Mechanisms of vaporization for silver and gold using electrothermal atomization

S ectrochimica Acta, Vol. 48B, No. 1, pp. 7!9-69, 1993 k&ted in Great Britain.

05ed-ss47/93 $6.00 + .oo @ 1992 Pergamon Press Ltd

Mechanisms of vaporization for silver and gold using electrothermal atomization RODNEYW. FONSECA,JOHN MCNALLY*and JAMESA. HOLCOMLBE~ Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, TX 78712, U.S.A. (Received 10 April 1992; accepted 28 July 1992) Abstract-Electrothermal atomization (ETA) of Ag and Au within graphite tubes was studied at atmospheric pressure. Vaporization for Ag and Au appears to occur from varying sixes of microdroplets or from adsorbed atoms, depending on the conditions of the analysis. Using as criteria the shifting of the peaks to later times with increasing concentrations, a fractional order of release for both metals was suggested under normal ETA analytical conditions. However, a first order process was obtained when formation of adsorbed atoms was expected. The activation energies for desorption (EJ of Ag and Au are mass dependent. The decreasing intbtence of the graphite as the droplet size becomes larger might explain the increase in E. observed at increasing concentrations. The highest E. values obtained for both metals approach the respective AH, and indicate desorption from the surface of large microdroplets. The lowest E, might represent the interaction between individual adsorbed atoms and the graphite surface. The dry and thermal pretreatment stages affect Ea and the shape of the absorption profiles because they probably produce changes in the surface topology of these metals before atomization takes place. Chemisorbed O2 seems to reduce the size of the microdroplets in the case of Ag, but the effect on Au has not been completely rationalized. The tendency of the Ag and Au microdroplets to disperse on graphite seems to be reduced by mechanical roughening of the furnace surface, which probably increases the number of active sites on graphite.

MECHANISTIC studies

in electrothermal atomization (ETA) seek a better understanding of the atom formation process and the type of interactions taking place between the analyte and the surface of the atomizer. This type of information can lead to logical approaches for improvement of the technique or the method of analysis. Previous studies suggest there are differences in the degree of interaction that various metals show for graphite [l-9]. Cu, for example, has been proposed to show strong affinity for graphite surfaces [l-S] while Ag and Au have been viewed as metals that exhibit weak interactions with graphite [7-91. Therefore, it has been suggested that Cu vaporizes from adsorbed atoms and that the vapor has a high probability of readsorbing to the surface. Ag and Au tend to form microdroplets and produce homogeneous vapor distributions across the furnace diameter, indicative of low readsorption affinities at the temperatures where these metals are vaporized [5, 7-91. While extensive studies have explored Cu vaporization and present strong evidence for desorption from dispersed adsorbed atoms, less effort has been extended to the vaporization of metals, like Ag and Au, that appear to form microdroplets on the surface. Studies by GANZet al. [lo] using scanning tunneling microscopy (STM) support the existence of adatoms, microdroplets and two dimensional islands on graphite for several metals including Ag and Au. Their study shows images of static monomers and microdroplets as well as illustrates the ability of small clusters to migrate on the graphite surface. ARTHURand CHO [l] have also provided evidence of a large sticking coefficient for Cu(g) on graphite and have illustrated that Au and Cu adatoms can move on single crystal graphite and form droplets or caps. TAMMANN [ll] established that there is a minimum temperature at which a solid would interact with a surface of the same kind. BAKER [12] studied the relationship between particle motion on a graphite surface and the “Tammann temperature”, i.e. * Current address: Chevron, 1862 Kingwood Drive, Kingwood, TX 77339, U.S.A. t Author to whom correspondence should be addressed. 79

