EPR Investigations of the Effects of Inorganic Additives on the

Energy & Fuels 2004, 18, 1049-1065

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EPR Investigations of the Effects of Inorganic Additives on the Charring and Char/Air Interactions of Cellulose Ji-Wen Feng, Shaokuan Zheng, and Gary E. Maciel* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received August 13, 2003

EPR (electron paramagnetic resonance) behaviors of cellulose chars during exposure to air at room temperature strongly depend on the identities of any inorganic additives and on the charring temperature. The air exposure of low-temperature (300-400 °C) cellulose/alkali carbonate chars or alkali bicarbonate chars appears to involve two different chemical processes, the production and annihilation of free radicals. The “new” radicals formed during air exposure have larger g-values than those generated by pyrolysis. In contrast, the corresponding low-temperature chars of pure cellulose and cellulose/NaCl show only an apparently small, slow decrease in radical concentration upon air exposure. For the high-temperature (>400 °C) chars of cellulose and cellulose/additive mixtures used in this study, air exposure is accompanied by a large reduction in apparent radical concentration. However, this decrease is found to depend dramatically on the presence of water in the exposure environment. Adding Na2CO3 into the cellulose not only causes a higher aliphatic-to-aromatic ratio of carbons in the char (assessed by 13C NMR), but also changes the distribution of functional groups. Carboxyl groups appear in the cellulose/Na2CO3 char prepared at 350 °C for 1 h, while in the corresponding pure-cellulose char and cellulose/ NaCl char they were not detected.

I. Introduction The catalytic effects of inorganic additives or indigenous inorganic material on the gasification of carbonaceous materials such as coal, char, graphite, etc., have been known for a long time.1-4 Recently, the effects of inorganic additives on pyrolysis, gasification, and combustion of biomass, such as cellulose, wood, agricultural residues, and municipal solid wastes, have received much attention.4-8 The implications of catalytic gasification have practical importance in the conversion of carbonaceous solid materials to gaseous or chemical feedstocks and in the yield and composition of volatile components in cigarette smoke. In the past, most of the research on catalytic gasification was focused on investigations of the efficiency of various inorganic salts, metals, and minerals. There is now a consensus that the most effective catalysts are alkali and alkaline earth metal salts, metallic nickel, and iron oxides.1,3 So far, it has been shown that inorganic additives can promote or inhibit the pyrolysis, gasification, and combustion of cellulosic materials. * Author to whom correspondence should be addressed. Tel: 970491-6480. Fax: 970-491-1801. E-mail: [email protected]. (1) Wood, B. J.; Sancier, K. M. Catal. Rev.sSci. Eng. 1984, 26, 233279, and references therein. (2) McKee, D. W. Chem. Phys. Carbon 1981, 16, 1-118; Mckee, D. W. Fuel 1983, 62, 170-175. (3) Wen, W. Y. Catal. Rev.sSci. Eng. 1980, 22, 1-28. (4) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; John Wiley & Sons Inc.: New York, 1988; pp 187-193. (5) Kanan, M. P.; Richards, G. N. Fuel 1990, 69, 747-753; Devi, T. G.; Kanan, M. P. Fuel 1998, 77, 1825-1830. (6) DeGroot, W. F.; Shafizadeh, F. Carbon 1983, 21, 61-67. (7) Yokoyama, S.; Ogi, T.; Koguchi, K.; Nakamura, E. Chem. Lett. 1983, 151-176. (8) Morterra, C.; Low, M. J. D. Carbon 1985, 23, 335-341.

