Comparison of alkali-promoted ZrO2 catalysts towards carbon black oxidation

The effect of alkali metals deposition on zirconia has been studied in the oxidation of carbon black, considered as a model of diesel soot. The study of the influence of alkali content and alkali precursor was undertaken for K/ZrO2, evidencing a better activity for a catalyst prepared with an atomic ratio K/Zr = 0.14 and from a nitrate precursor. Using the latter preparation conditions, alkali/ZrO2 are found to be active in the oxidation of carbon black according the sequence ZrO2 < Li/ZrO2 < Na/ZrO2 < K/ZrO2 < Rb/ZrO2 < Cs/ZrO2. Alkali metals have an influence on the tetragonal–monoclinic crystalline modification. Alkali metals ions with low size tend to stabilize the tetragonal ZrO2 phase whereas those with higher ionic radius favour the tetragonal–monoclinic modification. Fourier transform-infrared spectroscopy (FTIR) and temperature programmed reduction (TPR) measurements show that the catalytic activity partially depends on the presence of nitrate species stabilized in alkali/ZrO2 even after calcination treatment at 600 °C. Nitrate species are more stable in the presence of alkali with high ionic radius than those of low size.