Immobilization of Iron Minerals on a Layered Silicate for Enhancing its Solar Photocatalytic Activity toward H2 Production

The development of efficient and cost-effective solar photocatalysts capable of producing hydrogen from formic acid as a hydrogen storage medium is still a challenging issue. Herein, we report that iron minerals, ferric iron hydroxy sulfates (FHS), immobilized on a natural layered silicate, magadiite, can be used as a photocatalyst to produce hydrogen from formic acid under irradiation with solar simulator. The material exhibits the hydrogen production rate of 470 μmol g⁻¹ h⁻¹, which is considerably higher than that obtained on other iron minerals and comparable to that obtained on precious metal-based photocatalyst ever reported. The present result may open a way to design efficient photocatalyst for hydrogen production from formic acid in an economically and environmentally friendly way.

Introduction

Numerous endeavors have been done to develop new strategies that can store and provide hydrogen, an alternative energy source to non-renewable resources including fossil fuels, at acceptable costs (Loges et al., 2010; Grasemann and Laurenczy, 2012; Yadav and Xu, 2012; Li and Xu, 2013; Singh and Xu, 2013). Due to its availability, non-toxicity, and safe handling in aqueous solutions, formic acid (FA) is one of the most widely investigated hydrogen storage materials and can generate a molecular hydrogen via dehydrogenation reaction (HCOOH ↔ H₂ + CO₂) (Mori et al., 2013; Bulushev et al., 2016; García-Aguilar et al., 2016; Navlani-García et al., 2018; Podyacheva et al., 2018; Navlani-García et al., 2019). The catalytic hydrogenation of FA based on precious metals such as Pt, Pd, and Rh and its alloys have been extensively investigated because they can be operated under relatively mild conditions (Mori et al., 2013; Singh and Xu, 2013; Bulushev et al., 2016; Doustkhah et al., 2018; Podyacheva et al., 2018; Doustkhah et al., 2020). On the other hand, efforts have been recently directed to developing new catalytic systems that can decrease or replace precious metals used (Flaherty et al., 2010; Yi et al., 2013).

Photocatalytic FA hydrogenation has been considered to offer an alternative because the reaction can be done at room temperature using solar energy, available in an unlimited supply. Solids photocatalysts such as TiO₂ and CdS have been used for the reaction after the modification with precious metals such as Ru, Pd, Au, and Pt (Matsumura et al., 1984; Zhang et al., 2010; Li et al., 2011; Cai et al., 2013; Zhang Z et al., 2015). Recently, plasmonic photocatalysts like AuPd nanoalloys supported on carbon nitride have been reported to show a relatively high photocatalytic activity for FA hydrogenation (Zhang et al., 2019). While acknowledging these pioneering works, the development of precious-metal-free photocatalytic systems for FA hydrogenation is still challenging.

Here we report that ferric iron hydroxy sulfate (FHS) minerals, hydronium jarosite and volaschioite (Umetsu et al., 1977; Biagioni et al., 2011; Najorka et al., 2016), supported on a natural silicate can be used as a solar photocatalyst for FA hydrogenation. Although iron minerals and iron (oxyhydr)oxides materials, including hematite and akaganeite, have been extensively investigated as photocatalysts toward different reactions due to its low-cost and biocompatibility (Bora et al., 2013; Mishra and Chun, 2015; Ide et al., 2016a; Mani et al., 2018; Ide et al., 2019), to the best of our knowledge, this is the first report to show the solar photocatalytic activity of FHS toward FA hydrogenation. We use a natural layered silicate, magadiite, as the support of FHS because it can be prepared by a simple hydrothermal reaction and possesses a significantly larger density of surface silanol groups (for immobilizing nanoparticles on the surface) than other silicates (Rojo et al., 1988; Ide et al., 2018; Doustkhah and Ide, 2020).

Experimental Section

Materials and Chemicals

The original (natural) form of magadiite containing sodium cations in the structure, Na-magadiite was purchased from Nippon Chemical Industrial Co., Ltd. Fe (NO₃)₃⋅9H₂O (99%) was bought from Nacalai Tesque. H₂SO₄ (97%) and acetonitrile (99.5%) were obtained from Wako Pure Chemical Corporation. P25 TiO₂ was kindly supplied from Nippon Aerosil Co., Ltd. All chemicals and materials were used without any further purification.

Preparation of H-Magadiite

Na-magadiite (Na-mag, 10 g), was mixed with an aqueous solution of HCl (1,000 mL, 0.2 M) and the dispersion was stirred for 3 days. After separation of the product by centrifugation (3,500 rpm, 30 min), the solid was washed with pure water for several times and then dried under vacuum at room temperature. The product was named H-mag.

