GB2555814A

GB2555814A - Improvement in and relating to catalysis

Info

Publication number: GB2555814A
Application number: GB1619008.4A
Authority: GB
Inventors: Huang Zhaorong; Achour Abdenour
Original assignee: Cranfield University
Current assignee: Cranfield University
Priority date: 2016-11-10
Filing date: 2016-11-10
Publication date: 2018-05-16
Anticipated expiration: 2036-11-10
Also published as: WO2018087540A1; GB2555814B

Abstract

A catalyst system comprises a heat exchanger 310 and a thermoelectric material 330 for changing the rate of a chemical reaction, wherein: the catalyst system comprises a reaction surface 350, and a first portion of the thermoelectric material is arranged to be nearer to the reaction surface than a second portion of the thermoelectric material; and the heat exchanger is arranged to produce a spatial temperature difference between the first portion and the second portion of the thermoelectric material, whereby the thermoelectric material alters an activation energy on the reaction surface to change the rate of the chemical reaction. A process for creating a chemical product using the catalyst system, the use of a thermoelectric material as a catalyst or a reaction inhibitor for a chemical reaction, and the use of a composite comprising a noble metal or oxide-based catalyst and a thermoelectric material is also disclosed.

Description

(54) Title of the Invention: Improvement in and relating to catalysis Abstract Title: Thermoelectric catalyst system (57) A catalyst system comprises a heat exchanger 310 and a thermoelectric material 330 for changing the rate of a chemical reaction, wherein: the catalyst system comprises a reaction surface 350, and a first portion of the thermoelectric material is arranged to be nearer to the reaction surface than a second portion of the thermoelectric material; and the heat exchanger is arranged to produce a spatial temperature difference between the first portion and the second portion of the thermoelectric material, whereby the thermoelectric material alters an activation energy on the reaction surface to change the rate of the chemical reaction. A process for creating a chemical product using the catalyst system, the use of a thermoelectric material as a catalyst or a reaction inhibitor for a chemical reaction, and the use of a composite comprising a noble metal or oxide-based catalyst and a thermoelectric material is also disclosed.

At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

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Improvement in and relating to catalysis

FIELD [01] The present invention relates to a catalyst system, a method, a process, a reaction medium, or use for changing a rate of a chemical reaction.

BACKGROUND [02] Catalysts (i.e. reaction promoters) and reaction inhibitors are widely used to increase or decrease a chemical reaction rate, to improve desired selectivity, or to lower/increase process temperature in many industrial processes.

[03] A catalyst promoter can improve the activity, selectivity or lifetime of a catalyst, and can generally be divided into structural and electronic promoters. Classical promoters, for example adding of alkalis (K) on Fe for the ammonia synthesis reaction, are added in during catalyst preparation so can be subjected to degradation during catalytic process.

[04] On the other hand, electrochemical promotion, also called a non-Faradaic electrochemical modification of catalytic activity (NEMCA) which refers to a reversible change of catalytic properties of metal catalysts deposited on a solid electrolyte, can be obtained by applying a small external electric current or voltage. This allows for a controlled in-situ introduction of promoters on a catalyst surface under operating conditions. The NEMCA is due to the ionic species back-spilling over from the electrolyte to form a double layer at the catalyst surface, which in turn leads to a change in work function and chemisorption properties of the catalyst. However, the NEMCA is often not used in commercial reactors due to its low mass efficiency (i.e. low catalytic activity per mass) of the catalyst material (which are often noble metals), the need to maintain electrical connection at often harsh conditions, and the incompatibility of its reactor configuration with typical chemical reactors (fixed bed, monolithic or fluidized bed).

[05] Embodiments of the present invention aim to address problems associated with a conventional catalyst, a conventional reaction inhibitor, a conventional catalyst promoter/inhibitor, a conventional chemical reactor, or catalysis, whether mentioned herein or not.

SUMMARY [06] In a first aspect, the present invention provides a catalyst system comprising a heat exchanger and a thermoelectric material for changing a rate of a chemical reaction, wherein: the catalyst system comprises a reaction surface, and a first portion of the thermoelectric material is arranged to be nearer to the reaction surface than a second portion of the thermoelectric material; and the heat exchanger is arranged to produce a spatial temperature difference between the first portion and the second portion of the thermoelectric material, whereby the thermoelectric material alters an activation energy on the reaction surface to change the rate of the chemical reaction.

[07] Suitably, the heat exchanger is arranged to apply to the second portion of the thermoelectric material: if the thermoelectric material is a p-type and the chemical reaction is an electrophobic reaction, a lower temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a higher temperature than the temperature of the first portion to decrease the rate of the chemical reaction; if the thermoelectric material is a ntype and the chemical reaction is an electrophobic reaction, a higher temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a lower temperature than the temperature of the first portion to decrease the rate of the chemical reaction; if the thermoelectric material is a p-type and the chemical reaction is an electrophilic reaction, a higher temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a lower temperature than the temperature of the first portion to decrease the rate of the chemical reaction; and if the thermoelectric material is a n-type and the chemical reaction is an electrophilic reaction, a lower temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a higher temperature than the temperature of the first portion to decrease the rate of the chemical reaction.

[08] Suitably, the thermoelectric material comprises an oxide based thermoelectric material. Suitably, the oxide based thermoelectric material comprises at least one of intrinsic or doped ZnO, SrTiO₃, CaMnO₃, layered cobaltites Ca₃Co₄O₉, NaCoO₂, SnO₂, ln₂O₃, BiCuSeO oxyselenides, and Bi₂Sr₂Co₂O_y. Suitably, the thermoelectric material comprises BiCuSeO oxyselenides.

[09] Suitably, the thermoelectric material provides at least some of the reaction surface, and the thermoelectric material acts a catalyst or a reaction inhibitor for the chemical reaction.

[10] Suitably, the catalyst system further comprises a noble metal or oxide based catalyst for providing at least some of the reaction surface, wherein the thermoelectric material is arranged to alter the activation energy of the noble metal or oxide based catalyst to promote or inhibit the catalysis. Suitably, a thin film layer of the noble metal or oxide based catalyst is deposited on a surface of the first potion of the thermoelectric material. Suitably, particulates of the noble metal or oxide based catalyst are dispersed in a matrix of the thermoelectric material. Suitably, the noble metal or oxide based catalyst comprises Pt.

[11] In a second aspect, the present invention provides a method of changing a rate of a chemical reaction using a catalyst system according to the first aspect, the method comprising: producing a spatial temperature difference between a first portion and a second portion of a thermoelectric material, wherein the first portion is nearer to a reaction surface than the second portion, whereby the thermoelectric material alters an activation energy on the reaction surface to change the rate of the chemical reaction.

[12] Suitably, the producing comprises applying to the second portion of the thermoelectric material: if the thermoelectric material is a p-type and the chemical reaction is an electrophobic reaction, a lower temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a higher temperature than the temperature of the first portion to decrease the rate of the chemical reaction; if the thermoelectric material is a n-type and the chemical reaction is an electrophobic reaction, a higher temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a lower temperature than the temperature of the first portion to decrease the rate of the chemical reaction; if the thermoelectric material is a p-type and the chemical reaction is an electrophilic reaction, a higher temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a lower temperature than the temperature of the first portion to decrease the rate of the chemical reaction; and if the thermoelectric material is a n-type and the chemical reaction is an electrophilic reaction, a lower temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a higher temperature than the temperature of the first portion to decrease the rate of the chemical reaction.

[13] Suitably, the chemical reaction is an oxidation reaction or a reduction reaction. Suitably, the chemical reaction is an ethylene C₂H₄ oxidation.

[14] In a third aspect, the present invention provides a process for producing a chemical product, the process comprising contacting at least two reactants with a catalyst or a reaction inhibitor comprising a thermoelectric material, wherein the thermoelectric material alters an activation energy on a reaction surface.

[15] Suitably, a first portion of the thermoelectric material is nearer to the reaction surface than a second portion of the thermoelectric material, and the process further comprises exposing the second portion to: if the thermoelectric material is a p-type and the chemical reaction is an electrophobic reaction, a lower temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a higher temperature than the temperature of the first portion to decrease the rate of the chemical reaction; if the thermoelectric material is a n-type and the chemical reaction is an electrophobic reaction, a higher temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a lower temperature than the temperature of the first portion to decrease the rate of the chemical reaction; if the thermoelectric material is a p-type and the chemical reaction is an electrophilic reaction, a higher temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a lower temperature than the temperature of the first portion to decrease the rate of the chemical reaction; and if the thermoelectric material is a n-type and the chemical reaction is an electrophilic reaction, a lower temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a higher temperature than the temperature of the first portion to decrease the rate of the chemical reaction.

[16] Suitably, the process comprises an oxidation or a reduction reaction. Suitably, the process comprises an ethylene C₂H₄ oxidation, and the at least two reactants comprises ethylene C₂H₄ and oxygen.

[17] Suitably, the thermoelectric material comprises an oxide based thermoelectric material. Suitably, the oxide based thermoelectric material comprises at least one of intrinsic or doped ZnO, SrTiO₃, CaMnO₃, layered cobaltites Ca₃Co₄O₉, NaCoO₂, SnO₂, ln₂O₃, BiCuSeO oxyselenides, and Bi₂Sr₂Co₂O_y. Suitably, the thermoelectric material comprises BiCuSeO oxyselenides.

[18] Suitably, the thermoelectric material provides at least some of the reaction surface.

[19] Suitably, the catalyst or the reaction inhibitor further comprises a noble metal or oxide based catalyst for providing at least some of the reaction surface, wherein the thermoelectric material is arranged to alter the activation energy of the noble metal or oxide based catalyst to promote or inhibit the catalysis. Suitably, a thin film layer of the noble metal or oxide based catalyst is deposited on a surface of the first potion of the thermoelectric material. Suitably, particulates of the noble metal or oxide based catalyst are dispersed in a matrix of the thermoelectric material. Suitably, the noble metal or oxide based catalyst comprises Pt.

[20] In a fourth aspect, the present invention provides use of a thermoelectric material as a catalyst or a reaction inhibitor for a chemical reaction. Suitably, the thermoelectric material comprises an oxide based thermoelectric material. Suitably, the oxide based thermoelectric material comprises at least one of intrinsic or doped ZnO, SrTiO₃, CaMnO₃, layered cobaltites Ca₃Co₄O₉, NaCoO₂, SnO₂, ln₂O₃, BiCuSeO oxyselenides, and Bi₂Sr₂Co₂O_y. Suitably, the thermoelectric material comprises BiCuSeO oxyselenides.

