Functional nanoscale metal oxides for stable metal single atom and cluster catalysts

Abstract

A nanocomposite catalyst includes a support, a multiplicity of nanoscale metal oxide clusters coupled to the support, and one or more metal atoms coupled to each of the nanoscale metal oxide clusters. Fabricating a nanocomposite catalyst includes forming nanoscale metal oxide clusters including a first metal on a support, and depositing one or more metal atoms including a second metal on the nanoscale metal oxide clusters. The nanocomposite catalyst is suitable for catalyzing reactions such as CO oxidation, water-gas-shift, reforming of CO.sub.2 and methanol, and oxidation of natural gas.

Claims

1. A nanocomposite catalyst comprising: a support; a multiplicity of nanoscale metal oxide clusters coupled to the support; and 1-100 metal atoms coupled to each of the nanoscale metal oxide clusters, wherein the support is negatively charged, the nanoscale metal oxide clusters are positively charged, the 1-100 metal atoms are negatively charged, and the support is free of direct contact with the 1-100 metal atoms.

2. The catalyst of claim 1, wherein the support comprises a refractory material having a surface area of at least 50 m.sup.2/g or at least 100 m.sup.2/g.

3. The catalyst of claim 2, wherein the support comprises silica, alumina, magnesia, zirconia, cordierite, mullite, perovskite or any combination thereof.

4. The catalyst of claim 2, wherein the support is powdered.

5. The catalyst of claim 1, wherein the nanoscale metal oxide clusters comprise CeO.sub.2, Co.sub.3O.sub.4, Fe.sub.2O.sub.3, TiO.sub.2, CuO, NiO, MnO.sub.2, Nb.sub.2O.sub.5, ZrO.sub.2 or any combination thereof.

6. The catalyst of claim 1, wherein the 1-100 metal atoms independently comprise one or more transition metal atoms.

7. The catalyst of claim 6, wherein the 1-100 metal atoms independently comprise one or more precious metal atoms.

8. The catalyst of claim 7, wherein the 1-100 metal atoms comprise Pt, Pd, Rh, Au, Ru, Ir, or any combination thereof.

9. The catalyst of claim 1, wherein the support comprises SiO.sub.2 and the metal oxide clusters comprise, CeO.sub.2, Co.sub.3O.sub.4, CuO, Fe.sub.2O.sub.3, or any combination thereof.

10. The catalyst of claim 1, wherein the nanoscale metal oxide clusters have a dimension in a range of 0.5 nm to 10 nm.

11. A method of catalyzing a reaction comprising contacting the nanocomposite catalyst of claim 1 with reactants, wherein the reaction comprises CO oxidation, water-gas-shift reaction, reforming of CO.sub.2 and methanol, or oxidation of natural gas.

12. A method of fabricating a nanocomposite catalyst, the method comprising: forming nanoscale metal oxide clusters comprising a first metal on a support; and depositing 1-100 metal atoms comprising a second metal on the nanoscale metal oxide clusters, wherein the support is free of direct contact with the second metal.

13. The method of claim 12, wherein the nanoscale metal oxide clusters comprise CeO.sub.2, Co.sub.3O.sub.4, Fe.sub.2O.sub.3, TiO.sub.2, CuO, NiO, MnO.sub.2, Nb.sub.2O.sub.5, ZrO.sub.2, or any combination thereof.

14. The method of claim 12, wherein the 1-100 metal atoms independently comprise one or more transition metal atoms.

15. The method of claim 12, wherein the 1-100 metal atoms independently comprise one or more precious metal atoms.

16. The method of claim 12, wherein the support comprises a refractory material having a surface area of at least 50 m.sup.2/g or at least 100 m.sup.2/g.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIGS. 1A-1C depict a single metal atom, two different single metal atoms, and a metal cluster, respectively, anchored onto the surface of a reducible metal oxide nanoisland bound to the surface of a refractory oxide support.

(2) FIG. 2A depicts solution deposition of positively charged metal complexes onto the negatively charged surfaces of a refractory oxide support. FIG. 2B depicts formation of uniformly deposited metal complexes onto the surface of a refractory oxide support. FIG. 2C depicts formation of individually isolated reducible metal oxide nanoisland by a high temperature calcination process.

(3) FIG. 3A depicts a process for solution deposition of negatively charged metal species onto the positively charged surfaces of the reducible metal oxide nanoisland, but not onto the negatively charged surfaces of the refractory oxide support. FIG. 3B depicts a single metal atom which deposits on the reducible metal oxide nanoisland which is supported on the refractory oxide support after washing, drying, and calcining processes.

(4) FIG. 4A shows a representative low-magnification high-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM) image of CeO.sub.x nanoislands uniformly distributed throughout a high-surface-area SiO.sub.2 support. FIG. 4B shows a representative high-magnification HAADF-STEM image of CeO.sub.x nanoislands.

(5) FIG. 5 shows a histogram of the particle size distribution of the CeO.sub.x nanoislands obtained by analyzing high resolution HAADF-STEM images.

(6) FIG. 6 shows powder X-ray diffraction (XRD) patterns obtained from 12 wt % CeO.sub.x nanoislands supported on SiO.sub.2 and 12 wt % CeO.sub.2 nanoparticles supported on SiO.sub.2.

