Development of Highly Active Co-Mo Catalysts with Phosphorus and ...

114

Journal of the Japan Petroleum Institute, 48, (2), 114−120 (2005)

[Regular Paper]

Development of Highly Active Co-Mo Catalysts with Phosphorus and Citric Acid for Ultra-deep Desulfurization of Diesel Fractions (Part 2) Characterization of Active Sites Takashi FUJIKAWA†1)*, Masahiro KATO†1), Takeshi EBIHARA†1), Kazuhiko HAGIWARA†1), Takeshi KUBOTA†2), and Yasuaki OKAMOTO†2) †2)

†1) Research and Development Center, Cosmo Oil Co., Ltd., 1134-2 Gongendo, Satte, Saitama 340-0193, JAPAN Dept. of Material Science, Interdisciplinary Faculty of Science and Engineering, Shimane University, Matsue 690-8504, JAPAN

(Received August 31, 2004)

Mo K-edge EXAFS, TEM and FT-IR of adsorbed NO were performed to further investigate the nature of the active sites on the CoMo/HY-Al2O3 catalyst containing citric acid and phosphorus, which had significantly higher HDS activity compared to the conventional CoMoP/Al2O3 catalyst. The results showed that the new catalyst has multiple layers of MoS2 slabs and the edges of MoS2 are mainly occupied by Co_Mo_S phases. XPS and FT-IR were used to investigate the sulfiding behavior of Co and Mo in the formation process of the active sites during sulfidation. The results showed that addition of citric acid to the impregnation solution postponed the sulfidation of Co at low temperatures, thereby increasing formation of the Co_Mo_S phase.

Keywords Hydrodesulfurization, Citric acid addition, Phosphorus addition, Co_Mo_S phase, Characterization, Sulfidation process

1.

Introduction

Sulfur is an undesirable component in diesel fuels because of the adverse effect on the durability of the after-treatment devices required for the control of diesel exhaust emissions, such as nitrogen oxide (NOx) reduction catalysts and catalyzed diesel particulate filters (CDPF). Proposed legislation aimed at reducing air pollution will require most Japanese refiners to begin producing ultra low-sulfur fuels, 300 m2/g). After impregnation, the catalyst was air-dried (without calcination). The Co/Mo molar ratio was ca. 0.3 and the loading of Mo was ca. 13 mass% Mo. The molar ratio of citric acid monohydrate to Co was 0 or 0.7. The molar ratio of P/Mo was 0 or ca. 0.1. The catalyst with both citric acid and phosphorus was designated as C-605A in the first study1). For comparison, a conventional Al2O3-supported CoMoP catalyst was prepared by wet incipient poreVol. 48,

No. 2, 2005

115

volume impregnation using an aqueous solution containing the required amounts of CoCO3, MoO3, and orthophosphoric acid on a support consisting of Al2O3 (specific surface area 364 m2/g). After impregnation, the catalyst was air-dried and calcined at 500°C. The Co/Mo molar ratio was ca. 0.3 and the Mo loading was ca. 13 mass% Mo. 2. 2. Characterization of Catalysts 2. 2. 1. TEM Observations Transmission electron microscope (TEM) measurements were carried out using an electron microscope (JEM-2010, JEOL), with an accelerating voltage of 200 keV. The catalyst was placed in the reactor and 5% H2S/H2 (100 ml/min (STP)) was introduced into the reactor at room temperature. Under a flow of 5% H2S/H2, the catalyst was heated at 5°C/min to 400°C and maintained at that temperature for 1 h. After sulfidation, the reactor was cooled to 200°C under the flow of 5% H2S/H2, then N2 was introduced into the reactor to purge the 5% H2S/H2, and cooled to room temperature. The catalyst was then taken out from the reactor. The presulfided sample was finely ground and suspended in acetone, then placed on a collodion film mounted on a specimen grid. The direct magnification was 200,000 diameters, and five fields of view were examined. The electron micrographs were enlarged to provide a magnification of 2,000,000 diameters. The number of MoS2 laminated layers visually recognized on the photograph was counted, and the lateral direction length of the layers was measured. 2. 2. 2. EXAFS Measurements The Mo K-edge XAFS spectra of sulfided catalysts and reference compounds were measured in the transmission mode at room temperature with a BL-10B station in the Photon Factory of the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK-IMSS-PF), with 2.5 GeV ring energy and 250-290 mA stored current2). The catalysts were sulfided under the same conditions as for the TEM measurements. The synchrotron radiation was monochromatized by a Si (311) channel-cut monochromator. The number of scans was three for each sample and the sum spectrum was used for further analysis. The EXAFS data were analyzed assuming a spherical wave approximation and a scattering model. The EXAFS data were converted from the k-space (40-150 nm−1) to the R-space by Fourier transform. The empirical backscattering amplitude and phase shift for the Mo_S and Mo_Mo pairs were extracted from the EXAFS data for the polycrystalline MoS2. 2. 2. 3. FT-IR Measurements of Adsorbed NO To evaluate the amount of adsorbed NO on the coordinatively unsaturated Co and Mo sites, a sulfided catalyst after NO adsorption was analyzed by diffuseJ. Jpn. Petrol. Inst.,

