PLENARY ACCOUNT
CARBON AS ADSORBEh AND CATALYST R O B E R T W.
COUGHLIN
Center for Marine and Environmental Studies and Department of Chemical Engineering, Whitaher Laboratory, Lehigh Uniuersity,Bethlehem, Pa. 18015
CARBONS of large specific surface area (sometimes called
A s associate professor of chemical engineering at Lehigh Uniwrsity, R. W. COUCHLIN has a B.S. in chemistry from Fordham (1956) and a Ph.D. in chemical engineering from Cornell (1961). He did postdoctoral work on chemistry of carbon at the University of Heidelberg in Germany, which was the origin o f his interest in carbon. A s a Fulbright Fellow, he worked with H . P. Boehm at the Institute for Inorganic Chemistry on enthalpy difference between hexagonal and rhombohedral graphite. Past experience includes work in polymer extrusion, oil-water separation and other techniques for control o f pollution from oil, simulation o f systems by digital and analog computer, and linear programming at Esso Research and Engineering. A t Isotopes Inc., he was responsible for all company data-processing activities and directed DOD projects in underground nuclear test detection. I n addition to his teaching activities at Lehigh, he is the Associate Director for Enuironmental Studies of the Center for Marine and Environmental Studies. He is a member of the American Chemical Society, American Institute for Chemical Engineers, New York Academy of Science, and American Association for the Advancement of Science.
12
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
active, amorphous, or microcrystalline carbon) are assuming increasing importance in the control of air and water pollution, in purifying and controlling the general chemical environment, and in certain biomedical applications. Many of these newer uses of carbon, as well as well known older applications, involve adsorption and catalysis, physical-chemical processes which may depend upon carbon's crystalline structure, microscopic physical structure, electronic properties, surface chemistry, and, sometimes, the presence of impurities within the carbon. Of the myriad uses of the common element, carbon, a thorough and complete survey of only its adsorptive and catalytic behavior could entail volumes, rather than a single article. For this reason, the present paper is limited to discussing only certain salient features of carbon as adsorbent and catalyst. The intent is to present some of the characteristics of carbon adsorbents and catalysts within a logical framework based on fundamentals of the physics and chemistry of carbon. Wherever possible, material has been selected to illustrate relationships between theoretical models or hypotheses on the one hand and experimentally observed phenomena on the other hand. This approach has the disadvantage of neglecting many experimental facts that may he poorly understood or not easily explained and it may also appear to oversimplify a complex subject. Nevertheless, i t is hoped that the presentation here will motivate more research, not only into the adsorptive and catalytic behavior of carbon for which explanations have been suggested, but also into those phenomena which may appear to defy explanation a t this time. The structure and surface chemistry of carbon are discussed first, followed hy an attempt to relate these properties to catalytic and adsorptive behavior, Excluded from consideration are phenomena believed to arise from "unusual" impurities sometimes present within the carbons of interest. However, account is taken of the "usual" impurities, hydrogen and oxygen, generally believed to he chemically incorporated within the high-surface-area carbons under consideration here.
The catalytic and adsorptive behavior of carbon is discussed primarily from the point of view of the carbon substrate. The crystalline structure, electronic properties, surface chemistry, and microcrystalline physical structure of carbon are reviewed and related to the catalytic and adsorptive power of that element. M a n y specific examples of such uses of carbon are drawn from the chemical literature for discussion to illustrate the general ideas and relationships. Crystal Structure
Within the three-dimensional lattice of diamond the carbon atoms are joined by covalent sp" bonds in tetrahedral geometry. The other well-defined crystalline form of elemental carbon is graphite, which possesses a well known layered structure. Within these layers the carbon atoms are joined by sp' covalent bonds in a fused ring system. such that each layer, if perfect, might be thought of as a huge polynuclear aromatic molecule. The layers are held together by relatively weak van der Waals forces. The interlayer distance in graphite is about 3.35 A. (Franklin, 1951) and the stacking sequence is . . . A B A B . . . sequence may also occur (Boehm and Coughlin, 1964). Figure 1 shows the structure of graphite. At the edges of the graphitic layer planes are free valences which may be expected t o combine chemically with any suitable. active, foreign atoms present upon the genesis of the carbon structure. These surface-bound atoms or surface group:s are discussed further below. As catalyst or adsmrbent, only black, microcrystalline (amorphous) carbon need be considered. This material has many commercial forms like active carbon, carbon blacks, coke, etc. The structure of microcrystalline carbon has features in common with graphite, as was shown by Hofmann and Wilm (1936) using x-ray diffraction. It consists of graphite-like layers stacked in packets of 3 to 30 layers about 10 t o 100 A. thick. I n microcrystalline carbon, the interlayer spacing is larger than in graphiteLe.. about 3.6 A.-and the stacking sequence is greatly perturbed. with the result that many graphitic layer planes are tilted with respect to one another. I n addition there may be present within microcrystalline carbon a considerable content of disorganized, tetrahedrally bonded carbon (Alexander and Sommler, 1956; Hofmann and Groll, 19329, often crosslinking nearby layers. Moreover, the graphitic layer planes may be expected to possess various carbonnetwork defects like holes, edge and claw defects, bond isomerism defects, and chemical impurities within the layer
planes. Ubbelohde and Lewis (1960) give a more extensive discussion of these defects. Surface Chemistry and Surface Groups on Carbon
The unsatisfied bonds at the edges of the graphitic layer planes of microcrystalline carbon are very reactive and form compounds with any suitable foreign atoms present. Therefore, functional groups or surface compounds can be expected almost exclusively at the layer edges; foreign atoms or molecules can be only weakly adsorbed on the basal faces by means of the graphitic a-electron system, except where they are bound at lattice defects. Most important and best known among the surface compounds of carbon are those with oxygen and sulfur, although other elements, such as chlorine and hydrogen, can also combine with elemental carbon. Of these compounds, the acidic surface oxides of carbon have received the most study, and the role of these oxides in adsorption and catalysis is considered below. S o t nearly so much is known about the other surface compounds or their role in adsorption and catalysis. Acidic surface oxides of carbon are formed under the most usual conditions of treatment and manufacture of microcrystalline carbon products like active carbon. Basic surface oxides also occur, but less frequently and their nature and structure have not yet been elucidated very thoroughly. Garten and Weiss (1957a) claim that basic oxides are produced during oxidation at high temperatures, but Boehm (1966) has pointed out that they are formed after an outgassed carbon surface comes into contact with oxygen after cooling in an inert atmosphere. In an extension of earlier work (Garten and LVeiss. 195Saqb;Studebaker et ai., 19563, Boehm et ai., (1964) used typical identification reactions of organic analysis to characterize oxygen chemisorbed on carbon as comprising four different types of acidic surface groups: I , a strongly acidic carboxyl group; 11, a more weakly acidic carboxyl group; 111, a phenolic hydroxyl group; and IV, a carbonyl group. Figure 2 shows a schematic structure
