Electrocatalytic decomposition mechanism of hydroxyamine nitrate on the Cu(111) surface

2.1. Reaction path of electrocatalytic decomposition

Under water-solvent conditions, HAN decomposes into NH₃OH⁺ and NO₃^– ions [22]. The aqueous solvent condition is simulated using the implicit solvent SMD (Solvent Model based on Density) model. The configuration changes of HAN under vacuum and water-solvent conditions have been shown in Figure 2, and the energies were calculated using Gaussian 16 software [16]. Under vacuum and water-solvent conditions, the HAN molecule is configured as NH₂OH·HNO₃ and NH₃OH⁺·NO₃^–, respectively. As NH₃OH⁺·NO₃^– has a lower free energy of configuration than NH₂OH·HNO₃, HAN easily decomposes into NH₃OH⁺ and NO₃^– ions under water-solvent conditions. The electrocatalytic decomposition mechanism of HAN has been shown in Figure 3, where ‘*’ represents adsorbed molecules, and radicals are identified with ‘·’ (for example, ‘·OH’ denotes OH radicals). Under a potential difference, anions and cations will move toward the anode (which tends to lose electrons) and cathode (which tends to gain electrons), respectively. As the NO₃^– ions move toward the anode, they lose one electron and adsorb as *NO₃ on the anode surface Eq. (1). Figure 4(a) shows the configuration of *NO₃. The two O atoms of NO₃ are adsorbed on adjacent Cu atoms at the anode surface with different bond lengths (2.040 Å and 2.045 Å) and a bond angle of 119.337° between the adsorbed O and N atoms. *NO₃ dissociates into *NO₂ and *·O on the anode surface Eq.(2). *NO₂ is configured as shown in Figure 4(b). The two O atoms of NO₂ are adsorbed on adjacent Cu atoms on the anode surface with the same bond lengths as *NO₃ (2.040 Å and 2.045 Å). The bond angle between the adsorbed O and N atoms is slightly smaller in *NO₂ than the bond angle between the adsorbed O and N atoms in *NO₃. Figure 4(c) shows the configuration of *·O. The bond lengths between the O and adjacent Cu atoms on the anode surface are 1.906 Å and 1.907 Å, with a bond angle of 88.604°. *NO₂ can desorb from the anode surface as NO₂ see Eq. (3):

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Figure 2.

Configurational changes of HAN under vacuum and water-solvent conditions.

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Figure 3.

Diagram showing the electrocatalytic decomposition mechanism of HAN.

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Figure 4.

Configurations of the adsorbed molecules involved in electrocatalytic decomposition: (a) *NO₃; (b) *NO₂; (c) *·O; (d) *·OH; (e) *·OOH; (f) *NH₂OH; (g) *(·NH₂ + ·OH); (h) *·NH₂.

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H₂O molecules lose one electron, decomposing into *·OH and H⁺ ions on the anode surface Eq. (4). Figure 4(d) shows the configuration of *·OH. The bond lengths between the O atom and adjacent Cu atoms on the anode surface are 2.027 Å and 2.032 Å, with a bond angle of 84.646°. *·OH loses one electron on the anode surface, forming *·O and H⁺ Eq. (5). The *·O radicals react with H₂O molecules, losing one electron to form H⁺ and *·OOH Eq. (6). Figure 4(e) shows the configuration of *·OOH. The two O atoms of ·OOH are adsorbed on adjacent Cu atoms on the anode surface with bond lengths of 2.189 Å and 2.033 Å and bond angles of 89.832° and 109.330°, respectively. *·OOH loses one electron on the anode surface, forming H⁺ and O₂ Eq. (7). The reaction equations are as follows:

