## 2 Reaction Mechanisms and Kinetics of Electrocatalytic Nitrate Reduction
### Advanced in Situ Characterization Techniques  
Electrode materials gained particular attention for their ability to ensure the eNO\({}_{3}\)R mechanism. Hence, to establish a link between the electrode surface structure, (including its chemical composition, crystal structure, coordination number, and interatomic distance) and the eNO\({}_{3}\)R performance, in situ characterization techniques have been employed during the electrochemical reaction. These techniques mainly include X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), in situ electrochemical scanning tunneling microscopy (EC-STM), grazing-incidence X-ray diffraction (GIXRD), and shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) [57; 97; 98; 99; 100; 101]. These techniques allow the real-time control of the reaction and the characterization of the catalysts under reaction conditions. Thanks to the use of these techniques, a qualitative understanding of the eNO\({}_{3}\)R reaction mechanism is possible. In addition, they can provide information on dynamic changes in the catalyst's stereo geometry and electronic structure. Thus, the development of in situ techniques is of utmost importance to identify the active species involved in the eNO\({}_{3}\)R mechanism and provide a qualitative understanding of the reaction mechanism. The various in situ techniques are summarized in the following Table 3, which outlines the detected products and intermediates, principles, advantages, and disadvantages of each method. \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Characterization Techniques** & **Detected Products and Intermediates** & **Principles** & **Advantages** & **Disadvantages** & **Refs.** \\ \hline \hline \multicolumn{6}{p{56.9 pt}}{The principle is based on measuring the inelastic scattering of light caused by molecular vibrations involving changes in the polarization of the molecule.} \\ Shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) & All the intermediates adsorbed on different faces of catalyst material. & It consisted of a gold nanoparticle with an insulating thin SiO\({}_{2}\) shell (Au/SiO\({}_{2}\)) to eliminate the interference of deposited Au or other substrates (Ag, Au), which have a surface plasmon resonance effect, and to inhibit the signal from the analytes adsorbed on the Au core. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: In situ Characterization Techniques for eNO\({}_{3}\)R.  
\begin{table}
\begin{tabular}{p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt} p{42.7 pt}} \hline \hline
**Characterization Techniques** & **Detected Products and Intermediates** & **Principles** & **Advantages** & **Disadvantages** & **Refs. ** \\ \hline  & & The incident infrared ray is guided by the prism and optical stage toward the chemical bonds of intermediates formed at the surface of the electrode. & Rapid determination of adsorbed intermediates, which was then diffused and reflected in the detector. The FT-IR measurements were performed on a spectrometer equipped with a liquid-nitrogen-cooled detector. & FTIR instruments have only a single beam. & [104; 105; 106] \\  & & The method involves selecting the desired energy of X-ray radiation using a monochromator and measuring the resulting XAS spectrum, which is composed of two parts: the X-ray absorption near-edge structure (SAANES) and the extended X-ray absorption fine structure (EXAFS). The XANES spectrum provides information on the electronic structure of the element being studied, while the EXAFS spectra give information on the local geometry of the atom. This technique is well suited for identifying active sites in multi-element systems and can measure elements at low concentrations due to their high flux properties. & & XAS data represents an average of the entire sample and Soft XAS requires an ultra-high or high vacuum environment. XAS may not provide detailed information about specific regions of the sample.