1308
ROBERT B. ANDERSON AND P. H. EMMETT
SURFACE COMPLEXES ON CHARCOAL' GAS EVOLUTION AS A FUNCTION OF VAPOR ADSORPTION AND HIGH-TEMPERATURE EVACUATION
OF
ROBERT B. ANDERSONz A N D P. H. EMMETT3
Department of Chemical Engineering, The Johns Hopkins University, Baltimore 18, Maryland Received June 1 1 , 1.947
As part of the research program undertaken by Division 10 of the Kational Defense Research Committee on the study of charcoal for gas mask use, a considerable amount of information was obtained relative to the nature and amount of surface complexes on various charcoals. Since such information is of interest to all of those concerned with the employment of activated carbon, charcoals, or carbon black, the present summary of the results is being presented. The type of work described here is not new. Rhead and Wheeler ( 8 ) , who studied the oxygen complex on a highly purified charcoal, showed that the complex was not adsorbed or occluded carbon monoxide or dioxide, since these gases did not react with the charcoal a t temperatures at which the complex was formed. Lowry and Hulett ( 5 ) after World War I presented the results of hightemperature evacuation experiments on a number of American, English, and German charcoals of that war. The work reported here extends the application of the high-temperature evacuation technique to a larger variety of charcoals and to the study of several factors in the activation and oxidation of charcoal or carbon surfaces. In addition, a few data are presented on the question of the extent of gas evolution from surface complexes that occurs as a result of exposure of charcoals to vapors such as carbon tetrachloride. EXPERIMENTAL
Apparatus The apparatus for evacuating the samples which is shown in figure 1 was similar to that described by Lowry and Hulett ( 5 ) . The degassing tube D consisted of a quartz tube of 2.5 cm. inside diameter which was joined by a graded seal to Pyrex tubing. A platinum crucible (J. L. Smith type) containing about 1 g. of charcoal was suspended by platinum wires to a position near the bottom of the quartz tube. The degassing tube was connected to a large stopcock having a bore 7 mm. in diameter, and this, in turn, to a ground-glass joint so that the tube could be removed from the degassing system for weighing. The degassing tube was connected t o a McLeod gauge and to a Stimson mercury pump P (lo), which continuously pumped the gas from the sample. This pump 1 Presented at the Symposium on the Adsorption of Gases which was held under the auspices of the Division of Colloid Chemistry a t the 110th Meeting of the American Chemical Society, Chicago, Illinois, September 11-12, 1946. 2 Present address: Central Experiment Station, Bureau of Mines, Pittsburgh 13, Pennsylvania. 3 Present address: Mellon Institute, Pittsburgh 13, Pennsylvania.
SURFACE COMPLEXES ON CHARCOAL
1309
was designed t o operate efficiently against back pressures as high as 10-20 mm. of mercury. From the mercury pump, the gases passed through a liquid-nitrogen trap F to an automatic Toepler pump T of the type described by Urry (12). The gas was collected in bulb S, the mercury being set a t N to act as a check valve. When the evacuation was completed, the volume of gas was determined by compressing it into calibrated bulb B by raising mercury to line L, and determining the pressure on the scale M: Then the gas was transferred by an evacuated bulb G to the gas-analysis apparatus. For temperatures to 1000°C., a resistance furnace was used, the temperatures being measured by a chromel-alumel thermocouple, and controlled automatically to less than f 5°C. Above 1000°C. the crucible was heated by an induction furnace, the temperature being measured by an optical pyrometer. The pyrome-
FIG.1. Apparatus for high-temperature evacuation experiments
ter was sighted through plane Pyrex window W of the degassing tube into the platinum cone within the crucible, the cone approximating black-body conditions. The temperature could be maintained to within f 50OC. with the induction furnace. The gases evolved were separated into three fractions by using liquid nitrogen, dry ice, and warm water in succession on trap F. The fraction that passed through liquid nitrogen contained hydrogen, carbon monoxide, and methane; the part volatile a t -78°C. contained carbon dioxide; and that volatile a t room temperature contained water, which was determined volumetrically as a vapor by expanding into a large calibrated bulb at about three-fourths of the saturation pressure, The gases were analyzed by a semimicro method as developed by Taylor and Saunders (11). The method was checked by using known mixtures of gases and found to be satisfactory. Difficulty was encountered in separating carbon dioxide-carbon monoxide mixt.ures; hence the liquid-nitrogen separation was
1310
ROBERT B. ANDERSON AND P. H. EMMETT
used. The analysis of methane by the combustion method gave uncertainties similar to those reported by Taylor and Saunders (ll),but since methane mas present to only a few per cent, this was not important. Analysis for oxygen was accomplished by using dry yellow phosphorus, the phosphorus being melted and solidified between each analysis to insure a fresh surface. A check on the experimental methods is given in table 1, in which the weights of gases calculated from gas volumes and analyses are compared with weight loss of the degassing tube. The tube was weighed against a counterpoise on a large balance t o an accuracy of f 0.5 mg. The tube and crucible were washed with dilute hydrochloric acid after each run, rinsed with water, dried, and pumped a t 1200OC. to a good vacuum. The samples were dried over phosphorus pentoxide for at least 1 day, and then placed in the degassing tube and pumped to a vacuum of mm. of mercury before gas collection was started.
TABLE 1 Comparison of weight loss of samples by direct weighing with weight loss computed from gas analysis wE1CET LOSS SAMPLE
BY pa! analysis
1
By,direct weighing
mg.
CWSN S5 extracted with hydrofluoric acid and evacuated to
12Oo0C... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
242.8
245.8
125.5 63.8 49.6
135.5 64.7 50.2
P C I 1042 extracted with hydrofluoric acid and evacuated to
120O0C... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CWSN 44 evacuated to SOO’C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CWSN 196 B1 evacuated to 900°C... . . . . . . . . . . . . . . . . . . . . . . . . . .