Under the action of the potential difference, NH₃OH⁺ ions move toward the cathode, where they dissociate into *NH₂OH and H⁺ ions on the cathode surface Eq. (8). Figure 4(f) shows the configuration of *NH₂OH. The N atom of NH₂OH adsorbs with a bond length of 2.159 Å near the Cu atom on the cathode surface. The bond angles are 107.311° between the Cu, N, and H atoms and 113.946° between the Cu, N, and O atoms. *NH₂OH dissociates into *(·NH₂ + OH) Eq. (9) with the configuration shown in Figure 4(g). The bond lengths between the N atom of ·NH₂ and the two adjacent Cu atoms on the cathode surface are 2.000 Å and 1.994 Å with a bond angle of 113.946°. Meanwhile, the bond lengths between the O atom of · OH and the two adjacent Cu atoms on the cathode surface are 2.042 Å and 2.024 Å, with a bond angle of 83.845°. *·NH₂ obtains an electron at the cathode and combines with H⁺ ions to form NH₃ Eq. (10). Figure 4(h) shows the configuration of NH₂. The N atom of ·NH₂ bonds with two adjacent Cu atoms on the cathode surface with bond lengths of 1.990 Å and 1.991 Å and a bond angle of 82.757°. *·OH obtains an electron at the cathode and combines with H⁺ ions to form H₂O Eq. (11). The reaction equations are as follows:

3.1. Anodic reaction mechanism

Table 1 lists the thermodynamic parameters of the relevant molecules during the anodic reaction process, calculated using CP2K software. As analyzed in the previous section, NO₃^– ions near the anode first lose one electron and adsorb to the anode surface. To avoid the CP2K calculation of the free energy of NO₃^– ions, the formation free energy of Eq. (1) was obtained by summing the formation free energies of Eqs. (12), (13), and (14); that is, $\Delta {\text{G}}_1=\Delta {\text{G}}_{12}+\Delta {\text{G}}_{13}+\Delta {\text{G}}_{14}$ , where $\Delta \text{G}$ represents the formation free energy and the subscript denotes the number of the reaction equation. In this expression, $\Delta {\text{G}}_{12}=\text{G}\left({\text{*NO}}_3\right)+0.5\text{G}\left({\text{H}}_2\right(\text{g}\left)\right)-\text{G}\left({\text{HNO}}_3\right(\text{g}\left)\right)-\text{G}\left(\text{*}\right)$ , where G represents the free energy and H⁺ + e^– is replaced by half the free energy of H₂. According to Calle-Vallejo et al. [14], $\Delta {\text{G}}_{13}=0.317\text{ eV}$ and $\Delta {\text{G}}_{14}=0.075\text{ eV}$ . As only the vibrations of adsorbed molecules are relevant, the surface free energy of the anode is replaced by the single-point energy [23]. From the data in Table 1, $\Delta {\text{G}}_1\text{was}\text{determined}\text{as }0.001\text{ eV}$ . Eqs. (12)–(14) are computed as

The formation free energy of *NO₃ dissociation into *NO₂ and *·O on the anode surface can be expressed as

