Morphologically tailored facet dependent silver nanoparticles supported α -Al 2 O 3 catalysts for chemoselective reduction of aromatic nitro compounds

The nanoparticles surface area, intrinsic sites, exposed microcrystal shapes and lattice planes are some of the key factors in nanocatalysis. The influence of nanoparticles shape dependent had been profound effect on its catalytic activity. This study is focused on the synthesis of morphologically shape-controlled silver (Ag) nanoparticles supported on α -Al 2 O 3 catalysts were performed. The correlation of Ag NPs with varied facets and lattice planes on the catalytic activities in chemoselective reduction of nitro compounds was investigated. Engineering the silver nanoparticles with different shapes and facets i.e., nanocubes (AgNCs), nanowires (AgNWs) and nano spheres (AgNSPs) were synthesized by using modified polyol method. It is demonstrated that there is a significant difference in their activities with respect to the shape and nanocrystal facets. The evolution of nanoshapes and the structural properties of Ag nanoparticles were analysed by SEM, TEM, HR-TEM and P-XRD techniques. From XRD, Ag nanocubes exhibited high percentage of low index (100) facets which are favourable active centers than (111) plane in nitro reduction. We observed that the silver nanocubes selectively exposed (100) facets, which are highly favorable for the enhanced catalytic activity in nitro reduction. The reaction rate of nitro phenol to amino phenol over different Ag nanoshapes are 35.01 × 10 (cid:0) 3 min (cid:0) 1 (AgNCs/Al 2 O 3 ), 8.28 × 10 (cid:0) 3 min (cid:0) 1 (AgNWs/Al 2 O 3 ), 0.65 × 10 (cid:0) 3 min (cid:0) 1 (AgNSPs/Al 2 O 3 ), respectively. The calculated thermodynamic parameters of the E a values 23.6, 28.6 and 29.4 for the AgNC, AgNWs and AgNSP respectively.


Introduction
Nanocatalysis made significant breakthroughs in the field of sustainable and green chemistry [1][2][3][4].The active phase has been synthesised in various forms to gain highest activity and stability with high dispersion.There are many strategies and novel approaches in designing and engineering the metal-based nanocatalysts at relatively low loadings [5,6].Further, synthesizing the catalysts with different morphologies at nano scale such as with different shapes and sizes have gained immense interest in the scientific community.The best way to decorate the active metal with excellent catalytic behavior is to design the synthetic route by changing their morphology and successfully managing their structures with various crystallographic exposed facets and shapes [7][8][9][10].The remarkable activities by nanocatalysts are attributed to its high surface to volume ratio and site accessibility.Typically, nanosized metal particles tend to have high surface energy, which are highly unstable and lead to microaggregates.Recently, significant progress had been made in the field of nanoparticle synthesis (e.g., Au, Pt, Ag, & Pd nanocatalysts) with advanced techniques and methods [11][12][13][14][15].This developments in metal nanoparticles synthesis opened many opportunities in tuning, modifying, and restructuring the surface microcrystal lattice.Further, this differentiation between the effect of size, specific crystallographic planes, and other features such as defects and active sites at edges had profound outcome on their catalytic activities.
Silver is a group IB transition metal with a 4d 10 5s 1 electronic structure [16].Silver-based nanocatalysts studied with different morphologies and structures such as nanowires, nanorods, nanocubes, and nanospheres have been proven as effective due to its better specific physical and chemical structures and applied in many fields example in heterogeneous catalysis [17].However, most of the works have been focused on catalysis application i.e., mostly related to the silver clusters or nanoparticles studied in structure-activity correlations.Nevertheless, the Ag catalysis is still yet to be fully explored in nanocatalysis like that of Au catalysis, which is widely studied with various Au facets from nano level to single atom.The surface structure and localized active sites are important features and typically determining factors in silver based nanocatalysts and their respective performance in various reactions.Moreover, Ag is highly sensitive to the reaction conditions, preparation methods, and the size variations.Thus, we considered all factors to design and modify the Ag surface with different morphologies and shape-controlled facets and their correlation with activity in reduction of nitro compounds were studied.
In this context, few studies have been reported on the tailoring and engineering the silver nanoparticles (AgNPs) with different shapes and sizes for the catalytic applications [18][19][20][21][22][23].The facet-dependent on distinctive Ag indexed exposed lattices with varied shapes has not explored widely in catalysis.Shaped-controlled and exposed indexes lattice planes exhibit different activities and solely depends on the reaction characteristics.One such study on supported silver nanocatalysts have been reported, e.g., a low indexed Ag (1 0 0) facet planes are highly selective in gas phase ethylene epoxidation [24].Recently, Linic et al. presented on how silver nanocubes (AgNCs) size and shape controlled had significant influence on the epoxidation performance [25].Moreover, the AgNCs exhibited improved catalytic activity compared to their nanospherical counterparts.The AgNCs have shown to be more selective for (1 0 0) planes for this reaction than (1 1 1) because of the set of planes that form the external facets [24,25].However, there are no studies reported were found related to the selective Ag nanoparticles facet dependent catalytic system for the liquid phase reactions.
Reduction of nitro compunds is critical from environmental point of view.The nitro compounds are by-products of pharma ingredients found in many aquatic streams such as industrial waste waters [26][27][28][29][30]. Generally, even in low concentrations of nitro compounds in waste waters are harmful and carcinogenic to human life.Nevertheless, aromatic amines are versatile synthetic intermediates due to their wide range of application in the preparation of dyes, agricultural products, pharmaceuticals, polymers and surfactants [31][32][33].At both industrial and laboratory scale, the aromatic amines are typically prepared by conventionally manufactured by the reduction of corresponding nitro compounds via catalytic hydrogenation and other different reductive pathways.There are numerous reducing agents were utilized for the reduction of nitro compounds owing their pros and cons.However, the most common being metallic reagents in the presence of an acid.As the existing reagents and solvents are highly complex and environmentally hazardous, thus, there is a need for cleaner solvents with high atom economy, high dissolution rates, efficient hydrogen transfer and cheaper alternatives are considered [11].In this context, NaBH 4 is moderately efficient reducing agent, better alternative and promotes faster hydrogen transfer.In this work, NaBH 4 in water is used as the hydride source for the liquid phase reduction experiments.However, the reduction of nitro groups in NaBH 4 solvent can be promoted in the absence of catalyst, but with hydrogenation rate.The reductive amination in the presence of metal-based catalysts is highly active and selective with high atom economy was reported.Especially, noble metalbased nanoparticles (such as Au, Pt, Pd,) were reported extensively in the literature and prepared with wide range of novel methods in tailoring size and shape-controlled catalytic materials with ultra-low loadings in catalytic reduction of nitro compounds [11].However, silver (Ag) nanoparticles are not well studied in reduction of nitro compounds.Exploring the Ag nanoparticles with different shapes and low indexed facets planes in reductive aromatic nitro compounds to respective aromatic amines is a novel idea.Herein, we demonstrated the synthesis and catalytic behaviour of different facet-oriented silver nanoparticles in the form of nanocubes -AgNCs, nanospheres -AgSPs, and nanowires -AgNWs and their respective α-Al 2 O 3 supported catalytic systems in chemoselective reduction of aromatic nitro compounds were reported.

