A Green Desulfurization Technique: Utilization of Flue Gas SO2 to Produce H2 via a Photoelectrochemical Process Based on Mo-Doped BiVO4

Introduction

Sulfur dioxide (SO₂), as one of the acid gases, could transform into some atmospheric products (e.g., sulfate, sulfuric acid aerosol) through the chemical process in the atmosphere. It and its atmospheric products are detrimental to human health, and cause lots of environmental problems such as smog formation, acid deposition, and degradation of visibility. As is well-known, the SO₂ mainly enters the atmosphere through the anthropogenic processes, e.g., combustion and release of petroleum, fossil fuel, etc. (Lelieveld and Heintzenberg, 1992; McDonald-Buller et al., 2016). Till now, to remove SO₂ released from the burning of fossil fuels, several effective methods has been developed (Srinivasan and Grutzeck, 1999; Bashikova et al., 2001; Xia et al., 2011; Kaplan et al., 2013; Yang et al., 2013). Among these technologies, Wet Flue Gas Desulfurization (WFGD) has been one of the state-of-the-art technologies for SO₂ removal with high efficiency, simple equipment and obtaining multi-useful byproducts (Yang et al., 2012; Lu et al., 2013). Unfortunately, there are some inevitable drawbacks with the WFGD process: for example, the oxidation process energy of ${\text{SO}}_3^{2-}$ to final ${\text{SO}}_4^{2-}$ is wasted and it is not consistent with the sustainable principles. In order to solve this problem, our group proposed a solar-to-H₂ energy conversion with SO₂ removal simultaneously via a solar water splitting process for the first time (Han et al., 2017). Since the oxidation of ${\text{SO}}_3^{2-}$ (formed during desulfurization process) needs much lower activation energy and 2 electrons than that of water, acted as a sacrificing reagent during the process, thus it could significantly improve the evolution rate of H₂. This method not only utilizes the waste energy during the desulfurization process, also realizes the energy production by using air pollutants, which could achieve the zero release of SO₂.

Though we have reported a photoresponse semiconductor (BiVO₄) for H₂ generation with SO₂ removal, but it is still a challenge to facilitate the performance due to the intrinsic low mobility of photogenerated charges of BiVO₄. Lots of methods have been proposed to solve this problem, such as controlling the morphology (McDonald and Choi, 2012; Kim and Choi, 2014; Zhou et al., 2014), metal and nonmetal doping (Jo et al., 2012; Chen et al., 2015; Huang H. et al., 2015; Huang H. W. et al., 2015; Huang et al., 2017), forming heterojunctions (Hong et al., 2011; Luo et al., 2011; Seabold and Choi, 2012; Zhang et al., 2016). These modifications have improved the properties of BiVO₄ greatly by reducing the band-gap energy or improving the charge carrier transport. It has been reported that Mo⁶⁺ ion substitute the V site in monoclinic sheelite BiVO₄ could improve the photoinduced carriers, which could facilitate the oxidation while in water splitting reaction system theoretically and experimentally (Luo et al., 2011, 2013; Parmar et al., 2012; Ding et al., 2014; Park et al., 2014; Seabold et al., 2014; Zhou et al., 2014; Jiang et al., 2015, 2017; Kuang et al., 2016; Nair et al., 2016; Pattengale and Huang, 2016). However, Mo-doped BiVO₄ has not been studied as photoanode for flue gas SO₂ removal. Here, we prepared Mo-doped BiVO₄ for enhancing H₂ generation with simultaneous SO₂ removal. Moreover, the importance of the amount of Mo-dopants on the performance of BiVO₄ with SO₂ removal was also studied. Besides we prepared a series of Mo-doped BiVO₄ films with different content. The structure characterizations of the obtained films are investigated by XRD, SEM, Raman, UV-vis. Furthermore, we studied the performance of H₂ generation and efficiency of SO₂ removal.

Experiment

Synthesis of the Catalysts

In this work, the deionized water (DI water) was used throughout the whole experiment, and all the chemical reagents are analytical grade and used without any further purification.

