Nitrogen Fixation with Water on Carbon-Nitride-Based Metal-Free Photocatalysts with 0.1% Solar-to-Ammonia Energy Conversion Efficiency

Abstract

Ammonia (NH 3 ), which is an indispensable chemical, is produced by the Haber−Bosch process using H 2 and N 2 under severe reaction conditions. Although photocatalytic N 2 fixation with water under ambient conditions is ideal, all previously reported catalysts show low efficiency. Here, we report that a metalfree organic semiconductor could provide a new basis for photocatalytic N 2 fixation. We show that phosphorus-doped carbon nitride containing surface nitrogen vacancies (PCN-V), prepared by simple thermal condensation of the precursors under H 2 , produces NH 3 from N 2 with water under visible light irradiation. The doped P atoms promote water oxidation by the photoformed valence-band holes, and the N vacancies promote N 2 reduction by the conduction-band electrons. These phenomena facilitate efficient N 2 fixation with a solar-to-chemical conversion (SCC) efficiency of 0.1%, which is comparable to the average solar-tobiomass conversion efficiency of natural photosynthesis by typical plants. Thus, this metal-free catalyst shows considerable potential as a new method of artificial photosynthesis.

Photo(electro)catalytic Nitrogen Fixation: Problems and Possibilities

“…As mentioned in the previous sections, many studies claimed that the vacancy defects such as oxygen vacancies, nitrogen vacancies, and sulfur vacancies58c–e with their abundant localized electrons can donate their electron for absorbed N 2 , forming chemical bonding between N 2 and surrounded atoms and activating N 2 molecule, therefore, enhancing catalytic performance. The photocatalytic nitrogen fixation performance of NVs assisted P doped carbon nitride have been determined with the high solar conversion efficiency of 0.1% which is comparable with the average efficiency of natural photosynthesis by general plant6a ( Figure a). Moreover, the six‐electron transfer process can also be facilitated in S vacancies supported MoS 2 nanosheets.…”

“…For photocatalytic reaction, a high specific surface area photocatalyst performs the better activity as compared to the similar photocatalyst with low surface area, particularly for H 2 evolution, and CO 2 conversion . However, it seems that the photocatalytic performance of nitrogen fixation has less dependence on this factor . Hirakawa and coworkers compared the activities of commercial TiO 2 catalysts with different surface areas .…”

“…Referred to these progresses in the field of electrocatalytic nitrogen fixation, we propose the strategies for developing advanced photocatalyst for nitrogen reduction. Ideally, engineering the physical and chemical surface structure of semiconductors such as creating anion vacancies, dopant sites composite heterojunctions, or N 2 active cocatalysts can promote both N 2 adsorption and activation and prevent hydrogen generation reaction 6g. Moreover, it is essential to understand the thermodynamic and kinetic mechanism for the further development of photo(electro)catalyst system.…”

“…In addition, this process also allows fixing nitrogen at ambient conditions with zero carbon dioxide emission, which is environmentally benign. Recently, the scientific community has achieved some convincing progress toward proving the possibility of photo(electro)catalytic nitrogen reduction into the generation of NH 3 . However, before being practically viable, the challenges such as low quantum efficiency, electron–hole separation, and mechanistic processing, such as N 2 activation need to be completely addressed.…”

Formation of BNC Coordination to Stabilize the Exposed Active Nitrogen Atoms in g‐C3N4 for Dramatically Enhanced Photocatalytic Ammonia Synthesis Performance

“…Based on the above discussions, developing effective strategies to achieve high‐efficiency g‐C 3 N 4 ‐based photocatalyst, capable of improving the N 2 adsorption/activation and visible‐light utilization, simultaneously effectively inhibiting the recombination of photogenerated carriers in g‐C 3 N 4 , is critically important and highly needed. Recently, Shiraishi and co‐workers have demonstrated that heteroatom doping could be an effective means to regulate the electronic structures of g‐C 3 N 4 , thus obtaining new catalytic active sites (N vacancy) for N 2 adsorption and activation, leading to an high‐efficiency NH 3 synthesis performance 17. More importantly, Wang et al theoretically predicted that B‐doping in g‐C 3 N 4 can not only improve the visible‐light utilization efficiency, also reaching rather low onset potential for photocatalytic NRR based on the concept of electron “acceptance–donation” on B atoms, similar to that of transition metals 5,18.…”