MS-1-P-1970 High resolution HAADF-STEM imaging of MoS2 nanolayers in industrial hydrotreating catalysts
Current severe environmental legislations constrain a strong decrease of sulfur concentration in fuels. Thus, the improvement of hydrotreating catalysts is of major importance. Co-promoted MoS2 based catalysts supported on alumina are known to be industrially used in the selective gasoline hydrodesulfurization (HDS) process. The challenge is to increase the selectivity of these catalysts. Catalyst performance (in particular selectivity) is suspected to be related to the local structure (ie 2D morphology) of the active phase, composed of MoS2 nanolayers promoted by cobalt. The equilibrium morphology is usually well predicted by theoretical approaches based on density functional theory (DFT) [1,2]. For since, it has also been visualized in model materials supported on gold or graphite by STM [3] and HAADF-STEM [4].
For that study, we observed MoS2 and CoMoS industrial catalysts, supported on delta-alumina by HAADF-STEM using a JEOL TEM 2100F with a Cs-corrected condenser. MoS2 and CoMoS catalysts were prepared by incipient wetness impregnation and sulfided under pure H2S at atmospheric pressure, either at 550°C or 700°C.
In MoS2 catalyst sulfided at 550°C, nanolayers present mainly a truncated triangle shape, in good accordance with DFT predictions [1]. Nevertheless, the morphologies are quite irregular : some nanolayers present a more isotropic shape. Some clusters are also observed. In MoS2 catalyst sulfided at 700°C (Fig 1), nanolayers are larger, well crystallized and morphologies are more homogeneous : mainly truncated triangles and some isotropic multi-facetted slabs.
CoMoS catalyst sulfided at 500°C present mainly hexagonal or irregular shape. No truncated triangle morphology is present. Some clusters are present. In addition, slabs are more stacked and aggregated than in non-promoted catalyst. At higher sulfidation temperature (Fig 2), the morphology of the nanolayers is homogeneous : all slabs are large, well crystallized, isotropic with many edges. No cluster is present. This observation is attributed to a combined effect of temperature and promoter edge decoration impacting the resulting 2D morphologies of CoMoS slabs [2].
In conclusion, this study highlights that HR-HAADF-STEM is a powerful technique to observe MoS2 nanolayers, even supported on alumina in industrial catalysts. In perspective of this work, changes of 2D morphology of nanolayers will be correlated to selectivity measured by catalytic tests.
[1] H. Schweiger, P. Raybaud, G. Kresse, H. Toulhoat. J. Catal. 207, 76-87 (2002).
[2] E. Krebs, B. Silvi, P. Raybaud. Catal. Today 130, 160-169 (2008).
[3] J. V. Lauritsen et al. Journal of Catalysis 197, 1–5 (2001)
[4] L. P. Hansen et al., Angew. Chem. Int. Ed. 2011, 50, 10153-10156
The authors thank P. Raybaud for helpful discussions about DFT.
Fig. 1: MoS2/alumina catalyst sulfided at 700°C under pure H2S |
Fig. 2: CoMoS/alumina sulfided at 700°C under pure H2S |