J. Phys. Chem. 1985,89, 2299-2304
2299
Watching Silver Clusters Grow in Zeolites: Direct Probe Fourier Transform-Far-Infrared Spectroscopy of the Red Form of Fully Silver Ion Exchanged Zeolite A Mark D. Baker,* John Godber, and Geoffrey A. Ozin* Lash Miller Chemistry Laboratories, University of Toronto, Toronto, Ontario, Canada M5S 1Al (Received: September 17, 1984)
The red form of vacuum thermally dehydrated AgI2-A has been studied for the first time by FT-far-IR spectroscopy. Silver-silver skeletal modes are observed in the wavenumber range 175-50 cm-l and are assyiated with the IR-active skeletal vibrations of a pseudocluster located in the cuboctahedral cages, in accord with crystallographic, spectroscopic,and quantum chemical published work on silver-zeolite A. The far-IR spectra anticipated for various structural models of two (DZh,D,, C2) and four (Oh)loosely coupled, cuboctahedrally located Ag3x+linear clusters, themselves having C2 site symmetry, are investigated, and although not unequivocal in terms of cluster size, shape, and charge, favor a Dlh or DZ1!(Ag3*)2,formulation. Controlled H2 reduction of (Ag,*), in the low-temperature range room temperature to 130 OC,as monitored by in situ far-IR spzctroscopy, appears to proceed through at least two detectable steps to yield finally what is probably best described as a cuboctahedrallyentrapped, reduced form of (Ag3*),. The interaction of this reduced cluster with O2at 200-400 OC reverses the reduction process to the original (Ag3X+)n cuboctahedrally located pseudocluster. By contrast, more severe H2 reduction of the (A&),, red form at 200 OC also monitored by in situ far-IR spectroscopy appears to transform the cluster to external silver microcrystals. A prolonged 450 OC O2oxidation of these samples partially reinstates the original (A&+), pseudocluster. A collection of far-IR data for zeolite A and Y entrapped Ag,9+ clusters ( n = 2-13) indicates that silver-silver stretching frequencies tend to decrease with the cluster nuclearity n toward a value around 60-65 cm-I for approximately 50-8, silver microcrystallites. This behavior is consistent with the idea that the silver-silver bond stretching force constant per atom (probably paralleling the bond energy per atom) decreases with coordination number up to the bulk silver value of 12 nearest neighbors, an observation considered to be in line with current quantum chemical cluster model theory.
Introduction Understanding the reduction and oxidation processes of metal guests immobilized on the external surface or in the pores of solid supports is of paramount importance in the assembly of selective heterogeneous catalysts.*a Starting with metal cations, complex ions, organometallics, or solvated metal atoms loaded into, for example, oxide, carbon, polymer, or zeolite substrates, subjecting them to either chemical, photochemical, or thermal pretreatments yields materials containing entrapped metal atoms, metal clusters, and/or metal microcrystals. These are often the active sites in supported metal catalysts, and it remains a major challenge to discover ways of generating specific metal centers, means of stabilizing them under operating conditions, and methods of improving their resistance to poisons. Progress in the design of such high technology catalyst systems hinges to a considerable extent on the experimentalist's ability to directly observe the embryonic events leading to and following the birth of the active metal center, as well as to establish its fate when placed on a stream. At present, no single physicochemical technique provides a satisfactory solution to this problem and a picture of the catalyst system is usually pieced together from the patient and painstaking accumulation of data from a variety of distinct sources.lb Although very low frequency (10-400 cm-') vibrational spectroscopy has long been recognized for its ability to diagnose the skeletal motions of metal clusters,Ic it has been completely neglected for the study of ligand-free supported metal catalysts where its strengths could in principle be exploited to the greatest advantage. In this context the far-IR probe is expected to be sensitive to the metal and support type, oxidation state, nuclearity, structure, and site symmetry and moreover offers the opportunity to gain access to such data in a single straightforward measurement. The (1) (a) "Metal Microstructures in Zeolites", P. A. Jacobs, N. I. Jaeger,
P.Jim, and G. Schulz-Ekloff, Eds., Elsevier, Amsterdam, 1982; Stud. Surf. Sci. Catal., 16 (1983); "Structure of Metallic Catalysts", J. R. Anderson, Ed., Academic Press, New York, 1975, and references cited therein; (b) 'Spectroscopy in Heterogeneous Catalysis", W. N. Delgass, G. L. Haller, R. Kellerman, and J. H. Lunsford, Eds., Academic, Press, New York, 1979, and references cited therein; (c) D. F. Shriver and C. B. Cooper, 111, A d a Infrared Raman Spectrosc., 5 (1978). and references cited therein; (d) P. R.Griffiths, Ado. Infrared Raman Spectrosc., 10, 277 (1983). and references cited therein; (e) M. G. Baldecchi and B. Melchiorri, Infrared Phys., 13, 189 (1973).
