Catalyzed by Cuprous Oxide
3.3. Results and discussion
To determine the highest electrochemical reduction of CO2 among three different electrolytes and Cu2O particles in three different morphologies, we analyzed the electrochemical performances by cyclic voltammetry and potentiostatic method.
3.3.1. Results from cyclic voltammetry
A. The influence of three different electrolytes on the electrochemical reaction of CO2
Figure 3.6 exhibits the current-potential curves with the Cu2O particles (diameter of 1570 nm) catalyzed gas diffusion electrode in three different electrolytes under N2 gas flowing. The current density from these three electrolytes at the potential of -1.7 V revealed substantial differences. The electrolyte of NaOH presented the highest current density of -15 mAcm-2, while the electrolyte of CaCl2 showed a current density of -8.82 mAcm-2. In contrast, the electrolyte of NaHCO3 exhibited the lowest current density of -4.6 mAcm-2. The current-potential curves in figure 3.7 were the electrochemical performances of the Cu2O particles (diameter of 1570 nm) catalyzed gas diffusion electrode in three different electrolytes under CO2 gas flowing.
Similarly as above, the electrolyte of NaOH demonstrated the highest current density of -5.14 mAcm-2, while the electrolyte of CaCl2 obtained a current density of -4.64 mAcm-2. Likewise, the electrolyte of NaHCO3 showed a current density of -4.14 mAcm-2. From figure 3.6 to 3.11 we arrived at some conclusions. First, the NaOH as the electrolyte revealed the highest current density at -1.7 V. Second, the current density of CO2 bubbling at -1.7 V was lower than that of N2 bubbling at identical conditions and setups.
B. The influence of Cu2O particles in three different morphologies on the electrochemical reaction of CO2
The Cu2O particles with a diameter of 170 nm revealed the highest current density at -1.7 V in NaHCO3 aqueous solution (shown in figure 3.12). Figure 3.13 exhibits that the Cu2O particles with a diameter of 170 and 640 nm demonstrating similar densities of -5.5 mAcm-2 at -1.7 V in NaOH aqueous electrolyte. When the electrochemical reactions were conducted in the CaCl2 electrolyte, the Cu2O particles with a diameter of 1570 nm showed the highest current density. This is to our surprise that the larger particles of Cu2O resulted in higher current densities in this case (figure 3.13). Table 3.1 lists the current density at -1.7 V with different sizes of Cu2O particles in three different electrolytes under constant N2 or CO2 bubbling.
C. The catalytic ability of the Cu2O particles on the electrochemical reaction of CO2
We observed that the electrolysis under constant N2 bubbling arrived at higher current densities than under CO2 bubbling. We realized that when the electrolysis was conducted under the N2 atmosphere, at the working electrode exclusively H2 was evolved and no CO2 was being reduced. Hence, the current density recorded under N2 gas bubbling was attributed solely to water reduction. The CV curves of figure 3.15 demonstrate two interesting points. First, the Cu2O is not the catalyst for water electrolysis because there was negligible current response observed. Second, the Cu2O itself is not reduced in the potential range from -0.4 to -1.7 V since there was no reduction peak recorded. Figure 3.16 presents the electrochemical reduction of CO2 with and without Cu2O in 0.5 M NaOH aqueous solution. From the CV curves we observed that the Cu2O catalyzed the electrochemical reduction of CO2 because a much larger current was resulted over that of non-Cu2O electrode.
3.3.2. Result from potentiostatic measurement
A. The influence of three different electrolytes on the stability of the electrochemical reaction of CO2
Through figure 3.17, 3.18, and 3.19 we determined that the electrochemical reactions occurring in the CaCl2 electrolyte were
unstable. When the electrolyte was NaHCO3 instead, the current density revealed negligible change after electrochemical reactions for 5 hours. The current density for the electrochemical reactions in the NaOH at -1.7 V became unstable for the first hour and then the current density was stabilized. From EDX we determined that the electrode in the CaCl2 electrolyte was covered with calcium after 5 hours, which effectively decreased the current response (shown in figure 3.20). Table 3.2 lists the average current densities for three different Cu2O particles in three different electrolytes under the CO2 gas flowing at -1.7 V for 5 hours. When the NaOH was used as the electrolyte, most of the current densities recorded were higher than those of NaHCO3 and CaCl2. The only exception was the Cu2O particles with a diameter of 170 nm in NaHCO3, which demonstrated marginally higher current density than that in NaOH.
