Reverse Microemulsion-mediated Synthesis of Monometallic and Bimetallic Early Transition Metal Carbide and Nitride Nanoparticles

Summary

A “removable ceramic coating method” is presented in visual format for the synthesis of non-sintered and metal-terminated monometallic and bimetallic early transition metal carbide and nitride nanoparticles with tunable sizes and crystal structures.

Abstract

A reverse microemulsion is used to encapsulate monometallic or bimetallic early transition metal oxide nanoparticles in microporous silica shells. The silica-encapsulated metal oxide nanoparticles are then carburized in a methane/hydrogen atmosphere at temperatures over 800 °C to form silica-encapsulated early transition metal carbide nanoparticles. During the carburization process, the silica shells prevent the sintering of adjacent carbide nanoparticles while also preventing the deposition of excess surface carbon. Alternatively, the silica-encapsulated metal oxide nanoparticles can be nitridized in an ammonia atmosphere at temperatures over 800 °C to form silica-encapsulated early transition metal nitride nanoparticles. By adjusting the reverse microemulsion parameters, the thickness of the silica shells, and the carburization/nitridation conditions, the transition metal carbide or nitride nanoparticles can be tuned to various sizes, compositions, and crystal phases. After carburization or nitridation, the silica shells are then removed using either a room-temperature aqueous ammonium bifluoride solution or a 0.1 to 0.5 M NaOH solution at 40-60 °C. While the silica shells are dissolving, a high surface area support, such as carbon black, can be added to these solutions to obtain supported early transition metal carbide or nitride nanoparticles. If no high surface area support is added, then the nanoparticles can be stored as a nanodispersion or centrifuged to obtain a nanopowder.

Introduction

Early transition metal carbides (TMCs) are low-cost, earth-abundant materials that exhibit high thermal and electrochemical stability as well as unique catalytic activities.^1-3 In particular, tungsten carbide (WC) and molybdenum carbide (Mo₂C) have been studied extensively for their catalytic similarities to the platinum group metals (PGMs).^4,5 Due to these favorable properties, TMCs have been identified as candidates for replacing expensive PGM catalysts in emerging renewable energy technologies, such as biomass conversion, fuel cells, and electrolyzers.^6,7

To maximize catalytic activity, commercial catalysts are almost always formulated as ultrasmall nanoparticles (diameters <10 nm) dispersed on a high surface area support, such as carbon black.⁸ However, the synthesis of TMCs requires temperatures higher than ~700 °C. This leads to extensive sintering of the nanoparticles (NPs), excess surface carbon deposition (coke), and thermal support degradation. Both particle sintering and support degradation lead to decreased material surface areas. Excess surface impurity deposition blocks active metal sites, which has been shown to greatly reduce or in some instances completely eliminate the catalytic activity of TMCs.^9,10 As such, the fundamental study of TMC reactivity is predominantly performed on bulk microparticles or thin films with finely controlled surfaces rather than on high surface area TMC nanomaterials.

Many methods have been developed to synthesize TMC NPs, but these methods are not suitable for synthesizing catalytically active TMC NPs. Traditional wet impregnation techniques use metal salt solutions impregnated on a high surface area support. On heating, wet impregnation methods can expose the catalyst support to destructive carburizing conditions leading to support degradation. Furthermore, sintering can only be mitigated at low wt% loadings of the metal on the support, and it is also not possible to synthesize unsupported TMC nanopowders using wet impregnation. Several newer methods involve mixing a metal precursor with a carbon precursor and applying conventional and unconventional heating techniques.^11-18 Excess carbon is used to prevent sintering, but this excess carbon results in extensive surface carbon, making these materials not suitable for catalytic applications.

