Method of forming supported doped palladium containing oxidation catalysts

This application is a divisional application of U.S. Non-Provisional patent application Ser. No. 13/588,779 entitled “DOPED PALLADIUM CONTAINING OXIDATION CATALYSTS”, filed on Aug. 17, 2012, which is now U.S. Pat. No. 8,652,993, which claims the benefit of Provisional Application Ser. No. 61/524,937 entitled “DOPED PALLADIUM CONTAINING OXIDATION CATALYSTS”, filed Aug. 18, 2011, both of which are herein incorporated by reference in their entirety.

This invention was made with Government support under Florida Hydrogen Initiative Contract #DEFC3604GO14225 awarded by the U.S. Department of Energy. The Government has certain rights in this invention.

Disclosed embodiments relate to methods of forming palladium containing catalysts or gas sensors.

One of the future alternatives to current fossil-based transportation fuels has been centered on hydrogen gas (H 2 ). Currently, H 2 is the primary energy source of today's space exploration projects (e.g., as rocket propellant). It is also used in fuel cells that power a variety of machinery including automobiles. Furthermore, H 2 is an important industrial commodity produced and used in many industries. For example, it is used for the reduction of metal oxides (e.g. iron ore), ammonia synthesis, and production of hydrochloric acid, methanol and higher alcohols, aldehydes, hydrogenation of various petroleum, coal, oil shale and edible oils, among others. However, H 2 is a colorless, odorless gas, and is also a flammable gas with a lower explosive limit of about 4% in air. Therefore reliable H 2 sensors are required to detect H 2 leaks wherever H 2 is produced, stored, or used.

To detect H 2 , sensors comprising a palladium alloy Schottky diode formed on a silicon substrate are known. These sensors are based on metal-oxide-semiconductor (MOS) technology that is used in the semiconductor industry. The gas sensing MOS structures comprise a H 2 -sensitive metal (palladium or its alloy) on a dielectric (e.g., an oxide) adherent to a semiconductor. This H 2 sensor has been commercialized and exploited for detecting H 2 leaks during pre-launches of space vehicles. Others have also used palladium or the like as a sensing element for detecting H 2 . A H 2 sensor containing an array of micromachined cantilever beams coated with palladium/nickel has also been disclosed.

Semiconductor materials with a wide band-gap (e.g. gallium nitride) have also been used to make diodes for H 2 detection. One of the concerns for all of these types of sensors using palladium or the like is the requirement of a high operating temperature (greater than 200° C.) and further elevated temperatures (greater than 500° C.) to reactivate the sensing element, bringing about lengthy analysis. Another issue is sensitivity of the sensing element to unintended compounds that are commonly found in the atmosphere, including water vapor, various hydrocarbons, and various reducing gases such as carbon monoxide and hydrogen sulfide. Although not conventionally used, chemochromic H 2 sensors are known. Some chemochromic H 2 sensors lack field stability and have a tendency to crack and peel and some can be washed off by precipitation and/or condensation. Moreover, some chemochromic H 2 sensors do not show selectivity to H 2 .

Thus, there remains a need for an improved, reliable and durable chemochromic H 2 sensor, or more generally an oxidation catalyst for a chemochromic reducing gas sensor or catalyst, for a variety of applications, including space, transportation, oil refineries, and chemical plants.

This Summary is provided to introduce a brief selection of disclosed concepts in a simplified form that are further described below in the Detailed Description including the drawings provided. This Summary is not intended to limit the claimed subject matter's scope.

A method of forming a supported oxidation catalyst includes providing a support comprising a metal oxide or a metal salt, and depositing first palladium compound particles and second precious metal group (PMG) metal particles on the support while in a liquid phase including at least one solvent to form mixed metal comprising particles on the support. The PMG metal is not palladium. The mixed metal particles on the support are separated from the liquid phase to provide the supported oxidation catalyst.

As used herein, a “support” refers to a material in which the palladium compound particles and PMG metal particles are deposited onto in the formation of the supported oxidation catalyst. The support can be in layer form on another layer, such as a layer comprising a plurality of bound particles on a solid surface, or be in discrete particle form.

Disclosed supported oxidation catalysts can be used as a pigment for hydrogen sensing/detection or other sensing/detection of other reducing gases where they may be encapsulated in a reducing gas permeable polymer matrix such as silicone rubber or silicone resin. In another embodiment, the supported oxidation catalyst is used as a catalyst without polymer encapsulation.

