WO2015011680A1

WO2015011680A1 - A method of synthesizing intermetallic compounds and applications thereof

Info

Publication number: WO2015011680A1
Application number: PCT/IB2014/063418
Authority: WO
Inventors: Sebastian Chirambatte PETER; Pradeep Prasannamurthy SHANBOGH; Udumula SUBBARAO
Original assignee: Jawaharlal Nehru Centre For Advanced Scientific Research
Priority date: 2013-07-25
Filing date: 2014-07-25
Publication date: 2015-01-29
Also published as: IN2013CH03317A

Abstract

The present disclosure relates to a method of synthesizing intermetallic nanoparticle with a crystal structure of NiAs type, wherein said method is a modified polyol method. The disclosure particularly discloses NiSb, CoSb, NiPb PdSb and other intermetallic compounds having a similar structure type, synthesized by the above said method and also describes the catalytic activity of the said intermetallic nanoparticle. The modified polyol method of the instant disclosure is rapid and cost effective when compared to the conventional methods employed to synthesize the intermetallic nanoparticles.

Description

"A METHOD OF SYNTHESIZING INTERMETALLIC COMPOUNDS AND

APPLICATIONS THEREOF"

TECHNICAL FIELD

The present disclosure relates to a method of synthesizing intermetallic nanoparticle compounds having a crystal structure of NiAs type, wherein said method is a modified polyol method. The disclosure particularly discloses NiSb, CoSb, NiPb PdSb and other intermetallic nanoparticle compounds having a similar structure type, synthesized by the above said method and also describes the catalytic activity of the said intermetallic nanoparticle.

BACKGROUND AND PRIOR ART OF THE DISCLOSURE

Catalyst is a substance that increases the rate of a chemical reaction without, itself, being consumed in the reaction. Catalyzed reactions proceed through a mechanism that is not apparent in the stoichiometry of the reaction. For example, catalysts may be used in the rate determining step of a reaction and later re-formed, so there is no net change in the concentration of the catalyst during the reaction. Specifically, catalysts lower the activation energy associated with the rate determining step and accelerate the chemical reaction. Many types of materials may function as catalysts for different types of reactions.

Catalysts may be used in numerous applications. For example, catalytic reduction process, where catalyst reduces the activation energy of the intermediate structures and increases the rate of the reaction. Morphology, stoichiometry and a selected chemical structure will play a crucial role in tuning the activity and selectivity of the reaction. Though there are numerous catalysts used for different chemical reactions for the improved activity, selectivity and conversion, the ultimate goal is to find an appropriate catalyst for the selected reactions.

Typical heterogeneous catalysts currently being used are transition metals, bimetallic, alloys or intermetallic compounds. An enhanced activity is often observed for these materials when they are in the low dimensional nature due to the more surface area, which enhances catalytic activity of these compounds, which is further increased with aid of proper support. Generally, noble metals based catalysts Pd, Pt, Rh and Au were extensively studied for the catalytic reduction process. Though, mono metallic and multimetallic systems have shown excellent activity and selectivity in the selected processes, the production cost is enormously high for long run in the future. Thus, there is a need to design suitable low cost and equally efficient or even better than the best materials reported for the above mentioned process in order to have an efficient chemical reaction incurring lesser cost and better productivity. Designing a low cost catalyst material can be done by casting the catalyst particles on oxide supports or by converting the metal particles into an alloy or ordered intermetallic form. Alloys and solid solutions do not have an ordered atomic structure; as they have metal atoms which are arranged randomly. On the other hand, ordered intermetallic compounds generally have a more stable atomic arrangement in comparison to alloys and solid solutions. This results in an enhanced lifetime of the catalyst under reaction conditions. In alloys and solid solutions, atoms are prone to migration with an associated reduction of catalytic performance. It is worth to mention that in majority cases alloys and solid solution were reported for the catalysis compared to the well-ordered intermetallics due to the difficulty in the synthesis of the later by existing synthetic strategies. Further, convential process involved in designing of intermetallic compounds in the prior art carry many disadvantages such as, process involving longer reaction time and high temperature, leading to single phase NiSb nanoparticles making the process tedious and time consuming process leading to a process which is not cost effective.

In view of the above observed disadvantages, it becomes a necessity to arrive at a process which is cost effective and circumvents all the limitations of the conventional process. The Applicant in the specification herein below discloses a method of synthesizing an intermetallic compound/nanoparticle which has a significant value and is technically advanced.

STATEMENT OF THE DISCLOSURE

Accordingly, the present disclosure relates to a method for synthesizing an intermetallic compound having a transition metal and p-block element, said method comprising acts of: a) co- reducing the transition metal precursor and the p-block element precursor in presence of polyol and reducing agent to obtain a mixture, and b) subjecting the mixture to temperature variations to synthesize said intermetallic compound; an intermetallic compound having a transition metal and p-block element, wherein the intermetallic compound is deficient in the p-block element; and a method of catalyzing a reaction for conversion of a reactant to a product, said method comprising act of subjecting the reaction to an intermetallic compound having a transition metal and p-block element obtained by the method as mentioned above.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

Figure la: depicts the crystal structure of NiAs along the [001] direction, Ni atoms are occupied in the octahedron cage of Sb atoms.

Figure lb: depicts the single layer of NiAs6 octahedron along the [100] direction fitted to show the Ni occupying in the pockets of octahedron cage.

Figure lc: depicts the NiAs structure along the [100] direction.

Figure 2: depicts the comparison of p-XRD pattern of CoSb and NiSb catalyst nanoparticles synthesized by modified polyol method.

Figure 3: depicts the TEM characterization of NiSb (a) and CoSb (b).

Figure 4a: depicts the typical UV-Vis plot for the catalytic reduction of p-NP to p-AP by NiSb nanoparticles.