Nitrogen, steam, and ammonia treatments were carried out in a quartz tube small enough t o be weighed on an analytical balance. Transfer of the samples to the degassing tube was done in a stream of pure nitrogen in a manner which precluded the possibility of exposure of the sample to air. For steam treatments, nitrogen saturated with water vapor a t 75°C. was used. To control the level of the mercury in the Toepler pump, the apparatus shown in figure 2 was used. The details may be of interest, since the device can be constructed from readily available laboratory supplies. The plugs of the two glass stopcocks are connected bj. a wooden tube W which fits over the handles of the plugs, the two plugs being set with an angle of about 25’ between the open position of each. Lever L attached to the middle of W is connected‘to an iron core in solenoid S. When the solenoid is not energized, the stopcock B is open to the vacuum line and A is closed. When the solenoid is energized, A is open to atmospheric pressure through a leak with B closed. The solenoid was activated by a controller of the type described by Rowley and Anderson (9). Some of the charcoal samples were treated with nitrogen at llOO°C. in a
SURFACE COMPLEXES OS CHARCOAL
1311
sillimanite tube in a Globar furnace. Samples aged in moist air were prepared a t Korthwestern University. Analyses of the ash contentc: of charcoals were made by E. 0. Wiig and J. F. Flagg a t the UniIersity of Rochester. Yitrogen-adsorption measurements on various charcoals before and after hightemperature evacuation were made on the standard apparatus of the type that has been described many times (3). Samples of evacuated charcoals nere transferred in air to the adsorption tube. For the study of gases displaced during the adsorption of vapors on charcoal a 10- to 20-g. sample of charcoal \yas evacuated to a pressure of mm. of mercury. The adsorbate was repeatedly frozen out a n d pumped to remove dissolved gases. The liquid was then permitted t o adsorh on the charcoal and
FIG.2 . Apparatus for controlling the level of the mercury in the Toepler pump
to equilibrate for varying lengths of time. The charcoal bulb was then evacuated through a dry-ice trap by the regular gas-collecting system. Charcoals The charcoals used include most of the standard varietie9 upon which research was done in World War I1 by the Sational Defense Research Committee. The relative effectiveness of these charcoals for removing gases by adsorption or for acting as supports for chemicals and catalysts that remove gases by chemical means is irrelevant t o the present discussion and will not be presented here. The essential information as to the origin and methods of preparation of the charcoals may be summarized as follow: Charcoals from wood: Charcoals CWSS 19, CWSK 55, CWSS 44, CWSX 196 B1, CWSN 196 B1X were made by the zinc chloride activation process from
1312
ROBERT B. ANDERSON AND P. E. EMMETT
wood sawdust. Charcoals CWSN 19, S5, and 44 were obtained by using a dryweight mixture ratio of 0.90 for zinc chloride: wood sawdust. CWSN 19 was heated to 850°C. in a rotating furnace and was characterized by a low ash content of 0.2 per cent. The ash contents of CWSN 55 and 44 were 6.67 and 3.8 per
'1
TABLE 2 Analyses of ash of typical zinc chloride and coal charcoals i n weight per cent
I
COXPONKNT
SiOz. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AlzOa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fe20s.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heavy metals. . . . . . . . . . . . . . . . . . . . . . . CaO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CWSN S5
I
0.12 0.42 0.24 0.45 6.38
PCI 1042
10.6 8.2 2.0
0.8 0.7
TABLE 3 Degassing experiments on various charcoals
1
VOLUXE OF GASES EVOLVED, CC.(N.T.P.) PEP G R A Y
SAMTLE
........
........ 3. NS 5 (extracted with
HF . . . . . . . . . . . . . . . . 4. N 4 4 . . . . . . . . . . . . . . . . . 5. N 1 5 6 B 1 . . . . . . . . . . . . . 6. N 1 9 6 B l X . . . . . . . . . . .
........ ........ 9. P c I P 5 8 . . . . . . . . . . . . .
10. P C I 2 5 . . . . . . . . . . . . . . 11. PCI 1 0 4 2 . . . . . . . . . . . . with HF) . . . . . . . . . . . . 25-3Oo'C.
-~
cent, respectively. CWSN 196 B1 and 196 B l X were made by using a zinc chloride: wood sawdust ratio of 1 . 1 : l . They differed primarily in that CWSN 196 B1X had a final calcination in a rotary furnace a t 850"C., whereas CWSN 196 B1 was merely dried at 400°C. as a h a 1 step. Most of the ash was zinc oxide, as indicated by the analyses of CWSN 55 given in table 2.
1313
SURFACE COMPLEXES ON CHARCOAL
CWSC 1242 was made from wood sawdust by calcination and steam activation. CWS B-X2 \vas a steam-activated charcoal made from nut shells. Coal charcoals: Samples made from coal include PCI P58, PCI 25, PCI 1042, and CFI "CC." The PCI charcoals were made by briquetting finely ground coal, carbonizing the briquetted material by heating it in two rotary furnaces a t a series of temperatures that gradually increased to 515"C., and finally steam activating it in rotary furnaces a t temperatures between 870" and 980°C. The details for making CFI "CC" are not available. The samples made from coal TABLE 4 S u m m a r y of degassing experzmenls on various charcoals GSES
EVOLVED TO 930"C., GRAM
cc (X.T.P.)PEP
I
1
GASES EVOLVED
I n 12n0°c,
cc fY,'L,P ) PER
GR.iM
1
WEIGHT PERrrVr4GE
I-
I
'
___~___ * The evolved hydrogen FVY&Spartly free and partly combined; all evolved oxygen was in a combined form. t Ash assumed t o be zinc oxide. f Analyses of E. 0. Wiig and J. F. Flagg (see table 2 ) . had high ash contents. The percentages of ash in PCI P58, PCI 25, and PCI 1042 were 19.4, 20.6, and 20.3, respectively. An analysis of the ash in PCI 1042 is given in table 2. One sample of PCI 1042 that had been extracted with hydrofluoric acid had an ash content of only 0.58 per cent. RESULTS
In table 3 are presented the results obtained on degassing the various charcoals a t temperatures up to 1200°C. The results have been summarized into two groups in table 4 : the gas evolution in the range from room temperature to 900°C., and the gas evolution from room temperature to 120OoC. Included in
1314
ROBERT B. ANDERSON AXD P. H. EMhIETT
table 4 also are the values on a weight basis for the per cent of hydrogen evolved (free and combined) and the per cent of combined oxygen evolved. KO free oxygen was given off by any of the samples. The ash content of the samples and also the per cent of oxygen in the ash are also shorn for comparison. In most of the experiments, the samples mere degassed over temperature intervals TABLE 5
Run 14: Sample of CWSN S6 degassed to 960°C. i n 60°C.intervals
1
TEMPERATURE RANGE
GASES E Y O L M D , CC.
