CO Adsorption on Pd Nanoparticles: Assignment of Experimental C-O Vibrational Frequencies by DFT Calculations

Adsorption of CO probe molecules on metal catalysts is widely used to characterize the surface reactivity and morphology of these nanomaterials by assigning measured C-O vibrational frequencies to particular surface sites. Density-functional calculations of the corresponding CO adsorption complexes provide key complementary data for such characterization. However, even for the adequate structural models, the calculated frequencies do not quantitatively match the experimental values due to approximations in conventional generalized-gradient exchange-correlation functionals. We proposed a frequency-dependent scaling of the density-functional C-O frequencies for adsorption on different sites of nanostructured Pd catalysts, enabling quantitative agreement with the reference experimental values. Then, we computationally studied coverage-dependent bridge CO adsorption on edge sites of Pd nanoparticles, which revealed the energetic feasibility of the full CO occupation of these sites. Due to the static and dynamic CO-CO interactions, the calculated C-O stretching frequency grows by as much as 100 cm^-1 from the singleton CO adsorbed value with the number of coadsorbates at the neighboring bridge-edge sites. The saturation frequency approaches 1990 cm^-1, quantitatively matching the value experimentally observed for moderately large Pd particles. Using our frequency scaling, such particles are estimated to be at least 3 nm large.