Ni₂P Nanoparticles Embedded in Mesoporous SiO₂ for Catalytic Hydrogenation of SO₂ to Elemental S

Highly active nickel phosphide (Ni 2 P) nanoclusters confined in a mesoporous SiO 2 catalyst were synthesized by a two-step process targeting tight control over the Ni 2 P size and phase. The Ni precursor was incorporated into the MCM-41 matrix by one-pot synthesis, followed by the phosphorization step, which was accomplished in oleylamine with trioctylphosphine at 300 °C so to achieve the phase transformation from Ni to Ni 2 P. For benchmarking, Ni confined by the mesoporous SiO 2 (absence of phosphorization) and 11 nm Ni 2 P nanoparticles (absence of SiO 2 ) was also prepared. From the microstructural analysis, it was found that the growth of Ni 2 P nanoclusters was restricted by the mesoporous channels, thus forming ultrafine and highly dispersed Ni 2 P nanoclusters (<2 nm). The above approach led to promising catalytic performance following the order u-Ni 2 P@m-SiO 2 > n-Ni 2 P > u-Ni@m-SiO 2 > c-Ni 2 P in the selective hydrogenation of SO 2 to S. In particular, u-Ni 2 P@m-SiO 2 exhibited SO 2 conversions of 94% at 220 °C and ∼99% at 240 °C, which are higher than the 11 nm stand-alone Ni 2 P particles (43% at 220 °C and 94% at 320 °C), highlighting the importance of the role played by SiO 2 in stabilizing ultrafine nanoparticles of Ni 2 P. The reaction activation energy E a over u-Ni 2 P@m-SiO 2 is ∼33 kJ/mol, which is lower than those over n-Ni 2 P (∼36 kJ/mol) and c-Ni 2 P (∼66 kJ/mol), suggesting that the reaction becomes energetically favored over the ultrafine Ni 2 P nanoclusters.