OH
OPEN FORM
Figure 1. Structure of hexagonal form of graphite
L A C T O N E FORM
Figure 2 . Possible structures of carbon surface oxides VOL. 8 NO. 1 M A R C H 1 9 6 9
13
representation of these groups, in which the difference between the two kinds of carboxyl groups is related to their ability to form a lactone or lactol. These acidic functional groups can be identified by their reaction (or failure t o react) with bases of different strength. Thus, group I is neutralized by each of the bases NaHCO,, I\ja?COi,and I\ja0C2Hi; group I1 is neutralized by S a C O I or stronger bases hut not by NaHCO.{,etc. Basic oxides predominate on the surface after carbon is heated in vacuum or in an inert atmosphere and comes into contact with oxygen only after cooling to low temperatures. I t is probable that both acidic and basic oxides are always present on the surface in varying proportions depending on the history of the carbon. Garten and Weiss (1957a,b) have suggested possible models for the basic reactions of a carbon surface (Figure 3). This is based on the experimental fact that oxygen is adsorbed and hydrogen peroxide liberated when carbons hind HC1. The nature and identification of the surface oxides of carbon have been reviewed by Boehm (1966) and Donnet (1968). Electronic Properties
Since microcrystalline carbon may be visualized as composed of graphitic layer planes, more or less poorly oriented, it is interesting t o consider the strong anisotropic conductance exhibited by graphite itself. This anisotropy, also exhibited in the thermal and mechanical behavior of graphite, may be regarded as a consequence of structure and bonding. Measurements on “single crystals” of natural graphite and pyrolytic graphite yield resistivity values a t room temperature of about 10~-‘ohm-cm. parallel to the layer planes and values on the order of 1 ohm-cm. perpendicular to the hexagonal networks (Dutta, 1953; Ganguli and Krishman, 1939; Grown et al., 1936; Primak and Fuchs, 1954, 1956). The low resistivity parallel to the layer planes of graphite ranks within the resistivity range found for common metals; indeed, experiments show that positive temperature coefficients of resistivity values perpendicular t o the layers lie within the range for semiconductors. Both positive and negative coefficientsof resistivity have been reported (Ganguli and Krishman, 1939; Primak and Fuchs. 1954, 1936) for this perpendicular Chrcmene-like s t r j c t u r e at layer edge:
direction where measurements are less reliable because of the effects of screw dislocations which could shortcircuit the conductance path. More detailed investigations as to correlation between specific resistance and thermoelectric power of graphite (Blackman et al., 1960) suggest that conduction in the hexagonal layer planes of graphite is primarily by r-electrons, while conduction by positive holes predominates in the perpendicular direction. In the K-electron band structure of the layer planes the positive and negative carriers are balanced closely in concentration, and the total concentration is small relative to that in metals. These factors cause the thermoelectric power to be very sensitive to impurities like chemisorbed oxygen which can trap electrons (Walker et al., 1965). Carbons useful as adsorbents and as contact catalysts are available in a variety of forms and can have their properties altered in many ways by different kinds of treatment. This is widely recognized with regard to pore structure and specific surface area, hut the variation in surface chemistry and electronic properties may be frequently overlooked. The electronic properties of wellordered graphite described above represent one extreme. For an appreciation of the possible variations in the electronic properties of other carbons it is useful to consider the model of Mrozowski and coworkers (McMichael et al., 1934). Figure 4 represents the various stages reached on passing from aromatic hydrocarbons through grossly defective graphites (cokes) to near ideal graphite, as involving progressive reduction of a finite energy gap between the full a-electron band and the conduction band. In any given microcrystalline carbon we might expect to find islands of order, differing in degree of perfection and, therefore, corresponding to several of the stages shown in Figure 4. Thus, we might find, within a typical sample, well-ordered carbon whose electronic behavior approaches that of a metal, as well as some highly disordered carbon with electronic behavior similar to that of an insulator. Further evidence of the strong anisotropic characteristics of graphite comes from the work of Hennig (Hennig, 1965, 1966; Hennig and Kanter, 1960) on the chemical reactivity of this material. Using the novel method of etch decoration, he was able to show that in several different oxidizing atmospheres the rate of reaction of graphite parallel to the layer planes was up to several orders of magnitude larger and proceeded with lower activation energies than the rates of reaction perpendicular to the layers. As part of these observations he showed that the chemical reactivity a t the center of screw dislocations is enormous and causes their cores to burn out quickly in most oxidizing atmospheres. Microstructure
Free radical at layer edge:
Figure 3. Possible basic behavior of oxygen chemisorbed on’ carbon 14
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
Graphite has often been used for fundamental studies of adsorption and catalysis. Little more need be said about its well known structure of flakelike crystals, for which Figure 1 shows the paragon. I t can be inferred that impurity atoms are bound a t the edges of layer planes in graphite as well as a t any imperfections which expose such layer-plane boundaries. However, such bound impurities make up a very small fraction of the surface of a typical, well crystallized graphite. More important than graphite are the black microcrystalline carbons formed by incomplete combustion or by thermal decomposition. Carbon blacks are formed by nucleation and growth during incomplete combustion of
4Ak COKE
POLECULAR SCLI3
BAKED CAREON
POLYCRYSTALLIUE GRAFLil T E
'PAPHITE
~~
Figure 4. Band model of electronic structure of different carbons
hydrocarbons-for example, natural gas; these materials find widest application as pigments and fillers for rubber but they also have been used as catalysts and adsorbents in fundamental studies (Clauss et al., 1957; Kipling, 1964). Carbons commonly employed as practical catalysts and adsorbents possess specific surface areas on the order of about 1000 sq. meters per gram, compared to carbon blacks with specific surface areas on the order of a few hundred square meters per gram. The higher-surface-area materials are usually formed by thermal decomposition of organic matter, followed by an "activation" process to increase porosity and thereby develop very large specific surface area. Carbon blacks consist of spherical particles frequently aggregated into chains. Besides elementary carbon, these materials always contain small amounts of hydrogen and oxygen and frequently also some sulfur, depending on the raw material. Carbon blacks of rather different properties, like particle size and degree of aggregation, can be obtained by variation in the production process. A simple visualization of the structure of carbon black is provided by the left side of F:igure 5, which represents a section through a carbon black particle. The short dashes within the spherical shell signify graphitic layer planes formed during growth of the particle from a central nucleus. The structure of carbon black particles has been investigated widely. Heckman (1964) provides an extensive review. An interesting material, and one widely employed in fundamental adsorption research, results from the graphitization of carbon black. I n the process, the carbon black is held under vacuum or in an inert atmosphere a t 2000" to 3000" C. Thereby, the carbon black can lose some volatile impurities and its; graphitic layer planes grow (some a t the expense of others) larger and simultaneously undergo a reorientation. As a result of this reorientation the graphitized product no longer has spherical boundaries but assumes the shape of a polyhedron, as shown on the right-hand side of Figure 5. Thus the graphitized carbon black possess'es a higher degree of crystallinity and a rather uniform surface made up mainly of basal graphitic planes. Such a surface is hydrophobic, relatively nonpolar, and presents high specificity for the adsorption of many organic molecules. These are some of the reasons
