CHARACTERIZATION OF OXYGEN-CONTAINING ... - Science Direct

Curbon, Vol. 31. No. I,pp. Printed in Great Bntan.

109-121.

0008-6223193 $6.00 + .OO Copyright 0 1993 Pergamon Press Ltd

1993

CHARACTERIZATION OF OXYGEN-CONTAINING SURFACE COMPLEXES CREATED ON A MICROPOROUS CARBON BY AIR AND NITRIC ACID TREATMENT YOSHINOBU OTAKE* and ROBERT G. JENKINS? Fuel Science Program, Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, U.S.A. (Received 5 December 199 1,accepted in revised,fbrrn 8 July 1992) Abstract-The nature and extent of oxygen functional groups present on the surface of a microporous carbon has been studied. The carbon was prepared by the carbonization ofa porous phenol-formaldehyde resin in NZ at I273 K. Following preparation, samples of the carbon were carefully oxidized in either air or concentrated HN03 at various temperatures and times to form oxygen complexes. The complexes were characterized by use ofboth a linear temperature programmed desorption (LTPD) technique and selective neutralization. Four characteristic groups of oxygen complexes are defined on the basis of the decomposition product gases and the thermal stabilities of the complex. Detailed studies are made of the nature and structure of the acidic, COz-evolving surface complexes. Key Words-Carbon

surfaces, oxygen surface functionality, neutralization, LTPD.

1. INTRODUCTION

The surfaces of carbons can be characterized by their ability to chemisorb oxygen at low temperatures (e.g., near room temperature), resulting in the formation of oxygen-containing complexes of varying thermal stability. It is well established that the chemisorption of oxygen is strongly dependent on the crystalline nature of the carbon. That is, under the same conditions, well-ordered carbons that contain a relatively small concentration of edge sites will chemisorb less oxygen than more amorphous carbons, such as activated carbons. Once oxygen is chemisorbed it can only be removed as an oxide of carbon (CO and COJ at temperatures generally above 500 K. Complete removal of the chemisorbed oxygen requires temperatures as high as 1250 K[ 11. The presence of oxygen surface complexes influences the surface behavior of carbons to a great extent. As examples, the wettability[2] and adsorptive behavior of a carbon[3-51, as well as its catalytic[6], and its electrical properties[7,8], are influenced by the nature and extent of the complexes. Analyses of the oxygen functionality have been performed by a wide range of specific chemical reactions and spectroscopic and potentiometric methods[5]. However, it should be noted that different investigators, using differing methods, often report conflicting results, which is probably due to variations in carbon crystallinity and location of the complex on the carbon’s surface. It needs to be emphasized that carbon-oxygen surface complexes are unlikely to behave as their analogs in simple organic

*Current address: Osaka Gas Co., Osaka, Japan. tCurrent address: Department of Chemical Engineering, University of Cincinnati, OH, U.S.A.

compounds. It is also important to recognize that one is dealing with energetically diverse systems and, thus, observing distributions of sites rather than unique entities. The specific purpose of this study was to characterize the carbon-oxygen functionalities created on a microporous carbon’s surface by air and HNO,-oxidation. The methods selected for characterization combined selective neutralization and linear temperature programmed desorption (LTPD). Through the application of these methods, it was anticipated that differences between the two types of oxidation would be discerned. 2. EXPERIMENTAL 2.1

Materials

A proprietary porous phenol-formaldehyde resin (PF-polymer) was used as the carbon precursor throughout this study. The polymer was specifically selected in order that it would yield a carbon with extensive porosity in the meso- and microporous ranges. The PF-polymer was heated in a flowing stream of N? at 10 K/min up to 1273 K. It was held at maximum temperature for 1 h before cooling slowly in N2. The carbon produced was somewhat novel in that its total surface area (COz, 298 K, Dubinin-Radushkevich approach) was determined to be 890 m’/g and its mesopore area (from Hg-porosimetry and Nz desorption) was 55 m*/g[3]. This char was then used to prepare all other carbons utilized in this investigation. Two methods were used to form the oxygen complexes on the carbon’s surface. In one method, the char was oxidized, at varying temperatures (473-723 K) and for varying times (0.5-4 1 h) in flowing air in a small, fluidized bed reactor. The second type of oxidation was with concentrated HNO?

109

110

Y. OTAKE and R. G. JENKINS Table 1. Concentration

of CO2 and CO-yielding complexes on air and HN03-oxidized chars

Amount gasified (wW Air chars (temp, time) 473 K, 12 h 573 K, 0.5 h 573 K, 12 h 648 K, 0.5 h 648 K, 3 h 648K, 13h 648 K,41 h 673 K, 3 h 698 K, 3 h 698 K, 1I h 723 K, 0.8 h HNO, chars (temp, time) 320 K, 1 h 340K,O.l h 340 K, 0.25 h 340 K, 1 h 340 K, 5 h 340 K, 28.5 h 360 K, I h

