J. Phys. Chem. 1982, 86. 2714-2717
2714
the ZnTPPS solubilized CTAC microemulsion media decreases the porphyrin's triplet lifetime from 50 to 5.6 ps. The second-order rate constant for the anion decay is k / ( t l ) N 5.7 X lo6 s-l. The decay time of ZnTPPS" is -450 ps, approximately 30 times shorter than in micellar solution and, thus, a diffusion-controlled back-reaction. Charge separation in microemulsions is consequently not as efficient as in micelles due to the lower charge density at the interface, although charge ejection from the interior is apparently achieved.
Conclusion This paper reports for the first time the photoinduced oxidation of tetrathiafulvalene to its radical cation by two metal complexes, Ru(bpy)gP+ and ZnTPPS4-. As has previously been mentioned, both the reduced species, R ~ ( b p y ) and ~ + ZnTPPS" are powerful reductants, and, thus, could afford the reduction of water to hydrogen or could promote other desired reductions. An important result of this study is the achievement of charge separation with organized assemblies.w31 The photoinduced electron transfer from T T F to ZnTPPS4- to form redox products does not occur in methanol. ZnTPPS" formation in micellar and microemulsion media is, however, quite sub(29)M. Griitzel in 'Micellization, Solubilization and Microemulsions", Vol. 2,K. L. Mittal, Ed., Plenum, New York, 1977,pp 531-48. (30)M. Gritzel, Zsr. J. Chem., 18, 364 (1979). (31)We wish to draw attention to two recent publications on lightinduced charge separation in colloidal assemblies that appeared after completion of this work, i.e., (a) I. Willner, J. W. Otvos, and M. Calvin, J. Am. Chem. SOC.,103,3203 (1981);(b) S. S. Atik and J. K. Thomas, ibid., 103, 3550 (1981).
stantial. This aspect is important for light energy conversion devices where the formation of long-lived redox products in high yield is desirable. On thermodynamic grounds alone there is much more driving force for the formation of redox products in the R u ( ~ ~ ~ ) , ~ + /system T T F than in the ZnTPPS"/TTF system. Electrostatic factors may also influence the course of these two reactions. Since both redox products in the light-induced reduction of Ru(bpy),2+ by TTF to Ru( b ~ y )and ~ + TTF+are positive ions, static repulsion aids the completion of the charge-transfer process, whereas the Coulombic attraction between ZnTPPS" and TTF+ inhibits the formation of redox products. Other authors have noted the quenching of a zinc porphyrin triplet with no evident free ion production. These reacitons were explained in the light of exciples formation in nonpolar media.32 However, this study hypothesises that the two species stay in a caged ion configuration from whence a rapid back-reaction can take place in a relatively polar solvent such as methanol. The present study of the light-induced one-electron oxidation of tetrathiafulvalene makes a contribution to the vast effort presently underway in the field of conducting salts. Acknowledgment. Support of this research was provided by the European Research Standardization Group of the United States Army. We also express our appreciation to Dr. P. P. Infelta for his aid in the evaluation of the intramicellar kinetic events. (32)I. G. Lopp, R. W. Hendren, P. D. Wildes, and D. G. Whitten, J. Am. Chem. SOC.,92,6440 (1970).
