This invention relates to generation and enhancement of a catalytic effect through a gated voltage bias.
A catalyst is a substance that increases the rate of a reaction or reduces the activation energy of the reaction and can be recovered chemically unchanged at the end of the reaction. A catalyst provides an alternate mechanism that is faster or lower energy than the mechanism in the absence of the catalyst. Although the catalyst participates in the mechanism, it is not consumed during the chemical reaction. Many catalytic effects in order to work necessitate a surface energy that basically will raise the energy of electrons in the catalytic layer in order for the catalytic effect to take place. For example, metal oxide sensors that use a catalytic effect (e.g., a tin oxide CO (carbon monoxide) sensor) need to be heated to over 300° C. in order for the catalytic effect to take place, oxidize CO to CO2, and CO gas to be sensed. Another example is TiO2 (titanium oxide) that needs to be irradiated with UV (ultraviolet) light in order for a catalytic effect to take place.
The catalytic process generally follows a standard path. In either a liquid or gas phase, a process or source molecule lands on the catalyst, then a reaction takes place that activates the molecule. The activated species react with other molecules or the catalyst and then moves away from the catalyst. If the catalyst is consumed in the initial reaction, it can be refreshed from other molecules in the ambient through follow-on reactions, thus bringing the catalyst to its initial state.
There are many factors that affect the rate of reaction. Initial reactions may require an activation energy that can be delivered by heating the catalyst and reactant molecules to high temperatures or by exposing it to light or other electromagnetic energy. Thus, the effectiveness of a catalyst has been shown in the prior art to increase with the addition of heat (changing the temperature) and/or the addition of light energy (photo-excitation of the catalyst or reactants at the catalyst surface). A problem with heating and UV radiation is that they complicate the overall system, possibly even causing the system to be unusable or lower the efficiency of the system.
Embodiments of the present invention use a source of energy that is created by a voltage gated bias to a catalytic layer Due to the fact that heating or UV irradiation or other types of external energy sources complicate the system and sometimes even make the applications unusable, using a solid gated bias simplifies catalytic applications, and also reduces costs and miniaturizes the system.
Referring to
The conductivity of the sensor 300 is monitored during exposure to the gas by monitoring the current between the source and drain electrodes (301) using a current meter (307). In this embodiment, the gas is carbon monoxide (CO). At the surface of the sensor, the CO gas is catalytically converted to CO2 by oxidation reaction in the presence of air or oxygen. This oxidation reaction provides an extra electron in the metal oxide film 302 that is detected as a change in conductivity, and thus a change in the current measured by the current meter (307) when the applied voltage (305) is held constant. This change in conductivity, and thus sensor response, is dependent on the voltage 306 applied to the gate. The output of the catalyst is the CO2 gas that is generated by the catalytic reaction. This reaction is monitored by the change in conductivity across the catalytic film 302 as a result of the donated electron per CO molecule that is converted to CO2. From one point of view, the sensor is measuring the presence of CO, but from another point of view, it is measuring the performance of the catalyst in the presence of CO. Thus, the sensor, in effect, is measuring the effectiveness of the molybdenum oxide catalyst 302. The reaction takes place even if the change in conductivity of the catalyst film is not measured.
The performance of sensor 300 is shown in
The response of the sensor 300 to CO at −60° F. is shown in
Some of this increased efficiency may be tied to thermal transport and reaction energy levels. Hot surfaces (for heated catalysts) can provide diffusion input due to temperature differentials within a given system. A reactant will start to heat up as it diffuses toward the heated surface. As this reactant absorbs energy in the form of heat, it has a greater probability of diffusing away without having reacted. A non-heated, gated catalyst surface does not create these diffusion conditions, increasing the likelihood of the reaction taking place.
It is well known that catalysts are used to reduce the energy level (or energy barrier) for a reactant to become a product. This energy barrier is usually overcome by application of heat that creates a new population of electrons in an energy level within a materials band structure. This newly popullated energy level is where the catalytic reaction takes place. The temperature-based distribution of electrons follows Fermi-Dirac Statistics. The Fermi-Dirac distribution function describes the probability that an available energy state (E) will be occupied by an electron at a given temperature T. The distribution function is:
f(E)=1/(1+e(E-Ef)k(b)*T)
where k(b) is Boltzmann's constant and Ef is the Fermi-level energy. For any semi-conducting material Ef describes the probability of an electron to occupy its lowest energy band, the highest occupied molecular orbital (HOMO) or the next higher energy band, the lowest unoccupied molecular orbital (LUMO). Two scenarios of energy levels are considered for the catalyst embodiments of this patent. The first scenario is shown in
In another scenario, the catalyst has a smaller difference in energy between the HOMO and LUMO but the catalytic reaction may take place in a different molecular that is, for example, a higher energy than the LUMO. This is shown in
The scope of this disclosure is not limited to the observed phenomena of this sensor An applied field or gate bias on a catalyst surface may increase the effectiveness of the catalyst surface or film. This may arise as a result of the applied bias shifting the energy levels of the catalyst and making open states (unoccupied states) available to the reactant molecule that are not available without the applied bias.
There are many industrial manufacturing processes that also depend on semiconducting catalysts.
1. A SOHIO (Standard Oil of Ohio) process involving oxidation/ammoxidation of propylene to make acrolein and acrylonitrile. One of the catalyst used in this case is bismuth molybdenum oxides, although multi-component catalysts (including Bi, Mo, Fe, Ce, etc.) are also used. Typically, this catalyst is heated to 300° C.-400° C.
