1520
J . Phys. Chem. 1990, 94, 1520-1526
Distribution of Molybdenum Oxidation States in Reduced Mo/TiO, Catalysts: Correlation with Benzene Hydrogenation Activity Roger B. Quincy, Marwan Houalla, Andrew Proctor, and David M. Hercules* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania I5260 (Received: April 24, 1989; In Final Form: August 28, 1989)
A 5 wt % Mo03/Ti02catalyst was reduced in hydrogen at various temperatures to produce a surface with average Mo oxidation states between +6 and 0. The changes in molybdenum oxidation states as a function of the extent of reduction were monitored by gravimetric analyses and X-ray photoelectron spectroscopy (XPS, ESCA), and the results were correlated with benzene hydrogenation activity. ESCA Mo 3dSl2binding energy values for the various Mo oxidation states on a 5 wt 75 Mo03/Ti02 catalyst show a shift of 5.1 eV between M o + ~(232.7 eV) and Moo (227.6 eV). The benzene hydrogenation activity was found to depend strongly on the extent of reduction of the Mo phase. When the catalyst was reduced at temperatures of 5350 'C (average Mo oxidation state L4), the benzene hydrogenation rate was very low (550 'C. Comparison of benzene hydrogenation activity with the distribution of Mo oxidation states determined by ESCA suggests that molybdenum ions with an oxidation state of +2 are the most active species.
Introduction Titania-supported molybdenum (Mo/Ti02) catalysts are active for a wide range of important industrial reactions. For example, Tanaka et al.' showed that olefin metathesis occurs most favorably on Mo/TiO, catalysts in the oxide form or when mildly reduced to an average Mo oxidation state higher than +4. Catalysts with a more reduced surface (Mo oxidation state lower than +4) were shown' to be active for olefin hydrogenation and isomerization. Segawa et alS2reported similar results for Mo/Ti02; hydrogenation of 1,3-butadiene required lower Mo oxidation states than metathesis of propene or isomerization of cyclopropane. Mo/Ti02 catalysts in the sulfided form have also been shownM to be active for hydrodesulfurization (HDS). One important piece of information lacking in the studies of supported Mo catalysts is a correlation of specific molybdenum oxidation states with activity. Knowing the specific molybdenum oxidation state responsible for catalysis of a particular reaction allows the pretreatment of the catalyst to be better adjusted to generate the active sites. The objectives of this paper were to produce a series of Mo/Ti02 catalysts with a variety of molybdenum oxidation states between +6 and 0, to quantitatively determine the distribution of these oxidation states with X-ray photoelectron spectroscopy (XPS, ESCA) and gravimetric analyses, and to correlate the results with benzene hydrogenation activity measurements. Experimental Section Catalyst Preparation. Degussa P-25 Ti02 (78/22 anatase/ rutile ratio, pore volume -0.5 cm3/g, BET surface area 50 f 5 m2/g) was first mixed with deionized water, dried (120 "C) and calcined (400 " C ) in air for 12 h, and ground and sieved to 100 mesh before molybdenum (5 wt % Moo3) was deposited by incipient wetness impregnation using ammonium heptamolybdate (Fisher Scientific) solution. The resulting 5 wt % MoO3/TiOz catalyst was then dried at 120 "C for 16 h followed by calcination in air at 500 'C for 16 h. This catalyst has been previously shown6 to consist of uniformly dispersed surface molybdate species; bulk ( I ) Tanaka, K.; Miyahara, K.; Tanaka, K . Bull. Chem. SOC.Jpn. 1981, 54, 3106.
(2) Segawa. K.; Kim, D. S.; Kurusu, Y.; Wachs, I. E. In Proceedings of the 9th International Congress on Caalysis; Phillips, M. J., Ternan, M., Eds.; The Chemical Institute of Canada: Ottawa, 1988; p 1960. (3) MuraliDhar, G.; Massoth, F. E.; Shabtai, J. J . Coral. 1984, 85, 44. (4) Ng, K. Y. S.; Gulari, E. J . Caral. 1985, 95, 3 3 . ( 5 ) Shimada, H.; Sato, T.; Yoshimura, Y.; Hiraishi, J.; Nishijima, A. J . Catal. 1988, 1 IO, 275. (6) Quincy, R. B.; Houalla, M.; Proctor, A,; Hercules, D. M. J . Phys. Chem. 1989, 93, 5882.
