J . Phys. Chem. 1990, 94, 756-760
756
Formatlon of Electronically Excited Ag,O from the Oxidation of Small Silver Clusters T. C. Devore,+J. R. Woodward, P. N. Le, J. L. Cole,* High Temperature Laboratory, Center for Atomic and Molecular Sciences, and School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332
and D. A. Dixon E . I . duPont de Nemours and Company, Central Research and Development Department, Experimental Station, Wilmington, Delaware 19898 (Received: September 23, 1988; In Final Form: June 9, 1989)
The chemiluminescent reactions of silver clusters, M,, n I 3, with ozone have been studied under multiple-collision conditions. As a function of variations in the silver flux, a minimum of five distinct electronic emissions associated with Ago, Ag,O (x 2), and Ag2 are observed as well as features that are tentatively associated with the higher silver cluster oxides. The energetics of the observed spectral features combined with supplementary thermodynamic and kinetic information demonstrate that it is unlikely that electronically excited products with enough energy to account for the observed chemiluminescence can be produced through the reaction of either Ag or Ag2 with 0 3 . The smallest cluster whose reactions can readily yield the observed Ago emission features is the trimer. The formation of Ag20* can also be achieved through reaction of the trimer; however, it may better be accounted for via the reaction of higher clusters. At moderate silver fluxes, the observed chemiluminescenceis dominated by the Ago A211-X211 (400-420 nm) and B211-X211 (320-370 nm) emission features. At higher silver fluxes, leading to greater agglomeration, both the Ago A-X and B-X emissions are quenched and the spectrum is dominated by a combination of Ag,O (x 2) and Ag2 emission features extending from 420 to 700 nm. At even higher silver flux further spectral features at X > 680 nm emerge. The chemiluminescent spectrum between 500 and 700 nm has been found to contain two distinct emission band regions which have been assigned as the A-X and B-X band systems of Ag20. The A-X transition which onsets at -630 nm is well fit by the expression v (cm-I) = 15670 - 1 6 5 ~ ~ ”O . ~ ( U ~ ” ) ~ . The B-X transition which originates at 506 nm is well fit by the expression Y ~ , ~(cm-’) ~ , = ~ 19766 ~ , ~- 4~ 4 2 ~ ~ ~~ - ”1651,’’256~3”+ 6 ( ~ 1 ”+ ) ~~ u I ” u ~ ” + 25vl“v2/1. The observed frequenciesare consistent with a nonlinear Ag-Ag-O structure, emission from the asymmetric cluster oxides characterizing these kinetically controlled oxidation experiments in contrast to the thermodynamically more stable symmetric species.
-
-
+
Introduction Much evidence now exists to indicate that the reactions of metal clusters can provide valuable insights toward an understanding of the fundamental mechanisms of surface catalysts and numerous chemical conversion processes.’ As a result, many experimental and theoretical studies are now using a variety of techniques to explore the basic properties and chemical reactivities of small metal clusters.’ The chemistry of silver derives its technological importance from its use in photography* and catalytic proces~es.~ The epoxidation of ethene is catalyzed by supported silver3 while the dehydrogenation of methanol is catalyzed by bulk silver. Hence, in order to better understand the oxidation of silver surfaces and to provide useful modeling data for the silver cluster oxides, a study of the reactions between small silver clusters and ozone was Previous publications have presented discussions of the changes in the emission spectra (AX = 280-1000 nm) observed as the optical signature of the Ag,-03 reactions varies with increasing silver flux. A kinetic and thermodynamic evaluation of the chemical reactions occurring was given.’,4 These investigations showed that, at low silver flux, the spectrum was dominated by Ago. As the metal flux increased, new features between 400 and 700 nm appeared in the spectrum and increased in intensity relative to the A g o bands. This behavior clearly showed that these bands resulted from the oxidation of silver clusters and most likely emanated from band systems associated with the silver cluster oxides. However, no attempts were made to provide a detailed analysis of the observed band systems or provide a specific indication of the product molecules carrying the spectrum. This paper presents the results of a further analysis of these metal cluster oxide band systems. Experimental Section The apparatus used for these studies has been described in detail p r e v i ~ u s i y . Silver ~ - ~ was heated in a specially designed graphite ‘Permanent address: Department of Chemistry, James Madison University.
