J. B. PERI
220
A Model for the Surface of ?-Alumina’
by J. B. Peri Research and Dwelopment Department, American Oil Company, Whiting, Indiana
(Received J u l y 20, 1964)
To account for the surface hydration and catalytic properties of y-alumina, a statistical model is proposed on the assumption.of preferred exposure of a single crystal face. The behavior of the model was investigated by a Monte Carlo method using an IBAI 705 computer. Given a square lattice initially filled with hydroxyl ions, adjacent ions are assumed to combine randomly, provided only that local order is preserved in the residual oxide lattice, to form water, which is desorbed. This restriction permits removal of only 67% of the original hydroxyl layer and creates a two-domain surface lattice of residual oxide ions. Further dehydration, made possible by removal of the local order restriction, creates defects of various types in the domain boundaries, and leaves five types of isolated hydroxyl ions, differing in nearest neighbor configuration and covering about 10% of the surface. Mobility of surface ions, required to remove the final 10% of the hydroxyl ions, tends to minimize defects and leads to predominance of three types of hydroxyl ions. Five previously reported infrared bands between 3700 and 3800 em.-’ are tentatively assigned to the isolated hydroxyl ions. Defects produced in the domain boundaries can account for the catalytically active “strain” sites generated on y-alumina by surface dehydration.
Introduction Various crystallographically distinct forms of alumina exist. Among these, two high-area transition forms, 1- and y-alumina, are of greatest catalytic interest. They are widely thought to have rather similar defect spinel lattices. Which crystal planes are preferentially exposed and how aluminum ions are distributed a t the surface are not known. Electron and X-ray diffraction studies indicate, however, that y-alumina may preferentially expose the 100 plane of spineL2 The surface hydration and catalytic activity of yalumina have been discussed in previous papers. -5 Ionic surfaces normally terminate in Oxide surfaces, unless highly dried, are usually covered with hydroxyl groups formed by chemisorption of water. Removal of such groups from alumina leaves a strained surface on which strained oxide linkages have been postulated as active sites.8 Active sites on y-alumina have also been identified with cation defects arising from its presumed defect spinel ~ t r u c t u r e with , ~ such defects which have captured protons,lo or with aluminum ions abnormally exposed as a result of surface dehydration. Various chemically distinct types of hydroxyl groups also persist on the surface of alumina even after drying at 800-1000°.5 The Journal of Physical Chemistry
Accumulating evidence on the relationship between surface hydration and the nature of active sites of y-alumina justifies the development of a model to aid interpretation and correlation of observations. A preliminary version has been briefly outlined. The proposed model has already proved useful, and it embodies certain concepts which may be applicable to other surfaces. The work of Langmuir,” Roberts,12 and others on (1) Presented in part at Combined Southeast-Southwest Regional Meeting, American Chemical Society, New Orleans, La., Dec. 1961. (2) B. C. Lippens. “Structure and Texture of Aluminas,” Thesis, Technische Hogeschool of Delft, The Netherlands, 1961. (3) J. B. Peri and R. B. Hannan, J . Phys. Chem., 64, 1526 (1960). (4) J. B. Peri, Actes Congr. Intern. Catalyse, l e , Paris, 1, 1333 (1961). (5) J. B. Peri, J . Phys. Chem., 69, 211 (1965). (6) W. A. Weyl, Ann. N . Y . Acad. Sci.. 12, 245 (1950); “Structure and Properties of Solid Surfaces.” R. Gomer and C. S. Simth, Ed., University of Chicago Press, Chicago, Ill., 1953, p. 147. (7) K. Moliere, W. Rathje, and I. N. Stranski, Discussions Faraday SOC., 5 , 21 (1949). (8) E . B. Cornelius, T . H. Milliken, G. A. Mills, and A. G. Oblad, J . Phys. Chem., 59, 809 (1955). (9) A. Eucken, Discussions Faraday Soc., 8 , 128 (1950). (10) D. A. Dowden, J . Chem. SOC.,242 (1950). (11) I. Langmuir, ibid., 511 (1940). (12) J. K. Roberts, Proc. Roy. SOC.(London), A152, 445 (1935).
A MODEL FOR THE SURFACE OF ?-ALUMINA
statistical effects in adsorption of vapors on metals is relevant to the present subject although the processes involved differ considerably.
