WO2013188589A2 - SPINEL CATALYST OF LixMn2O4 (0<x<0.2) FOR OXIDATION REACTIONS - Google Patents

SPINEL CATALYST OF LixMn2O4 (0<x<0.2) FOR OXIDATION REACTIONS Download PDF

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WO2013188589A2
WO2013188589A2 PCT/US2013/045500 US2013045500W WO2013188589A2 WO 2013188589 A2 WO2013188589 A2 WO 2013188589A2 US 2013045500 W US2013045500 W US 2013045500W WO 2013188589 A2 WO2013188589 A2 WO 2013188589A2
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catalyst
water
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WO2013188589A3 (en
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Kandalam Ramanujachary
Adibhatia Kali Satya Bhujanga RAO
Archana JAIN
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Kandalam Ramanujachary
Rao Adibhatia Kali Satya Bhujanga
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    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02Impregnation, coating or precipitation
    • B01J37/03Precipitation; Co-precipitation
    • B01J37/031Precipitation
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/005Spinels
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/16Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of arsenic, antimony, bismuth, vanadium, niobium, tantalum, polonium, chromium, molybdenum, tungsten, manganese, technetium or rhenium
    • B01J23/32Manganese, technetium or rhenium
    • B01J23/34Manganese
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00Catalysts, in general, characterised by their form or physical properties
    • B01J35/002Catalysts characterised by their physical properties
    • B01J35/004Photocatalysts
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/06Washing
    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25BELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B1/00Electrolytic production of inorganic compounds or non-metals
    • C25B1/003Electrolytic production of inorganic compounds or non-metals by photo-electrolysis with or without external current source
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02PCLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P20/00Technologies relating to chemical industry
    • Y02P20/10General improvement of production processes causing greenhouse gases [GHG] emissions
    • Y02P20/12Energy input
    • Y02P20/133Renewable energy sources
    • Y02P20/134Sunlight
    • Y02P20/135Photoelectrochemical processes

Abstract

The invention relates to transition metal oxide spinel catalysts, particularly LixMn2O4 compositions having a cubical Mn4O4 core, wherein 0<x<0.2, and methods of catalytic oxidation including oxidation of water. The invention also relates to methods of controlling the synthesis of the catalyst to ensure that 0<x<0.2.

Description

SPINEL CATALYST OF LixMn2O4 (0<x<0.2) FOR OXIDATION REACTIONS

CROSS-RERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 61/658,783 filed on June 12, 2012, the contents of which are incorporated herein in their entirety by reference.

FIELD OF THE INVENTION

The present invention relates to, but is not limited to, transition metal oxide spinel catalysts, particularly LixMn204 compositions having a cubical Mn404 core, wherein

0<x<0.2, and methods of catalytic oxidation including oxidation of water. The present invention also relates to Li2Co204 and LiMn204 compositions and catalysts.

BACKGROUND OF THE INVENTION

The efficient conversion/storage of solar energy into chemical bond energy via the splitting of water into its elements - hydrogen (H2) and oxygen (02) - represent a clean source of renewable fuel. Water oxidation is one of the half reactions of water splitting:

2H20→ 02+4H++4e" Oxidation (generation of oxygen gas or molecular oxygen)

4H++4e"→ 2H2 Reduction (generation of hydrogen gas or molecular hydrogen)

2H20→ 2H2 + 02 Total reaction

Of the two water splitting half cell reactions (H20/02 and H20/H2), the H20/02 reaction is considerably more complex. This reaction requires a four electron oxidation of two water molecules coupled to the removal of four protons requiring a large overpotential. In addition to controlling this proton coupled electron transfer (PCET), a catalyst must tolerate prolonged exposure to the oxidizing conditions. Even at the thermodynamic limit, water oxidation requires an oxidizing power that causes most chemical functional groups to degrade. Accordingly, the generation of oxygen from water presents a significant challenge towards realized artificial photosynthesis.

This H20/02 oxidation process occurs naturally in plant's photosystem II to provide protons and electrons for the photosynthesis process and releases oxygen to the atmosphere. Since hydrogen can be used as an alternative clean burning fuel, there has been a need to split water efficiently. The water splitting reaction opens a very promising pathway for the development of systems that use artificial photosynthesis to store solar energy on a large scale in the form of oxygen and hydrogen for subsequent use in a fuel cell. Conventional electrolytic cells not only require a high pH, but also require operation at an overpotential that makes them unfeasible. Further, the oxygen evolution is sensitive to the surface on which the reaction takes place. A catalyst system may be used to reduce the overpotential to commercially practical levels. In a conventional platinum catalyzed reaction, under acidic conditions, water binds to the surface of the catalyst with the reversible removal of one electron and one proton to form platinum hydroxide. In an alkaline solution a reversible binding of hydroxide ion coupled to one electron oxidation results in removal of one electron and one proton to form metal oxide. Thus, oxygen evolution reaction does not take place on clean metal surface, but instead an oxide surface is formed prior to oxygen evolution.

