod for a range of Ni compositions [18].

Table 1 also shows that the addition of citric acid gave rise to asignificant increase in the surface area (>100 m2/g) of thephosphides compared to the samples prepared without CA(<10 m2/g). The Ni2P catalyst also had a higher surface area(216 m2/g) than that reported for unsupported Ni2P prepared fromprecursor salts mixed with ethylene glycol and a surfactant [16].The MoP sample had a surface area of 139 m2/g, similar to the valuereported by Cheng et al. [17], who first reported the use of citricacid mixed with the precursor salts to obtain high surface areaMoP. The Ni0.25MoP catalyst also had a much higher surface area(115 m2/g) than that of the Ni0.25MoP catalyst prepared without CA(4 m2/g), indicating that the bimetallic phosphides could also beprepared with high surface area by adding CA to the precursor saltsolutions. There was a gradual decrease in the surface area uponaddition of more Ni to the MoP. Note that the pore volumes weresimilar over all the MoP and NixMoP samples, whereas the porevolume of the Ni2P catalyst was significantly higher than that of theNi2P prepared without CA. In addition, all the samples preparedwith CA had lower average pore size than the correspondingsamples prepared without CA.

The catalyst particle size was estimated from the BET surfacearea using the equation dP = 6/[SBETr], assuming cubic or sphericalgeometry, where dP is the particle size, SBET is the BET surface areaand r is the density of the phase of interest. The particle size of theNi2P and MoP catalysts prepared without citric acid were

Table 1Textural properties of different metal phosphides prepared by TPR of calcined precursor salts mixed with citric acid (CA).

Sample XRD BET area (m2/g) Pore volume (cm3/g) Average pore size (nm)

Phase Crystallite size (nm)

MoP MoP 14 139 0.07 2

MoP – no CA MoP 17 5 0.04 18

Ni2P Ni2P 20 216 0.28 5

Ni2P – no CA Ni2P 33 2 0.03 19

Ni0.125MoP MoP 15 121 0.07 2

Ni0.25MoP MoP, NiMoP 13, 6 115 0.06 2

Ni0.25MoP – no CA MoP, NiMoP 18, 22 4 0.04 18

Ni0.5MoP MoP, NiMoP 16, 10 107 0.06 2

Ni0.75MoP MoP, NiMoP 30, 22 23 0.03 7

NiMoP MoP, NiMoP 32, 24 16 0.03 7

R. Wang, K.J. Smith / Applied Catalysis A: General 361 (2009) 18–25 21

estimated at 206 and 172 nm, respectively, significantly largerthan the crystallite size value calculated from the XRD data usingScherrer’s equation (33 and 17 nm), suggesting significantagglomeration of the metal phosphide crystallites preparedwithout citric acid. In contrast, the calculated particle size of theNi2P and MoP samples prepared with citric acid was only 4 and6 nm, respectively, somewhat smaller than the crystallite sizevalues obtained from the XRD data (20 and 14 nm, respectively).These differences suggest that the CA prepared metal phosphidesinclude very small particles not identified by XRD. TEM was used tofurther investigate the structure of the samples, and the results areshown in Fig. 5. In the case of the MoP catalyst prepared without CA(MoP – no CA; Fig. 5a), the adjacent small particles aggregated toform large nanoclusters with a size of 50–60 nm, whereas for theMoP sample (Fig. 5b) separate particles of size 6–10 nm wereclearly evident.

Fig. 5. TEM images of (a) MoP – no

The TEM picture of the Ni2P catalyst (Fig. 5c) showed thepresence of amorphous, small and large particles in the sample andhence the high surface area of the nickel phosphide was likely fromamorphous nickel phosphide. Yang et al. [16] also observed that,for the high surface area Ni2P prepared using ethylene glycol and apolymer surfactant (Triton X-114) in the precursor mixture, thesmall particles were amorphous and the large particles werecrystalline. Hence we conclude that the high surface area Ni2Pcatalyst included small particles of amorphous Ni2P, whereas theMoP was composed of separate, nanocrystallites.