0022-3654/90/2094- 1520$02.50/0
molybdenum compounds (e.g., M a , ) were not detected. In order to duplicate several experiments in this study, a second batch of 5 wt % MoOJTiO, catalyst was prepared by using the same procedure described above. Both preparations produced similar catalysts. The anatase/rutile ratio and BET surface area for the oxidic catalysts were the same as those measured for the Ti02 carrier. The nomenclature used for the catalysts will be "Mo5X" where 5 represents the molybdenum loading calculated as weight percent MOO, and X corresponds to the reduction temperature in Celsius. Gravimetric Analyses. Gravimetric analyses were carried out using a Cahn 113 microbalance. Catalysts (50-100 mg) were heated to the desired temperature of reduction (300-650 "C) in 1 h under 40 cm3/min of ultrahigh-purity (99.999%) 10% 0 2 / H e and maintained a t this temperature until constant weight was obtained. The system was then purged for 30 min with ultrahigh-purity H e before switching the gas flow to ultrahigh-purity hydrogen (95 cm3/min). Both He and H2 gases were introduced through an activated oxyabsorbent. The gas flow exited the microbalance through a liquid nitrogen trap to collect the water formed during reduction (12-13 h). After reduction, the system was again purged with He for 30 min before reoxidizing the catalyst with 10% 0 2 / H e (40 cm3/min). The weight loss due to reduction was taken from the measured difference of the reoxidized and reduced weights for all catalysts r e d u d below 650 'C. The catalyst reduced at 650 'C could not be reoxidized to the initial weight measured after drying. Therefore, the weight loss due to reduction for this catalyst was taken from the difference between the initial weight after drying and the reduced weight. Gravimetric analyses were repeated for several reduction temperatures; the results obtained agreed to within 2%. X-ray Diffraction and BET Surface Area Data. A Diano 700 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 28 deg). The powdered samples were pressed (2000 psi) into -8 X 18 mm2 rectangular pellets and mounted on a probe for reduction treatment. (The reduction treatment is described below in the ESCA section.) After reduction and ESCA analysis the reacted pellets were removed from the probe and mounted on a glass side with vacuum grease for XRD 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 reacted catalyst. When the anatase reflections were very weak (encountered at reduction temperatures of 550-650 "C), the anatase (101) and rutile ( 1 10) reflections at 25.4" and 27.5' (in 0 1990 American Chemical Society
Oxidation States in Reduced Mo/Ti02 Catalysts 28), respectively, were used to determine the composition. An equation relating the intensity ratio for either set of these reflections to the percent anatase was obtained from ref 7. Peak areas were measured with a planimeter or from digitized data; the reproducibility for a measurement was *5%.8 BET surface area measurements were made with a Model 4200 automatic surface area analyzer (Beta Scientific Corp.) using nitrogen as the adsorbing gas. Reacted pellets were powdered by mortar and pestle and placed in a Pyrex U-tube for drying and surface area measurement. ESCA Analyses. ESCA spectra were obtained with a Leybold-Heraeus LHS- 10 photoelectron spectrometer using an aluminum anode (A1 K a = 1486.6 eV) with a hemispherical analyzer operating in the constant pass energy mode (50 eV). The X-ray source power used was 240 W (12 kV and 20 mA), and the operating pressure of the spectrometer during data acquisition Torr. The spectrometer was interfaced was typically 5 X to an HP-1000 microcomputer. Data were subsequently transferred to an IBM PC environment where damped nonlinear least-squares curve fitting"' was used to help resolve overlapping peaks. The methodology to curve fit the envelopes for Mo5 after reduction treatments involved the use of parameters (e.g., binding energy values, full width at half-maximum) derived from curve fitting the Mo 3d doublet of oxidic Mo5 and Mo metal foil. For example, the parameters obtained from the fit of oxidic Mo5 for M o + ~were used to fit M o + ~to the Mo 3d envelopes of Mo5 after reduction. In a similar manner, parameters for M o + ~were obtained from the fit of the envelope for Mo5 after reduction at 207 'C; this mild reduction treatment produced a Mo 3d envelope that consisted primarily of M O + ~More . severe reduction treatments produced envelopes that were curve fitted to obtain parameters for the other Mo oxidation states (Le., M o + ~M , o + ~M , O + ~ ) It . must be stressed that parameters obtained for the Mo oxidation states were always kept constant in the fit of envelopes for all reduction treatments. Also, the relative 3d5 2*3d3/2area ratio for each set of spin-orbit doublets fitted to a L o 3d envelope was fixed at the theoretical value of 3:2 and the background contribution was accounted for by assuming an integral type backgroundg which was included in the basic peak shape. A goodness-of-fit parameter (x2value)" and reference to gravimetric measurements of average Mo oxidation state found for a particular reduction temperature were used to assist curve fitting. Several of the Mo 3d envelopes, particularly those resulting from hightemperature reductions (>500 "C), contained a small S 2s peak at -225.5 eV. This peak was due to sulfur in the stainless steel probe used for mounting catalyst pellets and was thus curve fitted and removed from the overall Mo 3d envelope. ESCA peak area were used to monitor the degree of intensity ratios (IMo3d/ITiZp) Mo dispersion on the TiO, support as a function of reduction treatment. The reproducibility in peak area intensity ratios for an oxidic Mo5 catalyst was within f l % . Binding energy values for the catalysts were referenced to the Ti 2~312line of Ti02 (458.7 eV, ref 6). For reduction experiments the catalysts were mounted on a stainless steel probe as pressed (2000 psi) 12 X 18 mmz rectangular pellets and introduced into a stainless steel bolt-on reactor attached directly to the LHS-IO electron spectrometer. Each reduction experiment was performed with a new sample pellet in the oxidized form. The reactor was first evacuated with a turbomolecular pump at 100-120 OC and then heated to the desired reduction temperature in -4 h under a flow of 100 cm3/min of ultrahigh-purity H2. The H 2 gas was introduced through an activated oxyabsorbent. After the reduction treatment of 12 h the reactor was cooled to -50 'C under H2 flow and then evacuated for at least 15 min. The probe was then transferred directly from the reactor (7) Spurr, R. A.; Myers, H. Anal. Chem. 1957, 29, 760. (8) Quincy, R. B.; Houalla, M.; Hercules, D. M. J . Catal. 1987, 106,85. (9) Proctor, A.; Hercules, D. M. Appl. Spectrosc. 1984, 38, 505. (10) Hughes, A. E.; Sexton, B. A. J . Electron Spectrosc. Relat. Phenom. 1988. 46. 3 I . (1'1) Ansell, R. 0.; Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J . Electroanal. Chem. 1979, 98, 79.
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1521
'
0300
400
500
600
700
Reduction Temperature (C)
Figure 1. Average Mo oxidation state versus reduction temperature from gravimetric analyses (0,A) and from curve-fitted ESCA Mo 3d spectra (0).
to the analyzing chamber via a gate valve without being exposed to air. Activity Measurements. The hydrogenation of benzene was carried out at 1 atm and 160 "C in a fixed-bed, flow microreactor operated under differential conditions. The catalyst bed consisted of 75 mg of crushed pellet packed against a quartz wool plug in a quartz reactor tube. The catalysts were prereduced at the desired temperature (300-650 "C) for 12 h under 50 cm3/min of ultrahigh-purity H,. After the catalysts were cooled to the reaction temperature of 160 OC, a bubbler containing benzene (Aldrich 99.9%) at 10 OC was opened to hydrogen flow. The flow rate of the hydrogen/benzene reactant mixture was 50 cm3/min. The effluent gas was sampled at 10-min intervals and analyzed with a Perkin-Elmer Sigma 2000 gas chromatograph equipped with a flame ionization detector. The reaction product (cyclohexane) was separated from benzene with a 6-ft stainless steel column (1/8-in. diameter) packed with 80/100 mesh chromosorb W-HP. Peak areas for cyclohexane and benzene were obtained with a Perkin-Elmer LCI- 100 integrator which was interfaced to the chromatograph. The rate of benzene hydrogenation expressed as mmol of cyclohexane product/(h g of MOO,) is an average value calculated for analyses obtained between 200 min (steady state) and -360 min of time on-stream. The reproducibility in benzene hydrogenation rate for a typical catalyst was *9%.
Results and Discussion Gravimetric Analyses. Figure 1 shows a plot of average Mo oxidation state versus reduction temperature obtained from gravimetric analyses. Note that a sharp decrease in average Mo oxidation state from ca. 4.8 to 0.3 is observed as the reduction temperature is increased from 300 to 650 O C . Note also that two different values are shown from gravimetric analyses for Mo5-600 and Mo5-650 (i.e., Mo5 reduced at 600 and 650 OC, respectively). The larger value was calculated taking into account the small weight loss observed when the TiOz support alone was subjected to 650 O C reduction. It will be shown later from ESCA data that the larger value for these two reduction experiments represents an upper limit. The data in Figure 1 suggest that the different reduction treatments have produced catalysts with a variety of Mo oxidation states between +6 and 0. The sharp decline in average Mo oxidation state versus reduction temperature (Figure 1) is consistent with previously published results for Mo/Ti02 catalysts;' Tanaka et a1.l stated that Mo can be substantially reduced (presence of zerovalent Mo) at mild reduction temperatures (500 "C). The ease of Mo reduction for Mo/TiO, catalysts is quite different from that reported for Mo/Al2O3.I2J3 Naka(12) Nakamura, R.; Pioch, D.; Bowman, R. G.; Burwell, R. L., Jr. J . Catal. 1985, 93, 388. (13) Redey, A.; Goldwasser, J.; Hall, W. K. J . Catal. 1988, 113, 82.