I T
CO
'C.
2560120180240-
60 120 180 240 300
0.00 0.00 0.00 0.00 0.00
Totals.. . 25- 300
0.00
3003& 420480540-
0.00 0.01 0.03 0.09
1
(N.T.P.) PER 0 1 A M
1
C&
I
0.00 0.00 0.00 0.00
'
1
CHARCOAL
0.32 0.67 0.24 0.46 0.82
1.55
2.49
0.63 0.54 0.34 0.50 0.60
0:73 0.76 0.64 0.98 1.51
2.60
4.62
3.96 2.82 0.41 0.00 0.00
0.00 0.03 0.03 0.07 0.32
0.21 0.28 0.37 0.84 2.06
Totals.. ,300- 600
0.45
3.79
0.10
3.29 21.48 30.82 31.55 16.41
4.76 8.39 5.86 2.38 0.59
,
0.00 0.09 0.14 0.00 0.00
0.44 1.22 0.47 0.06 0.03
105.05
22.18
1
0.24
2.21
960-1200
30.49
0.38 0.67
[
0.00 0.16
Totals
900-1200
43.66
Totals
25-1200
147.65
~
600660720780840-
-
_
660 720 780 840 900
Totals.. ,600- 900
__-____----
1
~
_
IIY
0.00 0.11 0.14 0.50 0.82
360 420 480 540 600
0.00 0.00 0.00 0.00 0.10
PISAL PRESSCRE
IH~O (vapor) X '0' --coz
0.3 0.2 0.6 1.4 1.4
1.4 1.0 1.5 2.0 2.3
-~~ ~ _ _ _ - -
1
7.19
4.5 8.0 7.0 5.0 5.0
[
of about 3OO0C., as indicated in tables 3 and 4. However, in one experiment a sample of CWSN 55 was degassed and 'the gas collected for analysis at intervals of G0"C. The results of this run are summarized in table 5. In a few experiments with one of the zinc chloride-activated samples, CWSN 196 BlX, an attempt was made t o ascertain in detail the effect of exposing a highly degassed sample to oxygen both at room temperature and at 300°C. The final amounts of complex left on the surface after flushing with nitrogen at
1315
SURFACE COMPLEXES ON CHARCOAL
1000°C. and at 1110°C. were also desired. All of these results are summarized in table G according t o the gas evolution at 300’ intervals and in table 7 over the range 25-90O0C. and 25-1200°C. The weight per cent of evolved hydrogen and TABLE 6 Effect of heat-treatment on zinc chloride charcoals
I SAMPLE
GASES EVOLVXD, CC.
25-3W”C.
1
300-6OO’C.
I
( S . T . P . )PER
60-PW’C.
GRAY
i
WO-12W”C.
-1
6. S 196 B1X 15. S 196 B1X heated in ni-
trogen a t 1000°C.; cooled and transferred in nitrogen
1
16. Same as S o . 15 exposed
t o air for one week 17. S 196 B1S TH 410 heated in nitrogen for 10 hr. a t 1100°C.; cooled in nitrogen and then exposed to air
18. Same a8 No. 17 exposed to oxygen a t 300’C.
19. X 196 B1X exposed to oxygen a t 300°C. 20. X 196 B1X extracted with HF and heated in nitrogen at 1000°C.; cooled and transferred in nitrogen
oxygen, the ash content, and the weight per cent of oxygen in the ash are also shown. I n the course of developing a chemically treated charcoal called whetlerite for removal of certain gases that would not have been removed by a straight adsorption process, some indication was obtained that the effectiveness of the final product depended upon the extent of exposure of the raw charcoal to air and water vapor prior t o the whetlerization. A number of samples of charcoal were therefore artificially “aged” by exposing them t o a relative humidity of
1316
ROBERT B. ANDERSON AKD P. H. EYMETT
80 per cent at 50°C. for an extended period of time. At one stage of the aging work it was reported that heating "aged" samples to 115°C. for an hour restored them t o a condition in which they would make satisfactory whetlerites of the type developed in World War 11. As a means of ascertaining the extent to which Summary of data on
I
TABLE 7 reel of heat-treatment of zinc chloride type charcoals
SAMPLE
6. N 196 B1X 15.
N 196 B1X heated in nitrogen a t 1000°C.; cooled and transferred in nitrogm
16. Same as KO.15 exposed to air for one week
17. K 196 B1X TH410 heated in nitrogen for 10 hr. a t llOO°C.; cooled in nitrogen and then exposed to air 18. Same as No. 17 exposed t o oxygen at 300°C. for 30 niin.
19.
N
196 B1X exposed to oxygen at 300°C. for 30 min.
20. iV 196 B1X extracted with HF and heated in ni trogen at llOO°C.; cooled and transferred in nitrogen
* See footnote t o table 4. t Ash assumed t o be zinc oxide. these various aging treatments were reflected in the composition of the surface complex, a series of experiments was carried out on aged and unaged samples of CWSN 55, CWSS 44,and PCI 58. The results are shown in table 8 by 300" intervals between 25" and 900°C., and between 25' and 12OOOC. In tables 3, 6, and 8 the'detailed analysis for methane below 900°C. is omitted, since it was usually quite small. The methane content can, however, be obtained
1317
SURFACE COMPLEXES OK CHARCOAL
TABLE 8 Effect of aging at 80 per cent relative h u m i d i t y and 5OOC. on oxygen content
I 2.
GASES EVOL\ZD, CC.
25-300°C. '
SAMPLE
300-5OO'C.
(S.T.P.) PER
600-9OO'C.
