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W. E. Wallace, R. F. Karllcek, and H. Imarnura
The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
Mechanism of Hydrogen Absorption by LaNi5 W. E. Wallace,* R. F. Karllcek, Jr., and H. Imamura Department of Chemistv, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received December 1 1, 1978; Revised Manuscript Received March 5, 1979) Publicailon costs asslsted by the Universlty of Pittsburgh
A model is presented for the mechanism of absorption (or release) of hydrogen by LaNiS. The surface of LaNi, is extensively coated by La203. The surface oxide forms in two ways: (1)by reaction of gaseous oxygen with LaNi5or (2) by diffusion of dissolved oxygen to the surface where it reacts with LaNi5to form oxide and precipitate Ni, some of which is also at the surface. Hydrogen reaches the underlying LaNis,by diffusion in monatomic form along the La203-Ni interface. The dissociation of H2 at the point of entry into the interfacial region is rate controlling for absorption. Release of hydrogen involves diffusion of monatomic hydrogen along the interface followed by recombination to form molecular hydrogen at the top of the interfacial region. The latter is rate controlling. The model is in accord with the known surface features of the intermetallic and is consistent with the observed kinetics of absorption and desorption of H2 by LaNi,. It is also consistent with the kinetics of hydrogenation of C2H4 over LaNi6 and LaNi5H2.&In the latter, interfacial diffusion of monatomic hydrogen is rate controlling. The present model differs from earlier models which regarded either the phase transformation or mass transport through macroscopic cracks or fissures as the rate-controlling step.
I. Introduction LaNi, absorbs hydrogen to the approximate composition LaNi5H6a t mild conditions of temperature and pressure (-25 “C and -2 atm pressure).l The mass of hydrogen contained in this material per unit volume exceeds that of liquid hydrogen by about 40%. The hydrogen in this material is very labile. The sample can be “loaded” or “unloaded” to 90-95% completion in a matter of minutes by raising or lowering pressure of the hydrogen gas applied to the system.ls2 The rapidity of the sorption process is quite remarkable since hydrogen is absorbed dissociatively. Many metals, e.g., La or Zr, absorb hydrogen but do not release it at a significant rate except at temperatures in excess of 500 “C, and then only at very reduced pressures. The rapid kinetics of the LaNi5-H2 system are its truly remarkable feature. The factors which are responsible for the rapidity of hydrogen absorption or release have not yet been adequately elucidated. It is the purpose of this article to reveal those factors by making use of the rather considerable body of information which has been accumulated for the LaNi5-H system, particularly results that have been provided in several significant studies which have been carried out in the past 2 years. In section I1 we present a brief summary of the pertinent experimental features of the LaNi5-H system. The model is presented in section I11 together with an identification of the rate-determining steps for hydrogen uptake and release of hydrogen. A summary of the experimental features of the LaNi5-H system which have been interpreted by using the model is provided in section IV. 11. Summary of Results Obtained i n Recent Relevant Studies A. Boser’s S t u d y . While the first study of the hydrogenation and dehydrogenation of LaNi, was described in 1970, the first satisfactory quantitative information was not available until the work of Boser,2 published in 1976. He worked with activated materials, Le., samples which had been pressurized to 20 atm or more at room temperature and put through 10 in-and-out hydrogen cycles. He studied the uptake and release of hydrogen in a closed system and found that in both processes the time dependence of pressure was given by the relationship 0022-3654/79/2083-1708$01 .OO/O
Pf --- Po - A t + l Pf - P where Po and Pfrepresent initial and final pressures, respectively, P is the pressure a t time t , and A is a constant. Since in the desorption process Pf - Po is proportional to Co, the initial hydrogen concentration in LaNi,, and Pf - P is proportional to the concentration C remaining a t time t , eq 1 is equivalent to the statement that 1/C is linear with time, This implies that the desorption process is second order with respect to C. This is the behavior observed by Goudy et al.3 for a number of related rare earth intermetallics and for which it was postulated that the rate-determining step (rds) was the recombination rate of atomic hydrogen at the surface. We propose that this is the rds for desorption of hydrogen from LaNiS. The full meaning of this statement will be brought out in the discussion that follows. The assertion that surface recombination is rate determining is at variance with the point of view of Boser, who suggested that phase transformation was rate controlling. As will be pointed out later, the recent studies of Soga, Imamura, and