J. Phys. Chem. 1983, 8 7 , 3441-3445
measured q. Lack of resolution of the spectral contribution from inequivalent CH bonds in the halo-substituted benzenes indicates that all of the CH bonds have similar lengths. Studies of the gas-phase overtone spectra of substituted benzenes and fully optimized geometry calculations using split valence basis sets should help in the identification of inequivalent CH bonds, and provide more information for the analysis of the various substituent effects.
Acknowledgment. We are grateful to the National Sciences and Engineering Research Council of Canada for
3441
financial research support. K.M.G. is also grateful to the University of Manitoba for a fellowship. Registry No. C$&F, 462-06-6; C&CI, 108-90-7;C&&, 108-86-1;C6H51,591-50-4;C6H5NO2,98-95-3;1,2-difluorobenzene, 367-11-3; 1,3-difluorobenzene,372-18-9; 1,4-difluorobenzene, 540-36-3; 1,2-dichlorobenzene,95-50-1; 1,3-dichlorobenzene, 541-73-1; 1,4-dichlorobenzene, 106-46-7; 1,2-dibromobenzene, 583-53-9; 1,3-dibromobenzene,108-36-1; 1,4-dibromobenzene, 106-37-6;1,3,5-trifluorobenzene,372-38-3;1,3,5-trichlorobenzene, 108-70-3;1,2,4-trichlorobenzene,120-82-1;1,3,5-tribromobenzene, 626-39-1;1,2,4,5-tetrachlorobenzene,95-94-3;pentafluorobenzene, 363-72-4.
Secondary Ion Mass Spectrometry of Metal Halides. 3. Ionic Radii Effects in Alkali Halide Clusters Thomas M. Barlak,+Joseph E. Campana, Jeffrey R. Wyatt, and Rlchard J. Colton" Naval Research Laboratw, Chemistry Dlvlsbn, Washlngton, D.C. 20375 (Received: October 4, 1982; I n Flnal Form: Merch 9, 1983)
Secondary ion mass spectrometry (SIMS) results for the alkali halides (MX) show the emission of intense [M(MX),]+cluster ions. The anomalous ion intensity behavior at n = 13-15,22-24,37-39, and 62-64 corresponds to the formation of stable "cubiclike" structures. This report presents experimental evidence for ionic radii effects on the stability of the sodium halide cluster ions. These SIMS results show that (1)the cluster ion intensity decreases over several orders of magnitude as n (the cluster size) increases, (2) the slopes of the cluster ion intensity distributions are a measure of the relative stability of the cluster ions and can be correlated to the size of the anion, (3) an enhanced ion intensity at n = 4 corresponding to the square-planar 3 X 3 X 1structure occurs when the ionic radius of the anion is small, and (4) bombarding the surface with xenon primary ions enhances certain structural features of the ion intensity distribution. Theoretical calculations concerning the stability of cluster ions are compared with measured SIMS ion intensities.
Introduction The positive ion spectra of the alkali halides (MX), obtained by secondary ion mass spectrometry (SIMS) or fast atom bombardment (FAB), show numerous cluster ions of the form [M(MX),]+ for n < 100. we have recently reported the SIMS/FAB spectra of the alkali using SIMS/FAB instruments with exceptional high-mass ~apabilities.~7~ For example, the SIMS instrument (a modified, 20-year-old, double-focusing mass spectrometer) has detected ions up to mass-to-charge ratio (m/z) 18320, i.e., [ C S ( C ~ I ) ~ and ~ ] + the ; ~ FAB instrument (a state-ofthe-art, reverse-geometry mass spectrometer) has detected ions up to m/z 25854, Le., [ C S ( C S I ) ~ ] + . ~ General features of the [M(MI),]+ cluster ion distributions are as follows: (1)an ion intensity which decreases pseudoexponentially with increasing cluster number, n; (2) an anomalous intensity behavior at n = 13-15 and 22-24 for NaI, KI, RbI, and CsI' and at n = 37-39 and 62-64 for Cs13where the cluster ions at n = 13,22,37,and 62 have enhanced intensities and the cluster ions immediately following them (Le., n = 14 and 15,23 and 24, etc.) are absent or have dramatically reduced intensities; (3) the notable ion intensity enhancements at n = 13,22,37, and 62 which are attributed to the formation of stable 3 X 3 X 3, 3 X 3 X 5, 3 X 5 X 5, and 5 X 5 X 5 "cubiclike" structures, re~pectively,'-~ and (4) the ion intensity enhancements at n = 6,9, and 12 which are attributed to the formation of stacked-hexagonal ring structures.' In addition, we have confirmed that variations in the ion in-
'
+ Presently
with Geo-Centers, Inc., Suitland, MD 20746.