(7) FIG. 7A shows Ce 3d X-ray photoelectron spectroscopy (XPS) spectra recorded over SiO.sub.2 supported CeO.sub.x nanoislands. FIG. 7B shows Ce 3d XPS spectra recorded over SiO.sub.2 supported CeO.sub.2 nanoislands.

(8) FIG. 8 shows Raman spectra obtained over SiO.sub.2 supported CeO.sub.x and CeO.sub.2 nanoislands.

(9) FIG. 9A shows a representative low-magnification HAADF-STEM image of CoO.sub.x nanoislands uniformly distributed throughout the high-surface-area SiO.sub.2 support. FIG. 9B shows a representative high-magnification HAADF-STEM image of CoO.sub.x nanoislands.

(10) FIG. 10A shows a representative low-magnification HAADF-STEM image of CuO.sub.x nanoislands uniformly distributed throughout the high-surface-area SiO.sub.2 support. FIG. 10B shows a representative high-magnification HAADF-STEM image of CuO.sub.x nanoislands.

(11) FIG. 11A shows a representative low-magnification HAADF-STEM image of FeO.sub.x nanoislands uniformly distributed throughout the high-surface-area SiO.sub.2 support. FIG. 11B shows a representative high-magnification HAADF-STEM image of FeO.sub.x nanoislands.

(12) FIG. 12A shows a representative low-magnification HAADF-STEM image of a typical Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst revealing the absence of Pt nanoparticles and nanoclusters. FIG. 12B shows a representative high-magnification HAADF-STEM image of a typical Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst showing the absence of Pt nanoparticles and nanoclusters.

(13) FIG. 13A shows a representative HAADF-STEM image of a typical CeO.sub.x—SiO.sub.2 supported Pt cluster catalyst. FIG. 13B shows a representative HAADF-STEM image of a typical CeO.sub.x—SiO.sub.2 supported Pt nanoparticle catalyst.

(14) FIG. 14A shows a representative low-magnification HAADF-STEM image of a typical Pd.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst showing the absence of Pd nanoparticles and nanoclusters. FIG. 14B shows a representative high-magnification HAADF-STEM image of a typical Pd.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst showing the absence of Pd nanoparticles and nanoclusters.

(15) FIG. 15 shows conversion rate of CO oxidation over the 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst and the CeO.sub.x—SiO.sub.2 support control.

(16) FIG. 16 shows conversion rate of CO oxidation over the 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 before and after H.sub.2 reduction treatment.

(17) FIG. 17 shows conversion rate of CO oxidation over the 0.05 wt % Pd.sub.1/CeO.sub.x—SiO.sub.2 before and after H.sub.2 reduction treatment.

(18) FIG. 18 shows conversion rate of CO oxidation over Au/CeO.sub.x—SiO.sub.2 catalyst.

(19) FIG. 19 shows a stability test of CO oxidation over the 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst.

(20) FIG. 20 shows a stability test for CO oxidation over the 0.05 wt % Pd.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst.

(21) FIG. 21 shows a stability test for CO oxidation over the Au/CeO.sub.x—SiO.sub.2 catalyst.

(22) FIG. 22 shows a conversion rate of water-gas-shift reaction (WGS) over the 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst.

(23) FIG. 23 shows a stability test of WGS over the 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst.

(24) FIG. 24 shows a conversion rate for methanol steam reforming over the 0.2 wt % Pt/CeO.sub.x—SiO.sub.2 catalyst.

(25) FIG. 25A depicts high-number density of Pt.sub.1 atoms supported on CeO.sub.x—SiO.sub.2 nanoislands which are supported on high-surface-area refractory material. FIG. 25B shows HAADF-STEM images of a typical Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst showing spatial distribution of the CeO.sub.x nanoislands. FIG. 25C shows the atomic structure of the CeO.sub.x clusters. The Pt.sub.1 atoms cannot be reliably identified due to lack of proper image contrast. FIG. 25D shows catalytic testing data for CO oxidation on various types of catalysts.

DETAILED DESCRIPTION

(26) Extremely stable supported metal atom and cluster catalysts have been developed by judicially integrating metal atoms (e.g., noble metal atoms), reducible metal oxides, and refractory high-surface-area supports. Atomically dispersed metal atoms and clusters are stabilized by use of nanoscale metal oxides (“nanoislands”) attached to refractory support materials. The reducible metal oxides serve as a binder to confine the movement of supported metal atoms or clusters during catalytic reactions. The reducible nanoscale metal oxides not only stabilize metal atoms and clusters during a catalytic reaction at high temperatures but also provide desirable functions to enhance the activity of a desired catalytic reaction. The nanoscale metal oxides typically have a dimension (e.g., diameter or height) of 0.5 nm to 10 nm. In some cases, the nanoscale metal oxides having a dimension of 0.5 nm up to 3 nm are referred to as “nanoglues,” while the nanoscale metal oxides having a dimension of 3 nm to 10 nm are referred to as “nanoparticles” or “nanocrystals.”

