SCALABLE PREPARATION OF OXYGEN CARRIERS FOR CHEMICAL LOOPING

Abstract

Oxygen carriers for chemical looping and scalable methods of preparation thereof. Wet impregnation of active metal precursors into porous substrates, together with selective adsorption of the precursors on the pore surfaces, enables transition metal oxides derived from the precursors to disperse throughout the substrate, even at the nanoscale, without increased sintering or agglomeration. The porous substrate can be an oxide, for example SiO.sub.2. The oxygen carriers can comprise relatively large oxide loadings of over about 20 wt % and exhibit high reactivity over many regeneration cycles with substantially no loss in oxygen transport capacity or decrease in kinetics. The use of multiple transition metals, for example NiO in addition to CuO, can greatly enhance chemical looping performance.

Claims

1. A method for making a chemical looping oxygen carrier, the method comprising: wet impregnating a porous substrate with one or more active metal precursors; selectively adsorbing the one or more active metal precursors onto a porous substrate; and thermally converting the one or more active metal precursors to one or more metal oxides.

2. The method of claim 1 wherein the one or more metal oxides are present in the oxygen carrier in an amount greater than approximately 20 wt %.

3. The method of claim 2 wherein the one or more metal oxides are present in the oxygen carrier in an amount greater than or equal to approximately 33 wt %.

4. The method of claim 3 wherein the one or more metal oxides are present in the oxygen carrier in an amount greater than approximately 40 wt %.

5. The method of claim 1 wherein the one or more active metal precursors comprise ammonia.

6. The method of claim 5 wherein the one or more active metals comprise transition metals and the ammonia creates a transition metal coordination complex for each oxygen carrier.

7. The method of claim 6 wherein at least one of the transition metal coordination complexes comprises an ammine or a chlorine complex.

8. The method of claim 7 wherein one of the one or more transition metals comprises CuO and the corresponding transition metal coordination complex comprises tetraammine copper nitrate (TACN).

9. The method of claim 7 wherein one of the one or more transition metals comprises NiO and the corresponding transition metal coordination complex comprises hexaammine (HANN).

10. The method of claim 1 comprising approximately uniformly dispersing each metal oxide on the porous substrate and within the pores of the porous substrate.

11. The method of claim 1 wherein the porous substrate comprises an oxide.

12. The method of claim 11 wherein the oxide comprises Al.sub.2O.sub.2, TiO.sub.2, or ZrO.sub.2.

13. The method of claim 1 wherein the porous substrate is macroporous, mesoporous, microporous, or nanoporous.

14. An oxygen carrier for chemical looping, the oxygen carrier comprising: a porous substrate; and one or more metal oxides each approximately uniformly dispersed on the porous substrate and within the pores of the porous substrate.

15. The oxygen carrier of claim 14 wherein the one or more metal oxides are present in the oxygen carrier in an amount greater than approximately 20 wt %.

16. The oxygen carrier of claim 15 wherein the one or more metal oxides are present in the oxygen carrier in an amount greater than or equal to approximately 33 wt %.

17. The oxygen carrier of claim 16 wherein the one or more metal oxides are present in the oxygen carrier in an amount greater than approximately 40 wt %.

18. The oxygen carrier of claim 14 wherein the one or more metal oxides comprise transition metal oxides.

19. The oxygen carrier of claim 18 wherein one of the one or more transition metal oxides comprises CuO.

20. The oxygen carrier of claim 18 wherein one of the one or more transition metal oxides comprises NiO.

21. The oxygen carrier of claim 14 wherein the porous substrate comprises an oxide.

22. The oxygen carrier of claim 21 wherein the oxide comprises Al.sub.2O.sub.2, TiO.sub.2, or ZrO.sub.2.

23. The oxygen carrier of claim 14 wherein the porous substrate is macroporous, mesoporous, microporous, or nanoporous.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings, which are incorporated into and form a part of the specification, illustrate the practice of embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating certain embodiments of the invention and are not to be construed as limiting the invention. In the drawings:

[0019] FIG. 1 is a schematic showing chemical looping combustion with CuO as the oxygen carrier.

[0020] FIG. 2 is a schematic of molecular interactions for selective adsorption, showing regimes for anionic and cationic metallic species.

[0021] FIG. 3A is a schematic for quartz fluidized bed setup.

[0022] FIG. 3B is a detail of the reactor in the fluidized bed setup of FIG. 3A.