CR.iM
900-12OoT
3 s 5 original
21. SS 5 aged 22. S S 5 aged and heated in air at 110°C. 4. S 44 original
23. S 44 aged
S u m m a r y of degassing data of aged samples
2. IiS 5 original 21. SS 5 aged
22. SS 5 aged and heated in air at 110°C.
4. N 44 original 23. N 44 aged
9. P C I P58 original 24. P C I P58 aged
* See footnote to table 4. t Ash assumed to be zinc oxide. t by subtracting the values in column 4 from those in column 9 in tables 4,6, and the lower half of 8. No appreciable amount of methane was evolved below 60O0C.
1318
ROBERT B. ANDERSON AND P. H. EMMETT
TABLE 9 Complex jornied d u h g steam activation of C W S S 196 BIX extracted with hydrofluoric acid and heated at 1000°C. f o r 3 hr. 1
j
GASES EVOLVED DUPING DEGASSING, CC. TEYPERAIORE
~
1
~2
I
co
I
CH,
(s T.P ) PER GRAY
I
I
coI
Ij
0.0
0.6
0.1 0.1
0.2
H,O (vawr)
~~
A . Extracted and heated charcoal T.
20. . . . . . .
1
25- 300 300- 600 900-1200
~
Total. . . . . . . . . . . . . . . . . . . . . .
25. . . . . .
0.0 0.3
15.6
25- 300 300- 600 600- 900 900-1200
1.1
11.4
0.1 0.2
0.0 0.0 0.1 0.3
0.3
0.4
0.6 0.4
0.0 0.0 0.1 0.2
0.0
~
0.4
~
0.6
~
i
,
0.1 0.9 0.2 0.2
0.1 0.0 0.9
0.8 0.8 0.1 0.0
C. Treated as in A, than exposed t o water vapor at 6OOOC. for 3 1 hr. (weight loss = 1.0 per cent); g. H20 per g. charcoal = 0.99 26.. . . . .
0.1 0.0 1.8 2.3
300- 600 600- 900
27. , . . , . . 900-1200
28. . . . . . .
25- 300 300- 600
~
1
i
!
0.0 0.0 0.0 0.2
I
I
0.0 0.3 0.1
~
,
1.3 0.6
0.0
0.0
0.3
1.7
16.9
1.9
0.2
0.1
0.0
0.0 0.6
0.0 0.1 1.3
0.0
0.4
2.2
0.0 0.2 0.5
1
0.6 1.1
~
1319
SURFACE COMPLEXES UN CHARCOAL
Since most charcoals are activated by treating them with steam it seemed worthwhile to ascertain the influence of steamicg upon the composition of the surface complex of a charcoal. A sample of CIVSS 196 B l X was used after being extracted with hydrofluoric acid to remove the ash and being heated in nitrogen a t 1000°C. for 3 hr. to remove most of the sirface complex. Steaming experiments were carried out at 300°C. for 33 hr. with a 0.4 per cent weight loss; a t 600°C. for 31 hr. with a 1 per cent weight loss; at ;rjO"C. for 1 hr. with a 1.6 per cent weight loss; and a t 900°C. for 31 hr. with a 56 per cent weight loss. Samples from these individual steaming experiments were then degassed and analyzed in the usual way, the gas being collected over 300" temperature intervals. 'These experiments are summarized in table 9. il few experiments n-ere made with a view to determining whether or not marked evolution of gas from the surface complex of charcoal occurs when a vapor such as carbon tetrachloride is adsorbed on the sample. Such gas evoluTABLE 10 Displacement of complex b y adsorptton oj vapors at 26-"C. VOLUME OF
CHAPCOAL
\'%POP CHAPCOAL
1
AGE$&
'
hours
CWSN 196 B l . . . . . . . CWSN 196 B1. . . . . . . . CWSN 196 B l . . . . . . . CWSN 196 B1. . . . . . CWSX 196 B l . . , . . , . PCI P 5 8 . . . . . . . . . . . . . PCI P58.. . . . . . . . . . . .
0.5 0.4 0.4 0.2
18 5
41 2t
'
GASES EVOLVED
cc' (S'T'P'' PER O R A M
co
cot
0.0028 0.00036 0.0014 0.0024 0.0016 0.0016 0.0025
0.0036 0.00039 0.495 0.205 0.235 0.00003 0.0094
tion had been reported in numerous experiments carried out by Almand (1). Samples of a zinc chloride charcoal, CWSX 196 31, were treated with carbon tetrachloride, chlorobenzene, and water. After each vapor treatment, the gas that had been evolved i n s pumped off and analyzed. Similar experiments with PCI P58 were made using chlorobenzene and water vapor, respectively. The results are summarized in table 10. Finally, in table 11 data are reported on the formation of a nitrogen complex by treating charcoal with ammonia at GOO" and 900°C. The results as summarized in tables 1-11 \Till non' be briefly discussed. DISCUSSION AND COSCLCSIOSS
The experimental results taken as a whole are in agreement with thosereported by Lowry and Hulett ( 5 ) but are much more estensive. It will be convenient to discuss them under a number of separate headings as follows : Nature and extent of gas evolution f r o m surface complexes The carbon monoxide from samples produced by the zinc chloride activation process was evolved primarily a t temperatures below 900°C. This is illustrated
1320
ROBERT B. ANDERSON AND P. H. EMMETT
by tables 3 and 4 and also by table 5 . Simultaneously with the burst of carbon monoxide in the temperature range of 600-840°C. the deposition of a mirror of zinc, formed presumably by the reduction of the zinc oxide present in the ash was observed on the cool parts of the tube. This reduction of zinc oxide by carbon is entirely consistent with known values for the equilibrium constant of the reaction (6): C
+ ZnO = Zn + CO
(1)
For this reaction, if the pressures of zinc and carbon monoxide are assumed to be equal, the equilibrium pressures are 0.06 mm. of mercury at 527"C., 1.3 mm. TABLE 11 Complex formed durang ammonia activation on CWSN 196 B I X , ash-extracted and heated in nztrogen at 1000°C. 'IEYPERATL'RE
'
GASES EVOLVED DUURING DEG.4SSlbC, CC.
iS.T.P.) PER
8 . Sample exposed t o ammonia at 750°C. for 3 h r . ; weight loss "C.
