Gold nanoparticles decorated two-dimensional TiO₂ nanosheets as effective catalyst for nitroarenes and rhodamine B dye reduction in batch and continuous flow methods

Highlights

• AuNPs modified on TiO₂ nanosheets were successfully obtained.

• Small AuNPs were uniformly deposited onto surface of TiO₂ nanosheets.

• The AuNPs-TiO₂NSs were highly efficient for the reduction of nitroarenes and dyes.

• The catalyst was active for 12 cycles in fixed bed reactor.

Abstract

Our environment is greatly endangered by the accumulation of various toxic organic pollutants that are continually produced through unavoidable human needs and the industrialization process. Herein, we report highly active gold nanoparticles (AuNPs) immobilized on two-dimensional (2D) TiO₂ nanosheets (AuNPs-TiO₂NSs) as a catalyst for the catalytic reduction of nitroarenes (NAs) such as 4-nitroaniline (4-NA), 4-(4-nitrophenyl)morpholine (4-NM), 4-(2-fluoro-4-nitrophenyl)morpholine (4-FNM) and rhodamine B (RhB) dye in the presence of sodium borohydride (NaBH₄) medium. Initially, TiO₂NSs are prepared by the hydrothermal treatment followed by the modification with 3-aminopropyl-trimethoxysilane (APTMS) coupling agent for strong anchoring of the AuNPs. HR-TEM images exhibit that AuNPs (2.30 ± 0.06 nm) are immobilized on the surface of ultrathin 2-dimensional TiO₂NSs. AuNPs-TiO₂NSs catalyst shows excellent catalytic activity towards the reduction of various NAs (4-NA, 4-NM and 4-FNM) and RhB dye with maximum conversion efficiency of >98 %. Moreover, the pseudo-first-order rate constants are estimated as 5.50 × 10⁻³ s⁻¹, 7.20 × 10⁻³ s⁻¹, 6.40 × 10⁻³ s⁻¹ and 4.30 × 10⁻³ s⁻¹ for the reduction of 4-NA, 4-NM, 4-FNM, and RhB, respectively. For large-scale industrial applications, AuNPs-TiO₂NSs catalyst embedded in a continuous flow-fixed bed reactor for the catalytic reduction of 4-NA and RhB dye under optimized reaction conditions. AuNPs-TiO₂NSs catalyst shows high conversion rates for 4-NA (>99 %) and RhB (>99%) along with excellent recyclability over 12 cycles in continuous flow fixed bed reactor. The mechanism of synthetic pathway and catalytic reduction of NAs and RhB dye over AuNPs-TiO₂NSs catalyst are also proposed. This study may lead to the use AuNPs-TiO₂NSs catalyst with superior recyclable catalytic efficiency in various catalytic reactions.

Introduction

Water is the most essential source for the existence of life on earth. Owing to the rapid growth of the population and the technological explosion, water pollution has increased by various highly toxic pollutants such as pesticides, dyes, heavy metals, insecticides, and herbicides [1], [2], [3], [4], [5], [6]. Accordingly, the discharge of polluted water in larger quantities is a severe threat to the environment and is considered one of the primary challenges to environmental sustainability. The enormous amount of industrial effluents mainly contains various toxic compounds, including organic dyes and nitroarenes (NAs) [5]. Amongst, NAs are the primary source of water contaminants that are found to be environmentally hazardous substances that are widely used in the production of pharmaceuticals, anilines, agrochemicals, dyes, pesticides, explosives, and so forth that cause serious threats to living beings [7], [8], [9].

Among the various NAs, 4-nitroaniline (4-NA) has been utilized as an intermediate compound in the manufacture of dyes, medicines, pesticides, etc [10]. Due to the excellent chemical stability and high spread utility, 4-NA is easily enriched in living organisms and eventually stays in the environment for a prolonged time leading to severe ecological issues and causing carcinogenic or toxic effects on living beings [10], [11]. Long-term exposure to 4-NA can induce several health issues in humans, such as irritability, dyspnea, diarrhea, vomiting, respiratory arrest, jaundice, and anemia. Consequently, 4-NA has been listed as one of the priority toxic pollutants controlled by the Environmental Protection Agency [12]. There is extensive attention on how to effectively remove 4-NA from wastewater [10], [11], [12]. Additionally, 4-(4-nitrophenyl)morpholine (4-NM) is another kind of nitro-compounds, which is widely used as a starting material for the synthesis of Reversine anticancer drugs [13]. The analog of 4-NM, 4-(2-fluoro-4-nitrophenyl)morpholine (4-FNM) is also utilized in the production of the Linezolid® antibiotic drug [14], [15]. Currently, >100 morpholinoanilines- based products are available in the global market, and also significant research is in progress for efficient synthetic methods [16], [17]. Therefore, the reduction of nitro compounds to the less toxic and biodegradable respective amines is highly needed for the synthesis of various aniline-based materials for various industrial applications [10].