(27) The type of metal can be any transition metal (e.g., precious metal). Suitable metal oxides include CeO.sub.x (e.g., CeO.sub.2), CoO.sub.x (e.g., Co.sub.3O.sub.4), FeO.sub.x (e.g., Fe.sub.2O.sub.3), TiO.sub.x (e.g., TiO.sub.2), CuO.sub.x (e.g., CuO), NiO.sub.x (e.g., NiO), MnO.sub.x (e.g., MnO.sub.2), NbO.sub.x (e.g., Nb.sub.2O.sub.5), ZrO.sub.x (e.g., ZrO.sub.2) combinations of these oxides, and other appropriate meal oxides. A typical dimension for the nanoscale reducible metal oxide (e.g., diameter or height) is in a range of 0.5 nm to 10 nm. Suitable high-surface-area refractory support materials include SiO.sub.2, Al.sub.2O.sub.3, MgO, ZrO.sub.2, combinations of these oxides, and other appropriate support materials (e.g., mullite, cordierites, or perovskites).

(28) The utilization of such manufactured stable catalysts has been tested for CO oxidation, water-gas-shift reaction, reforming of CO.sub.2 and methanol, oxidation of natural gas, and the like. The catalyst design and synthesis strategy described herein is schematically illustrated in FIGS. 1-3.

(29) FIG. 1A depicts a single metal atom 3 preferentially anchored onto the surface of a reducible nanoscale metal oxide 2, which strongly binds onto the surface of a high-surface-area refractory oxide support 1. As used herein, “high surface area” generally refers to at least 50 m.sup.2/g or at least 100 m.sup.2/g. FIG. 1B depicts a single metal atom 3 and a different single metal atom 4 which are preferentially anchored onto the surface of a reducible nanoscale metal oxide 2 that strongly binds onto the surface of a refractory oxide support 1. The single metal atoms 3 and 4 can be associated with each other or independently anchored onto the surface of a reducible nanoscale metal oxide 2. FIG. 1C depicts a cluster of metal atoms (e.g., 2 to 100 metal atoms) 5 which is preferentially anchored onto the surface of a reducible nanoscale metal oxide 2 that strongly binds onto the surface of a high-surface-area refractory oxide support 1. The cluster 5 can include a single metal, a bimetallic alloy, or combinations of two or more metals. Suitable refractory oxide supports include SiO.sub.2, Al.sub.2O.sub.3, MgO, ZrO.sub.2, other appropriate materials, or any combination thereof (e.g., mullite and cordierite). Suitable nanoscale metal oxides include reducible oxide nanoclusters such as CeO.sub.x, CoO.sub.x, TiO.sub.x, FeO.sub.x, CuO.sub.x, MnO.sub.x, NbO.sub.x, ZrO.sub.x, other appropriate oxides, or any combination thereof. The reducible oxide nanoclusters typically have a dimension (e.g., diameter or height) of 0.5 nm to 10 nm. In some cases, nanoclusters having a dimension of 0.5 nm up to 3 nm are referred to as “nanoglues,” while nanoclusters having a dimension of 3 nm to 10 nm are referred to as “nanoparticles” or “nanocrystals.” Suitable metals for the metal atoms include precious metals (e.g., Pt, Pd, Rh, Au, Ru, Ir, Ag, and the like), transition metals, or any appropriate metal or alloy cluster with a total number of atoms in a range of 2 to 200 (e.g., 2 to 100 or 3 to 10).

(30) FIG. 2A depicts solution deposition of positively charged metal complexes 7 onto the surface of a high-surface-area refractory oxide support 1 that are negatively charged 6 due to the presence of surface species in aqueous solution at the appropriate pH. FIG. 2B depicts formation of uniformly deposited metal complexes 8 onto the surfaces of the high-surface-area refractory support 1 via strong electrostatic adsorption processes. FIG. 2C depicts formation of individually isolated reducible metal oxide nanoisland glue 2, by a high temperature calcination process, uniformly covering the surfaces of the high-surface-area refractory oxide support 1.

(31) FIG. 3A depicts a process for preferential solution deposition of metal atoms 9 onto the surfaces of the reducible metal oxide nanoisland glue 2, but not onto the surface of the high-surface-area refractory oxide support 1. By tuning the solution pH value, the refractory oxide support 1 can be made negatively charged while the reducible metal oxide nanoisland glue 2 becomes positively charged. The negatively charged metal complex 9 then can typically only deposit onto the positively charged reducible metal oxide nanoisland glue 2 due to strong electrostatic attraction but cannot typically deposit onto the negatively charged refractory oxide support 1 due to strong repulsion. FIG. 3B depicts single metal atom 3 which deposits on the nanoscale metal oxide 2 which are attached to the refractory oxide support 1 after washing, drying, and calcining processes.

(32) The specific synthesis examples illustrated below follow the general principles of the design strategy. Reducible metal oxide nanoislands are used as functional nanoglues. The reducible nanoscale metal oxides are synthesized by a facile wet chemical synthesis route. Specifically, metal complexes are solution deposited onto the refractory support surfaces by a strong electrostatic adsorption method. High temperature calcination of the deposited species produces isolated individual nanoscale metal oxide islands strongly attached to the refractory support surfaces. The metal atoms and/or clusters are preferentially deposited onto the surfaces of the isolated individual nanoscale metal oxides but not onto the surfaces of the refractory support materials by fine tuning the solution pH so that the nanoscale metal oxide surfaces maintain a surface charge that is opposite to that of the deposited metal complexes, and the refractory support surfaces maintain a surface charge similar to that of the deposited metal complexes.

