1425
SORPTION AND MAGNETIC SUSCEPTIBILITY STUDIES
Sorption and Magnetic Susceptibility Studies on Metal-Frele-Radical Systems: Nitric Oxide on Palladium by Richard W. Zuehlke, Maurice Skibba, and Carl Gottlieb Department of Chemistry, Lawrence University, Appleton, Wisconsin 64911
(Received January 24, 1967)
Sorption and simultaneous magnetic-susceptibility measurements were made on a nitric oxide-palladium black system from - 196 to 42". The results suggest that N O is adsorbed in one (possibly two) chemisorption form, a low-energy state reached through physisorbed NO as a precursor state. In addition, a transition temperature of - 120" is observed for equilibrium between mobile and immobile forms of chemisorbed NO. Pertinent energetics for the system are summarized in an energy profile, thereby justifying an observed case of high-temperature physisorption.
Introduction The investigation described below was undertaken to shed some light on the nature of the interaction between an "odd molecule" and a free-metal surface unperturbed even by a support matrix. Adsorption !studies involving nitric oxide are limited in number but are rather varied in adsorbent and technique. Included are a series of oxide gels by straight isotherms, potassium chloride by straight isotherms,2 iron and nickel by infrared, transition metals and oxides by infrared,4 and several refractory oxides and palladium by magnetic methods referred to below. Several catalytic-decomposition studies have been made at elevated temperature^,^ but they will not be considered here. The picture that emerges from these studies is that NO, by virtue of its free-radical character, can adsorb in a variety of different ways, depending upon the nature of the adsorbent and, for a given adsorbent, upon the exact position of adsorption on the surface and upon the temperature of adsorption. This paper does not alter that picture at all and further supplies some details of the energetics of adsorption of nitric oxide on an unsupported palladium surface. Experimental Section Palladium samples used were all supplied in one lot by Engelhard Industries, Inc. Typical analyses have been presentedj6and all samples have been shown to be purely paramagnetic through a susceptibility-field strength investigation. Nitric oxide came from Matheson cylinders and was fractionally distilled through a tetratrap at - 139" until the frozen product was snow-white in color (usually five distillations). Tank hydrogen (99.9%) was purified by passage through a Deoxo purifier, t h r o u h hot magnesium perchlorate, and finally through activated charcoal at - 196". Tank oxygen (99.5%) was passed through ascarite, over hot CuO,
and finally through a Dry Ice-ether trap for purification. The experimental system used was a classical gravimetric-sorption system employing a Cahn RG electrobalance, a vacuum system capable of regular production of 10-a torr, and a needle valve for dosing. The null-seeking electrobalance was also used for simultaneous magnetic-sorption determinations through the use of the Faraday susceptibility technique; the pole-tip configuration used with the 4.5-in. magnet has been described.6 Sorption-rate studies were made a t constant pressure using a Pirani gauge as a monitor and the needle valve as a regulator. During a series of magnetic-sorption determinations, the maximum nitric oxide pressure used was about 1 torr. Since the investigation was, in part, designed to study the influence of palladium surface states on the adsorption process, it was necessary to retain a fixed particle size throughout; consequently, only a mild cleaning process could be used in pretreating the samples. The pretreatment process finally selected (and referred to as CP-11) was: (1) hydrogen scrubbing (300 p pressure, room temperature), (2) evacuation and pump off at 80", (3) oxygen (40 p > SO"), (4)evacuation at 80, (5) hydrogen (300 p , room temperature), (6) evacuation at 80", (7) nitric oxide (300 p, room temperature), (8) evacuation at 80", and (9) repeat steps 1-6. After a sample had been exposed to nitric oxide during a run, the nitric oxide was pumped off at room (1) L. I. Kuznetsov-Fetisov and E. 5. Krasnyi, Tr. Kazanst. Khim.Tekhnol. Inst., 106 (1958); Chem. Abstr., 53, 21029e (1959). (2) A. Granville and P. G . Hall, J . Phys. Chem., 70, 937 (1966). (3) G. Blyholder and M. C. Allen, ibid., 69, 3998 (1965). (4) A. Terenin and L. Roev, Spectrochim. Acta, 946 (1959). (5) J. T. Yates, Jr., and T . E. Maday, J. Cham. Phys., 45, 1623 (1966), and references therein. (6) R. W. Zuehlke, Ph.D. Thesis, University of Minnesota, -Minneapolis, Minn., 1960. Volume 72, iVumber 6
