Probing Active Sites on Pd/Pt Alloy Nanoparticles by CO Adsorption.

We studied the adsorption of CO on Pd/Pt nanoparticles (NPs) with varying compositions using polarization-dependent Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) and theoretical calculations (DFT). We prepared PtPd alloy NPs via physical vapor codeposition on α-AlO(0001) supports. Our morphological and structural characterization by scanning electron microscopy and grazing incidence X-ray diffraction revealed well-defined, epitaxial NPs. We used CO as a probe molecule to identify the particles' surface active sites. Polarization-dependent FT-IRRAS enabled us to distinguish CO adsorption on top and side facets of the NPs. The role of the Pd/Pt alloy ratio on CO adsorption was investigated by comparing the experimental CO stretching band frequency for different alloy arrangements to the results for pure Pd and Pt NPs. Moreover, we studied the influence of hydrogen adsorption on the NP surface composition. We determined the dependence of the IR bands on the local atomic arrangement via DFT calculations, revealing that both bulk alloy composition and neighboring atoms influence the wavenumber of the bands.