relative to that of surface transport processes (sur-
face migration and vapor-phase transport) as the t,einperaturr! of aging increases. Aging a t the highest teinperatures results in very little change in the pore diameter. This observation is consistent with a nearly complete predominance of volume transport processes over surface processes, with the result that a kind of local fusion or coalrscence destroys both surface area and pore volumr simui taneously and in constant proportion. The oociirrence of such a process is strongly indica,ted hy the observations of Adams.12 As is shown in Fig. 6, the aging process approaches this course more mid more closely as the temperature of aging in rreaws.
Steam has a strong accelerating effect on the aging process a t all temperatures. The relatively small dependence of dTr,/dS on the partial pressure of steam (Fig. 8) suggests that both volume transport processes and surface transport processes are accelerated to nearly the same extent by steam, presumably by a process of hydrolytic attack on the silicon-oxygen or aluminum-oxygen bonds in the gel network. Acknowledgments.-The authors wish to thank H.’ H. Voge and C. R. Adams for stimulating discussion and for their interest in this study, E. E. Roper for placing a t our disposal some of his data, and Miss J. R. Olsen for aid in performing the experiments.
AGING OF SILICA-ALUMINA CRACKING CATALYST. 11. ELECTRON MICROSCOPE STUDIES BY C. R. ADAMSAND H. H . VOGE Shell Development Company, Emeryville, California Received November 6 , 1966
‘ l h Id1ysic:tl striictrire of an unagcd silica-alumina gel consists of coherent aggregates of dense, smooth, spherical particles, :iI~wit45 h;. i n diamcter, with a relatively narrow distribution of particle sizes. The specific surface area IS the eometrical s i i r f n w :wca of these particlcs and the pore structure is the void structure resulting from the packin8 to ether of these par-
ticlvs. Aging in the presence of steam a t low temperatures (below about 800’) results in an increase in t f e ultimate particle !liainctctr, with a widening of the distribution. The decrease in surface area is quantitatively accounted for by this increase i n RIZO. Aging at high tem eratures is characterized by a complete collapse of microscopic regions of the gel, the remaining regions having essentially tRe same physical structure as that of the fresh gel. This high temperature sintering is strongly accclrmtcd by the presence of steam.
Introduction The fine structure of inorganic gels has been the subject of int,ensive investigation and discussion for many years. The inherest in, and importance of, these gels is due; in part, to thosc properties which result from this structure: namely, pore structure, surface area and the subsequently important surface chemical activity. It is the purpose of this paper t o present the results of a high magnification electron microscope study of the structure of a silica-base gel and the strnctjural changes produced in this gel by treat,ments with steam and high temperatures. A systematJic study of the changes in pore volume and surface area of this gel, as a function of temperature, steam pressure and time, has been reported by Schlaffer, Morgan and Wilson.’ Experimental Thc gr1 riclcd i l l thcse studies was a samplr! or a cominercial powderctl cracking catalyst manufactured by the American Cyanamid Company by the hydrolysis of an aluminum salt in the presence of H silica hydrogel,. The powder was in the form of microsphcrcs about A0 p in diameter, obtained by spray-drying the composited hydrogel. This gel contained 12.4y0 AIZO3,had :t surfacc nren of 608 n~.2/g.,and a ore volume of 0.73 cc./g. The esperimrntd techniques ofdeartivation are given in detctil by Sr,lilaffer, Morgan and Wilson . I Serious experimental prohlcms arc poscd in an electron microscope study of sucli n gel, inasmurli as the diameters of the ultimate particles are about 30-50 h;. , a range requiring high resolution and magnification. Thc grcatmt @ingle problem was that of specimen contamiiistion in the olcct,roii (1) W . I:. SolilaRer. C. Z , illorpan and J . N. W i l w n , T H I JNo w R N A L , 61, 714 (1957).