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lack of experimentation in this field can be traced in part to instrumental impediments and sample preparation techniques. Howevbr, with major advances in Fourier transform-far-IR spectroscopy hardware and dedicated interactive softwareld coupled with a knowledge of the far-IR windows of conventional catalyst supports cast into thin-wafer form,Ic rapid advances with the in situ study of immobilized metal guests can be anticipated. In this spirit we have recently demonstrated that FT-far-IR spectroscopy of metal zeolites subjected to a range of autoreduction, hydrogen reduction, and oxygen oxidation pretreatments can yield site-specific information on entrapped metal atoms, metal ions, and metal clusters. Our early reports focused attention on Co2+ ion exchanged faujasites (Si/Al = 1/1 to 20/1),* Agnq+in zeolite Y (n = 1, q = 0; n = 2, q = 1; n = 3, q = 2); Ag3* (where x is probably close to 2) in zeolite A,4a M2+crystal field effects in zeolite Y (M = Ca, Mn, Fe, Co, Ni, Cu, Zn),4bfrequency and intensity considerations (experiment and theory) in the far-IR spectroscopy of metal ion exchanged fanjasites," and direct probe far-IR studies of the deammination of ammonium and transition metal ammine complex cation exchanged zeolites.4d The Ag3X+ cluster, which has been proposed to exist in the yellow form of silver-zeolite A, is produced by mild thermal vacuum dehydration of partially or fully Ag+ ion exchanged zeolite A.5,6a The purely (2) G. A. Ozin. J. Godber. and M. D. Baker in "Frontiers of Heteroeeneous Catalysis", B. Shapiro,'Ed., Texas A & M University Press, College TX. 1984. Station _ _ - (3) G. A. Ozin, J. Godber, and M. D. Baker, Angew. Chem., Int. Ed. Suppl., 1075 (1983); J . Phys. Chem., 88, 4902 (1984). (4) (a) G. A. Ozin, J. Godber, and M. D. Baker, J. Phys. Chem., 89, 305 (1985); (b) G. A. Ozin, J. Gcdber, W. Shiua, and M. D. Baker, J. Am. Chem. SOC.,in press; (c) G. A. Ozin, J. Godber, and M. D. Baker, J. Am. Chem. SOC.,in press; (d) G. A. Ozin, J. Godber, M. D. Baker, and K. Helwig, J. Phys. Chem., in press. (5) L. R. Gellens, W. J. Mortier, R. A. Schoonheydt, and J. B. Uytterhoeven, J. Phys. Chem. 85, 2783 (1981). (6) (a) L. R. Gellens, W. J. Mortier, and J. B. Uytterhoeven, Zeolites, 1, 11 (1981); (b) B. D. Cullity, "Elements of X-Ray Diffraction", AddisonWesley, Reading, MA, 1967, Appendix 13. (7) Y.Kim and K. Seff, J . Phys. Chem., 82, 921 (1978); J . Am. Chem.
---.
SOC.,99, 7055 (1977); 100, 175 (1978). (8) L. R.Gellens, W. J. Mortier, and J. B. Uytterhoeven, Zeolites, 1, 85 (1981); P. A. Jacobs, J. B. Uytterhoeven, and H. K. Beyer, J . Chem. SOC., Faraday Trans. I, 75, 56 (1979), and references cited therein.
