PREFERENTIAL OXIDATION OF CO IN H2-CONTAINING GAS

Abstract

A method and apparatus for: providing a ceria aerogel and copper nanoparticle catalyst, flowing a hydrogen, carbon monoxide, and water vapor source gas from an inlet into contact with the catalyst to produce a product gas, and flowing the product gas to an outlet. The concentration of carbon monoxide in the product gas is no more than 50% of the concentration of carbon monoxide in the source gas. The concentration of hydrogen in the product gas is no less than 90% of the concentration of hydrogen in the source gas.

Claims

1. A method comprising: providing a catalyst comprising: a ceria aerogel; and copper nanoparticles; flowing a source gas comprising hydrogen, carbon monoxide, and water vapor from an inlet into contact with the catalyst to produce a product gas; and flowing the product gas to an outlet; wherein the concentration of carbon monoxide in the product gas is no more than 50% of the concentration of carbon monoxide in the source gas; and wherein the concentration of hydrogen in the product gas is no less than 90% of the concentration of hydrogen in the source gas.

2. The method of claim 1, wherein the concentration of carbon monoxide in the product gas is no more than 10% of the concentration of carbon monoxide in the source gas.

3. The method of claim 1, wherein the catalyst comprises up to 10 wt. % of the copper nanoparticles.

4. The method of claim 1, wherein the ceria is doped with up to 20 mol % gadolinium.

5. The method of claim 1, further comprising: heating the catalyst to an elevated temperature.

6. The method of claim 5, wherein the elevated temperature is no more than 200° C.

7. The method of claim 1, wherein the outlet directs the product gas into a hydrogen storage vessel.

8. The method of claim 1, wherein the outlet directs the product gas into a fuel cell reaction chamber.

9. An apparatus comprising: a catalyst chamber; a catalyst within the catalyst chamber comprising: a ceria aerogel; and copper nanoparticles; an inlet for flowing a source gas comprising hydrogen, carbon monoxide, and water vapor in contact with the catalyst; an outlet for flowing a product gas from the catalyst chamber.

10. The apparatus of claim 9, wherein the catalyst comprises up to 10 wt. % of the copper nanoparticles.

11. The apparatus of claim 9, wherein the ceria is doped with up to 20 mol % gadolinium.

12. The apparatus of claim 9, further comprising: a heater that can heat the catalyst.

13. The apparatus of claim 9, further comprising: a hydrogen storage vessel coupled to the outlet.

14. The apparatus of claim 9, further comprising: a fuel cell having a reaction chamber coupled to the outlet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] A more complete appreciation will be readily obtained by reference to the following Description of the Example Embodiments and the accompanying drawings.

[0011] FIG. 1 schematically illustrates CeO.sub.2 aerogel synthesis and photodeposition of Cu nanoparticles to create Cu/CeO.sub.2 aerogels (PPO=propylene oxide).

[0012] FIG. 2 shows nitrogen isotherms used to calculate surface area.

[0013] FIG. 3 shows cumulative pore-volume as a function of pore diameter.

[0014] FIG. 4 shows CO and CO.sub.2 concentration as a function of temperature.

[0015] FIGS. 5-7 show CO oxidation and hydrogen consumption for Cu/CeO.sub.2 and Cu/GCO under FIG. 8 schematically illustrates the covalently networked aerogel nano architecture with well dispersed Cu NPs in intimate contact with the supporting oxide.

[0016] FIG. 9 shows XRD patterns of CeO.sub.2 (lower curve) and GCO (upper curve) aerogels with inset showing FWHM of the CeO.sub.2 (111) peak.

[0017] FIG. 10 shows particle size distributions calculated from TEM data (GCO: higher peaks ≤6.5, CeO.sub.2 higher peaks ≥7).

[0018] FIG. 11 shows BJH pore size distributions with inset N.sub.2 isotherms and calculated BET surface area indicated.

[0019] FIG. 12 shows a scanning electron micrograph of CeO.sub.2 aerogel.

[0020] FIG. 13 shows transmission electron micrographs of CeO.sub.2 aerogel. Fast Fourier Transforms of the micrograph (inset enlarged on right) show clear polycrystallinity. Lattice parameters for the CeO.sub.2 (111) plane are indicated.

[0021] FIG. 14 shows a scanning electron micrograph GCO aerogel.

[0022] FIG. 15 shows transmission electron micrographs of GCO aerogel. Fast Fourier Transforms of the micrograph (inset enlarged on right) show clear polycrystallinity. Lattice parameters for the CeO.sub.2 (111) plane are indicated.

[0023] FIG. 16 shows the X ray photoelectron spectra of the O1s region for CeO.sub.2 and GCO aerogels before and after photodeposition of Cu nanoparticles.

[0024] FIG. 17 shows the X ray photoelectron spectra of the Ce3d region for CeO.sub.2 and GCO aerogels before and after photodeposition of Cu nanoparticles.

[0025] FIG. 18 shows the X ray photoelectron spectra of the Cu2p region for CeO.sub.2 and GCO aerogels before and after photodeposition of Cu nanoparticles.

