WO2013012989A2

WO2013012989A2 - Method and system for production of hydrogen and carbon monoxide

Info

Publication number: WO2013012989A2
Application number: PCT/US2012/047303
Authority: WO
Inventors: Siu-Wai Chan
Original assignee: The Trustees Of Columbia University In The City Of New York
Priority date: 2011-07-19
Filing date: 2012-07-19
Publication date: 2013-01-24
Also published as: US20140130415A1; WO2013012989A3

Abstract

A method for preparing a fuel using oxygen-storing compound nanoparticles is provided, in which the nanoparticles is heated at a first temperature to release an amount of oxygen, thereby producing a reduced oxide compound, and the reduced oxide compound is exposed to a gas at a second temperature to produce the fuel. The gas can include carbon dioxide and water vapor, and the fuel can include carbon monoxide and/or hydrogen. The oxygen-storing compound nanoparticles can be nano ceria or nano ceria doped with one or more metals, such as Cu and/or Zr. A system for carrying out the method is also disclosed.

Description

Method and System for Production of Hydrogen and Carbon

Monoxide

CROSS REFERENCE TO RELATED APPLICATIONS This application claims priority from U.S. Provisional Application No.

61/509,370, filed July 19, 2011 and U.S. Provisional Application No. 61/638,960, filed April 26, 2012, the disclosure of each of which is herein incorporated by reference by its entirety.

BACKGROUND

Converting ¾0 and C0₂ gases into syngas (¾ and CO) can be a useful strategy for carbon sequestration and as a source of renewable energy. One technique for realizing energy conversion involves heating porous monolithic cerium dioxide (cena) to about 1600 °C to release oxygen, from the micron-sized (bulk) crystals/grains. When the resulting reduced ceria is cooled, ¾0 or C0₂ can be converted to H₂ and CO, respectively, while ceria is re-oxidized and ready for another thermal-gas cycle. This high reduction temperature of about 1600 °C is dictated by the reduction thermodynamics of bulk ceria (i.e. ceria grains in micron-size and larger). However, the high temperature can increase the cost of the process and reduce material lifetime. Therefore, it is desirable to lower the ceria reduction temperature to improve the economics and material stability of the process.

SUMMARY

The disclosed subject matter provides techniques for preparing a fuel using an oxygen-storing compound. In accordance with one aspect of the disclosed subject matter, methods of preparing a fuel using an oxygen-storing compound in the form of nanoparticles is provided. An exemplary method includes heating the nanoparticles at a first temperature to release an amount of oxygen, thus producing a reduced oxide compound, and exposing the reduced oxide compound to gaseous carbon dioxide and/or water vapor at a second temperature to produce CO and/or ¾, which are also referred as the gaseous fuel, or fuel. As liquid fuels such as gasoline and diesel oil can be prepared from CO and ¾, the mixture of CO and H2 is also referred as syngas. The first temperature can be about 700°C or lower, 450°C or lower, or 300°C or lower (e.g., between about 150°C-300°C). The second temperature can be, but not necessarily be, lower than the first temperature. The fuel can include at least one of carbon monoxide and molecular hydrogen, or mixture thereof.

The oxygen-storing compound nanoparticies can include cerium oxide (ceria) nanoparticies. The oxygen-storing compound can be cerium oxide doped with one or more transition metals, such as Cu, Zr, Pd, Hf, Fe, Cr, Co, Zn, Ni, Au, Ti, Pt, Rh, and Ru, as well as a rare earth metal such as Y, Gd, and the like. The oxides of these metals can also be used as dopants. In some embodiments, the transition metal is Cu. The amount of Cu can be such that it replaces about 0.01 to about 0.16 of cenum in the ceria, or about 0.05 to about 0.10, or about 0.08 of cerium (cation- atomic ratio). The oxygen-storing compound nanoparticies can have an average size (diameter) of about 1 to about 100 nm, about 2 to about 50 nm, about 2 to about 20 ran, about 2 to about 15 nm, 5 to about 15 nm, and about 5 to about 10 nm.

