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Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts
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Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts Kyong-Hwan Chung a, Byung-Joo Kim b, Sun-Jae Kim c, Young-Kwon Park d, Sang-Chul Jung a,* a
Department of Environmental Engineering, Sunchon National University, 255 Jungang-ro, Sunchon, Jeonnam, 57922, Republic of Korea b Korea Institute of Carbon Convergence Technology, R&D Division, 110-11 Banryong-ro, Jeonju, 54853, Republic of Korea c Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, 209 Neungdong-ro, Gwangjingu, Seoul, 05006, Republic of Korea d University of Seoul, School of Environmental Engineering, 163 Seoulsiripdaero, Dongdaemun-gu, Seoul, 02504, Republic of Korea
highlights Photo-decomposition
graphical abstract by
liquid
phase plasma was assessed in organic pollutants solution. The effects of irradiation of the liquid phase plasma were evaluated over TiO2 photocatalyst. Optical emission properties were measured with various conditions of plasma discharging. CaTiO3 perovskite was employed as a photocatalyst with Ni loading. Hydrogen evolution was increased significantly in the aqueous acetaldehyde solution.
article info
abstract
Article history:
Photocatalytic decomposition of organic materials-contained aqueous solution is assessed
Received 30 November 2019
using a plasma discharged into the liquid directly. The correlation of H2 generation and
Received in revised form
optical emission spectroscopy is discussed in terms of photocatalytic H2 production using
6 January 2020
plasma and photocatalysts. Variations of the active species are evaluated according to the
Accepted 13 January 2020
conditions of the plasma in the liquid phase. The optical emission spectra vary according
Available online xxx
to the plasma discharging conditions in the liquid phase. The intensities of the OH$ peaks at 309 nm increase with the addition of ethanol or acetaldehyde in water. The highest
* Corresponding author. E-mail address: [email protected] (S.-C. Jung). https://doi.org/10.1016/j.ijhydene.2020.01.089 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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Keywords: Liquid phase plasma Optical emission Hydrogen evolution Photocatalytic decomposition Photocatalysts
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intensities and rate of H2 evolution are observed at a 10% acetaldehyde concentration in the aqueous solution. The rates of H2 evolution in the ethanol or acetaldehyde solution correspond to the concentration of OH$ in the solution. The photocatalytic reaction using liquid plasma generates hydrogen at the same time as the decomposition of the organic chemicals. The rate of hydrogen evolution in aqueous solutions containing the organic chemicals is higher than that in pure water. This is because hydrogen is further generated due to hydrogen generation by photolysis of the organic chemicals. CaTiO3 perovskite photocatalyst shows better photocatalytic activity than TiO2. Ni loading on the photocatalyst lead to an increase in H2 production. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Plasma discharges have attracted considerable attention because they have the ability to cause various chemical reactions [1e3]. Plasma is generated by high voltage discharges at various phases. Plasma is typically plasma generated in the gas phase, which is generally applied to produce fine crystals for thin films and semiconductors. Plasma generated in solid phase is called plasmon and is applied to various fields such as medical. On the other hand, even in the liquid phase, the plasma may be generated by the high voltage. However, the technique of applying plasma to the liquid phase has not been developed so far, and is still limited to the research field. Liquid phase plasma (LPP) is generated by high voltages emitted directly into the liquid. Plasma induced by highvoltage discharges emits dense ultraviolet and visible light, producing many active species [4,5]. Discharge in liquid phase leads to higher plasma density and larger spatial distribution than high voltage UV lamp illumination [6,7]. Therefore, it is expected that LPP can be effectively applied to the underwater photolysis reaction for H2 generation [8]. LPP simultaneously causes five effects: electric field, strong ultraviolet light, high pressure shock wave, various free radical generation and ozone generation. Such an effect can destroy harmful chemicals and eliminate microorganisms in the liquid. In addition, the strong release of LPP can produce H2 by photocatalytic separation of water. LPP has been applied to many industries. High technology materials such as fullerenes, carbon nanotubes and graphene can be easily manufactured using LPP technology from organic compounds such as bisphenol A, benzene and toluene. Highly crystalline carbon nanocrystals are produced uniformly and quickly by LPP irradiation. Carbon nanoparticles are also produced quickly by irradiating carbon source materials with LPP. LPP can also be applied as a more powerful light source in the photocatalytic hydrolysis for H2 production. Photocatalytic decomposition of water is an efficient solution for converting solar energy into H2 energy. In particular, the light source as well as the photocatalyst is the core of the photochemical reaction. Photocatalytic activity for H2 production was mainly investigated under ultraviolet and visible light irradiation. Few studies have investigated photocatalysts using the LPP system by direct irradiation in water.
Photo-electrochemical separation of water is of interest as a method of producing hydrogen [9]. Recently, attention has been focused on the development of a semiconductor that can be applied to a technique for producing hydrogen by photochemical reaction using solar energy [10,11]. TiO2 has been applied as a representative photocatalyst with high photocatalytic activity and chemical stability. However, the band gap is large and limited to photocatalysts that are effective only in ultraviolet light [12,13]. Therefore, various studies on the development of a new visible light sensitive photocatalyst showing high reaction efficiency to visible light have been conducted [14e16]. In recent years, in order to develop photocatalysts that achieve high reactivity under visible light, semiconductor TiO2 has been developed by several methods such as doping metal or nonmetal ions, improving surface sensitivity of TiO2, bonding with narrow bandgap semiconductors, and doping of precious metals on TiO2 surfaces. Much research has been carried out on the modification of TiO2 [17e23]. Doping the transition metal to the TiO2 structure improves the photocatalytic efficiency. Transition metals such as V, Cr, Fe, Mg, Co, Zn and Mo have enhanced photocatalytic activity when doped to TiO2 lattice [24e28]. Incorporation of the dopant results in an increase or decrease in the band gap. Perovskite materials are one of the best photocatalysts in the field of photochemical reactions. Perovskite is known to exhibit photocatalytic properties even in visible light. Improving photocatalytic properties with perovskite materials has been evaluated as a particular challenge [29]. Recently, CaTiO3 attracts attention because of its easy synthesis and high stability [30]. Heterogeneous photocatalysts have been applied as an effective technique for water treatment in various wastewater treatment techniques by photocatalytic reaction [31]. Photolysis of organic wastewater using the LPP process promotes purification and hydrogen production. Plasma reactions are generally carried out in gas phase [1,32]. Recently, photoreactions by plasma emitted from pure water have been reported [33e35]. LPP is produced by direct release as a liquid at high voltages. High pressure pulsed plasma discharge in liquid phase is an effective technique for wastewater treatment [5,6,36]. LPP technology by high pressure discharge quickly removes organic substances from waste water and there is no secondary pollution. This process has the advantages of high decomposition efficiency of pollutants, simple process and
Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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low energy consumption. LPP produces higher plasma density and larger spatial distribution than UV lamp radiation [8]. LPP technology effectively eliminated organic pollutants and hydrogen production in previous studies [37]. Research has also been made to purify wastewater and produce hydrogen at the same time through the process of decomposing hardly decomposable wastewater by a photocatalytic reaction using LPP and photocatalyst. Photocatalytic reactions with LPP and photocatalysts have been found to be effective in producing hydrogen from wastewater treatment. However, the photochemical reaction of LPP in the liquid phase, which is the basic reaction principle, has not been considered. It is necessary to elucidate the correlation with the photocatalytic reaction according to the optical properties of the LPP. This study evaluated the optical emission properties of LPP in liquid phase reactants over the range of emission conditions for LPP. Various active species produced under various LPP release conditions were also evaluated. Various additives were injected into the water to investigate changes in the optical emission spectrum (OES). Investigating the correlation between H2 production and OES by the reaction of LPP and photocatalyst, the method to improve photocatalytic reaction efficiency is discussed. TiO2 and metal/TiO2 nanocrystals were used as photocatalysts. The photochemical reaction ability of the CaTiO3 perovskite photocatalyst was also evaluated in the photocatalytic decomposition of water for H2 production. The influence of organic reagents injected into water is also assessed in relation to the H2 evolution rate.