The catalyzed conversion of water into 02 and protons (H+) can be used to make H2 or to chemically reduce other molecules including carbon dioxide (C02). This technology can be applied in fuel cells for electricity production, and in electrolyzers and solar cells for production of 02, H2, and other hydrocarbon fuels. For example, a photoelectrochemical (PEC) cell or reverse fuel cell is a device for splitting water with energy from sunlight. The use of water as a source and sunlight as energy implies this technology is inherently sustainable and globally scalable, and could provide vast amounts of fuel (hydrogen), oxygen, and other hydrogenic precursors for reduction of carbon dioxide to hydrocarbon fuels from ordinary water.

Development of cheap water oxidation catalysts to replace expensive noble metals in commercial electrolyzers and solar fuel cells has been an unmet need preventing global development of hydrogen fuel technologies. Several metal oxides including Ir02 and Ru02 are already in use in industrial electrolyzers, but are made from rare and expensive metals that are not globally scalable. Accordingly, there is a need for inexpensive electrodes made from earth- abundant elements.

Recent advances in methods for synthesizing transition metal oxide (TMO) nano- particles with the spinel structure in contact with proton conduction sites have produced more efficient catalysts for water oxidation that are suitable for renewable hydrogen production, when coupled with a proton reducing cathode. Such advances are applicable to energy storage problems inherent to intermittent solar energy conversion (i.e., photovoltaic (PV) and wind). One catalytic system capable of oxidizing water to molecular oxygen is the photosystem II water- oxidizing complex (PSITWOC) found within photo synthetic organisms. PSITWOC is expressed by the following equation (1):

2H20→ 02 + 4H+ + 4e (1) The catalytic core of this enzyme is a CaMn4Ox cluster, which is conserved across all known species of oxygenic photo trophs. Many attempts to develop a biological water oxidation catalysts with a modest overpotential (E0 = 1.23 V at pH = 0) have focused on Ru and Ir based compounds, which are inherently resource limited.

The chemical principles that govern the PSITWOC, specifically the Mn-0 bonding, have been studied through catalytic water oxidation capabilities of structurally related synthetic molecular manganese-oxo complexes. Patent Application Publication No.

US2010/0143811 discloses Mn4 04L6, where Mn404 is a manganese-oxo cubane core and L is a ligand stabilizing core such as (C6H5)2P02 or MeO(C6H5)2P02, as demonstrating catalytic activity. Recently, spinel-type Co304 nanoparticles have demonstrated catalytic capabilities. However, water oxidation activity by spinels has exhibited a strong dependence on crystallite size and surface area, frequently necessitating high overpotentials and alkaline conditions to accelerate the rate.

WO 2011/163626 generally discloses a catalyst that includes a catalytic group comprising A1_xB2-yB' 04 spinels having a cubical M404 core, wherein A is Li or Na, B and B' are independently any transition metal or a main group metal, M is B, B' or both and x is a number between 0 to 1. In some embodiments, B and B' are selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and combinations of these. In other

embodiments, B and B' are selected from the group consisting of Al, Ga, In, Sn, Sb, and Bi. In this publication, LiMn204 is converted to λ-Μη02 with the catalyst being free of lithium. However no evidence is given for the absence of lithium and the characterization aspects of the spinel materials are not fully disclosed.

The inventors have identified a need in the art for efficient water-oxidizing catalysts made from low-cost earth-abundant materials, particularly those used in connection with PECs. There also remains a need for a greater understanding of what limits the rate of turnover of reactants to products at photoelectrodes with complex three-dimensional architecture. Applicants have recognized a need for TMOs exhibiting high activities, simpler synthetic routes, and compatibility with PEC device fabrication. The present invention addresses these needs, among others.

SUMMARY OF THE INVENTION

Provided herein is a catalyst for the photo-electrolysis of water molecules, the catalyst including a catalytic group comprising LixMn204 spinels having a cubical Mn404 core, wherein 0<x<0.2.

According to another aspect of the invention the catalyst comprises: (a) a catalytic group comprising LixMn204 spinels having a cubical M404 core, wherein 0<x<0.2; (b) a conductive support substrate supporting a plurality of the catalytic groups and capable of incorporating water molecules; and (c) wherein at least some of the catalytic groups supported by the support substrate are able to catalytically interact with water molecules incorporated into the support substrate.

Also provided herein is an anode for the electrolysis of water comprising: (a) an electrode substrate; (b) a catalytic group comprising LixMn204 spinels having a cubical M404 core, wherein 0<x<0.2; and (c) a conductive support substrate supporting a plurality of the catalytic groups and capable of incorporating water molecules. At least some of the catalytic groups supported by the substrate are able to catalytically interact with water molecules incorporated into the support substrate. In certain embodiments the anode is a photo-anode, wherein the catalyst is for the photo-electrolysis of water.