Characteristics of the Model For simplicity, the assumptions are initially more restrictive than necessary and are in most cases indefensible in detail. Modifications will be discussed after the basic concepts have been established. Ideal Surfme Structures. The proposed "surface" includes two outer layers of a n ionic crystal. Only one face is assumed to be exposed. On "ideal" dry alumina the top layer contains only oxide ions, regularly arranged, as shown in Figure la, over aluminum ions in octahedral sites in the next lower layer. Only half as many oxide ions are present in the top layer as in the next lower layer, which represents the 100 plane of a cubic, close-packed, oxide lattice where each oxide ion occupies an area of about 8 and .aluminum ions are located in all interstices between oxide ions. The stoichiometry of the two upper layers combined corresponds to AlzOl. At 100' or somewhat higher, depending on the method of rehydration, sufficient chemisorbed water is held to convert the top layer to a filled, square lattice of hydroxyl ions, aa represented in Figure lb. Each hydroxyl ion is assumed to be directly over an aluminum ion in the next lower layer. When the top layer is thus filled, the two upper layers combined correspond stoichiometrically to Al2OJ.H20. Removal of water from this surface must expose oxide ions. Dehydration without Migration of Hydroxyl Ions. During dehydration of the surface, adjacent hydroxyl ions combine to form water molecules, which are then desorbed. For each molecule of water formed, one
221
0
20
40
80
60
IW
NUMBER OF TRIALS (THOUSANDS1
Figure 2. Removal of hydroxyl pairs from square lattice (IBM 705 computer).
oxide ion is left in the top layer, and one aluminum ion is left in an incomplete octahedral site in the next lower layer. Complete regular removal of the surface hydroxyl ions would ultimately produce the surface lattice of oxide ions shown in Figure la. Random combination of adjacent hydroxyl ions leaving an oxide ion on either of two sites with equal probability would yield a completely disordered surface oxide lattice. About 8% of the original hydroxyl monolayer would be left as isolated hydroxyl ions (having no adjacent hydroxyl ion neighbor) on several types of sites differing in nearest neighbor configuration.
Figure 3. Surface showing 3Ooj, removal:
0 , O H - ; 0, 0-*.
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40
39
a
37 36 35 34
33 32 31
30
29 28 27 26 23 26
23 22 21 20 19 18 11 16 15 14
13 12 11 10 0
a
? 6 5 4
3 2 1
To reduce the disorder that would result from a purely random process, the assumption is made that water must initially be removed in such a way that two (or more) oxide ions are not left on immediately adjacent sites, and two (or more) immediately adjacent sites are not left vacant (Le., local order must be preserved in the residual oxide lattice). So long as this requirement is maintained, a large fraction of the original hydroxyl monolayer cannot be removed. The removal of water from a hydroxyl monolayer in accordance with this limitation was studied with an IBM 705 computer. “Pseudo-random” numbers13 generated by the computer were used to select a pair of adjacent hydroxyl ions on a square lattice containing 10,000 sites. The edges of the lattice were joined, right to left and top to bottom, to avoid edge effects. If the selected pair could be removed without leaving the residual oxide ion adjacent to another oxide ion or leaving a vacant site adjacent to another vacant site, it was removed. If this could not be done, the computer randomly selected another pair and again tested of the entire lattice for possible removal. 650’), surface ions must be mobile. Implications as to the degree of such mobility are not clear, however. Even at lOOO’, as indicated by infrared spectra, surface hydroxyl ions on alumina apparently do not possess the mobility of a two-dimensional gas but are normally attached to characteristic sites of different type^.