System and method for co 2 utilization and applications of same

Abstract

This invention discloses system and method for CO2 utilization. The system comprises a gas condition; and an oxygen carrier containing iron (Fe) and tungsten (W), exposed to the gas condition for an appropriate period of time under an appropriate temperature.

Description

[0001] SYSTEM AND METHOD FOR CO2 UTILIZATION AND APPLICATIONS OF SAME

[0002] STATEMENT AS TO RIGHTS UNDER FEDERALLY-SPONSORED RESEARCH

[0003] This invention was made with government support under grant number CMMI-2015641 awarded by the National Science Foundation. The government has certain rights in the invention.

[0004] CROSS-REFERENCE TO RELATED PATENT APPLICATION

[0005] This application claims priority to and the benefit of U.S. Provisional Patent Application Serial No. 63 / 634,557, filed April 16, 2024, which is incorporated herein in its entirety by reference.

[0006] FIELD OF THE INVENTION

[0007] The present invention generally relates to the material science, particularly to system and method for CO2 utilization and applications of the same.

[0008] BACKGROUND OF THE INVENTION

[0009] The background description provided herein is to present the context of the invention generally. The subject matter discussed in the background of the invention section should not be assumed to be prior art merely due to its mention in the background of the invention section. Similarly, a problem mentioned in the background of the invention section or associated with the subject matter of the background of the invention section should not be assumed to have been previously recognized in the prior art. The subject matter in the background of the invention section merely represents different approaches, which in and of themselves may also be inventions. Work of the presently named inventors, to the extent it is described in the background of the invention section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the invention.

[0010] The global goal to limit CO2 emissions requires carbon capture, utilization, and storage technologies to both limit CO2 emissions from transitional energy technologies and support CO2 producing chemical reactions, such as concrete production. Carbon utilization is the process of converting CO2 into a more valuable product, often CO or other organic molecule. In this way many processes can be rendered carbon neutral, with all CO2 emissions being upcycled into alternate products.

[0011] One carbon utilization process of interest is the use of inorganic materials as oxygen carriers to be used in chemical looping: CO2 emissions are routed through a bed of oxygen carrier powder at high temperature, where the CO2 oxidizes the oxygen carrier, producing CO. The oxygen carrier is then reduced back to its original state with hydrogen H2 (or any H carrier such as CH^rNHs). Due to its low cost, reversible redox cycling capability, and non-toxicity, Fe is viewed as an ideal oxygen carrier material. The oxidation of Fe by CO2 (and subsequent reduction of iron oxide with H2) require relatively high temperatures to be practically useful, 550 °C and above. This technology has been held back by the fast sintering and densification of iron and iron oxide powder beds at these elevated temperatures, the same mechanism that occurs with H2O / H2 cycling of Fe. This sintering and densification process causes subsequent cycles to be much more sluggish due to poor gas access to the densified areas.

[0012] Several efforts have been made to remedy the sintering and densification of Fe beds at high temperature. Use of inert additives such as AI2O3 and SiCh produce strong results, with the supporting oxides physically preventing sintering between nearby Fe particles. Fluidized beds produce a similar result, with the unique environmental conditions limiting the contact and sintering of particles. Sintering and densification can still pose a challenge after many cycles, however. An additive that not only slows sintering and densification, but cyclically regenerates porosity has the potential to transform the process.

[0013] Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.

[0014] SUMMARY OF THE INVENTION

[0015] In one aspect, this invention discloses a system for CO2 utilization, comprising a gas condition; and an oxygen carrier containing iron (Fe) and tungsten (W), exposed to the gas condition for an appropriate period of time under an appropriate temperature.

[0016] In one embodiment, the gas condition contains a gas that is still or flowing at an appropriate flow rate.

[0017] In one embodiment, the gas condition is alternatively switchable between a CO2 condition in which oxidation occurs and an H2 condition in which reduction occurs.

[0018] In one embodiment, the CO2 condition contains CO2 or a CCh-containing gas blend, and the H2 condition contains H2 or an FF-containing gas blend. In one embodiment, the C Ch-containing gas blend includes CO2 and another gas including N2, CO, and / or like.

[0019] In one embodiment, the CO2-containing gas blend contains 66% CO and 34% CO2.

[0020] In one embodiment, the CO2-containing gas blend contains CO2 or CO-CO2 mixed with an inert gas including N2 or Ar.

[0021] In one embodiment, the H2-containing gas blend includes H2 and another gas including N2, CO, Ar, and / or like.

[0022] In one embodiment, the H2-containing gas blend further includes certain leftover CO2.

[0023] In one embodiment, the H2-containing gas blend further includes an active gas that helps reducing, such as CO, CH4, NH3.

[0024] In one embodiment, the oxygen carrier is cyclable between the oxidation and the reduction hundreds of times.

[0025] In one embodiment, when the gas condition is in the CO2 condition, the appropriate period of time is an oxidizing period, and the appropriate temperature is an oxidizing temperature; and when the gas condition is in the H2 condition, the appropriate period of time is a reducing period, and the appropriate temperature is a reducing temperature.

[0026] In one embodiment, in different oxidation-reduction (redox) cycles, the oxidizing period is same or different, and the oxidizing temperature is same or different; and the reducing period is same or different, and the reducing temperature is same or different.

[0027] In one embodiment, the oxygen carrier contains a mixture of F 626)3 and WO3 precursors, in a powder form.

[0028] In one embodiment, the oxygen carrier contains a Fe-W alloy with about 5-50 at%, with sub-ranges of 10-33, 18-30, and 20-25 at%, of W in the Fe-W alloy.

[0029] In one embodiment, the oxygen carrier contains a porous Fe-25W(at%) alloy, which is packed powder beds or foams.

[0030] In one embodiment, the Fe-25W alloy has hierarchical porosity: a continuous microporous network that does not sinter due to the sintering inhibition of W, and submicron pores formed by the chemical vapor transport during the reduction period of FeWC .

[0031] In one embodiment, during redox cycling at about 550-1100 °C, the oxygen carrier is reversibly oxidized by CO2 and reduced by H2 with minor or no densification.