M a y 1968
1426 temperature, and the sample was subjected to step 9. Starting weights for a given sample (usual weight, 150 mg) agreed within 50 pg. When discrepancies greater than this occurred, the sample was discarded. While this procedure admittedly does not yield a "clean" surface, the data obtained suggest that there are substantial areas of bare metal formed, and, at the very least, they are formed reproducibly. Return of the magnetic susceptibility to the precleaning value after cleaning indicated that neither appreciable particle-size change nor hydrogen uptake occurred as a result of the cleaning. Occasionally, reference will be made to samples cleaned with procedure CP-I. This consisted of several consecutive exposures of the sample to hydrogen at low pressures and room temperature, with evacuation and pump off at room temperature occurring between each exposure. Data. The adsorption of NO on palladium is generally irreversible, except near monolayer coverage (monolayer coverage for NO is defined with respect to nitrogen BET areas) ; i.e., at this coverage an added increment of gas can be readily removed by pumping at constant temperature. Therefore, isosteric heats of adsorption obtained by a Clausius-Clapeyron analysis are generally invalid except in the region of monolayer coverage. Figure 1 shows such an analysis in the 0-28' temperature range for the NO-Pd system cleaned with procedure CP-I, where the minimum in the curve occurs at monolayer coverage. I n this case, the observed heat of adsorption of 3.56 kcal/mol corresponds very closely to the heat of liquefaction of nitric oxide, 3.29 k ~ a l / m o l . ~For the samples used in this study (cleaned by procedure CP-11), however, the adsorption at monolayer coverage is not entirely reversible, and the Clausius-Clapeyron heats (of questionable validity) at roughly the same temperatures range from 4.5 to 6.0 kcal/mol. These observations suggest that extensive physical adsorption occurs at monolayer coverage, either exclusively or concurrently with a more energetic chemisorption, depending upon the exact nature of the substrate surface. Studies of the desorption kinetics of the nitric oxide indicate that two types of nitric oxide binding are involved. Thus in the temperature range 25-50', nitric oxide, preadsorbed at 25', desorbs with an energy of 4.07 kcal/mol. After removal of the weakly bound NO, which typically amounts to about 50%) desorption in the temperature range 69-162' occurred with an energy of 10.5 kcal/mol. These observations are in good agreement with the postulates presented in the preceeding paragraph. In Figure 2, redrawn recorder traces show the rate of adsorption of nitric oxide on a freshly cleaned surface under a constant pressure of 75 p in the temperature range from -22.5 to -77.6'. It will be noted that a t The Journal of Physical Chemistry
R. W. ZUEHLKE, M. SKIBBA, AND C. GOTTLIEB
. IO 12
\
-. \
\
\
\ 8 -
WD,, KCALIMOLG
\
\ \
.
\ I
\ \
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Figure 1. Isosteric heats of adsorption (after Clausius-Clapeyron) of nitric oxide on palladium black (sample weight, 150 mg).
" L l 00
20
30
40
50
TIMEl MINUTES
Figure 2. Time dependence of nitric oxide adsorption on palladium black (sample weight, 150 mg). Initially clean surfaces exposed to a constant pressure.
these temperatures an initial rapid adsorption occurs, followed by a much slower sorption, the rate of which increases with decreasing temperature. It seems reasonable to assume that the slow and linear increase (with time) of adsorption represents a chemisorption process. Should this be true, an extrapolation back to zero time would give the amount of physisorbed NO. An Arrhenius plot for the chemisorption rates leads to an apparent negative activation energy of -3.39 kcal/ mol. Since this situation is identical with that found in the nitric oxide-alumina system (vide infra), it is taken as further support for the postulate of coexisting chemisorption and physisorption, even a t temperatures exceeding the critical temperature of nitric oxide. Figure 3 shows two sets of isobars obtained a t 75 p (7) D. M. Yost and H. Russell, "Systematic Inorganic Chemistry," Prentice-Hnll, Inc., Englewood Cliffs, N. J., 1944, p 26.
