Quantification of the active site density and turnover frequency for soot combustion with O2 on Cr doped CeO2

The soot combustion on Cr-doped CeO2 mixed oxides was studied. The catalysts were characterized by X-ray powder diffraction, N2 adsorption/desorption, X-ray photoelectron spectroscopy and temperature-programmed reduction with H2. The active sites were quantified using isothermal anaerobic titrations with soot as the probe molecule. The turnover frequency (TOF) was calculated and used to evaluate the activity. The samples with a Cr/(Cr + Ce) ratio within 1–3 at.% show an improved activity on the basis of reaction rates and TOF values. The active sites were determined to be composed of Ce–O–Ce species, which is a little more active compared with CeO2 due to the promotion by the doping of Cr. Although the similar reaction rates were observed, the doping of 5 at.% Cr results in a detectable decrease of the TOF compared with CeO2. This is because the active sites are composed of Cr–O–Ce species. The strong interaction of the supported CrOx species with CeO2 results in an increase in the activity of Cr2O3. The promotional effect of Fe to CeO2 on soot combustion is more significant than that of Cr. The TOF is strongly recommended to be the basis of the activity comparison.