Yangen
Zhou
,
Huaxiang
Lin
,
Quan
Gu
,
Jinlin
Long
and
Xuxu
Wang
*
Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou 350002, P. R. China. E-mail: xwang@fzu.edu.cn; Fax: +86-591-83779251; Tel: +86-591-83779251
First published on 19th October 2012
Herein, red lead (Pb3O4) is firstly reported to be able to degrade organic pollutants efficiently and simultaneously fix CO2 in aqueous solution under visible light irradiation. CO2 bubbling accelerates both the organic contaminant decomposition and the CO2 fixation. Pb3O4 was partially transformed to Pb3(CO3)2(OH)2 or PbCO3 in the process. Thermal decomposition of the solid products can obtain high concentration of pure CO2 and PbO. Pb3O4 can be regenerated easily by calcinations of PbO in O2 atmosphere. This work provides a possible route to purify organic wastewater and fix CO2 synchronously with visible light illumination.
Pb3O4 is a material with interesting physicochemical characteristics, owing to its mixed valence of Pb ions and unique electronic structure.17,18 Pb3O4 has been studied mainly as an anti-corrosive paint,19 as an electrode in batteries,20,21 and as a selective oxidant.22 Pb3O4 has been rarely studied as a photocatalyst due to the toxicity of Pb,23 although it is a semiconductor responding to visible light (band gap of ca. 2.1 eV).18 The present work shows that Pb3O4 is an excellent material for the photodegradation of organic pollutants and simultaneous fixation of CO2 in aqueous solution under visible light irradiation.
The sample Pb3O4 was prepared by calcining Pb(NO3)2 in flowing oxygen (20 mL min−1) at 753 K for 30 h. Fig. 1A is the XRD pattern of the as-synthesized sample. All the diffraction peaks can be perfectly indexed to the Pb3O4 phase (JCPDS No. 08-0019), indicating that pure Pb3O4 can be obtained by the facile thermal decomposition method.17 SEM and TEM images of the sample show that the prepared powder has an irregular morphology in the size range of ca. 100–800 nm (Fig. 1B and 1C). Further HRTEM analysis (Fig. 1C, inset) indicates that the Pb3O4 sample is a crystal with a d-spacing of 0.295 nm for the (110) plane. The BET surface area of the as-synthesized Pb3O4 powder was ca. 0.7 m2 g−1.
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Fig. 1 XRD patterns (A), SEM image (B), TEM image (C), and UV-Vis diffusive reflectance spectrum (D) of the Pb3O4 samples. Inset in (C) is the HRTEM image of the Pb3O4 sample; Inset in (D) is the (αhv)2versus hv curve. |
The UV-Vis DRS spectrum, as shown in Fig. 1D, suggests that the Pb3O4 sample can absorb solar energy with a wavelength shorter than ca. 580 nm. The optical band-gap energy of the sample was determined by the equation:24
αhν = A(hν − Eg)n/2 | (1) |
Where α, h, v and Eg are absorption coefficient, light frequency, proportionality constant, and band gap, respectively. In the equation, a semiconductor has a direct transition for n = 1 and an indirect transition for n = 4. Only in the case of n = 4 does the (αhv)2versus hv curve show linear behaviour near the band edge, indicating that Pb3O4 is an indirect transition semiconductor. By extrapolating the linear portion of the curve to the hv axis, the Eg of Pb3O4 was estimated to be 2.25 eV (inset in Fig. 1D), which is in good agreement with that reported previously in the literature.18 The position of the conduction band edge of Pb3O4 could be calculated using the following equation:25
ECB = X − EC − 0.5Eg | (2) |
Where EC is the energy of free electrons on the hydrogen scale, X is the electronegativity of the semiconductor, and Eg is the band-gap energy of the semiconductor. The edge of the conduction band (ECB) of Pb3O4 is estimated at −0.08 V (vs NHE) and the edge of the valence band (EVB) is at +2.17 V (vs NHE), which are close to the data determined by electrochemical measurements in the previous literature.21 This EVB is more positive than the oxidation potentials of H2O2 and O3 (1.35 and 2.07 V, respectively), suggesting that photogenerated holes of Pb3O4 have a stronger oxidation ability than H2O2 and O3.
The photodegradation activity of Pb3O4 was evaluated by the degradation of several organic pollutants including Rhodamine B (RhB), Methyl Orange (MO) and salicylic acid (SA) under visible light irradiation (420 nm ≤ λ ≤ 800 nm). Fig. 2A shows the concentration of organic pollutants as a function of time with and without light irradiation. It is seen that the concentrations of all the pollutants have no significant change without irradiation. However, under visible light irradiation, both the RhB solution and the MO solution were discolored completely within 40 min, showing an efficient photooxidation process occurring over Pb3O4. As the organic dyes can adsorb visible light, leading to a self-sensitized reaction,26 the SA solution without any absorption in the visible region was also tested. As shown in Fig. 2A, SA is decomposed efficiently within 100 min. Additionally, phenol can be photooxidized by Pb3O4 under visible light (Fig. S1, ESI†). These results indicate that the degradation of organic compounds is due to the excitation of Pb3O4 by visible light. We compared the performance of Pb3O4 with the well-known TiO2 photocatalyst P25, and Pb3O4 exihibits a much better activity than P25, as shown in Table 1.
