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Catalysis Today 251 (2015) 60–65

Contents lists available at ScienceDirect

Catalysis Today

j o ur na l ho me page: www.elsev ier .com/ locate /ca t tod

emarkable effect of soft-templating synthesis procedure on catalyticroperties of mesoporous carbon supported Ni inydrodechlorination of trichloroethylene in liquid phase

nna Srebowataa,∗, Izabela I. Kaminskaa, Damian Gizinskia, Dariusz Widełb,erzy Oszczudłowskib

Institute of Physical Chemistry, PAS, PL-01224 Warszawa, PolandJan Kochanowski University, Institute of Chemistry, PL-25406 Kielce, Poland

r t i c l e i n f o

rticle history:eceived 31 July 2014eceived in revised form 21 October 2014ccepted 5 November 2014vailable online 29 November 2014

eywords:ickel

a b s t r a c t

Ordered mesoporous carbon (CST-A) obtained by soft-templating method under acidic condition was usedfor preparation of nickel containing catalyst by incipient wetness impregnation with an aqueous solutionof NiCl2·6H2O. The reduction of as prepared catalysts by 10% H2/Ar at 673 K led to formation of welldispersed active metallic phase with average nickel particles size ∼11 nm. 2 wt.% Ni/CST-A was examinedas a catalyst in hydrodechlorination (HDC) of trichloroethylene in liquid phase, without presence of anadditional organic solvent in the reaction mixture. Ethane and ethylene were the predominant productof reaction. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies for catalysts

richloroethyleneydrodechlorinationiquid phaseesoporous carbons

after reduction and after HDC reaction indicated that use of hydrophobic, mesoporous active carbon asthe support for nickel led to formation of resistant for sintering and leaching catalyst and the effectivematerial for catalytic removal of trichloroethylene from water. Post-reaction deposits investigated bythe temperature programmed hydrogenation (TPH) showed substantial amounts of carbon-containingdeposit, with C2-hydrocarbons desorbing at a relatively low temperature (∼650 K).

. Introduction

Water is one of the most important substance in the worldut the access to the clean resources becomes problematic dueo, e.g. contamination of ground and drinking water by volatilerganic compounds (VOCs), like chlorinated hydrocarbons. Thus,he essential challenge for 21st century is access to clean water.herefore, there is still an increasing need for technologies whichould convert harmful (usually carcinogenic and mutagenic) chlo-

inated hydrocarbons into more useful or environmentally benignroducts [1,2]. Attractive and environmentally friendly method ofutilization” of these chemicals is hydrodechlorination (HDC), espe-ially because catalytic HDC is considered to be one of the mostniversal resource-saving methods for reprocessing chlorinatedydrocarbons [3].

Carbon materials are considered as the most suitable catalystupports for hydrodechlorination, due to their high adsorptionapacity for organic compounds and the high stability under the

∗ Corresponding author. Tel.: +48 1 22 343 3320.E-mail addresses: asrebowata@ichf.edu.pl, aniasr@wp.pl (A. Srebowata).

ttp://dx.doi.org/10.1016/j.cattod.2014.11.005920-5861/© 2014 Elsevier B.V. All rights reserved.

© 2014 Elsevier B.V. All rights reserved.

aggressive conditions of the reaction [4,5]. In industry, carbonmaterials are prepared in a variety of ways allowing a wider varietyof grain sizes, specific surface areas, porosities, impurity contentsand other characteristics of the carbon [6].

Very interesting group of carbons are soft-templated meso-porous materials. The resulting soft-templated carbons possesslarge and ordered mesopores with geometries analogous to thewell-known siliceous materials such as SBA-15, SBA-16, etc. [7,8],which distinguish them from carbon structures such as CMK-3 [9]and CMK-5 [10,11], which are inverse replicas of the silica tem-plates used [12]. Mesoporous structure, is desirable especially incase of the reaction in liquid phase. It was noticed that metal sup-ported on mesoporous carbon exhibited a much higher catalyticactivity for liquid phase HDC of chloroorganic compounds thanthat supported on microporous activated carbons [13,14]. So, itseems interesting to test the ability of ordered mesoporous car-bon as the support for the metallic catalyst in hydrodechlorinationof chloroorganic compound.