(33) Atomically dispersed metal atoms and clusters are stabilized by use of nanoscale metal oxides. The synthesis processes include dispersing metal oxide clusters (e.g., 1 nm to 2 nm CeO.sub.x clusters) on a support (e.g., SiO.sub.2) and then depositing single metal atoms (e.g., Pt) onto the metal oxide clusters. Extremely stable supported metal atom and cluster catalysts can be prepared by judicially integrating metal atoms (e.g., noble metal atoms), reducible metal oxide nanoglues, and refractory high-surface-area supports. The use of reducible nanoscale metal oxides stabilizes metal atoms and clusters during a catalytic reaction at high temperatures and provides desirable functions (e.g., providing readily available active surface and/or lattice oxygen species) to enhance the activity of a desired catalytic reaction.

(34) Specific examples of facile and scalable wet chemistry methods to manufacture supported metal atom and cluster catalysts that are resistant to sintering, even at elevated temperatures and under various gas environment, are described below. In some embodiments, reducible metal oxide nanoislands strongly glue the metal atoms/clusters to a high-surface-area refractory support which can resist sintering at high temperatures. The zeta potential of different materials can be utilized to preferentially deposit metal atoms or clusters only to the reducible metal oxide nanoislands. The synthesis process is low cost, scalable, and ready for large scale manufacturing.

(35) CO oxidation is used as a probe reaction to evaluate the stability of the prepared supported metal atom and cluster catalysts. For a Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst (SAC) system, results demonstrate that the CeO.sub.x clusters stabilize the Pt.sub.1 single atoms during the CO oxidation and also enhance the activity, presumably due to the redox capability of CeO.sub.x clusters that facilitate CO oxidation.

EXAMPLES

(36) Synthesis of CoO.sub.x—SiO.sub.2

(37) 180 mg of fumed SiO.sub.2 powder (surface area of 278 m.sup.2/g) was dispersed into 30 mL of water by sonication. 54 mg of hexamminecobalt(III) chloride was dissolved into 20 mL of ammonia solution (concentration of NH.sub.3.Math.H.sub.2O was 5 mol/L). Under rigorous stirring, the Co precursor was quickly injected into the SiO.sub.2 solution. The mixture was aged under stirring for 1 h and then the precipitate was collected by vacuum filtration. The resultant orange Co—SiO.sub.2 precipitates were removed for air dry overnight at room temperature. The dried powder was ground with a pestle and annealed at 400° C. for 12 h in a muffle furnace to obtain the dark-green CoO.sub.x—SiO.sub.2 powder.

(38) Synthesis of CuO.sub.x—SiO.sub.2

(39) 180 mg of fumed SiO.sub.2 powder was dispersed into 30 mL of water by sonication. 48 mg of copper(II) nitrate hydrate was dissolved into 20 mL of ammonia solution (the concentration of NH.sub.3.Math.H.sub.2O was 5 mol/L). Under rigorous stirring, the Cu precursor solution was quickly injected into the SiO.sub.2 solution. The mixture was aged under stirring for 1 h and then the blue precipitate was collected by vacuum filtration. Then the resultant Cu—SiO.sub.2 precipitates were removed for air dry overnight at room temperature. The dried powders were ground with a pestle and annealed at 400° C. for 12 h in a muffle furnace to obtain the final dark-green CuO.sub.x—SiO.sub.2 powder.

(40) Synthesis of FeO.sub.x—SiO.sub.2

(41) 180 mg of fumed SiO.sub.2 powder was dispersed into 50 mL of water by sonication. 40 mg of iron(III) nitrate was added into the SiO.sub.2 solution. Under rigorous stirring, 0.2 mL of ammonia solution (the concentration of NH.sub.3—H.sub.2O was 2 mol/L) was quickly injected to the mixture solution. The mixture solution was aged under stirring for 1 h and then the orange precipitate was collected by vacuum filtration. Then the resultant Fe—SiO.sub.2 precipitates were removed for air dry overnight at room temperature. The dried powder was ground with a pestle and annealed at 400° C. for 1 h in a muffle furnace to produce the final orange FeO.sub.x—SiO.sub.2 powder.

(42) Synthesis of 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 Single-Atom Catalyst

(43) 300 mg of CeO.sub.x—SiO.sub.2 powder was dispersed into 72 mL DI water under sonication for 20 min. Then the pH of the solution was adjusted to below 4 by using HCl (0.1 mol/L). 530 L of platinum precursor solution (2.82 mg/mL of Pt) was diluted into 50 mL DI water and the pH was adjusted to below 4. Under rigorous stirring, the Pt precursor solution was slowly pumped into the CeO.sub.x—SiO.sub.2 solution under stirring over 4 h. After aging under stirring for another 2 h, the precipitates were filtered using vacuum filtration and washed with DI water 3 times to remove any non-adsorbed ions and any other residue species. The resultant precipitates were dried in air overnight and then were calcined in air at 600° C. for 12 h.

(44) Synthesis of Reduced 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 Single-Atom Catalyst

(45) 300 mg of CeO.sub.x—SiO.sub.2 powder was dispersed into 72 mL DI water under sonication for 20 min. Then the pH of the solution was adjusted to below 4 by using HCl (0.1 mol/L). 530 μL of platinum precursor solution (2.82 mg/mL of Pt) was diluted into 50 mL DI water and the pH value was adjusted to below 4. Under rigorous stirring, the Pt precursor solution was slowly pumped into the CeO.sub.x—SiO.sub.2 solution under stirring over 4 h. After aging under stirring for another 2 h, the precipitates were filtered using vacuum filtration and washed with DI water 3 times to remove any non-adsorbed ions and any other residue species. The resultant precipitates were dried in air overnight and then were calcined in air at 600° C. for 12 h. Prior to catalytic CO oxidation reaction, the as-calcined catalyst was reduced in 10 sccm (standard cubic centimeter per minute) of 5% H.sub.2/He at 300° C. for 1 h. Such reduced Pt.sub.1/CeO.sub.x—SiO.sub.2 SACs significantly improve CO oxidation activity.

