Characterization of the surface chemistry of carbon materials by potentiometric titrations and temperature-programmed desorption

CARBON

4 6 ( 2 0 0 8 ) 1 5 4 4 –1 5 5 5

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Characterization of the surface chemistry of carbon materials by potentiometric titrations and temperature-programmed desorption Hono´ria F. Gorgulhoa,b,*, Joa˜o P. Mesquitaa, Filomena Gonc¸alvesb, Manuel Fernando R. Pereirab, Jose´ L. Figueiredob a

Universidade Federal de Sa˜o Joa˜o del Rei, Departamento de Cieˆncias Naturais, Prac¸a Dom Helve´cio 74, CEP 36300-000, Sa˜o Joa˜o del Rei, MG, Brazil b Laborato´rio de Cata´lise e Materiais (LCM), Laborato´rio Associado LSRE/LCM, Departamento de Engenharia Quı´mica, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal

A R T I C L E I N F O

A B S T R A C T

Article history:

Two methods currently used to characterize oxygen-containing functional groups on the

Received 23 January 2008

surface of carbon materials are compared, namely potentiometric titration (PT) and tem-

Accepted 20 June 2008

perature-programmed desorption (TPD). Two materials were used, activated carbon and

Available online 27 June 2008

multi-walled carbon nanotubes, which were subsequently modified by oxidative treatments in order to produce samples with different amounts of surface groups. The concentrations of carboxylic acid groups determined by both techniques are in good agreement, but the quantitative result obtained by TPD is closer to the analytical concentration of the species obtained by elemental analysis. Discrepancies between the quantitative results are more pronounced at higher pKa values (weak acids), where the concentrations determined by PT are lower than those obtained by TPD. This directly reflects the effects of neglecting the electrostatic interaction parameter. The TPD method was particularly suited for the characterization of samples modified with ethylenediamine, which is anchored to specific oxygenated groups. PT results are useful to describe the material behaviour in aqueous solutions, where the activity of the surface groups depends not only on their concentrations, but also on their environment. Ó 2008 Elsevier Ltd. All rights reserved.

1.

Introduction

The unusual properties of carbon materials are of scientific and technological interest due to their extended range of potential applications beyond their traditional uses as catalyst supports and adsorbents. Most applications of carbon materials harness their ability to interact with specific atoms, ions, and molecules through chemical surface groups. The acidic and basic surface properties of activated carbons (AC) make

this material an effective adsorbent for the removal of pollutants from both the gas and liquid phase, as well as a promising catalyst or catalyst support system for a variety of reactions [1,2]. These applications are heavily influenced by the heteroatoms on the carbon surface, particularly oxygen. The concentration and nature of the surface functional groups on AC can be modified by suitable thermal or chemical treatments. In the case of carbon nanotubes, the functionalization of the carbon surface through the introduction of

* Corresponding author: Address: Universidade Federal de Sa˜o Joa˜o del Rei, Departamento de Cieˆncias Naturais, Prac¸a Dom Helve´cio 74, CEP 36300-000, Sa˜o Joa˜o del Rei, MG, Brazil. Fax: +55 32 33792483. E-mail address: [email protected] (H.F. Gorgulho). 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.06.045

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carboxyl groups is important as it facilitates the anchoring of different species, including proteins and other compounds to form key building blocks in nanotechnology [3,4]. Thus, the accurate analytical description of the surface functional groups present on a given carbon material is indispensable for developing further applications of these materials. Studies have shown that the surface of carbon materials contains functional groups formed by the chemical bonding of carbon atoms with heteroatoms (oxygen, nitrogen, and sulfur) [5,6]. The structure of graphitic carbon can be described in terms of the graphene layers, which are characterized by the basal plane and the edges, where the functional groups prevail. Among them, the most important contain oxygen, and include carboxyl, carbonyl, phenol, and lactone functional groups. The acidic or basic character of the carbon materials depends on the surface concentration of these groups [5]. A variety of experimental techniques have been used to characterize these functional groups, each presenting its own advantages and disadvantages. Potentiometric titrations (PT) can be used in combination with a nonlinear treatment of the experimental data for studying the acid–base properties of carbon materials [7,8]. Among the other analytical methods available, temperature-programmed desorption (TPD) is becoming increasingly utilized. This method is based on the thermal decomposition of the surface groups, and is particularly useful when coupled with mass spectrometric analysis, as this can provide information relevant to the mechanism of thermal decomposition of the surface groups [9]. Both these methods have been widely applied to study the surface chemistry of AC and in the last decade also of carbon nanotubes. Nevertheless, a quantitative comparison of the amounts of surface groups determined by these techniques has not yet been done. The comparison of these two techniques, which are carried out under completely different atmospheres and conditions, is also important because they are generally used to characterize carbon materials that may be applied in conditions different from those of the technique used. In fact, TPD (carried out in gas-phase) is often used to characterize carbon samples to be used as catalysts or adsorbents in liquid phase, and potentiometric titrations (carried out in the liquid phase) are often used to characterize carbon samples to be used in gas-phase applications [10–12]. The present study compares the distributions of functional groups of carbon materials obtained by PT and by TPD. This topic has been previously explored by Salame and Bandosz who describe the effects of varying treatments on the surface of AC [13]. Herein, we aim to improve and expand the results of these techniques to two different types of carbon materials and surface treatments. Samples of AC and multi-walled carbon nanotubes (MWCNT) were employed in this study to examine the effect of oxidation in the gas-phase and by nitric acid. MWCNT have been mainly characterized by dry methods to investigate their structural and morphologic properties [14]. However, wet methods, such as PT, can give information about the chemical surface interaction with the aqueous solutions. This is specially important for electrochemical applications, where the electrochemical double layer is dependent on surface hydrophilicity, which is usually improved by introducing oxygen groups [15].

