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, commerc....

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 co....

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