Figure 4b: depicts the typical UV-Vis plot for 1st reusability for the catalytic reduction of p-NP to p-AP by NiSb nanoparticles

Figure 5a: depicts the plot of InA at 400 nm Vs reduction time for the reduction of p-NP to p- AP by NiSb nanoparticles.

Figure 5b: depicts the plot of InA at 400 nm Vs reduction time for the 1st reusability for the reduction of p-NP to p-AP by NiSb nanoparticles.

Figure 5c: depicts the plot of InA at 400 nm Vs reduction time for the reduction of p-NP to p-AP by CoSb nanoparticles.

Figure 5d: depicts the plot of InA at 400 nm Vs reduction time for the ist reusability for the reduction of p-NP to p-AP by CoSb nanoparticles. Figure 6: Schematic representation of the mechanism for the reduction of p-nitrophenol in aqueous phase in the presence of NiSb and CoSb catalysts.

Figure 7: illustrates the p-XRD comparison of main peak corresponding to 101 plane of synthesized CoSb and NiSb by co-reduction modified polyol method with reported bulk material.

Figure 8: illustrates the ED AX spectrum of NiSb-p nanoparticle Figure 9: illustrates the ED AX spectrum of CoSb-p nanoparticle.

Figure 10: illustrates the powder X-ray diffraction pattern of NiSb synthesized by co-reduction modified polyol method. The solid line through the experimental points is the Rietveld refinement pattern.

Figure 11: illustrates the XRD pattern of NiSb phase synthesized by modified polyol method at different temperatures; (a) 110 °C, (b) 150°C, (c) 180°C and (d) simulated pattern of NiSb.

Figure 12: illustrates the p-XRD characterization of NiSb nanoparticles prepared by modified polyol method at temperature value of about 100°C, 150°C and 180°C without capping agent and SDS as capping agent at 180°C.

Figure 13: illustrates TEM characterization of NiSb nanoparticles prepared by modified polyol method at temperature values of about 100°C, 150°C and 180°C without capping agent and SDS as capping agent at 180°C.

Figure 14a: illustrates the absorbance plot of degradation of p-nitro phenol by the NiSb-SDS nanoparticles.

Figure 14b: illustrates the InA vs Reduction time plot of the catalytic degradation of p-nitro phenol at 400 nm by NiSb-SDS nanoparticles.

Figure 15a illustrates the p-XRD characterization of NiSb nanoparticles prepared by

solvothermal method at 240°C without capping agent and reaction duration is 72 hours. Figure 15b: illustrates the TEM characterization NiSb nanoparticles.

Figure 16: illustrates the absorbance plot of degradation of p-nitrophenol by NiSb- 100, NiSb- 150 and NiSb- 180 nanoparticles and the reusability plot of the said nanoparticles.

Figure 17: illustrates the InA vs Reduction time plot of the catalytic degradation of p- nitrophenol at 400 nm by NiSb-100, NiSb-150 and NiSb-180 nanoparticles and the reusability plot for the said nanoparticles.

Figure 18: illustrates the p-XRD characterization of NiSb and CoSb isolated post 72 hours incubation of the nanoparticles in the colloidal solution.

Figure 19 (a): illustrates the absorbance plot of degradation of p-nitro phenol by the NiSb synthesised by solvothermal method.

Figure 19(b): illustrates the InA vs Reduction time plot of the catalytic degradation of p-nitro phenol at 400 nm by NiSb synthesised by solvothermal method.

Figure 20 (a): illustrates the PXRD comparison of ball milled and arc melted NiSb compared with simulated pattern.

Figure 20 (b): illustrates the Gaussian fitting of the selected peak at 2Θ =~32.

Figure 21 (a): illustrates the TEM image of NiSb indicates nano particles are mono disperse.

Figure 21(b): illustrates the Electron diffraction pattern indicates compound is in polycrystalline nature.

Figure 22 (a): illustrates the UV- Visible absorption spectra of P-nitro phenol to P-amino phenol conversion using as NiSb nano particles obtained from the ball milling method.

Figure 22 (a): illustrates the Natural logarithmic plots of absorbance at 400 nm Vs reduction time for the catalytic reaction with NiSb nanoparticles obtained from the ball milling method.

DETAILED DESCRIPTION OF THE DISCLOSURE The present disclosure relates to a method for synthesizing an intermetallic compound having a transition metal and p-block element, said method comprising acts of:

a) co-reducing the transition metal precursor and the p-block element precursor in presence of high boiling polyol and reducing agent to obtain a mixture; and b) subjecting the mixture to temperature variations to synthesize said intermetallic compound.

In an embodiment of the present disclosure, the polyol is diethylene glycol, triethylene glycol, polyethylene glycol or a combination thereof.

In another embodiment of the present disclosure, the reducing agent is sodium borohydride, Lithium Tetraethyl borohydride or a combination thereof

In still another embodiment of the present disclosure, the co-reduction is carried out under gaseous atmosphere; and wherein the gas is argon, nitrogen or a combination thereof.

In yet another embodiment of the present disclosure, the temperature variation of step (b) comprises heating the mixture to a temperature of about 180°C to 220°C, maintaining the said temperature for a time period of about lhr to about 2hrs and cooling the mixture to a temperature ranging from about 15°C to about 30°C.

In yet another embodiment of the present disclosure, the synthesized intermetallic compound is a nanoparticle, having particle size ranging from about 8nm to about 15nm, preferably in the range of about 8nm to about 12nm

In yet another embodiment of the present disclosure, after the temperature variation in step (b), the synthesized intermetallic compound is isolated by centrifuging the mixture at 15000rpm with water and organic solvent, followed by drying under vacuum at a temperature of about 50°C to about 80°C for a time period ranging from about 3 hours to about 6 hours.

In yet another embodiment of the present disclosure, the organic solvent is an alcohol, selected from a group comprising methanol, ethanol, 2-propanol or any combination thereof. In yet another embodiment of the present disclosure, the synthesized intermetallic compound has a conventional NiAs type crystal structure.

