Two Birds, One Stone: Coupling Hydrogen Production with Herbicide Degradation over Metal–Organic Framework-Derived Titanium Dioxide

Abstract

Consumption of contaminated water can have detrimental effects on the health of every living organism on earth. There is, thus, a need to develop novel materials and technologies to purify water. Water is also a source of hydrogen, a clean renewable fuel that can be generated through the action of a photoactive catalyst and the earth's abundant solar energy. Using photocatalysis, we can purify water by removing organic pollutants through the photodegradation reaction (oxidation) and produce hydrogen (H 2 ) through the hydrogen evolution reaction (reduction). However, we can combine these two reactions in a single process to achieve an efficient photocatalytic system. Herein, we report the dual-functional photocatalysis (DFP) on herbicidecontaminated water using TiO 2 polymorphs derived from the amino-functionalized metal−organic framework (MOF), MIL-125-NH 2 . Heteroatom TiO 2 doping led to the generation of N-and N,S-doped TiO 2 . Of all the pristine and doped TiO 2 phases synthesized, the N,S-doped anatase (NSTA) was the best dualfunctional photocatalyst in degrading glyphosate (PMG) with simultaneous H 2 production at a rate of 660 μmol g −1 h −1 . Nuclear Magnetic Resonance studies indicate the preferential cleaving of PMG's C−N bonds, here referred to as α and β C−N bonds, leading to the formation of glycine, formic acid, and phosphoric acid as the major degradation products. Density functional theory calculations indicate PMG's activation through the carboxy and phosphoric acid groups on the surface of NSTA and through the phosphoric acid group on the surface of TiO 2 -rutile. Our results suggest that the catalytic activity of NSTA can be attributed to the templated impact of the parent MOF, owing to its porosity, redshifting of the band absorption edge toward the visible region, reduced energy bandgap, surface defects, and the presence of oxygen vacancies. The binding mode of PMG to NSTA and its degradation allowed us to test the photodegradation of other herbicides, such as glufosinate ammonium and 2,4dichlorophenoxyacetic acid. Interestingly, our MOF-derived NSTA proved to be active in purifying water when all three herbicides were combined and produced H 2 with a rate of 329 μmol g −1 h −1 simultaneously.

Advanced photocatalytic materials based degradation of micropollutants and their use in hydrogen production – a review

“…Consequently, addressing the generation of AMPA during glyphosate degradation has become a critical concern in mitigating the environmental consequences associated with glyphosate. 165 In their study, Musa et al 166 opted to use glyphosate ( N -phosphonomethylglycine or PMG) as the herbicide of choice for their photocatalytic tests. The investigators proceeded to analyze the relative proportions of PMG and the decomposition products, namely glycine, formic acid, and AMPA, generated in the solution.…”

“…While the performance of both pristine and doped mixed phases (consisting of both rutile and anatase) was enhanced, the resultant mixed products included formic acid, glycine, and AMPA, with PMG persisting in the system. 166 AMPA, an intermediate of PMG, is known for its adverse environmental impact, primarily involving damage to the carbon–nitrogen bond in PMG. The N, S-doped anatase (NSTA) phase demonstrated both efficient degradation of PMG and the generation of environmentally safe by-products, namely glycine and formic acid.…”

“…Specifically, the herbicides PMG, exhibiting high solubility and hydrophilic properties, demonstrated significantly greater efficiency in hydrogen production. 166 This emphasizes the need for developing dual-functional photocatalytic devices that prioritize the characteristics of the pollutant and treatment requirements as the starting point for development.…”

Elucidation of the role of metals in the adsorption and photodegradation of herbicides by metal-organic frameworks

“…The majority of studies on photocatalytic degradation of GP employ the metal oxide semiconductor TiO 2 , with a few studies focused on MnO 2 30 . Recently, our group demonstrated complete GP degradation in 5 mM (845 ppm) GP solution after 8 h of irradiation using a nitrogen and sulfur-doped TiO 2 anatase (NSTA) derived from the metal-organic framework (MOF) MIL-125-NH 2 54 . While kinetic investigations revealed that AMPA was formed initially, AMPA was almost entirely degraded after the reaction.…”

CdS and Ti3C2 Quantum Dot-Modified TiO2–x Nanotubes as Highly Efficient Photocatalysts for Selective Oxidation of Benzyl Alcohol to Benzaldehyde

“…In the past decades, efforts have been made to harness solar energy more efficiently with efficient photocatalysts such as Cu 2 O/TiO 2 , Fe 2 O 3 /TiO 2 , CdS/TiO 2 , and WO 3 /TiO 2 . However, TiO 2 has a wide band gap and absorbs only ultraviolet (4% of the entire solar spectrum), which largely limits its utilization of sunlight, and many solutions have been proposed to solve this problem. − For example, it is possible to extend its optical absorption to visible light by doping with various ions, but this usually results in a lower photogenerated charge lifetime. − Since photocatalysts modified by vacancies can reduce the complexation rate of photogenerated carriers and enhance photocatalytic performance, the introduction of oxygen vacancies in TiO 2 is expected to be one of the effective strategies to optimize the photocatalytic activity of TiO 2 . ,− In addition, combinations with other semiconductor materials have emerged. Zhang et al dispersed CdS particles on the surface of MIL-125­(Ti), resulting in a conversion of BA to BAD of about 20.1% .…”