In situ green synthesis of Au/Ag nanostructures on a metal-organic framework surface for photocatalytic reduction of p-nitrophenol

Highlights

• In situ immobilization of GNPs and SNPs over MOF-76 surface to yield NCs.

• The highest catalytic efficiency (96.3%) for p-NP degradation was calculated for MOF-76(1a) NCs.

• MOF-76 (1a) NCs also exhibited rate constant ($K_{app}^{\text{'}}$) of 0.33 min⁻¹.

• The NCs showed substantial recyclability and superiority over commercial photocatalysts.

Abstract

In this manuscript, we report the insitu growth of gold nanoparticles (GNPs) and silver nanoparticles (SNPs) over Ce/Tb-doped Y-benzene tricaroboxylate metal organic frameworks (MOF-76). The photocatalytic potential of these nanocomposites (NCs) was then assessed for the photocatalytic reduction of p-nitrophenol (p-NP) in aqueous medium. The NCs had large surface area to favorably showcase the efficient absorption of light energy. Correspondingly, it facilitated the electron transfer between the valance and conduction bands to promote the reductive removal of p-NP. Among the tested catalysts, GNPs@MOF-76 (Ce) (MOF-76(1a)) (dosage: 10 μl of 5 mg.ml⁻¹) recorded the maximum catalytic efficiency (96.3%) with an apparent rate constant ($K_{app}^{\text{'}}$: 0.33 min⁻¹). Further, MOF-76 (1a) NCs exhibited good recyclability (e.g., a catalytic efficiency of 77.45% over 3 cycles). Moreover, the proposed NCs exhibited superior performance over commercial photocatalysts (e.g., p25 TiO₂ nanopowder) towards p-NP reduction. As such, the results of this experimental study successfully demonstrated the superiority of the MOF immobilized nanoparticles (NPs) as an efficient platform for the removal of industrial grade pollutants.

Introduction

Wastewater is a combinatorial term used collectively for the untreated effluents generated from agricultural, industrial, commercial, and domestic origins [1]. In recent years, excessive pollution of water resources caused by harmful anthropogenic activities has substantially devastated both human and ecological systems. Consequently, it has become necessary to introduce benign and effective wastewater management practices including recycling of wastes containing harmful substances within aqueous ecosystems. The major components of wastewater include both chemical (e.g., organic dyes, toxic metals, and pesticides along with their precursors and degradation by-products) and biological origins (e.g., pathogenic microbes and plant nutrients) [1], [2]. Among the various organic pollutants, p-nitrophenol (p-NP) are of major environmental concern due to their recalcitrant nature and their removal difficulties [3], [4]. Specifically, p-NP is a potent carcinogen classified as a pollutant of prime concern by the United States Environment Protection Agency (USEPA) [5]. It is routinely used to manufacture herbicides, insecticides, synthetic dyestuffs, and wood preservatives [5]. Furthermore, p-NP is also released into the environment through partial degradation of insecticides such as ethyl and methyl parathion. In light of the health and environmental concerns of these pollutants, it is imperative to develop innovative treatment methods for their high removal efficiency.

Among various remediation approaches, photocatalytic reduction of organic pollutants has generated substantial interest. Primarily this process is carried out using a photocatalyst (e.g., metal or semiconductor NPs in presence of light and alkaline environment. During the course of the reaction, the pollutant p-NP is reduced into a relatively useful material called p-aminophenol (p-AP). In the presence of OH- ions, the p-NP molecules undergo deprotonation, thereby producing intermediate nitrophenolate ion [4]. Subsequent addition of NaBH₄ molecules reduces the nitrophenolate ion into p-AP. The progress of this reduction reaction can be easily monitored by the fading of the yellow color of p-NP solution. In presence of NaBH₄, the reaction proceeds spontaneously in forward direction owing to reduction in free energy (E₀ for p-NP/p-AP = −0.76 V). However, the kinetic energy barrier for this reaction is very high owing to large potential difference between donor (H₃BO₃/NaBH₄) and acceptor molecules (nitrophenolate ion) [4]. As such, the feasibility of the reaction decreases as a function of time. However, the reaction can be realized in presence of an appropriate catalyst.

