Hydrogen peroxide (H2O2) is a widely used chemical used in consumer products, medical settings and industrial processes - for example, it is used as a bleaching agent in paper and fabric production. Each year, more than 3.5 million metric tons of the chemical are produced through the reduction and oxidation of anthraquinone, a method that comes with drawbacks such as the high costs of anthraquinone and palladium (Pd) catalysts, and the carbon dioxide (CO2) emitted in the steam-methane reforming process used to produce hydrogen gas (H2) for the reaction. Researchers at the São Paulo Research Foundation have now developed a new method that lowers the cost and carbon footprint of H2O2 production, using a sustainable photocatalyst and alternative reagents to achieve high yields of the chemical.
The researchers used crystalline versions of carbon nitrides, known as poly(heptazine imides), or PHIs, as photocatalysts in the reaction, which are activated by light in the visible spectrum (around ~410 nm). The team experimented with metal-doped PHIs (M-PHIs), sodium-functionalized PHI (Na-PHI) and fully protonated PHI (H-PHI). Glycerin, a relatively inexpensive chemical that is a byproduct of biofuel production, was selected as the sacrificial electron donor reagent and hydrogen source. M-PHIs were found to cause degradation of the produced H2O2, but Na-PHI and H-PHI showed favorable catalytic activity.
H-PHI was found to have the highest catalytic activity, producing up to 1556 mmol L–1 h–1 g–1 H2O2 under visible light and mild conditions with as little as 5 mg of catalyst. Additionally, PHIs are inexpensive and straightforward to synthesize and have good recyclability. The use of visible light opens up the possibility of using sunlight in the reaction, further increasing sustainability and reducing costs. This research was published in ACS Applied Materials & Interfaces.
“The road we had to travel in our investigation until we arrived at the results described in the published article was a long one because we discovered that at the same time as H2O2 was produced on the surface of the photocatalyst, it could also be degraded. We had to perform several tests and keep modifying the photocatalyst in order to promote the formation of H2O2 and avoid its decomposition,” explained corresponding author Ivo F. Teixeira. “Understanding the mechanism whereby H2O2 decomposes on the surface of carbon nitride was extremely important to enable us to develop the ideal photocatalyst for this reaction.”
The authors note that the H2O2 yield of this process could be improved by optimizing conditions including the intensity of the radiation or engineering of the reactor. They also write that the production of glycerin in the biofuel industry is predicted to increase in the future, potentially increasing the abundance and lowering the cost of the sacrificial electron donor.