0 1985 American Chemical Society
2300
The Journal of Physical Chemistry, Vol. 89, No. 11, 19'85
Baker et al. that silver gradually accumulates in the cuboctahedral cages in the form of a collection of weakly interacting Ag,X+units from two up to a maximum of four after prolonged pretreatment at 400-500 OC. An illustration of three possible structures for cuboctahedrally located (Ag3"+)2 (DZh,DZd,C,) and (Ag3"+), (D4) as suggested by Gellens et al." and Uytterhoeven et aL5is shown in Figure 2 (note the coplanar array of the four central silver atoms of the three Ag3X+clusters giving D, overall symmetry to the Ag12'+ aggregate). It is one of the purposes of the present study to record the far-IR spectra for the interesting silver cluster species giving rise to the red center(s) in vacuum thermally dehydrated Ag12-A as well as that of the products resulting from its interaction with H2 and O2 at different temperatures.
Figure 1. A schematic view of the AglX+cluster in dehydrated NaAg-A. The silvers of the site G-C-G cluster are shaded for clarity, and the zeolite framework is represented by open circles. T on the figure corresponds to a Si or AI atom (ref 6 ) .
crystallographic evidence for Ag3X+in zeolite A appears to be equivocal. Some authors have been unable to obtain conclusive evidence for the size, shape, charge, and location of the cluster giving rise to the yellow hue of these samples.10 Others have proposed that the cluster exists and is located in the cuboctahedra @-cages), fixed symmetrically (CZusite symmetry) across two hexagonal 6-rings and a bridging 4-ring as illustrated in Figure l.5,6a In the latter view the cluster is considered to be essentially linear with a site G site C site G occupancy and a silver-silver bond length of 2.85-3.00 A compared to 2.89 A for bulk silver6b (because of these interatomic distances the cluster is formally written as Ag+-Ago-Ag+ although it is recognized that the charges on the central and terminal silver atoms may not be exactly zero and unity, r e s p e ~ t i v e l y ~ * ~ ~ Consistent ' ~ * ' ~ ) . with this picture of Ag3AX+, we have found that the far-IR spectrum of the yellow form of silver-zeolite A displays the anticipated silversilver stretching (Xu+)and deformational (nu) vibrational modes centered at 149 and 110 cm-I, respectively, where the latter II, mode experiences a splitting of about 20 cm-' due to the C , site symmetry of the cluster in the @-cage.4a It is also significant that the reduction of Ag3*+in partially exchanged zeolite Ab* at room temperature ~ ~ v ~(where y C x and y is to 140 O C produces in its p l a ~ e Ag3Y+ probably zero) which can be oxidized by O2 a t 350 OC back to Ag;+. This redox cycle can be repeated indefinitely without any deleterious effects, the entire process being monitored by in situ far-IR s p e c t r o ~ c o p y Le. ,~~
+
Ag3x+ (149/110 cm-I)
+
+ -(xH2 - Y ) 2
(183/91 cm-')
+ (X - y ) H +
Success with the direct observation of the redox processes of zeolite A encapsulated Ag3*+/Ag3Y+clusters suggested to us that similar investigations of "fully" exchanged silver zeolite A, Ag12-A, could provide new insights into the silver clusters responsible for the well-known "red" color of the autoreduced samples, as well as additional information concerning their H2 reduction and O2 oxidation reactions and products. It is recognized that the crystallography of autoreduced AgI2-A5-' is also equivocal,I0 but when taken in conjunction with the physicochemical properties of the system, is consistent with the formation of "some kind of metal cluster" where it is visualized (9) R. P. Messmer in "Nature of the Chemical Bond", T. N. Rhcdin and G. Ertl, Eds., ,North-Holland Publishing Co., Amsterdam, 1979, and references cited therein. (10) L. R. Gellens, J. V. Smith, and J. J. Pluth. J . Am. Chem. SOC.,105, 51 (1983). (1 1) L.R. Gellens, W. J. Mortier, R. Lissillour, and A. Le Beuze, J. Phys. Chem., 83, 2509 (1982).