Two important effects of citric acid have been suggested in theliterature [19]. Firstly, citric acid has the ability to form complexeswith Ni and Mo and crystallization of the formed nickel citrate andmolybdenum citrate during drying is minimal, thereby inhibitingaggregation of the metal species. Secondly, during calcination, thespace occupied by citric acid is converted to mesopores that do not

CA (b) MoP (c) Ni2P catalysts.

Fig. 6. TPR profiles of different catalyst precursors prepared with CA: (a) Ni2P (b)

MoP (c) Ni0.25MoP (d) Ni0.5MoP (e) Ni0.75MoP (f) NiMoP.

R. Wang, K.J. Smith / Applied Catalysis A: General 361 (2009) 18–2522

collapse after elimination of the citric acid, and are also likelycomposed of amorphous metal phosphide. The TEM micrographsand low pore volumes reported herein suggest that the formationof metal complexes that inhibit metal aggregation is the morelikely explanation of the role of citric acid in the synthesis of thesehigh surface area metal phosphides.

The TPR profiles of the calcined catalyst precursors are shown inFig. 6. The TPR profile of the MoP precursor showed two main peaksat 907 and 985 K. The lower-temperature peak is attributed to thereduction of Mo6+ to Mo4+, and the higher-temperature peak is acomposite of different peaks (reduction of Mo4+ to Mo0 and P5+ toP0) [18]. For the Ni2P precursor, two reduction peaks wereobserved at about 959 and 994 K. No reduction of bulk NiO, thatoccurs at 673 K, was observed. The two peaks are attributed to thelow-temperature reduction of the nickel oxide-phosphate, fol-lowed by the reduction of the nickel phosphate to Ni2P at highertemperature [9]. Compared to the catalysts prepared with CA, theTPR profiles of both the MoP – no CA and Ni2P – no CA (Fig. 7) weresimilar, but the reduction peak temperature of the sampleswithout CA were higher, indicating that the presence of citric acid

Fig. 7. TPR profiles of catalyst precursors prepared with and without CA.

may favor the reduction of the metal phosphide precursors. Themodel of the Mo–O–P precursor suggests a high occurrence ofMoO6 octahedra separated by single PO4 groups [20]. We suggestthat the use of CA promotes the homogeneity of metal and P atomsin the amorphous oxidic precursor, implying a lower diffusion pathlength for P atoms [20], to account for the lower reductiontemperature of the CA samples. For the NixMoP precursor (Fig. 6),the reduction temperature significantly decreased as the Nicontent increased, indicating the presence of an interactionbetween the Ni and the MoP in the calcined precursors, promotingthe reduction of the MoP precursor. Also note that a new shoulderpeak occurred at approximately 773 K for the Ni0.25MoP precursor,indicative of the presence of a new NiMoP precursor species.Moreover, the intensity of this peak increased gradually as the Nicontent increased, in agreement with the increased formation of aNiMoP phase, as demonstrated by the XRD results.

In previous studies, correlations have been demonstratedbetween the electronic properties and the HDS activity of metalsulfide and phosphide catalysts. In this study, XPS was used tocharacterize the electronic properties of the surfaces of thepassivated metal phosphide catalysts. The spectra of the Ni (2p),Mo (3d) and the P (2p) regions are shown in Fig. 8 and thecorresponding binding energies (BEs) are reported in Table 2. In allcases, the spectra showed oxidized Ni, Mo and P species as well asreduced species associated with the phosphide phase, becauseafter TPR synthesis, the catalysts were passivated using 1% O2 in Heto protect them from deep oxidation. The binding energies of theoxidized Ni 2p (855.8–856.7 eV), Mo 3d (232.3–232.6 eV) and P 2p(133.5–133.9 eV) are consistent with assignments by others forNi2+, Mo6+ and P5+ species, respectively [21]. In the XPS spectrum ofthe MoP catalyst prepared using CA (Fig. 8), the peaks at 228.4 and129.8 eV were assigned to reduced Ni and P species, respectively.The binding energy of Mo was higher than metallic Mo0 species,indicating that the Mo in MoP had a partial positive charge (d+),where 0 < d < 4, which gave rise to a decrease in electron densityof Mo 3d. The binding energy of P was lower than that of P0 species,suggesting the P had a partial negative charge (d�), where0 < d < 1. Therefore, a small transfer of electron density fromthe metal to the P occurred in MoP, consistent with reports byothers [22]. For the Ni2P catalyst (Table 3), two peaks at 853.5 and129.9 eV were ascribed to the Ni

d+ (0 < d < 2) and Pd� (0 < d < 1)

species, suggesting a transfer of electron density from Ni to P alsooccurred.

