J. Phys. Chem. 1980, 84. 1825-1829
Figure 6. Decay of transient observed at 460 nm at long time (1 ms/dw) after the laser pulse: [ZnP] = [ZnTPP] = 5 X M; [CI1DQ] = 3.5 x 10-4 M.
its exit from the cationic aggregates requires more time than the back electron transfer, rendering the back reaction quantitative.
Acknowledgment. We wish to thank G. Rothenberger for helpful discussioins. We are grateful to Ciba Geigy Ltd., Basel, Switzerland, for financial aid. Thanks are due to Miss Elizabeth Irving for the preparation of the drawings. References and Notes (1) J. R. Bolton, Ed., “Solar Power and Fuels”, Academic Press, New York, 1977.
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(2) B. V. Koryakin, T. S. Dzhabiev, and A. E. Shllov, Dokl. Akad. Nauk SSSR, 238, 620 (1977); J. M. Lehn and J. P. Sauvage, Now. J. Chim., 1, 449 (1977); K. Kalyanasundaram, J. Kiwi, and M. Gratzel, Helv. Chim. Acta, 81, 2720 (1978); A. Moradpour, E. Amouyal, P. Keller, and H. Kagan, Nouv. J. Chim., 2, 547 (1978); B. 0. Durham, W. J. Dressick, and Th. J. Meyer, J . Chem. SOC.Chem. Commun., 381 (1979); P. J. De Laive, B. P. Sullivan, T. J. Meyer, and D. 0. Whitten, J. Am. Chem. Soc., 101, 4007 (1979); A. I. Krasna, Photochem. Photobiol., 29, 267 (1979); T. Kawai, K. Tanlmura, and T. Sakada, Chem. Lett., 137 (1979); M. Kirsch, J. M. Lehn, and J. P. Sawage, Helv. Chlm. Acta, 62, 1345 (1979); J. Kiwi and M, Qakel, J. Am. Chem. Soc., 101,7214 (1979); J. Kiwi and M. Grtitzel, Nature (London),281, 657 (1979); I. Okura, S. Nakamura, N. Klm-Thuan, and K. I. Nakamura, J. Mol. Catal., 8, 261 (1979). (3) J. Kiwi and M. aatzei, Angew. Chem. Int. Ed. E@., 17, 860 (1978); J. KIWIand M. Qatzel, Chimia, 33,289 (1979); M. Qatzel, in “Dahlem Conferences 1978 on Light-InducedCharge Separatlon”, H. Gerlsd-w and J. J. Katz, Ed., Verlag Chemie, 1979, p 299; J. Kiwi and M. oriitzel, Angew. Chem., Int. Ed. Engl., 18, 624 (1974); J.M. Lehn, J. P. Sauvage, and R. Zlessel, Nouv. J. Chim., 3, 423 (1979); K. Kalyanasundaram and M. Gratzel, Angew. Chem., Int. Ed. Engl., 18, 701 (1979); K. Kalyanasundaram, 0. Mlclc, E. Pramauro, and M. Gratzel, Helv. Chim. Acta, 62(7), 2432 (1979). (4) K. Kano, K. Takuha, T. Ikeda, D. Nakalirna, Y. Tsutsui, and T. Matsuo, Photochem. Photobiol., 27, 695 (1978). (5) M. P. Pileni, A. Braun, and M. Qatzel, photochem. photobbl., In press. (6) L. Pekkarinen and H. Linschitz, J. Am. Chem. Soc., 82,2407 (1960); G. R. Seely and H. Calvin, J. Chem. Phys., 23, 1065 (1955). (7) U. Lachlsch, A. Shafferman, and G. Stein, J. Chem. Phys., 64, 4205 (1976); U. Lachisch, P. P. Infeita, and M. Gratzel, Chem. Phys. Lett., 62, 317 (1979). (8) P. P. Infelta, M. Gratzel, and J. K. Thomas, J. Phys. Chem., 78, 190 (1974); 0. Rothenberger, P. P. Infelta, and M. Gratzel, IbM., In press. (9) M. Maestri, P. P. Infelta, and M. Gratzel, J. Chem. Phys., 69, 1522 (1978). (10) Y. Moroi, P. P. Infelta, and M. Gratzel, J. Am. Chem. SOC.,101, 573 (1979). (1 1) M. Almgren, F. Wiser, and J. K. Thomas, J. Am. Chem. Soc. 101, 279 (1979). (12) C. Woiff and M. Qatzel, Chem. Phys. Lett., 52,542 (1977); Y. Waka, K. Hamamoto, and N. Mataya, ibid., 53, 242 (1978). (13) B. Razem, M. Wong, and J. K. Thomas, J. Am. Chem. Soc., 100, 1679 (1978).
