Controlled Nitridation Of Tantalum(Oxy)nitrides And The Catalytic Application

Transitional metal（oxy）nitrides are a class of promising functional materials, due to their noble-metal-like electronic structure, rich physicochemical properties, excellent catalytic performance and outstanding stability. Recently, the energy crisis and serious environment pollution have driven the development of efficient and green catalysts, especially metal nitrides that are promising as the alternative for noble metals. The study dealing with the facile synthesis of tantalum（oxy）nitrides and the catalytic behaviors relying on nitridation is expected to advance the physical chemistry of nitrides.This thesis introduced a controlled nitridation via alkaline-earth-metals mediated ionothermal processes to tune the crystalline and electronic structure of tantalum（oxy）nitrides. Such controlled nitridation strenghthened the metal-support interactions and optimized the catalytic performance in hydrogenation of nitrobenzene. Meanwhile, for the supported-nitrides, the synergic nitridation of active sites and the supports are studied, which led to the N-Ta/RGO electrocatalysts. The synergic nitridation remarkably enhanced the efficiency of electrocatalytic oxygen-reduction-reaction（ORR）. Specifically, the following parts are discussed,Mediated by SrCl₂, pure TaON and Ta₃N₅ nanoparticles could be synthesized in mild condition. It is found that the SrTa₄O₁₁ intermediate generated from reacting Sr²⁺ with Ta₂O₅, while urea was polycondensed to CNx. And finally, the corresponding TaON and Ta₃N₅ nanoparticles could be produced upon heating. The strontium-tantalate intermediates is proved to be crucial for realizing controlled nitridation. In the intermediate of SrTa₄O₁₁, parts of Ta-O bonds are weakened which benefits the replacement of O by N for generating TaON, avoiding over-nitridation to Ta₃N₅. This mechanism is universal for ionothermal processes using other alkaline-earth-metal ions. Such controlled nitridation obviously varies the electronic properties of tantalum（oxy）nitrides to express electronic metal-support interactions and optimize the cayalysis. For example, the electrons on the surface of supported gold nanocatalysts could be tailored by nitridation of supports. The obtained catalysts showed the tendency of Au/TaON > Au/Ta₃N₅ > Au/Ta₂O₅ in terms of the activity and selectivity to aniline in the probe reaction of nitrobenzene（NB） reduction which is ascribed to the suitable adsorption and effective activation of substrates by moderately negative Auδ-. The work proposes a feasible strategy to accomplish controlled nitridation to optimize the metal-support interaction and the activity in hydrogenation, providing new ideas in designing catalysts.Considering the method of controllable nitridation presented in previous work, we accomplish the synergic nitridation in catalytic center（Ta） and supports（RGO） to uncover the relationship between catalytic performance and nitridation level. Meanwhile, the changes of structure and properties of the supports doped with nitrogen are also considered. It is found that with the increaseing urea, Ta would be nitridated as amorphous TaON gradually and the N-RGO mainly proved to be graphitic-N. With excessive addition of urea, the nitridation of Ta is depressed due to the insufficient of SiO2 which is to provide a surface supporting the production of CNx for further nitridation. While the appropriate amount of urea（Urea/Ta=4） is capable of achieving the effective doping in both Ta and RGO, exhibiting excellent ORR catalytic performance that is prone to be a four-electron pathways. Compared with the commercial Pt/C（40%）, N-Ta/RGO shows better capability in stability and selectivity with high methanol tolerance. This illustrates that controlled nitridation could tune the active center and supports simutaniously to optimize the catalytic properties, suggesting a novel way to develop hybrid materials and competitive substitution of Pt-based catalysts for green energy application.In conclusion, our thesis presents a principle of controlled nitridation with tantalum（oxy）nitrides compound as an example, and use the law to guide the liquid-phase hydrogenation, the design of catalysts in ORR. It provides an alternative method to fabricate highly efficient and economic transition metal nitrides catalyst.