(46) Table 1 shows specific reaction rates of Pt (mmol CO/(gPt*s)) at different reaction temperatures.

(47) TABLE-US-00001 TABLE 1 Specific reaction rates of Pt (mmol CO/(g.sub.Pt*s)) at different reaction temperatures Catalyst 150° C. 160° C. 170° C. E.sub.a 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 9.2 14.9 23.6 −67.8 kJ/mol 0.05 wt % Pt NPs/CeO.sub.x—SiO.sub.2 0.46 1.1 1.8 −79.3 kJ/mol 0.05 wt % Pt/SiO.sub.2 0.045 0.056 0.077  −140 kJ/mol CeO.sub.x—SiO.sub.2 (control) — — — — The specific rates of Pt.sub.1 atoms of Pt nanoparticles were measured with feed gas of 1.0 vol. % CO, 4.0 vol. % O.sub.2 and He balance, pressure was 0.1 MPa. The apparent activation energy (E.sub.a) of the 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 single atom catalyst is the lowest, indicating most active. The conversion of CO over CeO.sub.x—SiO.sub.2 (control) was practically zero at the reaction temperatures evaluated.
Synthesis of 0.05 wt % Pd.sub.1/CeO.sub.x—SiO.sub.2 Single-Atom Catalyst

(48) 300 mg of CeO.sub.x—SiO.sub.2 powder was dispersed into 72 mL DI water and the solution was then sonicated for 20 min. Then the solution pH was adjusted to below 4 using HCl (0.1 mol/L). 747 μL of palladium (II) chloride solution (2.0 mg/mL of Pd) was diluted into 50 mL DI water while the solution pH was maintained below 4. Under rigorous stirring, the Pd precursor solution slowly pumped into the CeO.sub.x—SiO.sub.2 solution under stirring over 4 h. After aging under stirring for another 2 h, the resultant precipitates were filtered using vacuum filtration and washed with DI water for 3 times to remove non-adsorbed ions or other residue species. The precipitates were then dried in air overnight and further calcined in air at 400° C. for 3 h.

(49) Synthesis of 2 wt % Pt/CeO.sub.x—SiO.sub.2 Cluster Catalyst

(50) 300 mg of CeO.sub.x—SiO.sub.2 powders were immersed into 72 mL DI water under sonication for 20 min. The solution pH was maintained below 4 by using HCl (0.1 mol/L). 2.12 mL of platinum precursor solution (contains 2.82 mg/mL of Pt) was diluted into 50 mL DI water while maintaining the pH below 4. Under rigorous stirring, the Pt precursor solution was pumped into the CeO.sub.x—SiO.sub.2 solution over 4 h. After aging (under stirring) for another 2 h, the precipitates were filtered using vacuum filtration and washed with DI water 3 times to remove non-adsorbed ions or other residue species. The precipitates were dried in air overnight at room temperature and were then calcined in air at 600° C. for 12 h. Finally, the 2 wt % Pt/CeO.sub.x—SiO.sub.2 powders were reduced in 5 vol % CO at 400° C. for 5 h to produce uniformly distributed Pt nanoclusters that are attached to the CeO.sub.x nanoglues.

(51) FIG. 4A shows a representative low-magnification high-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM) image of CeO.sub.x nanoislands 400 uniformly distributed throughout a high-surface-area SiO.sub.2 support 402. FIG. 4B shows a representative high-magnification HAADF-STEM image of CeO.sub.x nanoislands 400, revealing the shape, size and spatial distribution of the CeO.sub.x nanoislands that are strongly attached to the SiO.sub.2 surface 402.

(52) FIG. 5 shows a histogram of the particle size distribution of the CeO.sub.x nanoglues obtained by analyzing high resolution HAADF-STEM images. The average size of the CeO.sub.x nanoglues is 1.8 nm.

(53) FIG. 6 shows powder X-ray diffraction (XRD) patterns obtained from defect-rich 12 wt % CeO.sub.x nanoglues (<3 nm) supported on SiO.sub.2 and from stoichiometric 12 wt % CeO.sub.2 nanoparticles (3-10 nm) supported on SiO.sub.2. The sizes of the CeO.sub.x nanoglues are too small to give observable peaks in the XRD pattern while the larger CeO.sub.2 nanocrystals show clearly recognizable peaks in the XRD pattern.

(54) FIG. 7A shows Ce 3d X-ray photoelectron spectroscopy (XPS) spectra recorded over SiO.sub.2 supported CeO.sub.x nanoglues. FIG. 7B show Ce 3d XPS spectra recorded over SiO.sub.2 supported CeO.sub.2 nanocrystals. The amount of Ce.sup.3+, reflecting the defect state, is more dominant in the CeO.sub.x nanoglues (˜26%) than that in the CeO.sub.2 nanoparticles (˜10%).

