April, 1956
CARBON-OXYGEN AND CARBON-HYDROGEN SURFACE COMPLEXES
The heats of formation of gaseous HCI, CH4 and CzHs are given as9 -22,063, -17,889 and -20,236 cal. per mole, respectively. Using this information and the present data, we can calculate the heats of formation of methyl, ethyl and vinyl chlorides. The results are given in Table VI. Bichowsky and RossiniIb give 20.1 ked. for the heat of formation of methyl chloride. For ethyl chloride they give 25.7 kcal. Recently Casey and FordhamlBmeasured the heat of combustion of (15) "Thermochemiatry of Chemical Substances," F. R. Bichowsky and F. 0. Rossini, Reinhold Publ. Co., New York, N. Y . . 1936. (16) D. W. H. Casey and 5. Bordhnm, J . Chem. SOC.. 2513 (1951).
495
ethyl chloride and obtained a value of -341 f 2.5 kcal. From this they calculate the heat of formation to be 24 kcal. The present method has an advantage over the combustion experiment in that the total heat effect is small. Bichowsky and RossinilS give -9 kcal. as the heat of formation of vinyl chloride. Using the present data, one can calculate the heat of addition of hydrogen chloride to ethylene to be -16,173 cal. and the heat of addition of hydrogen to yinyl chloride to be -34,567 cal. These are shown as the last two entries in Table VI. About 2000 cal. more heat is liberated when hydrogen is added t o vinyl chloride than when it adds to ethylene.
CARBON-OXYGEN AND CARBON-HYDROGEN SURFACE COMPLEXES' BY R. NELSONSMITH,JACKDUFFIELD,ROBERT A. PIERO'M'I AND JOHNMoor Chemistry Department, P o "
College, Chremont, California
Received Octobar 17, 1066
Experimental results show that hydrogen-treatment of carbon surfaces at 1000° is effective in removing carbon-oxygen complexes and that carbon-hydrogen complexes are not formed in their place. HAreatment reduces the h drogen content of carbon samples, probably a result of the thermal treatment. The hydrogen content of a carbon sample is distributed throughout the sample and is not bound to the surface alone. Outgassing at 100" ie effective in reducmg the amount of carbon-oxygen complex on Graphon, a black, but it is not effective for a charcoal. Hs-treated surfaces rapidly pick up carbon-oxygen complexes on exposure to air at room temperature. The extent of carbon-oxygen complexes resulting from treatment of carbon surfaces with air, nitrous oxide, nitric oxide and water is given and related to previously published work.
I n an effort to correlate some observed surface phenomena with the amount of carbon-oxygen complexes present on the surface, a microanalytical method was developed for the determination of these complexes. The method is published elsewhere,2but the results obtained with a few carbons are published and discussed here for the information of those who have used the same or similar carbons. In addition, there is presented here some information about carbon-hydrogen surface complexes. There has been some question in the minds of the authors and others concerning the nature of carbon surfaces remaining after the hydrogen treatment used to remove carbon-oxygen complexes. Hztreatment, as usually performed by the authors, consists of heating the sample in vacuo to 1000", then adding an atmosphere of hydrogen. The hydrogen atmosphere is removed after 15 minutes and followed by a pumping period of about onehalf hour. This addition of hydrogen followed by pumping is repeated two more times. After the third addition the sample is allowed to cool slowly from 1 0 o 0 O with continuous pumping by a high vacuum system. Pumping is usually continued for several hours or overnight. The question is: does removal of carbon-oxygen surface complexes by this treatment leave in their place a surface with carbon-hydrogen complexes? (1) This ia a progress report of work done under Contract N h n r 54700 with the Office of 'Naval Research. Reproduction in whole or in part ia perniitted for nny purpose of the United States Go,vernment. (2) R . N. Smith, J . DiiHield. R. A. Piprotti and J. RIooi. A n a l . Chem., 28, in press (1950).
