rexresearch.com
Martin DEMUTH, et al.
Titanium Disilicide Photocatalyst
Titanium Disilicide Photocatalyst
http://www.keelynet.com/indexsep907.htm
09/26/07
http://phys.org/news109941196.html
Sep 25, 2007
Splitting Water with Sunlight
Hydrogen is one of the most important fuels of the future, and the sun will be one of our most important sources of energy. Why not combine the two to produce hydrogen directly from solar energy without any detours involving electrical current? Why not use a process similar to the photosynthesis used by plants to convert sunlight directly into chemical energy?
Researchers from the German Max Planck Institute have now developed a catalyst that may do just that. As they report in the journal Angewandte Chemie, titanium disilicide splits water into hydrogen and oxygen. And the semiconductor doesn’t just act as a photocatalyst, it also stores the gases produced, which allows an elegant separation of hydrogen and oxygen.
Researchers from the German Max Planck Institute have now developed a catalyst that may do just that. As they report in the journal Angewandte Chemie, titanium disilicide splits water into hydrogen and oxygen. And the semiconductor doesn’t just act as a photocatalyst, it also stores the gases produced, which allows an elegant separation of hydrogen and oxygen.
“The generation of hydrogen and oxygen from water by means of semiconductors is an important contribution to the use of solar energy,” explains Martin Demuth (of the Max Planck Institute for Bioinorganic Chemistry in Mülheim an der Ruhr). “Semiconductors suitable for use as photocatalysts have been difficult to obtain, have unfavorable light-absorption characteristics, or decompose during the reaction.”
Demuth and his team have now proposed a class of semiconductors that have not been used for this purpose before: Silicides. For a semiconductor, titanium disilicide (TiSi2) has very unusual optoelectronic properties that are ideal for use in solar technology. In addition, this material absorbs light over a wide range of the solar spectrum, is easily obtained, and is inexpensive
At the start of the reaction, a slight formation of oxide on the titanium disilicide results in the formation of the requisite catalytically active centers. “Our catalyst splits water with a higher efficiency than most of the other semiconductor systems that also operate using visible light,” says Demuth.
One aspect of this system that is particularly interesting is the simultaneous reversible storage of hydrogen. The storage capacity of titanium disilicide is smaller than the usual storage materials, but it is technically simpler. Most importantly, significantly lower temperatures are sufficient to release the stored hydrogen.
The oxygen is stored as well, but is released under different conditions than the hydrogen. It requires temperatures over 100°C and darkness. “This gives us an elegant method for the easy and clean separation of the gases,” explains Demuth. He and his German, American, and Norwegian partners have founded a company in Lörrach, Germany, for the further development and marketing of the proprietary processes.
Angewandte Chemie International Edition 2007, 46, No. 41, 7770–7774,
doi: 10.1002/anie.200701626
http://onlinelibrary.wiley.com/doi/10.1002/anie.200701626/abstract
A Titanium Disilicide Derived
Semiconducting Catalyst for Water Splitting under Solar
Radiation—Reversible Storage of Oxygen and Hydrogen
Martin Demuth et al.
Martin Demuth et al.
Divide and separate: Photocatalytic splitting of water into hydrogen and oxygen is achieved with a catalyst which is formed on the surface of titanium dicilicide (see picture). The two product gases are reversibly physisorbed by the catalyst. Desorption of hydrogen occurs at ambient temperature, but oxygen is entirely stored up to 100?°C in light and can be released upon heating at this temperature in the dark, which allows convenient separation of the gases.
Metallic or non-metallic silicides for
photo-electrochemical decomposition of water...
DE102008051670
US2011303548
DE102008051670
US2011303548
Electricity is generated simultaneously or separately from water decomposition. In the case where only electricity is produced, the silicide is illuminated but there is no contact with water. Otherwise contact is made with water, under illumination. Metallic and non-metallic silicides are used. The silicides have the molecular formula RSi. R is an organic, organometallic, and/or inorganic radical (or a mixture). Si represents silicon. In listed formulae, R is present as one or more atoms of Ti, Ni, Fe, Th, B (silicon tetraboride), Co, Pt, Mn, CSi/poly-CSi, Ir, N, Zi, Ta, V and Cr. The silicide contains at least one silicon atom with increased electron density. The silicide acts as a catalyst and receiver of artificial and solar light. The light participates in a photo-electrical process e.g. water decomposition, and/or a photovoltaic process generating electricity. The silicides serve as electrodes in a conductive-electrolyte system. They are connected with one or more opposite electrodes. Current is generated, with or without water decomposition. The opposite electrode is conductive. It is a: metal, transition metal, metal oxide, transition metal oxide, non-metallic structure and/or mixed structure. The two types of operation, photoelectric and photovoltaic, proceed separately or in combination. The silicide is in crystalline form and the artificial light and/or sunlight is concentrated and/or diffuse. With silicide electrodes connected to conductive opposite electrodes, each with suitable doping, electrolyte can be dispensed with and the electrodes are in direct connection. The light sources and/or additional thermal energy sources emit energy in the waveband 200-15000 nm. Silicide light absorption is increased by one or more dyes. These are coupled, complexed, coated or bonded to the silicide. The dyes are perylenes and their analogs. High temperatures and light concentrations enhance reaction. Energy introduced for this purpose comes from e.g. electrical-, microwave- and/or geothermal heating. Other oxide semiconductors are listed as agents actively assisting the process. The silicides are immobilized in e.g. listed polymers or glasses. These are conductive. Numerous possible electrode dopants are listed.
