surface chemistry of carbon blacks and other carbons

Pergamon

Carbon, Vol. 32, No. 5, pp. 759-769, 1994 CoDyright 0 1994 Elsevier Science Ltd

Printedk &eat Britain. All rights reserved 0008~6223/94 $6.00 t .OO

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REVIEW ARTICLE

SOME ASPECTS OF THE SURFACE CHEMISTRY OF CARBON BLACKS AND OTHER CARBONS

H. P. BOEHM Institut fi.ir Anorganische Chemie der Universitiit Miinchen, Meiserstrasse 1,

80333 Miinchen, Germany

(Received 4 January 1994)

Abstract-A review is given on the surface chemistry of carbon blacks and other carbons, in particular, activated carbons. The main part is devoted to surface oxides with emphasis on the chemical methods used in the assessment and identification of surface functional groups. Their formation under mild conditions and the influence of water vapor and metal catalysts on the reaction with air (aging of carbons) are described. Reaction with free organic radicals can be used for the functionalization of carbon surfaces.

Key Words-Activated carbon, carbon black, surface chemistry, surface functionalization, surface oxides.

1. INTRODUCTION

Carbon blacks consist of spheroidal particles with a pronounced ordering of the carbon layers (graphene layers). The layers are wrapped around a very disor- dered nucleus with a preferential orientation parallel to the particle surface[i,2].

High-resolution TEM showed[3] that the layers are bent and curved, following the surface. They are larger, therefore, than the crystallite dimension L, of 1 S-2.5 nm, as determined from line broaden- ing in X-ray diffraction. The spherical primary particles are fused to branched chain-like structures by deposition of such layers.

Since carbon blacks are produced from hydrocar- bons, the dangling bonds at the edges of the carbon layers are saturated mostly by hydrogen. Often, one finds large polycyclic aromatic ring systems on the surface that can be extracted with hot solvents (e.g., xylene). One suspects, therefore, that there are also still larger molecules on the surface that are insolu- ble, and that there is a gradual transition in size to the layers that can be recognized in HRTEM photographs.

Other elements than hydrogen are also found in carbon blacks. The most important of these is oxygen. Whereas sulfur and nitrogen originate from the oil precursor, oxygen can also be taken up during carbon black formation or storage. Much more oxygen is chemisorbed on heating carbon blacks in air (or oxygen or by treatment with oxidizing media such as HNO, or NaOCl solution. The surface oxides formed in these reactions have a pronounced effect on the surface properties of the carbons.

Activated carbons consist of small layers stacks that are less regularly organized. They are also

curved in part, and there is pronounced cross-link- ing. Due to the activation process, the layer packets are separated by micropores, most of which seem to be slit-shaped. Depending on the precursor, there may also exist meso- and macropores. The color blacks of high apparent surface area (BET surface area) are microporous in a similar way.

The surface oxides are bound to the edges of the carbon layers. It has been shown that basal planes of graphite are attacked by molecular oxygen only at their periphery or at defect sites such as vacan- cies[4-61. Many oxygen-containing functional groups have been detected in the surface oxides of carbon. Other elements, in particular halogens, can be chemisorbed on carbon surfaces.

The surface properties ofcarbon blacks and other types of carbon are influenced to a large extent by the foreign elements fixed on the surface, in particular by oxygen. This also affects the behavior of carbon blacks in practical applications.

The present paper gives an overview of the functional groups in surface oxides. The emphasis is on the methods used for their identification because some of the pertinent literature is now no longer readily available. The experiments were performed in part with carbon black, but activated carbons have also been used because of their larger surface areas and larger concentrations of surface groups. Activated carbons were prepared from carbonized sugar char, resulting in very pure carbon materials. Charcoals produced from wood (Eponit) or peat (Norit) were also used: they were extracted with hot hydrochloric acid and washed with hot water until no chloride could be detected. The carbon blacks Corax 3 (furnace black) and CK3 (similar to channel black) were obtained from Degussa. The

759

760 H.P. BOEHM

experimental methods are outlined briefly; they are quite simple, and details are described in the original literature.

2. SURFACE OXIDES

2.1 General Carbons, including carbon blacks, can show ba-

sic or acidic pH values in aqueous dispersions, A good correlation between pH and oxygen content of carbon blacks has been found[7]. The dispersion is the more acidic, the higher the oxygen content is. The acidic surface properties are due to the presence of acidic surface groups. Such carbons have cation exchange properties. Carbons with a low oxygen content show basic surface properties and anion exchange behavior. The basic properties are ascribed to the presence of basic surface oxides, but it has been shown that the 7~ electron system of the basal planes of carbon is sufficiently basic to bind protons from aqueous solutions of acids&-IO].

