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Fabrication and Testing of Catalytic Aerogels Prepared Via Rapid Supercritical Extraction
Summary August 31st, 2018
Here we present protocols for preparing and testing catalytic aerogels by incorporating metal species into silica and alumina aerogel platforms. Methods for preparing materials using copper salts and copper-containing nanoparticles are featured. Catalytic testing protocols demonstrate the effectiveness of these aerogels for three-way catalysis applications.
Transcript
This method can lead to the development of new materials for automotive catalysis applications without reliance on precious metals. The main advantage of this technique is that a variety of high-performing catalytic aerogels can be prepared and tested relatively quickly. We first had the idea for this application of the RSCE method and development of a catalytic test bed when working with a former student to develop an undergraduate thesis project.
Visual demonstration of this method is valuable, as the catalytic test bed is not a commercially-available instrument. Using a calibrated digital balance, weigh out 5.92 grams of aluminum chloride hexahydrate and add to a 250 milliliter beaker. Add 40 milliliters of reagent-grade ethanol and a stir bar to the beaker.
Cover the beaker with paraffin film and place on a magnetic stir plate for stirring at moderate speed, until the aluminum salt has dissolved. Then, remove the beaker from the magnetic plate and uncover. Using a 10 milliliter syringe, add eight milliliters of propylene oxide to the beaker containing the aluminum chloride solution.
Replace the paraffin film on the beaker and place on the magnetic stir plate for stirring at moderate speed until the solution has gelled. Following this, remove the beaker from the magnetic stir plate and allow the gel to age at room temperature for 24 hours. Using the calibrated digital balance, weigh out 1.4 grams of copper nitrate trihydrate.
Add this salt to 40 milliliters of absolute ethanol in a beaker. Then, place the beaker in a sonicator and sonicate until the copper salt dissolves. Now, pour any excess solvent off the alumina sol gel.
Remove the stir bar and break the gel into several pieces using a spatula. Transfer the copper solution to the 250 milliliter beaker containing the gel pieces. Then, cover the beaker with paraffin film and allow the gel to age at room temperature for 24 hours.
On the following day, remove the excess solvent and add 40 milliliters of fresh absolute ethanol. Replace the paraffin film on the beaker, and allow the gel to age for another 24 hours at room temperature. Obtain an appropriately-sized stainless steel mold.
To prepare the gasket material, cut ceiling gaskets sufficient in size to fully cover the mold from a 1.6 millimeter thick graphite gasket sheet and a 0.012 millimeter thick stainless steel foil. Following this, program the hot press for ethanol extraction. Following preparation and ethanol exchange of the wet sol gels, decant the excess solvent.
Now, distribute the wet sol gels into the wells of the mold and center the mold on the hot press heating plate. Top off each well with absolute ethanol. Place the stainless steel foil, followed by the graphite sheet on to the top of the mold to seal it.
Then, begin the hot press extraction program. Once the process is complete, remove the mold from the hot press. Then, remove the gasket material from the mold and transfer the aerogels into sample containers.
Place a clean 250 milliliter beaker on the calibrated digital balance and pipette approximately 13 milliliters of TMOS into the beaker. Add additional TMOS as needed for a total of 13.04 grams of TMOS. Next, pipette 32.63 grams of methanol and 3.90 grams of deionized water into the beaker.
Shake the previously prepared 20 weight percent copper two oxide nanodispersion to resuspend any nanoparticles that have settled to the bottom. Pipette 1.5 grams of the nanodispersion into the beaker of precursor solution. Then, pipette 200 microliters of 1.5 molar ammonia solution into the beaker.
Cover the beaker with paraffin film and sonicate the mixture for five-10 minutes, until it is a monophasic solution. After heating the aerogel in a furnace, pour 20 milliliters of the cooled material in a clean, day, UCAT test section and insert an end screen to retain the sample in place during testing. Following this, load the test section into the UCAT assembly using copper washers and clamps to seal.
Then, securely close the UCAT oven. After setting up the five gas analyzers, set the desired oven temperature and start the oven. Ensure that the bypass valve is set to deliver air through the test cell.
Next, adjust the mass flow rate controllers to deliver the correct quantities of air used during warm-up and simulated exhaust used during testing to maintain the desired space velocity. Turn on the warm-up air flow to purge the test cell and wait for the flow through the test cell to stabilize at the desired test temperature, typically, 30 minutes. Now, re-zero the five gas analyzer and set the bypass valve to send the flow to bypass the test section.
Turn off the air. Turn on the simulated exhaust flow. Allow the five gas analyzer readings to stabilize for approximately 90 seconds and record the bypass pollutant concentrations.
Next, set the bypass valve to direct the flow through the test section. Allow the five gas analyzer readings to stabilize for approximately 360 seconds and record the treated no-oxygen exhaust pollutant concentrations. Turn on the oxygen addition to the blend.
Allow the five gas analyzer readings to stabilize for approximately 90 seconds and record the treated-with-oxygen exhaust pollutant concentrations. Following this, set the bypass valve to send the flow to bypass the test section. Allow the five gas analyzer readings to stabilize for approximately 90 seconds and record the bypass pollutant concentrations again.
Finally, turn off the simulated exhaust flow. Representative physical characteristics of the as-prepared copper-containing aerogels are listed here. A 400 nanometer-diameter nanoparticle containing copper is observed in the EDX image of the silica copper nanoparticle aerogel, prepared using the copper two nanodispersion.
This indicates that some agglomeration of the 25-55 nanometer nanoparticles in the original nanodispersion has occurred. Smaller nanoparticles are observed in the alumina copper impregnated aerogel. The XRD pattern of the as-prepared silica copper impregnated and nanoparticle aerogels contain peaks corresponding to metallic copper.
This indicates the alcohol thermal reduction of the copper species occurred during RSCE processing of the gels. The as-prepared alumina copper-impregnated aerogel pattern shows XRD peaks consistent with the pseudo-boehmite form of alumina and a copper two-containing species. Copper-containing alumina aerogels are capable of catalyzing reactions that can eliminate each of the three major pollutants of concern in gasoline engine exhaust under the conditions tested.
The catalytic ability in copper-containing silica aerogels is shown here and provides evidence that the catalytic capabilities of metal-doped aerogels are robust and tailorable. The catalytic activity appears to depend on how copper is introduced to the sol-gel mixture and the underlying aerogel itself. Once mastered, catalytic aerogel preparation takes about one to two hours, followed by as little as three hours to process the mixture to yield aerogels.
Following this procedure, other types of metal-containing catalytic aerogels can be fabricated for automotive pollution mitigation or other applications. After watching this video, you should have a good understanding of how the test bed can be used to gain information about the aerogel's catalytic performance. Working with TMOS, propylene oxide, the hot press, and simulated automotive exhaust can be extremely hazardous.
Precautions such as wearing personal protective gear and working under proper ventilation should always be taken when performing these procedures.
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