Also provided herein is an electrochemical cell for the electrolysis of water comprising: ( 1) a chamber capable of containing an aqueous electrolyte; (2) a cathode in contact with the aqueous electrolyte when the chamber contains the aqueous electrolyte; and (3) an anode. The anode includes: (a) an electrode substrate; and (b) a catalyst for the electrolysis of water molecules. The catalyst includes: (i) a catalytic group comprising LixMn204 spinels having a cubical M404 core, wherein 0<x<0.2; and (ii) a conductive support substrate supporting a plurality of the catalytic groups and capable of incorporated water molecules. At least some of the catalytic groups supported by the support substrate are able to catalytically interact with water molecules incorporated into the support substrate. The anode is capable of being electrically connected to the cathode in contact with the aqueous electrolyte when the chamber contains the aqueous electrolyte. In certain

embodiments, the electrochemical cell is a photo-electrochemical cell, wherein the anode is a photo-anode, and the catalyst is for the photo-electrolysis of water.

Also provided herein is a method for preparing an electrochemical cell for use in the electrolysis of water, the method including the steps of: (i) providing a conductive support substrate capable of incorporating water molecules; (ii) allowing catalytic groups comprising a catalytic group comprising LixMn204 spinels having a cubical M404 core, wherein 0<x<0.2, to self-assemble on the support substrate so that at least some of the catalytic groups are able to catalytically interact with the water molecules; (iii) coating the support substrate having the catalytic groups assembled thereon onto an electrode substrate to provide an anode; (iv) providing a cathode and forming an electrical connection between the anode and the cathode; and (v) providing an aqueous electrolyte between the anode and the cathode to provide a photo-electrochemical cell.

Also provided is a method for preparing a photo-electrochemical cell for the light driven catalysis of water oxidation. The method includes the steps of: (i) providing a semiconductor layer; (ii) coating a layer comprising a photo -electrochemical relay system onto the semi-conductor layer; (iii) coating a layer of a conductive support substrate capable of incorporating water molecules onto the semiconductor layer having the chemical relay system thereon; (iv) allowing catalytic groups comprising LixMn204 spinels having a cubical M4O4 core, wherein 0<x<0.2, to self- assemble on the support substrate so that at least some of the catalytic groups are able to catalytically interact with the water molecules thereby forming a photo-anode; (v) providing a cathode and forming an electrical connection between the photo-anode and the cathode; and (vi) providing an aqueous electrolyte between the photo-anode and the cathode to provide a photo- electrochemical cell.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective view of (A) the spinel type structure of L1B2O4, (B) an extended three-dimensional framework structure of L1B2O4, and (C) λ-Β02·

FIG. 2 is a scheme for the preparation of LixMn204.

FIGS. 3 and 4 are magnetic measurements for LixMn204.

FIG. 5 is an XRD of LixMn204.

DETAILED DESCRIPTION OF THE INVENTION

The following abbreviations are used in this application and have the meanings listed below:

Abbreviation Term

PCET proton coupled electron transfer

PEC Photoelectrochemic al

TMO transition metal oxide

PV Photovoltaic

PSII-WOC photosystem II water- oxidizing complex

FTO fluoride doped tin oxide ((F)Sn02)

ITO indium tin oxide

PEDOT poly(3,4-ethylenedioxythiophene) Heterogeneous Inorganic Spinel Catalysts. The present invention relates to new classes of TMO (transition metal oxide) spinel phases with nano-particle sizes or porous properties. More particularly, the present invention relates to TMO based heterogeneous catalysts capable of oxidizing water to oxygen, and in particular water oxidation from the cubical Mn404 clusters of LixMn204 spinels, wherein 0<x<0.2. The spinels are defined by their crystal structure which has a repeating cubic Mn404 core.

It has been found that, in one embodiment, the cubical Mn404 units become highly active water oxidation catalysts when absorbed within a suitable proton-conducting polymer membrane that is immersed in an aqueous medium, illuminated with light and placed in contact with a suitable electrolysis cell. Such hybrid homogeneous-heterogeneous catalysts are active as thin layers in single layer arrangements and are incorporable into multi-layer arrangements.

In one embodiment of the present invention, LiMn204 has a spinel type structure AB204 with Mn(III) and Mn(IV) ions occupying the octahedral B sites and the Li ions in the tetrahedral A sites as seen in FIG. 1. The present invention further relates to the exchange properties of Li+ from LiMn204. Li+ can be partially removed from the LiMn204 spinel framework yielding a polymorph of LixMn204, wherein 0<x<0.2. These materials retain the spinel framework but with empty or partially occupied A sites, resulting in a uniquely open structure. LixMn204 is not found naturally and differs from the common polymorph βΜη02 (rutile structure, all O atoms tricoordinate). The B cations in LixMn02 and LiMn204 are organized as cubical Mn404 subunits that are linked to the other B site cations via oxo bridges (exclusively dicoordinate in λΜη02). The cubical Mn404 units in LixMn204 are topologically similar to the Mn404 core found in the molecular "cubane" catalysts used for water oxidation and indirectly the CaMn404 core of the PSITWOC structure, as highlighted in FIG. 1. The partial delithiation of LiMn204 to LixMn204 where 0<x<0.2 by multiple methods creates an active water oxidation catalyst that is unusually robust and inexpensive compared to noble metals.

Support Substrates. According to another aspect of the invention the catalyst comprises a catalytic group and a conductive support substrate supporting a plurality of the catalytic groups. The support substrate is capable of incorporating molecules of compounds selected from a group consisting of water, aliphatic or aromatic hydrocarbons, ammonia, and at least some of the catalytic groups supported by the support substrate are able to

catalytically interact with water molecules incorporated into the support substrate. In certain embodiments, the support substrate is a nano-porous substrate.

