Electronic Structure Modulation of Unusual Phase of Metal Nanomaterials for Catalysis

The present invention relates to a heterostructured electrocatalyst for example particularly, but not exclusively, a heterostructured electrocatalyst for carbon dioxide reduction reaction; and a preparation method and use of the heterostructured electrocatalyst.

Various methods for facilitating carbon dioxide (CO) fixation and fossil resource consumption reduction have been developed for tackling the increasing atmospheric greenhouse gas emissions. Among those methods, the electrocatalytic COreduction reaction (CORR) is considered as one of the ustainable approaches to generate value-added chemicals and fuels to promote carbon-neutral energy cycle. For example, carbon monoxide (CO) converted from COmay be used as essential feedstock to produce high-value chemicals through Fischer-Tropsch reaction.

In this regard, various noble metal nanomaterials such as gold (Au), silver (Ag) and palladium (Pd) have been used for electrocatalytic CORR toward CO production. However, because of the weak adsorption of COmolecules on nobel metal surfaces, it is believed that it remains challenging to achieve highly efficient and selective COelectroduction to a single product, particularly in a broad potential window.

The present invention thus seeks to eliminate or at least mitigate such shortcomings by providing a new or otherwise improved electrocatalyst for CORR.

In a first aspect of the present invention, there is provided a heterostructured electrocatalyst for carbon dioxide reduction reaction comprising a lanthanide oxide nanomaterial deposited on a gold-containing nanosupport.

In an optional embodiment, the gold-containing nanosupport is physically distinguishable from the lanthanide oxide nanomaterial.

It is optional that structure of the lanthanide oxide nanomaterial is different from that of the gold-containing nanosupport.

Optionally, the lanthanide oxide nanomaterial comprises a nanoparticle of cerium oxide.

In an optional embodiment, oxidation state of the gold in the gold-containing nanosupport is different from that of the lanthanide in the lanthanide oxide nanomaterial.

Optionally, the oxidation state of the gold is 0 and the oxidation state of the lanthanide is +3 or +4.

It is optional that the gold-containing nanosupport has a hetero-crystal phase of 4H/fcc.

It is optional that the lanthanide oxide nanomaterial has a homo-crystal phase of fcc.

In an optional embodiment, the gold-containing nanosupport is a 4H/fcc gold nanorod.

In an optional embodiment, the lanthanide oxide nanomaterial is a fcc cerium oxide nanoparticle

Optionally, the 4H/fcc gold nanorod is partially covered by the fcc cerium oxide nanoparticles to provide a metal-oxide interface as reactive sites for electrocatalytic carbon dioxide reduction reaction

It is optional that the gold-containing nanosupport has a deposit of about 3 nm to about 10 nm of fcc cerium oxide nanoparticles.

Optionally, the lanthanide oxide nanomaterial include cerium (IV) oxide nanoparticles and cerium (III) oxide nanoparticles at a ratio of about 2:1.

It is optional that atomic ratio of gold:cerium is about 3:1 to about 5:1.

Optionally, the 4H/fcc gold nanorod has a diameter of about 12 nm to about 25 nm and a length of about 400 nm to about 900 nm.

In a second aspect of the present invention, there is provided a method of preparing the heterostructured electrocatalyst in accordance with the first aspect, comprising the steps of: a) providing a reaction mixture including a hetero-crystal phase gold-containing nanosupport, a lanthanide precursor and a first reducing agent; b) heating the reaction mixture at a temperature of about 100° C. for about 5 mins; and c) isolating the electrocatalyst from the reaction mixture.

In an optional embodiment, the gold-containing nanosupport is suspended in ethanol.

Optionally, the gold-containing nanosupport comprises a 4H/fcc gold nanorod formed from the steps of: providing a closed reaction mixture including gold (III) chloride hydrate, n-heptane, a surfactant including oleylamine, and a second reducing agent including N-ethylcyclohexylamine; heating the closed reaction mixture at about 68° C. for about 48 h to form crude 4H/fcc gold nanorod; isolating the crude 4H/fcc gold nanorod from the closed reaction mixture; purifying the isolated 4H/fcc gold nanorod by successively washing the isolated 4H/fcc gold nanorod with cyclohexane, a cyclohexane/ethanol mixture (1:1, v/v), and ethanol; and resuspending the washed 4H/fcc gold nanorod in ethanol to obtain an ethanol suspension of the 4H/fcc gold nanorod.

It is optional that the lanthanide precursor comprises cerium nitrate hexahydrate, and the first reducing agent comprises hexamethylenetetramine.

