1. Introduction {#sec1}
===============

Reduction process is a fundamental and important chemical transformation in organic synthesis^[@ref1]−[@ref3]^ and industrial chemistry,^[@ref4]−[@ref6]^ the key step being that the electrons transform from a donor to the target substance. The reduction of nitroaromatics (NAs) is a common and facile route to produce aminoaromatics (AAs), which are very significant intermediates for the synthesis of several nitrogen-containing compounds, such as agrochemicals, pharmaceuticals, polymers, dyes, pesticides, and cosmetics.^[@ref7]−[@ref12]^ Several toxic NAs are responsible for serious environmental pollutions.^[@ref13]^ However, they can be transformed into AAs, that is, nitrophenol (NP) conversion into harmless aminophenols (AP), which are potential intermediates for accessing pharmaceuticals and dyes via the reduction process ([Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}a). Accordingly, various NAs can be reduced to their amino counterparts through the use of catalysts wherein the applied catalysts play a significant role.^[@ref14]−[@ref19]^ Consequently, the suitable design of the catalyst structure and their prudent selection can remarkably improve the reduction efficiency, thus providing better catalytic sustainability and recoverability.

![(a) Schematic design for the reduction of nitrobenzene with aqueous NaBH~4~ using nanocatalysts. (b) Classification of various nanocatalysts applied for the reduction of NAs to AAs.](ao-2018-03051w_0029){#fig1}

The description of the catalysts these days can be simply and ideally stated as nanoparticles (NPs) with or without supports. Nanosized catalysts with high specific surface area and without supports provide a ready contact with the reactants, thereby improving the catalytic activity.^[@ref20]−[@ref22]^ However, the high surface energy of nanostructures escalates their instability and leads to aggregation, which results in the loss of catalytic activity.^[@ref23],[@ref24]^ An inevitable loss of nanocatalysts appears during their tedious separation from the products. Therefore, the deployment of supports effectively prevents their aggregation and undesirable lose, thereby enlarging the total surface area and assuring their sustained catalytic activity and reusability; the positioning of supports with a high specific surface area generally provides a promoted adhesion to reactants.^[@ref25]−[@ref27]^

Regarding the greener aspect of the catalytic processes, it is highly desirable to develop environmentally benign procedures that can be conducted preferably in aqueous media, thus avoiding the use of volatile organic solvents; sodium borohydride (NaBH~4~) is one such favored water-soluble reductant for representative reductions.^[@ref28]−[@ref30]^ In the reduction process of NA to AA with aqueous NaBH~4~, electrons from BH~4~^--^ transfer to NA when both of the species are absorbed on the surface of the catalyst.^[@ref31]−[@ref33]^

X-ray diffraction (XRD), scanning electron microscopy (SEM), field emission SEM (FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and infrared radiation (IR) are the most commonly used characterization techniques for the synthesized catalysts, whereas ultraviolet--visible spectroscopy (UV--vis) is a convenient way for monitoring the conversion process; 4-NP has a typical absorption peak at 317 nm. However, after adding aqueous NaBH~4~, a red shift of the maximum absorption peak occurs immediately from 317 to 400 nm because of the formation of 4-nitrophenolate ions under alkaline conditions. Thereafter, these peaks at 400 nm are reduced over time.^[@ref34]−[@ref38]^ Moreover, gas chromatography--mass spectrometry is needed for the measurement of accurate conversion percentage and for verifying the ensuing reaction products.^[@ref39],[@ref40]^

Consequently, the compatibility of the applied catalysts affects the overall reduction productivity.^[@ref41]^ In view of their importance, we present recent methods for the synthesis of nanocatalysts as directed for the reduction of NAs with focus on the advances in nanostructured catalysts for the hydrogenation of NA with aqueous NaBH~4~. The general strategies for the synthesis of representative heterogeneous nanocatalysts, their advantages, limitation, and challenges are also discussed. For the sake of differentiation and ease of understanding, the nanocatalysts are classified with or without supports for monometal and bi- or multimetal nanostructured catalysts. Additionally, we have categorized separately the supported nanocatalysts with and without magnetic properties in view of their ease of separation and reuse ([Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}b). The first category of nanocatalysts in this article, monometallic nanocatalysts, possesses solely one metal element as the catalyst for expediting the reduction of NAs to AAs. Subsequently, bimetallic and solid supported non-magnetic nanocatalysts are discussed, emphasizing on their designs to overcome the low efficiency and aggregation issues of the monometal nanocatalysts.^[@ref42]^ Although these catalysts demonstrate considerably improved catalytic activity, their reusability is still a challenge. Consequently, the magnetic nanostructured catalysts are presented to facilitate easy separation and as sustainable catalysts for reduction of NAs.^[@ref43]^

