Motivated by increased nitrate contamination of surface waters and its deleterious effects in public health, we sought non-noble metal catalysts for the chemical degradation of nitrate. First row transition metal phosphides have become excellent alternatives for hydrogen evolution and hydrotreating reactions. We find that a key member of this family, Ni2P, readily hydrogenates nitrate (NO3–) to ammonia (NH3) near ambient conditions with very high selectivity (96%). One of the few non-precious metal-based catalysts for this transformation, and among ca. 1% of catalysts with NH3 selectivity, Ni2P can be recycled multiple times with limited loss of activity. Density functional theory (DFT) indicates that a variety of adsorbates (H, O, N, NO) induce surface reconstruction with top-layer Ni-rich surface stoichiometry. Critically, H saturation coverage on Ni2P(001) is only ca. 3 nm–2, significantly less than that on Pd(111) and Ni(111) of ca. 15–18 nm–2, which may play a key role in allowing coadsorption of NOx–. Based on these results, we synthesize and compare the activity of Ni2P-modified Ta3N5 (Ni2P/Ta3N5), TaON (Ni2P/TaON), and TiO2 (Ni2P/TiO2) toward photocatalytic NO3− reduction. As made Ni2P/Ta3N5 and Ni2P/TaON display as high as 79% and 61% NO3− conversion under 419 nm light within 12 h, which correspond to apparent quantum yields of 3-4%. In contrast, only 24% NO3− conversion is achieved with Ni2P/TiO2. Our observations suggest 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. The ability of Earth-abundant, binary metal phosphides such as Ni2P to catalyze nitrate hydrogenation could transform and help us to better understand the basic science behind catalytic hydrogenation and, in turn, advance the next generation of oxyanion removal technologies. Self-sustaining photocatalytic NO3− reduction systems could become ideal NO3− removal methods.

Mild and selective reduction of nitrate in the absence of noble metals

ACS Spring Meeting 2021

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