1427
SORPTION AND MAGNETIC SUSCEPTIBILITY STUDIES
t
0.2 0'4
0'
-1o ;
-100
-80 -6b
20 40
-40
-20 TEMPERATURE, ( O C )
Figure 3. Isobars (at 75 p pressure) for nitric oxide on palladium black (sample weight, 150 mg): curve A, single exposures of initially clean surface; curve B, points selected from isotherms.
1,1~,
, ,
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,
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-
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Figure 5 . Magnetotherms (-104 to - 122') for nitric oxide on palladium black (sample weight, 150 me). The inclusion of two curves a t - 122' shows the reproducibility of a typical magnetotherm. Cleaning procedure I1 used.
ceptibility, x, to the susceptibility of the clean sample,
xo) ' Mms> MG.
Figure 4. Magnetotherm (- 196") for nitric oxide on palladium black (sample weight, 150 mg). Cleaning procedure I1 used.
pressure for the nitric oxide-palladium system. Data for isobar A were obtained by following the extent of sorption when an initially clean sample was exposed to a constant NO pressure of 75 p . The temperature sequence was more or less random, and a given point 20 pug. could be reproduced within approximately The points shown represent the amount of gas adsorbed after an exposure of 1 hr; a t temperatures below -85" and above +21", this is an apparent equilibrium amount, while in the temperature range -85to +21°, a slow adsorption is still taking place after this time (see Figure 2). Isobar B is formed from points taken from isotherm data. The disagreement between the two isobars is striking and will be shown below to be intimately related to the observed irreversibility of adsorption. Isobars having the shape of curve A have been observed8 and are taken to be an indication of several different forms of sorption which can take place on the surface. Magnetic Xtudies. The magnetic data obtained are summarized in the form of "magnetotherms" (plots of amount adsorbled against the ratio of observed sus-
At -196" (Figure 4), a positive slope suggesting physisorptiong is observed. It should be noted that the maximum NO pressure used at this temperature was 31 p , well below the expected equilibrium liquidvapor value of about 75 p . While this evidence for physisorption is reassuring, the nature of the adsorption as deduced from the details of the magnetotherms (see below) is not as simple as one might expect. For temperatures above - 120") the magnetotherms (Figures 5 and 6) are seen to be essentially independent of temperature. Below -120", a new slope is observed, one which has a slightly steeper slope than that of an equivalent palladium-silver alloy curve (the "Hoarel0 slope"). The Hoare slope represents the decline in susceptibility observed when palladium is alloyed with silver; the slope shown in Figure 5 is drawn for corresponding electron concentrations in a palladium-silver and a palladium-nitric oxide system. Needless to say, the parallels between the two systems are striking and form a basis for discussing the mechanism of chemisorption of NO at these temperatures. With several exceptions, all the magnetotherms are (8) P. H.Emmett and R. W. Harkness, J . Amer. Chem. SOC.,5 7 , 1631 (1935). (9) A. Solbakken and L. H. Reyerson, J . Phys. Chem., 63, 1622 (1959). (10) F. E.Hoare, J. C. Matthewa, and J. C. Walling, Proc. Rov. SOC. (London), A216, 602 (1963).
Volume 78, Number 6 M a y 1068
1428
R. W. ZUEHLKE,M. SKIBBA,AND C. GOTTLIEB
0
(MG/GMl
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Figure 6. Magnetotherms (0-40”) for nitric oxide on palladium black (sample weight, 150 mg). Cleaning procedure I1 used.
seen to consist of a steep, initial negative slope, followed by a declining slope at higher coverages, and a final “bottoming out” at monolayer coverages. The continuing decline beyond monolayer coverage at -122 and +25” is taken to be an indication of the nonreproducibility of the surfaces. I n approximately 30 suns taken during this series, only three were observed to exhibit this behavior.