that the graphitized carbon black, Graphon (Cabot Corp.), has enjoyed such popularity as a substrate in adsorption research. Active carbon which finds the most widespread practical use as adsorbent and catalyst is usually manufactured by carbonization of a natural product followed by activation with an oxidizing gas. Certain source materials like bones and coconut shells appear to yield a product more suitable for some applications than others; this specificity is thought to be related to impurities in the source material. Carbonization is frequently carried out in the absence of air but the presence of inorganic materials like metallic chlorides appears to be necessary to obtain products with specific properties-for example, decolorizing ability. The carbonized material is subsequently activated by oxidation with gases like air, COz, and steam. The latter process is thought to burn pores in the carbon and thereby greatly increase its specific surface area. Variations in both carbonization and activation processes may be expected to influence the microstructure of the resulting carbon as well as the nature of the surface functional groups bound a t the edges of graphitic laver planes. There is very little published information about the microstructure, pore shape, pore-size distribution, and surface functional groups of commercial carbon adsorbents, even though these properties can be expected to influence
Figure 5. Section through carbon black particle Based on many electron micrographs
VOL. 8 N O . 1 M A R C H 1969
15
adsorptive and catalytic behavior strongly. Perhaps one explanation for the dearth of studies of pore size and porosity as they affect adsorption is the obvious intuitive conclusion that molecules too large to fit into pores of the average size will be adsorbed with difficulty; however, such a consideration neglects the fact (discussed in more detail below) that ordinary dispersion forces between atomic centers are additive. This means that, in general, the larger the molecule the more easily it is adsorbed. Still another reason for lack of information on the influence of pore size distribution is the difficulty in measuring this characteristic. Moreover, the pores are tortuous, irregular paths, not regular geometric passages as assumed in most models used for estimating pore-size distribution from adsorption hysteresis measurements. The monograph by Hassler (1963) provides a broad review of many aspects of active carbon. Pore Structure
I n microcrystalline carbon, the small graphitic crystallites can be joined together by various kinds of carbon structures, which can generally be described as “amorphous” with respect to the graphite lattice. Thus pores can be thought of as arising quite naturally as empty spaces between crystallites as well as pores deliberately produced in a carbon by activating with an oxidizing gas. The arrangement of atoms a t internal surfaces of pores can have important effects on physicochemical properties. Compared with adsorption on an open surface, a molecule adsorbed mainly through dispersion interactions will be more strongly held in a fine pore because of the additivity of force fields from the opposing walls. This enhanced adsorption may be offset, however, by the inability of the molecule to reach part of the internal structure through ultrafine entrances. These entrances present potential-energy barriers which molecules must surmount if they are to continue their passage toward the interior of a porous pellet. Hence the diffusion of molecules into such pores is strongly temperaturedependent. Kiselev (1961, 1964a,b) found that pore narrowing has the greatest influence on adsorption of large molecules of high polarizability-e.g., long-chain n-alkanes-is much less pronounced with K a and C H 3 0 H where adsorption is not due to dispersion forces alone, and is least with water, which is sorbed mainly through hydrogen bonding. When a carbon is employed as an adsorbent or catalyst, the presence of pores inaccessible to the molecular species in question is wasteful. These pores can be enlarged by activation with an oxidizing gas as described above.
However, it is also possible t o cause pore contraction by heat treatment. The influence of pore structure on the catalytic and adsorptive behavior on carbon has received perhaps somewhat more attention than the role of the chemical and electronic properties of carbon. Therefore, the ensuing discussion is focused primarily on chemical and electronic properties. Carbon as Catalyst
The use of carbon as a carrier for other catalysts is not considered here. Rather, catalytic properties arising from the carbon itself are discussed, including the influence of hydrogen and oxygen impurities found in all commercial carbons. Care will be taken, however, to exclude from discussion those cases where catalysis might be caused by other kinds of impurities-metal oxides, for examplenot normally present in the purest microcrystalline carbons. I t is interesting t o consider how carbon fits into a rudimentary ranking of catalysts based on electrical conductivity. Table I presents such an organization (Bond, 1962) that can be used as the basis for predicting catalytic activity in a very broad and diffuse way (Coughlin, 1967). In this table, the primary catalytic functions are shown in capital letters and the less important functions are written in lower case. As mentioned above, carbon may display electronic properties of conductor, semiconductor, or insulator depending on the method of preparation and pretreatment. Different portions of material within a single sample of microcrystalline carbon could conceivably possess these separate different electronic properties. Moreover, the strong electronic anisotropy of graphitic carbon further complicates the simplistic idea of electrical conductance under consideration here as one possible predictor of catalytic activity. This possible broad spectrum of crystalline properties within a given carbon catalyst may be one explanation of the poor selectivity sometimes observed in reactions over carbon. An example of this behavior is the decomposition of formic acid which takes place over carbon by both dehydrogenation and dehydration. This is discussed below in more detail. On the one hand we are faced with a dilemma about assigning carbon a position in Table I. However, Table I might be used as a guide in predicting the activity of a particular carbon having a certain history and, therefore, certain electronic properties. Alternatively, Table I might suggest treatment of a carbon catalyst to influence its electronic properties and thereby alter its activity and selectivity for a particular kind of reaction.