The formation and decomposition of surface oxidation products are believed to be important processes during the gasification of carbonaceous materials and biomass. The formation of surface oxidation products on carbonized materials is presumably a significant factor in controlling the gasification and combustion of a solid and it is believed to be the initial step leading to formation of the ultimate gaseous products.6 Many investigations of chemisorption on carbonaceous materials have been made, and a large amount of information about the structures of surface structures on carbonaceous solids has been obtained by direct or indirect methods.9,10 The main surface functional groups are believed to be carboxyl groups, phenolic groups, lactone moieties, and quinone-like carbonyls. However, the precise mechanisms for surface oxidation reactions and catalytic gasification are not clear. Several possible mechanisms have been proposed to explain the roles of catalysts on gasification of carbonaceous materials and biomass. These include the following: (1) redox oxygen transfer by catalysts (this mechanism, in which the catalyst is alternately reduced by carbon and oxidized by the gaseous reactant, has been widely accepted); (2) topographical changes induced by the presence of inorganic species; and (3) changes in chemical functional groups on the material surface due to the interaction with the inorganic species during the pyrolysis or gasification. Probing active sites and intermediates during oxidation or gasification may be very helpful for understand(9) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; John Wiley & Sons Inc.: New York, 1988; pp 86-173. (10) Bansal, R. C.; Donnet, J. B. Carbon Black; Marcel Dekker Inc., New York, 1993; pp 67-88.

10.1021/ef030151y CCC: $27.50 © 2004 American Chemical Society Published on Web 06/15/2004

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ing mechanisms of catalytic gasification. Various experimental methods, such as titration reactions, infrared (IR), nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), etc., have been used to study the structures of surface oxidation products or surface intermediates on carbonaceous materials. EPR may be of special interest for studying the charring and char oxidation of carbonaceous materials because these systems usually contain free radicals. Such free radical centers are potentially highly active chemically and they are candidates for involvement in a variety of chemical processes, including oxidation and gasification. Since carbonaceous materials usually have large surface areas, it may be possible to probe the microscopic environment around unpaired spins near/ on the surface, so EPR might seem promising for providing valuable information on chemical processes occurring on these surfaces. In principle (depending on concentrations and lifetimes), EPR can be used to study free-radical intermediates directly during the oxidation or gasification reactions of carbonaceous materials. Oxygen-centered free radicals have been found as intermediates during the reaction of polycyclic aromatic hydrocarbons with oxygen in solution,11 and EPR may be well suited to study intermediates in catalysis. Oxidation-reduction cycles have been proposed to explain the catalytic behavior of alkali metal carbonates involved as intermediates in the formation of peroxides.2 Numerous carbonaceous materials have been studied by EPR. These include coal, coke, carbon black, graphite, and char from various types of biomass or organic polymers.12,13 However, only a few EPR studies on oxidation or gasification of carbonaeous materials with catalysts have been reported. Wood and Sancier1 and Sancier14 found an irreversible increase in line width of the electron spin resonance due to free radicals in carbon black, when mixtures of certain salts and carbon black were heated in helium to temperatures above 600 K. This was considered to be indicative of a strong interaction between the carbon material and the catalyst. However, in their investigation, KCl, which is ineffective as a gasification catalyst, generates a larger increase in EPR line width than does K2CO3. DeGroot and Shafizadeh found that radical concentrations and oxidation behavior do not differ significantly between the chars prepared by heating pure cellulose and cellulose mixtures with inorganic additives (NaCl, Na2B4O7, and H3BO3) at various temperatures.6 They concluded that combustion behavior cannot be explained strictly in terms of changes in free radical concentration. However, we note that the inorganic additives (NaCl, Na2B4O7, and H3BO3) used in their study are largely ineffective catalysts for gasification. In the work described in this paper, we investigated the charring (pyrolysis under He) of cellulose mixtures with various inorganic additions and the air-exposure behaviors of their chars. We have found that addition (11) Lewis, I. C.; Singer, L. S. J. Phys. Chem. 1981, 85, 354-360. (12) Lewis, I. C.; Singer, L. S. Chem. Phys. Carbon 1981, 17, 1-88. (13) Singer, L. S. Proc. Conf. Carbon, 5th, University Park, PA, 1961, 1963; pp 37-64. (14) Sancier, K. M. Fuel 1983, 62, 331-335; Sancier, K. M. Fuel 1984, 63, 679-685.