[0032] In one embodiment, the system has a carbon monoxide (CO) production of about 19 mol CO / kg Fe-25W. In one embodiment, a rate of reaction, effectively the CO2 utilization rate, increases with cycle number due to formation and expansion of porosity in the oxygen carrier.

[0033] In one embodiment, the oxygen carrier converts CO2 to CO, without degrading due to the microstructure and phase evolution of the oxygen carrier.

[0034] In one embodiment, under a desired flow rate of CO2, the oxygen carrier is oxidized to a combination of FeW04 and Fe20.3 rather than FeW04 and Fe.304.

[0035] In one embodiment, the oxygen carrier is oxidized to Fe20s under CO2.

[0036] In another aspect, this invention relates to a method for CO2 utilization, comprising: providing an oxygen carrier containing iron (Fe) and tungsten (W); and exposing the oxygen carrier to a gas condition for an appropriate period of time under an appropriate temperature, to convert, fully or partially, CO2 and carbon to carbon monoxide (CO).

[0037] In one embodiment, the gas condition contains a gas that is still or flowing at an appropriate flow rate.

[0038] In one embodiment, said exposing the oxygen carrier to the gas condition comprises alternatively switching the gas condition between a CO2 condition in which oxidation occurs and an H2 condition in which reduction occurs.

[0039] In one embodiment, the CO2 condition contains CO2 or a CCh-containing gas blend, and the H2 condition contains H2 or an H2-containing gas blend.

[0040] In one embodiment, the CCh-containing gas blend includes CO2 and another gas including N2, CO, and / or like.

[0041] In one embodiment, the CO2-containing gas blend contains 66% CO and 34% CO2.

[0042] In one embodiment, the CO2-containing gas blend contains CO2 or CO-CO2 mixed with an inert gas including N2 or Ar.

[0043] In one embodiment, the H2-containing gas blend includes H2 and another gas including N2, CO, Ar, and / or like.

[0044] In one embodiment, the Fh-containing gas blend further includes certain leftover CO2.

[0045] In one embodiment, the Ft-containing gas blend further includes an active gas that helps reducing, such as CO, CH4, NH3.

[0046] In one embodiment, the oxygen carrier is cyclable between the oxidation and the reduction hundreds of times.

[0047] In one embodiment, when the gas condition is in the CO2 condition, the appropriate period of time is an oxidizing period, and the appropriate temperature is an oxidizing temperature; and when the gas condition is in the H2 condition, the appropriate period of time is a reducing period, and the appropriate temperature is a reducing temperature.

[0048] In one embodiment, in different oxidation-reduction (redox) cycles, the oxidizing period is same or different, and the oxidizing temperature is same or different; and the reducing period is same or different, and the reducing temperature is same or different.

[0049] In one embodiment, the oxygen carrier contains a mixture of FC2O3 and WO3 precursors.

[0050] In one embodiment, the mixture of Fe20s and WO3 precursors is produced by preparing a slurry of the Fe O3 and WO3 powders in a solvent, which is then ball milled for 24 hours to mix the powders and then dried, wherein the dried powders are reduced at high temperature to produce a metallic powder bed which is unsintered or partially sintered.

[0051] In one embodiment, the oxygen carrier contains a Fe-W alloy with about 5-50 at%, with sub-ranges of 10-33, 18-30, and 20-25 at%, of W in the Fe-W alloy.

[0052] In one embodiment, the oxygen carrier contains a porous Fe-25W (at%) alloy, which is packed powder beds or foams.

[0053] In one embodiment, the Fe-25W alloy has hierarchical porosity: a continuous microporous network that does not sinter due to the sintering inhibition of W, and submicron pores formed by the chemical vapor transport during the reduction period of FeWC .

[0054] In one embodiment, during redox cycling at about 550-1100 °C, the oxygen carrier is reversibly oxidized by CO2 and reduced by H2 with minor or no densification.

[0055] In one embodiment, a rate of reaction, effectively the CO2 utilization rate, increases with cycle number due to formation and expansion of porosity in the oxygen carrier.

[0056] In one embodiment, the oxygen carrier converts CO2 to CO, without degrading due to the microstructure and phase evolution of the oxygen carrier.

[0057] In one embodiment, under a desired flow rate of CO2, the oxygen carrier is oxidized to a combination of FeWO4 and Fe2O3 rather than FeWO4 and Fe3O4.

[0058] In one embodiment, the oxygen carrier is oxidized to Fe20s under CO2.

[0059] In yet another aspect, the invention relates to a method for enhancing the efficiency of chemical looping CO2 utilization. The method comprises optimizing particle size of Fe-25W powder to maximize surface area for reaction; controlling temperature and pressure conditions of the reaction; incorporating additional transition metals into the Fe-25W powder to modify redox kinetics; and periodically regenerating the Fe-25W powder to maintain catalytic activity over extended cycles. In one embodiment, the additional transition metals include molybdenum (Mo) or cobalt (Co) in concentrations up to 10% by weight.

[0060] In a further aspect, the invention relates to a composition for CO2 utilization. The composition comprises a metallic powder mixture including 75% by weight Fe and 25% by weight W; optionally, up to 10% by weight of additional transition metals selected from Mo and Co; and a particle size distribution configured to optimize CO2 reduction efficiency in a chemical looping process.

[0061] These and other aspects of the present invention will become apparent from the following description of the preferred embodiment taken in conjunction with the following drawings, although variations and modifications therein may be affected without departing from the spirit and scope of the novel concepts of the invention.

[0062] BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The accompanying drawings illustrate one or more embodiments of the invention and together with the written description, serve to explain the principles of the invention. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like elements of an embodiment.

[0064] FIG. 1 shows in situ X-ray diffraction (XRD) patterns for the first two redox cycles of Fe-25W under CO2 as oxidizing gas and 4%Ar-H2 as reducing gas, according to embodiments of the invention, (a) time-resolved XRD patterns showing the overall system transformation, (b) Normalized peak integrals for metallic phases Fe (blue), p-Fe?W6 (purple), and X-Fe2W (black), (c) Normalized peak integrals for oxide phases FeO (orange), FeWC (green), and FeaCU (red).

