Ab initio study of bilateral doping within the MoS2-NbS2 system

We present a systematic study on the stability and the structural and electronic properties of mixed molybdenum-niobium disulphides. Using density functional theory we investigate bilateral doping with up to 25 % of MoS2 (NbS2) by Nb (Mo) atoms, focusing on the precise arrangement of dopants within the host lattices. We find that over the whole range of considered concentrations, Nb doping of MoS2 occurs through a substitutional mechanism. For Mo in NbS2 both interstitial and substitutional doping can co-exist, depending upon the particular synthesis conditions. The analysis of the structural and electronic modifications of the perfect bulk systems due to the doping is presented. We show that substitutional Nb atoms introduce electron holes to the MoS2, leading to a semiconductor-metal transition. On the other hand, the Mo doping of Nb2, does not alter the metallic behavior of the initial system. The results of the present study are compared with available experimental data on mixed MoS2-NbS2 (bulk and nanoparticles).Comment: 7 pages, 6 figure

The Bond Energy Model for Hydrotreating Reactions: Theoretical and Experimental Aspects

Recently, we have shown that the periodic variations in the HDS activities of different transition metal sulfides can be explained by a Bond Energy Model (BEM) based on theoretically calculated metal sulfur bond energies. The model differs from many previous explanations for the observed Balandin-type volcano curves. Specifically, the BEM model suggests that under most conditions, the variations in HDS activities are dominated by differences in the concentration of coordinatively unsaturated sites (CUS). The model has also been used to estimate the metal sulfur-bond strength of Co-Mo-S and NL-Mo-S and the promotion is explained in terms of the estimated bond energies for the different types of surface structures. EXAFS and TPR-S results are seen to provide experimental evidence for the BEM model. Besides coordinatively unsaturated sites, surface SH groups are also present and new evidence is presented based on 1H MAS NMR measurements. SH groups may also play an important catalytic role in the supply of hydrogen and for acid catalyzed reactions. The relative importance of the SH groups and the CUS sites appears to depend critically on the metal-sulfur bond energy. Furthermore, the BEM model is shown to provide a useful starting point for understanding various reactivity trends observed under different reaction conditions