Methods Materials TiO2 (Degusa, Aeoroxide P25) was introduced as a standard photocatalyst. Ni, Co and Fe metals were used as non-noble metal ions. These were loaded onto the TiO2 surface. As precursor reagents for metal ions, Ni(NO3)2$6H2O (Daejung, 99%), CoCl2$6H2O (Daejung, 99%), FeCl2$4H2O (Daejung, 99%) were used. Metal ions were loaded onto TiO2 according to a typical wet impregnation method. The concentration of metal ions incorporated into TiO2 was adjusted to 2% by weight. CaTiO3 (CTO) was synthesized using the sol-gel method according to the formula CaTiO3:Eu3þ. Distilled water was used as the basic solvent. Zn(NO3)2$6H2O, Ca(NO3)3$4H2O and Eu(NO3)3$6H2O were used as starting materials and prepared in stoichiometric amounts. Ca(NO3)3$4H2O (94.45 g), Zn(NO3)2$6H2O (29.75 g), and Eu(NO3)3$6H2O (8.90 g) were injected into distilled water, followed by vigorous stirring at 65 C for 3 h. Then Ti(OC4H9)4 (170.15 g) was injected into the solution and stirred for 2 h. This solution was dried at 115 C for 12 h. The resulting solid was calcined at 1100 C for 5 h in a muffle furnace at 10 C/min. The calcined solid was ground to make a fine powder. Nickel incorporated CTO were prepared according to a typical impregnation method [38]. Ni(NO3)2$6H2O (Daesin, 99%) was used as a precursor of Ni ions. This precursor was dissolved in distilled water. CTO was then injected into this solution and stirred at 80 C for 12 h. The catalyst was then
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dried and calcined to remove volatile components in the solution to deposit metal on the CTO surface. Ni incorporated CTO photocatalyst was dried at 120 C and calcined at 1100 C. The calcined photocatalyst was ground to fine particles. The content of Ni loaded in the CTO photocatalyst was adjusted to 2 wt%.
Photocatalytic hydrogen generation by LPP irradiation Photocatalytic decomposition of water by LPP irradiation was carried out in pure water and water containing organic matter. Schematic of a photochemical reactor equipped with an LPP system is shown in Fig. S1 (Supplementary material). The photocatalytic decomposition reaction was carried out in a photo-reactor under LPP irradiation. Distilled water or aqueous reactants (200 mL) containing organic matter were injected into the photochemical reactor together with the photocatalyst (1 g). Gas products resulting from the photoreaction were analyzed by gas chromatograph (GC; Youngllins, Y600D) by a thermal conductivity detectors (TCD) connected to a column packed molecular sieve 5 A. To supply a constant flow rate, the flow rate of the carrier gas was adjusted to 20 mL/min using a mass flow controller (MFC; MJT, MR300). The internal temperature of the LPP reaction system equipped with the needle electrode was maintained to 25 C by the water bath.
Analysis of optical emission OES was obtained during LPP discharge into the reactants using an optical fiber and an optical fiber spectrometer (Avantesie, AvaSpec-3600) installed perpendicular to the axis of the electrode. LPP was generated by a pulse discharge using two needle-type tungsten electrodes. The gap between the two needle electrodes was controlled to 0.3 mm. OES was estimated at various concentrations in the reactants, aqueous ethanol and aqueous acetaldehyde. Measurement conditions during the OES acquisition were controlled with various LPP emission conditions. The voltage of LPP discharge was 220e250 V. The frequency of the LPP discharged by the power source was 25 kHze30 kHz. The pulse width was 3 mse5 ms The change in peak of optical emission with plasma discharge conditions of the plasma generator at 309 nm, 486 nm and 656 nm in Fig. S2. OES also changed with changing plasma generation conditions such as voltage, frequency and pulse width. The peak changed according to the variation of plasma generation conditions. As the voltage increased at all three wavelengths, the intensity of the OES increased. Higher frequencies and wider pulse widths result in higher intensity of OES. The highest intensities to the three optical emission peaks were seen under LPP discharge conditions of 250 V, 40 kHz, and 5 ms The plasma generation conditions were then fixed at these values.
Characterization of photocatalysts The structure and crystallinity of the photocatalyst were analyzed by high resolutions X-ray diffraction (XRD; Rigaku, D/max-II/PC1) using Ni filtered CuKa X-ray radiations (l ¼ 1.54056 A). The N2 adsorption isotherm of the
Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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photocatalyst was obtained using a volume adsorption measuring device (Porosity-HG, M SI) at 197 C. By pretreatment the photocatalyst was exposed to N2 gas while preheating at 180 C. for 3 h. Their apparent surface area was determined using the BET equation. The morphology of the photocatalyst was observed by scanning electrons microscopy (SEM; Varian, S-4750). Their elemental composition was analyzed using energy dispersive X-ray spectroscopy (EDS; Norans Z-MAXI 350). Transmission electrons microscopy (TEM; FEII Technaii 200) was measured using LaB6 filaments with an accelerating voltage of 200 kV. The composition of the photocatalyst component was investigated using an EDS micro analyzer (PGIS IMIXI PC1) mounted on a microscope. UVevisible diffuse reflection spectroscopy (DRS) was measured using a UVevisible spectrometer (Hittachii, UV-3000) in the region of 200e1000 nm using BaSO4 as the reflection standard. The optical band gap (Egap) was obtained by adopting the proposed Ku¨belka and Mu¨nk methods for indirect electron transitions [39]. The optical emission spectrum (OES) of the PLPs was measured while directly discharging from the reaction using a fiber optical spectrometer (Avantessi, AvaSpecis-3500). Optical spectra were collected by installing an optical fiber perpendicular to the axis of the electrode. The estimation conditions were 5 ms pulse width, 250 V discharge voltage and 30 kHz frequency while collecting the spectra. Fourier transform infrareds (FT-IR; JASCOS 600 plus) spectroscopy was first diluted to 10% in KBr and then measured in the frequency range of 400e4000 cm1 at ambient temperature. Metal states of Ni-loaded CTO samples were analyzed by X-ray photoelectron spectroscopy (XPS; MultoLab 2000). All samples were irradiated with a monochromatic Al Ka X-ray source at a laboratory pressure of 106e107 Pa. The resolution of the instrument was 0.35 eV wide using silver Fermi edge.
Results and discussion Characteristics of optical emission by LPP Fig. 1 illustrates the OES results measured during LPP irradiation in pure water. LPP irradiation in pure water produced active species and strong UV light and visible light illumination. The strong atomic peaks expressed as active species in the photoreaction resulted in Hb at 486 nm, Ha at 656 nm, OI at 777 nm, and OH (A-X) radical molecular peaks at 309 nm. This indicates that the light source of LPP can cause photochemical reactions in both ultraviolet and visible range (300 nme900 nm). In particular, the emission of UV light at 309 nm and the visible light at 609 nm were large. This is indicated to be excellent in the reaction activity of the photosensitive photocatalyst in the two wavelength range. In the absence of the photocatalyst, H2 was produced by photolysis of water under LPP irradiation due to the active species. However, the photo-reaction on the photocatalyst under LPP irradiation is mainly caused by strong UV and visible light. Thus, H2 gas generation from photochemical reactions by LPP radiation in the presence of photocatalysts is governed by stronger UV and visible light irradiation than the active species produced.
Fig. 1 e OES of LPP measured at various wavelength with discharging conditions of LPP generation in distilled water. (a) 309 nm, (b) 486 nm, (c) 656 nm.