25- 600
I
0.6 3.3 26.6
Total , . . . . . . . . , . . . . . , . 1. 3 0 . 5
600-900 900-1200
'
I I 0.0
I
1
0.1 0.4
I
0.1 0.2 5.2
1
0.5
1
5.5
~
1
~
_~
0.2 0.1 2.3 2.6
i ~
~
=
GRAM
0.4 per cent 0.1 0.1
0.3
0.5
~
i 1
0.5 0.5 0.1 1.1
B. Sample exposed to ammonia a t 900°C. for 3 hr.; weight loss = 1i.l per cent 25- 300 300- 600 600-900 900-1200
1 1
Total . . . . . . . , . . . . . . . . ~
0 0
:;
0 0 2 3 5 2 318
1
0 1 0 7
1
0 1 6 3
39.3
j
0.8
1
6.5
0 0
1I
1
I
1
0 0 0 3
0.0) 0 2 0 2 0 8
0 2 3 5
2.9
~
1.2
0 0 0 0
1
6 6 3 2
1.7
of mercury at F27"C.. and 100 mm. of mercury at 82iCC. Since the zinc and carbon monoxide are continually removed to pressures less than 0.1 mm. of nizxury, this reaction is thermodynamically possible at temperatures above about 000°C. In zinc smelting, it has been sho\vn that the important reduction reaction is between zinc oxide and carbon monoxide, the ieaction of zinc oxide with solid carbon being too slow. However, since the zinc oxide and.carbon were very intimately mixed in the charcoals and the carbon monoxide pressure was very lorn, the reduction probably proceeds by reactlon 1. All samples of charcoal made by the zinc chloride process evolve very little carbon monoxide in the temperature range 900-1200°C. This is very much in contrast to all the samples made from coal and having a considerable ash content of silica, alumina, iron oxide, and oxides of other elements. Whereas 94-98 per cent of
SURFACE COMPLEXES ON CHARCOAL
1321
the carbon monoxide evolved from the zinc chloride-sawdust charcoals was given off below 9OO"C., about 90 per cent of the carbon monoxide from the PCI and CFI charcoals was given off above 900°C. This is consistent with the ivork of Brunner (21, in xhich the carbon monoxide pressures of the system silica-silicon carbide-carbon were found to be 0.1 mm. of mercury a t 900°C. and 10.7 mm. of mercury a t 12OO0C., and of the system alumina-aluminum carbide-carbon n-ere found t o be mm. of mercury a t 900°C. and 0.1 mm. of mercury at 1200°C. With both silicon and alumina, the carbon monoxide pressure was lower for reduction t o silicon and aluminum than for the reduction to the carbides. The evolution of carbon monoxide from charcoals that were prepared in such a n.ay as t o have IOIY ash contents (not the samples extracted with hydrofluoric acid, however) appears to h a w a maximum in the carbon monoxide evolution in the GOO-900°C. range. This is illustrated by the data on samples CWSK 19 and CWSC 1242, as s h o w in tables 3 and 4. This is in general agreement with previous work that has been done on charcoal. It probably also is consistent with the well-known fact that carbon blacks lose most of their volatile matter on being heated to temperatures of about 750" to 800°C. The samples washed ivith hydrofluoric acid appear t o merit special consideration and will be discussed below. Carbon dioxide evolution seems to reach a maximum in the temperature range 300-600°C. in all of the charcoals. This does not necessarily mean that most of the evolved carbon dioxide is given off over this temperature range, for it is possible that some of the carbon monoxide from the GOO-900°C.region results from secondary reaction of carbon dioxide with the hot carbon surface while the gas is passing through the sample. The chance of reaction is minimized by the fact that the gas \vas continually pumped doivn t o a pressure of 10-2 mm. or less, but it cannot be said with certainty that it \vas entirely eliminated. In almost every esperiment, the amount of carbon dioxide was greater than that in equilibrium n.ith carbon monoxide and carbon. Hydrogen evolution occurs for the most part at, a temperature above 900°C. This, too, is in agreement with the findings of Loivry and IIulett. However, a close examination of the data in tables 3 and 4 makes it apparent that samples which, in the procese of preparation, were not heated above about 600°C. evolve more hydrogen in the GOO--9OOOC. than in the 900-1200°C. region, the maximum being attained at about 750-8OO0C. Actually, samples heated to 850°C. during activation evolve about the same amount of hydrogen above 900°C. as those that were not so heated. There is no evidence in the present paper as t o the extent to which the evolved hydrogen is surface hydrogen and the extent to which it is hydrogen from carbon-hydrogen complexes deeper nithin the charcoal particles. When one remembers, hoivever, that the adsorption isotherms of nitrogen on these charcoals correspond to surface areas between 1000 and 2000 sq. m. per gram (see figures 3 and 4), it will be realized that most of the carbon atoms, and hence the carbon-hydrogen complex, must lie in the surface of the charcoal.
1322
ROBERT B. PLNDERSON .LVD P . H. E>I>IETT
Water vapor and methane are both evolved in comparatively small amounts, Water vapor evolution usually reaches a maximum in the 300-600°C. temperature range, though for some of the samples activated irith zinc chloride, there is a further evolution of water a t about 60O-72O0C. Perhaps this higher-tempera500
400
9 0 K
L 300 0. c
z
I
0 Y
n W
E 200
s a w
5
B I00
0
02
04 RELATIVE
0.6
0.8
I
.o
PRESSURE
FIG.3. Kitrogen adsorption isotherms at -195%. charcoal; 6 ,charcoal after degassing.
on charcoal CWSN 44.
0, original
ture water is produced by the reduction of some of the zinc oside ash by the hydrogen that is beginning to be evolved in this same temperature range. Zinc oxide should be reducible by hydrogen under the conditions of the degas&.
Effect of extraction with hydrojluoric acid on the surface complex Tables 3 and 4 show the results obtained in two experiments in which the ash content of the samples was reduced nearly t o zero by the extraction with hydro-
1323
SURFACE COMPLEXES ON CHARCOAL
fluoric acid. The gas evolved on heating these extracted samples is very surprising. The amount of complex on the charcoal seems t o be greatly increased by the extraction process. For example, CWSN S5 after the washing with hydrofluoric acid evolves 116 cc. of carbon monoxide and 44.8 cc. of carbon diSO0
400
z K 4 W LT
-2
300
a +.