[21] Suitably, the chemical reaction is an oxidation reaction or a reduction reaction. Suitably, the chemical reaction is an ethylene C₂H₄ oxidation.

[22] In a fifth aspect, the present invention provides use of a thermoelectric material as a catalyst promoter, a catalyst inhibitor, a reaction inhibitor promoter, or a reaction inhibitor inhibitor for a chemical reaction, wherein the thermoelectric material alters an activation energy of a catalyst or a reaction inhibitor to promote or inhibit the catalysis. Suitably, the thermoelectric material comprises an oxide based thermoelectric material. Suitably, the oxide based thermoelectric material comprises at least one of intrinsic or doped ZnO, SrTiO₃, CaMnO₃, layered cobaltites Ca₃Co₄O₉, NaCoO₂, SnO₂, ln₂O₃, BiCuSeO oxyselenides, and Bi₂Sr₂Co₂O_y. Suitably, the thermoelectric material comprises BiCuSeO oxyselenides.

[23] Suitably, the chemical reaction is an oxidation reaction or a reduction reaction. Suitably, the chemical reaction is an ethylene C₂H₄ oxidation.

[24] Suitably, the catalyst or the reaction inhibitor comprises a noble metal or oxide based catalyst. Suitably, the noble metal or oxide based catalyst comprises Pt.

[25] In a sixth aspect, the present invention provides use of a composite comprising a noble metal or oxide based catalyst and a thermoelectric material as a catalyst or a reaction inhibitor for a chemical reaction. Suitably, the composite comprises a thin film layer of the noble metal or oxide based catalyst deposited on a surface of the thermoelectric material. Suitably, the composite comprises particulates of the noble metal or oxide based catalyst dispersed in a matrix of the thermoelectric material.

[26] Suitably, the thermoelectric material comprises an oxide based thermoelectric material. Suitably, the oxide based thermoelectric material comprises at least one of intrinsic or doped ZnO, SrTiO₃, CaMnO₃, layered cobaltites Ca₃Co₄O₉, NaCoO₂, SnO₂, ln₂O₃, BiCuSeO oxyselenides, and Bi₂Sr₂Co₂O_y. Suitably, the thermoelectric material comprises BiCuSeO oxyselenides.

[27] Suitably, the noble metal or oxide based catalyst comprises Pt.

[28] Suitably, the chemical reaction is an oxidation reaction or a reduction reaction. Suitably, the chemical reaction is an ethylene C₂H₄ oxidation.

[29] In a seventh aspect, the present invention provides use of a catalyst system according to the first aspect, wherein the chemical reaction is an oxidation reaction or a reduction reaction. Suitably, the chemical reaction is an ethylene C₂H₄ oxidation.

[30] In an eighth aspect, the present invention provides a reaction medium comprising one or more reactants, and a catalyst system according to the first aspect.

[31] According to the present invention there is provided a system, an apparatus, a method, a process, a reaction medium, and use as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows.

BRIEF DESCRIPTION OF DRAWINGS [32] For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which:

[33] Figure 1A shows a catalyst system according to an embodiment of the invention;

[34] Figure 1B shows a catalyst system according to an embodiment of the invention;

[35] Figure 1C shows a catalyst system according to an embodiment of the invention;

[36] Figure 2A shows a flow chart of a method of changing a rate of a chemical reaction according to an embodiment of the invention;

[37] Figure 2B shows a partial flow chart method of a spatial temperature difference production step of the method of Figure 2A according to an embodiment of the invention;

[38] Figure 3A shows a chemical reactor according to an embodiment of the invention;

[39] Figure 3B shows the chemical reactor of Figure 3A being used for ethylene oxidation reaction characterisation according to an embodiment of the invention;

[40] Figure 4 shows a comparison of an ethylene oxidation rate of a catalyst system according to an embodiment of the invention which comprises a Pt thin film deposited on BiCuSeO (Pt/BCSO), and an ethylene oxidation rate of a Pt thin film on a Y₂O₃-stabilized ZrO₂ (YSZ) solid electrolyte (Pt/YSZ);

[41] Figure 5 shows ethylene oxidation reaction rates of a catalyst system according to an embodiment of the invention under a normal temperature gradient (NTG) condition and under a reduced temperature gradient (RTG) condition;

[42] Figure 6A shows a measured voltage between a hot and cold surfaces of a Pt/BCSO disc sample (Pt/BCSO B) prepared according to an embodiment of the invention;

[43] Figure 6B shows a Ln(r) vs -eV/k_bT plot for the Pt/BCSO B of Figure 6A;

[44] Figure 7 shows a schematic of energy bands for a p-type and a n-type thermoelectric (TE) material with a metal particle supported on the TE material on a hotter portion according to an embodiment of the invention;

[45] Figure 8A shows characteristics of a catalyst system according to an embodiment of the invention which comprises a thermoelectric composite comprising Pt nanoparticles and BiCuSeO (Pt(NP2)/BCSO), after having used the catalyst system for an ethylene oxidation;

[46] Figure 8B shows how a change in Seebeck voltage of Pt(NP2)/BCSO of Figure 8A affects the ethylene oxidation rate;

[47] Figures 9A and 9B show an ethylene oxidation rate and a Ln(r) vs -eV/k_bT plot for a bare BCSO disc sample (BCSO B) prepared according to an embodiment of the invention;

[48] Figure 10 shows an ethylene oxidation rate for the Pt/BCSO B prepared according to an embodiment of the invention;

[49] Figure 11 shows a comparison of ethylene oxidation rates for different example embodiments of the invention;

[50] Figure 12 shows ethylene oxidation rate and conversion dependence on temperature for a bare BCSO sample;

[51] Figure 13A shows characteristics of a catalyst system according to an embodiment of the invention which comprises a thermoelectric composite comprising a Pt thin film deposited on BiCuSeO (Pt/BCSO), after having used the catalyst system for an ethylene oxidation;

[52] Figure 13B shows an Scanning Electron Microscope (SEM) image of Pt/BCSO of Figure 13A after having used it for the ethylene oxidation for more than 10 hours; and [53] Figure 14 shows characteristics of a catalyst system according to an embodiment of the invention which comprises a bare BiCuSeO (BCSO) but no catalyst such as Pt, after having used the catalyst system for an ethylene oxidation.

DESCRIPTION OF EMBODIMENTS [54] Figures 1A to 2B shows different embodiments of the present invention.

[55] Figure 3A and Figure 3B show a chemical reactor according to an embodiment of the invention.

[56] Figure 4 to Figure 14 show example embodiments of the invention being used on an ethylene oxidation, wherein the example embodiments are based on a thermoelectric material called BiCuSeO oxyselenides (BCSO).

[57] A thermoelectric (TE) material converts a temperature difference directly into an electrical voltage (a potential difference) via Seebeck effect represented by S= -V/ΔΤ, wherein V is the voltage between two ends of the TE material, ΔΤ is the temperature difference applied, or created, between the two ends of the TE material, and S is the Seebeck coefficient of the TE material. Performance of a TE material is measured by its figure of merit ZT=S²oT/k, wherein σ is its electrical conductivity, κ is its thermal conductivity, and T is the absolute temperature. An ideal TE material possesses a high Seebeck coefficient, a high electrical conductivity and a low thermal conductivity.

[58] Figure 1A shows a catalyst system according to an embodiment of the invention, wherein the catalyst system comprises a heat exchanger 310 and a thermoelectric material 330 for changing a rate of a chemical reaction.

[59] The catalyst system also comprises a reaction surface 350 for engaging reactants of the chemical reaction. A first portion 331 of the thermoelectric material 330 is arranged to be nearer to the reaction surface 350 than a second portion 332 of the thermoelectric material 330 so that there is a coupling for transferring electrical energy, e.g. a conductive coupling through a physical contact, between the first portion 331 and the reaction surface 350.

[60] For example, the embodiment shown in Figure 1A has an external surface of the first portion 331 as the reaction surface 350. Because thermoelectric materials 330 often comprises a transition metal, noble metal or oxide based material which can be used as a catalyst in its own composition, the external surface of the thermoelectric material 330 is able to function as the reaction surface 350, functioning as the catalyst for the chemical reaction.

[61] It is understood that instead of a physical contact, any other type of coupling capable of transferring an electrical characteristic of the first portion 331 to the reaction surface 350 may be provided between the first portion 331 and the reaction surface 350 according to another embodiment.

[62] The heat exchanger 310 is arranged to produce a spatial temperature difference between the first portion 331 and the second portion 332 of the thermoelectric material 330, whereby the thermoelectric material 330 alters an activation energy on the reaction surface 350. The thermoelectric effect produced by the thermoelectric material 330 alters Fermi level on the reaction surface 350, which leads to an alteration in activation energy on the reaction surface 350. As the rate of chemical reaction depends on the activation energy on the reaction surface 350, this alteration in the activation energy changes the rate of the chemical reaction.

[63] This change in the chemical reaction rate is achieved by at least one reactant 390 engaging the reaction surface 350, which has the altered activation energy, so that the chemical reaction produces at least one product 395 based on the altered activation energy level.

[64] Further, as the thermoelectric material 330 is able to both increase or decrease the activation energy level based on the sign of the spatial temperature difference applied between the first portion 331 and the second portion 332, the catalyst system can be controlled, depending on the requirements and needs of a user, to function as a promoter (i.e. a catalyst) for the chemical reaction by increasing the rate, or an inhibitor for the chemical reaction by decreasing the rate of the chemical reaction.

[65] For example, the catalyst system can be controlled so that the heat exchanger applies to the second portion 332 of the thermoelectric material 330: if the thermoelectric material 330 is a p-type and the chemical reaction is an electrophobic reaction, a lower temperature than the temperature of the first portion 331 to increase the rate of the chemical reaction, or a higher temperature than the temperature of the first portion 331 to decrease the rate of the chemical reaction; if the thermoelectric material 330 is a n-type and the chemical reaction is an electrophobic reaction, a higher temperature than the temperature of the first portion 331 to increase the rate of the chemical reaction, or a lower temperature than the temperature of the first portion 331 to decrease the rate of the chemical reaction; if the thermoelectric material 330 is a p-type and the chemical reaction is an electrophilic reaction, a higher temperature than the temperature of the first portion 331 to increase the rate of the chemical reaction, or a lower temperature than the temperature of the first portion 331 to decrease the rate of the chemical reaction; and if the thermoelectric material 330 is a n-type and the chemical reaction is an electrophilic reaction, a lower temperature than the temperature of the first portion 331 to increase the rate of the chemical reaction, or a higher temperature than the temperature of the first portion 331 to decrease the rate of the chemical reaction.