[0001] The present invention relates to a process for the photoelectrochemical production of hydrogen and oxygen from water and of electricity, this in the presence of silicides (silicides) in general and especially in presence of metal silicides and nonmetallic silicides, such as borosilicides, carbon-containing silicides and nitrogen-containing silicides, i.e., compounds containing silicon and having the composition RSix. R represents in this connection an organic, metallic, organometallic, non-metallic or inorganic residue and Si stands for the element silicon (silicon) with an increasing number of atoms X>0. In the following text, these classes of substances are named silicides. The silicide subunits of these substances are characterized by enhanced electron density. The silicides in the aforementioned processes catalytically active wherein these processes can occur with or without light. However, when illuminated, using artificial light and sunlight, an increase of gas evolution is observed. Higher reaction temperatures are often accelerating the reaction. Silicides are mostly semiconductor materials.
[0002] The silicides are used as electrode materials (alternatively as anode or cathode), coupled with a counter electrode (e.g., a metal, a metal oxide or another conducting material and/or as a light-harvesting material as a part of a photoelectronic/electrical process. In this way, they can be employed as part of a photovoltaic system. The silicides serve to (a) produce hydrogen and oxygen from water in the presence of light and (b) to also simultaneously or separately produce electricity (electrical energy). In case of (b), liquid and/or non-liquid electrolytes are employed and for the purely photovoltaic application, with suitable doping of the electrodes, electrolytes can be eliminated entirely.
[0003] Furthermore, it was found that coupling or complexing of a dye, such as perylenes or analogs thereof, to the silicides is favorable for the light absorption of these substances as well as the charge separation and hence the reactivity of the silicides.
[0004] Further, it was found that reactions with silicides for splitting water into hydrogen and oxygen in presence of light for the purpose of generating hydrogen and oxygen but also for generating electricity can be performed also with immobilized silicides, i.e., with silicides which are located on or in polymeric materials and/or on or in glasses or glass-like materials as well as on or in electrically/electronically conducting materials. This applies also to the photovoltaic application of the silicides. The process of the invention delivers aside from hydrogen and oxygen also electricity, i.e., electric energy.
BACKGROUND
[0005] For carrying out photochemical reactions to produce hydrogen and oxygen from water with metallic catalysts, several processes have been disclosed. These are lanthanide-like photcatalysts such as, for example, NaTaO3:La, catalysts from rare earth metals, such as R2Ti2O7 (R=Y, rare earth metal), or TiO2-derived semiconductor systems, so-called tandem cells, wherein up to now no use of silicides has been disclosed for the application according to the title.
[0006] The processes for production of hydrogen and oxygen from water are based on reduction and/or oxidation of water with semiconductor materials and light. Such processes are also named water splitting processes. The hitherto disclosed processes use UV light. Although in some cases an appreciable development of hydrogen and oxygen has been found, the required illumination conditions are not suitable for solar-based application of the process. In addition, the production of the catalysts is laborious and requires the use of uneconomically high temperatures, starting from expensive basic materials of extremely high purity. Furthermore, for performing the aforementioned processes the use of very pure (triple distilled) water is required. In most cases there is no mention with respect to long-term applicability and the related stability of the catalysts.
[0007] The only promising approach so far employs silicide powder wherein the semiconductor itself splits the water into hydrogen and oxygen gas in presence of light. Oxygen has to be separately removed from the system. All these systems can be used only to split water and not for simultaneously or separately occurring production of electric power (photovoltaic principle).
[0008] Silicides have so far not been used for the configuration of photovoltaic systems. Only in individual cases (i.e., IrSi2 and beta-FeSi2) electrical and optical properties of films were measured.
DESCRIPTION OF THE INVENTION
[0009] Surprisingly, it has now been found that these disadvantages can be avoided when using silicides (silicides), i.e. metal silicides and non-metallic silicides such as borosilicides, carbon-containing silicides and nitrogen-containing silicides, i.e., compounds that contain silicon and have the composition RSix, when these processes are carried out according to a photoelectric principle, i.e., when the silicides are used as light absorbers and/or electrode. R can be an organic, metallic, organometallic or inorganic moiety and Si stands for the element silicon (silicon) with an increasing number of atoms X>0. In the following text these classes of substances are named silicides. The silicide subunits of these compounds are characterized by an enhanced electron density, i.e., a negative charge density, or they have a negative charge.
[0010] Non-metallic silicides such as borosilicides, carbon-containing silicides and nitrogen-containing silicides are also called silicon borides, silicon carbides and silicon nitrides, respectively.