1545

PT in an aqueous medium and numerical analysis of data by nonlinear regression techniques were used in order to distinguish acidic groups with different acid ionization constants (Ka) on the carbon surface. The approach used is based on the work of Masini et al., which was developed for humic acids and polyelectrolytes [16–18]. Functional groups on the surface of AC are often involved in acid–base equilibria, and can be modified at lower pH providing a protonated carbon surface by the addition of a strong acid such as HCl. As a result, basic groups become the acid conjugate species, which can be neutralized with a strong base. The adopted model is based on this neutralization reaction, where the protonated groups on the surface can be described by a discrete distribution of acidity constants [19]. The effects of charge accumulation on Ka were neglected in this model. The versatility of the TPD technique was also explored in this work to investigate the anchoring of ethylenediamine on AC. The introduction of an amino group on the AC surface is useful for a wide range of applications [20,21]. Amine groups immobilized on AC through alkyl chains are useful as functional groups, since they have a high reactivity. In the case of ethylenediamine, it is an electron donating ligand, which may reduce the valence of a metal that has been coordinated. This property can be used not only for anchoring metals in adsorption processes, but also to improve catalyst activities [22].

2.

Experimental

2.1.

Materials

2.1.1.

AC samples

The AC (Darco G-60, Fluka) was washed with distilled water and dried at 373 K for 24 h. Next, the sample was cooled in a desiccator containing silica gel, and stored in a bottle labeled raw activated carbon (RAC). RAC was used to produce the oxidized activated carbon by refluxing with HNO3 (VETEC) solution (50% m/v) for 2 h, washing with distilled water, and refluxing with 1.0 mol L1 of NH4OH (SINTH) solution for 1 h. The sample was rinsed in a Soxhlet extractor at 373 K with distilled water to remove the residual base and byproducts. The resulting material was dried at 373 K for 24 h (sample HAC). HAC was used as the starting material to produce the sample modified by ethylenediamine. HAC (30.0 g) was added to a solution of ethylenediamine (300 mL, 50% w/v). The mixture was refluxed for 24 h. The solids were collected by filtration and rinsed with distilled water. The modified carbon was then dried at 373 K for 24 h (sample NAC).

2.1.2.

MWCNT samples

MWCNT, prepared by a catalytic chemical vapour deposition process (CCVD), were obtained from commercial sources (specified carbon purity >95%). They have an average diameter of 9.5 nm and an average length of 1.5 lm. Two additional samples were prepared by oxidation: (a) with HNO3 (7.0 mol L1) at 403 K for 3 h, subsequently washed with distilled water to neutral pH, and dried in an oven at 383 K for 48 h (sample MWCNT-H) and (b) with 5% O2/N2 at 773 K for 3 h, up to a burn-off of 11% (sample MWCNT-O).

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CARBON

2.2.

Characterization

2.2.1.

Elemental analysis

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Determination of C, N, H and O content on the carbon materials was carried out using a CARLO ERBA EA 1108 Elemental Analyser. The samples were previously dried at 400 K for 5 h under vacuum to eliminate any physisorbed water.

2.2.2.

Thermogravimetry



Thermal analysis was carried out using a Mettler TA 4000 thermal analyzer, and proximate analysis was based on the method described by Ottaway [23].

2.2.3.

2.2.5.1. Potentiometric titration modeling. The nonlinear regression program used in this work has been described elsewhere [19]. A general equation that describes the titration of the mixture of a strong acid with N weak acids (surface groups) is used to fit the potentiometric titration data   Kw f ðVi ; ½Hþ i Þ ¼ ðVi  VHA0 ÞCb þ ½Hþ i  þ ðV0 þ Vi Þ ½H i

Textural properties

The textural characterization of the materials was based on the N2 adsorption isotherms, determined at 77 K with a Coulter Omnisorp 100 CX apparatus. The micropore volumes and mesopore surface areas were determined by the t-method, using the standard isotherm for carbon materials.

j¼1

KHAn KHAn þ ½Hþ i

ð1Þ

where [H+]i is the hydrogen ion concentration, Vi is the titrant volume added, and V0 is the initial volume in the potentiometric cell before starting titration. V HA0 is the equivalent volume for the excess strong acid. V HAn and K HAn are the equivalent volume and the ionization constant, respectively, for the N acids present on the surface, and Cb is the concentration of base (mol L1). The parameters V HAn and K HAn are adjusted through the minimization of the function S S¼

2.2.4.

N X ðVHAn  VHAn1 ÞCb

m X

TPD

½pHi ðExpÞ  pHi ðcalcÞ2

ð2Þ

i¼1

TPD profiles were obtained with an AMI-200 (Altamira Instruments) apparatus. Samples were heated under an atmosphere of He (69 lg s1) at 5 K min1 to 1373 K, and CO and CO2 evolution was monitored by mass spectrometry (Dymaxion 200 amu, Ametek). The TPD spectra were deconvoluted using the nonlinear least-squares procedure based on the Levenberg–Marquardt (LM) algorithm assuming a multi Gaussian peak shape, described by Figueiredo et al. [9,24].

where m is the total number of data measurements (V, pH). Calculation begins with the input of the initial values of V HAn and K HAn (guesses). With Quasi-Newtonian methods, the best approximations for the values of pHi (calc) are obtained with the recurrence equation [25] pHi;kþ1 ¼ pHi;k 

f ðpHi;k ÞðpHi;k  pHi;k1 Þ f ðpHi;k Þ  f ðpHi;k1 Þ

ð3Þ

where the values of the functions f(pHi,k1), f(pHi,k), pHi,k1 and pHi,k are chosen according to the following conditions:

if f ðpHi;kþ1 Þ  f ðpHi;k Þ < 0; then ½pHi;k1 ; f ðpHi;k1 Þ is exchanged for ½pHi;k ; f ðpHi;k Þ if f ðpHi;kþ1 Þ  f ðpHi;k Þ > 0; then

"

½pHi;k1 ; f ðpHi;k1 Þ is exchanged for pHi;k1 ; f ðpHi;k1 Þ 

2.2.5.

