Changes of MoS2 Morphology and the Degree of ... - ACS Publications

Dec 20, 2006 - The Netherlands, and Research and Technology Chemicals, Akzo Nobel, P. O. Box 9300, 6800SB Arnhem,. The Netherlands. A series of commer...
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Ind. Eng. Chem. Res. 2007, 46, 3945-3954

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Changes of MoS2 Morphology and the Degree of Co Segregation during the Sulfidation and Deactivation of Commercial Co-Mo/Al2O3 Hydroprocessing Catalysts Sonja Eijsbouts,*,† Leon C. A. van den Oetelaar,† Jaap N. Louwen,† Rob R. van Puijenbroek,‡ and (Kees) G. C. van Leerdam‡ Research Centre, Albemarle Catalysts Corporation B.V., P. O. Box 37650, 1030BE Amsterdam, The Netherlands, and Research and Technology Chemicals, Akzo Nobel, P. O. Box 9300, 6800SB Arnhem, The Netherlands

A series of commercial Co-Mo/Al2O3 catalysts, freshly sulfided as well as used in different trickle phase and gas phase reactions, has been characterized by using (scanning) transmission electron microscopy combined with energy dispersive X-ray analysis [(S)TEM-EDX)] and X-ray diffraction (XRD) analysis. While liquid phase sulfided Co-Mo/Al2O3 catalysts contain only MoS2 single layers, some MoS2 stacking is observed following H2S/H2 sulfidation. Liquid phase sulfided catalysts as well as catalysts partly sulfided with liquid prior to H2S/H2 sulfidation have higher activities than those sulfided only with H2S/H2. MoS2 sintering and Co sulfide segregation are observed in catalysts after commercial use under severe conditions. Compared to what is typically found for Ni-Mo catalysts, metal-rich agglomerates in Co-Mo/Al2O3 catalysts contain more Mo while the Co sulfide crystals are less well developed. 1. Introduction

Table 1. Catalyst Description

The loss of MoS2 dispersion and the segregation of the promoter atom have often been associated with the deactivation of sulfidic hydroprocessing catalysts.1-4 MoS2 stacking in CoMo catalysts has been observed on several occasions.5-9 This stacking was in some cases associated with the formation of the type 2 active phase, i.e., with the catalyst preparation,5,6,9 and in other cases with the conditions of H2S/H2 sulfidation8 or with extended use in a commercial reactor.7 In a previous paper,10 we showed that MoS2 stacks are not prominent in liquid phase sulfided commercial type 2 Ni-Mo catalysts but are formed after H2S/H2 sulfidation. MoS2 stacks were reported to “destack/exfoliate” if H2S/H2 sulfided catalysts were used for reactions in the liquid phase for an extended period of time.11 The formation of Co9S8 crystals was frequently observed in sulfided and used Co-Mo catalysts.12-26 Co9S8 crystals were, among others, detected in catalysts having high Co-to-Mo ratios (higher than 0.5 Co per 1 Mo),13,14 in catalysts after prolonged/ high-temperature sulfidation or reduction treatments,13-16,24 or after use in a commercial reactor.25,26 Previously we have shown that, after a relatively short exposure to reaction conditions, CoMo catalysts remain essentially free of Co9S8 crystals.10 In the present paper, it will be shown that the Co-Mo catalysts need to be exposed longer to more extreme conditions to form such Co9S8 crystals. 2. Experimental Section 2.1. Samples. The samples are listed in Tables 1 and 2 together with the annotations used in the text, which refer to the sulfidation procedure and reaction conditions. CoMo1 and CoMo2 catalysts are based on proprietary type 2 and 1/2 preparation technologies,10 respectively. * To whom correspondence should be addressed. Tel.: 31-206347360. Fax: 31-20-6347653. E-mail: [email protected]. † Albemarle Catalysts Corporation B.V. ‡ Akzo Nobel.

metal loading (atoms/nm2) catalyst annotation CoMo1 CoMo2

description

Co

Mo

type 2 Co-Mo/Al2O3 type 1/2 Co-Mo/Al2O3

1.7 1.7

4.9 3.6

The samples were subjected to different presulfiding procedures (Table 2). Liquid phase sulfidation (L) was carried out using dimethyl disulfide (DMDS) spiked straight run gas oil (SRGO) containing in total 3.7 wt % S [total pressure ) 30 bar; stage 1, 3 h soaking at 100 °C; stage 2, 30 °C/h to 250 °C, 8 h at 250 °C; stage 3, 20 °C/h to 320 °C, 5 h at 320 °C].10 For catalysts CoMo2-LS1/G3 and CoMo2-LS2/G3, the liquid phase sulfidation was stopped after stage 1 (S1) and stage 2 (S2), respectively. Three different gas phase H2S/H2 sulfidation procedures were applied (RT, room temperature):

G1: 120 °C/h from RT to 300 °C, 4 h at 300 °C G2: 360 °C/h from RT to 450 °C, 2 h at 450 °C G3: 360 °C/h from RT to 400 °C, 2 h at 400 °C Samples CoMo2-LS1/G3, -LS2/G3, -L/G3, -L/D/G3, and -L/ D4/G3 were first partly or fully presulfided in the liquid phase or even used in diesel hydrodesulfurization (HDS) prior to H2S/ H2 sulfidation at 400 °C. If used in trickle flow test reactions, the gas phase sulfided samples were further treated using the above liquid phase sulfidation procedure, except that the SRGO feedstock (1.2 wt % S) was not spiked with DMDS. Some of the samples were characterized directly after sulfidation. Other samples were used in short trickle flow tests in small reactors using 100 mL of catalyst, in commercial HDS units, or in a gas phase thiophene HDS test.5,18 The reaction conditions used in the different trickle flow test reactions and commercial HDS units are listed in Table 3. The thiophene HDS test was carried out in a microflow reactor using a catalyst