Due to these synthetic challenges, TMCs have traditionally been studied as co-catalysts¹¹ for PGMs, catalyst supports for PGMs,^19-22 or supports for active PGM monolayers.^23-25 The method presented here offers the ability to synthesize both non-sintered and metal-terminated TMC NPs as well as transition metal nitride (TMN) NPs with tunable sizes, crystalline phases, and metallic composition.²⁶ The method presented also offers the ability to obtain TMC or TMN nanodispersions or deposit the TMC and TMN NPs on a high surface area catalyst support at room-temperature, thereby mitigating thermal support degradation. This method is therefore suitable for standalone catalytic applications of TMC and TMN NPs, the development of advanced multimetallic TMC and TMN NPs, or other applications requiring finely controlled particle sizes and surfaces.²⁶

The method presented here uses a three step protocol to synthesize TMC and TMN NPs. In the first step, a reverse microemulsion (RME) is used to coat early transition metal oxide (TMO) NPs in silica nanospheres. The emulsion is prepared by dispersing water droplets in a nonpolar medium using a commercial nonionic surfactant. The silica-encapsulated TMO NPs are then subjected to either carburizing or nitridizing heat treatments. Here, the silica prevents particle sintering at high temperatures while allowing the reactive gases to diffuse to the TMO NPs and convert them to TMC or TMN NPs. In the final step, the silica shells are removed using either acidic or alkaline treatment to obtain TMC or TMN nanodispersions that can be dispersed on a high surface area support, such as carbon black.

Protocol

1. Synthesis of Silica-encapsulated Monometallic or Bimetallic Metal Oxide Nanoparticles

1. Prepare the reverse microemulsion
   1. Add 240 ml of anhydrous n-heptane to a clean, oven-dried 1 L round bottom flask (RBF) containing an oven-dried magnetic stir bar using a clean, oven-dried graduated cylinder.
   2. Add 54 ml of polyoxyethylene (4) lauryl ether to the n-heptane under constant stirring.
      Note: Because of the high viscosity and low surface tension of this surfactant, a clean, dry 60 ml syringe should be used to obtain accurate volumes instead of a graduated cylinder.
   3. Add 7.8 ml of ultrapure, deionized (DI) water under constant stirring using a pipette. Seal the RBF with a rubber stopper and Parafilm wax. Let the RME mix for at least 10 min.
   4. Add 0.1 to 0.5 ml of reagent-grade NH₄OH to the emulsion if it is desired to reduce the hydrolysis time.
      Note: Some metal alkoxide precursors hydrolyze slowly at neutral pH. Let the RME mix for at least another 10 min after adding NH₄OH. As a representative example, adding 0.5 ml of NH₄OH is recommended for syntheses involving titanium (IV) isopropoxide.
2. Prepare a metal alkoxide precursor, alcohol, and n-heptane solution
   1. Connect a clean, oven-dried 250 ml RBF to a Schlenk line and flush with nitrogen. Alternatively, place the clean, oven-dried 250 ml RBF into a dry nitrogen glovebox.
   2. Add 12 ml of 5% w/v tungsten isopropoxide (WIPO) in isopropanol using a clean, dry syringe. At this stage, other metal alkoxides can also be added, such as tantalum (V) isopropoxide (TaIPO), molybdenum (V) isopropoxide (MoIPO), niobium (V) isopropoxide (NbIPO), titanium (IV) isopropoxide (TiIPO), nickel (II) methoxyethoxide (NiMEO), cobalt (II) isopropoxide (CoIPO), etc.
      Note: Metal alkoxides can be purchased commercially or synthesized by the addition of an anhydrous alcohol to the corresponding metal chloride salt using a Schlenk line as described elsewhere (Caution: metal alkoxide synthesis from the metal chloride releases gaseous HCl and must be performed in a well-ventilated fume hood).²⁷
   3. Using a clean, oven-dried cannula, transfer 120 ml of anhydrous n-heptane to the 250 ml RBF containing the metal alkoxide solution
3. Using a clean, oven-dried cannula, transfer the metal alkoxide-alcohol-n-heptane solution into the RME under constant stirring over the span of 10 min.
   Note: For metal alkoxides that react vigorously with water, such as tellurium (IV) isopropoxide, a much slower drop-wise rate of addition is required. The final solution should be optically clear regardless of the metal alkoxide precursor used, but can be various colors depending on the metal alkoxides used and their oxidation states. We note that whether the emulsion is transparent or translucent is dependent on particle size while the color is dependent on metal oxidation state.
4. After 4 hr, use a clean, dry syringe to inject 1.4 ml of reagent-grade NH₄OH into the solution dropwise. Then, using another clean, dry syringe, inject 1.2 ml of reagent-grade tetraethyl orthosilicate (TEOS) dropwise.
   Note: Sodium hydroxide (NaOH) should not be used as it can lead to densification of the silica shells during carburization/nitridation and result in sodium impurities in the lattice of the final nanoparticles. Residual NH₄OH on the other hand conveniently decomposes during the heat treatments.
5. After 16.5 hr, remove the rubber stopper and use a clean, dry graduated cylinder to add 300 ml of methanol to the solution under constant stirring.
6. After 10 min of stirring, remove the stir bar and allow the solution to settle.
   Note: There will be two liquid phases present: a heptane-rich upper phase and a methanol-rich lower phase. The silica-encapsulated metal oxide nanoparticles, designated as SiO₂/MO_x will sediment to the bottom of the flask from the lower phase.
7. After 1 hr, decant the liquid phases into an organic waste container and collect the solid phase SiO₂/MO_x in clean 50 ml centrifuge tubes.
   Note: if polypropylene or polystyrene centrifuge tubes are being used, the precipitant cannot be stored long-term in this state as the residual heptane will slowly dissolve the centrifuge tube, causing it to elongate. Furthermore, residual NH₄OH and TEOS can cause the SiO₂ to continue to grow beyond what was desired.
8. Centrifuge the SiO₂/MO_x phase at 2,056 x g for 5 min, then decant the supernatant into an organic waste container.
9. Add 10-30 ml of acetone to each 50 ml centrifuge tube and disperse the powder into the acetone by shaking or sonicating. This helps remove excess surfactant before calcination.
10. Centrifuge the SiO₂/MO_x and acetone mixture at 4,626 x g for 10 min and then decant the supernatant into an organic waste container. Allow the SiO₂/MO_x gel to dry overnight.