tensity distributions, particularly in the anomalous regions, originate from unimolecular decompositions of the unstable cluster ions, e.g., n = 14 and 15 cluster ions decompose to the n = 13 cluster Our results therefore indicate that the most stable alkali halide cluster ion configurations correspond to cubiclike structures that are reminiscent of the bulk s t r u c t ~ r e This . ~ ~ ~conclusion is further supported by theoretical developments in our laboratory based on statistical and cluster surface energy consideration~.~J-~ As previously noted1p3an enhanced ion intensity for the n = 4 species which is postulated to have the planar 3 X 3 X 1 structure was not observed for the alkali iodides. Rabalais and co-workers,1° however, have observed a relatively strong signal for the [Na(NaF),]+ ( n = 4) cluster ion compared to n = 3 or 5; and Martin's calculations" ~~~
~-
(1)Barlak, T. M.;Campana, J. E.;Colton, R. J.; DeCorpo, J. J.; Wyatt, J. R. J . Phys. Chem. 1981,85,384C-4. (2)Campana, J. E.;Barlak, T. M.; Colton, R. J.; Decorpo, J. J.; Wyatt, J. R.; Dunlap, B. I. Phys. Rev. Lett. 1981,47,1046-9. (3)Barlak, T. M.;Wyatt, J. R.; Colton, R. J.; DeCorpo, J. J.; Campana, J. E.J . Am. Chem. SOC.1982,104,1212-5. (4)Barlak, T. M.;Campana, J. E.; Wyatt, J. R.; Dunlap, B. I.; Colton, R. J. Znt. J. Mass Spectrom. Zon. Phys. 1983,46,523-6. (5)Colton, R.J.; Campana, J. E.; Barlak, T. M.; DeCorpo,J. J.; Wyatt, J. R. Rev. Sci. Instrum. 1980,51,1685-9. ( 6 ) Campana, J. E.; Colton, R. J.; Wyatt, J. R.; Bateman, R. H.; Green, B. N., submitted for publication. (7) Dunlap, B. I. Surf. Sci. 1982,121,260-74. (8)Dunlap, B. I.; Campana, J. E.;Green, B. N.; Bateman, R. H. J. Vac. Sci. Technol. 1983,Al, 432-6. (9)Dunlap, B. I.; Campana, J. E.,submitted for publication. (10)Honda, F.; Lancaster, G. M.; Fukuda, Y.; Rabalais, J. W. J. Chem. Phys. 1978,69,4931-7.Taylor, J. A,; Rabalais, J. W. Surf. Sci. 1978,74, 229-36. (11)Martin, T. P.J. Chem. Phys. 1980,72, 3506-10.
This article not subject to U S . Copyright. Published 1983 by the American Chemical Society
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Barlak et al.
The Journal of Physical Chemistry, Vol. 87, No. 18, 1983
In
0.0
=
INTENSITY OF [ M iMII,]'
Ar' BOMBARDMENT 4.0 keV
1
*
I 1
l
I
I
I
l
I
l
3
5
7
9
11 13 15 n
l
1
1
17 19 21
1
Nal KI Rbl 13.4 keV) Csl
1-
a h
1
23 25
1
1
l
!
1
3
5
7
9
l
,
l
.
11 13 15 17
l
/
!
l
19 21 23 25
n
Flgwe 1. Plot of the log of relative cluster ion intensities of the sodium halides vs. the cluster number, n , during Xe+ ion bombardment.
