Abstract
The photocatalyzed water splitting reaction in aqueous methanol solution is an efficient preparation method for hydrogen and methanal under mild conditions. In this work, metal sulfide-loaded TiO2 photocatalysts for hydrogen and methanol production were synthesized by hydrothermal method (180°C/12 h) and characterized by X-ray diffraction (XRD), UV-visible diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). The crystal structures of the samples are the typical anatase phase of TiO2 and exhibit a spherical morphology. When TiO2 was loaded with CoS, ZnS, and Bi2S3, respectively, the resulting catalysts showed photocatalytic activities for water decomposition to hydrogen in aqueous methanol solution under 300 W Xe lamp irradiation. Among the photocatalysts with various compositions, the 20 wt% CoS/TiO2 sample with a 2.1 eV band gap showed the maximum photocatalytic activity for the photocatalytic reaction, which indicated that CoS improved the separation ratio of photoexcited electrons and holes. The enhanced activity can be attributed to the intimate junctions that are formed between CoS and TiO2, which can reduce the electron-hole recombination. The production rate of hydrogen with 20 wt% CoS/TiO2 photocatalyst was about 5.6 mmol/g/h, which was 67 times higher than that of pure TiO2. The formation rate of HCHO was 1.9 mmol/g/h with 98.7% selectivity. Moreover, the CoS/TiO2 photocatalyst demonstrated good reusability and stability. In the present study, it is demonstrated that CoS can act as an effective cocatalyst to enhance the photocatalytic hydrogen and methanal production activity of TiO2. The highly improved performance of the CoS/TiO2 composite was mainly ascribed to the efficient charge separation.
1. Introduction
Photocatalytic water splitting into hydrogen, a renewable, clean-burning, and environmental-friendly fuel for future energy sources, is considered as one of the most significant and attractive solutions to solve the global energy and environmental problems [1–3]. A previous study found that adding methanol (CH3OH) to pure water can dramatically enhance H2 production, suggesting that CH3OH plays a crucial role in H2 production [4]. Methanol is used as a raw material for the industrial production of methanol through an oxidation reaction using Ag, Cu, or V2O5 as catalysts. However, this process requires high temperatures of 700–900 K and expensive catalysts. Photocatalytic production of both hydrogen and methanol from aqueous methanol solution using photocatalysts is an efficient approach to address the above problems. Moreover, the photocatalytic reaction conditions are mild compared to industrial methods. Fujishima and Honda first observed the splitting of water at a TiO2 electrode under the irradiation of ultraviolet (UV) light in 1972 [5]. Since then, TiO2 is considered one of the most promising semiconductor photocatalysts due to its superior photo reactivity, nontoxicity, long-term stability, and low cost [1, 6]. TiO2 has also received a lot of attention as a photocatalyst for hydrogen production [7, 8]. However, the photocatalytic decomposition of water on pure TiO2 photocatalyst is ineffective. One reason is that the production of hydrogen is limited by the rapid recombination of photoexcited holes and electrons. To improve the photocatalytic efficiency, one of the effective strategies is to develop cocatalyst-modified photocatalysts [9–13]. According to previous research, CdS photocatalysts facilitate the production of H2 by promoting the separation of photoexcited electrons and holes [14–16]. However, CdS is noxious, environmentally hazardous, and costly [17]. Therefore, developing suitable photocatalysts for H2 production is important and extremely urgent. Our research is focused on the development of nontoxic, environmentally friendly, and inexpensive promoters, such as CoS, ZnS, and Bi2S3. Although metal sulfides have demonstrated high activity in H2 involving reactions in heterogeneous catalysis, CoS has rarely been used as a cocatalyst in photocatalytic H2 production. In our research, different photocatalysts were successfully prepared through the hydrothermal method and were characterized using XRD, UV-visible DRS, SEM, and EDX analyses. CoS, ZnS, and Bi2S3 were investigated as cocatalysts for photocatalytic H2 and methanal production from methanol solution under 300 W Xe lamp irradiation. The stability and reusability of the catalyst were also evaluated.