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crucible to temperatures between 1400 and 1700 K, yielding a silver vapor pressure between 10 and 150 Pa. The silver flux was entrained in a flow of rare gas (He or Ar) at a total pressure between 10 and 65 Pa. Agglomeration in the system occurs both as a result of ( I ) the high metal flux and (2) the cooling of the silver vapor by the room-temperature entrainment gas. At a suitable point above the furnace assembly, ozone was introduced into the flow and a chemiluminescent flame was formed. The optical spectra were taken in first order with a Spex 1704 1-m scanning monochromator equipped with a 1200 groove/mm grating blazed at 500 nm and a dry ice cooled EM1 9808 photomultiplier tube. The photomultiplier signals were fed to a Keithley 41 7 fast picoammeter whose partially damped output signal drove a Leeds and Northrup strip chart recorder. All bands were measured at the peak maxima and were wavelength calibrated. The absolute positions of the band maxima are estimated to be good to f0.2 nm. Some preliminary mass spectrometric investigations were done on the Ag,-O3 system at considerably lower background pressures (Ar) than are associated with the entrainment configurations described in the present study. Results Kinetic and preliminary spectroscopic studies’s4 showed that at least two distinct band systems are produced when small silver clusters are oxidized by ozone. The stronger system centered at 460 nm was assigned at least in part to AgzO based upon its behavior with changes in metal flux as inferred on the basis of the monitored oven source temperature and the total time required for silver sample depletion (verses an effusive source yielding only metal atoms and a small concentration of silver dimer and pro( 1 ) See: Woodward, J. R.; Le, P. N.; Temmen, M.; Gole, J . L. J . Phys. Chem. 1987, 91, 2637-45, and references therein. (2) (a) The Physics of Latent Image Formation in the Silver Halides; Baldereschi, A., Czaja, W., Tosatti, E., Tosi, M., Eds.; World Scientific: Singapore, 1984. (b) The Theory of the Photographic Process; James, T. H.,
Ed.; MacMillan: New York, 1977. ( 3 ) Ethylene and Industrial Derivatives; Miller, S. A,, Benn, E., Eds.: MacMillan: New York, 1977. (4) Gole, J. L.; Woodward, J . R.; Hayden, J . S.; Dixon. D. A. J . Phys. Chem. 1985.89, 4905-08.
0 1990 American Chemical Society
Formation of Electronically Excited Ag,O
A-
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 757
x Bx = 4
X 3
2
1 I
0 1 ; ’ 1:2: 1:3:
040
600
580
6 %O
480
Figure 1. Chemiluminescent spectrum for the reaction of small silver clusters with ozone. Scan A depicts the long-wavelength end of the -440-nm system. Scan B depicts the relative intensities of the tail of the 440-nm system and the 500-nmsystem correlated with Ag,O. Scan C depicts the 500-nm band system transition region. The assignments for the first 12 bands are noted. This system displays the Ag-0 stretching, Ag-Ag stretching, and AgAgO bending frequencies (uI = 442, u2 = 165 cm-I, and u3 N 256 ( u &)). Scan D depicts the continuation of the 500-nmsystem which becomes more complex with emission to higher vibrational quantum levels and is%minated by combination bands. This system blends into the 630-nm system (E) correlated also with Ag20. Scan E depicts the 630-nm band system (correlated with Ag,O) which may extend to shorter wavelength blending with the 500-nm system. This system is associated with a long progression in the A g 2 0 bending mode (-165 cm-I). See text for discussion.
