FIELD OF THE INVENTION

The present disclosure relates to catalytic technologies for use in a carbon dioxide (CO2) hydrogenation reaction. Specifically, the present disclosure relates to a supported multimetallic catalyst system for tuning and enhancing selectivity in hydrogenation of CO2, and a method of synthesizing the same. The present disclosure further relates to a method of hydrogenation of carbon dioxide into CO and Cl compounds using said supported multimetallic catalyst system.

BACKGROUND OF THE INVENTION

Carbon-neutral technologies are intensely pursued by the research community to negate the effects of anthropogenic CO2 emission and its catastrophic impact on the environment. A shift from fossil fuel sources to green fuels is a work in progress and might have several hurdles due to a shift in existing energy infrastructure and geopolitical issues. The inherent stability of the CO2 molecule means any process requires high energy to activate the molecule or the development of smart catalysts for bringing down the energy barriers. The thermodynamic barrier for C-C coupling from CO2 is higher, making high energy density C2+ products more challenging. The catalytic conversion of CO2 to C 1 products (i.e., CO, CH4, CH3OH, Formic acid, etc.) is one of the potential routes to utilize CO2 to value-added products. Among the molecules, “Sabatier Reaction” producing methane and reverse water gas shift reaction (RWGS) producing CO have been widely explored. RWGS reaction producing CO is widely appreciated from two fronts: (1) highest catalytic technology readiness level (TRL) and (2) diverse platform molecules that can be formed from CO as feedstock molecules.

Ni supported over SiO2, AI2O3, and other active supports is well known for CO2 methanation reaction but suffers deactivation at higher temperatures due to aggregation of nickel particles. On the other side, Noble-metal based monometallic catalysts are also reported for the same reaction having high catalytic activity, selectivity, stability, and excellent resistance to coke deposition, but are not cost- effective from an industrial point of view.

Bimetallic nanomaterials have been extensively used by the heterogeneous research community to improve the stability and activity of catalytic reactions. In a pioneering work, Besenbacher et al. showed using model single crystal surfaces that when Au is alloyed to the first layer of Ni, the barrier for the CH4 dissociation increase by more than twice. So, over the pure Ni (111) CH4 dissociation rate is higher whereas when Au is in the vicinity the same hep Ni (111) facet has a lower CH4 dissociation rate and lowers C adsorption or graphite formation. The reactivity of Au modified Ni surfaces was investigated by our group which showed synergistic effects apart from imparting stability for the catalysts. The coking behaviour was studied over Ni and NiCo systems via experiments and DFT study. They calculated the amount of coke deposition over different Ni and NiCo catalyst facets and found that Ni facets are easily accessible for carbon deposition. Co as a promoter increases carbon migration and makes Ni sites available for the main reaction.

As discussed above, conventional Ni-based catalysts are extensively used for hydrogenation of molecules like CO and CO2 but faces stability issue when reaction are done for a longer period and at a higher temperature.

Therefore, there is a clear need for a catalyst system for enhancing selectivity in CO2 hydrogenation reaction via RWGS reaction and methanation reaction at lower temperatures.

OBJECTS OF THE INVENTION

An objective of the present invention is to provide a supported multimetallic catalytic system for carbon dioxide (CO2) hydrogenation.

Another objective of the present invention is to provide a method for synthesizing a supported multimetallic catalytic system for tuning and enhancing the selectivity of CO2 hydrogenation.

Another objective of the present invention is to provide a supported bimetallic catalyst system for enhancing the selectivity, stability and conversion of CO2 hydrogenation into methane as C 1 product via methanation reaction at ambient pressure.

Another objective of the present invention is to provide a supported trimetallic catalyst system for enhancing and tuning the selectivity of CO2 hydrogenation from methanation into CO via RWGS reaction at lower temperatures and ambient pressure.

Another objective of the present invention is to provide a supported trimetallic catalytic system for tuning and enhancing the selectivity of CO2 hydrogenation from methane to CO at lower temperature and ambient pressure, and methanol at higher pressure.

Another objective of the present invention is to provide a method for synthesizing a supported trimetallic catalytic system for enhancing the selectivity, stability and conversion in CO2 hydrogenation reaction to CO and methanol.

Yet another objective of the present invention is to provide a method for synthesizing a supported bimetallic catalytic system for enhancing the selectivity, stability and conversion in CO2 hydrogenation reaction to methane. Still another objective of the invention is to provide a method for producing CH4 and CO at ambient pressure and Cl products such as methanol by tuning and enhancing the selectivity of CO2 hydrogenation using said supported multimetallic catalytic system at high-pressure reaction conditions.

SUMMARY OF THE INVENTION

An aspect of the present disclosure relates to a supported multimetallic catalyst system for carbon dioxide (CO2) hydrogenation comprising: a support material which is selected from a group consisting of an inorganic oxide, a mixed metal oxide, a metal sulfide, a metal oxide, a chalcogenide, an oxide of spinel, an oxide of wuestite structure (FeO), an oxide of olivine clay, an oxide of perovskite, a zeolite, carbon black, graphitic carbon, and a carbon nitride; and a multimetallic mixture which is selected from a group consisting of copper, aluminum, magnesium, manganese, zinc, chromium, lead, cadmium, cobalt, nickel, gold, silver, platinum, tin, palladium, indium, iron, tungsten, molybdenum, ruthenium, and bismuth or combination thereof, salts or alloys thereof, wherein said multimetallic mixture is supported on the support material.