[0065] FIG. 2 shows in situ XRD patterns for the first two redox cycles of Fe-25W under CO2 as oxidizing gas and 4%Ar-H2 as reducing gas, according to embodiments of the invention, (a) time-resolved XRD patterns showing the overall system transformation, (b) Normalized peak integrals for phases Fe (blue) and X-FeaWaC (purple), (c) Normalized peak integrals for oxide phases FeO (orange), FeWO4 (green), FeaO4 (red), and Fe20a (cyan).

[0066] FIG. 3 shows microstructural evolution of Fe-25W foams under CO2 / H2 cycling, after (a,b) 1 cycle, (c,d) 10 cycles, (e,f) 100 cycles, and (g,h) 165 cycles, in the (a,c,e,g) reduced and (b,d,f,h) oxidized states, according to embodiments of the invention. Cyan arrows mark large Fe regions in the first cycle, yellow arrows mark micropores, and red arrows mark submicron pores. Insets in (e-h) show high magnification to better observe submicron features. FIG. 4 shows electron images of pure Fe powder bed surface (a) before oxidation, (b) after oxidation, in a region with both oxide (white dashed region) and metal (pink dashed region), and (c) after oxidation, in a region with heavily sintered oxide surface, according to embodiments of the invention.

[0067] FIG. 5 shows thermogravimetric analysis (TGA) data for CO2 oxidation of foams: 1 cycle, 10 cycles, 100 cycles, 165 cycles, and 1 cycle Fe-only with temperature program on the right axis, according to embodiments of the invention. Masses normalized to stabilization of gas flow mass gain.

[0068] DETAILED DESCRIPTION OF THE INVENTION

[0069] The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. However, this invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this specification will be thorough and complete and fully convey the invention's scope to those skilled in the art. Like reference numerals refer to like elements throughout.

[0070] The terms used in this specification generally have their ordinary meanings in the art, within the context of the invention, and in the specific context where each term is used. Certain terms used to describe the invention are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner regarding the description. For convenience, certain terms may be highlighted, for example using italics and / or quotation marks. The use of highlighting has no influence on the scope and meaning of a term; the scope and meaning of a term are the same, in the same context, whether or not it is highlighted. It will be appreciated that same thing can be said in more than one way. Consequently, alternative language and synonyms may be used for any one or more of the terms discussed herein, nor is any special significance to be placed upon whether or not a term is elaborated or discussed herein. Synonyms for certain terms are provided. A recital of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification including examples of any terms discussed herein is illustrative only, and in no way limits the scope and meaning of the invention or of any exemplified term. Likewise, the invention is not limited to various embodiments given in this specification.

[0071] It will be understood that, as used in the description herein and throughout the claims that follow, the meaning of “a”, “an”, and “the” includes plural reference unless the context clearly dictates otherwise. Also, it will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0072] It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, or section without departing from the invention's teachings.

[0073] Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element’s relationship to another element as illustrated in the figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. For example, if the device in one of the figures, is turned over, elements described as being on the “lower” side of other elements would then be oriented on “upper” sides of the other elements. The exemplary term “lower”, can, therefore, encompasses both an orientation of “lower” and “upper,” depending on the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as “below” or “beneath” other elements would then be oriented “above” the other elements. Therefore, the exemplary terms “below” or “beneath” can encompass both an orientation of above and below.

[0074] It will be further understood that the terms “comprises” and / or “comprising,” or “includes” and / or “including” or “has” and / or “having”, or “carry” and / or “carrying,” or “contain” and / or “containing,” or “involve” and / or “involving, and the like are to be open-ended, i.e., to mean including but not limited to. When used in this specification, they specify the presence of stated features, regions, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and / or groups thereof.

[0075] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and this specification, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0076] As used in this specification, “around”, “about”, “approximately” or “substantially” shall generally mean within 20 percent, preferably within 10 percent, and more preferably within 5 percent of a given value or range. Numerical quantities given herein are approximate, meaning that the term “around”, “about”, “approximately” or “substantially” can be inferred if not expressly stated.

[0077] As used in this specification, the phrase “at least one of A, B, and C” should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0078] The description below is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. The broad teachings of the invention can be implemented in a variety of forms. Therefore, while this invention includes particular examples, the true scope of the invention should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. It should be understood that one or more steps within a method may be executed in a different order (or concurrently) without altering the principles of the invention.

[0079] Carbon emissions inherent to materials industries such as cement and steel need to be captured, utilized, or sequestered in order to curtail greenhouse gas release, both for societal improvement and to meet increasingly stringent international regulations. While capture and sequestration can be costly, utilization can accomplish the same goal while providing a value-added product that can offset the costs of emission. Because of its low cost, natural abundance, non-toxicity, and versatile reactivity, iron (Fe) is an ideal material for carbon utilization. This process converts waste CO2 to CO in order to both upcycle emissions into more valuable chemical feedstocks and mitigate CO2 emissions. Fe oxidizes under flowing CO2 at high temperatures (> 600 °C), first to FeO and then to FesCh (further oxidation to Fe20a is theoretically possible but not observed). The oxidized Fe can then be reduced back to metallic Fe under H2. However, at the high reaction temperature, severe sintering and densification strongly hinders this reaction after just 1 cycle. In addition, current Fe based beds may require costly inputs such as fluidized beds or nanoparticles and nano-scaffolds to prevent sintering.

[0080] One of the objectives of this invention is to provide an iron-tungsten (Fe-W) system to solve the sintering and densification by preventing pores from sintering and by forming new pores during cycling and maintains fast reaction kinetics. Additionally, the Fe-W system shows formation of Fe20.3 under sufficient CO2 flow, increasing the overall CO2 utilization of the material beyond what is possible for other chemistries. The Fe-W system is much more robust, as relatively impure micron-sized particles can be used and the bed will still resist and even reverse sintering and densification.