R. W. FONSECAet al.

80

one half the melting point of the metal (in Kelvin). BAKER [12] concluded that the rate of movement of the metal ions or atoms shows a rapid change in the vicinity of the Tammann temperature. In the case of Ag and Au, aggregate or droplet mobility is suggested to occur at temperatures below 400°C. Au decoration techniques demonstrate that the mobility of metal atoms on graphite surfaces can occur at temperatures close to 300°C [13]. Several studies indicate that the form of the analyte on the surface of the atomizer prior to vaporization can impact the absorbance-time profiles [7-91. MCNALLY[8] was the first to conclude that the dry temperature could alter the shape of absorbance profiles in ETA-AAS and the effect is element dependent. They observed that Ag absorbance profiles, as well as the activation energies for these processes, are strongly affected; Au is moderately affected, and Cu is not affected at all. AgN03 is believed to decompose thermally to Ag20 or perhaps AgOH, both of which decompose to Ag at temperatures below 180°C [14, 151. Au prepared by dissolution in aqua regia produces [(H,O)+(AuCl,)-1, which is thermally unstable and decomposes producing Au [15]. In a study that compares the appearance temperature and the heat of carbide dissociation, L’vov [16] suggested that metals like Ag and Au do not form stable carbides. Thus, elemental particles and not other chemical forms of the analyte are likely precursors to gas phase atom formation. In this work, the behavior of Ag and Au samples vaporized from graphite tubes was studied to provide additional insights into the mechanism of desorption at atmospheric pressure. The experiments were designed for monitoring changes in E, associated

with changes

in surface

topology

for these metals.

EXPERIMENTAL

Apparatus The atomic absorption optical system employed for these studies was designed in this laboratory and described in detail elsewhere [17]. A 9-mm long pyrolytically coated graphite furnace (Varian CRA-90) was enclosed in an air-excluding aluminum box into which Ar was introduced as the sheath gas. This furnace exhibits nearly isothermal heating along the length of the tube. An Ar flow of 50 ml min-’ was controlled using a rotameter. Stop flow conditions were used during the atomization step. Ag and Au hollow cathode lamps (Perk&Elmer) were operated in a direct current mode according to manufacturer specifications. The 328.1 nm and 242.8 mu lines were used for Ag and Au, respectively. An automatic optical pyrometer (IRCON, series 1100, model 11x20) was focused through the dosing hole on the bottom of the furnace and was used for temperature monitoring. All temperatures are accurate to within +5O”C. The temperature and HCL intensity data were collected by a microcomputer interfaced through an A/D converter (Keithley; model 570) to the ETA-AAS system by means of a program written in ASYST.

Reagents Stock solutions of Ag and Au were prepared from high purity metal wires in a procedure similar to that used by DEAN and RAINS [18]. Working standards of Ag were prepared daily by dilution from the stock solution using distilled, deionized water.

Procedure A lO+l Hamilton syringe equipped with a PFTE needle was used to deposit a 2-l~l aliquot of the working standards onto the furnace wall. The heating program consisted of drying at either 80 or 300°C (50 s), thermal pretreatment at 300°C (40 s), and atomization at 2200°C (2 s), with heating rates of either 800 or 400 K s-l. Atomization of the sample was performed from the wall of the graphite tube. Temperature profiles and blanks were run for each set of conditions.

81

Ag and Au vaporization mechanisms using ETA 0.65

j o” 0.32 s 0.16

,

,

,

0.50

0.70

0.90

0.00

,

,

,

1.10

1.30

/

, 1.50

time (s) Fig. 1. Absorbance-time

profiles for Ag with masses varying between 0.02 and 0.10 ng in increments of 0.02 ng.

RESULTS AND DISCUSSION

Concentration study Figure 1 presents absorbance-time profiles for Ag samples at different concentrations. It is evident from this figure that as the concentration increases, the position of the peaks shifts to later times. This is consistent with the results of MCNALLY and HOLCOMBE [7] and indicates a fractional order of release, assuming constant activation energies. Figure 2 shows similar results for Au and also suggests a fractional order of release with a value 0
0.8

Fig. 2. Absorbance-time

1.0

tink2(s)

1.4

profiles for Au solutions with concentrations and 0.60 ng in increments of 0.10 ng.

1.6

varying between 0.10

R. W. FONSECAet al.

Mass (ng) Fig. 3. Dependence

200

of the activation energy on Ag mass.

