The authors are grateful to Stephen Steiner at Aerogel Technologies for his inspiration and technical insights, and to Dr. Deryn Chu at the Army Research Laboratory-Sensors and Electron Devices Directorate, Dr. Christopher Haines at the Armament Research, Development and Engineering Center, U.S. Army RDECOM-ARDEC, and Dr. Stephen Bartolucci at the U.S. Army Benet Laboratories for their assistance. This work was supported by a Faculty Development Research Fund grant from the United States Military Academy, West Point.

CAUTION: Consult all relevant safety data sheets (SDS) before use. Use appropriate safety practices when performing chemical reactions, to include the use of a fume hood and personal protective equipment. Rapid hydrogen gas evolution can cause high pressure in reaction tubes causing caps to pop and solutions to spray out. Ensure that reaction tube caps remain open as specified in the protocol.
1. Metal Gel Preparation
<ol>
	<li>Preparation of metal ion solutions. 
	<ol>
		<li>Prepare 2 mL of 0.1 M solutions of the following salts: HAuCl4&#8226;3H2O and Na2PdCl4 in deionized water. Prepare 2 mL of 0.1 M K2PtCl6 in a 1:1 (v/v) water and ethanol solvent. Vigorously shake and vortex solutions to aid in the dissolution of the salts.</li>
	</ol>
	</li>
	<li>Preparation of reducing agent solutions. 
	<ol>
		<li>Prepare 10 mL of 0.1 M solutions of the following reducing agents: dimethylamine borane (DMAB) and NaBH4 (sodium borohydride).</li>
	</ol>
	</li>
	<li>Preparation of Au gels. 
	<ol>
		<li>Pipette 0.5 mL of 0.1 M HAuCl4&#8226;3H2O solution into a 1.7 mL or 2.0 mL microcentrifuge tube. Forcefully pipette 0.5 mL of DMAB into the microcentrifuge tube with gold solution to ensure a rapid mix of salt and reducing agent solutions. Once the solutions are mixed, place the microcentrifuge tube vertically in a tube rack with the tube cap open. 
		Note: If the tube cap is left closed, hydrogen gas evolution will cause the pressure inside to force the cap to pop open and potentially spray the reduction mixture.</li>
	</ol>
	</li>
	<li>Preparation of Pd gels. 
	<ol>
		<li>Pipette 0.5 mL of 0.1 M Na2PdCl4 solution into a 1.7 mL or 2.0 mL microcentrifuge tube. Forcefully pipette 0.5 mL of NaBH4 into the microcentrifuge tube with palladium solution. Place the microcentrifuge tube vertically in a tube rack with the tube cap open.</li>
	</ol>
	</li>
	<li>Preparation of Pt gels. 
	<ol>
		<li>Pipette 0.5 mL of 0.1 M K2PtCl6 solution into a 1.7 mL or 2.0 mL microcentrifuge tube. Forcefully pipette 0.5 mL of DMAB into the microcentrifuge tube with platinum solution. Place the microcentrifuge tube vertically in a tube rack with the tube cap open.</li>
	</ol>
	</li>
	<li>Tube inversion. 
	<ol>
		<li>At approximately 5 min, cap the microcentrifuge tubes and gently invert 3-5 times to aid in the coalescence of metal particles not part of the metal gel. Ensure that the tube caps are immediately uncapped after inverting tubes, and replace tubes in a rack to maintain the vertical orientation of the tube.</li>
	</ol>
	</li>
	<li>Equilibration.
	<ol>
		<li>While Au, Pd, and Pt gels initially form within minutes, leave nascent gels in reducing agent solution for 3 &#8211; 6 h to allow for complete reduction of metal ions and for surface free energy minimization to occur. 
		Note: Metal gels occupy a smaller volume than the initial volume of mixed metal ion and reducing agent solution. Some additional slight volume contraction may be observed during equilibration time, and is more pronounced for gold gels and believed to be due to Ostwald ripening25.</li>
	</ol>
	</li>
	<li>Gel rinsing.
	<ol>
		<li>For Au, Pd, and Pt gels after the equilibration period, remove excess reducing agent solution, but leave enough solution volume so that the metal gel remains submerged. Ensure that the solution meniscus does not come in contact with the metal gel. 
		Note: Although the metal gels are stable enough to transfer between solutions with a spatula, capillary forces due to contact with the solution meniscus will deform and compress the gels resulting in an increase in the final aerogel density. This requires that some reducing agent solution remains in the tube with the gel submerged when transferring to deionized water.</li>
		<li>Slowly pipette deionized water to the top of the reaction microcentrifuge tubes. Submerge the microcentrifuge tube in a 50 mL conical tube full of deionized water and allow the gel to slide out of the microcentrifuge tube.</li>
