S-Scheme-Mediated NiCo2S4–P–g-C3N4 Heterojunction for Selective Nitrate Reduction to Ammonia

Photocatalysis offers a promising avenue for the efficient removal of nitrate compounds from water by harnessing solar energy and transforming them to ammonia. The constraints, such as rapid charge carrier recombination and comparatively low selectivity, however, hinder the widespread application of the technology. In this study, an inexpensive bimetal sulfide (NiCo2S4, NCS) is anchored over the phosphorus (P)-doped g-C3N4 (PCN) sheets. The fabricated NCS-PCN heterojunction is tested for the photocatalytic nitrate-reduction activity. Notably, a dose of NCS-PCN composite with 30% (w/w) NCS shows an optimal nitrate reduction, achieving over 99% removal in 4 h, with ammonia selectivity increasing from ∼56 to 96%, compared to PCN alone under the identical conditions. The photocatalyst also shows remarkable stability, experiencing a marginal decrease of less than 1% in the activity after 5 reaction-regeneration cycles. The isotope labeling test using nuclear magnetic resonance spectroscopy confirms that nitrate salt supplied in the feedstock is the nitrogen source for ammonia formed during photoreduction. The observed enhanced activity, supported by radical scavenging tests, electronic paramagnetic resonance spectroscopy measurements, X-ray photoelectron spectroscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopic analyses, provides evidence for the S-scheme-mediated charge transfer mechanism for the NCS-PCN heterojunction. This study not only contributes valuable insights into an efficient light-harvesting photocatalyst but also paves the way for sustainable nitrate-reduction processes on a large scale.