Eggshell catalyst structure having increased near-surface concentration of active metals

Abstract

An eggshell catalyst support has a porous support structure that is impregnated with an active metal catalyst. The metal catalyst is heterogeneously distributed so that the catalyst is concentrated at or near the exterior surface of the porous support structure. The catalyst may be concentrated by impregnating the support structure using a molten metal catalyst to contact bulk catalyst supports. A relatively smaller volume of molten catalyst relative to the pore volume of the support may be utilized to concentrate the catalyst in a band near the exterior surface of the supports. The viscosity of the molten metal catalyst may be adjusted to control the depth of the metal catalyst within the porous supports.

Description

PATENT 650.34-100EGGSHELL CA TALYST STRUCTURE HA VING INCREASED NEAR-SURFACECONCENTRA TION OF ACTIVE METALS byPawarat BootpakdeetamWilaiwan ChanmaneeBrian H. DennisFrederick M. MacDonnellCROSS REFERENCES

[0001] This application claims the benefit of U.S. Provisional Application No.63 / 704,671, filed on October 8, 2024, which application is incorporated herein in its entirety by reference.FIELD OF THE DISCLOSURE

[0002] The subject matter of the present disclosure refers generally to catalysts, and more particularly to catalysts having a heterogeneous distribution of active metals.BACKGROUND

[0003] Catalysts are commonly prepared by impregnating a porous support structure with one or more active metals. In the case of exothermic reactions, catalyst that is located deep within the interior of the support will be exposed to higher temperatures due tobuildup of heat energy released by the reaction than catalyst located near the surface of the support, where heat can rapidly escape. This may result in a runaway temperature increase that can damage the support due to the resulting thermal stresses and also cause deactivation of the metal catalyst, thereby reducing efficiency and increasing catalyst costs. Further, the increase in temperature may cause the reaction to shift to undesirable products. For instance, in the case of Fischer-Tropsch synthesis (FTS) reactions, elevated temperatures generally result in the undesired products of methane and soot.

[0004] The performance of supported cobalt-based FTS catalysts have been enhanced by numerous techniques, including the use of catalyst promoters, support shape, and impregnation techniques, including “eggshell” catalysts in which a thin layer of metal is impregnated onto the support only at the outer surface of the support, as well as coreshell catalysts in which a shell surrounds an inner core, each of which has considerable influence on the thermal properties of the catalyst. In the case of eggshell catalysts, many mass transport restrictions within a large catalyst pellet can be reduced, leading to a higher synthesis rate and C5+ selectivity and perhaps better temperature control of the reaction. Other catalyst production techniques include a cobalt on alumina core-shell (CS) support in which the support is made by partial oxidation of an aluminum powder via oxidation / corrosion with sodium hydroxide solution. These CS nanoparticle catalysts showed an increase in thermal conductivity over similarly prepared cobalt / alumina catalysts and better CO conversion and C5+ selectivities. Other catalyst production techniques have included a yolk / shell catalyst support from a high thermal conductive core-shell but used phase change material to manage the heat from catalytic reactions. Both eggshell and core-shell catalysts generally have better mass-transfer properties asthe catalytic reaction is constrained to a portion of the support containing active metal, thus minimizing differences in the intra particle diffusion rates of CO and resulting in an increase in C5+ selectivity. However, this comes with a proportional loss in catalyst loading.SUMMARY

[0005] In one aspect, an eggshell catalyst and a method of producing an eggshell catalyst having a heterogeneous distribution of active metal catalyst is provided. The catalyst comprises a support structure, which is preferably porous silica or alumina, that is impregnated with an active metal catalyst, such as cobalt and / or ruthenium. The metal catalyst is heterogeneously distributed within pores of the support structure so that there is a greater concentration of the active metal catalyst near the exterior surface of the support structure. The catalyst support structure preferably comprises a plurality of pellets, each of which may have a cylindrical shape or other suitable shape. Each pellet is porous but generally has an exterior surface. A molten form of the metal catalyst is poured over the pellets to impregnate the catalyst support with active metal catalyst. In one preferred embodiment, the molten catalyst solution is an aqueous solution of cobalt nitrate hexahydrate.

