interaction of carbon dioxide with carbon adsorbents below 400°c

Carbon 1964, Vol. 1, pp. 245-254. Pergamon Press Ltd. Printed in Great Britain

INTERACTION OF CARBON DIOXIDE

WITH CARBON ADSORBENTS BELOW 400°C

V. R. DEITZ*, F. G. CARPENTERt and R. G. ARNOLD1

National Bureau of Standards, Washington, D.C.

(Received 29 July 1963)

Abstract-The adsorption of COs on carbon adsorbents (FT carbon, coconut charcoal, acid-washed bone char) and adsorbents containing basic calcium phosphate (hydroxylapatite, bone char, ash of bone char) was studied. Special consideration was given to the pretreatment of the materials. The carbons equilibrated as rapidly as the temperature; the basic calcium phosphates showed a rapid initial adsorption followed by a very slow rate which continued for days. Linear adsorption isotherms were found on FT carbon and the isosteric heats varied slightly with coverage. The isotherms for the remaining materials had varying curvature and were for the most part in the same sequence as the estimated surface areas. The isosteric heats of CO, correlated very well with the magnitude of surface hydroxyl groups, an estimate of which was made from the chemical composition. There appeared to be three increasing levels of interaction: (1) pure physical adsorption; (2) an adsorption complex having “bicarbonate structure” and (3) an adsorption complex having “carbonate structure”.

1. INTRODUCTION

SINCE the investigations of TITOFF(~) and HOMFRAY@* 3, in 1910, there have been many studies on the adsorption of carbon dioxide by carbon adsorbents.(4) The adsorbents were derived from a variety of carbonaceous materials and many of the non-carbon constituents (H, 0, N, S) in the product were derived from the original source material. Some, however, because of the strong adsorptive properties of the product, were taken up from the environment. After the outgassing treatment, always employed preliminary to adsorption studies, these constituents as impurity centers must play a role in the formation of specific sites for the adsorption of CO, below a monolayer. The part that surface oxygen and hydrogen play in the interaction with CO, is the subject of this paper.

*Present address : Naval Research Laboratory, Washington, D.C.

tResearch Associate representing the Bone Char Research Project, Inc. ; present address: Southern Regional Laboratory, Dept. of Agriculture, Orleans, La.

fResearch Associate representing the Bone Research Project, Inc.; present address: Melpar Springfield, Va.

New

Char Inc.,

For the most part a carbon adsorbent consists of very small crystallites and a valid assumption in such amorphous structures is a random distribution of the constituents. Assuming such a random distribution of the C, H and 0, the surface concentration of these constituents may be calculated from the ultimate analysis. In a high-area adsorbent such as coconut charcoal, almost all of the constituents are already in the surface. In this paper a simple correlation is reported between the OH group as a surface constituent and the isosteric heats of adsorption of CO, on six adsorbents. The significance of these observations is discussed.

2. MATERIALS

The sample of Sterling FT carbon (2700°C) was kindly provided by W. R. Smith of Godfrey L. Cabot, Inc., Cambridge, Mass. The properties of this carbon black have been described;(5) it is essentially nonporous and is believed to have a homogeneous carbon surface. The coconut charcoal was a steam-activated commercial product. It was treated with hydrochloric acid and the product heated to 1000°C in helium. The bone char (Char 100) was received from a cane sugar refinery; this sample has been the subject of a number of investigations. (6) Bone char is viewed as a mixed ad-

245

246 V. R. DEITZ, F. G. CARPENTER and R. G. ARNOLD

sorbent, the carbonaceous residue being intimately mixed with a basic calcium phosphate. An acid- washed bone char was prepared by removing the basic calcium phosphate from bone char with repeated treatments with hydrochloric acid which left the carbonaceous matter with a small acid- insoluble ash. A sample of the acid-insoluble ash was obtained by ignition of acid-washed bone char at 500°C; it consisted of finely divided silica and some calcium silicates. The ash of bone char (basic calcium phosphate plus the acid insoluble residue) was obtained by slow combustion of the bone char in a rotary laboratory retort in air at 500°C for 3 days, followed by 6 hr in oxygen at 500°C. The hydroxylapatite was Sample No. 122 previously described;(‘) the fraction insoluble in hydrochloric acid was 0.007 per cent and the sulfate content 0.006 per cent.

The principal chemical constituents in the six adsorbents used in this research are given in Table 1. Carbon, hydrogen, nitrogen, and ash were determined directly in the carbonaceous adsorbents; the oxygen was determined by difference. Oxygen in non-ash materials is usually determined by a heat treatment to 1200°C and the oxygen calculated in the products H,O, CO and CO,;(‘) this method, however, is not feasible in the presence of those ash constituents which react chemically with carbon.