Another aspect of the present disclosure relates to a method for synthesizing a supported multimetallic catalyst system comprising the steps of: a) providing a dispersion comprising a support material in deionized water, followed by sonication and then stirring to obtain a dispersed support; b) providing individual solutions of two or more metal precursors dissolved in water to obtain a metal precursor solution, respectively; c) adding a first metal precursor solution to the dispersed support from step a) dropwise, followed by stirring to obtain a pre-final solution; d) adding a solution of PVP in water to the pre-final solution from step c) dropwise followed by stirring to prevent aggregation and to control the particle size of metal nanoparticles and to obtain a PVP-treated pre-final solution; e) adding remaining metal precursor solution dropwise to the PVP-treated pre-final solution from step d) dropwise followed by stirring to obtain a final solution having cationic form of metals; f) adding an ice cold solution of sodium borohydride to the final solution from step e) dropwise followed by stirring for reducing the cationic form of metals to obtain a non-ionic metals final solution; g) aging the non-ionic metals final solution, followed by centrifugation to obtain a supported multimetallic catalyst pellets; h) washing the supported multimetallic catalyst pellets with a solvent, followed by drying to obtain a dried supported multimetallic catalyst pellets; and i) calcining the dried supported multimetallic catalyst pellets to obtain a supported multimetallic catalyst system. Still further aspect of the present disclosure relates to a method of hydrogenation of CO2 in the presence of a supported multimetallic catalyst system comprises the steps of : i) adding a supported multimetallic catalyst system in a reactor; ii) pre-treating the supported multimetallic catalyst system under H2 flow to reduce the catalyst prior to reaction; and iii) adding and contacting CO2 stream and H2 stream through the reactor under conditions to form CH4, CO or Cl products or methanol, wherein the ratio of CO2 stream and H2 stream are in the range of 1: 1 to 1:4 for a total flow rate in the range of 6 mL/min to 18 mL/min.

ABBREVIATIONS USED

CP: Ceria nanopolyhedra, NCP: Nickel-Ceria nanopolyhedra, NACP: Nickel-Gold-Ceria nanopolyhedra, NAICP: Nickel-Gold-Indium-Ceria nanopolyhedra.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 relates to the XRD patterns for CP and different compositions of catalysts.

FIG. 2 relates to the graph of N2 adsorption-desorption isotherm for CP fresh 2.0 NCP and 1.5 NACP catalysts.

FIG. 3 relates to HR-TEM images of fresh catalyst 2.0 NCP (a), 1.5 NACP (b) and 1.5 NAICP (c).

FIG. 4 relates to the graphs of temperature-dependent catalytic performance for CO2 hydrogenation reaction of catalyst 2.0NCP, 1.5ACP and 1.5NACP a) Graph for CO2 conversion, b) Graph for CO and CH4 selectivity.

FIG. 5 relates to the graph of comparison of CO2 conversion by time-dependent study of catalyst 2.0 NCP and 1.5 NACP at 360 °C and atmospheric pressure.

FIG. 6 relates to the graph of CO2 hydrogenation activity and CO and CH4 selectivity of catalyst 1.5 NAICP with respect to temperature.

FIG. 7 relates to the graph of comparison of CO2 hydrogenation activity and CO and CH4 selectivity of catalyst 0.5 NAICP, 1.5 NAICP and 2.0 NAICP at 400 °C.

FIG. 8 relates to the graph of long-term catalytic test for CO2 hydrogenation reaction for catalysts 1.5 ICP, 1.5 NICP and 1.5 NAICP at 360 °C and atmospheric pressure.

FIG. 9 relates to the Graph for catalyst 1.5 NAICP showing CO2 conversion at different CO2:H2 ratio in the temperature range of 200-400 °C.

FIG. 10 (A-C) relates to the ED AX mapping images of the catalyst showing uniform distribution of metal(s) over ceria support (10A) 2.0NCP. (10B)1.5NACP and (10C) 1.5NAICP. FIG. 11 relates to the graph of CO and CH4 selectivity of catalysts 1.5 NACP and 2.0 NCP with respect to time at 360 °C and atmospheric pressure.

FIG. 12 relates to the graph of catalyst xNACP synthesized with varying Au wt% (x= 0.25, 0.5, 1.5 and 2.0) for optimal loading and their function in CO2 conversion.

FIG. 13 (A-B) relates to the graphs of catalytic activity of xNACP synthesized with varying Au wt% (x= 0.25, 0.5, 1.5 and 2.0).

FIG. 14 (A-B) relates to CO2 hydrogenation activity and selectivity data with respect to temperature at atmospheric pressure for the catalyst (14A) 0.5 NAICP, (14B) 2.0 NAICP.

FIG. 15 (A-B) relates to CO2 hydrogenation activity and selectivity data with respect to temperature at atmospheric pressure for the catalyst (15A) 1.5 ICP, (15B) 1.5 NICP.

FIG. 16 relates to CO2 hydrogenation activity and selectivity reaction data at temperature and atmospheric pressure conditions for the catalyst 2.0 NAICP.

FIG. 17 relates to the graph of CO selectivity of 1.5NAICP (trimetallic catalyst) with varying CO2:H2 ratio in the temperature range of 200 - 400°C.

FIG. 18 (A) relates to CO2 hydrogenation to methanol activity using 1.5NAICP trimetallic catalyst and (B) relates to the stability study of 1.5NAICP trimetallic catalyst.

DETAILED DESCRIPTION OF THE INVENTION

The following is a detailed description of embodiments of the disclosure. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.

Unless the context requires otherwise, throughout the specification which follow, the expression “nano sized particles” and variations thereof, such as, “nano particles” and “nano shaped particles” relate mainly to the “nanoparticles”.

The present disclosure relates to a supported multimetallic catalyst system, for tuning and enhancing selectivity in CO2 hydrogenation reaction from methane to CO in RWGS reaction at ambient pressure and to methanol at high pressure. The invention further relates to a method for synthesizing of said supported multimetallic catalyst system. Furthermore, the present invention provides a method of preparation of CO and Cl compounds by CO2 hydrogenation using said multimetallic catalyst system at lower temperatures, respectively. In some embodiment, the supported multimetallic catalyst system includes the multimetallic catalysts and a support material comprising a surface, wherein the multimetallic catalysts are deposited on the surface of the support system.

An aspect of the present disclosure provides a supported multimetallic catalyst system for carbon dioxide (CO2) hydrogenation comprising: a support material which is selected from a group consisting of an inorganic oxide, a mixed metal oxide, a metal sulfide, a metal oxide, a chalcogenide, an oxide of spinel, an oxide of wuestite structure (FeO), an oxide of olivine clay, an oxide of perovskite, a zeolite, carbon black, graphitic carbon, and a carbon nitride; and a multimetallic mixture which is selected from a group consisting of copper, aluminum, magnesium, manganese, zinc, chromium, lead, cadmium, cobalt, nickel, gold, silver, platinum, tin, palladium, indium, iron, tungsten, molybdenum, ruthenium, and bismuth or combination thereof, salts or alloys thereof, wherein the said multimetallic mixture are supported on the support material.