Preparation of Iron Minerals Deposited on H-mag

H-mag (200 mg) was mixed with acetonitrile solution (60 mL) containing Fe(NO₃)₃⋅9H₂O (1800 mg). Water (600 μL) and H₂SO₄ (1,320 μL) were added to the mixture and then the mixture was stirred for 1 h. Subsequently, the obtained dispersion was transferred to Teflon-lined stainless-steel autoclave (100 mL) and solvothermally treated at 100°C for 24 h. After the reaction, the solid product was separated by centrifugation (3,500 rpm, 30 min) and subsequent decantation, and finally dried at room temperature under vacuum overnight. The obtaining product was named FHS/H-mag. A control sample without H-mag (named FHS) was also prepared by the similar procedure except for adding H-mag. Likewise, α-Fe₂O₃ was prepared by the similar procedure except for adding H-mag and H₂SO₄.

Characterization

Powder X-ray diffraction (XRD) patterns were taken utilizing a Rigaku SmartLab diffractometer, with Cu Kα radiation at 40 kV and 30 mA at a scan rate of 1°min⁻¹. Fourier transform infrared (FTIR) spectra were measured on a Shimadzu FTIR-4200 spectrometer. UV−vis spectra were recorded with a JASCO V-570 spectrometer. N₂ adsorption/desorption was carried out at −196°C using a MicrotracBel BELMAX after the samples had been evacuated at 60°C for 12 h. Field emission scanning electron microscope (FE-SEM) images were observed with a HITACHI S- 4800 microscope and a Hitachi SU-8230 microscope equipped with energy dispersive X-ray (EDX) spectroscopy analyzer. X-ray photoelectron spectroscopy (XPS) was performed using a PHI Quantera SXM instrument, operated with Al Kα radiation at 20 kV and 5 mA. The binding energy shift was calibrated using the C1s level at 285.0 eV.

Photocatalytic Decomposition/Dehydrogenation of FA

Oxidative decomposition and dehydrogenation of FA was performed under O₂ and Ar atmospheres, respectively, in a Pyrex glass tube (34 mL) as follows: the powder sample (15 mg) was added into an aqueous solution (5 mL) containing formic acid (5 vol%) and then bubbled with O₂ or Ar for 30 min. Subsequently, the mixture was ultrasonicated for 2 min and then irradiated via a solar simulator (San-Ei Electric, λ > 300 nm, 1,000 Wm⁻²) under stirring. The headspace gas in the glass tube was withdrawn with a gas-tight syringe and quantified using a BID gas chromatograph (Shimadzu BID-2010 plus) equipped with a Micropacked ST column. For apparent quantum yield (AQY) calculation, a Pyrex glass tube was irradiated with a monochromated light using an Ushio 500 W Xe lamp equipped with a Bunkoukeiki SM-25 monochromator. The number of incident photons was determined using a Bunkoukeiki S1337–1010BQ silicon photodiode. AQY (%) was defined as [number of H₂ evolved] × 2/[number of incident photons] × 100.

Results and Discussion

FHS minerals, hydronium jarosite and volaschioite with chemical formulas of (H₃O)Fe₃(SO₄)₂(OH)₆ and Fe₄(SO₄)O₂(OH)₆(H₂O)₂, respectively, are metastable phases relative to hematite and converted to hematite at elevated temperature (Umetsu et al., 1977; Biagioni et al., 2011; Najorka et al., 2016). Thus, we had investigated the effect of temperature on the formation of FHS on H-mag in the reactions of H-mag with Fe(NO₃)₃⋅9H₂O solution containing H₂SO₄. At room temperature and 50°C, no particles were deposited on H-mag. In contrast, iron species, which later was confirmed to be FHS, was deposited on H-mag (FHS/H-mag) when the reaction was conducted at 100°C.

Figure 1A shows the XRD patterns of FHS/H-mag and H-mag. FHS/H-mag gave diffractions peaks due to hydronium jarosite and volaschioite, in addition to those of H-mag. The position and intensity of the peak due to the basal spacing for FHS/H-mag observed at around 2θ of 7.5° was not significantly different from that of H-mag. These results suggest the formation of FHS phases mainly outside H-mag particles.

FIGURE 1

FIGURE 1. (A) XRD patterns and (B) FT-IR spectra of H-mag, FHS and FHS/H-mag samples.