Nanomaterials (NMs) possess unique morphological and structural characteristics (such as ultrafine dimensions, high specific surface area, tunable functionalities, and appreciable pore properties) for photocatalytic applications [6]. Specifically, GNPs and SNPs have been investigated extensively for p-NP removal owing to their well-known features (e.g., tunable morphology, photo-thermal effects, surface plasmon resonance, and high resistance towards oxidation/corrosion in air) [7], [8]. These characteristics facilitate increased adsorption of target analytes on the NPs’ surfaces [9]. For instance, Ag NPs are well known to reduce the kinetic energy barrier during photocatalytic oxidation of p-NP to p-AP owing to enhanced adsorption capacities for the reactants. As such, the kinetic energy barrier is minimized through enhanced charge transfer from BH₄⁻ to −NO₂⁻ groups of p-NP [4]. The reaction of Ag NPs mediated heterogeneous catalytic reduction of p-NP to p-AP is thermodynamically favourable in aqueous NaBH₄ [6]. Furthermore, the enhanced reusability of NMs can be accounted for by facile separation of p-AP from the active sites [9].

In addition to GNPs and SNPs, many other types of NMs (such as TiO₂ NPs [10], SnO₂ NPs [11], carbon based NMs (e.g., carbon nanotubes, graphene) [12], [13], [14], silica NPs [15], covalent organic frameworks [16], and metal blends [17], [18], [19]) have also been tested for their photocatalytic potentials. However, due to strong affinity and high surface energy, these NMs are found to exhibit the reduced catalytic response with the formation of large aggregates (macrostructures) with [20]. To enhance their stability and catalytic activity (or to resolve such drawbacks), numerous approaches (e.g., steric and electrostatic stabilization and immobilization over solid supports) have been proposed [18], [21], [22], [23]. For instance, CuInS₂-ZnS porous spherical crystals were synthesized using wet chemical approach [24]. The prepared NMs showcased high catalytic efficiency towards removal of NO gas. Likewise, the use of magnesium silicate and hydrothermal carbon composites was recommended to improve adsorption of Cd²⁺ ions and methylene blue (MB) dye [18]. These composites showed enhanced adsorption rates of Cd²⁺ ions via electrostatic bonding and ionic bonding interactions. The adsorption of MB was also augmented owing to enhanced electrostatic, hydrogen bonding, and π-π bonding interactions in aqueous medium [18]. The aforementioned chemical interactions were also adopted to describe intensified adsorption of diclofenac sodium by Mg/Al layer double hydroxide-(Poly (m-phenylenediamine) composites [23]. In light of the enhanced performance of NCs for pollutant removal, they are also expected to have enhanced potential for p-NP mitigation.

To date, the emergence of porous coordination polymers called metal organic frameworks (MOFs) has opened up new avenues for pollutant mitigation in view of their tailorable porosities, high surface area, and open metal sites [25], [26], [27]. The presence of pendant functional groups on their surface is favorable for the immobilization of various guest species such as MNPs. Further, in situ growth or immobilization of the MNPs on MOFs can enhance their catalytic potential for the abatement of pollutant compounds in comparison to their pristine forms [28]. For the preparation of NCs between MOFs and MNPs, many processing options are available such as in situ encapsulation [28], metal organic chemical vapor deposition [29], post-synthetic covalent surface attachment [30], solid grinding [31], and solution infiltration [32]. Unfortunately, in the majority of case studies, the NCs were demonstrated to suffer from agglomeration of MNPs or from degradation of the MOF crystal structure. Further, the complex procedures required for the purification and recycling of the catalysts are major challenges encountered during the synthesis and application of NCs.

In light of these limitations, we investigated the synthesis of NCs between GNPs/SNPs and Ce- and Tb-doped Y-benzene tricarboxylic acid (BTC) MOFs (MOF-76) for the photocatalytic reduction of p-NP. The MOF-76 family holds an intersecting 3D structure, high thermal/chemical stabilities [33], [34], and a large pore size/surface area [35]. In addition, it has also been reported to form stable suspensions in water for the removal of pollutants such as uranium [35]. All of these characteristics render MOF-76 a desirable candidate for the maximum possible deposition of MNPs for the formation of highly functional NCs. The Ce- and Tb-doped MOF-76 samples are referred to as MOF-76 (1) and MOF-76 (2), respectively. GNPs and SNPs were synthesized by a green approach using Momordica charantia fruit extract. The MNPs were immobilized over the MOF template using an in situ synthesis approach to yield NCs. The as-prepared NCs were subsequently investigated for the reduction of p-NP. As such, the NCs exhibited a synergistically remarkable catalytic activity through the immobilization of the MNPs over the MOF substrates. Moreover, the catalytic efficiency of the NCs was considerably enhanced compared to the pristine forms of the individual components. Thus, this manuscript reports the efficacy of the MOF-MNP NCs for the catalytic removal of pollutant organic compounds.