Photocatalytic Reduction of Aqueous Nitrate with Hybrid Ag/g-C₃N₄ under Ultraviolet and Visible Light

Abstract

The concentration of nitrate in natural surface waters by agricultural runoff remains a challenging problem in environmental chemistry. One promising denitrification strategy is to utilize photocatalysts, whose light-driven excited states are capable of reducing nitrate to nitrogen gas. We have synthesized and characterized pristine and silver-loaded graphitic carbon nitrides and assessed their activity for photocatalytic nitrate reduction at neutral pH. While nitrate reduction does occur on the pristine material, the silver cocatalyst greatly enhances product yields. Kinetic studies performed in batch photoreactors under both UV and visible excitation suggest that nitrate reduction to produce aqueous nitrite, ammonium, and nitrogen gas proceeds via a cooperative water reduction on the silver metal domains to produce adsorbed H atoms. By varying the percentage of silver loading onto the g-C 3 N 4 , the density of metal domains can be adjusted, which in turn tunes the reduction selectivity toward various products.

Microtubular Nanoporous g-C3N4@Ag as an Efficient Photocarrier Transfer Agent and Visible Light-Harvesting for Photodegradation of Methylene Blue

“…The wide peaks in the range of 3300–3500 cm –1 are related to the stretching vibration methods of NH, NH 2 , and OH methods . By comparing the FT-IR pattern related to pristine MN-g-C 3 N 4 and MN-g-C 3 N 4 @Ag, it can be concluded that some index peaks, which are related to the structure of MN-g-C 3 N 4 , are common to both, and in fact, the structure of MN-g-C 3 N 4 was not significantly changed by the addition of Ag NPs to its surface. , …”

“…43 By comparing the FT-IR pattern related to pristine MN-g-C 3 N 4 and MN-g-C 3 N 4 @Ag, it can be concluded that some index peaks, which are related to the structure of MN-g-C 3 N 4 , are common to both, and in fact, the structure of MN-g-C 3 N 4 was not significantly changed by the addition of Ag NPs to its surface. 44,45 The Brunauer−Emmett−Teller (BET) and Barret−Joyner− Halenda (BJH) analyses were done (Figure 1d). The specific surface areas and pore sizes of g-C 3 N 4 , MN-g-C 3 N 4 , and MNg-C 3 N 4 @Ag were estimated to be 6.7, 71.6, and 62.9 m 2 /g and 6.9, 14.5, and 8.2 nm, respectively.…”

“…g-C 3 N 4 has a suitable bandgap (2.70 eV) and conduction band energy −0.89 V vs standard hydrogen electrode (SHE) and valance band (1.80 V vs SHE), high thermal and chemical stability, non-toxicity, surface modification, cheapness, and fast transfer of charge carriers and can be useful photocatalysts. ,− Also, compared with other semiconductors, facile synthesis, ability to be modifiable as a platform for designing hybrid materials, hydrophilicity, high thermal and chemical stability, high biocompatibility, excellent ability to collect visible light, ability to interact π–π, and formation of hydrogen bonds as well as electrostatic interactions with organic dyes are the unique properties of g-C 3 N 4 . − Also, the high capacity for the adsorption of toxic ions and other synthetic pollutants over g-C 3 N 4 has caused its wide use in the field of surface absorption . The mentioned properties have led to the synthesis and use of its various forms such as two-dimensional structures, shell cores, uniform spherical, hierarchical multilayer, rings, and nanotubes. ,− Today, microtubular nanoporous g-C 3 N 4 (MN-g-C 3 N 4 ) has been studied due to its porous morphology, high surface area, unique charge transfers in the axial direction, and so on. At the same time, the existence of some defects such as nitrogen and carbon defect (nitrogen or carbon vacancy) can adjust the electronic structure, modify the surface characteristics, and act as a charge trap site to improve electron and hole separation.…”

Ultrafast Charge Injection in Silver-Modified Graphitic Carbon Nitride

“…Photoactive organic semiconductors are increasingly important frameworks for photocatalytically activated chemical reclamation, organic synthesis, and energy production due to their high degree of tunability and abundance of precursors. − One particularly promising photocatalytic material is graphitic carbon nitride (gCN), with its tunable electronic structure, activity under visible light, and low toxicity. − Despite these advantages, gCN’s photocatalytic activity is limited by short excited-state lifetimes and low charge mobility. − To address these limitations, significant efforts have been directed toward optimization of the chemical and structural characteristics of gCN through, for example, leveraging different precursors , or via tuning of synthetic and exfoliation approaches. ,, Another widely utilized strategy functionalizes gCN with metal nanoparticle co-catalysts, including Zn, Pt, Fe, Co, and Ag. ,,− The observed activity enhancement observed in these hybrid materials is typically attributed to charge separation across the gCN/metal interface, which is believed to mitigate charge recombination and yield long-lived redox-active excited states. − …”

Photochemical conversion of nitrate to ammonium ions by a newly developed photo-reductive titanium dioxide catalyst: implications on nitrogen recovery

“…This is mitigated by hybrid structures such as those synthesized via silver loading, which also demonstrates the promising denitrification strategy to reduce aqueous nitrate to nitrogen gas and ammonium. 32,33 However, the conversion yield of nitrate to ammonium is very low (<25%) and a majority of nitrate is converted to nitrite (>80%), which is even more toxic than nitrate. Other technological options, such as metal-organic-frameworks (MOFS) and metal-sulfide compounds, provide alternatives to nitrate reduction.…”