# H2 Fuels from Photocatalytic Splitting of Seawater Affected  by Nano-TiO2 Promoted with CuO and NiO

Authors: A.-J. Simamora; Fang-Chih Chang; H. Paul Wang; T.-C. Yang; Y.-L. Wei; W.-K. Lin
Journal: International Journal of Photoenergy
 (2013)

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## Abstract

To enhance H2 yields from the photocatalytic splitting of seawater, better photocatalysts such as nanostructured TiO2 promoted with NiO (2.5%) and CuO (2.5%) have been studied in the present work. The accumulated H2 yielded from the photocatalytic splitting of seawater containing oxalic acid (50 mM) as the sacrificial reagents on CuO/nano-TiO2 is 8.53 μmol/gcat after the 5 h radiation. On the NiO/nano-TiO2 photocatalyst, the H2 yield is relatively low (i.e., 1.46 μmol/gcat). Note that the hole scavenging with chlorides in seawater may be associated with the less H2 yielded from the seawater photocatalysis (on CuO/nano-TiO2) if compared with that from water (42.4 μmol/gcat).


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## Body

## 1. Introduction

Utilization of fossil energy which may cause air pollution or global warming has negative impacts on the human health and environment. Today’s world is also facing an urgency need in developing alternative fuels. Renewable hydrogen energy (H2) is becoming one of the better alternatives. Combustion of H2 for energy does not emit pollutant gases such as CO, NOx, or SOx. H2 has also been widely used in many sectors such as food, metallurgical, electronic, chemical, petroleum, and refinery industries. H2 has been technically demonstrated for transportation, heating, and power generation [1].H2 yielded from the photocatalytic splitting of water has extensively received attention recently [2–4]. Since the H2 generation by the water electrochemical method was demonstrated, splitting of water photocatalyzed by TiO2 for the H2 fuel has been considered to be one of the alternates [5]. The TiO2-based photocatalysts for H2 generation could be promoted by codoping of antimony and chromium [6], nickel and either tantalum or niobium [7], cobalt [8], Pt [9], and nitrogen [10]. During photocatalysis, photo-excited holes can irreversibly oxidize electron donor compounds, and thus facilitate water reduction by conduction band electrons if the bottom of the conduction band of the photocatalyst is located at a more negative redox potential than the reduction potential of water [11]. Inorganic reagents such as Na2S and Na2SO3 are generally used as the sacrificial compounds [12, 13].In the present work, CuO and NiO were used as the promoters on the nanosize TiO2 to enhance catalytic splitting of water and seawater. Oxalic acid which may be formed from the photocatalytic reduction of CO2 in H2O was used as the sacrificial reagent during photocatalysis.


## 2. Materials and Methods

The CuO (2.5%) and NiO (2.5%) supported on nanosize TiO2 (P25) (UR, ITIT001) were prepared by the impregnation method. The samples were dried and calcined at 673 K for two hours to form CuO and NiO dispersed on TiO2 (CuO/nano-TiO2 and NiO/nano-TiO2), respectively. The X-ray diffraction (XRD) patterns of the photocatalysts were recorded on a XRD spectrometer using Cu Kα radiation (λ=1.5418Å) ranged from 10° to 80° (2θ) at the scanning speed of 5°/min.The XANES spectra of the TiO2 photocatalysts were also recorded on the Wigler 17C beam line at the Taiwan National Synchrotron Radiation Research Center (NSRRC). The electron storage ring was operated at the energy of 1.5 GeV (ring current = 120–200 mA). A Si(111) double crystal monochromator was used for selection of energy at an energy resolution (ΔE/E (eV/eV)) of 1.9×10-4. The absorption spectra were collected in ion chambers that were filled with helium and nitrogen mixed gases. Beam energy was calibrated by the adsorption edges of nickel and copper foils at the energy of 8335 and 8979 eV, respectively.The photocatalytic splitting of water and seawater experiments was carried out in a quartz reactor (45 mL) having a total reflection mirror system. About 100 mg of the catalyst samples were dispersed in the simulated seawater (about 35 g of NaCl in the 1 L of pure water) under magnetic stirring for five hours. A 300 W Xenon arc lamp (Oriel Instruments, Model 6259) equipped with a water filter was used as the photocatalytic light source. The H2 gas generated from the photocatalytic splitting of water and seawater was analyzed by a gas chromatography (Varian 430-GC) equipped with a thermal conductivity detector. The apparent quantum efficiency (QE) of the photocatalysts was obtained by the equation QE (%) = (number of reacted electrons)/(number of incident photons) × 100 = (number of evolved H2 molecules × 2)/(number of incident photons) × 100.