The present disclosure further relates to an intermetallic compound having a transition metal and p-block element, wherein the intermetallic compound is deficient in the p-block element.

In an embodiment of the present disclosure, the compound is a nanoparticle, having particle size ranging from about 8nm to about 15nm, preferably in the range of about 8nm to about 12nm; and wherein the compound has a conventional NiAs type crystal structure.

The present disclosure further relates to a method of catalyzing a reaction for conversion of a reactant to a product, said method comprising act of subjecting the reaction to an intermetallic compound having a transition metal and p-block element obtained by the method as mentioned above.

The present disclosure relates to method of synthesizing intermetallic nanoparticle with a crystal structure similar to NiAs type. The said method comprises the step of combining transition metal and p-block element along with a solvent comprising polyethylene glycol and reducing agent.

The present disclosure thus pertains to a rapid method of preparing low dimensional (nanometer range) and size controlled intermetallics within the NiAs structure type. The present disclosure also pertains to the use of these compounds as low cost catalysts for the hydrogenation reaction with a better activity compared to the best reported compounds, especially precious metals such as gold, platinum etc., and their corresponding alloys.

In an embodiment, the synthesis of metal-containing compounds in polyethyleneglycol (PEG) is called polyol method. The ethylene glycol acts as the solvent and as a mild reducing agent during the synthesis process. In the present disclosure, the polyol method has been modified by employing sodium borohydride as an external reducing agent and therefore the instant method of synthesizing intermetallic nanoparticle is referred to herein as 'modified polyol method'.

In an embodiment, the nanoparticle compounds of the instant disclosure contains catalytically active metal atom, surrounded by the catalytically inactive or less active element. This unique nature of crystal structure results in the isolation of catalytically active centers which promotes the selective interaction of substrate molecules to undergo reduction, and enhances the selectivity of the reaction in various processes. In the present disclosure, modified polyol method for the synthesis of equiatomic compounds (listed in Table 1) is developed which provides for thermodynamically stable compounds, and therefore preparation of these compounds by the method of the present disclosure provides several advantages over the conventionally followed solvothermal method.

The processes involved in designing of intermetallic compounds in the prior art carry many disadvantages and thus there is a need to arrive at an improved methodology to arrive at such intermetallic compounds. Some of the disadvantages of the prior art and the corresponding advantages of the instant disclosure are:

a. Long reaction time and high temperature are required to produce ordered intermetallic nanoparticles by solvothermal process; a typical example is the single phase NiSb nanoparticle was previously obtained by carrying out the reaction at 240°C for 48-72 hours. On the contrary, the present disclosure takes about 2 hours to complete, at much lesser temperature, ranging from about 100 to about 220°C.

b. The existing methods generally yield poly dispersed nanoparticles and multiphase products. On the other hand, simple and rapid co-reduction method with well controlled particle size is employed to produce the single phase of the compounds through the methodology of the instant disclosure. The synthesis technique of the instant disclosure is also used for the large scale production of single phase nanoparticles.

c. In the heterogeneous hydrogenation process, the best catalysts reported are expensive noble metal systems (Pt, Pd, Au and Rh), which result in the overall increases of the production cost for the synthesis of commercial products. On the other hand, the starting material used in the present disclosure is inexpensive and naturally abundant and thereby making the instant method cost-effective

d. Generally, the catalysts in elemental form and alloys require a very good support to uniformly distribute them to increase the surface area and improve their catalytic activity. In some cases, support also plays a crucial role in the catalytic activity and selectivity of the reaction, which may lead to undesired products and yield. Further, large scale (gram level) preparation of single phase and size controlled catalyst nanoparticles is a challenging task by the existing methods. Most of the intermetallic nanoparticles are not stable in water.

On the contrary, the methodology of the instant disclosure provides for the compounds to be highly stable at ambient conditions and storable for long duration even as colloidal solution. No capping agent/surfactants are required to stabilize the nanoparticles and materials show excellent stability towards the aerial oxidation. It is a common observation that surface of a material oxidizes in the presence of air when stored for a longer duration. However, as mentioned above the nanoparticles of the present disclosure do not undergo aerial oxidation even in the bulk compounds. Further, one of the unique features of the nanoparticles synthesized by the method of the present invention is that the nanoparticles are not susceptible to aerial oxidation and surface oxidation without presence of any additional components in the nanoparticles such as capping agents or surfactants. However, the nanoparticles synthesized by the polyol method are not susceptible to aerial oxidation and surface oxidation only in the presence of capping agents or surfactants.

The materials also do not need any support for the selected organic reactions. Active sites are isolated in the cage made up of mostly non-active materials. The compounds from the method of the instant disclosure further show improved catalytic activity over the existing expensive catalysts. The observed rate constants for the chemical reactions by catalysts such NiSb and CoSb obtained by the instant method are better than the noble metal catalysts. Further, the catalysts provided herein are easily isolated and reusable. The activity of the catalysts is retained even after several catalytic cycles.

In an embodiment of the present disclosure, the transition metal precursor employed in the method of the instant disclosure is halide, acetyacetonate, nitrate or any combination thereof.

In an embodiment of the present disclosure, the transition metal precursor employed in the method of the instant disclosure is metal halide, metal nitrates, metal acetylacetonate salts or any combination thereof. In an embodiment of the present disclosure, the transition metal precursor employed in the method of the instant disclosure is selected from a group comprising nickel chloride (NiCl₂), antimony trichloride (SbCl₃), cobalt chloride (CoCl₂) or any combination thereof.

In an embodiment of the present disclosure, the p-block element precursor is a halide, preferably chloride.