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
1522
Quincy et al.
100 7
24
N
2 X
I
\
I
0 0
'1 i
LZi 6~ 0 0
30-
*
Is;
j
~
0 1
0
400
500
600
700
versus reduction temperature.
I\'
u
t
300
200
Reduction Temperature (C) Figure 3. ESCA IMo3d/ITi2p peak area intensity ratio of Mo5 catalyst
;
!l
100
100
I
I
200
300
1
400
500
456.7
458.7
@ c 1
600
-
1
b
700
Reduction Temperature (C) Figure 2. (a) Anatase/rutile composition of Mo5 catalyst ( 0 )and Ti02 (0)versus reduction temperature from XRD data. (b) BET surface area values versus reduction temperature for Mo5 ( 0 )and Ti02 (0).
mura et al.Iz showed for Mo/A1203 catalysts that the average Mo oxidation state decreased to only +2 after 650 O C H2 reduction and reached zero at 800-950 OC. Redey et al.13 reported an average Mo oxidation state of -+4 after 500 OC H2 reduction and a value of + 1 after reduction at 900 O C for Mo/AI2O3 catalysts. Thus, it is evident that Mo is more easily reduced when supported on T i 0 2 than on A1,03. X-ray Diffraction and BET Surface Area Data. X-ray diffraction (XRD) data were collected after the various reduction treatments to see whether any molybdenum phases could be observed and to check whether the Ti02support changed from its original 78/22 anatase/rutile composition. Diffraction patterns due to molybdenum compounds were not seen under any circumstances. Figure 2a shows a plot of anatase/rutile composition as a function of reduction temperature for the 5 wt % Mo03/Ti02 catalyst and for the Ti02 support alone. It can be seen that for reduction temperatures of 600 OC the Ti 2~312peak of T i 0 2 and Mo5 differs from that of oxidic Ti02 (Figure 4a). Note that the Ti 2p3/2peak for TiO, and Mo5 reduced at 648 OC (large dashed line and small dashed line, respectively, in Figure 4a) shows the presence of a small shoulder on the low binding energy side of the peak. The shoulder on the low binding energy side of the Ti 2~312peak suggests that partial reduction of Ti+4has occurred. Overlaying the Ti 2~312regions of oxidic Ti02, Ti02-648, and Mo5-648, as shown in Figure 4a, also suggests that the presence of molybdenum may inhibit the reduction of TiO,. Thus, the correction made for Ti02reduction in the calculation of average Mo oxidation state from gravimetric analysis at reduction treatments of 600 and 650 OC (Figure 1) represents an upper limit. An estimate of the Ti 2p3/2 peak position for the reduced Ti species can be made by substracting the Ti 2p envelope of oxidic Ti02 from the envelope of Ti0,-648; Figure 4b shows the resulting difference spectrum. To help bring out this feature more clearly, Figure 4c shows the quotient spectrum, obtained by dividing the Ti 2p envelope of Ti02-648by the envelope of oxidic Ti02. Note that the Ti 2p3 peak position for the reduced Ti species is centered at -456.7 e\! Thus, a shift in Ti 2p3/2 binding energy of -2 eV is observed between Ti+4(458.7 eV) and the reduced Ti species (456.7 eV). A shift in the Ti 2p3 peak of Ti+4to lower binding energy by 2 eV is consistent with the formation of Ti+3(ref 16 and references therein). Figure 5 shows ESCA spectra of the Mo 3d region for Mo5 after various treatments. Also shown for comparison is the spectrum for molybdenum metal foil (Alfa Products) after treatment in H2 at 657 OC (Figure Sa). Note that the shape of the Mo 3d envelope for Mo5 in the oxidic form (Figure 5f) is characteristic of MO'~;'~the Mo 3d5 binding energy is centered at 232.7 eV. Reduction at 304 OC (Figure 5e) results in broadening of the Mo 3d envelope and a shift to lower binding energy; the peak at the lowest binding energy is now centered at -229.6 eV. More severe reduction conditions cause the envelope to shift to still lower binding energies (Figure 5 ) . For the highest reduction temperature used (662 "C), the shape of the M o 3d doublet for Mo5 (Figure 5b) is identical with that of molybdenum foil (Figure
,
( I 5 ) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-ray Photoelectron Spectroscopy, Eden Prairie, MN, 1979. (16) Raupp, G. B.; Dumesic, J. A. J . Phys. Chem. 1985, 89, 5240.
