WO2006021009A2

WO2006021009A2 - Nano-catalytic spontaneous ignition and method

Info

Publication number: WO2006021009A2
Application number: PCT/US2005/029712
Authority: WO
Inventors: Zhiyu Hu; Vassil Boiadjiev; Thomas G. Thundat
Original assignee: Ut-Battelle, Llc; University Of Tennessee Research Foundation
Priority date: 2004-08-19
Filing date: 2005-08-19
Publication date: 2006-02-23
Also published as: WO2006021009A3

Abstract

A thermoelectric generator includes a heat source comprising a plurality of nanocatalyst particles, an inlet for fuel and oxygen for contacting the fuel and oxygen with the nanocatalyst particles, and a heat sink. A thermoelectric generating unit having n- and p-type semiconductors is provided between and in thermal contact with the heat source and the heat sink, and an electrical connection is provided between the n-and p-type semiconductors. A method for spontaneously combusting a fuel is also disclosed.

Description

NANO-CATALYTIC SPONTANEOUS IGNITION SYSTEM AND METHOD

BACKGROUND OF THE INVENTION

[0001] Catalysis is a well-known process in which a catalyst aids in the attainment of chemical equilibrium by reducing the free energy of transition-complex formation in a reaction pathway. Catalytic reactions have very wide and vitally important applications in the chemical, petroleum, energy, and automobile industries as well as in many other fields . Normally, however, heterogeneous catalysts are not sufficiently active at room temperature and require external heat for ignition (generally, reaction temperatures are >200 ⁰C) or high pressure. The requirement of preheating or high pressure for a catalytic reaction can be very costly for industrial applications. The search for catalysts that are sufficiently active at room temperature and pressure and that are also able to rapidly convert large amount of reactants has a long history.

[0002] Energy stored in the form of chemical bonds (i.e., gasoline or methanol) is most often converted to thermal energy first rather than directly to other forms of energy such as mechanical or electrical energy. Conventionally, such energy conversion occurs at high temperature, and external ignition generally is required to initiate the combustion or burning process. Modern internal combustion engines (ICEs) were designed on the basis of the Carnot cycle and built by taking advantage of such energy conversion processes in which sophisticated and complex ignition systems govern the performance and quality of the engine. Such ignition processes require an external energy source (such as a battery) and other supporting systems (such as a controller) that greatly increase the complexity of each system and potentially reduce the energy utilization efficiency. On average, the efficiency of an ICE is ~21%. In contrast, an advanced fuel cell converts chemical energy directly to electrical energy and can yield higher energy conversion efficiency (~51%). Nevertheless, temperatures of a few hundred degrees are typically required to carry out such reactions, while heat-related energy losses represent -20% of total energy produced. However, high reaction temperatures may not be an absolutely necessary step in energy conversion processes. New developments in the area of thermoelectric devices have shown promising results.

[0003] Heterogeneous catalytic reactions are reactions that take place on the surface of a solid catalyst during which reactants adsorb and react on the surface catalyst, followed by the reaction products desorbing from the catalyst surface. Flameless combustion is a heterogeneous process in which the fuel reacts with oxygen on a catalyst surface providing carbon-dioxide, water and thermal energy. This process proceeds at much lower temperatures (1270- 1570K) than conventional combustion, and hence less NOx (<5 ppm) is formed. Conventional catalysts consist of ceramic or metal monoliths covered by alumina stabilized with barium oxide and bearing finely dispersed precious metals, particularly Pt and Pd. However, at ambient conditions conventional catalytic reactions require external energy such as heat, ignition or high pressure to initiate that leads to complex system design, energy waste and other drawbacks.

[0004] Recent developments in the methods for synthesis and characterization of nano- particles are opening new prospective approaches for studies of heterogeneous catalytic activity and preparation of novel highly active catalysts. Studies have demonstrated the importance of structure, nano-particle size and morphology on catalytic activity. For the oxidation of hydrogen on platinum nanoparticles, the structure of nanoparticle agglomerates has been found to have substantial influence on their catalytic activity. [0005] The influence of the size of nanoparticles on their catalytic activity has been investigated for systems on unsupported, i.e. gas borne nanoparticles. For the oxidation of hydrogen on Pt nanoparticle agglomerates, transport processes have shown significant dependency on nanoparticle size. [0006] Nanosized gold/platinum and palladium/platinum bimetallic nanoparticles have been obtained and have shown higher activities for the hydrogenation of 4-pentenoic acid, compared to monometallic nanoparticles or random alloys with a corresponding bimetallic ratio.