[0006] Unlike a conventional incipient wet impregnation technique that uses a volume of solution having a volume that is approximately equal to the pore volume of the catalyst support structure and that is typically at room temperature, the present method utilizes a solution that is heated above room temperature and also utilizes a volume of solution that is substantially less than the volume used in incipient wet impregnation. In a preferredembodiment, the present method utilizes a volume of solution that is in the range of approximately 10% to 30% by volume of the volume of solution that would typically be utilized for an incipient wet impregnation process. The penetration depth of the solution into the catalyst support may be controlled by adjusting the viscosity of the molten catalyst solution. For instance, the viscosity may be decreased by adding water or increased by adding cellulose or a similar type of polymer in a solution. The present catalyst preparation method results in the active catalyst metals being concentrated in a narrow band at or just below the exterior surface of the catalyst support structure.

[0007] Alternatively, the metal catalyst may be dissolved in a solvent to produce a metal catalyst solution, which is preferably an aqueous solution. The catalyst solution may be sprayed onto the catalyst support structure, which may be heated to a temperature sufficient to evaporate a portion of the solvent upon contact with the support structure. The temperature of the catalyst support structure may be adjusted to control the rate of evaporation of the solvent. By evaporating the solvent of the catalyst solution, the average catalyst penetration depth into the porous support structure may be limited, thereby also producing a catalyst structure having an increased near-surface concentration of metal catalyst.

[0008] In the case of exothermic reactions, the active catalyst being concentrated at or near the surface allows for heat produced by the reaction to escape from the catalyst support rapidly and efficiently, which will minimize buildup of heat energy within the catalyst support. By minimizing heat buildup, catalyst produced in accordance with the present method also minimizes catalyst deactivation, which reduces the amount of active metals required, thereby reducing the catalyst cost. In addition, by minimizing thetemperature of the catalyst support, catalyst produced in accordance with the present method also minimizes the potential for damage to the catalyst support due to thermal stresses and minimizes undesirable products, such as methane and soot in the case of FTS reactions.

[0009] The foregoing summary has outlined some features of the system and method of the present disclosure so that those skilled in the pertinent art may better understand the detailed description that follows. Additional features that form the subject of the claims will be described hereinafter. Those skilled in the pertinent art should appreciate that they can readily utilize these features for designing or modifying other structures for carrying out the same purpose of the system and method disclosed herein. Those skilled in the pertinent art should also realize that such equivalent designs or modifications do not depart from the scope of the system and method of the present disclosure.DESCRIPTON OF THE DRAWINGS

[0010] These and other features, aspects, and advantages of the present disclosure will become better understood with regard to the following description, appended claims, and accompanying drawings where:Fig. 1 shows optical micrographs of silica pellets impregnated in cobalt-ruthenium solution in accordance with the present disclosure.Fig. 2 shows optical micrographs of silica pellets impregnated in cobalt solution in accordance with the present disclosure.DETAILED DESCRIPTION

[0011] In the Summary above and in this Detailed Description, and the claims below, and in the accompanying drawings, reference is made to particular features, including method steps, of the invention. It is to be understood that the disclosure of the invention in this specification includes all possible combinations of such particular features. For example, where a particular feature is disclosed in the context of a particular aspect or embodiment of the invention, or a particular claim, that feature can also be used, to the extent possible, in combination with / or in the context of other particular aspects of the embodiments of the invention, and in the invention generally.

[0012] The term “comprises” and grammatical equivalents thereof are used herein to mean that other components, steps, etc. are optionally present. For example, a system “comprising” components A, B, and C can contain only components A, B, and C, or can contain not only components A, B, and C, but also one or more other components.

[0013] Where reference is made herein to a method comprising two or more defined steps, the defined steps can be carried out in any order or simultaneously (except where the context excludes that possibility), and the method can include one or more other steps which are carried out before any of the defined steps, between two of the defined steps, or after all the defined steps (except where the context excludes that possibility).

