Infrared studies of aromatic compounds adsorbed on the surface of carbon films

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Vol. 26, No. 5, pp. 603-611, in Great Britain.

1988 CopyrIght

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0008-6223/8X $3.00 + .OO iD 198X Pcrgamon Press plc

STUDIES OF AROMATIC COMPOUNDS ON THE SURFACE OF CARBON FILMS

JERZY ZAWADZKI Institute of Chemistry, Nicolaus Copernicus University, 87-100 Torud, Poland (Received 27 February

1987; accepted in revised form 12 ~~vernb~~ 1987)

Abstract-IR spectroscopic studies were carried out on the bond character of benzene, phenol, and pnitrophenol molecules adsorbed on the surface of carbons. Interaction between the carbon surface and the a-electron system of an adsorbed aromatic molecule leads to changes in the IR spectrum (the value of the absorption coefficient). Oxygen chemisorbed on a carbon surface appears to strongly influence adsorption of aromatic compounds, especially when the adsorptive properties of carbon are partly determined by its molecular-sieve properties. Also, IR studies were carried out to determine the effect of ion exchange I-I+ + Na’ on the mechanism of phenol and p-nitrophenol adsorption. Key Words-Carbon

films, benzene, phenol, p-nitrophenol,

1. INTRODUCTION

adsorption,

infrared spectroscopy

cussed the application of internal reflection technique to the study of carbon surfaces. The present paper describes an IR spectroscopic study of the interaction between adsorbed aromatic molecules and the surface functional groups of carbon.

Active carbon has received considerable attention as a means for removing pollutants from both liquid and gaseous waste streams[l]. There have been a number of studies dealing with the adsorption of organics, especially of phenol and its derivatives, on carbons[l-231. Coughlin e# al. [2,3,7] have found that adsorption of phenol from dilute solution is markedly lowered by prior oxidation of the carbon. Epstein et al. [ 181 have shown that p-nitrophenol is preferentially adsorbed on an oxidized carbon surface rather than on a reduced surface. It is interesting to consider the possible explanation for the role of surface oxides in the adsorption mechanism of phenol and F-nitrophenol molecules from an aqueous solution. Magne and Walkerf2lj have suggested that phenol adsorbs on active carbon both by physisorption and chemisorption, although the actual mechanism through which carbon removes phenol and p-nitrophenol from aqueous solutions has not been well defined. There are several effects that influence the nature and magnitude of the adsorption of aromatic compounds from aqueous solution. These include changes in adsorption capacity due to the variations of pH, ionic strength, solubility, and dissociation of weak aromatic electrolytes. There have been a number of investigations of the adsorption of benzene on carbons[24-371. Isirikyan and Kiselev[25] consider the adsorption of benzene to be influenced by the surface---OH groups, whereas Puri et ccl. 1271 have shown that surface oxygen complexes affect the adsorptive properties of carbon. Direct information concerning the nature of the adsorbed phase may be obtained by means of IR spectroscopy. An infrared spectroscopy study of pnitrophenol adsorption on active carbon has been undertaken by Mattson et a1.[9,16,17] who have dis-

2. EXPERIMENTAL

Carbon films were prepared by carbonization of cellulose[38]. Cellophane purified in 20% HCl, and washed repeatedly with distilled water, was heated at 600°C for 1 h under a vacuum of 10 -.’ Pa. Carbon films differing in the chemical structure of surface functional groups were produced by posttreatment with oxygen at 300°C for 2 h. Adsorption isotherms for phenol or p-nitrophenol were obtained by equilibrating known weights of carbon (about 0.1-0.5 g portions) with 100 cm3 of an aqueous solution in the concentration range 1 to 50 mmole/dm’. The suspensions were shaken for 24 h after which the decrease in solute concentration was measured by UV s~ctrophotometry. Phenol was determined by spectrophotometry using 4-aminoantipyrine as a reagent[39]. IR spectra were recorded using the vacuum cuvette described previously[40,41]. Spectral changes accompanying benzene adsorption were established by comparing IR spectra of the same carbon film recorded in vacua (lo-* Pa) with spectra recorded under a known pressure of benzene vapor. For the adsorbate pressures under study, the intensity of absorption bands of the gaseous phase was very low (Fig. 1, spectrum 4). Adsorption of phenol or p-nitrophenol from aqueous solution was carried out in a separate vessel. The carbon films were dried in air and then placed in the vacuum cuvette. Spectral changes caused by 603