The XPS spectra of the NixMoP catalysts prepared using CA(Fig. 8) were similar to that of the MoP catalyst but for someimportant differences in the BEs. The addition of Ni led to anincrease in the binding energy of Ni 2p and a decrease in the BE ofMo 3d, suggesting that a transfer of electron density from Ni to Mooccurred. This may be associated with the interaction between Niand MoP identified by the XRD and TPR measurements. Thetransfer of electron density from Ni to Mo is consistent withPauling’s electronegativity (Ni = 1.91 and Mo = 2.16). Moreover, asmore Ni was added to the NixMoP the Mo 3d BE decreased and theNi 2p BE increased gradually, as a consequence of the stronger Ni–Mo interaction discussed previously in regards to the reported TPRdata.

Table 2 also summarizes the measured catalyst bulk composi-tions and the surface compositions determined by XPS. The Ni2Pcatalyst had a bulk P/Ni ratio equal to the stoichiometric ratio (0.5),but much lower than the initial ratio of 2.0. Previous studies haveshown that phosphorous is removed from the catalyst as PH3

during reduction [8], but Table 2 shows that the surface remainedenriched in P relative to the bulk composition. As shown in Table 2,the MoP and Ni2P catalysts prepared with CA had P-rich surfacescompared to the bulk composition. For example, surface and bulkmolar ratios of Ps/Mes = 0.79 and P/Me = 0.5 (Me = Ni + Mo),

Fig. 8. XPS of (a) Ni 2p, (b) Mo 3d and (c) P 2p region for metal phosphides prepared

using CA.

Table 2Summary of XPS analysis of the reduced metal phosphide catalysts.

Catalyst Binding energy (eV) Composition (atom ratio)

Ni 2p Mo 3d P 2p Nominal

P/Me

Surface

P/Me

Bulk

P/Me

MoP – 228.4 129.8 1 1.33 1.0

232.3 133.9

MoP – no CA – 228.7 130.1 1 1.24 –

232.5 133.9

Ni0.25MoP 853.6 228.2 129.8 0.8 1.04 –

232.3 133.7

Ni0.5MoP 853.7 228.3 129.7 0.67 0.99 –

231.5 133.5

NiMoP 853.9 228.2 130.2 0.5 0.79 0.5

232.6 133.5

Ni2P 853.5 – 129.9 2 1.54 0.5

Ni2P – no CA 853.6 – 130.1 2 1.91 –

Me = Ni, Mo or Ni + Mo.

R. Wang, K.J. Smith / Applied Catalysis A: General 361 (2009) 18–25 23

respectively, were observed for the NiMoP catalyst. The surface P/Me ratio of the MoP sample was similar to the sample preparedwithout CA (1.33 vs 1.24) whereas the surface P/Me ratio of theNi2P sample was lower than that of the Ni2P prepared without CA.These observations suggest more P loss during the preparation ofNi2P in the presence of CA compared to the preparation withoutCA. Note that the surface phosphorous species reported in Table 2include phosphide and phosphate, but the phosphide wasdominant. The surface phosphate has been related to the acidicproperties of the catalyst [18].

Burns et al. [23] have reported that the monometallicphosphide catalysts (Co2P/SiO2 and Ni2P/SiO2) both have metal-rich surface compositions relative to the measured bulk composi-tions. This contrasting result may be due to the interactionbetween P and the silica support that anchor the P to the support.