2. Monometallic Nanocatalysts {#sec2}
=============================

The decrease in size of metal catalysts having diameters in the nanometer range ensures high surface area, enhancing their catalytic performances. Metal NPs have remarkable catalytic susceptibility because of their adjustable size, shape-associated activity, and selectivity. Monometal nanocatalysts used in NaBH~4~ reduction of NAs, generally, noble- and transition-metal NPs such as palladium (Pd), platinum (Pt), ruthenium (Ru), and silver (Ag), are synthesized and directly employed as nanocatalysts in the hydrogenation of various NAs using aqueous NaBH~4~ as the reductant.^[@ref44]−[@ref49]^ Effective catalytic achievement can be attained using monometallic NPs with high surface potential determined by the valence amount of unsaturated surface atoms and by the Fermi energy level in redox.^[@ref50]−[@ref52]^ The catalytic activity of the Pt NPs has been reported to be highly dependent on the morphology, porosity, size distribution, and phase composition; significant enhancement could be discerned in the catalytic activity when monodispersed Pt NPs are linked into Pt nanowires.^[@ref53]^

Mahmoud et al.^[@ref54]^ concluded, while reducing 4-NP, that the reaction yield is a function of free surface area of the NPs. In this study, the mechanism of 4-NP reduction by Au nanocatalysts was investigated using NaBH~4~ in solution. The reduction process can be achieved either on the NP surface following the heterogeneous mechanisms or by the leached atoms or ions from the NPs (homogeneous mechanism) in the solution. Considering the plasmonic effect of Au NPs and measuring the dependence of the reaction yields on the surface area of the nanocatalyst by the adsorption isotherm of binding 4-NP and the changes of the plasmonic extinction band position of the NPs during the reactions, the researchers concluded that it is the surface heterogeneous-type mechanism and not the homogeneous path by the leached atoms in the solution ([Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}) that is involved.

![Schematic explanation for the reduction of 4-NP by NaBH~4~ catalyzed by Au nanocatalysts. (a) The reaction is driven rapidly in the absence of 4-NP. (b) The reaction rate is slower when the surface of Au nanocatalysts is decreased because of 4-NP binding resulting in the heterogeneous catalytic mechanism.](ao-2018-03051w_0028){#fig2}

Kaur et al*.*^[@ref55]^ also demonstrated this premise by reducing NA using reusable silver nanocatalysts and size-controlled copper (Cu) NPs in aqueous medium; hydrogenation proceeded on the surface of Cu NPs, and the catalytic efficiency increased as the size of particles decreased ([Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}). It is believed that the activity of nanocatalysts would decrease once the exposed surface area is covered.^[@ref56]^ However, a long shelf life of Cu NPs could be attained by coating polyacrylic acid without inhibiting the reduction activity.

![Schematic demonstration of Cu nanocatalysts synthesis.](ao-2018-03051w_0027){#fig3}

3. Bi- or Multimetallic Nanocatalysts {#sec3}
=====================================

This section describes bi-metals of the various alloys as they appear to exhibit an improved activity compared to the corresponding monometallic nanocatalysts because of synergetic effects.^[@ref57]^ In the case of bi- or multimetallic nanocatalysts, various electronic interactions between the atomic orbitals of different metals result in the "volcano-shape" relationship, which clearly indicates synergistic effects between the metallic sources.^[@ref58]^ The extra electrons in the outer orbitals of the metallic sources having relatively higher activity are capable of transferring to the adjacent different metallic atoms with relatively lower activity, culminating in coordination between those different metallic elements. Consequently, the formed electron-rich metallic atoms facilitate the electron transfer from the adsorbed BH^4--^ to the 4-NP, enhancing the process efficiency. For instance, Chu et al.^[@ref58]^ reported that polyelectrolyte multilayer-supported AuPt alloy NPs exhibit higher catalytic activity than Au and Pt monometallic catalysts for the reduction of 4-NP by NaBH~4~.

3.1. Cu--Pd Nanocatalysts {#sec3.1}
-------------------------

Saikia et al.^[@ref59]^ synthesized Cu--Pd alloy NPs by stirring CuCl~2~ and PdCl~2~ in aqueous NaOH, followed by injecting the mixed solution into a Teflon cup in a stainless steel autoclave ([Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"}).

![Schematic procedural details for the synthesis of Cu--Pd nanoalloy.](ao-2018-03051w_0026){#fig4}

The prepared Cu-Pd NPs were characterized using XRD, SEM, energy-dispersive spectroscopy (EDS) elemental mapping analyses, and TEM. [Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"} shows TEM images of the formed Cu--Pd NPs with below 20 nm size; more than 90% of the NPs fall in the size range 3--4 nm, while [Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"}d presents the histogram of the NPs with an average size of 3.3 nm. The reduction activity was characterized by the hydrogenation reaction of 4-NP to 4-AP in aqueous NaBH~4~ wherein the use of bimetal Cu--Pd nanocatalysts remarkably improved the catalysis activity compared to the use of only either Cu or Pd.