FTIR also shows the presence of FHS in the FHS/H-mag sample (Figure 1B). FHS/H-mag had an absorption band at 447 cm⁻¹ assigned to the vibrations in FeO₆ of FHS (Ristić et al., 2005; Wei and Nan, 2011), in addition to those at 543, 574, and 611 cm⁻¹ assigned to the Si-O-Si bending vibration, those at 707 and 784 cm⁻¹ attributed to the Si-O-Si stretching vibration, and those from 1,000 to 1,260 cm⁻¹ assigned to the Si-O-Si asymmetric stretching vibration (Kooli et al., 2001). Note that the characteristic absorption band due to the sulfate (SO₄^2-) moiety of FHS, which should be observed in the range of 900–1,300 cm⁻¹ (Ristić et al., 2005), are overlapped by the absorption bands due to the silicate framework. Importantly, FHS/H-mag, moreover, had an absorption band at 651 cm⁻¹ assignable to the Si-O-Fe vibration (Szostak et al., 1987). All these results demonstrate that the FHS phases are immobilized on the surface of H-mag particles. XRD and FTIR data also confirmed that a control sample, sole FHS, is composed of hydronium jarosite and volaschioite.

To investigate the location of the immobilized FHS, SEM-EDX analysis was performed. As shown in Figure 2A, H-mag is composed of rosette-like aggregates of large plate-like crystals with a size of ca. 2 μm. On the other hand, FHS/H-mag is also composed of similar rosette-like aggregates but their surface is partially rough and disordered due to the presence of smaller particles attached. From the EDX elemental mapping of FHS/H-mag (Figure 2B), Fe and S elements are entirely distributed on each rosette-like aggregate. Moreover, the EDX spectrum of FHS/H-mag indicates that the Fe, S, and Si contents in FHS/H-mag is 1.5, 1.4, and 33.5 wt%, respectively. Note that Fe and S were also detected on particles other than rosette-like aggregates while the amount of the former particles were considerably smaller than that of the latter particles.

FIGURE 2

FIGURE 2. (A) SEM images of H-mag and FHS/H-mag and (B) EDS elemental mappings and EDX spectrum of FHS/H-mag.

Nitrogen adsorption-desorption isotherms measurements further confirmed the morphology of FHS/H-mag. As shown in Figure 3, FHS/H-mag showed a significantly decreased external surface area compared to H-mag. This result means that small FHS segments covers rosette-like aggregates or fill the angularities of the aggregates. All the results described above indicate that FHS are immobilized in a highly dispersed state on the particle outer surface of H-mag for FHS/H-mag.

FIGURE 3

FIGURE 3. N₂ adsorption/desorption isotherms of H-mag and FHS/H-mag.

The optical properties of FHS/H-mag were investigated (Figure 4). In contrast to H-mag that is white, FHS/H-mag is light yellow. UV-Vis absorption spectra revealed that FHS/H-mag had an absorption onset around 550 nm like sole FHS. We thus expected that FHS/H-mag could be used as photocatalysts working under solar light irradiation.

FIGURE 4

FIGURE 4. UV-Vis spectra of H-mag, FHS and FHS/H-mag. Inset shows the photographs of each powder.

The photocatalytic activity of FHS/H-mag was firstly evaluated via the oxidation of FA to evolve CO₂, a representative reaction to check the performance of the synthesized photocatalysts (Kominami et al., 2010; Ide and Komaguchi, 2015; Ide et al., 2016b; Saito et al., 2016). As shown in Figure 5A, the amount of evolved CO₂ required for FA oxidation on FHS/H-mag increased linearly with the irradiation time, showing the photocatalysis of FHS/H-mag toward this reaction. This result, considering that H-mag was inactive for this reaction, indicates the photocatalytic activity of the FHS component of FHS/H-mag. Importantly, with the same sample amount (15 mg), FHS/H-mag, having only 1.5 wt% of the immobilized Fe, showed a high CO₂ evolution rate comparable to that of P25, a benchmark TiO₂ photocatalyst, and significantly higher than that of a typical iron mineral, α-Fe₂O₃ (hematite) and sole FHS. These results demonstrate the high photocatalytic activity of the immobilized FHS for FHS/H-mag. Furthermore, to our surprise, FHS/H-mag produced H₂ even in the presence of O₂ as shown in Figure 5B. Generally, the photocatalytic decomposition of organic compounds, including FA, into CO₂ proceeds efficiently with O₂ probably because O₂ is reduced with photoexcited electrons to produce a relatively stable superoxide radical anion and then electron–hole recombination is retarded (Nosaka et al., 1997; Ohtani et al., 2008). FHS/H-mag could reduce FA in competition with O₂; thus, we also expected its high photocatalytic activity toward FA dehydrogenation (reduction).