Effect of Support Material on Rh Catalysts S. D. Worley,*t C. A. Rlce,t 0. A. Maltson,' C. W. Curtls,t J. A. Guln,t and A. R. Tarred Depertmnt of Chemistry and Department of Chemical Engineering, Auburn Univm/ty,Auburn Uniwdty, Alabama 36849 (Received:January 5, 1982: I n Final Form: February 26, 1982)
The effect of support material on Rh/X catalysts has been studied by using CO as a probe molecule for chemisorption and infrared spectroscopy as the analytical method. Support materials including TiOz,AlZO3, SO2,kaolinite, and montmorillonitehave been compared as to their tendencies to produce the various CO/Rh/X species generally attributed to this catalytic system. The TiOzand SiOzsupports enable the most facile reduction of a rhodium precursor material to rhodium metal. Although all of the Rh/X catalysts contain some Rh(1) sites, alumina-supported ones contain the most nonmetallic rhodium. Kaolinite and montmorillonitemay contain impurities which tend to poison the Rh sites for CO chemisorption. The CO/Rh/SiOz infrared bands are much less intense than for alumina- or titania-supported rhodium. This is probably due to weak metal/support interaction for Rh/Si02 because the silica employed was of high purity. Introduction Recent work in these laboratories has focused on the use of infrared spectroscopy as a means of probing the interaction of CO with supported Rh catalysts. Our first paper on the subject reported the identification of eight different CO/Rh/Alz03species for catalysts containing various Rh loadings and following different reduction conditions;' the primary species are I-III.233 In that work infrared spectroscopy was also used as a means of identifying the oxidation state of Rh in its various site distributions on A1203 by use of CO as a probe adsorbate molecule.' A second 'Department of Chemistry. Department of Chemical Engineering. f
0022-3654/82/2086-27 14$0 1.2510
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paper has recently appeared which discusses an extension of the work to a systematic study of the effect of rhodium precursor material on the CO/Rh/AlZO3~ y s t e m . ~Ex(1) C. A. Rice, S.D. Worley, C. W. Curtis, J. A. Guin, and A. R. Tarrer, J . Chem. Phys., 74,6487(1981). (2) A. C. Yang and C. W. Garland, J. Phys. Chem., 61, 1504 (1957). (3)J. T. Yates, T. M. Duncan, S. D. Worley, and R. W. Vaughan, J . Chem. Phys., 70,1219 (1979).
0 1982 American Chemical Society
Effect of Support Material on Rh Catalysts
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982
2715
tensive prior work on the CO/Rh/A1203system from other tem, the support materials being zeolite and A1203. The H2/CO/Rh/Ti02 system has been the focus of a recent laboratories was cited and reviewed in these two papers'*4 NMR study,21and Rh/Ti02 has been shown to promote and thus will not be cited again here except where directly nitrogen chemisorption, whereas other supports are not relevant. Metalaupport interaction is a topic of great importance active in this respect.22 Guerra and Schulman have in current research involving supported transition-metal studied the CO/Rh/Si02 system at high CO pressures catalysts. To cite a few recent examples, Kellner and Bell using infrared spectro~copy.~~ have shown that the principal oxygenated products of the The purpose of this work was to extend our infrared reaction of H2 and CO over supported ruthenium are acstudies of the CO/Rh/X system, so as to observe the effect etaldehyde and methanol for silica- and alumina-supported of a systematic variation of support material. An extensive catalysts, re~pectively.~Several laboratories have shown similar infrared investigation of the CO/Pd/X system, that the nature of the support can alter the behavior of where X = A1203,Si02, Si02-A1203,and TiO,, has been the CO hydrogenation reaction over n i ~ k e l ; in ~ ~fact ' Ni/ reported recently by Vannice and c o - ~ o r k e r salthough ,~~ Ti02 catalysts are more active in methanation than are the emphases of the two investigations were quite different. Ni/Al2O3 or Ni/Si02 catalysts. Furthermore, methanation proceeds more rapidly on Ru/Si02 than on R U / A ~ ~ O ~ Experimental ,~ Section whereas the opposite has been reported for the water gas The supported catalyst samples used in this study were shift reaction for Pt/A1203and