Enhancement of Hydrogen Evolution from Water Photocatalysis Using

Research Article pubs.acs.org/journal/ascecg

Enhancement of Hydrogen Evolution from Water Photocatalysis Using Liquid Phase Plasma on Metal Oxide-Loaded Photocatalysts Sangmin Jeong,† Kyong-Hwan Chung,† Heon Lee,† Hyunwoong Park,‡ Ki-Joon Jeon,§ Young-Kwon Park,∥ and Sang-Chul Jung*,† †

Department of Environmental Engineering, Sunchon National University, 255 Jungang-ro, Sunchon, Jeonnam 57922, Republic of Korea ‡ School of Energy Engineering, Kyungpook National University, 80 Daehakro, Bukgu, Daegu 41566, Republic of Korea § Department of Environmental Engineering, Inha University, 100 Inharo, Nam-gu Incheon 22212, Republic of Korea ∥ School of Environmental Engineering, University of Seoul, 163 Seoulsiripdaero, Dongdaemungu, Seoul 02504, Republic of Korea ABSTRACT: This study examined hydrogen evolution by water photocatalysis using liquid phase plasma on metal oxide-loaded photocatalysts. Metal-loaded TiO2 nanocrystallites supported on mesoporous materials were introduced as photocatalysts. SBA-15 and MCM-41 mesoporous materials were applied as a support for the metal-loaded TiO 2 nanocrystallites. The photocatalytic activities of the photocatalysts were estimated for hydrogen production from water. Hydrogen was generated in the photodecomposition of water through liquid phase plasma irradiation. The rate of hydrogen evolution was increased by the metal loading on the TiO2 surface. Photocatalytic activity was improved significantly with Ni loading on the TiO2. The TiO2 nanocrystallites prepared by sol−gel method were incorporated above 50 wt % on the MCM-41 mesoporous support. The mesoporous materials acted as an efficient photocatalytic support for the fixation of TiO2. Hydrogen evolution was enhanced with Ni incorporation on the TiO2 supported on the mesoporous supports. The addition of formaldehyde to water induced an apparent enhancement of hydrogen evolution. The formaldehyde assists to improve the hydrogen production with adding of hydrogen by its decomposition. KEYWORDS: Liquid phase plasma, Photocatalytic decomposition, Hydrogen evolution, Metal-loaded TiO2, Mesoporous supports

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INTRODUCTION Photocatalysis is an efficient method for hydrogen production because it can be obtained sustainably using solar energy. This process is attractive economically compared to other methods, such as a steam reforming process and water electrolysis.1,2 Photocatalytic water splitting is effective for converting solar energy to hydrogen as a clean and renewable hydrogen energy.3−5 The development of high photosensitive catalysts has been studied under UV and visible light illumination.6−9 In particular, visible light sensitive photocatalysts for hydrogen generation from water have attracted considerable attention.10−14 In addition, light sources are as important as the photocatalysts in a photochemical reaction. Although a range of light sources have been employed in photocatalysis, few studies have examined photocatalysis for hydrogen generation using liquid phase plasma (LPP) by irradiation into water directly.15 Plasma reforming is generally performed in the gas phase. Discharge in a liquid has been used in water treatments based on its simple electrical configuration.16−18 Similar technical approaches can be applied to liquid hydrocarbon reforming.19,20 On the other hand, there are few reports on its application to hydrogen production by photocatalysis. Dis© 2017 American Chemical Society

charge in a liquid can generate a higher density plasma and larger spatial distribution compared to UV lamp irradiation.21,22 These advantages can lead to the decomposition of raw materials and hydrogen production. The plasma in a liquid produces strong UV and visible light simultaneously. Hydrogen production from alcohol-containing water using a pulsed discharge in liquid has been reported.23 Although the plasma in a liquid can lead to an effective photocatalytic reaction with photocatalysts, there are few reports on the characteristics of hydrogen production from water using liquid phase plasma with photocatalysts. The development of highly photoresponsive and low-cost photocatalysts is also important. Photocatalysts using a noble metal show high activity and resistance to photocorrosion, but their high cost is a significant obstacle to future applications. Therefore, it is necessary to prepare a highly active and low-cost Special Issue: Asia-Pacific Congress on Catalysis: Advances in Catalysis for Sustainable Development Received: November 30, 2016 Revised: February 28, 2017 Published: March 20, 2017 3659

