EFFECT OF PARTICLE-SIZE ON THE REACTIVITY OF OXYGEN-ADSORBED PLATINUM SUPPORTED ON ALUMINA

A Pt/Al2O3 catalyst was aged in a methane-oxygen-nitrogen mixture at 873 K with an oxygen-to-methane ratio of 4. The metal particle size increased from 2 to 12 nm. The catalytic activity for methane oxidation was slightly increased for reaction temperature below 700 K and not modified at higher temperatures. As a consequence, the turnover number (i.e., the activity per surface platinum atom) is much higher on the large platinum particles. Microcalorimetric measurements showed a decrease in the heat of oxygen chemisorption when the platinum particle size increased. Titration of chemisorbed oxygen with gaseous hydrogen at 195 K indicated changes in the adsorbed oxygen reactivity: the reactivity of chemisorbed oxygen increased with increasing metal particle size. Changes in the strength of the PtSO bond are proposed to take into account the increase in turnover number that accompanies a decrease in the dispersion of the metallic phase. © 1990.