Feng et al. Table 1. Content of Inorganic Additive in Cellulose Mixtures sample additive wt % mmol/g (cellulose)

Na2cela Na2CO3 6.7 0.63

NaHcela NaHCO3 10 1.2

K2cel K2CO3 5.2 0.37

NaClcela NaCl 6.9 1.2

Li2cel Li2CO3 8.9 1.2

a Na content of 2.7 wt % is constant in the cellulose mixtures with Na2CO3, NaHCO3, or NaCl.

of alkali carbonates dramatically changes the airexposure behaviors of cellulose-derived chars and that the presence or absence of H2O in the flowing gas is important. The chars produced at low pyrolysis temperatures show a pronounced initial increase in free radical concentration when exposed to air or molecular oxygen at room temperature, indicating that free radicals are formed as intermediates during air exposure of cellulose chars. Our results may be helpful for understanding the complicated EPR-observed oxidation behaviors in various carbonaceous materials that usually contain inorganic constituents. For example, an increase of EPR intensity was also observed in fresh Victorian brown coal when it was exposed to air,15 which may be related to the effect of indigenous inorganic constituents. II. Experimental Section Avicel cellulose was obtained from FMC International. Cellulose mixtures with various inorganic additives (see Table 1), NaHCO3, Na2CO3, Li2CO3, K2CO3, and NaCl, were prepared as follows: 1 g of cellulose powder was dispersed in 40 mL of dilute aqueous Na2CO3 (NaHCO3, Li2CO3, K2CO3, NaCl) solution with concentrations of 1.8 mg/mL; these mixtures were then dried under stirring in air for 24-48 h at room temperature. Table 1 summarizes the compositions of the cellulose/additive mixtures in terms of the wt % of the inorganic content. These cellulose mixtures were each charred for 1 h at various high temperatures (300 to 600 °C) in a flowing helium atmosphere and then cooled to room temperature under flowing helium; the chars thus obtained were transferred to an EPR tube under N2 without any air exposure. In-situ EPR measurements of air-exposure, O2-exposure, or H2O/O2-exposure effects at room temperature were carried out by exposing a char to air, to a mixture of O2 (6 vol %) and He, or to a H2O-saturated O2-He mixture. X-band EPR measurements were made at room temperature using a Bruker EMX-200 spectrometer with dual cavities. Except for saturation experiments, a microwave power of 0.2 mW was employed to avoid power saturation effects. The concentrations and Lande g-values of the detected free radicals were calculated by comparison with a sample of 2,2-diphenylpicrylhydrazyl (DPPH). The g-value of DPPH was taken as 2.0036.16 The EPR measurements on any specific sample were highly reproducible, i.e., with (3% for intensities, line widths, or g-values in repetitions. When a specific type of sample was re-prepared and re-measured, reproducibility was found to be within 10 percent, except for samples prepared by exposure to air. In those cases, variations up to 30% were sometimes encountered among repetitive preparations, presumably reflecting variations in the composition (e.g., humidity) of air.

III. Results 1. General EPR Features of Cellulose-Mixture Chars (no air exposure). (a) Line Shape and Line Width. Unexposed chars of cellulose or cellulose mix(15) Dack, S. W.; Hobday, M. D.; Smith, T. D.; Pilbrow, J. R. Fuel 1984, 63, 39-42. (16) Wertz, J. E. Chem. Rev. 1955, 55, 829-955.

