ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT i1741

2001 © The Japan Society for Analytical Chemistry

Analysis of Active Sites on Synthetic Carbon Surfaces by Various

Methods

Krisztina LÁSZLÓ

1, Katalin JOSEPOVITS

2, and Etelka TOMBÁCZ

3

1†

Department of Physical Chemistry, Budapest University of Technology and Economics, H-1521 Budapest, Hungary (E-mail: klaszlo.fkt@chem.bme.hu) 2 Department of Atomic Physics, Budapest University of Technology and Economics, H-1521 Budapest,

Hungary 3 Department of Colloid Chemistry, Szeged University, Aradi vértanúk tere1, H-6720 Szeged, Hungary

The surface composition of polymer based porous carbons containing O and N heteroatoms was studied by X-ray Photoelectron Spectroscopy (XPS), Boehm titration and ”equilibrium” potetiometric titration, in order to study the chemistry of the surface. The PET derived carbon contains less surface hetero atom (5.7 %w/w), than the APAN (12.9 %w/w), according to XPS data. Both carbons exhibit an acid/base character. The net proton surface excess

becomes 0 at pH 6.4 and 7.1 in case of APET and APAN, respectively. The PET derived carbon has 93.7 µequiv/g

acidic and 416.8 µequiv/g basic surface groups, while APAN carbon contains 112.9 and 336.3 µequiv/g acidic and basic

groups, respectively. Considering the surface area of these carbons, the APAN carbon has almost twice as high active surface site concentration (49.7/100 nm2), than the APET sample (25.7/100 nm2). The same tendency has been concluded from XPS analysis. The acid/base property plays a distinguished role when porous carbons are applied as adsorbents in aqueous media.

(Received on August 9, 2001; Accepted on September 13, 2001)

It was in the late ′80s when the until then neglected importance

of carbon surface chemistry was first analyzed in depth, as neither the surface area nor the pore structure were sufficient to explain many of the properties of carbon-supported catalysts. The chemistry of the surface admittedly is and can be influenced by the presence of hetero atoms, either built into or situated at the edges of the turbostratic graphite layers. The hetero atoms like hydrogen, oxygen, nitrogen, halogens, sulfur, phosphorus, etc., form non-stoichiometric stable surface compounds. They derive either from the precursor or may be introduced by additional treatment(s). The polarities of these functional groups are strongly influenced by the neighboring chemical structures as

,

O

HO

OH C O

OH

COHO

O

O

H

HH

H

H

H

H

H

H

H

H

Fig. 1 Schematic representation of a graphene layer including the oxygen containing functional groups at the edges. Dot and dot+*

mean unpaired σ � e lectron and in-plain σ pair (where * is

a localized π electron), respectively

well. Therefore, the functional groups on the carbon surfaces differ from the same group of organic molecules; moreover there are no two groups behaving exactly alike on a carbon solid.1 Due to these phenomena the surface analysis of carbonaceous materials may need a special approach. The diversity of the O- and N-containing functional groups on the active carbon surfaces is illustrated in Figs 1 – 2.2, 3

N

N

N

N N

O

O OH

O

N

H

N-6

N-5

N-5

N-X

N-Q Fig. 2 Schematic representation of N-containing functional groups on carbon surfaces The presence of these groups results in an acid/base character of the carbon surface. As for the acidic groups, surface oxides such as carbonyl, carboxyl, phenolic hydroxyl, lactone and quinone groups are representative. The basic behavior is associated e. g. with chromenes, ethers and carbonyls, mentioning only the oxygen containing groups. Nevertheless, part of the basicity of

the carbon surface is explained in terms of the π sites of the

carbon basal plane. The Lewis basicity of π electrons is highly

i1742 ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT

influenced by the aromatic system and the localizing effects of oxygen containing groups. There are several sophisticated analytical techniques to