ducing no chemiluminescent emission upon reaction with 03). Further, the nature of the source flux and its apparent reactivity were studied in a preliminary fashion using mass spectrometric sampling which appeared to indicate the presence of Ag and A g o doublets as well as broader less structured peaks at -216 (Ag2), -232 (Ag,O), and -325 amu (“Ag3”), these latter peaks and the chemiluminescence increasing with an increasing noneffusive silver flux (albeit in a noisy spectrum). A second weaker system, which ranged between 500 and 700 nm, also clearly arose from cluster oxidation. However, the identity of this emitter was not considered in detail. The chemiluminescence spectrum observed between 480 and 680 nm when a higher silver flux, entrained in argon, was oxidized with ozone is depicted in Figure 1 . It is assigned to Ag,O ( x 2). This spectrum is a collage of several scans taken using a number of silver flux, carrier gas, and ozone variations. The spectrum shows a long progression of reproducible bands ranging over the entire region. The bands between 500 and 600 nm are irregularly spaced and show marked intensity fluctuations. However, at -630 nm, the spectrum simplifies to regularly spaced bands with smoothly varying intensity. This marked difference suggests that there are at least two band systems in this region. The kinetic and thermodynamic data presented indicate that they either arise from the same molecule or, at the very least, are produced from the same general set of silver molecule chemical reactions. The 630-nm System. The bands that can readily be assigned to the 630-nm system are shown in Figure 1E. This system blends with the 500-nm system at shorter wavelength so that it is difficult to determine whether some bands from this system have been obscured toward this short-wavelength limit. The frequencies and assignments for the 10 bands that can readily be assigned to this system are given in Table I. The spacing between the features is nearly constant, and all of the bands can be fit by using one ground-state vibrational frequency. A least-squares fit of the observed band positions was made using the expression given by Herzberg5
-
v =
+ Cwo:V: + Ci Ck>iX o i i u i ) U k ’ i
-
ci w o / u /
-
CxxoikffU/uk)f i k>i
(1)
where um is the band origin, the w o i s are the vibrational frequencies, the x o i k ’ s are the anharmonic constants, the u’s are the ( 5 ) Herzberg, G. Molecular Spectra and Molecular Structure. III. Electronic Spectra of Polyatomic Molecules; Van Nostrand Reinhold: New
York, 1966.
TABLE I: Spectral Assignments Made for the Observed Bands in the 630-nmSystem of Ag2W assignment band position, cm-I v,ll v2/1 v,fl obs calcb A: cm-I 0 0 0 15680 15670 -10 0 1 0 15 508 15 505 -3 15341 +7 0 2 0 15534 +I 0 15178 15 179 0 3 0 4 0 15014 15016 +2 0 5 0 14845 14855 +IO 0 14691 I4 694 +3 0 6 14535 -7 0 7 0 14 542 0 8 0 14381 14376 -5 0 14212 14217 +5 0 9 ‘All bands originate from the (O,O,O) level in the excited state. bBand positions were calculated using Y = 15670 - 165~; + O . ~ ( U ? ) ~ . A = ~~l~ - ~ , b .
vibrational quantum numbers, and the single and double primes imply excited and ground states, respectively. The least-squares fit yielded the ground-state vibrational constants given in Table
I. The shortest wavelength band that can clearly be assigned is at 15 680 cm-I, and this feature is tentatively associated with the origin. However, since this system blends with the 500-nm system, it is certainly possible that the true band origin is at shorter wavelength and has not been identified. If so, the numbering in Table I will need to be revised accordingly. However, since the observed anharmonicity is small, any change in the assignments will not have a large effect o n the determined vibrational frequency. The long progression indicates that there is a difference in geometry between the two states involved in this transition. The band splittings (165 cm-’) are consistent with the frequency expected from either a bending mode or a silver-silver stretching frequency. The regular spacings indicate that if this is a bending mode, the band system must be associated with a nonlinearnonlinear transition resulting in a significant change in bond angle. This finding is consistent with theoretical calculations for the isoelectronic Cu206molecules which indicate that both the Cu(6) (a) Devore, T. C.; Bauschlicher, Jr., C. W.; Langhoff, S. R.; Siegbahn, Per E. M.; Sulkes, M.; Gole, J. L. Formation, Electronic Spectra, and Electronic Structure of the Low-Lying Singlet States of Symmetrical Cu,O, to be submitted for publication. (b) Devore, T. C.; Bauschlicher, Jr., C. W.; Burkeholder, T.; Gole, J. L. A ComparativeStudy of the Oxidation of Atomic Copper and Higher Copper Clusters Under Single and Multiple Collision Conditions: Electronic Structure of the Asymmetric Copper Clustered Oxides, Cu,O, to be submitted for publication.
