Two Ways to more NH2-Groups: Formation of Polymeric Carbon Nitride via Melem Tetramer Nano Sheets or Supramolecular Assembly of Melamine and Cyanuric Acid for Applications as Photocatalyst

Polymeric carbon nitride (PCN) can be prepared by heating of nitrogen rich precursors and is used as a photocatalyst for hydrogen evolution and transformation of organic molecules. This work gives a deeper understanding of the different properties of PCN and the observed intermediates when melamine or urea are chosen as precursors and heated to temperatures in the range of 350–575 °C. Melem or a melem-tetramer are the main intermediates when melamine is used as starting material, while urea-based PCN is formed via a supramolecular assembly of melamine and cyanuric acid. This leads to crucial differences in properties such as surface area, amount of NH2-groups and photocatalytic activity. PCN from urea, loaded with Pt nanoparticles, shows higher activities for photocatalytic hydrogen evolution due to more NH2-groups. The melem-tetramer shows nanosheetlike structure combined with a high amount of NH2-groups and has the highest activity of all prepared catalysts for the photocatalytic reduction of 5-(hydroxymethyl)furfural (HMF) to 2,5-bis(hydroxymethyl)furan (BHMF). Thus, this research demonstrates the formation mechanism and the design of PCN based catalysts with a large amount of NH2-groups and a high photocatalytic reduction ability.