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S.B. Lyubchik----活性炭

Intercalation as an approach to the activated carbon preparation

from Ukrainian anthracites

S.B.Lyubchik a,*,L.Ya.Galushko a ,A.M.Rego b ,Yu.V.Tamarkina a ,O.L.Galushko a ,

I.M.Fonseca c

Institute of Physical-Organic and Coal Chemistry,National Academy of Science,Donetsk,Ukraine

Centro de Quimica-Fisica Molecular,Instituto Superior Te

′cnico,Complexo Interdisciplinar,Lisboa,Portugal c

Faculdade de Cie

?ncias e Tecnologia,Universidade Nova de Lisboa,Lisboa,Portugal Abstract

Recently developed approach to the activated anthracites preparation combining a chemical pre-treatment and a physical activation was

applied to Ukrainian anthracites (C daf 92–95%).Anthracites were pre-treated under intercalation-like conditions either with HClO 4or with HNO 3vapours.Chemically pre-treated samples were afterwards submitted to physical activation either with H 2O or CO 2at 8508C for 1–8h.The in?uence of the pre-treatment conditions was analysed by means of X-ray photoelectron spectroscopy,XRD,elemental analysis and gas adsorption techniques.Under the experimental conditions investigated,the optimum conditions for producing of activated carbons from chemically modi?ed anthracites were identi?ed.Chemical pre-treatment with nitric acid of the low rank anthracite of C daf 92%followed by its activation with steam for 2h at 8508C looks like to be the best way for activated anthracites preparation.By abovementioned approach the high microporous activated carbon of 0.7cm 3/g and extended surface area of 1600m 2/g was obtained from Ukrainian anthracite.q 2003Elsevier Ltd.All rights reserved.

Keywords:A.Microporous materials;B.Chemical synthesis;C.Photoelectron spectroscopy;D.Surface properties

1.Introduction

Chemical pre-treatment followed either by physical (H 2O,CO 2)or chemical (KOH)activation is widely used for improving the textural properties of activated carbons.For this purpose different oxidants (O 2[1],HNO 3in aqueous solutions [2]or mixtures with HClO 4[3],Ac 2O [4])were https://www.wendangku.net/doc/f112404843.html,st years we have studied in detail the in?uence of different kinds of coals pre-treatment,either with HNO 3or HClO 4[5–7]under conditions like the ones used for graphite intercalation compound (GIC)synthesis.It was observed that there is a strong in?uence on the pore size distribution of resulted activated carbons and on the activation process in whole.

In the present study our developed approach to the activated carbon preparation,combining a chemical pre-treatment via intercalation-like reactions and a physical activation,has been applied to Ukrainian anthracites from the main coal region of Ukraine,i.e.Donbass region.The optimization of the synthesis conditions and of the textural parameters of the resulted activated anthracites has been

done under control of the standard solid-state physics background.According to our recent studies,the resulted activated carbons from Ukrainian anthracites were found to be suitable for the liquid phase application,namely for the selective adsorption of precious metals [11].2.Experimental

Representative group of the anthracites in a wide range of carbon content,i.e.C daf 92–95%,with 0.4–1mm particle sizes were chosen for the investigation (Table 1).Sample of Ceylon graphite was used as a reference.Chemical pre-treatment was carried out either with HClO 4or with HNO 3vapour.The HClO 4pre-treatment was performed by ‘step by step’heating (up to 1608C),the details are described elsewhere [6,7].The HNO 3pre-treatment was carried out by exposure of dried anthracite in HNO 3vapour at room temperature during different soaking time (1–72h).After accomplishing the contact time,samples were evacuated from the reactor and weighted.Then they were washed to remove nitric acid,dried,and weighted again.

The kinetics experiment was carried out to evaluate the effective constants of weight loss of the chemically

doi:10.1016/j.jpcs.2003.10.006

Journal of Physics and Chemistry of Solids 65(2004)127–132

https://www.wendangku.net/doc/f112404843.html,/locate/jpcs

*Corresponding author.Fax:t380-622-558425.

E-mail address:s_lyubchik@https://www.wendangku.net/doc/f112404843.html, (S.B.Lyubchik).

pre-treated anthracites.The deintercalation process was investigated by time-based analyses,when the intercalated samples were placed in dry atmosphere at 208C and analysed for certain time until the values of weight loss in the studied systems became unchanging.