758
Devore et al.
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990
TABLE 11: A Comparison of the Calculated and Observed Band Positions for AgzO assignment band position, cm-’ c,f‘ 0; U3ff obs calc“ 0
0 0 I 1 I
2 2 2 3
3 3 4 4 4 5 5 5 6 6 7 7
8 8 8 8 9
0 0
0
1
0 0
19 766 19510 19 600 19331 19081 19 191 18906 18 662 18791 18 494 18256 18 404 18 094 17 862 18 029 17 706 17 480 17 626 17330 17 080 I6 568 17 006
1
0 0 1 0 0
1
0 0 1
1
0 0
0 0
1
1
0 0 1 0 0 1 0 0
0 0 1
0 0 2 0 0 3 4 0
1
1
2 0 0 0
0 1 2
1 1 0
19766 19508 19 600 19331 19083 I9 197 18910 18 672 18793 18 504 18261 18412 18 099 17 870 18012 17 703 17462 17 620 17331 17073 16 570 17012
E} E:;} 16334
I6 628 16 426 16 345
“The calculated positions were determined using u = 19766 - 4 4 2 ~ ~ ”
+ 6 ( ~ 1 ” -) ~256~3“+ 6uI”u3” - 165~2”+ 25Ulf’V2‘‘.
TABLE 111: Observed Spectroscopic Constants for AgzO transition T, WI” W,” A-X
B-X
-15670 -19766
442
-I63 165
Wd’
256
Cu-0 and Cu-0-Cu isomers are nonlinear with highly bent ground states. The possibility that this transition involves a Ag-Ag stretching frequency with a substantial change in the Ag-Ag bond length cannot be eliminated unequivocally on the basis of the available information. However, it seems unlikely that a large change in the Ag-Ag bond length could occur without a correspondingly significant change in the Ag-0 bond length. Thus, we favor the assignment of the 165-cm-’ frequency as a bending mode. The 500-nm System. The bands that have been assigned to the 500-nm system are depicted in Figure 1C. Unlike the 630-nm system, the bands in this system display variable spacings and fluctuating intensities, indicative of a polyatomic molecule where more than one vibrational frequency is excited. The region near 500 nm contains a distinctive repeating three-band pattern consisting of strong, weak, and intermediate intensity bands. The repeating pattern can be fit with three vibrational frequencies, -440, -160, and -250 cm-I. Assignments were made for the first 12 bands, and approximate molecular constants were determined by fitting the observed frequencies for these bands to eq 1. These frequencies were then used to estimate the positions for additional bands in the system, leading to further assignments. The molecular constants were then refined by using the additional assignments, and the process was iterated. The final set of assignments and the molecular constants derived from these are given in Tables I1 and 111. All of the observed bands can be assigned by using only three ground-state vibrational frequencies. The 442-cm-’ separation can readily be assigned as a silver oxide stretch. The 165- and 256-cm-I separations are consistent with either a Ag-Ag stretch or an Ag20 bend. The 165-cm-’ frequency agrees well with that found for the 630-nm system. This agreement strongly supports the conclusion reached from an initial kinetic analy~isI*~ that both systems arise from the same molecule.
~~
~~__.______~_.-_I____~___~_ 480
460
44.3
7
~~~
4na
420
Figure 2. Observed chemiluminescence spectrum for the oxidation of small silver clusters with ozone showing Ago, Ag,, and Ag,O ( x L 2) emission features. The numbers associated with the Agz spectrum denote sequence groupings Au = u’- u”, where u’is the upper state vibrational quantum number and u“ is the corresponding ground-state quantum number. See text for discussion.
