The critical factors of photocatalytic H₂ production from seawater by using TiO₂ as photocatalyst

The solar H₂ generation directly from natural seawater is a sustainable way of green energy. However, it is limited by a low H₂ generation rate even compared to fresh water. In this report, TiO₂ is chosen as a model photocatalyst to disclose the critical factor to deteriorate the H₂ generation rate from seawater. The simulated seawater (SSW), which is composed of eight ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, Br⁻, SO₄²⁻, and CO₃²⁻), is investigated the effect of each ion on the H₂ production. The results indicate that all ions have a negative effect at the same concentration as in the seawater except Br⁻. The CO₃²⁻ has the most serious deterioration, and the H₂ production rate lowers near 40% even at [CO₃²⁻] of 1.5 mmol·L⁻¹. The H₂ production rate can be recovered to 85% if the CO₃²⁻ is excluded from the SSW. To understand the reason, the zeta potential of the TiO₂ treated with different ions aqueous solution reveals that the zeta potential decreases when it is treated with CO₃²⁻ and SO₄²⁻ due to they can adsorb on the surface of TiO₂ nanoparticles. Fourier transform infrared (FTIR) and thermogravimetric analysis-mass spectroscopy (TGA-MS) further confirm that the adsorbed ion is mainly from CO₃²⁻. Since the pH of seawater is about 8.9 between pK_a1 (6.37) and pK_a2 (10.3) of H₂CO₃, the CO₃²⁻ should exist in the form of HCO₃⁻ in the seawater. We proposed a simple method to remove the adsorbed HCO₃⁻ from the TiO₂ surface by adjusting the pH below the pK_a1. The results indicate that if a trace amount of HCl (adjusting pH ~ 6.0) is added to the SSW, the H₂ production rate can be recovered to 85% of that in pure water.