The nitrate pollution from fertilizer and animal waste has become a severe environmental problem in recent years. High concentration of NO^3-ions in surface water could impair the health of human beings.¹The global trend for environmental compatibility has led to increasing demand for removing nitrates using environmentally friendly approaches that meet energy conservation standards.²Due to the low cost and controllable selectivity, the electro-reduction of NO^3-ions promoted by metallic or bimetallic catalysts has been considered the among the most promising means for denitrification.³

In order to enhance activity and durability, and minimize the use of expensive catalytic materials, deposition of ultrathin metal/alloy films with controllable thickness and composition has attracted broad attention in last decades. Electrochemical atomic layer deposition (E-ALD) approaches for the growth of single metal films such as Pt and Pd have been developed by our group based on surface-limited redox replacement (SLRR) in one cell configuration.^4,5The application of SLRR for deposition of alloy films has been taken as challenge and thoroughly studied in this report. Cu_xAu_(1–x)bimetallic ultrathin-film catalysts have been synthesized using E-ALD by SLRR of Pb underpotentially deposited layer.⁶ The composition of synthesized alloy ultra-thin film could be controlled by the ratio of [Cu²⁺] ions and [AuCl₄^–] complex in the deposition solution along with the application of specifically designed routine facilitating the administration of SLRR cycles. The alloy film composition has been determined by X-ray photoelectron spectroscopy and indirectly estimated by anodic stripping voltammetry.

The nitrate electroreduction activity and durability of Cu_xAu_(1–x)thin films, Cu thin film, and bulk Cu have been studied by one- and multiple-cycle voltammetry. The synthesized Cu_xAu_(1–x)thin films feature up to two times higher nitrate electroreduction activity in acidic solution compared to bulk and thin-film Cu counterparts. Highest activity has been measured with a Cu_0.70Au_0.30alloy catalyst. Durability tests have demonstrated that Cu thin films undergo rapid passivation losing 65% of its peak activity for up to 100 cycles, whereas Cu_0.70Au_0.30catalysts developed in this study lose about 40% of their top performance. The significantly better durability of alloy films can be attributed to effective resistance to poisoning and/or minimized passivation of Cu active centers. It has been also found that both Cu_xAu_(1–x)and pure Cu thin films show best electroreduction activity at lowest pH.

References

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2. Martínez, J.; Ortiz, A.; Ortiz, I. State-of-the-art and perspectives of the catalytic and electrocatalytic reduction of aqueous nitrates. Appl. Catal., B 2017, 207, 42−59
3. Duca, M.; Koper, M. T. M. Powering denitrification: the perspectives of electrocatalytic nitrate reduction. Energy Environ. Sci. 2012,5,9726−9742.
4. Dimitrov, N. Recent Advances in the Growth of Metals, Alloys, and Multilayers by Surface Limited Redox Replacement (SLRR) Based Approaches. Electrochim. Acta 2016, 209, 599−622.
5. Dimitrov, N.; Achari, I.; Ambrozik, S. Palladium Ultrathin Film Growth by Surface-Limited Redox Replacement of Cu and H UPD Monolayers: Approaches, Pros, Cons, and Comparison. Electrochem. Soc. Interface 2018, 27, 65−69.
6. Xie, Y.; Dimitrov, N. Highly Active and Durable Cu _x Au _(1–x) Ultrathin-Film Catalysts for Nitrate Electroreduction Synthesized by Surface-Limited Redox Replacement. ACS Omega. 2018, 3 (12), 17676–17686.