Experimental Section High-purity zeolite A used in this study was supplied in its hydrated sodium form from Union Carbide and IBM. Care was taken to remove any sodium defect by slurrying the zeolite with NaCl followed by washing. Ion exchange with aqueous silver nitrate (in a darkened laboratory) was performed by using standard procedures. In a typical preparation, a stoichiometric excess of 0.01 N A g N 0 3 solution (pH approximately 6.5) was stirred with zeolite at room temperature for 48 h. Filtration and washing with distilled water until the washings were free of Ag' were performed in the absence of light. Samples were then dried at 100 "C and then rehydrated by storage over saturated NH4Cl solution. Analysis of silver content was performed by neutron activation analysis at the University of Toronto Slowpoke reactor facility. For far-IR studies the silver zeolite A was pressed into selfsupporting wafers using between 5 and 8 t o n s / h 2 . The wafers were carefully clamped into the sample holder of an in situ zeolite vacuum cell which was compatible with the vacuum chamber of a Nicolet 2OOSXV FT-far-IR ~pectrorneter.'~ The samples could be moved in their own high-vacuum system within the interferometer vacuum chamber to intersect the IR beam or to a furnace area where dehydration, thermal treatment, and reaction chemistry could be performed over the range 10-600 OC.I3 Far-IR spectra were recorded with a 6.25-km Mylar beam splitter, TGS detector, and globar source. The spectra shown are the result of the coaddition of 500 Happ-Genzel apodized interferograms resulting from a 30-min collection time, although survey spectra of useable quality could be obtained in less than 1 min. The spectral resolution is 4 cm-', and the spectra are not smoothed in any way, although in most cases the spectra are base line corrected. After thermal or chemical treatment the samples were cooled to room temperature or below before the spectra were recorded. Results and Discussion Recall that the far-IR spectrum of dehydrated Na12-A displays four major absorptions at 270, 216, 180, and 100 cm-' which are respectively assigned to a zeolite framework mode and Na+ site A, E, and H cation Eight of the twelve Na+ ions reside in site A. Of the remaining four, three occupy site E (a-cage) and one occupies site H (a-cage); interestingly the site A Na+ cation band is about twice as intense as that of site E Na+. By contrast, the corresponding data for partially Ag+ ion exchanged Ag6Na6-A (leaving aside the pore-opening band region) show considerably depleted site A and site E cation bands (essentially unshifted) with the striking appearance of two new intense bands at 149 and 110 cm-I. It is under precisely these experimental conditions that the crystallography in combination with the physicochemical properties of the system is consistent with the presence of the 8-cage entrapped "yellow" Ag,*+ cluster comprised of a linear arrangement of silver atoms located in sites G, C, and G, respectively,5-8,10~11 as shown in Figure 1. The most straightforward assignment of the far-IR bands of the yellow (12) W. J. Mortier, "Compilation of Extra Framework Sites in Zeolites", Butterworths, London, 1982. (13) G. A. Ozin, M. D. Baker, and J. Godber, Cotal. Rev.-Sci. Enn., in press.
The Journal of Physical Chemistry, Vol. 89, No. 11, 1985 2301
FT-Far-IR Studies of Silver Zeolite A
@@@ i.I ,... . L
.. ... .
.......
Q Figure 2. Possible arrangements for two and four linear Ag3"+clusters per cuboctahedron in vacuum thermally dehydrated Ag12-A (ref 5 and 10). Note that the cuboctahedron is represented for clarity as a cube.
n
I' w , w
25 :HZ
300 4 50'
4 00'
300'
2$0
WAVENUMBER 200 150
250
ZbO
150
100
SO
WAVENUMBER
Figure 3. The far-IR absorbance spectra of Ag12-A during vacuum dehydration at the indicated temperature in OC. Letters identifying each spectrum are referred to in the text.
cluster at 149 and 110 cm-' is to the silversilver skeletal stretching (2,+)and deformational (nu) modes, respectively, as anticipated for a linear Ag,"' cluster.4a On passing to fully Ag+ ion exchanged Ag,,-A, the far-IR spectrum of the 25 O C yellow sample displays (apart from the pore-opening band) two intense absorptions at 150 and 106 cm-' but with the noticeable absence of any significant intensity at 212 and 180 cm-' (Figure 3A) consistent with the essentially fully Ag+ ion exchanged nature of the starting zeolite. The overall similarity of this far-IR spectrum to that of Ag3"+ in partially Ag+ ion exchanged Ag6Na6-A argues that the @age is hosting a single Ag3"+ moiety in both cases.
k0
Figure 4. The far-IR absorbance spectra of vacuum thermally dehydrated Aglz-A showing the reduction-oxidation behavior discussed in the text. The reversibility of the Hzreduction at 130 OC is not observed after more severe reduction at 200 OC due to formation of microcrystals of silver external to the @-cage.
