(71e) Surface Modification of Pd Nanocube By Cu UPD for Electrochemical Nitrate Reduction

Nitrogen loss from fertilizer runoff results in a high concentration nitrate in groundwater ^[1]. Loss of fixed nitrogen from agricultural sites contributes to low nitrogen use efficiencies by plants, and furthermore is harmful to public health. Nitrate contamination can result in several health issues such as methemoglobinemia (“blue baby syndrome”), and can result in various forms of cancer. For these reasons removing nitrate from groundwater is a global concern. Currently, nitrate removal from drinking water occurs primarily using ion exchange based processes. These processes while effective produce a significant amount of waste brine that is difficult to dispose of. Catalytic strategies for nitrate remediation, if possible could allow for a more sustainable path for nitrate remediation.

Current thermal catalytic strategies for nitrate remediation utilize a hydrogenation metal (Pd, Pt, Rh) and a promoter metal (In and Cu) in tandem^[2,3]. A chief catalytic challenge is that nitrate can be reduced to both nitrogen gas and ammonium limiting the selectivity. In addition, hydrogen gas must be supplied as an electron donor. While ammonium is a valuable byproduct, complete remediation to nitrogen gas is preferred for treatment. Furthermore, the cost of hydrogen makes it cost prohibitive. Here, we aim to explore the potential for electrochemical nitrate reduction or electrocatalytic denitrification. We specifically aim to explore the role catalyst surface structure, electrochemical potential, and a copper based co-catalyst play in promoting selective conversion to nitrogen gas.

The main challenge with electrocatalytic denitrification is the low activity and selectivity of the NO₃^- to nitrogen (N₂) process because this conversion results in undesirable products such as nitrite (NO₂^-) and ammonium (NH₄⁺). To achieve an efficient activity and selectivity for the denitrification, here, we modify Pd nanocube surfaces by using a Cu UPD method, in which the electrochemically deposited Cu atoms promote the reduction of NO₃^- to NO₂^- and Pd (100) surfaces catalyze the reduction of NO₂^- to N₂. Under the Cu desorption area, sweeping stopped at selected potentials can control the surface coverage of Cu atoms. We then verify the several redox reactions of the intermediates that result from the reduction of NO₃^- to N₂ or NH₄⁺ by conducting RRDE tests. The activity and selectivity of NO₃^- to N₂ could be improved by the shape-controlled Pd with its surface modified by Cu atoms. This work demonstrates that modifying the surface of Pd nanocubes with Cu on the carbon support could play an important role as an efficient electrocatalyst for removing NO₃^- from water. Electrocatalytic nitrate reduction provides a new avenue to reduce nitrate to nitrogen gas using electricity as the electron donor. This could reduce the cost and environmental impacts associated with nitrate removal from groundwater.

1. Hansen, B., Thorling, L., Schullehner, J., Termansen, M., & Dalgaard, T. (2017). Groundwater nitrate response to sustainable nitrogen management. Scientific Reports, 7(1), 8566.
2. Barrabés, Noelia, and Jacinto Sá. "Catalytic nitrate removal from water, past, present and future perspectives." Applied Catalysis B: Environmental104, no. 1-2 (2011): 1-5
3. Li, Miao, Chuanping Feng, Zhenya Zhang, Zhaoli Shen, and Norio Sugiura. "Electrochemical reduction of nitrate using various anodes and a Cu/Zn cathode." Electrochemistry communications11, no. 10 (2009): 1853-1856.