Oxygen complex (mmol/g”) co2

co

Total

[01c0*/P1,

0.26 0.36 3.47 2.60 4.48 13.04 38.80 7.44 nd nd nd

0.13 0.13 0.28 0.22 0.40 0.58 0.96 0.51 0.79 1.49 0.98

0.43 0.49 1.28 I .08 2.17 3.52 5.25 3.35 4.94 7.68 5.89

0.56 0.62 1.56 1.30 2.57 4.10 6.21 3.86 5.73 9.17 6.87

0.38 0.35 0.30 0.29 0.27 0.25 0.27 0.26 0.24 0.28 0.24

3.3 3.8 4.6 4.9 5.4 6.1 5.5 6.6 6.3 5.2 6.0

nil nil nil nil nil nil nil

0.91 0.63 1.06 1.24 1.78 3.00 2.65

1.83 1.34 1.96 2.31 3.1 I 4.22 4.06

2.74 1.79 3.02 3.55 4.89 7.22 6.71

0.50 0.49 0.52 0.52 0.53 0.59 0.57

2.0 2.1 1.9 1.9 1.8 1.4 1.5

co/co*

aBased on weight of oxidized char. bFraction of oxygen as CO2 complexes.

nd = not determined.

for, again, different times (0.25-28.5 h) and temperatures (320-360 K). After HNOj treatment the samples were washed extensively in distilled water to constant pH. It should be noted that the HNOJ treatment did give rise to a small increase in the nitrogen content of the chars. For example, the original char contained 0.08 wt% nitrogen but the HNO,-oxidized char (at 340 K for 5 h) contained 0.65 wt%. As will be seen, this increase is small relative to the amount of chemisorbed oxygen on the HN03 char. All samples were dried under vacuum and stored over Drierite until further use. Full details of the oxidation methods are described elsewhere[9]. A complete listing of the modified carbon samples is given in Table 1.

2.2 Linear temperature programmed desorption (LTPD) experiments The LTPD experiments were carried out in a conventional horizontal furnace. The procedure employed consisted of first drying about 1 g of sample at 383 K for at least 12 h in flowing Nz. The temperature was then increased at 5 K/min to 1273 K, again, in a flowing N2 atmosphere. Selected experiments were conducted at a lower heating rate (2 K/min) to improve peak resolution. A gas chromatograph was used to monitor, continuously, the evolution of gases (COz, CO, HZ, and H20 vapor) during all LTPD runs.

2.3 Selective neutralization method In almost all cases, 0.25 g of oxidized carbon sample was immersed in separate 25-ml solutions (0.2 N) of NaHC03, Na2C03, NaOH, and 75 ml (0.075 N) Ba(OH),. Each flask was then sealed and shaken mechanically for 48 h to reach equilibrium. A prior in-

vestigation had determined that equilibrium was actually achieved well before that period of time had elapsed. The concentrations of Na+ and Ba++ remaining in solution were then measured by titrating aliquots of the supernatant liquids with 0.12-N HCl solution. By knowing the concentration of these cations in solution one calculates the amounts of acidic oxygen complexes that had been neutralized by each alkali solutions. 3. RESULTS AND DISCUSSION

3.1. Thermal decomposition of oxygen complexes by L TPD As was just stated, it is known that all of the oxygen complexes on the carbon’s surface decompose as CO? and CO at temperatures up to 1273 K. The amounts and temperatures at which these gases are evolved are characteristic of the oxygen containing surface groups only if the primary products of decomposition do not undergo further external reactions prior to detection. If the evolving gases do not react externally then the gaseous products have the same number of oxygen atoms as the original surface complex[ lo]. To test for the absence of secondary reactions, a series of preliminary experiments was performed in which both air and HNO,-oxidized chars were subjected to LTPD at two heating rates (2 and 5 K/min). The results clearly indicated that the desorption rate (cc/g K) curves for each char at both heating rates were superimposable[9] and that the total amounts of each gas evolved was the same for the two rates. Thus, the gas evolution data are essentially free of secondary reactions, and are representative of the

Oxygen-containing

surface complexes

surface groups. Ofcourse, the rate ofgas evolution, as expressed as cc/min g, for the SK/min runs is 2.5 times larger than those determined at 2 K/min. Figures I and 2 present duplicate LTPD profiles for an HNO,-oxidized char (340 K, 5 h) And an airoxidized char (673 K, 3 h). As would be anticipated, thedominant gasesevolved areC0, and COand their evolution is essentially complete at - 1273 K. Hydrogen is generated from both samples at temperatures in excess of 1,200 K. Inspection of the CO desorption profiles for both materials (Figs. 1 and 2) indicates that the CO-generating complexes exist as two types energetically and/or chemically. The first CO peak is seen at -900 K and the other at - 1,100 K. For the rest of this paper the two types will be referred to as the low and high temperature CO complexes. The situation for the CO?-generating groups on these two samples is quite different. The CO* complex decomposed most rapidly on the HNOl char at near to 575 K, while that for the air char has it peak at -900 K. Close examination ofthe HNO,-oxidized sample profile does, however, indicate that there is a higher temperature CO?-evolution regime (shoulder or tail). Likewise, the air-oxidized char exhibits a small. low temperature CO,-evolving complex (Fig. 2). The apparent presence of both high and low temperature complexes which generate CO1 and CO is indicative of the occurrence of chemically different complexes (e.g., phenol and carbonyl groups as COevolving complex) and/or the same oxygen complex

existing on energetically different sites (e.g., carbony groups on edge carbon atoms in the zig-zag configuration versus those on armchair sites). Of course, functionalities such as carboxyl anhydrides will evolve both CO and COZ, presumably concurrently and not at two different temperature ranges. Water evolution was not observed for the air oxidized char (Fig. 2) but was measured for the HNOi char LTPD (Fig. I), occurring in the temperature range of 400-600 K. However, the amount of oxygen evolved as Hz0 accounts for only 3% of the total chemisorbed oxygen, It is feltf9] that this evolved water originated from H-bonded Hz0 associated with acidic oxygen complexes, and from condensation of any adjacent phenolic groups present on this sample. The oxidized samples studied are listed in Table 1; they are identified by the medium of oxidation and the temperature and time of treatment. All the air treated materials. to varying degrees, were gasified by the oxidation. For these samples, the amount of carbon gasified was determined from the CO? and CO evolved during oxidation. The extent of gasification is as would be expected (i.e., the more severe the treatment the greater the amount of carbon gasified). In the case of the HNO? chars, no measurable amounts of gasification were determined during preparation. Attempts were made to estimate gasification at these low temperatures by a carbon/oxygen mass balance for the preparation of the most highly HNO,-oxidized char (360 K, 1 hf. The result indicated, even in

600 800 1000 Temperature ( K )

Fig.