[0014] In one aspect, a catalyst having an increased near-surface concentration of active metal catalyst and a method of producing the catalyst are provided. The catalyst comprises a support structure, which is preferably porous silica or alumina, that is impregnated with an active metal catalyst, such as cobalt and / or ruthenium. The support structure is porous and has a pore volume. The metal catalyst is heterogeneouslydistributed within the pores of the support structure so that there is a greater concentration of the active metal catalyst near the exterior surface of the support. The catalyst support preferably comprises a plurality of pellets, each of which may have a cylindrical shape or other suitable shape. Each pellet is porous but generally has an exterior surface. In a preferred embodiment, the support comprises porous silicon dioxide (SiCh) or alumina (AI2O3). In other embodiments, the support may comprise other suitable types of catalyst support structures, including, but not limited to, zirconia (ZrCh), silicon carbide (SiC), or other porous structures suitable for use as a catalyst support, or any combinations thereof.

[0015] A solid metal catalyst is heated to produce a volume of a molten metal catalyst.The volume of the molten metal catalyst is then contacted with the plurality of pellets so that the volume of molten metal catalyst is distributed evenly onto the exterior surface of each of the pellets to impregnate the support structure with the active metal catalyst. In one embodiment, the molten metal catalyst may be poured over the pellets to impregnate the catalyst support structure with active metal catalyst. Alternatively, the molten catalyst may be sprayed onto the pellets or otherwise contacted with the exterior of the pellets. In one preferred embodiment, the molten catalyst solution is a solution of cobalt nitrate hexahydrate. Solid cobalt nitrate hexahydrate may be heated above its melting point to form the molten metal catalyst solution. Additional water may optionally be added to the aqueous molten cobalt nitrate hexahydrate solution.

[0016] Unlike a conventional incipient wet impregnation technique that uses a volume of solution having a volume that is approximately equal to the pore volume of the catalyst support and that is typically at room temperature, the present method utilizes a solution that is heated above room temperature to melt the catalyst and also utilizes a volume ofmolten catalyst that is substantially less than the volume used in incipient wet impregnation. In a preferred embodiment, the present method utilizes a volume of molten metal catalyst that is in the range of approximately 10% to 30% by volume of the pore volume of the support structure comprising the plurality of pellets. The volume of solution utilized for an incipient wet impregnation process is typically approximately the same as the total pore volume of the support structure. The penetration depth of the molten metal catalyst into the catalyst support may be controlled by adjusting the viscosity of the molten catalyst solution. For instance, the viscosity of the molten catalyst may be decreased by adding water or increased by adding cellulose or a similar type of polymer in a solution. The present catalyst preparation method results in the active catalyst metals being concentrated in a narrow band at or just below the surface of the catalyst support.

[0017] In the case of exothermic reactions, the active catalyst being concentrated at or near the surface allows for heat produced by the reaction to escape from the catalyst support rapidly and efficiently, which will minimize buildup of heat energy within the catalyst support. By minimizing heat buildup, catalyst produced in accordance with the present method also minimizes catalyst deactivation, which reduces the amount of active metals required, thereby reducing the catalyst cost. In addition, by minimizing the temperature of the catalyst support, catalyst produced in accordance with the present method also minimizes the potential for damage to the catalyst support due to thermal stresses and minimizes undesirable products, such as methane and soot in the case of FTS reactions.

[0018] The heterogeneous catalyst with increased near-surface metal concentration was prepared by mixing 5 grams of SiO? support (surface area of 210 m2 / gram and pore volume of 1.5 cm3 / gram) with 4.94 grams (16% by weight) of cobalt nitrate hexahydrate (CO(NOS)2 6H2O) melted at 70-90°C by placing on a hot plate. Molten cobalt nitrate hexahydrate was poured uniformly over the SiCh support (~1 cm bed height). The SiC>2 bed was stirred with a glass rod in order to facilitate even absorption of the molten cobalt nitrate on the SiOs support. The sample was then placed in ambient temperature for 2 hours and then dried overnight at 90°C. After drying, the sample was impregnated again with 0.15 grams (1.5% wt) ruthenium chloride (RuCh XH2O) in an aqueous solution. The sample was dried again at room temperature and 90°C overnight. The calcination process then heated the sample at 350°C with a heating rate 10°C / minute and kept at 35O°C for 4 hours in ambient air in order to form cobalt oxide and ruthenium oxide.