		<li>Leave the gel in the deionized water for 24 h, and replace the water at 12 h. Do not to allow a liquid meniscus to come in contact with the gel.</li>
	</ol>
	</li>
</ol>
2. Electrochemical Surface Area (ECSA) Characterization of Wet Metal Gels
Note: Electrochemical characterization is performed on wet metal gels prior to conducting freeze drying. The resulting ECSA is then an estimate of the surface of the final aerogel structure. Nitrogen adsorption measurements are used to estimate the surface area of the dried aerogels.
<ol>
	<li>Solvent exchange.
	<ol>
		<li>Remove as much of the deionized water from the Au, Pd, and Pt rinse solutions as possible and ensure that the liquid meniscus does not come in contact with the gel.</li>
		<li>Add 50 mL, 0.5 M KCl to the conical tubes in order to exchange the deionized water with supporting electrolyte within the gel pores. Leave the gels in KCl solution for 24 h.</li>
	</ol>
	</li>
	<li>Working electrode preparation.
	<ol>
		<li>Coat a 1 mm platinum wire electrode with non-reactive lacquer using a fine bristle brush or other application device leaving a 4-5 mm length of the wire tip exposed.</li>
		<li>Allow 20 minutes for the lacquer to dry.</li>
		<li>Apply at least two coats of lacquer.</li>
	</ol>
	</li>
	<li>3-electrode cell set-up.
	<ol>
		<li>Use a 3-electrode cell set-up with an Ag/AgCl (3 M saturated) reference electrode, a 0.5 mm diameter Pt wire auxiliary/counter electrode, and the lacquer coated working electrode.</li>
		<li>Cut a plastic 50 mL conical tube in half and use as an electrochemical vial.</li>
		<li>Contact the gel with the working electrode with one of two methods: 1) impaled gel, or 2) contact mode.
		<ol>
			<li>Working electrode - impaled gel.
			<ol>
				<li>With the gel at the bottom of the modified 50 mL conical tube, gently insert the lacquer coated electrode into the gel. 
				Note: The impaled gel method proves more effective with Au gels, whereas Pd and Pt gels fracture more frequently upon electrode insertion.</li>
			</ol>
			</li>
			<li>Working electrode - contact mode.
			<ol>
				<li>Insert the lacquer coated working electrode into the conical tube along the inner surface and rest the metal gel on the top of the exposed Pt wire of the working electrode.</li>
			</ol>
			</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Electrochemical impedance spectroscopy (EIS).
	<ol>
		<li>Perform potentiostatic EIS scans with frequencies between 100 MHz and 1 mHz using a 10 mV amplitude sine wave. In the event of current overflows, use galvanostatic EIS with the same frequency range and a 100 &#8211; 200 mA amplitude sine wave.</li>
	</ol>
	</li>
	<li>Determination of electrochemical surface area (ECSA) from EIS data.
	<ol>
		<li>For Z&#34;, the imaginary component of impedance, at the lowest EIS frequency f of 1 mHz, and dividing by the sample mass, m, use the following equation to determine specific capacitance, Csp: 
		Csp = 1/(2&#960;fZ&#34;m)&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;(1) 
		Note: Given that the ECSA is determined from a wet gel prior to freeze drying in Step 3 below, determine mass by assuming all of the metal ions in solution are reduced to form the gel. Based on this assumption, any actual yield less than 100% will result in underestimating Csp.</li>
	</ol>
	</li>
	<li>Cyclic voltammetry (CV).
	<ol>
		<li>Use scan rates of 100, 75, 50, 25, 10, 5, and 1 mV/s for CV measurements. Use voltage ranges of -0.2 to 0.2 V (vs Ag/AgCl) for Au gels, and select 0.1 to 0.4 V for Pd and Pt gels to avoid hydrogen adsorption and desorption, and oxidation-reduction of the metals.</li>
	</ol>
	</li>
	<li>Determination of electrochemical surface area (ECSA) from CV data.
	<ol>
		<li>Use the slowest CV scan rate of 1 mV/s, and calculate specific capacitance with the equation: 
		Csp = (&#8747;ivdv) / (2&#956;m&#916;V)&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;(2) 
		Note: Here i and v are the current and potential in the CV scan (A and V), scan rate is &#956; (V/s), mass of the gel is m (g), and &#916;V is the potential window of discharge (vs Ag/AgCl).</li>
	</ol>
	</li>
</ol>
3. Aerogel Preparation and Characterization.
<ol>
	<li>Remove the deionized rinse water for Au, Pd, and Pt gels in Step 1.8 and ensure that the water meniscus does not come in contact with the metal gels.</li>
	<li>Place the gels in a -80 &#176;C freezer for no less than 30 min. Transfer the frozen metal gels to a freeze dryer with a set point pressure of 4 Pa or lower.</li>
</ol>