"Catalytic groups" include catalytic spinels that are able to catalyze the oxidation of water molecules, aliphatic or aromatic hydrocarbon molecules, or ammonia molecules by interacting with the water, aliphatic or aromatic hydrocarbons or ammonia molecules. By "catalytically interact" it is meant that the oxidation of at least some of the water molecules, aliphatic or aromatic hydrocarbon molecules, or ammonia molecules that contact the catalytic groups is catalyzed by the catalytic groups. In certain embodiments, the catalytic groups include a conductive binder within which the catalytic spinels are dispersed. The conductive binder enables the application of a cohesive catalyst coating on the support substrate.

Suitable binders include carbon paste or other nanoporous conducting material.

The support substrate is conductive to electrons so that when an electric potential difference is present across separate points on the support substrate, the mobile charges within the support substrate are forced to move, and an electric current is generated between those points. In one embodiment, the support substrate is rendered conductive by applying a thin layer of the support substrate onto a conductive material. Suitable conductive materials include glassy carbon, carbon nanotubes and nanospheres, fluoride doped tin oxide (FTO or ((F)Sn02)) coated glass and indium tin oxide (ΠΌ) coated glass, and multilayer structures having nano- structured semiconductor films coated onto the conductive substrates. Other means of causing the support substrate to be conductive are within the scope of the invention. For example, in one embodiment, the support substrate contacts a sensitized semiconductor.

Preferably, the support substrate has hydrophobic regions and hydrophilic regions. While not wishing to be limited by theory, it is thought that at least some of the catalytic groups can be supported in the hydrophobic regions of the support substrate and once supported are able to catalytically interact with water molecules in the hydrophilic regions. Effectively, the support substrate is thought to act as an interface between water molecules and the hydrophobic catalytic groups which are otherwise insoluble in aqueous solution. In one embodiment, the hydrophobic regions are formed by a hydrophobic polymeric backbone and the hydrophilic regions are regions of ionizable functional groups, preferably on the polymer backbone that can serve as sites for proton conductance. Preferably the ionizable functional groups are sulfonic acid groups (-SO3H) that lose a proton to form negatively charged sulfonate groups. Alternatively, the ionizable functional groups can form positively charged functional groups if preferred.

The support substrate can be, for example, selected from polysulfones, polysulfonates, and polyphosphonates. In certain preferred embodiments, the supports substrate comprises a sulfonated fluoro-polymer (sold under the trade mark of Nafion(R)). The hydrophobic CF2CF(CF3)0- polymer backbone of Nafion(R) forms a hydrophobic solid that is penetrated by aqueous channels lined with the hydrophilic ionizable sulfonic acid groups.

Other support substrates that could be used include, for example, per-fluorinated sulfonic acid polymer cation-exchange membranes such as F- 14100, F-930 and F-950, the GEFC perfluorinated proton exchange membranes, polysulfone ionomers, nanostructured films formed by metal oxide nanoparticles suitably decorated with organic acids including perfluorinated sulfonic acids, nanostructured films formed by the hydrolysis of alkoxysilanes suitably decorated with organic acids including perfluorinated sulfonic acids. Also within the scope are heterogeneous-homogeneous colloidal systems, two-phase mixtures (stabilized and unstabilized with surfactant), conducting polymers (e.g., poly(3,4-ethylenedioxythiophene) (PEDOT)), surface-modified silica and titania.

Any means of contacting the catalyst with water is within the scope of the invention. In one embodiment, the catalyst is immersed in a solution containing water molecules. The solution can be an aqueous solution containing an electrolyte. In another embodiment, the aqueous solution can be a solution from which the water is preferentially removed (i.e., solid liquid separation). For example, where the aqueous solution is salt water or sea water the water could be removed leaving the salt behind (i.e., desalination). In one embodiment about 0.1 M electrolyte is sufficient.

Synthesis. Spinels of the present invention, such as LixMn204, wherein 0<x<0.2, are synthesized as nanoscopic particles using low temperature methods and mild treatments, for comparison to traditional high temperature aerobic oxidation. The synthesis of nano-porous spinels is also carried out by partial removal of the A site cation, which leaves channels capable of water/proton transport and enables new soft modes of lattice displacements that flex the [B404] core. For example, Li+ is removed from the spinel LiMn204 by treatment with concentrated acid (pH < 2.1). This results in conversion to LixMn204 where 0<x<0.2. The resulting material, designated LixMn204, preserves the structural framework of the original spinel, but with most of the Li+ removed from the tetrahedral sites while the octahedral sites become oxidized completely to Mn+4. This transformation produces the

[Mn404] 8+ redox state at the B site cluster. The resulting LixMn204 may be used as a water oxidation catalyst.