F-doped SnO₂ coated glass (FTO) were purchased from China Southern Glass Co. Ltd, and the FTO glasses were sonicated by immersing in acetone, ethanol and DI water for removing the impurities on the surface of the glass. For comparison, the pristine BiVO₄ was also prepared. All of the electrodes were synthesized by drop-coating method. The precursor solution was dropped onto the conducting side of FTO, followed by annealing in air. The precursor solutions were synthesized by the following procedure (Zhang et al., 2014): diethylene-etriaminepentaacetic acid (DTPA) and ammonia in water (13.0 mol L⁻¹) were added into hot deionized water. After dissolution, the stoichiometric Bi(NO₃)₃•5H₂O, V₂O₅ powder and moderate ammonium molybdate tetrahydrate (H₂₄Mo₇N₆O₂₄·4H₂O) were added into sequence as listed. The resulted mixture was stirred and heated for an hour to promote the dissolution and reaction (complexation of Bi³⁺, V⁵⁺, and Mo⁶⁺ with DTPA) until the mixture turned into a transparent solution. Here, we prepared three different samples with the content of Mo ranging from 1, 3, to 5. The amounts of doped Mo were 1 atom%, 3 atom%, 5 atom%, and were denoted as BiVO₄(Mo-1), BiVO₄(Mo-3), BiVO₄(Mo-5), respectively. Then, 40 μl prepared solutions were dropped onto the conducting side of FTO (1 × 2 cm) respectively. After dried at 60° C in oven, the films were annealed at 500° C for 3 h with a ramping rate of 2° C min⁻¹ in air. The above process was repeated by three times for the synthesis of the electrode.

Characterization of the Samples

The crystal structures of the as-prepared samples were determined using an X-Ray Diffraction (XRD) with Cu Kα radiation ranging from 10 to 60°. The crystallite sizes of the samples were calculated using the Scherrer formula:

$\begin{array}{ll}D=\frac{K\lambda }{\beta \text{cos}\theta }& (1)\end{array}$

Where D is the average crystallite size (nm), λ is the wavelength of the X-ray radiation (0.154 nm), K is the shape factor (0.9), β is the peak width at half-maximum height, corrected for instrumental broadening, and 2θ = 28.7°. The micromorphology and the microstructure of the samples were determined by using field emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan). UV-vis transmission spectra of the as-prepared catalysts were measured using a UV-Vis spectrophotometer (SHIMADZU UV-2600) with an integrating sphere attachment. BaSO₄ used as a standard. Raman spectra were recorded with a Raman spectrometer (HORIBA, X-plo RA Plus), a green laser (532 nm) were used as excitation sources.

PEC Measurement

The apparatus for the PEC tests was a gastight photoreactor. The PEC performance were conducted with a typical three-electrode configuration by using a potentiostat (CHI 660E, Shanghai Chenhua Co. Lid. China). The synthesized pure BiVO₄ or Mo-doped BiVO₄ electrode was used as working electrode, Pt wire was used as counter electrode and Hg/HgO electrode was used as reference electrode. The electrolyte solutions for the PEC tests were prepared by absorbing SO₂ gas with NaOH solutions of certain concentration (detailed in Table 1). We bubbled SO₂ of concentration is 1,000 ppm successively to NaOH aqueous with the flow rate of 200 ml min⁻¹ to form Na₂SO₃ with a specific concentration. Eventually, the obtained electrolytes for PEC tests were 0.1 M NaOH−0.025 M Na₂SO₃ [denoted as NaOH(aq)+SO₂(g)-1], 0.1 M NaOH-0.05 M Na₂SO₃ [denoted as NaOH(aq)+SO₂(g)-2], 0.1 M NaOH-0.075 M Na₂SO₃ [denoted as NaOH(aq)+SO₂(g)-3]. For comparison, the NaOH (aq, 0.1 M) solution was also prepared. The PEC tests were measured under illumination by using a 300 W Xe lamp solar simulator with AM 1.5 G filter (100 mW cm⁻²) from the back side of the working electrode, as well as in dark conditions. Liner Sweep Voltammetry (LSV) was measured with the sweep rate of 10 mV s⁻¹.

TABLE 1

Table 1. The detailed information of electrolyte and SO₂ absorbing efficiency.

