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
0
-1.4
21 1
413
LOG In + 11, VALUES (THIS WORK) LOG I, + ,/I, VALUES (FROM HONDA, ET. AL., J. CHEM. PHYS., 1978,
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
3446
Ozin et al.
The Journal of Physical Chemistry, Vol. 87, No. 18, 1983
hydrated silver-exchanged zeolite A, when subjected to a mild vacuum thermal treatment, leads to the linear cluster cation Ag32+located in the sodalite cage. Following a thermal treatment at higher temperature, these trinuclear clusters dimerize to Ags4+which are also located in the sodalite cages. Completely silver-exchanged Y and X zeolites, when submitted to dehydration a t elevated temperature and to a subsequent oxygen treatment, were shown by X-ray crystallography to contain the clusters cations Ag2X+and linear Ag3Y+,arising from the simultaneous occupancy of a site I and one or two adjacent site
TABLE I
designation Ag,,Na-Y Ag,Na-Y Ag,Na-Y Ag,,Na-Y Ag,,Na-Y Ag,,Na-Y Ag,Na-X Ag,,Na-X
degree of Ag' per exchange, % unit cell
0.5 5.5 11.0 23.5 63.5 100.0 7.0 100.0
0.3 3.0 6.0 13.0 35.0 55.0 6.0 86.0
7% Ag w t in t h e d r y zeolite
0.25 2.5 5.0 10.0 24.0 34.0 5.0 45.0
r.4
Hermerschmidt and Haul5 reported the observation of a seven-line ESR spectrum following the low-temperature hydrogen reduction of silver-exchanged zeolite A which is attributed to a Ag6" cluster cation. The isotropic ESR spectrum obtained with g N 2 indicates the equal coupling of a single unpaired electron to six equivalent silver centers, consistent with an 2A1, electronic ground state for an octahedral Agz+cluster cation. This species could be related to Zeff s X-ray structural report of a perfectly octahedral Ag6 c1uster2a~b and the Ag64+cluster mentioned earlier,3 both entrapped within the sodalite cages of zeolite A. An optical reflectance investigation of the autoreduction of partially silver-exchanged zeolite Y by Kellerman and Texter6 led to the observation of UV and visible adsorptions whose growth behavior and energies appeared to be charateristic of isolated silver atoms and low nuclearity silver clusters encaged in the cavities of the zeolite. Similar diffuse reflectance studies3 on silver-exchanged zeolite A led to the identification of optical transitions of yellow and red Ag?+ and Ag64+,respectively. In both of these studies, silver atom and cluster absorptions were assigned by comparison with the known optical spectra of neutral Ag, ( n = 1-6) in rare gas solids.' The chemical properties of neutral and charged silver cluster guests in zeolites are particularly interesting as they have been implicated in the photochemical/thermal cleavage of water to H2 and 02* and more recently in the photodimerization of alkane^.^ The photoheterogeneous conversion of C,H2n+2to C2,H4,+2 is especially intriguing, because photoexcited copper and silver atoms, as well as small clusters, have recently been found to selectively activate the carbon-hydrogen bond of alkanes in lowtemperature matrices.'O In an earlier paper1' we presented our spectroscopic findings for Ag,*+ located in site I-site I' of silver-loaded X and Y zeolites (where n = 1, q = 0; n = 2, q = 1; n = 3, q = 2). As an extension of this work, we now present our optical spectroscopic and photochemical observations for higher nuclearity silver cluster cations located in the (3) C. R. Gellens, W. J. Mortier, R. A. Schoonheydt, and J. B. Uytterhoeven, J. Phys. Chem., 85,2783 (1981). (4)L. R. Gellens, W. J. Mortier, and J. B. Uytterhoeven Zeolites, I, 11 (1981). ( 5 ) D. Hermerschmidt and R. Haul, Ber. Bumenges. Phys. Chem., 84, 902 (1980). (6)R. Kellerman and J. Texter, J. Chem. Phys., 70,1562 (1979). (7)G.A. Ozin,Faraday Symp. Chem. Soc., 14,7(1981),and references cited therein. (8)(a) S. Leutwyler and S. Schumacher, Chimia, 31,475(1977). (b) P. A. Jacobs, J. B. Uytterhoeven, and H. K. Beyer, J . Chem. SOC.,Chem. Commun., 128 (1977). (9)G. A. Ozin and F. Hugues, J. Phys. Chem., 86,5174 (1982). (10)G. A. Ozin, D. F. McIntosh, S. Mitchell, and J. Garcia-Prieto, J. Am. Chem. Soc., 103, 1574 (1981);G.A. Ozin, "Dynamic Processes of Metal Atoms and Small Metal Clusters", in 'Chemistry in the 21st Century" M. Chisholm, Ed., American Chemical Society, Washington, DC, 1982,ACS Symposium Series; G. A. Ozin, S. Mitchell, and J. Garcia-Prieto, Angew. Chem., Int. Ed., SuplZ., 798 (1982);M. J. Parnis, G. A. Ozin, S. A. Mitchell, and J. Garcia-Prieto, Pure Appl. C h m . , in press. (11) G. A. Ozin and F. Hugues, J . Phys. Chem., 87,94 (1982).