Raman Spectr(a of Cobalt Molybdenum Oxide Supported on Silica H. Jeziorowski, H. Knoringer,” Instifut fur Physikalische Chemle, Universitat Munchen, 8000 Munchen 2, West Germany
P. Grange, and P. Gajardo Univors/t6 Catholique de Louvain, Groupe de Physico-Chlmle Min6rale et de Catalyse, 1348 Louvah-la-Neuve,Belgium (Rec48ivedNovember 7, 1979)
Three series of cobalt molybdenum oxide catalysts supported on silica have been studied by Raman spectroscopy. All iaeries had been characterized previously by various techniques. An interaction species which is described as a two-dimensional polymolybdate is observed, which is relatively weakly bound to the support. Addition of C!o leads to a detachment of this species and to the formation of CoMoOl. “Free” Moo3 and Co304are also detected, the relative amounts of the various supported compounds depending on the atomic ratio r = Co/(Co + &Io) and the total transition metal oxide loading. The present Raman data principally confirm the previously reported characteristics of the catalysts, but in addition Raman spectroscopy demonstrates the existence of supported compounds which were not previously detected by X-ray analysis because of their small particle size and high dispersion. 1. Introduction The nature of the carrier very strongly influences the interactions between the oxidic Co-Mo phase and the support. Moreover, the strength of this interaction governs the compound formation in the various systems. This behavior becomes evident on comparing y-A1203-and Si02-supportedmolybdena catalysts. Thus, it has been proposed that a CoMo bilayer was being formed in y0022-3654/80/2084-1825$0 1.OO/O
A1203-supportedsystems: this bilayer probably being the precursor of the active sulfided phase of hydrodesulfurization ~atalysta.~ When SiOzis used as the carrier, the interaction of the molybdena phase with the support has been found to be much The interaction species have been described as polymolybdate-like twodimensional structures, in which Mo6+most probably occurs in octahedral coordination,lV2though the occurrence 0 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 14, 1980
of tetrahedrally coordinated Mo6+ cannot be excluded. Because of the weakness of the Si02support-molybdena interaction, the relative amount of the interaction species formed with respect to the theoretical monolayer capacity is small as compared to Alz03-supported catalysts, and even at low loadings has the formation of “free” Moo3 been detecteda2 On addition of cobalt to the Si02-supported molybdena phases, CoMo04 in its A and B forms was observed; this compound was formed almost stoichiometri~ally.~ Interestingly, the addition of cobalt led to a detachment of the molybdena interaction species from the Si02support and to the formation of COM004.3 Co304was detected only when cobalt was present in excess. Some of these interpretations were the result of more or less indirect conclusions from X-ray diffraction, diffuse reflectance spectra, XPS, and H2-reductionmeasurements. In order to elucidate further the molybdena-SiOz interaction and the compound formation in cobalt-promoted catalysts, we have now undertaken a Raman spectroscopic study of the same series of Si02-supported molybdena catalysts which had been characterized previously by the above-mentioned and other techniques. Raman spectroscopy has been applied successfully for the characterization of A1203-supportedmolybdena catalysts by several research groups in recent yeam6-14 SO2-supported catalysts have been studied much less extensively by this t e c h n i q ~ e . ~Medema J~ et ala8reported the direct observation by Raman spectroscopy of a polymolybdate phase on SiOz for the first time. Besides this phase large relative amounts of “free” Moo3were present in molybdena on Si02catalysts. The authors concluded from their Raman data that the Mo species interacted to a lesser extent with the SiOz surface than with A1203 surfaces. The crystalline modifications (A and B forms) of CoMo04 were formed on addition of cobalt at lower loadings than on A1203 as reported in the same paper.s Also Co304was formed in one of the two studied catalysts. These results are in qualitative agreement with those reported by Cheng et al.13