Figure 2. Plot of the log of relative cluster ion intenstties of the alkali iodides vs. the cluster number, n , during Ar' ion bombardment.
showed that [Na(NaCl),]+ "has a highly symmetric planar form which turns out to be unusually stable". We how have experimental evidence showing that the relative stability of certain cluster ions (includingthe n = 4 species) is dependent on the ionic radii of the constituent atoms. SIMS data from sodium halides (NaX) and other alkali iodides (MI) are used to evaluate the ionic radii effects. In addition, cluster ion intensities (stabilities) are compared to Martin's theoretical calculation of cluster ion dissociation energies."J2
TABLE I: Ionic Radii Effects o n Cluster Ion Stability
alkali halides
ionic radius,a A
uo,
slope of SIMS intensity distributionC
r+
r-
kcal/mol
k,
k,
NaF NaCl NaBr NaI
0.95 0.95 0.95 0.95
1.36 1.81 1.95 2.16
-214.4 -182.6 -173.6 -163.2
-2.6 -3.0 -3.7 -4.2
-2.8 -3.2 -3.8 -4.4
Li I NaI KI RbI CSI
0.60 0.95 1.33 1.48 1.69
2.16 2.16 2.16 2.16 2.16
Experimental Section The experimental conditions and instrumentation have been described elsewhere'" and apply here unless otherwise noted. Xenon instead of argon was used as the primary ion for all sodium halide experiments, because it provides a higher secondary ion yield.13 The xenon primary ion energy and current density were 4.0 keV and 1 X loi, A/cm2, respectively. Isotopically enriched salts (Oak Ridge National Laboratory) were used to obtain monoisotopic mass spectra. NaCl was 99.35% enriched in 36Cl and NaBr was 98.61% enriched in 79Br.
-177.7 -5.7 -6.6 -163.2 -4.3 -4.5 -149.9 -4.1 -4.3 -144.9 -5.0 -5.4 -142.4 -5.6 -6.2 a After ref 16. Lattice energy. Data from various tables in ref 17. Slope of SIMS intensity distribution for NaX data (in Figure 1 for n = 2-13 formed by Xe' ion bombardment) and MI data (in Figure 2 for n = 2-13 ( n = 1-3 for Lil) formed by Ar+ ion bombardment). Slopes k, and k, are from log I vs. log n and log I vs. log m (mass), respectively. The correlation coefficient for the fit is >0.95.
Results and Discussion Ionic Radii Effects on Cluster Ion Stability. The secondary ion intensity distributions of the [M(MX),]+ cluster ions for the sodium halides and the alkali iodides are shown in Figures 1 and 2, respectively. The relative ion intensities, log I,,, are normalized to n = 1 ( n = 2 for NaF) and plotted against the cluster number n. Although the absolute or relative secondary ion yields were not determined in this experiment, we found for the sodium halide data that NaI is the most emissive with typical ion count rates of 2 X lo6 cps for the n = 2 species while NaBr is the least emissive with ion count rates of 2 X lo4 cps for the n = 2 species. The sodium halide (Figure 1)data show the same general cluster ion intensity distributions observed previously for the alkali iodides;1-3 i.e., enhanced cluster ion intensities occur for n = 13 and 22 corresponding to stable cluster ions with 3 X 3 X 3 and 3 X 3 X 5 cubiclike structures, respectively, followed by cluster ions with decreased ion intensity. Other ion intensity enhancements are also noted
in Figure 1 for the n = 4 and 7 species of NaF. The secondary ion intensity distribution can be approximated by a log-log fit of the data. Table I gives the slopes, k,,and k,, of the fit of the intensity data (plotted as log I,, vs. log n (cluster number) or log I,, vs. log m (cluster mass)) in Figures 1and 2 for the n = 2-13 species. The correlation coefficient for the fit is typically >0.95. As shown in Table I the slopes of the distributions increase in the order NaF < NaCl < NaBr < NaI and remain relatively steep for all of the MI data. The ion intensity distributions for LiI and CsI give the steepest slopes. Since the ion intensity distribution falls off differently for each of the alkali halides even when the distributions are plotted against the cluster mass, the differences are taken to be real and not an artifact of the spectrometer transmission. The secondary ion intensity distribution can be correlated to several properties of the alkali halides since it is well-known in SIMS that the secondary ion intensity is directly proportional to the secondary ion yield and sputtering yield,14and from sputtering theory that sput-
(12)Martin, T. P.J. Chem. Phys. 1978,69, 2036-42. (13)Benninghoven, A.; Bispinck, H.; Mod. Phys. Chem. 1979, 2, 391-421. Staudenmaier, G.Radiat. Eff. 1972,13, 87-91.