The structures of prepared BiVO₄ and Mo-doped BiVO₄ bulk were optimized by using the CASTEP code (Ding et al., 2014). The primitive cell of pure monoclinic sheelite BiVO₄ was relaxed using 400 eV energy cut off for the plane-wave expansion. The structural model of Mo-doped monoclinic sheelite BiVO₄ was built by substituting one V atom in a relaxed (2 × 1 × 2) supercell of monoclinic sheelite BiVO₄ with one Mo atom.

Hydrogen Evolution

The reactor for hydrogen evolution experiments is identical with the apparatus used for PEC tests, which use a two-electrode configuration. The electrolyte was NaOH solution bubbled with SO₂ gas. The experiments were conducted under illumination using a 300 W Xe lamp with AM 1.5 G filter (100 mW cm⁻²) from the back side of the photoanodes in a 150 ml reactor with 100 ml electrolytes filled in and the external bias was 1.6 V. The amount of H₂ was analyzed by gas chromatography using a thermal conduction detector (TCD) once an hour. The pH values of the solution during the PEC tests were detected by Ohaus (STARTER 2100). The theoretical evolution rate of H₂ is calculated according to the following equation:

$\begin{array}{ll}v=\frac{I\times t}{Z\times F\times A\times 3600}& (2)\end{array}$

Where, v indicates the evolution rate of H₂ (mol cm⁻² h⁻¹); I indicates the average current (A); t indicates the time (s); Z indicates the transferred electron number (1); F is the Faraday's constant (96,500 C mol⁻¹); A is the area of the film (cm²).

The equation for the calculation of Faradaic efficiency is:

$\begin{array}{ll}Faradaicefficiency(\%)=\frac{m\times n\times F}{I\times t}\times 100\%& (3)\end{array}$

Where, m is the experimental value of H₂ (mol); n is the reacted electron number (1); F is the Faraday's constant (96,500 C mol⁻¹); I is the average current (A); t is the time (s).

Results and Discussion

Structure and Physical Properties of the Photoelectrode

The SEM (Scanning Electron Microscopy) images of pristine BiVO₄ and Mo-doped BiVO₄ films are shown in Figure 1. All of the films present nanoparticle structure while a rougher, more disordered structure can be observed in the BiVO₄ films. Besides, the incorporation of Mo could decrease the average size of BiVO₄ with decreasing aggregation of the particles. The average diameter of pure BiVO₄ is about 280 nm (Figure 1a), and the average diameter of Mo-doped BiVO₄ are ranging from 200 to 250 nm, which are smaller than that of pure BiVO₄ (Figures 1b–d). Besides, the smaller particle size of the as-prepared photocatalyst could provide more available active sites, which could be in favor of the PEC performance.

FIGURE 1

Figure 1. SEM images and particle range of (a) pure BiVO₄; (b) BiVO₄(Mo-1); (c) BiVO₄(Mo-3); (d) BiVO₄(Mo-5).

Figure 2A compares the XRD (X-Ray Diffraction) patterns of pure and Mo-doped BiVO₄ films with different concentrations of Mo dopant. All the diffraction peaks are assigned to phase-pure monoclinic sheelite BiVO₄ structure (JCPDS No. 14-0688). Since the films are very thin, the diffraction peaks of FTO substrate are also observed in XRD. No noticeable peaks of MoO₃ is detected in the Mo-doped BiVO₄, the results are consistent with previous study (Berglund et al., 2012; Luo et al., 2013; Chen et al., 2015; Nair et al., 2016; Thalluri et al., 2016). Additionally, the main peaks of the monoclinic BiVO₄ structure shift to lower intensity and higher scattering angles in the Mo-doped BiVO₄ films since the radius of Mo is higher than that of V. This represents a shrinkage or an enlargement of the d spacing of corresponding crystal planes due to incorporation of dopant cations into V sites of BiVO₄ (Parmar et al., 2012). As the pure phase and modification in d spacing, it can be calculated that the Mo have been effectively incorporated into the crystal lattice of BiVO₄ with the monoclinic phase unchanged (Parmar et al., 2012). Furthermore, the crystallite sizes calculated by using the Scherrer formula are 110, 117, 83, 85 nm for the BiVO₄, BiVO₄(Mo-1), BiVO₄(Mo-3), BiVO₄(Mo-5), respectively.