This depositio of amorphous matter was 80 great (often as high as 5 - 6 1 per minute) a t the higher beam intensities necessary for high magnification exposures that the utmost haste had to be employed to obtain a true picture. Exposures were usually made within 16-20 seconds after beam illumination, and many pictures of each sample were taken to ensure accurate representation of the true structure. Another major problem was the instability of the sup ort film. Nitrocellulose films were used in most cases. d e s e films have a tendency to stret,ch and move for a few seconds after beam illumination but, because of the contamination problem, it was impossible to wait until the film became completely stabilized. Evaporated silicon monoxide films were used in somc instances, but the advantage of the greater stability of this film was offset by the great tendency to charge up in the electron beam. Evaporated carbon films6 have been found in later work to be very superior supports. Samples were usually lightly ground by hand for a few seconds in a small mortar and either suspended in water or mounted dry by dusting onto the su port film. Measurements on specimens prepared by bot: techni ues indicated that the suspension in water and subsequent %rying had no effect upon the physical structure of the gel. This is in agreement with other studies’ on aggregated, calcined BeO-InzOl gels which have shown that aqueous suspension and subsequent evaporation to dryness had no effect upon the pore size distribution within the aggregate. Electronic manifications of 20,OOOX were obtained in an RCA EMU-2C microscope, with optical enlargements of the resulting negatives employed to obtain prints a t a total magnification of 200,OOOX. The particle size diRtributions were obtained from the photographic priets by direct meas-
.
( 2 ) R. L. Stewart, Phus. Rev., 45, 488 (1934). (3) .I. H. 1,. Watson, J . A p p l . Phys., 18, 153 (1947). (I) . I . Hillipr, ibl’d., 19, 220 (1948). ,(R) .A. IC. Fhnos, B r i t . . I . A p p l . Ph?/.s.,5, 27 (1954). . . (6) I). E. Urndlcg, 3 p ) . Furthermore, there appeared to tion of ultimate particle sizes given in Fig. 2 has a be an endless hierarchy of higher aggregation.9 It is postulated that the primary aggregates are number-average diameter of 214 A. and a surfaceformed during the aging of the sol. The secondary area-average diameter of 262 8. This corresponds aggregate structure is at least partially built up to an electron microscope surface area of 100 m.Z/g., when the gel sets. This secondary (and higher in excellent agreement with adsorption measureorder) aggregation is certainly strengthened and ments of 95 m.2/g. The observation was made above that the aggrebuilt up further during the drying of the gel. The effect of lightly grinding the dried powder is to gates appeared similar a t various stages of steaming. This observation allows an explanation for
(8) W. G . Schlaffor, rinpublished data. (10) €1. E. Rics, Jr., “Advances in Catalynis.” Vol. IV, Academic (9) Idcas on tho cliistciing of tiltiniatc pnrticlcs of gels into ~~riiiitrry Press, New York, N. Y., 1053,p . 87, and earlicr papers cited there. aggregates and of the further aggregation into rouglily isometric (11) K. D. Ashley and W. B. Innes, I n d . E n g . Chern., 44, 2857 secondary aggregates were proposed independently here by J. N. Wilson (1952). in 1053.
Steam to 78O c.
d) Heated in Air to 26 at 90O9C.
.
Fig, I.-Electr.on
micrographs of silic:i-alumina gel; magnification: 200,000 diameters.
400
Junc, 1957
Aci X N C OF ~ SILICA-ALIJMI N A C ILACTCINC,CATALYST
725
the relatively constant pore volume and increasing I pore diameter observed over the course of low tem4 perature steam deactivation. It is postulated that the aggregates retain their approximate shape and Frcah G e l . 608 m‘/g size during the process of steaming. This is strongly Heat Dencttvaled to 100 m’/g demanded by the appearance of the aggregates of Heat Deactlvaled lo 269 m’/g 0 4 the sample steamed to 95 m.Z/g., where the ultimate k particles are obviously strongly bound into the aga gregates, yet the irregular shape of the ultimate ags\” gregates clearly shows that they could not be fraga2 team Deactlvated to 116 m’/g ments broken off mechanically from a large homogeneous mass. Arguments will be advanced below t o indicate that only two mechanisms are possible 1 for low temperature steam deactivation : surface migration or vapor phase transport. Either mechanism will require that essentially all material transport occur within the aggregate. Since, by 0 either mechanism, transport of matter to or from a 0 50 100 150 200 250 300 350 400 450 500 given ultimate particle will be incoherent with reDiameter, A. spect to transport to and from all particles a t least Fig. 2.