determine these groups, but carbon analysis is very often beyond the technical limits of these methods, due to a very intensive matrix effect. Therefore, generally several methods are concurrently applied to characterize the surface chemistry of a carbonaceous surface. XPS, TDP, FTIR, solid 13C-NMR or “classical” acid/base titrations are among the most frequently applied methods. The methods reported in this paper have precious virtues and serious limitations. Elementary analysis does not say anything about the chemical form of the elements in the carbon sample. High resolution XP analysis is capable to detect the different bonding states of both the carbon and the hetero atoms, but can

not distinguish C – C from C – H , for H can not be detected at all. An other disadvantage of this method is, that due to the interaction between the electrons emitted and the sample being analyzed, only electrons from the uppermost few nm of the probe have the chance to reach the detector. That is, XPS provides information only about the uppermost surface layer. “Equilibrium” acid-base titration is considered to reveal the acid/base feature of the carbon surface by titrating the base/acid consumption of the surface continuously. However, in this case we have to face other kind of limitations. Porous probes, especially when micro or ultra-micro pores are dominant

(nanoporous carbons), may exhibit a kinetic hindrance due to the diffusion limited transport in the narrow pores.21 This may result a hysteresis loop in the titration curve. The so called Boehm titration eliminates this inconvenience, as the contact time can be chosen arbitrarily, up to several days. This case the pKa

resolution is the limiting factor. Only groups of ∆pKa> 4.7 can

be distinguished. That is the reason, why the basic nature of the active carbons is not fully explored yet. Other drawback is that samples of relatively high specific surface area are needed to obtain reproducible titration results.

Experimental Materials The carbons used in this work were prepared from polyethyleneterephthalate (PET) and polyacrylonitrile (PAN)

precursors by a two-step physical activation process. They are labeled as APET and APAN. The preparation and the physical characterization, including low temperature nitrogen adsorption and small angle X-ray scattering data have been reported recently.4, 5 Both are highly microporous and their BET surface area is 1190 and 544 m2/g, respectively.

Methods The surface chemical composition of the samples was determined by XPS (X-ray Photoelectron Spectroscopy) using an XR3E2 (VG Microtech) twin anode X-ray source and a Clam2

(X-ray Photoelectron Spectroscopy) hemispherical electron energy analyzer. The base pressure of the analysis chamber was

5x10-9 mbar. The used Mg Kα radiation (1253.6 eV) was non-

monochromatized. Wide scan spectra in the 1000 - 0 eV binding

energy range were recorded with a pass energy of 50 eV for all samples. High resolution spectra of the C 1s, O 1s and N 1s signals were recorded in 0.05 eV steps with a pass energy of

20 eV. After the linear base line was subtracted, the curve-fitting was performed assuming a Gaussian peak shape. Boehm titration method was used to determine the number of the oxygenated surface groups6. The carbon samples were immersed into 0.05M HCl, NaHCO3, Na2CO3 and NaOH solutions according to the method developed by Boehm et al..6 The number of the basic sites was calculated from the amount of HCl that reacted with the carbon. The various free acidic groups were derived using the assumption that NaOH neutralizes carboxyl, lactone and phenolic groups, Na2CO3 neutralizes carboxyl and lactone and NaHCO3 neutralizes only carboxyl groups. A continuous “equilibrium” potentiometric titration in the pH 3 -

11 range was also applied to study the acid-base properties. Details of equilibrium acid-base titration for surface charge characterization of amphoteric solid particles are given elsewhere.7 - 9

Results and Discussion Two regions of the XP survey spectra are evaluated here. At the typical asymmetric peak of the C 1s core level spectrum the optimum fitting was achieved by resolving each C 1s spectrum into five peaks: graphitic carbon (Peak I, binding energy, BE=284.0 - 284.3 eV), carbon present in phenolic, alcohol, ether or C=N groups (Peak II, BE=285.3 - 285.7 eV), carbonyl or quinone groups (Peak III, BE=286.8 - 287.4 eV), carboxyl or

ester groups (Peak IV, BE= 288.5 - 289.2 eV), and shake-up

satellite peaks due to π - π* transitions in aromatic systems

(Peak V, BE=290.2 - 291.1 eV) (Fig. 3)1, 10-12. Data obtained from this spectrum range are listed in Table 1.