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Fig. 1. Infrared spectra of benzene and phenol adsorbed on a nonoxidized carbon film. (1) Carbon film outgassed at 6OO”C,(2) after the adsorption of benzene (pc# = 5 hPa), (3) after evacuation for 1 h at room temperature, (4) spectrum of the gaseous benzene under the pressure of 5 hPa, (5) carbon film after outgassing at 2OV’C,(6) after adsorption of phenol from 0.05 m aqueous solution (spectrum was recorded in air after drying at room temperature), (7) carbon film after outgassing at 200°C. Spectra 1, 3, 5, and 7 were recorded under a vacuum of lo-* Pa. Spectra 1 to 3 and 5 to 7 were recorded with transmission scale expanded 1.43 times. adsorption were established by comparing IR spectra of the same film before and after contact with an aqueous solution. A Specord 71 IR spectrophotometer was used for measurements.

3. RESULTS AND DISCUSSION

Infrared spectra of benzene adsorbed on carbon films are presented in Figs. 1 and 2. The spectral features of a carbon film obtained at 600°C in vacua were described earlier[38,42]. When such a sample was exposed to benzene vapor at room temperature, some new bands attributed to adsorbed benzene appeared in the spectrum (Fig. 1, spectrum 2). The positions and assignments for the absorption bands of benzene adsorbed on the surface of carbon are summarized in Table 1. The band positions presented in Table 1 can be found in ref. 43. Benzene, though nonpolar, has an uneven density distribution of rr electrons. For this reason, adsorption of benzene should be sensitive to the presence of surface functional groups (particularly hydroxyl groups) as are all molecules with large quadrupole moments[44,4.5]. Changes in the spectrum of both the surface functional groups and aromatic molecules when they adsorb provide information about the actual molecular interaction that takes place during the adsorption process[46-481. For example, Galkin et al. [49] have observed changes in location of the absorption bands of a benzene ring on passing from the liquid to the adsorbed state. A more sensitive indicator of the disturbance of the aromatic r-electron system on adsorption is the

value of the coefficient of absorption, which is proportional to the square of the change in the dipole moment of a molecule when it vibrates along the corresponding normal coordinate. Thus, when benzene adsorbs on carbon (Fig. 1, spectrum 2) there is no shift in the benzene band positions but the intensity of the out-of-plane CH deformation at 673 cm-’ increases markedly relative to the intensity of the C-H stretching mode at 3064 cm-‘. The changes in the intensity of CH out-of-plane vibrations can serve as a direct indication of the disturbance of the electron system of the benzene ring on adsorption. It follows from Fig. 1. that the intensity of the bands attributed to adsorbed benzene (673,1037,1482, and 3064 cm-‘) decreases only slightly after outgassing at room temperature. These bands disappeared after outgassing at 200°C (spectrum 5). Oxidizing the same carbon film (Fig. 2, spectrum 1) with oxygen at 300°C produces acidic surface oxides[50,51] that yield a broad OH stretching band at 3500 cm-’ (carboxyl, phenol) that does not disappear in vacuum at 200°C. The presence of a doublet at 1830 and 1760 cm-’ in the region of C=O stretching vibrations, as well as bands at 900 and 730 cm-’ indicates that some of the surface oxides are cyclic anhydrides. The IR spectrum of this carbon film recorded in the presence of benzene vapor (pcgHg= 5 hPa) does not show any changes caused by adsorbed benzene molecules (Fig. 2, spectrum 2). However, Puri et al. [27] observed that CC&-evolving surface oxides that impart polar and hydrophylic character to the surface inhibit the sorption of benzene. With the elimination of the majority of these surface oxides,