[0023] FIG. 4A is a scanning electron microscope (SEM) micrograph of the WI-20-SiC sample.

[0024] FIG. 4B is an SEM micrograph of the DI-20-SiC sample.

[0025] FIG. 4C is an SEM micrograph of the DI-20-SiC sample of FIG. 4B set in epoxy and polished, showing the internal matrix.

[0026] FIG. 5A is an SEM micrograph of the DI-20-SiO.sub.2 sample.

[0027] FIGS. 5B and 5C are SEM micrographs of the WIFM-20-SiO.sub.2 sample at higher and lower magnifications, respectively.

[0028] FIG. 5D is an SEM micrograph of an SAWI-25-SiO.sub.2 sample produced without a wash step.

[0029] FIG. 5E is an SEM micrograph of the sample of FIG. 5D at higher magnification showing CuO sub-particles.

[0030] FIG. 5F is an SEM micrograph of an SAWI-25-SiO.sub.2 sample produced using a wash step, showing fewer visible CuO subparticles.

[0031] FIG. 6A shows a thermogravimetric analyzer (TGA) profile for CuO loading estimates for DI-20-SiC, 15-minute cycles at 900° C., CuO/Cu.sub.2O cycling. Shaded regions are N.sub.2 atmosphere. The Y-axis is the wt % change from the oxidized mass at the start of the first cycle.

[0032] FIG. 6B shows a TGA profile for CuO loading estimates for DI-20-SiO.sub.2, 15-minute cycles at 900° C., CuO/Cu.sub.2O cycling. Shaded regions are N.sub.2 atmosphere. The Y-axis is the wt % change from the oxidized mass at the start of the first cycle.

[0033] FIG. 6C shows a TGA profile for CuO loading estimates for SAWI-25, 15-minute cycles at 900° C., CuO/Cu.sub.2O cycling. Shaded regions are N.sub.2 atmosphere. The Y-axis is the wt % change from the oxidized mass at the start of the first cycle.

[0034] FIG. 6D shows a TGA profile for CuO loading estimates for SAWI-25, 15-minute cycles at 900° C., Cu/CuO cycling. Shaded regions are 4 vol % H.sub.2/N.sub.2 reducing atmosphere. The Y-axis is the wt % change from the oxidized mass at the start of the first cycle.

[0035] FIG. 7A is an SEM/EDS image of an SAWI-25-SiO.sub.2 particle.

[0036] FIGS. 7B and 7C are Cu and Si elemental analysis maps, respectively, for the particle of FIG. 7A.

[0037] FIG. 8 shows a TGA profile of SAWCI-33, CuO/Cu.sub.2O cycling, 8-minute cycles for both decomposition in inert N.sub.2 atmosphere and oxidation in air at 950° C.

[0038] FIG. 9 shows x-ray diffraction (XRD) patterns of the oxygen carrier after impregnation and calcination at 1000° C. The top curve is for bimetallic SAWCI-25-SiO.sub.2. The bottom curve is for monometallic SAWI-25-SiO.sub.2. CuO peaks are denoted with (.box-tangle-solidup.), α-quartz (SiO.sub.2) peaks are denoted with (o), and α-cristobalite (SiO.sub.2) peaks are denoted with (.circle-solid.).

[0039] FIG. 10A is an SEM micrograph SAWCI-33-SiO.sub.2 after impregnation and NO.sub.2 decomposition at 350° C.

[0040] FIG. 10B is an SEM micrograph of SAWCI-12-SiO.sub.2 after calcination at 1000° C.

[0041] FIG. 100 is an SEM micrograph of SAWCI-12-SiO.sub.2 after fluidized bed testing at 975° C. with CO reducing gas.

[0042] FIG. 11A shows the recorded TGA weight change for SAWCI-33-SiO.sub.2 over 100 cycles at 950° C.

[0043] FIG. 11B is a detail of FIG. 11A showing decomposition and oxidation for SAWCI-33-SiO.sub.2 at the selected TGA cycles.

[0044] FIG. 11C shows TGA decomposition conversion curves for SAWCI-33-SiO.sub.2 at selected cycles at 950° C.