^ Loss of surface area does not necessarily indicate high mobility of the ions in plane surfaces. Oxide and aluminum ions a t crystal edges and corners undoubtedly migrate more easily than those in plane surfaces, and such migration could explain loss of area without requiring mobility of most surface ions, nor does the exchange of oxygen atoms between the surface of alumina and oxygen or water require much mobility of oxide ions. Although protons on “dry” alumina appear to be mobile only above 400’, protons can transfer between hydroxyl ions to form water and an oxide ion (or vice versa) a t 200’ or lower. Explanation of this apparent anomaly must lie in the transfer processes involved. To permit further consideration of the migration of hydroxyl ions on the surface, the following assumptions were made. (1) Migration occurs through singlespace moves of an ion to an adjacent vacant site or through proton transfer from a hydroxyl ion to an The Journal of Physical C h e m a r y
adjacent ion in the top layer. (2) Defect minimization is favored, and moves which reduce defects occur very readily. (3) Proton transfer usually occurs more readily than migration of a hydroxyl ion, per se, and the transfer occurs more readily to an adjacent hydroxyl than to an adjacent oxide. (4) Hydroxyl ions migrate more readily than oxide ions. (5) The relative ease of any move depends additionally on the nature of the resultant surface defect. Restriction of proton transfer t o moves between ions in the top layer may be justified by the “electrical double-layer” characteristic of the dry surface or, in any case, by the factors which cause residual hydroxyl ions to remain with high preference on the surface rather than in the interior of alumina crystals. On the basis of the above assumptions, a set of rules was established to govern removal of the final 9.6% monolayer of hydroxyl ions. In order of decreasing priority, these rules are as follows. (1) Water is removed from any adjacent hydroxyl pair in a way which leaves the least possible defect. (The degree of defect is taken as the maximum number of oxide contacts per oxide ion in an oxide defect or as the maximum number of vacant site contacts per vacancy in a vacancy defect. Where a choice exists, oxide defects are avoided rather than vacancy defects. When no choice is involved, removal is random.) (2) Wherever possible, defects are reduced or eliminated by single-space moves of protons from hydroxyl ions to adjacent oxide ions or of hydroxyl or oxide ions to adjacent vacant sites. (3) Migration of a proton to an adjacent oxide ion is permitted as an intermediate step if no defect more serious than a “triplet” oxide defect (two oxide ions immediately adjoining another) results. (4) Migration of a hydroxyl ion to an adjacent vacant site is permitted if no increase in defect character results. The move having the highest priority was always chosen, and on this basis hydroxyl ions were removed from the surface depicted in Figure 7 by inspection. Edge effects were avoided by examining the surface beyond the area shown to assess possible moves near the edges. Rules were invoked, in order, as necessary to permit continued removal of hydroxyl ions. Less than 200 single-space moves were needed to reduce the coverage to 2.4% monolayer, but the complexity of the necessary move sequences increased very greatly as the number of hydroxyl ions decreased. The surface obtained after such removal is illustrated in Figure 9. The remaining hydroxyl ions can be removed only by moves not permitted under the four existing rules. A three-dimensional model of a portion of the surface represented by Figure 9 is shown in
A MODELFOR
THE
SURFACE OF 7-ALUMINA
22.5
0 0 1.0 0 0 o a o o o 0 0 ~ 0 0 O O D O O 0 0 ~ 0 0 0 0:o0 0 0 0 ~ 0 0 oolooo 0 0 ~ 0 0
o @ o o lljl
0 ~ 0 0 0 0 O D 0 0 0 00 0 ~ 0 0 0 00000 00 0 0 0 0 0 m..o 00 0 0 0 : 0 0 0 0 CXL 00 0 0 0 0 0 0 0 0 0 le,
Figure 12. Defects and hydnnyl ions in domain boundaries.
Figure 10. The defects remaining in Figure 9 are either oxide or vacancy defects of special types. These will be discussed later. The various types of hydroxyl ions were removed a t differcut rates during the process. Those in the
Figure 11. Changes in distribution of hydroxyl types during progressive removal.