1.

ChS

1200

evolution profiles (CO?, CO, HlO. and HJ for HNO,-oxidized char (340 K, 5 h), 5 K/min, Nz.

112

Y. OTAKE and R. G. JENKINS 1.2

I-

1200 Temperature

( K )

Fig. 2. Gas evolution profiles (CO?, CO, HzO, and HZ) for air-oxidized char (648 K, 3 h), 5 K/min, Nz.

this most severe case, the amount was negligible.

of carbon

gasified

Table 1 also contains the data describing the amounts of CO1 and CO-yielding complexes, expressed as mmol/g of oxidized char, for each sample. Three trends in these data should be noted. First, as the oxidation temperature is raised, for materials produced by either oxidant at comparable times, the total amount of oxygen complex is increased. For example, air oxidation for 3 h at 648, 673, and 698 K produces chars containing total oxygen complex of 2.57, 3.86, and 5.73 mmol/g, respectively. Comparable data are shown for the HNO,-t&led materials. A significant difference between chars prepared by the two oxidants is revealed if the ratio of oxygen present as CO,-yielding complex to total oxygen complex is calculated [O],,/[O],. This trends shows that this ratio for the air-oxidized samples is between 0.24 and 0.38. As air treatment temperature increases there is somewhat of a pattern that [O]c,,/[O], decreases. On the other hand, the same ratio for the HNO, char is significantly larger, somewhere between 0.49 and 0.59, thus indicating the higher propensity for HNOJ to create CO,-yielding complex than does air oxidation. It is also interesting to note that a comparison of the data obtained for 1-h treatment in HNOX indicates a modest increase in [O]c,,/[O], as the oxidation temperature increases, the opposite trend to that seen for the air results. Another important trend observed from the values given in Table 1 is the ratio of the oxygen con-

tained in the CO to CO,-yielding complexes. As the extent of air oxidation increases, this CO/CO2 ratio increases from initially 3.3 to about 6 when the total oxygen complex is approximately 4 mmol/g. Further addition of oxygen results in the ratio becoming more or less constant (6.0 k OS), with a slight decline at the highest level of oxidation. In other words, in the early stages of air oxidation, more CO-yielding complexes are formed relative to CO, complexes. However, further oxidation in air produces, more, or about the same, amount of CO, complexes, thus causing the ratio to become quite constant. The trend for the HNO,-oxidized material is very different, in that, as more oxygen complex is formed this CO/CO2 ratio decreases monotonically from a value of 2 to about 1.5.

3.2 Acidic oxygen surface complexes A definite correlation between the acidity of carbon and oxygen complexes on carbon surfaces has been found by many researchers(51. However, several interpretations of this relationship have been made in regard to the specific chemical structures of acidic oxygen complexes. The HNO,-treated char (340 K, 5 h) and the airoxidized char (648 K, 3 h) were further heat treated in Nz for 8 h at various temperatures to progressively decompose the oxygen complexes. The total acidities of the resulting chars, measured by NaOH neutralization, are listed in Table 2 as well as the amounts of the CO- and CO,-yielding complexes obtained by

oxygen-containing Table 2. Total amounts (mmol/g) of acidic oxygen complexes neutralized by NaOH on air (648 K, 3 h) and HNOJ (340 K, 5 h) chars heat treated for 8 h at different temperatures in relation to amount of oxygen complexes mmol/g Oxygen complex Heat treatment temp (K)

CO2

CO

Total

Acidic groups

As prep, air char (648 K, 3 h) 673 773 973 As prep. HNOJ char (340 K, 5 h) 673 773 973

0.40

2.17

2.57

0.77

0.34 0.27 0.14 1.78

2.10 1.85 0.86 3.11

2.44 2.12 1.00 4.89

0.64 0.52 0.27 1.88

0.58 0.41 0.20

2.98 2.54 1.20

3.57 2.95 1.40

0.64 0.35 0.18

LTPD. As heat-treatment temperature is increased, the oxygen-containing complexes progressively decompose thermally under these conditions. Clearly, the amount of acidic oxygen group neutralized by NaOH also decreases with higher heat-treatment temperature. Some 90% of the original acidic oxygen groups on the HNO,-treated char are eliminated by treatment to 973 K, yet the total oxygen complexes are diminished by about 70%. Under the same subsequent heat-treatment conditions (973 K, 8 h) for the air char, the acidic oxygen content is reduced by about 75Ohand total oxygen groups reduced by 60%. For both samples, under these conditions, the relative reduction of CO complex is less than that for CO?yielding complex.