10.1021/ie061131x CCC: $37.00 © 2007 American Chemical Society Published on Web 12/20/2006

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Table 2. Catalyst Origin (Pretreatment and Reaction) catalyst annotationa

sulfidation/pretreatment

test reactionb

CoMo1-G1 CoMo1-L CoMo1-L-U CoMo1-L-D1 CoMo1-L-D2 CoMo1-L-D3 CoMo2-G1 CoMo2-G2 CoMo2-G1-D CoMo2-L-D CoMo2-G3-T CoMo2-LS1/G3-T CoMo2-LS2/G3-T CoMo2-L/G3-T CoMo2-L/D/G3-T CoMo2-L/D4/G3-T

H2S/H2 300 °C liquid liquid liquid liquid liquid H2S/H2 300 °C H2S/H2 450 °C H2S/H2 300 °C liquid H2S/H2 400 °C liquid; stage 1 f H2S/H2 400 °C liquid; stage 1 + 2 fH2S/H2 400 °C liquid f H2S/H2 400 °C liquid f diesel HDS fH2S/H2 400 °C liquid f diesel HDS commercial unit fH2S/H2 400 °C

ULSD commercial 1; diesel HDS commercial 2; diesel HDS commercial 3; diesel HDS diesel HDS diesel HDS thiophene HDS thiophene HDS thiophene HDS thiophene HDS thiophene HDS commercial 4; diesel HDS and thiophene HDS

a Liquid phase sulfidation (L); gas phase sulfidation (G); ULSD (U); diesel HDS (D); thiophene HDS (T); stage of liquid phase sulfidation (S). b Ultralowsulfur diesel (ULSD); hydrodesulfurization (HDS)

Table 3. Reaction Conditions commercial diesel HDS unitsa D1 temperature (°C) pressure (bar) H2:oil (N L/L) LHSV (h-1) product S (ppm) feedc feed S (wt % )

340 50 250 0.6 500 diesel blend 1.8

model reactionsb

D2

D3

D4

360 40 135 1.5 10 diesel blend 1.0

325 10 130 0.5 10 treated diesel blend 0.4

340 30 120 4.0 500 SRGO 1.2

U 340 45 200 2.5 10 SRGO 1.2

D 340 30 120 4.0 500 SRGO 1.2

a Typical cycle length of 2 years. b Test duration 5 days. c SRGO, straight run gas oil; diesel blend consists typically of light gas oil (LGO) and light catalytic cycle oil (LCCO).

amount equivalent to 200 mg of fresh oxidic catalyst. The catalysts were H2S/H2 sulfided at 400 °C (G3) prior to the thiophene HDS test, which was carried out at 400 °C, using a mixture of 6.2 mol % thiophene in H2. The catalyst performance is compared on an equal fresh catalyst weight basis, using the relative weight activity (RWA) with the activity of one of the catalysts of the series defined as 100% and the activities of other catalysts calculated relative to this catalyst. 2.2. Analytical Methods. 2.2.1. Sample Unloading and Storage. The sulfided and used samples were unloaded into a container filled with SRGO and were kept submerged in SRGO in closed containers to prevent air oxidation.27 The samples were further stored and handled in a glovebox. Residual SRGO had to be removed from the catalyst particles prior to further analysis. This was done by rinsing the catalyst particles briefly with toluene. 2.2.2. (Scanning) Transmission Electron Microscopy Combined with Energy Dispersive X-ray Analysis [(S)TEMEDX]. The samples were extracted with toluene, powdered, evacuated for at least 2 h at 150 °C, and subsequently vacuum impregnated with the standard mixture Ultra Low Viscosity Kit, hard version (Polaron Instruments Inc.). The mixture with catalyst was transferred into a polyethylene capsule (BEEM) and mixed with fresh embedding medium. The epoxy embedding medium was hardened at least 48 h under N2 (0.2 MPa, 338 K). Sections of about 60 nm thickness were prepared using a Leica Reichert Ultracut-S ultramicrotome, collected on a water surface and transferred to a slightly etched (Ar plasma) Cu grid and dried. The thin sections on the TEM grid were covered with a thin layer of carbon to prevent charging during TEM analysis. Shortly after preparation, the sections were investigated with a JEOL JEM-2010F-HR TEM, with a 200 kV electron beam (field emission gun, FEG), equipped with a STEM unit and a Thermo Noran EDX system. The MoS2 morphology was

studied by TEM imaging under slightly underfocused conditions (-48 nm). Semiquantitative evaluation of the MoS2 features in the TEM micrographs was carried out using a method described elsewhere.1,28 The promoter segregation was studied by STEMEDX spectral imaging under analytical probe (1.0 nm) conditions.10 Typically, the crystals are inhomogeneously distributed throughout the specimen. The minimum size of Co9S8 crystals detectable by STEM-EDX is ca. 5 nm. 2.2.3. X-ray Diffraction (XRD). The samples were extracted with toluene prior to being powdered and measured using a Bruker AXS D5000 reflection diffractometer with monochromated Cu KR radiation at 40 kV and 35 mA. A silicon single crystal was used as sample holder. Samples were rotated during the measurements. The XRD patterns were recorded using a step size of 0.02°. Phase identification was checked using the Powder Diffraction File from the International Centre for Diffraction Data (ICDD). 3. Results 3.1. Dependence of MoS2 Morphology on Sulfidation Procedure. Liquid phase sulfided CoMo1 and CoMo2 catalysts typically contain single layers of MoS2 (Figure 1a, CoMo1-L, Table 4). Some MoS2 stacks, however, appear after H2S/H2 sulfidation especially when a high temperature is used (Figure 1b, CoMo1-G1, the micrograph quality is somewhat lower due to the higher/less uniform specimen thickness; Table 4, compare CoMo2-G1 and -G2). The stacking is not very significant; i.e., the stacks consist of two to three layers and are on average 3-4 nm long. None of the freshly sulfided samples contains distinct CoSx crystals visible in TEM micrographs. When quantified,1,28 the dispersion and morphology differences observed by TEM in catalysts sulfided by different procedures are only 10-20% (Table 4, compare Mo (e+c) values for

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Figure 1. TEM micrographs of (a) liquid phase sulfided CoMo1-L and (b) gas phase sulfided CoMo1-G1 materials. The gas phase sulfided sample contains MoS2 stacks, consisting of two to three layers.

