The preparation chemistry and phase transitions under N2, H2 and air of mixed oxides of Zn, Mn, and Cr is investigated by means of chemical analysis, XRD, IR, DTA-TG, and UV-Vis DR. By reaction of Zn and Mn carbonates with a (TMA)2CrO4 solution, a spinel-related phase with formula ZnII(Mn1.5IIICr0.5III)O4 is formed already at 70–80°C. After drying at 100°C a precursor is obtained consisting of a core made of the spinel-like phase, Zn5(OH)6(CO3)2 and ZnO and covered by a shell constituted by TMAHCO3 and unreacted (TMA)2CrO4. The spinel-like phase is stable upon activation in N2 and air up to 400°C, wherease it transform into a rock-salt type structure under reducing atmospheres. Calcination at high temperature yields a tetragonal distortion of the cubic spinel phase due to Jahn-Teller stabilization of octahedrally coordinated Mn(III) along with an orthorhombic distortion due to the ordering of Mn(III) and Cr(III) ions in two different crystallographic sites. The degree of ordering depends on the Mn(III)/Cr(III) ratio. Pyrophoric effects, observed when the reduced samples are exposed to air, have been associated with the rapid reoxidation of Mn(II) formed by reduction of Mn(III).
Synthesis of Alcohols From Carbon Oxide and Hydrogen .12. Preparation Chemistry and Phase-transformations In the Zn-Mn-Cr-O System
CRISTIANI, CINZIA;FORZATTI, PIO;LIETTI, LUCA;
1989-01-01
Abstract
The preparation chemistry and phase transitions under N2, H2 and air of mixed oxides of Zn, Mn, and Cr is investigated by means of chemical analysis, XRD, IR, DTA-TG, and UV-Vis DR. By reaction of Zn and Mn carbonates with a (TMA)2CrO4 solution, a spinel-related phase with formula ZnII(Mn1.5IIICr0.5III)O4 is formed already at 70–80°C. After drying at 100°C a precursor is obtained consisting of a core made of the spinel-like phase, Zn5(OH)6(CO3)2 and ZnO and covered by a shell constituted by TMAHCO3 and unreacted (TMA)2CrO4. The spinel-like phase is stable upon activation in N2 and air up to 400°C, wherease it transform into a rock-salt type structure under reducing atmospheres. Calcination at high temperature yields a tetragonal distortion of the cubic spinel phase due to Jahn-Teller stabilization of octahedrally coordinated Mn(III) along with an orthorhombic distortion due to the ordering of Mn(III) and Cr(III) ions in two different crystallographic sites. The degree of ordering depends on the Mn(III)/Cr(III) ratio. Pyrophoric effects, observed when the reduced samples are exposed to air, have been associated with the rapid reoxidation of Mn(II) formed by reduction of Mn(III).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