[0019] The volume of molten cobalt nitrate utilized was in the range of approximately 10% to 30% of the pore volume of the bulk catalyst support pellets. In the present preparation, a volume of about 1 ml to 2 ml of molten cobalt nitrate hexahydrate was utilized for an amount of catalyst support that would typically require about 7.5 ml of cobalt solution for an IWI method for the same amount of catalyst support. The volume of molten cobalt nitrate utilized in the present preparation was generally a volume sufficient to cover all exterior surfaces of all of the catalyst pellets without a significant excess of molten solution.

[0020] Optical micrographs of the eggshell Ru-Co / SiO2 catalysts are shown in Figs. 1(a)-(d). The present melt impregnation technique allows the synthesis of the eggshell catalyst with an average penetration depth of the catalyst, which in this case is cobalt andruthenium, of less than 0.5 mm from the exterior surface of the pellet, as shown in Figs. 1(c) and (d), which show measured penetration depths of approximately 400 microns and 430 microns in Fig. 1(c) and 467 microns in Fig. 1(d). The catalyst support structure preferably has an average penetration depth of less than 30% of a diameter of each pellet, and more preferably less than 25%. The local metals oxide (cobalt and ruthenium) content in the shell area of the catalyst shown in Fig. 1 is about 30%, and about 70% of the silica support contains no metal oxide. The synthesis of thin or thick eggshell can be adjusted by altering the viscosity of the molten cobalt nitrate solution. The viscosity may be adjusted to adjust the penetration depth of the molten metal catalyst into the pore volume of the support structure by adding water to decrease the viscosity or by adding an organic solution, such as cellulose in an organic solvent, to increase the viscosity of the molten cobalt nitrate solution. Alternatively, other polar solvents, including, but not limited to, alcohols or acetone, or any combinations thereof, may be utilized to decrease the viscosity. Alternatively, other compositions may also be utilized to increase the viscosity, including, but not limited to, sugar solutions, polyethylene glycol, glycerol, polyols, polyvinyl alcohol, any suitable types of thickening agents, or any combinations thereof. Altering the viscosity of the solution will then control the penetration depth of the solution into the pores of the support, with the depth of penetration being greater as the viscosity decreases. The volume of liquid added to increase or decrease the viscosity should be minimized in order to minimize the overall volume of the impregnation solution.

[0021] After impregnating the support structure with catalyst, the impregnated support structure is dried, preferably utilizing an inert gas, which is preferably nitrogen gas.After drying, the impregnated support structure is calcinated to produce the catalyst structure.

[0022] In an alternative method of producing a catalyst structure having an increased near-surface metal concentration, the solid cobalt nitrate hexahydrate may be dissolved in a solvent to produce a metal catalyst solution, which may be sprayed onto the plurality of pellets to impregnate the support structure with active metal catalyst. The solvent is preferably water, though other solvents suitable for dissolving solid catalyst may be utilized. In this embodiment, the support structure, which is preferably a bulk mass of pellets, may be heated to a temperature sufficient to evaporate a portion of the solvent upon contact with the support structure when the metal catalyst solution is sprayed onto the pellets. The temperature of the catalyst support structure may be adjusted to control the rate of evaporation of the solvent. By evaporating the solvent of the catalyst solution, the average catalyst penetration depth into the porous support structure may be limited, thereby also producing a catalyst structure having an increased near-surface concentration of metal catalyst. To distribute the metal catalyst solution on the exterior surfaces of the pellets, the pellets may be placed into a container, and the container may then be rotated to tumble the pellets within the container during the spraying step of the method. The metal catalyst solution may also be independently heated before spraying onto the pellets to increase the efficiency of the evaporation of the solvent from the surfaces of the pellets.

