Mixed-Metal Amorphous and Spinel Phase Oxidation Catalysts: Characterization by X-ray Diffraction, X-ray Absorption, Electron Microscopy, and Catalytic Studies of Systems Containing Copper, Cobalt, and Manganese

Mixed-metal carbonates CuMn₂(CO₃)₃, CuMn(CO₃)₂, CoMn₂(CO₃)₃, and CuCoMn(CO₃)₃ possessing the rhodocrosite structure have been prepared by coprecipitation. Extended X-ray absorption fine structure (EXAFS) analysis indicates that cobalt substitutes fully and without distortion into the rhodocrosite structure in CoMn₂(CO₃)₃, whereas copper is present in CuMn₂(CO₃)₃ in a distorted environment. Decomposition at temperatures of ca. 600 K for CuCoMn(CO₃)₃ and ca. 690 K for the other carbonates yields amorphous or (for the CoMn₂ system) very poorly crystalline oxides. EXAFS analyses of these solids reveals that cobalt and manganese ions order to the second and higher coordination shells in proto-spinel structures faster than do ions of copper. The majority of manganese and cobalt cations have 6-fold and most copper 4-fold coordination in the just-decomposed oxides, although in the poorly crystalline CoMn₂ oxide around one-third of the cobalt ions are in tetrahedral sites. Heating the carbonates to 773 K in air produces spinels that are phase pure by X-ray diffraction: whereas the copper-bearing spinels are cubic, CoMn₂O₄ is tetragonal. Copper occupies mainly tetrahedral sites in these spinels, although the presence of some octahedral copper is clearly revealed by EXAFS. This increases in the order CuCoMnO₄ < CuMn₂O₄ < Cu_1.5Mn_1.5O₄. Manganese is predominantly octahedral in all of the spinels (Jahn-Teller distortion in CoMn₂O₄) and cobalt is octahedral in CuCoMnO₄ and occupies both sites in CoMn₂O₄, ordering on to the tetrahedral site upon heating. On the basis of EXAFS and X-ray absorption near-edge spectral (XANES) analyses and assuming the spinels are stoichiometric, we infer that copper is present in the spinels as a mixture of 1+ and 2+ ions. In CuMn₂O₄ and CuCoMnO₄ the manganese is present as a mixture of 3+ and 4+ and in CoMn₂O₄predominantly as 3+. Cobalt is 3+ in CuCoMnO₄ and a mixture of 2+ and 3+ in CoMn₂O₄. Many of the oxides catalyze the complete reaction of an undiluted 2:1 CO/O₂ gas mixture at room temperature, so that dilution of both the gas mixture and the catalyst itself was performed to give conversions less than 100% between 60 and 160 °C. Amorphous oxides in the Cu/Mn and Co/Mn systems have slightly higher specific activities than the corresponding spinels. Of all the mixed-metal oxides prepared, the CuCoMnO₄ spinel has both the highest specific activity, probably due to the cobalt being largely trivalent, as well as the highest surface area.