Operando X-ray Diffraction Study of Mn–Ce Catalysts for CO Oxidation

Abstract

We prepare a series of MnO_x–CeO₂ catalysts with a molar ratio of Mn : Ce = 3 : 7 by coprecipitation and varying the calcination temperature from 300 to 800°C. The catalysts are characterized by powder X-ray diffraction, low-temperature nitrogen adsorption, and X-ray photoelectron spectroscopy, and the catalytic activity of all samples is tested in the CO oxidation reaction. A (Mn,Ce)O₂ solid solution with the fluorite structure forms in all catalysts. Based on the studies performed, a catalyst calcined at 600°C is selected for further studies of the effect of topochemical reduction on the catalytic activity in the CO oxidation reaction by X-ray diffraction in the operando mode. The experiment is carried out sequentially in a stepwise mode: stepwise heating/cooling in a reaction mixture of 1% CO + 2% O₂ in the mode 150–175–200–175–150°C (stages 1, 3, and 5); reduction of the sample in a mixture of 10% CO + He at 400°C (stage 2); reduction of the sample in a mixture of 10% H₂ + He at 400°С (stage 4). The reductive treatment leads to segregation of the initial (Mn,Ce)O₂ solid solution and the appearance of dispersed manganese oxides on the surface, while enrichment of the surface with manganese oxide increases its activity in the CO oxidation reaction.