Japanese and Chinese researchers have created a copper-cobalt catalyst capable of autonomously changing its structure during the electrochemical reduction of nitrates. This reconstruction ability was a decisive factor in achieving high efficiency in ammonia production from nitrates, with a Faraday efficiency of 95.4 percent. The results of this work were published in the journal Advanced Science.
The team of Japanese and Chinese chemists, led by Takeshi Fujita from Kochi University of Technology, managed to solve these problems. They modified copper oxide by adding between 10 and 50 mol% cobalt oxide, resulting in a series of CuO/CoOX type catalysts. It was hypothesized that the cobalt component would promote the formation of active hydrogen species, while the copper component would ensure the binding and sequential reduction of nitrogen-containing compounds.
The catalysts were tested in an electrochemical cell for nitrate reduction, followed by a comparison of their performance against catalysts containing only copper or only cobalt. Structural changes during the reaction were monitored using in situ Raman, infrared, and electron paramagnetic spectroscopy. Before and after the reaction, the catalysts were examined using an electron microscope, and quantum chemical calculations were also performed.
The best performance was demonstrated by the catalyst with 10 mol% cobalt content (Co:Cu ratio = 1:9). Further increasing the proportion of cobalt led to an increase in the competing reaction of molecular hydrogen evolution, which slowed down ammonia formation.
It was found that the initial catalyst Co1Cu9OX transforms over time, forming a new functional structure. This structure includes metallic copper Cu(0), cobalt oxide, as well as oxidized copper compounds Cu(I) and Cu(II) saturated with hydroxyl groups on the surface. This rearrangement promoted faster nitrate binding on the copper and stabilized intermediate products. Moreover, the proximity of copper allowed the generated hydrogen particles to immediately interact with nitrogen for its reduction. Thanks to this reconstruction, the ammonia yield reached 54.68 milligrams per hour per milligram of catalyst, significantly higher than the figure for the copper catalyst, which yielded about 35 milligrams per hour per milligram under the same conditions.
Thus, Fujita and his team proved that the catalyst's efficiency depends not only on its initial composition but also on the ability of its surface to spontaneously rearrange during the reaction. The authors presented the concept of a proto-catalyst—a material that forms the most active configuration during operation. This approach may serve as a basis for developing heterostructured catalysts for various electrochemical processes.
Japanese scientists have developed a method for selectively extracting polyurethane from composite materials using an iridium catalyst. This method allows for the degradation of polyurethane specifically, leaving other polymers such as polyester and polyamide untouched, which opens prospects for simplifying the chemical recycling of mixed waste, including sportswear.
Currently, less than 12 percent of all plastic waste is recycled. Mixed plastic waste presents a particular challenge, such as materials for swimsuits and sportswear, which often consist of a combination of elastane (polyurethane), polyester (polyethylene terephthalate), and nylon (polyamide). Since these polymers require different recycling methods, and separating interwoven fibers is practically impossible, such clothing is most often sent to landfills or incinerated in countries with developed recycling infrastructure.
Kiyoko Nozaki from the University of Tokyo led the team that proposed this original solution. The scientists applied hydrogenolysis technology, which involves breaking down polymer chains using hydrogen activated on a special catalyst. To achieve selectivity for polyurethane, an iridium catalyst was tested in combination with various bases. It was found that weak bases did not provide sufficient catalytic effect, while strong bases caused not only the breakdown of polyurethane but also an undesirable side reaction of transesterification. Cyanomethyl potassium phenolate was identified as the optimal choice, effectively suppressing side processes while maintaining high catalytic activity towards polyurethane.
After fine-tuning the conditions, Nozaki and her staff tested the method on actual materials. When working with a sports shirt fabric containing 88% polyester and 12% elastane, they managed to almost completely remove the polyurethane component while the structure of the polyester remained unchanged. Gas chromatography analysis confirmed the presence of polyurethane decomposition products, but ethylene glycol, one of the monomers of polyester, was absent even in trace amounts, indicating no polyester degradation. Similar positive results were obtained when testing fabrics mixed with elastane and nylon, as well as kitchen sponges made from polyurethane and polyethylene terephthalate.
In the future, researchers plan to focus on the efficient recovery of products obtained from polyurethane decomposition, as well as on scaling up the entire process. Successful realization of these tasks could transform selective hydrogenolysis into a new, promising strategy for the chemical recycling of mixed plastics.