In accordance with one aspect of the disclosed subject matter, a syste for preparing a fuel from a gas using nanoparticies of an oxygen-storing compound is provided. The system includes a reactor adapted to hold nanoparticies of the oxygen- storing compound in a predetermined location and to receive the gas through a gas intake, and a heater adapted to heat the oxygen-storing compound nanoparticies to a first temperature to reduce the nanoparticies to form a reduced oxygen-storing compound. The oxygen-storing nanoparticies can be thermally reduced at the first temperature, and then an oxygen-carrying gas such as CO? and/or 3¾0 can be introduced through the gas intake to react with the reduced nanoparticies at a second temperature to form the fuel. The fuel can be carbon monoxide, molecular hydrogen, or mixture thereof. The heater can be a passive heater or exchanger, coupled with a heat source having a temperature of about 700°C or lower, 450°C or lower, or 300°C or lower. The heat source can be waste heat (e.g., in the form of a flue gas) from an industrial process or facility, for example, a refinery, a chemical plant, a nuclear plant or other power plants. Alternatively, the heater can utilize coal-burning, electric or other forms of energy, such as being coupled with a solar concentration device. The oxygen-storing compound nanoparticles can be ceria doped with one or more metals, as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a diagram depicting an example system for preparing a fuel from a gas using nanoparticles of an oxygen-storing compound according to the disclosed subject matter.

Figure 2 is a plot showing the relationship between lattice parameter of Ce0₂ nanoparticles and the size of the Ce0₂ nanoparticles.

Figure 3 is a plot of ¾ -Temperature Program Reduction (TPR) results of Cu-Ce0₂ nanoparticle samples having different loading levels of Cu (1.6%, 8.2% and 1 .6%). The area under the peak corresponding to the amount of ¾ converted to F1₂0 by the oxygen in the sample. The larger the area under the curve means a larger reduction capacity of the sample. Temperature program simply means a constant heating rate.

Figure 4 is a plot showing the results of a therm ©gravimetric analysis (TGA) of a sample of Cu-Ce0₂ nanoparticles.

Figure 5 is a plot showing the time course of the temperature and the weight of a sample of Cu-Ce0₂ nanoparticles during a TGA test.

Figure 6 is a TGA plot of a sample of undoped Ce0₂ nanoparticles.

Figure 7 is a TGA plot of a sample of Zr-Ce0₂ nanoparticles.

Figure 8 is a plot showing the changes in the lattice parameter and particle size of a nano ceria sample having a starting average size of 6.7 nm when the sample is heated.

Figure 9 is a diagram depicting an example support structure for supporting nanoparticles of an oxygen-storing compound according to some embodiments of the disclosed subject matter. DETAILED DESCRIPTION

The disclosed subject matter provides methods and systems for preparing a fuel, such as H₂ and/or CO, using oxygen-storing compound

nanoparticles. The process involves reducing the oxygen- storing compound nanoparticles at a first temperature, e.g., by heating, and exposing the reduced nanoparticles to a gas including C0₂ and/or water vapor to produce the fuel. Upon reacting with the gas, the reduced nanoparticles are oxidized and the original oxygen- storing compound can be restored. As will be further described below, because the reduction of the oxygen-storing compound nanoparticles can be performed at much lower temperatures than previously known, the disclosed techniques can offer significant benefit in utilizing waste heat in the production of valuable gaseous fuel.

As illustrated in Figure 1 , an exemplary system for preparing a fuel using nanoparticles of an oxygen-storing compound includes a reactor 100, which has an inlet 1 15 and an outlet 120. The oxygen-storing compound nanoparticles 105 can be loaded in a support structure 107 at a predetermined position in the reactor, such as a fixed bed of porous inert supports (e.g., porous ceramics) as those commonly used in catalytic reactions or as wash-coats on monoliths as in catalytic converters of automobiles. The heater 1 10 can be located in the reactor, e.g., near the support structure 107, to provide heat of sufficiently high temperature for the reduction of the oxygeivstoring compound nanoparticles 105. Alternatively, the heater 1 10 can be positioned external to the reactor to preheat a gaseous medium (such as N₂ or 0₂) to be introduced into the reactor to contact the oxygen-storing nanoparticles, thereby regulating the temperature of the oxygen-storing nanoparticles. Such an external heater can also be used to preheat C0₂ and/or water vapor to be introduced to the reactor.