[0045] FIG. 11D shows TGA oxidation conversion curves for SAWCI-33-SiO.sub.2 at selected cycles at 950° C.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0046] Embodiments of the present invention are novel selective adsorption wet co-impregnation methods for the manufacture of Cu-based OC on SiC and SiO.sub.2 supports, enhancing the metal-support interaction to increase dispersion and reduce CuO sintering. In other embodiments one or more other transition metal oxides may be homogeneously bound on the surface and within the pores of a porous, for example microporous, mesoporous, or nanoporous, substrate using selective adsorption wet impregnation or co-impregnation. As used throughout the specification and claims, the term “selective adsorption” means any method by which a compound, for example as a metal coordination complex, is attracted to and bound to a substrate, including but not limited to selective adsorption, electrostatic adsorption, strong electrostatic adsorption, charge-enhanced impregnation, and the like, and the term “selectively adsorbed” correspondingly means any state in which a compound, for example as a metal coordination complex, is attracted to and bound to a substrate, including but not limited to selectively adsorbed, electrostatically adsorbed, strongly electrostatically adsorbed, charge-enhanced impregnated, and the like.

[0047] Particles prepared using these methods can comprise relatively large oxide loadings, preferably over about 20 wt %, and exhibit agglomeration resistance and high reactivity over many regeneration cycles. In one example CuO/NiO@SiO.sub.2 oxygen carrier (33 wt % CuO, 1 mol Ni/99 mol Cu) showed no signs of agglomeration up to 975° C. in fluidized bed testing, cycling between CuO/Cu.sub.2O. Thermogravimetric analyzer (TGA) analysis showed no loss in oxygen transport capacity or decrease in kinetics over 100 cycles. SiC is a high-strength, relatively low-cost substrate, and SiO.sub.2 achieves better CuO metal dispersion. Capillary migration during drying, metal-support interaction, and alloying with <1 wt % NiO were studied to meet the requirements of a high-quality oxygen carrier material. Particles were characterized using N.sub.2 sorption tests and SEM, and they were screened based on TGA and fluidized bed performance. Samples that exhibited good reactivity and agglomeration resistance were examined with crush strength testing, x-ray diffraction (XRD), and multicycle TGA tests.

Example

[0048] Raw Materials. Two support raw materials were investigated, nonporous, relatively inexpensive SiC, and porous SiO.sub.2 (Cariact Q-10C from Fuji Silysia). The properties of these two materials are summarized in Table 2. The SiC support comprising up to 20 wt % CuO content was studied. The following two copper precursors were used in this study: (i) copper nitrate trihydrate (Sigma Aldrich, 99%) to prepare copper nitrate solution using deionized water; and (ii) tetraammine copper nitrate (TACN), prepared with the addition of excess ammonia (99%) until all copper hydroxide precipitate had dissolved, followed by dilution with deionized water. For co-impregnation of CuO and NiO, nickel nitrate hexahydrate (Sigma Aldrich, 99%) was dissolved into solution, then excess ammonia was added to produce hexaammine nickel nitrate (HANN), again followed by dilution. An aliquot of this solution was added to TACN to produce TACN/HANN for co-impregnation on SiO2.

TABLE-US-00002 TABLE 2 Measured characteristics of the support materials Support SiC SiO2 Cost ($/kg, August 2018) 3 95 Size Range (μm) 106−250 295−425 Crush Strength (MPa) 408 22.3 BET Surface Area (m.sup.2/g) 0.1 330 Bulk Density (g/mL) 1.4 0.5

[0049] Oxygen Carrier on SiC. CuO supported on SiC OCs were prepared via wet impregnation (WI) or dry impregnation (DI). Lab scale WI was investigated in a rotary vaporizer (RV) and a rotary kiln (RK-1). Samples consisted of approximately 50 g of OC with 20 wt % CuO. All batches were prepared by wetting the support with excess Cu precursor solution, drying, and nitrate decomposition and oxidation to CuO at 350° C. Wetting was performed with copper nitrate and vigorous stirring. In both reactors, drying was performed at ambient pressure at 85° C. Drying and decomposition were carried out in a heating mantle without stirring. Four additions, which each included wetting, drying and nitrate decomposition steps, were performed to reach the desired value of 20 wt % CuO.