[0007] Catalytic oxidation of methanol has been demonstrated recently on a Pt/BN catalyst in a steady-state flow reactor to evaluate the catalyst for use in the catalytic convertor of internal combustion engines using methanol as an alternative fuel. The reaction temperature has been maintained between -10 and 200 ⁰C at very low concentrations of methanol (1000- 4000 ppmv) in excess of oxygen (5 to 80 %) being balanced with nitrogen. Under such conditions, a 50 % methanol conversion has been achieved at room temperature (20 ⁰C), but ignition or self-supporting combustion has not been observed at these low methanol concentrations.

SUMMARY OF THE INVENTION

[0010] A method for spontaneously combusting a fuel includes the steps of providing a plurality of nanocatalyst particles, and contacting the nanocatalyst particles with a fuel and oxygen at substantially ambient conditions, wherein the fuel and oxygen spontaneously combust. The combustion can occur in the absence of flame.

[0011] The nanocatalyst particles can have a diameter of less than 500 nm. In another aspect, the nanocatalyst particles have a diameter of less than 100 nm. The nanocatalyst particles can be at least one selected from the group consisting of group VIII B materials, group I B materials, and group II B materials, and mixtures, alloys and/or oxides of these materials. The nanocatalyst particles can be at least one selected from the group consisting of Pt, Ru, Rh, Pd; Ag, Cu Zn, Fe-Pd, Ni, and Mn. [0012] The fuel can be a hydrocarbon-based fuel, such as natural gas, methane, and propane. The fuel can also be an alcohol, such as methanol and ethanol, The fuel can also comprise other materials, such as hydrogen gas and carbon monoxide.

[0013] The nanocatalyst particles can be entrained in reactant gases, and can be provided on a support. The support can be at least one selected from the group consisting of glass, quartz, silicon, ceramics, alumina, zeolites, MgO, TiO₂, Cr₂O₃, La₂O₃, ZnO; Ru/CeO₂, and Rh/alumina.

[0014] A thermoelectric generator according to the invention includes a heat source comprising a plurality of nanocatalyst particles, and an inlet for fuel and oxygen for contacting said fuel and oxygen with said nanocatalyst particles. A heat sink is also provided. A thermoelectric generating unit comprising n- and p-type semiconductors is in thermal contact with the heat source and the heat sink. An electrical connection is provided between the n-and p-type semiconductors. The thermoelectric generator can include a plurality of thermoelectric generating units. The thermoelectric generating units can be electrically connected in series and/or in parallel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] There is shown in the drawings embodiments which are presently preferred, it being understood, however, that the invention can be embodied in other forms without departing from the spirit or essential attributes thereof.

[0016] Fig. 1 is a plot of temperature (⁰C) vs. time for the combustion of methanol/air according to the invention.

[0017] Fig. 2 is a plot of temperature (⁰C) vs. time for the combustion of methanol/air with various mixture ratios.

[0018] Fig. 3 is a plot of maximum temperature (⁰C) vs. time for the combustion of methanol/air with different mixture ratios, for a heterogeneous-only catalytic reaction (γ) and homogeneous methanol gas phase burning (β).

[0019] Fig. 4 is a schematic depiction of a reactor system for the spontaneous combustion of a fuel.

[0020] Fig. 5 is a schematic diagram of a solid-state thermoelectric heater/cooler.