DOI: 10.1021/acssuschemeng.6b02898 ACS Sustainable Chem. Eng. 2017, 5, 3659−3666

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic diagram of photocatalytic reaction apparatus using LPP system. Ni, Fe, Co, and Li ions were introduced as the metal ions loaded onto the TiO2 photocatalysts. Nickel(II) nitrate hexahydrate (Daejung, 98%), iron(II) chloride hydrate (Daejung, 98%), lithium chloride(II), and cobalt(II) chloride hexahydrate (Aldrich, 98%) were used as the precursors of the metal-loaded TiO2 photocatalysts. The metal-loaded TiO2 photocatalysts were prepared using the typical incipient wetness impregnation method. The metal ions were loaded onto TiO2 at a 2 wt % theoretical content. SBA-15 and MCM-41 mesoporous materials were introduced as a photocatalytic support for the metal-loaded TiO2 photocatalysts in the reaction. The MCM-41 mesoporous material was synthesized using hexadecyltrimethylammonium bromide (99.5%, Aldrich) as a structure-directing agent according to a procedure in the literature.38 The SBA-15 mesoporous material was prepared using poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) [mean Mn ∼ 5800, Aldrich] as a structure-directing agent.39 The structure-directing agent retained in the mesopores was removed through calcination at 550 °C for 12 h. The TiO2/SBA-15 and TiO2/MCM-41 photocatalysts were prepared by the wetness impregnation method for the mesoporous materials in the TiO2 sol. The mesoporous materials were soaked in a TiO2 sol. After stirring for 4 h, the sample was calcined at 500 °C for 5 h. The Ni/TiO2/SBA-15 and Ni/TiO2/MCM-41 photocatalysts were prepared from the impregnation of Ni ions on the surface of the TiO2/ SBA-15 and TiO2/MCM-41 photocatalysts. Distilled water was employed as the reactant. Formaldehyde (Daejung, 35%) was introduced as an additive in the photocatalytic reaction. Photocatalysis of Water Using LPP. Figure 1 presents a schematic diagram of the apparatus of the photocatalytic reaction using the LPP system. Photocatalysis was performed for water in an air-free system connected to a gas chromatograph (GC). The gas products produced during the reaction were carried by an N2 carrier gas at a continuous flow to the GC at a 20 mL/min flow rate adjusted by a mass flow controller (MFC). The temperature of the LPP reactor was maintained at 25 °C with cooling water, which was controlled by a circulating water bath. The gas products were defined by GC (Younglin, M600D) equipped with a thermal conductivity detector (TCD) and a molecular sieve 5A packing column. The liquid products were also analyzed by GC (Shimadzu, 8A) using an FID analyzer and HP-1 capillary column, 50 m length.

metal photocatalyst for hydrogen production, which can be a substitute for noble metals.24,25 TiO2 has attracted considerable attention because of its low cost,26−29 high photocatalytic efficiency, and chemical stability.30−33 Recently, studies have also focused on the development of stable and efficient low-cost metal-loaded TiO2 photocatalysts for hydrogen generation.34−37 Nanocrystalline TiO2 photocatalysts are suspended in water or hydrogen-containing substances during the photocatalytic reaction. Supports for nanocrystalline photocatalysts have been developed to separate and recover the photocatalysts from the reactant solution. The photocatalytic supports can fix the nanocrystalline photocatalysts appropriately. Some of mesoporous materials can be introduced as a photocatalytic support due to their large pore structure and recovery properties. On the other hand, there are few reports on the employment of the mesoporous materials as a photocatalytic support. This paper reports on the enhancement of hydrogen evolution by water photocatalysis using LPP with metal oxide photocatalysts. TiO2 and metal-loaded TiO2 nanocrystallites were employed as the photocatalysts. SBA-15 and MCM-41 mesoporous materials were introduced as a support for TiO2 nanocrystallites. The photocatalytic activities of the photocatalysts were estimated from the rate of hydrogen production from water. The increase in hydrogen evolution by the formaldehyde addition was also evaluated.