3. PRETREATMENT In all cases the adsorbents were evacuated at

400°C overnight previous to the determination of the isotherms. The samples of FT carbon, coconut charcoal, and acid-washed bone char degassed readily and soon attained an undetectable residual pressure (<10T5 torr) at 400°C. Bone char, ash from bone char, and hydroxylapatite evolved a gaseous product in steady quantities. Analyses

showed only the presence of CO, and this continued to come off the char for many days under high vacuum and high temperature. A “well- outgassed sample” entails an evacuation for a convenient temperature and time, usually overnight. Obviously, such procedures are inadequate and may not give a reproducible surface for a gas-solid reaction.

In an attempt to monitor outgassing, it has been the practice in this laboratory to record at frequent intervals the rate of gas evolution of the hot sample with the vacuum pumps disconnected. In applying this technique to the desorption of CO, on bone char, instead of obtaining a gradual pressure increase, the pressure rapidly (10 min) reached a steady state as shown in Fig. 1. This steady state

I I II I 4-

3-

k k 2 E ir

V- :-’ Pump on

Pump off

Pressure(torr) by thermocouple gage

FIG. 1. Recording showing steady outgassing pressure with bone char at 400°C.

TABLE 1. COMPOSITION OF THE ADSOREIENTS STUDIED (WEIGHT PER CENT)

C H N 0 Acid ~0 L Ca2+ Po,~ insol. ash ’

OH-

~___ --- FT carbon 99.99 - - - - - - -

Coconut charcoal 96.3 0.5 0.18 1.9 1.10 - - - - Acid-washed bone char 66.6 1.5 3.2 10.7 18.0 - - Hydroxylapatite - - - - - - 39.9 56.7 3.4 Bone char 4.5 0.4 0.38 0.7 0.9 0.6 36.2 49.2 7.1 Ash of bone char - - - - 1.0 0.7 38.2 52.0 8.1

INTERACTION OF CARBON DIOXIDE WITH CARBON ADSORBENTS BELOW 400°C 247

pressure decreased progressively with the duration

of outgassing. In one case, after outgassing at

400°C for 17 hr, the pressure rose to 0.07 torr; after 10 days of evacuation the outgassing pressure was 0.008 torr and after four months it was 0.0001 torr. In another case, the char was outgassed

at 500°C for 15 hr then cooled to 400”; the outgassing pressure was 0.0002 torr. Thus, as far as

the extent of outgassing is concerned, 15 hr at 500” is about equivalent to 4 months at 400”. The temperature effect on outgassing is well known, but the attainment of a steady state outgassing pressure is unusual. This will be discussed in a

later paragraph.

4. RESULTS

The adsorption of CO, on FT carbon, coconut

charcoal, and acid-washed bone char attained a

steady state within 30 min at all temperatures studied. Within this time the temperature of the

adsorbent, indicated by a thermocouple inserted in

a thin-walled glass well located coaxially in the sample tube, fell to within a few tenths of a degree of the bath temperature and the gas pressure attained a constant value. For the remaining three adsorbents, namely, hydroxylapatite, bone char,

and ash of bone char, there was a small but steady

increase in the adsorption with time and this

continued far beyond the period when the samples had definitely reached temperature equilibrium with the bath. A typical behaviour is shown in

Fig. 2 for bone char. At 65°C the amount adsorbed increased at about the rate of 0.002 PM/g min and

at 0” it increased at 0.001 PM/g min. Accepting this background change as steady-state behavior

for these three materials, isotherms are reported using the pressure readings obtained when the temperature of the adsorbent has reached to within a few tenths of a degree of the bath tempera-

ture. This required only 5-15 min/point; most

isotherms consisted of about 10 points and, therefore, the measurements ordinarily occupied a time

of 100-200 min. This slow increase, therefore,

amounted to at most 0.4 yM/g out of a total of the

order of 200 @I/g adsorbed and for the purposes of this paper it was negligible.

Linear adsorption isotherms of carbon dioxide were observed on the FT carbon as shown in

Fig. 3. Analogous behavior has been reported for isotherms at 0°C of Ne, A and Kr on a similar material, P33 carbon black (2700”C).(9’ The isotherms for the five other materials at 0°C are

1

FIG. 2. Time-dependent behavior in the adsorption of CO, by bone char at the indicated temperatures.