Dyes are considered to be another important organic pollutant, which is directly discharged into the environment [4], [5], [6]. Among the various dyes, rhodamine B (RhB) is used extensively in the printing, textile, and photographic industries [18], [19]. This dye is directly discharged into the natural environment, which is aesthetically not favorable [4]. In general, many organic dyes are carcinogenic, and mutagenic and pose a major threat to the natural environment. Therefore, considering the toxicity and carcinogenic effects of both NAs and dyes, it is imperative to degrade such hazardous chemicals into less toxicity compounds [4].

Numerous chemical, physical and biological approaches have been utilized in the past decades for the reduction of nitro compounds and degradation of hazardous organic dyes in the water bodies [4], [5], [6]. There are different technologies for water remediation of dyes and NAs, such as membrane filtration, adsorption, and catalytic degradation, into less hazardous chemicals. Besides, these conventional treatment methods have many constraints and do not effectively remove the hazardous contaminants [20]. On the other hand, chemical reduction routes have been greatly employed to reduce NAs and degradation of the dyes into less toxic products [21], [22]. These reactions have a negligible impact on the environment and are generally performed using a metal-based catalyst which initiates the electron transfer and sodium borohydride (NaBH₄) as a greener reducing agent [23], [5], [24].

To date, several nanostructured and heterogeneous catalysts have been reported for the reduction of NAs and degradation of dyes [25], [26], [27], [28]. Among those, metal nanoparticles (MNPs) exhibit unique catalytic performance owing to their high surface area-to-volume ratio and versatile electronic/geometric structures [25], [26], [27], [28], [29], [30], [31]. When compared to various MNPs, gold NPs (AuNPs) have been extensively utilized for catalysis due to their higher stability and catalytic activity for several chemical reactions [32], [33]. However, one of the key issues in the synthesis of Au-based catalysts is the control of particle size to create the most active, stable, and selective systems in the catalytic reduction reactions. For this purpose, the selection of suitable support is highly desired which plays a vital role in controlling of NPs growth, dispersity, and stability which are mainly based on metal-support interactions [34]. Metal oxides are considered ideal supports to design catalysts, particularly, TiO₂ nanosheets (TiO₂NSs) is a potential candidates for various catalytic reactions due to their chemical stability, abundance, and nontoxicity. Unique two-dimensional (2D) TiO₂NSs are one of the primary conditions for the synthesis of highly efficient catalysts as a carrier of immobilizing MNPs. And also the synthesis of TiO₂NSs is easy and simpler than that of other two-dimensional materials. The selective amine-functionalization over TiO₂NSs is also a novel strategy to introduce highly dispersed and ultra-small metallic NPs over TiO₂NSs [15], [17], [34]. The excellent advantages of TiO₂NSs make it a promising catalyst support used widely in electrochemical catalysis [35], photocatalysis [36], solar cell [37], and fuel cells [38], etc. So far, there is no reported works on TiO₂NSs-supported metal NPs for catalytic reduction of NAs/dye in the presence of a green reducing agent.

In this study, the synthesis of ultra-small AuNPs on the surface of 2-dimensional TiO₂ nanosheets (AuNPs-TiO₂NSs) by hydrothermal and followed by the wet chemical methods has been reported. The synthesized material is subjected to structural and morphological studies by employing various analytical techniques. Moreover, the catalytic activities of synthesized AuNPs-TiO₂NSs are assessed by the chemical reduction of NAs and the degradation of RhB dyes. The obtained AuNPs-TiO₂NSs catalyst exhibits superior catalytic activity, and degradation efficiency of NAs and RhB can reach almost >98 % within ∼ 10 min. In addition, AuNPs-TiO₂NSs catalyst shows excellent operational stability over 12 recycles in a continuous flow fixed bed reactor for the reduction of 4-NA and RhB dye. The catalytic mechanism for the reduction of NAs and dyes by the catalyst is discussed in detail. Finally, the obtained catalyst is tested for the reduction of NAs and RhB dye in various environmental water samples.