B. The influence of Cu2O particles in three different morphologies on the stability of the electrochemical reaction of CO2
When the NaHCO3 was used as the electrolyte, the Cu2O particles with a diameter of 170 nm resulted in the highest current density. The larger Cu2O particles obtained the higher current densities when the NaHCO3 was used as the electrolyte (shown in figure 3.21). The current densities for the Cu2O particles with different sizes showed negligible change when the NaOH was used as the electrolyte (shown in figure 3.22). The current density for the Cu2O particles with a diameter of 170 nm in the CaCl2 electrolyte demonstrated the highest current
density about -7.3 mAcm-2 at -1.7 V and its current decreased rapidly with the precipitation of calcium on the electrode. In the CaCl2 electrolyte, the Cu2O with particle sizes of 640 nm demonstrated more stable current density than the particle sizes of 1570 and 640 nm (shown in figure 3.23).
3.4. Conclusions
The electrochemical reductions of CO2 with the Cu2O catalyzed gas diffusion electrode in different electrolytes of 0.5 M at ambient temperature were studied. Combinations with Cu2O particles in three different sizes (1570, 640, and 170 nm) and three distinct electrolytes (NaHCO3, NaOH, and CaCl2) were explored. The highest current density recorded was -4.82 mAcm-2, with the condition of the Cu2O particles at a diameter of 640 nm in the 0.5 M NaOH aqueous electrolyte for 5 hours electrochemical reduction of CO2 at -1.7 V.
The electrolyte of NaHCO3 demonstrated the most stability in which the current density revealed negligible change during the electrochemical testing.
The smaller Cu2O particles resulted in higher average current density when the NaHCO3 or CaCl2 was used as the electrolyte. The Cu2O particles presented notable catalytic abilities for electrochemical reduction of CO2.
Figu elec
(a)
(b)
ure 3.1.
ctrolysis ce )
The photo ell.
ograph (a)) and thee schematic diagramm (b) of the
Figure 3.2. Illustration of step involved in gas diffusion electrode fabrication.
(a) red (b) orange (c) yellow
Figure 3.3. The images for the gas diffusion electrode catalyzed with the Cu2O particles of (a) 1570, (b) 640, and (c) 170 nm.
(
ctrode catallyzed withh the
Figure 3.6. The CV curves of the electrochemical reactions catalyzed by the Cu2O particles with a diameter of 1570 nm in different electrolytes under constant N2 gas flowing.
Figure 3.7. The CV curves of the electrochemical reactions catalyzed by the Cu2O particles with a diameter of 1570 nm in different electrolytes under constant CO2 gas flowing.
Figure 3.8. The CV curves of the electrochemical reactions catalyzed by the Cu2O particles with a diameter of 640 nm in different electrolytes under constant N2 gas flowing.
Figure 3.9. The CV curves of the electrochemical reactions catalyzed by the Cu2O particles with a diameter of 640 nm in different electrolytes under constant CO2 gas flowing.
Figure 3.10. The CV curves of the electrochemical reactions catalyzed by the Cu2O particles with a diameter of 170 nm in different electrolytes under constant N2 gas flowing.
Figure 3.11. The CV curves of the electrochemical reactions catalyzed by the Cu2O particles with a diameter of 170 nm in different electrolytes under constant CO2 gas flowing.
Figure 3.12. The CV curves of the electrochemical reactions catalyzed by the Cu2O particles with different diameters in NaHCO3 acted as the electrolyte under constant CO2 gas flowing.
Figure 3.13. The CV curves of the electrochemical reactions catalyzed by the Cu2O particles with different diameters in NaOH acted as the electrolyte under constant CO2 gas flowing.
Figure 3.14. The CV curves of the electrochemical reactions catalyzed by the Cu2O particles with different diameters in CaCl2 acted as the electrolyte under constant CO2 gas flowing.
Figure 3.15. The CV curves of the electrochemical reactions with and without the Cu2O particles in NaOH acted as the electrolyte under constant N2 gas flowing.
Figure 3.16. The CV curves of the electrochemical reactions with and without the Cu2O particles in NaOH acted as the electrolyte under constant CO2 gas flowing.
Table 3.1. The current density at -1.7 V with Cu2O particles in different sizes in three different electrolytes under N2 or CO2 constant bubbling.
gas Particle
size NaHCO3 NaOH CaCl2
N2
1570 nm -4.61 mAcm-2 -15 mAcm-2 -8.82 mAcm-2 640 nm -4.75 mAcm-2 -14 mAcm-2 -6.68 mAcm-2 170 nm -4.97 mAcm-2 -13.6 mAcm-2 -8.36 mAcm-2
CO2
1570 nm -4.14 mAcm-2 -5.14 mAcm-2 -4.64 mAcm-2 640 nm -4.01 mAcm-2 -5.51 mAcm-2 -4.28 mAcm-2 170 nm -4.75 mAcm-2 -5.5 mAcm-2 -3.85 mAcm-2
Figure 3.17. The current-time curves for the electrochemical reactions catalyzed by the Cu2O particles with a diameter of 1570 nm in the different electrolytes under constant CO2 gas flowing at -1.7 V.