[0026] FIG. 19 shows catalytic performance as a function of temperature of 5 wt. % Cu/CeO.sub.2 (left curve) and Cu/GCO (right curve) aerogels under CO and O.sub.2 with no other reactants.

[0027] FIG. 20 shows catalytic performance as a function of temperature of 5 wt. % Cu/CeO.sub.2 (left curve) and Cu/GCO (right curve) aerogels under CO and O.sub.2 with H.sub.2 added. Open symbols and dashed lines correspond to O.sub.2 selectivity as a function of temperature.

[0028] FIG. 21 shows catalytic performance as a function of temperature of 5 wt. % Cu/CeO.sub.2 (right curve) and Cu/GCO (left curve) aerogels under CO and O.sub.2 with H.sub.2 and H.sub.2O added. Open symbols and dashed lines correspond to O.sub.2 selectivity as a function of temperature.

[0029] FIG. 22 shows catalytic stability of Cu/CeO.sub.2 and Cu/GCO aerogels evaluated over a 16 h period 100° C.

[0030] FIG. 23 shows X-ray photoelectron spectra of the Cu2p region after catalytic testing of Cu/CeO.sub.2 aerogel.

[0031] FIG. 24 shows X-ray photoelectron spectra of the Cu2p region after catalytic testing of Cu/GCO aerogel.

[0032] FIG. 25 shows X-ray photoelectron spectra of the Cu2p region after catalytic testing of Cu/CeO.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0033] In the following description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of the present disclosure. However, it will be apparent to one skilled in the art that the present subject matter may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and devices are omitted so as to not obscure the present disclosure with unnecessary detail.

[0034] Disclosed herein is a catalytic material that selectively oxidizes carbon monoxide (CO) at low temperature in a feedstream comprising excess hydrogen (H.sub.2) and retains its selectivity and activity even in the presence of water vapor. Selective CO oxidation in excess H.sub.2, a reaction colloquially referred to as COPROX, is an important process for preferential removal of CO from hydrogen feedstocks. Existing COPROX catalysts typically suffer from incomplete CO oxidation and/or poor selectively towards CO, i.e., oxidizing the desired H.sub.2 along with the target CO. These materials are also prone to deactivation in the presence of water vapor, a common constituent in H.sub.2 feedstocks. Disclosed herein is the use of low-valent copper nanoparticles (Cu NPs) supported on a covalently networked ceria (CeO.sub.2) aerogel to facilitate complete oxidization of CO at low temperature while leaving gaseous H.sub.2 largely unaffected. Catalytic activity and stability persist even in the presence of water vapor. The CeO.sub.2 support stabilizes the active Cu species against oxidation, maximizes interfacial contact with the NPs, and supplies lattice oxygen to drive CO oxidation.

[0035] Based on the current understanding of both CO oxidation in general and the specific considerations of COPROX catalysts, the following criteria were used for designing highly active and stable catalysts: [0036] Excellent dispersion of Cu on or within the CeO.sub.2 to promote the maximum number of active interfacial sites and increase the amount of low-valent Cu [0037] Effective transport of water through the catalysts to mitigate blocking of active sites by adsorbed molecular water [0038] A hydroxyl-rich surface to promote formation of bicarbonates over carbonates

[0039] FIG. 1 illustrates the steps involved in the synthesis of copper nanoparticles (Cu NPs) supported on nanoscale ceria (Cu/CeO.sub.2) aerogels. CeO.sub.2 aerogels are prepared using a sol-gel method followed by supercritical drying of the wet gel (Laberty-Robert et al., Chem. Mater. 18 (2006) 50-58). This method preserves the co-continuity of the pore-solid networks of the covalently networked, ultraporous oxide because supercritical extraction prevents surface tension (and thus compressive forces) being imposed in the wet gel as fluid is removed. Preventing collapse of the pores during drying of the wet gel is essential to the transport of both reactants and water molecules into the interior of the high surface-area oxide while the covalently bonded CeO.sub.2 nanoparticles that define the framework of the architected oxide minimize sintering and particulate agglomeration. After supercritical drying, the aerogel is calcined in air to crystallize CeO.sub.2 domains and improve the mechanical properties. The calcined aerogel is then dispersed into an alkaline solution and mixed with copper (II) nitrate in an ethanol/water mixture under broadband irradiation to photodeposit Cu NPs onto the aerogel surface (DeSario et al., Nanoscale 9 (2017) 11720-11729). The solution is then vacuum filtered over a membrane or centrifuged gently to recover the aerogel from solution and the recovered solid is dried at low temperature (<100° C.) overnight to remove residual water.