To the best of our knowledge, prepared by soft-templating

method active carbon has been never used as the support for nickelcatalysts in aqueous-phase HDC process.

Therefore, the goal of this work is to elucidate the properties ofthe new ordered mesoporous carbon supported nickel catalyst in

A. Srebowata et al. / Catalysis T

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cheme 1. Illustration of the soft-templating synthesis of mesoporous carbon –ST-A.

ydrogen-assisted chlorine removal reaction – a potential clean upoute for chloroorganic compounds such as trichloroethylene.

. Experimental

.1. Catalyst preparation and characterization

Mesoporous carbon was synthesized according to modifiedang recipe [15] by soft-templating method carried out under

cidic conditions. Scheme 1 shows the steps of the soft-templatingynthesis of mesoporous carbon. Thus, during synthesis triblockopolymer Lutrol F127 and resorcinol were dissolved in deion-zed water and ethanol. After complete dissolution of copolymerhe small amount of concentrated HCl was added and solutionas stirred for 30 min. Next, proportional amount of formaldehydeas added and the solution was stirred until it became white andilky; stirring was continued for another 30 min. Subsequently,

he magnetic bar was removed and the solution was left for totalravitational separation (decantation). Following separation, therganic layer was transferred into a Petri dish and dried at openir and room temperature until uniform film was formed. Afterhat, the dish was transferred into an oven and dried at 373 K for4 h. Afterward, the sample was transferred from the Petri dish to

quartz boat and carbonized in a tube furnace in nitrogen flow.he temperature program was as follows: first, the sample waseated from the room temperature to 453 K and held for 5 h. Next,emperature was elevated from 453 K to 1123 K. The sample wasept at 1123 K for 4 h. At the end, the sample was labeled as CST-Aefers to soft-templated (ST) carbon prepared in acidic solution (A).ow temperature nitrogen adsorption and desorption isotherms at7 K for CST-A were determined. According to the adsorption andesorption isotherms, specific surface area, pore distribution and

esoporosity were calculated.2 wt.% Ni/CST-A was synthesized by incipient wetness impreg-

ation of soft-templating carbon by NiCl2·6H2O solution. Duringmpregnation and preliminary drying using infrared lamp, a proper

oday 251 (2015) 60–65 61

mixing was assured by the rotary motion of a beaker containingthe catalyst precursor. Then, the solid was dried overnight in anair oven at 363 K and next was reduced in 10% H2/Ar (25 cm3/min)flow, increasing the temperature from room to 673 K and kept at673 K for 3 h. Catalyst after pretreatment by this was investigatedin hydrodechlorination of trichloroethylene in liquid phase.

Temperature-programmed reduction (TPR) of 2 wt.% Ni/CST-Acatalyst precursor was carried out using the glass-flow system. TPRruns were performed in flowing 10% H2/Ar (25 cm3/min), ramp-ing the temperature at 10 K min−1 and using a Gow-Mac thermalconductivity detector (TCD) and alternatively by AutoChem II 2920from Micromeritics. Additionally TPR were performed by massspectrometry (MA200, Dycor-Ametek, Pittsburgh). During moni-toring released species in the course of TPR, attention was focusedon changes in m/z 2 (hydrogen consumption) and a variety ofmasses originating from released/decomposed/hydrogenated pre-cursor (from chloride).

Catalytic tests in liquid phase were performed in a 500 mL roundbottomed flask equipped with the pH-meter, the magnetic stirringbar and the temperature controller, using 350 mL MiliPore waterand 2.0 �L of trichloroethylene. Before adding the substrate, thewater had been saturated with hydrogen for 30 min. Reaction sam-ples were taken at 2, 5, 10, 15, 20, 60, 90, 120 and 150 min. Thesubstrate concentration was monitored using gas chromatographymethod (Bruker 456-GC with ECD and FID, Headspace SHS-40).

The controlled experiments without the catalysts were per-formed to confirm that the reaction depends on the presence ofthe catalyst. To check activity of used sample in the repeat experi-ments, CST-A supported nickel was recovered by filtration and thenwashed with distilled water and dried in the oven at temperature393 K for the subsequent catalytic run. The catalytic procedure wasrepeated three times.

