SYNGAS PRODUCTION FROM BINARY AND TERNARY CERIUM-BASED OXIDES

Abstract

Metal oxides having a lower activation temperature and enhanced oxygen mobility are disclosed. The metal oxides comprise oxygen (O), cerium (Ce) and one or both of iron (Fe) and uranium (U). Also disclosed are methods for producing hydrogen or carbon monoxide from water or carbon dioxide using the metal oxides.

Claims

1. A metal oxide capable of producing hydrogen from water or carbon monoxide from carbon dioxide comprising oxygen (O), cerium (Ce) and one or both of iron (Fe) and uranium (U).

2. The metal oxide of claim 1, having the following structure:
Ce.sub.wFe.sub.xU.sub.yO.sub.z, where 0<w<1; where 0.1x<1(w+y); where 0y<0.09; and where 1.5<z<2.5.

3. The metal oxide of claim 2, wherein the metal oxide has a fluorite lattice structure.

4. The metal oxide of claim 3, wherein the metal oxide maintains its fluorite lattice structure when subjected to a temperature of 300 to 1400 K or 900 to 1400 K.

5. The metal oxide of claim 2, wherein the metal oxide is a binary metal oxide.

6. The metal oxide of claim 5, wherein the binary metal oxide comprises Ce and Fe.

7. The metal oxide of claim 5, wherein the binary metal oxide comprises Ce and U.

8. The metal oxide of claim 2, wherein the metal oxide is a ternary metal oxide.

9. The metal oxide of claim 2, wherein Ce is Ce (IV), Fe is Fe (III) or Fe(II) or a combination thereof, and U is U (IV), U (V), or U (VI), or a combination thereof.

10. The metal oxide of claim 1, wherein the oxygen to metal ratio of the metal oxide is equal to or less than 2.5.

11. The metal oxide of claim 1, wherein the metal oxide has been calcined at a temperature of 400 to 600 C.

12. The metal oxide of claim 1, wherein the metal oxide has been reduced and contacted with water or carbon dioxide or a combination thereof.

13. The metal oxide of claim 12, wherein the metal oxide has been reduced with heat.

14. The metal oxide of claim 1, wherein the metal oxide is capable of producing hydrogen from water or carbon monoxide from carbon dioxide with solar radiation as an energy source.

15. A water splitting system comprising the metal oxide of claim 1, a heat source, and a water feed or a carbon dioxide feed or both.

16. A method for producing hydrogen gas from water or carbon monoxide from carbon dioxide, the method comprising: (i) reducing the metal oxide of claim 1 to form a reduced material; and (ii) contacting the reduced material with a feed comprising water under reaction conditions sufficient to produce hydrogen gas from the water or contacting the reduced material with a feed comprising carbon dioxide under reaction conditions sufficient to produce carbon monoxide from the carbon dioxide.

17. The method of claim 16, wherein the feed comprises water and wherein hydrogen gas is produced from the water.

18. The method of claim 17, wherein the water is in a gaseous or vapor phase.

19. The method of claim 16, wherein the feed comprises carbon dioxide and wherein carbon monoxide is produced from the carbon dioxide.

20. The method of claim 19, wherein the feed comprises water and carbon dioxide and wherein hydrogen gas is produced from the water and carbon monoxide is produced from the carbon dioxide.

21. A method for making a metal oxide of claim 1, the method comprising mixing cerium nitrate and one or both of iron nitrate or uranyl nitrate with ammonium hydroxide to form a mixture, and co-precipitating the mixture to produce the metal oxide.

22. A method for increasing oxygen mobility in cerium (Ce) oxide catalysts that are capable of producing hydrogen from water or carbon monoxide from carbon dioxide, the method comprising substituting a portion of Ce cations with iron (Fe) cations or uranium (U) cations or both.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1: Illustration of the fluorite structure of CeO.sub.2. Ce (IV) cations are at the center of a cube. Every other cube does not contain Ce (IV) to maintain the CeO.sub.2 stoichiometry.