In an embodiment, the metals are Nickel, gold, and indium. In further preferred embodiment, M 1 is Nickel, M2 is Gold and M3 is Indium.

In an embodiment, the support material is a metal oxide. The metal oxide is selected from a group consisting of cerium oxide (CeCh), zirconium dioxide (ZrCh), zinc oxide (ZnO), aluminium trioxide (AI2O3), titanium dioxide (TiCh), magnesium aluminate (MgAhC ), silicon dioxide (SiCh), magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO), strontium oxide (SrO), vanadium pentaoxide (V2O5), chromium (III) oxide (CnCh), Niobium pentoxide (Nb2O5), and tungsten trioxide (WO3) or combinations thereof.

In an embodiment, the support material is selected from a nanopolyhedra, a nanorod, a nanocube or a nanotube. Preferably, the support material is nanopolyhedra. More preferably, the support material is CeCh metal oxide or Ceria nanopolyhedra. In a preferred embodiment, the support material is Ceria nanopolyhedra (Cerium oxide) synthesized from cerium nitrate hexahydrate. Ceria nanopolyhedra is synthesized by grinding Cerium nitrate hexahydrate to fine white powder, followed by calcination. In an embodiment, the calcination of the fine white powder is effected at 400 °C to 600 °C for about 4 to 8 hours in static air with a heating rate of 2 °C/min to 7 °C/min. Preferably 500 °C for 4 hours, with a heating rate of 5 °C/min.

In an embodiment of the present invention, the support material is desirably in the nanometer size range. For example, in an embodiment, the support material has an average particle size in the range of about 1 to about 1000 nm. Preferably, from about 50 to about 500 nm, or about 100 to about 200 nm, or about 1 to about 100 nm. Most preferably, 4 nm to 15 nm. Catalyst for this process is selected from a supported bimetallic and trimetallic nanoparticles. “Bimetallic” are compounds that contain stoichiometric amounts of two metal elements (such as alkali metal, alkaline earth metal, noble metal, transition metal, lanthanide, actinide, and metalloid). “Trimetallic” are compounds that contain stoichiometric amounts of three metal elements (such as alkali metal, alkaline earth metal, noble metal, transition metal, lanthanide, actinide, and metalloid). In an embodiment, the supported multimetallic catalyst is selected from a group consisting of a supported bimetallic catalyst system (M1M2) or a supported trimetallic catalyst system (M1M2M3). The supported multimetallic catalysts are supported bimetallic catalysts. In an embodiment of the present disclosure, the supported bimetallic catalysts are synthesized from two metals, wherein first metal and second metal are different. In one embodiment, the supported bimetallic catalysts are nanoparticular compounds that contain stoichiometric amounts of two metal (alkali metal, alkaline earth metal, transition metal, lanthanide, actinide, and metalloid) elements. In an embodiment, the supported bimetallic catalyst system (Ml M2) comprising Ni and Au metal nanoparticles supported on ceria nanopolyhedra.

The bimetallic catalysts synthesized are desirably in the nanometer size range. In an embodiment, the supported bimetallic catalyst system has an average particle size in the range from about 1 to about 1000 nm. Preferably from about 50 to about 500 nm, or about 100 to about 200 nm, or about 1 to about 100 nm. Most preferably, the particle size is 4 nm to 15 nm.

The multimetallic catalysts are trimetallic catalysts. In an embodiment of the present disclosure, the trimetallic catalysts are synthesized from three metals, wherein first metal, second metal, and third metal are different. In one embodiment trimetallic catalysts are nanoparticular compounds that contain stoichiometric amounts of three metal (alkali metal, alkaline earth metal, transition metal, lanthanide, actinide, and metalloid) elements. In an embodiment, the supported trimetallic catalyst system (M1M2M3) comprising Ni, Au and In metal nanoparticles supported on ceria nanopolyhedra. The trimetallic catalysts synthesized are desirably in the nanometer size range. The supported trimetallic catalyst system has an average particle size in the range from about 1 to about 1000 nm. Preferably from about 50 to about 500 nm, or about 100 to about 200 nm, or about 1 to about 100 nm. Most preferably, the particle size is 4 nm to 15 nm.

Another embodiment of the present disclosure provides a method for synthesizing a supported multimetallic catalyst system comprising the steps of: a) providing a dispersion comprising a support material in deionized water, followed by sonication and then stirring to obtain a dispersed support; b) providing individual solutions of two or more metal precursors dissolved in water to obtain a metal precursor solution, respectively; c) adding a first metal precursor solution to the dispersed support from step a) dropwise, followed by stirring to obtain a pre-final solution; d) adding a solution of PVP in water to the pre-final solution from step c) dropwise followed by stirring to prevent aggregation and to control the particle size of metal nanoparticles and to obtain a PVP-treated pre-final solution; e) adding remaining metal precursor solution dropwise to the PVP-treated pre-final solution from step d) dropwise followed by stirring to obtain a final solution having cationic form of metals; f) adding an ice cold solution of sodium borohydride to the final solution from step e) dropwise followed by stirring for reducing the cationic form of metals to obtain a non-ionic metals final solution; g) aging the non-ionic metals final solution, followed by centrifugation to obtain a supported multimetallic catalyst pellets; h) washing the supported multimetallic catalyst pellets with a solvent, followed by drying to obtain a dried supported multimetallic catalyst pellets; and i) calcining the dried supported multimetallic catalyst pellets to obtain a supported multimetallic catalyst system.

In an embodiment, the sonication in step a) is carried out for a period in the range of 5-15 min and stirred at a speed in the range of 400-600 rpm for a period in the range of 5-15 min.

In an embodiment, the stirring in step c) is carried out at a temperature in the range of 20-35 °C for a period in the range of 5 to 15 min.

In an embodiment, the stirring in step d) is carried out at a temperature in the range of 20-35 °C for a period in the range of 5 to 15 min.

In an embodiment, the stirring in step e) is carried out at a temperature in the range of 20-35 °C for a period in the range of 5 to 15 min.