2. Experimental Section 2.1. Catalysts. Three series of Si02-supported molybdena catalysts are studied. The SiOzused as support was Spherosil R.P. with a specific surface area of 179 m2 g-’. The details of the catalyst preparation have been described previously,lP2 The first series of catalysts only contains molybdena at varying loadings between 0 and 20.6 w t %. These catalysts are designated SiMo x , where x indicates the molybdena loading expressed as wt % of Moo3. The second series contains additionally cobalt as a promoter. The catalysts are designated SiMoCo y , where y indicates the total transition metal oxide loading expressed as wt % of (Moo3+ Co304). The atomic ratio r = Co/(Co Mo) was kept constant for all catalysts of this series and was equal to 0.36. In the third series finally the total transition metal oxide loading was maintained constant and equal to 12 wt % (Moo3 Co304),while the atomic ratio r was varied between 0 and l. These catalysts are designated Si r. 2.2. Raman Spectroscopy. The Raman spectra were recorded on a Cary 82 spectrometer equipped with a triple monochromator. The 514.5-nm line of a Spectra Physics Model 165 Ar+ laser was used for excitation. The laser power was limited to appoximately 60 mW (measured at the sample position). The spectral slit width was typically 4 cm-l, and wavenumbers obtained from the spectra were accurate to within f 2 cm-l. The oxide samples were mounted into a stainless steel frame which was rotated at
+
+
Jeziorowski et al.
TABLE I: Raman Bands of Reference Compoundsa compd wavenumbers, cm-’ ref MOO, 996(vs), 820(vs), 667(m), this work 380(w), 367(w), 337(m), 292(m), 284(s), 247(w), 218(w), 199(w), 158(s), 129(s), 117(s) c0304 695(s), 528(w), 489(w) this work CoMo04(A) 946(s), 895(vw), 704(m) 9 940(s), 880(w), 700(m) this work CoMoO,(B) 939(s), 879(m), 817(m) 9 939(s), 873(m), 819(m) this work a vs = very strong, s = strong, m = medium, w = weak, vw = very weak.
I 1
SiMo 6.7
,
1000
,
/
800
,
I
,
,
600 LOO AGR Icm-’I
,
,
200
0
Figure 1. Raman spectra of the SiMo x series. Spectra were recorded with the following count rates: (1) SiMo 2.8,S = 2 X lo3 counts s-‘; (2) SiMo 7, S = 5 X lo3 counts s-‘; (3) SiMo 11.6,S = 5 X lo3 counts s-’; (4)SiMo 20.6,S = 10’ counts s-‘.
a frequency of approximately 60 Hz to avoid any damage of the samples by laser-induced effects as described by Medema et aL8l9 3. Results and Discussion In order to facilitate the description and discussion of the Raman spectra, characteristic Raman bands of authentic reference compounds, namely, Moos, Co304,and the A and B forms of CoMo04,are summarized in Table I. 3.1. SiMo x Series. The Raman spectra of catalysts of the SiMo x series are shown in Figure 1. Sample SiMo 2.8 only gave rise to a weak broad feature at 970 cm-l. This band occurred a t 956 cm-l on sample SiMo 7 and was accompanied by an additional broad band at 884 cm-l. Following the band assignments on alumina-supported molybdena>lO.ll these bands can be ascribed to a molybdena interaction species which must be considered as an analogue of a polyanion chemically interacting with the support surface. The bands between 900 and 1000 cm-l are due to symmetric and antisymmetric MFO stretching
Cobalt Molybdenum Oxide Supported on Silica
modes, whereas that, between 850 and 900 cm-I must be assigned as the antis:ymmetric stretching mode of Mo-OMo bridges. The width of the bands indicates that a number of structurailly different, probably more or less distorted polyanionic species are being formed on the support surface. The detection of this interaction species is in agreement with previous Raman work8J3and also with the previously suggested structural modeL2 The interaction species is also present at higher loadings, as indicated in Figure 1 by the broad band near 960 cm-’ on sample SiMo 11.6, whereas the low sensitivity which was necessary to