Modified Poly(Heptazine Imides): Minimizing H₂O₂ Decomposition to Maximize Oxygen Reduction

Area of Science:

• Materials Science
• Chemical Engineering
• Sustainable Chemistry

Background:

• Hydrogen peroxide (H₂O₂) is a key industrial chemical traditionally synthesized via the anthraquinone process.
• Photocatalysis offers a sustainable alternative for H₂O₂ production through oxygen reduction reaction (ORR).
• Two-dimensional carbon nitrides show promise as photocatalysts due to their selectivity for the two-electron ORR pathway.

Purpose of the Study:

• To investigate the photocatalytic activity of functionalized poly(heptazine imides) (PHIs) for H₂O₂ synthesis.
• To evaluate the role of protonation and transition metal functionalization in PHI photocatalysts.
• To advance the development of efficient and scalable photocatalytic H₂O₂ production.

Main Methods:

• Synthesis and characterization of sodium-functionalized PHIs and fully protonated H-PHIs.
• Photocatalytic experiments under visible light irradiation (410 nm) using glycerin as a sacrificial electron donor.
• Transient photoluminescence spectroscopy to study excited-state dynamics.

Main Results:

• Fully protonated H-PHIs demonstrated high H₂O₂ production rates (up to 1556 mmol L⁻¹ h⁻¹ g⁻¹).
• Transition metal functionalization was found to be detrimental, catalyzing H₂O₂ decomposition.
• H-PHIs exhibited enhanced activity attributed to a longer excited-state lifetime.

Conclusions:

• Rarely examined fully protonated PHIs are highly active photocatalysts for H₂O₂ synthesis.
• Protonation enhances photocatalytic performance, while transition metals are unsuitable for this application.
• This work represents a significant step towards utilizing inexpensive covalent materials for sustainable H₂O₂ production.