Preparation and characterization of highly dispersed cobalt oxide and

J . Phys. Chem. 1991, 95, 310-319

310

Room-temperature 129XeNMR is a convenient means of studying heterogeneous distributions of HMB in N a y , providing the heterogeneity length scales are larger than about 10 pm. Such situations may lend themselves to NMR chemical shift imaging methods in which a linear magnetic field gradient might be used to image xenon profiles in samples possessing macroscopic adsorbate heterogeneities. The suitability of multiple-quantum NMR spectroscopy for probing adsorbate distributions quantitatively is due primarily to the sensitivity of the technique to the number of dipole-dipole coupled spins in a collection of isolated molecules. Counting the number of proton spins in clusters of chemisorbed organic species yields information on their spatial distributions and, thus, about microstructural features of the adsorption sites themselves. Because of the central importance of these sites to the reaction process, multiple-quantum NMR represents a po-

tentially valuable means by which a catalyst's microscopic adsorbate structure can be correlated with its chemical reaction properties. Such information, used in conjunction with the macroscopic adsorbate distributions measured by '29Xe N M R spectroscopy, is key to characterizing intracrystalline mass transport and adsorption of reactant species within zeolitic catalysts.

Acknowledgment. We thank D. N. Shykind and M. Trecoske for assistance with the multiple-quantum experiments and with zeolite sample preparation. M. G. Samant kindly provided the NaY zeolite used. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U S . Department of Energy, under Contract No. DE-AC03-76SF00098.

Preparation and Characterization of Highly Dispersed Cobalt Oxide and Sulfide Catalysts Supported on SiOz Yasuaki Okamoto,**tKozo Nagata,t Toshinori Adachi,+ Toshinobu Imanaka: Kazuhiro Inamura,t and Toshiyuki Takyu* Department of Chemical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan, and Central Research Laboratories, Idemitsu Kosan Co., Ltd., 1280, Kamiizumi, Sodegaura-machi, Kimitsu-gun, Chiba 299-02, Japan (Received: June 13, 1990)

Physicochemical characterization of calcined and sulfided CoO/SiO, catalysts were carried out to reveal the interaction modes between cobalt and SiOz using XPS, TPR, TEM, DRS-VIS, and XRD techniques. The CoO/Si02 catalysts were prepared by an impregnation method using cobalt acetate as well as cobalt nitrate and by an ion-exchange technique. It was found that several kinds of cobalt species are formed on CoO/SiO,. These species are assigned to Co304,Co-Si4 mixed oxide, surface Co3+species, surface silicate, surface Co2+species in the order of the TPR reduction temperature. Their proportions strongly depended on the starting salt, cobalt content, and preparation method. Cobalt acetate was found to provide highly dispersed CoO/Si02 catalysts with a uniform distribution of cobalt species throughout the catalyst particles as compared to conventionally employed cobalt nitrate. The proportion of Co3+greatly decreased when cobalt acetate was used instead of cobalt nitrate. All the cobalt species interacting with SiO, were found to be sulfided at 673 K. It was demonstrated that sulfided CoO/Si02 catalysts prepared from cobalt acetate show several times higher hydrogenation activity than the catalysts from cobalt nitrate. On the basis of the XPS characterization of uncalcined precursors, the effects of starting salt on the cobalt-Si0, interaction modes and cobalt dispersion and distribution are discussed.

Introduction Supported cobalt-molybdenum sulfide catalysts have been widely employed for hydroprocessing of petroleum feedstocks and extensively investigated by many workers.'v2 Recently, Topsae et alS3q4have proposed that the formation of so-called Co-Mo-S phases generate catalytic synergies between Co and Mo. The Co sites in the Co-Mo-S phases are considered to form catalytically active centers. In addition, de Beer and Prins et aI.>* have shown that carbon-supported cobalt sulfides exhibit very high hydrodesulfurization (HDS)activities as compared to carbon- or alumina-supported molybdenum sulfide catalysts, suggesting that sulfided cobalt species show a high HDS activity in a specific configuration as the Co species in the Co-Mo-S phases do. Besides hydrotreating processes, supported cobalt metal catalysts are extensively used for the hydrogenation of carbon monoxide to produce hydrocarbon^.^^'^ The physicochemical characterization of Alz03-supportedcobalt catalysts has shown the presence of several cobalt species in oxide states by using XPS."-'6 X-ray absorption spectroscopy (XAS)," temperature-programmed reduction (TPR),'8q19and other techn i q u e ~ . ~Detailed ~ ~ ~ characterization of cobalt species on Si0,-supported catalysts are, however, very scarce as compared 'Osaka University. Idemitsu Kosan Co. f

with CoO/AI2O3 systems. Very recently, Castner et al.24325have shown by using TPR, XPS, AEM, XAS, and XRD that after 1978. 27. 265 (2) Grange, P. Catal. Reu.-Sci. kng. 1980, 21, 135. (3) Topsoe, H.; Clausen, B. S. Coral. Reu.-Sci. Eng. 1984, 26, 395. (4) Wievel, C.; Candia, R.; Clausen, B. S.; Morop, S.; Tops%, H. J. Coral. 1981, 68, 453. (\.,.._........ 1 ) Massoth. F . E. Adu. Catal. ~... .~

~1

~

I

~~~

( 5 ) Prins, R.; de Beer, V. H. J.; Somorjai, G.A. Cafal. Reo.-Sci. Eng. 1989. 31. - , 1. (6) Vissers, J. P. R.; de Beer, V. H. J.; Prins, R. J. Chem. SOC.,Faraday Trans. I 1987.83, 2145. (7) Duchet, J. C.; van Oers, E. M.; de Beer, V. H. J.; Prins, R. J . Catal. 1983, 80, 386. (8) Bouwens, S.M. A. M.; Koningsberger, D. C.; de Beer, V. H.J.; Prins, R. Catal. Left. 1988, 1, 5 5 . (9) Vannice, M. A,; Garten, R. L. J . Carol. 1979, 56, 236. (10) Anderson, R. B. The Fischer-Tropsch Synthesis; Academic Press: New York, 1984. ( 1 I ) Okamoto, Y.; Nakano, H.; Imanaka, I.; Teranishi, S. Bull. Chem. Soc. Jpn. 1975, 48, 1163. (12) Okamoto, Y . ; Imanaka, I.; Teranishi, S.J. Catal. 1980, 65, 448. (13) Grimblot, J.; Bonnelle, J. P.; Beaufils, J. P. J. Electron Specrrosc. Relat. Phenom. 1976, 8, 431. (14) Bonnelle, J. P.; Grimblot, J.; D'Huysser, A. J . Elecfron Spectrosc. Relat. Phenom. 1975, 7 , 151. ( 1 5 ) Declerck-Grimee, R. I.; Canesson, P.; Friedman, R. M.; Fripiat, J. J. J . Phys. Chem. 1978, 82, 885. (16) Chin, R. L.; Hercules, D. M. J . Phys. Chem. 1982, 86, 360.

.

_ .-

~

0022-3654191 12095-0310%02.50/0 0 1991 American Chemical Society

SiOz-Supported Cobalt Oxide and Sulfide Catalysts

iI /A”* \

120 a

Rp /nm Figure 1. Pore size distribution of S O 2 (JRC-SIO-4).