-Ultimate particle size distributions of fresh, steamed two particle diameters away, and since each aggreand heat deactivated gel. gate contains a very large number of ultimate particles, the size and shape of the aggregate would be ing from an equivalent amount of pore volume expected to remain essentially unchanged. The above results clearly show the effect of destroyed when the surface area decreases. An elecsteam deactivation a t low temperatures upon sil- tron micrograph is given in Fig. Id of the silicaica-alumina gel: namely, the regular growth of alumina gel heated in a stream of dry air for 7 hours individual spheres of the solid, a t the expense of the a t 900”. The resulting nitrogen surface area was smaller particles. Thus the larger particles are con- 269 rna2/g. The secondary aggregate structure of stantly growing larger and the smaller ones are this high temperature deactivated sample is disconstantly getting smaller and disappearing. This tinctively different from that of the fresh or would result in size distributions spreading out to- steamed samples. The secondary aggregates of ward the larger sizes, as steam deactivation pro- this sample are considerably larger, usually several ceeded, and yet still having one side of the distribu- microns in size. The aggregates are regularly tion extend into the small particle range. These are shaped, having a shard-like appearance, with freexactly the facts observed here and displayed in quent fracture edges present in the ground material. Fig. 2. The concept of collapse, sintering or amal- In contrast to the low temperature steam deacgamation of two or more particles is not consistent tivated samples, the ultimate particles of this heat with the observations of (a) round particles having deactivated sample have not changed in size from (b) a roughness factor of essentially unity, (c) in- that of the fresh gel, as shown in Fig. 2. An electron dividually observable particles, (d) broadening dis- micrograph is given in Fig. l e of the gel heated in tributions as deactivation continues, (e), the pres- air for 7 hours a t 940”, with a resulting surface area ence of an appreciable number of small particles of 100 m.2/g. The aggregate appearance of this even after prolonged aging, and (f) the continuous, sample is quite similar to that of the sample heated smooth increase of the average particle size with a t 900”. The ultimate particle size distributions, steaming. The consistently spherical shape of the given in Fig. 2, are unchanged from that of the particles indicates that surface tension forces are fresh gel, with the exception of slight shoulders on prominent in such growth. Only two mechanisms the larger particle side. It is postulated that these of growth are consistent with all the above ob- shoulders are due to only a few particles fused toservations : surface migration or molecular diffu- gether. I t has been shown by Schlaffer, Morgan and Wilsion in the gaseous state. I n either case surface son’ that the deactivations in “dry air” were intension forces would become important and the a t least in part by steam generated a t the presence of steam would be expected to accelerate fluenced high temperatures from the water originally comthe growth. It should be pointed out that this bined in the gel. Electron micrographs were, theretype of deactivation does not result from movefore, also obtained of a sample of fresh gel which ment of silica from the surface of wider capillaries had been calcined in vacuo a t 1012’ for 1.7 hours by to fill in the smaller capillaries or pores, as has been E. E. Roper. This material had a surface area suggested elsewhere. l 2 Aging at High Temperatures.-A second method of 104 m.2/g. The aggregate appearance of this of deactivating silica-base gels consists of heating sample was identical with that of the gel heated in the gel a t high temperatures This type of destruc- “dry air.” The ultimate particle size distribution, tion of surfme area is characterized by a constancy given in Fig. 3, is the same as that of the fresh gel, with the exception of a slight increase in the of pore size and pore size d i ~ t r i b u t i o n , ~result~ ~ ~ ~ lagain ~ number of larger particles. Thus conclusions (12) R. K . Iler. “The Colloid Chemistry of Silica and Silicates,” drawn about the mechanism of deactivation of the Cornell University Press, Ithaca, N. Y.,1955, p . 270. samples heated in air will apply equally well to (13) W. 0. Milligan and H. H. Rachford, Jr., THISJOURNAL, 61, deactivation in vacuo. It appears that the presence 333 (1947).
