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Applied Catalysis A: General 469 (2014) 81– 88

Contents lists available at ScienceDirect

Applied Catalysis A: General

j ourna l h om epage: www.elsev ier .com/ locate /apcata

ilicon carbide foam decorated with carbon nanofibers as catalytictirrer in liquid-phase hydrogenation reactions

ai Truong-Phuoca, Tri Truong-Huua,b, Lam Nguyen-Dinhb, Walid Baaziza,hierry Romeroa, David Edouardc, Dominique Begina, Izabela Janowskaa,uong Pham-Huua,∗

Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), ECPM, CNRS-Université de Strasbourg (UDS) UMR 7515, 25,ue Becquerel, 67087 Strasbourg Cedex 08, FranceThe University of Da-Nang, University of Science and Technology, 54, Nguyen Luong Bang, Da-Nang, Viet NamLAGEP, Université Claude Bernard Lyon 1, UMR CNRS 5007, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France

r t i c l e i n f o

rticle history:eceived 20 April 2013eceived in revised form4 September 2013ccepted 17 September 2013vailable online 26 September 2013

a b s t r a c t

Silicon carbide solid foams decorated with a dense, homogeneous and mechanically strength layer ofcarbon nanofibers were successfully employed as catalyst support for the palladium active phase in theliquid-phase hydrogenation of C C bond. The carbon nanofibers provide a high density of anchoragesites for improving the dispersion of the metal nanoparticles compared to the pure SiC support wherelow metal–support interaction leads to the formation of larger palladium particles. The catalyst wasdirectly used as a rotating foam catalytic stirrer and has shown a better catalytic performance in the

eywords:arbon nanofibersilicon carbide foamatalytic stirrerydrogenation

liquid-phase selective hydrogenation compared to that obtained on a palladium deposited on a pristineSiC support. The macroscopic shaping of the catalyst also allows an easy product/catalyst separationafter the reaction. The catalyst also displays an extremely high stability as a function of cycling tests asno hydrogenation activity lost was observed after ten cycling tests.

© 2013 Elsevier B.V. All rights reserved.

atalysisalladium

. Introduction

Solid foams (or open cell foams) have received an over increas-ng scientific and industrial interest as catalyst support since theast decade owing to their exceptional properties, namely highccessible porosity and effective surface area, low pressure dropnd good mixing behavior [1–3]. The use of the foam structuredatalysts instead of slurry catalysts allows one to avoid the costlynd time consumption filtration process to separate the catalystrom the final product. Structured catalysts also prevent the prob-em of fine formation as encountered with slurry reactor due to thettrition of small particles under vigorous stirring which lead tohe plugging of the filtration device. It is expected that the designf new catalytic generation containing smaller amount of catalyticctive phase with higher selectivity, high efficient recovery fromeaction medium and high durability will significantly contribute

o the development of the more environmental friendly catalyticrocesses with a high cost-effectiveness.

∗ Corresponding author. Tel.: +33 3 68 85 26 75; fax: +33 3 68 85 26 74.E-mail address: cuong.pham-huu@unistra.fr (C. Pham-Huu).

926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apcata.2013.09.032

Solid foam as catalyst support has been proposed by Twigg andRichardson [4] in the early of 2000 to replace honeycomb ceramicmonoliths. Compared to the honeycomb monolith foam displays anextremely high degree of radial mixing of the flow passing throughit porous matrix, being it gaseous or liquid, which considerablyimprove the reactant mixing and also the heat transfer for thereaction. In addition, the open structure of the foam also signifi-cantly reduces the pressure drop along the catalyst bed. Solid foamemployed as catalytic stirrer has long been developed by the groupof Schouten and co-workers [5–9] during the last decade for sev-eral gas–liquid phase reactions. The catalytic stirrer developed wasbased on aluminum metal foam coated with an alumina wash-coatlayer. The reactant and product diffusion rates are closely dependedto the thickness of the alumina wash-coat layer [6,7]. In addition,the high reactivity of the alumina wash-coat and the metallic phaseunderneath render impossible any attempts to recover the activephase deposited on the support surface due to the problem linkedwith corrosion. The use of such composite support is also hamperedin some processes such as the Friedel-Crafts reaction where acidic

compounds were formed during the course of the reaction whichrender the reaction medium corrosive and thus, could becomedetrimental for the aluminum support. Therefore, it is of interestto find new kinds of support with better performance along with

8 Cataly

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mproved resistance in harsh conditions. The drawback linked withommercial available solid foams, i.e. aluminum, �-alumina, metal,s linked with their relatively low specific surface area and thus, a

ash-coat layer was necessary in order to enhance the dispersionf the deposited active phase.