Figure 2. XRD patterns of (from top to bottom) H2S/H2 sulfided CoMo1-G1, liquid phase sulfided used CoMo1-L-U, Co-Mo1-L-D1, Co-Mo1-L-D2, and Co-Mo1-L-D3, and the γ-alumina reference. Reference stick pattern of Co9S8 is included at the bottom.

different samples inside the CoMo1 and CoMo2 series). The MoS2 dispersion is so high that only between 13 and 31% of the total MoS2 present is actually visible in the TEM micrographs. The nonvisualized part of MoS2 is assumed to be present as MoS2 clusters consisting of seven Mo atoms.1,28 An example of the XRD pattern of the H2S/H2 sulfided CoMo1 sample is shown in Figure 2. The pattern contains contributions of MoS2 and γ-Al2O3 (Figure 2). No Co9S8 reflections are present. As the MoS2 stacking is not very significant, there is no distinct MoS2 002 peak at 2θ of about 14.4° (d spacing of 6.1 Å). In addition to the above features, there is a broad peak with a maximum at 2θ of about 18° (d spacing of 4.9 Å) that was not yet assigned to any specific compound. [A broad peak at the same position observed in Nicontaining catalysts was attributed to an amorphous halo

corresponding to an almost amorphous Ni sulfide phase, which ordering resembles that of a millerite-like structure (β-Ni-S). However, as there is no Co analogue of millerite and none of the known Co sulfides has a reflection at this position, we have not been able to attribute this reflection to a specific crystal structure.] To analyze the XRD spectra in terms of MoS2 slab stacking, the powder XRD pattern of MoS2 clusters of various sizes was simulated using the Debye interference function.29 A similar approach has been useful in the analysis of XRD of pseudoboehmite.30 Consequently, with reference to a rhombohedral unit cell with axes of a ) b ) 3.163 Å and c ) 18.37 Å,31 simulated clusters of 10 unit cells along both the a- and b-axes and 1/3, 2/3, and 1 unit cell along the c-axis were created (Figure 3a). Addition-

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Table 4. MoS2 Dispersion Calculated from TEM Micrographs visualized1,28 catalyst annotationb

stacks/1000 nm2

average no. layers/ stack

average length (nm)

visualized Mo (%)

calculated1,28 Mo (e+c)a,b atoms/10000 nm3

CoMo1-G1 CoMo1-L CoMo1-L-D1 CoMo1-L-D2 CoMo1-L-D3 CoMo2-G1 CoMo2-G2 CoMo2-G1-D CoMo2-L-D

32 36 36 32 36 24 15 25 18

1.5 1.1 1.2 1.2 1.5 1.3 1.7 1.3 1.4

3.3 2.7 3.5 3.7 3.3 4.3 6.3 5.1 3.5

22 13 20 20 25 25 31 31 14

30 600 32 700 30 600 30 600 29 900 24 300 22 200 22 800 27 100

a

(e+c) ) edge + corner atoms. b The accuracy of the calculated number of Mo (e+c) atoms is (10%.1,28

Figure 3. Simulated powder diffractograms for MoS2 slab and stack clusters: (a) orthorhombic stacking and (b) hexagonal stacking.

Figure 4. Composite of simulated powder diffractograms for the 10-101/3 (50%) and 10-10-2/3 (50%) MoS2 clusters: (a) orthorhombic stacking and (b) hexagonal stacking.

ally, one-, two-, and three-layer clusters were created assuming hexagonal stacking (Figure 3b). These simulations imply a single slab, a stack of two MoS2 layers, and a stack of three MoS2 layers, respectively, with 100 Mo atoms in each layer. Two reflection positions are reasonably well resolved in every simulated diffractogram. These are the (100) and (110) reflections at 2θ ) 33.5° and 58.5° (d spacings of 2.67 and 1.58 Å), respectively. Since the planes of reflection are parallel to the crystallographic c-axis, the line broadening of these reflection peaks depends on the layer size only. The simulated diffractograms for the rhombohedral (Figure 3a) and hexagonal stacking (Figure 3b) show essentially the same features, but the signals at 2θ of about 40° and 50° (d spacings of about 2.3 and 1.8 Å) are more prominent in the case of hexagonal stacking. The hexagonal stacking leads to a better resolved signal as it has the simpler ABABAB stacking as compared to the orthorhombic ABABCBCAC. The lowest angle peak (002) in well-crystallized MoS2 has a d value corresponding to the distance between two layers and lies at 2θ ) 14.4° (d spacing of 6.1 Å). Obviously, this peak is absent in the simulated diffractograms for the 10-10-1/3 clusters (Figure 3). Instead, from about 2θ ) 20° downward,