Optionally, the lanthanide precursor and the first reducing agent have a concentration ratio of 1:1.

In an optional embodiment, the lanthanide precursor includes a concentration of 1 mg/mL to 3 mg/mL.

In an optional embodiment, the first reducing agent includes a concentration of 1 mg/mL to 3 mg/mL.

In a third aspect of the present invention, there is provided an electrode for carbon dioxide reduction reaction comprising an electrocatalytically active mixture including the heterostructured electrocatalyst in accordance with the first aspect provided on a conductive substrate.

In an optional embodiment, the conductive substrate comprises glassy carbon.

Optionally, the electrocatalytically active mixture further includes carbon black and tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer.

It is optional that the electrode has a working area of about 0.5 cm.

Optionally, the electrode has a catalyst loading of about 400 μg cm.

As used herein, the forms “a”, “an”, and “the” are intended to include the singular and plural forms unless the context clearly indicates otherwise.

The words “example” or “exemplary” used in this invention are intended to serve as an example, instance, or illustration. Any aspect or design described in this disclosure as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the words “example” or “exemplary” is intended to present concepts in a concrete fashion. As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, if X employs A; X employs B; or X employs both A and B, then “X employs A or B” is satisfied under any of the foregoing instances.

As used herein, the phrase “about” is intended to refer to a value that is slightly deviated from the value stated herein. Examples have been described throughout the present disclosure.

Without wishing to be bound by theory, the inventors have, through their own research, trials, and experiments, devised that electronic structure modulation of metal nanomaterials may be effective for improving the electrocatalytic carbon dioxide reduction reaction (CORR) performance. In particular, it is believed that by constructing a metal-oxide interface, such as by introducing a lanthanide oxide (LnO) to the metal, the resultant metal-oxide interface may feature superior active and highly selective sites for the electrocatalytic CORR. Thus, it is also believed that the present invention may provide an effective strategy for rational electronic structure regulation of the metal nanomaterials to enhance the electrocatalytic performance thereof.

In a first aspect of the present invention, there is provided a heterostructured electrocatalyst for carbon dioxide reduction reaction comprising a lanthanide oxide nanomaterial deposited on a gold-containing nanosupport. As used herein, the term “nanosupport” generally refers to a nanostructured material that acts as a physical support for another material, in particular, for active compounds such as catalysts, which might thereby enhance the surface area of the active compounds for chemical reaction. For example, in some embodiments of the present invention, the lanthanide oxide nanomaterial may form an interface with the gold-containing nanosupport and the interface may reduce the energy barrier of the rate-limiting (rate-determining) step, such as the formation of *COOH species, upon the CORR process, and therefore the CORR process may be optimized.

In some embodiments, the gold-containing nanosupport may be physically distinguishable from the lanthanide oxide nanomaterial. For example, the gold-containing nanosupport may have different structures, dimensions, size, density, mass, hardness, ductility, conductivity, etc. as compared with the lanthanide oxide nanomaterial. In an embodiment, the gold-containing nanosupport may be a zero-dimensional, one-dimensional, two-dimensional, or three-dimensional nanomaterial. Examples of zero-dimensional nanomaterials may include nanoparticles, quantum dots, fullerenes and the like. Examples of one-dimensional nanomaterials may include nanowires, nanorods, nanotubes and the like. Examples of two-dimensional nanomaterials may include graphene, nanofilms, nanocoatings and the like. Examples of three-dimensional nanomaterials may include nanocomposites, nanoporous materials, nanoprism and the like. In some particular embodiments, the gold-containing nanosupport may be a two-dimensional nanomaterial such as a gold-containing nanorod.

In some embodiments, the oxidation state of the gold-containing nanosupport may be different from that of the lanthanide in the lanthanide oxide nanomaterial. In some particular embodiments, the oxidation state of the gold may be 0 (i.e., in its metallic state) and the oxidation state of the lanthanide may be +3 and/or +4. Examples of such a lanthanide may include cerium, praseodymium, lanthanum, etc.

The lanthanide oxide nanomaterial may be a nanomaterial with the zero-dimension, one-dimension, two-dimension, or three-dimension as described herein. In some embodiments, the structure of the lanthanide oxide nanomaterial may be different from that of the gold-containing nanosupport. For example, in an embodiment where the gold-containing nanosupport may be a two-dimensional nanomaterial, such as a nanorod, the lanthanide oxide nanomaterial may have both a different structure and a different dimension compared to the gold-containing nanosupport, such as a nanoparticle of the lanthanide oxide. In an alternative embodiment, the gold-containing nanosupport and the lanthanide oxide nanomaterial may have the same dimension but different structures. For example, the gold-containing nanosupport may be a nanorod and the lanthanide oxide nanomaterial may be a nanotube.