reflectance FT-IR. The FT-IR was measured using an FT-IR-8100M (Shimadzu Corp.) and an in-situ diffusereflectance type cell (Spectratech Co.). The finely ground catalyst was sulfided in an in-situ diffusereflectance type cell in a gas flow consisting of 5% H2S in H2 at 400°C for 2 h, followed by He flush treatment at 400°C for 30 min. After cooling to room temperature, NO was introduced for adsorption for 30 min. After flushing with He for 30 min, the IR spectra were recorded. 2. 2. 4. XPS Measurements The X-ray photoelectron spectra (XPS) of the catalysts were measured on an XPS-7000 (Rigaku Corp.) photoelectron spectrometer using Mg K α 1,2 radiation (10 kV, 300 mA). Measurements were recorded with a constant pass energy of 20 eV. The sulfided catalyst sample, which had been stored in an evacuated ampul glass, was mounted on a piece of double-sided adhesive tape in a glove-box (vacuum-type) filled with Ar and transferred to a pretreatment chamber using a specially designed cell without exposure to air. The sample was evacuated at room temperature in the pretreatment chamber (1 × 10−5 Pa) prior to the XPS measurement. The base pressure of the spectrometer was better than 1 × 10−6 Pa during the measurement. Sample charging was compensated using the Al 2p line of Al2O3 at 74.7 eV as the internal standard3). Background signals were removed using the Shirley-type integral4). The ratios of Mo6+, Mo5+, and Mo4+ were quantitatively calculated from the peak deconvolution of the Mo 3d XPS spectrum. The S 2p/Mo 3d peak area intensity ratios (S 2p binding energy, 162.0 eV, referenced to Al 2p, 74.7 eV) were converted to the S/Mo molar ratio using the atomic sensitivities obtained from the MoS2 and Co9S8 powders as standards, which were prepared by grinding in a N2 atmosphere5). 2. 2. 5. FT-IR Measurements The IR spectra were recorded on a FT-IR 730 (Horiba Ltd.) using the KBr pellet technique. The catalyst was placed in the reactor and 5% H2S/H2 (100 ml/min (STP)) was introduced at room temperature. Under a flow of 5% H2S/H2, the catalyst was heated at 2°C/min to the desired temperature (150, 200, 225, 300, and 350°C), then cooled to room temperature under the flow of 5% H2S/H2 and taken out from the reactor. The sample was finely ground, mixed with KBr powder, and then ground again. Thin 13 mm diameter wafers were made by pressing a 1 mg sample homogenized in 200 mg KBr powder under high pressure of 8 t/m2 for 3 min. The thin wafer was placed in a ringtype sample holder. The IR spectrum was acquired in the absorbance mode at room temperature in the range of 4600-400 cm−1 with 4 cm−1 resolution. A dry KBr pellet was used as the reference.

Vol. 48,

No. 2, 2005

116

3.

Results and Discussion

3. 1. Nature of the Active Site 3. 1. 1. TEM Measurements TEM measurements were carried out to investigate the characteristics of the active site on the developed CoMoP/HY-Al2O3 catalyst (C-605A) and the conventional CoMoP/Al2O3 catalyst. Figure 1 shows the TEM micrographs of the sulfided catalysts. C-605A has multiple layers of MoS2 slabs with more than two observed layers. The stacking of the MoS2 slabs may be indicative of the formation of the higher activity Type II sites of the Co_Mo_S phase6). These layers of MoS2, which are formed on the support, not only increase the contact efficiency of the reactants but also contain active sites, e.g., the Co_Mo_S phase7),8). On the other hand, the number of such laminating layers on the conventional catalyst averages less than two, so this catalyst has a larger proportion of the lower activity Type I sites of the Co_Mo_S phase. TEM measurements of the uncalcined catalyst with citric acid and phosphorus, the uncalcined catalyst with phosphorus (without citric acid), and the uncalcined catalyst with citric acid (without phosphorus) were carried out to ascertain whether the presence of phosphorus or citric acid leads to greater stacking of the MoS2

