Carbon Vd. 18, pp. 281-285 6 PUPUIMI Pms Lid.. 1980. Printed in Great Britain

IR SPECTROSCOPIC INVESTIGATIONS OF THE MECHANISM OF OXIDATION OF CARBONACEOUS

FILMS WITH HNO, SOLUTION

JERZY ZAWADZKI Institute of Chemistry, Nicholas Copernicus University, Torun, Poland

(Received 30 April 1979)

Abstract-The investigations were carried out on carbonaceous films prepared by carbonization of polyfurfuryl alcohol and of cellulose. The chemical structure of carbons oxidized with HNOj solution was studied using an IR spectroscopic technique. The same method was also used to examine the intermediate stages of oxidation process and the thermal stability of formed surface functional groups.

1. INTRODUCTION Surface oxides are formed on carbons both by the activation with oxidizing gases or by the reaction with solutions of oxidizers[l, 21. One of the most often used oxidizing solutions is the concentrated nitric acid. The mechanism of reaction between HNOs and the carbon surface, as well as the chemical structure of surface oxides formed, have not been clarified completely until now.

Ubbelohde [3] studied the reaction between graphite and the HNO, vapour at room temperature and found the formation of graphite nitrate C,NO,.3HNO, during the process. Other authors[4,5] state that the oxidation of carbons and graphite with HNO, solution leads to the formation of benzopolycarboxyl acids (including mellitic acid) and organic nitro compounds (e.g. pi&c acid). Their presenci is claimed to be found in HNO, solution. The oxidation of graphite with a mixture of concentrated HNO, and H2S04 acids results in the formation of graphite oxide of a chemical composition not exactly determined but with structures (C602H),, (C,O.+H&, (Cs04H,),, most often attributed to it[6]. According to the literature[3-6] it seems that the structure of an oxidation product depends on the conditions of the reac- tion (especially temperature and the concentration of solution) and the chemical structure of oxidized carbons.

Some direct information concerning the chemical structure of carbon surfaces and the mechanism of oxi- dation process and its intermediate stages can be obtained from IR spectroscopy investigations. The amount and character of the information depend on the choice of a proper examination method. The technique worked out for IR studies on carbonaceous films creates a perspective for broader application of IR spectroscopy for the investigations of surface phenomena on carbons.

The present paper deals with IR spectroscopy in- vestigations of the mechanism of carbonaceous films oxidation with HNOs solution with films prepared by the carbonization of polyfurfuryl alcohol and of cellulose.

2. EXPERIMENTAL

The carbonaceous films used were prepared from polyfurfuryl alcohol and cellulose. The carbonization

processing of these films has been described previously [7,8].

The desorption of the carbonaceous films and their carbonization were carried out in a vacuum cell des- cribed previously[9]. The films were then taken out from the cell and oxidized by dipping in a 63% HNO, solution. After oxidation, the films were washed many times with water and dried in an air stream at 70°C. Then they were placed back in the cell and desorbed under low pressure lo-* Pa (lo-* Tr). The thermal stability of sur- face functional groups formed was examined by desorp- tion at increasingly high temperature.

Gravimetric measurements of the oxidation process and of the thermal stability of surface functional groups were made using a McBain sorption balance. The measurements were carried out for films carbonized and oxidized under the same conditions as the films for IR spectroscopy investigations. The films were prepared in the form of spirals weighing about 1OOmg. IR spectra were recorded using a spectrophotometer Specord 71 IR.

3. RESULTS AND DISCUSSION

The formation of surface functional groups during the reaction between carbon and the HNO, solution is a complex process. The reaction of HNO, with low-tem- perature carbons (containing organic functional groups) differs from the case of high-temperature carbons. At the initial stage of carbon oxidation, the oxidizing agent is a nitric acid solution (HNOs, (OH)*NO’, NO*+, NO,-, HsO+, H20) which during reduction forms among others, nitrogen oxides. According to the literature[6], these oxides can oxidize carbons and then they are reduced to

N2.