P (torr)

FIG. 3. Linear adsorption isotherms of COS on FT carbon (27000).

compared with FT carbon in Fig. 4, and show much greater adsorption of CO,. These isotherms have varying degrees of curvature and were for the most part in the same sequence as the estimated

TABLE 2. SURFACE AREA FROM NITROGEN ADSORPTION

I Method I Area

FT carbon Hydroxylapatite Ash of bone char Bone char HCl-washed bone char Coconut charcoal

Point B B.E.T. B.E.T. B.E.T. B.E.T.

Point B

m’lg 9.8

37 55 69

570 1200

surface areas. Table 2 contains the values for surface area determined from nitrogen isotherms at either 78 or 90°K. The B.E.T. and “point B” methods were used to estimate the surface area.

When the isotherms were plotted on a per unit area basis, as in Fig, 5, quite a different sequence was obtained than in Fig. 4. This is shown in Table 3. Adsorption per unit area serves to bring out the specificity in surface activity, to a degree that depends on the accuracy with which the area is known. The three carbon adsorbents, FT carbon, acid-washed bone char and coconut charcoal, were found to increase in that order and the remaining three adsorbents showed even superior adsorption for CO,.

TABLE 3. SEQUENCE OF RESULTS FOR THE ADSORPTION OF CARBON DIOXIDE

Increasing area

CO2 isotherms at 0”

Wt. basis Area basis @M/g) @M/m3

Isosteric heats

OH surface fraction

FT carbon 1 1 1 1 Hydroxylapatite 2 5 4 4 Ash of bone char 4 6 6 6 Bone char 3 4 5 5 HCl-washed bone char 5 2 3 3 Coconut charcoal 6 3 2 2

INTERACTION OF CARBON DIOXIDE WITH CARBON ADSORBENTS BELOW

kc&d CO, mok/e.chor)

Bone char r

_I J _I 3 Hydroxyla~tite

I I- i FT Cc&on I _ loo 200

P(torr) 3m 400 5

FIG. 4. Adsorption isotherms of CO, at O’Cjg of sample.

The isosteric heats of adsorption were calculated from the isotherms obtained at several temperatures. These are given in Fig. 6 where the heat is plotted as a function of the extent of surface coverage with CO,. Monolayer coverage with CO, was calculated from the surface area and a value of 17.0 A2 for the cross-section of adsorbed CO,. There is a wide range in the magnitude of the isosteric heats ranging at 0.05 coverage from 2.5 to 12 kcal/mole. As shown in Table 3 these are ordered in almost the same sequence as the iso-

therms expressed on a per unit area basis. The isosteric heat for FT carbon was constant (2.52& 0.02 kcaljmole) over the fractional coverage range 0.01-0.07.

The adsorbents bone char, ash from bone char and hydroxylapatite have in common a large per- centage of basic calcium phosphate, the main constituents of which are Ca2+, OH- and POh3-. As a result, OH ions are contained in the boundary surface and for reasons that will be discussed below, it is of interest to compare the isosteric heats

2.50 V. R. DEITZ, F. G. CARPENTER and R. G. ARNOLD

dswbed Co,

pmole/ m*)

Coconut chorcool

P (tom)

FIG. 5. Adsorption isotherms at 0”C/m2 of surface.

of adsorption with the estimated heat of the surface area is estimated to be about 1300 m*/g reaction for both sides of a graphite sheet two atoms thick.

OH-+ CO,(g)+HCO,- (1) The surface area obtained for coconut charcoal

Values for the heats of formation(“) of the ions was about this value and indicated that in such a

are available (referred to the aqueous state) and the model nearly every carbon atom is in the surface.

calculated heat for reaction (1) is -12.6 kcal/mole. The oxygen and hydrogen in carbon adsorbents

This value is remarkably close to the observed may be in the surface or protruding therefrom, and

isosteric heats of adsorption at CO, coverages of some may be buried in the solid. In materials like

0.01-0.02 for hydroxylapatite. coconut charcoal of large area, however, almost all of the hydrogen and oxygen, no matter how distri-

5. DISCUSSION buted, must be present in the surface-in the same From the dimensions of the graphite lattice mole ratio calculated from the analysis of the

(1.42 A for the C-C separation) the maximum complete material. In the other carbon adsorbents

INTERACTION OF CARBON DIOXIDE WITH CARBON ADSORBENTS BELOW 400°C 251

i-7

15

:“::-I,,

0 one char

E c Hydroxv I- sh of bone char

Acid- washed bone char

Coconut chorcool

,FT_ orbon

/ 1 1 0 0.05 0.10 0.15 0.20 0.25

CO,- coverage

FIG. 6. Isosteric heats of adsorption as a function of surface coverage by COz.