Synthesis of Ag nanowires (AgNWs)
Silver nanoparticles with different shapes i.e., AgNSPs, AgNWs and AgNCs nanocatalysts were successfully synthesized by modified polyol method which was developed by Xia's group [19][20][21]34].A modified seedless polyol process method was used in synthesizing the silver nanowires (AgNWs).Wherein, silver precursor AgNO 3 was reduced in ethylene glycol (EG) at 160 • C in the presence of poly(vinyl pyrrolidone) (Mol.Wt. = 58 000) [6,7].For the preparation of AgNWs, 5 mL of EG was pre-heated to 160 • C and continued under reflux for 1 hr in a three round bottom flask equipped with a magnetic stirrer and a condenser, which is submerged in an oil bath to allow homogeneous heating.A twochannel syringe pump was used for introducing 3 mL of AgNO 3 solution (94 mM in EG) and 3 mL of PVP solution (144 mM in EG) at a rate of 0.375 mL/min and the solution was heated simultaneously.After introducing the reagents into the solution, it was refluxed under vigorous stirring at 160 • C for 3 h.During the synthesis process, after adding few drops of AgNO 3 to the heated EG, immediately the solution turns into yellow colour and then after 5-10 min, the solution turns to reddish colour and then gradually turn to grey and then finally it forms opaque solution, indicating the beginning of nanowires growth.The final solution was completely in turbid and grey colour.The reaction solution was diluted with 10-fold in acetone and centrifuged at 2500 rpm for 20 min in order to separate the nanowires from the ethylene glycol.The seedless growth of nanowires differentiated with colour variations.This phenomenon was witnessed at each stage during the synthesis steps.

Synthesis of silver nanocubes (AgNCs)
For a standard synthesis [19,20], a 5 mL of diethylene glycol (DEG) was added to a flask, and then suspended in an oil bath under magnetic stirrer at 150 • C for 30 min.Further, simultaneously, other reagents were separately dissolved in DEG and then introduced into the flask sequentially using a pipette.At first, a 0.06 mL of NaCl solution (3 mM) was added and after 4 min, a 0.5 mL of HCl (3 mM) was added, followed by 1.25 mL of PVP (Mol.Wt. = 58 000) (20 mg/mL).In the next step, after 2 min, a 0.4 mL of CF 3 COOAg solution (282 mM) was introduced into the first solution.The flask was capped with glass stoppers during the entire reaction process.The synthesis was quenched in flask under ice bath and the resultant products were centrifuged followed by repeated washing with acetone and de-ionized water to remove the remaining pre-cursor, DEG and excess PVP.The AgNCs centrifuged and separated from the solution and dried under vacuum.

