Boosting Carbon Nitride Photoactivity by Metal-Free Functionalization for Selective H₂O₂ Synthesis under Visible Light

Abstract

Carbon nitride (CN) is a polymer-based material thoroughly studied for different photocatalytic applications due to its optical, electronic, and chemical properties. Its main limitation is fast electron/hole recombination. To overcome this drawback, several functionalization methods have been applied using organic solvents and severe operating conditions. In the present work, a thermally exfoliated CN was functionalized for the first time via a hydrothermal route using glucose (G), perylene (P), and anthraquinone (A) by implementing water as solvent at a mild temperature (120 °C). The materials were tested for the photocatalytic production of hydrogen peroxide (H 2 O 2 ) in aqueous solutions saturated with dissolved oxygen and in the presence of a sacrificial agent (isopropyl alcohol). Improved evolution rates of H 2 O 2 were confirmed from 9.7 mmol g −1 h −1 for the unmodified CN material, up to 11.1, 14.2, and 25.0 mmol g −1 h −1 for the functionalized photocatalysts CN-G, CN-P, and CN-A, respectively. The surface chemistry studied by XPS, EDX, and FTIR revealed the specific functionalities allowing for improved energy transfer, faster redox reactions, and enhanced photoactivation. The optical and electrochemical characterizations corroborate the photocatalytic results since photoluminescence was severely quenched after functionalization, and the electronic properties were enhanced, both indicating reduced recombination of charge carriers. The band structure was also investigated, proving that the generation of H 2 O 2 was thermodynamically favored for the functionalized materials, while H 2 O 2 decomposition was suppressed.

Enhanced Visible-Light H2O2 Production over Pt/g-C3N4 Schottky Junction Photocatalyst

“… ,,,,,, How does the • O 2 – radical change into H 2 O 2 in eq ? There are three different opinions/routes in literature. ,,, Route one is that • O 2 – reacts with H + to form • OOH ( • O 2 – + H + → • OOH), and then it further reacts with H + and e – to generate H 2 O 2 ( • OOH + H + + e – → H 2 O 2 ). , Route two is that • O 2 – reacts with H + to form • OOH ( • O 2 – + H + → • OOH), and then two • OOH radicals combine to generate H 2 O 2 ( • OOH + • OOH → O 2 + H 2 O 2 ) . Route three is that • O 2 – reacts with H + to form • OOH ( • O 2 – + H + → • OOH), and then • OOH acquires an electron to form HO 2 – ( • OOH + e – → HO 2 – ), finally, HO 2 – reacts with H + to generate H 2 O 2 (HO 2 – + H + → H 2 O 2 ) .…”

“…There are three different opinions/routes in literature. 3,36,45,74 . 36 Route three seems to be more feasible to occur than Route one and two, owing to only two reactants involved.…”

Anthraquinone-Based Conjugated Organic Polymers Containing Dual Oxidation Centers for Photocatalytic H2O2 Production from H2O and O2 under Visible-Light Irradiation

“…7−9 The narrow absorption range limits the efficient utilization of sunlight. 10 At the same time, the confined variability of inorganic-based photocatalyst materials is unfavorable for the modulation of optical and electrical properties. 11 significantly suppress the efficiency and selectivity of transformation.…”

“…Photocatalytic H 2 O 2 generation from H 2 O and O 2 is a promising strategy which provides a safe, low-energy consumption, sustainable, and eco-friendly alternative approach 5. The continuous and intense efforts have given rise to the rapid development of various semiconductor-based photocatalyst materials,6 but they mostly focus on inorganic materials such as titanium dioxide, cadmium sulfide, bismuth vanadate, metal−organic frameworks, etc.7−9 The narrow absorption range limits the efficient utilization of sunlight 10. At the same time, the confined variability of inorganic-based photocatalyst materials is unfavorable for the modulation of optical and electrical properties 11.…”

Sustainable and Continuous Flow Preparation of High-Concentration H2O2 by Visible-Light-Activated Molecular Oxygen Oxidation of Lower Alcohols

“…In addition, semiconductor materials with photocatalytic activity have been applied to produce H 2 O 2 under visible light or ultraviolet light, in which H 2 O 2 is directly synthesized from water or other hydrogen atom donors (HADs) via a cleaner method of combing the photocatalytic two-electron reduction of O 2 . Over the past decades, the field of photocatalytic H 2 O 2 production has witnessed remarkable development, with reported catalysts ranging from homogeneous , to heterogeneous, single component − to multicomponent systems, , metal oxide or organic-based materials , to their hybrids, and covalent organic frameworks ,,, (COFs) to metal–organic frameworks (MOFs) …”