[66] Here, an electrophilic reaction is a chemical reaction wherein the reaction rate decreases with an increase in the work function (which can be changed by the alteration in the Fermi level) of the reaction surface 350, and an electrophobic reaction is a chemical reaction wherein the reaction rate increases with an increase in the work function (which can be changed by the alteration in the Fermi level) of the reaction surface 350. So for given partial pressures of electron acceptor reactants (for example, O₂ or NO) and electron donor reactants (for example, C₂H₄ or C₆H₆), the electrophilic reaction has < 0 and the eletrophobic reaction has > 0, wherein r is the reaction rate and Φ is the work function of the reaction surface.

[67] In other words, a catalytic chemical reaction is an electrophilic reaction if an electron acceptor is weakly bound and an electron donor is strongly bound on the reaction surface, and the catalytic chemical reaction is an electrophobic reaction if an electron acceptor is strongly bound and an electron donor is weakly bound on the reaction surface.According to an embodiment, the catalyst system is used to control the rate of the chemical reaction by increasing, decreasing or maintaining a rate of the chemical reaction based on an input from a user and/or a computer system for managing the chemical reaction.

[68] According to an embodiment, the thermoelectric material 330 provides only a first part of the reaction surface 350, and the thermoelectric material 330 acts as a catalyst (i.e. a reaction promoter) or a reaction inhibitor for the chemical reaction by altering the activation energy of the first part.

[69] According to an embodiment, a coupling for transferring electronic energy, e.g. a conductive coupling through a physical contact, is provided between the first portion 331 of the thermoelectric material 330 and a second part of the reaction surface 350, wherein the second part is not a part of the thermoelectric material 330, and the thermoelectric material 330 is able to alter the activation energy of the second part of the reaction surface 350 as well.

[70] Suitably, the second part of the reaction surface 350 is provided by a catalyst or a reaction inhibitor for the chemical reaction, such as a noble metal or oxide based catalyst 380, and the thermoelectric material 330 is arranged to alter the activation energy of the noble metal or oxide based catalyst 380 or the reaction inhibitor to promote or inhibit the catalysis, whereby the rate of the chemical reaction is changed.

[71] According to another embodiment, the thermoelectric material 330 comprises an oxide based thermoelectric material. Preferably, the oxide based thermoelectric material comprises at least one of intrinsic or doped ZnO, SrTiO₃, CaMnO₃, layered cobaltites Ca₃Co₄O₉, NaCoO₂, SnO₂, ln₂O₃, BiCuSeO oxyselenides, and Bi₂Sr₂Co₂O_y, wherein the at least one for forming the thermoelectric material 330 is selected based on an operational temperature range for the intended chemical reaction, and the above list is in an order from a higher operational temperature range (i.e. a higher working temperature of the thermoelectric material 330) to a lower operational temperature range (i.e. a lower working temperature of the thermoelectric material 330).

[72] According to an embodiment, the oxide based thermoelectric material comprises at least one of p-type layered BiCuSeO oxyselenides, Cobalt oxide AxCoO2 (A=Na, Ca, Sr), or n-type CaMnO3 and SrTiO3 based perovskites.

[73] Figure 1B shows a catalyst system according to an embodiment of the invention, wherein the catalyst system comprises a catalyst for the chemical reaction, and the catalyst provides whole of the reaction surface 350.

[74] A thin film layer of the catalyst, for example the noble metal or oxide based catalyst 380, is deposited on a surface of the first portion 331 of the thermoelectric material 330 so that at least some of, or the whole, reaction surface 350 is provided by this thin film layer of the noble metal or oxide based catalyst 380 instead of the first portion 331 of the thermoelectric material 330 as shown in Figure 1A. So the catalyst system comprises a thermoelectric composite, wherein the thermoelectric composite comprises the thin film layer of the catalyst and the thermoelectric material 330.

[75] This has an advantage in that a chosen thermoelectric material 330 can be used with a number of different, and possibly more suitable and/or effective, catalysts for that particular chemical reaction to achieve more efficient chemical reaction.

[76] It is understood that any number of known techniques for depositing the thin film layer of catalyst can be used to form the catalyst system. For example, according to an embodiment, the thin film layer of the catalyst is deposited by sputtering particulates of the catalyst onto a surface of the first portion 331, whereby the reaction surface 350 is formed.

[77] It is also understood that according to an embodiment, the thin film layer of the catalyst is merely a sporadically distributed particles and/or nanoscale particles of the catalyst deposited on a surface of the first portion 331 of the thermoelectric material 330. So the thin film layer is not a continuous layer, and a sporadic spots of the catalyst deposited on the surface of the first portion 331 acts as the catalyst layer. It is understood that any number of known techniques for depositing such catalyst layer comprising sporadic spots of the catalyst can be used to form the catalyst system.

[78] Figure 1C shows a catalyst system according to an embodiment of the invention, wherein a catalyst for the chemical reaction is provided in a particulate form embedded inside and/or on a surface of the thermoelectric material 330.

[79] A plurality of particulates of the catalyst, namely the noble metal or oxide based catalyst 380, are dispersed in a matrix of the thermoelectric material 330. So the catalyst system comprises a thermoelectric composite, wherein the thermoelectric composite comprises particulates of the catalyst dispersed in and/or on the thermoelectric material 330.

[80] This has an advantage in that a composite of the thermoelectric material 330 and the noble metal or oxide based catalyst 380 can be formed and used to more efficiently control the rate of the chemical reaction than, for example using only the thermoelectric material 330 as shown in Figure 1A.

[81] It is understood that any number of known techniques for forming such a thermoelectric composite may be used. For example, according to an embodiment, a plurality of particulates of the catalyst are mixed with a solution containing the thermoelectric material 330 in a powder form, after which the solvent is removed from the mixture (for example by heating or sintering) so that the thermoelectric composite is left behind.

[82] It is also understood that a thermoelectric composite formed according to an embodiment of the invention can also function as a thermoelectric material in the example embodiments of the invention as long as the thermoelectric composite is able to generate the thermoelectric effect such as the Seebeck effect when a spatial temperature difference is created therein.

[83] So according to an embodiment, a thermoelectric material or a thermoelectric composite is used as a catalyst (i.e. a reaction promoter) or a reaction inhibitor for the chemical reaction.

[84] According to yet another embodiment wherein a separate catalyst is also used and a reaction surface is provided on the separate catalyst, a thermoelectric material or a thermoelectric composite is used as a catalyst promoter or a catalyst inhibitor for the chemical reaction, wherein the thermoelectric material or the thermoelectric composite alters an activation energy of the separate catalyst to promote or inhibit the catalysis.

[85] According to yet another embodiment wherein a separate reaction inhibitor is also used and a reaction surface is provided on the separate reaction inhibitor, a thermoelectric material or a thermoelectric composite is used as a reaction inhibitor promoter (promoting the reaction inhibition effect provided by the reaction inhibitor, i.e. further decrease the reaction rate) or a reaction inhibitor inhibitor (inhibiting the reaction inhibition effect provided by the reaction inhibitor, i.e. lessen the decrease in the reaction rate) for the chemical reaction, wherein the thermoelectric material or the thermoelectric composite alters an activation energy of the separate reaction inhibitor to promote or inhibit the catalysis.

[86] It is understood that according to an embodiment of the invention wherein a separate catalyst or a separate reaction inhibitor is used, a thermoelectric material or a thermoelectric composite can be used as at least one of a catalyst, a reaction inhibitor, a catalyst promoter, a catalyst inhibitor, a reaction inhibitor promoter, and/or a reaction inhibitor inhibitor as appropriate, wherein the thermoelectric material or the thermoelectric composite alters an activation energy on a reaction surface to promote and/or inhibit the catalysis.

[87] Figure 2A shows a flow chart of a method of changing a rate of a chemical reaction according to an embodiment of the invention, wherein the method uses the catalyst system of any one of Figure 1A to 1C to change the rate of the chemical reaction. The method comprises producing a spatial temperature difference between a first portion and a second portion of a thermoelectric material S130, wherein the first portion is nearer to a reaction surface than the second portion, whereby the thermoelectric material alters an activation energy on the reaction surface to change the rate of the chemical reaction.

[88] According to an embodiment, the method further comprises a step of preparing a reaction medium comprising one or more reactants, and the catalyst system of any one of Figure 1A to 1C for the chemical reaction S110.

[89] It is understood that the method may also further comprise the steps of: receiving an input from a user or a computer system for managing the chemical reaction; and controlling to produce a suitable spatial temperature difference based on the input at the spatial temperature difference production step S110.

[90] It is also understood that the method may also further comprise the steps of: forming a thermoelectric composite; and producing the spatial temperature difference between a first portion and a second portion of a thermoelectric composite, wherein the first portion is nearer to the reaction surface than the second portion, whereby the thermoelectric composite alters the activation energy on the reaction surface S130 to change the rate of the chemical reaction.

[91] Figure 2B shows a partial flow chart method of a spatial temperature difference production step S110 of the method of Figure 2A according to an embodiment of the invention, wherein the spatial temperature difference production step S110 comprises a chemical reaction type determination step S210 and a thermoelectric material determination step S230, S250 so that a suitable spatial temperature difference for the desired increase or increase in the rate of the chemical reaction can be determined, and applied accordingly.

[92] At the chemical reaction type determination step S210, whether the chemical reaction is an electrophobic reaction or an electrophilic reaction is determined.

[93] If the chemical reaction is determined to be an electrophobic reaction, the method proceeds to the thermoelectric material determination step S230.

[94] If the chemical reaction is determined to be an electrophilic reaction, the method proceeds to the thermoelectric material determination step S250.

[95] At the thermoelectric material determination step S230, whether the thermoelectric material is a p-type or an n-type is determined.

[96] If the thermoelectric material is determined to be a p-type, a lower temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a higher temperature than the temperature of the first portion to decrease the rate of the chemical reaction, is applied to the second portion of the thermoelectric material at a temperature application step S232.

[97] If the thermoelectric material is determined to be a n-type, a higher temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a lower temperature than the temperature of the first portion to decrease the rate of the chemical reaction, is applied to the second portion of the thermoelectric material at a temperature application step S234.