2.2 Acidic s~r~uce oxides The acidic surface oxides have been the subject

of many studies that have been summarized in several reviews[l l-181. Figure 1 presents several structures of oxygen functional groups that might be found at the edges of graphene layers. Carboxyl groups (a) might give carboxylic anhydrides (b) if they are close together. In close neighborhood to hydroxyl groups or carboxyl groups, carbonyl groups might condense to lactone groups (c) or form lactols (d). Single hydroxyl groups (e) on the edge of aromatic layers would be of phenolic character. The existence of carbonyl groups is very plausible; they could come either isolated (f) or arranged in quinone-like fashion (g). Obviously, other ar- rangements could be envisaged for quinone-type functions. Finally, oxygen could simply be substi- tuted for edge carbon atoms (h); such xanthene- or ether-type oxygen is very di~c~t to detect.

The groups (a) to (e) react more or less weakly acidic. Evidence for their existence has been found

(a) lb)

by classical chemical detection methods, such as esterification with hot methanol (acid-catalyzed), formation of acyl chlorides with thionyl chloride or formation of methyl esters of the carboxyl groups, and methyl ethers of the phenolic hydroxyl groups with diazomethane; see below. Methyl esters and methyl ethers can be differentiated by their different resistance towards hydrolysis. These reactions were supplemented by observation of the changes in neutralization adsorption behavior.

Obviously, the individual functional groups, such as carboxyl groups, will exhibit a spread of their dissociation constants, depending on the neighboring groups, the size of the graphene layers, etc. Yet the acidity constants of carboxyl groups, lactones, or phenols differ over several orders of mag- nitude (see ref. [I8]), and it was established that the various types ofgroups can be distinguished by their neutralization behavior. At a given pH of the adjoin- ing aqueous medium, practically all carboxyl groups will be dissociated to carboxylate with the counter ions dispersed in the diffuse double layer. It has been found that the most convenient way of de- termining the concentration of free carboxyl groups is to perform a neutralization adsorption experiment with 0.05 M NaHCO, solution, separate the solution, and titrate the remaining Na+ ions (by adding excess 0.05 M HCl to an aliquot, boiling off the COZ, and back-titrate with standard NaOH). It was shown that practically identical results are found as with a pH-static titration to pH 8.2[19,20], or by reaction with a solution of KI and KIO, and titration of the liberated iodine[l2]. Such solutions establish a constant pH of 7.5. With nonporous carbons, the adsorption of diphenylguanidine agrees, too, with NaHCO, consumption[8], Direct potentiometric titration suffers from very slow establishment of the ion exchange equilibria, particularly at higher pH values[l9-211. However, the fact that the acidity constants cluster around discrete values can be clearly seen in conductometric titration curves[22]. The best results were obtained with dilute NaOCH,

0 0 \\ *

C-O c-o 011

(e) if) fd (h) Fig. 1. Possible structures of surface oxygen groups (see text).

The surface chemistry of carbon blacks and other carbons 761

Fig. 2. Conductometric titration of oxidized graphite wear dust with 0.05 M NaOMe in MeOH.

solutions in methanol; an example is shown in Fig. 2. There are distinct breaks in the curves, and as shown in Table 1, they agree quite well with the neutralization values found with NaHCO, and Na,C!03. In a very recent paper, Bandosz ef af. described the analysis of titration curves that leads to a resolution of the various acidity constants[23].

Lactones are weaker acids than free carboxyl groups. This is clearly demonstrated by the fact that the lactone ring in phenolphthaleine is opened (purple color) by sodium carbonate, but not by bi- carbonate. Therefore, 0.05 N Na,C03 solution is suitable for the determination of carboxyl groups in lactone-like binding. The formation of lactols (Fig. Id) from aromatic carboxylic acids with neighboring carbonyl groups is well known, for example, for 2- benzoylbenzoic acid[24]:

The presence of analogous structures seems quite plausible on the edge of a graphene layer:

0 C-O

OH

-;:x!:-:l:: It is generally assumed that phenolic hydroxyl

groups on the carbon surface react with strong alkali (e.g., NaOH) analogously to free phenols. Attempts have been made to verify by chemical reactions the presence of such groups. An example of such reactions is shown in Table 2. On reaction with an etheric solution of diazomethane, CH2N2, carboxylic acids form methyl esters of the acids, and methyl ethers are produced from phenols. Lactols form methyl esters, too[251. Ethers are stable to hydroIysis by dilute acids, in contrast to esters that are saponified. As shown in Table 2, the neutralization values with NaOH have decreased to those found with NaZCOj, and the difference is equal to the methoxyl content remaining after hydrolysis.

