A CATALYST FOR THE CONVERSION OF CO2 TO CO AND PROCESS FOR THE PREPARATION THEREOF

Abstract

The present invention relates to catalyst. Co.sub.3O.sub.4 nanocube or In.sub.2O.sub.3 with novel characterization features for the synthesis of CO, which is used as a reducing agent in the production of direct reduced metal from metal ore or mixture of metal oxides.

Claims

1. A metal oxide catalyst of formula M.sub.nO.sub.m for a selective production of CO from CO.sub.2 wherein M is selected from Co or In; n=2, m=3 when M is In and n=3, m=4 when M is Co, wherein particle size of the Co.sub.3O.sub.4 nano-cube (NC) and In.sub.2O.sub.3 is in a range of 18-35 nm and 8-10 nm respectively.

2. The metal oxide catalyst as claimed in claim 1, wherein said catalyst is selected from i. Co.sub.3O.sub.4 nano-cube (NC) having XRD peaks at 2=19.3, 31.5, 37, 38.8, 45, 47.91, 52.08, 55.8, 59.5, 65.4, 76.3; ii. In.sub.2O.sub.3 has XRD peaks at 2=21.7, 30.76, 35.51, 38.00, 41.92, 45.43, 51.05, 56.03 and 60.74.

3. The metal oxide catalyst as claimed in claim 1, wherein the Co.sub.3O.sub.4 nano-cube (NC) has surface area in the range of 20 to 30 m.sup.2 g.sup.1.

4. A process for preparation of the catalyst Co.sub.3O.sub.4 nanocube (NC) as claimed in claim 1, wherein said process comprising the steps of: a) dissolving cobalt precursor in water followed by stirring at a temperature in the range of 298-303 K for a period in the range of 5-10 mins to obtain a solution; b) adding aqueous ammonia solution dropwise into the solution as obtained in step (a) to make pH 9.0 and stirring for a period in the range of 20 to 60 mins to obtain a reaction mass; c) transferring the reaction mass as obtained at step (b) into an autoclave with teflon liner and maintaining a temperature in a range of 433 to 473 K for 10 hours to obtain a solution; d) filtering and washing the solution as obtained at step (c) with water to obtain a reaction mass; e) calcining the reaction mass as obtained at step (d) at a temperature in the range of 573 to 673 K for a period in the range of 2 to 4 hours in the air to obtain Co.sub.3O.sub.4 nano cube (NCs); and f) optionally, calcining the Co.sub.3O.sub.4 nano cube as obtained in step (e) in oxygen atmosphere at temperature in the range of 523-673 K for a period in the range of 12-24 hours to obtain calcined Co.sub.3O.sub.4 nano cube.

5. The process as claimed in claim 3, wherein the cobalt precursor is Co(OAC).sub.2.Math.4H2O.

6. A process for preparation of the catalyst In.sub.2O.sub.3 cube as claimed in claim 1, wherein said process comprising the steps of: a) dissolving indium nitrate precursor in a mixture of water and ethanol to obtain a solution; b) adding ammonia solution in ethanol into the solution as obtained in step a) at temperature in the range of 298-303 K to get the hydroxide precipitate; c) aging the precipitate as obtained in step b) at a temperature in the range of 343 to 363 K for a period in the range of 5 to 15 minutes to obtain a slurry; d) cooling the slurry as obtained in step c) at temperature in the range of 298-303 K and washing the slurry with water and ethanol to obtain a mass; e) drying the mass as obtained in step d) at a temperature in a range of 383 to 423 K for a period in the range of 6 to 14 hours followed by calcining at a temperature in the range of 673 to 773 K for a period in the range of 2 to 12 hours to afford the catalyst.

7. The process as claimed in claim 6, wherein the indium precursor is In(NO.sub.3).sub.3.Math.5H2O.

8. A process for the selective production of CO from CO.sub.2 using the catalyst as claimed in claim 1 comprising the steps of: a) pre-treating the catalyst as claimed in 1 to 2 in air at temperature in the range of 673 to 773 K for a period in the range of 2 to 6 h at a ramping rate in the range of 5 K.Math.min.sup.1; b) loading the catalyst to a fixed bed catalyst reactor and feeding CO.sub.2:H.sub.2 gas mixture in a ratio ranging between 1:0.67-1:7 using two different mass flow controllers; c) reducing CO.sub.2 at atmospheric pressure in reverse water gas shift (RWGS) reaction in the fixed bed catalyst reactor at a temperature in the range of 373 K to 923 K with constant gas hourly space velocity (GHSV) in a range of 15000-192000 h.sup.1 to obtain the CO.

9. The process as claimed in claim 8, wherein CO gas is useful to convert metal oxide(s)/metal ore(s) to a reduced metal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG. 1 shows XRD patterns of the (left) fresh and (right) spent Co.sub.3O.sub.4 catalysts. A spent catalyst was obtained after the CO.sub.2 reduction reaction carried out at 773 K with 3:2 ratio of CO.sub.2:H.sub.2 feed after 12 h.

[0045] FIG. 2 illustrates (a-c) TEM and HRTEM images of Co.sub.3O.sub.4 which shows cubic morphology and the average particle size is found to be 18-35 nm. HRTEM of single (NC) is shown suggesting the NC catalysts are faceted in the (110) orientation and the lattice fringes corresponding to the interplanar distance of the (110) facet (d.sub.110=0.45 nm) and (222) facet (d.sub.222=0.25 nm). (d) TEM of spent catalyst is shown, and it exhibits near cubic or spherical morphology with same particle size as that of fresh catalyst.