[0081] In some exemplary embodiments, a powder mixture of FeiO and WO3 precursors is produced by preparing a slurry of the Fe2Os and WO3 powders in a solvent, which is then ball milled for 24 hours to mix the powders and then dried. The dried powders are reduced at high temperature to produce a partially sintered metallic bed. During redox cycling at 550-1100 °C, the bed is reversibly oxidized by CO2 and reduced by H2 with minor or no densification due to sintering, which would block gas access and slow reaction rates.

[0082] The invention, among other things, has the advantageous effects: inherent sintering inhibition; in situ pore formation; oxidation to Fe2Ch rather than FesC , regenerative microstructural evolution, fast reaction kinetics, all components redox active, micron-scale powders can be used.

[0083] The invention may also have applications in carbon utilization, chemical looping combustion, and the like.

[0084] Without intent to limit the scope of the invention, exemplary embodiments of the invention are given below.

[0085] In one aspect, this invention discloses a system for CO2 utilization, comprising a gas condition; and an oxygen carrier containing iron (Fe) and tungsten (W), exposed to the gas condition for an appropriate period of time under an appropriate temperature in a range of about 550-1100, preferably, ranges of about 600-900, and about 750-850 °C.

[0086] In one embodiment, the gas condition contains a gas that is still or flowing at an appropriate flow rate.

[0087] In one embodiment, the gas condition is alternatively switchable between a CO2 condition in which oxidation occurs and an H2 condition in which reduction occurs. In one embodiment, the CO2 condition contains CO2 or a CCh-containing gas blend, and the H2 condition contains H2 or an FF-containing gas blend

[0088] In one embodiment, the C Ch-containing gas blend includes CO2 and another gas including N2, CO, and / or like.

[0089] In one embodiment, the CO2-containing gas blend contains 66% CO and 34% CO2.

[0090] In one embodiment, the CO2-containing gas blend contains CO2 or CO-CO2 mixed with an inert gas including N2 or Ar.

[0091] In one embodiment, the H2-containing gas blend includes H and another gas including N2, CO, Ar, and / or like.

[0092] In one embodiment, the H2-containing gas blend further includes certain leftover CO2.

[0093] In one embodiment, the H2-containing gas blend further includes an active gas that helps reducing, such as CO, CH4, NH3.

[0094] In one embodiment, the oxygen carrier is cyclable between the oxidation and the reduction hundreds of times.

[0095] In one embodiment, when the gas condition is in the CO2 condition, the appropriate period of time is an oxidizing period, and the appropriate temperature is an oxidizing temperature; and when the gas condition is in the H2 condition, the appropriate period of time is a reducing period, and the appropriate temperature is a reducing temperature.

[0096] In one embodiment, in different oxidation-reduction (redox) cycles, the oxidizing period is same or different, and the oxidizing temperature is same or different; and the reducing period is same or different, and the reducing temperature is same or different.

[0097] In one embodiment, the oxygen carrier contains a mixture of F 626)3 and WO3 precursors, in a powder form.

[0098] In one embodiment, the oxygen carrier contains a Fe-W alloy with about 5-50 at%, with sub-ranges of 10-33, 18-30, and 20-25 at%, of W in the Fe-W alloy.

[0099] In one embodiment, the oxygen carrier contains a porous Fe-25W (at%) alloy, which is packed powder beds or foams. In one embodiment, the Fe-25W foam has hierarchical porosity: a continuous microporous network that does not sinter due to the sintering inhibition of W, and submicron pores formed by the chemical vapor transport during the reduction period of FeWC .

[0100] In one embodiment, during redox cycling at about 550-1100 °C, the oxygen carrier is reversibly oxidized by CO2 and reduced by H2 with minor or no densification.

[0101] In one embodiment, the system has a carbon monoxide (CO) production of about 19 mol CO / kg Fe-25W.

[0102] In one embodiment, a rate of reaction, effectively the CO2 utilization rate, increases with cycle number due to formation and expansion of porosity in the oxygen carrier.

[0103] In one embodiment, the oxygen carrier converts CO2 to CO, without degrading due to the microstructure and phase evolution of the oxygen carrier.

[0104] In one embodiment, under a desired flow rate of CO2, the oxygen carrier is oxidized to a combination of FeW04 and Fe20s rather than FeW04 and FesO4.

[0105] In one embodiment, the oxygen carrier is oxidized to Fe2Os under CO2.

[0106] In another aspect, this invention relates to a method for CO2 utilization, comprising: providing an oxygen carrier containing iron (Fe) and tungsten (W); and exposing the oxygen carrier to a gas condition for an appropriate period of time under an appropriate temperature to convert, fully or partially CO2 and carbon to carbon monoxide (CO).

[0107] In one embodiment, the gas condition contains a gas that is still or flowing at an appropriate flow rate.

[0108] In one embodiment, said exposing the oxygen carrier to the gas condition comprises alternatively switching the gas condition between a CO2 condition in which oxidation occurs and an H2 condition in which reduction occurs.

[0109] In one embodiment, the CO2 condition contains CO2 or a CCh-containing gas blend, and the H2 condition contains H2 or an FF-containing gas blend.

[0110] In one embodiment, the C Ch-containing gas blend includes CO2 and another gas including N2, CO, and / or like.

[0111] In one embodiment, the CO2-containing gas blend contains 66% CO and 34% CO2.

[0112] In one embodiment, the CO2-containing gas blend contains CO2 or CO-CO2 mixed with an inert gas including N2 or Ar.

[0113] In one embodiment, the Fh-containing gas blend includes H2 and another gas including N2, CO, Ar, and / or like.

[0114] In one embodiment, the Fh-containing gas blend further includes certain leftover CO2.

[0115] In one embodiment, the FL-containing gas blend further includes an active gas that helps reducing, such as CO, CH4, NH3.

[0116] In one embodiment, the oxygen carrier is cyclable between the oxidation and the reduction hundreds of times.

[0117] In one embodiment, when the gas condition is in the CO2 condition, the appropriate period of time is an oxidizing period, and the appropriate temperature is an oxidizing temperature; and when the gas condition is in the H2 condition, the appropriate period of time is a reducing period, and the appropriate temperature is a reducing temperature.

[0118] In one embodiment, in different oxidation-reduction (redox) cycles, the oxidizing period is same or different, and the oxidizing temperature is same or different; and the reducing period is same or different, and the reducing temperature is same or different.