Acyf chlorides are formed in the reaction with thionyl chloride, SOCl,. At reflux temperature, SOCI, decomposes slowly to Cl? and other products, and therefore some chlorine is also bound by addition or by hydrogen substitution. Only part of the fixed chlorine can be hydrolyzed with alkali; the larger part is resistent even to hot I M NaOH. Only the chloride that can be recovered by hydrolysis with alkali was equivalent to the carboxyl groups (Table 3). This is what one would expect for the reaction

Table 1. Comparison of the breaks in conductometric titration with neutralization values (titration with 0.05 M Na+OMe- in MeOH)

Sample

Corax 3a, H.T. 1400C ox. with (NH,),S,Os

Corax 3 H T. 3000C ox. v&h &MnO,

Graphite wear dust ox. with air at 420C

1st break peq/g

65

10s

205

NaHC03 uptake Feq/g

56

99

181

2nd Na?CO, break uptake peq/g CLeqg

85 92

145 134

350 363

Corax 3 = furnace black (Degussa).

762 H. P.BOEHM

COCI ;:k + 2NaOt-l - COO-No+ ir- t Na+CI-

However, with NaOEt in some cases the consumption was less than with the carbon before chlorina- tion (see Table 3), although one would expect an unchanged consumption, as was observed with the other samples:

COCI

i_x + NaOEt -

COOEt

h

+ Na+CI-

A plausible explanation is that anhydrides of two neighboring carboxyl groups had formed, which form ester plus carboxylate:

0

~

Z,

,O + NaOEt -

c II 0

cx

COO-No+

COOEt

The loss in NaOEt consumption corresponded quite well to half the content of carboxyl groups

determined with NaHCO,. It is known that phthalic acid is converted to phthalic anhydride in the reaction with thionyl chloride. The acyl chloride must have been formed in this case from the lactone-type carbonyl functions.

Another attempt to identify carboxyl groups used the Friedel-Crafts reaction with dimethylaniline and the Schmidt rearrangement, a special type of the Curtius rea~angementr26~. In both reactions, the carboxyl groups were first converted to acyl chloride by reaction with thionyl chloride. After treatment with dimethylaniline and anhydrous aluminum chloride in hot nitrobenzene as a solvent, and ex- traction with dilute hydrochloric acid, water, and ethanoi, the nitrogen content corresponded to the acyl chloride content after ~hlorination[Z6~. The neutralization value with NaHCO, was decreased by an equivalent amount, whereas the neutralization of the other groups was unchanged within experimental limits. The results presented in Table 4 can be explained by the reaction sequence

Ph-NMe, R-COOH + R-COCl-

3

R-CO-C,H,-NMe,.

The Schmidt rea~angement involves reaction of the acyl chloride with an aicoholic solution of sodium azide, NaN,. The resulting acyl azide decomposes to form a urethane, which is stable in acidic media, but decomposes with alkali to give an amine. Thus, again, the carboxyl groups are destroyed:

NaNl R-COOH + R-COCl----+ R-CON, 2

+H,O R-NH-COOEt -

-CO,,-EtOH R-NH2'

The reaction was followed by determinations of the nitrogen and ethoxy contents, as well as the changes in neutralization behavior after each step.

Table 2. Influence of reaction with di~omethane on the neutralization behavior of oxidized carbons (the activated carbons from carbonized sugar char were oxidized with 0, at 400C)

Treatment

Neutralization in peqig 0CH3 content

NaHCO, Na$O, NaOH pmol/g

Sugar char, H.T. 950C after oxidation 200 430 720 0 methylated with CHzNz - - 720 methylated and hydrolyzed with 210 410 410 290

hot HCI solution diff.: 310

Sugar char, H.T. 1100C after oxidation 160 320 690 0 methylated with CH,N, - - 720 crethylated and hydrolyzed with 170 320 330 380

hot HCl solution diff.: 360


Table 3. Neutralization behavior of oxidized carbons after reaction with thionyl chloride (oxidation with OZ at 400C)

Change in consumptn.