In an embodiment, the step f) is carried out at a temperature in the range of 20 to 50 °C. Preferably at a room temperature of about 20 to 35 °C.

In an embodiment, the aging in step g) is carried out at a temperature in the range of 20 to 50 °C for a period in the range of 15 min to 1 hr. Preferably at a room temperature of about 20 to 35 °C for a period of 30 min.

In an embodiment, the centrifugation in step g) is carried out at a speed in the range of 6000 to 10000 rpm for a period in the range of 5 to 15 min. Preferably, at a speed of 8000 rpm for a period of 10 min.

In an embodiment, the solvent in step h) is selected from a group consisting of water, methanol, ethanol, n-propanol, butanol and combination thereof. Preferably the solvent is water and ethanol. The first three washings were done with water and then last washing was done with ethanol. In an embodiment, the drying in step h) is carried out at a temperature in the range of 50 to 100 °C for a period in the range of 8-14 hrs. Preferably, at a temperature of 70 °C for a period of overnight about 10-12 hrs.

In an embodiment, the calcination in step i) is carried out at a temperature in the range of 400 to 900 °C for a period in the range of 4 to 18 hrs in a static air and a heating rate in the range of 2 °C/min to 7 °C/min. Preferably, at a temperature in the range of 400 to 600 °C for a period in the range of 5 to 10 hrs with a heating rate in the range of 2 °C/min to 5 °C/min. More preferably, at a temperature of 400 °C for a period of 5 hrs with a heating rate 2 °C/min.

In an embodiment, the metal precursors of step b) are selected from a group consisting of metal sulfates, metal halides, metal nitrates, metal acetates, metal nitrites, metal oxides, metal carbonates, metal hydroxides, metal oxalates, metal pyrazolyl borates, metal azides, metal fluoroborates, metal carboxylates, metal halogencarboxylates, metal hydroxycarboxylates, metal aminocarboxylates, metal aromatic and nitro and/or fluoro substituted aromatic carboxylates, metal aromatic and nitro and/or fluoro substituted aromatic carboxylates, metal beta diketonates, metal sulfonates, metal acetylacetonate and combination thereof. In some embodiment, the metal halides are selected from metal chlorides or metal bromides.

In an embodiment of the present disclosure provides a method for synthesizing a supported bimetallic catalyst system comprising the steps of: a) providing a dispersion comprising ceria nanopolyhedra support material in a deionized water, followed by sonication for a period in the range of 5-15 min and then stirring at a speed in the range of 400-600 rpm for a period in the range of 5-15 min to obtain a dispersed support; b) providing individual solutions of Ni precursor and Au precursor dissolved in deionized water to obtain a Ni precursor solution and an Au precursor solution respectively; c) adding the Au precursor solution to the dispersed support from step a) dropwise, followed by stirring at a temperature in the range of 20-35 °C for a period of 5 to 15 min to obtain a pre-final solution; d) adding a solution of PVP in water to the pre-final solution from step c) dropwise followed by stirring at a temperature in the range of 20-35 °C for a period of 5 to 15 min to prevent aggregation and to control the particle size of Au nanoparticles and to obtain a PVP- treated pre-final solution; e) adding the Ni precursor solution to the PVP-treated pre-final solution from step d) dropwise, followed by stirring at a temperature in the range of 20-35 °C for a period of 5 to 15 min to obtain a final solution comprising cationic Ni and Au; f) adding an ice cold solution of sodium borohydride to the final solution from step e) dropwise followed by stirring at a temperature in the range of 20 to 50°C for reducing the cationic Ni and Au to obtain a non-ionic Ni and Au final solution; g) aging the Ni and Au final solution from step f) at a temperature in the range of 20 to 50 °C for a period in the range of 15 min to Ihr, followed by centrifugation at a speed 6000 to 10000 rpm for a period in the range of 5 to 15 min to obtain a supported bimetallic catalyst pellets; h) washing the supported bimetallic catalyst pellets from step g) with water and ethanol, followed by drying at a temperature in the range of 50 to 100 °C for a period in the range of 8-14 hrs to obtain a dried supported bimetallic catalyst pellets; and i) calcining the dried supported bimetallic catalyst pellets at a temperature in the range of 400 to 900 °C for a period in the range of 4 to 18 hrs in a static air and a heating rate in the range of 2 °C/min to 7 °C/min to obtain a supported bimetallic catalyst system.

In an embodiment, bimetallic catalyst contains Ni and Au. The Ni precursor has an amount in the range of about 0.1% wt to 5.0 % wt; For example, about 0.1% wt; about 0.5% wt; about 1% wt; about 1.5% wt; about 2% wt; about 2.5% wt; about 3% wt; about 3.5% wt; about 4% wt; about 4.5% wt; or about 5% wt. Preferably 2% wt. In an embodiment, Au precursor has an amount in the range of about 0.1% wt to 5.0 %wt. For example, about 0.1% wt; about 0.5% wt; about 1% wt; about 1.5% wt; about 2% wt; about 2.5% wt; about 3% wt; about 3.5% wt; about 4% wt; about 4.5% wt; or about 5% wt. Preferably 1.5% wt.

In an embodiment, the nickel precursor is nickel (II) chloride hexahydrate.

In an embodiment, the gold precursor is hydrogen tetrachloroaurate (III)) trihydrate.

