基于表面氧空位的光催化固氮材料

Surface Oxygen Vacancy Based Photocatalysts for Nitrogen Fixation

合成氨是基本的化工过程,对地球能源、环境和生命过程至关重要。Harbor-Bosch过程利用Fe催化剂开创性地实现了氮气还原固定为氨。由于氮气N≡≡N三键化学性质稳定,该反应需要在高温高压条件才能实现有效氮气固定。近年来,太阳能驱动的光催化固氮以其绿色温和的反应条件受到了广泛关注。最近,作者团队以卤氧铋和钌(Ru)/氢化氧化钛为模型催化剂,提出氧空位电子可有效活化氮气,即氧空位对氮气的直接电子传递或者氧空位电子经由Ru纳米晶传递给吸附态分子氮能有效弱化氮气N≡≡N三键,结合高效太阳光全光谱吸收材料,将光催化合成氨效率提高至接近传统热催化效率,为高效太阳光驱动固氮提供了新途径。从氧空位活性位点的构建出发,总结概括了含缺陷(尤其是氧空位)材料在光催化固氮的前沿进展,归纳了氧空位、氧空位和传统过渡金属中心耦合对氮气活化的机理。最后,基于对以上机理的理解,总结展望了光催化固氮面临的机遇与挑战,提出了可行的新型高效光催化固氮材料设计思路。

Ammonia synthesis is a process of vital importance to energy, environment and lives on earth. The industrial paradigm of Harbor-Bosch process utilizes Fe as catalyst realizing large-scale nitrogen fixation. Because of the robust N≡≡N triple bond of the molecular nitrogen, this reaction (N 2 +3H 2 = 2NH 3 ) occurs at high pressure and temperature. In recent years, the facile and "green" alternate of photocatalytic nitrogen fixation has been paid much attention. Recently, we reported that the excess electrons in oxygen vacancies (OV) could activate molecular nitrogen effectively via ① the direct electron transfer from surface oxygen vacancy to OV adsorbed N 2 on catalysts such as BiOCl and BiOBr, and ② the indirect electron donation from OV to Ru and then to N 2 bonded on Ru within the K/Ru/TiO 2- x H x catalyst. By embedding the OV into materials of broad-spectrum light absorbance, we obtained a light-driven reactivity comparable to that of thermal Harbor-Bosch process using the K/Ru/TiO 2- x H x catalyst, which provides a new avenue for efficient solar ammonia synthesis. In this paper, we reviewed the recent advances in defects (especially the OV) promoted photocatalytic nitrogen fixation, including the nitrogen fixation mechanism of OV within various metal oxides and the coupling effect of OV to traditional active centers such as transition metals, aiming to reach a fundamental understanding of OV based nitrogen fixation. At last, we summarize the opportunities and challenges in the photocatalytic nitrogen fixation and propose possible pathways for the design of highly efficient photocatalysts.