PALLADIUM-LANTHANUM INTERACTION PHENOMENA IN PD-LACL3 SIO2 AND PD-LA2O3/SIO2 CATALYSTS/

The effect of La addition and the nature of the precursors on the surf ace properties of Pd/SiO2 are studied. Samples containing 0.5wt% Pd we re prepared by incipient wetness impregnation and characterized by H-2 and CO chemisorption, TEM, TPR, EDX, XPS, Ar+-sputtering and CO/FTIR. The use of nitrate precursors improves both reducibility and dispersi on of Pd. Lanthanum addition makes the metal reduction more difficult, indicating that a Pd-La interaction takes place. When starting from C l- precursors, Pdn+ species are formed at the surface, whereas only Pd -0 is found when nitrate precursors are used. The addition of LaCl3 in creases the Pd dispersion and hinders the formation of Pd-nt species t hrough a dilution effect. In a sample prepared from Pd(NO3)(2) + La(NO 3)(3), the La2O3 formed upon calcination originates a SMSI-like effect on the palladium. This effect is suggested by the strong reduction of the H-2 and CO chemisorption capacity of Pd in this sample, in spite of the fact that the dispersion of palladium calculated from TEM incre ases in the presence of La. From XPS results this SMSI state appears t o be due to a physical decoration of Pd by La2O3 rather than to an ele ctronic Pd-La2O3 interaction. The ''decoration model'' is further conf irmed by the progressive increase of the Pd/La atomic ratio observed u pon XPS/Ar+-sputtering at different times the sample containing La. Th e hypotheses of both dilution and decoration effects caused by LaCl3 a nd La2O3, respectively, are strengthened by the CO/FTIR results.