Pt-K/Al2O3 NSR catalysts: characterization of morphological, structural and surface properties

Morphological, textural, and surface properties of a NSR (NOx storage reduction) Pt-K/Al2O3 model catalyst (Pt 1 wt %; K 5.4 wt %) were characterized by means of XRD, HRTEM, and FT-IR spectroscopy. Thin crystalline K-containing layers, in the form of cubic K2O and monoclinic K2CO3 and very small roundish Pt particles with a mean diameter of 1.5 nm, have been observed. Monoclinic K2CO3 disappears, and a certain degree of Pt sintering occurs (dPt ≈ 3.4 nm) after use. However, the presence of potassium limits the Pt sintering which occurs on the Pt/Al2O3 reference sample (Pt 1 wt %). FT-IR spectra of CO adsorbed at RT, compared with those recorded for Pt/Al2O3, revealed a marked interaction between the Pt and K phases that is much higher than the interaction between the Pt and Ba phases observed for the classic Pt-Ba/Al2O3 catalyst. CO2 adsorption at RT indicated a high heterogeneity of the K phase, evidenced by the formation of a variety of surface-carbonate-like species (mainly bridging carbonates on K sites). Minor amounts of nitrites and nitrates were formed at RT under NO admission, while the uptake was sensibly higher under NO/O2 or NO2 admission; nitrites (mono- and bidentate) and nitrates (ionic and bidentates) were formed in different amounts, both relative and absolute, and the nitrate to nitrite ratio increased in parallel with the NO/O2 ratio. Also, at each contact time, the amount of the stored NOx species increased upon increasing the NO/O2 ratio.