Eva M. Vogel

The nature and effects of platinum in perovskite catalysts

It has been observed that the catalytic activity of La0.7Pb0.3MnO3 for CO oxidation is greatly enhanced by the addition of platinum. The enhancement is up to 100 times greater than that expected for homogeneously distributed Pt whose specific activity is that of metallic Pt on Al2O3. X-Ray photoemission spectroscopy (ESCA) experiments show that Pt in these perovskites is in the form of the dissolved tetravalent ion. Catalytic activity measurements on BaPtO3 show that the specific activity of Pt4+ in that structure is about twice that of metallic Pt on Al2O3. Various observations of Pt concentrating at the surfaces of La0.7Pb0.3MnO3 can, in part, account for the enhanced activity. However, Pt4+ dispersed in La0.7Pb0.3MnO3 may still have a higher activity than Pt4+ in BaPtO3 since the measured surface concentration of Pt4+ cannot completely account for the observed activity. Additions of Pt to supported La0.5Sr0.5MnO3 and LaMn0.5Cu0.5O3 affect the activity of the former only slightly and may degrade the activity of the latter for CO oxidation. On the other hand, the Pt renders both compounds resistant to SO2 poisoning, the more so for the La0.5Sr0.5MnO3.

Effects of the Pt content of La0.7Pb0.3MnO3 on its catalytic activity for the oxidation of CO in the presence of SO2

Abstract The catalytic activity of La 0.7 Pb 0.3 MnO 3 , prepared by various methods and supported on monoliths of cordierite, for the oxidation of CO in the presence of 50 ppm of SO 2 is observed to increase with increasing Pt content of the samples. Crushed single crystals with Pt contents of 0–5200 ppm and polycrystalline materials with 0–2400 ppm behaved qualitatively similar in this respect. As little as 200 ppm of Pt imparted a significant resistance to poisoning by SO 2 .

Synthesis and structure of BaPtO3

Abstract BaPtO3 was prepared by the reaction of BaO2 + PtO2 at high O2 pressures and by the thermal decomposition of BaPt(OH)6 at 500–700°C in O2 at 1 atm. The powder pattern can be indexed based on a hexagonal cell with a = 5.64, c = 27.44A, corresponding to a 12-layer perovskite structure. The Oxygen content of BaPtO3−x materials was determined by firing the samples in H2 to form Ba(OH)2 + Pt and calculating x from the weight loss.

Effects of additions of TiO2, SnO2, Ga2O3, and MgO on the phase equilibria, stoichiometry, and lattice parameter of Ni0.89Fe2.11O4

Abstract Additions up to a mole fraction of 0.1 of TiO 2 , SnO 2 Ga 2 O 3 , and MgO were made to Ni 0.89 Fe 2.11 O 4 . Pellets were equilibrated in O 2 , 0.1% O 2 in N 2 , and N 2 at 1150, 1250, and 1350°C. Chemical and microstructural analysis enabled the determination of phase boundaries and the mechanisms of charge compensation. Charge compensation for the solubility of the additives in the spinel was predominantly (70%) by Fe 2+ formation, as opposed to cation vacancy formation, under the more oxidizing conditions and approached 100% under the reducing conditions. Variations in the lattice parameter of the spinel were noted as a function of additive, vacancy, and Fe 2+ concentrations.