[0040] Structural characterization reveals that the underlying architecture of the aerogel is maintained after immersion into the water/ethanol suspension used to photodeposit Cu NPs. X-ray diffraction (XRD) patterns of CeO.sub.2 and 5 weight percentage of photodeposited Cu NPs on calcined ceria aerogel (5Cu/CeO.sub.2), have peaks corresponding to the fluorite crystal structure CeO.sub.2 in both materials. Peaks for potential Cu-phases, such as metallic Cu, Cu.sub.2O, or CuO, do not appear in 5Cu/CeO.sub.2, indicating small particle size and/or amorphous regions. Nitrogen isotherms (FIG. 2) and pore-volume distribution plots (FIG. 3) confirm the structural preservation, where the calculated BET surface area is 79 and 83 m.sup.2 g.sup.−1 for CeO.sub.2 and 5Cu/CeO.sub.2 aerogels, respectively.

[0041] X-ray photoelectron spectroscopy (XPS) was used to evaluate the chemical states of the materials. Cerium in both CeO.sub.2 and Cu/CeO.sub.2 appears as primarily Ce.sup.4−, an expected result as the calcination in air tends to heal most oxygen vacancies. In the O1s region, two peaks are designated: O.sub.a and O.sub.b. O.sub.a is stoichiometric (lattice) oxygen in CeO.sub.2, (Sohn et al., Catal. Lett. 147 (2017) 2863-2876) while O.sub.b contains overlapping contributions from oxygen vacancies in CeO.sub.2 (Sohn) (i.e., O in Ce.sub.2O.sub.3) as well as hydroxyl groups on either the oxide or the Cu surface (Liu et al., Phys. Chem. Chem. Phys. 18 (2016) 16621-16628). The higher intensity of the O.sub.b peak in Cu/CeO.sub.2 compared to CeO.sub.2 is likely a product of Cu activating water at the interfaces to form hydroxyl groups, similar to that reported for Cu NPs photodeposited on TiO.sub.2 aerogel (McEntee et al., ACS Appl. Nano Mater. B (2020) 3503-3512).

[0042] XPS of the Cu2p region confirms that Cu exists entirely in a low-valent state (Cu.sup.1+ or metallic Cu). Satellite peaks that appear prominently in CuO are not detected. This high proportion of low-valent Cu is facilitated by intimate contact between Cu nanoparticles and the reducing oxide support (Pitman et al., Nanoscale Adv. 2020, 2, 4547-4556; Laberty-Robert et al., Adv. Mater. 2007, 19, 1734-1739).

[0043] A typical COPROX reaction was conducted in a packed bed reactor in a programmable ceramic tube oven, where 50 mg of the catalyst was diluted with 200 mg of CeO.sub.2 aerogel to increase space time. Prior to measurements, the sample was pretreated in a reducing atmosphere (CO without O.sub.2) during the 1-hour ramp from room temperature to 250° C. During the experiment, the total flow of gases was held constant at 80 mL min.sup.−1 with 20% O.sub.2 and 1% CO (He balance). Products were analyzed using an in-line GC (GC-2014, Shimadzu) as the temperature was decreased in increments of 15° C., with 5 injections at each temperature point to ensure there were no residuals in the column.

[0044] As shown in FIG. 4, the generated concentration of CO.sub.2 mirrors CO consumption, and no other products are detected. FIGS. 5-7 show the CO oxidation performance and stability of Cu/CeO.sub.2 under various conditions. Without H.sub.2 or H.sub.2O added, the temperature to achieve 50% CO conversion (T.sub.50) is only 65° C., shifting to 71° C. with the addition of H.sub.2, and 95° C. with both H.sub.2 and H.sub.2O. Despite the high activity towards CO oxidation, no consumption of H.sub.2 above the error range for this peak (˜500 ppm) was observed until 145° C. In terms of O.sub.2 consumption, this indicates that selectively towards CO is ≥95.3% at temperatures of 130° C. and below. Previously reported CeO.sub.2-based materials are unable to achieve this combination of >90% selectivity and >90% CO conversion under humid conditions (Avgouropoulos et al., Chem. Eng. J. 2006, 124, 41-45; Han et al., Appl. Catal. B 108-109 (2011) 72-80; Park et al., J. Power Sources 132 (2004) 18-28).

[0045] Cu/CeO.sub.2 aerogels doped with 10 mol % gadolinium (Gd) were also evaluated for the COPROX reaction. Compared to CeO.sub.2 aerogels, Gd-doped CeO.sub.2 (GCO) aerogels have a higher proportion of Ce.sup.3+ states and higher surface area (146 m.sup.2 g.sup.−1). Cu/GCO aerogels are slightly less active than Cu/CeO.sub.2 under dry CO oxidation conditions, perhaps due to fewer Ce.sup.4+ sites available for 3+/4+ redox cycling, but do not suffer as much loss in activity when exposed to water. Upon addition of H.sub.2 and H.sub.2O to the feedstream, the T.sub.50 for Cu/GCO shifts only 15° C. compared to 30° C. for Cu/CeO.sub.2. The improved selectivity for Cu/GCO is further evidence that CO oxidation with H.sub.2O in the feedstream is a transport-limited reaction: the higher surface area of Cu/GCO and its higher number density of chemically fixed oxygen vacancies enable the substituted ceria aerogel to more effectively distribute water as well as react with it to hydroxylate the surface, both of which mitigate blockage of active sites.

