MoS2 morphology and promoter segregation in commercial Type 2 Ni–Mo/Al2O3 and Co–Mo/Al2O3 hydroprocessing catalysts

doi:10.1016/j.jcat.2004.11.011

Journal of Catalysis

Volume 229, Issue 2, 25 January 2005, Pages 352-364

https://doi.org/10.1016/j.jcat.2004.11.011 Get rights and content

Abstract

A series of commercial liquid-phase-sulfided Type 2 Ni–Mo/Al₂O₃ and Co–Mo/Al₂O₃ catalysts used in different trickle-phase reactions has been characterized by (scanning) transmission electron microscopy combined with energy-dispersive X-ray analysis ((S)TEM-EDX) and X-ray diffraction (XRD) analysis. In contrast to what is reported for H₂S/H₂ sulfided Type 2 model catalysts, MoS₂ stacking is not prominent in liquid-phase sulfided Type 2 commercial catalysts. The presence or absence of MoS₂ stacking has been shown to be highly dependent on the sulfidation procedure applied. Although Type 2 commercial Ni–Mo catalysts have high MoS₂ dispersion, there is a significant Ni sulfide segregation. This is not the case for their Co-containing counterparts.

Introduction

Gasoline and diesel specifications continue to tighter, moving toward ultraclean fuels ( $< 10 ppm$ S) as the standard by the end of this decade. The commercial Type 2 catalysts discussed in the present paper are high-performance catalysts, having been specifically developed to meet these future fuel targets in existing refinery processes and units.

It is an accepted concept that the active phase in hydroprocessing catalysts consists of MoS₂ structures decorated with the promoter (Ni or Co) sulfide, the so-called Ni–Mo–S or Co–Mo–S phase [1], [2], [3], [4], [5], [6], [7]. Two types of these phases are described, Types 1 and 2 [3], [4], [5], [6], [7], but there is still confusion about the definition of these phases [8]. Type 2 is characterized by lower active phase-support interaction and, consequently, by a more complete sulfidation [3], [4], [5], [6], [7]. Type 2 model catalysts studied in the literature are mostly prepared by coimpregnation with nitrilo triacetic acid (NTA) followed by H₂S/H₂ sulfidation [2], [3], [4], [9], [10], [11], [12], [13], [14], [15], [16]. Ni sulfidation is delayed because of the application of NTA, resulting in the simultaneous sulfidation of the promoter and Mo and, therefore, in less promoter segregation [12], [13], [16]. In addition, these catalysts often contain MoS₂ stacks [1], [3], [4], [17], [18]. The intrinsic activity of the Type 2 active sites is higher compared with Type 1 [2], [3], [4], [5], [6], [7].

The commercial Type 2 catalysts are different from the NTA model catalysts. They have much higher metal loadings and are mostly liquid phase sulfided. As in NTA model catalysts, no strong interaction with the support can be formed because of preparation conditions. The commercial Type 2 catalysts are also fully sulfided; their sulfidation is complete at lower temperatures, and the sulfidation of Ni is delayed compared with conventional catalysts [19]. The intrinsic activity of the commercial Type 2 catalysts is higher than those of conventional Type 1 and mixed Type 1/2 catalysts [20], [21].

The promoter and support effects as well as the structure and morphology of the active phase in sulfidic Type 1 and Type 2 catalysts have been the subject of numerous studies [1], [18], [22], [23], [24], [25]. Among other things, it was found that Co₉S₈ [26], [27], [28] and Ni sulfide [29], [30], [31], [32] segregate under reaction conditions. Different Ni sulfides can be formed, depending on the reaction conditions; Ni₃S₂ is the most stable [11], [30], [31], [33], [34], [35], [36], [37]. The segregated Co₉S₈ and Ni₃S₂ are proposed to have a distinct catalytic role, providing spillover hydrogen for the MoS₂ active phase, in the “Remote Control” concept [26], [30], [38]. Different MoS₂ morphologies, including straight and curved MoS₂ single slabs and stacks, are observed in supported and unsupported catalysts [18], [22], [24], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48]. The MoS₂ structures can be parallel [39], [46], [47], [49], [50], [51], [52], [53] or perpendicular [44], [49], [51], [54] to the support. The observed morphology and orientation differences are related to active phase composition, type of support, presence of additives (such as phosphorus), preparation variables, and sulfidation and reaction conditions applied [1], [18], [22], [23], [24], [25].

