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
Reviews 1995, 95, (3), 661-666.
4. J. Kibsgaard, T. F. Jaramillo, F. Besenbacher. In preparation 2013.