Discussion Physisorption. The physisorption state is typified by the -196” magnetotherm of Figure 4. While the initial 0.2 mg of sorbed NO causes no change in the magnetic properties (a presently unexplainable fact), the following positive slope is indicative of sorption in a paramagnetic state similar to that found on both silica and a l ~ m i n a . ~ ! lIln this investigation, the slope of Figure 4 indicates that the magnetic moment of sorbed NO is 1.23 BRI. This may be compared with a predicted value1*of 1.34 BIT at the same temperature; it is thus apparent that a palladium surface apparently physisorbs NO in a state electronically comparable with that of the gas at the same temperature. It will also be noted that in Figure 4 a slight decline at the end of the magnetotherm is followed by a “bottoming out.” It appears that after a significant amount of physisorption has occurred, a cooperative rearrangement follows which involves either dimerization or freezing into a diamagnetic layer. It was mentioned earlier and will be further discussed below that the initial rapid adsorption shown in Figure 2 is attributable to physisorption. Although it is extremely unusual to observe physisorption at such high temperatures, the evidence for this is unmistakable. The enthalpy of adsorption, the energy of desorption at such high temperatures, the evidence for The Journal of Physical Chemistry
this is unmistakable. The enthalpy of adsorption, the energy of desorption, and the observed isobars are strong evidence in favor of this concept. I n addition, it can be shown that the temperature dependence of the amount of physisorbed NO a t constant pressure (see Figure 2) is in accord with the BET adsorption model. (A plot of In ?“zadsorp against 1/T at “high” temperatures and low pressure is linear.) Thus the initially physically adsorbed XO apparently loads up portions of the surface “piggy-back” style, leaving appreciable sections of bare surface open for subsequent chemisorption. The observation of the existence of infrared bands around 1876 cm-I (the gas-phase stretching frequency) for NO adsorbed on a number of transition elements (including palladium) at room temperature4~l3 and their disappearance upon evacuation also constitute evidence for this “anomalous” physisorption. It is to be noted that similar observations have not been recorded for sorption systems involving molecules with singlet ground states (i.e., carbon monoxide). This suggests that such physisorption is uniqae to free-radical adsorbates, and this is discussed at length in a later section. Chemisorption. For temperatures below - 120°, the similarities between the magnetotherm slopes and the Hoare slope suggests that chemisorption follows roughly the same mechanism observed in the process of alloying silver with palladium. Thus it seems apparent that a net of approximately 1.5 electrons/NO molecule is transferred to the bulk palladium d band. A detailed analysis of this electron transfer is treated in a forthcoming publication.14 The temperature-independent slopes of magnetotherms obtained above - 120” suggest that extensive chemisorption also occurs at these temperatures; however, if it involves electron transfer, it involves less than 1 electron/NO. Comparison of these magnetotherm slopes with the Hoare slope suggests several possible mechanisms regarding the effect of nitric oxide on the magnetic susceptibility of palladium. The following conclusions are possible, all of which are designed to account for the transfer of less than one electron per KO molecule. 1. (a) The “alloy mechanism’’ is followed in the case of nitric oxide; however, a polymer of NO is formed and donates 1 electron/polymer to the palladium bulk band. (b) Alternatively, each nitric oxide molecule gives up one-half an electron to the palladium bulk band ( i e . , a covalent bond is formed with, perhaps, the odd electron). (11) A. Solbakken and L. H. Reyerson, J. Phys. Chem., 64, 1903 (1960). (12) J. H. Van Vleck, “The Theory of Electric and Magnetic Susceptibilities,” Oxford University Press, London, 1932, p 269. (13) K. H. Rhee, private communication. (14) R. W. Zuehlke, t o be published.
1429
SORPTION AND MAGNETIC SUSCEPTIBILITY STUDIES 2. Nitric oxide is adsorbed in several different states, one of which involves the donation of 1 or more electrons/NO molecule in that state to the bulk band of palladium. 3. Nitric oxide is adsorbed as in 1 or 2, but electron transfer involves surface states of palladium rather than bulk electronic states." The third conclusion is significant but is probably not applicable in this particular study. All palladium samples used had average particle sizoes (normal to the (111) plane) ranging from 150 to 250 A; according to a surface-state analy~is,'~ surface states in these particles are empty and only bulk states would be involved in determining magnetic susceptibility. Indeed, if surface states were involved (as they should be for much smaller particles), the nature of nitric oxide adsorption should be changed considerably. Conclusion l a is of questionable value because the only reasonable polymer to be expected is NZOZ, a molecule which has only been observed at very low temperatures. On the other hand, conclusions l a and lb, which involve one electron covalence, have some credibility, since both mechanisms predict temperature independence of the magnetotherms. This follows because a, ratio of two susceptibilities is involved, each of which ,would be expected to have essentially the same temperature dependence. The second conclusion, which is hinted at by the shapes of the isobars and the low-temperature magnetotherms, is probably the most reasonable one. With this mechanism, any effect of temperature on the magnetotherms is simply explained through its influence on the distribution of nitric oxide among the several postulated sorption states. The chemisorption state following from these arguments is assumed to be exactly equivalent to that observed below - 120". However, if the magnetotherm slopes are plotted against temperature as in Figure 7, it is apparent that a marked discontinuity appears a t -120". This is indicative of a first-order phase transition and, in the case of adsorption, is commonly attributed to a transition from mobile to immobile adsorption. If this is indeed the case, -120" is the equilibrium temperature for the transition at 75 p pressure. If the two states differ only by the gain or loss of one degree of translational freedom, the twodimensional Sackur-Tetrode equation may be applied. l6 With the assumption that the area available per molecule is 1.07 X cm2 (i.e., calculation made at halfcoverage), the entropy change is found to be approximately 13 kcal/mol deg, from which an energy difference (AH) of approximately 2.0 kcal/mol follows. It is assumed that when transition to the mobile chemisorbed phase occurs, extensive physisorption can also occur. This results in a decreased slope of the magnetotherms above - 120", and this decreased slope
TEMPERATURE,
"C.
