Promotion effects in the oxidation of CO over zeolite-supported Pt nanoparticles

Pt particles with an average diameter of 1 nm supported on a series of zeolite Y samples containing different mono- and divalent cations have been used to investigate the effect of promotor elements in the oxidation of CO. IR allowed to study the TPD of CO from supported Pt particles in order to monitor the electronic changes in the local environment of adsorbed CO. It was found that the red shift of the linear Pt-coordinated C equivalent to O vibration compared to that of gas-phase CO increases with an increasing cation radius-to-chargc ratio. In addition, a systematic shift from linear to bridge bonded C equivalent to O was observed for decreasing Lewis acidity. A decreasing Lewis acidity results in an increasing electron charge on the framework oxygens and therefore an increasing electron charge on the supported Pt particles. This observation was confirmed with X-ray absorption spectroscopy and the intensity of the experimental Pt AXAFS correlates with the Lewis acidity of the cation introduced. Furthermore, it was found that the CO coverage increases with increasing electron density on the Pt particles. This increasing electron density was found to result in an increased CO oxidation activity; i.e., the T-50% for CO oxidation decreases with decreasing Lewis acidity.