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EXPERIMENTAL SECTION

Photocatalysts. TiO2 (P25, Degussa) was used as the parent photocatalyst. To incorporate TiO2 nanocrystallites on the MCM-41 and SBA-15 mesoporous supports, the TiO2 sol (TS) was prepared by dissolving titanium tetraisopropoxide (Daejung, 99.9%) and 2propanol (Aldrich, 99%) into distilled water. The content of titanium tetraisopropoxide was adjusted to 10 wt % of the mass of TiO2. After stirring the solution at ambient temperature for 5 h, nitric acid was added to the solution. The TiO2 sol was used in the impregnation on the mesoporous photocatalytic supports. Nanocrystalline TS was obtained by drying and calcining the TiO2 sol at 500 °C. 3660


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ACS Sustainable Chemistry & Engineering The electric discharge was generated from a needle-to-needle electrode system in a double annular tube reactor in a liquid. Tungsten electrodes with a ceramic-coated insulator were introduced with a 0.3 mm interelectrode gap. The plasma in the liquid reactant was generated by the plasma power supply. A bipolar pulse power supply with a high frequency (Nano Technology, Inc., NTI-1000W) was used to generate the electrical plasma discharge in the liquid directly. The range of applied voltages, frequency, and pulse width were 230−250 V, 25−30 kHz, and 3−5 μs, respectively. Characterization of Photocatalysts and Optical Analysis. The crystallinity of the photocatalysts were confirmed by high resolution Xray diffraction (XRD Rigaku D/MAX-2500) using Ni-filtered Cu Kα X-ray radiation (λ = 1.54056 Å). The shape and size of the photocatalyst particles were measured by transmission electron microscopy (TEM, JEOL JEM-2100F). TEM was carried out using an LaB6 filament and operated at 200 kV. The photocatalysts were analyzed using a selected energy dispersive X-ray spectroscopy (EDS) microanalyzer (PGI IMIX PC) mounted on the microscope. UV−vis diffuse reflectance spectroscopy (DRS) was performed using a UV−vis spectrometer (Shimadzu, UV-2450) in the region of 200−800 nm using BaSO4 as the reflectance standard. The optical bandgap (Egap) was calculated using the method proposed by Kubelka and Munk for indirect electronic transitions.40 The N2 isotherms of the photocatalysts were estimated using the volumetric adsorption apparatus (MSI, Nanoporosity-XQ) at liquid nitrogen temperature. The photocatalysts were pretreated at 200 °C for 2 h before exposure to N2 gas. Their surface areas were calculated using the BET equation.41 Optical emission spectroscopy (OES) of LPP was performed during electrical discharge in a distilled water and formaldehyde solution using a fiber optical spectrometer (Avantes, AvaSpec-3648). The optical emission spectra were recorded using an optical fiber protruding perpendicular to the axis of the electrodes. The measurement conditions during acquisition of the spectra were a 250 V discharge voltage, 30 kHz frequency, and 5 μs pulse width.

at 656 nm. This suggests that the light source of LPP can induce photoevents in the UV and visible light ranges. The strength of optical emission was apparently enforced totally with the addition of formaldehyde to the water. In particular, the optical emission peaks were highly enforced in the range from 250 to 350 nm in the formaldehyde solution. Figure 3 shows TEM images of TiO2 (P25) and TS obtained by the calcination of a TiO2 sol. The particle size of TiO2 (P25)

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RESULTS AND DISCUSSION Characteristics of Optical Emission and Photocatalysts. Figure 2 presents the optical emission spectra of LPP measured during discharge in a distilled water and formaldehyde solution. Strong atomic lines were observed in the water discharge, such as OI (777 nm), Hα (656 nm), and Hβ (486 nm), and the molecular band of the OH radical, which peaked at 309 nm. A strong emission peak appeared at 309 nm. Another emission peak in the visible light region was observed Figure 3. TEM images of (a) TiO2 (P25) and (b) TS nanocrystallines.