0.10

0.20

0.30

0.40

0.50

0.60

Maas (ng) Fig. 4. Dependence

of the activation energy on Au mass.

also contribute to the shifts. It is important to mention that the pre-exponential factor must also increase as E, increases in order to explain the near alignment of the rising edges for the absorption profiles at different concentrations. Considering the changes observed in E,, the peaks would appear at significantly different temperatures if the pre-exponential factor remained constant. Explanations for the dependence of the E, on concentration should include several considerations. First, it is necessary to recognize that a fractional order of release persists at elevated concentrations; but at low concentrations a highly dispersed state (i.e. adsorbed atoms) might also be realized. Thus, reasonable explanations should be based on the assumption of multiple mechanisms for release with a continuous transition from one form to another as the concentration is altered. The most likely sites for release of the metal are from: (a) the surface of the microdroplets; (b) the metal-graphite interface; and (c) adsorbed atoms. Desorption from the surface should approach AH, of Ag(1) (72 kcal mol-l) as the concentration increases and the influence of the graphite surface becomes less strongly felt by the metal on the droplet surface. This is consistent with the approach to a larger, more constant E, at elevated concentrations in Fig. 3. Interestingly, the nearly constant E, (viz. 27 kcal) at low concentrations is considerably lower than AH, as was observed for adsorbed Cu atoms on the surface [7, 201. As will be mentioned in a later section, the alignment of the Ag peaks near these concentrations suggests a first order desorption and is also in agreement with the morphology of dispersed atoms. One possibility is the retention of the same release mechanism (i.e. metal release from the droplet surface) with the influence of the graphite decreasing as the droplet size increases. In other words, the limit of infinitely small droplets (e.g. individual

Ag and Au vaporization mechanisms using ETA

83

Table 1. E. (kcal mot-l)’ for Ag vaporization Heating rate (K SK’) Condition Conventional surface Chemisorbed 0, Roughened surface

Drying temperature (“C)

800

4cuI

80 300 80 300 80 300

35 23 29 24 49 23

36 24 30 24 49 24

*An uncertainty of 4 kcal mol-1 can be considered in these values.

adatoms), metal-graphite desorption is observed; and with sufficiently large droplets, the metal takes on bulk properties (viz. E,= A&) and the graphite influence is negligible. A second possibility is that the transition between these two regions reflects the increased importance in the release at the interface between microdroplet and graphite. The contribution of this region for desorption will increase in proportion to the cube root of the droplet diameter [8, 211, assuming the contact angle [22, 231 is not significantly changed with increasing particle size. Drying temperature and heating rate In this work, two different drying temperatures, 80 and 3OO”C, and two different heating rates (800 or 400 K s-l) were used. From Table 1, it can be observed how E, is significantly affected by changes in the drying temperature but not affected by the heating rate. E, decreases from 35 to 24 kcal molV2 when the drying temperature is changed from 80 to 300°C. The possibility of formation of microdroplets for Ag and Au has been suggested previously [8, 91. These authors have concluded that Ag and Au show weak affinities for graphite and consequently form microdroplets on the graphite surface prior to the atomization cycle. A high drying temperature (i.e, “explosive” drying) is expected to produce smaller particle sixes and a larger degree of dispersion than at low drying temperature because of differences in the rate of crystal growth [24]. The influence of the graphite surface on E, is probably more noticeable at smaller particle sixes, with a maximum interaction achieved when only adatoms are present on the surface. An alternative view is that the metal-metal cohesive interactions are minimized as the droplet size decreases. Figure 5 shows how the difference in E, is reflected in the shape of the absorbance profiles with the lower E, associated with the higher drying temperature (dispersed

0.64-

0.48-

0.32-

0.16-

time (s) Fig. 5. Absorbance-time profiIes for Ag solutions (0.08 ng) using (a) 80°C and (b) 300°C drying temperatures.

R. W. FONSECA et al.

84

Table 2. E. (kcal mol-I) *for Au vaporization Heating rate (K s-l) Condition

Drying temperature (“C)

800

400

80 300 80 300 80 300

52 33 64 42 63 42

57 39 47 33 69 44

Conventional surface Chemisorbed O2 Roughened surface

*An uncertainty of 4 kcal mol-l can be considered in these values.