Experimental One black and two charcoals were used in this work. Their properties are as follows. Graphon .-A partially gra hitised carbon black (supplied through the courtesy of the &dfrey L. Cabot Co.) was made by heating Spheron Grade 6 (a medium processing channel black) to approximately 3000" in an electric furnace. The surface area is about 80 sq. m. per g. and its ash content is about 0.020/0. Su-W.-A sugar charcoal of extremely low ash content was prepared, starting with Confectioners AA sugar furnished through the courtesy of the California and Hawaiian Sugar Refbing Corporation. This sugar was used because it had an ash content of 0.0008%. After activation8 this charcoal had a BET area of 1020 sq. m. per g., using ethyl chloride, and its ash content was less than 0.0050/0. B.-An activated commercial nut charcoal, de-ashed with HC1 in a Soxhlet extractor, dried and heated to 1000" tn u m o . The BET surface area is 1050 sq. m. per g. and it8 ash content is about 0.2%. Nitrous Oxide.-Obtained from the American Medical Gas Co. and used from the cylinder without further purification. Precautions were taken to prevent contamination on removal from the c linder. Nitric Oxide.-dbtained from the Matheson Co. and used from the cylinder without further purification. Precautions were Gken to prevent contamination on removal from the cylinder. Analytical Apparatus and Procedure.-The carbon-oxygen complexes were determined by a micro ter Meden method.* The details concerning the treatment and handling of carbon samples are also included in this analytical article. Each analysis was alternated with a blank run, and a sample of pure benzoic acid (Parr Calorific Grade) was used occasionally as a check on catalyst performance. Each sample waa outgassed in uacuo at llOo for 12 hours prior to analysis except where noted. After outgassing the samples were always stored, transferred and weighed in a nitrogen atmosphere. The hydrogen contentn were determined by burning samples in a stream of pure oxygen. The conventional
method of Niederl and Niederl wa,s not used because of the great length of time required for complete burning of samples. Instead the fast-flowrate and unpacked-tube method of Belcher and Spooner' was used with silver gauze as recommended by them for removal of sulfur. Only an Anhydrone absorption tube (in addition to a guard tube and mariotte bottle) was used after the train since water was the onl product whose determination was desired. Nitrogen, ur and ash do not interfere with this determination and, in fact, sulfur could be determined simultaneously if desired. Technique and procedure were checked using pure benzoic acid (Parr Calorific Grade). The carbon samples were prepared for analysis as indicated in Table 11.
Treated with NtO at 500" for 3 hr. (3 exposures of 1hr. each) Treated with NO a t room temp. for 3 hr. (3 exposures of 1 hr. each) Heated a t 1000" for 2 weeks with continuous pumping. Then, a t loo", it was exposed to about 25 mm. HtO vapor for 10 days. This was followed by 2 days of outgassing a t 100' As supplied; not outgassed
sd
Results The results obtained with carbon-oxygen complexes are given in Table I and those for carbonhydrogen complexes given in Table 11. The sample treatments shown in Table I were chosen because of other work carried out in this Laboratory. For reasons previously published2 it is believed that the results in Table I are not seriously in error because of the presence of sulfur, nitrogen or ash in the various samples. The nitrogen content was assumed to be negligibly small, though if it had not been, the same analytical method could have been used with h c a r i t e instead of h h y d r o n e in the TABLE I Sample
Sample wt., Treatment before analysis mg.
O x y p
oxygen av..