The OES of the LPP in pure water and ethanol or acetaldehyde-added water is presented in Fig. S3. The intensities of OES at 309 nm, which generates a OH$ radical, increased after the addition of ethanol or acetaldehyde into the water. In particular, the intensity at 309 nm was enhanced significantly with ethanol addition into water. Other main peaks at 486 nm and 656 nm were also increased with ethanol and acetaldehyde addition into water. The intensities of the OES increased strongly in the aqueous acetaldehyde solution. This suggests that the emission characteristics of plasma in aqueous solution were improved by the injection of ethanol or
Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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acetaldehyde organic chemicals. This improvement in luminescence results in an improvement in photo-reactivity by LPP. Fig. 2(a) presents the OES of the LPP in an ethanol solution with different ethanol concentrations. Fig. 2(b) shows variations of the OES at main peaks wavelength, such as 309 nm, 486 nm, and 656 nm. The Ha peaks at 656 nm in the OES did not change with ethanol concentration. The Hb peaks at 486 nm increased slightly with increasing ethanol concentration. The highest intensity of the OES at 309 nm, which generates OH$ radicals, was observed with the 10% ethanol solution. The OH$ radicals induce mainly a photodecomposition reaction. Therefore, the OES intensities were highest in the 10 vol% ethanol solution. Fig. 3(a) presents the OES by LPP irradiation with different acetaldehyde concentrations in solution. The addition of acetaldehyde into water resulted in a significant increase in the intensities of the OES. Fig. 3(b) presents the variations of OES intensities at 306 nm (OH$), 486 nm (Hb), and 656 nm (Ha). The intensities of the OES at 486 nm were unchanged despite the injection of acetaldehyde into water. The intensities of OES at 309 nm and 656 nm increased with increasing acetaldehyde concentration in water. On the other hand, the intensities were not enhanced further when the acetaldehyde concentration as more than 20 vol%. The highest intensities of the OES were obtained at an acetaldehyde concentration of 10 vol% in water. The luminescence properties of the plasma are improved by the injection of ethanol or acetaldehyde organic chemicals, and the injection concentration is 10 vol%, which suggests the best ultraviolet or visible light emission.
Physicochemical properties of the photocatalysts The XRD pattern of TiO2 and Ni/TiO2 photocatalyst is shown in Fig. S4 (a). XRD peaks of the anatase properties of TiO2 and weak rutile XRD peaks were observed. XRD peaks for titanium dioxide were found to be predominantly anatase phase. According to the data provided by the manufacturer, the ratio of anatase to rutile was ca. 80:20. A weak XRD peak for Ni at 33 2q was found in the Ni/TiO2. This indicates that Ni is incorporated into TiO2. However, as a result, it is difficult to
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determine whether Ni exists as a metal or in the form of NiO. The TEM analysis and Ni map results of the Ni/TiO2 photocatalyst is also shown in Fig. S4 (b). TiO2 crystal size was measured to be about 40 nm. Ni ions incorporated into the TiO2 surface were observed in the images of Ni maps. Ni atoms appeared as dots in the elemental map of the Ni/TiO2 photocatalyst. This is because Ni atoms are incorporated on the TiO2 surface as very small particles. Metal nanoparticles of Ni on the TiO2 surface were difficult to distinguish from TEM images. At lower metal loading, the size of the Ni nanoparticles was less than 1 nm, which was at the resolution limit of the TEM instrument. The amount of Ni incorporated onto the TiO2 surface was estimated to be 1.92% by weight, as defined from the EDS results. The XRD pattern of CTO photocatalyst is shown in Fig. S5 (a). Reference XRD data of the CTO photocatalyst is presented together in the figure for comparison with the prepared photocatalyst. As shown in Fig. S5 (a), the position and intensity of the primary peak was in good agreement with the data of the JCPDS card (No. 22-0153), which is the reference data. XRD data was indexed to tetragonal CaTiO3. Although Zn2TiO4 and Ca2ZnTi15O36 peaks were observed, their strength was very weak. In the first stage of CaTiO3, the strength was not affected. Fig. S5 (b) shows representative EDS results of CTO photocatalyst. EDS of CTO showed the first peak related to Zn, Ca and Eu. Ti appeared as a small peak at 6 keV, because the amount contained in the compound was very small. The EDS results were in good agreement with the data in Ref. [40]. The ionic radii of Ca2þ, Ti4þ and Eu3þ ions at the octahedral site are 1.00, 0.605 and 0.947 A, respectively [41]. This represents Eu3þ ions that replaced Ca2þ ions more easily than Ti4þ ions in the CaTiO3:Eu3þ lattice because Ca2þ and Eu3þ ions have similar ion radii. In addition, a simulated tetragonal lattice of CaTiO3 Eu3þ has been reported, indicating the substitution of Eu3þ for Ca2þ at the octahedral site [42]. These results suggest that the synthesized CTO is in good agreement with the standard CaTiO3 containing small amounts of Zn. Fig. S5 (c) presents the surface morphology of CTO measured by SEM. The CTO particles were under micron-sized crystallites, which exist with conglomerations among the crystallites because of the high sintering temperatures. The
Fig. 2 e OES of LPP irradiation in aqueous ethanol solution with various ethanol concentrations (a) and variation of main peaks of OES with various ethanol concentrations (b). Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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Fig. 3 e OES of LPP irradiation in aqueous acetaldehyde solution with various acetaldehyde concentrations (a) and variation of main peaks of OES with various acetaldehyde concentrations (b).
CTO photocatalyst had morphology of conglomerated irregular cubes. The small crystallites were loaded onto aggregated crystallites. The particles are spherical polyhedral, and size is ca. 3 mm. Fig. S5 (d) shows the FT-IR spectrum of the CTO photocatalyst measured at 400e4000 cm1. Broadbands of 2900e3700 cm1 with a peak of 3423 cm1 show characteristic stretching vibrations of the hydroxyl (-OH) group of H2O. OH bending bands appeared at 1650 cm1. The peak at 877 cm1 is due to symmetrical stretching vibrations along the CaeOeCa bond. The band at 1439 cm1 is represented by the asymmetric stretching vibration of the hydroxylate of the OHeCa bond. The band at 560 cm1 is represented by the CaeO vibration bonds of CaTiO3. A wide and strong band, characteristic of alkaline titanates, appeared at 460 cm1. The band at 421 cm1 corresponds to the asymmetric stretching vibration of TieO. These results suggest that the CTO produced in this study is in good agreement with the standard CaTiO3. Fig. 4 shows Ni2p and O1s spectra of XPS for Niincorporated CTO. The Ni2p spectrum of Ni-loaded CTO was very similar to that for NiO powders reported in the literature [32]. Satisfactory fitting was consistent with the literature results by showing a change in the binding energy position (853.7 eV) of the multiple contributions to the main peak of NiO. The three peak intervals that make up the main peak
were changed from 0.95 eV to 1.4 eVe0.5 eV and 1.6 eV, respectively. O1s spectrum was also consistent with the NiO spectrum presented in the literature [43]. Therefore, the state of Ni loaded onto CTO was confirmed as NiO state. The DRS of TiO2 and metal/TiO2 converted to Ku¨belkaMu¨nk expression are shown in Fig. S6 (a) [39]. Optical properties of the photocatalysts are initiated by light absorption in photochemical reactions. The adsorption edge of TiO2 was about 380 nm in the DRS results. In contrast, those of the metal/TiO2 photocatalysts were moved to the upper range. That is, when TiO2 was loaded with the metals, the adsorption wavelength was extended to the range of visible light. The adsorption edges in the DRS results of various metal-loaded TiO2 appeared in the upper light range compared to TiO2. This means that the adsorption range is extended by the metal incorporation. The adsorption edges of Ni/TiO2, Fe/TiO2 and Co/TiO2 were 398 nm, 400 nm and 405 nm, respectively. The band gap ranges from 2.7 eV to 2.9 eV. No significant difference was observed in the band gap between the metal/ TiO2 photocatalysts. The DRS results for CTO and Ni/CTO photocatalysts converted in Kubelka-Mu¨nk units are shown in Fig. S6 (b). The optical properties of the photocatalyst are mainly due to photo adsorption in the photochemical reaction process. The spectrum of the CTO appeared in the upper
Fig. 4 e XPS results on (a) Ni2p scan and (b) O1s with Ni-loaded CTO perovskites. Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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wavelength range. The absorption edge of CTO was about 380 nm. Ni doping on the CTO extended the absorption range to the visible light range. The band gaps of the CTO and Ni/ CTO were 3.0 eV and 2.8 eV, respectively.