2. Experimental
2.1. Catalyst Preparation
All the chemicals were of reagent grade and used as received without any further purification.
The metal sulfide samples were prepared by the hydrothermal method [18]. In a typical procedure, 5 mL deionized water and 20 mL ethyl alcohol were stirred at room temperature for 0.5 h. Different metal salts (Co(NO3)3·6H2O, 1.46 g; Zn(CH3COO)2, 0.92 g; Bi(NO3)3·5H2O, 1.62 g) and thiourea ((NH4)2S, 0.38 g), which were used as the Co, Zn, Bi, and S precursors, were added to the above solution. The mixture was continuously stirred for 0.5 h and ultrasonicated for 0.5 h to obtain a well-mixed solution. The mixture was then transferred to a Teflon-lined autoclave and heated at 180°C for 12 h. The resulting precipitate was collected by centrifugation and washed successively with distilled water and ethanol three times to remove unbound impurities. It was then dried at 60°C in air for 12 h and ground for 1 h.
The metal sulfide-loaded TiO2 samples were prepared by the hydrothermal method [19]. In a typical procedure, different amounts of CoS powder were dissolved in 1 mL tetrabutyl titanate and 5 mL ethyl alcohol, and the mixture was stirred at room temperature for 0.5 h. Then, 20 mL deionized water and ammonia water (to adjust pH = 10) were added to the above solution. The solution was continuously stirred for 0.5 h and ultrasonicated for 0.5 h to achieve a well-mixed solution. The mixture was then transferred to a Teflon-lined autoclave and heated at 180°C for 12 h. The resulting precipitate was collected by centrifugation and washed successively with distilled water and ethanol three times to remove unbound impurities. The product was then dried at 60°C in air for 12 h and ground for 1 h and labeled as CoS/TiO2.
The procedure for the preparation of TiO2, ZnS/TiO2, and Bi2S3/TiO2 was the same as that for CoS/TiO2, except for the different precursors.
2.2. Characterization of Catalysts
The phase compositions of the samples were determined from their XRD patterns, which were obtained using an X’Pert X-ray diffractometer (PANalytical, Netherlands) using Cu Kα radiation ( nm) at a scan rate of 2°/min from 20° to 80° (). The accelerating voltage and applied current were 40 kV and 30 mA, respectively [20].
The micro structures of the samples were determined using SEM images obtained at an accelerating voltage of 20 kV using a ZEISS SIGMA instrument.
The UV–vis diffuse reflection spectra (DRS) were recorded using a Varian Cary 500 Scan UV–vis–NIR spectrometer with BaSO4 as the reference sample. The reflectance spectra were transformed into absorption intensity by using Kubelka-Munk method.
2.3. Catalytic Performance
The photocatalytic reactions were carried out in a sealed quartz tube reactor (volume, 25 mL). The light source was a 300 W Xe lamp. The solid catalyst powder (25 mg) was ultrasonically dispersed in 5.0 mL of mixed solution containing 76 wt% CH3OH and 24 wt% H2O. Then, the reactor was evacuated and filled with high-purity (99.999%) nitrogen. The photocatalytic reaction was carried out at room temperature for 12 h. After the reaction, the liquid products were analyzed by high-performance liquid chromatography (HPLC, Shimadzu LC-20A) with both refractive index and UV detectors. The stationary phase was a Shodex SUGARSH-1011 column (8 × 300 mm) and the mobile phase was a dilute H2SO4 aqueous solution. H2 contents were analyzed by an Agilent Micro GC3000 equipped with a molecular sieve 5A column and a high-sensitivity thermal conductivity detector [21].