calcination at 723-8 I3 K major cobalt species are Co304 on CoO/Si02 prepared from cobalt nitrate (5-10 wt % Co) and that the Co304phase is moderately dispersed with a uniform particle size of 5-20 nm, depending on the pore size of SiO? These cobalt oxide species were completely reduced by H2 below 723 K. Pepe et a1.26have reported that at a lower calcination temperature (523 K) c0304 of a particle size of 12-1 5 nm is formed on CoO/Si02 prepared from cobalt nitrate. These observations suggest that the dispersion of cobalt is moderate on Si02. On the other hand, Pepe et a1.26have noticed that CoO/Si02 catalysts prepared by using cobalt oxalate contains X-ray amorphous phases besides crystalline Co304after calcination at 523 K. Recently, Sugi et al.27*28 have shown that in contrast to cobalt-SiO, catalysts prepared from cobalt nitrate, cobalt catalysts (uncalcined) from cobalt acetate are not reduced by H2 at 723 K. Their brief XPS characterization suggested the presence of Coz+ after the H, reduction.29 The formation of cobalt species unreducible at 673 K has also been reported by Reuel and Bartho lo me^^^ with a CoO/Si02 catalyst (3 wt % Co) prepared by a pH-controlled precipitation method.31 Accordingly, it is inferred that CoO/SiOz catalysts contain several cobalt species interacting with Si02 besides crystalline c0304 particles and that the cobalt-Si02 interaction modes depend strongly on preparation variables such as a starting material. No detailed investigation of CoO/Si02 has been carried out concerning the characterization of the interaction species on CoO/Si02 and the reduction and sulfiding behaviors of these species. In the present study, CoO/SiOz catalysts were prepared by an impregnation of cobalt acetate as well as conventional cobalt nitrate and by an ion-exchange technique and characterized by TPR, XPS, transmission electron microscope (TEM), diffuse reflectance visible spectroscopy (DRS-VIS), and XRD to reveal the effects of preparation variables on cobalt-Si02 interaction modes and on the dispersion and distribution of cobalt species. Reduction and sulfidation behaviors and catalytic properties of cobalt were investigated to provide a clue for the preparation of (17) Greegor, R. B.; Lytle, F. W.; Chin, R. L.; Hercules, D. M. J . Phys. Chem. 1981.85, 1232.

(18) Arnoldv. P.;Mouliin. J . A. J . Card. 1985. 93, 38. (19) Arnoldy, P.;de Brkys, J. L.; Scheffer, B.; Moulijn, J. A. J . Carol. 1985, 96, 122. (20) Chung,-K. S.; Massoth, F. E. J . Card 1980, 64, 320 and 332. (21) Burggraf, L. W.; Leyden, D. E.; Chin, R. L.; Hercules, D. M. J . Card. 1982, 78, 360. (22) Aschley, J. H.; Mitchell, P. C. H. J . Chem. SOC.A 1968, 2821. (23) Lo Jacono, M.; Cimino, L.; Schuit, G. C. S. C a n . Chim. Ira/. 1973, 103, 1281. (24) Castner, D. G.; Santilli, D. S. ACS Symp. Ser. 1984, 248, 39. (25) Castner, D. G.:Watson, P. R.; Chan, 1. Y . J . Phys. Chem. 1989, 93, 3 188. (26) P e p , D.:Walker, D. S.; Whalley, L.; Moss,R. L.J . Card 1973,31, 775

(27) Takeuchi, K.; Matsuzaki, T.; Arakawa, H.; Sugi, Y . Appl. Card 1985, 18, 325.

(28) Arakawa, H.; Takeuchi, K.; Matsuzaki, T.; Sugi, Y . Sekiyu Gak-

The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 311 highly dispersed cobalt oxide and sulfide catalysts. It was found that several kinds of interaction species are present on S O 2 surfaces in addition to crystalline c0304 and that cobalt acetate provides the catalysts with significantly higher degree of the cobalt dispersion on Si02than cobalt nitrate. Similar results have been obtained for CoO/Al,03 catalysts that will be reported elsewhere.32

Experimental Section The silica employed in the present study was supplied by Catalysis Society of Japan as a reference catalyst (JRC-SIO-4). The BET surface area (3 16 m2 g-I), pore volume (0.6 14 cm3 g-I), and pore size distribution in Figure 1 were measured by an N2 adsorption and desorption methods at 77 K using a gas adsorption apparatus (BELSORP 28, Bell Japan, Inc.,) after evacuation at 343 K for 2 h. The mean pore size of Si02was calculated to be 3.9 nm. Supported cobalt catalysts were prepared by impregnating SiO, with an aqueous solution of cobalt salt. The amount of the aqueous solution was adjusted to be 1.3 cm3 per gram of Si02when cobalt nitrate was used to prepare CoO/Si02 containing up to 20 wt % COO. The supported amount of COO is represented here with respect to the support. In the case of cobalt acetate, the catalysts containing 2 and 5 wt % COOwere prepared in a way similar to the nitrate. As for the catalysts with 10 and 20 wt % COO,a wet impregnation method was applied, in which the catalyst slurry was evaporated to dryness at 343-353 K over a water bath while being vigorously stirred, because of a low solubility of cobalt acetate. The pH values of the impregnation solutions (5 wt % COO)were 4.3 and 6.8 for cobalt nitrate and acetate, respectively. After the catalyst precursor was dried at 383 K for 16 h, it was calcined at 673 K for 5 h in air by using an electric furnace. The CoO/Si02 catalysts prepared from cobalt nitrate and acetate are denoted CoO/Si02(N) and CoO/SiO,(A), respectively. A Colt ion-exchanged Si02catalyst was also prepared by using cobalt acetate at a pH value of 8.3 adjusted by adding an aqueous ammonium solution.33 The final pH value was 7.7 after 4.5 h. The ion-exchanged sample was thoroughly washed with distilled water, followed by drying and calcination. The cobalt content was 8.9 wt % CoO by chemical analysis. CoO/Si02 thus prepared is denoted CoO/Si02(I). Co2Si04was hydrothermally prepared by using Na2Si03-9Hz0 and CoBr,dH,O at 568 K for 20 h according to NOHet al.” The Co/Si atomic ratio was adjusted to 1.5 to avoid a formation of free c0304. The formation of Co2Si04was confirmed by XRD. Sulfidation of CoO/Si02 catalyst was carried out at 673 K in a stream of atmospheric pressure of H2S/H, (l/lO, 550 cm3 min-I) for 90 min, followed by an H2 treatment (1 atm, 500 cm3 min-]) at the same temperature for 60 min. The sulfidation temperature was raised at a uniform rate of 6 K m i d from room temperature to 673 K. The hydrogenation of butadiene was conducted over the sulfided CoO/Si02 catalyst at 473 K and 14.6 kPa of a reaction gas (H2/butadiene = 2) by using a closed circulation system (220 cm3). The reaction gas was analyzed by using GLC. In TPR experiments, after evacuation at 623 K for 60 min CoO/SiOz (0.2 g) was reduced in a 5% H2/Ar stream (72 cm3 min-I), which was purified by being passed through a deoxygenation catalyst layer, followed by a zeolite trap cooled at a methanol-dry ice temperature. The reduction temperature was uniformly raised from 300 to ca. 920 K at a heating rate, /3, of 2.5 K min-I. The H2 consumption in the effluent gas was analyzed by a TCD cell at 303 K after removal of evolving water vapor by a cold trap at a methanol-dry ice temperature. TPR experiments a t higher reduction temperature were conducted on a TPR-TPS apparatus (ID-200, Idemitsu Kosan Co., Ltd.). The reduction temperature was raised from 300 to 1350 K at /3 = 10 K mi& in a stream of 64% H2/Ar (20 cm3 mi&) after purging the sample (44 mg) in a pure Ar flow for several hours.

kaishi 1988, 31, 335.

(29) Kintaichi. Y.;Takeuchi, K.; Matsuzaki. T.: Arakawa, H.; Hanaoka, T.;‘Ito, T.; Sugi, Y . Chem. Express 1989, 4, 129. (30) Reuel, R. C.: Bartholomew, C. H. J . Card. 1984, 85, 63. (31) Richardson, J. T.; Dubus, R. J. J . Card 1978, 54, 207.

~~

(32) Okamoto, Y . ;et al., to be submitted. (33) Taniguchi, K. Thesis, Kyoto University, 1970. (34) Noll, W.; Kircher, H.; Sybertz, W. Eeritr. Minerd. fetr. 1960, 7, 232.