Synthesis of silver nanospherical (AgNSPs)
For a typical AgNSPs synthesis [22], ethylene glycol (5 mL) was heated in an oil bath at.90 • C for an hour.In the next step, a solution of silver nitrate (2.5 mM) in ethylene glycol (3 mL) and a solution of PVP (25 mm, Mol.W. 40,000) in ethylene glycol (3 mL) were simultaneously added under vigorous stirring under hot ethylene glycol solution.The reaction solution was stirred for an hour at 90 • C and then allowed to cooldown to room temperature.The silver nano particles were centrifuged (30 min at 11,000 rpm) followed by redispersion in acetone and water then filtered and dried under vacuum at 60 • C.

Preparation of AgNPs supported α-alumina catalysts
AgNPs (silver nanoparticles) with different shapes i.e., nanospheres, nanocubes and nanowires of desired amount was dispersed in ethanol solution and mixed with dried α-alumina and stirred vigorously at 600 rpm for 24 h.The alumina supported AgNPs were then separated by mild centrifugation and then dried in vacuum overnight.Efficient adsorption of AgNPs over alumina was only evident when the AgNPs were dispersed in ethanol [16].

Catalytic reaction
2.6.1 Method for chemo-selective reduction of aromatic nitro compounds of various halo nitroarenes using spherical, cube and wire shaped silver nanoparticles supported on alumina, in the presence of aqueous NaBH 4 .All the catalysts were calcined at 350 • C for 5 h to apply the aromatic nitro reduction.
In a typical reaction, a 50 mg of catalyst (2 wt% AgNPs/α-Al 2 O 3 after calcined 350 • C for 5 h) and 1 mmol of halonitro compound were added to 50 mL of water [35].In next step, a 10 mmol of NaBH 4 was added slowly to the reaction mixture under stirring at room temperature.The organic reactions were monitored on a thin layer chromatography (TLC) plate and the reaction mixture was quenched by extracting the organic derivatives with ethyl acetate.In the next step, the solvent was evaporated under vacuum and the corresponding product i.e., amine compound solute left out.Finally, the reduced amine products were confirmed by FT-IR and 1 H NMR spectroscopic analysis.The conversion and product yield were measured by the GC-MS.

Activity in reduction of p-nitrophenol using 2 wt.% AgNPs/α-Al 2 O 3 after calcined at 350 • C for 5 h
The evaluation of catalytic activities over 2 wt% AgNPs/α-Al 2 O 3 after calcined 350 • C for 5 h with different shapes, were studied in the reduction of nitrophenol [26,27].The model reaction kinetic experiments were conducted in UV quartz cuvette in the presence of aqueous NaBH 4 as follows: 3 mL of 0.01 mM aqueous solution of 4-nitrophenol, 0.1 mM (50 μL) of NaBH 4 and a 5 mg of 2 wt% AgNPs/α-Al 2 O 3 were introduced into the UV cuvette chamber and the reaction was monitored by Ocean optics spectrophotometer at 400 nm at different time intervals.The continuous H 2 generation (BH 4 ‾ kept the solution mixing throughout the reaction time [35].Thus, hydrogen transfer from NaBH 4 efficiently done during nitro reduction kinetics.

Characterisation techniques
The morphology and structure of the as-synthesized materials was analyzed by using Field-emission scanning electron microscopy (FE-SEM, Philips FEI Quanta 200F model) operated at 20-30 kV electron volts.The samples were fixed on a conductive carbon tape complied with an aluminum sample holder.The average particle diameter of the Ag samples was determined by counting at least 10 silver particles of each sample.The X-ray power diffraction (XRD) patterns were analysed and documented by D/Max-2500/PC diffractometer (Rigaku, Japan) using a CuKα (λ = 0.154 nm) radiation source operated at 40 kV and mA.The oxidation state of the synthesised catalysts was determined by using X-ray photoelectron spectroscopy (XPS) analysis and performed by a Thermofisher ESCALAB 250Xi spectrometer using an Al K α radiation source operated at an accelerating voltage of 15 kV.The core level binding energies (BE) were aligned with respect to the C1S binding energy of 284.6 eV from carbon contamination contacted during the handling in air.Transmission electron microscopy (TEM) was performed by using FEI Tecnai G2 Spirit microscope operated at 120 kV and a highresolution TEM (HR-TEM) images were recorded on a FEI Tecnai G2 F30S-Twin microscope instrument operated at 300 kV.The samples were prepared ultrasonically by dispersing them into ethanol, and then depositing the droplets of the suspensions onto a carbon-enhanced copper grid and finally, dried under air.The UV visible spectra were recorded for the reduction kinetics of p-nitrophenol was investigated by a spectrophotometer Ocean optics (HR4000) in the range of 200-800 nm wavelength.(Fig. 1).