The present disclosure thus focuses on synthesizing the ordered compounds with low cost transition metals to replace the expensive noble metal based catalysts. Although these compounds were synthesized by conventional solvothermal method, with a particle size distribution of 30-70 nm, as mentioned, the existing methods needs long reaction time (24-72 hours) and high temperature (220°C -240°C) to arrive at the compounds. In the instant modified polyol method, the compounds NiSb and CoSb in pure phase have been successfully synthesized at relatively low temperature (about 100°C to about 220°C) and within shorter reaction time (2 hours).

In an embodiment of the present disclosure, the particle size of the intermetallic nanoparticle ranges from about 8nm to about 15nm, preferably in the range of about 8nm to about 12nm, wherein the desired particle size of the intermetallic nanoparticle is achieved by optimizing the process parameters such as temperature of the reaction and reaction time in the present disclosure.

In another embodiment, particle size of the nanoparticle synthesized at a temperature of about 100°C is about 8nm. Similarly particles size of the nanoparticle synthesized at temperatures of about 150°C and 180°C is about lOnm and l lnm, respectively.

In an embodiment of the present disclosure, the intermetallic nanoparticle is highly stable towards the aerial oxidation.

In an embodiment, the intermetallic nanoparticle of the instant disclosure has high catalytic activity, as the nanoparticle is highly stable without addition of any surfactant or capping agent, as addition of surfactant or capping agent covers the catalytic active site of the nanoparticle, thereby reducing catalytic activity. The nanoparticles synthesized by the instant disclosure have a -3 -1 -3 -1

rate constant ranging from about 3.1x10^" S^" to about 3.6x10^" S^" . Further, the nanoparticles of the instant disclosure employed in a catalytic reaction can be reused, wherein the rate constant of such a nanoparticle is in the range of about 1.93x10 -^"3 ^J S--1¹ to about 3.2XKT -3 S-^"1¹.

In a preferred embodiment, the rate constant of NiSb-100, NiSb-150 and NiSb-180 is about

3.1x10 -^"3 S-^"1 , 3.3x10 -^"3 S-^"1 and 3.6x10 -^"3 S-^"1 , respectively. In another preferred embodiment, the rate constant of the recycled NiSb-100, NiSb-150 and NiSb-180 is about 3.2xl0^"3 S^"1, 3.04xl0^"3 S^"1 and 1.93x10 -^"3 S -^"1 respectively.

In an embodiment of the present disclosure, the method is a modified polyol method, which involves a co-reduction step, which is achieved by the addition of external reduction agent, wherein the reducing agent is selected from a group comprising sodium borohydride, lithium tetraethyl borohydride or a combination thereof. Addition of reducing agent in the modified polyol method aids in co-precipitation of the metal precursors and faster reduction of said precursors, which in turn yields to the formation a nanoparticles of desired size, ranging from about 8nm to about 15nm. On the other hand, polyol method performed without addition of reducing agent, showed no precipitation even after about 24hours.

In another embodiment, the modified polyol method is superior over the existing polyol method, because polyol method does not yield pure compounds at the end of the process, whereas modified polyol method yields pure intermetallic nanoparticle which is catalytically active.

In an embodiment of the present disclosure, method for synthesis of intermetallic nanoparticle comprises acts of:

a) contacting transition metal precursor and p-block element precursor;

b) dissolving the components of step a) in polyethylene glycol, followed by reducing the components; and

c) heating solution of step b) to obtain intermetallic nanoparticle, followed by subjecting the nanoparticle to centrifugation and drying.

In a preferred embodiment, method for synthesis of intermetallic nanoparticle comprises acts of- a) contacting transition metal precursor and p-block element precursor; b) dissolving components of step a) in polyethylene glycol, followed by addition of reducing agent under a gaseous environment;

c) heating the solution of step b) to a temperature ranging from about 180° C to about 220°C at the rate of about 5° C/min and holding the temperature for a time period ranging from about lhr to about 2hrs to obtain the intermetallic nanoparticle;

d) centrifuging the nanoparticle twice with water and thrice with alcohol; and

e) drying the intermetallic nanoparticle at a temperature ranging from about 50°C to about 80°C for a time period ranging from about 3hrs to about 6hrs.

In an embodiment of the present disclosure, the gaseous environment is provided by gases argon, nitrogen or a combination thereof.

In an embodiment of the present disclosure, the alcohol is methanol, ethanol, 2-propanol or any combination thereof.

In an embodiment, the method of synthesis of the instant disclosure is extrapolated to large scale production of single phase nanoparticles. In a preferred embodiment, large scale production of nanoparticle can be carried out by increasing the volume of the polyethylene glycol alongside proportionally increasing the volume of the other components during production. In an alternate embodiment, polyethylene glycol concentration can be greater than 20ml for large scale production of nanoparticle

In an embodiment, the intermetallic nanoparticle has an isolated active site in the cage made up of mostly non-active materials.

In an embodiment, the chemical reaction driven by the intermetallic nanoparticle obtained by the method of the instant disclosure has higher rate constant under a controlled condition.

In an embodiment, the intermetallic nanoparticle obtained by the method of the instant disclosure is easily isolated and is reusable.

In an embodiment, the intermetallic nanoparticle obtained by the method of the instant disclosure is extrapolated in the preparation of efficient anode materials for fuel cell application, particularly formic acid oxidation. Also to methanol oxidation and oxidation-reduction reactions which are carried out electro catalytically.

In an embodiment of the present disclosure, intermetallic nanoparticles NiSb and CoSb are known to crystallize in the NiAs type crystal structure. The crystal structure is described as hep arrangement of antimony atoms with the nickel or cobalt atoms filling the octahedral voids created by antimony atom resulting in the site isolation of catalytically active transition metal atoms. A detailed representation of the crystal structure with Ni in the pockets of Sb cages is shown in Figure 1. This ordered arrangement of atoms in the crystal structure distributes the active metal sites uniformly in the compound, which enhances the catalytic properties for selective chemical reactions.