3.2. HDS of 4,6-DMDBT

The hydrodesulfurization of 4,6-DMDBT was performed at583 K, 3.0 MPa, and Fig. 9 and Table 3 show the HDS conversionsobtained over the MoP and Ni2P prepared with CA and without CA,as a function of time-on-stream. The data show that the initialconversions of the catalysts prepared with CA were uniformly high,with 74.7% and 87.7% for MoP and Ni2P, respectively, and theseconversions were significantly higher than those of the catalystsprepared without citric acid (30–55%). The higher conversions areno doubt a result of the higher surface area of the catalystsprepared using citric acid. CO chemisorption data measured atroom temperature for each of the catalysts was used to estimatethe number of active sites, and the data shown in Table 3 clearlydemonstrate that the catalysts prepared with CA had much higherCO uptake than the samples prepared without CA. The Ni2Pprepared herein using CA had similar activity, on a Ni2P mass basis,to that reported for a Ni2P/SiO2 catalyst that was shown to be moreactive than a commercial NiMoS/Al2O3 catalyst [24].

For the MoP catalysts, a significant deactivation with time-on-stream was observed and stable activities, measured afterapproximately 4 h time-on-stream, were similar for both the CAprepared catalysts and the conventionally prepared samples. Forthe Ni2P prepared with citric acid, the catalyst activity was stable,whereas the conversion gradually decreased with time-on-streamover the Ni2P catalyst prepared without CA.

With increasing Ni content, the initial conversion over NixMoPgradually decreased, while the stability of the catalyst wasenhanced. The decrease in initial conversion is likely associatedwith an increased crystallite size of the MoP and NiMoP phases anda decrease in surface area. For the NiMoP catalyst, the HDSconversion during reaction was very stable, although the initialconversion was low.

Shu et al. [24] have proposed that the HDS of 4,6-DMDBT is astructure-sensitive reaction. Therefore, we have compared catalyticactivity using turnover frequency (TOF) based on the COuptake measured on the fresh catalysts (Fig. 10). The HDS activityof the catalysts prepared using CA decreased in the order:

Table 3CO uptake and 4,6-DMDBT HDS performance over metal phosphide catalysts measured at 583 K and 3.0 MPa.

Catalyst CO uptake

(mmol/g)

Initial

conversion (%)

Final

conversion (%)

TOF

(10�3 s�1)

Selectivity (%) Ratio

THDMDBT +

HHDMDBT

DMBP MCHT DMBCH (THDMDBT +

HHDMDBT)/MCHT

MCHT/

DMBP

Ni2P 30.1 87.7 80.6 7.11 4.2 8.5 66.6 19.3 0.06 7.83

Ni2P – no CA <1 35.2 21.6 – 35.3 13.4 51.3 – 0.69 3.83

MoP 42.4 74.7 34.3 3.33 12.2 12.2 52.3 22.8 0.23 4.28

MoP – no CA <1 54.5 31.5 – 34.4 3.5 26.5 3.6 1.86 7.57

Ni0.125MoP 26.6 59.2 26.7 3.44 11.7 7.8 62.1 18.4 0.19 7.96

Ni0.25MoP 18.3 51.1 26.1 4.06 8.7 16.9 64.1 9.7 0.14 3.79

Ni0.25MoP – no CA <1 34.2 24.3 – 22.0 6.6 44.7 26.7 0.49 6.77

Ni0.5MoP 27.8 65.1 23.9 3.86 3.8 23.8 67.1 9.0 0.07 2.91

Ni0.75MoP 18.5 56.2 39.5 4.54 4.0 10.6 57.8 15.4 0.06 5.45

NiMoP 7.1 31.9 28.9 5.52 16.0 16.0 51.9 6.1 0.31 3.24

THDMDBT: 4,6-tetrahydro-dimethyldibenzothiophene; HHDMDBT: hexahydro-dimethyldibenzothiophene.