In preferred embodiments of the preparation of LiMn204 and LixMn204, LiMn204 can be synthesized from Mn(OAc)2 and L1NO3 wherein Mn(OAc)2 is combined with L1NO3 at a temperature of about 350°C in the presence of urea and citrate in acidic solution, which enables the formation of a nanoscopic material during degassing of H20, NH3, and C02. The reaction of Mn(OAc)2 and LiN03 in acidic solution generates the LiMn204 spinel crystal phase. In order to ensure nanoparticle size distribution, crystal growth is controlled using urea and citrate in solution forming a porous polymeric network. Partial removal of the Li atoms is performed by concentrated HN03 solution treatment. Partial removal of lithium atoms can be achieved by controlling the reactions conditions such as solvent system, concentration of the acid used, varying the acid used, using a different treatment agent (e.g., bromine or iodine), or controlling the temperature in the bromination reaction, although controlling the temperature is expected to have more of a kinetic effect rather than controlling the amount of lithium removed during the reaction. The acid treatment dissolves the Li20 and MnO products of the reaction and yields a solid that analyzes gravimetrically as

LixMn204 with partial removal of Li+ with 0<x<0.2. The complete delithiation reaction described in the literature is expressed in the following equation:

2LiMn204+HN03→ Li20 + 3λΜη02 + MnO

However, in contrast to the above complete delithiation reaction reported in the literature, the present invention controls the reaction to provide a partially delithiated product, LixMn204 where 0<x<0.2.

Catalytic Oxidation of Water and Hydrocarbons. One aspect of the present invention relates to methods of catalysis of oxidation reactions, where the catalysts comprise nanoparticulate spinels and their delithiated or partially delithiated analogs, and wherein the oxidation uses energy in the form of light, electricity or heat. The spinels are used either as free unmodified materials or supported on another material. Examples of co-supports include electrically conducting, semi-conducting and non-conducting supports such as metals, metal oxides, semiconductors, conducting and non-conducting organic polymers, and so forth.

In another aspect, the present invention is related to partial or selective oxidation of water, hydrocarbons, or other sources of hydrogen fuel with oxygen gas as the oxidant, heat as the energy source and the spinel material as the catalyst. In certain preferred

embodiments, the thermal conversion of the hydrogen fuel occurs via a flow reactor incorporating a catalyst of the present invention. Those of ordinary skill in the art would understand how a catalyst of the present invention can be incorporated in a flow reactor. In one embodiment, the hydrogen fuel comprises hydrocarbons. In particularly preferred embodiments methane is converted to methanol. In another preferred embodiment, propane is converted to propanol. The spinel catalyzes the selective transfer of an oxygen atom to the hydrocarbon, while air provides the source of oxygen. In this regard, gaseous reactants may be oxidized at temperatures of up to about 500°C.

When dissolved in organic solvents, the spinels of the present invention can act as a powerful catalysts for the oxidation of a range of organic substrates. Accordingly, in certain embodiments, the spinels of the present invention are capable of catalyzing the following reactions:

• Water oxidation anode reaction: 2H20→ 02+4H++4e~

• Water reduction cathode reaction: 4H++4e"→ 2H2

• Water splitting (anode and cathode reactions): 2H20→ 02 + 2H2

• Partial deoxygenation of carbon dioxide: 2C02→ 02 + 2CO

• Reduction of carbon dioxide to formic acid: H20 + C02→ HCOOH + [l/2]02

• Reduction of carbon dioxide to methanol: 2H20 + C02→ CH3OH + [3/2]02

• Reduction of carbon dioxide to methane: 2H20 + C02→ CH4 + 202

• Partial oxidation of methane to methanol: CH4 + [1/2] 02→ CH OH

(CH2)x + [l/2] 02→(CHOH)x

• Partial oxidation of CH3OH to formaldehyde: CH3OH+[ 1/2] 02→ HCHO+H20

• Formation of hydrogen peroxide for use in further oxidations: H2 +02→ H202

• Oxidation of paraxylene by-partial or complete oxidation of methyl groups leading to p-toluic acid (or) terephthalic acid: C6H4(CH3)2→ C6H4(COOH)2 (or) C6H4(CH3) (COOH)

• Oxidation of ammonia to nitric acid: NH + 202→ HN03+H20

• Oxidative coupling of-alkanes and arenes: 2CH4 +02→ C2H4+2H20

2CH4 +l/202→ C2H6+H20 ArH +l/202→ Ar-Ar +H20 Phenols→ Bis Phenols • Electrochemical decomposition of methanol: CH3OH→ 2H2+CO

• Gas separation reactions

• Air/water purification reactions

• Carbohydrate oxidation

• Substituted furans to furan mono and di-carboxylic acids

Electrolysis and Photoelectrochemical Cells. In yet another aspect, the present invention is related to methods of use of the spinel catalysts of the present invention in anodes and photoanodes for electrolysis and photoelectrochemical cells. The spinels are permanently bonded to conductive metal surfaces and electrically biased at electrical potentials that support the electro-oxidation of the aforementioned sources of hydrogenic fuels. The spinel catalysts are used in both polymer membrane type electrolyzers and solid oxide fuel cells. Such catalytic oxidation is carried out by electricity from any source, preferably generated from solar or wind sources. In embodiments comprising solar cells, the spinel-coated anodes are driven by electricity generated from a photovoltaic or

semiconductor source in an integrated photoelectrochemical cell. The electrolysis and photoelectrochemical (PEC) applications are carried out in electrolytes having a pH ranging from alkaline to acidic.