In an embodiment of the present disclosure provides a method for synthesizing a supported trimetallic catalyst system comprising the steps of : a) providing a dispersion comprising ceria nanopolyhedra support material in deionized water, followed by sonication for a period in the range of 5-15 min and then stirring at a speed in the range of 400-600 rpm for a period in the range of 5-15 min to obtain a dispersed support; b) providing individual solutions of Ni precursor, Au precursor and In precursor dissolved in deionized water to obtain a Ni precursor solution, an Au precursor solution and an In precursor solution respectively; c) adding the Au precursor solution to the dispersed support from step a) dropwise, followed by stirring at a temperature in the range of 20-35 °C for a period in the range of 5 to 15 min to obtain a pre-final solution; d) adding a solution of PVP in water to the pre-final solution from step c) dropwise followed by stirring at a temperature in the range of 20-35 °C for a period in the range of 5 to 15 min to prevent aggregation and to control the particle size of Au nanoparticles and to obtain a PVP-treated pre-final solution; e) adding the Ni precursor solution dropwise followed by stirring at a temperature in the range of 20-35 °C for a period in the range of 5 to 15 min and then adding In precursor solution drop wise followed by stirring at a temperature in the range of 20-35 °C for a period in the range of 5 to 15 min to the PVP- treated pre-final solution from step d) to obtain a final solution comprising cationic Ni, Au and In; f) adding an ice cold solution of sodium borohydride to the final solution from step e) dropwise followed by stirring at a temperature in the range of 20 to 50 °C for reducing the cationic Ni, Au and In to obtain a non-ionic Ni, Au and In final solution; g) aging the Ni, Au and In final solution from step f) at a temperature in the range of 20 to 50 °C for a period in the range of 15 min to Ihr, followed by centrifugation at a speed in the range of 6000 to 10000 rpm for a period in the range of 5 to 15 min to obtain a supported trimetallic catalyst pellets; h) washing the supported trimetallic catalyst pellets from step g) with water and ethanol, followed by drying at a temperature in the range of 50 to 100 °C for a period in the range of 8-14 hrs to obtain a dried supported trimetallic catalyst pellets; and i) calcining the dried supported trimetallic catalyst pellets at a temperature in the range of 400 to 900 °C for a period in the range of 4 to 18 hrs in a static air and a heating rate in the range of 2 °C/min to 7 °C/min to obtain the supported trimetallic catalyst system.

In an embodiment, trimetallic combination contains Ni, Au and In. Ni precursor has an amount in the range of about 0.1% wt to 5.0 % wt; For example about 0.1% wt; about 0.5% wt; about 1% wt; about 1.5% wt; about 2% wt; about 2.5% wt; about 3% wt; about 3.5% wt; about 4% wt; about 4.5% wt; or about 5% wt. Preferably 2% wt. In an embodiment, Au precursor has an amount in the range of about 0.1% wt to 5.0 %wt. For example about 0.1% wt; about 0.5% wt; about 1% wt; about 1.5% wt; about 2% wt; about 2.5% wt; about 3% wt; about 3.5% wt; about 4% wt; about 4.5% wt; or about 5% wt. Preferably 1.5% wt. In an embodiment, In precursor has an amount in the range of about 0.1% wt to 5.0 %wt. For example about 0.1% wt; about 0.5% wt; about 1% wt; about 1.5% wt; about 2% wt; about 2.5% wt; about 3% wt; about 3.5% wt; about 4% wt; about 4.5% wt; or about 5% wt. Preferably 1.5% wt. Most preferably Ni 2.0% wt, Au 1.5% and In 1.5% wt. Moreover, in some embodiments, the all three metal precursors in the trimetallic combination is about 0.5% wt to 10 % wt.

In an embodiment, the nickel precursor is nickel (II) chloride hexahydrate. In an embodiment, the gold precursor is hydrogen tetrachloroaurate(III) trihydrate . In an embodiment, the indium precursor is indium (III) chloride. Also, preferred metal loading in catalyst where it acts efficient is 5.0% with broader range of 1-10%.

Another embodiment of the present disclosure provides a method of hydrogenation of CO2 in the presence of a supported multimetallic catalyst system comprises the steps of : i) adding a supported multimetallic catalyst system in a reactor; ii) pre-treating the supported multimetallic catalyst system under H2 flow to reduce the catalyst prior to reaction; and iii) adding and contacting CO2 stream and H2 stream through the reactor under conditions to form CH4, CO or Cl products or methanol, wherein the ratio of CO2 stream and H2 stream are in the range of 1 : 1 to 1:4 for a total flow rate in the range of 6 mL/min to 18 mL/min.

The multimetallic means for bimetallic and trimetallic. Bimetallic gives methane at ambient pressure and trimetallic gives CO at ambient pressure and methanol at high pressure.

In some embodiments of the present invention, the Cl compound is methane, carbon monoxide, formic acid, methanol and the like.

In an embodiment, the pre-treatment in step ii) is carried out at a temperature in the range of 400 °C - 600 °C for a period in the range of about 4 to 8 hours under H2 flow with a heating rate in the range of 2 °C/min to 15 °C/min. The condition in step iii) includes bimetallic catalyst at ambient pressure and at a temperature in the range of 200 to 400 °C to obtain CH4 as Cl products. The condition in step iii) includes trimetallic catalyst at ambient pressure at a temperature in the range of 200 to 400 °C to obtain CO as Cl products. The condition in step iii) includes trimetallic catalyst at a temperature in the range of 200 to 500 °C with pressure in the range of 1 to 50 bar to obtain methanol.

In some aspects, the supported trimetallic catalyst system as mentioned above can be used in said RWGS reaction for obtaining higher selectivity for CO at ambient pressure and also, to produce lower alcohols (e.g. methanol) at high pressure. Whereas the supported bimetallic catalyst system as mentioned above shows higher stability towards methanation reaction at ambient pressure, where methane is selectively formed and the catalyst was found stable for 48h at lower temperatures of 200-400 °C.

In some aspects, the supported trimetallic catalyst system as mentioned above shows selectivity for CO production by 75-100%, specifically 75%, 80%, 85%, 90%, 95%, 99%, 99.9%, or higher. Preferably 97-100% selectivity for CO production at lower temperatures of 200-400 °C. For 1.5 NAICP and 2.0 NAICP, the selectivity is 97 % and 100% respectively.

In some aspects, the supported trimetallic catalyst system as mentioned above can be used in CO2 hydrogenation for obtaining higher selectivity for methanol at higher pressure and temperature. Preferably in temperature range 200-500 °C and pressure 5-50 bar.

In some aspects, the supported trimetallic catalyst system as mentioned above shows selectivity for production of methanol with high CO2 conversion at higher pressure and temperature. Preferably 50- 90% selectivity for methanol and 8-50% CO2 conversion at temperatures of 200-500 °C and in the pressure range of 5-50 bar.

In some aspects, the supported bimetallic catalyst system as mentioned above shows selectivity for methane production by 75-100%, specifically, 75%, 80%, 85%, 90%, 95%, 99%, 99.9%, or higher. Preferably 99.4 -100% selectivity for methane production at lower temperatures of 200-400 °C.