The samples were characterized before and after each of the experimental step-treatment by elemental analysis and proximate analysis with Automatic CHNS-O Elemental Analyser Flash EATM 1112following standard procedures;X-ray diffraction with X-ray diffractometer DRON UM1;gas adsorption technique using Nitrogen adsorption measurements at 77K with Surface Area and Porosimetry Analyzer,Micromeritics ASAP 2010;and X-ray photo-electron spectroscopy using a XSAM800(KRATOS)X-ray Spectrometer operated in the ?xed analyzer transmission mode with pass energy of 20eV and the non-monochro-matized Mg K a X-radiation.Samples are labelled as ‘Index (R)A ’,where ‘Index’is listed in Table 1,and R,chemical agent (HClO 4or HNO 3);A,activation agent (CO 2or H 2O).For example,Pr(HNO 3)A(H 2O),anthracite of coal mine ‘Progress’(Table 1),pre-treated by HNO 3and activated by steam.

3.Results and discussion

We found that the interaction between anthracite and HNO 3produces coal swelling,weight uptake,and evolution of nitrogen dioxide as a product of one-electron reduction of nitronium-cation (NO 2t).This phenomena is resembled to the one observed for the interaction between graphite and HNO 3vapour at room temperature and it was explained by Forsman et al.[8]according to the following reaction

2HNO 3$NO t2tNO 2

3tH 2O C n tNO t2!C tn tNO z 2"

C tn tNO 23tm HNO 3!C tn NO z 3·m HNO 3eGICs T

Weight uptakes (D m ;%)of anthracites and graphite were found to be different (Fig.1).The minimal values are found for the samples with carbon content at C daf 95–96%.Under given conditions,graphite was transformed into the GICs (with D m ?50%),which can be then thermally exfoliated.The weight uptakes of low-rank anthracite (C daf 92%)were the highest,i.e.D m values ca.100%,which is comparable to graphite behaviour.We suppose that both modi?cation and vapour condensation in bulk,as well as at the external surface are responsible for the weight uptakes in this case.

Generally,under studied experimental conditions anthra-cite reactivity towards HNO 3-intercalation decreases with carbon content.Furthermore,we observed for low rank anthracite anomalous increase of weight uptake from 100to 300%increasing the reaction time from 24to 72h (Fig.1).

Table 1

Proximate and ultimate analysis of the studied anthracites Coal mine

Sample index W a (%)A d (%)C daf

(%)

H daf (%)N daf (%)S daf (%)

O daf dif (%)

Ilovaiskaya Il

1.4 1.29

2.1

3.93 1.680.58 2.41

Kommunist Ko 1.9 2.393.9 2.58 1.140.65 2.38Progress Pr 2.2

5.7

95.0

1.91

0.90

0.75

1.34

Fig.1.In?uence of the time of chemical pre-treatment on weight uptake for the anthracites with different carbon content.

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128

It was observed that samples became visually wet.This is caused by appearance of liquid phase containing nitric acid and water formed as reaction product according to Forsman [8]mechanism.However,we have not observed such a wetting for high rank anthracites and graphite.

When the anthracites pre-treated with HNO3were placed in dry atmosphere at208C,we observed a decrease of weight,probably,due to deintercalation and desorption of physically bonded species,such as HNO3,NO2,H2-O.During the?rst6h the process follows the?rst-order kinetics.The kinetic effective constants of weight loss depended on anthracite rank.Thus,the pre-treated anthra-cite with carbon content of92.1%was found to be unstable, whereas high rank anthracite and graphite nitrate were more stable.

For further investigations we have chosen two anthra-cites with highest and lowest reactivity towards intercala-tion reactions(Pr and Il indexes from Table1).We have studied in detail the in?uence of the anthracites pre-treatment(either with HClO4or HNO3)along changing the activation agents(CO2or H2O)on the structural properties of the resulted activated carbons.

Whatever the activation agent,thermal treatment results in splitting of anthracite plates to the more thin layers with the formation of pores with a slit-like shape.They have the average width around0.77–0.92nm.The microporous volume is varied between0.2and1.0cm3/g and surface area of300–1600m2/g depending on the conditions of preparation of porous anthracite materials.

It has been found that the activation of the pre-treated samples with steam appears to be more effective then that with CO2(Table2).Thus,activated carbons with S BET upto1600m2/g and total pore volume ca.1cm3/g have been obtained from more reactive anthracite Il(C daf92.1%)for two h of activation at8508C.Less reactive towards intercalation anthracite Pr(C daf95.0%)under the same conditions gives activated carbons with S BET of800–1100m2/g.

In order to compare the effect of CO2and steam the same anthracites were activated with CO2.It has been concluded that CO2activation was not so ef?cient for the development of the surface area and pore structure of resulted activated carbons even for8h at8508C(Table2).Activated carbons of low surfaces and porosity,i.e.300–400m2/g and pore volume of0.1–0.15m3/g were obtained.Despite on that, pre-treatment effect is clearly revealed.Pre-intercalation of the anthracites improves the properties of the resulted activated carbons(Table2).

Generally,the reactivity of the pre-treated anthracites was higher during activation,and the S BET of the resulted activated carbons was found to be higher by1.5–3times for low-and high rank anthracites,Il and Pr,in comparison with the parent samples.

