Combined DFT and microkinetic modeling study of S O 2 hydrodesulfurization reaction on N i 5 P 4 catalyst

Highlights: • ${SO}_2$ and $H_2$ adsorptions were examined on low-index facets of ${Ni}_5{P}_4$using DFT. • Microkinetic model was developed for ${SO}_2$ HDS over ${Ni}_5{P}_4$(0 0 1) surface. • The kinetic barriers of products formation influence selectivities. • $H_{2}S$ formation and desorption were found to be the rate-determining steps. Optimizing catalysts for the ${SO}_2$ hydrodesulfurization (HDS) is a crucial step toward conforming with the environmental requirements concerning ${SO}_2$ 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 ${SO}_2$ HDS reaction over ${Ni}_5{P}_4$surfaces. The adsorption of both reactants, ${SO}_2$ and $H_2$ molecules, on low-index facets of ${Ni}_5{P}_4$ crystal, namely (0 0 1), (0 1 1), (1 1 1), (1 1 0) , (1 0 1), (0 1 0) and (1 0 0) surfaces, were investigated using density functional theory (DFT) calculations. The stability of ${Ni}_5{P}_4$ surfaces was examined and (0 0 1) surface was found to be the most stable surface. Therefore, microkinetic modeling was conducted on ${Ni}_5{P}_4$(0 0 1) surface to predict the catalytic preferred pathways. Reaction towards $H_{2}S$, main product, exhibited 100% selectivity at reaction temperatures below 700 K, however the selectivity towards $H_{2}O$ became dominant at higher temperatures. This is because the barrier for HS^- hydrogenation to $H_{2}S$ (1.20 eV) is lower than that of the OH hydrogenation to $H_{2}O$ (2.37 eV). The model revealed that conversion of HS^- ions to $H_{2}S$ was the rate-controlling step at reaction temperature below 500 K, whereas $H_{2}S$ 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 ${SO}_2$ and $H_2$ increased with rising temperature, reaching 0.15 and 1.0, respectively, at 800 K.