Table I. Classification of Catalysts
16
Metal f Conductors)
Metal Oxides and Sulfides (Semiconductors)
Salts and Acid-Site Catalysts (Usually Insulators)
HYDROGENATION DEHY DROGEKATIOS HY DROGESOLYSIS Oxidation Reduction
OXIDATIOK REDUCTION DEHY DROGENATIOS CYCLIZATIOK Hydrogenation
POLY MERIZATIOS ISOMERIZATION CRACKIXG DEHYDRATION ALKYLATION HALOGENATIOS DEHALOGEKATIOS HYDROGEN TRANSFER
l & E C PRODUCT RESEARCH A N D DEVELOPMENT
Table II. Some Reactions Catalyzed by Carbon
lieactions IncolLzng Hydrogen
References
H? + D,+ 2H D O-H?-JJ-H;
Burstein, 1938; Smith, 1959 Bonhoeffer et al., 1933 Turkevich and Laroche, 1958 Dulou, 1945 Clauss et al , 1957; Stumpp and Rudorff, 1962 Baladin and Patrikeev. 1941 Stumpp, 1965b Kotelow, 1952
Abietic acid -_dehydroabietic acid H2+ Br,- 2H.Br R X + H2- RH + H X (X = C1 or Br) HCOOH CO? + Hr C H < C H O H C H + CHiCOCHi + Hr
-
Oxidation-reduction 2H20>- 2H:O + O?
so, + so; KO + ‘_.o?--I\Jo3 ‘20?*
CHO + CsHjCOOH
+ 2H2O
Brinkmann, 1951 Dratwa, 1966; Pearl and Benson, 1942; Siedlewski, 1965a,b,c; Siedlewski and Trawinski, 1965 Rao and Hougen, 1952 Demougin and Landon, 1935 I. G . Farbenindustrie, 1899
Halogenation H.
+ Br2-
H13r
co + Clr-COC1r + 5Cl?--C?Cls + 4HC1 CGHjCH, + (211- CcHjCHrCI + HC1 SO, + Clr 83,Cli CIH,
-
Clauss et al., 1957; Stumpp and Rudorff, 1962 Dulou, 1945; Swab, 1957 Kipling and L$-right, 1963 Dulou, 1945 Dulou. 1945
Polymerization, :Isomerization, dehydration HCOOH * H20 + CO 3C2H2- CeH, Ci,HiCOCC6H;(a-oxime) CsH;COCCGHi (d-oxime) NOH HOX Alpha-olefins -+ poly(a1pha-olefins)
-
In the face of this dilemma, it is not surprising to find carbon catalysts apparently fitting into each column of Table I , with different parts of the carbon surface corresponding to the different magnitudes of conductivity covered in the table. Moreover, these considerations suggest how one might treat a carbon catalyst in an attempt to enhance a particular kind of activity-for example, heat treatment and graphitization would be expected to favor metallic behavior. Oxidation and the formation of oxides on the edges of the layer planes would be expected to localize K-electrons in surface states and lead to more semiconductivity. Treatments or methods of manufacture which result in highly disordered carbon might be expected to produce more insulator-type behavior. Reactions Catalyzed by Carbon
Some reactions catalyzed by carbon (Table 11) have been selected rather arbitrarily from the chemical literature. KO serious attempt has been made to sample the patent literature. I n the main, the reactions appear t o fit some categories in each of the columns of Table I . Reactions ranging from hydrogenation through oxidation and reduction to polymerization and chlorination are catalyzed by carbon. Thus the possible behavior of carbon as a catalyst appears to bear out, a t least in a very gross way, its possible electronic properties ranging from metallic behavior through semiconductor to insulator. I n the following discussion, several of the reactions listed in Table I1 are considered in some detail with regard to mechanism and, in particular, the role of the catalyst. Reactions for discussion have been selected on the basis
Stumpp, 1965b Dulou, 1945; Clemo and McQuillen, 1935 Dulou, 1945 Hill, 1965
of available information and with a view to treating reaction types listed in each column of Table I. Pure charcoals stringently outgassed a t high temperature are active catalysts for hydrogen-deuterium exchange (Burstein, 1938). The reaction is inhibited by oxygen in and on the carbon and activity increases with carbonizing temperature from about zero a t 650” C. to high activity a t 950°C. (Smith, 1959). I t is thought that the a-electron system of the carbon plays an important role here. Two mechanisms have been distinguished in the case of the ortho-para hydrogen exchange (Bonhoeffer et al., 1933), a paramagnetic and a chemical mechanism. I n the paramagnetic mechanism, conversion is thought to be due to the magnetic field caused by the unpaired electrons associated with free radicals. The latter probably are to be attributed to large condensed resonating ring structures with unsaturated edges. The so-called chemical mechanism of ortho-para hydrogen exchange is supposed to be similar t o that for H2-D2exchange and to involve dissociation of hydrogen molecules. The synthesis of HBr over carbon catalysts has been widely investigated. I n a recent study, Stumpp and Rudorff (1962) concluded that this is an “acceptor” reaction-i.e., it proceeds via an adsorbed Br intermediate whose formation depends on the presence of electrons in the conduction band of the catalyst. This was based on studies of the reaction over pure graphite and over metal chloride-graphite intercalation (or lamellar) compounds in which the metal chloride, embedded between the graphite layer planes, presumably acts as an electron acceptor, thereby decelerating the HBr synthesis. These notions were strengthened further by VOL. 8 NO. 1 M A R C H 1 9 6 9
17
experiments (Stumpp, 1965,) that showed weaker chemisorption of bromine on the intercalation compounds than on pure graphite. On the other hand, experiments by Clauss et al (1957) indicate that HBr synthesis is enhanced by the presence of surface compounds that can be removed by graphitization; one would expect surface complexes of oxygen to deplete electrons in the conduction band of carbon and, thus, inhibit a reacticn proceeding via Br . Stumpp based his conclusions on activation energy alone, whereas Clauss measured rate constants a t only one temperature. I n agreement with Stumpp, Smith (1959) states that the HBr synthesis is inhibited by surface oxygen complexes on carbon catalysts. A possible objection to the reasoning of Stumpp is that the formation of lamellar compounds distorts the crystal lattice of graphite, thereby affecting the electronic properties. I n agreement with Stumpp, Kmetko (1953) observed positive changes in the thermoelectric power with increased oxidation of the lamellar bisulfate compounds formed between graphite and sulfuric acid. He attributed this to removal of