[0119] In one embodiment, the oxygen carrier contains a mixture of Fe20s and WO3 precursors, in a powder form.

[0120] In one embodiment, the mixture of I76263 and WO3 precursors is produced by preparing a slurry of the Fe2<D3 and WO3 powders in a solvent, which is then ball milled for 24 hours to mix the powders and then dried, wherein the dried powders are reduced at high temperature to produce a metallic powder bed, which is unsintered or partially sintered.

[0121] In one embodiment, the oxygen carrier contains a Fe-W alloy with about 5-50 at%, with sub-ranges of 10-33, 18-30, and 20-25 at%, of W in the Fe-W alloy.

[0122] In one embodiment, the oxygen carrier contains a porous Fe-25W (at%) alloy, which is packed powder beds or foams.

[0123] In one embodiment, the Fe-25W foam has hierarchical porosity: a continuous microporous network that does not sinter due to the sintering inhibition of W, and submicron pores formed by the chemical vapor transport during the reduction period of FeWC .

[0124] In one embodiment, during redox cycling at about 550-1100 °C, the oxygen carrier is reversibly oxidized by CO2 and reduced by H2 with minor or no densification.

[0125] In one embodiment, a rate of reaction, effectively the CO2 utilization rate, increases with cycle number due to formation and expansion of porosity in the oxygen carrier.

[0126] In one embodiment, the oxygen carrier converts CO2 to CO, without degrading due to the microstructure and phase evolution of the oxygen carrier.

[0127] In one embodiment, under a desired flow rate of CO2, the oxygen carrier is oxidized to a combination of FeWO4 and Fe20s rather than FeWO.4 and FesO4.

[0128] In one embodiment, the oxygen carrier is oxidized to Fe2O3 under CO2.

[0129] In yet another aspect, the invention relates to a method for enhancing the efficiency of chemical looping CO2 utilization. The method comprises optimizing particle size of Fe-25W powder to maximize surface area for reaction; controlling temperature and pressure conditions of the reaction; incorporating additional transition metals into the Fe-25W powder to modify redox kinetics; and periodically regenerating the Fe-25W powder to maintain catalytic activity over extended cycles.

[0130] In one embodiment, the additional transition metals include molybdenum (Mo) or cobalt (Co) in concentrations up to 10% by weight.

[0131] In a further aspect, the invention relates to a composition for CO2 utilization. The composition comprises a metallic powder mixture including 75% by weight Fe and 25% by weight W; optionally, up to 10% by weight of additional transition metals selected from Mo and Co; and a particle size distribution configured to optimize CO2 reduction efficiency in a chemical looping process.

[0132] These and other aspects of the invention are further described below. Without intent to limit the scope of the invention, exemplary instruments, apparatus, methods, and their related results according to the embodiments of the invention are given below. Note that titles or subtitles may be used in the examples for convenience of a reader, which in no way should limit the scope of the invention. Moreover, certain theories are proposed and disclosed herein; however, in no way they, whether they are right or wrong, should limit the scope of the invention so long as the invention is practiced according to the invention without regard for any particular theory or scheme of action.

[0133] EXAMPLE:

[0134] Fe-25W POWDER BEDS FOR HIGH TEMPERATURE CO2 UTILIZATION

[0135] CO2 utilization technology converts CO2 to more valuable products, like CO, for use as chemical feedstocks. This helps convert processes such as steel or cement production, which have high CO2 emissions, to become carbon neutral or negative. Iron (Fe) can be used as a CO2 utilization material because at high temperatures, Fe will be oxidized by CO2, producing CO. The Fe-W system can also convert CO2 to CO, but will not degrade due to the microstructure and phase evolution of the system. Additionally, under high CO2 flow rates, this material can be oxidized to a combination of FeWO4 and Fe2O3 rather than FeWO4 and FesO4. The formation of Fe20a under CO2 at 800 °C is unexpected and improves the overall CO2 utilization capability of the material.

[0136] We previously showed the excellent cycling characteristics of Fe-25W powder beds under H2O / H2 cycling, a related process of high temperature oxidation and reduction, relevant to electrical energy storage. Because of its high melting point, W is an excellent sintering inhibitor. The Fe-25W foam shows hierarchical porosity: a continuous microporous network that does not sinter due to the sintering inhibition of W, and submicron pores formed by the chemical vapor transport during the reduction period of FeWC .

[0137] In this exemplary study, we explore the behavior of the Fe-25W system when oxidized by CO2, rather than H2O. A theoretical carbon monoxide production of 19 mol CO / kg Fe-25W makes this system worthwhile to investigate, even though the use of W as an additive increases the mass of the foam.

[0138] In situ X-ray diffraction (XRD) and thermogravimetric analysis (TGA) results show that the rate of reaction, effectively the CO2 utilization rate, increase with cycle number due to the formation and expansion of porosity in the powder. Additionally, we show for the first time a Fe-based system that is oxidized to Fe2Ch under CO2, going beyond the limits of the pure Fe system (FeiC ). The combination of high CO2 utilization, microstructural stability, and redox cycling stability indicate that the Fe-25W system can be used for effective carbon utilization.

[0139] In situ XRD spectra detailing the first two cycles of CO2 oxidation followed by H2 reduction are shown in FIG. 1. Panel (a) of FIG. 1 shows time-resolved diffraction patterns for the first two CO2 / H2 cycles. Paneled (b)-(c) of FIG. 1 show calculated normalized peak integrals for each phase’s strongest peak. To slow the reduction reaction for higher temporal resolution, 4% H2-bal Ar was used as the reducing gas.

[0140] Initially, the power bed includes a two-phase mixture of p-FeyWe and a-Fe(1.3W), the equilibrium phases at the higher initial sintering temperature of 1200 °C. When exposed to CO2 at t = 3 min, this initial state quickly oxidizes. FeWC>4 is first observed at t = 4 minutes, taking approximately 60 min to fully form. In this slow first oxidation period, a significant amount of transient a-W is formed as the initial intermetallic state oxidizes into two different oxides (FeO and FeWC>4). The incorporation of this standalone W phase is sluggish as compared to later cycles when it does not form. FeO is observed at t= 18 min and is gradually converted to FesC over a 50 minute period. In total, the first oxidation reaction takes 70 min.

