Hydrogen peroxide is also an essential chemical in various applications such as pulp and paper bleaching, wastewater treatment, chemical synthesis, textile industry and other fields (Figure 1.1) because hydrogen peroxide is considered an excellent oxidizing agent that is only released water. as a by-product (Table 1.1)6. As shown in Figure 1.2, the anthraquinone molecule is first reacted with hydrogen in the presence of a noble metal such as palladium, then it generates hydroquinone molecule. Now, many researchers have studied the production of hydrogen peroxide by a green process such as direct synthesis (Figure 1.3a), electrochemical synthesis (Figure 1.3b), and particulate synthesis for H2O2 (Figure 1.3c).
The electrochemical synthesis is that hydrogen peroxide can be electrically formed via a two-electron reduction of oxygen (ie the oxygen reduction reaction, ORR). Many researchers have studied the development of electrocatalysts to improve their activity, selective and very stable catalysts (Figure 1.5)21.
The SEM image of rutile TiO2 shows the single-crystal oriented nanocrystal on the conducting FTO substrate (Figure 2.15b). The junction current between the cathode and the photoanode was determined from the point of intersection of the LSV curves between the photoanode and the (10-2 Co)- MgAl LDH cathode (Figure 2.17c). Furthermore, the ORR performance of CoAl LDH was also performed in the same condition with (10-2 Co)-MgAl LDH for comparison (Figure 2.18a).
To quantify the amount of H2O2 produced, we observed the concentration of H2O2 over time for 12 hours using the colorimetric method (Figure 2.17d). As shown in PXRD patterns (Figure 3.6), Cd(EDDA) could successfully convert to CdS nanocomposite (M-CdS-xh), which was consistent with the reference pattern JCPDS No. Time (h) . Figure 3.11 Photocatalytic H2O2 production by M-CdS-10h a) with different sacrificial agents and b) with different amounts of 2-propanol.
EDS mapping also indicated that the carbon materials were still well distributed on its M-CdS-10h-A (Figure 3.16). In addition to contact angle measurements, photocatalytic H2O2 degradation tests were performed to further identify the effect of hydrophobicity on the three samples (Figure 3.18a). As shown in Figure 3.18, 47% of the initial concentration of H2O2 on commercial CdS was degraded after 1 hour.
However, the carbon layer in the presence of S dopant showed that the overall band position of the CdS shifted positively towards the valence band (VB) of the S-C structure (Figure 3.21b, bottom).
The selectivity for 2 e-pathways (*-OOH cleavage) is not determined by thermodynamic energy gap (free energy level of O* and H2O2 (l)), but by free energy barriers (ΔGǂ) for each cleavage process measured by CI-NEB method (figure 2.12b). From the linear sweep voltammetry (LSV) curve (Figure 2.17a), the generation current of H2O2 started from 0.74 V (vs. RHE), suggesting a well lower overpotential. In PXRD (Figure 3.4b), it is in line with the simulated pattern representing the as-synthesized Cd(EDDA) has the same structure.
Further, Figure 3.12 shows the performance in alkaline 1 M KOH solution, DI aqueous solution and 0.1 M acidic HCl solution. As shown in Figure 3.13b, it shows the good linearity of the K-L graphs and the n value of M-CdS- 10h is approximately 2.5, indicating that M-CdS-10h is preferred by the two-electron pathway to produce H2O2. Further, the kinetics of H2O2 production and decomposition were estimated by the equation of [H2O2] = (kf/kd), where kf and kd are the rate constants of H2O2 formation and decomposition (Figure 3.18b).
For the simulation model structure, sp3-type carbon was placed on bare CdS, carbon-doped CdS (C@CdS) and surfur-doped carbon CdS (110) surface structure (S-C@CdS) (Figure 3.20). In CdS/C, the valence band states (electron occupied region) of carbon partially overlapped in the conduction band of CdS (Figure 3.21b, middle), indicating that the generated holes could be transferred to the carbon layer and trapped on this layer. PXRD pattern and XPS spectra also showed that M-CdS-10h-A was partially oxidized but still retained their state and surface properties after the recyclability test (Figure 3.23).
To identify the saturated concentration of H2O2 for three photocatalyst samples, the reaction time was extended to 24 hours (Figure 3.22b).
Result & Discussions
Electrocatalytic performance of CoMgAl LDH
As synthesized, the Co-controlled MgAl LDH was tested for its electrocatalytic capabilities to confirm their activities and selectivities of H2O2 by RRDE. The ORR was performed in 0.1 M KOH at pH 13, rotating at 1600 rpm. ORR mechanisms can follow a two- or a four-electron path44. The remarkable H2O2 selectivity can be determined by depending on whether the oxygen bond is broken during the ORR process, thereby suppressing water formation (U0O2/H2O = 1.23 V)27.
To accelerate the 2e pathway for ORR to H2O2, which is the goal of this study, we used (Co)-MgAl LDHs as atomically dispersed electrocatalysts. Among these electrocatalysts (Figure 2.8a), the (10-2 Co)-MgAl LDH sample exhibited -0.8 mA/cm2 at 0.45 V (vs. RHE), which has the lowest activity than others due to low active places. However, the current activity was improved as the content of the cobalt atom increased, because transition metals such as cobalt are very efficient for ORR.