FIGURE 2

Figure 2. (A) XRD patterns; (#: FTO) (B) Raman spectra of pure BiVO₄ and Mo doped BiVO₄ with different concentration.

Though XRD shows some trace that Mo have been doped in the crystal of pure BiVO₄. However, it is too rough to identify the doping sites in the crystal lattice because of the low doping concentration. To probe the doping sites and local distortions of Mo-doped BiVO₄, the Raman spectra were measured and the results are shown in Figure 2B. The Raman mode located at 829 cm⁻¹ is assigned to the symmetric stretching mode of ${\text{VO}}_4^{3-}$ units. It is clear that the symmetric stretching mode in Mo-doped BiVO₄ shifts to a lower wave number, which suggests Mo⁶⁺ substitutes V⁵⁺ in the ${\text{VO}}_4^{3-}$ tetrahedron (Luo et al., 2013; Zhang et al., 2014).

Optical properties of the films are very important for the PEC performance. Figure 3A displays the UV-Vis DRS absorption spectra of the four films. All of the films present a strong absorption in the UV-Vis range, and the incorporation of Mo can hardly affect the absorption edges of BiVO₄. Besides, there isn't any peak shift in the UV-Vis spectra of Mo-doped films, which indicates there is not existence of the phase transfer from monoclinic to tetragonal structures with doping of Mo (Figure 3A; Pattengale and Huang, 2016). As the pure BiVO₄ shows a larger optical absorbance at wavelengths (>325 nm), thus the pure BiVO₄ are less porous than the Mo-doped BiVO₄ (Nair et al., 2016).

FIGURE 3

Figure 3. (A) UV-Vis DRS spectra and (B) Tauc plots of pure BiVO₄ and Mo doped BiVO₄ with different concentration.

The optical band gap energy can be calculated by the following equation:

$\begin{array}{ll}\alpha h\nu =A{(h\nu -E_{\text{g}})}^{\text{n}/2}& (4)\end{array}$

where, α, hν, A, and E_g are the absorption coefficient, photo energy, constant and band gap energy, respectively. The value of n depends on whether the transition is direct (n = 1) or indirect (n = 4). The bandgap of all the films are about 2.5 eV (Figure 3B), which is consistent with the reported band gap of BiVO₄ (Zhang et al., 2014). This result indicates that the incorporation of low amount of Mo could hardly influence the bandgap of BiVO₄, and it is considered to be the characteristic band gap of monoclinic phase of BiVO₄.

In the experiments, the SO₂ gas was absorbed by NaOH solutions and the concentration of ${\text{SO}}_3^{2-}$ was detected. Through analyzing, the removal efficiencies of SO₂ gas were about 98%, it indicated that the absorption method could removal SO₂ completely. In order to determine the PEC properties of the photoelectrodes, the Linear Sweep Voltammograms (LSV) were measured both in dark and under AM 1.5 G illumination (100 mW cm⁻²; Figure 4). After analyzing the photocurrent densities of the photoanodes in different electrolyte systems after absorbing SO₂ (Figure 4A), it is concluded: (1) the photocurrent densities are negligible under dark conditions in different electrolyte systems; (2) the photocurrent densities could be significantly enhanced after inletting SO₂ into electrolyte. Since the introduction of SO₂ into electrolyte, the concentration of ${\text{SO}}_3^{2-}$ could be increased, and the oxidation reaction of ${\text{SO}}_3^{2-}$ needs lower activation energy and kinetically faster than that of water, thus the formed ${\text{SO}}_3^{2-}$ consumes the photogenerated holes instantaneously and generates a higher photocurrent density than that of water. Though the doping of Mo could decrease the light-absorbing slightly (Figure 3A), but it could significantly enhance the bulk charge carrier transportation of BiVO₄. Therefore, the PEC performance of Mo-doping BiVO₄ is higher than that of BiVO₄. The photocurrent density is greatly dependent on the amount of SO₂ absorbed in the electrolyte. In NaOH(aq)+SO₂(g)-3, the photocurrent density is improved by 1.2 times, 1.6 times, and 5 times than that of NaOH(aq)+SO₂(g)-2, NaOH(aq)+SO₂(g)-1, NaOH(aq) at 0.5 V vs. Hg/HgO, respectively. The photocurrent density is significantly improved by 5 times as compared with our previous research on porous BiVO₄ (Han et al., 2017).