[0141] Reduction is comparatively quick, starting at t = 83 min. First, Fe3O4 rapidly reduces to FeO, in a 2 minute period. FeO reduces to Fe relatively quickly as well, in a 10 minute period. Reduction of FeWO4 is the slowest step, occurring over a 20 minute period. When FeWO4 reduces to X-Fe2W, it does so by the chemical vapor transport reduction mechanism discussed above. Because of this reduction mechanism, the A-Fe2W formed during cycling is nanocrystalline, as evidenced by broad, poorly resolved diffraction peaks. Because of the broad shape of these peaks, the Fe2W normalized integral signal is comparatively low.

[0142] The second cycle, beginning at t = 108 minutes, is significantly faster than the first due to both the different phase composition and the nanocrystalline nature of the -Fe2W present. The same sequence of oxidation is observed, but it occurs over just a 20 minute period, rather than the sluggish 70 minute period of the first oxidation. The second reduction is very similar to the first, again following the same sequence and occurring over a 20 minute period.

[0143] To assess the formation of F 626)3 under CO2 conditions, in situ XRD redox cycling was performed on a foam that had been pre-cycled to 10 cycles. The results are shown in FIG. 2, with stacked diffraction patterns in panel (a) of FIG. 2 and normalized peak integrals in panels (b)-(c) of FIG. 2. CO2 was used as the oxidizing gas, and pure H2 was used as the reducing gas. The oxidation at cycle ten proceeds by the same path and reaction times as seen in cycle 2 in FIG. 1. The initial mixture of -Fe2W and a-Fe(1.3W) first oxidizes to FeWC>4, fully forming after 5 minutes. Transient FeO is formed at the same time, which undergoes a slower transition to F 636)4 over 7 minutes. At t = 10 minutes into the oxidation observed in FIG. 2, oxidation would typically be considered complete, and the gases would be switched to H2 to begin reduction. If CO2 conditions are held, however, a gradual oxidation of F 636)4 to Fe2O is observed, occurring over a 40 minute period between t = 20 and t = 60 minutes. CO2 conditions are maintained for another 80 minutes, but no further changes to the phase composition were observed. The slow rate of oxidation indicates that the formation of F 626)3 observed is due to CO2 as opposed to an air leak or other unintended oxygen exposure: an air leak would cause extremely rapid (<1 minute) oxidation to F 626)3. The formation of F 626)3 is attributed to the combination of a highly reactive foam subjected to a high flow rate of pure CO2. Such conditions, though not usually achieved, can produce Fe26)3 under CO2 conditions so long as the CO2:CO ratio is very high. A repeat experiment with lower flow rate of CO2 did not show Fe2O3 formation, indicating that the supply of CO2 is vital to the observed Fe2O3 formation.

[0144] Reduction under pure H2 occurs rapidly, forming first the expected two phase composition of X- Fe2W and a-Fe after 2 minutes of H2 exposure. This phase composition is short-lived, however, as it rapidly transforms after 1 minute into a mixture of Fes W3C' and a-Fe. This transformation is indicative of the presence of solid C in the powder. This carbon is present due to the Boudouard reaction, The occurrence of this reaction is attributed to the combination of the reverse water-gas shift reaction H20(5) + when H2 is introduced into the CO2 environment, combined with the rapid reduction producing metal surfaces onto which C can readily deposit. A repeat experiment with N2 flushes between oxidizing and reducing conditions did not show carbide formation, indicating that the reverse water-gas shift reaction is responsible for the carbon deposition and carbide formation observed in FIG. 2.

[0145] The carbide phase can be re-oxidized however, as shown in the second oxidation starting at t = 140 minutes. This releases the CO that was originally disproportionated in the Boudouard reaction. A final reduction at t = 160 minutes again shows carbide formation. Carbide formation could be suppressed by reacting at higher temperatures, shifting the Boudouard equilibrium to further favor CO, and future work will detail the effect of higher temperature on the reaction process.

[0146] The microstructural evolution of foams undergoing CO2 / H2 cycling is remarkably similar that of H2O / H2 cycling, since the phases formed are the same in both cases. Examples of the foam surface microstructure after 1, 10, 100, and 165 cycles in both the reduced and oxidized states are shown in FIG. 3. After 1 cycle, the reduced state (panel (a) of FIG. 3) shows a two phase composition of a-Fe and X-Fe2W. The Fe regions are 2-5 pm in size (larger regions marked with cyan arrows), and relatively dense, while the regions are < 1 pm in size, interrupted by submicron porosity (red arrows). Throughout the foam, microscale porosity is prevalent (orange arrows). In the oxidized state (panel (b) of FIG. 3), the volume expansion associated with oxidation results in most pores being closed in the first cycle, leaving a mostly densified oxide surface.

[0147] After 10 cycles, the microstructure is quite different, shown in panel (c) of FIG. 3 for the reduced state. The large Fe regions have shrunk and homogenized, with no clear segregated regions apparent, indicating both phases are in the micron or submicron size range. Micropores (orange arrows) are more prevalent, and are consistently 1-2 pm in diameter. Submicron pores (red arrows) are seen in all ligaments of the foam, as expected from the nanocrystalline A-Fe?W phase formed during H2 reduction. The 10 cycle oxidized state, shown in panel (d) of FIG. 3, shows that the micropores are now present even after oxidation (orange arrows). The submicron pores have closed, but the combination of micropores and homogeneously mixed FeWO4 and Fe3O4 prevents sintering or segregation from occurring.

