J . Phys. Chem. 1989, 93, 5882-5888
5882
above picture would not be applied for the present case, since the values of 8, 24" and 29' for r I 1/ 1, seem to be too small for H aggregates according to the exciton theory,,' although the more realistic extended dipole shows that the peak position of the aggregate can change depending also on the distance between the chromophores. The effect of introducing phenylcyclohexyl group, a mesogenic unit, into the hydrophobic part is discussed in terms of cluster formation at the air-water interface and the lower miscibility of the modified dye with C20 than that of a usual amphiphilic molecule having normal hydrocarbon chain as a hydrophobic part. Surface pressure-area isotherms of a dye mixed with C20 as well as a pure dye changed significantly, probably due to the afore-
mentioned two factors. LB films mixed with C20 showed a different set of absorption spectra depending upon the presence or absence of the mesogenic units. LB films of CN-PH with C20 showed large in-plane spectral anisotropy, whose limiting value for the infinite dipping speed is explained by the flow orientation of the crystallites at the air-water interface during the deposition process. This requires that the crystallite should be elongated along one of its axes, which suggests a possible role of anisotropic interaction caused by the benzene ring in the phenylcyclohexyl group. The results described above are closely related to each other and are to be attributed to the larger dispersion force between the mesogenic units. This method, modifying the hydrophobic part, will be useful especially when the functions of the films are to be enhanced or to be controlled externally.
(27) Sturmer, D. M.; Heseltine, D. W. In The Theory ofrhe Photographic Process, 4th ed.; James, T. H . , Ed.; Macmillan: New York, 1977, Chapter 8. (28) Czikkey, V.; Forsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 1 I , 207.
Acknowledgment. We thank Drs. N. Minari, K. Saito, K. Ikegami, and S. Kuroda for informative comments and helpful discussions on the flow orientation. Registry No. CN-C18, 34344-25-7; CN-PH, 117204-99-6;SQ-C18, 98987-52-1;SQ-PH, 117191-82-9;C 2 0 , 14923-81-0.
Conclusion
Surface Structure and Thiophene Hydrodesulfurization Activity of Mo/TiO, Catalysts Roger B. Quincy, Marwan Houalla, Andrew Proctor, and David M. Hercules* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: November 3, 1988; In Final Form: March 16, 1989)
Thiophene hydrodesulfurization (HDS) activity was measured for a series of Mo/TiO, catalysts having Mo loadings of 0.5-1 3.5 wt % MOO,. Raman data and thiophene HDS activity measurements suggest that three molybdenum species are present on oxidic Mo/Ti02 catalysts. A surface Mo interaction species is formed up to Mo loadings of 2.5 wt % MOO, and is the oxidic precursor to the most active species for thiophene HDS (intrinsic thiophene HDS activity of 816 cm3 of C4product/(h g of MOO,)). For higher Mo loadings, a second surface Mo interaction species is formed, which contributes to the background on the low-frequency side of the broad 950-960-cm-' Raman peak and is less active for thiophene HDS (intrinsic activity of 250 cm3 of C4 product/(h g of MOO,)). X-ray photoelectron spectroscopicdata show that both Mo surface species have similar dispersions. Catalysts with Mo loadings of >8-9 wt % MOO, also contain bulk MOO,. The distribution of the three Mo species for the oxidic Mo/Ti02 catalysts is derived from Raman and HDS measurements.
Introduction
The major emphasis in studies of hydrodesulfurization (HDS) catalysts is to correlate HDS activity with the structural characteristics of the catalyst. Having obtained this information, catalyst preparation (Le., type of metal and support, metal loading, method of impregnation, heat treatment, etc.) can be better adjusted to produce the most effective catalyst. Most research in HDS catalysis has focused on Mo/AI,O, and cobalt- and nickel-promoted Mo/AI2O3catalysts. Few previous investigations of HDS activity for M o / T i 0 2 catalysts have appeared in the literature.'-, MuraliDhar et aI.,l in a study of support effects on HDS activity, reported that an 8 wt % Mo/Ti02 catalyst was less active for the HDS of thiophene than a Mo/AI20, catalyst with the same molybdenum loading. Ng and Gulari, showed that thiophene HDS activity depends on the molybdenum loading for Mo/Ti02 catalysts and that this dependence is quite different from that observed for the Mo/AI2O3 system. More recently, Shimada et aL3 reported that, for a Mo loading of 10 wt % MOO,, titania-supported molybdenum was 50% more active for the HDS of dibenzothiophene than the corresponding Mo/ A1203catalyst. ~I~
~~
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In a previous study,4 we varied the molybdenum loading for titania-supported molybdenum catalysts and developed a methodology based on Raman spectroscopy to measure quantitatively the distribution of species on the catalyst surface. In the present paper we report X-ray photoelectron spectroscopic (XPS, ESCA) data and thiophene HDS activity for sulfided Mo/Ti02 catalysts. The major objective of this paper is to correlate the thiophene HDS activity with the distribution of species measured on the oxidic catalysts. Experimental Section
Catalyst Preparation. The T i 0 2 support was obtained from the Degussa Corp. (P-25 TiO,) and had a pore volume of 0.5 cm3/g, a BET surface area of 50 f 5 m2/g, and a 78/22 anatase/rutile ratio. Several additional catalysts were also prepared from a different batch of TiO, which had a slightly different anatase/rutile ratio (67/33). All catalysts were prepared in an identical manner. The support was first mixed with deionized water, dried (120 "C), and calcined (400 "C) in air for 8 h and then ground and sieved to 100 mesh before molybdenum was deposited by incipient wetness impregnation using ammonium heptamolybdate (Fisher Scientific) solutions. The resulting Mo/Ti02 catalysts were then dried at 120 "C for 16 h followed
G u l a r i ~ ~JE. .Cutal. 1985. 95, 33. ,
~
-~raishi,J.; Nishijima, A . J . ( 4 ) Quincy, R. B.; Houalla, M.; Hercules, D. M. J . Card. 1987, 106, 85.