a-cages of silver-exchanged X and Y zeolites. Experimental Section Preparation of the Silver Zeolite Samples. The zeolites X and Y in the Na+ form were supplied by the Linde Division of Union Carbide Corp. The Na+ ions were partially exchanged by Ag+ ions in silver nitrate aqueous solution according to conventional ion-exchange procedures. Preparation, handling, and storage of the samples were performed in the dark. The silver content of the different zeolites was obtained by a neutron activation analysis using the University of Toronto Slowpoke reactor facility. Table I gives the composition of the Ag,Na-Z samples used in this study. Pretreatment of the Samples. The hydrated silverloaded zeolites were thermally activated under vacuum (around lo4 torr) stepwise, by 100 "C increments, each for approximately 2 h, up to 500 "C. This treatment was performed in the absence of light to avoid a photoreduction of the silver ions. The samples pretreated in this way are denoted with the subscript 500, e.g., Ag,Na-Y,. In some cases samples were oxidized by O2 at pressures ranging from 300 to 760 torr and temperatures between 250 and 500 "C. Such samples were denoted with the subscript oxT where T is the temperature of the oxidation, e.g., Ag6Na-Y5D0 ox 500. Interaction of the Samples with Gases. All of these operations were performed by conventional high vacuum handling procedures. The gases used (02,CHI, Ar) were of research purity grade from Matheson of Canada. Spectroscopic Techniques. The spectroscopic measurements were all performed in situ on the zeolite samples at different steps in the thermal vacuum activation, of the oxidation, and of the photolysis treatment. The sample was held in a quartz cylindrical cell (diameter 25 mm, thickness 3 mm) connected to a Pyrex glass tube to which a vacuum valve was attached. Diffuse reflectance spectra were recorded on a Cary 17 UV-visible spectrometer using a diffuse reflectance attachment working in the type I1 illumination mode. Only the visible range 320-700 nm could be recorded with the attachment used. Photolysis of Zeolite Samples. Irradiation of the zeolite samples in the UV-visible range was performed directly on the diffuse reflectance cell by means of a 450-W Hanovia high-pressure mercury lamp equipped with a water cell to filter out infrared radiation. The irradiation was performed either without specific filters, the wavelength range being 220-900 nm, or with filters to select specific wavelength ranges. During the photolysis, the temperature of the sample was close to room temperature and never in any case exceeded 40 "C. The samples submitted to irradiation were denoted with the subscript phot, e.g., Ag6Na-Y500 phot. Experimental Results Diffuse Optical Reflectance Spectra of Silver-Loaded Zeolites X and Y. The hydrated samples are found to be
The Journal of Physical Chemistry, Vol. 87, No. 18, 1983 3447
Low Nucleartty Silver Clusters in Faujasites
Aq3NaY
Ag6NaY
A
voc.
25%
c 02
D
300F
700
600
500
400
300
Ahm)
700
600
500
400
300
Nnm)
Figure 2. Diffuse optical reflectance spectra of Ag,,Na-Y: (A) after vacuum treatment up to 500 OC; (B) after UV photolysis under argon (1 atm) for 20 h followlng A; (C) after evacuation at 500 OC following B.
Figure 1. Diffuse optical reflectance spectra of Ag,Na-Y after the following treatments: (A) dehydratlon under vacuum at 25 OC; (B) dehydration under vacuum up to 500 OC; (C) UV photolysis for 16 h under methane (1 atm) after B; (D) oxidatlon by O2 (400 torr) at 300 OC after B or B C.