Potentiometric titration

PT was performed in triplicate for each sample with three different amounts: 25.00, 50.00 and 100.00 mg. The mass of carbon was added to the electrochemical cell with 20 mL HCl (0.0895 mol L1) under N2 atmosphere, and the ionic force adjusted to 0.10 mol L1 (value previously-optimized) with KCl (0.50 mol L1) to a total volume of 25 mL. The pH was monitored until equilibrium was reached, and then titration was started with NaOH (MERCK, 0.108 mol L1). The glass electrode calibration was performed in terms of hydrogen ion concentration [16,17], and repeated for each titration procedure. The experiment was performed with an automatic Titroline ALPHA microburette system (SCHOTT SA) interfaced with a microcomputer. Titrations were conducted with 0.0100 mL injections at intervals of 100 s. All solutions were made with degassed water, prepared by boiling and cooling milliQ water (resistivity > 18 MX cm) under a stream of N2.

# f ðpHi;k Þ f ðpHi;k Þ þ f ðpHi;kþ1 Þ

These iterations are carried out until the condition [(pHi,k+1  pHi,k)/pHi,k] < 0.00001 is reached. The parameters V HAn and K HAn are adjusted using an iterative approximation subroutine based on a Quasi-Newtonian method and the LM algorithm [26]. The calculation sequence is repeated until a minimum S value is obtained. The program was developed in MATLAB 6.5.

3.

Results and discussion

3.1.

Textural properties

Table 1 summarizes the results of textural analysis obtained from N2 adsorption isotherms. The result showed that the specific surface areas and microporosity of AC materials decrease due to destruction of pore walls by the oxidation treatment [27]. For NAC sample, the decrease on surface areas and

CARBON

Table 1 – Textural properties of all materials

RAC HAC NAC MWCNT MWCNT-H MWCNT-O a b c d

2

(m /g)

758 725 726 196 209 259

Smb

2

(m /g)

306.3 274.6 210.5 – – –

Vtc

3

(cm /g)

Vld

0.65 0.58 0.45 – – –

3

(cm /g) 0.28 0.23 0.20 – – –

BET surface area. Mesopore surface area by t-method. Total pore volume by BJH. Micropore volume by t-method.

Sample RAC HAC NAC MWCNT MWCNT-H MWCNT-O

C (%)

H (%)

94.0 ± 0.2 82.0 ± 0.3 78.0 ± 0.2 98.1 ± 0.6 96.5 ± 0.2 97.5 ± 0.3

0.42 ± 0.04 1.25 ± 0.08 1.0 ± 0.1 0.11 ± 0.01 0.10 ± 0.01 0.07 ± 0.01

Oa (%)

N (%)

0.43 ± 0.03 3.65 ± 0.03 2.40 ± 0.05 14.3 ± 0.2 5.1 ± 0.1 14.1 ± 0.2 0.03 ± 0.02 1.2 ± 0.1 0.15 ± 0.01 3.1 ± 0.1 0.07 ± 0.03 1.4 ± 0.1

Ob (%) 2.32 10.3 10.0 1.00 2.60 1.40

a Obtained from dedicated elemental analysis. b Obtained from CO and CO2 TPD spectra.

microporosity can be also attributed to blocking of the narrow pores by the surface complexes that were introduced by the ethylenediamine functionalization [28]. The surface areas of the MWCNT samples do not change significantly after treatment with nitric acid, but increase by about 30% after gaseous oxidation. This was expected, since this kind of treatment is known to lead to the opening of the nanotubes [29].

3.2.

Table 3 – C, N, H and O elemental analysis: dry and ashfree basis

Proximate and elemental analysis

Tables 2 and 3 indicate the results of proximate and elemental analyses, respectively. The chemical modification of AC materials leads to an increase in volatile matter content and a decrease in ash content. Since these volatiles include products released from the decomposition of the surface groups, the observed results are in agreement with the increase in oxygen and nitrogen observed by elemental analysis. The nitrogen content increases in samples oxidized with nitric acid (Table 3), which is generally attributed to the ability of nitric acid to nitrate aromatic rings on the carbon surface [10]. However, as the AC was treated with NH4OH after oxidation to leave surface free from soluble acids, the introduction of ammonium carboxylate groups on HAC surface may be also considered. For NAC, the twofold increase in nitrogen content confirms the anchoring of ethylenediamine. The increase in oxygen content, primarily observed in HAC, results directly from an increase in carboxylic acid, lactone and phenol functional groups. Proximate analysis of MWCNT samples also shows an increase in the volatile matter content after nitric acid and gas-phase treatment, which is accompanied by a slight decrease in fixed carbon and ash. Elemental analysis shows that nitrogen was incorporated into the nanotube structures after

nitric acid oxidation, similar to the observations made for AC samples. Elemental analysis indicates an increase in nitrogen and oxygen content of these samples, and the chemical nature of these modifications was subsequently investigated by PT and TPD to obtain more detailed information about the type and distribution of surface oxygen complexes.

3.3.