The heater can be coupled with a heat source 160, which can be waste heat from an industrial process or facility, for example, refinery, chemical plant, nuclear plant or other power plants. The waste heat from these sources can have various grades (i.e. different maximum temperatures). The heat source can also include a solar concentration device or other heat generation devices.

After the reduction of the oxygen-storing compound nanoparticles 105, the reduced oxide can be contacted with a gas 125 (C0₂, ¾0 or a mixture thereof) introduced from the inlet 1 15. The reaction product of the reduced oxygen- storing compound nanoparticles with the gas can include a fuel 130 to be released from outlet 120.

The reactions involved in the process can be carried out in a single reaction vessel, and the oxygen-storing nanoparticles need not be moved during the process. However, although shown as a single-vessel structure, the reactor herein can include multiple reaction vessels. The oxygen-storing compound nanoparticles can be loaded in different vessels at different reaction stages. For example, after the initial heating of the oxygen-storing compound nanoparticles in a first vessel at a first temperature, the reduced oxygen-storing nanoparticles can be transferred to a second different vessel to carry out the fuel production reaction at a second temperature. Thereafter, the regenerated oxygen- storing compound nanoparticles can be transferred back to the first vessel for re-heating. The second vessel can also be provided with a mechanism to periodically or continuously replace portions of the regenerated oxygen-storing nanoparticles with reduced oxide transferred from the first reactor, thus allowing for continuous operation of the process. For example, the mechanism can include a supporting structure for accommodating the oxygen-storing

nanoparticles, and having an inlet for receiving freshly reduced nanoparticles (e.g., from the first reaction vessel) and an outlet for releasing the "spent nanoparticles" (the nanoparticles after the fuel production reaction has run for a period of time). The inlet and the outlet can be located at opposite ends of the supporting structure. A retrieving device can be used periodically or continuously to remove a portion of the spent nanoparticles from the supporting structure in the second reaction vessel.

In an alternative embodiment, the support structure can include at least one flow channel having a wall loaded with the oxygen-storing particles. Figure 9 depicts an example support structure which includes an array of flow channels with the adjacent channels plugged at alternative ends. The walls of the flow channels can be made from a porous material to retain the oxygen-storing particles 105 (e.g., as a wash coat on the channel walls) while allowing a gas to pass through. In this manner, a gaseous medium (e.g., pre-heated by an externally positioned heater) can be introduced into the structure to regulate the temperature of the oxygen-storing nanoparticles. After the oxygen-storing nanoparticles are reduced, C0₂ gas and/or water vapor can be introduced in the structure to produce a fuel at a suitable second temperature, as explained further below.

For illustration and understanding, the method and system of the disclosed subject matter are described below in conjunction with each other. It is appreciated that the various embodiments of the methods concerning the oxygen- storing nanoparticles and operating conditions are also applicable to the system, and vice versa.

Using water vapor as an example gas, the reactions occurring during the above described process can be written as:

Me_pO_q -» Me_pO_q-y + y/2 0₂ (1) y H₂0 + Me_pC y -» Me_pO_q + y H₂ (2)

y H₂0 -> y/2 0₂ + y ¾ (net reaction) where Me_pO_q is the oxygen-storing compound nanopariicles, Me is a metal in the oxygen-storing compound. As will be further described below, Me can include more than one metals. For convenience, the first reaction can be referred to as the nanoparticle-reduction reaction, and the second reaction can be referred to as the fuel- production reaction. The net reaction of the overall process is water vapor being split into oxygen and hydrogen, the latter can be used as a fuel. Using a gas containing CO?, the second reaction would produce CO and original Me_pO_q. The regenerated Me_pO_q can be reused in reaction (1) and the process can be repeated. As such, although the oxygen-storing compound is involved in the chemical reactions during the process, it is not consumed, and its role in the overall process can be considered catalytic.

In the above process, the first temperature (or reduction temperature) can be selected to be about 700°C or lower, 450°C or lower, or 300°C or lower. The second temperature (the gas-nanoparticle reaction temperature) can be same as or different from the first temperature, e.g., from about 200 to about 900°C, from about 250 to about 600°C, from about 300 to 500°C, etc. The second temperature can be selected according to the pressure, purity, or flow speed of the gas fed into the reactor, and the like.

