Fabrication and Testing of Catalytic Aerogels Prepared Via Rapid Supercritical Extraction

Summary

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.

Abstract

Protocols for preparing and testing catalytic aerogels by incorporating metal species into silica and alumina aerogel platforms are presented. Three preparation methods are described: (a) the incorporation of metal salts into silica or alumina wet gels using an impregnation method; (b) the incorporation of metal salts into alumina wet gels using a co-precursor method; and (c) the addition of metal nanoparticles directly into a silica aerogel precursor mixture. The methods utilize a hydraulic hot press, which allows for rapid (<6 h) supercritical extraction and results in aerogels of low density (0.10 g/mL) and high surface area (200-800 m²/g). While the work presented here focuses on the use of copper salts and copper nanoparticles, the approach can be implemented using other metal salts and nanoparticles. A protocol for testing the three-way catalytic ability of these aerogels for automotive pollution mitigation is also presented. This technique uses custom-built equipment, the Union Catalytic Testbed (UCAT), in which a simulated exhaust mixture is passed over an aerogel sample at a controlled temperature and flow rate. The system is capable of measuring the ability of the catalytic aerogels, under both oxidizing and reducing conditions, to convert CO, NO and unburned hydrocarbons (HCs) to less harmful species (CO₂, H₂O and N₂). Example catalytic results are presented for the aerogels described.

Introduction

Silica- and alumina-based aerogels have remarkable properties, including low density, high porosity, high surface area, good thermal stability and low thermal conductivity¹. These properties render the aerogel materials attractive for a variety of applications^1,2. One application that exploits the thermal stability and high surface area of aerogels is heterogeneous catalysis; several articles review the literature in this area^2,3,4,5. There are many approaches....

Protocol

Safety Considerations: Wear safety glasses or goggles and laboratory gloves at all times when performing preparatory work with chemical solutions and when handling wet gels or catalytic aerogel materials. Handle propylene oxide, tetramethyl orthosilicate (TMOS), ethanol, methanol, ammonia, nanoparticles and solutions containing any of these within a fume hood. Read Safety Data Sheets (SDS) for all chemicals, including nanoparticles, prior to working with them. Wear a particulate mask when crushing aerogel samples and during loading and unloading of the test cell. Wear safety glasses or goggles when operating the hydraulic hot press or catalytic test bed. Tie back long....

Results

Photographic images of the resulting aerogels are presented in Figure 2. Because the wet gels were broken into pieces prior to solvent exchange, the Al-Cu IMP and Si-Cu IMP aerogels are in small, irregularly shaped monolithic pieces. It is clear from the coloration of these samples that the aerogels contain copper species and that variations in copper speciation and/or ligand structure occur within the materials. Al-Cu IMP aerogels (Figur.......

Discussion

The utility of the RSCE method for fabrication of catalytic aerogels and the UCAT system for demonstrating catalytic ability has been demonstrated herein. Major advantages of these protocols over other methods are the speed of RSCE aerogel fabrication and the relatively inexpensive approach to catalytic testing by UCAT.

Gels to be extracted can be prepared via a variety of methods, including impregnation of metal salts into an alumina or silica wet gel matrix, inclusion of metal salts as co-pr.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Variable micropipettor, 100-1000 µL	Manufactured by Eppendorf, purchased from Fisher Scientific www.fishersci.com	S304665	Any 100-1000 µL pipettor is suitable.
Variable Pipettor, 2.5-10 mL	Manufactured by Eppendorf, purchased from Fisher Scientific www.fishersci.com	21-379-25	Any variable pipettor is suitable.
Pasteur pipettes	FisherScientific	13-678-6A
Syringe	Purchased from Fisher Scientific	Z181390 syringe with Z261297 needle
Digital balance	OHaus Explorer Pro		Any digital balance is suitable.
Beakers	Purchased from Fisher Scientific		Any glass beaker is suitable.
Graduated Cylinder	Purchased from Fisher Scientific		Any glass graduated cylinder is suitable.
Magnetic Plate/Stirrer	FisherScientific Isotemp	SP88854200P	Any magnetic plate/stirrer is suitable.
Ultrasonic Cleaner	FisherScientific FS6	153356	Any sonicator is suitable.
Mold	Fabricated in House		Fabricate from cold-rolled steel or stainless steel.
Hydraulic Hot Press	Tetrahedron www.tetrahedronassociates.com	MTP-14	Any hot press with temperature and force control will work. Needs maximum temperature of ~550 F and maximum force of 24 tons.
UCAT (Union Catalytic Testbed)	Fabricated in House		Described in detail in reference #21:  Bruno, B.A., Anderson, A.M., Carroll, M.K., Brockmann, P., Swanton, T., Ramphal, I.A., Palace, T. Benchtop Scale Testing of Aerogel Catalysts. SAE Technical Paper 2016-01-920 (2016).
Bar 97 Gas	Praxair	MS_BAR97ZA-D7