electrons from the conduction band due to the repulsion of the intercalated bisulfate ions. However, Kmetko, also, reported that residue compounds (with the foreign species bound a t lattice defects only) display larger increases in thermoelectric power than do the corresponding intercalation compounds. I n similar work on the decomposition (dehydrogenation) of formic acid, Stumpp (196513) has shown that the graphite intercalation compounds catalyze the dehydrogenation reaction better than does pure graphite. Another dehydrogenation, that of 2-propanol, proceeds faster over carbons more strongly wetted by benzene and which are better adsorbents for methylene blue (Kotelkow, 1952). The latter finding indicates either that polar surface complexes inhibit the dehydrogenation reaction, or that more perfect crystallites are better catalysts for this reaction as well as better adsorbents for benzene and methylene blue. I n this regard, there is evidence that oxidized carbon surfaces display markedly reduced capacities for adsorbing phenol and nitrobenzene from aqueous solution, but that chemical reduction can restore the adsorptive capacity of the carbon surface (see below). On the other hand, Graham (1955) found that methylene blue adsorption was little affected by chemisorbed oxygen groups as compared to adsorption of metanil yellow. Dehydrogenation reactions are well catalyzed by metals in general, and well ordered graphite crystals have conductivities similar to metals. In oxidation reactions over carbon, surface oxygen complexes on the catalyst participate in many reactions (Brinkmann, 1951). Hydrogen peroxide decomposition is an interesting related case. This decomposition has been shown to be catalyzed by basic surface oxides on carbon but inhibited by the acidic functional groups of chemisorbed oxygen (Brinkmann, 1951; Dratwa, 1966). The following mechanism accounts for many of the known phenomena with regard to H 2 0 ?decomposition:
Here C - ] indicates the carbon lattice. The OOH ion situated on a positive site of this lattice has an increased oxidation potential and, therefore, can oxidize an H,O, molecule. 18
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
I n the case of SO, oxidation over carbon, the surface oxygen complexes are an active intermediate and it has been shown that carbon pretreated with oxygen is a far better catalyst than untreated material (Dratwa, 1966). In a series of detailed experiments Siedlewski showed that, although the total adsorption of SO? on carbon does not depend on the concentration of free radicals, electrons with unpaired spins nevertheless appear to be active centers for S O r chemisorption on carbon, that oxygen chemisorption involves surface free radicals, and that the quantity of electrochemically active chemisorbed oxygen corresponds to the quantity of oxygen participating in the catalytic reaction (Pearl and Benson, 1942; Siedlewski, 1965a,b,c; Siedlewski and Trawinski, 1965). This suggests that surface oxides participate directly in the reaction. Carbon catalysts are commonly used for halogenation reactions and apparently there is more information about this class of reactions in the patent literature than in the chemical journals. Few fundamental studies have been published as to the mechanisms of these important catalytic reactions and as to the nature of the active sites which catalyze these reactions on carbon. I n the absence of more recent studies, it is interesting to speculate that the electronic considerations pertaining to the adsorption of bromine as discussed above for the HBr reaction (Stumpp and Rudorff, 1962) also may apply to various reactions involving chlorine and other halogens. Alternatively, the activity of carbon in catalyzing halogenation and dehalogenation reactions may be thought of as related to the polarity associated with surface compounds or functional groups on the carbon surface. For example, the mechanism of the reaction:
has been explained (Xoller and Ostermeier, 1956) in terms of the distance between charged anions and cations on the XaC1 surface. With regard to polymerization, isomerization, dehydration, and cracking over carbon catalysts, there are not many examples in the chemical literature (though older patent literature contains a number), as there are in some of the other categories discussed above. The explanation for this lies in the fact that better catalysts than carbon are available today for these reactions. The dehydration of formic acid over carbon is an interesting case, since it occurs simultaneously with the dehydrogenation reaction discussed above as a reaction involving hydrogen. Stumpp (19658) found that this reaction was catalyzed equally well by graphite and by its CdCL intercalation compound. Fine graphite powder was a better catalyst for the dehydration than graphite flakes but the graphite was deactivated by pretreatment with chlorine or formic acid itself. The dehydration of formic acid on oxides is generally attributed to surface hydroxyl groups (Schwab, 1957). Therefore, there is good reason to believe this is also the case for the dehydration over carbon. Finely powdered graphite would be expected to have a larger concentration of these groups on the edges of layer planes and it is easy to visualize these groups being split off by pretreatment of the catalyst with formic
acid or chlorine. Formation of the CdClz intercalation compound can be assumed to leave these surface OH groups relatively undisturbed with little concomitant alteration of activity in dehydration. Stumpp's observations as to catalyst deactivation agree well with the model used by Kipling and Wright (1963) to explain their observations on the reaction of formic acid with the acidic surface oxides of a carbon black:
ECOH
+ HCOOH
4
ECH
+ H?O + COj