The oxygen-storing nanoparticles can be, for example, ceria nanoparticies (or nano ceria). As used herein, oxygen-storing nanoparticles, such as nano ceria (or doped nano ceria), can have an average particle size of about 1 to about 100 nm, from 2 to about 50 nm, about 5 to about 20 nm, about 5 to about 15 nm, or from about 5 to about 10 nm. The first reaction where oxygen-storing nanoparticles lose oxygen can also be referred to as the "reduction" of the oxygen-storing nanoparticles. For example, before reduction, nano ceria (Ce0₂) can have a small amount of oxygen deficiency (appearing as a few missing oxygen ions, i.e. oxygen vacancies), i.e., in the form of CeC _y where y is a number from 0 to about 0.06.

When ceria is doped, y can increase accordingly. Nano ceria remains in the cubic fluorite crystal-structure without collapsing or changing into a different crystal- structure. After reduction, the oxygen deficiency increases, and y can be a number about 0.5 or smaller, e.g., about 0.15 or smaller. In some embodiments, y can be approximately 0.1. For convenience, "Ce0₂" as used herein includes cerium dioxide having some oxygen vacancies.

Unlike micron-sized or bulk ceria, nano ceria can release oxygen at much lower temperature, e.g., about 800 °C or below. While not wishing to be bound by any particular theory, it is surmised that this property of nano-ceria is due to its increased lattice parameter and a few missing oxygen ions, which results in not only greater o ygen -storage capacity compared with bulk or micro-sized ceria, but lower activation energy for the oxygen to move in and out of the lattice. As shown in Figure 2, with decreasing particle size of ceria nanoparticles, the lattice parameter of Ce0₂ increases. The increase is particularly pronounced when the particle size is smaller than 20 nm. Compared with micron-sized ceria particles, the increase of lattice parameter of cena nanoparticles having an average size of about 6 nm is about 0.45%, and with about 1.5% of O^2" ions missing (i.e., oxygen vacancies) from the close- packed nature of O^2" ions. These seemingly small numbers, however, are significant considering the virtual incompressibility of crystal structure of ceria. The bulk modulus of ceria is reported to be greater than 200 GPa. The oxygen-storing nano articles, such as nano ceria, can further include one or more dopants, such as a metal or an oxide of the metal, e.g., transition metal such as Cu, Zr, or Pd, or their oxides. Doped oxygen-storing nanoparticles can have increased oxygen-storage capacity, and can also have reduced thermal reduction temperature. In some embodiments, Cu-doped nano ceria where 1 - 16 % (cation-atom ratio), or 5-12% of Ce is replaced by Cu can be used, which can release oxygen at a temperature of below 700 °C. in particular, nano Cu-Ceria having about 8% of Cu doping level appear to have high oxygen-storing and release capacity, as is demonstrated in Figure 3, which is a H2-Temperature Program Reduction (TPR) results of Cu-Ce0₂ samples having different doping amounts (or loading levels) of Cu ( 1 .6%, 8.2% and 19.6%). In Figure 3, the intensity (arbitrary units) signifies the amount of oxygen released from the Cu-doped oxygen-storing nanoparticles at different temperatures as the H₂ is being consumed.

Other dopants, such as zirconium (e.g., 10-60 % of Ce is replaced by Zr) or palladium (e.g., 0.1 -5% of Ce is replaced by Pd), can also be used. Other transition metal dopants, such as Hf, Fe, Co, Cr, Zn, Ni, Ti (or their oxides) can also be used, For example, Hf can be used to replace 0.5-20%o Ce in nano ceria. Rare earth metal dopants, such as Y and Gd and their oxides, can also be used, as well as those metals useful as catalysts in the water-gas shift (WGR) reaction, such as Pt, Rh, Au, and Ru or their oxides.

The dopant can be incorporated into the oxygen-storing nanoparticles by co-precipitation of precursor solutions of a salt of the dopant metal with cerium- containing precursor solutions. For example, methods for preparing Cu-doped nano ceria, Pd-doped nano ceria, and Zr-doped nano ceria, are disclosed in International patent application publications WO201 0045484, WO2010062694 and US Patent No. 6,449, 163 , the disclosures of all of which are incorporated by reference herein in their entireties. For low levels of doping, e.g., 0.01 -5% of transition metal dopant in nano ceria, impregnation of the dopants onto the nano ceria can also be used.

