W. H. SLABAUOH AND B. C. SEILER
396
Vol. 66
INTERACTIONS OF AMMONIA WITH GRAPHITE OXIDE BY W. H. SLABAUGH AND B. C. SEILER Department of Chemistry, Oregon Slate University, Corvallis, Oregon Received Azsoust 8, 1051
Certain as ects of the structure of graphite oxide and the mechanism by which it adsorbs ammonia have been investigated througff observation of the phenomena: adsorption isotherms and isobars, infrared absorption spectroscopy, and X-ray diffraction. From the apparent stepwise process of adsorption and intercalation of ammonia, the graphite oxide is shown to consist of laminar platelets that are populated by enol and keto groups. The resulting rough surface possesses holes or cavities that accommodate the early portions of adsorbed ammonia. Further adsorption produces a monolayer that separates the platelets by distances proportional to the size of the ammonia molecule. The adsorption process is not simple, but involves condensation of the film, chemical interaction with the acidic sites, and physical adsorption.
Introduction
samples, the graphite oxide was electrodialyzed for approxi-
48 hr. in a Mattson type cell. The electrode polarity Although graphite oxide has been known since mately was reversed periodically to prevent excessive accumulation 1859, when Brodie' first oxidized graphite with of material on the cellophane membranes. The dialyzed fuming HXOa and KC103, little has been reported products were freeze-dried and stored at ambient temperaabout its gaseous adsorption properties. Graphite tures in clear glass bottles. The compositions of the oxide has two features which make it a potentially graphite oxides are given in Table I. good adsorbing agent. First, graphite oxide reTABLE I tains a laminar structure similar to graphite itself,2 COMPOSITION OF GRAPHITE OXIDE and therefore, it should have approximately the %C %O %H %ash C:O ratio same surface area as graphite. Second, graphite Conventional Brodie method (vacuum dried) oxide has been found by Thiele,S Hoffman14 and 50.05 46.92 3.03 0.1 1.43:1 others t o have radicals such as carboxyl, hydroxyl, 50.71 46.19 3.09 .1 1.46:l epoxy, and possibly other groups attached to the 51.71 45.70 3.13 .1 1.51:l hexagonal platelets. These groups constitute ener51.15 45.97 2.87 .1 1.48:1 getic sites for chemical adsorption, especially for Modified Brodie method (vacuum dried) polar molecules. The adsorption of water and certain non-polar 55 * 53 42.24 2.23 0.01 1.75:l gases, such as nitrogen and hexane, on graphite 55.48 42.00 2.52 .01 1.76:l oxide has been studied by several investigator^.^^^ I n early ex eriments the basis of weight was graphite oxide They found that, while there was considerable vacuum driejfor 24 hr. at less than 0.1 mm. pressure. Howwater adsorption, the adsorption of non-polar gases ever, this basis was subject to error although the weight rewas comparatively small. The adsorption of water mained constant after this period. Therefore, the graphite is attributed t o the intercalation of water molecules oxide was stored over a saturated Ca(NOs)z solution, the relative humidity of which was 51%. The moisture content between the graphite oxide platelets, which is possi- of gra hite oxide kept in this manner was approximately ble because of the high dipole moment of water and 18-19k. This percentage in turn was based on samples oven-dried for 3 hr. at 160". The temperature, 160", was the hydrophilic nature of graphite oxide. Because of the acidic nature of graphite oxide chosen as the standard drying temperature because differthermal analysis* revealed that graphite oxide appears and the high polarizability of NHI, the adsorption ential to lose all of its adsorbed water between 115-130°, while of this gas should be even more interesting than complete decomposition does not occur below 200". Furbherthat of mater. This paper shows the results of an more, X-ray diffractiong showed that the basal spacing a minimum at approximately 140 to 150". investigation of the graphite-NH3 system, in- reaches Finally, pryrolysislo of graphite oxide had shown a definite volving adsorption and desorption isotherms, in- break in the loss of weight when graphite oxide was heated frared absorption spectra, and X-ray diffraction. to 150-160". Little difference appeared between the strucAn examination of the adsorption properties of ture and composition of vacuum-dried and oven-dried graphite oxide. This was demonstrated by infrared analysis graphite oxide salts also is included.