the signal level rises rapidly. For the clusters with two layers, the 10-10-2/3 clusters, a peak at about 2θ ) 12° (d spacing of 7.4 Å) is found. The simulated diffractograms for the 1010-1 clusters have a peak at 2θ ) 13.8° (d spacing of 6.4 Å), which is already quite close to 2θ ) 14.4° (d spacing of 6.1 Å). In fact, a satisfactory match between the simulated and experimental MoS2 patterns is found by compositing, e.g., 50% of the simulated signal for the 10-10-2/3 and 50% of the simulated signal for the 10-10-1/3 orthorhombically stacked clusters. At a decreasing angle starting from 2θ ) 20°, the signal rises to a sort of plateau between 2θ ) 10° and 15° and rises steeply again at lower angles, as is shown in Figure 4. (Comparing Figure 4a to Figure 2, a signal due to γ-alumina is present at angles 2θ g 20° in the latter scan.) The average stack height for this simulated cluster is 1.5, which is on the same order of magnitude as the height estimated by the quantification of the TEM micrographs of the CoMo1-G1 catalyst (Table 4). Due to the more prominent signal at about 2θ ) 40° and 50° (d spacings of about 2.3 and 1.8 Å), the match between the simulated and experimental MoS2 patterns is somewhat less good for the hexagonal stacking (Figure 4b).

Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 3949 Table 5. Thiophene HDS Results catalyst

RWA HDSa

RWA HYDb

CoMo2-G3-T CoMo2-LS1/G3-T CoMo2-LS2/G3-T CoMo2-L/G3-T CoMo2-L/D/G3-T CoMo2-L/D4/G3-T

44 68 77 100 96 29

93 86 93 100 84 49

a RWA HDS, relative weight activity for the conversion of thiophene to hydrocarbons. b RWA HYD, relative weight activity for the consecutive hydrogenation of butadiene and butenes to butane.

3.2. HDS Activity of Co-Mo Catalysts Subjected to Different Start-Up Procedures. The MoS2 dispersion differences visible in TEM or XRD may be fairly limited; however, the differences in catalyst performance are relatively large. There is a clear activity advantage for the liquid phase sulfided catalyst. Our ultralow-sulfur diesel (ULSD; test conditions are listed in Table 3) test showed that CoMo1-G1 has a relative weight activity (RWA) HDS of 57 compared to CoMo1-L (RWA HDS ) 100). Furthermore, in a series of CoMo2 catalysts sulfided by different combined gas and liquid phase procedures prior to the thiophene HDS test, the highest RWA HDS is obtained for the sample that was fully liquid phase sulfided (Table 5). The fully gas phase sulfided sample has the lowest activity among the freshly sulfided samples. kHDS declined only slightly if the catalyst passed the 5 days diesel HDS test prior to being tested in thiophene HDS (Table 5, CoMo2-L/D/G3-T). Interestingly, the RWA HYD does not change considerably with the sulfidation procedure. As one would expect, a very low activity is observed for a commercially used catalyst tested in thiophene HDS (Table 5, CoMo2-L/D4/G3-T), which has the lowest RWA HDS and RWA HYD in the present series of catalysts. 3.3. Changes of Catalyst Morphology on Extended Use in a Commercial Reactor. Liquid phase sulfided CoMo1 samples used in short test reactions have high MoS2 dispersion, i.e., MoS2 single layers, which are on average 3-4 nm long, and do not contain distinct CoSx crystals visible in the TEM micrographs. While MoS2 dispersion is relatively high and CoSx crystals are absent after sulfidation or a short test reaction, profound changes can occur after extended catalyst use in a commercial unit. The extent of MoS2 sintering and Co segregation obtained depends on the unit operating conditions and reaction severity. It should be noted that the TEM micrographs and STEM-EDX maps included in this section were selected in order to show the (less abundant) segregated features in the catalysts and not the high dispersion areas, which are still dominant. The CoMo1-L-D1 catalyst was used to treat a diesel blend from about 1.8 wt % S to 500 ppm S at low temperature (340 °C), low liquid hourly space velocity (LHSV) (0.6 h-1), high pressure (50 bar), and high H2:oil ratio (250 N L/L). The combination of low LHSV, high H2:oil, and high product S concentration makes this catalyst application relatively mild. Figure 5 shows that the MoS2 dispersion is still relatively high and there is only a limited amount of small Co- and Morich entities, being 20-50 nm in diameter. Metal-rich domains with MoS2 stacks were observed in ca. 30% of randomly selected regions investigated by TEM. The Co- and Mo-rich areas contain MoS2 stacks, but distinct CoSx crystals with clear lattice spacings have not been observed (Figure 5a). Small metalrich areas (5-10 nm in diameter) without any distinct morphological features were visible in ca. 45% of investigated regions. Short MoS2 single layers were the dominant feature in ca. 70%