Since the 500-nm system is overlaid by the 400-500-nm systems, there is also some uncertainty about the location of the origin band for this transition. However, there are several reasons for choosing the 506-nm band as the origin. There is a large coupling between the 442- and 165-cm-’ modes. An increase in the numbering for those features associated with u1 (440 cm-I) causes the calculated frequencies to fit poorly with the observed frequencies. For an increase of one in the numbering for u2, the value of u2 increases from 165 to -190 cm-I. Since the kinetic analysis indicates that the 630- and 500-nm systems arise from the same molecule and the 630-nm system clearly displays a lower state vibrational frequency of 165 cm-I, this data supports the present assignment. Finally, there is a significant increase in the relative intensities of the observed emission bands for X < 500 nm. This increase appears inconsistent with the intensities of the bands observed in this system. Consequently, the 506-nm band has been chosen as the origin. The 400-nm Region. The spectrum observed in the 400-5Wnm region is shown in Figure 2. At the shortest wavelengths, features attributable to the A g o A-X transition are observed. The remaining features have a deceptively simple appearance in that the bands follow a smooth progression in intensity. However, they are not regularly spaced. Rather, the splitting increases from 130 to 260 cm-’ as the system or systems progress to the red. We believe that the bands in this region must be assigned to at least two and possibly three emitters. The region between 420 and 450 nm shows spacings on the order of 150 and 195 cm-I. These are reminiscent of the observed spacings for the first excited state and the ground state of Ag2.’-I2 A comparison of the spectrum observed here to that obtained recently by Shulze et aI.l3 demonstrates that the dominant emitter in the 420-450-nm region is the silver dimer molecule. Kinetic studies and thermodynamic considerations suggest that the dimer results from the multicentered reaction of silver polymers, viz., Ag, + O3 Ag2* + A g o 02.The broad bands observed here are also, at least in part, unresolved Ag, sequence structure where the emitter would appear to be characterized by a surprisingly high internal excitation. However, the appearance of a high effective rotational temperature might also be manifest in a background spectrum resulting from an additional system underlying the clearly observable features. At wavelengths longer than 450 nm, the supplementary kinetic and mass spectrometric data that has been obtained thus far would indicate the onset of a deceptively simple optical signature for a complex region. Here both Ag2 and a relatively weakly bound Ag20 complex may contribute to the observed spectral emission. Shin-Piaw et aI.* in studying the silver dimer system have reported some complex bands between 480 and 500 nm which
-
-
+
-
-
(7) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure I V . Constants f o r Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (8) Shin-Piaw, Choong; Loong-Seng, Wang; Yoke-Sang, Lim Nature 1966, 209, 1300-1302. (9) Ruamps, J. Ann. Phys. (Paris) 1959, 4 , 1111-1157. (IO) Ruamps, J . C. R. Acad. Sei. 1954, 238, 1489. (11) Kleman, B.; Lindkvist, S . Ark. Fys. 1955, 9, 385. (12) Makeshwaii, R. C . Indian J . Phys. 1963, 37, 41. (13) Schulze, W. Unpublished data.
Formation of Electronically Excited Ag,O could not be fit using Ag, assignments. Similar bands are observed here (see Figure 1). While these bands could not be assigned with certainty, some of the band spacings are similar to those observed for the 500-nm system. If so, this implies that the emitter in this region contains a bond to oxygen since the 500-nm system provides clear evidence for an Ag-0 stretch. The origin of these 480-nm Ag,O bands is uncertain, and the system may extend several nanometers to the blue. While there is some meager evidence that they may be an extension of the 500-nm system, it is not convincing. Some bands match the positions predicted by extending the 500-nm system, but many do not fit convincingly or are blended by the Ag, bands. Hence, it appears that the region between 400 and 500 nm may contain bands arising from three molecules, A g o , Ag,, and AgxO, which are at least partially blended at this level of resolution and internal excitation. A more complete analysis is not possible at this time. Summary and Discussion A minimum of five distinct electronic emissions are associated with the products of the oxidation of small silver clusters with ozone. The region between 400 and 500 nm contains three band systems, two of which are readily assigned to A g o and Ag,. A portion of at least one additional band system has been identified in this region, with band spacings indicating that the molecular emitter contains oxygen. Because preliminary mass spectral data indicate a correlation with the growth of intensity for this emission system and mass peaks associated with Ag,O, these bands have been tentatively associated with A g 2 0 or a weakly bound Ag,O system. However, further study of this region will be necessary to verify this assignment. Two distinct band systems were observed in the region between 500 and 700 nm. The vibrational analysis of these