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Effect of Sulfidation Temperatures on the Bulk Structures of Various Molybdenum Precursors Hamdy Farag† National Institute of Advanced Industrial Science and Technology (AIST), Institute for Energy Utilization, 16-1 Onogawa, Tsukuba, Ibaraki, 305-8569, Japan Received December 19, 2001. Revised Manuscript Received March 30, 2002
Sulfiding is a necessary preliminary treatment for the hydrotreating supported CoMo and NiMo catalysts. Sulfidations of two bulk molybdenum precursors, ammonium heptamolybdate and molybdenum acetylacetonate, with 5 wt % H2S/H2 have been investigated as a function of temperatures. The course of the reactions was followed and studied by means of X-ray diffraction and gravimetric analysis. The sulfidation extent of bulk ammonium heptamolybdate was found to be highly dependent on temperature. Three main successive steps are suggested to be proceeded through sulfidation. These are calcination to MoO3, reduction to MoO2, and ultimately direct sulfidation to MoS2-2H structure. Results show that crystalline MoO2 is an intermediate in sulfidation of ammonium heptamolybdate. In addition, a fully sulfided phase to a highly crystalline MoS2-2H was obtained at ca. 800 °C. However, sulfidation of molybdenum acetylacetonate occurs quantitatively at ca. 400 °C and leads likely to the formation of a nanocrystalline single layer of MoS2. A sulfiding scheme is proposed.
Introduction Molybdenum forms the crucial basis for the hydrotreating catalyst sector. Although over 30% of the petroleum extracted in the world is transformed by some hydrotreating process, there is still ambiguity concerning the chemistry of the corresponding catalysts.1,2 Alumina is widely used as a support for NiMo and/or CoMo catalysts for hydrodesulfurization reactions. However, carbon has recently appeared and received much attention as a competitive support against Al2O3 due to its availability of a large variety of chemical and physical properties.3 It is known that molybdenum in these catalysts is present in the form of MoS2 crystallites, but no consensus has been reached yet for the promoters of cobalt and/or nickel structure. MoS2 is recognized to be primary support for the promoting catalysts, i.e., Ni and/or Co.4-6 Because such transition metals sulfide loaded on a support have a relatively minor concentration of ∼10 wt %, it appears as a highly disordered phase that creates characterization difficulties. Massoth7 is the first who claimed the uncertainty of description of the sulfided molybdenum catalysts in terms of only the MoS2 phase. †
Tel: 81-298-61-8437. Fax: 81-298-61-8408. (1) Delmon, B. Recent Approaches to the Anatomy and Physiology of Cobalt Molybdenum Hydrodesulfurization Catalysts. In Proceedings of the Third International Conference on Chemistry and Uses of Molybdenum, Ann Arbor, Michigan, 1979. (2) Whitehurst, D. D.; Isoda, T.; Mochida, I. Adv. Catal. 1998, Vol. 42, 345-471. (3) Farag, H.; Whitehurst; D. D., Sakanishi, K.; Mochida, I. Catal. Today 1999, 50, 9-17. (4) Farag H.; Whitehurst, D. D.; Mochida I. Ind. Eng. Chem. Res. 1998, 37, 3533-3539. (5) Barath, F.; Turki, M.; Keller, V.; Maire, G. J. Catal. 1999, 185, 1-11. (6) Topsøe, N.-Y.; Topsøe, H. J. Catal. 1983, 84, 386-401. (7) Massoth, F. E J. Catal. 1975, 36, 164-184.