[0046] FIG. 3 shows Temperature dependence CO.sub.2 reduction activity of spinel Co.sub.3O.sub.4 NC evaluated with three CO.sub.2:H.sub.2 ratios, namely 3:2, 1:1 and 1:3. Panels a to d shows, CO.sub.2 conversion, H.sub.2 conversion, and selectivity of (c) CO and (d) CH.sub.4 respectively.

[0047] FIG. 4 shows Time on stream study of CO.sub.2 reduction with and H.sub.2 on Co.sub.3O.sub.4 NC for (a) 1:5 and (b) 3:2 ratio of CO.sub.2:H.sub.2 at temperature 723 and 723 K, respectively. Reactants are shown in square (CO.sub.2) and triangle (H.sub.2) symbols and product selectivity is shown in dense (CO) and sparse (CH.sub.4) hash-line bars.

[0048] FIG. 5 provides Temperature dependent CO.sub.2 reduction activity of oxygen treated CO.sub.3O.sub.4 nano-cube evaluated with four CO.sub.2:H.sub.2 ratios, namely 1:0.67, 1:1, 1:2, and 1:3. (a) CO.sub.2 conversion (b) CO selectivity (c) H.sub.2 conversion (d) CH.sub.4 selectivity

[0049] FIG. 6 (a) XRD patterns of fresh and spent catalyst, (b) H.sub.2 TPR study of fresh In.sub.2O.sub.3 catalyst, and (c & d) HRTEM study of fresh and spent In.sub.2O.sub.3 catalyst respectively. Catalyst collected after the reaction with 1:3 CO.sub.2:H.sub.2 ratio at 773 K for 12 h is termed as spent catalyst.

[0050] FIG. 7 provides Temperature dependence of (a) CO.sub.2 Conversion, (b) H.sub.2 conversion, (c) CO selectivity (d) CH.sub.4 selectivity. Reaction Conditions: Pressure: 1 bar, GHSV: 15000 H.sup.1, Gas ratio: CO.sub.2:H.sub.2=1:X (X=0.67, 1, 3, 5, 7)

[0051] FIG. 8 provides Time Aon stream studies of reactants conversion (CO.sub.2 and H.sub.2) on In.sub.2O.sub.3 with products selectivity (CO and CH.sub.4) for (a) CO.sub.2:H.sub.2=1:0.67 (b) CO.sub.2:H.sub.2=1:3 ratios at 773 K temperature and atmospheric pressure; GHSV=15000 h.sup.1.

[0052] FIG. 9 shows Valence band spectrum of In.sub.2O.sub.3 recorded in the presence of 1:0.67 ratio of CO.sub.2:H.sub.2 at a total pressure of 0.1 mbar at 295 and 773 K. Note the shift in valence band at 773 K due to the oxygen vacancy formation and subsequent broadening of valence band due to electron filling.

[0053] FIG. 10 shows Valence band spectrum of Co.sub.3O.sub.4 recorded in presence of 1:3 ratio of CO.sub.2:H.sub.2 at a total pressure of 0.1 mbar at 375 and 675 K. Note the shift in valence band to lower binding energy at 675 K. This is possibly due to oxygen vacancy formation and (200) and (400) stepped facets formation due to reaction conditions.

[0054] FIG. 11 depicts the reactor system for CO.sub.2 hydrogenation and its application for Iron ore reduction.

DETAILED DESCRIPTION OF THE INVENTION

[0055] The present invention provides a catalyst with novel characterization features for the selective production of CO from CO.sub.2.

[0056] The present invention provides Co.sub.3O.sub.4 NC and In.sub.2O.sub.3, catalyst for the selective production of CO from CO.sub.2, wherein the catalysts Co.sub.3O.sub.4 NC and In.sub.2O.sub.3 are characterized with x-ray diffraction (XRD), transmission electron microscopy (TEM), H.sub.2-temperature programmed reduction (H.sub.2-TPR), and valence band shift by near-ambient pressure photoelectron spectroscopy (NAPPES) under simulated reaction conditions.

[0057] Further, present invention provides a process for the preparation of catalyst for the selective production of CO from CO.sub.2. The Co.sub.3O.sub.4 nano-cube was synthesized by the wet chemical synthesis method reported in the literature. The template-free hydrothermal method has been adopted to prepare nano-crystalline and cubic Co.sub.3O.sub.4 by using Co(OAC).sub.2.Math.4H.sub.2O as a cobalt precursor. In.sub.2O.sub.3 catalyst is prepared by using Indium nitrate, In(NO.sub.3).sub.3.Math.5H.sub.2O precursor.

[0058] The present invention relates to a process for the preparation of Co.sub.3O.sub.4 NC catalyst is provided, wherein said process comprises the steps of: [0059] a) dissolving cobalt precursor in water and stirring at 298-303 K for 5-10 mins; [0060] b) adding aqueous ammonia solution dropwise into the solution obtained at step a) to make pH 9.0 and stirring for 30 mins; [0061] c) transferring the reaction mass obtained at step b) into autoclave with Teflon liner and maintaining at 453 K for 10 hours; [0062] d) filtering and washing the resulting solution obtained at step c) with water; [0063] e) Calcining the reaction mass at 623 K for 3 hours in air to obtain Co.sub.3O.sub.4 NCs; and [0064] f) optionally calcining the Co.sub.3O.sub.4 NCs in oxygen atmosphere at 573 K for 24 h.