## 3. Results and Discussion

The XRD patterns of the CuO/nano-TiO2 and NiO/nano-TiO2 photocatalysts are shown in Figure 1. The main diffraction peaks of the photocatalysts at (101), (200), (004), and (105) are observed, suggesting that the photocatalysts have mainly anatase crystallites. The XRD peaks of CuO and NiO are barely observed in the CuO/nano-TiO2 and NiO/nano-TiO2, which may be due to the existence of subnanosize CuO and NiO crystallites that are well dispersed on the nano-TiO2.Figure 1
XRD patterns of (a) CuO and (b) NiO promoted nano-TiO2.The XANES spectra of nickel and copper in the photocatalysts are shown in Figure2. Metallic nickel (Ni) and copper (Cu) are not found. The absorption features observed at 8335 and 8979 eV suggest that NiO and CuO are the main nickel and copper species on the TiO2, respectively.Component-fitted XANES spectra of (a) nickel and (b) copper in the NiO/nano-TiO2 and CuO/nano-TiO2 photocatalysts, respectively.
(a)
(b)Figure3 presents the photocatalytic splitting of water and seawater containing 0.05 M oxalic acid as the sacrificial reagent. On the nano-TiO2 photocatalyst, a very small amount of H2 is formed from the photocatalytic splitting of water and seawater. Notably, the accumulated H2 yielded from the photocatalytic splitting of water on CuO/nano-TiO2 and NiO/nano-TiO2 is about 32.4 and 3.07 μmol/gcat after the 5 h radiation, respectively. Compared to the photocatalytic splitting of seawater on CuO/nano-TiO2 and NiO/nano-TiO2, the accumulated H2 yields are less than those from water by 3.8 and 2.1 times, respectively. Hole scavenging with chlorides may occur during photocatalysis [14, 15]. Many hydroxyl groups such as TiOH2, TiOH on TiO2 may absorb Cl− to form TiCl, which may decrease the H2 yield in the photocatalysis [16, 17]. Generally, CuO on TiO2 can be reduced to Cu+ (E0=0.16 V) or Cu0 (E0=0.34 V) by attracting the excited electrons from the valence band of TiO2 during photooxidizing [18–21]. The reduction potential of NiO (E0=-0.25 V) is slightly less than the TiO2 conduction band gap (E0=-0.26 V) [22, 23]. Additionally, the reduction potential of H+/H2  (E0=-0.00 V) is less than that of CuO or NiO. CuO and NiO can thus promote TiO2 in the photocatalytic H2 formation.Photocatalytic splitting of (a) water and (b) seawater with 50 mM oxalic acid sacrificial reagent affected by (A) nano-TiO2, (B) NiO/nano-TiO2, and (C) CuO/nano-TiO2.
(a)
(b)In the separate experiments, oxalic acid was formed from simultaneously photocatalytic reduction of H2O and CO2. It is of great interest to study the behavior of acetic acid as the sacrificial compound for photocatalytic splitting of water and seawater for the H2 fuel. Note that addition of sacrificial reagents in water or seawater may cause water pollution.Effects of the oxalic acid sacrificial reagent concentrations on the photocatalytic splitting of water and seawater on CuO/nano-TiO2 are shown in Figure 4. Without oxalic acid, H2 may not be formed in the photocatalysis. A small amount of oxalic acid can enhance the H2 generation. Note that oxalic acid is a strong reductive reagent which may consume the photogenerated holes. After a 5 h UV-Vis irradiation, a better H2 yield from the photocatalytic splitting of water is about 42.4 μmol/gcat when the initial concentration of oxalic acid is 12.5 mM. However, the photocatalytic H2 formation from water and seawater affected by CuO and NiO promoted TiO2 was less than that of related studies, mainly due to the fact of the much less amount of the sacrificial reagents used in the photocatalysis. With an increase of the initial oxalic acid concentration by two times, the photocatalytic H2 generation is slightly decreased. As the initial concentration of oxalic acid is increased by four times, the H2 yields from the photocatalytic splitting of water are decreased by 24%. It seems that excessive oxalic acid sacrificial reagent may not have a positive effect on the photocatalytic splitting of water.Photocatalytic splitting of (a) water and (b) seawater affected by CuO/nano-TiO2 with (A) 0, (B) 12.5, (C) 25, and (D) 50 mM of the oxalic acid sacrificial reagent.
(a)
(b)


## 4. Conclusions

Photocatalytic splitting of seawater for H2 formation on CuO/nano-TiO2 and NiO/nano-TiO2 is feasible experimentally. The sacrificial reagent (oxalic acid) can enhance the H2 yields in the photocatalytic splitting of seawater. However, the excessive sacrificial reagent may not favor the photocatalysis. It is worth noting that a better H2 can be yielded from the photocatalytic splitting of seawater affected by CuO/nano-TiO2 (8.53 μmol/gcat) than that by NiO/nano-TiO2 (1.46 μmol/gcat). In particular, the hole scavenging with chlorides in seawater may be associated with the less H2 yielded from the seawater photocatalysis (on CuO/nano-TiO2) if compared with that from water (42.4 μmol/gcat).



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