[0021] Fig. 6 is a schematic diagram of a thermoelectric generator according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0022] Stable and reproducible spontaneous self-ignition and self-supporting combustion have been achieved at room temperature by exposing nanometer-sized catalytic particles to suitable fuel/oxygen reactants such as methanol/air or ethanol/air gas mixtures. Without any external ignition or other stimuli capable of initiating combustion, and preferably at substantially ambient conditions, nanocatalyst particles react with the fuel and oxygen mixtures. As defined herein, substantially ambient conditions refer to reaction initiation conditions of no more than ambient pressure with some allowance for a pressure increase upon flow of the reactants into the reactor, and a temperature of reactants no more than about 40 ⁰C. The reaction releases heat and can produce oxidation products such as CO₂ and water. Such reactions starting at ambient temperature have reached both high (>600 ⁰C) and low (a few tenths of a degree above room temperature) reaction temperatures. The reaction is controlled by varying the fuel/air mixture. It has been discovered that catalytic activity can be dramatically improved by reducing particle size and changing particle morphology. [0023] Nanometer-scale catalyst supported self-ignition requires no external energy (heat, flame, or high pressure) to start fuel oxidation or combustion process. This invention provides a nanometer-scale catalytic reaction system in which highly active nanometer-sized catalytic particles (NCP) can cause spontaneous and self-supporting heterogeneous catalytic combustion at room temperature when a favorable mixture of fuel and oxygen is present. Such NCP-induced spontaneous combustion could start at room temperature without pre¬ heating or external ignition or high pressure once optimal fuel vapor and oxygen gas mixture is exposed to NCP. The combustion process ends once fuel vapor and oxygen gas mixture is no longer in contact with NCP. Such heterogeneous catalytic reactions have been demonstrated by introducing methanol/air and ethanol/air gas mixtures into a glass tube reactor filled with structurally supported platinum nanoparticles on quartz glass wool. Regimes with different reaction rates (heterogeneous catalytic reaction, gas-phase ignition, and steady self-supporting combustion) were obtained by simply controlling the fuel/air mixture ratio and gas flow rate. The reactions have been found to be highly reproducible and stable.

[0024] Platinum (Pt) nanoparticles (50-700 run) were mechanically loaded onto -10 μm diameter quartz glass fibers, which were then packed into a tubular quartz reactor (2.5 cm diameter). A K-type thermocouple was imbedded at its center for monitoring temperature as an indicator of reaction intensity. As shown in Fig. 1, when only pure air was sent into the reactor at room temperature (-20-30 °C), no temperature increase was observed (zone I). When a methanol/air or ethanol/air mixture was introduced into the reactor at the same total flow output rate, the reactor temperature increased until a thermal equilibrium maximum was reached (zone II) at a relatively low methanol content or a much faster secondary reaction took place at a transition temperature of 43 °C (zone III) when a richer fuel mixture was introduced. The rate of temperature increase, an indicator of reaction rate, depended on the fuel type, the fuel/air ratio, and the gas flow rate. The reaction could take place in a temperature range from a few tenths of a degree above room temperature to more than 600 °C, at which point parts of the quartz fibers turned to an orange-reddish color but did not produce a visible flame. However, another type of combustion is also observed at which visible flame and instantaneous gas-phase ignition are produced when certain fuel/air mixtures contact Pt nanoparticle-loaded quartz fibers.

[0025] If the fuel/air supply is maintained at a certain level (i.e., <14.4% mixture in this case), the reactor temperature can be maintained steadily at a rather low temperature (<40 ⁰C) thermal equilibrium in zone II for hours without transition to zone III. The reaction almost stops instantly when the fuel is removed from the gas flow (zone IV). The entire reaction process can be reproduced by reintroducing the fuel/air mixture into the reactor. Water condensation has been observed at the venting end of the glass tube reactor. Gas chromatographic analysis of the combustion gas products in zone III confirms clean burning of the fuel and identifies only O₂, N₂, and CO₂. This confirms the complete combustion of methanol.