Ikeda make Boser’s viewpoint in this regard untenable. To appreciate the implications of eq 1with regard to the mechanism of hydrogen absorption, dealt with in section 111, it is necessary to note that eq 1 can be rearranged to give Po - P = At(P - Pf) (2) A t small times P Po and hence under these conditions eq 2 can be written as Po - P f f t (2’) where cy is a constant = A(Po- Pf). Thus initially one has a nearly linear dependence of Po - P on t . B. Hydrogenation of C2H4with LaNi5 and Its Hydride. Soga, Imamura, and Ikeda (SII)have recently published* results which are of very considerable significance in regard to hydrogen absorption by LaNi,. Their study was concerned with the hydrogenation of C2H4with LaNi6 and LaNi6H2,4.Most of their work involved the hydrogenation of C2H4 with LaNi,H2,4. The strong affinity of C2H4 for hydrogen was made use of to extract hydrogen from the
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0 1979 American Chemical Society
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Hydrogen Absorption by LaNi,
hydride. This affinity is the rough counterpart of the pressure reduction (below the equilibrium pressure of hydrogen) involved in Boser’s work. Both serve to abstract hydrogen from the hydride and in this respect they are similar. However, as is pointed out below, there are very significant differences between these studies. The most important findings of SI1 were the following: (1)When mixtures of hydrogen and ethylene are passed over LaNi5H2.4,hydrogen is extracted from the hydride with the hydrogen concentration in the solid (CH)varying with time according to the relation dCH/dt = - k l C ~ . Hence, hydrogen release is first order with respect to hydrogen concentration. (2) kl is independent of the pressure of ethylene flowing over the catalyst but depends upon P H . For example, kl measured at -78 “C is 0.52 min-l (g of EaNi5)-l for P H=~ 0 and decreases to a limiting value of -0.03 (in the same 1 95 torr. Thus gaseous hydrogen inhibits units) for PH2 the formation of C2H6. (3) When ethylene and deuterium are passed over L B N ~ ~ Hethylene ~ . ~ , is largely (-90%) converted to C2H6. The reaction product contains deuterium only to the extent of -8% as C2H5Dand -2% as C2H4D2. Thus, hydrogenation comes primarily from the hydride. (4)Hydrogenation of CzH4over LaNi5 was observed to be about two orders of magnitude slower than that over LaNi5H2,4. Soga et al. postulated on the basis of their data that “migration of the hydride from the bulk to the surface sites is rate determining”. The present authors hold the opinion that this postulate is essentially correct for their experiments; however, their postulate is not specific in regard to either the nature of the migration or the character of the surface sites. Surface chemistry studies described below make it possible to supply additional information in regard to these issues, whereas, as is brought out below, the kinetics of hydrogenation of ethylene can be made use of to deal with the questions left unanswered (vide infra) in previous studies of the surface chemistry of LaNi5., In a second study Soga, Imamura, and Ikeda6 showed that LaNi5H3.5catalyzes isotopic exchange in a mixture of D2 and H2. They find that this exchange takes place very rapidly even a t 150 K. This indicates that molecular hydrogen or deuterium is adsorbed and dissociated into atoms very rapidly at the surface of LaNi,. At room temperature the speed of this overall process is at least lo3 times faster than the measured rate of uptake of hydrogen by LaNiS. This is a point of great significance in the model proposed. C. Surface Chemistry of LaNi5. Recently, Siegmann, Schlapbach, and Brundle (SSB) examined5 the surface of LaNi5 by Auger spectroscopy and found that it was enriched in the rare earth constituent compared to the bulk. Similar results have been obtained for this compound and the closely related rare earth intermetallics PrNi,, CeCo5, PrCo5, SmCo5, and ErFez by Moldovan, Sankar, and Wallace,’ also using quantitative Auger spectroscopy. SSB also examined LaNi5 by photoelectron spectroscopy, and found that the surface of LaNi, consists of La(OH)3and elemental nickel. These studies are highly significant in characterizing the surface of LaNi,. However, they have left many important questions unanswered. For example, since hydrogen is not soluble in Ni and presumably not soluble in La(OH)3, how does it reach the underlying LaNi5? What is the rate-determining step for hydrogen uptake or release? Is it surface dissociation, penetration of the surface barrier, or diffusion in the bulk? What are the factors which are responsible for the formation of the
The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
--_---_---_-_-_-_- LO,^, -____
and
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La ( O H ) 3
NI
55%
LON’,
Flgure 1. Representation of the surface of LaNi5. The top 50-100 A of LaNi, is comprised of oxide and Ni, in accord with ref 5. The oxMe becomes hydroxide in the presence of atomic hydrogen.
hydroxide? What limits the amount of hydroxide formed?