of evaluated regions. Overall, the variations of local Co and Mo concentrations are relatively limited and the local Co:Mo ratio deviates only slightly from the bulk value in CoMo1-LD1 catalyst (Figure 6). The CoMo1-L-D2 catalyst was used under rather severe conditions to treat a diesel blend to obtain ULSD product, i.e., from about 1 wt % S to 10 ppm S, at high temperature (360 °C), high LHSV (1.5 h-1), high pressure (40 bar), and low H2: oil ratio (135 N L/L). The combination of high LHSV, low H2: oil, and low product S level makes this catalyst application very demanding. High reaction temperature is required to reach the low product S level. This leads to extensive sintering of MoS2 and to the segregation of Co. The metal-rich domains (20-50 nm in diameter) observed in the used CoMo1-L-D2 catalyst consist of MoS2 stacks (Figure 7a) and, attached to them, separate Corich areas (Figure 7b). Significantly, distinct CoSx crystals with clear lattice spacings have not been observed. These metal-rich domains with MoS2 stacks were observed in ca. 10% of randomly selected regions investigated by TEM. Small metalrich areas (5-10 nm in diameter) without any distinct morphological features were visible in ca. 55% of investigated regions. Short MoS2 single layers were the dominant feature in ca. 90% of evaluated regions. The composition of the metal-rich areas is highly variable, being Co rich compared to the bulk composition while the areas surrounding them are strongly Co and Mo depleted (Figure 8). The CoMo1-L-D3 catalyst was used to desulfurize a pretreated diesel blend to ULSD product, i.e., from about 0.4 wt % S to 10 ppm S at low temperature (325 °C), low LHSV (0.5 h-1), very low pressure (10 bar), and low H2:oil ratio (130 N L/L). The combination of low reaction pressure, low H2:oil, and low product S level makes this catalyst application very demanding. Low LHSV is required to reach the low product S at such a low reaction temperature and pressure. The metal-rich domains observed in the used CoMo1-L-D3 catalysts are very large (0.3-3 µm in diameter), clearly larger than in CoMo1-L-D1 and -D2 catalysts, and consist of conglomerates of large MoS2 stacks intermixed with small (1015 nm in diameter) CoSx crystals (Figure 9). These metal-rich domains with MoS2 stacks were observed in ca. 30% of randomly selected regions investigated by TEM. Small metalrich areas (5-10 nm in diameter) without any distinct morphological features were visible in ca. 55% of investigated regions. Short MoS2 single layers were the dominant feature in ca. 70% of evaluated regions. The variations of local Co and Mo concentrations are rather large and the metal-rich areas are Co rich compared to the bulk composition in CoMo1-L-D3 catalyst (Figure 10). When comparing the calculated MoS2 dispersion data of the above three samples from the commercial units and to the number calculated for the freshly sulfided CoMo1-L catalyst, the differences are very small, only about 10% (Table 4). This implies that the MoS2 stacking observed by TEM is a very localized phenomenon. MoS2 monolayers are still the dominant morphology even in samples after extended use in the commercial reactor. This is also confirmed by the XRD results. The changes of Mo and Co dispersion on catalyst deactivation are only to a very limited extent reflected in the XRD patterns (Figure 2). There is a very broad MoS2 peak at about 2θ ) 14° (d spacing of 6.1 Å) and very small peaks at about 2θ ) 30° (d spacing of 2.98 Å) and 2θ ) 52° (d spacing of 1.76 Å) due to the presence of Co9S8 crystals (pentlandite) in the samples from

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Figure 5. (a) TEM micrograph and (b) STEM-EDX composite maps of Co (red) and Mo (blue) of commercially used CoMo1-L-D1 catalyst indicating that small metal-rich areas contain MoS2 stacks mixed with an amorphous Co sulfide phase.

Figure 6. Quantitative TEM-EDX results of CoMo1-L-D1 catalyst: magenta squares, alumina areas; blue triangles, crystals; green circles, large areas. The dashed line marks the Co:Mo bulk ratio.

the three commercial diesel units. The lack of more pronounced Co9S8 peaks is in agreement with the absence of distinct large CoSx crystals in the TEM micrographs of these catalysts. There is also a broad peak with a maximum at about 2θ ) 18° (d spacing of 4.9 Å) in the XRD pattern of CoMo1-L-U that was not yet assigned to any specific compound. 4. Discussion 4.1. Dependence of MoS2 Morphology on Sulfidation Procedure. In the present study, we have shown that the CoMo catalysts essentially duplicate the sulfidation behavior seen with Ni-Mo catalysts with respect to MoS2 morphology changes. While liquid phase sulfided catalysts typically contain single layers of MoS2, some MoS2 stacks appear after H2S/H2 sulfidation, especially when carried out at high temperature. The stacking is not very significant. This trend is similar to that observed for Ni-Mo catalysts10 and is related to the fact that the liquid present during the sulfidation helps to absorb the heat generated by the sulfidation reactions.32-36 When quantified, the dispersion and morphology differences observed by TEM in Co-Mo catalysts sulfided by different procedures are only ca. 10-20% inside a catalyst series. The lower Mo surface loadings of the present set of Co-Mo catalysts make the differences less pronounced than in the Ni-Mo catalysts evaluated in the previous study.10 An additional factor may, however, be the lower tendency of Co to segregate from the “Co-Mo-S” phase and/or to form separate CoSx crystals.10

Consequently, Co still present on the MoS2 edges may help to stabilize the MoS2 dispersion.37 The MoS2 stacking observed in the present series of freshly sulfided Co-Mo catalysts is rather limited when compared to other H2S/H2 sulfided Co-Mo catalysts.5,8,9,11 Obviously, the catalyst morphology at the end of the sulfidation procedure is the product of the catalyst preparation/composition and the exact start-up conditions. High metal surface loading,6 low metalsupport interaction,5 gas phase sulfidation,5,8,9 high sulfidation temperature,8 and high exotherm during the sulfidation32-36 facilitate the MoS2 stacking observed. It may seem surprising that the XRD pattern of the H2S/H2 sulfided Co-Mo sample does not provide any evidence for the presence of small MoS2 stacks, a pronounced peak at about 2θ ) 14° (d spacing of 6.1 Å) being absent. The reason is that only a small part of MoS2 is really visualized in the TEM micrographs and the actually observed MoS2 stacks are very small (two to three layers, 3-4 nm long). Overall, only an insignificant fraction of the MoS2 clusters has a stacking of three or more, while the major (nonvisualized) part is likely to be present as very small clusters (i.e., MoS2 slabs shorter than 1.5 nm which are not recognizable as lines in TEM micrographs).1,28 The simulated XRD patterns of MoS2 clusters of various sizes lead to the same conclusion concerning MoS2 stacking. The (001) peak in well-crystallized MoS2 lies at 2θ ) 14.4° (d spacing of 6.1 Å). This peak is absent in the simulated diffractograms for the single slab. Instead, from about 2θ ) 20° downward, the signal level rises rapidly. This is usually described as the “background signal” in scans of material containing MoS2 with very low stacking. For the clusters with two and three layers, the (001) peak is found at about 2θ ) 12° and 13.8° (d spacing of 7.4 and 6.4 Å), respectively. A satisfactory match for the measured XRD patterns of freshly sulfided catalysts is found by compositing 50% of the simulated signal for the double layer and 50% of the simulated signal for the single slab (rhombohedral stacking). Thus, the XRD results confirm that there is a very low degree of stacking, a significant part of MoS2 apparently not being stacked at all. All in all, as all the samples have a very high dispersion, neither TEM nor XRD makes it possible to really quantify the stacking of MoS2. 4.2. HDS Activity of Co-Mo Catalysts Subjected to Different Start-Up Procedures. The MoS2 dispersion differences visible in TEM or XRD for the present set of Co-Mo