Figure 3.18. The current-time curves for the electrochemical reactions catalyzed by the Cu2O particles with a diameter of 640 nm in the different electrolytes under constant CO2 gas flowing at -1.7 V.
Figure 3.19. The current-time curves for the electrochemical reactions catalyzed by the Cu2O particles with a diameter of 170 nm in the different electrolytes under constant CO2 gas flowing at -1.7 V.
Figure 3.20. The EDX images before (a) and after (b) electrochemical reaction for 5 hours in the CaCl2 electrolyte.
Figure 3.21. The time-current curves for the electrochemical reactions catalyzed by the Cu2O particles with different diameters in the NaHCO3 electrolyte under constant CO2 gas flowing at -1.7 V.
Figure 3.22. The current-time curves for the electrochemical reactions catalyzed by the Cu2O particles with different diameters in the NaOH electrolyte under constant CO2 gas flowing at -1.7 V.
Figure 3.23. The current-time curves for the electrochemical reactions catalyzed by the Cu2O particles with different diameters in the CaCl2 electrolyte under constant CO2 gas flowing at -1.7 V.
Table 3.2. The average current densities for the Cu2O particles in three different sizes in three different electrolytes under the constant CO2 gas flowing at -1.7 V for 5 hours electrochemical reaction.
NaHCO3 NaOH CaCl2
1570 nm -4.19155 mAcm-2 -4.61581 mAcm-2 -2.61968 mAcm-2 640 nm -4.41833 mAcm-2 -4.81755 mAcm-2 -3.02033 mAcm-2 170 nm -4.73211 mAcm-2 -4.72106 mAcm-2 -3.533 mAcm-2
Chapter 4.
Photocatalytic Properties of Cuprous Oxide
4.1. Introduction
Photocatalysis is the phenomenon that occurs on the surface of a semiconductor by the photoinduced electron-hole pairs. Photocatalyst is a material that is used to accelerate the desired photoreaction. In chapter 1 we have already described some semiconductors that could act as photocatalysts.
The most widely known photocatalyst is TiO2, which promotes the photochemical reaction of some organic and inorganic compounds only under UV light region. The Cu2O as a semiconductor with a direct bandgap of 2.14 eV is expected to accelerate the photoreaction of some organic or inorganic compounds under visible light irradiation.
We have already discussed methods in fabricating different sizes and shapes of Cu2O particles in chapter 2. In this chapter, we concentrate on the effect of the photocatalysts morphologies and light sources on the photocatalysis processes in Cu2O. The Cu2O particles with diameters of 1570, 640, and 170 nm were chosen as photocatalysts. Photocatalytic degradation of methyl orange in an aqueous solution containing the Cu2O was investigated under one 150 W halogen lamp (yellow light), or two to four 27 W fluorescent lamp (white light). The photocatalytic efficiencies were estimated by recording the primary absorption peak of methyl orange with a UV-Vis spectrophotometer. We determined that the photochemical process catalyzed by Cu2O particle with a diameter of 170 nm under single 150 W halogen lamp
demonstrated the highest photocatalytic efficiency.
The materials and the photocatlytic processes of Cu2O particles are mentioned in the experimental section (Section 4.2). The degradation abilities of the photocatalysts with three different morphologies are discussed in section 4.3. The conclusions for this work are provided in section 4.4.
4.2. Experimental
4.2.1. Reagents
1. As-prepared Cu2O particles with a diameter of 1570 nm Method: method A and aging for 6 hours
Reagents: CuCl2 (0.005 M), PEG (200) (2 M), NaOH (2 M), and LAAS (0.05 M)
2. As-prepared Cu2O particles with a diameter of 640 nm Method: method A and aging for 6 hours
Reagents: CuCl2 (0.005 M), PEG (200) (0.002 M), NaOH (0.2 M), and LAAS (0.05 M)
3. As-prepared Cu2O particles with a diameter of 170 nm
Method: method A and aging for 6 hours
Reagents: CuCl2 (0.005 M), PEG (200) (0.002 M), NaOH (0.02 M), and LAAS (0.05 M)
4. Methyl Orange FluKa
4.2.2. Determination in photocatalytic ability
10 mg of the Cu2O particles was put into the vessel covered with an aluminum foil. 100 mL of 10 mgL-1 methyl orange (MeO) solution was added to the vessel followed by ultrasonication for 30 min in darkness. The samples were irradiated with light from a 150 W halogen lamp or four to two 27 W fluorescent lamps. The distance between the sample and light source was 5 cm. Photocatalysis results were evaluated by detecting the absorbance of the solutions with a dual beam UV-Vis spectrophotometer (HITACHI U-3300 Spectrophotometer) at 464 nm, which is the maximum absorbance of MeO.