Temperature-controlled vacuum thermal dehydration of the Ag12-A sample in the range 25-500 OC is known to induce intrazeolitic autoreduction (via water and lattice oxygen) of the a-cage Ag' ionss which migrate to and accumulate in the &cage, resulting after prolonged pretreatment Ag3X+clusters 400-500 O C in a maximum of four weakly interacting Ag3X+clusters (Figure 2) considered by some authors to be responsible for the red color of these ~ a m p l e s . ~ * ~ *Following *J~J exactly the literature prescription that affects this reduction-agglomeration process, we have observed the sequence of far-IR spectral changes depicted in Figure 3B-F. In essence, the two-band spectrum of the linear "yellow" Ag3"+ cluster monotonically transforms into a totally distinct one showing bands which are best ascribed to silversilver skeletal modes at 168, 153, 110, and 63 cm-'. (The very weak band around 214 cm-' is considered to be residual Na' site A; cf. Figure 1 of ref 4a). The final spectrum shown in Figure 3E (2 h, 500 O C pretreatment) is essentially invarient to an additional 15-h, 500 OC pretreatment and consequently is most appropriately associated with the red (Ag3X+),majority cluster species existing under these experimental conditions (Figure 3F).5-7J0J' This far-IR spectrum will be discussed in more detail later. The response of the red (Ag3X+)nmoiety to a H, treatment is most intriguing. For example, exposure of the red form of Ag,,-A to 200 torr of H2at 25-100 "C causes the far-IR spectral alterations illustrated in Figure 4A-D. Here one can see loss of the 168, 153 cm-' doublet and red-shifting and broadening of the 1IO-cm-' band to 102 cm-' with a slight blue shift of the 63-cm-' band. When this "intermediate" sample is subjected to a slightly more severe H2 pretreatment, 200 torr, 130 O C , the far-IR spectrum experiences yet another even more dramatic transfor-
'
3b0
100
2302 The Journal of Physical Chemistry, Vol. 89, No. 11, 1985
Baker et al.
Scheme I /Ag2(Gas
phase)
i (6-cage)
I
H2, 100°C, 200 torr '20;
O2 4oooc,
200torr
I
ICO1
H2, 13OoC, ZOO torr i
1 ~
H ~ ,Z O O O C ZOO , torr
i O2 -
45OoC,
200torr
(external surface)
mation to a spectrum displaying a single broad band centered around 80 cm-' (note also the concomitant red-shifting and broadening of the pore-opening band in Figure 4D). A particularly important observation concerns the far-IR spectral transformation of the sample shown in Figure 4D,following O2 treatment at 200 torr, 400 OC, shown in Figure 4E. Amazinglpl (but not inconsistent with crystallographic proposals and physicochemical measurements on the system5-'), the far-IR spectrum of the original (Ag,*+), red cluster is almost completely reinstated. The slight perturbation of the far-IR spectrum of the 02-oxidized sample compared to the initially autoreduced sample (cf. spectra A and F in Figure 4) is ascribed to the presence of traces of residual intrazeolitic "product water" formed in the oxidation process in the presence of protons and not completely removed from the sample by the preceding vacuum treatment, in line with literature e v i d e n ~ e . ~Moreover, -~ this H2/02 redox process can be recycled without any serious loss of intensity of the far-IR absorptions shown in Figure 3 and 4. Following a more severe H2 reduction of the red silver cluster (200 torr, 200 "C), the far-IR spectrum shown in Figure 4D converts to yet a different picture, showing weak broad absorptions centered around 150 and 80 cm-I, which in O2 at 450 OC, 200 torr only with difficulty shows signs of transforming back to the red (Ag3*+),,@cage moiety. Clearly this is not as facile or complete an interconversion as that preceding the less severe H2 reduction described earlier (Figure 4). However, the observations are consistent with the prevailing physicochemical and crystallographic conclusions, that under these conditions, loss of the crystalline form of the zeolite occurs with concomitant production of large silver microcrystals on the external surface of the zeolite, which subsequently requires a severe O2 treatment to repair the zeolite lattice and to partially restore the silver to the intracavity regions, with some accompanying loss of silver to Ag20.5-s Collecting together the far-IR clues for the autoreduction, H2 reduction, and O2oxidation steps applied to Ag12-A and working within the constraints of the existing literature proposals for these ~ a m p l e s , ~ - ~we ~ ' would ~ ~ ' ' contend that we have directly observed by far-IR spectroscopy the embryonic (autoreductive) growth stages leading to &cage encapsulated (Ag3*+),,, identified its