[0050] Oxygen Carrier on SiO.sub.2. OC particles of 20 wt % CuO supported on SiO.sub.2 were prepared by DI, following the same steps as those for the SiC support, except the volume of precursor used was the pore volume of the substrate as determined by Brunauer, Emmett and Teller (BET) analysis. A novel drying method was examined by preparing CuO on SiO.sub.2 supports via wet impregnation with filtration and microwave drying (WIFM). The high pore volume of SiO.sub.2 allowed high precursor retention after filtration, in contrast to SiC. Microwave drying was used to reduce copper precursor migration. Copper nitrate precursor was added slowly to the SiO.sub.2 support in a Buchner funnel with stirring until excess volume was observed, to ensure homogeneous wetting. The funnel was then filtered of excess precursor, dried via microwave, followed by nitrate decomposition and oxidation at 350° C. in the heating mantle. A total of three additions of wetting, drying, and decomposition was required for the desired 20 wt % CuO material.

[0051] Novel selective adsorption methods with the copper precursor TACN were also explored for the SiO.sub.2 support. Cu-based OCs with 25 wt % CuO were prepared via the selective adsorption wet impregnation (SAWI) method by wetting approximately 20 g of SiO.sub.2 with 4.7 mL of 0.45 M TACN/g SiO.sub.2, drying in the RV over several hours in a 120° C. oil bath, followed by washing with de-ionized water and nitrate decomposition at 350° C. OCs consisting of bimetallic CuO/NiO with 33.3 wt % CuO on SiO.sub.2 were prepared via selective adsorption wet co-impregnation (SAWCI). The same procedure as SAWI was used, but the precursor solution contained a minor amount of nickel nitrate (NN) which complexes with excess ammonia in the TACN to form HANN, for a molar ratio of 1 mol Ni/99 mol Cu. For 33.3 wt % CuO, a precursor volume of 4.7 mL/g SiO.sub.2 was used for three additions. SAWI and SAWCI preparation methods produced free-flowing particles after the drying stage, in contrast to impregnation with copper nitrate. A summary of the materials examined in this study is shown in Table 3, which includes the following codes: W=wet; D=dry; I=impregnation; F=filtration; M=microwave dry; SA=selective adsorption; CI=co-impregnation; NN=nickel nitrate; TACN=tetraammine copper nitrate.

TABLE-US-00003 TABLE 3 Summary of prepared oxygen carriers. Code Desired CuO Loading (wt %) Precursor Phase separation Additions WI-20-SiC 20 Copper nitrate No 4 DI-20-SiC 20 Copper nitrate No 4 DI-20-SiO.sub.2 20 Copper nitrate No 2 WIFM-20-SiO.sub.2 20 Copper nitrate Filtration 3 SAWI-25-SiO.sub.2 25 TACN No 2 SAWI-40-SiO.sub.2 40 TACN No 4 SAWCI-12-SiO.sub.2 12 TACN/NN No 1 SAWCI-25-SiO.sub.2 25 TACN/NN No 2 SAWCI-33-SiO.sub.2 33 TACN/NN No 3 SAWCI-40-SiO.sub.2 40 TACN/NN No 4

[0052] One pilot-scale batch (3 kg) was produced using the SAWCI technique using a rotary kiln (40 cm diameter, 1.8 m long). The surface of the kiln was externally heated with natural gas burners, and NO.sub.2 was captured in a NaOH absorbing column. Bimetallic OC with 12 wt % CuO and 1 mol Ni/99 mo Cu was produced over one addition. Drying was performed at 120° C. at 3 rpm over 2 h. Free-flowing particles were then retrieved, washed with tap water, and loaded into the kiln for nitrate decomposition and oxidation at 350° C.

[0053] Particle Characterization. To measure the quality of produced materials and screen for impregnation methods with potential for scale-up, particle size distribution (wt %) was analyzed by sieving. CuO loading was determined by TGA cycling (see below), and in some cases confirmed by inductively coupled plasma mass spectroscopy (ICP-MS) elemental analysis. The crush strength of individual particles was analyzed with a Shimpo FGE-5X with 25 replicates. The crush force was normalized to crush pressure by dividing by the average cross-sectional area of the particles, resulting in units of MPa. Bulk density was determined by performing the tapped bulk density test for powders. Surface area and pore volume was examined with a Micromeritics Tristar II surface area and porosity analyzer using N.sub.2 physisorption at 77 K. Surface morphology, in particular CuO crystallite size and dispersion, was analyzed with a FEI Quanta 600 scanning electron microscope (SEM) with elemental composition analysis by energy dispersive X-ray spectroscopy (EDS or EDX). To examine penetration to the interior of the pore structure of the support, particles were set in epoxy, polished, and examined with SEM-EDX map scans. X-ray powder diffraction (XRD) was used to determine the crystalline phases, using a DT Bruker instrument.