Charring and Char/Air Interactions of Cellulose

Figure 1. EPR line shape for two typical cellulose/Na2CO3 (6.7% Na2CO3) chars at the two indicated temperatures. See text for the definitions of terms.

tures with various inorganic additives show EPR behaviors that are overall similar for the collection of samples of this study, in terms of line shape, line width, g-value and variation in radical concentration with variation in the pyrolysis temperature or pyrolysis time. EPR spectra (microwave power absorption) of chars prepared with charring temperatures >400 °C show a typical Lorentzian line shape (Figure 1), while the spectra of chars prepared at low temperatures (T e 400 °C) cannot be described by only a single Lorentzian or single Gaussian component. To analyze the line shape variation with charring temperature, we used a linear combination of two components, Lorentzian and Gaussian, to simulate the experimental data, as detailed in eq 1:

f(x) ) a{c(1/{1 + [(x - x0)/b] 2}) + (1 - c) exp{-0.5[(x - x0)/d]2}} (1) where c represents the fraction of a Lorentzian component; 1 - c is the fraction of a Gaussian component, and b (or d) is the line width parameter of a Lorentzian (or Gaussian) component. We have found that all of our experimental curves can be fitted well by this model (for example, see Figure 1). Variation with charring temperature of the Lorentzian component fraction, c, thus obtained for the various chars is shown in Figure 2(a). From this figure, it is clear that the Gaussian component is a major contribution to the spectra for lowtemperature chars. The Lorentzian component fraction c increases steadily with increasing charring temperature below 450 °C, e.g., from c ) 0.35 for a 300 °C/1 h cellulose/Na2CO3 char to c ) 1 (pure Lorentzian line shape) for the corresponding 450 °C/1 h char. Also, with increasing charring temperature, the peak-to-peak EPR line width (from the derivative of the power absorption spectrum) decreases first and then increases. A minimum of 0.7 G in the peak-to-peak line width appears

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Figure 2. Variations of EPR parameters with charring temperature for cellulose/Na2CO3 (6.7% Na2CO3) char (no air exposure). Charring temperatures indicated.

around a charring temperature of 500-550 °C for the cellulose/Na2CO3 char (Figure 2). (b) Free Radical Concentration and g-Value. The dependences of free radical concentration and g-value on pyrolysis temperature for a cellulose/Na2CO3 char are also shown in Figure 2(a). The free radical concentration increases as the temperature is increased from 300 to 550 °C, and then decreases for a charring temperature of 600 °C. A maximum spin concentration of 1.6 × 1020 spins/g can be found at ∼550 °C, which also corresponds approximately to the temperature of minimum line width. On the other hand, the g-value of the char decreases from 300 to 500 °C and stays nearly constant above 500 °C; g ) 2.0038 at 300 °C and g ≈ 2.0027 for T g 500 °C. The latter value is only slightly larger than the free-electron g-value, 2.0023. (c) Microwave Power Saturation. The effects of microwave power saturation have been examined for chars of various cellulose-additive mixtures. Some typical results for a cellulose/Na2CO3 char are shown in Figure 3. It is evident that a low-temperature char (for example, the 300 °C/1 h cellulose/Na2CO3 char) shows saturation behavior, i.e., a nonlinear intensity dependence on P1/2, where P is the microwave power. As seen in Figure 3, lowering the pyrolysis temperature gives rise to a lower saturation power. Over the range of microwave powers employed in these saturation experiments, the EPR line width does not change with microwave power. Saturation behavior is not observed over the entire microwave power range examined for high-temperature chars (Figure 3(a)). Similar behaviors have also been found in our experiments on cellulose and tobacco chars.17 (d) Effects of Additive Content on EPR Behaviors of Cellulose/Additive Chars. To understand the effects of (17) Feng, J.-W.; Zheng, S. K.; Maciel, G. E. Energy Fuels 2004, 18, 560-568.


Feng et al.

Figure 5. Char yields obtained by heating cellulose/Na2CO3 vs Na2CO3 content. Table 2. Char Yield Obtained by Heating Various Samples at 350 °C for 1 h sample pure cellulose Li2cel Na2cel NaHcel K2cel NaClcel residue 11 33 31 29 30 24 (wt %)

Figure 3. EPR saturation behavior of cellulose/Na2CO3 (6.7% Na2CO3, no air exposure) chars at the indicated charring temperatures.