(55) FIG. 8 shows Raman spectra obtained over SiO.sub.2 supported CeO.sub.x nanoglues (upper) and CeO.sub.2 nanocrystals (lower). Compared to the peak at 462 cm.sup.−1 of CeO.sub.2 nanoparticles (NPs), the corresponding peak of the 12 wt % CeO.sub.x—SiO.sub.2 redshifts to 448 cm.sup.−1 and becomes much broader, reflecting the increase in lattice constant due to their smaller sizes and the strong interaction between the CeO.sub.x nanoglues and the SiO.sub.2 support.

(56) FIG. 9A shows a representative low-magnification HAADF-STEM image of CoO.sub.x nanoislands 900 uniformly distributed throughout the high-surface-area SiO.sub.2 support 902. FIG. 9B shows a representative high-magnification HAADF-STEM image of CoO.sub.x nanoislands 900 showing the shape, size, and spatial distribution of the CoO.sub.x nanoislands attached to the SiO.sub.2 surfaces 902.

(57) FIG. 10A shows a representative low-magnification HAADF-STEM image of CuO.sub.x nanoislands 1000 uniformly distributed throughout the high-surface-area SiO.sub.2 support 1002. FIG. 10B shows a representative high-magnification HAADF-STEM image of CuO.sub.x nanoislands 1000 showing the shape, size, and spatial distribution of the CuO.sub.x nanoislands attached to the SiO.sub.2 surfaces 1002.

(58) FIG. 11A shows a representative low-magnification HAADF-STEM image of FeO.sub.x nanoislands 1100 uniformly distributed throughout the high-surface-area SiO.sub.2 support 1102. FIG. 11B shows a representative high-magnification HAADF-STEM image of FeO.sub.x nanoislands 1100 showing the shape, size, and spatial distribution of the FeO.sub.x nanoislands attached to the SiO.sub.2 surface 1102.

(59) FIG. 12A shows a representative low-magnification HAADF-STEM image of a typical Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst 1200 revealing the absence of Pt nanoparticles and nanoclusters. FIG. 12B shows a representative high-magnification HAADF-STEM image of a typical Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst 1202 showing the absence of Pt nanoparticles and nanoclusters. Single Pt atoms cannot be reliably imaged because of the lack of image contrast since the atomic number differences between Pt and Ce is not large enough and that the thicknesses of the CeO.sub.x nanoislands change rapidly.

(60) FIG. 13A shows a representative HAADF-STEM image of a typical CeO.sub.x—SiO.sub.2 supported Pt cluster catalyst 1300 revealing the presence of many Pt nanoclusters 1302 with sizes ranging from ˜0.4 nm to 1.0 nm. FIG. 13B shows a representative HAADF-STEM image of a typical CeO.sub.x—SiO.sub.2 supported Pt nanoparticle catalyst 1300 showing the presence of many Pt nanoparticles 1304 with sizes ranging from ˜0.5 nm to 2.0 nm.

(61) FIG. 14A shows a representative low-magnification HAADF-STEM image of a typical Pd.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst 1400 showing the absence of Pd nanoparticles and nanoclusters. FIG. 14B shows a representative high-magnification HAADF-STEM image of a typical Pd.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst 1400 showing the absence of Pd nanoparticles and nanoclusters. Single Pd atoms cannot be reliably imaged because of the lack of image contrast since the atomic number differences between Pd and Ce is small and that the thicknesses of the CeO.sub.x nanoislands change rapidly.

(62) CO Oxidation Reaction

(63) The CO oxidation reaction over the fabricated catalysts was conducted in a fixed-bed plug-flow reactor at atmospheric pressure. Typically, 30 mg of catalyst was used for each catalytic test. For CO oxidation, the reaction temperature was ramped up with a heating rate of 1° C./min. The feed gas, containing 1 vol % CO, 4 vol % O.sub.2 balanced with He, passed through the catalytic bed at a flow rate of 10.0 mL/min (corresponding to weight hourly space velocity (WHSV) of 20,000 mL/g.Math.h). Outlet gas composition was measured by an online gas chromatograph (Agilent 7890A) equipped with a thermal conductivity detector (TCD).

(64) FIG. 15 shows conversion rate of CO oxidation over the 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst and the CeO.sub.x—SiO.sub.2 support control. Reaction conditions: 1 vol % CO and 4 vol % O.sub.2 balanced with He, pressure=0.1 MPa, space velocity=20,000 mL/g.Math.h. The high CO oxidation activity originates from the Pt single atoms.

(65) FIG. 16 shows conversion rate of CO oxidation over the 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 before and after H.sub.2 reduction treatment. Reaction conditions: 1 vol % CO and 4 vol % O.sub.2 balanced with He, pressure=0.1 MPa, SV=20,000 mL/g.Math.h. The H.sub.2 reduction process removes some oxygen ligands which are weakly bound to the Pt atoms. The reduction of the oxidation state of the Pt atoms promoted their CO oxidation activity. The modification of the CeO.sub.x by the H.sub.2 reduction treatment may affect the observed CO oxidation activity as well.

(66) FIG. 17 shows conversion rate of CO oxidation over the 0.05 wt % Pd.sub.1/CeO.sub.x—SiO.sub.2 before and after H.sub.2 reduction treatment. Reaction conditions: 1 vol % CO and 4 vol % O.sub.2 balanced in He, pressure=0.1 MPa, SV=20,000 mL/g.Math.h. The calcined 0.05 wt % Pd.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst is intrinsically active for CO oxidation. The H.sub.2 reduction process did not appreciably change the CO oxidation activity.