[0023] To prepare the catalyst structure using this method, cobalt eggshell catalysts were prepared using 100 g of SiO? support (3 mm pellet diameter, surface area 200 m2 / g, pore volume 0.85 cm3 / g) impregnated with 98.8 g of cobalt nitrate hexahydrate(CO(NOS)2-6H2O), corresponding to 20 wt% cobalt. The cobalt nitrate was dissolved in the minimum required amount of water, based on its solubility (134 g / 100 mb of water at 20 °C), resulting in a 75 mL solution. This solution was uniformly sprayed onto the SiO? pellets using a 0.3 mm nozzle brush sprayer operated with nitrogen gas at a pressure of 15 psig. During spraying, the SiC>2 support was continuously tumbled in a rotating bowl at a speed of 50 revolutions per minute (rpm) to enhance cobalt distribution.

[0024] Nitrogen gas was used during the spraying process to assist drying, which promotes a more uniform cobalt distribution compared to air drying, which typically results in medium to large cobalt aggregates (60-120 nm). A heat gun was used to maintain the rotating bowl at 70-80 °C, allowing the cobalt nitrate solution to evaporate rapidly upon contact with the warm SiCh surfaces. This evaporation of the solvent ensured that cobalt was concentrated at or near the exterior surfaces of the support structures, thereby achieving the desired concentrated egg-shell distribution. The heat gun was used to blow the nitrogen gas onto the support structure during the spraying step to increase the evaporation rate of the solvent of the metal catalyst solution.

[0025] During the drying process subsequent to the spraying step to impregnate the support structure, the sample was placed in a gas washing bottle, and nitrogen gas was passed through at a flow rate of 200 ccm. The setup was placed in an oven and dried overnight at 90 °C. The dried sample was heated to 350 °C at a rate of 5 °C / min and held at this temperature for 4 hours in ambient air to convert cobalt nitrate into cobalt oxide.

[0026] The eggshell thickness can be controlled by adjusting the volume of the cobalt slurry sprayed onto the SiCh pellets. This slurry volume is typically matched to the calculated void volume of the silica pellets to achieve uniform impregnation. In addition,the temperature during the spraying process plays a significant role in determining the final coating quality. At higher temperatures, the cobalt nitrate solution tends to evaporate rapidly upon contact with the warm SiO? surface, which can influence the uniformity of the cobalt layer.

[0027] Another factor is the rotation speed of the mixing bowl during spraying. If the rotation speed is too low, the silica pellets may not move uniformly, causing the slurry to deposit only on a limited area rather than evenly over all particles. In opposition, an optimal rotation speed ensures continuous movement of the pellets, thereby promoting even distribution of the slurry and consistent coating thickness.

[0028] The pressure of the spray nozzle also affects the cobalt deposition. Proper control of the pressure ensures a fine and steady spray pattern, avoiding excessive slurry accumulation. Additionally, maintaining the airbrush nozzle at a slightly warm temperature during spraying helps prevent clogging caused by the partial drying or crystallization of cobalt nitrate, which is the cobalt precursor used in this process. To this end, the aqueous cobalt solution is preferably heated to 60-80 °C prior to spraying onto the pellets.

[0029] Optical micrographs of the eggshell Co / SiO2 catalysts produced in accordance with this alternative method are shown in Fig. 2, which shows a measured penetration depth of approximately 360 microns plus / minus 21 microns and 378 microns plus / minus 15 microns for two representative catalyst structures, respectively. Preferably, each respective pellet of the plurality of pellets of the catalyst structure has an average catalyst penetration depth of less than 0.5 mm and also preferably less than 30% of a diameter of the pellet.

[0030] The catalyst structure produced by this alternative method may be dried and / or calcinated in the same manner as previously described herein or in any other suitable manner. In alternative embodiments, this method may be utilized to impregnate a catalyst support structure with any other catalyst, including ruthenium or a combination of cobalt and ruthenium.

[0031] It is understood that versions of the present disclosure may come in different forms and embodiments. Additionally, it is understood that one of skill in the art would appreciate these various forms and embodiments as falling within the scope of the invention as disclosed herein.