In the literature,7,8 almost all the sulfidation processes of the Mo, CoMo, and/or NiMo supported Al2O3 catalysts are carried out via the flowing of a gas mixture of H2 with 5-10 wt % H2S started at ambient temperature that increased gradually up to 300-400 °C. Despite the numerous studies on hydrodesulfurization process over molybdenum-based catalysts, the mechanism of sulfiding is not fully understood in a fundamental systematic way. The sulfiding process of a diversity of molybdenum precursors has received little attention up to now.4,9-11 Ammonium heptamolybdate is the starting precursor of the industrial Mo based catalysts, and further it is the main one for a broad literature investigation. However, just little attention has been forward to the investigation of the Mo-complexes as preliminary precursors for Mo-supported catalysts, although they generally show higher activity for hydrodesulfurization.9,10,12 For example, the addition of chelating ligands such as nitrilotriacetic acid (NTA) or ethylenediaminetetraacetic acid (EDTA) has beneficial effect on the catalytic activity of NiMo/SiO2 catalysts.9 Also, molybdenum acetylacetonate leads to a catalyst of higher activity in comparison the classic ammonium heptamolybdate.4 Yet, there is a doubt about whether the molybdenum reactive species in hydrodesulfurization reaction is related to molybde(8) Arnoldy, P.; Van Den heijkant, J. A. M.; De Bok, G. D.; Moulijn, J. A. J. Catal. 1985, 92, 35-55. (9) Cattaneo, R.; Shido, T. and Prins, R. J. Catal. 1999, 185, 199212. (10) Ishihara, A.; Qian, W.; Kabe, T. Sekiyu Gakkaishi 2001, 44, 80-91. (11) Farag, H.; Mochida, I.; Sakanishi, K. Appl. Catal., A 2000, 194195, 147-157. (12) Medici, L.; Prins, R. J. Catal. 1996, 163, 38.
10.1021/ef0102972 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/21/2002
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Table 1. Effect of Various Heat Treatment Conditions of Molybdenum Precursors on the Final Mo Speciesa
molybdenum precursor A molybdenum acetylacetonate B molybdenum acetylacetonate C molybdenum acetylacetonate D ammonium heptamolybdate E ammonium heptamolybdate F ammonium heptamolybdate G ammonium heptamolybdate a
treatment temperature
experimental weight ratio of treated sample to the start precursor
atmospheric condition
ambient temperature ∼ flow of 5 wt %H2S/H2 400 °C, 10 °C/min from the starting heat treatment ambient temperature ∼ flow of Ar until reaching to 400 °C, 10 °C/min 400 °C then, change the gas to 5 wt %H2S/H2 ambient temperature ∼ calcination in air 400 °C, 10 °C/min ambient temperature ∼ calcination in air 400 °C, 10 °C/min ambient temperature ∼ calcination in continuous 400 °C, 10 °C/min flow of O2 gas ambient temperature ∼ flow 5 wt %H2S/H2 from 400 °C, 10 °C/min the starting heat treatment ambient temperature ∼ flow of Ar until reaching to 400 °C, 10 °C/min 400 °C then, change the gas to 5 wt %H2S/H2
theoretical estimated ratio based on Mo species
texture and color of treated sample
48.94%
49.04% as for MoS2 amorphous; spongy bright black
49.25%
49.04% as for MoS2 amorphous; spongy bright black
44.20%
44.13 for MoO3
81.52%
81.53% for MoO3
81.52%
81.53% for MoO3
73.46%
72.47% for MoO2
73.46%
72.47% for MoO2
crystalline; white-faint blue ccrystalline; white-faint blue crystalline; white-faint blue crystalline; black ccrystalline; black
Note: Factor of errors (0.20 ∼ 1.30%.