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Figure 7. (a) TEM micrograph and (b) STEM-EDX composite maps of Co (red) and Mo (blue) of commercially used CoMo1-L-D2 catalyst showing that small metal-rich areas consist of large MoS2 stacks and, attached to them, separate Co-rich areas without distinct morphological features.

Figure 8. Quantitative TEM-EDX results of commercially used CoMo1L-D2 catalyst: magenta squares, alumina areas; blue triangles, crystals. The dashed line marks the Co:Mo bulk ratio.

catalysts with different presulfiding procedures may be limited; however, the differences in catalyst performance are relatively large. This feature is related to the fact that a major part of the active phase that is responsible for the catalytic performance has a too-high dispersion and in fact cannot be probed/ characterized by TEM or XRD. That is why the activity tests and chemisorption measurements will always be crucial techniques in the characterization of sulfidic hydroprocessing catalysts. Partly liquid phase sulfided catalysts activated in the gas phase and especially the fully liquid phase sulfided catalysts have higher activity than the gas phase sulfided catalysts. The performance (kHDS) differences observed for CoMo2 in the thiophene HDS test are larger than those observed for CoMo1 catalysts with different start-up procedures in the ULSD test. The very simple thiophene HDS test proves a quick, very discriminative test procedure for the characterization of used catalysts from various liquid phase reactions or even commercial units. Previous work has shown that the dibenzothiophene HDS test can be used for characterization of catalysts from other test reactions or commercial units.11 4.3. Changes of Catalyst Morphology on Extended Use in Commercial Reactor. In agreement with our previous study,10 none of the freshly sulfided Co-Mo samples or CoMo samples used in short test reactions contains distinct CoSx crystals visible in TEM micrographs. This is different from some observations in the literature where Co9S8 crystals were detected already after short tests, e.g., for catalysts having high Co-to-

Mo ratios (higher than 0.5 Co per 1 Mo)13,14 and for catalysts after prolonged/high-temperature sulfidation or reduction treatments.13-16,24 However, the CoMo1 catalyst (type 2, 0.35 Co per 1 Mo), apparently, needs to be exposed for a longer time to more extreme conditions to exhibit extensive MoS2 sintering and formation of segregated Co sulfide phase. The extent of catalyst changes depends on the unit operating conditions and reaction severity. Co and Mo remain well mixed, and only a few large MoS2 stacks are observed in the CoMo1L-D1 catalyst from a unit operated at mild conditions. The highest degree of Co segregation, large MoS2 stacks, and, attached to them, separate Co-rich amorphous areas, are observed in the CoMo1-L-D2 catalyst from a unit operated at high temperature. Very large conglomerates of large MoS2 stacks intermixed with small CoSx crystals are observed in the CoMo1-L-D3 catalyst from a unit operated at low pressure. This is in agreement with the general notion38 that high-temperature/ high-LHSV and low-pressure ULSD applications are very demanding and lead to rapid catalyst deactivation. More examples of MoS2 stacking6,7 and Co9S8 formation25,26 after use in commercial units are described in the literature. The formation and growth of Co9S8 crystals depend on catalyst properties such as the type of support26 and are further stimulated by high reaction temperature.25 This is in agreement with our observation that the highest degree of Co segregation was found in the catalyst operated at the highest reaction temperature (CoMo1L-D2). It should be noted that segregated features such as large MoS2 stacks intermixed with small CoSx crystals have lower active surface area than the surrounding highly dispersed sulfides. However, they are still catalytically active and may even exhibit a certain degree of synergy,3 thereby contributing to the overall catalyst performance. The changes of Mo and Co dispersion during the catalyst use are only to a very limited extent reflected in the XRD patterns. There is again a very broad MoS2 peak at about 2θ ) 14° (d spacing of 6.1 Å) related to MoS2 stacking and very small peaks at 2θ ) 30° (d spacing of 2.98 Å) and 2θ ) 52° (d spacing of 1.76 Å) due to the presence of Co9S8 crystals (pentlandite) in samples from the three commercial diesel units. The absence of a well-defined reflection peak at about 2θ ) 14° (d spacing of 6.1 Å) implies that the MoS2 stacking observed by TEM is a very local phenomenon. Even after extended use in a commercial reactor, the major (nonvisualized) part of MoS2

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Figure 9. (a) TEM micrograph and (b, c) STEM-EDX composite maps of Co (red) and Mo (blue) of commercially used CoMo1-L-D3 catalyst showing that metal-rich areas are much larger than in CoMo1-L-D1 and -D2 catalysts and consist of MoS2 stacks mixed with a crystalline Co sulfide phase.

