Researchers develop technique to create new tailored molecule with high density of active catalytic sites; potential low-cost alternative to platinum for splitting water
Researchers with the US Department of Energy’s Lawrence Berkeley National Laboratory have developed a technique for creating a new molecule that structurally and chemically replicates the active part of the widely used industrial catalyst molybdenite. This technique holds promise for the creation of catalytic materials with high densities of active sites that can serve as effective low-cost alternatives to platinum for generating hydrogen gas from water that is acidic.
Molybdenite (molybdenum disulfide, MoS2) is the crystalline sulfide of molybdenum and the principal mineral from which molybdenum metal is extracted. Molybdenite is one of the most widely used catalysts in industry today as the standard for hydrodesulfurization (HDS) of petroleum and natural gas. However, recent studies have shown that in its nanoparticle form, molybdenite also holds promise for catalyzing the electrochemical and photochemical generation of hydrogen from water.
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 synthesized 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.
As is the case with many inorganic solids, the catalytic activity of MoS2 is localized to rare surface sites, whereas the bulk material is relatively inert. High-resolution scanning tunneling microscopy studies and theoretical calculations performed on nano-particulate MoS2 structures that form under sulfiding conditions implicate the formation of disulfide linkages or triangular MoS2 units along the fully sulfided catalytically active edges of the layered structure. However, the precise molecular structures and modes of action of these sites remain elusive. Because of the bulk material’s layered structure, which favors the growth of plate-like crystals, a single crystal with a large edge dimension is extremely challenging to prepare.
Here, we report the synthesis of a well-defined molecular analog of the proposed MoS2 edge structure, a side-on bound MoIV-disulfide complex. Electrochemical reduction of this molecule leads to the catalytic generation of hydrogen from acidic organic media as well as from acidic water, lending support to the proposed active site morphology in the more active heterogeneous catalyst.—Karunadasa et al.
Since the bulk of molybdenite crystalline material is relatively inert from a catalytic standpoint, molecular analogs of the catalytically active edge sites could be used to make new materials that are much more efficient and cost-effective catalysts.
Chang and Long are the corresponding authors of a paper in the journal Science describing this research; other authors are Hemamala Karunadasa, Elizabeth Montalvo, Yujie Sun and Marcin Majda.
Using molecular chemistry, we’ve been able to capture the functional essence of molybdenite and synthesize the smallest possible unit of its proposed catalytic active site. 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.—Christopher Chang
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 analogs of these sites that are tailored for a specific purpose. This represents a conceptual path forward to improving future catalytic materials.—Jeffrey Long
Preparing molybdenite with a high density of functional triangular molybdenum disulfide edges in a predictable manner is extremely challenging, the authors note. Chang, Long and their research team met this challenge using a pentapyridyl ligand known as PY5Me2 to create a molybdenum disulfide 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 synthesized molecules can form a layer of material that is analogous to constructing a sulfide edge of molybdenite. The synthesized molecules performed robustly in evolving hydrogen from water, even using crudely filtered California seawater.
The electronic structure of the molecular analog can be adjusted through ligand modifications, Long says, suggesting that the material’s activity, stability and required over-potential for proton reduction can be tailored 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 catalyze the generation of hydrogen from neutral buffered water or even sea water. Molybdenite complexes synthesized from this new molecular analog can just as effectively and efficiently catalyze hydrogen gas from acidic water.
The ability to prepare, characterize, and evaluate molecular analogs of the active components of inorganic solids has broad implications for the design and optimization of functional metal sites, not the least of which is control over the density of these units. For example, recent electronic structure calculations conducted on nanoparticulate MoS2 indicate that only a quarter of the edge sites are used for hydrogen production. Increasing the number of active edge sites per unit volume by tailoring progressively smaller nano-structures or changing the electronics of the system to increase the enthalpy of hydrogen adsorption is a major challenge in inorganic materials and nanoscience. We present an alternative strategy using discrete molecular units, which in principle can be tailored to give a high density of catalytically active metal sites without the rest of the inactive bulk material.—Karunadasa et al.
This research was supported by the DOE Office of Science, in part through the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub.
Hemamala I. Karunadasa, Elizabeth Montalvo, Yujie Sun, Marcin Majda, Jeffrey R. Long, and Christopher J. Chang (2012) A Molecular MoS2 Edge Site Mimic for Catalytic Hydrogen Generation. Science 335 (6069), 698-702. doi: 10.1126/science.1215868
Hemamala I. Karunadasa, Christopher J. Chang and Jeffrey R. Long (2010) A molecular molybdenum-oxo catalyst for generating hydrogen from water. Nature 464, 1329–1333 doi: 10.1038/nature08969