num oxysulfide or MoS2.2,8 Thus, sulfiding manipulation is a crucial factor for determining the activity of such catalysts. The objective of the present study is to investigate the H2S/H2 sulfidation process of various bulk molybdenum precursors with respect to temperature influence. We believe it is of great importance to analyze first the bulk sulfidation process of molybdenum salts and extract to a certain extent quantitative data of such a process. In this way, the sulfidation procedure of the molybdenum-supported catalysts can be easily simulated. Experimental Section Materials. All chemicals were used as commercially received without further purification. Molybdenum acetylacetonate, ammonium heptamolybdate, (NH4)6Mo7O24‚4H2O, MoO3, MoO2, and MoS2 with high purity grade, more than 99.9 wt %, were purchased from the Dojindo company. Sulfidation Procedure. A gas mixture of 5 wt % H2S in H2 was used with a fixed rate of 50 SCCM for all of the runs. A quartz reactor tube with a length of 30 cm and diameter of 3.5 cm was used for the heat treatment experiments. The reactor was charged with 3-4 g of the sample. The molybdenum precursors were sulfided at various temperatures with a constant heating rate of 9 °C/min and kept at a distinct temperature for ∼4 h. Flowing of the sulfiding gas mixture through the reactor starts either from the beginning of the heating process or after reaching the steady temperature. Some samples were first heated in air for ∼4 h at the temperature of the run and then the H2/H2S gas mixture was admitted for ∼4 h. All samples were then flushed with argon for 20 min at the sulfiding temperature, thereafter cooled to ambient temperature with continuous flowing of argon gas. This helps to eliminate all excess H2S, which might otherwise be physically adsorbed at room temperature. The chemical composition of calcined and/or sulfided molybdenum samples was determined gravimetrically using an ex situ microbalance and further followed by X-ray diffraction technique. From the weight difference of the samples during the sulfidation and/or the calcination manipulation and the starting one, the final composition of samples were calculated and compared with the theoretical estimated values. X-ray Diffraction Procedure. A portion of the solid samples was analyzed by X-ray powder diffraction (XRD) using a Rigaku Geigerflex diffractometer using Cu KR radiation (λ
) 1.542 Å). Crystallite sizes were calculated based on the Debye-Scherrer equation.13
Results Ammonium Heptamolybdate Precursor. (a) Calcination of Ammonium Heptamolybdate. Two samples of molybdenum precursor were calcined, one in air and the other under continuous flowing of O2 gas (purity of 99.9%) at 400 °C. The final chemical structure of these treatments leads to MoO3 with good agreement between the theoretical and the experimental weight loss. Rows A, D, and E of Table 1 give the conditions under which the various specimens were obtained, the means by which they are identified, and their gravimetric weight losses. Thus, whether the calcination at 400 °C has been carried out in air or oxygen atmosphere, the pure highly crystalline MoO3 structure is the ultimate product. X-ray diffraction pattern of MoO3 produced in such procedure is depicted in Figure 1. The d spacings match a typical pattern from the powder diffraction file (PDF) number 5-0508C for MoO3 orthorhombic structure,14 Table 2. However, the peak intensities differ from those on the PDF card, especially at hkl’s (110) and (040). This indicates the high percentage of such crystalline faces. (b) Sulfidation of Ammonium Heptamolybdate. Two samples of ammonium heptamolybdate were sulfided at 400 °C, one through the continuous flow of H2/H2S (5wt %) from the beginning of heat treatment and the other first calcined in argon atmosphere until the temperature reached 400 °C and then thereafter exposed to the H2/H2S (5wt %) gas mixture. The sulfiding gas was kept flowing over the samples at 400 °C for about 3 h. Then, the sulfiding gas was turned off and argon gas was allowed to flush the samples for 20 min. Ultimately, the samples were cooled to the ambient temperature under flow of argon. Results are demonstrated in Table 1F,G. The weight change and X-ray diffraction data of Figure 2 show that there is incomplete sulfidation under these conditions. The main species produced is Mo(IV)O2 with a very tiny amount of MoS2 that is hard to be detected by XRD but (13) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures; Wiley: New York, 1954. (14) JCPDS (Joint Committee for Powder diffraction Studies), International Center for Diffraction Data: Swarthmore, PA, 1986.
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Figure 1. X-ray diffraction patterns of the calcined molybdenum precursors in air at 400 °C. (A) Ammonium heptamolybdate; (B) molybdenum acetylacetonate. Table 2. Indexing of X-ray Diffraction Pattern of MoO3 Obtained after Calcination of Ammonium Heptamolybdate and Molybdenum Acetylacetonate in Air at 400˚C and Their Corresponding from JCPDS MoO3a hkl 021 110 040 020 111 060 002 081
d
I/Io
3.26 100 3.80 54 3.46 78 6.92 52 2.65 21 2.31 39 1.85 16 1.57 13
MoO3b crystallite size,d nm 9.14
d
I/Io
3.26 72 3.81 53 3.46 100 6.92 57 2.65 16 2.31 50 1.85 12 1.57 15
crystallite size,d nm 11.87
MoO3, syn (5-508)c d
I/Io
3.26 3.81 3.46 6.93 2.65 2.31 1.85 1.57
100 82 61 34 35 31 21 16
a Extracted from ammonium heptamolybdate. b Extracted from molybdenum acetylacetonate. c PDF number of JCPDS database. d XRD crystallite size from line broadening (Scherrer equation).
