Photocatalytic turnover number & turnover frequency of 4-HNB under solar light by ‘1’ photocatalyst with & without reducer

Abstract

4-hydroxynitrobenzene (4-HNB) is a highly effective industrial pollutant that causes adverse effects to human beings. In this regard, detoxification of noxious water is utmost indispensable. Highly efficient metal-free photocatalytic degradation and reduction of 4-HNB with and without reducing agent still challenge. Additionally, for this role, largely expensive reagents that can create inauspicious impacts on the environment are utilized. Herein, we developed a ‘1’ photocatalyst that has the excellent ability for the H₂O₂-mediated degradation and reductant-free reduction of 4-HNB. The ‘1’ photocatalyst has an excellent turnover number (TON) 0.644×10²⁰ molecules and turnover frequency (TOF) 0.0035×10²⁰ molecules /min.

Keywords

4-HNB degradation reduction 4-HAB ‘1’ potocatalyst

The progress of the technology in 19^th to 21^st centuries has increased crucial environmental troubles that must be resolved in the coming future. These troubles consider the depletion of environmental pollution and the energy crisis [1]. Therefore, the development of highly active solar light materials that could potentially degrade noxious pollutants from air and wastewater is desperately needed [2]. Since the first manifestation of water splitting in the form of oxygen and hydrogen by graphitic carbon nitride (GCN), this 2D light-harvesting semiconductor material has been broadly utilized as a photocatalyst for the fixation of carbon dioxide [3], the separation of dangerous gases from the air [4] and degradation of inorganic and organic pollutants from use less water, and so on [5].

But like another broadly explored solar light active materials, it suffers from shortcomings like negligible electron transfer ability, marginal solar light-harvesting ability, the fast possible recombination rate of solar light-created charge carriers, and restricting its significant application in the area of photocatalysis [6, 7]. Therefore, to overcome the aforesaid limitations, the synthesis of nanocomposite with other photoactive expensive materials have been considered one of the utmost auspicious methodologies. By opted this, various nitrogen-rich graphitic carbon nitride-based solar light active expensive materials such as Au@GCN, TiO2@GCN, CdS/GCN, and so forth have been designed by different researchers [8 –10], but in all reported cases the basic circumstance, that is, matching the energy levels in between two reported semiconductors [11, 12] to form a close heterojunction has been unsatisfied. In earlier reported research work [13], Synthesis of the GCN-based heterojunction by using urea and thiourea as precursors. They have explained that this type of heterojunction exhibits highly efficient charge separation and transfer across the designed close interface. Therefore, it exhibits increased photocatalytic removal activity of pollutant compounds than neat GCN [13]. Furthermore, some current reported [14] monomers demonstrate that the in-situ preparation of the porous GCN polymeric material can not only improve its reactant adsorption and solar light-harvesting abilities but also possess improved electron transfer rate, charge recombination, and charge separation. Many researchers have prepared a derivative of porous GCN, which exhibits a promising photocatalytic ability for water splitting, organic pollutants, rhodamine dyes, and bacteria degradation [14, 15] under solar light. In this context, porous GCN hetero-structures exhibit excellent photocatalytic ability in comparison to bulk GCN, till date, and to the best of our cognition, no reports are present on the synthesis of light-harvesting GCN-based hetero-structures [16, 17]. Apart from the modification of GCN with anthraquinone is an additional highly efficient and in-expensive, methodology for the improved ability of the photocatalyst which can increase the optical properties of GCN in the UV-visible range and prolong the change in the efficiency of the photo-generated holes and electrons. Considering the excellent optical, solar light harvesting, stability, and porous properties of anthraquinone-based GCN materials have attained remarkable ability in the research area of photocatalysis. In earlier work [18, 19(a–d)], we developed an in-situ copolymerized 1,4-diaminoanthraquinone (AQ) thermally along with urea utilizing a modified metal-free carbon nitride (CN) photocatalyst (AQBCN ’1’) and utilized it in a photo-catalytic–biocatalytic platform for regeneration & formation of NADH and chemical from CO₂ respectively [18].

