Mining Science and Technology (China) 21 (2011) 599–603

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Mining Science and Technology (China)

journal homepage: www.elsevier .com/locate /mstc

Role of Ni(NO3)2 in the preparation of a magnetic coal-based activated carbon

Zhang Jun, Xie Qiang ⇑, Liu Juan, Yang Mingshun, Yao XingSchool of Chemical and Environmental Engineering, China University of Mining & Technology, Beijing 100083, China

a r t i c l e i n f o

Article history:Received 7 December 2010Received in revised form 5 January 2011Accepted 26 January 2011Available online 16 July 2011

Keywords:Magnetic coal-based activated carbonNi(NO3)2

Magnetic propertiesPore structure

1674-5264/$ - see front matter � 2011 Published bydoi:10.1016/j.mstc.2011.01.003

⇑ Corresponding author. Tel.: +86 10 62331014.E-mail address: dr-xieq@cumtb.edu.cn (Q. Xie).

a b s t r a c t

The role of Ni(NO3)2 in the preparation of a magnetic activated carbon is reported in this paper. Magneticcoal-based activated carbons (MCAC) were prepared from Taixi anthracite with low ash content in thepresence of Ni(NO3)2. The MCAC materials were characterized by a vibrating sample magnetometer(VSM), X-ray diffraction (XRD), a scanning electric microscope (SEM), and by N2 adsorption. The cylindri-cal precursors and derived char were also subjected to thermogravimetric analysis to compare theirbehavior of weight losses during carbonization. The results show that MCAC has a larger surface area(1074 m2/g) and a higher pore volume (0.5792 cm3/g) with enhanced mesopore ratio (by about 10%). Italso has a high saturation magnetization (1.6749 emu/g) and low coercivity (43.26 Oe), which allowsthe material to be magnetically separated. The MCAC is easily magnetized because the nickel salt is con-verted into Ni during carbonization and activation. Metallic Ni has a strong magnetism on account ofelectrostatic interaction. Added Ni(NO3)2 catalyzes the carbonization and activation process by accelerat-ing burn off of the carbon, which contributes to the development of mesopores and macropores in theactivated carbon.

� 2011 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

1. Introduction

Activated carbon is among the important carbon based materi-als that find wider applications in almost every industrial field.They are employed in liquid phase adsorption to remove contam-inants, recover products, and as catalysts, or catalytic supports,due to the large internal surface area and specific pore structure.However, difficulties encountered when separating spent activatedcarbon limit its application in many occasions. Filtration, the tradi-tional method for separating activated carbon, may result in theblockage of filters or the loss of carbon causing secondary pollu-tion. Magnetic separation is considered a quick and effective tech-nique for separating magnetic particles and has been used formany applications in biochemistry, analytical chemistry, and oremining [1–4]. Therefore, magnetization of activated carbon is aneffective approach for separating and recovering spent activatedcarbon.

Synthesis of magnetic activated carbon has recently become afocus in the activated carbon industry, and in metallurgy, environ-mental, chemical, and pharmaceutical areas [5–8]. Usually, a mag-netic activated carbon is prepared from a suspension ofcommercial activated carbon in a solution of an iron salt by addingalkali to precipitate magnetic iron oxides.

Most of these magnetic activated carbons have a small surfacearea and very poor porosity compared to common activated

Elsevier B.V. on behalf of China Un

carbons. Moreover, the preparation of the magnetic activated car-bon requires several steps and special chemicals. Incorporation ofthe magnetic particles inside the pores of the activated carbon,while at the same time keeping a high surface area, is the key pointfor preparing magnetic activated carbons used as adsorbents.

A simple one-step method for preparing magnetic activated car-bon with a regulated pore size from coal in the presence of Fe3O4

has recently been published by our teams [9–11]. In the presentwork the preparation of another new magnetic coal-based acti-vated carbon with a high surface area, appropriate pore size, andmagnetic separability is described. This carbon is prepared fromanthracite with a low ash content in the presence of Ni(NO3)2.

2. Experimental

2.1. Preparation of the magnetic, coal based activated carbon

Synthesis of magnetic activated carbon from coal can be dividedinto three stages, i.e., preparing cylindrical precursors, carboniza-tion, and activation [12,13]. An anthracite coal (Taixi, China) con-taining 7 wt.% volatile material and about 2.52 wt.% ash contentwas crushed, ground, and sieved to particles sized smaller than0.074 mm. A blender was used to prepare a mixture of the anthra-cite powder (68 wt.%), coal tar (28 wt.%), and Ni(NO3)2�9H2O(4 wt.%), in water. Then the thoroughly mixed feedstock was ex-truded in the form of 1 cm cylinders.