confirmed by gravimetric analysis. MoO2 and a very minor amount of MoS2 are the produced species, regardless of the gas treatments applied. Therefore, MoO2 represents over than 90% of the species produced after sulfiding the ammonium heptamolybdate under the prescribed conditions at 400 °C. Mo(IV)O2 is produced in a perfect monoclinic crystalline form in agreement with the PDF card number 5-0452 of JCPDS. The black
color is probably an indication of the surface formation of MoS2. (c) Sulfidation of Ammonium Heptamolybdate at High Temperature. It was previously shown that the sulfidation of ammonium heptamolybdate precursor by H2S/ H2 at 400 °C is uncomplete. Then, we decided to follow up the formation ratio of MoS2 with various treatments of temperature. Sulfidations of ammonium heptamolybdate with a mixture of 5wt % H2S/H2 (flow rate of 100 SCCM) that flowed through the quartz reactor from the beginning of the heat process at 1 atm and 600, 700, and 800 °C have been carried out. Then, the reactor was flushed by argon gas as described previously. The data are presented in Table 3. X-ray diffraction pattern of ammonium heptamolybdate sulfided by a gas mixture of 5wt % H2S/H2 at 400, 600, 700, and 800 °C are depicted in Figure 2. The patterns illustrate typical scans that are characteristic of crystalline MoO2 and MoS2 structure. They are similar to those reported in JCPDS data for MoO2 and MoS2, PDF 5-0452 and PDF 6-97i, respectively. The products thus obtained are typically highly crystalline forms. As the sulfidation temperature increased the intensity of (100) Bragg peak of MoO2 decreased and simultaneously, the intensity of (002) Bragg peak of MoS2 increased. Furthermore, sulfidation of ammonium heptamolybdate at 800 °C leads to the formation of exclusively MoS2 as can be seen
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Energy & Fuels, Vol. 16, No. 4, 2002 947
Figure 2. X-ray diffraction patterns of molybdenum species obtained after sulfidation of ammonium heptamolybdate by flowing of 5 wt % H2S/H2 gas mixture at 400, 600, 700, and 800 °C. Table 3. Temperature Influence on the Bulk Structure of Mo-Species Starting from Various Precursorsa precursor ammonium heptamolybdate
molybdenum acetylacetonate (A) molybdenum acetylacetonate (B)
temperature treatments
produced wt % of MoO2
produced wt % of MoS2
crystallite sze,b nm MoS2 d(002)
400 °C 600 °C 700 °C 800 °C 400 °C 400 °C
98.65% 39.35% 26.27% 0 0 0
1.35% 60.65% 73.73% 100% 100% 100%
NDc 2.63 4.79 6.68 6.10 Å 5.97 Å
a Sulfidation has been carried out for all samples via flowing of 5 wt % H S/H gas mixture from the beginning of heat treatment with 2 2 flow rate of 100 SCCM. b XRD crystallite size from line broadening (Scherrer equation). c Not determined.
from the X-ray diffraction pattern (Table 3) and the gravimetric analysis (Table 1F,G). It is worthwhile to note that the crystallite sizes grew for this sample from 2.63 to 6.68 nm as the temperature raised from 600 to 800 °C. For MoS2, all of the identified Bragg peaks are indicated in Table 4 in comparison the ideal hexagonal layer structure of MoS2, molybdenite 2H.