crystallites. Table 1 shows that the average stacking number and slab length of MoS2 of the uncalcined catalyst with citric acid and phosphorus approximately agreed with those of the uncalcined catalyst with phosphorus, whereas the morphology of MoS2 was unaffected by the addition of citric acid to the catalyst. Addition of phosphorus to calcined Ni(Co)MoP/ Al2O3 catalysts may increase the number of stacks of the MoS2 crystallites9)∼13). However, C-605A showed much higher stacking of MoS2 crystallites than the conventional CoMoP/Al2O3 catalyst. C-605A catalyst containing phosphorus, which is uncalcined before sulfidation, has much higher stacking of the MoS2 crystallites than the conventional catalyst with phosphorus, which is calcined before sulfidation. By not calcining, the interaction of the Mo and the Al2O3 support is weaker, so the stacking of MoS2 progresses further. 3. 1. 2. XAFS Measurements The TEM results showed that C-605A has multiple layers of MoS2 slabs with more than two observed layers. However, TEM may not show all the characteristics of the catalyst, for example, fine crystals, which are not fully grown, cannot easily be detected and the characterisitcs of the entire sample are difficult to average since the observation domain is minute. Thus, Mo K-edge EXAFS measurements of C-605A and the conventional CoMoP/Al2O3 catalyst were performed. The structural parameters of the coordination number and Debye_Waller factor were obtained by the curve-fitting technique are shown in Table 2. C605A was characterized by higher Mo_Mo and Mo_S coordination numbers and lower Debye_Waller factor14)∼16) for the Mo_Mo shell than the conventional CoMoP/Al2O3. This is consistent with the higher Table 2 Structural Parameters Resulting from the Mo K-edge Fourier-filtered k3-weighted EXAFS Function of Sulfided C-605A Catalyst and Sulfided Conventional CoMoP/Al2O3 Catalyst Catalyst

Shell

Ncoord

∆σ2 [10−3Å]

Dark fringes represent basal planes of MoS2.

C-605A

Mo_S Mo_Mo

5.7 3.4

0.0065 0.0069

Fig. 1 High Resolution TEM Micrograph of the Developed CoMoP/HY-Al2O3 Catalyst, C-605A, and the Conventional CoMoP/Al2O3 Catalyst after Sulfidation

Conventional CoMoP/Al2O3

Mo_S Mo_Mo

5.5 3.1

0.007 0.0071

Table 1 Average Stacking Number and Lateral Length of MoS2 on Uncalcined Catalyst Containing Citric Acid and Phosphorus, Uncalcined Catalyst Containing Phosphorus (without citric acid), and Uncalcined Catalyst Containing Citric Acid (without phosphorus) Stacking number

Lateral length [nm]

Phosphorus + citric acid Phosphorus Citric acid

Catalyst

3.2 2.9 1.9

2.9 2.8 3.3


1.9

3.6

J. Jpn. Petrol. Inst.,

Vol. 48,

No. 2, 2005

117

crystallinity of the MoS2 structure of C-605A than the conventional CoMoP/Al2O3. The TEM results and Mo K-edge EXAFS analysis indicate that the MoS2 of C-605A has a higher stacking structure than that of the conventional catalyst. 3. 1. 3. FT-IR of Adsorbed NO To investigate the dispersion of the Co_Mo_S phases, FT-IR experiments were carried out to analyze the adsorption of NO on the catalysts sulfided at 400°C. NO is the one of the most frequently used molecules to characterize HDS catalysts17),18). Figure 2 shows the IR spectra of NO adsorbed on the sulfided catalysts. Compared with the conventional CoMoP/Al2O3 catalyst, C-605A shows a strong intensity of the Co-associated symmetric NO vibration (1840 cm−1) but a very weak intensity of the Mo-associated symmetric NO vibration (1700 cm−1). This indicates that the edges of MoS2 on C-605A are likely to be occupied to a great extent by the Co_Mo_S phases, as indicated by the increased HDS activity. 3. 2. Formation of Active Sites 3. 2. 1. XPS Measurements The sulfiding behavior of Mo on the catalyst was investigated by semi in-situ XPS. The XPS spectra of