111. Proposed Model for Hydrogen Absorption or Release The authors hold the view that the results obtained in the studies summarized in the preceding section, together with general knowledge of the thermodynamics of the substances involved, provide a means for establishing the essential features of hydrogen uptake by or release from LaNi,. A. Origin of the Surface Species. The surface oxide develops in one of two ways: (1) migration of dissolved oxygen to the surface and reaction with LaNi5 or (2) reaction of LaNi5 with atmospheric oxygen. In regard to the first process it is well known that rare earth intermetallics such as LaNi5 contain oxygen as an impurity to the extent of hundreds or thousands of parts per million. At the temperatures of formation, -1500 “C, the oxygen is partially or perhaps totally in solid solution. There is powerful chemical affinity of La for oxygen (standard free energy of formation is -1700 kJ/mol) and hence, upon cooling, the dissolved oxygen migrates to the surface and unites with La in LaNi5 to form La203. Oxide formation is unlikely to occur in the interior because of the -25% volume change in the process. This process will stop when LaNi5 is sufficiently depleted of dissolved oxygen. When LaNi, is in contact with gaseous oxygen a similar process will occur: 2LaNi5 + 3/202 La203 + lONi (3)
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As the oxide is formed, Ni is “squeezed out” and the aggregates in some cases intersect the surface, and so one has a mixture of Ni and La203at the surface. If the system is in contact with HP,atomic hydrogen will be formed on the exposed Ni and this will rapidly migrate to the oxide grains and will convert them to La(OH),. This is in qualitative agreement with recent results of Flanagan et al., which demonstrates the affinity of the activated LaNi, surface for hydrogenas These are the mechanisms which give rise to the species observed by SSB6 and Moldovan et al.’ Our concept of the top 5C-lo0 A. of LaNi5 is represented in Figure 1. Von Waldkirch and Zurcher have recently reached a similar concl~sion.~ La(OH)3 and Ni form a protective coating over LaNi5 and hence this material is relatively unaffected by impurities such as COz, H20, or O2 present in hydrogen, which is being absorbed into the intermetallic. The underlying LaNi,, which might be affected by these impurities, is inaccessible. Ni, the only oxidizable species at the surface, is not affected by these impurities in the strongly reducing 99.9+ 7% hydrogen atmosphere which
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W. E. Wallace, R. F. Karlicek, and H. Imamura
The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
prevails when LaNi, is used for hydrogen storage. Accordingly, the surface chemistry makes it clear why LaNi, is not poisoned by these oxidizing impurities. These ideas are slightly at variance with suggestions put forth by SSB, who invoked a “self-restoring’’ cleaning mechanism to account for the behavior of LaNi,. We regard the protective nature of the surface species to be the essential point in regard to the durability of LaNi,. B. T h e Rate-Determining S t e p s f o r Sorption and f o r Formation of Ethane. In regard to the hydrogen absorption process, a sequence of steps is obviously involved in transporting hydrogen from the gas phase into bulk LaNi,. We propose, on the basis of general information about H2-transition metal interactions and in agreement with what was proposed by SSB, that H2 chemisorbs on the exposed Ni and very rapidly dissociates there into atomic hydrogen. Since hydrogen has little solubility in Ni and probably also in La(OH)3, penetration to the underlying LaNi, must occur along the hydroxide-Ni interface. We can then describe the process as follows: (a) H2(g) H2 (adsorbed on Ni) (b) H2 (adsorbed on Ni) 2H (adsorbed on Ni) (c) interfacial diffusion to reach LaNiS (d) diffusion in LaNiS (e) a-LaNi,-H P-LaNi,-H. The work of SI16indicates, as noted above, that steps (a) and (b) and the desorption of Hz are faster by a factor of lo3 than the rate of entry of hydrogen into LaNi? In regard to step (c), the observed rate of ethylene formation from LaNi5H2.4is observed to be about lo2faster than the rate of Hz absorption into LaNi,. Consequently, neither step (b) nor step (c) can be rate