FIG. 1A is a cross sectional depiction of an example supported oxidation catalyst comprising a support, and mixed metal comprising particles on the support including first particles comprising a palladium compound and second particles comprising a precious metal group (PMG) metal, where the PMG is not palladium, according to an example embodiment.

FIG. 1B is a cross sectional depiction of an example encapsulated supported oxidation catalyst comprising the oxidation catalyst in FIG. 1A along with an optional reducing gas permeable polymer that forms a continuous phase which provides complete encapsulation for the support and metal comprising particles, according to an example embodiment.

FIG. 2 shows ΔE vs. exposure time to 100% H 2 gas sensing performance for various disclosed platinum-doped Pd/TiO 2 supported oxidation catalysts used as pigments encapsulated in DOW 734® silicone resin that were synthesized as compared to a known PdO on TiO 2 control (no Pt doping).

FIG. 3 shows ΔE vs. exposure time to 100% H 2 gas sensing performance for various disclosed platinum-doped Pd/TiO 2 supported oxidation catalysts used as pigments encapsulated in DOW 734® silicone resin that were synthesized having different Pt contents as compared to a known PdO on TiO 2 control (no Pt doping).

FIG. 4 shows ΔE vs. exposure time to 100% H 2 sensing gas for various disclosed platinum-doped PdO/TiO 2 supported oxidation catalysts used as pigments encapsulated in DOW 734® silicone resin that were synthesized using different solvents during sonication processing as compared to known PdO on TiO 2 controls (no Pt doping):

FIG. 5 shows ΔE vs. exposure time to 100% H 2 sensing gas for various disclosed PdO/BaSO 4 oxidation catalysts (BaSO 4 as supports) used as pigments encapsulated in DOW 734® silicone resin that were synthesized as compared to a known PdO on TiO 2 oxidation catalyst control (no Pt doping).

Disclosed embodiments in this Disclosure are described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. FIGS. 1A and 1B are not drawn to scale, and they are provided merely for illustration. Several aspects are described below with reference to example applications for illustration.

It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the disclosed embodiments. One having ordinary skill in the relevant art, however, will readily recognize that the subject matter disclosed herein can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring structures or operations that are not well-known. This Disclosure is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with this Disclosure.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of this Disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all sub-ranges subsumed therein. For example, a range of “less than 10” can include any and all sub-ranges between (and including) the minimum value of zero and the maximum value of 10, that is, any and all sub-ranges having a minimum value of equal to or greater than zero and a maximum value of equal to or less than 10, e.g., 1 to 5.

Some disclosed embodiments are based on the discovery that a palladium compound oxidation catalyst such as PdO on a support when “doped” with particles comprising a PMG metal provides supported oxidation catalysts which oxidize a reducing gas (e.g., hydrogen, carbon monoxide, or hydrocarbons) with oxidation kinetics sped up significantly and sensitivity increased significantly as compared to the palladium compound (PdO) on the support oxidation catalyst alone. The PMG metal particles feature the metal in its atomic/elemental form which may be clearly contrasted with the form of the metal in metal compounds, such as compounds selected from the group consisting of oxides, hydroxides and hydrated oxides of platinum group metals. As well known in metallurgy, physics and chemistry, an elemental (or atomic) metal is in its elemental uncharged state while metals in metal compounds are in an ionic (generally cationic) state. For example, for the conventional PdO pigment, Pd is in the +2 state (Pd +2 ), and as known in the art, and upon exposure to a reducing gas such as H 2 , Pd +2 reduces to its elemental/atomic form Pd 0 which results in a visible color change (and the generation of water). An elemental metal such as Pd 0 cannot itself accept electrons.

Other disclosed embodiments are based on the discovery that the support selection of a material other than titania such as BaSO 4 also improves oxidation kinetics and detection sensitivity. These embodiments may be combined, by combining doping of the palladium compound (e.g., PdO) with a PMG metal and BaSO 4 supports, which may further improve oxidation kinetics and detection sensitivity.

The support is generally selected so that interaction of the metal particles with the surface of support surfaces minimizing the total energy required for palladium compound (e.g., PdO) reduction. This suggests a small chemical interaction between support and the metal particles and hence destabilization of the palladium compound particles on the surface of the support. For PdO on a TiO 2 support surface, the second PMG metal “dopant” (e.g. Pt) lowers the activation energy required for PdO reduction.