+
devoid of absorption bands in the region 320-700 nm. A typical example, Ag6Na-Y hydrated, is shown in Figure 1A. However, on subjecting the Ag6Na-Y hydrated sample to a vacuum thermal treatment up to 500 "C and recording the optical reflectance spectrum at different stages of this treatment, a gradual and monotonic growth of three broad (bandwidth at half-height e 50-75 nm) bands at 580,460,and 390 nm is observed (Figure 1B). The energy and intensity ratio of these bands are found to remain essentially the same throughout the dehydration process. These spectra are in close agreement with those observed by Kellerman and Tester6 for a Ag8Na-Y sample, except that their data extended further into the UV region. When a A g 6 N a - y ~sample was exposed, in a methane atmosphere, to broad-band irradiation from a mercury lamp and the optical reflectance spectrum recorded after every hour of irradiation, the spectrum gradually transformed from one initially displaying the three optical transitions at 580,460 and 390 nm to one with two optical transitions at 600 and 410 nm (Figure lC),that is, the central component at 460 nm has decayed to zero while the high-energy (390 nm) and low-energy (580 nm) bands have red shifted to 410 and 600 nm, respectively. Wavelength-dependent photochemical studies demonstrated that only the 220-300-nm range was effective in promoting the phototransformation described above. Furthermore, this transformation could not be induced thermally, even at 500 "C. A photoinduced transformation similar to that observed in methane was also found to occur in argon with the sample Ag13Na-Ym (Figure 2A,B) except that the effect was thermally reversible a t 500 "C in argon (Figure 2C) but irreversible in methane. Additional information that is of assistance in the characterization of the silver guest absorptions at 580/ 4601390 nm and 6001410 nm in the aforementioned samples has been obtained in Ag6Na-X and higher silverloaded Ag,Na-Y samples. Although not as well resolved as for Ag,Na-Y,, the optical reflectance spectrum of Ag6Na-XW also displayed three visible hands around 600,
I
L
eo0
25?
A -
700
600
500
100
300
h (mi
Flgure 3. Diffuse optlcal reflectance spectra of AS,-Y: (A) to (E) at different stages of the vacuum dehydration up to 500 O C and (F) to (I) at different stages of the oxygen treatment (400 torr).
450 and 400 nm, whose growth behavior, relative intensities, and bandwidths were essentially identical with those observed for Ag6Na-Y,. The three-line and the two-line spectra are completely bleached by an oxygen treatment of the sample at 300 OC. Together these observations imply that a common silver species is produced in both zeolites X and Y. As shown in Figures 1 and 2 similar spectra are obtained for Ag13Na-Y5, and Ag6Na-Y5,. However, for a completely silver-exchanged zeolite Y, Ag5,-Y, some significant differences were noticed. To begin with the three-band spectrum observed for Ag6NaY5, and Agl3Na-Y, was now obtained at 325 "C rather than 500 "C in Ag5,Y samples, together with an intense and narrow band at 340 nm (Figure 3C). Further heating
3440
The Journal of Physical Chemistry, Vol. 87, No. 18, 1983
to 400 "C induced the growth of only the 460-nm band (Figure 3D). Moreover, heating to 500 "C led to the production of just one broad band at around 450 nm. The bands at 580 and 390 nm were bleached, whereas the 340-nm band continued to increase in intensity (Figure 3E). Subsequent oxygen treatment at 300 "C completely removed the absorption at 450 nm, leaving the 340-nm band unaffected (Figure 3F). Further oxygen treatment at 400-480 "C caused the 340-nm band to decay with concomitant growth of an intense new band at 402 nm (Figure 3 G I ) . We have observed similar effects for a completely silver-exchanged zeolite X, namely, Ags-X, where the corresponding bands showed small but reproducible red spectral shifts to 350 and 410 nm, respectively.