[0026] As shown in Figure 2, a higher fuel/air mixture ratio not only increased the maximum temperature of reaction (higher reaction rate) but also changed the reaction regimes. In this experiment, at a methanol/air mixture ratio of 14.4% at first exposure to catalyst, a two-stage process was observed with a transition point at ~42 °C (between zones II and III). When the experiment was repeated at the same mixture ratio, the reactions exhibited a much lower transition point at a temperature of <32 °C. This implies that Pt nanopartides became more active after the first transition to gas-phase combustion. All four curves at the 14.4% mixture ratio reached about the same maximum temperature of ~85 °C (same steady-state rate of combustion). The slight differences of maximum temperature among those four curves are within the thermocouple measurement errors and might have been caused by unstable air flow inside the venting hood where the experiment was carried out. By adjusting the fuel/air mixture ratio, reactor temperature could be increased as little as a few tenths of a degree or more than 600 ⁰C (the maximum measurement limit provided in the experiment). [0027] When different fuel ratios were introduced, the maximum temperatures (maximum steady-state rates of combustion) could be grouped in two clusters that may represent two different reaction mechanisms (see Figure 3): (1) a slower, heterogeneous-only catalytic reaction (γ) and (2) a faster, mixed-reaction regime that includes both a heterogeneous catalytic reaction on the nanoparticle surfaces and a homogeneous combustion of the methanol in the gas phase (β).The changes in these two slopes reflect the changes of the maximum steady-state combustion rates at corresponding temperatures with increasing fuel/air mixture ratios. [0028] Bulk platinum cannot produce similar results. It is likely that particle size may play a significant role in autoignition at room temperature. This view is supported by the experimental observation that Pt nanoparticles (-100 nm) are more active than larger nanoparticles (>500 nm). Particle coverage on the fibers, fiber packing density, particle surface morphology, and particle agglomeration all can change catalytic activity. Also, high humidity can reduce the activity of the nanoparticles. It is theorized that a water layer barrier may form on the surface, or a water-driven structural transformation takes place. Similar effects have been reported to occur on ZnS nanoparticles. The activity of the Pt nanoparticles diminished at high humidity but returned after the catalyst was baked in a vacuum oven for 2 h at 125°C. No evidence of a reaction was observed when only air flowed through the reactor, and it is very unlikely that any metallic pyrophoric reaction is the cause. The Pt ^• nanoparticles were stored in air before and after the reaction.

[0029] Quartz glass wool not only provides a good mechanical supporting structure for nanoparticles but also is an excellent thermal isolation material that allows the nanoparticles to reach higher local temperatures, which could be responsible for rapid self-ignition. The reaction most likely starts at isolated nanoparticles on the quartz glass fibers where undissipated heat accelerates the reaction further until spontaneous ignition occurs. Clustered nanoparticles and tightly packed quartz fibers appear to have much lower activity than evenly dispersed nanoparticles and loosely packed quartz glass fibers. [0030] Appropriate combination of optimal particle size and morphology can strongly enhance catalytic activity and allow spontaneous ignition at room temperature. Practical use of such spontaneous room-temperature catalytic reactions will reduce operational costs, reduce system complexity, and improve reaction efficiency and may introduce new low- temperature catalytic reactions. Other possible applications might be found when localized heating at micro- or nano-scale is required. [0031] Optimal reactant/catalyst surface contact is achieved by reducing catalyst particles size to less than 1 micrometer in diameter with various shapes & morphology using mechanical, chemical or other methods, hi another aspect, the catalyst particles are less than 500 nm in diameter. In still another aspect, the catalyst particles are less than 100 run in diameter, hi yet another embodiment, the catalyst particles are 50nm in diameter or less. [0032] The invention can be utilized with a variety of different catalysts. Suitable catalysts include group VIII B - Pt, Ru, Rh, Pd; group I B - Ag, Cu; group II B - Zn, as well as Fe-Pd, Ni, Mn, as well as mixtures, alloys and/or oxides of these materials. Other catalysts are possible. The selection of the catalyst will be made commensurate with the reaction that is being catalyzed. The catalyst should preferably not suffer substantial degradation, and should therefore be tolerant of the reactants and reaction products.

[0033] The catalyst particles can be supported or gas-borne nano-particles. Any suitable catalyst support can be used. The catalyst particles can be attached to various structure support materials such as alumina, quartz or glass mechanically, electrically or chemically. In one aspect, the support is quartz fibers. Ceramic materials can also be used. The nano- catalyst particles can be provided in solid powder-like form and can be applied in various patterns to the surface of a variety of inert supporting substrates. Suitable substrates can include as glass, quartz, silicon, ceramics and other active support material, such as alumina, zeolites, MgO, TiO₂, Cr₂O₃, La₂O₃, ZnO; Ru/CeO₂, Rh/alumina.

[0034] The invention can utilize a variety of different fuels. A fuel vapor such as methanol, ethanol, and other alcohols, natural gas, methane, propane,or other hydrocarbon-based fuels, and carbon monoxide and hydrogen gas can be used. Other fuels are possible. Oxygen gas, either alone or in mixtures, and air are preferred oxidizing agents for reacting with the fuels, although others are possible. [0035] As noted above, the combustion reaction according to the invention can be adjusted by changing the NCP' s morphology, particle size, material, composition, fuel/oxygen(air) mixture and surrounding environmental conditions such as temperature, pressure and humidity. The thermal (heat) conductivity of the reactor walls/supporting material can also be varied to control the reaction. The main combustion products of a well-optimized reactor are generally water and carbon dioxide.