Disclosed supported oxidation catalysts embodied as chemochromic sensors (pigments) change color in a presence of at least one reducing gas. Embodied as chemochromic reducing gas sensors, disclosed reducing gas sensors are irreversible sensors. Disclosed PMG's can utilize the PMG metals of gold, silver, or platinum group metals, such as ruthenium, rhodium, osmium, iridium, and platinum.

The response time of disclosed chemochromic reducing gas sensors toward H 2 gas by including nanosized PMG dopant particles, wherein the PMG is not palladium, with palladium compound particles (e.g., PdO) has been found to unexpectedly be decreased by about 10 fold while encapsulated in a silicone resin. As noted above, disclosed supported oxidation catalysts can also be used as catalysts, such as to reduce undesirable emissions from fossil fuel powered vehicles.

FIG. 1 is a cross sectional depiction of an example supported oxidation catalyst 100 comprising a support 101 that can comprise a plurality of metal oxide or metal salt (e.g. sulfate or carbonate) particles, first particles 102 comprising palladium compound (e.g., PdO), and second particles 103 comprising a PMG metal wherein the PMG is not palladium, on the support 101 , according to an example embodiment. In typical applications, the supported oxidation catalyst 100 can be adhered to a substrate where it can provide its chemochromic reducing gas sensing or catalyst function, such as on a metal surface (e.g. a metal wall). The shapes of the support 101 , first particles 102 and second particles 103 shown in FIG. 1A are arbitrary.

As noted above, although the support 101 is shown in FIG. 1A in layer form, such as comprising a polycrystalline layer, support 101 may also be in discrete particle form. Typically the support 101 comprises metal oxide or metal salt particles, mixed metal oxide particles, but can also comprise their salts. For pigment (chemochromic) applications, the support 101 comprises particles which are generally colorless, white, or slightly colored. The metal oxide particles can comprise transition metal oxide particles and the metal salt particles can comprise alkaline earth salt particles.

The transition metal oxide particles can comprise Group IV metal oxide particles, such as such ZrO 2 , SrTiO 3 , AlTiO 3 or SrZrO 3 . The metal salt particles can comprise alkaline earth metal salt particles, such as BaSO 4 . The support can also comprise titanium (e.g., embodied as TiO 2 ), and cerium (e.g., embodied as CeO 2 ). In one embodiment, the support 101 comprises particles having a size in a range from 0.1 μm to 1.0 μm, and in one particular embodiment from 0.2 μm to 0.25 μm for pigment applications to maximize opacity.

The first particles 102 are palladium compound particles that can comprise palladium oxide, palladium hydroxide, or a palladium salt. The first particles 102 can have a median size in the range of 2 nm to 8 nm. The second particles 103 are PMG metal particles that can comprise gold, silver, or a PGM metal other than palladium, such as ruthenium, rhodium, osmium, iridium, or platinum. In one embodiment the second particles 103 have a median size in the range from 5 nm to 10 nm. The relative concentration ratio of the second particles 103 to first particles 102 generally range from 1:10 to 1:25 by weight.

The supported oxidation catalyst 100 can further comprise an ultraviolet (UV) absorber or UV Blocker, or a mixture thereof to remedy possible adverse environmental effects. The UV blocker can comprise ZnO 2 or TiO 2 . The UV absorber can comprise compounds from the triazine family, such as benzotriazol or a benzopheneone. The UV absorber or UV Blocker can be in the range of 1-10 wt. % of the supported oxidation catalyst 100 . Although not shown in FIG. 1A , UV absorber or UV Blocker particles if shown would be on the support 101 analogous to first particles 102 or second particles 103 , or be physically mixed with supported oxidation catalyst 100 .

FIG. 1B is a depiction of an example encapsulated supported oxidation catalyst 150 that comprises the supported oxidation catalyst 100 shown in FIG. 1A along with an optional reducing gas permeable polymer 104 that forms a continuous phase (matrix) which provides complete encapsulation for the support 101 , first particles 102 , and second particles 103 , according to an example embodiment. The gas permeable polymer 104 continuous phase can be formed by admixing a suitable material, such as a moisture curable or heat curable silicone sealant with dry catalyst particles or its slurry comprising support particles 101 , first particles (palladium compound) 102 , and second particles (PMG metal, not palladium) 103 in a liquid phase before drying, and then curing the silicone sealant to form a rubbery gas permeable silicone polymer that forms a continuous phase that provides complete encapsulation.