After 3 days of drying in the open air the cylindrical precursorswere put into a tube furnace and carbonized under a flow of N2

iversity of Mining & Technology.

10 20 30 40 50 60 70 80

AC

CM-Ni

NiNiO

MCAC-Ni

2θ (°)

Inte

nsity

Fig. 2. X-ray diffraction patterns of AC, CM–Ni, and MCAC–Ni.

600 J. Zhang et al. / Mining Science and Technology (China) 21 (2011) 599–603

(80 mL/min) at a heating rate of 5 �C/min to a final temperature of600 �C. The samples were carbonized at 600 �C for 45 min. Thensamples were heated to 880 �C at a heating rate of 10 �C/min andwith a nitrogen flow of 80 mL/min. Steam activation at 880 �C witha steam flow of 0.77 mL H2O/(h g) was then carried out for 3 h. Theactivated carbon prepared in the presence of Ni(NO3)2 was labeledMCAC–Ni and the one without Ni(NO3)2 was labeled AC. RM andCM represent the cylindrical precursor and the carbonized mate-rial, respectively.

2.2. Characterization of the samples

The magnetic properties of the samples were measured using avibrating sample magnetometer (LakeShore VSM-7307, USA). Thefield dependence of the magnetization was measured at room tem-perature. X-ray diffraction of the samples was performed with anX-ray diffractometer (Rigaku D/Max-RB, Japan) using Cu Ka radia-tion at a scan speed of 9�/min. The morphology of the activated car-bon was characterized with a scanning electron microscope (JEOLJSM6700F, Japan).

The activated carbon was characterized by N2 adsorption/desorption at 77 K with a NOVA-1200 instrument from Quanta-Chrome (USA). The specific surface area (SBET) was derived fromthe N2 adsorption isotherms by the BET equation. The total porevolume was calculated from a single point on the nitrogen adsorp-tion isotherm at a relative pressure of 0.99. The micropore volumeswere calculated using the t-plot method. The mesopore volumeswere calculated from the volume of N2 adsorbed at a relative pres-sure of 0.99 minus the corresponding micropore volume. The BJHmethod was employed to determine the mesopore distribution.

Thermogravimetric analysis (NETZSCH STA 409C, Germany)was conducted on the cylindrical precursors using these condi-tions: N2 flow of 80 mL/min; heating rate of 5 �C/min; temperaturerange from room temperature to 600 �C. Thermogravimetric anal-ysis (Versa Therm HS, USA) of the carbonized material was per-formed under these conditions: N2 flow of 100 mL/min; heatingrate of 10 �C/min from room temperature to 880 �C; a hold temper-ature of 880 �C for 1 h under a flow of steam (0.1 g/min). The flowof steam was controlled by a trace constant-flow pump (SolventDelivery Module 501, USA).

3. Results and discussion

3.1. Mechanism of magnetization of MCAC–Ni

A characteristic hysteresis loop could be observed (Fig. 1) whichindicates the ferromagnetic character of the MCAC–Ni. The AC wasnearly diamagnetic. Fig. 1 shows the saturation magnetizationvalue of the MCAC–Ni to be around 1.6749 emu/g, which was highenough to enable the samples to be manipulated with conven-tional magnets. The values for both coercive force and residual

- 10000 - 5000 0 5000 10000- 2

- 1

0

1

2MCAC-Ni

CM-Ni

Field (Oe)

AC

Mom

ent m

ass

(em

u/g)

Fig. 1. Hysteresis loop of AC, CM–Ni, and MCAC–Ni.

magnetization were very low suggesting that the MCAC–Ni couldnot only rapidly respond to changes in external magnetic fieldbut could also be easily demagnetized.

XRD patterns of AC, CM–Ni, and MCAC–Ni are shown in Fig. 2.The MCAC–Ni contains nickel mainly in the form of metallic nickel(2h = 44.51�, 51.85�, 76.37�). Diffraction peaks were also observedat 2h = 37.25� and 43.28�, suggesting the presence of smallamounts of NiO. These peaks were also observed weakly in theXRD diffraction patterns of CM–Ni, which showed lower quantitiesof nickel than the MCAC–Ni sample.

The magnetic properties of MCAC–Ni are attributed to Ni that isformed by the thermal decomposition and reduction of the nickelsalt during the carbonization and activation process. The 3d elec-trons of Ni are arranged in accordance with Hund’s rules and thePauli exclusion principles and these orbitals have unpaired elec-trons. The electrons of the adjacent atoms have magneticmoments. The unpaired electrons contribute to the magnetic mo-ment of 0.6 on Ni. The direct exchange interaction of these elec-tronic magnetic moments causes the atomic magnetic momentsto have planar alignment.