[98] At the thermoelectric material determination step S250, whether the thermoelectric material is a p-type or an n-type is determined.

[99] If the thermoelectric material is determined to be a p-type, a higher temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a lower temperature than the temperature of the first portion to decrease the rate of the chemical reaction, is applied to the second portion of the thermoelectric material at a temperature application step S252.

[100] If the thermoelectric material is determined to be a n-type, a lower temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a higher temperature than the temperature of the first portion to decrease the rate of the chemical reaction, is applied to the second portion of the thermoelectric material at a temperature application step S254.

[101] It is understood that the order of the chemical reaction type determination step and the thermoelectric material determination step can be changed, or can be performed simultaneously, according to another embodiment of the invention. For example, according to another embodiment the chemical reaction type determination step is performed at S230, S250 in Figure 2B and the thermoelectric material determination step is performed at S210 in Figure 2B.

[102] According to an embodiment, the chemical reaction is either an oxidation reaction or a reduction reaction.

[103] According to an embodiment, the chemical reaction is one of a reduction of NO_X gases (for example, NO_X gases from an automobile exhaust), a hydrocarbon C_nH_m reforming reaction, a reverse water-gas shift reaction, or a low temperature fuel cell reaction.

[104] According to an embodiment, the chemical reaction is one of a reduction of NO_X gases (for example, NO_X gases from an automobile exhaust), a hydrocarbon C_nH_m reforming reaction, a reverse-water gas reaction, or a low temperature fuel cell reaction, and a catalyst comprising at least one of a noble metal or oxide based catalyst is used therewith for the catalysis.

[105] According to an embodiment, the chemical reaction is an ethylene C₂H₄ oxidation.

[106] According to an embodiment of the present invention, there is provided a process for producing a chemical product, the process comprising contacting at least two reactants with a catalyst comprising a thermoelectric material, wherein the thermoelectric material alters an activation energy on a reaction surface. According to an embodiment, the process also comprises performing steps of the method described in relation to either Figure 2A or Figure 2B.

[107] For the avoidance of any doubt, each and every feature described hereinbefore is equally applicable to any or all of the various aspects of the present invention as set out herein, unless such features are incompatible with the particular aspect or are mutually exclusive.

[108] The following examples further illustrate the present invention. There examples are to be viewed as being illustrative specific materials falling within the broader disclosure presented above and are not to be viewed as limiting the broader disclosure in any way.

[109] Examples - the thermoelectric material (TE) [110] Most TE materials used today are heavy-metal-based, for example Bi2Te3 and PbTe due to their high figure of merit (ZT). However, some of these TE materials are not best suited for a high temperature, large scale applications because of their low melting or decomposition or oxidation temperatures, potentially harmful effect on the environment, and scarcity of constituent elements. Further, some heavy metals can cause catalyst poisoning to some catalysts.

[111] Oxide-based TE materials, such as p-type layered BiCuSeO, Cobalt oxide AxCoO2 (A=Na, Ca, Sr), and n-type CaMnO3 and SrTiO3 based perovskites overcome these disadvantages. For example, BiCuSeO oxyselenides (BCSO) TE material is able to produce the thermoelectric effect up to temperatures of over 900 K, and possesses an extra-ordinarily low intrinsic thermal conductivity of less than 0.5 Wm¹K¹. This means a high spatial temperature difference can be built up across two portions of this material more readily, which leads to more robust and efficient catalyst system, method and/or process according to an embodiment of the invention. Also, BCSO possesses a high Seebeck coefficient of up to 500 pVK¹ at room temperature, which remains to be greater than 300 pVK¹ even at relatively high temperatures of around 773K, and suffers no decomposition below 773 K. Therefore, BCSO is a suitable thermoelectric material for an example of the invention.

[112] Examples - the chemical reaction and the catalyst [113] An ethylene (C₂H₄) oxidation converts ethylene (C₂H₄) into carbon dioxide (CO₂) and water (H₂0). Platinum (Pt) can be used as a catalyst for the ethylene oxidation reaction. The ethylene oxidation finds many practical applications, for example in reduction of residue hydrocarbons from a catalyst converter of an automobile. Therefore, ethylene (C₂H₄) oxidation is a suitable chemical reaction for an example of the invention, and Platinum (Pt) is also a suitable catalyst for the ethylene (C₂H₄) oxidation when a catalyst is used in an example of the invention.

[114] Examples [115] - TE 330 : BiCuSeO oxyselenides (BCSO) [116] - Solid electrolyte : YSZ [117] - Catalyst 380 (the noble metal or oxide based catalyst) : Pt [118] - Chemical reaction : ethylene (C₂H₄) oxidation [119] - Reactants 390 : ethylene (C₂H₄) and oxygen [120] - Products 395 : carbon dioxide (CO₂) and water (H₂O) [121] - How a rate of the chemical reaction is measured : by measuring a rate of carbon dioxide production and ethylene conversion from the outlet partial pressures of C₂H₄ and CO₂ [122] The thermoelectric material BiCuSeO (BCSO) [123] The thermoelectric material BiCuSeO (BCSO) was synthesized by a solid-state reaction using a B₂O₃ flux method in air. Stoichiometric amounts of Bi₂O₃ (Alfa Aesar, 99.9%), Bi (Alfa Aesar, 99.999%), Cu (Alfa Aesar, 99.99%), and Se (Alfa Aesar, 99.999%) powders were weighted, homogeneously ground in an agate molar, and then transferred into an alumina crucible. The latter was covered by B₂O₃ powders and compacted; the crucible was placed into a chamber furnace, heated at 873 K for 10h and finally cooled down to room temperature. During the flux synthesis, B₂O₃ melts and serves as a liquid-seal on the top of the crucible because it is lighter. The calcined sample was densified using a cold press system (150 MPa) to form a dense BiCuSeO (BCSO) pellet of 20 mm in diameter and 2-3 mm in thickness. The pellet was then sintered at 923 K for 10h under an argon (Ar) atmosphere.

[124] To establish an electrical contact with opposing surfaces (one on a first portion and another on a second portion) of BCSO, a solid electrolyte based on dense YSZ pellets were prepared from 8 mol% YSZ (CoorsTeK powder, 99.99%; average grain size, 0.5-0.7 pm), and sintered at 1773 K for 2 hours in air (density higher than 98%). YSZ discs were 16 mm in diameter and 1.5-2.0 mm thick.

[125] Pt catalysts preparation [126] Pt films and nanoparticles were prepared by a magnetron sputtering using pure Pt (99.99%) as a target. The discharge current was maintained at a constant value of 0.2 A. Prior to achieving the desired thickness, deposition rates at different locations were determined, which was then used as a guide for determining the deposition time. The deposition thickness was also measured using profilometer (Dektak XT, Broker).

[127] For samples Pt/BCSO B and Pt/BCSO A, Pt catalyst of 80 nm thin film (Pt/BCSO B and A - with a continuous thin film of Pt deposited on a BCSO surface) and nanoparticles with Pt nominal thickness 15 nm (Pt(NP1)/BCSO - with sporadically distributed spots of Pt nanoparticles deposited on a BCSO surface to form average thin film thickness of 15 nm, which is not a continuous film, i.e. a rough surface of Pt nanoparticles sporadically distributed on the BCSO surface) were deposited by a DC magnetron sputtering on one side of a pellet in inert (Ar) atmosphere at room temperature. For another Pt nanoparticle sample (Pt(NP2)/BCSO with Pt nanoparticles embedded in BCSO as shown in Figure 1C), BCSO powders were mixed with Pt containing solution H₂PtCI₆ (Sigma-Aldrich, 8wt% in water) using ultrasonic at 313 K for 60 min. Then, the excess water was evaporated overnight at 383 K. Finally, a pellet was formed under the same conditions as those for BCSO and then calcined at 823 K for 1 hour under argon atmosphere. So for these samples, BCSO acts as a catalyst support for affixing a catalyst.

[128] A chemical reactor [129] Referring to Figure 3A, a chemical reactor 400 according to an embodiment of the invention is shown. The chemical reactor 400 is a single chamber reactor with sensors for detecting its chemical reaction performance so that the chemical reaction can be managed by controlling a rate thereof. The chemical reactor 400 enables use of a thermoelectric effect with a catalytic chemical reaction. The chemical reactor 400 comprises a chemical reaction chamber 450 where the catalytic chemical reaction occurs. The chemical reaction chamber 450 is housed inside a stainless steel cylinder of 65 cm³ volume, capped with a cover plate and tightened with O-ring and screws.

[130] A heat exchanger 310 comprising a heat sink such as a cooling chamber with an inlet and an outlet for circulating a relatively (in relation to a temperature inside the reaction chamber 450) cold water is provided so that a second portion of a thermoelectric material 330 away from the reaction chamber 450 can be cooled. According to an embodiment, the heat exchanger 310 also comprises a heat source such as a hotplate 470 arranged to heat the reaction chamber 450 so that the temperature inside the reaction chamber 450 can be increased by the heat source, whereby a first portion of the thermoelectric material 330 near the reaction chamber 450 can be heated.

[131] It is understood that the heat exchanger 310 may be arranged in a number of different ways as long as a spatial temperature difference can be created across the thermoelectric material 330 by the heat exchanger 310. It is also understood that according to an embodiment the heat exchanger 310 may comprise a single component to perform both a heat source and a heat sink function, for example a peltier device.

[132] Gold wires (Agar Scientific, 0.2 mm diameter) are used as electrodes W, C, R and temperatures (both second and first portions) were measured with a K-type thermocouples T_h(temperature at a hotter side). T_c (temperature at a cooler side) placed on surfaces thereof. The Gold was selected because of its negligible catalytic activity in ethylene oxidation reactions. The electric potentials were recorded according to the procedure generally used in conventional three-electrode electrochemical cells using a potentiostat-galvanostat (VersaStat 3F, Princeton Applied Research). A catalyst potential or Seebeck voltage was measured between the working electrode (Pt) and the reference electrode (Au).

[133] The thermoelectric material 330 is arranged so that the second portion is nearer to, and/or is exposed to the heat exchanger 310, and a first portion is nearer to the chemical reaction chamber. A sample holder is arranged to hold the thermoelectric material 330 so that a reaction surface 350 thereof is exposed in the chemical reaction chamber. Reactants 390 are introduced via a gas inlet of the chemical reaction chamber so that the reactants 390 can engage the reaction surface 350, and products 395 of the chemical reaction after the engagement are output via a gas outlet of the chemical reaction chamber. According to an embodiment, the sample holder is arranged so that a sample pellet comprising the thermoelectric material 330 is placed into a specific glass ceramic holder (MACOR - the sample holder), and be detachably attached to a cooling surface of the cover plate, whereby the cover plate can be arranged to be in contact with the second portion of the thermoelectric material 330 to cool the second portion.