Figure 10. Quantitative TEM-EDX results of commercially used CoMo1L-D3 catalyst: magenta squares, alumina areas; blue triangles, crystals; green circles, large areas. The dashed line marks the Co:Mo bulk ratio.

is apparently still present as very small clusters.1,28 XRD patterns of the CoMo1 catalysts from different commercial units are similar to the patterns reported elsewhere11 for a Co-Mo catalyst after extended commercial use. However, as the CoMo1 catalyst was in all cases liquid phase sulfided and thus contained predominantly single layers of MoS2 at the end of the start-up procedure, the present study provides no evidence for the hypothesis that MoS2 would destack/exfoliate during the use of the catalyst in the liquid phase for a long period of time.11 The lack of more pronounced Co9S8 peaks in the XRD

patterns is in agreement with the absence of distinct large CoSx crystals with clear lattice spacing in TEM micrographs of these catalysts, the majority of the Co-rich particles having small size and amorphous appearance. Unfortunately, neither the local (S)TEM-EDX analysis nor the bulk XRD analysis makes it possible to really quantify the amount of segregated Co9S8. It is therefore meaningful to look for alternative methods, which may be more quantitative. For example, Mo¨ssbauer emission spectroscopy (MES)39,40 offers the possibility to study the formation of Co9S8 in cobalt-based catalysts under realistic hydrotreatment conditions (in a high-pressure in situ reactor). However, MES is not widely available and the need to use 57Co-impregnated samples in the investigations makes it almost impossible to use this method for the characterization of commercially prepared and used catalysts. Nevertheless, the CoMo1 catalyst used in our study does not have the tendency to segregate Co and form larger well-defined CoSx crystals. This may have several reasons. First, the CoMo1 catalyst has a low bulk Co-to-Mo ratio of 0.35 Co to 1 Mo. Second, the MoS2 dispersion is high so that a high amount of Co (and perhaps all Co present) can be accommodated on the MoS2 edges. Third, the maximum reaction temperatures the different CoMo1 samples were exposed to (340-360 °C) are still clearly lower than the Tamman temperature of Co9S8.41

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The surface mobility and the tendency to agglomerate for different compounds can be assessed based on the melting point (Tmelt), using the Tamman temperature (TT) and the Hu¨ttig temperature (TH). TT, defined as 0.5Tmelt in kelvin, is a temperature sufficient to make atoms or ions of the bulk of a solid adequately mobile for bulk to surface migration.42 TH, defined as 0.33Tmelt in kelvin, is a temperature sufficient to make the species already located on the surface adequately mobile to undergo agglomeration or sintering.42 For Co9S8 (Tmelt ) 1080 °C),41 the Tamman and Hu¨ttig temperatures are TT ) 404 °C and TH ) 178 °C. The reaction temperatures applied in our study (340-360 °C) are well above the Hu¨ttig temperature but below the Tamman temperature of Co9S8. For comparison, the Tamman and Hu¨ttig temperatures of Ni3S2 (TT ) 257 °C and TH ) 80 °C) are much lower,41 far below the reaction temperature of any of the hydrotreating applications. 5. Conclusions Co-Mo catalysts mimic the sulfidation behavior of the NiMo catalysts, with liquid phase sulfided catalysts typically containing MoS2 single slabs. MoS2 stacks appear after H2S/ H2 sulfidation especially when carried out at high temperature. The stacking is not very significant. The differences inside a catalyst series are fairly small due to the relatively low Mo surface loadings and the lower tendency of Co to segregate from the “Co-Mo-S” phase. None of the Co-Mo samples, which were freshly sulfided or used in short test reactions, contain distinct CoSx crystals. Partly liquid phase sulfided catalysts activated in the gas phase, and especially the fully liquid phase sulfided catalysts, have higher activity than the gas phase sulfided catalysts. The performance differences in the thiophene HDS test are larger than those in the ULSD test. The thiophene HDS test does appear to be a rapid, discriminative procedure for the characterization of used catalysts from liquid phase reactions or even commercial units. Co-Mo catalysts need to be exposed for a longer time to more extreme conditions to exhibit extensive MoS2 sintering and formation of segregated Co sulfide phase. The extent of catalyst changes depends on the unit operating conditions and reaction severity. MoS2 single layers remain the dominant MoS2 morphology in all cases. Co and Mo remain well mixed in the catalyst used at mild conditions. The highest degree of Co segregation, large MoS2 stacks, and, attached to them, separate Co-rich amorphous areas, are observed for the catalyst used at high temperature. Very large metal-rich conglomerates of large MoS2 stacks mixed with small CoSx crystals are observed in the catalyst from the unit operated at low pressure. The changes of Mo and Co dispersion during the catalyst use are only to a very limited extent reflected in the XRD patterns. There is a very broad MoS2 peak at about 2θ ) 14° (d spacing of 6.1 Å), implying that the MoS2 stacking observed by TEM is a very local phenomenon. There are only very small peaks at 2θ ) 30° (d spacing of 2.98 Å) and 2θ ) 52° (d spacing of 1.76 Å) due to the presence of Co9S8 crystals (pentlandite) in samples from the three commercial diesel units. The lack of more pronounced Co9S8 peaks is in agreement with the absence of distinct large CoSx crystals; the majority of the Co-rich particles have small size and amorphous appearance. While Co segregation takes place, the CoMo1 catalyst does not have the tendency to form well-defined Co9S8 crystals. This is due to the low bulk Co to Mo ratio, high MoS2 dispersion, and the maximum temperatures the Co-Mo samples were exposed