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The authors have nothing to disclose.

The noble metal aerogel synthesis method presented here results in the rapid formation of porous, high surface area monoliths that are comparable to slower synthesis techniques. The 1:1 (v/v) metal ion solution to reducing agent solution ratio is critical in facilitating the proposed gel formation model. The rapid hydrogen gas evolution as a by-product of the electrochemical reduction of metal ions serves as a secondary reducing agent and facilitates the densification, and fusion of growing nanoparticles during gel forma...

A wide range of energy storage and conversion, catalysis, and sensor applications benefit from three-dimensional metallic nanostructures which provide control over chemical reactivity, and mass transport properties1,2,3,4,5. Such 3-dimensional metallic nanostructures further enhance conductivity, ductility, malleability, and strength8,9. Integration into devices necessitates that materials be free-standing or combined with support materials. Incorporation of nanomaterials onto support structures provides a means of minimizing active material, but may suffer from weak adsorption and eventual agglomeration during device operation10,11.
While there are a variety of synthesis methods to control individual nanoparticle size and shape, few approaches enable control over contiguous 3-dimensional nanomaterials12,13,14. Noble metal 3-dimensional nanostructures have been formed through dithiol linkage of monodisperse nanoparticles, sol-gel formation, nanoparticle coalescence, composite materials, nanosphere chains, and biotemplating15,16,17,18. Many of these approaches require synthesis times on the order of days to weeks to yield desired materials. Noble metal nanofoams synthesized from the direct reduction of precursor salt solutions have been prepared with a faster synthesis timescale and with short-range order of hundreds of micrometers in length, but require mechanical pressing for device integration19,20.
First reported by Kistler, aerogels provide a synthesis route to achieve porous structures with high specific surface areas that are orders of magnitude less dense than their bulk material counterparts21,22,23. Extending 3-dimensional structures to the macroscopic length scale of bulk materials offers an advantage over nanoparticle aggregates or nanofoams that require support materials or mechanical processing. While aerogels provide a synthesis route to control porosity and particle feature size, however, extended synthesis times, and in some cases the use of capping agents or linker molecules, increases overall processing steps and time.
Here a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented24. Combining various precursor noble metal ions with reducing agents in a 1:1 (v/v) ratio results in the formation of metal gels within seconds to minutes compared to much longer synthesis times for other techniques such as sol-gel. The use of a microcentrifuge tube or small volume conical tube takes advantage of the vigorous hydrogen gas evolution as a by-product of the reduction step facilitating a proposed nucleation, growth, densification, fusion, equilibration model for gel formation. A close correlation in aerogel nanostructure feature sizes is determined with scanning electron microscopy image analysis, X-ray diffractometry, nitrogen gas adsorption, electrochemical impedance spectroscopy, and cyclic voltammetry. The solvent accessible specific surface area is determined with both electrochemical impedance spectroscopy and cyclic voltammetry. This synthesis method for noble metal aerogels demonstrates that high specific surface area monoliths may be achieved with a rapid and direct reduction approach.