Sample

Sugar char H.T. 1100C

Sugar char H.T. 1100C

Eponit H.T. 1100C

Activ. sugar char H.T. 950C

Activ. sugar char H.T. 950C

Eponit, add. activation

Group Group I II

peq/g peqig

160 150

230 210

870 430

600 460

600 350

660 680

Hydrolyzable Cl

peqlg

160

210

480

350

520

after SOCI? reaction

NaOH NaOEt peq/g peq/g

+I80 0

+200 0

+410 0

+450 -340

+350 -310

+580 -340

Group I = NaHCO, neutralization. Group II = NarCO,-NaHCO, neutralization

The results are a bit complicated because lactones may form ethyl esters,and carbonyl groups may react with NaN, in hot ethanol. When the reaction was performed at room temperature, one nitrogen atom was taken up, and one carboxyl function was lost for each acyl chloride group formed origi- nally[26,27].

Phenols are very weak acids, neutralized by NaOH solutions. In contrast to carboxylic acids, they can be reacted with 2,4-dinitrofluorobenzene (DNFB) or with p-nitrobenzoyl chloride (p- NBC)[26]. As shown in Table 5, the bound quantities agree very well with the difference between NaOH and Na&Os consumption.

Using a still stronger base than NaOH, sodium ethoxide, Na+OEtt, in ethanol, additional groups are detected. We have shown[26] that equivalent quantities of Nat and OEt- are bound by the carbons in this reaction. The assumption that this is due to the formation of sodium salts of hemiacetals from carbonyl groups

is based on identical conversions after reaction with hydroxylamine that lead to oximes (Table 6).

The carbonyl groups are susceptible to reduction with nascent hydrogenl261. This results in a de- crease of NaOEt consumption. After reaction with zinc and hydrochloric acid, the sodium ethoxide uptake was reduced to that of sodium hydroxide in some cases. In one such case, the weakly acidic carboxyl function (neutralized by Na*CO,) had increased in acidity and reacted with NaHCO,, too. This observation points to an interrelationship between the carbonyl groups and neighboring carboxyl groups. The quantity of active carbonyl groups was also reduced after reaction with NaBH, or LiAlH,.

Pairs of carbonyl groups arranged at the periphery of the graphene layers in such a way that a system of conjugated double bonds can be formally drawn in, will behave similar to quinone functions. Reduction peaks in cyclovoltammetry experiments have been ascribed to such quinone functions[28-311. Also, some chemical evidence for their existence has been presented[32,33].

In our early experiments we observed that frequently equivalent quantities of the various groups are found after strong oxidation that results in partial gasification of the carbons. This observation led us to speculate on a model in which carboxyl groups, lactols, and phenols are grouped in immediate prox- imityl341. It was thought that this surface complex is an intermediate step in the removal of carbon atoms from the layer edge. It must be stressed,

Table 4. Results of the Friedel-Crafts reaction with dimethylaniline (from refs. 126,271)

NaHCO? N content neutralization Cll after after

reaction w. reaction w. before after SOClz

Sample Ph-NMe*

peqig yeqlg peq/g pmol/g

Eponit ox. with OZ 460 280 230 220 Eponit 590 280 310 330

ox. with (NH&&Ox

764 H.P. BOEHM

Table 5. Reactions of phenolic surface groups on oxidized carbons (oxidation with O2 at 400C)

Sample

Difference Hydrol. NaOH- resistent Conversion with Na2C03 OCH, uptake gr. DNFB p-NBC weoig fimol/g wmol/g ~molig

Carbon black CK3 Activ. sugar char

H.T. 950C Sugar char, H.T. 1100C Activ. char sugar

H.T. 1100C

320 330 130 290 610 620 620 -

420 430 420 - 390 - 370 400

however, that very frequently other distributions of acidic surface functions are observed, especially after relatively mild oxidation treatments. In the more recent literature, such equivalences of different surface groups are rather rare. With carbon blacks, there are often fewer carboxyl groups than reactive carbonyl groups. In the early stages of oxidation with solutions of (NHJ2S205, KMnO,, etc., at room temperature, only weakly acidic carboxyl groups and reactive carbonyl groups are formed (lactol functions)[271. Oxidation with concentrated nitric acid often produces acidic compounds of relatively small size that go into homogenous solution on treatment with dilute alkali[20].

