Study of the electrical and catalytic properties of spinels with CuFe2−xMnxO4 composition (x = 0, 0.4, 0.8, 1.6 and 2)

Abstract

CuFe2−xMnxO4 spinels (x=0, 0.4, 0.8, 1.6 and 2) were used as catalysts for the total oxidation of methane in the temperature range 450–800°C and characterized by in situ electrical conductivity measurements. Their electrical conductivity was studied as a function of temperature and oxygen partial pressure and temporal responses during sequential exposures to air, methane–air mixture (reaction mixture) and pure methane in conditions similar to those of catalysis were analyzed. The catalytic activity decreased with increasing value of x in the CuFe2−xMnxO4 spinels as follows: CuFe2O4>CuFe1.6Mn0.4O4>CuFe1.2Mn0.8O4>CuFe0.4Mn1.6O4>CuMn2O4. CuFe2O4 appeared to be a p-type semiconductor, while CuMn2O4 a n-type semiconductor under air in the reaction temperature range but CuFe2O4 became n-type while CuMn2O4 p-type when contacted with methane. Both types of conduction mechanisms are present in CuFe2−xMnxO4 spinels with x=0.4, 0.8 and 1.6, being predominant one or another as a function of temperature. Nevertheless, they were of p-type under air in the reaction temperature range and became n-type when contacted with methane. The conducting behavior and redox properties of the spinels in the reaction temperature range were correlated with their catalytic behavior. For CuMn2O4 catalyst, the reaction mainly involves surface lattice O– species, whereas for the other surface lattice O2– anions are mainly involved. In all cases, the overall reaction mechanism can be assimilated to a Mars and van Krevelen mechanism.