Figure 4. Variations of (a) radical concentration (b) and (b) EPR line width (9) with Na2CO3 content for 350 °C/1 h cellulose/Na2CO3 chars.

additives on cellulose/additive pyrolysis, we investigated by EPR a series of cellulose/Na2CO3 chars (350 °C/1 h) with different Na2CO3 contents. Figures 4(a) and (b) show pronounced variations in the free radical concentration and EPR line width, respectively, of cellulose/ Na2CO3 chars (unexposed) with Na2CO3 content. When the Na2CO3 content is low (e1.6 wt %), increasing the Na2CO3 content in cellulose/Na2CO3 increases the radical concentration in the char. In contrast, adding Na2CO3 at high content (g2.4 wt %) into cellulose produces chars of low radical concentration. The chars from cellulose/Na2CO3 exhibit larger EPR line widths than those of the corresponding chars from pure cellulose. These results show that a strong interaction between cellulose and Na2CO3 exists during pyrolysis. 2. Weight Loss Measurements. The effects of inorganic additives on cellulose pyrolysis are also revealed by weight loss measurement. Figure 5 shows plots of the char yield against Na2CO3 content in 350 °C/1 h cellulose/Na2CO3 chars. It is clear that the char

Table 3. Char Yield of Na2Cel and Pure Cellulose under Different Heating Temperatures (for 1 h) 300 °C 350 °C 400 °C 450 °C 500 °C 550 °C Na2cel 41 31 28 27 23 20 (residue wt %) pure cellulose 11 8.4 (residue wt %)

yields increase steadily with sodium carbonate content. Table 2 shows the char yields obtained by heating cellulose, unmixed or mixed with various inorganic additives (NaHCO3, Na2CO3, Li2CO3, K2CO3, NaCl), at 350 °C (under He). From this table we can see that alkali metal carbonate (or bicarbonate) additives (NaHCO3, Na2CO3, Li2CO3, and K2CO3) enhance the char yield substantially. NaCl was found to be the least effective at enhancing char yield. Table 3 summarizes char yields for unmixed cellulose and cellulose/Na2CO3 at various charring temperatures. Again, one sees that the char yield is larger when the inorganic additive Na2CO3 is present. 3. EPR Investigations on the Air Exposure of Cellulose-Mixture Chars. We have found that the EPR behavior of cellulose chars during exposure to air (or O2) at room temperature strongly depends on the type of any inorganic additive present. Some lowtemperature cellulose chars, pectin chars, and tobacco chars show sensitive air-exposure EPR behavior, while others do not.17 Also, different charring temperatures lead to completely different EPR variations when these chars are exposed to air. In the study reported here, the roles of certain inorganic additives (Na2CO3, NaHCO3, K2CO3, Li2CO3, NaCl, and calcium acetate) in cellulose pyrolysis and char/air behavior were examined. Except for NaCl, all these additives are regarded in the literature as effective catalysts for gasification or combustion.1 (a) Effects of Various Inorganic Additives on EPR Behavior of Low-Temperature Chars during Air Exposure. Figures 6A and 6B show variation of the EPRmeasured free radical concentration, EPR line width, and g-value of 350 °C/1 h chars of pure cellulose and of



Figure 6. Effects of air exposure on EPR characteristics of 350 °C/1 h chars prepared from (A) pure cellulose and (B) cellulose/ Na2CO3 (6.7%).

Figure 7. Effects of air exposure on EPR characteristics of 350 °C/1 h chars prepared from cellulose mixtures with 5.2% K2CO3 and 8.9% Li2CO3.

a cellulose/Na2CO3 (6.7%) mixture under exposure to air. Figure 7 shows analogous results for chars prepared from mixtures with two additional carbonates, Li2CO3 and K2CO3. Figure 8 presents corresponding airexposure EPR results on chars prepared from cellulose/

NaHCO3 and cellulose/NaCl mixtures. Figure 9 shows air-exposure EPR results on 350 °C/1 h cellulose/Na2CO3 chars with various Na2CO3 contents. The unpaired electron spin concentration of the unexposed cellulose/NaCl char is 2.8 × 1019 spins/g,


Feng et al.