[0054] Fluidized Bed Testing. A screening test was performed to identify OCs with good resistance to sintering using a quartz fluidized bed, a schematic of which is shown in FIG. 3A. As shown in FIG. 3B, the reactor consisted of the following zones: (1) inlet, (2) bed, supported by a sintered quartz frit, (3) freeboard expansion zone, and (4) outlet. The bed inner diameter (ID) was 2.54 cm and the superficial gas velocity was 0.15 m/s, approximately three times the minimum fluidization velocity. The OC bed volume was 20 cm.sup.3 for each test and the bed was heated by a Carbolite VST 12/600 clamshell furnace. Agglomeration temperature was measured for each prepared sample. The onset of defluidization was detected by a sharp decrease in the pressure drop variation, and agglomeration was confirmed by visual observation. Particles were heated to 850° C. in air at 30° C./min, then CLOU cycling was initiated. One cycle was five-minute decomposition in N.sub.2 followed by five-minute oxidation in air (21 vol % O.sub.2). If no de-fluidization was detected after five cycles, the temperature was ramped up 25° C. in air to 875° C., and the five-cycle test was repeated. The maximum temperature tested, due to quartz reactor suitability, was 975° C. If agglomeration was detected below 950° C., the impregnation method for that sample was not examined further. Attrition was analyzed with downstream filters, but the amount collected was too small to be accurately studied.

[0055] Thermogravimetric Analysis. The ability to maintain high reaction rate over many regeneration cycles, or cyclability, was assessed by operation in TA Instruments Q-500 TGA at CLOU reactor temperatures (900-950° C.). Multicycle tests analyzed the OTC for up to 100 cycles by switching between oxidizing and inert environments. The atmospheres were air and nitrogen for the oxidation and decomposition reactions, respectively. Cycles ranged from 5 to 15 min to allow full conversion, as the rate was highly dependent on the CuO loading and impregnation method. OC samples of 8-15 mg were placed as monolayers on platinum pans and ramped at 30° C./min to the desired temperature. Preliminary tests were performed to control for external mass transfer effects, which were deemed negligible at 120 mL/min gas flow rate.

[00001]$\begin{array}{cc}\mathrm{OTC}=\frac{m_{\mathrm{ox}}-m_{\mathrm{red}}}{m_{\mathrm{ox}}}& \left(2\right)\\ \mathrm{CuO}\mathrm{loading}=9.94\frac{ℊ\mathrm{CuO}}{ℊO_2}\times \mathrm{OTC}& \left(3\right)\\ x_{\mathrm{ox}}\left(t\right)=\frac{m\left(t\right)-m_{\mathrm{red}}}{m_{\mathrm{ox}}-m_{\mathrm{red}}}& \left(4\right)\\ x_{\mathrm{dec}}\left(t\right)=\frac{m_{\mathrm{ox}}-m\left(t\right)}{m_{\mathrm{ox}}-m_{\mathrm{red}}}& \left(5\right)\end{array}$

[0056] OTC was estimated by examining the fully oxidized (m.sub.ox) and reduced (m.sub.red) masses for an individual cycle, shown by Equation (2). The fifth cycle was selected for analysis. Equation (3) shows the experimental CuO loading, which was estimated from the OTC and the theoretical oxygen capacity for pure CuO from Equation (1). This measured value was compared with the desired CuO loading to assess the quality of impregnation batches. For clarity, the normalized wt % change is reported, defined as the instantaneous mass divided by the mass at the start of the first cycle prior to decomposition. For a 20 wt % CuO content OC, decomposition would proceed from 100% to 98% over the first cycle, corresponding to an OTC of 2%. FIG. 6 shows that occasionally oxidation never reached full conversion. Since tests indicated that complete conversion in such cases was not reached even after an hour, 15 min was selected as the cycle time to estimate a reasonable OTC in a real chemical looping reactor. Conversion over was examined for both oxidation and decomposition reactions using Equations 4 and 5, respectively. X.sub.ox defines the percentage of Cu.sub.2O converted to CuO, while X.sub.dec defines the opposite reaction.