R. Wang, K.J. Smith / Applied Catalysis A: General 361 (2009) 18–2524

Ni2P > NixMoP > MoP, and the TOF value of the NixMoP seriesincreased with increased Ni content (Fig. 10), in agreement with theresults of DBT HDS over supported NixMoP/SiO2 catalysts reportedby Sun et al. [7]. The trend in HDS activity can be understood if onetakes into consideration the different electronic properties shown inTable 3 and Fig. 8 for the metal cations of MoP, Ni2P, NixMoP. Theelectron density of the Mo increased gradually with increased Nicontent. It is well known that the LUMO of thiophene, DBT and 4,6-DMDBT is C–S anti-bonding. Surfaces that are able to transferelectron density to this orbital facilitate the dissociation of thethiophene, DBT and 4,6-DMDBT molecules. Therefore the increasedelectron density of the Mo surface over NixMoP catalysts mayaccount for the higher TOF value of NixMoP compared with MoP. TheHDS activity of the metal atoms has been theoretically correlatedwith the order of density of metal d states near the Fermi level:Ni2P > NiMoP > MoP [10] and the present data confirm this trend.Although the theoretical calculations of Rodriguez et al. [10] supportthe activity trends observed herein, no promotional effect of Ni onthe HDS activity of supported MoP catalysts was observed by theseauthors. They reported that the HDS activity of thiophene on NiMoP/SiO2 was lower than MoP/SiO2 catalysts [10] and concluded that thelow activity of NiMoP could not be traced to the relative dispersion orsurface area of the catalysts, but may be a result of interactions withthe silica support, that is clearly absent in the present work. Henceour results confirm that the differences in activities among thecatalysts reported herein are associated with the electronicproperties of the metal phosphides.

Fig. 9. 4,6-DMDBT conversion measured at 583 K and 3.0 MPa for catalysts prepared

with and without CA: (a) MoP (b) Ni2P (c) Ni0.25MoP.

3.3. Reaction pathway

The reaction network of 4,6-dimethyldibenzothiopheneinvolves two parallel reactions that lead to desulfurized products[25–27]. One is denoted the direct desulfurization pathway (DDS)which, in the case of 4,6-DMDBT, results in the formation of 3,3-dimethylbiphenyl (3,3-DMBP). The second is referred to as thehydrogenation pathway (HYD) and involves the pre-hydrogena-tion of 4,6-DMDBT to 4,6-tetrahydro- and hexahydro-dimethyldi-benzothiophene (THDMDBT and HHDMDBT), which aresubsequently desulfurized to methylcyclohexyltoluene (MCHT)and dimethylbicylcohexane (DMBCH). The ratio of the principalproducts of the HYD and DDS routes (MCHT/DMBP) shows that thedominant pathway is via the HYD route for all the catalysts of thepresent study (Table 3).

To elucidate the effect of citric acid and Ni addition on thereaction pathway of 4,6-DMDBT, the initial reaction productdistribution and HDS conversions were compared for the differentcatalysts of the present study. Table 3 shows that the DMBPselectivities of the MoP and Ni0.25MoP catalysts prepared using CAwere higher than the same catalysts prepared without CA. Thesame comparison of the Ni2P catalysts is influenced by the fact thatDMBP selectivity increases as HDS conversion decreases withincreased space velocity. Hence it is important to compare thedifferences in DMBP selectivity of the CA prepared catalysts andthe catalysts prepared without CA at approximately the sameconversion. This is very different to sulfide catalysts, on which

Fig. 10. Initial TOF at 583 K versus Ni content of unsupported Ni2P, MoP and NixMoP

catalysts prepared with citric acid.

R. Wang, K.J. Smith / Applied Catalysis A: General 361 (2009) 18–25 25

DMBP selectivity hardly changes with a variation in space velocity[29]. Data obtained at the same conversion (35%) confirmed thatthe DMBP selectivities of the catalysts prepared using CA werehigher than those prepared without CA. For the MoP prepared withand without CA, the DMBP selectivities were 2.7% and 0%,respectively, whereas the DMBP selectivities of the Ni2P catalystswere 18.4% and 13.4%, respectively.