Figure 8. Effects of air exposure on EPR characteristics for cellulose/NaHCO3(10.0%), cellulose/NaCl (6.9%), and pure-cellulose chars (350 °C/1 h).

which is about 20-30% higher than for the corresponding pure-cellulose or cellulose/NaHCO3 char. From Figures 6-9 it can be seen that for different chars the free radical concentration, line width, and g-value of different chars behave differently with increasing air exposure time. For the pure-cellulose char and cellulose/ NaCl mixture char, the EPR is largely insensitive to air exposure; only a small decrease in free radical concentration is observed as a result of air exposure, and the line width and g-value are almost constant. In contrast, the free radical concentration in the cellulose/NaHCO3 char increases rapidly in the initial air-exposure period, followed by a very slow decrease. The maximum unpaired electron spin concentration is about twice the initial spin concentration. From the EPR-reported air-exposure behaviors observed for 350 °C/1 h chars of cellulose mixtures with other inorganic components (including Na2CO3, K2CO3, Li2CO3, and CaAc), shown partially in Figures 6-9, the following patterns are observed: compared to purecellulose char, which is apparently insensitive to air exposure, chars of cellulose mixed with alkali metal carbonates or bicarbonates (Na2CO3, NaHCO3, K2CO3, Li2CO3), as well as with CaAc, show very high EPR sensitivity to exposure to air. For the chars of these mixtures, the free radical concentrations, EPR line

widths, and g-values increase rapidly during the initial air exposure period. On further air exposure, the free radical concentration of all the above samples decreases slowly; correspondingly, a small line narrowing is observed. However, during this gradual process, the g-values do not change. EPR study of the room-temperature exposure of a 350 °C/1 h cellulose/Na2CO3 char in an atmosphere of pure O2 was also made; it was found (not shown here) that the EPR behaviors of char exposure in air and in O2 are qualitatively similar. As expected for an oxidation process, the initial increase in free radical concentration was more abrupt in pure O2 than in air, but the subsequent slow decrease in radical concentration was not accelerated by the increase in O2 concentration. This behavior is rationalized below in terms of the role of moisture in these experiments. Free radical decay due to air exposure can last more than a week, as seen in Figure 6. Similar phenomena have also been observed in our experiments on pectin char and tobacco char.17 The former contains 2 wt % Na and the later contains comparable amounts of various inorganic components, such as compounds of Ca, K, and Mg;18 both K and Ca compounds are regarded as active catalysts.1 NaCl, although it also contains the metal ion Na+, is considered an ineffective gasification



Figure 9. EPR variations with air exposure time for 350 °C/1 h cellulose/Na2CO3 chars with different Na2CO3 contents.

catalyst. No substantial EPR changes were revealed in this study during air exposure of the char prepared by heating cellulose/NaCl at 350 °C for 1 h, as seen in Figure 8. In Figure 9, one sees qualitatively similar EPR behaviors (radical concentration, line width ,and gvalue) during air exposure for all cellulose/Na2CO3 chars, with quantitative differences. As mentioned above, short-time air exposure of cellulose/Na2CO3 char increases the free radical concentration, i.e., produces “new” radicals. From Figure 9 we can see that the concentration of “new” radicals formed during air exposure, defined as the maximum increase, δc ) cmax cpyro, in radical concentration, is closely related to the Na2CO3 content. Here cmax corresponds to the maximum radical concentration measured during air exposure and cpyro represents the radical concentration resulting directly from pyrolysis (no air exposure). The dependence of the “new” free radical concentration on Na2CO3 content is more clearly shown in Figure 10. Obviously, The “new” free radical concentration increases with increasing Na2CO3 content and then becomes saturated at high Na2CO3 content (x g 3.3 wt %) . (18) Bokelman, G. H.; Ryan, William S., Jr. Beitr. Tabak. Int. 1985, 13, 29-36.