t
1 I!--
726
C. R. ADAMSAND H. H. VOGE
Vol. 61
while vacuum calcination would scarcely have changed the area at all from the original value of 608 m.2/g. The ultimate particle size distribution for the sample steamed a t 863” is given in Fig. 3. Although there appears to be some (about 20%) increase in ultimate particle size, due to the low temperature steam mechanism, it is apparent that the major mechanism is that of high temperature deactivation. The averag: particle size would have increased to above 200 A. if the decrease of area had been due solely to the low temperature steaming mechanism. The mechanism of high temperature deactivation appears to be that of heterogeneous sintering, resulting in local collapses of structure which spread through the mass in a process similar to fusion, This conclusion is based upon (a) the linear decrease of pore volume with respect to surface area, and the constancy of (b) average pore size, (c) pore size distribution, (d) average ultimate particle size, and (e) ultimate particle size distribution for heatdeactivated silica-base gels. Although this evidence is somewhat indirect in that the data describe 0 20 40 60 80 100 120 the properties of the uncollapsed regions, it is posDiameter, A. Fig. 3.-Ultimate particle size distributions of fresh gel, after sible to get direct information about the collapsed vacuum cttlcination at 1 0 1 2 O , and after steaming at 863’. regions. This has been done by dying samples of heat-deactivated gels with a benzene solution of methyl red. It has been shown14 that the methyl red forms a monolayer on the surface, so that particles having appreciable internal surface area will be dark red, while particles having no Internal surface will be clear. A photomicrograph of a particle of gel, heat-deactivated to 100 ma2/g.,is shown in Fig. 4. The dark regions of the particle shown in Fig. 4 are regions having a high internal surface area while the clear regions have essentially no internal area. It should be mentioned that dyed particles of the fresh gel appear uniformly dark. Furthermore, skeletal density measurements of the heat-deactivated gels indicated no occluded voids so that the clear regions visible in Fig. 4 represent solid, fused masses, rather than glazing. The number of clear particles visible in the light microscope is not great enough (only about 5y0) to account for all of the deactivation. A majority of the fused domains must therefore be below the resolution of the light microscope. This is understandable in terms of the aggregate structure discussed above. The extent of these fused domains would be expected to be limited to the primary aggregates, until essentially all of the ultimate particles had fused together. The mechanism given here for thermal sintering of silica-base gels is not new. Van Nordstrand, Kreger and Riesls arrived a t the same conclusion on the basis of pore size measurements. However, Ries’O Fig. 4.--Photomicrograph of heat deactivated particle of gel in a later review of the structure and sintering properties of cracking catalysts, attempted to disprove dyed with methyl red, approx. 1OOOX. of steam a t high temperatures merely accelerates this mechanism by showing that physically sepathe mechanism that occurs when the gel is heated rated shell and core portions of a heat-deactivated in vacuo. An examination of a sample deactivated catalyst bead had the same surface area as that of to 98 m.”/g. by 0.5 hour at 863” in one atmosphere the whole bead. However, since the domains of of steam has shown that this accelerating effect of fusion are limited by the size of the very small pri(14) I. Shapiro and I. M. Kolthoff, J . Ant. Chem. Soc., 7 8 , 776 steam can be very marked. Heating in “dry air” at (1950). this temperature and for this length of time would (15) R. A. Van Nordstrand, W. E. Kreger and H. E.Ries, Jr., THIE have reduced the area to only about 400 me2/g., JOURNAL, 65, 621 (1951).
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STRUCTURE OF THE RHENIDE ION
June, 1957
mary aggregates, and since the state of aggregation wm probably consistent throughout the bead, one would not expect to find any variation in surface area for various macroscopically large regions of the bead.
727
Acknowledgment.-The authors wish to thank their colleagues A. M. Cravath, E. E. Roper and W. G. Schlaffer for their aid and cooperation in this study.
ON THE STRUCTURE OF THE RHENIDE ION’ BY J. W. C O ~ B L E Contribution from the Department of Chemistry, Purdue Univemity, Lajayelte, Indiana Received November 10, 1068
The thermodynamic factors controlling the stability of the rhenide ion have been examined. The conclusion is reached that rhenide exists in aqueous solution as an oxygenated complex, 2nd thnt solid rllenidcs must be “hydrated” to be stable. Other ions of this general type are also exnmiricd.