0022-3654/89/2093-5882$01.50/0
0 1989 American Chemical Society
Thiophene HDS Activity of M o / T i 0 2 Catalysts by calcination in air at 500 'C for 16 h. When the TiOz support alone was subjected to the same impregnation (using only distilled water), drying, and calcination steps employed for the preparation of the Mo/Ti02 catalysts, we observed a decrease in the anatase/rutile ratio (78/22 to 69/31). A decrease in the anatase/rutile ratio was also observed for catalysts with molybdenum loadings of 1 2 . 5 wt % MOO,. This effect was more pronounced for catalysts with the lowest molybdenum loadings; the 0.5 wt % M o 0 3 / T i 0 2 catalyst had an anatase/rutile value 20% lower than the value measured for Ti02 (400 "C calcination) while the value for the 2.5 wt % Mo03/Ti02 catalyst was only 7% lower. Anatase/rutile values for catalysts with Mo loadings of >2.5 wt % Moo3 did not change appreciably (13%) from the value measured for the TiO, support. BET surface areas measured for catalysts with Mo loadings of 1 2 . 5 wt %I M o o 3 were generally lower (lO-l5%) than that measured for T i 0 2 . By comparing the results of the BET surface area measurements with the observed changes in anatase/rutile values, we can postulate that the transformation in anatase to rutile is responsible for the decrease in surface area. The nomenclature to be used for the catalysts will be "MoX" where X represents the molybdenum loading calculated as weight percent Moo3. The range of catalysts studied is from MOOS to Mo13.5, i.e., 0.5-13.5 wt % Moo3. Spectral Data Acquisition. Raman spectra were recorded on a Spex Ramalog spectrometer interfaced to a Spex Datamate computer. Details of the spectrometer and the sample preparation for analysis have been described earlier.4 The Raman data were transferred from the Datamate computer to an IBM PC. This facilitated detailed analyses of the data (e.g., use of background subtraction and damped nonlinear least-squares curve fitting). The methodology employed to quantitate the molybdenum species has been described in an earlier paper.4 As noted in the previous section, several of the M o / T i 0 2 catalysts differ in the anatase/ rutile content of the support (range is 59-77% anatase). Since the Raman peaks due to molybdenum species are referenced to the T i 0 2 anatase peak at 516 ~ m - ' the , ~ measured area of the 5 16-cm-l peak was normalized to a constant anatase content (65%). This allows direct comparison of Mo intensities for all catalysts. A Diano diffractometer equipped with a graphite monochromator and a copper X-ray tube was used to obtain X-ray diffraction (XRD) data. The X-ray tube was operated at 50 kV and 25 mA, and the scan rate was 0.4 deg/min (in 20 deg). The powdered samples were packed into a 2 X 2 X 0.2 cm hollowed-out plastic slide for analysis. The peak areas of the anatase (200) and rutile ( 1 11) reflections at 48.1" and 41.2' (in 28), respectively, were measured to determine the anatase/rutile composition of the catalysts. An equation relating the intensity ratio for these reflections to the percent anatase was derived from ref 5. Peak areas were measured with a planimeter; the reproducibility for a measurement was *5%. ESCA spectra were obtained with an AEI ES200A electron spectrometer equipped with an aluminum anode (AI K a = 1486.6 eV). The anode was operated at 12 kV and 20 mA; the operating pressure of the spectrometer during data acquisition was - 5 X IO-* Torr. The spectrometer was interfaced to an Apple I1 microcomputer. Data were subsequently transferred to an IBM PC where damped nonlinear least-squares curve fitting6*'was used to help resolve overlapping peaks. ESCA peak area intensity ratios (Imetal/lsupprt) were used to monitor the degree of metal dispersion on the TiOz support. As noted in the previous section, several of the Mo/Ti02 catalysts differed in surface area (40-56 mZ/g). Since the ESCA intensity ratio is a function of the surface area of the support,* the measured I,,,,/I,, zp ratios were normalized to a constant surface area (46 m2/g). This allows direct comparison of the molybdenum dispersion on the Ti02 support for ( 5 ) Spurr, R. A.; Myers, H. Anal. Chem. 1957, 29, 760. (6) Proctor, A.; Hercules, D. M. Appl. Specrrosc. 1984, 38,505. (7) Hughes, A . E.; Sexton, B. A . J . Electron Spectrosc. Relat. Phenom. 1988, 46, 3 1 . (8) Kerkhof, F. P. J . M.; Moulijn, J. A . J . Phys. Chem. 1979, 83,1612.