In continuation of this research work, 4-hydroxynitrobenzene (4-HNB) is a highly effective industrial pollutant that causes adverse effects on the human being. In this context, detoxification of infected water is utmost indispensable. The metal-free highly efficient photocatalytic degradation of 4-HNB is pic-up an efficient route. Furthermore, for this role, largely in-expensive reagents that can create inauspicious effects on the environment are utilized. Therefore, for the crucial treatment of infected water with 4-HNB, an eco-friendly route is substantially needed. Furthermore, the ability of the photocatalyst ‘1’ (AQBCN) has been examined by the H₂O₂-mediated degradation and reluc-tant-free reduction of 4-HNB.

The outcomes of the ‘1’ ability advise that the photo-catalytic pursuit is strongly pursued by the existence of AQ content. For the 1, the highest photocatalytic activity is determined, which is 12-fold higher than that of AQ toward the degradation and reduction of 4-HNB. The reactive species also engaged during the photo-chemical reactions have been examined, and plausible mechanistic pathways have been proposed for degradation and reduction reactions. The stability and durability of the ‘1’ photo-catalyst have been examined, and the received results explain that the ‘1’ can suffer the experimental situations even after many successive cycles [18]. Therefore, this move opens up an aspect for the fabrication of a metal-free heterogeneous ‘1’ photocatalyst with the utmost photocatalytic ability for degradation of 4-HNB with H₂O₂ and reduction of the same without reducing agent (Scheme 1).

Scheme 1

Illustrative pictorial diagram for the degradation (Deg.) and reduction (Red.) of 4-HNB with and without reducing agent under solar light.

Oxidative photocatalytic degradation of a few organic compounds by reducing agents such as hydrazine, sodium borohydride, and hydrogen peroxide is utilized as highly effective green scientific pathways for the detoxification of wastewater in till date [20(a–c)]. Generally, the rate of decomposition at the ambient temperature of hydrogen peroxide (H₂O₂) is very poor. Therefore, a highly efficient catalyst is needed to activate the reducing agent such as H₂O₂. In the photocatalytic procedure (Scheme 2), frequently H₂O₂ is utilized as a sacrificial agent/electron scavenger that creates ●OH radicals by ensnaring the solar light-generated electrons. Likewise, ●OH radicals can also be generated through the self-decomposition of H₂O₂ under solar light irradiation, such types of radicals are highly very reactive and can oxidize noxious organic compounds into less innocuous products. Therefore, in this current study, the degradation of 4-HNB via photo-catalyst was also investigated utilizing 25 mg/L of H₂O₂, when ‘1’ is utilized as the catalyst under solar light irradiation. Schemes 1 and 2 show, the time-dependent study of the reaction-mediated solution in the existence of the ‘1’ photo-catalyst. This scheme explains that the scenario of the wavelength at maximum absorption for 4-HAB (4-Hydroxyaminobenzene) and 4-HNB (4-Hydroxynitrobenzne) centered near about at 317 and 400 nm (Scheme 1 and Scheme 3) increases unsteadily with solar light illumination time, and the color of the working solution simultaneously undergoes a continuously change from light pale yellow to colorless.

Scheme 2

Degradation of 4-HNB with reducing agent (H₂O₂) by ‘1’ under solar light, and its mechanism. Reaction conditions for Degradation of 4-HNB: 25 mL 4-HNB (15 ppm stock solution), 25 mg/L ‘1’photocatalyst, 1 mL 30% H₂O₂, absorption spectra were measured by UV-Visible spectrophotometer (UV-Vis 1900i).

Scheme 3

The reductant-free photocatalytic reduction of 4-HNB to 4-HAB by 1 under solar light. Reaction conditions for reduction of 4-HNB to 4-HAB: 25 mg/L ‘1’photocatalyst, and 25 mL 4-HNB (15 ppm) during the photocatalytic reaction, at a particular interval of time, about 3 ml of the reaction solution was carried and filtered by the membrane, absorption spectra were measured by UV-Vis. spectrophotometer (UV-Vis 1900i).