[0148] After 100 cycles, the microstructure has continued to homogenize and become more porous. The ligaments of the foam, shown in the reduced state in panel (e) of FIG. 3, are approximately equiaxial particles with no clear sintering or segregation, and ample microporosity. The small size of the particles makes distinguishing submicron pores more difficult, but the inset (red border) shows nano-scale particles with submicron pores, as expected. The oxidized state (panel (f) of FIG. 3) shows similar particle size, and the inset (red border) shows that submicron pores (red arrow) are present in the oxidized state as well, though each phase’s size is microcrystalline rather than nanocrystalline. There are no significant changes to the microstructure after 165 cycles in either the reduced state (panel (g) of FIG. 3) or the oxidized state (panel (h) of FIG. 3). In each micropores and submicron pores are still observed.

[0149] These microstructures can be compared with pure Fe microstructure before (panel (a) of FIG. 4) and after (panels (b)-(c) of FIG. 4) oxidation with CO2 at 800 °C. The fresh Fe was prepared by 4 h reduction under pure H2 at 600 °C of the same Fe20s powders used in the Fe- 25W foams. The pure Fe powder bed (FIG. 4a) is initially highly porous, due to the small size of the Fe20a powders and the relatively low reduction temperature. Two regions of the oxidized surface are shown in panels (b)-(c) of FIG. 4. Panel (b) of FIG. 4 shows a region of incomplete oxidation, where the growing oxide layer (white dashed region) is growing inwards towards a metallic Fe region (pink dashed region). The oxide grains are much larger than the initial Fe powders, due to both expansion (110% molar volume expansion) and sintering at high temperatures. In other regions (panel (c) of FIG. 4) the oxide surface is completely densified, trapping unreacted Fe inside and resulting in incomplete reaction. These heavily sintered regions contrast starkly with the porous structures observed even in the oxidized state for Fe-25W.

[0150] The microstructural evolution observed in FIG. 3 supports the boost to redox kinetics observed in the in situ XRD experiments. This was further explored with TGA under pure CO2, shown in FIG. 5, to foams pre-cycled to 1, 10, 100, and 165 cycles, as well as an Fe-only powder for comparison. A constant flow rate of 30 seem CO2 was applied to each sample during a 20 °C / min ramp to 800 °C, followed by a 4 hour hold at 800 °C. A control run with no sample was used to determine the time at which mass gain due to only gas flow stabilizes, mass values before this point (t = 32 min) are masked due to run-to-run irregularities in the gas flow mass gain pattern. The TGA shows that the Fe-25W foam is sluggish to react during the first cycle (yellow curve), reaching only 24% mass gain in 4 hours, compared to an expected maximum mass gain of 30%. The Fe-25W foam is in fact slower to react than the Fe-only foam, which reaches 23% mass gain in the same period, out of an expected maximum mass gain of 38% for full conversion to FesO4. The microstructure of the pure Fe powder, shown in FIG. 4, shows the formation of a dense oxide shell, and an initially present microporous network that is completely filled with oxide, slowing further reaction, as expected.

[0151] After 10 cycles, however, the Fe-25W foam is much more reactive, rapidly reaching 30% mass gain between t = 40 min and t = 80 min. This is followed by a gradual mass increase to 31% by t = 240 minutes. The increase above the expected mass gain is likely due to the formation of Fe2Ch in the foam, confirmed by ex situ XRD.

[0152] The mass gain curves for the 100 cycles and 165 cycle foams are very similar to the 10thcycle foam, characterized by a rapid mass gain between t = 40 (T = 575 °C) and t = 80 minutes (T = 800 °C). The 165thcycle foam reacts faster than the 10thand 100thcycles foams, indicating that the microstructural refinement continues between the 100thand 165thcycle, and likely continues for additional cycles pending additional investigation.

[0153] The foregoing description of the exemplary embodiments of the invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.

[0154] The embodiments were chosen and described to explain the principles of the invention and their practical application to enable others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the invention pertains without departing from its spirit and scope. Accordingly, the scope of the invention is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.

[0155] Some references, which may include patents, patent applications, and various publications, are cited and discussed in the description of this invention. The citation and / or discussion of such references is provided merely to clarify the description of the invention and is not an admission that any such reference is “prior art” to the invention described herein. All references cited and discussed in this specification are incorporated herein by reference in their entireties and to the same extent as if each reference was individually incorporated by reference.

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Claims

CLAIMSWhat is claimed is:

1. A system for CO2 utilization, comprising: a gas condition; and an oxygen carrier containing iron (Fe) and tungsten (W), exposed to the gas condition for an appropriate period of time under an appropriate temperature.

2. The system of claim 1, wherein the gas condition contains a gas that is still or flowing at an appropriate flow rate.

3. The system of claim 2, wherein the gas condition is alternatively switchable between a CO2 condition in which oxidation occurs and an H2 condition in which reduction occurs.

4. The system of claim 3, wherein the CO2 condition contains CO2 or a CCh-containing gas blend, and the H2 condition contains H2 or an FF-containing gas blend.

5. The system of claim 4, wherein the CCh-containing gas blend includes CO2 and another gas including N2, CO, and / or like.

6. The system of claim 5, wherein the CO2-containing gas blend contains 66% CO and 34% CO2.

7. The system of claim 5, wherein the CO2-containing gas blend contains CO2 or CO-CO2 mixed with an inert gas including N2 or Ar.

8. The system of claim 4, wherein the FL-containing gas blend includes H2 and another gas including N2, CO, Ar, and / or like.

9. The system of claim 8, wherein the FL-containing gas blend further includes certain leftover CO2.

10. The system of claim 8, wherein the Fh-containing gas blend further includes an active gas that helps reducing, wherein the active gas includes CO, CH4, and / or NH3.

11. The system of claim 3, wherein the oxygen carrier is cyclable between the oxidation and the reduction hundreds of times.

12. The system of claim 3, wherein when the gas condition is in the CO2 condition, the appropriate period of time is an oxidizing period, and the appropriate temperature is an oxidizing temperature; and when the gas condition is in the H2 condition, the appropriate period of time is a reducing period, and the appropriate temperature is a reducing temperature.

13. The system of claim 12, wherein in different oxidation-reduction (redox) cycles, the oxidizing period is same or different, and the oxidizing temperature is same or different; and the reducing period is same or different, and the reducing temperature is same or different.