regular domain regions (A- and C-sites) were removed most slowly because under the rules they could not migrate, either through proton or hydroxyl ion ~rroves. Of the three remaining types, two (D aud E) were readily converted, through defcct miuiiriization, into the third (B), which was eliminated fairly readily through further moves. Changes irr the relative numbers of the five types a t various stages of the removal process are showu in Figure 11. The details of surface rearrangement are very spcculative. In reality, the niovcs governed by t,he rules would probably occur concurrently and pnssihly in different order. The rules obviously do not cover all possible moves but only those assuuied to occur most easily. llobility of ions on the surface is prohahly less restricted a t high temperatures than suggested. If oxide ious may migrate where 110defect more serious than a t,riplet would result., many ncm m n v c ~hecnme possible, and the domain regious in the oxide latlicc may change appreciably in size and shape. All such facton would cause some change in the hydroxyl removal process. So loug as the principles of prcservation of local order and defect miiiinrizatioii arc upheld, however, generally similar rcsults would he obtaincd. The surface lat,tice is probahly itidistiuct at very high temperat~ures,with intis vihrat iiig strongly ahout regular sites of at,tachmcnt and frcquenlly moving to adjacent vacant sites. Vacancies aud “iuterstitial” oxide ions must exist, to sonic extent, i n rcgular domain areas of t,hesurface oxide lat,tice,aud domain hoiitidaries coirstantly shift iu position. Hydroxyl ions and oxidc ious are probably frecly intcrcouvcrlihlc as protons transfer from hydroxyl to oxidc on closr approach of the ions. Protons may even rnigratc through lower
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oxide layers or be desorbed as water and readsorbed to form hydroxyl ions again. As the alumina cools, however, all protons are again captured by oxide ions to form hydroxyl ions on characteristic lattice sites, preferentially, in such a way as to minimize surface defects. A close relation exists between the types of hydroxyl ions arid the defects pictured by the model. Triplet defects ( L e . , an oxide ion with two oxide nearest neighbors or a vacant site adjoining two vacant sites) capture protons or hydroxyl ions to form B-site hydroxyl ions. Pair defects (two oxide ions or two vacancies on ininiediately adjoining sites) siniilarly form E- or D-site hydroxyl ions. Attachment of protons a t interstitial oxide ions or of hydroxyl ions at vacant oxide sites in regular domain areas produces A- or Csite hydroxyl ions. Pair and triplet defects are found only in the boundaries between the two domains of the surface-oxide lattice and must exist in such boundaries as long as the two doinains remain, unless sufficient water is cheniisorbed to eliminate them. In general, odd and even domains can adjoin at regular boundaries of two types, as illustrated in Figure 12 (A and B). Boundaries of both types must coexist because either type occurs a t corners of a domain region bounded principally by boundaries of the other type. (Boundaries of both types can (easily be found in Figure 9.) One of these can be characterized as a “pair-defect”; the other, as a “triplet-defect” boundary. A pair-defect boundary is static, and no transfer of oxide ions can occur across it without creation of triplet defects. Rearrangement of the surf:tce oxide lattice, thus, requires formation of triplet-oxide defects, even though a pair-defect boundary may represent a less defective configuration. Defects in either boundary could theoretically be eliminated by retention of sufficient cheniisorbed water, held as hydroxyl ions, but the boundaries cannot retain hydroxyl ions to this extent at high temperatures. Chemisorption of water as isolated B-site hydroxyl ions eliiiiiriates triplet defects which are higher energy configurations than the pair defects which would be eliniiriated by D- or E- site hydroxyl ions. This is illustrated in Figure 12 (C and D). Retention of B-site in preference to D- or E-site hydroxyls is thus understandable. Refinement of Asswi~ptions. Nost of the assumptions for the developnient of the simple model are oversimplified. Experinlentally, a surface compktely filled with hydroxyl ions appears to be unrealizable. Whenever sufficient water is cheniisorbed to produce a filled hydroxyl inonolayer, sonie of the water seenis to be held molecularly. Because hydroxyl ions are charThe Journal of Physical Chemistry
acteristically somewhat larger than oxide ions, a coniplete hydroxyl monolayer possibly could not be superimposed on a close-packed oxide layer. To the niaximum possible extent, however, hydroxyl ions would probably occupy normal lattice positions. When “crowding” became too great, water could be held, as such, to minimize surface energy. (A surface equilibrium 20H- @ HzO 0-2 could be involved.) The assumption of random removal of pairs (subject to the restriction that local order is preserved) is probably inconsistent with differences expected in the ease of removal of various hydroxyl pairs as a result of differences in near-neighbor configuration. Rather than occur randomly, reinoval of hydroxyl pairs might spread froni a relatively few (randomly chosen) initial points, or occur through systeniatic removal of pairs in other ways. Xevertheless, if the requirement for preservation of local order is retained, it matters relatively little whether removal proceeds regularly from comparatively few or from niany points selected a t random. The sites on which the various types of isolated hydroxyl ions have been pictured require at least minor modification. Owing to electrostatic repulsion between ions of like charge, we should expect some distortion of the regular lattice spacings in the vicinity of these ions. Thus, for example, the oxide nearest neighbors of A- and B-site hydroxyl ions would probably be slightly further, and the oxide ions closest to C-site hydroxyl ions would be slightly closer, than pictured.
