Carbon 43 (2005) 2857–2868

www.elsevier.com/locate/carbon

Surface chemistry of phosphorus-containing carbonsof lignocellulosic origin

Alexander M. Puziy a,*, Olga I. Poddubnaya a, Amelia Martınez-Alonso b,Fabian Suarez-Garcıa b, Juan M.D. Tascon b,*

a Institute for Sorption and Problems of Endoecology, Naumov St. 13, 03164 Kyiv, Ukraineb Instituto Nacional del Carbon, CSIC, Apartado 73, 33080 Oviedo, Spain

Received 11 March 2005; accepted 10 June 2005Available online 1 August 2005

Abstract

Activated carbon adsorbents were prepared by phosphoric acid activation of fruit stones in an argon atmosphere at various tem-peratures in the 400–1000 �C range and at different acid/precursor impregnation ratios (0.63–1.02). The surface chemistry of thecarbons was investigated by elemental analysis, cation exchange capacity (CEC, measured by neutralization of NaOH with acidicsurface groups), infrared spectroscopy and potentiometric titration. Porous structure was derived from adsorption isotherms (N2 at�196 �C and CO2 at 0 �C). It was demonstrated that all carbons show considerable cation exchange capacity, the maximum(CEC = 2.2 mmol g�1) being attained at 800 �C, which coincides with the maximum contents of phosphorus and oxygen. The cationexchange properties of phosphoric acid activated carbons from fruit stones are chemically stable in very acidic and basic solutions.Proton affinity distributions of all carbons show the presence of three types of surface groups with pK at 2.0–3.3, 4.6–5.9 and7.6–9.1. These pK ranges were ascribed primarily to: (a) phosphorus-containing and carboxylic groups; (b) lactonic groups, and(c) phenolic groups, respectively. Phosphoric acid activated carbons are microporous with a relatively small contribution of mesop-ores. A maximum BET surface area of 1740 m2 g�1 was attained at 400 �C.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Activated carbon; Activation; Infrared spectroscopy; Functional groups; Surface properties

1. Introduction

Activation with phosphoric acid is a well-establishedmethod for the preparation of activated carbons. Muchinformation is available concerning the pore structure ofthe resulting materials [1–3], but much less is knownabout their surface chemistry. In previous joint workfrom our teams [4–6], we have shown that carbons pre-pared by phosphoric acid activation of a styrene–

0008-6223/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2005.06.014

* Corresponding authors. Tel.: +38 44 4529328; fax: +38 444529327(A.M. Puziy), fax: +34 98 5297662 (J.M.D. Tascon).

E-mail addresses: alexander.puziy@ispe.kiev.ua (A.M. Puziy),tascon@incar.csic.es (J.M.D. Tascon).

divinylbenzene copolymer exhibit acidic character andconsiderable ion exchange capacities, which made themuseful for the removal of heavy metals from aqueoussolutions. The source of these unique cation-exchangeproperties are phosphorus-containing surface groupswhich are introduced during the activation process, themaximum amount being attained at 800 �C.

The outstanding surface chemical properties of theabove polymer-derived carbons prompted us to investi-gate whether incorporation of phosphorus imparts sim-ilar characteristics to carbons obtained from other morecommon and inexpensive precursors. Accordingly, inthis work, fruit stones (taken as representative for ligno-cellulosic feedstocks) were chemically activated with

2858 A.M. Puziy et al. / Carbon 43 (2005) 2857–2868

phosphoric acid. The resulting materials were character-ized in terms of surface chemistry and porous texture inorder to gain information regarding the effects of activa-tion temperature and acid/precursor impregnation ratioon their performance as ion exchangers.

2. Experimental

2.1. Materials

Phosphorus-containing carbons were prepared by aphosphoric acid activation method described previously[4–7]. Briefly, crushed and sieved fruit stones wereimpregnated with 60% phosphoric acid to the desiredacid/precursor ratio (weight of H3PO4/weight of precur-sor, dry basis), dried in air and then carbonized in aquartz glass reactor at temperatures of 400–1000 �Cfor 30 min in argon atmosphere (the flow rate was about1 L min�1). Fruit stones were a mixture of apricot andpeach stones, particle size 0.315–1.0 mm, and moisturecontent 7.4%. After heat treatment, the carbon wascooled to room temperature in the same flow of argon.To remove the excess of phosphoric acid the carbonswere extensively washed with hot water in a Soxhletextractor until the pH of the wash water became neutral.After the washing, carbons were dried in an oven at110 �C. Phosphorus-containing carbons are referred toin the paper as APPXXX/YY where XXX is the temper-ature (in �C) at which they were heat treated and YY isthe impregnation ratio (IR). Two samples were obtainedat the same conditions, APP800(1)/0.63 and APP800(2)/0.63, to check the reproducibility of the preparationtechnique.

2.2. Methods

Chemical stability tests were conducted by boiling of2 cm3 of carbon in 100 mL of either 0.5 M H2SO4 or1 M NaOH for 2 h. After boiling, the NaOH-treatedcarbon was transformed into its H-form with 1 M HCland then extensively washed with water in a Soxhletextractor until neutral pH was attained.

Elemental analysis was performed using an LECOCHNS-932 microanalysis apparatus with a VTF-900accessory for oxygen. Phosphorus content was deter-mined by means of the molybdenum blue method afterdigesting the carbon material in a mixture of concen-trated H2SO4 and HNO3.

Fourier transform infrared (FTIR) spectra were ac-quired using a Magna-IR 560 spectrometer (NicoletInstrument Corporation, USA) by adding 256 scans inthe 4000–400 cm�1 spectral range at 4 cm�1 resolution.Pressed KBr pellets at a sample/KBr ratio of 1:400–1:1200 were used. All spectra were normalized to 1 mgcm�2 of carbon adsorbent.