[0029] FIG. 2: Illustration of various products that can be produced from syngas.

[0030] FIG. 3: In situ X-Ray Diffraction (XRD) of as prepared Ce.sub.0.5U.sub.0.5O.sub.2 at room temperature and then heated at the indicated temperatures.

[0031] FIG. 4: In situ X-Ray Diffraction (XRD) of as prepared Ce.sub.0.75Fe.sub.0.25O.sub.2 at room temperature and then heated at the indicated temperatures.

[0032] FIG. 5A: X-ray Photoelectron Spectroscopy (XPS) of Fe2p region for Ce.sub.0.75Fe.sub.0.25O.sub.2, as a function of reduction time (by Ar.sup.+ sputtering).

[0033] FIG. 5B: X-ray Photoelectron Spectroscopy (XPS) of Fe2p region for Ce.sub.0.95Fe.sub.0.05O.sub.2, as a function of reduction time (by Ar.sup.+ sputtering).

[0034] FIG. 6A: Thermal Gravimetric Analysis (TGA) of CeO.sub.2.

[0035] FIG. 6B: Thermal Gravimetric Analysis (TGA) of Ce.sub.0.75Fe.sub.0.25O.sub.2.

DETAILED DESCRIPTION OF THE INVENTION

[0036] The currently available materials that are used to produce hydrogen or carbon monoxide gas from water or carbon dioxide, respectively, require a high activation energy. In particular, and as illustrated above in equation 1, the reduction step of metal oxide catalysts requires heat input.

[0037] The present invention relates to doped or modified cerium dioxide catalysts that decrease the temperature/heat input needed for the reduction step to occur. Without wishing to be bound by theory, this reduced energy input is believed to be due to the introduction of Fe or U cations or both into the cerium oxide lattice structure. The Fe and U cations act to (1) reduced the energy needed to remove the lattice oxygen anions, O.sup.2, (2) enhance O.sup.2 mobility and (3) stabilize the fluorite structure when the catalysts of the present invention are subjected to a reducing reaction.

[0038] These and other non-limiting aspects of the present invention are discussed in further detail in the following sections.

A. Cerium-Based Oxides

[0039] The cerium-based oxides of the present invention includes oxygen (O), cerium (Ce) and one or both of iron (Fe) and uranium (U). The cerium-based oxides are capable of producing hydrogen from water and carbon monoxide from carbon dioxide. In one embodiment, the metal oxide comprises Ce (IV) and one or both of Fe cations (e.g., Fe (II) or Fe (III) or both) and uranium cations (e.g., U (IV), U (V), or U (VI)) or any combination or all of said cations. The cerium-based oxides of the present invention are capable of being activated via a reduction reaction. In this context, the term activated refers to a change in the material to a state in which the metal oxide optimally performs its desired function. In particular, Ce (IV) in the fluorite lattice structure is activated/reduced to Ce (III); in the case of Fe doped ceria, Fe can also exist in its metallic form (Fe.sup.0) in the reduced material. Once activated, the Ce (III) can then be oxidized via oxygen from water, leaving behind the desired H.sub.2 gas or via oxygen from carbon dioxide, leaving behind the desired carbon monoxide gas.

[0040] As illustrated in FIG. 1, cerium dioxide (also referred to as cerium (IV) oxide, ceria, ceric oxide) has a fluorite lattice structure and a chemical formula of CeO.sub.2. In one non-limiting embodiment of the present invention, cerium dioxide is commercially available in nano-powder and powdered forms from Sigma-Aldrich, St. Louis, Mo. (USA). The cerium dioxide can be modified or doped with Fe cations via iron (II) oxide (i.e., FeO) or iron (III) oxide (i.e., Fe.sub.2O.sub.3). Both iron (II) oxide and iron (III) oxide are commercially available in nano-powder and powdered forms from Sigma-Aldrich, St. Louis, Mo. (USA). In preferred embodiments, iron (III) oxide is used. The cerium dioxide can also be doped or modified with uranium cations via uranium (IV) oxide (i.e., UO.sub.2) or uranium (VI) oxide (i.e., UO.sub.3). Both uranium (IV) oxide and uranium (VI) oxide are commercially available in nano-powder and powdered forms from Sigma-Aldrich, St. Louis, Mo. (USA). In preferred embodiments, uranium (IV) oxide is used.