U
n W
6 eo0
v)
4
Y -1
0
>
100
0
02
04 RELATIVE
06
0.e
IO
PRESSURE
FIG.4 . Nitrogen adsorption isotherms at -195°C. on PCI 1042 charcoals. 0 , original PCI 1042, 0 , P C I 1042 after degassing; 6 ,ash-estractedPCI 1042; 4 , ash-estracted PCI 1042 after degassing
oxide on being evacuated t o 1200°C. compared to 27 and 5.7 cc., respectively, evolved by the unextracted sample. Similarly, PCI 1012 after the washing with hydrofluoric acid evolved 72.1 cc. of carbon monoxide and 12.1 cc. of carbon dioxide on being evacuated t o 900°C., compared to 9.2 and 1.7 cc., respectively, for the unextracted samples. It seems that in some as yet unexplained way the washing process catalyzes the formation of complex on the charcoal surface.
1324
ROBERT B. ANDERSON AND P. H. EMMETT
However, these charcoals were not degassed until about two years after their preparation and, although they were kept in stoppered bottles, they may have been able to pick up oxygen slowly over this extended period. Even below 600°C., surprisingly large quantities of carbon monoxide and carbon dioxide are evolved from the extracted samples. Thus, CWSN 55 evolved 88.1 cc. of carbon monoxide and 45.7 cc. of carbon dioxide below 600°C. after extraction, compared t o 5.1 and 4.6 cc., respectively, before extraction. The corresponding figures for PCI 1042 were 51.2 cc. of carbon monoxide and 9.4 cc. of carbon dioxide after extraction, compared to 2.6 cc. of carbon monoxide and 1.4 cc. of carbon dioxide before extraction. In the range 900-1200°C. the extracted sample of PCI 1042 evolved only 0.8 cc. of carbon monoxide, compared with 93.1 cc. for the unextracted sample, confirming our conclusion that most of the carbon monoxide evolution from the original PCI 1042 in this temperature range came from the reduction of ash by carbon.
Changes of adsorption isotherms on degassing In the high-temperature evacuation the pore structure of the charcoal as evidenced by nitrogen-adsorption isotherms was changed. The interpretation of isotherms as to pore structure has been discussed by Holmes and Emmett (4). Nitrogen isotherms at -195°C. on original and degassed charcoals CWSN 44, PCI 1042, and PCI 1042 ash-extracted, are presented in figures 3 and 4, where the volume of nitrogen adsorbed per gram is plotted against relative pressure. With CWSN 44 in figure 3, the high-temperature evacuation caused the volume adsorbed a t a relative pressure of 0.2 to drop from 390 t o 264 cc./g. (to 61 per cent of the original),.the entire isotherm being decreased by about this ratio. The change in the isotherms is interpreted as a sintering of an equal fraction of all of the pores. With ash-extracted PCI 1042 in figure 4, the entire isotherm was decreased t o a constant fraction (89 per cent) of that of the original extracted sample on hightemperature evacuation. As with CWSN 44, this may be interpreted as the removal of an equal fraction of all the pores by sintering. The degree of sintering is less than with N 44. The original PCI 1042 sintered to a greater extent than the two other charcoals, and the shape of the isotherm changed to a more pronounced S-type. The degassed charcoal has a lower surface area and probably a smaller number of small pores (less than 16 A. in diameter) and a larger number of large pores (greater than 50 A. in diameter) than the original charcoal. Thus, some of the pore walls may have been removed by reduction of the oxides to form larger pores, while probably other pores were sintered. With CWSS 44 and the ash-extracted PCI 1042 the high temperature caused sintering, but there was little change in the pore distribution. The CWSN 44, which contains zinc oxide, sintered more than the ash-free PCI 1042. With the original PCI 1042 the ash, either because of the large amount or the composition, caused changes in the distribution of the pores. Emmett and Holmes (4) found that certain oxides, such as ferric oxide, nickelous oxide, chromic oxide, and
SURFACE COMPLEXES ON CHARCOAL
1325
cupric oxide had pronounced effects in altering pore structure on nitrogen, hydrogen, or steam treatments.
Oxygen pickup by degassed samples Lowry and Hulett showed that oxygen pickup by charcpal at room temperature is of two distinct types: namely, that which is chemically bound to the charcoal surface and that which is physically bound. The chemically bound oxygen is picked up slowly over a long period of time, whereas the physically adsorbed oxygen is taken up and given off readily by the charcoal at room temperature. Experimental n-ork in the present research confirms these results as regards the chemical adsorption of oxygen; no observations were made relative to physical adsorption. In run 15 of tables 6 and 7 it is shown that the treatment of CWSS 196 B1X with nitrogen at 1000°C. is about as effective in removing oxygen complex and hydrogen as evacuation at 900°C. When this sample in run 15 was exposed to air for a week a t room temperature, it picked up sufficient oxygen so that the carbon dioxide evolved on degassing t o 120OOC. was as large as that from the original sample and the carbon monoxide evolution was two-thirds as large. At the same time, more than the original complement of chemisorbed water was picked up by the charcoal, though it is possible that most of this water mas held by the 5 per cent ash content that the sample contained. Heating CWSN 196 B1X in a stream of nitrogen even to 1100°C. did not prevent the sample on re-exposure to air a t 25°C. from picking up enough oxygen to evolve, on heating, 50 per cent of the original carbon monoxide and 100 per cent of the original carbon dioxide content. Heating a sample of CWSS 196 B1X T H 4104 to 300°C. in oxygen for 3 hr. put more complex on it than mas present on the original CWSS 196 B1X. The total evolved oxygen given off increased from 2.59 per cent by weight on the original sample to 2.82 per cent on the oxidized sample. The carbon monoxide evolution became 50 per cent greater than on the original sample, whereas the carbon dioxide evolution was approximately the same as on the original. Heating one of the original samples of CWSX 196 B1X in oxygen to 300°C. for 3 hr. (run 19) produced a product similar to that obtained on heating the degassed sample, except that the oxygen in the complex became 3.74 per cent by weight of the sample, in contrast to 2.82 per cent that had been reached by oxygen treatment of a degassed sample (run 19, tables 6 and 7 in comparison to run 18). Extracting a sample of this same charcoal with hydrofluoric acid and then heating it t o 1000°C. in a stream of pure nitrogen produced the most complex-free sample that was encountered. The total free and combined hydrogen evolved on heating this sample t o 1200°C. was only 0.15 per cent by weight of the charcoal, and the combined oxygen was 0.17 per cent by weight. Furthermore, the CWSN 196 B I X TH 410 was prepared by heating sample CWSN 196 B I X in a stream of nitrogen a t 1ooO"C. for 4 hr. t o a 4.6 per cent weight loss and then cooling the sample to room temperature in nitrogen.