IR spectral changes that accompany the process of carbonaceous film oxidation with HNO, solution, observed for carbons prepared from polyfurfuryl alco- hol, are shown in Figs. 1 and 2. The band at 16OOcm-’ present in the IR spectrum of the carbonizate before its oxidation (Fig. 1) has been observed by many authors [l&12] studying various carbon samples in IR and has not been interpreted unequivocally until now. According to some authors, this band is caused by oxygen compounds of carbon. The band at 16OOcm-’

281

282 J. ZAWADZKI

Lb I II I I/I I I ,,,,,,,,,,,,,,I 3600 iooo 2LOO 2000 1700 1400 1100 800

cti'

Fig. 1. IR spectra of a carbonaceous film prepared from polyfurfuryl alcohol and oxidized with HNO, solution. (1) film prepared by the carbonization of polyfurfuryl alcohol in CO2 atmosphere at 550°C “thickness” of the film = 1.83 m&m*; (2) same film after oxidation with 63% HNOl solution at room temperature. Spectra 1 and 2 were recorded in air with transmission scale expanded 1.45

times.

Lj #It II I Id I I/I I I I I I I I I I I II 3600 3000 2400 2000 1700 1400 1100 800

cm-'

Fii. 2. IR spectra of a carbonaceous film prepared from poly- furfuryl alcohol and oxidized with HNOJ solution. (1) film oxi- dized with HN03 solution (spectra of the same film are presented in Fig. 1) after desorption at room temperature; (2) after desorp- tion at 200°C; (3) after desorption at 400°C. Spectra l-3 were recorded for the film in a vacuum cell with NaCl windows.

Transmission scale expanded 1.6 times.

and mutually overlapping absorption bands within the 1150-1450 cm-’ range were attributed previously[7,81 to thermally stable carboxylocarbonate structures.

The increase in absorption caused by oxidation (Fig. 1) in the range of 1700, 1600 and 820cm-’ and the for- mation of new absorption bands at 1640, 1560, 1530 and 1330cm-’ indicate formation of surface oxygen struc- tures and of structures containing nitrogen-oxygen bonds. These oxygen structures are probably iono-radi- cai structures [17] C-O absorbing within the 1600 cm-’ range and groups C=O absorbing at 17OOcm-‘. The car- bonaceous aromatic structures reacting with HNO, form nitro-groups absorbing at 1530 cm-’ (v..NOJ and 1330 cm-’ (v,NO~). According to Ubbelohde [31 the car- bon-HNOX reaction at room temperature leads to the formation of surface nitrate structures. Surface organic structures -O-NO* form the absorption band at 1640 cm-’ (v..NO,) and the broad band at 1250 cm-’ (v,NO*) in the spectrum of the oxidized car- bonizate. The surface nitrate complexes[l8] give the band V~ of NO,- ion at 1560 cm-’ (v.J *and the band at 1330 cm-’ (vJ that overlaps with the band of nitro

groups. The intensity increase of the band at 820cm-’ (Fig. 1) is caused by the above mentioned structures containing nitro-oxygen bonds.

The formed surface structures do not decompose dur- ing the desorption at room temperature (Fig. 2). A partial decomposition of these structures takes place when desorbed at 200°C. Then, the bands at 1640 and l%Ocm-’ decay and the absorption in the range of 1200-1400 and around 820 cm-’ decreases. These obser- ved spectral changes (Fig. 2) show that organic nitrates and surface nitrate complexes are the relatively least stable. The nitro compounds decompose when desorbed at 400°C. The disappearance of the bands at 1530 and 1330 cm-’ can be seen in the spectrum 3, Fig. 2.

The thermal decomposition of surface structures con- taining nitrogen-oxygen bonds is accompanied by in- tramolecular rearrangements with the formation of car- bony1 groups. An increase in absorption at 17OOcm-’ is connected with these rearrangements.

The results of IR investigations of the reaction of HNO, with the carbonaceous film prepared from cel- lulose are presented in Fig. 3.