4.3. Results and discussion
In this section, we discuss the influence of the particles morphologies and light sources on the photocatalytic process.
4.3.1. The influence of the catalysts on the degradation ability of methyl orange
The Cu2O particles in three different diameters of 1570, 640, and 170 nm were used as the photocatalysts. As shown in figure 4.2, 4.3, and 4.4 we determined that the Cu2O particles with a diameter of 1570 nm did not possess the ability to accelerate the desirable photochemical reaction since there was negligible reduction in the absorption peak at 464 nm. Fesult from figure 4.5, 4.6, and 4.7 suggested that the Cu2O particles with a diameter of 640 nm revealed limited catalytic ability because the MeO absorption peak was not
reduced notably after 6 hours of irradiation under different light sources. In contrast, we obtained the information that the Cu2O particles with a diameter of 170 nm exhibited considerable photocatalytic abilities from figure 4.8, 4.9, and 4.10. The Cu2O particles with a diameter of 170 nm revealed the highest photocatalytic ability under the irradiation of one 150 W halogen lamp. After 6 hours in irradiation, 31.7 % of MeO was decolorized. Result from figure 4.2 to 4.10 indicated that the morphologies of the Cu2O particles played an important role in the photochemical reaction. As expected, the particles with the smallest size provided the highest photocatalytic ability.
4.3.2. The influence of light on the degradation ability of methyl orange
Three different light sources were explored to estimate the influence of the irradiation energy on the degradation ability of MeO. Figure 4.11 provides the results for the degradation of MeO catalyzed by Cu2O particles with a diameter of 170 nm under a single 150 W halogen lamp. We determined the degradation of MeO was 68.3 %. The Cu2O particles with a diameter of 1570 and 640 nm did not possess photocatalytic ability under one 150 W halogen lamp. As shown in figure 4.12 and 4.13 we obtained the results that there was minimum photochemical reaction when the Cu2O particles with diameters of 1570 and 640 nm were tested under four or two 27 W fluorescent lamps irradiations. The 27.7 % MeO decolorization was obtained by using the Cu2O particle with diameter of 170 nm under four florescent lamps irradiation.
When the lamps were reduced from four to two, the decolorization of MeO was decreased from 27.7 to 5.5 %. As shown in figure 4.14, higher irradiation energy led to higher degradation ability, which is entirely expected.
4.4. Conclusions
After careful analysis, we arrived at several conclusions. First, the Cu2O particles with a diameter of 170 nm presented higher photocatalytic abilities than those of diameters in 640 and 1570 nm. Second, the irradiation energy influenced the photochemical reaction. When the Cu2O particles with diameters of 640 and 1570 nm were employed as photocatalysts, the colors of the MeO solutions revealed negligible change after 6 hours of irradiation under different light sources. The highest degradation of MeO was acquired when the MeO solution containing the Cu2O particle with a diameter of 170 nm was irradiated under one 150 W halogen lamp for 6 hours.
Figure 4.1. Illustration of experimental determination in photocatalytic ability.
Figure 4.2. The photocatalytic ability of Cu2O particles with a diameter of 1570 nm under one 150 W halogen lamp.
Figure 4.3. The photocatalytic ability of Cu2O particles with a diameter of 1570 nm under four 27 W fluorescent lamps.
Figure 4.4. The photocatalytic ability of Cu2O particles with a diameter of 1570 nm under two 27 W fluorescent lamps.
Figure 4.5. The photocatalytic ability of Cu2O particles with a diameter of 640 nm under one 150 W halogen lamp.
Figure 4.6. The photocatalytic ability of Cu2O particles with a diameter of 640 nm under four 27 W fluorescent lamps.
Figure 4.7. The photocatalytic ability of Cu2O particles with a diameter of 640 nm under two 27 W fluorescent lamps.
Figure 4.8. The photocatalytic ability of Cu2O particles with a diameter of 170 nm under one 150 W halogen lamp.
Figure 4.9. The photocatalytic ability of Cu2O particles with a diameter of 170 nm under four 27 W fluorescent lamps.
Figure 4.10. The photocatalytic ability of Cu2O particles with a diameter of 170 nm under two 27 W fluorescent lamps.
Figure 4.11. The degradation of MeO with Cu2O particles as the photocatalysts under one 150W halogen lamp.
Figure 4.12. The degradation of MeO with Cu2O particles as the photocatalysts under four 27 W fluorescent lamps.
Figure 4.13. The degradation of MeO with Cu2O particles as the photocatalysts under two 27 W fluorescent lamps.
Figure 4.14. The degradation of MeO catalyzed by Cu2O particles with diameter of 200 nm under different light sources.