--MOLfCULRR2 4 6 81012
BULK
-m
n ( Cnuclear1 luster ) ty Figure 5. Plot of observed far-IR silversilver stretching modes (cm-I) for zeolite A and Y encapsulated Agf silver clusters as a function of
cluster nuclearity. IR-active skeletal vibrations, and observed some P-cage, H2/02 induced cluster redox processes as illustrated in Scheme I. Recognizing the lack of certainity from the purely crystallographic viewpoint of the size, shape, and charge of the (Ag3*+),, silver cluster(s) responsible for the red hue of vacuum thermally dehydrated silver-zeolite A, we will work within the guidelines of the combined physicochemical, quantum chemical, and crystallographic information available for the system, which are consistent with the formation of some kind of cluster, comprised of weakly interacting Ag3*+moieties. Keeping in mind the rather large internuclear distances proposed to exist" between the @-cage entrapped Ag3*+ clusters in (Ag3"+)2 and (Ag3*+)4, namely 3.00-4.24A, we will use as a first approximation vibrational analysis of these clusters a weakly coupled oscillator model. This approach involves consideration of the local site symmetry of an individual linear DmhAg3*+cluster, that is, C2 in the 0-cage, and the outcome of coupling Zg+, Xu+,and II, skeletal modes of two and four of these Ag3"+ clusters to produce the appropriate combination of vibrational modes of (Ag3"+)2 in three of its (Ag3"+),, symmetries, DZh,D2d, and C,, and (Ag3x+)4under D4 symmetry, all in the &cage. (Note that 0,A&@ is not considered in this analysis, as it is expected to display only one TI, IR-active skeletal m ~ d e . ~ , ~The " ) result of this weakly coupled oscillator approximation is sketched out in Scheme 11. From these analyses, one expects to observe at most four, four, and eight IR-active modes for D2d, DZh, and C2 (Ag3*+)2, respectively, and six IR-active modes for D4 (Ag3*+)4as indicated in Scheme 11. Simply on the basis of the numbers and frequencies of the observed IR-active stretching (168, 153 cm-I) and deformational (109, 62 cm-I) modes alone, one is drawn toward an assignment in terms of either DZd,or DZh(Ag3"+)2. However, because of the uncertainty in the relative intensities anticipated for the various IR-active Ag-Ag stretching and deformational motions of the red cluster, which correlate with those of the Ag3*+ yellow cluster in Scheme 11, it seems best to regard the present IR data as interesting and supportive of the formation of some kind of (Ag3*+),,silver cluster but inconclusive in terms of being able to define the shape and size of the cluster giving rise to the red hue. On a final note it is fascinating that a plot of the silver-silver stretching frequencies observed for silver clusters (average values used where appropriate), Ag2 (gas phase), Ag3"+/Ag3Y+/A (@cage), Ag,+/Y (0-cage), Ags9+/Y (@-cage),A ~ w ~ + / Y (a-cage),
The Journal of Physical Chemistry, Vol. 89, No. 11, 1985 2303
FT-Far-IR Studies of Silver Zeolite A Scheme 11‘
Ag-
c+ vsAsAg
g
/ *\
/
A*
\A2*
A1
B2*
B
-A
3u
/ B1
*-B*
\ B2
/ g
A2*
A*\
B
3u
*-B*
/B1
\ BZ
Asterisk denotes IR-active skeletal modes. (AgjX+),/A (@-cage),and Ag;/Y (-50 A external s ~ r f a c e ) , ~ , ~ tends to decrease with increasing cluster nuclearity (Figure 5). This can be taken to imply that the silversilver bond stretching force constant per silver atom (perhaps synonymous with cohesive (bond) energy per silver atom for these very small aggregates n 5 13) monotonically decreases with the number of atoms in the cluster, leveling out when the bulk silver coordination number of 12 is achieved for the majority of atoms in the cluster. This experimental observation is certainly in line with current quantum chemical calculations of metal-metal bond strengths for bare metal clusters on passing from metal atom to cluster to bulk.g