The electrolytes carry protons between the anode and cathode. In certain

embodiments in which water is oxidized, the electrolyte is separated from the water. To this end, the support substrates (described above) comprises, on a first side, flow fields allowing water to flow to the anode (as well as removal of 02 gas), and on a second side, and water flow fields allowing to the cathode to provide cooling and removal of H2 gas. Accordingly, protons travel through the support substrate. In certain other embodiments, the electrolyte is not separated from the water, thereby enabling protons to travel through the water phase.

In one embodiment, the catalyst of the invention is formed on an electrode substrate to provide a photo-anode. The electrode substrate can be any suitable substrate, for example, glass. As mentioned above, the glass could be coated with, for example, indium tin oxide to render the support substrate conductive. In some embodiments there are multiple layers between the electrode substrate and the catalyst. These layers can replace the conductive material (e.g., indium tin oxide) applied to the electrode substrate. The layers can include a semiconductor and a chemical relay system material. The incorporation of a photo-electrochemical relay system into the photo-anode improves the overall efficiency of the catalysis of water oxidation. The chemical relay system may be a photo-electrochemical relay such as a dye that absorbs light and facilitates electron transfer. A thin layer of the catalyst can be in contact with the chemical relay system. The chemical relay includes polymers possessing cation exchange groups (e.g., sulfonates) that facilitate proton exchange with water and photo-active dyes such as ruthenium N-donor dyes that absorb in regions of the electromagnetic spectrum that are not absorbed by the catalytic clusters. The ruthenium N-donor dyes absorb visible light and then electrochemically oxidize the catalytic groups. This enhances the efficiency with which light in the visible region is converted into chemical energy overall, since the catalytic groups typically do not absorb visible light strongly.

The photo-anode of the invention can be used in a photo-electrochemical cell for the electrolysis of water. The cell can be in the form of a chamber capable of containing an aqueous electrolyte. The chamber can be bounded by walls so as to contain the aqueous electrolyte within it, or open to allow the aqueous electrolyte to flow through it. The photo- electrochemical cell includes the photo-anode in combination with a cathode, both of which are able to contact the aqueous electrolyte when it is present. When the cell is used, the photo-anode is electrically connected to the said cathode in order to complete the electrical circuit.

Examples of suitable cathode materials include supported platinum nanoparticles, supported nickel nanoparticles and supported nickel alloys. The supports include electrically conductive substrates such as carbon paste, carbon nanoparticles, or intrinsically conductive polymers including, for example, polyanilines, polythiophenes (PEDOT), and

polyacrylamides.

The photo-electrochemical cell can be used in a method of generating hydrogen and oxygen. Effectively, the cell is exposed to light radiation in order to activate the catalytic groups. When the cell is absent a photo-electrochemical relay system, an electric potential is applied to encourage the regeneration of the catalytic groups once they have undertaken one catalytic cycle. The cell is capable of producing hydrogen and oxygen gases which can be collected or immediately used in a further application as desired. In certain embodiments collection is capable wherein the cell is operated at pressures of greater than or equal to about 1 atm. within a pressure secure reactor that is capable of pressures in the range of about 1 to about 400atm. Alternatively, the photo-electrochemical cell can be used in a method of generating electricity (i.e., an electric current). Where a chemical relay system is present in the photo- anode, to generate electricity all that is required is exposure of the cell to light radiation such as sunlight.

Optionally, the hydrogen and oxygen generated by the photo-electrochemical cell described above can be passed into a fuel cell for the generation of electrical energy.

In some embodiments, a plurality of photo-electrochemical cells are arranged in a series.

The invention also provides a method for preparing an electrochemical cell for use in the electrolysis of water, the method including the steps of: (i) providing a conductive support substrate capable of incorporating water molecules; (ii) allowing catalytic groups comprising LixMn204 spinels having a cubical Mn404 core, wherein 0<x<0.2, to self- assemble on the support substrate so that at least some of the catalytic groups are able to catalytically interact with the water molecules; (iii) coating the support substrate having the catalytic groups assembled thereon onto an electrode substrate to provide an anode; (iv) providing a cathode and forming an electrical connection between the anode and the cathode; and (v) providing an aqueous electrolyte between the anode and the cathode to provide an electrochemical cell. In certain embodiments, the electrochemical cell is a photo- electrochemical cell for use in the photo-electrolysis of water, wherein the anode is a photo- anode.

According to yet a further aspect of the invention there is provided a method for preparing a photo-electrochemical cell for the catalysis of water, the method including the steps of: (i) providing a semiconductor layer; (ii) coating a layer comprising a photo- electrochemical relay system onto the semi-conductor layer; (iii) coating a layer of a conductive support substrate capable of incorporating water molecules onto the

semiconductor layer having the chemical relay system thereon; (iv) allowing catalytic groups comprising LixMn204 spinels having a cubical Mn404 core, wherein 0<x<0.2, to self- assemble on the support substrate so that at least some of the catalytic groups are able to catalytically interact with the water molecules thereby forming a photo-anode; (v) providing a cathode and forming an electrical connection between the photo- anode and the cathode; and (vi) providing an aqueous electrolyte between the photo-anode and the cathode to provide a photo-electrochemical cell.