atoms), producing a broader peak with a more gentle rising edge. The area under the peaks is the same in both cases, indicating that there is no loss of material during the high temperature drying stage. Similar results were obtained for Au, and they are summarized in Table 2. The results for these elements contrast with those obtained by MCNALLY [8] for Cu where no alteration in the absorption profiles was observed with changing the drying temperature. Since Cu atomization from graphite is believed to proceed by desorption of individual metal atoms, changing the drying temperature should not change the appearance of the sample on the surface and, as a consequence, should not exhibit any changes with altered drying conditions, as is the case for Ag and Au. Oxygen chemisorption The effect of O2 chemisorption on E, was also studied. In addition to altering the chemical binding character of the surface, chemisorbed O2 also increases the hydrophilic character of pyrolytic graphite [25]. This should assist in solution sample spreading to promote smaller metal particle sizes after drying that, in turn, should be reflected in decreased Ea. This decrease in interfacial tension between graphite and water produced by chemisorbed O2 can be easily seen by projecting the image of the furnace onto a screen and observing the appearance of the sample solution droplet. Table 1 presents E, data for this altered surface. O2 was chemisorbed onto the surface prior to sample deposition by rapidly heating the furnace to approximately 1000°C and letting it cool down while introducing a jet of air into the furnace through the dosing hole. The volume was then flushed thoroughly with Ar, the sample deposited and the normal heating programs followed in an Ar atmosphere. As seen from Table 1, O2 decreases E, of Ag when the lower drying temperature is employed, but the low value of 24 kcal mol-’ recorded for 300°C drying is not influenced by the presence or absence of the surface oxide. This suggests that a lower limit for E, has been reached at 24 kcal mol-‘. The results obtained for Au (Table 2) indicate that the effect of O2 on Au vaporization is different from that observed for Ag. E, tends to decrease similar to Ag when the slower heating rate was employed. However, the opposite effect (i.e. an increase in E,) was observed for both drying temperatures when the 800 K s-l heating rate was used. Some insight can be gleaned from ancillary studies of the metal-graphite-oxygen system. Studies on catalytic graphite oxidation [26, 271 also suggest differences in the reactivity of Ag and Au. MCKEE [27] concluded that Au does not show any catalytic behavior for particles in the l-10 km range, while Ag is very active. He also indicates that the reactivity will probably increase for smaller particle sizes, and HENNIG [28] also has reported catalytic channeling of graphite by colloidal gold. On the other hand, HARRIS et al. [26] indicates that while Ag is a good catalyst of the graphite-oxygen

reaction at temperatures below 88O”C, it can act as an inhibitor above this temperature. If this is true, then the heating rate will also be a factor on the catalytic effect. According to BAKER [12] the exothermic catalytic gasification of graphite can transfer heat to the metal particles,

thereby

increasing

their mobility

on the surface.

Ag and Au vaporization mechanisms using ETA

288 0

20

40 time

60

85

80

(s)

Fig. 6. Dependence of activation energy on thermal pretreatment time in the case of Ag (0.08 ng). Drying and thermal pretreatment temperatures are 80 and 300°C respectively.

Let us consider that catalytic activity increases for small particle sixes and that it might be inhibited at high heating rates. In the case of Au, slow heating probably increases channeling by Au microdroplets leaving behind smaller particles or atoms adsorbed to active sites as they move on the surface. At high heating rates, Au, to a greater extent than Ag, might become an inhibitor with only smaller particles becoming mobile. These mobile, small particles can then combine with larger particles to produce large static droplets that have an associated large E, approaching AH, in the limit. Thermal pretreatment: time study Figures 6 and 7 present the effect of varying the thermal pretreatment times on E, for Ag and Au. A thermal pretreatment temperature of 300°C was used after drying the sample at 80°C. It is clear that E, tends to decrease as the thermal pretreatment time is increased. Assuming that we start with a given size of microdroplets after the low temperature drying, the decrease in E, might be explained as a reduction in the particle size as the larger droplets begin to dissociate and form particles of smaller average diameter during the thermal pretreatment cycle. This process might be driven by a purely thermodynamic tendency of the system to increase its entropy by existing in a more disperse state. These smaller particles can produce a decrease in surface tension at the metal-graphite interface or lessen the metal cohesive interactions. Other research confirms the ability of metals to move on graphite surfaces, especially at temperatures close to the Tammann temperatures, which are lower than 400°C in the case of Ag and Au [lo-121. As mentioned earlier, another illustration of gold mobility

time (s) Fig. 7. Dependence of activation energy on thermal pretreatment time in the case of Au (0.3 ng). Drying and thermal pretreatment temperatures are 80 and 3OOY, respectively.