![TEM images of (a--c) Cu--Pd NPs at different resolutions and (d) the particle size distribution obtained from (c). Reproduced with permission from ref ([@ref59]).](ao-2018-03051w_0025){#fig5}

3.2. Pt--Au Nanocatalysts {#sec3.2}
-------------------------

Fu et al.^[@ref60]^ synthesized uniformly nanosized Pt--Au alloy NPs via a one-pot hydrothermal approach for the reduction of 4-NP and compared their catalytic activity with Pt NPs. The prepared Pt--Au NPs were characterized by TEM and EDS techniques ([Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"}). The uniform square nanocatalysts had an average size of ∼10 nm as shown in [Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"}a,b. The EDS elemental mapping results indicate that the Pt--Au NPs consist of alloys with a uniform elemental distribution rather than a core--shell structure ([Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"}c,d).

![(a, b) Typical TEM images of the Pt--Au alloy nanocubes. (c, d) EDS elemental mapping images of Pt and Au in the Pt--Au NPs. Reproduced with permission from ref ([@ref60]).](ao-2018-03051w_0024){#fig6}

The catalytic activity of the Pt--Au nanocatalysts was ascertained by the reduction of 4-NP solutions with 0.1 M NaBH~4~ ([Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}) using a UV--vis instrument. The reduction period by Pt--Au bimetallic nanocatalysts (33 min) was shorter than the 51 min by Pt monometallic nanocatalysts, exhibiting a better catalytic activity for reducing 4-NP.

![(a) UV--vis spectra of 4-NP before and after adding NaBH~4~ solution. (b) UV--vis spectra for successive reduction of 4-NP with NaBH~4~ using Pt--Au nanocatalysts. Reproduced with permission from ref ([@ref60]).](ao-2018-03051w_0023){#fig7}

3.3. Pt--Ni Nanocatalysts {#sec3.3}
-------------------------

Ghosh et al.^[@ref61]^ synthesized the bimetallic Pt-Ni alloyed nanocatalysts by the addition of NiSO~4~ and H~2~PtCl~6~ in cetyltrimethyl ammonium bromide (CTAB) micellar medium followed by injection of hydrazine (N~2~H~4~) and KOH solution. The catalytic activity of the Pt--Ni nanocatalysts was evaluated for the reduction of 4-NP to 4-AP using aqueous NaBH~4~ as the reductant. The catalytic reduction was found to be equally successful for other nitroarenes such as 2-NP and 4-nitroaniline. The comparative catalytic activity between Raney Ni and Pt--Ni nanocatalysts showed that the activity of the Pt--Ni alloy is much higher than that of only Ni and, similarly, when compared to only Pt nanocatalysts.

3.4. Pt--Ni--Fe Nanocatalysts {#sec3.4}
-----------------------------

Chen et al.^[@ref62]^ synthesized Pt--Ni--Fe multimetal heterogeneous nanocatalysts wherein the as-synthesized Ni--Fe branched-dumbbell-shaped material was synthesized via electromagnetic wave assistance ([Figure [8](#fig8){ref-type="fig"}](#fig8){ref-type="fig"}).

![Schematic view for the fabrication of branched-dumbbell Pt--Ni--Fe nanocatalysts.](ao-2018-03051w_0022){#fig8}

The morphology and structure of the Pt--Ni--Fe nanocatalysts were characterized by SEM and TEM. In addition, powder XRD was employed to characterize the phase structure of the prepared nanocatalysts. The branched-dumbbell-shaped Pt--Ni--Fe nanocatalysts presented efficient catalytic activity compared to Ni/Fe branched-dumbbells and Ni nanocatalysts for the reduction of *p*-NP; at least 10 successive cycles of reaction with a conversion efficiency of ∼97% could be successfully achieved ([Figure [9](#fig9){ref-type="fig"}](#fig9){ref-type="fig"}). The as-prepared Ni--Fe and Pt--Ni--Fe NPs could be easily separated from the solution and recovered using an external magnet and redispersed in solution for subsequent use.