In an alternate embodiment, intermetallic nanoparticles including but not limiting to NiPb and PdSb are synthesized by the modified polyol method of the instant disclosure, wherein the rate constant of NiPb and PdSb is 2.4x10 -^"3 S-^"1 and 3.2x10 -^"3 S-^"1 , respectively for a reduction reaction.

In a non-limiting embodiment, one of the unique aspects of the modified polyol method of instant disclosure is that the entire process of synthesizing intermetallic nanoparticle is carried out at normal pressure. On the other hand, intermetallic nanoparticle synthesized by the conventional solvothermal method employs very high pressure. Therefore, the instant method for synthesizing intermetallic nanoparticle is cost effective when compared to the conventional processes.

In an embodiment, the compounds mentioned in the below table are synthesized by the process of the instant disclosure and all the compounds are intermetallic nanoparticle with NiAs crystal structure type.

6 MnBi 15 NiTe 24 PtSb

7 MnSb 16 NiBi 25 PtSn

8 MnTe 17 CuSn 26 AuSe

9 FeSb 18 RhTe 27 AuSn

Table 1 : The compounds reported in the NiAs crystal structure type.

In an alternate embodiment, the intermetallic nanoparticle is synthesized by planetary ball miller method, wherein the synthesized nanoparticle has a particle size of about lOnm. The ball miller method of synthesizing nanoparticle comprises the act of loading the metals in the jar of planetary ball miller along with balls with a size of about 10mm diameter, followed by milling at lOOOrpm for about 30minutes, figure 20a illustrates the PXRD comparison of ball milled and arc melted NiSb compared with simulated pattern.

In an embodiment, the intermetallic nanoparticle obtained by the method of the instant disclosure is used as a catalyst in the preparation of analgesic and antipyretic drugs such as phenacetin, acetanilide and p-acetamol. It is also used as a dye and as a photographic developing agent. Further, the intermetallic nanoparticle is also used as a scavenging agent in the degradation of nitro compounds present in water bodies.

As described herein the description, NiSB-100 or CoSb-100 refers to NiSb or CoSb synthesized at a temperature of about 100°C. Similarly NiSB-150, CoSb-150, NiSb- 180 and CoSb-180 refer to NiSB or CoSb synthesized at a temperature of about 150°C and 180°C, respectively.

The present disclosure is further elaborated by way of the following examples and accompanying figures herein. However, these examples should not be construed to limit the scope of the present disclosure.

In the examples, the activity of the intermetallic nanoparticle synthesized by the method of the present disclosure is demonstrated by evaluating the catalytic activity of the nanoparticle towards reduction of nitro group in p-nitrophenol (p-NP) to produce p-aminophenol (p-AP). The mentioned reduction process is chosen as a representation of several reduction processes or other catalytic reactions to which the process of the present disclosure shall be applicable. Therefore, the activity of the intermetallic nanoparticle must not be construed to be limited to only this reduction reaction. Here, a person skilled in the art upon learning catalytic activity of the nanoparticle w.r.t general reduction process, provided in this disclosure will be able to employ the nanoparticles of the present invention to other reaction types without any undue burden.

EXAMPLES:

Example 1: Preparation of intermetallic nanoparticle

For the synthesis of equiatomic TX compounds (T = transition metals; X = p-block elements) within the NiAs structure type, about 1 mmole transition metal precursor (halides, acetyacetonates and nitrates) and about 1 mmole of p-block element precursor (chlorides) are used in a single batch reaction. Metal precursors are dissolved in about 20 ml of tetraethylene glycol taken in a three neck round bottomed flask connected to schlenk line apparatus. Argon gas was purged to maintain air free condition. A freshly prepared sodium borohydride solution (stoichiometric amount of NaBH₄ dissolved in about 5 ml of TEG) is injected in to the flask at room temperature under magnetic stirring. The color of the solution turns black indicating the formation of nanoparticles. Solution is then heated in an oil bath to about 180°C out at the rate of about 5°C/min and the solution is held at this temperature for about two hours. After the reaction is completed, the samples are cooled down slowly to the room temperature. The black colored colloidal particles start to settle down at the bottom of the round bottom flask, which are isolated by centrifugation, thereupon the particles are washed twice with water and thrice with ethanol. Finally, particles are dried in an oven under vacuum at a temperature of about 50° C for about 3 to 6 hours. Nano powders are stored in glass vials in ambient conditions. Using this synthesis procedure, the CoSb and NiSb nanoparticles are synthesized.

The above procedure can be scaled up to gram level synthesis for NiSb and CoSb in one step, by increasing the volume of the TEG. Use of excess of TEG (about >20ml) for 1 mmole metal precursor solutions has no effect on the phase purity and particle size, aforesaid is asserted by analyzing the XRD patterns, which shows more or less similar full width at half maximum suggesting no change in the particle size of the nanoparticle synthesized with excess TEG. The below table describes the metal precursor and reaction condition employed in the synthesis of NiSb and CoSb

Table 2: Precursor used and reaction conditions employed in the synthesis of NiSb and CoSb nanoparticles.

Synthesized nanoparticles are characterized through powder X-ray diffraction (XRD) which shows broad peaks, which corresponds to the small crystallite size of the material (Figure 2). Further characterizations are carried out using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Absorption spectroscopy (EDAX), Transmission Electron Microscopy (TEM) and Electron Diffraction spectroscopy (EDS). Particle size is controlled between 10-12 nm (Figure 4) based upon the reaction time and temperature. Figure 3 shows TEM characterization of NiSb (a) and CoSb (b).

