The identification of the active sites in heterogeneous catalysis requires a combination of surface sensitive methods and reactivity studies. We determined the active site for hydrogen evolution, a reaction catalyzed by precious metals, on nanoparticulate molybdenum disulfide (MoS 2 ) by atomically resolving the surface of this catalyst before measuring electrochemical activity in solution. By preparing MoS 2 nanoparticles of different sizes, we systematically varied the distribution of surface sites on MoS 2 nanoparticles on Au(111), which we quantified with scanning tunneling microscopy. Electrocatalytic activity measurements for hydrogen evolution correlate linearly with the number of edge sites on the MoS 2 catalyst.

The electrochemical hydrogen evolution reaction is catalyzed most effectively by the Pt group metals. As H2 is considered as a future energy carrier, the need for these catalysts will increase and alternatives to the scarce and expensive Pt group catalysts will be needed. We analyze the ability of different metal surfaces and of the enzymes nitrogenase and hydrogenase to catalyze the hydrogen evolution reaction and find a necessary criterion for high catalytic activity. The necessary criterion is that the binding free energy of atomic hydrogen to the catalyst is close to zero. The criterion enables us to search for new catalysts, and inspired by the nitrogenase active site, we find that MoS2 nanoparticles supported on graphite are a promising catalyst. They catalyze electrochemical hydrogen evolution at a moderate overpotential of 0.1-0.2 V.

The pace of materials discovery for heterogeneous catalysts and electrocatalysts could, in principle, be accelerated by the development of efficient computational screening methods. This would require an integrated approach, where the catalytic activity and stability of new materials are evaluated and where predictions are benchmarked by careful synthesis and experimental tests. In this contribution, we present a density functional theory-based, high-throughput screening scheme that successfully uses these strategies to identify a new electrocatalyst for the hydrogen evolution reaction (HER). The activity of over 700 binary surface alloys is evaluated theoretically; the stability of each alloy in electrochemical environments is also estimated. BiPt is found to have a predicted activity comparable to, or even better than, pure Pt, the archetypical HER catalyst. This alloy is synthesized and tested experimentally and shows improved HER performance compared with pure Pt, in agreement with the computational screening results.

The hydrogen evolution reaction (HER) on carbon supported MoS2 nanoparticles is investigated and compared to findings with previously published work on Au(111) supported MoS2. An investigation into MoS2 oxidation is presented and used to quantify the surface concentration of MoS2. Other metal sulfides with morphologies similar to MoS2 such as WS2, cobalt-promoted WS2, and cobalt-promoted MoS2 were also investigated in the search for improved HER activity. Experimental findings are compared to density functional theory (DFT) calculated values for the hydrogen binding energies (deltaGH) on each system.

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Evolution -[DFT calculations]. -(HINNEMANN, B.; MOSES, P. G.; BONDE, J.; JOERGENSEN, K. P.; NIELSEN, J. H.; HORCH, S.; CHORKENDORFF, I.; NOERSKOV*, J. K.; J. Am. Chem. Soc. 127 (2005) 15, 5308-5309; Dep. Phys., Tech. Univ. Den., DK-2800 Lyngby, Den.; Eng.) -Schramke 25-015

Electrocatalytic properties of biomimetic supported incomplete cubane-type [Mo 3 S 4 ] 4+ clusters are investigated. The activity toward the hydrogen evolution reaction (HER) is evaluated on both a high surface area gas diffusion electrode in a membrane electrode assembly and on highly orientated pyrolytic graphite (HOPG) supports. Sub-monolayers of the clusters were imaged by means of scanning tunnelling microscopy (STM) prior to electrochemical characterization. This enabled the quantification of the activity on a per cluster basis for the HER and the comparison of the activity with other HER catalysts. We find that the HER activity of the [Mo 3 S 4 ] 4+ is comparable with that of the edge sites of MoS 2 . The supported [Mo 3 S 4 ] 4+ molecules were also characterized by X-ray photoelectron spectroscopy (XPS), and the observed deterioration in electrocatalytic activity with time was assigned to slow [Mo 3 S 4 ] 4+ cathodic desorption from the catalyst support.

Density Functional Theory (DFT) calculations of the adsorption energy of CO, for a platinum overlayer on Ru(0001), have been performed. For all coverages a significant reduction in the binding energy of up to 0.5 eV has been observed compared to that obtained on Pt(111). In addition, a Steady‐State Isotopic Transient Kinetic Analysis (SSITKA) study has been performed to determine the desorption rate dependence on the partial pressure of CO over commercial Pt/Ru electrocatalysts. As expected, no significant difference in the rate of exchange of CO at any given pressure is observed on going from Pt to Pt/Ru electrocatalysts when the diluted gas used was argon since the CO states will be filled to the same desorption energy for the two catalysts. However on changing the diluent gas to hydrogen, a reduction in the exchange rate for CO is observed clearly reflecting the lower CO binding energy and the increased competition for sites at the surface of the catalyst. The reduction efficiency of the Pt/Ru electrocatalyst was also studied and found to be highly dependent on whether CO or hydrogen was used. These results will be discussed with reference to the anode catalysis of the Polymer Electrolyte Membrane Fuel Cell (PEMFC).