[0041] A general stoichiometric structure of the cerium-based oxides of the present invention includes the following:

Ce.sub.wFe.sub.xU.sub.yO.sub.z,

where 0<w<1, where 0.1x<(1(w+y), where 0y<0.09, and where 1.5<z<2.5. However, and in preferred embodiments, z is 1.5<z<2, or more preferably 2. The following Table 1 provides some non-limiting examples of the various amounts of w, x, and y that can be used with the catalysts of the present invention:

TABLE-US-00001 TABLE 1 w x y sum 0 1 0 1 0.1 0.89 0.01 1 0.2 0.78 0.02 1 0.3 0.67 0.03 1 0.4 0.56 0.04 1 0.5 0.45 0.05 1 0.6 0.34 0.06 1 0.7 0.23 0.07 1 0.8 0.12 0.08 1 0.9 0.01 0.09 1 1 0 0 1

[0042] Generally, preparation of the cerium-based catalysts of the present invention involves the steps of preparing a primary solid, processing the primary solid, for example by heat treatment, to obtain a metal oxide precursor, and activation of the precursor to give the activated metal oxide. The heat treatment of the metal oxide solids or precursors may include steps of drying, thermal decomposition of salts, and/or calcination. The term calcination refers to a heat treatment of a material in an oxidizing atmosphere for a certain period of time.

[0043] In particular, the initial preparation of the primary solid can be performed by a variety of methods known in the art. By way of example only, such methods can include co-precipitation from a solution of salts of the desired products, flame spray synthesis, and flame spray pyrolysis. In one particular embodiment, the cerium-based oxides are prepared by co-precipitation from their nitrate salts. In a further embodiment, the metal oxides are precipitated at a pH of 8-9. In certain instances, the precipitation agent used in the preparation of the metal oxides is ammonium hydroxide (NH.sub.4OH). The co-precipitation steps generally conform to the following parameters: Metal oxide materials were synthesized by the precipitation method. For example Ce0.5U0.5O.sub.2 can be prepared as follows. An aqueous solution of cerium (III) nitrate hexahydrate (Fluka) and zirconyl chloride octahydrate (Fluka) can be prepared with 50 mol % Ce.sup.4+ and 50 mol % U.sup.4+ cations. Then, ammonium hydroxide can be added until the pH of the solution is about 9 where cerium and uranium hydroxides have co-precipitated. The precipitate can be washed with distilled water until neutral pH then dried over night at 100 C. followed by calcination at 500 C. in air for 5 hours. The same method can be used to prepare all of the catalysts of the present invention. Following co-precipitation, the single or mixed hydroxides may undergo a series of washing and drying steps. The material may be heated in a dry inert atmosphere at sufficiently high temperature to remove substantially all activity-affecting amounts of water and carbon dioxide. In certain instances, the washing steps are undergone at a neutral pH with water. The drying step may comprise drying the material in a heated environment (e.g. 100 C.). The drying step may comprise drying the material for at least 6, at least 8, at least 10, at least 12, or at least 14 hours. In some embodiments, the material may be dried for 6-18 hours, 8-15 hours, or 10-15 hours. The drying step may be done in a heated environment of at least 100 C., or at least 200, 300, or 400 C.