In some embodiments, the gold-containing nanosupport may have a hetero-crystal phase (i.e., unconventional phase) of 4H/face-centered cubic (4H/fcc) whereas the lanthanide oxide nanomaterial may have a homo-crystal phase of fcc. It is believed that unconventional phase metal nanomaterials may have unique electronic structures, and such electronic structures may be tuned by electronic transfer between the metal and the lanthanide oxide, as it is also believed that the lanthanide oxide may have unique electronic structures, abundant oxygen vacancies as well as special redox properties, thereby enhancing the electrocatalytic CORR performance towards practical applications. Detailed electrocatalytic reaction mechanism of the heterostructured electrocatalyst will be discussed in the later part of the present disclosure.

In some preferred embodiments, the gold-containing nanosupport may be a 4H/fcc gold nanorod whereas the lanthanide oxide nanomaterial may be a fcc cerium oxide (CeO) nanoparticle. The gold nanorods may act as seeds for which the cerium oxide may grow thereon (i.e. for the overgrowth of cerium oxide). In particular, the 4H/fcc gold nanorod may be partially covered by the fcc cerium oxide nanoparticles to provide a metal-oxide interface such as a gold-cerium oxide (Au—CeO) interface as reactive sites for electrocatalytic carbon dioxide reduction reaction. As used herein, the subscript x of CeOgenerally denotes that the numbers of oxygen atom in CeOvary in accordance with the oxidation state(s) of the cerium. In other words, there are more than one type of cerium oxide exist in the lanthanide oxide nanomaterial. In some particular embodiments, the lanthanide oxide nanomaterial may include cerium (IV) oxide nanoparticles and cerium (III) oxide nanoparticles at a ratio of about 2:1. In some other embodiments, the heterostructured electrocatalyst may also have atomic ratio of gold:cerium of about 3:1 to about 5:1 such as about 83.9:16.1 to about 75.2:24.8.

The gold-containing nanosupport such as the 4H/fcc gold nanorod may have a deposit of about 3 nm (e.g., from 2.90 nm . . . 2.91 nm . . . 2.95 nm . . . 2.99 nm, 3 nm, 3.01 nm . . . 3.03 nm . . . to 3.10 nm) to about 10 nm (e.g., from 9.90 nm . . . 9.92 nm . . . 9.99 nm, 10.00 nm, 10.01 nm . . . 10.04 nm to 10.10 nm) of fcc cerium oxide nanoparticles. In an embodiment, the 4H/fcc gold nanorod may have a deposit of about 3 nm of the fcc cerium oxide nanoparticles. In another embodiment, the 4H/fcc gold nanorod may have a deposit of about 10 nm of the fcc cerium oxide nanoparticles.

In some other embodiments, the 4H/fcc gold nanorod may have a diameter of about 12 nm (e.g., from 11.9 nm . . . 11.95 nm . . . to 11.99 nm, 12.00 nm, 12.01 nm . . . 12.05 nm . . . to 12.10 nm) to about 25 nm (e.g., from 24.9 nm . . . 24.95 nm . . . to 24.99 nm, 25.00 nm, 25.01 nm . . . 25.05 nm . . . to 25.10 nm) and a length of about 400 nm (e.g., from 399.0 nm . . . 399.5 nm . . . 400.0 nm . . . 400.2 nm . . . 400.5 nm to 401 nm) to about 900 nm (e.g., from 899.0 nm . . . 899.5 nm . . . 900.0 nm . . . 900.2 nm . . . 900.5 nm to 901.0 nm).

The method of preparing the heterostructured electrocatalyst as disclosed herein is now described. The method may comprise the steps of: a) providing a reaction mixture including a hetero-crystal phase gold-containing nanosupport, a lanthanide precursor and a first reducing agent; b) heating the reaction mixture at a temperature of about 100° C. for about 5 mins; and c) isolating the electrocatalyst from the reaction mixture.

