Dynamic active sites over binary oxide catalysts: In situ/operando spectroscopic study of low-temperature CO oxidation over MnOx-CeO2 catalysts

Abstract

The determination of the dynamic active sites over binary oxide catalysts is of great challenge in heterogeneous catalysis. In this work, the origin of active sites toward low-temperature CO oxidation (<200°C) were thoroughly studied using MnOx-CeO2 composite oxide catalysts with different Mn/Ce molar ratios, synthesized by a redox co-precipitation method. The optimum Mn1Ce1 catalyst (T100=190°C), which showed excellent activity, has found to be composed of three phases: (1) CeO2; (2) amorphous MnOx; (3) MnOx-CeO2 solid solution (active sites). With the combination of kinetics and characterization results, including Temperature-Programmed-Desorption/Reduction (TPD/TPR), operando Raman spectroscopy and in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS), the dynamic structures of catalysts were rationalized with the identification of the interface of MnOx and CeO2. The mechanism for CO oxidation over MnOx-CeO2 in the temperature range 100–190°C were proposed that the direct and the formate routes were followed at T<130°C, and the carbonate route became dominant at T>130°C. Notably, the Mars-van Krevelen mechanism was proceeded in the whole temperature range. We speculate that cheap binary oxides will substitute for noble metal as catalysts for the removal of CO and other toxic gases, especially operating under mild conditions.