surface complexes

113

The nature of the acidic oxygen is also of much interest. If plots are made of the data in Table 2 of the total oxygen complex concentration (mmol/g) for both sets of samples versus the acidic functional groups neutralized by NaOH, nonlinear graphs are obtained (Figs. 3 and 4). Of course, one might anticipate nonlinearity because not all of the oxygen-containing surface groups are acidic (e.g., carbonyls[5]). The picture becomes clearer if plots are made of the concentrations of each of the CO- and CO,-yielding complexes versus the neutralized acidity for each sample suite, as in Figs. 3 and 4. Examining first the HNO,-oxidized, heat-treated char data (Fig. 3), it is clear that there is a linear relationship between the COZ complex concentration and the groups neutralized, strongly suggesting that it is these functionalities that are primarily responsible for the acidic character of these carbons. The slope of the straight line is 1.07, with a correlation coefficient of (R’) 0.996. There is no such relationship between the concentrations of the CO complexes and the acidic sites (Fig. 3). The observation that there is substantial residual CO complex concentration, even when the acidic groups have been almost completely neutralized, certainly is evidence that they (CO groups) are not acidic or phenolic groups. Similar plots made for the air-oxidized set of heat-treated samples (Fig. 4) show a linear relationship between the CO? complexes and total acid sites (slope = 1.92, R’ = 0.999), and nonlinearity in the case of CO producing groups. The CO-yielding complexes barely cont~bute to the acidic nature of these chars because (i) the acid sites neutralized do not relate to COgroup concentration, and (ii) no H20 was detected during LTPD of air-oxidized chars that

A

A

0 y 0

Total O-Complex r

i

i

1

2

8

3 Oxygen Complex Concentration

t

,

4

5

(mmollg)

Fig. 3. Relationship between total acidity measured by NaOH and concentrations of total oxygen complex, CC&-complex, and CO-complex for HNOj-oxidized char (340 K, 5 h), heat treated for 8 h at various temperatures.

Y. OTAKEand R. G.

114

JENIc~Ns

A

I I 1 0.5 1.0 1.5 Oxygen CompIex Concentration

Total O-Complex

i 2.0 (mmol/g)

t 2.5

Fig. 4. Relationship between total acidity measured by NaOH and concentrations of total oxygen complex, C02-complex, and CO-complex for air-oxidized char (648 K, 3 h), heat treated for 8 h at various temperatures.

could have been derived from condensation of phenolic groups[ 111. Table 3 presents the results of the selective neutralization by NaOH, Na2C03, and NaHCO, of the two series of samples oxidized at different temperatures and for different times. These data show that for all these samples the percentage of total oxygen functionality that is acidic (as defined by NaOH neutralization) ranges from about 20% to nearly 50%. For the HNO,-oxidized carbons, the relative range of total acid sites is, overall, somewhat higher (30 to 50%) than that seen for the air-treated samples (2 1 to 38%). As was the case for the heat-treated samples

Table 3. Amount of acidic oxygen complexes neutralized by selective neutralization Na ions exchanged (mmol/g)

Air chars 473 K, 12 h 573 K, 12 h 648 K, 3 h 648 K,41 h 673 K, 3 h 698 K, 3 h 698 K, 11 h HNOr chars 340K,O.l h 340 K, 0.25 h 340 K, 1 h 340 K, 5 h 340 K, 28.5 h 360 K, 1 h

NaHCOI

NarCOr

NaOH

Acidic/ total?

nil 0.03 0.13 0.68 0.27 0.56 1.49

nil 0.12 0.34 1.19 0.59 1.07 1.75

0.12 0.36 0.77 2.33 1.16 1.91 3.22

0.2 1 0.23 0.30 0.38 0.30 0.33 0.35

0.07 0.23 nd 0.64

0.22 0.45 0.58 1.09 1.91 1.78

0.55 0.97

0.31 0.32 0.30 0.38 0.47 0.45

1.47 nd

1.08 1.88 3.39 3.02

TRatio of total acidities to total site concentrations oxygen complexes.

of

(see Figs. 3 and 4), if a plot is made of the total acidity versus the site concentrations of the CO,-yielding complexes, linear relationships are observed for both suites of samples, as depicted in Fig. 5. Least squares analyses of the graphs indicate that the slope of the data for the HNO,-treated samples is 1.20 (R*= 0.999) and that for the air-oxidized samples is 2.34 (R*= 0.995). That is, for the HNO,-oxidized series of carbons the relationship between total acidity and CO,-generation from the oxygen functional groups is about one to one. On the other hand, the equivalent relationship for the air-oxidized samples is approximately two to one. These are essentially the same relationships just reported for the heat-treated samples in Figs. 3 and 4. The data obtained by selective neutralization (Table 3) are presented graphically in Fig. 6 as the amount of acidic groups neutralized by differing strengths of alkali as a function of the total concentration of acidic groups. Obviously, the line for the NaOH results are, in essence, meaningless because it is the neutralization by this alkali that defines total acidity. The plots for the other two alkali solutions are quite interesting. For both the air and HNO,-oxidized carbons the data for each of the neutralizing solutions are very similar. Within experimental error, single lines can be used to pass through the data points shown for neutralization by NaCO, and NaHCOj solutions, regardless of the origins of the oxidized chars. The plot of the NaHCOJ results are distinctly nonlinear, and there is a hint that the Na,CO, data are poorly fitted by a straight line at the higher acid site concentrations. It would appear, therefore, that the specific acidity of the CO,-yielding complexes changes with surface coverage.

Oxygen-containing

1

2 Amount

115

surface complexes

3

of CO, Complex

(mmol/g)

Fig. 5. Relationship between total acidities, measured by NaOH neutralization, and the site concentrations of C02-yielding groups for samples of chars oxidized different temperatures and times.