In an embodiment, the present disclosure provides a method of hydrogenation of CO2 into CO via RWGS reaction using said trimetallic catalyst system comprises the steps of: i) adding the supported trimetallic catalyst system as discussed above in a reactor; ii) pre-treatment the catalyst system under H2 Flow to reduce the catalyst prior to reaction; and iii) passing and contacting CO2 stream and H2 Stream through the reactor under ambient reaction conditions to form CO by the catalyst system.

In an embodiment, the present disclosure provides a method of hydrogenation of CO2 into CPU via methanation reaction using said bimetallic catalyst system comprises the steps of: i) adding the supported bimetallic catalyst system as discussed above in a reactor; ii) pre-treatment the catalyst system under H2 Flow to reduce the catalyst prior to reaction; and iii) passing and contacting CO2 stream and H2 Stream through the reactor under ambient pressure and temperature reaction conditions to form CPU by the catalyst system.

In an embodiment of the present invention, the pre-treatment is effected at 400 °C - 600° C. for about 4 to 8 hours under H2 Flow with a heating rate of 2 °C/min to 15 °C/min. Preferably 400 °C for 4 hours, with a heating rate of 10 °C/min. The H2 Flow is maintained at 10 mL/min -30 mL/min, preferably 20 mL/min.

The ratio of flow between hydrogen gas and carbon dioxide, for example, can also affect selectivity and yield of the process. In an embodiment of the present invention, the preferred reaction parameters for RWGS reaction in present invention is: i) CO2:H2 ratio ranging from 1: 1 to 1:4 for a total flow rate ranging from 6 mL/min to 18 mL/min, preferably in the ratio of 1:4 with 15 mL/min total flow rate; ii) temperature in range of 200 °C - 800 °C and preferably 200 °C - 400 °C, under ambient pressure conditions, and; iii) WHSV of 6000 - 25000 mL h ¹ gcat ¹ , preferably 15,000 mL h ¹ _n "I gcat .

The ratio of flow between hydrogen gas and carbon dioxide, for example, can also effect selectivity and yield of the process. In an embodiment of the present invention, the preferred reaction parameters for CO2 hydrogenation to methanol in present invention is: i) CO2:H2 ratio is 1:1 to 1:4 ii) temperature in range of 200 ° C - 500 °C iii) High pressure ranging from 5-50 bar, and; iv) WHSV of 1000 - 25000 mL h ¹ gcat ¹ . In an embodiment of the present invention, the hydrogenation of CO2 is carried out at atmospheric pressure, and higher pressures of up to 50 bar. The CO2 hydrogenation to methanol can also be carried out at pressures ranging from about 1 bar to 50 bar.

In an embodiment of the present invention, the reactor can be a standalone system for chemical synthesis, or incorporated into a renewable chemical synthesis scheme, or incorporated into a gas purification scheme. In either case, the hydrogenation of carbon dioxide with hydrogen gas, is accomplished using the catalytic process described herein.

In an embodiment, the bimetallic and/or trimetallic nanoparticles of the present invention maybe used in any manner known to a person skilled in the art.

EXAMPLES

Example 1: Synthesis of Ceria nanopolyhedra (CP)support

Ceria nanopolyhedra (CP) were synthesized by grinding Cerium nitrate hexahydrate to fine white powder. Finally, the resulting white powder was calcined at 500 °C for 4h in static air with a heating rate of 5 °C/min to obtain ceria nanopolyhedra (CP) support material.

Examples 2: Synthesis of Bimetallic catalyst (xNACP) system

The bimetallic catalyst was synthesized via the chemical reduction method. First, the support was dispersed in 50 mL of MQ water and sonicated for 10 min and then stirred for 10 min at 500 rpm. After that, 1.5wt. % of Au and 2 wt. % of Ni precursor were dissolved in MQ water separately. Then Au solution was added dropwise to the dispersed support after complete addition the solution was kept for stirring for 10 min after that PVP solution was added dropwise and kept for stirring for 10 min. Then Nickel precursor solution was added to it and stirred for 10 min. To reduce Au ³⁺ and Ni ²⁺ to Au° and Ni° respectively, ice-cold (about 0 °C) 0.1 M NaBH4 solutions was added drop wise there will be a change in colour of the overall solution from yellow to pink to brown. After that aged for 30 min, at room temperature (about 20-35 °C) then centrifuged it at 8000 rpm for 10 minutes and washed with water 3 times and further washing was done with absolute ethanol. The catalyst was kept at 70 °C overnight (about 10-12 hrs) for drying and then calcined at 400 °C in static air for 5h with a ramp rate of 2 °C/ min to obtain the bimetallic catalyst system. A series of catalyst with different Au loading was synthesized and labelled as xNACP (where x= theoretical gold loading in wt %).

Example 3: Synthesis of Trimetallic catalyst (yNAICP) system

Trimetallic catalyst was synthesized via the same chemical reduction method. First, the support was dispersed in 50 mL of MQ water and sonicated for 10 min then stirred for 10 min at 500 rpm. After that 1.5wt% of Au and 2wt% of Ni with 1.5wt% In precursor were dissolved in MQ water in separate beakers. Then Au solution was added dropwise to dispersed support after complete addition, the solution was kept for stirring for 10 min after that PVP solution was added drop wise and kept for stirring further 10 min. Nickel was added to it and kept for stirring for 10 min and then Indium precursor solution were added to it and kept for stirring for 10 min. To reduce Au ³⁺ , Ni ²⁺ and In ³⁺ to Au°, Ni° and In ⁰ respectively, ice-cold (about 0 °C) 0.1M NaBkk solution was added dropwise there will be a change in colour of the overall solution from yellow to pink to black. After that aged for 30 min, at room temperature (about 20-35°C) then centrifuged it at 8000 rpm for 10 minutes and washed with water 3 times and further washing was done with absolute ethanol. The catalyst was kept at 70 °C overnight for drying and then calcined at 400 °C in static air for 5h with a ramp rate of 2 °C/min. In the same way a series of catalysts with different Indium loading was synthesized and labelled as yNAICP (where y= theoretical Indium loading in wt%).

