Relationship Between Catalytic Activity and Electrical Conductivity in the Nonstoichiometric Nickel Manganite Spinels
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Abstract Nonstoichiometric nickel manganite spinels NiMn3_.03,404+ , have been synthesised by calcination of mixed oxalates in air at 350'C. The variation of the electrical conductivity cy with partial pressure of 02 shows that these oxides are n-type semiconductors; a also varies with the nickel content and has a maximum at x = 0.6. The intrinsic catalytic activity of these oxides for CO/CO 2 conversion varies with nickel content and the most active catalyst is at x=0.6 where the conversion starts at room temperature. The variation of the catalytic activity and the electrical conductivity and the nickel amount are correlated. Apparent activation energies are very low (less than 20 kJ/mol) and remain the same for all these mixed oxides. Similarly, the reaction order with respect to 02 and CO does not depend on the nickel content (order/O2 Z0; order/CO • 0.6). A reaction mechanism involving formation of oxygen ions adsorbed and carbonate species is discussed. Introduction: Nickel manganite spinels, widely studied for their application as thermistor ceramics, are usually prepared by thermal decomposition of mixed manganese-nickel oxalates in air at 900'C. These oxides have a low specific surface area (1 m2 g-1) and are stoichiometric (1,2). Recently, thermal decomposition of manganese-nickel oxalate at low temperature (T < 3502C) has been investigated (3-5) and the formation of cation deficient spinels NixMn3-xD3W/4O4+m has been shown. These oxides are finely divided materials. Their BET specific surface area is close to 100 m g1 and the average diameter of their crystallites is about 10 nm (6). The cation deficiency 6 has been evaluated by TGA under controlled atmosphere. It varies with nickel content and reaches a maximum between x = 0.4 and x = 0.6 (6,7). The marked nonstoichiometry of these oxides can be linked to their high reactivity towards molecules likes 02, CO or hydrocarbons. The goal of this study is to correlate the catalytic activity of CO oxidation and electrical conductivity at different temperatures and under different atmospheres with the nickel content. Synthesis: Nonstoichiometric nickel manganites were prepared by decomposition of mixed oxalates at 350'C in air for 6 hours. The oxalates Nix/ 3Mn(3 _x)/ 3(C20 4).2H 2 0 (0 • x < 1) were obtained at room temperature by coprecipitation of aqueous solution of nickel and manganese nitrates with a solution of ammonium oxalate. After one hour, the mixture was filtered, washed several times with deionized water and dried at 25°C in air.
47 Mat. Res. Soc. Symp. Proc. Vol. 497 ©1998 Materials Research Society
Structure: X-ray powder diffraction patterns show that the cation deficient nickel manganites are poorly crystallised, but the most intense lines of the spinel structure clearly appear. Significant information on the structural architecture of these materials have been obtained from a Wide Angle X-ray Scattering (WAXS) study (8), which show that in the amorphous state, the structural arrangement of manganese, nickel and oxygen atoms follows a spinel
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