It also should be understood that the invention relates, but is not limited, to catalysts of the compositions of L12C02O4 and LiMn204. These catalysts are believed to operate or function similarly to the LixMn204 where 0<x<0.2 catalyst compositions described above. Thus the Li2Co204 and LiMn204 catalysts can be formed in analogous reactions to those for forming LixMn204 where 0<x<0.2 and are expected to have applications similar to those of LixMn204 with 0<x<0.2.

Example 1

Preparation of LiMn204

Figure 2 provides a route of synthesis scheme for preparation of LiMn204 and conversion of the LiMn204 into LixMn204 where 0<x<0.2. Initially an aqueous solution of L1NO3 and Mn(OAc)2 4H20 (40 mL) is prepared and separately an aqueous solution of citric acid and urea (200 mL) is prepared. These solutions are then mixed.

Next, 30 ml of concentrated HNO3 is added to the above mixture of 40 ml of aqueous solution of L1NO3 and Μη(ΟΑΰ)2.4Η20 and 200 ml of an aqueous solution of citric acid and urea. The resulting solution is evaporated at 80°C with continuous stirring to provide a yellowish gel. The yellowish gel is dried to a porous black resin by heating at 170°C for 12 hours. The resulting porous black resin is calcined at 350°C for 12 hours to obtain nanocrystalline LiMn204 as a black powder.

Example 2

Preparation of LixMn204 (Partial delithiation step)

Referring still to Figure 2, the nanocrystalline LiMn204 of Example 1 is made into an aqueous solution to which 3M HNO3 is added drop wise until the pH of the solution is approximately 2. It was determined that if the pH was too much below 2, e.g., below pH = 1.5, the structure of the catalyst structure disintegrates. It also was determined that if the pH was too much above 2, e.g., above 2.5, the formation of the catalyst required significantly increased temperature or time to partially delithiate the LiMn204. The resulting precipitate is repeatedly washed with distilled water and centrifuged until the washing has a pH of approximately 7, demonstrating that no acid remains with the precipitate. The washed precipitate is dried overnight at 80°C to obtain LixMn204 (where 0<x<0.2) as a dark brown powder. Example 3

ICP analysis of LiMn204 and LixMn204

The compositions of the catalysts, LiMn204 and LixMn204 (obtained after the acid treatment of LiMn204 at pH of approximately 2) were analyzed by ICP (Inductively

Coupled Plasma) analysis. A weighed amount of the sample was digested in warm Aqua Regia and then diluted to 100 mL with deionized water. The resulting solution was used for making a standard solution for ICP analysis.

The ICP data is provided in Table 1.

Table 1

Figure imgf000016_0001

ICP analysis of the LixMn204 sample shows its composition to be Lio.2oMn204 with a lithium content of approximately 10%. The data demonstrates that contrary to what is understood from the literature, one can obtain partial removal of Li ions upon acid treatment of LiMn204.with the resulting catalyst having lithium present at 10% (i.e., Lio.2oMn204).

Example 4

Magnetic measurements

For the LixMn204 sample, the Curie-Weiss curve was plotted using the equation, 1/5C

= (T-0)/C = (1/C)T - Θ/C; where m = Molar magnetic susceptibility. The Curie-Weiss law is obeyed above -125 K (Fig. 3), with best fit parameters C =1.92 emu-K/mol and Oc = -158.5 (K) (Fg. 4). The Curie constant is slightly larger than the spin only value of 1.87 emu-K/mol for Mn4+ and is well consistent with the ICP results that show a lithium content of 10%, which suggests an equivalent concentration of Mn3+.

Example 5

XRD analysis of LixMn204

Figure 5 provides an X-ray diffractogram for LixMn204. The diffractogram shows that the spinel pattern is preserved in LixMn204 after delithiation.

While several particular forms of the invention have been illustrated and described, it will be apparent that various modifications and combinations of the invention detailed in the text and drawings can be made without departing from the spirit and scope of the invention. For example, the LixMn204 spinels (0<x<0.2) catalyst can be used in a variety of applications including industrial air and water purification, oxidation of aromatics to produce chemical products, oxidation of paraffins and olefins to produce oxygenated compounds,

electrochemical oxidation, photochemical oxidation, gas separation, biomass conversions, fuel cells, and batteries. Similarly references to methods of construction, specific

dimensions, shapes, utilities or applications are also not intended to be limiting in any manner and other materials and dimensions could be substituted and remain within the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.