(67) FIG. 18 shows conversion rate of CO oxidation over Au/CeO.sub.x—SiO.sub.2 catalyst. Reaction conditions: 1 vol % CO and 4 vol % O.sub.2 balanced with He, pressure=0.1 MPa, SV=20,000 mL/g.Math.h.

(68) FIG. 19 shows a stability test of CO oxidation over the 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst. The CO oxidation reaction was conducted at 140° C. The 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst did not show any trend of deactivation during the long-term test. Reaction conditions: 1 vol % CO and 4 vol % O.sub.2 balanced with He, pressure=0.1 MPa, SV=20,000 mL/g.Math.h.

(69) FIG. 20 shows a stability test for CO oxidation over the 0.05 wt % Pd.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst. The CO oxidation reaction was conducted at 120° C. Reaction conditions: 1 vol % CO and 4 vol % O.sub.2 balanced with He, pressure=0.1 MPa, SV=20,000 mL/g.Math.h.

(70) FIG. 21 shows a stability test for CO oxidation over the Au/CeO.sub.x—SiO.sub.2 catalyst. The CO oxidation reaction was conducted at 80° C. Reaction conditions: 1 vol % CO and 4 vol % O.sub.2 balanced with He, pressure=0.1 MPa, SV=20,000 mL/g.Math.h.

(71) Water-Gas-Shift Reaction (WGS)

(72) The WGS reaction over the fabricated catalysts was conducted in a fixed-bed plug-flow reactor at atmospheric pressure. Typically, 30 mg of catalyst was used for each test. Catalyst powders were pretreated in 10 sccm (standard cubic centimeter per minute) of 5% H.sub.2/He at 300° C. for 1 h. The reaction temperature was ramped up with a heating rate of 2° C./min. The feed gas, containing 1 vol % CO balanced with He, passed through a water reservoir which was heated to 33° C. The gas mixture went through the catalytic bed at a flow rate of 10.0 mL/min (corresponding to a weight hourly space velocity (WHSV) of 20,000 mL/g.Math.h). Outlet gas composition was measured by an online gas chromatograph (Agilent 7890A) equipped with a thermal conductivity detector (TCD).

(73) FIG. 22 shows conversion rate of WGS reaction over the 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst. Reaction conditions: 1 vol % CO and 5 vol % H.sub.2O balanced with He, pressure=0.1 MPa, space velocity=20,000 mL/g.Math.h.

(74) FIG. 23 shows a stability test of WGS over the 0.05 wt % Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst. The WGS reaction was conducted at 300° C. Reaction conditions: 1 vol % CO and 5 vol % H.sub.2O balanced with He, pressure=0.1 MPa, space velocity=20,000 mL/g.Math.h.

(75) Methane Combustion Reaction

(76) The methane combustion reaction over the fabricated catalysts was conducted in a fixed-bed plug-flow reactor at atmospheric pressure. Typically, 30 mg of catalyst was used for each test. Before reaction test, the catalyst was pretreated with 10 sccm (standard cubic centimeter per minute) of 5% H.sub.2/He at 300° C. for 1 h. For methane combustion, the reaction temperature was ramped up with a heating rate of 1° C./min. The feed gas, containing 1 vol % CH.sub.4, 4 vol % O.sub.2 balanced with He, passed through the catalytic bed at a flow rate of 10.0 mL/min (corresponding to a weight hourly space velocity of 20,000 mL/g.Math.h). Outlet gas composition was measured by an online gas chromatograph (Agilent 7890A) equipped with a thermal conductivity detector (TCD).

(77) Methanol Reforming Reaction

(78) The methanol reforming reaction over the synthesized catalysts was conducted in a fixed-bed flow reactor at atmospheric pressure. Typically, 30 mg of catalyst was used for each test. Before reaction, the catalyst was pretreated with 10 sccm (standard cubic centimeter per minute) of 5% H.sub.2/He at 300° C. for 1 h. For methanol reforming reaction, the reaction temperature was ramped up with a heating rate of 1° C./min. The feed gas, containing 10 vol % CH.sub.3OH, 7 vol % H.sub.2O balanced with He, passed through the catalytic bed at a flow rate of 10.0 mL/min (corresponding to weight hourly space velocity of 20,000 mL/g.Math.h). Outlet gas composition was measured by an online gas chromatograph (Agilent 7890A) equipped with a thermal conductivity detector (TCD).

(79) FIG. 24 shows a conversion rate for methanol steam reforming over the 0.2 wt % Pt/CeO.sub.x—SiO.sub.2 catalyst. Reaction conditions: 1 vol % methanol and 1 vol % H.sub.2O balanced with He, pressure=0.1 MPa, space velocity=20,000 mL/g.Math.h.

(80) Synthesis of CeO.sub.x—SiO.sub.2

(81) CeO.sub.x clusters were used as nanoglues to anchor Pt single atoms onto high surface area, inexpensive, and abundant SiO.sub.2 supports, as depicted in FIG. 25A. The highly reducible CeO.sub.x clusters act as an oxygen gateway to store/release oxygen during selected catalytic reactions. CO oxidation was used as a probe reaction to test the catalytic performance of Pt.sub.1/CeO.sub.x clusters dispersed onto SiO.sub.2 nanoparticles.