[0065] The materials prepared and obtained at the end of step as well as step (f) were utilized as catalyst. Specifically, the inventor surprisingly found that the catalyst obtained after step f shows highly desired activity of 100% CO selectivity at relatively lower temperatures and the results are described in FIG. 5.

[0066] The present invention relates to a process for the preparation of Co.sub.3O.sub.4 [NC] catalyst is provided, wherein said process comprises the steps of: [0067] a) dissolving cobalt precursor in water and stirring at 298-303 K for 5-10 mins; [0068] b) adding aqueous ammonia solution dropwise into the solution obtained at step a) to make pH 9.0 and stirring for 30 mins; [0069] c) transferring the reaction mass obtained at step b) into the autoclave with Teflon liner and maintaining at 453 K for 10 hours; [0070] d) filtering and washing the resulting solution obtained at step c) with water; and [0071] e) calcining the reaction mass at 623 K for 3 hours in the air to obtain Co.sub.3O.sub.4 NCs.

[0072] The present invention relates to a process for the preparation of Co.sub.3O.sub.4 NC catalyst is provided, wherein said process comprises the steps of: [0073] a) dissolving cobalt precursor in water and stirring at 298-303 K for 5-10 mins; [0074] b) adding aqueous ammonia solution dropwise into the solution obtained at step a) to make pH 9.0 and stirring for 30 mins; [0075] c) transferring the reaction mass obtained at step b) into autoclave with Teflon liner and maintaining at 453 K for 10 hours; [0076] d) filtering and washing the resulting solution obtained at step c) with water; [0077] e) calcining the reaction mass at 623 K for 3 hours in air to obtain Co.sub.3O.sub.4 NCs; and [0078] f) calcining the Co.sub.3O.sub.4 NCs in oxygen atmosphere at 573 K for 24 h.

[0079] The present invention relates to a process for the preparation of In.sub.2O.sub.3 catalyst is provided, wherein said process comprises the steps of: [0080] i. dissolving indium nitrate precursor in a mixture of water and ethanol; [0081] ii. adding ammonia solution in ethanol into the solution obtained at step i) to get the hydroxide precipitate at 298-303 K; [0082] iii. aging the obtained slurry at step ii) at 353 K for 10 mins; [0083] iv. cooling the slurry obtained at step iii) to 298-303 K and washing with water and ethanol; [0084] v. drying the obtained mass at step iv) at 383 K for 12 hours and calcining at 723 K for 3 hours to afford the catalyst.

[0085] The present invention provides a process for the selective production of CO from CO.sub.2. The process comprises of reducing CO.sub.2 at atmospheric pressure in RWGS reaction by using catalyst (Co.sub.3O.sub.4 NC or In.sub.2O.sub.3) in a fixed bed catalyst reactor at a temperature in the range of 373 K to 923 K with constant gas hourly space velocity (GHSV) in the range of 15000-17000 h.sup.1, wherein CO.sub.2:H.sub.2 ratio is in the range of 1:0.67-1:7.

[0086] In another embodiment, the present invention relates to a process for the selective production of CO from CO.sub.2 comprising the steps of: [0087] a) pre-heating a catalyst as claimed in any one of the claims 1 to 5 in air at 723 K for 3 h at a ramping rate of 5 K.Math.min.sup.1; [0088] b) loading the catalyst to a fixed bed catalyst reactor and feeding CO.sub.2:H.sub.2 gas mixture using two different mass flow controllers; [0089] c) reducing CO.sub.2 at atmospheric pressure in reverse water gas shift (RWGS) reaction in the fixed bed catalyst reactor at a temperature in the range of 373 K to 923 K with constant gas hourly space velocity (GHSV) to obtain CO gas; and [0090] d) treating the CO gas obtained in step (d) to convert metal oxide(s)/metal ore(s) to a reduced metal.

[0091] In another embodiment of the present invention, the constant gas hourly space velocity (GHSV) used in the process of the selective production of CO is in a range of 15000-192000 h.sup.1.

[0092] In another embodiment of the present invention, the CO.sub.2:H.sub.2 ratio used in the process of the selective production of CO is in the range of 1:0.67-5:3.

[0093] In another embodiment of the present invention, the metal oxide(s)/metal ore(s) comprises iron metal oxides or iron metal ores, cobalt oxides, manganese oxides and so on.

[0094] In another embodiment of the present invention, the treating step (d) is done at 10% H.sub.2. 10% CO or 5% H.sub.2+5% CO under inert conditions (N.sub.2, He, Ne, etc.) at a heating rate of 5 K/min, attaining the temperature up to from around 530 K to 900 K to obtain reduced metal. In preferred embodiment, the metal oxide(s)/metal ore(s) reduction starts from around 530 K and completes the reduction at 900 K, more preferably, the reduction completes at 650 K, 673 K or 900 K.

Characterization of Co.sub.3O.sub.4 Nano-Cube

[0095] The XRD analysis of the fresh and spent catalysts (reaction performed at 773 K with 3:2 CO.sub.2:H.sub.2 for 12 h) sample shown in FIG. 1. The XRD pattern of fresh catalyst shows different features (FIG. 1a) at 20=19.3, 31.5, 37, 38.8, 45, 55.8, 59.5, 65.4 correspond to (111), (220), (311), (222), (400), (422), (511), (400) crystal facets of CO.sub.3O.sub.4 respectively. The XRD patter observed in FIG. 1a is identical to those reported in the literature (JCPDS 65-3103), supporting the catalyst is cubic (spinel) in nature. However, after the reaction at 773 K, some new facets have appeared along with few of the originally observed crystallographic facets. The intensity of the (400) facet increased and appeared as parent peak; further, peaks at 47.91, 52.08, 76.3 correspond to (101), (200), and (110) crystal facets were observed in FIG. 1 (right). Indeed, this observation suggests a restructuring of the surface and high intensity (400) facet and the growth of moderate (200) facet suggests a surface with step like structure on the surface. Above observed restructuring is retained, even after repeated reactions for at least six cycles of the reaction results shown in FIG. 3 for 1:0.67 ratio of CO.sub.2:H.sub.2. This observation also reiterates that the virgin (or as prepared) nanocube, which contains predominant (100) facet undergoes restructuring under the reaction conditions to the above found facets, which forms the basis for the active phase of the catalyst. This observation supports the recyclability and sustainability of the catalyst for several cycles.

