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 < Cu1.5Mn1.5O4. 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 CoMn2O4 predominantly 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-degrees-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.