The cation-exchange capacity (CEC) of carbonadsorbents was determined using a modified Boehm�smethod [4,8]. Potentiometric titration experiments wereperformed using a 672 Titroprocessor combined with655 Dosimat (Metrohm, Herisau, Switzerland). Protonconcentration was monitored by means of an LL pHglass electrode (Metrohm, Herisau, Switzerland). Priorto experiments, the electrode electromotive force wascalibrated to proton concentration by blank titration.The adsorbed amount of protons was calculated usingthe equation:

Q ¼ V 0 þ V t

mð½Hþ�i � ½OH��i � ½Hþ�e þ ½OH��eÞ ð1Þ

where V0 and Vt are the volumes of background electro-lyte and titrant added, and m is the mass of adsorbent.Subscripts i and e refer to initial and equilibriumconcentrations.

Proton binding constants were calculated by solvingthe integral adsorption equation using the CONTINmethod [4,6,9]:

Qtð½Hþ�Þ ¼Z Kmax

Kmin

Qlð½Hþ�;KHÞf ðKHÞdKH þ Q0 ð2Þ

where Qt is the experimentally measured proton bind-ing isotherm, Ql is the local isotherm, f(KH) is the pro-ton affinity distribution (PAD) function, and Kmin andKmax are the limits of integration. Q0 is a constantbackground term, which accounts for surface groupswith binding constant outside the experimental win-dow. The distribution function, f(KH), describes thesite concentration as a function of the binding con-stant, and is a unique characteristic of the adsorbentmaterial.

As a local isotherm, which reflects the underlyingmechanism of ion binding, the Langmuir equation wasused

Ql ¼KH½Hþ�

1 þ KH½Hþ� ð3Þ

where KH is the binding constant and [H+] is the protonconcentration. For the case of adsorption on a chargedsurface, the proton concentration in the surface layerð½Hþ

s �Þ should be used rather than the bulk protonconcentration [H+]. The surface layer concentration isrelated to the bulk concentration by a Boltzmann cor-rection factor as

½Hþs � ¼ ½Hþ� expð�Fw=RT Þ ð4Þ

where w is the electric potential at the surface, F isFaraday�s constant, R is the universal gas constant,and T is the absolute temperature. The relation betweenthe surface charge and the electric potential in a diffuselayer model is given by

A.M. Puziy et al. / Carbon 43 (2005) 2857–2868 2859

w ¼ 2RTF

sinh�1 � rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi8ee0RTI

p� �

ð5Þ

where r is the surface charge, e is the relative permittiv-ity of the solution, e0 is the permittivity of vacuum, and Iis the ionic strength of the solution.

A Micromeritics ASAP 2010 volumetric adsorptionanalyzer was used to measure N2 adsorption–desorptionisotherms at �196 �C in the 10�6 to 1 relative pressurerange. CO2 adsorption was used to assess the narrowmicroporosity (size < 0.7 nm) where N2 adsorption at�196 �C can be kinetically restricted [10]. Adsorptionisotherms of CO2 at 0 �C were measured in a semiauto-matic adsorption apparatus (NOVA-1200, Quanta-chrome). Prior to the measurements, all samples weredegassed overnight at 250 �C under vacuum. The repro-ducibility of the isotherms in different runs was betterthan 1.5% (N2) or 2% (CO2).

The surface area of the samples under study was as-sessed by the standard BET method, using nitrogenadsorption data in the relative pressure range from0.01 to 0.10. The total pore volume was calculated fromthe amount of nitrogen adsorbed at a relative pressureof 0.975. N2 adsorption isotherms were analyzed bymeans of the high-resolution as method, based on theoriginal as method [11,12] and reviewed by Kanekoet al. [13,14]. Non-graphitized Spheron 6 carbon blackwas used as a reference. The reference adsorption iso-therm on this material showed a good agreement withprevious data for other carbon blacks [13,15]. Micro-pore-related parameters were also calculated by fittingthe Dubinin–Radushkevich (DR) equation to the N2

and CO2 adsorption isotherms. A comparison of resultsobtained with both adsorptives allowed us to assess thenarrow microporosity [16,17]. Pore size distributions(PSD) were calculated from N2 adsorption isothermsusing the DFT Plus Software (Micromeritics InstrumentCorporation), based on the non-local density functionaltheory (NLDFT) [18].

Table 1Experimental conditions, bulk density, yield and cation exchange capacity (C

Sample T, �C IRa Yieldb, %

APP400/0.89 400 0.89 48.5 (32.7APP500/0.89 500 0.89 45.3 (30.3APP600/0.89 600 0.89 45.1 (26.2APP700/0.89 700 0.89 39.8 (26.8APP800/0.89 800 0.89 43.1 (22.0APP900/0.89 900 0.89 36.0 (18.1APP1000/0.89 1000 0.89 31.9 (20.9APP800(1)/0.63 800 0.63 43.0APP800(2)/0.63 800 0.63 40.0APP800/0.76 800 0.76 43.0APP800/0.89 800 0.89 43.1APP800/1.02 800 1.02 62.9

a IR = phosphoric acid/carbon precursor impregnation ratio.b Values in parenthesis are for carbons carbonized without H3PO4.