Discussion The above observations alert one to the existence of a silver species (A), characterized by three absorption bands at 580,460, and 390 nm, which is accessible to molecular oxygen and therefore located in the a-cage (supercage) of zeolite Ag,Na-Ym and Ag13Na-Ym The position of the absorption bands indicates that it is most probably a low nuclearity silver cluster.' The same cluster (A) is also present in Ag55-Y325,and appears to be involved in the production of a higher nuclearity silver cluster (species B), characterized by a single absorption maximum around 450 nm. The silver species C giving rise to a narrow band at 340 nm in AgM-Y (formed during the vacuum treatment), together with the silver species D characterized by a band at 402 nm (formed at the expense of species C by oxygen treatment at 400-480 "C) have been discussed earlier" in terms of a two-step formation of the cluster cation Agl+ (species D). The first stage involves the production of cluster Agz+(species C) under vacuum thermal treatment from the simultaneous occupancy of a silver atom in site I and a silver ion in site 1'. The second stage is thought to proceed via the migration of a silver ion under oxygen treatment toward the vacant site I' adjacent to the cluster Agz+,thereby leading to Ag32+.20In the present study, it is the mode of formation, the estimated size, the chemical/photochemical behavior, and the most probable location of the small silver cluster A that will form the main subject of discussion. Properties of Silver-Exchanged Zeolites. In this study we begin with silver ion exchanged zeolites X and Y at different levels of cation replacement. The hydrated samples are then subjected to vacuum thermal treatment up to 500 "C. It is well documented12 that the physicochemical transformations that ensue under these conditions include (a) removal of molecular water from the zeolite; (b) partial reduction of the silver ions by water with concomitant evolution of oxygen and formation of lattice hydroxyl groups, in a low-temperature region (300-525 K) according to 2(Ag+,ZO-)+ HzO l/zOz + Agzo+ 2ZOH ( c ) further reduction of the silver ions by lattice oxygens with additional evolution of oxygen and formation of Lewis acid centers, in a high-temperature region (525-650 K) according to 2(Ag+,ZO-) l/zOz + Agzo + ZO- + Z+ (Lewis site)
-
-
The degree of silver ion reduction can be quantified by measuring the amount of oxygen evolved per equivalent (12) P. A. Jacobs, J. B. Uytterhoeven, and H. K. Beyer, J . Chem. SOC., Faraday Trans. I , 7 6 , 56 (1979).
Ozin et ai.
of silver in the zeolite. From the crystallographic work of Costenoble and Maes,13the initial distribution of silver ions among the various cationic sites for hydrated samples, as a function of the extent of ion exchange, has been established and is summarized as follows: site
I I' I1 unlocalized
Ag,Na-Y
1.9 0 0 0
Ag,Na-Y
3.9 0.3 1.8 1.25
Ag,,Na-Y
4.4 1.8 6.0 1.8
It is understood that vacuum thermal activation of hydrated silver-exchanged zeolites X and Y can lead to a redistribution of the silver ions, and partial reduction and agglomeration of the silver ions, involving molecular water and lattices oxygens. Clearly the nature and the location of the various silver guests 90 formed within the zeolite host is likely to be a sensitive function of the degree of cation exchange, the Si/A1 ratio, the details of the vacuum thermal treatment, and whether or not further treatment, for example, under hydrogen or oxygen, is applied to the sample. Estimation of the Size of the Ag,Q+Cluster, Species A. The optical reflectance spectrum and the thermal, oxidative, and photochemical properties of species A strongly suggest that it be associated with a low nuclearity silver cluster cation Ag,,q+ with n greater than 3. By analogy with Kasai's work on Na43+in zeolite Y14 and the optical estimation of its size by approximating the cluster as an electron-in-a-boxproblem, we will assume that Ag,Q+is also a highly charged one-electron cluster that can be treated in a similar manner. Using the observed 580-nm visible band as the HOMO-LUMO transition of Ag,q+ one calculates 7.26 A as the edge dimension of a cube which will just enclose the cluster. With a silver-silver bond distance of 3 A (close to the distance observed in the cluster Agp 3,4) one finds that the edge dimensions of the smallest cubic box that will just enclose tetrahedral Agh3+,trigonal bipyramidal Ag$+, octahedral Age5+,cubic Ag;+, and cubooctahedral AgI3l2+are 5.12,6.46,6.46,6.00, and 7.24 A, respectively. One can therefore propose that, within the framework and limitations of an electron-in-a-box calculation, the optical spectroscopy is consistent with the idea that species A is a low nuclearity silver cluster cation Ag,@ with n probably in the range of 5-13. With such a size, the cluster can easily be accommodated in the large a-cage of the faujasite-type zeolites. Formation, Location, and Stability of the Ag,q+ Cluster, Species A . According to the results of Costenoble and Maes,13in AgGNa-Yhydrated and Ag13Na-Yhydrated,4 Ag+ occupy site I, and the others are mainly located in site 11. During the vacuum dehydration process the silver ion in site I can either migrate to another site, stay as silver ions in site I, or be reduced in its place to a silver atom, as indicated by Kellerman and Texter's results6as well as by our own.ll However, in the absence of molecules other than water, it is less likely that silver ions in site I will move out of this site thereby providing pseudo-octohedral coordination for the cation and shown to be highly preferred by Ag+.13 By contrast, silver ions in site I1 and unlocalized silver ions, the latter probably being in the a-cage associated with water molecules, having a more exposed location and lower symmetry site (for example Ag+(II) is associated with three oxygens and certainly with water molecules in the hydrated form), are likely to be sensitive
-
(13) M. Costenoble and A. Maes, J. Chem. SOC.,Faraday Tram. 1 , 7 4 , 131 (1978). (14)P.H.Kasai, J. Chem. Phys., 43, 3322 (1965).