[0046] The long-term stability of transition metal-based COPROX catalysts under humid conditions is rarely reported, and the few published results that exist suggest it is problematic. Notably, Gongalves et al. observed that Cu nanoparticles were stable under dry feedstreams for CO oxidation, but experienced continuous deactivation when exposed to 1% water vapor (Gongalves et al., ACS Appl. Mater. Interfaces 2015, 7, 7987-7994). Other transition metals used in conjunction with ceria, such as Mn and Co, are also known to generally be unstable under humid conditions (Basu et al., ChemCatChem 2020, 12, 3753-3768). One report that processed a Cu—CeO.sub.2 catalyst using freeze-drying, also experienced deactivation in the presence of H.sub.2O (g) (Arango-Diaz et al., Appl. Catal. A 477 (2014) 54-63).

[0047] To test the long-term catalytic stability of Cu/CeO.sub.2, the CO oxidation activity and selectively was monitored for 16 hours. The hold temperatures are selected to ensure significant CO oxidation activity, but less than 100% conversion to ensure that potential changes in activity are quantifiable. Under all tested conditions, no significant change in activity or selectivity is observed. Deactivation of COPROX catalysts under humid conditions is often regarded as a transport issue, with accumulated water blocking active sites. The bonded oxide network in manganese oxide (Doescher et al., Anal. Chem., 77 (2005) 7924-7932) and titanium oxide aerogels prevents flooding of the mesopores until reaching tropical levels of humidity (>80%) because the water adsorbed at the oxide network is distributed along that network and serves as a transport wire for proton diffusion. The expression of CeO.sub.2 and substituted ceria in aerogel form provides the through-connected mesoporosity to effectively mitigate these transport limitations and more effectively shuttle adsorbed species away from these sites.

[0048] In a first step, a ceria aerogel/copper nanoparticle catalyst, such as that described above, is provided. The catalyst may comprise, for example up to 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 wt. % of the copper nanoparticles. Further, the ceria may be doped with up to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, or 20 mol % gadolinium. The catalyst is disposed in a catalyst chamber.

[0049] Next, a source gas comprising hydrogen, carbon monoxide, and water vapor flows from an inlet, into the chamber, and in contact with the catalyst. Optionally, the catalyst is heated to an elevated temperature by a heater. The catalyst alters the composition of the source gas to make a product gas. The elevated temperature may be no more than 200, 180, 160, 145, 130, 115, or 100° C.

[0050] Next, the product gas flows out of the chamber through an outlet. The outlet may direct the product gas into further apparatus, such as a hydrogen storage vessel or a fuel cell reaction chamber.

[0051] Due to the catalyst, the concentration of carbon monoxide in the product gas is no more than 50%, 40%, 30%, 20%, or 10% of the concentration of carbon monoxide in the source gas. Also, the concentration of hydrogen in the product gas is no less than 90% or 95% of the concentration of hydrogen in the source gas. The concentrations can be measured by methods known in the art. When the catalyst is heated, the concentration measured may be used to determine a suitable temperature.

[0052] By redesigning the arrangement of Cu NPs with reducing oxide NPs by imposing an aerogel morphology, the degree of intimacy at the Cu//oxide interface is markedly enhanced, allowing preferential oxidation of CO in a H.sub.2-rich feedstream. The intimacy of the Cu//oxide junction enabled by an architected catalyst may accrue the following technological advantages: (1) high activity for CO oxidation at modest temperature (T<100° C.); (2) selective CO oxidation in the presence of H.sub.2 at T<100° C.; and (3) selective and stable CO oxidation in the presence of H.sub.2 and H.sub.2O at T<100° C.

[0053] The performance of these catalysts under practical feedstreams enables refinement of hydrogen feedstocks to the purity levels needed for such applications as PEMFCs. Architected catalysts as a design metaphor should also provide effective activity, selectivity, and durability for other catalytic oxidations compromised by the presence of water.

[0054] The following examples are given to illustrate specific applications. These specific examples are not intended to limit the scope of the disclosure in this application.

[0055] Synthesis of CeO.sub.2 aerogels—CeO.sub.2 and GCO aerogels were prepared using a modification of previous methods (Pitman et al., Nanoscale Adv. 2020, 2, 4547-4556; Laberty-Robert et al., Adv. Mater. 2007, 19, 1734-1739). First, 2.39 g of CeCl.sub.3.Math.7H.sub.2O (Sigma-Aldrich, 99.9%) was dissolved in 10 g of anhydrous methanol (Fisher, 99.9%), followed by adding 6 g of propylene oxide (Sigma-Aldrich, ≥99%) [Safety Note: this epoxide is carcinogenic]. In the case of GCO, 10 mol % of GdCl.sub.3.Math.6H.sub.2O was substituted for 10 mol % CeCl.sub.3.Math.7H.sub.2O to form a Gd.sub.0.1Ce.sub.0.9O.sub.x sol; Gd.sub.0.05Ce.sub.0.95O.sub.x was also prepared by substituting 5 mol % GdCl.sub.3.Math.6H.sub.2O. The mixture was stirred for 20 min and left to gel overnight. The wet, aged gels were rinsed several times with isopropanol followed by acetone and then loaded into a Leica EM CPD300 autoclaved and programmed for 99 CO.sub.2 flushes. After supercritical drying at 42° C., the autoclave was gradually vented to atmospheric pressure. The aerogels were calcined in air at 500° C. (5° C. min-1 ramp, 2 h dwell) to crystallize the networked CeO.sub.2 nanoparticles (NPs).

