Comparative study of the solid-state and catalytic properties of La2–xSrxNiO4λ perovskites (x= 0.00–1.50) prepared by the nitrate and citrate methods

Abstract

Perovskites of the form La_2–xSr_xNiO_4–λ(x= 0.00, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50) have been prepared using both the nitrate and citrate methods. The solids obtained were examined by X-ray diffraction at various temperatures to examine the development of their crystal structure. Surface area determinations showed that the citrate method yields solids with surface areas that are on average three times as high as high as those of the nitrate species, a result that is due to the lower temperature required for the formation of the perovskite phase in the former case. Iodometric determination of the nickel oxidation state showed that this is similar for the two sets of samples in the range 0.00 < x < 0.75 but appreciably higher for nitrate solids for x > 0.75. These results are corroborated by magnetic measurements. The catalytic activity for both sets of solids, probed by N₂O decomposition, increases from x= 0.00 up to x= 0.75–1.00 if measured per unit mass, but up to 1.25 if measured per unit area of the solids. A comparison of the catalytic activity per unit mass for the two series indicates that citrate solids are about three times more active than the nitrate ones. This effect can be attributed to the higher surface area of the citrate solids. Nevertheless, the comparison per unit surface area shows that the citrate solids are still twice as active as the nitrates. Both series show a unique dependence of activity on oxygen deficiencies. Kinetic analysis of the catalytic results showed that this last factor alters the kinetics of the surface reaction and the apparent activation energies, a fact which is reflected in the reaction rates.