<table><tbody><tr><td>HAuCl&lt;sub&gt;4&lt;/sub&gt;&amp;Yuml;&amp;bull;3H&lt;sub&gt;2&lt;/sub&gt;O&amp;nbsp;</td><td>Sigma-Aldrich</td><td>16961-25-4</td><td></td></tr><tr><td>Na&lt;sub&gt;2&lt;/sub&gt;PdCl&lt;sub&gt;4&lt;/sub&gt;</td><td>Sigma-Aldrich</td><td>13820-40-1</td><td></td></tr><tr><td>K&lt;sub&gt;2&lt;/sub&gt;PtCl&lt;sub&gt;6&lt;/sub&gt;</td><td>Sigma-Aldrich</td><td>16921-30-5</td><td></td></tr><tr><td>Pd(NH&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;4&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt;</td><td>Sigma-Aldrich</td><td>13933-31-8</td><td></td></tr><tr><td>K&lt;sub&gt;2&lt;/sub&gt;PtCl&lt;sub&gt;4&lt;/sub&gt;</td><td>Sigma-Aldrich</td><td>10025-99-7</td><td></td></tr><tr><td>Pt(NH&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;4&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt;&amp;Yuml;&amp;bull;H&lt;sub&gt;2&lt;/sub&gt;O</td><td>Sigma-Aldrich</td><td>13933-31-8</td><td></td></tr><tr><td>dimethylamine borane (DMAB)</td><td>Sigma-Aldrich</td><td>74-94-2</td><td></td></tr><tr><td>NaBH&lt;sub&gt;4&lt;/sub&gt;</td><td>Sigma-Aldrich</td><td>16940-66-2</td><td></td></tr><tr><td>NaH&lt;sub&gt;2&lt;/sub&gt;PO&lt;sub&gt;2&lt;/sub&gt;&amp;Yuml;&amp;bull;H&lt;sub&gt;2&lt;/sub&gt;O</td><td>Sigma-Aldrich</td><td>10039-56-2</td><td></td></tr><tr><td>Ethanol</td><td>Sigma-Aldrich</td><td>792780</td><td></td></tr><tr><td>Snap Cap Microcentrifuge Tubes, 2.0 mL</td><td>Cole Parmer</td><td>UX-06333-70</td><td></td></tr><tr><td>Snap Cap Microcentrifuge Tubes, 1.7 mL</td><td>Cole Parmer</td><td>UX-06333-60</td><td></td></tr><tr><td>Conical Centrifuge Tubes 15mL</td><td>Stellar Scientific</td><td>T15-101&amp;nbsp;</td><td></td></tr><tr><td>Ag/AgCl Reference Electrode</td><td>BASi</td><td>MF-2052</td><td></td></tr><tr><td>Pt wire electrode</td><td>BASi</td><td>MF-4130</td><td></td></tr><tr><td>Miccrostop Lacquer</td><td>Tober Chemical Division</td><td>NA</td><td></td></tr><tr><td>Potentiostat</td><td>Biologic-USA</td><td>VMP-3</td><td>Electrochemical analysis-EIS, CV</td></tr><tr><td>Freeze Dryer</td><td>Labconco</td><td>Freezone 2.5 Liter</td><td>Aerogel freeze drying</td></tr><tr><td>XRD</td><td>PanAlytical</td><td>Empyrean</td><td>X-ray diffractometry</td></tr><tr><td>Surface and Pore Analyzer</td><td>Quantachrome</td><td>NOVA 4000e</td><td>Nitrogen gas adsorption</td></tr><tr><td>ImageJ, Image analysis software</td><td>National Institute of Health</td><td>NA</td><td>SEM image analysis</td></tr></tbody></table>

a rapid synthesis method for au, pd, and pt aerogels via direct solution-based reduction

