Ren Wenju, Mei Zongwei, Zheng Shisheng, Li Shunning, Zhu Yuanmin, Zheng Jiaxin, Lin Yuan, Chen Haibiao, Gu Meng, Pan Feng
School of Advanced Materials, Peking University, Shenzhen Graduate School, China.
School of Advance Manufacturing Engineering, Chongqing University of Posts and Telecommunications, Chongqing, China.
Research (Wash D C). 2020 May 29;2020:3750314. doi: 10.34133/2020/3750314. eCollection 2020.
Solar-driven N fixation using a photocatalyst in water presents a promising alternative to the traditional Haber-Bosch process in terms of both energy efficiency and environmental concern. At present, the product of solar N fixation is either NH or NO . Few reports described the simultaneous formation of ammonia (NH ) and nitrate (NO ) by a photocatalytic reaction and the related mechanism. In this work, we report a strategy to photocatalytically fix nitrogen through simultaneous reduction and oxidation to produce NH and NO by WO nanowires in pure water. The underlying mechanism of wavelength-dependent N fixation in the presence of surface defects is proposed, with an emphasis on oxygen vacancies that not only facilitate the activation and dissociation of N but also improve light absorption and the separation of the photoexcited carriers. Both NH and NO can be produced in pure water under a simulated solar light and even till the wavelength reaching 730 nm. The maximum quantum efficiency reaches 9% at 365 nm. Theoretical calculation reveals that disproportionation reaction of the N molecule is more energetically favorable than either reduction or oxidation alone. It is worth noting that the molar fraction of NH in the total product (NH plus NO ) shows an inverted volcano shape from 365 nm to 730 nm. The increased fraction of NO from 365 nm to around 427 nm results from the competition between the oxygen evolution reaction (OER) at W sites without oxygen vacancies and the N oxidation reaction (NOR) at oxygen vacancy sites, which is driven by the intrinsically delocalized photoexcited holes. From 427 nm to 730 nm, NOR is energetically restricted due to its higher equilibrium potential than that of OER, accompanied by the localized photoexcited holes on oxygen vacancies. Full disproportionation of N is achieved within a range of wavelength from ~427 nm to ~515 nm. This work presents a rational strategy to efficiently utilize the photoexcited carriers and optimize the photocatalyst for practical nitrogen fixation.
在水体系中使用光催化剂进行太阳能驱动的固氮，在能源效率和环境问题方面，是传统哈伯-博施法颇具前景的替代方案。目前，太阳能固氮的产物要么是NH 要么是NO 。鲜有报道描述通过光催化反应同时生成氨（NH ）和硝酸盐（NO ）以及相关机制。在本工作中，我们报道了一种策略，即通过WO纳米线在纯水中进行光催化固氮，同时进行还原和氧化反应以生成NH 和NO 。提出了在存在表面缺陷时波长依赖性固氮的潜在机制，重点关注氧空位，其不仅促进N的活化和解离，还改善光吸收和光生载流子的分离。在模拟太阳光下，甚至直到波长达到730 nm时，在纯水中都能生成NH 和NO 。在365 nm处最大量子效率达到9%。理论计算表明，N分子的歧化反应在能量上比单独的还原或氧化反应更有利。值得注意的是，在总产物（NH 加NO ）中NH 的摩尔分数在365 nm至730 nm呈现倒火山形状。从365 nm到约427 nm，NO 分数增加是由于无氧空位的W位点上的析氧反应（OER）与氧空位位点上的N氧化反应（NOR）之间的竞争，这是由本质上离域的光生空穴驱动的。从427 nm到730 nm，由于NOR的平衡电位高于OER，且氧空位上存在局域光生空穴，NOR在能量上受到限制。在约427 nm至约515 nm的波长范围内实现了N的完全歧化。这项工作提出了一种合理的策略，以有效利用光生载流子并优化光催化剂用于实际固氮。
