Directly Heated Ceramic Catalyst Support

This invention relates generally to catalysts and, in particular, relates to a catalyst for reconstituting CO.sub.2, the catalyst comprising a catalytic material, such as a noble metal, deposited upon a directly heated ceramic substrate.

CO.sub.2 lasers typically contain a mixture of gases such as carbon dioxide, nitrogen and other gases An electrical discharge between a pair of electrodes within the laser cavity induces the lasing of the CO.sub.2 and also dissociates the CO.sub.2 into CO and O.sub.2. This dissociation of CO.sub.2 tends to reduce the volume of the CO.sub.2 within the laser, resulting in the gradual decrease in the output power of the laser and, possibly, in the complete failure of the laser after a period of time

In order to replenish the CO.sub.2 it has been known to provide an external gas canister of CO.sub.2. However, in some applications, such as military applications, such an external gas canister may be undesirable.

It has also been known to provide a catalyst within the laser cavity to recombine the dissociated CO and O.sub.2 inasmuch as CO and O.sub.2 do not readily recombine at room temperatures or at typical laser operating temperatures. A variety of catalysts have been known which promote this reconstitution of CO.sub.2 from CO and O.sub.2. An example of such a catalyst is metallic platinum (Pt) in wire form which is resistively heated to a temperature of about 1000.degree. C. It has also been known to recombine CO and O.sub.2 by pumping the laser gas through a bed of hopcalite (60% MnO.sub.2, 40% CuO, and trace quantities of other oxides), the hopcalite typically being provided in granular or powder form. However, a significant pressure drop may occur within such a bed of hopcalite if it is required that the laser gas be pumped through the bed. Also, hopcalite often requires periodic treatment in order to maintain its activity and must also be suitably contained such that the powder does not contaminate the inner surface of the laser cavity.

In a metallic platinum catalyst it is known that the number of CO.sub.2 and O.sub.2 molecules that are recombined to CO.sub.2 during a given interval of time is directly related to the surface area of the catalyst in contact with the molecules and, also, is exponentially related to the temperature of the platinum Thus, an increase in the surface area results in a significant decrease in the temperature required to accomplish a desired CO.sub.2 recombination rate.

Conventional methods of heating catalysts include upstream heating of the laser gas and direct thermal contact of the catalyst structure with an embedded heating element. The upstream heating of laser gas may be undesirable in some applications due to subsequent heat removal operations possible, inefficiency in heat transfer, and possible structural complexity and increased cost. Direct thermal contact of the catalyst structure with an embedded heating element may also prove disadvantageous due to inefficient thermal transport between the element and the catalytic structure and the difficulty in implementing this technique in a compact geometry.

It is therefore an object of the invention to provide a catalytic structure which employs a coating comprised of a catalytic material deposited upon an electrically conductive ceramic substrate, the substrate being resistively heated in order to elevate the catalytic coating to its activation temperature.

It is a still further object of the invention to provide a catalyst for a CO.sub.2 laser having a high catalytic activity per unit volume at a reduced operating temperature or a minimum power requirement, the catalyst having a resistively heated substrate for elevating a catalytic coating to a desired temperature.

One further object of the invention to provide a catalyst for a CO.sub.2 laser having a high catalytic activity per unit volume at a reduced operating temperature or input power requirement, the catalyst having a resistively heated ceramic substrate for elevating a catalytic coating to a desired temperature, the catalyst further having a temperature control coupled thereto for maintaining the coating at the desired temperature.

The foregoing problems are overcome and other advantages are realized by a catalytic structure which is comprised of a coating of a catalytic material deposited upon a surface of an electrically conductive ceramic substrate. In one illustrative embodiment of the invention a high surface area form of platinum (Platinum Black) is deposited upon an electrically conductive ceramic substrate. The substrate is resistively heated by an electrical current thereby elevating the platinum coating to its activation temperature. Gas is flowed over the catalyst which, due to the high platinum surface area provided by the Platinum Black coating, achieves a high catalytic activity with a moderate degree of heating. The substrate may be any electrically conductive ceramic material such as silicon carbide or tin oxide.

In accordance with the invention the ceramic catalyst support may also be used to provide a temperature sensing means for indicating the temperature of the catalytic coating by noting the resistance of the support. The temperature sensing means may further be coupled to a current controlling means for controlling the amount of heater current through the substrate.