Pt/Si02.g Moody et al. prepared by spraying a slurry of RhCl3.3H20 (Rhhave shown that isotope exchange in CO is rapid over (N03)3.2H20in one experiment), a support material, nine alumina and have suggested that the alumina may be parts spectroscopic grade acetone, and one part distilled playing an active role in activating CO in the methanation water onto a 25-mm CaF2 window maintained at 353 K. reaction over alumina-supported transition-metal cataUpon evaporation of the solvents, a film of rhodium prelysts.'O cursor/support was obtained on the window. The sample Although much of the work performed to date on the was mounted in a Pyrex infrared cell containing CaF2 Rh/X system has been for X = A1203,11other support windows and treated on a metal vacuum line as described materials are being studied as well. Solymosi and copreviously.' Reduction was accomplished by means of workers have worked extensively on methanation of C02 heating in the presence of Matheson-grade hydrogen with over various supported Rh ~atalysts.'~-'~ They found that a small "wrap-around" ceramic tubing furnace.' The exthe support exerts a marked influence on the specific acperimental procedures used in preparing the Rh/X catativity of Rh, the order being Ti02 > A 1 2 0 3 > Si02.13They lysts and in introducing CO were the same as those depostulated that Ti02, being an n-type semiconductor, scribed in detail in ref l. The support materials employed provides much greater electronic interaction with Rh than in this study were alumina (Degussa Aluminum Oxide C, does A1203 or Si02.13 Infrared studies of the H2/C02/ 100 m2 g-l), titania (Degussa Titanium Dioxide P25,50 m2 Rh/X system indicated that Si02caused different behavior g-'), silica (Degussa Aerosill30,130 m2 g-l; Degussa Aerosil than did the supports Ti02, A1203,or Mg0.12 Andersson OX50,50 m2 g-' ; Cabot Cab-0-si1 M-5,200 m2 g-'), kaolinite and Scurrell have shown that the highest activity for (A14Si4010(OH)E, Burgess Pigment mineralogical grade, 13 carbonylation of methanol to acetic acid for a supported m2 g-'), and montmorillonite (A14Si8020(OH)4.nH20, rhodium catalyst is obtained when a molecular sieve zeolite Georgia Kaolin mineralogical grade, 17 m2g-'). In all cases is the upp port.'^ They have also reported that the oxithe commercial support materials were used without furdation state of Rh is dependent on the nature of the ther purification. support material, with Rh(II1) being present initially on Infrared spectra to be discussed in this work were obzeolite or MgO supports, while Rh(1) is present initially tained on a Perkin-Elmer 580 spectrometer which was on Si02or Sn02~upports.'~ Several infrared studies have operated at a resolution of 2.8 cm-' in the 2000-cm-l region. been reported involving Rb(CO)16as a precursor on a The ordinate scales for Figures 1-3 represent relative abvariety of supports.lG18 Primet and co-workers have emsorbances. ployed XPSlg and infrared20to study the CO/Rh/X sys(4)S. D. Worley, C. A. Rice, G. A. Mattaon, C. W. Curtis, J. A. Guin, and A. R. Tarrer, J. Chem. Phys., 76,20 (1982). ( 5 ) C. S. Kellner and A. T. Bell, J. Catal., 71,288 (1981). (6) M. A. Vannice, J. Catal., 37,449(1975);M. A. Vannice and R. L. Garten, ibid., 66,242 (1980). (7)R. Burch and A. R. Flambard, J. Chem. Soc., Chem. Commun., 123 (1981). (8)E. Zagli and J. L. Falconer, J. Catal., 69,1 (1981). (9)D. C. Grenoble, M. M. Estadt, and D. F. Ollis, J. Catal., 67,90 (1981). (10)D. C. Moody, M. Goldblatt, B. B. McInteer, and T. R. Mills, J. Catal., 67,240 (1981). (11) See extensive citations in ref 1 and 4. (12)F.Solymosi, A.Erdohelyi, and T. B h i g i , J.Chem. Soc., Faraday Trans. 1, 77, 2645 (1981). (13)F. Solymosi, A. Erdohelyi, and T. Bhs&gi, J. Catal., 68,371 (1981). (14)F. Solymosi and A. Erdohelyi, J. Catal., 70,451 (1981);see also T. Iizuka and Y. Tanaka, ibid, 70, 449 (1981). (15)S.L.T. Andersson and M. S. Scurrell, J . Catal., 71,233(1981). (16)G.C. Smith, T. P. Chojnacki, J. R. Dasgupta, K. Iwatate, and K. L. Watters, Inorg. Chem., 14, 1419 (1975). (17)A. K.Smith, F. Hugues, A. Theolier, J. M. Basset, R. Ugo,G. M. Zanderighi, J. L. Bilhou, V. Bilhou-Bougnol, and W. F. Graydon, Inorg. Chem.. 18. 3104 (1979). (18)E. 'W. Thornton, H. Knbzinger, B. Tesche, J. J. Rafalko, and B. C. Gates, J. Catal., 62,117 (1980).