The oxidation with HNO, solution (spectrum 2 in Fig. 3) of the film prepared from cellulose leads to the for- mation of bonds having the same absorption maxima as those described in the case of the films prepared from polyfurfuryl alcohol (Fig. 1).

The oxidation of the film at 80°C (Fig. 3, spectrum 3) leads to an increase in the intensity of the absorption bands of nitro groups (1530 and 1330cm-‘). The for- mation of the band at 1665 cm-’ under these conditions indicates the presence of surface quinone groups. These structures are formed probably as a result of an in-

t

14

14

;tl

3600 3000 2400 2000 1700 1400 1100 800

-1

Fig. 3. IR spectra of a carbonaceous film prepared from cell:se and oxidized with HNO, solution. (1) cellulose carbonized in C@ at 400°C (1.23 m&m? after desorption for 1 hr at 600°C; (2) film oxidized with HN03 solution for 1 hr at room temperature; (3) film oxidized with HNOg solution for 1 hr at 80°C; (4) film reoxidii with HN09 solution for 1 hr at 1WC. Spectra l-4 were recorded in a vacuum cell with NaCl windows. Trans- mission scale expanded 1.43 times. Spectra 2-4 were recorded after water washing, drying and desorption of films at room

temperature.

IR spectroscopic investigations of the mechanism of oxidation 283

tramolecular rearrangement of iono-radical forms of chemisorbed oxygen. An absorption increase at 1720 cm-’ together with the parallel intensity increase of the band of OH stretching vibrations (35OOcm-‘) is caused by the formation of carboxyl structures. The density of these structures on the surface rises rapidly after oxidation of the same sample at 100°C (for the third time). There is a visible single intense band at 1730 cm-‘, in the range of C=O stretching vibrations (Fig. 3, spec- trum 4). The spectrum of the sample oxidized with HNO, solution at 100°C reveals the intense bands of nitro groups (1530 and 1330cm-‘). In this process the quinone structures undergo decomposition (the band at 1665 cm-’ disappears).

The results of IR investigations concerning the thermal stability of the surface functional groups formed by oxidation with HNO, solution are shown in Fig. 4.

The spectral changes caused by the desorption at 200°C of the film prepared from cellulose and oxidized with HNO, at room temperature (Fig. 4) are similar to those described for the film prepared from polyfurfuryl alcohol (Fig. 2). The compounds undergoing decom- position are organic nitrates and surface nitrate com- plexes. Iono-radical structures C=O probably undergo intramolecular rearrangements forming carbonyl groups. Similar rearrangements were observed previously[ 171 during the desorption at 200°C of the chemisorbed oxygen at room temperature on the carbonaceous film. The desorption at 200°C of the carbonic film oxidized with HNOs solution for 0.5 hr at 100°C (spectrum 5) causes the absorption to decrease in the range of O-H stretching vibrations relative to the sample desorbed at room temperature (spectrum 4). A decrease in the con- tent of hydroxyl groups in the carbonizate is accom- panied by changes in the range of C=O stretching vibra- tions. Instead of one single band at 1730cm-‘, three bands at 1830, 1770 and 174Ocm-’ are formed. The formation of a doublet of bands at 1830 and 1770cm-’ and an absorption increase at 900 and 730cm-’ are caused by the formation of cyclic anhydrides structures. During the desorption at 200°C (spectrum 5), a part of the quinone and nitro groups is undergoing a thermal decomposition or an intramolecular rearrangement and because of that the absorption at 1665, 1530 and 1330 cm-’ is decreasing. During the desorption at 200°C of the film oxidized twice at 100°C structures of cyclic anhydrides are formed. The density of anhydride struc-

/I 3600 3000 2400 2000 1700 1400 ii00 600

-1

Fii. 4. IR spectra of a carbonaceous film prepared from celluike and oxidized with HNO, solution. (1) cellulose carbonized in CO2 at WC (0.86mg/cm2) after desorption at 600°C; (2) the same film oxidized with HNOl solution for 0.5 hr at room temperature after desorption at room temperature; (3) after desorption at 200°C; (4) film oxidized for 0.5 hr at loo” and desorbed at room temperature; (5) desorkd at 200°C; (6) film reoxidized for 0.5 hr (1 hr altogether) at 100°C and desorbed at room temperature; (7) desorbed at 200°C. Spectra l-7 were recorded in vacuum with

transmission scale expanded I. 11 times.

