Hydrogen Activation on the Promoted and Unpromoted ReS₂ (001) Surfaces under the Sulfidation Conditions: A First-Principles Study

Hydrogen activation on the promoted and promoter-free ReS₂(001) surfaces under the sulfidation conditions is studied by means of periodic density function theory (DFT) calculations within the generalized gradient approximation. First, surface-phase diagrams are investigated by plotting the surface free energy as a function of the chemical potential of S (μ_S) on the unpromoted and promoted ReS₂ (001) surfaces with different loadings of nickel, cobalt, tungsten, and tantalum. The results show that on the unpromoted surface sulfur coverage of 25% and on the promoted surfaces sulfur coverage of 25% as well as 25% promoter modification are the most stable conditions, respectively, under hydrodesulfurization (HDS) reaction conditions. Second, hydrogen adsorption and dissociation are explored on these preferred surfaces. It is found that hydrogen adsorbs weakly on all the surfaces studied. The physical adsorption character makes its diffusion favorable, resulting in various adsorption sites and dissociation pathways, i.e., dissociation at surface Re or promote atom, at the interlayer, as well as at the adsorbed S atom. Calculated results show that hydrogen dissociation at the surface Re site is always kinetically favorable. All of the studied dopants can largely activate the adsorbed S but display distinct roles toward the activity of the nearest Re atom; i.e., Co/Ni dopant passivates the nearest surface Re while W/Ta activates it. The activity difference is found to be closely associated with the difference in the bond strength of metal–S and the resultant difference in the induced surface geometry. Moreover, promoter effect is localized because it seems nominal when the reaction occurs at a Re atom with one dopant atom separation. The present results provide a rational understanding of the activity difference between the promoter-free and the promoted surfaces, which would be helpful to further understand the mechanism of HDS and to enhance the development of highly active and selective hydrotreating catalysts

Density Functional Theory Investigation on Thiophene Hydrodesulfurization Mechanism Catalyzed by ReS₂ (001) Surface

We present density functional theory calculations on the reaction mechanism of thiophene hydrodesulfurization (HDS) over ReS₂ (001) surface under typical HDS reaction conditions. It is found that thiophene adopts an “upright” adsorption configuration with the binding energy of 1.26 eV. Considering the factors such as Bader charge, two reaction mechanisms, named direct desulfurization (DDS) to the product of butadiene and hydrogenation (HYD) to 2-butene, 1-butene, and butane, are systematically investigated. Results show that H prefers to attack thiophenic C before the first C–S bond rupture but begins to hydrogenate S_T (S atom of thiophene) after ring-opening. Prehydrogenation has different effect on the activity of C–S bond breaking. When the ring is intact, it has nominal effect; but when the ring is open, appropriate prehydrogenation can dramatically decrease the energy barrier while complete hydrogenation makes the barrier rise again due to stereohindrance effect. The DDS mechanism is proved to be kinetically unfavorable while 2-butene is suggested to be a predominated product for HYD mechanism. The role of S_a (preadsorbed S) is a “ladder” which helps H approach the thiophenic molecule while S_T acts as an “intermediary” for H exchange. Changing reaction conditions through partial pressure of H₂ can only alter the rate-determining step but has nothing to do with the catalytic selectivity