In some embodiments, the hetero-crystal phase gold-containing nanosupport may be suspended in ethanol. For example, in an embodiment where the hetero-crystal phase gold-containing nanosupport comprise a 4H/fcc gold nanorod, it may be formed from the following steps. First, a closed reaction mixture including gold (III) chloride hydrate, n-heptane, a surfactant including oleylamine, and a second reducing agent including N-ethylcyclohexylamine may be provided by sealing a glass vial containing a homogeneous reaction mixture of the gold (III) chloride hydrate, the n-heptane, the surfactant, and the second reducing agent with a sealing agent such as a parafilm. Then, the closed reaction mixture may be heated, such as in an oil bath, at about 68° C. for about 48 h to form crude 4H/fcc gold nanorod. Next, the crude 4H/fcc gold nanorod may be isolated from the closed reaction mixture by way of precipitation and/or centrifugation (e.g., at about 4000 rpm to about 4500 rpm for about 5 mins). After that, the isolated 4H/fcc gold nanorod may be purified by successively washing it with cyclohexane, a cyclohexane/ethanol mixture (1:1, v/v), and ethanol. Finally, the washed 4H/fcc gold nanorod in ethanol may be suspended in ethanol to obtain an ethanol suspension of the 4H/fcc gold nanorod.

It is believed that the lanthanide precursor and the first reducing agent may be selected in accordance with practical needs. For example, in an embodiment where the electrocatalyst is a heteronanostructure of Au—CeOcomprising the 4H/fcc gold nanorod and the fcc CeOnanoparticle as described herein, the lanthanide precursor may comprise cerium nitrate hexahydrate, and the first reducing agent may comprise hexamethylenetetramine.

In some embodiments, the hetero-crystal phase gold-containing nanosupport, the lanthanide precursor, and the first reducing agent in the reaction mixture may have a concentration ratio of about 1:1.67-5:1.67-5, such as 1:1.67:1.67, 1:2:2, 1:2.5:2.5, 1:3:3, 1:4:4, 1:4.5:4.5, 1:5:5 and the like. For example, in an embodiment where the concentration of the hetero-crystal phase gold-containing nanosupport, the lanthanide precursor, and the first reducing agent are 600 ppm, 1000 ppm, and 1000 ppm, respectively, their concentration ratio is 1:1.67:1.67. In some other embodiments, the lanthanide precursor and the first reducing agent may have a concentration ratio of 1:1. For example, in some embodiments where the lanthanide precursor may include a concentration of 1 mg/mL to 3 mg/mL, the first reducing agent may also include a concentration of 1 mg/mL to 3 mg/mL. As a specific embodiment, when the lanthanide precursor such as cerium nitrate hexahydrate has a concentration of 1 mg/mL, the first reducing agent such as hexamethylenetetramine may also have a concentration of 1 mg/mL upon conducting the synthesis.

It is also believed that by varying the concentrations of the lanthanide precursor and the first reducing agent, the thickness of the lanthanide oxide deposited on the gold-containing nanosupport would be varied/adjusted accordingly. For example, in an embodiment where the concentration of the lanthanide precursor such as cerium nitrate hexahydrate is 1 mg/mL and the concentration of the first reducing agent such as hexamethylenetetramine is 1 mg/mL, the resultant CeOnanoparticles deposited on the gold-containing nanosupport may have a thickness of about 3 nm. In another embodiment, where the concentrations of the lanthanide precursor and the first reducing agent are each 3 mg/mL, the resultant CeOnanoparticles deposited on the gold-containing nanosupport may have a thickness of about 10 nm.

In step b), the reaction mixture may be heated in an oil bath at, for example 100° C. for 5 mins to obtain the crude product of the electrocatalyst, such as, in an embodiment, crude Au—CeO. Then, the crude product may be, in step c), isolated from the reaction mixture by centrifugation (e.g., at about 4000 rpm to about 4500 rpm for about 5 mins), followed by purifying the isolated product by washing it with suitable solvent such as ethanol for, e.g., three times. Optionally or additionally, the purified product may be resuspended in ethanol for storage.

Another aspect of the present invention is an electrode for carbon dioxide reduction reaction comprising an electrocatalytically active mixture including the heterostructured electrocatalyst as described herein provided on a conductive substrate. The conductive substrate may be selected from one or more of the followings according to practical needs: glassy carbon, indium doped oxide glass (ITO/glass), fluorine-doped tin oxide glass (FTO/Glass), indium doped tin oxide polyethylene terephthalate (ITO/PET), etc. In an embodiment, the conductive substrate may comprise glassy carbon. The glassy carbon may have an area of about 0.5 cm(e.g., from 0.48 cm. . . 0.482 cm. . . 0.485 cm. . . 0.49 cm. . . 0.493 cm. . . 0.499 cm, 0.5 cm. . . 0.501 cm. . . 0.505 cmto about 0.51 cm). In other words, the electrode in this embodiment may have a working area of about 0.5 cm.