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Abstract:
The atomic structure and composition of a catalyst's surface have a
major influence on its performance regarding activity and selectivity.
In this respect, intermetallic compounds are promising future catalyst
materials, as their surfaces exhibit small and well-defined ensembles of
active metal atoms. In this study, the active adsorption sites of the
3-fold-symmetric surfaces of the PdGa interrnetallic compound were
investigated in a combined experimental and computational approach using
CO as a test molecule. The PdGa(111) and (-1-1-1) surfaces exhibit very
similar electronic structures, but have Pd sites with very different,
well-defined atomic coordination and separation. They thereby serve as
prototypical model systems for studying ensemble effects on bimetallic
catalytic surfaces. Scanning tunneling microscopy and Fourier transform
infrared spectroscopy show that the CO adsorption on both surfaces is
solely associated with the topmost Pd atoms and Ga acts only as an
inactive spacer. The different local configurations of these Pd atoms
dictate the CO adsorption sites as a function of coverage. The
experimental results are corroborated by density functional theory and
illustrate the site separation and ensemble effects for molecular
adsorption on intermetallic single crystalline surfaces.
