Improving the Ca-assisted Urea-Glass Method for the Synthesis of Zirconium Nitride as Potential Electrocatalyst for the Nitrogen Reduction Reaction

Kurzfassung

Zirconium mononitride (ZrN), among other nitrides, is of interest as a catalyst in general and especially for the electrochemical nitrogen reduction reaction (NRR). Synthesis of nanoparticulate ZrN is challenging due to the formation of undesired other phases and the need for hazardous precursors or high temperatures. The urea glass method (UGM) was used in this work to synthesise nanoparticulate zirconium nitride. A focused parameter variation of the precursor composition and process conditions lead to an optimised synthesis, greatly improving material purity in regard to minor phase and residual carbon content. The nitride starts to form at a pyrolysis temperature of 900 °C, likely by irreversible decomposition of previously-formed oxynitride, which forms from amorphous ZrO2 and urea during pyrolysis. An increased pyrolysis temperature improves product purity but slightly lowers nitride purity. Calcium admixture slows the urea decomposition, yielding a more phase-pure product with less residual carbon at low amounts. A classical carbothermal nitridation (CN) was used in comparison to the UGM to synthesise ZrN. This was successful in synthesising zirconium carbonitride Zr(N, C) without minor phase, but significant carbon remains if added overstoichiometrically. Using less carbon and a longer dwelling time yielded ZrN without significant carbide (ZrC) incorporation or residual carbon, but with some minor ZrO2 phase, proving a promising synthesis route. A limited electrochemical investigation of a synthesised ZrN material was carried out to scope the general material behaviour and to qualitatively derive the potential catalytic activity for the NRR. Higher reductive currents in nitrogen-saturated electrolyte qualitatively point to NRR activity, but further and more in-depth examination is required.