312 The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 c a l c ined

sulf id e d

a

a/ I

40

35

I

1

55

50

45

28/degree Figure 2. X-ray diffraction diagrams of CoO/SiO, in calcined and sulfided forms: (a) IO wt 9% CoO/SiO,(A), (b) 20 wt 9% CoO/Si02(A), (c) 2 wt B CoO/Si02(N), (d) 5 wt % CoO/Si02(N), ( e ) 10 wt % CoO/SiO,(N), and (f) 20 wt 5% CoO/Si02(N). The XPS measurements for CoO/Si02were made on a Hitachi 507 photoelectron spectrometer using AI Kal,*radiation (1486.6 eV). The anode was operated at 9 kV and 50 mA. The sample was mounted on a copper holder by using a double-sided adhesive tape. The XP spectra of CoO/Si02 were measured also after grinding the sample by hand in a mortar and pestle typically for about I O min. The XP spectra of the sulfided catalysts were recorded after the hydrogenation reaction (reaction time: 60 min) without exposure to air by using an N,-filled glovebox. Assuming linear backgrounds, XPS intensity ratios were calculated by using the planimetered peak areas of the C0(2p), Si(2p), and N( Is) bands. Binding energies (BE) were referenced to the Si(2p) level (103.1 eV) of the support, which had been separately determined by using the C( 1 s) band at 285.0 eV due to adventitious carbon.35 For electron-microscope observations, the catalyst particles were embedded in epoxy resin (Quetol-812, Nisshin EM Co.) and ultrathin sections were cut out by using an LKB type-V microtome (LKB Productor, Bromma) equipped with a diamond knife. The sections were then mounted on microgrids and observed in a transmission electron microscope (H-800, Hitachi Ltd.) operated at an accelerating voltage of 200 kV. DRS-VIS spectra of CoO/SiOz were recorded in the range 370-800 nm by use of a Hitachi 200-20 spectrophotometer against a standard of alumina. XRD patterns of CoO/SiO, were obtained with a Shimadzu VD-I diffractometer using Cu K a radiation with a Ni filter. The XRD measurements of sulfided catalysts were made in air soon after exposure to air.

Results The XRD results are shown in Figure 2 for oxidic CoO/Si02 prepared by using cobalt nitrate and acetate. CoO/Si02(N) clearly exhibited the presence of crystalline Co304(28 = 36.9O) even at 2 wt % COO. The XRD peak intensities due to CO304 increased with increasing COO content. On the other hand, no distinct XRD peaks due to CO& were detected even for I O wt % CoO/SiO,(A), although weak peaks were observed at 20 wt % COO. These findings suggest that cobalt is well dispersed when cobalt acetate was used as a starting material. The crystalline size of Co304was estimated from the line width of the XRD peak at 28 = 36.9' using Sherrer's equation (K= 0.9). As can be seen in Figure 2, the size of Co304was independent of COO content

Okamoto et al. and about 7 nm for CoO/Si02(N). The formation of a similar size of Co3O, was observed also for 20 wt % CoO/SiO,(A). Representative TEM results are presented in Figure 3 for typical CoO/Si02(N) and (A). With 2 wt % CoO/Si02(N), cobalt oxide was dispersed over Si02, forming two-dimensional aggregates with an almost uniform size of 80-100 nm. Each of the aggregates consisted of oxide particles of 5-8 nm. An excellent agreement of the particle size of cobalt oxide with the crystalline size of c0@4 suggests that cobalt oxide particles in TEM micrographs are crystalline Co304. When the COOcontent increased to 10 wt %, the number of aggregates increased as shown in Figure 3b, while the aggregate size and constituent particle size of cobalt oxide did not vary and were 80-100 and 5-10 nm (7 nm in average), respectively. The sizes of aggregate and oxide particle were essentially identical at 20 wt % COO(not shown). Scrutinizing the TEM micrographs, some of the cobalt oxide particles were found to conglomerate, forming larger cobalt oxide particles of ca. 25 nm in size in the concentration range 2-20 wt % COO. Some of them are shown in Figure 3b by arrows. With 2-10 wt % CoO/Si02(A), the cobalt oxide particles and aggregates were hardly detected as presented in Figure 3c. A small amount of aggregates (20-50 nm) consisting of cobalt oxide particles of ca. 5 nm were detected in 20 wt % CoO/SiOz(A) as in CoO/Si02(N). These TEM observations are obviously indicative of a higher dispersion of cobalt oxide on CoO/Si02(A) than on CoO/SiO,(N) in line with the XRD results in Figure 2. The TPR profiles of CoO/Si02(N) catalysts are shown in Figure 4 as a function of COOcontent. Several distinctly different reduction peaks were observed at 538 (Co I), 563 (Co II), 593 (Co III), 673 (Co IV), and 800-860 K (Co V). They are denoted Co I-V with increasing reduction temperature. The intensities of Co I-V increased independently as the COO loading increased from 2 to 20 wt %. The reduction peak of c0@4consisted of a slightly asymmetric single peak as shown in Figure 5 due, probably, to a stepwise reduction of Co3+ species. The reduction temperature depended on the particle size of Co304and the samples with particle sizes of 50 and 1200 nm (SEM observations) showed the TPR peaks at 570 and 590 K, respectively. The TPR profiles for CoO/Si02(A) are also presented in Figure 5. As with CoO/SiOz(N), Co I was observed around 540 K, although the intensity was significantly small. Co I1 and I11 were hardly detected and the intensity of Co IV was very low, in contrast to CoO/SiO,(N). A peak broadening and temperature rise (1 5 K) of Co I at 20 wt % COOare considered to be induced by a superposition of Co 11. Co V was not obviously detected in CoO/Si02(A). Strong H2 consumptions at >920 K indicate the presence of the reduction peaks at the higher temperature for CoO/Si02(A) and -(I). Figure 6 illustrates the TPR profiles for 10 wt % CoO/SiO,(A) and CoO/SiOz(I) obtained at j3 = 10 K min-'. The CoO/SiO,(A) showed a reduction peak at 1030 K (Co VI), while CoO/SiO,(I) exhibited two peaks at 1030 (Co VI) and 1081 K (Co VII). With 10 wt % CoO/Si02(N), Co VI or VI1 was not detected at all. It is obvious from Figures 4 and 5 that the area intensity of the TPR profile strongly depends on the starting salt, preparation method, and COOcontent. Figure 7 presents the reduction areas below 870 K as a function of COOcontent for CoO/Si02(N), -(A), and -(I). It was found that the TPR area linearly increased with increasing COOcontent and that the TPR area for CoO/SiO,(A) is considerably smaller than that for CoO/Si02(N). The H2 consumption for CoO/SiO,(N) was close to or even higher than that for Co304as compared to Figure 7. The low TPR intensities of CoO/SiO,(A) and, in particular, of CoO/Si02(I) are apparently ascribed to considerably high proportions of Co VI and VI1 species in these samples. The DRS-VIS spectra of CoO/Si02 are shown in Figures 8 and 9. Before calcination, CoO/SiO,(I) showed a strong absorption band at 528 nm characteristic of Co2+in an octahedral symmetry (fT,, 4bT,,).36 The other weak bands at 630, 500,

-

(35) Okamoto, Y . ;Ogawa, M.; Maezawa, A.; Imanaka, T. J . Caral. 1988. 112, 427.

( 3 6 ) Reinen,

D.Srruct. Bonding

1970, 7, 114.

SiO2-Supported Cobalt Oxide and Sulfide Catalysts 8

- --

-.

The Journal of Physical Chemistry, Vol. 95, No. I , 1991 313 b

-

-

~

-.--

.

d

C

Figure 3. Transmission electron micrographs of CoO/Si02: (a) 2 wt ICoO/Si02(N), (b) IO wt ICoO/Si02(N), (c) (d) 20 wt ICoO/Si02(A).

600 700 800 900 Reduction Temperature/K Figure 5. Temperature-programmedreduction profiles of C0O/SiO2and Co30, (B = 2.5 K min-I): (a) 5 wt % CoO/Si02(A), (b) IO wt I CoO/Si02(A), (c) 20 wt % CoO/Si02(A), (d) Cd/SiO2(1), and (e) Co30, (particle size: 1200 nm). The magnifications of the spectra are shown with respect to the spectra in Figure 4.

400

600 700 800 900 Reduction Temperature/ K 'Figure 4. Temperature-programmedreduction profiles of CoO/Si02(N) at B = 2.5 K min-I: (a) 2 wt S, (b) 5 wt 96, (c) IO wt 96, and (d) 20 wt 400

% coo.

