Deposited Thin-Film Nanoelectrocatalysts of Non-Noble Metals for Co-Capture of CO₂ and Reduction of Nitrates ^†

Abstract

The co-electrolysis of nitrate and CO₂ can contribute to urea production with low carbon-oxide emission rate and at the same time reduce NO₃⁻ to extremely low permissible concentrations. It was found that Ag and Fe particles, as thin catalytic layers, can potentially be used for the joint reduction of NO₃⁻ and CO₂ under benign ambient conditions. The linear voltammetry, chronoamperometry, electrochemical impedance spectroscopy, and electron scanning microscopy were used. The Fe/C electrocatalyst exhibits superior current density stability at −1.2 V vs. Ag/AgCl, whereas Ag/C electrocatalyst shows noticeable degradation over time. This reaction is necessary both for the removal of nitrates from wastewater and for the capture of carbon dioxide, which makes it one of the important applications of sustainable chemistry.

1. Introduction

The co-electrolysis of nitrate and CO₂ can contribute to urea production with low carbon-oxide emission rate and at the same time reduce NO₃⁻ to extremely low permissible concentrations [1,2]. The synthesis of thin-layer nanoelectrocatalysts containing transition-metal nanoparticles is a promising venture. This study proposes the use of precipitated electrocatalysts from base metals. Such a method makes it possible to obtain an electrocatalyst selective to the reduction reaction of CO₂ or NO₃⁻, whose joint reduction product is urea (CO(NH₂)₂) [3,4]. The electrocatalyst coating should firmly bind C-N and proceed with the formation of intermediate compounds (such as *CONH₂) [5,6].

Due to the relevance and modernity of this topic, the latest research can be noted. Here, ref. [7] report a Copper (Cu) single-atom catalyst (Cu-N₃ SAs) with a Cu–N₃ coordination structure for the electrochemical co-reduction of CO₂ and NO₃⁻ into urea. In their perspective review, ref. [8] highlight the importance of investigating structure-sensitivity and electrolyte effects on electrochemical C–N coupling through complementary in situ spectroscopy and online techniques. Recent advances in electrocatalytic C−N coupling for urea synthesis were reviewed in detail [9,10].

The unique electronic structure of transition metals allows them to be active catalysts in the co-reduction reaction of nitrate and carbon dioxide. The aim of the study was to create a thin-layer electrocatalysts that would be effective in the reaction of combined reduction in nitrite ions and CO₂ to produce urea. This required preliminary and relatively simple measurements, such as determining the possibility of such a reaction itself (which can be designated as (NO₃⁻ + CO₂)RR), determining reaction potentials, as well as an initial explanation of these results.

2. Methods

A standard three-electrode electrochemical cell with separated chambers was used, with a volume of 60 mL as the working space. The electrolyte contained 0.1 M KNO₃ and was degassed with Ar for more than 30 min, after which the electrolyte was purged with CO₂ for 40 min. The reference electrode was Ag/AgCl; the auxiliary electrode was a Pt-plate.

The Autolab 302N potentiostat galvanostat with Nova 2.1.5 software (Metrohm, Netherlands-Switzerland) was used for electrochemical measurements and electrochemical synthesis in ambient conditions. The impedance spectra were measured using a PS-20 galvanostat potentiostat with an FRA electrochemical impedance module (SmartStat, Chernogolovka, Russia). Scanning electron microscopy (SEM) was performed using a LEO EVO 50 XVP electron microscope (Carl Zeiss, Germany). A more detailed description of the experiments can be found in a recent study by the authors [11].

3. Results and Discussion

To solve the research problem, iron and silver were selected to create a catalytic thin layer with graphite as the substrate. The following electrolytes were used for electrodeposition of the catalytic layer of the corresponding metal: 5 mM AgNO₃ + 0.1 M KNO₃ at a constant potential of −0.6 V (Ag/AgCl) for 300 s for silver; and 0.1 M FeSO₄ + 0.3 M Na₂SO₄ at a constant current of −1.0 mA for 1 h for iron. Such different conditions served the purpose of obtaining a similar thickness of the catalytic deposited layer.

Figure 1 shows the linear voltammograms (LVs curves) illustrating the possibilities of the urea synthesis process on both Ag/C and Fe/C electrocatalysts. An increase in current is clearly noticeable for these samples compared to an inert graphite substrate, especially with an increase in cathodic potential, which can be attributed to the synthesis of NH₃ (nitrate reduction reaction, NO₃RR) according to the following equation:

NO₃⁻ + 2H₂O → NH₃ + 2O₂ + OH⁻

(1)

Moreover, with the addition of CO₂ purging, the current changes slightly due to the synthesis of urea according to the following equation:

CO₂ + 2NO₃⁻ + 3H₂O → CO(NH₂)₂ + 4O₂ + 2OH⁻

(2)

Thus, the process (1), i.e., NO₃RR can be considered as a competitive reaction with respect to (2), as well as the hydrogen evolution reaction (HER) described in the literature. The dotted lines indicate the selected potentials at which urea synthesis was carried out.

Figure 2 shows the process of urea synthesis (NO₃⁻ + CO₂)RR for the selected curves (used to obtain LVs and based on Figure 1) for electrocatalysts Ag/C (Figure 2a) and Fe/C (Figure 2c). The surface images (SEM) are shown for Ag/C (Figure 2b) and Fe/C (Figure 2d), respectively.

As can be seen from the chronoamperometry curves, the Ag/C sample (Figure 2a) does not show high current densities, and at the best for the (NO₃⁻ + CO₂)RR potential (−1.4 V), there is even a decrease in current density over time, which does not indicate success in conducting (NO₃⁻ + CO₂)RR. On the contrary, good performance at −1.2 V is clearly visible when using an Fe/C electrocatalyst, while maintaining stability in current density (Figure 2c). This can be attributed to the different morphology of the surface of these two samples. Thus, for Ag/C, the Ag precipitate is strongly grouped and is not essentially a nanolayer (light large aggregations in Figure 2b). And the Fe/C sample shows good dispersibility of Fe nanoparticles over the entire surface and, therefore, excellent development of the catalytic layer (scaly light NPs over the entire surface in Figure 2d).

Electrochemical impedance spectra (EIS) are a very informative method for characterization of the electrical conductivity of the catalyst surface. It is believed that the smaller arc radius unambiguously allows us to judge the best conductivity of the sample. Figure 3 shows the EIS for graphite, Ag/C, and Fe/C. A well-dispersed Fe NPs electrocatalyst shows the best conductivity, exceeding both the bare substrate and the Ag/C electrocatalyst. In contrast, the Ag/C sample is inferior in conductivity even to an inert bare graphite substrate.

4. Conclusions

The selected metals can potentially be used for the co-reduction of NO₃⁻ and CO₂ as catalysts. Preliminary studies of the (NO₃⁻ + CO₂)RR reaction have been conducted on two electrocatalysts representing deposited silver and iron coatings on a graphite substrate, and the possibility of urea synthesis under benign environmental conditions has been demonstrated. The superior electrochemical stability of Fe/C (at −1.2 V) compared to Ag/C should be explicitly emphasized. Furthermore, the fundamental advantages of the catalyst design include the uniform dispersion of Fe nanoparticles and improved electrical conductivity. Finally, the study’s limitations are the lack of quantified urea yield and the absence of long-term stability tests, which will be the focus of our following studies. Since this research is a starting point, further studies are necessary, as well as the confirmation of the yields of other possible reaction products such as NH₃ and H₂ (competitive and undesirable). This reaction is very necessary both for the removal of nitrates from wastewater and for the capture of carbon dioxide, which makes it one of the most important applications in modern green chemistry.