IN-SITU ELLIPSOMETRIC STUDY OF A PALLADIUM CATALYST DURING THE OXIDATION OF METHANE

Ellipsometry is used to follow the growth of a PdO layer on the surfac e of a thick Pd-film catalyst during methane oxidation at approximatel y 500-degrees-C. The oxide layer that develops under rich conditions ( excess CH4) is quite porous and roughnes with time. Little CO is forme d during this period, but the CO2 formation rate increases until spont aneous oscillations develop, which correlate with changes in the ellip sometric data. These changes indicate that the porous oxide rapidly co nverts to a metal-rich state, which has decreased catalytic activity, and then slowly reoxidizes.