Figure 7. Temperature dependence of magnetotherm slopes (cleaning procedure 11).
will be discussed more extensively when the detailed model is presented below. Activated Complex. The negative activation energy of -3.39 kcal/mol for adsorption into the mobile chemisorbed state has already been presented. The existence of such a negative activation energy frequently implies a two-step reaction mechanism, with the ratedetermining second step involving a species in temperature-dependent equilibrium with the original reactants. The apparent mechanism in this case is NO(g)
NO (physisorbed) + NO (chemisorbed)
Following the classical argument of SoIbakken and Reyerson'l developed for X O on alumina, one sees that is the apparent negative activation energy, E',,,, composed for the energy of physisorption, AH,,, and the true activation energy for transition from physisorption to chemisorption, E
*
Eiapp= E'
+ AH,,
Substituting the Clausius-Clapeyron heat of physisorption into the above expression gives a true activation energy of 170 cal/mol. (Existing experimental data force the use of low-temperature rate data with roomtemperature equilibrium data; however, it is likely that the heat capacity of liquid NO is slightly greater than that of gaseous NO. This being the case, the heat of physisorption a t low temperatures, would be slightly more negative, thus making the true energy of activation slightly higher than the calculated 170 cal/ mol.) It is most interesting to note that the observed negative activation energy is exactly equal to that found for NO on alumina. It therefore seems apparent that the activated complexes on the two surfaces are essentially identical, This fact, along with a discussion of the bonding mechanisms in the chemisorption and R.W. Zuehlke, J. Chem. Phys., 45, 411 (1966). (16) C. Kernball, Proc. Roy. SOC.(London), A187, 73 (1946). (15)
Volume 78,Number 6 May 1968
1430
R. W. ZUEHLKE, 1M. SKIBBA,AND C. GOTTLIEB
-Y
0
0
A
z:-2.0
s 0 Y
>
4
ANCE FROM SURFACE (no scale)
-
\ E
-
-4.0
(3
-14.0
Figure 8. Energy profile for nitric oxide adsorption on palladium black.
