Characterization of Pd-based FCC CO/NO x control additives by in situ FTIR and extended X-ray absorption fine structure spectroscopies

Abstract

Pd n+ /Ce n+ /Na +/γ-Al 2O 3-type materials used as FCC additives for CO/NO x control were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and in situ FTIR. The EXAFS data indicate that in freshly prepared samples palladium is present in the form of highly dispersed PdO species. Reduction with H 2 at 500 °C leads to the formation of small Pd clusters incorporating on average approximately six to eight metal atoms at a Pd−Pd bond distance of 2.76 Å. All components of these materials can interact with NO and promote the formation of nitrate/nitrite species, essentially “trapping” NO x species on the catalyst surface. However, the Na + species dominate the surface chemistry and readily form sodium nitrates with a characteristic IR band at 1370–1385 cm −1. Finally, hydroxyls from the support are also actively participating in the formation of HNO x type compounds with characteristic stretching vibrations in the 3500–3572 cm −1 region.