(14)Blaise, G. In 'Material Characterization Using Ion Beams"; Thomas, J. P., Cachard, A., Eds.;Plenum Press: New York, 1978;pp 143-238.
Secondary Ion Mass Spectrometry of Metal Halides
tering yields increase as the lattice or binding energy of the material decreases.15 In addition, since the size of the constituent atoms directly affects the lattice or binding energy of the compound, the ionic radii can also be correlated with the relative intensity and stability of the cluster ions. In this paper we shall examine the effect that these properties have on the stability of the alkali halide cluster ions; but first, we will review their effect on the properties of the bulk crystals. The arrangement of monovalent ions in a crystal depends on the relative sizes of the ions.16 As cations are generally smaller than anions, the crystal structure is usually determined by the number of anions which can be packed around the smaller cation. I t is the anion-anion repulsion effect then which largely determines the structure and relative stability of the crystal (and cluster ions). These repulsive forces are known to increase (1)as the size of the anion increases, (2) as the size of the cation decreases, and (3) as the coordination number increases. For the alkali halides, therefore, the anion-anion repulsion is at a maximum for the alkali iodides, especially LiI, and at a minimum for the alkali fluorides. The stability of the ionic lattice is determined from the Coulombic interactions among the ions. The interactions consist of the strong Coulombic attractive forces between oppositely charged ions and the strong repulsive forces that occur when the ions get too close to each other. Summing these energy components together with two weaker interactions-the van der Waals force and the zero-point energy of the crystal-gives the lattice energy of the crystal. In general terms, the lattice energy of the crystal is a measure of the binding energy or net stability of the crystal. Table I lists the experimentally determined lattice energies" for some of the alkali halides. The alkali fluorides have higher lattice energies compared to the alkali iodides since in fluorides the anion-anion repulsion effects are minimal due to the smaller anion size. The slope of the seconary ion intensity distribution becomes steeper (or the relative stability of the cluster ions decreases) as the lattice energy decreases. For example, NaF, which has the highest lattice energy in the series studied, yields the most stable cluster ions as is evident from their higher relative ion intensity. As the size of the anion increases from F to I-, the anion-anion repulsion increases dramatically, thereby destabilizing the cluster. The net loss in cluster stability causes the relative ion intensity to drop rapidly when going from NaF to NaI (Figure 1). In fact, the steep slopes of the secondary ion intensity distribution for all of the alkali iodides (Figure 2) demonstrate their overall lower relative stability. Figures 1 and 2 also show a change in the relative intensity (stability) of certain cluster ions. For example, ion intensity enhancements at n = 4 and 7 are evident particularly for the NaF data in Figure 1. In fact, the ion intensity ratio 14/13ranks the relative stability of the [Na(NaX),]+ cluster ion as F > C1> Br > I, which is the same ranking discussed above for the overall cluster ion stability. Again, the n = 4 cluster ion for NaF is particularly stable because of the minimal anion-anion repulsion and the higher lattice energy associated with the structure of the cluster ion. Futhermore, due to its enhanced intensity, the [Na(NaF)4]+cluster ion is assigned the (15) Behriseh, R.,Ed. 'Sputtering by Particle Bombardment I"; Springer-Verlag;New York, 1981; Top. Appl. Phys. Vol. 47. (16) Pauling, L.'The Nature of the Chemical Bond", 3rd, ed.;Comell University Press: Ithaca, NY, 1960. (17) Tosi, M. P. Solid State Phys. 1964, 16, 1-120.