The Journal of Physical Chemistry, Vol. 93, No. 15, 1989 5883
1
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WAVENUMBER Figure 1. R a m a n spectra of Ti02 and four Mo/Ti02 catalysts scanned under ambient conditions: (a) Degussa KO2, (b) Mo1.5, (c) Mo6, (d) Mo8, (e) Mo10.5.
all catalysts. The reproducibility in peak area intensity ratios was *3%. Binding energy values for the catalysts were referenced to the Ti 2~312line of Ti02 (458.7 eV, ref 9-12). The calcined (oxidic) catalysts were analyzed as powders dusted onto sticky tape or as pressed (2000 psi) 6 X 15 mmz rectangular pellets. There were no differences in binding energies or peak area intensity ratios for catalysts scanned as powders vs those scanned as pressed pellets, indicating that pellet pressing does not affect the catalysts. Catalyst pellets were also mounted on a sealable probe and placed in a reaction furnace for sulfidation experiments. The sulfidation procedure involved using conditions necessary to reach steady state in the hydrodesulfurization of thiophene. The probe was first heated to the reaction temperature of 300 "C in 1 h under a flow of 50 cm3/min Ar. Ultrahigh-purity hydrogen (99.999%) was bubbled (50 cm3/min) for 12 h through a bubbler containing thiophene at 0 'C. The reaction was quenched at 300 " C for 1 h under 100 cm3/min Ar. The probe was then sealed and cooled under Ar before being transferred to the ESCA spectrometer. This ESCA probe system allows transport of samples from the reaction chamber to the spectrometer without exposure to air.I3 HDS Activity Measurements. The HDS of thiophene was carried out at 1 atm and 300 "C in a fixed-bed, flow microreactor operated under differential conditions. The catalyst bed consisted of 75 mg of crushed pellet, which was packed against a Pyrex wool plug in the Pyrex reactor tube. The catalysts were heated to the reaction temperature (300 "C) in 1 h under a flow of 50 cm3/min argon. The catalysts were then exposed to H2 (99.999%) bubbled through thiophene at 0 "C. The flow rate of the hydrogen/ thiophene reactant mixture was 50 cm3/min. The reaction products were sampled at 30-min intervals and analyzed with a Perkin-Elmer Sigma 2000 gas chromatograph equipped with dual thermal conductivity detectors. The reaction products (hydrogen sulfide, n-butane, 1-butene, butadiene, cis- and trans-2-butene) were separated with a 6-ft stainless steel column (l/s-in. diameter) packed with n-octane/porasil C. Peak areas of these products were obtained with a Perkin-Elmer LCI-100 integrator which was interfaced to the chromatograph. The total C4 area was used to calculate the thiophene conversion, and the ratio of n-butane area to total C4 area was used as an indication of hydrogenation se(9) Ramqvist, L.; Hamrin, K.; Johansson, G.; Fahlman, A,; Nordling, C. J . Phys. Chem. Solids 1969, 30, 1835. ( I O ) Fung, S . C. J . Caral. 1982, 76, 225. ( I 1) Tanaka, K.; Miyahara, K.; Toyoshima, I. J . Phys. Chem. 1984,88, 3504. (12) Tanaka, K.; Tanaka, K. J . Chem. Soc., Faraday Trans. I 1988.81, 601. (13) Ng, K. T.; Hercules, D. M. J . Phys. Chem. 1976, 80,2094.
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Mo Loading (wt% MOO,) Figure 2. Raman peak area intensity ratio of Mo interaction species/ TiOl anatase (1950/15,6)vs Mo loading for Mo/TiO, catalysts scanned under ambient conditions. Bars represent standard deviation.
M06 I
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lectivity. The reproducibility in percent conversion for a typical catalyst was f8%.
Results Raman Data. Figure 1 shows the Raman spectrum of Degussa TiO, and of four Mo/TiOz catalysts for the 765-1025-cm-' wavenumber region. The Raman spectrum of TiO, (Figure la) shows only one weak band at -795 cm-I in this region, assigned to the first overtone of the 396-cm-I anatase band.14 Figure I b shows the spectrum for Mol .5 (1.5 wt % M o 0 3 / T i 0 2 )catalyst. Note that a broad peak approximately 40 cm-' wide and centered at 945 cm-' is present on a background very similar to the TiO, spectrum (Figure la). This peak has been attributed to a Mc=O of a surface molybdate spe~ies'~,''or Mo stretching interaction specie^.^^'* Figure I C shows the Raman spectrum of the Mo6 catalyst. Note that the broad peak attributed to the Mo interaction species is now centered at 96.5 cm-l. This increase in peak frequency with Mo loading for the Mo interaction species has been observed previously when Mo/Ti02 catalysts have been scanned under ambient condition^.^*^-'^^" The position of this peak was independent of Mo loading and was located at 1000 cm-I when catalysts were scanned after 400 OC in situ 0, ~ a l c i n a t i o n . ~ Another important feature to note in the spectrum of Mo6 (Figure IC) is the change in background on the low-frequency (cm-') side of the broad peak. The background is now nearly horizontal and quite different from those of Ti0, (Figure la) and Mo1.S (Figure lb). The Raman spectrum of the Mo8 catalyst (Figure Id) shows a further increase in background on the low-frequency side of the broad peak. Figure l e shows the spectrum of Mo10.S. In addition to the broad peak attributed to Mo interaction species and the high background to the low-frequency side of this peak, sharp peaks centered at 820 and 996 cm-I are observed. These peaks are characteristic of Figure 2 shows a plot of the Raman peak area intensity ratio of Mo interaction species/TiO, anatase (1950/15]6) as a function of molybdenum loading for catalysts scanned under ambient conditions. Note that the intensity of the Mo interaction species increases with Mo loading to about 8 wt % MOO, and levels off at higher Mo loadings. Close examination of Figure 2 reveals, however, that the increase in intensity is linear with Mo loading only to about 2 wt % MOO,. Catalysts with Mo loadings of >8 wt % MOO, (Le., Mo10.5 and Mo13.S) contain MOO, in addition to the interaction species. The absolute amount of MOO, present (14) Balachandran, U.;Eror, N. G. J . Solid State Chem. 1982, 42, 276. ( 1 5 ) Stencel, J. M.; Makovsky, L. E.; Sarkus, T. A.; De Vries, J.; Thomas, R.;Moulijn, J. A. J . Cural. 1984, 90, 314. (16) Kg, K. Y . S.; Gulari, E. J . Catal. 1985, 92, 340. ( 1 7 ) Liu, Y. C.; Griffin, G. L.; Chan, S. S.; Wachs, I. E. J . Catal. 1985, 94. 108. ( I 8 ) Zingg, D. S.; Makovsky, L. E.; Tischer, R. E.; Brown, F. R.: Hercules, D. M . J . Phys. Chem. 1980, 8 4 , 2898.