[134] It is understood that the chemical reactor according to an embodiment of the invention can have more than one chamber but for an illustrative purpose only a single chamber reactor is described herein.

[135] Measuring rates of the ethylene oxidation reaction [136] Figure 3B shows the chemical reactor 400 of Figure 3A being used for an ethylene oxidation reaction characterisation according to an embodiment of the invention.

[137] According to an embodiment, ethylene oxidation reaction is carried out under atmospheric pressure (101.3 KPa). A set of three gas mass flow controllers (MFCs) is used to control the gas composition and flowrate. The reactants 390 (C₂H₄, O₂) and products 395 (CO₂) were continuously monitored using an online Gas Chromatography (GC8340, CE instruments, TCD detector, packed column with Porapak Q) and an on-line infrared (G150 CO₂, Gem Scientific) analyser to quantify the concentrations of C₂H₄, O₂ and CO₂. The carbon mass balance for ethylene oxidation measurements are within 6%.

[138] According to an embodiment, the reactants are BOC certified standards of O₂ (99.996%) supplied as a 20% mixture in He (99.996%), 4% C₂H₄ mixture in He (99.996%), and the gases are diluted using helium gas (BOC, 99.996%) to obtain desired concentrations.

[139] The catalytic properties of all example samples are tested at an overall flow rate of 200 ml_ min¹ and a reactive mixture is Pc₂h₄= 0.189 KPa and P₀₂ = 3.01 KPa using He as carrier. For a structural stabilization of Pt, Pt film is pre-treated in 5% H₂/Ar at 603 K for 2 h in order to reduce platinum. Then, the example sample is cooled down to room temperature under a flow of helium. And then, the example sample is pre-treated at 723 K for 4h under the reaction mixture of 3.01 KPa O₂ and 0.189 KPa C₂H₄, and total gas flow rate of 200 ml_ min'¹. Finally, the temperature is decreased to room temperature for the catalytic reaction activity measurements.

[140] Each ethylene oxidation rate measurement is repeated for three cycles to test the stability and reproducibility. At each temperature, the catalyst is allowed to reach a steady state for 2030 min and then three gas injections for GC measurement are performed. The average of three GC measurements at each temperature is reported herein.

[141] Rates of the reactions are evaluated using a rate of carbon dioxide production and ethylene conversion from the gas outlet’s outlet partial pressures of C₂H₄ and CO₂ according to a stoichiometry of the ethylene oxidation reaction: C₂H₄ + 3O₂ A 2CO₂ + 2H₂O. The carbon dioxide partial pressure at the Gas outlet was converted to an area specific molar flow rate in n mol s¹ by using the following equation: rC02 = ^2400^βΓ ^χ//^χ1θ^θ , where f, is the volumetric flow rate at the outlet of the reactor in ml min¹. The rate of consumption of O (r0) is simply: ^ro = 3xr_CO2.

[142] The ethylene conversion into CO₂ was defined as:

Pc 0 [143] % C₂H₄ Conversion = -x 100, where Pco₂ and P_CzH4 are the partial pressure of

CO₂ and C₂H₄ in the outlet, respectively.

[144] Surface charge density at surfaces [145] For a p-type thermoelectric (TE) disc sample of 20 mm in diameter and thickness L (2 mm) with its colder side at T_c, its hotter side at T_h, and Seebeck coefficient S, the total Seebeck voltage is V_o = -S(T_h-T_c). If a z-axis is along the thickness direction, with origin at the colder side, at any point z within the TE material, V(z) = V_oz/L, assuming the spatial temperature gradient within the TE material is uniform. From Poisson’s equation, we know the free charge density within the TE material is zero, and the electrical field E = V_o/L is a constant. The surface charge density is σ = ε_οεΕ =ε_οεν_ο/Ι_, where ε₀ is the dielectric constant of vacuum, and ε the relative permittivity of the TE material. So at a hot surface (a surface on the hotter side), the net charge density is a_h = -s_osS(T_h-T_c)/L, and at a cold surface (a surface on the colder side), the net charge density is c_c = e_oeS(T_h-T_c)/L. The above equations are valid for n-type TE materials as well, only that S is negative.

[146] Measurements from the example samples [147] Figures 4-6B show a thermoelectric material (BCSO) being used as a promoter or an inhibitor of a catalytic ethylene oxidation when Pt is used as a catalyst.

[148] Figure 4 shows a comparison of ethylene oxidation reaction rates of a catalyst system according to an embodiment of the invention comprising a Pt thin film on a thermoelectric material BCSO (Pt/BCSO B), and a 80 nm Pt thin film on a Y₂O₃-stabilized ZrO₂ (YSZ) solid electrolyte (Pt/YSZ). The comparison shows the ethylene oxidation reaction rate and its conversion to CO₂ and H₂O as functions of a temperature.

[149] For Pt/YSZ, the rate was 9.6 n Mol (O)/s ((0) is omitted hereinafter) and the conversion was 0.37% at 603 K, and reached 11.7 n Mol/s and 0.68% at 673 K, respectively. For Pt/BCSO B, the rate was 1202 n Mol/s and the conversion was 100% at 648 K. This rate of Pt/BCSO B is 102 times of the rate of Pt/YSZ at 673 K. Considering that a reaction surface of Pt/YSZ was very rough but a reaction surface of Pt/BCSO B was much smoother, the total Pt reaction surface area for Pt/BCSO B is likely to be smaller than for Pt/YSZ so the catalytic activity per surface area for Pt/BCSO B may be increased by an even higher magnitude than the rate increase.

[150] Figure 5 shows ethylene oxidation reaction rates of a catalyst system according to an embodiment of the invention under a normal temperature gradient (NTG) condition and under a reduced temperature gradient (RTG) condition. Sample Pt/BCSO A, which is made in accordance with the same embodiment as Pt/BCSO B, is used for the measurements. The ethylene oxidation rates are shown as a function of temperature.

[151] The NTG condition is when the cover plate is in contact with a cold surface or a back surface (i.e. a second portion) of Pt/BCSO A to generate a higher Seebeck voltage, and the RTG condition is when the cover plate is not in contact with the cold surface or the back surface of Pt/BCSO A so that less thermoelectric effect is generated. This is because a much higher temperature difference can be created across the thickness of Pt/BCSO A (hence across BCSO of Pt/BCSO A as well) under NTG than under RTG condition. As the reaction chamber 450 is placed on a hot-plate 470, and its cover plate is water cooled by the heat exchanger 310, a large temperature gradient can exist between opposite surfaces of the reaction chamber 450.

[152j Under a normal operation using the NTG (Pt/BCSO NTG), a back surface (on a second portion) of Pt/BCSO A was in contact with the water cooled stainless steel cover plate (with a mica sheet in between for an electrical insulation) so that the temperature of the back surface was never higher than 373 K. When a hot surface (on a first portion) of Pt/BCSO A was at 666 K, its back surface was at 349 K, so a temperature difference of 317 K existed across the thickness of Pt/BCSO A, which produced a Seebeck voltage of -71 mV. The corresponding ethylene oxidation rate and conversion rate were 2257.6 n Mol/s and 99.9% respectively.

[153] Under a reduced temperature operation using the RTG (Pt/BCSO RTG), the back surface was not in physical contact with the water cooled cover plate, therefore the temperature difference, and also the Seebeck voltage, between the back surface and the hot surface was much smaller. The measured Seebeck voltage was -8.6 mV when the hot surface was at 713K, and the corresponding ethylene oxidation rate and conversion rate were 7.1 n Mol/s and 1.2%, respectively. This means the ethylene oxidation rate under NTG condition (with the hot surface at 666 K) was 318 times of the rate for under RTG condition (with the hot surface at 713 K).

[154] Figure 6A shows a measured voltage between a hot and cold surface of a Pt/BCSO (Pt/BCSO B) disc sample made according to an embodiment of the invention as a function of the temperature difference between the two surfaces. The negative voltage means the potential was lower at the hot surface than at the cold surface. The voltage is zero when both hot and cold surfaces are at the same temperature (i.e. the temperature difference is zero), for example both at room temperature, and it increased linearly with the temperature difference ΔΤ when ΔΤ<150 K (when the hot and cold surface temperature was T_h < 473 K and T_c < 323 K respectively). The gradient of this linear relationship, i.e. the magnitude of the Seebeck coefficient, was 410 pV/K, which is within a known range of BCSO’s Seebeck coefficient. This indicates that the measured voltage was mainly, if not entirely, due to the thermoelectric effect of BCSO, and other factors, such as potential difference in gas concentrations between the hot and cold surface, did not significantly affect the measured voltage. When ΔΤ> 150 K, the measured voltage still increased with ΔΤ but at a reduced rate. This is as expected since a Seebeck coefficient of a thermoelectric material tends to be temperature dependent.

[155] Figure 6B shows a plot of Ln(r) vs -eV/k_bT for Pt/BCSO B disc sample of Figure 6A. Within each of region I and II, a linear relationship exists between Ln(r) and -eV/k_bT.

[156] When no thermoelectric effect is present, a rate of a chemical reaction with temperature usually follows an Arrhenius law:

[157] r = k exp(-E_a/k_bT)......(1) [158] , where E_a is the activation energy for the reaction, k_b the Boltzmann constant and k a constant.

[159] Ln(r) v.s 1/T for Pt/YSZ shows a linear relationship, demonstrating that the ethylene oxidation using Pt as a catalyst obeys the Arrhenius law.

[160] In a Ln(r) v.s -eV/k_bT plot for Pt/BCSO B sample of Figure 6A, -e is the charge of an electron and the introduction of e/k_b makes the term -eV/k_bT dimensionless. Three distinctive regions can be observed: (a) region I at a low -eV/k_bT from 0.289 to 0.863, a large gradient (4.48) linear relationship; (b) at region II a smaller gradient (2.5) linear relationship with -eV/k_bT from 1 to 1.56; and (c) at region III with a nearly vertical line when -eV/k_bT is above 1.56.

[161] This behaviour can be explained by considering what happens when a Pt/BCSO B sample is heated up as discussed in relation to Figure 7 below.