Low Nuclearity Silver Clusters in Faujasttes
to reduction processes and migration effects induced by the increase in the temperature and the presence of water during the vacuum thermal activation. Therefore it can be tentatively argued that cluster formation will involve silver ions in site I1 as well as unlocalized silver ions. Thus one can envisage the migration of silver ions, simultaneous reduction of some of them, and agglomeration to a small cluster Ag,q+. The question arises as to why such a small cluster is stable under vacuum up to temperatures as high as 500 "C? It has already been assumed that small charged silver clusters can be formed by hydrogen reduction of silverexchanged zeolite at moderate temperature (-350 "C) and, providing that hydrogen is removed, those clusters do not agglomerate to large particles when the samples are subjected to vacuum treatment up to 350 "C.lCJ5 Furthermore, these charged clusters are assumed to be inside the zeolite supercages, subject to strong Coulombic interaction with the zeolite lattice, and easily oxidized by molecular oxygen at temperatures lower than 200 oC.1cJ5 These observations therefore provide a strong foundation on which to build our argument that the cluster Ag,,q+ of the present study is highly charged (q might take any value from 1 to n - 1) and is subject to strong Coulombic interactions with the negatively charged zeolite lattice which stabilizes the silver cluster cation probably on the wall of the zeolite supercage. Formation of Larger Clusters in Completely Exchanged Zeolite Y, Species B. In the case of Ag5,-Y, we have observed after vacuum thermal activation at 325 "C the formation of the cluster Ag,q+, associated with the three bands at 580/460/390 nm. At higher temperatures, this cluster is consumed and the spectrum shows a single broad absorption around 450 nm. The thermal behavior of this new species suggesta that it is associated with a larger silver cluster formed at the expense of Ag,q+. With the available information, it is impossible to draw any definite conclusions regarding the size, charge, and location of the larger cluster B. All one can say is that the region of absorption around 450 nm is characteristic of electronic transitions of low nuclearity molecular silver clusters, as well as collective electronic excitations (plasmon resonance) in very small silver particle^.^ Recent X-ray crystallographic analysis of analogous samples indicates the presence of silver metal particles located on the outside surface of the zeolite crystallites following vacuum activation at 420 Therefore the absorption band around 450 nm can be associated either with small particles located inside the zeolite cages, or to larger particles located on the outside surface. However, the easy oxidation of these particles, evidenced by the bleaching of the 450-nm band after oxygen treatment at 300 "C for 1 h, suggests that they are quite small in size, because complete oxidation of large silver particles requires temperatures in excess of 300 "C.15 Photochemistry of Ag,q+ and Its Relationship to the Photodimerization of Alkanes. We have obtained optical spectroscopic evidence that under UV irradiation (220-300 nm) the silver cluster cation Ag,q* is transformed into another cluster most likely having the same nuclearity. In a methane atmosphere, this photochemistry is accompanied by the selective and stoichiometric production of ethane.g Similarly, other alkanes, such as ethane and propane have been photodimerized to n-butane and hexanes respectively with better than 90% sele~tivity.~ The (15)H.Beyer, P.A. Jacobs, and J. B. Uytterhoeven, J. Chem. SOC., Faraday Trans. 1 , 72,674 (1976). (16) L. R. Gellens, W. J. Mortier, and J. B. Uytterhoeven, Zeolites, 1, 85 (1981).