record the spectrum of sample SiMo 20.6 does not allow the detection of these bands. There was some indication of a transformation 0.5. The wavenumbers of the characteristic bands of the various detected supported compounds coincide within the limits of experimental error with those of the bulk reference compounds, as was the case for Moo3in the SiMo x series. Again are the relative intensities, namely 1996/1820, of characteristic Raman bands changed as compared to those of bulk Moo3, the ratios being very close to those calculated for the SiMo x series. For Co304in its bulk form, = 0.14 and 1489/1698 = 0.14 are intensity ratios 15528/1698 found, whereas the corresponding values for Si 0.36, Si 0.49, and Si 0.75 are approximately 0.22. This may indicate that also the Co304crystallites are interacting with the support. Though these interactions appear to be weak, they are probably the reason for the reported high dispersion’ of the supported compounds. This is corroborated by comparing the detectability of the various compounds by X-ray diffraction and Raman spectroscopy. Table I1 summarizes this information in a qualitative manner. The comparison shows that the crystallite sizes must be rather small for Moo3 at r L 0.27, for Co304at r I0.75, and for CoMo04(B) at r 5 0.36, whereas CoMo04(A)is not detectable by X-ray diffraction at any composition. The most interesting case is that of sample Si 0.27, for which X-ray diffraction did not detect any of the possible compounds. Gajardo et a1.l have concluded that this sample contains Moo3 and CoMoo4but that the corresponding crystallites are very small and that the resulting high dispersion is respqnsible for the observed maximum activity of this sample which is observed in plots of raw conversion of hydrodesulfurization of thiophene vs. r.I6 The present results confirm the presence of MOO, and CoMo04(B)in Si 0.27 and demonstrate the potential value of Raman spectroscopy as an additional technique for the characterization of this class of catalysts.
Acknowledgment. This work was financially supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. This work is part of a program supported by the “Services de la Programmation de la Politique Scientifique” in the frame of the “Actions concertdes Interuniversitairs de Catalyse”.
References and Notes ( 1 ) P. Gajardo, P. Grange, and B. Delmon, J . Phys. Chem., 83, 1771
(1979).
J. Phys. Chem. 1980, 8 4 , 1829-1832
(2) P. Gajardo, D, Pirotte, P. Grange, and B. Delmon, J . Phys. Chem., 83, 1780 (1979). (3) P. Gajardo, D. Pirotte, C. Defosse, P. Grange, and B. Delmon, J. Nectron Spectrosc. Relat. Phenom., 17, 121 (1979).
(4)P. Gajardo, P. Grange, and B. Delmon, J . Catal., in press. (5) P. Grange, Catal. [lev., in press (6) F. R. Brown, L. E. Makovsky, and K. H. Rhee, J . Catal., 50, 162, 385 (1977). (7) F. R. Brown, R. Tischer, L. E. Makovsky, and K. H. Rhee, paper presented before the Division of PetroleumChemistry, 175th National Meeting of the American Chemical Society, Anaheim, CA, March
12-17, 1978. (8) J. Medema, C. van Stam, V. H. J. de Beer, A. J. A. Konings, and D. C. Koningsbergetr, J . Catal., 53, 386 (1978). (9) J. Medema, R. Bosman, V. H. J. de Beer, A. J. A. Konings, and D.
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C. Koningsberger, submitted for publication.
(10) H. Knozinger and H. Jeziorowski, J . Phys. Chem., 82,2002 (1978). (11) H. Knozinger and H. Jeziorowski, J . Phys. Chem., 83,1166 (1979). (12) E. Payen, J. Barbilht, J. Grimblot, and J. P. Bonnelle, Spectrosc. Lett., 11, 997 (1978). (13)C. P. Cheng, M. S. Bohner, and G. L. Schrader, Jr., paper presented at the S i North American Meeting of the Catalysis Society, Chicago, IL, March 18-22, 1979;J . Catal., 80, 276 (1979). (14) H. Knozinger, H. Jeziorowski, and E. Taglauer, Int. Congr. Catal., 7th, Tokyo, 1980,accepted. (15) H. Jeziorowski and H. Knozinger, Chem. Phys. Lett., 43,37(1976). (16) P. Gajardo, R. J. DeclerckGrim6e, G. Dehraux, P. Olcdo, J. M. Zabala, P. Canesson, P. Grange, and B. Delmon, J . Less-Common Met.,
54, 311 (1977).