According to the X-ray photoelectron spectroscopy (XPS)and elemental analysis data,differences between parent and pre-treated samples were observed for carbon, oxygen chlorine and nitrogen contents,which reveal the gasi?cation ability of the modi?ed sample.Positions of C 1s,O1s,Cl2p and N1s core level signals,assignment and area percentage are given in Table3.Fitting Voigt pro?les to the peak regions gives show seven components for all C 1s regions and three components for O1s regions(Figs.2 and3)The seventh component in the C1s regions,centred at,7.5eV from the main peak,is assigned to a shake-up satellite corresponding to a p–p p excitation typical of aromatic systems.

The?rst main difference between samples is the ratio of aliphatic and aromatic carbons at the surface.The anthracites pre-treated by HNO3are rich in aromatic carbons(aliphatic/aromatic carbons ratio was found to be 0.27)and the insertion reactions slightly change initial aromaticity of the anthracites(Fig.2a).Further activation of the pre-treated samples results in increase of aliphatic/ aromatic carbons ratio up to2.8.However,this tendency was found to be opposite for the anthracite pre-treated with HClO4(Fig.2b).Due to strong interaction of the parent anthracites with HClO4during the stage of chemical pre-treatment the amount of aromatic groups drastically decreases(the aromatic/aliphatic carbons ratio is0.15). Then during activation this ration increases to0.41.This fact reveals that surface of the pre-treated by HClO4 anthracites is much richer in aliphatic carbons resulting from graphitic bonds rupture.

Such different behaviour is caused by features of the high-and low-reactive anthracites interaction with the intercalation agents of different nature,when one could observe either remarkable destruction of the aromatic system or its relative stability(Table3).

Fitted at287.9^0.1eV and at ca.289.3eV peaks could be attributed to the carbonyl and carboxylic bonds(Fig.2). The presence of those bonds is considerable con?rming a strong oxidation of the parent anthracites during chemical

Table2

Surface parameters of resulted activated anthracites

Sample S BET

(m2/g)Burn-off

(%)

V total

(cm3/g)

V micro

(cm3/g)

V meso

(cm3/g)

Activation with CO2,8h at8508C

Pr277110.110.080.03

Pr(HClO4)425280.160.120.04

Pr(HNO3)597450.210.180.03

Il410520.150.110.04

Il(HClO4)463720.170.140.03

Il(HNO3)613800.210.170.04

Activation with H2O,2h at8508C

Pr422320.160.110.05

Pr(HClO4)789600.150.120.03

Pr(HNO3)1139780.640.380.26

Il1131480.680.390.28

Il(HClO4)1524700.890.390.50

Il(HNO3)1601820.980.530.46

S.B.Lyubchik et al./Journal of Physics and Chemistry of Solids65(2004)127–132129

pre-treatment.However,the carboxylic bonds contribution is weaker and less evident for the pre-treated samples than for those after activation (Fig.2).That fact is one more con?rmation of the interaction of the intercalation species with the anthracite surface groups.

The conclusion,extracted from the carbon peak analysis,is con?rmed by the analysis of O 1s region.The following oxygen functionalities:carbonyl/quinine (C y O),alcoho-l/ether (C–O)and carboxylic acid/ester (O y C–O p ),con-tribute with peaks centred at 531.1,532.3and 533.3eV,respectively [10];chemisorbed water contributes with peak at ca.535.9eV (Fig.3a and b )[10].

The global quantitative results (Table 3)con?rm the ef?ciency of HNO 3treatment.The degree of surface oxidation expressed as the ratio of total oxygen to total carbon,O total /C total ,was found to be higher (O total /C total ?0.09)than for the anthracites treated with HClO 4(O total /C total ?0.07).

The differences between studied samples are also revealed in the position of the peaks for O 1s regions.In fact,we may notice that the O 1s region in pre-treated by HNO 3samples is shifted towards lower binding energy than in pre-treated by HClO 4ones.This is compatible both with oxygen included in groups bound to aromatic groups or included in ether groups instead of alcohol groups [10]con?rming the same qualitative conclusions reached above:activated surface is richer in aromatic carbons and C–X–R groups,whereas pre-treated by HClO 4samples are richer in C–X–H groups bound to aliphatic carbons.

We found the ratio oxidized carbon/electronegative elements of ,3for activated anthracites and of ,2for pre-treated ones.It means that the number of groups C–X–H (where X is O,N or Cl)is larger in pre-treated samples,whereas they are alkyl terminated after activation.