1326
ROBERT B. ANDERSON AND P. H. EMMETT
residual 0.8 per cent ash (presumably zinc oxide) could account for practically all of the combined osygen evolved from this sample (run 20, tables G and 7 ) . Complex jormation by steaming or by aging in m i s t air I t has been reported a number of times in the literature that charcoal changes its properties as a result of aging. In the aging for GO days at 50°C. and 80 per cent humidity, the charcoals shown in table 8 increased their osygen contents by 50 to 120 per cent. The increased osygen content represents an increase in the volume of evolved carbon monoxide, carbon dioside, and ivater vapor. However, on the samples macle by the zinc chloride process ( C R S S S5 and CWSS 44) care must be used in interpreting thi? result. There is a sufficiently large ash content on both of these charcoals t o account for the entire carbon dioxide content by asuming the ash to be in the form of zinc carbonate after the aging. Furthermore, the total water picked up by these two charcoals is no more than equivalent to the ash holding 2 moles of n-ater per mole of zinc oside. ;iccordingly, it is difficult t o state xvith certainty the extent to which the aging has actually changed the composition of the surface comples, in contraat to merely causing an increase in the carbon dioxide and xater content of the ash. Hoivever, the similarity between the results obtained on the PCI charcoal with 20 per cent ash and the CWSS charcoals with 3.8 and G.0i per cent ash (probably zinc oxide) makes one believe that a major portion of the added oxygen pickup is in the form of a surface complex. The drying of an aged ssmple a t 115°C. for 3 hr. produced no significant change in the amount of surface complex. The results of experiments on the steamed samples shown in table 9 are of considerable interest. For the most part, it is evident that steaming even at temperatures as high as 900°C. produces very little oxygen-carbon complex on the charcoal surface. Steaming a t and above 000°C. causes a slight increase in the amount of complex that yields carbon monoxide on heating in the range G00-120OoC., but the total oxygen complex is equivalent to only 1 or 2 per cent of the surface area even after prolonged steaming. Run E in table 9 indicates that steaming is capable of causing the hydrogen content of the charcoal to increase. After the 900°C. steaming, the hydrogen content evolved t o 1200°C. was about 47 cc. per gram compared t o 15-20 cc. for the initial samples and for samples steamed to 750OC. It is not certain, however, whether this extra hydrogen is t o be attributed t o the building up of a surface complex or to the uncovering of a certain amount of hydrogen-carbon complex by the 5G per cent removal of carbonaceous material by steaming. It is also possible that the accumulation of hydrogen might result from the preferential removal of carbon atoms not attached to hydrogen atoms. In general, the results obtainid on the steamed charcoals are in agreement with those reported by Rluller and Cobb ( 7 ) ,who studied the chemisorption of water vapor on acid-extracted charcoal at temperatures from 300" to 1100°C. In both their experiments and ours, a small amount of water n-as chemisorbed and was removed as carbon monoxide, carbon dioxide, or water vapor on degassing
SURF.iCE COVPLEXES ON CHARCOU
1327
t o a high temperature. They noted, as did me, that in the temperature range 7O0-9OO0C., steaming results in the fixation of more hydrogen than oxygen.
Displacement of oxygen complex by adsorption Allmand (l),who studied the sorption of vapors such as carbon tetrachloride, carbon disulfide, and water on charcoal at room temperature and pressures less than 0.1 mm. of mercuiy (relative pressures less than 0.001), found that in the adsorption process a portion of the oxygen complex was displaced as carbon monoxide and carbon dioxide. This was accompanied by “drifting” of the isotherm, the change in the amount adsorbed being many times the volume of complex displaced. The “drift” could be eliminated only by exposing the sample t o the adsorbate a t pressures of about 0.1 mm., this procedure being more effective than evacuation a t 800°C. I n no case have n e been able to find any statement by Allmand (1) as t o the volume of the displaced gases, but v e have noted that the pressure of the evolved gas was reported t o be as high as 0.04 mm. in the system. Our experiments (table 10) show that the amount of complex displaced from our samples in the adsoiption of vapors was very small, usually less than 0.00G cc. (S.T.P.) per gram. The carbon monoxide evolution n a s equivalent t o less than 0.001 per cent of a monolayer, and t o less than 0.02 per cent of the total carbon monoyide that is evolved on evacuating t o 1200’C. Similarly, the carbon dioxide evolution is small in all experiments except those in nhich water was adsorbed on a charcoal made by the zinc chloride process. It does not seem unreasonable that for these charcoals, the carbon dioxide evolution could be due to the reaction of water vapor with zinc carbonate that might be present in the ash. The largest carbon dioxide evolution n as equivalent t o only about 8 per cent of the total carbon dioxide that is normally evolved n hen the sample is evacuated t o 1200°C. It is equivalent to only a small fraction of the total zinc oxide in the sample if one assumes, as is usual for samples prepared by the zinc chloride process, that the ash is entirely zinc oxide or some product such as zinc carbonate that could be formed during the contact of the sample with air. Since the extent of the surface uncovered by displacing the complex nas so small, it is probable that no changes in the adsorption isotherms such as reported by Allmand (1) would have been obtained on these samples. It is obvious that adsorption of a vapor t o “clean up” the surface has very little effect in these cases on the oxygen complex compared t o the effect of evacuating the sample at high temperatures. Formation of a nitrogen complex Although, as shov n in previous experiments, molecular nitrogen does not react with charcoal, i t has been reported (13) that a nitrogen comple.: can be prepared by passing ammonia over charcoal at 700-900°C. In the process some of the charcoal nas gasified, with the formation of hydrogen cyanide. The nitrogen complex TI as reported t o be more stable t o thermal decomposition than an osygen complex; for example, cokes containing nitrogen can be heated t o 1100°C. without an appreciable decrease in the nitrogen content.