Results and discussion
XRD patterns of silver nanoparticles (i.e., nanospherical, nanowires and nanocubes) which are prepared by the controlled-polyol synthesis method is shown in Fig. 2. For all three types of Ag nanoparticles, the diffraction peaks corresponding to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes are observed, and they can be indexed to a cubic structure JCPDS-04-0783 [24,25,36].In Fig. 2 (c, d), the sharp peaks at 38.23 (2θ) assigned to the high degree of crystallinity with a noticeably enhanced (1 1 1) reflection due to the Ag spherical nanoparticles.This is the common feature for the silver nanoparticles due to the strongly retarded growth in the (1 1 1) direction portraying the orientation in the predominant surface facet.Silver nanowires normalized peak intensity of the (1 1 1) facet is slightly reduced, as nanowires grow in the longitudinal direction (see Table 1) showing that these are terminated on all sides of the (1 0 0) planes.In contrast, in the case of silver nanocubes Fig. 2 b, the (1 1 1) facet reduces significantly and whereas, (1 0 0) facet strongly increases, which indicates the rich and enhanced facets of (1 0 0) surface in nanocubes (see Table 1).Furthermore, the Fig. 2 (f, g,  h) represents the powder XRD patterns of alumina supported silver nanocubes, silver nanowires and silver nanospheres shaped catalysts, respectively.There is an evident that the XRD patterns of alumina (Al 2 O 3 ) found to be in alpha (α) phase (JCPDS-46-1212) and there are no major changes in the silver nanoparticles phases and facets.Thus, the Ag nanoshapes are intact even after the incorporation into the α-Al 2 O 3 .
The Table 1 summarizes the relative intensities of different XRD reflections (hkl) obtained for silver nanoparticles i.e., ( Es) of AgNPs and the corresponding B.Es of supported α-Al 2 O 3 (AgNPs/ α-Al 2 O 3 ) catalysts was appeared at 367.941 and 373.92 eV (Fig. 3).The values are in good agreement with unbond metallic silver values [37,38].The metallic nature of silver nanoparticles intact after the application in catalytic reduction of nitroaromatics.This phenomena can be confirmed by the appearance of Ag 3d 5/2 and Ag 3d 3/2 core level binding energies (B.Es) at 368.15 and 374.15 eV for the AgNCs/α-Al 2 O catalyst after the reaction (Fig. S1) [37,38].The shift to higher binding energy values may be due to the reduction of exposed surface oxide layer of the silver nanoparticles by the NaBH 4 reducing agent, which was present in the reaction mixture during the catalytic reaction.To confirm the stability and the metallic nature of the AgNCs/α-Al 2 O 3 catalyst, further analysed by P-XRD after the catalytic reaction (Fig. S6).One can see from Fig. S6, the crystalline nature of the silver catalyst still intact and it does not undergo oxidation.The obtained binding energy values of the Ag, Al, and O atoms for all the catalysts were presented in the table S1.There is a shift in the Al 2p and O 1 s core levels binding energies after loaded silver nanoparticles on the α-Al 2 O 3 support without    S2 and Fig. S3) .[37,38].The supported AgNPs/α-Al 2 O 3 catalyst doesn't undergo any chemical and structural changes even after reaction at atomic level (Fig S1, Fig. S2 and Fig. S3).The exact Ag loading and the ratios were calculated by ICP-OES and the metal content values are found to be identical with that of XPS results.To confirm the metallic nature of the silver nanoparticles of both catalysts, we also analysed by using UV-vis absorbance spectroscopy.From Fig. S4, it should be noted that the synthesised AgNSPs, AgNCs and AgNWs exhibits strong surface plasmon resonance peak at 431, 430 and 432 nm respectively, it may be due to strong metallic nature of AgNPs formation.
The size homogeneity and the morphological properties of the prepared silver nano particles were confirmed by SEM analysis and are consistent with the XRD results.The morphology of all three shapes: nanospheres (Fig. 4 a), nanowires (Fig. 4 b,) and nanocubes (Fig. 4c) silver particles are clearly presented, and their corresponding particles size distributions are shown in Fig. 4d, 4e and 4f, respectively.The size distribution of the nanoparticles for all the three shapes are almost identical and the average particle size was found to be ~ 30 nm.We noticed that the silver nanoparticles have retained the similar size and shape even after incorporation into the alumina support Fig. S5.Thus, different shapes with similar size distributions are clearly presented in the SEM images [23,24].
Moreover, the TEM analysis also confirmed that the average size of the nanoparticles (NPs) is found to be ~ 30 nm.Thus, the TEM results revealed that the synthesis method reproduced and yielded all nanoparticles with similar morphologies and narrow size distribution with similar in average particle size.The HR-TEM and the selected area electron diffraction (SAED) patterns of the representative as-prepared silver nanospheres, nanowires and nanocubes are shown in Fig. 5.The HR-TEM image of the nanocubes (Fig. 5f) was attained by aligning the electron beam parallel to the (1 0 0) plane.The analysis results shown that the average distance between the Ag planes was 0.204 nm, which is consistent with the (1 0 0) surface termination [39][40][41][42].Note that the (2 0 0) peak is relatively stronger to that of (1 1 1), which dominates the JCPDS patterns mainly due to the nanocubes bonded by low indexed {1 0 0} facets, whereas the nanospheres and nanowires shapes were predominantly by the lower energy {1 1 1} facets.This indicates that majority of the nanocubes were aligned flat on the substrate with their (1 0 0) planes and being oriented upward.The Fig. 5e depicted a HR-TEM micrograph of the edge of a pentagonal Ag nanowire obtained with an electron beam aligned parallel to the (1 1 1) plane (arrow on Fig. 5e, inset).The distance between successive planes was measured to be 0.235 nm.The measured interplanar distance value agrees with reported HR-TEM analysis and DFT calculations, which indicated that the bulk lattice structure of the nanowires is unstrained face centered cubic (FCC) structure, and the nanowires terminated with the (1 0 0) plane [7].The measured selected area electron diffraction (SAED) patterns (Fig. 5e, inset) are also consistent with the proposed (1 0 0) surface terminations of the Ag nanowires and nanocubes.Further, the HR-TEM images (Fig. 5d) of the spherical nanoparticles provided us further insights into the nanosphere structure and crystallinity of the as-prepared silver nanospheres.The clear lattice fringes confirmed that the nanospheres are highly crystalline and localized over the surface.The lattice spacing of 0.235 nm corresponds to the (1 1 1) planes of Ag.Thus, the results confirmed that the dominant facets of the silver nanospheres are (1 1 1) planes.The SAED pattern was obtained by directing the electron beam perpendicular to one of the nanospheres.The hexagonal symmetrical diffraction patterns spots were displayed in the inset of Fig. 5 and confirmed the nanospheres formation with well-defined crystalline structure and its facet indexed to (1 1 1) planes.Both HR-TEM images and SAED patterns, confirm that each silver nanosphere visible in the image is a single nanocrystal.