The Journal of Physical Chemistry, Vol. 87, No. 18, 1983 3449
species responsible for the activation of the C-H bond of, for example, CH4 is thought to be a lattice V center (an electron hole associated with a lattice oxygen) obtained in the zeolite under UV irradiationg and probably formed via the charge transfer electronic e~citation,'~ A13+-02- + A12+-0-, the formation of which is enhanced by the presence of the cluster cation Ag,q+, as follows: 0
T
/*\Si
220-300 hv
"IllC
(T= Si or Al) xe-
+
pg,,Q+
+
T /o\s
-
e-
~g,,~+ H
In this scheme it is proposed that the photogenerated electron is trapped by the Ag,q+ cluster cation, simultaneously producing the hole and the reduced silver cluster Ag,P+. We have reported earlier that the occurrence of such an electron transfer process can explain why only the Ag,Na-Y zeolites containing the cluster Ag,q+ exhibit higher activity than a zeolite Na-Y in the photodimerization of alkanes. The photolytic stability of the cluster Ag,P+ under these conditions is thought to stem either from the fact that only a limited number of V centers can be produced by UV light (note that y-irradiation of Na-Y zeolite produces a maximum of about 1019 V centerslg, which represents only 0.1% of the total number of framework T-0-Si groups; -1022/g18), or from the possibility that Ag,P+ is a much poorer electron acceptor than Ag,q+. However, because of the uncertainties in the evaluation of the number of Ag,q+ clusters and V-centers per unit cell of zeolite, it is difficult to give an unequivocal explanation for the apparent stability of Ag,P+ to further reduction by photogenerated electrons. The Reverse Ag,P+ Ag,@ Redox Reaction. Based on the foregoing discussion, we can begin to understand why the Ag,q+ + hv Ag,,p+ photoreaction can be thermally reversed at 500 "C only when the original photoproduction of Ag,p+ is performed in an atmosphere of Ar but not in CH4. In essence this relates to the chemical quenching of the photogenerated V center by its reaction with methane and the anticipated high thermal stability of the surface hydroxyl group(s)lgso formed. By contrast, when Ag,p+ is generated in an argon atmosphere the V center, possibly in close proximity to the Ag,p+ cluster, may be involved in the thermally induced reverse redox process Ag,P+ -, Ag,q+, although one cannot be sure that other electron traps, such as, Lewis acid sites formed during the first thermal activation procedure do not participate in the reduction process. Finally, it is interesting to note that the optical spectroscopic alterations for Ag,q+ following 220-300-nm photolysis can be considered to support the idea of a phototransformation to Ag,*+ in which the cluster integrity is maintained but the overall charge on the cluster diminishes. With the assumptions of an electron-in-a-box model,14the HOMO-LUMO 580- to 600-nm red spectral
-
-+
~~
~
(17)E.D. Garbowski and C. Mirodatos, J. Phys. Chem., 86,97(1982). (18)A. Abou-Kais, J. C. Vedrine, and J. Massardier, J. Chem. SOC., Faraday Trans. I , 71, 1697 (1975). (19)J. W. Ward in 'Zeolite Chemistry and Catalysis",J. A. &bo, Ed., American Chemical Society, Washington, DC, 1976,ACS Monograph No. 171,p 118.
J. Phys. Chem. 1983, 87, 3450-3455
3450
shift corresponds to an increase in the size of the cluster from 7.27 i!for Ag,@ to 7.39 i!for Ag,p+.
Conclusion Besides the low nuclearity silver cluster cations Ag2+and Ag+ : which can be formed in zeolites X and Y,20we have obtained evidence for a higher nuclearity cluster cation Ag,Q+, the optical spectroscopy of which suggests that n is in the range of 5-13. The thermal and oxidative stabilities of this cluster are consistent with a cationic formulation and location probably on the wall of the a-cage. Under conditions of high silver ion exchange, agglomeration to larger silver clusters occurs apparently at the expense of Ag,q+. The 220-300-nm induced photochemistry observed for Ag,q+ can be understood in terms of pho(20) In view of the consistency between the crptallyraphic data3*'and the electronic absorption and emission spectroscopf for the proposed intrazeolitic guests, Ago, Ag2*+,and Ag$+ it is surprising that, on one hand, there is no observable EPR signal attributable to these paramagnetic species (at least Agf, the charges p and p for Ag2*+and Ag$' being not precisely known) and, on the other hand, considering the extensive previous EPR work done, that no spedra have been reported for the vacuum dehydrated forms of AgNa-X and AgNa-Y zeolites. Note that for n = 1,2,3 ...) have the EPR spectra of similar species (Agf and been observed in hydrated or partially dehydrated silver exchanged zeolites A, X, Y following y- or X-ray irradiation.* The EPR silence for the species may be due to several factors, such as, saturation of the paramagnetic species at low microwave powers at the recording temperature, due to spin-lattice relaxation effects, coupled with broadening effecta due to the existence of other paramagnetic species in the vicinity of the silver guests.