$\Delta {\text{G}}_2=\text{G}\left({\text{*NO}}_2\right)+\text{G}(\text{*}·\text{O})-\text{G}\left({\text{*NO}}_3\right)-\text{G}\left(\text{*}\right)$. From the data in Table 1, $\Delta {\text{G}}_2\text{was}\text{obtained}\text{as }-0.811\text{eV}$. Given the formation free energy of NO₂ on the anode surface $\Delta {\text{G}}_3=\text{G}\left(\text{*}\right)+\text{G}\left({\text{NO}}_2\right(\text{g}\left)\right)-\text{G}\left({\text{*NO}}_2\right)$ and the data in Table 1, $\Delta {\text{G}}_3$was obtained $\text{as}1.125\text{eV}$. The formation free energy of H₂O molecules decomposing into *·OH and H⁺ ions on the anode surface can be expressed as $\Delta {\text{G}}_4=\text{G}(\text{*}·\text{OH})+0.5\text{G}\left({\text{H}}_2\right(\text{g}\left)\right)-\text{G}\left({\text{H}}_2\text{O}\right(\text{l}\left)\right)-\text{G}\left(\text{*}\right)$, giving $\Delta {\text{G}}_4=0.310\text{ eV}$ based on the data in Table 1. The free energy of *·O and H⁺ formation on the anode surface after *·OH adsorption is computed as $\Delta {\text{G}}_5=\text{G}(\text{*}·\text{O})+0.5\text{G}\left({\text{H}}_2\right(\text{g}\left)\right)-\text{G}(\text{*}·\text{OH}),\text{ from which}\Delta {\text{G}}_5=0.782\text{ eV}$ from the data in Table 1. The formation free energy of H⁺ and *·OOH formed by the reaction between *·O and H₂O molecules is $\Delta {\text{G}}_6=\text{G}(\text{*}·\text{OOH})+0.5\text{G}\left({\text{H}}_2\right(\text{g}\left)\right)-\text{G}(\text{*}·\text{O})-\text{G}\left({\text{H}}_2\text{O}\right(\text{l}\left)\right)$. From the data in Table 1, $\Delta {\text{G}}_6\text{was obtained as}2.799\text{ eV}$. Finally, the formation free energy of H⁺ and O₂ on the anode surface of *·OOH, given by $\Delta {\text{G}}_7=\text{G}\left(\text{*}\right)+\text{G}\left({\text{O}}_2\right(\text{g}\left)\right)+0.5\text{G}\left({\text{H}}_2\right(\text{g}\left)\right)-\text{G}(\text{*}·\text{O})-\text{G}\left({\text{H}}_2\text{O}\right(\text{l}\left)\right)$, was obtained as $\Delta {\text{G}}_7=1.028\text{ eV}$ using the data of Table 1.

From the above analysis, it was concluded that O₂ on the anode surface can be formed through two pathways: electrocatalytic decomposition of NO₃^– ions and electrocatalytic decomposition of H₂O molecules. Figure 5 shows the potential barrier diagram of NO₃^– ion decomposition on the anode surface. The potential barrier of electron adsorption of NO₃^– ions is almost 0, and that of *NO₃ decomposition into *NO₂ and *·O is negative, indicating that the process can easily spontaneously occur. Meanwhile, NO₂ desorption, *·O to *·OOH conversion, and *·OOH to O₂ conversion must cross potential barriers of 1.125, 2.799, and 1.028 eV, respectively. Figure 6 shows the potential barrier diagram of H₂O decomposition on the anode surface. The conversion of adsorbed H₂O molecules to *·OH, conversion of *·OH to *·O, and subsequent conversion to *·OOH and O₂, like NO₃^– ions, must cross potential barriers of 0.310 eV, 0.782 eV. As NO₃^– conversion to O₂ has a lower potential barrier than H₂O conversion to O₂ on the anode surface, the decomposition rate of NO₃^– exceeds that of H₂O, and NO₃^– decomposition is the main pathway of O₂ generation at the anode.

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Figure 5.

Reaction barrier diagram of NO₃^– ion decomposition on the anode surface.

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Figure 6.

Reaction barrier diagram of H₂O molecule decomposition on the anode surface.

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3.2. Cathodic reaction mechanism

Table 2 lists the thermodynamic parameters of the molecules involved in the cathodic reaction process, calculated using CP2K software. As analyzed in the previous subsection, NH₃OH⁺ ions first dissociate into *NH₂OH and H⁺ ions on the cathode surface. To avoid the CP2K calculation of the free energy of NH₃OH⁺ ions, the formation free energy of the acid-dissociation reaction of NH₃OH⁺ ions Eq. (15) was first calculated using Gaussian software. Next, the formation free energy of Eq. (16) was calculated in CP2K software to obtain the formation free energy of Eq. (8), namely, $\Delta {\text{G}}_8=\Delta {\text{G}}_{15}+\Delta {\text{G}}_{16}$, where the formation free energy of Eq. (15) was calculated as $\Delta {\text{G}}_{15}=0.349\text{ eV}$ and that of Eq. (16) was obtained as $\Delta {\text{G}}_{16}=\text{G}\left({\text{*NH}}_2\text{OH}\right)-\text{G}\left({\text{NH}}_2\text{OH}\right)-\text{G}\left(\text{*}\right)$ $=0.310\text{ eV}$ using the data in Table 2. Therefore, ${\text{G}}_8$was obtained as $0.279\text{ eV}$. The formation free energy of NH₂OH dissociation into *(·NH₂ + ·OH) is $\Delta {\text{G}}_9=\text{G}\left(\text{*}\right(·{\text{NH}}_2+·\text{OH}\left)\right)-\text{G}\left({\text{*NH}}_2\text{OH}\right)$. From the data in Table 2, $\Delta {\text{G}}_9$ was obtained as $-1.954\text{ eV}$. The reaction equations are given by