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Infrared studies of surface adsorption

after outgassing at 400°C the adsorption of benzene increases appreciably. According to Puri ei a1.1271, the samples outgassed at 600°C contain a CO complex that includes oxygen present as surface quinonic groups. They suggested that the enhanced sorption is due to the interaction of rr electrons of a benzene ring with the partial positive charge on the carbonyl carbon atom. It is possible, however, that a carbon film displays molecular-sieve effects whereby molecules exceeding a certain size cannot be adsorbed in narrow pores or in larger diameter pores having narrow entrances (i.e. ink-bottle shaped pores). The chemisorption of oxygen on a carbon film probably leads to decreases in the effective diameter of pores. The importance of simultaneous consideration of surface chemistry together with specific surface area and pore-size distributiou is pointed out. The effect of substitution in the benzene ring on adsorption and interaction with a carbon surface was studied using phenol and p-nitrophenol. The IR spectrum of a nonoxidized carbon film after adsorption of phenol shows only a very weak out-of-plane vibration band at 750 cm-’ due to adsorbed phenol molecules (Fig. 1, spectrum 6). Adsorption of phenol leads to a decrease in the background noise level and to an increase in the intensity of the 1590 cm--* band and overlapping bands at 1450 to 1150 cm-‘. These changes result from interactions between the carbon surface and an aqueous solution of phenol and/or oxygen IR spectra of phenol adsorbed on the same carbon film after oxidation are presented in Fig. 2. A diminished intensity of anhydride bands (doublet at

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1830 cm-’ and 1760 cm-’ in the region of C=O stretching vibrations as well as bands at 900 and 730 cm-j) indicates that some surface cyclic anhydrides undergo hydrolysis in an aqueous solution of phenol. A broad O-H stretching band (Fig. 2, spectrum 3) is formed as a consequence of the hydrolysis of anhydride structures and the adsorption of phenol and water molecules. The IR spectrum of phenol adsorbed on a carbon film is identical with the spectrum of pure phenol. The band positions and assignments are summarized in Table I. Adsorbed phenol remains on the surface of the oxidized carbon after outgassing at room temperature (Fig. 2, spectrum 4). Spectral changes brought about by outgassing at room temperature indicate that absorption within the range of O-H stretching vibrations has been partly caused by physically adsorbed water. A decrease in the intensity of the O-H stretching band at 3600 cm-’ is accompanied by a decrease in the H,O deformation band near 1600 cm-‘. The shape of the O-H stretching band is indicative of hydrogen bonding. Comparison of IR spectra in Figs. 1 and 2 show that the adsorption capacities of the same carbon film sample for aromatic compounds display a strong effect of oxidation. Benzene adsorption appears to be considerably smaller after surface oxidation, yet the oxidized film retains significantly more phenol molecules after outgassing at room temperature. This is in contrast to reports by many investigators[2,3,7,9,11.12] that phenol uptake decreases appreciably on carbon surface oxidation. Coughlin and Ezra[7] suggested that oxygen chemisorbed on carbon appears to strongly influence

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Fig. 2. Infrared spectra of benzene and phenol adsorbed on an oxidized carbon film. (1) Carbon film outgassed at 600°C (Fig. 1) oxidized for 2 h at 300°C with oxygen, after outgassing at 200°C (2) after adsorption of benzene (pcsHs = 5 hPa), (3) spectrum of a carbon film outgassed at 200°C after adsorption of phenol from 0.05 m aqueous solution (spectrum was recorded in air after drying at room temperature), (4) after evacuation I h at room temperature. Spectra 1-4 were recorded with transmission scale expanded 1.11 times.