[0057] WI vs. DI. Two wet impregnation reactor types were tested for 20 wt % CuO on SiC supports. Regardless of the reactor, rotary kiln or rotary vaporizer, the desired CuO content was not reached due to uneven deposition. The measured CuO loading was approximately 50% of the expected value within the range of 106-250. This indicates that a significant fraction of CuO did not bind to the support surface. A sizable portion of fines was observed below the initial size range (106 m) of the support material SiC. TGA analysis indicated that the fines were composed of nearly 100% CuO, and this was confirmed visually by SEM micrographs as shown in FIG. 4A. On the other hand, DI samples were identified to deposit over 90% of the desired CuO onto SiC, and thus DI was explored further.

[0058] For DI samples, CuO was almost entirely located on the external surface, due to the low surface area of the SiC support. FIGS. 4B and 4C show a thick, uneven shell of CuO attached to SiC. The poorly stabilized CuO is undesirable due to the high risk of sintering and associated problems. Fluidization tests indicated a high agglomeration risk for DI particles and regardless of the method of preparation (number of additions, copper nitrate concentration), since particles always agglomerated below 900° C. Agglomeration was detected soon after the first gas switch to air, indicating that the temperature rise associated with the exothermic oxidation led to particle sintering. SiC supported OC material was very difficult to retrieve from the reactor due to the severity of agglomeration. Table 4 summarizes the measured BET surface area and agglomeration temperatures of the OC materials investigated in this work.

TABLE-US-00004 TABLE 4 BET surface area of fresh samples, agglomeration temperatures determined in fluidized bed testing, and OTC estimated with TGA cycling. Code BET SA (m.sup.2/g) T.sub.agglomeration (C.) OTC (g O.sub.2/g OC) WI-20-SiC 0.1 900 0.009 DI-20-SiC 0.1 900 0.019 DI-20-SiO.sub.2 200 925 0.020 WIFM-20-SiO.sub.2 276 950 0.018 SAWI-25-SiO.sub.2 300 >975 0.001 SAWCI-33-SiO.sub.2 320 >975 0.033

[0059] SiC vs. SiO.sub.2. The results of DI with SiO.sub.2 supports were compared with those with SiC supports. The greater voidage of SiO.sub.2 supports allowed for higher precursor volume, so fewer additions were required to reach 20 wt % CuO content. Surface area after deposition and calcination at 350° C. of CuO typically dropped from roughly 300 to 200 m.sup.2/g for SiO.sub.2. A typical SEM image of CuO on SiO.sub.2 supports is shown in FIG. 5A. The brightly shaded CuO crystallites were relatively large (about 10-20 μm) and unevenly dispersed on the SiO.sub.2 surface. TGA cycling tests for SiC and SiO.sub.2 are shown in FIGS. 6A and 6B, respectively. SiC-supported OC exhibited fast and complete reactions (the curves for each cycle reached 100% or greater); however the aforementioned agglomeration problems preclude using SiC as practical substrate material. In contrast, SiO.sub.2-supported OC showed only partial re-oxidation. Fluidization testing indicated slight improvement over the SiC supported material. The particles could be fluidized at a higher temperature and the agglomerated material could easily be broken apart, signaling less severe agglomeration. Agglomeration was still detected at 925° C. (see Table 4), again during the oxidation stage.

[0060] Novel impregnation methods on SiO.sub.2. SEM images of the WIFM-20-SiO.sub.2 in FIGS. 5B-5C indicate that microwave drying improved CuO dispersion. The CuO crystallite size on the external surface was also notably smaller. Agglomeration testing showed good fluidization up to 950° C., at which point agglomeration occurred. The aggregates were easily fragmented, indicating less severe agglomeration than what occurred with the SiC supported material. The images and agglomeration temperature can be contrasted with the dry impregnation samples, DI-20-SiC and DI-20-SiO.sub.2 (shown in FIGS. 4B and 5A, respectively), which were dried in the heating mantle.

[0061] An alternate copper precursor, TACN, was investigated to improve CuO dispersion via improved metal-support interaction for the batch SAWI-25-SiO.sub.2. Improved affinity of Cu to the SiO.sub.2 surface was qualitatively observed during preparation, as the support material turned dark blue. After decomposition at 350° C., the BET surface area was still very high, 300 m.sup.2/g. As the SEM micrographs in FIGS. 5D and 5E show, the CuO crystallites were very small when the wash step was not incorporated. The maximum observed crystallite size was 1-2 μm. With the wash step, external CuO crystallites could not be identified on the particle surface, as shown in FIG. 5F.