Here, a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented. The combination of various precursor noble metal ions with reducing agents in a 1:1 (v/v) ratio results in the formation of metal gels within seconds to minutes compared to much longer synthesis times for other techniques such as sol-gel. Conducting the reduction step in a microcentrifuge tube or small volume conical tube facilitates a proposed nucleation, growth, densification, fusion, equilibration model for gel formation, with final gel geometry smaller than the initial reaction volume. This method takes advantage of the vigorous hydrogen gas evolution as a by-product of the reduction step, and as a consequence of reagent concentrations. The solvent accessible specific surface area is determined with both electrochemical impedance spectroscopy and cyclic voltammetry. After rinsing and freeze drying, the resulting aerogel structure is examined with scanning electron microscopy, X-ray diffractometry, and nitrogen gas adsorption. The synthesis method and characterization techniques result in a close correspondence of aerogel ligament sizes. This synthesis method for noble metal aerogels demonstrates that high specific surface area monoliths may be achieved with a rapid and direct reduction approach.

The addition of metal ion and reducing agent solutions together results in solutions immediately turning a dark black color with vigorous gas evolution. Observation of the reaction progress suggests the proposed gel formation mechanism shown in Figure 1. Gel formation proceeds through five steps of 1) nanoparticle nucleation, 2) growth, 3) densification, 4) fusion, and 5) equilibration. The first four steps are observed to occur during the first few minutes o...

Watch this Scientific Journal Video about A Rapid Synthesis Method for Au, Pd, and Pt Aerogels Via Direct Solution-Based Reduction at JoVE.com

A Rapid Synthesis Method for Au, Pd, and Pt Aerogels Via Direct Solution-Based Reduction

A rapid, direct solution-based reduction synthesis method to obtain Au, Pd, and Pt aerogels is presented.

Here, a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented. The combination of various ...

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8.6K Views.  United States Military Academy, West Point. A rapid, direct solution-based reduction synthesis method to obtain Au, Pd, and Pt aerogels is presented.

Video: A Rapid Synthesis Method for Au, Pd, and Pt Aerogels Via Direct Solution-Based Reduction

Preparing Silica Aerogel Monoliths via a Rapid Supercritical Extraction Method

A procedure for the fabrication of monolithic silica aerogels in eight hours or less via a rapid supercritical extraction process is described. The procedure requires 15-20 min of preparation time, during which a liquid precursor mixture is prepared and poured into wells of a metal mold that is placed between the platens of a hydraulic hot press, followed by several hours of processing within the hot press. The precursor solution consists of a 1.0:12.0:3.6:3.5 x 10-3 molar ratio of tetramethylorthosilicate (TMOS):methanol:water:ammonia. In each well of the mold, a porous silica sol-gel matrix forms. As the temperature of the mold and its contents is increased, the pressure within the mold rises. After the temperature/pressure conditions surpass the supercritical point for the solvent within the pores of the matrix (in this case, a methanol/water mixture), the supercritical fluid is released, and monolithic aerogel remains within the wells of the mold. With the mold used in this procedure, cylindrical monoliths of 2.2 cm diameter and 1.9 cm height are produced. Aerogels formed by this rapid method have comparable properties (low bulk and skeletal density, high surface area, mesoporous morphology) to those prepared by other methods that involve either additional reaction steps or solvent extractions (lengthier processes that generate more chemical waste).The rapid supercritical extraction method can also be applied to the fabrication of aerogels based on other precursor recipes.

This article describes a rapid supercritical extraction method for fabricating silica aerogels. By utilizing a confined mold and hydraulic hot press, monolithic aerogels can be made in eight hours or less.

A procedure for the fabrication of monolithic silica aerogels in eight hours or less via a rapid supercritical extraction process is described. The ...

Synthesis and Catalytic Performance of Gold Intercalated in the Walls of Mesoporous Silica

As a promising catalytically active nano reactor, gold nanoparticles intercalated in mesoporous silica (GMS) were successfully synthesized and properties of the materials were investigated. We used a one pot sol-gel approach to intercalate gold nano particles in the walls of mesoporous silica. To start with the synthesis, P123 was used as template to form micelles. Then TESPTS was used as a surface modification agent to intercalate gold nano particles. Following this process, TEOS was added in as a silica source which underwent a polymerization process in acid environment. After hydrothermal processing and calcination, the final product was acquired. Several techniques were utilized to characterize the porosity, morphology and structure of the gold intercalated mesoporous silica. The results showed a stable structure of mesoporous silica after gold intercalation. Through the oxidation of benzyl alcohol as a benchmark reaction, the GMS materials showed high selectivity and recyclability.