In previous studies added Fe3O4 was mostly converted into FeOduring activation and residual Fe and Fe3O4 created the magnetismin that magnetic activated carbon [10,11]. The non-magnetic nat-ure of FeO required a larger addition of Fe3O4 in that case. The Niformed by thermal decomposition and reduction of the nickel saltis present in greater relative amounts. Further, if the content of Niwere to be increased the magnetic properties of the resulting acti-vated carbon would be stronger.

3.2. Mechanism of pore size regulation in MCAC–Ni

3.2.1. Pore characterizationFig. 3a shows a micrograph of AC without added Ni(NO3)2. The

AC has a surface morphology that shows deflected zonal or sheet-like graphite that affords a large number of micropores. Fig. 3bshows the magnetic activated carbon at a magnification of 5000.Observe that small aggregates of nickel, which appear brighter,are present on the darker surface of the activated carbon. The finenickel particles tend to cover specific parts of the activated carboninstead of being dispersed evenly over the whole surface. The mor-phology of the MCAC–Ni, shown in Fig. 3c, contains surface poros-ity that contributes to an increase in the surface area. There arealso some irregular macropores on the surface of the activated car-bon. Nickel has a catalytic effect during carbonization and activa-tion that accelerates the burning off of the carbon wall andenlarges the pore size. This leads to weakening of the carbon skel-eton and the formation of a loose structure [14,15].

Fig. 4 shows the N2 adsorption isotherms for both AC andMCAC–Ni. The samples show type VI behavior, as defined by the

Fig. 3. SEM photographs.

0.0 0.2 0.4 0.6 0.8 1.0200220240260280300320340360380

Vol

ume

(cm

3 /g)

Relative pressure (p/p0)

ACMCAC-Ni

Fig. 4. N2 adsorption–desorption isotherms of AC and MCAC–Ni.

J. Zhang et al. / Mining Science and Technology (China) 21 (2011) 599–603 601

International Union of Pure and Applied Chemistry (IUPAC). Thesample AC shows a small hysteresis loop in the adsorption–desorp-tion isotherm that indicates a microporous structure with a lowmesopore ratio and a fairly large surface area is present. TheMCAC–Ni sample shows an obvious hysteresis loop that indicateslarger pores varying from 2 to 10 nm in radius are present(Fig. 5). At relative pressures of higher than 0.9 the adsorption iso-therms of the MCAC–Ni increase without reaching equilibriumindicating the presence of small macropores.

The data shown in Table 1 suggest that the BET surface area, andthe microporous and mesoporous volume were significantlyaffected by the added Ni(NO3)2. An increase of surface area from817.6 m2/g (for AC) to 1074 m2/g (for the nickel doped carbon)was observed. The microporous volume changed from 0.3720 to0.4421 cm3/g and the mesoporous volume changed from 0.0575to 0.1371 cm3/g and the ratio of mesopore doubled because of theadded nickel. These results show that adding nickel salt helps con-trol the pore structure in the coal-based activated carbon prepara-tion [16–18]. AC, without nickel salt, has a typical microporous

dV(c

m3 /(

nm·g

))

1 2 3 4 5 6 7 8

0

0.01

0.02

0.03

0.04

0.05

Pore diameter (nm)

AC

MCAC-Ni

Fig. 5. Pore size distributions in activated carbons AC and MCAC–Ni.

activated carbon structure. Meanwhile, MCAC–Ni had an enhancednumber of mesopores as well as more macropores. Compared toadded Fe3O4, the ratio of mesopore increased significantly becauseof the catalytic performance of the nickel, which is higher than thatof iron [11].

3.2.2. Effects of Ni(NO3)2 in the preparation of magnetic activatedcarbon

The mechanism of pore size development during the carboniza-tion and activation of MCAC–Ni was studied by thermal analysisunder nitrogen and steam atmospheres. Fig. 6 shows the thermo-gravimetric results. Over the temperature range of 150–300 �Cthe weight loss of RM–Ni was higher than that of RM becausenitrogen oxide from decomposition of the nickel nitrate (above110 �C nickel nitrate decomposes) affords a more oxygen richatmosphere that accelerates the carbonization.