This model is based on the known oxidation of formic acid by triphenylcarbinol. Isomerization, cracking, and polymerization reactions over acid-site catalysts generally are thought to take place by carbonium ion mechanisms. The acidic groups on the surface of carbon are logical candidates as active sites for these kinds of reactions. I n a study of polymerization of liquid alpha-olefins over carbon blacks, Hill (1965) observed a definite correlation between catalytic activity and surface acidity of the carbon. The absence of abundant evidence of cracking over carbon is probably related to the elevated temperatures frequently required for cracking-at these temperatures carbon begins to lose acidic oxygen complexes from its surface. Carbon as Sorbent
This function of carbon also depends on specific affinity as well as the accessible surface area. Selectivity is determined not only by pore geometry, but also by various specific contributions to the physical bond between carbon and adsorbate. This behavior can depend to some extent on the degree of adsorption: When a sorbent is relatively free of sorbate, sorbent-sorbate interaction predominates; however, as the capacity of the sorbent is approached, interaction between sorbate molecules themselves becomes increasingly important. The latter kinds of interaction are not considered in the discussion which follows. Widely known general references on adsorption are the books by Brunauer (1945) and de Boer (1953), which primarily treat sorption from the gas phase, and the book by Kipling (1964) which is concerned with sorption from liquid solutions of nonelectrolytes. The interaction energies most commonly associated with the physical bond between sorbent and sorbate are: Dispersion energy and close-range repulsion energy, generally considered nonspecific. Polarization energy (E,J= - f2cuF'),which arises when a local electrostatic field, F , generated by positive and negative ions in a sorbent polarizes a sorbate molecule of polarizability LY. Field-dipole energy and field gradient-quadrupole energy, which can arise when local fields interact with molecules having permanent dipole or permanent quadrupole moments-e.g., S ? CO, , Cor. Dipole-dipole, dipole-quadrupole, quadrupole-quadrupole, etc., energies. With the possible exception of dipoledipole energies, these terms are usually small and contribute but little to bond energies. Since electrostatic energies may or may not be important, according t n the nature of sorbent and sorbate, these contributions niight be regarded as specific components of a physical hond. On the other hand, dispersion and close-range repuleion energies might be regarded as nonspecific because of their universality and approximate additivity. Thus a carbon possessing a polar surface con-
a on s J rfac e
b w thin
pore
C
d
ir
t
hevispherical p0cK-t
So?tOV of pore
e withif-
cavtty
Figure 6. Sorbed molecule in various idealized positions on or in pores of sorbent
taining many oxide functional groups might be expected to form stronger bonds with sorbed molecules having permanent electric moments-e.g., CO?, CO, "3, H20, S02-as compared with nonpolar molecules like methane. However, the same molecules might be expected t o adsorb less tenaciously upon carbon rendered nonpolar by graphitization or outgassing. For multiatomic molecules like the normal paraffins, on polar or nonpolar substrates, the heats of adsorption increase approximately linearly with the number of carbon atoms (Barrer and Sutherland, 1956; Kiselev, 1961). Thus the rough additivity of dispersion energies for atom pair interactions ensures that large molecules will be adsorbed by any kind of carbon a t low temperatures. I t is the smaller, nonpolar molecules which adsorb with more difficulty. Although dispersion and close-range repulsion energies have been described above as nonspecific components of the physical bond, nevertheless these contributions to the physical bond can, in a certain sense, display a specificity associated with surface topography. I n this regard, the five different situations shown schematically in Figure 6 illustrate differences in coordination number of the adsorbed molecule with respect to atoms of the sorbent. Here the sorbed molecule is shown ( a ) tangent to the sorbent surface; ( b ) fitting between parallel surfaces of sorbent; (c) in a hemispherical pocket of sorbent; ( d ) fitting snugly within a cylindrical pore in the sorbent; and ( e ) in a cavity within the sorbent. The energetic relationships among these modes of adsorption have been computed by de Boer and Custers (1934) as:
Actual enhancement in strength of adsorption due to pores, pockets, etc., will be somewhat smaller, however, because the computations were based on perfect fits, uniform surfaces, and dense atomic distributions within the sorbents. Nevertheless, variations of the initial heats of adsorption among porous and nonporous sorbents are significant even for nonpolar molecules. For example, Beebe and Young (1954) have demonstrated enhancement of dispersion energy (initial heats) of argon on porous carbons with progressive lowering in initial heat as porosity was destroyed by graphitization (Figure 7 ) . T o deduce electrostatic energy components of the physical bond, Barrer (1966) developed a correlation for estimating the contribution to the initial heat of adsorption, IE, from dispersion, repulsion, and polarization. Accordingly, Barrer plotted initial heat of sorption against polarizability for a number of nonpolar, structurally simple small molecules as shown in Figure 8. This graph then allows approximately for dispersion, repulsion, and polarization, and the sum of these components can be read VOL. 8 N O . 1 M A R C H 1 9 6 9
19
Table Ill. Contributions to Initial Heats of Adsorption, AHa, on Graphitic Carbons (Barrer, 1966)
Gas
N. H,O
" CH OH SO?
Initial Heats, -1Ha. Cai. Moie
Dispersion and Repulsion Energ?,, Cui. ,Vole
Mirror Image or Other Energ?, Cui. .Mole
3,100
2750-2500'
3;i0-60Li
2,600L
2750-Z500'
10.500 7.200
6.200" 1Z.000 10.500' 6.000
2650 3,500 4850 5350-4200"
-
7850 3700
7150 ,5650 650-1800
Ranges for S,and SO, arise from anisotrop? of polarirabiiit,v for these moiecuics; smaiier calues refer to direction normal to linear axis of -Y2or plane of SO,; large figure is mean d u e . ' L o u e r values of -1Ha marked b refer to flat portions of curue of -1Ha cs. amount adsorbed.