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Table 1. Absorption

bands observed for aromatic compounds adsorbed on surface of carbon films Band positions cm-’

The classification of the vibrations

Normal vibrations

Benzene

Phenol

Radial, skeletal, vibrations

VI

810

Out-of-plane

v4

688

skeletal vibrations

C-H stretching modes

p-Nitrophenol

694 1111

% 1604

%b

1619 1602

C-H in plane vibrations

UPa

1174

C-H out of plane vibratjons

VU

C-H stretching modes

VI3

C-H out of plane vibrations

Ml

C-H in plane bending modes

VIBb

1037

1071

C-C stretching vibrations

%a %?b

1482

1497 1465

1513 1449

C-H stretching vibrations

%Ml

3064

VOH

3623

3363

I3nu

1220

1207

C-C stretching vibrations

_

%

673

749 1259

1281 852

~80,

1341

~30,

1.506

WO,

857

-YWL

755

The band positions were taken from ref. 43.

phenol adsorption only at low concentration when the molecules are thought to be adsorbed parallel to the surface due to nonpolar forces operating over the entire phenol nucleus. At higher concentrations, phenol molecules are thought to be adsorbed in the vertical or “end-on” position[8] due to intramolecular interaction with the hydroxyl groups directed away from the carbon surface. Under these conditions, surface oxygen appears to exert very little infl uence on phenol adsorption. Coughlin et al. [2] have also suggested that phenol adsorption on carbons involves dispersive forces between the phenol n-electron system and the 7r-electrons in carbons. They attribute their findings that phenol adsorption is lowered by the oxidation of carbon to the removal of electrons from the n-electron system of the basai planes as a result of oxygen chemisorption. It can be seen in Fig. 3 that surface oxidation has little influence on the shape of the phenol and p-nitrophenol adsorption isotherms and on the adsorption capacity of the carbons. The results show that the quantity of p-nitrophenol adsorbed from an aqueous solution is higher than the quantity of phenol.

Mattson et al.[9] have found major differences for the adsorption of phenol and its derivatives on carbons. The mechanism hypothesized for the adsorp tion of phenols and nitrophenols on active carbon involves a donor-acceptor complex between a surface carbonyl oxygen acting as an electron donor and the aromatic ring of the solute acting as an acceptor. The electron density of an aromatic ring is strongly influenced by the nature of the substituent groups on the ring. Nitro group substitution can enhance the donor-acceptor interaction by acting as a strong electron-withdrawing group. Thus, p-nitrophenol molecules act as acceptors in such complexes and form stronger donor-acceptor complexes with a given donor than phenol does. IR spectral changes caused by p-nitrophenol adsorption on a nonoxidized carbon film (Fig. 4, spectrum 2) indicate very little irreversible adsorption. The adsorption of p-~trophenol on the same carbon film, after oxidation gives a spectrum (Fig. 4, spectrum 5) in which the locations of the absorption bands are identical with the bands of solid, p-nitrophenol (Table 1). Based on the IR spectroscopic data, it is concluded

Infrared studies of surface adsorption

that surface oxidation increases the binding energy between phenol or p-nitrophenol molecules and the carbon surface. From these IR spectroscopic data, some definite conclusions regarding the structure of the adsorbedp-nitrophenol molecules can be drawn. If the specific interaction of the surface of carbon is with the aromatic nucleus, then it would be expected that the vibrations that are most perturbed would be those associated with the aromatic ring of the adsorbed compound. Vibrations due to substltuent groups would be very little affected. The IR spectra clearly show that the phenolic ~~~(3363 cm-‘) and po,(1207 cm-‘) as well as nitro group peaks, v,N0,(1341 cm-‘) u,,N02(1506 cm-‘), P,N0,(857 cm-‘) and y,N0,(755 cm-‘) are essentially unchanged with respect to their position before adsorption. The intensity ratios observed for the physically adsorbed species are a little different from those of solid, pure p-nitrophenol. The C-C stretching band at 1449 cm-’ is more intense in the adsorbed state than in the solid state. Some p-nitrophenol remains on the surface after outgassing at room temperature (Fig. 4, spectrum 5) but no spectroscopic evidence has been found for the formation of chem-

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Fig. 3. Adsorption isotherms of phenol and p-nitrophenol on carbon films in an aqueous solution (at 25°C). (1) Adsorption of phenol on a carbon film outgassed at 6WC, (2) adsorption of p-nitrophenol on a carbon film outgassed at 6OO”C, (3) adsorption of phenol on a carbon film outgassed at 600°C and then oxidized for 2 h with oxygen at 3Oo”C, (4) adsorption of p-nitrophenol on a carbon film outgassed at 600°C and then oxidized for 2 h with oxygen at 300°C.