3.3. Exchange of acidic oxygen surface groups It is clear from the preceding discussion that this study has shown that the thermal stabilities of the CO,-yielding complexes are dependent on the oxidation medium, air or HNO,, and that the acidic groups present on these samples display similar acid-

0

Na,C03 ( Air Chars)

.

ities in aqueous alkali solutions. In an effort to link these observations, and to clarify the nature of the CO?-producing groups, another investigation was made in which the two procedures of neutralization and LTPD were combined. New types of samples were prepared from the

Na,C03 WN03 Chars)

0 0

0.5

1.5 1 Total Site Concentration

2.5 2 of Acidic Groups (mmol/g)

3

Fig. 6. Amounts of acidic oxygen groups neutralized by solutions of NaOH, Na$O,, function of total concentration

of acid sites.

and NaHCO, as a

116

Y.OTAKE~~~R.G.JENKINS

HN03 (340 K, 5 h) and air(673 K, 3 h) oxidized carbons by exchanging the protons associated with the acidic groups with Na+ cations from 0.2-N NaOH solution. The resulting carbons will now be referred to as Na-form chars, the original, oxidized chars being in the H-form. LTPD runs were made on each of the Na- and H-form carbons (HN03 and air-oxidized). Figure 7, presents the CO, desorption profiles for each of the chars and the total amount of CO and CO, evolved from the chars, up to 1273 K, is listed in Table 4. The CO, desorption profiles (Fig. 7) for the HNOr oxidized chars show that the behavior of the H- and Na-forms are identical below 700 K. However, once above that temperature, more CO, is evolved from the Na-form. The total amount of CO, desorbed up to 1,273 K for the Na-form was 2.04 mmol/g, whereas that for the H-form was I .78 mmol/g. Concurrently, though, the Na-form yielded signifi~ntly less CO (2.64 mmolfg) than the H-form (3.11 mmol/ g). Consequently, the total amount of oxygen which evolved as CO, plus CO from both forms of the HNO,-oxidized char was essentially identical (3.34 mmol/g from the H-form and 3.36 mmol/g from the

Na-form). These observations immediately evoke two questions; one is related to the fate of Na+ associated with the CO1 complexes, and the other to the possibility of catalytically enhanced secondary gas reactions in the presence of sodium ions at temperatures in excess of 700 K. The first question will be dealt with later in conjunction with discussions on the chemical structure of the CO? complex and its decomposition behavior. In regard to the second question, there are two secondary gas-phase reactions that must be examined. They are: (a) CO i- Hz0 -

CO1 + Hz, the water-gas shift reaction, and the CO dispro(b) 2 CO - CO, + C, portionation reaction. Both reactions are favorable thermodynamically up to about 1,000 K and are catalyzed by species such as sodium[l2]. According to reaction (a), the molar quantity of CO, produced is the same as the molar quantity of CO consumed. However, for reaction (b),

0.8

jQs

HN03Char l H-Form

'0 t

0 Na-Form

t0,

I

\ '0 s '0

~

l

f C?

IL1

0

z 2

0.4

I

Air Char l H-Form 0 Na-Form 0 Na-H-Form

600

800

1000

1200

Temperature ( I ) Fig, 7. CO2 evolution profiles for the H-form, Na-form, and Na-H-form, air- (673 K, 3 h) and HN03(340 K, 5 h) oxidized chars.

Oxygen-containing

117

surface complexes

Table 4. Total amount of CO2 and CO evolved from H-form, Na-form, oxidized chars in HNOs (340 K, 5 h) and air (673 K, 3 h) Total amount of gases evolved (mmol/g)

HNOX char Air char

Na-H-Form

Na-form

H-form CO?

co

ozt

coz

co

02t

co*

co

o*t

1.78 0.51

3.1 I 3.35

3.34 2.19

2.04 1.13

2.64 2.61

3.36 2.44

nd 0.55

nd 3.32

nd 2.21

tTotal amount of oxygen evolved as CO2 and CO.

the molar quantity of CO, produced is half that of the

CO consumed. Table 4 shows that the amount of CO which was possibly reacted in secondary reactions was 0.47 mmol/g (3.1 l-2.64) and that the quantity of CO? produced was 0.26 mmol/g (2.04- 1.78) strongly suggesting that the CO disproportionation reaction is the main secondary reaction occurring in the presence of Na+ above 700 K. Another intriguing result found in Fig. 7 is for the evolution of CO, from the Na-form of the air-oxidized char. The temperature at which the maximum rate of CO,-evolution occurred was shifted from 900 to 575 K simply by exchanging a proton on an acidic site with a Na-ion. The CO2 profile of the Na-form, air-oxidized char is somewhat similar to that found for both H- and Na-forms of the HNOroxidized char, in terms of peak position. It seems unlikely that the shift of the CO, peak for the air-oxidized char was caused by the CO disproportionation reaction just discussed since it occurs at temperatures above 700 K. As seen in Table 4, the yield of CO* from the Naform air-treated char (1.13 mmol/g) was just over twice that from the H-form char (0.5 1 mmol/g). The total amount of oxygen evolving as CO, plus CO from the Na-form of the air-treated char (2.44 mmol/ g) was slightly higher than that from the H-form material (2.19 mmol/g). These quantitative observations on the air-oxidized chars are very different than those found for the HNO,-oxidized chars (Table 4). These differences will be elaborated later in this paper. A final sample type was prepared by removing the Na-ions from the Na-form of the air-oxidized char by treatment with 0.2-N acetic acid, followed by washing in distilled water, thus exchanging the Na+ with H+. This material is referred to as the Na-H form of the air-oxidized char. The LTPD, CO, profile (Fig. 7) on this sampie shows clearly that the temperature at which the CO, peak appears has been shifted back to 900 K, the same as the original H-form. In addition, the CO?, CO, and oxygen evolution data, as listed in Table 4, show the great similarity between the Hform and the back-exchanged Na-H-form. All these data suggest that the nature and structure of the CO*yielding complexes on the air-oxidized char can be markedly modified by exchange with protons or Naions. This is not the case for the HNO,-treated char. Further interpretation of these data and observations will be discussed in the next section.