tures rises with the increase of the oxidation degree (spectra 5 and 7). The content of those structures depends on the number of carboxyl groups being formed during the oxidation and on their distribution on the surface. Only few nitro groups undergo decomposition during the desorption at 200°C (Fig. 4, spectra 6,7).

The gravimetric measurements of the process oxida- tion of carbons with HNO, solution were carried out together with IR examination (Fig. 4). The measurements were conducted with the film carbonized and activated under the same conditions as those for IR investigations. The results of the gravimetric measurements are given in Table 1.

The values listed in Table 1 show a summary gain in mass due to the formation of surface functional groups (assuming that the mass of an initial sample does not decrease as a result of formation of CO, and CO or other compounds soluble in the HNO, solution).

Table 1. The influence of oxidation with HNOJ solution on the carbonaceous film mass and on the nitrogen content in the film

Carbon oxidized with HNOs solution (IR spectra in Fig. 4)

Gain in mass (relative to a sample desorbed at

600°C) after desorption at Room temp. 200°C

Nitrogen content after desorption at

Room temp. 200°C

For 0.5 hr at room temperature (spectra 2 and 3)

For 0.5 hr at 100°C (spectra 4 and 5)

For 1 hr at 100°C (spectra 6 and 7)

6.9 3.9 0.7 0.5

20.8 11.3 - -

28.8 16.5 1.6 1.4

284 J. ZAWADZKI

The comparison between the gain in mass of a sample result of oxidation, the transmittance of the IR radiation shown in Table 1 and the nitrogen content for the film increases in the long wave range of the spectrum (below oxidized with HNOS solution at room temperature, lOOOcm-‘). As the oxidation degree rises, the trans- shows a considerable contribution of oxygen groups (not mittance of IR radiation increases in the range of shorter containing nitrogen) to the total gain in mass. These waves. The IR spectrum reveals the absorption bands of groups are probably iono-radical structures C”o being formed surface functional groups. The absence of bands an intermediate stage in the process of formation of C=O attributed to the presence of aromatic hydrogen (bands groups (carboxyl) strongly bonded to the surface. The of C-H stretching vibrations and bands of C-H defor- adjoining iono-radical C”0 groups formed as a result of mation out-of-plane vibrations) indicates both a thermal oxygen chemisorption can form resonance structures decomposition of C-H structures and a high degree of which are shown below. substitution of aromatic rings by oxygen groups. The

I

Since the values of force constants and the lengths of bonds are probably close to those for the COO- structure, these structures can cause an absorption increase observed in Figs. l-3 within the 1600 and 135fL13OOcm-’ ranges.

surface oxygen groups show their absorption maxima at 3600, 1730, 1620 and 126Ocm-‘. The intense and broad band of O-H stretching vibration, as well as the intense

A rise in the carbonization temperature causes both a shift of continuous absorption limit in the direction of longer waves and an increase of a background level related to the free electron absorption. This phenomenon is the result [ 191 of a decrease in energy gap between the filled and the conduction r-bands. As it was shown previously[l7], the oxygen chemisorption on the car- bonic film causes a decrease in the background level and a shift of continuous absorption limit in the direction of shorter waves.

IR spectroscopic investigations of the carbonaceous films oxidation with HNOs solution give some infor- mation about the interactions of 7~ electrons with surface functional groups formed on the carbons. Oxidation with HN03 solution at room temperature (Figs. 1, 3 and 4), causes an increase in the background level (especially in the short wave range of a spectrum). The surface func- tional groups formed in this process can be both ac- ceptors and donors of electrons. A strong decrease in the background levels (Figs. 3 and 4) and a parallel increase in the sample mass (Table 1) result from oxidation with the HNO, solution at 100°C. The formation of intense ab- sorption bands in the range of C=O stretching vibrations (anhydrides, lactones and carboxyl groups) occurs in oxidation. The functional groups absorbing in this region, when desorbed at 2OO”C, undergo a partial decomposition and an intramolecular rearrangement (Fig. 4) causing an increase of the background level (especially in the short wave range of the spectrum).

