Unraveling the role of surface termination in Ni2P(001) for the direct desulfurization reaction of dibenzothiophene (DBT): A density functional theory (DFT) and microkinetic study

Abstract

Achieving ultradeep desulfurization of transportation fuels dictates the removal of the last fragments of sulfur present in bulky refractory molecules such as dibenzothiophene (DBT). Improving the hydrodesulfurization (HDS) performance of the next-generation Ni2P catalyst is crucial; however, it is still unclear how a DBT direct desulfurization (DDS) reaction proceeds on different surface terminations of this material. This work aims at elucidating the influence of Ni3P-, Ni3P2-, and partially sulfided Ni3P2 (Ni3PS)-terminated surfaces of the Ni2P(001) crystal on the DDS reaction of DBT using density functional theory (DFT) computations. The scission of the first C-S bond in DBT was found to be kinetically hindered compared to the second C-S bond cleavage over the three surfaces owing to the highly stable structure of adsorbed DBT compared to the C12H9SH intermediate. Notably, the overall DBT desulfurization process is thermodynamically favorable only on the Ni3PS surface, due to the presence of the active phosphosulfide (Ni-P-S) phase. Microkinetic modeling was conducted to probe the reaction orders, apparent activation energy, and rate-controlling steps as a function of temperature. Notably, the ring-opening step of DBT, via the first C-S bond cleavage, is the rate-controlling step on Ni3P and Ni3PS surfaces, which is rationalized by the high energy barrier of this step. The activation energy of the reaction over the surfaces followed the order Ni3P < Ni3PS < Ni3P2. Compared to the bare Ni3P2 surface, the lower activation energy on the Ni3PS surface is explained by the destabilization effect of the reaction intermediates on the latter. Thus, the partially sulfided Ni3P2 provides a better model for the DBT desulfurization process than the fresh Ni3P2 surface.