[0056] Photodeposition of Cu nanoparticles—Copper NPs were photodeposited as previously described (Pitman et al., Nanoscale Adv. 2020, 2, 4547-4556; DeSario et al., Nanoscale 2017, 9, 11720-11729) using ceria aerogel ground through a 45 m sieve. For 5 wt. % Cu/CeO.sub.2, 38 mg of Cu(NO.sub.3).sub.2.Math.2.5H.sub.2O was dissolved in 200 mL of 10 vol. % ethanol in water into which 200 mg of CeO.sub.2 aerogel was dispersed. An equivalent 200 mg of GCO aerogel or commercial CeO.sub.2 powder (Aldrich, <50 nm particle size) was used for preparation of Cu/GCO aerogel and Cu/CeO.sub.2—COM. Aqueous NaOH was added to adjust the suspension pH to 9.5±0.1; the initial adjustment is made with 1 M NaOH with a final adjustment using 0.1 M NaOH. The pH-adjusted suspension was then purged with N.sub.2 before irradiating for 48 h using a 500 W Xe arc lamp (Newport-Oriel). The blue-green solids were collected by vacuum filtration over a 0.1 m PVDF filter, rinsed with 18 MQ cm water, and dried overnight under ambient conditions.

[0057] Characterization—Brunauer-Emmett-Teller (BET) surface area and Barrett-Joyner-Halenda (BJH) pore size distributions were calculated using the adsorption and desorption curves, respectively, of the N.sub.2 physisorption isotherms (Micromeritics ASAP2020). Samples were degassed under vacuum at 150° C. prior to N.sub.2 physisorption. X-ray diffraction (XRD) data were collected using a Rigaku Smartlab (40 kV, 44 mA) at a 4° min.sup.−1 scan speed. X-ray photoelectron spectra were taken using a Thermo Scientific Nexsa (Al Kα) and a flood gun to prevent charging. X-ray absorption near-edge spectroscopy (XANES) measurements were executed using an in-lab X-ray absorption spectrometer (easyXAFS300) with a 1.2 kW liquid-cooled X-ray tube operating at 30 kV and 12 mA. The spectrometer was configured with a Si (440) spherically bent crystal analyzer to investigate the Ce L3 absorption edge. Three consecutive scans were averaged for each material to improve the signal-to-noise ratio. Raman spectra were collected on a Renishaw inVia Raman microscope with a 514 nm laser source and imaged through a 50× objective. Micrographic imaging was performed on a LEO Supra 55 field-emission scanning electron microscope operating at 10 kV. Samples analyzed by transmission electron microscopy (TEM) were sonicated and drop-cast onto conductive “lacey” carbon. High-resolution TEM (HRTEM) and high-angle annular dark-field (HAADF) images and EDS maps were obtained using a JEOL JEM2200FS TEM operating at 200 kV with 0.7 nm nominal probe size.

[0058] FIG. 8 illustrates the networked Cu/aerogel structure for the COPROX reaction. Both the CeO.sub.2 and GCO aerogels are mesoporous structures comprising covalently networked nanocrystalline oxide domains. The X-ray diffraction patterns of calcined CeO.sub.2 and GCO are consistent with the fluorite crystal structure (FIG. 9), but the GCO diffraction peaks are broader relative to CeO.sub.2 (FIG. 8, inset), revealing smaller primary crystallite diameters for GCO. Crystallite sizes based on the Scherrer equation are ˜9 and ˜6 nm for CeO.sub.2 and GCO, respectively. Diffraction peaks for Gd.sub.2O.sub.3 are absent, indicating that Gd(III) substitutes into the CeO.sub.2 lattice rather than phase segregating. Particle size distributions derived from TEM analysis (FIG. 10) confirm the smaller average particle sizes in calcined GCO (4-5 nm) compared to CeO.sub.2 (7-8 nm), in agreement with the broadened XRD peaks for GCO.

[0059] The porosimetry-derived Barrett-Joyner-Halenda (BJH) pore size distributions (FIG. 11) and Brunauer-Emmett-Teller (BET) surface-areas (FIG. 11, inset) reveal that GCO has a smaller average pore size and higher surface area than CeO.sub.2. The CeO.sub.2 and GCO aerogels with either 5 or 10 mol % Gd have essentially identical surface area prior to calcination (˜300 m.sup.2 g.sup.−1) and more of this surface area is retained post-calcination as the Gd mol % increases, consistent with suppressing crystallite growth upon substituting Gd(III) for Ce(IV) (Inaba H et al., Solid State Ionics 1998, 106, 263-268; Durgasri et al., J. Chem. Sci. 2014, 126, 429-435). The peak broadening in XRD for GCO and the corroborating shift to smaller particle size distributions measured by TEM confirm that the higher surface area of GCO is related to the presence of finer crystallites.