I
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228
/ eV
220
Figure 3. Illustration of damped nonlinear least-squares curve fitting of ESCA spectra for (a) Mo6 catalyst and (b) MoS2 (1 = Mo4+ doublet, I1 = MoS+doublet).
in these two catalysts was calculated from a Raman calibration curve derived from MoOJTiO, physical mixtures4 Also, analogous to a previous paper,4 the amount of MOO, determined from Raman measurements (1.9 and 4.9 wt % MOO, for Mo10.5 and Mo13.5, respectively) was compared to values calculated from X-ray diffraction (XRD) data (1.7 and 4.6 wt % MOO,). Good agreement between Raman and XRD measurements of MOO, content was observed. The amount of molybdenum not present as MOO, can be calculated by subtracting the absolute amount of MOO, determined from Raman measurements from the total Mo loading for Mo10.5 and Mo13.5 catalysts. This amount of remaining molybdenum (8.6 wt % MOO,) is consistent with the observed variation in Raman intensity of the Mo interaction species for these catalysts (Figure 2). Thus, the intensity of Mo interaction species levels off at a Mo loading concomitant with the formation of MOO,. This was also observed for a previous series of Mo/Ti02
catalyst^.^ ESCA Data. X-ray photoelectron spectroscopy (XPS, ESCA) was used to provide information about the Mo dispersion and speciation for catalysts in both the oxidic and sulfided (thiophene/H,, 300 "C) states. The Mo 3d5/, binding energy was constant (232.4 f 0.12 eV) for oxidic catalysts with Mo loadings of >2.5 wt % MOO,. For the five oxidic catalysts with Mo loadings of 0.5-2.5 wt % MOO,, the measured Mo 3d5/2 binding energy was consistently lower; the Mo 3d5/, binding energy increased gradually from 23 1.7 eV for Mo0.5 to 232.2 eV for 4 wt % Mo03/TiOz. This trend of increasing Mo binding energy with increasing Mo content has been reported previously for oxidic M o / T i 0 2 catalyst^.^ The Mo 3d binding energies and peak areas for the sulfided catalysts were obtained by curve fitting the Mo 3d-S 2s envelope. Figure 3a shows the envelope for a representative catalyst (Mo6). Also shown for comparison is the spectrum for MoS, (Figure 3b). A Mo 3d5/2 binding energy of 229.1 eV and a S 2s binding energy of 226.3 eV were measured for MoS,; these values are consistent with binding energies reported in the literature for M O S , . ' ~ - ~ ~ Note that the Mo 3d envelope for the Mo6 catalyst was fitted with two sets of Mo doublets (Mo 3dSi2and 3d,/2) corresponding (19) Patterson, T. A,; Carver, J. C.; Leyden, D. E.; Hercules, D. M . J . Phys. Chem. 1976, 80, 1700. (20) Duchet, J. C ; van Oers, E. M.; de Beer, V. H. J.; Prins, R.J . Cutal.
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(21) Shuxian, Z.; Hall, W. K.; Ertl, G.; Knozinger, H . J . C a t ~ /1986, . 100, 167. (22) Roxlo, C. B.;Deckman, H. W.; Gland, J.; Cameron, S. D.; Chianelli, R. R. Science 1987, 235, 1629.
Thiophene HDS Activity of Mo/Ti02 Catalysts Q
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Mo Loading (wtX MOO,) Figure 7. Specific thiophene HDS rate (cm' of C4 product/(h g of MOO,)) vs Mo loading at steady state (650-1200 min). Bars represent standard deviation. For reaction conditions see Figure 6 .