[0044] Following precipitation and drying, the material may be calcined to make the oxides. The calcination step may be performed at a temperature of 500 C. until the desired product is formed. In some instances, the materials are calcined for five hours. In further instances, the materials may be calcined for 6, 7, 8, 9, or more hours. In one embodiment, calcination of the materials takes place at a temperature that is higher than that of the metal oxide operating temperature. In certain embodiments, calcination takes place at a temperature of 400, 500, 600, 700, 800, 900 or 1000 C. In further embodiments, calcination takes place at a temperature of 300-1000 C., 400-1000 C., 400-900 C., 400-800 C., 400-700 C., or 400-600 C.

[0045] After calcination, the metal oxides may be activated. Activation of the material may include reduction of the metal oxide. Activation may be performed, for example, with hydrogen gas. In another instance, activation may be performed with an inert gas. Inert gases include, for example, nitrogen, helium, neon, argon, krypton, xenon, and radon gases. In some embodiments, activation of the metal oxide is performed under inert atmosphere. In further embodiments, activation of the metal oxide is performed in a vacuum. When activation is performed in a vacuum, the pressure may be 0.01 torr. In further embodiments, the pressure in the vacuum may be 0.005, 0.02, 0.03, or 0.05 torr. The metal oxides described herein are particularly useful since they may be activated at a lower temperature. In certain embodiments, the metal oxides are activated at temperature of 100-1000 C., 100-900 C., 100-800 C., 200-800 C., 200-700 C., 300-700 C., 300-600 C., 300-500 C., 300-400 C., or 100-500 C., 100-400 C., 100-300 C., or 100-200 C. In particular instances, the activation can take place at about 500 C. or above in the presence of hydrogen, at about 700 C. or above in the presence of methane, and about 1300 C. and above in the presence of an inert environment (e.g., N.sub.2, He, or Ar, or any combination thereof).

B. Methods for Producing Syngas

[0046] Aspects of the disclosure relate to methods for producing syngas (i.e. hydrogen and carbon monoxide). The metal oxides described herein may be in contact with water and capable of producing hydrogen gas from the water. In another instance, the metal oxide may be in contact with carbon dioxide and be capable of producing carbon monoxide from the carbon dioxide. In a further embodiment, the metal oxide is in contact with water and carbon dioxide and is capable of producing hydrogen gas and carbon monoxide from the water and the carbon dioxide. In certain embodiments, the metal oxide is capable of producing hydrogen from water or carbon monoxide from carbon dioxide with solar radiation as the energy source.

[0047] Further aspects of the disclosure relate to a metal oxide in a water-splitting system. The water-splitting system may comprise a composition comprising a metal oxide described herein and water or carbon dioxide or both. In one embodiment, there is a water splitting system and a heat source. In a further embodiment, the heat source is sunlight. The heat source may also be produced mechanically or electrically by, for example, an oven, a microwave, a heat gun, an electric high temperature furnace, or other common laboratory source of heat.

[0048] Methods of the disclosure include a method for producing hydrogen gas from water, the method comprising contacting the metal described herein with water under reaction conditions sufficient to produce hydrogen gas from the water and the metal oxide. A further method relates to a method for producing carbon monoxide from carbon dioxide, the method comprising contacting a metal oxide of the disclosure with carbon dioxide under reaction conditions sufficient to produce carbon monoxide from the carbon dioxide and the metal oxide. In certain instances, the methods are conducted in a reactor. The reactor may be, for example, a tank, a pipe, or a tubular reactor. The reactor may be used as a continuous reactor or a batch reactor and may accommodate one or more solids, fluids, or gases (e.g. reagents, catalysts, or inert materials). The reactors may run at a steady-state or operated in a transient state. When a reactor is first brought into operation (after maintenance or in operation) it would be considered to be in a transient state, where key process variable change with time. The process variables may be estimated according to models such as batch reactor model, continuous stirred-tank reactor model, or plug flow reactor model. Key process variables may include, for example, residence time, volume, temperature, pressure, concentration of chemical species, and heat transfer coefficients. The reactor may be a packed bed in which the packing inside the bed comprises the metal oxide. The chemical reactor may also be a fluidized bed. The chemical reactions in the reactor may be exothermic or endothermic. In certain instances, the activated metal catalyst is used in the methods described herein. In further instances, the method comprises the catalyst in a non-activated state, and the activation of the catalyst occurs just before the reaction with water or carbon dioxide. In certain instances, the activation of the catalyst may be done in the reactor. In further instances, the catalyst is activated prior to reactor loading.

