H 2 Fuels from Photocatalytic Splitting of Seawater Affected by Nano-TiO 2 Promoted with CuO and NiO

1 Department of Environmental Engineering, National Cheng Kung University, Tainan 70101, Taiwan 2 Sustainable Environment Research Center, National Cheng Kung University, Tainan 70101, Taiwan 3 Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan 4Department of Environmental Science and Engineering, Tunghai University, Taichung 40704, Taiwan 5Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan


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 (H 2 ) is becoming one of the better alternatives.Combustion of H 2 for energy does not emit pollutant gases such as CO, NO  , or SO  .H 2 has also been widely used in many sectors such as food, metallurgical, electronic, chemical, petroleum, and refinery industries.H 2 has been technically demonstrated for transportation, heating, and power generation [1].
H 2 yielded from the photocatalytic splitting of water has extensively received attention recently [2][3][4].Since the H 2 generation by the water electrochemical method was demonstrated, splitting of water photocatalyzed by TiO 2 for the H 2 fuel has been considered to be one of the alternates [5].The TiO 2 -based photocatalysts for H 2 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 Na 2 S and Na 2 SO 3 are generally used as the sacrificial compounds [12,13].
In the present work, CuO and NiO were used as the promoters on the nanosize TiO 2 to enhance catalytic splitting of water and seawater.Oxalic acid which may be formed from the photocatalytic reduction of CO 2 in H 2 O was used as the sacrificial reagent during photocatalysis.

Materials and Methods
The CuO (2.5%) and NiO (2.5%) supported on nanosize TiO 2 (P25) (UR, ITIT001) were prepared by the impregnation method.The samples were dried and calcined at 673 K for two International Journal of Photoenergy hours to form CuO and NiO dispersed on TiO 2 (CuO/nano-TiO 2 and NiO/nano-TiO 2 ), respectively.The X-ray diffraction (XRD) patterns of the photocatalysts were recorded on a XRD spectrometer using Cu K radiation ( = 1.5418Å) ranged from 10 ∘ to 80 ∘ (2) at the scanning speed of 5 ∘ /min.The XANES spectra of the TiO 2 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 (Δ/ (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 H 2 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 H 2 molecules × 2)/(number of incident photons) × 100.

Results and Discussion
The XRD patterns of the CuO/nano-TiO 2 and NiO/nano-TiO 2 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-TiO 2 and NiO/nano-TiO 2 , which may be due to the existence of subnanosize CuO and NiO crystallites that are well dispersed on the nano-TiO 2 .
The XANES spectra of nickel and copper in the photocatalysts are shown in Figure 2. 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 TiO 2 , respectively.
Figure 3 presents the photocatalytic splitting of water and seawater containing 0.05 M oxalic acid as the sacrificial reagent.On the nano-TiO 2 photocatalyst, a very small amount of H 2 is formed from the photocatalytic splitting of water and seawater.Notably, the accumulated H 2 yielded from the photocatalytic splitting of water on CuO/nano-TiO 2 and NiO/nano-TiO 2 is about 32.4 and 3.07 mol/g cat after the 5 h radiation, respectively.Compared to the photocatalytic splitting of seawater on CuO/nano-TiO 2 and NiO/nano-TiO 2 , the accumulated H 2 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 TiOH 2 , TiOH on TiO 2 may absorb Cl − to form TiCl, which may decrease the H 2 yield in the photocatalysis [16,17].Generally, CuO on TiO 2 can be reduced to Cu + ( 0 = 0.16 V) or Cu 0 ( 0 = 0.34 V) by attracting the excited electrons from the valence band of TiO 2 during photooxidizing [18][19][20][21].The reduction potential of NiO ( 0 = −0.25 V) is slightly less than the TiO 2 conduction band gap ( 0 = −0.26V) [22,23].Additionally, the reduction potential of H + /H 2 ( 0 = −0.00V) is less than that of CuO or NiO.CuO and NiO can thus promote TiO 2 in the photocatalytic H 2 formation.
In the separate experiments, oxalic acid was formed from simultaneously photocatalytic reduction of H 2 O and CO 2 .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 H 2 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-TiO 2 are shown in Figure 4. Without oxalic acid, H 2 may not be formed in the photocatalysis.A small amount of oxalic acid can enhance the H 2 generation.Note that International Journal of Photoenergy oxalic acid is a strong reductive reagent which may consume the photogenerated holes.After a 5 h UV-Vis irradiation, a better H 2 yield from the photocatalytic splitting of water is about 42.4 mol/g cat when the initial concentration of oxalic acid is 12.5 mM.However, the photocatalytic H 2 formation from water and seawater affected by CuO and NiO promoted TiO 2 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 H 2 generation is slightly decreased.As the initial concentration of oxalic acid is increased by four times, the H 2 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.

Conclusions
Photocatalytic splitting of seawater for H 2 formation on CuO/nano-TiO 2 and NiO/nano-TiO 2 is feasible experimentally.The sacrificial reagent (oxalic acid) can enhance the H 2 yields in the photocatalytic splitting of seawater.However, the excessive sacrificial reagent may not favor the photocatalysis.It is worth noting that a better H 2 can be yielded from the photocatalytic splitting of seawater affected by CuO/nano-TiO 2 (8.53 mol/g cat ) than that by NiO/nano-TiO 2 (1.46 mol/g cat ).In particular, the hole scavenging with International Journal of Photoenergy chlorides in seawater may be associated with the less H 2 yielded from the seawater photocatalysis (on CuO/nano-TiO 2 ) if compared with that from water (42.4 mol/g cat ).