3. Results and discussion

3.1. Pyrolysis

Table 1 shows the experimental conditions, bulk den-sity, yield and cation exchange capacity (CEC) of phos-phoric acid activated carbons from fruit stones. For animpregnation ratio of 0.89, with increasing pyrolysistemperature the yield of carbonaceous residue decreasesup to 700 �C, then increases at 800 �C and again de-creases at 900 and 1000 �C. The increase of yield at800 �C is attributed to a higher amount of incorporatedphosphorus and as a result, the highest amount of ash isformed (see next section). The carbons obtained at800 �C also show the highest cation exchange capacity,suggesting a relationship between phosphorus speciesand this feature. It should be noted that the yield ofphosphoric acid activated carbons is on average about16% over that for carbons carbonized at the same tem-peratures without H3PO4 (Table 1). For carbons ob-tained at 800 �C, the yield does not vary much withimpregnation ratio in the range from 0.63 to 0.89, butit increases when the impregnation ratio is increased to1.02.

It is noteworthy that the cation exchange propertiesof phosphoric acid activated carbons from fruit stonesare chemically stable in very acidic and basic solutions.Fig. 1 shows that boiling of carbons in either 0.5 MH2SO4 or 1 M NaOH solutions for 2 h does not changethe cation exchange capacity very much, except for acidtreatment of low temperature carbon adsorbents (400–500 �C), for which a slight decrease is observed. Theabove result suggests that surface groups are chemicallystable and tightly bound to the carbon lattice.

3.2. Chemical composition

The chemical composition of carbon adsorbents ob-tained at different conditions is presented in Table 2.

EC) of phosphorus-containing carbon adsorbents from fruit stones

Bulk density, cm3 g�1 CEC, mmol g�1

) 0.41 1.47) 0.41 1.18) 0.44 1.13) 0.50 1.91) 0.52 2.21) 0.46 2.16) 0.41 2.05

0.56 2.200.64 2.240.66 2.240.52 2.210.72 2.16

0.0

0.5

1.0

1.5

2.0

2.5

300 400 500 600 700 800 900 1000 1100Temperature, °C

CEC

, mm

ol g

-1

parentNaOHH2 SO4

Fig. 1. Change in cation exchange capacity of phosphorus-containingcarbon adsorbents from fruit stones after boiling in either 0.5 MH2SO4 or 1 M NaOH solutions for 2 h.

2.5

0

2

4

6

8

10

12

14

16

18

300 400 500 600 700 800 900 1000 1100

Con

tent

,%

0.0

0.5

1.0

1.5

2.0OPCEC

Temperature, °C

CEC

, mm

ol g

-1

Fig. 2. Temperature dependence of oxygen and phosphorus contentand cation exchange capacity of phosphorus-containing carbonadsorbents from fruit stones.

2860 A.M. Puziy et al. / Carbon 43 (2005) 2857–2868

As in the case of polymer based carbons [4–6], phospho-ric acid activation applied to fruit stones led to theincorporation of a significant amount of phosphorusin the resulting carbon adsorbents. With the increaseof pyrolysis temperature, the content of phosphorus in-creases reaching 8.5% at 800 �C and then decreasing.The decrease in phosphorus content above 800 �C is as-cribed to volatilization of phosphorus-containing com-pounds, most probably in the form of polyphosphoricacid, phosphorus pentoxide or even as elemental phos-phorus [19–21]. For carbons obtained at 800 �C withan increasing impregnation ratio, the content of phos-phorus virtually remains the same up to IR = 0.89 witha slight decrease at 1.02 (Table 2).

Phosphorus-containing carbons from fruit stonescontain a relatively high amount of oxygen (8–17%,Table 2). The oxygen content decreases as pyrolysistemperature increases up to 500 �C. With further in-crease of the pyrolysis temperature, the oxygen contentincreases reaching a maximum at 700–800 �C and de-creases at higher temperatures (Fig. 2). The oscillatingbehavior of the temperature dependence of oxygen con-tent implies several processes proceeding during the

Table 2Elemental analysis of phosphorus-containing carbon adsorbents from fruit s

Sample C H N S

APP400/0.89 78.72 2.71 0.19 0.03APP500/0.89 80.71 2.34 0.11 0.03APP600/0.89 80.75 1.92 0.18 0.02APP700/0.89 72.43 1.80 0.19 0.02APP800/0.89 69.48 1.49 0.18 0.03APP900/0.89 68.98 1.59 0.19 0.03APP1000/0.89 78.94 0.81 0.16 0.01APP800(1)/0.63 69.60 1.39 0.21 0.01APP800(2)/0.63 70.27 1.38 0.17 0.02APP800/0.76 69.66 1.32 0.18 0.02APP800/0.89 69.48 1.49 0.18 0.03APP800/1.02 70.24 1.39 0.20 0.03

pyrolysis. At low temperatures (400–500 �C) an initiallyreach rich in oxygen carbonaceous material loses itsoxygen due to dehydrating effect of phosphoric acid.At this stage, the phosphorus content is relatively low,suggesting that the most of oxygen in carbonaceousmaterial is retained from the precursor material. Inother words, the major part of oxygen is bound to thecarbonaceous material. When pyrolysis temperature in-creases from 600 to 800 �C, the oxygen follows much thesame temperature dependence as phosphorus. This sug-gests that at higher temperatures the proportion of oxy-gen bound to phosphorus increases progressively. Somedeparture from this trend is observed at the highestpyrolysis temperatures 900–1000 �C most probably dueto possible evaporation of phosphorus and/or reductionof pentavalent phosphorus with carbon [22].