In some embodiments, the supported oxidation catalyst can comprise a composite layer. A composite layer is known in the material arts and is defined herein as a composite engineered material made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct on a macroscopic level within the finished structure. In principle, composites can be constructed of any combination of two or more materials, metallic, organic, or inorganic; but the constituent forms are typically more restricted. The matrix provided by the gas permeable polymer 104 is the body constituent, acting as a continuous phase having the other materials referred to as the additive phases embedded therein, with the gas permeable polymer 104 serving to enclose the other composite components and give it bulk form.

Disclosed embodiments also include methods of forming disclosed doped supported oxidation catalysts. A support is provided, such as comprising a plurality of metal oxide or metal salt particles. A palladium compound (e.g., PdO) and a PMG metal other than Pd are deposited on the support(s) in a liquid phase slurry including a solvent to form nanosized palladium compound (e.g., PdO) particles and PMG metal particles other than Pd particles. A solid-liquid separation process such as filtering the slurry allows removal of the liquid phase to form dried solid supported oxidation catalyst which may be referred to as composite particles.

The depositing can comprise co-depositing the palladium compound and PMG metal using a precursor for the palladium compound and a precursor for the PMG metal. As described below, depositing the palladium compound after depositing the PMG metal may provide enhanced sensing performance.

In one embodiment, the method further comprises admixing a moisture curable silicone sealant with the particles in the liquid phase, and then curing the moisture curable silicone sealant to form a rubbery gas permeable silicone polymer. In this embodiment, the gas permeable polymer provides a continuous phase that completely encapsulates the palladium compound particles, the PMG metal particles and the supports.

As noted above, disclosed supported oxidation catalysts can be used in a variety of applications including in the development of chemochromic passive sensors for detecting reducing gases such as hydrogen, carbon monoxide, and hydrocarbons (e.g., natural gas or propane). These gases/liquids are widely used and produced in many industries and a safe operation is a top priority for all of them. Disclosed supported oxidation catalysts can also provide a layer of protection against accidental releases of these gases and therefore reinforces a safe manufacturing/utilization environment. Furthermore, disclosed supported oxidation catalysts can be used as catalysts in the automotive industry to reduce hydrocarbon, carbon monoxide, and NOx emissions, or in any industry that carry methane combustion such as natural gas-powered vehicles. Other example applications include depollution of natural gas-powered vehicles, and catalytic processes for energy production from natural gas.

The effectiveness of gas sensors, such as disclosed supported oxidation catalysts used as chemochromic reducing gas sensors, is typically evaluated by measuring the time necessary to reach a given level of color change, and by determining the total amount of color change. The latter is expressed as ΔE and is measured by a colorimeter. ΔE measures a difference in color by measuring specific parameters of the film (L, a, b). These parameters refer to a color system for measuring absolute chromaticity, L*a*b* and color difference Δ(L*a*b*) or ΔE. Color is defined in three dimensions: hue, chroma (saturation) and lightness. L*=the gradient from light to dark, a*=the gradient from red to green, and b*=the gradient from yellow to blue, and ΔE*={(ΔL*) 2 +(Δa*) 2 +(Δb*) 2 } 1/2 . This equation gives a standard measurement technique by which one can compare color changes from different chemochromic reducing gas sensor film samples. The greater the ΔE* value, the greater the color contrast. Chemochromic reducing gas sensor films can be analyzed both before and after exposure to the reducing gas, allowing quantification of the intensity of the color change.

Disclosed embodiments are further illustrated by the following specific Examples, which should not be construed as limiting the scope or content of this Disclosure in any way.

For example, although Pt metal is generally used as the “dopant” in these examples, Pt can be replaced by gold, silver, or other platinum group metal. Example methods of preparation of disclosed supported oxidation catalysts are now described. Samples referred to as “base” in FIGS. 2-5 such as PK-1-135 NM1 described below refer to known PdO on TiO 2 controls (no Pt metal doping). For testing, the samples were all placed inside a vial and were exposed to 100% H 2 gas with a 45 ml/min flow rate.