[162] Figure 7 shows a schematic of an energy band and Fermi level for a p-type and an n-type thermoelectric (TE) material with a metal particle supported on the TE material on a relatively hotter portion according to an embodiment of the invention. T_c and T_h are the temperatures at the colder and hotter portion (for example, the cold and hot surface of Pt/BCSO B) respectively. The arrow heads point the electric field direction within the TE material.

[163] BCSO of Pt/BCSO B is a p-type TE material, with holes at the hotter portion diffusing into the colder portion upon heating, whereby an internal electrical field is generated. Once an equilibrium is reached, the Fermi level (also called electrochemical potential) at the hotter portion s_{F h} is higher than at the colder portion s_Fc, and:

[164] s_F,_h - s_F,_c - -eV...... (2).

[165] If a metal particle is in contact with the TE materials at the hot surface, its Fermi level ε_Ρ:Γγιmust be the same as the Fermi level of the TE supporting material, i.e., ε_Ρ:Γγι = ε_Ρ>

[166] For an n-type TE materials, the Fermi level at the colder portion is higher than at the hotter portion, but the relationship s_{F h} - s_Fc = -eV is still valid, as V is positive for an n-type TE material and negative for a p-type TE material between the hot and cold portions. Referring to Figure 6B, in regions I and II, Ln(r) is linear with -eV/k_bT, i.e.

[167] Ln(r/r₀) =-aeV/k_bT ......(3).

[168] Here a is a dimensionless positive constant, r₀ is the reaction rate when V equals zero, i.e. when T_c=T_h =T so that r₀ = k_oexp(-E_a/k_bT), wherein k₀ is a constant.

[169] As chemical reaction rate usually follows the equation (1), equation (3) indicates that the activation energy is reduced by -aeV/k_bT. -eV/k_bT can be regarded as a ratio of thermoelectric energy to thermal energy of an electron. At room temperature, the thermal energy k_bT is about 25 meV and at 600 K it is ~50 meV. This means if a TE voltage of -100 mV is generated at 600K, the difference between the reaction rate with the TE voltage and the reaction rate without the TE voltage will be a factor of exp(2a). Taking data points from region I and II, with (-V,T) values being (28.5 mV, 383 K) in region I and (69.3 mV, 506 K) in region II, the Ln(r/r₀) values are 3.87 and 3.97 respectively, meaning that a rate change factor between the reaction rate with a TE voltage and without a TE voltage was 47.9 at 383 K, and 53 at 506 K. This is a thermoelectric energy induced catalytic reaction rate increase or a thermoelectric promotion of catalysis (TEPOC).

[170] The influence of this TEPOC (or the thermoelectric voltage) on a chemical reaction rate can also be observed from measurements made from other samples such as Pt(NP2)/BCSO.

[171] Pt(NP2)/BCSO is prepared by mixing BCSO powders with Pt containing solvent H₂PtCI₆to form a green ceramic, which is then sintered at 823 K for 1h before being used as a catalyst for the ethylene oxidation. So the not fully sintered BCSO of Pt(NP2)/BCSO continues its sintering process while the measurements are taken (see Figure 8A’s hot surface, i.e. hot side, discussed below). This means Pt(NP2)/BCSO experiences changes in the thermoelectric properties thereof inside the reaction chamber as the sintering process progresses, and the effect of this TE properties change, such as the change in the Seebeck coefficient and/or the electric conductivity whilst the chamber temperature is kept constant, on the reaction rate can be monitored.

[172] Figure 8A shows characteristics of a catalyst system according to an embodiment of the invention which comprises a thermoelectric composite comprising Pt(NP2)/BCSO, after having used the thermoelectric composite in an ethylene oxidation. The characteristics of the cold side (i.e. the cold surface) shown on top, and the hot side (i.e. the hot surface) shown at the bottom show significant difference. During the ethylene oxidation measurement, the cold side’s temperature never exceeded 350 K because it was being cooled by the heat exchanger 310 so no further sintering took place on the cold side. This meant the cold side retained the structure of the as-prepared green ceramics, which contains some second phases. However, during the ethylene oxidation measurement, the hot side did experience temperature of over 773 K for more than 1 hr, therefore underwent further sintering in the reaction chamber. This meant at least some second phases were transformed into the BiCuSeO structure, which lead to an increase in the Seebeck coefficient for Pt(NP2)/BCSO during the catalytic reaction measurement as shown in Table I below.

[173] Table I summaries the temperatures, the measured Seebeck voltages, and the reaction rates and conversions at different times while the temperatures of the hot surface (T_h) and the cold surface (T_c) were kept at a constant value of 339 and 705 K respectively. An increase in Seebeck voltage was observed, which is an increase due to a change in the Seebeck coefficient as the Pt(NP2)/BCSO undergoes the sintering process during the ethylene oxidation.

[174] Table I: Summary of Seebeck voltages (V), reaction rates (r_C02) and ethylene conversion (C₂H₄Conv %) at a constant temperature T_h = 705 K and T_c = 339 K for Pt(NP2)/BCSO.

[175]

V (-mV)	104	105	106	107	107.5
r_C02 (nMol (O)/s)	123.7	157.0	204.3	249.8	281.2
C₂H₄ Conv %	0.2407	0.2431	0.2454	0.2477	0.2488

[176] The change in the Seebeck coefficient leads to the change of the Seebeck voltage, which in turn resulted in the increased ethylene conversion (C₂H₄Conv %), i.e. catalytic reaction rate.

[177] Figure 8B shows how a change in the Seebeck voltage of Pt(NP2)/BCSO of Figure 8A affects the ethylene oxidation rate. As discussed above, this change in the measured Seebeck voltage is due to the change in the Seeback coefficient as Pt(NP2)/BCSO undergoes further sintering process during the ethylene oxidation. A very good fit linear dependence of Ln(r) on eV/k_bT with a gradient of 8.7 can be observed, demonstrating that the change of Seebeck voltage alone can increase the reaction rate significantly.

[178] Combining equation (3) and the definition of Seebeck coefficient S gives [179] Ln(r/r₀) = a-e-S-AT/k_bT= a-e-S· (T_h-T_c)/k_bT_h, at the hot side (hot surface)......(4) [180] Ln(r/r₀) = a-e-S-AT/k_bT= a-e-S-(T_c-T_h)/k_bT_c, at the cold side (cold surface)......(5) [181] When S is not a constant, S-(T_h-T_c) should be replaced by f^^hS(T)dT. Equation (5) indicates that if the chemical reaction is taking place at the cold surface of the catalyst (e.g. Pt or metal particles) supported on a p-type TE material, the reaction rate will be smaller than that when V is zero so the TE effect acts as a reaction inhibitor. Equations (4-5) are also applicable to n-type TE materials. So when an n-type TE material is used, a reaction at the cold surface will be promoted, and a reaction at the hot surface will be inhibited since S is negative for a ntype TE material.

[182] Based on Equations (4-5), there are four different methods for using a TE material to affect a catalytic reaction rate, i.e. a promotion of the reaction at the hot surface of a p-type and also at the cold surface of an n-type, and an inhibition at the cold surface of a p-type and at the hot surface of an n-type.

[183] It is understood that a “cold” surface (or a “cold” side) and a “hot” surface (or a “hot” side) are defined in relation to one another. So a “cold” side can be obtained by heating up the opposite (hot) side, and the “hot” side can be obtained by cooling the opposite (cold) side, for example using liquid nitrogen. For example, to promote a reaction on a reaction surface (or a reaction side) at room temperature (298 K), one can either heat up the other side of an n-type TE material to a temperature higher than the room temperature (e.g. 498 K), or cool down the other side of a p-type TE material to a temperature lower than the room temperature (e.g. 98 K) to create a spatial temperature difference (e.g. 200 K) across the TE material. In this way, any TE material, no matter what their operational temperature range for the thermoelectric effect is, can potentially be used for the thermoelectric promotion of catalysis. As catalytic reactivity at around room temperature can be promoted, this raises the possibility that some biochemical reactions can also be promoted using thermoelectric materials if the reaction is a surface process. Equations (4-5) also suggest it is possible to tune the selectivity of a process by using the p-type and n-type promotions and inhibitions together so that some reactions are promoted and some other reactions are inhibited.

[184] Based on the empirical equations (3) to (5), mechanisms for the observed reaction promotion and/or inhibition phenomena can be provided.

[185] For an ethylene oxidation to happen on a catalyst surface (i.e. a reaction surface), C₂H₄and O₂ have to be adsorbed onto the catalyst surface first. Then, the donor C₂H₄ gives up electron(s) to the catalyst, and the acceptor O receives electron(s) from the catalyst. After that, the adsorbents combine to form CO₂ and H₂O molecules, before being desorbed into gases. At any particular temperature, any one of the steps of this process may become a reaction rate limiting step, and this will determine the overall reaction rate. When the temperature changes, another step may become the rate limiting step, therefore change the rate-limiting mechanism for the whole reaction.

[186] If an internal electrical field within a TE material increases the electrochemical potential (or Fermi level) of electrons in the catalyst, this reduces the activation energy for the steps carried out at the catalyst surface. This can then lead to an exponential increase in the reaction rate. If however, the TE effect reduces the electrochemical potential (or Fermi level) of electrons, the reaction rate is decreased since the chemical reaction will be inhibited.

[187] For the case discussed above in relation to Figure 6B, the constant a is bigger than 1, the reduction in activation energy -aeV is larger than the energy increase -eV due to the TE effect. Between the region I and II, a different mechanism was the rate limiting step, hence the rate gradient a was different for these two regions. The first two data points in region I in Figure 6B do not fit well within the linear relationship, most likely due to the influence of the term exp(E_a/k_bT) since when T is small, the term exp(-E_a/k_bT) cannot be approximated as a constant.

[188] Regarding Equation (4), in region III when the hot surface (i.e. the reaction surface) temperature T_h was at 538 K (the corresponding cold surface temperature T_c was 329 K), the TE voltage was -72.38 mV, the reaction rate was 65 n Mol/s, C₂H₄ conversion was 3.85%, and -eV/k_bT was 1.56. When T_h was increased to 648 K, the corresponding values were T_c =341 K, V= -92.63 mV, r =1202.7 n Mol/s, 100% conversion and -eV/k_bT = 1.66. Although a linear relationship between Ln(r) and -eV/k_bT is not a perfect fit for this region, it was still sufficient as an approximation for the behaviour. In fact, this approximation is a conservative one since it can be seen from Figure 6B that a gradient from this approximate linear relationship is in fact smaller than a differential for the data points in region III as -eV/k_bT increase. The gradient of this approximately linear relationship was 28.5 (see region III in Figure 6B), which suggests that the factor of the reaction rate increases between with a TE voltage and without a TE voltage can be very high in the region III. So the promoted rate of the reaction can be very high for Pt/BCSO B.

