Surface interactions in the Pt/γ-Al2O3 system. II. Dynamics of CO chemisorption by infrared fluctuation spectroscopy

Abstract

A consistent picture relating the nature of the surface phases of Pt on g-Al2O3 with the asymmetric shapes of the infrared profiles of chemisorbed CO is given. Observed quantities are (i) the absorption coefficient of the profile of the CO stretching fundamental as a function of temperature-controlled desorption steps, (ii) spectral second and third band moments, (iii) frequency shifts of the peak absorption, and (iv) particle size distribution of Pt on the alumina. Derived quantities are correlation times for the mobilities of the CO molecules on their adsorption sites, based on a simple model of vibrational dephasing determined by essentially 1:1 nearest-neighbor interactions (breakdown of the central limit theorem). It is concluded that the motional freedom (’’wagging’’ or ’’nearest-site hopping’’) of the chemisorbed CO molecules increases with increasing predominance of the particulate phase over the dispersed phase of Pt on g-Al2O3, reflecting the stronger platinum–alumina interactions in the dispersed phase.