High-Speed Synthesis of Early Transition Metal Oxyfluoride or Oxynitride Photocatalysts from Waste Products by Spark Plasma SinteringFriday (25.09.2020) 12:50 - 13:20 P: Processing and Synthesis 1 Part of:
Solid state reactions are notoriously slow because the interdiffusion of the reactants is the rate-limiting step. Therefore, solid state reactions need high temperatures and reaction times of days to weeks. Metal oxides are particularly unreactive, because they have high lattice energies and high melting points. We report a generalized method to carry out high-speed fluorinations and nitridations of early transition metal oxides (e.g. Ta2O5) to TaO2F or Ta3O7F and TaON by spark plasma sintering. Spark plasma sintering allows to perform fluorinations on a minute scale with Teflon scrap, nitridations were carried out with alkaline earth nitrides. A conventional synthesis of oxyfluorides and oxynitrides required gas reactions with caustic HF or NH3 taking hours to days with special equipment.
The potential of this new approach is demonstrated by the following results. (i) The tantalum oxyfluorides TaO2F and Ta3O7F are obtained in a “trash to treasure” approach from plastic scrap without using toxic or caustic chemicals for the fluorination . (ii) Niobium and tantalum oxynidrides NbON and TaON were synthesized from Nb2O5 or Ta2O5 and Mg3N2. (iii) Short reaction times of 5-10 minutes reduce the process time and also the energy costs by more than two orders of magnitude. (iv) The oxyfluorides TaO2F and Ta3O7F and the oxynidrides NbOxNy and TaOxNy were produced in bulk amounts as monoliths of nanoparticles with large specific surface areas. The synthesis can be upscaled easily to the kg range with appropriate sintering equipment. (v) NbOxNy and TaOxNy were obtained as new and metastable intermediates with defect Nb2O5 and Ta2O5 structure before the conventional products NbON and TaON were formed as stable products. (vi) TaO2F and Ta3O7F or TaOxNy exhibit high activity for photocatalytic oxygen evolution, reaching photoconversion efficiencies up to 24.7 % and applied bias to photoconversion values of 0.86 % for TaO2F, comparable to the values for pristine hematite.
Thus, this study is a proof of concept for the energy saving production of new and valuable photocatalysts for water splitting from plastic waste or inexpensive and abundant starting compounds.