From 400 to 600 �C RM–Ni has more weight loss compared toRM. This may be due to the catalysis of Ni in the carbonizationand reductive decomposition of nickel oxide. In the process of car-bonization as the cylindrical precursors are heated the coal parti-cles undergo thermal decomposition and form solid semicoke or/and coke, tar, and light gases. This happens between the tempera-tures of 420 and 460 �C. CH4 is released primarily between thetemperatures of 450 and 700 �C, and H2 and CO are released ateven higher temperatures. The latter process supplies a source ofreductant. Ni has a catalytic effect on anthracite pyrolysis duringthe carbonization process and accelerates the rate of free radicalgeneration [19,20]. Ni in the carbonization process also hindersthe condensation of free radicals by absorbing electrons formedduring pyrolysis. This eventually leads to carbon having a crystalstructure with short range order but long range disorder, whichis beneficial to forming initial porosity in the activated carbon.

Fig. 7 shows thermogravimetric analysis results from scans onCM and CM–Ni: the curves have a distinctly different appearance.After 60 min activation the burned off weight of CM–Ni was 15%lower than the weight of the burned off CM. Carbon gasificationunder H2O occurs at temperatures above 880 �C to produce COand H2 that may then reduce nickel oxides. Formation of carbonmonoxide and water is represented by:

CþH2O! H2 þ CO ð1Þ

Reduction of nickel oxides in a direct reduction occurs by theaction of gaseous reductants like CO and H2 rather than by the ac-tion of solid carbon. The present study shows that NiO undergoesstepwise reductions by CO and H2, which is represented by:

NiOþ CO�Niþ CO2 ð2ÞNiOþH2�NiþH2O ð3Þ

NiO reacts with the carbon wall to enlarge the pores accompa-nied by its reduction to the free metal. Metallic Ni has a catalyticeffect during H2O gasification on the carbon wall of the activated

Table 1Pore structure of AC and MCAC–Ni.

Sample BET, surface area (m2/g) Pore volume (cm3/g) Mesopore ratio (%)

Vtotal Vmicro Vmeso

AC 817.6 0.4295 0.3720 0.0575 13.39MCAC–Ni 1074.0 0.5792 0.4421 0.1371 23.70

100 200 300 400 500 6000.75

0.80

0.85

0.90

0.95

1.00

1.05

TG

(%)

TGDTG

Temperature ( )

-1.0

-0.8

-0.6

-0.4

-0.2

0

0.2

RM

RM-Ni

DTG

(%

/min

)

RM

RM

Fig. 6. Thermogravimetric analyses in a nitrogen atmosphere: samples RM andRM–Ni.

0 10 20 30 40 50 6070

75

80

85

90

95

100

Bur

n-of

f (%

)

Time (min)

CM

CM-Ni

Fig. 7. Thermogravimetric analyses of CM and CM–Ni in steam.

602 J. Zhang et al. / Mining Science and Technology (China) 21 (2011) 599–603

carbon. The catalytic mechanism of nickel can be elucidated as acarbon transfer mode where the carbon atom is transferred tothe gasification agent by nickel, which promotes the reaction ofcarbon and water vapor.

The mechanism proposed for the modification of the surfaceproperties and pore size of the formed MCAC–Ni includes: (1)Thermal decomposition of a nickel salt to nickel oxide, which thenreacts with the reducing gases to enlarge the pore; (2) Metallicnickel catalyzes the carbonization and activation process by accel-erating burn off of the carbon wall to enlarge the pores.

4. Conclusions

(1) MCAC–Ni having a large surface area (1074 m2/g) and a highpore volume (0.5792 cm3/g) with an enhanced ratio of mes-opores (about 10%), a high saturation magnetization(1.6749 emu/g), and a low coercivity (43.26 Oe) is reportedherein. The material can be prepared in a simple one stepprocedure starting from anthracite with a low ash contentand pyrolyzing it in the presence of Ni(NO3)2.

(2) The magnetic properties of the MCAC–Ni derive from free Ni,which is formed by thermal decomposition and reduction ofthe nickel salt during the carbonization and activation pro-cess. Electrostatic interaction gives Ni a strong magnetismthat enhances the magnetization of the coal based activatedcarbon.

(3) Nickel catalysis increased the rate of carbonization and acti-vation. The greatest effect was on the activation processwhere accelerated burn off of the carbon wall to formnumerous macropores and mesopores in the activated mate-rial was observed.

Acknowledgments

The authors are thankful for the financial support by theNational Natural Science Foundation of China (No. 20776150),the National Hi-Tech Research and Development Program of China(No. 2008AA05Z308) and the Special Fund for Basic Scientific Re-search of Central Colleges (No. 2009QH15).

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