IN-SITU INFRARED REFLECTION-ABSORPTION SPECTROSCOPY STUDIES OF SULFURIC-ACID FORMATION ON PLATINUM AND PALLADIUM SURFACES

This work describes in situ infrared reflection absorption spectroscop y (IRAS) studies of the initial interaction of platinum and palladium to humidified air into which SO2 has been introduced. The formation of surface species could be followed in situ from 9 min to 20 h of expos ure. Supported by X-ray photoelectron spectroscopy and glancing incide nce X-ray diffraction analysis after exposure, the IRAS bands from sur face species formed in the aqueous adlayer on both metals has been ass igned to sulfuric acid, H2SO4, with no evidence of chemical reaction w ith the metal. When changing from humid to dry exposure conditions, th e aqueous adlayer with sulfuric acid transforms into one or more phase s of hydrated crystalline sulfuric acid, H2SO4 . nH(2)O. On platinum t here is observed an initially fast formation rate of sulfuric acid fol lowed by a slower formation rate with prolonged exposure. The initial formation rate on palladium, however, is much slower than on platinum. After about 7 h of exposure, a sudden but well-reproduced increase oc curs, whereafter the formation rate levels off.