[0011] Examples of silicides, metal silicides and non-metallic silicides, such as borosilicides, carbon-containing silicides and nitrogen-containing silicides are nickel silicide (Ni2Si), iron silicide (FeSi2, FeSi), thallium silicide (ThSi2), borosilicide, also called silicon tetraboride (B4Si), cobalt silicide (CoSi2), platinum silicide (PtSi, Pt2Si), manganese silicide (MnSi2), titanium carbon silicide (Ti3C2Si), carbon silicide/polycarbon silicide or also named silicon carbide/poly-silicon carbide (CSi/poly-CSi or SiC/poly-SiC), iridium silicide (IrSi2), zirconium silicide (ZrSi2), tantalum silicide (TaSi2), vanadium silicide (V2Si), chromium silicide (CrSi2), beryllium silicide (Be2Si), magnesium silicide (Mg2Si), calcium silicide (Ca2Si), strontium silicide (Sr2Si), barium silicide (Ba2Si), aluminum silicide (AISi), gallium silicide (GaSi), indium silicide (InSi), hafnium silicide (HfSi), rhenium silicide (ReSi), niobium silicide (NbSi), germanium silicide (GeSi), tin silicide (SnSi), lead silicide (PbSi), arsenic silicide (AsSi), antimony silicide (SbSi), bismuth silicide (BiSi), molybdenum silicide (MoSi), tungsten silicide (WSi), ruthenium silicide (RuSi), osmium silicide (OsSi), rhodium silicide (RhSi), palladium silicide (PdSi), copper silicide (CuSi), silver silicide (AgSi), gold silicide (AuSi), zinc silicide (ZnSi), cadmium silicide (CdSi), mercury silicide (HgSi), scandium silicide (ScSi), yttrium silicide (YSi), lanthanum silicide (LaSi), cerium silicide (CeSi), praseodymium silicide (PrSi), neodymium silicide (NdSi), samarium silicide (SmSi), europium silicide (EuSi), gadolinium silicide (GdSi), terbium silicide (TbSi), dysprosium silicide (DySi), erbium silicide (ErSi), thulium silicide (TmSi), ytterbium silicide (YbSi), lutetium silicide (LuSi), copper-phosphorus silicide (CuP3Si2, CuP3Si4), cobalt-phosphorus silicide (Co5P3Si2, CoP3Si3,) iron-phosphorus silicide (Fe2PSi, FeP4Si4, Fe20P9Si), nickel-phosphorus silicide (Ni2PSi, Ni3PeSi2 NiP4Si3, Ni5P3Si2), chromium-phosphorus silicide (Cr25P8Si7), molybdenum-phosphorus silicide (MoPSi), tungsten-phosphorus silicide (WPSi), titanium phosphorus silicide (TIPSi), cobalt-boron silicide (Co5BSi2), iron-boron silicide (Fe5B2Si), nickel-boron silicide (Ni4BSi2, Ni6BSi2, Ni9B2Si4), chromium-boron silicide (Cr5BSi3), molybdenum-boron silicide (Mo5B2Si), tungsten-boron silicide (W2BSi), titanium-boron silicide (TiBSi), chromium-arsenic silicide (CrAsSi), tantalum-arsenic silicide (TaSiAs), titanium-arsenic silicide (TiAsSi) or mixtures thereof. Notably, the elemental compositions given in parentheses (empirical formulas) are exemplary only and the ratios of the elements relative to each other are variable.
[0012] Silicides are abundant and easily accessible materials (mostly semiconductor materials) and so far have not been applied for the applications according to the title in photoelectrochemical as well as photovoltaic application.
[0013] Silicides are mainly semiconductor-like materials with high electron densities (negative charge densities) at the silicon, carbon, nitrogen and boron. The claimed processes for the production of hydrogen and/or oxygen by means of silicides proceed efficiently with light. The employed light energy can be in this connection artificial or solar-generated (emission range: 200-15,000 nm) and can be of diffuse or concentrated nature. The thermal energy which accompanies photonic energy of a light source or also thermal energy in general can accelerate the gas-producing claimed processes. In general terms, the application of higher temperatures as well as higher light concentrations can lead to a higher efficiency of the claimed processes. This applies not only to water splitting into hydrogen and oxygen but also to the production of electricity via photovoltaics, i.e., electric energy, which can proceed jointly or separately with water splitting.
[0014] The silicides are applied as electrode materials (alternatively as cathode or anode) in these photoelectrochemical and photovoltaic processes and are coupled to one or several counter electrode(s) (alternatively as anode(s) or cathode(s)) in an electrically conducting manner. The counter electrodes have to be of metallic or non-metallic but electrically conducting nature. In this arrangement an electrolyte is used between the electrodes. For the purely photovoltaic application of the silicides, with suitable p-/n-doping of the electrode materials the presence of an electrolyte is not necessary and the electrodes can be brought in direct contact.
[0015] When used as part of a photovoltaic device undoped or doped silicides (see doping examples below) are used that are brought into electrically conducting contact. In this connection, also other photoelectrically/photovoltaically active materials can be employed; this also external to the system as a light absorber.
[0016] The silicides absorb themselves usually enough solar or artificial radiation energy without requiring larger surface modifications in order to effect splitting of water for generating hydrogen and oxygen or for simultaneously or separately occurring generation of electricity (photovoltaics).
[0017] Surprisingly, it was found also that the quality and purity of the employed water is not important or can be neglected for carrying out the processes according to the title, i.e., relative to the oxidation and reduction of water and/or the simultaneously or separately occurring generation of electricity (photovoltaics).
[0018] The reactivity of the silicides with respect to water splitting claimed in this application for generation of hydrogen and oxygen and/or the simultaneously or separately occurring generation of electricity (photovoltaics) is primarily of a catalytic nature.
[0019] Further, it was found that the processes that are performed with the silicides for water splitting to produce hydrogen and oxygen and/or simultaneous or separate generation of electricity (photovoltaics) can also be conducted with silicides in immobilized form, i.e. the processes can also be performed with compounds that are bound/fixed on or in polymeric surfaces or materials as well as on or in glasses or glass-lice materials as well as on or in other solid surfaces or also on nano particles, and especially also when these materials are electrically conducting, i.e., are charge-conducting. Further, the silicides can be existing as a solid composite, preferentially crystalline, but can also be of amorphous constitution.
[0020] The processes described above can be conducted at higher or lower temperatures than room temperature and high as well as low light intensities.
[0021] In case of the production of electricity, it is also possible to eliminate the use of an aqueous/liquid electrolyte and instead a viscous, solid and/or gel-type electrolyte can be used. In case of suitable p-/n-doping of the electrodes, the electrolyte can also be eliminated and the electrodes can be brought into direct contact.
[0022] Further, it was found that coupling/complexing/attaching/bonding of a dye or an agglomeration of dyes on silicides is favourable for the light absorption and charge separation and, in turn, for the reactivity of these compounds (so-called dye-sensitized reactions with semiconductors). Especially suited for this purpose are dyes such as perylenes and analogs thereof. These dye-complexed silicides can also be used in thermally conducted reactions, even at higher temperatures, because perylene dyes are thermally stable.
[0023] The higher temperatures mentioned above can be achieved electrically, by geothermal energy, light energy, solar energy, heating devices, microwave discharge or any other source of thermal energy.