[0060] Scanning electron microscopy (SEM) confirms the mesoporous nature of both CeO.sub.2 and GCO aerogels (FIGS. 12, 14) while the transmission electron micrographs reveal that the networked oxide NPs comprising both aerogel compositions are crystalline (FIG. 13 (left), 14 (left)), with a measured lattice parameter of 3.11 Å corresponding to the (111) plane of CeO.sub.2 (FIG. 13 (right), 14 (right)). Gadolinium appears atomically dispersed in EDS mapping and the measured Gd content (8 at. %) closely matches the synthetically targeted value of 10 at. %.

[0061] X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) were used to monitor the chemical states of the CeO.sub.2 and GCO aerogels before and after Cu photodeposition. The presence of Gd in GCO aerogels is confirmed by the prominent shoulder at −9 eV in valence-band XPS, as well as the appearance of peaks in the Gd.sub.4d region at binding energies characteristic of Gd.sup.3+ (˜140.5 eV) (Zatsepin et al., Appl. Surf Sci. 2018, 436, 697-707). XANES of the cerium L3 edge reveals the corollary change in average Ce valance with Gd substitution, from 9% Ce.sup.3+ in CeO.sub.2 to 11% in GCO.

[0062] The lower binding energy peak (˜528.5 eV) in the XPS O1s spectra (FIG. 16, designated O.sub.a) is primarily assigned to lattice oxygen in stoichiometric CeO.sub.2 (Liang et al., J. Mater. Chem. A 2015, 3, 634-640; Sohn et al., Catal. Lett. 2017, 147, 2863-2876), although some contribution from O in CuO is also possible (Zheng et al., J. Mater. Sci. 2016, 51, 917-925). The higher binding energy peak at ˜531 eV (O.sub.b) is a combination of contributions from oxygen-deficient ceria (i.e., G in Ce.sub.2O.sub.3) (Sohn) or surface hydroxyl groups (Liu et al., PhysChemChemPhys 2016, 18, 16621-16628) on Cu or CeO.sub.2. A more prominent O.sub.b peak is seen for GCO, an expected result because of the oxygen vacancies created to charge compensate Gd.sup.3+ substitution into Ce.sup.4+ lattice sites. Raman spectroscopy also indicates the presence of Gd-induced oxygen vacancies through the emergence of α and β peaks not seen in pristine CeO.sub.2 (Durgasri et al., J. Chem. Sci. 2014, 126, 429-435).

[0063] The photoelectron intensity of the O.sub.b peak increases after Cu photodeposition for both CeO.sub.2 and GCO aerogels; this peak was previously assigned to an increased density of surface hydroxyl groups (McEntee et al., ACS Appl. Nano Mater. 2020, 3, 3503-3512). Although some Ce.sup.31 is discernable (FIG. 17), the binding energy of Ce3d peaks are predominately Ce.sup.4+. For both CeO.sub.2 and GCO, the weak Ce.sup.3+ peaks decrease in intensity after Cu photodeposition, likely because the photodeposited metal nucleates at oxygen vacancies (Pitman et al., Nanoscale Adv. 2020, 2, 4547-4556).

[0064] The Cu2p region reveals that Cu is present entirely in its low-valent speciation (metallic Cu or Cu.sup.1+) when supported on either CeO.sub.2 or GCO aerogels, with the characteristic Cu.sup.2+ satellite feature completely absent in spectra (FIG. 18). Intimate contact between the Cu NP nestled on the reducing oxide nanoparticulate network helps to stabilize low-valent Cu (Pitman et al., Nanoscale Adv. 2020, 2, 4547-4556), which stands in contrast to the primarily CuO speciation of most Cu/CeO.sub.2 catalysts reported in COPROX studies (Tiscornia et al., Int. J. Hydrogen Energy 2020, 45, 6636-6650; Tiscornia et al., Catal. Today 2021, 10.1016/j.cattod.2021.08.008; Gómez-Cuaspud et al., Int. J. Hydrogen Energy 2013, 38, 7458-7468; Mariño et al., Int. J. Hydrogen Energy 2008, 33, 1345-1353; Jung et al., Appl. Catal. B 2008, 84, 426-432). No distinct Cu phases (metallic Cu, Cu.sub.2O, or CuO) are detected in the XRD patterns of Cu/CeO.sub.2 or Cu/GCO, indicating that the particles are small and well dispersed and/or amorphous. Imaging by TEM could not resolve individual Cu NPs, but EDS mapping reveals Cu to be uniformly distributed on both Cu/CeO.sub.2 and Cu/GCO.