This informs that the almost 4-HNB organic compound is degraded due impact of oxidative ●OH radicals, which is obtained by H₂O₂ decomposition after taking solar light. Therefore, the ‘1’ photocatalyst is of utmost efficiency to the variously reported photocatalysts [21, 22]. Due to this reason, we performed the control experiment in absence of a ‘1’ photocatalyst (Fig. 1). It has been determined that nearly about 4% degradation of 4-HNB is only in the existence of H₂O₂. It shows a good correlation between time and –ln (C/C₀) for the ‘1’ photocatalyst received, and the outcome is explained in Fig. 1a and b, intimating the degradation 4-HNB through photocatalytic routes follows the order of kinetics reactions [23]. The appear rate constant (k) was determined as per the reported paper [23]. The ‘K’ of the ‘1’ photocatalyst sample is 0.0667 min^-1 and half-life for the degradation of 4-HNB is 10.38 min (Fig. 1b).

Fig. 1

(a) Photocatalytic degradation rates of 4-HNB under solar light, (b) First-order kinetic study for 4-HNB degradation and K = 0.067min^-1, (c) Photocatalytic degradation with H₂O₂ (d) Photocatalytic reduction rates of 4-HNB to 4-HAB, (e) First-order kinetic study for 4-HNB reduction to 4-HAB and K = 0.013 min^-1, (f) Photocatalytic reduction without photocatalyst.

Fig. 2

(a) Photocatalytic degradation of 4-HNB in the absence of light, (b) photocatalytic reduction of 4-HNB in the absence of light (c) Long-term stability of the ‘1’ photocatalyst for the degradation of 4-HNB and, Long-term stability of the ‘1’ photocatalyst for the reduction of 4-HNB into 4-HAB.

In continuation to investigate the reductive and mechanistic behavior of the ‘1’ photocatalyst, the solar light reduction of 4-HNB without the existence of unstable and carcinogenic redcurrants in Table 1 was chosen by different researchers as a model reaction. As a reported literature [24] solution of 4-HNB has a strong absorption peak near about at 316 nm. Furthermore, in the existence of the ‘1’ photocatalyst, the color of the solution of 4-HNB changes from pale yellow to colorless with a spatial absorption peak shift from 316 to 400 nm. This type of information was also authenticated by different researchers via UV-visible absorption spectroscopy [25, 26]. Because the adsorption between ‘1’ photocatalyst and 4-HNB can reduce the photocatalytic reaction duration by increasing the 4-HNB concentration on the photocatalyst surface, so under dark reaction conditions, the effect of adsorption was examined. But when the ‘1’ photocatalyst was mixed with the reaction solution, the fixation of 4-HNB to 4-HAB was initiated, which could be envisioned by a continuous change in the solution color from bright yellow to colorless.

Table 1

Comparison table for the photocatalytic performance of different photocatalyst

Photocatalyst	Reducer	Reaction Time (min)	Conversion (%)	Reference
Pd/SiO₂- TiO₂-0.2	NaBH₄	3.35sec	100	28(a)
BiPO₄/g-C₃N₄	NaBH₄	1 min	95%	28(b)
Fe₃O₄/SiO₂@PDA/Pd	NaBH4	2 min	100%	28(c)
AgPd/rGO	NaBH₄	3 min	95.6	28(d)
N/graphene	NaBH₄	18 min	100	28(e)
Cu@g-C₃N₄	NaBH₄	25 min	98%	28(f)
g-C₃N₄/CuS	NaBH₄	50 min	99.09	28(g)
AQBCN	No one	180 min	89.36	Present work

Scheme 3 shows the UV-visible absorption spectrum on time-dependent of a typical reduction procedure using ‘1’ as a photocatalyst. As shown in Scheme 1, the UV-visible absorbance peak near about 400 nm corresponding to 4-HNB diminutions steadily, and meanwhile, an additional UV-visible absorption peak related to 4-HAB at near about 317 nm continuously strengthens and increases with time under solar light illumination (Schemes 1 and 3). Additionally, to compare the reduction efficiency of the ‘1’ photocatalyst with the reported photocatalyst in Table 1, the reduction procedure time profile was estimated by C/C₀ (Fig. 1d) as per the reported literature method [27], where C₀ and C are the final and initial absorbance of the 4-HNB reaction mixture near about at 400 nm, respectively.