The present paper will demonstrate that Ni sulfide segregation occurs in both commercial Types 1 and 2 Ni–Mo catalysts under reaction conditions, even though the Ni sulfidation is delayed in Type 2 catalysts. In addition, it will be shown that MoS₂ stacks are not prominent in liquid-phase-sulfided commercial Type 2 catalysts.

Section snippets

Samples

The categorization Types 1, 1/2, and 2 used in the present paper is based on the catalyst preparation procedure. Type 1 catalysts are typically P-free calcined catalysts. Most of the conventionally prepared calcined catalysts contain in fact a mixture of Types 1 and 2 phases [8] and are therefore referred to as Type 1/2 in the text. Type 1/2 catalysts are, for example, low-temperature calcined or dried catalysts with a low P content, which contain a mixture of Types 1 and 2 phases. Catalysts

Segregation of Ni sulfide

Commercial Types 1/2 and 2 Ni–Mo/Al₂O₃ hydroprocessing catalysts typically have a high MoS₂ dispersion, as measured by TEM [29], [55]. Fig. 1 shows one example of the high MoS₂ dispersion of Type 2 Ni–Mo catalysts (NiMo3-T2-L-U). NiMo1-T2-L-F and NiMo2-T2-L-F catalysts have TEM characteristics similar to those of NiMo3-T2-L-U. The three catalysts contain predominantly short MoS₂ monolayers (Table 3).

Fig. 2 shows the XRD patterns of the NiMo1-T2-L-F and NiMo2-T2-L-F catalysts. Fig. 2 confirms

Segregation of Ni sulfide

Commercial Type 2 Ni–Mo/Al₂O₃ hydroprocessing catalysts that are liquid-phase sulfided typically have a high MoS₂ dispersion, as shown by TEM analysis (Fig. 1). Ni₃S₂ crystals (5–50 nm in diameter), some decorated with MoS₂ layers, are also present (Fig. 3, Fig. 4). MoS₂ stacks and fewer but much larger (200 nm to 1 μm in diameter) Ni₃S₂ crystals are observed in TEM after gas-phase sulfidation. Ni sulfide segregation is also observed in Type 1/2 Ni–Mo catalysts. STEM-EDX spectral imaging is an

Conclusions

I.
Commercial Type 2 Ni–Mo/Al₂O₃ liquid-phase-sulfided hydroprocessing catalysts typically exhibit a high MoS₂ dispersion. However, a significant part of Ni segregates and is present as separate Ni-rich particles. The remaining (not visibly segregated) Ni is still sufficient for moderately high Ni/Mo edge + corner atom ratios. The Ni sulfide segregation is also observed in Type 1/2 Ni–Mo catalysts. Most importantly, the absence of Ni₃S₂ peaks in the XRD pattern does not indicate the absence of Ni

Acknowledgment

The authors thank Mrs. Caroline van der Meij for the TEM sample preparation.

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MoS2 morphology and promoter segregation in commercial Type 2 Ni–Mo/Al2O3 and Co–Mo/Al2O3 hydroprocessing catalysts

Abstract

Introduction

Section snippets

Samples

Segregation of Ni sulfide

Segregation of Ni sulfide

Conclusions

Acknowledgment

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Chem. Eng. Sci.

MoS₂ morphology and promoter segregation in commercial Type 2 Ni–Mo/Al₂O₃ and Co–Mo/Al₂O₃ hydroprocessing catalysts