Example 2: Powder XRD profile illustrating the deficiency of Antimony in intermetallic nanoparticle

Powder XRD profile of antimonide nanoparticles synthesized by the co-reduction modified polyol method at similar conditions is compared with the reported bulk compound as illustrated in figure 7. There is a substantial shift in the main peak corresponding to the 101 reflection towards higher two theta in the case of NiSb, whereas negligible shift is observed in the case of CoSb. The lattice parameters reported for CoSb are a = b = 3.89 A c = 5.17 A and for NiSb are a = b = 3.94 A c = 5.14 A. Since the covalent radii of Co (126 pm) is less than Ni (128 pm), the lattice parameters for NiSb is higher than CoSb. The peak 101 in the experimental data of NiSb has shifted towards the higher two theta value, and thereby can be interpreted to be due to the deficiency at the antimony site, but no shift is observed in case of CoSb. This is further confirmed by the energy dispersive x-ray spectroscopy (EDS). The composition obtained from the EDX measurements (illustrated in figures 8 and 9) are Ni_1-14Sbo_.86 and Coo.99Sbi_.oi respectively, which is in line with pXRD data.

Reitveld profile analysis is carried out in the fullprof suite to refine the X-ray diffraction data of the NiSb nanoparticles (illustrated in figure 10). The refined lattice parameters obtained are a = b = 4.024(2) A and c = 5.261(3) found to be in good agreement with the reported lattice constants of Ni rich composition Nii_+xSbi__x, which confirms that the synthesized nanoparticle is Sb deficient. Further it is deciphered that the Standard reduction potential for Co²⁺ is -0.28 and for Ni²⁺ it is -0.25 (in Volts). So Co will reduce Sb faster than Ni, which resulted in the reduction of whole Sb precursor in the CoSb synthesis, but not in the case of the NiSb synthesis, which resulted in the deficiency of Sb. Example 3: Temperature studies for the synthesis of intermetallic nanoparticle.

The intermetallic nanoparticle is synthesised by the process as described in Example 1 , but with varying temperature in order to analyze the optimum temperature to obtain an intermetallic with lesser nanoparticle. NiSb is synthesized at a temperature of 110°C, 150°C, 180°C, 220°C, respectively. The crystallite size of the nanoparticle is calculated using Sherrer formula. The 101 peak is used for the calculation and it is observed that the crystallize size has increased for the material synthesized at higher temperature (220°C) (as illustrated in Table 3). The intermetallic nanoparticles synthesized at low temperature is amorphous in nature with poor crystal quality (as illustrated in figure 11), whereas the optimal temperature identified for the synthesis of the intermetallic nanoparticle with lesser particle size is 180°C.

Example 4: Catalytic activity of intermetallic nanoparticle- Heterogeneous hydrogenation of 4-nitrophenol to 4-amniophenol

Catalytic activity of the synthesized ordered intermetallic nanoparticles are tested for the reduction of nitro group in p-nitrophenol (p-NP) to produce p-aminophenol (p-AP) by sodium borohydride in aqueous phase and the reaction progress is monitored through UV-Vis spectrophotometer (described in figure 4).

In a catalytic reaction, about 1 mL (1.0x10^ M) aqueous solution of p-NP and about 1 mL (100 mg/L) aqueous colloidal suspension comprising nanoparticles of the intermetallic catalyst obtained by the process of the present disclosure are mixed together in a 1 cm quartz cuvette. Freshly prepared about 1 mL (6x 10 M) aqueous NaBH₄ solution is added to the reaction mixture and time-dependent absorption spectra are recorded in the UV-Vis spectrophotometer at 25° C.

The reduction of aromatic nitro group under mild condition is thermodynamically favorable only with the aid of proper catalysts. Para-nitro phenol in aqueous phase exists as nitro phenolate ion with absorbance maximum ( _max) at 317 nm under neutral or acidic condition. Addition of NaBH₄ enhances the basic condition of the solution and in alkaline medium phenolate ions are predominating with absorbance maximum shifts to 400 nm which can be observed by change in the color intensity of the solution. Upon addition of the catalyst nanoparticles, the color intensity of the solution gradually decreased which is recorded through UV spectrophotometer. A new peak is observed at 300 nm as a shoulder to the main peak which corresponds to p-AP. Several isosbestic points in the spectra of the reacting mixtures reveal that p-AP is the predominant species without any side products. Since the peak at 400 nm is much stronger than the peak at 300 nm, the concentrations of p-NP ions were measured, and the kinetics of the reaction is monitored by recording the absorbance at 400 nm.

Figure 4 shows the typical time dependent UV absorption plot of reduction of nitro group with NiSb and CoSb catalyst loading of 100 mg/L, respectively. Figure 5 shows the natural logarithmic plots of absorbance at 400 nm versus reduction time for the catalytic reaction with NiSb and CoSb nanoparticles respectively, wherein the plots yield a good linear correlation that indicates the reaction follows a pseudo first order kinetics. This is expressed by the following equation:

[At] = [Ao]. e ^~kt 2

Where, A_t, absorbance at time t represents corresponding concentration of the reactant; Ao, initial concentration of the reactant and k, pseudo-first order rate constant. The observed rate constant of the reaction is compared with reported alloy and metallic nanoparticles under the same reaction conditions (Table 3). Both NiSb and CoSb show a substantial improvement in the catalytic activity of p-NP to p-AP reaction. Use of NaBH₄ (6 x 10^" M) generates small bubbles of gas, which evolves out of the system, helps in the mixing of the catalyst particles uniformly throughout the reaction mixture, facilitating majority of catalyst particles available for the reaction. Increase in pH due to large amount of NaBH₄ slows down the dissociation of BH ⁴ facilitating slow evolution of H2 gas, which in turn protects the products from areal oxidation. The table below showcases the comparison of rate constant of CoSb and NiSb in the reduction of p-NP to p-AP with the reported or existing compounds.

Table 3: Rate constant of CoSb and NiSb in the reduction of p-NP to p-AP compared with the reported compounds or materials.

Recyclability of the intermetallic nanoparticle.

After the complete conversion of p-NP to p-AP in the Example 2 above, about 1 ml of p-NP solution is introduced to the cuvette with the parameters mentioned in example 2 and the recyclability of NiSb and CoSb is evaluated.