Hydrogen evolution by LPP irradiation Fig. 5(a) shows the H2 generation rate obtained by decomposition of water by LPP irradiation in the absence of a photocatalyst and a condition in which the TiO2 photocatalyst was injected. From the photolysis reaction of water, H2 gas and a small amount of O2 gas were obtained in the gaseous product. No new liquid product was observed during the photochemical reaction. Even in the absence of a photocatalyst, a small amount of H2 was produced by LPP irradiation. The hydrogen production appeared to be due to the active species generated by the LPP irradiation. This is because LPP irradiation generates various active species such as H$, OH$, O$, HO2, H2O2, O 2 and O3 from water [35]. The active species leads to the generation of H2 during LPP irradiation. OH$ radicals and Hþ ions were generated by LPP irradiation in water as shown in Fig. S2. Injecting TiO2 photocatalyst into this reaction significantly increased the rate of H2 generation. Fig. 5(b) shows the rate of H2 evolution by LPP irradiation on metal/TiO2 photocatalyst. The rates were increased by loading the metals on TiO2. Ni/ TiO2 photocatalyst caused the most H2 generation. As mentioned above, the metal-incorporating TiO2 absorbed light at about 400 nm. Their bandgaps were ranged from 2.7 eV to 2.9 eV. Therefore, when TiO2 is loaded with metal, photosensitivity is increased and H2 generation rate is increased. Fig. 6(a) illustrates the rate of H2 evolution according to the ethanol concentration in water after 30 min of photo-reaction time. The rates of H2 evolution increased remarkably after the addition of ethanol to water. The highest H2 evolution was observed at 10 vol% ethanol in water. Fig. 6(b) represents variation of OES at 309 nm, which generates a OH$ radicals. The highest intensity of the OES at 309 nm was observed with the 10% ethanol solution. The OH$ radicals induce mainly a photodecomposition reaction. Therefore, the rate of H2 generation and OES intensities were highest in the 10 vol% ethanol solution. This is in accord with the excellent result of
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OES emission characteristics when the concentration of the organic chemical in the aqueous solution containing ethanol or acetaldehyde organic chemicals is 10 vol%, the hydrogen generating activity is excellent. Fig. 7(a) presents the rate of H2 evolution with acetaldehyde injection into water. The rates of H2 evolution were improved by the injection of acetaldehyde to water than that of pure water. The rates of H2 generation were higher in the acetaldehyde solution than ethanol solution. The highest rate of H2 generation was obtained in the 10% acetaldehyde concentration in water. This showed the same tendency as in the ethanol aqueous solution. In acetaldehyde aqueous solution, when the concentration is 10 vol%, the hydrogen production rate is the highest. Fig. 7(b) shows the variation of OES at 309 nm, which generates a OH$ radicals. The highest intensity of the OES at 309 nm revealed with the 10% acetaldehyde solution. The OH$ radicals induce mainly a photodecomposition reaction. Therefore, the rate of H2 evolution and OES intensities were highest in the 10 vol% ethanol solution. This suggests that the superior intensity of the OES under the conditions of the LPP photo-reaction improved the H2 generation properties. Fig. 8 shows the rates of H2 evolution in pure water and aqueous acetaldehyde solution on various photocatalysts. CTO showed a higher rate of H2 evolution than TiO2. Rate of H2 generation was improved with Ni incorporating on CTO than that of bare CTO. In the acetaldehyde solution, the rates were increased remarkably because of addition of hydrogen generation derived from acetaldehyde decomposition. The decomposition of ethanol and acetaldehyde by photocatalytic degradation using LPP is shown in Fig. S7. As the photochemical reaction proceeded, the concentrations of ethanol and acetaldehyde decreased gradually. The decomposition rate of acetaldehyde on Ni/CTO photocatalysts was improved. Ni loading on CTO improved ethanol and acetaldehyde degradation. The photocatalytic degradation activity of acetaldehyde was higher than that of ethanol in the Ni/CTO photocatalyst. This is because acetaldehyde is more optically sensitive than ethanol, as confirmed by OES results. That is, when the OES light emission characteristics are excellent, excellent photoreaction activity is exhibited. In addition, the
Fig. 5 e Rate of hydrogen evolution without photocatalyst and with TiO2 photocatalyst addition (a). Rate of hydrogen evolution on various metal-loaded TiO2 photocatalysts (b). Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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Fig. 6 e Variation of OES intensities of OH· radical peak at 309 nm (a) and rate of hydrogen evolution with various ethanol concentrations (b).
Fig. 7 e Variation of OES intensities of OH· radical peak at 309 nm (a) and rate of hydrogen evolution with various acetaldehyde concentrations (b).
doping of Ni ions to the CTO extends the light absorption region to the visible light region, and thus, hydrogen production characteristics are improved by the LPP emission effect and the photocatalytic reactivity of the visible light region.
Photocatalytic reaction pathway The reaction mechanism was estimated from the gas and liquid products obtained in the photocatalytic reaction. H2 and a small amount of CO were only detected in the gaseous product from the ethanol-added reactant, but no O2 was detected. A small amount of HCHO was observed in the liquid product in the photocatalytic reaction to the C2H5OH solution. Based on these results, a typical photochemical reaction route to C2H5OH solution is proposed by the following equation:
Fig. 8 e Rates of hydrogen evolution in pure water and aqueous acetaldehyde solution on various photocatalysts.
H2O / H2 þ ½O2
(1)
C2H5OH þ ½O2 / CH3OH þ HCHO
(2)
Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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CH3OH / HCHO þ H2
(3)
HCHO / CO þ H2
(4)
Reaction Eq (1) indicates a typical photocatalytic reaction of water. As shown in Eq (2), the ethanol would be converted to CH3OH and HCHO. The CH3OH might be decomposed to HCHO and H2. The HCHO decomposed to CO and H2. Therefore, the C2H5OH added to water accelerates H2 production. In the photocatalytic splitting of the aqueous CH3CHO solution, H2 was also produced as the main gaseous product. On the other hand, no O2 was observed. Some CH4 and CO were detected in the gaseous products. In addition, a small amount of C2H5OH, HCHO, and CH3COOH were defined newly. The pathway of the photocatalytic reaction of water can be suggested from the product distributions of the gas and liquid as shown in the reaction equations below, H2O / H2 þ ½O2
(5)
CH3CHO / CH4 þ CO
(6)
CH3CHO þ ½O2 / CH3COOH
(7)
CH3COOH / 2CO þ 2H2
(8)
9
improved by the ratio of these organic chemicals. TiO2 and CTO perovskite showed photocatalytic activity in photocatalytic water decomposition for H2 production. The CTO perovskite showed higher photocatalytic activity than TiO2. Ni loading on CTO led to an increase of H2 production.
Acknowledgment This research was supported by Nano$Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4908162).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.01.089.
references
Eq. (5) indicates the typical photo-chemical reaction of water. A small amount of O2 was generated in the gaseous products. The oxygen might be consumed in the reaction with CH3CHO, as described in Eq. (7). Large H2 generation would be observed according to Eq. (8). Therefore, the H2 generation was enhanced significantly owing to the additional H2 generation by the photo-decomposition of CH3CHO. The weak secondary reactions can be suggested as Eqs (9)e(11). C2H5OH, and HCHO might be formed according to these reactions. These suggestions were made from the observation of new products in the liquid products. CH3COOH /CH3OH þ CO
(9)
CH3OH / HCHO þ H2
(10)
HCHO þ CH4 / C2H5OH
(11)
Conclusion The correlation between H2 generation and OES in H2 generation by photocatalytic reaction using LPP was evaluated. OES was changed according to the plasma discharge conditions in the liquid phase. The intensity of the OH radical peak at 309 nm increased with the addition of ethanol or acetaldehyde in water. The highest OES intensity was found at 10 vol% ethanol concentration and 10 vol% acetaldehyde concentration in aqueous solution. The highest rate of H2 generation also occurred at 10 vol% in aqueous solutions containing ethanol or acetaldehyde. The rate of H2 generation in the ethanol or acetaldehyde aqueous solution is improved because the light emission characteristics of the plasma are
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Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts Kyong-Hwan Chung a, Byung-Joo Kim b, Sun-Jae Kim c, Young-Kwon Park d, Sang-Chul Jung a,* a
Department of Environmental Engineering, Sunchon National University, 255 Jungang-ro, Sunchon, Jeonnam, 57922, Republic of Korea b Korea Institute of Carbon Convergence Technology, R&D Division, 110-11 Banryong-ro, Jeonju, 54853, Republic of Korea c Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, 209 Neungdong-ro, Gwangjingu, Seoul, 05006, Republic of Korea d University of Seoul, School of Environmental Engineering, 163 Seoulsiripdaero, Dongdaemun-gu, Seoul, 02504, Republic of Korea
highlights Photo-decomposition
graphical abstract by
liquid
phase plasma was assessed in organic pollutants solution. The effects of irradiation of the liquid phase plasma were evaluated over TiO2 photocatalyst. Optical emission properties were measured with various conditions of plasma discharging. CaTiO3 perovskite was employed as a photocatalyst with Ni loading. Hydrogen evolution was increased significantly in the aqueous acetaldehyde solution.