[0024] Further, it was found that the silicides can be used for the applications according to the title individually or in combination of two or more silicides. It is also possible to carry out the title processes with one or more silicide(s) together with application of additional semiconductor materials that are not of silicide-type structure such as e.g. ruthenium dioxide (RuO2), manganese dioxide (MnO2), tungsten trioxide (WO3) and generally other semiconductor material in order to assist/enhance the processes according to the title.
[0025] Furthermore, it was found that the processes according to the title can be enhanced that are with silicides doped/combined/alloyed with lithium, sodium, magnesium, potassium, calcium, aluminum, boron, carbon, nitrogen, silicon, titanium, vanadium, zirconium, yttrium, lanthanum, nickel, manganese, cobalt, gallium, germanium, phosphorus, cadmium, arsenic, technetium, alfa-SiH and the lanthanides up to 50 percent by weight (relative to the silicides and the silicide-type compounds). As dopant (p- and n-doping) generally the elements usually applied in photovoltaics are conceivable that have a chemical valance that is different from the surrounding material.
[0026] This new technology, based on the applications of silicides described above, can be employed in the following fields: New heating systems, fuel cell technology and/or production of electricity in general. There will be also applications in terrestrial and extra-terrestrial area for moving as well as static constructions and devices; this to replace, support or supplement these constructions and devices that up to now are driven by devices that utilize conventional fossil energy sources.
EXAMPLES
Example 1
[0027] Crystalline titanium disilicide (TiSi2) in solid form (“sputtering target”, 5 cm in diameter) is placed into a vessel (cylindrical shape and coolable and with free gas space, reaction temperature: 25-30° C.) and electrically connected with a counter electrode (e.g., IrO2 or RuO2).
[0028] A membrane, e.g. of nafion or Teflon that is permeable for hydrogen and oxygen is placed between the electrodes. An electrolyte is added to the water phase (e.g., acidified with sulphuric acid to pH 2) and irradiation is done along the longitudinal axis of the cylindrical apparatus (white light, 500-1,000 W or sunlight); the silicide is irradiated in doing so. The gas analyses are conducted by gas chromatography. The employed water used can be purified by ion exchange material; but normal water can be used also. The silicide in this arrangement serves as a cathode (hydrogen generation) and the transition metal oxides as anode (generation of oxygen). An appreciable electrical current can be measured in addition.
Example 2
[0029] Instead of the silicides mentioned in example 1, other silicides were employed such as cobalt silicide (CoSi2), platinum silicides (PtSi, Pt2Si), titanium carbosilicide (Ti3C2Si), carbosilicide/poly-carbosilicide (also named silicon carbide/polysilicon carbide (CSi/poly-CSi or SiC/poly-SiC)), zirconium silicide (ZrSi2), or chromium silicide (CrSi2). The reactions were carried out in analogy to example 1. In principle any silicide is suitable in this application.
Example 3
[0030] Same experimental set-up as in example 1, but titanium disilicide (TiSi2) serves as anode and platinum as counter electrode (cathode). Less oxygen and hydrogen are formed but a higher electrical current is measured that can be used for example for drives and other energy-dependent systems.
Example 4
[0031] If in the processes described in experiments 1 and 2, TiSi is employed as cathode material, no gas evolution is observed but a significant electrical current is measured.
Example 5
[0032] Example 4 can also be conducted without water contact In this connection, instead of the aqueous sulphuric acid as electrolyte, an electrolyte gel has to be used between the electrodes as a contact.
Example 6
[0033] When in the processes described in experiments 1 and 2, higher reaction temperatures are applied (45-100° C.), a more vigorous gas evolution is observed. Practically, such temperatures can be reached upon use of flat-bed solar reactors and sunlight as a radiation source.
Example 7
[0034] A perylene such as N,N′-bis-phenyl-ethyl-perylene-3,4,9,10-tetracarboxyl-diimide (2 g) which is soluble in chloroform (but not in water), is dissolved (in chloroform, 5 ml), 5 g of titanium silicide (TiSi2 or Ti5Si3) are added, and the shiny stirred and irradiated (see example 1) at room temperature for 2 hours.
[0035] After removal of the solvent in vacuum the residue is used for further reaction according to the conditions as described in example 1. A higher hydrogen and oxygen development was measured in this connection.
Example 8
[0036] As an alternative to the reaction conditions that have been described in connection with examples 1 and 2, flat-bed solar reactors or a sunlight concentrator system (parabolic troughs or Fresnel optics) can be used.
[0037] Heating of the silicide to, for example, 200° C. under these conditions is no problem for the success of the reaction according to the title and even process-promoting. This applies also to the use of concentrated light energy.
Example 9
[0038] The silicide (e.g., TiSi2) was provided with (doped) according to standard techniques with Pt and a reaction carried out in analogy to example 1. A higher gas yield and higher electrical current compared to example 1 were measured.
Example 10
[0039] Both water splitting for generating hydrogen and oxygen as well as the production of electricity is achieved upon external use of a silicide (e.g., as a plate) electrically connected to the platinum electrode and a transition metal electrode as a counter electrode. In this connection, the electrode spaces can be separated for water splitting by a membrane (nation or Teflon) and an electrolyte, as mentioned in example 1, can be used
Example 11
[0040] For the production of electricity (photovoltaics) water contact can also be eliminated and an electrolyte-gel in analogy to example 5 is used between the electrodes.
Example 12
[0041] Carried out as in example 11, but with several electrodes in series (connected electrically) and provided with an electrolyte (as in examples 5 and 11) As electrodes TiSi/beta-FeSi2/RuO2)alfa-FeSi2) were employed.
Example 13
[0042] Upon suitable p-/n-doping of the electrodes the use of an electrolyte is not necessary and the electrodes can be brought into direct contact upon electrical connection. For p-doping aluminum on TiSi2 and for n-doping phosphorus on TiSi were chosen and the two layers were brought into contact. A considerable electrical current was measured. It is also possible to bring several layers into contact wherein a significantly higher current flow was measured when this arrangement was exposed to artificial light as well as solar radiation.