Silicon carbide (SiC) is a covalent material, with a tetrahedraltructural unit with an excellent thermal stability, high mechanicaltrength, excellent resistance to oxidation and corrosion, low coef-cient of thermal expansion and rather high thermal conductivity.ince the last decades, silicon carbide support, pure or doped, withedium to high specific surface area (25–110 m2 g−1), synthesized

y a gas–solid reaction [10–12] has been employed as catalyst sup-ort in several catalytic processes such as selective oxidation [13],

somerization of long chain hydrocarbons [14], Fischer-Tropschynthesis [15–18], fine chemical production [19,20], and photo-atalysis [21]. The gas–solid synthesis allows one to obtain SiCupports with different size and shape depending to the down-tream application [11]. Among the different SiC materials, theost appropriate for gas–liquid phase reactions is the foam struc-

ure with a high voidage (>87%) and high accessible surface area22–24]. The specific surface area of the SiC foam is in the range of5–30 m2 g−1, which allows the reasonable dispersion of the metalr oxide active phase on its surface. However, for liquid-phase reac-ions, the effective surface area of the support and the size of thective phase particle size should be optimized in order to offer

maximum contact surface between the reactant and the cata-yst and to reduce as much as possible the diffusion length of theeactant toward the active site [25].

More recently, carbon nanofibers (CNFs) with high aspect ratiothe aspect ratio is defined as the length of the major axis dividedy the width or diameter of the minor axis), have received an over

ncreasing scientific interest in the field of heterogeneous catalysisuring the two last decades due to their exceptional physical andhemical properties [26–29]. Indeed, in the heterogeneous catalysispplication, the nanoscopic dimension of the CNFs provides a highffective and fully accessible surface area to the reactant. Further-ore, the size of the active phase particle size should be optimized

n order to reduce as much as possible the diffusion length of theeactant toward the active site and to improve the density of theatalytic site [29–31].

However, for catalytic applications, the direct use of CNFs in powder form is limited due either to the problems linked withhe high pressure drop across the catalytic bed (fixed bed gas-hase reactor) or to the problem of catalyst recovery (liquid-phaseeactor). In order to solve this problem, attempts to make CNFnd/or CNT with controlled macroscopic shape have been reportedy supporting them on a macroscopic host matrix, i.e. foam, felt,pheres [24,25,32–34]. In addition, the nanoscopic dimension andhe absence of micropores of the supported CNFs significantlyeduce the mass transfer limitations, especially in liquid-phaseeactions [35–37].

In this context, the aim of the present article is to combine thehysical properties of the SiC foam and the nanoscopic dimension ofhe carbon nanofibers (CNFs), with a high intrinsic specific surfacerea, i.e. >100 m2 g−1 [38–40], inside a single hierarchical compos-te which significantly improve the dispersion of the active phasearticles and increase the contact surface between the reactantnd the catalyst. For the detail description of the different physicalnd chemical properties of these nanocarbon materials as supportor metal in the selective hydrogenation reactions, a recent well-ocumented review has been published [41]. The advantage of aierarchical structured material made of carbon nanofibers deco-

ating a macroscopic host matrix, i.e. SiC foam, is to provide, on oneand, a support with high effective surface area and short diffu-ion paths for liquid phase reactions and on the other hand, an easyecovery of the catalyst from the reaction medium.

sis A: General 469 (2014) 81– 88

2. Experimental

2.1. SiC foam synthesis

Silicon carbide (�-SiC) in a foam shape was synthesized via agas–solid reaction between the SiO vapor and solid carbon [9]. Thedetailed synthesis of the SiC-based materials is summarized in areview [11].

