DFT Investigation of MoS2 Nanoclusters Used as Desulfurization Catalysts

摘要

Supported single-layer MoS2 is a well-established catalyst used by the petroleum industry to remove sulfur from fossil fuels. It is believed that the catalytic activity occurs at the edge of the MoS2 nanoparticles. Recently, atomic-scale images of MoS2 nanoclusters under catalytic working conditions have been obtained with scanning tunneling microscopy (STM). These images show that certain triangular-shaped "magic clusters" are formed, where the triangular shape is attributed to stabilization from excess sulfur at the cluster edges. Much of the recent theoretical rationalization of the MoS2 cluster structures has been formulated by using plane-wave DFT calculations. However, the dusters observed in the recent STM images are small enough that their structures and properties can be evaluated using direct space DFT calculations. We present the theoretically optimized structures obtained with DFT calculations for the series of Mo10Sx clusters with x = 12, 18, 24, 30, and 36. From consideration of their relative energies and simulated STM images, we find the cluster with the Mo-10 core most likely being imaged in the STM experiments is the Mo-edge Cluster Mo10S24. This contrasts with the assignment to the S-edge cluster Mo10S24 by the experimentalists and their suggestion that Mo-edge clusters are less favored than S-edge clusters when there are 21 or less Mo atoms. Furthermore, despite being able to build initial Mo10Sx structures with a high degree of symmetry, we find the fully optimized Mo10Sx dusters to have essentially no symmetry, and we discuss how this could be playing a role in the MoS2 catalytic activity.