Generation of hydrogen and oxygen from
water and storage thereof with silicides
WO2007036274
WO2007036274
Also published as: DE102005040255 / EP1928782 / JP2009505927 / CA2619515 / WO2007036274
The invention relates to a method for the photo- and thermochemical generation of hydrogen and/or oxygen from water in the presence of silicides, silicide-like compositions, metallosilicides and non-metallic silicides such as borosilicides, carbosilicides and nitrosilicides, i.e. all compositions containing silicon and being of the molecular formula RSix and/or RSixOy wherein R represents an organic, metallic, organometallic and/or inorganic residue and/or oxides thereof, and Si being silicon and specifically a silicide moiety with X > zero and O representing oxygen with Y zero. The silicide moieties in these compositions exhibit characteristically a high electron density on silicon all of which can also be oxidized. The silicides and silicide-like compositions and/or oxides thereof can react catalytically in these aforementioned processes proceeding with or without light. However, upon irradiation of the reactions an increase of gas evolution is observed, this notably applying to artificial light as well as sunlight. Higher reaction temperatures are often favourable for these processes. Silicides and silicide-like compositions and/or oxides thereof are mostly semiconductor-type materials. Furthermore, these compositions are able to absorb/desorb hydrogen and oxygen reversibly wherein oxygen absorption/desorption is favourable but can occur simultaneously with hydrogen absorption and desorption. The desorption of hydrogen and oxygen can occur at ambient or higher temperatures, especially the processes concerning hydrogen, depending on the nature of the silicides and silicide-like compositions and/or oxides thereof employed.
The present invention relates to a process for the photo- and thermochemical production/generation of hydrogen and/or oxygen wherein water is brought into contact with silicides and silicide-like compositions and/or oxides thereof.
Background
Several procedures for the oxidation and/or reduction of water to yield hydrogen and oxygen by the aid of metallic catalysts have been disclosed. The catalysts employed so far for this latter purpose are lanthanide-type photocatalysts, such as NaTa03:La, catalysts based on rare earth metals, such as R2Ti207(R = Y, rare earth), or Ti02-derived semiconductor materials arranged in a so-called tandem cell. Notably, in these procedures no mention of the use of suicides and silicide-like compositions and/or oxides thereof has been made for the title applications.
The processes for the generation of hydrogen and oxygen from water comprise reduction and/or oxidation processes using semiconductors and light. These processes are also called in summa water splitting processes. The hitherto disclosed procedures employ UV light. Although in some cases remarkable amounts of hydrogen and oxygen evolution is observed, the irradiation conditions are not suitable for solar applications. Further, the preparations of the catalysts are laborious and require uneconomically high temperatures, starting from expensive materials of very high purity. Furthermore, these processes require water of very high purity, i.e. tri-distilled water. Of the cases no indication concerning longer time applications including the consequences for the stability of the catalysts is made.
Therefore subject matter of present invention is a process for the photo- and thermochemical production/generation of hydrogen and/or oxygen wherein water is brought into contact with suicides and silicide-like compositions and/or oxides thereof., i.e. compositions all containing silicon and oxides thereof and being of the molecular formula RSixOywherein R represents pure or mixed organic, metallic, organometallic and/or biochemically derived residues and/or inorganic residues, and Si being silicon and specifically a suicide moiety with X > 0 and O is oxygen with Y 0. The suicide moieties in these compositions exhibit characteristically a high electron density at silicon. The suicides and silicide-like compositions and/or oxides thereof can react catalytically in these aforementioned processes proceeding with or without light. However, upon irradiation of the reactions an increase of gas evolution is observed, this notably applying to artificial light as well as sunlight. Higher reaction temperatures are often favourable for these processes. Suicides and silicide-like compositions and/or oxides thereof are mostly semiconductor-type materials. Furthermore, these compositions are able to store/release and/or absorb/desorb hydrogen and oxygen reversibly wherein oxygen storage/release and/or absorption/desorption is favourable but can occur simultaneously with hydrogen storage/absorption and desorption/release. The release/desorption of hydrogen and oxygen can occur at ambient or higher temperatures, especially the processes concerning hydrogen, depending on the nature of the suicides and silicide-like compositions and/or oxides thereof employed.
Furthermore, it was found that coupling/complexing/attaching/binding of a dye such as perylenes, perylene dyes and perylene congeneers/analogs to suicides and silicide-like compositions and/or oxides thereof is favourable for the light absorption and hence reactivity of the suicides and silicide-like compositions and/or oxides thereof.
Further, it was found that the reactions using suicides and silicide-like compositions and/or oxides thereof for the purpose of water reduction and/or oxidation to yield hydrogen and/or oxygen, respectively, can be carried out by employing the suicides and silicide-like compositions and/or oxides thereof in immobilized form, i.e. when these compositions are attached/fixed onto or in a polymeric surface or material, as well as onto or in a glass or glass-like material, especially when the polymeric and/or glass-type material is electrically conducting.
Further, it was found that the storage/release and/or absorption/desorption of hydrogen and/or oxygen using suicides and silicide-like compositions and/or oxides thereof when these compositions are immobilized, i.e. when these materials are attached/fixed onto or in a polymeric surface and/or glass and/or glass-like material, this in processes carried out with or without light.
Furthermore, processes wherein oxygen is transformed to polyoxygen of the formula On(n > 3) and/or hydogenpolyperoxides of the formula H2On(n > 2) including the back reactions to form oxygen again have not been described in literature so far, but have been found experimentally here; theoretical studies based on calculation predicting a shallow energy minimum and hence low to questionable stability for polyoxygen and hydrogenpolyperoxides in the gas phase. However, the experienced stability of polyoxygen and hydrogenpolyperoxides is seemingly due to stabilization in solution and/or by a metal.