![The catalytically recyclable reduction of 4-NP by Pt--Ni--Fe nanocatalysts. Reproduced with permission from ref ([@ref62]).](ao-2018-03051w_0021){#fig9}

4. Solid-Supported Nonmagnetic Nanocatalysts {#sec4}
============================================

Solid supports effectively inhibit the aggregation of active nanocatalysts compared with unsupported monometal and multimetal nanocatalysts.^[@ref63]−[@ref66]^ In addition, the supports generally offer vast surface area, which is crucial for the improvement of catalytic efficiency.^[@ref67]−[@ref70]^ Accordingly, significant development has been made in the field of heterogeneous catalysis by applying strong metal--support interactions to strengthen catalytic performance.^[@ref71]^ The selection of supports is an important issue because of the synergetic effects between the supports and nanocatalysts, empowering catalytic processes. For example, conductive support-containing catalytic NPs may assist electron transfer between nanocatalysts and reactants/reductants.^[@ref72]^ In NA reduction, the supports also facilitate the adherence of nitro moieties and reductants to the nanocatalysts, supplying more electron transport channels. This section explores various supported nanocatalysts applied in the reduction of NAs in water. In practice, the supported nanocatalysts display enhanced recycling performance in the reduction of NAs because of their retainable mechanical properties; supports play an important role in the proficiency of the catalytic systems.

4.1. Carbon Nanotube-Supported Nanocatalysts {#sec4.1}
--------------------------------------------

Wang et al.^[@ref73]^ synthesized carbon nanotube (CNT)-supported Au NPs for the degradation of pollutants via a multistep approach as shown in [Figure [10](#fig10){ref-type="fig"}](#fig10){ref-type="fig"}, including (1) activation of stainless steel mesh, (2) growth of CNT arrays, (3) synthesis of Au NPs, and (4) attachment of Au NPs to CNTs. Au NP-decorated CNT-supported materials promoted the catalytic degradation of *p*-NP in aqueous NaBH~4~ solution.

![Preparation of CNT-supported Au nanocatalysts.](ao-2018-03051w_0020){#fig10}

4.2. Mesoporous Silica-Supported Nanocatalysts {#sec4.2}
----------------------------------------------

Han and co-workers introduced a facile one-step method for the preparation of Ag-NP-loaded mesoporous silica SBA-15.^[@ref74]^ Mesoporous silica and Ag NPs were spontaneously synthesized with the Ag NPs embedded in the channels, and their catalytic activity was examined for the reduction of 4-NP in the presence of NaBH~4~. In an interesting work, Zhang et al.^[@ref75]^ synthesized nanotube-shaped silica loaded with silver NPs using electrospinning technology. TEM images ([Figure [11](#fig11){ref-type="fig"}](#fig11){ref-type="fig"}) exhibited the amorphous and extremely long nanotubes with uniform diameters (250--350 nm) independent of whether there is loading or nonloading of other active species; these nanotube catalysts exhibited excellent catalytic performances for the hydrogenation of 4-NP to 4-AP, which has been ascribed to their high specific surface area.

![(a) TEM image of the as-prepared nanotube-shaped silica and (b) nanotube-shaped silica-supported Ag NPs. Reproduced with permission from ref ([@ref75]).](ao-2018-03051w_0019){#fig11}

4.3. Graphene Oxide-Supported Nanocatalysts {#sec4.3}
-------------------------------------------

Graphene oxide is one of the most widely used supports for loading of nanocatalysts such as Pd, Au, and Ag.^[@ref76]^ Ye et al.^[@ref72]^ designed Pt--Au NPs, which were supported on functionalized graphene oxide ([Figure [12](#fig12){ref-type="fig"}](#fig12){ref-type="fig"}); the as-synthesized Pt--Au alloys exhibited a dendrimer-like nanostructure with a small size of ∼50 nm. The size and morphology of Au NPs could affect the hydrogenation catalysis.^[@ref77],[@ref78]^ However, the hydrogenation activity of Au is usually much lower than Pt-group metals.^[@ref60]^ The introduction of engineered graphene oxide-supported Pt--Au could provide active catalytic sites, improving the electron transfer, thereby promoting the catalytic activity of the NA reduction.

![Synthetic scheme for Pt--Au nanocatalysts on functionalized graphene oxide supports for the efficient reduction of 4-NP.](ao-2018-03051w_0018){#fig12}

Chen et al.^[@ref79]^ synthesized a graphene oxide-supported CdS hybrid photosensitive catalyst via the electrostatic interaction of negatively charged graphene and positively charged CdS NPs for the light-assisted reduction of NA ([Figure [13](#fig13){ref-type="fig"}](#fig13){ref-type="fig"}). The CdS/graphene hybrid nanocatalysts demonstrated a high reductive activity presumably via the synergistic effect emanating from CdS and graphene oxide, which provided a high transfer rate of electrons for the reduction of NA to AA. The intimate interfacial contact via an electrostatic self-assembly strategy promoted the carrier transfer at the CdS/graphene hybrid nanocatalysts upon irradiation of visible light, thus providing enhancement of the photocatalytic performance for the selective reduction of 4-NAs. The introduction of graphene increased the work function of the electrons, which were generated from CdS NPs upon irradiation of visible light. In addition, the deployment of graphene also contributes to the enhancement in the concentration of the NAs on the graphene surface, which effectively accelerates the catalytic hydrogenation.

