MoS2 morphology and promoter segregation in commercial Type 2 Ni–Mo/Al2O3 and Co–Mo/Al2O3 hydroprocessing catalysts
Introduction
Gasoline and diesel specifications continue to tighter, moving toward ultraclean fuels ( 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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