+
Interpretation of Experimental Observations Assignment of Infrared Bands to Hydroxyl Ions of Various Types. The five types of hydroxyl ion sites shown in Figure 8 differ in local charge density because of their nearest neighbor configurations. The most negative (A-site) has four oxide ion nearest neighbors; the most positive (C-site) , four ininiediately adjacent vacant sites. If sites in the regular oxide lattice are regarded as neutral, an “interstitial” hydroxyl ion (A-site) would be associated with an extra electron, while a hydroxyl ion replacing an oxide ion on a regular site would represent a local deficiency of one electron. The other sites would fall between these limits, B-sites being approximately neutral. Other factors being equal, the frequencies of the corresponding stretching bands would probably decrease with decreasing electron density (A > D > B > E > C). The chemical properties of the hydroxyl ions should vary similarly with type of site, the A-site ions being the most basic and the C-site ions the most acidic. The five isolated hydroxyl bands observed in infrared spectra of dry aluniina are,
A MODELFOR
THE
227
SURFACE OF y-ALUMINA
Table I Band
Wave number, em. -1
A
3800 3744 3700 3780 3733
B C D
E
Site
No. of oxide nearest neighbors
A B
c D
E
4
2 0 3 1
therefore, tentatively assigned to hydroxyl ions on the five types of sites as given in Table I. At surface coverages above 9.6% monolayer, some hydroxyl ions have one or more hydroxyl ion nearest neighbors, and hydroxyl bonding is assumed to occur between them. Since both protons in a pair of adjoining hydroxyl ions cannot simultaneously form bonds to the other hydroxyl ion, one of them should give rise to an “isolated” hydroxyl infrared band. Each hydroxyl ion in a pair of adjacent hydroxyl ions may have four significant nearest neighbor configurations, the number of oxide ion nearest neighbors ranging from 0 to 3. Paired hydroxyl ions inay thus contribute four isolated hydroxyl bands, as well as bands corresponding to vibrations in which the proton is directly involved in hydrogen-bond formation. The frequencies
r
1
26
24
22 I
W LJ
F 20 I-
< -I 18 In
ru
X In ‘ 6
1
ru
2
14
0
ISOLATED COMPONENT OF PAIRS
12 X
0
6* IO
OF T R I P L E T S
I
be K
W
Sa 6 z
4
2
I
0
-NUMBER
OF O X I @ E NEIGHBORS
Distribution of hydroxyl types in Figure 6.
of the bands should, as for hydroxyl ions having no hydroxyl nearest neighbors, increase with the number of adjacent oxide ions. The influence of the adjacent (hydrogen-bonding) hydroxyl cannot be exactly assessed. On the basis of formal charge, an adjacent hydroxyl should be equivalent to half an oxide ion, but, because the bond from this hydroxyl tends to remove an electron from the free hydroxyl ion, the net effect would probably be roughly equivalent to that of an adjacent oxide vacancy. The hydroxyl ions of all types (single, pair, triple) on the final computer printout section shown in Figure 6 have been counted and plotted in Figure 13. An adjacent hydroxyl ion has been assumed equivalent to an adjacent vacant site. Half of the paired hydroxyls and one-third of the hydroxyl ions with two nearest hydroxyl ion neighbors have been considered as isolated, in addition to those having no hydroxyl ion nearest neighbors. The remainder are assumed to be directly bonded through their protons. Hydrogenbonded hydroxyl groups produce broad bands a t frequencies below those of isolated hydroxyl groups. They have been indicated in Figure 13 as a block a t the low-frequency side of the isolated hydroxyl ions. At high coverages with hydroxyl groups, hydrogen bonding would be expected to be extensive on the model surface although an isolated band or bands would be expected to persist. Spectral changes observed as alumina is dried can be explained by the model. The assumption is made that relative band intensities are roughly proportional to the nuniber of corresponding hydroxyl ion types. Comparison of Figures 11 and 13 with published ~ p e c t r a ~and - ~ with data