MIT researchers develop process to create inexpensive transition-metal carbide catalysts to replace platinum
Researchers at MIT have developed a method to produce inexpensive catalysts that can replace platinum catalysts in renewable energy technologies such as fuel cells. In a paper in the journal Angewandte Chemie, they describe producing metal-terminated transition-metal carbides (TMCs) exhibit catalytic activities similar to platinum group metals (PGMs). TMCs, however, are orders of magnitude more abundant and less expensive.
The drawback has been current TMC synthesis methods, which lead to sintering, support degradation, and surface impurity deposition, ultimately precluding their wide-scale use as catalysts.
A method is presented for the production of metal-terminated TMC nanoparticles in the 1–4 nm range with tunable size, composition, and crystal phase. Carbon-supported tungsten carbide (WC) and molybdenum tungsten carbide (MoxW1−xC) nanoparticles are highly active and stable electrocatalysts. Specifically, activities and capacitances about 100-fold higher than commercial WC and within an order of magnitude of platinum-based catalysts are achieved for the hydrogen evolution and methanol electro-oxidation reactions. This method opens an attractive avenue to replace PGMs in high energy density applications such as fuel cells and electrolyzers.—Hunt et al.
The researchers set out to determine if it was possible to modify the electron density of earth-abundant early transition metals [groups IV to VI on the periodic table] to catalytically mimic PGMs.
Conceptually, tungsten, with six valence electrons, can be electronically modified to mimic platinum, which has 10 valence electrons, by reacting it with carbon (four valence electrons) to give the ceramic material tungsten carbide (WC).
Numerous studies have shown that WC is indeed platinum-like, and able to catalyze important thermo- and electrocatalytic reactions that tungsten metal cannot—such as biomass conversion, hydrogen evolution, oxygen reduction, and alcohol electro-oxidation. Tungsten is more than three orders of magnitude more abundant than platinum in the Earth’s crust, making it a viable material for a global renewable-energy economy.
However, both WC and platinum are heterogeneous catalysts, meaning that they require nanoparticle formulations to create high surface areas and invoke quantum confinement effects to maximize the rates of chemical reactions. While platinum nanoparticles are relatively easy to synthesize, until now, there have been no known methods to synthesize WC nanoparticles less than 5 nanometers and devoid of surface impurities.
Tungsten carbide forms at very high temperatures, typically over 800 degrees Celsius [1,500 ˚F]. These high temperatures cause nanoparticles to sinter into large microparticles with low surface areas. Methods to date that alleviate this agglomeration instead result in nanoparticles that are covered with excess surface carbon. These surface impurities greatly reduce, or completely eliminate, the catalytic activity of WC.—Sean Hunt, lead author
To solve this problem, the MIT team developed a “removable ceramic coating method” by coating colloidally dispersed transition-metal oxide nanoparticles with microporous silica shells. At high temperatures, they show that reactant gases, such as hydrogen and methane, are able to diffuse through these silica shells and intercalate into the encapsulated metal oxide nanoparticles. This transforms the oxide nanoparticles into transition metal carbide (TMC) nanoparticles, while the silica shells prevent both sintering and excess carbon deposition. The silica shells can then be easily removed at room temperature, allowing the dispersal of non-sintered, metal-terminated TMC nanoparticles onto any high-surface-area catalyst support. This is the first method capable of this result.
The team has also been successful in synthesizing the first non-sintered, metal-terminated bimetallic TMC nanoparticles. Electrocatalytic studies have shown that these materials are able to perform hydrogen evolution and methanol electro-oxidation at rates similar to commercial PGM-based catalysts, while maintaining activity over thousands of cycles. The catalytic activities obtained were more than two orders of magnitude better than commercial WC powders and WC nanoparticles made by current state-of-the-art synthesis methods that do not prevent sintering or surface carbon deposition.
Next steps include the synthesis of other bimetallic TMCs, as well as transition metal nitride (TMN) nanoparticles. The team is investigating these materials for other electrocatalytic reactions as well as thermal catalytic reactions, such as hydrodeoxygenation for biomass reforming.
This research was supported by the Department of Energy’s Basic Energy Sciences Division.
Hunt, S. T., Nimmanwudipong, T. and Román-Leshkov, Y. (2014), “Engineering Non-sintered, Metal-Terminated Tungsten Carbide Nanoparticles for Catalysis,” Angew. Chem. Int. Ed., 53: 5131–5136 doi: 10.1002/anie.201400294