Here, we present a protocol with a sol-gel process to synthesize gold intercalated in the walls of mesoporous materials (GMS), which is confirmed to possess a mesoporous matrix with gold intercalated in the walls imparting great stability and recyclability.

As a promising catalytically active nano reactor, gold nanoparticles intercalated in mesoporous silica (GMS) were successfully synthesized and properties ...

A Simple Method for the Size Controlled Synthesis of Stable Oligomeric Clusters of Gold Nanoparticles under Ambient Conditions

Reducing dilute aqueous HAuCl4 with sodium thiocyanate (NaSCN) under alkaline conditions produces 2 to 3 nm diameter nanoparticles. Stable grape-like oligomeric clusters of these yellow nanoparticles of narrow size distribution are synthesized under ambient conditions via two methods. The delay-time method controls the number of subunits in the oligoclusters by varying the time between the addition of HAuCl4 to alkaline solution and the subsequent addition of reducing agent, NaSCN. The yellow oligoclusters produced range in size from ~3 to ~25 nm. This size range can be further extended by an add-on method utilizing hydroxylated gold chloride (Na+[Au(OH4-x)Clx]-) to auto-catalytically increase the number of subunits in the as-synthesized oligocluster nanoparticles, providing a total range of 3 nm to 70 nm. The crude oligocluster preparations display narrow size distributions and do not require further fractionation for most purposes. The oligoclusters formed can be concentrated &#62;300 fold without aggregation and the crude reaction mixtures remain stable for weeks without further processing. Because these oligomeric clusters can be concentrated before derivatization they allow expensive derivatizing agents to be used economically. In addition, we present two models by which predictions of particle size can be made with great accuracy.

We describe a simple method for producing highly stable oligomeric clusters of gold nanoparticles via the reduction of chloroauric acid (HAuCl4) with sodium thiocyanate (NaSCN). The oligoclusters have a narrow size distribution and can be produced with a wide range of sizes and surface coats.

Reducing dilute aqueous HAuCl4 with sodium thiocyanate (NaSCN) under alkaline conditions produces 2 to 3 nm diameter nanoparticles. Stable grape-like ...

Generation of Zerovalent Metal Core Nanoparticles Using n-(2-aminoethyl)-3-aminosilanetriol

In this work, a facile one-pot reaction for the formation of metal nanoparticles in a water solution through the use of n-(2-aminoethyl)-3-aminosilanetriol is presented. This compound can be used to effectively reduce and complex metal salts into metal core nanoparticles coated with the compound. By controlling the concentrations of salt and silane one is able to control reaction rates, particle size, and nanoparticle coating. The effects of these changes were characterized through transmission electron microscopy (TEM), UV-Vis spectrometry (UV-Vis), Nuclear Magnetic Resonance spectroscopy (NMR) and Fourier Transform Infrared spectroscopy (FTIR). A unique aspect to this reaction is that usually silanes hydrolyze and cross-link in water; however, in this system the silane is water-soluble and stable. It is known that silicon and amino moieties can form complexes with metal salts. The silicon is known to extend its coordination sphere to form penta- or hexa-coordinated species. Furthermore, the silanol group can undergo hydrolysis to form a Si-O-Si silica network, thereby transforming the metal nanoparticles into a functionalized nanocomposites.

Generation of Zerovalent Metal Core Nanoparticles Using n-2-aminoethyl-3-aminosilanetriol

A novel method for metal core nanoparticle synthesis using a water stable silanol is described.

In this work, a facile one-pot reaction for the formation of metal nanoparticles in a water solution through the use of ...

Fabrication and Testing of Catalytic Aerogels Prepared Via Rapid Supercritical Extraction

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 (&#60;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.

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.

