Probing Active Sites\non Pd/Pt Alloy Nanoparticles\nby CO Adsorption

We studied the adsorption of CO on Pd/Pt nanoparticles\n(NPs) with\nvarying compositions using polarization-dependent Fourier transform\ninfrared reflection absorption spectroscopy (FT-IRRAS) and theoretical\ncalculations (DFT). We prepared PtPd alloy NPs via physical vapor\ncodeposition on α-Al<sub>2</sub>O<sub>3</sub>(0001) supports.\nOur morphological and structural characterization by scanning electron\nmicroscopy and grazing incidence X-ray diffraction revealed well-defined,\nepitaxial NPs. We used CO as a probe molecule to identify the particles’\nsurface active sites. Polarization-dependent FT-IRRAS enabled us to\ndistinguish CO adsorption on top and side facets of the NPs. The role\nof the Pd/Pt alloy ratio on CO adsorption was investigated by comparing\nthe experimental CO stretching band frequency for different alloy\narrangements to the results for pure Pd and Pt NPs. Moreover, we studied\nthe influence of hydrogen adsorption on the NP surface composition.\nWe determined the dependence of the IR bands on the local atomic arrangement\nvia DFT calculations, revealing that both bulk alloy composition and\nneighboring atoms influence the wavenumber of the bands.