[0049] In the methods described herein, the water may be in a liquid, gaseous, or vapor phase. In one embodiment, the water is in a gaseous or vapor phase. In one non-limiting instance, the carbon dioxide can be obtained from a waste or recycle gas stream (e.g. from a plant on the same site, like for example from ammonia synthesis) or after recovering the carbon dioxide from a gas stream. A benefit of recycling such carbon dioxide as starting material in the process of the invention is that it can reduce the amount of carbon dioxide emitted to the atmosphere (e.g., from a chemical production site). In a further embodiment, the carbon dioxide is from a feed stream comprising carbon dioxide and water, and wherein hydrogen gas is produced from the water and the metal oxide. The chemical reaction may be carried out such that the reduction step under inert conditions is typically about 350 C. to 450 C. (or more preferably about 400 C.) above that of the reaction step (contact with water). Therefore, and by way of example, a reduction at about 1400 C. can be followed by a reaction at about 1000 C. The reduction step under a reducing environment (e.g., such as with a hydrocarbon) can be done at the same temperature as with an inert environment.

[0050] In some embodiments, the methods described herein may further comprise isolating the produced hydrogen or the produced carbon monoxide.

[0051] The resulting syngas can then be used in additional downstream reaction schemes to create additional products. FIG. 2 is an illustration of various products that can be produced from syngas. Such examples include chemical products such as methanol production, olefin synthesis (e.g. via Fischer-Tropsch reaction), aromatics production, carbonylation of methanol, carbonylation of olefins, the reduction of iron oxide in steel production, etc.

EXAMPLES

[0052] The present invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes only, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of noncritical parameters which can be changed or modified to yield essentially the same results.

Example 1

[0053] Preparation of metal oxides. CeO.sub.2, Fe.sub.2O.sub.3, Ce(Fe)O.sub.2-x, and Ce(Fe,U)O.sub.2-x (where x is less than 0.5) were prepared by the co-precipitation method from their nitrate salts at pH 8-9. Ammonium hydroxide was used as a precipitating agent. The single or mixed hydroxides were washed with de-ionized water until neutral pH, dried overnight at 100 C. then calcined to make the oxides at 500 C. for five hours or more. X-ray diffraction, temperature programmed reduction, BET surface area, and X-ray photoelectron spectroscopy were conducted to further identify and study the materials.

[0054] Activation and reaction of metal oxides. Reactions were conducted in a tubular reactor capable of working up to 1600 C. Prior to reactions, catalysts were either reduced with hydrogen or under inert atmosphere using N.sub.2 gas or vacuum (ca. 10.sup.2 torr). After the reduction step, the material was exposed to steam using N.sub.2 as a carrier gas and hydrogen was monitored using a GC equipped with a thermal conductivity detector (TCD).