was ca. 20−50 nm. In contrast, that of TS was smaller than TiO2 (P25), below 10 nm. Figure 4 shows XRD patterns of the SBA-15 and MCM-41 mesoporous materials and TiO2 loaded on the mesoporous materials photocatalysts. Anatase TiO2 showed XRD peaks at 25° 2θ. This suggests that TS had been incorporated onto the SBA-15 and MCM-41 mesoporous materials. Figure 5 presents TEM images and Ti maps of TiO2/ SBA-15 and TiO2/MCM-41. The cluster of TiO2 nanocrystallites appeared in the TEM images. This was also confirmed by Ti mapping. The loading of TiO2 on SBA-15 and MCM-41 defined from EDS measurement was 43.6 and 58.4 wt %, respectively. Figure 6 shows the variation of the N2 isotherm and physical properties of SBA-15 and MCM-41 with the TiO2 loading onto the mesoporous materials. The specific surface area and pore volume were reduced significantly with the TiO2 loading on the mesoporous materials. In particular, the pore volume of TiO2/ SBA-15 was reduced by 37.5% compared to that of SBA-15. The mean pore diameter of SBA-15 was estimated to be ca. 9.4

Figure 2. Optical emission spectra of LPP measured in distilled water and formaldehyde solution. 3661


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many TiO2 particles were loaded onto the surface of MCM-41 compared to SBA-15. The large pore size and wide surface area of the mesoporous materials could derive a large TiO2 loading. Therefore, the SBA-15 and MCM-41 mesoporous materials can be used as efficient photocatalytic supports. Figure 7 presents TEM images and elemental maps of the Ni/TiO2 loaded on the mesoporous supports. The metal atoms appeared as dots in the element maps, which suggest that Ni atoms were incorporated on the TiO2 supported on the mesoporous materials surfaces without conglomeration. Figure 8 presents the DRS data of the photocatalysts expressed as Kubelka−Munk units. The optical properties of the photocatalysts were initiated by light absorption in the photochemical processes. The spectrum of TiO2 exhibited an adsorption edge at ca. 380 nm. In contrast, those of the metalloaded TiO2 photocatalysts had shifted to the upper range. Regarding the metal loading on TiO2, the absorption wavelength was increased to the visible light range. The DRS of various metal-loaded TiO2 photocatalysts appeared in the upper light range compared to that of TiO2. The adsorption ranges were increased by the metal loading. The adsorption edges of Ni/TiO2, Fe/TiO2, and Co/TiO2 were 395, 400, and 405 nm, respectively. The bandgaps of the metal-loaded TiO2 photocatalysts were similar, ranging from 2.7 to 2.9 eV. In contrast, the DRS of TiO2/SBA-15 and TiO2/MCM-41 were shifted to a lower range compared to that of TiO2; its adsorption edge was ca. 365 nm, which corresponds to a bandgap of 3.4 eV. With the loading of Ni atoms on TiO2/SBA15 and TiO2/MCM-41, their DRS values were shifted to a higher range compared to TiO2. This suggests that the photosensitivities of TiO2/SBA-15 and TiO2/MCM-41 were improved by Ni loading. Hydrogen Evolution by Water Photocatalysis Using LPP. Hydrogen was obtained in the gas products with a small amount of oxygen from the photocatalysis of water. No new liquid products were obtained during the photochemical reaction. Figure 9 presents the rate of hydrogen evolution from water using LPP without photocatalysts, with Na2CO3 addition, and with TiO2 photocatalyst addition. A small amount of hydrogen was evolved by LPP irradiation despite the lack of a photocatalyst. The rate of hydrogen evolution increased with increasing irradiation time. The hydrogen evolution by LPP irradiation was attributed to the active species. This is because the LPP irradiation generates various active species (OH•, H•, O•, HO2, O−2, H2O2, O3, etc.), as reported in a previous work.43 The active species make it possible to generate the hydrogen evolution during LPP irradiation. As confirmed in Figure 2, OH radicals and H+ ions were produced by LPP irradiation in water. The decomposition of water induced hydrogen evolution. Na2CO3 (0.1 g) was added to water to improve the conductivity of the reactant. A high conductivity of the reactant enhances plasma irradiation, leading to an increase in hydrogen evolution. The amount of hydrogen production was increased significantly on the TiO2 photocatalyst. The oxygen was also observed in the products, but its amount was small, under half of that of hydrogen. When an Hg−UV lamp (500 W) was used as a light source in the water photocatalysis, we obtained the rate of hydrogen evolution as 0.07 mmol/h·g on Ni/TiO2 photocatalyst. The rate of hydrogen evolution has been reported as ca. 0.11 mmol/ h·g on Pt/TiO2 in the similar photoreaction conditions.44 The value was even 0.02 mmol/h·g on TiO2 (P25) in the photocatalysis using a full solar light (UV+visible) by solar

Figure 4. XRD patterns of mesoporous supports (SBA-15 and MCM41) and TiO2 supported on mesoporous materials (TiO2/SBA-15 and TiO2/MCM-41).