to two different molybdenum species. The relative 3d5/?:3d312area ratio was fixed at 3:2, and the background contribution was accounted for by assuming an integral type background6 which was included in the basic peak shape. The Mo 3d5/2 binding energies are constant for all sulfided catalysts: 228.7 f 0.05 eV for the major species and 230.7 f 0.09 eV for the minor species. The molybdenum binding energy for the major species (228.7 eV) is comparable to that for MoS2 (229.1 eV). Note that the Mo 3d5p binding energy of 230.7 eV for the minor species is between the binding energies for Mo6+ and Mo4+ (Le., 232.4 and 228.7 eV, respectively). This value was therefore attributed to Mo5+. The distribution of the two molybdenum species fitted to the Mo 3d envelope was constant for catalysts with Mo loadings of >0.5 wt % MOO,; the major species constituted 83 f 3% of the total area. Sulfidation of the Mo0.5 catalyst resulted in a slightly different ESCA Mo 3d species distribution; the major species (binding energy of 228.7 eV) accounted for 73% of the total area. Sulfur to molybdenum atomic ratios were calculated for the catalysts by comparing the ESCA 1, zp/IMo 3d intensity ratios with the value measured for MoS2 (0.41). The S 2p intensities for the M o / T i 0 2 catalysts were corrected for the amount of sulfur adsorbed on the TiOzsupport. This was accomplished by removing the S 2p intensity measured for the TiOz support alone after sulfidation with the thiophene/H2 reaction mixture. The S/Mo atomic ratios were constant for all catalysts (1.20 f 0.16). Figure 4 shows a plot of the ESCA M o 3d/Ti 2p peak area intensity ratio vs Mo loading for catalysts in the oxidic state and after sulfidation with the thiophene/H2 reaction mixture. Note that for both oxidic and sulfided catalysts the Mo/Ti intensity
ratio increases linearly with M o loading to -6-8 wt % M o o 3 and levels off for further Mo additions. Also, it should be noted that sulfidation does not result in a significant decrease (average of 11%) in Mo/Ti intensity ratio. Thiophene HDS Actiuity Dura. Figure 5 shows a plot of thiophene H D S percent conversion vs time for three M o / T i 0 2 catalysts. The percent conversion decreases with time and reaches an apparent steady state after approximately 650 min. The steady-state activity (>650 min) for all catalysts was on the average 51 f 3% of the initial activity (e100 min). The thiophene HDS deactivation with time has been reported previously for M o / T i 0 2 catalysh2 A plot of thiophene percent conversion vs Mo loading is shown in Figure 6. The percent conversion [ 100% X total C4 area/(total C4 area thiophene area)] is an average value taken for analyses obtained between 650 min (steady state) and 1200 min of time on-stream. A very small thiophene conversion (-0.05%) was measured when the TiOz support alone was subjected to the same reaction conditions. This value is considered insignificant and is not subtracted from the results shown in Figure 6 . Note that the percent conversion for the catalysts increases with Mo loading up to 8 wt % MOO, and levels off for higher molybdenum loadings. However, as noted in Figure 2 for the Raman intensity ratios, the increase in percent conversion is linear only up to 2-2.5 wt % MOO, and increases at a slower rate for catalysts with Mo loadings between 2.5 and 8 wt % Moo3. The effect of molybdenum loading on thiophene conversion can be better illustrated by plotting the specific rate (cm3 of C4 product/(h g of MOO,)) vs loading as shown in Figure 7 . The highest rate (816 & 44 (3111, of C4 product/(h g of MOO,)) is
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The Journal of Physical Chemistry, Vol. 93, No. 15, 1989
obtained for the five catalysts having the lowest molybdenum loadings (i.e., Mo0.5-Mo2.5). Note that increasing the Mo loading above 2.5 wt % M o o 3 leads to a pronounced decrease in the specific rate. The shape of the curve for the specific rate vs Mo loading (Figure 7 ) is in good agreement with data published by Ng and Gulari., The product distribution was measured for the Mo/TiO, catalysts. Under our HDS reaction conditions the amount of butane produced (expressed as butane area/total C4 area) was constant at 36 i 2% for all catalysts.
Discussion Ouemiew. In a previous paper4 we showed that the molybdenum species (Mo interaction species, MOO,) on M o / T i 0 3 catalysts could be monitored by Raman spectroscopy as a function of molybdenum loading. For a series of M o / T i 0 2 catalysts (Mo loadings 1-15 wt % MOO,), the Raman intensity of the Mo interaction species increased to about 6 wt % MOO, and leveled off at higher Mo loadings. M o o 3 was detected at 7.5 wt % MoO,/TiO, and increased linearly with further Mo additions. The distribution of Mo species determined from Raman data correlated with ESCA and X-ray diffraction measurements4 The series of Mo/TiOz catalysts prepared for the present study differ slightly in the species distribution from the catalysts described above; the Raman intensity of Mo interaction species increased to about 8 wt % MOO, (Figure 2) before leveling off with further Mo additions. Also, the formation of MOO, was suppressed until molybdenum loadings of >8-9 wt % MOO,. The differences in Mo species distribution for the two series of Mo/TiO, catalysts can be attributed to the two different batches of Degussa TiO, used for catalyst preparation. The series of M o / T i 0 2 catalysts used for our previous study4 was prepared from a Degussa TiOz which had a lower anatase-to-rutile ratio (67:33 vs 78:22) and a 14% lower surface area than the Ti0, used for the present study. TiO, of lower surface area would be expected to accommodate less total surface molybdenum, and thus formation of MOO,should occur at lower molybdenum loadings. Similar behavior in the Raman intensity of the Mo interaction species has recently been published by Shimada et al., Thiophene HDS Activity and Surface Characterization, The decrease in HDS specific rate (i.e,, activity expressed per gram of Moo3) for molybdenum loadings above 2.5 wt % MOO, (Figure 7 ) suggests a change in the nature of molybdenum species on the catalysts. This is consistent with a previous studyZwhich showed that the thiophene HDS activity per gram of Mo (Le., intrinsic activity) decreased as the molybdenum content was increased. Ng and Gulari proposed,2 from an earlier Raman and FTIR study of oxidic M o / T i 0 2 catalysts,I6 that tetrahedrally coordinated molybdenum is the oxidic precursor to the most active species for thiophene HDS and is the major species for catalysts with low molybdenum loadings. The decrease in intrinsic activity with increase in Mo loading was explained by formation of a less active octahedral surface molybdate species., Further evidence for a change in molybdenum species distribution at a Mo loading of 2.5 wt % MOO, is suggested from the Raman data. If a single Mo species was being formed up to 8 wt % MOO,, one would expect the plot of 1950/1516 vs M o loading to be linear. Since the intensity ratio is only linear to 2 wt % M o o 3 and increases at a slower rate for Mo loadings between 2.5 and 8 wt % MOO,, the Raman data suggest formation of a second species between 2.5 and 8 wt % MOO,, with a lower Raman scattering cross section. This finding is consistent with recent Raman and X-ray absorption near-edge spectroscopic (XANES) data for W/AI20, catalysts which showed that the Raman scattering cross section of a tetrahedral tungsten oxide component (formed at low W loadings) is much higher than the cross section for the octahedral component.23 An alternate explanation for the linear increase in intensity of the broad Raman peak at 950-960 cm-I to only -2 wt % MOO,, and the slower rate of increase in intensity for catalysts with Mo (23) Horsley, J . A,; Wachs, 1. E.; Brown, J. M.; Via, G. H.; Hardcastle, F. D. J . Phys. Chem. 1987, 91, 4014.