3.4 Chemical structure of the acidic oxygen complexes Figure 8 is a depiction of the proposed structures of the acidic oxygen functional groups that exist on the HNOJ- and air-oxidized chars, as well as possible decomposition products evolved during LTPD. For the HNO,-oxidized carbons, the proposed carboxyl groups will decompose on LTPD to yield a molar equivalent of CO,. This carboxyl group can be neutralized with one equivalent Na’ from NaOH solution. In its Na-form, the carboxylate salt ( - COONa) will decompose to a molar equivalent of CO,. By comparison, for the air-oxidized chars, the proposed carboxylic anhydride will neutralize two equivalent Na’ ions, through hydrolysis, to form two moles of carboxyl groups in aqueous NaOH solution. Two moles of the carboxylate salt (-COONa), thus formed will decompose, on LTPD, to yield two moles of CO>. Removal of Na+ ion by H+ from acetic acid, will again form the original carboxylic acid groups. Thus, it is expected that the Na-form air-oxidized char will yield twice as much COZ as produced from the H-form and the Na-H forms. All quantitative decomposition and neutralization behavior predicted above on both the HNOj- and air-treated carbons have been found experimentally, and are reported in Table 4. The proposed chemical structure of the dominant acidic oxygen groups on the air-oxidized chars is based on the assumption that the hydrolysis of carboxylic anhydride occurs in aqueous solutions. A justification of that assumption is as follows. If the hydrolysis occurs in an aqueous solution then the anhydride groups will form carboxylic acid groups, that is, the same acidic chemical group that is present in the HNO,-oxidized chars. In this sense, both the HNOX- and air-oxidized chars would be expected to display similar acidities in aqueous solutions on the basis of the number of carboxylic acid groups. This in fact confirmed by the selective neutralization experiments. From plots of acidic groups neutralized by the alkalis of varying strength (Fig. 6) versus the total site concentration of acidic groups, it has been shown that there is a distinct correlation between theses two properties, irrespective of the medium of oxidation. In other words, the relative acidities ofthe CO,-yielding groups in aqueous solutions are similar, strongly suggesting that the chemical nature of the CO*-com-

118

Y. OTAKE and HNO,-

Char and Low-Temperature

R. G. JENKINS

Air Char

Air - Char After Gasification

LTPD

2co2

Fig. 8. Proposed chemical structures of CO/COl-yielding surface groups for the HNO, and air-oxidized chars.

plexes in water are the same for both air and HN03oxidized chars. Further proof for the possibility of hydrolysis is given by the following comparison. If hydrolysis occurs, it would be expected that an extra half mole of oxygen would be introduced to one mole of carboxylic anhydride by H,O) in order to form two moles of carboxylic groups. For example, since the concentration of carboxylic anhydride on the Hform, air-oxidized char (673 K, 3 h) was 0.5 1 mmol/ g, one would expect that 0.26 mmol/g of 0, would be introduced into carboxylic groups formed through hydrolysis. The oxygen balance for the H-form and Na-forms of the air-oxidized char indicates that the total amounts of oxygen evolved as CO, plus CO, were 2.19 and 2.44 mmol/g, respectively (Table 4). The expected value of 0.26 mmol/g is in good agreement with the observed difference of 0.25 mmol/g (2.44-2.19) again strongly implying hydrolysis of carboxylic anhydride had occurred in aqueous alkali solution. This also suggests that the sodium was not in the form of NazO at 1273 K, as will be discussed later. Condensation of two adjacent carboxyl groups to form a carboxylic anhydride plus the evolution of an Hz0 molecule was proposed because the total amounts of CO, produced from dried Na-H-form and H-form air-oxidized chars were identical (see Table 4). The thermal stabilities of acidic oxygen complexes present in the HN03- and air-oxidized chars are thought to be different from the standpoint of the proposed chemical structures. The resonance energy of carboxylic anhydride appears to be greater than that of carboxylic acids, although the exact values are somewhat uncertain[ 131. This delocalization of ?r

electrons on the planar ring structure may result in difficulty in breaking the C-C bond (between a Catom in aromatic rings and a dangling C-atom associated with oxygen atoms). In fact, the COz-evolving groups on air-oxidized carbons predominantly decomposed at a higher temperature range than those on the HNO,-treated chars. Contributions from low temperature CO,-evolving sites are very small for the air-oxidized sample (Fig. 2). Carbonization studies of model compounds also suggest differences in thermal stabilities of both types of CO*-evolving groups (i.e., carboxylic acids and carboxylic anhydrides). For example, benzoic acid decomposes at 600 K, releasing CO2 while phthalic anhydride produces both CO? and CO in large quantities at about 950 K[ 141. The fate of Na+ associated with acidic oxygen complexes is also important to consider. As seen in Table 4, the amounts of oxygen which evolved (as CO, plus CO, up to 1273 K) for the H-form and Naform of the HNO,-treated chars are identical. The same result is found for the air-oxidized char (673 K, 3 h) if account is made for the quantity of oxygen introduced by hydrolysis. These quantitative observations indicate that sodium, associated with the CO*complex, was not in the oxide form (Na,O) at 1273 K. From the CO, desorption profiles the fate of the sodium can be deduced. It will be recalled that when the higher-temperature CO1 group (carboxylic anhydride) on the air-oxidized chars decomposed its peak rate for CO,-evolution was near to 900 K. If two sodium carboxylates (- COONa) condensed to form Na,O plus the anhydride, the anhydride produced would be expected to evolve CO, at around 900 K during LTPD. However, the Na-form of both the air