The changes of chemical structure during the pro-gress of pyrolysis are also reflected in the temperaturedependence of the O/P atomic ratio (Fig. 3). Interest-ingly, the O/P ratio follows the same trend for carbonsprepared from two dissimilar precursors: styrene/divi-nylbenzene copolymer (SP series [4]) and mixture of fruitstones (APP series, this study). A first drastic decrease in

tones (wt%, dry basis)

O P Ash CHNSOP + ash

12.23 1.69 5.33 100.908.34 2.59 7.65 101.779.25 3.17 9.53 104.82

12.77 5.95 16.87 110.0316.92 8.52 21.76 118.3816.94 7.09 18.39 113.218.01 4.15 13.23 105.31

13.75 8.56 22.32 115.8414.93 8.45 21.91 117.1314.66 8.42 21.47 115.7316.92 8.52 21.76 118.3814.61 7.84 20.30 114.61

02468

101214161820

300 400 500 600 700 800 900 1000 1100

O/P

ato

mic

ratio

APPSP

Temperature, °C

Fig. 3. Temperature dependence of oxygen to phosphorus atomicratio for phosphoric acid activated carbon adsorbents from fruit stonesAPP (this study) and polymer-based carbons SP [4].

1000

900

800

700

600

500

400

0.000.050.100.150.200.250.300.350.400.450.50

0.00 0.05 0.10 0.15 0.20 0.25O/C atomic ratio

H/C

ato

mic

ratio

slope=2

Fig. 4. van Krevelen diagram of phosphorus-containing carbonadsorbents from fruit stones. Closed symbols, carbons obtained atdifferent temperatures indicated in the graph, open symbols, carbonsobtained at 800 �C with different impregnation ratios.

A.M. Puziy et al. / Carbon 43 (2005) 2857–2868 2861

O/P ratio at 500 �C is followed by a much slower de-crease reaching a minimum at 800 �C. At this point,the O/P atomic ratio is equal to 4.1 for the SP seriesand 3.8 for the APP series. The actual O/P ratio ofP-containing species is obviously lower when it is con-sidered that a portion of oxygen is not connected withphosphorus species. It is intriguing that linear polyphos-phates of general formula Hn+2PnO3n+1 show an O/Patomic ratio from 4 for n = 1 (orthophosphoric acid)to 3 for n = 1. This fact argues for polyphosphates asprobable chemical structure of phosphorus species inphosphoric acid activated carbons. Increase of the O/Patomic ratio for SP carbons at 900–1000 �C is attribut-able to the reduction of phosphorus compounds, evapo-ration of white phosphorus (P4) and concomitantoxidation of the carbonaceous material. Oxygen-con-taining surface groups, though thermally unstable athigh temperature, may survive for a short period ofpyrolysis (30 min). An additional source of oxygen inthe carbons obtained at high temperature is post-pyroly-sis self-heating, which most probably is due to oxidationof carbons by air. The decrease in phosphorus contentand less pronounced decrease in the content of oxygenlead to increase in O/P ratio at high temperatures. Thesame mechanism was proposed for phosphoric acid acti-vation of Nomex fiber [19]. Unlike polymer-based car-bons SP [4] or Nomex [19], APP carbons fromlignocellulosic precursor contain a relatively highamount of ash. Consequently much phosphorus inAPP carbons is bound to ash components (Na, K, Ca,Mg, etc.) in the form of corresponding phosphates, mostof which have melting and boiling points higher than1100 �C. Moreover, some of them are insoluble in waterand can thus remain retained in the final activated car-bon after washing. On the other hand, these phosphatesare more difficult to reduce to P4. For instance, phos-phorus is produced industrially by reduction of calciumphosphate at 1500 �C.

Interestingly, the sum of all measured elementsamounts to more than 100% (Table 2). This can be ex-plained by the fact that both phosphorus and oxygenare likely to be present in the ash and thus are accountedfor twice—as element and as part of the ash.

The cation exchange capacity roughly follows thetrend of both oxygen and phosphorus contents, suggest-ing that the cation exchange properties are governed bythe presence of oxygen- and phosphorus-containingsurface groups (Fig. 2). Fig. 2 also shows that, even at400–500 �C, when phosphorus is relatively absent butthe carbonaceous material is still rich in oxygen, theCEC correlates with oxygen content.

A suitable representation of the elemental composi-tion of phosphorus-containing carbons from fruit stonesis given by the van Krevelen diagram (H/C atomic ver-sus O/C atomic [23]). Such representation clearly showsthe chemical changes occurring during pyrolysis of fruitstones in the presence of phosphoric acid. It is evidentfrom the diagram (Fig. 4) that the pyrolysis processcould be divided into three parts. First is the pyrolysistemperature interval from 400 to 500 �C where both Hand O atoms are lost. The second interval, from 500to 900 �C, is characterized by a relative enrichment ofO, while H continues to decrease. Finally, when thepyrolysis temperature increases from 900 to 1000 �C,both H and O are lost again. At this step, the materialtends to reach the origin of the diagram (representativepoint for a pure carbon). Increasing the impregnationratio does not show any departure from this trend forthe carbons obtained at 800 �C (Fig. 4). It is interestingto note that low temperature carbon adsorbents (up to500 �C) do not fall on a straight line with slope 2 aswas observed for wood carbons [2,24]. This suggeststhat reactions other than dehydration take place dur-ing chemical activation of fruit stones with phosphoricacid.