[0032] What is claimed is:1) A method of producing a catalyst structure comprising a support structure impregnated with active metal catalyst, said method comprising the steps of: providing a support structure, wherein the support structure is porous and has a pore volume, wherein the support structure comprises a plurality of pellets, wherein each respective pellet of the plurality of pellets has an exterior surface; heating a solid metal catalyst to produce a volume of a molten metal catalyst; contacting the volume of the molten metal catalyst with the plurality of pellets and distributing the volume of the molten metal catalyst onto the exterior surface of each respective pellet of the plurality of pellets to impregnate the support structure with active metal catalyst, wherein the volume of the molten metal catalyst contacted with the plurality of pellets is in a range of 10% to 30% by volume of the pore volume of the support structure;drying the impregnated support structure; and after drying, calcinating the impregnated support structure to produce the catalyst structure. ) The method of claim 1, wherein each respective pellet of the plurality of pellets of the catalyst structure has an average catalyst penetration depth of less than 30% of a diameter of the pellet. ) The method of claim 1, wherein each respective pellet of the plurality of pellets of the catalyst structure has an average catalyst penetration depth of less than 0.5 mm. ) The method of claim 1, further comprising the step of adjusting the viscosity of the molten metal catalyst to adjust a penetration depth of the molten metal catalyst into the pore volume of the support structure. ) The method of claim 4, wherein the step of adjusting the viscosity of the molten metal catalyst comprises adding water to the molten metal catalyst to decrease the viscosity of the molten metal catalyst or adding an organic solution to the molten metal catalyst to increase the viscosity of the molten metal catalyst. ) The method of claim 1, wherein the organic solution comprises cellulose. ) The method of claim 1, wherein the support structure comprises silica, alumina, zirconia, or silicon carbide. ) The method of claim 1, wherein the metal catalyst comprises cobalt, ruthenium, or a combination of cobalt and ruthenium. ) The method of claim 1, wherein the step of drying the impregnated support structure comprises drying the impregnated support structure with an inert gas. 0) The method of claim 9, wherein the inert gas is nitrogen.) The method of claim 1, wherein the step of calcinating the impregnated support structure comprises calcinating the impregnated support structure in the presence of oxygen. ) A method of producing a catalyst structure comprising a support structure impregnated with active metal catalyst, said method comprising the steps of: providing a support structure, wherein the support structure is porous and has a pore volume, wherein the support structure comprises a plurality of pellets, wherein each respective pellet of the plurality of pellets has an exterior surface; dissolving a metal catalyst in a solvent to produce a metal catalyst solution; heating the support structure; spraying the metal catalyst solution onto the plurality of pellets and distributing the metal catalyst solution onto the exterior surface of each respective pellet of the plurality of pellets to impregnate the support structure with active metal catalyst, wherein the support structure is heated to a temperature sufficient to evaporate a portion of the solvent of the metal catalyst solution upon contact of the metal catalyst solution with the support structure, thereby limiting an average catalyst penetration depth of the metal catalyst solution into the pore volume of the support structure during the spraying and distributing step; drying the impregnated support structure; and after drying, calcinating the impregnated support structure to produce the catalyst structure. ) The method of claim 12, further comprising the step of heating the metal catalyst solution before spraying the metal catalyst solution onto the plurality of pellets.) The method of claim 12, wherein the support structure is disposed within a container, wherein the step of distributing the metal catalyst solution onto the exterior surface of each respective pellet of the plurality of pellets comprises rotating the container during the spraying step. ) The method of claim 12, further comprising the step of blowing nitrogen gas onto the support structure during the spraying step to increase an evaporation rate of the solvent of the metal catalyst solution. ) The method of claim 12, wherein each respective pellet of the plurality of pellets of the catalyst structure has an average catalyst penetration depth of less than 30% of a diameter of the pellet. ) The method of claim 12, wherein each respective pellet of the plurality of pellets of the catalyst structure has an average catalyst penetration depth of less than 0.5 mm.) The method of claim 12, wherein the support structure comprises silica, alumina, zirconia, or silicon carbide. ) The method of claim 12, wherein the metal catalyst comprises cobalt, ruthenium, or a combination of cobalt and ruthenium.