3. Results and Discussion
3.1. Characterization of the Samples
The crystalline phases of the samples were characterized by their XRD patterns. Figure 1 shows the XRD patterns of CoS/TiO2, ZnS/TiO2, and Bi2S3/TiO2 nanoparticles. For all the samples, the peaks at , 37.6°, 48.0°, 53.8°, 55.0°, and 62.7° can be attributed to the typical anatase phase of TiO2 (JCPDS: 21–1272) [22]. The XRD patterns show that the loading of metal sulfide nanoparticles did not change the crystal structure of TiO2. The peaks at and 29.9° with low intensity can be attributed to CoS [23]. The peak at with a low intensity is due to ZnS [24]. The peaks at , 29.3°, and 32.5° with low intensity can be attributed to Bi2S3 [25].
3.2. UV–Vis Diffuse Reflection Spectra (DRS)
Figure 2 shows the DRS of TiO2, CoS/TiO2, ZnS/TiO2, and Bi2S3/TiO2 samples. It can be observed from the spectra that the metal sulfide-loaded TiO2 samples have enhanced absorption in the visible-light region compared to pure TiO2. Specifically, CoS/TiO2 showed stronger absorption than ZnS/TiO2 and Bi2S3/TiO2. Compared to pure TiO2 and metal sulfide-loaded TiO2, the broader absorption bands can be attributed to the type of loaded metal sulfide nanoparticles. The (ahv)1/2 vs (hv) spectra were obtained from the corresponding diffuse reflectance spectra by means of the Kubelka-Munk function [26]. Figure 3 shows the curves of (ahv)1/2 vs (hv) for the samples. By extrapolating the linear portion of the curves to (ahv)1/2 = 0, the values of TiO2, CoS/TiO2, ZnS/TiO2, and Bi2S3/TiO2 were determined to be 3.4 eV, 2.1 eV, 3.2 eV, and 2.4 eV, respectively. As a result, CoS/TiO2 has the largest visible light absorption capacity and the smallest band gap energy. This result is consistent with the fact that the increase in wavelength range of absorption edge in semiconductors is related to the decrease in optical absorption edge energy.
3.3. Morphologies of Samples
SEM and EDX analyses of the samples were carried out to determine the morphologies, polycrystalline structure, and elemental composition of the samples. The SEM images of 20 wt% CoS/TiO2, 40 wt% CoS/TiO2, and 60 wt% CoS/TiO2 samples are presented in Figures 4(a)–4(c), respectively. It can be seen in Figure 4(a) that most of the crystallites are spherical, and their morphologies are almost the same. It can be seen in Figures 4(b) and 4(c) that the crystallite shape transforms from particles to platelets with increase in content of CoS. The SEM images indicate that 20 wt% CoS/TiO2 nanoparticles showed the best dispersion among all the samples. The EDX spectrum in Figure 4(d) for 20 wt% CoS/TiO2 sample shows the signals of Ti, O, Co, and S elements.
(a)
(b)
(c)
(d)
3.4. Photocatalytic Performance of Samples
The photocatalytic activities of TiO2, CoS/TiO2, ZnS/TiO2, and Bi2S3/TiO2 samples were evaluated using the photocatalytic hydrogen generation reaction in aqueous methanol solution. The results are shown in Figure 5. As can be seen from the figure, the loaded metal sulfides have a significant influence on the photocatalytic activity of TiO2. When there was no metal sulfide, pure TiO2 showed low photocatalytic activity because of the rapid recombination between Conduction Band (CB) electrons and Valence Band (VB) holes [27]. Moreover, we found that CoS is a better cocatalyst for H2 production than ZnS and Bi2S3. The photocatalytic activity of the samples decreased in the following order: CoS/TiO2 > Bi2S3/TiO2 > ZnS/TiO2 > TiO2. In the liquid phase reaction, HCHO was the major product along with H2.