determining. Since the dehydrogenation of LaNi5H2,4by ethylene is 100-fold faster than the rate induced by changes in H2 gas pressure (the Boser-type experiment), it is evident that steps (d) and (e) are also not rate controlling. This is seen from the fact that the rate can be influenced by means of hydrogen removal at the surface. This establishes that processes occurring deep within the bulk, viz., steps (d) and (e), cannot be rate controlling. Obviously the slow step is not represented in the five processes cited above. The inhibitory effect of gaseous hydrogen on the reaction of LaNi5H2.4with CzH4 observed by SI1 provides a powerful clue in regard to the rate-determining step. Let us now consider this reaction. The diagram in Figure 2 gives a possible representation of the hydroxide-Ni interface. In regard to the hydrogenation of ethylene, the following steps are envisioned: (f) bulk diffusion of H in LaNi, to point B (g) H (point B) H (point A) (interfacial diffusion) (h) H C2H4 (adsorbed adjacent to point A) CzH6 (adsorbed) (i) C2H6 (adsorbed) C2H6(g) The SI1 results to the effect that the rate of reaction to form CzHs is proportional to CH indicate that (8) is rate controlling. Again we note that the process in which dissolved hydrogen is extracted by reaction with ethylene is 100-fold faster than dehydrogenation by pumping. In the latter case, Le., removal of hydrogen by pumping, interfacial diffusion is too fast to be rate determining. However, in the faster process of hydrogen removal by reaction with C2H4interfacial diffusion is rate controlling. As indicated earlier, SI1 found that gaseous H2 inhibits the hydrogenation of C2H4by LaNi5H2,4.We propose that this occurs because regions at the top of the interfacial region preferentially accommodate molecular hydrogen, thereby reducing the accessibility of atomic hydrogen to CzH4 (or vice versa), and hence reducing the rate of C2H4
-
+
-
-
+
-
-
-
H
I
H
Ofi
Figure 2. One of the possibilities for the blocking effect of molecular hydrogen which is referred to in the text. The configuration shown for H, could result if H, attempted to chemisorb on Ni atom 2 when Ni atoms 1 and 3 are already bonded to atomic hydrogen. Alternatively, failure to interact with Ni-3 could be due to its inaccessibility because of geometrical constraints.
hydrogenation. This viewpoint is consistent with Boser’s findings, which indicate a much slower rate of hydrogen removal by pumping, inasmuch as this method of dehydrogenation must also involve the presence of molecular hydrogen near the interfacial region. That the form of the blocking hydrogen is molecular may also be concluded from the results of SII, since the presence of D2 suppresses the rate of ethane formation, but results in the formation of only small amounts of deuterated ethane. The blockage of the interfacial region by molecular hydrogen may occur in a variety of ways, such as chemisorption of molecular hydrogen along diffusion paths of atomic hydrogen in the interface, as depicted in Figure 2. From this viewpoint, molecular hydrogen is a mild poison for the sorption process. FlanaganlO has stated that helium also inhibits hydrogen uptake. Very likely it adsorbs in the interfacial region and represents an impediment in the diffusion path of atomic hydrogen. Experiments involving the ethylene reaction clearly indicate that there is some sort of blockage a t the top of the interfacial region, as described in the preceding paragraph. We hold that it is this blockage which restrains the rate of ethane formation, and we also hold that this blockage is rate determining for hydrogen absorption or release by LaNi& Atomic hydrogen is formed very rapidly at the exposed Ni surface. It would, if unimpeded, enter LaNiS very rapidly by diffusion along the interface, as represented in Figure 2. If there were no blockage and the rate were controlled by the rate of interfacial diffusion, hydrogenation of LaNi, would be two orders of magnitude faster. In the actual situation, atomic hydrogen experiences an impediment originating with blocking molecular H2 and the rate of absorption is slowed accordingly. The rate of absorption is controlled by the dissociation rate of the blocking molecular hydrogen molecules. If we make the reasonable postulate that the number of such molecules is proportional to pressure, we will obtain the kinetic expression given in eq 2’. On desorption, formation of diatomic molecules a t the head of the interfacial region (point A in Figure 2) is the rate-determining step. Diffusion of atomic hydrogen in the interfacial region is high because the H in La(OH)3is rather acidic and the OH bond is easily broken. Moreover, H probably interacts with nearby Ni to further weaken the OH bond. These combined effects make H in the interface