2. Heat Treatment of the Synthesized Powder

Caution Statement: Hydrogen, methane, and ammonia gas are extremely flammable. Ammonia gas is toxic. A thorough leak-check should be performed before all heat treatments, and the treatments should be performed in quartz-tubular furnace kept in a well-ventilated fume hood with an oil bubbler at the gas outlet. All gas cylinders should be equipped with flame arrestors and stored away from the furnace. If required by law or institutional regulations, excess ammonia gas should be bubbled through a large volume of water and either discarded as base waste or neutralized to pH 7 ammonium chloride solution using HCl.

1. Calcination of the SiO₂/MO_x powder in air
   1. Crush the dried SiO₂/MO_x gel into a fine powder using a glass stir bar or mortar and pestle. Put the powder in alumina crucible boats and load into a quartz-tube furnace.
   2. Using a 2 °C/min heating rate, calcine the SiO₂/MO_x powder under 100 sccm of air at 450 °C for 1 hr. This removes any residual surfactant and also increases the microporosity of the silica shells. After the furnace has cooled, remove the calcined SiO₂/MO_x powders and store in ambient conditions.
2. Carburization of the SiO₂/MO_x powder in a methane/hydrogen atmosphere
   1. Load the calcined SiO₂/MO_x powder into an unglazed alumina crucible boat and place into a quartz-tube furnace. Flush the quartz-tube furnace with nitrogen for at least 30 min to remove oxygen. Perform a leak check by spraying all joints with soapy water.
   2. Using a 2 °C/min heating rate, carburize the SiO₂/MO_x powder under 120 sccm of hydrogen and 33 sccm of methane at 850 °C for 4 hr to form SiO₂/MC_x.
   3. After 4 hr, stop the flow of methane and hold the powder at 850 °C for 1 hr in just 120 sccm of hydrogen to scavenge any excess surface carbon.
   4. Allow the furnace to slowly cool to room temperature under 120 sccm of hydrogen. Then, flush the furnace with nitrogen for at least 30 min to remove hydrogen.
   5. Flow 95 sccm of nitrogen and 5 sccm of air over the powder for 3 hr to slowly passivate the carbides before exposure to air.
   6. Remove the SiO₂/MC_x powder from the furnace and store in a vacuum dessicator or in a dry nitrogen glovebox to prevent long-term bulk oxidation of the metal carbide nanoparticles in air.
3. Nitridation of the SiO₂/MO_x powder in an ammonia atmosphere
   1. Load the calcined SiO₂/MO_x powder into an alumina crucible boat and place into a quartz-tube furnace.
   2. Flush the quartz-tube furnace with nitrogen for at least 30 min to remove oxygen.
   3. Using a 2 °C/min heating rate, nitridize the SiO₂/MO_x powder under 100 sccm of ammonia at 800 °C for 4 hr to form SiO₂/MN_x.
   4. Allow the furnace to slowly cool to room temperature. Then, flush the furnace with nitrogen for at least 30 min to remove ammonia.
   5. Flow 95 sccm of nitrogen and 5 sccm of air over the powder for 3 hr to slowly passivate the nitrides before exposure to air.
   6. Remove the SiO₂/MN_x powder from the furnace and store in a vacuum dessicator or in a dry nitrogen glovebox to prevent long-term bulk oxidation of the metal nitride nanoparticles in air.

3. Removing the Silica Shells and Supporting the Nanoparticles