Figure 4 depicts the results of a thermogravimetric analysis (TGA) of a sample of Cu-doped nano ceria (29.9 mg, average size ~5 nm) with 8% of Ce replaced by Cu, i.e., Cu_xCei-_x0₂._y with x=0.08 (also referred to as 8%> Cu-Ce0₂ herein.

Similarly, r% Me-Ce0₂ means that the loading level of the metal Me in Ce0₂ is r%). The analysis was conducted in three stages: (1) Heating the sample from room temperature to 700°C in 100% 0₂. The weight loss from room temperature to 300°C can be from the loss of surface adsorbed gases including water vapor and residual organic or volatile compounds from the synthesis of nano Cu-Ce0₂; the weight loss from 450 to 700°C can be attributed to the reduction of the nano-oxide (i.e., loss of lattice oxygen); (2) Cooling the sample down from 700°C to 50°C in 100% 0₂: the weight gain can be attributed to re-oxidation of the nano Cu-Ce0₂; and (3) Re-heating the sample from 0°C to 700°C in 100% N₂. At the end of stage 3 (i.e., as shown by the third curve in Figure 4), the weight of the reduced nano Cu-CeOi was the same as that of the reduced nano Cu-Ce0₂ at the end of stage 1 , Figure 5 is an alternative view of the results of another experiment with the same experimental setup and 8% nano Cu-Ce0₂, but a different start sample weight, with time course of the heating- cooling cycle shown on the horizontal axis.

Approximately 1 wt% loss of nano Cu~Ce0₂ (in stage 3) from Figures 4 and 5 can be attributed to the reversible loss of lattice oxygen in the Cu-Ce0₂ which is thermodynainically prohibited in bulk or micron-size particles of ceria unless the temperature is raised to above 1500°C or higher. Furthermore, it is noted that the gain of lattice oxygen in. stage 2 (cooling in 0₂ ) occurred at very low temperature (below 300°C), and more significantly, the loss of lattice oxygen in stage 3 (heating in N₂ atmosphere) primarily occurred below 450°C. Most of the oxygen loss occurred below 300 °C. This means that in the H₂ or CO production process described above, the temperature for the reduction of nano Cu-Ce0₂ can be carried out at a temperature below 700°C, or below 450°C (or even below 300°C), depending on the atmosphere in which the nano Cu-Ce0₂ is heated. For example, if the nano Cu-Ce0₂ is heated in the air, which includes approximately 21 % of oxygen, the reduction can occur between 450°C and 700°C.

As a comparison with Figure 4, Figure 6 shows a similar heating- cooling-reheating cycle of a pure (i.e., undoped) nano-Ce0₂ sample. The oxygen loss at the reheating stage in N2 also occurs primarily below 450°C. However, the amount of lattice oxygen released from pure nano-Ce0₂ is less than one-third of that of the 8% nano Cu-Ce0₂. Likewise, Figure 7 shows a heating-cooling-reheating cycle of a Zr-doped CeC sample (Zro.3gCeo.62O2). The oxygen loss for this sample at stage 3 is comparable to that of 8% nano Cu-Ce0₂ (about 1 wt%).

The lower temperature and forgiving requirement for the atmosphere for thermal reduction of the oxygen-storing nanoparticles (e.g., ceria or doped nano ceria) can enable the ¾ or CO production process to utilize lower grade heat, such as the exhaust or flue gas produced from industrial process (from power plants, chemical plants, petrochemical plants, etc. that produces waste heat at low temperatures such as 700°C or lower ). Furthermore, conducting the reduction of the oxygen-storing nanoparticles at a lower temperature, such as below 550°C or below 400°C can reduce, inhibit, or avoid crystal growth or coarsening at higher temperatures, which negatively impacts the oxygen storage capacity of the material and hinders reuse of the oxygen-storing nanoparticles. As shown in Figure 8, a sample of undoped nano ceria of a starting average size of 6.7 nm can coarsen (indicated by the particle-size increase) when heated, and the coarsening is more pronounced when the temperature is greater 550 °C. Below 550°C, the coarsening effect is insubstantial. Transition metal doped ceria nanoparticles can have further improved stability against coarsening when heated. For example, below 600°C, nano Zr-Ce0₂ can maintain long term particle size stability and oxygen-storing capacity over repeated heating without coarsening. The foregoing merely illustrates the principles of the disclosed subject matter. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the disclosure herein. It will thus be appreciated that those skilled in the art will be able to devise numerous methods which, although not explicitly shown or described herein, embody the principles of the disclosed subject matter and are thus within its spirit and scope of the appended claims.