on surface hydroxyl coverage as a function of drying temperature5 shows that, in addition to accounting fairly well for general changes in band intensities and in the nuniber and relative frequencies of types of isolated bands on dry alumina, the model can explain the following points: (1) disappearance of hydrogen-bonded hydroxyl bands a t a surface coverage of -loyo monolayer (no paired hydroxyl groups left a t 9.6Y0 coverage); (2) marked reduction in D and E bands as surface coverage changes froin 10 to 8% monolayer; (3) faster disappearance of the R band as compared to the A arid C bands below 8% monolayer. At 2.4% monolayer coverage the B band persists in infrared spectra although, according to the reiiioval scheme, no B-site hydroxyl ions should remain. Permitting additional surface migration of oxide ions or less restricted proton migration would avoid this result. Such moves must, in any case, be permitted to reinove the final 2.4% nionolayer. Agreement between the Volume 69, Y i i m b e r 1
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model and the changes in the infrared bands can evfdently be made as good as the experimental observations permit. This, although purely a curve-fitting process, demonstrates that the model can explain the spectral data. Dejects on Dry Alumina; Catalytically Active Sites. Of the types of defects created during dehydration, those presently imagined to have the greatest catalytic importance are the triplet vacancies, found in the boundaries between regular oxide domain regions. These vacancy defects characteristically adjoin pair or triplet oxide defects or B-site hydroxyl ions. They provide unusual exposure of the aluminum ions in the underlying layer and should constitute strong (‘acid)’ sites for adsorption of unsaturated hydrocarbons, ammonia, and other “electron-rich” molecules. Larger vacancy defects, such as oxide ions missing from regular surface oxide domain sites, may coexist to a slight extent, and these should be very strongly acidic. In a broad sense, any irregularity in charge distribution in the surface layer constitutes a defect of some sort. When every aluminum ion in the underlying layer is covered with a hydroxyl ion in the top layer, the surface presumably represents a low-energy configuration wherein the electrostatic repulsion between adjacent hydroxyl ions is minimized by hydrogen bonding. Such a configuration is defect-free. On an idealized, completely regular, dry surface, however, the oxide ions in the top layer cover only half of the aluminum ions in the next lower layer, resulting in adjoining sites of charge excess and charge deficiency. Description of the oxide ions in the top layer as O W 2 is, of course, an approximation. Some degree of electron sharing with the aluminum ions lying below the adjoining vacant sites undoubtedly occurs. The oxide ions in the top layer are not regarded, however, as being located midway between two‘aluminum ions in the next lower layer with equal sharing of electrons with both aluminum ions. Single vacant sites are, thus, weak Lewis acids, and oxide ions in the surface lattice are weak Lewis bases. Both are “defects” which can be minimized by chemisorption of water or various other molecules. The character of such defects is, moreover, affected by neighboring sites, and a broad spectrum of defect energies should result from the many possible nearest neighbor (and next nearest neighbor) configurations (as can be seen in Figure 3). Isolated vacant sites differ significantly from other vacancy defects, however, because the aluminum ions in the next lower plane are more completely coordinated and less accessible to adsorbed molecules. The nuinber of defects of all types could be obtained from the COii~pUter printouts, but, in view of the The Jozirnal of Physical Chemistry
Table I1
______ OH coverage,
8.6 5.6 2.4 a
Yo”
Defects/ 1000 . L 2 - -
--
7
--Vacancy-Pair
Oxide--
Triplet
Pair
Triplet
4.8 5.1 7.1
3.4 3.1 2.5
7.1 6.9 5.9
2.5 4 8
1.1
100% = 8 A.Z/OH.