Caution Statement: ammonium bifluoride (ABF) is highly toxic. It should only be handled in a well-ventilated fume hood using proper personal protective equipment, including butyl-rubber gloves, a face shield, and an HF-resistant labcoat with HF-resistant sleeves. Glass and metal lab equipment should never be used when handling ABF.

1. Silica Dissolution in Ammonium Bifluoride for Acid-Stable Metal Carbides and Nitrides (such as WC and W₂N)
   1. Weigh out 200 mg of SiO₂/MC_x or SiO₂/MN_x and put in a 30 ml sealable polypropylene container with a Teflon-coated magnetic stir bar.
   2. If it is desired to support the nanoparticles on a high surface area catalyst support, such as carbon black or carbon nanotubes, weigh out the material and add it to the sealable polypropylene container.
      Note: The mass weighed out will depend on the desired loading of nanoparticles on the support. For catalyst supports that are not stable in ABF, such as alumina, the support can be added after the silica has been removed and the ABF is neutralized. If no support is added, a nanodispersion will be obtained.
   3. Add 20 ml of ultrapure DI water and begin mixing to form a suspension. Alternatively, add 20 ml of pre-made 20 wt% ABF solution.
   4. Weigh out 5 g of ABF and then add to the stirring mixture. Once added, seal the polypropylene container.
      Note: Dissolution of ABF in water is endothermic, so the temperature of the solution will drop.
   5. To ensure complete dissolution of the silica and good dispersion of the nanoparticles on the catalyst support, stop the reaction after 16 hr by adding reagent-grade NH₄OH dropwise to neutralize the ABF solution to a pH of 6-7. Caution: this reaction is exothermic.
   6. Empty the neutralized mixture into a centrifuge tube and centrifuge at 2,056 x g for 10 min.
   7. Empty the supernatant into an alkaline waste container (to ensure total neutralization) and redisperse the catalyst powder in 20 ml of ultrapure DI water.
   8. Centrifuge at 4,626 x g for 10 min, discard the supernatant, and do one final 20 ml DI water rinse. Up to 12,850 x g may be required for unsupported nanoparticles.
   9. Dry the powder under vacuum and then store in a vacuum dessicator or a dry nitrogen glovebox to prevent long-term bulk oxidation of the metal carbide or nitride nanoparticles.
2. Silica Dissolution in Sodium Hydroxide for Alkaline-Stable Carbides and Nitrides (such as TaC and Ta₃N₅)
   1. Weigh out 200 mg of SiO₂/MC_x or SiO₂/MN_x and put in a 50 ml glass RBF with a magnetic stir bar. Weigh out and add a catalyst support if desired.
   2. Add 30 ml of at least 0.1 M aqueous NaOH solution. Attach a condenser to the RBF or seal with a rubber stopper and Parafilm wax.
   3. Heat at 60 °C under constant stirring for 16 hr. After cooling, centrifuge the solution at 2,056 x g for 10 min.
   4. Empty the supernatant into an alkaline waste container and redisperse the catalyst powder in 20 ml of ultrapure DI water.
   5. Centrifuge at 4,626 x g for 10 min, discard the supernatant, and do one final 20 ml DI water rinse. Up to 12,850 x g may be required for unsupported nanoparticles.
   6. Dry the powder under vacuum and then store in a vacuum dessicator or a dry nitrogen glovebox to prevent long-term bulk oxidation of the metal carbide or nitride nanoparticles.