[0036] Nano-catalytic particles can greatly enhance the performance of any related devices and allow localizing significant amounts of heat on extremely confined surface/volume. This in turn, will allow a variety of applications in micro- and nano-scale devices and arrays. The invention can have utility in NCP-inducted automatic ignition systems for combustion engine design, energy utilization, micro-/nano-scale heating or fuel efficiency improvement. [0037] An experimental system is shown in Figure 4. Pt nanoparticles (50-700 run) were mechanically loaded (-0.1-0.5 wt %) onto -10 μm diameter quartz fibers. Quartz fibers were then loosely packed into the quartz tube 10. A thermocouple 14 (K-type) was placed inside the quartz tube 10, near the center of the reactor for temperature measurement. Because of the different sizes between nanoparticles and the thermocouple 14, the thermocouple's readout only indicates the average temperature inside the tube reactor filled with the Pt nanoparticles and quartz fibers. Actual local temperatures of individual nanoparticles might be much higher than the reading from the thermocouple. Here, reactor temperature serves as an indicator of reaction rate since heat production is proportional to reaction rate. One line of compressed air (medical grade, 21% O₂) from source 18 passes the methanol or ethanol saturator 22. Flow meters 26 monitor the flow of air. The methanol-rich or ethanol-rich air mixes with another line of air 30 before the reactor. When sufficient amounts of fuel/air are provided to the reactor, Pt nanoparticle-loaded quartz glass fibers heat up very rapidly and can exhibit orange-reddish color at high temperature. The gas mixture ratio was controlled by adjusting the flows passing through two 1000 ccmv gas flow controllers (model 1479A13CS1AM) and MKS four-channel power supply/readout unit (type 247). During the experiment the total gas flow into the reactor inlet was maintained at a fixed rate of 1000 ccmv. The end products were vented into the chemical fume hood directly or collected into the gas sample bag for gas chromatographic (GC) analysis. The small size catalytic reactor (glass tube; i.d. = 2 cm, o.d. = 2.2 cm, length = 5 cm) and absence of any thermal insulation facilitate rapid heat exchange with the surrounding ambient environment. [0038] Upon contact with the air/methanol mixture the catalysts instantly release heat with or without visible flame and rapidly reach the ignition temperatures (visible flame or red- glowing) within seconds. The reaction is self-maintained in a suitable fuel/air mixture environment and the reaction is highly exothermic. A large amount of heat was produced within seconds by a very small amount of methanol or ethanol vapor. Such catalytic combustion stops almost instantaneously once the fuel source is withdraw. Then catalytic combustion resumes almost instantaneously again once the fuel/air source is provided. Initial observations show these catalysts are robust and could be subjected to multiple and prolonged use without apparent poisoning or loss of activity.

[0039] An electrical power generator according to the invention utilizes the Peltier-Seebeck effect. The Peltier-Seebeck effect, also known as the thermoelectric effect, hereafter referred to as the "Seebeck effect", is the direct conversion of heat differentials to electric voltage as well as the inverse process. Applied to the invention, thermal power is created by breaking down fuel molecules, and this thermal power is converted into electrical power. A temperature gradient can be created by combustion, as of an alcohol, at one side of a Seebeck materal and ambient temperature at the second side of the Seebeck material. The generated power is proportional to the temperature difference, and the configuration of the device. Such an electrical power generator converts chemical energy directly into electrical power by thermal decomposition without any moving parts and without a requirement for external ignition. The generator can utilize alcohols and related fuels, and avoids issues related to environmental pollution that come with the use of oil.

[0040] Devices that operate by temperature and electrical currents are generally referred to as thermoelectric devices. In thermoelectric devices, the generation of movable charge carriers is affected by temperature, and the flow of charge carriers due to temperature gradient creates a voltage difference or pumps heat from one side of the material to the other. There exist three effects that influence such devices, the Seebeck effect, the Peltier effect, and the Thompson effect. When a temperature gradient, T, is applied to a material, it will be accompanied by an electric field in the opposite direction. This is known as Seebeck effect, and the ratio V/T is defined as the Seebeck coefficient (S). The reverse effect, where electrical current is used to produce temperature difference, is known as the Peltier effect and is used in cooling or heating without moving parts. The ratio of heat flow to the electrical current for a particular material is known as Peltier coefficient, P. Its value is related to the Seebeck coefficient, S. The Thomson effect is a thermoelectric effect and involves a relationship between the Seebeck and Peltier coefficients. This effect relates the heating or cooling in a single homogenous conductor when a current passes along it in the presence of a temperature gradient. These three effects are related to each other by