R. W. FONSECAet al.

86 0.35

I

,

I

# 0.75

I 1.00

I 1.25

1.50

1.25

1.50

a 0.28 -

time (s)

0.28

0.00 0 50

0 75

1 00

time (s) Fig. 8. Comparison of the alignment of absorbance-time profiles for Ag using a mechanically roughened furnace (0.02, 0.04 and 0.06 ng) and two drying temperatures: (a) 80°C and (b) 3oOT.

on graphite surfaces is gold decoration techniques, which are successful at temperatures close to 300°C [13]. Mechanically created active sites Without exposing the electrographite, the graphite surface was mechanically roughened by abrasion to increase the number of active sites. The walls of the furnace were carefully sanded using a small roll of silicon carbide sanding paper (Grit no. 180). If the electrographite were exposed, soaking of the solution would occur producing a delay and further broadening of the absorption peak. Ag E, values shown in Table 1 are slightly larger for the lower drying temperature, but remain constant for the high temperature drying. This suggests that larger particles are favored with the more active surface, perhaps through the inability of Ag to disperse to form smaller particles from the initial solution precipitate. The highly dispersed nature with rapid drying appears to foster small particles that are unable to move and coalesce due to the diffusion barrier set up by the increase in active sites. The effect on Au from surface roughening (Table 2) is very similar to the case of Ag, except that the E, also increases when the high drying temperature is employed. It is possible that a stronger Au-Au interaction or a weaker Au-C attraction (relative to Ag) favors the formation of microdroplets to a larger degree and at lower effective surface coverages. It is assumed that a smaller E, is characteristic of desorption from either adatoms, metal at the droplet-graphite interface or droplets whose desorption barriers have been lowered by the near vicinity of the surface [l]. This suggests that if the microdroplets tend to dissociate as indicated by varying the thermal pretreatment time (previous section), the particles at the interface would tend to be released more readily than the ones from the bulk. If the sample is initially deposited on a surface

Ag and Au vaporization mechanisms using ETA

87

very rich in active sites, such as the surface generated during the sanding operation, metal particles at the interface might not easily diffuse across the surface because of stronger metal-graphite interactions produced by the presence of the active sites and, as a result, remain bound to the microdroplet. To further discuss the effect of surface roughening the next two equations are included: MW -Z M(ads) (M&-C*

s M(ads).

(I) (2)

Equation 1 shows a metal microdroplet, Me,,,, located on a graphite surface where interface. When the not many active sites, C*, are found at the metal-graphite temperature at the graphite surface allows the atoms of the microdroplet to start moving [ll, 121, small clusters or atoms might leave the microdroplet and stick to active sites in the vicinity of the microdroplet so that vaporization will take place from a larger fraction of adatoms. Equation 2 shows a microdroplet that is located on a surface that is enriched in active sites, (Mo+C*. The atoms at the metal-graphite interface are now interacting strongly with the support material and their mobility is reduced, thereby preserving larger microdroplets. This is equivalent to a shift of the equilibrium to the left in Eqn 2. Desorption from larger microdroplets might explain the higher activation energies obtained when the sanded surface and the lower drying temperature were employed. In the case of the higher drying temperature, the sample is widely dispersed after the drying step and the smaller particles formed are not expected to recombine because of the same effect. Hence, E, is not predicted to change. Order of release at low concentrations using a roughened furnace surface

Figure 9 shows a comparison of peak alignments at different concentrations under situations that favor the formation of (a) adatoms and (b) microdroplets, respectively. By a qualitative examination of the alignment of the peaks and the appearance of the leading edges of the absorption profiles [7, 211, first order release is observed for the conditions expected when adatoms are present on the surface, and fractional order is seen where vaporization from microdroplets would be expected. Similar results for Au were observed.

CONCLUSIONS Vaporization for Ag and Au appears to take place from both microdroplets of varying size and adsorbed atoms, depending on the conditions of analysis. Under normal ETA analytical conditions, fractional orders of release are observed for both metals. This is supported by shifting of the peaks to later times at increasing concentrations and the close alignment of the leading edge of the absorbance trace. However, a first order process was observed for conditions expected to favor the formation of adsorbed atoms, i.e. high drying temperatures, and/or very low concentrations. E, for the desorption of Ag and Au are mass dependent. The increase in E, observed at increasing concentrations probably reflects the decreasing influence of the graphite as the droplet size becomes larger. The highest E, values obtained for both metals approach the respective AH, and probably represent desorption from the surface of large microdroplets. At the other extreme, the lowest E, might represent the limit of “infinitely small droplets” (individual adsorbed atoms) and the graphite surface, i.e. the true desorption energy of the metal from graphite. E, and the shape of the adsorption profiles are affected by the dry and thermal pretreatment stages, since they produce changes in the surface distribution of these metals prior to the atomization cycle. O2 chemisorbed onto the graphite fosters a reduction in the size of the microdroplets