systems supports the conclusion reached from the kinetic analysis that both transitions arise from the same molecule. Three vibrational frequencies were established for this molecule, indicating that it contains at least three atoms. The 442-cm-' frequency signals the presence of oxygen in the molecule. The similarity of the 165and 256-cm-I spacings observed for the Ag,O system in the 450-500-nm region suggests that they may arise from this molecule. Further, the observation of only three vibrational frequencies and the magnitude of the measured frequencies versus those for Ag, and A g o are consistent with the assignment of the emitter as Ag2O.I4 Like Cu20:J7J8 the nonlinear symmetric structure ( A g U A g ) is expected to be the most stable isomer observed for Ag20; however, the observed spectrum is not consistent with this assignment. The lone long progression in the 165-cm-' mode observed for the 630-nm system is consistent with the assignment of this frequency to the bending mode of this molecular emitter. This suggests that the 442- and 256-cm-' modes which accompany the excitation of the 165-cm-I mode in the 500-nm system correspond to Ago-based stretches if they are to be ascribed to the symmetric isomer.19 However, given this assumption, force
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 759
constant calculations performed at a bond angle of 105' required a stretching interaction constant approximately 60% of that for the stretching constant itself and a bending force constant larger than the stretching constant. This seems unreasonable. Further, the excitation of the asymmetric stretch mode is usually not observed for transitions between electronic states in nonlinear symmetric molecules. Thus, the symmetric Ag-O-Ag structure was not considered as a feasible source of the observed emission. Evidence is accumulating which indicates that the thermodynamically less stable asymmetric isomers can be formed during the kinetically controlled oxidation reactions of metal clusters.lJ6M Preliminary quantum chemical calculations completed for CuCu06J7 indicate that this molecule is nonlinear, suggesting that the Ag-Ag-0 isomer is also nonlinear. The observed spectra strongly imply that the 442-cm-I mode is the signature of the A g o stretch, the 165-cm-' mode corresponds to the bending mode, and the 256-cm-' mode correlates primarily with the Ag-Ag stretch. Force field calculations that assume weak mode-mode coupling do not produce force constants within the expected limits unless an acute angled structure is assumed.19 However, if a significant stretching-bending interaction is assumed, the force constants are consistent with those expected for the Ag-Ag and Ag-0 bonds in the asymmetric isomer. The large value of the uIu2 anharmonicity constant (x12)implies that a significant bend-stretch interaction characterizes the Ag,O molecule. A precise rationale for this large interaction cannot be presented; however, large anharmonic constants appear to be characteristic of other floppy metal cluster based species.,I Hence, we suggest that the 500and 630-nm band systems arise from nonlinear Ag-Ag-O. More data will be needed in order to establish further details of the structure of the Ag,O molecule. The dynamical processes that result in the formation of Ag,O have been discussed in detail p r e v i o ~ s l y .The ~ ~ ~most reasonable reactions for its formation are Ag4
+0 3
AgzO*
-
+ Ag202
Similarly, the excited states of A g o must be formed via Ag,
or, more generally Ag,
+ O3
+0 3
-
Ago*
Ago*
+ Ag20,
+ Ag,,O,
x2 3
Reactions to form Ag, have not been considered in detail although complex reactions such as Ag,
+0 3
Ag,
+ 03
-
+ AgO + Ago2 Ag2* + AgO + AgZO,
+
Agz*
could also produce the dimer. However, another contributing formation mechanism might correspond to an intermolecular energy-transfer process such as Ago*
(14) It is possible that the molecular emitter could be a larger metal clustered oxide. The molecules A g 3 0 or Ag,O, would each have six vibrational modes which could be associated in part with the observed vibrational frequencies. However, the evidence that is now apparent from several studies indicates that kinetically controlled oxidation leads to the formation of the asymmetric metal clustered oxides with a considerable reduction in symmetry. The molecular emitters are therefore expected to display additional vibrational frequency increments. Since there is no indication of these additional modes, it is mast reasonable to assign the molecular emitter as Ag,O. We adopt here a bent Ag-Ag-O structure; however, if future theoretical calculations or experimental studies should indicate the possibility of a stable ringed structure, a considerably different, but reasonable, set of assignments would be required. (1 5) All calculations used the Wilson FG method as modified by: Cotton, F. A. Chemical Applications of Group Theory, 2nd ed.; Wiley: New York, 1971. For Ag-O-Ag, the bond angle was 105" and the bond lengths were 225 pm. For Ag-Ag-O, bond angles of 180°, 120", 105'. and 66' and bond lengths of 225 pm for Ag-O and 250 pm for Ag-Ag were used. (16) Devore, T. C.; Woodward, J. R.; Gole, J. L. J. Phys. Chem. 1988, 92, 69 19. (17) Bauschlicher, C. W. Private communication. (18) Tevault, D. E. J . Chem. Phys. 1982, 76. 2859.