[0096] The particle size and morphology of the as-synthesized nano-crystal are identified by Transmission electron microscopy (TEM). FIG. 2 (a, b and c) reveals nano-cube (NC) morphology and facets of the Co: 04 sample. The as-prepared NC possesses particle size in the range of 18 and 35 nm, while the cubic morphology remains observed. Selected area electron diffraction result shown in FIG. 2 (c) demonstrates the crystalline nature the Co.sub.3O.sub.4 NCs, which is in good agreement with spectra of the sample. The TEM image demonstrates the growth of nano-cube with 0.25 nm and 0.45 nm d-spacing value obtained along (222) and (111) facet respectively, which is shown in XRD of the sample. TEM image shown for spent catalyst in FIG. 2d shows change in morphology from perfect cubic to near cubic and/or spherical shape, while the particle size remains in the range of 18-35 nm. In fact, it is to be noted that the spent catalyst results shown for XRD in FIG. 1 and the TEM in FIG. 2d are the active catalyst. Even after repeated cycling of catalyst for CO.sub.2 reduction, no further change in the morphology or particle size was observed. This demonstrated the sustainability of the catalyst with same activity for several cycles or for long hours.

Analysis of CO.sub.2 Reduction with H.sub.2 Over Co.sub.3O.sub.4 Nano-Cube

[0097] CO.sub.2 reduction with H.sub.2, which is also known as reverse water gas shift reaction (RWGS), is carried out in a fix bed catalytic reactor at atmospheric pressure with spinel Co: 04 (nanocube) and temperature between 100 to 823 K with different CO.sub.2:H.sub.2 ratios (1:0.67 to 1:5) at gas hourly space velocity of 17000 h.sup.1. The catalyst sample (1 cm.sup.3) retained between the plug of quartz wool and ceramics bead. The results obtained from the reactor are shown in FIG. 3 for three CO.sub.2:H.sub.2 ratios, namely 1:0.67, 1:1 and 1:3. In the RWGS reaction CO is a desired product as it can be used directly in Fischer-Tropsch (FT) reaction, iron-ore reduction to metallic iron, and many metal making processes. Though, the methane is an undesired product in FT, it is not an issue for iron-ore reduction. Gaseous products from the outlet of the fixed bed reactor are analyzed by using Gas chromatography (GC) with both FID and TCD detectors. The CO formation is observed to be increasing with increasing reaction temperature from 523 K and above with all ratios. Maximum conversion of CO.sub.2 and H.sub.2 is observed around 64 and 70%, respectively, with 1:3 ratio of CO.sub.2:H.sub.2 at 823 K on spinel Co: 04. CH.sub.4 shows 100% selectivity upto 673 K, and then it decreases with increase in CO selectivity above 673 K. Although CO.sub.2 conversion is observed to be 25-35% for 1:0.67 and 1:1 CO.sub.2:H.sub.2 ratios above 773 K, CO selectivity is observed to be more than 94%. Indeed 100% CO selectivity was observed with 1:0.67 ratio above 773 and up to 823 K. It is to be noted that CO.sub.2 conversion decreases marginally to 22% above 823 K, CO selectivity remains observed to be 100%. CO.sub.2 conversion increases linearly with temperature with CO.sub.2-rich compositions, at least up to 923 K and a marginal decrease is observed above 823 K. Hydrogen conversion also decreases above 823 K.

[0098] In another variation, Co: 04 nano-cube calcined at 573 K under pure oxygen at 573 K for 24 h and then the reaction was performed with four different CO.sub.2:H.sub.2 ratio, namely 1:0.67, 1:1, 1:2, and 1:3. The maximum CO.sub.2 conversion was observed at 723 K for any CO.sub.2:H.sub.2 ratio. In contrast to the results shown in FIG. 3, H.sub.2 consumption decreases from 623 K and above for all CO.sub.2:H.sub.2 compositions. CH.sub.4 shows more than 90% selectivity below 673 K with CO.sub.2:H.sub.2=1:3 ratio. Critically, 100% CO selectivity (with no methane formation) was observed above 673 K with CO.sub.2:H.sub.2=1:0.67; 1:1 ratio also shows more than 90% CO selectivity from 723 K and above. 20-25% CO.sub.2 conversion and high CO selectivity observed for CO.sub.2:H.sub.2=1:0.67 above 623 K demonstrates its superior performance over the results shown in FIG. 3. The CO.sub.2 and H.sub.2 conversion and products selectivity is shown in FIG. 5.

