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1. Ni ₂ P active site ensembles tune electrocatalytic nitrate reduction selectivity

   https://doi.org/10.1039/d4cc01834f  

   Nishiwaki, Emily; Rice, Peter S; Kuo, Ding-Yuan; Dou, Florence Y; Pyka, Anthony; Reid, Bryce; Nguyen, Hao A; Stuve, Eric M; Raugei, Simone; Cossairt, Brandi M (July 2024, Chemical Communications)

   Intrinsic active site ensembles on Ni₂P nanocrystal surfaces direct the selective reduction of nitrate to ammonia through the potential-dependent co-adsorption of H* and NO_x*. 

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2. Hydrogen Evolution by Ni ₂ P Catalysts Derived from Phosphine MOFs

   https://doi.org/10.1021/acsaem.9b02109  

   Mabayoje, Oluwaniyi; Dunning, Samuel G.; Kawashima, Kenta; Wygant, Bryan R.; Ciufo, Ryan A.; Humphrey, Simon M.; Mullins, C. Buddie (December 2019, ACS Applied Energy Materials)
3. Understanding the Early Stages of Nickel Sulfide Nanocluster Growth: Isolation of Ni ₃ , Ni ₄ , Ni ₅ , and Ni ₈ Intermediates

   https://doi.org/10.1002/smll.202003133  

   Touchton, Alexander J.; Wu, Guang; Hayton, Trevor W. (September 2020, Small)

   Abstract Addition of sub‐stoichiometric quantities of PEt₃and diphenyl disulfide to a solution of [Ni(1,5‐cod)₂] generates a mixture of [Ni₃(SPh)₄(PEt₃)₃] (1), unreacted [Ni(1,5‐cod)₂], and [(1,5‐cod)Ni(PEt₃)₂], according to¹H and³¹P{¹H} NMR spectroscopic monitoring of the in situ reaction mixture. On standing, complex1converts into [Ni₄(S)(Ph)(SPh)₃(PEt₃)₃] (2), via formal addition of a “Ni(0)” equivalent, coupled with a CS oxidative addition step, which simultaneously generates the Ni‐bound phenyl ligand and the μ₃‐sulfide ligand. Upon gentle heating, complex2converts into a mixture of [Ni₅(S)₂(SPh)₂(PEt₃)₅] (3) and [Ni₈(S)₅(PEt₃)₇] (4), via further addition of “Ni(0)” equivalents, in combination with a series of C–S oxidative addition and CC reductive elimination steps, which serve to convert thiophenolate ligands into sulfide ligands and biphenyl. The presence of1–4in the reaction mixture is confirmed by their independent syntheses and subsequent spectroscopic characterization. Overall, this work provides an unprecedented level of detail of the early stages of Ni nanocluster growth and highlights the fundamental reaction steps (i.e., metal atom addition, CS oxidative addition, and CC reductive elimination) that are required to grow an individual cluster. 

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4. Ni ₂ P‐Modified Ta ₃ N ₅ and TaON for Photocatalytic Nitrate Reduction

   https://doi.org/10.1002/cnma.202000174  

   Wei, Lin; Adamson, Marquix_A_S; Vela, Javier (May 2020, ChemNanoMat)

   Abstract Self‐sustaining photocatalytic NO₃⁻reduction systems could become ideal NO₃⁻removal methods. Developing an efficient, highly active photocatalyst is the key to the photocatalytic reduction of NO₃⁻. In this work, we present the synthesis of Ni₂P‐modified Ta₃N₅(Ni₂P/Ta₃N₅), TaON (Ni₂P/TaON), and TiO₂(Ni₂P/TiO₂). Starting with a 2 mM (28 g/mL NO₃⁻−N) aqueous solution of NO₃⁻, as made Ni₂P/Ta₃N₅and Ni₂P/TaON display as high as 79% and 61% NO₃⁻conversion under 419 nm light within 12 h, which correspond to reaction rates per gram of 196 μmol g⁻¹ h⁻¹and 153 μmol g⁻¹ h⁻¹, respectively, and apparent quantum yields of 3–4%. Compared to 24% NO₃⁻conversion in Ni₂P/TiO₂, Ni₂P/Ta₃N₅and Ni₂P/TaON exhibit higher activities due to the visible light active semiconductor (SC) substrates Ta₃N₅and TaON. We also discuss two possible electron migration pathways in Ni₂P/semiconductor heterostructures. Our experimental results suggest one dominant electron migration pathway in these materials, namely: Photo‐generated electrons migrate from the semiconductor to co‐catalyst Ni₂P, and upshift its Fermi level. The higher Fermi level provides greater driving force and allows NO₃⁻reduction to occur on the Ni₂P surface. 

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5. Composition-Dependent Electrocatalytic Activity of Zn-Doped Ni ₅ P ₄ Nanocrystals for the Hydrogen Evolution Reaction

   https://doi.org/10.1021/acs.chemmater.3c01229  

   Graves, Lisa S; Sarkar, Rajib; Lao, Ka Un; Arachchige, Indika U (September 2023, Chemistry of Materials)

   Electrocatalytic water splitting presents an exciting opportunity to produce environmentally benign hydrogen fuel to power human activities. Earth abundant Ni5P4 has emerged as an efficient catalyst for the hydrogen evolution reaction (HER) and its activity can be enhanced by admixing synergistic metals to modify the surface affinity and consequently kinetics of HER. Computational studies suggest that the HER activity of Ni5P4 can be improved by Zn doping, causing a chemical pressure-like effect on Ni3 hollow sites. Herein, we report a facile colloidal route to produce Ni5-xZnxP4 nanocrystals (NCs) with control over structure, morphology, and composition and investigate their composition-dependent HER activity in alkaline solutions. Ni5-xZnxP4 NCs retain the hexagonal structure and solid spherical morphology of binary Ni5P4 NCs with a notable size increase from 9.2-28.5 nm for x = 0.00-1.27 compositions. Elemental maps affirm the homogeneous ternary alloy formation with no evidence of Zn segregation. Surface analysis of Ni5-xZnxP4 NCs indicates significant modulation of the surface polarization upon Zn incorporation resulting in a decrease in Niδ+ and an increase in Pδ- charge. Although all compositions followed a Volmer-Heyrovsky HER mechanism, the modulated surface polarization enhances the reaction kinetics producing lower Tafel slopes for Ni5-xZnxP4 NCs (82.5-101.9 mV/dec for x = 0.10-0.84) compared to binary Ni5P4 NCs (109.9 mV/dec). Ni5-xZnxP4 NCs showed higher HER activity with overpotentials of 131.6-193.8 mV for x = 0.02-0.84 in comparison to Ni5P4 NCs (218.1 mV) at a current density of -10 mA/cm2. Alloying with Zn increases the material’s stability with only a ~10% increase in overpotential for Ni4.49Zn0.51P4 NCs at -50 mA/cm2, whereas a ~33% increase was observed for Ni5P4 NCs. At current densities above -40 mA/cm2, bimetallic NCs with x = 0.10, 0.29, and 0.51 compositions outperformed the benchmark Pt/C catalyst, suggesting that hexagonal alloyed Ni5-xZnxP4 NCs are excellent candidates for practical applications that necessitate lower HER overpotentials at higher current densities. 

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