Oxygen~ontaining

Case A: “Adjacent”

Case B: “Distant’

119

surface complexes

Surface Carboxylic

Surface Carboxylic

Acid Group

Acid Groups

0 + 2Ba(OH)2 _.+, Fig. 9. Neutralization of acidic oxygen containing surface complexes (aAer Boehm[ 151).

and HNO,-oxidized chars actually exhibit an LTPD CO, peak at near to 600 K (Fig. 7). This observation strongly implies that NazO was not formed during thermal decomposition of sodium carboxylates that existed on the Na-form carbons. 3.5 Relative distance between dangling carbon atoms responsiblefor the formation of CO, evolving groups This investigation has concluded that CO,-evolving groups are responsible for the acidic character of the carbons being studied. As the CO* complex is formed on a “dangling” carbon atom, it is important to examine this phenomenon more closely. In this regard, one must consider differences between mechanisms of the exchange of monovalent and divalent ions. The specific cations utilized in this work are barium and sodium. The mechanisms of exchange of Ba++ [from Ba(OH),] with carboxylic acids are depicted in Fig. 9 (after Boehm[ 151). If the - COOH groups are close together (i.e, adjacent), then neutralization will occur as shown in scheme A (Fig. 9) in which a single mole of barium hydroxide is reacted with two -COOH groups. For the case when the groups are distant, then scheme B will represent the exchange. In this instance, neutralization is equimolecular and the extra positive charges are balanced by OH-[ 151. Thus, from considerations shown in Fig. 9, a compa~son of neutralization behavior for both NaOH and Ba(OH& will lead to an indication as to the proximity of the acidic groups. Results for such a study are presented in Table 5. The HNO,-oxidized chars show an equivalent relationship between the concentrations of CO,-complex and the amounts of acidic groups neutralized by both NaOH and BafOH),. Given the reaction schemes presented in Fig. 9, these results would indicate that the acidic groups on the HNOjoxidized carbons are distant (nonadjacent). Thus, the evolution of CO2 from acid sites on the HNO,-oxi-

dized materials is the result of the decomposition of a single -COOH group. The results generated for the air-oxidized samples are quite different. With one exception (the char oxidized in air at 473 K for 12 h), each CO+omplex results in neutralization of two ions of sodium and one barium ion. It will be recalled that on hydrolysis, a single carboxylic anhydride group forms two carboxylic acids, which will require two ions of sodium for exchange (Fig. 9). Assuming that the neutralization mechanisms by barium hydroxide are valid, if the two acid groups are neutralized by one barium ion then case A holds. The conclusion being that the acid groups on carbons oxidized in air at temperatures greater than - 6.50 K are present as adjacent -COOH sites. These observations are completely consistent with the models depicted in Fig. 8 because the carboxylic anhydride groups proposed for the higher temperature air-oxidized chars require the involvement of adjacent carbon atoms. As just described, the char oxidized in air at 473 K for 12 h behaved very differently in terms of the neutralization to the other air-oxidized samples. Figure IO shows the CO/CO,-evolution profiles for this char. The CO2 evolution pattern is not at all similar to

Table 5. Acidic sites neutralized by NaOH and BatOH)? compared to site concentmtion of CO2 complexes

Air chars 473 K, 12 h 648 K, 3 h 698 K, I I h HNOj chars 340K,O.l h 340K,Sh

Cations neutralized, mmol/g

CO2 complex mmol/g

NaOH

Ba(OHh

0.13 0.40 I .49

0.12 0.77 3.22

0.15 0.44 1.55

0.63 1.78

0.55 1.88

0.66 1.61

120

Y. OTAKE and R. G. JENKINS 0.10,

^,

. c ..-i c :

-0.20

0.08

-

0.06

-

-0.04

8 u-l O J lz

on .

.

0’ .r( Y ,’ g 0.04 w N 8 Iu O J y”

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co2 -

f

ON*

l

t

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I

;; 400

600

-0 ‘0

.* Of

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I

800

1000

c:

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1200

Temperature ( K ) Fig. 10. CO and CO2 evolution

from the char oxidized in air at (473 K, 12 h).