240
235
230
Binding Energy
225
220
/ eV
Figure 6. Curve-fitted Mo 3d spectra of Mo5 in the oxidic form and after two reduction treatments: (a) MoS-662, (b) Mo5-304, (c) Mo5-oxidic. Doublets A, B, C, and F refer to Mo oxidation states +6, +5, +4, and 0, respectively. See Table I and text for details. TABLE I: ESCA Parameters Derived from Curve Fitting the Mo 3d Envelooe of MoS after Various Treatments binding energy, eV fwhm, eV doublet Mo 3d3/2 Mo 3ds12 assignt Mo 3d3/2 Mo 3d5/, A 235.9 232.7 M o + ~ 2.07 2.07 B 234.6 231.4 M O + ~ 1.98 1.98 C 232.8 229.6 M o + ~ 1.87 1.87 D 232.0 228.8 Mo+' 1.63 1.63 E 231.4 228.2 M O + ~ 1.53 1.53 F 230.8 227.6 Moo 1.41 1.23
Sa). The measured Mo 3d5/2 binding energy of -227.6 eV for Mo5-662 is within experimental error the same as that measured for Mo metal foil (227.5 eV referenced to Fermi edge, Figure Sa) or reported for Mo metal (227.7 eV, ref 15). Thus, the shape of the Mo 3d doublet and the Mo 3d5 binding energy value of -227.6 eV for Mo5-662 (Figure 5b$ suggest the presence of predominantly Mo metal. A shift in binding energy of -5.1 eV between M o + ~(232.7 eV) and Moo (227.6 eV) for Mo Ti02 is consistent with previous ESCA studies of molybdenum.15J -22 The presence of significant amounts of Mo metal at reduction temperatures of 2650 OC is consistent with gravimetric data (Figure 1); an average M o oxidation state of 0.3 was observed by gravimetric analysis at 650 OC reduction. The distribution of Mo oxidation states at each reduction temperature and the binding energy values for the Mo oxidation states were obtained by curve fitting the ESCA Mo 3d envelopes. The methodology used to curve fit the envelopes is described in detail in the Experimental Section. Figure 6 shows the fitted M o 3d envelope of oxidic Mo5 catalyst and of Mo5 after two different reduction treatments. Table I lists the parameters derived from
-/
(17) Aptekar', E. L.; Chudinov, M. G.; Alekseev, A. M.; Krylov, 0. V. React. Kinet. Catal. Lett. 1974, I , 493. (18) Cimino, A.; De Angelis, B. A. J . Catal. 1975, 36, 11. (19) Zingg, D. S.;Makovsky, L. E.; Tischer, R. E.; Brown, F. R.; Hercules, D. M. J . Phys. Chem. 1980,84, 2898. (20) Holl, Y . ; Touroude, R.; Maire, G.; Muller, A,; Engelhard, P. A.; Grosmangin, J. J . Catal. 1987, 104, 202. (21) Brox, B.; Olefjord, I . Surf. Interface Anal. 1988, 13, 3. (22) Goldwasser,J.; Fang, S. M.; Houalla, M.; Hall, W. K. J . Catal. 1989, 115, 34.
1524 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
Quincy et al.
curve fitting the Mo 3d envelope for all treatments. The fitted envelope for oxidic Mo5 (Figure 6c) shows the Mof6 doublet (Mo 3d5 2, 3d312). The Mo 3d5,, and 3d3/2binding energies of 232.7 and235.9 eV, respectively, and the peak widths of 2.07 eV (Table I) derived from the fitted envelope of oxidic Mo5 (Figure 6c) were used whenever the M o + doublet ~ was fitted to envelopes of reduced Mo5. Figure 6b shows the curve-fitted Mo 3d envelope for MoS after reduction at 304 "C. Note that three sets of doublets (labeled A, B, and C in Figure 6 and Table I) were required to fit the experimental data. Doublet A corresponds to M o + and ~ constitutes only 10% of the total Mo 3d envelope area. Doublet B contains about 34% of the total area; the Mo 3d5/2 and 3d3/2 binding energies of 23 1.4 and 234.6 eV, respectively, for doublet B (Table I) are consistent with M O + ~ . ' Doublet ~ ~ ' ~ C is the major species fitted to the Mo 3d envelope of Mo5-304 (Figure 6b); it contains 56% of the total Mo 3d envelope area. Mo 3d5 and 3d3 binding energies of 229.6 and 232.8 eV, respectiveiy, were #ound for doublet C (Table I). Note that a difference in binding energy of 3.1 eV is observed between doublet A (Mof6) and doublet C. The shift in Mo 3d binding energy of 3.1 eV observed between Mof6 and doublet C is consistent with the shift reported between MOO, and Mo02.17-19~23-25 Thus, doublet C was assigned to M o + ~ (Table I). The assignment of doublets B and C in the envelope ~ M o + ~ respectively, , is of Mo5-304 (Figure 6b) to M O + and consistent with gravimetric data (Figure 1); an average Mo oxidation state of 4.8 was observed when Mo5 was reduced at 300 "C. Figure 6a shows the curve-fitted Mo 3d envelope for MoS after reduction at 662 "C. Note that only one doublet (labeled F in Figure 6 and Table I) is fitted to the envelope for Mo5-662. Note also the peak tailing at the high binding energy side of the 3dji2 and 3d5/2 peaks. The tailing factor was set equal to the value found in the fit of Mo metal foil and can be attributed to conduction band interactions." The Mo 3d5/2 and 3d3/2 binding energies of 227.6 and 230.8 eV, respectively, for doublet F (Table I) are consistent with M0°.15922It can also be seen in Table I that the full width at half-maximum (fwhm) for the Mo 3d3i2peak of doublet F (1.41 eV) is slightly larger than the value for the 3d5j2peak (1.23 eV). This can be attributed to differences in the 3djj2 and 3d5/2 core hole lifetimes as a result of Coster-Kronig transitions.26 The same Mo 3d3j2:3d5 peak width ratio (1.14) was found in the fit for Mo metal foil. $he assignment of doublet F to Moo is consistent with gravimetric data (Figure 1); an average Mo oxidation state of 0.3 was observed for Mo5 after reduction at 650 "C. The presence of molybdenum oxidation states of +6, +5, and +4 is well-established for supported Mo catalysts (e& ref 17 and 19). The existence of Moo has also been reported for Mo/A1203 catalysts.'2,20*22-27*28 The curve-fitted ESCA spectra shown thus far in this study have also revealed the presence of Mo oxidation states of +6, +5, +4, and 0 (Figure 6). The existence of intermediate Mo oxidation states between +4 and 0 for supported Mo catalysts has not been unequivocally demonstrated by ESCA; complicating factors can preclude a straightforward assignment of these intermediate states. For example, a difference of only 2 eV is observed (Table I) between M o + ~(229.6 eV) and Moo (227.6 eV). This shift in Mo 3d binding energy of only 2 eV between the four molybdenum oxidation states of +4 and 0 is consistent with the shift reported between MOO, and Mo Thus, for oxidation states between M o + and ~ Moo, a unit change in oxidation number (e.g., M o + ~ to M o + ~would ) be expected to cause an average binding energy shift of only 0.5 eV. It is apparent that a systematic approach and extensive data analyses (e.g., curve fitting) are necessary to reveal the existence of intermediate states
A
(23) Kim, K. S.; Baitinger, W. E.; Amy, J. W.; Winograd, N. J . Electron Spectrosc. Relat. Phenom. 1974, 5. 35 1 . (24) Patterson, T. A,; Carver, J. C.; Leyden, D. E.; Hercules, D. M. J . Phys. Chem. 1976, 80, 1700. ( 2 5 ) Grzybowska, B.; Haber, J.; Marczewskii, W.; Ungier, L. J . Cutal. 1976, 42, 321. (26) Maartenson, N.; Nyholm, R. Phys. Reu. E 1981, 24, 7121. (27) Brenner, A.; Burwell, R. L., Jr. J . Cafal. 1978, 52, 353. (28) Burwell, R.L., Jr.; Chung,J. React. Kiner. Catal. Lett. 1987, 35, 381.
2
2
c
A" *
5-416 238
234
230
Binding Energy
/
ria eV
212
5-416 238
23,
230
Binding Energy
/
zis
ziz
eV
Figure 7. Curve-fitted Mo 3d spectra of Mo5 after two reduction treatments: (a) Mo5-512, (b) Mo5-512, (c) Mo5-416, (d) Mo5-416. Two different curve-fitting procedures were used to fit the Mo 3d envelope of Mo5 for each reduction treatment. See text for details. Doublets D and E refer to Mo oxidation states +3 and +2, respectively.
