Chemochromic hydrogen sensors

Disclosed embodiments relate to chemochromic-based hydrogen 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.

Semiconductors with 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 for a variety of applications, including space, transportation, oil refineries, and chemical plants.

FIG. 1 is a depiction of an exemplary two layer H 2 sensor comprising a composite layer including supports comprising a plurality of metal oxide particles and a PGM compound on the supports, and a gas permeable polymer that forms a continuous phase that completely encapsulates both the supports and the PGM compound which are both embedded in the polymer, according to an embodiment of the invention.

FIG. 2 is color contrast vs. exposure time data for a variety of chemochromic H 2 sensors, according to various embodiments of the invention.

FIG. 3 is a table that provides a chemochromic H 2 sensor preparation matrix for a plurality of different embodiments of the invention.

Disclosed embodiments in t hi s Disclosure are described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate the disclosed embodiments. 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.

This Disclosure includes new chemochromic H 2 sensors comprising supports including a plurality of metal oxide particles and a PGM compound that functions as a H 2 sensitive pigment on the supports. The Inventors have discovered sensitive, reliable, and durable chemochromic H 2 sensors can utilize a wide variety of supports including support materials other than titania, and the PGM compounds can include PGM salts or PGM complexes. In one embodiment, the chemochromic H 2 sensor comprises supports comprising a plurality of metal oxide particles exclusive of titania, and a PGM compound on the supports. In another embodiment, the chemochromic H 2 sensor comprises supports comprising a plurality of metal oxide particles, and a PGM salt or a PGM complex on the supports.

Typically the supports comprise metal oxides, mixed metal oxides, and their salts that are colorless, white, or slightly colored. The metal oxides, mixed metal oxides and their salts can comprise at least one transition metal, such as titanium, zirconium and cerium (e.g., embodied as CeO 2 ). In one embodiment the supports comprise a plurality of Group IV metal oxide particles, such as ZrO 2 , SrTiO 3 , or SrZrO 3 particles. In one embodiment the particle size for the support particles is in a range from 0.1 to 1.0 μm, and in one particular embodiment, from 0.2 to 0.25 μm to maximize opacity.

The PGM for the PGM compound can comprise one or more of palladium, iridium, ruthenium, platinum, rhodium, gold, and silver. The PGM compounds generally comprise PGM oxides, hydroxides, and hydrated oxides. In certain embodiments the PGM compounds comprise PGM salts (e.g., acetates, or other carboxylates such as propionates), or PGM complexes such as acetylacetonates, dichlorodiamine, tetraamine, and tetrachloropalladate.

The PGM salt can be an organic salt or an inorganic salt. In one particular embodiment the PGM salt comprises a carboxilate, such as palladium acetate, Pd(OCOCH 3 ) 2 , or other carboxylates such as propionates. Exemplary inorganic salt examples include chlorides, sulfates, and iodide-based salts.

Using a PGM salt or PGM complex instead of a conventional PGM compound such as a PGM oxide, hydroxide, or hydrated oxide for the pigment can allow significant simplification in H 2 sensor preparation. The H 2 sensor preparation method using a PGM salt or PGM complex can be simplified because PGM salts or PGM complexes can be dissolved in a suitable solvent to form a PGM solution which can then be added to a slurry solution comprising support particles, followed by drying of the resulting solution. Accordingly, the neutralization and filtration stages generally needed when using conventional PGM compounds (e.g., PGM oxides, hydroxides, hydrated oxides) to form H 2 sensors are effectively eliminated. When the PGM salt or PGM complex has low solubility in water, such as the organic PGM salt palladium acetate, solvents such as weak organic acids (e.g., acetic acid) may be used.

The H 2 sensor can further comprise an accelerant or contrast additive. For example, the accelerant or contrast additive can comprise MoO 3 , (NH 4 ) 6 Mo 7 O 24 , or polyoxometalates that include V, Nb, Ta, Cr, Mo or W.

One embodiment (a) comprises adding a PGM salt or PGM complex to a slurry of support particles comprising metal oxide particles exclusive of titania, and then neutralizing the resulting mixture by adding an alkali or an acid, as required. In another embodiment (b), PGM salts or PGM complexes are added to an alkali or an acid, as needed, to a slurry of the support particles comprising metal oxide particles exclusive of titania, while maintaining the pH of the slurry from 3 to 11, such as from 6 to 11, and in the particular case mineral acid salts of the PGM are used, from 8 to 11, to neutralize the mixture. In yet another embodiment (c), a slurry of the support particles comprising metal oxide particles exclusive of titania is premixed with a higher amount of an alkali or an acid, as required, than required to neutralize the PGM salts or PGM complex, and then a PGM salt or PGM complex is added to neutralize. Embodiment (b) generally allows the fine PGM compounds to be present more uniformly on the surfaces of the support particles with a lower amount of the PGM compounds needed to be used. The alkalis for neutralization, if needed, can include sodium hydroxide, potassium hydroxide, sodium carbonate, and ammonia, for example. The acids for neutralization, if needed, can include hydrochloric acid, sulfuric acid, nitric acid, and acetic acid. The temperature for the neutralization reaction can be in the range of 50° C. to 90° C.

