Nickel Enhances\nInPd-Catalyzed Nitrate Reduction Activity\nand N<sub>2</sub> Selectivity

Palladium–indium (PdIn) is a well-established\nbimetallic\ncomposition for reductively degrading nitrate anions, one of the most\nubiquitous contaminants in the groundwater. However, the scarcity\nand the variable price of these rare-earth and platinum group critical\nmetals may hinder their use for water treatment. Nickel (Ni), a nonprecious\nmetal in the same element group as Pd, could partially replace and\nlower Pd usage if the resulting trimetallic composition is sufficiently\ncatalytically active. Herein, we report the synthesis and nitrate\nreduction catalysis of activated carbon-supported “In-on-Pd-on-Ni”\ncatalysts (InPdNi/AC). While bimetallic InPd/AC (0.05 wt % In, 1.3\nwt % Pd) was expectedly active, trimetallic InPdNi/AC containing the\nsame In amount, much less Pd (0.1 wt %), and 1 wt % Ni was >17\nmore\nactive (<i>k</i><sub>cat</sub> ≈ 20 vs 349 L min<sup>–1</sup> g<sub>surface metal</sub><sup>–1</sup>). X-ray photoelectron spectroscopy (XPS) and density functional\ntheory (DFT) calculations showed that Pd gained electron density from\nNi, correlating to the increased nitrate reduction activity. Ammonium\nbyproduct selectivity for InPdNi/AC (18% at 50% nitrate conversion)\nwas lower compared to that of InPd/AC (48%), suggestive of the higher\nsurface coverage of NO or its greater reactivity with NO<sub>2</sub><sup>–</sup>, which led to more N<sub>2</sub>. Accounting\nfor the catalyst precursor, manufacturing costs, and spent metal recovery,\nwe calculated that Ni incorporation lowered the net catalyst cost\nsignificantly (from $1028/kg to $170/kg). The trimetallic composition\nlowered, by ∼26 times, the catalyst cost of a stirred tank\nreactor sized to the same treatment capacity as that for the bimetallic\ncase. The results demonstrate that the partial replacement of the\nprecious metal with an earth-abundant one leads to a higher efficiency\nand lower cost denitrification catalyst, via a material strategy that\nshould be beneficial for other clean-water catalytic systems.