Mo1.5 8;O
896
980
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WAVENUMBER
Figure 8. Raman spectra of five Mo/Ti02 catalysts after subtraction of Ti02 Raman spectrum: (a) Mo1.5, (b) Mo2.5, (c) Mo6, (d) Mo8, (e) Mo10.5. 140 3
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10.5
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Mo Loading (wt%Moo3) Figure 9. Raman peak area intensity ratio of the 780-1020-cm-' region/Ti02 anatase (1780-1020/f5!6) vs Mo loading for Mo/Ti02 catalysts scanned under ambient conditions. Bars represent standard deviation. loadings between 2.5 and 8 wt % MOO,, is that the intensity of this peak alone may not represent all Raman-active molybdenum species. Recall that the background on the low-frequency side of the broad peak at 950-960 cm-l increased as the molybdenum loading was increased (Figure 1). This increase in background can be seen more clearly by subtracting the TiO, Raman spectrum from spectra of the Mo/Ti02 catalysts. Figure 8 shows the resulting Raman spectra of five Mo/TiO, catalysts for the 7651025-cm-' region after T i 0 2 subtraction. Note the pronounced increase in overall intensity for the region 780-1020 cm-I and also the change in shape of the broad peak centered at 950-960 cm-' as the Mo loading is increased from 2.5 wt % Moo3 (Figure 8b) to 8 w t % MOO, (Figure 8d). The peak shape and background do not change significantly for catalysts with Mo loadings 1 8 wt % Mo03/TiOz (Figure 8d,e). The variation of total Raman intensity for the region 780-1020 cm-' (i.e., peak at 950-960 cm-I low-frequency background) is shown in Figure 9. Note that the total Raman intensity is now approximately linear with Mo loading to 8 wt % MOO, and levels off for further Mo additions. Recalling that the plot of thiophene HDS rate vs Mo loading (Figure 7 ) showed a decrease in activity after 2.5 wt % MOO,, which suggests a change in the nature of Mo species, we can attribute the change in the shape of the 950-960-cm-' peak and the increase in the Raman intensity of the low-frequency background for catalysts with Mo loadings of 2.5-8 wt % MOO, to formation of an oxidic Mo interaction species which is the precursor to a phase of low thiophene HDS activity.
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The Journal of Physical Chemistry, Vol. 93, No. 15, 1989 5887
Thiophene HDS Activity of M o / T i 0 2 Catalysts Mo10.5 aMo8
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Mo6 Mol35
a Mo4
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Mo/Ti02 catalysts.
0
3.5
7
10.5
14
Mo Loading (wtW MOO,) Figure 11. Distribution of Mo species as a function of molybdenum loading for Mo/Ti02 catalysts.