14. The system of claim 1, wherein the oxygen carrier contains a mixture of Fe2C>3 and WO3 precursors, in a power form.

15. The system of claim 14, wherein the oxygen carrier contains a Fe-W alloy with about 5- 50 at%, with sub-ranges of 10-33, 18-30, and 20-25 at%, of W in the Fe-W alloy.

16. The system of claim 15, wherein the oxygen carrier contains a porous Fe-25W (at%) alloy, which is packed powder beds or foams.

17. The system of claim 16, wherein the Fe-25W alloy has hierarchical porosity: a continuous microporous network that does not sinter due to the sintering inhibition of W, and submicron pores formed by the chemical vapor transport during the reduction period18. The system of claim 17, wherein during redox cycling at about 550-1100 °C, the oxygen carrier is reversibly oxidized by CO2 and reduced by H2 with minor or no densification.

19. The system of claim 17, having a carbon monoxide (CO) production of about 19 mol CO / kg Fe-25W.

20. The system of claim 17, wherein a rate of reaction, effectively the CO2 utilization rate, increases with cycle number due to formation and expansion of porosity in the oxygen carrier.

21. The system of claim 17, wherein the oxygen carrier converts CO2 to CO, without degrading due to the microstructure and phase evolution of the oxygen carrier.

22. The system of claim 17, wherein under a desired flow rate of CO2, the oxygen carrier is oxidized to a combination of FeWO4 and Fe20.3 rather than FeWO4 and Fe.304.

23. The system of claim 17, wherein the oxygen carrier is oxidized to Fe2O3 under CO2.

24. A method for CO2 utilization, comprising: providing an oxygen carrier containing iron (Fe) and tungsten (W); and exposing the oxygen carrier to a gas condition for an appropriate period of time under an appropriate temperature to convert, fully or partially, CO2 and carbon to carbon monoxide (CO).

25. The method of claim 24, wherein the gas condition contains a gas that is still or flowing at an appropriate flow rate.

26. The method of claim 25, wherein said exposing the oxygen carrier to the gas condition comprises alternatively switching the gas condition between a CO2 condition in whichoxidation occurs and an H2 condition in which reduction occurs.

27. The method of claim 26, wherein the CO2 condition contains CO2 or a CO2 -containing gas blend, and the H2 condition contains H2 or an H2-containing gas blend.

28. The method of claim 27, wherein the CCh-containing gas blend includes CO2 and another gas including N2, CO, and / or like.

29. The method of claim 28, wherein the CO2-containing gas blend contains 66% CO and 34% CO2.

30. The method of claim 28, wherein the CO2-containing gas blend contains CO2 or CO-CO2 mixed with an inert gas including N2 or Ar.

31. The method of claim 27, wherein the H2-containing gas blend includes H2 and another gas including N2, CO, Ar, and / or like.

32. The method of claim 31, wherein the H2-containing gas blend further includes certain leftover CO2.

33. The method of claim 31, wherein the Th-containing gas blend further includes an active gas that helps reducing, wherein the active gas includes CO, CH4, and / or NH3.

34. The method of claim 26, wherein the oxygen carrier is cyclable between the oxidation and the reduction hundreds of times.

35. The method of claim 26, wherein when the gas condition is in the CO2 condition, the appropriate period of time is an oxidizing period, and the appropriate temperature is an oxidizing temperature; and when the gas condition is in the H2 condition, the appropriate period of time is a reducing period, and the appropriate temperature is a reducing temperature.

36. The method of claim 35, wherein in different oxidation-reduction (redox) cycles, the oxidizing period is same or different, and the oxidizing temperature is same or different; and the reducing period is same or different, and the reducing temperature is same or different.

37. The method of claim 24, wherein the oxygen carrier contains a mixture of Fe2O3 and WO3 precursors, in a power form.

38. The method of claim 37, wherein the mixture of Fe2O3 and WO3 precursors is produced by preparing a slurry of the Fe2Ch and WO3 powders in a solvent, which is then ball milled for 24 hours to mix the powders and then dried, wherein the dried powders are reduced at high temperature to produce a metallic powder bed, which is unsintered or partially sintered.

39. The method of claim 37, wherein the oxygen carrier contains a Fe-W alloy with about 5- 50 at%, with sub-ranges of 10-33, 18-30, and 20-25 at%, of W in the Fe-W alloy.

40. The method of claim 39, wherein the oxygen carrier contains a porous Fe-25W (at%) alloy, which is packed powder beds or foams.

41. The method of claim 40, wherein the Fe-25W alloy has hierarchical porosity: a continuous microporous network that does not sinter due to the sintering inhibition of W, and submicron pores formed by the chemical vapor transport during the reduction period ofFeWC .

42. The method of claim 41, wherein during redox cycling at about 550-1100 °C, the oxygen carrier is reversibly oxidized by CO2 and reduced by H2 with minor or no densification.

43. The method of claim 41, wherein a rate of reaction, effectively the CO2 utilization rate,increases with cycle number due to formation and expansion of porosity in the oxygen carrier.

44. The method of claim 41, wherein the oxygen carrier converts CO2 to CO, without degrading due to the microstructure and phase evolution of the oxygen carrier.

45. The method of claim 41, wherein under a desired flow rate of CO2, the oxygen carrier is oxidized to a combination of FeW04 and Fe2O3 rather than FeW04 and Fe3O4.

46. The method of claim 41, wherein the oxygen carrier is oxidized to Fe2O3 under CO2.

47. A method for enhancing the efficiency of chemical looping CO2 utilization, comprising: optimizing particle size of Fe-25W powder to maximize surface area for reaction; controlling temperature and pressure conditions of the reaction; incorporating additional transition metals into the Fe-25W powder to modify redox kinetics; and periodically regenerating the Fe-25W powder to maintain catalytic activity over extended cycles.

48. The method of claim 47, wherein the additional transition metals include molybdenum (Mo) or cobalt (Co) in concentrations up to 10% by weight.

49. A composition for CO2 utilization, comprising: a metallic powder mixture including 75% by weight Fe and 25% by weight W; optionally, up to 10% by weight of additional transition metals selected from Mo and Co; and a particle size distribution configured to optimize CO2 reduction efficiency in a chemical looping process.

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