Investigation of coverage dependence of the stretching frequency of CO adsorbed on Pd surfaces at low coverage limits

•Metal-mediated interaction between CO molecules remain strong at long distances.•Adsorbed CO requires a greater scaling factor for the calculated stretching frequency than in gas phase.•The stretching frequency of adsorbed CO on Pd surfaces shows distinct coverage dependences due to both the choice of adsorption site and the change of COCO distances in the calculations.•Calculations support the possibility of ‘island formation’ at low coverages that can lead to a plateau in the frequency dependence as shown in some experiments. The stretching frequency of the CO bond is a sensitive probe of the local environment of a surface-bound CO molecule, including the adorption site and density, i.e. surface coverage. In this work, we extend our analysis beyond the frequency shift due to differences in adsorption configurations. Using density functional theory (DFT) calculations, we directly explore the correlations between surface coverage and the stretching frequency of adsorbed CO on Pd surfaces. We also perform constant pressure infrared reflection absorption measurements of CO on Pd(111) and use existing relations between pressure and coverage to derive coverage dependency. Both results are compared to previously reported experimental data. Our derived correlations of peak frequency and area with surface coverage can help interpret experimental IR spectra in real time and extract time-dependent concentration data from transient kinetic experiments. [Display omitted]