Protocols for preparing and testing catalytic aerogels by incorporating metal species into silica and alumina aerogel platforms are presented. Three ...

Synthesis Method for Cellulose Nanofiber Biotemplated Palladium Composite Aerogels

Here, a method to synthesize cellulose nanofiber biotemplated palladium composite aerogels is presented. Noble metal aerogel synthesis methods often result in fragile aerogels with poor shape control. The use of carboxymethylated cellulose nanofibers (CNFs) to form a covalently bonded hydrogel allows for the reduction of metal ions such as palladium on the CNFs with control over both nanostructure and macroscopic aerogel monolith shape after supercritical drying. Crosslinking the carboxymethylated cellulose nanofibers is achieved using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) in the presence of ethylenediamine. The CNF hydrogels maintain their shape throughout synthesis steps including covalent crosslinking, equilibration with precursor ions, metal reduction with high concentration reducing agent, rinsing in water, ethanol solvent exchange, and CO2 supercritical drying. Varying the precursor palladium ion concentration allows for control over the metal content in the final aerogel composite through a direct ion chemical reduction rather than relying on the relatively slow coalescence of pre-formed nanoparticles used in other sol-gel techniques. With diffusion as the basis to introduce and remove chemical species into and out of the hydrogel, this method is suitable for smaller bulk geometries and thin films. Characterization of the cellulose nanofiber-palladium composite aerogels with scanning electron microscopy, X-ray diffractometry, thermal gravimetric analysis, nitrogen gas adsorption, electrochemical impedance spectroscopy, and cyclic voltammetry indicates a high surface area, metallized palladium porous structure.

A synthesis method for cellulose nanofiber biotemplated palladium composite aerogels is presented.The resulting composite aerogel materials offer potential for catalysis, sensing, and hydrogen gas storage applications.

Here, a method to synthesize cellulose nanofiber biotemplated palladium composite aerogels is presented. Noble metal aerogel synthesis methods often ...

A Salt-Templated Synthesis Method for Porous Platinum-based Macrobeams and Macrotubes

The synthesis of high surface area porous noble metal nanomaterials generally relies on time consuming coalescence of pre-formed nanoparticles, followed by rinsing and supercritical drying steps, often resulting in mechanically fragile materials. Here, a method to synthesize nanostructured porous platinum-based macrotubes and macrobeams with a square cross section from insoluble salt needle templates is presented. The combination of oppositely charged platinum, palladium, and copper square planar ions results in the rapid formation of insoluble salt needles. Depending on the stoichiometric ratio of metal ions present in the salt-template and the choice of chemical reducing agent, either macrotubes or macrobeams form with a porous nanostructure comprised of either fused nanoparticles or nanofibrils. Elemental composition of the macrotubes and macrobeams, determined with x-ray diffractometry and x-ray photoelectron spectroscopy, is controlled by the stoichiometric ratio of metal ions present in the salt-template. Macrotubes and macrobeams may be pressed into free standing films, and the electrochemically active surface area is determined with electrochemical impedance spectroscopy and cyclic voltammetry. This synthesis method demonstrates a simple, relatively fast approach to achieve high-surface area platinum-based macrotubes and macrobeams with tunable nanostructure and elemental composition that may be pressed into free-standing films with no required binding materials.

A synthesis method to obtain porous platinum-based macrotubes and macrobeams with a square cross section through chemical reduction of insoluble salt-needle templates is presented.

The synthesis of high surface area porous noble metal nanomaterials generally relies on time consuming coalescence of pre-formed nanoparticles, followed ...

