Nitrate (NO₃^-) and nitrite (NO₂^-) are common water contaminants that can originate from natural and anthropogenic sources, including agricultural runoff, sewage, and industrial waste. Exposure to high levels of NO₃^- and NO₂^- can pose serious health risks to humans including methemoglobinemia and an increased risk of certain types of cancer. Due to the harmful effects of these contaminants, the U.S. EPA has set the federal regulation on the maximum level of nitrate and nitrite in drinking water to be 10 and 1 mg/L as nitrogen, respectively. Over a few decades, the catalytic treatment of NO₃^- and NO₂^- using bimetallic palladium (Pd) catalysts (with indium (In) as a secondary metal) has been widely studied due to its effectiveness in removing NO₃^- and NO₂^- with high activity and selectivity. However, increasing material costs and the scarcity of Pd and In still hinders the commercialization of the InPd catalysts. To address this issue, we investigated the research to substitute the majority of Pd mass with nickel (Ni) mass which is cheaper than Pd and possesses similar properties to Pd, for thermocatalytic reduction NO₃^-. We synthesized the activated carbon supported InPdNi (InPdNi/AC) catalyst for nitrate reduction and compared its performance to the benchmark InPd/AC catalyst. Whereas InPd/AC (0.05 wt% In, 1.3 wt% Pd) had an initial rate constant k_cat of ~18.7 Lmin^-1g_{surface-metal}^-1, InPdNi/AC (0.05 wt% In, 0.3 wt% Pd, 1 wt% Ni) catalyst was ×18.3 more active (k_cat ~ 341.5 Lmin^-1g_{surface-metal}^-1). We propose that electron donation from Ni to Pd, as demonstrated by X-ray photoelectron spectroscopy (XPS) and DFT calculations, resulted in enhanced nitrate removal due to the abundance of electron density of Pd. Based on our CatCost analysis, we anticipate a reduction of 70% in catalyst expenses by substituting 1 wt% of Pd with Ni. This study provides a new opportunity to universally commercialize catalyst technology for nitrate/nitrite treatment by significantly reducing the cost and usage of precious metals while enhancing the catalytic activity.

Yimo Han

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