Abstract
Highly active nickel phosphide nano clusters (Ni2P) confined in 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 oC 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), were also prepared. From the microstructural analysis, it was found that the growth of Ni2P nano clusters was restricted by the mesoporous channels, thus forming ultrafine and highly dispersed Ni2P nano clusters (< 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 an SO2 conversion of 94 % at 220 oC and ~99 % at 240 oC, which is higher than the 11 nm stand-alone Ni2P particles (43 % at 220 oC and 94 % at 320 oC), highlighting the importance of the role played by SiO2 in stabilizing ultrafine nanoparticles of Ni2P. The reaction activation energy Ea over u-Ni2P@m-SiO2 is ~33 kJ/mol, which is lower than over n-Ni2P (~36 kJ/mol) and c-Ni2P (~66 kJ/mol), suggesting that the reaction becomes energetically favored over the ultrafine Ni2P nano clusters.