However, the selectivity decreased with increasing cobalt content due to oxygen coordination45. For CoAl LDH, its cobalt coordination allowed the breaking of the O=O bond due to the coordination change of the oxygen bonds. For transition metal nanoparticle sites, the O=O double bond can be dissociated with agglomerated active sites.
Understanding the reaction of CoMgAl LDH
∆E is the total energy change from DFT calculations, ∆ZPE are zero point energies and T∆S are entropic energies. Further investigation could verify that the number of Al atoms right next to the active O atom increases the overpotential of OOH*. The center of the unit cell was occupied by the active oxygen site where the oxygen reduction reaction took place.
The red atom indicates O, the light blue and green atoms are OH and CO32-, respectively.). After OOH* has been adsorbed, there are 2 possible sequential reactions: 4 e- process through (II) reaction producing O* and OH- molecule (O*-OH cleavage) and 2 e- process through (II') reaction , which desorbs OOH- molecules from the active site (*-OOH cleavage) shown in the figure. When it comes to Co-MgAl LDH with a low Co ratio, the 3Co sample would occupy a small amount, giving a high percentage of the total selectivity of 2e.
However, the population with 3Co composition grows for Co-MgAl LDH with a high ratio, so that the global 2e selectivity decreases. Thus, inhibiting Co-aggregation could improve H2O2. selectivity by controlling the Co ratio infinitely small compared to Mg and Al atoms. The blue line indicates the 4e path with O*–OH bond cleavage and the red line means *–OOH cleavage 2e path.
Solar-assisted production of hydrogen peroxide
The photoanode creates an electron under 1 solar illumination and the electron produced provided the photocatalytic production of H2O2 in the cathode cell. The cathode and photoanode were then combined in a two-compartment system with a copper wire to generate hydrogen peroxide without any bias. It produced only 4.68 mM H2O2 which was 4.4 times lower, indicating that the clustering of the Co metal atom could suppress the oxygen reduction reaction to generate H2O2.
Metal-Organic Framework (MOF)
High-performance H2O2 production achieved by sulfur-doped carbon on CdS photocatalyst via inhibition of reverse H2O2 decomposition. Metal-organic frameworks (MOFs) have received a lot of attention today due to their morphological properties as porous materials that have a high surface area. MOFs are porous materials with a network structure composed of metal ions or clusters and organic ligands56.
For example, MOFS have great potential to capture CO2 gas due to their controllable pore size and high selectivity57. The carbon matrix on CdS could suppress the decomposition of H2O2 due to their hydrophobic nature. These nanocomposite catalysts show improved generation performance and retained the self-decomposition of H2O2 compared to commercial CdS.
Result & discussions
- Photocatalytic performance of photocatalyst
- Understanding the effect of sulfur-doped carbon
- Stability of photocatalyst
Different types of sacrificial agent, different amounts of sacrificial agent and the effect of pH on M-CdS-10h were obtained to further optimize the performance. To verify these properties, the contact angle DI of water and H2O2 solution (35 wt%) was measured to estimate the hydrophobicity of the photocatalysts (Figure 3.14). In addition, M-CdS-10h was re-annealed to further improve the hydrophobicity because the first temperature during thermolysis, 270℃, could not be effective to stabilize the polymeric carbon matrix on their surface.
Therefore, M-CdS-10h was additionally annealed at high temperature under Ar atmosphere for the stabilization of carbon on its surface completely, which showed some dark brown-green solid (denoted M-CdS-10h-A). In spectra of Cd 3d5/2, the main peak of cadmium ions in CdS nanocomposites appeared at 405.1 eV and the second peak of 404.1 eV could be indicated to cadmium oxide which was similar from M-CdS-10h. In spectra of S 2p, the peaks at 169.3 eV (C- SOx) are significantly reduced compared to M-CdS-10h due to reduction at high temperature.
From the XPS spectra, it could be found that the sulfur atoms in M-CdS-10h-A, especially the C-S-C form, become largely visible in the carbon residue. In contrast, M-CdS-10h-A showed the lowest H2O2 degradation after 1 h, resulting in only 8.1% degradation for the same conditions, and M-CdS-10h showed degradation of 20% of the initial H2O2 concentration. Interestingly, M-CdS-10h showed a large decrease in efficiency during the recyclability test, while M-CdS-10h-A still showed similar H2O2.
However, the concentration of H2O2 in M-CdS-10h showed 17.1 mM, which shows 9.3 times much higher than the performance of commercial CdS.
The carbon matrix could also limit the rapid saturation of the H2O2 concentration, showing a gradual increase in the formation of H2O2. Hydrogen peroxide (H2O2) is an environmentally friendly liquid material due to its good oxidizing properties, so it can be used for various purposes. Here, an electrocatalyst for high selectivity of oxygen reduction reaction to 2 e-pathway and photocatalyst for high performance of H2O2 generation under visible light are introduced.
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