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ical bonds between the adsorbed molecules and the carbon surface. IR spectra (Figs. 1, 2, and 4) show that adsorption of the aromatic compounds is partly a specific interaction with the surface of carbon and occurs with the participation of IT electrons of the adsorbed molecules. Another phenomenon that could be expected to play some role in the adsorption mechanism is the bonding of water molecules to polar oxygens on the

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Fig. 4. Infrared spectra ofp-nitrophenol adsorbed on a carbon film. (1) Carbon film outgassed at 6WC, (2) after adsorption of p-nitrophenol from 0.05 m aqueous solution (spectrum was recorded in air after drying at room temperature), (3) after outgassing at 2OOT, (4) carbon film outgassed at 600°C and then oxidized for 2 h with oxygen at 3OO”C,(5) after adsorption of p-nitrophenol from 0.05 m aqueous solution and after subsequent evacuation at room temperature. Spectra l-3 were recorded with transmission scale expanded 1.43 times. Spectra 4 and 5 were recorded with transmission scale expanded 1.11 times.

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Fig. 5. Infrared spectra of benzene and p-nitrophenol adsorbed on a carbon film with pores of larger sizes. (1) Carbon film oxidized with oxygen at 300°C and outgassed at 600°C three times alternately, (2) after adsorption of p-nitrophenol from 0.05 m aqueous solution and after subsequent evacuation at room temperature, (3) carbon film outgassed at 600°C and oxidized with oxygen at 300°C three times alternately, (4) after adsorption of benzene (pcsH6= 5 hPa). Spectra 1 and 2 were recorded with transmission scale expanded 1.43 times. Spectra 3 and 4 were recorded with transmission scale expanded 1.11 times.

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800

-4

Fig. ^.. 6. Infrared spectra of phenol adsorbed on carbon film prior and after ion exchange. (1) Carbon film outgassed at 600°C and then oxidized for 2 h with oxygen at 3OO”C,(2) after adsorption of phenol from 0.05 m aqueous solution, (3) after washing in water, (4) after ion exchange in 0.05 m NaOH solution and hydrolysis of sodium salts in water, (5) after adsorption of phenol from 0.05 m aqueous solution. Spectra 1 to 5 were recorded in vacuum (p = 10m2Pa) with transmission scale expanded 1.11 times.

Infrared studies of surface adsorption

10 76

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Fig. 7. Infrared spectra of p-nitrophenol adsorbed on carbon film prior and after ion exchange. ( 1) Carbon film outgassed at 600°C and then oxidized for 2 h with oxygen at 3OO”C,(2) after adsorption of p-nitrophenol from 0.05 m aqueous solution, (3) after washing in water, (4) after ion exchange in 0.05 m NaOH solution and hydrolysis of sodium salts in water, (5) after adsorption of p-nitrophenol from 0.05 m aqueous solution. Spectra 1 to 5 were recorded in vacuum (p = 10 ?Pa) with transmission scale expanded 1.11 times.