Claims

1 . A method for preparing a fuel from an oxygen-storing compound in the form of nanoparticles, comprising: heating the nanoparticles at a first temperature to release an amount of oxygen, thereby producing a reduced oxide compound; and exposing the reduced oxide compound to a gas at a second temperature to produce the fuel, wherein the gas is selected from the group consisting of carbon dioxide and water vapor.

2. The method of claim 1 , further comprising selecting the second temperature to be less than the first temperature.

3. The method of claim 1 , further comprising selecting the first temperature to be about 700°C or lower.

4. The method of claim 1 , further comprising selecting the first temperature to be about 450°C or lower. 5. The method of claim 1 , further comprising selecting the first temperature to be about 150°C to about 300 °C.

6. The method of claim 1 , wherein the fuel comprises at least one of carbon monoxide and molecular hydrogen.

7. The method of claim 1 , wherein the oxygen-storing compound comprises cerium oxide.

8. The method of claim 1, wherein the oxygen-storing compound comprises cerium, oxide doped with a transition metal or an oxide thereof.

9. The method of claim 1, wherein the oxygen-storing compound comprises a cerium oxide doped with a rare earth metal or an oxide thereof. 10. The method of claim 8, wherein the transition metal comprises a metal selected from the group consisting of Cu, Zr, and Pd.

1 1. The method of claim 8, wherein the transition metal comprises a metal selected from the group consisting of Hf, Fe, Co, Cr, Zn, Ni, Au, Ti, Pt, Rh, and Ru.

12. The method of claim 10, wherein the transition metal comprises Cu such that Cu replaces about 5% to about 10% of cerium in the cerium oxide.

13. The method of claim 12, wherein the Cu replaces about 8% of cerium in the cerium oxide. 4. The method of claim 1, wherein the average size of the nanoparticles of the oxygen-storing compound is about 2 to about 15 nm.

15. A system for preparing a fuel using nanoparticles of an oxygen-storing compound and a gas selected from the group consisting of carbon dioxide and water vapor, comprising: a reactor adapted to receive nanoparticles of the oxygen-storing compound in a predetermined location therein and the gas through a gas intake; a heater adapted to heat the oxygen-storing nanoparticles to a first temperature to reduce the nanoparticles to form a reduced oxygen-storing compound; wherein the gas intake is positioned to deliver the gas to contact the reduced oxygen-storing compound at a second temperature, thereby converting at least a portion of the gas to the fuel.

16. The system of claim 15, wherein the fuel comprises at least one of carbon monoxide and molecular hydrogen.

17. The system of claim 15, wherein the heater is configured to deliver heat having a maximum temperature of about 450°C.

1 8. The system of claim 17, wherein the heater is coupled with a heat source having a maximum temperature of about 450°C or lower.

19. The system of claim 17, wherein the heat source is waste heat from an industrial process.

20. The system of claim 17, wherein the heater is coupled with a solar concentration device.

21. The system of claim 15, wherein the oxygen-storing compound comprises cerium oxide. 22. The system of claim 15, wherein the oxygen-storing compound comprises cerium oxide doped with a transition metal or an oxide thereof.

23. The system of claim 21 , wherein the transition metal comprises a metal selected from the group consisting of Cu, Zr, and Pd.

24. The system of claim 15, wherein the average size of the nanoparticles of the oxygen-storing compound is about 2 to about 15 nm.

25. The system of claim 15, further comprising a support structure having at least one flow channel having a gas-permeable wall, the oxygen -storing nanoparticles loaded on the support structure.