Harnessing metal-free photocatalysts for renewable hydrogen and hydrogen peroxide

Abstract

Undesirable climate change due to fossil-fuel emissions has created a strong impetus for the exploration of renewable energy sources, particularly from the abundant solar power via photocatalysis. However, the majority of semiconductive photocatalysts still suffer from low efficiency - with a high rate of photoinduced electron-hole (e-h) recombination and poor visible light absorption. The main aim of this thesis is to develop functional metal-free photocatalysts so as to generate renewable energy carriers under visible light with high efficiency without extensive use of costly metallic elements. Two renewable energy/chemical carriers were considered here: hydrogen for its application in highly efficient fuel cells and H2O2 for its versatility as chemical reactants, bleaches, and disinfectants. Graphitic carbon nitride (g-C3N4) has garnered significant attention as a metal-free and visible-light-responsive photocatalyst in the areas of solar energy conversion. Nonetheless, its photocatalytic efficiency is severely affected by the recombination process and the limited active sites. Here, in Chapter 3, an effective approach was demonstrated for enriching active sites and inhibiting the e-h recombination process of g-C3N4, leading to a significant increase in photocatalytic hydrogen evolution reaction (HER) activity. The approach involved the calcination of urea to obtain the bulky g-C3N4, which was then exfoliated into holey g-C3-xN4 sheets with a larger specific area and carbon deficiency. Subsequently, Au single atoms were anchored onto the surface of the holey g-C3-xN4. The designed Au1-Ho@g-C3-xN4 shows a higher visible-light-driven photocatalytic HER performance (3200 µmol·h-1·g-1) than the bulky g-C3N4 (trace amount). The atomically dispersed Au-N3 sites serve as active domains for hydrogen evolution, which enhances the e-h separation and hinders the recombination process so as to achieve high catalytic reactivity. Another approach (Chapter 4) involves the modification of hydrothermal carbonaceous carbon (HTCC) for photocatalytic H2O2 generation. HTCC can be produced on an industrial scale from cost-effective precursors and non-toxic manufacturing processes from various biomasses by hydrothermal treatment. It possesses suitable band structures for solar-driven photocatalysis. Developing from previous publications, this work focused on both an in-depth investigation of the influence of pH values on the photocatalytic process and the structure of HTCC, which are particularly important to H2O2 generation but have not been well studied before. It was identified that, in the basic condition, HTCC will generate the H2O2 at a higher rate via two different pathways: oxygen reduction reaction (ORR) at the LUMO (Lowest Unoccupied Molecular Orbital) site and water oxidation reaction (WOR) at the HOMO (Highest Occupied Molecular Orbital) site. Compared with the pH=7 condition, the H2O2 generation rate of HTCC at pH=11 is enhanced from 0.53 to 1.35 mmol gcat-1 h-1. The hydroxide ion in the alkaline solution reacts with the carboxyl-terminal groups of HTCC to form carboxylate anions, which effectively enhances the hydrophilicity and dispersion of HTCC. Moreover, the hydroxide ion also boosts the effectiveness of the indirect WOR pathway of H2O2 evolution. The findings pave the way for developing non-metal-based photocatalysts with high efficiency for scaling up and industrial production of renewable energy or chemical carriers.