Effect of NOx adsorption/desorption over ceria-zirconia catalysts on the catalytic combustion of model soot

CeO2, ZrO2 and CexZr1−xO2 mixed oxides with different Ce/Zr ratios were prepared by (co-)precipitation and 500 °C-calcination, and were characterized by Raman spectroscopy, XRD and N2 adsorption at −196 °C. The catalytic activity of these materials for soot oxidation by NOx/O2 has been correlated with the catalytic activity for NO2 production, and the surface processes occurring upon NO + O2 interaction with the different catalysts have been studied by in situ DRIFTS and NOx adsorption and further He-TPD. The catalytic activity for soot combustion of cerium-containing catalysts depends on the NO2 production capacity, and a linear relationship between the temperature of maximum NO2 production and the temperature of half soot conversion has been obtained. In a first step, surface nitrites are formed upon catalysts interaction with NO + O2, for all the catalysts. These nitrites are progressively converted to nitrates, and NO2 is yielded after these adsorption/oxidation/desorption processes. For CexZr1−xO2 mixed oxides, the catalytic activity for NO2 production depends on the cerium content, the higher the better, rather than on the BET area of the catalyst. The whole results demonstrate that the production of NO2 in the range of temperatures relevant to soot oxidation not only depends on the NO/nitrites oxidation capacity of the catalyst used but also probably on the adsorption strength of the nitrates under NO oxidation conditions. The thermostability of the nitrates adsorbed on zirconium sites is higher than that of nitrates adsorbed on cerium sites.