Potentiometric titration

The experimental and fitted data obtained from PT on the AC samples are shown in Fig. 1. It is significant to note that the titration curves (experimental and calculated) are in good agreement. However, a clear difference is seen in individual curve profiles. The observed HAC and NAC titration curves show an enlargement in the buffer zone relative to RAC, due to an increase in the number of carboxylic acid sites. In order to quantitatively compare the pH curves, a nonlinear program was used to calculate the pKa distribution and the acid site concentrations. The number of sites was chosen based only on the statistical test described by Eq. (2). As the AC surface groups are expected to have acid strengths similar to the corresponding organic acids with similar structures,

12

10

RAC HAC NAC HCl

8

pH

Sample

SBETa

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6

4

Table 2 – Proximate analysis: dry basis results Sample RAC HAC NAC MWCNT MWCNT-H MWCNT-O

Volatiles (wt%) 5.50 18.7 20.0 4.4 5.3 5.6

Cfixed (wt%) 87.51 75.0 74.4 93.2 93.1 92.6

Ash (wt%) 7.0 6.3 5.6 2.4 1.6 1.8

2

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

volume of NaOH (ml)

Fig. 1 – PT curves (symbol) obtained employing masses of 50 mg for each sample. The solid lines show the corresponding results of theoretical calculations.

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Table 4 – Results of potentiometric titration for AC samples Number of sites (mmol g1) pKa < 3 RAC HAC NAC

– 0.60 ± 0.08 –

pKa 4–5

pKa 6–7

pKa 8–9

pKa 10–11

pKa > 11

Total

– 0.30 ± 0.05 0.22 ± 0.01

0.06 ± 0.01 0.20 ± 0.05 0.34 ± 0.04

– 0.4 ± 0.1 0.65 ± 0.04

– 0.95 ± 0.23 0.40 ± 0.06

0.40 ± 0.02 – –

0.46 ± 0.03 2.45 ± 0.51 1.61 ± 0.15

the acid strengths of various organic acids can be referred for evaluation of the acid–base behaviour of the surface functional groups on AC [30]. Carboxylic acids generally have a pKa between 2.0 and 5.0 [7]. Lactone and lactol groups are formed by cyclic esters or ketones, and tend to hydrolyze in the presence of acids and bases [31]. The acid dissociation constant depends on the size of the ring. For example, phenolphthalein has a pKa  8.5, and butyrolactone has a pKa  5.85 [32,33]. Phenols such as phenol, resorcinol, and aminophenol have pKa’s above 9.0. Hence, data fitting was started by assuming the presence of all three functional groups (carboxylic acid, lactone, and phenol). After the preliminary results, more sites were added to the model until no statistically significant improvement was observed. The results obtained by fitting the experimental data are presented in Table 4. Table 4 shows an increase in the distribution and concentration of the acid sites for the modified samples, HAC and NAC. This behavior results from the oxidation of RAC, which not only increases the amount of carboxylic acid sites on the carbon surface, but also alters their pKa range distribution. Indeed, as a result of the carbon surface heterogeneities, the introduced carboxylic groups exhibit a range of acid dissociation constants (Ka) that depend on the neighbouring groups and the size of the graphene layer. Thus, for the HAC sample, it is possible to observe two types of carboxylic acids, strong and weak, at pKa below 5. In addition to the carboxylic acids, lactones and carboxylic anhydride groups may be present, and could be ascribed to the pKa range between 6 and 7, observed for all AC samples. For pKa ranges above 7, the distribution of acid sites on the HAC sample was divided into two groups: pKa range 8–9, and pKa range 10–11. These species can be attributed to very weak acids such as phenolic and carbonyl groups. It should be noted that the phenolic groups may have lower pKa’s due to electrophilic substitution of NO2 on the aromatic ring. Similar behavior is observed for the sample modified by ethylenediamine, NAC, which was prepared from HAC. In this case, the carboxylic acid groups that showed a pKa below 3 on the corresponding HAC (0.60 ± 0.08 mmol g1), vanish after

modification. The other acid group affected was that with pKa between 10 and 11, attributed to phenol groups, which decrease 57% in relation to HAC. In general, the total number of acid sites on NAC decreases by approximately 30% in comparison to HAC. These results suggest that anchored ethylenediamine increases the basic character of the carbon surface, which is supported by an increase in the number of species with pKa between 8 and 9, characteristic of the protonated ethylenediamine. The RAC sample was characterized by a low concentration of acidic groups on the surface. The high concentration of sites with pKa above 11 is not necessarily related only to the presence of phenol and carbonyl surface groups, but also to other basic structures protonated by the initial condition of PT. The previously-discussed procedure was adopted for the MWCNT samples, and resulted in better-defined pKa values than those observed with AC, as indicated in Table 5. This behavior probably results from the fact that nanotube surfaces are significantly more homogeneous than those of AC. Similarly to AC, MWCNT exhibits increased acidity upon HNO3 oxidation, with the presence of two main acid sites with pKa below 5.2 (Table 5). These groups can be attributed to carboxylic acids present on different sites, at the tube ends and on the side walls [4]. The opposite trend was observed with the samples oxidized by O2, which displayed lower acid site concentrations and higher concentrations of sites with pKa above 8.0. These can be attributed primarily to phenol and carbonyl groups. These groups behave like very weak acids, a property that is intensified by electrostatic interactions resulting from negative charge accumulation at high pH values. As a consequence, the majority of these groups could not be completely characterized by titrimetry in aqueous solution, and a low number of sites are observed for this pKa range. The differing behaviors observed between MWCNT-H and MWCNT-O samples are in good agreement with earlier studies that show that gas-phase oxidation of AC mainly increases the concentration of hydroxyl and carbonyl surface groups, while oxidation in the liquid phase increases the concentration of carboxylic acid surface groups [9].