Description of the Invention
Surprisingly, it has now been found that these disadvantages can be avoided by employing silicides, silicide-like compositions, metallosilicides and non-metallic silicides such as borosilicides, carbosilicides and nitrosilicides, i.e. compositions all containing silicon and being of the molecular formula RSixOywherein R represents an organic, metallic, organometallic or inorganic residue, and Si being silicon and specifically a suicide moiety with X > zero and O is oxygen with Y zero (this ensemble of silicide-type compositions is in the following text named silicides and silicide-like compositions and/or oxides thereof). The suicide moieties in these compositions exhibit characteristically a high electron density at silicon, i.e. higher than in the parent silicon atom.
The non-metallic silicides such as borosilicides, carbosilicides and nitrosilicides are also called silicon borides, carbides and nitrides, respectively.
Examples of silicides, silicide-like compositions, metallosilicides and non-metallic silicides are silicides of the formula RSixOywherein R represents an organic, metallic, organometallic, biochemically derived and/or inorganic residue, and Si being silicon and specifically a suicide moiety with X > zero and O is oxygen with Y zero wherein a choice of R can be lithium, beryllium, sodium, potassium, calcium, copper, zinc, rhodium, scandium, rubidium, gallium, selenium, rhodium, palladium, cadmium, lead, osmium, antimon, iridium, tungsten, tin, strontium, barium, titanium, nickel, iron, thallium, boron, cobalt, platinum, manganese, titanium, silicon, carbon, carbon in form of nanotubes, iridium, molybdenum, nitrogen, zirconium, tantalum, vanadium, chromium, silver, gold, lanthanides, actinides, organic residues such as dyes, i.e. perylenes, alkoxy residues and/or oxides of these residues R as well as mixtures of these residues R. Selected examples are titanium silicides (TiSi2, Ti5Si3), nickel suicide (Ni2Si), iron silicides (FeSi2, FeSi), thallium suicide (ThSi2), borosilicide or also silicon tetrabohde named (B4Si), cobalt suicide (CoSi2), platinum suicide (PtSi, Pt2Si), manganese suicide (MnSi2), titanium carbosilicide (Ti3C2Si), carbosilicide/poly-carbosilicide or also silicon carbide/poly-silicon carbide named (CSi/poly-CSi or SiC/poly-SiC), iridium suicide (lrSi2), nitrosilicide or also named silicon nitride (N4Si3), zirconium suicide (ZrSi2), tantalum suicide (TaSi2), vanadium suicide (V2Si) or chromium suicide (CrSi2) and/or oxides thereof, perylene titanium or vanadium silicides, methoxy or ethoxy titanium or vanadium or iron silicides and oxides thereof. The silicides and silicide-like compositions and/or oxides thereof are cheap, abundant and have so far not been claimed for the use with respect to the title applications. They have been used for transistor technique and photovoltaic devices and applications thereof.
The silicides and silicide-like compositions and/or oxides thereof can be used for the generation of hydrogen and/or oxygen from water by conducting the reactions with or without light, i.e. photonic and/or thermal processes, respectively.
The silicides and silicide-like compositions and/or oxides thereof are materials containing also silicon atoms with enhanced electron densities as compared to elemental silicon. Such effect happens when silicon is brought into contact with other elements and/or oxides thereof which can be of metallic and/or non-metallic nature.
It is also important to note that silicides and silicide-like compositions oxidize upon contact with water, oxygen and other oxidizing media to various degree, i.e. 0-100% dependent on the reaction conditions.
Silicides and silicide-like compositions and/or oxides thereof can be prepared by bringing into contact the individual elements and/or the oxides thereof in various ratios in solution/suspension as well as in solid and/or melted and/or gaseous form.
The silicides and silicide-like compositions and/or oxides thereof are mainly semiconductortype materials with high electrondensities at silicon, carbon, nitrogen and boron, respectively. The claimed processes for the generation of hydrogen and/or oxygen using silicides and silicide-like compositions and/or oxides thereof can be achieved with or without light, but are significantly more efficient when running under irradiation. The light and thermal energy can be artificial or of solar origin (200 - 15000 nm emission of the light and thermal source) and can be diffuse or concentrated. The thermal energy being produced by the light source, besides the photonic energy and heat in general, can accelerate the gas evolution processes. In general, higher reaction temperatures are usually promoting the processes rather favourably.
The silicides and silicide-like compositions and/or oxides thereof are mostly absorbing sufficient solar or artificial radiation by themselves without the need for major surface engineering to effect reduction and/or oxidation of water to generate hydrogen and/or oxygen, respectively. Furthermore, the herein claimed title processes are occurring concomitantly, but can be steered by temperature and the nature of silicides and silicide-like compositions and/or oxides thereof.
Surprisingly it was also found that the water quality and purity is of minor importance or even negligible for carrying out the title processes, i.e. oxidation and/or reduction of water as well as storing/abrorbing and releasing/desorbing hydrogen and oxygen, respectively, using silicides and silicide-like compositions and/or oxides thereof.
It has also to be noted that the herein claimed activity of the silicides and silicide-like compositions and/or oxides thereof for the purpose of water reduction and/or oxidation to yield hydrogen and/or oxygen, respectively, is predominantly of catalytic nature, this refers to dark reactions as well as to reactions using light (artificial and/or solar light).
Further, it was found that the reactions using silicides and silicide-like compositions and/or oxides thereof for the purpose of water reduction and/or oxidation to yield hydrogen and/or oxygen, respectively, can be carried out by employing the silicides and silicide-like compositions in immobilized form, i.e. when these materials are inbedded in, attached/fixed onto a polymeric material (such as polyamid, macrolon or plexiglass) or surface or glass or glass-like material, especially when the polymeric and/or glass-type material is electrically conducting.
The reactions such as described above can also be conducted at elevated temperatures.