The Journal of Physical Chemistty, Vol. 87, No. 18, 1983
to,6t
3443
o'8 06
x
04
CsF
0
I 02Csl 0.0O O10 .10
KI
I
I
I
I
12
14
16
18
I
I
20
22
I/ao-
Figure 3. Plot of the [M(MX),]+ cluster ion intensity ratio, 1,113, for the alkali halides vs. the reciprocal of their respective lattice spacing llao.
square-planar 3 X 3 X 1 structure which is the smallest member in the cubiclike structure series previously proposed for the cluster ions.'J Likewise, the intense cluster ion signal at n = 7 for NaF is assigned the planar 3 X 5 X 1 structure. To summarize this part of the discussion, we conclude from the above results and discussion that the slope of the secondary ion intensity distribution is a measure of the relative stability of the cluster ions and that the size of the ions (particularly the anions) directly affects the stability of the cluster ions. In addition, the NaF data show an enhanced ion signal for the n = 4 cluster ion which is assigned the planar 3 X 3 X 1structure. Figure 3 is a plot of the ion intensity ratio 14/13 for the alkali halides against the reciprocal lattice spacing, l/ao, of the crystal. The l/ao term is proportional to the lattice or binding energy of the crystal which increases as a. (or the M-X bond distance) becomes small. The plot clearly shows the added stability of the [Na(NaF),]+ cluster ion compared to the others and how the stability of the ions correlates with the anion size and M-X distance. Primary Ion Beam Effects o n Cluster Ion Formation. It is well-known in SIMS that, as the mass of the bombarding ion increases, the secondary ion yield will also increase.13 This increased yield has led to several significant results in our SIMS study of the alkali halide cluster ion^.^^^ For example, the ion intensity distributions of [Cs(CsI),]+ cluster ions obtained with three different primary ions are shown in Figure 4. The primary ion beams consist of 4.0-keV Xe+, Ar+, and Ne+ ions, each beam with a current density of -1 x A/cm2. From Figure 4, it is readily apparent that the nature of the primary ion beam influences the overall appearance of the secondary ion intensity distribution. The ion intensity
3444
Barlak et ai.
The Journal of Physical Chemistry, Vol. 87, No. 18, 1983 [CS
6.0r
00
(CSll,]+
,
I
I
1
1
[ M lMll,li Xe+ BOMBARDMENT
1
c\ h
I
'I 2.0
-c
3.0
(3
9 4.0 50+ Nal 14.0 keV1
1
l
l
l
l
l
l
l
l
1
3
5
7
9
11
13 n
15
1
17
1
I
I
19 21 23 25
Figure 4. Plot of the log of secondary ion intensity distribution for [Cs(CsI),]+ cluster ions for Xe+, Ar+, and Ne+ ion bombardment.
5
9
13
alkali iodide NaI KI RbI CSI NaI CSI
16/Is 0.72 1.07 0.83 0.85 0.70 1.58
19118
0.46 0.71 0.85 0.95 0.53 1.04
'25
29
33
37
41
Figure 5. Comparison of the secondary ion intensity distributions for CsI and NaI cluster ions under Xe+ ion bombardment.
