Green chemistry: Producing hydrogen peroxide more efficiently

The versatile substance can be produced sustainably and with high yield using a custom-designed photocatalyst.

To the point:

Many applications: Hydrogen peroxide is used as an environmentally friendly bleaching and disinfecting agent, as well as in numerous chemical processes.

Sustainable Production: A catalyst composed of carbon nitride, lithium, and potassium enables the generation of hydrogen peroxide using light instead of fossil fuels.

More Efficient Conversion: A novel synthesis method allows precise control over the structure of the photocatalyst, thereby increasing the yield of hydrogen peroxide production.

Hydrogen peroxide (H₂O₂) serves, among other applications, as an eco-friendly bleaching agent primarily in the paper industry, a disinfectant in medical settings and private households, and an oxidising agent in chemical processes. The chemical industry produces approximately eight million tonnes of this reactive substance annually, with demand on the rise. To date, hydrogen peroxide is mostly manufactured via an energy-intensive process known as the anthraquinone method, which utilises oxygen and relies on fossil energy sources. A significantly more climate-friendly alternative involves producing it using a catalyst based on carbon nitride that is activated by light. In an international research project involving the Max Planck Institute for Colloids and Interfaces, researchers have now developed a synthesis pathway whereby hydrogen peroxide can be produced three times more efficiently with such a photocatalyst than previously possible. The team published their findings in the journal Advanced Materials.

In photocatalysis employing carbon nitride, the energy from light temporarily generates mobile positive and negative charges within the catalyst. The efficiency of the photocatalytic process depends, among other factors, on how effectively these charges move through the material and how long they remain available for reactions. It is already known that atoms of alkali metals such as lithium, sodium, and potassium, incorporated into carbon nitride, enhance charge separation and thus improve catalytic efficiency. The researchers have now amplified this effect in their current work by synthesising the catalyst so that the alkali metal atoms can be deliberately positioned within the carbon nitride. In previous manufacturing methods, the metal atoms were distributed more or less randomly throughout the catalyst. “Instead of changing the chemical composition of a photocatalyst to make it more efficient, we tried to change how the building blocks are arranged,” says corresponding author Christian M. Pelicano, who leads the “SunThesis” research group at the Max Planck Institute in Potsdam and has been researching this topic since 2023.

To achieve this, the researchers used organic-inorganic ionic cocrystals composed of a carbon- and nitrogen-containing compound and lithium chloride as starting materials for the photocatalyst. In these crystals, the organic and inorganic components are linked into a regular three-dimensional network. The hybrid crystal was heated together with potassium chloride, resulting in the formation of the final catalyst, which they named CALiK, where the metal ions occupy defined positions. “These crystals were already known, but we were the first to use them as precursors for carbon nitrides”, says Pelicano. “What caught our interest was that they contain the same chemical components as the precursors we normally use. The key difference is that the ions are already arranged in a specific way, and we wondered whether this arrangement could be transferred to the carbon nitride material.”

A defined arrangement of ions improves photocatalytic performance

In subsequent experiments, CALiK produced approximately three times as much hydrogen peroxide as a conventionally manufactured reference material and about 40 percent more than the most efficient tested material CALi, which contains only lithium. “The spatial arrangement of the ions in the optimized material makes a considerable difference,” says first author Anna Lo Presti, a chemist pursuing her PhD on this topic at the Max Planck Institute. “We were able to demonstrate for the first time that it is not only the type or quantity of ions that matters, but also their position in the lattice.”

To investigate the reason for the improved performance, the researchers combined structural analyses with spectroscopic and electrochemical measurements. They found that the charges generated by light separate more effectively in CALiK, and therefore remain available for the reaction with oxygen for longer. Additional calculations suggest that the spatial distribution of lithium ions changes the electronic environment within the material, thereby facilitating oxygen activation. It should be noted, however, that the calculations are based on simplified model structures and therefore do not yet fully capture the processes in the real material.