Figure 6 shows a comparison of the photocatalytic H2 production activities of the 5 wt%, 10 wt%, 20 wt%, 40 wt%, and 60 wt% CoS/TiO2 samples in aqueous methanol solution. As can be seen from the figure, the content of CoS has a significant influence on the photocatalytic activity of TiO2. The photocatalytic activity of the samples increased as the content of CoS increased from 5% to 20%. The highest hydrogen and methanal production rates were obtained for the 20 wt% CoS/TiO2 sample. The H2 formation rate was 5.6 mmol/g/h, which is 67 times higher than that of pure TiO2. The formation rate of HCHO was 1.9 mmol/g/h with 98.7% selectivity. As shown in Figure 6, further increase in CoS content resulted in reduced photocatalytic activity. Based on the Debye-Scherrer equation, the calculated crystalline lattice sizes are summarized in Table 1. It is clear that the lattice size increased with the content of the CoS composite, indicating that the introduction of CoS can accelerate the aggregation and growth of TiO2 nanocrystals. As a result, although an appropriate CoS content plays a role in increasing the photocatalytic activity, the larger TiO2 nanocrystals lead to decreased photocatalytic activity. Moreover, we speculate that the reaction mechanism involves the activation of C–H bond and O-H bond in methanol by photoexcited holes on CoS/TiO2 surface. The photogenerated electrons will transfer to the surface of CoS/TiO2 and reduce protons to H2.
The capability for reuse is one of the most important factors for an ideal photocatalyst. Hence, the reusability and stability of the 20 wt% CoS/TiO2 sample were investigated. The sample was collected after each photocatalytic H2 production experiment and reused for five times. Figure 7 shows the results of five successive H2 production runs under the same experimental conditions. It can be seen that 20 wt% CoS/TiO2 does not exhibit a significant loss in photocatalytic activity in the five recycles.
3.5. Reaction Mechanism
A possible mechanism for the H2 and HCHO production over the CoS/TiO2 photocatalyst proposed is shown in Figure 8. Obviously, the CoS/TiO2 sample as an oxidation and reduction semiconductor can be excited under simulated solar light irradiation. Subsequently, the photogenerated holes will migrate to the host photocatalyst surface, react with methanol, and drive the generation of HCHO. Photogenerated electrons in the CB of TiO2 could quickly transfer to CoS and recombine with holes in the VB of CoS. Then, the electrons in the CB of CoS with stronger reduction ability could drive the generation of H2. Clearly, TiO2 lacks the active sites for H2 evolution, so the rate of H2 evolution on pure TiO2 is extremely low. However, when the photogenerated electrons transfer from TiO2 to the CoS particles, protons can be efficiently reduced to produce H2 because CoS is a good cocatalyst for the reduction of protons. Moreover, intimate junctions can be formed between CoS and TiO2, which can facilitate the electron transfer from TiO2 to CoS and reduce the electron and hole recombination.
4. Conclusion
Metal sulfide-modified TiO2 catalysts were synthesized using the hydrothermal method. We found that H2 formation on CoS/TiO2 is considerably more efficient than on ZnS/TiO2 and Bi2S3/TiO2. The results showed that CoS/TiO2 had the best photocatalytic activity in the H2 and HCHO production reactions under 300 W Xe lamp irradiation. The enhanced activity can be attributed to the intimate junctions that are formed between CoS and TiO2, which can facilitate the electron transfer from TiO2 to CoS and reduce the electron-hole recombination. The experimental results showed that a suitable amount of CoS could significantly enhance the photocatalytic activity of TiO2 for H2 and methanal production. This result is consistent with SEM analysis of the samples, which showed the highly dispersed nature of the 20 wt% CoS/TiO2 sample. The maximum photocatalytic activity was obtained for 20 wt% CoS/TiO2, with hydrogen formation rate of 5.6 mmol/g/h and HCHO formation rate of 1.9 mmol/g/h with selectivity of 98.7%. Moreover, the CoS/TiO2 photocatalyst showed good reusability and stability.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work was financially supported by the Fujian Provincial Collaborative Innovation Center for Clean Coal Gasification Technology, the Key Project Young Natural Science Foundation of Fujian Provincial University (JZ160478), Outstanding Youth Scientific Research Talent Incubation plan in Universities of Fujian Province ([2017]52), and the National Natural Science Foundation of China (21707055).