Figure 10. New radical concentration (produced by air exposure) and char yield vs Na2CO3 content for 350 °C/1 h cellulose/Na2CO3 chars.

(b) Dependence on Pyrolysis Temperature of the Effects of Air Exposure on the EPR Characteristics. The above sections present EPR results primarily on cellulosemixture chars prepared at only one specific charring temperature, 350 °C. In addition, a systematic EPR investigation of the dependence of air exposure on charring temperature was carried out on the cellulose/


Feng et al.

Figure 11. EPR variation with air-exposure time for cellulose/Na2CO3 (6.7%) chars obtained for various low charring temperatures, as indicated.

Na2CO3 system. For cellulose/Na2CO3 chars obtained in 1 h at low pyrolysis temperatures (300, 350, and 400 °C), the following air-exposure results were found: EPR measurements on the effects of air exposure on these chars show an initial rapid increase of free radical concentration. Correspondingly, the EPR line width and g-value also increase rapidly. On further air exposure, the free radical concentration decreases slowly, accompanied by a slow and small decrease in line width, but the g-value does not change during this slow process. These results are summarized in Figure 11. It can also be noted that the increase of free radical concentration for the 400 °C/1 h cellulose/Na2CO3 char lasts for only several tens of minutes, while corresponding increases for the 350 °C/1 h chars persist for a thousand minutes or longer. To verify that the observed increases in EPR intensity upon air exposure are due to increased free radical concentrations and not to microwave power saturation effects (e.g., the presence of paramagnetic O2, enhancing relaxation of free radicals on the char surface), EPR power saturation experiments were carried out on the cellulose/Na2CO3 sample. It was demonstrated that the microwave power of 0.2 mW used in the experiments reported above is well below the saturation power. Thus, the air-exposure results imply that chars with higher

pyrolysis temperature, but e 400 °C, are more readily modified and that this modification involves, or is accompanied by, formation of new free radicals. Figure 12(a) shows two EPR (power) absorption spectra, taken before and after air exposure (for 800 min) of the 350 °C/1 h cellulose/Na2CO3 char; the corresponding difference spectrum is shown in Figure 12(b). One can note that the broadening and substantial enhancement of EPR peak intensity due to air exposure occur mainly in the low-field (high-g) side. This implies that the newly formed free radicals have a larger g-value than those produced directly by pyrolysis. From the difference spectrum, which is mainly due to the contribution of “new” free radicals, a g-value of 2.0043 is obtained. The EPR line shape of the new free radicals is accounted for by a mixture of 38% Lorentzian and 62% Gaussian, compared to roughly half and half before air exposure. For all of the cellulose/Na2CO3 (and pectin)17 chars obtained with pyrolysis temperatures in the range of 250 to 400 °C, the “new radical” g-values obtained by this difference spectrum method are found to be in the range of 2.0030 to 2.0048. This range of g-values is consistent with those of small, oxygen-containing aromatic radicals, such as those derived from anthracene, 1,2-benzpyrene,11 and semiquinones.19



Figure 12. (a) EPR spectra taken before and after 800-min air exposure for 350 °C/1 h cellulose/Na2CO3 (6.7%) char, and (b) difference spectrum from (a).

The results presented above suggest that the free radicals produced during the initial air exposure of the chars are oxygen-centered free radicals and that the airexposure behavior of low-temperature chars is due to two chemical processes: formation and annihilation of free radicals. This hypothesis is also consistent with NMR measurements, which are discussed below. The nature of these phenomena was investigated by EPR experiments on oxygen desorption of oxidized chars. For example (Figure 13), after a 350 °C/1 h cellulose/Na2CO3 char was exposed to air for a long time (65 days), it was evacuated (

EPR Investigations of the Effects of Inorganic Additives on the

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