-+
+ Ag,
-
Ago
+ Ag2*
It is also possible that electronically excited Ag,O could predissociate to form electronically excited Ag, and a ground-state oxygen atom, the excited Ag, then emitting to yield the Ag, A-X emission spectrum. With an improved mass spectrometer system, these processes will be the subject of further study in our laboratory. Finally, we note that at even higher silver fluxes and increased entrainment cooling (dry ice cooled helium or argon') further ( 1 9) An important consideration in the analysis of the molecular electronic structure of the Cu,O and Ag,O isomeric forms is the small bond angle which characterizes the equilibrium form of both the symmetric and asymmetric cluster oxides. The situation, which appears more critical for the copper oxides, may result in a significant probability for the conversion of the symmetric to the asymmetric isomer, and vice versa. (20) Gole, J. L.; Devore, T. C.; Woodward, J. R. J . Phys. Chem. 1989, 93, 4920. (21) See for example: Morse, M. Chem. Phys. Lert. 1987, 133, 8.
760
J . Phys. Chem. 1990, 94, 760-165
Dartially resolvable features at wavelengths h > 680 nm emerge. These features which are believed to emanate from higher silver clustered oxides will also be the subject of further study in our laboratory. Conclusion The oxidation of small silver clusters, Ag, (x L 3), generates an emission signature that can be assigned to Ago, Ag,, and Ag20. Two and possibly three emission band systems have been found for Ag,O. These systems seem to have the ground state in common and vibrational frequencies have been established for this lower stale. The structure of the Ag,O ground state is yet in doubt. The observed spectra suggest that the molecule has a Ag-Ag bond. The values observed for the frequencies, the observed relative intensities, and the large value for the anharmonicity constants suggest that a ringlike, highly bent, or extremely floppy structure can represent alternate possibilities which must be considered.
Further study will be needed to assess the structural framework.
Note Added in Proof. Recently we have attempted to excite LIF spectra corresponding to the silver clustered oxide emission observed in the present study. We have used an Ar+-pumped R6G dye laser to indice transitick in the region of the Ag,O A-X and B-X band systems, Ah = 562-612 nm. Employing the dark Ag, N 2 0 oxidation reactions to form the ground-state metal cluster oxide, we have successfully excited an LIF spectrum consisting of a Au 40 cm-' sequence structure superimposed on short progressions displaying the 440- and 165-cm-' frequency separations characteristic of the spectra considered in the present study.
+
-
Acknowledgment. The authors are grateful to Prof. Schulze for providing data on the Ag, molecule prior to publication. T.C.D. thanks NSF-RUI for support during this project. J.L.G. thanks the National Science Foundation (Grant CHE-8604471); the donors of the Petroleum Research Fund, administered by the American Chemical Society; and the Eastman Kodak Co.
