A RIETVELD ANALYSIS OF THE TRANSFORMATION OF (LA-SR-V-O)(REDUCED) TO (LA-SR-V-O)(OXIDIZED) SOLIDS AND THE EFFECT ON THEIR SURFACE CATALYTICPROPERTIES

The transformation of the crystal phases of the system (La-Sr-V-O)(red uced), having the general formula La1-xSrxVO3 (x=0.0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9, 0.95, 1.0) and the perovskite structure, towards t he (La-Sr-V-O)(oxidized) one, was studied using the Rietveld analysis. The oxidized system contains the crystal phases LaVO4, Sr2V2O7 and Sr 3V2O8 which originate from the crystal phases LaVO3 and SrVO3 existing in the reduced system. The oxidized solids exhibits surface areas (BE T) around 8-10 m(2) g(-1) while the reduced ones much lower similar to 1 m(2) g(-1). SEM studies showed that the oxidized particles exhibits a much more irregular surface with pores, cracks, steps and similar f eatures which lack in the reduced solids. The oxidized solids rich in Sr show also a platelet-like structure. The acid/base function of the surface was studied using the isopropanol decomposition as a test reac tion towards propene or acetone. The oxidized forms of the solids exhi bits stronger acidic properties as compared to the reduced one and lea d to dehydration towards propene while the reduced solids exhibit also considerable dehydrogenation activity. The rest features of catalytic action, like reaction rate and apparent activation energies appear ra ther similar and a common compensation effect, for the reduced and oxi dized solids, is observed. The variation of the reaction path towards dehydrogenation or dehydration is related to the intermediate electron egativity of the solids for both the reduced and the oxidized samples. (C) 1998 Elsevier Science B.V.