Controlled growth of silver nanocubes, nanowires and nanospheres
This work focuses on the growth of silver nanocrystals with welldefined and shape-controlled for the applicaiton in nitro reduction model reaction.The Fig. 6 illustrates typical model images of growth of silver nanocubes shape formation.In silver nanocubes (AgNCs) synthesis, there are several parameters to optimize such as the precursor concentration, temperature, and the reducing power of polyols to achieve a brief spurt of nucleation growth during polyol synthesis.We employed different polyols during the synthesis procedure and fixed the precursor concentration and temperature.Consequently, the reducing power of polyols is likely to play an important role in the nucleation and nanocubes growth formation.It is well-known that polyol method is employed in growth of nanocrystals with different shapes and sizes by varying the type of polyol and its composition [19].It has been confirmed by earlier studies, that the reducing power of polyols follows a decreasing trend: EG > DEG > TEG > TTEG [19], since a polyol has  longer hydrocarbon chain, thus, its reducing power weaken with chain length.Therefore, it is easy to recognize the uniformed Ag seeds are obtained in high yields for both EG and DEG polyols [19].Typically, a faster and rapid nucleation of nanocrystals exhibited with stronger polyol reducing power.In addition, twinning is only favourable when the surface energy of the {1 0 0} facets are greater than that of the {1 1 1} facets.Moreover, the presence of PVP can act as to reduce the driving force for twin formation through its selective interaction with the {1 0 0} planes.Once the large proportion of single-crystal seeds formation took place, the selective adsorption of PVP over {1 0 0} facets can lead to preferential addition of silver atoms to the {1 1 1} facets.As the growth rate in the 〈1 1 1〉 direction is greater than the 〈1 0 0〉 direction, and the {1 0 0} sides of the cube will become enlarged at the expense of the {1 1 1} corners [41].After the cubic shape was formed, each face of the silver nanocubes will have the similar growth rate, and further growth will mainly increase the size with no significant morphological variation.On the contrary, in the case of TEG and TTEG due to their relatively weak reducing powers, the nucleation of Ag nanoparticles is slower and delayed over the period of time and finally obtained with broad size distributions.Furthermore, in TEG and TTEG, the slow nucleation led to the formation of seeds with twin defects to reduce the total surface free energy [19].Even though single-crystal Ag nanocubes could be attained in both EG and DEG under suitable conditions.One could only make ultra-fine and smaller Ag nanocubes with edge lengths ranging below 30 nm in DEG (see SI Fig. S7).It is noteworthy in measuring the ratio between (2 0 0) and (1 1 1) planes of XRD diffraction peaks (Fig. 2) intensities of AgNCs samples.For AgNCs sample, the measured ratio value is 1.68 which is much higher than the conventional values (1.68 vs 0.5), suggested that our synthesized AgNC were found to be in abundant with (1 0 0) facets, and it indicates that the (1 0 0) planes likely to be preferentially oriented.Similarly, the measured ratio between (2 2 0) and (1 1 1) values appeared to be in higher than the conventional value (0.56 vs 0.36), indicates that the synthesized AgNC contains relatively higher (1 1 0) facets on the AgNC surface.The Fig. 5i is a typical electron diffraction pattern recorded by directing the electron beam perpendicular to one of the square faces of an individual cube.The exactly square symmetry of this pattern confirms that each silver nanocube was a single crystal bonded primarily to {1 0 0} facets.Researchers have used several techniques to control the size and shape of silver nanowires.However, there are only very few reports available on shape and size-controlled selective Ag nanoparticles synthesis.Typically, the formation of different shapes from Ag precursor occurs mainly due to etching of silver metal.In order to overcome this issue, several etchants such as Fe and Cu complexes were introduced by different research groups [43,44].However, this complexity may create the secondary effects on the reactant substrate.To circumvent these problems, we planned to prepare shape-controlled and precise size selective Ag nanoshapes by optimizing the temperature, precursors concentration and molecular weights of PVP were implemented during synthesis steps.The primary stages of the synthesis steps and reactions can be recognized by their distinctive colours.During the synthesis of AgNWs, after the injection of AgNO 3 and PVP solutions, it turns to dark yellow coloured solution.Further, dark yellow colour denotes the presence of AgNPs seeds in the reaction mixture (20-40 nm) (see SI TEM image Fig. S8).After 30 min, the solution turns into dark grey coloured sample and after 60 min, the dark grey colour gradually disappears and faded, and the solution appears ochre coloured sample, implying the formation of AgNWs [40,42].The SEM and TEM images depicted the formation of silver nanowires with different PVP molecular weights (i.e., 24 000, 40 000, and 58 000 respectively) as shown in Fig S8(a) -(d).From Fig S8, it is evident that the nanowire like morphology is not seen in the sample prepared with molecular weight 24 000 PVP, whereas only few particles are seen with wire like morphology in the sample prepared with 40 000 molecular weight PVP, but still the percentage of wire morphology is relatively low.The Ag sample prepared with PVP 58 000 molecular weight was found to be the most optimum for the formation of AgNWs and further, precise and its uniform size that can be seen from fig.S8(d).The effect of co-catalyst (i.e., Fe complex) in nanowire formation was also studied using preferred 58 000 molecular weight PVP.The percentage of nanowires like morphology was found to be lower than the sample prepared without the catalyst (Fig. S8(c)).
The Fig. 7a represents the evolution of a nanowire shaped nanorods from a decahedral silver particle with the assistance of PVP by the polyol method.The nanorod ends are terminated by {1 1 1} facets, and the side surfaces are bonded by {1 0 0} facets.The light indico colored facet indicates the preferential adsorption of PVP on the {1 0 0} facets, and the sandal-colored part stands for the weaker interactions with the {1 1 1} facets.The black lines at the end surfaces denotes the twin boundaries that can act as active sites for the addition of silver atoms.The plane marked in light maroon denotes one of the five twin planes that can act as the internal confinement for the evolution of nanorods from MTP.The Fig. 7(b) represents the diffusion of silver atoms toward the two ends of a nanorod, with the side surfaces completely passivated by PVP.These illustrated diagrams show the projection in perpendicular to one of the five side facets of a nanorod, and the arrows represent the diffusion fluxes of silver atoms.Furthermore, the HR-SEM image of silver nanowires obtained by the polyol method is shown in Fig. 7 (c).From Fig. 7c, it can be clearly visible the cross-sections of the nanowires displayed in pentagonal shape.