Conclusion: Similar reduction time is observed from the recyclability reaction when compared with the initial run, as seen in Table 3 above.

Example 5: Comparative data illustrating the catalytic activity of capped intermetallic nanoparticle vs. non-capped intermetallic nanoparticle.

For the purpose of illustration, sodium dodecyl sulfate (SDS) and polyvinyl pyrolliodone (PVP) capped NiSb are prepared by the modified polyol method described in example 1. At a temperature of about 180°C, pure phase of NiSb nanoparticles with SDS capping are obtained, similarly unknown phase of NiSb with PVP capping are obtained. Upon synthesis of capped NiSB, catalytic activity of such capped nanoparticles is evaluated in comparison to the NiSB nanoparticles devoid of capping agent. The catalytic activity is evaluated by analyzing the rate constant of the said nanoparticles while reducing p-nitro phenol to p-amino phenol as described in example 4 above.

Result and discussion:

The rate constant for the reduction of p-nitro phenol by NiSb-SDS is about 1.4x10 -^"3 S -^" 1. On the other hand, the rate constant for the reduction of p-nitro phenol by NiSb without capping agent is in the range of about 3.1xl(F -3 S-^"1¹ to about 3.6χ1(Γ -3 S -^"1¹. Fi gure 14 illustrates the reduction of p- nitro phenol by NiSb-SDS, and the said reduction of p-nitro phenol is measured at 400nm. Further, the reduction in the rate constant of capped nanoparticle (NiSB-SDS and NiSb-PVP) is attributed to blockage of active sites by the capping agent and formation of monodispersed particles and thereby resulting in the increase agglomeration of the formed nanoparticles, which yielded lesser reaction rate when compared to the NiSb nanoparticles without capping agent. Figures 12 and 13 illustrates p-XRD and TEM characterization, respectively of NiSb nanoparticles synthesized by modified polyol method at 100°C, 150°C, 180°C without capping agent and NiSb nanoparticle with SDS as capping agent at 180°C. The TEM characterized image of the NiSb-SDS illustrated in figure 13 clearly indicates the agglomeration.

Example 6: Demonstrating the superiority of the modified polyol method of the instant disclosure over the conventional solvothermal method for the synthesis of intermetallic nanoparticle.

About lmmol of NiCl₂ and about lmmol of SbCl₃ are dissolved in 40ml (80% volume) of tetraethylene glycol under constant stirring and transferred to a Teflon lined stainless steel autoclave. 5mmol of NaBH₄ is added to the autoclave. The autoclave is immediately sealed upon completion of reduction reaction and placed in an oven and heated to a temperature of about 240°C for about 72hrs. Post completion of 72hrs, particles are isolated and washed several times with water and ethanol, individually.

The particle size of NiSb synthesized by solvothermal method is about 50nm. On the other hand, as described in the embodiments of the instant disclosure, the particle size of NiSb synthesized with the modified polyol method of the present disclosure yields a particle size in the range of about 8nm to about 12nm. Further, the solvothermal method needs longer period for the synthesis of nanoparticle and leads to limited formation of smaller monodispersive nanoparticles. On the other hand, modified polyol method synthesis the nanoparticle in short period of time, in this method the morphology and size of the nanoparticle can be controlled unlike in the solvothermal method.

Figure 15 illustrates the p-XRD and TEM characterization, respectively of NiSb nanoparticles prepared by solvothermal method. Further, figure 19 illustrates the catalytic activity of the nanoparticle synthesized by solvothermal method and it is observed that the rate constant of the nanoparticle for reduction of p-nitro phenol is about 0.77x10^" . The overall reduction time observed for the catalytic reduction of p-nitro phenol by 50nm NiSb is about 40minute, whereas the reduction time observed with 1 lnm NiSb synthesized by the method of the present disclosure is about 8minute.

The aforesaid clearly indicates that the particle size of the intermetallic nanoparticle has a significant effect on the catalytic activity of the nanoparticle. Thus the nanoparticles synthesized by the modified polyol method of the present disclosure are efficient and cost-effective when compared to the nanoparticles synthesized by the conventional solvothermal method.

Example 7: Demonstration of catalytic activity of the NiSb nanoparticle synthesized at temperatures lOO C, 150 C, 180°C and 200°C.

NiSb nanoparticle is synthesized as per the procedure of the example 1, however at varied temperature of about 100°C, about 150°C, about 180°C and about 200°C. For the easy of understanding NiSb synthesized at aforementioned temperatures are referred herein as NiSb- 100, NiSb- 150, NiSb- 180, and NiSb-200.

The catalytic activity of the nanoparticles is evaluated for the reduction of p-nitro phenol. It is observed that irrespective of the different synthesizing temperature of the nanoparticles, all the nanoparticles showcased similar activities. However, any temperature value less than 100°C or greater than 200°C tends to increase the particle size of the nanoparticle and thereby increasing the agglomeration of the formed nanoparticle and in turn decreases the catalytic activity of such nanoparticle.

Table 4 below illustrates the rate constant of NiSb nanoparticles synthesized at various temperatures during reduction of p-nitro phenol to p-amino phenol, wherein the rate constant is

-3 -3

in the range of about 3.1 x 10^" to about 3.6 x 10^" . Table 4 also demonstrates reusable property of the NiSb for the reduction of p-nitro phenol by illustrating the rate constant of reused nanoparticle, wherein the rate constant of such nanoparticles for the reduction reaction are in the

-3 -3

range of about 1.93 x 10^" to about 3.2 x 10^" . Further, figures 16 and 17 illustrate that above mentioned temperature varition during synthesis of nanoparticles does not have any impact on the catalytic activity in the form of absorbance patterns for the degradation of p-nitro phenol by NiSb- 100, NiSb- 150, NiSb- 180. The figures also illustrate the rate constant of the re -used nanoparticles and it is observed that the catalytic activity of such re-used nanoparticles is similar to the freshly synthesized nanoparticles.

