Temperature-Limited Synthesis of Copper Manganites along the Borderline of the Amorphous/Crystalline State and Their Catalytic Activity in CO Oxidation.

Abstract

Copper manganese oxides (CMO) with CuMn2O4 composition are well-known catalysts, which are widely used for the oxidative removal of dangerous chemicals, e.g., enhancing the CO to CO2 conversion. Their catalytic activity is the highest, close to those of the pre-crystalline and amorphous states. Here we show an easy way to prepare a stable CMO material at the borderline of the amorphous and crystalline state (BAC-CMO) at low temperatures (<100 °C) followed annealing at 300 °C and point out its excellent catalytic activity in CO oxidation reactions. We demonstrate that the temperature-controlled decomposition of [Cu(NH3)4](MnO4)2 in CHCl3 and CCl4 at 61 and 77 °C, respectively, gives rise to the formation of amorphous CMO and NH4NO3, which greatly influences the composition as well as the Cu valence state of the annealed CMOs. Washing with water and annealing at 300 °C result in a BAC-CMO material, whereas the direct annealing of the as-prepared product at 300 °C gives rise to crystalline CuMn2O4 (sCMO, 15–40 nm) and ((Cu,Mn)2O3, bCMO, 35–40 nm) mixture. The annealing temperature influences both the quantity and crystallite size of sCMO and bCMO products. In 0.5% CO/0.5% O2/He mixture the best CO to CO2 conversion rates were achieved at 200 °C with the BAC-CMO sample (0.011 mol CO2/(m2 h)) prepared in CCl4. The activity of this BAC-CMO at 125 °C decreases to half of its original value within 3 h and this activity is almost unchanged during another 20 h. The BAC-CMO catalyst can be regenerated without any loss in its catalytic activity, which provides the possibility for its long-term industrial application.