The oxidation of the carbon surface increases the affinity of the sorbent for water. According to literature data[2,3,11], the carbon surface competes for both phenol and water molecules. Preferential uptake of water on the oxide functional groups could result in a lower uptake of phenol. This, however, is contrary to the IR spectroscopic data obtained in this study. Here, it is seen that the effect of acidic surface oxides on the carbon film is to increase the affinity of the sorbent for phenol and pnitrophenol. It is clear that this behavior cannot be attributed to the competition between water and organic solute for the carbon surface. Therefore, the formation of hydrogen bonds with the polar oxygen centers is one plausible explanation for the influence of chemisorbed oxygen. The shape of the O-H stretching band indicates that phenol (Fig. 2) or pnitrophenol (Fig. 4) can form H bonds with the polar oxygen centers on the surface of carbon. The following experiment has been carried out in order to check whether some adsorptive properties can be explained by the differences in relative pore size distributions. The carbon films after adsorption of benzene and phenol (Fig. 2) orp-nitrophenol (Fig. 4) have been outgassed at 600°C and oxidized with oxygen at 300°C. three times alternately. The formation and decomposition of surface oxides leads to the widening of existing micropores and to the increase of their volume. The results of IR spectroscopic studies of benzene carbon.

and p-nitrophenol adsorption on the surface of carbon films with pore diameters increased by oxidation are presented in Fig. 5. IR spectra (Fig. 5) of pnitrophenol adsorbed on a nonoxidized carbon film (spectrum 2) as well as of benzene adsorbed on an oxidized carbon film (spectrum 4) are evidence that some of the micropores previously inaccesible to aromatic compounds become available for adsorption. In order to regenerate carbon films containing adsorbed phenol or p-nitrophenol, they have been washed with distilled water. IR spectra presented in Figs. 6 and 7 clearly show that carbon films can be regenerated efficiently by washing in water. Mahajan et af. (111 showed that phenol uptake on activated carbons decreased on exchange of H + ions of carboxylic acid groups with Na’ ions. This was thought to be so because carbons with exchangable cations adsorb more water than the unexchanged samples. The effect of the change in chemical character of acidic surface groups as a result of ion exchange, on phenol and p-nitrophenol adsorption He+Na+, is shown in Figs. 6 and 7. Carboxyl groups formed as a result of hydrolysis undergo ion exchange, bringing about a rise in absorption at the 1590 cm ’ region (Y,, COO-) and the formation of a band at 1380 cm ’ (u, COO-). The IR spectra (Fig. 6 and 7) show that instead of the expected decrease in the intensity of the OH

JERZY ZAWADZKI

610

stretching band, due to cation exchange, a rise in the intensity of this band is observed. If it is assumed that, after outgassing at room temperature, the quantity of physically adsorbed water before and after the reaction with NaOH solution is the same, then the increase in intensity of the OH band could be ascribed to the formation of structural hydroxylic groups. Our previous IR spectroscopic investigation[53] showed that these groups do not undergo methylation and that the vOHband is not due to physically adsorbed water. The chemical properties of these OH groups are affected by conjugation with the carbon surface and the presence of bound Na+ ions. Surface sodium salts resistant to hydrolysis in water are hydrolyzed in phenol solution, Figs. 6 and 7 demonstrate that, as a result of hydrolysis of sodium salts, decreases in absorption at 1590 cm-’ and at 1380 cm-’ are accompanied by the formation of a 1720 cm-’ band and a rise in the intensity of the OH stretching band. These spectral changes and increased absorption at 1200 to 1250 cm-’ are consistent with the formation of carboxyl groups. After the adsorption of phenol, the IR spectrum (Fig. 6) exhibits bands of physically adsorbed molecules (Table 1). A comparison of IR spectral changes brought about by phenol adsorption on the sample prior and post ionic exchange (spectra 2 and 5) indicates differences in the quantity of adsorbed phenol. The substitution of a nitro group for a para hydrogen atom in phenol produces a great increase in acidity. As a consequence of the substitution, p-nitrophenol is 690 times as acid as phenol itself[54]. As a consequence, more hydrolysis and higher adsorption are observed with p-nitrophenol (Fig. 7) than with phenol (Fig. 6). Although infrared spectroscopy has been used very successfully in explaining adsorption phenomena on a number of adsorbents and catalysts, it has had limited application to the carbon surface studies. The IR spectra obtained in this investigation demonstrate the utility of the carbon film technique in the study of adsorption of organic pollutants on carbons.

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