[0065] Catalyst testing—Oxidation of CO was performed in a ⅜″ glass tube flow-through reactor in a programmable ceramic tube furnace, with conditions chosen to minimize heat/mass transport effects and facilitate comparison to other published studies on CO oxidation (DeSario et al., Appl. Catal. B 2019. 252, 205-213; Pitman et al., Nanoscale Adv. 2020, 2, 4547-4556; Pennington et al., ACS Catal. 2020, 10, 14834-14846; Wu et al., Mater. Chem. Front. 2017, 1, 1754-1763). A packed bed comprising 50 mg of catalyst diluted with 200 mg of native CeO.sub.2 aerogel was sandwiched between glass wool and conditioned during the 1 h ramp from RT to 250° C. under a continuous flow of 80 mL min.sup.−1 of 1.75% CO (Airgas, 10000 ppm) and 98.25% He (Praxair, 5.0 UHP), giving a gas hourly space velocity (GHSV) of 39000 h.sup.−1. The downstream gas mixtures were analyzed with an in-line gas chromatograph (GC-2014, Shimadzu) equipped with a pulsed discharge detector for product analysis. The GC detector was calibrated with known concentrations of CO and CO.sub.2 in UHP He to ensure accurate quantitative analysis of CO conversion percentage and CO.sub.2 yield. Temperature was decreased in 15° C. steps at 3° C. min.sup.−1 and four replicate injections were performed at each set point. Additional reactions were performed with 10% H.sub.2 added (He balanced to maintain a flow rate of 80 mL min-), and in a humidified stream using the same gas composition as the dry runs, but the mixed feedstream passed through a bubbler to produce 100% RH at RT.

[0066] Oxidation of CO and H.sub.2 by O.sub.2 proceeds by reactions 1 and 2, respectively:

CO+1/2O.sub.2=CO.sub.2  1

H.sub.2+1/2O.sub.2=H.sub.2O  2

CO oxidation activity (or H.sub.2 oxidation activity) is determined as the percent CO (H.sub.2) conversion at a given temperature and the temperature at which 50% conversion is achieved (T.sub.50). COPROX selectivity is determined according to the mol percentage of O.sub.2 consumed by CO oxidation relative to the mol percentage of O.sub.2 consumed by H.sub.2 oxidation (Eq. 1):

[00001]$\begin{array}{cc}\mathrm{selectivity}\left(\%\right)=100\times \frac{\mathrm{mol}{\mathrm{CO}}_2\mathrm{produced}}{2\times \mathrm{mol}{O}_2\mathrm{consumed}}& \left(\mathrm{Eq}.1\right)\end{array}$

[0067] With a 1:1 mol ratio of CO to O.sub.2 in the reaction feed, the minimum selectivity according to (Eq. 1) is 50%.

[0068] FIGS. 19-21 summarize the activity and selectivity of Cu/CeO.sub.2 and Cu/GCO aerogels in different feedstreams. Both catalysts are highly active towards CO oxidation without H.sub.2 or H.sub.2O added and achieve 100% CO conversion by ˜125° C. (FIG. 19). The temperature at which Cu/CeO.sub.2 and Cu/GCO convert 50% of CO (T.sub.50) is ˜73° C. and ˜88° C., respectively. The lower activity of Cu/GCO may be related to the persistence of oxygen vacancies at the surface: the MvK mechanism requires lattice oxygen to be extracted, but at Gd-induced oxygen vacancy sites the oxide cannot be further reduced. The calcined CeO.sub.2 and GCO aerogels show minimal CO oxidation activity without photodeposited Cu NPs. The concentration of CO.sub.2 produced closely matches CO consumption during trial runs and no other products are detected, implying a product selectivity toward CO.sub.2 of effectively 100% under dry, H.sub.2-free conditions.

[0069] With H.sub.2 in the feedstream (FIG. 20), only minor shifts in CO oxidation activity for Cu/CeO.sub.2 and Cu/GCO were observed. Detectable consumption of H.sub.2 does not occur until >130° C., therefore, selectivity below this temperature is 100%. Above this temperature, H.sub.2 consumption increases with increasing temperature until all the residual oxygen is consumed and selectivity hits a minimum of 50%. For both catalysts, the disparate temperatures at which CO and H.sub.2 oxidation occur create an operating window where effectively 100% of CO is oxidized with >95% selectivity. Catalytic testing was also performed with sufficient O.sub.2 present to consume all CO and H.sub.2 and it was found that even with excess O.sub.2 in the feedstream (20%), H.sub.2 consumption is negligible until 145° C. The Tso for H.sub.2 oxidation with 20% O.sub.2 in the feedstream is 176 and 185° C. for Cu/CeO.sub.2 and Cu/GCO aerogels, respectively, ˜100° C. higher than the T.sub.50 for CO oxidation.

[0070] The remarkable selectivity is attributed to the nature of active sites facilitated by the morphology characteristic of Cu NPs well-dispersed in the ceria network. Expressing the supporting oxides as aerogels maximizes intimate contact between the supported metal NP and the networked nanoparticulate oxide, as indicated for Cu/CeO.sub.2 and Cu/GCO by the lack of evidence for discrete Cu phases in TEM or XRD. Because CO oxidation is preferred at Cu//oxide interfacial sites (Gamarra et al., J. Am. Chem. Soc. 2007, 129, 12064-12065), the aerogel morphology is a synthetically opportune method to maximize those sites. Although Cu/GCO has a higher T.sub.50 than Cu/CeO.sub.2, it demonstrates higher selectivity (FIG. 20), perhaps a result of greater Cu//oxide intimacy facilitated by the smaller crystallites of the GCO network.