500

-

and 470 nm can be assigned to the transitions TI: 4Aze.:TI,, and 2AIgrrespectively. However, the color of the sample changed from light pink to blue on grinding using mortar and pestle (ca. 10 min) at room temperature. The DR spectrum (b) of the ground sample is characteristicof Cd+in a tetrahedral configuration and

10 wt ICoO/Si02(A), and

500

-

the absorption bands at 637,585, and 525 nm are ascribed to the transitions 4A2 2Tlr4TI,and 2T2, respectively.% The increase in the absorbance for uncalcined CoO/Si02(1) on grinding is in conformity with the change in the configuration of Cd+ ions from

Okamoto et al.

314 The Journal of Physical Chemistry, Vol. 95, No. 1, 1991

(u

1 ,

400

600 800 loo0 Reduction Temperature/K

,

I

1200

Figure 6. Temperature-programmedreduction profiles of CoO/Si02 at

p = IO K m i d : (a) CoO/Si02(1) and (b) IO wt 3'% CoO/Si02(A).

l

400

I

I

I

I

1

500 600 Wavelength/nm

I

700

I

I

8 00

Figure 9. DRS-VIS spectra of CoO/Si02(A) and -(N): (a) 2 wt %, (b) 5 wt %, (c) sample b after grinding, (d) IO wt %, and (e) 20 wt % CoO/SiO,(A), and (f) 2 wt %, and (8) 5 wt 7% CoO/Si02(N). The spectra are vertically shifted for clarity.

COO c o n t e n t / w t o / o Figure 7. TPR area in the temperature range 300-870 K as a function of COO content, preparation method, and starting salt: (A) COO/ SiO,(N), (0)CoO/SiO,(A), and (0)CoO/SiO,(I). The dashed line

indicates the TPR area for Co304. A

U

\

1

400

I

I

500

1

I

600

I

I

700

I

I

800

Wavelength/ n m Figure 8. DRS-VIS spectra of CoO/SiO,(I): (a) before calcination, (b) sample a after grinding, (c) after calcination at 673 K, and (d) sample c after grinding. The spectra are vertically shifted for clarity.

octahedral to tetrahedral symmetries, since it is generally observed that Co2+species in tetrahedral symmetries have much stronger absorption coefficients than those in octahedral coordination^.^^,^^

These findings suggest that the configuration of Co2+ species ion-exchanged on Si02changes mechanochemically from octahedral to tetrahedral symmetries on grinding, probably because of desorption of H 2 0 molecules by the heat of friction or local high pressure. On wetting again with water and subsequent drying at 383 K, the blue CoO/Si02(I) restored the original DRS spectrum in Figure 8a. Calcination of the ion-exchanged sample at 673 K brought about a change in color from light pink to light brown, showing a DR spectrum in Figure 8c. On grinding CoO/SiO,(I), the triplet bands due to Co2+in a tetrahedral configuration appeared, suggesting that some ion-exchanged Co2+species remained intact after the calcination. Grinding procedures were found to be a useful diagnosis for the detection of ion-exchanged Co2+species on SO2. The DR spectra of CoO/Si02(A) and -(N) did not change on grinding by mortar and pestle as exemplified in Figure 9 for 5 wt % CoO/Si02(A), suggesting the absence of ion-exchanged Co2+ species on the impregnation catalysts after the calcination. All the DR spectra in Figure 9 for CoO/Si02(A) and -(N) were recorded after grinding. CoO/Si02(A) showed two bands at 580 and 650 nni. They are attributed to Co2+species in tetrahedral configurations with slightly different symmetries from that of ion-exchanged Co2+ tetrahedral species. Water treatments of CoO/Si02(A) did not alter the DR spectra in Figure 9 at all, suggesting that H20molecules do not strongly coordinate to the tetrahedral Co2+species in contrast to the tetrahedral Coz+species loaded by ion exchange. The broad bands around 7 10 and 4 10 nm are due to co304.22923 The DRS peak intensity ratio of Co304 and the tetrahedral Co2+species did not significantly vary with COO content for CoO/Si02(A). With CoO/SiO,(N), strong bands due to c0304 were predominant and no tetrahedral Co2+ species and ion-exchanged Co2+ species were discernible in the DRS-VIS spectra. Co2Si04(purple in color) showed DRS bands at 730m,645w, 56Y, 52OW,51 5", 487m,and 465w. These bands are consistent with the spectrum reported by Reinen." C O ( N O ~and ) ~ CO(CH~COO)~ supported on Si02(5 wt % COO) showed the sets of bands at 50Y, 490m, and 465m nm and at 56Y and 53oE nm, respectively. These results indicate that the tetrahedral Co2+species in CoO/Si02(A) (37) Cotton, A. L.; Wilkinson, G. Advanced Inorganic Chemistry, 2nd ed.; Interscience: New York, 1966; p 869. (38) Greenwood, N. N.; Earnshaw. A. Chemistry o f t h e Elemenfs;Pergamon Press: Oxford, 1984; p 1313.

SiO,-Supported Cobalt Oxide and Sulfide Catalysts

A

calcined

sulfided

The Journal of Physical Chemistry, Vol. 95, No. I, 1991 315 2

fi

0-

,4'

.-0 c m

L

A

c VI

El1

c

.-c

n

800

790

780

800

Bi n d in g

790

780

Energ y/eV

Figure 10. X P spectra of the C02p band for 5 wt 9% CoO/SiO, in calcined and sulfided forms: (a) CoO/SiO,(A) and (b) CoO/SiO,(N).

N .-

LA

k

N 0 U

TABLE 1: XPS Results of CoO/Si02 Calcined at 673 K COOwt % 2

Co(2p3,,) BE, eV' as prep.b groundb CoO/Si02(N) 779.7 779.9 779.8 779.3

(14.8)c (15.3) (15.4) ( 1 5.3)

20

781.3 78 I .6 781.8 781.3

(15.8) ( I 5.8) (15.9) (16.1)

8.9

781.4 (16.0)

CO,O,~

779.6 (1 5.0) 78 1.3 ( 15.9)

5

IO 20

779.9 (15.5) 779.7 (15.0) 779.9 (1 5.1)

CO(~P)/S~(~P) ratio as prepb groundb 0.065 0.13 0.29 0.48

0.099 0.13 0.29

CoO/Si02(A) 2 5

IO

0.088 781.4 (1 5.8) 781.5 (16.0) 781.5 (15.6)

0.29 0.63 1.64

0.24 0.59 1.35

3.10

1.34

CoO/Si02(I) 781.4 (16.0)

Reference Compounds Co2Si04

"Referenced to Si(2p) = 103.1 eV. bAs prep. (as prepared) and ground (after grinding using mortar and pestle). cSpin-orbit splitting (ev). dReferenced to C(ls) = 285.0 eV. are not attributed to the unreacted Co2+ salt or to bulk CozSi04. Ion-exchanged Co2+species of 4.1 wt % COOwas found to remain on the surface after washing thoroughly uncalcined 20 wt % CoO/Si02(A) with water at room temperature. On the other hand, washing uncalcined 20 wt % CoO/SiO,(N) removed almost completely the cobalt species. The difference between the two catalyst systems results from the difference in pH of the impregnation solutions: the extent of ion exchange steeply increases as the pH of a solution i n c r e a ~ e s . ~ ~ * ~ ~ , ~ The chemical state and dispersion of cobalt supported on SiO, were examined by using XPS. Typical Co(2p) XP spectra are presented in Figure IO for 5 wt % CoO/Si02. The BE of Co(2p3 ,) band and the Co(2p)/Si(2p) intensity ratio are summarized in fable I for CoO/Si02(N), -(A), and -(I) calcined at 673 K. The Co(2p3/J BE values for Co304and Co2Si04are also presented in Table I for comparison. Both BE and satellite structure of Co(2p) level for 2-20 wt % CoO/SiO,(N) indicate that major cobalt species are Co304-likespecies. On the other hand, the high Co(2p3,,) BE in Table I and relatively high satellite intensity (Figure IO) for CoO/SiO,(A) suggest that paramagnetic Coz+ species"*4'dominate on CoO/SiO,(A) irrespective of CoO content. (39) Bell, A. T. Catalyst Design; Hedgedus, L. L., Ed.; John Wiley & Sons: New York, 1987; p 103. (40) Anderson, J. R. Strucrure of Meralfic Catafysrs;Academic Press: New York, 1975. (41) Frost, D. C.; McDowell, C. A,; Woolsey, I. S. Mol. Pfiys. 1974, 27, 1473.