The PGM compounds can be applied in an amount of 0.5 to 10% by weight, such as 1 to 5% by weight as expressed by the PGM based on the weight of the supports. The Inventors have recognized that an amount of PGM compound lower than 0.5% by weight may cause insufficient color change to detect gas leakage, while an amount of higher than 10% by weight can become uneconomical with no further improvement in the sensing function being expected.

The effectiveness of the H 2 gas sensors, such as H 2 sensors disclosed herein, 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*=gradient from light to dark, a*=gradient from red to green, and b*=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 H 2 sensor film samples. The greater the ΔE* value, the greater the color contrast. Chemochromic H 2 sensor films can be analyzed both before and after exposure to H 2 , allowing quantification of the intensity of the color change.

In one embodiment, a disclosed H 2 sensor includes a moisture-cured silicone polymer that forms a continuous/matrix phase that completely encapsulates the PGM compound and supports in a specified ratio to give a composition that responds in a controllable way to the presence of H 2 . In this embodiment, the PGM compound, support particles and optional accelerant is generally 1 to 50 wt. % of the overall H 2 sensor, and is 2-20 wt. % in one embodiment. Upon evaluating a series of alternate supports for the chemochromic H 2 sensor including a silicone matrix, all, with the exception of those with very small particle size supports, gave ΔE* values equal or better to those of known PdO/TiO 2 chemochromic H 2 sensors as described below relative to FIG. 2 .

FIG. 1 is a depiction of an exemplary two layer H 2 sensor 10 according to an embodiment of the invention. H 2 sensor 10 comprises a composite layer 1 including supports comprising a plurality of metal oxide particles and a PGM compound on the supports, and a gas permeable polymer that forms a continuous phase that completely encapsulates both the supports and the PGM compound which are both embedded in the polymer. An optional clear silicone layer 2 is also shown. The clear silicone layer 2 does not generally include any PGM compound.

An example of the impact of support particle size on chemochromic H 2 sensor performance is demonstrated with samples containing ZrO 2 . The particle size of ZrO 2 is much less than the value of approximately 0.22 microns which the Inventors have found generally maximizes the opacity or hiding power of a chemochromic pigment. In this example, the resulting sample was translucent although the chemochromic color change was observable with H 2 exposure. Such a chemochromic H 2 sensor system can be an advantage since the events occurring behind the chemochromic system can be observed. With silicone chemochromic tapes the overcoat system developed for H 2 from behind an observer can be eliminated since the initial color change is readily apparent.

As described above, another disclosed embodiment comprises the addition of an accelerant to give color change rates comparable with Pd/TiO 2 systems. In this embodiment, the H 2 sensor further comprises an accelerant or contrast additive mixed with the PGM compound selected from MoO 3 , (NH 4 ) 6 Mo 7 O 24 , and polyoxometalates that include V, Nb, Ta, Cr, Mo, and W. When the level of accelerant is optimized, the rate of color change can be increased by a factor of 2-5, and modest increases in ΔE* values are also observed.

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.

The H 2 sensor preparation matrix is shown as Table 1 in FIG. 3 for the Examples described below. Data from the H 2 exposure tests performed as described in the Examples is shown in FIG. 2 .

Chemochromic H 2 sensors to be tested were tape casted. The tape-casting process included weighing out the pigment powder that is a mixture of metal oxide support particles and PGM compound(s), RTV 3145 silicone, and ammonium molybdate ((NH 4 ) 6 Mo 7 O 24 ) if used as an accelerant or contrast additive. The PGM compound mixed with a metal oxide support, (and optionally ammonium molybdate) were grinded in a mortar, and admixed with the RTV 3145 silicone. This mixture was then scraped up and applied to a flat piece of taped-down wax paper. A doctor blade was used to slowly draw out the silicone-pigment paste into a thin film. The film thickness of the samples prepared was typically 10 mils.

After letting the sheets cure overnight, the sheets were cut into sample sheets for the H 2 exposure tests. Each sample was cut into a 1⅜ inch by 1⅜ inch square. The wax paper was allowed to remain on the back side of each sample during the tests, due to the thinness of the sheet and its saran-wrap like tendency to stick together. Before testing, each sample was marked with a small cut near its upper left-hand corner, and then subjected to colorimetric measurements using a Konica-Minolta CR-10 instrument that gave the ΔL*, Δa*, and Δb* values at five locations on the sample sheet—each of its four corners, and in the middle. The sample was then placed (exposed side of membrane down) onto a badge-holder. H 2 produced by a H 2 generator was passed through a narrow tube into the bottom of the badge-holder. Samples were placed onto the badge-holder, and then capped at the top.

A shut-off valve was used to ensure a consistent and immediate exposure to the H 2 flow. Before each test, the H 2 flow was allowed to stabilize before switching to flow into the badge sample holder. Each sample was exposed to H 2 for a set length of time, and then taken out of the badge-holder and extent of color change measured by the Konica-Minolta CR-10 instrument at five locations on the sample sheet. This process repeated for as long as the sample color change was measurable. Collective testing times ranged from a few minutes to well over an hour, based on the kinetics of coloration.