Usually the carbons contain more oxygen than can be explained by the detected functional groups[12,35,36]. This oxygen is usually ascribed to ether-type oxygen (Fig. lh) without further proof.

Determination of active hydrogen by reaction with methyl magnesium iodide or lithium methyl was found to be low compared to the content of carboxylic and phenolic groups on carbon black Spheron 6, whereas good agreement was found with agraphite[37]. The reason is that part of the methane may be adsorbed on the carbon black surface. Therefore, with higher-surface-area carbons, active hydrogen can only be determined by isotope exchange. Table 7 shows that the contents of active hydrogen agreed quite well with the NaOH uptake with carbon black, as well as with a graphite wear dust produced by milling graphite under argon[22].

2.3 Basic surface oxides Basic surface oxides are always present on a

carbon surface. When a carbon is heated to ca. 1000C in vacua or under an inert gas, the existing surface compounds are almost quantitatively de- composed. When this carbon is exposed to dry oxygen after cooling to room temperature, some oxygen is chemisorbed. After submersing this carbon under aqueous acids, the same quantity of oxygen again is taken up, and approximately one equivalent of acid per chemisorbed oxygen atom is bound at the same time (Fig. 3)[39]. The bound anion of the acid can be exchanged for other anions. Water is a suffi- cantly strong acid, and OH- ions are bound when the reaction is conducted in pure water, giving rise to an alkaline pH of the dispersion. Some H202 is formed during the second chemisorption reaction, but carbon surfaces catalyse its decomposition and it decomposes rapidly[39,40]. The chemisorbed HCl or H,O are desorbed on outgassing in vucuo, even at room temperature[41].

Garten and Weiss, who studied the basic surface oxides in the 195Os[42,43], ascribed the basic properties to chromene-like structures. Voll and Boehm concluded on the basis of a few chemical reactions that y-pyrone-like structures, as shown in Figs. 4 and 5, are more plausible[44]. The ether-type oxygen can easily be replaced by nitrogen in the reaction with ammonia. The hydroxyl groups can be methylated with diazomethane, whereas the anion exchange property is preserved.

Table 6. Reactions of carbonyl groups on oxidized carbons (oxidation with 0, at 400C if not stated otherwise)

Samnle

Difference NaOEt-NaOH

uptake ueolg

Sugar char, H.T. 95OC, CO*-activated

Sugar char, H.T. 1100C Eponit, H.T. 1100C

(ox. with (NH&S20s) Carbon black CK3

670

480 630

210

Bound -0Et groups pmol/g

680

460 430 640 580

180 -

Fixed N after react. with

NH20H peq/g

660

The surface chemistry of carbon blacks and other carbons

Table 7. Active hydrogen on oxidized carbon surfaces. In the Zerewitinov method, the volume of methane that forms in the reaction with a solution of CH,MgI is measured. The isotope exchange was

performed by exchange with deuterium from D20, as described in detail in ref. [38].

765

Sample

Surface area m?!g

Method of Active H determination fimolig

NaOH uptake i-=q&

Graphite wear dust ox. with air at 420C ox. with NaOCl ox. with (NH4)2S?08

Corax 3 ox. with (NH&SI08

Corax 3, H.T. 1100C ox. with (NH&O8

Corax 3, H.T. 1400C ox. with (NH&O8

275 isotope exch. 670 580 345 isotope exch. 1040 1025 330 isotope exch. 1440 1650 n.d. Zerewitinov 130 200

n.d. Zerew~tjnov 228 206

n.d. Zerewitinov 94 123

As mentioned earlier, hydrochloric acid is also adsorbed on the basal planes of carbons and graphite[8-IO]. This is due to the basic character of aromatic 7r electrons. Also in Fig. 3, the HCI adsorption isotherm is a little higher than the oxygen-uptake curve. By potentiomet~c titration two types of proton-binding sites were found on a carbon fiIm1451. One corresponded to a base with a mean basicity constant pKb = 6.6, while the second site was a very weak base (pKb > 11)[451. Table 8 shows that some hydrochloric acid is adsorbed even on graphite powders with a very small surface area of the prism faces. The acid uptake did not correlate well with the BET surface areas, however. Papirer et nl.[36] observed, in contrast, that the concentration of basic groups on oxidized carbon blacks was propor- tional to the surface area. They concluded further that one of the oxygen atoms of a pyrone group is quite heat-resistant, and is desorbed as CO and CO1 only between 800C and 950C. It was confirmed that one oxygen atom is fixed at 100C for each basic site formed.