FIGURE 4

Figure 4. Linear Sweep Voltammograms (LSV) curves (A) BiVO₄(Mo-3) in different electrolyte systems; (B) different photoanodes in NaOH(aq)+SO₂(g)-3 electrolyte. (Scan speed: 10 mV s⁻¹).

Compared with the four different electrode films in NaOH(aq)+SO₂(g)-3 electrolyte, it is clear that BiVO₄(Mo-3) displayed an outstanding PEC performance. The photocurrent density of BiVO₄(Mo-3) is 1.7 times than that of BiVO₄(Mo-1), 2 times than that of BiVO₄(Mo-5), and 5 times than that of BiVO₄ at 0.5 V vs. Hg/HgO (Figure 4B). The current densities of BiVO₄(Mo-3) in NaOH(aq) with and without SO₂(g)-3 at 0.6 V vs. Hg/HgO are ca. 0.6 and 0.1 mA cm⁻², respectively; the current density in NaOH(aq)+SO₂(g)-3 is 6 times higher than that in NaOH(aq). On the other hand, the evolution rate of H₂ in NaOH(aq)+SO₂(g)-3 (39.4 μmol h⁻¹ cm⁻²) is more than 40 times higher than that in NaOH (0.92 μmol h⁻¹ cm⁻²) in the Table 2. The above results indicate that the amount of Mo at 3 atom% could be an optimal choice.

TABLE 2

Table 2. The current density, theoretical and experimental evolution rate of H₂, and the Faradaic efficiency in different electrolytes [A−0.1 M NaOH; B—NaOH(aq)+SO₂(g)-3].

The experiments for H₂ evolution in different systems were measured with a two-electrode configuration at a bias of 1.6 V under AM 1.5G irradiation. Figure 5A shows the amount of H₂ evolved in different systems and Table 2 summarized the data of current densities, theoretical/ experimental evolution rates of H₂ and the Faradaic efficiencies in the different systems. Comparing each photoanodes in different solution systems, it is clear that the SO₂ removal could significantly facilitate the current density and evolution rate of H₂, which is in good agreement with our previous research (Han et al., 2017). Besides, the current density and evolution rates for BiVO₄(Mo-1, 3, 5) under light irradiation are higher than that under the dark condition. Furthermore, all of the Mo doped BiVO₄ photoanodes show more attractive performance than that of pure BiVO₄, and the BiVO₄(Mo-3) performed the best H₂ generation activities. As calculated, a highest H₂ evolution rate of 39.4 μmol h⁻¹ cm⁻² is realized in NaOH(aq)+SO₂(g)-3 with BiVO₄(Mo-3) as photoanode, and the H₂ evolution rate is only 0.19 μmol h⁻¹ cm⁻² in NaOH(aq) with the BiVO₄ as photoanode, suggesting the H₂ production can be enhanced about 200 times with the removal of SO₂ simultaneously with 3 atom% Mo-doped BiVO₄. Furthermore, the theoretical evolution rates of H₂ are close to theoretical rates in each system, indicating a high Faradaic efficiency of H₂ production (higher than 95%). Besides, the catalysts could not dissolve although the solution is a strong basic aqueous solution.

FIGURE 5

Figure 5. (A) Hydrogen generation at an applied potential of 1.6 V in two-electrode configuration for 3 h; (B) The variation of pH values of electrolytes during the process.

In order to better understand the structure reconstruction with/without Mo doping, the geometric structures of pristine and the Mo-doped BiVO₄ were compared in Figure 6. It is clear that the doping of Mo could increase the length of Bi-O bonds. This phenomenon indicates that the original coordinated O atoms are “shifted” toward the doped Mo sites, and introduce “oxygen vacancies.” As well-known, the oxygen vacancies could play an important role in improving the photocatalytic performance. Besides, the Mo⁶⁺ ion substitute the V site in monoclinic sheelite BiVO₄ could improve the transportation of photoinduced carriers, which could facilitate the charge carrier separation and leads to suppressed bulk recombination (Ding et al., 2014). Therefore, the photocatalytic activity is enhanced significantly after Mo-doping, as shown in Figure 4.

