Chukwunenye Precious, Ganesan Ashwin, Gharaee Mojgan, Balogun Kabirat, Adesope Qasim, Amagbor Stella Chinelo, Golden Teresa D, D'Souza Francis, Cundari Thomas R, Kelber Jeffry A
Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, TX 76203-5017, USA.
Phys Chem Chem Phys. 2023 Jul 26;25(29):19540-19552. doi: 10.1039/d3cp01330h.
The production of ammonia for agricultural and energy demands has accelerated research for more environmentally-friendly synthesis options, particularly the electrocatalytic reduction of molecular nitrogen (nitrogen reduction reaction, NRR). Catalyst activity for NRR, and selectivity for NRR over the competitive hydrogen evolution reaction (HER), are critical issues for which fundamental knowledge remains scarce. Herein, we present results regarding the NRR activity and selectivity of sputter-deposited titanium nitride and titanium oxynitride films for NRR and HER. Electrochemical, fluorescence and UV absorption measurements show that titanium oxynitride exhibits NRR activity under acidic conditions (pH 1.6, 3.2) but is inactive at pH 7. Ti oxynitride is HER inactive at all these pH values. In contrast, TiN - with no oxygen content upon deposition - is both NRR and HER inactive at all the above pH values. This difference in oxynitride/nitride reactivity is observed despite the fact that both films exhibit very similar surface chemical compositions - predominantly Ti oxide - upon exposure to ambient, as determined by X-ray photoelectron spectroscopy (XPS). XPS, with transfer between electrochemical and UHV environments, however, demonstrates that this Ti oxide top layer is unstable under acidic conditions, but stable at pH 7, explaining the inactivity of titanium oxynitride at this pH. The inactivity of TiN at acidic and neutral pH is explained by DFT-based calculations showing that N adsorption at N-ligated Ti centers is energetically significantly less favorable than at O-ligated centers. These calculations also predict that N will not bind to Ti centers due to a lack of π-backbonding. XPS measurements and electrochemical probe measurements at pH 3.2 demonstrate that Ti oxynitride films undergo gradual dissolution under NRR conditions. The present results demonstrate that the long-term catalyst stability and maintenance of metal cations in intermediate oxidation states for pi-backbonding are critical issues worthy of further examination.
为满足农业和能源需求而生产氨的过程加速了对更环保合成方法的研究，特别是分子氮的电催化还原（氮还原反应，NRR）。NRR的催化剂活性以及相对于竞争性析氢反应（HER）的NRR选择性是关键问题，而对此的基础知识仍然匮乏。在此，我们展示了溅射沉积的氮化钛和氮氧化钛薄膜在NRR和HER方面的活性和选择性结果。电化学、荧光和紫外吸收测量表明，氮氧化钛在酸性条件（pH 1.6、3.2）下表现出NRR活性，但在pH 7时无活性。氮氧化钛在所有这些pH值下均不具有HER活性。相比之下，沉积时不含氧的TiN在上述所有pH值下对NRR和HER均无活性。尽管通过X射线光电子能谱（XPS）测定，两种薄膜在暴露于环境时都表现出非常相似的表面化学成分——主要是TiO，但仍观察到氮氧化物/氮化物反应性的这种差异。然而，通过在电化学和超高真空环境之间转移进行的XPS表明，这种TiO顶层在酸性条件下不稳定，但在pH 7时稳定，这解释了氮氧化钛在该pH值下的无活性。基于密度泛函理论（DFT）的计算解释了TiN在酸性和中性pH下的无活性，计算表明在N配位的Ti中心吸附N在能量上明显不如在O配位中心有利。这些计算还预测，由于缺乏π反馈键，N不会与Ti中心结合。在pH 3.2下的XPS测量和电化学探针测量表明，氮氧化钛薄膜在NRR条件下会逐渐溶解。目前的结果表明，长期催化剂稳定性以及维持用于π反馈键的中间氧化态金属阳离子是值得进一步研究的关键问题。