R. W. FONSECAet al.

88

091

i i2 time (s)

1.34

1.55

b 0.20

0 io

0

I

91

I

1.12

I

1.34

time (s)

Fig. 9. Comparison of the alignment of absorbance-time profiles for Au using a mechanically roughened furnace (0.05, 0.10 and 0.15 ng) and two drying temperatures: (a) 80°C and (b) 300%

formed in the case of Ag, but the effect on Au cannot be completely explained at this time. Mechanical roughing of the furnace surface increases the number of active sites on the graphite producing a reduction in the tendency of Ag and Au to diffise on the graphite surface. The main analytical implication of the study is the fact that changes in morphology of the sample before desorption takes place affects the shape of the absorption profiles but the peak area remains constant, making peak area a more accurate tool than peak height for quantitative considerations. Acknowledgements-This work was partially supported by the National Science Foundation (CHE-9020591) and the Robert A. Welch Foundation (F-1108). R.W.F. acknowledges the supplemental support provided by the University of Costa Rica.

REFERENCES [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13]

J. R. Arthur and A. Cho, Surf. Sci. 36, 641 (1973). S. S. Black, M. R. Riddle and J. A. Holcombe, Appl. Spectrosc. 40, 925 (1986). 0. A. Gtiell and J. A. Holcombe, Spectrochim. Acta 43B, 459 (1988). 0. A. Giiell and J. A. Holcombe, Spectrochim. Acta 44B, 185 (1989). R. W. Fonseca, 0. A. Gtiell and J. A. Holcombe, Spectrochim. Acta 45B, 1257 (1990). J. McNally and J. A. Holcombe, Anal. Chem. 63, 1918 (1991). J. McNally and J. A. Holcombe, Anal. Chem. 59, 1105 (1987). J. McNally, Dissertation, University of Texas at Austin, U.S.A. (1988). S. Lynch, R. E. Sturgeon, V. T. Luong and D. Littlejohn, Spectrochim. Acta 43B, 1187 (1988). E. Ganx, K. Sattler and J. Clarke, Surf. Sci. 33, 219 (1989). G. Tammann, Lehrbuch d. Metallkunde, 4th Edn. L. Voss, Leipzig (1932). R. T. K. Baker, J. Catal. 78, 473 (1982). G. R. Hennig, Chembtry and Physics of Carbon, vol. 2, p. 1. Marcel Dekker, New York (1967).

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[14] R. C. Weast, Handbook of Chemistry and Physics, 52nd Edn. The Chemical Rubber Co., Cleveland (1971). [15] F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, chap. 19, p. 937. Wiley, New York (1988). [16] B. V. L’vov, Spectrochim. Acta 33B, 153 (1978). [17] D. K. Eaton and J. A. Holcombe, Anal. Chem. 55, 946 (1983). [18] J. A. Dean and T. C. Rains, Flame Emhsion and Atomic Absorption Spectrometty, vol. 2, chap. 13, p. 334. Marcel Dekker, New York (1971). [19] B. Smets, Spectrochim. Acta 35B, 33 (1980). [20] P. Wang, V. Majidi and J. A. Holcombe, Anal. Chem. 61, 2652 (1989). [21] P. A. Redhead, Vacuum 12, 203 (1%2). [22] J. B. Newkirk, High Temperature Materials, Coatings and Surface Interactions, pp. 257-301. Freund Publishing House, Tel-Aviv (1980). [23] D. A. Bass and J. A. Holcombe, Spectrochim. Acta 43B, 1473 (1988). [24] A. W. Adamson, Physical Chemistry of Surfaces, chap. 9, p. 319.Wiley, New York (1982). [25] W. E. Gamer, Chemisorption, p. 251. Academic Press, New York (1957). [26] P. S. Harris, F. S. Feates and B. G. Reuben, Carbon 12, 189 (1974). [27] D. W. McKee, Carbon 8, 623 (1970). [28] G. R. Hennig, Proc 3rd Carbon Conf, BuflaZo, p. 265. Pergamon Press, Oxford (1958).