The formation of the oxides of the silicides and silicide-like compositions can be carried out in water and/or oxygen containing atmosphere or in presence of other oxidants wherein the speed of oxide formation depends on the reaction conditions, such as temperature, presence of inert gas, pH of the reaction media and other physical conditions such as stirring, shaking or not moving the reaction media at all. The growth of the oxide layers (0-100%) can conveniently be followed and analyzed by XPS and XRD spectroscopy. The same types of analyses are applied when bringing into contact already oxidized elements and components prior to reaction and employing these silicides and silicide-like oxides and/or partially oxidized compositions for the above described purposes.
Furthermore, it was found that coupling/complexing/attaching/binding of a dye or an agglomeration of dyes to silicides and silicide-like compositions and/or oxides thereof is favorable for the light absorption and hence reactivity of these compositions (so-called dye sensitized semiconductor reactions). Most favorably dyes such as perylenes and analogs thereof are employed. These dye-complexed silicides and silicide-like compositions and/or oxides thereof can also be applied when running thermal reactions, this even at elevated temperatures, since the perylene dyes are thermally stable.
Additionally, it was found that the silicides and silicide-like compositions and/or oxides thereof can store/release and/or absorb/desorb hydrogen and/or oxygen reversibly. The storage/release and/or absorption/desorption of oxygen is therein most favourable but can occur together with the storage/release of hydrogen. The release/desorption of hydrogen and oxygen can occur at ambient temperatures, especially the release/desorption of hydrogen, but these processes are more favourable at higher temperatures. The rates of these processes depend on the reaction temperature and the nature of the semiconductor-type material employed, i.e. of the silicides and silicide-like compositions and/or oxides thereof.
The higher temperatures stated above can be created electrically, by earthem temperature, solar energy, furnaces, microwave discharge or any other source of thermal energy.
Further, it was found that the reactions using silicides and silicide-like composrf/<'>ons and/or oxides thereof for the purpose of water reduction and/or oxidation to yield hydrogen and/or oxygen, respectively, can be carried out by employing the silicides and silicide-like compositions and/or oxides thereof in immobilized form, i.e. when these compositions are attached/fixed onto or in a polymeric surface or material, as well as onto or in a glass or glass-like material, especially when the polymeric and/or glass-type material is electrically conducting.
It was also found that the storage/release and/or absorption/desorption of hydrogen and/or oxygen using silicides and silicide-like compositions and/or oxides thereof when these compositions are immobilized, i.e. when these materials are attached/fixed onto or in a polymeric surface or glass or glass-like material, this in processes carried out with or without light.
The photochemical and thermal processes stated above can be conducted with silicides and silicide-like compositions and/or oxides thereof in catalytic amounts.
Both, the photochemical and thermochemical processes stated above can be conducted at elevated temperatures which is even beneficial for the course of the processes. The processes stated above leading to storage/absorption of oxygen are concomitant with the storage/absorption of hydrogen but it is found that the selectivity and the speed of such processes are dependent on the reaction conditions, such as temperature, concentration, pressure, light vs. dark reactions, pH, physical conditions such as stirring, ultrasound treatment, shaking etc. The presence of other gas storage material can help to improve the selectivity and speed of the absorption/desorption of hydrogen and/or oxygen on and/or in the silicides and silicide-like compositions and/or oxides thereof. Absorption and/or adsorption (storage) of oxygen is found in most cases to be very efficient and even more efficient than of hydrogen.
The processes stated above leading to the release/desorption of hydrogen are concomitant with the release/desorption of oxygen but are found to be predominant dependent on the reaction conditions applied (such as nature of the suicide or silicide-like composition and/or oxides thereof used, temperature and pressure): E.g. when processing with titanium suicide at ambient temperature and pressure. At higher temperatures and in light the release/desorption of oxygen can be forced.
Furthermore, it was found that the silicides and silicide-like compositions and/or oxides thereof can be employed for the title applications individually or in combinations of two or more silicides or silicide-like compositions and/or oxides thereof. It is also possible to conduct the title processes with one or more of the silicides or silicide-like compositions together with additional semiconductor materials of non-silicide structures such as ruthenium dioxide (Ru02), manganese dioxide (Mn02), tungsten trioxide (W03) and other semiconducting materials in order to enforce the title processes. The ratio of hydrogen-to-oxygen evolution and storage thereof varies with the semiconductor mixtures, temperature and pressure employed. The same is true for silicides or silicide-like compositions and/or oxides thereof which are doped (see below).
It was also found that storage/release and/or absorption/desorption of hydrogen and/or oxygen using silicides and silicide-like compositions is active when the contact of the silicides and silicide-like compositions and/or oxides thereof to water is disrupted or cancelled, i.e. when the storage device was attached to the reaction vessel externally via a pipe.
Further, it was found that the title processes can be forced by doping/mixing/alloying the silicides and silicide-like semiconductors and/or oxides thereof with any of the previously mentioned elements/residues for the choice of R and/or oxides thereof as well as mixtures thereof in the third paragraph of this chapter. The silicides and silicide-like compositions and/or oxides thereof can be prepared by bringing into contact the individual elements and/or the oxides thereof as well as other derivatives thereof, all in various ratios in fully or partlially oxidized form or in non-oxidized form, this in solution, suspension as well as in solid, e.g. by milling or by alloying/melting, or in liquid or any other chemical and/or physical form.
Polyoxygen of the formula On(n > 3) and/or hydogenpolyperoxides of the formula H2On(n > 2) are formed in the above described reactions from oxygen and preferentially under light and in connection with the above described catalysts reversibly. These methods can also include biochemical transformations such as the application of a peroxidase. Examples of polyoxygen and hydrogenpolyperoxides, i.e. a selection of ring size and chain lengths, show UV-absorptions with maxima in the region of 221 (016), and 202 nm (H08) (both species with tailing up to 350-400 nm), respectively, and mass peaks/fragmentation peaks in mass spectroscopy at m/z 256 (O[iota]6), 129 (H08), 97 (H06), 81 (H05) and 32 (02) in water.