In accordance with the invention there is also disclosed a flowing gas laser, comprising means for containing a flow of gas, the gas comprising CO.sub.2 ; means for flowing the gas through the containing means; means for energizing the CO.sub.2 for causing the CO.sub.2 to emit laser radiation, the energizing means disassociating some of the CO.sub.2 gas into CO and O.sub.2. The laser further comprises a directly heated catalyst for catalyzing the recombination of the CO and O.sub.2 into CO.sub.2, the catalyst comprising: an electrically conducting ceramic substrate having an electrical resistance associated therewith; a coating comprised of a catalytic material overlying and thermally coupled to the substrate, the coating being operable for catalyzing, at a predetermined activation temperature, the recombination of CO and O.sub.2 into CO.sub.2, the catalyst being disposed within the flow of the gas; and means for supplying current to the substrate for resistively heating the substrate whereby the coating is heated to the predetermined activation temperature.

Further in accordance with the invention there is disclosed a method of fabricating a catalytic structure, comprising the steps of providing a substrate having a desired shape, the substrate comprising an electrically conductive ceramic; and forming a coating over the substrate, the coating being comprised of a catalytic material.

These and other aspects of the present invention will become more apparent in conjunction with the ensuing detailed description of the invention read in conjunction with the accompanying drawing wherein:

FIG. 1 shows, in accordance with one embodiment of the invention, a catalytic structure comprised of a layer of catalytic material, such as a noble metal, deposited upon a surface of an electrically conducting ceramic substrate;

FIG. 2 shows a block diagram of a flowing gas CO.sub.2 laser having, in accordance with the invention, a directly heated catalyst for promoting the recombination of CO and O.sub.2 into CO.sub.2 ; and

FIG. 3 is a graph which plots oxygen recombination rate versus temperature for a catalytic structure constructed in accordance with FIG. 1, the graph being plotted for two gas flow rates.

Referring now to FIG. 1 there is shown a side view of a catalyst constructed in accordance with the invention. Catalyst 10 can be seen to be comprised of a substantially circular cylindrical substrate 12 having a coating 14 comprised of a catalytic material operable for catalyzing the recombination of CO and O.sub.2 into CO.sub.2. Substrate 12 is preferably comprised of an electrically conductive ceramic, such as silicon carbide (SiC), tin oxide (SnO.sub.2) or any suitable ceramic material which has a sufficient degree of electrical conductivity whereby the substrate may be resistively heated. The coating 14 comprises a catalytic material, such as a noble metal, which in a preferred embodiment of the invention comprises a coating of a high surface area form of platinum such as Platinum Black. Platinum Black is a finely divided form of metallic platinum which typically is derived from a reduction of a solution of a platinum salt with zinc or magnesium. In general, it has the form of a black powder, is soluble in aqua regia and has a specific gravity of 15.8 to 17.6 (apparent). The layer 14 of Platinum Black may be deposited upon the surface of the substrate 12 by immersing the substrate 12 within an electroplating solution of Platinum Black and thereafter applying a current to the substrate 12. The current causes the electrodeposition of the Platinum Black upon the substrate Prior to use the catalyst structure is preferably conditioned by being placed in an atmosphere of helium, the structure being heated to a temperature of about 350.degree. C.

The substrate may also have a pair of electrical terminals 16 and 18 for electrically coupling the substrate 12 to a source of heater power, such as alternating or direct current. There may also be provided a heat control means such as a heat sensor or thermostat 20 which is thermally coupled to the substrate 12 and which also comprises terminals 22 and 24 for coupling the thermostat 20 to a suitable temperature controller.

Of course, the invention may be practiced by other catalytic substances deposited upon the substrate, such as by depositing a layer of palladium (Pd). In general any catalytic substance, including noble metals and other substances, may be employed so long as the catalytic material is amenable to being deposited as a layer or coating upon the substrate 12. Examples of non-noble metal catalytic substances include manganese, magnesium, nickel and copper. Also, the coating may be formed by other than electrodeposition. For example, the ceramic material of the substrate 12 may be impregnated with the catalytic substance. This impregnation may be accomplished by a number of known techniques. For example, an insipient wetness technique may be employed, especially with a relatively porous substrate such as tin oxide, wherein the voids within the porous substrate are filled with a saturated solution of catalytic material, the liquid portion thereafter being evaporated. Also, a precipitation and baking technique may be employed to form the coating.

The electrical conductivity of substrate 12 is determined such that the substrate may be resistively heated to a temperature where the activation of the catalytic layer 14 is achieved. For example, if the substrate is coated with Platinum Black a temperature within the range of about 100.degree. C. to about 350.degree. C. may be sufficient.