TABLE 111: Calculated Energy Barriers (in e V ) for (NaCl)' Neutral and Na' Ion Emission from [ Na(NaI),]+ Cluster Ionsa
TABLE 11: Cationic Radius Effects on Cluster Ion Stability primary ion Ar' Ar' Ar' Ar' Xe' Xe'
21 n
17
D
A (NaCl);
I,,lIl, 0.64 0.56 0.56 0.54
0.76 1.00
distribution obtained with Ar+ and Ne+ ions falls off much more rapidly than the distribution obtained with Xe+ ions. The higher ion yield caused by Xe+ ion bombardment allowed measuring the distribution of [Cs(CsI),]+ cluster ions to n = 70 with Xe+ ion b ~ m b a r d m e n t .These ~ differences in the ion intensity distributions are undoubtedly the result of the different momentum and collisional cross section of the primary ions. The data in Figure 4 also show that the more massive primary ions tend to enhance certain structural features of the ion intensity distribution. For example, the cluster ions at n = 6,9, and 12 (which correspond to the proposed stacked-hexagonal rings with an extra cation positioned above or within the hexagonal structure') are more pronounced when Xe+ primary ions are used. Table I1 lists the ion intensity ratios &/I5,&/Is, and Ilz/Ill for the alkali iodides as a function of the alkali metal. Although the ion intensity ratios do not show an overwhelming correlation with the size of the cation, the n = 9 cluster ion does consistently increase in intensity as the cation gets larger. This phenomenon is illustrated by the data in Figure 5 which show intense ions at n = 6,9, and 12 only in the CsI data. These observations suggest that the cluster ions at n = 6,9, and 12 are apparently more stable when the M-I bond distance is large (unlike what we observed for the n = 4 planar structure). The data in Figure 5 also show some subtle differences in the ion intensities within the anomalous intensity regions, n = 13-16 and n = 22-25. First of all, the increased secondary ion yield produced by Xe+ ion bombardment allows us to observe the cluster ions (particularly at n = 14,15and 23,241 which are below detectable levels with Ar+ primary ions. Second, the NaI data in Figure 5 clearly show that the ion intensity ratios Z16/Z14 and 124/Z23 are LO. A possible explanation for this observation is that the higher intensity
1 2 3 4 5 6
7 8
5.75 13.01 20.50 28.03 35.15 43.07 50.25 57.97
B [ Na( NaCl),]'
C Na+ + E Na+ C1(NaCl)' emission emission emission
7.75 15.03 22.51 30.29 37.39 44.90 52.65 60.32
2.00 2.02 2.01 2.26 2.24 1.83 2.40 2.35
7.28 7.48 7.78 7.10 7.51 7.75 7.67
2.00 1.53 1.73 2.03 1.35 1.76 2.00 1.92
a A : Energy required to dissociate neutral clusters into gaseous ions, ''J' B: Energy required to dissociate charged clusters into gaseous ions.lz C: Energy for Na' ion emission found by subtracting column A from column B. D : Energy for Na' ion t C1- ion emission found by subtracting the n - 1value from the n value in column B. E : Energy for (NaCl)" emission found by subtracting the NaCl binding energy from column D data.
for the n = 15 species in the CsI data may correspond to a 5-stacked hexagonal ring structure which is stable only when M-I distances are large as for CsI. Dissociation of Cluster Ions. Martin1'J2 has performed extensive energy calculations for the ion and neutral clusters of NaC1. The most energetically favorable configurations for [Na(NaCl),]+ cluster ions have been determined for n = 1-8. Martin suggests that particularly stable cluster ions are observed when energy barriers prevent unimolecular dissociation via the emission of ions, atoms, or simple molecules. We have used the magnitude of the energy barriers derived from Martin's work to correlate our experimental cluster ion intensities to his predictions. The two unimolecular dissociation channels are (1) Na+ ion emission [Na(NaCl),]+
-
-
(NaCl),
+ Na+
(1)
and (2) (NaC1)O molecular emission [Na(NaCI),]+
[Na(NaCI),-J+
+ (NaC1)O
(2)
Table I11 contains the data relevant to the energy barrier calculations. The SIMS experimental data for [Na(NaCl),]+ cluster ion are correlated in Figure 6 with the predicted energy (18) Bruner, P.; Karplus, M. J. Chem. Phys., 1973,58, 3903.
J. Phys. Chem. 1983, 87, 3445-3450
3445
barriers is to the right of the plot. As shown in Figure 6, the predicted energies follow closely the experimental results. In particular, the energy calculations predict a higher stability for the n = 4 cluster ion which we have shown to be stable.
0
LOG In + 11, VALUES (THIS WORK)
0
LOG I, + ,/I, VALUES (FROM HONDA, ET. AL., J. CHEM. PHYS., 1978,
-1.4
21 1
413
615
Conclusions The sodium halide cluster ion intensity distribution exhibits the general features noted earlier for the alkali iodides. We found that the slope of the secondary ion intensity distribution is a measure of the relative stability of the cluster ions and can be correlated to the size of the ions (particularly the anion). In addition, the NaF data show an enhanced ion signal for the n = 4 cluster ion which is assigned the square-planar 3 X 3 X 1 structure. Finally, the comparison of the theoretical results of Martin with our SIMS data shows that the probable dissociation mechanism of the cluster ion is through the emission of (NaC1)O neutrals.