Diffractograms, obtained by X-ray diffraction method (XRD), ofthe samples after reduction step and after kinetic run were recordedon a Siemens D5000 diffractometer using Ni-filtered Cu Ka radia-tion.

HRTEM and HAADF STEM investigations were carried out usingFEI TITAN Cubed electron microscope operated at an accelerationvoltage of 300 keV and equipped with an energy dispersive X-ray(EDX) EDAX spectrometer. The samples were prepared by dispers-ing in pure alcohol using ultrasonic cleaner and putting a drop ofthis suspension on carbon films on copper grids and purified withplasma cleaner.

After catalytic reactions, nickel catalysts were investigated by atemperature-programmed hydrogenation (TPH) to detect speciesthat can be removed by hydrogen from spent catalysts. Progress ofTPH runs, using a 20% H2/He flow (25 cm3/min) at a 10 K min−1

ramp, was followed using mass spectrometry (MA200, Dycor-Ametek, Pittsburgh).

Energy dispersive X-ray fluorescence analysis (EDXRF) was car-ried out using MiniPal 4 equipment from PANalytical Co., withan Rh-tube and silicon drift detector (resolution 145 eV). EDXRFmethod was used to gain information on the elemental compositionof liquid samples after catalytic experiments.

3. Results and discussion

The soft-templating method under acidic conditions led to for-mation of carbon material with small microporosity, while themesoporosity contribution to the total pore volume (determinedby BJH method) may be as high as 75%. The specific surface

of CST-A calculated from nitrogen adsorption isotherms is ratherhigh (770 m2/g) to compare with more expensive commerciallyavailable active carbons (with typical surface area 150–250 m2/g).Additionally, this method has a very high reproducibility of

62 A. Srebowata et al. / Catalysis Today 251 (2015) 60–65

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Fig. 3. Plot of ln (C/C ) versus reaction time for hydrodechlorination of TCE on

Fig. 1. TPR pattern of 2 wt.% Ni/CST-A.

he structural parameters of carbonaceous materials obtained inepeated syntheses [16].

Temperature-programmed reduction experiments (Fig. 1) werearried out to determine the reducibility of nickel ions on CST-A. Lowetal loading (2 wt.%) and application of nickel chloride as precur-

or would suggest that conditions of reduction (H2, 673 K, 3 h) areot sufficient for effective elimination of chlorine from nickel pre-ursor [15,17]. On the other hand, available data indicate that evenn the absence of H2 (e.g. in inert gas) active carbon itself is capablef reducing nickel nitrate already at 723 K [18].

To be sure that the conditions of reduction of nickel chloriderecursor are sufficient, results were obtained by application ofhree independence devices for TPR experiments. Two of themased on TCD signal (results not shown) showed summary pro-esses demanding of the consumption of hydrogen. Application ofass spectrometry opened the possibility of monitoring m/z 2 and/z 36 corresponding to hydrogen consumption and HCl liberation.

herefore, some effects of the H2 consumption for another surfacepecies which may be removed by hydrogen from the catalystsere eliminated. Identification of the amounts of H2 consumed forickel precursor reduction was estimated.

According to Fig. 1 the reduction conditions used in this studyhould lead to complete reduction of carbon-supported nickel chlo-ide. Additional TPR experiment for pre-reduced sample showedot further reduction of nickel chloride precursor (results not

ig. 2. Kinetics of TCE hydrodechlorination in the presence of CST-A and Ni/CST-A.

0

Ni/CST-A with subsequent 1st, 2nd and 3rd reuse. TCE conversion after 150 min ofthe 1st, 2nd and 3rd reuse.

shown). Our results are in agreement with the study obtained forsibunit carbon supported nickel chloride precursor [19].