The overall shape of the catalytic structure 10 is determined such that a large surface area of the catalytic material is presented to a gas flow passing over and through the catalytic structure. As can be appreciated, the greater the catalytic surface area in contact with the flowing gas stream the greater will be the rate of recombination of CO and O.sub.2 to CO.sub.2. The actual shape and dimensions of a particular catalytic structure is typically application dependent and may be varied within a wide range while yet achieving the beneficial results made possible by the invention. Thus, it can be appreciated that the circular cylindrical helical shape illustrated in FIG. 1 is but one suitable shape for a catalytic structure constructed in accordance with the invention.

Referring now to FIG. 2 there is shown an illustrative pulsed, flowing gas CO.sub.2 laser 30 having the catalyst 10 of FIG. 1 disposed within a gas flow path, indicated by the arrow designated as A, contained within a gas conduit 31. Laser 30 comprises a pair of electrodes 32 and 34 disposed opposite one another across the gas flow path A and defining a discharge region therebetween. A pulsed source of high voltage (indicated by V+ and ground) or other form of discharge energy is coupled to the electrodes for inducing a discharge, the discharge causing the flowing CO.sub.2 within the discharge region to lase. Pulses of laser radiation are coupled from the laser 30 by a suitable output coupling means (not shown). A gas flow means, such as a fan 36, is also typically disposed within the conduit 31 for circulating the laser gas at a rate sufficient to replenish, before each discharge, the gas within the discharge region.

As has been previously stated, each discharge disassociates some of the CO.sub.2 into CO and O.sub.2. If the CO and O.sub.2 are not recombined a gradual decrease in laser pulse power will occur, leading to the eventual failure of the laser. The catalyst 10 is provided, therefore, to recombine the CO and O.sub.2 gases to ensure the continued operation of the laser 30 at a desired pulse power and pulse repitition rate.

In accordance with the invention, the catalyst 10 is provided with a power source 38 coupled to the terminals 16 and 18 for directly heating the substrate 12 whereby the catalytic layer 14 is heated to a desired activation temperature. Power source 38 may be any suitable source, such as an AC or DC current source, which is operable for providing sufficient current flow through the resistance of the substrate 12 to elevate and maintain the temperature of the noble metal layer 14 at a desired temperature.

If desired, the invention also advantageously provides for coupling a heat control means such as a thermocouple 40 and a temperature controller 42 to the catalyst 10 and to the power source 38 for maintaining the temperature of the layer 14 at a desired temperature. The temperature control means may comprise a number of well known devices which are operable for responding to a change in temperature of the catalytic structure for increasing or decreasing the current flow through the substrate 12. Thus, variations in gas flow rates over the catalyst 10, variations in laser pulse rate and other gas temperature influencing factors may be compensated for in order to maintain the catalyst at a desired temperature.

Furthermore, inasmuch as the resistance of the substrate is related to the temperature of the substrate by a known function, the resistance may be employed as a direct indication of substrate and, hence, the catalytic coating temperature. The resistance is determined by the application of Ohm's law as the ratio of voltage to current. That is, the ratio of the voltage output of power source 38 to the current provided by the power source may be determined by well known circuits to derive a signal indicative of resistance and temperature.

It can be appreciated that the directly heated catalyst of the invention may also be advantageously employed in continuous output flowing gas lasers. The invention may also be employed for other applications wherein a heated catalyst recombines disassociated gases. Thus, the invention is not limited solely for use within a laser.

Referring now to FIG. 3 there is shown a graph wherein the oxygen disappearance rate, that is the volume of molecular oxygen converted per unit time, is plotted against surface temperature of the catalyst 10 of FIG. 1. The graph shows data for two different gas flow rates. The intersection of the two curves with the horizontal operating lines for 10, 25 and 50 Hz laser operation defines the minimum operating temperature for achieving continuous operation. The leveling off of the oxygen disappearance rate at approximately 200.degree. C. for the lower gas flow rate of 5 SCFH is believed to be related to a limitation imposed by mass transport of the reactants to the catalytic site. At the higher flow rate of 10 SCFH the oxygen disappearance rate does not exhibit such a leveling off at 200.degree. C., indicating that for a given application it may be desirable to empirically determine a gas flow rate relative to laser pulse repetition rate and catalyst activation temperature.

A presently preferred embodiment of the invention has herein been disclosed, it being appreciated that a number of modifications to this embodiment may occur to those having skill in this art. For example, it can be realized that the helical shaped structure illustrated in FIG. 1 is but one suitable shape. Thus, the invention is not to be considered to be limited to this presently preferred embodiment but is instead only to be limited by the appended claims.