Figure 7. Calorimetrically measured heats of adsorption of argon on Spheron carbon blacks a t 195" C. ----Untreated black Temperatures of graphitizotion indicated
He
Ne
H2
from the characteristic curve for other molecules which have intermediate polarizabilities but also permanent electric moments. This sum can be subtracted from measured initial heats to estimate the energy contribution from permanent electric moments. The results of this procedure are shown in Table 111. I t is evident that dipolar sorbates like HjO, N H , , and CH ;OH generate the largest electrostatic terms. I n this table, the ranges of energies given for S , and SO2 arise from the anisotropy of these molecules; the smaller value results if ' 1 ' s normal to the length of N ? or the plane of SO? are used, whereas the larger values correspond to the mean values of 3: for each molecule. As mentioned earlier, surface polarity of carbons can
H20
Figure 8. Characteristic curves of initial heat v s . polarizability of sorbate for small, relatively symmetrical adsorbed species (Burrer, 1966) 20
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
6
0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.a 0 0.1 0.2 0.3 0.0 0.5 relative p r e s w r t - ,
pjps
Figure 9. Isotherms of n-hexane, benzene, methanol, and water
------ O n initial channel black - O n channel black oxidized with mixture of nitric and sulfuric ocids 9 . 0
*
*
Methanol on a hydroxylated silico surface
depend strongly on the presence of chemisorbed oxygen in the form of surface functional groups. Removal of these groups by thorough outgassing and graphitization can render a carbon surface hydrophobic. On the other hand, chemical oxidation can make carbon hydrophilic by increasing the concentration of surface oxides. The effect of such treatment upon adsorption is shown in Figure 9 for n-hexane, benzene, methanol, and water (Kiselev, 1964a). Water, the most polar of these sorbates, shows the greatest increase in adsorption affinity for the oxidized surface. Similar behavior is shown by normal and dehydroxylated silica surfaces (Kiselev, 1964a). Adsorption from Solution
In adsorption from solutions the situation becomes more complex because frequently all components, including solvent, are adsorbed by carbon to some extent. Then the competition among components for the carbon surface becomes important. This competition is influenced by factors similar to those discussed above for sorbate-sorbent interaction in single-component adsorption. However, increasing importance can be assumed by interactions among different adsorbate components on the carbon surface. In sorption from solution, the competition among components can be strongly influenced by the nature of the carbon surface-for example, charcoals usually adsorb benzene in preference to alcohols. However, if a charcoal is very strongly oxidized, its preference for ethyl alcohol relative to benzene increases with oxidation until it approaches that shown by silica gel (Bartell and Lloyd, 1938). Conversely, preferential adsorption of alcohol can be reduced by removal of the oxygen groups from the surface (Puri et a[.,1963). Similar preference for competitive adsorption of alcohol over benzene was shown by a carbon black containing considerable chemisorbed oxygen, as compared to Graphon which showed the converse behavior (Gasser and Kipling, 1960). The influence of surface oxygen has also been observed in adsorption of phenol, nitrobenzene, and benzene sulfonate from aqueous solution (Coughlin and Ezra, 1968a; Coughlin et al.. 1969). I n these studies, adsorption of
the organic component was decreased markedly by increased concentration of acidic oxygen groups on the carbon surface. However, adsorption capacity could be restored in part by reducing the acid surface oxides with hydrogen (Figure 10). When high-temperature outgassing was employed to destroy surface functional groups it was possible, in some cases, to improve the adsorptive capacity of an active carbon beyond its original value (Coughlin and Tan, 196810). In general. good correlations were found between the measured surface acidities and adsorption capacity of the carbons on a "per unit B E T surface area" basis. Undoubtedly, preferential adsorption of strongly polar water molecules plays a leading role in this phenomenon. As shown in Figure 11, decreased adsorption of phenol on oxygenated carbon is strongly in evidence a t the first plateau of the adsorption isotherm, but not a t the higher-concentration plateau. This two-plateau isotherm for phenol on carbon has been interpreted (Giles et al., 1960) as resulting from flat orientation of phenol
Figure 10. Adsorption isotherms of phenol on activated Columbia carbon LC325 from aqueous solution 30' C., 1-week equilibrium time VOL. 8 NO. 1 MARCH 1969
21
Dt'EI.IX COPICEIITE4Tl011.
ri
ticLEs/L:TER
Figure 1 1 . Adsorption of phenol on Black Pearls 607 carbon black from aqueous solutims a t 30" C.
molecules on the carbon surface corresponding to the first plateau; as phenol concentration is increased the molecules are thought to reorient from the prone to the end-on position which corresponds to the second plateau. Since Figure 11 would then indicate that surface oxygen groups inhibit adsorption of phenol in the prone position but not in the upright position, this suggests that the oxygen groups a t the edges of graphitic layer planes may remove electrons from the graphitic r--electron system (Walker et al., 1965) and influence adsorption by weakening the interaction between the a-electron systems of phenol and the graphitic layer planes. The surface functional groups on carbon confer ion exchange properties on this material, which therefore possess the ability to bind acids and bases. Frequently, carbons liberate acids or bases when they adsorb dyes or other organic salts-for example, the adsorption of methylene blue (a cation) liberates acid and the adsorption of potassium benzoate liberates base. Adsorption accompanied by such pH change has been termed hydrolytic adsorption because it has been assumed that the salt enters into a hydrolysis reaction with water and that the hydrolyzate is most strongly adsorbed. Thus, according to this view, potassium benzoate hydrolyzes according to:
