A Mechanism Study On IR Spectra Properties Of CO Adsorption On Nanostructure Pt Surfaces

This dissertation is concerned with the chemical adsorption of CO-Pt system, from many aspects, such as polarizability, interaction energy, energy transfer, through the interactions among atoms (or molecules), as well as atoms with external field near the CO-Pt surface. It is found that the changes of these interactions may give rise to the changes of the polarizabilities, the coverage and the IR absorption spectra of the adsorbed molecules. The results of theoretical calculations show that the nanostructure of metallic adsorption surface would cause the changes of the IR spectrum properties, which lead to abnormal IR effects (AIREs). We analyzed the forms of interaction, and calculated the polarizability of effective dipoles, the local fields near the surface of nanoparticles, the distributions of adsorption CO molecules, IR vibrational spectra, and so on. The main contributions of this dissertation are: (1) The potential energy function of a molecule was studied, by using the Embedded Atom Method (EAM) to set up a model for CO-Pt system, and the interactions among atoms(or molecules), as well as atoms with external field near the CO-Pt surface were analyzed. Then, we chose reasonable potential functions for different cases and did the calculations. (2) Based on the mechanism of electron-hole damping, we discussed the influence of interparticle interaction on the polarizability of adsorbed molecules, which leads to an abnormal structure of IR spectrum through computer simulation. (3) Based on classical electromagnetic field theory, a model for metallic nanostructured surfaces was established to calculate the local field near the surface. The interaction acting on CO molecules by the local field was also analyzed. (4) Using Monte-Carlo simulation with various interactions taking into consideration, the stable distribution of CO molecules on a nanoparticle surface were obtained. We analyzed the symmetry of the distribution of CO molecules, the coverage range, and the influence among nanoparticles. It is shown that the structures of metal nanoparticle make the CO molecules agglomerate on the particle surface. The smaller the size of particles is, the more agglomerative the CO molecules are. (5) By theoretical calculation, the physical picture of AIREs is unveiled. The nanostructured metal surface leads to the agglomeration of the adsorbed CO molecules, which increases the internal field as well as the interaction among CO molecules. As a result, the agglomeration make the IR spectra transform from normal to Fano-like (positive and negative-going peak) line shape, then to anti-absorption AIREs. (6) Based on the embedded-atom energy model and the finite difference method, the IR vibrational spectra were calculated. It is found that the vibration states of the CO-Pt system are similar to those of the harmonic oscillation, the eigenvibration energies of CO-Pt system are lower than those of the free state of CO, and the spectra are red-shift. Besides, we discussed the influences of the potential function parameters on the theoretical calculations.