physisorption states, are dealt with in another publication.14 Model. The energy profile for the adsorption of nitric oxide on palladium as deduced from the study above is shown in Figure 8 (the ordinate is drawn to scale; the abscissa is not). The curve representing physisorption is shown to have a minimum at 3.6 kcal/ mol; this is in accord with the Clausius-Clapeyron heats observed on samples cleaned with CP-I. The minima in the mobile and immobile chemisorption curves have already been discussed. The points a t which these curves cross the physisorption curve are of interest, however. The negative activation energy discussed above was found in connection with the mobile chemisorption and the physisorption states. According to classical kinetic analyses, the crossing of these two curves occurs a t the point which represents the energy of the activated complex ( i e . , - 3.39 kcal/mol). Reyerson and Solbakken have shownll that the transmission coefficient for the corresponding process on alumina gel is of the order of lo+. A simple analysis shows that an equally small coefficient is observed in this case. This situation is related17,1*to the nature of the crossing of these two curves and suggests that the resonance energy of the activated complex (which is related to the physiand chemisorption states) is very small. I n effect, this means that the two curves actually do cross and are not rounded off into two separate, noncrossing curves with a small energy gap between them. This is not at all surprising, because the magnetic data show a spin multiplicity change between the two states. Hence the resonance energy at the curve intersection is essentially zero and the two curves effectively cross at the activated complex and give rise to a very small transmission coefficient. Thus even at relatively high temperatures, extensive physisorption of NO (in a The Journal of Physical Chemistry
paramagnetic state) may take place, and a very large fraction of the molecules may have sufficient kinetic energy to surmount the activation energy barrier. However, since the physisorption and chemisorption curves cross, the probability of a molecule bound in the physisorption well changing its position to be trapped in a chemisorption well is very small. Therefore, one observes an anomalously high percentage of physisorption. The crossing of the immobile chemisorption and physisorption curves is somewhat more difficult to portray. The fact that physisorption alone is observed at very low temperatures suggests that an activation-energy barrier exists between the two states, although it is very small. Therefore, the crossing must occur to the left of the physisorption minimum. However, since a spin multiplicity change is involved here as well, the transmission coefficient might also be expected to be low. If two potential energy curvm cross, the transmission coefficient might be expected, intuitively, to be related to the angle a t which the curves cross ( i e . , the sharper the angle of crossing, the lower the transmission coefficient). The crossing shown in Figure 8 is a manifestation of these principles, with the crossing shown just beyond the minimum in the physisorption curve, where the small negative slope of the physisorption curve gives rise to both a small activation energy and a reasonable transmission coefficient. With the aid of the model and the data presented, the shapes of the magnetotherms and isobars may be readily understood. Above - 120°,the magnetotherm (17) S. Glasstone, K. J. Laidler, and H. Eyring, “The Theory of Rate Processes,” McGraw-Hill Book Co., Inc., New York, N. Y., 1941, Chapter 111. (18) K. J. Laidler, “Chemical Kinetics,” McGraw-Hill Book Co., Inc., New York, N. Y., 1960, Chapter 13.
SORPTION AND MAGNETIC SUSCEPTIBILITY STUDIES slopes suggest that both physisorption and chemisorption occur in the early stages of the process, and, when monolayer coverage is approached, an increasing percentage of physisorption occurs. The Clausius-Clapeyron data are useful here, for they suggests that, when a relatively high percentage of oxide coverage is present ( i e . , surface cleaned by CP-I), the heat of physisorption approaches very closely the heat of liquefaction of nitric oxide. On the other hand, when a larger percentage of the surface is clean ( i e . , CP-11 is used), the adsorption process occurring near monolayer coverage is considerably more energetic. These observations suggest that something like an oxide substrate is necessary for true physisorption of NO to occur. The kineCic data, however, indicate that above - 120” only a small portion of the initially physisorbed NO is converted to the chemisorbed state during the time intervals of observation. The chemisorption layer then forms an ideal base for relatively pure physisorption and effectively blocks out additional chemisorption sites for future doses of gas. Therefore, as successive increments of gas are added, the observed percentage of chemisorption resulting from each dose will be observed to decrease. Ultimately, when nearly all chemisorption sites are blocked, pure physisorption results, and the magnetotherm slopes bend to the horizontal or become slightly positive. The bottoming out mentioned earlier apparently results from a co-
1431 operative dimerization of the entire physisorbed layer. The adsorption process is seen to be “autocatalytic” with respect to the promotion of physisorption. The difference in shapes of the two isobars of Figure 3 can also be explained by this model. The magnetotherms show only one discontinuity a t - 120°, and this agrees rather well with the break shown in isobar B. Curve A, on the other hand, is markedly different: however, it was obtained using single doses of NO, while curve B was obtained from isotherm data. Since the character of NO adsorption has been shown to be dependent on the sample’s history of prior exposure to NO and since the sapples used in the magnetotherms and isobar B have similar (essentially identical) histories, one might expect to see agreement between the magnetotherms and isobar B and substantial disparity between isobars A and B. It can perhaps be concluded that the shape of isobar A reflects the nature of the oxide contamination on the clean surface at the start of adsorption. Acknowledgment. The authors are grateful to the Petroleum Research Fund, administered by the American Chemical Society, and to the Directorate of Chemical Sciences of the United States Air Force Office of Scientific Research (Grant No. AF-AFOSR-114-63) for their support of this work. This work was presented in part before the Division of Colloid and Surface Chemistry a t the 150th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1965.
Volume 78, Number 6 Maw 1968