The novel technology based on the use of silicides and silicide-like compositions and/or oxides thereof as stated above can find applications e.g. for the purpose of novel heating systems, in fuel cell technology which will be ultimately applied for and in terrestrial and nonterrestrial traffic and static constructions and devices replacing or supporting or supplementing such constructions and devices driven so far by devices based on the use of conventional fossil energy.
Examples
Note, in all examples the silicides and silicide-like compositions are oxidized by water and/or oxygen to various degrees (0-100%) dependent on the reaction conditions. But in most cases oxidation stops at a layer size of 1-5 nm depth which protects the catalyst from further (rapid) oxidation. Such effects can be controlled for example by temperature and pH as well as by other physical and chemical conditions. The same or silmilar effects can be achieved by bringing into contact individually oxidized (0-100%) elements and components of the catalyst's compositions prior to reaction. Analyses of the state of oxidation of the catalysts and the respective components have been performed by XRD and XPS spectroscopy.
Example 1: 3-5 g of a titanium suicide (TiSi2or Ti5Si3) are stirred in 200 - 400 mL of water (filtered over ion exchange resin, a slightly lower gas yield was determined when using plain water without purification) in a vessel which is transparent for solar radiation or radiation of an artificial light source. As light source served a Heidelberg irradiation system with lamps having emission maxima at 415, 525, or 660 nm and emission ranges from 300-550, 490600 or 610-700 nm, respectively, or halogen lamps with emossions in the range of 350-800 nm. This reaction set-ups yield 25 mL and more of hydrogen and oxygen per day at room temperature (gas evolution and ratios depend on the nature of the catalyst used, temperature and pressure). Most of the oxygen is in such reactions absorbed by the catalysts to give hydrogen/oxygen molar ratios of 2/1 up to 20/1. The oxygen and hydrogen evolution is measured volumetrically in conjunction with gas chromatography and mass spectrometry. The experiment can be continued up to at least 3 months if the gas volume of the reaction vessel is emptied and flushed with air after periods of 2-3 days. Alternatively, a solar flatbed reactor made of macrolon or plexiglass and sunlight irradiation can be employed.
Example 2: Instead of the silicides mentioned in example 1 , also nickel suicide (Ni2Si), iron silicides (FeSi2, FeSi), thallium suicide (ThSi2), boron suicide (B4Si), cobalt suicide (CoSi2), platinum suicide (PtSi, Pt2Si), manganese suicide (MnSi2), titanium carbosilicide (Ti3C2Si), carbosilicide/poly-carbosilicide (also named silicon carbide/poly-silicon carbide (CSi/poly-CSi or SiC/poly-SiC), iridium suicide (lrSi2), nitrosilicide or also named silicon nitride (N4Si3), zirconium suicide (ZrSi2), tantalum suicide (TaSi2), vanadium suicide (V2Si) or chromium suicide (CrSi2) can be employed. The reactions are carried out as described in example 1.
Example 3: Same experimental set-up as in example 1 , but using nickel suicide (Ni2Si). A hydrogen/oxygen molar ratio of approx. 20/1 was measured.
Example 4: If in reactions given in examples 1 and 2, higher temperatures (30-45 degrees Celcius) were applied, more vigorous gas evolution was observed. Conveniently this temperature can be reached by using solar flatbed reactors and sunlight.
Example 5: The same conditions as in examples 1 and 2, but without the application of light gave at higher reaction temperatures (30-40 degrees Celcius) more vigorous gas evolution.
Example 6: A chloroform soluble perylene (but not soluble in water), such as N,N'-bisphenyl ethyl perylene-3,4,9,10-tetracarboxylic diimide (2 g), was dissolved (in 5 mL of chloroform) and stirred with a titanium suicide (3 g, TiSi2or Ti5Si3) during 2 hours at room temperature. The solvent was then removed in vacuo and the residue subjected to the conditions stated in example 1. An increase of hydrogen and oxygen evolution (> 30 mL per day) was observed.
Example 7: Alternatively to the reaction conditions stated in example 1 , a flatbed reactor made of macrolon or plexiglass can be employed wherein the reactor material macrolon or plexiglass was heated (50-100 degrees Celcius) prior to the reaction and in the presence of the semiconductor material (Ni2Si) to achieve immobilization of the catalyst on the polymer surface of the reactor. Otherwise the experiments were conducted as in 1.
Example 8: If for reactions such as stated in example 1 , a closed reaction vessel was employed, storage of hydrogen and oxygen is exercised when opening the vessel after two weeks. Vigorous release of hydrogen and oxygen (20/1) at room temperature is observed and the amount of gas collected and measured corresponded to a continuous experimental set-up which includes the collection of the gases repeatedly after 2-3 days. The reason for a lack of oxygen in these reactions has been identified. Oxygen is continuously consumed under the given reaction conditions to form polyoxygen and hydrogenpolyperoxides of the formula On(n > 3) and H2On(n > 2), respectively. Polyoxygen and hydrogenpolyperoxides can be converted back to oxygen by treatment with metal oxides (such as with mixtures of Mn02, CuO and suicide oxides) and light or thermal activation.
Example 9: The same reaction set-up as in example 1 was employed here, but 1 g of W03was added to the reaction slurry. A more vigorous gas evolution than in example 1 resulted (> 30 mL per day).
Example 10: 3 g of TiSi2were doped with Pt using standard techniques. A reaction run according to example 1 gave a higher yield of gases than in the latter example (> 25 mL per day).
METHOD FOR PRODUCING PROTECTIVE LAYERS
CONTAINING SILICIDES AND/OR OXIDIZED SILICIDES ON SUBSTRATES
WO2014019571
WO2014019571