(82) 180 mg of fumed SiO.sub.2 powder (surface area of 278 m.sup.2/g) was mixed with 50 mL of water and then sonicated to obtain a uniform suspension. Under rigorous stirring, 86 mg of Ce(NO.sub.3).sub.3.6H.sub.2O was added into the SiO.sub.2 solution. Subsequently, 0.4 mL of NH.sub.3—H.sub.2O (concentration 2 mol/L) was quickly injected into the mixed solution. After stirring for 3 min, the mixture was collected by vacuum filtration. The resultant light-brown Ce—SiO.sub.2 precipitate was dried in air overnight at room temperature. The dried powder was ground with a pestle and then annealed at 600° C. for 12 h in a muffle furnace to obtain the light-yellow colored CeO—SiO.sub.2 powders. The loading of the CeO.sub.x was 12 wt % by inductively coupled plasma mass spectrometry (ICP-MS) measurement. Through the same procedure, the 6 wt % CeO—SiO.sub.2 was synthesized by using 43 mg of Ce(NO.sub.3).sub.3.6H.sub.2O and 0.2 mL of ammonia (2 mol/L). This synthesis process was successfully scaled up to 10 times, in which 1800 mg of SiO.sub.2, 500 mL of H.sub.2O, 860 mg of Ce(NO.sub.3).sub.3.6H.sub.2O and 4 mL of NH.sub.3—H.sub.2O were used, respectively.

(83) A strong electrostatic adsorption method was used to disperse Pt salt precursors onto the surfaces of the as-prepared CeO.sub.x—SiO.sub.2 nanocomposite powders. The Pt/CeO—SiO.sub.2 precipitates were then filtered, washed and dried at 60° C. for 5 h. The Pt.sub.1/CeO.sub.x—SiO.sub.2 powders, with a nominal loading of 0.05 wt. % of Pt, were calcined and/or reduced to form the final Pt.sub.1 SACs.

(84) FIG. 25A depicts high-number density CeO.sub.x—SiO.sub.2 supported Pt.sub.1 catalyst 2500, with platinum atoms 2502 on CeO.sub.x cluster 2504 and SiO.sub.2 support 2506. FIG. 25B shows HAADF-STEM images of a typical Pt.sub.1/CeO.sub.x—SiO.sub.2 single-atom catalyst showing spatial distribution of the CeO.sub.x nanoislands 2508. FIG. 25C shows the atomic structure of the CeO.sub.x clusters 2510. The Pt.sub.1 atoms cannot be reliably identified due to lack of proper image contrast. FIG. 25D shows catalytic testing data for CO oxidation. The conversion rates for the Pt.sub.1/SiO.sub.2 and the CeO.sub.x—SiO.sub.2 controls were displayed as well. Reaction conditions: 1 vol % CO and 4 vol % O.sub.2 balanced with He, pressure=0.1 MPa, SV=20,000 mL/g.Math.h.

(85) FIG. 25B shows representative HAADF-STEM images of the as-synthesized Pt.sub.1/CeO.sub.x—SiO.sub.2 SAC. The bright patches (indicated by the yellow arrows) represent uniformly dispersed CeO.sub.x clusters with an average size of 2 nm and Pt nanoclusters are not observable. DRIFTS and EXAFS results show that the Pt species exists as isolated single Pt atoms. The Ce 3d XPS spectrum of the Pt.sub.1/CeO.sub.x—SiO.sub.2 SAC reveal enrichment of oxygen vacancies in the CeO.sub.x nanoclusters since the Ce.sup.3+/(Ce.sup.4++Ce.sup.3+) ratio is 26% (FIG. 7A) vs 10% for CeO.sub.2 nanocrystals (FIG. 7B). CO oxidation over the as-synthesized Pt.sub.1/CeO.sub.x—SiO.sub.2 SAC demonstrates that the CeO.sub.x anchored Pt.sub.1 atoms are not only highly active but extremely stable during the three reaction cycles (FIG. 25D). The CO-DRIFTS results of the used Pt.sub.1/CeO.sub.x—SiO.sub.2 SAC demonstrate that the localized Pt.sub.1 atoms did not sinter and maintained their catalytic property during the CO oxidation reaction. The crystalline nature and the dispersion of the CeO.sub.x clusters are clearly shown in FIG. 25C. Some small CeO.sub.x clusters are almost atomically dispersed and do not possess a crystalline phase. Although it is difficult to distinguish single Pt atoms from those of the highly dispersed Ce atoms/clusters we have not detected, in the as-prepared and used SACs, any Pt particles/clusters which are distinguishable from the CeO.sub.x clusters. FIG. 25D shows the catalytic testing data clearly demonstrating the high activity and stability of the Pt.sub.1/CeO.sub.x—SiO.sub.2 SAC for the CO oxidation reaction.

(86) Although this disclosure contains many specific embodiment details, these should not be construed as limitations on the scope of the subject matter or on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in this disclosure in the context of separate embodiments can also be implemented, in combination, in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments, separately, or in any suitable sub-combination. Moreover, although previously described features may be described as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.

(87) Particular embodiments of the subject matter have been described. Other embodiments, alterations, and permutations of the described embodiments are within the scope of the following claims as will be apparent to those skilled in the art. While operations are depicted in the drawings or claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed (some operations may be considered optional), to achieve desirable results.

(88) Accordingly, the previously described example embodiments do not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure.