Localisation of Alkali Metal Ions in Sodium-Promoted Palladium Catalysts as Studied by Low Energy Ion Scattering, and Transmission Electron Microscopy

Three series of palladium-based catalysts have been studied by Low Energy Ion Scattering (LEIS) and Transmission Electron Microscopy (TEM). The first series is comprised of Na-Pd/SiO2 catalysts, obtained by addition of palladium to a silica support and by further addition of sodium ions with a Na/Pd atomic ratio (R) equal to 0, 6.4, and 25.6. The second series consists of palladium catalysts supported on natural pumice, in which, due to a different loading of supported palladium, R0, the (NaCK)/Pd atomic ratio, is equal to 17.0 and 39.4. The third series is represented by two palladiumbased catalysts supported on "model pumices," synthetic silicoaluminates, obtained by sol-gel techniques, with a different amount of sodium, and R equal to 2.1 and 6.1, respectively. LEIS experiments and electron microscopy demonstrate a different location of alkali metal ions in the first two series: in the Na-Pd/SiO2 catalysts sodium is distributed in a way which is not uniform on the support and on the palladium metal, which is partly decorated with Na ions, whereas in the Pd/natural-pumice series the palladium surface is sodium-free. The results on the third series of catalysts, Pd/model pumice, are not definitive on the basis of the LEIS and TEM analyses, but by FTIR study of CO and CO2 adsorption, the decoration of palladium by sodium ions could be excluded. The results confirm the importance of the alkali metal ion location in alkali-promoted palladium catalysts and open new possibilities in the design of palladium-supported catalysts by a better control of promoter location.