2.2. Carbon nanofibers/SiC composite synthesis

The carbon nanofibers decorated SiC foam (CNF/SiC) compositewas synthesized as follows: the SiC foam, with a medium specificsurface area of 20 m2 g−1, was impregnated with an aqueous solu-tion containing a nickel nitrate salt (Ni(NO3)2·6H2O). The nickelcontent was fixed at 2 wt.%. The as-impregnated solid was oven-dried at 110 ◦C overnight and then calcined in air at 350 ◦C for2 h to decompose the nickel salt into the corresponding nickeloxide. The nickel oxide precursor was reduced in situ under H2flow at 400 ◦C for 2 h. Then, the reactor temperature was raisedfrom 400 ◦C to 680 ◦C (heating rate of 30 ◦C min−1) and the H2 flowwas replaced by a mixture of ethane and hydrogen (total flow rate,20 and 100 mL min−1) and the synthesis was lasted at 680 ◦C for2 h. The reactor was cooled down under the reactant mixture flowdown to 400 ◦C and then, the reactant mixture flow was replacedby an argon flow and the product was discharged from the reac-tor at room temperature. Details of the synthesis can be found inprevious reports [24,33]. The deposition of a thin layer of carbonnanofibers on the SiC foam surface was evidenced by a color changefrom green to dark green between the starting SiC foam and the SiCfoam decorated with carbon nanofibers (Fig. 1A). The compositewas treated in an aqueous HNO3 (15 vol.%) in order to remove thegrowth catalyst. It is worthy to note that part of the nickel cata-lyst was encapsulated within carbon layers and thus some residualnickel is still remaining inside the composite even after an acidtreatment. However, these remained nickel particles are no longeraccessible during the subsequence catalytic evaluation. In addition,nickel is not active under the reaction conditions used in the presentwork and thus, residual activity linked with nickel can be neglected.Catalytic test carried out on the SiC-CNFs, after acid treatment toremove the nickel growth catalyst, shows marginal hydrogenationactivity which confirms the inactivity of the remained nickel insidethe support matrix. The absence of any nickel nanoparticles on thesurface of the material was also confirmed by the TGA experiment(see below).

2.3. Palladium deposition

The palladium was deposited via an incipient wetness impreg-nation method using a palladium nitrate aqueous solution as saltprecursor (Pd(NO3)2·6H2O). The impregnated solid was oven driedat 110 ◦C for 2 h and calcinated in air at 350 ◦C for 2 h in order todecompose the palladium nitrate into its corresponding oxide. Thecatalyst was further reduced under hydrogen flow at 350 ◦C for 2 h.The catalyst was passivated with a flow of O2 (1 vol.%), diluted inhelium, at room temperature for 30 min, before discharging fromthe reactor in order to avoid an excessive oxidation of the palladiumnanoparticles.

2.4. Characterization techniques

The morphology of the composite was characterized by scan-

ning electron microscopy (SEM) on a JEOL F2600 microscopeequipped with a CCD camera. The sample was covered with a thinlayer of gold before analysis in order to avoid the problem of charg-ing effects.

L. Truong-Phuoc et al. / Applied Catalysis A: General 469 (2014) 81– 88 83

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ig. 1. (A) Silicon carbide foam, before (left) and after (right) decorated with carbof carbon nanofibers on its surface. (B) Pd/CNF-SiC foam catalyst mounted inside aeaction.

The microstructure of the carbon nanofibers and the depositedetal active phase was analyzed by transmission electronicroscopy (TEM). The TEM analyze was carried out on a TOP-

ON EB-002 HR working at 200 kV accelerated voltage and aoint-to-point resolution of 0.2 nm. The sample was dispersed byltrasounds in an ethanol solution for 5 min and a drop of the solu-ion was deposited on a copper grid covered with a holey carbon

embrane for observation.The thermal gravimetric analysis (TGA) was carried out on a TA

nstruments Model Q-5000. The known amount of sample (50 mg)as loaded into a platinum crucible and heated up in air from room

emperature to 1000 ◦C with a heating rate of 5 ◦C min−1.BET surface areas were measured using a commercial BET unit

Tristar, Micromeritics) using N2 adsorption at 77 K. Before the N2dsorption, samples were heated at 300 ◦C for 3 h under dynamicacuum to desorb surface impurities. SBET is the surface area ofhe sample calculated from the nitrogen isotherm using the BET

ethod. Micropore surface area was calculated using the t-plotethod. The pore size distribution was determined from the results

btained on the desorption branch of the N2 isotherm.