[189] A relatively small TE energy, often less than 100 meV, can change a chemical reaction rate so by a great magnitude. Although 100 meV is not a great amount of energy, it is still greater than the thermal energy of an electron at any temperature below 1200 K, and twice as large as the thermal energy at 600 K. The fact that the value of the dimensionless constant a is larger than 1, sometimes reaching a few tens, means that the thermoelectric effect is a very efficient way of changing the activation energy involved in a catalytic chemical process. Figures 9A and 9B show an ethylene oxidation rate and a Ln(r) vs -eV/k_bT plot as functions of temperature for a bare BCSO disc sample (BCSO B) prepared according to an embodiment of the invention, illustrating use of the thermoelectric material itself, namely BCSO B, as a catalyst or a reaction inhibitor for the ethylene oxidation without Pt present as a catalyst.

[190] Some similarities can be observed between the Ln(r ) vs -eV/k_bT plots of Pt/BCSO B (see Figure 6B) and bare BCSO B (see Figure 9B). Specifically, for -eV/k_bT smaller than 1.86, there was a good linear relationship between Ln(r) and -eV/k_bT, but for -eV/k_bT greater than 1.86, Ln(r) increased very rapidly for a very small change in -eV/k_bT. In fact, further increases in T actually lead to slight decrease in -eV/k_bT.

[191] Figures 9A and 9B demonstrate that a very rapid chemical reaction takes place on the reaction surface of the TE material, without any noble metals present as a catalyst. This suggests that as long as there is a large temperature difference, therefore a large Seebeck voltage between the hot and cold surfaces, and the reactants can be adsorbed onto the reaction surface of the TE materials, the TE effect will promote the reaction rate to large values even if the un-promoted (without TE effect) reaction rate without using any catalyst is very small. We call this phenomenon thermoelectrocatalysis (TEC).

[192] For some reactions, the reactant(s) cannot be adsorbed onto the reaction surface of the TE material so catalysts (such as metal particles) may still be required to enable the adsorption of these reactants. However, the TE materials will be able to promote the catalysis of the catalysts (for example, the metal particles) regardless of the size of the catalyst. This is because whatever the particle size of the metal particle, the Fermi level or the electrochemical potential of the metal particle will be the same as that of the TE material supporting the metal particles. Therefore, a thermoelectric energy induced catalytic reaction rate increase or a thermoelectric promotion of catalysis (TEPOC) is applicable to most catalytic chemical reactions and/or surface processes wherein a conductive catalyst is used. It is understood that the chemisorption properties will also be affected as some surface charges will be produced from the TE effect. For TEPOC, small or nano particles will be preferred since this will ensure that the supporting TE material have enough free-moving electrons to change the Fermi level of the catalyst, i.e. the metal particles.

[193] As discussed before, for region III in Figure 6B the plot of Ln(r) vs -eV/k_bT for Pt/BCSO B did not follow linear relationship. However, as shown in Figure 10, a plot of the reaction rate against the measure TE voltage (-V or Seebeck voltage) for Pt/BCSO B showed a good fit for a linear relationship in region III of Figure 10.

[194] Figure 10 shows ethylene oxidation rates for the Pt/BCSO B disc sample prepared according to an embodiment of the invention, wherein the reaction rate r is shown as a function of Seebeck voltage -V for the ethylene oxidation using Pt/BCSO B. In region III, the reaction rate r increases linearly with the Seebeck voltage.

[195] Figure 11 shows a comparison of the ethylene oxidation rates for different embodiments of the invention, showing the ethylene oxidation rate r as a function of -V for different Pt/BCSO and bare BCSO example embodiments. These curves resemble an l-V curve for a semiconductor p-n diode under forward bias, suggesting an intrinsic similarity between how a TE material behaves when experiencing a temperature gradient, and a p-n diode.

[196] Figure 11 also shows the dependence of a reaction rate on the Seebeck voltage for six different samples: two samples (Pt/BCSO A and Pt/BCSO B) of 80 nm Pt film deposited on BCSO by sputtering; two bare BCSO samples (BCSO A and BCSO B); and two samples of Pt nanoparticles deposited on BCSO (Pt(NP1)/BCSO and Pt(NP2)/BCSO), with Pt(NP1)/BCSO prepared by sputtering, and Pt(NP2)/BCSO prepared by mixing Pt solution with BCSO powders.

[197] It can be observed from Figure 11 that all six samples show similar trends, i.e. at the beginning the reaction rate increases slowly with -V (as discussed before in relation to Figure 6B) but after reaching a certain -V value r increases rapidly. This rapid increase in r is linear with increase in -V. In fact, three different samples, BCSO A, Pt/BCSO B, and Pt(NP1)/BCSO all have almost the same rate dependence on the TE voltage, suggesting that at this range of V values, the TE effect dominate the catalytic activity, and the exact nature of the reaction surface, i.e. whether it is on a bare BSCO as in BCSO A or on a catalyst Pt as in Pt/BCSO B, does not matter very much.

[198] As evident from Figures 10 and 11, r vs (-V) trends shown in Figures 10 and 11 resemble an l-V curve for a semiconductor p-n diode in a forward bias. So there is an intrinsic similarity between how a TE material behaves when experiencing a temperature gradient, and a space charge region of a p-n diode. As discussed earlier, the temperature gradient in a p-type TE material BCSO drives holes toward the cold surface, just like a charge carrier concentration difference drives holes in a p-type into the space charge region of the n-type in a p-n diode. On the hot surface of BCSO, there are holes and electrons, and the donor adsorbents will donate electron(s) to hole(s), and acceptor adsorbents will accept electron(s) from the electron sites. This is also similar to a recombination and generation of charge carriers at the boundary of the space charge region in a p-n diode. Assuming a uniform temperature gradient along BCSO thickness, a linear Seebeck voltage with temperature difference and Poisson’s equation imply a uniform electric field between the hot and cold surfaces, and net negative and positive charges (proportional to the Seebeck voltage V over the sample thickness L, i.e. V/L) at the hot and cold surfaces respectively. So another effect of the Seebeck voltage is to change the hole and electron concentrations at the hot and cold surfaces, which changes the rates of electron transfer between the catalyst and the donor adsorbents, and also between the catalyst and the acceptor adsorbents, whereby the chemical reaction rate is altered.

[199] Regarding Equation (5), a bare BCSO was placed directly on a stainless steel floor of the reaction chamber. As the bare BCSO was in direct contact with the hot floor (i.e. on an inside surface of the reaction chamber that is nearest to the hotplate), the hot surface of the BCSO was effectively connected to earth (since the reaction chamber is earthed) so that any surface charges produced by the TE effect are neutralised by the earth. This means no promotion of the catalytic activity at the hot surface, and all the ethylene reaction (and conversion) was due to a reaction carried out on the cold (top) surface of the BCSO. Table II summarises the measured reaction rates and conversion of the ethylene oxidation at different (cold surface) temperatures T_c, together with the corresponding measured Seebeck voltage at the cold surface (relative to the hot surface).

[200] According to Equation (5), the ethylene oxidation rate can be expressed as r = k₀-exp(E_a/k_bT)-exp(-aeV/k_bT ), wherein V is the measured Seebeck voltage at the cold (top) surface and is a positive value.

[201] Table II: Summary of the measured Seebeck voltages, reaction rates and ethylene conversion at different cold surface temperatures T_c for the bare BCSO sample. The calculated reaction rates were obtained by using Equation (5) r = k₀-exp(-E_a/k_bT)-exp(-aeV/k_bT ) with parameters k₀ =7.127 x10⁸, E_a = 9.32 x 10²⁰, and a = 27.8.

[202]

Tc (K)	458	503	538	583	623	658	703	743
V (mv)	7.361	9.508	11.348	13.188	15.335	17.485	20.243	23.923
r CO2 (nMol/s)	1.146	1.363	2.153	4.917	6.559	5.208	3.007	2.219
C2H4 Conv %	0.068	0.081	0.127	0.289	0.384	0.308	0.177	0.131
Calculated rate (n Mols/s)	1.566	2.355	2.786	4.489	4.969	4.707	4.438	2.487

[203] Figure 12 shows an ethylene oxidation reaction rate and conversion as a function of temperature for bare BCSO TE materials when the BCSO is placed directly onto the hot floor of the reaction chamber. The calculated reaction rate was obtained using the equation r = k₀-exp(E_a/k_bT)-exp(-aeV/k_bT ) with the parameters k₀ =7.127 x10⁸, E_a = 9.32 x 10²° (J), and a = 27.8. The left axis is for the reaction rate and right axis for the conversion.

[204] The reaction rate increased with temperature initially, because initially the Seebeck voltage was very small so that the reaction rate behaved more like exp(-E_a/k_bT). The reaction rate then reached a peak of 6.6 n mol/s at 623K, before starting to decrease with increasing temperature. This is because the Seebeck voltage becomes larger when temperature is increased, and the reaction rate is mainly determined by the term exp(-aeV/k_bT) at high temperatures whilst exp(-E_a/k_bT) is kept approximately constant. Although the temperature of the cold surface was actually very high at above 700 K, the reaction rate was very low. The calculated reaction rate using the above equation with the parameters k₀ =7.127 x10⁸, E_a = 9.32 χ 1O²⁰ (J), and a = 27.8 are also shown in Figure 12 with triangular markers. The agreement between the calculated and the measured ones are good, except for the highest reaction rate point. This is because when such low reaction rates are measured, contributions to the reaction rate from surfaces on sides of the sample cannot be ignored.

[205] All the catalyst systems according to example embodiments, either bare BCSO, Pt 80 nm film on BCSO, or Pt nano particles on BCSO, did not show any activity degradation during the measurements, suggesting a stable and sustained catalytic activity of the example embodiments in ethylene oxidation reactions at temperatures up to 773 K for up to10 hrs. X-ray Diffraction (XRD) image did not show any change after the ethylene oxidation reactions were carried out (see Figure 13A).