2. Molybdenum Acetylacetonate. (a) Calcination of Molybdenum Acetylacetonate. Molybdenum acetylacetonate has been calcined in air at 400 °C for 2 h. X-ray diffraction pattern of Figure 1B and the gravimetric analysis indicate the exclusive formation of MoO3 structure. However, there are some Bragg peaks that have different intensities than the related one extracted
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Table 4. Indexing of X-ray Diffraction Pattern of MoS2 Obtained after Sulfidation of Ammonium Heptamolybdate with 5 wt % H2S/H2 at 800 °C and Its Corresponding from JCPDS MoS2
MoS2 molybdeite-2H, syn
(6-97i)a
hkl
d
I/Iο
d
I/Iο
002 004 006 008 103 101 110 100
6.146 3.075 2.049 1.531 2.271 2.67 1.579 2.73
100 1 5 5 8 7 10 10
6.15 3.08 2.049 1.538 2.277 2.674 1.581 2.737
100 4 14 12 45 10 12 16
a
presented as follows: calcination/400 °C
(NH4)6 Mo7O24‚4 H2O 98 MoO3
(1)
5wt % H2S/H2 400 °C
MoO3 98 MoO2 + MoS2 (trace) (2) 5wt % H2S/H2 400 °C
(NH4)6 Mo7O24‚ 4 H2O 98 MoO2 (98.6%) + MoS2 (1.4%) (3) 5wt % H2S/H2 600 °C
MoO2 98 MoO2 (39.4%) + MoS2 (60.6%) (4)
PDF number of JCPDS database.14 5wt % H2S/H2 700 °C
from ammonium heptamolybdate precursor or even the MoO3 in JCPDS database,14 Table 2. (b) Sulfidation of Molybdenum Acetylacetonate. Two types of sulfiding experiments were made: (a) direct sulfiding through flowing the sulfiding gas from the beginning of the heat treatment and b) sulfidation of the precalcined sample in argon atmosphere. In the first type, molybdenum acetylacetonate was sulfided at 400 °C by flow of 5 wt % H2S/H2 gas mixture at atmospheric pressure from the beginning of the temperature rise. After passing the predetermined sulfidation time, it was found that prolonged time of sulfidation had no effect on the weight change and/or the phase of the ultimate obtained molybdenum species, the sample was flushed by argon gas for 20 min and then cooled to ambient temperature. Another sample (second type) was first calcined in an argon flow until the temperature reached 400 °C, then the 5 wt % H2S/H2 gas mixture was admitted for a given reaction time. This was followed by an argon purge before cooling down to ambient temperature. Figure 3 displays the X-ray diffraction patterns of samples obtained in direct sulfidation and the sulfidation of the precalcined procedures, respectively. The gravimetric analysis in Table 1A,B indicates quantitatively the exclusive formation of MoS2 structure. However, this MoS2 prepared in this manner gives an X-ray diffraction pattern with no recognizable Bragg reflections: only a broad hump at 2θ ≈ 36° can be observed. The crystallite size determined from XRD corresponds to a stacking of only one layer within 1-2% factor of error. One can see that the single layer of MoS2 is formed regardless the type of sulfiding procedures applied. Discussion 1. Sulfidation Nature of Ammonium Heptamolybdate Precursor. It is seen from the present results that the course of sulfiding of ammonium heptamolybdate occurs quite differently from that of molybdenum acetylacetonate. Calcination of ammonium heptamolybdate either in air or oxygen atmosphere leads to the formation of crystalline MoO3 with orthorhombic structure. However, ammonium heptamolybdate and/or its bulk MoO3 undergo a rapid reduction to MoO2 in the presence of 5 wt % H2S/H2 gas mixture at a moderate temperature. This is followed by slow sulfiding to MoS2, which is significantly dependent on the temperature. The proposed sequence of sulfidation reaction can be
MoO2 98 MoO2 (26.3%) + MoS2 (73.7%) (5) 5wt % H2S/H2 800 °C
MoO2 98 MoS2 (100%)
(6)
This sulfiding scheme is in agreement with literature as far as steps 1 and 2 are concerned.7,8,15 There was no indication on the formation of molybdenum metal clusters. Sulfidation of the bulk MoO3 phase passes through first the reduction to MoO2 phase and ultimately to MoS2. The reduction of MoO3 to MoO2 almost quantitatively takes place in the presence of 5wt % H2S/ H2 gas mixture at 400 °C. Thus, MoO2 is an intermediate for the sulfidation of MoO3 extracted from ammonium heptamolybdate precursor. However, the results of X-ray diffraction and the gravimetric analysis show that the rate of sulfidation of such intermediate is very low especially at moderate temperature of 400 °C under the prescribed conditions. This indicates that MoO2 is so resistant to sulfiding by 5wt % H2S/H2 gas mixture until reaching the very high temperature of ca. 800˚C. Probably the formed texture of MoO2 is not very porous, so diffusion limitations of 5wt % H2S/H2 gas mixture are being concerned.8 This could be supported by the surface black color of such material that is very probably due to the sulfiding of minor amounts of surface species. Both MoO3 and MoO2 are formed in a highly crystalline form. It can be concluded that sulfidation of bulk MoO3 extracted from ammonium heptamolybdate by 5wt % H2S/H2 to form MoS2 is extremely hard at a temperature of ca. 400 °C. Direct sulfidations of ammonium heptamolybdate and/or MoO3 extracted from this precursor via flowing of 5wt % H2S/H2 gas mixture are dependent on temperature. Only, two species, MoO2 and MoS2, are detected and confirmed by X-ray diffraction results. As the temperature increased, the production of MoS2 moves from yield of ca. 60% at 600 °C to 100% at 800 °C (Table 3). MoS2 is formed as crystalline MoS2-2H (PDF: 6-97I). It exhibits a strong maximum corresponding to the (002) Miller index in the ideal hexagonal structure. Indeed, this index is associated with the number of stacking layer in this structure. Sulfiding of Mo(IV)O2 to Mo(VI)S2 takes place via an O-S exchange mechanism. Sulfiding is quantitatively completed at ca. 800 °C. Below this temperature, Mo(IV) and Mo(VI) (15) de Beer, V. H. J.; Schuit, G. C. A. In Preparation of Catalysts; Elsevier: Amstrdam, 1976.