[0099] Indeed the reactivity observed with H.sub.2-lean compositions with exclusive CO selelctivity 673 K is the most favorable condition for iron ore reduction to metallic iron, as there is no methane in the product. Further, smaller amount of hydrogen and large amount of CO.sub.2 employed in the input feed is very attractive from the economical point as well as reducing the carbon footprint. H.sub.2 being an expensive fuel, using smaller amount of the same for exclusive CO production is very attractive for commercial applications. Indeed, either the product stream (CO along with unspent H.sub.2+CO.sub.2) may be used as such for iron ore reduction; rather unspent CO.sub.2 and/or H.sub.2 may be recycled to produce CO in the subsequent cycles. Later step comes at a cost of separation of CO.sub.2 and/or H.sub.2 from the product stream, but leading to higher aggregate of CO.sub.2 utilisation.

Time on Stream

[0100] To evaluate the sustainability of the air calcined Co: 04 NC material for the reaction, time on stream study (TOS) study was performed for the CO.sub.2 reduction reaction with 1:5 and 1:0.67 ratio of CO.sub.2:H.sub.2 at temperature 673 and 723 K for 24 and 12 h, respectively, and the result obtained are shown in FIGS. 4a and b. TOS studies has been carried out with hydrogen rich and lean compositions to measure the sustainability of the catalyst. With 1:5 ratio, CO.sub.2 conversion drops marginally from 62% to 57% in the initial hours and then exhibit stable conversion. Similarly, H.sub.2 conversion decreases from 57% in the initial hours to 48%; nonetheless, methane is the only product produced selectively under this conditions for 24 h. However, with 1:0.67 ratio of CO.sub.2:H.sub.2, CO production was observed with >90% selectivity and the remaining was methane. A marginal reduction on CO.sub.2 and H.sub.2 conversion was observed in in the first 4-5 hrs. and thereafter stable activity was observed (FIG. 4b). From the time on stream study with different ratios, it is clear that Co.sub.3O.sub.4 nano-cube catalyst is highly active and stable under CO.sub.2 reduction at least up to 24 h.

Characterization of In.sub.2O.sub.3 Catalyst

[0101] XRD analyses of fresh and spent In.sub.2O.sub.3 catalysts are carried out to understand the bulk structure of the catalyst and the impact of CO.sub.2 reduction reaction on it. XRD results are shown in FIG. 6a. Fresh In.sub.2O.sub.3 shows several diffraction features and all of them are assigned. Diffraction from (211), (222), (400), (411), (332), (431), (440), (611) and (622) facets are observed at 2 value of 21.7, 30.76, 35.51, 38.00, 41.92, 45.43, 51.05, 56.03 and 60.74, respectively. Diffraction pattern matches very well with the cubic crystalline phase of In.sub.2O.sub.3 (JCPDS file no. 71-9529). XRD pattern of spent In.sub.2O.sub.3 catalysts, collected after carrying out the CO.sub.2 reduction reaction with 1:0.67 and 1:3 ratio of CO.sub.2:H.sub.2 at 773 K for 12 h, are recorded and the results are shown in FIG. 6a. Both the spent catalysts, (spent In.sub.2O.sub.3 1:0.67 and spent In.sub.2O.sub.3 1:3) also show In.sub.2O.sub.3, and suggesting that there is no change in crystalline structure. However, diffraction features are narrower for the spent catalysts, compared to that of fresh one, indicating a possible growth of crystallites due to reaction. To confirm the growth of crystallites, average crystallites size was calculated by using the Scherrer equation. For fresh In.sub.2O.sub.3 sample, it was found to be 10.8 nm, however for spent In.sub.2O.sub.3 1:0.67 and 1:3 catalysts, it was measured to be 19.6 and 18.3 nm, respectively. Preserving the cubic phase confirms the catalyst stability under the working reaction conditions. It is to be reiterated that after the above change observed in crystallite size, for subsequent reactions carried out on the same spent catalyst with any CO.sub.2:H.sub.2 reaction did not make any significant change in the XRD pattern or crystallite size. Intermittent air calcination at 623 K for 30 min was carried out, when the CO.sub.2:H.sub.2 ratio changes. This observation supports the recyclability and sustainability of the catalyst for several cycles, and any carbon deposition can be removed by intermittent air-calcination. However, no peak is observed for the metallic indium, hinting that catalyst did not undergo any reduction under the present reaction conditions employed for activity evaluation. H.sub.2-TPR study is carried out to understand the reducibility of the In.sub.2O.sub.3 catalyst and the result is shown in FIG. 6b. H.sub.2-TPR studies show a major and sharp reduction at 460 K. The onset of the major reduction begins at 420 K and ends at 475 K. Nonetheless, low intensity and a broad reduction feature continued up to 670 K. The sharp reduction feature observed at 460 K corresponds to the oxygen vacancy formation i.e., partial reduction of the surface sites. However, the low-intensity broad feature observed is attributed to the onset of bulk of oxygen vacancies. Under the present measurement conditions, no bulk reduction of In.sub.2O.sub.3 is observed.

[0102] HRTEM analysis is carried out for both the fresh as well as spent catalysts, and the results are shown in FIG. 6 (c and d). For the fresh In.sub.2O.sub.3 catalyst, lattice fringes for the two different planes, namely (222) and (211), are observed with lattice d-spacing values of d=0.29 nm and d=0.41 nm respectively. Same crystallographic facets ((222) and (211)), as that of fresh In.sub.2O.sub.3, are observed on spent catalyst also (FIG. 6d). Compared to fresh In.sub.2O.sub.3, an Increase in the particle size was observed for both spent catalysts. For fresh In.sub.2O.sub.3, average particle size was 8.5 nm; however, for spent 1:0.67 and 1:3 In.sub.2O.sub.3 catalysts, particle size was observed to be 26.7 and 22.2 nm, respectively. Particle size observed for spent catalyst was retained even after several cycles and no further change in particle size was observed. This reiterates the robust character of the catalyst.