![Schematic procedure for the electrostatic self-assembly of CdS/graphene hybrid nanocatalysts.](ao-2018-03051w_0017){#fig13}

In another effort, Zhang and colleagues introduced a facile method to decorate Pd nanocatalysts on the graphene oxide surface^[@ref80]^ by a simple mixing process in aqueous Pluronic F--127 as a mild reductant at room temperature ([Figure [14](#fig14){ref-type="fig"}](#fig14){ref-type="fig"}). TEM images confirmed the good distribution of Pd NPs on graphene oxide ([Figure [15](#fig15){ref-type="fig"}](#fig15){ref-type="fig"}). The prepared catalyst converted NAs to AAs in aqueous NaBH~4~ in a short reaction time.

![Synthetic procedure for the preparation of Pd NPs decorated on graphene oxide.](ao-2018-03051w_0016){#fig14}

![(a) TEM image of graphene oxide. (b) TEM and (c, d) HRTEM images of Pd nanocatalysts decorated on graphene oxide. Reproduced with permission from ref ([@ref80]).](ao-2018-03051w_0015){#fig15}

4.4. Carbon Nitride-Supported Nanocatalysts {#sec4.4}
-------------------------------------------

Bhowmik et al.^[@ref81]^ reported the synthesis of carbon nitride-supported ultrasmall Au NPs for the reduction of 4-NP in aqueous medium, which showed an excellent catalytic activity and good stability. The nanocatalysts were characterized by TEM ([Figure [16](#fig16){ref-type="fig"}](#fig16){ref-type="fig"}), selected area electron diffraction, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and X-ray photoelectron spectroscopy methods.

![(a) TEM images of carbon nitride and (b) Au--carbon nitride nanocomposite catalyst. Reproduced with permission from ref ([@ref81]).](ao-2018-03051w_0014){#fig16}

The morphology of the formed carbon nitride ([Figure [16](#fig16){ref-type="fig"}](#fig16){ref-type="fig"}a) shows two-dimensional carbon nitride sheets. [Figure [16](#fig16){ref-type="fig"}](#fig16){ref-type="fig"}b exhibits the uniform dispersion of Au NPs with a mean size of 1.5 nm on the carbon nitride sheets demonstrating highly loaded Au NPs. The reduction measurement of 4-NP to 4-AP was completed by employing aqueous NaBH~4~ as the reductant, monitored by UV-vis spectra absorption, with the reduction time being remarkably shortened, ∼15 s to complete the reduction. Li and his research group^[@ref78]^ reported a similar finding for the reduction of 4-NP with mesoporous carbon nitride-supported Au NPs, showing 96% conversion using aqueous NaBH~4~ in 5 min. The catalyst could be recycled by centrifugation, exhibiting at least five successive cycles with high conversion efficiency. Graphitic carbon nitrides are readily obtainable starting from urea or melamine or a mixture thereof.^[@ref82]^

4.5. Polymer-Supported Nanocatalysts {#sec4.5}
------------------------------------

Sreedhar et al.^[@ref83]^ synthesized Pt NPs supported on gum acacia, which were characterized by TEM, XRD, XPS, and Fourier transform infrared spectroscopy (FT-IR); the amount of Pt NPs in the formed heterogeneous catalyst was determined by using inductively coupled plasma atomic emission spectrometry (ICP-AES). The strong interactions of NPs with functional molecular groups of gum acacia resulted in the formation of monodispersed NPs; catalytic activity of the catalyst was also compared with Pt/C, Pt-Al~2~O~3~, and Pt-ZrO~2~ in the reduction of nitrobenzene. The results ([Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"}) showed an enhanced activity of gum acacia-supported Pt nanocatalysts because of the high surface area and strong hydrogen trapping property of Pt NPs. Furthermore, the nanocatalysts could be reused several times with moderate loss in catalytic activity ([Table [2](#tbl2){ref-type="other"}](#tbl2){ref-type="other"}). Stable anchoring of Pt NPs on gum acacia was robustly maintained after recyclization measurement. The renewable gums obtained from various trees are relatively untapped resources, although they have been used as food additives for a while.^[@ref84]^

###### Comparative Study of Different Supported Pt Nanocatalysts in the Reduction of Nitrobenzene

![](ao-2018-03051w_0001){#gr26}

  entry   nanocatalyst     time (min)   yield (%)
  ------- ---------------- ------------ -----------
  1       gum acacia--Pt   6            91
  2       Pt/C             12           47
  3       Pt-Al~2~O~3~     12           0
  4       Pt-ZrO~2~        12           0