Fig. 2

FT-IR Spectra of NO Adsorbed on Sulfided Catalysts

Fig. 3

C-605A and the conventional CoMoP/Al2O3 catalyst in Fig. 3 show the effect of sulfidation temperature. The Mo 3d signals of both fresh catalysts consist of only a Mo 3d doublet with Mo 3d5/2 binding energy of 232.7 eV, which is characteristic for Mo in the formal 6+ oxidation state19),20). At 100°C, a shoulder peak appeared at a binding energy of 229.0 eV, which indicates that Mo was reduced and sulfided. At 400°C, most Mo was transformed to a species with a binding energy of 228.9 eV, indicating that Mo was mostly present as the sulfide. In the region between 100-400°C, the Mo 3d spectra of the sulfided catalysts can be described in terms of the Mo6+ and Mo4+ doublets and one additional doublet with a Mo 3d5/2 binding energy of 231.9 eV, which we attributed to Mo5+ present in the oxysulfide intermediate phases. Table 3 shows the distribution of the Mo species, Mo6+, Mo5+, and Mo4+, as a function of the sulfiding temperature. The ratio of Mo4+ on C-605A was 89% at 200°C, 88% at 225°C, and 94% at 400°C. On the other hand, the ratio of Mo4+ on the conventional CoMoP/Al2O3 catalyst was 75% at 200°C, 78% at 225°C, and 88% at 400°C. Also, Mo6+ was present on the conventional catalyst at 200°C and 225°C. These results show that C-605A is easier to sulfide than the conventional catalyst. Table 4 shows the sulfidation degree of the catalysts expressed as the S 2p/Mo 3d ratio (S 2p, 162.0 eV). Since the S/Mo molar ratio on C-605A at 175°C is greater than 2, which implies the formation of almost stoichiometric MoS2, Mo was completely sulfided at less than ca. 200°C. On the other hand, the S/Mo molar ratio on the conventional catalyst at 175°C was less than 2. Apparently the calcination procedure promotes the formation of strong Mo_O_Al linkages. Mo species on C-605A had only weak interaction with the Al2O3 support in the absence of calcining, so the sulfidation degree increased compared with the conventional catalyst.

Mo 3d XPS Spectra of (A) C-605A and (B) Conventional CoMoP/Al2O3 during Sulfidation at Various Temperatures J. Jpn. Petrol. Inst.,

Vol. 48,

No. 2, 2005

118 Table 3 Percentage Abundance of the Mo Species, Mo6+, Mo5+, and Mo4+ on the Catalysts as a Function of the Sulfiding Temperature Catalyst

Sulfiding temperature [°C]

Sulfiding time [h]

100 175 200 225 400

1 1 1 1 1

100 175 200 225 400

1 1 1 1 1

C-605A


Table 4 Sulfidation Degree of the Catalysts Expressed as the S2p/Mo3d Ratio (S2p, 162.0 eV) Sulfiding temperature [°C]

Sulfiding time [h]

S/Mo

C-605A

100 175 200 225 400

1 1 1 1 1

1.5 2.3 2.3 1.9 2.2


100 175 200 225 400

1 1 1 1 1

1.6 1.7 1.7 1.7 2.0

Catalyst

3. 2. 2. FT-IR Measurements on Fresh Catalyst FT-IR was carried out to investigate the formation of the complex of Co and citric acid on C-605A. Citric acid and cobalt citrate, which was the complex of Co and citric acid, were also measured using FT-IR. Figure 4A shows the FT-IR spectra of citric acid and cobalt citrate. Cobalt citrate has two bands at 1558 cm−1, which can be attributed to the asymmetric vibration of RCOO−, and at 1415 cm−1, which can be attributed to the symmetric vibration of RCOO− 21). Figure 4B shows the FT-IR spectra of C-605A and the conventional CoMoP/Al2O3 catalysts. C-605A also showed bands at 1412 cm−1. However, C-605A did not show the asymmetric vibration band of RCOO−, probably because the band at ca. 1640 cm−1, which is assigned to the H_O_H bending mode of adsorbed water22), overlaps with the asymmetric vibration band of RCOO−. The fact that the H_O_H bending band on C-605A broadens in the direction of lower wave number strongly supports the overlap with the asymmetric vibration band of RCOO−. On the other hand, the conventional catalyst showed no band at ca. 1415 cm−1, only a sharp band at 1665 cm−1, which was assigned to the H_O_H bending mode of adsorbed water. Furthermore, since C-605A did not show the bands attributed to citric acid, presumably all the citric acid is J. Jpn. Petrol. Inst.,