.5. Selective hydrogenation of C C bond in a rotating foamtirrer reactor

The selective hydrogenation of cinnamaldehyde (CALD) leads tohe formation of three reaction products namely hydrocinnamalde-yde (HALD) which was formed by the selective hydrogenationf the C C bond, cinnamyl alcohol (CALC) which was formed byhe selective hydrogenation of the C O bond and hydrocinnamyllcohol (HALC) which was formed by a complete hydrogenation ofoth unsatured C C and C O bonds. The selective hydrogenationf the cinnamaldehyde was carried out in a glass reactor (vol-me = 250 mL). The foam catalyst was mounted on a glass holderxed to a mechanical stirrer and thus, acting both as a catalyst sup-ort and as stirrer (Fig. 1B). The catalyst foam was directly fixedertically at the end point of the mechanical stirrer (Fig. 2B). Works ongoing to evaluate the influence of the other stirrer-catalystonfiguration such as those reported by Schouten and co-workers7], i.e. T-shaped stirrer, on the hydrogenation performance. Theeaction medium contains 0.72 mol L−1 of cinnamaldehyde dilutedn a dioxane solvent and was heated up from room temperature tohe reaction temperature (80 ◦C) with a heating rate of 5 ◦C min−1.ioxane was used instead of alcohol in order to avoid any homo-eneous reactions which could lead to the formation of heavier

y-products [42–44]. At 80 ◦C the foam catalyst was immersed intohe reaction medium and rotated at different speed (200–450 rpm).he hydrogen was continuously supplied into the reactor mediumt a flow rate of 60 mL min−1. The hydrogen stream was regulated

ofibers. The support becomes darker due to the formation of a homogeneous layerholder fixed vertically at the end of the stirrer for the liquid-phase hydrogenation

by a Brooks 5850 TR mass flow controller. The liquid medium waswithdrawn at a regular time and analyzed by a gas chromatogra-phy (GC) on a Varian 3800 CX equipped with a PONA column anda Flame Ionization Detector (FID). The products were calibrated byusing pure components diluted in a dioxane solution. The conver-sion and product distribution were calculated from the GC results.

3. Results and discussion

3.1. CNF/SiC composite characteristics

The SEM analysis of the morphology of the SiC foam decoratedwith a layer of carbon nanofibers (CNFs) is presented in Fig. 2A andB. The CNFs content determined by TGA is amounted to 20 wt.%.It is worthy to note that the macroscopic shape of the host matrixwas completely retained after deposition of a CNF layer on its sur-face which allows one to control the macroscopic shape of thecatalyst by controlling the shape of the host matrix [32–34]. Thelow magnification SEM micrograph (Fig. 2A) indicates the homo-geneous coverage of the SiC host matrix by a dense layer of CNF.The formed CNFs have an average diameter of around 80 ± 10 nm(Fig. 2B) and a length up to several micrometers which indicatesthe significant high aspect ratio of these materials. The completecoverage of the SiC foam structure by a layer of CNF was attributedto the exceptional length of the formed CNFs which rapidly cov-ered the entire surface of the host matrix similarly to the case ofcarbon nanofibers decorated graphite felt [29,33]. The thickness ofthe CNFs layer determined by SEM was about 20 �m which afforda high accessibility of the liquid to the active sites localized on thesurface of the CNF.

The amount of the CNFs deposited onto the SiC macroscopichost structure, after acid treatment, was investigated by means ofthe TGA technique and the results are presented in Fig. 3. Accord-ing to the TGA curves (not shown) the amount of the CNFs can befinely tuned by varying the synthesis conditions. The CNFs oxida-tion starts at about 450 ◦C and becomes complete at 650 ◦C whichis in good agreement with the CNFs oxidation reported in the lit-erature. The single derivative peak also accounted for the presenceof a single carbon species in the material, i.e. CNFs, and could allowus to discard the presence of some residual nickel nanoparticles onthe CNFs surface. It is speculate that if some nickel nanoparticleswere presence on the surface of the sample one should expected tofind at least two combustion peaks in the TGA curve: one is linkedto the uncatalytic combustion of the CNFs and the other is linked

with the catalytic combustion of the CNFs by the nickel catalyst.

The CNFs were extremely well anchored on the SiC host matrixas almost no matter lost was observed on the composite after a son-ication treatment for 30 min. Such high mechanical anchorage has

84 L. Truong-Phuoc et al. / Applied Catalysis A: General 469 (2014) 81– 88

F s layen TEM ma

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ig. 2. (A, B) SEM micrographs of the SiC support decorated with a homogeneouanofibers was centered at around 80 ± 10 nm. (C) Representative high-resolution

ngle of 76 ± 5◦ along the fiber axis.

lready been reported in previous works and was attributed to theartially penetration of the CNFs into the SiC matrix which signifi-antly improves the anchorage between the two materials [24,25].imilar results have also been reported by Pham-Huu et al. [33]uring the synthesis of carbon nanofibers on macroscopic graphiteelt host matrix.