Hydrogen Absorption by LaNi,
very mobile. However, the system cannot utilize the exceptional mobility of H for desorption since monatomic hydrogen cannot desorb at the top of the interfacial region. It is too tightly bonded to desorb as a unimolecular event. A hydrogen atom must bond with another atom to form H2 before desorption can occur. This gives rise to the observed2i3second-order desorption kinetics. For absorption we then have dP/dt = -0P (44 which leads to P = Poe-Pt (4b) At small times this reduces to Po - P = -P& (5) This equation is in accord with Boser's results as expressed in eq 2. For desorption dC,/dt = -kCH2 (6) where CHrepresents monatomic hydrogen concentration either in bulk LaNi, or in the interface. Since interfacial diffusion is extremely rapid, there is proportionality between bulk and interfacial hydrogen concentration. These features account for the second-order desorption kinetics observed by Boser. The exceedingly rapid dehydrogenation of LaNi5Hz.4by C2H4when no H2 is present is a consequence (1) of the absence of the blocking Hz and (2) of there being no need to form Hz at the head of the interfacial region. Chemisorbed C2H4is a t the head of the interfacial region and can react immediately with monatomic hydrogen which has diffused through the interfacial region. To summarize, the rate-determining steps are as follows: (a) for Hz absorption by LaNi5 it is the rate of dissociation of Hz adsorbed at the top of the interfacial region; (b) for hydrogen desorption it is the rate of recombination of atomic hydrogen at the top of the interfacial region; (c) for ethane formation it is the rate of interfacial diffusion of atomic hydrogen. C. Other Hydrogen Hosts. The proposed model not only accounts for the characteristics of LaNi, just described but it is also consistent with observations on the kinetics of hydrogenation observed for several related intermetallic compounds: PrCo,, ErCo3, HoCo3, and ErFez. Gualtieri, Narasimhan, and Takeshita establishedll that the rates of hydrogen uptake at 25 "C were in the order ErFe2 > HoCO~, ErCo3 > PrCo, > LaNi5. It has been established by quantitative Auger spectroscopy12that the surface of Er consists of Er203[which undoubtedly becomes Er(OH)3 in the presence of hydrogen] and Fe. It therefore seems almost a certainty that the surfaces of the several intermetallics consist of an oxide or hydroxide plus Fe, Co, or Ni. Since the activity of the surfaces is in the order Fe > Co > Ni, it is to be expected, if dissociation of H2 at the entry into the interfacial region is the rate-determining step, that hydrogenation of ErFe2 will be fastest and LaNi, slowest. Thus, the model provides a plausible explanation for the experimental observations of Gualtieri, Narasimhan, and Takeshita. D. Relationship to LaNi, as a Methanation Catalyst. A number of intermetallic compounds, including LaNi,, have been used to form CHI from CO and H2.13-18 It has been established by a variety of techniques that during the reaction the surfaces of these materials are transformed into a rare earth or actinide oxide plus nodules of transition metal approximately 0.5 wm in diameter. Thus, in regard to surface features the materials after use as methanation catalysts bear a general similarity to the ma-
The Journal of Physical Chemistty, Vol. 83, No. 13, 1979
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terials which have been subjected to many hydrogenation-dehydro enation cycles. When used as methanation catalysts, the material initially has rather low activity. Its activity grows during the course of the reaction, presumably because the surface is progressively being transformed to form more and more free Ni. Thus, the surface of hydrogen-cycled LaNi, is qualitatively similar to the surface of LaNi,, which has been used as a methanation catalyst.