Claims

CLAIMSWhat is claimed is:1) A method of producing a catalyst structure comprising a support structure impregnated with active metal catalyst, said method comprising the steps of: providing a support structure, wherein the support structure is porous and has a pore volume, wherein the support structure comprises a plurality of pellets, wherein each respective pellet of the plurality of pellets has an exterior surface; heating a solid metal catalyst to produce a volume of a molten metal catalyst; contacting the volume of the molten metal catalyst with the plurality of pellets and distributing the volume of the molten metal catalyst onto the exterior surface of each respective pellet of the plurality of pellets to impregnate the support structure with active metal catalyst, wherein the volume of the molten metal catalyst contacted with the plurality of pellets is in a range of 10% to 30% by volume of the pore volume of the support structure; drying the impregnated support structure; and after drying, calcinating the impregnated support structure to produce the catalyst structure.2) The method of claim 1, wherein each respective pellet of the plurality of pellets of the catalyst structure has an average catalyst penetration depth of less than 30% of a diameter of the pellet.3) The method of claim 1, wherein each respective pellet of the plurality of pellets of the catalyst structure has an average catalyst penetration depth of less than 0.5 mm.4) The method of claim 1 , further comprising the step of adjusting the viscosity of the molten metal catalyst to adjust a penetration depth of the molten metal catalyst into the pore volume of the support structure.5) The method of claim 4, wherein the step of adjusting the viscosity of the molten metal catalyst comprises adding water to the molten metal catalyst to decrease the viscosity of the molten metal catalyst or adding an organic solution to the molten metal catalyst to increase the viscosity of the molten metal catalyst.6) The method of claim 1, wherein the organic solution comprises cellulose.7) The method of claim 1, wherein the support structure comprises silica, alumina, zirconia, or silicon carbide.8) The method of claim 1, wherein the metal catalyst comprises cobalt, ruthenium, or a combination of cobalt and ruthenium.9) The method of claim 1, wherein the step of drying the impregnated support structure comprises drying the impregnated support structure with an inert gas.10) The method of claim 9, wherein the inert gas is nitrogen.11) The method of claim 1, wherein the step of calcinating the impregnated support structure comprises calcinating the impregnated support structure in the presence of oxygen.12) A method of producing a catalyst structure comprising a support structure impregnated with active metal catalyst, said method comprising the steps of: providing a support structure, wherein the support structure is porous and has a pore volume, wherein the support structure comprises a plurality of pellets, wherein each respective pellet of the plurality of pellets has an exterior surface;dissolving a metal catalyst in a solvent to produce a metal catalyst solution; heating the support structure; spraying the metal catalyst solution onto the plurality of pellets and distributing the metal catalyst solution onto the exterior surface of each respective pellet of the plurality of pellets to impregnate the support structure with active metal catalyst, wherein the support structure is heated to a temperature sufficient to evaporate a portion of the solvent of the metal catalyst solution upon contact of the metal catalyst solution with the support structure, thereby limiting an average catalyst penetration depth of the metal catalyst solution into the pore volume of the support structure during the spraying and distributing step; drying the impregnated support structure; and after drying, calcinating the impregnated support structure to produce the catalyst structure.13) The method of claim 12, further comprising the step of heating the metal catalyst solution before spraying the metal catalyst solution onto the plurality of pellets.14) The method of claim 12, wherein the support structure is disposed within a container, wherein the step of distributing the metal catalyst solution onto the exterior surface of each respective pellet of the plurality of pellets comprises rotating the container during the spraying step.15) The method of claim 12, further comprising the step of blowing nitrogen gas onto the support structure during the spraying step to increase an evaporation rate of the solvent of the metal catalyst solution.16) The method of claim 12, wherein each respective pellet of the plurality of pellets ofthe catalyst structure has an average catalyst penetration depth of less than 30% of a diameter of the pellet.17) The method of claim 12, wherein each respective pellet of the plurality of pellets of the catalyst structure has an average catalyst penetration depth of less than 0.5 mm.18) The method of claim 12, wherein the support structure comprises silica, alumina, zirconia, or silicon carbide.19) The method of claim 12, wherein the metal catalyst comprises cobalt, ruthenium, or a combination of cobalt and ruthenium.

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