817
(n + l l l n
Flgure 6. Correlation between the SIMS experimental data for [Na(NaCI),]' cluster ions and the predicted energy barriers for (NaCI)' neutral emission.
barriers for (NaC1)O neutral emission (Table 111, column E). The scale for log of the ion intensity ratio In+l/In is shown on the left for our SIMS results and for the results of Honda et al.'" The energy scale for the predicted energy
Acknowledgment. We thank our co-workers Brett Dunlap, for his many helpful discussions, and Steven Schneider, for technical assistance. T.M.B. thanks the National Research Council for support as a Resident Research Associate. Registry No. NaF, 7681-49-4;NaC1,7647-14-5;NaBr, 764715-6;NaI, 7681-82-5;LiI, 10377-51-2;KI, 7681-11-0;RbI, 7790-29-6;
CSI,7789-17-5.
Low Nuclearlty Sllver Clusters in Faujasite-Type Zeolites: Optical Spectroscopy, Photochemistry, and Relationship to the Photodlmerization of Alkanes Geoffrey A. Ozln,' Francols Hugues, Saba M. Mattar, and Douglas F. McIntosh Lash Miller Chemistry Laboratories, UniversiW of Toronto, Toronto, Ontario, Canada M5S 1A 1 (Received:November 2, 1982)
A diffuse optical reflectance spectroscopicinvestigation of low nuclearity silver clusters generated and trapped within the cavities of silver ion exchanged X and Y zeolites is reported. The present study focuses attention on a small cluster cation Agnq+ in which n is thought to be in the range of 5 to 13 and which probably resides on the wall of the zeolite supercage. A UV-induced (220-300 nm) intrazeolitic phototransformation of Ag,q+ is found to occur and to be thermally reversible in an argon atmosphere but irreversible in a methane atmosphere. These effects are discussed in terms of a UV-photogeneratedV center and concomitant reduction of the Agnq+ cluster and are related to the recently observed photodimerization of alkanes on silver-loaded Y zeolites.
Introduction A knowledge of the electronic and geometrical properties, site locations, and support interactions of atomically dispersed metal and small neutral or charged metal cluster guests in the lattice confines of zeolites is of paramount importance for understanding the performance of these systems in catalysis.' Silver has been widely studied in (1)(a) Kh. M. Minachev and Ya. I. Isakov in "Zeolite Chemistry and Catalysis", J. A. Rabo, Ed., American Chemical Society, Washington, DC, 1976,ACS Monograph No. 171,p 552. (b) P.A. Jacobs, "Carboniogenic Activity of Zeolites", Elsevier, Amsterdam, 1977. (c) J. B. Uytterhoeven, Acta Phys. Chem., 24,53 (1978). (d) P. Gallezot, Catal. Rev. Sci. Eng., 20, 121 (1979).
this connection.2 Silver species encaged in A, X, and Y zeolites have recently been the focus of considerable structural, spectroscopic, and chemical attention. Uytterhoeven et al.3have shown by X-ray crystallography that (2)(a) Y. Kim and K. Seff, J. Am. Chem. Soc., 100,175 (1978). (b) J. Phys. Chem., 82,1307(1978). (c) K. Tsutsumi and H. Takamashi,Bull. Chem. SOC.Jpn., 45,2332(1972). (d) P.A. Jacobs, J. B. Uytterhoeven, Faraday Trans. 1,73,1755 (1977);75, and H. K. Beyer, J. Chem. SOC., 109 (1979). (e) M. Narayana, A. S. W. Li, and L. Kevan, J.Phys. Chem., 85,132(1981).(0 M. Narayana and L. Kevan, J. Chem. Phys., 76,3999 (1981). (9) A.Abou-Kais, J. C. Vedrine, and C. Naccache, J. Chem. Soc., Faraday Trans. 1, 74,959 (1978). (h) N. Giordano, J. C. Bart, and R. Maggiore, 2.Phys. Chem., 124,97(1981). (i) M. Iwamoto, T. Mashimoto, T. Hamano, and S. Kagawa Bull. Chem. SOC.Jpn., 54, 1332 (1981).
0 1983 American Chemical Society