*(·NH₂ + ·OH) reacts with H⁺ ions to remove an H₂O molecule Eq. (17). Next, *·NH₂ and H⁺ ions combine to form NH₃ Eq. (17) with a formation free energy of $\Delta {\text{G}}_{17}=\text{G}(\text{*}·{\text{NH}}_2)+\text{G}\left({\text{H}}_2\text{O}\right(\text{l}\left)\right)-\text{G}\left(\text{*}\right(·{\text{NH}}_2+·\text{OH}\left)\right)-0.5\text{G}\left({\text{H}}_2\right(\text{g}\left)\right)$. From the data in Table 2, $\Delta {\text{G}}_{17}\text{was obtained as}-0.343\text{ eV}$. Meanwhile, the formation free energy of Eq. (10) is $\Delta {\text{G}}_{10}=\text{G}\left(\text{*}\right)+\text{G}\left({\text{NH}}_3\right)-\text{G}(\text{*}·{\text{NH}}_2)-0.5\text{G}\left({\text{H}}_2\right(\text{g}\left)\right)=-0.388\text{ eV}$ using the data in Table 2. *(·NH₂ + ·OH) reacts with H⁺ ions to remove an NH₃ molecule Eq. (18). The *·OH and H⁺ ions then combine to form H₂O with a formation free energy of $\Delta {\text{G}}_{18}=\text{G}(·\text{OH})+\text{G}\left({\text{NH}}_3\right)-\text{G}\left(\text{*}\right(·{\text{NH}}_2+·\text{OH}\left)\right)-0.5\text{G}\left({\text{H}}_2\right(\text{g}\left)\right)$. From the data in Table 2, $\Delta {\text{G}}_{18}\text{was obtained as}-0.422\text{ eV}$. The formation free energy of Eq. (11) is $\Delta {\text{G}}_{11}=\text{G}\left(\text{*}\right)+\text{G}\left({\text{H}}_2\text{O}\right(\text{l}\left)\right)-\text{G}(\text{*}·\text{OH})-0.5\text{G}\left({\text{H}}_2\right(\text{g}\left)\right)$. From the data in Table 2, $\Delta {\text{G}}_{11}\text{was determined as}-0.310\text{ eV}$. The reaction equations are as follows:

As shown in the above analysis, NH₃OH⁺ ions gradually decompose on the cathode surface. Figure 7 shows the reaction potential-barrier diagram of NH₃OH⁺ decomposition on the cathode surface. The potential barrier of electron adsorption by NH₃OH⁺ ions and subsequent *NH₂OH decomposition is 0.279 eV. As the energy increases during conversion, the reaction cannot easily proceed spontaneously. However, when *NH₂OH decomposes into *(NH₂ + OH), the energy barrier is lowered and this reaction can spontaneously proceed. The energy also decreases when *(·NH₂ + ·OH) converts to *NH₂ and *·OH, but the larger decrease in energy during conversion to *·OH indicates a faster reaction rate of conversion to *·OH than of conversion to *NH₂. Finally, the desorption of adsorbed molecules ·NH₂ and ·OH from the cathode surface can also spontaneously proceed. Therefore, the decisive step in NH₃OH⁺ decomposition is electron adsorption and decomposition of NH₃OH⁺ ions to *NH₂OH. All subsequent decomposition processes can spontaneously occur.

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Figure 7.

Reaction barrier diagram of NH₃OH⁺ ion decomposition on the cathode surface.

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