[0055] FIG. 3 presents XRD of as prepared Ce.sub.0.5U.sub.0.5O.sub.2 that has been heated to the indicated temperatures while FIG. 4 presents similar results for Ce.sub.0.75Fe.sub.0.25O.sub.2. The numbers at the right hand side of the figure are those of crystallites size based on the 111 diffraction line of the fluorite structure at 2=28.54 (d=3.12 ). The numbers encompassed by a circle on top of the dashed vertical lines in the figure are the diffraction pattern of -U.sub.3O.sub.8. The numbers encompassed by a box on top of the dashed vertical lines are the diffraction pattern of the fluorite solid solution. In both cases the fluorite structure is maintained for the as prepared materials as well as after high temperature treatment, although the presence of U.sub.3O.sub.8 (FIG. 3, above 800 K) and Fe.sub.2O.sub.3 (FIG. 4, above 1000 K) could be observed. FIG. 4 is an in situ X-Ray Diffraction (XRD) of as prepared Ce.sub.0.75Fe.sub.0.25O.sub.2 at room temperature and then heated at the indicated temperatures. The numbers encompassed by a circle are those of the diffraction pattern of CeO.sub.2. The numbers encompassed by a box are those of Fe.sub.2O.sub.3. The inset is an expansion of the 2 x-axis for the (111) and (200) lines.

[0056] FIG. 5A presents an X-ray Photoelectron Spectroscopy (XPS) of Fe2p region for Ce.sub.0.75Fe.sub.0.25O.sub.2 as a function of reduction time (by Ar.sup.+ sputtering). FIG. 5B presents an X-ray Photoelectron Spectroscopy (XPS) of Fe2p region for Ce.sub.0.95Fe.sub.0.05O.sub.2 as a function of reduction time (by Ar.sup.+ sputtering). The XPS fitted peaks are: line 502 is Fe.sup.02p.sub.3/2, line 504 is Fe.sup.2+2p.sub.3/2, line 506 is Fe.sup.3+2p.sub.3/2, line 508 is Fe.sup.2+2p.sub.3/2 satellite, line 510-Fe.sup.3+2p.sub.3/2 satellite, line 512 is Ce Mn1 and CE Mn2; line 514 is Fe.sup.02p.sub.3/2, line 516 is Fe.sup.2+2p.sub.1/2, line 518 is Fe.sup.3+2p.sub.1/2, line 520 is F.sup.2+2p.sub.1/2 satellite, line 522 is Fe.sup.3+2p.sub.1/2 satellite. The behavior of Fe reduction as a function of sputtering follows a consecutive reduction Fe.sup.3+.fwdarw.Fe.sup.2+.fwdarw.Fe.sup.0. In this sequence, Fe.sup.3+ decreases upon reduction to Fe.sup.2+ which in turn is further reduced to Fe.sup.0.

[0057] FIG. 6A is a thermal Gravimetric Analysis (TGA) of CeO.sub.2. (Top) TGA profile of CeO.sub.2 at different heating rates (10, 15 and 20 C.) which exhibit four regions: (i) the first one was between 100-200 C. due to the loss of water; (ii) the second region was between 300-700 C. due to the loss of carboxylates and carbonates; (iii) the third was a flat region between 800-1100 C.; and (iv) the fourth region, the most important one, was between 1400-1600 C. due to the reduction of CeO.sub.2 (loss of O.sub.2). Kinetics of the reduction of CeO.sub.2 was also extracted from TGA (bottom) by plotting In (rate) vs 1/T, where the rate is dm/dT. dm was the change in mass; dT was the change in temperature. Moreover, the activation energy (E.sub.a) was calculated (values on the plot) from the slope of the plot. Within the investigated heating rate it appeared that the activation energy of 2.1 eV could be extracted. The fact that E.sub.a did not increase between 15 C./min and 20 C./min may indicate that extraction of oxygen anions from the lattice is not kinetically limited. In other words, there was enough residence time to remove oxygen anions during heating. FIG. 6 B is a thermal Gravimetric Analysis (TGA) of Ce.sub.0.75Fe.sub.0.25O.sub.2. The TGA profile of CeFe material at different heating rates (10, 15 and 20 C.), and was similar to that of CeO.sub.2, however, there was one more region appearing between 1100-1300 C. (in boxed on the figure), which was due to the reduction of Fe.sup.3+ cations in Fe.sub.2O.sub.3. The more pronounced response to the heating rate in this region may indicate that the rate at which this mass loss occurs was higher as the heating rate was increased.