10 15 20 COO c o n t e n t / w t % Figure 11. Co(2p)/Si(2p) XPS intensity ratio for calcined CoO/Si02 as a function of COOcontent, preparation method, and starting salt: (0) CoO/SiO,(A), (A)CoO/SiO,(N), and (0)CoO/.SiO,(l). Closed sym0

5

bols show the ratios after grinding. The spectral features of the Co(2p) level were not altered by grinding the catalyst sample in a mortar and pestle within the experimental accuracy as summarized in Table I. The Co(Zp)/Si(2p) XPS intensity ratio is presented in Figure 11 for CoO/SiO, as a function of COOcontent. According to Kerkhof and moulijn,"2 the XPS intensity ratio is correlated with the dispersion of cobalt in the following equation, assuming a uniform particle size, d, of cobalt species (eq l), where (Co( C O ( ~ P ) / S ~ ( ~=P )(CO(2P)/Si(2P))m(X/d)[l ) - exp(-d/X)l (1) (2p)/Si(2p)), represents the XPS intensity ratio for a catalyst with a monolayer dispersion of cobalt and X is the escape depth of photoelectrons. Higher XPS intensity ratios suggest higher dispersions or higher surface concentrations of cobalt. The Co(2p)/Si(2p) XPS intensity ratio for CoO/SiO,(N) was found to linearly increase up to 10 wt % COOand then become deviated downward from the linear line at 20 wt % COO. With CoO/SiO,(A), the ratio linearly increased up to 10 wt % COO, while it increased more steeply at 20 wt % COO. It is noteworthy that the Co(2p)/Si(2p) XPS intensity ratios for CoO/Si02(A) are steadily higher than those for CoO/SiOz(N). CoO/Si02(I) showed a significantly higher XPS ratio than CoO/SiO,(N) and (A) as presented in Figure 11. It was found that grinding the sample significantly affected the Co(Zp)/Si(Zp) ratio. As depicted in Figure 11, the ratio for CoO/SiO,(N) considerably decreased on grinding, whereas that for CoO/Si02(A) did not appreciably vary at 5 and IO wt % CoO and slightly decreased at 20 wt % COO. Castner et aLZ5also noted the decreases in the Co(2p)/Si(2p) XPS intensity ratio on grinding 10 wt % CoO/SiOz catalysts prepared from cobalt nitrate and calcined at 723 K. The Co(Zp)/Si(2p) ratio for CoO/Si02(I) was Significantly reduced by grinding but was still higher than those for the impregnation catalysts. These results in Figure 11 indicate that both cobalt dispersion and cobalt distribution throughout the Si02 particles strongly depend on the starting material and preparation method. The XP spectra of uncalcined samples (after drying at 383 K) were measured in order to reveal the determining stages of the (42) Kerkhof, F. P. J. M.; Moulijn, J. A. J . Pfiys. Cfiem. 1979,83, 1612.

Okamoto et al.

316 The Journal of Physical Chemistry, Vol. 95, No. 1, 1991

781.6 (15.5)b 781.6 (1 5.8) 782.0 ( 1 5.8) 782.0 (15.8) 781.9 (15.5) 781.9 (16.0) 781.7 (15.6)

2 5 5c

IO I oc

20 20c

*.

I

0.13 0.37 0.35 0.79 0.65 1.70 1.39

20 20'

CoO/Si02(A) 781.6 (15.8) 0.13 0.34 78 1.6 (1 5.8) 0.36 78 I .7 (1 5.8) 781.9 (16.0) 0.67 781.9 (16.0) 0.90 782.0 (15.6) 1.92 2.26 781.8 ( 1 5.7)

8.9 8.9' 8.9d

CoO/Si02(I) 781.3 (15.8) 3.42 1.10 781.4 (16.2) 781.7 (15.6) 1.10

Co(NOJ2 Co(CH,C00)2

Reference Compounds' 781.8 (16.0) 781.1 (16.1)

2 5 5'

IO I 0'

406.2 406.8 406.7 406.8 407.0 407.0 407.1

0.040 0.035 0.065 0.079 0.10

0.1 1 0.12

0L

407.0

0.20

-3 .-0 c

0 .-+

L

L

m

m 5r

0-

2 1

.-

CL N

.-v, \

CL

N ..

I/''

0-

-1

2

t'

J/

0

5

10

15

20

O

COO c o n t e n t / w t % Figure 13. Co(2p)/Si(2p) XPS intensity ratio for sulfided CoO/Si02: (0)CoO/SiO,(A), (A) CoO/Si02(N), and (0) CoO/sio2(1).

0

c

d

R 0,s 0

"Referenced to Si(2p) = 103.1 eV. bCo(2p) spin-orbit splitting (eV). cAfter grinding using mortar and pestle. dAfter grinding again. eReferenced to C(ls) = 285.0 eV.

c

L

I

I

I

I

5

10

15

20

10

COO c o n t e n t / w t % Figure 12. C0(2p)/Si(Zp) XPS intensity ratio for CoO/SiO, before

calcination as a function of COO content, preparation method, and starting salt: (0)CoO/SiO,(A), (A) CoO/SiO,(N), and (0)COO/ Si02(l). Closed symbols show the ratios after grinding. dispersion and distribution of cobalt. The Co(2p3/,) BE values and Co(2p)/Si(2p) intensity ratios are summarized in Table 11. On the basis of the BE, satellite structure (very close to the spectrum for oxidic CoO/Si02(A) in Figure IO), and spin-orbit splitting of Co(2p) level,"-4' it is concluded that Co2+ species dominate on all the series of catalysts before the calcination. The Co(Zp)/Si(2p) ratios for uncalcined CoO/SiO,(A) and -(N) are presented in Figure 12 as a function of COOcontent. In contrast to the calcined catalysts, the Co(2p)/Si(2p) ratio of uncalcined CoO/Si02(N) linearly increased up to 20 wt % with increasing COO content. The ratio slightly decreased on grinding in a mortar and pestle but the linear correlation was kept, sug-

gesting a uniform distribution of cobalt nitrate throughout the Si02particles. With uncalcined CoO/Si02(A), the Co(2p)/Si(2p) ratio was very close to that of uncalcined CoO/Si02(N) at 2-10 wt % COO but the ratio was much larger than that of the corresponding nitrate sample at 20 wt %. It was found with CoO/Si02(A) that the ratio was not changed by grinding at 5 wt 5% COO, whereas it was considerably increased at 10 and 20 wt %. The Co(2p)/Si(2p) ratio of CoO/Si02(I) significantly decreased on grinding but it was still considerably higher than the ratios for uncalcined impregnation catalysts. This suggests that the outer surface of the Si02 particle is preferentially ion-exchanged with Coz+ in the present procedure. The C0(2p3 ,) BEs in Tables I and I1 indicate that major cobalt species are divalent in CoO/SiO,(I) before and after the calcination. It is worthy of note that the Co(2p)/Si(2p) ratios enormously decrease on calcination at 673 K for CoO/Si02(N), whereas the ratios diminish to a much lesser extent for CoO/Si02(A). It appears that with CoO/Si02(N) significant changes in the cobalt dispersion and distribution take place on the calcination. The N( 1s) BE and N( ls)/Co(2p) intensity ratio are presented in Table I1 for uncalcined CoO/SiO,(N). The N(1s) BE was consistent with that for Co(N03),. The N(ls)/Co(Zp) ratio was lower than that for the starting salt and decreased with decreasing COO content. A decrease in the N(ls)/Co(2p) ratio for uncalcined precursor COO/Si02 samples (10 wt %) was also noted by Castner et al.z5 The N ( Is)/Co(2p) ratio was found to increase on grinding the sample, suggesting a larger concentration of the anions in the inner part of Si02particles. No N ( 1s) peak was discernible for calcined CoO/Si02(N). The C( 1s) peak due to acetate anions was detected at 288.7 eV for CoO/Si02(A) before calcination. The intensity analysis was hindered by a superposition of adventitious carbon at 285.0 eV. Acetate carbons disappeared on the calcination. CoO/Si02(I) showed no appreciable C( Is) peak around 289 eV. Typical XP spectra of sulfided CoO/Si02(A) and -(N) are ) shifted to 777.8 0.2 shown in Figure 10. The C 0 ( 2 p ~ / ~BE eV on sulfidation at 673 K and no peak due to residual oxides was detected at 780-782 eV, indicating that the cobalt species on Si02was sulfided irrespective of the starting salt. The Co(2~312)BE was consistent with the value for co&, in the literaCOO/Si02(I) was also found to be completely sulfided at 673 K. As shown in Figure 13, the Co(Zp)/Si(2p) XPS intensity ratio for CoO/Si02(A) was still higher than that for CoO/SiO,(N), although the ratios for both catalyst systems slightly decreased. The XRD results for sulfided CoO/Si02 are

*

(43) Alstrup, Ib; Chorkendorff, Ib; Candia, R.; Clausen, B. S.; Topsee, H. J . Catal. 1982, 77, 391. (44) Chin, R. L.; Hercules, D.M. J . Phys. Chem. 1982, 86, 3079.