294 292 290 288 286 284 282 280

V IVIII

II

I

Inte

nsity (

arb

itra

ry u

nit)

Binding energy, eV

Fig. 3 Typical C 1s core spectrum. I: graphitic carbon, II: hydroxyl or ether, III: carbonyl, IV: carboxyl or ester, V: shake-

up satellite peaks due to π-π* transitions in aromatic rings

Table 1 Distribution of carbon structures* (atomic %)

I II III IV V

APET 60.0 18.1 8.1 5.3 4.2 APAN 50.4 22.3 7.9 5.5 3.4

*see the capture of Fig. 3.

ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT i1743

406 404 402 400 398 396

Binding energy, eV

IV

III

II I

Rela

tive in

tensi

ty

Fig. 4 The N 1s core spectrum of the APAN carbon. I: pyridine-like structures, II: pyrrolic and/or pyridon-N, III quaternary N, IV: N-oxide

The N 1s spectra were deconvoluted and fitted by considering pyridinic, pyrrolic and quaternary nitrogens and N-oxides (Fig. 4). Peak I (BE=398.0 - 398.1 eV) can be ascribed to N-6 or pyridine-like structures, Peak II (BE=400.1 - 400.7 eV) to N-5, i.e. pyrrolic and/or pyridon-N moieties, Peak III (BE=401.4 ± 0.5 eV) to N-Q or quaternary nitrogen and Peak IV (402 - 405 eV) to N-oxides, respectively (Fig. 4, Table 2).3, 10, 13 - 16 Table 2 Distribution of nitrogen structures* ( atomic %)

I II III IV

APAN 1.7 1.8 1.3 0.5

*see the capture of Fig. 4. The potentiometric titration was limited into the 3 – 12 pH range.

The specific net proton surface excess amount (∆nσ, mmol/g),

was derived directly from the initial and equilibrium

concentrations of the solute.7 The values of nσH+ and nσOH- were

calculated in each point of the titration from the electrode output using the actual activity coefficient derived from the slope of H+/OH- activity vs. concentration straight lines of the

background electrolyte titration. ∆nσ = nσH+ - nσ

OH- was plotted as

a function of the equilibrium pH. Positive values indicate acid consumption, i.e. proton binding on the surface, while the

negative ones mean base consumption, i.e. release of protons or binding of OH--ions.

4 6 8 10

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

APET

APAN

nσ

H+ -

nσ

OH-,

mm

ol/g

pH

Fig. 5 Proton binding isotherms of the polymer derived carbon samples

The concentration of the surface functional groups determined by Boehm titration is listed in Table 3.

Table 3 Results of the Boehm titration, µequiv/g

APAN APET

pKa < 6.37* 30.1 n.d.****

6.37 < pKa < 10.25** 12.3 17.7

10.25 < pKa < 15.74*** 70.5 76.0

Total acidic groups 112.9 93.7 Total basic basic groups 336.3 416.8

*carboxylic, **lactonic, ***phenolic OH group on the APET

surface; ****below the detection limit

The XPS data reflect the sample composition only over a depth of about a few nanometers. The PET derived carbons show a surface carbon content of about 94.3 %(w/w), what is slightly higher than the 93.1 %(w/w) derived from the bulk composition.5 The surface composition of APAN carbon from XPS is 87.1 %(w/w) C, 6.9 %(w/w) O and 6.0 %(w/w) N. The data obtained from bulk analysis are 86.1, 5.4 and 8.0, respectively.