Table 5 – Result of potentiometric titration for MWCNT samples Number of sites (mmol g1) pKa

3.2 ± 0.1

5.2 ± 0.2

6.5 ± 0.3

8.0 ± 0.4

10 ± 0.2

Total

MWCNT MWCNT-H MWCNT-O

– 0.09 ± 0.02 –

– 0.10 ± 0.01 0.03 ± 0.01

0.040 ± 0.007 0.07 ± 0.01 –

– – 0.04 ± 0.02

0.30 ± 0.08 0.30 ± 0.04 0.40 ± 0.01

0.34 ± 0.09 0.56 ± 0.08 0.47 ± 0.04

CARBON

TPD results

a

0.20

0.15

-1

-1

Concentration (μmol g s )

CO CO2

0.10

0.05

0.00 400

600

800

1000

1200

T (K)

b 0.7

0.6

CO2 CO

-1 -1

Although H2O, CO2, and CO evolution are mainly observed in TPD experiments, other molecules resulting from the decomposition of various oxygen- and nitrogen-containing groups present on the carbon surface (NO and NH3) were explored in this work. There is some controversy in the literature with respect to the assignment of TPD peaks to specific surface groups, because the peak temperatures are known to be affected by the porous texture of the material, the heating rate, and the geometry of the experimental system used [24,34]. However, some general trends have been established for the oxygen-containing groups. The CO2 peak results from carboxylic acids at low temperatures (T < 700 K), and from lactones at higher temperatures (700–900 K). Carboxylic anhydrides produce both CO and CO2 peaks, while phenols and carbonyls produce a CO peak at temperatures above 900 K. The TPD spectra obtained from carbon materials display overlapping data from different contributions, and should thus be deconvoluted to determine each individual contribution from specific oxygen surface groups. The procedure adopted in this work was previously described by Figueiredo et al. [9,24]. It was assumed that the CO2 desorption curve contains at least four peaks – two from carboxylic acids and two from carboxylic anhydrides and lactones. If a peak does not exist, the parameter obtained from the fitting procedure would tend to zero. The carboxylic anhydride temperature peak was assigned after inspection of the range where CO and CO2 desorption curves overlap. Thus, the amounts of CO and CO2 from decomposition of anhydrides are equal. The CO and CO2 TPD spectra obtained for RAC, shown in Fig. 2a, indicate that CO evolution is more significant than CO2 evolution. Due to the low concentration of surface acid groups for this sample, the corresponding TPD profile shows significant noise for the CO2 data. Thus, the correlation coefficient (R2 = 0.88) observed for this fitting (Table 6) is relatively low as a consequence of the CO2 TPD data deviation relative to the values achieved for the entire curve. The deconvolution of the CO2 spectra shows two contributions attributed to carboxylic acid groups (peak at 503 K) and carboxylic anhydride groups (peak at 795 K) at temperatures where CO evolution is simultaneously observed. The amount of CO2 released from carboxylic acid decomposition detected at 503 K was very small (0.10 mmol g1). The comparison with PT shows that this species was not observed for pKa’s below 5, but was observed for pKa values between 6 and 7 (0.06 mmol g1). In addition, the carboxylic anhydrides thermal decomposition was observed at 795 K, contributing additionally with

Concentration (μmol g s )

3.4.

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0.5

0.4

0.3

0.2

0.1

0.0 400

600

800

1000

1200

T (K) Fig. 2 – CO and CO2 TPD spectra and deconvolution for (a) RAC and (b) HAC.

0.15 mmol g1 of CO2. This represents 0.30 mmol g1 of carboxylic acid, taking into account that carboxylic anhydride groups yield two carboxyl groups in aqueous solutions. Other species observed for the CO TPD spectra of RAC were

Table 6 – TPD deconvolution of CO2 spectra for AC samples Sample

T w A T w A T w A T w A (K) (K) (mmol g1) (K) (K) (mmol g1) (K) (K) (mmol g1) (K) (K) (mmol g1)

T w A (K) (K) (mmol g1)

RAC HAC NAC

– – – – 496 53

– – 946 147 880 195

– – 0.30

503 139 530 120 – –

0.10 0.58 –

– – 654 126 590 160

– 0.23 0.83

795 300 770 150 730 140

0.15 0.40 0.22

T = temperature corresponding to peak maximum; w = width at half height; and A = integrated peak area.

– 0.27 0.48

Total (mmol g1)

R2

0.25 1.48 1.85

0.88 0.99 0.99

1550

CARBON

4 6 ( 2 0 0 8 ) 1 5 4 4 –1 5 5 5

2.5 TPD CO (945-1070K)

Concentration (mmol / g-1)

2

TPD CO (932-1070) 1.5

TPD CO 2 (795 - 946K) TPD CO 2 pKa 10-11 (503-654K)

1

0.5

TPD-CO TPD CO2 (T>795) (795K) pKa > 11 TPD CO 2

pKa 6-9

TPD CO 2 (730-880K)

TPD CO 2 (590K)

pKa < 5 pKa 6-9

pKa 10-11

(503K) pKa 6-7

pKa < 5

0 RAC

HAC

NAC

Fig. 3 – Comparison between PT (black) and TPD (white) results obtained for RAC, HAC and NAC samples.