'
toionization of a lattice oxygen to produce a V center, with simultaneous reduction of Ag,q+ to AgnP+. Support for this idea stems from the observation that the photoredox process Ag,q+ Ag,P+ can be reversed thermally at 500 "C in an Ar atmosphere. By contrast, in a CH4atmosphere the photogenerated V center is thought to be chemically quenched, forming a surface hydroxyl group and methyl radicals, the latter dimerizing to gaseous C2H6. Under these circumstances the Ag,P+ Ag,q+ reverse redox process is thermally impeded. From this and earlier studies it is clear that one must now address the problem of the charge dependence of the electronic and magnetic properties of low nuclearity silver clusters in weakly and strongly interacting supports. In this spirit, we have initiated an SCF-Xa-SW MO study of model silver clusters Ag,*+ (where n = 2 to 6 and q = 0 to n - 1). The results of these calculations will be presented in a later publication.21
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Acknowledgment. Financial assistance from the Natural Sciences and Engineering Research Council of Canada's Strategic Energy Programme and a Special Research grant from the Connaught Foundation of the University of Toronto are both greatly appreciated. Registry No. Ag, 7440-22-4. (21) G. A. Ozin, F. Hugues, D. F. McIntosh, and S. M. Mattar in 'Intrazeolite Chemistry", G. D. Stucky and F. G. Dwyer, Ed., American Chemical Society, Washington, DC,1983,ACS Symp. Ser., No. 218, p 409; J . Phys. Chem., manuscript in preparation.
Electron Spin Resonance Study of N-Alkyl-N-(a1kylthio)aminyl Radicals' Yo20 Mlura, Hldetsugu Asada, Masayoshl KlnorhHa, Department of A p p W Chemistry, Faculty of Engineerlng. Osaka CW UniversW, SumiyoshUru, Osaka 558, Japan
and Katsuhlsa Ohta Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto Unlversky, Sakyo-ku, Kyoto 606, Japan (Received: February 14, 1983)
A variety of N-alkyl-N4alkylthio)aminylradicals (2), R1NSR2,were generated by hydrogen-atom abstraction from N-(alky1thio)alkylamines and were studied by electron spin resonance (ESR) spectroscopy. When both R1and in 2 were tert-alkyl, the corresponding radicals were extremely persistent in oxygen-free hydrocarbon solvents and showed no tendency to dimerize even at low temperatures. In the ESR signal of t-BuNSBu-t radical the satellite (5.93 G)due to 33Sin natural abundance could be detected. From a plot of 33Shyperfine splitting (hfs) constants for three N-thioaminyl radicals vs. the a-spin densities on the sulfurs of the radicals calculated by the Huckel method or McLachlan perturbation treatment, a magnitude of 23 G for Q, of the McConnell type equation (a = Qp") was obtained. Ab initio molecular orbital calculations on a model radical, HNSH,with the double-zeta basis set predicted a trans-coplanar structure as the most stable conformation.
Introduction Recently, a variety of N-alkoxy-N-alkylaminyl radicals (3), R1NOR2,have been widely studied by electron spin resonance (ESR) spectro~copy.~-~ The generation was (1)Part 19 in the series "ESR Studies of Nitrogen-Centered Free Radicals". For part 18, see: Miura, Y.; Yamamoto, A.; Kinoshita, M. Bull. Chem. SOC. Jpn. 1983,56, 1476. (2) Danen, W. C.; West, C. T.; Kensler, T. T. J.Am. Chem. SOC.1973, 95, 5716. (3) Kaba, R. A.; Ingold, K. U. J . Am. Chem. SOC.1976, 98,7375.
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achieved by photolysis of R,NHCOOOBu-t and RINHO&, and oxidation of RINHO& with inorganic oxidizing agents. It w e found that INDO molecular orbital calculations on CH3NOCH, radical predicted a trans-coplanar structure as the most stable conformation2 and that, when R1 and R2 in 3 were both tert-alkyl, the corresponding radicals showed no tendency to dimerize and persisted in oxygenfree conditions with no detectable decay.4 (4) Woynar, H.; Ingold, K. U. J . Am. Chem. SOC.1980, 102, 3813.
0 1983 American Chemical Society