IV. Concluding Remarks A model has been proposed to account for the kinetics of hydrogenation or dehydrogenation of LaNi,. Surface analysis shows that LaNi, is overlaid with La203 [or La(OH)3 when hydrogen is present] and elemental Ni. Since neither of these is penetrable to hydrogen, it appears that hydrogen reaches the underlying LaNi, by migration along the interface between Ni and La(OH)3. In experiments in which hydrogen is added (or withdrawn) by an increase (or decrease) of pressure of the hydrogen gas which is in contact with the solid, the rate is controlled by dissociation into (or recombination from) monatomic hydrogen at the top of the interfacial region between La203 and Ni. When hydrogen is withdrawn by reaction with C2H4 to form C2H6,the faster reaction entails another rate-determining step. In this case migration along the interface is rate controlling. The proposed model accounts for the essential features of the hydrogenation of C2H4 over LaNi5 and LaNi5H2.* carried out by Soga, Imamura, and Ikeda. I t also is consistent with the rates of hydrogenation and dehydrogenation observed by Boser. The model is applicable only to activated, i.e., fine particle, LaNis. It is inapplicable to hydrogenation of nonactivated LaNi,, which was examined recently in a careful study by Tanaka, Clewley, and Flanagan.lg It is also probably inapplicable to the study by the same authors20of the sorption kinetics of activated LaNi5, since in that study only the a phase was involved. All of the studies used to establish the present model involved either the j3 phase, which involves faster diffuor a mixture of a and 0 phases. Tanaka et al. proposed a model to account for their observations in which mass transport through cracks and fissures was regarded as rate controlling. The unliklihood of the latter is attested to by the greatly enhanced hydrogen abstraction rate when ethylene rather than reduced pressure is used to extract hydrogen. This sensitivity of extraction rate to the nature of the surface, including the species adsorbed there, provides strong evidence that neither phase transformation nor mass transport is rate controlling. The following features of the LaNi5-H system find a ready interpretation with the model proposed: (1) The reaction orders of the desorption and absorption of hydrogen by LaNi,. (2) The enhanced H extraction rate from LaNi,H2,4 with ethylene, compared to that achieved by depressurizing the system. (3) The source of the inhibitory effect of H2 gas on the formation of C2Hs from C2H4 and LaNi5Hz,4. (4) The relatively slow rate of hydrogen uptake by LaNi, compared with the rates of (a) dissociation of H2 on Ni and (b) interfacial diffusion. (5) The fact that hydrogen rapidly penetrates the surface coating to reach the underlying LaNi,. (6) The observation that hydrogenation rates for rare earth (R) intermetallics are in the order RFe, > RCo, > RNi,. It is noted that the restraint imposed by the presence of molecular hydrogen in the interfacial region significantly
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The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
M. D. Croucher and M. L. Hair
slows the H2 sorption rate of LaNi5. If this could be prevented, the rate of H2 uptake or release by this material might be increased 100-fold. A model has been proposed specifically for the sorption of hydrogen by LaNi5. Studies on related intermetallic compounds, PrCo5, PrNi5, and ErFe2, indicate surface features similar to that exhibited by LaNi5. The general similarity of the kinetic features of hydrogen sorption by the several rare earth intermetallics which have been studied, for examples, the second-order kinetics observed by Goudy et al.,3 the extremely rapid rate of reaction of C2H4with hydrogenated ErFe2,22etc., suggests that the present model may be of rather general validity.
K. Soga, H. Imamura, and S. Ikeda, Nippon Kagaku Kaishi, g, 1304 (1977). A. G. Moldovan, Ph.D. Thesis, University of Pittsburgh, 1978; A. G. Moldovan, S. G. Sankar, and W. E. Wallace, to be submitted for publication. S. Tanaka, J. D. Clewley, and T. B. Flanagan, J. Catal., 51, 9 (1978). Th. Von Waklkirch and P. Zurcher, Appl. Phys. Lett., 33,669 (1978). T. B. Flanagan in “Hydrides for Energy Storage”, A. F. Andresen and A. J. Maeland, Ed., Pergamon Press, New York, 1978, p 135. Also, private communicationfrom Professor Fianagan to W. E. Wallace. D.M. Gualtieri, K. S.V. L. Narasimhan, and T. Takeshita, J. Appl. Phys., 47, 3432 (1976). A. G. Moldovan, H. K. Smith, and W. E. Wallace, to be submitted for publlcation. V. T. Coon, T. Takeshita, W. E. Wallace, and R. S. Craig, J . Phys. Chem., 80, 1878 (1976). A. Elattar, T. Takeshita, W. E. Wallace, and R. S. Cralg, Sclence, 196, 1903 (1977). A. G.Moldovan, A. Elattar, W. E. Wallace, and R. S. Craig, J. Solhl State Chem., 25, 23 (1978). A. Elattar, W. E. Wallace, and R. S.Craig, “The Rare Earths In Science and Technology”, G. J. McCarthy and J. J. Rhyne, Ed., Plenum Press, New York, 1978, p 63. V. T. Coon, W. E. Wallace, and R. S. Craig, ref 16, p 93. A. Elattar, W. E. Wallace, and R. S.Cralg, Adv. Chem. Ser., in press. S.Tanaka, J. D. Clewley, and T. B. Flanagan, J. Phys. Chem., 81, 1684 (1977). S.Tanaka, J. D.Clewley, and T. B. Flanagan, J. Less-Common Met., 56, 137 (1977). W. E. Wallace, R. S.Craig, and V. U. S. Rao, Adv. Chem. Ser., in press. H. Imamura and W. E. Wallace, unpublished measurements.