Si02-Supported Cobalt Oxide and Sulfide Catalysts

The Journal of Physical Chemistry, Vol. 95, No. 1 , 1991 317

+ 0

I

I

I c

EL:,; m

a.

5

10

15

.-0

t u

j

20

COO content/wt% Figure 14. Catalytic activity of sulfided CoO/Si02 for the hydrogenation of butadiene at 473 K: (0)CoO/Si02(A), (A)CoO/Si02(N),and (0)

CoO/SiO,( I). presented in Figure 2. With sulfided CoO/Si02(N), the peaks due to CogSs were observed (20 = 52.2' and 47.6') even at 2 wt % COO at the expense of c0304 diffractions. The line width of Cogss was broad as compared with that of cOg04, suggesting a decrease in the crystal size on sulfidation. As for CoO/Si02(A), no diffractioin peaks were detected at 10 wt % and, probably, even at 20 wt 5% COO. These XRD results suggest that the dispersion of cobalt sulfides is higher on CoO/SiO,(A) than on C o o / Si02(N). The catalytic activity of sulfided CoO/Si02 catalysts is presented in Figure 14 for the hydrogenation of butadiene at 473 K. The hydrogenation products were only butenes. It is evident that CoO/Si02(A) catalysts show more than six times higher hydrogenation activity than CoO/Si02(N) catalysts. CoO/Si02(I) was found to show a hydrogenation activity similar to that of CoO/Si02(A). The hydrogenation activities of both COO/ Si02(A) and -(N) linearly increased up to 10 wt % and leveled off at 20 wt 7% as the COO content increased. These findings are considered to correlate to the linear increase of the Co(2p)/Si(2p) ratio for the sulfided CoO/Si02(A) or -(N) in the range of 2-10 wt % c o o .

Discussion I . CobaIt Species on CoO/Si02. The TPR results indicate the existence of several kinds of cobalt species on CoO/Si02 calcined at 673 K. The formation of Co304was detected by XRD and DRS-VIS for CoO/SiO,(N). TEM observations suggest a bimodal distribution of c O j 0 4 particles (7 and 25 nm). On the basis of these observations and the reduction temperatures, which are relatively close to those of Co304,Co I and I1 species in TPR are assigned to c0304 phases with different particle sizes of Co304, that is, 7 and 25 nm, respectively. The TPR peak temperature of Co304 increases with increasing particle or crystalline size because of a limited diffusion rate of oxygen anions through the oxide particles during H2 reduction. This was confirmed by the TPR profiles of CO304 at the particle sizes of 50 and 1200 nm. In Figure 15, the reduction temperature of c0304 in TPR (B = 2.5 K m i d ) is plotted against the particle size of Co304for the unsupported Co304samples and Co I and I1 on CoO/SiO,(N). The TPR and XRD results32 for CoO/A1203with varying COO content are also included in Figure 15. It is likely that a good correlation in Figure 15 supports the above assignments of Co I and I 1 to the c0@4particles with different sizes. Only a small amount of Co I was found on CoO/Si02(A) by TPR. This is in accordance with the TEM observations in Figure 3. It is considered that Co 1 and I 1 have no strong interactions with S O z surfaces. Castner et aLzs have suggested with CoO/Si02 prepared from cobalt nitrate that the particle size of Co304depends on the pore size of Si02: the particles sizes of 20 and 5-10 nm were observed for silicas with the mean pore radii of 15 and 3 nm, respectively. The present TEM and XRD observations that Co304 with an averaged particle or crystallite size of 7 nm forms on the Si02 with a mean pore radius of 3.9 nm are consistent with their findings. The formation of uniform particle size of Co304,almost

'0 3 W

LL

5001 1

5

10

50

100

d/nm Figure 15. Correlation between the TPR temperature (6 = 2.5 K min-') and particle or crystallite size of co304: (A)unsupported Co3O4,(0) CoO/SiOz, and ( 0 )CoO/AI2O3. being independent of COOcontent and starting salt, suggests the pore size to be a determining factor in the extent of agglomeration of cobalt species. The c0@4particles of 25 nm are considered to be formed through the pores of ca. 10 nm (Figure 1). Co 111 cannot be attributed to Co304with a larger particle size, such as 1200 nm on the basis of the TPR peak position, since cobalt oxide particles larger than 25 nm were hardly observed by TEM. Co 111 is assigned to Co-Si-0 mixed oxides, Si4+x/2C02+I-,Co3+204 with a spinel structure, produced by a solid state reaction with cobalt and S O 2surfaces. It is well known4s that Sil,2C0204has a normal spinel structure (extent of inversion: 0.014) and that Si4+and Co3+ occupy tetrahedral and octahedral sites, respectively. Another possible Co-Si-0 mixed oxide is Co2Si04in which cobalt is divalent. The XPS and TPR (Figure 7) results negate the formation of a considerable amount of Co2+ species on CoO/Si02(N). Accordingly, Co 111 cannot be assigned to Co2Si04. Replacement of Co2+in c0304 with Si4+is considered to increase the polarization of Co-0 bonds, decreasing the reducibility and hence increasing the reduction temperature in the TPR profiles (50-60 K). With CoO/A1203, incorporations of AI3+ into octahedral interstices of c0304 greatly increase the reduction temperature (ca. 150 The replacement of Co2+in Co304 with Zn2+ increased the reduction temperature by 40 K.32 CoO/Si02(A) exhibited nearly no formation of Co 111. A considerable amount of Co IV was observed in COO/ Si02(N). As deduced from the TPR area in Figure 7 and the XPS results in Figure 10 and Table I, major cobalt species are trivalent on CoO/Si02(N). On the basis of an increased reduction temperature as compared with that of cOg04, it is inferred that Co IV is a surface Co3+species interacting with the Si02surface. Taking into account the crystal field strength18 of Co3+ ions, it is likely that the surface Co3+species are in octahedral symmetries. Co V may also be attributed to surface interaction species, probably, Co2+ in octahedral configurations or surface silicate as discussed below. The interaction strength of Co V with Si02is close to those of Co VI and VII. Unfortunately, no further information concerning Co IV and V was obtained in the present study. Ion-exchanged Co2+ species are differentiated from Co VI simply by grinding. A reversible transformation of Co2+structure between octahedral and tetrahedral coordinations is achieved by H 2 0 adsorption and grinding for ion-exchanged Co2+ species. After calcination at 673 K, CoO/Si02(I) contains a considerable fraction of ion-exchanged Co2+ species together with Co VI. N o spectral change of CoO/Si02(A) on grinding (Figure 9) indicates the absence of ion-exchanged Co2+species. The TPR peak Co VI1 is, accordingly, assigned to ion-exchanged Co2+ species. The DRS-VIS spectra of CoO/Si02(A) catalysts are completely different from that of cobalt silicate, Co2Si04,where Co2+occupies octahedral sites,36and rather close to that of ion-exchanged Co2+ in a tetrahedral symmetry. Accordingly, Co VI is assigned to K).18332

(45) Hill, R.J.; Craig, J . 4, 317.