It has been reported that an increase in the Brønsted acidity ofmetal phosphides can facilitate the DDS pathway of HDS of 4,6-DMDBT. Our previous studies [28] showed that the DDS pathwayfor the HDS of 4,6-DMDBT correlated with the surface phosphorouscontent of a series of CoxNi2P catalysts. The source of the acid sitesis likely a consequence of the incomplete reduction of phosphatespecies and an increase in the surface P/Me (Me – metal) ratio ofthe phosphides can result in an increase in Brønsted acidity. In thepresent work, the surface P/Me ratio (Table 2) from XPS was similarover the MoP and Ni2P catalysts for both the CA prepared catalystsand the catalysts prepared without CA. Furthermore the P/Me ratioreported herein was significantly lower than that reportedpreviously for CoxNi2P catalysts, suggesting significantly loweracidity of the NixMoP catalysts. Measurement of acidity by n-propylamine uptake confirmed this observation with uptakes of<1.5 mmol/g for the MoP and Ni2P catalysts.

Hence, we conclude that the acceleration of the DDS pathwayover the catalysts prepared using CA must be associated with thecrystallite size of the catalysts. The CA catalysts had much smallercrystallite size than the catalysts prepared without CA, and thismay lead to less steric hindrance during s adsorption of reactants,resulting in a higher DMBP selectivity. Rothlisberger and Prins [29]reported a similar trend over Pt-based catalysts and similar resultswere also observed over the Ni2P/C catalysts with differentdispersion by Shu and Oyama [30]. In contrast, however, Oyamaand Lee [31] have more recently reported a catalytic activitysequence of Ni2P/MCM-41 > Ni2P/SiO2-H > Ni2P/SiO2-L as thecrystallite size decreased, whereas the trend in DMBP selectivity(DDS pathway) was opposite. Oyama and Lee [31] have proposedthat the HDS pathway is determined by the presence of two typesof sites in Ni2P catalysts: tetrahedral Ni(1) sites and squarepyramidal Ni(2) sites. They concluded that the Ni(1) sites wereresponsible for DDS while the Ni(2) are highly active sites for theHYD route. These same sites have been invoked by Burns et al. [23]to explain the high activity of CoxNi2�xP catalysts, in which theNi(2) sites result in P enriched surfaces that are resistant to siteblockage by S. In the present study, however, it appears that theNi2P and NixMoP are dominated by Ni(2) sites, based on theproduct selectivities, even though the surface composition of thecatalysts showed P enrichment of the surface.

In the case of NixMoP catalysts, the activity in the DDS pathwaywas only marginally affected, since no significant change in theselectivity of DMBP was observed as Ni was added to the MoP.However, the (THDMDBT + HHDMDBT)/MCHT ratio decreasedwith addition of Ni, although the conversion also decreased(which should lead to an increase in the value of this ratio),suggesting that Ni increased the rate of sulfur elimination fromhydrogenated intermediates of the HYD route. For sulfidedcatalysts, it has been reported that Ni strongly enhances theactivity of MoS2/alumina for the direct desulfurization of DBT and4,6-DMDBT and the final sulfur-removal step in the hydrogenation

pathway, while the hydrogenation was moderately promoted [27].For the metal phosphides reported in the present study, theenhancement in the C–S bond breaking ability was much morepronounced for the HYD pathway, where the hydrogenatedintermediates yield products with less steric hindrance and easieradsorption.

4. Conclusions

Results presented herein demonstrate that unsupportedmonometallic phosphides (MoP and Ni2P) and bimetallic nickelmolybdenum phosphides (NixMoP) can be prepared with highsurface area by adding citric acid to precursor metal salt solutionsbefore drying, calcination and temperature-programmed reduc-tion. The catalyst surface areas were significantly greater than thatobtained when CA was not used in the preparation. The HDSactivity of the catalysts was enhanced and among the studiedphosphides, Ni2P exhibited the highest HDS activity. It was alsofound that the electronic properties of Ni2P, NixMoP and MoPcorrelated with their HDS TOF and for the NixMoP catalysts the TOFincreased with increased Ni content.

Acknowledgement

Financial support from the Natural Sciences and EngineeringResearch Council of Canada is gratefully acknowledged.

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