2862 A.M. Puziy et al. / Carbon 43 (2005) 2857–2868

3.3. IR spectroscopy

IR spectroscopy provides information on the chemi-cal structure of carbon adsorbents. Fig. 5 shows FTIRspectra of carbons obtained by phosphoric acid activa-tion of fruit stones at different temperatures. The spectrawere interpreted using vibrational group frequenciescharacteristic of the most common functional groupsand structural components [25]. All spectra show apronounced band at 1580 cm�1. This band is character-istic of the aromatic ring stretching mode indicating theexistence of single or multiple aromatic rings in thestructure of the carbon adsorbent. A small shift of thisband to lower frequencies for carbons obtained athigh temperatures indicates enlargement of aromaticring structures with increasing pyrolysis temperature.The aromatic character of the carbons is supportedby aromatic C–H stretch (3060 cm�1) and aromaticC–H out-of-plane bend vibrations (890, 820, 760 cm�1)which are evident at 400–500 �C and vanish at highertemperatures. The disappearance of aromatic C–H bandsdemonstrates a progressive elimination of hydrogendue to substitution by functional groups or formationof fused-ring structures. This fact is in line with ele-mental analysis data, which showed a decrease inhydrogen content with increasing pyrolysis tempera-ture (Table 2). The spectra also show aliphatic C–H

05001000150020002500300035004000

Wavenumber, cm-1

Abso

rban

ce

400

500

600

700

800

900

1000

1580

1085

1180

1700

3420

1400

3060

2960

2920

2850

890

820

760

Fig. 5. FTIR spectra of phosphorus-containing carbon adsorbentsfrom fruit stones obtained at different temperatures (in �C). Thevertical lines correspond to the most significant band wavenumbers (incm�1).

stretching vibrations (methyl C–H asymmetric stretchat 2960 cm�1 for APP400 carbon adsorbent and methy-lene >C–H asymmetric/symmetric stretch at 2920/2850 cm�1 for all carbons) suggesting the presence ofaliphatic structures.

All spectra show a strong wide absorption band at3600–3200 cm�1 with a maximum at about 3420 cm�1.The position of the band is characteristic of the stretch-ing vibration of hydroxy compounds while broadeningof the band is indicative of high degree of associationbecause of extensive hydrogen bonding. Thus, it isexpected that the investigated carbons contain hydroxylgroups from carboxyls, phenols or alcohols. The exis-tence of phenols is supported by O–H bending(1400 cm�1) and C–O stretching (1200–1180 cm�1)vibrations characteristic for phenols [25]. Low temper-ature carbon adsorbents (400–600 �C) show a smallpeak at 1700 cm�1 characteristic of C@O absorptionin carboxylic acids. The relatively low frequency ofthe band indicates conjugation of C@O to the aro-matic ring system. The small intensity of this peak sug-gests a relatively low content of carboxylic groups ascompared to other oxygen groups of the carbonadsorbents.

All carbons showed a broad band in the main finger-print spectral region between 1300 and 900 cm�1. Themaximum of this band appeared at 1240 cm�1 forAPP400 carbon adsorbent and shifted to 1180 cm�1

for carbons obtained at higher temperatures. The spec-tra of carbon adsorbents obtained at 500–800 �C alsoshowed a shoulder at 1085 cm�1 that transformedinto a distinct peak for the carbons obtained at900–1000 �C. Absorption in this region is usually foundin oxidized carbons and has been assigned to C–Ostretching in acids, alcohols, phenols, ethers and esters[26,27]. It is also characteristic of phosphorus and phos-phocarbonaceous compounds present in phosphoricacid activated carbons [28, see also web service1]. Thepeak at 1180 cm�1 can be assigned to the stretchingvibration of hydrogen-bonded P@O groups [30–32]from phosphates or polyphosphates, to the O–C stretch-ing vibration in the P–O–C (aromatic) linkage [30, seealso web service1], and to P@OOH [web service1]. Thepeak at 1085 cm�1 could be due to P+–O� in acid phos-phate esters [30,31] and to the symmetrical vibration inpolyphosphate chain P–O–P [33,34].

It should be noted that the spectra do not show anybands characteristic of P–H (2500–2225 cm�1) andC–P bonding (795–650 cm�1) [32] which allows rulingout the existence of phosphine derivatives and directC–P bonding.

1 Spectroscopic Tools, http://www.chem.unipotsdam.de/tools/index.html.

A.M. Puziy et al. / Carbon 43 (2005) 2857–2868 2863

3.4. Proton binding

The acid–base properties of phosphorus-containingcarbons from fruit stones were investigated by meansof a potentiometric titration method. Fig. 6 shows theproton binding isotherms calculated from potentiomet-ric titration data. All proton binding isotherms lie inthe negative region showing that only proton dissocia-tion occurs in the investigated pH range. With increas-ing pyrolysis temperature from 400 to 800 �C theproton binding isotherms shift to more negative valuesof proton binding, Q. Pyrolysis of fruit stones at highertemperatures leads to a decrease in the cation exchangecapacity and consecutively to a shift in the isotherms tomore positive values of Q. This finding is in line with thecation exchange capacity increasing with increasingpyrolysis temperature up to 800 �C and decreasing athigher temperatures (Table 1).

Proton affinity distributions (PADs) (Fig. 7) were cal-culated from proton binding isotherms by solving the

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

2 3 4 5 6 7 8 9 10 11 12

pH

Q, m

mol

/g

APP400/0.89APP500/0.89APP600/0.89APP700/0.89APP800/0.89APP900/0.89APP1000/0.89

Fig. 6. Proton binding isotherms by phosphorus-containing carbonadsorbents from fruit stones.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1 2 3 4 5 6 7 8 9 10 11pK

F(pK

)

400

500

600

700

800

900

1000

Fig. 7. Proton affinity distributions for phosphorus-containing carbonadsorbents from fruit stones obtained at different temperatures (in �C).Curves are offset by 0.5.

adsorption integral equation (2) using the CONTINmethod [9]. The solution yields the constant backgroundterm, Q0 (see Table 3), and the distribution function, F(pK), describing site density as a function of pK (seeTable 3 and Fig. 7). The constant background term cor-responds to the concentration of very acidic surface sitesthat are fully deprotonated even at very low experimen-tal pHs. Since very acidic groups are fully deprotonatedin the experimental conditions it was possible to deter-mine only the amount of these groups but not their dis-sociation constant. With the increase in pyrolysistemperature the amount of very acidic groups, Q0,follows the trends of oxygen and phosphorus content(Table 2)—that is, they increase up to 800 �C and thendecrease. This fact allows one to ascribe this group tovery acidic hydrogen of polyphosphates. Relativelystrongly acidic groups, Q0, are present in minor amountsin the investigated carbon adsorbents obtained at 400–500 �C. The contribution of these groups becomes sig-nificant at 600 �C and higher temperatures with a max-imum at 800 �C. This suggests that intensive formationof polyphosphates occurs at 700–900 �C.

