Partially oxidized niobium carbonitride as a non-platinum catalyst for the reduction of oxygen in acidic medium

Nanostructured Niobium and Titanium Carbonitrides as Electrocatalyst Supports

Nanostructured niobium-titanium carbonitrides, (Nb,Ti)C1-xNx, with the cubic-rock salt structure are prepared without the use of reactive gases via thermal treatment (700-1200 °C) under nitrogen of mixtures of guanidine carbonate and ammonium niobate (V) oxalate hydrate, with addition of ammonium titanyl oxalate monohydrate as a titanium source. The bulk structure and chemical composition of the materials are characterized using powder X-ray diffraction (XRD) and powder neutron diffraction, elemental homogeneity is studied using energy dispersive spectroscopy (EDS) mapping using transmission electron microscopy (TEM), and surface chemical analysis is examined using X-ray photoelectron spectroscopy (XPS). Nanoscale crystallites of between 10 and 50 nm are observed by TEM, where EDS reveals the homogeneity of metal distribution for the mixed-metal materials. Titanium carbonitrides are found to be air sensitive, reacting with air under ambient conditions, while titanium-niobium carbonitrides are found to degrade in aqueous sulfuric acid. The niobium carbonitrides, however, show some stability toward acidic solutions. Materials are produced with composition NbC1-xNx with x between 0.35 and 0.45, and more carbon-rich materials (x ≈ 0.35) are found as the synthesis temperature is increased, as proven by Rietveld refinement of crystal structure against powder neutron diffraction data. Despite phase purity seen by diffraction and negligible bulk carbon content, XPS shows a complex surface chemistry for the NbC1-xNx materials, with evidence for Nb2O5-like oxide species in a carbon-rich environment. The NbC1-xNx prepared at 900 °C has a surface area around 50 m2 g-1, making it suitable as a catalyst support. Loading with iridium provides a material active for the oxygen evolution reaction in 0.1 M sulfuric acid, with minimal leaching of either Nb or Ir after 1000 cycles.

Steering from electrochemical denitrification to ammonia synthesis

The removal of nitric oxide is an important environmental issue, as well as a necessary prerequisite for achieving high efficiency of CO2 electroreduction. To this end, the electrocatalytic denitrification is a sustainable route. Herein, we employ reaction phase diagram to analyze the evolution of reaction mechanisms over varying catalysts and study the potential/pH effects over Pd and Cu. We find the low N2 selectivity compared to N2O production, consistent with a set of experiments, is limited fundamentally by two factors. The N2OH* binding is relatively weak over transition metals, resulting in the low rate of as-produced N2O* protonation. The strong correlation of OH* and O* binding energies limits the route of N2O* dissociation. Although the experimental conditions of varying potential, pH and NO pressures can tune the selectivity slightly, which are insufficient to promote N2 selectivity beyond N2O and NH3. A possible solution is to design catalysts with exceptions to break the scaling characters of energies. Alternatively, we propose a reverse route with the target of decentralized ammonia synthesis.

Cobalt-zinc nitride on nitrogen doped carbon black nanohybrids as a non-noble metal electrocatalyst for oxygen reduction reaction

Bimetallic nitrides are now being considered as one of the emerging advanced functional materials due to their characteristic features and remarkable physicochemical properties. Herein, we report a new crystalline bimetallic nitride (Co₃ZnN) that belongs to the cubic crystal phase, which was successfully synthesized through direct nitridation of metallic salts as precursors. Co₃ZnN nanoparticles were then supported on nitrogen-doped XC-72 carbon black (N-CB), and this typical Co₃ZnN/N-CB nanohybrid discovered can serve as an efficient non-noble metal electrocatalyst with a 4e^- reaction pathway for ORR, and demonstrated excellent electrocatalytic performance with high activity and stability.