FTIR SPECTROSCOPY OF NITRIC-OXIDE ADSORPTION ON PD/AL2O3 - EVIDENCE OF METAL-SUPPORT INTERACTION

Automotive three-way catalysts containing Pd as the only precious metal have certain advantages over current formulations consisting of various mixtures of Pt, Pd, and Ph. In this study, the adsorption of NO on 2 wt% Pd/Al2O3 was investigated using infrared spectroscopy. Bands for NO on Pd were assigned to linear (1753-1750 cm(-1)), twofold bridged (1615-1599 cm(-1)), and threefold bridged (1580-1572 cm(-1)) adsorption modes. For Pd supported on alumina, linear NO was favored on the reduced catalyst while two- and threefold bridged NO were favored on the oxidized catalyst. Upon heating, a decrease in the linear form was accompanied by increases in the two- and threefold bridged forms up to the highest temperature measured (300 degrees C). This result may indicate a conversion of the linear to bridged adsorption modes upon heating. NO adsorption on pure alumina revealed a band near 1807 cm(-1) attributable to weak, linear adsorption. This band increased drastically upon oxidation of the Pd/Al2O3 catalyst, indicating enhanced NO adsorption on alumina due to the presence of palladium. Conventional spillover from Pd to alumina was ruled out. The increased intensity is explained by adsorption of NO on alumina sites involved in a metal-support interaction brought about by catalyst oxidation. NO adsorption on La-promoted Pd/Al2O3 was less than on the unpromoted catalyst. Possible explanations are an increase in NO dissociation and coverage of Pd by the modifier. (C) 1995 Academic Press, Inc.