article info
abstract
Article history:
Photocatalytic decomposition of organic materials-contained aqueous solution is assessed
Received 30 November 2019
using a plasma discharged into the liquid directly. The correlation of H2 generation and
Received in revised form
optical emission spectroscopy is discussed in terms of photocatalytic H2 production using
6 January 2020
plasma and photocatalysts. Variations of the active species are evaluated according to the
Accepted 13 January 2020
conditions of the plasma in the liquid phase. The optical emission spectra vary according
Available online xxx
to the plasma discharging conditions in the liquid phase. The intensities of the OH$ peaks at 309 nm increase with the addition of ethanol or acetaldehyde in water. The highest
* Corresponding author. E-mail address: [email protected] (S.-C. Jung). https://doi.org/10.1016/j.ijhydene.2020.01.089 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
2
Keywords: Liquid phase plasma Optical emission Hydrogen evolution Photocatalytic decomposition Photocatalysts
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intensities and rate of H2 evolution are observed at a 10% acetaldehyde concentration in the aqueous solution. The rates of H2 evolution in the ethanol or acetaldehyde solution correspond to the concentration of OH$ in the solution. The photocatalytic reaction using liquid plasma generates hydrogen at the same time as the decomposition of the organic chemicals. The rate of hydrogen evolution in aqueous solutions containing the organic chemicals is higher than that in pure water. This is because hydrogen is further generated due to hydrogen generation by photolysis of the organic chemicals. CaTiO3 perovskite photocatalyst shows better photocatalytic activity than TiO2. Ni loading on the photocatalyst lead to an increase in H2 production. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Plasma discharges have attracted considerable attention because they have the ability to cause various chemical reactions [1e3]. Plasma is generated by high voltage discharges at various phases. Plasma is typically plasma generated in the gas phase, which is generally applied to produce fine crystals for thin films and semiconductors. Plasma generated in solid phase is called plasmon and is applied to various fields such as medical. On the other hand, even in the liquid phase, the plasma may be generated by the high voltage. However, the technique of applying plasma to the liquid phase has not been developed so far, and is still limited to the research field. Liquid phase plasma (LPP) is generated by high voltages emitted directly into the liquid. Plasma induced by highvoltage discharges emits dense ultraviolet and visible light, producing many active species [4,5]. Discharge in liquid phase leads to higher plasma density and larger spatial distribution than high voltage UV lamp illumination [6,7]. Therefore, it is expected that LPP can be effectively applied to the underwater photolysis reaction for H2 generation [8]. LPP simultaneously causes five effects: electric field, strong ultraviolet light, high pressure shock wave, various free radical generation and ozone generation. Such an effect can destroy harmful chemicals and eliminate microorganisms in the liquid. In addition, the strong release of LPP can produce H2 by photocatalytic separation of water. LPP has been applied to many industries. High technology materials such as fullerenes, carbon nanotubes and graphene can be easily manufactured using LPP technology from organic compounds such as bisphenol A, benzene and toluene. Highly crystalline carbon nanocrystals are produced uniformly and quickly by LPP irradiation. Carbon nanoparticles are also produced quickly by irradiating carbon source materials with LPP. LPP can also be applied as a more powerful light source in the photocatalytic hydrolysis for H2 production. Photocatalytic decomposition of water is an efficient solution for converting solar energy into H2 energy. In particular, the light source as well as the photocatalyst is the core of the photochemical reaction. Photocatalytic activity for H2 production was mainly investigated under ultraviolet and visible light irradiation. Few studies have investigated photocatalysts using the LPP system by direct irradiation in water.
Photo-electrochemical separation of water is of interest as a method of producing hydrogen [9]. Recently, attention has been focused on the development of a semiconductor that can be applied to a technique for producing hydrogen by photochemical reaction using solar energy [10,11]. TiO2 has been applied as a representative photocatalyst with high photocatalytic activity and chemical stability. However, the band gap is large and limited to photocatalysts that are effective only in ultraviolet light [12,13]. Therefore, various studies on the development of a new visible light sensitive photocatalyst showing high reaction efficiency to visible light have been conducted [14e16]. In recent years, in order to develop photocatalysts that achieve high reactivity under visible light, semiconductor TiO2 has been developed by several methods such as doping metal or nonmetal ions, improving surface sensitivity of TiO2, bonding with narrow bandgap semiconductors, and doping of precious metals on TiO2 surfaces. Much research has been carried out on the modification of TiO2 [17e23]. Doping the transition metal to the TiO2 structure improves the photocatalytic efficiency. Transition metals such as V, Cr, Fe, Mg, Co, Zn and Mo have enhanced photocatalytic activity when doped to TiO2 lattice [24e28]. Incorporation of the dopant results in an increase or decrease in the band gap. Perovskite materials are one of the best photocatalysts in the field of photochemical reactions. Perovskite is known to exhibit photocatalytic properties even in visible light. Improving photocatalytic properties with perovskite materials has been evaluated as a particular challenge [29]. Recently, CaTiO3 attracts attention because of its easy synthesis and high stability [30]. Heterogeneous photocatalysts have been applied as an effective technique for water treatment in various wastewater treatment techniques by photocatalytic reaction [31]. Photolysis of organic wastewater using the LPP process promotes purification and hydrogen production. Plasma reactions are generally carried out in gas phase [1,32]. Recently, photoreactions by plasma emitted from pure water have been reported [33e35]. LPP is produced by direct release as a liquid at high voltages. High pressure pulsed plasma discharge in liquid phase is an effective technique for wastewater treatment [5,6,36]. LPP technology by high pressure discharge quickly removes organic substances from waste water and there is no secondary pollution. This process has the advantages of high decomposition efficiency of pollutants, simple process and
Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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low energy consumption. LPP produces higher plasma density and larger spatial distribution than UV lamp radiation [8]. LPP technology effectively eliminated organic pollutants and hydrogen production in previous studies [37]. Research has also been made to purify wastewater and produce hydrogen at the same time through the process of decomposing hardly decomposable wastewater by a photocatalytic reaction using LPP and photocatalyst. Photocatalytic reactions with LPP and photocatalysts have been found to be effective in producing hydrogen from wastewater treatment. However, the photochemical reaction of LPP in the liquid phase, which is the basic reaction principle, has not been considered. It is necessary to elucidate the correlation with the photocatalytic reaction according to the optical properties of the LPP. This study evaluated the optical emission properties of LPP in liquid phase reactants over the range of emission conditions for LPP. Various active species produced under various LPP release conditions were also evaluated. Various additives were injected into the water to investigate changes in the optical emission spectrum (OES). Investigating the correlation between H2 production and OES by the reaction of LPP and photocatalyst, the method to improve photocatalytic reaction efficiency is discussed. TiO2 and metal/TiO2 nanocrystals were used as photocatalysts. The photochemical reaction ability of the CaTiO3 perovskite photocatalyst was also evaluated in the photocatalytic decomposition of water for H2 production. The influence of organic reagents injected into water is also assessed in relation to the H2 evolution rate.
