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.

Multi-Centered Solid-Phase Quasi-Intramolecular Redox Reactions of [(Chlorido)Pentaamminecobalt(III)] Permanganate—An Easy Route to Prepare Phase Pure CoMn2O4 Spinel

“…The solid phase formed in the thermal decomposition of compound 3-Mn under refluxing toluene in 1 h consisted of amorphous phases (Figure 10), which could be crystallized by heating above 400 °C with the formation of CoMn2O4 and gaseous (N2, H2O, NH3, NH4Cl) products. The crystallite size of the CoMn2O4 depended on the heating time and temperature, similarly to other spinels prepared from ammine complexes of transition metal permanganates [1,[3][4][5]. Separating the amorphous reaction product into water-soluble and insoluble (oxide) parts and evaporating the aqueous leachate, we assigned the water-soluble components by powder XRD and IR as NH4NO3, [Co(NH3)5Cl]Cl2 (compound 4) and a small amount of NH4Cl (Figure 10).…”

“…It is well known that ammonium nitrate forms in the solid-phase quasi-intramolecular redox reaction of the ammonia complexes of divalent metal permanganates with the formation of solid Mn (III) type spinel compounds [1][2][3][4][5]:…”

“…Nanosized mixed transition metal-manganese oxides can be prepared by the controlled thermal decomposition of transition metal permanganate complexes that have reducing inorganic [1][2][3][4][5][6] or organic ligands [7,8]. For example, Mansouri et al prepared a cobalt manganese oxide spinel (CoMn 2 O 4 ) with excellent activity in the Fischer-Tropsch fuel synthesis, by the thermal decomposition of the [Co(NH 3 ) 4 CO 3 ]MnO 4 complex [6,9]; however, [Co(NH 3 ) 4 CO 3 ]MnO 4 contains Co and Mn in a 1:1 atomic ratio.…”

Solid-Phase “Self-Hydrolysis” of [Zn(NH3)4MoO4@2H2O] Involving Enclathrated Water—An Easy Route to a Layered Basic Ammonium Zinc Molybdate Coordination Polymer

“…The PXRD and SEM meaurements were performed on a Philips Bragg Brentano parafocusing goniometer (Philips, Eindhoven, The Netherlands) and Zeiss EVO40 microscope (Carl Zeiss AG, Oberkochen, Germany) as descsribed in [31][32][33].…”

AgNO3⋅NH4NO3 – an enigmatic double‐salt type “decomposition intermediate” of diamminesilver(I) permanganate

“…1) During the decomposition reaction of compound 1 only one of the ammonia ligand is oxidized with the permanganate ion into nitrate in consistent with ammine complexes of transition metals. [23,25] As a result, the permanganate anion disappears, but equivalent amount of nitrate ion forms to neutralize the charge of the silver ions remained back as Ag + . The equation can be summarized as:…”