Analysis of CO.sub.2 Reduction with H.sub.2 Over In.sub.2O.sub.3 Catalyst

[0103] Various CO.sub.2:H.sub.2 ratios employed are from 1:0.67 to 1:7, ranging from lower than the stoichiometric amount to excess amount of hydrogen by using In.sub.2O.sub.3 catalyst. In a typical CO.sub.2 reduction reaction, apart from water and CO, CH.sub.4 formation also occurs. Methane is not the desired product, due to several reasons, such as the high cost of production, transportation issues. It is well-known that one mole of methane formation from CO.sub.2 requires four moles of hydrogen gas, which makes it a costly process (CO.sub.2+4H.sub.2.fwdarw.CH.sub.4+2H.sub.2O). Global warming potential is 84 and 72 for methane and CO.sub.2, respectively, and hence the former traps the heat effectively and contributes more to global warming. Thus, the production of methane in CO.sub.2 reduction should be minimized.

[0104] FIG. 7a shows the catalytic conversion of CO.sub.2 with H.sub.2 and the product selectivity of CO and CH.sub.4 due to CO.sub.2 reduction using In.sub.2O.sub.3 catalyst as a function of temperature. Irrespective of the ratio of the reactants, there is no CO.sub.2 conversion below 573 K is observed. However, with an increase in the reaction temperature, the CO.sub.2 conversion also increases linearly from 573 to 973 K. Some of the salient features worth underscoring are listed below: (a) The maximum CO.sub.2 conversion is observed at 873 K for all the reactants ratios. The maximum CO.sub.2 conversion of 72% is observed at 873 K with a 1:7 ratio, which is higher than any other ratio at the same reaction temperature. (b) Between 773 and 873 K, 1:0.67 and 1:1 ratios show a similar CO.sub.2 conversion with the maximum conversion at 37% at 873 K. (c) Surprisingly, less than stoichiometric (1:0.67) ratio, assuming strict CO.sub.2 reduction reaction, shows the promise of a possible best catalytic activity. Indeed, this is supported by high CO selectivity. (d) Likely the high CO.sub.2 contact time possible with 1:0.67 (and 1:1) CO.sub.2:H.sub.2 ratio helps for the same conversion as that of 1:1 ratio.

[0105] FIG. 7b shows the H.sub.2 conversion data for the CO.sub.2 reduction reaction by using In.sub.2O.sub.3 catalyst. Like CO.sub.2, H.sub.2 conversion also shows a linear increase with increasing temperature; however, H.sub.2 conversion decreases with the increase in the H.sub.2 amount in the reactant ratio. Maximum H.sub.2 conversion (51%) is observed with a 1:0.67 reactants ratio at 873 K. 573 and 623 K data shows comparable H.sub.2 conversion for any ratio employed. Results show that CO.sub.2 conversion increases with an increase in the H.sub.2 in the reactant feedstock and the reaction temperature. This increase in the CO.sub.2 conversion can be correlated with the generation of more active sites over the In.sub.2O.sub.3 surface with more H.sub.2 in the reactant ratio. The ratio of conversion of CO.sub.2:H.sub.2 is 2:3 for 1:0.67 reactants ratio, between 723-873 K, underscoring a possible dynamic change on the catalyst surface.

[0106] FIGS. 7c and 7d show the catalytic selectivity data for CO and CH.sub.4, respectively. The CO selectivity shows an increase with the reaction temperature for all reactants ratio; however, interestingly, a decrease in the H.sub.2 content in the reactants leads to an increase in the CO selectivity. The maximum CO selectivity (98%) was observed with 1:0.67 ratio at 873 K and higher temperatures. An increase in the reaction temperature leads to a decrease in methane formation due to the high desorption rate of hydrogen, which prevents hydrogenation of carbon. Selectivity value and trend for both products show comparable for 1:0.67 and 1:1, and 1:3 and 1:5; indeed, CO.sub.2 conversion also shows a similar trend. Very high (low) selectivity for CO (CH.sub.4) with 1:0.67 also suggests removing the oxygen atom of CO.sub.2 in the form of water rather than methane formation. Here it is to be noted that all the temperature dependent reaction study were carried out over the same catalyst. After every reaction, the catalyst is treated in air for 3 h at 773 K and used for new reaction. 100% CO selectivity is observed up to 873 K, but with a marginal decrease in CO.sub.2 and hydrogen conversion.

Time on Stream

[0107] In.sub.2O.sub.3 catalyst surface with a CO.sub.2:H.sub.2=1:0.67 and 1:3 ratio shows the economically attractive catalytic activity and CO.sub.2 reduction to CO without and with methane, respectively. Time on stream (ToS) studies is carried out for these two ratios at 773 K for 12 h, and the results obtained are shown in FIGS. 8a and 8b; this is specially to understand the stability aspects of the catalyst. For CO.sub.2:H.sub.2=1:0.67 ratio, stable CO.sub.2 and H.sub.2 conversion at 24% and 37%, respectively, is observed; under the above reaction conditions, CO and CH.sub.4 selectivity are observed to be 98% and 2%, respectively. Same studies with 1:3 reactants ratio show a CO.sub.2 and H.sub.2 conversion of 49% and 32%, respectively; nonetheless, CO and CH.sub.4 selectivity is observed to be 87% and 13%, respectively. Interestingly, the catalyst shows sustainable activity and selectivity for the entire reaction period and for both reactant's ratio. From the present ToS study, it is clear that the In.sub.2O.sub.3 catalyst is highly active and stable under CO.sub.2 reduction reaction conditions. Sustainable activity with H.sub.2 rich feeds also demonstrates the catalyst's resilient nature and not vulnerable to a total reduction of the catalyst.