Example 4: Characterization of the supported bimetallic and trimetallic catalyst system

The ex-situ X-ray diffraction (XRD) patterns of the samples were recorded on a powder X-ray diffractometer (Bruker D8Advance powder), using Cu-Ka radiation (k = 0.15405 nm) in the 20 range of 10-80 °C at a scanning rate of 3 °C min ¹ .

The specific surface area of the catalyst was measured by the low-temperature liquid nitrogen adsorption and desorption method with a Quantachrome instrument. A quartz tube with 50 mg of the sample was dewatered and degassed for 3 h at 250 °C and under a 1 MPa vacuum. After the pretreatment, the adsorption and desorption experiment were carried out with N2 as the adsorbed gas and He as the balance gas. The structures and morphologies of catalysts were observed by scanning electron microscopy FE-SEM characterization was carried out on a NOVA NANO SEM field emission scanning electron microscope. HR-TEM analysis has been carried out by the JEOL F-200 HRTEM instrument. The samples for HR-TEM analysis were prepared in isopropyl alcohol and dried completely before the analysis.

The catalyst surface analysis was performed by X-ray photoelectron spectroscopy (XPS) over a Thermo Scientific Kalpha+ spectrometer equipped with micro-focused and monochromatic Al Ka (ho =1486.6 eV). The pass energy for the spectral acquisition was kept at 50 eV for individual core levels. The electron flood gun was utilized for providing charge compensation during data acquisition. The peak fitting of the individual core levels was done using Avantage software with a smart type background. The binding energy (BE) was calibrated based on the line position of C Is (284.50 eV). The XRD patterns (FIG. 1 ) showed crystalline nature of the catalyst having characteristic of the facecentered cubic fluorite structure of ceria (JCPDS No.34-0394). After loading Ni, Au and In. There were no significant changes in the original structure of ceria. The peaks at 20 value 28.5°, 33.1°, 47.5°, 56.4°, 59.2°, 76.8°, 79.3° and 88.5° correspond to CeO ₂ (111), (200), (220), (311), (222), (400), (331) and (420) planes, respectively. The reflections corresponding to Ni, NiO, Au and In ₂ O3 were absent which refers to higher dispersion of metal over support.

The catalyst was further characterized with N ₂ -adsorption desorption analysis and the isotherm for CP, 2.0 NCP and 1.5 NACP are shown by FIG.2. The specific surface area of Ceria nanopolyhedra support (CP), 2.0 NCP and 1.5 NACP were found from BET analysis is 96.2 m ² /g, 95.9 m ² /g and 80.1 m ² /g. The decrease in value of specific surface area of bimetallic catalyst is due to introduction of Au metal in 2.0 NCP. The wide hysteresis loop shows that catalyst is mesoporous.

FIG. 3 shows HR-TEM images of fresh catalyst 2.0 NCP, 1.5 NACP and 1.5 NAICP. In all catalyst the lattice fringes correspond to exposed plane (111) for which the interplanar d- spacing value is found 0.31 nm. The morphology of Ceria nanopolyhedra was confirmed with exposed (111) and (200) plane with d-spacing value of 0.31 and 0.27 nm respectively. The morphology of catalyst has not changed after reaction which indicates that the active sites of catalyst are quite stable. EDAX mapping images shows homogenous distribution of metals over ceria support (FIG. 10(A-C)).

Example 5: CO ₂ hydrogenation at atmospheric pressure using the supported bimetallic and trimetallic catalyst system.

The catalytic performances for CO ₂ hydrogenation for all the catalysts of different compositions were carried out with a plug flow tubular quartz reactor of 40 cm length having an inner diameter of 8 mm. The reactor was fixed inside a carbolite vertical furnace equipped with a programmable temperature controller. 60 mg of powder catalyst was loaded into a fixed bed reactor (FBR) without mixing any diluent. Before the reaction, the catalysts were pre-treated at 400 °C for 4 h under H ₂ flow (20 mL/min) with a ramp rate of 10 °C/min and then cooled to 100 °C under a similar H ₂ stream. The flow of CO ₂ was set to 3 mL/min while H ₂ was set to 12 mL/min to maintain the ratio of CO ₂ :H ₂ to 1:4 using Mass Flow Controller (MFC). While catalyst was also tested for different CO ₂ :H ₂ ratios. As the reactant gas was introduced to FBR the temperature of the catalyst was also increased to reaction temperature in intervals of 40 °C. The products at the outlet from the reactor were analysed by using an online gas chromatograph (Nucon 5765, GC) equipped with two columns in parallel: one is a DB-624 capillary column connected to a flame ionization detector (FID) for the analysis of CFU and other hydrocarbon, and the other a carbon sieve packed column connected to Methanizer through flame ionization detector (FID) for the analysis of CO, CPU and C02.The stability tests were performed under the same space velocity at 360 °C for 24-48 h.

The conversion of CO2 (Xco2) was calculated using equation- 2a

Where [CO2]in and [CO2]out is the concentration of CO2 in the inlet and outlet gas, respectively.

The selectivity of a specific product (S specific product) was calculated using equation- 2b

> %spedf . product ,

Specific product ~ _{% totaj products} ^{X i0U} . ^2b

Where % specific product was calculated from standard calibration

The Space time yield was calculated by equation-2c where Xco2 is the conversion of CO2, [CO2]totai is the total concentration of CO2 (mol/sec), and mcataiystis the mass of the catalyst.

The turnover frequency (TOF) was calculated using equation -2d

Where Xco2 is the conversion of CO2, [CC ]totai is the total concentration of CO2 (mol/sec), mcataiyst is the mass of catalyst, and [M] surface is the content of surface Metal in the catalyst based on ICP analysis and M’ is the relative atomic mass of the metal (in g/mol).

All the catalytic tests were conducted in a Fixed bed reactor at a higher space velocity of 15,000 mL.gcat^.h ¹ . FIG. 4(A-B) compares the steady-state catalytic CO2 hydrogenation over monometallic and bimetallic catalyst. When the reaction was carried out with a monometallic catalyst i.e., 2.0 NCP lower CO2 conversion was achieved with methane selectivity. The same reaction was done with Au- modified bimetallic catalyst 1.5 NACP which showed enhanced CO2 conversion with higher methane selectivity (FIG. 4A, 4B). To determine the effect of Au over monometallic catalyst Time- on stream study was done at 360 °C while all the reaction condition was kept the same. It was found that the catalyst 1.5 NACP was stable for more than 48h whereas for 2.0 NCP it gets deactivated (FIG. 5). Whereas selectivity was maintained throughout the reaction time (FIG.l 1).