Catalytic hydrodechlorination of TCE in liquid phase were car-ried out in rather narrow pH range (6.24–5.71). To eliminate theeffect of organic solvent on HDC rate and products distributions,catalytic removal of TCE from water were performed without addi-tional organic chemicals. Fig. 2 shows kinetics of TCE conversion asC/C0 versus time in the presence of CST-A and Ni/CST-A. Carbon pre-pared by soft-templating method have shown very good efficiencyof trichloroethylene adsorption in water. The results obtained bychromatographic analysis of reaction mixture showed that dur-ing 150 min only the loss of trichloroethylene was observed forCST-A without formation of any products. This phenomenon derivedfrom the properties of CST-A, rather high specific surface area andespecially the presence mostly mesoporosity in carbon material.

Addition of nickel to CST-A led to productivity increasing ofthe water purification process from TCE. The presence of 100% ofhydrocarbons (mainly ethane and ethylene) as reaction productsindicates that hydrodechlorination process occured. Thus, the cat-alytic behavior of Ni/CST-A depends on the high adsorption affinityof CST-A for low weight polar organic compounds like TCE and thepresence of well dispersed nickel active phase (XRD and HRTEMresults described below).

As it was shown in Fig. 3 Ni/CST-A exhibit relatively high stabilityduring three cycles of TCE HDC. Only negligible differences in TCEconversion obtained after 150 min of subsequent reactions could

be related rather to catalyst mass loss during filtration and dryingbetween the cycles than the deactivation by leaching or formationdeposits on catalyst support.

Fig. 4. Plot of ln (C/C0) versus reaction time for hydrodechlorination of TCE onNi/CST-A.

A. Srebowata et al. / Catalysis T

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ig. 5. XRD patterns of 2 wt.% Ni/CST-A after reduction step (red-Ni/CST-A) and aftereaction in liquid phase (spent-red-Ni/CST-A).

The hydrodechlorination of trichloroethylene was assumed toe pseudo first-order due to predominant amount of hydrogenompared to TCE, thus, Fig. 4 plot the kinetic data in according

o the linearized pseudo first order rate equation. Additionally, bysing the reaction kinetics ln(C/C0) = −kt, the calculated reactionate constant k was 1254 × 10−2 min−1 for the reaction with 2 wt.%i/CST-A.

Fig. 6. HRTEM images and nickel particle size distribution of 2 wt.% Ni/CST-A af

oday 251 (2015) 60–65 63

XRF analysis for the portion of water after reaction did not showany amounts of nickel within the limits of detection. Therefore,we can supposed that the effect of leaching of nickel to reactionmixture in case 2 wt.% Ni/CST-A is negligible.

The X-ray diffractograms of the catalysts after reduction andafter HDC reaction indicate that the reaction conditions did notaffect the structure of mesoporous carbon material prepared bysoft-templating method (Fig. 5). Nickel particles size in both sam-ples (after reduction and after HDC) obtained from the broadeningof XRD reflections have shown that Ni particles size in Ni/CST-A donot change significantly upon hydrodechlorination.

Careful analysis of the results obtained by STEM and HRTEMmeasurements for red-catalyst (Fig. 6a and c) confirmed XRDresults, while for the spent – sample the decreasing of nickel par-ticles size were observed (Fig. 6b and d).

Thus, nickel particles size in the samples after reduction andafter catalytic reaction indicated that the structure of CST-A preventsof metal particles sintering or even conducive to the re-dispersionof active phase (Fig. 6a and d).

The effects of leaching and sintering are not new and they areextensively discussed for both palladium and less noble metals inhydrodechlorination processes [13,14,20–23]. Generally this effectis prevented by small amounts of base addition like, e.g. NaOH[24]. Thanks to this the shift in metal particle size to larger values

under HDC conditions attributed to metal chlorides creation (whichare responsible for migration and agglomeration of the metal par-ticles) [25,26] is strongly restricted. We suppose that the effectof mesoporous carbon prepared by soft-templating method plays

ter reduction at 673 K (a) and (c) and HDC trichloroethylene (b) and (d).