%
Graphon As supplied; not out- 504.3 0.0176 826.4 .0129 0.0153 gassed Outgassed only 623.2 .0043 553.9 .0032 .0038 Hrtreated at 1000" for 504.3 .OOO .OOO .OOO 15 min. No subse- 826.4 quent exposure to air Hrtreated a t 1000" for 389.0 .0228 15 min. Subsequent 389.0 .0251 .0240 exposure to air at room temp. for 2 hr. Poured through air at 390.4 .312 700" 260.1 .342 316.1 .309 377.2 .358 .330 .0130 Treated with N2O a t 478.0 .0123 .0127 500" for 3 hr. (3 ex- 432.6 posures of 1hr. each) Treated with NO at 566.5 .0235 .0294 room temp. for 3 hr. 542.7 (3 exposures of 1 hr. 578.7 .0292 458.8 .0291 .0278 each) 189.8 .412 Su-60 As made; not out195.1 .428 g=d .415 197.1 .415 221.8 .368 Outgassed only 236.4 ,402 .385 Hrtreated a t 1000" for 170.9 .Ooo 15 min. No subse- 202.6 .000 .Ooo quent exposure to air Hptreatad a t 1000" for 170.9 .0676 .0570 15 min. Subsequent 202.6 exposure to air at 193.8 .0687 .0644 room temp. for 24 hr. R. Belcher and C. E. Spooner, J . Cham.
Soc., 313 (1943).
178.2 1.19 146.0 1.22
1.21
152.1 2.72 133.8 2.86
2.79
274.1 0.434 182.0 .439
0.437
60.7 50.6 57.1 68.5 114.4 Outgassed only 90.1 164.1 168.5 Hptreated a t 1000" for 60.7 15 min. No subse- 50.6 quent exposure to air Hptreated a t 1000' for 153.7 15min. Subsequent 157.3 exposure to air for 2 hr. at room temp. Hptreated at 1000" for 156.8 15 min. Subsequent 150.1 exposure to air a t 155.2 room temp. for 7 days
PERCENT.OXYQEN IN CARBON SAMPLES
(4)
Vol. 60
R. N. SMITH, J. DUFFIELD, R. A. PIEROTTI AND J. Moo1
496
4.46 4.23 4.25 4.15 4.41 4.20 4.38 4.55 0.000
4.27
4.39
,000
.ooo
.150 .141
,145
.351 .297 .315
.321
TABLE I1 PERCENT.HYDROGEN IN CARBON SAMPLES Sample HydroSample
Treatment prior t o analysis
wt.. mg.
gen.
%
drogen HYav..
%
Graphon Outgassed a t 450" for 4 499.1 0.038 557.0 .033 hr . 454.6 .033 0.035 Hrtreated at 1000" for 1 551.9 .026 .026 hr. Pumped continu- 542.4 .026 ously while cooling SU-60 Outgassed a t 450" for 4 116.0 .550 111.0 .510 hr. 102.9 .522 127.8 .525 .527 Hrtreated a t 1OOO" for 1 129.8 .360 .350 hr. Pumped continu- 159.2 128.2 .356 .355 ously while cooling Outgassed at 1OOO" for 2 118.4 .296 hr. Pumped continu- 95.42 .304 91.75 .295 .298 ously while cooling Hrtreated at 1250' for 8 135.9 .167 .179 hr. Cooled to room 117.0 temp. in HSa t 1 atm., 80.5 .175 .174 then outgassed a t 450" for 4 hr.
April, 1956
CARBON-OXYGEN AND CARBON-HYDROGEN SURFACE COMPLEXES
absorbent tube. The results in Tables I and I1 may be compared with the data given by Studebaker516for a variety of carbon blacks. His oxygen values were determined by the Unterzaucher method with suitable correction made for ash; his hydrogen values were determined by combustion analysis. Discussion Carbon-Oxygen Complexes.-Table I shows quantitatively the magnitude of some commonlyobserved effects. Outgassing of the Graphon at 110" is effective in removing the majority of the oxygen, but in the case of the charcoals it has negligible effect. H2-treatment of carbon surfaces a t 1000" is very effective in removing carbonoxygen complexes, at least as determined by a method which is itself a Hz-treatment. After Hztreatment, carbon surfaces rapidly pick up oxygen from the air-Graphon an amount greater than that which it originally possessed, but the charcoals an amount which is only a fraction of what they originally possessed. Perhaps, in the case of the charcoals, much of the oxygen is associated with some of the hydrogen in a form related to the molecular structure of the material from which they were made (sugar or coconut shells). When this oxygen is removed and the sites destroyed by H2-treatment molecular oxygen may not be able to be rebound to these sites again when the carbon is exposed to air. There is an additional slow u p take of oxygen over a period of time. It is interesting to note that the oxygen uptake per unit surface area is considerably greater on the hydrogen-treated Graphon than on the hydrogen-treated charcoals. The extent of surface oxidation by NzO a t 500" is also given in Table I. At this temperature carbon surfaces rapidly catalyze the NzO oxidation of CO. It has been demonstratedSthat the carbonoxygen complexes formed to the extent shown poison the surface for this reaction, but if CO is present in equal or larger quantity than N20 this poisoning effect is essentially eliminated. The effect of these oxides on the reaction of NzO with carbon surfaces will be published separately, but it may be said now that the rate of reaction is increased by their presence. The extent of the room temperature oxidation of carbon surfaces by nitric oxide is about the same as by air on Graphon, but on Su-60 nitric oxide is about forty times more effective than air. The influence of this oxidation on the isotherms and heats of absorption of nitric oxide on carbon surfaces will be discussed in a separate paper. It has been shown previously7 that as water (5) M. L. Studebaker, India Rubber World. 197, 215 (1952). (6) M. L. Studebaker. Kaulschuk und Gummi, 6, 193 (1953).
