A technique for creating a new molecule that structurally and chemically replicates the active part of the widely used industrial catalyst molybdenite has been developed by researchers with the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). This technique holds promise for the creation of catalytic materials that can serve as effective low-cost alternatives to platinum for generating hydrogen gas from water that is acidic.
Christopher Chang and Jeffrey Long, chemists who hold joint appointments with Berkeley Lab and the University of California (UC) Berkeley, led a research team that synthesised a molecule to mimic the triangle-shaped molybdenum disulfide units along the edges of molybdenite crystals, which is where almost all of the catalytic activity takes place. Since the bulk of molybdenite crystalline material is relatively inert from a catalytic standpoint, molecular analogues of the catalytically active edge sites could be used to make new materials that are much more efficient and cost-effective catalysts.
“Using molecular chemistry, we’ve been able to capture the functional essence of molybdenite and synthesise the smallest possible unit of its proposed catalytic active site,” says Chang, who is also an investigator with the Howard Hughes Medical Institute (HHMI). “It should now be possible to design new catalysts that have a high density of active sites so we get the same catalytic activity with much less material.”
Says Long, “Inorganic solids, such as molybdenite, are an important class of catalysts that often derive their activity from sparse active edge sites, which are structurally distinct from the inactive bulk of the molecular solid. We’ve demonstrated that it is possible to create catalytically active molecular analogues of these sites that are tailored for a specific purpose. This represents a conceptual path forward to improving future catalytic materials.”
Chang and Long are the corresponding authors of a paper published in the journal Science. Other authors are Hemamala Karunadasa, Elizabeth Montalvo, Yujie Sun and Marcin Majda.
Molybdenite is the crystalline sulphide of molybdenum and the principal mineral from which molybdenum metal is extracted. Although commonly thought of as a lubricant, molybdenite is the standard catalyst used to remove sulphur from petroleum and natural gas for the reduction of sulphur dioxide emissions when those fuels are burned. Recent studies have shown that in its nanoparticle form, molybdenite also holds promise for catalysing the electrochemical and photochemical generation of hydrogen from water. Hydrogen could play a key role in future renewable energy technologies if a relatively cheap, efficient and carbon-neutral means of producing it can be developed.
Currently, the best available technique for producing hydrogen is to split water molecules into molecules of hydrogen and oxygen using platinum as the catalyst. However, with platinum going for about R15 200 per 28 g, the market is wide open for a low cost alternative catalyst. Molybdenite is far more plentiful and about 1/70th the cost of platinum, but poses other problems.
“Molybdenite has a layered structure with multiple microdomains, most of which are chemically inert,” Chang says. “High-resolution scanning tunnelling microscopy studies and theoretical calculations have identified the triangular molybdenum disulfide edges as the active sites for catalysis; however, preparing molybdenite with a high density of functional edge sites in a predictable manner is extremely challenging.”
Chang, Long and their research team met this challenge using a pentapyridyl ligand known as PY5Me2 to create a molybdenum disulphide molecule that, while not found in Nature, is stable and structurally identical to the proposed triangular edge sites of molybdenite. It was shown that these synthesised molecules can form a layer of material that is analogous to constructing a sulphide edge of molybdenite.
“The electronic structure of our molecular analogue can be adjusted through ligand modifications,” Long says. “This suggests we should be able to tailor the material’s activity, stability and required over-potential for proton reduction to improve its performance.”
In 2010, Chang and Long and Hemamala Karunadasa, who is the lead author on this new Science paper, used the PY5Me2 ligand to create a molybdenum-oxo complex that can effectively and efficiently catalyse the generation of hydrogen from neutral buffered water or even sea water. Molybdenite complexes synthesised from this new molecular analogue can just as effectively and efficiently catalyse hydrogen gas from acidic water.
“We’re now looking to develop molecular analogues of active sites in other catalytic materials that will work over a range of pH conditions, as well as extend this work to photocatalytic systems” Chang says.
Adds Long, “Our molecular analogue for the molybdenite active site might not be a replacement for any existing catalytic materials but it does provide a way to increase the density of active sites in inorganic solid catalytic materials and thereby allow us to do more with less.”