Introduction
pressure of rhenium metal and have reported AWo as 187 A 1 kcal. Correctedll to 25’ this beable in literature which help t o elucidate the struc- comes 183 f 2 kcal. very close to Brewer’s original ture of the rhenide ion.2 That this ion exists seems estimate of 189 kcal. There is no evidence either no longer subject to question. It has been con- way for the existence of dimeric or polymeric gasefirmed by a number of investigator^,^-^ including ous rhenium species. However, the closeness of this author; further, recent reports have appeared the estimated and experimental heats of vaporization leads one to believe that these species, if preson the preparation of solid p o t a s ~ i u m ,lithium* ~ and thallous r h e n i d e ~ . ~The former two solid ent, are a minor constituent and have not greatly rhenides appear to be stable (as hydrates), charac- affected the value for AHV. B. The Heat of Oxidation of Re-(aq) ( A H o x ) . terized mainly by great ease of oxida.tion. Further, aqueous solutions of rhenide ion appear to reduce -A close estimate of the heat of oxidation of hydrogen ion slowly. It is the purpose of this aqueous rhenide comes from the chemical observacommunication to demonstrate that the aqueous tion that aqueous rhenide solutions oxidize hydrorhenide ion is almost certainly not a simple halogen- gen ion moderately fast. This propcrty is common to other aqueous ions whose oxidation-reduction like ion, but some type of an oxygenated complex. potentials are around 0.4 volt (the practical oxidaThe Thermodynamics of Aqueous Rhenide.Consider the thermodynamic cycle in Fig. 1 ttion stability limit of unit activity Hf in aqueous solutions) for the reaction: M(reduced) = M AHe(oxidized) e-. Thus Latimer12gives Eofor ReRe(& -+R e - W = Re(c) e- as 0.4 volt. This estimate is almost certainly accurate to a few tenths of a volt, and is AH.1 /AHhya independent of the structure of the aqueous species. Ro(c) +-Re-(as) However, following our previous assumption that AHox the structure is Re-(aq), a value of A H can be esFig. 1.-Cycle for the aqueous rhenide ion. timated from the ,730 of 0.4 volt as follows: the enI n this figure the assumption is made initially that tropy of R.e(c) has been determined as 8.89 e.u.I3; the structure of the aqueous ion is simple, Le., Re-. the entropy of Re-(aq) can be estimated to be 28 A careful examination of the four A H values, for i 5 e . ~ . if’ ~the ionic radius is taken as approximately16 2.3 f 0.3 A. ASo then becomes - 3 which the cycle must close, is necessary. A. The Heat of Vaporization of Rhenium ( A H , ) . f 5 B.U. for the reaction -The heat of vaporization of rhenium has been Re-(aq) H+(aq) = Re(c) ‘/ZHz(g) (1) estimated by Brewerg to be 189 kcal. a t 25”. ReEO = $0.4 f 0.2 V. cently Sherwood, et aZ.,’O have measured the vapor AFO = -9 i 5 kcal.
It is not generally recognized that data are avail-
+ +
+
(1) This research was supported b y the Unitcd States Air Force Office of Scientifio Research of the Air Research and Development Command under contract No. A F 18(600)-152R. (2) 0. E. F. Lundell and H. B. Knowlea, J . .Research N a t l . Bur. Standards, 18, 629 (1937). (3) 0.Tomicsk and F. Tomicek, Collection Ciech. Chem. Communn., 11, 626 (1939). (4) J. J. Lingane. J . Am. Chem. Boc., 64, 2182 (1942). (5) E. K. Maun and N. Davidson, ibid., ‘IS,3509 (1950). (6) .C. L. Rulfs and P. J. Elving, ibid.. 7 8 , 3287 (1951). (7) J. B. Bravo, E. Griswold and J. Kleinberg, ibid., 68, 18 (1954). (8) A. V . Grosse, 2. Naturforsch., 8b, 533 (1953). (9) L. Brewer,, “Chemistry and Metallurgy of Misrellaneous Materials: Thermodynamios,” National Nuclear Energy Series, \’oL IV-19B, edited by L. L. Quill, McGraw-Hill Book Co., New York, N. Y.,1950, p. 26. (IO) E. M. Sherwood, D. M. Rosenbaum, J. M. Blocker, J r . , and I. E. Campbell, J. Elaclrochsm. Soc., 109,650 (1955).
-
+
(11) Using Hu Hipa datn for Re(g) from reference 9, and from reference 13 for Re(c). (12) W. M. Latimer, “Oxidation-Potentials,’’ Prentice-Hall, Inc., New York, N. Y.,1952, pp. 12. 243. (13) Wm. T.Smith, Jr., G. D. Oliver and J. W. Cobble, J . Am. Cham. Soc., 76, 5785 (1953). (14) R. E. Powell and W. M. Latimer, J . Chem. P h y s . , 19, 1139 (1951). (15) The ionic radius of R e - is estimated by a comparison of the covalent and ionic radii of the halogens. The difference between the covalent and univalent radii of CI, Br and I is 0.82,0.84 and 0.8R A., reapectively. J t is plausible that the difference between the covalent radius of Re and R e - will also be 0.88 A. The oovalent radius of Re woiild appear to be almost the same a s the observed metallic radius of 1.37 b. Thus the radius of R e - is 2.3 A., which should be accurate to a t least 0.3b Radii were taken from L. Paulina. ”Nature of the Chemical Bond,“ Cornell University Press, Ithacs, NT Y.,1946.
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