Conclusion The major findings of the present in situ far-IR study of the autoreduction, H2 reduction, and O2oxidation of fully exchanged Ag12-A can be summarized as follows: (i) The 25 “C vacuum dehydration of Ag,,-A produces the “yellow” form of the zeolite, the far-IR spectrum of which is consistent with the presence of a single Ag3X+linear cluster entrapped in the @-cage. (ii) Prolonged vacuum thermal dehydration of the yellow form in (i) up to 500 “C yields as the final product the well-known “red” form of silver-zeolite A whose far-IR spectrum is inconsistent with the existence of a strongly bound octahedral Ag,q+ cluster but is consistent with the presence of a weakly coupled DZdor D,, (Ag3”’)z cluster. However, a t this stage, the data must be con-
sidered equivocal, in that it is not presently possible to confidently discount other weakly coupled clusters such as C, (Ag3”+)2 or D4 (Ag3”+)4. (iii) Mild H2 reduction of the red form of (Ag3*+),,appears to occur with at least two far-IR detectable steps to produce a silver cluster which can be easily oxidized by O2back to the original (Ag3X+)n@-cage cluster. This redox process can be recycled without any significant loss of (Ag,*),. These observations can be interpreted in terms of a @ a g e localized redox process in which the nuclearity of the (Ag3+),, silver cluster is probably maintained. (iv) A collection of far-IR data for zeolite A and Y entrapped Ag,q+ clusters (n = 2-1 3) indicates that silver-silver stretching frequencies tend to decrease with cluster nuclearity n toward a value of around 60-65 cm-’ for approximately 50-Asilver microcrystallites. This behavior is consistent with the idea that the silver-silver bond stretching force constant per atom (probably paralleling the bond energy per atom) decreases with coordination number up to the bulk silver value of 12 nearest neighbors, an observation in line with current quantum chemical cluster model the~ry.~ (v) The results of this study emphatically demonstrate that FT-far-IR spectroscopy is a high-sensitivity, informative in situ probe for ‘watching metal clusters grow in real catalyst supports”. Clearly, problems still exist with the ability to straightforwardly assign observed cluster skeletal modes to a particular cluster size, shape, and charge. This is a challenging area with considerable
J . Phys. Chem. 1985,89, 2304-2309
2304
scope for further development. Our study represents a first step in this direction. In view of the experimental simplicity of collecting far-IR metal cluster vibrational data, its application to a multitude of longstanding and new problem involving supported metal clusters is likely to grow rapidly in the near future. Acknowledgment. The financial assistance of the Natural
Sciences and Engineering Research Council of Canada's Strategic Grants Programme and the Connaught Foundation of the University of Toronto is gratefully appreciated. We are also indebted to Drs. Edith Flanigen (Union Carbide) and Paul Kasai (IBM) for supplying various ultrahigh-purity zeolites. Registry No. Ag, 7440-22-4.
Ab Initio Molecular Orbital Calculations on Phosphates: Comparison with Silicates M. O'Keeffe,* B. DomengL, Department of Chemistry, Arizona State University, Tempe, Arizona 85287
and G. V. Gibbs Department of Geological Sciences, Virginia Polytechnic Institute, Blacksburg, Virginia 24061 (Received: October 26, 1984)
The results of molecular orbital calculations using several different basis sets are reported for a number of phosphate and silicate molecules. In order to reproduce equilibrium P-O bond lengths correctly, d orbitals are necessary on P. d orbitals are necessary on 0 also to reproduce observed T-0-T bond angles (T = P or Si) at atoms bridging two tetrahedra. Energies are derived for hydrolysis of T-0-T linkages (-23 kJ mol-' for T = P and -20 kJ mol-' for T = Si), for hydrolysis of the monomeric metaphosphate ion (-150 kJ mol-') and a number of other hydrolysis reactions, for the difference in energy between a P=O double bond and two P-O single bonds (-238 kJ mol-'), and for deprotonation of phosphoric and silicic acids. Bond length-bond angle relationships are derived that closely mimic behavior observed in crystalline silicates and phosphates.