Nanoparticles with quasi-spherical shapes have conventionally been obtained when the molar ratio.between PVP and AgNO 3 was relatively high Fig.8(a-c).In this case, any surface specific adsorption is overwhelmed and the entire surface of the initially formed seeds is covered with a thick coating of PVP (as indicated in the Fig. 8 c).As the resistance surface addition by the silver atoms is now nearly isotropic, growth of the seeds proceeds in an isotropic manner as well.In Fig. 8c shows the TEM image of typical nanospheres evolution, and these particles are essentially spherical in shape.One of the activity evaluations in a catalyzed chemical reaction was the correlation with the active phase morphology [45][46][47] and facetdependency [48][49][50].Herein, we present the synthesis of shapecontrolled and highly selective silver nanoparticles with different morphologies at low indexed facets over the alumina support, and its improved catalytic activity in the reduction of p-nitrophenol (p-NP) into p-aminophenol (p-AP) was demonstrated.Environmentally, nitroarenes are highly toxic, thus, there is a need of efficient methods such as catalytic reduction to transform the nitroarenes to amino arenes is essential.The reduction of nitrophenol to aminophenol has been carried out extensively over noble metal-based catalysts [31][32][33].This work is the first of its kind, where morphologically tailored silver nanoparticles in shape-controlled environments using modified polyol synthesis method and achieved enhanced activity in reducing nitro compounds to corresponding amines.Comparison analysis of the time-dependent absorption spectra for the catalytic reduction of 4-NP to 4-AP using NaBH 4 as reducing agent was also performed.
First, we investigated the reaction of p-NP (4-NP) in a solution.A pale yellow-colored 4-NP solution exhibited an absorption peak at ~317 nm in an aqueous medium (Fig. 9).However, the solution turns to bright yellow in the presence of NaBH 4 reducing agent and presenting a strong, red-shifted absorption peak at ~400 nm due to the formation of an extended conjugation of nitrophenolate anions in alkaline media.While the reduction mechanism initiated by hydrogen transfer and proceeds under the presence of catalyst, this is evident by the disappearance of absorption band at 400 nm.Further, a concomitant characteristic peak was observed at ~300 nm, which is assigned to the colorless 4-AP reactant product.Moreover, two isosbestic points (points at which wavelength remains constant) were observed at ~311 nm and ~280 nm, respectively, which confirms the complete conversion and The mechanisms proposed based on the analysis as follows for the reductive amination of 4-NP reaction pathways occurs in two steps, first, adsorption of the p-NP, proceeds the formation of intermediates in transition state via hydrogen transfer over Ag sites from reducing agent and second step, the desorption of reaction product p-AP (Fig. 10).In this research hypothesis, the factors essential for elucidating the efficiency of a catalyst are the morphology of the nanocatalysts, active surface area for catalysis, facets involved, and the existing surface charge.A fine tuning of the morphology and facet, of a nanoparticle consequently increases the active site of the catalyst in the reaction medium and hence it has direct effects on the catalytic activity.The comparative study of the experiential reduction rate of the spherical, wires and cubes silver nanoparticles supported alumina are given in Table 2.
There are some critical studies reported in the literature on Ag catalysts with different shapes and sizes and their respective performance  The activity of AgNPs/Al 2 O 3 towards the chemoselective reduction of aromatic nitro groups in the presence of other reducing functionalities were investigated.Few attempts have been made towards the selective reduction of nitroarenes in the recent past, using copper NPs [73], palladium NPs nanocatalysts [74] and the Pt nanoparticles act as a membrane catalyst [75].However, all the aforementioned studies on selective aromatic nitro reduction methods were associated with few drawbacks such as low yield, high temperature (above ~ 120 • C) and high pressures and thus, making the reduction process highly tedious where E a is the activation energy, R is the universal gas constant, and A is the pre-exponential factor.From the apparent rate constants at different temperatures.We determined the activation energy of this reduction reaction (Fig. 11).A linear relationship between ln k app and the reciprocal of temperature was calculated for the catalysts AgNC/  4. Notably, for the E a measured values for AgNW/ α-Al 2 O 3 and AgNSP/α-Al 2 O 3 are slightly higher than that of AgNC/ α-Al 2 O 3 catalyst.Typically, the activation energy of this reaction is the function of nanoparticles size, however, in this case the shape and facet dependent are profoundly visible for AgNPs.Thus, it is clearly indicates that the morphology of the AgNPs plays an important role in determining the reaction performance [76,77].Moreover, the activation parameters such as the enthalpy of activation (ΔH # ) and entropy of activation (ΔS # ) are calculated by using the Eyring equation (eq (2)): where k B is the Boltzmann constant (1.38x10 − 23 J K − 1 ), h is the Plank's constant (6.626 × 10 − 34 J s).
By plotting ln(k app /T) versus (1/T), the values of entropy and enthalpy of activation are evaluated from the intercept and the slope by considering the k app value of at least four different temperatures starting from 288 K as shown in Fig. 11.
The Table 4 implies the thermodynamic constraints for activation i. e., energy barrier E a , enthalpy ΔH # and entropy ΔS # activation at elevated temperatures.The ΔH # for AgNC is the least positive magnitude, whereas highest positive magnitude for AgNW and AgNSP.Further, the ΔS # is the largest negative value for the AgNC and the least negative value for AgNW and AgNSP.A comparison of the apparent rate constants for the AgNC, AgNW and AgNSP nanocatalysts was depicted in Fig. 11 and it is followed the k app values order as: AgNC ≫ AgNW > AgNSP.This thermodynamic parameters values can be considered as a direct proof and substantiate the presence of more and preferred active sites over AgNC.The outcome of rate constant dependency and correlation with temperature was clearly presented that the AgNC is a more efficient catalyst than other counterparts.Therefore, it is concluded that the Ag nanoparticles with cubic morphology exhibit better catalytic properties and performance toward p-nitrophenol reduction reaction then the nanowires and nanospheres.