The microstructure of the carbon nanofibers was investigated

y TEM and the representative TEM micrographs are presented inig. 2C. The TEM micrograph shows the peculiar orientation of theraphite planes forming an angle of about 76 ± 5◦ with respect to

ig. 3. TGA curve obtained in air medium on the SiC decorated with 17 wt.% of CNFs.he sample was previously treated with acid medium in order to remove the nickelrowth catalyst.

r of carbon nanofibers (20 ± 2 wt.% loading). The average diameter of the carbonicrograph of the CNFs showing the presence of prismatic planes oriented with an

the nanofiber axis. TEM analysis also evidences the presence ofa channel-like in the middle of the fiber. The less dense graphiteplanes in the middle of the fiber can be explained by the differ-ent growth rates at different points of the growth metal catalyst[45,46].

The formation of a layer of CNFs on the SiC foam surface leadsalso to a significant increase of the composite surface area com-pared to that of the pristine SiC, i.e. 47 m2 g−1, with a loading ofabout 17 wt.% of CNFs, instead of 20 m2 g−1 for the pristine SiC. Suchsurface area increases was attributed to the nanoscopic dimensionof the CNF with a high intrinsic effective surface area [47]. Simi-lar results dealing with the specific surface area improvement afterthe CNFs and/or CNTs deposition have also been reported by sev-eral groups in the literature [25,48–50]. It is worthy to note thatfor the SiC-based composite, the optimal loading of the CNFs waskept around 20 wt.%, as with higher loading part of the CNFs wasdetached from the SiC host matrix during sonication treatment andalso during the reaction. On other host matrix such as graphite feltor cloth the maximum loading could be significantly increased upto 100 wt.%.

3.2. Pd/CNF/SiC and Pd/SiC catalysts characteristics

The representative TEM micrographs of the palladium sup-ported on the SiC foam and the same decorated with a layer ofCNFs are presented in Fig. 4 with different magnifications. The TEMresults indicate that the palladium particle size is smaller on the

CNF than the one deposited on the SiC surface which is confirmedby the particle size distribution measured on a population of morethan 600 particles (Fig. 5). The average particle size distributionon the Pd/CNF/SiC catalyst is centered at around 3 ± 1 nm while for

L. Truong-Phuoc et al. / Applied Catalysis A: General 469 (2014) 81– 88 85

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ig. 4. Representative TEM micrographs of the (A) Pd/SiC and (B, C) Pd/CNF/SiC cathe Pd/CNF/SiC showing a profile view of the Pd NPs forming a flat interface with th

he Pd/SiC the particle size distribution is higher at around 7 ± 1 nm.uch observation was in line with the high metal–support interac-ions between the palladium precursor and the prismatic planes ofhe CNF [35]. Vieira et al. [51] have reported high iridium dispersionn the carbon nanofibers surface and attributed such dispersion tohe strong interaction between the metal precursor and the pris-

atic planes of the fiber. Similar results have also been reportedy de Jong and co-workers [52] and Chen and co-workers [53] forifferent metals deposited on carbon nanofiber structures.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 150

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ig. 5. Metal particle size distribution on the Pd/SiC and Pd/CNF/SiC catalysts. Thearticle size distribution was calculated from statistical measurements performedn a population of >600 particles.

after reduction under H2 at 400 ◦C for 2 h. (D) High-resolution TEM micrograph ofmatic planes (indicated by arrows).

The high metal–support interaction between the palladium NPsand the CNFs surface is also confirmed by the high-resolution TEMmicrograph (Fig. 4D) where palladium particles exhibit a flat inter-face with the prismatic planes of the carbon nanofibers (indicatedby arrows). In addition, it should be noted the apparent connectedpores inside the CNFs network will significantly enhances the diffu-sion of the reactant toward the active sites and also for the escapingof the intermediate products allowing a significant reduction ofsecondary reactions [51].