The graphitic carbon is the dominating component of the surface in both samples. The concentration of the various forms of O related carbons decreases in both samples, as BE increases, i.e. C – O single bonds are the most prevalent forms on the basal plane edges. According to the XPS analysis, the APAN surface has a significantly higher hetero atom concentration, than the APET surface. Four nitrogen species were identified in APAN carbon. It should be premised, that within the accuracy of XPS measurements, pyridone-N and pyrrolic-N can not be distinguished from each other.15 Several further forms of nitrogen may exist in the

carbonized PAN sample as well, but, it is extremely difficult to separate these peaks due to their small fractions on the one hand and the resolution of the spectra on the other hand.13 The formation of pyridones by oxidation of carbonized samples has recently been reported. The combination of the ring expansion and this oxidation may result in the almost constant proportion of the N-5 functional form13 The results of the equilibrium acid - base titration performed under CO2-free condition, but not with special precaution against O2, show both acidic and basic, i.e. amphoteric character of these activated carbons. The basicity of the samples is demonstrated

by the definite amounts of bound protons with limiting values of 0.16 and 0.12 mmol/g at low pH. The significant hydroxide

consumption occurring above a pH > 11 were interpreted by the

formation of CO2 surface complexes being responsible for surface acidity.17 However, slow hydrolysis of surface esters and/or lactones contributes to the small pH changes above pH 9 during all continuous titrations.8 Our equilibrium titration procedure took sufficient time, therefore any slow alkaline hydrolysis process may occur. The distinct inflection points apparent in the net proton binding isotherms of the APAN and APET carbons at pH 6.1 and 7.1, respectively, indicate that protolytic processes take place in that narrow pH range which may be assigned to pyrilium - chromenol type equilibria (3 < pK

< 6).9 At pH > 8, where the quinone - hydroquinone equilibrium may exist, no obvious inflection point appears in our

experimental curves. Pyrone-type functionalities can not be excluded as well, for ab initio calculations reported by Suárez et al.14 have shown recently, that a broad spectrum of base strength can be predicted for pyrone-type structures. The carbons studied contain both Brönsted- and Lewis-type surface sites (see also Table 3). The Brönsted-type sites take part in proton acceptor - donor reactions, belonging to the oxygen and nitrogen functionalities having developed on the edges of carbon layers and identified by XPS analysis. As the graphitic

part of the surface is significant (Table 1), the π electrons of the

i1744 ANALYTICAL SCIENCES 2001, VOL.17 SUPPLEMENT

graphite planes are of great importance. They may act as Lewis basic sites accepting protons.9 However, the electron localizing effect of the hetero atomic functional groups (Tables 1 – 2) may

considerably decrease the basic strength of the π electrons. The proton transfer reactions may be accompanied by a simultaneous

redox transformation.9, 18 In the presence of physically adsorbed

oxygen a carbon surface with π electrons may act as a reversible

oxygen electrode, as due to their highly microporous structure they may contain entrapped oxygen.19, 20 The functional groups on the surfaces were selectively determined by titration according to Boehm (Table 3). Their classification in this context, however, is necessarily different from the XPS analysis. Therefore, XPS and Boehm titration results can not be directly compared with each other. The data derived from the latter are representative to all the surface sites available to the reactants. Both titrated surfaces are dominated by basic groups. Two different types of acidic functional characters were found on the APET surface, and three acidic

ranges were distinguished on the APAN surface. The acidic ranges titrated on this latter carbon can not be assigned, as Boehm’s assignation relates only to O-functionalities. The APAN surface is more densely populated by surface groups, than the APET, as it can be concluded from the combination of data in Table 3 and BET surface area values (Table 4). The same tendency was concluded from XPS analysis. The higher concetration of the basic functionalities may explain also that intersection of the APAN titration curve with the pH axis takes place at higher pH value, than that of the APET carbon. The acid/base property plays a very definitive role when porous carbons are applied as adsorbents in aqueous media.

Table 4 Number of the titrated species/100 nm2

APAN APET

pKa < 6.37* 3.33 n.d.****

6.37 < pKa < 10.25** 1.36 0.90

10.25 < pKa < 15.74*** 7.81 3.85

Total acidic groups 12.50 4.74 Total basic basic groups 37.23 21.10 Total functionalities 49.73 25.74

*carboxylic, **lactonic, ***phenolic OH group on the APET surface; ****below the detection limit

Acknowledgement This research was supported by the OTKA Fund (Hungary) No.

T 025581. The experimental work of Ms. Emese Fülöp and Mr. György Bosznai is gratefully acknowledged. KL thanks Ms. Edina Csibi for the fruitful discussions.

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