arbitrary assumptions employed, little present purpose would be served by such tabulation. During the final stages of hydroxyl ion removal the pair and triplet defects, which seem of greatest interest, were present to approximately the extent shown in Table 11. Defects are apparently considerably more riunierous than the sites reponsible for strong adsorption of butene4 l 4 or ethylene15 on alumina. Previous work has indicated that roughly 0.6 site/1000 A.2 holds olefins strongly. If such sites can only be triplet vacancies, the model still provides 4 to 5 times as many sites as needed. Several explanations are possible. Reorganization of the surface with higher retention of hydroxyl ions in shortened domain boundaries could markedly reduce the number of defects. Possibly, only certain of the triplet vacancies can strongly adsorb olefins. Differences exist in the configurations of adjoining oxide or hydroxyl ions. These differences may, for example, restrict strong adsorption to those triplet vacancies which suitably adjoin triplet oxides. Finally, experimental data on olefin adsorption may not correctly indicate the n~aximumnumber of possible sites for strong adsorption. Additional evidence is clearly needed. Rehydration of a Dry Surface. Results achieved in rehydration of dry y-alumina depend on the experimental conditions.6 Above 600°, isolated hydroxyl ions are created by chemisorption of water a t low pressures. This requires either sufficient mobility to permit hydroxyl ions (or protons) originally on adjacent sites to separate, or simultaneous chemisorption of two water molecules on neighboring sites in such a way that proton transfer between hydroxyl ions can desorb water and leave two isolated hydroxyl ions. Either mechanism can produce isolated ions of all five types to an extent limited by the rate of recombination of the hydroxyl ions. Such behavior is easily explained by the model. At much lower temperatures, adsorbed water tends (14) 1’.Amenomiya and R. J. Cvetanovi;, J . Phys. Chem., 67, 2046 (1963). (15) Y. Amemomiya and R. J. Cvetanoviit, ibid., 67, 144 (1963).
A MODELFOR
THE
SURFACE OF 7-ALUMINA
to be held either as such or as hydrogen-bonded hydroxyl ions. Chemisorption of water vapor at 100’ rapidly covers the surface with, the equivalent of about 50% hydroxyl monolayer (32 A.2/water molecule) , but further chemisorption is much slower. This suggests that a “packing” effect niay be involved. This could arise if, for example, a given hydroxyl ion cannot easily immediately adjoin niore than two other hydroxyl ions. Random chemisorption to form pairs of hydroxyl ions would give &50% hydroxyl monolayer. Slow reorganization (not difficult with high surface hydroxyl coverage and appreciable water vapor pressure) would permit additional adsorption up to 66-67% monolayer with no more than two hydroxyl nearest neighbors, and up to 75% monolayer with no more than three neighbors. Coniplete coverage (probably impossible) with hydroxyl would require four neighbors. llolecular water would also be strongly held on a surface less than 1 0 0 ~filled o with hydroxyl ions. A similar argument might explain the increased hydration a t 100’ if alumina is first exposed to water vapor at room temperature. At room temperature enough water is strongly held to permit attachment of one molecule a t eacb oxide and vacant site on the niodel surface (8 A. 2/water molecule). Desorption might require a ‘(condensation” reaction between two adjacent water molecules to desorb one water molecule arid leave two hydroxyl ions in the surface. Adjoining water molecules could conibine easily until removal of additional water would require more than two hydroxyl ion nearest neighbors per hydroxyl ion. At this point (-50% of the surface covered with water molecules and 50% with hydroxyl ions) surface reorganization would be required to permit removal of additional water. The total coverage would correspond to 125% monolayer (12.8 .$.2/water molecule). However, some explanation is needed for the failure of molecular water to readily adsorb strongly a t 100’ to produce this coverage. Near 300’, adsorbed water readily forms hydroxyl ions, but these are mostly paired (hydrogen-bonded). Because surface coverage is only partial, hydroxyl ions would tend to desorb on subsequent dehydration through recombination with their original partners. Fewer isolated hydroxyl ions (and fewer new defects) should therefore be formed on subsequent drying than if the surface had first been completely rehydrated a t 100” or below. Dehydration Isotherms. The simple, random renioval process employed to illustrate the model might, a t first glance, be expected to yield a dehydration isotherm somewhat resembling Figure 2, i.e., exhibiting Some type of break near 33y0 surface coverage. This
229