Valence Band (VB) Spectral Measurements on Co.sub.3O.sub.4 and In.sub.2O.sub.3 Under CO.sub.2 Reduction Conditions

[0108] VB measurement was carried out in a near-ambient pressure photoelectron spectrometer (NAPPES) with He I radiation on In.sub.2O.sub.3 in the presence of 1:0.67 CO.sub.2:H.sub.2 mixture at a total pressure of 0.1 mbar, and the result is shown in FIG. 9. Spectral measurements are shown for 295, and 773 K. Interesting results are observed. The following points are worth underscoring: (i) Entire VB broadened up to 0.6 eV towards Fermi level at high temperatures (773 K), and highlighting a possible electron filling of VB; it is to be noted that a similar VB shift was observed on reduction of ceria [R. Jain, A. J. Dubey, M. K. Ghosalya, C. S. Gopinath, Gas-Solid Interaction of H.sub.2Ce.sub.0.95Zr.sub.0.05O.sub.2: New Insights on Surface Participation in Heterogeneous Catalysis, Catalysis Science and Technology 6, 1746-1756 (2016)] and this lead to a change in work function too; (ii) First and low BE VB feature gains in intensity at 773 K and above and shift to low BE by 0.7 eV, at the expense of second VB feature; (iii) VB features reverts to the spectral pattern observed at 295 K on cooling in reaction atmosphere. Last point suggests the changes observed with VB are fully reversible, and it can be observed exclusively with in-situ spectral methods; no post-reaction analysis would reveal these changes. CO.sub.2 vibrational features observed around 9 and 13 eV exhibits shifts to higher BE by 0.5 eV underscoring the nature of the catalyst surface is very different under the reaction conditions.

[0109] NAPPES recorded on fresh Co.sub.3O.sub.4 (without any treatment) with 1:0.67 CO.sub.2:H.sub.2 ratio as a function of temperature, and the results are shown for 375 and 675 K in FIG. 10. Gas-phase CO.sub.2 is also plotted for reference. Main VB shifts by 0.35 eV from 1.2 eV at 375 K to 0.85 eV at 675 K. CO.sub.2 vibrational features also shift by 0.45 eV from 375 to 675 K. Further, vibration features also broadens at high temperature, indicating a possible heterogeneity of the surface, possibly due to oxygen vacancies created. However, overall, Co.sub.3O.sub.4 feature is maintained even under reaction conditions, underscoring the spinel phase is the active phase. Corresponding Co 2p core level spectra exhibited typical 2:1 ratio for Co(III):Co(II) oxidation states. However, the VB shift under measurement conditions is attributed to a dynamic oxygen vacancy creation due to reaction atmosphere.

[0110] Several iron oxides and mixtures thereof were employed as starting material to reduce with H.sub.2, CO and in a 1:1 mixture of H.sub.2+CO. Up to 20% of H.sub.2, CO or 1:1 H.sub.2:CO and the rest as N.sub.2 (as carrier gas) was employed for reduction of iron oxide(s). These experiments were carried out in a typical temperature programmed reduction (TPR) unit with 10-100 mg iron oxides. It is generally found that the reduction starts from around 523 K and up to 973 K. Various factors affects the reduction, such as nature of reductant(s), ramping rate of reduction. Employment of 100% reduction agent would facilitate the reduction at lower temperatures. Advantage of the present CO.sub.2 reduction selectively to CO, along with unspent H.sub.2 can be the real input reduction agent for iron oxide reduction to iron. Additionally, the product mixture is at the reaction temperature (773-823 K), which is exactly required for iron oxide reduction. This is likely to save significant amount of energy in the iron oxide reduction.

[0111] FIG. 11 depicts the reactor system for CO.sub.2 hydrogenation and its application for Iron ore reduction.

EXAMPLES

[0112] Following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention.

Example 1: Synthesis of Co.SUB.3.O.SUB.4 .Nano-Cube

[0113] 2.5 mmol of Co(OAC).sub.2.Math.4H.sub.2O was dissolved in 125 ml water and stirred the solution at room temperature for 5 min. The aqueous ammonia was added dropwise to the cobalt precursor solution to achieve pH=9 and the solution turns blue. Afterwards, the solution obtained was further stirred for 30 mins and transferred into a 200 ml autoclave with Teflon liner and kept it at 453 K for 10 h. The resulting solution was filtered and washed with water multiple times. Finally, the sample was calcined in air at 623 K for 3 h, and Co.sub.3O.sub.4 nano-cubes were formed.

Example 2: Synthesis of In.SUB.2.O.SUB.3 .Catalyst

[0114] Initially, the indium hydroxide was prepared by dissolving 3.05 g of Indium nitrate In(NO.sub.3).sub.3.Math.5H.sub.2O (99.99% Sigma Aldrich) in a mixture of deionized water (12 ml) and ethanol (35 ml). The ammonia solution (9 ml of 25 wt. % in H.sub.2O) in ethanol (27 ml) was added drop-wise under stirring conditions to get the hydroxide precipitate at 298 K. The slurry obtained was kept for aging at 353 K for 10 mins. After the aging, the slurry was kept for cooling to 298 K, washed with the water and ethanol, followed by drying at 383 K for 12 h. The dried powder was calcined at 723 K for 3 h to afford the catalyst.