to (340-360 °C) being well below the Tamman temperature of Co9S8 (ca. 404 °C). Literature Cited (1) Eijsbouts, S.; Heinerman, J. J. L.; Elzerman, H. J. W. MoS2 structures in high activity hydrotreating catalysts 2. Evolution of the active phase during the catalyst life cycle. Deactivation model. Appl. Catal., A: Gen. 1993, 105, 69. (2) Marinkovic-Neducin, R.; Hantsche, H.; Micic, R.; Boskovic, G.; Kis, E.; Lomic, G.; Pavlovic, P. Deactivation of industrial hydrotreating catalyst for middle petroleum fractions processing. Appl. Catal., A: Gen. 1994, 107, 133. (3) Karroua, M.; Matralis, H.; Grange, P.; Delmon, B. Unsupported NiMo catalysts. Influence of the sulfiding temperature and evolution of the unsupported NiMoS phase during reaction. Bull. Soc. Chim. Belg. 1995, 104, 11. (4) Eijsbouts, S. On the flexibility of the active phase in hydrotreating catalysts. Appl. Catal., A: Gen. 1997, 158, 53. (5) Bouwens, S. M. A. M.; van Dijk, M. P.; van der Kraan, A. M.; Koningsberger, D. C.; van Zon, F. B. M.; de Beer, V. H. J.; van Veen, J. A. R. On the structural differences between alumina-supported CoMoS Type I and alumina-, silica-, and carbon-supported CoMoS Type II phases studied by XAFS [(EXAFS)], MES [(Mo¨ssbauer emission spectroscopy)], and XPS. J. Catal. 1994, 146, 375. (6) Qabazard, H.; Abu-Seedo, F.; Stanislaus, A.; Andari, M.; AbsiHalabi, M. A comparison between the performance of conventional and high-metal (CoMo and NiMo)/Al2O3 catalysts in the deep desulfurization of Kuwait atmospheric gas oil. Fuel Sci. Technol. Int. 1995, 13, 1135. (7) Yokoyama, Y.; Ishikawa, N.; Nakanishi, K.; Satoh, K. Deactivation of Co-Mo/Al2O3 hydrodesulfurization catalysts during a one-year commercial run. Catal. Today 1996, 29, 261. (8) Kooyman, P. J.; Buglass, J. G.; Reinhoudt, H. R.; van Langeveld, A. D.; Hensen, E. J. M.; Zandbergen, H. W.; van Veen, J. A. R. Quasi in situ sequential sulfidation of CoMo/Al2O3 studied using high-resolution electron microscopy. J. Phys. Chem. B 2002, 106, 11795. (9) Usman Kubota, T.; Araki, Y.; Ishida, K.; Okamoto, Y. The effect of boron addition on the hydrodesulfurization activity of MoS2/Al2O3 and Co-MoS2/Al2O3 catalysts. J. Catal. 2004, 227, 523. (10) Eijsbouts, S.; van den Oetelaar, L. C. A.; van Puijenbroek, R. R. MoS2 morphology and promoter segregation in commercial Type 2 NiMo/Al2O3 and Co-Mo/Al2O3 hydroprocessing catalysts. J. Catal. 2005, 229, 352. (11) de la Rosa, M. P.; Texier, S.; Berhault, G.; Camacho, A.; Yacaman, M. J.; Mehta, A.; Fuentes, S.; Montoya, J. A.; Murrieta, F.; Chianelli, R. R. Structural studies of catalytically stabilized model and industrialsupported hydrodesulfurization catalysts. J. Catal. 2004, 225, 288. (12) Gil-Llambias, F. J.; Rodriguez, H.; Bouyssieres, I.; Escudey, M.; Carkovic, I. Hydrodesulfurization catalysts electrophoretic study of Mo(or W)-Co, Mo(or W)-Ni, and Mo(or W)-Ca sulfided phases. J. Catal. 1986, 102, 37. (13) Chiu, N. S.; Johnson, M. F. L.; Bauer, S. H. Co/Mo/alumina catalyst structure determination by EXAFS (extended X-ray absorption fine structure spectroscopy) 4. Co-K edge in the oxide and sulfided states. J. Catal. 1988, 113, 281. (14) Guenter, J. R.; Koranyi, T. I.; Marks, O.; Paal, Z. High-resolution electron microscopy (HREM) of cobalt-molybdenum catalysts: Structural changes and activity. Appl. Catal. 1988, 39, 285. (15) Prada Silvy, R.; Grange, P.; Delmon, B. Activation of cobaltmolybdenum hydrodesulfurization catalysts: Influence of the sulfidation procedure on the physico-chemical properties and catalytic activity. Stud. Surf. Sci. Catal. 1990, 53, 233. (16) Koranyi, T. I.; Paal, Z. Activation of unsupported and Al2O3 supported Co-Mo catalysts in thiophene hydrodesulfurization. Stud. Surf. Sci. Catal. 1990, 53, 261. (17) Diaz, G.; Luna, R.; Rios-Jara, D.; Banos, L. X-ray diffraction study of a CoMo sulfide obtained by the impregnated thiosalt decomposition method. Catal. Lett. 1990, 7, 377. (18) Bouwens, S. M. A. M.; van Veen, J. A. R.; Koningsberger, D. C.; de Beer, V. H. J.; Prins, R. Extended X-ray absorption fine structure [(EXAFS)] determination of the structure of cobalt in carbon-supported Co and Co-Mo sulfide hydrodesulfurization catalysts. J. Phys. Chem. 1991, 95, 123. (19) Permana, H.; Lee, S.; Ng, K. Y. S. Scanning tunneling microscopy investigations of cluster sizes of molybdenum-based catalysts on graphite. Catal. Lett. 1994, 24, 363.

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ReceiVed for reView August 28, 2006 ReVised manuscript receiVed October 23, 2006 Accepted October 24, 2006 IE061131X