Introduction Molecular orbital calculations on suitably chosen molecules have provided valuable insights into the factors determining local geometries in crystals; a prime example is provided by recent studies of silicates and related materials.' In this paper we are concerned mainly with phosphates, although we will be particularly interested in comparing and contrasting their behavior with that of silicates. As for silicates, a striking feature of phosphates containing P207 groups (and more-condensed groups) is the large and characteristic angle at the bridging 0 atom which results in an almost constant d(P-P) in the same configurations.* Phosphates differ from silicates in one important way however, in that they exhibit a wider range of bond orders (and hence a wider range of bond lengths3) than the analogous silicates. This results in PO4and PzO, groups in crystals being more adaptive to the bonding requirements of other groups in ternary, etc. crystals than are the analogous silicates. A striking example, which interests us, is the large number and variety of tungsten phosphates4 in contrast to the apparent nonexistence of tungsten silicates. Calculations have been made with the GAUSSIAN-80 computer program5 using the STO-3G minimal basis set and the 6-31G split-valence basis sets6 It was found necessary to use polarization functions (ad = 0.55) on phosphorus to reproduce known geometries accurately. We identify basis sets employing such functions by adding a suffix of one of two asterisks according to whether there are polarization functions on phosphorus only or on phosphorus and the other atoms (excluding hydrogen). Unless otherwise stated, the basis set used for a particular calculation was 6-31G*. Care was taken to optimize the geometry of the mole(1) G. V. Gibbs, Amer. Mineral., 67,421 (1982), and references therein. (2) M. O'Keeffe and B. G. Hyde, Trans. Am. Crystallogr. Assoc., 15, 65 (1979).
(3) G. A. Lager and G. V. Gibbs, Amer. Mineral., 58, 756 (1973). (4) B. Domengts, Thtse, Universite de Caen, 1983. (5) J. S . Binkley, R. A. Whiteside, R. Krishnan, H. B. Schlegel, R. Seeger, D.J. DeFrees, and J. A. Pople, Quantum Chemistry Program Exchange, Indiana University, Bloomington, IN. (6) W. J. Pietro, W. H. Hehre, J. S. Binkley, M. S. Gordon, D.J. DeFrees, and J. A. Pople, J . Chem. Phys., 77, 3654 (1982), and references therein.
0022-3654/85/2089-2304$01.50/0
TABLE I: P-0 Bond Lengths (A) and Stretching Force Constants (N m-') and SCF Energies (in hartrees) at the Optimized Geometries for PO4* as a Function of Basis Set basis set bond length force constant SCF energy STO-3G STO-3G* 6-31G 6-31G* 6-3 1G**
1.713 1.547 1.653 1.574 1.572
372 1055 670 821 840
-630.835 28 -631.30637 -639.479 98 -639.649 38 -639.681 47
cules, a procedure that is efficiently implemented in GAUSSIAN-80' (although it might be mentioned that large stretch-bend force constants (in the phosphates particularly) make these molecules unusually difficult to optimize). Structures thus found often differ significantly from those assumed in some earlier studies. In what follows we first present the results of the calculations for each molecule individually; we then assess the significance of these data collectively. Some of the species have been studied before; however, we report our results even when the previous work has employed larger basis sets as it is useful to have a set of data (particularly total energies) all calculated at the same level of approximation. Where possible, we compare local geometries calculated for molecules with those observed in crystals, as we are concerned to establish the extent to which molecular calculations can be used to predict conformations to be expected in the solid state. The work here complements to some extent recent ab initio c a l c ~ l a t i o n son ~ ~other phosphorus-containing molecules.
Calculated Properties of Molecules ( a ) PO:-. Table I lists the P-0 bond lengths, stretching force constants, and S C F energies at the optimized geometry of this species ( Td symmetry assumed). The bond length calculated by using polarization functions is reasonably close (see below) to that found in orthophosphates; however omitting these functions results in dramatically longer bonds and smaller stretching force constants. (7) M. O'Keeffe and G.V. Gibbs, J . Chem. Phys., 81, 876 (1984).
0 1985 American Chemical Society
