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

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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.