FIGURE 5

FIGURE 5. (A) CO₂ evolution and (B) H₂ evolution profiles on different materials from an aerated aqueous solution containing FA under solar simulator irradiation.

As expected, FHS/H-mag showed a high H₂ production rate that is considerably higher than that obtained on P25 and α-Fe₂O₃ with the same sample amount (Figures 6A,B). We could not detect CO at all but CO₂, confirming the photocatalytic FA dehydrogenation ability of FHS/H-mag. To compare the photocatalytic activity of FHS/H-mag objectively, we calculated the hydrogen evolution rate as 470 μmol g⁻¹ h⁻¹. This value was comparable to that reported for Rh-N-TiO₂ (750 μmol g⁻¹ h⁻¹), Cu-TiO₂ (830 μmol g⁻¹ h⁻¹) and Ru-CdS supported on a zeolite (540 μmol g⁻¹ h⁻¹) (Zhang et al., 2010; Halasi et al., 2012; Lanese et al., 2013). The photocatalytic activity of FHS/H-mag was significantly higher than that of sole FHS. FHS/H-mag showed the durability against the photocatalytic reaction by approximately 3 h-irradiation (Figure 6C).

FIGURE 6

FIGURE 6. (A) H₂ evolution profiles (B) H₂ evolution rates on different materials from a deaerated aqueous solution containing FA under solar simulator irradiation (C) Durability test for FHS/H-mag and (D) Action spectrum in photocatalytic dehydrogenation of FA on FHS/H-mag.

We investigated the reason for the high activity of FHS/H-mag. It has been reported that Fe(III)-salen complex integrated with CdS photocatalytically yields H₂ from FA. The proposed photocatalytic mechanism involves 1) the adsorption of FA on CdS to produce formate (HCOO⁻), 2) the band gap excitation of CdS generates an electron/hole pair and the photogenerated hole oxidizes formate to produce CO₂⁻ and H⁺, 3) the Fe(III) complex rapidly accepts the photogenerated electron from the excited CdS to produce Fe(II) species, 4) transfer of another electron from CO₂⁻ to Fe(II) species produces Fe(I) species which effectively reduce H⁺ to yield H₂ (Irfan et al., 2020). In the present case, the immobilized FHS can act as a semiconductor photocatalyst enabling to both oxidize formate to produce H⁺ and reduce H⁺ to yield H₂. This scenario is suggested by the action spectrum of FHS/H-mag in FA dehydrogenation (Figure 6D), in which AQY on FHS/H-mag for H₂ evolution well-correlated with the photo-adsorption of FHS/H-mag.

We, moreover, investigated the effect of the hybridization between FHS and H-mag on the high activity of FHS/H-mag. The Fe 2p XPS spectra for FHS and FHS/H-mag revealed that the oxidation state of Fe ions is mainly trivalent (Zhang X et al., 2015) and the Fe 2p peak for FHS/H-mag appeared at a higher binding energy region than that for FHS (Figure 7). This result suggests that Fe(III) sites in the FHS component (Figure 8) of FHS/H-mag becomes more cationic (Eguchi et al., 2017) and thus the adsorption of FA (formation of formate) is enhanced, in addition to that the FHS component is immobilized on the surface of H-mag particles.

FIGURE 7

FIGURE 7. Fe 2p-region XPS spectra of FHS and FHS/H-mag samples.

FIGURE 8

FIGURE 8. Crystal structure of hydronium jarosite showing an example of Fe(III) cites in FHS that can interact with the surface of H-mag. Color coding: pink = H, red = O, yellow = S, brown = Fe.

Conclusion

We have reported the synthesis of iron minerals, ferric iron hydroxy sulfates (hydronium jarosite and volaschioite) supported on a layered silicate, magadiite, via a simple hydrothermal reaction of the silicate with a solution containing iron nitrite Fe(NO₃)₃⋅9H₂O and H₂SO₄. We demonstrated that the iron minerals were immobilized in a highly dispersed state and showed an impressive solar photocatalytic activity toward FA dehydrogenation comparable to that obtained on precious metal-based photocatalysts ever reported. The material design presented here is flexible to design supported photocatalysts based on different iron minerals. This paper thus may open a way to create highly efficient precious-metal free solar photocatalysts for H₂ production from H₂ storage materials.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author Contributions

HE: Methodology, Investigation, Writing—original draft. RT: Investigation. ME: Investigation. ME: Supervision, Investigation. YI: Conceptualization, Writing—review andamp; editing—original draft, Supervision.

Conflict of Interest

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.

Acknowledgments

We would like to gratefully acknowledge the financial support by joint supervision scholarship from Cultural Affairs and Missions Sector, Egyptian Ministry of Higher Education.

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