Potassium Poly(Heptazine Imide): Transition Metal-Free Solid-State Triplet Sensitizer in Cascade Energy Transfer and [3+2]-cycloadditions.

Polymeric carbon nitride materials have been used in numerous light-to-energy conversion applications ranging from photocatalysis to optoelectronics. For a new application and modelling, we first refined the crystal structure of potassium poly(heptazine imide) (K-PHI)-a benchmark carbon nitride material in photocatalysis-by means of X-ray powder diffraction and transmission electron microscopy. Using the crystal structure of K-PHI, periodic DFT calculations were performed to calculate the density-of-states (DOS) and localize intra band states (IBS). IBS were found to be responsible for the enhanced K-PHI absorption in the near IR region, to serve as electron traps, and to be useful in energy transfer reactions. Once excited with visible light, carbon nitrides, in addition to the direct recombination, can also undergo singlet-triplet intersystem crossing. We utilized the K-PHI centered triplet excited states to trigger a cascade of energy transfer reactions and, in turn, to sensitize, for example, singlet oxygen (1 O2 ) as a starting point to synthesis up to 25 different N-rich heterocycles.

Micro-Blooming: Hierarchically Porous Nitrogen-Doped Carbon Flowers Derived from Metal-Organic Mesocrystals.

Synthesis of 3D flower-like zinc-nitrilotriacetic acid (ZnNTA) mesocrystals and their conformal transformation to hierarchically porous N-doped carbon superstructures is reported. During the solvothermal reaction, 2D nanosheet primary building blocks undergo oriented attachment and mesoscale assembly forming stacked layers. The secondary nucleation and growth preferentially occurs at the edges and defects of the layers, leading to formation of 3D flower-like mesocrystals comprised of interconnected 2D micropetals. By simply varying the pyrolysis temperature (550-1000 °C) and the removal method of in the situ-generated Zn species, nonporous parent mesocrystals are transformed to hierarchically porous carbon flowers with controllable surface area (970-1605 m2 g-1 ), nitrogen content (3.4-14.1 at%), pore volume (0.95-2.19 cm3 g-1 ), as well as pore diameter and structures. The carbon flowers prepared at 550 °C show high CO2 /N2 selectivity due to the high nitrogen content and the large fraction of (ultra)micropores, which can greatly increase the CO2 affinity. The results show that the physicochemical properties of carbons are highly dependent on the thermal transformation and associated pore formation process, rather than directly inherited from parent precursors. The present strategy demonstrates metal-organic mesocrystals as a facile and versatile means toward 3D hierarchical carbon superstructures that are attractive for a number of potential applications.

Enhanced Electrocatalytic N2 Reduction via Partial Anion Substitution in Titanium Oxide-Carbon Composites.

The electrochemical conversion of N2 at ambient conditions using renewably generated electricity is an attractive approach for sustainable ammonia (NH3 ) production. Considering the chemical inertness of N2 , rational design of efficient and stable catalysts is required. Therefore, in this work, it is demonstrated that a C-doped TiO2 /C (C-Tix Oy /C) material derived from the metal-organic framework (MOF) MIL-125(Ti) can achieve a high Faradaic efficiency (FE) of 17.8 %, which even surpasses most of the established noble metal-based catalysts. On the basis of the experimental results and theoretical calculations, the remarkable properties of the catalysts can be attributed to the doping of carbon atoms into oxygen vacancies (OVs) and the formation of Ti-C bonds in C-Tix Oy . This binding motive is found to be energetically more favorable for N2 activation compared to the non-substituted OVs in TiO2 . This work elucidates that electrochemical N2 reduction reaction (NRR) performance can be largely improved by creating catalytically active centers through rational substitution of anions into metal oxides.

Template- and Metal-Free Synthesis of Nitrogen-Rich Nanoporous "Noble" Carbon Materials by Direct Pyrolysis of a Preorganized Hexaazatriphenylene Precursor.

The targeted thermal condensation of a hexaazatriphenylene-based precursor leads to porous and oxidation-resistant ("noble") carbons. Simple condensation of the pre-aligned molecular precursor produces nitrogen-rich carbons with C2 N-type stoichiometry. Despite the absence of any porogen and metal species involved in the synthesis, the specific surface areas of the molecular carbons reach up to 1000 m2 g-1 due to the significant microporosity of the materials. The content and type of nitrogen species is controllable by the carbonization temperature whilst porosity remains largely unaffected at the same time. The resulting noble carbons are distinguished by a highly polarizable micropore structure and have thus high adsorption affinity towards molecules such as H2 O and CO2 . This molecular precursor approach opens new possibilities for the synthesis of porous noble carbons under molecular control, providing access to the special physical properties of the C2 N structure and extending the known spectrum of classical porous carbons.