1328
ROBERT B. ANDERSON AND P. H. EMMETT
An extracted CWSN 196 BlX, similar but not identical to the samples used in the steam activation runs of table 10, was exposed to a slow flow of ammonia air being excluded throughout the experiments. The a t 750°C. and 900"C., results are given in table 11. In each case some of the sample was gasified and some nitrogen and hydrogen fixed as a complex. In both experiments, about 1 per cent of free and combined nitrogen \vas removed in the degassing; most of this mas evolved in the range 900-1200aC. in the form of nitrogen, and hydrogen cyanide or cyanogen. The gas analysis method was not particularly suitable for these mix%ures,but since the gases were fractionated into three parts, it is believed that the errors are not serious. It is interesting to note that the oxygen complex decomposed in the range 600-8OO0C., whereas the nitrogen comples decomposed principally above 900°C. SUMbIARY
1. Samples of all major types of charcoals studied by the Sational Defense Research Committee in World War I1 were evacuated over 300°C. intervals to 1200°C., the evolved gases being collected and analyzed. Oxygen-containing gases, carbon monoxide, carbon dioxide, and water vapor were formed by the decomposition of the oxygen complex and reduction of the oxides in the ash. Large amounts of hydrogen were evolved. In all charcoals, the amount of oxygen evolved would cover less than 50 per cent of the surface. 2. With zinc chloride-wood charcoals, the zinc oxide was reduced in the temperature range GOO-9Do°C., as indicated by the formation of a mirror of metallic zinc in the cooler portions of the degassing tube. The oxides in coal charcoals, silica and alumina, were chiefly reduced in the 900-1200°C. interval. The coal charcoals had less oxygen complex and less hydrogen on them initially than most of the zinc chloride-wood charcoals. 3. Aging of the charcoal in moist air at 50°C. and 80 per cent relative humidity for a month increased their oxygen content by 50-120 per cent. 4. Steam activation was shown to form very little oxygen complex. At GOOaC. and below, the sample was not gasified. Above 000°C. gasification occurred, and the hydrogen content of the sample increased. 5. Studies on the amount of oxygen complex displaced on adsorption of carbon tetrachloride, chlorobenzene, or mater vapor on charcoal a t room temperature indicated that the gas evolved was less than 0.01 cc. per gram of charcoal. 6. While molecular nitrogen does not react with charcoal, ammonia a t 750°C. and 900°C.formed a nitrogen complex which was more stable thermally than the oxygen complex. REFEREKCES
(1) ALLYAND et al.: Proc. Roy. SOC.(London) Al29,235,252 (1930);A130,610 (1931);Al69, 25 (1938). (2) BRUNNER: Z. Elektrochem. 38, 55 (1932). (3) EMMETT:Advances in Colloid Sei. 1, 1-36 (1942). ( 2 ) HOLMES AND EMMETT: J. Phys. Colloid Chem. 61, 1276 (1947). AND HULETT:J. Am. Chem. SOC.42, 1408 (1920). (5) LOWRY
SURFACE AREAS OF METAL SPHERES AND CARBON BLACKS
1329
(6) MIXER: U. S. Bur. Mines, Bull. No. 924 (1930). (7) MULLERAND COBB:J. Chem. Soc. 1940,177. (8) RHEADAND WHEELER:J. Chem. SOC. 103, 461, 1210 (1913); Proc. Chem. Soc. 29, 51, 193 (1913). (9) ROWLEY AND AHDERSON: Ind. Eng. Chem., Anal. Ed. 11, 397 (1939). (10) STIMSON:J. Washington Acad. Sci. 7,477 (1917). AND SAUNDERS: J. Chem. Phys. 9, 616 (1941). (11) TAYLOR (12) URRY:J . Phys. Chem. 38, 1831 (1932). (13) VANDER LEYA N D WIBAUT:Rec. trav. chim. 61, 1143 (1932). (14) BENTEAND WALTON: J . Phys. Chem. 47, 133,329 (1913).
SURFACE-AREA MEASUREMENTS OK METAL SPHERES AND ON CARBOS BLACKS' P. H. EMMET'P
AND
MARTIN CINES*
Department of Chamicul Engineering, The Johns Hopkins Unzuersity, Baltimore 18, Maruland Received June If, 1847
During the past ten years a great many (2, 4, 5, G, 7, 9, 10, 11, 13, 15) apparently successful measurements have been made of the surface areas of finely divided materials by means of physical adsorption isotherms of nitrogen and other gases near their boiling points. It seems to have been well established (5,7) that a plot of the volume of gas, T', adsorbed at the relative pressure p l p o according to the equation
yields a straight line over a sufficiently large relative pressure range to permit the determination of I,,,,, the volume of gas required to form a monolayer on the solid adsorbent. A simple multiplication of the number of molecules in the monolayer by their average cross-sectional areas yields an absolute surface area value in square meters per gram. The present paper presents additional evidence of the usefulness of the method in summarizing briefly a number of adsorption results for finely divided porous and non-porous solids that were selected either because of size and shape uniformity or because of the extremely small particle size. The adsorption measurements for the most part have been restricted to the use of nitrogen as an adsorbate at - 195"C., though some runs have been made with carbon disulfide and butane. 1 Presented at the Symposium on the Adsorption of Gases which was held under the auspices of the Division of Colloid Chemistry at the 110th Meeting of the American Chemical Society, Chicago, Illinois, September 11-12, 1916. 2 Present address: Mellon Institute, Pittsburgh 13, Pennsylvania. Present address: Phillips Petroleum Company, Phillips, Texas. J