Example 3: General Process for the CO.SUB.2 .Reduction in Fixed Bed Catalyst Reactor

[0115] CO.sub.2 reduction was performed at atmospheric pressure in RWGS reaction by using catalyst (Co.sub.3O.sub.4 nano-cube or In.sub.2O.sub.3) in a fixed bed catalyst reactor at a temperature in the range of 373 K to 823 K with constant gas hourly space velocity (GHSV) in the range of 15000-19200 h.sup.1, wherein CO.sub.2:H.sub.2 ratio is in the range of 1:0.67-1:3. The catalyst performance was tested with a continuous flow fixed bed reactor. 1 cm.sup.3 of the catalyst was loaded in the uniform heating zone of the tubular reactor. Before the reaction, the catalyst was pre-treated in the air at 723 K for 3 h at a ramping rate of 5 K.Math.min.sup.1. The CO.sub.2:H.sub.2 gas mixture was fed to the reactor using two different mass flow controllers. The temperature was set to the desired reaction temperature and it was measured with a K-type thermocouple placed at the center of the catalyst bed in the reactor tube. About 30 minutes was allowed to stabilize the reaction temperature as well as to reach the steady state, before any reaction measurement/GC analysis of the products. The gas products were analyzed using the online GC (Model: Trace 1110; Thermo scientific).

Example 4:02 Calcined Co.SUB.3.O.SUB.4 .Nano-Cube Used for the CO.SUB.2 .Hydrogenation Reaction in Fixed Bed Catalyst Reactor

[0116] CO.sub.2 reduction with H.sub.2 was performed at atmospheric pressure by using Co.sub.3O.sub.4 nano-cube calcined under 02 at 573 K. RWGS was carried out in a fixed bed reactor between 523 and 823 K at a constant gas hourly space velocity 19200 h.sup.1 wherein CO.sub.2:H.sub.2 ratio was maintained in the range of 1:0.67-1:3.

[0117] The catalyst was tested with a continuous flow fixed bed reactor with 1 cm.sup.3 of the catalyst. Catalyst evaluation was carried out, as described in example 3.

Example 5: Reduction of Iron Oxides to Metallic Iron in H.SUB.2

[0118] FeO, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4 and 1:1:1 mixture thereof oxides were reduced in TPR setup with 10% H.sub.2 in N.sub.2 at a heating rate of 5 K/min. It is generally found that the reduction starts from around 500 K and complete reduction to metallic iron was observed between 673 and 973 K.

Example 6: Reduction of Iron Oxides to Metallic Iron in CO

[0119] FeO, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4 and 1:1:1 mixture thereof oxides were reduced in TPR setup with (a) 10% CO in N.sub.2 at a heating rate of 5 K/min. It is found that the reduction starts from around 550 K and complete reduction to metallic iron was observed between 650 and 900 K.

Example 7: Reduction of Iron Oxides to Metallic Iron in CO:H.SUB.2.1:1 Mixture

[0120] FeO, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4 and 1:1:1 mixture thereof oxides were reduced in TPR setup with 5% H.sub.2+5% CO in N.sub.2 at a heating rate of 5 K/min. Iron oxide reduction starts from around 530 K and complete reduction to metallic iron was observed between 673 and 900 K.

Example 8: Comparative Study of Co.SUB.3.O.SUB.4 .Nanocube Particles

Reaction Condition: Temperature: 450 C.; Pressure: 1 Atm; Reactant Molar Ratio CO.sub.2:H.sub.2=5:3

TABLE-US-00001 Catalyst Name CO.sub.2 conversion Surface area (m.sup.2g.sup.1) Co.sub.3O.sub.4 Nano Cube 26.2% 26.1 Co.sub.3O.sub.4 Nano-rod 23.7% 23.5 Co.sub.3O.sub.4 Nano -sphere 22.1% 22.2
Reaction Condition: Temperature: 450 C.; Pressure: 1 atm; Reactant Molar ratio CO.sub.2:H.sub.2=1:3

TABLE-US-00002 Catalyst Name CO.sub.2 conversion Surface area (m2g1) Co.sub.3O.sub.4 Nano Cube 49.4% 26.1 Co.sub.3O.sub.4 Nano-rod 40% 23.5 Co.sub.3O.sub.4 sphere 38% 22.2

Advantages of the Invention

[0121] The conversion of CO.sub.2 activation reaction to highly selective CO, and exclusive CO production at 673 K and above, with lower than stoichiometric amount of hydrogen (CO.sub.2:H.sub.2=1:0.67) is achieved. This is a unique and commercially important aspect for exploitation of CO.sub.2 to value added CO, to be employed for many different applications. [0122] Low temperature and ambient pressure activation of CO.sub.2 with wide range of CO.sub.2:H.sub.2 ratios and catalysts. [0123] Byproduct of this reaction, water, will help carbon/Coke gasification reaction, which in turn will help the reduction reaction [0124] CO.sub.2 footprint can be minimized to a large extent by using this process. Present process also contributes to carbon-neutral economy through FT synthesis process to value added chemicals. [0125] Catalyst composition with dynamic oxygen vacancy formation under the reaction conditions for the conversion of CO.sub.2 to CO is novel, Scalable, and cost-effective [0126] Nanocrystalline catalyst with well-controlled cell parameters are responsible for the selective CO formation with lower activation energy requirements. [0127] Simple and novel process flow scheme and reactor system for the continuous use and recycle of CO.sub.2.