Table 3Parameters of proton affinity distributions of phosphorus-containingcarbon adsorbents from fruit stones (IR = 0.89)

Carbon Surfacegroup

Q, mmol g�1 Error(%)

pK Error(%)

APP400 Q0 �0.063 206Q1 0.379 33 2.85 54Q2 0.394 5 5.92 9Q3 0.805 1 8.72 2

APP500 Q0 0.037 173Q1 0.335 17 2.89 26Q2 0.314 5 5.7 9Q3 0.505 2 8.84 3

APP600 Q0 0.107 75Q1 0.363 23 2.95 36Q2 0.275 7 5.56 15Q3 0.575 2 8.43 3

APP700 Q0 0.534 1Q1 0.452 2 2.64 6Q2 0.426 5 5.15 9Q3 0.443 4 8.40 7

APP800 Q0 0.548 3Q1 0.625 3 1.97 8Q2 0.652 3 4.63 6Q3 0.453 3 7.56 5

APP900 Q0 0.541 1Q1 0.336 2 2.73 5Q2 0.598 2 5.09 3Q3 0.611 1 8.15 3

APP1000 Q0 0.108 28Q1 0.184 14 3.32 36Q2 0.202 9 5.9 19Q3 0.59 1 9.14 3

2864 A.M. Puziy et al. / Carbon 43 (2005) 2857–2868

Proton affinity distributions of all carbon adsorbentsshow three peaks with maxima at pK values of 2.0–3.3,4.6–5.9 and 7.6–9.1. The maximum dissociation con-stant at pK 2.0–3.3 may correspond to phosphorus-con-taining groups in polyphosphates [35] and to carboxylicgroups [36]. The next surface group with the maximumpK of 4.6–5.9 may correspond to lactones while the leastacidic groups with pK values of 7.6–9.1 may be phenolgroups. Using a similar scheme based on comparisonof experimental pK values with dissociation constantsof organic acids, the surface groups of oxidized carbonswere also assigned to carboxylic (pK 3–8) and phenolicgroups (pK > 9) [37,38]. It should be noted that func-tional groups on the carbon surface might show moreacidic character as compared to their homogeneous ana-logs due to the influence of the surface charges and theinductive effect of neighboring groups. The protonationconstants of surface groups of phosphorus-containingcarbons from fruit stones increase (decrease of pK) withincreasing pyrolysis temperature up to 800 �C and thendecrease.

3.5. Porous texture

Fig. 8 shows selected adsorption–desorption iso-therms of N2 at �196 �C for carbon adsorbents pre-pared with an impregnation ratio of 0.89 at differenttemperatures. All the isotherms belong to a mixed typein the IUPAC classification, type I at low relative pres-sures and type IV at intermediate and high p/p0. In theirinitial part, they are type I, with an important uptake atlow relative pressures, characteristic of microporousmaterials. However, the knee of the isotherms is wide,no clear plateau is attained and a certain slope can beobserved at intermediate and high relative pressures,all these facts indicating the presence of large microp-

0

100

200

300

400

500

600

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

P/P0

Volu

me,

STP

cm

3 g-1

APP400/0.89APP600/0.89APP800/0.89APP1000/0.89

Fig. 8. Selected adsorption–desorption N2 isotherms at �196 �C onactivated carbons from fruit stones prepared by chemical activationwith an impregnation ratio of 0.89 of H3PO4 at different pyrolysistemperatures. Open symbols: adsorption; full symbols: desorption.

ores and mesopores (type IV). The isotherms exhibittype H4 hysteresis, typical of slit-shaped pores. Nitrogenuptake progressively decreases as the pyrolysis tempera-ture increases from 400 to 800 �C, increasing againabove 800 �C.

Fig. 9 shows high-resolution as plots for selected sam-ples. As Kaneko et al. [13,14] have shown, high-resolu-tion as curves for carbonaceous adsorbents can exhibittwo departures from linearity at as < 1, named fillingswing (FS) and cooperative swing (CS), which are dueto an enhancement in the surface-molecule interactionsfor certain pore sizes. The FS occurs at as < 0.5 and isassociated with the ‘‘primary micropore filling’’ pro-posed by Gregg and Sing [11, p. 94], This phenomenonindicates the presence of ultramicropores, where theinteraction between the pseudo-graphitic surface andthe adsorbate molecules is strongly enhanced by theoverlap with the potential of the opposite pore wall.The CS is typically observed at as > 0.7 and is attributedto the presence of wide micropores, where adsorption inthe empty space between the two monolayers adsorbedon either side of the pore wall is enhanced regardingmultilayer adsorption on the flat surface. As Fig. 9shows, the studied carbon adsorbents exhibit deviationsfrom linearity for as < 0.5 due to FS, indicating the pres-ence of ultramicroporosity. The magnitude of the FS ef-fect increases with increasing activation temperature,suggesting that the average pore size decreases as thetemperature increases.