has not been observed. Closer appraisal of the partially dehydrated surface shows that a very large number of different site configurations, undoubtedly differing in energy, are created during dehydration. Dehydration behavior would thus probably reflect the progressively increasing severity of defects of all types created by removal of hydroxyl ions. This should reduce or eliminate discontinuities otherwise expected in the dehydration curves. Dehydration can also be regarded as analogous to charging an electrical condenser, since the dry surface is assumed to have a niuch more pronounced “double-layer” character than the hydroxylfilled surface. The concentration dependence of the dehydration rate would, thus, appear to be of higher order than otherwise expected. As ionic migration beconies possible in the course of hydroxyl removal, the entropic factor should become increasingly important. Ultimately, the rate of dehydration is probably governed by a large entropy factor, secondorder kinetics, and possible readsorption of water. Although quantitative interpretation is not possible a t present, the model, with moderate refinement, can be reconciled with observed dehydration isotherms. Exchange Reactions of Hydroxyl Ions. The three principal types of isolated hydroxyl ions on dry alumina (A-, B-, and C-site hydroxyl ions according to the niodel) exchange hydrogen with deuterium, deuterium oxide, butene, etc., at different ratesa3z4 Such “exchange” does not, however, require substitution of deuterium on the same site from which the hydrogen was removed. A proton (or hydroxyl ion) may be removed from one site while a deuteron (or deuteroxyl ion) is added a t a similar neighboring site. Because hydrogen exchange occurs readily between surface hydroxyl ions and adsorbed molecular water (or deuterium oxide), traces of water niay play an intermediate rolein exchange between hydroxyl groupsand other molecules (e.g., deuterium and butene) which exchange more slowly. The exchange behavior of the various types of hydroxyl ions with deuterium oxide can be plausibly explained by the model. Deuterium oxide adsorbs on “dry” alumina with transient formation of two deuteroxyl ions through random addition of a deuteron to an oxide ion, the second deuteroxyl ion being attached at an adjacent vacant site. If one of the deuteroxy1 ions adjoins a pre-existing hydroxyl ion, transfer of a proton from the hydroxyl ion to the deuteroxyl ion can form HDO. Desorption of HDO leaves a deuteroxyl ion on the surface, usually on a site similar to that holding the original hydroxyl group. This process should occur readily at A- or C-site hydroxyl ions (and desorption of HDO should normally leave Volume 69,.\‘umber
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either an A- or C-site deuteroxyl ion), but less often with B-site hydroxyl ions. Usually, however, formation of only one new deuteroxyl ion is possible in an exchange reaction. Thus, presumably, Dz 0-2 a ODD-. Exchange can occur in the manner described above only if Dcan easily recapture a proton from a hydroxyl ion on a site corresponding to that on which the deuteroxyl ion was formed. The niost common oxide ions in the surface are the regular doniairi oxide ions. If a deuteroxyl can be transiently formed under exchange conditions at any surface oxide ion, it would be formed niostly from regular domain oxide ions. Exchange could result, however, only if the D- ion immediately adjoined a hydroxyl ion. This is possible near C-site (or B-site) hydroxyl ions. S o A-site hydroxyl should be exchangeable through such a niechanisni because initial attachment of D f to an interstitial oxide ion (presumably very rare) would be needed, and the Dion cannot in any case, adjoin an A-site hydroxyl ion. Exchange with B-site hydroxyl ions should be much less frequent than with C-site hydroxyl ions. Experinientally, the C-band (3700 cni.-l) is, as expected, usually most easily replaced by the corresponding OD-band in exchange reactions (e.g., deuterium or butene) but in some cases the A-band (3800 cni.--l) also shows rapid change. This can be readily understood if traces of water can play an intermediate role, as suggested by recent evidence.16 Other mechanisms for exchange are possible, however. Observation of “exchange” of surface hydroxyl ions is seldom uIlalllbiguous because resolution of the bands often leaves much to be desired, new types of hvdroxyl ions may be formed by chemisorption of
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The Journal of Physical Chemistry
+
actions is needed before results are used to evaluate the model.
Conclusion Observations to date are plausibly explained by a simple model for the surface of y-alumina. Although inherent defects, sites on crystal edges or corners, and various exposed crystal faces may exist, they arc not needed to explain either the various types of hydroxyl groups or the generation of active sites on dry alumina. Rather than strained oxide linkages,8 high energy “strain” sites on dry alumina can be defects which persist in boundaries between odd and even surface oxide domains. The present niodel is, of course, speculative. The surface may be very different from that depicted and possibly much more complex. Faces other than the 100 face may be exposed to a major extent. The model should prove applicable to other faces which can be approxiniated by a square lattice, but could not be readily applied, for example, to a 111 face. Serious efforts should be made to determine whether the 100 face is actually preferentially exposed on yalumina and whether known faces of other crystalline forms of alumina exhibit similar characteristics. Further evidence, obtained from infrared and related studies of the adsorption of ammonia on y-alumina, is discussed in the following paper.
Acknowledgment. Special acknowledgment is due to JIr. W. B. Traver, who programmed the model for the IBRI 705 computer.