Preparation of Silver-Palladium Alloyed Nanoparticles for Plasmonic Catalysis under Visible-Light Illumination

Localized surface plasmon resonance (LSPR) in plasmonic nanoparticles (NPs) can accelerate and control the selectivity of a variety of molecular transformations. This opens possibilities for the use of visible or near-IR light as a sustainable input to drive and control reactions when plasmonic nanoparticles supporting LSPR excitation in these ranges are employed as catalysts. Unfortunately, this is not the case for several catalytic metals such as palladium (Pd). One strategy to overcome this limitation is to employ bimetallic NPs containing plasmonic and catalytic metals. In this case, the LSPR excitation in the plasmonic metal can contribute to accelerate and control transformations driven by the catalytic component. The method reported herein focuses on the synthesis of bimetallic silver-palladium (Ag-Pd) NPs supported on ZrO2 (Ag-Pd/ZrO2) that acts as a plasmonic-catalytic system. The NPs were prepared by co-impregnation of corresponding metal precursors on the ZrO2 support followed by simultaneous reduction leading to the formation of bimetallic NPs directly on the ZrO2 support. The Ag-Pd/ZrO2 NPs were then used as plasmonic catalysts for the reduction of nitrobenzene under 425 nm illumination by LED lamps. Using gas chromatography (GC), the conversion and selectivity of the reduction reaction under the dark and light irradiation conditions can be monitored, demonstrating the enhanced catalytic performance and control over selectivity under LSPR excitation after alloying non-plasmonic Pd with plasmonic metal Ag. This technique can be adapted to a wide range of molecular transformations and NPs compositions, making it useful for the characterization of the plasmonic catalytic activity of different types of catalysis in terms of conversion and selectivity.

Presented here is a protocol for the synthesis of silver-palladium (Ag-Pd) alloy nanoparticles (NPs) supported on ZrO2 (Ag-Pd/ZrO2). This system allows for harvesting energy from visible light irradiation to accelerate and control molecular transformations. This is illustrated by nitrobenzene reduction under light irradiation catalyzed by Ag-Pd/ZrO2 NPs.

Localized surface plasmon resonance (LSPR) in plasmonic nanoparticles (NPs) can accelerate and control the selectivity of a variety of molecular ...

Aesthetically Enhanced Silica Aerogel Via Incorporation of Laser Etching and Dyes

A procedure for aesthetically enhancing silica aerogel monoliths by laser etching and incorporation of dyes is described in this manuscript. Using a rapid supercritical extraction method, large silica aerogel monolith (10 cm x 11 cm x 1.5 cm) can be fabricated in about 10 h. Dyes incorporated into the precursor mixture result in yellow-, pink- and orange-tinged aerogels. Text, patterns, and images can be etched onto the surface (or surfaces) of the aerogel monolith without damaging the bulk structure. The laser engraver can be used to cut shapes from the aerogel and form colorful mosaics.

This protocol describes a method for etching text, patterns, and images onto the surface of silica aerogel monoliths in native and dyed form and assembling the aerogels into mosaic designs.

A procedure for aesthetically enhancing silica aerogel monoliths by laser etching and incorporation of dyes is described in this manuscript. Using a rapid ...

Engineering

Gold Nanoparticle Synthesis

Gold nanoparticles (Au nanoparticles) that are ~12 nm in diameter were synthesized by rapidly injecting a solution of 150 mg (0.15 mmol) of tetrachloroauric acid in 3.0 g (3.7 mmol, 3.6 mL) of oleylamine (technical grade) and 3.0 mL of toluene into a boiling solution of 5.1 g (6.4 mmol, 8.7 mL) of oleylamine in 147 mL of toluene. While boiling and mixing the reaction solution for 2 hours, the color of the reaction mixture changed from clear, to light yellow, to light pink, and then slowly to dark red. The heat was then turned off, and the solution was allowed to gradually cool down to room temperature for 1 hour. The gold nanoparticles were then collected and separated from the solution using a centrifuge and washed three times; by vortexing and dispersing the gold nanoparticles in 10 mL portions of toluene, and then precipitating the gold nanoparticles by adding 40 mL portions of methanol and spinning them in a centrifuge. The solution was then decanted to remove any remaining byproducts and unreacted starting materials. Drying the gold nanoparticles in a vacuum environment produced a solid black pellet; which could be stored for long periods of time (up to one year) for later use, and then redissolved in organic solvents such as toluene.

A protocol for synthesizing ~12 nm diameter gold nanoparticles (Au nanoparticles) in an organic solvent is presented. The gold nanoparticles are capped with oleylamine ligands to prevent agglomeration. The gold nanoparticles are soluble in organic solvents such as toluene.