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

Highly active nickel phosphide (Ni2P) nanoclusters confined in a mesoporous SiO2 catalyst were synthesized by a two-step process targeting tight control over the Ni2P 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 degrees C so to achieve the phase transformation from Ni to Ni2P. For benchmarking, Ni confined by the mesoporous SiO2 (absence of phosphorization) and 11 nm Ni2P nanoparticles (absence of SiO2) was also prepared. From the microstructural analysis, it was found that the growth of Ni2P nanoclusters was restricted by the mesoporous channels, thus forming ultrafine and highly dispersed Ni2P nanoclusters (<2 nm). The above approach led to promising catalytic performance following the order u-Ni2P@m-SiO2 > n-Ni2P > u-Ni@m-SiO2 > c-Ni2P in the selective hydrogenation of SO2 to S. In particular, u-Ni2P@m-SiO2 exhibited SO2 conversions of 94% at 220 degrees C and similar to 99% at 240 degrees C, which are higher than the 11 nm stand-alone Ni2P particles (43% at 220 degrees C and 94% at 320 degrees C), highlighting the importance of the role played by SiO2 in stabilizing ultrafine nanoparticles of Ni2P. The reaction activation energy E-a over u-Ni2P@m-SiO2 is similar to 33 kJ/mol, which is lower than those over n-Ni2P (similar to 36 kJ/mol) and c-Ni2P (similar to 66 kJ/mol), suggesting that the reaction becomes energetically favored over the ultrafine Ni2P nanoclusters.