**Si and *'AI NMR Study of Zeolite Formation from Alkali-Leached Kaolinites. Influence of Thermal Preactivation A. Madani: A. Aznar, J. Sanz,* and J. M. Serratosa Instituto de Ciencia de Materiales, CSIC, calle Serrano 115 bis, E-28006 Madrid, Spain (Received: December 27, 1988; In Final Form: July 25, 1989)
The alkali treatment of kaolinites heated between 400 and 1000 O C has been studied by NMR (29Siand 27AIsignals), IR, and X-ray diffraction techniques. The dissolution of kaolinite samples produces AI(OH), and Si04' species which participate in the zeolite germ formation. The nature of zeolites generated (hydroxysodalite,Na-A, and tetragonal PI)varies with thermal activation of the starting silicates, but the prolonged leaching of the samples favors the transformation of initially formed phases into the most stable PI zeolite. The synthesis of this compound (Si/AI > 1) requires the previous polymerization of dissolved silica species, and the amount of incorporated Si depends on the temperature of pretreatment of kaolinite.
Introduction Numerous works referring to zeolite synthesis have been published in which the influence of experimental conditions, chemical composition of reactants, and type of ions used as exchangeable cations has been analyzed.] In particular, it has been demonstrated that the alkalinity, the temperature, and the reactivity of the silicon source affect the dispersion of silica and as a consequence the nature of the aluminosilicate precursor species., The induction period associated with the formation of each zeolite depends not only on the experimental conditions but also on kinetic aspects related to the nucleation process. In the synthesis of zeolites from alkali-leached aluminosilicates, nuclei corresponding to different phases can be simultaneously present in the reactional medium and those less stable thermodynamically are substituted by the more stable ones as the reaction p r o g r e ~ s e s . ~ - ~ In a previous work on thermal decomposition of kaolinite6 we have shown that the dehydroxylation of the kaolinite is completed at 750 OC, the partial breaking of the tetrahedral sheet takes place at 850 "C, and the segregation of amorphous silica occurs at 980 OC. The aim of this work is to study the influence that the thermal treatment of the kaolinite has on the nature of zeolites obtained during the alkali attack of this aluminosilicate. With this purpose, IR and X-ray diffraction techniques have been used to identify the new phases and to follow the progress of their formation. NMR spectroscopy (29Siand 27Alsignals) has allowed monitoring of the local transformations in the environment of these two nuclei and characterization~ofthe atomic associations in the liquid phase that precede the formation of zeolite nuclei. 'Present address: E.N.S. Zarzouna, Bizerte 7021, Tunisia.
0022-3654/90/2094-0760$02.50/0
Techniques and Experiments Samples. A kaolinite with a high purity (Caobar, Spain) has been used as starting material. Dehydroxylated form of kaolinite (metakaolinite) has been obtained by heating the sample at 750 OC for 24 h. In order to modify the silicon source reactivity, two samples of kaolinite heated 2 h at 850 and 980 "C were prepared. In alkali treatment of kaolinites, concentrations of NaOH solutions were chosen in order to have Na2O/AI2O3 ratios close to 2.5. Temperature of reaction was maintained constant at 90 f 0.1 "C by a proportional temperature controller. Liquid and solid phases in each experiment were separated by filtration, and the solid phase was then dried at 160 OC during 3 h. Techniques. High-resolution 29Siand 27AlMAS-NMR spectra of powdered samples were recorded at 79.5 and 104.3 MHz, respectively, by spinning the sample at the magic angle 5 4 O 44' in a Bruker MSL-400 spectrometer equipped with a FT unit. The spinning frequency was in the range of 4000-5000 cps. Crosspolarization and proton decoupling were not used. A time interval (1) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. (2) Aiello, R.; Collela, C.; Sersale, R. Molecular Sieves; Advances in Chemistry Series 101; American Chemical Society: Washington, DC, 1971;
p 51. (3) Aiello, R.; Collela, C.; Sersale, R. Molecular Sieves; Advances in
Chemistry Series 101; American Chemical Society: Washington, DC, 1971; p 102. (4) Flanigen, E. M. Molecular Sieues; Advances in Chemistry Series 121; American Chemical Society: Washington, DC, 1973; p 119. (5) Zhdanov, S. P. Molecular Sieves; Advances in Chemistry Series 101; American Chemical Society: Washington, DC, 1971; p 20. (6) Sanz, J.; Madani, A.; Serratosa, J. M.; Moya, J. S.;Aza, S. J . Am. Ceram. SOC.1988, 71, (2-418.
0 1990 American Chemical Society