Chlorine is present in a consequent proportion in the anthracite pre-treated with perchloric acid (Fig.4).The atomic Cl total /C total ratio was found to be 0.003.The Cl 2p core level signal is composed of two contributions,which are attributed to chlorine bound to aromatic carbons (peak centred at ,201eV)and to chlorine in perchloric acid (peak centred at ,208eV)(referred to Au4f 7/2?84eV)(Fig.4)[9].Chlorine is mainly bound to carbon with some contribution of chlorine belonging to HClO 4:the Cl (bound to carbon)/Cl (in HClO 4)ratio is 3.45.

From the XPS data,a consequent amount of nitrogen is present in anthracites chemically pre-treated by HNO 3(Fig.5).The N 1s core level signal also con?rms the presence of carbon/nitrogen terminated groups of O–C(y O)N,CN(H)C,whereas carbons are mostly alkyl terminated after activation.After ?tting,peaks could be attributed to protonated amine functions (398.4eV),O–C(y O)N or C(y O)NC(y O)(400.6eV),–N(CH 3)3t(402.7eV)and nitrous groups (NO 2)(405.7eV)(Fig.4).The contribution of protonated amine functions is weak

T a b l e 3C o m p o n e n t o f t h e p e a k s f o r C 1s ,O 1s ,C l 2p a n d N 2p r e g i o n s

C 1s

O 1s

C l 2p

P o s i t i o n

P r (H C l O 4)A (C O 2)284.4285.0286.5287.9289.3290.7531.3533.3536.7––P r (H C l O 4)284.4285.0286.5287.9289.3290.8532.2533.4535.9200.6207.2A t o m i c %

P r (H C l O 4)A (C O 2)20.750.310.64.53.23.41.93.10.5––P r (H C l O 4)

9.8

66.08.7

2.9

2.6

1.9

3.6

2.60.4

0.17

0.05

C 1s

O 1s

N 2p

P o s i t i o n

I l (H N O 3)A (H 2O )284.5285.0286.5287.9289.3290.9531.5533.1535.3–––I l (H N O 3)284.5285.0286.5287.9289.3291531.8532.9536.2398.6400.6402.7405.7A t o m i c %

I l (H N O 3)A (H 2O )8.057.29.95.84.25.53.61.90.8––––I l (H N O 3)

51.00

22.5

7.2

3.9

3.1

2.4

3.43.80.5

0.19

0.49

0.14

0.57

A s s i g n m e n t

C –C s p 2

C –C s p 3

C –O H ,C –O –C ,C –S C y O ,O –C –O ,O –C –N ,C –C l C O O H ,C O O C ,O –C O O 1

O y C O O C y O

C –O –C ,C –O –H

W a t e r

C –C l

C l O 4

C N (H )C

O O C N

–N (C H 3)3t

N O 2

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130

(up to 0.19at.%),whereas the presence of nitrogen/oxygen functionalities is remarkable (up to 1.20at.%).

Thus,the pre-treatment of anthracite with HNO 3and HClO 4acids leads to a well-balanced contribution of insertion and oxidation reactions to the process of anthracite modi?cation,resulting in noticeable increasing reactivity and accessibility of activating gas to interior anthracite surface during activation.

The conclusion,extracted from the XPS analysis,is con?rmed by the XRD.According to X-ray diffraction data parameters of spatial structure are also changed.Generally,for the obtained activated carbons the average diameter of crystallites decreases (from 3.543to 2.712nm)because of partial oxydestruction of the edges of polyaromatic layers;whereas height of the crystallites increases (from 7.297to 7.831nm),probably due to partial spatial disordering.4.Conclusions

Combined chemical pre-treatment via intercalation-like reactions and a physical activation could be successfully applied to the activated carbons preparation from natural anthracites of different carbon content.For the activated

carbons production with extended surface area and porous structure one must carefully combine the conditions of the synthesis.Thus,steam activation was found to be better for the anthracite in all range of carbon content then that with CO 2.However,the choice of the chemical agents

for

Fig.2.C 1s XPS spectra obtained on chemically modi?ed anthracites:(a)by HNO 3;(b)by HClO 4

Fig.3.O 1s XPS spectra obtained on chemically modi?ed anthracites:(a)by HNO 3;(b)by HClO 4

Fig.4.Cl 2p XPS spectra obtained on an anthracite chemically modi?ed by HClO 4.

S.B.Lyubchik et al./Journal of Physics and Chemistry of Solids 65(2004)127–132131

the stage of pre-treatment must be given taking into account the initial carbon content of the https://www.wendangku.net/doc/f112404843.html,ing the mentioned above approach,the microporous activated carbons with extended surface area have been prepared from Ukrainian anthracites of wide range carbon content.

Acknowledgements

The authors are grateful to the INTAS Programme for the ?nancial support of this work,INTAS project 00-750.

Dr S.Lyubchik is thankful to the FCT Programme,Portugal for the grant SFRH/BPD/7150/2001.

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