Figure 5. TEM images and Ti maps of (a) TiO2/SBA-15 and (b) TiO2/MCM-41.

nm using the N2 isotherm applied BJH method.42 Therefore, TS particles can enter the pores of SBA-15 because the particle size of TS was smaller than the pore size of SBA-15. This suggests that reducing the pore volume of SBA-15 is due to the loading of TiO2 in the pores of SBA-15. On the other hand, reducing the pore volume of MCM-41 was lower than that of SBA-15 with TS loading. This resulted in a smaller pore size of MCM-41 than SBA-15. However, the composition of TiO2 supported on MCM-41 evaluated TEM-EDS, which is estimated mainly because the exposed element on the surface of sample was higher than that of SBA-15. This indicates that 3662


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Figure 6. N2 isotherms and structural properties of (a) SBA-15, (b) TiO2/SBA-15, (c) MCM-41, and (d) TiO2/MCM-41 photocatalysts.

improved by the metal loading on the TiO2 surface compared to that of the TiO2 photocatalyst. The Ni-loaded TiO2 photocatalyst exhibited the highest hydrogen evolution in the metal-loaded TiO2 photocatalysts. As shown in Figure 8, the metal-loaded TiO2 photocatalysts adsorbed light to ca. 400 nm, and the bandgap ranged from 2.7 to 2.9 eV. Therefore, the metal loading on TiO2 brought out extended photosensitivity resulting in a high rate of hydrogen evolution. Figure 11 presents the rate of hydrogen evolution on metal oxide photocatalysts supported on the mesoporous materials. The samples, TiO 2 and Ni/TiO 2 , supported on the mesoporous supports were adopted as 0.5 g in the photocatalysis. The amount of TiO2 nanocrystallites incorporated on the SBA-15 and MCM-41 supports were ca. 43.6 and 58.4 wt %, respectively, as defined from the TEM-EDS results. This suggests that the mesoporous materials can serve as a photocatalyst support incorporating a considerable amount of TiO2 photocatalyst. In order to compare the photocatalytic acitivity with TiO2, 0.25 g of bare TiO2 was introduced in the photocatalytic reaction to adjust with the loading amount of TiO2 supported on the mesoporous materials. The amount TiO2 nanocrystallites incorporated on the supports were ca. 50%. The rate on TiO2 supported onto the supports was slightly higher than on the TiO2 photocatalyst. This is because the crystal size of TiO2 (TS) loaded on the supports was smaller than that of pure TiO2 (P25). On the other hand, the

Figure 7. TEM images and element maps (Ti and Ni) of (a) Ni/ TiO2/SBA-15 and (b) Ni/TiO2/MCM-41.

simulation.45 The rate of hydrogen evolution (1.52 mmol/h·g) on the photocatalysis using the LPP was higher than that obtained from photocatalysis by the UV lamp irradiation on Pt/ TiO2 photocatalyst. This indicates that the LPP irradiation leads a remarkable improvement of photocatalytic activity compared with the irradiation of UV lamp. Figure 10 presents the rate of hydrogen evolution using LPP on various metal-loaded TiO2 photocatalysts. The rates were 3663


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Figure 9. Hydrogen evolution from water photocatalysis using LPP on various photoreaction conditions: (a) TiO2 with Na2CO3 addition, (b) addition of Na2CO3 without photocatalyst, and (c) pure water.

