LOCALIZATION 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 Ene rgy Ion Scattering (LEIS) and Transmission Electron Microscopy (TEM). The first series is comprised of Na-Pd/SiO2 catalysts, obtained by add ition of palladium to a silica support and by further addition of sodi um ions with a Na/Pd atomic ratio (R) equal to 0, 6.4, and 25.6. The s econd series consists of palladium catalysts supported on natural pumi ce, in which, due to a different loading of supported palladium, R', t he (Na + K)/Pd atomic ratio, is equal to 17.0 and 39.4, The third seri es is represented by two palladium-based catalysts supported on ''mode l pumices,'' synthetic silico-aluminates, obtained by sol-gel techniqu es, with a different amount of sodium, and R equal to 2.1 and 6.1, res pectively. LEIS experiments and electron microscopy demonstrate a diff erent 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 su rface 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 ana lyses, but by FTIR study of CO and CO2 adsorption, the decoration of p alladium by sodium ions could be excluded. The results confirm the imp ortance of the alkali metal ion location in alkali-promoted palladium catalysts and open new possibilities in the design of palladium-suppor ted catalysts by a better control of promoter location. (C) 1996 Acade mic Pres, Inc.