As displayed in Fig. 1f, the demonstrates control experiment without a ‘1’ photocatalyst, in this context 4-HNB cannot be reduced after 180 min under solar light irradiation, but the excellent reduction efficiency is measured for the ‘1’ photocatalyst. The major benefits lying behind this impressive gain in the performance of ‘1’ photocatalyst are (1) the network crosslinking justification in terms of the change in the band structure electronic properties for fantastic utilization of solar light; and (2) the existence of incorporating monomer in ‘1’ photocatalyst provides competent sites for the slow recombination charge carriers and electron transport properties. To further examine the conversion rate of 4-HNB using a ‘1’ photocatalyst, the graph of ln(C/C₀) vs time for reaction is monitored for 180 min. As an outcome, Fig. 1e, exhibits a good correlation, this relation intimating that the nature of the kinetic reaction is pseudo-first-order. The apparent rate constant (k) was obtained at 0.0132 min^-1 for the ‘1’ photocatalyst from Fig. 1e and the half-life for the reduction of 4-HNB is 52.30 min.

Additionally, we checked the degradation of 4-HNB and reduction of 4-HNB to 4-HAB in the absence of light by the ‘1’ photocatalyst. In Fig. 2, we got a negligible change in the degradation and reduction spectra. Along with it, we checked the reusability of the ‘1’ photocatalyst. We performed the recycling experiment for checking the chemical stability of the ‘1’ photocatalyst during the degradation of and reduction of 4-HNB, shown in Fig. 2. In this experiment, we have re-used the same photocatalyst for five successive runs (i.e., four recycles) under the same reaction conditions. We observed that the photocatalytic degradation and reduction efficiency is almost constant during each recycles which confirms the highest stability of the ‘1’ photocatalyst.

To address the superiorities of the ‘1’ photocatalyst, the photocatalytic ability of ‘1’ was compared with the reported literatures (Table 1).

As entries one–seven cases, either cyanogenic reducing agent [28] such as hydrazine, sodium borohydride, sodium thiosulfate, and hydrogen sources were used. Additionally, as per the reported method [28] during a photocatalytic procedure, different active species like ●OH radicals, electrons, holes, and ●O₂^– radicals are generated. Therefore, it is confirmed that electrons are the primary active species participating in the fixation of 4-HNB to 4-HAB. Herein we demonstrate first time as per my knowledge photocatalytic fixation from the ‘1’ photocatalyst of 4-HNB to 4-HAB without utilizing any reducing reagents, making our photocatalyst superior to the reported light harvesting ‘1’photocatalyst [18]. Because radical trapping mechanism and photocatalytic active sites on the surface of the photocatalysts [29, 30] explains that electrons are working as the main reactive species during the fixation process of 4-HNB.

In summary, the ‘1’ photocatalyst has excellent network cross-linking electronic properties along with competent sites for the slow recombination charge carriers, and electron transport properties for fantastic utilization of solar light. Apart from the particular results, this research work confirms the important role of the edge or surface of the ‘1’ photocatalyst in photo-fixation and degradation reactions, proposing new strategies for effective noxious pollution remediation. Therefore ‘1’ photocatalyst has a remarkable turnover number (TON) and turnover frequency (TOF).

Footnotes

Acknowledgments

Thanks for the financial support to MMMUT Gorakhpur and Mokpo National University, Jeollanam-do, South Korea.

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