Interaction of carbon monoxide with small metal clusters: a DFT, electrochemical, and FTIR study

The adsorption of CO molecules onto small metal clusters was studied using density functional theory (DFT) calculations, and experimental electrochemical and attenuated total reflection-Fourier transform infrared spectroscopic (ATR-FTIR) techniques were used to examine CO adsorbed onto nanostructures of similar composition. The adsorption strengths and CO vibrational stretching frequencies were calculated and analyzed for clusters of the form M CO for all of the period 4, 5, and 6 d-block transition metals. A direct link between the v(CO) and the population of d orbitals of the metal was observed. All possible binding sites for CO on clusters of the form Pd-4-CO, Pd2Pt2-CO, and Pd2Au2-CO were determined and the corresponding adsorption energies and CO stretching frequencies were examined. Pure Pd and bimetallic PdPt and PdAu nanostructures were fabricated and used as catalysts for the adsorption and electrochemical oxidation of CO. The relative quantities of CO molecules adsorbed to surface of the catalysts decrease in the order of PdPt > Pd > PdAu, consistent with our DFT results. The location of v(CO) bands of CO adsorbed onto the nanostructured catalysts were determined by means of ATR-FTIR spectroscopy and were found to have values close to that predicted by DFT. This paper shows that DFT calculations on very small metal clusters M-n-CO (n <= 4) can be a simple but effective way of screening catalysts for their adsorbing properties.