between +4 and 0 for supported Mo catalysts. The approach we used to determine whether ESCA can monitor Mo oxidation states between +4 and 0 is to reduce the Mo5 catalyst at temperature increments of -50 "C and curve fit the resulting Mo 3d envelopes with the assistance of gravimetric data. Reduction temperatures of 400-500 "C were of particular interest because they produced catalysts with average Mo oxidation states in the range of 3-2 (Figure I). Thus, the envelopes of Mo5 after reduction treatments of 400-500 "C must contain either some combination of the well-established oxidation states +6, +5, +4, and 0 or intermediate states between +4 and 0. We first attempted to curve fit the Mo 3d envelopes of the Mo5 catalyst after reduction treatments at 400-500 "C with combinations of MO oxidation states of +6, +5, +4, and 0. Figure 7a shows the results ~ and Moo doublet to the ESCA of curve fitting the M o + doublet spectrum of the Mo5 catalyst after 512 "C reduction. It is obvious that the curve-fitted envelope does not accurately represent the experimental data. Other combinations of M o + ~M , o + ~M , o+~, and Moo did not accurately fit the data. A good fit to the envelope of Mo5-512 is obtained by including a doublet at a binding energy between 229.6 eV ( M o + ~and ) 227.6 eV (Moo). Figure 7b shows the curve-fitted envelope of Mo5-512 with the inclusion of this doublet (labeled E). Table I lists the parameters for doublet E. Note that the Mo 3d512 and 3d3/, binding energies of doublet E are centered at 228.2 and 231.4 eV, respectively. Note also that the Mo 3d binding energies of doublet E are shifted to higher binding energy by only 0.6 eV from the values determined for Moo (doublet F). On the basis of an average Mo oxidation state of 2.2 from gravimetric analysis at 500 "C reduction treatment (Figure 1) and that doublet E is the dominant species fitted to the total 3d envelope of Mo5-512 (Figure 7b), it is reasonable to assign doublet E to M o + ~ . The same approach was used to fit the Mo 3d envelope of the Ma5 catalyst after reduction at 416 "C. Figure 7c shows the results of curve fitting the M o + ~and Moo doublets to the 3d envelope of MoS-416. A doublet of intermediate binding energy between 229.6 ( M o + ~and ) 227.6 eV (Moo) is clearly needed to improve the fit. Figure 7d shows the curve-fitted envelope of Mo5-416 with the inclusion of this doublet (labeled D). Table I lists the parameters for doublet D. Note that the Mo 3d5/2 and 3d3/, peak positions of doublet D are centered at 228.8 and 232.0 eV, respectively, and thus located approximately intermediate between the values for M o + ~and M O + ~ This . suggests an assignment of oxidation state +3 to doublet D. An assignment of M o + ~to doublet D is consistent with gravimetric data; an average Mo oxidation state of 3.2 was found for Mo5 after reduction at 400 "C (Figure 1). Figure 8 shows a plot of the distribution of Mo oxidation states determined from the curve-fitted ESCA Mo 3d envelope of the
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1525
Oxidation States in Reduced Mo/Ti02 Catalysts
P 2331
0
t70
.d
a
4
rd
4X 0
200
400
300
500
600
700
Reduction Temperature (C) Figure 8. Mo oxidation state distribution versus reduction temperature: ( 0 )M o + ~ (0) , M O + ~(A) , Mo'~, (0) Mo'),
(m) Mo+*, (V)Moo.
Mo5 catalyst after each reduction treatment. It can be seen that the relative abundance of M o + ~( 0 )decreases to 400 OC, Figure loa). This can be seen more effectively by plotting the distribution of M o + ~M , O + ~and , Moo versus prereduction temperature for benzene hydrogenation activity measurements, as shown in Figure lob. It can be seen that the distribution of Mo metal (Figure lob) as a function of prereduction temperature does not follow the curve for benzene hydrogenation activity (Figure loa); Mo metal is seen to increase sharply only at reduction temperatures which correspond to a decrease in activity (i.e,, 7550 "C). Thus, the data suggest that Mo metal formed by high-temperature reduction of Mo/Ti02 catalysts is not the most active species for the hydrogenation of benzene. It is evident that the relative abundances of both M o + ~and M o + ~(Figure lob) show maxima like the benzene hydrogenation activity (Figure loa) as the temperature of reduction is increased. However, M o + ~reaches a maximum at 400-450 OC (Figure lob) which is 100 OC lower than the reduction temperature of 500-550 OC required for maximum benzene hydrogenation activity (Figure loa). Thus, M o + ~does not appear to contribute significantly to the activity. Comparison of the distribution of M o + ~(Figure lob) with benzene hydrogenation activity (Figure loa) as a function of prereduction temperature suggests that M o + ~is the most active species. One cannot rule out, however, that Mo metal present as a minor component is responsible for the activity and that the decrease in benzene hydrogenation activity at prereduction temperatures of >550 OC is due to coverage of the active sites by reduced Ti moieties or by impurities migrating to the catalyst surface on reduction or sintering of the Mo metal phase. However, the fact that the ESCA Mo (metal)/Ti intensity ratio for the Mo5 catalyst actually increases by a factor of 2.4 when the reduction temperature is increased from 550 to 600 OC makes it unlikely that the observed 30% decrease in benzene hydrogenation activity (Figure loa) is due to sintering of the Mo phase. It is worth mentioning that the maximum benzene hydrogenation activity observed at intermediate Mo oxidation states for the Mo5/Ti02 catalyst is similar to results reported for 1,3-butadiene hydrogenation over Mo/Ti02 catalysts2 and for isoprene hydrogenation over sulfided Mo/AI2O3 catalysts.30 Segawa et (30) Wambeke, A,; Jalowiecki, L.; Kasztelan, S.;Grimblot, J.; Bonnelle, J. P. J . Card. 1988, 109, 320.