The as method has been typically employed to calcu-late the external or non-microporous surface, Aext, andthe micropore volume, Vlp(as) from the slope and inter-cept, respectively, of the straight line fitting as values >1(dashed line in Fig. 9). Kaneko et al. [13,14] proposedthe determination of the specific surface area (As) ofthe material from the slope of the straight line that fitsas values < 1 and goes through the origin of coordinates.The interval for fitting depends on the sample under

0

100

200

300

400

500

600

0 0.5 1 1.5 2αs

APP400/0.89APP600/0.89APP800/0.89APP1000/0.89

Volu

me,

STP

cm

3 g-1

Fig. 9. High-resolution as curves of selected activated carbonsprepared at different temperatures with an impregnation ratio of 0.89.

A.M. Puziy et al. / Carbon 43 (2005) 2857–2868 2865

concern. For materials exhibiting only FS, as is the caseof this work, data at as > 0.5 are used (solid line inFig. 9). This method, called ‘‘subtracting pore effect’’,is very useful for determining the real surface area ofultrahigh surface area materials, for which the standardBET method overestimates the surface area [13].

Fig. 10 shows the PSDs obtained by the NLDFTmethod for selected samples prepared with an impregna-tion ratio of 0.89 at different pyrolysis temperatures. Allthe curves exhibit systematically a minimum around1 nm, which is well known to be an artifact introducedby modeling assumptions [39]. The PSDs exhibit twomaxima at 0.7 and 1.2 nm, as well as another smallshoulder at 2–2.5 nm. The porosity range extends topore widths >20 nm, but the contribution of mesoporesis relatively small, as indicated by Fig. 10. The materialprepared at the lowest temperature (400 �C) presents amaximum in the PSD at 1.2 nm; as the treatment tem-perature increases a decrease in the contribution ofthis pore size occurs, the maximum in the PSD being

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.1 1 10 100Pore width, nm

Diff

eren

tial p

ore

volu

me,

cm

3 g-1 APP400/0.89

APP600/0.89APP800/0.89APP1000/0.89

Fig. 10. Pore size distributions calculated by the DFT method forselected activated carbons prepared at different temperatures.

Table 4Textural parameters deduced from N2 adsorption at �196 �C and CO2 adchemical activation with H3PO4

Sample ABET Vtp as method

As Aext V

APP400/0.89 1737 0.89 1742 287 0APP500/0.89 1366 0.74 1369 260 0APP600/0.89 1262 0.67 1276 220 0APP700/0.89 1094 0.56 1115 181 0APP800/0.89 1055 0.54 1070 162 0APP900/0.89 1135 0.61 1147 222 0APP1000/0.89 1211 0.68 1216 259 0APP800(1)/0.63 746 0.38 783 132 0APP800(2)/0.63 801 0.39 838 119 0APP800/0.76 959 0.48 980 138 0APP800/0.89 1055 0.54 1070 162 0APP800/1.02 1196 0.62 1205 188 0

Surface areas in m2 g�1; pore volumes in cm3 g�1.

progressively displaced to 0.7 nm. This evolution ofthe PSDs confirms the tendency to lower pore sizes withincreasing pyrolysis temperature observed by the as

method.Table 4 reports different porous textural parameters

deduced from the isotherms of N2 at �196 �C andCO2 at 0 �C for the carbon adsorbents prepared withan impregnation ratio of 0.89 at different pyrolysis tem-peratures, and also for the series of samples prepared at800 �C with different impregnation ratios. Equivalent re-sults are obtained for the surface area as calculated bythe BET method (ABET) or by the as method (As); in-deed, the pressure range in which the BET methodwas applied was 0.01–0.1, which is the interval in whichthe error in applying the BET method to type I iso-therms is minimal [13].

It has been found with other lignocellulosic precur-sors studied previously [1–3,24,28,40,41] that porositydevelopment by chemical activation with phosphoricacid reaches a maximum at moderate temperatures(400–500 �C interval). In the present case this occurs at400 �C, the corresponding carbon adsorbent exhibitinga surface area of 1737 m2 g�1, a total pore volume of0.89 cm3 g�1 and a micropore volume of 0.64 cm3 g�1.As the activation temperature increases from 400 to800 �C a decrease is produced in all textural parametersexcept the volume of micropores <1 nm (Vlp<1 nm

(DFT)). Fig. 11 shows the activation temperaturedependence of the mean micropore width as determinedby the DR method (see experimental section) from theN2 and CO2 isotherms. The decrease in pore volumewith increasing temperature during chemical activationof other lignocellulosic precursors with H3PO4 has beenattributed to contraction of the material [2,24]. Duringthe pyrolysis of the lignocellulosic precursor, phosphoricacid combines with organic species forming phosphateand polyphosphate bridges that connect the biopolymerfragments avoiding contraction of the material by effect

sorption at 0 �C on activated carbons prepared from fruit stones by

DFT method DR method

lp Vlp<1 nm Vlp Vmp Vlp (CO2)

.64 0.16 0.53 0.23 0.35

.50 0.15 0.41 0.23 0.28

.47 0.17 0.38 0.19 0.28

.40 0.18 0.33 0.15 0.28

.40 0.18 0.32 0.14 0.26

.42 0.17 0.33 0.19 0.26

.45 0.21 0.34 0.24 0.31

.26 0.14 0.23 0.09 0.22

.29 0.17 0.25 0.08 0.23

.36 0.16 0.29 0.11 0.25

.40 0.18 0.32 0.14 0.26

.45 0.19 0.35 0.17 0.29

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

300 400 500 600 700 800 900 1000 1100

Temperature, ºC

Mea

n m

icro

pore

wid

th, n

m

From N2 isoterms

From CO2 isotherms

Fig. 11. Mean micropore width (calculated by the DR method fromadsorption isotherms of N2 at �196 �C and CO2 at 0 �C) as a functionof activation temperature.