Methods Materials TiO2 (Degusa, Aeoroxide P25) was introduced as a standard photocatalyst. Ni, Co and Fe metals were used as non-noble metal ions. These were loaded onto the TiO2 surface. As precursor reagents for metal ions, Ni(NO3)2$6H2O (Daejung, 99%), CoCl2$6H2O (Daejung, 99%), FeCl2$4H2O (Daejung, 99%) were used. Metal ions were loaded onto TiO2 according to a typical wet impregnation method. The concentration of metal ions incorporated into TiO2 was adjusted to 2% by weight. CaTiO3 (CTO) was synthesized using the sol-gel method according to the formula CaTiO3:Eu3þ. Distilled water was used as the basic solvent. Zn(NO3)2$6H2O, Ca(NO3)3$4H2O and Eu(NO3)3$6H2O were used as starting materials and prepared in stoichiometric amounts. Ca(NO3)3$4H2O (94.45 g), Zn(NO3)2$6H2O (29.75 g), and Eu(NO3)3$6H2O (8.90 g) were injected into distilled water, followed by vigorous stirring at 65 C for 3 h. Then Ti(OC4H9)4 (170.15 g) was injected into the solution and stirred for 2 h. This solution was dried at 115 C for 12 h. The resulting solid was calcined at 1100 C for 5 h in a muffle furnace at 10 C/min. The calcined solid was ground to make a fine powder. Nickel incorporated CTO were prepared according to a typical impregnation method [38]. Ni(NO3)2$6H2O (Daesin, 99%) was used as a precursor of Ni ions. This precursor was dissolved in distilled water. CTO was then injected into this solution and stirred at 80 C for 12 h. The catalyst was then
3
dried and calcined to remove volatile components in the solution to deposit metal on the CTO surface. Ni incorporated CTO photocatalyst was dried at 120 C and calcined at 1100 C. The calcined photocatalyst was ground to fine particles. The content of Ni loaded in the CTO photocatalyst was adjusted to 2 wt%.
Photocatalytic hydrogen generation by LPP irradiation Photocatalytic decomposition of water by LPP irradiation was carried out in pure water and water containing organic matter. Schematic of a photochemical reactor equipped with an LPP system is shown in Fig. S1 (Supplementary material). The photocatalytic decomposition reaction was carried out in a photo-reactor under LPP irradiation. Distilled water or aqueous reactants (200 mL) containing organic matter were injected into the photochemical reactor together with the photocatalyst (1 g). Gas products resulting from the photoreaction were analyzed by gas chromatograph (GC; Youngllins, Y600D) by a thermal conductivity detectors (TCD) connected to a column packed molecular sieve 5 A. To supply a constant flow rate, the flow rate of the carrier gas was adjusted to 20 mL/min using a mass flow controller (MFC; MJT, MR300). The internal temperature of the LPP reaction system equipped with the needle electrode was maintained to 25 C by the water bath.
Analysis of optical emission OES was obtained during LPP discharge into the reactants using an optical fiber and an optical fiber spectrometer (Avantesie, AvaSpec-3600) installed perpendicular to the axis of the electrode. LPP was generated by a pulse discharge using two needle-type tungsten electrodes. The gap between the two needle electrodes was controlled to 0.3 mm. OES was estimated at various concentrations in the reactants, aqueous ethanol and aqueous acetaldehyde. Measurement conditions during the OES acquisition were controlled with various LPP emission conditions. The voltage of LPP discharge was 220e250 V. The frequency of the LPP discharged by the power source was 25 kHze30 kHz. The pulse width was 3 mse5 ms The change in peak of optical emission with plasma discharge conditions of the plasma generator at 309 nm, 486 nm and 656 nm in Fig. S2. OES also changed with changing plasma generation conditions such as voltage, frequency and pulse width. The peak changed according to the variation of plasma generation conditions. As the voltage increased at all three wavelengths, the intensity of the OES increased. Higher frequencies and wider pulse widths result in higher intensity of OES. The highest intensities to the three optical emission peaks were seen under LPP discharge conditions of 250 V, 40 kHz, and 5 ms The plasma generation conditions were then fixed at these values.
Characterization of photocatalysts The structure and crystallinity of the photocatalyst were analyzed by high resolutions X-ray diffraction (XRD; Rigaku, D/max-II/PC1) using Ni filtered CuKa X-ray radiations (l ¼ 1.54056 A). The N2 adsorption isotherm of the
Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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photocatalyst was obtained using a volume adsorption measuring device (Porosity-HG, M SI) at 197 C. By pretreatment the photocatalyst was exposed to N2 gas while preheating at 180 C. for 3 h. Their apparent surface area was determined using the BET equation. The morphology of the photocatalyst was observed by scanning electrons microscopy (SEM; Varian, S-4750). Their elemental composition was analyzed using energy dispersive X-ray spectroscopy (EDS; Norans Z-MAXI 350). Transmission electrons microscopy (TEM; FEII Technaii 200) was measured using LaB6 filaments with an accelerating voltage of 200 kV. The composition of the photocatalyst component was investigated using an EDS micro analyzer (PGIS IMIXI PC1) mounted on a microscope. UVevisible diffuse reflection spectroscopy (DRS) was measured using a UVevisible spectrometer (Hittachii, UV-3000) in the region of 200e1000 nm using BaSO4 as the reflection standard. The optical band gap (Egap) was obtained by adopting the proposed Ku¨belka and Mu¨nk methods for indirect electron transitions [39]. The optical emission spectrum (OES) of the PLPs was measured while directly discharging from the reaction using a fiber optical spectrometer (Avantessi, AvaSpecis-3500). Optical spectra were collected by installing an optical fiber perpendicular to the axis of the electrode. The estimation conditions were 5 ms pulse width, 250 V discharge voltage and 30 kHz frequency while collecting the spectra. Fourier transform infrareds (FT-IR; JASCOS 600 plus) spectroscopy was first diluted to 10% in KBr and then measured in the frequency range of 400e4000 cm1 at ambient temperature. Metal states of Ni-loaded CTO samples were analyzed by X-ray photoelectron spectroscopy (XPS; MultoLab 2000). All samples were irradiated with a monochromatic Al Ka X-ray source at a laboratory pressure of 106e107 Pa. The resolution of the instrument was 0.35 eV wide using silver Fermi edge.
Results and discussion Characteristics of optical emission by LPP Fig. 1 illustrates the OES results measured during LPP irradiation in pure water. LPP irradiation in pure water produced active species and strong UV light and visible light illumination. The strong atomic peaks expressed as active species in the photoreaction resulted in Hb at 486 nm, Ha at 656 nm, OI at 777 nm, and OH (A-X) radical molecular peaks at 309 nm. This indicates that the light source of LPP can cause photochemical reactions in both ultraviolet and visible range (300 nme900 nm). In particular, the emission of UV light at 309 nm and the visible light at 609 nm were large. This is indicated to be excellent in the reaction activity of the photosensitive photocatalyst in the two wavelength range. In the absence of the photocatalyst, H2 was produced by photolysis of water under LPP irradiation due to the active species. However, the photo-reaction on the photocatalyst under LPP irradiation is mainly caused by strong UV and visible light. Thus, H2 gas generation from photochemical reactions by LPP radiation in the presence of photocatalysts is governed by stronger UV and visible light irradiation than the active species produced.
Fig. 1 e OES of LPP measured at various wavelength with discharging conditions of LPP generation in distilled water. (a) 309 nm, (b) 486 nm, (c) 656 nm.