3.3. Catalytic hydrogenation performance

3.3.1. Influence of the stirring rateFirst, the catalytic test was evaluated by using a magnetic stirrer

(625 rpm) while the catalyst foam was kept unstirred (Fig. 6). Thehydrogenation activity was expressed in terms of the cinnamalde-hyde conversion in the reaction medium as a function of time onstream. Next, the catalytic activity was evaluated by rotating thefoam stirrer with different speeds, 200–450 rpm without a mag-netic stirrer, and the results are presented in the same figure forcomparison. Increasing the stirring rate from 200 rpm to 400 rpmleads to a significant improvement of the catalytic activity. It is wor-thy to note that higher stirring speed up to 450 rpm only leads to

a marginal improvement of the hydrogenation activity. The resultsobtained indicate that the reaction rate was more efficient with thecatalytic foam stirrer than with the magnetic stirrer, indeed, for thefoam stirrer, the same conversion rate was obtained at much lower

86 L. Truong-Phuoc et al. / Applied Catalysis A: General 469 (2014) 81– 88

Fig. 6. Hydrogenation activity expressed in terms of molCALD/L/h/gPd ontR[

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Fig. 7. (A) Hydrogenation activity, in terms of moles/L/h/gPd of cinnamaldehydeconverted on the Pd/CNF/SiC (filled symbol) and Pd/SiC (open symbol) catalysts.The stability of the catalysts as a function of the cycling tests is also presented inthe same figure for comparison (Pd/SiC catalyst: circle: first cycle, square: secondcycle, diamond: third cycle; Pd/CNFs-SiC catalyst: circle: first cycle, square: fifthcycle, diamond: tenth cycle). Reaction conditions: [CALD]initial = 0.72 mol L−1, reac-

◦ −1

dissolution rate as if it is the case, one should observed a similarhydrogenation rate between the two catalysts keeping the reac-tion conditions similar. In addition, taken into account the relatively

he Pd/CNF/SiC as a function of the stirring speed of the catalyst foam.eaction conditions: reaction temperature = 80 ◦C, H2 flow rate = 60 mL min−1,CALD] = 0.72 mol L−1.

tirring rate (300 rpm) compared to that of the magnetic stirrer625 rpm). According to the results one can state that at a stirringate of 400 rpm the reaction is not controlled by the diffusion ofhe reactant from the reaction volume to the catalytic sites on theurface. For the subsequence tests, a stirring speed of 400 rpm wille used unless specify. The influence of the foam stirring rate onhe hydrogenation activity has also been reported by Tschentschert al. [54] for the oxygen/water system with rotating foam catalystnd could be explained by the following fact: at increasing stirringate, the number of bubbles breaking when passing the foam win-ow is increased and could led to the formation of finer dispersedubbles which enhance the gas–liquid mass transfer and also theontact between the reactant and the active sites localized on theNFs surface. It is worthy to note that the size of the bubbles formedannot be measured in the setup employed in this present work anddditional work with a modify setup is needed in order to confirmuch statement.

.3.2. Influence of the supportThe hydrogenation activity, expressed in terms of the specific

ate of converted cinnamaldehyde, obtained on the Pd/SiC andd/CNF-SiC catalysts is presented in Fig. 7 as a function of timen stream. The experimental results of the catalytic hydrogenationhow that the curve of cinnamaldehyde conversion as a function ofhe reaction time is linear; it means that the variation of conversionver the time is constant. We can therefore consider that the reac-ion rate does not change over time in the operating conditions. Thealues of the reaction rate can be easily derived from the slope of theonversion line and the initial concentration of cinnamaldehyde.eing the zero order with respect to the cinnamaldehyde concen-ration, these results are evidence for supporting the fact that theydrogen diffusion is the limiting step of the reaction rate.

According to the results, the hydrogenation rate on thed/CNF/SiC composite catalyst is about twice of the one obtainedn the Pd/SiC catalyst under similar reaction conditions. Suchn improvement in terms of the hydrogenation activity can bettributed to the high dispersion of the palladium particles on therismatic planes of the CNF and also to a higher effective sur-ace area of the catalyst which provides a higher contact surfaceetween the reactants and the active sites. The nanoscopic dimen-ion of the CNFs also significantly reduces the diffusion path of theeactant toward the active site especially in the liquid-phase reac-

ions where mass transfer phenomenon is predominant. It is alsoorthy to note that in this present work, the whole metal active

ite on the CNFs surface was available for reaction as internal massransfer limitations was significantly lowered due to the open and

tion temperature = 80 C, H2 flow rate = 60 mL min (STP), stirring speed = 400 rpm.(B) The hydrogenation rate taken after 6 h of reaction for different tests showing theclear stability of the CNFs-SiC catalyst compared to the SiC catalyst.