S = P /T where S is the Seebeck coefficient, P is the Peltier coefficient and T is the temperature difference. It is well established that Seebeck effect can be used to generate voltage by creating a difference in temperature between the ends of a conductor, and transporting the extra charge carriers using a second material. The Seebeck effect is routinely used in thermocouples to monitor the temperature of an object. [0041] The Seebeck coefficient S is expressed in volts per degree. Conventional metals have very low Seebeck coefficients, which are measured in microvolts per degree raise in temperature. The power generation efficiency is around 1 % and is uneconomical as a source of electrical power. However, at present there exist many materials with Seebeck coefficients in the range of tens of milli volts per degree raise in temperature. These materials can improve this efficiency, and efficiencies as high as 60% have been reported. [0042] The dimensionless thermoelectric figure of merit, ZT, determines the dependence of device efficiency upon material properties, and is defined as follows,

ZT = TS 2σ/ K

where σ and K are electrical and thermal conductivity, respectively, and the latter consist of

electronic Ke and lattice KL components at absolute temperature T(K) . A good thermoelectric material must have a large Seebeck coefficient, S, to produce the required

voltage, high electrical conductivity, σ, to reduce the thermal noise (joule heating, PR), and a

low thermal conductivity, K, to decrease thermal losses from the thermocouple junctions. A

low thermal conductivity in a good thermoelectric material means a low value of its lattice

component KL as a major contributor since its electronic component Ke is a minor

contributor and is proportional to the electrical conductivity.

[0043] Another impediment in using the Seebeck effect for power generation is the creation of a suitable temperature difference. The present invention can create a temperature difference of hundreds of degrees by nano-catalytic combustion of fuels such as alcohols, natural gas, propane, and hydrogen.

[0044] A simplified schematic diagram of an exemplary thermoelectric generator is shown in Fig. 5. The thermoelectric generator 40 includes a p-type semiconductor 44 and a n-type semiconductor 48, which generates electrical current through conductor 52 upon exposure to a temperature difference. Heat rejection occurs at a heat generator 56, which contacts fuel with nanocatalyst particles 57, and cooling occurs at a heat sink 60. By combining a p- and n-type semiconductor, voltage and therefore electrical power are generated. Because the Seebeck coefficient, S, has an opposite sign for p- and n-type materials, contributions from both pieces are added to nearly double the generator voltage as compared to that of a single element.

[0045] The p- and n-type semiconductors are arranged electrically in series and thermally in parallel, as shown in Fig 6. Individual thermoelectric generator units 74 can be connected both in series and/or in parallel, and are connected by conductors 76, which can be provided on a common substrate. The units 70 are provided between heat source 80 and heat sink 84. The heat sink 84 is thermally shielded from the high temperature of the heat source 80. The heat source 80 is a nano-catalytic combustion device comprising nano-catalyst particles according to the invention. A mixture of a fuel such as alcohol and air is allowed come in contact with the nanop articles. The nanoparticles are distributed such that the particles catalytically burn alcohol. The uniform and efficient distribution of the nanoparticles can be accomplished by immobilizing the nanoparticles on structures such as glass wool immobilized on a support plate. The combustion raises the temperature of the heat source 80 from room temperature to hundreds of degrees centigrade without the need for a preheating environment.

[0046] The invention provides devices and methods of producing electrical power with many advantages. Since such devices can be based on alcohol there is very little pollution from the operation of the device. Therefore the devices can be used indoors. The expected byproducts are carbon dioxide and water vapor. The fuels such as alcohols, natural gas, propane or hydrogen can be produced by agricultural products such as com. A high temperature is achieved with flameless nano-combustion. A fast start up time and short shut-down time are possible. There is no need for initiation of the catalytic reaction by applying external stimuli such as temperature, pressure, vibration, or spark. The lack of moving parts reduces friction losses and improves efficiency, and also lowers maintenance costs and increases product life. Also, the invention can be scaled from low power to high power applications. Particularly in the low power application, a generator according to the invention can be implanted in other devices or can be portable. The thermoelectric generator 70 is completely scaleable. A device can be designed to produce micropower for nanosensors. It can also be designed to produce power for devices such as computers and small appliances. The process can also be scaled for localized, large-scale power production for buildings and houses. [0047] This invention can be embodied in other forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be had to the following claims rather than the foregoing specification as indicating the scope of the invention.