Structure analysis through X-ray powder refinement and EDAX data shows the presence of Sb deficiency in all NiSb nanomaterials synthesized by the instant modified polyol method. NiSb- 100 and NiSb-150 is established as Ni₁.23Sbo.-7-7 and Ni₁.42Sbo.58 respectively. Further, the transmission electron microscopy analysis of the nanoparticles illustrates that the particle sizes of NiSb- 100, NiSb- 150 and NiSb- 180 is about 8nm, about lOnm, about 1 lnm, respectively.

Composition Catalyst loading Rate constant (s ¹) Rate constant (s ¹)

(mg/niL) (Reusability Data)

NiSb- 100 0.1 3.1 x 10^"3 3.2 x 10^"3

NiSb- 150 0.1 3.3 x 10^"3 3.04 x 10^"3

NiSb- 180 0.1 3.6 x 10^"3 1.93 x 10^"3

Table 4

Based on above observations it can be concluded that the particle size of the NiSb nanoparticles is organized within 8 to 12 nm by tuning the temperature between 100°C to 200°C. Also, Sb deficiency of the NiSb nanoparticles is an important factor for the enhanced catalytic activity of the said nanoparticles, wherein Sb deficiency is in favor for the exposure of more active Ni nuclei and thereby increasing the catalytic activity. Example 8: Demonstrating the stability of the synthesized nanoparticle by the instant modified polyol method.

50mg of the nanoparticles (NiSb and CoSb) synthesized by the present disclosure are dispersed in 10ml of distilled water and sonicated for about 30minutes and followed by incubating the sonicated solution for about 72hours at about room temperature. Post incubation, the particles are isolated by centrifuging and dried to yield powders. The dried powder is analyzed by powder X- ray diffraction.

Figure 18 illustrates the p-XRD characterization patterns for NiSb and CoSb isolated from 72hours colloidal solution, it is observed that the nanoparticles synthesized by the instant modified polyol method are stable even after sonication and incubation for about 72hrs in a solution.

Example 9: Demonstrating the cost effectiveness of the intermetallic compound synthesized by the instant modified polyol method.

Firstly, the modified polyol method of the instant disclosure employs very low pressure in the reaction when compared to the conventional solvothermal method, and also the reaction time for synthesis of intermetallic compound by modified polyol method is comparatively less, when compared to the solvothermal method. Owing to this advancement in the modified polyol method of the present disclosure towards synthesis of intermetallic compound, the cost of synthesizing is reduced drastically when compared to the solvothermal method and other conventional process for synthesis of said intermetallic compounds.

Further, the intermetallic nanoparticle compound of the present invention is cost-effective when compared to the noble metal compounds. The below table provides for a comparison of the cost of noble metal compounds and the intermetallic compounds (NiSb and CoSb) of the present disclosure.

Pt nanopowder (<50 nm) 17,974 (100 mg)

Platinum powder 31,916 (lg)

NiSb (of present disclosure) ~340 (lg)

CoSb (of present disclosure) ~340 (lg)

Table 5

From the above table 5, it can be understood that the cost of intermetallic compound (NiSb and CoSb) of the present disclosure is approximately Rs 340/g. On the other hand, the cost of the noble metal compound (e.g.- 0.5 wt.% Pd NPs on alumina) is 5 times greater than the cost of intermetallic compound of the present disclosure. Even though the intermetallic compound of the present disclosure is cost effective, the catalytic activities of the cost-effective intermetallic compounds (NiSb and CoSb) are better than the noble metal compounds (see table 3).

Claims

We Claim:

1. A method for synthesizing an intermetallic compound having a transition metal and p- block element, said method comprising acts of:

a) co-reducing the transition metal precursor and the p-block element precursor in presence of polyol and reducing agent to obtain a mixture; and

b) subjecting the mixture to temperature variations to synthesize said intermetallic compound.

2. The method as claimed in claim 1, wherein the polyol is diethylene glycol, triethylene glycol, polyethylene glycol or a combination thereof; and wherein the reducing agent is sodium borohydride, Lithium Tetraethyl borohydride or a combination thereof.

3. The method as claimed in claim 1, wherein the co-reduction is carried out under gaseous atmosphere; and wherein the gas is argon, nitrogen or a combination thereof.

4. The method as claimed in claim 1, wherein the temperature variation of step (b) comprises heating the mixture to a temperature of about 180°C to 220°C, maintaining the said temperature for a time period of about lhr to about 2hrs and cooling the mixture to a temperature ranging from about 15 °C to about 30 °C; and wherein after the temperature variation in step (b), the synthesized intermetallic compound is isolated by centrifuging the mixture with water and organic solvent, followed by drying under vacuum at a temperature of about 50 °C to 80°C for a time period ranging from about 3 hours to about 6 hours.

5. The method as claimed in claim 1, wherein the synthesized intermetallic compound is a nanoparticle, having particle size ranging from about 8nm to about 15nm.

6. The method as claimed in claim 4, wherein the organic solvent is an alcohol, selected from a group comprising methanol, ethanol, 2-propanol or any combination thereof.

7. The method as in claim 1, wherein the synthesized intermetallic compound has a conventional NiAs type crystal structure.

8. An intermetallic compound having a transition metal and p-block element, wherein the intermetallic compound is deficient in the catalytically inactive p-block element.

The intermetallic compound as claimed in claim 8, wherein the compound is a nanoparticle, having particle size ranging from about 8nm to about 15nm; and wherein the compound has a conventional NiAs type crystal structure.

A method of catalyzing a reaction for conversion of a reactant to a product, said method comprising act of subjecting the reaction to an intermetallic compound having a transition metal and p-block element obtained by the method as claimed in claim 1.