of somewhat lesser area, the assumption of a Using the known covalent radii (ri) for the con- random distribution of the hydrogen, oxygen, and stituent atoms bonded to carbon,(l’) the area other constituents was made, so that the surface fraction (Si) for each constituent was calculated as has the same molecular composition as the solid. follows :

The mole fraction of the ith constituent ( fi) was calculated from the observed weight composition (Wi) and the molecular weight (Mi) as follows:

The results are given in Table 4. The appropriate radii for Ca2+, OH- and POb3- in hydroxylapatite are 1.051, 1.305, and 2.96 A, respectively.(12) The

f, = WilMi ’ ZwilMi

TABLE 4. COMPOSITION AND SURFACE AREA FRACTIONS OF THE CHEMICAL CONSTITUENTS OF THE ADSORBENTS

~. C H N 0

Insol. ash (silica)

Ca2+ Poda- OH-

FT carbon Coconut Acid-washed charcoal bone char

Fraction Fraction

Mole Area Mole Area -~~~

0.9999 0.9999 0.925 0.969 - 0.058 0.010

- - 0.001 0.001 - - 0.014 0.009

- 0.002 0.011 ~~

- - -, - - - - - -

I I

Hydroxyl- apatite

Bone char

Fraction Fraction

Mole Area Mole Area -~~-

0.673 0.695 - - 0.182 0.031 - - 0.028 0.016 - - 0.081 0.052 - -

0.036 0.206 - -~

- - 0.556 0.165 - - 0.333 0.784 - - 0.111 0.051

Fraction

Mole Area ~-

0.144 0.035 0.153 0.006 0.010 0.0014 0.017 0.0025

0.005 0.0067 --

0.346 0.155 0.198 0.706 0.126 0.087

-

_.

_.

_.

_.

-

Ash of bone char

Fraction

Mole 1 Area

- -

0.516 0.164 0.296 0.744 0.188 0.092

252 V. R. DEITZ, F. G. CARPENTER and R, G. ARNOLD

radii used for carbon-bonded atoms in the carbonaceous solids were as follows: C, 0.77; H, 0.31; 0, 0.60, and N, 0.58 A.

Unpublished observations in the infrared have indicated the presence of OH groups in dried samples of coconut charcoal and in acid-washed bone char by the characteristic absorption band at 2.7 p. Carbon adsorbents containing oxygen and hydrogen have frequently been assumed to contain a keto-enol isomerization(4) and recently it has been applied to the structure of graphitic oxide.(‘3*14) The migration of an a-hydrogen atom to an adjacent carbonyl group would account for the observed OH in carbon adsorbents. In known organic compounds the mobility of the hydrogens and the degree of enolization is greater with stronger negative side groups.(i5) It is suggested that such an OH group might possibly be the origin of the strong interaction with CO, in excess of that due to physical adsorption alone.

The isosteric heat of adsorption of CO, on the six adsorbents correlates very well with the magnitude of surface OH groups. It is assumed that the minimum quantity of hydrogen or oxygen in the carbonaceous residue determines the maximum present as OH; the validity of this assumption is yet to be fully explored. Figure 7 is a plot of the isosteric heats at 0.05 fraction coverage of CO, as a

‘F-----J -10

-5

0 0.05 0.10 0.15 Fraction of surfocr ~o”e,ed by OH

FIG. 7. Correlation of isosteric heats with surface fraction covered by OH groups.

function of the surface fraction containing OH groups. A similar good correlation of the magni- tudes of adsorption with surface OH is also valid, see Table 3.

There appear to be, in general, three levels of interaction of CO, with carbons: (I) pure physical adsorption, (II) an adsorption complex having “bicarbonate structure”, and (III) strong adsorption having properties which may be termed “carbonate structure”. The behavior of adsorbed CO, on the FT carbon is a good example of physical adsorption and the observed isosteric heat of 2.3 kcal is typical for such processes.