The catalyst xNACP was synthesized with varying Au wt% (x= 0.25,0.5,1.5 and 2.0) to get the optimum loading (FIG. 12) and catalytic activity was tested under similar reaction conditions in which 1.5 wt% Au modified catalyst 1.5 NACP was found to show best results (FIG. 13 (A-B)). Au provides stability and enhances the activity of catalyst so, further to tune the selectivity of product in CO2 hydrogenation reaction a small fraction of Indium is introduced to 1.5NACP bimetallic catalyst. Thus, a combination of these three metals leads to a unique trimetallic catalyst 1.5NAICP. The same reaction was carried out with a trimetallic catalyst to optimize the change in selectivity trend (FIG. 6). Where the selectivity has completely tuned towards CO (97%) even at lower temperatures. So, moreover from a typical CO2 methanation reaction the reaction turns to be RWGS reaction.

To see the effect of Indium, same reaction was evaluated with catalyst 0.5 NAICP and 2.0 NAICP (0.5wt% and 2.0 wt% of Indium). The reaction data concludes that with lower indium loading the CO selectivity has been decreased and there is an increase in methane selectivity as the reaction temperature increased FIG. 14 (A-B), which may be due to more exposure of Ni over the surface with the reactant effluent. Whereas with the increased indium loading, the selectivity (97-100%) and conversion (47-50%) were almost the same as of 1.5NAICP (FIG. 7).

For comparative study catalyst 1.5NICP (without Au) and 1.5ICP (without Ni and Au) were also tested for same reaction with similar reaction conditions. Where 1.5ICP showed poor activity towards CO2 conversion and 1.5NICP showed comparatively higher conversion (FIG. 15 (A-B)).

Both the catalyst 1.5NICP &1.5ICP were evaluated for Time on stream study to see the effect of gold loading over the stability of 1.5NICP. The activity for 1.5NICP has decreased after 24h showing the improved stability of 1.5 NAICP (FIG. 8). As RWGS reaction is an endothermic reaction so favoured at higher temperatures above 500 °C so the reaction was carried out for catalyst 2.0NAICP at higher temperatures up to 600 °C (FIG. 16) where CO2 conversion and CO selectivity have been increased to -80% and -99% respectively.

As for a typical methanation reaction H2:CO2 is 4: 1, in order to see the effect of H2 concentration over product selectivity and CO2 conversion the same reaction was carried out with varying H2:CO2 ratio in the temperature range of 200-400 °C. From the obtained reaction data, it was concluded that the conversion (~ 45-47%) was almost the same at different ratios except 1H2: 1CO2 the conversion was 32% (FIG.9). The CO selectivity was maintained at different ratios (FIG. 17), so it can be concluded that modified trimetallic catalyst has shifted towards RWGS even in a typical methanation reaction condition. H2:CO2 is 4: 1 at 400 °C in presence of Ni, Au, Ni and Au and Ni, Au and In showed the CO2 conversion , CO selectivity and CPU selectivity as follows:

The CO2 hydrogenation to methanol activity using 1.5NAICP trimetallic catalyst is shown in Fig. 18 A, where the reaction conditions are as follows:

Pressure: 40 bar

WHSV: 3000 ml gV CO2: 15 ml/min; H2: 45 ml/min; N2: 3 ml/min.

The CO2 conversion, methanol selectivity and STY (space-time yield) at temperature 250, 275 and 300 °C are given below:

The stability study of 1.5NAICP trimetallic catalyst is shown in Fig. 18 B, where the reaction conditions are as follows:

Pressure: 40 bar

WHSV: 3000 ml gV

CO2: 15 ml/min; H2: 45 ml/min; N2: 3 ml/min.

The CO2 conversion, methanol selectivity and STY (space-time yield) at temperature 275 °C is given below:

ADVANTAGES OF THE PRESENT INVENTION

CO2 conversion for bimetallic catalyst is 94% with methane selectivity of 99.4%; and for trimetallic catalyst system, CO2 conversion was 47- 50% with CO selectivity of 97-100% even at the lower reaction temperatures.

Reduction and resistance in coke deposition in said catalyst system providing preferable, reliable and improved catalyst system.

The multimetallic catalyst system is highly stable upto 48 hrs of reaction.

Highly re-usable upto numerous cycles.

Provides Bimetallic catalyst for methanation reaction forming methane as major product, and the trimetallic catalyst gives activity for RWGS reaction at ambient pressure whereas same catalyst when subjected to high pressure given Methanol as major product.

Provides specific weight loading of Au in Bimetallic and Indium in trimetallic catalysts with optimized optimal loading of Au & In respectively, thus achieving highest conversion of CO2 and selectivity at lower temperatures.

In case of bimetallic catalyst, the CO2 conversion is enhanced as well as the long-term activity test shows its high stability for more than 48hr.

When trimetallic catalyst is used, the selectivity tuned towards CO or RWGS reaction rather than forming methane under same reaction conditions.

Both bimetallic and trimetallic catalysts are synthesized by chemical reduction method. Provides conversion of CO2 via hydrogenation into methane (in case of bimetallic catalyst), and CO & methanol in case of trimetallic catalyst.

As per XPS analysis, the Ni (in bimetallic catalyst) is in 2+ oxidation state, and Indium (in trimetallic catalyst) has In(0) and In (0<X<1.5), the binding energy indicates In species comes around 444.16 eV which can originate from InOx (0<x<1.5). So there is presence of synergistic effect or charge transfer.

Provides the maximum CO production i.e., 61058 pmol gcat ^-1 h ^-1 with 47% of CO2 conversion and 97-99% CO selectivity at a temperature 400 °C and ambient pressure. The literature reported catalysts have very high metal loadings of upto 40wt%, whereas in the present disclosure, the catalyst used around ~ 5-5.5wt% of active metals.