2866 A.M. Puziy et al. / Carbon 43 (2005) 2857–2868

of the temperature. Above 450 �C these phosphate andpolyphosphate bridges become thermally unstable, lead-ing to a decrease in porosity by contraction of the char.This is reflected in a decrease in the average microporesize (Figs. 10 and 11) and in the fact that, unlike the restof the parameters, the volume of narrow micropores(Vlp<1 nm (DFT)) increases from 500 to 800 �C. AsTable 2 shows, the amount of P retained in the carbonadsorbents increases up to 800 �C; these P compoundscan block a certain fraction of porosity, contributingto the decrease of the measured pore volume. Above800 �C the porosity increases again, showing a behaviorsimilar to that reported recently by Suarez-Garcıa et al.[42] in phosphoric acid activation of Nomex polyaramidfibers. Thermogravimetric study of Nomex pyrolysis inthe presence of phosphoric acid showed a mass loss be-tween 800 and 900 �C that did not take place whenNomex was carbonized alone, and that was ascribedto volatilization of P compounds [42]. This volatilizationcan produce new channels (pores) and contribute to theobserved increase in porosity above 800 �C.

Table 4 also includes porous texture parameters formaterials prepared at 800 �C using different impregna-tion ratios. In this case, all the parameters measured in-crease with increasing impregnation ratio. Therefore,increasing the impregnation ratio leads to increases inboth pore volume and pore size. Similar effects havebeen reported in the preparation of porous carbonsby phosphoric acid activation of other precursors[1–3,5,29,43–45].

4. Concluding remarks

Chemical activation of a mixture of fruit stones withphosphoric acid produced carbon adsorbents withstrongly acidic surface groups, the maximum acidity

being attained at 800 �C. It is obvious that this effect isdue to chemical reactions taking place during heat treat-ment of the impregnated precursor. It is generallybelieved that carbons obtained by pyrolysis of lignocel-lulosic precursors without impregnation show a slightlybasic character. The acid surface of phosphoric acidactivated carbons is obviously due to phosphorus-containing species along with carboxylic and phenolicgroups. This and previous studies have shown that upto 8.5–8.7% of phosphorus can be incorporated in car-bons obtained by phosphoric acid activation of sty-rene/divinylbenzene copolymer [4–6] and fruit stones(Table 2).

It is of interest to elucidate the chemical state of phos-phorus in phosphorus-containing carbons. Consideringthe chemical state of the phosphorus species one musttake into account the manufacturing technique of thesample as well as the specific properties of the carbon.These properties are acidic character of the surface (con-nected with highly acidic surface groups), cation ex-change properties, and chemical stability of the cationexchange properties. The source of phosphorus com-pounds in our experiments was phosphoric acid, whichin the presence of the carbon precursor undergoes chem-ical changes at high temperatures. The fundamentalreactions of phosphoric acid at elevated temperaturesare dehydration and formation of condensed phos-phates [46,47]. Phosphoric or polyphosphoric acids canreact with the carbonaceous precursor to form phos-phate or polyphosphate esters [24]. Very acidic surfacegroups on phosphorus-containing carbons (see Section3.4) may be related to polyphosphates bound to carbonvia P–O–C linkages (see Section 8). It is known that, inany condensed phosphoric acid, there is one stronglyacidic hydrogen atom (pK < 2) for each phosphorusatom. In addition, chain phosphoric acids have a weaklyacidic hydrogen atom (pK = 6–9) at each of the ends ofthe chain. The previously observed chemical stability ofpolymer-based phosphoric acid activated carbons [4] isan indication of a high degree of polymerization of con-densed phosphates. Progressive increase of stability forpolyphosphates with an increasing number of tetrahe-dral [PO4] units has been reported [48]. An additionalexplanation of chemical stability may be chemical bond-ing to carbon surface and steric hindrance in pore space.

The cation exchange properties of phosphoric acidactivated carbons may also be associated with carbox-ylic and phenolic groups as in oxidized carbons in addi-tion to the phosphorus-containing surface groups. Thesource of these oxygen-containing groups may be phos-phoric acid or oxygen from air. All carbons investigatedin this work showed a self-heating effect upon contactwith air after cooling down to room temperature; thesame phenomenon was observed for polymer-based car-bons [4]. Most probably phosphorus is reduced by car-bon at high temperature to form highly dispersed

A.M. Puziy et al. / Carbon 43 (2005) 2857–2868 2867

elemental phosphorus, which upon contact with air atroom temperature, oxidizes giving off heat. Indeed, ele-mental phosphorus is industrially obtained by heatingphosphates in an electrical furnace in the presence ofcarbon [22]. The formation of elemental phosphorusduring phosphoric acid activation of Nomex fiber [19]or pyrolysis of phosphorus-containing phenol resins[20,21] has also been reported.

Phosphoric acid activation of a lignocellulosic precur-sor (fruit stones) produced carbon adsorbents with well-developed porous structure, the surface area beingseveral times higher than that of polymer-based carbons[4]. Well-developed porosity together with high cationexchange capacity makes phosphoric acid activated car-bons from lignocellulosic precursors promising materi-als for applications involving metal ion adsorption.

Acknowledgement

This research was made possible in part by theNATO Collaborative Linkage Grant EST.CLG.979588.The constructive suggestions from two anonymous ref-erees are gratefully acknowledged.

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