Heterogeneous Nucleation of Ice on Surfaces of Liquids J. Rosinski National Center for Atmospheric Research,t Boulder, Colorado 80302 (Received October 29, 1979) Publication costs assisted by the National Center for Atmospheric Research
It was demonstrated that heterogeneous nucleation of ice can take place at a liquid-liquid interface. A liquid (supercooledwater) -* solid (ice) phase transition seems not to require the presence of ice-nucleating sites on the surface of a solid. It is resonable to assume the existence of well-defined interfacial structures at the liquid-liquid interface inducing a liquid solid phase transition; the configuration of dipole moments in an ice-nucleatingliquid at the interface is likely responsible for changing the random orientation of water molecules in supercooled water into an icelike orientation, leading to the subsequent phase transition.
-
Introduction Experimental evidence shows that ice nucleation on an ice-nucleating solid surface takes place through adsorption of water molecules at a specific site. The mere presence of ice-nucleating sites indicates that solid surfaces are not energetically homogeneous. That is, different minute sites on the surface have different free energies of interaction with water molecules; thus the surface is randomly covered with different adsoription potentials. Adsorption of water molecules at a site ton the surface is a necessary but insufficient criterion for ice nucleation; the orientation of the adsorbed water molecules must result in an icelike structure to produce an ice embryo and to subsequently extend the ice surface into space for macroscopic ice growth. The energetically nonuniform distribution of adsorption sites, together with the specific orientation of adsorbed water molecules at each potential ice-nucleating site, gives each site a different probability of becoming an active ice-nucleating site. And this probability should increase with decreasing temperature and with increasing concentration of waiter molecules in the vicinity of a solid surface. Extensive theoretical studies of adsorption sites and dipole orientations for water molecules on silver iodide crystal surfaces were undertaken by B. Hale at the University of Missouri-Rolla. Both experimental work (Bryant and Mason,l Hallett and Shrivastava,2 Rosinski et al,,3 Rosinski and Nagarnoto4) and theoretical work (Fukuta and Paik,5 Kiefer and Hale: Hale et al.7!8)indicate that effective ice-nucleating sites are associated with chargedsurface regions such as the area immediately surrounding an Ag+ ion and that surface defects and irregularities play T h e National Center for Atmospheric Research is sponsored by the National Science Foundation.
an important role in orienting the adsorbed water molecules in icelike structures. However, most of this work has assumed an underlying substrate with a lattice structure similar to that of ice. I t was suggested (by Rosinski in Langer et aL9) that a theory of ice nucleation should consider the spatial distribution of electrical vectors corresponding to the constituent-link dipole moments in an organic molecule, rather than crystallographic similarity. Experiments were performed to find out whether the random orientation of water molecules in a supercooled liquid water could be changed into an icelike orientation in the presence of a liquid organic compound. Formation of the solid phase (ice) under such a condition should take place at the liquid-liquid interface, which in this case was an interface between supercooled water and a liquid organic compound.
Experimental Section The experimental procedure consisted of placing 1-10 drops of distilled water on the surface of a liquid organic compound. The temperatures of the water drops and the liquid were -0 and -1 "C, respectively, to minimize dissolution of the water in the liquid or the liquid in the water. The liquid organic compound was kept in an open pan (6 cm in diameter and 1.5 cm high) which was placed on an electrically cooled plate located in a particle-free hood. Vaseline or thermal joint compound was used to ensure uniform thermal contact between the cold place and the pan. The rate of cooling was -0.7 "C/min. Activated silica gel was kept in the apparatus to eliminate condensation of water vapor and formation of ice on the internal sides of the pan, which could produce spurious results. Water drops (0.0136 f 0.0014 g) of ca. 3-mm diameter were used in all experiments. The density of the naturally occurring organic compounds used was lower than that of
0022-3654/80/2084-1829$01 .OO/O . O 1980 American Chemical Society