up to Mo loadings of 2.5 wt % M o o 3 as surface species A and to the species formed at higher Mo loadings as surface species B. Note also that the linear increase in overall Raman intensity with Mo loading to 8 wt 5% MOO, implies that species A and B have essentially the same Raman scattering cross sections. Furthermore, the linearity of ESCA Mo 3d/Ti 2p intensity ratios vs Mo loading to 6-8 wt % MOO, (Figure 4) indicates that these two species both have similar dispersions. The contribution of the two oxidic precursor Mo interaction species (Le., surface species A and B) to thiophene HDS activity can be examined in more detail by plotting the thiophene percent conversion vs the overall Raman intensity (178&1020/15]6), as shown in Figure 10. Note that the HDS activity is linear with overall Raman intensity up to molybdenum loadings of 2.5-4 wt % MOO, and that the activity increases at a much slower rate with further increase in the Raman intensity. This clearly illustrates the low HDS activity of species B, noted earlier. Speciation. Raman data and thiophene HDS activity measurements indicate that three molybdenum species are present on oxidic M o / T i 0 2 catalysts. A surface Mo interaction species (surface species A) is formed up to Mo loadings of 2.5 wt % MOO, and is the oxidic precursor to the most active species for thiophene HDS. For higher Mo loadings, a second surface Mo interaction species (surface species B), which contributes to the background on the low-frequency side of the broad 950-960-cm-’ Raman peak and is less active for thiophene HDS, is formed. Finally, catalysts with Mo loadings of >8-9 wt % Moo3 contain MOO,. The contribution of MOO, to thiophene HDS activity is negligible (see Figure 6). The following methodology was employed to construct a plot of the distribution of the three Mo species as a function of molybdenum loading. First, as previously outlined, the absolute amount of MOO,present in catalysts with high Mo loadings (i.e., >8 wt % MOO,) was determined from Raman measurements. Second, we assumed that catalysts with molybdenum loadings of 1 2 . 5 wt % Moo3 contain only surface species A and that the amount of this species remains constant for molybdenum additions above 2.5 wt % MOO,. This can be rationalized on the basis of the surface characteristics of the Ti02 support. Ng and GulariI6 recognized that two types of surface hydroxyl groups are present on Degussa P-25 TiO,; a terminal hydroxyl group is associated with one Ti4+ ion, and a bridging O H group is coordinated to two Ti4+sites. The reactivities of these two hydroxyl groups differ; the terminal O H group is more labile to exchange with other anions due to its basicity. Van Veen et al.24925developed a method for quantitating the two types of hydroxyl groups (referred to as
basic and acidic). They showed that for a 50 m2/g P-25 Ti02 (total hydroxyl content of 0.47 mmol of OH/g of Ti02) the basic OH groups constituted 0.14 mmol of OH/g of T i 0 2 of the total. Thus, it is possible that molybdenum will preferentially react first with the basic O H groups of Ti02,26forming surface species A, and that this species will continue to form until all of the basic hydroxyl groups are exhausted. Finally, the amount of surface species B formed for catalysts with Mo loadings of >2.5 wt % Moo3was determined by difference ( e g , total Mo loading minus Moo3 minus surface species A). Figure 11 shows the distribution of the three Mo species as a function of molybdenum loading. Note that species B increases to 8 wt % MOO, and levels off, concomitant with MOO, formation, for higher Mo loadings. It is interesting to mention that if one assumes that each M o atom binds to only one Ti02 hydroxyl group, the ratio of species B to species A (2.2, Figure 11) is in good agreement with the ratio of acidic to basic hydroxyl groups (2.4, see ref 24). On the basis of the proposed distribution of Mo species for oxidic Mo/Ti02 catalysts, we can estimate the intrinsic thiophene HDS activity of species A (expressed as cm3 of C4 product/(h g of species A)) and compare it to species B. To do so, one must assume that the surface structure of the sulfided catalyst (Le., the working HDS catalyst) is related to that of the corresponding oxidic catalyst (e.g., ref 27). As mentioned above, it was assumed that catalysts with Mo loadings of 1 2 . 5 wt % MOO, contain only surface species A. The intrinsic activity of surface species A is therefore calculated to be 816 cm3 of C4product/(h g of species A), since this was shown to be the average specific rate for Mo0.5-Mo2.5 catalysts (Figure 7 ) . The intrinsic activity of surface species B can be calculated from the data shown in Figures 6 and 11. With the assumption mentioned above that catalysts with Mo loadings of >2.5 wt % MOO, contain a constant amount of surface species A (Le., 2.5 wt % Moo3), the difference in thiophene conversion for Mo0.8 and Mo2.5 catalysts (- 1.5% Figure 6) represents the conversion attributed to 5.5 wt % M o o 3 of surface species B (Figure 11). This leads to an intrinsic activity of 250 cm3 of C4 product/(h g of species B) for surface species B. Note that the intrinsic thiophene H D S activity of surface species A (816 cm3 of C4product/(h g of species A)) is more than 3 times the value calculated for surface species B (250 cm3 of C4 product/(h g of species B)). The exact nature of surface species A and B is unknown at the present time, but one can speculate about the identity of these species from published data. As previously mentioned, N g and GulariI6 showed for M o / T i 0 2 catalysts that tetrahedral surface molybdate species are formed for low Mo loadings and at higher loadings polymeric surface Mo species form, having octahedral coordination. More recently, Mensch et a1.28proposed from an
(24) Van Veen, J. A. R.; Veltmaat, F. T. G.; Jonkers, G. J . Chem. SOC., Chem. Commun. 1985, 1656. (25) Van Veen, J . A. R.; Hendriks, P. A. J. M. Polyhedron 1986, 5 , 7 5 .
(26) Van Veen, J. A. R.; de Wit, H.; Emeis, C. A,; Hendriks, P. A. J. M. J . Catal. 1987, 107, 519. (27) Topsoe, H.; Clausen, B. S. Appl. Catal. 1986, 25, 273.
In the following sections we will refer to the Mo species formed
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J . Phys. Chem. 1989, 93, 5888-5894
EXAFS study of Mo/AI2O3 catalysts (prepared by equilibrium adsorption) that the basic hydroxyl groups of the A1203lead to the adsorption of tetrahedral molybdenum species and other sites on the A1203,presumably coordinatively unsaturated AI3+ sites, are responsible for the adsorption of octahedrally coordinated Mo species. Thus, it is reasonable to tentatively assign surface species A to a tetrahedral surface molybdate species and surface species B to surface polymolybdate species with octahedral coordination. Finally, it is necessary to see whether the Raman spectral features are consistent with the tentative assignments for surface species A and B. It must first be realized, however, that Raman bands characteristic of tetrahedral and octahedral Mo species overlap extensively in the 900-1000-~m-~region.2g Thus, the change observed in the 950-960-cm-l peak shape as molybdenum loading is increased from 2.5 to 8 wt % MOO, (Figure 8) may be due to formation of an additional Raman band characteristic of octahedral polymolybdate species. Features in the Raman spectra of supported Mo catalysts have also been observed in the 800-900-cm-' region (e.g., ref 29) and have been attributed to either MeO-Mo or Mo-0-X modes or to a combination of both (X refers to the support, e.g., X = Ti for Ti02) (ref 29 and references herein). The broad background on the low-frequency side of the 950-960-cm-' peak that we observe for Mo/TiO, catalysts with Mo loadings of >2.5 wt % MOO, suggests a species with a high degree of disorder. Thus, it is possible that this broad "peak" on the low-frequency side of the 950-960-cm-' peak is due to a Mo-0-Mo mode, characteristic of polymolybdate, and that the high degree of disorder is a result of the binding of a welldefined polymolybdate species to heterogeneous sites on TiO, (presumably acidic hydroxyl groups) or to the interaction of an (28) Mensch, C. T.J.; van Veen, J. A. R.; van Wingerden, B.; van Dijk, M. P. J . Phys. Chem. 1988, 92, 4961. (29) Payen, E.: Grimblot, J.; Kasztelan, S. J . Phys. Chem. 1987, 91, 6642.
