The new mechanism helps electrocatalytic nitrate to efficiently reduce synthetic ammonia

On April 20, 2023, the team of Professor Zhang Bing/Yifu Yu of Tianjin University published a research result entitled “Ultralow overpotential nitrate reduction to ammonia via a three-step relay mechanism” in the journal Nature Catalysis.

This achievement reports a new mechanism of electrocatalytic nitrate reduction, a three-step relay mechanism, including spontaneous redox reaction, electrochemical reduction and electrocatalytic reduction process. Ru15Co85 alloy catalyst was designed and prepared by this mechanism, which significantly reduced the overpotential of nitrate electroreduction and improved energy efficiency. In addition, a series of controlled experiments, in situ characterization and theoretical calculations demonstrate the reliability and universality of the three-step relay mechanism. It provides new ideas for the design of high-efficiency catalysts and the in-depth understanding of catalytic mechanisms.

The corresponding authors of the paper are Professor Yu Yifu and Professor Zhang Bing of Tianjin University, and the first authors are Dr. Hanshuhe of Tianjin University and Associate Researcher Li Hongjiao of Sichuan University (responsible for theoretical calculation).

Ammonia is an important bulk chemical that is essential for agricultural development. In recent years, ammonia has attracted much attention as a new generation of carbon-free energy carriers. At present, ammonia synthesis technology mainly relies on the traditional Haber–Bosch process, which is driven by fossil fuels and not only releases more than 400 million tons of greenhouse gases per year, but also the centralized, continuous production model is difficult to match the distributed and fluctuating characteristics of renewable energy (solar, wind, etc.). Therefore, it is essential to explore and develop new ammonia synthesis strategies.

In recent years, the strategy of nitrate reduction to ammonia has attracted much attention, which can use nitrate in industrial wastewater as raw material to synthesize ammonia driven by clean energy, so it is also regarded as an effective “waste into treasure” strategy. Existing studies have shown that catalysts such as copper and cobalt can achieve high Faraday efficiency ammonia synthesis, but their electrocatalytic overpotential is high, resulting in low energy efficiency. The limiting step of the electrocatalytic nitrate reduction reaction is the reduction of nitrate to nitrite. According to the Nernst equation, nitrate, as a strong oxidizing acid, can undergo spontaneous redox reactions with active metals. Taking the metal cobalt (Co) as an example, the reaction equation can be written as: NO3– + Co + H2O → NO2– + Co(OH)2 (ΔE = 1.57 V). Considering the subsequent deoxidation reduction process of cobalt hydroxide and the hydroreduction process of nitrite, another metal element to be introduced should have excellent active hydrogen production capacity (such as Ru).

Based on this, the Zhang Bing/Yu Yifu team designed and prepared RuxCoy alloy catalysts in different proportions as model catalysts. The results of electrochemical experiments show that Ru15Co85 has the best catalytic performance: the highest FE is 97%, the initial reduction potential is +0.4V vs. RHE, and the highest energy efficiency is 42%. Through preliminary technical and economic analysis, it can be seen that the price of ammonia synthesized by this strategy is 0.49 US dollars / kg, which is much lower than the current market price (1 US dollars / kg). In addition, they designed a series of controlled experiments, combined with electrochemical in situ tests (in situ infrared, in situ Raman, in situ XAS, in situ DEMS, etc.) and theoretical calculations to prove the existence of a three-step relay mechanism, which can effectively promote the conversion of nitrate to ammonia. The Co in the alloy can undergo spontaneous redox reaction with nitrate to form cobalt hydroxide and nitrite. Subsequently, cobalt hydroxide can be reduced to Co0 under electrochemical action to realize the valence cycle of the catalyst. Finally, nitrite is produced by the dissociation-hydrogenation pathway to ammonia. The introduction of ruthenium can effectively improve the catalyst’s ability to form active hydrogen, thereby facilitating the second and third steps. The proposal of this mechanism breaks through the traditional electrocatalytic cognition of the one-way conversion or static catalysis mechanism of catalysts, and provides guidance for the design of efficient catalysts.

Figure 1: Catalyst design.

Figure 2: Structural characterization of catalysts.

Figure 3: Characterization of electrochemical performance.

Figure 4: Demonstration of the three-step relay mechanism.

Figure 5: Exploration of nitrite hydrogenation pathway.

Figure 6: Theoretical calculations.

In summary, the research results provide new ideas for the design of efficient catalysts and the in-depth understanding of the catalytic mechanism. (Source: Science Network)

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