2. Supported molybdenum oxide (MoO3/Al2O3) catalyst was studied for the oxidative dehydrogenation of ethane. The demand for olefins remains a challenge for the refining and petrochemical industry. The classical commercial processes applied for the production of olefins are energy-intensive; more economic sources are sought. (See “an operando Raman study of structure and reactivity of alumina-supported molybdenum oxide catalysts for the oxidative dehydrogenation of ethane,” A. Christodoulakis, E. Heracleous, A. A. Lemonidou, and S. Boghosian J. Catal. 2006, Vol. 242, pp 16-25.)
3. Tin oxide (SnO2) is used as an oxidation catalyst for carbon monoxide (CO). (See “The surface and materials science of tin oxide,” M. Batzill and U. Diebold Progress in Surface Science 2005 Vol. 79, pp. 47-154.) SnO2 is also used in many heated metal oxide sensors.
4. Vanadium oxide (V2O3) with various loadings of titanium oxide (TiO2) is used for selective oxidation of methanol to formaldehyde (See “In situ IR, Raman, and U-Vis DRS spectroscopy of supported vanadium oxide catalysts during methanol oxidation.” L. J. Burcham, G. Deo, X. Gao, and I. E. Wachs Top. Catal. 2000 11/12, 85) and selective reduction (See “Reactivity of V2O5 Catalysts for the Selective Catalytic Reduction of NO by NH3: Influence of Vandadia loading, H2O and SO2,” M. D. Amiridis, E. E. Wachs, G. Deo, J.-M. Jehng, and D. S. Kim J. Catal. 1996 Vol. 161, p. 247) of NOx by NH3.
5. Zinc oxide (ZnO2) is used for the production of H2 via steam reformation of ethanol (See “Current Status of Hydrogen Production Techniques by Steam Reforming of Ethanol: A Review,” A. Haryanto, S. Fernando, N. Murali, and S. Adhikari Energy & Fuels 2005 19, 2098)
These processes may take place at lower temperature and be more efficient or the production of the reactant product (e.g., acrolein and acrylonitrile in example 1 above) may be more complete if the catalyst was biased with a gate voltage or if an electric field was applied to the catalytic film.
There are also some catalytic reactions processes where the catalyst material creates different products when not heated. In this example, a normal catalyst produces a mixture of enantiomers of a chiral molecule. The heat of reaction for the catalyst's activation provides enough energy to overcome the formation of both enantiomers of the chiral product. The application of a gate bias may reduce the temperature enough so that only one of the two chiral products is produced. The reduction or elimination of heat and addition of a gate bias to the catalyst may also direct the reaction intermediates that take place and thus products from a directed chemical mechanism. In other words, it may be used to steer the reaction in one direction or another. This may be useful in the creation of bio-molecules or natural products synthesis.
Thus, there are other configurations that may allow a gate bias or electric field to be applied to a semiconducting or wide band gap catalytic film. The polarization of the applied gate bias may also be important; for the example of the CO sensor 300 of
These configurations may be used in a gas phase system or in a liquid phase system, as long as a bias is able to be maintained between the various electrode surfaces.
Thus, the effectiveness of a catalyst has been shown in prior art to increase with addition of heat (changing the temperature) and/or the addition of light energy (photo-excitation of the catalyst or reactants at the catalyst surface). A catalyst material may also demonstrate increased effectiveness (higher catalytic response and greater product yield) by adding a gate bias or electric field to the catalyst.
Referring to
Particle catalyst 207 is a bed of particles between layers of conducting mesh 208 divided by insulating spacers 206. Particles 207 may be vanadium oxide, tin oxide or other semiconducting or wide band gap material. Particle sizes may range from 10 microns to 1 nm (nanometer). The mesh 201-205 is constructed to contain the catalyst particles 207. The catalyst particles 207 may be mixtures of different materials (e.g., tin oxide particles mixed with vanadium oxide). The catalyst particles 207 may be one material coated with another material (e.g., Al2O3 coated with vanadium oxide). The configuration can be heated or cooled to further control reaction processes. In operation, this assembly is exposed to gas or fluid flow and a reaction takes place to form product chemicals. The catalyst particles 207 promote this reaction. The presence of the proper bias to the mesh electrodes 201-205 enhances the performance of the catalyst, resulting in reduction of heat or other energy applied to the catalyst (not shown in
For any of the embodiments described in
Titania is a wide-band-gap semiconductor with the energy gap of 3.03 eV or 3.18 eV for rutile and anatase phases, respectively. One of the factors dictating the electron distribution between the conductive and valence bands is dictated by Fermi-Dirac statistics. The electron distribution may be affected by shifting the Fermi energy level. The closer the Fermi energy level is to a particular energy band (e.g., conductive band), the more electrons will occupy this band. If there are more electrons in the conduction band, then there are more electrons available to perform chemical reactions. One way to adjust the Fermi energy level is to apply the bias potential as described herein. As stated before, this may also affect the direction of a reaction towards one product or another.
Another way to affect the Fermi energy level and thus further increase the effectiveness of the titania photocatalyst, is to dope the titania with n-type material fabricating for example titania-doped stabilized tetragonal zirconia (TiO[2]—ZrO[2]—Y2O[3]) material which is a n-type semiconductor. This method may be combined with the application of the bias potential to achieve a synergistic effect.
Typically, UV light is required to achieve the activation of the titania photocatalyst. Decreasing the activation energy requirements to allow use of visible light commonly available in sunlight, one would need to substantially decrease a band-gap of titania. This may be achieved by doping titania with nitrogen as widely discussed in literature. Therefore, to utilize all of the above enhancements one would synthesize nitrogen-doped (n-type) titania material and subject it to the bias potential.
This application claims priority to U.S. Provisional Application Ser. No. 60/735,604.
Number | Date | Country | |
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60735604 | Nov 2005 | US |