(771a) Maximizing Active Sites On Molybdenum Sulfide Nanomaterials: Hydrogen Evolution On Thiomolybdate [Mo3S13]2- Clusters

In order to develop improved catalysts, it is important to identify and
understand the active sites responsible for reaction turnover in order to
produce catalysts with a greater fraction of those sites and possibly even
improve upon their turnover frequency. Oftentimes, the most active sites of a
solid-state catalyst surface are those with special local structure and
stoichiometry such as edges, corners, and kinks.¹

MoS₂ is one example of such a catalyst: For
the hydrogen evolution reaction (HER)² as well as for
hydro-desulfurization (HDS)³,
MoS₂ edge sites are known to be catalytically active unlike the MoS₂
basal planes which are catalytically inert. In an effort to develop a scalable
HER catalyst with an increased number of active sites, herein we report on a
new type of Mo-S catalyst ? supported thiomolybdate
[Mo₃S₁₃]^2- nanoclusters⁴ ? which are particularly interesting as
most sulfur atoms in the cluster exhibit a similar structural motif to those
found at MoS₂ edges, see Figure 1.

Moreover the thiomolybdate [Mo₃S₁₃]^2- nano-clusters are synthesized by a facile, scalable route,
and can be deposited onto a wide range of electrode surfaces by means of a
simple drop-casting method using methanol as a solvent. The ability to deposit
onto a wide range of supports in such a straightforward manner enables ready
integration of these nanoclusters onto different
device architectures and materials for electrochemical applications.

We evaluated the HER activity of the clusters on two
types of substrates: (1) a high surface area graphite paper similar to that
used in commercial electrochemical devices such as water electrolyzers
and fuel cells, and (2) a highly orientated pyrolytic
graphite (HOPG) substrate which allowed for fundamental studies on a
sub-monolayer of nanoclusters by imaging them at the
atomic-scale with scanning tunneling microscopy (STM), see Figure 1.

In a strong acid environment, these active and stable [Mo₃S₁₃]^2-nanoclusters exhibit unprecedented turnover frequencies
for the HER compared to all other molybdenum sulfide
catalyst ever synthesized by non-vacuum methods, see Figure 2. We attribute
this high activity to the fact that these small [Mo₃S₁₃]^2-nanoclusters inherently expose a significant number
of active edge sites.

REFERENCES

1.             M. Boudart. Chemical
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2.             T. F. Jaramillo, K. P. Jørgensen, J. Bonde, J. H.
Nielsen, S. Horch, I. Chorkendorff. Science
2007, 317, (5834), 100-102.

3.             J. V. Lauritsen, M. Nyberg, J. K. Norskov, B. S.
Clausen, H. Topsoe, E. Laegsgaard, F. Besenbacher. Journal of Catalysis 2004,
224, (1), 94-106.

4.             J. Kibsgaard, T. F. Jaramillo, F. Besenbacher. In preparation 2013.