###### Reusing Gum Acacia--Pt NPs in the Reduction of Nitrobenzene

  entry       first   second   third   fourth   fifth
  ----------- ------- -------- ------- -------- -------
  yield (%)   91      88       86      85       84

4.6. Polyelectrolyte Multilayer-Supported Nanocatalysts {#sec4.6}
-------------------------------------------------------

Chu et al.^[@ref85]^ synthesized Au--Pt alloy NPs supported on polyelectrolyte multilayers (PEMs) for the reduction of 4-NP using aqueous NaBH~4~; the ensuing nanocatalysts were analyzed by ICP-AES and were found to be well-dispersed Au--Pt alloy NPs with a narrow size distribution in the polymer matrices. The reduction process was tracked using UV--vis absorption spectroscopy, taking ∼6 min to achieve the hydrogenation of 4-NP. The optimal synergistic effect of Au/Pt was identified to be 2:1. The PEM-supported Au--Pt alloy NPs exhibited higher catalytic activity than Au and Pt monometallic NPs for the reduction of 4-nitrophenol by NaBH~4~, presenting synergistic effects between Au and Pt.

5. Supported Magnetic Nanocatalysts {#sec5}
===================================

The magnetic nanocatalysts have a comparative advantage as they provide easier separation from the reaction media with less catalyst loss by simply using an external magnet.^[@ref86]^ The nanocatalyst separation and reuse by magnetic field is an important factor for commercial manufacture and attaining cost effective reductions.^[@ref87]^ However, there are also some disadvantages including the material selection for synthesizing appropriate nanocatalysts. The limited availability of model magnetic nanocatalysts for the catalytic processes restricts their practical applications. Therefore, many scientists have pursued various strategies to develop ideal magnetic nanocatalysts.^[@ref88]^

5.1. Nanocatalysts Decorated on Iron Oxide NPs {#sec5.1}
----------------------------------------------

Patra et al.^[@ref89]^ synthesized Ag/Fe~2~O~3~ nanocatalysts for the hydrogenation of nitroarenes; hydrothermally formed Fe~2~O~3~ NPs were obtained by admixing sodium salicylate with NaOH, followed by injection of aqueous Fe(NO~3~)~3~. Subsequently, the Ag NPs were deposited on the surface of the Fe~2~O~3~. The FESEM images ([Figure [17](#fig17){ref-type="fig"}](#fig17){ref-type="fig"}) of the prepared magnetic nanocatalysts indicated the identical bitruncated-octahedron-shaped Fe~2~O~3~ NPs with a length of ∼310 nm, a width of ∼220 nm, and a height of ∼150 nm and a well-dispersed coating of Ag NPs on the surface.

![FESEM images of magnetic NPs (a) before and (b) after Ag NP decoration. Reproduced with permission from ref ([@ref89]).](ao-2018-03051w_0013){#fig17}

The catalytic activity was explored via the hydrogenation of 2 mL of aqueous 0.1 mmol/L 4-NP with 200 μL of 10 mmol/L aqueous NaBH~4~ at 500 μL of 1 mg/mL aqueous nanocatalysts. The reduction process required ∼10 min for the complete conversion of 4-NP to 4-AP. The reusability of Ag/Fe~2~O~3~ NPs in the hydrogenation reaction was measured for 10 cycles using 4-NP as a reference, which exhibited a good catalytic activity and almost no obvious deactivation after 10 times of cyclic hydrogenation. The Ag/Fe~2~O~3~ nanocatalysts also successfully reduced other functionalized NAs such as 4-nitrobenzoic acid, which is a challenging proposition because of the presence of carboxylic acid groups ([Table [3](#tbl3){ref-type="other"}](#tbl3){ref-type="other"}). Similarly, Pelisson et al.^[@ref90]^ prepared maghemite-supported Pd NPs nanocatalysts for the reduction of nitro aromatics to amino aromatics with aqueous NaBH~4~.

###### Magnetic Ag/Fe~2~O~3~ Nanocatalysts for Hydrogenation of Nitroarenes in the Presence of NaBH~4~ for 30 min

![](ao-2018-03051w_0002){#gr27}

5.2. Nanocatalysts Decorated on Carbon-Coated Magnetic NPs {#sec5.2}
----------------------------------------------------------

An et al.^[@ref91]^ synthesized Fe~3~O~4~\@carbon-supported Ag--Au nanocatalysts and studied the effect of the Ag/Au ratio on the catalytic activity in the hydrogenation of NAs ([Figure [18](#fig18){ref-type="fig"}](#fig18){ref-type="fig"}). Remarkably, this work showed that the use of carbon can contribute to both the improvement of catalytic performance of the noble metals and in situ preparation of Ag-Au bimetallic nanocatalysts. Catalytic reduction of 4-NP by aqueous NaBH~4~ using Fe~3~O~4~\@C\@Ag--Au nanocatalysts, monitored by UV--vis spectra absorption, showed significant enhancement in the catalytic activity (4.5 min).