ill-defined polymeric phase with one type of TiO, site.
Conclusions Raman data and thiophene HDS activity measurements suggest that three molybdenum species are present on oxidic M o / T i 0 2 catalysts. A surface Mo interaction species, tentatively assigned to tetrahedral surface molybdate species, is formed up to Mo loadings of 2.5 wt % MOO, and is the oxidic precursor to the most active species for thiophene HDS (intrinsic thiophene HDS activity of -816 cm3 of C4 product/(h g of MOO,)). A second Mo interaction species, tentatively assigned to surface polymolybdate species, is formed for higher M o loadings. The polymolybdate species contributes to the background on the low-frequency side of the broad 950-960-cm-l Raman peak and is less active for thiophene HDS (intrinsic activity of 250 cm3 of C4 product/(h g of MOO,)). ESCA data showed that both of these Mo surface species have similar dispersions. Catalysts with Mo loadings of >8-9 wt % MOO, also contain bulk MOO,. The distribution of these three Mo species as a function of molybdenum loading can be derived from Raman and H D S measurements. Acknowledgment. We gratefully acknowledge Dennis Finseth and Leo Makovsky from the Pittsburgh Energy Technology Center for stimulating discussion and for use of the Raman spectrometer. W e also thank Douglas P. Hoffmann for assistance with data analysis and Thomas Gasmire for machine shop work. This work was supported by the Department of Energy under Grant DEAC02-79ER10485. R.B. Quincy acknowledges the Gulf Oil Corp. and the A. W. Mellon Educational and Charitable Trust for Predoctoral Fellowships. Registry No. Moo3, 1313-27-5; MoS,, 1317-33-5;TiO,, 13463-67-7; thiophene, 110-02-1; n-butane, 106-97-8; I-butene, 106-98-9;butadiene, 106-99-0;cis-2-butene,590-18-1;trans-2-butene, 624-64-6; molybdenum, 11098-99-0.
Threadlike Micelles from Cetyltrimethylammonium Bromide in Aqueous Sodium Naphthalenesulfonate Solutions Studied by Static and Dynamic Light Scattering Wyn Brown,* Karin Johansson, and Mats Almgren Institute of Physical Chemistry, Box 532, University of Uppsala, 751 21 Uppsala, Sweden (Received: November 4 , 1988; In Final Form: January 31, 1989)
Dynamic and static light scattering measurements have been made on both dilute and semidilute solutions of the threadlike micelles formed in equimolar mixtures of CTAB and sodium naphthalenesulfonate in aqueous solutions. In dilute solution (1 mM), the first normal mode of the chains (7,)was isolated from the CONTIN decay time spectra as a function of measurement angle. The value of T~ agreed with that estimated by the free-draining Zimm model. At higher, semidilute concentrations (C > 0.02 M) above which the solutions become viscoelastic, the decay time distribution is bimodal with well-separated components on the time scale. The faster (q2-dependent)component reflects the cooperative motions of the transient network formed through interchain entanglements. The slow (q-independent) component of large amplitude apparently reflects the disruption/coalescence kinetics of the micellar aggregates, which are characterized by a strong positive concentration dependence of the measured relaxation rate (i.e., a relaxation time which decreased with increasing concentration of the CTAB/ naphthalenesulfonate complex) and an activation energy of about 84 kJmol-'.
Introduction ~n recent years, considerable interest has been taken in the solution properties of the extended micellar structures formed when salts are added to suspensions of cationic surfactants. When the salt is a simple one (e.g., NaBr) and is present at high concentration, long, flexible, threadlike micelles are formed. This was shown, for example, by ~ ~et ~ using ~ magnetic l birefrinl ( I ) Porte, G.; Appell, J.; Poggi, Y . J . Phys. Chem. 1980, 84, 3105. (2) Porte, G.; Appell, J . J . Phys. Chem. 1981, 85, 2511.
0022-3654/89/2093-5888$01.50/0
gence and light scattering on dilute solutions of cetylpyridinium bromide in solutions of NaBr of high concentration. It was shown that there are similarities between the properties of such solutions and those of conventional Polymer solutions. Subsequently, there have been numerous studies highlighting the properties of both the dilute surfactant solutions and also more concentrated ones, exploring the dynamical behavior above the overlap concentration where network behavior similar to that of semidilute solutions
(e)
( 3 ) Appell, J.; Porte, G.; Poggi, Y. J . Colloid Interface Sci. 1982, 87, 492.
0 1989 American Chemical Society