connected pores inside the CNFs network. Similar results have alsobeen reported by de Loos et al. [55] on a microchannel reactorcoated with a layer of carbon nanofibers. Such catalytic perfor-mance was attributed to an enhancement of the liquid–solid masstransfer. The significantly hydrogenation activity change betweenthe SiC and SiC-CNFs catalysts reported in Fig. 7A also indicatesthat the hydrogenation activity was not limited by the hydrogen

Fig. 8. The selectivity of the HALD (�, �) and HALC (�, ©) on the Pd/SiC (full symbol)and Pd/CNFs-SiC (open symbol) catalysts.

Catalysis A: General 469 (2014) 81– 88 87

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L. Truong-Phuoc et al. / Applied

hin layer of the CNFs, i.e., 20 �m, one should expect that such layeras steadily filled-up with the liquid during the reaction.

The Pd/CNF/SiC exhibits a constant hydrogenation activity as function of the cycling tests, up to tenth (Fig. 7B), which indi-ate that deactivation by palladium leaching or sintering is unlikelyo happen. Similar results have already been reported on palla-ium supported on carbon nanofibers and was attributed to theigh reactivity of the prismatic planes of graphite to anchor metalanoparticles [35]. On the other hand, a slight deactivation wasbserved on the Pd/SiC catalyst which indicates that some deacti-ation occurred during the cycling tests which could be attributedo the lower metal–support interactions which ultimately leads tohe sintering or leaching of the metal particles into the reaction

edium.TEM analysis of the catalysts after third cycling tests indi-

ates that the palladium particle size distribution remains almostnchanged on both catalysts (not reported). According to the TEMesults, one should expected that the deactivation observed in thease of the SiC-based catalyst was mostly linked with pore pluggingy the intermediate products which reduce the number of activeite. Leaching could also be responsible for the activity declinebserved on the bare SiC-based catalyst.

On the contrary with the CNF-SiC catalyst, the problem of leach-ng is unlikely to occur thank to the presence of a strong interactionetween the metal phase and the prismatic planes of the carbonupport. Similar results have already been reported during the cat-lytic decomposition of hydrazine on iridium supported on carbonanofibers catalyst [51]. Baker’s group has also reported that theorphology of the nickel particles when deposited on CNFs was

ignificantly modified leading to a large impact on the catalytic per-ormance of nickel for the hydrogenation of crotonaldehyde intorotyl alcohol [57]. The existence of electronic perturbations haseen advanced by the authors to explain such results. The highetal–support interactions between the deposited metal particles

nd the CNFs surface have also been reported by Sanz-Navarrot al. [56] on the Pt supported on fishbone and platelet CNFs usingeaxFF molecular-dynamics simulations. According to the resultshe adsorption energy of Pt on carbon surface is too strong that itauses a large restructuration of the metal particles and induces theppearance of strain in the Pt Pt bond.

The leaching test was carried out as follows: the catalyst was fil-ered from the reaction solution and a fresh cinnamaldehyde wasdded to the filtered solution. Catalytic test indicates that less than% of hydrogenation was occurred in this filtered solution after sev-ral hours of reaction which indicates that the leaching of the activehase in the solution is negligible.

The Pd/CNF-SiC catalyst also exhibits a higher selectivity towardhe C C bond hydrogenation compared to that of the Pd/SiC cat-lyst. On the Pd/CNF-SiC catalyst, the C C bond hydrogenationelectivity, i.e. 90%, remains almost unchanged regardless the totalonversion whereas on the Pd/SiC, a loss of the C C bond hydro-enation selectivity was observed as increasing the conversion,0%. Such result could be again explained by the high rate of inter-ediate products desorption on the Pd/CNF-SiC catalyst with a

anoscopic dimension compared to the bare SiC support. On thisater, the long residence time could favor the consecutive hydro-enation pathway and thus, lower the C C bond hydrogenationelectivity. However, it is worthy to note that the hydrogena-ion selectivity toward the C C bond is relatively high on bothiC-based catalysts compared to that obtained on the activatedharcoal catalyst according to previous report [35]. The productistribution as a function of the reaction duration and the cin-

amaldehyde conversion is presented in Fig. 8. At low conversionoth the C C and total hydrogenation (HALC) were occurred on theatalyst. However, as the cinnamaldehyde conversion increases the