Results and Discussion It has been reported that CO is weakly chemisorbed on Ti02 at room temperature (infrared bands were obtained near 2200 cm-' which disappeared rapidly upon evacuati or^).^^ At higher CO pressures for several hours, bands appear at 2050-2000 cm-' for CO/Si02 also.23 For these reasons CO exposure experiments were performed on films of all of the support materials in this work. A t the pressures of CO in our experiments (5 torr maximum), no infrared bands in the 2200-1800-cm-' region which could be attributed to CO adsorption on the support materials were detected. Thus, it is apparent that the CO bands to (19)M. Primet, J. C. Vedrine, and C. Naccache, J. Mol. Catal., 4,411 (1978). (20)M. Primet, J. Chem. SOC., Faraday Trans. 1 , 74, 2570 (1978). (21)T. M. Apple and C. Dybowski, J . Catal., 71,316 (1981). (22)D. Resasco and G. L. Haller, J . Chem. SOC.,Chem. Commun., 1150 (1980). (23)C. R. Guerra and J. H. Schulman, Surf. Sci., 7,229 (1967). (24)M.A. Vannice and S. Y. Wang, J . Phys. Chem., 85,2543(1981); M. A. Vannice, S. Y. Wang, and S. H. Moon, J. Catal., 71,152 (1981). (25) D. J. C. Yates, J. Phys. Chem., 65,746 (1961).
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Worley et ai.
The Journal of Physical Chemistry, Vol. 86,No. 14, 1982
f
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and If). In fact CO bands were not even detected for Rh/montmorillonite. As mentioned in the Experimental Section, several silica supports were employed from two different suppliers; all of the background spectra contained the band structure shown in Figure Id. It has been suggested that these background bands for SiOz are due to S i 4 combinations of lattice vibrations in that they cannot be removed by purification or deuterati~n.~'We believe that the background spectra in at least spectra l b and IC might be caused in part by some impurity(ies) (although kaolinite and montmorillonite do contain silica) and that the Rh may be partially poisoned toward CO chemisorption, although other explanations such as surface sites inhibiting reduction, etc., might be possible for the clays. Such poisoning might well affect the activity of supported Rh catalysts when these two supports are employed. The alumina- and titania-supported catalysts contained no such background spectral bands, and, as noted earlier, spectra l e and If do contain by far the most intense CO band structure. In spite of the apparent poisoning phenomenon for kaolinite and montmorillonite, all four supported Rh catalysts for which CO band structure was obtained contained the four bands generally attributed to CO/Rh/X catalysts, although the relative integrated band intensities do vary among the four catalysts. The bands at ca. 2030 and 2100 cm-' have been assigned to the antisymmetric and symmetric stretching modes for species I (see introd~ction).'-~The oxidation state of Rh is most certainly +1for species I;1,4J7J98*28 the nature of the site for species I remains controversial with some workers believing that species I corresponds to isolated atomic ~ites,1,~7~3J" while others have suggested that I corresponds to small twodimensional rafts.31 The band near 2060 cm-' for the 2.2% Rh/X catalysts corresponds to species 11. This band, in contrast to the 2100- and 2030-cm-' bands, does shift to higher frequency as the CO coverage is increased and has generally been attributed to a zero valent Rh cluster species. The broad band near 1900 cm-' which also shifts with coverage has always been attributed to a bridged carbonyl species (species 111). The relative integrated band intensities in Figure 1are of interest. It is evident that the integrated intensities for the species I1 and I11 bands relative to the twin species I bands are significantly greater for Rh/Ti02 than for Rh/A120,. We believe that this indicates that RhC13.3H20 is more efficiently reduced on Ti02 than on A1203, Le., there is relatively less Rh(1) on Ti02 than on Al2O,. This undoubtedly will affect the activities of the two supported catalysts. D. J. C. Yates has suggested that there is "more multiple CO adsorption" on a 1% Rh/A1203catalyst than on a 1% Rh/Si02 The band structures in spectra Id and le are in accord with this suggestion, for the relative amounts of species I1 and I11 to species I are greater for the Rh/Si02 catalyst than for Rh/A1203. It would appear that RhC13.3H20is also reduced more easily on Si@ than on A1203. Figure 2 illustrates the effect of changing the Rh loading on a Rh/Ti02 catalyst which has been prepared by re-
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I.IAMVU'1BER (m") Figure 1. Infrared spectra for CO adsorbed at rcom temperature on 2.2% Rh/X catalysts prepared from RhC13.3Hz0and prehydrogenated at 493 K. The spectra correspond to a 5.0-torr dose of CO following sequential doses at 0.01, 0.05, and 0.5 torr: (a) 0.08 mg cm-' unsupported Rh; (b) 4.8 mg cm-2 Rh/montmorlllonlte; (c) 4.4 mg cm-* Rh/kaolinlte; (d) 4.4 mg cm-' Rh/S02; (e) 4.4 mg cm-' Rh/AIz03; (f) 4.8 mg cm-' Rh/TiO,.