Conclusions
Exploring the Ag morphology by tailoring the shape-controlled synthesis was successfully investigated.The evolution and the growth of Ag nanoshapes in confined environments are substantiated by XRD and microscopic analysis.The effect of different Ag shapes i.e., nanowires, nanospheres and nanocubes was studied in the chemoselective reduction of nitro compounds.Shape and facet dependent activity were determined and the FCC Ag nanocubes with low indexed facets {0 0 1} exhibited highest activity in reductive amination.The optimal precursor PVP molecular weight, temperature and solvent are critical parameters in forming the shape selective synthesis.The Ag nanocubes with low indexed facet catalyst exhibited the highest rate constant values compared to other counterparts.The higher reduction rate of the nanocube catalysts can be attributed to its higher concentration of the Ag(1 0 0) surface facets and higher adsorption sites compared to the nanospheres and nanowires.This work is the novel approach in controlled synthesis of uniform nanostructures with well-defined surface facets might provide an important platform in designing the highly active catalysts.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 1 .
Fig. 1.Schematic illustration of shape selective synthesis procedure of silver nanoparticles using different reagents and conditions.

Fig. 6 .
Fig. 6.Schematic representation of nanocubes growth evolution.The nucleation of seeds and the growth of seeds into nanocubes (a) model diagram of single nanocrystal with cuboctahedron shape (b) model figure of perfect cubic structure and (c) obtained TEM images of synthesized silver nanocubes.The surfaces marked in light and dark red represents the {1 1 1} and {1 0 0} facets, respectively.