CcHjCOOK
+ H20 2 CcHiCOOH + KOH
and the ChHjCOOH is adsorbed, leaving KOH in solution. However, another possible interpretation is that carbon simultaneously binds C6H5COO and liberates OH -i.e., exchanges CeH5COO~-for OH-^. A similar alternative interpretation would be the exchange of a methylene blue cation for H-. The precise mechanisms remain in doubt, however, as is evidenced by the fact that Graham (1955) found no correlation between acidic oxygen on various carbons and their ability to adsorb methylene blue cations; however, Graham did find that chemisorbed oxygen on carbon impeded the adsorption of metanil yellow, an anion, and attributed this to electrostatic repulsion between anion and surface oxygen groups. ~
22
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
Conclusions
Many of the catalytic and adsorptive properties of carbon can be understood in terms of structure, electronic behavior, and surface chemistry. Depending on treatment and preparation, carbon manifests different structural, electronic, and chemical properties which can markedly influence its behavior as catalyst or adsorbent. I n many instances, these considerations suggest how selecting and pretreating a carbon might enhance its activity, capacity, and selectivity as a catalyst or adsorbent. Acknowledgment
The author is grateful for support by the Federal Water Pollution Control Administration under Grant WP-0096902, by the National Center for Air Pollution Control under Grant 3 R01 AP00738-01S1 and by a Lehigh University Grant-in-Aid for Environmental Studies. Literature Cited
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Burstein, R., Acta Physicochim. U S S R 8, 857 (1938). Clauss, A,, Boehm, H. P., Hofmann, U., 2. anorg. allgem. Chem. 290, 35-51 (1957). Clemo, G. R., McQuillen, A., J . Chem. Soc., 1935, 851. Coughlin, R. W., Ind. Eng. Chem. 59 (9), 45 (1967). Coughlin, R. W., Ezra, F. S., Environ. Sci. Technol. 2, 291-7 (1968). Coughlin, R. W., Tan, :R. N.: Chem. Eng. Progr. Symp. Ser. 64, (go), 207-14 (1968). Coughlin, R. W., Tan, R. N., Ezra, F . S., J . Colloid Interface Sei., in press, 1969. de Boer, J. H., “The Dynamical Character of Adsorption,” Oxford university Press, London, 1953. deBoer, J. H., Custers, J . F. H., 2 . physih. Chem. (Leipzigj B25,225 (1934). Demougin, P., Landon, Bull. Soc. Chim. 5 (2), 27 (1935). Donnet, J. B., Carbon 6, 161-76 (1968). Dratwa, H., doctoral thesis, “Removal of the Oxides of Sulfur from Stack Gases,” Technische Hochschule, Aachen, June 1966. Dulou, R., Chim. Ind. (Paris) 54, 396-403 (1945). Dutta, A. K., Phys. Rev. 90, 187 (1953). Franklin, R. E., Acta Cryst. 4, 253 (1951). Ganguli, X., Krishman, K. S., Nature 144, 667 (1939). Garten, V. A., Weiss, D. E., Australian J . Chem. 10, 309 (1957a). Garten, V. A., Weiss, I>. E., Rev. Pure Appl. Chem. 7, 69 (1957b). Gasser, C. G., Kipling, J. J., Proceedings of IVth Conference on Carbon, p.55, Pergamon Press, New York, 1960. Giles, C. H., MacEwan, T. H., Nakhwa, S. N., Smith, D., J . Chem. Soc. 1960, 3973. Graham, D., J . Phys. Chem. 59, 896 (1955). Grown, A. R. G., Watt, W., Powell, R. W., Tye, R. R., Brit. J . Appl. Phys. 7, 73 (1956). Hassler, J. W., “Activated Carbon,” Chemical Publishing Co., New York, 1963. Heckman, F. A., Rubber Chem. Technol. 37 (5), 1245-98 (1964). Hennig, G. R., Carbon 3 ( 2 ) , 107 (1965). Hennig, G. R., “Chemistry and Physics of Carbon,” P. L. Walker, Jr., Ed., Vol. 2, p. 1, Marcel-Dekker, New York, 1966. Hennig, G. R., Kanter. M. A., “Reactivity of Solids,” J. H . de Boer, Ed., p. 649, North-Holland, Amsterdam, 1960. Hill, L. W., Ph.D. thesis, Pennsylvania State University, 1965. Hofmann, M., Groll, E . , Ber. Deut. Chem. Ges. 65, 1257 (1932).
Hofmann, U., Wilm, D., 2. Elektrochem. angeu:. physih. Chem. 42, 504 (1936). I. G. Farbenindustrie, A. G., German Patent 203,848. (1899). Kipling, J. J., “Adsorption from Solutions of NonElectrolytes,” Academic Press, London and Kew York, 1964. Kipling, J. J., Wright, E . H. M., J . Phys. Chem. 67 (9), 1789-93 (1963). Kiselev, A. V., Quart. Rev. (London) 15, 99 (1961). Kiselev, A. V., Rev. gen. Caoutchouc. 41, 377 (1964a). Kiselev, A. V., Zh. Fiz. Khim. 38, 2753 (196413). Kmetko, E. A., J . Chem. Phys. 21, 2152 (1953). Kotelkow, N. Z., Zh. Priklod. Khim. 25, 337-41 (1952). McMichael, B. D., Kmetko, E . A., Mrozowski, S., J . Opt. Soc. A m . 44, 26 (1954). Noller, H., Ostermeier, K., 2. Elektrochem. 60, 921-9 (1956). Pearl, I. H., Benson, H. K., Ind. Eng. Chem. 34, 4368 (1942). Primak, W., Fuchs, L. H., Phys. Rev. 95, 22 (1954); 103, 541 (1956). Puri, B. W., Kumar, S.,Sandle, N. K., Indian J . Chem. 1 , 4 1 8 (1963). Rao, M. N., Hougen, 0. H., Chem. Eng. Progr. Symp. Ser. 48, No. 4, 110-24 (1952). Schwab, G. M., “Handbuch der Katalyse, Vol. V, pp. 322, 323, 329, Vienna, 1957. Siedlewski, J., Intern. J . Chem. Eng. 5 (4), 293-7, 295301 (19658). Siedlewski, J., Intern. J . Chem. Eng. 5 (4), 608-12 (1965b). Siedlewski, J., Intern. J . Chem. Eng. 5 (4), 616-22 ( 1 9 6 5 ~ ) . Siedlewski, J., Trawinski, S.,Intern. J . Chem. Eng. 5 (a), 289-93 (1965). Smith, R. N., Quart. Rec. ilondon) 13 (4), 287 (1959). Studebaker, M. L., Huffman, K. W. D., Wolfe, A. C., Kabors, L. G., Ind. Eng. Chem. 48, 162 (1956). Stumpp, E., 2. anorg. allgem. Chem. 337, 285 (19658). Stumpp, E., 2. anorg. allgem. Chem. 337, 292-300 (196513). Stumpp, E., Riidorff, W., 2. Elektrochem. 66, 648-52 (1962). Turkevich, J., Laroche, J., 2. phys. Chem. 15, 399 (1958). Ubbelohde, A. R., Lewis, F. A., “Graphite and Its Crystal Compounds,” Oxford University Press, London, 1960. Walker, P., Jr., Austin, L., Tietjen, J., “Oxygen Chemisorption Effects on Graphite, Thermoelectric Power,” in “Chemistry and Physics of Carbon,” Vol. 1, Marcel Dekker, New York, p. 327, 1965. RECEIVED for review October 8, 1968 ACCEPTED January 14, 1969
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