The Formation of Mn-Ce Oxide Catalysts for CO Oxidation by Oxalate Route: The Role of Annealing Conditions

Abstract

A series of Mn-Ce oxide catalysts with Mn:Ce = 1 was synthesized by oxalate route under different annealing conditions. The physicochemical properties were characterized by means of XRD, HRTEM, N2 adsorption, TPR, XPS and DD technique. All the catalysts were investigated in the CO oxidation reaction. The annealing conditions exhibited a great impact on the structural properties, which resulted in different catalytic performance. The activity of the samples calcined in air exceeds the activity of the samples prepared in argon at a similar temperature. In the inert at 300–400 °C, the oxalate precursor does not completely decompose to oxide; at 500–600 °C, MnO and ceria are formed. For the Mn-Ce oxide catalyst obtained in air, the combination of XRD, DD and TPR showed that the catalyst consists of a MnxCe1-xO2-δ (x ~ 0.2) solid solution, amorphous and crystalline manganese oxides (Mn3O4, Mn2O3, Mn5O8). XPS analysis indicates that the catalyst surface contains Mn cations in the form of Mn2+, Mn3+ and Mn4+. An increase in the calcination temperature results in the oxide sintering. During the variation of the calcination time at 300 °C, the composition of the MnxCe1-xO2-δ solid solution does not change, while transformations occur in Mn oxides. The most active Mn-Ce catalyst was obtained by annealing at 300 °C for 10 h in air and consists of a MnxCe1-xO2-δ solid solution and Mn5O8.