R.;Gibbs, G . V. Phys. Chem. Minerols 1979,

Okamoto et al.

318 The Journal of Physical Chemistry, Vol. 95, No. 1 , 1991

TABLE 111: Cobalt Species on CoO/SiO1 Calcined at 673 K' cobalt speciesb Si-Co-0 surface surface ionCoO/Si02 mixed Co3+ surface Co2+in exchanged Co2+ catalyst Co304 oxide species silicate Td N A

I

+++

+ -

++

-

+

+

f -

i

-

+++ +++

-

-

+

'Presented for IO wt 70COO. bCo304:CoI and 11, Si-Co-0 mixed oxide:Co I l l , surface Co3+ species:Co IV, surface silicate:Co V, surface Co2+ in tetrahedral symmetries:Co VI, and ion-exchanged Co2+:Co VII. The proportion of Co species decreases in the order +++ (major) > ++ > + > f > - (not detected). surface Co*+ species in a (distorted) tetrahedral symmetry. The formation of surface cobalt silicate is also possible due to an expected high stability of the silicate and hence Co V is tentatively assigned to surface silicate. A considerable proportion of the ion-exchanged Co2+species is converted to the surface Co2+species in a tetrahedral symmetry on calcination at 673 K. It is inferred, however, that a majority of the surface Co2+ species on COO/ Si02(A) are produced from well-dispersed Co2+acetate species during the calcination, since the amount of ion-exchanged cobalt was only 4.1 wt 7% COO for 20 wt 7% CoO/Si02(A) under the present impregnation conditions. The cobalt species on CoO/Si02(N), -(A), and -(I) are qualitatively summarized in Table 111. Their proportions depend on COO content as deduced from the TPR results in Figures 4-6. The assignment of cobalt species in Table I11 is in conformity with ,the C0(2p3/,) BEs in Table I and Co(2p)/Si(2p) intensity ratios (vide infra). It is deduced that the above remarkable differences in the cobalt species between CoO/Si02(A) and -(N) result from the presence of NO3-anions on CoO/Si02(N), which effectively oxidize Co*+ to Co3+and hence facilitate the formations of Co30,, Co-Si-0 mixed oxide, and Co3+ interaction species. 2. Dispersion and Distribution of Cobalt on CoO/SiO,. The XRD and TEM observations in Figures 2 and 3 indicate that cobalt species are much more highly dispersed on CoO/Si02(A) than on CoO/Si02(N). The Co(2p)/Si(2p) XPS intensity ratios in Figure 1 1 agree with these observations, taking into account eq I . The decreases in the Co( 2p)/Si(2p) ratio for CoO/SiO,(N) on grinding strongly suggest that cobalt species considerably segregate on the outer surface of Si02. The intense segregation of cobalt on CoO/Si02(N) is not caused during the impregnation processes, since cobalt species on uncalcined CoO/Si02(N) distributes in a considerably uniform manner throughout the SiO, particles as suggested by a comparison of the Co(2p)/Si(2p) ratios (Figure 12) measured before and after grinding. The C0(2p)/ Si(2p) ratios of uncalcined CoO/SiO,(N) samples significantly diminished on calcination at 673 K. This indicates intense agglomeration of cobalt, forming Co304 and Si,,zCo3-x04. It is obvious that these strong surface segregation and agglomeration of cobalt on CoO/SiO,(N) are brought about by the calcination and hence by the oxidation of Co2+ to Co3+by NO< anions. The downward deviation of the Co(Zp)/Si(2p) ratio from the linear correlation (Figure 1 1) implies that the extent of the agglomeration of cobalt strongly increases at 20 wt % COOon the calcination. In contrast to CoO/SiO,(N), no significant surface segregation and agglomeration of cobalt are observed for 2-10 wt % COO/ Si02(A), indicating a uniform cobalt distribution and a high dispersion of cobalt species. The formation of the surface Co2+ species (Co VI) is responsible for the high dispersion of cobalt. A significant surface segregation of cobalt acetate observed for uncalcined 20 wt % CoO/SiO,(A) is considered to result from a surface deposition of cobalt acetate due to a low solubility as compared to cobalt nitrate. An increase in the Co(2p)/Si(2p) ratio on grinding might suggest that cobalt acetate is deposited in pores slightly deeper than the surface layer sampled by XPS (2-5 nm). On calcination at 673 K, the distribution of cobalt becomes considerably uniform and the dispersion of cobalt is kept

high, suggesting a considerable surface mobility of Co2+cations at 673 K. With CoO/SiO,(I), it is apparent from Figure 12 that ionexchange takes place from the outer surface to the inner surface of SO,. A surface segregation of cobalt is still remarkable after the calcination. On the basis of the XPS ratio after grinding (Figure 1 l ) , it is concluded that the dispersion of cobalt is very high on calcined CoO/SiO,(I). This is consistent with the TPR results in Figures 5 and 6. 3. Sulfided CoO/Si02 Catalysts. It is evident from Figure 10 that all the cobalt species on SO,, Co I-VII, are sulfided under the present sulfiding conditions. Taking into account the fact that the reduction temperatures of the surface Co2+species in a tetrahedral symmetry and ion-exchanged Co2+species are significantly higher than the sulfiding temperature (673 K), it is deduced that sulfiding of cobalt species takes place via 0 - S anion exchanges rather than via H2 reductions of Co2+ followed by sulfidation. Arnoldy et aI.l9 reached the same conclusion in the temperature-programmed sulfidation (TPS) study of CoO/AI2O3. With supported molybdenum oxide, a similar sulfidation mechanism has been proposed by M a ~ s o t h . ~ ~ From comparisons of the XPS and XRD results between oxidic and sulfided catalysts, it is concluded that sulfidation of COO/ SiO,(A) and -(N) did not appreciably alter the distribution and dispersion of cobalt in the range 2-10 wt % COO. At 20 wt % COO,however, considerable decreases in the Co(2p)/Si(2p) ratio for both catalyst systems suggest enhanced agglomeration of cobalt on the sulfidation, probably, due to a conjestion of the aggregated cobalt phases in the oxidic state. It is evident that an intense agglomeration of cobalt is induced by sulfidation also for CoO/SiO,(I), in which a surface accumulation of cobalt is very remarkable in the oxidic form as stated above. However, the extent of dispersion of cobalt is still high on sulfided CoO/SiO,(I) even after the agglomeration of cobalt because of inherently high dispersion of cobalt as compared with that of CoO/SiO,(N). The high catalytic activities of CoO/Si02(A) and -(I) for the hydrogenation of butadiene in Figure 14 obviously result from high dispersions of sulfided cobalt species as compared with those on CoO/SiO,(N). The leveling off of the activity at 20 wt % COO for CoO/SiO,(A) and -(N) is a consequence of the increase in the extent of agglomeration of sulfided cobalt and hence of the leveling off in the number of active sites. This was corroborated by NO adsorption experiment^.^' The hydrogenation activity of CoO/SiO,(I) was very close to that of CoO/SiO,(A) in spite of a high Co(2p)/Si(2p) ratio for CoO/SiO,(I). This is ascribed to the surface segregation of cobalt as discussed above.

Conclusions The interaction modes between cobalt and SiO, were characterized by using XPS, TPR, TEM, DRS-VIS, and XRD techniques. CoO/SiO2 was prepared by an impregnation method or an ion-exchange technique. It was found that several kinds of cobalt species are formed on CoO/SiOz. These cobalt species were assigned to c0304, Six,zCo3-x04,surface Co3+ species, surface silicate, surface Co2+ species in tetrahedral coordinations, and ion-exchanged Co2+ in the order of the TPR reduction temperature. Their proportions were found to strongly depend on starting salt, COOcontent, and preparation method. It was revealed that CoO/SiO,(A) prepared from cobalt acetate contains predominantly surface Co2+ species in tetrahedral symmetries, while conventional CoO/SiO,(N) prepared from cobalt nitrate contains ill-dispersed c0304 and Co-Si4 mixed oxide phases. The cobalt species on CoO/SiO,(A) were demonstrated to be highly dispersed and uniformly distributed throughout the catalyst particles as compared with those on CoO/SiO,(N). A considerable proportion of ion-exchanged Co2+ species in CoO/SiO,(I) was found to transform to the Co2+species in a tetrahedral symmetry. All the cobalt species interacting with S O , were found to be sulfided at 673 K. Sulfided CoO/SiO,(A) and -(I) exhibited several times (46)Massoth, F. E. J . Cutul. 1975, 36, 164. (47) Okamoto, Y . ;et a]., unpublished work.