Mo6+ [%]

Mo5+ [%]

Mo4+ [%]

0 0 0

not identifiable not identifiable 11 12 6

89 88 94

9 8 0

not identifiable not identifiable 16 14 12

75 78 88

complexed with Co during the catalyst preparation. Based on these results, the Co complex with citric acid was completely formed in the developed C-605A catalyst. 3. 2. 3. FT-IR Measurements on the Developed Catalyst during Sulfidation FT-IR measurements were carried out to investigate the sulfiding behavior of the Co-complex on C-605A as shown in Fig. 5. No differences were observed between the samples sulfided at 150°C and 200°C. On the other hand, the RCOO− band at 1412 cm−1 gradually decreased at over 200°C. Decomposition of the Co complex started at around 200°C. Above the sulfidation temperature of 350°C, the RCOO− band disappeared, indicating complete decomposition of the Co-complex. Ni or Co on the HDS catalyst is sulfided below 200°C before the MoS2 phase is formed13),23). However, Mo on the C-605A catalyst is sulfided below 200°C (Tables 3 and 4) before the Co complex is decomposed. Therefore, the main function of citric acid is to postpone the sulfidation of Co at low temperatures. Chelating agents such as NTA, EDTA, and CyDTA stabilize Ni or Co against sulfidation at low temperatures and retard sulfidation of Ni or Co at temperatures at which Mo or W is already in the form of MoS2 or WS224)∼ 27). Our XPS and FT-IR studies support the idea that the addition of citric acid to the impregnation solution during catalyst preparation prevents the sulfidation of Co at low temperature, thereby increasing the formation of the Co_Mo_S phases. 4.

Conclusions

The main conclusions of our characterization of the CoMoP/HY-Al2O3 catalyst containing phosphorus and citric acid, C-605A, are as follows. (1) TEM and EXAFS measurements showed that C605A has multiple, more than two, layers of MoS2 slabs. Vol. 48,

No. 2, 2005

119

Fig. 4

(A) FT-IR Spectra of Citric Acid and Cobalt Citrate, (B) FT-IR Spectra of C-605A and Conventional CoMoP/Al2O3

References

Fig. 5 FT-IR Spectra of C-605A during Sulfidation at Various Temperatures

(2) FT-IR measurements of adsorbed NO indicated that the edges of MoS2 on the developed catalysts was mainly occupied by the Co_Mo_S phases. (3) FT-IR measurements of C-605A showed that the Co2+ ion on the catalyst is complexed with citric acid. (4) FT-IR clearly demonstrated that the favorable effect of the addition of citric acid results from increased thermal stability of Co by the formation of Co citrate. (5) Our new catalyst preparation method can increase the formation of the Co_Mo_S phases and provide more highly active Co_Mo_S Type II, which is located at the edges of the MoS2 multi-layers. Acknowledgments This R&D, which was carried out under the participating contract with the Petroleum Energy Center (PEC), was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the sponsorship of the Ministry of Economy, Trade and Industry (METI) of Japan.