aL2 showed for Mo/Ti02 catalysts that 1,3-butadiene hydrogenation activity begins at average Mo oxidation states of -+4 and rapidly increases to a maximum at an average oxidation state of -+3. A sharp decrease in hydrogenation activity was observed for lower average Mo oxidation states2 Wambeke et aLMshowed that isoprene hydrogenation activity is a strong function of reduction temperature for sulfided Mo/AI2O3catalysts. When the sulfided Mo/Al,03 catalyst was prereduced in hydrogen at temperatures of -250 to 550 OC, a sharp increase in activity was observed. The isoprene hydrogenation activity was shown to decrease sharply when the sulfided catalyst was prereduced at temperatures of >550 OC, and no activity was observed above 800 OC reduction.30 On the basis of these findings, Wambeke et aL30 proposed that for sulfided Mo/AI2O3catalysts intermediate Mo oxidation states were most active for isoprene hydrogenation. Conclusions Gravimetric analyses and ESCA data showed that Mo oxidation states between +6 and 0 are produced on a 5 wt % Mo03/Ti02 catalyst after hydrogen reduction at different temperatures. It was shown that reduction temperatures of 1650 O C are necessary to generate a catalyst containing predominantly Mo metal. The Mo 3d5 binding energy of supported Mo metal on T i 0 2 (227.6 eV) is shifted 5.1 eV from the value measured for supported Mo in the +6 oxidation state (232.7 eV). Curve-fitted ESCA results for reduced Mo5 catalysts suggest the presence of intermediate Mo oxidation states between +4 and 0, specifically +2 and +3. The distribution of Mo oxidation states was determined for each reduction treatment. Benzene hydrogenation activity was found to depend strongly on the extent of reduction of the Mo phase. Comparison of activity with the distribution of Mo oxidation states suggests that the M o + ~oxidation state is the most active species for benzene hydrogenation.
Acknowledgment. We gratefully acknowledge Professor W. K. Hall for stimulating discussions. We also thank Douglas P. Hoffmann for assistance with data analysis and Thomas M. Gasmire for machine shop work. This work was supported by the Department of Energy under Grant DE-AC02-79ER10485. R. B. Quincy acknowledges the A.W. Mellon Educational and Charitable Trust for a Predoctoral Fellowship.
Polymer-Induced Lateral Phase Separation in Mixed Lipid Membranes: A Theoretical Model and Calorimetric Investigation Antonio Raudino,* Francesco Castelli, and Salvatore Gurrieri Dipartimenfo di Scienze Chimiche, Universitci di Catania, Male Andrea Doria, 8-951 25 Catania, Italy (Received: April 25, 1989: In Final Form: August 4, 1989)
A theoretical model describing the effect of polymer adsorption onto the surface of a mixed lipid membrane has been developed. The model investigates the ability of the adsorbed polymer to induce lateral phase separation of the membrane components forming microdomains richer in one-lipid species. Moreover, the reciprocal influenceof these domains on the polymer spreading over the membrane surface as well as on the lipid-polymer binding constant was investigated. The model is based on a variational procedure that minimizes the total energy of the system with respect to some parameters describing the size and composition of the microdomains, the number of adsorbed polymer units, and the polymer spreading over the membrane surface. The resulting equations have been solved by a perturbation technique that yields simple analytical results. Most of the theoretical predictions have been confirmed by differential scanning calorimetry (DSC) measurements performed on a membrane model containing charged (phosphatidic acid) and neutral (phosphatidylcholine)lipids and interacting with a water-solublepoly(amino acid) (polylysine).
Introduction The formation of microdomains richer in one-lipid components is an interesting phenomenon that influences several properties of the biological membranes. Properties such as water permeability,' membrane potential: rate of fusion between adjacent lipid
0022-3654/90/2094- 1526$02.50/0
bilayer^,^ and protein lateral diffusion4 have been supposed to be influenced by the clustering of lipids into inhomogeneous domains. ( 1 ) Carruthers, A,; Melchior, D. L. Biochemistry 1983, 22, 5797. (2) McLaughlin, S.;Brown, J. J . Gen. Physiol. 1981, 475.
0 1990 American Chemical Society