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Figure 3. X-ray diffraction patterns of molybdenum species obtained after: (A) Direct sulfidation of molybdenum acetylacetonate at 400 °C through flowing of 5wt % H2S/H2 gas mixture from the begining of heat treatment. (B) Sulfidation of the precalcined molybdenum acetylacetonate (argon, 400 °C) through flowing of 5wt % H2S/H2 gas mixture at 400 °C.
phases are present. Most authors agree on the presence of Mo(IV), Mo(V), and Mo(VI) phases after sulfiding at 360 °C.16,17 Naturally occurring MoS2 has been found in the 3R as well as 2H sandwich stacking.18 The two types belong to different space groups. Our present crystals of MoS2 in fact were always of the 2H form, in which the layered material arise from the stacking of hexagonally packed planes in the sequence of Mo-layer sandwiched between two layers of S. The S-Mo-S layer form is packed with each other through the van der Waals gap. The coordination around the metal atoms is trigonal prismatic. 2. Sulfidation Nature of Molybdenum Acetylacetonate Precursor. Calcination of molybdenum acetylacetonate in air led to the formation of the crystalline MoO3 phase. However, the peak intensities of the X-ray diffraction pattern of this phase differ either from those on the PDF card 5-508 or the one produced from ammonium heptamolybdate precursor (Table 1). Such differences are due to the development of some certain planes in the crystal. Sulfidation of molybdenum acetylacetonate is one that shows interesting results. Sulfidation either through the flow of the sulfiding gas, i.e., 5wt % H2S/H2, whether the sulfiding began at the heat treatment or after calcination in argon flow at 400 °C, leads to the formation of a nanocrystalline MoS2 phase. The trans(16) Li, D.; Sato, T.; Imamura, M.; Shimada, H.; Nishijima, A. J. Catal. 1997, 170, 357-365. (17) Ratansamy, P.; Sivasanker, S. Cata. Rev.-Sci. Eng. 1980, 22 (3), 401-429. (18) Wilson, J. A.; Yoffe, A. D. Adv. Phys. 1969, 18, 193.