FIGURE 6

Figure 6. The conventional cell of pure monoclinic sheelite BiVO₄ (left) and supercell of Mo doped monoclinic sheelite BiVO₄ (right).

With the existence of flue gas SO₂ removal, ${\text{SO}}_3^{2-}$ was formed in electrolyte solution. Then the photo-generated holes could be more quickly consumed by the formed ${\text{SO}}_3^{2-}$ than that without ${\text{SO}}_3^{2-}$ (pure NaOH solution), because oxidation of ${\text{SO}}_3^{2-}$ has a much lower activation energy and kinetically much faster than water oxidation or OH⁻ oxidation (McDonald and Choi, 2012; Seabold and Choi, 2012). The oxidation of SO₂ is thus proposed to replace the oxygen evolution reaction in water splitting, which can improve the water splitting efficiency and reduce the cost of H₂ production. The whole process for SO₂ removal with simultaneous production of H₂ is thus summarized and illustrated in Figure 7. Mo-doped BiVO₄ is first excited to generate e⁻ in conduction band (CB) and create hole (h⁺) in the valence band (VB) at the same time. With the assistance of space charge layer and extra bias, the e⁻ transfer to the cathode and participate in cathode reaction for H₂ production. While the formed of ${\text{SO}}_3^{2-}$ after flue gas SO₂ absorption is oxidized by the holes on the photoanode. As the Mo incorporated in BiVO₄ could improve the electronic conductivity of pure BiVO₄ (Luo et al., 2011; Pilli et al., 2011), therefore, the e⁻ produced in Mo doped BiVO₄ moves faster than that produced in BiVO₄, and shows a significantly increased photocurrent density (Figure 4) and rate of H₂ production (Figure 5). The reactions are summarized as following:

$\begin{array}{ll}{\text{SO}}_2(\text{g})+{\text{OH}}^{-}\to {\text{HSO}}_3^{-}& (5)\end{array}$

$\begin{array}{ll}{\text{HSO}}_3^{-}+{\text{OH}}^{-}\to {\text{SO}}_3^{2-}+{\text{H}}_2\text{O}& (6)\end{array}$

Reaction on photoanode (Mo-doped BiVO₄ film):

$\begin{array}{ll}{\text{SO}}_3^{2-}+2{\text{OH}}^{-}\to {\text{SO}}_4^{2-}+{\text{H}}_2\text{O}+2e^{-}& (7)\end{array}$

Reaction on cathode (Pt wire):

$\begin{array}{ll}2{\text{H}}_2\text{O}+2e^{-}\to {\text{H}}_2(\text{g})+2\text{O}{\text{H}}^{-}& (8)\end{array}$

The total reaction:

$\begin{array}{ll}\text{S}{\text{O}}_2(\text{g})+2\text{O}{\text{H}}^{-}\stackrel{\text{h}\text{v}}{\to }{\text{H}}_2(\text{g})+\text{S}{\text{O}}_4^{2-}& (9)\end{array}$

Conclusions

In summary, the effect of Mo doping on BiVO₄ used as photoanode for H₂ production with simultaneously flue gas SO₂ removal is investigated. The 3 atom% Mo-doped BiVO₄ (BiVO₄(Mo-3)) possessed the best PEC activity due to its better charge carrier transportation. With the help of Mo, the H₂ evolution rate and SO₂ removal rate of Mo-doped BiVO₄ almost 3 times higher than pristine BiVO₄. Through this process, the SO₂ in flue gas is removed and collected to produce H₂, which could greatly reduce the cost of desulfurization process, and even make it profitable.

FIGURE 7

Figure 7. Schematic illustration of H₂ generation during the PEC process with SO₂ removal.

Author Contributions

JH: assisted in design of the experiments and wrote the manuscript; KL: performed the experiments and wrote the manuscript; HC: assisted in the analysis and interpretation of the data; LZ: planned the project, designed the experiments, and also wrote the manuscript.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The reviewer, GJ, and handling Editor declared their shared affiliation.

Acknowledgments

The authors gratefully acknowledge financial support from National Natural Science Foundation of China (No. 21507011 and No. 21677037), and Ministry of Science and Technology of the People's Republic of China (2016YFE0112200 and 2016YFC0203700).

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