Fig. 7 .
Fig. 7. (a) Illustration of the evolution of nanowires from decahedral silver nanoparticles, (b) Indicates the diffusion of silver atoms toward the two ends of a nanorod, with the side surfaces completely passivated by PVP and (c) HR-SEM image of silver nanowires obtained by the polyol method.

Fig. 8 .Fig. 9 .
Fig. 8. Schematic illustration of the seeds nucleation and the growth of seeds into nanospheres (a) model figure of quasi-spherical multiple twined particles (QS-MTP) (b) model figure of perfect silver nanospherical structure and (c) obtained TEM images of silver nanospherical particles.

Fig. 10 .
Fig. 10.Illustration of plausible mechanism of reduction of p-nitrophenol (p-NP) to p-aminophenol (p-AP) over shape-controlled silver nanoparticles dispersed on Al 2 O 3 support.
a Silver nanosphere, b silver nanowires and c silver nanocubes respectively.d Standard card of silver XRD reflection intensities (JCPDS-04-0783).b a Fig. 3. Representation of fitted XPS spectra of (a) AgNW/α-Al 2 O 3 and (b) AgNCs/α-Al 2 O 3 nanocatalysts after calcination at 350 • C for 5 h.changing the oxidation state, it is due to the strong electrostatic interaction between AgNPs and α-Al 2 O 3 support (see Fig.

Table 2
Comparison of reaction rate constants over various Ag nanoparticles with different shapes and sizes published in the literature.Al 2 O 3 , b Silver nanowires supported α-Al 2 O 3 .cSilvernanocubessupported α-Al 2 O 3 .dAllsilvernanocatalystswere synthesized using PVP polymer support.eSize of the silver nanoparticles measured by TEM analysis.fApparentrate constants obtained under optimized conditions.NaBH 4 was used as the reducing agent in all these cases.nitroreductionwas summarised (Table2).There is a huge variation in the reported rate constant values over Ag-based nanocatalysts.The size and shape had tremendous effects on the reduction rate constant.In this work, we gained promising results beyond the state of art on Ag nanocatalysis with shape-controlled facets performance in nitro reduction model reaction.The stability of the current AgNCs presented a moderately high in the studied reaction cycles and this outcome is somewhat like that of reported values.
a Reactions were catalyzed by Silver nanosphere supported α-in

Table 3
[76,77]ic reduction of different nitro aromatics over AgNCs/α-Al 2 O 3 , AgNWs/α-Al 2 O 3 and AgNSPs/α-Al 2 O 3 systems.unsustainable.Hence, in this work, we studied the engineered nanocatalysts for the reduction different nitro substituted aromatics under mild conditions.The efficacy of the prepared catalyst AgNWs/ α-Al 2 O 3 , AgNCs/ α-Al 2 O 3 and AgNSPs/α-Al 2 O 3 in reducing the various nitro compounds are presented in Table3.The versatile character of Ag shaped-controlled nanocatalysts exhibited a highly selective reduction of halo-substituted nitrobenzenes without any dehalogenation (Table3, entries 6-9).All the tested mononitroarene derivatives have resulted in complete conversion to corresponding amine compounds, whereas the dinitroarene derivative has resulted in only 55-70 % conversion (Table3, entry 8).The remaining 30-45 % probably have resulted in the formation of monoamine derivatives, which can be further reduced to diamine via consecutive reduction step.Al 2 O 3 , AgNW/ α-Al 2 O 3 and AgNSP/α-Al 2 O 3 catalysts, the reduction of p-nitrophenol was carried out at different temperatures as shown in Fig.S11 (a-c)[76,77].The apparent rate constants (k app ) at different temperatures were calculated from the linear portion of ln(A) versus the reaction time plot at each temperature.The values of apparent rate constants at different temperatures for AgNC/α-Al 2 O 3 , AgNW/α-Al 2 O 3 and AgNSP/α-Al 2 O 3 nanocatalysts are presented in Fig.S11d.It is depicted that for all the catalysts, the reaction rate constant gradually increases linearly with temperature at (288 K-323 K) due to the faster molecular motion and greater probability of collisions for bond scissions.The rate constant remains unchanged with further temperature increasing, thus, indicating the compensation of increased collision by lower adsorption coefficients at higher temperature.
a Reaction conditions: 50 mg of catalyst, 1 mmol of substrate, 10 mmol of NaBH 4 , 50 mL of water, stirring at room temperature.bReused catalyst after separation from the reaction mixture.cGC was used to measure the conversion yield.R. Rajendiran et al.and

Table 4
The activation energy, enthalpy, and entropy values of different Ag shapes.