The rapid depletion of unsustainable resources has sparked global research on renewable-energy technologies, such as fuel cells, electrolyzers, and lithium-air batteries.
Unfortunately there is a common unsustainable thread that links these burgeoning technologies: a dependence on platinum-group metals (PGMs).
These elements — platinum, palladium, rhodium, iridium, ruthenium, and osmium — are the six least-abundant in the Earth’s lithosphere, yet are the most stable and active catalysts.
Even with efficient recycling, numerous studies have indicated that the Earth simply does not contain enough PGMs to support a global renewable-energy economy. Thus, PGMs can be considered unsustainable resources that are currently needed to enable renewable energy technologies.
MIT researchers believe is is possible to replace PGMs with metals that are more plentiful.
Rather than finding new materials to replace PGMs in specific reactions, is it possible to modify the electron density of earth-abundant early transition metals [groups IV to VI on the periodic table] to catalytically mimic the PGMs.
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).
WC is indeed platinum-like, and able to catalyze important thermo- and electrocatalytic reactions that tungsten metal cannot.
Importantly, 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.
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. 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.
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.
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.