![Schematic diagram for the fabrication of magnetic metal oxide\@C\@Ag--Au nanocatalysts.](ao-2018-03051w_0012){#fig18}

The recycling ability of the Fe~3~O~4~\@C\@Ag--Au nanocatalysts was verified with high yields in six reaction cycles. Similarly, Zhang et al.^[@ref92]^ prepared Pt--Pd nanoalloys supported on Fe~3~O~4~\@C core--shell NPs using a facile two-step synthesis method ([Figure [19](#fig19){ref-type="fig"}](#fig19){ref-type="fig"}) and characterized them by high-resolution TEM; Pt--Pd nanocatalyst decoration on a carbon layer coating on the surface of the Fe~3~O~4~ NPs was discerned. In another exploration, Du et al.^[@ref93]^ synthesized Pt--Pd NPs on super-paramagnetic core--shell nanocatalysts for the reduction of 4-NP to 4-AP using aqueous NaBH~4~. The hydrogenation required 22 min to completely convert the 4-NP to 4-AP using 13.63 wt % of the prepared nanocatalysts; the yield and selectivity were obtained to be 96% and 99%, respectively.

![Schematic procedure for the preparation of Pt--Pd NPs on magnetic core--shell nanocatalysts.](ao-2018-03051w_0011){#fig19}

5.3. Nanocatalysts Decorated on Polymer-Encapsulated Magnetic NPs {#sec5.3}
-----------------------------------------------------------------

Various nanocatalysts can be adorned on the polymer shell containing magnetic NPs. The polymer layers can protect the magnetic NPs from dissolution and corrosion in the reaction environment. Ayad et al.^[@ref94]^ synthesized silver nanocatalysts decorated on a polyaniline--chitosan--magnetite (Ag\@PANI-CS-Fe~3~O~4~) nanocomposite catalyst for the hydrogenation of 4-NP by aqueous NaBH~4~ ([Figure [20](#fig20){ref-type="fig"}](#fig20){ref-type="fig"}).

![Synthetic strategy for the preparation of Ag\@PANI-CS-Fe~3~O~4~ nanocomposite catalyst.](ao-2018-03051w_0010){#fig20}

The Ag\@PANI-CS-Fe~3~O~4~ nanocomposite catalyst successfully reduced 4-NP under aqueous NaBH~4~ reaction conditions as monitored by UV--vis spectrophotometry, which confirmed rapid hydrogenation of 4-NP to 4-AP in less than 10 min; the magnetic catalysts remained active with an efficacy of 95% in the fourth cycle. By using a similar technique, Zeng et al.^[@ref95]^ synthesized Fe~3~O~4~\@polydopamine (PDA)--Au nanocatalysts, which were characterized using TEM, indicating the presence of Au NPs on the core surface of the polymer shell. The aqueous *o*-nitroaniline solution could be reduced in 7 min using a small amount of Fe~3~O~4~\@PDA--Au nanocomposite catalyst. In this work, PDA effectively protected the iron oxide and Au NPs from the aggregation in the solution. The core--shell catalyst also displayed good catalytic activity for various nitrobenzene reductions ([Table [4](#tbl4){ref-type="other"}](#tbl4){ref-type="other"}).

###### Reduction of Various Nitrobenzenes Using Fe~3~O~4~\@PDA--Au Nanocatalysts

![](ao-2018-03051w_0003){#gr28}

Shokouhimehr and colleagues^[@ref96]^ prepared a magnetically retrievable nanocomposite adorned with Pd nanocatalysts, which was applied for the reduction of NAs in aqueous NaBH~4~ solution. Pyrrole monomers were polymerized in the presence of Pd precursors and iron nanopowder forming Pd NPs on the polypyrrole framework without the requirement of an additional reductant ([Figure [21](#fig21){ref-type="fig"}](#fig21){ref-type="fig"}).

![Procedure for the synthesis of the magnetically retrievable nanocomposite adorned with Pd nanocatalysts. Reproduced with permission from ref ([@ref96]).](ao-2018-03051w_0009){#fig21}

TEM and FESEM images of the prepared magnetic catalysts showed uniform ∼2 nm Pd NPs accommodated discretely on the polypyrrole layer of the prepared nanocomposite ([Figure [22](#fig22){ref-type="fig"}](#fig22){ref-type="fig"}). The nanocomposite catalyst could be easily separated and recycled using a small magnet and reused for seven consecutive cycles of high-yield reduction of nitrobenzene (99--95%) in aqueous NaBH~4~ solution.