formation to this MoS2 phase is quantitatively complete at 400 °C (Table 1A,B). It was observed that the resultant texture of MoS2 is spongy with bright black color. Additionally, sulfidation of the crystalline MoO3 extracted from molybdenum acetylacetonate produces the nanocrystalline MoS2 phase. This is in contrast to what happened for MoO3 produced from the calcination of ammonium heptamolybdate precursor. The reason for this difference is unclear at present. No observation has been found for the formation of MoO2 during the sulfidation of molybdenum acetylacetonate under the prescribed experimental conditions. The above evidence suggests that the mechanism of sulfidation of molybdenum acetylacetonate precursor may proceed via two proposed possibilities. One is the formation of MoO2 as an unstable intermediate that is not detected due to either that the fast transformation to the sulfide phase or that this precursor was sulfided without passing through the MoO2 intermediate (this latter would mean that the terminal oxygen ions would be replaced by sulfur ions). An alternative explanation could be that the crystalline MoO3 phases formed from the two precursors are not typical in nature. This is clearly seen in Figure 1, where different peak intensities can be observed. One of the fundamental questions of the catalytic studies was concerned with the problem of the true composition of the surface and its difference with respect to the bulk. In a previous study4 using carbon as a support for CoMo hydrotreating catalysts, catalysts prepared from molybdenum acetylacetonate precursor
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exhibit much more hydrodesulfurization activity than those obtained from ammonium heptamolybdate. This may indicate the structure difference between the amorphous and the crystalline MoS2 phases. We switch now to basic and fundamental question of whether the active sites in the Mo-hydrotreating catalysts are related to the molybdenum oxy-sulfide species, amorphous MoS2, and/or the crystalline MoS2 phase. If the fundamental species of the catalytic active sites are accurately determined then it would be an easy task to tune the synthesis conditions appropriately. Generally, in industry, CoMo and/or NiMo catalysts sulfidations are proceeded via the flow of H2S/H2 gas mixture at ca. 400 °C. We wonder if this condition is the optimum one. Our results reflect the significant of the starting up molybdenum precursors and their treatments on the final observed species. The extent of sulfidation is dependent on the temperature for ammonium heptamolybdate precursor. Oxysulfide species are likely to be present at ca. 400 °C. In the literature, two types of CoMoS phases (type I and type II) were distinguished19-23 for the sulfided Mo, NiMo, and CoMo supported catalysts. Type I CoMoS is obtained by sulfidation at standard conditions (i.e., at a temperature of ca. 400 °C, atmospheric pressure using 10% H2S in H2). Type II, which can be obtained by sulfiding at temperature (600 °C), corresponds to the fully sulfided phase. Type I is supposed to have an interaction with the alumina support, while type II is almost free of interaction with the alumina support. The intrinsic activity is much higher for type II than for type I. These authors were able to prepare the pure type II (19) Bezverkhyy, I.; Afanasiev, P.; Geantet, C.; Lacroix, M. J. Catal. 2001, 204, 495-497. (20) Sakashita, Y. Surf. Sci. 2001, 489, 45-58. (21) Topsøe, H.; Clausen, B. S. Appl. Catal. 1986, 25, 273-293. (22) 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 Veen, J. A. R.; Koningsberger, D. C. J. Catal. 1994, 146, 375-393. (23) Weisser, O.; Landa, S. Sulfide Catalysts, Their Properties and Applications; Pergamon: New York, 1973.
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CoMoS at standard sulfidation conditions by taking the CoMo-nitrilotriacetic acid complex as the precursor to the active phase. Unsupported, poorly crystalline MoS2 exhibits catalytic activity in model and real hydrodesulfurization reactions.23 Thus, on the basis of this discussion and from our results, type II of CoMoS phase could be alternatively prepared by taking molybdenum acetylacetonate as the starting precursor. Conclusions An attempt has been made to follow up the sulfidation of various molybdenum precursors. The results obtained in this work indicate that sulfiding of ammonium heptamolybdate and molybdenum acetylacetonate precursors reveal some different features regarding the texture and the structure of the resultant species. Calcination of these precursors gives the crystalline MoO3 phase but with different peak intensities regarding to their X-ray diffraction patterns. Sulfidation of bulk ammonium heptamolybdate at standard conditions applied industrially (i.e., ca. 400 °C, flow of H2S/H2) leads mainly to the formation of a highly crystalline MoO2 phase. The extent of sulfiding is mainly correlated with temperature. A fully completed sulfidation to the highly crystalline MoS2-2H structure was obtained at ca. 800 °C. Below this temperature, two molybdenum species, MoO2 and MoS2, were identified with various ratios depending on the temperature of treatments. On the other hand, for sulfidation of molybdenum acetylacetonate at ca. 400 °C, the MoS2 produced are found to be in the amorphous state (nanocrystalline) as indicated from the X-ray diffraction data. Furthermore, quantitative transformation to MoS2 was observed under this condition. The nanocrystalline MoS2 phase was produced regardless either starting from molybdenum acetylacetonate or MoO3 extracted from it. In view of the two identified catalytic active sites, Type I and II, molybdenum acetylacetonate may lead to the much more favorable active sites of type II. Further studies in this area are now in progress. EF0102972