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Fig. 2 (A) Variation of RhB, MO, and SA concentrations as a function of time with or without visible light illumination (420 nm ≤ λ ≤ 800 nm). (B) Cycling runs for the photodegradation of RhB over Pb3O4 under visible light irradiation. (C) Photodegradation curves of RhB over Pb3O4 under different reactive atmospheres and under visible light irradiation. The concentration of RhB solution here is 10 μmol L−1, MO is 20 ppm, and SA is 500 μmol L−1. |
Photocatalyst | Specific surface area (m2 g−1) | Particle size (nm) | Light sourcea | Time required for decoloration (min) |
---|---|---|---|---|
a The light source: 500 W halogen lamp for P25, and 500 W halogen lamp equipped with an IR cut-off filter and a UV cut-off filter (420 nm ≤ λ ≤ 800 nm) for Pb3O4. | ||||
P25 | 55 | ∼21 | UV+Vis | 180 |
Pb3O4 | 0.7 | 100–800 | Vis | 40 |
A durability test for Pb3O4 was performed by the degradation of RhB under visible light irradiation. As shown in Fig. 2B, the degradation activity of Pb3O4 has no remarkable decrease in the first seven cycles, but obvious deactivation occurs in the eighth cycle. To understand the cause of the decrease in activity, the sample after the cyclic tests was characterized by XRD. Some new diffraction peaks, attributed to a hydrocerussite Pb3(CO3)2(OH)2 phase, were observed for the sample (Fig. S2, ESI†). It is estimated from the XRD peak intensity that 50% of Pb3O4 is transformed into the Pb3(CO3)2(OH)2. A controlling experiment proved that Pb3(CO3)2(OH)2 is inert for degradation of organic pollutants under visible light. Therefore, the decrease in the activity is due to the transformation of Pb3O4 to Pb3(CO3)2(OH)2 during the photodegradation processes.
Chemically, Pb3O4 is also considered as plumbous plumbate with the chemical formula Pb2[PbO4]. Two lead atoms in Pb3O4 are present in the +2 oxidation state and one lead atom is in the +4 oxidation state, while all the lead atoms in Pb3(CO3)2(OH)2 are in the +2 oxidation state. This implies that Pb3O4 undergoes a reductive reaction to Pb3(CO3)2(OH)2 in the presence of CO2 and H2O during the degradation process. In order to obtain further insight into the reaction mechanism over Pb3O4, a series of additional experiments were carried out. (i) When suspending in pure water, Pb3O4 is unchanged in crystalline structure after visible light irradiation for 10 h, revealing that Pb3O4 is photo-stable in water solution; (ii) in the case of bubbling pure N2 or O2, the photodegradation of RhB over Pb3O4 was considerably inhibited (Fig. 2C); (iii) When CO2 instead of N2 or O2 was bubbled, the photodegradation activity was enhanced remarkably (Fig. 2C). These results reveal that the presence of CO2 is a key factor leading to efficient organic degradation, and also a key factor for the deactivation of Pb3O4 due to the formation of Pb3(CO3)2(OH)2. The different activity of Pb3O4 in N2, O2, air and CO2 atmospheres may be in agreement with the difference in CO2 concentration, which will be discussed in the following section.
Since the redox potential of the ·OH/OH− couple (2.27 V vs. NHE at pH = 7)27 is more positive than the EVB of Pb3O4 (2.17 eV), the photogenerated hole cannot easily oxidize OH− to ·OH. Therefore, the organic photooxidation on Pb3O4 occurs mainly via the direct hole oxidation mechanism.28 This conjecture was well supported by the radical and hole trapping experiments (see ESI†). On the other hand, the redox potential of the O2/O2− couple (−0.16 V vs. NHE at pH = 7)27 is more negative than ECB (−0.08 eV) of Pb3O4, indicating that the reduction of O2 to the active oxygen species O2− was also not feasible. This is why the bubbling of O2 did not promote pollutant photodegradation over Pb3O4. However, we cannot exclude the possibility of O2 molecules capturing the photogenerated electrons. Accordingly, the possible mechanism of the photoreaction can be described as follows:
Pb3O4 + hv → h+ + e− | (3a) |
Organics + H2O + h+ → CO2 + H+ | (3b) |
Pb3O4 + e− + H+ → [PbO] + H2O | (3c) |
[PbO] + CO2 + H2O → Pb3(CO3)2(OH)2 | (3d) |
O2 + H+ + e− →H2O | (3e) |
Firstly, Pb3O4 is excited by visible light to form electrons and holes (3a). Then the organics are oxidized by the holes and mineralized to CO2 and H+ (3b). At the same time, Pb3O4 is reduced by the photogenerated electrons to form [PbO] (3c). With CO2 present in water, [PbO] would convert to Pb3(CO3)2(OH)2 (3d), and the partially photogenerated electrons could be consumed by O2 (3e). In reactions 3c and 3d, the photogenerated electrons reduce Pb4+ to Pb2+, while CO2 plays a crucial role in stabilizing Pb2+ by the formation of Pb3(CO3)2(OH)2. The cooperation of these two reactions leads to a high separation efficiency of the hole–electron pairs and thus efficient degradation of organics. This mechanism is also shown in Scheme 1.
