Scientists at Ulm University have succeeded for the first time in synthesizing colloidal nanoparticles of polymeric carbon nitride that are fully soluble in water and act as highly efficient and selective photocatalysts for light-driven chemical conversions. The research has been supported by the DFG and carried out within the Transregio Collaborative Research Center (Sonderforschungsbereich) SFB/TRR 234 "CataLight" (Light-driven Molecular Catalysts in Hierarchically Structured Materials - Synthesis and Mechanistic Studies) in collaboration with scientists from the University of Jena (Germany), University of Oviedo (Spain) and University of Aveiro (Portugal).
Bottom-up synthetic route
Photocatalysts can catalyse various important chemical reactions simply by irradiation with sunlight or low-cost LEDs, under otherwise completely mild conditions: at ambient pressure and temperature, and mostly using ubiquitous and sustainable reagents and solvents, such as water or aerial oxygen. Since more than ten years, one of the most promising and intensely investigated photocatalysts has been polymeric carbon nitride. However, its activity has typically been hampered by its low specific surface area and accordingly low amount of surface catalytic active sites. “For a long time, many researchers have tried to decrease the particle size and increase the solubility of carbon nitrides, mainly using various top-down methods, such as sonication or treatment with aggressive solvents,” says Dr. Igor Krivtsov, a Humboldt Fellow at Ulm University and the main author of the study. “We have discovered that it is much better to use a bottom-up synthetic approach, starting with a molecular precursor and using heat treatment in a melt of alkali hydroxides. By this way, we obtained tiny colloidal nanoparticles of polymeric carbon nitride that were fully soluble in water, without any gelation or precipitation over several months.”
Highly efficient photocatalysis
For initial testing, the scientists measured the activity of the newly prepared water-soluble carbon nitride in photocatalytic production of hydrogen peroxide via reduction of oxygen accompanied by selective oxidation of biomass-derived compounds. “This type of reactions is very attractive since out of two low-cost compounds we can obtain two high-value products at the same time,” says Dr. Krivtsov. “We were delighted to see that in all reactions we have studied so far the photocatalytic activity of our water-soluble carbon nitride was much higher than that of the conventional, non-soluble, carbon nitride. We envisage that this may hold also for many other reactions, only future studies can tell.”
“Conventional carbon nitride is a prototypical heterogeneous photocatalyst. Using our water-soluble colloidal nanoparticles, we were able, for the first time, to carry out photocatalysis with carbon nitride under practically homogeneous conditions,” says Professor Radim Beránek, the head of the research team. “Moreover, we found out that the dissolved photocatalyst can be easily precipitated and recovered simply by adding salt. After that, it can be re-dissolved again and used for photocatalysis, without any loss of activity or selectivity. Hence, the photocatalyst combines all advantages of homogeneous catalysts with easy recyclability that is the hallmark of heterogeneous catalysts. We believe that our work is the first step in establishing a completely new paradigm in photocatalysis with polymeric carbon nitrides, and opens up a route for other applications in which liquid aqueous operation or processing of carbon nitride can be useful.”
I. Krivtsov, D. Mitoraj, C. Adler, M. Ilkaeva, M. Sardo, L. Mafra, C. Neumann, A. Turchanin, C. Li, B. Dietzek, R. Leiter, J. Biskupek, U. Kaiser, C. Im, B. Kirchhoff, T. Jacob & R. Beranek "Water-Soluble Polymeric Carbon Nitride Colloidal Nanoparticles for Highly Selective Quasi-Homogeneous Photocatalysis" Angew. Chem. Int. Ed. 2019, DOI: 10.1002/anie.201913331.
Text: Radim Beránek
Mediacontact: Andrea Weber-Tuckermann