1) Fujikawa, T., Kato, M., Kimura, H., Kiriyama, K., Hashimoto, M., Nakajima, N., J. Jpn. Petrol. Inst., 48, (2), 106 (2005). 2) Okamoto, Y., Okamoto, H., Kubota, T., Kobayashi, H., Terasaki, O., J. Phys. Chem. B, 103, 7160 (1999). 3) Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., Muilenberg, G. E., “Handbook of X-ray Photoelectron Spectroscopy,” Perkin-Elmer, Palo Alto, CA (1979). 4) Shirley, D. A., Phys. Rev. B, 50, 4709 (1972). 5) Okamoto, Y., Tomioka, H., Imanaka, T., Teranishi, S., J. Catal., 66, 93 (1980). 6) Bouwens, S. M. A. M., van Zon, F. B. M., van Dijk, M. P., van der Kraan, A. M., de Beer, V. H. J., van Been, J. A. R., Koningsberger, D. C., J. Catal., 146, 375 (1994). 7) Daage, M., Chianelli, R. R., J. Catal., 149, 414 (1994). 8) Whitehurst, D. D., Isoda, T., Mochida, I., Adv. Catal., 42, 345 (1998). 9) Ryan, R. C., Kemp, R. A., Smegal, J. A., Denley, D. R., Spinnler, G. E., Stud. Surf. Sci. Catal., 50, 21 (1989). 10) Fierro, J. L. G., Lopez Agudo, A., Esquivel, N., Lopez Cordero, R., Appl. Catal., 48, 353 (1989). 11) Ramirez, J., Castano, V. N., Leclercq, C., Lopez Agudo, A., Appl. Catal. A: General, 83, 251 (1992). 12) van Veen, J. A. R., Hendriks, P. A. J. M., Andrea, R. R., Romers, E. J. G. M., Wilson, A. E., J. Phys. Chem., 94, 5282 (1990). 13) van Veen, J. A. R., Gerkema, E., van der Kraan, A. M., Hendriks, P. A. J. M., Beens, H., J. Catal., 133, 112 (1992). 14) Coulier, L., de Beer, V. H. J., van Veen, J. A. R., Niemantsverdriet, J. W., Topics in Catal., 13, 99 (2000). 15) Calais, C., Matsubayashi, N., Geantet, C., Yoshimura, Y., Shimada, H., Nishijima, A., Lacroix, M., Breysee, M., J. Catal., 174, 130 (1998). 16) Shido, T., Prins, R., J. Phys. Chem. B, 102, 8426 (1998). 17) Topsøe, N.-Y., Topsøe, H., J. Catal., 84, 386 (1983). 18) Okamoto, Y., Kawano, K., Kubota, T., J. Chem. Soc., Chem. Commun., 9, 1086 (2003). 19) Barr, T. L., J. Phys. Chem., 82, 1801 (1978). 20) Li, C. P., Hercules, D. M., J. Phys. Chem., 88, 456 (1984). 21) Nakamoto, K., “Infrared and Raman Spectra of Inorganic and Coordination Compounds,” (5th ed.), John Wiley, New York (1997). 22) Little, L. H., “Infrared Spectra of Absorbed Species,” Academic Press, London (1966). 23) Medici, L., Prins, R., J. Catal., 163, 38 (1996). 24) Ohta, Y., Shimizu, T., Honma, T., Yamada, M., Stud. Surf. Sci. Vol. 48,

No. 2, 2005

120 Catal., 127, 161 (1999). 25) Kishan, G., Coulier, L., van Veen, J. A. R., Niemantsverdriet, J. W., J. Catal., 200, 194 (2001). 26) Coulier, L., Kishan, G., van Veen, J. A. R., Niemantsverdriet, J.

W., J. Phys. Chem. B, 106, 5897 (2002). 27) Kubota, T., Hosomi, N., Bando, K. K., Matsui, T., Okamoto, Y., Phys. Chem. Chem. Phys., 5, 4510 (2003).

……………………………………………………………………

要旨リンとクエン酸を添加した高活性 Co-Mo 系軽油超深度脱硫触媒の開発（第 2 報）活性点のキャラクタリゼーション藤川貴志†1)，加藤勝博†1)，海老原猛†1)，萩原和彦†1)，久保田岳志†2)，岡本康昭†2） †1)

コスモ石油（株）中央研究所，340-0193 埼玉県幸手市権現堂 1134-2

†2)

島根大学総合理工学部物質科学科，690-8504 島根県松江市西川津 1060

第 1 報で，リンとクエン酸を添加した CoMo/HY-Al2O3 触媒は

のほぼ全体が活性点である Co−Mo−S 相で占められていると推

従来の CoMoP/Al2O3 触媒に比較し格段に脱硫活性の高いこと

測された。さらに，硫化過程での活性点の形成のメカニズムを

が確認された。本報では，この触媒の高活性の原因を究明する

X 線光電子分光，ならびに赤外分光で調べた結果，高活性の原

ため，硫化後の触媒について，Mo K-edge EXAFS，透過型電子

因は，Mo が先に硫化された後に，クエン酸とキレート化した

顕微鏡，NO 吸着赤外分光測定を行い，活性点の特徴について調べた。その結果，触媒上で MoS2 は積層化し，そのエッジ部

Co が徐々に分解され，効果的に MoS2 のエッジ部に活性点（Co−Mo−S Type II）が形成されるためと推察された。

……………………………………………………………………


Vol. 48,

No. 2, 2005