attributed to phenols (1010 K), and carbonyls (1163 K) with a total of 0.57 mmol g1. Fig. 3 compares the total concentration of surface groups obtained from PT and TPD methods. Both methods show that RAC has a basic character, which is due to its low content of oxygen surface groups. After oxidation, a significant increase in the quantity of evolved CO2 and CO was observed, as shown for HAC in Fig. 2b. Four contributions for CO2 TPD spectra were found after deconvolution of the data, and are detailed in Table 6. The first two species were found at temperatures of 530 and 654 K, which were attributed to two different types of carboxylic acid group decomposition, as expected for a heterogeneous surface. Previous work has shown similar results, and the two types of groups were assigned as strongly acidic (lower temperature) and less acidic (higher temperature) carboxylic acids [24]. Comparison with PT results shows a very similar acid concentration as showed by Fig. 3. Thus, for the oxidized sample (HAC), the total number of carboxylic acid sites at pKa’s below 5.0 as determined by PT (Table 4) was approximately 0.90 mmol g1. The total carboxylic acids sites, as determined by TPD analysis for that same sample, were 0.81 mmol g1. The other two TPD peaks were attributed to carboxylic anhydrides (770 K, 0.40 mmol g1) and lactones (946 K, 0.27 mmol g1), respectively, with a total carboxylic concentration of 1.07 mmol g1. This value represents 1.8-fold of the total amount of acid sites determined by PT with pKa between 6.0 and 9.0, which may include hydrolyzed species such as lactones and anhydrides. This means that the com-

plete hydrolysis of these groups did not happen in the PT experimental conditions used in this work. The primary CO evolution is observed at temperatures from 700 to 1273 K. Deconvolution of these spectra provide three important contributions from carboxylic anhydrides (770 K), phenols (945 K), and carbonyls (1070 K), with peaks and areas detailed in Table 7. It was observed that oxidation also increased the concentration of these surface groups, which contributed to an increase in the level of CO evolved. In Fig. 3, it can be observed that the total concentration of phenol and carbonyl species determined by deconvolution was 1.92 mmol g1, which is higher than the titratable sites obtained by PT at pKa values above 9 (0.95 mmol g1). Indeed, the PT result for higher pKa functional groups diverges more significantly from the values obtained by TPD analysis, which can result not only from the electrostatic interaction parameter effects, but also from the microporosity of the ACs samples. The response of PT methods is largely dependent on the access of the reactant to the active sites, which may be limited when the functional group is inside the narrow microporosity. In the case of the weak acid sites, pKa > 10, some oxygen groups may not react with the initial acid solutions in the experimental conditions used, yielding a not detectable acid site in titration. In the case of NAC, the spectrum cannot be solely attributed to CO and CO2 evolution. Other gas-phase compounds which originate from fragmentation of ethylenediamine may contribute to the mass spectrum. Thus, the temperature range where the evolutions are observed is the

Table 7 – TPD deconvolution of CO spectra for AC samples Sample

T w A T w A T w A (K) (K) (mmol g1) (K) (K) (mmol g1) (K) (K) (mmol g1)

RAC HAC NAC

– – – – 496 30

– – 0.06

– – – – 587 140

– – 0.20

795 300 770 150 730 140

0.15 0.40 0.22

T (K)

w A (K) (mmol g1)

1010 193 945 143 932 201

0.27 1.21 0.92

T (K)

w A (K) (mmol g1)

1163 140 1070 160 1070 240

T = temperature corresponding to peak maximum; w = width at half height; and A = integrated peak area.

0.30 0.71 0.62

Total (mmol g1)

R2

0.72 2.62 2.08

0.98 0.99 0.99

CARBON

1551

4 6 ( 20 0 8 ) 1 5 4 4–15 5 5

-9

7.0x10

m/z 16 m/z 17 m/z 18 m/z 28 m/z 30 m/z 44

-9

6.0x10

-9

MAS signal (Torr s-1)

5.0x10

-9

4.0x10

-9

3.0x10

-9

2.0x10

-9

1.0x10

0.0 400

600

800

1000

1200

T (K) Fig. 4 – TPD spectra for NAC sample.

O O

+

H2N NH2

OH

O

Δ

+

-

H3N

NH2

-

OH

+

O H N+ 3

H2N NH2

Δ

NH2

Fig. 5 – Examples of acid–base interactions between the ethylenediamine and oxide groups on AC.

0.7

0.6

CO2 Concentration (μmol g-1 s-1)

more important parameter to differentiate these species. Considering some hypotheses about ethylenediamine thermal decomposition, signals m/z, 16–18, 26–30, 42–45 and 60, were monitored by mass spectrometry during the TPD experiment. The TPD spectra for the major masses originating from the fragmentation of ethylenediamine groups, including CO (28) and CO2 (44), are shown in Fig. 4. The complete profiles are complex and composed of overlapping peaks (at low and high temperatures) and broad peaks. However, the peak centered at 496 K (shown by an arrow in Fig. 4) is observed for all masses, which is evidence of the simultaneous evolution of gases coming from thermal decomposition of ethylenediamine. Thus, at this temperature, the m/z = 28 signal is likely not due only to CO evolution, but also to some fragmentation of ethylenediamine. The same considerations can be made for the CO2 TPD spectra. TPD profiles of water (m/z = 18 and m/z = 17) show two very sharp peaks. The first is found at 393 K, and can be attributed to the desorption of surface H2O. The second overlaps with other masses and can be associated with the amine functional groups. Free ammonia is observed by the m/z = 16 and m/z = 17 signals, and both are present around 496 K, indicating that it originates from amine decomposition. The peak at 496 K for m/z = 30 is related to ethylenediamine, and is a clear indication that this temperature corresponds to the release of this molecule from a specific site. It is expected that the ethylenediamine molecules adsorbed on AC via dispersive forces are thermally unstable and decompose to alkylamine and free ammonia at temperatures below 390 K [21]. Some ethylenediamine complexes decompose between 508 and 633 K [21,35], therefore, the TPD peaks observed in Fig. 4 at temperatures below 400 K, can be attributed to ethylenediamine weakly adsorbed in the AC through dipole–dipole interactions. It is also expected that the decomposition of ethylenediamine at 496 K may

CO

0.5

0.4

0.3

0.2

0.1

0.0 400

600

800

1000

1200

T (K) Fig. 6 – CO and CO2 TPD spectra and deconvolution for NAC.