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Scheme 1 The photoreaction mechanism over Pb3O4 under visible light irradiation. |
It is noted that in the above process, the CO2 molecules produced by organics degradation were fixed to Pb3(CO3)2(OH)2. In this sense, the photoreaction over Pb3O4 can be a potential route to fix CO2 and simultaneously purify organic wastewater with high efficiency.
In the absence of any gas bubbling, Pb3O4 needs 5 h to completely remove the RhB (100 μmol L−1) in the first run and 7 h to remove ca. 90% RhB in the second run, under visible light illumination (Fig. 3A). However, with CO2 bubbling, the organics are completely degraded in ca. 1 h, and 90% of RhB is decomposed in ca. 3 h in the second run (Fig. 3B). As expected, the photodegradation activity of Pb3O4 was greatly enhanced by CO2 bubbling. Such a high removal efficiency under visible light illumination for the high concentration of organic contaminants is seldom seen in previous literature about photocatalytic degradation.15,16,29,30
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Fig. 3 (A) Photodegradation curves of RhB solution (100 μmol L−1) over Pb3O4 without any gas bubbling. (B) Photodegradation curves of RhB solution (100 μmol L−1) over Pb3O4 with CO2 bubbling. All the processes were performed under visible light illumination (420 nm ≤ λ ≤ 800 nm). The pH of the solutions was kept at ca. 7 during reactions. |
XRD analysis shows that Pb3O4 is partially transformed into Pb3(CO3)2(OH)2 without any gas bubbling, and to PbCO3 with CO2 bubbling after the second run (Fig. S4, ESI†). It shows that a measurable amount of CO2 is fixed to Pb3(CO3)2(OH)2 or PbCO3 along with the organic pollutants degradation under visible light illumination. The products contained Pb3(CO3)2(OH)2 or PbCO3 can be decomposed thermally below 773 K to PbO and CO2,31,32 which was confirmed by TPD experiments (Fig. S5, ESI†). The amount of CO2 released during the thermal decomposition of the solid products were calculated by the TPD plot of CO2, as listed in Table 2. After treating the same amount of RhB, no matter whether the degradation proceeded in air or CO2 bubbling, the amount of CO2 released from the products was much the same. This result shows that the CO2 fixed in the solid products is from that generated in the oxidation of RhB, although the adscititious CO2 can accelerate the reaction.
Pure phase Pb3O4 can be regenerated by calcination of the PbO in O2 atmosphere at 753 K for 10 h, which was proved by the XRD of the sample. Perhaps by using Pb3O4 as a medium, we could constitute a reaction system to more efficiently remove organic pollutants and synchronously fix CO2, in which Pb3O4 is able to recycle simply and with low-cost.
There is a problem to be mentioned. That is, there exists a Pb residue in the solution, which limits the application of Pb3O4. Therefore, the residual Pb(II) in solution after the purification processes was measured by ICP, as shown in Table 3. It is seen that the amount of Pb dissolved in solution is low, attaining Grade III of Environmental Quality Standards for Surface Water of China (GB3838-2002) (Grade I and II: ≤10 μg L−1, Grade III and IV: ≤50 μg L−1). This can be explained by the formation of Pb3(CO3)2(OH)2 and PbCO3, whose Ksp values are 3.16 × 10−46 and 7.9 × 10−14 respectively.33,34
In conclusion, Pb3O4 can be used for the photodegradation of organic pollutants under visible light irradiation and the reduced species produced in this process can fix CO2 in aqueous solution. Additional CO2 can greatly accelerate the two processes. The products of the reaction, Pb3(CO3)2(OH)2 or PbCO3, can be decomposed thermally to obtain a high concentration of CO2 and PbO. Pb3O4 can be regenerated easily by calcinations of the PbO in an O2 atmosphere. The Pb residue in the solution is very low after the reaction. This is perhaps a new strategy toward highly efficient organic wastewater purification and simultaneous CO2 fixation in mild conditions using sunlight.
Footnote |
† Electronic Supplementary Information (ESI) available: See DOI: 10.1039/c2ra21660d |
This journal is © The Royal Society of Chemistry 2012 |