be discussed in this work do arise from CO/Rh/X species. Figure 1shows the 2200-1800-~m~~ region of the infrared spectra for CO adsorbed on 2.2% Rh/X catalysts following a sequential dose of CO at the 5.0 torr level. Figures showing the spectra following each sequential dose of CO for 2.2% Rh/A1203at the 0.01,0.05,0.5,5.0, and 50.0 torr level have been presented ear1ier.l~~ Spectrum l a corresponds to unsupported Rh. In this experiment 0.08 mg cm-2 of Rh was deposited on the CaF2 infrared window; this represents approximately the same amount of Rh as is present in each 2.2% Rh/X catalyst (spectra lb-0. As can be seen, no CO bands were detected for the unsupported Rh film. Dubois and Somorjai have observed CO vibrational modes at 2070 and 1870 cm-' for full coverage of a Rh(ll1) single crystal using EELS,26but evidently transmission infrared is not sufficiently sensitive to detect bands corresponding to these two CO species on the unsupported Rh film prepared here. However, Harrod and co-workers3*have reported CO bands near 2000 and 1820 cm-I for CO adsorbed on 10 Rh thin films in series using a multiple transmission technique. The dashed curves in Figure 1represent a background infrared spectrum at 298 K for the Rh/X catalysts following reduction, but immediately preceding introduction of CO. The support materials montmorillonite, kaolinite, and silica all contained background bands in the 22001800-cm-' region; these were present for catalysts with or without Rh present. The CO spectra when these three supports were employed (spectra lb-d) were markedly less intense than those for Rh/A1203and Rh/Ti02 (spectra l e (26)L.H.Dubois and G. A. Somorjai, Surf. Sci., 91,514 (1980).
(27)J. B.Peri, J. Phys. Chem., 70,2937 (1966);H.A. Benesi and A. C. Jones, ibid., 63, 179 (1959). (28)K. W. Watters, R. F. Howe, T. P. Chojnacki, C. M. Fu, R. L. Schneider, and N.B. Wong, J . Catal., 66, 424 (1980). (29) J. T. Yates, T. M. Duncan,and R. W. Vaughan, J. Chem. Phys., 71,3908 (1979). (30)R. R. Cavanaugh and J. T. Yates, J.Chem. Phys., 74,4150(1981). (31)D.J. C. Yates, L. L. Murrell, and E. B. Prestridge, J. Catal., 57, 41 (1979). ._ ~ - .-- , . (32)J. F.Harrod, R. W. Roberts, and E. F. Rissmann, J . Phys. Chem., 71,343 (1967).
The Journal of Physlcal Chemisrry, Vol. 86, No. 14, 1982 2717
Effect of Support Material on Rh Catalysts
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Flgure 2. Infrared spectra for CO adsorbed at room temperature on X % Rhm02catalysts prepared frOm Rhckf3H20 and pfehybogenateed at 493 K. The spectra correspond to a 5.0-torr dose of CO followlng sequential doses at 0.01, 0.05, and 0.5 torr: (a) 0.5% Rh/TI02, 8.9 mg cm-2; (b) 2.2% Rh/TI02, 4.8 mg cm-*; (c) 10% Rh/TIO,, 3.1 mg cm-2.