1552

CARBON

4 6 ( 2 0 0 8 ) 1 5 4 4 –1 5 5 5

attributed to carboxylic anhydrides based on the same considerations as for the CO2 spectra. In the same manner, the other two peaks were assigned to phenols (932 K) and carbonyls (1070 K), with a total of 1.54 mmol g1, which is lower than the 1.92 mmol g1 observed for HAC. This result is in agreement with a decrease in the concentration of phenol groups (with pKa between 10 and 11) observed by PT, indicating that these groups correspond to ethylenediamine anchoring, which have changed their thermal stability. In the case of CO2 and CO TPD spectra for the nanotube samples, shown in Fig. 7, the evolution profile is similar to that obtained for the other AC samples. However, these TPD spectra present lower resolution than those obtained for AC samples, likely due to the very low concentrations of these species. Indeed, the deconvolution of the CO and CO2 TPD spectra, presented in Tables 8 and 9, yield considerably lower concentrations of surface groups than those observed for AC. An examination of the TPD profiles of the original MWCNT, shown in Fig 7a, reveals a very weakly-functionalized surface, with a total of 0.14 mmol g1 CO2 and 0.29 mmol g1 of CO. The comparison between PT and TDP results for MWCNT samples are summarized in Fig. 8. The deficiency of functional groups observed by TPD is in agreement with the large basicity observed by PT, as evidenced by the low concentration of acid sites with pKa < 6 and predominance of acid sites with pKa = 10 (basic sites). As discussed before, PT depends on the reactant accessibility and surface group dissociation. Because of their hydrophobicity, carbon nanotubes can only react with difficulty with the acid solution, yielding low acid concentration at the end of titration. However, similar to the AC samples after oxidation with HNO3, the MWCNT-H sample shows an increase in the concentration of the carboxylic acid groups (0.33 mmol g1) observed by the peaks at 539 and 692 K on the TPD spectra. In the same way, the PT results showed an increase in acid sites, at pKa’s below 6.5, reflecting also the decrease in surface hydrophobicity after acid oxidation (0.26 for pKa < 6.5). Low concentrations of carboxylic anhydrides (856 K) and lactones

result from interactions through hydrogen bonding with oxygen, or through Lewis acid–base interactions. Indeed, since the preparation of NAC was carried out without pH control, the ethylenediamine (pKa 8.5) is present in its protonated form, while the strong acids on the surface are dissociated. In this case, ethylenediamine can be bound to the carboxylic groups by a simple neutralization reaction, Fig. 5. In order to decompose the contributions to CO2 and CO, the TPD spectra of NAC were deconvoluted, and the results are shown in Fig. 6 and Tables 6 and 7. For the signal m/ z = 44, the deconvolution exhibits four contributions. Based on the previous considerations, the first of these (at 496 K) was not attributed to CO2 evolution. However, the second (at 590 K) was assigned to CO2 from carboxylic acids, both free and linked to ethylenediamine, with a total of 0.83 mmol g1. This is nearly the same order as the total CO2 obtained with HAC at temperatures below 700 K (0.81 mmol g1), as showed by Fig. 3. The other two peaks were attributed to carboxylic anhydrides (730 K) and lactones (880 K), with a total of 0.70 mmol g1, which is also close to the values obtained with HAC in the same temperature range (0.67 mmol g1), suggesting that the modification with ethylenediamine does not cause loss of these surface groups. The decrease in carboxylic anhydrides observed for the modified sample likely originates from the hydrolysis of these species during reaction with ethylenediamine, which behaves as a weak base. This conclusion is corroborated by the PT result that indicated an increase in the acid species with pKa between 8 and 9, and a decrease of the total concentration of acid sites after addition of ethylenediamine to the HAC surface as can be visualized in Fig. 3. For the signal m/z = 28, the deconvolution shows five contributions. The first is found at 496 K, and coincides with that observed for the signal m/z = 44, and thus is not attributed to CO evolution. The peak at 587 K, which is coincident with the CO2 TPD attribution to carboxylic acids, may have originated only from the decomposition of oxygenated groups linked to the ethylenediamine, leading to CO evolution at temperatures below those usually observed. The peak observed at 730 K was

Table 8 – TPD deconvolution of CO2 spectra for MWCNT samples Sample

T T w A (K) (K) (mmol g1) (K)

w A T (K) w (K) A T (K) w (K) A Total (K) (mmol g1) (mmol g1) (mmol g1) (mmol g1)

MWCNT MWCNT-H MWCNT-O

568 539 –

– 238 237

77 98 –

0.04 0.15 –

– 692 675

– 0.18 0.06

822 856 865

317 170 123

0.10 0.02 0.06

– 967 951

– 131 67

– 0.04 0.04

0.14 0.39 0.16

R2 0.89 0.99 0.95

T = temperature corresponding to peak maximum; w = width at half height; and A = integrated peak area.

Table 9 – TPD deconvolution of CO spectra for MWCNT samples Sample MWCNT MWCNT-H MWCNT-O

T (K)

w (K)

822 856 865

317 170 123

A (mmol g1) 0.10 0.02 0.06

T (K)

w (K)

990 980

330 122

A (mmol g1) 0.58 0.14

T (K)

w (K)

1076 1056 1065

263 101 74

A (mmol g1) 0.19 0.17 0.32

T = temperature corresponding to peak maximum; w = width at half height; and A = integrated peak area.