Results

In the first step of the protocol, the goal is to encapsulate the monometallic or bimetallic transition metal oxide (TMO) NPs within microporous silica spheres. Figure 1 shows images taken of representative syntheses before and after precipitation with methanol. Two reproducible morphological outcomes have been observed during this step that appear to be dependent on the metals used in the synthesis: the TMO NPs can be singly coated with a silica sphere (Figure 2b) or multiple TMO NPs ca...

Discussion

A procedure for synthesizing non-sintered, metal terminated transition metal carbide and nitride nanoparticles with tunable sizes and structure is presented here.²⁶ Critical steps in the method include: using a moisture-free RBF to contain the diluted metal alkoxide precursor, avoiding alkali metal impurities during all steps, precipitating the RME with excess methanol as opposed to acetone or isopropanol, performing a proper leak check before carburizing or nitridizing the components, and using proper PPE whe...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
n-heptane	Sigma-Aldrich	246654
polyoxyethylene (4) lauryl ether	Sigma-Aldrich	235989	Brij® L4
tungsten (VI) isopropoxide	Alfa Aesar	40247	W(VI)IPO
tungsten (VI) chloride	Sigma-Aldrich	241911	To prepare W(VI)IPO, homemade
tungsten (IV) chloride	Strem Chemicals	74-2348	To prepare W(IV)IPO, homemade
tantalum (V) isopropoxide	Alfa Aesar	40038	Ta(V)IPO
niobium (V) isopropoxide	Alfa Aesar	36572	Nb(V)IPO
nickel (II) methoxyethoxide	Alfa Aesar	42377	Ni(II)MEO
titanium (IV) isopropoxide	Sigma-Aldrich	87560	Ti(IV)IPO
molybdenum (V) isopropoxide	Alfa Aesar	39159	Mo(V)IPO
molybdenum (V) chloride	Sigma-Aldrich	208353	To prepare Mo(V)IPO, homemade
tetraethyl orthosilicate	Sigma-Aldrich	333859	TEOS
ammonium hydroxide	Sigma-Aldrich	320145
methanol	Sigma-Aldrich	34860
anhydrous isopropanol	Sigma-Aldrich	278475	To prepare homemade alkoxides
ammonium bifluoride	Sigma-Aldrich	224820
carbon black	Cabot Corp.	Vulcan® XC72R
Methane	AirGas	ME R300
Hydrogen	AirGas	HY UHP300
Ammonia	AirGas	AM AH80N705
Quartz Tube Furnace	MTI Corp.	OTF-1200X-S-UL