Acknowledgment. This work was supported by a grant from the Army Research Office. References and Notes J. H. N. Van Vucht, F. A. Kuijpers, and H. C. A. M. Bruning, Philips Res. Rep., 25, 133 (1970). 0. Boser. J. Less-Common Met.. 46. 91 (1976). A. Goudy,’ W. E. Wallace, R. S.Craig, and T. ‘Takeshita, Adv. Chem. Ser., No. 167, 312 (1978). K. S w , H. Imamura, and S.Ikeda, J. Phys. Chem., 81, 1762 (1977). H. C. Siegmann, L. Schhpbach, and C. d. Brundle, Phys. Rev. Lett., 40, 972 (1978).
Application of Corresponding States Theory to the Steric Stabilization of Nonaqueous Dispersions Melvln D. Croucher” and Michael L. Halr Xerox Research Centre of Canada, 2480 Dunwin Drive, Mississauga, Ontario L5L 1J9, Canada (Received December f 1 , 1978) Publicatlon costs assisted by Xerox Research Centre of Canada
The incipient flocculation behavior of sterically stabilized nonaqueous dispersions is discussed in terms of the corresponding states theories of polymer solutions. This has been achieved by incorporating a temperatureand pressure-dependent x parameter into the theory of steric stabilization. It is found that the free energy of interpenetration (AGIM) of such dispersions consists of (i) a combinatorial contribution (AGI’(comb)), (ii) a contact energy dissimilarity contribution (AGIM(contactenergy)),and (iii) a free volume dissimilarity contribution (AGIM(freevolume)). The AGIM(comb)term always acts to stabilize the particles against flocculation while the AGI’(contact energy) and AGIM(freevolume) terms act to flocculate the latices. In principle, flocculation at the LCFT is usually brought about by the AGIM(contactenergy) term while flocculation at the UCFT is dominated by the AGIM(freevolume) term. The theory is able to predict both the temperature and pressure dependence of incipient flocculation and has been applied to some recently reported experimental results.
Introduction The “protection” of colloidal particles against flocculation with nonionic macromolecules is known as steric stabilization.’ It has been found to be an especially useful method of stabilizing particles in a dispersion media of low dielectric constant where electrostatic stabilization appears to be relatively ineffective. Investigations of the repulsive forces between stable particle^,^" stability in polymer melts?5 and incipient flocculation behavior have been reported for such dispersions. Incipient flocculation has been the most extensively reported measurement1 and this can be induced by changing the solvent quality of the dispersion medium relative to that of the stabilizing polymer. It has been established that it is a reversible phenomenon1 and redispersion occurs if the solvency of the disperse medium 0022-3654/79/2083-1712$01 .OO/O
is improved with respect to that of the stabilizing polymeric moiety. Using thermodynamic arguments, Napper has distinguished‘ three types of steric stabilization in the vicinity of the critical flocculation temperature. They are (i) enthalpic stabilization which is characterized by flocculation on heating, (ii) entropic stabilization which is characterized by flocculation on cooling, and (iii) combined enthalpic-entropic stabilization where in principle the dispersion cannot be flocculated at any accessible temperature. It has been shown6 that poly(acrylonitrile) (PAN) latices stabilized by poly(isobuty1ene) (PIB) in 2-methylbutane flocculate on heating to the 0 point associated with the lower critical solution temperature (LCST) of the PIB-methylbutane solution. This corresponds to an enthalpically stabilized system, whereas Q 1979 American Chemical Society