Total (mmol g1) 0.29 0.77 0.52

R2 0.91 0.99 0.89

CARBON

a

0.07

CO2 CO

0.06

Concentration (μmol g-1 s-1)

4 6 ( 20 0 8 ) 1 5 4 4–15 5 5

0.05

0.04

0.03

0.02

0.01

0.00 400

600

800

1000

1200

T (K)

b

0.25

CO CO2

Concentration (μmol g-1 s-1)

0.20

0.15

0.10

0.05

0.00 400

800

1000

1200

T(K)

c

0.40

CO CO2

Concentration (μmol g-1 s-1)

0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 400

(967 K) were observed by TPD, with a total carboxylic concentration of 0.08 mmol g1. However, the acid treatment significantly increases the phenol (990 K) and carbonyl (1056 K) group concentrations, with a total 0.75 mmol g1 observed by TPD spectra. This high concentration attributed to phenol and carbonyl groups was not observed by PT analysis (0.30 mmol g1 for pKa = 10). The total amount of groups observed at temperatures above 990 K in TPD spectra likely behave as very weak acids (basic groups with pKa > 10) on MWCNT-H surface, and cannot be determined by the PT methodology. Oxidation with O2 showed varying effects on the nanotubes surfaces. In this case, the MWCNT-O TPD profiles (Fig. 7c) indicate that the quantity of oxygen-containing surface groups that decompose as CO is increased, with the peaks being assigned primarily to phenols (980 K) and carbonyls (1065 K). Similar to what was observed for MWCNT, PT result also showed very small concentration of acid sites at pKa’s below 8.0 and high proportion of the acid sites with pKa = 10. Comparison to O2 treatment with the original nanotubes indicates that the CO2 evolution increases by only 14%, while CO evolution increases by nearly 80%. This reflects an increase in the surface basicity, as shown by the PT analysis. The agreement between the two methods is supported by the similar values found for CO evolution from phenols and carbonyls (0.46 mmol g1) obtained by TPD, and by the total concentration of acid sites with pKa = 10 (0.40 mmol g1), as determined by PT analysis. Thus, the gas-phase treatment increases primarily the basic groups, including carbonyls and quinones, which behave as very weak acids in aqueous medium.

3.5.

600

600

800

1000

1200

T(K) Fig. 7 – CO and CO2 TPD spectra and deconvolution for (a) MWCNT, (b) MWCNT-H and (c) MWCNT-O.

1553

The oxygen content

The results discussed above indicate that the concentrations of surface groups determined by TPD are always higher than those determined by PT in the experimental condition used in this work. This is because all species on the surface can be decomposed upon heating, releasing simple gaseous molecules. Thus, the majority of oxygen complexes decompose into CO and CO2 at different temperatures. The oxygen content obtained from TPD (CO and CO2) can be compared with the oxygen determined by elemental analysis, which represents the total oxygen concentration in the sample. These data are shown in Table 3 (last column). The results obtained by TPD are in good agreement with those obtained by elemental analysis, considering that the AC samples have a significant percentage of nitrogen in their composition (Table 3), and that the oxygen can be present as several different species (NO and NO2), which were not accounted for. Thus, these species do not contribute to the total amount of oxygen from TPD. These considerations rationalize the lower oxygen percentage obtained by TPD in comparison to elemental analysis. The amount of oxygen obtained by TPD for the nanotube samples is in agreement with elemental analysis. As the nanotube structure is basically composed of carbon, there is no effect from other groups on the oxygen content obtained by TPD. However, the difference observed for MWCNT-H

1554

CARBON

4 6 ( 2 0 0 8 ) 1 5 4 4 –1 5 5 5

0.8

TPD CO (990-1056K)

0.7

0.6 TPD CO (980-1065K)

(mmol/g)

0.5

pKa 10

0.4

TPD CO 2 (539- 692K) pKa 10

pKa 10

0.3

pKa < 6.5 TPD CO2 (822K)

0.2

TPD CO (1076K)

TPD CO2 (568K) pKa 6.5

0.1

TPD CO 2 (865- 951K) TPD CO 2 (675K)

TPD CO2 (856-967K)

pKa (5-8)

0

MWCNT

MWCNT-H

MWCNT-O

Fig. 8 – Comparison between PT (black) and TPD (white) results obtained for MWCNT, MWCNT-H and MWCNT-O samples.

suggests that the nitro groups created by nitric acid oxidation contribute in this case to oxygen evolution.

4.

Tecnologia (MCT), Brazil, and FCT and FEDER through Project POCI/EQU/57369/2004 is gratefully acknowledged. FG thanks FCT for the Project POCI/N001/2005.

Conclusions

The two methods studied in this work, PT and TPD, yielded similar results when used to explain the behavior and differences among functional surface groups of various carbon samples. The determinations of carboxylic acid group concentrations by both techniques were in good agreement. However, the quantitative results obtained by TPD were closer to the analytical concentration if all oxygen surface groups are considered. Further, the TPD spectra obtained using a mass spectrometer can provide information about the identity of the species on the surface, while PT depends on the acidic and basic behavior of the species. The TPD method was particularly useful in characterizing samples modified with ethylenediamine, which is anchored to specific oxygenated groups. The results obtained for the surface chemistry of nanotube samples evaluated by TPD showed that acid treatment not only increases the carboxylic acid concentration, but also significantly changes the concentration of phenol and carbonyl groups. TPD analysis shows that the gas-phase oxidation of nanotubes preferentially induces the formation of phenol and carbonyl groups on the surface, as evidenced by the increase in basicity observed by PT. PT indicates that only a portion of the carbonyl groups obtained by TPD are detected as acid groups with pKa values below 10 under the titration conditions. Thus, PT has the advantage of describing these species in liquid medium, where the activity of these groups depends not only on their concentration, but also on their environment.

Acknowledgements Financial support from CNPq – Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico, Ministe´rio da Cieˆncia e

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