those generated for the other, higher temperature, airtreated carbons. This profile is very similar to those obtained for the HNOroxidized samples because it exhibits a maximum in CO,-evolution at near to 600 K (all the other air-oxidized samples have the maximum at about 900 K). The facts that this sample was the only one prepared by air oxidation at a temperature well below 600 K, and in the absence of significant gasification (Table l), probably account for its apparent “unusual” behavior. Thus, data generated for this sample indicate that the acidic sites responsible for its low tem~mture CO1 evolution are single -COOH groups. This supposition, of course, poses questions concerning the origin of hydrogen in the acidic group. Since there was no hydrogen-containing gas (water or hydrogen) present in the air used for the oxidation step it must be presumed that the hydrogen atoms originate from the chars surface (atomic C/H ratio for the char - 20: 1) or from some post-treatment exposure to moisture. Two major conclusions can be drawn from the observations made in this study. The first is that the thermal behavior of the CO, complex is closely related to its chemical behavior. In the opening part of these discussions it was stated that the COz-evolving groups should be divided into two types, based on the tem~ratures of the maximum evolution peaks in the CO, LTPD profiles. That is, there is a lower-temperature surface complex and a higher-temperature complex. Given the additional information generated about these groups each CO,-complex can be further defined as follows: l

The lower-temperature COz-generating complexes (carboxylic groups), which evolve CO, at

l

a maximum rate near to 600 K, are nonadjacent acidic oxygen complexes formed at dangling carbon sites. The higher-temperature CO*-generating complexes (carboxylic anhydrides), which evolve CO, at a maximum rate near to 900 K, are adjacent acidic oxygen complexes formed at dangling carbon sites.

The second conclusion is that oxidation temperature is a very important factor in determining the type of COrevolving complex that the carbon surface will form. It is quite clear that in the absence of substantial gasification, oxidation will favor the formation of the lower-temperature CO, complex. The basis for the selective formation is thought to be due to the presence, or removal, of inherent, nonadjacent dangling carbon atoms and the creation of freshly generated, adjacent dangling carbon atoms during oxidation[9]. The temperature at which the oxidation takes place is critical in determining whether or not the inherent dangling carbon atoms or the freshly generated one dominate. 4. CONCLUSIONS

Oxygen-confining surface complexes present on both HN03- and air-oxidized carbons are essentially completely decomposed by heat treatment to 1273 K in N2. These oxygen groups can be divided into four distinct groups based on the gas evolution profiles during LTPD. They are (a) lower- and higher-temperature CO-yielding surface complexes which evolve CO at maximum rates at 900 K and I100 K, respectively, and (b) lower- and higher temperature

Oxygen-containing

CO,-yielding

surface complexes which evolve CO, at

maximum rates at 600 and 900 K, respectively. The CO2 complexes present on both air and HNO1-oxidized chars are responsible for the acidic

nature of the surfaces. It is concluded that the highertemperature CO2 surface complexes found in chars air-oxidized above 600 K are in the form of carboxyl anhydrides. The predominant, lower-temperature CO2 complexes on the HNO,-treated carbons, and the chars oxidized in air in the absence ofgasification, are carboxylic acids. In addition, the HN03-treated materials do contain a relatively small amount of higher temperature C02-evolving groups, indicating the presence of some anhydride groups. The dominant oxygen-containing acidic groups on the chars oxidized in air at temperatures above - 650 K are carboxylic anhydrides. However, these samples do contain a small but measurable proportion of carboxylic acid groups. Evidence has been presented that suggests that the lower-temperature CO,-generating complexes (carboxylic groups) are nonadjacent acidic oxygen complexes formed at dangling carbon sites and the highertemperature CO?-generating complexes (carboxylic anhydrides) are adjacent acidic oxygen complexes formed at dangling carbon sites. The data presented in this work clearly show that oxidation temperature is a very important factor in determining the type of CO,-evolving complex that a carbon surface will form. Acknowledgements-The authors wish to acknowledge the financial support of the Teledyne Water Pik Corporation which made this investigation possible. Special thanks are

surface complexes

121

given to Professor P. L. Walker, Jr., whose encouragement and discussions were invaluable.

REFERENCES N. R. Laine, F. J. Vastola, and P. L. Walker, Jr., J. P&s. Chem. 67,203O (1963). F. H. Healey, Y-F. Yu, and J. J. Chessick, J. Phvs. Chem. 59,399 ( 1959). M. B. Rao, PhD Thesis, Pennsylvania State University, University Park, PA (1985). 0. P. Mahajan, C. Moreno-Castilla, and P. L. Walker, Sep. Sci. and Tech. 15, I733 (1980). B. R. Puri, In Chemistry and Physics QfCarbon (Edited by P. L. Walker, Jr.), Vol. 6, ~191. Marcel Dekker, New York ( 1970). 6. M. Smisek and S. Cerny, “ActiveCarbon Manufacture, Properties and Applications” Elsevier, Amsterdam, 1970. 7. H. Hirabayashi and H. Toyoda, Tanso 4.2 (1954). 8. R. W. Coughlin, Ind. Eng. Chem., Prod. Rex Develop. 8, I2 (1969). 9. Y. Otake, PhD Thesis, Pennsylvania State University, University Park, PA (1986). IO. C. D. Hurd, The Pyrolysis qf Carbon Compounds. J. J. Little and Ives Co.. New York (1969). II. E. Fitzer, K. Mueller, and W. ‘Schaefer, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr..). Vol. 7, p. 237. Marcel Dekker, New York (1979). 12. L. G. Massev. In Coal Conversion Technolow fEdited by C. Y. Wei and E. Stanley Lee), pp. 3 13-ii? Addison-Wesley, Reading, MA (1979). 13. G. W. Wheland Resonance in Organic Chemistry. Wiley, New York (1955). 14. E. K. Field and S. Meverson. Chem. Comm. 474 (1965). 1.5. H. P. Boehm, In Advances in Cutalysis(Edited by E. D. Eley. H. Pines, and P. B. Weisz), Vol. 16, p. 179. Academic Press, New York ( 1966).