Oxidized carbons always contain basic sites in addition to the acidic functions. However, their quantity is usually less than with the original carbon. It was found that the acid take-up increased when acidic groups on Pd- or Pt-loaded carbons were reduced in hydrogen at 80-500C (see Fig. 611461. Hy- drogen is chemisorbed as H atoms on Pt or Pd, and can diffuse from the metal surface to the surface of the support (hydrogen spillover)[47]. This provides a relatively mild way of reducing surface oxides, as compared to hydrogen treatment of the metal-free carbons, which becomes effective only above 4OO*C. The carboxyl groups/lactols were more susceptible to reduction than the phenofic groups[48]. Obviously, basic surface groups form at the same edge sites as acidic groups, but their quantity re- mains lower than that of the destroyed acidic sites.

2.4 Sp~ctroscopi& rn~~~o~s There have been numerous attempts to study the

surface groups by spectrometric methods, especially by infrared spectroscopy. This is made very difficult by the strong IR absorption of carbon. The first confirmation of the presence of carboxyl groups was found with dispersions of highly oxidized fine- particle-size color blacks[49-5 I I. Progress was made by application of the ATR technique[52] and especially by FT spectrometers[53,54]. Adsorption peaks at ca. 1000 cm- and ca. 1200 cm- indicate the presence of C-O single bonds. The spectrum of HOOK-oxidized graphitized Thornel carbon fibres showed peaks at 1705, 1730, and 1750 cm-,

Fig. 3. 0, and HCI consumption on immersion of outgassed aciivated carbon from sugar char in dilute hydrochloric acid. (Outgassing at 95OC, HCI adsorption by pH-

metric titration to constant pH).

(1) (n)

Fig. 4. Proton addition to y-pyrone-like structures.

746 H. P. BOEHM

Fig. 5. Possible structures of basic surface sites on a graphene layer, derived from the y-pyrone structure.

which were assigned to free carboxyl groups, esters, and lactone groups, respectively[54]. An absorption at 1640 cm- was explained by carbonyl groups situ- ated near hydroxyl groups (enols). However, we observed the same frequency with violanthrone (di- benzanthrone), a polycyclic system with nine con- densed rings and two carbonyl groups. An extended review on this subject has been published[45], but unfortunately there are not many studies on carbon black surfaces.

X-ray photoelectron spectroscopy (XPS, ESCA) has also been used for oxidized carbons, mainly carbon fibres. Due to the high electronegativity of oxygen, the 0 1s signal is not very sensitive to the

Acidic surface groups 1 psq/gf Basic surface groups [peq/gf

450 T-_q,i

350

: \

\

r;l t

I \ \

250 O--U Norit, ox

,50 ) y Nod:' ox-to,,, , 1 ~

0 100 200 300 400 500

Table 8. Adsorption of HCI from 0.05 M solution on graphites and carbon blacks

Surface area m*/g

Graphites Kropfmiihl AF (natural) Lonza KS 75 (synthetic) Lonza KS 15 Lonza KS 75/KM

Carbon blacks CK3 Corax 3 Corax 3, H.T. 9OOC Corax 3, H.T. 3000C

13 7.5

14 42

77 40 84 36 87 64 62 27

HCI uptake EiLeo/g

2s r 2 34

::

way in which the oxygen is bound. One fmds essen- tially two different binding energies for oxygen sin- gly and doubly bonded to carbon[SS]. More informa- tion can be gained from satellites of the C Is peak at higher binding energies[55,56]. The shifts from the main peak range from 1.6 eV for carbon atoms bonded to OH groups to 4.0 eV (carboxyl groups or esters) and even higher. XPS is not very accurate for quantitative determinations, and it is used mainly in studies of low-surface area carbons such as carbon fibers.

Thermodesorption spectroscopy is not very well suited for the determination of the functional groups. Carboxylic groups are least stable, and they decompose with formation of C02. However, this is not possible when carboxylic anhydrides are formed first, and they will produce CO2 plus CO in this

350 1

o.,.o Norlt, ox

250 1 TNorl:' ox-:,,t , , D

0 100 200 300 400 500

Temperature [C ]

Fig. 6. Reduction of acidic surface sites on treatment with hydrogen (left side) and concurrent increase in basic surface sites (right side). Open symbols: Activated carbon Norit, oxidized with 0,: Filled symbols: Norit loaded with 200 pmol/g of platinum (80% dispersion at 150C; there was some smtering

at higher temperatures).


case. The TDS patterns for CO* and CO are affected by the type of carbon used, the oxidation conditions, and the way the TDS experiment is carried out[35]. The probability of secondary reactions of the evolved gases is drastically enhanced with porous carbons. Especially in micropores, CO* might react with the carbon surface to give CO, and at lower temperatures CO might react with surface oxygen complexes to give CO,[57].