![(a) TEM image and (b) HRTEM image of iron nanopowders. (d) TEM image and (e) HRTEM image of the magnetic nanocomposite adorned with Pd nanocatalysts. (c) FESEM image of iron nanopowders. (f) FESEM image of the magnetic nanocomposite adorned with Pd nanocatalysts. Reproduced with permission from ref ([@ref96]).](ao-2018-03051w_0008){#fig22}

5.4. Magnetic Nanocatalysts Decorated on Mesoporous Silica Nanospheres {#sec5.4}
----------------------------------------------------------------------

Magnetic core--shell nanocatalysts have been utilized for hydrogenation reactions without any support.^[@ref97]^ For example, a Ag\@Ni nanocatalyst was prepared via a simple one-pot synthesis, which catalyzed the reduction of NAs.^[@ref98]^ However, the application of mesoporous silica (mSiO~2~) is an important issue for heterogeneous catalysis because of their excellent stability, high surface area, tunable pore size, and chemical inertness. Yao et al.^[@ref99]^ prepared the Fe*~x~*O*~y~*/Pd\@mSiO~2~ magnetic NPs consisting of a movable Fe*~x~*O*~y~* core and mesoporous mSiO~2~ ([Figure [23](#fig23){ref-type="fig"}](#fig23){ref-type="fig"}). The Fe~3~O~4~\@C NPs were prepared by mixing glucose and Fe~3~O~4~ NPs in water followed by heating at 200 °C in an autoclave for 8 h. The Fe~3~O~4~\@C/Pd was prepared by adding the Fe~3~O~4~\@C in PdCl~2~ ethanol solution, followed by injecting NH~2~NH~2~ and washing the ensuing products. Finally, the mSiO~2~ on the surface was formed by using tetraethyl orthosilicate and CTAB, followed by calcination at 600 °C. The obtained product was characterized by SEM, TEM, FT-IR, and UV--vis.

![Scheme of the preparation of Fe*~x~*O*~y~*/Pd\@mSiO~2~ nanocomposite catalyst.](ao-2018-03051w_0007){#fig23}

The catalytic activity of the core--shell nanocomposite catalyst was verified for the reduction of NAs, which was accomplished in ∼40 min; successive recycling of 4-NP reduction was achieved with the conversion of ∼100% for each cycle ([Figure [24](#fig24){ref-type="fig"}](#fig24){ref-type="fig"}).

![Conversion of 4-NP in 10 successive cycles using Fe*~x~*O*~y~*/Pd\@mSiO~2~ catalyst.](ao-2018-03051w_0006){#fig24}

Wang et al.^[@ref100]^ designed a mesoporous silica for loading core--shell Ni\@Pd nanocatalysts exhibiting high catalytic activity for the hydrogenation of 4-NP. Dong et al.^[@ref101]^ designed a silica sphere with a dandelion-like shape, which was decorated with Ni\@Pd nanocatalysts for the reduction of 4-NP and 4-chlorophenol ([Figure [25](#fig25){ref-type="fig"}](#fig25){ref-type="fig"}). The silica spheres with a size of 200--300 nm efficiently inhibited the aggregation of Ni\@Pd NPs providing an extensive accessibility for the hydrogenation of NAs. The magnetically separable catalysts can be promising candidates for important organic conversions and industrial applications.^[@ref102]^

![Preparation of Ni\@Pd nanocatalysts supported on mesoporous silica. Reproduced with permission from ref ([@ref101]).](ao-2018-03051w_0005){#fig25}

6. Conclusions {#sec6}
==============

Recent advancements in the hydrogenation of nitroaromatics to aminoaromatics, catalyzed by various nanocatalysts using aqueous NaBH~4~ as a reductant, are summarized with representative examples of the mono- and multimetal supported and magnetic nanocatalysts. Promising catalytic efficiency has been attained by designing the catalysts with high specific surface area and good protection of the active nanocatalysts. However, substantial challenges still persist for the large-scale production, which requires cost efficiency and a capability to sustain considerable recycling operation. The synthesis of sustainable nanocatalysts needs further innovative strategies and developments to solve the current limitations such as aggregation, recyclability, stability, and durability.

^‡^ K.Z. and J.M.S. contributed equally to this work.

The research presented was not performed or funded by the U.S. Environmental Protection Agency (EPA) and was not subject to EPA's quality system requirements. The views expressed in this article are those of the author(s) and do not necessarily represent the views or the policies of the EPA.

The authors declare no competing financial interest.

The financial support of the Future Material Discovery Program (2016M3D1A1027666), and the Basic Science Research Program (2017R1A2B3009135) through the National Research Foundation of Korea is appreciated.
