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.

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 P-31 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. Structural and morphological control of crystalline nanoparticles is crucial in heterogeneous catalysis. Applying DFT-assisted solid-state NMR spectroscopy, we determine the surface crystal and electronic structure of Ni2P nanoparticles, unveiling NMR nanocrystallography as an emerging tool in facet-engineered nanocatalysts.