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Highly mesoporous SiO2-encapsulated Ni x P y crystals, where (x, y) = (5, 4), (2, 1), and (12, 5), were successfully synthesized by adopting a thermolytic method using oleylamine (OAm), trioctylphosphine (TOP), and trioctylphosphine oxide (TOPO). The Ni5P4@SiO2 system shows the highest reported activity for the selective hydrogenation of SO2 toward H2S at 320 °C (96% conversion of SO2 and 99% selectivity to H2S), which was superior to the activity of the commercial CoMoS@Al2O3 catalyst (64% conversion of SO2 and 71% selectivity to H2S at 320 °C). The morphology of the Ni5P4 crystal was finely tuned via adjustment of the synthesis parameters receiving a wide spectrum of morphologies (hollow, macroporous-network, and SiO2-confined ultrafine clusters). Intrinsic characteristics of the materials were studied by X-ray diffraction, high-resolution transmission electron microscopy/scanning transmission electron microscopy-high-angle annular dark-field imaging, energy-dispersive X-ray spectroscopy, the Brunauer–Emmett–Teller method, H2 temperature-programmed reduction, X-ray photoelectron spectroscopy, and experimental and calculated 31P magic-angle spinning solid-state nuclear magnetic resonance toward establishing the structure–performance correlation for the reaction of interest. Characterization of the catalysts after the SO2 hydrogenation reaction proved the preservation of the morphology, crystallinity, and Ni/P ratio for all the catalysts.

Structural and morphological control of crystalline nanoparticles is crucial in the field of heterogeneous catalysis and the development of “reaction specific” catalysts. To achieve this, colloidal chemistry methods are combined with ab initio calculations in order to define the reaction parameters, which drive chemical reactions to the desired crystal nucleation and growth path. Key in this procedure is the experimental verification of the predicted crystal facets and their corresponding electronic structure, which in case of nanostructured materials becomes extremely difficult. Here, by employing 31P solid-state nuclear magnetic resonance aided by advanced density functional theory calculations to obtain and assign the Knight shifts, we succeed in determining the crystal and electronic structure of the terminating surfaces of ultrafine Ni2P nanoparticles at atomic scale resolution. Our work highlights the potential of ssNMR nanocrystallography as a unique tool in the emerging field of facet-engineered nanocatalysts.

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 °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 °C and ∼99% at 240 °C, which are higher than the 11 nm stand-alone Ni2P particles (43% at 220 °C and 94% at 320 °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 ∼33 kJ/mol, which is lower than those over n-Ni2P (∼36 kJ/mol) and c-Ni2P (∼66 kJ/mol), suggesting that the reaction becomes energetically favored over the ultrafine Ni2P nanoclusters.

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