(7) C. Pierce, R. N. Smith, J. W. Wiley and II. Cordea, J . Am. Chem. Sa.,78, 4551 (1951).
497
isotherms are determined the carbon surface is progressively oxidized, and as a result the adsorption of water is progressively increased. Air oxidation has the same effect.8 This oxidation is no doubt the cause of the apparent negative net heat of adsorption observed by Coolidgea for the adsorption of water on charcoals. It is probably the cause also of the initial adsorption (the BET value of V,) observed by Young, Chessick, Healey and Zettlemoyer, lo for Millard, Caswell, Leger and Millsll have shown that H2-treatment eliminates both the initial adsorption and the initial heat effect. Carbon-Hydrogen Complexes.-As mentioned in the Introduction, there has been some question as to whether high-temperature hydrogen treatment actually cleans up the carbon surface or merely replaces the carbon-oxygen complexes by carbonhydrogen complexes. The results in Table I1 indicate that Hz-treatment does not increase the amount of hydrogen held by the carbon. The hydrogen content actually decreases, but this is doubtless caused by the thermal treatment and not by the presence of hydrogen. In addition, a Graphon sample was burned stepwise, and the per cent. hydrogen calculated for the amount of carbon burned in each step. The results, using a 489.9-mg. sample, were as follows. Step 1: 86.6 mg. burned with 0.030% H. Step 2: 169.5 mg. burned with 0.026% H. Step 3: 186.2 mg. burned with 0.041% H. Step 4: 56.6 mg. burned with 0.078% H. If the whole sample had been burned a t once it would have shown 0.035% H as indicated in Table 11. These results show that the hydrogen which is present is not only on the surface but is distributed through the sample. This is reasonable since both blacks and charcoals are the pyrolyzed residues of organic compounds. It is also reasonable that the hydrogen content of Graphon should be so low since it is produced from Spheron Grade 6 (with 0.51% H) by heating to 3000" in an induction furnace. It is interesting to note that the hydrogen content of the interior of the Graphon particles is greater than the portion near the outside. It is also interesting to note that the ratio of the per cent. oxygen on the outgassed sample to that on the NzO-treated sample to that on the NO-treated sample is 1 m 3 . 3 4 : 7.32 on Graphon and 1.00:3.15:7.25 on Su-60. At present no real significance can be attached to this observation. (8) F. H. Healey, Y. Yu and J. J. Chessick, THISJOVRNAL, 69, 399 (1955). 49, 708 (1927). (9) A. 8. Coolidge, J . Am. Cham. SOC., (10) G. J. Young, J. J. Chessick, F. H. Healey and A. C. Zettlemoyer, THIEJOURNAL, 68, 313 (1954). (11) B. Millard, E. G. Caawell, E. E. Leger and D. R. Mills. ibid., S9, 976 (1955).