[0062] The dispersion across the internal matrix of an SAWI-25-SiO.sub.2 particle was analyzed using SEM/EDS of particles set in epoxy and polished, as shown in FIG. 7A-FIG. 7C. An even, uniform distribution of Cu and Si throughout the particle can be seen, indicating full penetration into the center of the particle at the scale, such as nanoscale, mesoscale, microscale, etc., of the pores in the particle. These results demonstrate that homogeneous dispersion of CuO in the substrate is feasible with this technique. The fluidization tests showed no signs of agglomeration for all temperatures tested, up to 975° C.

[0063] However, TGA reactivity was poor at 900° C. for the SAWI-25-SiO.sub.2 OC sample. CuO/Cu.sub.2O cycling with air and nitrogen environments resulted in large initial decomposition but incomplete re-oxidation, as shown in FIG. 6C. This was observed for many samples, regardless of calcination, particle size, and oxidation time. In order to clarify the reactions taking place, kinetic analysis was performed with a reducing gas. FIG. 6D shows Cu/CuO cycling in air and in a 4% H.sub.2/N.sub.2 environment, which resulted in a large weight loss, signaling the reduction reaction. After the gas switch to air, the weight gained about 50% of the original mass, indicating the sample was partially oxidized to Cu.sub.2O. The stable weight after partial re-oxidation indicated no diffusion or kinetic limitations for the oxidation reaction to CuO. ICP-MS was used to confirm the presence of elemental Cu. The measured Cu content corresponded to the initial TGA weight drop from decomposition. Poor re-oxidation was observed over the relevant temperature range of 900-950° C. The likely cause for this is inhibited diffusion through the product layer and pore blockage, as is discussed in greater detail below. Incomplete oxidation of Cu.sub.2O to CuO is unacceptable for CLOU processes, and thus the SAWI method was not examined further.

[0064] Selective adsorption wet co-impregnation (SAWCI) on SiO.sub.2. A bimetallic Cu/Ni OC, SAWCI-33-SiO.sub.2, was prepared using a mixed TACN/HANN precursor with 1 mol % Ni content. This batch demonstrated excellent reactivity, as shown by the TGA curve at 950° C. in FIG. 8. The instrument-induced drift above 100% normalized weight was observed frequently over the first few cycles and showed no dependence on oxygen carrier sample and preparation method. The oxidation and decomposition reactions were fast and complete. Fluidization tests confirmed excellent agglomeration resistance. No particle aggregation or sintering effects were observed at 975° C., the maximum temperature tested. One batch of the same composition was manufactured in a pilot scale rotary kiln to test the feasibility of scaling this method. Approximately 3 kg of 12 wt % CuO/NiO@SiO.sub.2 was successfully prepared over one addition in a large rotary kiln for pilot scale demonstration. No unattached CuO fines or particle aggregates were identified after the 350° C. calcination step, indicating similar quality material to the lab scale synthesis.

[0065] The internal surface area of SAWCI-33-SiO.sub.2 was high prior to high temperature fluidization or calcination. Material calcined below 350° C. had a surface area of 480 m.sup.2/g, about 50% greater than the uncoated SiO.sub.2 support material. This increase could stem from the roughness added by small CuO islands within the porous structure of the SiO.sub.2. Crush strength increased with CuO content, to 40 MPa (+/−11) for SAWCI-12-SiO.sub.2 and up to 62 MPa (+/−20) for SAWCI-33-SiO.sub.2. For the 12 wt % CuO sample, this value dropped after fluidized bed testing with CO reducing gas to 24.5 MPa (+/−6.4). XRD analysis was performed on SAWI-25-SiO.sub.2 and SAWCI-25-SiO.sub.2 samples, and the results are shown in FIG. 9. CuO phases were identified in both samples, whereas crystalline SiO.sub.2 was only seen for the SAWCI samples with NiO present. It was not observed for the monometallic samples. This indicates that the presence of NiO affected the crystallization of the initially amorphous SiO.sub.2, which occurred at lower temperature for the SAWCI samples than for the SAWI samples.

[0066] The effect of doping with NiO was examined using N.sub.2 sorption measurements. For the samples with 40 wt % CuO, the monometallic sample SAWI-40-SiO.sub.2 demonstrated a much higher surface area compared to the bimetallic sample SAWCI-40-SiO.sub.2 (Table 5). The average pore diameter was slightly greater for the bimetallic sample. Heat treatment was demonstrated to have a far greater influence on the porous structure, with a decrease in surface area from 300 to 10.7 m.sup.2/g after 1000° C. calcination in air environment. Fluidized bed operation in a fluidized bed reactor (FBR) at 950° C., cycling between air and hydrogen reducing atmosphere, further reduced the surface area, but made no change in the average pore diameter.

TABLE-US-00005 TABLE 5 Measured BET surface area and Barrett, Joyner, and Halenda (BJH) pore diameter of 12 and 40 wt% CuO samples prepared by the SAWI and SAWCI methods Sample BET SA (m.sup.2/g) Average Pore Diameter (nm) SAWCI-40-SiO.sub.2 321 3.9 SAWI-40-SiO.sub.2 (monometallic) 482 3.1 SAWCI-12-SiO.sub.2 307 4.1 SAWCI-12-SiO.sub.2 1000 °C. Calcine 10.7 0.5 SAWCI-12-SiO.sub.2 after 950 °C. FBR 5.3 0.5

[0067] The poor oxidation of Cu.sub.2O to CuO, as shown in FIGS. 6A-6D, has several possible explanations, but no definitive conclusions are available at this point. Literature suggests oxidation can be inhibited by slow diffusion caused by large CuO grain size. However, FIG. 5F shows no visible CuO crystallites for the washed SAWI sample, which demonstrated the most inferior oxidation to CuO. If no CuO crystallites are visible with the SEM, then large grain size is most likely not the issue, since solid state diffusion effects are negligible at small scale. SEM micrographs did not show a significant difference between SAWI (FIG. 5F) and SAWCI (FIGS. 10A-10C) samples. Pore size data does not conclusively explain the difference either, though the bimetallic sample was observed to have a larger pore diameter by about 1 nm. Analysis of SiO.sub.2 substrates with varying pore size could clarify if pore blocking plays a role. The presence of NiO clearly is a significant factor and could contribute by lowering the activation energy for adsorption or for the Cu.sub.2O to CuO oxidation reaction.

[0068] TGA was used to analyze the reactivity of the OC SAWCI-33-SiO.sub.2 over 100 redox cycles. FIG. 11A shows excellent cyclability over the 100-cycle test (Cu.sub.2O/CuO cycling) with no apparent deactivation. The OC material exhibited fast and complete decomposition and oxidation during the entire 16 hours of TGA operation. According to FIG. 11B, conversion rates over the multi-hour test increase with time (i.e. cycle number). Conversion curves in FIGS. 11C and 11D show decomposition and oxidation curves, respectively. As shown in FIG. 11C, the decomposition rate was slower than oxidation, but increased substantially with cycling. Near complete conversion was reached under 2 minutes after 16 hours of cycling at 950° C. As shown in FIG. 11D, complete conversion from Cu.sub.2O to CuO was reached in about 40 seconds after 100 cycles. The longevity of the SAWCI-33-SiO.sub.2 indicates it is a promising material for chemical looping applications.

[0069] In conclusion, a survey of several impregnation methods has been performed in order to identify the most promising methods for scalable preparation of CuO-based oxygen carriers for chemical looping technologies. Commercially available SiC and SiO.sub.2 support materials were investigated. For the purposes of pilot-scale manufacture of Cu-based oxygen carriers, the selective adsorption wet co-impregnation (SAWCI) method was identified to have the greatest potential and was explored further. Co-impregnation with NiO promoted the oxidation of Cu.sub.2O to CuO. Selective adsorption increased the metal-support interaction between Cu and the SiO.sub.2 support, resulting in a reduced crystallite size. SEM/EDS analysis has shown good dispersion and small CuO crystallites throughout the porous SiO.sub.2 sphere allowed high resistance to sintering. For 33 wt % CuO on SiO.sub.2 support, fluidization tests cycling between Cu.sub.2O and CuO did not show any signs of agglomeration, even at 975° C. Long TGA cycling has shown increasing reaction rates and no drop in oxygen transport capacity over 100 cycles.

[0070] Note that in the specification and claims, “about” or “approximately” means within twenty percent (20%) of the numerical amount cited. As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a functional group” refers to one or more functional groups, and reference to “the method” includes reference to equivalent steps and methods that would be understood and appreciated by those skilled in the art, and so forth.

[0071] Although the invention has been described in detail with particular reference to the disclosed embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover all such modifications and equivalents. The entire disclosures of all patents and publications cited above are hereby incorporated by reference.