2.5 Oxidation under mild conditions Formation of surface oxides is an activated pro-

cess. It has been shown that no oxygen is chemisorbed at low temperatures (below 200 K) on freshly produced carbon surfaces[58]. Surface oxidation with molecular oxygen is fairly rapid above 3OoC, but obviously surface oxides must be formed more slowly at lower temperatures. This phenomenon of aging of carbon materials (activated carbons and carbon blacks) is well known in the relevant indus- tries. The isoelectric point of a carbon black heat- treated at 1500C shifted from pH > 10.5 to pH 5.8 after one years storage in ambient air[59]. The presence of water vapor significantly accelerates the surface oxidation[60-621. The surface becomes more hydrophilic when covered with surface oxides, and the adsorption properties of activated carbons are greatly affected[60,62].

We have treated an activated carbon (Norit) and a furnace black (Corax 3) either in a drying oven at 110C in ambient air or in air of 70% relative humidity at 60C. The increase in acid surface functions was determined by titration with 0.05 N NaOH. As is shown in Fig. 7, acidic surface groups are formed in both cases with creation ofrelatively many pheno- lit groups[48]. The rate of their formation decreases gradually, but the surface was not saturated even after 70 days. With humid air the oxidation was much faster with the activated carbon, although the reaction temperature was lower. With the carbon black, slightly fewer acidic groups were found after reaction in moist air at 60C than in air of low relative humidity at 110C. Very likely this difference arises because much more water is

NaOH uptnke Norit NaOH uplnke @mWgl

comx .. @mWgl

&lo I .- l ,I

. . .

-time+

The carboxylic groups on oxidized carbon surfaces produce cation exchange properties, and carbons always exhibit an anion exchange capacity because basic surface oxides are always present when carbons have been exposed to the atmosphere. However, the concentration of basic surface sites is relatively small, whereas considerable cation exchange capacities can occur. This can have conse- quences for the application of color blacks in print- ing inks, paints, etc.

Fig. 7. Formation of acidic surface groups during low- temperature oxidation of an activated carbon (Norit) and a furnace black (Corax 3); open symbols. For comparison, the same carbons loaded with 200 pmol/g of palladium

were also studied (filled symbols). The surface charge of carbons and the zeta poten-

adsorbed in the micropores of the activated carbon than on the more hydrophobic surface of the nonporous carbon black.

For comparison, we also studied the same carbons after loading with 200 pmol/g of palladium by incipient-wetness impregnation with H2PdCl,. Al- though it is known that palladium catalyses the gasification of carbons with oxygen above 350C[63], the catalytic effect observed at 60-110C (Fig. 7) was impressive. It was very significant with the carbon black.

Liquid oxidizing agents are often used for the production of acidic surface oxides. Concentrated nitric acid is usually used at its reflux temperature, and-as shown above-solutions of (NHJ2S208, NaOCl, or KMnO, can be used at room temperature. Puri also found surface oxidation with KBrO, or even KNO, solutions[64]. Therefore, it is not surprising that carbon surfaces can also be oxidized with metal cations of a sufficiently high oxidation potential. It has been observed that hydrated Ag+ ions are reduced to metallic silver[65-671. Analo- gously, PdCI:- ions are reduced to Pd0[48], and iron(III) ions are reduced to iron(I1). Using a series of different metal ions, Fu et al. showed experimen- tally that the extent of oxidation depends on the redox potential of the system, which is dependent not only on the element, but also on temperature, pH, and the presence of complexing ions[67]. After oxidation with a weaker oxidizing agent, additional surface oxides can be produced by action of stronger oxidants[67].

2.6 Influence of surface oxides on the surface properties

Surface oxides provide hydrophilic sites on a hydrophobic surface. A high concentration of chemisorbed oxygen makes a carbon black hydrophilic, and it disperses very well in water. The dissociation of carboxylic surface groups facilitates the dispersion by creating a negative surface charge[l4]. Car- bon blacks oxidized with ozone form spontaneously colloidal dispersions on immersion in water[68]. The polar and the hydrophobic parts of a carbon black surface can be separately determined by microcalor- imetric determination of the heat of reaction for the preferential adsorption of n-butanol (for polar sites) and n-dotriacontane, C,*H, (for the unpolar surface), on the carbons immersed in n- heptane[69].