Figure 8. UV−vis diffuse reflectance adsorption spectra of (a) metalloaded TiO2 photocatalysts and (b) Ni-loaded TiO2 on mesoporous supports. Figure 10. Rate of hydrogen evolution from water photocatalysis on various metal-loaded TiO 2 photocatalysts and unloaded TiO 2 photocatalysts at 30 min of LPP irradiation.

hydrogen evolution on Ni/TiO2/MCM-41 was increased by the Ni loading. This result was derived from the improved photosensitivity with the Ni loading. This is due to the extended photoresponsible range. Effect of Formaldehyde Addition in Water Photocatalysis Using LPP. The effect of additive on the water photocatalysis using LPP was evaluated with addition of formaldehyde over Ni/TiO2/SBA-15 and Ni/TiO2/MCM-41 photocatalysts. Figure 12 presents the rate of hydrogen evolution from the photocatalytic reaction using LPP from distilled water and formaldehyde solution on the Ni/TiO2/ MCM-41 photocatalyst. The level of hydrogen evolution in the photocatalytic reaction was increased by the addition of formaldehyde. Hydrogen was obtained in the gas products, but no oxygen was present in the photocatalysis of a formaldehyde solution. On the other hand, some CO gas was observed in addition to hydrogen. Methanol and acetic acid were observed as new liquid products, but their amounts were very small.

The rate of hydrogen evolution was 1.52 mmol/h·g in the photocatalysis of pure water, whereas it increased apparently to 9.39 mmol/h·g in the photocatalysis of 5 vol % formaldehyde solution on Ni/TiO2/MCM-41 photocatalysts. However, the hydrogen evolution was not increased identically with the increment of formaldehyde addition. Formaldehyde in solution acts as an electron donor and thereby can improve the hydrogen evolution. This suggests that formaldehyde contributes as a kind of sacrificial reagent promoting the photocatalysis. We suggest the main reaction pathway of this photocatalysis reaction from the distribution of products. Water is decomposed into hydrogen and oxygen according to photodecomposition by LPP and the photocatalyst as shown in eq 1. Hydrogen and CO gas are generated from the photoreaction of formaldehyde through the reaction of eqs 2 and 3. It seems that the evolved oxygen was exhausted in the formaldehyde 3664



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CONCLUSION Hydrogen evolution from water photocatalysis using LPP on metal oxide-loaded photocatalysts was investigated. SBA-15 and MCM-41 mesoporous materials were used as supports for the metal-loaded TiO2 nanocrystallites. The photocatalytic activities of the photocatalysts were estimated for hydrogen production from water. Hydrogen was produced from the photodecomposition of water with LPP irradiation. The rate of hydrogen evolution was increased by the metal loading on the TiO2 surface. The mesoporous materials can be used as a useful photocatalytic support for the fixation of TiO2. Many TiO2 nanocrystallites were incorporated onto the mesoporous supports. Hydrogen evolution was enhanced by the Ni loading on the TiO2 nanocrystallites on the mesoporous supports. Hydrogen evolution was increased apparently with addition of formaldehyde which contributes as a kind of sacrificial reagent promoting the photocatalysis.

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Figure 11. Rate of hydrogen evolution on various metal oxide photocatalysts supported on the mesoporous materials.

AUTHOR INFORMATION

Corresponding Author

*Tel: +82-61-750-3814. Fax: +82-61-750-3810. E-mail: jsc@ sunchon.ac.kr. ORCID

Heon Lee: 0000-0001-8657-4635 Sang-Chul Jung: 0000-0002-7369-7879 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4908162).

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Figure 12. Rate of hydrogen evolution from photocatalytic reaction using LPP from pure water and various concentrations of formaldehyde solution on Ni/TiO2/SBA-15 and Ni/TiO2/MCM-41 photocatalysts.

oxidation. It can be presumed that the CO and H2O might be generated from decomposition of formic acid formed by formaldehyde oxidation

H 2O → H 2 + 1/2O2

(1)

HCHO → CO + H 2

(2)

HCHO + 1/2O2 → HCOOH → CO + H 2O

(3)

HCHO + H 2 → CH3OH

(4)

CH3OH + CO → CH3COOH

(5)

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Formaldehyde can react easily with hydrogen into methanol and acetic acid can be generated according to reactions 4 and 5. Methanol and acetic acid were observed in the final liquid products. Therefore, this suggests that the additive assists to improve the hydrogen production with the addition of hydrogen by its decomposition. 3665


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Enhancement of Hydrogen Evolution from Water Photocatalysis Using

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