Direct Z‐scheme among Niobium Pentoxide and Poly(heptazine imide) for NH₃ Photoelectrosynthesis under Ambient Conditions

Abstract

There is a crescent need to improve well‐known processes to environmentally friendly options, and NH3 production is not an exception. Herein, we describe the photoelectrochemical nitrogen reduction reaction under a carbon nitride, the poly(heptazine imide) (PHI) using mild conditions for NH3 synthesis. For this purpose, PHI was combined with Nb2O5 nanotubes, leading to a new photocathode with a charge transfer process in a Z‐scheme structure. The photoelectrocatalytic N2 reduction was carried out in Nb2O5Nt/PHI using solar‐simulated radiation and with an applied bias of −0.3 VAg/AgCl. The PHI presence in the photocathode showed not only superior stability compared to bare Nb2O5, but also an ammonia generation rate of 0.156 mmol L−1 h−1 cm−2, which is about 10‐fold higher than the amount obtained with niobium oxide alone. The mild and green conditions employed for ammonia generation place the obtained photocathode as a promising option for the currently used Haber‐Bosch process.

Carbon Nitrides in Photoelectrochemistry: State of the Art and Perspectives Beyond Water Splitting

“…The hierarchical structure improved the light harvesting from the material, in addition to providing a greater number of active sites. The second work was also a heterojunction, but with Nb 2 O 5 nanotubes (Nb 2 O 5 NT/PHI), once nanotubular structures present some advantages like high surface area, a large number of active sites, and especially for photoelectrocatalysis usually better charge separation and transfer, − and higher density of bulk states than for conventional nanoparticles . Nb 2 O 5 NT alone had energy potential enough to perform the ammonia PEC synthesis, however, the synergic effect observed on the heterojunction led to an ammonia generation rate of 0.156 mmol L –1 h –1 cm –2 , which was about 10-fold higher than just the niobium pentoxide.…”

“…Furthermore, is possible to find studies that indicate a variation of the CB potential for conventional C 3 N 4 from approximately −0.7 V to −1.53 V vs RHE, while for the VB the potential varies from 1.03 to 1.90 V vs RHE. ,,, In turn, ionic carbon nitrides typically achieve more positive conduction band potentials due to the organization of melam and melem intermediates during synthesis, , and for instance, can enable a higher production of • O 2 – radicals. In practical photoelectrocatalytic applications, these band values indicate that both conventional and ionic carbon nitrides possess a suitable band structure for performing many important reactions, such as water splitting, CO 2 and N 2 reduction, , among others…”

“…Proposed charge transfer mechanism for N 2 reduction over the Nb 2 O 5 Nt/PHI Z-scheme. Reproduced with permission from ref . Copyright 2023 John Wiley and Sons.…”

A Solar to Chemical Strategy: Green Hydrogen as a Means, Not an End

“…This catalyst yielded an NH 3 rate of 32.2 μmol h −1 g −1 (547.4 mg h −1 g −1 ) under full spectrum light and 20.8 μmol h −1 g −1 (353.9 mg h −1 g −1 ) under visible light illumination, surpassing Sb nanosheets and pure a-TiO 2 by factors of 2 and 16, respectively. [53] Various materials have been considered for photocatalytic nitrogen fixation (Table 1), such as metal oxides, metal sulfides, layered double hydroxides, and carbon-based materials including carbon nitride, [54,55] bismuth-based materials, [56] single atom catalysis, [55,57,58] and metal-organic frameworks (MOFs). [59] Among these materials, the most promising results for photocatalytic nitrogen fixation have been observed for bismuth-based materials.…”

“…Various materials have been considered for photocatalytic nitrogen fixation ( Table 1 ), such as metal oxides, metal sulfides, layered double hydroxides, and carbon‐based materials including carbon nitride, [ 54 , 55 ] bismuth‐based materials, [ 56 ] single atom catalysis, [ 55 , 57 , 58 ] and metal‐organic frameworks (MOFs). [ 59 ] Among these materials, the most promising results for photocatalytic nitrogen fixation have been observed for bismuth‐based materials.…”