\ 36OU 3000 2400 2000 1700 1400 1100 600

cm-'

Fig. 5. IR spectra of a carbonaceous film prepared from cellulose and carbonized at 800°C and oxidized with HN03 solution. (1) cellulose carbonized in CO2 at 4OOY (0.73 n&m’) desorbed for 0.5 hr at 800°C and oxidized with HNO, solution for 0.5 hr at 100°C; (2) film reoxidized at 100°C (for I.5 hr altogether); (3) film oxidized for the thud time at 100°C (for 2.5 hr altogether). Spec- tra l-3 were recorded in air after water washing and 70°C drying.

The decrease in the absorption coefficient is parti- cularly well visible during the oxidation of films car- bonized at MO“C.

The results of spectral studies of the process of oxida- tion of the carbonaceous film desorbed previously at 800°C and then oxidized with HNO, solution are presen- ted in Figs. 5 and 6.

Fig. 6. IR spectra of a carbonaceous film prepared from celluike carbonized at 800°C and oxidized with HNO? solution. (1) film oxidized with HNOs solution for 2.4hr at- IOCPC. Spectrum recorded in air for the tilm (Fig. 5) placed in the vacuum cell; (2) desorption at room temperature; (3) desorption at 2OPC. Spectra l-3 were recorded in a vacuum cell with NaCl windows. Trans- . . . .._.

The transmittance for the carbonaceous film desorbed at 800°C is 0% in the whole spectral range studied. As a

Transmission scale expanded 2 times.

II I I I I I I I I I II_8 I1,,,,,,,,~

3600 3000 2400 2000 J700 1400 1100 600

-1

mission scale expanded 1.6 times.

IR spectroscopic investigations of the mechanism of oxidation 285

bands at 1730cm-’ (C=O stretching vibrations) and at 1260 cm-’ (C-O stretching vibrations) indicate the presence of carboxyl structures. The desorption at room temperature (Fig. 6) decreases the intensity of the broad band of O-H stretching vibrations. The absorption max- imum of this band shifts to the region of 3350 cm-‘. The intensity of this band decreases after the desorption of water forming strong hydrogen bonds with surface oxides (those bonds give, among others, the band at 26OOcm-‘). The broad absorption band in the range of wave numbers below 37OOcm-’ is probably caused by the formation of hydrogen bonds of type (H20. . . H . . . OH*)’ with protons of acidic groups. As a result of desorption at room temperature, the intensity of the band of C=O vibrations at 1750cm-’ (carboxyl structures and lactones) increases. The formation of the bands at 1180cm- (C-O stretching vibrations) and at 1340 cm-’ (OH deformation vibrations) indicates the presence of phenol structures.

After the desorption at 200°C (Fig. 6) absorption coefficient of the film increases (especially in the short wave region of a spectrum). The desorption at higher temperature causes both a partial thermal decomposition of surface functional groups and their intramolecular rearrangements. As a result of dehydration of two neighbouring carboxyl groups, structures of cyclic anhydrides are formed. Their presence is shown by the formation of a doublet of bands at 1840 and 1770cm-’ and the formation of the bands at 900 and 730 cm-‘.

The spectrum of the carbonaceous film carbonized at 800°C and oxidized with HNO, solution does not reveal the absorption bands of nitro groups. This probably stems from a different chemical structure (in comparison with the films carbonized at 600°C) and from stronger oxidation of the film with HN09 solution.

The presented results of IR spectroscopic in- vestigations show that with the increase in the degree of oxidation one can observe not only variations in the density of various surface functional groups but, what is even more important, the changes in their specific chem- ical character.

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14.

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16. 17. 18.

19.

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