C bond hydrogenation remains almost unchanged compared to

Fig. 9. Influence of the cinnamaldehyde concentrations on the hydrogenationrate. Reaction conditions: reaction temperature = 80 ◦C, H2 flow rate = 60 mL min−1,[CALD] = 0.21–1.32 mol L−1.

the total hydrogenation pathway on the Pd/CNFs-SiC catalyst. Onthe other hand, on the Pd/SiC catalyst the HALC selectivity con-tinue to increase as increasing the total conversion. The C O bondhydrogenation was remained marginal for the whole test and isnot reported. This result showed that on the Pd/CNFs-SiC cata-lyst the C C hydrogenation product was rapidly desorbed fromthe active site without being re-hydrogenated. Such result couldbe attributed to the adsorption or electronic properties of the pal-ladium active site when it comes into contact with the prismaticplanes of graphite. Similar results have also been observed onthe Pd supported on the nitrogen-doped CNTs catalyst and wereattributed to be consecutive to the surface electron density changeon the particle due to the interactions between the deposited metaland the N-CNTs underneath. Asedegbega-Nieto et al. [58] have alsoreported that the ruthenium active site was significantly modifieddepending to the nature of the graphite nanofiber supports, i.e.stacked-up, platelet and bamboo-like.

3.3.3. Influence of the cinnamaldehyde concentrationsThe influence of the reactant concentration on the hydrogena-

tion rate is also evaluated and the results are presented in Fig. 9.The hydrogenation rate slightly increases as a function of the cin-namaldehyde concentration and remains almost stabilized at acinnamaldehyde concentration of 1 mol L−1. Such results indicatethat the active sites were almost saturated by the reactant at lowconcentration and confirm the high advantage of using carbonnanofibers-based catalyst for the liquid-phase selective hydrogena-tion with high effective surface area and nanoscopic dimension.

4. Conclusion

High surface area hierarchical structured silicon carbide foamdecorated with a homogeneous network of carbon nanofibershas been successfully prepared via a chemical vapor deposi-tion method. The conditions used in this work allowed one toprepare such hierarchical support with a controlled specific sur-face area ranged from 40 to more than 100 m2 g−1, dependingto the nature of the starting macroscopic host matrix and of thedownstream applications. The carbon nanofibers network forms adense and homogeneous layer covering the SiC host matrix sur-face with low diffusion path and high effective surface area. Theas-synthesized carbon nanofibers were consisted mostly with pris-matic planes with high reactivity for anchoring and stabilizingmetal nanoparticles for subsequence use as catalyst in both gas-

phase and liquid-phase reactions. The hydrogenation activity andselectivity were extremely high due to the short diffusion pathand the high efficiency of the foam structure to produce fine bub-bles which significantly increase the gas–liquid mass transfer. The

8 Cataly

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8 L. Truong-Phuoc et al. / Applied

atalyst also exhibits a high resistance toward deactivation as aunction of cycling tests and neither metal phase leaching norctive phase sintering, due to the high metal–support interaction,as observed. In addition, this hierarchical catalyst exhibits sev-

ral advantages compared to the slurry phase reactor such as anasy recovery of the catalyst from the reaction medium and a highesistance toward fine formation which significantly reduces theroblem of product contamination by the catalyst. It is expectedhat the use of this type of hierarchical support will provide a bet-er advance in the new generation of catalyst design with improvedelectivity and recyclability. Work is on going to evaluate thenfluence of the shape of the ceramic foam, i.e. flat versus cylin-rical, and the location of the foam with respect to the stirrer,nd the total pressure of H2 on the hydrogenation catalytic per-ormance and also to replace the undoped carbon nanofibers by aitrogen-doped carbon nanofibers with intrinsic basic properties

or selective hydrogenation and oxidation reactions.

cknowledgements

LTP would like to thank the EU for the grant during his post-octoral stay at the ICPEES through the FP7 FREECATS project. SICATompany (www.sicatcatalyst.com) is gratefully acknowledged forroviding silicon carbide foams for the experiments and for helpfuliscussion all along the project. The SEM and TEM experimentsere carried out at the facilities of the IPCMS (UMR 7504 CNRS).

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