Various well-defined bimetallic noble metal systems show exceptional synergistic effects, which are basically related with electronic, geometric and ensemble effects. After identifying optimum compositions and structures of such bimetallic model electrodes with unique catalytic activities in recent experiments, systematic studies of reactions mechanisms and elementary reaction steps of simple reactions will follow. In this way, different model reactions shall be studied in detail with single-crystalline bimetallic model systems, which are either obtained by electrochemical deposition on single crystal surfaces or by proper preparation of single crystal alloys. Studies of selected electrocatalytic reactions at structurally well-defined electrode surfaces are related to theoretical projects P3 (electron transfer in oxygen reduction) and P4 (elementary reaction steps in C1 electrooxidation).

The hydrogen evolution reaction (HER) will be studied for Au and Ag monolayers of different thickness electrodeposited onto fcc noble metal single crystal surfaces. Interestingly, Ag monolayers on Au(111) and Pt(111) reveal an electrocatalytic activity for glucose oxidation, while bulk Ag(111) is practically inactive for this reaction. In a similar way, huge variations of exchange currents are expected for the HER at Au and Ag monolayers, which shall be quantified.

The volcano relation for HER at Pd monolayers suggests a change in the rate-determining step, when the chemisorptions energy of adsorbed hydrogen is systematically varied. Kinetic studies shall provide evidence for this hypothesis. In relation to ensemble effects, the mechanism of HER at Pd monomers and dimers in Cu, Au and Ag hosts will also be addressed.

The oxygen reduction reaction (ORR) at Pt rich surfaces of PtRu single crystal alloys shows enhanced activities for certain bulk composition values. A strong influence of the electrolyte composition on ORR was found. In addition, clearly different Tafel slopes in the low overpotential regime indicate the decisive role of anions in the ORR. Besides theoretical DFT calculations, which allow for disentangling ligand and lattice strain effects, further calculations of adsorption energies of intermediates and energy barriers enable simulations of the reaction, which will be compared with experimental results.

Finally, it has been shown that structure and coverage of adsorbed formate are crucial for the electrooxidation of formic acid on Au(111). These findings shall be supported by theoretical DFT calculations regarding stability and reactivity of different formate adsorption configurations. Ordered formate structures will be imaged by in-situ STM. Studies of formic acid oxidation will be extended to technologically more important Pd monolayers deposited onto noble metal single crystals including alloys. Electrocatalytic effects of

• formic acid concentration
• pH value
• spectator anions
• structure and coverage of adsorbed formate and
• temperature
• etc.

on these systems will be examined.

Experimental and theoretical activities in this project shall highlight the importance of surface structure and composition in electrochemical kinetics.