0.0:
2:
duction of RhCl3-3H2O/TiO2at 493 K. Again the spectra obtained following a 5 torr sequential dose of CO are shown. In Figure 2a only the species I bands are obtained for a 0.5% Rh/Ti02 catalyst. A similar observation has been reported for 0.5% Rh/A1203 catalysts with both RhC13-3H201and Rh(N03)3-2H204 as precursors. The fact that only species I is detected on a 0.5% Rh/X catalyst, of course, provides circumstantial evidence for an isolated site for species I. It is notable also that the band intensities for species I on the 0.5% Rh/Ti02 catalysts are markedly lower than those for species I on a 0.5% Rh/A1203catalyst.' This provides further support for our belief that reduction of RhC13-3H20is more efficient on Ti02 than on A1203in that species I refers to Rh(1). Figure 2b for 2.2% Rh/Ti02 has been discussed already; figure 2c corresponds to the infrared spectrum of CO adsorbed on a 10% Rh/Ti02 catalyst. Clearly species I1 and 111corresponding to metallic cluster sites are enhanced relative to species I for this 10% Rh/Ti02 catalyst. Again this enhancement is considerably more pronounced for Rh/Ti02 than for Rh/A1203.' Earlier work in these laboratories showed that Rh/Al2O3 catalysts prepared from Rh(N03)3.2H20 yield more of species I1 and I11 relative to I than to analogous catalysts prepared from RhC13.3H20.4The present work has shown that the supports Ti02 and Si02 also lead to more of species I1 and I11 relative to I for a RhC13-3H20precursor. Figure 3 illustrates the combination of the two trends, namely, the infrared spectra of CO/Rh/Ti02 as a function of coverage for a 2.2% Rh/Ti02 catalyst prepared from Rh(N03)3.2H20. Although the reduction temperature was 673 K for the experiments in Figure 3, similar results are obtained for lower reduction temperatures. Species I1 and I11 grow in before species I and shift with coverage as is the case for catalysts prepared from a RhCl3*3H20precursor; however, it is clear from Figure 3 that there is a tremendous enhancement of species I1 and 111relative to I in this experiment. The obvious conclusion to be drawn from these data is that a significantly larger percentage of the Rh atoms on a support will be zero valent if the support is Ti02 and the precursor is Rh(N03)3.2H20than
0
WAVENUMBER ( c M - ~ ) Flgure 3. Infrared spectra for CO adsorbed In sequential doses at room temperature on a 2.2% Rh/Ti02 catalyst prepared from Rh(N03),.2H20 and prehydrogenatedat 673 K: (a) P, = 0.01 torr: (b) P, = 0.05 torr; (c) PF = 0.5 torr; (d) P, = 5.0 torr. Catalyst film contained 4.9 mg cm- .
for an A 1 2 0 3 support and a RhC13.3H20precursor. We would predict substantial differences in activity and behavior for Rh/X catalysts prepared from RhCl3.3H20as opposed to Rh(N03)3.2H20and for A1203 as opposed to Ti02as support materials. Studies along these lines are in progress in these laboratories.
Conclusions This investigation has shown that the nature of the support material in Rh/X catalysts is quite important in CO chemisorption behavior. Titania and silica supports provide more of the CO species characteristic of metallic Rh than does an alumina support. However, CO chemisorption is clearly more pronounced on Rh/X catalysts when X = A1203or Ti02 than for those when X = Si02, kaolinite, or montmorillonite. We believe that this could be due in part to a poisoning effect caused by impurities in the latter two supports (pure silica is expected to have weak background bands in the 2200-1800-cm-' region2'). In general, it has been shown that the activity of M/X catalysts is in the order M/Ti02 > M/A1203 > M/ Si02,617,9,12-14 The present infrared studies of CO chemisorption on Rh/X are certainly consistent with this order of activity. Acknowledgment. S.D.W., C.A.R., and G.A.M. gratefully acknowledge the support of the National Science Foundation through Grant No. CHE-7920825, the Research Corporation, and the Auburn University Energy Grant-In-Aid Program. C.W.C., J.A.G., and A.R.T. are grateful to the U.S. Department of Energy for equipment support of this work under contract No. EX76-S-01-2454. The authors also acknowledge helpful comments from two referees concerning possible impurities in support materials.