The OES of the LPP in pure water and ethanol or acetaldehyde-added water is presented in Fig. S3. The intensities of OES at 309 nm, which generates a OH$ radical, increased after the addition of ethanol or acetaldehyde into the water. In particular, the intensity at 309 nm was enhanced significantly with ethanol addition into water. Other main peaks at 486 nm and 656 nm were also increased with ethanol and acetaldehyde addition into water. The intensities of the OES increased strongly in the aqueous acetaldehyde solution. This suggests that the emission characteristics of plasma in aqueous solution were improved by the injection of ethanol or
Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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acetaldehyde organic chemicals. This improvement in luminescence results in an improvement in photo-reactivity by LPP. Fig. 2(a) presents the OES of the LPP in an ethanol solution with different ethanol concentrations. Fig. 2(b) shows variations of the OES at main peaks wavelength, such as 309 nm, 486 nm, and 656 nm. The Ha peaks at 656 nm in the OES did not change with ethanol concentration. The Hb peaks at 486 nm increased slightly with increasing ethanol concentration. The highest intensity of the OES at 309 nm, which generates OH$ radicals, was observed with the 10% ethanol solution. The OH$ radicals induce mainly a photodecomposition reaction. Therefore, the OES intensities were highest in the 10 vol% ethanol solution. Fig. 3(a) presents the OES by LPP irradiation with different acetaldehyde concentrations in solution. The addition of acetaldehyde into water resulted in a significant increase in the intensities of the OES. Fig. 3(b) presents the variations of OES intensities at 306 nm (OH$), 486 nm (Hb), and 656 nm (Ha). The intensities of the OES at 486 nm were unchanged despite the injection of acetaldehyde into water. The intensities of OES at 309 nm and 656 nm increased with increasing acetaldehyde concentration in water. On the other hand, the intensities were not enhanced further when the acetaldehyde concentration as more than 20 vol%. The highest intensities of the OES were obtained at an acetaldehyde concentration of 10 vol% in water. The luminescence properties of the plasma are improved by the injection of ethanol or acetaldehyde organic chemicals, and the injection concentration is 10 vol%, which suggests the best ultraviolet or visible light emission.
Physicochemical properties of the photocatalysts The XRD pattern of TiO2 and Ni/TiO2 photocatalyst is shown in Fig. S4 (a). XRD peaks of the anatase properties of TiO2 and weak rutile XRD peaks were observed. XRD peaks for titanium dioxide were found to be predominantly anatase phase. According to the data provided by the manufacturer, the ratio of anatase to rutile was ca. 80:20. A weak XRD peak for Ni at 33 2q was found in the Ni/TiO2. This indicates that Ni is incorporated into TiO2. However, as a result, it is difficult to
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determine whether Ni exists as a metal or in the form of NiO. The TEM analysis and Ni map results of the Ni/TiO2 photocatalyst is also shown in Fig. S4 (b). TiO2 crystal size was measured to be about 40 nm. Ni ions incorporated into the TiO2 surface were observed in the images of Ni maps. Ni atoms appeared as dots in the elemental map of the Ni/TiO2 photocatalyst. This is because Ni atoms are incorporated on the TiO2 surface as very small particles. Metal nanoparticles of Ni on the TiO2 surface were difficult to distinguish from TEM images. At lower metal loading, the size of the Ni nanoparticles was less than 1 nm, which was at the resolution limit of the TEM instrument. The amount of Ni incorporated onto the TiO2 surface was estimated to be 1.92% by weight, as defined from the EDS results. The XRD pattern of CTO photocatalyst is shown in Fig. S5 (a). Reference XRD data of the CTO photocatalyst is presented together in the figure for comparison with the prepared photocatalyst. As shown in Fig. S5 (a), the position and intensity of the primary peak was in good agreement with the data of the JCPDS card (No. 22-0153), which is the reference data. XRD data was indexed to tetragonal CaTiO3. Although Zn2TiO4 and Ca2ZnTi15O36 peaks were observed, their strength was very weak. In the first stage of CaTiO3, the strength was not affected. Fig. S5 (b) shows representative EDS results of CTO photocatalyst. EDS of CTO showed the first peak related to Zn, Ca and Eu. Ti appeared as a small peak at 6 keV, because the amount contained in the compound was very small. The EDS results were in good agreement with the data in Ref. [40]. The ionic radii of Ca2þ, Ti4þ and Eu3þ ions at the octahedral site are 1.00, 0.605 and 0.947 A, respectively [41]. This represents Eu3þ ions that replaced Ca2þ ions more easily than Ti4þ ions in the CaTiO3:Eu3þ lattice because Ca2þ and Eu3þ ions have similar ion radii. In addition, a simulated tetragonal lattice of CaTiO3 Eu3þ has been reported, indicating the substitution of Eu3þ for Ca2þ at the octahedral site [42]. These results suggest that the synthesized CTO is in good agreement with the standard CaTiO3 containing small amounts of Zn. Fig. S5 (c) presents the surface morphology of CTO measured by SEM. The CTO particles were under micron-sized crystallites, which exist with conglomerations among the crystallites because of the high sintering temperatures. The
Fig. 2 e OES of LPP irradiation in aqueous ethanol solution with various ethanol concentrations (a) and variation of main peaks of OES with various ethanol concentrations (b). Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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Fig. 3 e OES of LPP irradiation in aqueous acetaldehyde solution with various acetaldehyde concentrations (a) and variation of main peaks of OES with various acetaldehyde concentrations (b).
CTO photocatalyst had morphology of conglomerated irregular cubes. The small crystallites were loaded onto aggregated crystallites. The particles are spherical polyhedral, and size is ca. 3 mm. Fig. S5 (d) shows the FT-IR spectrum of the CTO photocatalyst measured at 400e4000 cm1. Broadbands of 2900e3700 cm1 with a peak of 3423 cm1 show characteristic stretching vibrations of the hydroxyl (-OH) group of H2O. OH bending bands appeared at 1650 cm1. The peak at 877 cm1 is due to symmetrical stretching vibrations along the CaeOeCa bond. The band at 1439 cm1 is represented by the asymmetric stretching vibration of the hydroxylate of the OHeCa bond. The band at 560 cm1 is represented by the CaeO vibration bonds of CaTiO3. A wide and strong band, characteristic of alkaline titanates, appeared at 460 cm1. The band at 421 cm1 corresponds to the asymmetric stretching vibration of TieO. These results suggest that the CTO produced in this study is in good agreement with the standard CaTiO3. Fig. 4 shows Ni2p and O1s spectra of XPS for Niincorporated CTO. The Ni2p spectrum of Ni-loaded CTO was very similar to that for NiO powders reported in the literature [32]. Satisfactory fitting was consistent with the literature results by showing a change in the binding energy position (853.7 eV) of the multiple contributions to the main peak of NiO. The three peak intervals that make up the main peak
were changed from 0.95 eV to 1.4 eVe0.5 eV and 1.6 eV, respectively. O1s spectrum was also consistent with the NiO spectrum presented in the literature [43]. Therefore, the state of Ni loaded onto CTO was confirmed as NiO state. The DRS of TiO2 and metal/TiO2 converted to Ku¨belkaMu¨nk expression are shown in Fig. S6 (a) [39]. Optical properties of the photocatalysts are initiated by light absorption in photochemical reactions. The adsorption edge of TiO2 was about 380 nm in the DRS results. In contrast, those of the metal/TiO2 photocatalysts were moved to the upper range. That is, when TiO2 was loaded with the metals, the adsorption wavelength was extended to the range of visible light. The adsorption edges in the DRS results of various metal-loaded TiO2 appeared in the upper light range compared to TiO2. This means that the adsorption range is extended by the metal incorporation. The adsorption edges of Ni/TiO2, Fe/TiO2 and Co/TiO2 were 398 nm, 400 nm and 405 nm, respectively. The band gap ranges from 2.7 eV to 2.9 eV. No significant difference was observed in the band gap between the metal/ TiO2 photocatalysts. The DRS results for CTO and Ni/CTO photocatalysts converted in Kubelka-Mu¨nk units are shown in Fig. S6 (b). The optical properties of the photocatalyst are mainly due to photo adsorption in the photochemical reaction process. The spectrum of the CTO appeared in the upper
Fig. 4 e XPS results on (a) Ni2p scan and (b) O1s with Ni-loaded CTO perovskites. Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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wavelength range. The absorption edge of CTO was about 380 nm. Ni doping on the CTO extended the absorption range to the visible light range. The band gaps of the CTO and Ni/ CTO were 3.0 eV and 2.8 eV, respectively.