Evidence for the “bicarbonate behavior” is indicated by the results of this paper. This type of adsorption, like physical adsorption, is reversible and has a negative temperature coefficient. One requirement is the presence of an OH group as a constituent of the boundary surface. A tendency to form a bicarbonate ion may be initiated from electronegativity considerations. The diagrammatic sketch below contains the pertinent values for the electronegativity, x, of the constituent atoms.(16)

\ /

/” C H 0 \

C H (0) 0 (1.4)

\

C+ . . . o/-C;

/ II / c - 0 . . . c (0.4) c

I I

0

$?.4) (1.4) II 1 0 (1.4)

Adsorbed phase 100% Ionic form

Although electronegativity measures the relative power of an atom within a molecule to attract electrons to itself, it may also serve to indicate the degree of attraction between constituent atoms of adsorbed molecules and the surface. The excess energy between constituent oxygen and carbon atoms using the electronegativity values above is estimated to be 23 kcal. This is even higher than the heat of formation of the bicarbonate ion, and it does indicate a definite orientating influence of the carbon atom in CO, towards an oxygen atom in the carbon surface. In an ash-free carbonaceous solid, a surface carbonium ion would be a necessary requirement. It would appear that there is suffi-

cient energy due to ionic resonance to form the bicarbonate ion, but other factors, such as the

INTERACTION OF CARBON DIOXIDE WITH CARBON ADSORBENTS BELOW 400% 253

0.0001’ , ,7.-3608 ,

500 ICC0 1500

FIG. 8. Steady-state outgassing pressures of COa over bone char at 400°C for various times of evacuation.

necessary constituent hydrogen and a suitable distance of close approach, are also necessary requirements.

The steady state outgassing pressure, reported in a previous section, can be interpreted as the decomposition pressure of a Type III adsorption complex. Type III adsorption of CO, requires no surface hydrogen, but does involve two electrons from the solid for the formation of CO’,-. The open circles in Fig. 8 give the steady-state outgassing pressures of CO, over bone char at 400°C as a function of the time of evacuation. During the period designated in Fig. 8 by A (300-350 hr) the temperature was varied in the sequence and temperatures noted by the points in closed circles. With the assumption that these data reflect the temperature coefficient of the steady-state pressure, it is possible to estimate the heat of the desorption.

The plot of log pressure vs. reciprocal absolute temperature, shown in Fig. 9, indicates an apparent heat effect of the order of 70 kcal/mole. This value corresponds to strong chemical bonding. Since the observed heat of a surface reaction depends upon coverage, different values would be expected for chars outgassed to different extents. The outgassing at 400°C is rather complete for CO, and the observed value of heat is probably near the maximum. A comparison with the reaction

o’ooo’ ‘. 1.40 1.44 1.46 I.52 1.56 I.60

loOO/ T’K

FIG. 9. Plot of log outgassing pressures of bone char reciprocal absolute temperature.

vs.


CaCO, 2 CO, + CaO suggests itself. However, while the calcium carbonate equilibrium is reversible and well behaved between 600-9OO”C, it is quite variable for different samples at temperatures below 500”. The dissociation pressures of CaCO,(“) extrapolated to the temperature range 360 to 420°C are given by the dotted line in Fig. 9 and the heat corresponding to this reaction is 43 kcaljmole. Data for the decomposition of CaCO, observed in the low temperature range show considerable scatter. At present, all that can be claimed is that the outgassing process at 400°C is associated with an apparent heat effect of the magnitude 70 kcal/mole and that it cannot be correlated with the classical calcium carbonate equilibrium.

6. CONCLUDING REMARKS

The adsorption of CO, in amounts greater than that associated with physical adsorption has been attributed to the additional specific interaction of CO, with the constituent OH group of a surface. There are, obviously, a number of points that need further clarification by future studies. The infrared evidence for the existence of hydroxyl and carboxyl groups on the surface of carbons has not been very conclusive@) and work in this direction is greatly needed. A deeper insight is needed into the nature of the bond of CO, with a carbon surface. The isosteric heats of adsorption of CO, on the ionic crystals KC1 and KI are 6.35 and 7.45 kcal/mole, respectively; these values have been attributed for the most part to the dispersion potential and the ion-quadruple electrostatic potentiaLfig) It may be a coincidence that these vaiues are of the same magnitude as the isosteric heats of carbon adsorbents such as coconut charcoal. However, the interaction between CO, and an hydroxyl group having a fair amount of ionic representation may also be expected to result in a high heat of adsorption comparable to that on KCl.

REFERENCES

1. TJTOFF A., 2. Pkys. C/~em. 74, 641 (1910).

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14. DE BOER J, H. and VAN DOORN A. B. C., Industrial Carbon and Graphite, p. 302. Sot. Chem. Ind. London (1958).

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19. LENEL, F. V., 2. Phys. CAem. P2& 379 (1933).

interaction of carbon dioxide with carbon adsorbents below 400°c

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