64 A. Srebowata et al. / Catalysis Today 251 (2015) 60–65

HCl Cl

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Cl CC

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Cl

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Scheme 2. Proposed probably mechanism

he crucial role in stabilization of nickel particles during HDC ofCE (inhibits of an agglomeration and leaching). Our results aren agreement with earlier works on OMC materials in HDC pro-esses with the exception that they usually observed the leachingffect in their catalysts [13,14,27]. Calvo et al. [14] showed thatorous structure of carbon support affect both activity and leachingffect. For mesoporous carbon supported palladium, a nearly com-lete removal of chloroorganic compound (diuron) was achieved,hereas conversion of around 70% were obtained with the micro-orous carbon based catalyst [14]. This phenomenon was explainedy a higher accessibility to the Pd active sites based on meso-orous material [14]. Shao et al. [13] have shown that this effect

s independence of the kind of chloroorganic compound used forDC. According to Letellier et al. [27] the activity of mesoporousarbons supported metallic phase have associated with the pres-nce of ordered mesopores with a narrow pore distribution, whereanoscale metal particles can be confined in the mesopores.

Fig. 2 shows that after first 10 min of reaction approximately 50%f TCE is removed from water by nickel containing catalyst. After50 min of reaction the conversion reached the value ∼90%.

It is very difficult to discuss our results in comparison with datavailable for hydrodechlorination of trichloroethylene, especiallyecause most of them concern HDC on palladium containing cata-

ysts [14,21,28,29,3]. However high expense of Pd catalysts mayestrict its wide application. Therefore the attractive alternativeor palladium catalyst can be nickel. Therefore, steadily increasingnterest of nickel as catalyst for HDC of trichloroethylene in liquidhase especially as the dopant for Fe catalysts [30–32]. The load-

ng of nickel was in the range between 2.5 and 25 wt.%, and the best

esults were obtained for the catalysts with more than 20% of metaloading, (2 wt.% in our catalyst) [31]. The reaction conditions, espe-ially the initial TCE concentration were usually less than in our

ig. 7. Temperature programmed hydrogenation profiles of post-reaction deposits:/z 15 methane evolution, m/z 28 evolution for showing the release of C2Hx species,/z 36 HCl evolution.

Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni

drodechlorination of TCE in liquid phase.

experiments, too [30–32]. Regardless of the differences, our resultsobtained for catalytic removal of TCE from water are satisfactory.

Inspired by studies of Wei et al. [30] and Li et al. [33] we have pro-posed a possible mechanism of the catalytic hydrodechlorinationof trichloroethylene in liquid phase on 2 wt.% Ni/CST-A (Scheme 2).The first step of mechanism shows the typical dissociative adsorp-tion of hydrogen on nickel centers. Meanwhile, trichloroethyleneadsorbed on nickel active sites via Cl atoms; C Cl bonds werebroken and Ni Cl bonds were formed. Simultaneously, TCE mayadsorbed on nickel active sites via C C double bond breaking, andformation of Ni C bonding. Next, all chlorine atoms in TCE may bereplaced by atomic hydrogens, and this step led to desorption ofethane from catalyst’s surface.

Additionally, seemingly inert support (mesoporous active car-bon) may play the role by its strong interaction with chlorine atomsand thanks to this metallic active phase is protected.

The presence of hydrogen in reaction mixture promotes theremoval of chlorine atoms from active sites and leads to their regen-eration.

The results obtained by the temperature programmed hydro-genation (TPH) of post-reaction deposits show a massive release ofC2-hydrocarbons (mainly ethene) at ∼650 K and no chlorine reten-tion (Fig. 7). It can suggest that adsorption of TCE on active centersvia creation of Ni C bonds is more probable.

Therefore, some accumulation of C2Hx surface species seemsimportant for shaping ethene selectivity. m/z 28 evolution in therange of higher temperatures seems less indicative here because itsmain release occurs resulting from support hydrogenation (Fig. 7).

4. Conclusions

Hydrophobic nature, and rather high mesoporosity of car-bon obtained by soft-templating method helps in more effectiveadsorption of trichloroethylene from wastewater. Incipient wet-ness impregnation of ordered mesoporous carbon by nickelchloride solution leads to formation of 2 wt.% Ni/CST-A – resistant forsintering catalysts and an effective material for catalytic removalof trichloroethylene from water.

Acknowledgement

This work was financially supported by the National ScienceCentre in Poland within the project Sonata based on decision DEC-2011/03/D/ST5/05516.

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