Hydrogen evolution by LPP irradiation Fig. 5(a) shows the H2 generation rate obtained by decomposition of water by LPP irradiation in the absence of a photocatalyst and a condition in which the TiO2 photocatalyst was injected. From the photolysis reaction of water, H2 gas and a small amount of O2 gas were obtained in the gaseous product. No new liquid product was observed during the photochemical reaction. Even in the absence of a photocatalyst, a small amount of H2 was produced by LPP irradiation. The hydrogen production appeared to be due to the active species generated by the LPP irradiation. This is because LPP irradiation generates various active species such as H$, OH$, O$, HO2, H2O2, O 2 and O3 from water [35]. The active species leads to the generation of H2 during LPP irradiation. OH$ radicals and Hþ ions were generated by LPP irradiation in water as shown in Fig. S2. Injecting TiO2 photocatalyst into this reaction significantly increased the rate of H2 generation. Fig. 5(b) shows the rate of H2 evolution by LPP irradiation on metal/TiO2 photocatalyst. The rates were increased by loading the metals on TiO2. Ni/ TiO2 photocatalyst caused the most H2 generation. As mentioned above, the metal-incorporating TiO2 absorbed light at about 400 nm. Their bandgaps were ranged from 2.7 eV to 2.9 eV. Therefore, when TiO2 is loaded with metal, photosensitivity is increased and H2 generation rate is increased. Fig. 6(a) illustrates the rate of H2 evolution according to the ethanol concentration in water after 30 min of photo-reaction time. The rates of H2 evolution increased remarkably after the addition of ethanol to water. The highest H2 evolution was observed at 10 vol% ethanol in water. Fig. 6(b) represents variation of OES at 309 nm, which generates a OH$ radicals. The highest intensity of the OES at 309 nm was observed with the 10% ethanol solution. The OH$ radicals induce mainly a photodecomposition reaction. Therefore, the rate of H2 generation and OES intensities were highest in the 10 vol% ethanol solution. This is in accord with the excellent result of
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OES emission characteristics when the concentration of the organic chemical in the aqueous solution containing ethanol or acetaldehyde organic chemicals is 10 vol%, the hydrogen generating activity is excellent. Fig. 7(a) presents the rate of H2 evolution with acetaldehyde injection into water. The rates of H2 evolution were improved by the injection of acetaldehyde to water than that of pure water. The rates of H2 generation were higher in the acetaldehyde solution than ethanol solution. The highest rate of H2 generation was obtained in the 10% acetaldehyde concentration in water. This showed the same tendency as in the ethanol aqueous solution. In acetaldehyde aqueous solution, when the concentration is 10 vol%, the hydrogen production rate is the highest. Fig. 7(b) shows the variation of OES at 309 nm, which generates a OH$ radicals. The highest intensity of the OES at 309 nm revealed with the 10% acetaldehyde solution. The OH$ radicals induce mainly a photodecomposition reaction. Therefore, the rate of H2 evolution and OES intensities were highest in the 10 vol% ethanol solution. This suggests that the superior intensity of the OES under the conditions of the LPP photo-reaction improved the H2 generation properties. Fig. 8 shows the rates of H2 evolution in pure water and aqueous acetaldehyde solution on various photocatalysts. CTO showed a higher rate of H2 evolution than TiO2. Rate of H2 generation was improved with Ni incorporating on CTO than that of bare CTO. In the acetaldehyde solution, the rates were increased remarkably because of addition of hydrogen generation derived from acetaldehyde decomposition. The decomposition of ethanol and acetaldehyde by photocatalytic degradation using LPP is shown in Fig. S7. As the photochemical reaction proceeded, the concentrations of ethanol and acetaldehyde decreased gradually. The decomposition rate of acetaldehyde on Ni/CTO photocatalysts was improved. Ni loading on CTO improved ethanol and acetaldehyde degradation. The photocatalytic degradation activity of acetaldehyde was higher than that of ethanol in the Ni/CTO photocatalyst. This is because acetaldehyde is more optically sensitive than ethanol, as confirmed by OES results. That is, when the OES light emission characteristics are excellent, excellent photoreaction activity is exhibited. In addition, the
Fig. 5 e Rate of hydrogen evolution without photocatalyst and with TiO2 photocatalyst addition (a). Rate of hydrogen evolution on various metal-loaded TiO2 photocatalysts (b). Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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Fig. 6 e Variation of OES intensities of OH· radical peak at 309 nm (a) and rate of hydrogen evolution with various ethanol concentrations (b).
Fig. 7 e Variation of OES intensities of OH· radical peak at 309 nm (a) and rate of hydrogen evolution with various acetaldehyde concentrations (b).
doping of Ni ions to the CTO extends the light absorption region to the visible light region, and thus, hydrogen production characteristics are improved by the LPP emission effect and the photocatalytic reactivity of the visible light region.
Photocatalytic reaction pathway The reaction mechanism was estimated from the gas and liquid products obtained in the photocatalytic reaction. H2 and a small amount of CO were only detected in the gaseous product from the ethanol-added reactant, but no O2 was detected. A small amount of HCHO was observed in the liquid product in the photocatalytic reaction to the C2H5OH solution. Based on these results, a typical photochemical reaction route to C2H5OH solution is proposed by the following equation:
Fig. 8 e Rates of hydrogen evolution in pure water and aqueous acetaldehyde solution on various photocatalysts.
H2O / H2 þ ½O2
(1)
C2H5OH þ ½O2 / CH3OH þ HCHO
(2)
Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089
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CH3OH / HCHO þ H2
(3)
HCHO / CO þ H2
(4)
Reaction Eq (1) indicates a typical photocatalytic reaction of water. As shown in Eq (2), the ethanol would be converted to CH3OH and HCHO. The CH3OH might be decomposed to HCHO and H2. The HCHO decomposed to CO and H2. Therefore, the C2H5OH added to water accelerates H2 production. In the photocatalytic splitting of the aqueous CH3CHO solution, H2 was also produced as the main gaseous product. On the other hand, no O2 was observed. Some CH4 and CO were detected in the gaseous products. In addition, a small amount of C2H5OH, HCHO, and CH3COOH were defined newly. The pathway of the photocatalytic reaction of water can be suggested from the product distributions of the gas and liquid as shown in the reaction equations below, H2O / H2 þ ½O2
(5)
CH3CHO / CH4 þ CO
(6)
CH3CHO þ ½O2 / CH3COOH
(7)
CH3COOH / 2CO þ 2H2
(8)
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improved by the ratio of these organic chemicals. TiO2 and CTO perovskite showed photocatalytic activity in photocatalytic water decomposition for H2 production. The CTO perovskite showed higher photocatalytic activity than TiO2. Ni loading on CTO led to an increase of H2 production.
Acknowledgment This research was supported by Nano$Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4908162).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.01.089.
references
Eq. (5) indicates the typical photo-chemical reaction of water. A small amount of O2 was generated in the gaseous products. The oxygen might be consumed in the reaction with CH3CHO, as described in Eq. (7). Large H2 generation would be observed according to Eq. (8). Therefore, the H2 generation was enhanced significantly owing to the additional H2 generation by the photo-decomposition of CH3CHO. The weak secondary reactions can be suggested as Eqs (9)e(11). C2H5OH, and HCHO might be formed according to these reactions. These suggestions were made from the observation of new products in the liquid products. CH3COOH /CH3OH þ CO
(9)
CH3OH / HCHO þ H2
(10)
HCHO þ CH4 / C2H5OH
(11)
Conclusion The correlation between H2 generation and OES in H2 generation by photocatalytic reaction using LPP was evaluated. OES was changed according to the plasma discharge conditions in the liquid phase. The intensity of the OH radical peak at 309 nm increased with the addition of ethanol or acetaldehyde in water. The highest OES intensity was found at 10 vol% ethanol concentration and 10 vol% acetaldehyde concentration in aqueous solution. The highest rate of H2 generation also occurred at 10 vol% in aqueous solutions containing ethanol or acetaldehyde. The rate of H2 generation in the ethanol or acetaldehyde aqueous solution is improved because the light emission characteristics of the plasma are
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Please cite this article as: Chung K-H et al., Correlation of hydrogen generation and optical emission properties of plasma in water photolysis on perovskite photocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.01.089