Combined DFT and microkinetic modeling study of SO2 hydrodesulfurization reaction on Ni5P4 catalyst

Optimizing catalysts for the SO2 hydrodesulfurization (HDS) is a crucial step toward conforming with the environmental requirements concerning SO2 emissions. Nickel phosphides have been reported as efficient catalysts in HDS reaction. However, how HDS reaction proceeds on nickel phosphides is not well understood. On this context, the present work focuses on the mechanistic understanding of SO2 HDS reaction over Ni5P4surfaces. The adsorption of both reactants, SO2 and H2 molecules, on low-index facets of Ni5P4 crystal, namely (001), (011), (111), (110) , (101), (010) and (100) surfaces, were investigated using density functional theory (DFT) calculations. The stability of Ni5P4 surfaces was examined and (001) surface was found to be the most stable surface. Therefore, microkinetic modeling was conducted on Ni5P4(0 0 1) surface to predict the catalytic preferred pathways. Reaction towards H2S, main product, exhibited 100% selectivity at reaction temperatures below 700 K, however the selectivity towards H2O became dominant at higher temperatures. This is because the barrier for HS- hydrogenation to H2S (1.20 eV) is lower than that of the OH hydrogenation to H2O (2.37 eV). The model revealed that conversion of HS- ions to H2S was the rate-controlling step at reaction temperature below 500 K, whereas H2S desorption dominates the overall reaction rate at higher temperatures. The apparent activation energy of HDS reaction decreased considerably from 195 to 48 kJ/mol at reaction temperature range of 400-800 K. The reaction orders in SO2 and H2 increased with rising temperature, reaching 0.15 and 1.0, respectively, at 800 K.

Automated summary

The study investigated the mechanism of SO2 HDS reaction on nickel phosphide surfaces, showing 100% selectivity towards H2S at temperatures below 700 K and dominance of H2O selectivity at higher temperatures. The control steps of the reaction rate at different temperatures and the change in activation energy were analyzed.