J . Phys. Chem. 1991, 95, 319-324 higher catalytic activity for the hydrogenation of butadiene than CoO/Si02(N). On the basis of the XPS characterization including uncalcined precursors, it is proposed that the differences between CoO/Si02(A) and -(N) in the cobalt species and in the dispersion and distribution of cobalt reside in the oxidation of Coz+ to Co3+ during calcination by residual NO3- anions on COO/ SiO,(N) prccursors.

319

Acknowledgment. We gratefully acknowledge Dr. Y. Nitta, Department of Chemical Engineering, Osaka University, for the supply of Co2Si04and Mr. M. Odawara, Department of Chemical Engineering, Osaka University, for the assistance in the XRD and catalytic activity measurements. We are deeply indebted to Mr. K. Nakai, BEL Japan, Inc., for the measurements of the pore size distribution of SiOz (JRC-SIO-4).

Pressure Dependence of the Excess Properties of Simple Molecular Mixtures. The CCI, CH2C12System

+

A. Compostizo, A. Crespo Colin, M. R. Vigil, R. G. Rubio, and M. Diaz Peiia* Departamento de Qujmica Fhica. Facultad de Ciencias Qdmicas, Universidad Complutense. 28040 Madrid, Spain (Received: March 7, 1990; In Final Form: May 17, 1990) The equation of state of CC14 + CH2CI2has been studied over the whole composition range and for 298.15 IT/K I318.15 and 0.1 Ip/MPa I40.0. As for the CCI, + CS, or CCI, CH2CI2systems, p decreases with increasing pressure, its magnitude for x = 0.5 being similar to that for CC14 + CS2 and less than half the value for CSz + CH2C12. HEhas been calculated as a function of p, and a reasonably good agreement with experimental data was found. H E ( x - 0 . 5 ) is again almost half that of CS2 + CHZCl2but twice that of CCI, + CS,. The change of GEover the pressure range follows a trend similar to p,but now GEincreases with p . The results have been analyzed in terms of two semiempirical generalized van der Waals equations of state, one due to Deiters and another due to Kohler and co-workers. Although both equations describe , fail in predicting for both pure components and binary mixtures. For the binary quite well the ( & ~ / r 3 p ) ~they systems formed with either CCl, or CH2Cl2,the binary adjustable parameters show almost negligible composition and temperature dependences.

+

(+/an,

Introduction The technological importance of having good theoretical tools for predicting the thermophysical properties of fluids is obvious. However, so far, rigorous statistical mechanical theories can only be applied to quite simple systems.I In fact, only for those molecular systems with relatively low anisotropies in the intermolecular potential can semiquantitative thermodynamic surfaces be obtained from different theoretical approachesz4 or computer simulation^.^ Moreover, the simultaneous existence of anisotropy of shape of the molecular core and multipolar moments has not been treated satisfactorily with any of the current perturbation theories, except for very low anisotropies of shape. The effect of electrical moments upon the thermodynamic properties of fluids with a spherical reference potential has been described in detail by Gubbins' group.6-8 More specifically, Flytzany-Stephanopoulos et aL9 have studied the effect of polar, quadrupolar, and overlap forces upon the excess properties of binary mixtures using a perturbation theory.z*6They found that these anisotropic forces led to positive deviations from ideality and caused the excess function curves to depart from symmetry with respect to the equimolar mixture, even leading to S-shaped curves for anisotropy strengths large enough. For mixtures with ( I ) Hansen, J. P.; McDonald, 1. R. Theory ofSimple Liquids; Academic Press: London, 1986. (2) Gray, C. G.;Gubbins, K. E. Theory of Molecular Fluids; Clarendon Press: Oxford, 1984. (3) Chandler, D. In The Liquid State of Matter; Montrol, E. W., Lebowitz, J. L.,Eds.; North-Holland Co.: Amsterdam, 1982. (4) Kohler, F.; Marius, W.;Quirke, N.; Perram, 1. W.; Hoheisel, C.; Breithenfelder-Manske, H. Mol. Phys. 1979, 38, 2057. ( 5 ) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Clarendon Press: Oxford, 1987. (6) Gubbins, K. E.; Shing, K. S.; Streett, W. B. J. Phys. Chem. 1983,87, 4573. (7) Clancy, P.; Gubbins, K. E.; Gray, C. G.Discuss. Faraday SOC.1978, 66, 116.

(8) Lobo, L. Q.;Staveley, L. A. K.; Clancy, P.; Gubbins, K. E. J. Chem. Soc., Faraday Trans. I 1980,76,1174. Calado, J. C. G.;Gomez de Acevedo, E. J. S.;Clancy, P.; Gubbins, K. E. J. Chem. Soc., Faraday Trans. I 1983, 7P 7 -6 -5 1. . . -,-

(9) Flytzany-Stephanopoulos,M.; Gubbins, K. E.; Gray, C. G. Mol. Phys. 1975, 30, 1649.

0022-3654/91/2095-03 19$02.50/0

highly polarizable molecules these effects were more intense as a consequence of many-body interactions.1° Using p-V-T data for pure fluids ranging from Ar to CC14, benzene, CHZCl2,or CHC13, we have confirmed the validity of the prediction of the Gubbins4ray theory for the bulk modulus." Similar conclusions were reached for CS2 + CH2Cl2.I2 However, the analysis of the excess properties at low pressures of several binary mixtures of simple molecular fluids, formed by molecules that were rigid and had different electrical moments and degrees of shape anisotropy, lead to the conclusion13 that the whole set of experimental data could not be fitted within the qualitative trends found by Flytzany-Stephanopoulos et ale9 This might be due to the fact that molecules such as benzene, CHJ, CH2C12,etc. are too aspherical to be described by a perturbation theory that uses a spherical reference system. The shape effects have been extensively discussed by Kohler's group using a perturbation theory based on t h e f e x p a n ~ i o n . ~ , ' ~ . ~ ~ In a recent paper, Bohn et a1.I6 have studied the excess properties of mixtures formed by fluids like CCl,, CF,, noble gases, etc. They concluded that, in general, when the parameters of the interaction between molecules of different species were fitted to an excess property, the predictions of other excess functions were not different whether a spherical or a site-site reference system was used. This result is in agreement with the fact that a good description of p-V-T data for several pure fluids was obtained using a Lennard-Jones fluid equation of state obtained by computer simulation." However, when combining rules were used for calculating the mixed interactions, excluding binary parameters, the inclusion of the anisotropy of shape through the use of site-site (10) Venkatasubramanian, V.; Gubbins, K. E.; Gray, C. G.; J o s h , C. G. Mol. Phys. 1984, 52, 141 I. (1 1) Compostizo, A,; Crespo Colin, A.; Vigil, M. R.; Rubio, R. G.;Diaz Peiia, M. Chem. Phys. 1989, 130, 177. (12) Compostizo, A,; Crespo Colin, A,; Vigil, M. R.; Rubio, R. G.;D h z PeRa, M. J. Phys. Chem. 1989, 93, 4973. ( I 3) Crespo Colin, A,; Lezcano, E. G.; Compostizo, A,; Rubio, R. G.;Diaz Peiia, M. J. Chem. Soc., Faraday Trans. I 1989, 85, 4295. (14) Bohn, M.; Lustig, R.; Fischer, J. Fluid Phase Equilib. 1986, 25, 251. ( 1 5 ) Bohn, M.; Fischer, J.; Kohler, F. Fluid Phase Equilib. 1986, 31, 233. (16) Bohn, M.; Lustig, R.; Fischer, J.; Kohler, F. Mol. Phys. 1988, 64, 595.

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