[0071] The addition of H.sub.2O to the reaction stream significantly lowers the CO oxidation activity of Cu/CeO.sub.2 and pushes its T.sub.50 from 76 to 98° C. (FIG. 21). In contrast, the activity of Cu/GCO is only mildly impacted by water vapor, with T.sub.50 shifting from 89 to 93° C. The loss of activity in the presence of water vapor is common for transition-metal COPROX catalysts, but it is notable that with the aerogel-expressed catalysts, an operational temperature window is observed that couples high activity (T.sub.50) to selectivity (see Eq. (1)). These architected COPROX catalysts demonstrate >95% CO conversion and >95% selectivity toward CO.sub.2 under humid conditions (Tiscornia et al., Int. J. Hydrogen Energy 2020, 45, 6636-6650; Cabello et al., Top. Catal. 2019, 62, 931-940; Avgouropoulos et al., Chem. Eng. J. 2006, 124, 41-45; Park et al., J. Power Sources 2004, 132, 18-28; Reis et al., Catal. Today 2020, 344, 124-128; Xu et al., Int. J. Hydrogen Energy 2019, 44, 4156-4166).

[0072] The observation that architected Cu/GCO experiences less deactivation in the presence of H.sub.2O than Cu/CeO.sub.2 holds important implications in catalyst design. Oxidizing CO in the presence of water is thought to be transport limited, where the rate-limiting step of Cu.sup.+-carbonyl formation is blocked by adsorbed molecular water at CuOx interfacial sites (Gamarra et al., J. Catal. 2009, 263, 189-195). Depletion of hydroxyls may be another contributing factor, as the relative abundance of hydroxyls dictate whether CO oxidation proceeds through the more favorable bicarbonate pathway or the less favorable carbonate pathway. Abundant hydroxyls promote the formation of bicarbonates in preference to carbonates, with the former species benefitting CO oxidation (Davó-Quiñonero et al., ACS Catal. 2016, 6, 1723-1731). The higher surface area of Cu/GCO more effectively distributes adsorbed water through the structure, while the higher concentration of oxygen vacancies ensures formation of abundant hydroxyl groups (Kundakovic et al., Surf Sci. 2000, 457, 51-62; Yang et al., J. Phys. Chem. C 2010, 114, 14891-14899), both of which mitigate blocking of active sites.

[0073] To further confirm this resilience in the presence of water, the long-term activity and selectivity of the architected catalysts at 100° C. was monitored (FIG. 22). Neither Cu/CeO.sub.2 nor Cu/GCO aerogel show a loss in activity; after 16 h, activity is within 1% of its initial value, well within the margin of error. Selectivity is maintained at 100% throughout the run. This stability is remarkable among non-noble metal catalysts: Cu catalysts are known to experience continuous deactivation under humidified feedstreams (Gongalves et al., ACS Appl. Mater. Interfaces 2015, 7, 7987-7994). Other transition metals commonly used for COPROX, such as Mn and Co, are similarly unstable in the presence of H.sub.2O (Basu et al., ChemCatChem 2020, 12, 3753-3768).

[0074] As a reference for the aerogel-based supporting oxides, 5 wt. % Cu was photodeposited over commercial CeO.sub.2 nanopowder (Cu/CeO.sub.2—COM) and this non-networked catalyst was evaluated under dry and humidified feedstreams. Cu/CeO.sub.2—COM shows a more severe activity loss in the presence of water than either Cu/CeO.sub.2 or Cu/GCO aerogels, with the T.sub.50 shifting from 101 to 140° C. Upon returning the sample to a dry feedstream and repeating the light-off curve, activity is lower than its initial activity, indicating that humidity has caused irreversible deactivation.

[0075] Ex situ XPS analysis of recovered catalysts (FIGS. 23-25) reveals stark differences between aerogel and non-aerogel supports in terms of Cu speciation. Copper in Cu/CeO.sub.2 and Cu/GCO aerogels persists as low-valent (0/1+), with minimal trace of the characteristic Cu.sup.2+ satellite. The recovered Cu/CeO.sub.2—COM catalyst displays prominent satellite peaks and broadening of the primary peaks with contributions from higher binding energy Cu.sup.2+. The poor water resilience, instability, and significant Cu.sup.2+ formation in Cu/CeO.sub.2—COM highlight the importance of the aerogel structure for stabilization of active low-valent Cu.

[0076] Many modifications and variations are possible in light of the above teachings. It is therefore to be understood that the claimed subject matter may be practiced otherwise than as specifically described. Any reference to claim elements in the singular, e.g., using the articles “a”, “an”, “the”, or “said” is not construed as limiting the element to the singular.