768 H. P. BOEHM

tials in aqueous dispersions are determined by the nature of the surface groups and the pH. Tempera- ture and the concentration of non-potential-de- termining ions have a smaller effect (provided there is no specific adsorption). The isoelectric point of a color black, Printex U, was found at pH 4.5 2 0.2. After heat treatment under N, at llOOC, that is, thermal destruction of most surface oxides, it shifted to pH 8.8; after treatment at 1500C it was at pH > 10.5, and even higher after graphitization at 27OOC[59]. On oxidation with air at 37OC, new acidic functions were formed with a consequent shift of the IEP to pH 5.7 after 2 h and pH < 3.0 after 60 h. These measurements were performed by elec- trophoresis, but mass titration provides a convenient way to determine IEPs if sufficient quantities of material are available[70].

3. BINDING OF NITROGEN AND HALOGENS

Carbon blacks contain small concentrations of nitrogen. This is very likely bound substitutionally at the edges of the graphene layers. Nothing is known about the solubility of nitrogen in the interior of graphene layers (e.g., in graphite). It is well known that diamonds frequently contain substitutionally dissolved nitrogen.

On treatment of carbons with ammonia at ele- vated temperatures (e.g., 600-9OOC), nitrogen is bound[71-731. This reaction has been studied with carbon blacks as well as with activated carbons. Photoelectron spectra show two main nitrogen N 1s signals. The peak with a binding energy of ca. 400 eV is assigned to amine-type nitrogen, and the signal at 398-399 eV is generally explained by pyri- dine- or acridine-type nitrogen[71,73]. These studies have been performed mainly because the catalytic activity of carbons (e.g., in oxidation reactions) is drastically increased by incorporation of nitrogen. At temperatures above 600C some carbon is gasi- fied in the reaction with NH,, and microporosity develops. But also at 6OoC, there is an increase in HCl neutralization, indicating that new basic surface sites, presumably amine groups, were formedl741.

Halogens react with carbon blacks by substitution of hydrogen, and hydrogen halide is evolved. Donnet reported that all hydrogen was removed from the surface of carbon black Vulcan 6 after treatment with chlorine at 75OC[15]. The maximum chlorine uptake was observed at 450- 5OOC[13,75,76]. The bound chlorine is hydrolyzed only to a small part by hot 1 M sodium hydroxide[76], but it can be removed by heating in hydrogen[77,781.

A study of chemisorption of chlorine at 500C has been performed using a carbon black and carbon cloth that had been outgassed at 1000C to remove all chemisorbed oxygen and most of the hydrogen[77]. It was concluded that chlorine reacts step- wise; first it is chemisorbed by addition, possibly to double bonds. Subsequently, in the major step,

it is bound substitutionally with HCl evolution, and finally it removes hydrogen as HCl without Cl chemisorption.

Bromine can be chemisorbed in a similar way, but the bound quantities are lower.

4. REACTIONS WITH FREE RADICALS

The surfaces of carbons react with free radicals, and this reaction can be used for the grafting of functional molecules or polymers to the carbon surface. The fixation of 2-isobutyronitrile radicals, (CH&-CN, on the carbon surface can be easily determined from their nitrogen content[79]. The radicals are produced by heating a solution of azo-di- isobutyronitrile. Similarly, the carbons react with 3,5-dichlorobenzoyl peroxide or lauryl peroxide. The radicals seem to attack especially quinone-type functions, and radicalic sites may be produced on the carbon surface[80]. Benzoate groups are found on the surface after reaction with dibenzoyl peroxide, and these can be hydrolyzed to surface phenolic groups[81].

Polystyrene can be grafted to carbon black surfaces by starting radicalic polymerization in styrene solutions in the presence of carbon blacks. It has been found that carbon blacks with quinone oxygen inhibit polymerization initially until all quinone is converted to hydroquinone[8 1,821. Prior hydroge- nation of the quinone groups eliminates the inhibi- tion period. These and other grafting reactions are well described in Donnets review[l5].

5.

6.

7.

8.

9. 10.

11.

12.

13.

14.

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surface chemistry of carbon blacks and other carbons

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