[206] Figure 13A shows an XRD image with characteristics of a catalyst system according to an embodiment of the invention which comprises a thermoelectric composite comprising a Pt thin film deposited on BiCuSeO (Pt/BCSO), after having used the catalyst system for a prolonged ethylene oxidation reaction. The X-ray diffraction (XRD) patterns confirm the existence of Pt film at the hot side (hot surface or top surface) and no Pt at the cold side (cold surface or bottom surface) for Pt/BCSO A and Pt/BCSO B samples. The cold side (cold surface or bottom surface) temperature never exceeded 353 K. Scanning Electron Microscope (SEM) image of Figure 13B illustrates that the Pt surface was still quite clean after more than 10 hrs of catalytic ethylene oxidation reaction for Pt/BCSO shown in Figure 13A. As expected, the cold side (cold surface) of Pt/BCSO has similar XRD patters as a cold side (cold surface or bottom surface) of a bare BCSO sample after ethylene oxidation reactions were carried out as shown in Figure 14.

[207] Therefore, when BCSO is used as a catalyst, a reaction inhibitor, a catalyst promoter, or a catalyst inhibitor for an ethylene oxidation in a catalyst system, a method, a process, a use of a thermoelectric material, a use of a composite, or a reaction medium for changing a rate of the ethylene oxidation according to an example embodiment of the invention, BCSO can be used without significant degradation in performance for a prolonged period.

[208] When an ethylene oxidation is taking place, the catalytic activities of Pt metal particles supported on the thermoelectric material BiCuSeO were increased by several hundred times through the thermoelectric effect, which resulted in a Seebeck voltage generated by a temperature difference between the reaction surface (of a Pt thin film layer or of BiCuSeO comprising Pt nanoparticles) and a back surface (not comprising Pt) of the BiCuSeO. Also, BiCuSeO by itself, without any Pt metal particles present as a catalyst on the reaction surface, shows a catalytic activity as high as that of Pt catalyst for the ethylene oxidation.

[209] This catalytic activity, or a promotion thereof, of the thermoelectric material BiCuSeO is attributed to its ability to generate surface charges and change an electrochemical potential (or Fermi level) of a reaction surface by the thermoelectric material itself, and also of any metal particles supported on it (for example Pt catalyst). This change in the Fermi level of the catalyst particles (for example Pt), leads to the change of the activation energy for the chemical reaction (for example ethylene oxidation). This controllable catalytic activity (controlled by altering the amount of thermoelectric effect through controlling, for example, the spatial temperature difference created across and/or in the thermoelectric material) means any catalytic chemical reaction can be tuned in-situ, independently from any changes in the conditions within a reaction chamber, for example controlling to achieve a higher reaction rate, or a comparable reaction rate at a lower temperature in the reaction chamber.

[210] For example, the catalytic activity of a catalyst such as Pt metal particles supported on a thermoelectric material BiCuSeO for an ethylene oxidation has been increased by several hundred times through the thermoelectric voltage generated by a temperature difference between a reaction surface and a back surface (a surface away from the reaction surface). Also, BiCuSeO thermoelectric material itself has been shown to be capable of producing a catalytic activity as high as that of Pt for an ethylene oxidation. The thermoelectric materials are also capable of inducing the catalytic effect on a surface of any metal particles supported on the thermoelectric materials. Intrinsic similarities between a thermoelectric material experiencing a temperature gradient and a space charge region of a semiconductor p-n diode at forward bias have been discussed. In particular, the similarities between redox reactions on a reaction surface of the thermoelectric material and recombination and generation of charge carriers in a p-n diode has been discussed, enabling many catalytic chemical reactions to be carried out with a higher reaction rate or at a similar reaction rate but at a lower reaction temperature. Also, use of a thermoelectric material as a catalyst support enables: better selectivity since the reaction rate and/or reaction temperature for the desired reaction rate can be altered using the thermoelectric material; and potential reduction, or even elimination, of the use of expensive noble metals as catalysts in catalytic chemical processes by using a cheaper thermoelectric material. For example, this use of the thermoelectric material also provides for a wider selection or choice of the catalyst through changing (increasing or decreasing) the backside temperature of the catalyst support (i.e. the thermoelectric material). Further, use of the thermoelectric material also enables promotion of a catalytic chemical reaction or a surface process even at around room temperature so that their potential applications can be expanded into other fields, for example to low temperature fuel cells and biochemistry.

[211] Further by controlling the temperature gradient applied to the thermoelectric material, it is possible to control, promote, inhibit, and/or significantly modify, in situ, a catalytic activity of both continuous thin film and highly dispersed (nanoscale particles) catalyst comprising Pt by using the thermoelectric material as a catalyst support. The use of thermoelectric material also enables exploitation of a usually large temperature difference between a chemical reaction chamber (for example due to an exothermic reaction taking place) and the ambient environment.

[212] It is understood that according to an embodiment of the invention, a catalyst system, an apparatus, a method, a process, a reaction medium, and use thereof controls, promotes and/or inhibits a catalysis using a thermoelectric material or a thermoelectric composite. The thermoelectric material or a thermoelectric composite can be used as at least one of a catalyst (a reaction promoter), a reaction inhibitor, a catalyst promoter, a catalyst inhibitor, a reaction inhibitor promoter, and/or a reaction inhibitor inhibitor as appropriate, wherein the thermoelectric material or the thermoelectric composite alters an activation energy on a reaction surface to promote and/or inhibit the catalysis.

[213] Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

[214] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[215] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

[216] Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[217] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A catalyst system comprising a heat exchanger and a thermoelectric material for changing a rate of a chemical reaction, wherein:

the catalyst system comprises a reaction surface, and a first portion of the thermoelectric material is arranged to be nearer to the reaction surface than a second portion of the thermoelectric material; and the heat exchanger is arranged to produce a spatial temperature difference between the first portion and the second portion of the thermoelectric material, whereby the thermoelectric material alters an activation energy on the reaction surface to change the rate of the chemical reaction.

2. The catalyst system of claim 1, wherein the heat exchanger is arranged to apply to the second portion of the thermoelectric material:

if the thermoelectric material is a p-type and the chemical reaction is an electrophobic reaction, a lower temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a higher temperature than the temperature of the first portion to decrease the rate of the chemical reaction;

if the thermoelectric material is a n-type and the chemical reaction is an electrophobic reaction, a higher temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a lower temperature than the temperature of the first portion to decrease the rate of the chemical reaction;

if the thermoelectric material is a p-type and the chemical reaction is an electrophilic reaction, a higher temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a lower temperature than the temperature of the first portion to decrease the rate of the chemical reaction; and if the thermoelectric material is a n-type and the chemical reaction is an electrophilic reaction, a lower temperature than the temperature of the first portion to increase the rate of the chemical reaction, or a higher temperature than the temperature of the first portion to decrease the rate of the chemical reaction.

3. The catalyst system of any preceding claim, wherein the thermoelectric material comprises at least one of intrinsic or doped ZnO, SrTiO3, CaMnO3, layered cobaltites Ca3Co4O9, NaCoO2, SnO2, ln2O3, BiCuSeO oxyselenides, and Bi2Sr2Co2Oy.

4. The catalyst system of claim 3, wherein the thermoelectric material comprises BiCuSeO oxyselenides.

5. The catalyst system of any preceding claim, wherein the thermoelectric material provides at least some of the reaction surface, and the thermoelectric material acts a catalyst or a reaction inhibitor for the chemical reaction.

6. The catalyst system of any preceding claim further comprising a noble metal or oxide based catalyst for providing at least some of the reaction surface, wherein the thermoelectric material is arranged to alter the activation energy of the noble metal or oxide based catalyst to promote or inhibit the catalysis.

7. The catalyst system of claim 6, wherein a thin film layer of the noble metal or oxide based catalyst is deposited on a surface of the first potion of the thermoelectric material.

8. The catalyst system of claim 6 or 7, wherein particulates of the noble metal or oxide based catalyst are dispersed in a matrix of the thermoelectric material.

9. The catalyst system of any one of claims 6 to 8, wherein the noble metal or oxide based catalyst comprises Pt.

10. A method of changing a rate of a chemical reaction using a catalyst system as claimed in any preceding claim, the method comprising: producing a spatial temperature difference between a first portion and a second portion of a thermoelectric material, wherein the first portion is nearer to a reaction surface than the second portion, whereby the thermoelectric material alters an activation energy on the reaction surface to change the rate of the chemical reaction.

11. The method of claim 10, wherein the producing comprises applying to the second portion of the thermoelectric material:

12. The method of claim 10 or 11, wherein the chemical reaction is an oxidation reaction or a reduction reaction.

13. The method of claim 12, wherein the chemical reaction is an ethylene C2H4 oxidation.

14. A process for producing a chemical product, the process comprising contacting at least two reactants with a catalyst or a reaction inhibitor comprising a thermoelectric material, wherein the thermoelectric material alters an activation energy on a reaction surface.

15. The process of claim 14, wherein a first portion of the thermoelectric material is nearer to the reaction surface than a second portion of the thermoelectric material, and the process further comprises exposing the second portion to:

16. The process of claim 14 or 15, wherein the process comprises an oxidation or a reduction reaction.

17. The process of claim 16, wherein the process comprises an ethylene C2H4 oxidation.

18. Use of a thermoelectric material as a catalyst or a reaction inhibitor for a chemical reaction.

19. The use of claim 18, wherein the thermoelectric material comprises BiCuSeO oxyselenides.

20. Use of a thermoelectric material as a catalyst promoter, a catalyst inhibitor, a reaction inhibitor promoter, or a reaction inhibitor inhibitor for a chemical reaction, wherein the thermoelectric material alters an activation energy of a catalyst or a reaction inhibitor to promote or inhibit the catalysis.

21. Use of a composite comprising a noble metal or oxide based catalyst and a thermoelectric material as a catalyst or a reaction inhibitor for a chemical reaction.

22. The use of claim 21, wherein the composite comprises a thin film layer of the noble metal or oxide based catalyst deposited on a surface of the thermoelectric material.

23. The use of claim 21 or 22, wherein the composite comprises particulates of the noble metal or oxide based catalyst dispersed in a matrix of the thermoelectric material.

24. The use of any one of claims 21 to 23, wherein the thermoelectric material comprises BiCuSeO oxyselenides, and the noble metal or oxide based catalyst comprises Pt.

25. Use of a catalyst system as claimed in any one of claims 1 to 9, wherein the chemical reaction is an oxidation reaction or a reduction reaction.

26. A reaction medium comprising one or more reactants, and a catalyst system as claimed in any one of claims 1 to 9.

27. A catalyst system, a method, a process, a use of a thermoelectric material, a use of a composite, or a reaction medium substantially as described herein with reference to accompanying Figures and/or examples.

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