Experimental Sample Preparation .-The graphite oxides were prepared from Canadian graphite No. 5, which was supplied by the Asbury Mills, Asbury, New Jersey. The material was 99% pure and its particle size was less than 35 p in diameter. No preliminary treatment of the graphite was made. Samples of graphite oxide were prepared by the conventional Brodie method,' and by a modified Brodie method suggested by Maire.' After the final oxidation step for all (1) B. C. Brodie, Trans. Royal Soc. (London), 149,249 (1859). (2) H.L. Riley, Fuel, 24, 43 (1954). (3) H. Thiele, liollozd-Z., 30, 1 (1937); 116, 167 (1949); 116, 1 (1950); 117, 144 (1960). (4) U. Hoffman and E. Konig, Z. anorg. u. allgem. Chem., 234,311 (1937). (5) J. H. de Boer and A , B. C. van Doorn, Proc. Koninkl. Ned. Akad. Wetenschap., 61B,242 (1958). (6) W.H.Slabaugh and Conrad Hatch, J . Chem. Eng. Dafq, 5, 453 (1960). (7) J. Maire, Compt. rend., 232, 61 (1951).
in the present study and by C and H determinations. The graphite oxide salts mere prepared by batch ion-exchange of the graphite oxide prepared above, using 10% salt solutions. The concentrations in meq. of metal ions per g. of the graphite salts were 2.00, 1.61, 1.09, and 0.83 for the Li, Na, K, and Rb salts, respectively. Adsorption.-The NH, adsorption isotherms were determined gravimetrically with quartz helixes maintained at constant temperature. Changes in weight of a 0.3000-g. sample could be detected with an accuracy of 1 0 . 0 2 mg. Samples were thermostated during the adsorption measurements to *0.2". Weighed samples of graphite oxide were suspended from the helixes and evacuated slowly for 24 hr. Then the pressure was lowered to 0.1 p or less, and outgassing was continued for two more days at room temperature before measurements were taken. The samples then were brought to the desired temperature, and appropriate increments of NHs
(8) C. V. Hatch, Ph.D. thesis, Orogon State College, 1960. (9) J. Cano-Ruin and D. M. C. MacEwan, Tercera Reunion International Sobre Reactivadad De Loa Solidoa, April, 227 (1956). (10) E. Matuysma, J . Phys. Chem., 08, 215 (1964).
March, 1962
INTERACTIONS or AMMONIAWITH GRAPHIT.E OXIDE
397
TABLE I1 ADSORPTION BEHAVIOR OF GRAPHITEOXIDESAND SALTS WITH NHa Graphitu oxides at - 3 6 O (Conventional Brodie samples)
W,-B.E.T. (mg./g.) (meq./g.) Area (m.a//s.) Resndual NH, after desorption a t 25" mg./g. meq./g. Residual NHa after desorption at 70' mg./g. meq./g.
164 9.7 750
170 10.0 778
Li
167 9.8 764
175 10.3 799
Graphite oxide salts at -35O Na K
143
144
144
649
657
658
658
..
..
42.3 2.49
42.8 2.52
34.6 2.04
35.1 2.06
47.9 2.82
..
21.7 1.28
13.4 0.79
13.8 0.81
42.8 2.52
..
were added. Equilibrium was obtained in 2 to 10 hr. depending on the sample, temperature, and pressure. Infrared Absorption.-Infrared absorption spectra of graphite oxide were obtained by the pressed KBr pellet technique with a Perkin-Elmer Model 21 double beam instrument. Both LiF and NaCl prisms covering the range from 4,000 to 2,000 om.-' were employed, but the difference in the spectra from these two prisms waa negligible in this region. Only the NaCl prism was used in the 2,000-650 cm. -1 region. Samples also were examined by Lippincottll by means of a diamond cell in a Model IR4 Beckman spectrophotometer with a NttCl prism. X-Ray Diffraction.-X-Ray diffraction studies were made on graphite oxide with NHa adsorbed a t room temperature. Other temperatures were not used because the proper equipment was not available to maintain the sample a t constant temperature while making the X-ray analysis. The X-ray diffraction gamples were mounted in Plexiglas sample holders and were protected with a covering of 0.012-mm. polyethylene film. The sample holders were placed in glass tubes which were attached to the vacuum system. The tubes could be readily sealed off from the system a t appropriate pressures. A minute hole in the Plexiglas holder allowed NHil to reach the sample, and this hole then was sealed with a silicone grease immediately after the glass tube was broken in an inert atmosphere. Within a few minutes the X-ray diffraction measurement waa made.
Discussion of Experimental Results A.dsorption.-NH3 adsorption isotherms were obtained for graphite oxide a t -45, -35, and -25"
Rb
I.42
..
.. .. ..
..
33.8 1.98
31.4 1.85
27.4 1.61
25.0 1.47
0 RRODIE 0 BRODlE
a
d
MODIFIED BRODIE
Desorption 9 BRODIE & M O D I F I E D BROOIE
10
2u
for the conventional Brodie samples. The isotherms for the four samples were almost identical in shape, although their vertical displacement varied over a range of 6%. Relatively little difference was noticed between isotherms at -35 and -25'. Because of their similarities only the isotherms for the fourth sample are shown in Fig. 1 for -45 and -35'. An isotherm of a modified Brodie sample at -35' also is shown in Fig. 1, demonstrating the similarity of NHa isotherms for graphite oxide made by the conventional and the modified IBrodie methods. From I3.E.T. plots which were regularly linear up to PIP0 of 0.25, the surface areas were calculated on the basis of an area of 12.9 A.2for the NH3molecule. The areas shown in Table I1 are much higher than the
