Influence of valence state of cerium ion on dimethyl carbonate synthesis from methanol and carbon dioxide over CeO2

The relationship between valence state of cerium ion and the catalytic activity for dimethyl carbonate (DMC) synthesis directly from carbon dioxide and methanol was investigated by using four CeO2 catalysts calcined at the different temperatures, which showed different concentrations of Ce4+ analyzed by XPS. In situ Fourier transform infrared spectroscopy (FTIR) was used to probe CO2 and methanol adsorption species on ceria surface. With the calcination temperature increasing from 323 to 973 K, the concentration of Ce4+ increased, leading to larger proportion of active methanol adsorptions (Types I and II methoxy) and CO2 adsorptions (bidentate carbonate). Meanwhile, high-calcination temperature also led to small surface area and more -OH group dehydration, which resulted in the less formation of active Types I and II methoxy and bidentate carbonate. As a result, the ceria calcined at 773 K showed the highest activity and the one calcined at 973 K performed the lowest.