Claims

We claim:

1. A method for spontaneously combusting a fuel, comprising the steps of: providing a plurality of nanocatalyst particles; contacting said nanocatalyst particles with a fuel and oxygen at substantially ambient conditions, wherein said fuel and oxygen spontaneously combust.

2. The method of claim 1, wherein said nanocatalyst particles have an average diameter of less than 500 nm.

3. The method of claim 1, wherein said nanocatalyst particles have an average diameter of less than 100 nm.

4. The method of claim 1, wherein said fuel is a hydrocarbon-based fuel.

5. The method of claim 1, wherein said fuel is an alcohol.

6. The method of claim 1, wherein said is at least one selected from the group consisting of methanol, ethanol, natural gas, methane, and propane.

7. The method of claim 1, wherein said fuel is at least one selected from the group consisting of hydrogen gas and carbon monoxide.

8. The method of claim 1, wherein said catalyst is at least one selected from the group consisting of group VIII B materials, group I B materials, and group II B materials, and mixtures, alloys and/or oxides of these materials.

9. The method of claim 1, wherein said nanocatalyst particles comprise at least one selected from the group consisting of Pt, Ru, Rh, Pd, Ag, Cu, Zn, Fe-Pd, Ni, and Mn.

10. The method of claim 1, wherein said nanocatalyst particles are entrained in reactant gases.

11. The method of claim 1, wherein said nanocatalyst particles are provided on a support.

12. The method of claim 1, wherein said support is at least one selected from the group consisting of glass, quartz, silicon, ceramics, alumina, zeolites, MgO, TiO₂, Cr₂O₃, La₂O₃, ZnO, Ru/CeO₂, and Rh/alumina.

13. The method of claiml, wherein said combustion is in the absence of flame.

14. A thermoelectric generator, comprising: a heat source comprising a plurality of nanocatalyst particles, an inlet for fuel and oxygen for contacting said fuel and oxygen with said nanocatalyst particles; a heat sink; a thermoelectric generating unit comprising n- and p-type semiconductors between and in thermal contact with said heat source and said heat sink, and an electrical connection between said n-and p-type semiconductors.

15. The thermoelectric generator of claim 14, comprising a plurality of thermoelectric generating units.

16. The thermoelectric generator of claim 15, wherein said thermoelectric generating units are electrically connected in series.

17. The thermoelectric generator of claim 15, wherein said thermoelectric generating units are electrically connected in parallel.

18. The thermoelectric generator of claim 14, wherein said fuel is a hydrocarbon- based fuel.

19. The thermoelectric generator of claim 14, wherein said fuel is an alcohol.

20. The thermoelectric generator of claim 14, wherein said fuel is at least one selected from the group consisting of methanol, ethanol, natural gas, methane, and propane.

21. The thermoelectric generator of claim 14, wherein said fuel is at least one selected from the group consisting of hydrogen gas and carbon monoxide.

22. The thermoelectric generator of claim 14, wherein said nanocatalyst particles have an average diameter of less than 500 nm.

23. The thermoelectric generator of claim 14, wherein said nanocatalyst particles have an average diameter of less than 100 nm.

24. The thermoelectric generator of claim 14, wherein said catalyst is at least one selected from the group consisting of group VIII B materials, group I B materials, and group II B materials, and mixtures, alloys and/or oxides of these materials.

25. The thermoelectric generator of claim 14, wherein said catalyst is at least one selected from the group consisting of Pt, Ru, Rh, Pd, Ag, Cu, Zn, Fe-Pd, Ni, and Mn.

26. The thermoelectric generator of claim 14, wherein said nanocatalyst particles are provided on a support.

27. The thermoelectric generator of claim 26, wherein said support is at least one selected from the group consisting of glass, quartz, silicon, ceramics, alumina, zeolites, MgO, TiO₂, Cr₂O₃, La₂O₃, ZnO, Ru/CeO₂, and Rh/alumina.