Claims

What is claimed is:
1. A catalyst for hydrocarbon oxidation, said catalyst comprising a catalytic group comprising LixMn204 spinels having a cubical Mn404 core, wherein 0<x<0.2.
2. The catalyst according to claim 1, further comprising conductive support substrate supporting a plurality of catalytic groups and capable of incorporating hydrocarbon molecules, wherein at least some of the catalytic groups supported by the support substrate are able to catalytically interact with hydrocarbon molecules incorporated into the support substrate.
3. The catalyst according to claim 2, wherein the support substrate has hydrophobic regions supporting at least some of the catalytic groups and hydrocarbon molecules; and
at least some of the catalytic groups supported in the hydrophobic regions are able to catalytically interact with hydrocarbon molecules.
4. The catalyst according to claim 3, wherein the hydrophobic regions are formed by hydrophobic polymer backbone.
5. The catalyst according to claim 2, further including a chemical relay system capable of electrochemically oxidizing the catalytic groups thereby assisting regeneration of catalytic groups.
6. The catalyst according to claim 5, wherein the chemical relay is a photo- electrochemical relay system in the form of a photo-active dye.
7. The catalyst according to claim 6, wherein the photoactive dye is a ruthenium polypyridyl dye.
8. A method for the oxidation of hydrocarbons comprising contacting hydrocarbons and oxygen with a catalyst of claim 1.
9. The method of claim 8, wherein the hydrocarbon comprises aliphatic or aromatic hydrocarbons.
10. The method of claim 9, wherein the aliphatic hydrocarbon comprises at least one alkane selected from the group consisting of methane and ethane.
11. The method of claim 9, wherein the aromatic hydrocarbon comprises paraxylene.
12. A method for the oxidation of substituted furans to furan mono and di-carboxylic acids by contacting the furans and oxygen with a catalyst of claim 1.
13. A method for the oxidation of alcohol comprising contacting alcohol and oxygen with a catalyst of claim 1.
14. The method of claim 13, wherein the alcohol comprises methanol.
15. The method of claim 14 comprising further oxidation of methanol to formaldehyde.
16. A method for purifying water of the organic hydrocarbons comprising contacting oxygen and impure water containing hydrocarbon impurities with the catalyst of claim 1.
17. The method of claim 16, wherein the purification of water includes removal of hydrocarbon impurities from water by photo-oxidation of hydrocarbons.
18. A photo-anode for the electrolysis of water comprising:
an electrode substrate; and
a catalyst for the photoelectrolysis of water molecules, the catalyst including:
a catalytic group comprising LixMn204 spinels having a cubical Μη404 core, wherein 0<x<0.2; and
a conductive support substrate supporting a plurality of catalytic groups and capable of incorporating water molecules,
wherein at least some of the catalytic groups supported by the support substrate are able to catalytically interact with water molecules incorporated into the support substrate.
19. The photo anode of claim 18, wherein there are multiple layers between the electrode substrate and the catalyst, the layers comprising:
a semiconductor; and
a photo-electrochemical relay system containing the semi-conductor and capable of electrochemically oxidizing the catalytic groups thereby assisting the regeneration of the catalytic groups.
20. A method for making hydrogen peroxide comprising contacting molecular hydrogen and molecular oxygen produced from a photo-electrochemical cell with spinel catalyst of claim 1, wherein the photo-electrochemical cell comprises:
a chamber capable of containing an aqueous electrolyte;
a cathode in contact with the aqueous electrolyte when the chamber contains aqueous electrolyte; and
a photo-anode comprising:
an electrode substrate; and
a catalyst for photo-electrolysis of water molecules, the catalyst comprising:
a catalytic group comprising LixMn204 spinels having a cubical M404 core, wherein 0<x<0.2;
and a conductive support substrate supporting a plurality of the catalytic groups and capable of incorporating water molecules; wherein at least some of the catalytic groups supported by the support substrate are able to catalytically interact with water incorporated into the support substrate;
said photo-anode capable of being electrically connected to said anode in contact with aqueous electrolyte when the chamber contains the aqueous electrolyte.
21. A method for oxidation of ammonia to nitric acid comprising contacting molecular ammonia and oxygen with the spinel catalyst of claim 1.
22. A method for partial deoxygenation of carbon dioxide comprising contacting molecular carbon dioxide with the spinel catalyst of claim 1.
23. A method for reducing carbon dioxide to formic acid comprising contacting molecular carbon dioxide and water with the spinel catalyst of claim 1.
24. A method for reducing carbon dioxide to methanol comprising contacting molecular carbon dioxide and water with the spinel catalyst of claim 1.
25. A method for forming ethene by oxidative coupling of methane, the method comprising contacting methane and oxygen with the spinel catalyst of claim 1.
26. A process for making a catalyst comprising LixMn204 having a cubical Mn404 core, wherein 0<x<0.2, the process comprising: providing an aqueous solution nanocrystalline LiMn204 ; adding concentrated HN03 drop wise to the aqueous solution of nanocrystalline LiMn204 until the pH of the solution is approximately 2 to form a precipitate; washing the precipitate with distilled water and centrifuging to remove the water one or more times until the removed water has a pH of approximately 7; and drying the washed precipitate overnight to obtain LixMn204 where 0<x<0.2.
27. The process of claim 26, wherein the concentrated HNO3 is 3M.
28. The process of claim 26, wherein the temperature for drying the washed precipitate overnight is approximately 80°C.
29. The process of claim 26, wherein x = about 0.1.
PCT/US2013/045500 2012-06-12 2013-06-12 SPINEL CATALYST OF LixMn2O4 (0<x<0.2) FOR OXIDATION REACTIONS WO2013188589A2 (en)

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